Microwave-activated Persulfate To Remediate Ethyl-parathion/polycyclic Aromatic Hydrocarbons From Contaminated Soil

In recent years,the relocation of contaminated sites left by enterprises has become an important issue that restricts the reuse of urban land resources in China,and"Carrying Out Pollution Control and Restoration"has been listed as the core task of the"Ten Articles of Soil".Among all kinds of industrially polluted sites,pesticide chemical sites were one of the main types of pollution,which already had posed a serious threat to the remaining plant areas and the surrounding urban ecosystems.To realize the efficient and safe remediation of soil,in-situ thermal desorption and chemical oxidation technologies have become common restoration techniques for this type of site.However,the traditional thermal desorption technology had some problems,such as slow heating speed,uneven soil temperature,and incomplete removal of pollutants.As an emerging heating technology in the field of soil remediation,microwave（MW）has attracted more and more attention in the field of soil remediation due to its unique heating mechanism and efficient and fast heating effect.However,the single microwave heating technology was the same as the traditional thermal desorption technology,and the collected exhaust gas needs to be re-oxidized,incinerated,or adsorbed.Combining microwave heating technology and advanced oxidation technology,using both advantages of the two technologies to form a new type of microwave-enhanced in-situ oxidation technology was a feasible path,which can reduce working procedure,save costs,and avoid secondary pollution.Based on this,the research object was organophosphorus pesticides（OPs）,the target pollutant was ethyl-parathion（PTH）.Using microwave（MW）activate persulfate（PS）remediation method to investigate the effect of reaction time,oxidant dosage,MW radiation temperature,soil physical and chemical properties,and other process parameters on PTH degradation.At the same time,biochar was used as a wave-absorbing material to improve the utilization efficiency of persulfate,and the effect and the mechanism of biochar combined with MW to activate PS to remove PTH were analyzed and discussed finally.Based on the above research,an in-situ simulation experiment was carried out on the remediation of organic contaminated soil by MW-activated PS technology.The main research results were as follows:（1）In the MW-PS system,when the MW radiation temperature is 60 ^oC,the change of PTH removal efficiency under different PS additions（1.19,2.38,7.14,14.28,23.8 g/Kg）was explored.The results showed that the removal efficiency of PTH increased with the addition of PS,and when the amount of persulfate added was 23.8 g/Kg,the removal efficiency of PTH could reach 75.3%within 80 min.At the same time,the removal efficiency of PTH increased significantly with the increase of temperature.When PS dosage was 23.8 g/kg and MW temperature was 100 ^oC,the removal efficiency of PTH can reach90.47%.Under the same conditions,the maximum removal efficiency of PTH could reach50%by the traditional thermal activated PS method（water bath thermal activation）,indicating that the efficiency of MW activation of PS was much higher than the traditional PS activation method,thereby promoting the removal efficiency of pollutants.Mechanism analysis showed that sulfate radicals(SO₄^·-)and hydroxyl radicals（·OH）play the main role in the PTH degradation process,followed by superoxide radicals(O₂^·-).In general,MW activated PS system can effectively degrade PTH,and the optimal reaction temperature of the system was 60 ^oC,and the optimal amount of PS added was 23.8 g/Kg.（2）The effect of biochar on the removal of PTH in the MW-activated PS system was explored.Five kinds of prepared biochar were used as MW absorbing materials.In the MW activated PS system,the PS addition amount was 23.8 g/Kg,the MW radiation temperature was 60 ^oC,and the soil biochar addition amount was 0.15 g as the reaction conditions.After80 min,the removal efficiency of PTH could reach 82.04-88.78%,respectively.Mechanism analysis showed that,on the one hand,the presence of biochar can increase the temperature of the reaction system;on the other hand,persistent free radicals（PFRs）in biochar can effectively activate PS,improve the utilization efficiency of PS,induce the production of active species,and finally remove the pollutants efficiently.In addition,the removal effect of biochar with different particle sizes on PTH was different.The smaller the average particle size,the higher the removal efficiency of the PTH.（3）In the MW-activated PS system,soil moisture content and soil humus had a greater impact on the removal of PTH.Among them,the pollutant removal efficiency will increase with the increase of soil moisture content.The reason was that the water can absorb MW energy,improve MW utilization efficiency,accelerate soil heating,and increase the PS decomposition rate;while soil humus will reduce the removal efficiency of pollutants.The reason was that too much soil humus will consume oxidants,resulting in a reduction in the amount of PS that participates in the degradation of pollutants,and ultimately reducing the removal efficiency of PTH.（4）Based on the above research,a microwave-enhanced oxidation in-situ remediation device was built by ourselves.According to the indoor simulation experiment parameters,the actual contaminated site soil was reconditioned and the soil temperature change during the process was monitored.The results show that the device has a good heating effect on the soil.The soil temperature could reach 60 ^oC after heating for 24 hours under the condition of500 W power.With the increase of microwave power,the soil temperature continues to rise,when the power reached 1000 W,after heating for 24 hours,the soil temperature can reach as high as 175 ^oC.This technology had a good remediation effect on the actual contaminated soil,and the pollutant removal efficiency can reach more than 90%.Increasing the MW power,increasing the amount of PS,adding water and biochar can improve the removal efficiency of pollutants.