Enhanced simultaneous removal of toxic (SeO4)2− and metals Cr3+ and Cu2+ using polysulfide intercalated Layered double hydroxide

Abstract It is usually very difficult to remove (SeO4)2−(Se(VI) from aqueous solution due to its high stability and mobility. Here, it was found enhanced removal efficiency of Se(VI) in the presence of metals cations such as Cr(III) and Cu(II) using trimetallic LDH (FeMgAl-MoS4 LDH) intercalated with MoS42− which abbr. as S-LDH. The removal efficiency of Se(VI) was relatively poor (67.9%) when treated alone. However, in the presence of Cr(III) and Cu(II) cations the removal efficiency of Se(VI) was enhanced to 99.32% and 96.02%, respectively under similar experimental conditions. The metal cations were also simultaneously removed (> 82%) and (> 99.99%) for Cr(III) and Cu(II) cations, respectively. Moreover, secondary water pollution was also inhibited in the presence of the metal cations with Se(VI) since the amount of Mo leached from S-LDH into the solution was decreased from 19.82 to 1.13 ppm when 140 ppm Se(VI) initial sample was treated alone and in the presence of 20 ppm Cr(III), respectively. Similarly, the amount of Mo leached was decreased from 21.34 to 2.37 ppm when 140 ppm Se(VI) was treated alone and with 20 ppm Cu(II), respectively. This could be due to co-precipitation reaction of the leached MoS42−with the cations in the solution. The pre- and post-adsorption characterization of S-LDH was conducted using XRD, FTIR, SEM-EDX, CHNS and XPS to determine the chemical structure and composition of the adsorbent as well as the reaction mechanisms. The reaction kinetics followed pseudo-second order which related with chemisorption and Langmuir reaction model was better fitted the isotherm data that indicated monolayer adsorption dominate in the reaction process. The evaluation of thermodynamic parameters with negative values of free energy, enthalpy and entropy indicated a spontaneous, exothermic and the solid/solution interface became less random and enthalpy driven processes. Based on the XPS result, the more toxic Se(VI) was mainly reduced to less toxic Se(IV) and Se(0) species due to the (-S-)2− in MoS42− and Fe(II) in the LDH while themselves oxidized to SO42− and Fe(III), respectively. We believe that this strategy provides a new direction to find enhanced removal of the reluctant Se(VI) and retarding secondary water pollution when MoS4 based LDHs used as sorbent materials.