Fractionation and kinetic processes of humic acid upon adsorption on colloidal hematite in aqueous solution with phosphate

Batch experiments were carried out to investigate fractionation and kinetics of humic acid (HA) during adsorption onto hematite and the effect of phosphate. The concentrations and weight-average molecular weight (Mw) of HA in solution were determined by total organic carbon (TOC) analyzer and high-performance size exclusion chromatography (HPSEC). Addition of phosphate (simultaneously or beforehand) decreased HA adsorption due to competition. Fractions of HA with Mw values larger than 4000 Da were preferably adsorbed. The adsorption kinetics can be described with Pseudo-second order model or Elovich kinetic model. Fractions of HA with relatively low Mw (3000–4000 Da) were quickly adsorbed in the first hour, and then were replaced slowly by larger ones (>5000 Da). This phenomenon could be explained by fast diffusion of relatively small HA particles from solution to mineral surface, but the overall binding affinity is higher for the bigger particles as a result of more reactive groups present on bigger particles compared to smaller particles. At last, molecular weight distribution results in adsorption isotherms and kinetics indicated that the replacement between relatively small and larger fractions of HA could last more than 24 h.

► We investigate size fractionation of humic acid by adsorption onto hematite. ► The addition orders of phosphate and humic acid influence the adsorption. ► Phosphate decreases humic acid adsorption strongly. ► Smaller size of humic acid were first adsorbed and then replaced by larger ones.