Glycine and Glucose Exert Divergent Concentration-Dependent Interfacial Regulatory Effects on Goethite-Catalyzed Polyphenol–Maillard Abiotic Humification

Iron oxide-mediated abiotic humification is critical for long-term soil organic carbon (SOC) stabilization. Goethite (α-FeOOH), the most thermodynamically stable iron oxyhydroxide in terrestrial ecosystems, is proposed to catalyze coupled Maillard reaction and polyphenol oxidation via surface hydroxyl and Fe(III) active sites. Nevertheless, the divergent interfacial regulatory mechanisms of glycine (Gly, N source) and glucose (Glu, C source) concentration gradients in goethite-catalyzed polyphenol-Maillard systems remain poorly understood. Two independent 360-h gradient incubation experiments were performed using synthetic goethite as a catalyst and catechol as a model polyphenol precursor to probe their concentration-dependent effects on short-term interfacial transformation kinetics and humic-like product properties. Kinetics, product properties, and underlying pathways were characterized via Gaussian and first-order asymptotic modeling, humic-like acid (HLA)/fulvic-like acid (FLA) fractionation, elemental analysis, Fourier-transform infrared (FTIR) spectroscopy, and partial least squares structural equation modeling (PLS-SEM). Results showed that 0.12 mol/L was the optimal concentration for both precursors to accelerate intermediate transformation. Gly elevated the baseline aromaticity parameter y₀ by 108.87% (vs. 20.38% for Glu), while Glu increased the asymptotic maximum dissolved organic carbon (DOC) by 322.66% (vs. 78.02% for Gly). Moderate Gly (0.03 mol/L) yielded the highest C_HLA. Excessive precursors reduced the C_HLA/C_FLA ratio via interfacial competitive adsorption and site occupation. The two precursors exerted opposing effects on the hydroxyl stretching peak of humic products, but both induced goethite surface reconstruction and activated iron-bearing surface functional groups. PLS-SEM confirmed precursor concentration as the dominant driver of humification kinetics. This study clarifies the distinct roles of Gly in aromatic cyclization and Glu as an aliphatic C donor during short-term mineral–organic interfacial processes, providing quantitative benchmarks for optimizing artificial humus production.