Zinc Uptake from Circumneutral Mine Drainage in Freshwater Biofilms: New Insights from In Vitro Experiments

Abstract

We present results from in vitro flask and flume experiments using freshwater biofilms sourced and cultivated from a mine-impacted stream in North Yorkshire, UK. Flask experiments showed rapid uptake of Zn from the water column into biofilms. This uptake was not light dependant and suggests that chelation of Zn by negatively charged functional groups in extracellular polymeric substances (EPS) secreted within the biofilm are the key sink for Zn. Solid state analysis of the biofilm from the mesocosm system by scanning electron microscopy highlights the presence of calcite precipitates within the EPS, which may provide another sink for Zn. Long-term monitoring of flume systems showed area-adjusted removal rates of ≈0.2 g Zn m−2 day−1, which is consistent with many other biologically-mediated mine water treatment systems. Diel (24 h) fluctuations in pH and Zn were observed over a 96 h intensive sampling period in the flumes. Practical considerations for establishing and maintaining biofilms under controlled conditions are also highlighted; these include regulation of light intensity and maintenance of flowing, low nutrient status waters.