PROGRAM | Civil and Environmental Engineering

By: Yanqing Lin Chair: Herb Allen

ABSTRACT

The purpose of the work is to develop a Terrestrial Biotic Ligand Model (TBLM) to predict Ni and Cu toxicities on barley root elongation (RE) grown in soil system, using controlled experiments in the inert sand culture with hydroponic sand solution. The chemical composition of the hydroponic sand solution can be varied univariately so the toxicity and the competition effect can be studied individually and independently while that of soil cannot. For Ni experiments EC50 (50% effective concentration) of free Ni activity increased as Mg2+ activity increased, whereas the effect of Ca2+ was insignificant. Toxicity from NiHCO3+ at high pH was considered to account for the effect of H+ on EC50. For Cu experiments linear relationships between EC50 of free Cu2+ and H+, Ca2+, and Mg2+ activities were found, meaning that the H+, Ca2+, and Mg2+ cations will alleviate the toxicity of Cu2+ in solutions. TBLM parameters were estimated and hydroponic sand solution culture bioassays were modeled by TBLM with root mean square error (RMSE) % root elongation (%RE) of 9.4% for Ni and 12.8% for Cu toxicity predictions.
Toxicity accompanying increasing concentration of solution ions other than Ni2+ or Cu2+ toxicity was observed and modeled as an osmotic effect and needed to be included for soil culture due to cations released into the soil solution. Including the osmotic effect improved soil culture toxicity prediction. TBLM parameters that were developed from hydroponic sand culture were validated by predicting soil toxicity response with RMSE %RE of 14.1% for Ni, and 13.0% for Cu toxicity bioassays, which are similar to the prediction of RMSE %RE of 10.5% for Ni and 14.0% for Cu using the TBLM parameters that were developed from soil culture itself. EC50 prediction by hydroponic sand solution TBLM parameters are also similar to those for soil parameters, so TBLM parameters for soils toxicity prediction can be derived from hydroponic sand solution culture experiments, and the TBLM parameters allow prediction of effects in systems other than those in which the TBLM constants were developed.
The effect of activity at the root plasma membrane surface was incorporated into the model for predicting the toxicities of Ni and Cu in the hydroponic sand solution and the soil culture. Using the activities at the plasma membrane surface as the toxicity dose did not improve the predictions over using activities in bulk soil solution to determine the metal toxicity, and using the value of f (fraction of biotic ligand sites occupied by the toxic metal) obtained from the TBLM resulted in the better prediction.
TBLM parameters developed from hydroponic water solution culture from previous reported work were also included for comparison for their ability to predict the Ni and Cu toxicity on barley root elongation in soil culture. Predictions of %RE and EC50 for respective Ni and Cu toxicity responses in soil culture result in larger errors with hydroponic water solution TBLM parameters than with hydroponic sand solution culture parameters. Parameters developed from hydroponic water solution systems were unable to accurately predict the Ni and Cu toxicities in soils.
As both the hydroponic sand solution culture and the hydroponic water solution culture are types of hydroponic methods, the same factors should affect both if plant response is dictated by solution chemistry. For the hydroponic water culture method there is no sand or other solid material present but in the hydroponic sand culture there is sand for the plants to grow in. Barley was grown in the 3 media of soil, hydroponic sand solution, and hydroponic water solution cultures at the same environmental conditions at same time. The difference of root morphology for barley grown in different growth media and its relationship with the Ni toxicity prediction were studied. Results showed that barley grown in hydroponic water solution had greater root hair length than those from soil and hydroponic sand solution which makes it more sensitive to the Ni toxicity, whereas root hair length of the barley grown in hydroponic sand solution culture are similar to that in soil culture. By including the effect of unit root surface area, the difference in root elongation and EC50 was diminished, and % root elongations can be improved with an error of approximately 10% for plants grown in different media.

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