Mobilization of heavy metals from urban contaminated soils under water inundation conditions

A microcosm experiment was conducted to investigate heavy metal release from the urban soils heavily contaminated by past industrial activities. The aim was to assess the mobility of various heavy metals under inundation with water. The results show that reductive dissolution of iron and manganese compounds was markedly enhanced by organic matter. However, mobilization of Fe and Mn was affected by the abundance of these metals in the soils. The dissolution of Fe and Mn oxides led to the release of As and Zn that were bound to them. However, mixed temporal variation patterns were observed for As, suggesting complication of As mobility by other factors. It is likely that the added organic matter played a role in the formation of organic matter-Fe(III)-arsenic association, leading to partial re-immobilisation of the liberated As at the latter stage of the experiment. Zn showed a consistent trend where it was initially released with reductive dissolution of Fe and Mn compounds and then re-immobilised, possibly through hydrolysis to form insoluble zinc hydroxide. In spite of abundant presence, release of Pb was limited due to its low solubility under less acidic conditions. It appears that anaerobic environment stabilized the soil-borne Cr by forming insoluble Cr(OH)3.