Release of Heavy Metals from Dolomite Phosphate Rock after Activation with Organic Agent

As high-grade phosphate reserves begin to diminish worldwide, low-grade phosphate rock such as that containing dolomite [CaMg(CO)] needs to be explored as a potential fertilizer. Activation of dolomite phosphate rock with humic acid substantially increased P availability by chelating Ca and Mg. However, the release potential of heavy metals during the activation process should be evaluated. In this study, an orthogonal experiment was designed to investigate the effects of major activating factors on the release of Cd, Cr, Cu, Mn, Ni, Pb, and Zn in activated fertilizers using a batch-equilibrium technique. The release potential of heavy metals from activated fertilizers was further compared with single superphosphate fertilizers made from the same dolomite phosphate rock. The activation process increased water-soluble Cr, Ni, and Zn by 39.8 to 65.7, 26.5 to 35.6, and 6.9 to 14.6 times, respectively, as compared with the original dolomite phosphate rocks, but there was a decrease in water-soluble Cd, Mn, Ni, Pb, and Zn by 1.4 to 5.4, 5.4 to 8.6, 2.3 to 3.7, 0.7 to 2.8, and 2.6 to 5.0 times, respectively, as compared with single superphosphate fertilizers. The release of heavy metals was affected by activating factors including dolomite phosphate rock type, particle size, humic acid dosage, and moisture content. Similar results were obtained with Mehlich-3 extraction, but levels of Mehlich-3-extractable heavy metals were less in activated fertilizers than in the original dolomite phosphate rocks, except Cu. These results indicate that activated fertilizers contain less bioavailable heavy metals than single superphosphate fertilizers and, therefore, are more environmentally friendly for application in agriculture.