Ind. Eng. Chem. Res. 2009, 48, 4495–4499
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Impregnating Zirconium Phosphate onto Porous Polymers for Lead Removal from Waters: Effect of Nanosized Particles and Polymer Chemistry Qingrui Zhang, Peijuan Jiang, Bingcai Pan,* Weiming Zhang, and Lu Lv State Key Laboratory of Pollution Control and Resource Reuse, School of the EnVironment, Nanjing UniVersity, Nanjing 210093, P.R. China
The subject study revealed several unique properties of polymer-based zirconium phosphate (ZrP) for lead removal from waters. ZrP particles were impregnated within two porous polymers, namely, a chloromethylated polystyrene (CP) and a polystyrene cation exchange resin (D-001). Both as-prepared hybrid sorbents (designated ZrP-CP and ZrP-001, respectively) were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and a N2 adsorption-desorption test at 77 K. As compared to the fresh particles, ZrP impregnated onto CP exhibits a slight increase in sorption capacity, which may result from their nanosized particles after impregnation. More attractively, ZrP-001 displays much higher sorption preference toward lead ions over calcium ions than ZrP, D-001, and ZrP-CP. Such significant phenomena is mainly attributed to the Donnan membrane effect exerted by the immobilized sulfonate groups on D-001 and further validates its potential application for enhanced removal of the toxic metal from contaminated waters. Introduction Water pollution by heavy metals still remains a serious environmental and public concern all over the world.1 Today, some of the toxic metals (e.g., lead and cadmium) even at trace levels are believed to pose adverse effects on human health.2 Thus, it is significant to develop efficient technologies for enhanced removal of heavy metal ions from contaminated waters.3 Up to now, various technologies have been proposed to trap heavy metals from waters, and ion exchange, which has been widely studied and applied in treatment of metalcontaminated waters,4,5 serves as one of the most attractive and efficient approaches. As an efficient inorganic ion exchanger, zirconium phosphate (denoted ZrP) was extensively studied for sorption of alkali and alkaline earth metal ions from aqueous systems, and it exhibits high capacity, fast kinetics, as well as remarkable thermal and radiolytic stabilities.6-9 Our earlier study also demonstrated that ZrP exhibits more preferable sorption toward toxic metal ions, e.g., Pb(II), Zn(II), and Cd(II), than a polystyrene cation exchange resin D-001.10,11 However, ZrP is present as fine or ultrafine particles and cannot be employed for direct use in fixedbed or any other flow-through systems due to the excessive pressure drop and poor mechanical strength. Similar difficulties are encountered for other inorganic sorbents like metal (hydr)oxides, e.g., ferric and manganese oxides. An effective approach to overcoming such technical bottlenecks is to impregnate these fine particles onto porous adsorbents,12-15 namely, activated carbon,12 alginate,13 and sand,14,15 to obtain hybrid sorbents. In recent years, highly cross-linked porous polystyrene adsorbents have proved to be ideal support materials, and hybrid sorbents based on surface charged polymeric supports16-20 (e.g., cation or anion exchangers) were found to exhibit more favorable sorption toward trace target metals. This is because the immobilized charged groups on a polymeric matrix would greatly enhance metal permeation and preconcentration prior to effective sorption by inorganic particles; the phenomenon is the so-called “Donnan membrane effect”.16,20 * To whom correspondence should be addressed. E-mail: bcpan@ nju.edu.cn. Tel: +86-25-8368-5736. Fax: +86-25-8370-7304.
The main objective of the current study is to elucidate different sorption performance of ZrP particles before and after impregnation onto polymeric adsorbents, as well as the effect of matrix chemistry of polymeric supports on the performance of the resulting hybrid sorbents. To achieve the goal, ZrP particles were first impregnated onto two different polymeric supports, i.e., a chloromethylated polystyrene (CP) and a sulfonated polystyrene (D-001), to fabricate two hybrid sorbents. Afterward, their sorption toward lead ion was evaluated from the background of calcium ions, as compared to the fresh ZrP particles and the polymeric support D-001. Materials and Methods Materials. All chemicals used were of analytical grade and purchased from Shanghai Reagent Station except as noted. Pb(II) stock solution was prepared by dissolving Pb(NO3)2 in doubledistilled water and filtering through a 0.22-µm membrane. D-001, a polystyrenesulfone cation exchanger with total capacity of 4.30 mequiv/g and cross-linking density of ∼8%, and CP, a chloromethlated polystyrene resin, were kindly provided by Zhenguang Resin Co., China. Both polymeric sorbents were obtained in spherical bead form with sizes ranging from 0.6 to 1.0 mm. Prior to use, they were subjected to DI flushing to remove the residue impurities until neutral pH (6.8-7.2) and then vacuum-desiccated at 348 K for 24 h until reaching a constant weight. Sorbents Preparation and Characterization. The amorphous ZrP particle was synthesized by a simple precipitation method according to refs 21 and 22, and the hybrid sorbents ZrP-001 and ZrP-CP were fabricated according to our previous study.23,24 In brief, 30 g D-001 or CP beads were immersed into ZrOCl2-HCl solution (40 g ZrOCl2 · 8H2O was dissolved in 100 mL 2 M HCl), and then the above mixture was evaporated off to ensure the dispersion of ZrOCl2 into the inner pores of D-001 or CP beads. Afterward, the resulting beads were added into a flask containing 300 mL 4 M H3PO4 solution and stirred at 150 rpm for 24 h. ZrP particles were then impregnated onto the inner surface of polymeric supports as
10.1021/ie8016847 CCC: $40.75 2009 American Chemical Society Published on Web 03/25/2009
4496 Ind. Eng. Chem. Res., Vol. 48, No. 9, 2009 HCl
2H3PO4(aq) + ZrOCl2(aq) 98 Zr(HPO4)2 V +HCl(aq) + H2O(aq) (1) The resulting hybrid sorbents were then filtered and rinsed by double-deionized water until the neutral pH and vacuum desiccated at 323 K for 24 h for further study. ZrP speciation within ZrP-001 and ZrP-CP was characterized by X-ray diffraction (XTRA, Switzerland) and a transmission electron microscope (Hitachi Model H-800, Japan). The fresh ZrP particle size distribution was determined using dynamic light scattering (DLS 90 Plus, Brookhaven Instrument Corp). N2 adsorption-desorption tests for the porous sorbents were performed by using a Micromertics 2010C automatic analyzer (USA). The amount of ZrP loaded onto D-001 was determined by digestion of the hybrid particles into HF solution, followed by ICP analysis (JA1100, USA). XPS analysis of a given sample was made with a spectrometer (ESCALAB-2, Great Britain) equipped with a Mg KR X-ray source (1253.6 eV protons). All binding energies were referenced to the C 1s peak at 288.75 eV to compensate for the surface charging effects. Batch Sorption Experiments. Batch sorption tests were carried out by the traditional bottle-point methods in 250 mL flasks. To start the experiments, desired amount of a given sorbent were added to a flask containing a 100-mL solution of known concentration of lead ion, and desired Ca(II) ions were introduced when necessary. All the sorption tests were conducted at pH 4.5-5.0 according to our earlier study (adjusted by HNO3).11 The flasks were then transferred to a G 25 model incubator shaker with thermostat (New Brunswick Scientific Co. Inc.) and shaken under 200 rpm for 24 h. The time was deemed sufficient to ensure apparent equilibrium as determined by
preliminary kinetic tests (data not shown). The lead amount loaded on a given sorbent is calculated by conducting a mass balance before and after the tests. Column Sorption Tests. Fixed-bed column tests were carried out with a column (12 mm diameter and 130 mm length) equipped with a bath to maintain a constant temperature. A 5-mL ZrP-CP or ZrP-001 sorbent was packed within two separate columns. A HL-2B pump (Shanghai, China) was used to ensure a constant flow rate. All the sorption column runs were performed under the same hydrodynamic conditions: the superficial liquid velocity (SLV) and the empty bed contact time (EBCT) were identical and equal to 0.50 m/h and 6 min, respectively. Analyses. The content of Pb(II) ions was usually determined by atomic absorption spectroscopy (AAS; Thermal Co. USA). When its content is less than 1 mg/L, it was determined by atom fluorescence spectrophotometry (AFS) with an online reducing unit (AF-610A, China) with NaBH4 and HCl solution. Results and Discussion Characterization of Sorbents. All the ZrP-based sorbent samples prepared in the study were well characterized, and some of the important properties are presented in Figure 1a-d and Table 1. It can be seen that ZrP has been successfully loaded on D-001 and CP beads respectively according to the ZrP content variation before and after loading. The Zr/P ratio in both sorbents determined as 1:2 by XPS analysis further indicated their structural formation as Zr(HPO4)2. Transmission electron microscopy (TEM; Figure 1a and b) of both sorbents shows that ZrP was dispersed onto the inner surface of the porous polymeric supports as nanoparticles of size less than 100 nm, while most of the as-prepared ZrP powders ranged from 300 to 750 nm in size by dynamic light scattering analysis
Figure 1. Characterization of the as-prepared sorbents in the study. (a) TEM of ZrP-CP; (b)TEM of ZrP-001; (c) size distribution of ZrP particles; and (d) XRD spectra of ZrP-based sorbent samples.
Ind. Eng. Chem. Res., Vol. 48, No. 9, 2009 4497 Table 1. Salient Properties of Amorphous ZrP Powders, a Strong-Acid Cation Exchanger (D-001), and the Resulting Hybrid Sorbents ZrP-001 and ZrP-CP designation
ZrP
D-001
ZrP-001
CP
ZrP-CP
matrix structure functional group BET surface area (m2/g) Pore volume (cm3/g) average pore diameter (nm) ZrP particle size (nm) ion-exchange capacity (mequiv/g) ZrP content (mass %)
NA ZrP NA NA NA 300-750 3.04a >99%
polystyrene -SO3H 25.1 0.241 34.1 NA 4.30 0
polystyrene -SO3H and ZrP 19.1 0.073 16.1 50-100 3.20a 33.0%
polystyrene -CH2Cl 30.6 0.104 13.6 NA 0 0
polystyrene -CH2Cl and ZrP 24.0 0.078 24.1 50-100 0.72a 21.2%
a
Determined at pH 7.0.
Table 2. Lead Distribution Coefficients (Kd) for ZrP-001 and the Corresponding Binary Sorbent in the Presence of Competing Cations (Experimental Data from Figure 4) Kd (L/g) at different initial Ca2+/Pb2+ ratios in solution (M/M) sorbent
0
2
4
8
16
32
64
ZrP-001 binary sorbent (D-001 + ZrP)
47.1 19.6
9.3 3.7
5.6 2.3
3.8 1.3
2.9 0.9
1.9 0.7
1.1 0.5
(Figure 1c). Though ZrP particle size before and after impregnation cannot be compared in a quantitative manner, impregnation onto both porous polymers is believed to further shorten the ZrP particle size. This is understandable because of the nanopores of the polymeric supports. In addition, XRD spectra (Figure 1d) indicated that all the ZrP particles, whether impregnated or not, are all amorphous in nature.25,26 Effect of ZrP Impregnation onto CP. Lead sorption onto ZrP before and after impregnation onto CP was performed, and the results are presented in Figure 2. Note that no lead sorption was detected onto the neutrally charged polymeric support CP, and ZrP dosages are same in both sorption systems. A slight increase in lead sorption capacity was observed for ZrP impregnated onto CP. This is possibly because ZrP impregnation would result in a lower nanoscale level of ZrP particle size (Figure 1a and b), and the nanosized particles are expected to possess higher surface area as well as more active sorption sites than the bulk ones.27,28 Effect of Polymeric Chemistry. Here we examined the effect of different polymeric support chemistry on lead sorption. Note that alkali or alkaline earth cations, such as Na(I), Ca(II), and Mg(II) ions, are ubiquitous in natural waters or industrial effluents, and these cations always exhibit competitive sorption over target toxic metals and thereafter decrease working capacity of a given sorbent. Lead sorption by two resulting hybrid
sorbents from the background solution of calcium ions was carried out and the results are depicted in Figure 3. D-001, a sulfonated cation exchanger, was also involved here because it can also sorb lead ions through electrostatic interaction, and its effect on sorption should be considered for comparison purpose. Note that sorption onto ZrP-001 consists of two parts: the impregnated ZrP particles and the cation exchanger support D-001. Furthermore, the result represented by a dash line (ZrP-001-D-001) in Figure 3 can be regarded as sorption only by the impregnated ZrP particles. From Figure 3, it can be found that ZrP impregnated onto D-001 exhibits more favorable lead sorption than that onto CP. Such an enhancement effect caused by D-001 is the so-called Donnan membrane effect, i.e., the negatively charged polymeric matrix D-001 would greatly enhance lead permeation and preconcentration from solution to the active sorption sites, and lead concentrations around ZrP particles are much higher than in solution. Similar results were also reported by our research group20,23,24 and Sengupta and his co-workers16,29,30 for hydrated ferric oxides impregnated onto an anion exchanger for arsenic removal. A detailed explanation of the Donnan membrane principle is available elsewhere.16,31 The effect of calcium ion on lead sorption by ZrP-001 was also examined by comparison with a binary sorbent of D-001 and ZrP. As seen in Figure 4, increase in the Ca(II)/Pb(II) ratio
Figure 2. Lead sorption isotherms onto ZrP-CP and amorphous ZrP particles (sorbent: 0.25 g ZrP-CP or 0.050 g ZrP. Note that ZrP contents of both sorbents are equal to each other).
Figure 3. Lead sorption onto ZrP-001, D-001, and ZrP-CP at 303 K (ZrP-001, 0.060 g; D-001, 0.040 g; ZrP-CP, 0.10 g. Test solution: 100 mL. Ca2+: 1000 mg/L. Note that ZrP content of each sorbent is equal).
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As observed, lead sorption onto ZrP-CP broke through quickly, while ZrP-001 exhibited more satisfactory performance in capacity. This was consistent with the results from batch sorption tests and further indicated the effect of polymeric matrix chemistry on sorption. Conclusions
Figure 4. Effect of calcium ion on lead uptake by ZrP-001 and the binary sorbents at 303 K (Solution: 100 mL. Pb2+: 200 mg/L. Note that chemical compositions of both sorbents are the same).
In the current study, we fabricated two hybrid sorbents by impregnating zirconium phosphate (ZrP) onto a chloromethylated polymer (CP) and a sulfonated polymer (D-001) for lead sorption from waters. The nanopore module of polymeric support renders the ZrP particle size to shift to a lower nanoscale level and thereby increase its sorption capacity. Moreover, the sulfonated polymeric support D-001 would greatly improve ZrP sorption preference toward lead ion as a result of the Donnan membrane effect. All the results imply that it is an effective approach to fabricating highly efficient hybrid sorbents for enhanced pollutant removal by impregnating other inorganic particles (e.g., metal hydr(oxide)s) onto suitable polymeric supports. Acknowledgment Prof. Bingcai Pan gratefully acknowledges the support from the Program for New Century Excellent Talents in University of China (NCET07-0421) and the Scientific Research Foundation of Graduate School of Jiangsu Province (CX08B-144Z). Literature Cited
Figure 5. Comparison of lead breakthrough curves onto ZrP-001 and ZrP-CP for separate column runs under identical conditions (EBCT ) 6 min; SLV ) 0.50 m/h).
would result in a decrease in lead capacity for both sorption systems due to the competitive sorption effect. However, ZrP impregnation onto D-001 would exhibit improved lead sorption preference over calcium ion than the bulk ones. To quantify the selectivity of both sorbents, the distribution ratio Kd (in liters per gram) was determined by the following equation.32 Kd )
mmol of heavy metals/g sorbent mmol of heavy metals/L solution
(2)
The Kd values thus defined provide a measure of the sorptive ability for lead ions per gram of sorbent. Table 2 lists the calculated Kd values of Pb(II) ions onto both sorbents. Substantially larger Kd values of ZrP-001 indicated that ZrP-001 exhibits more preferable lead sorption than the binary sorbent. Small-Bed Adsorption. To further examine the effect of polymeric support on lead sorption by such ZrP-loaded hybrid sorbent, small-bed lead sorption was carried out from multicomponent feeding solution onto two separate beds packed with ZrP-CP and ZrP-001, respectively, and their breakthrough curves are illustrated in Figure 5.
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ReceiVed for reView November 5, 2008 ReVised manuscript receiVed February 11, 2009 Accepted March 10, 2009 IE8016847
