Thallium Removal From Wastewater By Ferromanganese Oxide

Thallium?Tl?,known to be an extremely toxic heavy metal,is one of the priority heavy metal pollutants in China and many other countries worldwide.In recent years,the outbreak of Tl pollution in China has frequently reported,indicating that the situation of Tl pollution in water system is serious.It is of great significance to find a highly efficient,rapid and safe technology for the removal of Tl from wastewater.Based on the efficiency and ease of operation,the research and development of adsorbents with large adsorption capacity,fast adsorption rate,selective adsorption to heavy metal ions,easy separation,high reuse rate,simple preparation and no pollution to the environment were the hot topics in this field.Ferromanganese metal oxides have good pore structure and large specific surface area.They are excellent adsorbents for heavy metals with good efficiency and potential for Tl removal.Therefore,the adsorbents consisting of iron and manganese oxides through different modification methods were prepared for Tl removal from wastewater.Factors affecting the removal of Tl from wastewater by ferromanganese metal oxide based sorbents were examined,and the associated interfacial reaction mechanisms were also examined.First,the effect and mechanism of Fe-Mn composite oxide on Tl removal were investigated.Fe-Mncompositeoxideadsorbentwassynthesizedby oxidation-reduction precipitation method.The influence of the dosage,reaction time,initial pH,ionic strength and complex on the adsorption of Tl on Fe-Mn composite oxide was studied.The results show that when the initial Tl concentration was 10mg/L and the pH=10,the removal rate of Tl increased with the increase in dosage.When the dosage of adsorbent was 0.50 g/L,the adsorption capacity for Tl is 19.90mg/g and the removal efficiency was 99.49%.The removal efficiencyof Tl was almost unchanged with the increase in pH when the initial pH was more than five.Ionic strength and complex concentration have a low influence on Tl removal.The adsorption followed peudo-second-order kinetic and Langmuir isotherm.The maximum saturated adsorption capacity of Fe-Mn composite oxide reached 197.63mg/g at 298 K.The thermodynamic parameters confirm that the adsorption process of Fe-Mn complex oxide on Tl is a spontaneous and endothermic process.Analyses of SEM-EDS,XRD,FT-IR and XPS show that ion exchange and chemical oxidation of Fe-Mn binary oxide accounted for the Tl adsorption.Secondly,nanoscale Fe₃O₄ was prepared by hydrothermal precipitation method.The efficiency,mechanism and regeneration of nanometer iron oxide?Fe₃O₄?were investigated.The reversible adsorption/desorption characteristics of Tl by Fe₃O₄ and its associated mechanism of Tl removal and recovery were investigated.The study showed that fast and efficient removal of Tl was achieved by adsorption under alkaline conditions with pH above 11.0,while rapid and effective enrichment of Tl was obtained by desorption under acidic conditions with pH below 3.0.The saturation magnetization of Fe₃O₄ is 54.70 emu/g,and the adsorbent was quickly and effectively separated in the solution by external magnetic field.After adsorption,when the adsorbent concentrated at 10,12.5 and 25 times can led to an eluent Tl concentration of 90.01,112.60 and 202.40 mg/L respectively.The removal and recovery of Tl can be effectively and stably repeated without obvious loss of magnetite,indicating that the magnetite-based reversible adsorption-desorption process is a promising technique that can be further developed.Manganese dioxide has excellent adsorption properties for heavy metals.However,due to its small particle size,low solid-liquid separability,it is difficult for wide application to Tl removal in industry.In order to solve the obove-mentioned problem,core-shell structure of composite composed of magnetite cinder as the core coated and the MnO₂ as outer layer was prepared for removal and recovery Tl from wastewater.Experiments show that under the same conditions,when the initial concentration of Tl was 300 mg/L,the equilibrium adsorption capacity of the adsorbent prepared by the ambient temperature method is 55.25 mg/g,while the hydrothermal method was 293.66 mg/g.The adsorption of Tl by hydrothermal method was obviously better than that prepared by ambient temperature method.Analyses of XRD,FT-IR,XPS,VSM,SEM-EDS,TEM and BET indicate that the MnO₂@magnetite cinder was composed of amorphous and flower-like MnO₂ in the outer layer and magnetite-dominated magnetite cinders in the inner core.A mild and effective magnetic separation can be achieved because the saturation magnetization of this composite reached 15.40 emu/g.The physical adsorption,oxidation and the surface complexation was the main mechanisms for Tl adsorption.The adsorption capacity of MnO₂@magnetite cinder on Tl was enhanced with the increase in pH.Ionic strength and organic complexes were not significant.The adsorption process conforms to quasi-second-order kinetics,and isothermal adsorption can be described by Langmuir model.The composite adsorbed Tl could be effectively regenerated using 1 mol/L HCl solution within only 5 min.The adsorption capacity of the composite did not changed significantly even after the fifth regeneration.