Assessment of waste hop (Humulus Lupulus) stems as a biosorbent for the removal of malachite green, methylene blue, and crystal violet from aqueous solution in batch and fixed-bed column systems: Biosorption process and mechanism

The discharge of industrial textile waters into water bodies leads to the accumulation of pollutants which are a serious threat to humans and the environment. The development of an efficient adsorbent for the removal of toxic organic dyes from water is of crucial importance. For that reason, this work aimed to elaborate on the potential application of a biosorbent obtained from waste Humulus lupulus stems (Hlup-BA) for the elimination of malachite green, methylene blue, and crystal violet from aqueous solution in batch and fixed-bed column systems. The Hlup-BA was characterized using chemical composition analysis, by determination of particle size distribution, the point of zero charge (pHpzc), SEM, BET, EDS, XRD, and FTIR. Comprehensive analysis of Hlup-BA adsorption efficiency in a batch study showed that at the optimal conditions (e.g. contact time, 120 min; biosorbent dose, 2 g/L; dye concentration, 100 g/L; pH, 8 and temperature, 298 K), even 98.4 %, 98.8 %, and 98.5 % of MG, MB, and CV were removed, respectively. The results showed that the biosorption process followed pseudo-second-order kinetics. The equilibrium data were modeled using Langmuir, Freundlich, Sips, and Dubinnin-Radushkevich biosorption isotherms. High values of Hlup-BA biosorption capacities obtained from Langmuir isotherm were 398.9 mg g-1, 209.8 mg g-1, and 133.2 mg g-1 for MG, MB, and CV, thus proving HlupBA as an excellent biosorbent for removal of the tested dyes. The thermodynamic study indicated that the biosorption of these three dyes on Hlup-BA from aqueous solution is an endothermic, spontaneous, and thermodynamically stable process. The proposed mechanism of the biosorption process covers pi-pi interactions, electrostatic interactions, and hydrogen bonding. Furthermore, the data obtained by modeling in the fixed-bed column were also successfully correlated with the Thomas, Bohart-Adams, and Yoon-Nelson models. The very good fit of the results obtained in this way will greatly contribute to optimizing the pilot-scale operations for future wastewater treatment. Consequently, Hlup-BA can be an effective, cheap, and environmentally friendly biosorbent to eliminate methylene blue, malachite green, and crystal violet dyes from the aqueous solution in batch and column systems.