Performance and mechanism of Mg, Al layered double hydroxides and layered duble oxides for sulfide anion (S2-) removal

Based on the exchangeable property of the interlayer anions and the so-called "memory effect" of the layered double hydroxides (LDHs), the capacities and mechanisms for sulfide anion (S2-) removal were studied using chloride-containing MgAl-LDHs (MgAl-Cl-LDHs) and the layered double oxides (MgAl-LDO). Structures of the products obtained by anion exchange and reconstruction, and the removal capacities were characterized by XRD, FTIR, UV-Vis and ICP, and the different mechanisms for MgAl-Cl-LDHs and MgAl-LDO were proposed. The results indicate that the removal capacity for MgAl-LDO is much greater than that for MgAl-Cl-LDHs. In this work S2- removal test was proceed at pH of 9, and then S2- was hydrolyzed, and mainly existed as HS- in the solution. When MgAl-Cl-LDHs was used as the precursor to remove S-2(-), due to the weaker exchange capacity for HS- compared to Cl-, HS- could not enter the interlayer through anion exchange with Cl-. Adsorption of HS- to the sample's surface and oxidation to S2O32- are the removal mechanism for MgAl-Cl-LDHs. While when MgAl-LDO was used as the precursor, during the structure recovering, HS- entered the interlayer qickly, and was oxidized to thiosulfate anion (S2O32-) and polysulfide anions (S-2(-) and S-3(-)) simultaneously. Therefore, MgAl-LDO can remove S-2(-) efficiently. The products of LDHs with Mg/Al molar ratio of 2 calcined at 500 degrees C were shown to have a high degree of removal capacity at above 98%.