Immobilization of heavy metals in water and soil by phosphate treatment

Heavy metals such as Pb, Cd, Cu, Zn, Hg, Cr, and Ni are the main contaminants of surface water, groundwater, and soils. The heavy metals are of great concern because of their extreme toxicity even at low concentration and the tendency to accumulate in the food chain. Conventional methods for heavy metal removal from water and soil include chemical precipitation, filtration, solvent extraction, electrochemical technique, ion exchange, and adsorption. In particular, chemical immobilization is one of the most used for reducing the bioavailability of heavy metals. The aim is to immobilize the ions trapping them in minerals with low solubility and stable in the environment in a wide range of conditions. Synthetic or natural apatite and phosphate rock have been proved to immobilize divalent heavy metal ions such as Pb, Zn, Cd, Cu, Co, Cr, Ni, and Sb in contaminated waters and soils. The mechanisms of metal removal include ion exchange, surface complexation and dissolution/precipitation. In this study the removal of Cd, Pb, Zn, and Cu from aqueous solutions by both synthetic hydroxyapatite (HA) and natural phosphate rock (FAP) was investigated in batch conditions at 25 ± 2°C. The metals were applied both as single- or multi-metal (Cd + Pb + Zn + Cu) systems with initial concentrations from 0 to 8 mmol L-1. The removal capacity of phosphate amendments generally ranges between 50 and 99%. In the multi-metal systems competitive internal metal sorption reduced the removal capacity by 13–83% compared to the single-metal systems. The sorption of heavy metals by phosphate amendments follows the Langmuir model. Heavy metal immobilization occurs through a two-step mechanism: rapid surface complexation on the ≡POH sites followed by partial dissolution of phosphates and ion exchange with Ca resulting in the formation of heavy metal-containing phosphates.The sorption of Cu on HA was further investigated by means of the results of a combined structural simulation and EXAFS analysis. The EXAFS results suggest that the heavy metal is present in the Cu+2 form. The structural experimental and theoretical analysis shows that Cu is bond to about four O atoms at a distance of about 1.95Å. In all the studied cases the immobilization site of Cu is the same. The fixation of Cu occurs in the surface sites of hydroxyapatite whereas the sorption in the Ca sites in the inner part of the structure is unlikely.The effectiveness of phosphate treatment for Cd, Pb, Zn, and Cu immobilization in mine waste soils from sulfide mine areas (tailing dumps, ore stocking areas, streams, etc.) in Tuscany and Sardinia (Italy) was examined in batch conditions. Application of HA and FAP effectively reduced the heavy metals water solubility generally by about 84 to 99%. Between the two amendments evaluated, HA was slightly more effective with respect to FAP in immobilizing heavy metals. Although the lower effectiveness of phosphate rock, its application to reduce metal solubility in contaminated soils may minimize soil acidification and potential risk of eutrophication associated with the application of highly soluble phosphate sources.