Estimating the Acid-Base Properties and Electrical Charge of Organic Matter Using Spectrophotometry

Spectrophotometric acid-base titration is a simpleand powerfultechnique to evaluate the properties of proton binding sites of naturalorganic matter (NOM) at environmentally relevant concentrations. However,it is challenging to quantify the chemical charges (Q) carried by NOM at these concentrations. Based on a previous work,which relates the variation of Q with the specificUV-vis differential absorbance (& UDelta;A (& lambda;,pH)) at a given wavelength (& lambda;) and pH of a dissolvedNOM sample, the present work proposes a method to investigate anyNOM sample. It determines specific features in the absorbance spectraattributed to proton-inert chromophores (A (0,& lambda;)) and to the deprotonation processes of carboxylic (A (1,& lambda;)) and phenolic groups (A (2,& lambda;)). It enables to select sample-specific wavelength(& lambda;(mid)), where both these functional groups significantlycontribute to the variation of absorbance with pH. The linear regressionanalysis of A (& lambda;mid,pH) vs Q for various NOM reference samples evidenced that the sample-specificslope (S (NOM)) and intercept (I (NOM)) were related to the intrinsic spectroscopic propertiesof the sample (A (0,& lambda;mid), A (1,& lambda;mid), and A (2,& lambda;mid)). This approach can thus be used to approximate the Q values of the NOM samples at environmentally relevant concentrations:a pre-requisite for predicting the fate and behavior of metal ionsin natural systems. Thisarticle provides an empirical approach to approximatingthe charge of natural organic matter at environmentally relevant concentrationsusing a novel deconvolution method for the absorbance spectra.