Evaluation of the applicability of selected analytical techniques for determining the characteristics of humic substances sourced from by-products of the wastewater treatment process

Although humic substances (HSs) are among the most valuable compounds in the environment, they often constitute nuisance factors for wastewater treatment plants (WWTPs). However, their recovery from by-products of WWTPs opens up opportunities for their utilization. Therefore, this study aimed to evaluate the suitability of selected analytical methods for determining the structure, properties, and possible utilization of HSs originating from WWTPs based on model humic compounds (MHCs). As a consequence, the study proposed separate approaches for the initial and in-depth characterization of HSs. The results demonstrate that UV-Vis spectroscopy may be recognized as a cost-effective approach for the preliminary characterization of HSs. Such a method indeed gives similar information on the degree of complexity of MHCs as X-EDS and FTIR, and similarly to them allows for the differentiation of their particular fractions. In turn, X-EDS and FTIR techniques were recommended to be used for in-deep analysis of HSs due to their ability to detect heavy metals and biogenic elements in their structure. Contrarily to other studies, the presented research indicates that only selected absorbance coefficients - A₂₅₃/A₂₃₀, Q_4/6, and ΔlogK may help to distinguish particular humic fractions and evaluate changes in their behavior, independently of their concentration (coefficient of variation < 20%). Herein, the fluorescence capacities of MHCs were found to be equally affected as their optical properties by the changes in their concentration. Focusing on the obtained results, this study recommends that the quantitative comparison of the properties of HSs should be proceeded by the standardization of their concentration. Herein, the stability of other spectroscopic parameters characterizing solutions MHCs was achieved within a concentration from 40 to 80 mg L⁻¹. Among them, the analyzed MHCs were differentiated the most by the SUVA₂₅₄ coefficient, which value was almost 4 times higher for SAHSs (8.69) than for ABFASs (2.01).