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Soil Adsorption of Cadmium From Solutions Containing Organic Ligands¹

ABSTRACT

The adsorption of cadmium (Cd) from aqueous solutions containing acetate (Ac), oxalate (Ox), nitrilotriacetate (NTA) and ethylenediaminetetraacetate (EDTA) onto three soils from the northeastern United States was investigated. Experimental parameters were adjusted so that, under typical field pH conditions, roughly half of the soluble Cd was present as Cd²⁺ while the other half, depending on the system, existed as Cd²⁺, CdAc⁺, CdOx⁰, CdNTA^–, or CdEDTA^2–, Organic complexation reduced, or had little perceptible effect on, Cd adsorption when compared to the unbound Cd system. In general, the ability to reduce Cd retention followed the trend: EDTA > NTA > Ox Ac. Since the strength of the Cd-ligand stability constants follows the same order, it suggests that competition for the metal ion between the soil exchange sites and the added organic ligands is important in explaining the observed behavior. Despite the formation of cationic complexes (CdAc⁺), the presence of Ac interfered with Cd uptake under acidic conditions. This was apparently due to competitive adsorption of the free un-ionized acetate species on the soil surface. The ability of organic complexants to influence Cd adsorption decreased with increasing suspension acidity because of preferential binding of the ligands to metals (Fe³⁺ and Al³⁺) released by the acid-catalyzed dissolution of soil components. The coexistence of soluble metal chelators with heavy metals in a land-applied waste may increase the mobility of the metals in the soil, the effect being more pronounced in neutral or alkaline soils and for ligands that form stable, nonadsorbing complexes with the metals.

Key Words: metal speciation • zero point of charge • specific adsorption

¹ Published with approval of the Director of the Delaware Agric. Exp. Stn. as Sci. Contrib. no. 969. The work was part of Regional Research Project NE-96: Soil Properties Affecting Sorption of Heavy Metals from Wastes.

² Assistant Professor and Research Assistant, respectively, Agricultural Engineering Department, University of Delaware, Newark, DE 19711.

Received for publication December 7, 1981.

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