Review of electrochemical reactors for the efficient removal of heavy metals from wastewater

Heavy metals, such as cadmium, arsenic, lead and mercury, are a group of metalloids that can harm human wellness and the environment due to their persistence and accumulation. Hence, electrochemical reactions facilitated by multiple reactor types play a crucial role in the removal of metal contaminants from wastewater. Electrochemical reactors, especially in continuous mode, are beneficial for the removal of heavy metals, however the impacts of electrode materials, target-specific metals, efficiency, and performance are yet to be identified. Thus, this review aims to provide insight into various existing electrochemical reactors for wastewater treatment to optimally eliminate targeted hazardous metals. The membrane-based continuous flow-by reactor showcased exceptional efficiency in continuous extraction, with the lowest energy consumption of ∼6 Wh/mol of metal. However, hybrid reactors, that utilizes slag as an absorbent, are identified to be beneficial as a suitable sustainable approach for wastewater treatment. In addition, the review also demonstrates the ability of critical electrochemical reactor design with optimal operating parameters, energy consumption, and current densities to maximize heavy metal removal efficiency. It also highlights the potential of distinct reactors in specific wastewater treatment applications, which eventually leads to a pollution-free environment in the future. Major findings include the comparative benefits of integrating adsorption with electrochemical approaches and the identification of reactor designs with minimal energy consumption and high removal efficiency, such as electrocoagulation reactors that generate removal rates of over 98 % for Cu, Pb, and Cd. The novel contribution of this review lies in evaluating state-of-the-art technologies through a critical comparison of diverse electrochemical reactors, filling a crucial research void in heavy metal wastewater treatment. Unlike previous reviews, it uniquely assesses the real-world applicability of various systems, including novel and hybrid designs, covering their sustainability context. By synthesizing advancements, evaluating advantages and disadvantages, and suggesting future research directions covering scalability and cost-effectiveness, it supports the development of sustainable and efficient wastewater treatment technologies.