Metal retention mechanisms for the Aznalcóllar permeable reactive barrier (SW Spain)

A permeable reactive barrier, consisting of a mixture of compost and calcite gravel, was built in Aznalcóllar (SW Spain) to treat acidic groundwater containing zinc, aluminium and copper. Monitoring results show that practically no dissolved sulphate is reduced as water flows through the barrier, which has a residence time of about 1 to 2 days. Despite this lack of bacterial activity significant metal removal is achieved, making it clear that most of the metal retention can not be attributed to bacterially mediated sulphate reduction but to other mechanisms. In a laboratory study, a packed column was operated to evaluate the potential of municipal compost and calcite to promote sulphidogenesis in the remediation of a synthetic mine water at high flows (>0.5 m/d, 0.47 days average residence time). Results showed that acidity was neutralised and that metals were significantly removed. Rather than precipitation as metallic sulphides, metal removal was found mainly to be due to the combined result of precipitation as metal-oxyhydroxides, co-precipitation with these secondary phases, and sorption onto compost surfaces. The two latter mechanisms are especially significant for Zn: it can be roughly estimated that before saturation of compost sorption sites, 60% of the influent Zn was removed by coprecipitation with Feand Al-oxyhydroxides and 40% by sorption onto 2 Oriol Gibert1, Tobias Rötting2, Joan de Pablo1, José Luis Cortina1, José Bolzicco2,3, Jesús Carrera2 and Carlos Ayora3 compost. Sorption onto compost is only expected for the first months of the field barrier operation.