Biochar as novel medium for the Adsorption and Desorption of Emerging Pollutants

The experimental part of this work has been conducted at ICFAR, Institute of Institute for Chemicals and Fuels from Alternative Resources of the Western University (Ontario, Canada) from September 2022 to February 2023, thanks to the economic support of Politecnico di Torino under a “thesis on proposal” grant. Pharmaceuticals and dyes are relatively new toxic and hazardous contaminants, defined “emerging pollutants”, (EPs), in the aquatic ecosystems where they can cause detrimental effects even at minute concentrations. The novelty of their use poses a limit to the efficiency of state-of-the-art water and wastewater treatment technologies, which are insufficient for their effective removal. This thesis explored the removal of amoxicillin (AMO), tetracycline (TC), methylene blue (MB) and methyl orange (MO) from water through the adsorption on biochar (BC). BC is described by literature as sustainable and cost-effective sorbent due to the wide availability of parent biomasses, high surface area, developed pore structure and low cost. The experimental activities involved consequent phases: 1. Production of BC through pyrolysis at 450-500 °C in a lab-scale mechanically fluidized reactor of birchwood residues; 2. Selection of a standard BC, derived from miscanthus straw residues, from the UK Biochar Research Centre of the University of Edinburgh; 3. Activation of the BCs with CO2 at 900°C - to achieve larger specific surface area and higher porosity; 4. Optimization of the adsorption process parameters (contact time, solid-to-liquid ratio, pH) for AMO, TC, MB and MO on activated birchwood char (ABWC) and the activated miscanthus straw char (AMIS); 5. Adsorption tests in batch mode at 25°C and data analysis - investigation of isotherm models and kinetics; 6. Desorption tests via CO2 Supercritical Fluid Extraction and data analysis (only for MO with ABWC and AMIS). The four analytes were analyzed via UV-VIS spectrophotometry at wavelength equal to: 230 nm for AMO, 355 nm for TC, 454 nm for MO, and 668 nm for MB. The maximum adsorption capacity was found for solid-to-liquid ratio values equal to 50 mg/50 mL and 25 mg/50 mL mg for dyes and antibiotics, respectively. About the contact time, equilibrium was reached after 1 hour for AMO and TC, and after 17 hours for MB and MO. The adsorption of MO (anionic compound) was favoured in acidic conditions (2.9) and dominated by electron donor-acceptor (EDA) interaction and physical interactions in the π - π form. AMIS biochar exhibited the best adsorption performance for all the considered analytes: 432.47 mg/g for MO, 425.25 mg/g for MB, 253.23 mg/g and 167,94 mg/g for TC. The maximum adsorption capacity values obtained from ABWC were: 320.03 mg/g for MO, 164.09 mg/g for MB, 123.72 mg/g AMO and 158.74 mg/g for TC. A literature screening on the adsorption of these type of analytes, by GAC show the following results: 0.55 mg/g for AMO, 9.20 mg/g for TC, 21.5 mg/g for MB and 46.6 mg/g for MO confirming that biochar can be considered a novel and efficient medium for ECs adsorption. For all the four analytes, the best fitting of the experimental data was obtained with Freundlich isotherm models which consider heterogeneity in the distribution of active sites and their energies involved, even though the Langmuir model also agreed with the experimental data well. The kinetics were instead well-fitted by the pseudo-second order model, indicating that the reaction is more inclined towards chemisorption.