Synthesis Of Nano-magnetic Mineral Materials As Adsorbents For The Removal Of Heavy Metals From Aqueous Solutions

Heavy metals have been extensively released into the environment due to the rapid industrialization in recent years.Unlike organic contaminants,heavy metals are inclined to accumulate in living organisms as they are not biodegradable.Adsorption is generally preferred due to its high efficiency,facile operational conditions and the accessibility of different adsorbents,and the key point is to find a low-cost and high-efficiency adsorbent.Clay mineral is abundant in nature,and the controllable and changeable structure and physico-chemical characteristics make it an excellent material in adsorption and ion exchange fields.In this paper,we select cationic clay mineral（KC）and anionic layered double hydroxide clay minerals（LDH1 and LDH2）as matrix to modify their structures,synthesizing and characterizing these three kinds of nano magnetic clay minerals,and conduct a series of batch adsorption experiments to study the removal of heavy metals from aqueous solution.The results were shown as follows:（1）The Mag/KC was prepared via solvothermal method and characterized by X-ray diffraction（XRD）,Fourier transform infrared spectroscopy（FTIR）,Scanning electron microscopy（SEM）,Transmission electron microscopy（TEM）,X-ray photoelectron spectroscopy（XPS）and Zeta potential.The characteristic results showed that the Mag/KC displayed many lattice planes with perfect crystallinity,and indicated a successful loading of the Fe₃O₄ nanoparticles on the surface of KC.The as-prepared Mag/KC exhibited excellent adsorption performance compared with that of the original KC,and the adsorption capacity follow the order of Pb²⁺>Cd²⁺>Cu²⁺.The optimal adsorbent dosage were 4.0 g/L(Pb²⁺and Cu²⁺)and 2.5 g/L(Cd²⁺),respectively.Adsorption efficiency hardly changed when p H was lower than 10.The contact time were selected as 60 min.The adsorption kinetics fit well with the pseudo-second order kinetic model.In equilibrium isotherms fitting,the Langmuir model showed better suitability compared with the other two,and the maximum adsorption capacity was 86.1,16.5 and 22.1 mg/g,respectively.The adsorption mechanisms of Pb²⁺,Cu²⁺,and Cd²⁺could be a combined reaction of complexation between functional groups and metal ions and electrostatic attraction.（2）The magnetic Mg/Al-LDH（Mag/LDH1）was prepared via solvothermal method and characterized by XRD,FTIR,SEM,TEM,Brunner-Emmet-Teller measurements（BET）,XPS and Zeta potential.The results showed that the Fe₃O₄ nanoparticles had been loaded on the surface of Mg/Al-LDH successfully.The specific surface area of was Mag/LDH1 was 92.7m2/g,much higher than that of Mg/Al-LDH（48.6 m2/g）.The batch experiment proved that the maximum adsorption capacity of Mag/LDH1 was 176,33.2 and 20.5 mg/g for Pb²⁺,Cd²⁺and Cu²⁺,respectively.The ideal adsorbent dosage were 4.0 g/L(Pb²⁺and Cd²⁺)and 5.0 g/L(Cu²⁺),respectively.Adsorption efficiency hardly fluctuate below the p H value of 10,so there is no need to adjust the p H.The contact time was chosen as 210 min.The adsorption kinetics correspond to the pseudo-second order kinetic model.The equilibrium adsorption isotherm data showed better suitability to the Langmuir model,and the adsorption capacity followed the order of Pb²⁺>Cd²⁺>Cu²⁺.This paper analyzed the adsorption mechanism in detail by carrying out XRD,FTIR,Zeta potential and XPS measurement,which may be attributed to the following ways:（i）formation of outer-sphere surface complexes between the metal ions and deprotonated hydroxyl groups or electrostatic binding between the metal ions and the oppositely charged surface functional groups at a certain distance from the surface;（ii）formation of inner-sphere surface complexes between the metal ions and intercalated anions present in the solution(–NO₂ or OH^–);（iii）formation of Pb CO₃ precipitates by anion exchange;（iv）incorporation of Cd²⁺and Cu²⁺into the Mg/Al-LDH structure by isomorphic substitution of Mg²⁺in the layers.（3）The magnetic Mg/Zn/Al-LDH（Mag/LDH2）was also prepared via solvothermal process and characterized by XRD and FTIR analysis.The results showed that the Fe₃O₄nanoparticles had been adhered to the surface of Mg/Zn/Al-LDH successfully.The adsorption capacity of Mag/LDH2（176 mg/g and 30.2 mg/g）were higher than that of Mg/Zn/Al-LDH（139 mg/g and 22.3 mg/g）for Pb²⁺and Cu²⁺,while the adsorption capacity of Mag/LDH2（57mg/g）was lower than that of Mg/Zn/Al-LDH（78.9 mg/g）for Cd²⁺.The optimal adsorbent dosage were 5.0 g/L(Pb²⁺),3.0 g/L(Cd²⁺)and 6.0 g/L(Cu²⁺),respectively.There is no need to adjust the p H values under 10.The contact time was selected as 30 min.The adsorption kinetics gave better fitting to the pseudo-second order kinetic model.The isotherm data corresponded well to the Langmuir model,and the adsorption capacity followed the order ofThe results above show that the synthesized nano-magnetic clay minerals can efficiently remove heavy metal ions from aqueous solutions,and achieve quick solid-liquid separation and recycling of adsorbent by an external magnet,which will provide some theoretical support for removing heavy metals from wastewater.