Utilization of anion exchange resin Spectra/Gel for separation of arsenic from water

1 Introduction

Arsenic contamination of drinking water has been an environmental issue that requires serious attention in many parts of world. Although in certain situations it might be possible to switch the water supply source in the affected areas to a safer and easily available one, this solution is temporary at best and may only be possible during the part of the year when alternate water supplies are available. It may also be prohibitively expensive. Arsenic removal may be a more prudent option. In January 2001, EPA published a revised maximum contaminated level (MCL) for arsenic in drinking water that requires public water suppliers to lower the arsenic concentrations standard from 50 μg/L (part-per-billion, or ppb) to 10 ppb in 2006 (US EPA, 2000, 2001). The revised standards may affect arsenic cleanup goals for groundwater. Based on information from EPA’s CERCLIS 3 database (Comprehensive Environmental Response, Compensation, and Liability Information System) through fiscal year (FY) 1999 (US EPA Office of Water, 2001), arsenic is the second most common contaminant of concern (COC) cited in Records of Decision (RODs) for sites on the Superfund National Priorities List (NPL). Mining activities are responsible for arsenic poisoning in Thailand, while natural sources cause or have caused poisoning of populations, as has happened in Bangladesh, where nearly 30 million people were affected (Smedley and Kinniburgh, 2002). Because arsenic readily changes valence state and reacts to form species with varying toxicity and mobility, effective treatment of arsenic could be challenging. As(III) salts are generally more toxic to humans and 4–10 times more soluble in water than As(V) salts (US Occupational Safety and Health Administration, 2001). Most arsenic removal technologies are effective in removing the pentavalent form of arsenic. The trivalent form (arsenite) is predominantly non-charged below pH 9.2. Therefore, many treatment systems include an oxidation step to convert arsenite to arsenate. Various processes dealing with arsenic in hydrometallurgical treatments include; coagulation and filtration (Cheng et al., 1994; Kang et al., 2003), sorption onto metal oxides such as activated alumina (Clifford, 1999), etc. Granular ferric hydroxides were also used with high removal efficiency and capacity (Christopher, 2004; Pawel and Bartlomiej, 2004). Adsorption is one of the most commonly reported techniques for removal of As(V). This process regulates the fate and mobility of As in soil, and it is mainly linked to different environmental factors such as pH, redox potential, reaction time, oxidation state of arsenic, and adsorbent type. Synthetic ion exchange resins developed for removal of anions such as sulfate and nitrate have proven to be reasonably effective at removing arsenic from water (Clifford, 1999). Anion exchange resins (AER) offer an economical alternative to other As separation methods. The use of AER minimizes the adverse effects that sample preservatives can have on As species in environmental samples as the separation can be performed instantly (Christopher, 2004). Pawel and Bartlomiej (2004) used ion exchange column chromatography to speciate arsenic in solution. Different AER have been previously tested and evaluated for pentavalent arsenic removal (Thomas et al., 1996; Clifford and Ghurye, 2002; Pawel and Bartlomiej, 2004). Clifford et al. (2003) and Iesan et al. (2004) used a conventional sulfate-selective type 2 polystyrene strong-base-anion resin at pH of 7.5–8.0. Bissen et al. (2000) used Amberlite IRA-93, while Dominguez et al. (2003) synthesized anion exchange fibers from a vinylbenzyl chloride precursor. Kim et al. (2003) and Kim and Benjamin (2004) described an approach for operating an ion exchange process for separating As(V) selectively from sulfate or nitrate.

The aim of this work is to study the effectiveness of the anion-exchange (Spectra/Gel IE 1 × 8) resin for removal of the arsenic species {As(III) and As(V)} from aqueous solution (with As concentration close to that of the untreated drinking water). Different parameters affecting the adsorption process, such as solution pH, metal adsorption capacity and time are investigated.

2 Experimental

3 Results and discussion

The adsorption results of As(III) are shown in Fig. 1, As(III) concentration of 20 ppm was used with a resin concentration of 1 g/L, for adsorption times of 1 and 3 h. It can be seen that As(III) was poorly adsorbed, in general, the adsorption capability increases slowly with the increase of solution pH reaching a maximum As(III) removal of about 10 wt%. Increasing the adsorption time from 1 to 3 h did not greatly affect the removal efficiency. The adsorption behavior of As(V) on the Spectra/Gel resin and the effect of solution pH at different adsorption times are shown in Fig. 2. The pH of the solution has a strong effect on the adsorption and removal processes of As(V) from solution. Better arsenic removal efficiencies were achieved with As(V) as compared with that of As(III). Removal efficiency of As(V) was not measured to be significant at low pH values but increased rapidly with increasing pH, attaining a maximum value of 97.8% at pH of 7 after an adsorption time of 90 min. The removal % attained a maximum value of 99.2% at pH 9 after treatment for 30 min only. Further increase in pH, beyond 9, showed only a small increase in the removal efficiency. Fig. 3 shows the effect of adsorption time on the removal of As(V) by the Spectra/Gel resin at different pH values. A fast removal of As(V) was achieved in a short time. About 93.2% and 99.2% of As(V) was removed after 30 min of treatment at pH values of 7 and 9, respectively. However, increasing the time beyond 30 min did not affect the As(V) removal at lower pH in which case the removal efficiency values were about 51% and 85% at pH values of 3 and 5, respectively.

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Figure 1

Adsorption behavior of As(III) on Spectra/Gel IE 1 × 8 at different pH values. As(III) conc. 20 ppm, resin conc. 1 g/L.

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Figure 2

Adsorption behavior of As(V) on Spectra/Gel IE 1 × 8 at different pH values. As(V) conc. 16 ppm, resin conc. 1 g/L.

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Figure 3

Adsorption behavior of As(V) on Spectra/Gel IE 1 × 8 at different pH values. As(V) conc. 16 ppm, resin conc. 1 g/L.

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The difference in the adsorption capability of both As(III) and As(V) may be attributed to the difference in molecular structure between arsenite and arsenate, as shown in Fig. 4 (Pauling, 1970). The double bond oxygen in the arsenate molecule influences its ability to become ionized through the loss of hydrogen ions, the process is termed dissociation. A negative charge develops on the molecule when dissociation occurs. The double bond oxygen increases the capacity to delocalize that charge, easing the loss of hydrogen ions. The propensity for ionization is expressed by pK_a the constant of dissociation. For arsenate and arsenite pK_a values are as follows (Pauling, 1970):

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Figure 4

Differences in the molecular configuration of arsenate and arsenite in water.

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Figure 5

Control of arsenic speciation by Eh and pH conditions.

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Fig. 6 shows the effect of the resin dosage on the As(V) removal. The removal efficiency is increased by increasing the resin concentration. The As(V) removal reached a maximum value of 99.2% with resin dosage of 1 g/L after 90 min. Although further increase in the resin dosage only slightly increased the removal efficiency, it facilitated the As(V) removal by decreasing the treatment time. About 99.2% of the As(V) was removed after 30 min of treatment with 2 g resin, but it took only 20 min with 3 g resin to reach the same 99.2% removal efficiency.

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Figure 6

Adsorption behavior of As(V) on Spectra/Gel IE 1 × 8 as affected by resin concentration. As(V) conc. 16 ppm, pH 9.

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The effect of varying As(V) concentration on its removal rate is shown in Fig. 7. The lower the As(V) concentration, the higher the efficiency of it’s removal. The limitation of surface sites for the reaction may control the As(V) adsorption. At a concentration of 8 ppm, As(V) can be totally adsorbed (complete removal) within 30 min. The As(V) removal decreased with increasing concentration and the reaction was time independent. The removal attained its maximum after 90 min of treatment at any As(V) concentration. Maximum removal efficiency values of 99.2%, 96%, 78.8%, and 68.5% were achieved for As(V) concentration values of 16, 32, 48, and 64 ppm, respectively. The results clearly demonstrate that the As(V) adsorption process is promising at low concentrations of the pollutant. This is also typical for heterogeneous catalytic systems where the reaction occurs at the interface between two phases.

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Figure 7

Adsorption behavior of different concentrations of As(V) on Spectra/Gel IE 1 × 8. pH 9, resin conc. 1 g/L.

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The removal of As(V) by Spectra/Gel anion exchange resin can be explained as follows; the water containing As(V) passes through a bed of the chloride form resin (designated by RCl), during which the chloride arsenate ion exchange reaction (Eq. (3)) takes place to yield resin in the arsenate form (R₂HAsO₄). When the column capacity for arsenic is exhausted, the reaction is easily reversed and regeneration returns the resin to the chloride form according to Eq. (4) (Clifford, 1999).

The rate equation of the reaction was calculated by plotting ln C/C₀, for As(V) concentration of 16 ppm with 1 g resin at pH of 9, as a function of ln time, as shown in Fig. 8. Here C₀ and C are the initial and remaining As(V) concentrations in the solution, respectively. The plot gave a straight line which confirmed that the adsorption process follows a pseudo-first order reaction. Fig. 9 shows the As(V) equilibrium isotherm for the resin at the same experimental conditions of Fig. 8. In the lower concentration regime, the resin exhibits a higher exchange capacity, followed by a crossover. Although the resin is classified as strongly basic anionic, these systems may posses a combination of strong and weak groups due to side reactions that occur during resin synthesis (Dominguez et al., 2003). It is possible that as the concentration increases, these weaker groups begin to exert exchange capacity thus resulting in a crossover point.

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Figure 8

kinetics of As(V) adsorption on Spectra/Gel IE 1 × 8. As(V) conc. 16 ppm, resin conc. 1 g/L, pH 9.

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Figure 9

As(V) equilibrium isotherms for Spectra/Gel IE 1 × 8. As(V) initial conc. 16 ppm, pH 9.

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4 Conclusion

Strong base anion exchange resin Spectra/Gel IE 1 × 8 (type 1) with bead size of 75–150 μm was used and evaluated for removal of As(III) and As(V) species from water. As(III) was poorly adsorbed, whereas As(V) was retained on the resin. The adsorption process was optimized by using 1 g resin for 16 ppm As(V) at pH 9 for 30 min. The maximum removal efficiency of As(V) was 99.2%. The adsorption process follows a pseudo-first order reaction. The resin was regenerated and the adsorbed arsenic fractions were eluted by using 2 M NaCl.