Advanced Composites and Hybrid Materials https://doi.org/10.1007/s42114-018-0073-y
REVIEW
Inorganic-organic hybrid materials and their adsorbent properties Asgar Kayan 1 Received: 31 August 2018 / Revised: 5 December 2018 / Accepted: 7 December 2018 # Springer Nature Switzerland AG 2018
Abstract Owing to their unique chemical and physical properties, inorganic-organic hybrid materials have been used in many application fields. In this paper, I have summarized the synthesis and characterization of various inorganic-organic hybrid materials with representative examples from my previous studies. These hybrid materials were used as adsorbents in heavy metal adsorption processes in order to solve metal pollution which is one of the most important environmental problems in the world. This review highlights the operation conditions such as pH, required dose, initial concentration, temperature, and treatment performance. Also, adsorption isotherms and adsorption kinetics are reviewed. This knowledge will provide the basis for the researchers who seek the new synthesis and application of hybrid materials in the future. Keywords Hybrid material . Synthesis . Adsorbents . Application
1 Introduction There are many environmental problems caused by chemical substances occurring naturally or man-made discharged into surrounding environments as industrial wastes resulting in serious water and soil pollutions [1–3]. The water pollutions produced by humans and natural sources like weathering and erosion of rocks containing heavy metals increase the problems in our environments [3, 4]. The life of living creatures is being threatened by these pollutions. Therefore, it is vital to remove these toxic metal ions from drinking water and water resources. There are many studies on the removal of toxic metals from aqueous solution by various methods such as ion exchange [5, 6], precipitation [7, 8], solvent extraction [9, 10], chemical and electrochemical techniques [11], ultrafiltration [12], flotation [13, 14], coagulation [15, 16], and adsorption [17, 18]. Among these methods, adsorption process is the most used one because it is affordable, easy to apply, and effective for water decontamination [19–21]. For adsorption process, many natural [22, 23] or synthesized adsorbents such as activated carbon [24, 25], a nanocarbon bridged nanomagnetite network [26], polystyrene-based
* Asgar Kayan
[email protected];
[email protected] 1
Department of Chemistry, Kocaeli University, 41380 İzmit, Kocaeli, Turkey
carbon nanocomposites [27], modified clays or zeolites [28, 29], and polymeric resins [30–32] have been used, and scientists are still investigating new, low-cost, high adsorption capacities, and easily available adsorbents to remove the heavy metal ions from wastewater. In recent years, inorganic-organic hybrid materials have been used for the removal of heavy metal ions from wastewater [33–37] because they allow to combine materials with different properties existing in separate sources and turn it into one unique and accessible structure [38, 39]. This review describes preparation methods of inorganicorganic hybrid materials and focuses on their applications and performances for the removal of heavy metals from wastewater.
2 Synthesis of organic-inorganic hybrid materials 2.1 Hydrolysis of complexes synthesized from metal alkoxide or their derivatives with organic chelate ligands This process is based on the controlled hydrolysis and condensations of metal alkoxide complexes, which leads to the formation of mesoscaled particles and their arrangement to a nanoporous network after gelations. The inorganic-organic hybrid materials mentioned in this section were synthesized
Adv Compos Hybrid Mater
in two steps. In the first step, the preparation of metal-chelate complexes was performed (Scheme 1). In the second step, the hydrolysis and condensation reactions of metal-chelate complex were carried out by dilute HCl solutions. For example, in one study, titanium n-butoxide was complexed with tyrosine ligand to form stable inorganic-organic hybrid materials, (Tyr)4Ti2(OC4H9)2O [40], and then this hybrid material was hydrolyzed by dilute hydrochloric acid solution in alcohol. In hydrolysis and condensation reactions of these organicmodified metal alkoxide complexes, only the alkoxide group underwent substitution while chelated organic groups remained bonded to the metal as seen in Scheme 2. In other studies, organically modified metal alkoxides [ A l 6 ( O s B u ) 2 ( PA ) 8 O 4 ] , [ T i 4 ( O P r i ) 4 ( PA ) 8 O 2 ] , [ Z r 4 ( O n B u ) 4 ( PA ) 8 O 2 ] , [ Ti 4 ( O i P r ) 6 ( B C H O ) 4 O 3 ] , [Zr 4(O nBu) 4(BCHO)4O 4], and [Al6(OsBu) 2(BCHO)4O 6] were prepared from reactions between metal alkoxides (Al(OBs) 3, Ti(OPri) 4, Zr(OBun) 4) and 3-pentenoic acid (PAH) and 1-benzoylcyclohexanol (BCHOH) in alcohol at room temperature by sol-gel process [41].
2.2 Hydrolysis of materials formed from inorganic precursor or their derivatives with organic polymers from GPTS In this synthesis, firstly ethereal polymers were prepared from 3-glycidyloxypropyltrimethoxysilane (GPTS) [42] and then mixed in 1:1 or different mole ratios with metal alkoxide such as aluminum sec-butoxide, titanium isopropoxide, titanium butoxide, zirconium propoxide, and zirconium butoxide in alcohols. After that, the mixtures were hydrolyzed by dilute HCl 3.0/4.0 mol of water per mole of M(OR)3/4 (M: Al, Ti, Zr; OR: OPr, OPri, OBu, OBus…), and 3.0 mol of water per mole of GPTS was added dropwise to the solution and stirred for 24 h at room temperature. After stirring, the volatile parts of the mixture were removed under reduced pressure at 50 °C and a beige solid was obtained (Scheme 3) [43]. In other types of hybrid materials, metal chelate complexes were used instead of pure metal alkoxide precursors. For
H2N
Ti
CH H2C
C4H9O
O
O OC4H9
O
2.3 Hydrolysis of organic polymers formed from GPTS with bases in presence of preformed inorganic hybrid materials In this synthesis, firstly metal alkoxides (metals Zr, Ti, Al) and GPTS were hydrolyzed and condensed to form polymeric species composed of M–O–Si bonds. Then KOBut, KOH, or NaOSiMe3 bases were added to the polymeric species to open the rings of epoxides [44, 45]. For example, in the synthesis of Zr(OPrn)4-GPTS-KOBut-hydrolyzate, the epoxy rings were opened by addition of base KOBut to the pre-hydrolyzed product of Zr(OPrn)4-GPTS, as seen in Scheme 5.
CH O
CH2
2
2
OH
example, the compounds poly-GPTS and Ti2(Tyr)4(OnBu)2O (Scheme 4), [Al6(OsBu)2(PA)8O4], [Al6(OsBu)2(BCHO)4O6], [ Ti 4 ( O P r i ) 4 ( PA ) 8 O 2 ] , [ T i 4 ( O i P r ) 6 ( B C H O ) 4 O 3 ] , [Zr4(OnBu)4(BCHO)4O4], and [Zr4(OnBu)4(PA)8O2] were mixed in 1:1 mol ratio in alcohol and then the mixtures were hydrolyzed by dilute HCl.
NH2
O
Ti
Scheme 2 After hydrolysis reactions of (Tyr)4Ti2(OC4H9)2O complex
HO
Scheme 1 (Tyr)4Ti2(OC4H9)2O complex prepared from Ti(OBun)4 and Tyrosine
Scheme 3 Adsorbent prepared from poly-GPTS and Ti(OiPr)4
Adv Compos Hybrid Mater O-CHCH2
n
H2C O (CH2)3 OCH3
Si
H3CO
OCH3
hydrolysiswith
n
(Tyr)4Ti2(O Bu)2O
0.1MHCl
O-CHCH2
n
H2C O (CH2)3
O
Si
OH
O
HO
NH2
O
Ti
O
CH O
CH2 C6H4OH
Scheme 4 Adsorbent prepared from poly-GPTS and Ti2(Tyr)4(OnBu)2O [40]
3 Characterization of inorganic-organic hybrid materials Characterizations of these hybrid materials were carried out by a combination of 1H, 13C-NMR, mass spectroscopy, powder X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive X-ray (EDX), Brunauer-EmmettTeller (BET) analysis, Barrett-Joyner-Halenda (BJH) analysis, Fourier transform infrared (FTIR) spectroscopies, and elemental analysis. In general, 1H and 13C-NMR spectroscopies were used in the analysis of complex formations between metal alkoxides and appropriate organic ligands such as
Scheme 5 Hybrid materials Zr(OPrn)4/GPTS/ KOBut(NaOSiMe3)-hydrolyzate [45]
t y r o s i n e ( Ty r ) , 3 - p e n t e n o i c a c i d ( PA ) , a n d 1 benzoylcyclohexanol (BCHOH). These organic ligands influenced the functionality of the metal alkoxides and controlled the degree of condensation of the reaction products. Many of those complexes were soluble enough in deuterated solvents to give characteristic peaks in NMR spectra to determine the microstructure of metal alkoxide complexes. After addition of GPTS or poly-GPTS to organically modified metal alkoxide complexes, the other spectroscopy techniques were needed to characterize hybrid materials because of their less or no solubility in appropriate solvents. FTIR spectroscopy is one of the main instruments to support the hydrolysis and condensation reactions between two metal alkoxides. FTIR spectra showed the bond formation of Zr–O–Si and the changes in organic functional groups in hybrid material because of the hydrolysis and condensation reactions between Zr(OPrn)4 and GPTS. The monomer GPTS shows characteristic bands at ~ 1250 cm−1 for ring breathing, 910 cm−1 for asymmetric ring deformation, and 821 cm−1 for sym ring deformation, respectively [46, 47]. When GPTS undergoes polymerization reactions, the ring breathing and ring deformation peaks disappear in the FTIR spectrum. This disappearance supported the fact that the epoxy rings of GPTS underwent ring-opening reactions [41]. Peaks at 2970, 2934, and 2871 cm−1 in the FTIR spectrum of hybrid material Zr-GPTS-KOBut-H were due to C–H stretching in the propyl and etheral groups. The peaks at ~ 1190 and ~ 1092 cm−1 were due to the presence of Si–C and Si–O– bonds, respectively. These data confirmed the presence of an organic propyl group bonded to silicon atom after hydrolysis and condensation reactions. This hybrid material also showed a strong and broad band at 885–910 cm−1 for Zr–O–Si bonds. The Si–O–Si stretching band near 1100 cm−1 shifted toward lower values (885–910 cm−1) when Zr atoms replaced the Si atoms to form Zr–O–Si hetero-linkages [48].
Adv Compos Hybrid Mater
The elemental analysis results also supported the presence of condensation reactions between alkoxide groups of metal alkoxides and organoalkoxysilane (GPTS) in the hybrid materials after hydrolysis. The determination of elemental composition was made by using EDX and elemental analysis. The EDX spectra of hybrid materials showed strong peaks corresponding to zirconium, silicon, oxygen, carbon, and alkali elements. As it was expected, the EDX analysis spectrum of Zr-GPTS-KOBut-H included five elements, Zr, Si, C, O, and K, within approximately the expected weight percentage (Fig. 1). The EDX measurements of different parts of each sample showed that all parts have almost the same chemical compositions. These results supported the presence of unique condensation product (Zr–O–Si) in hybrid materials. Normally, three condensation products would have been expected between Zr–O–Zr, Si–O–Si, and Si–O–Zr since the precursors GPTS and zirconium propoxide contain three hydrolyzable methoxy and four propoxy groups, respectively. However, there was no evidence of self-condensation for Zr(OPrn)4 and GPTS in the FTIR spectra and EDX measurements. This was attributed to less reactivity of silicon alkoxides and their tendency to undergo to heterogeneous Zr–O–Si bond formation rather than self-condensation. SEM was used to observe the morphology of the inorganicorganic hybrid materials, which also supported the results mentioned above. SEM images of hybrid material Zr-GPTSH revealed almost uniform shapes (Fig. 2). SEM images of hybrid material Zr-GPTS-KOBu t -H (Fig. 3) including ethereal bonds were somewhat different from that of Zr-GPTS-H because of the potassium tertbutoxide basic catalyst they included. The KOBut addition to Zr-GPTS-H changed the morphology of hybrid material as they opened the epoxide rings. Since the catalyst additions to these hybrid materials at 50 °C caused not only ring opening of epoxide but also some polymerization reactions of epoxide groups which led more polymeric structure, the
decrease in pore volume (from 1.67 × 10−3 to 7.2 × 10−4 nm width) and the increase in surface area (from 0.581 to 0.641 m2/g) (Table 1). As it was in EDX results, there was no evidence of self-condensation or network in the SEM images exhibiting uniform particles or homogeneous phases. The XRD results suggested that the hydrolyzed materials contained amorphous zirconium-silicate structure. The amorphous pattern was expected considering the reaction conditions used for the preparation of inorganic-organic hybrid materials. The powder XRD analysis showed a broad band positioned at 2θ values of about 25.5–32.5° for the hydrolyzed hybrid material Zr-GPTS-KOBut-H. The other hybrid materials were characterized by the same or a similar way and published in international journals [44, 45]. The surface area, pore volume, and pore size developed by hydrolysis and condensation reactions were analyzed by the BET and the BJH methods. The most popular method to determine the specific surface area and pore volume of porous solids based on multilayer adsorption is the BET method (De
Fig. 1 EDX results of Zr-GPTS-KOBut-H [45]
Fig. 3 SEM image of Zr-GPTS-KOBut-H [45]
Fig. 2 SEM image of Zr-GPTS-H [45]
Adv Compos Hybrid Mater Table 1
Physical characteristics of hybrid materials [45] Zr-GPTS-H Zr-GPTS-KOBut-H Zr-GPTS-NaOSiMe3-H
Analysis adsorptive, N2 Analysis bath temp., 77.43 K Warm free space, 16.73 cm3 Cold free space, 48.75 cm3 Equilibration interval, 10 s Surface area, m2/g BET surface area Langmuir surface area
0.581 0.853
0.641 1.014
0.380 0.607
BJH adsorption cumulative surface area of pores between 1.7000 and 300.0000 nm width 0.243 BJH desorption cumulative surface area of pores between 1.7000 and 300.0000 nm width 0.660 Pore volume, cm3/g
0.083 0.238
0.043 0.049
Single point adsorption total pore volume of pores less than 126.0000 nm width Single point desorption total pore volume of pores less than 60.0000 nm width
0.00167 0.00135
0.00072 0.00054
0.00048 0.00026
BJH adsorption cumulative volume of pores between 1.7000 and 300.0000 nm width BJH desorption cumulative volume of pores between 1.7000 and 300.0000 nm width Pore size, nm Adsorption average pore width (4 V/A by BET)
0.00171 0.00180
0.00063 0.00079
0.00055 0.00055
11.473
4.4971
5.0674
Desorption average pore width (4 V/A by BET) BJH adsorption average pore width (4 V/A)
9.277 28.232
3.3726 30.514
2.7663 51.195
BJH desorption average pore width (4 V/A)
10.937
13.239
44.755
Zr, Zr(OPrn )4; GPTS, 3-glycidyloxypropyltrimethoxy silane; H, hydrolyzate
[49]. The surface areas of Zr-hybrid materials calculated by the BET method were about 0.38–0.64 m2/g. The most commonly applied method to determine the pore size distribution for porous solids based on the Kelvin equation is the BJH method. The pore sizes for hybrid materials calculated by the BJH adsorption average pore width (4 V/A) and BJH desorption average pore width (4 V/A) were 28.2–51.2 nm and 10.9–44.7 nm, respectively. Consequently, the hybrid inorganic-organic materials were nano-porous. The surface area, the pore volume, and the pore size of hybrid materials calculated by different methods are summarized in Table 1. When Al(OBus)3 was used as a precursor instead of Zr(OPrn)4, the surface area, the pore volume, and the pore size of hybrid materials changed as summarized in Table 2. The surface areas of Al-hybrid materials calculated by the BET method were about 0.45–0.61 m2/g. The BJH adsorption cumulative volume of pores between 1.7000 and 300.0000 nm width was 7.4 × 10−4−1.42 × 10−3 cm3/g. The pore size for hybrid materials calculated by the BJH adsorption average pore width (4 V/A) was 35.2–42.9 nm. The changes in the surface structure, surface area, pore size, and pore volume of hybrid materials played an important role in the efficiency of removing heavy metals from wastewater.
4 Removal of heavy metals by organic-inorganic hybrid materials Heavy metal ions such as Cu(II), Fe(III), Pb(II), Zn(II), Ni(II), and Cd(II) were removed from their aqueous solutions by using inorganic-organic hybrid materials. Stock metal ion solutions (1000 mg/L) were prepared from their nitrate or chloride salts in deionized water. The basic batch method was used to prepare samples for measurements. For batch conditions for adsorbents Al-GPTS-KOBut-H and Al-GPTS-KOH-H, a known amount of adsorbent and 25.00 mL of different metal ion concentrations were stirred in 50.00 mL flask at room temperature in different pH values. After equilibrium time, the adsorbents were separated from samples by filtration. Heavy metal ion concentrations in the filtrated solution samples were determined by FAAS (a Perkin Elmer model Flame AAnalyst 800 Atomic Absorption Spectrometer). The maximum adsorption capacity (qe, mg/g) and removal percentage (R%) were calculated by using the following Eqs. (1) and (2) [50, 51]: qe ¼ ðC 0 −C e Þ
V m
ð1Þ
Where C0 is the initial concentration of a metal ion; Ce is equilibrium concentration of adsorbed metal ions (mg/L), V is
Adv Compos Hybrid Mater Table 2
Physical characteristics of hybrid materials [44] Al-GPTS-H Al-GPTS-KOBut-H Al-GPTS-NaOSiMe3-H
Surface area, m2/g BET surface area Langmuir surface area
0.454 0.659
0.609 1.425
0.561 0.917
BJH adsorption cumulative surface area of pores between 1.7000 and 300.0000 nm width 0.084 BJH desorption cumulative surface area of pores between 1.7000 and 300.0000 nm width 0.118 Pore volume, cm3/g
0.127 0.302
0.133 0.276
Single point adsorption total pore volume of pores less than 126.0000 nm width Single point desorption total pore volume of pores less than 60.0000 nm width
0.00078 0.00051
0.00116 0.00086
0.00131 0.00085
BJH adsorption cumulative volume of pores between 1.7000 and 300.0000 nm width BJH desorption cumulative volume of pores between 1.7000 and 300.0000 nm width
0.00074 0.00085
0.00122 0.00129
0.00142 0.00153
Adsorption average pore width (4 V/A by BET) Desorption average pore width (4 V/A by BET)
6.8828 4.5118
7.6201 5.6498
9.3565 6.0795
BJH adsorption average pore width (4 V/A) BJH desorption average pore width (4 V/A)
35.196 28.946
38.462 17.070
42.874 22.117
Pore size, nm
the volume of the solution (L), m is the amount of adsorbents (g), and R% is removal percentage. Rð%Þ ¼
ðC 0 −C e Þ x100 C0
ð2Þ
The heavy metal ions removal by adsorption was optimized by using response surface methodology (RSM) equations and RSM-based CCD (central composite design). There were three steps in CCD: performing the designed experiments, estimating the coefficients in a mathematical model and predicting the response, and validating the model. The relationship between desired response and independent variables was written as in Eq. (3). y ¼ ðx1 ; x2 ; x3 ; …::xn Þ þ ε
ð3Þ
where y represents the response, x is the independent variable, n is the number of factors being studied, and ε is the experimental error [52, 53]. In RSM-based CCD, coded and uncoded form of variables were used, and the relation between them was written as in Eq. (4). xi ¼
X i −X i ΔX
ð4Þ
where xi is coded value, Xi is the actual value of the ith factor in the uncoded units, X i is the average of the low and high values for the ith factor, and ΔX represents the step change. Second-order polynomial model given in Eq. (5) was used to estimate the relationship between the variables and the response based on experimental results from CCD [54, 55].
k
k
k
i¼1
i¼1
1≤i≤ j
y ¼ β 0 þ ∑ βi xi þ ∑ βii x2i þ ∑ β ij xi x j þ ε
ð5Þ
where y is the predicted response, xi and xj are coded variables, the βo is the constant, the βi is the linear coefficient, the βii is the quadratic coefficient, the βij is the interactive coefficient, and ε is a random error. Experimental data sets for CCD were carried out for all metal ions for the given coded and actual values of the three independent factors and given at Table 1 in the original paper [56]. Based on these sets of statistical designs of experiments, the conditions that led to maximum metal removal for the metal ions were given below. The effects of initial pH (2.0– 6.0 for Cu(II) and Zn(II); 2.0–5.5 Pb(II) and Fe(III)), contact time (5.0–150 min), initial metal concentration of solutions (28–100 mg/L), and adsorbent quantity (1.8 × 10−3−5.0 × 10−3 g) on the adsorption efficiency (R%) were investigated successfully by applying the central composite design (CCD). pH values were adjusted with 0.1 mol/L and 0.01 mol/L NaOH and HCl solution before adding the adsorbents. The adjusted R-square (R2 (adj)) values indicated that the model was in good agreement with the data (Table 3). The adjusted correlation coefficient values (R2 adj) of 96% or higher indicated that there was a high correlation between the observed values and the predicted ones for two hybrid materials and metals (Table 3). These results also showed that there was no significant difference in the amount of heavy metal removal compared to the results for adsorbents Al-GPTS-H and Al-GPTSNaOSiMe3-H [57]. It is also important to note that the preliminary study showed that the heavy metal removal capacity of adsorbents Zr-GPTS-H and derivatives was close to that of adsorbents Al-GPTS-H and derivatives (Table 4).
86.58 ± 0.31 93.59 ± 1.60 96.82 ± 0.59 93.18 ± 0.83 Experimental removal (%) ± s (N = 3)
88.65 ± 0.91
81.03 ± 0.47
91.82 ± 0.67
95.21 ± 1.12
99.45 87.98 ± 1.08 98.32 95.90 ± 3.23 96.67 95.59 ± 3.14 99.88 91.82 ± 0.67 98.14 94.45 ± 3.32 98.65 90.75 ± 2.23
pH: 5 Co: 10 mg/L m: 0.0030 g
R2 (adj) (%)
Predicted removal (%) ± s
99.96 90.99 ± 3.66
99.37 79.24 ± 1.31
Cu Zn Fe Pb
Material A
Cu
Hybrid
Table 3
Results of some optimum adsorption conditions (material A: Al-GPTS-KOH-H; material B: Al-GPTS-KOBut-H) [56]
Material B
Pb
Fe
Zn
Adv Compos Hybrid Mater
The results of the experiments on the specified conditions were found to be quite compatible with the predicted results from the models. In these studies, pH was found to be the most important parameter and the largest positive main effect in hybrid materials for all the metal ions. This can be explained by the increase in the number of negatively charged active sites on the surface of inorganic-organic hybrid adsorbents at higher pH of initial solution and as a result, these negatively charged active sites caused an increase in the adsorption of the positively charged metal ions at metal-binding sites on adsorbents by electrostatic attractions and ion exchanges. The quantity and removal percentage of Pb2+, Cu2+, and Cd2+ metal ions at equilibrium were given in Table 5 for adsorbents Poly-GPTS and Poly-GPTS-Ti. The adsorption capacity and removal percentage of these adsorbents toward Pb2+ ions were in the range of 165–199 mg/g and 84–99%, respectively. This study showed that the results had a great importance for removing metal ions by using a very small amount of hybrid adsorbents. The quantity and removal percentage of Pb2+, Ni2+, Cu2+, and Cd2+ metal ions at equilibrium were calculated by the study of adsorption isotherms for adsorbent Poly-GPTS-Ti-Tyr. The study of adsorption isotherm was analyzed using Langmuir (Eq. (6)) and Freundlich isotherm equations (Eq. (7)) as follows [58, 59]: Ce Ce 1 ¼ þ qe Qo QobL
lnqe ¼ lnKf þ
1 þ lnCe nF
ð6Þ
ð7Þ
where Ce was the equilibrium concentration of metal ions (mg/L), qe was the quantity of metal ions adsorbed at equilibrium (mg/g). Qo (mg/g) and bL (L/mg) were maximum adsorption capacity and binding energy of adsorption of Langmuir, respectively. KF and nF were Freundlich constants measuring the adsorption capacity and the adsorption intensity, respectively. The parameters calculated from the two models were presented in Tables 6 and 7. When adsorbent Poly-GPTS-Ti (without tyrosine) was used, the parameters calculated from the two models were presented in Table 7. The correlation coefficients (R2) of the Langmuir isotherm model were close to 1.0, which indicated that the adsorption process of the metal ions on hybrid materials described by the Langmuir isotherm model fits the adsorption. As seen from Tables 6 and 7, the maximum adsorption capacity values obtained from isotherm models were consistent with the values calculated from Eq. (1).
Adv Compos Hybrid Mater Table 4 Results of specified adsorption conditions (hybrid A: Al-GPTS-KOH-H; hybrid B: AlGPTS-KOBut-H) [56]
Conditions
Metal ions
Predicted removal (%) ± s
Experimental removal (%) ± s (N = 3)
pH
C0 (mg/L)
m (g)
5.72
24
0.0026
Hybrid A
Cu(II)
95.66 ± 3.66
93.71 ± 1.96
5
10
0.0040
Hybrid A
Pb(II)
98.21 ± 2.23
99.97 ± 0.01
5 4
50 50
0.0020 0.0030
Hybrid A Hybrid A
Fe(III) Fe(III)
98.59 ± 2.27 98.21 ± 3.28
97.14 ± 0.44 93.12 ± 5.08
6
10
0.0050
Hybrid B
Cu(II)
95.60 ± 0.48
93.84 ± 1.50
6 5
10 50
0.0050 0.0020
Hybrid B Hybrid B
Pb(II) Fe(III)
97.90 ± 0.99 98.59 ± 2.27
96.23 ± 0.72 97.14 ± 0.44
The kinetic study was determined using two most accepted models namely, Lagergren’s pseudo-first-order kinetic (Eq. (8)) and pseudo-second-order kinetic models (Eq. (9)). The equations [60, 61] were as follows: log ðqe −qt Þ ¼ logqe −k 1 t=2:303
ð8Þ
t=qt ¼ 1=k 2 qe 2 þ t=qe
ð9Þ
Where k1 (1/min) and k2 (g/mg min) were rate constant of pseudo-first-order adsorption (Eq. (8)) and pseudosecond-order adsorption, respectively; qe (mg/g) was the equilibrium amount of metal ion adsorption and qt (mg/g) was the amount of metal ion adsorption at any time t. The plot of log (qe−qt) versus t was drawn to determine the k1 and the plot of t/qt versus t was drawn to determine k2. The value of the correlation coefficient was carried out by the plots correlations. Table 8 shows the computed results obtained from plots. It was found that the linear regression coefficient (R2) of pseudo-second-order kinetic model for metal ions adsorption gave a better fitting (R2 > 0.99). On the contrary, the linear regression coefficient (R2) of Lagergren first-order model for metal ions adsorption fitted worse (R2 < 0.9, except for Zn2+). Therefore, the adsorption of metal ions on these hybrid adsorbents showed the pseudo-second-order kinetic model, and the calculation of qe from plots and experimental qe was in agreement with each other. When adsorbents Al-GPTS-KOH-H and Al-GPTSKOBut-H were used to remove Cu(II), Fe(III), Pb(II), and Table 5 Results of the metal ions adsorption on PGPTS and PGPTS-Ti hybrid materials [43]
Materials
Zn(II) metal ions, the equilibrium amount of metal ions adsorption calculated by the pseudo-second-order kinetic model was in good agreement with the results of experiments conducted at 50 mg/L metal concentrations. The correlation coefficients for the pseudo-second-order kinetic model were greater than 0.998 indicating the applicability of this kinetic equation, and it also showed that the nature of the adsorption process of metal ions on inorganic-organic hybrid materials was pseudo-second order. This expression was used to describe chemisorption between the adsorbent and adsorbate. The obtained results supported that the adsorption mechanism was chemisorption.
5 Comparison of the removal performance of inorganic-organic hybrid materials This section includes the removal performance of inorganicorganic hybrid materials used as adsorbents for heavy metal removal from wastewater. The results of their removal performance are compared to that of other hybrid materials (Table 9). It is evident from my literature survey that these hybrid materials have demonstrated outstanding removal capabilities for certain metal ions as compared to other known adsorbents. The removal capability of hybrid materials depends on a few factors such as organic functional group, the number of electronegative atoms, porous/non-porous structures, the radius of central metal atom, the electropositivity of central metal, and other factors. It is also important to note that the sum of all the properties of the hybrid materials is not the outcome of properties individual contributions of their
Metal ion
Initial conc. mg/L
Poly-GPTS q(mg/g)
Poly-GPTS-Ti q(mg/g)
Poly-GPTS % Removal
Poly-GPTS-Ti % Removal
Pb2+ Cu2+ Cd2+
20.0 5.0 2.0
165.2 41.28 32.48
199.0 42.79 39.41
84.40 82.56 80.75
99.38 85.58 98.53
Adv Compos Hybrid Mater Table 6 The isotherm models for adsorption of metal ions on hydrolyzed-[poly-GPTS/Ti-Tyr] [40] Metal
Langmuir constant
Freundlich constant
R2 (%)
Qo (mg/g)
bL (L/mg)
R2 (%)
KF (mg/g)
nF
Pb2+
99.1
270.3
0.251
79.2
177.1
13.11
Ni2+
99.2 99.3 99.3
204.1 188.7 131.6
0.355 0.29 0.215
41.3 60.3 93.4
165.9 142.2 47.7
31.25 19.72 40.78
Cu2+ Zn2+
components but also from the strong synergy created by a hybrid interface. The nature of inorganic-organic interface, including the type of interactions present, the surface energy, and the existence of labile bonds, plays a strong role in controlling of a number of properties like adsorption capacity [74]. For example, adsorbents for high adsorption capacities were poly-GPTS/Ti-Tyr-hydrolyzate (270.3, 204.1, 188.7, and 131.6 mg/g of Pb2+, Ni2+, Cu2+, and Zn2+, respectively) and poly-GPTS/Ti-hydrolyzate (181.2, 44.6, and 35.8 mg/g of Pb2+, Cu2+, and Cd2+, respectively). For example, the adsorption of lead on P(A-O)/AT was found that 1 g P(A-O)/AT adsorbed 109 mg of Pb2+ ions [34, 75]. This result was significantly less than that obtained from poly-GPTS/Ti-Tyr-hydrolyzate [40]. This difference can be attributed to the reason mentioned above.
Table 7 The isotherm models for adsorption of metal ions on PGPTS-Ti [43]
Metal ions
Pb2+ Cu2+ Cd2+
Table 8 Kinetic parameters calculated from kinetic models for the metal ions on the adsorbent PGPTS-Ti(Tyr)-H [40]
Adsorbents
Pb2+ (20.0) Ni2+ (7.00) Cu2+ (7.00) Zn2+ (2.00)
Langmuir constant
Poly-GPTS Poly-GPTS-Ti Poly-GPTS Poly-GPTS-Ti Poly-GPTS Poly-GPTS-Ti
Metal ions conc. (mg/L)
Adsorbents for high removal percentage (R%) were AlGPTS-KOH-H (88.65, 93.18, 96.82, and 81.03 of Cu2+, Pb2+, Fe3+, and Zn2+, respectively), Al-GPTS-KOBut-H (91.82, 93.59, 95.21, and 86.58 of Cu2+, Pb2+, Fe3+, and Zn2+, respectively), and poly-GPTS/Ti-Tyr-hydrolyzate (99.9 for all ions Pb2+, Ni2+, Cu2+, and Zn2+). Their excellent adsorption efficiency was due to the rich oxy groups on the hybrid surface that allowed the electrostatic interactions between oxygen atoms and lead ions in addition to chemisorption. In addition to properties of the hybrid materials mentioned above, the other factors affecting adsorption capacities and removal percentage were experimental conditions such as pH, required dose, initial concentration, temperature, and treatment performance. For example, the adsorption capacity of Al-GPTS-KOBut-H adsorbent for Fe(III) ion was found to be 79.3 mg/g (where Co, 10 mg/ L; m (adsorbent dose), 0.0030 g; V (solution volume), 25 mL) and 607.1 mg/g (where C o , 50 mg/L; m, 0.0020 g; V, 25 mL), respectively [56]. Adsorption capacities and removal percentage of other hybrid materials synthesized by colleagues in my group were given in the previous sections and in the original papers. Adsorption capacities of other hybrid materials synthesized by other research groups were given in Table 9. These results provide evidence that inorganic-organic hybrid adsorbents have promising applications in the removal of metal ions from wastewater for environmental purposes.
Freundlich constant
R2 (%)
Qo (mg/g)
bL (L/mg)
R2
KF (mg/g)
nF
99.26 99.97 98.09 98.18 97.82 97.32
172.4 181.2 39.06 44.64 35.71 35.84
10.54 7.86 2.37 2.36 3.64 5.58
62.26 26.63 55.12 60.41 56.63 41.57
66.37 146.8 20.69 23.81 26.48 31.64
2.41 13.25 1.68 2.70 0.84 23.31
Pseudo-first order
Pseudo-second order
R2
k1 (1/min)
qe (mg/g)
R2
k2 (g/mg min)
qe (mg/g)
0.867 0.775 0.795 0.954
0.020 0.025 0.016 0.530
0.118 0.688 0.547 0.581
0.999 0.999 0.999 0.999
3.3 × 10−3 0.064 0.065 0.068
188.68 64.52 55.87 17.39
Adv Compos Hybrid Mater Table 9 Comparison of maximum adsorption capacities of similar adsorbents
qm (mg g−1)
Adsorbents
Pb2+ Si/Al-pr-N = salicyl aldehyde
91.0
Poly-GPTS/Ti-hydrolyzate Al-GPTS-NaOSiMe3-H Al-GPTS-KOBut-H
181 83.3 78.0
Lignin/MgO-SiO2 Jeffamine/diamino hexane PAN-MCM-48
Fe3+
Cu2+
75.3 79.3
44.6 73.9 76.5 84.0
Zn2+ [62]
35.8
68.3 72.2
100 230
Purolite ArsenXnp APAS-APTES Fe3O4@SiO2/Schiff base Poly(o-phenylenediamine)/hydrous zirconium oxide PDMS-net-P(VTMS-co-DMAEMA) Nickel ferrite/manganese MPTMS-PEG
5.30 6.0
13.4 12.4
13.7 12.3
10.0
185 270 81.8
204
189 21.5 10.8 97.2
132 10.1 10.8 87.0
30.0 9.98 81.6
[43] [57] [56] [63] [64] [65] [66] [67]
254
Poly-GPTS-Zr(PA)-hydrolyzate Poly-GPTS/Ti-Tyr-hydrolyzate
Inorganic-organic hybrid materials obtained by the sol-gel process were used as adsorbents. For the synthesis of these hybrid materials, environmentally friendly precursors, such as titanium, aluminum, zirconium alkoxides, and GPTS, polyGPTS, or organic chelate ligands were used. These hybrid materials combined the functional properties of organic compounds with the benefits of less-soluble and light-stable inorganic compounds. These hybrid materials had strong chemically and physically binding affinities toward heavy metal ions. Therefore, they had relatively high metal ion adsorption capacities. The structures of these hybrid materials were characterized by XRD, SEM, EDX, BET, and BJH analysis, mass, and FTIR spectroscopies. These physical instrument measurements showed that the structures of inorganic and organic moieties changed in hybrid materials, and synergy effects between them increased their adsorption and selective adsorption capacities compared to their original inorganic or organic fractions. In summary, synthesized hybrid materials may be good candidates to be effective adsorbents for the removal of heavy metal ions from wastewater.
Cd2+
119
PAO/SiO2 pgf-PVA/SiO2 G3-PAMAM-SPA-15 EDTA-G3 PAMAM-SPA-15
6 Conclusions
Ni2 +
Ref.
66.7 30.5 85.8 25.2
[68] [68] [42, 43] [40] [69] [70] [18] [71] [37] [72] [73]
Acknowledgements This work was supported by the research foundation of Kocaeli University (Project Number 107/2017).
Compliance with ethical standards Conflict of interest The author declares no conflict of interest.
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