Effect of partial oxidation of organic matter on cadmium leaching from phosphate

Abstract

Phosphate rocks are the main phosphorus source, a key nutrient for crop production in the tropics. Besides phosphorus and calcium, phosphate rocks may contain organic matter and heavy metals, such as cadmium and lead, depending on their geological origin. In general, phosphate rocks from sedimentary origin present higher content of heavy metal than igneous phosphate rocks. The world phosphate rock consumption of sedimentary and igneous origin is around 75 and 20%, respectively. The use of cadmium containing phosphate rocks as fertilizers can bring risks to the environment. The objective of this work was to investigate the influence of partial oxidation of organic matter with H2O2 as a pre-treatment to improve cadmium leaching from sedimentary phosphate with high organic matter content. As leaching agents, MgCl2, KCl, Na2-EDTA and citric acid solutions were tested. When cadmium removal was carried out with a 0.1molL−1 Na2-EDTA solution without partial oxidation of organic matter, its content in the sample was reduced from 49.5 to 38.5mgkg−1, after 5h, corresponding to a removal of only 23%. On the other hand, when cadmium leaching was carried out after organic matter oxidation with a 40mgL−1 H2O2 solution, the cadmium content in the sample was reduced from 49.5 to 22.5mgkg−1, corresponding to a removal of 54.5%, turning it more adequate to be used as fertilizer.