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Sorption and Release of Cadmium by Some Sewage Sludges¹

ABSTRACT

Sorption and release of Cd were determined in nine sewage sludges from Tuscany, Italy. The Cd adsorption data were in agreement with both the linear sorption isotherm and the Langmuir equation. The average of calculated adsorption maxima was about 50% of the mean CEC value, indicating that exchangeable cations present in the sludges were more effective in competing with the Cd ion for exchange sites. The calculated Langmuir adsorption maxima were found significantly correlated with fulvic acid C content and carboxyl and phenolic hydroxyl groups of fulvic acids.

The release of Cd retained at two initial concentrations as a function of different extraction solutions was indagated. Extraction experiments showed that Cd may be held in sludges by more than one type of site and that the strength of bonding decreases as the total Cd increases. Cadmium was retained mainly by exchange and complexing sites, the former assuming a relatively more important role as cadmium adsorption increased.

The effect of different exchangeable cations on the sorption of Cd by sludges was also studied. The amounts of Cd sorbed on the sludge surfaces decreased in the order: Ca-sludge > untreated sludge > Fe-sludge = Al-sludge.

Key Words: ion exchange • complexation • Langmuir adsorption model • exchangeable bases • heavy metal • soil pollution

¹ Joint contribution of the Institutes of Agricultural Chemistry, University of Viterbo, Pisa and Catania. This research was supported by M.P.I.

² Professor, University of Viterbo, Associate Professor and Graduate Research Assistant, University of Pisa, and Assistant Professor, University of Catania, respectively.

Received for publication February 12, 1982.

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