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The mobility and adsorption of sewage sludge-derived copper and zinc in forest soil Evans, Rhian Emma

Abstract

Forest land application of sewage sludge results in the recycling of nutrients essential for plant growth, and also provides organic matter important for soil structure. However, land disposal of sewage sludge is not an ideal solution. A major environmental concern associated with sewage sludge application to land, is the fate of sludge-derived heavy metals such as copper and zinc. Consequently, the mobility and retention mechanisms of sludge—derived metals in forest soils are enormously important to questions of plant uptake and groundwater contamination. The purpose of this study was to examine the mobility and adsorption of sludge-derived copper and zinc in a forest soil. To determine the mobility of the metals a column study was conducted (Phase I) with one main objective: to determine the effect of time and rate of sludge application on the movment and accumulation of copper and zinc in the underlying forest soil. The results of this study indicated significant differences in metal accumulation between application rates in the upper organic horizon (0—5 cm) of the soil. It also became evident that time had a significant effect in the redistribution of metals. Furthermore, and of considerable environmental significance, was the fact that more than 75 % of both copper and zinc remained in the sludge after 4 months. The main objective of Phase II of the research, the Adsorption Study, was to determine the adsorption behaviour of copper and zinc by FH and Bf horizons, and by woody material. The adsorption data was successfully fitted to the Langmuir isotherm. This model provided parameters which reflected both the adsorption capacity and bonding energy of the metal. Results showed that the FH material had a greater bonding energy constant and a higher adsorption capacity for both the metals, than did either of the other two materials. This is significant because the majority of the sludge, when applied to a forest soil, is primarily in contact with the FH horizon. As expected, copper had both a greater bonding energy constant and adsorption maximum than zinc for all three materials. The mobility of sludge-derived copper and zinc in a forest soil is highly dependent upon the adsorption characteristics of the metal with the adsorbate material. Therefore based on the results of this research, it is possible to conclude that in a forest soil with a considerable FH horizon, copper, and to a lesser extent zinc, will be readily adsorbed and retained by this highly organic material.

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