The feasibility of activated carbon derived from waste seaweed via molten salt activation in a eutectic mixture of ZnCl₂-NaCl-KCl for adsorption of anionic dyes

A recent development in activated carbon research is molten salt activation involving the combined use of activating agents. Preceding research indicated that a 60:20:20 mol% eutectic mixture of ZnCl₂-KCl-NaCl as activating agents on pinewood holds several advantages over pure ZnCl₂. Especially noteworthy was the characteristic pore development in the mesopores with the eutectic. Nevertheless, biomass precursors for activated carbon require sustainability and abundancy. Furthermore, a uniform product with similar porosity is desired. For this instance, this study investigates whether it is possible to generate activated carbon from waste feedstocks with a completely different biocomposition compared to pinewood such as macroalgae and if the employed ZnCl₂-KCl-NaCl eutectic stimulates the same pore development. Brown seaweed (Sargassum horneri) and green seaweed (Ulva pertusa) were treated with a 60:20:20 mol% mixture of ZnCl₂-KCl-NaCl as an activating agent at 400 and 600 °C. Characteristics of obtained carbons were determined via elemental and proximate analysis, FTIR, gas adsorption (N₂, CO₂) and chemical adsorption of iodine and several dyes (methylene blue, methyl orange, rose bengal and brilliant blue FCF). All ZnCl₂-KCl-NaCl treated samples treated obtained a tailored pore size distribution with a sharp distinctive peak around 22 nm, which is analogous to previous research with pinewood. This showcases the usefulness of eutectic salt mixtures as activating agents for tailor-made activated carbons. The best results were obtained at 400 °C with green seaweed; the surface area amounted to 504 m²/g with a yield of 36.1 wt% and the highest adsorption capacity of 175.7 mg/g with methyl orange.