Use of solid phase characterisation and chemical modelling for assessing the behaviour of arsenic in contaminated soils

A soil, containing waste material from an industrially contaminated site, w as found to be heavily contaminated with several heavy metals and As. A ris k assessment for As leaching from this material has been carried out in sev eral stages, collation and examination of historical records, solid-phase c haracterization and chemical modelling. The historical record indicates tha t the most probable source of As was arsenopyrite. However, the solid phase characterization of the soil, using X-ray diffraction (XRD), scanning elec tron microscopy (SEM), and energy-dispersive microanalysis (EDAX), did not yield any direct evidence for pyritic phases, although there was clear evid ence of known pyrite-weathering products, such as jarosite, The relative st ability of pyrite and arsenopyrite have been modelled for the range of acid ity and redox potentials likely to be encountered on the site. For adsorpti on modelling, a surface complexation model was used to predict arsenate des orption as a function of pH. It was assumed that the principal reactive ads orbent for As was hydrous ferric oxide (HFO) and this assumption was suppor ted by the results of direct and indirect measurements and by the mineral s tability calculations. This approach was successful at predicting the incre ased mobility of As at increasingly alkaline conditions. The modelling pred ictions were supported by results from batch equilibration experiments. Thu s, it was possible to link direct observations of mineralogy, mineral stabi lity calculations and adsorption models in order to predict the mobility of As. The success of this approach was dependent on identifying the reactive phase in this particular soil and having the appropriate data required for the adsorption modelling. (C) 2001 Elsevier Science Ltd. All rights reserv ed.