Site orientation, accessibility, and surface hydrophobicity control on AFC polymer to increase hexavalent chromium removal performance

Abstract

• Three versions of polymer, each version in acid salt and free base form. • Different versions have different surface properties due to the orientation of the functional group. • Hydrophilic salt forms showed a 20-fold increase in removal than the hydrophobic counterpart. • Salt forms remove 95%-98%, chromate concentrations 10–100 mg/L, dose 4 g/L. Aniline formaldehyde condensate (AFC) is a polymer with an amine functional group. It can exist in either an acid salt or free amine form. AFC Polymers in salt form, Poly-1s, and Poly-2s, were synthesized by switching the ratio between isopropanol to an aqueous solution in the reaction medium from 3:1 to 1:3. Another Poly-3s was prepared in a reduced volume using an intermediate solvent ratio. All three were hydrophilic with a contact angle of 0°. Treatment with a base produced the corresponding polymers in a free base form. Elemental analysis, FTIR, and MALDI mass analyses confirmed the formulation of the salt form and free base form without significant changes in the chemical identity of the polymers. Contact angle measurements showed that Poly-1b (from Poly-1s) was superhydrophobic (contact angle 154°), whereas Poly-2b (from Poly-2s) was hydrophilic (contact angle 0°). The reversal in hydrophobicity between Poly-1b and Poly-2b indicated a significant change in the orientation of the amine group. FESEM imaging and surface area measurements from the BET isotherm reflected the differences between them. In chromate removal experiments, all three salt forms showed 80%-93% chromate removal (initial chromate concentration 10–100 mg/L, adsorbent dose 1 g/L, pH 4). The adsorption capacity of salt forms, between 131 and 149 mg/g (dose 0.5 g/L, removal 66%-75%), is the highest reported for AFC. In contrast, the uptake of hydrophobic Poly-1b at 3.87 mg/g (dose 4 g/L, C i 100 mg/L) was the lowest found for AFC. This work demonstrates that the relationship between surface property and site accessibility profoundly impacts chromate binding property.