Diatomite-assisted sorption of humic acid from environmental waters: Kinetics and interparticle diffusion in a highly porous material

Humic acid (HA), a major fraction of natural organic matter, is a persistent contaminant that impairs water quality and contributes to the formation of disinfection byproducts. In this study, natural, unmodified diatomite was investigated as a cost-effective sorbent for HA removal, with particular emphasis on diffusion limitations within its hierarchical meso–macroporous structure derived from diatom frustules. Equilibrium data (2.5–30 mg L–1, pH 7) were described by the Langmuir and Freundlich models, indicating the coexistence of monolayer and heterogeneous sorption behavior. The uptake kinetics showed rapid initial removal, followed by a diffusion-limited regime. The pseudo-first-order model captured early-stage uptake, whereas the pseudo-mixed-order fractional (PMOF) model best represented the full kinetic profile. HA removal decreased from pH 4 to 9 owing to enhanced electrostatic exclusion above the point of zero charge and increased HA ionization. Divalent cations (Ca2+, Mg2+) enhanced removal through charge neutralization and cation bridging, while phosphate strongly inhibited uptake via site-specific competition. A comparative evaluation identified high-performing diatomite, and integrated evidence from BET, FT-IR, FE-SEM, and zeta potential analyses elucidated diffusion- and surface-controlled sorption pathways. Application to natural water (containing an HA-equivalent concentration of 9.4 mg L–1) achieved up to 87% removal at practical dosages. These results demonstrate that unmodified diatomite is a low-cost, environmentally benign sorbent for HA removal, providing a mechanistic basis for scaling up diatomite-based sorption in column or filtration-assisted configurations for practical water treatment.