Environmental Science and Pollution Research https://doi.org/10.1007/s11356-018-3367-2
RESEARCH ARTICLE
Date palm ash-MgAl-layered double hydroxide composite: sustainable adsorbent for effective removal of methyl orange and eriochrome black-T from aqueous phase Nawaf I. Blaisi 1 & Mukarram Zubair 1 & Ihsanullah 2 & Sadaqat Ali 3 & Taye Saheed Kazeem 4 & Mohammad Saood Manzar 1 & Walid Al-Kutti 5 & Mamdouh A. Al Harthi 4,6 Received: 9 April 2018 / Accepted: 27 September 2018 # Springer-Verlag GmbH Germany, part of Springer Nature 2018
Abstract Date palm ash (DPA) and MgAl-layered double hydroxide (LDH) composites were synthesized by the co-precipitation method and characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM), X-ray diffraction (XRD), and Brunauer–Emmett–Teller (BET). The DPA-MgAl-LDH (DPA/MgAl) composites were employed for the removal of methyl orange (MO) and eriochrome black-T (EBT) from aqueous phase. Incorporation of 33.33% (w/w) DPA into the layers of MgAl increased the surface area from 44.46 to 140.65 m2/g, which leads to the improved adsorption performance. The maximum adsorption capacity of DPA/MgAl (1:2) at 298 K was 242.98 and 425.16 (mg/g) for MO and EBT, respectively. The adsorption data of dyes were adequately fitted by a pseudo-second-order and Langmuir isotherm model. The composite showed excellent reusability performance up to three cycles. Addition of DPA into MgAl-LDH resulted in an effective low-cost adsorbent for decontamination of dyes from wastewater. Keywords Date palm ash . Layered double hydroxide . Dyes . Adsorption . Sustainable material
Introduction Contamination of synthetic dyes, mostly in industrial wastewater, is a global threat because of their serious adverse effects to environment and human health. Tons of dyes are discharged into wastewater from many industries such as textile, printing and dying, cosmetic, petroleum, and leather (Zheng et al. 2018). The annual dye consumption of textile industries is
around 1 × 104 tons, and over 100 tons/year of dyes is released into wastewater (Yagub et al. 2014). Methyl orange (MO) and eriochrome black-T (EBT) are anionic dyes and mainly consumed in textile industries. These dyes are stable, non-biodegradable, and thus potentially lethal, mutagenic, and carcinogenic. Presence of these dye molecules in water bodies, consequently, produces harm not only to ecosystems but also navigate through the food chain leading to adverse diseases to human
Responsible editor: Philippe Garrigues Electronic supplementary material The online version of this article (https://doi.org/10.1007/s11356-018-3367-2) contains supplementary material, which is available to authorized users. * Mukarram Zubair
[email protected] 1
2
Department of Environmental Engineering, College of Engineering, Imam Abdulrahman Bin Faisal University, 31982, Dammam, Saudi Arabia
Center for Environment and Water, Research Institute, King Fahd University of Petroleum & Minerals, 31261, Dhahran, Saudi Arabia
3
Department of Mechanical and Energy Engineering, College of Engineering, Imam Abdulrahman Bin Faisal University, 31982, Dammam, Saudi Arabia
4
Department of Chemical Engineering, King Fahd University of Petroleum & Minerals, 31261, Dhahran, Saudi Arabia
5
Department of Civil and Construction Engineering, College of Engineering, Imam Abdulrahman Bin Faisal University, 31982, Dammam, Saudi Arabia
6
Center of Research Excellence in Nanotechnology, King Fahd University of Petroleum & Minerals, 31261, Dhahran, Saudi Arabia
Environ Sci Pollut Res
health (Luo and Wang 2018). Effective decontamination of dyes in wastewater is extremely desirable in order to prevent potential hazards before releasing them into the environment (Zubair et al. 2018). Numerous treatment techniques have been reported such as adsorption (Song et al. 2017), ion exchange (Liu et al. 2007), ozonation (Liu et al. 2007), bio-sorption (Seema et al. 2018), and chemical oxidation (Mohamed et al. 2017) to treat dye-contaminated wastewater. Among all treatment techniques, adsorption is the promising technique due to low cost, ease of operation and efficient and fast removal tendency (Gondim et al. 2018). Researchers are always in a quest to explore novel lowcost and sustainable adsorbents with outstanding removal efficiencies for various pollutants (Riaz et al. 2017). During the last two decades, layered double hydroxides (LDHs), and its hybrids revealed promising adsorption characteristic for various toxic pollutants and attracted considerable attention in water/waste water treatment (Elanchezhiyan and Meenakshi 2017; Morales-Irigoyen et al. 2017; Zhang et al. 2018). These materials exhibited excellent interlayer ion exchange capability, high surface area, and lower toxicity with versatile composition. LDHs hybridization, i.e., the imbedding of LDHs with different materials such as anions, polymers and nanomaterial (graphene, CNT, etc), and others lead significant improvement in both surface and structure characteristics of LDHs (Guo et al. 2016; Zubair et al. 2017a). Several LDH hybrids have been reported for the effective remediation of wastewater. Cd-Al/carbon layered double hydroxide nanocatalyst showed efficient decolorize anionic dyes with superb regeneration ability (Khan et al. 2016). Hybridization of LDHs with fulvic acid resulted in abundant functional groups on the LDH surface leading to enhanced removal due to better interaction with dye and heavy metal. Yan et al. reported a high adsorption capacity of calcined glycerine-modified MgAl for methyl orange (Yang et al. 2013). Graphene oxide embedded on NiCr showed outstanding removal of methyl orange associated to the combined effect of both graphene and LDH (Ruan et al. 2016). Gao et al. fabricated biomorphic LDH hybrid by combining cotton fibers into the MgAl-LDH structure and reported effective deployment in the removal of methyl orange. Similar results for the enhanced removal of MO were also reported in our previous study when we incorporated starch into inter layers of NiFe-LDH (Zubair et al. 2018). The employment of cheap biomass can be regarded as a sustainable approach and demanding area of interest for water purification (Gola et al. 2017; Savova et al. 2001; Takam et al. 2017). Among other biomass sources, date palm tree is considered as one of the abundant trees on earth with an estimated population of around 105 million of different types (Agoudjil et al. 2011; Al-Kutti et al. 2018). It is estimated that typically each tree generates waste of 25 leaves per season (Abdelouahhab and Arias-Jimenez 1999). The Kingdom of Saudi Arabia has one of the largest cultivation of more than 300 types of date palm trees (Al-Kutti et al. 2017; Assirey 2015). Various studies have
been testified for the application of date palm for the removal of different pollutants from aqueous solutions (Banat et al. 2003). Date palm ash is a waste material produced from the process of recycling the palm or from burning palm dead fronds. Previous studies have reported the effective application of bottom ash, fly ash for the purification of wastewater (Nguyen et al. 2017; SecoReigosa et al. 2013). Ash produced from date palm is highly porous and recently employed as a replacement of cement material (Al-Kutti et al. 2017). Based on the detailed literature review and to the best of our knowledge, addition of date palm ash in layered double hydroxide and its application for the removal of pollutants from water have not been investigated yet. This incorporation of porous date palm ash is expected to change the surface properties of LDH and thereby enhanced its adsorption performance. Moreover, low cost and less toxicity of date palm ash (Blaisi 2018) provides a sustainable and ecofriendly adsorbent material for effective decontamination of hazardous pollutants from wastewater streams. The aim of the work is to hybridize different compositions of date palm ash with MgAl-LDH via a simple one-pot co-precipitation method and investigated it as adsorbent for the remediation efficiency of anionic dyes from aqueous phase. The characterization of composites was performed by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscope (SEM), and Brunauer–Emmett–Teller (BET) before and after adsorption to evaluate the changes in characteristics after addition of DPA and mechanism of dye adsorption. The influence of pH, contact time, initial dye concentration, dosage, salt content, temperature, adsorption equilibrium, and kinetics were studied by using batch mode experiments.
Experimental section Materials and methods Materials Date palm ash was obtained from a palm recycling plant located in the Eastern Province of Saudi Arabia. Aluminum (III) nitrate nonahydrate [Al(NO3)3·9H2O], and magnesium(II) nitrate hexahydrate [Mg(NO3)2·6H2O], were purchased from Sigma-Aldrich Co. (USA). All solvents and materials were of analytical grade and used as received. The stock solution of 1000 mg/L concentration of MO and EBT dyes was prepared and diluted to the required concentrations by using deionized water. The characteristics of the MO and EBT dyes are listed in Table 1. Preparation of date palm ash-MgAl (DPA/MgAl) composites The date palm ash/MgAl-LDH composites were prepared via a co-precipitation method. Initially, a known amount of 3:1
Environ Sci Pollut Res Table 1 Characteristics of MO and EBT
Chemical formula
Color index/type
ƛmax
Molecular weight (g/mol)
C14H14N3NaO3S C20H12N3NaO7S
13,025/anionic dye 14,645/azo dye
464 nm 530 nm
327.33 461.38
ratio of magnesium and aluminum salts (M2+: M3+) were dissolved in 50 mL of deionized (DI) water in a reactor equipped with a magnetic stirrer. Simultaneously, to achieve homogenous and effective dispersion of DPA into the interlayers of LDH, a known amount of DPA corresponding to the MgAlLDH ratio (Table 2) was also sonicated for 1 h in 100 ml deionized water and transferred to the reaction vessel. The mixture solution was stirred vigorously for 15 min at 90 °C. Subsequently, pH of the solution was adjusted to 10 ± 0.5 using 1 M NaOH. After maintaining the desired pH, the reactor was subjected to refluxing at 90 °C for 24 h. The resultant suspension was centrifuged and washed with DI water, followed by ethanol washing for the removal of impurities. The dense slurry was then dried at 40 °C in an oven for 48 h. The powder DPA/MgAl composites were then stored in a desiccator for further use. The composition of the DPA/ MgAl composites used for synthesis is listed in Table 2. Characterization of DPA and DPA/MgAl composites The synthesized DPA/MgAl composites and DPA were characterized by Fourier transform-IR ( Nicolet 6700, resolution 4 cm−1), X-ray diffraction (D8 advance x-ray instrument, wavelength = 0.1542 nm, and 2θ = 10 to 80°), scanning electron microscopy (SM-6460LV (Jeol)), and Brunauer– Emmett–Teller (Micromeritics, Tristar II series). MO and EBT adsorption experiments Initially, about 0.01 g of each DPA/MgAl composite and DPA was agitated in 50 ml round bottom plastic tubes at 298 K for 4 h at 275 rpm in 20 mg/L of MO and EBT dye solutions at pH of 3 and 2, respectively. Based on these experiments, the composite that showed the higher removal efficiency for both dyes was selected for batch mode experiments to evaluate the key influential adsorption factors such as initial dye concentration, pH, adsorbent dosage, salt concentration, and contact time via equilibrium and kinetics studies. Precisely 0.01 g of the selected composite was agitated for 360 min at 275 rpm in Table 2
Composition of DPA/MgAl-LDH composites
30 mL of MO and EBT solutions (20–100 mg/L by using 50 mL plastic tubes) at temperature ranges from 298 to 318 K. The appropriate pH of the mixture was adjusted by using 0.1 mol/L HNO3 and 0.1 mol/L NaOH solutions. After agitation, the mixture was centrifuged at 2000 rpm for 5 min to separate the spent adsorbent from the residual dye. The final concentration of the residual MO and EBT dyes was calculated by a Hach Lange spectrophotometer, which was set at a maximum wavelength of 464 and 530 nm, respectively. The amount of dye adsorbed on the adsorbent qe (mg/g) and percentage removal efficiency were estimated according to Eqs. (1) and (2), respectively: ðC0 −Ce Þ V W ðC0 −Ce Þ 100 Percentage removal ¼ Ce
Adsorption capacity ¼ qe ¼
ð1Þ ð2Þ
where Co and Ce are the initial and equilibrium concentration (mg/L) of MO and EBT in solution, respectively; qe (mg/g) is the equilibrium adsorption capacity; W (g) is the weight of adsorbent; and V (L) is the volume of solution. Adsorption isotherm To demonstrate the adsorption behavior of dye molecules on the surface of the DPA/MgAl composites, equilibrium data was applied to four isotherm models, namely the Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich isotherms. The Langmuir model, which assumes that the adsorbates are homogeneously adsorbed on equal-energy possessing monolayer surfaces, is given as: qe ¼
qmax b C e 1 þ b Ce
ð3Þ
where qmax and b are defined as maximum monolayer adsorption and the parameter related to free energy of adsorption, respectively, and are computed from regression analysis. The Freundlich isotherm that can be applied for non-ideal multilayer sorption on heterogeneous surfaces is given as:
Sample name
Date palm ash (g)
MgAl (3:1) (g)
qe ¼ K F C 1=n e
ð4Þ
DPA/MgAl-1 DPA/MgAl-2 DPA/MgAl-3
0.5 1 1.5
2 2 2
where n and Kf are the Freundlich constants obtained from non-linear regression of Eq. 4. Temkin isotherm, which considers the adsorbatesadsorbent interaction where the heat of adsorption
Environ Sci Pollut Res
reduces linearly with coverage for all molecules, is given as: qe ¼ B lnðAT C e Þ
ð5Þ
where B is Temkin constant related to the heat of sorption and AT is the Temkin isotherm constant. Dubinin–Radushkevich (D-R) considers the adsorption on a heterogeneous surface with Gaussian energy distribution, and it is given as: qe ¼ qs exp −K d ε2 ð6Þ where qs is the theoretical saturation capacity (mg/g), Kd is the Dubinin–Radushkevich constant, and ℇ2 is the Polanyi potential. Kd established a significant relation with the mean free energy of adsorption, E, given by Eq. (7). E can be used to distinguish between physical and chemical adsorption of a molecule on a surface: E ¼ 1=ð2 K d Þ1=2
ð7Þ
at 275 rpm for about 3 h. After adsorption, the spent adsorbent was regenerated by immersing in 50 ml 0.1 M NaOH solution and agitated for 3 h. The regenerated adsorbent was washed up to three times with deionized water to remove excess NaOH and dried at 60 °C for 3 h. The dried regenerated composite was again added in 20 mg/L MO and EBT dye solution for the second adsorption test. The adsorptiondesorption tests were repeated three times.
Result and discussion A preliminary experiment was performed to remove MO and EBT at pH 3 and pH 2, respectively, using all three assynthesized DPA/MgAl composites and DPA. The percent removal results are displayed in Fig. 1. It was found that DPA/MgAl-2 showed a higher percent removal of both dyes compared to other composites and DPA. Therefore, DPA/ MgAl-2 was selected to investigate the effect of influential adsorption parameters, equilibrium, and kinetics studies.
Adsorption kinetics
Characterization of DPA and DPA/MgAl composites
To further investigate the interaction between the adsorbents and adsorbates during the adsorption process, kinetics was examined using pseudo-first-order and pseudo-second-order models. The pseudo-first-order is given as:
Structure analysis
k1 t 2:303
ð8Þ
where qt and K1 define the adsorption capacity (mg/g) at contact time t and the first-order rate constant (min−1), respectively. The value of K1 is obtained from slope of linear plot of log (qe − qt) versus t. The pseudo-second-order model by Ho and Mckay, (Ho and McKay 1999), which is based on the assumption that the adsorption is predominantly chemisorption, is represented by: t 1 t ¼ þ qt k 2 q2e qe
ð9Þ
where K 2 is the pseudo-second-order rate constant (g mg−1 min−1) and its value is computed from the intercept of the linear plot of t/qt versus t.
100
MO EBT
80
% Removal of Dyes
logðqe −qt Þ ¼ logqe −
Figure 2a displays the FTIR spectrum of DPA and DPA/MgAl composites. The strong distinguished peak at 3449 cm− 1 in all the three DPA/MgAl composites is associated to the stretching vibration mode of the hydroxyl groups of the interlayer water molecules and/or hydrogen bonding in the layer of the composite surface (Zubair et al. 2014). The weak band at about 1643 cm−1 corresponds to the bending vibrations of C=C in
60
40
20
Regeneration test of DPA/MgAl composite A series of adsorption-regeneration tests were carried out to further evaluate the reusability performance of adsorbent. For adsorption, nearly 25 mg of adsorbent was added in a 200-ml flask containing 100 mL of 20 mg/L of MO and EBT dye solutions at pH 3 and 2, respectively. The flasks were shaken
0 DPA/MgAl-1
DPA/MgAl-2
DPA/MgAl-3
DPA
Adsorbents
Fig. 1 Percentage removal of MO and EBT on each adsorbent (Co = 20 mg/L; T = 298 K; 10 mg of each adsorbent, pH 3 for MO and pH 2 for EBT)
Environ Sci Pollut Res (a)
(b)
DPA/MgAl-3
DPA/MgAl-3
DPA/MgAl-1
1643
Intensity (a.u)
% Transmittance
DPA/MgAl-2
3449 1355
DPA
1437
DPA/MgAl-2
DPA/MgAl-1 975
3500
3000
2500
2000
1500
1000
DPA
10
20
30
-1
40
50
60
70
2 theta (degree)
Wavenumber (cm )
Fig. 2 FTIR of DPA and DPA/MgAl composites (a), XRD of DPA and DPA/MgAl composites (b)
DPA/MgAl composites. The sharp peak at 1355 cm−1 is associated with the symmetric stretching band of interlayer NO3 ions (Mu’azu et al. 2018). For DPA spectra, here are two main visible peaks at 1437 and 975 cm−1 that correspond to the C-H bond. These peaks are also found in DPA/MgAl composites, indicating good encapsulation of DPA in the interlayer of LDH. The FTIR spectrum clearly displays that the surface of DPA/MgAl composites is enriched with abundant functional groups which could facilitate effective binding of dye molecules. The XRD patterns of DPA and DPA/MgAl composites are shown in Fig. 2b. The X-ray diffraction pattern of the DPA/ MgAl-1 composite exhibits intense reflection peaks at 2 = 14.71, 31.76, 49.29, and 55.07° with interlayer spacing of 7.81, 3.92, 2.605, and 2.02°A, respectively. The XRD pattern of the DPA/MgAl-2 also showed a similar crystal structure to DPA/MgAl-1 but with a slight decrease in peak intensity, indicating negligible deviation in the crystal structure after incorporation of increased amount of DPA. However, the low and broad diffraction intensity of the DPA/MgAl-3 composite clearly suggests that the as-synthesized DPA/MgAl-3 has a poor crystalline structure compared to other DPA/MgAl hybrids. This is associated with the large amount of DPA which leads to the poor dispersion and agglomeration in the interlayer of the LDH surface and thus, poor crystallinity. A similar behavior was also reported when starch was incorporated into NiFe-LDH using the co-precipitation method (Zubair et al. 2018). Surface analysis Figure 3 displays the surface morphology of DPA and DPA/ MgAl-2 composites. The SEM image of DPA illustrates irregular-shaped particles with highly porous and rough surfaces. The SEM image of the DPA/MgAl-2 composite showed morphology of the ultra-fine composite with a porous and
rough surface due to the peculiar nature of DPA. This indicated the effective embedding of porous DPA particles into interlayers of LDH (Ahmad et al. 2017). Moreover, the composite structure at high magnification further confirmed that the porous DPA was uniformly deposited throughout the LDH layers. To further illustrate the surface characteristics, surface area, pore volume, and pore size of DPA, MgAl, and DPA/ MgAl-2 were calculated using BET surface analyzer and listed in Table 3. The surface area, pore volume, and pore size of DPA were estimated to be 1.034 m2/g, 0.003 cm3/g, and 21.908 A, respectively. It was observed that the surface characteristics (surface area, pore volume, and pore size) of MgAl after coupling with porous DPA has significantly increased from 44.46 m2/g, 0.009 cm3/g, and 40.88 A to 140.65 m2/g, 0.335 cm3/g, and 87.27 A. This indicates that incorporation of DPA into the MgAl surface has substantially improved the surface characteristics of the composite associated with the highly porous nature of DPA that facilitated the formation of a rough and porous composite. The rough surface and improved surface area are expected to enhance the active adsorption sites and binding capacity of the composite.
Effect of adsorption parameters Effect of initial pH on the dye solution Solution pH is an important factor that affects both aqueous chemistry and binding sites on the adsorbent surface. The effect of pH on the adsorption of MO and EBT by DPA/ MgAl-2 is presented in Fig. 4a. Solution pH was varied from 2 to 6. All other parameters, i.e., adsorbent dosage, contact time, and initial concentration of both MO and EBT, were kept constant at 10 mg, 3 h, 20 mg/L, and 298 K, respectively. The point zero charge (pHPZC) of DPA/MgAl-2 was estimated by using the pH drift method (Fiol and Villaescusa 2009) and is shown in S1. The pHPZC of DPA/MgAl-2 was found to be
Environ Sci Pollut Res Fig. 3 SEM image of DPA and DPA/MgAl-2 composite
9.81. The maximum removal of both MO and EBT was observed at pH 2–3 and 2, respectively. The percentage removal of EBT decreased almost linearly with the increase in pH from 2 to 6. However, in the case of MO, a rapid decrease was observed from pH 3 to 4, and beyond pH 4, no significant change in the removal was observed with further changes in pH. This behavior can be explained based on the ionic chemistry of the solution and surface charge of the adsorbents. At a lower pH (i.e., pH 2), surfaces of the adsorbents were protonated, and strong electrostatic interactions were obtained with both the anionic dyes MO and EBT. With increases in pH, the number of positively charged sites decreased which weakened the interaction between adsorbent surfaces and anionic dyes. It can be concluded that electrostatic interaction plays a critical role in the adsorption of MO and EBT on DPA/MgAl-2. However, other interactions such as hydrogen bonding, ion exchange, and chemisorption also contribute to the adsorption, as discussed in the BMechanism of adsorption of dyes on the DPA/MgAl composite^ section. A similar behavior of
Table 3 Surface area, pore volume, and pore size of DPA, MgAl, and DPA/MgAl-2 composite
BET surface area (m2/g) Average pore volume (cm3/g) Pore size (based on BJH) (A)
DPA
MgAl
DPA/MgAl-2
1.034 0.003 21.908
44.46 0.009 40.88
140.65 0.335 87.273
MO and EBT adsorption by different adsorbents has been reported in the literature (El Hassani et al. 2017; Yao et al. 2017). A maximum removal of 99 and 93% was observed at pH 2 and pH 3 by DPA/MgAl-2 for EBT and MO, respectively, after 3 h, and therefore, these were selected as the optimum pH for the later experiments. Effect of contact time Figure 4b shows the effect of contact time on adsorption of MO and EBT by DPA/MgAl-2 at MO and EBT concentrations of 20, 60, and 100 mg/L. All other parameters such as pH, temperature, adsorbent dosage, and agitation speed were kept constant at 3 (for MO) and 2 (for EBT), 298 K, 10 mg, and 275 rpm, respectively. It was observed that removal rate was extremely fast at the initial time until 60 min for all concentrations of MO and EBT. No further change in the removal was observed after 90 min. Maximum removals of 96 and 88% were observed for EBT and MO after 90 min by DPA/ MgAl-2 at initial concentrations of 20 mg/L for both dyes. The fast adsorption rate of DPA/MgAl-2 followed by slow adsorption may be associated with interactions between MO molecules and the active sites of hydrotalcite and surface functional groups, respectively. Effect of dye concentration To study the effect of initial dye concentration on the adsorption capacity of the DPA/MgAl-2, the initial concentration of
Environ Sci Pollut Res 100
(a)
100
(b)
90
% Removal of dyes
% Removal of dyes
80 80
MO
70 60 50
EBT
60
40
MO-20 mg/L MO-60-mg/L MO-100 mg/L EBT-20 mg/L EBT-60-mg/L EBT-100 mg/L
20
40 0
30 2
3
4
5
6
0
20
40
60
pH
80
100
120
140
160
180
Contact time (min)
(d)
(c)
EBT
100
350
90
% Removal of dyes
qe (mg/g)
280
210
140
70
MO EBT
MO
80
70
60
50
0
40 10
20
30
40
50
60
70
80
90
100
110
5
10
Initial concentration (mg/L)
100
15
20
Dosage (mg)
(e)
90
% Removal of dyes
80
EBT
70 60 50 40 30
MO
20 10 0 0
5
10
15
20
25
30
Salt (NaCl) concentration (g/L)
Fig. 4 Effect of initial pH on the removal of MO and EBT (Co = 20 mg/L; T = 298 K; 10 mg of each DPA/MgAl-2) (a). Effect of contact time on the removal of MO and EBT (Co = 20, 60 and 100 mg/L; T = 298 K; 10 mg; pH = 3 (MO) and 2 (EBT)) (b). Effect of initial concentration on the adsorption capacity (Co = 20–100 mg/L of MO and EBT; T = 298 K;
10 mg; pH = 3 (MO) and 2 (EBT)) (c). Effect of adsorbent dosage (Co = 20 mg/L; T = 298 K; pH = 3 (MO) and 2 (EBT), 2-20 mg DPA/ MgAl-2) (d). Effect of salt concentration (Co = 20 mg/L; T = 298 K; pH = 3 (MO) and 2 (EBT), 10 mg DPA/MgAl-2, salt concentration = 5–30 g/L) (e)
MO and EBT was varied from 20 to 100 mg/L during the batch adsorption experiments. All other parameters, such as pH, temperature, adsorbent dosage, and agitation speed, were
kept constant at 3 (for MO) and 2 (for EBT), 298 K, 10 mg, and 275 rpm, respectively. The results are shown in Fig. 4c. The adsorption capacity increases linearly in case of EBT
Environ Sci Pollut Res Table 4 Parameters of nonlinear Langmuir, Freundlich, Tempkin, and Dubinin-Reduskevich isotherm models for adsorption of MO and EBT onto DPA/MgAl-2 composite Dye
MO
EBT
T (K)
Langmuir
Freundlich
Tempkin
Dubinin–Radushkevich
qmax (mg/g)
KL
R2
KF
1/n
R2
B
AT
R2
qs
Kd × 10−5
R2
298
242.98
0.171
0.979
53.18
0.386
0.935
16.88
23.12
0.944
179.59
10.7
0.862
308
219.81
0.084
0.982
30.85
0.446
0.944
47.33
0.83
0.979
149.18
30.8
0.841
318 298
214.95 425.16
0.063 0.253
0.992 0.992
18.66 49.12
0.557 0.94
0.956 0.965
55.08 133.5
0.41 0.09
0.999 0.948
147.75 254.21
66.9 7.7
0.911 0.803
308 318
409.89 345.55
0.220 0.112
0.989 0.992
69.15 41.02
0.58 0.58
0.948 0.949
15.09 83.62
78.28 0.83
0.953 0.994
264.89 223.86
63.3 17.4
0.867 0.886
when the concentration range studied. While for MO, the adsorption capacity increases almost linearly up to an initial concentration of 84 mg/L, and no significant change was observed beyond this concentration. Increase in the adsorption capacity with an increase in concentration might be due to the fact that the adsorbent has the capacity to capture more dye molecules at higher concentrations. In case of MO, the adsorbent reached at maximum adsorption capacity at a concentration of 84 mg/L and hence upon further increase in concentration, no more molecules could be adsorbed on the surface. Maximum adsorption capacity of 342.6 and 215 mg/g was reached by DPA/MgAl-2 for EBT and MO under similar experimental conditions. The results demonstrated that the hybridization of DPA into interlayers of MgAl caused a substantial improvement in the removal efficiency for MO and EBT. This was associated with the presence of oxygen functionalities on the surface of composites higher surface area and surface roughness, as confirmed from FTIR, BET, and SEM results, respectively, which improved the interaction of composites with dye molecules. Similar behavior was also reported by Chang et al. (Chang et al. 2011) by using
starch-CNT composites for the removal of methyl orange and methylene blue. Effect of dosage The effect of adsorbent dosage (2–20 mg) on the removal efficiency of dyes is shown in Fig. 4d. It was observed that the removal efficiency of both EBT and MO dyes linearly improved with increase in the adsorbent amount from 2 to 10 mg. This can be associated with increase of adsorption active sites with the increase in amount of adsorbent. However, no significant increase in removal of both the dyes was noted with increase in adsorbed dosage beyond 10 mg. Consequently, 10 mg of dosage was considered enough for removal of both dyes and maintained in all the succeeding experiments. Effect of salt concentration The untreated water effluents containing colorants often discharged from industries have substantial magnitudes of
250
(a)
MO
(b)
400
EBT
350 200
qe (mg/g)
qe (mg/g)
300
150
298K 308K 318K Langmuir Freundlich
100
50
250 200
298K 308K 318K Langmuir Freundlich
150 100 50
0
10
20
30
Ce (mg/L)
40
50
60
0
10
20
30
Ce (mg/L)
Fig. 5 Adsorption isotherms on DPA/MgAl-2 for MO (a) and EBT (b), at 298, 308, and 318 K (error bars indicate the standard deviation of triplicate measurements)
Environ Sci Pollut Res Table 5 models Dye
Parameters of pseudo first and pseudo-second-order kinetic Pseudo-first order
Co
qe (exp) qe MO EBT
20 126.88 33.11
Pseudo-second order
R2
k1 0.06
Table 6 Thermodynamic parameters of adsorption of MO and EBT on DPA/MgAl-2 composite T (K)
k2 × 10−2 R2
qe
MO
0.681 131.57 0.130
0.999
100 417.36 243.22 0.090 0.823 476.19 0.012
0.992
20 148.96 106.59 0.144 0.959 153.84 0.164 100 686.08 539.15 0.090 0.931 833.33 0.003
0.999 0.990
salts, which has a deep impact on adsorption performance of adsorbents. Figure 4e demonstrates the effect of NaCl concentration (5–30) g/L on the removal of MO and EBT at an initial concentration of 20 mg/L at 298 K and solution pH 2 and 3, respectively. It can be perceived that removal of MO dye diminished with growing NaCl addition. Similarly, the percentage removal of EBT also eventually decreased with the increase in concentration of NaCl. For example, the removal of EBT and MO decreased from 99.78 to 64.45 5% and 88.68 to 12.16% with addition of salt from 0 to 30 g/L, respectively. This indicates a significant decline in the removal efficiency of DPA/MgAl-2 composites due to the presence of salt content. Considerable reduction in the adsorption capacity of composite with the rise in ionic strength of electrolytes may be attributed to the interference effect of Cl− with NO3− of LDH which may have resulted in the decrease in electrostatic attraction among dye and DPA/MgAl-2 molecules (George and Saravanakumar 2018). Similar behavior was also reported in the previous study (Boubakri et al. 2018).
Isotherm studies Adsorption isotherms are essential to discern the interaction between adsorbates and adsorbent at equilibrium.
EBT
298
31.78
− 8.56
308 318
14.71 10.82
− 6.88 − 6.29
298
92
− 11.20
308
73.31
− 10.99
318
31.61
− 9.13
ΔH (kJ/mol)
ΔS (J/mol K)
− 42.63
− 114.87
− 41.806
− 101.82
Hence, the equilibrium data were fitted to Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich (D-R) isotherm models via non-linear regression. Table 4 shows the isotherm parameters of all four models, while Fig. 5a, b displays the non-linear Langmuir and Freundlich isotherm plots of MO and EBT, respectively. It is vivid from Table 4 that the best correlation is, in increasing order, D-R < Freundlich < Temkin < Langmuir model. The results (Table 4) clearly confirm that at all temperatures (298– 318 K), the Langmuir isotherm (R2 > 0.978) model described the adsorption process better than Tempkin, Freundlich and D-R isotherms. It has been implied that the surface of DPA/MgAl-2 is more homogeneous than heterogeneous. Also, the value of KL (0 < KL < 1) signified the favorability of the adsorption process. The mean free energy of adsorption, E, values calculated using Eq. (7) from Table 4 for the D-R isotherm ranges from 28 to 88 KJ/mol. This is an indication that chemisorption and ion exchange dominates physisorption. In addition, it can also be seen from Table 4 that 1/n increased with increase in temperature, signifying the unfavorable adsorption process at higher temperatures. This assumption was further strengthened by decrease in maximum adsorption capacity with increases in temperature. The maximum adsorption
10
1.6
(a) 8
1.4
6
1.2
4
(b)
20 mg/L (MO) 20 mg/L (EBT) 100 mg/L (MO) 100 mg/L (EBT)
1.0
2
0.8
t/qt
ln(qe-qt)
ΔG (kJ/mol)
Kd
0
0.6
-2 0.4
20 mg/L (MO) 20 mg/L (EBT) 100 mg/L (MO) 100 mg/L (EBT)
-4 -6
0.2 0.0
-8 0
20
40
60
80
100
t (min)
120
140
160
180
200
0
20
40
60
80
100
120
140
160
180
200
t (min)
Fig. 6 Linear plots of the pseudo-first-order model (a) and the pseudo-second-order model (b) for the adsorption of EBT and MO on DPA/MgAl-2 at 20 and 100 mg/L dye solutions
Environ Sci Pollut Res
controlling dye removal by modified LDHs have been reported (El Hassani et al. 2017; Lu et al. 2016; Shan et al. 2015; Zubair et al. 2018).
Thermodynamic studies The thermodynamic parameters (Kd, ΔG, ΔH, and ΔS) of adsorption of EBT and MO on DPA/MgAl-2 were calculated using isotherm data at three temperatures (298, 308, and 318 K) using Eqs. 10 and 11: ΔG ¼ −RTlnK d Fig. 7 Plot of LnKd vs. 1/T for the valuation of thermodynamic parameters
capacity by DPA/MgAl-2 for MO and EBT at 298 K were 242.98 and 425.16 mg/g, respectively.
Kinetic studies Table 5 shows the parameters of both models for two concentration (20 and 100 mg/L) for DPA/MgAl-2 adsorption of MO and EBT. Figure 6a, b displays the plots of both models. The correlation coefficient of the pseudo-second-order model (R2 > 0.99) was significantly higher than the pseudo-firstorder model, indicating that the adsorption of EBT and MO onto DPA/MgAl-2 were adequately fitted by a pseudosecond-order model. In addition, the qe from the pseudosecond-order closely matched the experimental qe. This indicates that the mechanism controlling step for the adsorption process could be chemisorption. Moreover, the DPA/MgAlMO-EBT adsorption process also associated to electrostatic and ion exchange, as confirmed from the effect pH and salt concentration, respectively. Similar results on chemisorption
(a)
lnK d ¼
ΔS ΔH − R RT
ð10Þ ð11Þ
where ΔG is standard free energy in kilojoules/mole, ΔS is standard entropy in joules/mole kelvin, and ΔH is standard enthalpy in kilojoules/mole. R is the universal gas constant, T is the absolute solution temperature in kelvin, and Kd is the thermodynamic equilibrium constant that is determined using method of Xin et al. (Xin et al. 2011) by plotting ln(qe/Ce) vs. qe and extrapolating qe to zero. Values of ΔS and ΔH, listed in Table 6, were calculated from the plot of LnKd vs. 1/T (Fig. 7). The negative values of ΔG under all temperatures showed that the adsorption of EBT and MO on the DPA/MgAl-2 composite was spontaneous and favorable. Similarly, the negative values of ΔH for both MO and EBT showed that the adsorption process of both dyes on DPA/MgAl-2 is exothermic in nature. The value of ΔG increased from − 8.56 to − 6.29 (MO) and − 11.20 to − 9.13 (EBT) when the temperature was increased from 298 to 318 K, indicating an exothermic process. In addition, the values of ΔS for MO (− 114.87) and EBT (− 101.82) are negative, which suggests a greater order of reaction during the EBT-DPA/MgAl and MO-DPA/MgAl adsorption system (Zubair et al. 2017b).
(b)
Fig. 8 FTIR (a) and SEM (b) image of DPA/MgAl-2 after adsorption of EBT and MO
Environ Sci Pollut Res
100
% removal of dyes
high probability to be trapped and adsorbed inside these pores. The micrographs of composites after dye adsorption showed dark spots associated with the effective adsorption of dye molecules within the pores and cavities of the composite surface. The adsorbent surface has been covered by dye molecules over the whole surface. From the results, it can be concluded that the adsorption of dyes on the DPA/MgAl composite is mainly due to electrostatic attraction, chemical binding between functional groups of composite and anions molecules, and ion exchange capability of LDH.
EBT MO
75
50
25
Reuse of the DPA/MgAl composite 0 1
2
3
cycle number
Fig. 9 Reusability performance of DPA/MgAl-2 after regeneration
Mechanism of adsorption of dyes on the DPA/MgAl composite To further evaluate the adsorption mechanism of MO and EBT on DPA/MgAl-2 composite, FTIR and SEM analyses were performed after MO and EBT adsorption and it has been shown in Fig. 8a, b, respectively. In FTIR spectra, the C=C broad peak at 1643 cm−1 after EBT and MO adsorption has almost disappeared and narrowed, respectively, compared to unused DPA/MgAl-2 spectra (Fig. 2a). This might be due to chemical interactions of carbonyl groups of composites with anions of dye molecules. Similar conclusion was also obtained from isotherm and kinetic studies. Likewise, the band at 1355 cm−1 (associated to a symmetric stretching interlayer NO3−) shifted to lower intensity after EBT and MO adsorption, which can be attributed to the exchange of anions with MO and EBT molecules. Likewise, as discussed in the BEffect of salt concentration^ section, the adsorption of dyes decreased with increase in salt content, which might be associated to the interference of Cl− ions with NO3− ions of LDH. Figure 8b displays the SEM images of the DPA/MgAl-2 composite after dye adsorption. It is obvious that the dyes have a Table 7
To evaluate the economic feasibility of adsorbent for its industrial application, regeneration and reuse performance was carried out. It is believed that alkaline solution can successfully desorb the anionic dye molecules by weakening the electrostatic attraction between the surfaces of adsorbents and the dye. Therefore, 0.1 M NaOH solution was employed to regenerate the DPA/MgAl-2 composite. Figure 9 shows the adsorption-desorption cycle of MO and EBT dyes on DPA/ MgAl-2 composites. It was observed that removal efficiency of MO and EBT decreased from 88.23 to 60.55% and 99.23 to 68.66% in three cycles. These results indicate that DPA/ MgAl-2 exhibit good tendency of reusability and can be effectively applied commercially for removing hazardous dyes from wastewater.
Comparison with other adsorbents The adsorption capacities of the DPA/MgAl-2 composite and other previously investigated adsorbents for the adsorption of MO and EBT are listed in Table 7. The theoretical maximum adsorption capacity of the DPA/MgAl-2 composite for the adsorption of EBT and MO were 425.16 and 242.98 mg/g, respectively, that are better than most of the adsorbents. This comparison proposed that the DPA/MgAl-2 composite is a favorable adsorbent with exceptional potential for the elimination of pollutants from the aqueous medium.
Adsorption capacity and parameters of EBT and MO on other adsorbents
Adsorbent
pH
Time (h)/temp (°C)
Isotherm/kinetic
EBT qm (mg/g)
MO qm (mg/g)
References
Raw date pits
2–4
5/25
Langmuir/second order
–
46.0
(Mahmoudi et al. 2015)
MgAl-LDH
4
7/25
Langmuir/second order
128.2
–
(Yasin et al. 2010)
Graphene oxide/NiAl-LDH
7
16/25
Langmuir/second order
–
210.8
(Yang et al. 2013)
NiFe2O4
3–10
24/25
Langmuir/second order
82.04
–
(Moeinpour et al. 2014)
Starch/NiFe-LDH
3–7
2/25
Langmuir/second order
–
387.59
(Zubair et al. 2018)
Maize stem
2
−/−
Langmuir/second order
–
167.84
(Vučurović et al. 2014)
Date palm ash/MgAl
2–3
1.5/25
Langmuir/second order
425.16
242.98
This study
Environ Sci Pollut Res
Conclusion The MgAl-LDH-based date palm ash (DPA/MgAl) composite was produced by the co-precipitation method and investigated for the adsorption of anionic dyes. Addition of low-cost date palm ash into the surface of MgAl-LDH not only improved the surface area from 44.46 to 140.65 m2/g but also resulted in a novel cheap adsorbent for the adsorption of MO and EBT. The maximum removal of MO and EBT on DPA/MgAl was obtained at optimum pH 2 and 3, respectively, with adsorption equilibrium achieved at 90 min. The maximum adsorption capacity of DPA/MgAl-2 predicted by the Langmuir isotherm model was 242.98 and 425.16 mg/g for MO and EBT at 298 K, respectively. Thermodynamic analysis showed that adsorption of MO and EBT on DPA/MgAl was spontaneous and exothermic in nature. The adsorption of MO and EBT on DPA/MgAl was governed by several adsorption mechanisms including electrostatic attraction, ion exchange, and chemical bonding between MO and EBT molecules with functionalities of DPA/MgAl composites. The adsorption-desorption analysis demonstrated the effective adsorption performance of composite after three regeneration cycles. This study suggested that the coupling of date palm ash with MgAl-LDH is a promising approach to enhance characteristics and sustainability of LDH as a superior adsorbent material for removal of anionic dyes from the water phase.
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