Efficiency of humin immobilized in manganese–alginate beads for improving irreversible sorption and oxidative removal of 1-naphthol

Despite the high reactivity of birnessite for the oxidative removal of phenolic compounds, its radicals are persistent, and they adversely affect water systems. Alginate beads are widely used for secondary contaminant adsorption; however, they exhibit limited adsorption performance. To improve the efficiencies of birnessite and alginate for phenolic compound removal, humin was successfully immobilized in the sodium alginate beads to form birnessite–humin–alginate (Mn-Hu-AG) beads with various initial humin mass ratios (e.g. 1:0.25:1, 1:0.5:1, and 1:1:1). After the reaction between the Mn-Hu-AG beads and 1-naphthol, the products were analyzed using ultraviolet–visible spectrophotometry, high-performance liquid chromatography (HPLC), and infrared spectroscopy to evaluate the 1-naphthol-removal efficiency. The Mn-Hu-AG beads exhibited pseudo-second-order reaction kinetics, and the Mn-AG beads pseudo-first-order reaction kinetics. This indicates that both birnessite and humin are involved in the oxidative polymerization reaction between 1-naphthol and manganese oxide. The half-life of the Mn-Hu-AG beads (second-order reaction rate) was 3.7 times lower than that of the Mn-AG beads (first-order reaction rate). The Mn-Hu-AG beads exhibited a higher 1-naphthol removal rate than the Mn-AG beads, including further removal of the reaction products and Mn²⁺ elution from the reaction. In addition, the post-reaction HPLC results of the supernatant and the Mn-AG-Hu-bead extracts confirmed that humin immobilized the 1-naphthol reaction products via irreversible adsorption onto the Mn-AG-Hu-bead surface via birnessite-mediated cross-coupling. Because humin is eco-friendly and insoluble, its successful application in Mn-Hu-AG beads can be useful in developing effective materials for environmental remediation.