Heavy metal bioavailability in a soil affected by mineral sulphides contamination following the mine spillage at Aznalcollar (Spain)

A field experiment, lasting 14 months, was carried out in order to assess the effect of organic amendment and lime addition on the bioavailability of heavy metals in contaminated soils. The experiment took place in a soil affected by acid, highly toxic pyritic waste from the Aznalcollar mine (Seville, Spain) in April 1998. The following treatments were applied (3 plots per treatment): cow manure, a mature compost, lime (to plots having pH < 4), and control without amendment. During the study two crops of Brassica juncea were grown, with two additions of each organic amendment. Throughout the study, the evolution of soil pH, total and available (DTPA-extractable) heavy metals content (Zn, Cu, Mn, Fe, Pb and Cd), electrical conductivity (EC), soluble sulphates and plant growth and heavy metal uptake were followed. The study indicates that: (1) soil acidification, due to the oxidation of metallic sulphides in the soil, increased heavy metal bioavailability; (2) liming succeeded in controlling the soil acidification; and (3) the organic materials generally promoted fixation of heavy metals in non-available soil fractions, with Cu bioavailability being particularly affected by the organic treatments.