Mechanistic modeling of arsenic retention on natural red earth in simulated environmental systems
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Date
2004
Authors
Vithanage, Meththika Suharshini
Journal Title
Journal ISSN
Volume Title
Publisher
University of Peradeniya
Abstract
Arsenic is present mainly in water as a natural contaminant in the form of arsenate and arsenite. The natural release of arsenic species into groundwater results in detrimental health effects. Developing a cost-effective unit process to remove arsenic from drinking water is a pressing global need. Therefore. the final aim of this work was sharply focused at developing a removal method for As³⁺and As⁵⁺ utilizing red earth that is abundant in the Northwestern part of Sri Lanka. Potentiometric surface titrations of red earth were carried out by auto-titration system under inert environmental condition in 0.1. 0.01. 0.001 M NaNO₃. The retention of As species, both As³⁺and As⁵⁺. were examined as a function of pH and ionic strength in single- (As³⁺ or As⁵⁺ only) and dual-sorbate (As⁻³ and As⁻⁵) systems. The XRF analysis showed that the total chemical composition of red earth was found to be SiO₂(54.15%). Al₂O₃ (20.73%), Fe₂0₃ (12.39%) and TiO₂ (5.54%). According to XRD analysis. the red earth sample is dominated by α-SiO₂ phase exhibiting a characteristic peak at 2θ of 3.340. Proton titration data of red earth showed that though the titration curves intercept at pH = 8.8 exhibiting that the red earth surface is dominantly positive when pH < pHzpc (1.e. environmentally relevant conditions). Initial |As³⁺] = [As⁵⁺] = 0.385 μmol/L (50 μg/L of As³⁺) were fixed. Adsorption of arsenic species on the solid system was examined as a function of pH. Both As³⁺ and As⁵⁺ were adsorbed over 95% on to Red Earth when initial [As³⁺] = [As⁵⁺] = 0.385 μmol/L. In the dual sorbate system. when both As³⁺ and As⁵⁺ are present: the retention of As** decreases by about 80% above pH > 7. However, at pH < 7. adsorption of both species were shown strong adsorption. Modeling results showed a competition between arsenic species to the solid binding sites. However. both experimental and modeled data showed that there is no dependency of arsenic species on pH or ionic strength in both single and dual ion systems. which indicated that the affinity of arsenic species on to natural red earth was strong forming inner sphere complexes with the red earth surface. As determined by the simple diffuse layer model. the binding constants were very well fitted with experimental data. proving that red earth can be effectively used to remove arsenic from aqueous systems by modifying the available filtering systems.
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Keywords
Environmental sciences , Groundwater , Arsenic retention , Natural red earth