Determination Of Environmental Quality Guidelines For Typical Heavy Metals And In Situ Remediation Of Cd Pollution In Coastal River Sediments

Sediment acts as an important source and sink of heavy metals in aquatic ecosystem.In recent years,heavy metal pollution in sediments has become a global environmental problem that needs to be handled.At present,the pollution of heavy metals in sediments in China is not optimistic,threatening the safety of aquatic ecosystems and human health.Therefore,the effective controlling of heavy metals polluted sediments is very imminent.However,there is no river sediment environmental quality guidelines(or standards)to effectively supervise and evaluate heavy metal pollution in sediment until now,and research on effective treatment methods of heavy metals contaminated sediments is also relatively backward.In view of the above situation,from the perspectives of evaluation and remediation of heavy metal pollution in sediments,this study firstly explored the feasibility of the establishment of heavy metal environmental quality guidelines for river sediments(SQG)combined with modified equilibrium partitioning approach,and then carried out in-situ remediation of Cd polluted sediments combined with SQGs,aiming to provide scientific basis for the effective controlling of heavy metals contaminated sediments.The major findings are as follows:(1)Determination of quality guidelines for typical heavy metals in coastal river sediments: Taking sediments from Jiaolai River(JL)and Jiahe River(JH)as the research objects,the environmental quality guidelines of heavy metals(Cd,Cu,Pb and Zn)in sediments at watershed scale were established combining with the surface water quality standard(GB 3838-2002).Additionally,the spatial distribution,speciation,partition coefficient,and environmental risk of heavy metals in sediment and interstitial water in the two rivers were also studied.The results exhibited that the contents of heavy metals in sediments in JH were higher than those in JL,while the contents of heavy metals in interstitial waters showed a opposite trend.The speciation analysis of heavy metals showed that most heavy metals in the sediments were dominated by residual speciation.The distribution of heavy metals in sediments and interstitial water was mainly influenced by the properties of sediment and interstitial water,and the input of exogenous heavy metals.The results of contamination factor presented that Cd in sediments from JL and JH exerted a high or very high contamination level.The heavy metals in sediment interstitial water in most sites presented no toxicity to aquatic organisms according to the interstitial water criteria toxicity units and Nemerow index.Based on the interstitial water quality standards modified by surface water quality standards,the SQG of heavy metals at watershed scale were calculated by the modified Eq PA.It can be found that SQG at watershed scale can reasonably classify the sediments of different water function areas into different grades.Heavy metals in sediments exerted a low potential toxicity of the two rivers in view of SQG.Totally,it is feasible to establish SQG of heavy metals at watershed scale according to different water function regionalization.(2)In-situ immobilization of Cd contaminated sediments using different materials loaded with nano-zero-valent iron: zeolite,sepiolite,red mud(RM),and biochar(BC)were performed to load nano-zero-valent iron(n ZVI),and then used for in-situ immobilization of Cd polluted sediment(90 d).The physicochemical properties,remediation efficiency,and bacterial community of the sediments after the immobilization were explored.The experimental results showed that the sediment properties significantly changed after immobilization(P < 0.05).The proportions of weak acid soluble speciation of Cd in the treated sediments were reduced by 11% to47% compared to the control;conversely,and the proportions of residual speciation were increased by 50% to 1000%,indicating the Cd stability was enhanced.Among the four raw materials,the immobilization effects of RM and BC on Cd contaminated sediments were significantly higher than those of sepiolite and zeolite.In particular,the modified materials possessed higher immobilization efficiencies for Cd in sediments,and the leaching toxicities were reduced by 15%~22% compared with the raw material treatments.The remediation efficiencies of n ZVI/RM and n ZVI/BC were the best among all the materials,and their leaching toxicities were reduced by 42% and 44%,respectively,compared with the control.Immobilization treatment improved the richness and diversity of bacterial communities in sediments.In addition,the reductions of Fe(III)and sulfate were promoted by enhancing Fe(III)reducing bacteria and sulfate reducting bacteria after immobilization treatment,advancing the Cd immobilization in sediments.Overall,BC(n ZVI/BC)could be considered as the most ideal materials for the immobilization of Cd polluted sediments due to them showed better immobilization effects and little adverse effects on sediments.(3)Efficacies and microbial responses of BC and n ZVI/BC during in-situ immobilization of Cd contaminated sediments: To explore the effects of time and dose on the remediation efficacies of BC and n ZVI/BC,BC and n ZVI/BC with different doses were used to immobilize Cd polluted sediments in situ for a longer time(140 d).The remediation efficiencies of BC and n ZVI/BC under different p H values and responses of bacterial communities in sediments were studied.The results showed that Cd releases in overlying water and interstitial water were decreased by 31%~69% and26%~73% with BC and n ZVI/BC treatments compared to the control,respectively.Moreover,the Cd stability in the sediments was strengthened after treatment,and the leaching toxicities in treated groups were reduced by 7% to 29%.This directly proved that the risk of sediment Cd to overlying water and interstitial water was limited after in-situ immobilization,and the ecological risk of sediment Cd was reduced.It can be seen that after a period of remediation time,Cd in sediments may meet the corresponding SQG.The increase of Cd stability was intimately linked to the dosage of BC or n ZVI/BC,showing a dose-dependent manner.Notably,at different p H values,n ZVI/BC showed a higher inhibitory efficiency than BC on the release of Cd from the sediment.Under alkaline conditions(p H = 9),both BC and n ZVI/BC presented the higher remediation efficiencies compared to those in acidic and neutral conditions.Application of low-dose remediation materials increased the abundance and diversity of bacterial communities.However,high-dose remediation materials posed some adverse effects on bacterial communities because of the changes of sediment properties and toxicites of BC and n ZVI/BC.(4)In-situ capping of Cd highly contaminated sediments by BC loaded with nano-Fe2O3: Due to the high cost of n ZVI composite and the slow effect of immobilization,BC and nano-Fe2O3 modified BC(n Fe2O3@BC)were further performed to cover Cd-contaminated sediments in situ.The performances,applicable conditions,and mechanisms of BC and n Fe2O3@BC capping were investigated.The results showed that both BC and n Fe2O3@BC capping partly inhibited Cd release from sediment to overlying water(reduction rates > 99%)after 60 d capping.This indicated that the risk of sediment Cd to interstitial water and overlying water was reduced after in-situ capping.Under all different p H and interference intensity treatments of the overlying water,the effect of n Fe2O3@BC capping is superior to those of BC capping.Notably,both low p H value and high hydraulic disturbance could lower the effect of BC and n Fe2O3@BC in-situ capping.Given high toxicity of Cd,BC capping was inapplicable in acidic and neutral water bodies(p H = 3,5,and 7),while n Fe2O3@BC capping was available in all p H treatments.Both BC and n Fe2O3 were ineffective under high hydraulic disturbance(stirring speed = 150 rpm),but n Fe2O3@BC still worked in low hydraulic disturbance(stirring speed = 0 and 100 rpm).The analysis of diffusive gradient in thin films showed that BC and n Fe2O3@BC capping inhibited Cd release from sediment to porewater.Notably,most adsorbed Cd in BC(98.74%)and n Fe2O3@BC(98.10%)capping layers was potential to be re-released into water bodies.Therefore,it is necessary to treat the covering layer in time.Additionally,excessive application of n Fe2O3@BC capping could increase the risk of Fe release.