Sorption is often carried out in stirred batch reactors without any consideration of how much mixing is sufficient to avoid the effect of diffusion without compromising yield and cost due to overmixing. Therefore, the focus of this work was to study how the maximum sorption capacity, removal efficiency, kinetics and power consumption (P) of the studied process are affected by different mixing speeds, i.e., impeller speed/minimum impeller speed for complete suspension (N/N_JS) ratio values and zeolite suspension mass concentrations. Experiments were conducted in a baffled reactor with the propeller at a standard off-bottom clearance. In addition to the experimental studies, numerical modelling approaches were carried out to investigate the sorption process using a transient multiphase computational fluid dynamics model and fitting selected kinetic models. The results show that an increase in zeolite mass leads to a slight increase in the N_JS and consequently P_JS. The impeller speed affects the velocities, power consumption, kinetics, final amount and removal efficiency of copper sorbed. The experimentally determined kinetic data fit Ritchie’s kinetic model well. However, for two experiments, performed at N/N_JS ratios of 0.8 and 0.6, Mixed kinetic model fits better, suggesting that the second-order reaction is suppressed by diffusion. Due to the influence of diffusion, the experimentally determined sorption efficiency decreased from 59.377% to 54.486% and 46.372% for N/N_JS ratios of 0.8 and 0.6, respectively.

The batch sorption process is used to remove various species from wastewater and can be optimised by selecting adequate process parameters and reactor geometry. As sorption is a heterogeneous process, achieving the desired process outcomes in a batch reactor relies heavily on establishing conditions in which the influence of interphase diffusion is minimised while keeping the efficiency and cost of the process at acceptable values. These conditions can be managed by the selection of appropriate reactor geometries and mixing speed through examination of their influence on the sorption yield and cost. The relationship between mixing speed and power consumption is important, as excessive mixing can lead to increased energy costs without proportional gains in sorption kinetics and efficiency. For these reasons, the effect of reactor geometry and mixing speed on copper sorption kinetics, efficiency, and energy consumption was studied. The Ritchie model and Mixed surface reaction and diffusion-controlled sorption kinetic model were employed for the kinetic study. CFD simulations were carried out to identify optimal designs that enhance process efficiency and reduce energy consumption. Data obtained indicate that the sorption process generally follows second-order kinetics. Results demonstrate that sorption can be effectively conducted at impeller speeds lower than the critical suspension speed (N_JS), achieving almost equal removal efficiencies (after 30 min) while reducing energy consumption. From the perspective of energy consumption, reactors without baffles are a significantly better solution than baffled reactors, especially when using a PBT impeller. From a kinetic standpoint, better results are achieved at the highest N/N_JS or N_JS. In baffled reactors, considering both power consumption and process duration, the SBT impeller emerges as the most efficient choice. Considering the compromises between power consumption and process duration the choice of reactor geometry and specific operating conditions should align with process priorities, such as energy savings through lower power consumption or reduced mixing time. FTIR spectra did not reveal the differences in the zeolite structure after the sorption process occurred.

Proton exchange membrane fuel cells (PEMFCs) are among the most promising clean energy conversion technologies, offering high efficiency and zero emissions. However, their large-scale commercialisation is limited by the high cost and environmental impact of conventional perfluorosulfonic acid membranes such as Nafion. In recent years, increasing attention has been directed toward biopolymer-based membranes as sustainable, low-cost, and biodegradable alternatives. This review provides a comprehensive overview of recent advances in the development and modification of biopolymer membranes, including polysaccharide-based materials such as chitosan, cellulose, gellan gum, sodium alginate, and starch, as well as protein-based materials such as keratin and collagen. Various modification strategies, including sulfonation, phosphorylation, cross-linking, and incorporation of inorganic or hybrid fillers, are analysed for their impact on key parameters, including proton conductivity, methanol permeability, and power density. Comparative data indicate that several modified biopolymer membranes achieve proton conductivities of 50 mS/cm or higher. However, higher conductivity values are generally reported for membranes primarily composed of synthetic polymers, where the biopolymer is incorporated only as an additive. In addition, some biopolymer-based membranes exhibit significantly lower methanol permeability than Nafion. The lowest reported value among the membranes discussed in this article is 0.98 × 10⁻¹⁶, representing the best-performing biopolymer membrane in terms of methanol permeability alone. Although many biopolymer membranes demonstrate relatively poor performance in single PEMFC tests, several have achieved power densities comparable to Nafion, while simultaneously offering improved environmental compatibility and sustainability. Finally, current challenges and future directions are discussed, emphasising the potential of these renewable materials to advance PEMFC technology toward more sustainable and economically viable energy systems.