Metal Precipitation in Two Landfill Leachates

Abstract

Leachate from solid‐waste landfills poses a significant environmental threat to groundwater supplies, warranting capture and treatment. Treatment prior to biological co‐production to reduce heavy metal concentrations via sorption onto in situ precipitated hydrous ferric and manganese oxides may provide adequate reductions while minimizing hazardous residuals. This investigation studied the partitioning of cadmium, copper, zinc, and nickel with iron and manganese solid phase formed from leachate constituents at pH 9. Batch‐type partitioning experiments were conducted using sodium hydroxide and calcium hydroxide for pH control. Naturally produced leachate was obtained from the Snohomish County and King County solid‐state disposal facilities in Washington state. The cadmium‐sorbent ratios of this investigation were well below those found by previous researchers; however, removal rates were somewhat below those found in organic‐free systems. These lower removal rates might be attributed to the “complexation” of cadmium by leachate organic constituents. Copper‐sorbent ratios were also well below those found by previous researchers; conversely, removal rates were significantly below those found for organic‐free systems. Lower copper removal rates could be attributed to the complexation by organics and the formation of uncharged and negatively charged species at pH 9. Zinc‐sorbent ratios were much higher than those found by previous researchers; however, removal rates were similar. The mechanism of zinc partitioning was probably the formation of a zinc oxide solid phase. Nickel‐sorbent ratios were well below those found by previous researchers, while removal rates were far below those of organic‐free systems. Lower nickel removal rates cannot be attributed to either organic complex formation or speciation effects.