Trace metal speciation in natural waters: Computational vs. analytical

Abstract

Improvements in the field sampling, preservation, and determination of trace metals in natural waters have made many analyses more reliable and less affected by contamination. The speciation of trace metals, however, remains controversial. Chemical model speciation calculations do not necessarily agree with voltammetric, ion exchange, potentiometric, or other analytical speciation techniques. When metal-organic complexes are important, model calculations are not usually helpful and on-site analytical separations are essential. Many analytical speciation techniques have serious interferences and only work well for a limited subset of water types and compositions. A combined approach to the evaluation of speciation could greatly reduce these uncertainties. The approach proposed would be to (1) compare and contrast different analytical techniques with each other and with computed speciation, (2) compare computed trace metal speciation with reliable measurements of solubility, potentiometry, and mean activity coefficients, and (3) compare different model calculations with each other for the same set of water analyses, especially where supplementary data on speciation already exist. A comparison and critique of analytical with chemical model speciation for a range of water samples would delineate the useful range and limitations of these different approaches to speciation. Both model calculations and analytical determinations have useful and different constraints on the range of possible speciation such that they can provide much better insight into speciation when used together. Major discrepancies in the thermodynamic databases of speciation models can be evaluated with the aid of analytical speciation, and when the thermodynamic models are highly consistent and reliable, the sources of error in the analytical speciation can be evaluated. Major thermodynamic discrepancies also can be evaluated by simulating solubility and activity coefficient data and testing various chemical models for their range of applicability. Until a comparative approach such as this is taken, trace metal speciation will remain highly uncertain and controversial.