Monitoring Of Persulfate Oxidation Remediation Process And Secondary Corrosion Risk Of Soil Contaminated With Pahs

Polycyclic aromatic hydrocarbons have low water solubility,high fat solubility,carcinogenic,teratogenic,and mutagenic properties.They are defined as persistent organic pollutants and are among the most common organic contaminants in soil,natural water bodies,and wastewater.At present,the commonly used remediation technologies for organic contaminated sites include thermal desorption,bioremediation,and chemical oxidation remediation.Among them,the proportion of chemical oxidation in the remediation of organic contaminated sites has increased rapidly in China,but the potential corrosion risk of reuse caused by residual oxidants and by-products has also attracted the attention of researchers.Through practical investigation and literature study for the 137 domestic sites,the four main characteristics of chemical oxidation remediation technology could be concluded:（1）Mainly applications in small and medium-sized contaminated land.（2）Complex remediation media.（3）Excessive persulfate application and various by-products.（4）Overload of chemical oxidant.Then,for the most frequently used persulfate oxidation remediation technology,through literature review and sites case study,three main potential corrosion mechanisms were analyzed.The residual oxidant corrosion,by-product salt corrosion and microbial erosion would be generated in the redevelopment of post-remediated sites.Meanwhile,the qualitative and quantitative monitoring of corrosion risk were also studied.Finally,a conceptual model that can divide the potential post-remediation corrosion process into 3 different phases was proposed for chemical oxidation by persulfate based on the chemical oxidation process and the short-term,mid-term and long-term key factors,including pH,ORP,salt and microbial environmental conditions.Taking the surface polycyclic aromatic hydrocarbons contaminated soil of an abandoned coking plant in Hebei Province as the research object,through the laboratory simulated batch treatment experiment,the degradation effect of persulfate on polycyclic aromatic hydrocarbons was monitored under the conditions of oxidant to pollutant molar ratio of 5:1,10:1,25:1,50:1and 100:1.It was found that the degradation effect of persulfate on polycyclic aromatic hydrocarbons was mainly in the first week of oxidant application,and the degradation effect of oxidant on polycyclic aromatic hydrocarbons was no longer obvious after one week of treatment,The removal rates of pollutants were significantly correlated with the dosage of oxidant,which was 25.80%、29.06%、37.55%、47.01%and 51.95%,respectively.At the same time,the removal rates of monomer polycyclic aromatic hydrocarbons by persulfate are different.The removal rate of Indeno（1,2,3-cd）pyrene can reach more than 75%under the appropriate amount of oxidant,the removal rates of acenaphthene are over 42.29%,the removal rates of fluorene,anthracene,dibenzo（a,h）anthracene and monomers are 33.26%-37.86%,and the removal rates of naphthalene,benzoanthracene,benzo（b）fluoranthene and benzo（k）fluoranthene are less than 10%.The removal effect of persulfate in soil by HMW-PAHs was slightly better than that by HMW-PAHs.On the whole,low dose of activated persulfate can effectively remove PAHs in soil.Increasing the amount of oxidant can further improve the removal effect of PAHs,but excessive oxidant also further increases the potential corrosion risk in soil.Monitoring the physical and chemical properties of soil during persulfate treatment,mainly including long-term quantitative change monitoring of safety influencing factors such as soil salinity,pH and redox potential.After treatment,the soil pH decreased significantly,and the content of SO₄^2-in the soil increased significantly after the formation of sulfate radical in the process of oxidation reaction and pollutants,after one week,the content of SO₄^2-in the mixed solution was 8.56*10³、1.08*10⁴、1.70*10⁴、1.84*10⁴ and 4.03*10⁴mg·L^-1 respectively,an increase of 98.36%-833.46%compared with that before treatment,the content of SO₄^2-is positively correlated with the dosage of oxidant.The increase of sulfate content increases the risk of salt corrosion in soil.In addition,SO₄^2-combines with H⁺to form corrosive sulfuric acid under acidic conditions,which increases the risk of acid-base corrosion of soil.Persulfate has strong persistence.After 203 days of treatment,the persulfate concentration in the soil is3.98*10³mg·L^-1.The persulfate remaining in the soil still has high electrode potential,which will have a redox reaction with iron in the steel structure,resulting in potential corrosion risk.Oxidants have an impact on the dominant genera and abundance of microorganisms in the soil environment,but this effect is gradually weakened with the increase of treatment time,and the growth of abnormal microorganisms may bring a certain risk of biocorrosion.At the same time,persulfate remediation changes the form of metals in the soil,resulting in changes in the migration ability of heavy metals,increasing the content of metals such as Cr,Mn,Zn,As,and Pb in the soil,and even causing secondary heavy metal pollution.There are relatively few studies on the corrosion risk monitoring and evaluation methods of the repaired site.Two different standard iron sheets are added in 24 hours and 63 days of chemical oxidation treatment respectively to monitor and analyze the quality changes of standard iron sheets and the changes in surface cracks in the process.The erosion mechanism and kinetics of soil samples on standard iron flakes changed with time after persulfate treatment,which accelerated the formation and width of iron flake surface gaps.The gap width was significantly positively correlated with the treatment time,under the natural conditions without adding an oxidant,the surface gap width of the iron sheet varies from 384nm to 1537nm,from503nm to 7111nm in the low dose AP2 group,and from 520nm to 9367nm in the high dose AP5group.There is a very significant negative correlation between the surface gap and the quality,the mass-loss rates of iron sheets after one week were 1.23%,2.49%,4.70%,7.59%,and21.76%,respectively.The mass-loss rate changes with the treatment time.According to the standard iron sheet mass loss rate,the whole corrosion process is divided into three stages.The corrosion risk from high to low is the oxidant corrosion stage>the oxidant and salt corrosion stage>the salt and microbial corrosion stage.The oxidant itself is the main reason for the rapid corrosion of the iron sheets in the initial stage.In the middle stage,the consumption of oxidants and the increase of SO₄^2-and other concentrations reduce the corrosion rate,and the growth of bacteria such as sulfate-reducing bacteria in the later stage further accelerates the corrosion rate.The influence of the oxidant itself on the microbial community is weakened by time,and the rapid growth of abnormal microbial communities such as SRB has an accelerated effect on the corrosion of standard iron sheets.Through indirect monitoring of site corrosion risk after restoration and direct monitoring of standard iron sheet corrosion,the dosage of chemicals should be controlled when using persulfate for site restoration to reduce the high corrosion risk caused by excessive chemicals,and a small number of multiple chemicals can be used.Add the method to achieve the fine management of the soil.As far as possible,allow sufficient natural recovery time for the repaired site,infer the potential corrosion risk of the repaired site according to the amount and time of persulfate,and avoid redevelopment and utilization of the repaired site in the high corrosion risk stage.