- Email: [email protected]
Impregnating titanium phosphate nanoparticles onto a porous cation exchanger for enhanced lead removal from waters
Journal of Colloid and Interface Science 331 (2009) 453–457
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Impregnating titanium phosphate nanoparticles onto a porous cation exchanger for enhanced lead removal from waters Kun Jia a , Bingcai Pan a,∗ , Lu Lv a,∗ , Qingrui Zhang a , Xiaoshu Wang b , Bingjun Pan a , Weiming Zhang a a b
State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing 210093, PR China Modern Analysis Center, Nanjing University, Nanjing 210093, PR China
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 2 November 2008 Accepted 28 November 2008 Available online 6 December 2008 Keywords: Titanium phosphate Hybrid sorbent Cation exchanger Lead removal Nanocomposite
Titanium phosphate (TiP) exhibits preferable sorption toward lead ion in the presence of competing calcium ions at high levels, however, it is present as fine or ultrafine particles and cannot be directly employed in fixed-bed or any flow-through systems due to the excessive pressure drop and poor mechanical strength. In the present study a new hybrid sorbent TiP-001 was fabricated by impregnating titanium phosphate (TiP) nanoparticles onto a strongly acidic cation exchanger D-001 for enhanced lead removal from waters. D-001 was selected as a host material mainly because of the Donnan membrane effect resulting from the immobilized sulfonic acid groups bound on the exchanger matrix, which would enhance permeation of the target metal cation prior to effective sequestration. TiP-001 was characterized by transmission electron micrograph (TEM), X-ray diffraction (XRD), and pH-titration. Batch and column sorption onto TiP-001 was assayed to evaluate its performance as compared to the host exchanger D001. Lead sorption onto TiP-001 is a pH-dependent process due to the ion-exchange nature, and its sorption kinetics follows the pseudo-second-order model well. Compared to D-001, TiP-001 displays highly selective lead sorption in the presence of competing calcium cations at concentration of several orders higher than the target metal. Fixed-bed sorption of a synthetic feeding solution indicates that lead retention by TiP-001 results in a conspicuous decrease of this toxic metal from 0.50 to below 0.010 mg/L (drinking water standard recommended by WHO). Moreover, its feasible regeneration by dilute HCl solution also favors TiP-001 to be a feasible sorbent for enhanced lead removal from water. © 2008 Elsevier Inc. All rights reserved.
1. Introduction Heavy metals are generally considered as a threat toward human’s health and ecosystems because of their potentially high toxicity. Unlike organic pollutants, heavy metals do not undergo biological degradation and tend to accumulate in the organisms, thereby eventually enter the food chains [1]. Lead, copper, and cadmium belong to the group of serious hazardous heavy metals. It is believed that exposure to these toxic metals even at trace level is still a risk to human beings [2,3] and thus, more stringent environmental legislations have been established to restrict the maximum contaminant level (MCL) of heavy metals in waters [4]. For instance, the MCL of lead in natural or drinking water in China was regulated from 0.05 [5] to 0.01 mg/L [6], and in USA the maximum contaminant level goal (MCLG) of lead was set as zero [4]. Obviously, it is of particular significance to propose highly efficient processes to trap these toxic metals in contaminated waters to near-to-zero level.
*
Corresponding authors. Fax: +86 25 8370 7304. E-mail addresses: [email protected] (B. Pan), [email protected] (L. Lv).
0021-9797/$ – see front matter doi:10.1016/j.jcis.2008.11.068
©
2008 Elsevier Inc. All rights reserved.
Among various techniques applied for heavy metals removal from water, adsorption and ion exchange are the widely used options. Unfortunately, the traditional materials employed for both techniques, namely activated carbon [7,8], activated alumina [9], ion exchangers [8,10], and many low-cost materials [11–14], display little or insufficient specific sorption affinity toward toxic metals. Consequently, they cannot enhance heavy metals removal to meet these new and stringent regulations. As a family of inorganic cation exchangers, titanium phosphate [Ti(HPO4 )2 , hereafter denoted TiP] is generally used for sorption of alkaline or alkaline earth cations [15]. It is extremely insoluble in water and offers high capacity, fast kinetics, and is superior to the organic exchangers in terms of thermal stability and resistance to radiation and abrasion [15]. In our previous study [16], titanium phosphate was found to be a highly selective sorbent toward lead ion even in the presence of competing cations (Na+ , Ca2+ , and Mg2+ ) at greater concentration. Such sorption preference possibly mainly results from the synergetic effect of electrostatic interaction and formation of inner-sphere complexes [16]. However, TiP is present as fine or ultrafine particles and cannot be employed in fixed-bed or any other flow-through systems
454
K. Jia et al. / Journal of Colloid and Interface Science 331 (2009) 453–457
due to the excessive pressure drop and poor mechanical rigidity. As a continuation of our earlier study [16], the main objective of the current study is to fabricate a hybrid sorbent by impregnating TiP onto a polystyrenesulfone cation exchanger D-001 to overcome the above-mentioned technical bottleneck. D-001 was selected as a support material mainly because of the Donnan membrane effect exerted by the non-diffusible negatively charged sulfonic acid groups bound on the exchanger matrix [17–20], which is expected to enhance permeation and preconcentration of the trace target metal ion [18–20]. Effect of solution pH on sorption, sorption kinetics, competitive sorption, and fixed-bed sorption tests were involved herein to evaluate the performance of the resulting sorbent.
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). 1.0 M HNO3 solution was used to adjust solution pH throughout the experiment. One milliliter solution at various time intervals was sampled from the flasks to determine sorption kinetics. Lead uptake was calculated by conducting a mass balance before and after the test. Note that all the batch runs were performed in duplicate for data analysis, and detailed experimental conditions are available in the related figures or tables.
2. Materials and methods
Column experiments were carried out with a glass column (14 mm diameter and 120 mm length) equipped with a water bath to maintain a constant temperature. Ten milliliters of a desired sorbent was packed within the column before operation. A Lange-580 pump (Baoding, China) was used to ensure a constant flow rate. After sorption 2.0 M HCl solution was used as regenerant of the exhausted sorbent. The hydrodynamic conditions for sorption are as follows: the superficial liquid velocity (SLV), 0.70 m/h; the empty bed contact time (EBCT), 6 min, and those for regeneration are: SLV, 0.18 m/h and EBCT, 24 min.
2.1. Materials All chemicals used in the study are of analytical grade or purer, and were purchased from Nanjing Zhongdong Reagent Station. Lead nitrate was used as the source of lead ion during the experiment. D-001, a polystyrenesulfone cation exchanger (in H+ -type) with total capacity of 4.30 meq/g and cross-linking density around 8%, was kindly provided by Zhenguang Resin Co., China. It was obtained in spherical bead forms of size ranging from 0.6 to 1.0 mm. N2 adsorption–desorption test onto D-001 at 77 K indicated that most of its inner pores (larger than 95% of pore volume) range from 2–60 nm and its average pore size is 31.4 nm. Prior to use, it was subjected to flushing by deionized water to remove the residue impurities until neutral pH and then vacuum-desiccated at 343 K for 24 h until reaching the constant weight.
2.5. Fixed-bed column sorption and regeneration
2.6. Analysis
The hybrid sorbent TiP-001 was fabricated according to a slightly modified proprietary technique [21]. In brief, TiCl4 , a precursor of TiP, was firstly dispersed into the inner pores of D-001 by evaporating the TiCl4 /D-001 mixture. Afterwards, the TiCl4 -loaded D-001 beads were mixed with H3 PO4 solution to deposit TiP onto the inner surface of the D-001 beads as
Lead concentration in solution was obtained by atomic absorption spectroscope (Z-8100, HITACHI) except when its content was less than 1.0 mg/L, which was determined by atomic fluorescence spectrophotometer (AFS) with an online reducing unit (AF-610A, China) with NaBH4 and HCl solution [16,22]. The impregnated TiP onto D-001 was observed with a transmission electron micrograph (Hitachi model H-800, Japan). Speciation of TiP in TiP-001 beads was analyzed with an X-ray diffraction analysis instrument (XTRA, Switzerland). XPS analysis of the sorbent was performed with a spectrometer (ESCALAB-2, Great British) equipped with MgK α Xray source (1253.6 eV protons). All the binding energies were referenced to the C1s peak at 288.75 eV to compensate for the surface charging effects.
TiCl4 + H3 PO4 → Ti(HPO4 )2 ↓ + HCl.
3. Results and discussion
2.2. Sorbent preparation
The TiP-loaded D-001 beads were then rinsed and thermally treated at 343 K for 12 h, and we obtained the hybrid sorbent TiP001. It was then characterized by transmission electron micrograph (TEM), X-ray diffraction (XRD), and pH-titration. 2.3. pH-titration Portions (500 mg) of D-001 or TiP-001 were mixed with 100 mL of 0.10 M NaCl. This mixture was kept for 4 h and titrated against 0.100 M NaOH solution. The solution pH was recorded after each addition of 1.0 mL of the titrant till the pH became constant. Based on the pH values before and after the exchange process, the milliequivalents (meq) of OH− ion consumed were determined. Milliequivalents of OH− ions consumed by a given sorbent were then plotted against the corresponding pH values to obtain the pHtitration curves [15,16]. 2.4. Batch sorption experiments Batch sorption tests were carried out using traditional bottlepoint technique. To start the experiment, Varying amount of TiP001 or D-001 beads and lead solution were combined in 250 mL glass bottles. Calcium ion was introduced as the competing species by dissolving desired amount of Ca(NO3 )2 into the solution. The
3.1. Characterization of TiP-001 TiP impregnated within D-001 was 15.2% in mass according to its content variation before and after loading. The Ti/P ratio in sorbent phase determined as 1:2 by XPS analysis ensured its formula Ti(HPO4 )2 . As indicated by the TEM image (Fig. 1a) of TiP-001, TiP was dispersed into the inner surface of D-001 as nanoparticles of size around 10 nm. Note that nanosized inorganic particles are expected to display larger accessible surface areas and stronger activity than bulk ones [23–25]. The X-ray diffraction spectra (Fig. 1b) of the TiP-001 beads implied that TiP impregnated within D-001 is amorphous in nature [16,26]. TiP loading results in a dramatic decrease in the pore volume of the spherical beads from 0.225 to 0.083 cm3 /g. Similarly, the average pore diameter also decreases from 31.4 to 21.8 nm. However, BET surface area increases slightly from 25.1 to 27.8 m2 /g possibly due to the inclusion of the TiP nanoparticles of high surface area. A steeper pH-titration curve of D-001 than TiP-001 (Fig. 2) was mainly attributed to the specific ion-exchange property of amorphous TiP with the sorbent. This is because the acid groups within TiP are weakly dissociated and it is reluctant to exchange its H+ ion for Na+ . As a result, the hydrogen ion within TiP is released in a stepwise manner and the ion-exchange remains incomplete [16].
K. Jia et al. / Journal of Colloid and Interface Science 331 (2009) 453–457
455
(a)
Fig. 3. Effect of solution pH on lead uptake by TiP-001 at 303 K (initial lead concentration 1.0 mmol/L, S/L ratio 0.50 g/L, solution 100 mL).
than the calculated value (4.07 meq/g) at neutral pH, implying that part of the sulfonic acid groups or TiP within TiP-001 are inaccessible for lead uptake, which possibly results from pore blockage during TiP dispersion. 3.2. Effect of solution pH on sorption
(b) Fig. 1. Characterization of the as-prepared hybrid sorbent TiP-001. (a) TEM (the dark spots are TiP nanoparticles while the white and gray regions represent the background of the support material D-001); (b) XRD spectra.
Effect of solution pH on lead uptake by TiP-001 was examined and the results were presented in Fig. 3. Generally speaking, higher solution pH (under acidic or neutral conditions) is more favorable for lead uptake onto TiP-001. The specific pH-dependent trend can be explained by the ion-exchange mechanism between solution and TiP-001. In fact, there exist two different exchangeable sites for potential lead uptake within TiP-001. One is the negatively charged sulfonic groups bound on the host material D-001, and the other is TiP nanoparticles coated onto D-001. It is understandable that lead sorption by the host cation exchanger D-001 is a pH-dependent process due to the ion-exchange nature [10]. As for TiP, our previous study revealed that only half of the exchangeable hydrogen ion in amorphous TiP is accessible for ion exchange under neutral or acidic solution pH, and the rest is available only for the alkaline solution [16]. Therefore, lead uptake by TiP-001 can be realized according to the following two processes: Ti(HPO4 )2 + R–SO3 H +
1 2
1 2
Pb2+ TiPb0.5 H(PO4 )2 + H+ ,
Pb2+ R–SO3 Pb0.5 + H+ .
(1) (2)
In addition, negligible lead uptake at pH less than 0.50 suggests that the exhausted TiP-001 might be regenerated by strongly acidic solution, which was further demonstrated in the desorption experiments mentioned below. 3.3. Effect of Ca2+ on sorption
Fig. 2. Comparison of pH-titration curves of D-001 and TiP-001 using 0.100 M NaOH solution at 303 K.
Conversely, for a strong-acid cation exchanger, all the hydrogen ions in D-001 particles are readily released for ion-exchange with Na+ and then neutralization by the added OH− . The ion-exchange capacity of TiP-001 determined experimentally (3.00 meq/g) is less
Taken into account the fact that innocuous cations always coexist at high levels in waters and industrial waste water, we next tested their competitive effect on lead uptake onto TiP-001 by selecting Ca2+ as a competitive cation. D-001 was also involved here for comparison purpose. As shown in Fig. 4, the lead removal efficiency onto TiP-001 was slightly influenced by the added competing cations even at several orders higher than the target metal, whereas that onto D-001 was decreased dramatically under the otherwise identical conditions. To quantify the selectivity of both
456
K. Jia et al. / Journal of Colloid and Interface Science 331 (2009) 453–457
Fig. 4. Effect of calcium ion on lead uptake by TiP-001 and D-001 at 303 K (initial lead concentration 0.250 mmol/L; S/L ratio 2.50 g/L; pH 4.5–5.0).
Fig. 5. Sorption isotherm of lead ion onto TiP-001 at 303 K (TiP-001 50.0 mg, solution 100 mL, pH 4.5–5.0).
Table 1 Effect of Ca2+ on the distribution coefficients (K d ) of lead sorption onto TiP-001 and D-001 at 303 K. Metals
Sorbent
Ca2+ /Pb2+
TiP-001 D-001
K d (L/g) at different initial Ca2+ /M2+ ratios in solution 2
4
8
16
32
64
128
256
512
328 135
409 116
65.7 92
4.79 27.2
2.30 1.72
1.39 0.99
0.71 0.31
0.38 0.11
0.28 0.02
sorbents, the distribution ratio K d (in L/g) was determined by the following equation [19]:
K d = (C 0 − C e )/C e V /m,
(3)
where C 0 is the initial concentration of the solute, C e is the solute concentration in equilibrium, V is the volume of the solution, and m is the mass of the sorbent. In general, K d values (Table 1) of TiP-001 are larger than D-001 particularly at high Ca2+ /Pb2+ ratios, which further verifies its more preferable sorption toward lead ion. Note that such preferable sorption is not only attributed to specific sorption affinity between TiP and lead ion [16], but also to the immobilized sulfonic acid groups bound to D-001 matrix, which is expected to greatly enhance lead permeation and preconcentration prior to its effective sequestration by TiP nanoparticles. Such significant effect is so-called Donnan membrane effect, and its detailed explanation can be referred elsewhere [17–20]. 3.4. Sorption isotherms and kinetics Lead adsorption isotherm by TiP-001 was performed at 303 K and the results are correlated by the semi-empirical Freundlich model 1/n
qe = K F C e
,
(4)
where qe is adsorption capacity in equilibrium, and K F and n are constants to be determined. Results in Fig. 5 indicated that lead removal by TiP-001 can be represented by the Freundlich model well. Fig. 6 presents the plots of lead uptake versus contact time for TiP-001 and D-001, respectively. As expected, faster sorption kinetics of D-001 than TiP-001 was observed, which is possibly because of pore blockage as a result of TiP dispersion. However, the contact time of about 300 min is long enough for TiP-001 to achieve the sorption equilibrium. Kinetic data for both sorbents were then represented by the pseudo-second-order model [27]
Fig. 6. Sorption kinetics of lead ion onto D-001 and TiP-001 at 303 K (initial lead concentration 0.500 mmol/L, TiP-001 200 mg, solution 1000 mL). Table 2 The pseudo-second-order kinetic parameters for lead sorption onto TiP-001 and D001. Sorbents
k2
R2
TiP-001 D-001
0.0383 0.0778
0.992 0.998
t qt
=
1 k2 qe2
+
1 qe
t,
(5)
where qe and qt are the lead amount loaded (mg/g) at equilibrium and at time t (min), respectively, k2 is the pseudo-second-order rate constant (g/(mg min)). Higher correlation coefficients and the calculated qe values (Table 2) close to the experimental data indicate that lead uptake onto both TiP-001 and D-001 can be approximated favorably by the pseudo-second-order model. 3.5. Continuous batch sorption–regeneration cycles We were also interested in the integrity of TiP-001 during multiple cycles since any useful sorbent material must be robust to several cycles of sorption and regeneration. We next carried out
K. Jia et al. / Journal of Colloid and Interface Science 331 (2009) 453–457
457
(BV) of HCl solution with the desorption efficiency larger than 95%. As referred to Section 3.5, it is believed that TiP-001 can be employed for repeated use without any significant capacity loss. 4. Summary
Fig. 7. Results of continuous batch sorption–regeneration cycles at 303 K (sorption: 0.20-g TiP-001, 100 mL solution containing 100 mg/L Pb2+ ; regeneration: 10.0 mL 2.0 M HCl solution).
A novel hybrid sorbent TiP-001 was prepared by impregnating titanium phosphate (TiP) nanoparticles onto a strongly acidic cation exchanger D-001. Lead sorption onto TiP-001 was found to be pH-dependent due to the ion exchange mechanism. Compared to D-001, TiP-001 exhibits more preferable lead sorption even when other competing cations like Ca2+ coexists at much higher levels in solution. Fixed-bed column sorption indicated that lead sorption on TiP-001 could result in a conspicuous decrease of this toxic metal from 0.50 to below 0.01 mg/L, which is the drinking water standard recommended by WHO. Moreover, the exhausted TiP-001 beads are amenable to an efficient regeneration by 2.0 M HCl solution for repeated use without any significant capacity loss. All the results demonstrated that TiP-001 is a potential sorbent for enhanced trace lead removal from contaminated waters. Acknowledgments This research was supported by Program for New Century Excellent Talents in University (NCET07-0421), Engineering Center of Water Treatment and Environment Remediation, Ministry of Education (WTWER 0705), and the Scientific Research Foundation of Graduate School of Jiangsu Province (CX08B-144Z). References
Fig. 8. Comparison of breakthrough curves of lead sorption onto TiP-001 and D-001 during two separate fixed-bed column runs at 303 K.
5 cycles of lead sorption onto TiP-001 particles followed by regeneration with 2.0 M HCl solution. The results of the cycle runs summarized in Fig. 7 indicate that TiP-001 is effective and recyclable for lead removal at least for 5 cycles. 3.6. Fixed-bed column sorption and regeneration Fig. 8 illustrated an effluent history of separate fixed-bed columns packed with TiP-001 or D-001 for feeding solutions containing lead ion and competing cations (Na+ , Ca2+ , and Mg2+ ). As expected, lead broke through much earlier on D-001 due to its relatively poor selectivity towards lead ion. On the contrary, satisfactory breakthrough results were observed for TiP-001 under otherwise identical conditions. Note that TiP-001 retention resulted in a dramatic lead decrease from 0.50 to less than 0.01 mg/L, which is the allowance of drinking water recommended by WHO. Afterwards, we regenerated the exhausted TiP-001 column by 2.0 M HCl solution. Results (data not shown) show that almost all the sorbed lead were effectively rinsed by about 10 bed volumes
[1] F. Qin, B. Wen, X.Q. Shan, Y.N. Xie, T. Liu, S.Z. Zhang, S.U. Khan, Environ. Pollut. 144 (2006) 669. [2] J.O. Nriagu, J.M. Pacyna, Nature 333 (1988) 134. [3] X. Bosch, Science 609 (2003) 1. [4] US EPA, Edition of the drinking water standards and health advisories, EPA 822-R-06-013, 2006. [5] China MH (Ministry of Health), Standards for drinking water quality, GB574985, 1985. [6] China MH (Ministry of Health), Standards for drinking water quality, GB 57492006, 2006. [7] M. Imamoglu, O. Tekir, Desalination 228 (2008) 108. [8] J.P. Ruparelia, S.P. Duttagupta, A.K. Chatterjee, S. Mukherji, Desalination 232 (2008) 145. [9] D.G. Strawn, A.M. Scheidegger, D.L. Sparks, Environ. Sci. Technol. 32 (1998) 2596. [10] R.S. Juang, S.H. Lin, T.Y. Wang, Chemosphere 53 (2003) 1221. [11] S.E. Bailey, T.J. Olin, R.M. Bricka, D.D. Adrian, Water Res. 33 (1999) 2469. [12] R.H. Crist, J.R. Martin, D.R. Crist, Environ. Sci. Technol. 36 (2002) 1485. [13] A. Naidja, C. Liu, P.M. Huang, J. Colloid Interface Sci. 251 (2002) 46. [14] Z. Reddad, C. Gerente, Y. Andres, P. Le Cloirec, Environ. Sci. Technol. 36 (2002) 2067. [15] F. Helfferich, Ion Exchange, McGraw–Hill, New York, 1962. [16] K. Jia, B.C. Pan, Q.R. Zhang, W.M. Zhang, P.J. Jiang, C.H. Hong, B.J. Pan, Q.X. Zhang, J. Colloid Interface Sci. 318 (2008) 160. [17] L. Cumbal, A.K. SenGupta, Environ. Sci. Technol. 39 (2005) 6508. [18] B.C. Pan, Q.R. Zhang, W.M. Zhang, B.J. Pan, W. Du, L. Lv, Q.J. Zhang, Z.W. Xu, Q.X. Zhang, J. Colloid Interface Sci. 310 (2007) 99. [19] Q.R. Zhang, B.C. Pan, W.M. Zhang, B.J. Pan, K. Jia, Q.X. Zhang, Environ. Sci. Technol. 42 (2008) 4140. [20] P. Puttamraju, A.K. Sengupta, Ind. Eng. Chem. Res. 45 (2006) 7737. [21] B.C. Pan, Q.R. Zhang, Q.J. Zhang, W. Du, W.M. Zhang, Q.J. Zhang, Q.X. Zhang, Chinese Patent, 2008, ZL 2006 1 0041365.4. [22] F. Yu, T.L. Yu, Spectrosc. Spect. Anal. 20 (2000) 898. [23] J.T. Mayo, C. Yavuz, S. Yean, L. Cong, H. Shipley, W. Yu, J. Falkner, A. Kan, M. Tomson, V.L. Colvin, Sci. Technol. Adv. Mater. 8 (2007) 71. [24] G.V. Lowry, K.M. Johnson, Environ. Sci. Technol. 38 (2004) 5208. [25] N. Savage, M.S. Diallo, J. Nanopart. Res. 7 (2005) 331. [26] C. Trobajo, S.A. Khainakov, A. Espina, J.R. Garcia, Chem. Mater. 12 (2000) 1787. [27] Y.S. Ho, J. Hazard. Mater. 136 (2006) 681.
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Impregnating titanium phosphate nanoparticles onto a porous cation exchanger for enhanced lead removal from waters Kun Jia a , Bingcai Pan a,∗ , Lu Lv a,∗ , Qingrui Zhang a , Xiaoshu Wang b , Bingjun Pan a , Weiming Zhang a a b
State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing 210093, PR China Modern Analysis Center, Nanjing University, Nanjing 210093, PR China
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 2 November 2008 Accepted 28 November 2008 Available online 6 December 2008 Keywords: Titanium phosphate Hybrid sorbent Cation exchanger Lead removal Nanocomposite
Titanium phosphate (TiP) exhibits preferable sorption toward lead ion in the presence of competing calcium ions at high levels, however, it is present as fine or ultrafine particles and cannot be directly employed in fixed-bed or any flow-through systems due to the excessive pressure drop and poor mechanical strength. In the present study a new hybrid sorbent TiP-001 was fabricated by impregnating titanium phosphate (TiP) nanoparticles onto a strongly acidic cation exchanger D-001 for enhanced lead removal from waters. D-001 was selected as a host material mainly because of the Donnan membrane effect resulting from the immobilized sulfonic acid groups bound on the exchanger matrix, which would enhance permeation of the target metal cation prior to effective sequestration. TiP-001 was characterized by transmission electron micrograph (TEM), X-ray diffraction (XRD), and pH-titration. Batch and column sorption onto TiP-001 was assayed to evaluate its performance as compared to the host exchanger D001. Lead sorption onto TiP-001 is a pH-dependent process due to the ion-exchange nature, and its sorption kinetics follows the pseudo-second-order model well. Compared to D-001, TiP-001 displays highly selective lead sorption in the presence of competing calcium cations at concentration of several orders higher than the target metal. Fixed-bed sorption of a synthetic feeding solution indicates that lead retention by TiP-001 results in a conspicuous decrease of this toxic metal from 0.50 to below 0.010 mg/L (drinking water standard recommended by WHO). Moreover, its feasible regeneration by dilute HCl solution also favors TiP-001 to be a feasible sorbent for enhanced lead removal from water. © 2008 Elsevier Inc. All rights reserved.
1. Introduction Heavy metals are generally considered as a threat toward human’s health and ecosystems because of their potentially high toxicity. Unlike organic pollutants, heavy metals do not undergo biological degradation and tend to accumulate in the organisms, thereby eventually enter the food chains [1]. Lead, copper, and cadmium belong to the group of serious hazardous heavy metals. It is believed that exposure to these toxic metals even at trace level is still a risk to human beings [2,3] and thus, more stringent environmental legislations have been established to restrict the maximum contaminant level (MCL) of heavy metals in waters [4]. For instance, the MCL of lead in natural or drinking water in China was regulated from 0.05 [5] to 0.01 mg/L [6], and in USA the maximum contaminant level goal (MCLG) of lead was set as zero [4]. Obviously, it is of particular significance to propose highly efficient processes to trap these toxic metals in contaminated waters to near-to-zero level.
*
Corresponding authors. Fax: +86 25 8370 7304. E-mail addresses: [email protected] (B. Pan), [email protected] (L. Lv).
0021-9797/$ – see front matter doi:10.1016/j.jcis.2008.11.068
©
2008 Elsevier Inc. All rights reserved.
Among various techniques applied for heavy metals removal from water, adsorption and ion exchange are the widely used options. Unfortunately, the traditional materials employed for both techniques, namely activated carbon [7,8], activated alumina [9], ion exchangers [8,10], and many low-cost materials [11–14], display little or insufficient specific sorption affinity toward toxic metals. Consequently, they cannot enhance heavy metals removal to meet these new and stringent regulations. As a family of inorganic cation exchangers, titanium phosphate [Ti(HPO4 )2 , hereafter denoted TiP] is generally used for sorption of alkaline or alkaline earth cations [15]. It is extremely insoluble in water and offers high capacity, fast kinetics, and is superior to the organic exchangers in terms of thermal stability and resistance to radiation and abrasion [15]. In our previous study [16], titanium phosphate was found to be a highly selective sorbent toward lead ion even in the presence of competing cations (Na+ , Ca2+ , and Mg2+ ) at greater concentration. Such sorption preference possibly mainly results from the synergetic effect of electrostatic interaction and formation of inner-sphere complexes [16]. However, TiP is present as fine or ultrafine particles and cannot be employed in fixed-bed or any other flow-through systems
454
K. Jia et al. / Journal of Colloid and Interface Science 331 (2009) 453–457
due to the excessive pressure drop and poor mechanical rigidity. As a continuation of our earlier study [16], the main objective of the current study is to fabricate a hybrid sorbent by impregnating TiP onto a polystyrenesulfone cation exchanger D-001 to overcome the above-mentioned technical bottleneck. D-001 was selected as a support material mainly because of the Donnan membrane effect exerted by the non-diffusible negatively charged sulfonic acid groups bound on the exchanger matrix [17–20], which is expected to enhance permeation and preconcentration of the trace target metal ion [18–20]. Effect of solution pH on sorption, sorption kinetics, competitive sorption, and fixed-bed sorption tests were involved herein to evaluate the performance of the resulting sorbent.
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). 1.0 M HNO3 solution was used to adjust solution pH throughout the experiment. One milliliter solution at various time intervals was sampled from the flasks to determine sorption kinetics. Lead uptake was calculated by conducting a mass balance before and after the test. Note that all the batch runs were performed in duplicate for data analysis, and detailed experimental conditions are available in the related figures or tables.
2. Materials and methods
Column experiments were carried out with a glass column (14 mm diameter and 120 mm length) equipped with a water bath to maintain a constant temperature. Ten milliliters of a desired sorbent was packed within the column before operation. A Lange-580 pump (Baoding, China) was used to ensure a constant flow rate. After sorption 2.0 M HCl solution was used as regenerant of the exhausted sorbent. The hydrodynamic conditions for sorption are as follows: the superficial liquid velocity (SLV), 0.70 m/h; the empty bed contact time (EBCT), 6 min, and those for regeneration are: SLV, 0.18 m/h and EBCT, 24 min.
2.1. Materials All chemicals used in the study are of analytical grade or purer, and were purchased from Nanjing Zhongdong Reagent Station. Lead nitrate was used as the source of lead ion during the experiment. D-001, a polystyrenesulfone cation exchanger (in H+ -type) with total capacity of 4.30 meq/g and cross-linking density around 8%, was kindly provided by Zhenguang Resin Co., China. It was obtained in spherical bead forms of size ranging from 0.6 to 1.0 mm. N2 adsorption–desorption test onto D-001 at 77 K indicated that most of its inner pores (larger than 95% of pore volume) range from 2–60 nm and its average pore size is 31.4 nm. Prior to use, it was subjected to flushing by deionized water to remove the residue impurities until neutral pH and then vacuum-desiccated at 343 K for 24 h until reaching the constant weight.
2.5. Fixed-bed column sorption and regeneration
2.6. Analysis
The hybrid sorbent TiP-001 was fabricated according to a slightly modified proprietary technique [21]. In brief, TiCl4 , a precursor of TiP, was firstly dispersed into the inner pores of D-001 by evaporating the TiCl4 /D-001 mixture. Afterwards, the TiCl4 -loaded D-001 beads were mixed with H3 PO4 solution to deposit TiP onto the inner surface of the D-001 beads as
Lead concentration in solution was obtained by atomic absorption spectroscope (Z-8100, HITACHI) except when its content was less than 1.0 mg/L, which was determined by atomic fluorescence spectrophotometer (AFS) with an online reducing unit (AF-610A, China) with NaBH4 and HCl solution [16,22]. The impregnated TiP onto D-001 was observed with a transmission electron micrograph (Hitachi model H-800, Japan). Speciation of TiP in TiP-001 beads was analyzed with an X-ray diffraction analysis instrument (XTRA, Switzerland). XPS analysis of the sorbent was performed with a spectrometer (ESCALAB-2, Great British) equipped with MgK α Xray source (1253.6 eV protons). All the binding energies were referenced to the C1s peak at 288.75 eV to compensate for the surface charging effects.
TiCl4 + H3 PO4 → Ti(HPO4 )2 ↓ + HCl.
3. Results and discussion
2.2. Sorbent preparation
The TiP-loaded D-001 beads were then rinsed and thermally treated at 343 K for 12 h, and we obtained the hybrid sorbent TiP001. It was then characterized by transmission electron micrograph (TEM), X-ray diffraction (XRD), and pH-titration. 2.3. pH-titration Portions (500 mg) of D-001 or TiP-001 were mixed with 100 mL of 0.10 M NaCl. This mixture was kept for 4 h and titrated against 0.100 M NaOH solution. The solution pH was recorded after each addition of 1.0 mL of the titrant till the pH became constant. Based on the pH values before and after the exchange process, the milliequivalents (meq) of OH− ion consumed were determined. Milliequivalents of OH− ions consumed by a given sorbent were then plotted against the corresponding pH values to obtain the pHtitration curves [15,16]. 2.4. Batch sorption experiments Batch sorption tests were carried out using traditional bottlepoint technique. To start the experiment, Varying amount of TiP001 or D-001 beads and lead solution were combined in 250 mL glass bottles. Calcium ion was introduced as the competing species by dissolving desired amount of Ca(NO3 )2 into the solution. The
3.1. Characterization of TiP-001 TiP impregnated within D-001 was 15.2% in mass according to its content variation before and after loading. The Ti/P ratio in sorbent phase determined as 1:2 by XPS analysis ensured its formula Ti(HPO4 )2 . As indicated by the TEM image (Fig. 1a) of TiP-001, TiP was dispersed into the inner surface of D-001 as nanoparticles of size around 10 nm. Note that nanosized inorganic particles are expected to display larger accessible surface areas and stronger activity than bulk ones [23–25]. The X-ray diffraction spectra (Fig. 1b) of the TiP-001 beads implied that TiP impregnated within D-001 is amorphous in nature [16,26]. TiP loading results in a dramatic decrease in the pore volume of the spherical beads from 0.225 to 0.083 cm3 /g. Similarly, the average pore diameter also decreases from 31.4 to 21.8 nm. However, BET surface area increases slightly from 25.1 to 27.8 m2 /g possibly due to the inclusion of the TiP nanoparticles of high surface area. A steeper pH-titration curve of D-001 than TiP-001 (Fig. 2) was mainly attributed to the specific ion-exchange property of amorphous TiP with the sorbent. This is because the acid groups within TiP are weakly dissociated and it is reluctant to exchange its H+ ion for Na+ . As a result, the hydrogen ion within TiP is released in a stepwise manner and the ion-exchange remains incomplete [16].
K. Jia et al. / Journal of Colloid and Interface Science 331 (2009) 453–457
455
(a)
Fig. 3. Effect of solution pH on lead uptake by TiP-001 at 303 K (initial lead concentration 1.0 mmol/L, S/L ratio 0.50 g/L, solution 100 mL).
than the calculated value (4.07 meq/g) at neutral pH, implying that part of the sulfonic acid groups or TiP within TiP-001 are inaccessible for lead uptake, which possibly results from pore blockage during TiP dispersion. 3.2. Effect of solution pH on sorption
(b) Fig. 1. Characterization of the as-prepared hybrid sorbent TiP-001. (a) TEM (the dark spots are TiP nanoparticles while the white and gray regions represent the background of the support material D-001); (b) XRD spectra.
Effect of solution pH on lead uptake by TiP-001 was examined and the results were presented in Fig. 3. Generally speaking, higher solution pH (under acidic or neutral conditions) is more favorable for lead uptake onto TiP-001. The specific pH-dependent trend can be explained by the ion-exchange mechanism between solution and TiP-001. In fact, there exist two different exchangeable sites for potential lead uptake within TiP-001. One is the negatively charged sulfonic groups bound on the host material D-001, and the other is TiP nanoparticles coated onto D-001. It is understandable that lead sorption by the host cation exchanger D-001 is a pH-dependent process due to the ion-exchange nature [10]. As for TiP, our previous study revealed that only half of the exchangeable hydrogen ion in amorphous TiP is accessible for ion exchange under neutral or acidic solution pH, and the rest is available only for the alkaline solution [16]. Therefore, lead uptake by TiP-001 can be realized according to the following two processes: Ti(HPO4 )2 + R–SO3 H +
1 2
1 2
Pb2+ TiPb0.5 H(PO4 )2 + H+ ,
Pb2+ R–SO3 Pb0.5 + H+ .
(1) (2)
In addition, negligible lead uptake at pH less than 0.50 suggests that the exhausted TiP-001 might be regenerated by strongly acidic solution, which was further demonstrated in the desorption experiments mentioned below. 3.3. Effect of Ca2+ on sorption
Fig. 2. Comparison of pH-titration curves of D-001 and TiP-001 using 0.100 M NaOH solution at 303 K.
Conversely, for a strong-acid cation exchanger, all the hydrogen ions in D-001 particles are readily released for ion-exchange with Na+ and then neutralization by the added OH− . The ion-exchange capacity of TiP-001 determined experimentally (3.00 meq/g) is less
Taken into account the fact that innocuous cations always coexist at high levels in waters and industrial waste water, we next tested their competitive effect on lead uptake onto TiP-001 by selecting Ca2+ as a competitive cation. D-001 was also involved here for comparison purpose. As shown in Fig. 4, the lead removal efficiency onto TiP-001 was slightly influenced by the added competing cations even at several orders higher than the target metal, whereas that onto D-001 was decreased dramatically under the otherwise identical conditions. To quantify the selectivity of both
456
K. Jia et al. / Journal of Colloid and Interface Science 331 (2009) 453–457
Fig. 4. Effect of calcium ion on lead uptake by TiP-001 and D-001 at 303 K (initial lead concentration 0.250 mmol/L; S/L ratio 2.50 g/L; pH 4.5–5.0).
Fig. 5. Sorption isotherm of lead ion onto TiP-001 at 303 K (TiP-001 50.0 mg, solution 100 mL, pH 4.5–5.0).
Table 1 Effect of Ca2+ on the distribution coefficients (K d ) of lead sorption onto TiP-001 and D-001 at 303 K. Metals
Sorbent
Ca2+ /Pb2+
TiP-001 D-001
K d (L/g) at different initial Ca2+ /M2+ ratios in solution 2
4
8
16
32
64
128
256
512
328 135
409 116
65.7 92
4.79 27.2
2.30 1.72
1.39 0.99
0.71 0.31
0.38 0.11
0.28 0.02
sorbents, the distribution ratio K d (in L/g) was determined by the following equation [19]:
K d = (C 0 − C e )/C e V /m,
(3)
where C 0 is the initial concentration of the solute, C e is the solute concentration in equilibrium, V is the volume of the solution, and m is the mass of the sorbent. In general, K d values (Table 1) of TiP-001 are larger than D-001 particularly at high Ca2+ /Pb2+ ratios, which further verifies its more preferable sorption toward lead ion. Note that such preferable sorption is not only attributed to specific sorption affinity between TiP and lead ion [16], but also to the immobilized sulfonic acid groups bound to D-001 matrix, which is expected to greatly enhance lead permeation and preconcentration prior to its effective sequestration by TiP nanoparticles. Such significant effect is so-called Donnan membrane effect, and its detailed explanation can be referred elsewhere [17–20]. 3.4. Sorption isotherms and kinetics Lead adsorption isotherm by TiP-001 was performed at 303 K and the results are correlated by the semi-empirical Freundlich model 1/n
qe = K F C e
,
(4)
where qe is adsorption capacity in equilibrium, and K F and n are constants to be determined. Results in Fig. 5 indicated that lead removal by TiP-001 can be represented by the Freundlich model well. Fig. 6 presents the plots of lead uptake versus contact time for TiP-001 and D-001, respectively. As expected, faster sorption kinetics of D-001 than TiP-001 was observed, which is possibly because of pore blockage as a result of TiP dispersion. However, the contact time of about 300 min is long enough for TiP-001 to achieve the sorption equilibrium. Kinetic data for both sorbents were then represented by the pseudo-second-order model [27]
Fig. 6. Sorption kinetics of lead ion onto D-001 and TiP-001 at 303 K (initial lead concentration 0.500 mmol/L, TiP-001 200 mg, solution 1000 mL). Table 2 The pseudo-second-order kinetic parameters for lead sorption onto TiP-001 and D001. Sorbents
k2
R2
TiP-001 D-001
0.0383 0.0778
0.992 0.998
t qt
=
1 k2 qe2
+
1 qe
t,
(5)
where qe and qt are the lead amount loaded (mg/g) at equilibrium and at time t (min), respectively, k2 is the pseudo-second-order rate constant (g/(mg min)). Higher correlation coefficients and the calculated qe values (Table 2) close to the experimental data indicate that lead uptake onto both TiP-001 and D-001 can be approximated favorably by the pseudo-second-order model. 3.5. Continuous batch sorption–regeneration cycles We were also interested in the integrity of TiP-001 during multiple cycles since any useful sorbent material must be robust to several cycles of sorption and regeneration. We next carried out
K. Jia et al. / Journal of Colloid and Interface Science 331 (2009) 453–457
457
(BV) of HCl solution with the desorption efficiency larger than 95%. As referred to Section 3.5, it is believed that TiP-001 can be employed for repeated use without any significant capacity loss. 4. Summary
Fig. 7. Results of continuous batch sorption–regeneration cycles at 303 K (sorption: 0.20-g TiP-001, 100 mL solution containing 100 mg/L Pb2+ ; regeneration: 10.0 mL 2.0 M HCl solution).
A novel hybrid sorbent TiP-001 was prepared by impregnating titanium phosphate (TiP) nanoparticles onto a strongly acidic cation exchanger D-001. Lead sorption onto TiP-001 was found to be pH-dependent due to the ion exchange mechanism. Compared to D-001, TiP-001 exhibits more preferable lead sorption even when other competing cations like Ca2+ coexists at much higher levels in solution. Fixed-bed column sorption indicated that lead sorption on TiP-001 could result in a conspicuous decrease of this toxic metal from 0.50 to below 0.01 mg/L, which is the drinking water standard recommended by WHO. Moreover, the exhausted TiP-001 beads are amenable to an efficient regeneration by 2.0 M HCl solution for repeated use without any significant capacity loss. All the results demonstrated that TiP-001 is a potential sorbent for enhanced trace lead removal from contaminated waters. Acknowledgments This research was supported by Program for New Century Excellent Talents in University (NCET07-0421), Engineering Center of Water Treatment and Environment Remediation, Ministry of Education (WTWER 0705), and the Scientific Research Foundation of Graduate School of Jiangsu Province (CX08B-144Z). References
Fig. 8. Comparison of breakthrough curves of lead sorption onto TiP-001 and D-001 during two separate fixed-bed column runs at 303 K.
5 cycles of lead sorption onto TiP-001 particles followed by regeneration with 2.0 M HCl solution. The results of the cycle runs summarized in Fig. 7 indicate that TiP-001 is effective and recyclable for lead removal at least for 5 cycles. 3.6. Fixed-bed column sorption and regeneration Fig. 8 illustrated an effluent history of separate fixed-bed columns packed with TiP-001 or D-001 for feeding solutions containing lead ion and competing cations (Na+ , Ca2+ , and Mg2+ ). As expected, lead broke through much earlier on D-001 due to its relatively poor selectivity towards lead ion. On the contrary, satisfactory breakthrough results were observed for TiP-001 under otherwise identical conditions. Note that TiP-001 retention resulted in a dramatic lead decrease from 0.50 to less than 0.01 mg/L, which is the allowance of drinking water recommended by WHO. Afterwards, we regenerated the exhausted TiP-001 column by 2.0 M HCl solution. Results (data not shown) show that almost all the sorbed lead were effectively rinsed by about 10 bed volumes
[1] F. Qin, B. Wen, X.Q. Shan, Y.N. Xie, T. Liu, S.Z. Zhang, S.U. Khan, Environ. Pollut. 144 (2006) 669. [2] J.O. Nriagu, J.M. Pacyna, Nature 333 (1988) 134. [3] X. Bosch, Science 609 (2003) 1. [4] US EPA, Edition of the drinking water standards and health advisories, EPA 822-R-06-013, 2006. [5] China MH (Ministry of Health), Standards for drinking water quality, GB574985, 1985. [6] China MH (Ministry of Health), Standards for drinking water quality, GB 57492006, 2006. [7] M. Imamoglu, O. Tekir, Desalination 228 (2008) 108. [8] J.P. Ruparelia, S.P. Duttagupta, A.K. Chatterjee, S. Mukherji, Desalination 232 (2008) 145. [9] D.G. Strawn, A.M. Scheidegger, D.L. Sparks, Environ. Sci. Technol. 32 (1998) 2596. [10] R.S. Juang, S.H. Lin, T.Y. Wang, Chemosphere 53 (2003) 1221. [11] S.E. Bailey, T.J. Olin, R.M. Bricka, D.D. Adrian, Water Res. 33 (1999) 2469. [12] R.H. Crist, J.R. Martin, D.R. Crist, Environ. Sci. Technol. 36 (2002) 1485. [13] A. Naidja, C. Liu, P.M. Huang, J. Colloid Interface Sci. 251 (2002) 46. [14] Z. Reddad, C. Gerente, Y. Andres, P. Le Cloirec, Environ. Sci. Technol. 36 (2002) 2067. [15] F. Helfferich, Ion Exchange, McGraw–Hill, New York, 1962. [16] K. Jia, B.C. Pan, Q.R. Zhang, W.M. Zhang, P.J. Jiang, C.H. Hong, B.J. Pan, Q.X. Zhang, J. Colloid Interface Sci. 318 (2008) 160. [17] L. Cumbal, A.K. SenGupta, Environ. Sci. Technol. 39 (2005) 6508. [18] B.C. Pan, Q.R. Zhang, W.M. Zhang, B.J. Pan, W. Du, L. Lv, Q.J. Zhang, Z.W. Xu, Q.X. Zhang, J. Colloid Interface Sci. 310 (2007) 99. [19] Q.R. Zhang, B.C. Pan, W.M. Zhang, B.J. Pan, K. Jia, Q.X. Zhang, Environ. Sci. Technol. 42 (2008) 4140. [20] P. Puttamraju, A.K. Sengupta, Ind. Eng. Chem. Res. 45 (2006) 7737. [21] B.C. Pan, Q.R. Zhang, Q.J. Zhang, W. Du, W.M. Zhang, Q.J. Zhang, Q.X. Zhang, Chinese Patent, 2008, ZL 2006 1 0041365.4. [22] F. Yu, T.L. Yu, Spectrosc. Spect. Anal. 20 (2000) 898. [23] J.T. Mayo, C. Yavuz, S. Yean, L. Cong, H. Shipley, W. Yu, J. Falkner, A. Kan, M. Tomson, V.L. Colvin, Sci. Technol. Adv. Mater. 8 (2007) 71. [24] G.V. Lowry, K.M. Johnson, Environ. Sci. Technol. 38 (2004) 5208. [25] N. Savage, M.S. Diallo, J. Nanopart. Res. 7 (2005) 331. [26] C. Trobajo, S.A. Khainakov, A. Espina, J.R. Garcia, Chem. Mater. 12 (2000) 1787. [27] Y.S. Ho, J. Hazard. Mater. 136 (2006) 681.