Copper and zinc adsorption by sewage sludge-treated soil in Southern Spain

Heavy metal accumulation in soil due to the application of sewage sludge may induce changes in its ionic retention capacity. In this work, sludge application effects on copper (Cu) and zinc (Zn) adsorption by the surface horizon of a calcareous soil in Southern Spain have been studied. Sludge from the urban waste water treatment plant in Cordoba was applied at rates of 0, 40, and 80 Mg sludge ha(-1) for two consecutive years. Despite the low metal content of this sludge and the high amount of organic matter added, a decrease in the soil adsorption levels for both Cu and Zn was detected in the sludged samples. Differences were observed, though, in the behavior of each metal, relative to the main adsorption mechanism involved. Copper adsorption was related to carbonate surfaces in soil. Values up to 400 mmol adsorbed Cu kg(-1) soil were obtained for the three treatments, in equilibrium with 13.6, 16.7, and 17.4 mmol Cu m(-3) in solution for 0, 40, and 80 Mg sludge ha(-1), respectively, the first year, and 13.9, 21.2, and 52.2 mmol Cu m(-3) in solution the second year. However, an increase of soil Cu availability is not likely to arise during or after continuous application of these sludge doses, due to the high calcium carbonate content of this soil. In the case of Zn, adsorption took place in CEC sites, with values up to 40 mmol adsorbed Zn kg(-1) soil in the 0 and 80 Mg sludge ha(-1) samples, in equilibrium with 158.3 and 164.5 mmol Zn m(3) in solution, respectively the first year, and 196.1 and 505.8 mmol Zn m(3) in solution the second year, whereas in the 40 Mg sludge ha(-1) samples adsorption was below 25 mmol Zn kg(-1) soil, in equilibrium with 171.1 mmol Zn m(3) in solution the first year, and 231.0 mmol Zn m(-3) the second year. As soil CEC values and its organic matter content are positively related, Zn availability in this soil could be expected to increase after the cessation of sludge applications, due to the mineralization of the organic matter added.