Selective Degradation Effect,mechanism Of Naphthalene-sodium Dodecyl Sulfate Soil Washing Solution By Thermally Activated Persulfate And Its Recovery Performance Optimization

Surfactant enhanced in-situ soil washing technology has been widely used in the remediation of polycyclic aromatic hydrocarbon contaminated soil,but the processes typically produce various soil washing solutions that contain hydrophobic organic compounds and surfactants which require further treatment.How to effectively remove the target pollutants while minimizing surfactants degradation for reuse has become a major concern for the remediation project.The advanced oxidation technology based on SO₄^·-is more selective than the traditional Fenton oxidation technology,which provides basic conditions for improving the degradation rate of target pollutants and the recovery rate of surfactants.It is of great practical significance to explore the selective degradation effect and mechanism of SO₄^·-on pollutants and surfactants in soil washing solutions and optimize the recovery performance of surfactants for the treatment of soil washing solutions.In this paper,naphthalene,a common polycyclic aromatic hydrocarbon in contaminated soil,was used as the target pollutant,and sodium dodecyl sulfate（SDS）,an anionic surfactant commonly used in soil washing,was used as a surfactant representative.Firstly,the degradation effects of thermally activated persulfate（PS）system on the mixtures of naphthalene and SDS under different SDS concentrations were studied,and the preferential degradability of naphthalene by free radicals in the system was analyzed.The particle size,morphology of SDS micelles and surface tension changes before and after the reaction were determined,the main attack ways of free radicals on naphthalene were discussed,and the mechanism of selective degradation of free radicals was elucidated.Through electron paramagnetic resonance detection and free radical quenching experiments,the free radicals species and the dominant free radicals were identified,and the possible degradation path of naphthalene was discussed.The effects of PS dosage,thermal activation temperature and initial p H on naphthalene degradation rate,SDS recovery rate and naphthalene preferential degradation were clarified.Based on the response surface optimization method,an equation model was established with the concentration of naphthalene,PS and SDS as independent variables,and the ratio of the degradation rate constant of naphthalene to SDS(k_nap/k _SDS)as the response value to optimize the preferential degradation of naphthalene by free radicals.Finally,the washing removal effects of SDS solutions on naphthalene in contaminated soil were explored by dynamic washing experiments,the soil washing solutions were recycled after thermal activation PS oxidation,and the reusability of SDS under two different recycling methods was discussed.The main conclusions are as follows:（1）The degradation of naphthalene was inhibited due to the protection of SDS micelles.However,the k_nap/k _SDSvalues were all greater than 1 at all SDS concentrations,indicating that the degradation rate of naphthalene was higher than that of SDS,and free radicals preferentially degraded naphthalene in the mixtures of naphthalene and SDS.The degree of preferential degradation of naphthalene was significantly affected by SDS concentration.When SDS concentration was 5 times the critical micelle concentration（5 CMC）,the k_nap/k _SDSvalue was as high as 18,and it was as low as 3.16 at 10 CMC.Combined with the changes of the particle size and morphology of the micelles,it was found that the average particle size of the micelles increased with the increase of SDS concentration,and the average particle size of the micelles after the reaction was smaller than that before the reaction.When SDS concentration was 5 CMC,the micelles were spherical before and after the reaction.When SDS concentration was 10 CMC,there were vesicles with large particle size before the reaction,and the vesicles disappeared after the reaction.At 10 CMC,the micelle-water partition coefficient of naphthalene was large,and the main way for free radicals to attack naphthalene was to first break the outer layer of SDS surrounding naphthalene,and then exposing naphthalene for removal,resulting in the decrease of k_nap/k _SDSvalue and low recovery of SDS.SO₄^·-was dominant in the system,containing a small amount of·OH.Naphthalene was gradually ring-opened and oxidized to more stable alkanes under the combined action of SO₄^·-and·OH,and finally completely mineralized.（2）With the increase of PS dosage,the amount of free radicals production increased,k_nap/k _SDSvalue firstly increased and then decreased.When the concentrations of SDS and naphthalene were 5 CMC and 100 mg/L,respectively,the k_nap/k _SDSvalue was the highest when PS dosage was 50 m M,and excessive PS will lead to the reduction of SDS recovery.When the thermal activation temperature was30°C,the k_nap/k _SDSvalue was less than 1,which did not reflect the preferential degradation of naphthalene.When the thermal activation temperature was 60°C,the preferential degradability was stronger than that of 40°C.The initial p H value of the solution had little effect on the final degradation effect of naphthalene in the range of3～12,and as the reaction progresses,the p H value of the solution gradually decreased and finally became acidic.The response surface method was applied to construct the equation model,and the relationship between k_nap/k _SDSvalue and naphthalene,PS and SDS concentrations was established,so as to optimize the preferential degradation of naphthalene,which provided a reference for the treatment of actual soil washing solution.（3）SDS solution with initial concentration of 5 CMC was selected to wash naphthalene-contaminated soil column.The soil washing solution was collected for PS oxidation treatment and then directly reused in the next washing process or fill the SDS concentration to 5 CMC and then washed.After 4 times of soil washing and 3times of thermally activated PS oxidation treatments,the total washing removal rates of naphthalene in soil were 74.36%and 76.93%,respectively,which was similar to that of fresh SDS solution（77.52%）,indicating that SDS had well reusability.It is recommended that the soil washing solution should be directly recycled after oxidation in practical engineering applications.After soil washing,the soil was weakly acidic,and the residual organic matter in the soil included naphthalene and a small amount of alkaine compounds.