Microbial mediated zinc removal from mine water : a microcosm study

Abstract

The influence of cyanobacteria mats on the removal of zinc from mine waters and the nature of zinc precipitates were assessed using thirty batch microcosms. The cyanobacteria were isolated from white precipitate mats, composed of hydrozincite (Zn5(CO3)2(OH)6), collected from the Rio Naracauli creek, a stream affected by historical Pb-Zn mining in Sardinia, Italy. The cultured bacteria were inoculated into flasks containing sterilised dead leaves, as a source of carbon, immersed in a solution that simulated mine water with composition of SO4 745 mg/l, HCO3 27 mg/l, Ca 99 mg/l and Zn 300 mg/l. Epifluorescence microscopy showed that the microcosms were able to sustain an active population of bacteria for the duration of the experiment (226 days). Rising pH and falls in dissolved zinc concentrations by 40-60% were interpreted as being indicative of photosynthetic activity and precipitation of zinc from solution. SEM analysis of the particulate matter from the microcosm suspensions showed abundant fibres enriched in Zn and C, typically 10-50um in length. Saturation indices indicated that the microcosm solutions were undersaturated with respect to zinc carbonates, with both low pH and alkalinity limiting precipitation. Cell-surface microenvironments with steep gradients of pH and alkalinity, induced by photosynthesis, could drive localised precipitation of zinc carbonates and, together with absorption onto the cell matrix, account for the observed decrease of zinc in the microcosm solutions. The overall results produce more evidence supporting the role of cyanobacteria for bioremediation of metal contaminated waters. Sequential extraction studies are required to further understand the mobility and subsequent stability of metals associated with this type of microbial mats