Paper No. 3
Presentation Time: 2:05 PM
EFFECT OF ORGANIC AND INORGANIC ACIDS ON SERPENTINITE SOIL DISSOLUTION; RELEASE OF NICKEL AND MANGANESE TO THE ENVIRONMENT
The geological boundary between Highland and Vijayan Complexes in Sri Lanka is considered as a mineralized belt with several serpentinite occurrences, rich in toxic metals as Ni, Mn, Cr, and Co. Weathering and other pedogenic processes release these metals to the environment causing natural groundwater pollution leading to ecological and health problems. This study investigates the dissolution of serpentinite soil in the simulated environments with the presence of organic and inorganic acids to observe the release of Ni and Mn to the environment. The conventional potentiometric titration method yielded an average pHzpc around 8.57 for proton binding sites of the soil. The results of XRF studies show that the serpentinite soil is a Fe-Cr-Ni-rich aluminosilicate. Experiments were carried out over an acid concentration range of 0.05 to 10 mM with three organic (citric, acetic, oxalic), inorganic (H2SO4, HNO3, HCl) acids and also with distilled water. Nickel and Mn releasing rates increases in the order of nitric hydrochloric acetic < sulfuric < citric < oxalic acid. The maximum rate of release of Ni and Mn was observed in the presence of oxalic acid which was recorded as 5.84 and 2.56 mol m-2 s-1 respectively. In the presence of both organic and inorganic acids Ni released preferentially relative to Mn. The pH dependency of the dissolution was calculated by calculating H+ ion activity in the each acid and results showed the linear relationships between the dissolution rate and pH. Different acids demonstrated different dissolution rates although the pH was similar and that could be due to the accompanying anion of the acids. Therefore the effect of ligands (anion associated with the acid) was investigated by calculating the difference between the total dissolution rate and the rate promoted by H+ activity. This suggests that the pH as well as complex forming ligands accelerate the serpentinite dissolution rate.