Redistribution of metals in a New Caledonia ferralsol after microbial weathering

Ferralsols from southern New Caledonia developed on ultramafic rocks are very rich in Fe, Mn, and in other transition metals like Cr, Ni, and Co. Bacterial weathering of Fe and Mn oxides performed in batch experiments increases metal solubilization particularly under reducing waterlogged conditions and when organic compounds are available as nutrients. Moreover, bacterial weathering can modify the metal distribution through the geochemical compartments of the soil. Bacterial reduction of oxides led to the solubilization of Fe, Mn, Ni, and Co and brought about a significant metal redistribution into the different geochemical compartments of the soil. Selective sequential extractions showed an increase in the metal (Fe, Mn, Co, Ni, and Cr) content of the most labile compartments (water-soluble and exchangeable) after microbial weathering. Metal concentrations of amorphous and poorly crystallized Fe oxides also increased significantly. This mineral phase acted as a strong sorbent promoting the coprecipitation of metals with Fe. The Mn oxide compartment decreased significantly, especially under high microbial activity. The metal contents of the well-crystallized Fe oxide and residual fraction were stable showing that microbial weathering have a limited effect on these compartments. Although the soluble Cr was always nil, Cr was also significantly redistributed into the various geochemical compartments (highlighting the rapid sorption of this element versus its solubilization). Such redistribution processes are important to be known and to be quantified to define metal behavior and improve risk assessment.