Schwertmannite transformation to goethite and the related mobility of trace metals in acid mine drainage

In acid mine drainage (AMD), precipitated schwertmannite can reduce the trace metal concentration by adsorption and co-precipitation. With the geochemical changes in the water, the precipitated minerals are transformed into more stable goethite. However, no detailed systematic studies have been performed on the mobility changes of trace metals during iron-mineral transformation. The behaviors of trace metals during the transformation of schwertmannite to goethite are investigated for core samples from an AMD treatment. Schwertmannite had gradually transformed to goethite from the top to the bottom of the core samples. Among trace metals, Pb was highly retained in schwertmannite during precipitation, probably by co-precipitation with schwertmannite. Arsenate and chromate were also relatively well retained in schwertmannite, probably because of the substitution of sulfate during precipitation. Sequential extraction results showed that during the transformation of schwertmannite to goethite, most trace metals decreased their mobility by decreasing their exchangeable fraction. However, only Pb increased its mobility during transformation. Some elements, such as Cd and Co, had higher contents of exchangeable fractions compared to other metals and can be relatively easily released into water with slight geochemical changes, greatly affecting the environments of ecological systems.