Lability and bioavailability of toxic heavy metals in Ratnapura district gem sediments, Sri Lanka

Ratnapura district is composed of alluvial sedimentary gem deposits. These deposits have a variety of gem minerals which may contain toxic heavy metals. In this study, the fate of heavy metals associated with gem minerals was investigated through a quantitative determination. Sediments collected from 15 gem pits were analysed for total, exchangeable, and bioavailable fractions of toxic heavy metals. Air-dried, ground, and sieved samples were microwave acid digested using HNO3, HCl, and HF. For exchangeable metal fraction analysis, samples were extracted with 1 M MgCl2 solution. Bioavailable metal fraction was extracted by adding 0.01 M CaCl2 solution. The extractions were analysed using AAS. Results indicated high concentrations of Fe, Zn, and Pb in total heavy metal content which averaged 49 g kg-1, 223 mg kg-1, and 153 mg kg-1 respectively. Abundance of metals was recorded as Fe > Zn > Mn > Pb > As > Ni > Cu > Co > Cd. Exchangeable and bioavailable metal fractions showed more or less a similar order. Metal concentrations increased with the increase of soil depth. This analysis confirmed that a considerable amount of toxic heavy metals extracted and transported to the surrounding environment during the gem mining process causing contamination.