Release of Heavy Metals from Dolomite Phosphate Rock (DPR) after Activation with Organic Agents.

As high-grade phosphate reserves begin to diminish in Florida, lower-grade sedimentary phosphate rock containing dolomite (CaMg(CO₃)₂) has to be considered as a potential fertilizer resource. The addition of organic agent (HA) as an activating reagent to dolomite phosphate rock (DPR) remarkably increased the availability of phosphorus (P) by chelating Ca²⁺ and Mg²⁺ ions. However, few studies were conducted to evaluate the risk of heavy metals released during the activation process. The objective of this study was to investigate release of heavy metals (Cd, Cr, Cu, Mn, Ni, Pb and Zn) from three DPRs (DPR1, DPR5 and DPR7) at the absence and presence (3%, 5% and 8%) of HA using a batch-equilibrium technique. Superphosphates (SPs) made from the three DPRs were included for comparison. Minimal amounts of Cd, Cr, Cu, Mn, Ni and Pb were released from original DPRs in water or Mehlich 3 extraction. Activation with HA resulted in elevated concentrations for all the heavy metals, 4~7 times in Cd, 8~13 times in Cr, 2~4 times in Cu, 13~28 times in Mn, 3~5 times in Ni, 2~3 times in Pb, 7~17 times in Zn. However, much more metals were released from SPs than activated DPRs: an increase by 7~10 times in Cd, 16~36 times in Cr, 12~14 times in Cu, 48~84 times in Mn, 17~28 times in Ni, 5~18 times in Pb, and 42~62 times in Zn. These results indicated that activated DPRs have advantages over SP fertilizers in reducing the amounts of available heavy metals to crops and the environment.

Key words heavy metals, dolomite phosphate rock, organic agent