Toxic metals remediation in water and soil by phosphate treatment

Toxic metals pollution of water and soil is one of the most environmental concerns. Unlike organic contaminants, metals persist for a long time after their introduction in the environment adversely affecting water resources and agricultural productivity. Numerous studies demonstrated the effectiveness of phosphate treatment as a cost-effective and environmentally positive remediation technology of contaminated waters and soils. The goal is to reduce mobility and biovailability of toxic metals through the formation of metal phosphates with low solubility and which are stable under a wide range of environmental conditions. The possible reaction mechanisms for metal remediation include ion exchange processes, surface complexation, dissolution of phosphate materials and precipitation of new metal-bearing phosphates, and substitution by metals during recrystallization (coprecipitation) of phosphate minerals. Several natural and synthetic phosphate materials such as hydroxyapatite (HA), phosphate rock (PR), eggshell-derived HA, diammonium phosphate, triple superphosphate, etc. have been used. However, contaminated waters and soils are multi-component systems and the competition among toxic metals can affect the removal efficiency of phosphate amendments. Our results have shown that synthetic HA and PR can effectively immobilize toxic metals such as Pb, Cd, Cu, Zn, Ni and Co in polluted waters and soils. In particular, synthetic HA is able to remove Pb, Cd, Cu and Zn from aqueous solutions both in single- and multi-metal (Pb + Cd + Cu + Zn) systems. The removal efficiency generally ranges from 94 to 98% in single-metal solutions with reduced efficiency in multi-metal systems. Also in metal contaminated soils from sulfide mine areas the application of HA and PR results in the reduction of reactive and biovailable metal fractions in more stable forms. In general, our results suggest that the main mineralogical composition of the soils in mine areas may impact the effectiveness of metal immobilization. Although PR has lower removal capacity than synthetic HA, the application of PR in contaminated soils can minimize the potential risk of eutrophication due to the use of highly soluble phosphate sources. Phosphate-induced metal immobilization represents a cost-effective and environmentally friendly remedial strategy for contaminated waters and soils.