Evaluation of transparent low-critical-surface-tension coatings for potential easy-release of fouling from quartz surfaces of germicidal ultraviolet water purifiers
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During water treatment by transmitted germicidal UltraViolet (UV) light, quartz protective sleeves around illuminating UV lamps become encrusted with opaque inorganic deposits requiring removal by acidic chemicals and mechanical wiping to restore satisfactory UV transmittance. There have been successful prior modifications of quartz and other fouling-prone surfaces with Critical Surface Tension (CST)-controlled coatings that allow easier release of organic fouling components, mostly of biological origin, but these coatings have not been previously tested in UV-irradiated systems where oxidative degradation of organic matter occurs. Recent success with easier removal of charred deposits from electrosurgical cutting/coagulation blades bearing CST-controlled coatings encouraged this evaluation of similar coatings in the harsher, longer term operating environment of UV water purifier units. It was shown that initial fouling in absence of UV was dominated by glycoproteins. This was demonstrated in analytical flow cells containing uncoated and coated quartz and surrogate substrata subjected to 1-Pascal shear stress flows of water equilibrated with Lake Erie sediments, zebra mussel shells, rusting debris, and live snails in a 50-gallon laboratory water tank. Employing unique "window" flow cells that allow UV irradiation to pass through uncoated and coated quartz plates to parallel opaque germanium internal reflection prisms during water recirculation, it was demonstrated that UV-induced degradation of the initially deposited organic films was already substantial within 48 hours of continuous flow and UV illumination. Seven different CST-control coatings were examined. Primary CST-control coatings selected for extensive evaluation were ultrathin (less than 10 nanometers) covalently bound transparent layers of octadecylsilane (ODS) based on their display of a CST of 22 mN/m, at the optimum "easy-release" value for biological fouling, and ultrathin, covalently bound, transparent layers of (3-Hepta-fluoroisopropyl)-propoxy-methyl-dichlorosilane (3-Hept), based on their successful current use as coatings to maintain the transparency of quartz periscope windows on submarines. UV-transparent films, less than 200 micrometers thick, of a 22 mN/m CST commercial polymethylsilicone-based material were tested based on their successful current use in the electrosurgical blade char-shedding application. Additional coatings tested included commercial formulations developed for glass-cladding applications in biological research laboratories, and one of these coatings supplemented with photocatalytic titanium dioxide (TiO2) particles meant to assist in the breakdown of any remaining organic deposits at the quartz sleeve/water interface. Durability and efficacy of the coatings were tested by exposing the materials to UVC radiation at 254nm from mercury arc lamps. All the coatings were examined for initial and incremental changes in surface properties, UV transparency, and retention on quartz plates exposed to increasing times of UV irradiation at approximately 20mW/cm2 when dry or in flowing water manifolds in the laboratory. The coatings were also tested with 20 pairs of supplied quartz sleeves, one of each pair uncoated and exposed as-received and the other coated with a test material, and placed into an operating pilot UV-purifier unit for side-by-side evaluation in typically week-long once-through flows of Black Rock Channel water at the Great Lakes Laboratory, at the junction of Lake Erie and the Niagara River at Buffalo, NY. Quartz sleeve transparencies to UV emitted by the specific lamps they surrounded were monitored, and the sleeve surface properties and deposit retention strengths and chemistries were measured upon their return to the laboratory. It was found that the CST coatings were degraded by UV exposure. The ODS coatings were degraded by UV-induced scission of the bonds between their 18-carbon chains and silicon-oxygen anchoring groups, sustaining less than 2000 J/cm2 radiation doses in the presence of flowing water. The 3-Hept coatings were more durable, persisting through nearly 5000 J/cm2 irradiations before being degraded at their Si-O bonded interfaces. The methyl-silicone-based thicker coatings maintained their original CST values of 22 mN/m for greater than 10,000 J/cm2, eventually showing loss of UV transparency and embrittlement with increasing dosage of germicidal UV irradiation. The fine particle TiO2-modified coatings generally were too opaque to evaluate further, although they showed reduced fouling in some cases. Inorganic deposits of carbonates and silicates dominated in all cases, with no significant differences in susceptibility to mechanical removal.