The role of x-ray irradiation on the arsenic photo-oxidation and nucleation of arsenic-bearing phases in water

Abstract

The speciation of arsenic (As) in aqueous environments, particularly the relative concentration ratio of highly toxic and mobile arsenite (As(III)-bearing) anion and comparatively less toxic and less mobile As(V)-bearing arsenate anion, and their interconversion, if any, is a critical issue in terms of environment and health. In place of adsorption-based temporary removal, arresting these toxic species within stable crystalline cages of naturally occurring solids and minerals, made up of arsenic and other common elements, such as iron, is being considered as an important decontamination strategy in recent times. Evidently, designing such an environmental management plan requires information of nucleation of such desirable phases and consequently warrants several in-situ x-ray spectroscopic studies in the aqueous phase, containing arsenic and other common ions. But the present work warns against a barely noted photo-oxidation phenomenon of As(III) to As(V), while probing Fe-As aqueous solutions at certain pH ranges, that takes place under irradiation with high flux synchrotron x-ray. Our x-ray absorption spectroscopy (XAS) experiments on the Fe and As K-edges with the purpose of monitoring the in-situ nucleation of Fe(III)-As(III) clusters and growth, as a function of pH, revealed this unforeseen photo-oxidation as the pH of the solutions was raised above 3. Our combined experimental and density functional theory-based studies proved that the accelerated photo-oxidation is primarily driven by reactive free radicals, notably the hydroxyl radical (OH•), generated through the radiolysis of water by the high energy x-ray beam of very high brilliance, and this photo-oxidation consequently drives significant structural reorganization, in which Fe(III)-As(III) containing tooeleite mineral-like clusters get converted into As(V)-adsorbed ferrihydrite-like moieties, which act as the driving force for arsenic oxidation.