Treatment Of Wastewater Containing Heavy Metal Thallium By Sulfidized Zero-Valent Iron Materials And Detection Of Trace Heavy Metals By Carbon Quantum Dots

Thallium is a typical disperse heavy metal with extremely high toxicity.In recent years,thallium pollution incidents brike out frequently in our country,and our country and the related local regions have issused strict standards for the discharge of industrial wastewater containing thallium.Among the many techniques for thallium removal from wastewater,the adsorption method has been widely studied because of the advantages including simple operability and high thallium removal efficiency.Sulfidized zero-valent iron(S-ZVI)is a potential adsorptive material for thallium removal from wastewater,due to the high specific surface area,abundant surface chemical groups,excellent sulfidation precipitation efficacy and good solid-liquid separation performance.Therefore,this study was carried out through exploring the controllable synthesis methods of S-ZVI series materials,aiming at investigating the relationship between the morphology and structure of S-ZVI series materials prepared by different methods and the thallium removal efficiency.The synthesis method and thallium removal methodology regarding S-ZVI materials were also determined.On this basis,the key factors influenceing thallium removal was examined and the interfacial reaction mechanism of thallium removal were elucidated,in order to provide theoretical guidance and practical experience for the adanced removal of thallium from industial wastewater.In addition,this study examined the feasibility of extraction of extracellular polymer substances(EPS)from anaerobic ammonia oxidation granular sludge to synthesize carbon quantum dots(CQDs)for the detection of heavy metal ions,providing a new detection method for the monitoring of heavy metal ions from water.Firstly,the one-pot synthesis of S-ZVI materials with sodium thiosulfate as sulfiding agents was examined and optimized for thallium removal,the molar ration of S/Feat 3.0 was determined as the optimal one for thallium remocal:(1)the scanning electron microscope(SEM)images show that the S-ZVI material were spherical particle aggregates of zero-valent iron,stacking with flake-like pyrite compounds.The results of high resolution transmission electron microscopy(HRTEM)and selective area electronic diffraction(SAED)show that the S-ZVI was mainly composed o low crystalline materials.(2)The thallium removal process regarding S/Fe=3.0 S-ZVI material depended on the pH value of the water,and its thallium adsorption was mainly through inner layer adsorption,which was not affected by high electrolyte(NaNO₃).Isorhermal adsorption analyses showed that the adsorption of thallium by S-ZVI material can absorb high concentration of Tl(?)₀=100 mg/L,the reaction basically reached equilibrium within 10 min,and the adsorption process can be described by the pseudo-first-order kinetin model.(3)TEM,XPS,XRD,SEM-EDS and Fe-based M(?)ssbauer analyses were used to characterize the materials before and after adsorption.The results showed that the main mechanisms of thallium adsorption were as follows:(?)Tl(?)ions react with sulfides in the material to from thallium sulfide precipitates(Tl₂S);(?)surface complexation of Tl(?)with S-Fecompounds in the adsorbent;(?)electrostatic attration betwrrn Tl(?)ions and negatively charged S-ZVI.The contribution order of thallium removal mechanism was as folloews:sulfide precipitation>surface complexation>electrostatic attraction.Secondly,S-ZVI materials with Na₂S as sulfidizing agents,which have better thallium performance,were used for thallium removal study:(1)Among the S-ZVI series materials prepared by one-step method,the material with S/Femolar ration of2.0 had the best thallium removal efficiency.Among the S-ZVI materials prepared by two-step method,the material with S/Femolar of 3.0 had the best thallium removal efficacy.The SEM analyses showed that the material of one-step method with S/Feof2.0 had irregular surface due to the large amount of FeS precipitates.Naniscale chain structure of spherical particles appeared in the material of two-step with S/Fe=3.0.The HRTEM analyses showed that the material of one-step method with S/Feof 2.0 was amorphous,while the material of two-step with S/Fe=3.0 had apparent core-shell structure.(2)The isothermal adsorption for thallium removal using the above two materials all conformed to the Langmuir model,and the maxium thallium adsorption capacity is 930.0 mg/g(S/Fe=2.0 one-step method)and 890.0 mg/g(S/Fe=3.0 two-step method).Co-existing metal ions and the inhibition intensity was as followed:Ca²⁺>Na⁺=K⁺,high concentration>low concentration.(3)Various characterization methods were used to characterize and analyze the mechanisms of thallium removal by the S-ZVI materials.The order of the mechnisms was as followed:sulfide precipitation>surface complexation>electrostatic attraction.Furthermore,S-ZVI series materials were used to treat the industrial acid wastewater containing thallium.The optimized S-ZVI material alone had a good thallium removal efficiency,but it still needed durther treatment.Combined with alkali precipitation pretreatment and flower-like MnO₂ adsorption as advanced treatment,thallium in waste acids streams can be treated to less than 1?g/L.Other pollutants from the waste acids streams,such As,Cd and Zn,were also effecitively removed.Therefore,the S-ZVI series materials in this study have a very high application value for the deep purification of wastewater containing thallium.Finally,study on the exctraction of extracellular polymeric substances from anaerobic anmmonium oxidation granular sludge to synthesize carbon quantum dots(CQDs)were performed,order to detect the trace heavy metal ions.The CQDs can be used as a fluorescent probe,which contain N-doped groups,for the identification and quantitative detection of Mn(?)and Cr(?)ions.The detection limits for Mn(?)and Cr(?)are 5.8 nmol/L and 2.3 nmol/L,respectively,which are more sensitive than most previous reports on CQDs that were used for detection of Mn(?)and Cr(?)ions.