Lead toxicity to Lemna minor predicted using a metal speciation chemistry approach.

In the present study, predictive measures for Pb toxicity and Lemna minor were developed from bioassays with 7 surface waters having varied chemistries (0.5-12.5 mg/L dissolved organic carbon, pH of 5.4-8.3, and water hardness of 8-266 mg/L CaCO3 ). As expected based on water quality, 10%, 20%, and 50% inhibitory concentration (IC10, IC20, and IC50, respectively) values expressed as percent net root elongation (%NRE) varied widely (e.g., IC20s ranging from 306 nM to >6920 nM total dissolved Pb), with unbounded values limited by Pb solubility. In considering chemical speciation, %NRE variability was better explained when both Pb hydroxides and the free lead ion were defined as bioavailable (i.e., f{OH} ) and colloidal Fe(III)(OH)3 precipitates were permitted to form and sorb metals (using FeOx as the binding phase). Although cause and effect could not be established because of covariance with alkalinity (p = 0.08), water hardness correlated strongly (r(2) = 0.998, p < 0.0001) with the concentration of total Pb in true solution ([Pb]T_True solution ). Using these correlations as the basis for predictions (i.e., [Pb]T_True solution vs water hardness and %NRE vs f{OH} ), IC20 and IC50 values produced were within a factor of 2.9 times and 2.2 times those measured, respectively. The results provide much needed effect data for L. minor and highlight the importance of chemical speciation in Pb-based risk assessments for aquatic macrophytes.

Development of the terrestrial biotic ligand model for predicting nickel toxicity to barley (Hordeum vulgare): ion effects at low pH.

The focus of the present study was to investigate the potential for Al3+, Mg2+, and H+ to influence Ni2+ toxicity for barley seedlings grown in acidic aqueous solutions and to assess the capacity of a two-site terrestrial biotic ligand model (tBLM) to accurately predict 50% effect activities (EA50s). To accomplish these objectives, 48-h EA50Ni2+ values were obtained for three sets of exposures in which the pH and activity of Al3+ and Mg2+ were varied. Exposures contained both Al alone and in combination with Mg so that compound ion effects could be investigated. A tBLM was then constructed to predict EA50Ni2+ values from the exposure solution chemistry. The results show a slight protective effect of H+ against Ni2+ toxicity and a strong protective effect of Mg2+, as indicated by a 4.6- and 8.0-fold increase in the measured EA50Ni2+ values corresponding to changes in pH from 6.0 to 4.5 and {Mg2+} from 0 to 1.40 mM, respectively. Increasing solution {Al3+} from 0 to 0.5 microM had no effect on Ni2+ toxicity, although Al itself negatively affected root elongation. Comparison of EA50 values calculated as both Ni2+ and measured concentration of total Ni in the root ([Root-Ni]T) showed [Root-Ni]T to be a more normalized measure of Ni bioavailability. The strong correlation between root growth inhibition and tBLM-predicted root-Ni accumulation suggests that toxicity was influenced by Ni2+ binding to low-affinity ligands within the cell wall, in addition to Ni2+ uptake through Mg2+ transporters. Predicted EA50Ni2+ values generated with the model were all within a factor of +/-1.5 from measured values--a result that emphasizes the advantage of using the tBLM for risk assessment.