The effects of pH and ionic strength on fulvic acid uptake by chitosan hydrogel beads

The objective of this work is to investigate the effects of pH and ionic strength on the adsorption capacity for fulvic acid (FA) by chitosan hydrogel beads. The results indicated that the sorption amount increased with decreasing pH and increasing ionic strength concentration. The sorption isotherms were well described by using non-linear Langmuir, Freundlich and Redliche–Peterson equation. The adsorption kinetics of FA onto chitosan hydrogel beads could be described by pseudo-second-order rate model. The extent of FA removal in the presence of other ions decreases in the order Ca²⁺ > Mg²⁺ > Na⁺ ≈ K⁺ and Cl⁻ > NO₃⁻ > CO₃²⁻. FTIR along with XPS analyses revealed the amine groups on the beads were involved in the sorption of FA and the organic complex between the protonated amino groups and FA was formed after FA uptake. Sorption mechanisms including electrostatic interaction and surface complexation were found to be involved in the complex sorption of FA on the chitosan hydrogel beads.