Amino-modified hemp stem for high-capacity adsorption of Cr(VI) from aqueous solution

Two novel adsorbents, triethylenetetramine-grafted hemp stems (HS-TETA) and choline chloride-grafted hemp stems (HS-CL), were prepared by using epichlorohydrin as etherifying agent. The obtained adsorbents were comprehensively characterized, and the adsorption capacity for hexavalent chromium (Cr(VI)) was evaluated under various single-factor conditions, including adsorbent dosage, contact time, solution pH, initial hexavalent chromium concentration, and temperature. HS-TETA and HS-CL exhibit excellent adsorption performance, with maximum adsorption capacities of 396.88 and 335.04 mg/g, respectively, outperforming previous relevant studies. The adsorption behavior of Cr(VI) by HS-TETA and HS-CL was investigated using adsorption kinetics, adsorption isotherms, and adsorption thermodynamics. The adsorption process follows the pseudo-second-order kinetic model and the Langmuir isotherm model, suggesting a monolayer adsorption process with chemical adsorption. Thermodynamic analysis reveals that the adsorption of Cr(VI) on two adsorbents is spontaneous and exothermic. Combined with the characterization, it can be speculated that the adsorption of Cr(VI) on two adsorbents involves electrostatic effects, reduction, and complexation processes. HS-TETA and HS-CL exhibit good recyclability. After 5 cycles, HS-TETA and HS-CL maintain over 50% and 85% of their initial adsorption capacity, respectively. The cycling performance of HS-CL containing quaternary ammonium and amine groups is significantly superior to that of HS-TETA containing only an amine group, indicating that the introduction of the quaternary ammonium group enhances the cycling performance of the adsorbent. HS-TETA and HS-CL, as cost-effective and efficient adsorbents, offer a promising solution for the treatment of Cr(VI)-contaminated wastewater and facilitate the recycling of agricultural waste.