Process And Mechanism Study Of Persulfate Activation By Zero-valent For The Degradation Of Energetic Compounds

Energetic compounds are one of the main pollutants caused by the long-term widespread use of military and civilian ammunition and improper production management.Energetic compounds have the characteristics of difficult oxidative degradation,high toxicity,and strong accumulation,which pose serious threats to the ecological environment and human health.Persulfate chemical oxidation technology is widely used in organic contaminated site restoration due to its environmental friendliness,high remediation efficiency,long retention time,and strong selectivity.Persulfate can generate strong oxidizing free radicals(such as SO4-·)through zero valent iron activation,which can effectively degrade most organic pollutants.In this study,a series of studies on oxidative repair of TNT and RDX contaminated soil with different activated persulfate methods were conducted.The influences of experimental parameters on the effect of zero-valent iron-activated persulfate on the degradation of soil energetic compounds were ecplored and the process were optimized.At the same time,the reaction mechanism of Fe0/PS system oxidation of energetic compounds was clarified by free radical quenching experiments and electron spin resonance technology(ESR),which provided a scientific basis for the actual repair of persulfate oxidation degradation of energetic compounds.The main results of this study are as follows:(1)The influence of persulfate activation methods on energetic compounds(TNT and RDX)degradation was investigated and included thermal,ferrous ion,zero valent iron,nano-zero valent iron,humic acid and a hydrogen peroxide(H2O2)-persulfate binary mixture activation.ihe changes of properties in the Fe0/PS system before and after the reaction(pH,Eh,TOC,persulfate concentration)were compared.The results showed that the removal efficiency of energetic pollutants(TNT/RDX)was significantly enhanced after persulfate activation treatment.Among the six types of activation treatment methods,thermal activation,zero valent iron and nano-zerovalent iron activation resulted in the higher removal of energetic compounds(TNT>70%,RDX>75%).Compared with other activation methods,there was almost no residual energetic compound(TNT/RDX)in the supernatant of the system after Fe0 and NZVI activation treatment.In addition,the persulfate consumption in Fe0 and NZVI activation systems was the largest,and the acidity of the soil after Fe0,NZVI activation was relatively weak,which improved the safety of the soil after restoration to a certain extent,Fe0 activation was finally screened as the best way to activate energetic compounds due to its high activation efficiency,low cost,and environmental protection.(2)Studies on the influential factors of TNT and RDX degradation in Fe0/PS reaction system were performed in order to obtain the optimal reaction conditions,including water-soil ratio,PS and Fe0 dosing ratio,PS initial concentration,pH and temperature.The results showed that the removal efficiency of TNT and RDX increased first and then decreased with the increase of system water content.The dosage ratio of PS:Fe0 had the same effect with initial PS concentration:the pollutant removal efficiency also increased first and then decreased with the initial PS concentration/the dosage ratio of PS:Fe0 in the system.Under acidic conditions,the removal efficiency of TNT/RDX was higher than alkaline conditions,and the highest degradation efficiency of TNT/RDX could be obtained under weak acidic conditions(pH=6).Temperature can promote the degradation of TNT and RDX in Fe0/PS system.As the temperature increased,the removal efficiency of energetic compounds also increased.After screening,the optimal conditions for the degradation of TNT and RDX by the Fe0/PS process were:soil-water ratio=1:5,PS:Fe0=1:3,initial PS concentration=0.10mol/L,pH=6,and 55?,in this condition,the degradation efficiency of TNT and RDX were up to 97.0%and 96.2%.(3)The types and dynamic changes of active substances in Fe0/PS system oxidative energetic compounds were explored by radical quenching experiments and electron spin resonance technology(ESR)to elucidate the reaction mechanism.The effects of anions(chloride,bicarbonate ions)on the degradation of energetic compounds in the Fe0/PS system and the changes in free radicals were investigated.The results showed that the active substances in the Fe0/PS system were SO4-· and ·OH,and SO4·-had a higher reactivity to TNT and RDX compared with ·OH,it was the most active material for the degradation of energetic compounds in the system.There was a competitive consumption process between the two free radicals in the system.The signal intensity of SO4-· increased first and then decreased with increasing reaction time,but the signal strength of ·OH continued to increase with increasing reaction time.The degradation of TNT and RDX in Fe0/PS system was divided into two stages,the SO4-· attacked target pollutants in the system during the rapid degradation stage,achieving the rapid degradation of TNT and RDX.During the slow degradation stage,a large amount of SO4-· was converted to ·OH,and the two radicals also consumed each other to reduce the oxidation ability of the system.The system continued to react in a slow degradation state to 240min.The anions(chloride,bicarbonate ions)had a dual effect on the removal efficiency of TNT and RDX,that was,the presence of low concentrations of anions had a weak promotion effect on the removal of TNT and RDX,the high concentrations anions would inhibit the removal and degradation of TNT and RDX.