Immobilization of aqueous cadmium by addition of phosphates

In situ immobilization of heavy metals in polluted soils using phosphates leads to formation of products which are highly insoluble and thermodynamically stable over a broad pH and Eh range. In this research effectivity of Cd ions immobilization (initial [Cdaq]-4.800 mM) from aqueous solutions by different phosphorus compounds (K2HPO4, NH4H2PO4 and “Polifoska 15” fertilizer) was compared at pH in the range 4.00-9.00 and for reaction times 2-1440 h. The highest reduction of cadmium concentration (>99%), owing due to the formation of cadmium phosphates, was observed for all used phosphorus sources within pH range of 6.75-9.00. Uptake of cadmium for pH ≤ 5.00 did not exceed 80% and was lowest in the reaction with “Polifoska 15” fertilizer (28.25%). Identification of phases formed in the reactions using XRD, FTIR and SEM-EDS-EBSD was carried out. It was noticed that crystallinity of formed solid decreased with pH increase. Formation of Cd5H2(PO4)4·4H2O was observed in acidic conditions (pH ≤ 5.00), at pH ∼ 7.00 mixture of following cadmium phosphates Cd(H2PO4)2, Cd3(PO4)2 and Cd5H2(PO4)4·4H2O was formed. Amorphic cadmium phosphates were noticed in alkaline conditions (pH > 8.50).