| Chitosan is nature's most abundant biopolymer next to cellulose and a selective adsorbent for heavy metal ions. However, the chitosan raw material needs to be modified for use in low pH environments. To address this need, chemically modified porous chitosan beads were synthesized. Two chemical modifications of chitosan were considered, including partial N-acylation of 7% of the amine groups with C$/sb9$ hydrocarbon side chains before bead casting, and glutaric dialdehyde crosslinking of chitosan chains after bead formation but before freeze drying. N-acylation, crosslinking, and freeze drying steps must be combined to minimize acid solubility and maximize the internal surface area of the chitosan beads. Chemical modifications of acylation and crosslinking improved the material properties of the chitosan beads for use in waste water treatment applications, but did not improve the adsorption capacity for cadmium ions. Freeze drying without crosslinking increased the internal surface area to 44.2 m$/sp2$/g. Acylation improved the cadmium adsorption capacity from 169 mg Cd$/sp[+2]$/g to 216 mg Cd$/sp[+2]$/g but only at high cadmium concentrations exceeding also reduced the cadmium adsorption capacity. An extent of crosslinking equal to 0.81 mole GA/total mole -NH$/sb2$ increased the internal surface area from 44.2 m$/sp2$/g to 223.6 m$/sp2$/g and rendered the bead insoluble in acid solution, but reduced the cadmium adsorption capacity to 83 mg Cd$/sp[+2]$/g. A diffusion-limited modified shrinking core model was developed to describe the formation of the crosslinked layer within the gel bead. The effective diffusivity for glutaric dialdehyde in the gel beads was estimated to be $/rm 4.11/pm 0.082/cdot 10/sp[-8]/ (1s) cm/sp2/s$. The adsorbent can be regenerated by dilute acid treatment, and 100% cadmium recovery from the chitosan beads was feasible at pH less than 3.0. Decreasing the equilibrium pH increased the percentage of cadmium desorbed according to an S-shaped profile, consistent with the ion-exchange mechanism. A Langmuir-Freundlich model proposed that the desorption process is accomplished by displacing adsorbed cadmium ions with hydrogen ions. |