Experimental investigation of the impact of pH on colloid-associated metal mobility in mine-impacted wetland sediment

Metal mining operations generate waste rock that is affected by mineral
weathering reactions and can produce acidic metal-rich leachate. Leachate
pH can have direct impacts on metal mobility by changing metal solubility,
and through colloid-facilitated metal transport, potentially increasing
metal transport downgradient of the waste rock. This work presents a set
of laboratory column experiments that study the effect of colloids on
metal mobility in wetland sediment impacted by the drainage from a
sulfide-bearing abandoned waste-rock pile. The fully-saturated experiments
investigate pH conditions ranging from 2.5 to 7, varying flow rates 45 to
90 m/yr, and measured metal concentrations in leachate filtered at 10 μm,
1 μm, 0.1 μm, and 0.01 μm. Results indicate that metal mobility is higher
at lower pHs for Al, Mn, Fe, Cu, Cd, and Zn, but the impact of the
smallest colloids studied (i.e., <0.01 μm) is reduced as the pH
decreases and metal transport is dominated by the dissolved phase. These
findings suggest that colloid-facilitated transport will enhance metal
transport in areas receiving mine drainage until the pH of the drainage
decreases towards values of approximately pH ~2.5.