Light, in natural waters

S. Markager, W.F. Vincent (2000). Spectral light attenuation and the absorption of UV and blue light in natural waters. Limnol. Oceanogr., 45, 642-650. 

Mill et al. (1980) were the first to attempt an estimation of the concentrations of peroxy radicals in natural waters. Their technique was based on an analysis of the reaction products observed when cumene and pyridine were exposed to light in natural waters, together with the known kinetics and products (Figure 4.9) of ROO and HO- with these substrates. The calculated steady-state concentration for ROO-in a typical sunlit freshwater was about 10 M. Faust and Hoigne (1987) also proposed on kinetic grounds that transient peroxy radicals were formed when natural waters were irradiated with sunlight. They proposed that reactions with such species were the dominant sink for alkylated phenolic compounds in illuminated water bodies, and that the contributions of HO-, O2, and other potential oxidizing species were minimal. 

The procedure of simultaneous extracting-spectrophotometric determination of nitrophenols in wastewater is proposed on the example of the analysis of mixtures of mono-, di-, and trinitrophenols. The procedure consists of extraction concentrating in an acid medium, and sequential back-extractions under various pH. Such procedures give possibility for isolation o-, m-, p-nitrophenols, a-, P-, y-dinitrophenols and trinitrophenol in separate groups. Simultaneous determination is carried out by summary light-absorption of nitrophenol-ions. The error of determination concentrations on maximum contaminant level in natural waters doesn t exceed 10%. The peculiarities of application of the sequential extractions under fixed pH were studied on the example of mixture of simplest phenols (phenol, o-, m-, />-cresols). The procedure of their determination is based on the extraction to carbon tetrachloride, subsequent back-extraction and spectrophotometric measurement of interaction products with diazo-p-nitroaniline. 

Of major interest in geochemistry and in natural water systems are semiconducting minerals for which the absorption of light occurs in the near UV or visible spectral region and as a result of which redox processes at the mineral-water interface are induced or enhanced. Table 10.1 gives band gap energies of a variety of semiconductors. 

When an aqueous solution of aldrin (0.07 M) in natural water samples collected from California and Hawaii were irradiated (7, <220 nm) for 36 h, 25% was photooxidized to dieldrin. By comparison, no loss was reported when aldrin in deionized water was subject to UV light for 10 h. 

Photolytic. When an aqueous solution of p,//-DDE (0.004 pM) in natural water samples from California and Hawaii were irradiated (maximum X = 240 nm) for 120 h, 62% was photooxidized to jD,p -dichlorobenzophenone (Ross and Crosby, 1985). In an air-saturated distilled water medium irradiated with monochromic light (>. = 313 nm), p,//-DDE degraded to jo,//-dichloro-benzophenone, l,l-bis(p-chlorophenyl)-2-chloroethylene (DDMU), and l-(4-chlorophenyl)-l-(2,4-dichlorophenyl)-2-chloroethylene (o-chloro DDMU). Identical photoproducts were also observed using tap water containing Mississippi River sediments (Miller and Zepp, 1979). The photolysis half-life under sunlight irradiation was reported to be 1.5 d (Mansour et al., 1989). 

Light Absorption by Organic Chemicals in Natural Waters... 

Light and Light Attenuation in Natural Water Bodies... 

Let us use a practical example to get acquainted with some aspects of solar irradiance in natural waters, and to illustrate how to use light data as presented in Tables 15.3 and... 

Here is an extremely sensitive method for measuring nitrite (NOj) down to 1 nM in natural waters. The water sample is treated with sulfanilamide and N-( 1 -naphthylethylenediamine) in acid solution to produce a colored product with a molar absorptivity of 4.5 X ItHM" 1 cm- 1 at 540 nm. The colored solution is pumped into a 4.5-meter-long, coiled Teflon tube whose fluorocarbon wall has a refractive index of 1.29. The aqueous solution inside the tube has a refractive index near 1.33. The colored solution is pumped through the coiled tube. An optical fiber delivers white light into one end of the tube, and an optical fiber at the other end leads to a polychromator and detector. 

Photosynthesis is a major cause of nonequilibrium conditions in natural waters. By trapping light energy and converting it to chemical... 

Nitrate presented in natural waters is known to interact with UV light with the formation of hydroxyl radicals. Sorensen and Frimmel (1997) used the inner filter model to describe the nitrate influence on the degradation rate of the micropollutants. In the absence of H202, the degradation rate of the photolysis is enhanced in the presence of nitrate. It can be assumed that this... 

Natural substances (dissolved organic matter, DOM) also contribute to light absorption, and a, is used to account for light attenuation by DOM in natural water. Total light absorption in a natural water follows the relation in Equation (5) ... 

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