Metal speciation in soil at a contaminated glassworks site in southeastern Sweden

Abstract

Risk assessments of metal contaminated land are often based on total metal concentrations of the soil material, although it is well known that the potential for leaching is determined by the fractionation between labile metal forms and forms that are not to be assessed as geochemically active. In the present study, a 4-step sequential leaching procedure was applied to solid material from five subareas of a glass waste site in southeastern Sweden (including, for example, landfill areas and natural till material). The aim was to assess whether the major fraction of the total Cd and Pb content should be considered labile (easily mobilizable) or relatively stably sorbed, which is a question that is motivated by the fact that previous site-specific investigations have shown strongly elevated metal concentrations in soil, but very little impact on local groundwater. Although the total metal concentrations as well as the amount of visable glass particles differed between the subareas, the fractions extracted in the different steps of the sequential leach were similar for all areas. Only a minor part of the total content of both Cd and Pb was released in the final strong acid residual leach (9-19 %), which indicates that there has been a significant release of metal ions from the glass waste material in the past, followed by a transition to secondary phases. Of the two metals, Cd was assessed to be the most labile one. The first extraction, targeting the exchangeable and carbonate bound fraction, released 43-59 % of the total Cd content. So, although the relatively small effect seen on groundwater quality indicates an effective retention of these metal cations by the soil solid matrix, the relatively high fraction of potentially mobile forms highlights the need of further assessments of the stability in major retention mechanisms.