Environ. Sci. Technol. 2002, 36, 3405-3411
Arsenic Adsorption by Fe(III)-Loaded Open-Celled Cellulose Sponge. Thermodynamic and Selectivity Aspects J O S EÄ A N T O N I O M U N ˜ OZ, ANNA GONZALO, AND MANUEL VALIENTE* Departament de Quı´mica, Quı´mica Analı´tica, Universitat Auto`noma de Barcelona, 08193 Bellaterra, Barcelona, Spain
Nowadays there is a great concern on the study of new adsorbent materials for either the removal or fixation of arsenic species because of their high toxicity and the health problems associated to such substances. The present paper reports a basic study of the adsorption of arsenic inorganic species from aqueous solutions using an opencelled cellulose sponge with anion-exchange and chelating properties (Forager Sponge). Consequences of preloading the adsorbent with Fe(III) to enhance the adsorption selectivity are discussed and compared with the nonloaded adsorbent properties. The interactions of arsenic species with the Fe(III)-loaded adsorbent are accurately determined to clarify the feasibility of an effective remediation of contaminated waters. Arsenate is effectively adsorbed by the nonloaded and the Fe(III)-loaded sponge in the pH range 2-9 (maximum at pH 7), whereas arsenite is only slightly adsorbed by the Fe(III)-loaded sponge in the pH range 5-10 (maximum at pH 9), being that the nonloaded sponge is unable to adsorb As(III). The maximum sorption capacities are 1.83 mmol As(V)/g (pH ∼ 4.5) and 0.24 mmol As(III)/g (pH ∼ 9.0) for the Fe(III)-loaded adsorbent. This difference is explained in terms of the different acidic behavior of both arsenic species. The interaction of the arsenic species with the Fe(III) loaded in the sponge is satisfactorily modeled. A 1:1 Fe:As complex is found to be formed for both species. H2AsO4- and H3AsO3 are determined to be adsorbed on Fe(III) with a thermodynamic affinity defined by log K ) 2.5 ( 0.3 and log K ) 0.53 ( 0.07, respectively. As(V) is, thus, found to be more strongly adsorbed than As(III) on the Fe(III) loaded in the sponge. A significant enhancement on As(V) adsorption selectivity by loading Fe(III) in the sponge is observed, and the effectiveness of the Fe(III)loaded sponge for the As(V) adsorption is demonstrated, even in the presence of high concentrations of interfering anions (chloride, nitrate, sulfate, and phosphate).
Introduction Arsenic is a relatively scarce but ubiquitous element, with environmental substrates showing ranges of arsenic concentrations from ppb to ppm levels (1). The impact of natural and anthropogenic inputs of arsenic species in the environment, mainly in soils and waters, is considered to be one of * Corresponding author phone: +34-93-581-29-03; fax: +34-93581-19-85; e-mail:
[email protected]. 10.1021/es020017c CCC: $22.00 Published on Web 06/22/2002
2002 American Chemical Society
the major problems in pollution abatement, because of their high toxicity and the consequent risks for human health. The oxidation state of arsenic plays an important role since it determines the properties of the related species, i.e., the toxicity, the sorption behavior and the mobility in the aquatic environment. However, in natural waters arsenic is found mostly as As(III) and As(V). In January 2001, EPA published a new standard for arsenic in drinking water, requiring public water supplies to reduce arsenic from 50 to 10 ppb by 2006 (2). Thus, there is a growing interest in using low-cost methods and materials to remove arsenic from industrial effluents or drinking water before it may cause significant contamination. Although many different methods such as precipitation, coprecipitation (3), adsorbing colloid flotation (4), ion-exchange (5, 6), ultrafiltration, and reverse osmosis have been used for arsenic removal, the adsorption from solution has received more attention due to its high concentration efficiency. Many types of adsorbents have been used: activated alumina (7), gibbsite (8), aluminum-loaded materials (9), lanthanum compounds (10), fly ash (11, 12), natural solids (13), etc. However, Fe(III)bearing materials such as goethite (14, 15), hematite (16), iron-oxide-coated sand (17), ferrihydrite (18), and Fe(III)loaded resins (19-22) are the most used in arsenic adsorption because of the Fe(III) affinity toward inorganic arsenic species and consequent selectivity of the adsorption process. While most of the Fe(III) oxides present low arsenic adsorption capacities, Fe(III)-loaded chelating resins are not economically suitable for their use in a full-scale process. Besides these drawbacks, the adsorption process is rather slow, what compromises the effectiveness of the process and thus its applicability. In this paper, the arsenic adsorption on an economic, nonconventional ion exchange material (Forager Sponge, Dynaphore Inc.) based on an open-celled cellulose sponge incorporating a chelating polymer with selective affinity for dissolved heavy metals in both cationic and anionic states is presented. Forager Sponge and other adsorbent sponges have been successfully used in the treatment of heavy metal solutions (23-25). Their main advantages are their high porosity and flexibility which promote high rates of adsorption and their compressibility into an extremely small volume to facilitate disposal once the capacity of the material has been exhausted. Scarce attention has been paid up to now to the arsenic adsorption on sponge materials and much less to the corresponding thermodynamic aspects. In this study, Forager Sponge is chemically characterized. Fe(III) is loaded in the sponge to enhance the adsorption selectivity. The adsorption properties of arsenic are evaluated to determine the optimum conditions for the maximum arsenic loading capacity and the best selective removal of the arsenic species. An adsorption mechanism is proposed and it is thermodynamically evaluated. The interfering effect of common anions (chloride, nitrate, sulfate, and phosphate) in As(V) adsorption is also studied to evaluate the enhancement on the adsorption process selectivity by loading Fe(III) in the sponge.
Experimental Section Reagents and Apparatus. NaAsO2, Na2HAsO4‚7H2O, and FeCl3‚6H2O, ACS reagents from Aldrich (Milwaukee, U.S.A.), were used as As(III), As(V), and Fe(III) sources. All other chemicals used were of analytical grade. Forager Sponge, an open-celled cellulose sponge which contains a water-insoluble polyamide chelating polymer VOL. 36, NO. 15, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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SCHEME 1. Reactions Employed on the Characterization of Acidic Functional Groups on the Sponge
(formed by reaction of polyethyleneimine and nitrilotriacetic acid), was kindly supplied by Dynaphore Inc. (Richmond, VA, U.S.A.). This material is claimed to contain free available ethyleneamine and iminodiacetate groups to interact with heavy metals ions by chelation and ion exchange. Metal concentrations in solution were determined by the ICP-OES technique (Inductively Coupled Plasma Optical Emission Spectroscopy) using an ARL model 3410 with minitorch (Valencia, CA). The emission lines used for analysis were 197.262 nm (As), 259.940 nm (Fe), and 330.297 nm (Na), being the uncertainty of metal determination 10.5, Fe(III) hydrolysis takes place because of the hydroxyl ions competition for the active sites of the adsorbent, then Fe(III) precipitates, and the solution becomes turbid. At pH < 2.5, iron desorbed is quite important due to protons competition. Nevertheless, at pH range 3-9 iron desorption is not significant ( nitrate > chloride > phosphate, which is also found in anion-exchange resins such as Dowex-1 (36). The interfering effect is high as it corresponds to a nonselective anionexchange process. The Fe(III)-loaded sponge follows the selectivity pattern phosphate > sulfate > nitrate > chloride, being the interfering effect significantly lower than the corresponding to the blank sponge. Thus, the Fe(III) immobilization as an adsorption mediator by a selective ligand-exchange process enhances the As(V) adsorption selectivity against interfering anions with lower affinities for the immobilized metal ion (chloride, nitrate, and, especially, sulfate). On the other hand, the chemical similarity between arsenate and phosphate explains the high interfering effect of phosphate in the As(V) ligand3410
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exchange process when Fe(III) is loaded in the sponge. The knowledge of interactions between arsenic species and related adsorbents is needed for an accurate evaluation and selection of such adsorbents to their possible application for a remediation process. The results of this study showed that As(III) and As(V) are effectively adsorbed by the Fe(III)-loaded Forager Sponge adsorbent by ion exchange and ligand exchange interactions. Evidence from capillary electrophoresis suggested that As(V) and As(III) remains stable on the Fe(III)-loaded sponge surface. In this study, the thermodynamic parameters for chemical adsorption of arsenic species on Fe(III) loaded in the sponge have been determined by assuming the Fe-As interaction 1:1, which fits very well to the experimental data. The thermodynamic parameters obtained show the arsenate interaction to be stronger than the corresponding for arsenite. This results follows the well-known differences between As(V) and As(III) for Fe(III). Arsenic adsorption selectivity is a relevant aspect for the effective remediation of a contaminated water based on the removal of arsenic species. The present investigation has revealed that selectivity of arsenic adsorption is enhanced by loading Fe(III) in the adsorbent, even in the presence of high concentrations of the interfering anions (chloride, nitrate, sulfate, and phosphate).
Acknowledgments This work has been carried out under the support of the Spanish Commission for Research and Development CICYT (Project QUI1999-0749-C03-01). J. A. Mun ˜ oz acknowledges the “Departament d’Universitats, Recerca i Societat de la Informacio´ de la Generalitat de Catalunya” for the scholargrant received. Dynaphore Inc. (U.S.A.) is gratefully acknowledged for the samples supplied.
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Received for review January 30, 2002. Revised manuscript received May 7, 2002. Accepted May 21, 2002. ES020017C
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