Nickel and manganese release in serpentine soil from the Ussangoda Ultramafic Complex, Sri Lanka

Ultramafic rocks and their related soils and sediments are non-anthropogenic sources of metal contaminants. In the southeastern region of Sri Lanka, release of Ni and Mn into the surrounding areas and groundwater is an ecological, agricultural and human health concern. Here, we investigate the release and fate of Ni and Mn from serpentine sediment in the Ussangoda ultramafic complex by coupling interpretations garnered from chemical extractions. Sequential extraction experiments, utilized to identify 'elemental pools,' indicate Mn is mainly associated with oxides/(oxy)hydroxides, whereas, Ni is bound in silicates and spinels. Both Ni and Mn demonstrate rapid release rates in water (2.4×10 ^-12 and 2.0×10 ^-13molm ^-2s ^-1, respectively) and release rates increase with increasing ionic strengths. Sediments evaluated from 0.05 to 10mM with organic (citric, acetic and oxalic) and inorganic (H ₂SO ₄, HNO ₃ and HCl) acids show that the maximum rate of Ni and Mn release occurs with oxalic acid (10mM) at 7.11×10 ^-11 and 3.56×10 ^-11 molm ^-2s ^-1, respectively. Summarizing chemical extractions, Ni and Mn release rates increase in the order of HNO ₃≈HCl≈acetic<H ₂SO ₄<citric<oxalic acids and indicate that both protons (H ⁺) and ligands corroborate to accelerate metal release. Despite multiple phases capable of releasing Ni and Mn, the reaction kinetics demonstrate that antigorite (i.e., the silicate fraction) is responsible for a majority of the Ni and Mn release. Overall, our results support that serpentine sediments and soils offer a highly labile and chemically modifiable source of Mn and Ni.