Solubility product (Ksp)-controlled removal of chromate and phosphate by hydrocalumite

It has been revealed that hydrocalumite (Ca2Al–Cl–LDH) removes Cr (VI) via anion-exchange while P via dissolution–reprecipitation. In current work, the intercalation of Cr (VI) was relative to the remaining LDH, which resulted in the increase of Cr (VI) removal amount with increasing pH. In contrast, more Ca5(PO4)3OH was precipitated at initial pH 5.0 in the case of P removal, which was attributed to more hydrocalumite dissolved at such low pH. Moreover, the composition of the final solution in each case provided the evaluation of free ionic concentrations product (CP) which demonstrates that the formation of product was related to its CP value that was higher than or close to the corresponding Ksp. As Ca5(PO4)3OH or LDH–CrO4 has the lowest solubility product (Ksp) among all possible solid in each case, our results propose that the removal process of chromate and phosphate over hydrocalumite was controlled by Ksp of the product.

Graphical abstractChromate was intercalated into hydrocalumite via anion exchange to form LDH–CrO4, while phosphate was removed to form hydroxyapatite by dissolution–reprecipitation. The different removal processes of chromate and phosphate over hydrocalumite were controlled by the solubility product (Ksp) of final products.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► We examined the removal behaviors of P and Cr (VI) over hydrocalumite. ► Hydrocalumite removed Cr (VI) via anion-exchange while P via dissolution–reprecipitation. ► More Cr (VI) was intercalated to form LDH–CrO4 at high initial pH (9.0). ► More P was precipitated in hydroxyapatite at low initial pH (5.0). ► The different removal mechanisms were controlled by the solubility product (Ksp) of final products.