(177e) Achieve Circular Carbon Utilization in Wastewater Treatment through Electrochemical in-Situ Carbon Capture and Reduction Reaction (iC2R2) Technology

Wastewater treatment is a major contributor to greenhouse gas (GHG) emissions, primarily due to the release of carbon dioxide (CO₂) and methane during the microbial decomposition of organic contaminants. This environmental burden is further exacerbated by the intrinsic carbon imbalance within wastewater streams, where the availability of readily biodegradable carbon (e.g., 1C and 2 C products) is often insufficient to sustain efficient biological nutrient removal processes. As a result, external carbon sources such as methanol, acetate, or formic acid have been routinely added to maintain treatment efficiency, leading to huge operational costs and energy consumption. Ironically, despite the substantial CO₂ emissions generated within treatment systems, CO₂ remains largely underutilized as a potential in situcarbon source due to its low reactivity and poor solubility in aqueous environments. This disconnection between carbon release and carbon demand highlights a critical opportunity for re-engineering treatment processes toward carbon circularity. In this context, the development of high-efficiency in situ carbon (CO₂) capture and reduction reaction (iC2R2) holds promise for converting emitted CO₂ directly into value-added carbon intermediates that can serve as supplemental carbon sources. Nevertheless, the practical implementation of iC2R2 in wastewater treatment systems is constrained by several challenges, including the inherently low solubility of CO₂ in water, sluggish reaction kinetics governed by microbial respiration rates, and competitive interactions between CO₂ and dissolved oxygen for electron donors during reductive processes. Overcoming these limitations requires integrated strategies that couple advanced catalysis with engineered microbial pathways to enable efficient CO₂ valorization within the treatment infrastructure.

To address the multifaceted limitations associated with iC2R2 in wastewater treatment, we developed an integrated electrochemical system specifically engineered to facilitate a closed-loop carbon cycle within the treatment infrastructure. Central to this innovation is the strategic reconfiguration of the CO₂RR cell through the coupling of catalytic and membrane functionalities, aimed at selectively capturing, concentrating, and electrochemically converting CO₂ present in wastewater streams.

Specifically, to suppress the detrimental competition between CO₂ and dissolved oxygen (O₂) for electrons during the reduction reaction—a major bottleneck in achieving high Faradaic efficiency—we designed a CO₂-selective membrane interface to physically separate O₂ from the CO₂ reduction zone. This was accomplished by fabricating a thin-film composite membrane using a porous polyether sulfone (PES) support coated with a 10 μm-thick polyamide (PA) layer via electrospraying. The PA layer acts as a selective diffusion barrier that significantly retards O₂ permeation while permitting the passage of aqueous CO₂ species. This selective gating mechanism minimizes electron loss to competing O₂ reduction reactions and thereby improves the selectivity and efficiency of the CO₂RR.

To overcome the intrinsic limitation of low CO₂ solubility and to accelerate iC2R2 kinetics, we engineered the cathode interface by encapsulating single Sn atoms within a zeolitic matrix. The single-atom Sn catalyst not only enhances CO₂ adsorption capacity through local electronic modulation and confinement effects but also facilitates the selective electroreduction of CO₂ to formate (HCOOH) at elevated current densities. This carbon-rich intermediate can be directly reused within the treatment process as an endogenous electron donor or carbon source for biological denitrification, reducing the dependence on costly external carbon additions.

To synergistically utilize both CO₂ and O₂ within the wastewater matrix, we designed the iC2R2 system to spatially partition the cathodic reactions such that the captured O₂, initially excluded from the cathode side, can be redirected to support enhanced chemical oxygen demand (COD) oxidation in subsequent aerobic process for organic compound degradation. In this way we further support the growth of biofilms on the membrane-electrode system, which can simultaneously harvest CO₂ generated from wastewater and boost wastewater treatment efficiency. By achieving concurrent valorization of both dissolved gases—reducing CO₂ to a useful intermediate and directing O₂ to augment oxidative degradation pathways—the system maximizes the utility of endogenous chemical species, aligning with energy-efficient and resource-recovery-driven treatment paradigms.

Our innovative electrochemical design demonstrated successful implementation of iC2R2 directly within wastewater, offering a transformative pathway for both carbon circularity and enhanced treatment efficiency. In an 8-hour lab-scale operation using 20 mL of real wastewater, the system achieved a current density of approximately 20 mA/cm²—an order of magnitude higher than conventional microbial electrosynthesis cells—highlighting the effectiveness of the engineered cathode and membrane integration. During this process, over 150 mg/L of glucose was oxidized to CO₂, which was subsequently reduced in situ to HCOOH. This aqueous-phase conversion represents a significant advancement over traditional gas-phase CO₂RR approaches, which often suffer from low CO₂ availability and poor mass transfer in dilute aqueous environments.

Notably, the system exhibited a CO₂-to-HCOOH conversion efficiency exceeding 90%, underscoring its capacity to capture and valorize endogenous CO₂ with minimal energy loss. The generated HCOOH serves as a biodegradable carbon source that promotes biofilm development, thereby directly enhancing microbial activity and accelerating nutrient removal processes. As a result, the overall wastewater treatment efficiency was improved by more than 80% relative to conventional systems lacking iC2R2 functionality.

This advancement holds dual and far-reaching implications for chemical process engineering in wastewater treatment. First, by enabling the direct transformation of CO₂ within the aqueous matrix prior to atmospheric release, the system effectively mitigates greenhouse gas emissions at the source, aligning with carbon neutrality goals. Second, by converting waste-derived CO₂ into a usable carbon intermediate for downstream treatment processes, the system facilitates a closed-loop carbon cycle, thereby reducing reliance on external chemical additives and improving process sustainability.

Collectively, our electrochemical platform exemplifies a new direction in sustainable wastewater treatment by integrating gas-selective membranes, single-atom catalysis, and multifunctional reactor design to enable in situ carbon reuse, promote CO₂ circularity, and improve overall treatment efficiency with minimized external inputs. The iC2R2platform presents broad applicability for the integrated recovery of multiple valuable elements inherently present in wastewater, including nitrogen species. Its modular design, scalability, and compatibility with existing infrastructure render it a promising candidate for next-generation wastewater treatment systems that simultaneously achieve pollutant removal, resource recovery, and carbon reuse through a unified and chemically efficient process framework.