Fabrication of polymer-supported nanosized hydrous manganese dioxide (HMO) for enhanced lead removal from waters

Science of the Total Environment 407 (2009) 5471–5477

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Science of the Total Environment j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s c i t o t e n v

Fabrication of polymer-supported nanosized hydrous manganese dioxide (HMO) for enhanced lead removal from waters Qing Su a, Bingcai Pan a,⁎, Bingjun Pan a, Qingrui Zhang a, Weiming Zhang a, Lu Lv a, Xiaoshu Wang b, Jun Wu a, Quanxing 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

Article history: Received 18 April 2009 Received in revised form 27 May 2009 Accepted 29 June 2009 Available online 28 July 2009 Keywords: Hydrous manganese dioxide Polymeric resin Hybrid sorbent Lead Selective removal Heavy metals

a b s t r a c t In the current study, a new hybrid adsorbent HMO-001 was fabricated by impregnating nanosized hydrous manganese dioxide (HMO) onto a porous polystyrene cation exchanger resin (D-001) for enhanced lead removal from aqueous media. D-001 was selected as a support material mainly because of the potential Donnan membrane effect exerted by the immobilized negatively charged sulfonic acid groups bound to the polymeric matrix, which would result in preconcentration and permeation enhancement of lead ions prior to their effective sequestration by the impregnated HMO. HMO-001 was characterized by scanning electron micrograph (SEM), transmission electron micrograph (TEM), and X-ray diffraction (XRD). Lead adsorption onto HMO-001 was dependent upon solution pH due to the ion-exchange nature, and it can be represented by the Freundlich isotherm model and pseudo-first order kinetic model well. The maximum capacity of HMO-001 toward lead ion was about 395 mg/g. As compared to D-001, HMO-001 exhibited highly selective lead retention from waters in the presence of competing Ca2+, Mg2+, and Na+ at much greater levels than the target toxic metal. Fixed-bed column adsorption of a simulated water indicated that lead retention on HMO-001 resulted in a conspicuous decrease of this toxic metal from 1 mg/L to below 0.01 mg/L (the drinking water standard recommended by WHO). The exhausted adsorbent particles are amenable to efficient regeneration by the binary NaAc–HAc solution for repeated use without any significant capacity loss. All the results validated the feasibility of HMO-001 for highly effective removal of lead from contaminated waters. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Exposure to toxic metals even at trace levels is believed to be a risk to humanity (Nriagu and Pacyna, 1988; Bosch, 2003) and now, more stringent environmental regulations have been established to restrict the maximum contaminant level (MCL) of heavy metals in waters (US EPA, 2006). For instance, the MCL of lead in natural or drinking water in China was decreased from 0.05 mg/L (China MH, 1985) to 0.01 mg/L (China MH, 2006), and in USA the maximum contaminant level goal (MCLG) of lead was set as zero (US EPA, 2006). Thus, it is of particular significance to propose highly efficient techniques to trap toxic metals from the contaminated waters. Adsorption is one of the most widely used processes for toxic metals removal from contaminated waters. Traditional adsorbents for toxic metals include activated carbon (Imamoglu and Tekir, 2008), activated alumina (Strawn et al., 1998), ion exchangers (Juang et al., 2003), as well as many low-cost materials (Kurniawan et al., 2006). Unfortunately, these adsorbents usually trap heavy metals through nonspecific electrostatic interaction and thus display little or insufficient specific adsorption ⁎ Corresponding author. Tel.: +86 25 8368 5736; fax: +86 25 8370 7304. E-mail address: [email protected] (B. Pan). 0048-9697/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2009.06.045

affinity toward the target metals (Juang et al., 2003; Kang et al., 2004). Consequently they cannot trap the toxic substances to meet the standard regulated by new stringent regulations particularly in the presence of other competing ions (e.g. Ca2+, Mg2+, and Na+). Thus, it necessitates the development of new specific adsorbents for selective removal of toxic metals from waters. In the past decades many hydrous metal oxides, namely, hydrous Fe(III) (Xu et al., 2007), Al(III) (Trivedi and Axe, 1999), Zr(IV) (Mishra et al., 1996), and Mn(IV) oxides (Nelson et al., 2002; Tripathy et al., 2006), have been exploited as specific sorbents for toxic metal cations. Nevertheless, these hydrous metal oxides are usually present as fine or ultrafine particles (Savage and Diallo, 2005; Mayo et al., 2007), and cannot be employed for direct use in fixed-bed or any flow-through systems due to the excessive pressure drop and poor mechanical strength. To overcome the technical bottlenecks, hybrid sorbents were then designed by impregnating these metal oxides onto the conventional porous materials, namely, alginate(Min and Hering, 1998), activated carbon (Gu et al., 2005), zeolite (Hana et al., 2006), diatomite (Jang et al., 2006), cellulose (Brandao and Galembeck, 1990; Guo and Chen, 2005), and porous polymer (Yuchi et al., 2003). The host materials improved permeability in flow-through systems without influencing the adsorption behaviors of the metal oxides and

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thus, these resulting hybrid sorbents exhibit potential application in heavy metals retention. More recently, porous polymeric anion/cation exchangers were employed as more ideal alternatives to other host materials for metal oxides loading (Lenoble et al., 2004a,b; Cumbal and SenGupta, 2005; Zhang et al., 2008a). It is on the basis of the satisfactory mechanical strength of polymeric matrix, as well as the potential Donnan membrane effect caused by the immobilized charged groups bound to the polymeric matrix, where target metal ions would be subjected to preconcentration and permeation enhancement prior to effective sequestration by oxide particles (Cumbal and SenGupta, 2005; Zhang et al., 2008a,b). As for hydrous Mn(IV) oxides (HMOs), they have been impregnated onto polystyrene anion exchangers for anionic arsenic removal from waters (Lenoble et al., 2004a,b). To the best of our knowledge, similar polymerbased hybrid sorbents incorporated with HMOs were not well explored for efficient removal of heavy metal cations. In the current study, we fabricated a new hybrid adsorbent by impregnating nanosized hydrous manganese dioxide (HMO) within a porous polystyrene cation exchanger D-001. Lead, one of the toxic metals listed among the priority pollutants regulated by US EPA, was selected as a representative contaminant because of its ubiquity in water and toxicity to human health, such as serious lesions caused in the central nervous system and even permanent damage particularly for children (Moreira et al., 2001). The influence of solution pH, adsorption kinetics, competitive adsorption behaviors, and column experiments were performed to evaluate the newly developed adsorbent for lead removal from waters. 2. Materials and methods 2.1. Materials All chemicals are of analytical grade or more pure and were purchased from Shanghai Reagent Station (Shanghai, China). D-001, a polystyrenesulfone cation exchanger with total capacity of 4.30 meq/g and cross-linking density of 8 %, was kindly provided by Zhenguang Resin Co., China. Results of N2 adsorption onto D-001 at 77 K indicated that most of its inner pores (larger than 95% of pore volume) range from 2 to 60 nm and its average pore size is 25.7 nm. It was sieved in spherical beads with particle size from 0.6 to 1.0 mm. Prior to use, D-001 was subjected to flushing with the deionized water to remove the residue impurities until neutral pH (6.8 to 7.2), and then vacuum-desiccated at 348 K for 24 h until reaching a constant weight. 2.2. Preparation of HMO-001 The hybrid sorbent HMO-001 was fabricated according to a proprietary technique proposed by our laboratory (Pan et al., 2007a). In brief, its preparation procedure consists of the following three steps: Step 1: Mn(II) loading onto a polymeric cation exchanger D-001. 2R


− þ SO3 Na

2+

+ Mn

⇄ðR


− 2+ SO3 Þ2 Mn

þ

+ 2Na




2þ

+ NaOCl + NaOH→R
SO3 Na + HMOðsÞ + NaCl

2.3. Batch adsorption experiments 50.0 mg of a given adsorbent was introduced into 250-mL Erlenmeyer flasks containing solution of known Pb(II) concentration, except for being noted. The desired contents of competing cations such as Na(I), Mg(II), and Ca(II) were introduced into solution by dissolving their corresponding nitrates. 0.50-M HNO3 solution was used to adjust the solution pH throughout the experiment when necessary. The final solution volume was determined as 100 mL. The flasks were then transferred to a Model G-25 incubator shaker with thermostat (New Brunswick Scientific Co. Inc.) and shaken at 200 rpm for 24 h. The time was deemed sufficient to ensure apparent equilibrium as determined by preliminary kinetic tests (data not shown). For the kinetic determination, the initial solution volume was 1000 mL and 1-mL aliquot was sampled at various time intervals to determine adsorption 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. 2.4. Fixed-bed column adsorption and regeneration Column experiments were carried out with a polyethylene column (12 mm diameter and 130 mm length) equipped with a water bath to maintain a constant temperature. Five milliliter of a desired adsorbent was packed within the column before operation. A Lange-580 pump (Baoding, China) was used to ensure a constant flow rate. After adsorption a binary NaAc–HAc solution (8% NaAc and 10% HAc in mass, pH 4.5) was used as the regenerant of the exhausted sorbent. The hydrodynamic conditions including the superficial liquid velocity (SLV) and the empty bed contact time (EBCT) were described in related figures. 2.5. Analyses Lead analyses of samples were usually carried out by atom absorption spectroscope (Thermal Co. U.S.). When its content is less than 1 mg/L, it was determined by atom fluorescence spectrophotometer (AFS) with an online reducing unit (AF-610A,China) with NaBH4 and HCl solution. Amount of HMO loaded onto D-001 was determined by analyzing Mn(IV) content according to the oxidation– reduction titration method reported elsewhere (Ma et al., 2001). The loaded HMO particles were observed with a scanning electron microscopy (LEO 1530VP, Germany) and transmission electron microscopy (Hitachi Model H-800, Japan). The hybrid adsorbent was also subjected to an X-ray diffraction analysis (XTRA, Switzerland). 3. Results and discussion

ð1Þ

Step 2: Oxidation of the loaded Mn(II) as hydrous manganese dioxide (HMO). Here NaOCl was used as the oxidant and the reaction can be simplified as ðR
SO3 Þ2 Mn

water. Afterwards, they were dried under vacuum at 50°C for 12 h, and we obtained the hybrid sorbent HMO-001.

ð2Þ

As expected, the HMO particles were then dispersed within the inner surface of D-001. Step 3: Thermal treatment. The resulting composite material was washed first with 0.1 M HCl to neutralize the residual alkali and then rinsed with double-distilled

3.1. Characterization of HMO-001 The resulting hybrid adsorbent HMO-001 was well characterized, and HMO loaded within D-001 was about 7.33% in Mn mass. The successful loading of HMO particles was further demonstrated by comparing SEM images of D-001 and HMO-001 (Fig. 1). Moreover, TEM of HMO-001 indicates that HMO nanoparticles or nano-rods were loaded onto the inner surface of D-001. Note that nanosized metal oxides usually display larger accessible surface areas and stronger activity than the bulk ones (Cumbal and SenGupta, 2005; Savage and Diallo, 2005; Mayo et al., 2007). The X-ray diffraction pattern of the HMO-001 beads (Fig. 2) implies that most HMO impregnated within D-001 is amorphous, though some wide absorption peaks were observed in the pattern (Parida et al., 1981).

Q. Su et al. / Science of the Total Environment 407 (2009) 5471–5477

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Fig. 1. Scanning electron micrographs of (a) D-001, (b) HMO-001, (c) TEM of HMO-001, and (d) schematic illustration of HMO-001.

3.2. Effect of pH on lead retention Effect of solution pH on lead removal by HMO-001 was examined and the results are presented in Fig. 3. Obviously, lead uptake is increased with solution pH from 1 to 5.3, namely, higher solution pH is more favorable for lead removal from waters to HMO-001. The pHdependent trend could be reasonably explained by the ion-exchange mechanism between solution and HMO-001. As indicated by the schematic illustration of HMO-001 (Fig. 1d), there exist two types of active sites for possible lead adsorption, i.e., the negatively charged sulfonate groups bound to the polymeric matrix and HMO particles. It is understandable that lead uptake by the host cation exchanger D-001 is a

Fig. 2. XRD spectra of HMO-001 prepared in the current study.

pH-dependent process due to the ion-exchange nature. As for HMO, it also undergoes ion exchange with heavy metal cations from aqueous solution as: (Kanungo and Parida, 1984; Tamura et al., 1996) 2+


Mn
OH + Pb


Mn
ðOHÞ2 + Pb

þ

⇄−Mn
O
Pb + H

2+

þ

ð3Þ þ

⇄−Mn
ðOÞ2
Pb + 2H

ð4Þ

Obviously, higher acidity is not favorable for lead adsorption onto HMO according to the above equations, which is consistent with the results observed in Fig. 3.

Fig. 3. Effect of solution pH on the uptake of lead ions onto HMO-001 at 298 K.

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3.3. Effect of competing cations As some naturally occurring cations such as Na+, Ca2+, and Mg2+ may reach high levels in natural waters or industrial wastewater, and these cations always display competitive adsorption toward the target

Table 1 Lead distribution coefficients (Kd) for HMO-001 and D-001 in the presence of other competing metal cations. Competing metals

Sorbent

Ca2+

HMO-001 D001 HMO-001 D001 HMO-001 D001

2+

Mg

Na+

Kd L/g at different initial M2+/Pb2+ ratios in solution 10

50

100

150

200

5.46 2.58 8.23 5.23 941 295

3.08 0.75 3.37 1.43 180 59.5

2.81 0.23 3.34 1.22 54.8 25.7

2.77 0.10 2.55 0.66 19.9 9.24

2.71 0.05 2.40 0.63 13.5 5.97

heavy metals, we next tested the effect of these competing cations on lead uptake onto HMO-001. D-001 was employed for comparison purpose. As shown in Fig. 4, the lead removal efficiency onto HMO-001 was slightly influenced by the added competing cations even at several orders of magnitude higher than the target metal, whereas that onto D001 decreased greatly under otherwise identical conditions. To quantify the selectivity of both adsorbents, the distribution ratio Kd (L/g) was then determined (Pan et al., 2007b): Kd =

mmol of heavy metals=g sorbent mmol of heavy metals=L solution

ð5Þ

Table 1 lists the Kd values of HMO-001 and D-001 toward Pb2+ in the presence of other competing cations. Despite the competing effect of other cations on lead removal, a substantially larger Kd value of HMO-001 than D-001 indicates that HMO-001 favors more selective removal of lead than D-001. Of particular note is that competing capacities of the added cations yield the sequence as Ca2+ N Mg2+ ≫ Na+. Such a sequence can be explained by different preferences of both adsorbents for these cations. In general, divalent cations are adsorbed preferably over the monovalent ones, and divalent cations with lower hydration energies are adsorbed preferably over those with higher hydration energies (He and Huang, 1985). The Gibbs free energies of hydration are −1425 kJ/mol for Pb2+, −1505 kJ/mol for Ca2+, and −1830 kJ/mol for Mg2+ (Marcus, 1991). This explains their different competing capacities with Pb2+. All the above results demonstrated that HMO-001 exhibits more preferential adsorption than D-001 toward lead over these competing cations. It is mainly attributed to two aspects, the host material D-001 and HMO particles. Our recent studies proved that the non-diffusible negatively charged sulfonic acid groups bound to D-001 matrix would greatly enhance permeation and preconcentration of the target metal cations from solution to inner surface of the polymeric phase (Pan et al., 2007b; Zhang et al., 2008b), and in turn create favorable conditions for

Fig. 4. Effect of (a) Ca2+, (b) Mg2+, and (c) Na+ on lead retention by HMO-001 and D001 at pH 4.0 and 298 K.

Fig. 5. Adsorption isotherm of Pb2+ onto HMO-001 particles at 298 K (pH 4.4 ± 0.1).

Q. Su et al. / Science of the Total Environment 407 (2009) 5471–5477 Table 2 Isotherm constants for lead uptake onto HMO-001 at 298 K. Langmuir model

Table 4 Kinetic parameters for lead uptake onto HMO-001 and D-001 at 298 K.

Freundlich model 2

KL (L/mg)

R

21.0

0.849

5475

Sorbent

k1, 10− 2 min− 1

HMO-001 D-001

1.29 2.01

2

Kf

n

R

325

24.6

0.983

R2

qe, mg/g Calculated

Experimental

374 408

361 402

0.994 0.992

Table 3 Comparison of the maximum adsorption capacity of various adsorbents toward lead ion.

observed. It is possibly because HMO dispersion onto D-001 would partly block pore regions. Kinetic data for both sorbents were then represented by the pseudo-first-order model (Dogan et al., 2009)

Sorbent

Qmax (mg/g)

Ref.

Indonesian peat 8-hydroxy quinolineimmobilized bentonite Calcium alginate Lewatit CNP 80 Zeolite Chitosan functionalized with xanthate Activated carbon Clay/poly(methoxyethyl) acrylamide (PMEA) composite Manganese oxide-coated carbon nanotubes HMO-001

~ 80 ~ 143

Balasubramanian et al., 2009 Ozcan et al., 2009

339 420 530 322 13.5 81.2

Lagoa and Rodrigues, 2007 Pehlivan and Altun, 2007 Qiu and Zheng, 2009 Chauhan and Sankararamakrishnan, 2008 Imamoglu and Tekir, 2008 Solener et al., 2008

78.7

Wang et al., 2007

395

This study

logðqe −qt Þ = log qe −

k1 t 2:303

ð8Þ

where qe and qt are the amount adsorbed in equilibrium and at time t, respectively, k1 is the adsorption kinetic constants. High correlation coefficients and the calculated qe values (Table 4) close to the experimental data indicated that lead uptake onto both sorbents can be approximated favorably by the pseudo-first-order model. Higher k1 value of D-001 than HMO-001 may also result from the pore blockage caused by the HMO impregnation. 3.5. Fixed-bed column experiments

lead removal by HMO particles. Moreover, lead ion can be selectively sequestrated by HMO particles through inner-sphere complexation of Pb(II) on HMO (Villalobos et al., 2005; Xu et al., 2006). 3.4. Adsorption isotherms and kinetics Adsorption isotherm experiments of Pb(II) onto HMO-001 were performed at 298 K, and the results are illustrated in Fig. 5 and correlated by the Freundlich and Langmuir models (Parida et al., 2004), n

qe = Kf Ce

ð6Þ

1 1 1 = + qe KL qm Ce qm

ð7Þ

where Kf and n are the Freundlich constants to be determined, qm (mg/ g) is the maximal adsorption capacity, and KL (L/g) is a binding constant. The results in Fig. 5 and Table 2 indicated that lead removal by HMO-001 can be represented by the Freundlich model more reasonably. The maximum adsorption capacity of HMO-001 at 298 K is about 395 mg/g, which is also compared to other sorbents for lead ion (Table 3). Fig. 6 presents the plot of lead uptake versus contact time for HMO001 and D-001, and a faster kinetics of D-001 than HMO-001 was

Fig. 6. Lead adsorption kinetics onto D-001 and HMO-001 at 298 K.

Fig. 7 illustrated a complete effluent history of a separate fixed-bed column packed with either HMO-001 or D-001 for a feeding solution containing lead ion and competing cations (Na+, Ca2+, and Mg2+). Lead broke through quickly on D-001 due to its poor selectivity towards lead. On the contrary, satisfactory breakthrough results were observed for HMO-001 even when the competing cations were about 200 times more than the lead ion in mass concentration. The lead concentration in the effluent decreased dramatically from 1.0 to less than 0.01 mg/L, which is the allowance of drinking water recommended by WHO. Afterwards, we regenerated the exhausted HMO-001 column by a binary NaAc–HAc solution and the results are depicted in Fig. 8. As observed, most of the adsorbed lead can be effectively rinsed by using 5 BV regenerants. Also, we carried out a continuous adsorption– regeneration cycle runs for HMD-001 bed. Note that, to save the operation time, the lead concentration in the feeding solution increased from 1 mg/L to 200 mg/L. One can notice in Fig. 9 that the breakthrough curves from 2nd and 5th runs were overlapped, which further demonstrated that HMO-001 can be employed for repeated use without no significant capacity loss after regeneration by NaAc–

Fig. 7. Breakthrough curves of lead adsorption from synthetic waters onto HMO-001 and D-001 at 298 K.

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References

Fig. 8. A column desorption history of lead preloaded by HMO-001 at 298 K.

Fig. 9. Continuous HMO-001 fixed-bed operation results by using synthetic water as the feeding solution at 298 K.

HAc solution. Note that the dynamic adsorption capacity of HMO-001 column for lead with competing cations (Na+, Ca2+, and Mg2+) was about 82.8 mg/mL HMO-001. 4. Conclusions Polymer-supported hydrous manganese dioxide (HMO-001) prepared in the current study was demonstrated to be a specific adsorbent for lead retention from water. Lead adsorption onto HMO001 was pH-dependent. Compared to a polystyrene cation exchanger D-001, HMO-001 exhibited more favorable lead adsorption in the presence of other competing cations Na+, Ca2+, and Mg2+ at greater levels. Fixed-bed column results showed that lead adsorption on HMO-001 could result in a conspicuous decrease of this toxic metal from 1.0 to below 0.01 mg/L. Moreover, the exhausted HMO-001 beads are amenable to an efficient regeneration by NaAc–HAc solution for repeated use without any significant capacity loss. Acknowledgments The authors gratefully acknowledge the financial support from the Program for New Century Excellent Talents in University of China (NCET07-0421), the Ministry of Water Resources (200808104), the Ministry of Education of China (200802840034), and Jiangsu Department of Science & Technology (BE2009669). In addition, we are indebted to two anonymous reviewers for their insightful comments.

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