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

Spectrophotometric acid-base titration is a simple and powerful technique to evaluate the properties of proton binding sites of natural organic 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 specific UV-vis differential absorbance (ΔA_λ,pH) at a given wavelength (λ) and pH of a dissolved NOM sample, the present work proposes a method to investigate any NOM sample. It determines specific features in the absorbance spectra attributed to proton-inert chromophores (A_0,λ) and to the deprotonation processes of carboxylic (A_1,λ) and phenolic groups (A_2,λ). It enables to select sample-specific wavelength (λ_mid), where both these functional groups significantly contribute to the variation of absorbance with pH. The linear regression analysis of A_λmid,pH vs Q for various NOM reference samples evidenced that the sample-specific slope (S_NOM) and intercept (I_NOM) were related to the intrinsic spectroscopic properties of the sample (A_0,λmid, A_1,λmid, and A_2,λmid). 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 ions in natural systems.