Distribution of soluble heavy metals between ionic and complexed forms in a saturated sediment as affected by pH and redox conditions

Abstract

Research was undertaken to determine the effects of pH and redox potential on heavy metal speciation and on the size distribution of the organo-metal complexes in the soluble fraction of sediment-water systems. A cation exchange technique was effective in separating free and complexed metal ions. Fe, Mn and Zn differed in the extent of complexation with soluble organic matter. Under reducing conditions approximately two thirds of the soluble Fe was in a complexed form that was not sorbed in passing through the cation exchange resin. Soluble Mn on the other hand, was almost completely ionic under reducing conditions, with only a trace amount passing through the resin column. Over 90 percent of the soluble Zn was complexed under reduced conditions, with only 9 percent sorbed onto the resin. The complexed Fe and Zn were bound to soluble organic matter particles with equivalent molecular weight greater than 25,000 while Mn passed through this size filter. There were marked differences in the size distribution of the various organo-metal complexes under different redox and pH conditions. The soluble Fe was associated with both the largest and smallest size ranges of soluble organic matter. The effect of pH was most evident in the smallest size range with much more complexed iron being present at low pHs. Mn, on the other hand, was associated with only the smallest size range under all pH and redox conditions, reflecting its ionic nature. The greater solubility and mobility of Mn probably accounts for it being depleted relative to iron in Gulf Coast sediments. Hg and Pb were associated with only the largest size soluble complexes and were little affected by pH and redox conditions.