Metal Sulfides Functionalized Carbon Foam from Waste PET: An Advanced Photothermal Material Enabling Circular Economy for Interfacial Solar Desalination

Water scarcity is an increasingly pressing issue and, for this reason, developing a clean water production technologies is crucial. Interfacial solar desalination is a promising technology that produces clean water from bulk available seawater using renewable solar energy. However, finding a novel photothermal material is challenging as well. To operate this young technology efficiently, the material should be capable of capturing the maximum solar spectrum from UV/Vis to Near-IR to gain maximum photothermal efficiency, making the process more sustainable. An advanced photothermal material demands a rational design that meets the environmental, technological and economic needs of a sustainable process. In this framework, the development of advanced materials from waste offers a win-win solution by switching the concept of the circular economy. In addition to water scarcity, plastic pollution is another challenge for researchers, the environment and the industrial sector. In this study, we upcycled waste PET (Polyethylene Terephthalate) into carbon foam (CF) using eutectic salt (NaCl/ZnCl2) at low-temperature carbonization (340 ºC), compared to Flash Joule Heating carbonization (2000-3000 ºC). To enhance the optical activity and photothermal effect of CF, we further functionalized the CF with various metal sulfide nanostructures such as CuS, CuCo2S4, and Cu3SbS4-Sb2S3 via conventional hydrothermal synthesis, and CoxBi2S3 using microwave synthesis technology. The synthesized solar evaporators were characterized through a multi-technique approach using XRD (X-ray Diffraction), SEM (Scanning Electron Microscopy), XPS (X-ray Photoelectron Spectroscopy), and UV/Vis-NIR (Ultraviolet/Visible to Near-Infrared) spectroscopy. The metal sulfides-functionalized CF demonstrated a significant enhancement in average absorption, above 95%, compared to bare CF (68%) in the IR region. An interfacial solar desalination experiments were conducted under one-sun solar irradiation (1000 W/m²). The obtained results demonstrated an excellent evaporation rate of CuS/CF (1.92 kg m-2 h-1), CuCo2S4/CF (2.40 kg m-2 h-1), CoxBi2S3/CF (2.35 kg m-2 h-1), and Cu3SbS4-Sb2S3/CF (2.82 kg m-2 h-1) compared to the benchmark CF (1.58 kg m-2 h-1). These remarkable outperforming results were achieved due to the excellent photo-to-thermal conversion and the crucial role of morphology, both of CF and metal sulfides. To emphasize the role of hydrophilicity, contact angle analysis was systematically applied, demonstrating the excellent wettability of the photothermal material as evidenced by contact angle analysis. ICP-AES (Inductively Coupled Plasma Atomic Emission Spectroscopy) was applied to the evaporated water by collecting water vapors, demonstrating a 99% reduction in salt concentration. These results reflect the rational design of metal sulfides decorated CF as a potential nano-enhanced photothermal material for the interfacial solar desalination. Moreover, the outcomes of the work underscore the integral role of the circular economy in achieving sustainability goals, specifically focusing on SDG-6, SDG-7, and SDG-13, to address global plastic pollution and water scarcity in an eco-friendly and economically attractive manner.