Saturday, 15 July 2006

Phytostabilization of a Industrial Residue Contaminated with Zn and Cd.

Fabiana S. Santos, Márcio O.L. Magalhães, Nelson Mazur, and Nelson M.B. Amaral Sobrinho. Soil Department, Federal Rural Univ of Rio de Janeiro, BR 465 Km 7, Seropédica - Rio de Janeiro, 23890-000, Brazil

Phytostabilization and phytoextraction are the most usual phytoremediation techniques adopted for soils contaminated with heavy metals. Phytostabilization consists of using green plants to reduce the mobility of contaminant agents through revegetation strategies (Cunningham et al., 1995). In this case, the plant species tolerance, biological cycle, rusticity and ability to grown on unvegetated soils are characteristics that may contribute for the success of the stabilization of plants in soils contaminated with heavy metals. A greenhouse study was conducted to evaluate the effects of both chemical amendments (calcium silicate and brewery sludge), and phytoremediation using the grass Brachiaria humidicola, on an industrial residue contaminated with Zn and Cd. Residues from the extraction process of Zn from calamine ore in the Ingá Industry, situated at the Sepetiba Bay, in Rio de Janeiro State - Brazil. Ingá Industry, currently in bankruptcy situation, has been considered one of the companies that most have threatened local ecosystems. Residues accumulated over about 30 years may reach up to 2 million m3, and may contaminate soil, plants, marine sediments and million of fishes and mollusks with heavy metals, even banishing fishing activities.Industrial residue samples placed into 30-liter pots were amended with 20% brewery sludge, calcium silicate (2%, 3%), and 20% of brewery sludge + calcium silicate (2.5%, 4%), and were compared to the control treatment (non-amended residue). After pH stabilization, B. decumbens plants were grown on all the treatments, in order to evaluate the species ability to tolerate and extract Zn and Cd from the residue. Samples were collected twice, at planting and harvesting, for pH determination and simple extractions with water, sodium nitrate, acetic acid and DTPA. Differences in Zn and Cd concentrations in extracts allowed to estimate the concentrations of those elements in the most likely chemical forms they are found in the residue. High total concentrations of Zn, Cd, Fe, Mn and Pb were found in the Ingá residue. Soluble concentrations of Zn and Cd were found to be high as well, since those are elements that can be leached more easily, with a high risk in contaminating local environment, especially the groundwater. Alkaline and organic industrial amendments significantly reduced Zn and Cd percentages, both in the soluble and exchangeable fractions, as well as caused the predominance of Zn and Cd in the most stable chemical fractions, such as complexed and precipitated compounds. The Brachiaria humidicola biomass was affected by the Zn and Cd bioavailability, as well as by the type and doses of the amendments employed. Treatments with pH 7.0 (calcium silicate 3% and 20% of brewery sludge + calcium silicate 4%) showed high dry matter yield of B. humidicola, indicating that the pH value was important to reduce the solubility of heavy metals. B. humidicola showed tolerance and ability to extract Zn and Cd from the industrial residue. Key words: industrial residues, inertization, contamination, heavy metals.

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