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Effects of pH and Phosphate on Cadmium Sorption by a Hydrous Ferric Oxide¹

ABSTRACT

Sorption of cadmium by phosphate-free and phosphate-enriched hydrous iron oxides was investigated in 0.1M NaNO₃ solution. Significant Cd sorption by the phosphate-free oxide sample occurred at pH levels below the oxide zero point of charge (pH_ZPC), indicating that the sorption was sufficiently energetic to over come some electrostatic repulsion. Phosphate enrichment of the oxide enhanced its capability to retain Cd, and lowered the average pH of the sorption edge. The Langmuir sorption constant for Cd was increased by such enrichment. Such results could be attributed to reduction of the pH_ZPC and surface potential with phosphate enrichment, and were described satisfactorily by simplified electrostatic theory. Calculated activities of soluble Cd in the pH range of 4 to 7 were well below those calculated from the solubilities of Cd(OH)₂ or Cd₃(PO₄)₂. This suggests that precipitation of these two compounds was not a likely mechanism controlling the concentrations of soluble Cd for these systems.

¹ Scientific Paper no. 6503. College of Agriculture Research Center, Washington State Univ., Pullman. Project no. 0490.

² Associate Soil Scientist, Western Washington Research and Extension Center, Puyallup, WA, and Professor of Soils, Univ. of Florida, Gainesville, respectively.

Received for publication April 25, 1984. Accepted for publication April 27, 1984.

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