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20 mJ pulse"1) and analyzed at 370 nm through a Schott glass filter WG 345. The sample solution was replaced after each shot.Conventional flash photolysis was carried out by means of an Applied Photophysic K-20 apparatus, using solutions ca. 10"5 M of the substrates 1 or 2 in the appropriate solvent in cylindrical tubes, analyzing at 370 nm through the above-mentioned filters. The optical path was 10 cm; the sample solution was replaced after each shot. The kinetic constants for the decay of the transients were obtained manually from oscilloscope traces.

Oxygenated aqueous solutions of alginic acid, a model compound for polyuronic acids contained in surface waters, were photolyzed in UV light ( = 254 nm), treated with ozone, or reacted with radiolytically generated hydroxyl radicals. The average molecular weight decrease upon such treatment was measured by viscosimetry. At a fluence of 250 J m~2, which is generally considered sufficient to disinfect drinking water, 0.0005 strand breaks per macromolecule are effected. Alginic acid is capable of complexing ferric ions. Their presence increases photolytic strand-break formation. At an iron concentration of 10"6 mol dm"3, such as may prevail after flocculation with iron salts, 0.004 strand breaks per macromolecule are detected at the above fluence. Hydroxyl radicals, produced by subjecting the N20/02-saturated aqueous alginic acid solution to ionizing radiation from a 60Co y source, cause strand breakage with an efficiency of 22%, while superoxide radicals are released from the polymer peroxyl radicals with an efficiency of 71%. The efficiency of ozone in producing a strand break is 18%, relative to the total of the ozone consumed. The destruction of the alginic acid by ozone is mainly caused by the intermediate hydroxyl radicals. The polysaccharide peroxyl radicals that are formed by OH attack and subsequent addition of 02 eliminate superoxide radicals, which in turn stimulate further hydroxyl radical production by reacting rapidly with the ozone.

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