Laboratory evaluation of zero-valent iron to treat water impacted by acid mine drainage

This study examines the applicability and limitations of granular zero-valent iron for the treatment of water impacted by mine wastes. Rates of acid-neutralization and of metal (Cu, Cd, Ni, Zn, Hg, Al, and Mn) and metalloid (As) uptake were determined in batch systems using simulated mine drainage (initial pH 2.3-4.5; total dissolved solids 14000-16000 mgl(-1)). Metal removal from solution and acid-neutralization occurred simultaneously and were most rapid during the initial 24 h of reaction. Reaction half-lives ranged from 1.50+/-0.09 h for Al to 8.15+/-0.36 h for Zn. Geochemical model results indicate that metal removal is most effective in solutions that are highly undersaturated with respect to pure-metal hydroxides suggesting that adsorption is the initial and most rapid metal uptake mechanism. Continued adsorption onto or co-precipitation with iron corrosion products are secondary metal uptake processes. Sulfate green rust was identified as the primary iron corrosion product, which is shown to be the result of elevated [SO(4)(2-)]/[HCO(3)(-)] ratios in solution. Reversibility studies indicate that zero-valent iron will retain metals after shifts in redox states are imposed, but that remobilization of metals may occur after the acid-neutralization capacity of the material is exhausted.