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Chelator-mimetic multi-functionalized hydrogel: Highly efficient and reusable sorbent for Cd, Pb, and As removal from waste water
Accepted Manuscript Chelator-mimetic multi-functionalized hydrogel: Highly efficient and reusable sorbent for Cd, Pb, and As removal from waste water Zahra Mohammadi, Shangbin Sun, Cory Berkland, Jenn-tai Liang PII: DOI: Reference:
S1385-8947(16)31212-8 http://dx.doi.org/10.1016/j.cej.2016.08.121 CEJ 15687
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
13 July 2016 25 August 2016 27 August 2016
Please cite this article as: Z. Mohammadi, S. Sun, C. Berkland, J-t. Liang, Chelator-mimetic multi-functionalized hydrogel: Highly efficient and reusable sorbent for Cd, Pb, and As removal from waste water, Chemical Engineering Journal (2016), doi: http://dx.doi.org/10.1016/j.cej.2016.08.121
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Chelator-mimetic multi-functionalized hydrogel: Highly efficient and reusable sorbent for Cd, Pb, and As removal from waste water Authors: Zahra Mohammadi*, Shangbin Sun*, Cory Berkland**, Jenn-tai Liang*
*: Texas A&M university, Harold Vance Department of Petroleum Engineering **: University of Kansas, Chemical and Petroleum Engineering Department
Corresponding Authors: Zahra Mohammadi Email: [email protected] Tel: 979-845-150
Abstract:
Medically inspired hydrogels were successfully synthesized by mimicking the structure of current chelating agents. Thioglycolic acid modified Polyallylamine (PAAm/TGA) and Dihydroxybenzoic acid-Thioglycolic modified polyallylamine (PAAM/TGA/DHBA) were synthesized following a simple conjugation process. These hydrogels exhibit high affinity, low cost, and high re-usability efficiency. The maximum sorption capacities were significantly higher than those sorbents currently in use. The kinetic study of the hydrogels demonstrated that almost 50% removal occurred in less than 5 min. Additionally, the hydrogel exhibited excellent re-usability, maintaining their efficiency in 5 consecutive cycles. These results suggest that these hydrogels can be considered to be potential agents to be used in waste/produced water treatment.
Key words: Chelator mimetic, hydrogel, re-usable, toxic metal removal, high sorption capacity
1.
Introduction
Heavy metals, such as cadmium, lead, and arsenic, are highly toxic pollutants discharged into receiving waters as a result of different activities[1-3]. The increasing accumulation of heavy metals, has raised serious concerns in regard to human health[4, 5]. Wastewater discharge from mining,
agriculture and oil and gas industries is a primary source of environmental heavy metal contamination [3]. Due to the detrimental effects of the heavy metals to human body and microorganisms, it is necessary to remove such toxic metals from environment. This has led to an increasing interest in technologies that could effectively remove these contaminations from wastewater. Many methods have been used to decontaminate and remove heavy metals from wastewater; however most of these methods are not considered effective and suffer from several drawbacks.[4] As an example, the amorphous solubility of metal hydroxide leads to an incomplete precipitation, which makes this method ineffective and unreliable [6]. In Solvent extraction method, the eluent must be processed to avoid environmental contamination [7]. Similarly, subsequent treating of toxic sludge in chemical precipitation makes this method challenging and economically unfavorable [8]. Among these methods, adsorption has shown to be of interest due to its easy operation and high efficiency [4, 9]. Traditional adsorbents, such as activated carbon, only work best when heavy metals are present in high concentrations. Also, the adsorbent may produce toxic sludge which needs further processing and detoxification treatment [10]. Recently, nanomaterial-based adsorbents have been designed and synthesized, but their practical applications in treating wastewater are limited by their complex synthesis procedure and inconvenient recycle[11]. Although it is considered one of the most efficient heavy metal removal techniques, adsorption still has certain limitations. This method is yet to achieve an optimized removal capacity and subsequently a stable commercial level. Therefore, in practice, high sorption capacity, low cost, good
reusability, and lack of sludge formation are strongly recommended for efficient removal of heavy metals. Heavy metals perturb multiple enzyme systems in human body. Any enzyme that has a sulfhydryl ligand group is vulnerable and most probably would be malfunction upon exposure to any toxic metal [12]. Several chelating agents, such as calcium disodium ethylenediaminetetraacetate (CaNa2EDTA), deferoxamine, and D-penicillamine, have been used as antidotes for metal intoxication in humans[13, 14]. As shown in Fig 1, all of these drugs have O and S as donor groups, and they are rich in sulfur groups. In this work, hydrogels are used as adsorbents for removing heavy metals in aqueous medium. Hydrogels are three-dimensional cross-linked polymeric networks containing a large amount of water (50–90%). The density of the crosslinking agent controls the mechanical strength of hydrogels. Additionally, the high water content enables easy access to any foreign ions within hydrogel network. This work reports the synthesis of multi-functionalized hydrogels capable of removing toxic metals (i.e., Pb, Cd, and As) from an aqueous solution. Functionalized Polyallylamine (PAAm) hydrogels have been introduced previously as potential iron-specific chelating agents. These hydrogels were synthesized by mimicking the structure of a microorganism called entrobactin. Entrobactin has the highest known affinity and selectivity toward iron. In this study, Thioglycolic acids (TGA) in combination with the phenolic group of Dihydroxybenzoic acid (DHBA) were introduced onto PAAm to mimic the structure of current effective chelating agents. Conjugation of the thiol groups dramatically improved the binding affinity and selectivity of the hydrogels for toxic metals. The binding was fast and the binding capacity was relatively high for each metal ions. Moreover, functionalized hydrogels showed excellent reusability after being used in 5 consecutive batches. 2. Materials and Methods 2.1. Chemicals and materials
Poly(allylamine hydrochloride) (PAAm) with an average molecular weight of 56 kDa and an analytical grade reagent N,N’- methylenebisacrylamide (MBA) were obtained from Sigma-Aldrich and used without further modification. 2,3 dihydroxybenzoic acid, thioglycolic acid (TGA), N,N,N -triethylamine (TEA), dimethylformamide (DMF) and all metal chlorides were purchased from Fisher Scientific and used as received. Dicyclohexylcarbodiimide (DCC) and N-hydroxysuccinimide (NHS) were purchased from Thermo Scientific and used without further modification. Deionized water (DI) was obtained from a Barnstead EasyPure water purifier. 2.2. Preparation of multi-functionalized gel PAAm hydrogel was synthesized following the procedure reported in the literature[15, 16]. Briefly, a 20% w/v polymer solution containing a predetermined amount of MBA was prepared. The cross-linker was dissolved in deionized water and later added to the polyallylamine polymer. TEA, the cross-linking catalyst (300 μL), was subsequently added to the solutions and mixed thoroughly. Next, the precursors were transferred by pipette into small vials. The vials were held at ambient temperature for 1 h and later cooled to ca. 3 °C and held there for an additional 24 h. After this time period, hydrogels were removed from the vials and washed with 0.05 M sodium chloride for several days. A solution of TGA and NHS in 5 mL of DMF was mixed with a solution of DCC in 5 mL of DMF. The mixture was stirred at low temperature for 6 h to provide a white precipitate. The precipitate was filtered, and the filtrate was added directly to a known mass of ground hydrogel. The reaction mixture was held at room temperature for 3 days. PAAm conjugate hydrogel was later washed with water for several days. The same procedure was performed for the conjugation of 1:1 molar ratio of TGA/DHBA.
2.3. Characterizations 2.3.1.
Quantification of amine functional groups
Primary amine groups were quantified by potentiometric titration[17]. First, 40 mg of functionalized hydrogels were grounded and suspended in 35 mL of 0.2 M aqueous KCl solution. Next, 140 μL of 8 M KOH aqueous solution was added to polymer suspensions. This raised the pH to ~12. Titration was carried out using 0.1 M HCl standard solution. The solution was added until the pH was approximately 2.5 in all polymer suspensions. Free amine groups were quantified next.
2.3.2. Metal analysis Mono- and multi-elemental analysis of samples was quantified by Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) (Optima 8000 DV, PerkinElmer, USA) fitted with an AS 93plus autosampler (PerkinElmer, USA). The RF power was 1300 W and nebulizer and auxiliary flows were 0.8 and 0.2 L/min, respectively. Sample flow was set to 0.8 mL/min. The analytical curves used for sample analysis had coefficients of correlation >0.999. 2.4. Binding Kinetics Study 2 mg/mL metal chloride solutions were adjusted to pH 2.5 with ionic strength in the range of 0.02 M to 0.04 M and kept at room temperature for kinetic studies. 0.1M HCl and 1M NaOH were used to adjust the pH and ionic strength. Samples were taken from the solution at several time intervals to determine the rate of metal binding by the functionalized hydrogel.
2.5. Sorption experiments
In order to evaluate the ion sorption properties of the adsorbent, a series of sorption experiments, including the effect of environmental conditions (pH), contact time, initial concentration, temperature, selectivity, and recyclability, were conducted. 2.5.1. Binding experiments Known concentrations of metal chloride solutions (5, 20, 50, 100, 200, 400, 500) ppm were prepared. Binding experiments were conducted by taking 20 mL of the metal solution. Th pH was adjusted to 2.5 while the ionic strength was kept in a range of 0.02 M to 0.04 M. A known mass of functionalized hydrogel was added to the mixture and was held at room temperature for 2 h or until equilibrium was reached. At the end, the solutions were filtered, and the filtrates were analyzed for metal concentration. 2.5.2 Sorption isotherms Three isotherm models were applied at equilibrium to determine the distribution of the metal molecules between the liquid phase and the hydrogel. Sorption parameters of Langmuir, Freundlich, and Temkin models were calculated for each metal ion at a pH value of 2.5. The constants for each isotherm model were calculated, and the accuracy of the isotherm models were evaluated using linear correlation coefficient (R2) values. 2.5.3. Selectivity study The selectivity for Pb, Cd, and As by functionalized PAAm in the presence of competing metals was studied. A metal solution (10 mL, 2 mg/mL) containing all the competing metal ions was prepared. The pH of the solution was adjusted 2.5 and held at room temperature for 2 h after adding a known mass of functionalized dry gel. Ionic strength of all the solutions was between 0.02 M and 0.04 M.
3.
Results and Discussion
3.1. Synthesis and characterization Poly(allylamine hydrochloride) was cross-linked following a previously reported method[18]. In this experiment, an optimized reaction yield was chosen from the previously reported data[16]. 2,3 DHBA and TGA were covalently linked to available amino sites of PAAm hydrogel via DCC/NHC conjugation chemistry. Moreover, potentiometric titration data showed in Fig. 2 were used to calculate the degree of conjugation using the following equation. =
(1)
In this study, CH is the concentration of hydrogel. The conjugation efficiency was calculated to be 47% and 67% for PAAm/TGA and PAAm/TGA/DHBA hydrogels, respectively.
3.2. Binding kinetics Determination of the kinetics of metal removal is critical for understanding the performance of hydrogels and evaluating the potential applications of the gels. The kinetics of metal binding was studied by adding a known mass of dry hydrogels to a known initial concentration of metal solution (2 mg/mL, metal chloride). The concentration of metal in solution was measured over time. Approximately 40%50% of the total metal sorption was removed in less than 5 min for all of the functionalized hydrogels (Figure 3). The sorption was the fastest for As in the presence of different functionalized hydrogels and the slowest for Cd. This property might be due to the smaller atomic radius of As compared to that of Pb and Cd. The results published by Ozay et al. [19] showed that magnetic hydrogels are capable of removing less than 40% of the total metal within first 2 hrs. Literature shows that in most cases, the minimum required time to reach ~50% of the total removal is approximately 15-30 min. The rapid sorption of toxic metals by hydrogels reported in this study offers an important advantage, especially in
waste water treatment units, where the flow passes the column with a high rate and short contact time with the sorbent. To derive the rate constant and binding capacity, the kinetic data were modeled with pseudo-first-order (Lagergren model) and pseudo-second-order (Ho model) kinetic models, which are expressed in their linear forms as
log − = + log . #$
%
!
(2)
"
= #' + # & '
(3)
&
where k1 (L/min) and k2 (g/mg · min) are pseudo-first-order and pseudo-second-order rate constants, respectively. The model variables obtained by linear regression were compared. The results are shown in Table 1. The pseudo-second order reaction model showed the best fit because it had a R2 value close to unity in each of these cases. Table 1: Kinetic parameters for metal binding by functionalized hydrogels at pH=2.5. Ionic strength values ranged from 0.02 M to 0.04 M. (A) Constant for the kinetic sorption of different PAAm/TGA calculated with different models, (B) Constant for the kinetic sorption of different PAAm/TGA/DHBA calculated with different models. (A)
Metals
Pseudo-first order k1 (L/h)
qe (mg/g)
Pseudo-second order R2
k2(g/(mg h))
qe (mg/g)
R2
As(I)
0.01
0.8645
14.75
0.9999
Pb(II)
0.01
0.9858
16
0.997
Cd(II)
NA
NA
NA
NA
(B)
Metals
Pseudo-first order
Pseudo-second order
k1 (L/h)
qe (mg/g)
As(I)
NA
NA
Pb(II)
0.0203
Cd(II)
NA
R
2
0.9391 NA
k2(g/(mg h))
qe (mg/g)
R
2
13.48
1
5.46
0.9998
1.0788
0.9997
The equilibrium binding values were systematically higher for PAAm/TGA hydrogels compared to PAAm/TGA/DHBA. This finding might be observed because of a high density of thiol groups available for coordinating with toxic metals. Tofan et al. [21] have studied the sorption kinetic of sulfhydryl modified hemp when exposed to Cd and Pb. The sorption capacity of modified hemp was reported to be 14.0 and 23.0 mg/ g of fibers for Cd and Pb ions, respectively, at room temperature. In comparison, hydrogels reported in this study achieved ~ 20-fold and ~ 10-fold higher binding capacity when using PAAm/TGA to bind Cd and when using PAAm/TGA/DHBA gels to bind Pb, respectively. 3.3. Effect of pH pH of the aqueous solution is one of the most important factors affecting the metal sorption by the hydrogel. A very small change in pH would strongly affect the sorption phenomena as it controls the surface charge and functional group activity of hydrogel. In this work, the effect of pH on metal sorption was determined for a pH range of 2 to 8, but we mainly attempt to stay below a pH value of 6, primarily because a portion of the metal ions underwent hydrolysis in higher pH values. Additionally, the potential application of these hydrogels will be in waste water treatment where the pH ranges are mostly below neutral. The As uptake efficiency as a function of pH is shown in Fig. 4. The As sorption did not change significantly with an increase in the solution pH. It is assumed that a high concentration of hydronium ions (H3O+) near the surface of the gel could produce a repulsive force and hinder the interaction between the As ions and the hydrogel. This finding might account for the lower As sorption efficiency at low pH region. At pH ~ 6, the primary amine groups are not as protonated, and all coordinating sites are
anticipated to make a coordinate covalent bonding with metal ions. We believe that because the most of the surface sorption sites are deprotonated at pH >6, the electrostatic interactions required to adsorb As is slightly different than those observed for lower pH. 3.4. Binding Isotherms The relationship between the amount of metal absorbed by a unit mass of the hydrogel and the concentration of the remaining metal ions in solution represents the sorption isotherms. There are several empirical models such as Langmuir, Frendlich, and Temkin isotherm which are typically used to model experimental data and calculate the maximum binding capacity. In this study, the Frendlich model seems to fit the data almost accurately with a correlation coefficient (R2) close to unity (Table 2).
Table 2: Isotherm parameters for metal binding by functionalized hydrogels at pH=2.5. Ionic strength values ranged from 0.02 M to 0.04 M. (A) PAAm/TGA, (B) PAAm/TGA/DHBA) (A)
Model qmax
Langmuir isotherm As Pb Cd
664.50 NA NA
0.78 0.95 0.90
14078.44 806.57 1159.44 Parameters RT/B
A
Temkin isotherm As Pb Cd
32.20 NA NA Parameters kf
n
Frendlich isotherm As Pb Cd
Parameters kL
261.73 75.22 82.61
94.37 61.73 67.61
(B) Model
Parameters
R
2
0.92 NA NA
R
2
0.95 0.98 0.99
R
2
0.86 0.77 0.79
2
qmax
kL
R
As
NA
NA
NA
Pb
89.00
−
0.64
Cd
81.70
−
0.79
Langmuir isotherm
Parameters n
kf
R
2
Frendlich isotherm As
0.77
Pb
1.04
Cd
14256.95 0.958 544.57
0.908512765 1159.44
0.9978 0.99
Parameters 2
A
RT/B
R
As
261.72
94.05
0.86
Pb
36.60
80.00
0.81
Cd
82.61
67.61
0.79
Temkin isotherm
Even though a reciprocal plot of the data gave a straight line fit for Langmuir isotherms in most cases, the negative intercept values suggested that simple Langmuir did not occur. The Freundlich model assumes that there are many types of sites acting simultaneously; which is most likely the case in this study, each with a different free energy of sorption . Here, S and O donor groups along with protonated amine groups are coordinating with metal ions as different binding sites. The Temkin isotherm assumes that a decrease in the heat of sorption is linear and that sorption is characterized by a uniform distribution of binding energies. This model did not quite fit the experimental data, suggesting that multiple, complex binding mechanisms may be involved in metal sorption. Maximum sorption capacity of the hydrogel was compared to those previously reported and are shown in Table 3. The data indicates that the present hydrogels outperform many other sorbents in the case of the Cd and Pb removal.
Table 3: Comparison of different sorbent and their sorption capacities
Absorbent Alginated gel beads
Capacity (mg/g) Pb Cd
Papaya wood Sulfur-functionalized silica Brinessite Ionic imprinted silica Ulmus tree leaf Chitosan nanofiber Functionalized silica PVA/PAA gel PAAm/TGA/DHBA PAAm/TGA
673 379.2 201 118 61.9 194.99 452.25 345.00
Ref. [1]
17.22 116.7 31.4 80 60.85 115.88 294.08 291.73
[22] [23] [24] [25] [26] [27] [28] [4] This work This work
3.5. Effect of temperature The sorption isotherms of Pb(II), Cd(II), and As(i) on different hydrogels were obtained at three different temperatures (293, 303 and 313 K) and are shown in Table 4. Thermodynamic parameters, such as the standard free energy change (ΔG0), the standard entropy change (ΔS0) and the standard enthalpy change (ΔH0), were calculated according to the van't Hoff equation. The spontaneous nature of the sorption process was evident by the negative values of ΔG0 (Table 4). Furthermore, the positive value of ΔS0 indicated that the degree of freedom increases at the gel-liquid interface during the removal process, implying that there is an interaction between the metal ions and the hydrogel. The positive value of ΔH0 indicates that metal sorption on the gel is an endothermic process and which means a high temperature would be beneficial to the removal. This finding is consistent with the results from sorption isotherms.
Table 4: Values of thermodynamic parameters for metal sorption on a gel
Metal
T
Sorbent: PAAm/TGA
Sorbent: PAAm/TGA/DHBA
As
Pb
Cd
293
∆H (j/mol)
∆S (J/mol.K)
∆G (J/mol)
∆H (j/mol)
∆S (J/mol.K)
∆G (J/mol)
13238.39
113.760462
-20093.45
625.262684
67.22
-19070.05
303
-21231.04
-19742.25
313
-22368.65
-20414.44
293
4445.00
45.20
-8798.31
303
-9250.33
313
-9702.35
293
3002.78
59.4026986
-14402.22
NA
NA
751.868276
67.07
-18901.30
303
-14996.25
-19572.06
313
-15590.28
-20242.82
3.6. Selectivity Studies An important feature of metal chelating hydrogels is the ability to target the metal of interest and remove it from the solution. The metal selectivity of PAAm/TGA and PAAm/TGA/DHBA hydrogels was investigated using a multi-solute system. At an equal concentration of all toxic metals (i.e., Pb, Cd, As at 2 mg/mL), PAAm/TGA absorbed almost 100% of the lead present in the media while in the same experiment and in the presence of other competing metals, such as Fe and Zn. This value decreased to ~70% (Figure 5). The tendency of PAAm/TGA hydrogel for sorption of metals followed the order Pb > As> Cd> Zn> Fe. This trend was similar for PAAm/TGA/DHBA. Previously, a high Pb removal capacity using magnetic hydrogels (130 mg/g) has been reported[20]. The Pb removal capacity with PAAm/TGA and PAAm/TGA/DHBA was 345.6 and 291.7 mg/g, respectively. All data suggests that these hydrogels may have excellent potential in waste water treatment and a probable application in acute metal poisoning. 3.7. Reusability
For the sorption to be useful and economically favorable, the hydrogel must be desorbed easily. It is important that the spend gel could be reused for several cycles without significant changes in its original performance. Here, hydrogels were tested to see whether the absorbents could be chemically eluted and regenerated. As estimated, low concentration of HCl acid was able to elute the absorbed metal and to retain the functionality of the gels. From the experimental data, it was evident that the absorbed metal was sufficiently eluted from the gel (up to 60%) using 1 M HCl. The 1 M HCl was considered to be an optimal concentration to wash the gel. Both the PAAm/TGA and PAAm/TGA/DHBA hydrogels were simultaneously regenerated in their initial form for the next sorption operation after the washing process. The removal–wash–regeneration processes were performed five times, and the removal efficiency in each cycle was compared with the initial removal efficiency of the gel. The data are presented in Figure 6. The easy regeneration of hydrogels makes them strong candidate to be used as an absorbent agent in wastewater treatments plants.
4.
Conclusions
Cross-linked polymers with different functional groups were designed for the removal of toxic metals. Dihydroxybenzoic acid and thioglycolic acid were used as conjugated moieties to improve the affinity and selectivity of PAAm toward toxic metals. These hydrogels were able to remove and accumulate Pb, Cd, and As ions from aqueous solutions at relatively low metal concentrations. PAAm/TGA and PAAm/TGA/DHBA hydrogels demonstrated an almost instant metal sorption when equilibrated in the metal solution. The pseudo-second order Ho kinetic model offered an excellent fit for the sorption data. The inclusion of the thiol groups to hydrogels seemed to improve their selectivity toward Pb and As. The rapid binding and selectivity of these functionalized hydrogels for heavy metals provides important advances for the waste water/produced water treatment and may enable applications in acute metal poisoning.
5.
Acknowledgements
This research was supported by the Petroleum engineering department of Texas A&M University. We would also like to thank the team members of our laboratory and our colleges in the University of Kansas for valuable discussions.
References: [1] X. Guo, B. Du, Q. Wei, J. Yang, L. Hu, L. Yan, W. Xu, Synthesis of amino functionalized magnetic graphenes composite material and its application to remove Cr (VI), Pb (II), Hg (II), Cd (II) and Ni (II) from contaminated water, Journal of hazardous materials, 278 (2014) 211-220. [2] A.H. Hawari, C.N. Mulligan, Biosorption of lead (II), cadmium (II), copper (II) and nickel (II) by anaerobic granular biomass, Bioresource technology, 97 (2006) 692-700. [3] U. EPA, 2002 Edition of the Drinking Water Standards and Health Advisories, in, US Environmental Protection Agency Washington^ eDC DC, 2002. [4] L. Chu, C. Liu, G. Zhou, R. Xu, Y. Tang, Z. Zeng, S. Luo, A double network gel as low cost and easy recycle adsorbent: Highly efficient removal of Cd (II) and Pb (II) pollutants from wastewater, Journal of hazardous materials, 300 (2015) 153-160. [5] S. Ghorai, A. Sinhamahpatra, A. Sarkar, A.B. Panda, S. Pal, Novel biodegradable nanocomposite based on XG-g-PAM/SiO 2: application of an efficient adsorbent for Pb 2+ ions from aqueous solution, Bioresource technology, 119 (2012) 181-190. [6] J.A. Dyer, P. Trivedi, S.J. Sanders, N.C. Scrivner, D.L. Sparks, Lead sorption onto ferrihydrite. 3. Multistage contacting, Environmental science & technology, 37 (2003) 923-930. [7] H. Tokuyama, T. Yoshida, L. He, Preparation of novel emulsion gel adsorbents and their adsorption properties for heavy-metal ions, Industrial & Engineering Chemistry Research, 50 (2011) 10270-10277. [8] F. Fu, Q. Wang, Removal of heavy metal ions from wastewaters: a review, Journal of Environmental Management, 92 (2011) 407-418. [9] J.-L. Gong, B. Wang, G.-M. Zeng, C.-P. Yang, C.-G. Niu, Q.-Y. Niu, W.-J. Zhou, Y. Liang, Removal of cationic dyes from aqueous solution using magnetic multi-wall carbon nanotube nanocomposite as adsorbent, Journal of Hazardous Materials, 164 (2009) 1517-1522.
[10] V.M. Nurchi, I. Villaescusa, Sorption of toxic metal ions by solid sorbents: a predictive speciation approach based on complex formation constants in aqueous solution, Coordination Chemistry Reviews, 256 (2012) 212-221. [11] J. Saiz, E. Bringas, I. Ortiz, Functionalized magnetic nanoparticles as new adsorption materials for arsenic removal from polluted waters, Journal of Chemical Technology and Biotechnology, 89 (2014) 909-918. [12] B. Sharma, S. Singh, N.J. Siddiqi, Biomedical implications of heavy metals induced imbalances in redox systems, BioMed research international, 2014 (2014). [13] S.J. Flora, V. Pachauri, Chelation in metal intoxication, International journal of environmental research and public health, 7 (2010) 2745-2788. [14] O. Andersen, Principles and recent developments in chelation treatment of metal intoxication, Chemical reviews, 99 (1999) 2683-2710. [15] Z. Mohammadi, S.X. Xie, A.L. Golub, S.H. Gehrke, C. Berkland, Siderophore-Mimetic hydrogel for iron chelation therapy, Journal of Applied Polymer Science, 121 (2011) 1384-1392. [16] Z. Mohammadi, S.-X. Xie, E. Peltier, M. Veisi, C. Berkland, Enhancing the selectivity of an iron binding hydrogel, European Polymer Journal, 47 (2011) 1485-1488. [17] W. Evans, E. McCourtney, R. Shrager, Titration studies of phytic acid, Journal of the American Oil Chemists' Society, 59 (1982) 189-191. [18] É. Oliveira, S. Hirsch, R. Spontak, S. Gehrke, Influence of polymer conformation on the shear modulus and morphology of polyallylamine and poly ( -L-lysine) hydrogels, Macromolecules, 36 (2003) 6189-6201. [19] I. Alinnor, Adsorption of heavy metal ions from aqueous solution by fly ash, Fuel, 86 (2007) 853-857. [20] O. Ozay, S. Ekici, Y. Baran, N. Aktas, N. Sahiner, Removal of toxic metal ions with magnetic hydrogels, Water Research, 43 (2009) 4403-4411.
[21] L. Tofan, C. Paduraru, Sorption studies of Ag I, Cd II and Pb II ions on sulphydryl hemp fibers, Croat. Chem. Acta, 77 (2004) 581–586. [22] V.K. Singh, P.N. Tiwari, Removal and recovery of chromium (VI) from industrial waste water, Journal of Chemical Technology and Biotechnology, 69 (1997) 376-382. [23] O. Hakami, Y. Zhang, C.J. Banks, Thiol-functionalised mesoporous silica-coated magnetite nanoparticles for high efficiency removal and recovery of Hg from water, Water research, 46 (2012) 3913-3922. [24] D.C. Marcano, D.V. Kosynkin, J.M. Berlin, A. Sinitskii, Z. Sun, A. Slesarev, L.B. Alemany, W. Lu, J.M. Tour, Improved synthesis of graphene oxide, ACS nano, 4 (2010) 4806-4814. [25] X. Xin, Q. Wei, J. Yang, L. Yan, R. Feng, G. Chen, B. Du, H. Li, Highly efficient removal of heavy metal ions by amine-functionalized mesoporous Fe 3 O 4 nanoparticles, Chemical Engineering Journal, 184 (2012) 132-140. [26] S. Guo, D. Li, L. Zhang, J. Li, E. Wang, Monodisperse mesoporous superparamagnetic single-crystal magnetite nanoparticles for drug delivery, Biomaterials, 30 (2009) 1881-1889. [27] Z.-J. Fan, W. Kai, J. Yan, T. Wei, L.-J. Zhi, J. Feng, Y.-m. Ren, L.-P. Song, F. Wei, Facile synthesis of graphene nanosheets via Fe reduction of exfoliated graphite oxide, ACS nano, 5 (2010) 191-198. [28] N. Savage, M.S. Diallo, Nanomaterials and water purification: opportunities and challenges, Journal of Nanoparticle research, 7 (2005) 331-342.
Figure 1: Structure of some of the current chelating agents used to treat toxic metal poisoning.
pH
16 14 12 10 8 6 4 2 0
PAA/TGA PAAm/TGA/DHBA
0
1 2 3 4 Volume of HCl added (mL)
5
Figure 2: Titration data collected for PAAm/TGA and PAAm/TGA/DHBA. pH values represent the values of 3 readings differing by < 5%.
(A)
80 70 %Removal
60 50 40 30 20
Batch study PT-As
10
Batch study PDT-As
0 0
50
100
150 Time (min)
200
250
300
(B)
70 60 %Removal
50 40 30 20
Batch study PT-Pb
10
Batch study PDT-Pb
0 0
50
100
150 Time (min)
200
250
300
%Removal
(C)
90 80 70 60 50 40 30 20 10 0
Batch study PT-cd
Batch study PDT-cd 0
50
100
150 Time (min)
200
250
300
Figure 3: Ion binding by functionalized hydrogels, A) As, B) Pb, and C) Cd. Gels were equilibrated in 2 mg/mL metal solution (pH= 2.5). Ionic strength values ranged from 0.02 M to 0.04 M.
66
% removal
65 64
PT-As DPT-As
63 62 0
2
4
6
pH Figure 4: Effect of pH solution on the As removal efficiency of modified hydrogel.
8
100
PAAm/TGA/DHBA PAAm/TGA
% absorbed
75 50 25 0
Cd
100
Pb
As
PAAm/TGA/DHBA PAAm/TGA
% absorbed
75 50 25 0
Cd
Pb
Fe
Zn
As
Figure 5: Selectivity study of functionalized hydrogels toward toxic metal ions A) in the presence of competing toxic metals and B) in the presence of competing essential metals (pH=2.5).
Reusability test PTD-Pb 100
% removed
80 60 40 20 0
0
Reusability test 50.5048
1
2
3
4
54.4718
52.516
52.5095
57.3547
Reusability test PT-Pb
% removed
81 61 41 21 1
0
Reusability test 67.3796
1
2
3
4
65.323
65.1004
65.4929
62.4741
Figure 6: Reuse studies for PAAm/TGA (PT) and PAAm/ TGA/DHBA (PTD) after elution operation with pertinent eluent (1 M HCl). Five consecutive sorption-elution-reuse cycles were employed.
HIghlights •
Multi-functionalized hydrogels for toxic metal removal is presented.
•
Hydrogels are chelator-mimetic.
•
Modification of hydrogel dramatically improved the binding affinity.
•
Removal is fast, efficient and selective.
S1385-8947(16)31212-8 http://dx.doi.org/10.1016/j.cej.2016.08.121 CEJ 15687
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
13 July 2016 25 August 2016 27 August 2016
Please cite this article as: Z. Mohammadi, S. Sun, C. Berkland, J-t. Liang, Chelator-mimetic multi-functionalized hydrogel: Highly efficient and reusable sorbent for Cd, Pb, and As removal from waste water, Chemical Engineering Journal (2016), doi: http://dx.doi.org/10.1016/j.cej.2016.08.121
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Chelator-mimetic multi-functionalized hydrogel: Highly efficient and reusable sorbent for Cd, Pb, and As removal from waste water Authors: Zahra Mohammadi*, Shangbin Sun*, Cory Berkland**, Jenn-tai Liang*
*: Texas A&M university, Harold Vance Department of Petroleum Engineering **: University of Kansas, Chemical and Petroleum Engineering Department
Corresponding Authors: Zahra Mohammadi Email: [email protected] Tel: 979-845-150
Abstract:
Medically inspired hydrogels were successfully synthesized by mimicking the structure of current chelating agents. Thioglycolic acid modified Polyallylamine (PAAm/TGA) and Dihydroxybenzoic acid-Thioglycolic modified polyallylamine (PAAM/TGA/DHBA) were synthesized following a simple conjugation process. These hydrogels exhibit high affinity, low cost, and high re-usability efficiency. The maximum sorption capacities were significantly higher than those sorbents currently in use. The kinetic study of the hydrogels demonstrated that almost 50% removal occurred in less than 5 min. Additionally, the hydrogel exhibited excellent re-usability, maintaining their efficiency in 5 consecutive cycles. These results suggest that these hydrogels can be considered to be potential agents to be used in waste/produced water treatment.
Key words: Chelator mimetic, hydrogel, re-usable, toxic metal removal, high sorption capacity
1.
Introduction
Heavy metals, such as cadmium, lead, and arsenic, are highly toxic pollutants discharged into receiving waters as a result of different activities[1-3]. The increasing accumulation of heavy metals, has raised serious concerns in regard to human health[4, 5]. Wastewater discharge from mining,
agriculture and oil and gas industries is a primary source of environmental heavy metal contamination [3]. Due to the detrimental effects of the heavy metals to human body and microorganisms, it is necessary to remove such toxic metals from environment. This has led to an increasing interest in technologies that could effectively remove these contaminations from wastewater. Many methods have been used to decontaminate and remove heavy metals from wastewater; however most of these methods are not considered effective and suffer from several drawbacks.[4] As an example, the amorphous solubility of metal hydroxide leads to an incomplete precipitation, which makes this method ineffective and unreliable [6]. In Solvent extraction method, the eluent must be processed to avoid environmental contamination [7]. Similarly, subsequent treating of toxic sludge in chemical precipitation makes this method challenging and economically unfavorable [8]. Among these methods, adsorption has shown to be of interest due to its easy operation and high efficiency [4, 9]. Traditional adsorbents, such as activated carbon, only work best when heavy metals are present in high concentrations. Also, the adsorbent may produce toxic sludge which needs further processing and detoxification treatment [10]. Recently, nanomaterial-based adsorbents have been designed and synthesized, but their practical applications in treating wastewater are limited by their complex synthesis procedure and inconvenient recycle[11]. Although it is considered one of the most efficient heavy metal removal techniques, adsorption still has certain limitations. This method is yet to achieve an optimized removal capacity and subsequently a stable commercial level. Therefore, in practice, high sorption capacity, low cost, good
reusability, and lack of sludge formation are strongly recommended for efficient removal of heavy metals. Heavy metals perturb multiple enzyme systems in human body. Any enzyme that has a sulfhydryl ligand group is vulnerable and most probably would be malfunction upon exposure to any toxic metal [12]. Several chelating agents, such as calcium disodium ethylenediaminetetraacetate (CaNa2EDTA), deferoxamine, and D-penicillamine, have been used as antidotes for metal intoxication in humans[13, 14]. As shown in Fig 1, all of these drugs have O and S as donor groups, and they are rich in sulfur groups. In this work, hydrogels are used as adsorbents for removing heavy metals in aqueous medium. Hydrogels are three-dimensional cross-linked polymeric networks containing a large amount of water (50–90%). The density of the crosslinking agent controls the mechanical strength of hydrogels. Additionally, the high water content enables easy access to any foreign ions within hydrogel network. This work reports the synthesis of multi-functionalized hydrogels capable of removing toxic metals (i.e., Pb, Cd, and As) from an aqueous solution. Functionalized Polyallylamine (PAAm) hydrogels have been introduced previously as potential iron-specific chelating agents. These hydrogels were synthesized by mimicking the structure of a microorganism called entrobactin. Entrobactin has the highest known affinity and selectivity toward iron. In this study, Thioglycolic acids (TGA) in combination with the phenolic group of Dihydroxybenzoic acid (DHBA) were introduced onto PAAm to mimic the structure of current effective chelating agents. Conjugation of the thiol groups dramatically improved the binding affinity and selectivity of the hydrogels for toxic metals. The binding was fast and the binding capacity was relatively high for each metal ions. Moreover, functionalized hydrogels showed excellent reusability after being used in 5 consecutive batches. 2. Materials and Methods 2.1. Chemicals and materials
Poly(allylamine hydrochloride) (PAAm) with an average molecular weight of 56 kDa and an analytical grade reagent N,N’- methylenebisacrylamide (MBA) were obtained from Sigma-Aldrich and used without further modification. 2,3 dihydroxybenzoic acid, thioglycolic acid (TGA), N,N,N -triethylamine (TEA), dimethylformamide (DMF) and all metal chlorides were purchased from Fisher Scientific and used as received. Dicyclohexylcarbodiimide (DCC) and N-hydroxysuccinimide (NHS) were purchased from Thermo Scientific and used without further modification. Deionized water (DI) was obtained from a Barnstead EasyPure water purifier. 2.2. Preparation of multi-functionalized gel PAAm hydrogel was synthesized following the procedure reported in the literature[15, 16]. Briefly, a 20% w/v polymer solution containing a predetermined amount of MBA was prepared. The cross-linker was dissolved in deionized water and later added to the polyallylamine polymer. TEA, the cross-linking catalyst (300 μL), was subsequently added to the solutions and mixed thoroughly. Next, the precursors were transferred by pipette into small vials. The vials were held at ambient temperature for 1 h and later cooled to ca. 3 °C and held there for an additional 24 h. After this time period, hydrogels were removed from the vials and washed with 0.05 M sodium chloride for several days. A solution of TGA and NHS in 5 mL of DMF was mixed with a solution of DCC in 5 mL of DMF. The mixture was stirred at low temperature for 6 h to provide a white precipitate. The precipitate was filtered, and the filtrate was added directly to a known mass of ground hydrogel. The reaction mixture was held at room temperature for 3 days. PAAm conjugate hydrogel was later washed with water for several days. The same procedure was performed for the conjugation of 1:1 molar ratio of TGA/DHBA.
2.3. Characterizations 2.3.1.
Quantification of amine functional groups
Primary amine groups were quantified by potentiometric titration[17]. First, 40 mg of functionalized hydrogels were grounded and suspended in 35 mL of 0.2 M aqueous KCl solution. Next, 140 μL of 8 M KOH aqueous solution was added to polymer suspensions. This raised the pH to ~12. Titration was carried out using 0.1 M HCl standard solution. The solution was added until the pH was approximately 2.5 in all polymer suspensions. Free amine groups were quantified next.
2.3.2. Metal analysis Mono- and multi-elemental analysis of samples was quantified by Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) (Optima 8000 DV, PerkinElmer, USA) fitted with an AS 93plus autosampler (PerkinElmer, USA). The RF power was 1300 W and nebulizer and auxiliary flows were 0.8 and 0.2 L/min, respectively. Sample flow was set to 0.8 mL/min. The analytical curves used for sample analysis had coefficients of correlation >0.999. 2.4. Binding Kinetics Study 2 mg/mL metal chloride solutions were adjusted to pH 2.5 with ionic strength in the range of 0.02 M to 0.04 M and kept at room temperature for kinetic studies. 0.1M HCl and 1M NaOH were used to adjust the pH and ionic strength. Samples were taken from the solution at several time intervals to determine the rate of metal binding by the functionalized hydrogel.
2.5. Sorption experiments
In order to evaluate the ion sorption properties of the adsorbent, a series of sorption experiments, including the effect of environmental conditions (pH), contact time, initial concentration, temperature, selectivity, and recyclability, were conducted. 2.5.1. Binding experiments Known concentrations of metal chloride solutions (5, 20, 50, 100, 200, 400, 500) ppm were prepared. Binding experiments were conducted by taking 20 mL of the metal solution. Th pH was adjusted to 2.5 while the ionic strength was kept in a range of 0.02 M to 0.04 M. A known mass of functionalized hydrogel was added to the mixture and was held at room temperature for 2 h or until equilibrium was reached. At the end, the solutions were filtered, and the filtrates were analyzed for metal concentration. 2.5.2 Sorption isotherms Three isotherm models were applied at equilibrium to determine the distribution of the metal molecules between the liquid phase and the hydrogel. Sorption parameters of Langmuir, Freundlich, and Temkin models were calculated for each metal ion at a pH value of 2.5. The constants for each isotherm model were calculated, and the accuracy of the isotherm models were evaluated using linear correlation coefficient (R2) values. 2.5.3. Selectivity study The selectivity for Pb, Cd, and As by functionalized PAAm in the presence of competing metals was studied. A metal solution (10 mL, 2 mg/mL) containing all the competing metal ions was prepared. The pH of the solution was adjusted 2.5 and held at room temperature for 2 h after adding a known mass of functionalized dry gel. Ionic strength of all the solutions was between 0.02 M and 0.04 M.
3.
Results and Discussion
3.1. Synthesis and characterization Poly(allylamine hydrochloride) was cross-linked following a previously reported method[18]. In this experiment, an optimized reaction yield was chosen from the previously reported data[16]. 2,3 DHBA and TGA were covalently linked to available amino sites of PAAm hydrogel via DCC/NHC conjugation chemistry. Moreover, potentiometric titration data showed in Fig. 2 were used to calculate the degree of conjugation using the following equation. =
(1)
In this study, CH is the concentration of hydrogel. The conjugation efficiency was calculated to be 47% and 67% for PAAm/TGA and PAAm/TGA/DHBA hydrogels, respectively.
3.2. Binding kinetics Determination of the kinetics of metal removal is critical for understanding the performance of hydrogels and evaluating the potential applications of the gels. The kinetics of metal binding was studied by adding a known mass of dry hydrogels to a known initial concentration of metal solution (2 mg/mL, metal chloride). The concentration of metal in solution was measured over time. Approximately 40%50% of the total metal sorption was removed in less than 5 min for all of the functionalized hydrogels (Figure 3). The sorption was the fastest for As in the presence of different functionalized hydrogels and the slowest for Cd. This property might be due to the smaller atomic radius of As compared to that of Pb and Cd. The results published by Ozay et al. [19] showed that magnetic hydrogels are capable of removing less than 40% of the total metal within first 2 hrs. Literature shows that in most cases, the minimum required time to reach ~50% of the total removal is approximately 15-30 min. The rapid sorption of toxic metals by hydrogels reported in this study offers an important advantage, especially in
waste water treatment units, where the flow passes the column with a high rate and short contact time with the sorbent. To derive the rate constant and binding capacity, the kinetic data were modeled with pseudo-first-order (Lagergren model) and pseudo-second-order (Ho model) kinetic models, which are expressed in their linear forms as
log − = + log . #$
%
!
(2)
"
= #' + # & '
(3)
&
where k1 (L/min) and k2 (g/mg · min) are pseudo-first-order and pseudo-second-order rate constants, respectively. The model variables obtained by linear regression were compared. The results are shown in Table 1. The pseudo-second order reaction model showed the best fit because it had a R2 value close to unity in each of these cases. Table 1: Kinetic parameters for metal binding by functionalized hydrogels at pH=2.5. Ionic strength values ranged from 0.02 M to 0.04 M. (A) Constant for the kinetic sorption of different PAAm/TGA calculated with different models, (B) Constant for the kinetic sorption of different PAAm/TGA/DHBA calculated with different models. (A)
Metals
Pseudo-first order k1 (L/h)
qe (mg/g)
Pseudo-second order R2
k2(g/(mg h))
qe (mg/g)
R2
As(I)
0.01
0.8645
14.75
0.9999
Pb(II)
0.01
0.9858
16
0.997
Cd(II)
NA
NA
NA
NA
(B)
Metals
Pseudo-first order
Pseudo-second order
k1 (L/h)
qe (mg/g)
As(I)
NA
NA
Pb(II)
0.0203
Cd(II)
NA
R
2
0.9391 NA
k2(g/(mg h))
qe (mg/g)
R
2
13.48
1
5.46
0.9998
1.0788
0.9997
The equilibrium binding values were systematically higher for PAAm/TGA hydrogels compared to PAAm/TGA/DHBA. This finding might be observed because of a high density of thiol groups available for coordinating with toxic metals. Tofan et al. [21] have studied the sorption kinetic of sulfhydryl modified hemp when exposed to Cd and Pb. The sorption capacity of modified hemp was reported to be 14.0 and 23.0 mg/ g of fibers for Cd and Pb ions, respectively, at room temperature. In comparison, hydrogels reported in this study achieved ~ 20-fold and ~ 10-fold higher binding capacity when using PAAm/TGA to bind Cd and when using PAAm/TGA/DHBA gels to bind Pb, respectively. 3.3. Effect of pH pH of the aqueous solution is one of the most important factors affecting the metal sorption by the hydrogel. A very small change in pH would strongly affect the sorption phenomena as it controls the surface charge and functional group activity of hydrogel. In this work, the effect of pH on metal sorption was determined for a pH range of 2 to 8, but we mainly attempt to stay below a pH value of 6, primarily because a portion of the metal ions underwent hydrolysis in higher pH values. Additionally, the potential application of these hydrogels will be in waste water treatment where the pH ranges are mostly below neutral. The As uptake efficiency as a function of pH is shown in Fig. 4. The As sorption did not change significantly with an increase in the solution pH. It is assumed that a high concentration of hydronium ions (H3O+) near the surface of the gel could produce a repulsive force and hinder the interaction between the As ions and the hydrogel. This finding might account for the lower As sorption efficiency at low pH region. At pH ~ 6, the primary amine groups are not as protonated, and all coordinating sites are
anticipated to make a coordinate covalent bonding with metal ions. We believe that because the most of the surface sorption sites are deprotonated at pH >6, the electrostatic interactions required to adsorb As is slightly different than those observed for lower pH. 3.4. Binding Isotherms The relationship between the amount of metal absorbed by a unit mass of the hydrogel and the concentration of the remaining metal ions in solution represents the sorption isotherms. There are several empirical models such as Langmuir, Frendlich, and Temkin isotherm which are typically used to model experimental data and calculate the maximum binding capacity. In this study, the Frendlich model seems to fit the data almost accurately with a correlation coefficient (R2) close to unity (Table 2).
Table 2: Isotherm parameters for metal binding by functionalized hydrogels at pH=2.5. Ionic strength values ranged from 0.02 M to 0.04 M. (A) PAAm/TGA, (B) PAAm/TGA/DHBA) (A)
Model qmax
Langmuir isotherm As Pb Cd
664.50 NA NA
0.78 0.95 0.90
14078.44 806.57 1159.44 Parameters RT/B
A
Temkin isotherm As Pb Cd
32.20 NA NA Parameters kf
n
Frendlich isotherm As Pb Cd
Parameters kL
261.73 75.22 82.61
94.37 61.73 67.61
(B) Model
Parameters
R
2
0.92 NA NA
R
2
0.95 0.98 0.99
R
2
0.86 0.77 0.79
2
qmax
kL
R
As
NA
NA
NA
Pb
89.00
−
0.64
Cd
81.70
−
0.79
Langmuir isotherm
Parameters n
kf
R
2
Frendlich isotherm As
0.77
Pb
1.04
Cd
14256.95 0.958 544.57
0.908512765 1159.44
0.9978 0.99
Parameters 2
A
RT/B
R
As
261.72
94.05
0.86
Pb
36.60
80.00
0.81
Cd
82.61
67.61
0.79
Temkin isotherm
Even though a reciprocal plot of the data gave a straight line fit for Langmuir isotherms in most cases, the negative intercept values suggested that simple Langmuir did not occur. The Freundlich model assumes that there are many types of sites acting simultaneously; which is most likely the case in this study, each with a different free energy of sorption . Here, S and O donor groups along with protonated amine groups are coordinating with metal ions as different binding sites. The Temkin isotherm assumes that a decrease in the heat of sorption is linear and that sorption is characterized by a uniform distribution of binding energies. This model did not quite fit the experimental data, suggesting that multiple, complex binding mechanisms may be involved in metal sorption. Maximum sorption capacity of the hydrogel was compared to those previously reported and are shown in Table 3. The data indicates that the present hydrogels outperform many other sorbents in the case of the Cd and Pb removal.
Table 3: Comparison of different sorbent and their sorption capacities
Absorbent Alginated gel beads
Capacity (mg/g) Pb Cd
Papaya wood Sulfur-functionalized silica Brinessite Ionic imprinted silica Ulmus tree leaf Chitosan nanofiber Functionalized silica PVA/PAA gel PAAm/TGA/DHBA PAAm/TGA
673 379.2 201 118 61.9 194.99 452.25 345.00
Ref. [1]
17.22 116.7 31.4 80 60.85 115.88 294.08 291.73
[22] [23] [24] [25] [26] [27] [28] [4] This work This work
3.5. Effect of temperature The sorption isotherms of Pb(II), Cd(II), and As(i) on different hydrogels were obtained at three different temperatures (293, 303 and 313 K) and are shown in Table 4. Thermodynamic parameters, such as the standard free energy change (ΔG0), the standard entropy change (ΔS0) and the standard enthalpy change (ΔH0), were calculated according to the van't Hoff equation. The spontaneous nature of the sorption process was evident by the negative values of ΔG0 (Table 4). Furthermore, the positive value of ΔS0 indicated that the degree of freedom increases at the gel-liquid interface during the removal process, implying that there is an interaction between the metal ions and the hydrogel. The positive value of ΔH0 indicates that metal sorption on the gel is an endothermic process and which means a high temperature would be beneficial to the removal. This finding is consistent with the results from sorption isotherms.
Table 4: Values of thermodynamic parameters for metal sorption on a gel
Metal
T
Sorbent: PAAm/TGA
Sorbent: PAAm/TGA/DHBA
As
Pb
Cd
293
∆H (j/mol)
∆S (J/mol.K)
∆G (J/mol)
∆H (j/mol)
∆S (J/mol.K)
∆G (J/mol)
13238.39
113.760462
-20093.45
625.262684
67.22
-19070.05
303
-21231.04
-19742.25
313
-22368.65
-20414.44
293
4445.00
45.20
-8798.31
303
-9250.33
313
-9702.35
293
3002.78
59.4026986
-14402.22
NA
NA
751.868276
67.07
-18901.30
303
-14996.25
-19572.06
313
-15590.28
-20242.82
3.6. Selectivity Studies An important feature of metal chelating hydrogels is the ability to target the metal of interest and remove it from the solution. The metal selectivity of PAAm/TGA and PAAm/TGA/DHBA hydrogels was investigated using a multi-solute system. At an equal concentration of all toxic metals (i.e., Pb, Cd, As at 2 mg/mL), PAAm/TGA absorbed almost 100% of the lead present in the media while in the same experiment and in the presence of other competing metals, such as Fe and Zn. This value decreased to ~70% (Figure 5). The tendency of PAAm/TGA hydrogel for sorption of metals followed the order Pb > As> Cd> Zn> Fe. This trend was similar for PAAm/TGA/DHBA. Previously, a high Pb removal capacity using magnetic hydrogels (130 mg/g) has been reported[20]. The Pb removal capacity with PAAm/TGA and PAAm/TGA/DHBA was 345.6 and 291.7 mg/g, respectively. All data suggests that these hydrogels may have excellent potential in waste water treatment and a probable application in acute metal poisoning. 3.7. Reusability
For the sorption to be useful and economically favorable, the hydrogel must be desorbed easily. It is important that the spend gel could be reused for several cycles without significant changes in its original performance. Here, hydrogels were tested to see whether the absorbents could be chemically eluted and regenerated. As estimated, low concentration of HCl acid was able to elute the absorbed metal and to retain the functionality of the gels. From the experimental data, it was evident that the absorbed metal was sufficiently eluted from the gel (up to 60%) using 1 M HCl. The 1 M HCl was considered to be an optimal concentration to wash the gel. Both the PAAm/TGA and PAAm/TGA/DHBA hydrogels were simultaneously regenerated in their initial form for the next sorption operation after the washing process. The removal–wash–regeneration processes were performed five times, and the removal efficiency in each cycle was compared with the initial removal efficiency of the gel. The data are presented in Figure 6. The easy regeneration of hydrogels makes them strong candidate to be used as an absorbent agent in wastewater treatments plants.
4.
Conclusions
Cross-linked polymers with different functional groups were designed for the removal of toxic metals. Dihydroxybenzoic acid and thioglycolic acid were used as conjugated moieties to improve the affinity and selectivity of PAAm toward toxic metals. These hydrogels were able to remove and accumulate Pb, Cd, and As ions from aqueous solutions at relatively low metal concentrations. PAAm/TGA and PAAm/TGA/DHBA hydrogels demonstrated an almost instant metal sorption when equilibrated in the metal solution. The pseudo-second order Ho kinetic model offered an excellent fit for the sorption data. The inclusion of the thiol groups to hydrogels seemed to improve their selectivity toward Pb and As. The rapid binding and selectivity of these functionalized hydrogels for heavy metals provides important advances for the waste water/produced water treatment and may enable applications in acute metal poisoning.
5.
Acknowledgements
This research was supported by the Petroleum engineering department of Texas A&M University. We would also like to thank the team members of our laboratory and our colleges in the University of Kansas for valuable discussions.
References: [1] X. Guo, B. Du, Q. Wei, J. Yang, L. Hu, L. Yan, W. Xu, Synthesis of amino functionalized magnetic graphenes composite material and its application to remove Cr (VI), Pb (II), Hg (II), Cd (II) and Ni (II) from contaminated water, Journal of hazardous materials, 278 (2014) 211-220. [2] A.H. Hawari, C.N. Mulligan, Biosorption of lead (II), cadmium (II), copper (II) and nickel (II) by anaerobic granular biomass, Bioresource technology, 97 (2006) 692-700. [3] U. EPA, 2002 Edition of the Drinking Water Standards and Health Advisories, in, US Environmental Protection Agency Washington^ eDC DC, 2002. [4] L. Chu, C. Liu, G. Zhou, R. Xu, Y. Tang, Z. Zeng, S. Luo, A double network gel as low cost and easy recycle adsorbent: Highly efficient removal of Cd (II) and Pb (II) pollutants from wastewater, Journal of hazardous materials, 300 (2015) 153-160. [5] S. Ghorai, A. Sinhamahpatra, A. Sarkar, A.B. Panda, S. Pal, Novel biodegradable nanocomposite based on XG-g-PAM/SiO 2: application of an efficient adsorbent for Pb 2+ ions from aqueous solution, Bioresource technology, 119 (2012) 181-190. [6] J.A. Dyer, P. Trivedi, S.J. Sanders, N.C. Scrivner, D.L. Sparks, Lead sorption onto ferrihydrite. 3. Multistage contacting, Environmental science & technology, 37 (2003) 923-930. [7] H. Tokuyama, T. Yoshida, L. He, Preparation of novel emulsion gel adsorbents and their adsorption properties for heavy-metal ions, Industrial & Engineering Chemistry Research, 50 (2011) 10270-10277. [8] F. Fu, Q. Wang, Removal of heavy metal ions from wastewaters: a review, Journal of Environmental Management, 92 (2011) 407-418. [9] J.-L. Gong, B. Wang, G.-M. Zeng, C.-P. Yang, C.-G. Niu, Q.-Y. Niu, W.-J. Zhou, Y. Liang, Removal of cationic dyes from aqueous solution using magnetic multi-wall carbon nanotube nanocomposite as adsorbent, Journal of Hazardous Materials, 164 (2009) 1517-1522.
[10] V.M. Nurchi, I. Villaescusa, Sorption of toxic metal ions by solid sorbents: a predictive speciation approach based on complex formation constants in aqueous solution, Coordination Chemistry Reviews, 256 (2012) 212-221. [11] J. Saiz, E. Bringas, I. Ortiz, Functionalized magnetic nanoparticles as new adsorption materials for arsenic removal from polluted waters, Journal of Chemical Technology and Biotechnology, 89 (2014) 909-918. [12] B. Sharma, S. Singh, N.J. Siddiqi, Biomedical implications of heavy metals induced imbalances in redox systems, BioMed research international, 2014 (2014). [13] S.J. Flora, V. Pachauri, Chelation in metal intoxication, International journal of environmental research and public health, 7 (2010) 2745-2788. [14] O. Andersen, Principles and recent developments in chelation treatment of metal intoxication, Chemical reviews, 99 (1999) 2683-2710. [15] Z. Mohammadi, S.X. Xie, A.L. Golub, S.H. Gehrke, C. Berkland, Siderophore-Mimetic hydrogel for iron chelation therapy, Journal of Applied Polymer Science, 121 (2011) 1384-1392. [16] Z. Mohammadi, S.-X. Xie, E. Peltier, M. Veisi, C. Berkland, Enhancing the selectivity of an iron binding hydrogel, European Polymer Journal, 47 (2011) 1485-1488. [17] W. Evans, E. McCourtney, R. Shrager, Titration studies of phytic acid, Journal of the American Oil Chemists' Society, 59 (1982) 189-191. [18] É. Oliveira, S. Hirsch, R. Spontak, S. Gehrke, Influence of polymer conformation on the shear modulus and morphology of polyallylamine and poly ( -L-lysine) hydrogels, Macromolecules, 36 (2003) 6189-6201. [19] I. Alinnor, Adsorption of heavy metal ions from aqueous solution by fly ash, Fuel, 86 (2007) 853-857. [20] O. Ozay, S. Ekici, Y. Baran, N. Aktas, N. Sahiner, Removal of toxic metal ions with magnetic hydrogels, Water Research, 43 (2009) 4403-4411.
[21] L. Tofan, C. Paduraru, Sorption studies of Ag I, Cd II and Pb II ions on sulphydryl hemp fibers, Croat. Chem. Acta, 77 (2004) 581–586. [22] V.K. Singh, P.N. Tiwari, Removal and recovery of chromium (VI) from industrial waste water, Journal of Chemical Technology and Biotechnology, 69 (1997) 376-382. [23] O. Hakami, Y. Zhang, C.J. Banks, Thiol-functionalised mesoporous silica-coated magnetite nanoparticles for high efficiency removal and recovery of Hg from water, Water research, 46 (2012) 3913-3922. [24] D.C. Marcano, D.V. Kosynkin, J.M. Berlin, A. Sinitskii, Z. Sun, A. Slesarev, L.B. Alemany, W. Lu, J.M. Tour, Improved synthesis of graphene oxide, ACS nano, 4 (2010) 4806-4814. [25] X. Xin, Q. Wei, J. Yang, L. Yan, R. Feng, G. Chen, B. Du, H. Li, Highly efficient removal of heavy metal ions by amine-functionalized mesoporous Fe 3 O 4 nanoparticles, Chemical Engineering Journal, 184 (2012) 132-140. [26] S. Guo, D. Li, L. Zhang, J. Li, E. Wang, Monodisperse mesoporous superparamagnetic single-crystal magnetite nanoparticles for drug delivery, Biomaterials, 30 (2009) 1881-1889. [27] Z.-J. Fan, W. Kai, J. Yan, T. Wei, L.-J. Zhi, J. Feng, Y.-m. Ren, L.-P. Song, F. Wei, Facile synthesis of graphene nanosheets via Fe reduction of exfoliated graphite oxide, ACS nano, 5 (2010) 191-198. [28] N. Savage, M.S. Diallo, Nanomaterials and water purification: opportunities and challenges, Journal of Nanoparticle research, 7 (2005) 331-342.
Figure 1: Structure of some of the current chelating agents used to treat toxic metal poisoning.
pH
16 14 12 10 8 6 4 2 0
PAA/TGA PAAm/TGA/DHBA
0
1 2 3 4 Volume of HCl added (mL)
5
Figure 2: Titration data collected for PAAm/TGA and PAAm/TGA/DHBA. pH values represent the values of 3 readings differing by < 5%.
(A)
80 70 %Removal
60 50 40 30 20
Batch study PT-As
10
Batch study PDT-As
0 0
50
100
150 Time (min)
200
250
300
(B)
70 60 %Removal
50 40 30 20
Batch study PT-Pb
10
Batch study PDT-Pb
0 0
50
100
150 Time (min)
200
250
300
%Removal
(C)
90 80 70 60 50 40 30 20 10 0
Batch study PT-cd
Batch study PDT-cd 0
50
100
150 Time (min)
200
250
300
Figure 3: Ion binding by functionalized hydrogels, A) As, B) Pb, and C) Cd. Gels were equilibrated in 2 mg/mL metal solution (pH= 2.5). Ionic strength values ranged from 0.02 M to 0.04 M.
66
% removal
65 64
PT-As DPT-As
63 62 0
2
4
6
pH Figure 4: Effect of pH solution on the As removal efficiency of modified hydrogel.
8
100
PAAm/TGA/DHBA PAAm/TGA
% absorbed
75 50 25 0
Cd
100
Pb
As
PAAm/TGA/DHBA PAAm/TGA
% absorbed
75 50 25 0
Cd
Pb
Fe
Zn
As
Figure 5: Selectivity study of functionalized hydrogels toward toxic metal ions A) in the presence of competing toxic metals and B) in the presence of competing essential metals (pH=2.5).
Reusability test PTD-Pb 100
% removed
80 60 40 20 0
0
Reusability test 50.5048
1
2
3
4
54.4718
52.516
52.5095
57.3547
Reusability test PT-Pb
% removed
81 61 41 21 1
0
Reusability test 67.3796
1
2
3
4
65.323
65.1004
65.4929
62.4741
Figure 6: Reuse studies for PAAm/TGA (PT) and PAAm/ TGA/DHBA (PTD) after elution operation with pertinent eluent (1 M HCl). Five consecutive sorption-elution-reuse cycles were employed.
HIghlights •
Multi-functionalized hydrogels for toxic metal removal is presented.
•
Hydrogels are chelator-mimetic.
•
Modification of hydrogel dramatically improved the binding affinity.
•
Removal is fast, efficient and selective.