Photothermally-driven pyrolysis of agricultural by-products in NaOH-Na2CO3 molten salt for multifunctional biochar adsorbent: A life cycle assessment for sustainable production

Abstract

Molten salt-assisted photothermal pyrolysis has emerged as a promising approach due to the ability of molten salts to ensure uniform heat distribution and store thermal energy. The thermodynamic properties of these materials, such as thermal stability and high thermal conductivity, facilitate efficient heat transfer and continuous thermal transient, which is typically constrained in concentrated solar pyrolysis. During pyrolysis, the salts undergo phase transition, converting into a liquid at their melting point. This results in the dispersion of biomass particles throughout the salt reaction medium, facilitating efficient heat transfer. At the same time, the biomass can engage with the inorganic constituents present within a molten salt environment. This interaction can result in a distinct catalytic effect, which is contingent upon the chemical composition of the salt. However, despite its indispensable role in practical molten salts pyrolysis technology, the catalytic effect of molten salts on biomass remains poorly understood due to a paucity of information. This study investigated the physicochemical characteristics of biochar produced through molten NaOH-Na2CO3-based photothermal pyrolysis. The results demonstrated that the molten salts promoted the development of a porous structure with increased BET surface area, enhanced aromatic condensation, and enriched the oxygen-containing functional groups on the biochar surface. These modifications of biochar are closely related to improved adsorption performance, which results in biochar being endowed with chemically active sites and enhanced pore fillings. The adsorption capacity of the molten salt-assisted pyrolysed biochar was therefore evaluated against a range of pollutants, including methylene blue, sulfamethazine, and heavy metal ions (Pb(II), Cu(II), and Cd(II)). The molten salt-derived biochar exhibited superior adsorption performance compared to conventional pyrolysed biochar, thereby understanding its versatility as a multifunctional bio-adsorbent. Finally, a life cycle assessment (LCA) was conducted to assess the sustainability of molten salts-assisted photothermal pyrolysis, using the experimental adsorption data as a basis. This study offers novel insights into the potential of molten salt-based photothermal pyrolysis as a sustainable alternative to conventional pyrolysis, enhancing adsorption performance and evaluating its sustainability through a life cycle assessment.