Coupling interfacial thermodynamics and microbial ecology drives carrier-induced microgranulation in activated sludge...

High-concentration powder carrier bio-fluidized bed (HPB) technology provides a promising strategy for rapid cultivation of microgranules; however, the interfacial mechanisms governing carrier-induced granulation remain poorly explored. In this study, three typical carriers (diatomite, montmorillonite, and fly ash) were employed to investigate microgranule formation in HPB systems. By integrating expanded Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory, extracellular polymer substances (EPS) and electrochemical characterization, microbial community analysis, and nitrogen transformation functional genes, the mechanisms linking interfacial interactions to microgranule development were systematically elucidated. Among tested carriers, diatomite exhibited the strongest granulation-promoting effect, increasing particle size by 63.3% and biomass growth by 24.7% within 30 d. Surface thermodynamic analysis revealed that diatomite possessed the lowest adhesion Gibbs free energy (-16.90 mJ·m^-2), compared with -9.42 and -3.95 mJ·m^-2 for montmorillonite and fly ash, respectively. The Lewis acid-base (AB) interaction free energy of the diatomite-sludge interface reached -13.43 mJ·m^-2, accounting for more than 79% of the total adhesion free energy, indicating that AB interactions were primarily responsible for diatomite's superior microbial adhesion capacity. Consistent with thermodynamic advantage, the interaction energy barrier disappeared on day 30 in the diatomite system, 10 d earlier than in the fly ash system, with the reversible adhesion distance decreased to 5.61 nm, facilitating rapid aggregate formation. The accelerated microgranules' development was also accompanied by enhanced protein/polysaccharide (PN/PS) and electrochemical activity, with increasing from 1.40 to 2,29, 1185 to 1454 μF, respectively. Simultaneously, nitrifying, denitrifying, and granulation-associated microorganisms were selectively enriched. Correspondingly, the abundances of amoABC, hao, narB, and nosZ reached 188%, 179%, 163%, and 151% of control levels, respectively. These findings demonstrate the exceptional granulation performance of diatomite originates from its favorable interfacial thermodynamic properties, which promote microbial adhesion, extracellular electron transfer, and functional microbial assembly, thereby accelerating microgranule formation in HPB systems.