Evolution of Available Heavy Metals in Soils Amended with Sewage Sludge Cocompost

Abstract

In the present study a soil microcosm experiment was set up to investigate the evolution of the availability of 14 heavy metals as a function of sewage sludge and pruning waste cocompost dosage (0, 35 and 140 Mg ha⁻¹), incubation temperature (5, 20 and 35°C) and incubation time (0, 45 and 90 days) in four agricultural soils in a Mediterranean climate. Due to the multivariate nature of the experimental design, the results were mainly interpreted using factorial analysis. Soil type was the most influential variable studied within the system. Despite the complex network of relationships among the variables, it was possible to observe common trends in the evolution of available heavy metals (AHM) in the soils studied: Mn, Co, Ni, Cd, Zn, Mo, As, Sb and V, in most cases increased with time and with the amendment of compost, and decreased with incubation temperature, with the latter being especially pronounced in the amended soils (doses of 35 and 140 Mg ha⁻¹). Correlations were found between AHM and certain soil constituents and properties, some of which also evolved with the addition of compost and with incubation. Particularly noteworthy were negative correlations observed between organic materials and Ba, Se, Cu and Pb and between carbonates and Pb, Mn and Cd, positive correlations found between clays, Ba and Cu and pH that affected the evolution of Co and Mn. The addition of any dose of compost to the agricultural soils sampled did not raise AHM content to toxic levels. However, the results show that cocomposts containing sewage sludge must be used with caution. A prior study to optimize dosage addition, time and soil suitability (the most influential variable in the experimental system) is recommended in order to avoid problems from heavy metal toxicity arising.