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Environ Sci Pollut Res DOI 10.1007/s11356-017-9003-8

RECENT TRENDS FOR THE REMOVAL OF COLOURED PARTICLES IN INDUSTRIAL WASTEWATERS

An overview of nanomaterials applied for removing dyes from wastewater Zhengqing Cai 1 & Youmin Sun 1 & Wen Liu 2 & Fei Pan 3 & Peizhe Sun 4 & Jie Fu 1

Received: 30 October 2016 / Accepted: 7 April 2017 # Springer-Verlag Berlin Heidelberg 2017

Abstract Organic dyes are one of the most commonly discharged pollutants in wastewaters; however, many conventional treatment methods cannot treat them effectively. Over the past few decades, we have witnessed rapid development of nanotechnologies, which offered new opportunities for developing innovative methods to treat dye-contaminated wastewater with low price and high efficiency. The large surface area, modified surface properties, unique electron conduction properties, etc. offer nanomaterials with excellent performances in dyecontaminated wastewater treatment. For examples, the agarmodified monometallic/bimetallic nanoparticles have the maximum methylene blue adsorption capacity of 875.0 mg/g, which are several times higher than conventional adsorbents. Among various nanomaterials, the carbonaceous nanomaterials, nanosized TiO2, and graphitic carbon nitride (g-C3N4) are considered as the most promising nanomaterials for removing dyes from water phase. However, some challenges, such as high cost and poor separation performance, still limit their engineering application. This article reviewed the recent advances in the Responsible editor: Guilherme L. Dotto * Wen Liu [email protected] * Jie Fu [email protected]

nanomaterials used for dye removal via adsorption, photocatalytic degradation, and biological treatment. The modification methods for improving the effectiveness of nanomaterials are highlighted. Finally, the current knowledge gaps of developing nanomaterials on the environmental application were discussed, and the possible further research direction is proposed. Keywords Nanomaterials . Dyes . Organic pollutants . Adsorption . Photocatalytic degradation . Biological wastewater treatment

Abbreviations AFM Atomic force microscopy AOPs Advanced oxidation processes AQDS Anthraquinone-2,6-disulfonate CNFs Carbon nanofibers CNTs Carbon nanotubes CA Cellulose acetate CB Conduction band g-C3N4 Graphitic carbon nitride HS Humic substances NPs Nanoparticles NFW Nanofibrous webs PAC Powdered activated carbon SEM Scanning electron microscopy TNTs Titanate nanotubes UASB Upflow anaerobic sludge bed VB Valance band

1

Department of Environmental Science and Engineering, Fudan University, Shanghai 200433, China

2

School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA

3

School of Environmental Engineering, Wuhan Textile University, Wuhan 430073, China

Introduction

4

School of Environmental Science and Engineering, Tianjin University, Tianjin 300072, China

Dye molecules generally comprise of the chromophores and the auxochromes, of which the chromophores are responsible

Environ Sci Pollut Res

for producing color, and the auxochromes supplement the chromophores, render the solubility of dye, and increase their affinity toward the fibers. There are many structural varieties of dyes, such as acidic, basic, azo, diazo, anthroquinone based, and metal complex dyes. More than 100,000 dyes are commercially available and with a production amount of approximately 700,000 t per year (Crini 2006). Dyes are extensively used in various fields, such as textile (Sathishkumar et al. 2011), leather, paper, paint, cosmetics, etc. (Hong et al. 2009). During the application, large quantities of dyes were discharged into the water environment. It is estimated that approximate 10% of total spent dyes are eventually discharged into the effluent in the textile process. In the past decades, large percentage of industries with high contaminant discharges have been transferred to developing country. Nowadays, the most amount of dyes is manufactured and used in the developing countries, e.g., China, India, Thailand, Turkey, etc. (Boonyprapa et al. 2009; Güyer et al. 2016). Many of these developing countries do not have strict legislation, or the discharge standard is not mandatorily executed, which makes the dye pollution worse. And it is still challenging for the developing countries to eliminate the environmental pollution in the textile and related industries due to the cost and technical obstacles. However, the developed countries, like the USA, Canada, and Australia, have strict legislation to limit the effluent discharge of dyes, and they are capable to treat the dye-contaminated water from the aspects of technology and investment (Chequer et al. 2013). Most of dye molecules contain aromatic ring, which makes them carcinogenic, resistant to biodegradation, highly toxic, and mutagenic to both human beings and aquatic life (Zeng et al. 2009). In addition, the dyes cause undesirable esthetic problem (Luo et al. 2010), prevent the solar light penetration and retard the photosynthetic reaction, and affect the aquatic life. Therefore, it is essential to remove or minimize dyes to permissible concentration prior to discharge. A number of methods have been developed to reduce dyes in wastewater, such as adsorption, photo-oxidation, chemical oxidation, advanced oxidation processes (AOPs), biodegradation, filtration, coagulation, etc. (Alventosa-deLara et al. 2012; Cai et al. 2016b; Fu et al. 2009; Gupta and Suhas 2009; Yuan et al. 2012). However, these prevalent methods have respective deficiencies and defects. For example, the adsorption is considered as a globally acclaimed method, but the conventional adsorbents always show the limits of low adsorption capacity and kinetics, the high regeneration cost, and column fouling; thus, the new adsorbents are still required (Fu et al. 2011a; Liu et al. 2016a). The AOPs have been extensively studied and applied for treating the organic dyecontaminated wastewater (Fu and Kyzas 2014; Zhang et al. 2011). Among them, the heterogeneous photocatalytic oxidation processes are the most promising methods (Fu et al. 2010), especially when the solar light is used. It could rapidly

oxidize or even mineralize organic dyes without chemical consumption. However, for many photocatalysts, their photocatalytic activity still need to be enhanced, and which are always achieved by extending the visible light absorbance, inhibiting the excited electron-hole pair recombination, and enlarging the specific surface area. The conventional biological treatment methods are always ineffective due to the recalcitrant nature of the synthetic dyes and the high salinity of the dye-contaminated wastewater (Zhang et al. 2011). Therefore, many of conventional treatment methods still need to be modified to obtain the faster rate and higher efficiency, and most importantly with lower treatment cost. Nanomaterials refer to the materials with one or more external dimensions in the range between 1 and 100 nm (Kreyling et al. 2010). The small size offers nanomaterials extraordinary properties, e.g., extremely large surface area, more surface active sites, quantum effect, unique electron conduction property, etc. (Qu et al. 2013). These unique properties greatly benefit the performance of nanomaterials when used as adsorbents, catalysts, sensor, or in other application. Thus, nanomaterials bring new opportunities for the revolution of the dye-contaminated wastewater treatment technologies. Many nanomaterials, such as carbon nanotubes (CNTs) (Ma et al. 2012), graphene (Sui et al. 2013; Xiong et al. 2010b), TiO2 nanoparticles (Wang et al. 2017), MoS2 nanosheets (Li et al. 2014; Massey et al. 2016), etc., have been used to develop novel sorbents or photocatalytic materials for dye removal and showed superior performance than bulk materials. Some researchers found that the nanomaterials could promote the biodegradation of dyes in wastewater (Sathian et al. 2014). However, to the best of our knowledge, there is a lack of comprehensive review articles focused on the dye removal using nanomaterials. The aim of this work is to review the investigation/ application of different nanomaterials for the adsorption, photocatalytic degradation, and biological treatment of the dyes in water phase. In this review, the fundamental mechanism of the technologies using nanomaterials, the methods for improving the photocatalytic activity, or adsorption kinetic/capacity of the nanomaterials will be summarized. Moreover, the merits and limits of respective nanotechnologies will be discussed to reveal the application potentials. Finally, the current knowledge gaps of developing nanomaterials on the environmental application were discussed, and the possible further research direction are proposed.

Adsorption Adsorption is considered as a globally acclaimed method for treating the dye-contaminated water due to the high efficiency and wide applicability (Ali 2010, 2012, 2014; Ali et al. 2012, 2016b, 2016c; Ali and Gupta 2007; Cai et al. 2017; Kyzas


et al. 2013; Luo et al. 2010; Machado et al. 2011). A number of adsorbents, e.g., activated carbon, resins, mesoporous hybrid xerogel, pansil, mesoporous silica, clay, and sepiolite, have been investigated for removing water-soluble organic dyes like methylene blue, rhodamine B, methyl violet, procion red MX5B, etc. (Liu et al. 2012). Among these adsorbents, activated carbon and resins have been widely applied (Mahmoud et al. 2013); however, the relatively low adsorption capacity, high regeneration cost, and column fouling are the major demerits of these adsorbents. Therefore, the searching of better alternative adsorbents is currently ongoing. In recently years, the emerging of nanomaterials offers opportunities for developing new generation of adsorbents with high efficiency and capacity. In this part, an effort has been made to briefly describe the properties, recent advances, effectiveness, and merits of the most widely studied or used nanoadsorbents, i.e., carbonbased nanomaterials, titanate nanotubes (TNTs), nanoFe3O4, nano-MgO, etc.; the limitations and challenges of using these novel nanomaterials are discussed. Meanwhile, the adsorption mechanism and the adsorption kinetic and isotherm models by nanomaterials are discussed. Carbonaceous nanomaterials The carbonaceous nanomaterials generally refer to graphene, carbon nanofibers (CNFs), and carbon nanotubes (CNTs); they contain strong covalently bonded carbon molecules. As adsorbents, the carbonaceous nanomaterials adsorptively remove dyes through hydrophobic effect, hydrogen bonding, π-π stacking, electrostatic, and covalent interaction (Liu et al. 2012; Ma et al. 2012). The high adsorption kinetic and capacity and higher affinity make the carbonaceous nanomaterials superior adsorbents for dye removal from water phase when compared with conventional activated carbon. Graphene, CNFs, and CNTs have distinct geometry and show different spaces for adsorption. Graphene is a 2D sheet of sp2-hybridized carbon network with a honeycomb crystal structure (Liu et al. 2012); CNFs are cylindrical nanostructure with graphene layers arranged as stacked cones or cups (Rodriguez et al. 2010; Serp et al. 2003), while CNTs are rolled-up graphene sheets with single wall (SWCNTs) or multiwall (MWCNTs). For graphene, the adsorption occurs on both sides of the nanosheet; therefore, extremely large surface area is available to provide adsorption sites. For CNFs, only the external surface is available for the adsorption reaction. On the other hand, the adsorption by CNTs can take place at the external surface, inner cavity, and interwall spaces (only present in MWCNTs). However, the interwall spaces could only be accessible by the molecules with size smaller than the interwall distance, and the cavity could commonly be blocked by impurities or amorphous carbons. Therefore, the available spaces of carbonaceous nanomaterials are generally limited to

their external spaces (Yang and Xing 2010). It is worth noting that the theoretical surface area is always much larger than the measured surface area due to the aggregation of nanomaterials. Besides the surface area, the surface function groups greatly affect their adsorption performance. Different synthesis, purification, and postprocessing methods provide carbonaceous nanomaterials with different surface area and surface functional group, which will greatly change the adsorption performance. The long-range π-conjugation of graphene yields extraordinary mechanical strength and high thermal and electrical conductivity (Allen et al. 2010); in addition, the graphene owns extreme large specific surface area (2630 m2/g) and flat structure (Balapanuru et al. 2010; Liu et al. 2012). These ideal properties make graphene an excellent adsorbent and favored its application as an ideal support to anchor chemical functionalities or nanomaterials. The graphene-based nanocomposites are believed to own promising potential for adsorption process. Graphene oxide is the most popular graphene-based adsorbent with lower production cost; the high density of functional groups (hydroxyl, carboxyl, carbonyl, and epoxy) in the carbon lattice offers it with high adsorption performance (Perreault et al. 2015). Liu et al. (2012) applied a 3D graphene oxide nanostructure to remove methylene blue and methyl violet from water phase. The materials showed high dye removal efficiency, where 99.1% of methylene blue and 98.8% of methyl violet were removed within 2 min at the initial concentration of 800 and 946 mg/L, respectively (as shown in Fig. 1). Many efforts have been devoted to test the adsorption capacity of the graphene-based materials, and some interesting properties of graphene-based materials were observed. For instance, the cationic dye adsorption over a wide pH range (6–10) is mediated through electrostatic interaction between dyes and graphene oxide; however, the anionic dye adsorption is not favorable at the same pH range (Ramesha et al. 2011). It is because the adsorption of cationic dyes by graphene oxides is primary due to electrostatic interactions (Yang et al. 2013).

Fig. 1 The adsorption of dyes on the graphene oxide nanomaterials. Adapted from reference Liu et al. (2012)


Other than the excellent adsorption properties, graphene has good photocatalytic activity which gives it potential for the synergetic removal of dyes from water phase (Xiong et al. 2010b). More information will be discussed in the photocatalytic degradation part. It needs to mention that the graphenefamily nanomaterials had adverse impacts on aquatic organisms (e.g., algae, bacteria, fish, invertebrates, and plants) (Zhao et al. 2014). Thus, their toxicity should be evaluated prior to field application. The CNTs are one of most widely studied adsorbents. As early as 2004, the caged MWCNTs have been studied to remove acridine orange, ethidium bromide, eosin bluish, and orange G from water phase and exhibited high adsorptive capacity of 163, 170, 274, and 140 mg/g, respectively (Fugetsu et al. 2004). Thereafter, the CNTs have been extensively studied for treating dye-contaminated wastewater. Many methods have been applied to modify the materials to acquire higher adsorption efficiency or improve the recovery performance of the materials. Similar to other carbon-based materials, the activation of CNTs is effective to modify the surface morphology and surface functional groups and remove the amorphous carbon, thus enhance the adsorption capacity CNTs. Ma et al. (2012) applied the alkali-activated method to treat the synthesized MWCNTs; the obtained materials showed high specific surface area and large pore volume and possessed oxygen-containing functional groups. The modified MWCNTs exhibited excellent adsorption capacity for methylene blue (399 mg/g) and methyl orange (149 mg/ g). Introducing surface functional group is another way to promote the adsorption capacity of CNTs. For example, Rong et al. (2016) applied a glucose functionalized MWCNTs (MWCNT-COOH) to adsorptively remove various dyes (as shown in Fig. 2). The functionalized materials showed higher dye removal efficiency and faster adsorption kinetics than the original MWCNTs. The separation is a challenge for many nanomaterials. To facilitate the separation of CNTs from aqueous phase, a CNT-nano-Fe3O4 composite was synthesized. The prepared material can be easily separated by magnetic forces, and they also showed relatively high methylene blue and janus green adsorption capacity (48.1 and 250 mg/g, respectively) (Gupta and Suhas 2009; Madrakian et al. 2011). The development of synthesis technologies is decreasing the cost of the carbonaceous nanomaterials; in addition, the materials with relatively low purities could still fulfill the requirement as adsorbents. Therefore, the CNTs and graphene oxides are believed to be the attractive alternative for the conventional adsorbents for removing dyes from wastewater. However, the systematic comparative studies of the adsorption of different dyes over these nanomaterials are still lacking. Most published works explained the adsorption mechanisms based on the morphological properties, surface properties, adsorption kinetics, and isotherms. The import

thermodynamic parameters, such as enthalpy, entropy, adsorption free energy, and surface energies, are not generally characterized and discussed. This is not only for the carbonaceous nanomaterials but also for about all the following reviewed nanoadsorbents. Iron oxides The iron oxide nanoparticles can be synthesized in the forms of magnetite (Fe3O4), maghemite (γ-Fe2O3), and hematite (αFe2O3). The adsorption of dyes onto iron oxide primarily undergoes via surface exchange reactions, and the adsorption capacity is correlated to the amount of surface function groups. Similar to other nanomaterials, the large surface area of iron oxide nanoparticles provides them with tremendous surface active sites and high adsorption capacity. Meanwhile, the low cost, easy synthesis, and superparamagnetism property make them affordable and reusable materials. Now, they are believed to be promising materials for the environmental remediation (Ali et al. 2016a, 2016c, 2016d, 2016e; Zhang et al. 2013; Zhao et al. 2016b). Fe3O4 and γ-Fe2O3 nanomaterials are commonly used to remove dyes from water phase. A Fe3O4 hollow nanosphere was studied to remove neutral red dye from water phase; the adsorption of neutral red can reach equilibrium within 60 min at initial concentration of 200 mg/L, and the maximum dye adsorption capacity was 105 mg/g Fe3O4 hollow nanospheres (Iram et al. 2010). Nassar (2010) applied a type of γ-Fe2O3 nanoadsorbent to remove acid red dye from aqueous solutions; the acid red at initial concentration of 103 mg/L was completely removed within 4 min. The authors stated that the fast kinetic is due to the small size of γ-Fe2O3 nanoadsorbent; however, the high adsorbent dosage (10 g/L) should also be responsible to the high adsorption rate (Mahmoud et al. 2013). The surface modification is generally applied to enhance the adsorption capability of iron oxide nanomaterials. A type of 3-glycidoxypropyltrimethoxysilane (GPTMS) and glycine (Gly) modified Fe3O4 magnetic nanoparticles was applied to remove anionic and cationic dyes. The maximum adsorption capacity of five dyes, i.e., methyl blue, orange I, acid red 18, methylene blue, and azure I, was determined to be 158, 49, 45, 123, and 357 mg/g, respectively (Zhang et al. 2013). In addition, magnetic nanomaterials can be synthesized with other adsorbents to obtain both high adsorption capacity and separation property (Zhang and Kong 2011). However, the magnetic nanomaterials can be oxidized to Fe3+ ion by dissolved oxygen (Tang and Lo 2013). To overcome this problem, the coating of thin layer of polymer or other materials have been studied. For example, Patra et al. (2016) synthesized the agarmodified monometallic/bimetallic nanoparticles (Fe/Pd), which showed high stability and extremely high maximum adsorption capacity for methylene blue (875.0 mg/g) and rhodamine B (780.0 mg/g) removal. Meanwhile, the materials


Fig. 2 a Effect of MWCNT dosage on the adsorption of various dyes. b Comparison of the adsorption efficiency between MWCNTs and modified MWCNTs (MWCNT-COOH). c Zeta potential and adsorption

efficiency of materials at various pH. d Adsorption kinetics of various dyes. Adapted from reference Rong et al. (2016)

can be easily separated by magnetic force and regenerated without significant capacity loss. In summary, the superior adsorption performance and high separability make the iron oxide nanomaterials promising adsorbents for treating dye-contaminated wastewater. It is believed that the novel physical, chemical, and magnetic properties of iron oxide nanomaterials will facilitate the advanced applications in generating the lower cost and more efficient adsorptive technologies comparing with conventional technologies (Xu et al. 2012).

adsorption capacity of dyes due to the loss of cation exchange capacity. However, for the cationic dyes, e.g., methylene blue, the removal of methylene blue, the adsorption is primarily driven by the electrostatic attraction (Xiong et al. 2010a). Based on the adsorption mechanisms, some works have carried out for improving the adsorption performance of TNTs. Lee et al. (2007) investigated the adsorptive removal of two basic dyes (basic green 5 and basic violet 10) and two acid dyes (acid red 1 and acid blue 9) by TNTs synthesized by hydrothermal treatment and the hexadecyltrimethylammonium (HDTMA)-modified TNTs. The adsorption capacities of HDTMA-modified TNTs were improved by 10–20 times even though the surface area was decreased after the modification (Lee et al. 2007). It should be noted that the TNTs are photocatalytic active, especially when the TNTs was calcined. Therefore, the adsorption and photodegradation of dyes by TNTs could be combined to reach a more efficient and costsaving technology (Tang et al. 2012).

Titanate nanotubes Titanate nanotubes (TNTs), exhibiting multiwalled roll-type open-end structures, have relatively high pore volume (e.g., 0.67 to 0.89 cm3/g) and large surface area (i.e., 157.9 m2/g) (Lee et al. 2008b; Liu et al. 2016c; Xiong et al. 2010a; Zhao et al. 2016a). Very recently, the TNTs were found to have excellent adsorption ability to remove organic dyes from aqueous phase. Generally, the removal of basic dyes, e.g., basic violet 3, from aqueous phase is through the cation exchange mechanism, and adsorption capacity is dominated by the cation exchange capacity and the pore volume of TNTs (Lee et al. 2008a). According to the works done by Lee et al. (2008b), the sodium content was a key parameter for the adsorptive removal of basic dyes. The low sodium content increased the specific surface area and pore volume but reduced the

Magnesium oxide The MgO has been applied as a destructive adsorbent for toxic chemicals before (Sawai et al. 2000); however, the investigation of using nano-sized MgO to remove dye from aqueous was reported only in the past few years. Moussavi and Mahmoudi (2009) and Hu et al. (2010) investigated the removal of congo red and reactive brilliant red X3B by nano-MgO and reached the


maximum adsorption capacity of 297.0 and 254.3 mg/g, respectively, which are much higher than on activated carbon (Hu et al. 2010). The high adsorption capacity is primarily due to the large surface area (ca. 198 m2/g) of the MgO nanoplates prepared from Mg ribbon. Similarly, a type of MgO nanoparticles with surface area of 153.7 m2/g was synthesized by sol-gel method and showed the maximum adsorption capacity of 166.7 and 123.5 mg/g for reactive blue 19 and reactive red 198, respectively (Moussavi and Mahmoudi 2009). However, the literatures only show that MgO nanomaterials can effectively remove anionic dyes, whereas the treatment of other types of dyes is needed to further explore (Tan et al. 2015). Only limited literatures reported the effects of MgO nanomaterials on the dye removal. However, the low cost, relatively high adsorption capacity, and high adsorption rate are attractive properties of MgO nanomaterials for treating dye-contaminated wastewater.

Molybdenum disulfide Molybdenum disulfide (MoS2) nanosheet is a graphene-like 2D material. The MoS2 nanosheets are always vertically stacked, and the van der Waals interactions hold them together. The structure and atomic force microscopy (AFM) image of MoS2 are shown in Fig. 3 (Radisavljevic et al. 2011). The MoS2 nanosheets that synthesized by hydrothermal method show hierarchical structure, which makes the diffusion of dyes to material surface very fast. The MoS2 has rapid adsorption kinetic and large adsorption capacity. An experiment showed that the adsorption capacity of congo red dyes, malachite green, fuchsin acid, rhodamine 6G, and methylene blue can reach 146, 204, 183, 216, and 297 mg/g, respectively (Massey et al. 2016). The MoS2 is not commonly considered as adsorbents; however, they were more regularly reported as photocatalysts. The prominent adsorption property of MoS2 can greatly promote the photocatalytic degradation performance. As a good

photocatalyst, the MoS2 will be further discussed in the photocatalytic degradation part. Many other nanomaterials have been studied for removing dyes from water phase, e.g., AgBr (Tang et al. 2016b), Bi2WO6 (Tang et al. 2016b), CeO2-δ (Tomic et al. 2014), carbon nitride (Zhang et al. 2016), spherical ZnO (Chaudhary et al. 2013), etc. However, the further research or application has been retarded by either high material cost or low adsorption capacity/removal efficiency. The merits and limits of the discussed nanomaterials are summarized in Table 1.

Adsorption isotherm and kinetic models Adsorption isotherm models play an important role in predictive modeling for analysis and designing adsorptive systems and can also provide useful information about the possible interactions between adsorbents and adsorbates. Based on the literature analysis, the Langmuir and Freundlich isotherm models are the two most frequently models for fitting the dye adsorption data on nanoadsorbents (as shown in Table 2) (Luo et al. 2010; Peng et al. 2012). The Langmuir isotherm model is based on the assumptions that the adsorbent has a specific number of adsorption sites with equal affinity for the sorbate and only one layer of adsorbate molecules attached on the surface. Therefore, a saturation point is expected to appear at equilibrium, and no further adsorption will occur (Langmuir 1918). In contrast, the Freundlich isotherm model usually describes the adsorption of adsorbate onto heterogeneous surfaces with uniform energy distribution and reversible adsorption. The adsorption energy exponentially decreases on completion of the adsorptional centers of an adsorbent. The adsorbate amount on the adsorbent will increase as long as the adsorbate concentration in solution increases (Iram et al. 2010).

Fig. 3 a Structure and b AFM images of monolayer MoS2. Adapted from reference Radisavljevic et al. (2011)

Environ Sci Pollut Res Table 1

The merits and limits of various nanomaterials as adsorbents for dye removal

Material

Limits

Merits

References

CNTs and CNFs

Relatively high cost

Ma et al. (2012), Machado et al. (2011), Machado et al. (2012))

Graphene

Relatively high cost

Iron oxide nanoparticle

Aggregation limits the adsorption capacity Relatively high cost

Large specific surface area, hydrophobic wall, easy to be modified, high adsorption capacity Extremely large specific surface area, high adsorption capacity, easy to be modified Relatively high efficiency, low cost, easy to separate, easy to regenerate Large specific surface area, high adsorption capacity, can be combined with photocatalytic degradation Large specific surface area, high adsorption capacity, low cost Large specific surface area, can be combined with photocatalytic degradation

TNTs

MgO nanoparticles MoS2

Only effective for some dyes

Most of the studied adsorption isotherm data of dyes over nanomaterials can be better fitted by Langmuir model than Freundlich model (Xiong et al. 2010a), indicating the homogeneous nature of nanoadsorbent surface and the monolayer adsorption of dye molecules on nanoadsorbents. For example, a study applied the calcined TNTs to adsorptively remove methylene blue from aqueous phase; the adsorption isotherm data fitted by Langmuir and Freundlich models give R2 value of 0.9930 and 0.7708; obviously, the tested TNTs have uniform nanotubular structure and homogeneous distribution of active sites (Xiong et al. 2010a). Many artificially synthesized nanomaterials regularly show uniform crystal structure and homogeneous surface, and they are expected to show the adsorption with Langmuir model. However, some composite nanomaterials have heterogeneous surface; their adsorption data can be fitted by Freundlich model (Iram et al. 2010; Patra et al. 2016). For example, Patra et al. (2016) fitted the adsorption isotherm of methylene blue onto agar@Fe/Pd nanoparticles and found that the fitting by Freundlich model was much better than Langmuir model (with R2 of 0.99 and 0.82, respectively). Therefore, the heterogeneous surface with multilayer sorption can be presented on the agar@Fe/Pd nanoparticles. It is worth noting that many researchers fitted their experimental data by various adsorption isotherm models, then used the best fitted model to explain the underlying mechanism. However, due to the inevitable experimental errors and the limited adsorption isotherm data, some of the fitting can be deviated from the true isotherm (Yang and Xing 2010); thus, the conclusion derived from the fitted model can be problematic. Fitting the isotherm in narrow range of equilibrium concentration of adsorbate is the most commonly made problem, which will make it hard to find the true isotherm model and increase the possibility of misleading the data interpretations. For example, Mahmoud et al. (2013) fitted the adsorption data of remazol red RB-133 dye on three Fe 2 O 3 /MgO

Sui et al. (2013), Zhao et al. (2014) Mahmoud et al. (2013) Liu et al. (2016c), Xiong et al. (2010a) Hu et al. (2010) Massey et al. (2016)

nanomaterials and found that their data can be well described by both Langmuir and Freundlich models, with the R2 between 0.982 and 0.997. In their experiment, the equilibrium dye concentrations are in the very narrow range, i.e., from 1.2 to 1.8, from 2.0 to 3.8, and from 3.1 to 5.7 mg/L, respectively, for the three adsorbents. From our opinion, the equilibrium concentration of adsorbate is suggested to be in the range of >1 order of magnitude for the isotherm data fitting. Moreover, adequate experimental data points in the extended range of adsorbate concentrations are required. The adsorption rates of dyes on nanoadsorbents can be very fast at the initial stage due to the great concentration gradient and large surface area of nanomaterials. In most of the published literatures, the adsorption of dyes on nanomaterials follows the pseudo-second-order kinetic model (Hu et al. 2010; Peng et al. 2016; Zare et al. 2015), which indicate that the chemisorption is the rate-controlling mechanism (Reddad et al. 2002). Some examples are shown in Table 3. For instance, the adsorption of neutral red on halloysite nanotubes can be well described by pseudo-second-order kinetic model (Luo et al. 2010), and the adsorption of methyl orange and methylene blue on alkali-activated MWCNTs can be described by pseudo-second-order kinetic model (Ma et al. 2012). The intra-particle diffusion model based on the theory proposed by Weber and Morris could be used to identify the diffusion mechanism of the adsorption process (Fu et al. 2011b; Mahmoud et al. 2013); this model is occasionally used to describe the adsorption of dyes on nanomaterials. For example, the adsorption kinetics of remazol red RB-133 dye on three Fe2O3/MgO nanomaterials can be well described by the intra-particle diffusion model (Mahmoud et al. 2013). Photocatalytic degradation Photocatalytic degradation is one of the most studied methods for decomposition of dyes in water phase. Nowadays, the

þ

b

1 K L Qm

Orange G, indigo carmine Methylene blue, orange red, acid red, methylene blue Neutral red Methylene blue

AgBr-AgBr/CTAB

Fe3O4@GPTMS@Gly

Calcined TNTs

Remazol red 133 Neutral red Methylene blue Methylene blue

Agar-based bimetallic nanoparticles

MoS2

Methylene blue

Fe3O4 hollow spheres

613.5

Congo red

Mesoporous MgO

Cellulose-clay hydrogel

FeMgOIM, FeMgOCo, FeMgOHY

36.9, 23.1, 32.3

Remazol red 133

FeMgOIM, FeMgOCo, FeMgOHY

303.03, 277.78

18.79 –

–

1.2926

0.9835

0.99

0.994

0.989, 0.982, 0.997

0.99 4.3, 10.9, 1.86

0.999

0.37

0.982, 0.989, 0.995

0.994, 0.997

0.9999, 0.9974

0.9930

Massey et al. (2016)

Patra et al. (2016)

Iram et al. (2010)

Mahmoud et al. (2013)

Peng et al. (2016)

Li et al. (2012)

Mahmoud et al. (2013)

Venkatesha et al. (2012)

Hu et al. (2010)

Xiong et al. (2010a)

Iram et al. (2010)

Zhang et al. (2013)

– 0.992

Tang et al. (2016b)

Ma et al. (2012)

Zare et al. (2015)

References

0.9996, 0.9999

0.837, 0.911

0.9931

R2

–

1.13, 1.86, 2.58

0.220, 0.397

0.589, 0.279

1.06

0.0182, 0.0501, 0.0231, 0.0078 4.81

3.47, 12.69

2.45, 3.05

0.1132

KL or KF

0.9285

0.46

1.7857, 3.125, 1.8868

1065

92.16, 86.50

Congo red, reactive brilliant red X3B Levafix fast red CA, indanthren blue

MgO nanoparticle

133.33

105

158, 49, 45, 123

91.41, 139.28

149, 399

231.8

Qm or 1/n

MgO nanoplates

Fe3O4 hollow spheres

Congo red Methyl orange, methylene blue

MWCNTs

Alkali-activated MWCNTs

Dye

KF (L/mg(mg/L)1/n ) and 1/n are Freundlich constants related to adsorption capacity and adsorption intensity, respectively. The KF value indicates the affinity of adsorbate on adsorbent, and the empirical parameter 1/n (Ozcan et al. 2004)

b

Ce (mg/L) is the equilibrium adsorbate concentration, Qe (mg/g) is the equilibrium adsorbate concentration on adsorbent, Qm (mg/g) is the maximum adsorption capacity corresponding to monolayer coverage, and KL (L/mg) is the Langmuir constant

a

Freundlich Qe = KFCe1/n

¼

Ce Qm

Langmuira

Ce Qe

Adsorbent

Examples of dye adsorption isotherm parameters fitted by Langmuir and Freundlich adsorption isotherm models

Model

Table 2


þ

pﬃ tþcd

Fe2O3/MgO nanomaterials Agar-based bimetallic nanoparticles

MWCNTs AgBr-AgBr/CTAB nanomaterial

Calcined TNTs MoS2 nanosheets Cellulose-clay hydrogel CeO2−δ

Fe3O4@GPTMS@Gly

Halloysite nanotube-Fe3O4

MgO nanoparticle

Methylene blue

Congo red, reactive brilliant red Levafix fast red CA, indanthren Methylene blue, neutral red, methyl orange Methylene blue, orange red, acid red Methylene blue Methylene blue Methylene blue Methylene blue, reactive orange 16, mordant blue Congo red Orange G, indigo carmine, methyl orange, acid red 18 Remazol red RB-133

Dye

23.65

10.63

3.495 4.86, 1.99, 7.46, 1.42

13.2, 9.1 0.7 – 1.44, 9.09, 1.13

0.992

−28.35

0.99

0.9943 0.9989, 0.9984, 0.9994, 0.9988

– 89.29, 144.93, 105.26, 212.76

–

0.9999 0.9999 0.9981 0.9994, 0.9999, 0.9977

0.9954, 0.9892, 0.9911

0.999, 0.999, 0.980

0.997, 0.995

1, 0.9998

R2

93.46 303.0 50.02 106.3, 100, 96.3

119, 61, 31

18.49, 12.62, 0.70

0.208, 0.185, 2.349 2.9, 2.8, 5.9

103.84, 85.62

131.58, 131.58

Qe or c

1.45, 1.72

125.0, 149.25

k2 or kdif

1000

1000

1000 200

500 1000 50,000 2000

1000

2000 in all cases

500

750

M0b

5

50

200 –

50 300 10 200 in all cases

200 in all cases

34.7, 28.9, 32.7

50, 50

100, 100

C0

35

60

60 40 in all cases

∼90%

∼90%

92% ∼100% for acid red 18

Mahmoud et al. (2013) Patra et al. (2016)

Zare et al. (2015) Tang et al. (2016b)

Xiong et al. (2010a) Massey et al. (2016) Peng et al. (2016) Tomic et al. (2014)

∼95% 94% ∼95% 93, 90, 100% 60 480 3600 240, 120, 120

Zhang et al. (2013)

∼100% in all cases

cases 60 in all cases

Venkatesha et al. (2012) Xie et al. (2011)

Hu et al. (2010)

References

96, 95, 5.5%

98, 92%

97.7, 93%

Rc

120 in all

150, 180

10, 10

t

d

c

b

a

kdif is the intra-particular diffusion rate constant (mg/g min1/2 )

R is the removal efficiency at time t (min)

M0 (mg/L) is the dosage of adsorbents

Qe and Qt (mg/g) are the adsorbate adsorption capacity at equilibrium and at time t (min), k2 (min−1 ) is the rate constant of the pseudo-second-order model. C0 (mg/L) is the initial concentration of adsorbate, and R is the removal efficiency

Qt ¼ k dif

Intra-particle diffusion

¼

1 a Qe t

MgO nanoplates

1 k 2 Q2e

Pseudo-second-order

t Qt

Adsorbents

Examples of dye adsorption fitted by second-order or intra-particle diffusion kinetic models and dye removal efficiency by various nanoadsorbents

Model

Table 3



nanomaterials have become the pioneering photocatalysts and attracted most of the current research interests in this area. The nanomaterials provide abundant surface states, large surface area, diverse morphologies, and feasible device modeling, which are beneficial to photocatalysis (Tong et al. 2012). Semiconductor metal oxide nanomaterials, e.g., TiO2 (Aarthi and Madras 2007), ZnO (Hong et al. 2009), Cu2O (Kumar et al. 2016), CeO 2 (Tomic et al. 2014), MoS 2 (Massey et al. 2016), etc., have attracted great attention as photocatalysts for removal of organic pollutants, i.e., dyes, from aqueous phase (Wu et al. 2016). Other than the metal oxides, the organic semiconductors, e.g., carbon nitride (Wang et al. 2009b), graphene (Xiong et al. 2011b), etc., have attracted great attention as novel photocatalysts. Generally, a semiconductor photocatalytic degradation process includes the following steps (as shown in Fig. 4): The light irradiation induces the separation of electrons from the valance band (VB) to the conduction band (CB) and leaving the holes in the VB; then, the electrons and holes migrate to the surface of catalysts and react with the electron donors or electron acceptors directly (Tong et al. 2012). More importantly, the holes will react with surface adsorbed water molecules, forming the highly reactive hydroxyl radicals (•OH); the electrons react with molecular oxygen and produce superoxide radical (O2•−). The radicals can further react with pollutants. Great progress has been made in the photocatalytic remediation of dye-polluted wastewater; however, the efficiency of the photocatalytic degradation process still needs to be improved due to two inherent limitations of photocatalysts: rapid electron-hole pair recombination rate and limited light absorbance spectrum. In this part, the most studied nanophotocatalysts and their application on the dye removal are summarized and discussed. Since the dye removal efficiency of the photocatalytic degradation process is primary determined by the activity of the photocatalysts, the modification methods and examples of the photocatalysts are compared and discussed.

Fig. 4 Schematic illustration of basic mechanism of a semiconductor photocatalytic process. Adapted from reference Cai et al. (2016a)

TiO2 Since Fujishima and Honda (1972) demonstrated the potential of TiO2 on water splitting, the TiO2 has aroused great attention as a catalyst (Pelaez et al. 2012). The high photocatalytic activity, resistance to photocorrosion, low toxicity, and low cost of TiO2 favor its application; there is little doubt that the TiO2 is the most studied and used photocatalyst nowadays. As early as in 1996, Nasr et al. (1996) have studied the photocatalytic degradation of naphthol blue black dye by using TiO2 nanoparticles. Thereafter, many researchers investigated the effectiveness of TiO2 on the photocatalytic degradation of dyes, especially under UV irradiation (Aarthi and Madras 2007; Liu et al. 2016b). The removal efficiency of dyes is related to both surface properties of TiO2 and the molecular structure of dyes itself. For example, Vinu et al. (2010) compared the photocatalytic degradation of eight cationic, five anionic, and three solvent dyes by a combustion-synthesized nano-TiO2 and a commercial nano-TiO2 (P25) under UV light, where the combustion-synthesized nano-TiO2 showed higher photocatalytic degradation efficiency for all the anionic dyes, while the P25 was better on the degradation of most of the cationic dyes. However, the solvent dyes showed adsorption-dependent decolorization. As an ideal model photocatalyst, however, the efficiency of TiO2 is still limited by the low visible light activity and fast electron-hole pair recombination rate. In fact, ∼90% of the photo-excited electron-hole pairs were recombined according to the time-resolved spectroscopic studies (Guo et al. 2016; Salvador and Gutierrez 1984), which can be the main reason of the low quantum efficiency of TiO2. Meanwhile, the wide band gap (3.2 eV) makes the TiO2 only active under UV region. To overcome these limitations, efforts have been devoted to promote charge separation or lower the band gap energy of TiO2, including non-metal doping, transition metal doping, metal doping/deposition, dye sensitization, heterojunction, morphology control, etc. (Nagaveni et al. 2004; Pelaez et al. 2012; Yu et al. 2011). The incorporation of S, N, C, etc. into TiO2 is a promising method to continuously modulate the band gap for extending the visible light activity (Tong et al. 2012). Chaudhary et al. (2016) synthesized the S and C-doped TiO2 and evaluated their effectiveness by photocatalytic degrading reactive black 5, methylene blue, and methyl orange. The S and C doping on the catalysts decreased the complete dye removal time from 1–4 h to less than 5–20 min. Other than extending visible light absorption, the N doping on TiO2 was found to retard the electron-hole recombination and optimize electronic properties (Borges et al. 2016). The metal doping/deposition, e.g., using Au, Pt, Ag, and Pd, facilitates electron-hole separation and red shifts the light absorbance to visible region. For example, Sakthivel et al. (2004) compared the TiO2 that deposited with Pt, Au, and


Pd via the photo-oxidation of acid green 16 under UV irradiation and found that all the metal-deposited TiO2 behaved higher photocatalytic activity and with the order of Pt ≈ Au > Pd. The UV-Vis diffuse reflectance measurement demonstrated that the metal-doped TiO2 had absorption threshold extended into the visible region. In addition, metal deposition could induce the formation of Ti3+ defect sites which adsorb and photoactive oxygen. However, it should be noted that there is an optimum metal dosage that achieves the aim of enhanced photocatalysis efficiency but minimizes the block of surface active sites. The heterojunction of TiO2 with other semiconductors can reduce the electron-hole recombination thus promote the photocatalytic activity of the catalysts. It is known that the randomly dispersed ZnO on the surface of anatase TiO2 nanoparticles enhanced the electron injection into the conduction band of TiO2 by capturing electron, which promoted the production of reactive oxygen species (Zhao et al. 2008). Other than ZnO, the metal molybdates (MxMoxTi1-xO6) (Ghorai et al. 2007), gC3N4 (Tang et al. 2016a), Ag2S and Bi2S3 (Wang et al. 2017), etc. were reported to enhance the dye removal in aqueous phase. The hierarchical heteronanostructures of TiO2, i.e., 1D, hierarchical, facet-controlled nanostructure, could facilitate the photogenerated electron-hole pair separation, thus improve the photocatalytic activity (Tong et al. 2012). The exposed crystal facets play a critical role in determining the photoactivity of the nanomaterials; therefore, the synthesis of single crystals with exposed high reactive facets is considered to improve the photoactivity of the materials. For example, Yang et al. (2008) prepared anatase TiO2 single crystals with high percentage of the highly reactive {001} facets comprised surface. The 1D nanobelt structure of TiO2 behaves reduced electron-hole recombination rate, e.g., Wu et al. (2010) synthesized a single crystalline anatase TiO2 nanobelts, which showed improved charge mobility and fewer localized states near the band edges and in the bandgap due to fewer unpassivated surface states in the nanobelts. Other than improving the photoactivity of TiO2, many other methods can be applied to modify the catalysts indirectly, for example, enhancing the adsorption properties of pollutants on catalyst surface to increase the degradation rate and enlarging the surface area of catalysts by decreasing the size or manipulating the morphologies (1D, layer or hierarchical structure, etc.). These methods are not only suitable for TiO2 but can also be applied to most of other photocatalysts. It is a big challenge to separate the TiO2 nanoparticles, or most of other nanomaterials, from water phase for the reusing purpose. The manipulating of the synthesized TiO2 morphologies, e.g., nanotube, nanosheet, can promote aggregation properties and settleability of TiO2 (Cai et al. 2016a); however, the aggregation will inevitably cause the decrease of activity. The coupling of magnetite (Fe3O4) with TiO2-based

nanomaterials could greatly promote the separation performance (Abbas et al. 2016), and which is also a widely studied method for the separation of nanomaterials from water phase. The numerical simulation is developing rapidly in recent years, which could be applied to design the new materials, predict their properties, or demonstrate the mechanisms of reaction; however, very little work has applied the theoretical methods for the modification of TiO2-based photocatalysts, and they can even be applied for designing and modification of other nanomaterials for water treatment.

ZnO The nano-sized ZnO has attracted considerable interests as photocatalyst due to its low cost, large specific area, wide band gap (3.37 eV), and high potential to adsorb UV irradiation (Hong et al. 2009). The property of ZnO depends closely on the microstructures of the materials, e.g., morphology, crystal size, orientation, etc., and which can be manipulated by the synthesis methods (Tian et al. 2003). The nano-sized ZnO synthesized with various hierarchical structures, such as flower-like, sea-urchin shaped, dandelion-like, behaves higher photocatalytic activity than the monomorphological structures (Jing et al. 2013). Researches also showed that the ZnO nanoparticles prepared by codeposition and sol-gel methods have showed high activity on degrading organic dyes under UV light; the codeposited ZnO on SiO2 gave smaller particle size and showed higher activity than ZnO prepared by sol-gel method (Shen et al. 2008). The improved activity of ZnO can be primarily due to the larger surface area. Similar to other pristine semiconductor, the quantum efficiency of ZnO nanomaterials can be improved by extending the spectrum absorbance and retarding the excited electronhole recombination. The doping of metals and non-metals or coupling with other semiconductors is commonly applied to suppress the unwanted charge carrier recombination process (Hameed et al. 2009). For instance, the NiO-ZnO nanocomposites showed higher photocatalytic decolorization efficiency of organic dyes under UV-visible light than pristine ZnO, and they are comparable to Degussa TiO2 (P25) (Hameed et al. 2009). The incorporation of high electronical conductive compounds, i.e., graphene, with ZnO nanoparticles can facilitate the transport of photo-excited electrons and thus promote the photocatalytic activity (Allen et al. 2010). Meanwhile, the excited dyes can inject electrons to graphene due to the higher redox potential of graphene. Therefore, the electron-accepting properties of graphene can enhance the photocatalytic performance of the graphene-modified ZnO (Li and Cao 2011; Xiong et al. 2010b). Meanwhile, the photocatalytic activity of ZnO could be improved by immobilization of ZnO nanoparticles onto siliceous materials (Soltani et al. 2016).


It worth to mention that the coupling of ZnO-TiO2 could profoundly improve the catalytic activities and physical properties compared to pure ZnO or TiO2. The electron transfers from the conduction band of ZnO to the conduction band of TiO2 decrease the recombination of the electron-hole pairs and promote the photodegradation process, thus promote the photolysis rate of organic dyes (Agrawal et al. 2009). The high photocatalytic reactivity and low price (when compared with TiO2) favored the application of ZnO-based nanomaterials in water treatment process (Saikia et al. 2015). However, the anodic photocorrosion ZnO will cause the loss of activity of the material and also produce secondary pollution (Zn2+). The Zn2+ is known to be toxic to microbials (Baek and An 2011). Moreover, the LC50 of the ZnO nanoparticles has bee n reported as 2.3 mg/L to the n ematode Caenorhabditis elegans. Therefore, the dissolution of ZnO should be concerned, and the discharge of ZnO into water environment should be avoided (Soltani et al. 2016; Wang et al. 2009a).

orange than the pure g-C3N4 under visible light irradiation (Ge et al. 2011). The photocatalytic degradation of dyes occurs on the surface of g-C3N4; therefore, efforts could be devoted to enhance the adsorption of dyes on g-C3N4, thus improve the efficiency of the catalyst. Based on the recently disclosed photocatalytic properties and related researches, we believe that the exploration of potential g-C3N4 is just started. In the future researches, efforts could be partially devoted to the surface activation of g-C3N4 to derive the specific binding of functional groups, as well as the dispersion of semiconductor nanoparticles, which help in forming efficient g-C3N4/semiconductor heterostructures with improved interfacial contact for photocatalysis process. Although the mechanisms of photocatalysis process over gC3N4-based semiconductor are still not very clear yet, it is believed that the g-C3N4 will be a promising catalyst for treating the dye-contaminated or other organic pollutantcontaminated water.

Bi-based photocatalysts g-C3N4 The graphite carbon nitride (g-C3N4) is an organic semiconductor with the optical band gap of 2.7 eV. In recent years, specific attention has been paid to g-C3N4 since Wang et al. (2009b) found the high photocatalytic performance on water splitting over g-C3N4 under visible light, and then, Yan et al. (2009) observed high photocatalytic degradation of methyl orange dye over g-C3N4 under visible light. With the facile synthesis, low cost, non-toxicity, high photocatalytic activity, and stability of g-C3N4, g-C3N4 has triggered tremendous endeavors on its application as photocatalyst. Until now, many studies are focused on understanding the photochemical properties of g-C3N4 and improving its photoactivity; however, rare engineered application of g-C3N4 as photocatalyst is reported. The photocatalytic performance of pristine g-C3N4 is somewhat limited due to the low quantum efficiency. A large number of substances, e.g., Co (Han et al. 2014), Ag (Ge et al. 2011), B (Yan et al. 2010), Ag3VO4 (Wang et al. 2014), ZnO (Wang et al. 2011), polyaniline (Ge et al. 2012), etc., have been coupled with g-C3N4 to enhance the photocatalytic activity. The construction of heterojunction between semiconductors promotes the separation of electron-hole pair and becomes one of the most popular modification method for gC3N4. For instance, the Co3O4 (Han et al. 2014), Ag3VO4 (Wang et al. 2014), and ZnO (Wang et al. 2011) modified gC3N4 heterojunction photocatalysts greatly promoted their activity on degrading organic dyes under visible light irradiation. The doping/deposition of noble metal promotes the migration efficiency of the photogenerated carriers. For example, the Ag-loaded g-C3N4 nanoparticles showed ∼11.5 times higher photocatalytic decomposition rate for degrading methyl

The Bi-based oxides, such as BiVO4 (Dong et al. 2014), Bi2O2CO3 (Chen et al. 2012), BiWO6 (Zhang and Zhu 2005), Bi4Ti3O12 (Yao et al. 2004), Bi2O3 (Tseng et al. 2010), etc., behave high photocatalytic activity under visible light due to the relatively narrow band gap, which is derived from the more negative valence band that consists of the Bi6s and O2p orbitals (when compared with the O2p-consisted TiO2) (Madhusudan et al. 2012). The activities of nanostructured photocatalysts strongly rely on their shapes and size; efforts have been devoted to achieve higher photoactivity by manipulating the structures of synthesized Bi-based oxides. For example, the peanut, dumbbell, flower, olives, and rod sheave-shaped BiVO4 that synthesized at various pH condition are greatly differentiated in photoactivity over photocatalytic-degrading rhodamine B (as shown in Fig. 5). However, the photocatalytic performance did not correlate to the specific surface area or aspect ratios of the fabricated photocatalysts, but strongly associated with the unique shape configurations (Dong et al. 2014). More studies are required to well explain effects of shapes on the activities of BiVO4, as well as other similar photocatalysts. Comparing with titania, the scarcity and higher cost of Bibased photocatalysts can be a key parameter that limit their wide application recently, but their high photocatalytic activity still makes them promising photocatalysts in the future.

MoS2 Other than acting as adsorbent, the MoS2 has been demonstrated as an efficient photocatalyst for degrading dyes,


Fig. 5 TEM micrographs of BiVO4 hierarchical structures obtained at various synthesis pH with the precursors of NH3·H2O. a pH = 4.9. b pH = 6.26. c pH = 6.72. e pH = 7.3. TEM micrographs of BiVO4 hierarchical structures obtained at various synthesis pH with the

precursors of NaOH. f pH = 4.9. g pH = 6.26. h pH = 7 as the pH controlling agent. The photocatalytic rhodamine B removal under natural solar light over i A-BiVO4 and j S-BiVO4 samples synthesized at various pH values. Adapted from reference Dong et al. (2014, 2015)

especially when coupled with other semiconductors (Massey et al. 2016). For example, the incorporation of graphene with MoS2 provided many benefits such as facilitating the electron transfer, reducing the electron-hole pair recombination, and promoting the light absorbance and dye adsorption. Li et al. (2014) found that the photocatalytic activity of a MoS2-reduced graphene oxide composite behaved enhanced photocatalytic activity on degrading methylene blue under visible light. MoS2 quantum dots could be a prospective candidate as high-efficient photocatalysts for environmental remediation due to its high stability, large surface area and reaction sites, high visible light absorbance, and charge separation. The high in-plane electron transport abilities and large edge to volume ratios make the quantum dots stand out from other forms of MoS2 (Arul and Nithya 2016). It should be noted that the pristine MoS2 suffers from low activity due to its inert nature of basal plane and low edge sites; efforts still needed to be made to improve this aspect.

Graphene Graphene and graphene oxide are introduced as excellent adsorbents in the adsorption part, and they can also act as good photocatalysts. The reduced graphene oxide has been found to photocatalytic degrade rhodamine B under visible light irradiation at relatively low rate firstly (Xiong et al. 2011b), and then, researchers have tried various methods to improve the photocatalytic activity of the graphene series nanomaterials. For examples, the modification of graphene oxide by Au nanoparticles (Xiong et al. 2010b) and copper ion (Xiong et al. 2011b) promoted the photocatalytic activities of the graphene to surpass TiO2 on degrading rhodamine B under visible light. However, it is important to note that the graphene is much more commonly used as electron conductor to facilitate the photo-excited electron-hole pair separation, rather than act as a photocatalyst itself. Thousands of papers can be found about the researches that use graphene to promote the photocatalytic activities of semiconductors, e.g., TiO2 (Zhang


et al. 2011), ZnO (Xu et al. 2011), MoS2 (Li et al. 2014), and g-C3N4 (Liao et al. 2012). Until now, the cost of producing graphene is still relatively high. Considering only small amount of graphene is required to incorporate into other semiconductors to improve the electronic properties, and the application of graphene to modify other semiconductor will be a good aspect for synthesizing high active photocatalysts.

Ag-based photocatalysts The Ag series photocatalysts have aroused great attention due to the high visible light activity and potential application in environmental remediation. Their photocatalytic performance could be improved by morphology and facet-controlled processes, coupling with other functional materials. For example, when the Ag2O, Ag2CO3, and Ag2O/Ag2CO3 core-shell nanoparticles with TiO2 tested on methyl blue degradation under visible light, all these three materials showed much higher activity than TiO2 and their precursors (Yu et al. 2014). The hierarchical heteronanostructures of Ag-based nanomaterials could facilitate their separation of excited electron-hole pairs. And the ratio of Ag salts and the functional materials are key parameters that control the photocatalytic activity. A type of Ag/AgCl core-shell nanowire was synthesized by heteroepitaxial growth for photocatalytic degrading methyl orange under visible light (as shown in Fig. 6). The fast photodegradation was achieved at the optimum Ag/AgCl ratio of 8/92 for the core-shell nanowires, which could reach the complete methyl orange removal in 8 min under visible light (Bi and Ye 2010; Tong et al. 2012). The researches on the Ag-based photocatalysts have made considerable advances; however, further developments are needed, and they are still far from practical application. The detailed mechanism on the photocatalytic degradation of organics is unclear, and the cost of Ag-based photocatalysts is high. Moreover, due to the relatively low stability of Ag salt under irradiation, the loss of material mass and photocatalytic activity after reusing still need to be well evaluated.

Combination with adsorption process Many nanomaterials can act as both adsorbents and photocatalysts to remove organic dyes from water phase. The photodegradation of dyes primary conducted on the surface of photocatalysts; therefore, the adsorption is believed to affect the photodegradation process, most probably improving the photolysis rate. For example, a metal/WO2.72/rGO ternary nanocomposite was synthesized by a two-step in situ loading process for removing methylene blue from water. The graphene provided adsorption sites for dyes, and metal doping

and graphene incorporation extended visible light adsorption and increased lifetime of the photogenerated electron-hole pairs on the WO2.72 nanowires; the materials showed extremely high photocatalytic properties (Li et al. 2016). A novel polyacrylamide/Ni0.02Zn0.98O (PAM/NZP) nanomaterials that prepared by polymerization of acrylamide in the presence of Zn and Ni nanoparticles was investigated for the adsorptive and photocatalytic removal of malachite green and rhodamine B from aqueous phase. The tests showed that the simultaneous adsorption and photocatalysis could be better for the removal of the dyes under natural sunlight condition (Kumar et al. 2014a). However, the excessive adsorption of dyes on photocatalysts can weaken the screening effect of light on photocatalysts and decrease the number of active sites for generating electron-hole pairs in some cases (Xiong et al. 2011a); therefore, the adsorption properties should be well manipulated for synthesizing high-efficient photocatalysts. The examples of photocatalytic degradation of dyes by selected nanophotocatalysts and their effectiveness were summarized in Table 4. Most of the studied photocatalysts can help achieving much faster degradation/mineralization of dyes in water phase. It should be noted that the dyes are commonly used as model compounds to evaluate the photocatalytic activity of new photocatalyst. However, the use of dyes as probe can be problematic because some dyes can involve in the dyesensitized degradation process, and some degradation intermediate can have spectral interferences due to its light absorbance (Barbero and Vione 2016). Therefore, caution should be taken when using dyes to evaluate the photocatalytic activity of the nanophotocatalysts. It is worth to note that the large surface area of nanomaterials provides more surface adsorption sites, which favors the accumulation of dyes and enhances the photocatalytic degradation efficiency. Moreover, the larger surface area of nanomaterial provides more chances for light absorbance. This should be the primary mechanism of the enhanced photocatalytic activity of the nano-sized photocatalysts. Many nanoadsorbents and photocatalysts have been investigated by laboratory study, and most of the experiments were conducted under ideal conditions, i.e., in deionized water or distilled water (Moussavi and Mahmoudi 2009; Tang et al. 2016b; Wu et al. 2016; Zare et al. 2015). However, when dealing with real wastewater, the water parameters, e.g., pH, ionic strength, dissolved organic matters, etc., can greatly affect the effectiveness of the nanomaterials (Liu et al. 2016c; Patra et al. 2016). For example, the pH value alters the surface charge of nanomaterials and ionization of different pollutants; accordingly, the adsorption capacity of organic dyes by nanoadsorbents varies greatly under different pH (Iram et al. 2010; Liu et al. 2016c). Similarly, the photocatalytic process can be affected by pH due to the changes on dye adsorption, and the high pH favors the formation of hydroxyl radicals (Krishnakumar et al. 2012). The ionic strength affects the


Fig. 6 Scanning electron microscopy (SEM) images. a Ag nanowires. b Ag/AgCl core-shell nanowires. c Pt/AgCl heteronanotubes. d Photocatalytic degradation of methyl orange under visible light

(>400 nm). e Schematic illustration of the growth of Ag/AgCl coreshell nanowires and Pt/AgCl heteronanotubes. Adapted from references Bi and Ye (2010) and Tong et al. (2012)

activity coefficients of OH−, H3O+, and specifically the adsorbable dye ions on nanomaterials (Ma et al. 2012), thus affect the adsorption/photodegradation process. Organic matters are generally presented in wastewater and surface water, which can compete for active sites during the adsorption process or cover the sorption sites. During the photocatalytic degradation, dissolved organic matter can deactivate the surface sites and act as light screens to reduce the photon receiving efficiency and cause negative effects (Konstantinou and Albanis 2004; Liu et al. 2016c). Therefore, the water parameters should be carefully evaluated when testing the effectiveness of new nanomaterials, and the use of real wastewater is preferred.

effective and produce less harmful waste for disposal (Chen et al. 2005; Hao et al. 2012). However, due to the presence of complex aromatic molecular structures, the removal efficiency of biological processes on some specific refractory dyes is not satisfied. Thus, a biological process is usually modified (Sathian et al. 2014) or combined with other treatment processes (Azizi et al. 2015) for treating recalcitrant dye wastewater. In recent years, some scientists have attempted to apply nanotechnology or nanomaterials into the biological dye wastewater treatment, and Table 5 and Fig. 7 summarize their works. One of the most common uses of nanomaterials is immobilization matrix for functional bacteria/algal strains (Keskin et al. 2015; San et al. 2014; Yu et al. 2015), bioenzymes (Kumar et al. 2014b; Wang et al. 2013), and molecular groups (Alvarez et al. 2010; Cervantes et al. 2015). The unique characteristics such as large specific surface area, mass transfer resistance, and active loading sites make nanomaterials ideal support materials (Ahmad and Sardar 2015). Different metaloxide nanoparticles (e.g., Al2O3, Al(OH)3, and ZnO) are usually employed to directly absorb organic molecules such as anthraquinone-2,6-disulfonate (AQDS) and HS with different aims. Al(OH)3-AQDS served as effective redox mediators in the bioreductive decolorization of azo dyes (Alvarez et al. 2010); Al2O3-HS was used to accumulate humus reducing and other anaerobic microbes and to promote the granulation

Application of nanomaterials in biological dye wastewater treatment Although the physiochemical methods (e.g., adsorption by activated carbon and ozonation) could remove dyes from wastewater efficiently, however, the high cost, formation of hazardous by-products, and intensive energy requirement have limited their extensive application (Chang et al. 2009). Compared with physical and chemical treatments, biological processes which use biological species to degrade or transform toxic chemicals to less harmful forms are more cost-

Wang et al. (2014) Dong et al. (2014) Li et al. (2014) Xiong et al. (2011b) Kumar et al. (2016) Yu et al. (2014) 95% 97% ∼100% 91% 55% 100% 150 600 60 180 120 5

Yan et al. (2009) 95, 90% 300, 60

3000 in all cases 1000 500 1000 63 500 50

R is the removal efficiency at time t (min) a

Visible light Sunlight Visible light Visible light Visible light Visible light g-C3N4/Ag3VO4 Hierarchical BiVO4 MoS2/rGO Cu2+/rGO Cu2O hierarchical nanocubes Ag2O/Ag2CO3

Basic fuchsin Rhodamine B Methylene blue Rhodamine B Methylene blue Methyl orange

0.4 in all cases 20 5 60 2.5 10 20 Visible light

100 70 Methylene blue Acid red

ZnO nanoparticle ZnO embedded in biosilica nanobiostructure g-C3N4, Ag modified g-C3N4

Methyl orange

Shen et al. (2008) Soltani et al. (2016) 90 98.5% 60 30 100 1500

Liu et al. (2016b) Nasr et al. (1996) 90, 87% 92% 30 40 120 5 Si/SiC@C@TiO2 TiO2 nanoparticle

UV and visible light Visible light (λ > 380 nm) UV UV

200 2000

Vinu et al. (2010) 98, 99, 99, 100% 120, 90, 120, 30 1000 in all cases 50 in all cases UV TiO2 nanoparticle

Orange G, amido black, indigo carmine, alizarin cyanine green Methylene blue Naphthol blue black

C0 (mg/L) Light sources Dyes Photocatalyst

Table 4

Selected examples of photocatalytic degradation of dyes over various nanophotocatalysts

M0 (mg/L)

t (min)

Ra

References


process in an upflow anaerobic sludge bed (UASB) reactor (Fig. 7a) (Cervantes et al. 2015). For the immobilization of bacteria/algae, electrospun nanofibrous webs (NFW) are optimal choice due to their very large surface area to volume ratio along with nanoscale porosity (Wendorff et al. 2012). Several colleagues from Turkey have reported developing immobilized microbes by NFW to treat dye wastewater (Fig. 7b) and the adopted materials for synthesizing NFW included polysulfone (PSU) (Keskin et al. 2015) and cellulose acetate (CA) (San et al. 2014). For the immobilization of bioenzymes, the surface of nanoparticles should be prior modified with amino groups (e.g., by using 3-aminopropyltriethoxysilane) to create the active sites for coupling the bioenzymes (Fig. 7c) (Wang et al. 2013). Another important application of nanomaterials is to utilize the electron-donating capacity of nanometal to stimulate the microbial growth and activity (Fang et al. 2015) or to catalytic reduction of dyes (Johnson et al. 2011; Johnson et al. 2013; Quan et al. 2015). In situ formation of biogenic nano-Pd (bionano-Pd) is a promising technique to reductively degrade azo dyes, where Pd(II) ions were microbially reduced to form nano-Pd in the periplasmic space, cytoplasm, and on the cell walls of bacteria (Fig. 7d) (Quan et al. 2015). The formed nano-Pd can catalyze the extracellular microbial electron transfer to generate molecular hydrogen for reductive hydrogenation of the azo linkage (Johnson et al. 2013). Since some other transition metals (e.g., Ni, Pt, and Ru) with high catalytic capacity can also be microbially produced (Srivastava and Constanti 2012), in situ formation of these biogenic nanoparticles may be employed to reductively degrade azo dyes. Fang et al. (2015) have investigated the enhancement of ZVI on microbial azo reduction and found that the nanometer Fe precipitates can adsorb to the surface of the outer membranes and transport inside the microbial cells, improving cell growth and maintenance of viability (Fig. 7e). Thus, nZVI nanoparticles may be used as an additive to enhance the biodecolorization of dye wastewater. Besides nZVI, other nanoparticles such as FeS (Jiang et al. 2014) and KMn8O16 (Pan et al. 2015) have shown excellent promoting effects on the microbial diversity and activity, and these nanoparticles have great potentials for improving biodegradation of dyes as modulators of microbial community.

Conclusions and perspectives Huge amount of dye-contaminated wastewater were produced, but many conventional treatment methods cannot treat them efficiently. The application of nanomaterials offered opportunities for upgrading the conventional methods or generating innovative methods to deal with the dye contaminants with high efficiency.

10 mg/L

50, 100 mg/L

10–60 mM

∼370 mg TOC/L

1.2–4.8 mM

1%, w/v

20 mg

20 mg

0.2 U/mL

Bio-nano-Pd

Bio-nano-Pd

nZVI

γ-Al2O3-HS

Al(OH)3-AQDS

nano-TiO2

PSU-NFW

CA-NFW

M-CLEAs

Fe3O4/SiO2-laccase 1.6 mg/mL

5, 10 mg/L

Bio-nano-Pd

Trametes villosa laccase

Aeromonas eucrenophila, Clavibacter michiganensi, Pseudomonas aeruginosa Trametes versicolor laccase

Chlamydomona reinhardtii

Sludge bioreactor Bacillus spp.

UASB

Shewanella decolorationis S12

Sludge bioreactor

Clostridium pasteurianum BC1

Clostridium sp.

Species

Conc.

Type

Aerobic

Aerobic

Aerobic or facultative anaerobic

Aerobic

Anaerobic

Anaerobic

Anaerobic

Anaerobic

Anaerobic

Anaerobic

Anaerobic

Respiration

Microbe/bioreactor/enzyme

2h

2–4 h

24 h

1–14 d

1–24 h

12 h

12 h

29 h

12 h

7 min

25 h

Treating time/HRT

Remazol brilliant blue R, malachite green, reactive black 5 Acid red 1, reactive red 2

Methylene blue

Remazol black 5, reactive blue 221

Various azo dyes

Reactive red 2

Reactive red 2

Amaranth

Congo red, evans blue, orange II

Methyl orange

Evans blue

Type

Dye

Summarization of applications of nanomaterials in biological dye wastewater treatment

Nanomaterial

Table 5

∼100%

30–43%

Extracellular microbial electron transfer coupled to the catalytic reduction Extracellular microbial electron transfer coupled to the catalytic reduction Extracellular microbial electron transfer coupled to the catalytic reduction Stimulation of microbial growth and activity by supplementation with elemental iron and H2 as an additional electron donor Co-immobilization humus-reducing microorganisms and humic substances (HS) Effective redox mediator

Mechanism

5 mg/L

50 mg/L

Immobilization of biocatalyst

Immobilization of biocatalyst

61–96%

∼95%

32–100 mg/L average Immobilization of microbial cells 4.64 mg/g h 10–50 mg/L 72.97 ± 0.3% Immobilization of microbial cells for black 5, 30.2 ± 0.23% for blue 221 20–500 mg/L ∼95% Immobilization of microbial cells

0.3 mM

40–400 mg/L 67–98%

1 mM

0.29–0.49 h−1

∼90%

450 μM

100 mg/L

∼90%

45 μM

Conc.

Decolorization efficiency

Wang et al. (2013)

Kumar et al. (2014b)

San et al. (2014)

Keskin et al. (2015)

Alvarez et al. (2010) Yu et al. (2015)

Cervantes et al. (2015)

Fang et al. (2015)

Quan et al. (2015)

Johnson et al. (2013)

Johnson et al. (2011)

References


Environ Sci Pollut Res Fig. 7 Mechanisms of different applications of nanomaterials in the biological dye wastewater treatment. a Accumulation of humus-reducing microbes by humic substances (HS) absorbed on nanoparticles (NPs) (modified from reference Cervantes et al. (2015)). b Immobilization of microbial cells by nanofibrous webs (NFW) (adapted from reference Keskin et al. (2015)). c Immobilization of enzymes by amino-modified NPs. d In situ formation of biogenic nano-Pd (adapted from reference Johnson et al. (2011). d Nanometer Fe precipitates adsorbed to the outer membranes and inside the microbial cells (modified from reference Fang et al. (2015))

With extremely large surface area and modified surface properties, the nano-sized adsorbents, e.g., CNTs, graphene, TNTs, nano-MgO, etc., exhibit high selectivity and large adsorption capacity for dye removal. Among them, the carbonaceous materials show superior adsorption performance due to the large external surface area and high aspect ratio; most importantly, the comparable low cost makes them promising materials to be applied at large scale. The surface-modified iron oxide nanomaterials have excellent adsorption capacity and high separation performance, which are favorable properties for their further application. The large surface area offers the TNTs and MoS2 good adsorption properties for dye removal even with different adsorption mechanism, and they have the potential of combining with photocatalytic degradation process for treating dye-contaminated wastewater. However, the separation of some nanoadsorbents from aqueous phase is challenging, and the high cost of the materials need to be overcome.

The nano-sized photocatalysts, e.g., nano-TiO2, nano-ZnO, graphene-modified catalysts, g-C3N4, etc., provided much higher photocatalytic activity than bulk photocatalysts. Until now, the TiO2 is still in the preponderant position as the archetypical photocatalyst especially with the modified visible light utilization efficiency and electron-hole pair separation ability. As it has been stated in this review, the ZnO behaved high activity under UV light, but the anodic photocorrosion and toxicity of Zn ion make its use unviable in engineering application. Inspiring results have been obtained with g-C3N4based photocatalysts. With the high visible light activity and stability, the g-C3N4 is believed to be a promising catalyst, even although the mechanisms of photocatalysis process over g-C3N4-based semiconductor are still not very clear yet; therefore, more works are required for revealing this new catalyst. MoS2 is a visible light active semiconductor; it could be a prospective candidate of high-efficient photocatalysts for environmental remediation, especially in the forms of quantum


dots. The Ag-based photocatalysts have high visible light activity, and the related researches have made considerable advances; however, the detailed mechanism on the photocatalytic degradation of organics is unclear; the high cost and low stability of Ag salt under irradiation are still challenging for its practical application. The combination of nanotechnologies with biological process for treating recalcitrant dye wastewater is still in the initial stage. It is quite encouraging that some specific nanomaterials could enhance the biological process for biodegradation of dyes or have other positive synergetic effects on dye removal. However, most of the nanotechnologies are still in the laboratory study. The road of wide engineering application is still a long way. From the aspects of economy, safety, and feasibility, the following issues should be addressed for developing and application of nanomaterials: (1) The cost is a key parameter for nanomaterials’ wide application. Therefore, the materials and energy cost of synthesizing new nanomaterials and regenerating the used materials need to be minimized. In addition, the reusability and lifespan of nanomaterials should be improved. (2) The small size of nanomaterials makes their separation to be difficult; the immobilization of nanomaterials on supporters or incorporation of nanomaterials with magnetic composites can be developed to facilitate their separation properties. (3) The discharge of nanomaterials into environment during the wastewater treatment, as well as the toxicity of the nanomaterials, should be carefully evaluated. Cautions should be exercised to avoid the release of hazardous nanomaterials into the environment. (4) The laboratory tests need to be involved the real water environment parameters, e.g., ionic strength, organic matter concentration, pH, solar irradiation, etc., which will make the experimental results close to the real treatment process. Moreover, the following suggestions are proposed for the further investigation of nanomaterials on the dyecontaminated wastewater remediation: (1) The rapid development of numerical modeling methods and computational capability offer more opportunities for computational modeling in recent years. The theoretical methods using numerical simulation can be applied for developing or modifying new nanomaterials, predicting their properties, and demonstrating the mechanisms of reaction. (2) Most published works explained the adsorption mechanisms based on the morphological properties, surface properties, adsorption kinetics, and isotherms. However, the important thermodynamic parameters, such as enthalpy, entropy, adsorption free energy, and surface energies, are not generally characterized and discussed. Such works are suggested to conduct for investigating the mechanism of new nanoadsorbents. (3) The combination of band gap engineering with other modification methods is encouraged to improve the activity of photocatalysts. The comprehensive of surface/interface processes of the photocatalytic performance of nanomaterials

are suggested to be studied. The deep understanding of the photocatalytic degradation process will inevitably benefit the development of new generation of nanophotocatalysts. (4) Many researchers synthesized and evaluated the high-visible light active photocatalysts with the aim to obtain photocatalysts with high solar efficiency. However, considering that the solar light contains 5% of UV light, the solar light is highly encouraged to be used for testing the activity of nanophotocatalysts rather than visible light only. (5) The systematic comparative studies of the adsorption/photocatalytic degradation of different dyes over the nanomaterials are still lacking; this work will give us more visual ideas about the effectiveness of each materials. Acknowledgements Authors thank the financial support from the Startup Foundation by the Department of Environmental Science and Engineering, Fudan University (IDH1829053/001/006) and Talents Introduction Plan of Fudan University (IDH1829005).

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