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Water Quality Introduction Chlorate has been used historically as a herbicide to kill all terrestrial plants except mosses. The only other major source of chlorate input to the environment is pulp mill effluent where chlorine dioxide is used for bleaching. Minor amounts of chlorate are also produced when chlorine dioxide is used to disinfect drinking water. Chlorate toxicity is linked to nitrate concentrations and aerobic conditions. The most sensitive species to chlorate are marine brown algae, the kelps, i.e. Macrocystis and Fucus, which are vital components of the coastal ecosystem. Most other species are relatively insensitive. Brown algae are ubiquitous along coastal BC, are dominant components of the ecosystem and are the preferred sites for herring to spawn. The chlorate anion is not directly toxic; the mechanism of chlorate toxicity in plants is indirect. In simple terms an enzyme system in plants, evolved to reduce nitrate, also reduces chlorate to a toxic intermediate product, apparently chlorite or hypochlorite. These reduction products of chlorate inactivate the nitrate reductase system. The nitrate reductase enzyme system in plants is an inducible enzyme system that requires a certain minimum threshold level of nitrate in order to become activated. It then reduces nitrate to nitrite and ultimately to ammonium, the preferred form of nitrogen for use by the plant. If there is sufficient nitrate in the growth medium, the nitrate reductase system is activated and takes up chlorate, reducing it to the toxic chlorite. Generally, in BC, nitrate levels are low in large rivers and coastal marine waters which tend to be nitrogen-limited systems; rarely would there be enough nitrate available to tie up all the active sites in the nitrate reductase enzyme system. Often there will not even be enough nitrate to induce the enzyme system to activity. Chlorate is not very toxic in ammonium-based systems. There is no evidence of chlorate being mutagenic, carcinogenic or teratogenic. Chlorate is readily biodegraded by microorganisms under anaerobic conditions. There is no tendency for chlorate to accumulate or bioconcentrate in organisms and it is not magnified in the food chain. Chlorate
can be removed from bleach plant effluents by reduction with
sulphur dioxide but this is an incomplete process and consumes
large quantities of chemicals. Anaerobic bacteria can remove
chlorate from kraft bleach effluent with less than one-hour
retention time.
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