Experimental and theoretical study of multiple active site-functionalized Spirulina residue-based porous carbon as an economical adsorbent for NH3 and SO2 adsorption: Micro- and macro-mechanistic investigations

Abstract

Here, we successfully synthesized a cost-effective (7.88 USD/kg) multi-active site (CuCl2, CuO and KCl) functionalized Spirulina residue-based porous carbon (MMBC). This innovative synthesis method utilizes Spirulina residue, a sustainable and low-cost biomass source, making the production of MMBC economically viable and environmentally friendly. Under ambient conditions, MMBC exhibits competitive adsorption capacities for NH3 and SO2 compared to other adsorbents, reaching 3.01 mmol/g (i.e., 51.26 mg/g) and 0.70 mmol/g (i.e., 44.85 mg/g), respectively, and tolerable recoverability. These values are significant improvements over many existing adsorbents, highlighting MMBC's efficiency in removing toxic industrial chemicals (TICs). Additionally, even at low molar fractions of NH3 or SO2, such as 0.001, MMBC demonstrates significant ideal adsorption solution theory (IAST) selectivity for NH3/N2 and SO2/N2, with values of 12.02 and 4.19, respectively. This high selectivity at low concentrations underscores MMBC's potential for applications in environments with trace amounts of pollutants. Furthermore, extensive research has been conducted in this study to elucidate the microscopic and macroscopic adsorption mechanisms of NH3 and SO2 on MMBC. Specifically, investigations in statistical physics models and statistical thermodynamics reveal that the adsorption of NH3 and SO2 on MMBC adhere to a non-aggregative, multilayer, multi-anchorage, and spontaneous exothermic physicochemical behavior. And the adsorption orientation occurs either horizontally or both horizontally and non-horizontally, contingent upon the system and temperature. These findings provide a deeper understanding of the adsorption processes and provide innovative insights into the adsorption mechanisms at the molecular level and at the macroscopic scale. Besides, the material characterization results reveal that the above adsorption process involves both physical interactions (pore diffusion and van der Waals forces) and chemical interactions (CuCl2 with NH3, CuO and KCl with SO2), indicating a physicochemical adsorption mechanism. This dual mechanism of adsorption offers a comprehensive explanation of the high performance of MMBC. In summary, this study developed a cost-effective Spirulina residue-based adsorbent, exhibiting high removal efficiency and selectivity for TICs, and providing deep insights into adsorption mechanisms. The innovative use of Spirulina residue addresses waste management issues and promotes the development of sustainable materials in the field of environmental remediation, contributing to cleaner production.