Iron (Ⅱ) Activated Persulfate Oxidation Of Arsenic (Ⅲ) And The Following Adsorption In Aquatic System

Arsenic is a highly toxic element.In recent years,industrial mining waste water and groundwater have been seriously polluted by arsenic due to industrial development and mining.China is one of the countries suffering from arsenic pollution,especially in northern China,where groundwater is the main source of drinking water.Arsenic mainly exists in nature as As（Ⅲ）and As（V）,in which As（Ⅲ）has higher toxicity and migration,so it is more harmful.Because As（V）has stronger adsorption affinity on metal oxide surface,such as hydrous ferric oxides HFO,a pre-oxidation process often become necessary for arsenic removal.At present,there have been studies on oxidation of As（Ⅲ）in Fenton,UV/H₂O₂ and other systems.Some scholars have also reported photochemical and sonochemical oxidation of As（Ⅲ）using persulfate as an oxidizing agent,but few studies have been done on the use of transition metals to activate PDS to oxidize arsenic.This paper analyzed the arsenic oxidation characteristics of system by using the colorimetry of phospho-molybdenμMblue method;At the same time,the concentration of arsenic in the filtered water was measured by ICP-MS,in order to anlayze the adsorption capacity of in-situ HFO formed in the system.PDS has almost no oxidation effect to As（Ⅲ）,while the addition of Fe²⁺ can catalyze the oxidation of arsenic;Acidic conditions are favorable to the oxidation rate and yield of arsenic.Within a certain range,the increase of PDS and Fe²⁺ dosage accelerated the oxidation rate,but the excessive amount of ferrous iron would inhibit the oxidation yield,especially under neutral conditions.Meanwhile,the adsorption experiments showed that under the conditions of pH5 and pH6,the system has the best adsorption effect on arsenic.For 10μM target As（Ⅲ）,the removal rate of arsenic can reach 98% and 85% respectively at 80 min with 30μM PDS and Fe²⁺ dosage.Fe（Ⅲ）mainly exists in the presence of dissolved state,and the weak electrostatic interaction between Fe（Ⅲ）and the arsenic ion is the cause of poor adsorption under strong acid and neutral conditions.PMS can oxidize As（Ⅲ）alone,the oxidation rate is slow in acidic conditions,and the increase of pH is beneficial to the formation of As（V）.The kinetic experiments show that the reaction rate is in accord with the two order reaction kinetics at pH3 and pH5,and the rate constants are 4.89 and 13.65 M-1 s-1 respectively.The addition of Fe²⁺ rapidly accelerates the reaction rate and enables the reaction to be complete within 1min.However,the presence of ferrous iron competes with the As（Ⅲ）for free radicals and PMS,so the inhibition of yield is observed at high Fe²⁺ dosage.Similar to PDS system,pH5 has the best adsorption effect on arsenic,but due to the oxidation inhibition caused by ferrous ion,the adsorption effect of high Fe²⁺ dosage is not ideal.Methanol and tert butyl alcohol were used as free radical capture agents to consider their inhibitory effect on oxidation.Sulfate radical is the main oxidation species in the system,and there is a certain proportion of hydroxyl radical oxidation path in Fe（Ⅱ）/PDS system.For the Fe（Ⅱ）/PMS system,the inhibitory effect of tert butyl alcohol was observed only in pH3 conditions.In addition,under the pH9 conditions,methanol has little inhibitory effect on oxidation,indicating that the oxidation is completed in a non free radical pathway.