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In situ measurement of metal-ligand complex dissociation rates in cave dripwaters

The purpose of this study was to develop a new method for determining the trace metal binding properties of natural organic matter (NOM) in cave dripwaters. The methodology is based on the diffusive gradients in the thin-films (DGT) technique, but differs from DGT by using advective gradients along hydrogel to resolve temporal trends in dissociation. For this reason, it is referred to as advective gradients in thin-films (AGT). NOM is a ubiquitous component in natural waters. The complexation capacities of NOM ligands for metal ions in cave waters has a great impact on the transport of trace metals to, and their incorporation in, calcium carbonate cave deposits (speleothems). The relationship between the kinetics of metal-NOM complexes and the concentration of the metal-carbonate solid solution in actively forming cave precipitates, is the major focus in this study. The primary goal of this research is the determination of the mean kinetics of compositionally-unknown metal-NOM complexes in situ with the application of AGT. The first part of this study was to calibrate the self-assembled AGT device containing a resin gel (for the binding of metal ions). A range of drip rates was tested to establish the relationship between drip rates and flow velocity within the gel with the aim of establishing a calibration function between drip discharge and flow rate within the gel. The second part of this study was to validate the method using metal-ligand complexes of known character. Synthetic solutions containing organic ligands at known concentrations were measured. The experimental data showed acceptable reproducibility, and the modelling results agreed well with the literature. After the laboratory-based study of the AGT method, the device was deployed in Aranui Cave, which is a natural cave located in the Waitomo District, New Zealand. Four transition metals were found in both resin gel samples and the water sample. The accumulation behaviour of the metals in the Chelex-100 resin gel demonstrated the capability of the AGT method to measure trace metals bound to unknown NOM ligands in cave dripwaters. The dynamics of the dissociation of metal-NOM complexes can be reflected by the distinct dissociation patterns of the metals. This newly developed method can semi-qualitatively determine the dissociation rates of labile and partially labile metal-NOM complexes in natural waters. However, further optimisation of the method will be required to improve the precision and accuracy of measurements. In summary, this study provides the basic framework for further research of a new palaeoclimate proxy utilising trace meta-content in speleothems.
Type of thesis
Guo, X. (2019). In situ measurement of metal-ligand complex dissociation rates in cave dripwaters (Thesis, Master of Science (MSc)). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/13102
The University of Waikato
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