Selective removal of cationic organic pollutants using disulfide-linked polymer

Dyes are widely used in various industries, and the industrial use of dyes has increased steadily over the past few decades. However, it is difficult to remove these dyes from effluents by using conventional wastewater treatment processes owing to their chemical and physical stability. Herein, we propose the possibility of using disulfide-linked polymer (DiS-COP) as an effective adsorbent for selectively removing cationic dyes. The selective adsorption ability of DiS-COP was tested using two cationic dyes, namely methylene blue (MB) and crystal violet (CV), and two anionic dyes, namely methyl orange (MO) and orange G (OG). The adsorption equilibrium is reached in less than 4 h for all dyes with extremely fast removal for CV. The kinetic results were analyzed using the pseudo-first-order, pseudo-second-order, and intraparticle diffusion model. The maximum adsorption capacity for cationic dyes, as obtained using the Langmuir isotherm, was considerably higher than that for anionic dyes. DiS-COP exhibited 10 (Q_m,MB 211 mg/g) and 56 times (Q_m,CV 1181 mg/g) higher maximum adsorption capacities for MB and CV, respectively, than that for the anionic dye MO (Q_m,MO 21 mg/g). Selective adsorption of cationic dyes in the binary system was successfully realized. The results of the pH effect experiment revealed the selective adsorption mechanism. The electrostatic interaction between negatively charged DiS-COP and charged dye molecules induced the selective attraction of cationic dyes. Moreover, the effect of π-π stacking between DiS-COP and the dye molecules, as well as that between the adsorbed dye molecules and the free dye molecules in the solution, facilitated the dye adsorption. The removal mechanism was further elucidated using Fourier transform infrared spectrometry and X-ray photoelectron spectroscopy. In addition, the results of the adsorption experiments involving the selected pharmaceuticals indicated that DiS-COP was able to remove these cationic pharmaceuticals to a significantly greater extent than it was able to remove the those anonic and neutrally charged pharmaceuticals. The DiS-COP used in this study, which can be easily prepared via a one-pot reaction, has the potential for use as an adsorbent to selectively remove cationic organic pollutants.