Evaluation of municipal compost/limestone/iron mixtures as filling material for permeable reactive barriers for in-situ acid mine drainage treatment

The aim of the present study was to assess the potential of municipal compost as a carbon source for sulfate-reducing bacteria for acid mine drainage bioremediation for use in permeable reactive barriers at high flow rates (> 0.1 m d(-1)). Two different mixtures of municipal compost, limestone and zero-valent iron were assessed in two column experiments. The effluent solution was systematically analysed throughout the experiments. At the end of the experiments precipitates from both columns were withdrawn for scanning electron microscopy, energy-dispersive X-ray spectroscopy and X-ray diffractometry examination and solid digestion and sequential extraction were carried out. Results showed that the effluent was free of metals and acidity. It seems that metal removal was not due to biogenic sulfide generation but to pH increase, ie metal (oxy)hydroxides precipitation. These precipitates can sorb other metals onto the surface. Sorption to organic matter could also contribute to metal removal. When zerovalent iron was present, cementation of copper also occurred. It can be concluded that municipal compost was a poor carbon source to support continuous bacterial activity under high flow rates. (C) 2003 Society of Chemical Industry.