Acute Rhizotoxicity Of A Salt Marsh Plant (Scirpus Mariqueter) To Copper,Zinc And Nickel Using A Biotic Ligand Model (BLM)

Recently,with the development of tidal flats and the discharge of large amounts of industrial wastewater and sewage,the environment and the ecosystem of tidal flats acting as pools or sinks of basin pollutants have been facing great pressure.Heavy metals have serious negative impacts on living creatures,estuaries and the marine environment,and human health because of their widespread use and their non-biodegradability.Therefore,it is of vital significance to establish a rational evaluation standard of heavy metal toxicity for tidal flat wetlands.The bioavailability plays a key role in heavy metal risk assessment and environmental quality criteria.The Biotic Ligand Model(BLM)as a mechanistic bioavailability model has extensively been used for assessing the bioavailability of toxic metals on specific site basis in aquatic ecosystems and getting progresses in terrestrial ecosystems.It has not been reported for BLM application with a test plant as an indicator species in estuarine tidal flat sediments.In this thesis,the Yangtze River estuary tidal flat was chosen as the study area and Scirpus mariqueter,a typical salt marshes pioneer keystone species in the tidal flat deposition,was chosen as a test plant species.A series of single-factor,hydroponic root elongation experiments were carried out to investigate the effects of pH,calcium(Ca),magnesium(Mg),sodium(Na)and potassium(K)on toxicities of Copper(Cu),Zinc(Zn)and Nickel(Ni)to S.mariqueter and their quantitative relationships.Cu,Zn and Ni speciation was computed by the Visual MINTEQ.As a result,the bioavailable species of(Cu),(Zn)and(Ni)and their influence factors were estimated and a BLM predicting Cu,Zn and Ni toxicity toward S.mariqueter was developed.The toxic effects of Cu,Zn,and Ni on S.mariqueter and other terrestrial plants were compared on the acute toxic basis.The main results showed as follows:1)The increase of Na+ activities could significantly increase the median effective concentration(EC50[cu2+])of S.mariqueter,while no significant effect of Ca2+,K+,Mg2+ on EC50[Cu2+]was observed.There was no linear relationship between EC50 and H+ activity in the pH range of 5.55-8.22.Under higher pH conditions of 7.03-8.22,Cu2+ and CuCO3(aq),as toxic species,were able to compete binding sites of the biotic ligands(BL)with Na+.Under low pH conditions of 5.55-6.62,the increase of H+ did not significantly influence EC50[Cu2+].According to the BLM concept,the conditional binding constants for Cu2+,CuCO3(aq),and Na+ to biotic ligands were calculated.The values of logKCuBL,logKCuCO3BL and logKNaBL were 6.60,6.20 and 1.778 respectively and the fraction of biotic ligand sites occupied by Cu(fCuBL50%)was 0.492.2)The increase of Ca2+,Mg2+ and K+ activities could significantly increase the median effective concentration(EC50[Zn2+])of S.mariqueter,while no significant effect of Na+ and H+ on EC50[Zn2+]was observed.There was no linear relationship between EC50 and H+ activity in the pH range of 4.4-7.5.According to the BLM concept,Mg2+,K+ and Ca2+ were able to compete binding sites of the biotic ligands(BL)with Zn2+.The conditional binding constants for Mg2+,K+,Ca2+ and Zn2+ to biotic ligands were obtained.The calculated log KMgBL,log KKBL,log KCaBL and logKZnBL were 2.36,2.68,2.55 and 3.96 respectively and fZnBL50%was 0.378.3)The increase of Mg2+ activities could significantly increase the median effective concentration(EC50[Ni2+])of S.mariqueter,while no significant effects of Ca2+,Na+ and K+ on EC50[Ni2+]were observed.There was no linear relationship between EC50 and H+ activity in the pH range of 4.52-8.00.According to the BLM concept,Mg2+ was able to compete binding sites of the biotic ligands(BL)with Ni2+.The conditional binding constants for Mg2+ and Ni2+ to biotic ligands were obtained.The calculated log KMgBL and logKNiBL were 2.25 and 4.56 respectively and fNiBL50%was 0.48.4)Using the estimated parameters,the Cu-BLM,Zn-BLM,Ni-BLM equations,predicting Cu,Zn and Ni toxicity for S.mariqueter of tidal flats sediments,were successfully developed.5)The comparison between the toxicity threshold(EC50)of S.mariqueter and the binding constants(logKMeBL)of the acute toxicity showed the toxicity of Cu,Zn and Ni to S.mariqueter was in the order of Cu>Ni>Zn.The results indicated that f50%was positively correlated with metal toxicity,and Cu remained the greatest toxic among the three metals.The study successfully developed Cu-BLM,Zn-BLM,and Ni-BLM and revealed the pollution characteristics of heavy metals in tidal flat sediments.It provided a scientific method and basic data to estimate the toxicity and bioavailability of heavy metals on S.mariqueter.Also,it provided the basis for the establishment of environmental criteria for offshore and coastal zones.Therefore,the study method and developed BLM with a plant as an indicator can be potentially widely used in toxic risk assessment of other salt marsh plants.Therefore,this study has important significance for estuarine pollution management and ecosystem restoration.