Synthesis and Application of Magnetic Hydrogel for Cr(VI) Removal from Contaminated Water

Abstract

Many magnetic adsorbents reported in the literature, such as iron oxides, for Cr(VI) removal have been found effective only in low pH environments. Moreover, the application of polymeric hydrogels on heavy metal removal has been hindered by difficulties in separation by filtration. In this study, a magnetic cationic hydrogel was synthesized for Cr(VI) removal from contaminated water, making use of the advantages of magnetic adsorbents and polymeric hydrogels. The magnetic hydrogel was produced by imbedding 10-nm γ-Fe₂O₃ nanoparticles into the polymeric matrix via radical polymerization. Characterization of the hydrogel was undertaken with Fourier transform infrared and vibrating sample magnetometer; swelling properties were tested and anionic adsorption capacity was evaluated. The magnetic hydrogel showed a superior Cr(VI) removal capacity compared to commercial products such as MIEX^®. Cr(VI) removal was independent of solution pH. Results show that Cr(VI) removal kinetics was improved drastically by grinding the bulk hydrogel into powder form. At relevant concentrations, common water anions (e.g., Cl⁻, SO₄²⁻, PO₄³⁻) and natural organic matter did not exhibit significant inhibition of Cr(VI) adsorption onto the hydrogel. Results of vibrating sample magnetometer indicate that the magnetic hydrogel can be easily separated from treatment systems. Regeneration of the magnetic hydrogel can be easily achieved by washing the Cr(VI)-loaded hydrogel with 0.5 M NaCl solution, with a recovery rate of about 90% of Cr(VI).

Introduction

In aquatic environments, chromium exists in two common oxidation states, Cr(III) and Cr(VI). Cr(VI) is present mainly in the form of oxyanions (i.e., CrO₄²⁻ and HCrO₄⁻). Removal of Cr(VI) from contaminated water is essential because it is a type A human carcinogen and highly toxic to organisms (Bartlett, 1991; USEPA, 2000). Industrial wastewater from stainless-steel production, electroplating, corrosion control systems, leather tanning, and wood preservation contains high concentration of Cr(VI) (Barnhart, 1997). Accidental spillage or leakage and improper storage and discharge from these industrial processes and former industrial sites (Eary and Davis, 2007; Mukhopadhyay et al., 2007; Dong et al., 2009) and other sources such as coal fly ash landfills (Kingston et al., 2005) and acid mine drainage (Wilkin and McNeil, 2003) have resulted in serious Cr(VI) contamination in soil, surface water, and groundwater (Mukhopadhyay et al., 2007).

Conventional methods for removing Cr(VI) from wastewater, such as chemical precipitation, involve Cr(VI) reduction under acidic conditions to Cr(III), a relatively environmentally benign form of chromium, followed by precipitation of Cr(III) at basic pHs. However, this process results in a massive production of waste sludge.

During the past decade, a lot of research attention has been directed to the removal of Cr(VI) from contaminated water via adsorption. In principle, adsorption cannot only remove contaminants but can also recover and recycle them into industrial processes (Singh and Tiwari, 1997). Various types of adsorbents have been explored for their effectiveness in this regard, including activated carbon (Selvi et al., 2001; Fang et al., 2007), biosorbents (Ansari and Fahim, 2007; Sen et al., 2007), metal oxides (Hu et al., 2005; Wang and Lo, 2009), and synthetic polymeric adsorbents (DeMarco et al., 2003; Mukhopadhyay et al., 2007; Barakat and Sahiner, 2008). The focus of synthetic polymeric adsorbents for Cr(VI) removal is mainly on anion-exchange resins so far (Rengaraj et al., 2001; Galán et al., 2005). The commonly used commercial resins in this regard are strong-base (i.e., polyacrylic) resins with a macroporous structure, which are typically used with chloride as the exchangeable ion. The advantages of the anion-exchange resins lies in their high Cr(VI) removal capacity and easy regeneration when compared with other adsorbents (Rivas et al., 2001; Kaşgöz, 2006).

Hydrogels, on the other hand, are another type of synthetic polymeric adsorbents, which have three-dimensional networks of hydrophilic polymers capable of imbibing large amount of water (Ozay et al., 2009). In essence, resins and hydrogels have a lot in common: both are polymer-based materials and remove contaminants via electrostatic attraction. However, unlike resins, which have rigid structures, hydrogels have flexible structures and can accommodate more water into their matrix than the resins (Barakat and Sahiner, 2008). With a proper design of a hydrogel, even higher Cr(VI) removal capacity can be achieved.

In the past few years, magnetic property of iron oxides has been utilized in the context of environmental remediation. They facilitate the separation of adsorbents from contaminated water (Hu et al., 2004, 2005; Wang and Lo, 2009), which is desirable because it overcomes many issues present in filtration, centrifugation, or gravitational separation, such as fouling problems and operation cost, and requires much less energy to achieve a given level of separation. However, adsorption onto iron oxides was found to be only effective in acidic environments, and their maximum Cr(VI) adsorption capacity was reported to be about 15–20 mg/g (Hu et al., 2004, 2005; Wang and Lo, 2009).

A combination of a synthetic polymeric resin and a magnetic property has been reported for a commercial product: magnetic ion exchange (MIEX^®) resin, marketed by Orica Watercare of Victoria, Australia. MIEX was developed specifically for the removal of dissolved organic carbon from raw drinking water. The iron oxide provides the resin with magnetic characteristics that facilitates its separation. A similar idea has been recently proposed (Ozay et al., 2009) for the removal of cationic heavy metals, such as Cd(II), Co(II), and Cr(III), with magnetic application of the hydrogel. However, oxyanions [e.g., Cr(VI)] removal by magnetic hydrogels has not been reported to the best of the authors' knowledge.

In this study, the magnetic cationic hydrogel was synthesized by polymerization of (3-acrylamidopropyl)trimethylammonium chloride (APTMCl) in the presence of 10-nm γ-Fe₂O₃ nanoparticles to provide a magnetic property for fast separation, with ammonium groups providing positively charged sites for anion exchange. The magnetic hydrogel was characterized with Fourier transform infrared (FTIR), vibrating sample magnetometer (VSM), and swelling tests. The adsorption capability of the synthesized hydrogel for Cr(VI) was evaluated. The factors possibly affecting Cr(VI) removal performance, including solution pH and the presence of other anions and humic acids (HA), were examined. The regeneration and reusability of the magnetic cationic hydrogel were also investigated.

Materials and Approaches

Chemicals

Potassium dichromate (K₂Cr₂O₇), APTMCl (75 wt% solution in water), N,N′-methylenebisacrylamide (MBA), N,N,N′,N′-tetramethylethylenediamine (TEMED), and potassium persulfate (KPS) were purchased from Aldrich and used as received. γ-Fe₂O₃ was laboratory made and evaluated as described by Wang and Lo (2009). Other chemicals such as NaCl, HCl, NaOH, FeCl₃ · 6H₂O, FeSO₄ · 7H₂O, Na₂SO₄, and Na₃PO₄ were of reagent grade and also obtained from Aldrich Chemical. Commercial Aldrich HA were used for the batch studies.

Synthesis of 10-nm γ-Fe₂O₃

In this study, 10 nm γ-Fe₂O₃ nanoparticles were synthesized and imbedded into the hydrogel to provide the hydrogel with magnetic properties. Ten-nanometer Fe₃O₄ nanoparticles were first synthesized using a method modified from Kang et al. (1996). Briefly, 6.1 g FeCl₃ · 6H₂O and 4.2 g FeSO₄ · 7H₂O were dissolved in 100 mL of ultrapure water. A total of 25 mL of 6.5 M NaOH was then slowly added and mixed with the above solution. The system was mixed for a further hour after the addition of NaOH was completed. The formed black precipitates were washed with ultrapure water several times with the assistance of an external magnetic field. This procedure leads to Fe₃O₄ nanoparticles with a size of around 10 nm. The 10-nm Fe₃O₄ nanoparticles were then oxidized in air in an oven at 150°C for 2 h to obtain the 10-nm γ-Fe₂O₃.

Synthesis of magnetic hydrogel

Bulk magnetic hydrogel was synthesized via radical polymerization in the presence of a redox initiator (Barakat and Sahiner, 2008). In the synthesis, APTMCl served as the polymeric monomer, MBA as the crosslinker, TEMED as the accelerator, and KPS as the redox initiator. A typical magnetic hydrogel synthesis is as follows: in a 20-mL clear glass vial, 0.05 g of MBA was dissolved in 0.8 g of ultrapure water containing 2.5 g of APTMCl monomer (75 wt% solution in water) in the presence of 0.02 mL TEMED. Then, 0.3 g of 10-nm γ-Fe₂O₃ was added to the above mixture and mixed well under magnetic stirring to obtain a homogenous mixture. About 0.6 mL of the KPS solution was then added. The hydrogel precursor had a monomer-to-water ratio of 57:43 by mass (not including the mass of the 10-nm γ-Fe₂O₃). Once the solution was well mixed (∼10 min), the 20-mL vial was then kept in a 60°C water bath. The polymerization reaction was allowed to proceed at 60°C under magnetic mixing for 10 min for the reaction to be completed. The vial containing the bulk magnetic hydrogel was then taken out of the water bath and allowed to cool down in air. The bulk magnetic hydrogel was then taken out of the vial and was subsequently immersed in 2.0 L of ultrapure water. The water was replaced six times every 12 h to get rid of initiators, unreacted monomers, crosslinkers, and accelerators. The dry bulk magnetic hydrogel was obtained by freeze-drying the water-rinsed hydrogel. The dry hydrogel was stored in a desiccator prior to use. The powder magnetic hydrogel was produced by grinding the as-prepared bulk magnetic hydrogel in a household blender (Philips Electronics; Fig. 1). For comparison, the nonmagnetic hydrogel was synthesized using the same procedure, except that no 10-nm γ-Fe₂O₃ was added during the synthesis.

FIG. 1.

Photographs of the synthesized magnetic hydrogel: (a) bulk form; (b) powder form separated by a permanent magnet; (c) household blender used for grinding the hydrogel in powder form.

Characterization of magnetic hydrogel

The particle size of the powder magnetic hydrogel was measured using a laser diffraction particle size analyzer (Beckman Coulter LS13 320). The magnetic properties of the magnetic hydrogel were determined by VSM (LakeShore 7037/9509-P). FTIR (Bruker IFS-66 V) spectroscopy was used to examine the existence of organic groups in the prepared hydrogel samples. Infrared spectra of diluted samples in KBr were recorded between 4,000 and 500 cm⁻¹.

The swelling testing for the magnetic hydrogels was carried out at room temperature in deionized (DI) water. Hydrogels in DI water were weighed after wiping with KimWipes to remove the excess water on the surface. Dry weights were determined by weighing samples after 48 h of freeze-drying.

Batch experiments

A chromium stock solution was prepared by dissolving a known quantity of K₂Cr₂O₇ in ultrapure water. Batch Cr(VI) adsorption studies were performed by mixing 0.1 g of the bulk or powder magnetic hydrogel with 40 mL of Cr(VI) solution, at different concentrations, in 40-mL glass vials end-over-end. All the adsorption experiments were carried out at a room temperature of 22°C ± 2°C and were performed in duplicate. The adsorbent was separated at the end of each test by using a hand-held permanent magnet and the Cr(VI) concentration in the supernatant was measured using a flame atomic absorption spectrometer (Varian 220FS) in all tests. Sample dilution was conducted before atomic absorption spectrometer measurement, where necessary.

Kinetics of Cr(VI) adsorption onto the hydrogel

The kinetics of Cr(VI) adsorption onto both the bulk magnetic hydrogel and powder magnetic hydrogel were studied. Specifically, 0.1 g of the bulk or powder magnetic hydrogel was mixed with 40 mL Cr(VI) solution at a concentration of 200 mg/L. The aqueous Cr(VI) concentrations were measured at the end of 20, 40, 60, 120, 180, and 240 min to determine the Cr(VI) adsorption kinetics. In both cases, it took <2 min to magnetically separate all the bulk and powder hydrogels from the aqueous solution. However, as the kinetics of Cr(VI) adsorption by the powder magnetic hydrogel was much faster than the bulk one, the powder magnetic hydrogel was chosen thereafter for subsequent studies, unless otherwise specified.

Effect of pH on Cr(VI) adsorption onto the powder magnetic hydrogel

The Cr(VI) adsorption onto the magnetic hydrogel was studied at four different pH values (3.0, 5.0, 7.0, 9.0) to investigate the dependence of Cr(VI) adsorption on the solution pH. Standard acid (0.1 M HCl) and standard base (0.1 M NaOH) solutions were used for pH adjustment. Based on the findings of the pH independence on Cr(VI) adsorption, further Cr(VI) adsorption studies were conducted only at the pH of the initial Cr(VI) solution (around 4.2), unless otherwise specified.

Comparison of Cr(VI) adsorption onto the magnetic and nonmagnetic hydrogels

For the purpose of determining the contribution of the imbedded 10-nm γ-Fe₂O₃ to the total Cr(VI) adsorption by the magnetic hydrogel, Cr(VI) adsorption onto the magnetic hydrogel was compared with that onto the nonmagnetic sample under the same conditions. Three initial Cr(VI) concentrations (25, 50, and 100 mg/L) were tested.

Cr(VI) adsorption isotherm

Batch experiments were conducted to determine Cr(VI) adsorption by mixing 0.1 g of hydrogel with 40 mL of Cr(VI) solution with varying Cr(VI) concentrations (5–700 mg/L) to reach Cr(VI) adsorption equilibrium. No effort was made to adjust the solution pH as no significant change in the final pH was observed.

Effects of background electrolytes and natural organic matter on Cr(VI) adsorption by hydrogel

The competition for background electrolytes and natural organic matter in Cr(VI) equilibrium adsorption by the hydrogel was studied in the copresence of Cl⁻, SO₄²⁻, PO₄³⁻, and HA. The initial concentrations of each electrolyte anion were 0.5, 1.0, 1.5, and 2.0 mM, and two different initial concentrations of HA (10 and 20 mg dissolved organic carbon/L) were tested for their effects on Cr(VI) removal. In all cases, the initial Cr(VI) concentration of 100 mg/L (1.92 mM) was used.

Regeneration and reuse of the hydrogel

To study the regenerability and reusability of the magnetic hydrogel as an adsorbent for Cr(VI) removal, experiments pertaining to the magnetic hydrogel regeneration and Cr(VI) readsorption were carried out in three consecutive adsorption–desorption cycles. For each cycle, 40 mL of 100 mg/L Cr(VI) solution was adsorbed first by 0.1 g of magnetic hydrogel to reach the adsorption equilibrium. The supernatant was then decanted with the assistance of a permanent magnet. The adsorbed Cr(VI) on the magnetic hydrogel was then desorbed with 40 mL of 0.5 M NaCl. After each cycle of adsorption/desorption, the magnetic hydrogel was thoroughly washed with ultrapure water to neutrality and then used for adsorption in the succeeding cycle.

Results and Discussion

Characterization of the magnetic hydrogel

FTIR spectroscopy for the characterization of hydrogel with and without magnetic application was undertaken (Fig. 2). Both types of hydrogel exhibited similar characteristic peaks over the spectrum ranging from 500 to 4,000 cm⁻¹, which indicates that the γ-Fe₂O₃ nanoparticle modification does not change the structure of the hydrogel. The expected peak at around 3,000 cm⁻¹ (2,936 cm⁻¹) was due to the asymmetric stretching of alkane groups (Mansur et al., 2004). Peaks absorbance at 1,647 and 1,546 cm⁻¹ were assigned to an amide I (C = O) and II (C–N) stretch, respectively, from both the monomer (APTMCl) and the crosslinker (MBA) (Guiney et al., 2009). C–N stretching was characterized with the peaks occurring in the range of 900–1,350 cm⁻¹. The peak at 966 cm⁻¹, however, was attributed to the bending vibration of the trimethyl ammonium group [–N⁺(CH₃)₃], the characteristic group in the monomer used (APTMCl) (Chen et al., 2009).

FIG. 2.

Fourier transform infrared characterization of the magnetic hydrogel with and without magnetism.

Cationic density/anion adsorption capacity (assuming the nitrogen atoms at the quaternary ammonium in the monomer are the sites for adsorption) was calculated based on the hydrogel recipe. As water content is excluded from the ingredients, the dry mass ratio between the monomer (APTMCl) and the total dry hydrogel was found to be 0.851 g monomer/g hydrogel. The cationic density was then calculated as 4.12 mmol/g hydrogel, where the charge density is only dependent on the molar mass of the monomer.

The synthesized bulk magnetic hydrogel took on the shape of its container (Fig. 1a), and because of the relatively high content of the crosslinker, the swelling ratio of magnetic hydrogels in water was around 12.5 ± 0.5 (calculated based on the mass ratio of the wet hydrogel weight in DI water to its original dry weight after freeze-drying), which is much less than the ratio of 25–40 reported in the literature (Barakat and Sahina, 2008; Ozay et al., 2009) (Fig. 1a and b). Figure 3 shows the hydrodynamic particle size distribution of the powder magnetic hydrogel, in which the average particle size was around 10 μm. The particle sizes followed a symmetrical normal distribution, which can be probably attributed to a consistent mechanical powderization.

FIG. 3.

Particle size distribution of the powder magnetic hydrogel.

Magnetic separation

Detailed characterization of the synthesized 10-nm γ-Fe₂O₃ was reported by Wang and Lo (2009). The reason for using 10-nm γ-Fe₂O₃ instead of Fe₃O₄ is to avoid chemical adsorption. The highest iron valency (+3) in γ-Fe₂O₃ ensures that there is no redox reaction taking place to reduce Cr(VI) to Cr(III), which would otherwise render irreversible Cr(VI) adsorption. In the case with Fe₃O₄, where part of the iron is in an oxidation state of + 2, chemical adsorption of Cr(VI) has been reported, in that the formation of precipitated Cr(III) on the adsorbent surfaces led to irreversible adsorption (Hu et al., 2004).

The magnetic properties of the hydrogel were quantitatively determined by VSM measurement and were also compared with that of the pure nano γ-Fe₂O₃ (data not shown). The magnetic hydrogel is superparamagnetic (zero moment without magnetic field application) because its magnetic property is due to addition of the nano γ-Fe₂O₃ during polymerization, which is also superparamagnetic. The magnitude of the saturation magnetization of the hydrogel is around 6 emu/g, whereas it is 32 emu/g for the 10-nm γ-Fe₂O₃. As the γ-Fe₂O₃ accounts for only about 13.5% of the total weight of the magnetic hydrogel after freeze-drying, it is not surprising that much less saturation magnetic moment was found for the hydrogel. The VSM measurement was based on dry weight, but the magnetic hydrogel works in the aqueous phase. The weight of the magnetic hydrogel increased to more than one log magnitude of its dry weight, according to the swelling ratio of 12.5. Therefore, with the same magnetic composition and the same magnetic field application, the magnetic moment would be much less than its original one. To quantitatively verify the feasibility of the magnetic collection ability under wet conditions, a simple test was conducted by mixing 0.1 g of dry weight of the magnetic hydrogel with 40 mL DI water in a 40-mL vial, and the separation of the powder magnetic hydrogel by a household permanent magnet was demonstrated in Fig. 1b. A complete solid-aqueous separation can be achieved within 2 min. Thus, the magnetic property of the magnetic hydrogel is still applicable for rapid separation.

Cr(VI) adsorption onto the magnetic hydrogel

Cr(VI) adsorption onto bulk and powder magnetic hydrogels

The results of the kinetics of Cr(VI) adsorption on the bulk magnetic hydrogel are presented in Fig. 4a. As shown, it took 2 h to reach equilibrium for Cr(VI) adsorption onto the bulk magnetic hydrogel. Although the Cr(VI) adsorption onto the bulk hydrogel for Cr(VI) removal is high [98% of the Cr(VI) removal in this case], its slow kinetics makes it cost-ineffective for application in a large scale. It has been reported that slow kinetics is attributed to long diffusion paths of the contaminant molecules in reaching their adsorption sites within the hydrogel (Bajpai, 2001; Guilherme et al., 2009). Thus, it is expected that the hydrogel with a much smaller particle size would solve the problem of slow kinetics of pollutant removal.

FIG. 4.

Kinetics of Cr(VI) removal by (a) bulk magnetic hydrogel and (b) powder magnetic hydrogel.

To this end, the powder magnetic hydrogel was made by grinding the bulk magnetic hydrogel using a household blender. To test the Cr(VI) removal kinetics of the powder magnetic hydrogel, the same batch of adsorption experiments were conducted as for the bulk magnetic hydrogel, except that the powder magnetic hydrogel was used. It turned out that the grinding of the magnetic hydrogel dramatically shortened the Cr(VI) removal time and also maintained its Cr(VI) adsorption capacity. Figure 4b presents the Cr(VI) removal kinetics by the powder magnetic hydrogel. As can be seen, it took only a couple of minutes for reaching Cr(VI) adsorption equilibrium, when compared with 2 h for the bulk magnetic hydrogel. Based on a recycling point of view, only about 1/60 of the equivalent amount of hydrogel would be consumed for the powder magnetic hydrogel to achieve the same amount of Cr(VI) removal, in comparison to that for the bulk magnetic hydrogel. Considering the same treatment period, this would very much reduce the operation cost in the treatment processes. For this reason, the powder magnetic hydrogel was used for the rest of the study, unless otherwise specified.

Effect of pH on Cr(VI) removal by the magnetic hydrogel

To examine the effect of pH on Cr(VI) removal by the magnetic hydrogel, Cr(VI) adsorption was conducted at four different pH values (3, 5, 7, and 9). The results show that Cr(VI) removal by the magnetic hydrogel is nearly independent of the solution pH (Fig. 5), because the trimethyl ammonium group of the magnetic hydrogel provides positive surface charges. It is evidenced by the supplementary experiment on the zeta potential measurement of the magnetic hydrogel in a pH range of 3–12, which shows about +48 mV at pH 2 to about +30 mV at pH 12 (data not shown). The Cr(VI) removal mechanism is likely due to anion exchange, that is, Cr(VI) (oxyanion) exchanged with Cl⁻, which was originally present as the charge neutralizer in the magnetic hydrogel. The pH may affect the speciation of Cr(VI) to a certain extent (Bradl et al., 2005), but, apparently, the variation of Cr(VI) species within the pH range (3–9) does not significantly affect the overall Cr(VI) removal.

FIG. 5.

Cr(VI) removal capacity by the magnetic hydrogel under different solution pH.

Comparison of Cr(VI) adsorption onto the magnetic and nonmagnetic hydrogels

It has been reported that 10-nm γ-Fe₂O₃ is an effective adsorbent for Cr(VI), and the adsorption takes place via out-sphere complexation (Wang and Lo, 2009). Therefore, in this study, special efforts were taken to identify the contribution of 10-nm γ-Fe₂O₃, which was incorporated within the matrix of the hydrogel, to the total Cr(VI) removal by the magnetic hydrogel. To this end, the amounts of Cr(VI) adsorbed onto the magnetic and nonmagnetic hydrogels were compared. Figure 6 presents the results of Cr(VI) adsorption onto both the magnetic and nonmagnetic hydrogels and, as shown, no significant differences can be observed.

FIG. 6.

Comparison of Cr(VI) adsorption onto magnetic and nonmagnetic hydrogels at three different initial Cr(VI) concentrations.

Cr(VI) adsorption onto γ-Fe₂O₃ is highly pH dependent, as evidenced by a sharp decrease in Cr(VI) adsorption capacities with increasing solution pH (Illés and Tombácz, 2006; Wang and Lo, 2009). It has been reported by many researchers that the adsorption of Cr(VI) onto Fe₂O₃ is significant only at pH <3. In this study, the pH of the Cr(VI) solution was determined to be >4.2 in all cases. In the pH ranging from 4 to 6, where the adsorption of Cr(VI) by the hydrogel was studied, Wang and Lo (2009) found that the Cr(VI) saturation adsorption capacity onto the mesoporous γ-Fe₂O₃ was around 7.5 mg/g. The amount of Fe₂O₃ present within the hydrogel was very small (0.14 g γ-Fe₂O₃ per gram of magnetic hydrogel). In such a case, the maximum contribution of Cr(VI) adsorption by γ-Fe₂O₃ could only be 0.14 g/g hydrogel ×7.5 mg/g = 1.05 mg/g hydrogel, in comparison with the experimental hydrogel adsorption capacity of Cr(VI) of about 200 mg/g (Fig. 7). Thus, the contribution of γ-Fe₂O₃ to the total Cr(VI) removal by the magnetic hydrogel is negligible. The role of γ-Fe₂O₃ in the magnetic hydrogel is only to provide a magnetic property for magnetic separation.

FIG. 7.

Isotherm of Cr(VI) adsorption onto the magnetic hydrogel fitting with Langmuir model.

Isotherm of Cr(VI) adsorption onto the magnetic hydrogel

Figure 7 shows the Cr(VI) adsorption isotherm of the magnetic hydrogel. The maximum Cr(VI) removal capacity was determined to be 205 mg/g by fitting an isotherm using the Langmuir model. Recalling that the calculated theoretical anion capacity of the hydrogel is 4.12 mmol/g hydrogel, and as the main Cr(VI) species is HCrO₄⁻ within the studied pH (4–6) and the concentration ranges (<100 mg/L), the Cr(VI) removal capacity of 205 mg/g was thus converted to be the same as 4.12 mmol/g, or 214 mg Cr(VI)/g hydrogel. It means that up to 96% of cationic sites in the hydrogel can be utilized for ion exchange in the Cr(VI) adsorption, which is highly efficient. The maximum Cr(VI) removal capacity of the magnetic hydrogel is more favorable than that of MIEX. According to Jha et al. (2006), the Cr(VI) saturation capacity at the same conditions was 93.3 mg/g. Therefore, the magnetic hydrogel is twice as effective as MIEX in terms of Cr(VI) removal.

Effects of common background electrolytes on Cr(VI) removal by the hydrogel

The effects of background electrolytes for Cr(VI) adsorption onto hydrogel was studied by the copresence of Cl⁻, SO₄²⁻, and PO₄³⁻, respectively. The results of adsorption experiments are presented in Fig. 8a. Even at the highest concentration of the anions used (2.0 mM), which was comparable with Cr(VI) concentration (1.92 mM) in solution, the reduction was statistically insignificant. Therefore, the inhibition from these anions for Cr(VI) adsorption onto the magnetic hydrogel is insignificant within the anion concentration range studied.

FIG. 8.

Effect of (a) background electrolytes and (b) natural organic matter on Cr(VI) adsorption onto the magnetic hydrogel.

In the presence of 10 and 20 mg/L of HA, the Cr(VI) adsorption was also not affected (Fig. 8b). This can be attributed to the high adsorption capacities of the magnetic hydrogel for both Cr(VI) and HA. Further studies are being conducted in investigating the effectiveness of magnetic hydrogel for HA removal.

Regeneration and reusability of the spent hydrogel

NaCl was used to regenerate the magnetic hydrogel. The selection of NaCl is based on the following reasons: first, treatment using NaCl returns the hydrogel to its original state (i.e., Cl⁻ saturated) in one step. Other treatments would require more than one step to achieve the same results. Second, NaCl is a cheap salt and has almost no hazardous effects. Third, NaCl has a much higher solubility than Na₂SO₄ and Na₃PO₄, and higher concentrations of desorption solution ready for use can easily be obtained.

Figure 9a presents the results of the magnetic hydrogel regeneration during three consecutive Cr(VI) adsorption–desorption cycles using 0.5 M NaCl as the regeneration solution. For the first two cycles, about 90% of Cr(VI) was recovered, whereas it was slightly decreased to 85% in the third cycle, indicating that the majority of the adsorbed Cr(VI) could be easily recovered. It should be noted that the recovered Cr(VI) in the NaCl solution may need further treatment prior to recycling and reuse in industrial processes. The results of Fig. 9b show that even after three adsorption and desorption cycles, the magnetic hydrogel maintained almost the same Cr(VI) removal capacities, indicating its reusability.

FIG. 9.

Regeneration and reusability of the magnetic hydrogel after three consecutive cycles of adsorption–desorption process: (a) Cr(VI) recovery and (b) Cr(VI) adsorption capacity.

Conclusion

In this study, the magnetic cationic hydrogel was synthesized and applied for the removal of Cr(VI) from contaminated water. The magnetic hydrogel exhibited a high Cr(VI) adsorption capacity. To achieve fast Cr(VI) adsorption kinetics, the bulk magnetic hydrogel was ground into powder form. Further, the magnetic hydrogel can be easily separated from the treatment system. The results show that the magnetic hydrogel can be regenerated while maintaining almost the same Cr(VI) adsorption capacity after three consecutive adsorption–desorption cycles. Therefore, the magnetic hydrogel synthesized in this study represents a reusable adsorbent for fast, convenient, and highly efficient removal of Cr(VI) from contaminated water.

Footnotes

Acknowledgment

This work was supported by the Hong Kong Research Grants Council under grant HKUST RGC 617309.

Author Disclosure Statement

The authors declare that no competing financial interests exist.

References

Ansari

, Fahim

N.K.

2007. Application of polypyrrole coated on wood sawdust for removal of Cr(VI) ion from aqueous solutions. React. Funct. Polym., 67:367.

Bajpai

S.K.

2001. Swelling–deswelling behavior of poly(acrylamide-co-maleic acid) hydrogels. J. Appl. Polym. Sci., 80:2782.

Barakat

M.A.

, Sahiner

2008. Cationic hydrogels for toxic arsenate removal from aqueous environment. J. Environ. Manag., 88:955.

Barnhart

1997. Occurrence, uses, and properties of chromium. Reg. Toxicol. Pharmacol., 26:S3.

Bartlett

R.J.

1991. Chromium cycling in soils and water: links, gaps, and methods. Environ. Health Persp., 92:17.

Bradl

, Kim

, Kramar

, Stuben

2005. Heavy Metals in the Environment: Origin Interaction and Remediation. Amsterdam: Elsevier Academic Press.

Chen

, Chen

, Wang

, Luo

, Wang

2009. Preparation, characterization, and blood compatibility of polylactide-based phospholipid polymer. J. Mater. Sci., 44:6317.

DeMarco

M.J.

, SenGupta

A.K.

, Greenleaf

J.E.

2003. Arsenic removal using a polymeric/inorganic hybrid sorbent. Water Res., 37:164.

Dong

, Zhao

, Hua

, Liu

, Gao

2009. Investigation of the potential mobility of Pb, Cd and Cr(VI) from moderately contaminated farmland soil to groundwater in Northeast, China. J. Hazard. Mater., 162:1261.

10.

Eary

L.E.

, Davis

2007. Geochemistry of an acidic chromium sulfate plume. Appl. Geochem., 22:357.

11.

Fang

, Gu

, Gang

, Liu

, Ilton

E.S.

, Deng

2007. Cr(VI) removal from aqueous solution by activated carbon coated with quaternized poly(4-vinylpyridine) Environ. Sci. Technol., 41:4748.

12.

Galán

, Castañeda

, Ortiz

2005. Removal and recovery of Cr(VI) from polluted ground waters: a comparative study of ion-exchange technologies. Water Res., 39:4317.

13.

Guilherme

M.R.

, Moia

T.A.

, Reis

A.V.

, Paulino

A.T.

, Rubira

A.F.

, Mattoso

L.H.C.

, Muniz

E.C.

, Tambourgi

E.B.

2009. Synthesis and water absorption transport mechanism of a pH-sensitive polymer network structured on vinyl-functionalized pectin. Biomacromolecules, 190:190.

14.

Guiney

L.M.

, Agnello

A.D.

, Thomas

J.C.

, Takatori

, Flynn

N.T.

2009. Thermoresponsive behavior of charged N-isopropylacrylamide-based hydrogels containing gold nanostructures. Colloid Polym. Sci., 287:601.

15.

, Chen

, Lo

I.M.C.

2005. Removal and recovery of Cr(VI) from wastewater by maghemite nanoparticles. Water Res., 39:4528.

16.

, Lo

I.M.C.

, Chen

2004. Removal of Cr(VI) by magnetite nanoparticle. Water Sci. Technol., 50:139.

17.

Illés

, Tombácz

2006. The effect of humic acid adsorption on pH-dependent surface charging and aggregation of magnetite nanoparticles. J. Colloid Interface Sci., 295:115.

18.

Jha

A.K.

, Bose

, Downey

J.P.

2006. Removal of As(V) and Cr(VI) ions from aqueous solution using a continuous, hybrid field-gradient magnetic separation device. Separat. Sci. Technol., 41:3297.

19.

Kang

Y.S.

, Rishud

, Rabolt

J.F.

, Stroeve

1996. Synthesis and characterization of nanoparticle-size Fe₃O₄ and g-Fe₂O₃ particles. Chem. Mater., 8:2209.

20.

Kaşgöz

2006. New sorbent hydrogels for removal of acidic dyes and metal ions from aqueous solutions. Polym. Bull., 56:517.

21.

Kingston

H.M.

, Cain

, Huo

, Mizanur Rahman

G.M.

2005. Determination and evaluation of hexavalent chromium in power plant coal combustion by-products and cost-effective environmental remediation solutions using acid mine drainage. J. Environ. Monit., 7:899.

22.

Mansur

H.S.

, Oréfice

R.L.

, Mansur

A.A.P.

2004. Characterization of poly(vinyl alcohol)/poly(ethylene glycol) hydrogels and PVA-derived hybrids by small-angle X-ray scattering and FTIR spectroscopy. Polymer, 45:7193.

23.

Mukhopadhyay

, Sundquist

, White

2007. Hydro-geochemical controls on removal of Cr(VI) from contaminated groundwater by anion exchange. Appl. Geochem., 22:370.

24.

Ozay

, Ekicia

, Barana

, Aktasb

, Sahinera

2009. Removal of toxic metal ions with magnetic hydrogels. Water Res., 43:4403.

25.

Rengaraj

, Yeon

K.-H.

, Moon

S.-H.

2001. Removal of chromium from water and wastewater by ion exchange resins. J. Hazard. Mater, B87:273.

26.

Rivas

B.L.

, Pooley

S.A.

, Maturana

H.A.

, Villegas

2001. Metal ion uptake properties of acrylamide derivative resins. Macromol. Chem. Phys., 202:443.

27.

Selvi

, Pattabhi

, Kadirvelu

2001. Removal of Cr(VI) from aqueous solution by adsorption onto activated carbon. Bioresour. Technol., 80:87.

28.

Sen

, Dastidar

M.G.

, Roychoudhury

P.K.

2007. Biological removal of Cr(VI) using Fusarium solani in batch and continuous modes of operation. Enzyme Microb. Technol., 41:51.

29.

Singh

V.K.

, Tiwari

P.N.

1997. Removal and recovery of chromium(VI) from industrial wastewater. J. Chem. Technol. Biotechnol., 69:376.

30.

USEPA. 2000. Situ Treatment of Soil and Groundwater Contaminated with Chromium. Washington, DC: Office of Research and DevelopmentEPA 625/R-00/004.

31.

Wang

, Lo

I.M.C.

2009. Synthesis of mesoporous magnetic γ-Fe₂O₃ and its application to Cr(VI) removal from contaminated water. Water Res., 43:3727.

32.

Wilkin

R.T.

, McNeil

M.S.

2003. Laboratory evaluation of zero-valent iron to treat water impacted by acid mine drainage. Chemosphere, 53:715.

Synthesis and Application of Magnetic Hydrogel for Cr(VI) Removal from Contaminated Water

Abstract

Abstract

Introduction

Materials and Approaches

Chemicals

Synthesis of 10-nm γ-Fe2O3

Synthesis of magnetic hydrogel

Characterization of magnetic hydrogel

Batch experiments

Kinetics of Cr(VI) adsorption onto the hydrogel

Effect of pH on Cr(VI) adsorption onto the powder magnetic hydrogel

Comparison of Cr(VI) adsorption onto the magnetic and nonmagnetic hydrogels

Cr(VI) adsorption isotherm

Effects of background electrolytes and natural organic matter on Cr(VI) adsorption by hydrogel

Regeneration and reuse of the hydrogel

Results and Discussion

Characterization of the magnetic hydrogel

Magnetic separation

Cr(VI) adsorption onto the magnetic hydrogel

Cr(VI) adsorption onto bulk and powder magnetic hydrogels

Effect of pH on Cr(VI) removal by the magnetic hydrogel

Comparison of Cr(VI) adsorption onto the magnetic and nonmagnetic hydrogels

Isotherm of Cr(VI) adsorption onto the magnetic hydrogel

Effects of common background electrolytes on Cr(VI) removal by the hydrogel

Regeneration and reusability of the spent hydrogel

Conclusion

Footnotes

Acknowledgment

Author Disclosure Statement

References

Synthesis of 10-nm γ-Fe₂O₃