Environ. Sci. Technol. 1990, 24, 379-383
Degradation of the Polysaccharide Alginic Acid: A Comparison of the Effects of UV Light and Ozone M. Shahld Akhlaq, Helnr-Peter Schuchmann, and Clemens von Sonntag”
Max-Planck-Institut fur Strahlenchemie, Stiftstrasse 34-36, D-4330 Mulheim a d . Ruhr, West Germany Oxygenated aqueous solutions of alginic acid, a model compound for polyuronic acids contained in surface waters, were photolyzed in UV light (A = 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 lo4 mol dm-3, such as may prevail after flocculation with iron salts, 0.004 strand breaks per macromolecule are detected a t the above fluence. Hydroxyl radicals, produced by subjecting the N20/02-saturatedaqueous alginic acid solution to ionizing radiation from a 6oCoy 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 l a % , 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 O2 eliminate superoxide radicals, which in turn stimulate further hydroxyl radical production by reacting rapidly with the ozone. Introduction
With regard to the procedures employed in the disinfection of drinking water, there is a growing reluctance to continue the use of chlorine and chlorine dioxide. Ozone is often favored as an alternative. In Europe, several large cities such as Amsterdam, Munich, and West Berlin (the latter with a water-main network of 4600 km) have stopped relying on any residual disinfectant in their distribution lines. The prerequisites for this, of course, are high-quality water and well-maintained distribution lines. As the examples show, this can be achieved, and increasingly the question is raised whether it is possible to avoid the use of chemicals altogether and instead disinfect drinking water by means of UV irradiation. This has indeed proven to be a very suitable technique. Large-scale installations were in use already a t the turn of this century (cf. ref 1). At present this technique is mainly applied where the raw water carries only a small load of organic matter. It would be a breakthrough if one were to show that this method is also feasible for water from lakes and other sources that contain larger amounts of organic matter. A major component of the organic matter in lake water is the polyuronic acids, which originate from the debris of algae (2). These compounds appear to withstand attack by bacteria and other microorganisms reasonably well, but upon ozone treatment, their degradation products favor bacterial regrowth ( 3 ) . This is undesirable where drinking water is concerned. Indications are that these products also impair flocculation ( 4 ) . In the present work, we have set out to study the degradation of the polysaccharide alginic acid as a reasonably 0013-936X/90/0924-0379$02.50/0
well defined model compound of the more complex mixture of polyuronic acids found in lake water. Alginic acids are polymers that mainly consist of 1,4-linked P-D-mannuronic acid (A), with varying amounts of 1,4-linked a - ~ guluronic acid (B) ( 5 ) .
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It is of interest to compare, with respect to strand-break formation in alginic acid, the effect of UV radiation (254 nm) with that of ozone where hydroxyl radicals are generally thought to play a major role (6-8). For this reason, the OH-radical-induced reduction of the molecular weight of the alginic acids was investigated separately, applying the radiation-chemical approach, which is optimally suited to this purpose. It will be shown that UV light at a fluence of 250 J m-2, which is considered to be sufficient to disinfect drinking water (cf. references given in ref (9), barely affects the alginic acids, while typical ozone treatment (a few milligrams per cubic meter) leads to their extensive degradation. Experimental Section
Two different kinds of alginic acid were obtained from Sigma. According to our measurements (see below), the “high-viscosity”alginic acid had a molecular weight of 250 kdalton, and the “low-viscosity”one of 100 kdalton. For the most part, the experiments were carried out with the “high-viscosity” material. A typical concentration was 5 X mol dm-3 in terms of monomer units. The average molecular weight was determined by viscosimetry using a Ubbelohde capillary viscosimeter (Schott). The Hagenbach-Couette correction (10) was applied and the influence of the flow gradient was neglected. For the molecular weight determinations, after UV light, y radiation, or ozone treatment the solutions were made 5 X mol dm-3 in NaC1, so that the ionic strength was essentially the same in all of the samples. For the UV photolysis, a low-pressure mercury lamp (Hanau, Sterisol NN 30/89) was used. The quartz cell, which contained the air-saturated solutions to be irradiated, was placed at a distance of 5.5 cm. The fluence rate at 254 nm was 21.7 W m-2 as measured by the ferrioxalate actinometer (11). The contribution of the longer wavelength light to the total absorbed by the actinometer was determined a t some 10% of the 254-nm light, by inter-
0 1990 American Chemical Society
Environ. Sci. Technol., Vol. 24, No. 3, 1990 379
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