Direct Current Degradation Technology Of PFOA And PFOS In Groundwater And Soil

The perfluorinated compounds（PFASs）represents a new type of lasting synthetic organic pollutant with a certain degree of potential harms to human health.Owing to the strong chemical durability of PFAS s,the current degradation techniques designed to decompose and control the PFASs exiting in the soil and underground water are confronted with problems like high cost and demanding reaction conditions.For this reason,this paper will point out a kind of low voltage direct current degradation technique especially aimed at decomposing the perfluorooctanoate（PFOA）and perfluorooctane sulfonate（PFOS）in the soil and underground water,and will explore the degradation mechanism of this technique.The main findings and conclusions gained are as follows:（1）The fast performance liquid chromatography mass spectrometry（UPLC-MS）detection was established targeted at eight different PFASs in the soil and underground water so as to provide methodological support for the following degradation experiments.These eight PFASs have a sound linear relation within the concentration range in these experiments（R2>0.99）.In terms of the eight PFASs from the underground water,the method detection limit（MDL）was 0.06～1.07μg/L,the recovery rate was 75%～103%,the relative standard deviation（RSD）was 2.61%～9.19%except from PFBS and PFHxS being separately 11.34%and 14.70%.While the eight PFASs in the soil have a MDL of 2.69～7.86ng/g,a recovery rate of 77%～103%and a RSD of 1.96%～4.34%.（2）The results of the indoor simulation experiment showed that both PFOA and PFOS from the underground water were not degraded within 30 days without the addition of sodium perchlorate under the condition of pH being 7.5 and voltage 0-24V.In addition,with the addition of sodium perchlorate（2mmol/L）,the PFOA from the underground water saw a degradation rate of 0.003-0.042d^-1 with a voltage of 3-24V（pH=3）,and its removal rate and defluorination rate,within 20 days,were respectively 12%-59%and 5%-58%with the best degradation effect under the condition of a voltage of 24V.When the pH was 2.0-7.5（U=24V）,its degradation rate was 0.026-0.091d^-1 and its removal rate and defluorination rate were separately 22%-74%and 20%-67%within ten days with the best degradation effect when pH was 2.However,the PFOS from the underground water was not degraded with the addition of sodium perchlorate（2mmol/L）under the condition of a voltage of 3-24 V and a pH of 2.0-7.5.The addition of sodium perchlorate（2mmol/L）was able to drive the degradation rate of PFOA up by 11.3%when the pH was 7.5 and the voltage 24V,but had no impact on the degradation of PFOS.（3）The exploration on the degradation mechanism of PFOA and PFOS indicated that when the electric current density was adjusted to 30mA/cm² through the electrolyte of potassium chloride,the degradation rates of PFOA and PFOS were separately 0.035d^-1 and 0.049d^-1 when pH was 5.0-7.0 and the voltage was 24V,while their degradation rates were respectively 1.718d^-1 and 0.01d^-1 with a pH of 2.0 and a voltage of 24V.Therefore,it can be concluded that pH plays a much more essential role in the degradation of PFOA and PFOS from the underground water than the electric current density.（4）The indoor simulation experiment demonstrated that when the sodium chlorate was not added,the PFOA and PFOS from then soil had a total degradation rate of 0.077d^-1 and 0.042d^-1,a total degradation rate of 49%and 30%as well as a total defluorination rate of 46%and 27%after ten days with a pH of 7.0 and a voltage of 24 V.Besides,the degradation rates of PFOA and PFOS in the cathode and the middle position were all less than 92%,the rates of PFOA and PFOS in the anode were 55%and-37%,and the rates of PFOA and PFOS 8 cm outside of the cathode were-22%and-20%.It indicated that in the reaction system of low voltage direct current degradation,the ionic PFOA and PFOS in the cathode would be transferred to the anode and surrounding areas and then be degraded here.Moreover,thanks to the existence of electromigration,the degradation rates of PFOA and PFOS in the cathode and the middle position in the fourth day reached over 95%.（5）The study on the mechanism of low voltage direct current power degrading PFOA and PFOS turned out that the possible degradation mechanism of PFOA was C₇F₁₅COO-（?）C₇F₁₅COO·→C₇F₁₅·→C₇F₁₅OO·→C₇F₁₅O·→C₆F₁₃·+CO₂+2Hf,while that of PFOS was CaF₁₇SO₃·→CaF₁₇·→CaF₁₇OO·→CaF₁₇O·→CaF₁₅·CO₂+2HF.