High adsorption performance of synthesized hexametaphosphate green rust towards Cr(VI) removal and its mechanism explorations.

Hexametaphosphate intercalated green rust (hexa-P GR) was fabricated by a coprecipitation process in an anaerobic environment to improve the adsorption of hexa-P GR for Cr(VI) and the total Cr under various aqueous conditions. Three kinetic models including the pseudo-first-order, intraparticle, and Elovich were appropriate in describing the adsorption of hexa-P GR towards Cr(VI) and the total Cr. The maximum mono-layer adsorption capacities (mg/g) of hexa-P GR for Cr(VI) at pH of 2 and 7 were 87.64 and 92.25, respectively, with the theoretical maximum capacity (mg/g) of 52.73 being obtained at pH of 7. Some competing cations existing in solutions such as Al3+, Ca2+, and Mg2+ would consume more hexa-P GR to remove Cr species. The neutral and weak alkaline environment was conducive to the hexa-P GR reuse, while the strong alkaline environment was beneficial to the removal of the total Cr. The orthogonal variables including the initial pH, the flow rate, and the Cr(VI) concentration all significantly influenced Cr removal. The sequences of reaction pathways referring to the adsorption of hexa-P GR differently occurred in various pH conditions.