Effect Of Fulvic Acid On Removal Of Al(?)?Cd(?)and Pb(?)in Water By Ferrate(?)

Natural organic matter and various metal ions are widely present in river water.Natural organic matter can combine with metal ions as ligands,which will affect the toxicity and migration ability of metal ions,and also will change the environmental behavior of organic matter.Ferrate???has the function of oxidation and flocculation,and it's recognized as a“green”water treatment agent that removes organic matter and metal ions from water simultaneously.Therefore,this paper mainly studies the effect of fulvic acid,one of the natural organic substances,on the removal of aluminum,cadmium and lead in water by ferrate???.The research was mainly carried out in three aspects.Firstly,the complexation of metal ions and fulvic acid was investigated.The change of metal ions concentration in the water before and after addition of fulvic acid under different pH values was analysized.It was found that in the range of pH 3-11,the presence of fulvic acid increased the metal ions content in the water,indicating that fulvic acid increased the amount of metal ions dissolved in water and increased the migration of metal ions in water.Subsequently,the removal rate of metal ions by ferrate???in the presence and absence of fulvic acid was compared.The experiments were carried out at different pH values and different ferrate dosage?expressed as mass ratio of Fe?VI?/FA?TOC??.When fulvic acid was absent,the removal rate of metal ions increased with the increase of ferrate???dosage.The optimal conditions for the removal of Al by ferrate???were pH=7,Fe???/FA?TOC?=10:1,and removal rate of Al reached 100%;the removal rate of Cd and Pb is over 98%at pH=7-9,Fe???/FA?TOC?=10:1.The existence of fulvic acid hindered the ferrate on metal ions removal.The metal ions removal rate began to decrease when ferrate was added in large doses.In the presence of fulvic acid,the optimal conditions for aluminum removal by ferrate???were pH=7,Fe???/FA?TOC?=2:1,and the removal rate of Al was 99.9%.In the presence of fulvic acid,the best removal conditions for Cd were pH=7-9,Fe???/FA?TOC?=2:1,and the removal rates of Cd was 87%;the best removal conditions for Pb were pH=5-9,Fe???/FA?TOC?=2:1,and the removal rates of Pb was 94%.Excessive ferrate in the presence of fulvic acid is not conducive to the removal of metal ions.This was because the fulvic acid was excessively oxidized into fine fragments when the ferrate???dosage was more than the optimal value,thereby forming a water-soluble complex with the metal ions.Appropriate amount of ferrate???will oxidize fulvic acid to form substances with a larger molecular weight.Combined with metal ions,they will precipitate more easily,resulting in a relatively high metal ions removal rate.At the same time,the removal rate of fulvic acid in water was measured,and it was found that fulvic acid was also removed to a large extent?42%-70%?,indicating that ferrate can remove both metal ions and fulvic acid from water.Finally,in order to prove the effect of oxidized degree of fulvic acid on metal ions removal rate,FeCl₃ was used instead of ferrate???to investigate the removal of metal ions by flocculation in the presence of fulvic acid.It was found that when FeCl₃ was used to remove metal ions,the metal ions removal rate increased with the increase of FeCl₃ dosage.It was different to the rule when ferrate???was added,indicating that the oxidation degree of fulvic acid by ferrate???affects the removal of metal ions.In order to explain the effect of fulvic acid on metal ions removal,10 mg/L,20 mg/L,and 40 mg/L of ferrate???were added to the fulvic acid solution.to iInvestigate changes in fulvic acid molecules with different degrees of oxidation.Infrared spectroscopy analysis revealed that C-O bond stretching vibration and carboxyl group-OH stretching vibration were added to the fulvic acid molecule after oxidation,which shows that the structure of fulvic acid changes after oxidation,and more oxygen is combined to form more Lewis base sites that may bind metal ions.Gel chromatography?GPC?analysis found that when the dosage of ferrate was small?10mg/L,20mgL?,the small molecular weight substances decreased,and the large molecular weight substances increased.When the dosage of ferrate is too large?40mg/L?,more substances with smaller molecular weight appear.This indicates that the addition of excessive ferrate???causes the fulvic acid to be oxidized into smaller molecules,leading to small molecules to bind metal ions to form soluble complexes.This is not conducive to remove metal ions.In summary,the presence of fulvic acid increases the mobility of metal ions and hinders the removal of metal ions by ferrate???.However,the proper addition of ferrate can effectively remove both fulvic acid and metal ions simultaneously.Excessive ferrate???can cause excessive oxidation of fulvic acid,which is not conducive to the removal of metal ions.Therefore,in actual water treatment,it is necessary to use an appropriate amount of ferrate???according to the water quality.