Influence of Dissolved Organic Matter on the Solubility of Heavy Metals in Sewage-Sludge-Amended Soils

Authors: Ashworth, Daniel; ALLOWAY, B - UNIVERSITY OF READING

Submitted to: Communications in Soil Science and Plant Analysis
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/2/2006
Publication Date: 3/13/2008
Citation: Ashworth, D.J., Alloway, B.J. 2008. Influence of Dissolved Organic Matter on the Solubility of Heavy Metals in Sewage-Sludge-Amended Soils. Communications in Soil Science and Plant Analysis. Vol 39:538-550

Interpretive Summary: Sewage sludge is a waste by-product of the sewage treatment process and is often applied to agricultural soils because it is rich in organic matter and plant nutrients. However, because industrial and domestic waste are mixed in the sewage system, sewage sludge can also contain high levels of contaminants, e.g. heavy (potentially toxic) metals such as cadmium, nickel, lead, zinc and copper. The environmental significance of these metals is determined by their tendency to become soluble and move with the soil solution where they can be transported into surface or ground water, or taken up by crop roots. One process that may control the tendency of metals to enter the soil solution is solubilisation of organic matter, since many metals are chemically bound to this. This is especially important as organic matter becomes degraded over time and, as a result, may become more easily solubilised. In these experiments, the solubility of organic matter was artificially manipulated and the effect on the solubility of the heavy metals copper, nickel, lead and zinc studied. The solubility of copper, nickel and lead was found to increase in response to increasing organic matter solubility, particularly at high pH. Other processes controlled the solubility of zinc. The results suggest that under real conditions, the degradation and solubilisation of organic matter following sewage sludge application to soils may result in increased concentrations of certain heavy metals in the soil solution, and a consequent increase in their environmental significance

Technical Abstract: Sewage sludge-amended soils generally contain elevated levels of organic matter and heavy metals compared to control soils. Since organic matter is known to complex with heavy metals, the solubility behavior of the organic matter in such soils may exert a significant influence on the solubility of the metals. Little is known about such a process. Using batch experiments in which the solubility of organic matter in a heavily sludge-amended soil was artificially manipulated, we show that the solubility of the heavy metals Cu, Ni and Pb shows a strong positive relationship to the solubility of organic matter, particularly at high pH. The results suggest that under field conditions, spatio-temporal variations in the solid-solution partitioning of organic matter may have a bearing on the environmental significance (mobility and bioavailability) of these heavy metals.