The role of X-ray irradiation in 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 the highly toxic and mobile arsenite (As(III)-bearing) anion and the comparatively less toxic and less mobile As(V)-bearing arsenate anion, and their interconversion, if any, is a critical issue in terms of the 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, has been considered as an important decontamination strategy in recent times. Evidently, designing such an environmental management plan requires information on the nucleation of such desirable phases and consequently warrants several in situ X-ray spectroscopic studies of arsenic and other common ions in the aqueous phase. While probing Fe–As aqueous solutions at certain pH ranges, the present work found a barely noted photo-oxidation phenomenon of As(III) to As(V), which occurs 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.