POLYCHROMATIC ACTINOMETRY WITH FILTER SOLUTIONS

In the domain of photochemical processes for water treatment much of t he technology has been developed using u.v.-radiation in order to degr ade the microbiological and chemical contaminants in drinking water (L egrini et al., 1993). With regard to the removal of microbiological po llutants, the u.v.-disinfection of water is discussed as a promising a lternative to the use of chemical disinfectants (Von Sonntag and Schuc hmann, 1992). In addition, a number of papers have reported on advance d techniques for the oxidation of organic pollutants by the combinatio n of u.v.-light and oxidants such as O-3 and/or H2O2 (Von Sonntag et a l., 1993; Paillard et al., 1992; Hessler et al., 1993). These advanced oxidation processes (AOP) are particularly oriented towards commercia l application and have stimulated interest in the design of efficient reactors and construction of new light sources. Thus, the formulation of the photokinetic rates of a photochemical process is one of the mos t difficult parts of reactor design and involves determination of the absorbed light intensity causing chemical conversion, e.g. the generat ion of highly reactive hydroxyl radicals. Actinometric measurements ma y often cause problems due to the polychromatic emission of medium-pre ssure Hg-arcs, which are used in oxidative degradation procedures.Refe rring to polychromatic actinometry, this paper deals with the practica l use of filter solutions as an appropriate method for approaching mon ochromatic conditions with polychromatic radiation. The experiments we re carried out with the annular photoreactor of a pilot plant for drin king water treatment (Fig. 1). For evaluation of the u.v.-irradiance o f the medium-pressure Hg-arc (Fig. 2) in the 240-470 nm range, the azo benzene photoisomerization was used as a chemical actinometer (Gauglit z and Hubig, 1985; Gauglitz and Hubig, 1981; Kuhn et al., 1989) and fi lter solutions were taken to isolate certain parts of the radiation sp ectrum. The general problem of preparing solution filters is essential ly an empirical one of combining inorganic salts and organic, mainly a romatic, compounds in appropriate solvents like water. The selection r equirements for choosing a proper solution filter are as follows: spec ific transmittance to obtain wavelength bands or strong emission lines , which are within the range of the absorption of pollutants; removal of unwanted spectral bands for wavelength selective actinometry; photo chemical stability and no dark reactions. With respect to the determin ation of light fluxes over the entire irradiated volume, which could b e carried out inside the irradiation unit as internal actinometry, exp eriments with liquid filters are not practicable because filter and ac tinometric solutions must be separated (Table 2). Thus, only external actinometry was possible and cuvettes containing a filter and the acti nometer solution were placed outside the vessel (Fig. 3). As a consequ ence, the reactor geometry must be taken into account by theoretical e quations concerning the radiation field (Alfano et al., 1986). The ban d pass filters, which were used for actinometric measurements, are com posed of various filter components (Table 1) and gave a light distribu tion enriched with narrow bands of radiation [Fig. 4(a) and (b)]. The spectral overlapping of distinct absorption bands of the azobenzene ac tinometer (Fig. 5) with non-attenuated wavelengths of the are-spectrum [Fig. 6(a)(c)] is required for obtaining selectivity in actinometric measurements. However, the calculation of intensities by photokinetic rate equations is limited to monochromatic light and influences of sli ghtly polychromatic irradiation on fundamental parameters, such as qua ntum yields, still exist. These difficulties could be overcome if the photokinetic quantities, particularly the molar absorptivities of tran s- and cis-azobenzene and thus the pseudo quantum yield, were by weigh ted averages. Irradiation intensities in the range of 290-360, 335-400 and 375-470 nm could be measured with the filters I-III by assuming t hat the absorption of the actinometer solutions is negligible. Measure ment of the light intensity in the u.v.-C region could be achieved by using the filter solutions IV and V with different transmission charac teristics (Fig. 7). In the case when total absorption of the actinomet er is assumed, a linear relationship between Delta E and the irradiati on time is obtained for each filter respectively. The intensity of the irradiation source can finally be calculated by the difference of the slopes. In general, radiation powers of light sources measured by ext ernal actinometry must be related to the total intensity at the outsid e wall of the inner quartz tube of the u.v.-arc Experimental quantitie s, which decrease the u.v.-radiation in the actinometric solution like the transmittance of the filter solutions and the cooling water must therefore be considered.