Comparing a Fully Optimized ContinUouS (FOCUS) method with the analytical inversion of Non Ideal Competitive Adsorption (NICA) for determining the conditional affinity spectrum (CAS) of H and Pb binding to natural organic matter

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The Conditional Affinity Spectrum (CAS) is essential for describing the binding site heterogeneity of natural organic matter. A numerical method called ‘Fully Optimized ContinUouS’ (FOCUS) is described and employed to determine the CAS of H and Pb binding to Suwannee River Humic Acid (SRHA). The FOCUS-CAS is compared with the analytical solution of CAS based on the fitting of the experimental data to the Non-Ideal Competitive Adsorption (NICA) model. Although both methods give satisfactory results, their CAS distribution features are different, indicating that different CAS can reproduce the binding data at similar level of goodness-of-fit. Within a comparable, but not identical, affinity range, the FOCUS-CAS shows multimodal distributions, whereas the NICA-CAS displays bimodal distributions. These differences are due to the fact that the NICA-CAS assumes an a priori functionality (i.e., carboxylic and phenolic), while the FOCUS-CAS does not make this assumption. Without knowing the chemical reality of the binding site explicitly, both methods are viable. The NICA-CAS is theoretically superior to the FOCUS-CAS because it provides informative descriptions of competition between the H- and Pb-binding. Nonetheless, the FOCUS-CAS is practically more convenient because it does not require proton binding data. Additionally, the FOCUS-CAS can describe some strong metal-binding sites (e.g., thiol) that cannot be described by the NICA-CAS and, thus, constitutes a suitable alternative to NICA-CAS.