Sorption of Copper and Lead by Hydrous Ferric Oxide

Sorption of Cu²⁺ or Pb²⁺ on amorphous hydrous ferric oxide was unaffected by aging of the solid over several days, by variations in ionic strength from 0.005 to 0.5, or by changes in the nature of the background electrolyte from NaClCU to a complex artificial seawater mix. (Only Pb²⁺ sorption decreased because of complexation by Cl^- at high concentration.) Competition between Cu²⁺ and Pb²⁺ was difficult to demonstrate because saturation of the surface was not achieved with either metal before precipitation of the corresponding metal hydroxide. Competition was demonstrated between Cu²⁺ and Fe²⁺ in an oxygen-free system with a large excess of Fe²⁺, providing an estimate of total adsorption sites of ca. one site per Fe atom. Systematic variations in sorbing metal and hydrous ferric oxide concentrations showed that the sorption process can be approximately described as a simple mass action law. The corresponding adsorption constant which increases with pH with a slope of ca. 1.5 (pK vs. pH) appears applicable over a wide range of pH and metal and oxide concentrations. The high capacity of the iron oxide for metals is inconsistent with a purely surface phenomenon and suggests an open, permeable structure for the hydrous ferric oxide.