Whether metal (Pb, Cd, Zn) binding property of natural organic matter is source-dependent--A study based on spectroscopy, potentiometry, and voltammetry

Weibin Chen, Trent University, Environmental and Life Science Program, Peterborough, ON, Canada, Celine Gueguen, Trent University, Department of Chemistry, Peterborough, ON, Canada and Scott Smith, Wilfrid Laurier University, Department of Chemistry and biochemistry, Waterloo, ON, Canada
Abstract:
Natural organic matter (NOM) is a complex molecule of diverse ligands that essentially regulates metal speciation and toxicity. NOM source is heterogeneous, but can be operationally classified into allochthonous, autochthonous, and intermediate between these two groups. Whether the metal binding property (i.e. affinity and capacity) is NOM-source dependent however, remains unclear. The answer of this question is helpful for environmental modeling. If metal binding properties are sensitive to NOM source, then models used to determine metal speciation will need to be revised to take some measurement of NOM quality into consideration.

In this study, different sources of NOM were collected for the study of Pb, Cd, and Zn binding. The NOM properties were characterized by different techniques from different chemistry indexes, including acidity chemistry, optical spectroscopy for component and structure, and cathodic stripping voltammetry for thiol determination. Absence of Gradient and Nernstian Equilibrium Stripping was used to selectively determine the concentration of free metal ion for binding isotherm. A model based on continuous site distribution was used to describe this binding isotherm. By best fitting the binding isotherm into the model using a regularized optimization, a conditional affinity spectrum was derived.

Comparing binding isotherm and affinity spectrum between NOM samples showed that Pb binding was more dependent on NOM sources than Cd and Zn binding. Under the same binding condition (i.e. DOC, pH and ionic strength), allochthonous NOM mostly showed higher binding level (i.e. up to 3 log unit of bound species concentration) and stronger binding affinity for Pb than autochthonous NOM. This tendency however, was not obvious for Cd and Zn. Humic-like content probably contributed the most significant factor for Pb binding discrimination. Thiol content may be another important factor to differentiate metal binding affinity and capacity between NOM samples.

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