Characterization of Surface-Bound Zr(IV) and Its Application to Removal of As(V) and As(III) from Aqueous Systems Using Phosphonic Acid Modified Nanoporous Silica Polyamine Composites

Silica polyamine composites (SPC) made from silanized amorphous nanoporous silica gel and poly(allylamine) (BP-1) were functionalized with phosphorus acid using the Mannich reaction, resulting in a phosphonic acid modified composite (BPAP). Successful immobilization of the phosphonic acid ligand was confirmed by mass gain, elemental analysis, IR, and solid-state NMR. The modified composite had a ligand loading of 1.5 mmol/g, corresponding to N/P ratio of 0.73. Zirconium(IV) was immobilized on BPAP with a loading of 1.12 mmol/g. Zirconium loading was analyzed by mass gain, ICP/AES, and SEM/EDX. Arsenate anions strongly adsorbed on the ZrBPAP composite in the pH range 2−8, while arsenite only adsorbed well at pH 10. The sorption mechanism is a chelation between arsenate or arsenite and Zr(IV)−phosphonic acid complex of BPAP. Adsorption isotherm data were found to be well modeled by the Langmuir equation for As(V) at pH 4 with K_ads = 0.016 L/g and Q_m = 98 mg/g; and at pH 6 with K_ads = 0.018 L/g and Q_m = 56 mg/g. Regeneration of the resin was carried out successfully for As(V) and As(III) using 2M-H₂SO₄. Four adsorption/desorption cycles were performed for As(V) at pH 4 without significant decrease in the uptake performance. ZrBPAP capture capacity and kinetics for arsenate were tested for longevity over 1000 cycles with only a marginal loss of performance. This composite is highly selective for arsenate over sulfate (As/SO₄²⁻ = 50/1) and selenate (As/Se = 20/1); lower selectivity was observed with Fe(III) and Th(IV) loaded BPAP. The significance of the observed selectivity is discussed in terms of the chemical properties of the anion and the nature of the interactions with the immobilized metal site.