Efficiency And Mechanism Of In Situ Oxidation And Adsorption For Synergistic Removal Of Cadmium And Arsenic

In recent years,heavy metal pollution incidents such as cadmium and arsenic have occurred frequently in our country.The complex background components of water bodies such as natural organic matter and organic pollutants will cause difficulties in the removal of heavy metals by affecting the existence of heavy metals in the water.In situ oxidation and adsorption synergistic removal of heavy metal pollution in complex water bodies have very high research value.The"oxidation" process can oxidize many multivalent metals(As,etc.)into high valence and charge-rich forms,oxidatively decompose complexed heavy metals and organic heavy metals,and release the heavy metals in a free state;the"adsorption" process can simultaneously and efficiently adsorb and remove these heavy metals through the adsorbent formed in situ during the“oxidation" process.Therefore,this article studied the removal efficiency and mechanism of heavy metals represented by the free metal cation cadmium(Cd(Ⅱ)),complex metal cation(EDTA-Cd)and free metal anion arsenic(As),organic arsenic(p-AsA)in sodium thiosulfate/potassium permanganate system(TS/Mn(Ⅶ))and iron(Ⅱ)activated peroxydisulfate system(Fe(Ⅱ)/PDS),respectively.The article focuses on the influence of the reducing agent/oxidant molar ratio and pH on the removal of heavy metals as well as the physical and chemical proterties of the solid products formed in situ in the system.The MnO2 adsorbent formed in situ in the TS/Mn(Ⅶ)system is small in size and strong in electronegativity.It can adsorb and remove the heavy metal cation Cd(Ⅱ)efficiently under neutral and alkaline conditions.Under acidic conditions,excessive sodium thiosulfate(Na2S2O3)will consume MnO2 adsorbent and reduce the removal efficiency of Cd(II).Unexpectedly,at pH 5,the removal efficiency of Cd(Ⅱ)remained at a high level(85.2%and 96.0%)at high TS/Mn(Ⅶ)molar ratios(2 and 3)where negligible nano-MnO2 existed.Combined with the Cd(Ⅱ)removal kinetics and the analysis results of the solid products formed in the system,we found that the TS/Mn(Ⅶ)system can remove Cd(Ⅱ)by forming insoluble CdS,and speculated that the formation of S2-in the system might be related to the sulfur-containing intermediates(SxOyn-),which is generated by the redox reaction between KMnO4 and excess Na2S2O3.The TS/Mn(Ⅶ)system produces S2-in a gentle process,does not produce harmful gases such as H2S,and the process of combining with heavy metals to form insoluble sulfides is less affected by common coexisting cations,so it can be extended to the removal of many other heavy metals,such as Pb,Hg,Ag,Cu.In the process of removing EDTA-Cd in the TS/Mn(Ⅶ)system,it is found that the ability of MnO2 to adsorb Cd complexed by EDTA is greatly reduced compared to the adsorption of free Cd(Ⅱ),and it is difficult to effectively remove Cd in EDTA-Cd by MnO2 adsorption alone.However,the method of removing Cd in EDTA-Cd by forming CdS is still very efficient.Under the optimal conditions with a TS/Mn(Ⅶ)molar ratio of 3 and pH of 5-6,the removal efficiency of Cd in EDTA-Cd is higher than 99.5%.Ion chromatographic analysis found that during the removal of Cd in EDTA-Cd,a small part of EDTA was oxidized,and most of it was converted to EDTA,indicating that the S2-generated in the system can directly capture Cd from EDTA-Cd without degrading EDTA-Cd.Mn2+can greatly improve the efficiency of removing free heavy metal Cd(Ⅱ)and complexing heavy metal Cd in EDTA-Cd by forming CdS,indicating that intermediate trivalent manganese(Mn(Ⅲ))may be contained in the sulfurcontaining intermediates.The intermediate(Mn(Ⅲ)-SxOyn-)is gradually produced and accumulated in the TS/Mn(Ⅶ)system,and at the same time,it slowly disproportionates and decomposes into S2-,S2-will capture the free and complex heavy metal Cd in the system to form solid CdS and separate Cd from water phase.Below pH 9,As(Ⅲ),which mainly exists in the form of uncharged H3ASO3,has high toxicity and strong fluidity.The active oxidizing components in the Fe(Ⅱ)/PDS system can oxidize As(Ⅲ)to low toxicity and negatively charged As(V),which is simultaneously and efficiently adsorbed and removed by the positively charged iron(oxyhydr)oxide formed in situ in the system.The dosage of Fe(Ⅱ)and PDS in the system,and the pH of the solution all affect the removal efficiency of total As by affecting the oxidation and adsorption of As(Ⅲ),and the removal efficiency reaches the maximum at pH 5.Free radical quenching experiments show that the main active component in the system is SO4·一,and As(Ⅲ)is mainly adsorbed and removed in the form of As(V)by the iron oxyhydroxide formed in situ.Using the oxidation and adsorption characteristics of Fe(Ⅱ)/PDS system,the removal efficiency on p-AsA was further studied.Acidic conditions are conducive to the degradation of p-AsA.Under pH 3,the optimal dosage of Fe(Ⅱ)and PDS can completely degrade p-AsA within 10 minutes,and the arsenic-containing group will be cleaved in the form of As(Ⅲ),and then quickly oxidized to As(V);the organic part is oxidized to p-aminophenol,p-hydroquinone,p-benzoquinone,and finally to small molecule acids and CO2.Under near-neutral conditions(pH=5-6),the total arsenic removal efficiency reached more than 98.5%when only about 50%of p-AsA was oxidized.This is mainly due to the fact that the surface of the iron(oxyhydr)oxide adsorbent formed in situ is rich in hydroxyl groups,and the in situ formed iron(oxyhydr)oxide can simultaneously remove inorganic and organic arsenic through the formation of complexes between the hydroxyl groups and the As(V)products and the residual p-AsA.