Changes in metal speciation and pH in olive processing waste and sulphur-treated contaminated soil

Degradation of organic matter from olive mill waste and changes in the heavy metal fractionation of a metal-contaminated calcareous soil were studied in a laboratory experiment, in which the olive mill waste was mixed with the soil and then incubated under aerobic conditions. The soil was calcareous (15% CaCO3) with high Zn and Pb concentrations (2058 and 2947 mg kg−1, respectively). The organic amendment was applied at a rate equivalent to 20 g kg−1 soil, and unamended soil was run as a control. To discern if changes in metal solubility were due to the acidic character of the waste, elemental sulphur was applied to soil as a non-organic acidifying material. The S0 rates used were 3.14, 4.71 and 6.28 g kg−1. The mineralisation of total organic-C (TOC) from the waste reached 14.8% of the original TOC concentration after 56 days of incubation. The CO2–C produced from S0-treated soils showed the carbonate destruction by the H2SO4 formed through S0 oxidation. The organic waste increased EDTA-extractable Zn and Pb concentrations and CaCl2-extractable Mn levels in soil after two days of incubation. The changes in metal availability with time indicated that the oxidation of phenols from the waste reduced Mn (IV) oxides, releasing Zn and Pb associated with this mineral phase. Organic waste addition did not decrease soil pH; the acidifying effect of S0 did not change metal fractionation in the soil.