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Effect of Bentonite on Plant Availability of Sludge-Borne Heavy Metals in Soil¹

ABSTRACT

Agricultural use of industrial sewage sludge is limited by the presence of potentially toxic heavy metals within the sludge. Cation exchange capacity (CEC) has been identified as a soil property that is involved in inhibition of heavy metal mobility in the soil, and in subsequent plant uptake of these metals. In addition, other soil factors such as pH influence plant uptake of metals. This investigation was conducted to determine how modification of soil CEC would affect soil availability and plant uptake of selected sludge-borne heavy metals.

Cation exchange capacity and soil pH were altered by the addition of bentonite. Sewage sludge from a highly industrialized section of Atlanta, Georgia, was used as the heavy metal source. These materials were arranged in a 3 x 4 factorial using three bentonite and four sewage sludge rates and then combined with a Cecil sandy loam soil. Wheat (Triticum aestivum L. McNair 1813) was grown as an indicator crop under greenhouse conditions for 2 mo. Analyses of plant and soil samples indicate that addition of sewage sludge results in an increase in soil and plant concentrations of certain heavy metals. Concentrations of selected microelements extracted from soil mixes by DTPA were positively correlated with plant component levels. Introduction of bentonite resulted in increases in soil CEC and pH and reductions in concentrations of these same elements in the soil and plant. These data suggest that soils selected for land application of sewage sludge containing high heavy metal levels should have relatively high CEC and pH levels. Further work is necessary to differentiate between the CEC and pH effects of bentonite on the disposition of these and other heavy metals.

Key Words: cation exchange capacity • pH • plant uptake • microelement • montmorillonite • heavy metal fixation • 2:1 clays • wheat

¹ Contribution from the Dep. of Agron., Univ. of Georgia, Georgia Station, Experiment, GA 30212.

² Graduate Research Assistant and Professor, respectively.

Received for publication February 6, 1978.