Magnetic Hydrogels for Removal of Humic Acid from Aqueous Environment

Pinhua RAO School of Chemistry and Chemical Engineering

Shanghai University of Engineering Science, Shanghai 201620, China raopinhua@hotmail.com

Irene M. C. LO Department of Civil and Environmental Engineering, The Hong Kong University of Science and Technology, Hong

Kong, China

Abstract—Magnetic cationic hydrogels (MCH) were synthesized and its removal efficiency and mechanisms to humic acid (HA) from the aqueous environment were studied in this paper. MCH synthesized had a low swelling ratio and the strong magnetism, yielding extra potential for recycle and reuse of hydrogels. XRD analysis and dissolution experimental results indicated that γ-Fe2O3 was stably embedded into hydrogels. Batch studies indicated that the removal of HA by MCH was effective. Electrostatic adsorption was considered to be main removal mechanisms of HA. Solution environments with low pH and high ionic strength were unfavorable for HA removal. 10% NaCl solution was effective to regenerate MCH saturated by HA. MCH was found still to retain high adsorption capacity to HA after three cycles.

Keywords-Adsorption; Effect Factors; Humic Acid; Magnetic Hydrogels; Regeneration and Reuse

I. INTRODUCTION Humic acid (HA) is often found in aqueous environment

and recently has raised the public concern due to its effects on water quality[1-3]. HA itself is considered to be harmless. However, during the treatment and supply of drinking water, HA could cause numerous potential problems, including forming harmful disinfection by-products (DBPs) during disinfection, reducing water treatment efficiency during flocculation/coagulation, increasing the solubility of heavy metals due to its complexation with HA, and facilitating bacterial reproduction during drinking water distribution.

In order to minimize the negative effects of HA, many methods based on various principles have been developed to achieve HA removal from drinking water, such as coagulation, membrane separation, adsorption, etc [4-6]. Coagulation is a relative conventional method which is achieved by adding coagulants into aqueous environments. Recently, enhanced coagulation method, i.e., the addition of excess coagulants, was developed to enhance removal efficiency of HA. However, enhanced coagulation method was low effective in removing smaller molecules of HA and increased treatment cost and sludge production. Membrane separation is generally costly and the control of membrane fouling is a challenge faced for removal of HA. One promising method appears to be adsorption removal of HA from water due to its simplicity, reliability and safety. Various adsorbents (e.g., oxide, active

carbon, natural mineral, anion exchange resin etc.) and their effectiveness removing HA have been evaluated. Generally, effective separation and regeneration of adsorbents with high adsorption capacity and low cost are considered to be several important criteria for the selectivity of adsorbents.

On the other hand, hydrogel recently attracted much interest due to its many unique physicochemical properties and have been used for removal of organic pollutants and heavy metals from aqueous environments [7, 8]. Hydrogel is polymeric networks that can absorb large quantities of water. Functional groups in the hydrogel networks can be utilized for the removal of organic and/or inorganic pollutants. Hydrogel can also be modified by different materials to improve utilization properties. Magnetic hydrogel has recently been developed and showed a great superiority due to the combination of its magnetism with network property. Ozay et al.[9] indicated that magnetic hydrogel showed excellent property in the removal of heavy metal pollutants. However, few papers reported the effectiveness of magnetic hydrogel removing HA. In this paper, a magnetic cationic hydrogel (MCH) has been developed and its ability and mechanism in removing HA, together with its regeneration and reuse were evaluated.

II. MATERIALS AND METHODS

A. Materials and Chemicals Analytical (3-Acrylamidopropyl)trimethylammonium

chloride (APTMACl) (75 wt% solution in water), N,N’-methylenebisa-crylamide (MBA), N,N,N’,N’-tetramethylethylenediamine (TEMED), and ammonium persulfate (APS) were purchased from Aldrich Chemical Company, Inc. Commercially available HA obtained from Aldrich Chemical was used to represent NOM. HA stock solution was prepared by dissolving certain amount of HA into ultrapure water (>18.1 MΩ.cm) followed by filtering through 0.45-μm acetate cellulose membranes (ADVANTEC). Dissolved organic carbon (DOC) was employed to express the concentration of HA. Other chemicals used in this study were all reagent grade and obtained from Aldrich Chemical.

B. MCH Preparation MCH was synthesized by radical polymerization of

APMACl as monomers. Briefly, 0.05 g of MBA as cross-linker

This work was supported by Shanghai Municipal Natural Science Foundation (10ZR1412600) and the Hong Kong Research Grants Council under grant HKUST RGC 617309.

978-1-4244-4713-8/10/$25.00 ©2010 IEEE

was dissolved in 2.5 g of APMACl with the addition of 0.8 ml deionized (DI) water. After all the MBA was dissolved, 20μl of TEMED as accelerator and 0.2 g of nano γ-Fe2O3 with magnetism were then added into the matrix. When the matrix was homogenously mixed, initiator, 0.6 mL saturated potassium persulfate (KPS) solution, was added. The reaction proceeded at 600C water bath for 15 minutes. Afterwards, MCH generated was transferred into a glass bottle and immersed in deionized water for 5 days. Water was replaced every 8 hours to remove the unreacted species such as monomer, cross-linker, initiator and accelerator. Then, MCH was freeze-dried and ground into mesoporous particles with blender. MCH above was added into DI water to prepare MCH suspensions for batch study. Particle size distribution of MCH in the suspensions was measured using a laser diffraction particle size analyzer (LS-320, Beckman Coulter). Dissolution experiment of MCH was conducted using HCl or NaOH solutions (pH 2-12). A strong magnetism was observed for MCH and no high swelling was found after MCH was immersed in DI water.

C. Batch Experiments Experiments removing HA by MCH were conducted in 40-

mL glass vials. 20 ml of MCH suspensions and 20 ml of HA solution with desired pH and concentration were mixed and then were sealed using screw caps containing Teflon-lined rubber septa. All vials were shaken in end-over-end manner at 26 rpm and 23±1 0C. No pH adjustment was made during the process of experiments. After regular time, the caps of vials were opened and solutions were rapidly filtered through 0.45-μm membranes, followed by immediate measurement of pH and subsequent chemical analyses. The pH difference of filtrate and original solutions was found to be less than 0.3 unit for all experiments. HA concentrations in filtrate were determined by total organic carbon analyzer (TOC, Shimadzu 5000A) and UV-vis spectrophotometer (Ultrospec 4300 Pro) at a wavelength of 254nm.

Desorption experiments were conducted in 250-ml glass bottle. MCH firstly was adsorbed to saturation by HA solution of high concentration according to the procedure above. Then, MCH in HA solution was separated by filtration and the HA loaded MCH was taken for freeze-drying 48 hours. Afterwards, 0.2g of dried MCH with HA and 200 mL of 10% NaCl solutions were mixed in the bottle, followed by shaking in end-over-end manner at 26 rpm and 23±1 0C for 24 hours. (pre-experiment indicated that desorption might reach to equilibrium within 12 hours). Suspensions with MCH were filtered and HA concentration in filtrate was measured. MCH on the filter was washed for three times using DI water and then was freeze-dried again for reuse. All batch experiments were run in duplicate.

D. High-performance Size Exclusion Chromatography (HPSEC) Analysis

HPSEC was conducted using a high-performance LC (VP series, Shimadzu) equipped with a photodiode array UV-vis detector (SPD-M10A). A GPC column (100A 7u, Macrosphere) with a length of 300 mm and an inner diameter of 7.5 mm was used. The HPSEC mobile phase was a 0.004 M

phosphate buffer adjusted to an ionic strength of 0.1 M with NaCl at pH 6.8. The flow rate was 1.0 ml/min, and the sample injection volume was 25 µL.

III. RESULTS AND DISCUSSION

A. MCH Characterization

Fig. 1 Particle size distribution of MCH in suspensions

As shown in Fig. 1, the size of most of MCH particles ranges from 3 to 20 μm. Data analysis revealed that the average diameter of MCH was 10μm.

Fig. 2 XRD patterns of MCH (Cu Kα)

The XRD patterns of MCH are presented in Fig. 2. Peak positions correspond to those of γ-Fe2O3 (maghemite) reported in the reference [10]. Besides, Dissolution experiment of MCH confirmed that MCH was chemical stable in pH ranging from 2 to 12 (data not shown), indicating that γ-Fe2O3 has been stably embedded into hydrogels.

B. Dosage Effect and Removal Kinetics Fig.3 shows the effects of MCH dosage on the removal of

HA. 37.5 mg of MCH can achieve a complete removal to 3.74 mg of HA dissolved in 1 L water, indicating the strong ability of MCH removing HA. In drinking water treatment, the dosage of coagulants ranges from several to dozen of milligram per liter, which closes to MCH dosage used in this study. Also, MCH may be regenerated and reused, thus showing its great

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Fig. 3 Effects of MCH dosage on the removal of HA at pH 7.

Fig. 4 showed the removal kinetics of HA by MCH. About 70% of HA with 3.54 mg/L was removed within 30-minute reactions. Electrostatic adsorption could be main mechanism of HA removal by MCH, because HA carries negative charges at neutral pH which is pH range studied while MCH carries positive charges countered by chlorine ion. HA with strong exchange ability could substitute chlorine ion and adsorb onto MCH by electrostatic adsorption. However, due to netlike structure of MCH, HA need to diffuse into the pore of MCH depending on concentration gradient before adsorbed, which resulted in the relative long adsorption equilibrium time as observed in Fig.4.

Fig. 4 Kinetics of HA removal by MCH at pH 7. MCH concentration, 50

mg/L; Initial HA concentration, 3.74 mg/L

HPSEC was employed to study the preferential removal of of HA fraction of different relative molecular weights by MCH. As shown in Fig. 5, an obvious reduction of area under the curve was observed with the increase of reaction time, echoing the removal of HA observed in Fig. 4. Also, peak position shifted towards right, corresponding the longer retention time and indicating the mechanism that the fraction of HA with higher molecular weight was preferentially removed by MCH, which could be attributed that HA fraction with higher molecular weight has higher aromaticity.

Fig. 5 HPSEC profiles of residue HA after different reaction time. MCH

concentration, 50 mg/L; Initial HA concentration, 3.74 mg/L

C. Effect of Environmental Factors The effects of solution pH and background electrolyte concentration (i.e., ionic strength) on the removal of HA by MCH were studied. As observed in Fig. 6. Lower pH was unfavorable for removal of HA, which was different from the results of adsorption of HA on chitosan hydrogels [8, 11]. The adsorption of HA on hydrogels could be affected mainly by the following factors. i) charges carried by hydrogels; ii) charges carried by HA; iii) size and aromaticity of HA. Effects of pH on the adsorption of HA on hdyrogels depended on the factor playing a main role. Chitosan hydrogels carried variable charges with pH. Lower pH may facilitate protonation of amino groups in the chitosan, which could enhance the adsorption of HA by electrostatic interaction. However, MCH carries permanent positive charges within pH range studied. Change of solution pH mainly affected the properties of HA. At a lower pH, HA carried less negative charges and was easy to curl in solution, which could enlarger the size of HA, and thus inhibit HA to enter into the pores of MCH to some extent.

Fig. 6 Adsorption isotherms of HA on MCH at different pH. MCH concentration, 25 mg/L

HA=3.74mg/L

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Effects of background electrolyte concentration on the removal of HA by MCH are shown in Fig. 7. Concentration of NaCl was changed from 0.005M to 0.5M. Results showed, in the presence of 0.5M NaCl, the removal of HA was obviously inhibited compared to that in the presence of 0.005M NaCl, which could result from the competition of chlorine ion with HA for exchange sites.

Fig. 7 Effects of ionic strength on the removal of HA by MCH. MCH concentration, 25 mg/L

D. Regeneration and Reuse The reusability of adsorbent is very important as a cost-

effective process in water treatment. For the environmental sustainability of an adsorbent, its high regeneration capacity may enhance persuasion for economic feasibility. In order to regenerate and reuse MCH after adsorbing HA, 10% NaCl solution was selected as regeneration agent and three cyclic adsorption-desorption studies were carried out. As shown in Fig. 8, after first adsorption-desorption cycle, the adsorption capacity lost around 5%, which could be attributed that some HA molecules with large size were blocked in the pores of MCH. After consecutive three adsorption-desorption processes, over 85% recovery ratio was attained, indicating the high regeneration capacity of MCH.

Fig. 8 MCH regeneration and adsorption capacity during three

adsorption/desorption cycles.

REFERENCES [1] T. H. Boyer and P. C. Singer, “Bench-scale testing of a magnetic ion

exchange resin for removal of disinfection by-product precursors,” Wat. Res., 2005, 39, 1265-1276.

[2] H. Humbert, H. Gallard, H. Suty, and J. P. J. Croué, “Natural organic matter (NOM) and pesticides removal using a combination of ion exchange resin and powdered activated carbon (PAC),” Wat. Res., 2008, 42, 1635-1643.

[3] D. A. Fearing, J. Banks, S. Guyetand, C. M. Eroles, B. Jefferson, D. Wilson, et al., “Combination of ferric and MIEX for the treatment of a humic rich water,” Wat. Res., 2004, 38, 2551-2558.

[4] S. W. Krasner and G. L. Amy, “Jar-test evaluations of enhanced coagulation,” Journal of the American Water Works Association, 1995, 87, 93-107.

[5] S. Assemi, G. Newcombe, C. Hepplewhite, and R. Beckett, “Characterization of natural organic matter fraction separated by ultrafiltration using flow field-flow fractionation,” Wat. Res., 2004, 38, 1467-1476.

[6] J. Duan, F. Wilson, N. Graham, and J. H. Tay, ‘‘Adsorption of humic acid by powdered activated carbon in saline water conditions,’’ Desalination, 2002, 151, 53-66.

[7] M. A. Barakat and N. Sahiner, ‘‘Cationic hydrogels for toxic arsenate removal from aqueous environment,’’ J. Environ. Manage., 2008, 88, 955-961.

[8] W. L. Yan and R. Bai, ‘‘Adsorption of lead and humic acid on chitosan hydrogel beads,’’ Wat. Res., 2005, 39, 688-698.

[9] O. Ozay, S. Ekici, Y. Baran, N. Aktas, and N. Sahiner, “Removal of toxic metal ions with magnetic hydrogels,” Wat. Res., 2009, 43, 4403-4411.

[10] F. del Monte, M. P. Morales, D. Levy, A. Fernandez, M. Ocaña, A. Roig, E. Molins, K. O’Grady, and C. J. Serna, “Formation of γ-Fe2O3 isolated nanoparticles in a silica matrix,” Langmuir, 1997, 13, 3627-3634.

[11] X. Zhang and R. Bai, “Mechanisms and kinetics of humic acid adsorption onto chitosan-coated granules,” J. Colloid Interface Sci., 2003, 264, 30-38.

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