Recent advances in synthesis, characterization, and environmental applications of activated carbons and other carbon derivatives

Abstract

The historicity and versatility of activated carbons (ACs) and other carbon derivatives (OCDs) date back to antiquity. This article reviews the recent advances in synthesis, characterization, and environmental applications of these demand-driven adsorbents from biomass and non-biomass sources. It first identifies relevant literature sources and knowledge gaps and then segmentalizes and scrutinizes the theme to elucidate contemporary carbon-based adsorbents. Conventional and advanced syntheses are highlighted. Current trends in adsorbents' characterization and remediation are also presented. The conventional AC synthesis includes one-step or two-step chemical or physical activation or a combinatorial synthesis of the two. Issues in the combinatorics are examined. Advanced techniques such as hydrothermal carbonization/activation and microwave-assisted irradiation are described, which may also involve one-step or two-step procedures. OCDs, including carbon nanotubes, carbon nanofibers, and carbon dots, mainly employ advanced syntheses, for example, nanotechnology. Currently, ACs and OCDs are mainly characterized using advanced techniques like scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), nitrogen (N₂) adsorption, and X-ray diffraction (XRD), which elucidate their structures, properties, and potential efficacies. Their applications for removing water, soil, and air pollutants are evaluated. The highest percent removal efficiency was activation type-specific, reflecting the precursor's nature, synthesis method, product properties, and quality. For instance, the chemical activation of neem leaves by H₃PO₄ activation and Fe nanoparticles from tea extracts were 100% successful for Pb²⁺and Cr⁶⁺, respectively, whereas physical activation of rice husk produced 91.8% success for Cr³⁺. However, their differential adsorptivities for other metals were moderate, with H₃PO₄ activation the lowest. Due to the high cost, tedious processes in producing and restoring ACs, and their non-selectiveness, researchers continually search for suitable alternatives, which this review considers. Also, applying artificial intelligence (AI) techniques in advancing novel ACs and OCDs for environmental remediation is discussed. Finally, future research dimensions are proposed.