Persistence Of Persulfate And Its Effects In Karst Groundwater

Persulfate was a highly effective oxidant for ISCO technology to treat organic polluted groundwater.However,after injection into aquifer,PS may be influenced by other factors to reduce its durability and reduce its exposure to pollutants.Such problems have not been reported before in karst areas.In this study,by collecting limestone and lime soil samples in the field and carrying out microcosm experiment under static water condition and column experiment under flowing water condition in the laboratory,the persistence of PS in karst groundwater was studied,the factors affecting PS decomposition was explored and the influence of PS decomposition on hydrochemical indexes was evaluated,which has reference value for the use of PS-ISCO technology to remediation of organic contaminated karst groundwater.The following conclusions were obtained by the microcosm experiment with PS concentrations of 1,8 and 20 g/L respectively,and the column experiment with different PS:Fe²⁺concentrations:（1）The effect of different aquifer media on PS was different.Compared with lime soil,PS in limestone had better persistence.The decomposition rate constants of PS in limestone and lime soil were in the range of 0.34～0.68×10^-2/d and 1.35～3.57×10^-2/d respectively under the static water condition.The decomposition rate constant of PS in limestone was increased by an order of magnitude under flowing water condition.When the PS concentration was133.5 mg/L and 1335.0 mg/L,the decomposition rate constant was 9.27×10^-2and 7.63×10^-2/d respectively,with poor persistence.（2）Temperature,injection concentration,pH and OMC had obvious effects on PS persistence.Increasing the concentration of PS and lowering the temperature both increased persistence of PS.Low pH water environment would accelerate the decomposition of PS,limestone could keep the pH in the neutral range,strongly enhance the persistence of PS;lime soil could not maintain a neutral pH in the presence of high concentration of PS,which might promote the decomposition of PS.In the same aquifer media,the higher the concentration of PS,the more OMC decreased;the higher the initial OMC of aquifer media,the more it was consumed.The reaction between PS and OMC would lead to the increase of DOC concentration in water.OMC was able to promote PS decomposition,and the high OMC in lime soil was the main factor for the rapid decomposition of PS.（3）Excessive Fe²⁺was detrimental to the activation effect of PS.With the increase of PS concentration,the Fe²⁺in the system would be more inclined to activate PS than to consume SO₄^·-.The precipitation of iron-containing compounds formed by Fe²⁺would remain in the aqueous medium,which could form a long-term activation of PS and enhance the ability of PS-ISCO technology to deal with pollutants in the source area.（4）Compared with the static water state,the concentration of PS in the flowing water state was lower,and its influence on the hydrochemical indexes was small,and the influence degree gradually decreased under the action of water flow,indicating that the influence of PS-ISCO technology on the groundwater quality was small.However,the decomposition of PS would lead to the gradual increase of SO₄^2-concentration.Therefore,while using PS-ISCO technology to remove pollutants,it is also necessary to deal with SO₄^2-.