Colloid‐mediated transport of Pb through soil porous media

Intact soil column leaching experiments were used to assess the role of water dispersible soil colloids with diverse physicochemical and mineralogical composition in co‐transporting Pb in subsurface soil environments. There was essentially no elution of Pb (0 to < 1%) in 10mg/L control Pb solutions, suggesting a near complete attenuation by the soil column matrix. When the control Pb solutions were mixed with 300mg/L soil colloid suspensions, Pb transport increased by 10–3,000 times over that of control solutions. The presence of colloids increased the transport of both, the colloid bound and the soluble Pb fraction. Colloid‐induced transport was enhanced by increasing colloid surface charge, pH, organic carbon, and soil macroporosity and inhibited by increasing colloid size, Al, Fe, and quartz content. However, increased soil organic carbon content appeared to compensate for some of these limitations. Colloid binding and co‐transport appeared to be the dominant mechanism for increases in Pb transportability, but physical exclusion of soluble Pb species from matrix exchange sites blocked by colloids, competitive sorption, and organic complexation were also important. These findings suggest that the colloids play a dual role as Pb‐carriers and facilitators in the migration process and could have important ramifications on contaminant transport prediction and remediation applications.

Keywords:

• Surface charge
• mineralogy
• metal‐colloid sorption
• soil macroporosity
• soil columns
• co‐transport