Study On The Removal Of Heavy Metal Ions From Wastewater By Sulfide Zero-Valent Manganese And The Mechanism

With increasing attention on urban ecological environment and ecological civilization construction,the demand for safe and clean water resources is also growing.However,rapid population growth,extreme climate change,and accelerated large-scale industrial production have led to the generation of a large amount of wastewater worldwide.Among them,the discharge of heavy metals poses a serious threat to human health and the sustainable development of global ecosystems.Currently,adsorption is considered a promising method for the removal of heavy metal ions due to its simple operation,low cost,environmental friendliness,reusability,and high selectivity.In recent years,nanoscale zero-valent metal materials have been widely used for the removal of heavy metal ions and organic pollutants.Among them,zero-valent manganese（ZVMn）theoretically has high reducibility,but it has surface oxidation issues.Sulfides,on the other hand,exhibit strong affinity for a variety of heavy metals due to the low solubility of metal sulfides.Therefore,this study aimed to prepare efficient adsorbents for the removal of heavy metal ions from wastewater by modifying ZVMn with sulfides and to investigate their influencing factors and removal mechanisms.The main research contents are as follows:（1）Sulfide zero-valent manganese（SZVMn）was synthesized by reducing with sodium borohydride and modifying with sulfides.The removal performance of SZVMn before and after liquid nitrogen treatment on Tl（I）was compared,and its removal characteristics and mechanisms were studied.The results showed that liquid nitrogen modified SZVMn-1.0（LSZVMn-1.0）had more internal channels and pores than SZVMn-1.0 that had not undergone liquid nitrogen treatment,with a 65.3%increase in specific surface area and a 73.7%increase in pore volume,resulting in a 6.4～8.1%increase in Tl（I）adsorption at p H 4～10.At a coexisting ion concentration of up to 0.1 M,LSZVMn-1.0 exhibited good effectiveness and stability in removing Tl（I）.Tl（I）adsorption followed a pseudo-first-order kinetic model and the Langmuir isotherm model,with a maximum Tl（I）adsorption capacity of 264.9 mg·g^-1 at288 K.Thermodynamic studies showed that the adsorption of Tl（I）by LSZVMn-1.0 was spontaneous and exothermic.At the same time,cyclic adsorption experiments showed that LSZVMn had good reusability.Based on the characterization results,the mechanism of Tl（I）removal by LSZVMn-1.0 included sulfide-induced precipitation,ZVMn reduction,surface complexation,and electrostatic attraction.Liquid nitrogen pretreatment made the shell of the S/Mn compound brittle and fractured,forming a highly graded structure,enhancing manganese reduction and increasing the removal efficiency of Tl（I）.Based on these results,LSZVMn-1.0 is a new and effective environmental material that can be used to remove Tl（I）from wastewater,and are also worth further research for their application in removing other pollutants.（2）The SZVMn adsorbent prepared by ultrasound-assisted hydrothermal method was used to study the adsorption behavior and mechanism of Tl（I）,Pb（II）,Cd（II）,Ni（II）,Zn（II）,and Cu（II）heavy metal ions in monadic and mixture systems.The influencing factors of different S/Mn molar ratios,initial p H,coagulation p H,and material dosage on the removal of heavy metal ions were determined through experiments,and it was found that at coagulation p H 6 and temperature 298 K,the maximum adsorption capacities of Tl（I）,Pb（II）,Cd（II）,Ni（II）,Zn（II）,and Cu（II）in monadic system were 151.33 mg·g^-1,1022.91 mg·g^-1,148.13 mg·g^-1,159.39 mg·g^-1,160.89 mg·g^-1,and 1649.12 mg·g^-1,respectively,and the maximum adsorption capacity order was consistent with the hydrolysis constant values of the metal ions.In the mixture system,the maximum adsorption capacities were 21.09 mg·g^-1,212.5 mg·g^-1,43.89 mg·g^-1,43.78 mg·g^-1,41.31 mg·g^-1,and 256.00 mg·g^-1,respectively,and the order of maximum adsorption capacity was consistent with the polarization ability values of the six heavy metal ions.In terms of kinetics,both the monadic and mixture systems followed a pseudo-second-order linear equation,indicating that the adsorption is chemical adsorption.Through SEM,TEM,XRD,FT-IR,and XPS characterization methods,it was found that the reaction mechanism of SZVMn with heavy metal ions included ZVMn reduction,surface complexation,electrostatic attraction,and sulfide-induced precipitation.These results indicate that SZVMn is a promising adsorbent for the efficient removal of heavy metal ions from wastewater.