Solubility of arsenic in Swedish contaminated soils : experiments and modelling

Abstract: Even though arsenic can be found everywhere in the environment, it is phytotoxic and hazardous to human and animal health. Especially arsenic leaching produces contamination of groundwater. But a number of factors, such as the amount and type of adsorbing soil constituents, pH value, redox conditions and residence time have an important influence on the potential mobility and leachability of arsenic. Especially the presence of hydrated oxides and hydroxides of Fe, Al and Mn have an essential influence. In this study two heavily contaminated Swedish soils have been investigated on the arsenic solubility as a function of pH with the aid of batch experiments. The sites were a former wood impregnation site (Åsbro) and an area of a former glass factory (Pukeberg). The results of the batch experiments were evaluated using geochemical modelling (Visual MINTEQ). The objective of the study was to assess if a change in the soil pH could lead to an increased leaching of arsenic from the two contaminated soils. Furthermore, the plausibility of geochemical modelling as a tool in risk assessment was explored. The hypothesis of a low arsenic leachability for pH above 3 and that arsenic is mainly adsorbed to ferric (hydr)oxides were stated. The study showed that at least in one of the soils, ferric (hydr)oxides were not the main phase controlling As solubility. For the other soil no explicit statement on the role of ferric (hydr)oxides as adsorption sites for arsenic could be made. Furthermore, the assumption that arsenic adsorption results in a low leachability of As at pH > 3 was proved to be wrong. The depiction of the experimental results using geochemical modelling proved to be difficult. These results indicate that additional investigations need to be made concerning the reactive solid phases relevant for As in heavily contaminated soils before geochemical models can be used in risk assessment.