A Novel Technology For Removal And Fixation Of Arsenic By Ozone Oxidation Followed By Ferric Co-precipitation

Arsenic(As)is an acutely toxic element and chronic exposure to inorganic arsenic causes severe illness such as cancer.It naturally occurs with base and precious metal minerals and is commonly encountered in the solid or aqueous metallurgical industrial wastes.For extraction of As is not profitable,it is thought that is an ideal process which converts this element into a compact and inert residue for landfill.Arsenic is most effectively removed and stabilized when it is present in the pentavalent arsenate form,so usually the oxidation of As(?)into As(?)is required.Normally iron is used for arsenic removal and fixation due to its high affinity to arsenic.But in most cases the As-Fe co-precipitation occurred together with the neutralization with lime and the formed As-bearing amorphous iron oxy-hydroxides sludge has a large volume and high water content,which is unsuitable for landfill.Thus,the overall aim of this Ph.D.research was to devise a process for removal and fixation of arsenic by forming a crystal precipitate which can be disposed easily.This process consists of the oxidation of As(?)to As(?)using ozone in acidic solution and a two-stage enhancement of pH for precipitation.The oxidation of As(?)into As(?)with ozone was carried out at pH range of 0.5-2.The batch experiment results showed that the As(?)can be converted to As(?)by ozone oxidation in acidic solution rapidly.At pH 0.5 and 20?,the concentration of As(?)in the solution decreased from 5000 mg/L to less than 0.1 mg/L in 120 minutes.It was found that the ozone oxidation was inhibited at elevated temperature but catalyzed by Fe3+.In presence of Fe3+,97.2%of 5000 mg/L As(?)was oxidized at pH 2 in 2 hours.During two-stage arsenic co-precipitation process,the experimental results showed that arsenic can be removed effectively from a solution with the arsenic concentration of 5000 mg/L by forming crystalline scorodite through oxidizing ferrous with air at a pH of 2.While the pH value improved to 3 or 4,it found that almost all the residual arsenic was removed from the solution due to the adsorption of the amorphous ferrihydrite when the molar ratio of Fe/As 2,3 and 4 was applied respectively.The results of the TCLP tests showed that the complex precipitates generated under these conditions(Fe/As ratio of 4,final pH 3 or 5)were very stable,and no leached arsenic detected using acetic acid buffer solution(pH 4.93±0.05)in test period of 72 h.SEM-EDS results showed that the crystalline scorodite was formed at pH 2 in the first stage of 4 hours,and then the arsenical ferrihydrite(AsFH)deposited on the surface of scorodite particles when pH enhanced from 2 to 3 or 4.Experimental results suggest that a high Fe/As ratio was necessary,so that the excess iron promotes the formation a product of good stability.In some cases,the wastewater contained arsenic together with the ferric ions,which could interfere with the crystallization by forming amorphous ferric arsenate.In this research,we attempt to dispose the wastewater produced in bio-oxidation of arsenic-bearing gold ore using our proposed method earlier.In this wastewater,there was 1660 mg/L arsenic and 25000 mg/L ferric contained.It was found that in the two-stage precipitation process,99.96%of arsenic was removed when pH was improved from 2 to 3 and 99.99%when the pH increased from 2 to 4.The SEM results showed that the majority of the precipitate was crystalline grains but there was some amorphous product deposited on these grains.XRD pattern showed that the crystalline grains were natrojarosite instead of crystalline scorodite.There was no leached arsenic detected in the TCLP tests with these precipitate.