A study on the removal of arsenic ions from water by surface adsorption

Abstract

The potential for removing low levels of arsenic from water was investigated, initially, by studying the adsorption properties of chemically modified microsphere cysteine-coated silica particles. The results reported in this work suggest that these particles could be used for low-level arsenic removal from contaminated water. These observations are supported both by direct adsorption measurements using ICP-OES and through the analysis of surface zeta potential measurements. Measured zeta potentials and calculated surface charge densities of the particles in different pH and electrolyte solutions were used to improve basic ion adsorption models, which were then used to accurately describe the surface charging behavior of silica and aminated silica particles with pH. A modified approach was used which assumes that the bulk pH value remains constant throughout the solution, even close to the charged particle surfaces. A second process was developed to remove low levels of arsenic from drinking water using several specifically designed cysteine-based surfactants in an ion flotation process. Several studies were conducted into some of the chemical and physical properties of cysteine-based surfactants to find a suitable compound for optimum arsenic adsorption and bubble foaming characteristics. Several suitable surfactants were identified and synthesized and evaluated for the ion flotation process, although only one compound, single-chain octanoyl cysteine, produced high arsenic removal rates. The Na⁺ form of single-chain octanoyl cysteine surfactant was found to significantly remove low levels of arsenic ions from water, at a pH of around 8, in a simple, single stage ion flotation process. Once adsorbed, the arsenic oxyanions were effectively removed by the rising bubbles, which produced a high removal rate of 99.4% to 99.9% (using N₂ and air, respectively) in a 5 mg/l (ppm) arsenic feed solution, which indicates that this process is capable of reducing arsenic content in solution to lower than the recommended WHO limit, of 0.01 mg/l (ppm). This surfactant was also found to be suitable for the successful separation of arsenic from a natural water sample (i.e. fresh lake water) and from water containing a mixture of other ions; namely, cadmium, lead, and nickel.