Solar-Powered Cell for Electricity Generation and Wastewater Treatment

By Muhammad OsamaReviewed by Laura ThomsonJun 21 2024

In a recent article published in the journal water, researchers introduced an innovative device called a solar-driven photo-bioelectrochemical cell (s-PBEC) to simultaneously produce electricity and treat wastewater using solar energy and bacteria. Their approach employs a hybrid anode that combines a photocatalyst and an electroactive biofilm.

Background

Wastewater treatment is a significant environmental challenge, mainly for developing countries with inadequate infrastructure and resources. Conventional technologies such as pharmaceuticals, personal care products, and pesticides are often energy-intensive and inefficient at removing organic pollutants. Due to their persistence, toxicity, and bioaccumulation, these pollutants seriously threaten human health and ecosystems.

Microbial electrochemical systems (MESs) are emerging as a promising technology that integrates biodegradation and electrochemical processes to treat wastewater and generate renewable energy. MESs use electrochemically active bacteria (EAB) as biocatalysts to oxidize organic compounds and transfer the resulting electrons to an electrode. However, MESs face challenges like low energy output and substrate utilization rate, which depend on electrode materials and the efficiency of extracellular electron transfer (EET).

To overcome these limitations, researchers have proposed integrating bioanodes with abiotic photoanodes to enhance EET rates and degrade recalcitrant pollutants via photocatalysis. Photocatalysis uses light to activate a semiconductor material, such as titanium dioxide (TiO₂), to generate reactive oxygen species (ROS) that oxidize organic molecules. The intimate coupling of photocatalysis and biodegradation (ICPB) creates a synergy between the two processes, improving the performance of MESs.

About the Research

In this paper, the authors developed a novel ICPB photoanode-based s-PBEC using a two-dimensional (2D) cobalt phosphate (Co₃(PO₄)₂) and magnesium hydroxide (Mg(OH)₂) heterojunction photocatalyst, known for its narrow band gap and high visible-light absorption capacity. The photocatalyst was deposited on a three-dimensional (3D) carbon foam anode and then colonized by EAB from anaerobic digestion sludge.

The study utilized a double-chamber reactor with a quartz window for the anodic compartment and a carbon-felt cathode. The anolyte solution contained synthetic wastewater with acetate as the main carbon source, while the catholyte solution contained potassium hexacyanoferrate (III) as the electron acceptor. The reactor was operated with an external resistance of 1000 Ω and illuminated by a solar simulator.

The system's current densities, power outputs, and electrochemical activities were monitored under dark and light conditions. The morphology, optical properties, and crystal structures of the prepared photocatalysts were analyzed using various techniques, such as X-Ray diffraction (XRD), Raman spectroscopy, UV-Vis diffuse reflectance spectroscopy (DRS), photoluminescence (PL) spectroscopy, and high-resolution transmission electron microscopy (HR-TEM). Furthermore, high-throughput sequencing of the anodic biofilm was performed to investigate the effect of light exposure on the microbial community composition and diversity.

Research Findings

The outcomes showed that the s-PBEC with the ICPB anode exhibited a rapid increase in current density upon light illumination, rising from 0.15 A/m² to 0.32 A/m² within one hour and reaching a maximum of 0.38 A/m² after 2.5 hours. The power output increased from 66.0 mW/m² to 91.5 mW/m² under light conditions.

Electrochemical impedance spectroscopy (EIS) and linear sweep voltammetry (LSV) analyses indicated that the illuminated ICPB anode had lower ohmic and charge-transfer resistances, demonstrating enhanced EET efficiency.

The XRD, Raman, and HR-TEM results confirmed the successful formation of the 2D/2D Co₃(PO₄)₂/Mg(OH)₂ heterojunction photocatalyst, which exhibited a 2D sheet-like morphology with tiny spherical particles. The DRS and PL spectra revealed strong visible-light absorption and a low charge recombination rate for the photocatalyst.

Shotgun metagenomic analysis revealed significant changes in the microbial community composition of the anodic biofilm after illumination. The relative abundances of several phyla, such as Actinobacteria, Bacteroidetes, and Firmicutes, decreased, while the relative abundance of Proteobacteria increased. Under dark conditions, the most abundant genera were Geobacter, Rhodococcus, and Dechlorosoma, known for their EET and biodegradation capabilities.

However, their relative abundances decreased after illumination, suggesting that other EABs outcompeted them. Under light conditions, the most abundant genera were unclassified genera belonging to Neisseriales, Betaproteobacteria, and Alphaproteobacteria, correlating with enhanced current generation. These results indicated that photocatalysis may indirectly affect the microbial community structure by creating selective pressure, favoring those capable of a high rate of EET.

Conclusion

In summary, the novel s-PBEC proved effective for sustainable energy generation and wastewater treatment, particularly for recalcitrant organic pollutants. It efficiently harnessed solar energy to activate the photocatalyst, degrading pollutants into more biodegradable compounds that EAB could consume.

It also reduced the toxicity of pollutants on the EAB by protecting them from the ROS generated by the photocatalyst. The authors suggested optimizing their device by adjusting the dark/light cycles, photocatalyst composition, and microbial community composition.

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