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Synthetic bio-catalytic solutions for mercury pollution
thesisposted on 2022-03-28, 12:58 authored by Damien Nicholas McCarthy
Highly toxic mercury (Hg) causes many adverse biological impacts from exposure to even low concentrations. Mercury is subject to a complex biogeochemical cycle once released into the environment, many aspects of which are driven by living organisms. The extreme toxicity, together with the mobility of Hg once released, has profound impacts on biota, including at least two acute mass poisoning events for human populations in recent history. The transformative nature of the cycle to the chemical and physical characteristics of Hg means that there is no one solution readily amenable to solve the problem. The main inspiration for this work is derived directly from the bacterial world, whom, having evolved many varied ways of dealing with this toxicant, can now be employed both directly and indirectly to assist with reducing the environmental load of Hg. Some bacterial agents have long been known to possess the ability to enzymatically reduce divalent Hg to elemental Hg, whereupon it becomes volatile and passively diffuses as gaseous elemental mercury (GEM). Delivery of bacterial inoculants for remediation purposes to diverse, sometimes remote, and heterogeneous sites has proven logistically very difficult. This work shows that Pseudomonas veronii with this Hg2+ [to] Hg0 capability can be immobilized on a solid substrate via encapsulation in a biopolymer, and stored for extended periods, while retaining Hg transformation activity after international shipment and subsequent exposure to contaminated soils. Ideally one wants to capture those induced emissions. Elemental Hg has proven very difficult to capture in this GEM form, notwithstanding the rather complex and sometimes hazardous solutions previously employed. This body of work extends the existing research into capture of GEM in ambient conditions without relying on complex catalytic and physical separation and capture methods as constitutes the main body of current knowledge in this area. In this work, a modified coir fibre mat was used to show that GEM could be captured in ambient conditions without any prior physico-chemical alteration, by employing a semi-gas permeable silicone based fibre coating, the matrix of which is infused with copper(I) iodide crystals. The coating was applied to coir fibre prefabricated as matting. Upon contact with GEM, stable and insoluble copper(I) tetra iodide mercurate is formed and bound stably to the mat. It is envisioned this may be deployed as a geotextile over large terrestrial sites being remediated, or it could be configured for other GEM capture situations including numerous industrial settings. The final aspect of the work involves development of a biomimetic device for potential remediation of methylmercury in aquatic environments. A synthetic gene was designed, synthesized, and then expressed in a bacterial host, the product of which is a fusion protein consisting of an organo-mercurial lyase and a short tethering polypeptide at the C-terminal with very high affinity for silica. The tethering peptide allows one to directionally immobilize the lyase on a solid silica based substrate, so that the active site is not hampered by steric hindrance or other spatial considerations. The desired product was extracted, purified and tethered via the solid binding peptide (SBP) to synthetic zeolite particles. The enzyme produced is potentially capable of degrading methylmercury while bound. It is hoped that the combination of these three approaches can assist in reducing the environmental burden of mercury, and adds something valuable by extension to the existing body of knowledge in this area. Due to the distributive and transformational effect of the biogeochemical cycle on Hg, there is no single remedial solution that suits all forms and environmental conditions, but these approaches hopefully add low cost and readily employable solutions to a greater number of problematic contaminated sites -- abstract.