ENVIRONMENTAL SCIENTIFIC EVALUATIION OF PHOTOCHEMICAL REACTIONS IN NATURAL WATER

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Aquatic photochemical reactions caused by sunlight are important as a natural removal process for dissolved organic matter (DOM) in water. Little is known, however, about photo-degradation mechanism of humic substances (HS), a major constituent of DOM, and its influence on the photo-degradation of organic pollutants. We conducted field observations in Lake Biwa and inflowing rivers and made laboratory experiments to clarify following points;(a)contribution of photochemical reactions to the distribution of HS in Lake Biwa,(b)photo-degradation of nonyl phenols and other pollutants, (c) photo-degradation and distribution of pesticides in the lake, (d) photo-degradation of DOM in effluent from the waste-water treatment plants, and (e) influence of photo-reactions and DOM on the distribution of iron in the lake.Throughout the present study, we obtained following results.(1) Photochemical degradation of HS results in their lower concentrations at the surface water layer in Lake Biwa as compared to deeper layers.(2) The fluorophore of HS is of higher sensitivity to photo-degradation than chromophore, and photo-degradation of carbon bond take places later. These degradation reactions proceed in the first-order reaction kinetics with reaction rate constants ranging from 0.01 to 0.04.(3) Nonyl phenols and phosphate-esters in water are easily degradable on photo-reaction which is influenced by HS.(4) The pesticides being resistant to photo-reaction can survive for long time, and they are thus detected in Lake Biwa water throughout the whole year.(5) HS in the effluent from waste-water treatment plants emits strong fluorescence. They are easily photo-degraded, showing a possibility that photoreactions can be applicable to high-grade treatment of waste water.(6) Heavy metals like iron form complexes with DOM, which can be susceptible to the photochemical alteration.