UV spectra modelling

The use of UV spectrophotometry for organic pollution characterisation and estimation is obvious because of the presence of a lot of absorbing compounds in water and waste-water. Even if it is quite possible to find some nonabsorbing organic molecules (saturated hydrocarbons, small molecules, sugars, etc.), a UV spectrum of water and wastewater is very rarely flat. 

Starting from the study of several thousands of urban wastewater samples, a first universal basis of reference spectra has been proposed for one application concerning the wastewater quality monitoring [12,13]. The selected reference spectra are displayed in Fig. 10. 

The choice of reference spectra is carried out, on one hand, manually from real (deterministic) spectra, with the consideration of determined spectra (nitrate, nitrite, surfactants). It is completed automatically, on the other hand, with a mathematical procedure [15] allowing the selection of the more relevant spectra for the model, able to explain by a linear combination, the shape of UV spectra of water or wastewater. This last procedure can be replaced by any advanced statistical algorithms (PCA or PLS, for example) or by commercial software (such as UVPro from Secomam). 

This semi-deterministic approach is interesting because the basis of reference spectra used for the modelling of real spectra is made up of spectra mixtures and specific mineral or organic compounds, the optical properties of which often explain a part of the UV spectra. The reference spectra of mixtures are statistically representative of the different heterogeneous fractions of wastewater, because they are selected from wastewater fractionation. For each wastewater sample, several nitrations are carried out (1 and 0.025 ixm), and the spectra of the filtrate are acquired. The differential spectra are then considered for the basis constitution (Table 2). 

This procedure can be generalised for the study of surface or seawater, for example. In these cases, some reference spectra (suspended solids, colloids, surfactants) are replaced by more relevant spectra for the medium, such as humic substances, mineral-suspended solids or chloride) in order to constitute specific but rather universal basis of reference. 

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