Bayesian and unsupervised learning insights into pH- and temperature-driven sorption of fluoroquinolones and sulfonamides on marine algal biochar

Marine macroalgae are rich in nitrogen and oxygen functional groups, making their pyrolysis products promising adsorbents for antibiotic removal. This study investigated the brown algae biochar (SW400, SW600) for its ability to remove fluoroquinolones (FQs) and sulfamethoxazole (SMX) from water under various pH levels (3-10) and temperatures (20-32 °C). Nonlinear adsorption constants and thermodynamic parameters were estimated using Bayesian inference, while unsupervised pattern recognition revealed different adsorption mechanisms. Time-dependent adsorption was quantified via batch kinetic and continuous-flow fixed-bed breakthrough experiments, interpreted using six independent nonlinear models. Results found that SW400 maintained high concentrations of total phenolic and carboxylic groups (∼2.21 mmol g^-1), while SW600 exhibited a fivefold reduction in these functional groups. As the pH shifts from 3 to 7, the sorption FQs transitions from cation exchange and cation-π interactions to π-π EDA interactions, corresponding with a reduction in cationic species distribution. Hydrogen bonding is the primary mechanism for SMX sorption, with SW400 sorption surpassing biochar derived from land-based biomass by 7-26 times. Sorption is characterized as an endothermic and entropy-driven process. The sorption kinetics were primarily affected by the ionization, steric structure, and hydration state of the antibiotics rather than the properties of the biochar types. In continuous-flow fixed-bed columns, the kinetic sorption was predicted using the Chu-Hashim model and aligned closely with batch kinetics. Our findings demonstrate that seaweed biochar provides an environmentally sustainable method for the removal of antibiotics, thereby contributing to the advancement of Sustainable Development Goals 6, 7, 12, and 14 b.