The Treatment Of Heavy Metal Wastewater By Metal Sulfide Composites:Efficiency And Mechanism

In recent decades,with the rapid development of the industry,the discharge of industrial wastewater has also increased dramatically.Thse industrial wastewaters often contain a variety of pollutants,such as Cr（VI）and phosphate,and Cr（VI）and Ni（II）.The treatment of such multi-component wastewater is difficult than the treatment of single-component wastewater.Therefore,it is necessary to further develop more efficient adsorbents for wastewater treatment.Metal sulfides,such as pyrite（Fe S₂）and molybdenum disulfide（Mo S₂）,have been regarded as highly efficient adsorbents due to their excellent properties,including high surface reactivity,strong reducibility,good selectivity,and facile synthesis.At present,their application in the treatment of wastewater has attracted wide attention.In this study,pyrite-loaded magnetite（Fe₃O₄/Fe S₂）composite and cetyl trimethyl ammonium bromide intercalated molybdenum disulfide（Mo S₂/CTAB）composite with increased lattice spacing were synthesized.Transmission electron microscopy（TEM）,Brunauer-Emmett-Teller method（BET）,vibrating sample magnetometry（VSM）,X–ray diffraction（XRD）,Fourier transform infrared（FTIR）,and X–ray photoelectron spectroscopy（XPS）were applied to elucidate the simultaneous removal mechanism of Cr（VI）and phosphate by Fe₃O₄/Fe S₂and Cr（VI）and Ni（II）by Mo S₂/CTAB.The main conclusions of the study are as follows:（1）The magnetic Fe₃O₄/Fe S₂prepared by ultrasonic and co-precipitation method is composed of spherical particles with rough surface.The average diameter of the composite was about 20 nm.The magnetization saturation value of the composite was 60.7 emu/g,which indicates that the composite can perform good in magnetic separation.At p H 3.0,the removal efficiency of Cr（VI）and phosphate was nearly 100%.The removal efficiency of Cr（VI）and phosphate were still high during the fourth regeneration cycle.The maximum adsorption capacities for Cr（VI）and phosphate by Fe₃O₄/Fe S₂were 32.5 mg/g and 24.8 mg P/g,respectively.Langmuir isotherm model and the pseudo-second-order kinetic model can well describe this adsorption process which was controlled by chemisorption.（2）The removal of Cr（VI）by Fe₃O₄/Fe S₂was achieved through three aspects.First,Cr（VI）was adsorbed to the surface of Fe₃O₄/Fe S₂by electrostatic attraction.Secondly,part of Cr（VI）was reduced to Cr（III）by S₂^2-and Fe²⁺.Thirdly,Cr（VI）,Cr（III）,and Fe（III）can further form Fe（III）/Cr（III）/Cr（VI）hydroxides.The removal of phosphate by Fe₃O₄/Fe S₂was mainly achieved by electrostatic adsorption and the formation of Fe PO₄precipitate on the surface of Fe₃O₄/Fe S₂.（3）The Mo S₂/CTAB composite prepared by hydrothermal method was uniform sheets with good crystalline structure.XRD results showed that the d-spacing of Mo S₂/CTAB（9.5?）increased significantly due to the intercalation of CTAB.The increased d-spacing was conducive to exposing more S and Mo atoms,which was conducive to increasing the adsorption capacity for Cr（VI）and Ni（II）by Mo S₂/CTAB.（4）There was a synergistic effect in the simultaneous removal of Cr（VI）and Ni（II）by Mo S₂/CTAB.The adsorption capacities for Cr（VI）and Ni（II）by Mo S₂/CTAB from Cr（VI）-Ni（II）bimetallic system were obviously higher than those from Cr（VI）or Ni（II）single metal system.Langmuir isotherm model can well describe this adsorption process which was spontaneous and endothermic.The removal efficiency of Cr（VI）and Ni（II）by Mo S₂/CTAB still reached nearly 90%after 5 regeneration cycles.The maximum adsorption capacities for Cr（VI）and Ni（II）by Mo S₂/CTAB were 79.4 mg/g and 88.3 mg/g,respectively.Coexisting cations and anions have little effect on the removal of Cr（VI）.In contrast,coexisting anions have a slight effect on the removal of Ni（II）,and coexisting cations have a significant inhibitory effect on the removal of Ni（II）.（5）In the Cr（VI）-Ni（II）bimetallic system,the removal mechanism of Cr（VI）by Mo S₂/CTAB is mainly as follows.First,protonated Mo S₂/CTAB can adsorb Cr（VI）by electrostatic attraction.Then,Cr（VI）was in full contact with Mo and S atoms on Mo S₂/CTAB,and thus it can be reduced to Cr（III）by Mo⁴⁺and S^2-.The removal of Ni（II）by Mo S₂/CTAB was achieved through two aspects.First,Ni（II）was adsorbed on the surface of Mo S₂/CTAB by electrostatic attraction.Second,the adsorbed Ni（II）formed outer sphere surface complexation on the surface of Mo S₂/CTAB.