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 (Ca 2+ , Mg 2+ ) 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.