High Adsorption of Sulfamethoxazole by an Amine-Modified Polystyrene-Divinylbenzene Resin and Its Mechanistic Insight

Sulfamethoxazole (SMZ) adsorption by a series of amine-modified polystyrene-divinylbenzene resins (PSA/B/C/D) was investigated. All resins showed a similar pH dependent adsorption of SMZ but their capacities were linearly related with the contents of primary amines (-NH2) rather than secondary amines (-NH-). Mechanisms of SMZ adsorption by PSA (highest -NH2 content) were discussed as an example. Due to comparable pK(a) H-bonding interactions of -NH20 with SMZ(0) (regular H-bond) and SMZ(-) (negative charge-assisted H-bond, (-)CAHB) successively contributed most adsorption (pH 4-9). At weakly acidic pH, -NH20 was partially protonated and electrostatic attraction between -NH3+ and SMZ(-) occurred concurrently, but could be hindered by increased loading of SMZ(0). Hydrophobic/ pi-pi interactions were not major mechanisms as phenanthrene and nitrobenzenes had little effect on SMZ adsorption. At alkaline pH, where SMZ(-) and -NH20 prevailed, adsorption was accompanied by the stoichiometric (similar to 1.0) proton exchange with water, leading to OH- release and the formation of (-)CAHB [SO2N-center dot center dot center dot H center dot center dot center dot NH2]. The interaction and SMZ spatial distribution in the resin phase were further confirmed by FTIR and Raman spectra. SMZ was uniformly adsorbed on external and interior surfaces. SMZ adsorption by PSA had low-interference from other coexistent matter, but high stability after multiple regenerations. The findings will guide new adsorbent designs for selectively removing target organics.