Study On The Removal Performance Of Arsenic And Fluoride From Water By La-Based And Fe-Al-based MOFs

With the increasing problem of water pollution and the rapid consumption of freshwater resources,water pollution have attracted great attention from all over the world.Arsenic（As）and fluoride（F）are common toxic pollutants in water.Long-term exposure to arsenic-containing wastewater can cause lung disease,neurological disease,skin cancer,and impaired immune function.Drinking high-concentration fluoride water can cause dental fluorosis,skeletal fluorosis,laryngeal and bronchospasm and other diseases.Therefore,effectively controlling the content of arsenic and fluoride in water has become the key to ensuring the safety of drinking water.Based on the strong affinity of lanthanum,iron,aluminum and other metal elements for arsenic or fluoride,using lanthanum,iron and aluminum as metal sources,a series of lanthanum-based and iron-aluminum-based metal organic framework materials were synthesized,and their for the removal of arsenic and fluoride in water.The specific content of the paper are as follows:1.By adjusting the solvent composition,lanthanum-based metal-organic framework materials La-BHTA and La-FA were synthesized and used for arsenic removal in water.The adsorption isotherm results of arsenic show that La-FA generated by changing the solvent has better arsenic adsorption performance than La-BHTA MOF material.The maximum adsorption capacities of La-BHTA and La-FA at 298 K were 128.12 and 271.89mg g^-1 respectively,and the adsorption capacity of La-FA for arsenic was 2.12 times that of La-BHTA.Both have acceptable Langmuir model for arsenic,and the maximum adsorption capacity fit to the model was close to that of measurements.The adsorption reactions of La-BHTA and La-FA to arsenic conformed to the pseudo-second-order kinetic model,indicating that chemisorption played a major role in the process of arsenic removal.Meanwhile,the adsorption thermodynamic studies showed that increasing the temperature was favorable for the adsorption of arsenic by La-BHTA and La-FA,and La-BHTA and La-FA could efficiently remove arsenic in water in a wide p H range（5.0-11.0）.In addition,FTIR and XPS analysis results showed that arsenic was adsorbed through ligand exchange reaction with La-FA surface hydroxyl group to generate La-O-As inner complex.2.Fe/Al-BDC-NH₂ bimetallic MOFs were successfully prepared by a simple one-step solvothermal method,and the effects of different metal ratios on the morphology and microstructural characteristics of the samples were investigated by adjusting the Fe/Al molar ratio.The equilibrium isotherms,kinetics,thermodynamics,and effects of interfering ions and initial p H on the adsorption efficiency of arsenic by bimetallic MOFs were explored and discussed in detail.Adsorption results show that introducing aluminum into the iron-based material formed bimetallic Fe/Al-BDC-NH₂ MOFs that had much better adsorption performance for arsenic than the monometallic Fe-BDC-NH₂ or Al-BDC-NH₂ MOF.The isotherm adsorption of arsenic on bimetallic MOFs conforms to the Langmuir model,and the adsorption capability of Fe/Al-BDC-NH₂（1:1）at 298 K for arsenic can reach 146.8 mg g^-1.Thermodynamic studies confirmed that high temperature is favorable for the adsorption of arsenic by Fe/Al-BDC-NH₂（1:1）.Fe/Al-BDC-NH₂（1:1）is suitable for removal of arsenic from wastewater in a relatively wide p H range（5.0-9.0）.Within this p H range,Fe and Al are hardly detected in the water after adsorption,and this indicates that Fe/Al-BDC-NH₂（1:1）has excellent stability and will not cause secondary pollution,and is suitable for adsorption from water arsenic.Furthermore,to explore the adsorption mechanism of Fe/Al-BDC-NH₂（1:1）for arsenic,FTIR and XPS were used to analyze the adsorption process,and the results showed that the adsorption mechanism of arsenic was mainly ligand exchange and electrostatic adsorption.This work shows that Fe/Al-MOF materials are expected to be used as excellent adsorbents to remove arsenic in wastewater,and provide a certain technical reference for in-depth research on the application of bimetallic MOFs in the environmental field.3.With lanthanum as the metal source,trimesic acid（BTC）,biphenyl-4,4-dicarboxylic acid（BPDC）,2,5-dihydroxyterephthalic acid（BHT）,1,2,4,5-benzenetetracarboxylic acid（PMA）,and terephthalic acid（BDC）as ligands,a series of lanthanum-based metal-organic frameworks（La-MOFs）were synthesized and used for the removal of excess fluoride in water.The characterization of MOF materials shows that the microstructural characteristics and the adsorption performance of fluoride can be tuned by changing the organic ligands of La-MOF.The isothermal adsorption of fluoride shows that La-BDC has better fluoride adsorption performance compared with La-BTC,La-BPDC,La-BHT and La-PMA.The maximum capture capacities of La-BTC,La-BPDC,La-BHT,La-PMA,and La-BDC at 298 K are 105.2,125.9,145.5,158.9,and171.7 mg g^-1,respectively.According to the Langmuir isotherm model,the La-MOF adsorption isotherm for fluoride is well fitted,and the fitted maximum adsorption capacity is close to the measured value.Meanwhile,La-BTC,La-BPDC,La-BHT,La-PMA,and La-BDC can efficiently adsorb fluoride in a relatively wide p H range（4.0–9.0）.After adsorption,La-MOF can still maintain the original framework structure,and La hardly dissolves in the p H range of 4.0-9.0,indicating that La-BTC,La-BPDC,La-BHT,La-PMA and La-BDC have high water stability,and will not cause secondary pollution.The FTIR and XPS analysis confirmed that the mechanism of La-MOF adsorption of fluorine involves ligand exchange and complexation,that is,during the adsorption process,fluoride in water could exchange ligands with metal hydroxyl groups（La-OH）in MOF to generate La-F coordination bond to achieve the purpose of removal.The research results show that La-MOFs can be widely used in the removal of fluoride in water and have broad application prospects.