Elsevier

Microchemical Journal

Volume 81, Issue 1, August 2005, Pages 122-132
Microchemical Journal

Special article
Determination of adsorption and speciation of chromium species by saltbush (Atriplex canescens) biomass using a combination of XAS and ICP–OES

https://doi.org/10.1016/j.microc.2005.01.008Get rights and content

Abstract

Studies were performed to determine the effect of pH on chromium (Cr) binding by native, esterified, and hydrolyzed saltbush (Atriplex canescens) biomass. In addition, X-ray absorption spectroscopy studies were performed to determine the oxidation state of Cr atoms bound to the biomass. The amounts of Cr adsorbed by saltbush biomass were determined by inductively coupled plasma–optical emission spectroscopy (ICP–OES). For Cr(III), the results showed that the percentages bound by native stems, leaves, and flowers at pH 4.0 were 98%, 97%, and 91%, respectively. On the other hand, the Cr(VI) binding by the three tissues of the native and hydrolyzed saltbush biomass decreased as pH increased. At pH 2.0 the stems, leaves, and flowers of native biomass bound 31%, 49%, and 46%, of Cr(VI), respectively. The results of the XAS experiments showed that Cr(VI) was reduced in some extend to Cr(III) by saltbush biomass at both pH 2.0 and pH 5.0. The XANES analysis of the Cr(III) reaction with the saltbush biomass parts showed an octahedral arrangement of oxygen atoms around the central Cr(III) atom. The EXAFS studies of saltbush plant samples confirmed these results.

Introduction

Heavy metals tend to accumulate in the environment causing various diseases and disorders in living organisms [1]. Cr enters into water resources from industrial processes such as electroplating, tanning, dyeing, and printing [2]. This metal usually occurs in trivalent or hexavalent oxidation states in soil and water [3], [4]. Although both Cr species are toxic, hexavalent Cr poses a greater health risk than trivalent due to its carcinogenic activity [5], [6]. Because of this, many researchers have focused on Cr removal from soil and water [4], [7], [8], [9]. The conventional metal removal procedures include filtration, flocculation, reverse osmosis, solvent extraction, and ion exchange resins. In general, these techniques are associated with high energy requirements, incomplete removal, generation of wastes that requires special disposal, and potential health risks [10], [11]. The most common treatment for Cr in industrial effluents involves the reduction of Cr(VI) to the less toxic Cr(III), the recipitation of Cr(III) into Cr(OH)3 at high pH values, and the separation of the precipitate. This procedure has been found to be expensive and produces contaminated wastes [4], [7].

The use of dead plant biomass (some times referred to as phytofiltration) has appeared as an option for metal removal from contaminated waters. This material has a low cost, is abundant, and it could be selective for an easy metal removal [1], [10], [11], [12], [13], [14]. Previous studies on Cr removal indicate that certain biomasses such as alfalfa, hops, algae and others, have a high capacity for chromium binding. Experimental results demonstrated that Cr(III) and Cr(VI) bind to the stems and leaves of hops biomass through oxygen atoms [15]. Another study suggested that, while the binding for Cr(III) by oat biomass (Avena monida) occurs through negatively charged ligands such as carboxylic groups, the binding of Cr(VI) may occurs via positively charged ligands such as amino groups [16]. It has also been proposed that Cr binding to the biomass might be due to ligand exchange, ion exchange, and reduction mechanisms [17]. Nevertheless, the mechanism involved in Cr binding by biomass is not fully understood.

The use of saltbush plant (A. canescens) for phytofiltration has not been previously studied. This plant could be promising, since Atriplex species have special bladders in the leaves that act as salt sinks for the removal of the excess of salt [18]. In the present research study, we investigated the potential use of saltbush biomass for the removal of Cr(III) and Cr(VI) from aqueous solutions. Chemical modification was used to investigate the potential involvement of carboxyl groups on saltbush biomass in Cr(III) and Cr(VI) binding [14], [15], [16].

Extended X-ray absorption fine structure (EXAFS) was used to provide information about the coordination environment and the nearest neighboring atoms and the ligands involved in the binding of Cr by saltbush biomass. In addition, X-ray absorption near edge structure (XANES) was used to provide information about possible changes in the oxidation of Cr atoms bound to the biomass. The results of these studies are reported herein.

Section snippets

Collection and preparation of saltbush biomass

Saltbush plants were collected from wild areas around El Paso, Texas USA. The plants were washed with tap water and separated into flowers, stems, and leaves. All samples were oven dried at 90 °C for 1 week, and the different portions of the plant were then ground using a Wiley mill, to pass through a 0.149-mm (100-mesh) sieve.

Esterification of saltbush biomass

Carboxyl groups of the biomass were esterified following a similar method previously described [19], [20]. Nine grams of saltbush biomass were washed with 0.01 M HCl and

Effect of pH on Cr(III) and Cr(VI) binding by saltbush biomass

The percentages of Cr(III) and Cr(VI) bound by the native stems (NS), leaves (NL) and flowers (NF) of saltbush biomass are shown in Fig. 1A and B, respectively. As one can see in these figures, the binding for both chromium species was affected by the solution pH. For Cr(III), the percentages bound at pH values of 4.0 and 5.0 (greater than 90%) were statistically higher (P<0.05) than the percentages bound at pH 2.0 by the three tissues (35.7%, 49.6%, and 27.3% for the NS, NL, and NF,

Conclusions

The results of these studies demonstrated that native saltbush biomass can remove more than 90% of the Cr(III) from an aqueous solution at pH values of 4.0 and 5.0. The results also demonstrated that hydrolyzed saltbush biomass adsorbed more than 90% of the Cr(III) ions in a pH range from 3.0 to 6.0. In addition, the esterified biomass of leaves was able to adsorb more than 50% of the Cr(VI) present in the aqueous solution. The capacity experiments support the idea that the carboxyl groups

Acknowledgments

The authors would like to acknowledge the National Institutes of Health (grant S06 GM8012-33) and the University of Texas at El Paso's Center for Environmental Resource Management (CERM) through funding from the Office of Exploratory Research of the U.S. Environmental Protection Agency (cooperative agreement CR-819849-01). We also thank the financial support from the Southwest Center for Environmental Research and Policy (SCERP) program, and the HBCU/MI, Environmental Technology Consortium that

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