Study Of Biotoxicity Models For Predicting Nickel Toxicity To Wheat(Triticum Aestivum):Comparison Of Culture Method And Modeling Approaches

The consumption of nickel(Ni)in manufacture and life of human being often leads to the nickel contamination.Nickel in soil may be accumulated in the body of human being through the food chain.Consequently,the study of nickel toxicity is one of the most important parts of the field of heavy metal contamination.Biotoxicity models are the effective methods to describe the process of heavy metal toxicity to organisms.Free Ion Activity Model(FIAM),Biotic Ligand Model(BLM)and Gouy Chapman Stern Model(GCSM)are three classic biotoxicity models that were developed in recent years and the three models can quantitate the biotoxicity of heavy metals to assist the environmental risk assessment.Due to the complexity of soil component,hydroponic and sand culture experiments are the major methods to study the heavy metal biotoxicity models at the present but the study about the comparison of culture method and modeling approaches in biotoxicity models was inadequate and,at the same time,it is a question that the model parameters getting from hydroponic and sand culture were used into soil culture to predict the biotoxicity.In this study we focus on the nickel toxicity to wheat root elongation and compare the three models(FIAM,BLM and GCSM)in predicting the nickel biotoxicity through the hydroponic and sand culture experiment.Furthermore,soil culture experiment is used to validate the feasibility of using the model parameters getting from hydroponic and sand culture to predict the biotoxicity in soil.The main conclusions are as follows:(1)The results of the hydroponic and sand culture experiment showed that Mg2+had a strong competition with Ni2+ on the biotic ligand of wheat root and Mg2+ could alleviate the nickel biotoxicity.Increasing Mg2+ activities resulted in a significant linear increase in the EC50 with a high coefficient of determination R2 = 0.87(hydroponic culture)and 0.81(sand culture)but other three cations(Ko,Na+ and Ca2+)showed no effect on nickel toxicity.H+ had an influence on the speciation of nickel and exhibited a weak effect on nickel toxicity.When pH ranged from 8 to 9,BLM could successfully predict the nickel toxicity to wheat root elongation with considering the NiHCO3+ toxicity because the species NiHCO3+ could not be ignored if compared with free Ni2+.(2)In hydroponic and sand culture experiment,FIAM could not predict nickel toxicity very well when there were other competitive cations because this model only considered the toxicity of free ion activity.BLM not only focus on the toxicity of the free nickel activity but also the competition on the biotic ligand between Ni2+ and Mg2+ so it behaved much better than FIAM.GCSM underestimated the nickel toxicity when ionic strength in solution was very high while prediction of GCSM became better with the ionic strength decreasing.In general,GCSM could perform little better than FIAM when there were other competitive cations in solution.(3)We compared the toxicity predictions of three mechanistic models(FIAM,BLM and GCSM)based on two culture media to the Ni toxicity in 18 soils representing the major soil types in China.Result showed that model parameters estimated from the sand culture better predicted soil conditions than those from the hydroponic culture,probably because the structure of the sand culture more closely resembled that of soils.The best prediction was obtained with the BLM(RMSE=16.2,R2=0.79)because it considered the competing effect of Mg2+,followed by GCSM(RMSE=17.4,R2=0.70)and FIAM(RMSE=21.9,R2=0.75).(4)Dissolved organic matter(DOC)in soil solution may attenuate nickel toxicity by reducing free nickel activities in soil pore water.But in this study,DOC had a limited effect on the predictions of the three models,presumably due to its relatively low contribution to free nickel activity.The salinity of a soil solution can negatively affect plant root elongation by increasing the water potential.The negative effect of osmolarity was found to wheat root elongation and the dose-response relationship of osmolarity(expressed as the ionic strength,I)was well described by a log-logistic equation(RMSE=8.58,R2=0.90).After including DOC and the osmotic effect,in spite of the widely varying toxicity of nickel for root elongation in 18 soils,the variation of the prediction was substantially reduced.In summary,our study shows that sand culture will best estimate model parameters for use in root elongation bioassays and after including DOC and the osmotic effect BLM can be used to predict nickel toxicity to wheat root in different types of soil.The study results provide direct evidence that the BLM is a promising method for the risk assessment of nickel to wheat in terrestrial systems.