Comparative study of in-situ formation process of Mg-Al layered double hydroxides though two pathways to remove Cr(VI) and As(V) from wastewater

Abstract

The method of direct synthesis of layered double hydroxide (LDHs) in wastewater has attracted wide attention, but the formation process and removal mechanism are still unclear. In this study, two different paths of alkali solution (NaOH) dropped to salt solution (Mg2+/Al3+) (A-S process) and salt solution dropped to alkali solution (S-A process) were proposed to directly synthesize LDHs in wastewater and remove contaminants Cr(VI)/As(V) simultaneously. In A-S process, the formation of LDHs could be divided into three stages: initial formation of amorphous Al(OH)3, crystal phase transformation, and the complete formation of LDHs crystal. There was a gradual incorporation of Mg into Al(OH)3, and the removal of Cr(VI)/As(V) significantly increased upon the formation of LDHs structure. However, in S-A process, the formation of LDHs primarily involved crystal structure optimization. The layered structure was constantly growing and optimizing, accompanied by the removal of Cr(VI)/As(V). The Competition removal experiment of Cr(VI)/As(V) indicated that As(V) preferentially possess active sites and occupied more high-energy sites analyzed by molecular simulation and site energy distribution theory. The experimental investigation of co-existing ions, pH and recycle experiment for practical performance were also examined and demonstrated a high stability and a good recyclability.