Effect Of Structural Iron Reduction Of Nontronite On Adsorption Capacity Of Cd And Phytic Acid

Clay minerals are the main solid phase components of soil,and play an important role in regulating the migration and transformation of pollutants and nutrient elements.The interaction characteristics of pollutants and nutrient elements at mineral interfaces are closely related to the types and structures of clay minerals.Naturally occurring clay minerals generally contain different amounts of iron in crystal structure.In anaerobic environment,Fe（III）in the crystal structure of clay minerals is prone to reduction reaction,which changes the crystal structure and surface chemical properties of minerals,thus affecting their interaction with pollutants and nutrients.However,the effect of iron reduction on the adsorption of pollutants and nutrients in clay minerals under anaerobic conditions is not clear.In this paper,the difference of adsorption capacity of nontronite to cadmium and organophosphorus phytate before and after the reduction of structural Fe（III）with chemical reduction under anaerobic condition was studied.The main results are as follows:（1）The total iron content in the synthetic nontronite is 31.25%,and contains a small amount of Fe（II）,Fe（II）/Fe_total is 3.9%,and the structural iron content accounts for 98.63%of the total iron.After chemical reduction,the Fe（II）/Fe_total in the reduced nontronite was significantly increased to 67.4%.XRD,SEM-EDS,BET,FTIR and XPS were used to characterize the structure,morphology and composition of the two minerals.It is found that the reduction treatment increases the layer spacing of nontronite mineral structure,and the dispersion degree is higher.The specific surface area increases from 151.6 m²/g to 184.0m²/g,and the average pore radius of single point increases from 14.7?to 21.2?.A certain mineral structure rearrangement occurs,and the number of hydroxyl groups on the mineral surface increases.（2）The adsorption capacity of reduced nontronite for Cd（II）is lower than that of non-reduced minerals,while the desorption rate is slightly higher than that of non-reduced minerals.The reduction of nontronite structure reduces the adsorption and retention of cadmium of minerals.The Langmuir model can better fit the adsorption isotherms of Cd（II）before and after reduction,and the maximum adsorption amounts of Cd（II）for the fitted reduced and non-reduced minerals are 23.46 and 33.38 mg/g.The quasi-second-order kinetic model can better fit the adsorption kinetics of Cd（II）before and after reduction,and the equilibrium adsorption capacity of Cd（II）for reduced and non-reduced minerals is 11.02mg/g and 19.55 mg/g.The adsorption capacity for Cd（II）increases with p H and decreases with ionic strength before and after reduction.（3）The phytic acid adsorption capacity of reduced nontronite is higher than that of non-reduced minerals,and the desorption rate is lower than that of non-reduced minerals.The Langmuir model can better fit the adsorption isotherms of reduced and non-reduced minerals,and the maximum adsorption capacity of phytic acid for the fitted reduced and non-reduced minerals is 158.18μmol/g and 29.12μmol/g.The quasi-second-order kinetic model can better fit the phytic acid adsorption kinetics process before and after reduction,and the equilibrium phytic acid adsorption capacity of the fitted reduced and non-reduced minerals is 61.63μmol/g and 17.90μmol/g.The adsorption of phytic acid decreases with the increase of p H before and after reduction,and is less affected by ionic strength.（4）In the co-adsorption system,the adsorption capacity of phytic acid and Cd（II）on reduced and non-reduced nontronite increased significantly compared with that on phytic acid and Cd（II）alone.The Langmuir model can better fit the phytic acid adsorption isotherms before and after reduction in the presence of Cd（II）.The maximum phytic acid adsorption capacity of the fitted reduced and non-reduced minerals is 251.60μmol/g and142.63μmol/g.The addition of a certain amount of phytic acid can promote the adsorption of Cd（II）for the two minerals,and the adsorption capacity of Cd（II）for the unreduced minerals is stronger at a lower phytic acid concentration（≤107.6μmol/L）.However,at higher phytic acid concentration（≥161.4μmol/L）,Cd（II）adsorption capacity of reduced minerals is stronger.With the increase of solution p H value,phytic acid adsorption capacity of minerals decreases before and after reduction,Cd（II）adsorption capacity of reduced minerals increases slightly,and Cd（II）adsorption capacity of unreduced minerals fluctuates to a certain extent.Ionic strength of solution has little influence on phytic acid adsorption before and after reduction,but has certain influence on Cd（II）adsorption of minerals before and after reduction:With the increase of ionic strength,the Cd（II）adsorption capacity of the two minerals decreases to a certain extent.When the ionic strength is low,the Cd（II）adsorption capacity of reduced minerals is greater than that of non-reduced minerals,and with the increase of ionic strength,the Cd（II）adsorption capacity of non-reduced minerals is greater than that of reduced minerals.The phytic acid adsorption and retention capacity of nontronite after reduction is stronger.When the initial phytic acid concentration is 107.6μmol/L,the phytic acid adsorption capacity is slightly worse than that of non-reduced minerals,but the Cd（II）retention capacity is stronger.