THE REMOVAL OF ARSENIC FROM AQUATIC SYSTEMS BY IRON OXYHYDROXIDES

Abstract

Surveys of dissolved Fe, Mn and As in the River Tamar, Devon, indicated general removal from the water column at low salinities. Manganese and As showed evidence of remobilisation at higher salinities. Total and non-detrital sediment extracts confirmed a remobilisation chemistry for Mn and non-detrital As, and a redox couple between Mn and As was considered. Spatial-temporal analysis of sediment data indicated both physical and chemical processes were involved in the estuarine transport of these elements. Studies of synthetic and natural Fe precipitates indicated that speciation of the Fe source and temperature of formation influenced the surface area and nature of the precipitate formed. The presence of non-authigenic Fe coatings on sedimentary particles was demonstrated. Chemical models of synthetic and natural Fe oxyhydroxides were used to study As adsorption as a function of pH, temperature and precipitate age. Uptake of As (V) onto fresh Fe (III)-derived material was rapid and related to precipitate formation. Adsorption by fresh Fe (II) precipitates was pH dependent in freshwater and in seawater followed two kinetic regimes. Adsorption was 1st order at pH <7.6, dependent on the slow oxidation of Fe (II), and 2nd order at pH >7.6, where the adsorption process was rate determining. General agreement was found with natural precipitates. The overall adsorption behaviour of aged Fe (II) and Fe (III) precipitates suggested a physical process and an electrostatic adsorption model was proposed. Aged Fe (II) material did not adsorb As (III) and an Fe (III) - As (III) redox model was developed to account for the general slowness of As (III) adsorption. Natural aged precipitates showed complex adsorption behaviour, confirmed by precipitate characterisation studies which indicated a mixed oxide composition. The estuarine behaviour of As was interpreted in terms of the adsorption models and estuarine surveys. A model illustrating the timescales of As adsorption and desorption within an estuary was developed, and used to elucidate the anomalous behaviour of dissolved Fe, Mn and As in the River Carnon, Cornwall.