Geochemistry and partitioning of trace metals in paddy soils affected by metal mine tailings in Korea

Soil solution samples were extracted from paddy soil near mine tailing dumps in an abandoned mine in Korea. Trace metals in the soil solution and soil solid (As, Cd, Cr, Cu, Ni, Pb and Zn) were analysed, along with the mineralogical composition of the soil, using XRD. Sequential extraction was also undertaken for speciation of different fractions bound to the soil solid. Kaolinite was the main clay minerals and the CEC value was relatively low, recording between 11.0 and 27.5 meq/100 g. Comparison of the paddy soil with the control soil samples collected from an uncontaminated area revealed that nearly all the paddy soil samples were enriched for trace metal contents. Cadmium, Pb and Zn concentrations exceeded the intervention values of the Dutch standards, which requires remediation action. Arsenic, Ni and Cr were below the values, indicating little or no contamination for these metals. The trace metal contents in the paddy varied with distance from the mine tailing site. In addition to the changes with distance, higher proportions of relatively easily releasable fractions, such as exchangeable forms, were found to be proportional to the total trace metal contents in the soil sample. Although most paddy soil samples were enriched with respect to some trace elements, concentrations of Cd, Cu, Ni and Pb in the solution were very low, recording maximum levels of up to only several milligrams per liter. Geochemical equilibrium modelling using PHREEQC indicates the presence of solubility controlling solid phases for Cd and Pb, whereas Zn and Cu seem to be controlled by adsorption/desorption processes. We presumed that either iron sulphides or oxides removed dissolved trace elements. Under reducing conditions, iron may be dissolved as Fe2+ in the soil solution and then the reduced iron possibly would form iron sulphide precipitates with sulphur from the mine tailings. On the contrary, in an oxidizing condition, iron exists as the oxidised form, Fe3+, and again forms iron oxides, which is a good adsorbent for trace elements in soil environments. A Cycle of dry and flooded conditions is characteristic for paddy fields. Therefore, either iron sulphide or iron oxide is believed to provide a good adsorbing site for trace metals and so explains the subsequent low concentrations in the soil solution. Despite the low trace metal concentrations in the soil solution, higher portions of the trace metals still exist as relatively easily available form in the soil solid. It should be stressed that controlling of the redox condition and pH would be important in managing the transport of trace metals in the paddy soil that was affected by the mine tailings.