Directly Predicting Water Quality Criteria from Physicochemical Properties of Transition Metals

Abstract

Transition metals are a group of elements widespread in aquatic environments that can be hazardous when concentrations exceeding threshold values. Due to insufficient data, criteria maximum concentrations (CMCs) of only seven transition metals for protecting aquatic life have been recommended by the USEPA. Hence, it is deemed necessary to develop empirical models for predicting the threshold values of water quality criteria (WQC) for other transition metals for which insufficient information on toxic potency is available. The present study established quantitative relationships between recommended CMCs and physicochemical parameters of seven transition metals, then used the developed relationships to predict CMCs for other transition metals. Seven of 26 physicochemical parameters examined were significantly correlated with the recommended CMCs. Based on this, five of the seven parameters were selected to construct a linear free energy model for predicting CMCs. The most relevant parameters were identified through principle component analysis, and the one with the best correlation with the recommended CMCs was a combination of covalent radius, ionic radius and electron density. Predicted values were largely consistent with their toxic potency values. The present study provides an alternative approach to develop screening threshold level for metals which have insufficient information to use traditional methods.

Figure 1. Predictive model for Criteria Maximum Concentrations (CMCs) on a natural logarithmic scale and integrated radius (X₁) at 95% centile.

Data points of CMCs predicted from integrated radius (IR) are plotted as , and the data points for USEPA-recommended CMCs are plotted as . The purple, dashed line illustrated the 95% confidence interval.

Figure 2. Predicted Criteria Maximum Concentrations (CMCs).

(A) Periodic Table of CMCs for transition metals, showing CMCs recommended by US EPA and predicted by the integrated radius-PPCR (Physicochemical Properties-CMCs Relationships) model. (B) The predicted CMCs of the lanthanides. (C) The predicted CMCs of the actinides. (D) Comparison among the predicted CMCs in the forth (blue), fifth (red) and six period (green). The x axis of this graph is the group from IIIB to IIB, the y axis of the graph is the concentrations of the predicted CMCs, and the z axis is the periods.

Figure 3

Comparison among Criteria Maximum Concentrations (CMCs) predicted by the model based on integrated radius (IR) (), median lethal concentration (LC₅₀) for the fresh water amphipod (Hyalella azteca, Crustacea) in Lake Ontario (Burlington city tap, Canada) in soft water (nominal) () and soft water (measured) (), for seven transition metals in the fourth period (A), five transition metals in the fifth period (B), five transition metals in the sixth period (D), 14 lanthanide series metals (C) and two actinide series metals (E).