Removal of dissolved chromium from synthetic mine effluent: A mesocosm experiment

Dispersion of hexavalent chromium (Cr(VI)) in streams around nickel laterite mines, which are mostly located in the tropics, may pose serious risks for the environment and human health. In an earlier study, a local natural wetland effectively removed Cr from a nickel mine environment in Indonesia. In order to understand the processes and conditions that would facilitate the establishment of operational constructed wetlands that would remove Cr from mine water discharge, we used two native macrophyte species from the same wetland, Lepironia articulata and Machaerina rubiginosa, in a series of mesocosm experiments to follow the distribution of Cr species in water, substrate and plants. A 1 m³ mesocosm was charged with a sand/compost mixture to a depth of 0.5 m, filled to within 0.1 m from the top by water with Cr concentrations of about 1.0 mg L^-1, similar to mine discharge water, and plants were introduced to part of the substrate surface. Stage 1 of the experiment supplied and removed fresh water continuously by surface flow, maintaining a residence time of 12 h. In stages, the water was recirculated (Stage 2), more plants were added (Stage 3) and outflow conditions were changed from totally surface to partially from beneath the substrate (Stage 4). All stages lowered Cr concentrations at the surface water outflow, but Cr concentrations were lower again close to the sediment/water interface. Due to the reduction of Cr(VI), the Cr concentrations in substrate pore water were higher near the surface compared to those at depth, and the pore water concentrations of Cr(VI) and total Cr were higher in the vegetated area compared to the non-vegetated area. Higher plant density and mixed species composition of the macrophytes did not increase the efficiency of Cr(VI) removal from the system. The hybrid system, comprising surface and below-substrate outflow (Stage 4), removed hexavalent chromium at a much higher rate than surface outflow only.