Production of biochar and activated carbons from macroalgae biomass for the adsorption of water contaminants

This study aims to valorise the invasive brown alga Sargassum muticum, collected in the Portuguese coast, by converting it into biochar (BC) and activated carbon (AC), which can serve as biosorbents for removing contaminants from water. Effectively managing invasive alien species has become a pressing concern, as they pose significant threats to biodiversity, ecosystems, local economies, and human health in the affected regions. The primary objective is to assess the effectiveness of Sargassum-based adsorbents in removing VLF, the pharmaceutical compound of interest, as well as three different metals: Cr, Li, and Nd. For the metal tests, Nd was selected, as the produced carbon materials demonstrated the highest efficiency in removing this ion. The presence of VLF, a widely used antidepressant, in aquatic environments is of increasing concern due to its persistence and biological activity despite human biotransformation. Nd is a rare earth element essential to produce permanent magnets, holds high economic value, and poses a supply risk. The application of Sargassum-based biosorbents explored in the present study has not been previously reported, making it a novel contribution to the field. The BC was produced using a lab-scale pyrolysis installation at 800 °C for 2 hours, while ACs were prepared using chemical activation with K2CO3 and H3PO4 or physical activation with CO2. Several techniques, including CHNS analysis, TGA, N2 adsorption/desorption, pHPZC, SEM-EDS and FTIR were employed to characterize the physicochemical properties of the carbonaceous materials. The SM-K2CO3 sample exhibited a predominantly microporous and mesoporous structure (48% and 26%, respectively) with a high specific surface area of 1401 m²/g. In contrast, the SM-CO2-30min sample had a lower specific surface area of 209 m²/g, with few micropores. However, its high ash content (66.24%) proved beneficial for Nd adsorption. For comparison, a commercial activated carbon with a large specific surface area of 1030 m²/g was also analysed. Through batch adsorption experiments, the materials demonstrated exceptional adsorption capacities, surpassing the commercial adsorbent. The concentrations were determined using ICP-AES for Nd and UV-Vis spectroscopy for VLF, with the organic compound's maximum absorbance observed at 225.12 nm. SM-K2CO3 AC demonstrated the highest VLF removal efficiency equal to 73.44%, likely due to π-π electron-donor-acceptor (EDA) interactions or hydrogen bonding between VLF's aromatic rings and the carbon's surface functional groups. SM-CO2-30min AC excelled in removing Nd, with a percentage removal of 63.61%, potentially due to an ion exchange mechanism. Adsorption capacities of up to 217.1 mg/g for SM-K2CO3 AC and 204.9 mg/g for SM-CO2-30min AC were obtained. Kinetic studies revealed that the pseudo second-order model best fits the experimental data for both Nd and VLF, suggesting that the adsorption process is chemisorptive. Equilibrium data followed the Freundlich isotherm for both adsorbates, indicating multilayer adsorption on the adsorbent surfaces. Regeneration studies were conducted, but further research is needed to optimize these processes. In conclusion, the use of Sargassum-derived materials not only supports the principles of the circular economy but also offers a promising solution for the remediation of contaminated water resources.