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Title: Bioavailability-based environmental risk assessment of the impacts from metal toxicants and nutrients in Lake Tai
Author: Zhang, Xiaokai
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2020
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Environmental pollution has increasingly become a global issue in recent years. Heavy metals are the most prevalent pollutants and are persistent environmental contaminants since they cannot be degraded or destroyed. Environmental risk assessment (ERA) will pave the way for streamlined environmental impact assessment and environmental management of heavy metal contamination. Bioavailability is increasingly in use as an indicator of risk (the exposure of pollutants), and for this reason, whole-cell biosensors or bioreporters and speciation modelling have both become of increasing interest to determine the bioavailability of pollutants. While there is a great emphasis on metals as toxicants in the environment, some metals also serve as micronutrients. The same processes that introduce metals as pollutants into the environment also introduce metals that may function, in some cases, as micronutrients, which then have a role to play in eutrophication, i.e. excessive nutrient richness that is an impairment of many freshwater ecosystems and a prominent cause of harmful algal blooms. In this thesis, I cover a wide range of topics. A unifying theme is biological impacts of metals in the environment and what the implications are for environmental risk assessment. This thesis begins with my initial work in which I conducted laboratory experiments using a bioreporter, genetically engineered bacterial that can produce dose-dependent signals in response to target chemicals to test the bioavailability of lead (Pb) in aqueous system containing Pb-complexing ligands. Lead serves as a good model because of its global prevalence and toxicity. The studied ligands include ethylene diamine tetra-acetic acid (EDTA), meso-2,3 dimercaptosuccinic acid (DMSA), leucine (Leu), methionine (Met), cysteine (Cys), glutathione (GSH), and humic acid (HA). The results showed that EDTA, DMSA, Cys, GSH, and HA amendment significantly reduced Pb bioavailability to bioreporter with increasing ligand concentration, whereas Leu and Met had no notable effect on bioavailability at the concentrations tested. Natural water samples from Lake Tai (Taihu) were also been studied which displayed that dissolved organic carbon in Taihu water significantly reduced Pb bioavailability. Meanwhile, the bioreporter results are in accord with the reduction of aqueous Pb2+ that I expected from the relative complexation affinities of the different ligands tested. These findings represented a first step toward using bioreporter technology to streamline an approach to ERA. Dissolved organic matter (DOM) plays an important role in both speciation modelling and bioavailability of heavy metals. Due to the variation of DOM properties in natural aquatic systems, improvements to the exiting standard one size fits-all approach to modelling metal-DOM interactions are needed for ERA. My next effort was to investigate variations in DOM and Pb-DOM binding across the regional expanse of Taihu. Results show that different DOM components are highly variable across different regions of Taihu, and bivariate and multivariate analyses confirm that water quality and DOM characterisation parameters are strongly interrelated. I find that the conditional stability constant of Pb-DOM binding is strongly affected by the water chemical properties and composition of DOM, though is not itself a parameter that differentiates lake water properties in different regions of the lake. The variability of DOM composition and Pb-DOM binding strength across Taihu is consistent with prior findings that a one-size-fits-all approach to metal-DOM binding may lead to inaccuracies in commonly used speciation models, and therefore such generalised approaches need improvement for regional-level ERA in complex watersheds. Based on the findings from the investigation of Pb-DOM complexation, I compared a one-size-fits-all approach to different methods of implementing sitespecific variations in modelling. I was able to substantively improve the procedures to the existing speciation model commonly used in ERA applications. The results showed that the optimised model is much more accurate in agreement with bioreporter-measured bioavailable Pb. This streamlined approach to ERA that I developed has performed well in a first regional-scale freshwater demonstration. There is a close connection between environmental water and sediment contamination, and I also studied Pb bioavailability in lake sediemnt with a focus on the ramifications regarding environmental risk. For this work, I studied sediment samples from Brothers Water lake in the United Kingdom, a much simpler lake system than Taihu that is severly impacted by centuries of Pbmining in the immediate vicinity. The results showed that the total concentration of Pb in the sediment has an inverse relationship with bioavailable Pb in the test samples, has a positive relationship with sediment particle size and sand content and a negative relationship with clay content. I find that the relative amount of bioavailable Pb in the lake sediments are low, although surface sediments may have much higher bioavailable Pb than deeper sediments. To address the issues of metals and other micronutrients on algal growth, I performed small-scale mesocosm nutrient limitation bioassays using boron (B), iron (Fe), cobalt (Co), copper (Cu), molybdenum (Mo), nitrogen (N) and phosphorus (P) on phytoplankton communities sampled from different locations in Taihu to test the relative effects of micronutrients on in situ algal assemblages. I found a number of statistically significant effects for micronutrient stimulation on growth or shift in algal assemblage. The most notable finding concerned copper, which, to my knowledge is unique in the literature. However, I am unable to rule out a homeostatic link between copper and iron. The results from my study concur with a small and emerging body of literature suggesting that the potential role of micronutrientss in harmful algal blooms and eutrophication requires further consideration in ERA and environmental management. The findings from this work are not only of interest to academics, but represent feasible approaches from which environmental practitioners may evaluate risk. My work on Pb needs further validation, however would be validatable through impact assessment studies and is therefore directly and immediately extensible to environmental risk. I am therefore hopeful that my work on ERA will drive tangible outcomes in the work of environmental management. Likewise, though my work on the affect of micronutrients on algal growth is more fundamental than applied at present, there are important and immediate implications for environmental management: at present, copper is used as an algicide. My work suggests the long term effect of copper at 20 µg·L-1 could possibly encourage rather than inhibit harmful algal blooms. It is satisfying to arrive at a scientifically interesting, and at the same time practically useful outcome from my years' of work, however, I hope that this and other similar work on risk and management interventions could inspire a shift to pollution prevention rather than "end of pipe" solutions.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral