Study On The Efficacy And Mechanism Of Black-odor Sediments Remediation By CaO₂ Fenton-like And Hydroxyapatite Combined Process

In addition to Acid Volatile Sulfide（AVS）,there are many other pollutants such as persistent organic compounds（POPs）and heavy metals in the black-odor river sediments,which have a great impact on the safety of the river ecosystem.Calcium peroxide（CaO₂）,which has the characteristics of slow release of O₂ and H₂O₂,can effectively oxidize AVS to eliminate the black and odorous.It was widely applied in in-situ remediation of black-odors sediments.However,this method is not ideal for the removal of POPs and stabilization of heavy metals in the sediments.For this reason,we chose the common POPs of polycyclic aromatic hydrocarbons（PAHs）in black-odor sediments as the target pollutants,and Ni,Cu,Zn,and Pb as the representatives of heavy metals.Based on the elimination of the black and odorous by the addition of CaO₂,the CaO₂ Fenton-like degradation of PAHs and the remediation of heavy metals in black-odor sediment by CaO₂ compounded with hydroxyapatite（HAP）were investigated separately.The CaO₂ Fenton-like and HAP combined process was constructed to investigate its efficacy and reaction mechanism of simultaneously eliminating black and odorous from the sediment,degrading PAHs,and stabilizing heavy metals.Then,we evaluated the ecological risk of the remediated sediment.It is expected that this study can provide a theoretical basis and technical support for the engineering application of in-situ remediation of black-odor sediments.Using the difficult-to-degrade high molecular weight substance benzo（a）pyrene（BaP）as a representative pollutant of PAHs,the degradation effect of CaO₂/Fe（II）/citric acid（CA）Fenton-like reaction on BaP in simulated sediment was investigated and applied to remediate the actual black-odor sediment.The results showed that CaO₂/Fe（II）/CA Fenton-like could effectively degrade BaP in sediments,and the BaP removal efficiency could reach nearly 80%under the optimal conditions of CaO₂,Fe（II）,and CA dosing of 10,2.5 and 3 g/kg dw（dry weight）,respectively.The presence of CA broadened the p H range of the Fenton-like reaction and reduced BaP to the environmental Effect Range Low（ERL）value（430μg/kg）under neutral conditions.At the same time,CaO₂/Fe（II）/CA can simultaneously eliminate the black and odorous of actual sediment and degrade PAHs,with AVS removal efficiency of more than 98%and PAHs reduction below the ERL value（4022μg/kg）.For the types of heavy metals（Ni,Cu,Zn,Pb）in the contaminated sediment,the heavy metal stabilizer HAP was screened and compounded with CaO₂.The heavy metal leaching amount of the remediated sediment was determined by the heavy metal toxicity characteristic leaching procedure（TCLP）,and the changes in the physical and chemical properties of the sediment were monitored.The results showed that when the CaO₂ dosage was 5 g/kg dw,and the optimal dosage ratio of CaO₂compounded with HAP was 1:3,it could effectively stabilize the heavy metals in the sediment,reduce the active state content of heavy metals,and lower the toxicity of heavy metals in the sediment.The leaching amounts of heavy metals meet the requirements of the Technical Specification for the Disposal of River and Lake Sludge Treatment Plant Output（SZDB/Z 236-2017）Class I.Meanwhile,the addition of CaO₂effectively eliminated the black and odorous of the sediment and improved the anaerobic environment of the sediment,the AVS decreased to about 15 mg/kg dw,and the ORP increased by about 100 m V.To eliminate the black and odorous from sediment,degrade PAHs,and stabilize heavy metals simultaneously,a CaO₂ Fenton-like and HAP combined process was constructed.Its operating conditions were optimized,and the comprehensive efficacy and mechanism of the combined process for the remediation of black-odor sediment were investigated.The results showed that the combined process can effectively remove AVS,degrade PAHs,stabilize heavy metals and improve the water quality of the overlying water at the optimal dosage of CaO₂,Fe（II）,CA,and HAP of 20,2.5,3,and 14 g/kg dw,respectively.While the time interval between CaO₂ Fenton-like agents and HAP dosage must be greater than 1 h.The analysis of the reaction mechanism of the combined process showed that:the improvement of the anaerobic environment of black-odor sediment increased the ORP of the sediment,sulfide was oxidized and eliminated the black-odor sediment;HO·produced by CaO₂/Fe（II）/CA Fenton-like reaction could effectively oxidize the BaP in sediment,and the reaction process included three stages:rapid,fast and slow,and the contribution of each stage to BaP degradation was 29.4%,43.1%,and 2.4%,respectively,and the BaP degradation pathways mainly include step-by-step oxidation,ring opening,and deprotonation pathways;HAP plays a major role in heavy metal stabilization,and the presence of CaO₂ has a synergistic effect on heavy metal stabilization,the OH^-generated by its dissolution consumes the H⁺generated by HAP dissolution,which enhances the heavy metal stabilization effect.The effects of CaO₂ Fenton-like and HAP combined process on organisms were examined comprehensively from the perspective of acute and chronic toxicity of microorganisms,plants,and animals in three different trophic levels.Then,the ecological risks of the remediated sediments were also evaluated.The results showed that compared with the uncontaminated sediment,there was no acute and chronic toxicity to organisms in the remediated sediment by the CaO₂ Fenton-like and HAP combined process.The inhibition rate of luminescent bacteria and the mortality rate of Chironomus riparius in the remediated sediment were less than 5%.The germination rate of radish seeds increased by 13%,and the root length and stem length elongated by 1.2 cm and 2.1 cm,respectively.Compared with the black-odor sediment,the survival rate of Gambusia affinis and Bellamya Aeruginosa decreased significantly,and the growth rate of stem length and plant height of bitter grass increased by 64.2%and 50%,respectively.Moreover,the microbial community evolved in the remediated sediment（p<0.01）,and Proteobacteria and Firmicutes gradually recovered as the dominant genera.Meanwhile,AVS,PAHs,and heavy metals levels in the remediated sediments were below the threshold of ecological risk values.