Reclamation of a mine contaminated soil using biologically reactive organic matrices

Organic residues such as sewage sludge, biowastes and composts are increasingly used in land rehabilitation because they can improve the physical, chemical and biochemical properties of soil, and reduce the need for inorganic fertilization. Furthermore, their use contributes to an integrated approach to waste management by promoting recycling of nutrients and minimizing final disposal, especially of organic residues that, due to their composition, can pose problems to agricultural soils. In the present study, three different types of organic residues were considered as amendments to be used in the reclamation of a metal-contaminated mine soil from the Aljustrel mining area (a pyrite mine located in the SW Portugal in the Iberian Pyrite Belt), with high Cu, Pb and Zn total contents: sewage sludge from a municipal wastewater treatment plant (SS), compost from the organic fraction of unsorted municipal solid waste (MSWC), and garden waste compost (GWC), applied at 100 and 200 Mg ha^—1 . The soil and mixtures of soil and amendments were adjusted to 70% of the maximum water-holding capacity determined for each type of sample and incubated in a controlled-temperature room at 20 ± 1 °C. Sub-samples were taken prior to wetting (time zero), and after 7, 14, 21 and 28 days of incubation, and analysed for pH, electrical conductivity, organic matter content, effectively bioavailable Cu, Zn and Pb (extracted with 0.01 mol L^{— 1} calcium chloride) and potentially bioavailable metals (extracted with 0.5 mol L^—1 ammonium acetate, 0.5 mol L ^—1 acetic acid and 0.01 mol L^{— 1} EDTA, pH 4.7). In general, organic residues corrected soil acidity, and increased the total organic matter content of the soil. The SS and the MSWC amendments were roughly equivalent in their ability to correct soil acidity whereas the GWC had the smallest liming capacity and only with 200 Mg ha^—1 GWC did the soil pH reach acceptable values. As expected, all the tested organic residues, at both application rates, were effective in reducing the effectively bioavailable metals in the soil. The Zn bioavailability was the most affected by the addition of organic residues, whereas Pb bioavailability was small even in the unamended soil and was the least affected by treatments. Potentially bioavailable metals increased with SS and MSWC application and the opposite was true following amendment with GWC.