Study On The Application Of Biological Toxicity In Evaluation Of Urban Black And Odorous Water

The black and odorous water body is an extreme phenomenon in which a large amount of organic and inorganic pollutants are discharged to the water body.Among them,lots of unfully decomposed pollutants are settled and accumulated at the bottom of the water body,resulting in high concentration and complicated types of pollutants in the sediment.When the water body is disturbed,the release of the pollutants in the sediment to the overlying water makes the sediment become a secondary pollution source in the water environment.The quality evaluation of sediments at present is mainly based on chemical analysis,which is complicated and costly.Moreover,chemical analysis cannot directly reflect the impact of sediments on aquatic organisms,and lacks ecological relevance.Whole Sediment Toxicity（WST）describes the aggregate toxic effect of a sediment sample as measured by an organism’s response upon exposure to the sample（e.g.,lethality,impaired growth,or reproduction）.WST tests replicate the total effect of environmental exposure of aquatic life to toxic pollutants in a sediment without requiring the identification of the specific pollutants.Using the WST test can identify the biological toxicity of the sediment before treatment)and evaluate the ecological safety of the sediment after treatment,in order to achieve a full-cycle biological toxicity monitoring of the black and odorous water sediment.The test organisms in the WST test are mainly concentrated on benthic organisms with a long test period and a heavy workload.The standard aquatic indicator organisms with high fertility,high sensitivity,easy laboratory cultivation,and clear toxicity mechanism（Such as algae and Daphnia）cannot be used for sediment toxicity testing because they cannot be directly exposed to the whole sediment.In this study,a sediment immobilization technique was developed,which used sodium alginate mixed with freeze-dried sediments and hardened by Ca Cl₂ solution to obtain immobilized sediments.By using immobilized sediments,for the first time,the standard aquatic organisms,Chlorella vulgaris and Daphnia magna,were directly used to determine WST.The optimization of immobilized sediment was based on its stability,effect to algal growth and the diffusion ability of two model compounds of copper and diuron compared to free sediment,which sediment had been previously spiked by these compounds,individually.The immobilized sediment developed in this study has the following advantages:（i）does not affect the growth of C.vulgaris and biological safe to D.magna;（ii）excellent diffusion capacity and structure stability;These proved that the WST test using immobilized sediment is a reliable,accurate and efficient method.On this basis,using the immobilized sediments and their corresponding detoxified sediments（control）,the comprehensive toxicity changes and ecological safety of sediments in the black and odorous river（before and after treatment）were successfully evaluated.The results of this study are as follows:（1）The optimized sediment immobilization conditions are sediments mixed with 2%,3%,and4%sodium alginate and hardened in 4%Ca Cl₂ solution（indirect）or hardened in 2%Ca Cl₂-BG11medium.These immobilized sediments maintained an integral shape and formed no flocs in the solution during the experiment.The interference of the sediment particle in distinguishing and counting the algae cells was eliminated.Furthermore,C.vulgaris growth rate cultured with immobilized sediment met the 16-fold increase criteria after 72-h,which required by the quality controlof the algal toxicity test.In the D.magna-WST test,the detoxed immobilized sediments caused less than 10%mortality rate to D.magna,which fulfilled the requirement of D.magna acute toxicity test.（2）Immobilized sediment have efficient diffusion capabilities.Based on the desorption test of from sediment of spiked Cu and diuron,the immobilization of sediment does not obstruct the release of chemicals in the sediment,to some extent,the immobilization accelerates the releasing of the chemicals in the sediment to the overlying water.The sediment prepared by mixing with 3%alginate and hardening in 4%Ca Cl₂ solution was optimal to perform the toxicity bioassay due to its similar chemical diffusion ability to free sediment.（3）Algae bioassay using immobilized sediment could fully express the system response to toxic effects,compared to the use of immobilized algae（optimal sensitivity）.The EC₅₀value in the Cu-spiked sediment determined by immobilized sediment method was 506.23 mg/kg,which was significantly lower than that using the immobilized algae,which was 719.62 mg/kg.By using the immobilized sediment,the EC₅₀ value of diuron spiked sediment was detected as 2.37 mg/kg,which was also lower than that of using the immobilized algae,which was 3.12 mg/kg.（4）The immobilized sediment method has also achieved a more sensitive toxicity response in the D.magna–WST test than that using elutriate,which proves its practical application value.The LC₅₀value in the Cu-spiked sediment determined by immobilized sediment method was 826.20mg/kg,which was significantly lower than that using the elutriate phase,which was 3612.23 mg/kg.By using the immobilized sediment,the LC₅₀ value of diuron spiked sediment was detected as 10.06mg/kg,which was also lower than that of using the elutriate phase,which was 13.27 mg/kg.（5）The comprehensive sediment toxicity changes of a black and odorous river（before and after remediation）was successfully evaluated using the immobilized sediment and detoxed field sediment（control）.The toxic grades of sediments A1,B1,C1,D1（before remediation）to C.vulgaris are“T”（toxic）,“NT”（not toxic）,“NT”,and“ST”（slightly toxic）;and to D.magna are all“T”.After the river was remediated,the toxicity grades of A2,B2,C2 and D2 sediments to C.vulgaris are“NT”,“NT”,“NT”and“ST”;to D.magna are“ST”,“ST”,“NT”and“ST”.By the TIE（Toxicity Identification and Evaluation）manipulation,suspect toxicants in the sediment were also identified as organic pollutants.With a multi-nutrient-level bio toxicity experiment,a more comprehensive assessment of the toxic effects of sediments is achieved in this study.