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University of Balamand > Academics > Research > Seminars > Abdalla Shammas

bioavailability of cadmium in sewage sludge

SUBMITTED BY: Abdalla T. Shammas
University of Balamand

ABSTRACT: Present guidelines for sewage sludge disposal on land put a limit on the amount of cadmium (Cd) that may be added in any one year, and on the cumulative amount added over a number of years. The justification for this limit is that Cd enrichment of the food-chain must be restricted so that human exposure even to subtoxic levels of Cd can be avoided. However, the relation between sludge Cd addition and plant enrichment with Cd is still not well defined. This relation, bioavailability, must be correctly assessed in order to realistically determine what is a permissible level of Cd addition, and consequently how much sludge can be applied on a site.

In this study, three sludges of varying Cd and Zn concentrations as well as Cd(NO3)2 and Zn(NO3)2 were used to study the bioavailability of sludge Cd as affected by Cd concentration in sludge, liming regime, and the interactions between Cd and Zn. Ten soils of widely differing clay and organic matter contents were used to examine the effects of these variables on the uptake of sludge Cd.

High sludge Cd additions (74 kg Cd/ha) and high plant Cd concentrations (13 ug/g) were tolerated by the test plants without any reduction in growth. Hence, in the range of sludge Cd additions used, the problem of Cd uptake appears to be essentially a human health hazard rather than a phytotoxicity problem.

Sludges of low Cd content (19 and 9 ug/g) did not enrich the plants with Cd regardless of application rate, while increasing additions of the high Cd sludge (180 ug/g) increased plant Cd concentrations significantly but to a much lower extent than equivalent additions of Cd(NO3)2. Cadmium uptake from sludge was initially proportionate to application level with some drop in the relative rate of uptake at high applications. Soil properties had little effect on this uptake pattern. This difference in the magnitude and pattern of Cd uptake suggests that the availability of sludge-borne Cd is controlled mainly by the sludge, while salt Cd is more sensitive to reactions with the soil complex which affects its relative uptake. Soil properties other than pH seem to have limited effects on the availability of sludge Cd. Multiple regression analysis showed that variations in Cd uptake from sludge added to ten soils could not be attributed to variations in clay and organic matter contents or CEC. However, high soil pH was found to be an important factor in lowering the availability of sludge Cd.

Addition of Zn(NO3)2 enhances markedly the uptake of sludge or salt Cd probably due to displacement of adsorbed Cd with a resulting increase in Cd concentration in soil solution. Apparently, competition between Cd and Zn for adsorption sites on roots, which would lower Cd uptake, is outweighed by Cd displacement from sorption sites on the soil-sludge complex to the soil solution. Accumulation of soluble salts in pots was found to increase plant Cd concentration, probably because soluble Cd was increased as a result of replacement of surface-bonded Cd by soluble cations, particularly Ca. Also formation of soluble chloride complexes of Cd would contribute to increased solution concentrations of Cd. Thus, leaching of excess soluble salts from sludge application sites would be advisable.

Soil extraction with diethylenetriaminepentaacetic acid (DTPA) measures the enrichment of soil with complexible Cd, but does not necessarily reflect the availability of Cd for plant uptake. While high correlations between DTPA and plant Cd concentrations were found when a wide range of Cd applications was used, poor correlations resulted when several factors, such as soil, Zn addition, and time were investigated at one application rate of Cd. Concentration of Cd in soil solution would then be a better predictor of Cd availability than would the amount of Cd added or the DTPA extractable fraction.

These results suggest the high organo-metallic stability of sludge-borne Cd, and that its bioavailability is controlled mainly by the sludge humic substrate. Enrichment of the food-chain with Cd, and its subsequent threat or human health, would be avoided if its content in digested sewage sludge is kept at or below 20 ug/g. Guidelines for sewage sludge disposal on land need to consider the Cd content of the sludge prior to restricting the amount of Cd that may be added in any one year, and on the cumulative amount added over a number of years.
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