Selective Removal Of Heavy Metal Via Sulfate Doped Flower Like Layered Double Oxide In Water And The Mechanism Study

With the rapid development of industrial activities and technologies,water pollution by toxic heavy metals is becoming increasingly severe.Lead(Pb²⁺),regarded as one of most prevalent and hazardous contaminants,has become a hot pot in the field of water pollution control due to its high toxicity,easy bioaccumulation and difficult degradation.Among various available techniques,adsorption method with high efficiency,low cost,wide application range and simple operation is deemed as an effective and promising method in the treatment of Pb²⁺wastewater.However,the removal efficiency of Pb²⁺on traditional adsorbents is limited and cannot meet strict discharge standards.Since the lead-containing wastewater usually contains various coexistent metals(Ni²⁺,Cu²⁺,and Cd²⁺,etc.),causing serious disturbance to the removal of Pb²⁺,high selectivity as vital parameter should also be considered for the lead capture agents.In this study,we attempted to used sulfur-containing organic layered double hydroxide（S-FLDH）as precursor to prepare sulfur-rich layered double oxide（S-FLDO）via precursor-calcination at high temperature.And the effect of calcination temperature on samples was investigated.The chemical composition and structural morphology of S-FLDO at the optimum calcination temperature were explored by X-ray diffraction（XRD）,Fourier transform infrared spectroscopy（FT-IR）,X-ray photoelectron spectrum（XPS）,scanning electron microscopy（SEM）and Energy Dispersive Spectroscopy-Mapping（EDS）.Furthermore,the effects of initial concentration of solution,reaction time,reaction temperature and p H on the adsorption performance of S-FLDO were investigated by static batch adsorption experiments.The adsorption behavior of Pb²⁺on S-FLDO was studied by adsorption isotherm model,adsorption kinetics model and thermodynamic analysis.Meanwhile,the selective adsorption capacity of S-FLDO for Pb²⁺in single and mixed systems was investigated,and the selective adsorption mechanism of S-FLDO for Pb²⁺was revealed.Furthermore,the XRD,FT-IR,XPS and SEM were used to comprehensively explore the structural morphology,composition and functional group changes of the samples before and after Pb²⁺adsorption,and clarify the adsorption mechanism of Pb²⁺by S-FLDO.The detailed results are as follows:（1）The optimal calcination temperature for preparing S-FLDO is 700℃.Under the optimal conditions,the main component of S-FLDO is Mg Al₂O₄ and Mg SO₄.And A large number of SO₄^2-groups was anchored on the surface via Mg-S bonds.The morphology shows a typical flower-like microspheres consisting of numerous thin nanosheet.（2）The effects of p H,adsorbent dosage,initial concentration,reaction time and reaction temperature on the removal performance of Pb²⁺on S-FLDO were systematically investigated.The results show that S-FLDO possessed an excellent anti-p H-interference ability.The maximum adsorption capacity of S-FLDO to Pb²⁺can reach 875 mg·g^-1 when the dosage of adsorbent was 1.0 g·L^-1,the temperature was 318K,the contact time was 30 min,and the initial Pb²⁺concentration was 1000 mg·L^-1.The removal behavior of Pb²⁺on S-FLDO followed Langmuir model and pseudo-second-order kinetic model,indicating that the adsorption process was monolayer and chemisorption.Adsorption thermodynamic analysis revealed that the r removal reaction was spontaneous endothermic process.（3）The S-FLDO has excellent adsorption selectivity for Pb²⁺.In single system,the adsorption distribution coefficient（K_d）of S-FLDO on Pb²⁺is 10⁶,which is much higher than other common metal ions(such as Cu²⁺,Ni²⁺,Co²⁺and Cd²⁺).Even in the mixed system,S-FLDO still shows an outstanding selectivity for Pb²⁺.Moreover,the mechanism studies showed that the high selectivity is mainly related to the properties of metals（such as ionic radius and electronegativity）and hard and soft acid base theory.（4）The XRD,FT-IR,XPS and SEM analyses of the samples before and after Pb²⁺adsorption showed that the adsorption of Pb²⁺by S-FLDO is influenced by the initial concentration.At low concentration(10 mg·L^-1),Pb²⁺is removed by ion exchange with Mg²⁺in S-FLDO to form Pb SO₄ precipitation and complex with SO₄^2-group.At medium and high concentrations(100 mg·L^-1 and 1000 mg·L^-1),Pb²⁺can also be removed by hydration of LDO to form Pb₃（CO₃）₂（OH）₂ precipitation.（5）S-FLDO still maintain an excellent removal efficiency of Pb²⁺in tap water,electroplating wastewater and lake water.The removal efficiency of S-FLDO for Pb²⁺decreases slightly with the increase of the number of calcination cycles,indicating the well stability and recycling performance of S-FLDO.These results show that S-FLDO has potential application value in practical wastewater treatment and is an excellent adsorbent for lead-containing wastewater.