In Situ Conversion Of Refractory Organic Matter From River Sediment By Activated Sodium Persulfate

The accumulation of refractory organic matter in urban river ecosystem will cause ecological imbalance of water body.Sediment remediation in situ has the advantages of low cost,quick effect,good effect and not easy to rebound.In situ chemical remediation,the selection of agents should not only meet the requirements of in-situ remediation to degrade pollutants without changing the location of river sediment and overlying water,but also be efficient,low impact,low disturbance and low cost.Sodium persulfate（PS）is green,pollution-free,low-cost and excellent in the transformation of refractory organic compounds.Its reaction process and products have little harm to the original ecological environment of rivers,and PS has better degradation performance and a wider range of application after activation.In this paper,the transformation characteristics of common phthalate esters（PAEs）in four river ecological environments were studied by PS,and the activation mode,addition ratio and addition mode of sodium persulfate were compared and optimized,and the impact of this method on the ecological environment and the feasibility of engineering application were evaluated.The refractory organic compounds identified in this study are four KINDS of PAEs commonly found in the sediment of Zaohe River in Xi’an:dimethyl phthalate（DMP）,diethyl phthalate（DEP）,dibutyl phthalate（DBP）and butylbenzyl phthalate（BBP）.The main conclusions and achievements are as follows:（1）During static diffusion of activated sodium persulfate in sediment,after 7 days,the vertical diffusion is 1.71cm;The influence radius in the horizontal direction is 4.5cm,the static diffusion effect of the agent is limited,and it is necessary to increase the disturbance of external force to improve the diffusion effect.In the deep stirring method,when the speed is 20r/min and stirring for 10min,the agent is mixed with the bottom mud in the case of no bottom mud re-suspension,so as to improve the organic conversion efficiency of the agent in the bottom mud.（2）The total conversion rate of PAEs was more than 85%on the 12th day when the dosage of sodium persulfate was 11%of the quality of the sediment to be treated.The mixed addition of activation agent and sodium persulfate will lead to the low utilization rate of·SO₄^-,so activation agent is added first and sodium persulfate is added later.When Fe²⁺and sodium persulfate were added separately and the molar ratio of sodium persulfate to Fe²⁺was 1:1.2,the total conversion rate of PAEs was over 85%in the 4th day.When biochar and sodium persulfate were added separately,the total conversion rate of PAEs reached over 85%on the 9th day when biochar dosage was 50%of sodium persulfate.The addition of activation agent can accelerate the conversion rate of PAEs in sediment,and Fe²⁺has better activation effect on sodium persulfate.（3）The conversion of PAEs in river sediment by activated sodium persulfate mainly depends on the oxidation of·SO₄^-and·OH,and the intermediate products of the conversion of dimethyl phthalate（DMP）are mainly phthalic acid,salicylic acid,maleic acid,catechol,2-hydroxyisobutyric acid and malonic acid.The conversion process may be that·SO₄^-and·OH first oxidize DMP side chains to generate macromolecular organic compounds such as salicylic acid,and then further oxidize side chains or open cycles to generate small molecular organic compounds such as catechol and maleic acid,and then further mineralize to produce CO₂and H₂O.The intermediate products of DEP,DBP and BBP all have smaller molecular weight PAEs,which can explain and prove the fluctuation of PAEs concentration in the transformation process.The activation of sodium persulfate is affected by the initial pH of sediment.The conversion rate increases with the increase of pH between 5 and 9,and decreases with the increase of pH between 9 and 11.（4）Multiple dosing can effectively reduce the fluctuation of PAEs concentration in sediment and overlying water during the reaction process,thus reducing ecological and health risks.The ecological risk of PAEs to river organisms,algae,crustaceans and fish was significantly reduced,DMP,DEP and DEP no longer had potential risk,and BBP potential risk was reduced.In the process of transformation,ecological risks are steadily reduced.In health risk assessment,the non-carcinogenic risk of four kinds of PAEs in sediment was significantly reduced after the conversion of activated sodium persulfate.（5）In the PAEs treatment of Zaohe sediment,the optimal dosage ratio of activated sodium persulfate is 11%of the weight of the sediment to be treated,and the dosage of activated agent Fe²⁺is 1.2 times of the mole weight of sodium persulfate.Deep agitation method is used to add Fe²⁺and sodium persulfate separately for five times.The treatment process had a low impact on the original ecological environment,and the total conversion rate of PAEs in the sediment reached more than 85%on the 8th day.The in-situ addition of activated sodium persulfate has a significant effect on the transformation of refractory organic matter in sediment,and the restoration process is stable and efficient.It can effectively reduce the biological toxicity of sediment and overlying water,and improve the river ecological environment,which has a good application prospect.