Removal Of As(Ⅴ) And Cd(Ⅱ) In Water And Its Mechanism By 3D Flower-like Mg/Al/Fe-CLDH

At present,the pollution of heavy metals such as arsenic and cadmium has become a worldwide environmental concern.Dangerous concentrations of heavy metals in natural waters threaten the health of millions of people.Therefore,how to effectively prevent and control heavy metal pollution in wasterwater has become the focus of domestic and foreign researchers.According to statistics,industrial wastewater containing a large ammount of heavy metals such as arsenic and cadmium generally comes from non-ferrous metal smelting,sulfuric acid production,mineral processing and other industries.These wasetwater due to strong acidity,complex composition,contain a large number of highly toxic heavy metals,such as arsenic,cadmium,mercury,lead,copper,chromium,so it is difficult to treat.If it is not discharged into natural water bodies,it will cause serious threats to the life of human beings and various organisms through the enrichment of food chain.Adsorption is one of the most extensively used methods for low arsenic removal technologies.To date,researchers have developed new and high-efficiency adsorbents（e.g.,graphene oxide,carbon nanotubes,and metal organic frameworks,modified hydrotalcites,natural biological）.Hydrotalcites（LDHs）not only have the characteristics of larger interlayer space,larger anion capacity,good thermal stability and water resistance structure,but also have faster kinetics and full regeneration capacities compared with other adsorbents.Mg Al is the most commonly studied LDH owing to its non-toxic,environmentally benign,low cost and availability.However,the pH application range of traditional Mg/Al-LDH has great limitations.Recently,ternary or quaternary prepared LDHs have been gaining immense attention from researchers,because the redox and acid-base properties are improved after the isomorphic substitution of other metal.Considering the affinity of Fe to As and Cd,we propose an iron modified Mg/Al-LDH that partially replaces Al with Fe.From the practice point of view,ultrathin two-dimension nanomaterials often serve as highly efficient adsorbents due to their high specific areas with abundant active sites for target molecules.But two-dimension nanosheets are easy to aggregate,resulting in a decrease in surface active sites and catalytic activity.Fortunately,three-dimension nanocomposites are stable in structure and may allow easy recovery and reused.Nanocomposites with a three-dimension hierarchical architecture assembled from two-dimension nanosheets have several advantages:1)providing plenty of active sites for adsorption reaction;2)inhibiting the aggregation or accumulation of subunits;3)improving the diffusion efficiency of reaction molecules as well as the accessibility and availability of the inner surface due to the interconnected mesopores existed in the structures;and 4)being a micro-sized material that is easy to separate.Three-dimension nanocomposites have been extensively applied to photocatalysis,water treatment,and energy storage materials,and will be used in this study.The hydrothermal process serves as an ideal approach to synthesis three-dimension iron modified Mg/Al-LDH,because it possesses the advantage of controllable morphology,uniform particle size,simple operation and environmental friendliness.Especially calcination of LDHs（450～600℃）tend to produce poorly crystallized mixed metal oxides that have remarkable adsorption properties.This is possibly caused by the“memory effect”of hydrotalcite,in which the reconstructed hydrotalcite structure preferentially accommodate large anions.In light of these analyses,this study firstly constructed a novel three-dimension nanocomposite adsorbent Mg/Al/Fe-CLDH（CFMA）by“hydrothermal+calcination method”.Next,the single metal adsorption behavior of CFMA adsorbents for As（Ⅴ）and Cd（Ⅱ）in simulated wastewater was evaluated by single-factor experiments（e.g.,different adsorbent dosage,initial pH of solution,concentration of competing ions,initial concentration）.The results are as follows:（1）The removal efficiency of As（Ⅴ）by CFMA was above 99.9%when the pH of the solution was between 2 and 11,and the residual As（Ⅴ）concentration was less than1μg/L,which satisfied the standard of drinking water;The removal efficiency of Cd（Ⅱ）was over 99.6%when the pH was 4-7,and the residual Cd（Ⅱ）concentration was 40-70μg/L.（2）The adsorption kinetics and adsorption isothermal model showed that the adsorption behavior of CFMA on As（Ⅴ）followed the quasi-second-oeder kinetic model and Langmuir adsorption isothermal model,and the adsorption behavior of CFMA on Cd（Ⅱ）followed the quasi-second-oeder kinetic model and Freundlich adsorption isothermal model.The maximum adsorption capacity of As（Ⅴ）and Cd（Ⅱ）was 70 mg/g and 402 mg/g,respectively.（3）CFMA is suitable for a wide range of pH and has good recycling performance.After 4 cycles,the removal efficiency of saturated adsorbent（ACFMA）loaded with As（Ⅴ）could still be maintained at about 80%,which had practical application value.In fact,arsenic and cadmiun co-exist in most industrial wastewater.Due to the different physical and chemical properties such as surface charge and ion morphology between As（Ⅴ）and Cd（Ⅱ）,it is challenging to remove As（Ⅴ）and Cd（Ⅱ）,especially at low concentrations.In view of the excellent performance of CFMA in removing As（Ⅴ）and Cd（Ⅱ）,the third part of this study used different initial concentration ratios（Cd:As=1:2,1:1,2:1）to investigate the removal performance of CFMA on Cd（Ⅱ）and As（Ⅴ）,the main conclusions are as follows:（1）When the concentration ratio Cd:As=1:2,the residual concentrations of As（Ⅴ）and Cd（Ⅱ）were 8.7μg/L and 4.2μg/L,respectively,which met the drinking water standard;（2）In the co-adsorption system,As（Ⅴ）and Cd（Ⅱ）influence each other’s adsorption behavior due to the anionic bridgre and shielding effect of As（Ⅴ）on Cd（Ⅱ）,As（Ⅴ）gradually changed from monolayer adsorption to multi-layer adsorption dominant,while Cd（Ⅱ）gradually changed from multi-layer adsorption to monolayer adsorptiondominant.In the fourth part,the mechanism of removal As（Ⅴ）and Cd（Ⅱ）by CFMA was studied in depth by various characterization tachniques.The main conclusions are as follows:（1）After heavy metal adsorption,CO₃^2-re-enters the interlayer structure of hydrotalcite,which is the so-called“memory effect”;（2）As（Ⅴ）and Cd（Ⅱ）are specifically adsorbed by CFMA and coordinated by-OH groups to form multilayer complexes;（3）As（Ⅴ）and Cd（Ⅱ）are inserted into the onterlayer region of hydrotalcite,and As（Ⅴ）is more favorable to be exchanged into the interlayer.The inserted Cd（Ⅱ）combines with the interlayer CO₃^2-to generate precipitation;（4）Negatively charged As（Ⅴ）will provide shielding to balance the electric power between Cd（Ⅱ）,and coordinated deprotonated As（Ⅴ）will also act as anion bridge to enhance the attration of Cd（Ⅱ）;（5）Arsenic can replace-OH groups on metal oxides to formmonodentate mononuclear complexes and bidentate binuclear complexes.