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

Heavy metal accumulation in soil due to the application of sewage sludge ma y induce changes in its ionic retention capacity. In this work, sludge appl ication effects on copper (Cu) and zinc (Zn) adsorption by the surface hori zon of a calcareous soil in Southern Spain have been studied. Sludge from t he 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 meta l content of this sludge and the high amount of organic matter added, a dec rease 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 adsorpti on was related to carbonate surfaces in soil. Values up to 400 mmol adsorbe d Cu kg(-1) soil were obtained for the three treatments, in equilibrium wit h 13.6, 16.7, and 17.4 mmol Cu m(-3) in solution for 0, 40, and 80 Mg sludg e 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 availabili ty is not likely to arise during or after continuous application of these s ludge 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 equili brium 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, wherea s 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, a nd 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 b e expected to increase after the cessation of sludge applications, due to t he mineralization of the organic matter added.