Natural Organic Matter Characterization and Lead Speciation as Indicators of Potential Lead Bioavailability in Aquatic Environments
Master of Science (MSc)
Faculty of Science
Trace metals and natural organic matter (NOM) are ubiquitous in aquatic environments. Some trace metals are essential for aquatic life, while other non-essential metals (like lead) can be toxic if present in great enough concentrations. Natural waters contain a combination of inorganic, and organic ligands capable of binding metals. While the chemistry of inorganic Pb species is well understood, and National Institute of Standards and Technology (NIST) certified K values are readily available, the interactions between metals and organic matter is not so clearly defined. It is important to understand how NOM interacts with metals in the environment, as the properties of NOM vary with source. If these differences in NOM chemistry induce source-dependent Pb binding, the industrial and environmental implications would be significant. This study aimed to characterize a variety of NOM sources of diverse origin, measure Pb speciation in order to determine if Pb-NOM binding is indeed source-dependent, and to determine which property/ies would best explain Pb-NOM binding. NOM sources were characterized using: total organic carbon/dissolved organic carbon (TOC/DOC), fluorescence excitation-emission matrices (FEEM), parallel factor analysis (PARAFAC), fluorescence index (FI), specific absorption coefficient (SAC340), chromium-reducible sulfide (CRS), thiol, dissolved organic nitrogen (DON), and proton binding index (PBI). These methods allowed for a quantification of organic carbon; humic acid-, fulvic acid-, tyrosine-, and tryptophan-like components; origin; aromaticity; sulfide ligands; nitrogen ligands; and oxygen ligands. SAC340, FI, %HA, %FA, %Trp, %Tyr, CRS, thiol, DON, and PBI values ranged from: 7.76 – 40.84, 1.04 – 1.84, 46.41 – 82.41%, 13.32 – 39.21%, 1.02 – 16.21%, 1.34 – 14.99%, 2.03 – 89.0 nmol/mgC, 71.8 – 186.5 nmol/mgC, 35.76 – 253.8 μgN/mgC, and 0.33 – 1.72 respectively. No one parameter, or simple series of parameters was able to discriminate NOM source. However, CRS, Trp, and Tyr may be able to discriminate saltwater from freshwater sources, while SAC340, CRS, thiol, DON, Trp, Tyr, and PBI may be able to discriminate between freshwater sources. Sources of terrestrial origin had significant SAC340 and PBI, while sources of microbial origin had significant CRS, DON, Trp, and Tyr.
Free lead was then measured using flow-through titrations with a commercially available Pb ion-selective electrode (ISE) and an internal calibration method. To confirm that this method was applicable for trace-level analysis in NOM, ethylenediamine (EN) was used as a model ligand in both artificial freshwater (AFW) and artificial seawater (ASW), and the speciation modelled using certified logK values from NIST. In both AFW and ASW, the ISE accurately and reproducibly (within a factor of two) measured Pb2+ speciation with EN as a model ligand. However, when this method was applied to speciation measurement in NOM, measured values did not agree well with WHAM. This indicates that the fundamental assumption (that Pb-NOM binding will not occur at low pH) made by the internal calibration method is not effective at predicting speciation in NOM, as WHAM predicts that Pb-NOM binding will occur at low pH. An alternate calibration method was tested – forcing measured values to agree with WHAM at low pH – and gave much better agreement. However, speciation measurements in NOM demonstrated reproducible variation with source(indicating source-dependent Pb-NOM binding) which was not described by WHAM within a factor of two, regardless of the calibration method. DON and SAC340 for the titrated sources are significantly different, and may potentially explain differences in source-dependent Pb speciation in freshwater environments. This is of immense industrial and environmental importance, as current water quality guidelines (WQG) do not account for NOM, DON, or SAC340. Consequently, current guidelines could overestimate toxicity in highly aromatic sources, or underestimate toxicity in sources with high DON.
Cimprich, Giselle, "Natural Organic Matter Characterization and Lead Speciation as Indicators of Potential Lead Bioavailability in Aquatic Environments" (2017). Theses and Dissertations (Comprehensive). 1937.