Tuning the composition of new brushite/vivianite mixed systems for superior heavy metal removal efficiency from contaminated waters

Calcium phosphates (CPs) are excellent sorbents, with hydroxyapatite being the most commonly used for heavy metal removal from ecological samples. The use of more acidic brushite as a CP sorbent can be justified by its greater pH compatibility with particular types of soil, such as soil in the areas of high rainfall, but also iron-containing soil prone to oxidative weathering and soil nitrified with ammonium fertilizers. Here, brushite nanoparticles were supplemented with vivianite as another metal phosphate phase and the resulting material was studied at different concentrations of the respective phases (x = 0.0, 0.2, 0.4, 0.8 and 1.0 in FexCa1-xHPO4 center dot 2H2O). Structural refinement showed that the weight ratio between the two phases intensely impacted their crystallographic ordering. The modeling of thermal analysis data with the use of Coats-Redfern and Horowitz-Metzger formalisms showed a multifold reduction in the activation energy and in the changes of enthalpy and entropy for both dehydration and decomposition transitions with the introduction of vivanite to brushite. Increasing the vivianite content enhanced the efficacy of the removal of heavy metals from aqueous solutions, but only up to a certain limit. The maximal efficacy, along with hardness and microporosity, was obtained for the x=0.8 composition. For this composition, over 98 % of Pb(II) and 85 % of Cr(VI) were removed from the solution at the standard heavy metal concentrations of 2 ppm. The systematic analysis at different weight ratios between the two constitutive phases allowed for the delineation of a composition with ideal properties for environmental decontamination applications.