A sorbent containing pH-responsive chelating residues of aspartic and maleic acids for mitigation of toxic metal ions, cationic, and anionic dyes

Abstract

t-Butyl hydroperoxide-initiated cycloterpolymerization of diallylaminoaspartic acid hydrochloride [(CH₂CHCH₂)₂NH⁺CH(CO₂H)CH₂CO₂H Cl⁻] (I), maleic acid (HO₂CHCHCO₂H) (II) and cross-linker tetraallylhexane-1,6-diamine dihydrochloride [(CH₂CHCH₂)₂NH⁺(CH₂)₆NH⁺ (CH₂CHCH₂)₂ 2Cl⁻] (III) afforded a new pH-responsive resin (IV), loaded with four CO₂H and a chelating motif of NH⁺⋯CO₂⁻ in each repeating unit. The removal of cationic methylene blue (MB) (3000 ppm) at pH 7.25 and Pb(II) (200 ppm) at pH 6 by IV at 298, 313, and 328 K followed second-order kinetics with E_a of 33.4 and 40.7 kJ mol⁻¹, respectively. Both MB and Pb(II) were removed fast, accounting for 97.7% removal of MB within 15 min at 313 K and 94% of Pb(II) removal within 1 min. The super-adsorbent resin gave respective q_max values of 2609 mg g⁻¹ and 873 mg g⁻¹ for MB and Pb(II). IV was also found to trap anionic dyes; it removed 91% Eriochrome Black T (EBT) from its 50 ppm solutions at pH 2. The resin was found to be effective in reducing priority metal contaminants (like Cr, Hg, Pb) in industrial wastewater to sub-ppb levels. The synthesis of the recyclable resin can be easily scaled up from inexpensive starting materials. The resin has been found to be better than many recently reported sorbents.