Dynamics and thermodynamics for competitive adsorptive removal of methylene blue and rhodamine B from binary aqueous solution onto durian rind.

Concurrent adsorptive removal of methylene blue (MB) and rhodamine B (RhB) onto durian rind (DR) agricultural waste, from an aqueous binary solution as a model of wastewater containing multiple synthetic dyes, was investigated. The concurrent adsorption of the dyes followed pseudo-second-order kinetics. The adsorption isotherm was well simulated by the Langmuir model, implying a monolayer adsorption to the surface with a homogeneous binding energy. The adsorption process was governed by external mass transfer through two-step intraparticle diffusion of the dyes onto the adsorbent surface. The adsorption efficiency of MB (96.4%) is much higher than that of RhB (56.3%). This is attributed to the higher rate constant for the adsorption of MB (0.348 g mg-1 min-1) as compared to that of RhB (0.151 g mg-1 min-1). The adsorption behavior suggested that the two cationic dyes in the binary solution diffused and adsorbed independently and randomly onto the DR surface. The adsorption capacity of MB and RhB in the binary solution (47.4 mg g-1 and 32.9 mg g-1, respectively) is lower than those of their single solute solutions (93.3 mg g-1 and 62.8 mg g-1, respectively), suggesting a competitive effect in their concurrent adsorption. This was confirmed based on the adsorption characteristics of the binary solution with different molar ratios. The competitive effect was attributed to either non-interactive or repulsive electrostatic interactions between the positively charged dyes in the binary system. The domination of MB is attributed to its smaller molecular size, higher planarity, and faster adsorption kinetics compared with RhB.

Synergistic effect in concurrent removal of toxic methylene blue and acid red-1 dyes from aqueous solution by durian rind: kinetics, isotherm, thermodynamics, and mechanism.

In the present study, a synergistic effect between cationic methylene blue (MB) and anionic acid red 1 (AR1) on their concurrent adsorptive removal from aqueous binary solution onto durian rind (DR) was systematically investigated in batch mode across different parameters. The concurrent adsorption was pseudo-second-order kinetics and followed the Langmuir isotherm model, similar to their respective single component. The kinetics and intraparticle diffusion analyses demonstrated that the adsorption rate of MB was a 15-fold faster than AR1, and mass transports were governed by a combination of intraparticle and film diffusion. The synergistic effect was evidenced by an enhanced adsorption efficiency of AR1 from 27 to 42%, while that of MB was almost unchanged (97-98%). By changing the molar ratios of MB and AR1, it was found that the maximum adsorption capacity of MB and AR1 was 249 and 200 mg g-1, respectively, in the binary system higher compared with those in their respective single system (108 and 16 mg g-1). Overall data indicated that the synergistic effect was due to electrostatic interactions between cationic and anionic synthetic dyes, supported by negatively charged DR surface, leading to the formation of their stacking layers on the adsorbent surface. Novelty statement: A synergistic effect in concurrent adsorptive removal of synthetic dyes from multicomponent wastewater remains a critical research challenge. We believed that electrostatic interaction between ionic dyes could be explored to enhance their removal efficiency. This report is the first time that such a synergistic effect between cationic methylene blue (MB) and anionic acid red 1 (AR1) on their concurrent adsorption from aqueous binary system is systematically investigated. The kinetics, isotherm, thermodynamics, and mechanism of the concurrent adsorption of MB and AR1 attributed to the synergistic effect are elucidated in detail.