The transition towards sustainable water treatment and resource-efficient material production has increased interest in biomass-derived carbon materials obtained via thermochemical conversion processes. In this study, biochars were produced from selected biowaste precursors, including cherry stones and sunflower husks, using steam and CO2 gasification at 900 °C. The applied approach enables the valorisation of agricultural residues into functional carbon materials with potential applications in environmental protection. The obtained carbons were comprehensively characterised to establish relationships between their structure, composition and adsorption performance. Specific surface area and pore structure were determined using BET analysis, while surface functional groups were identified by Fourier-transform infrared spectroscopy (FTIR). Elemental composition and ash chemistry were analysed, providing insight into the degree of carbonisation and surface chemistry. Additionally, skeletal density was determined using helium pycnometry, allowing for a more detailed assessment of the material structure. The adsorption performance of the obtained carbons was evaluated using model organic contaminants from the phenolic group: phenol and p-cresol. Batch adsorption experiments were carried out under controlled conditions, and the influence of key parameters such as initial concentration and solution pH was assessed. The adsorption behaviour was analysed using commonly applied isotherm models to better understand the nature of the adsorption process and the interactions between adsorbate and adsorbent. The results indicate that both the type of biomass precursor and the activation conditions significantly affect the adsorption efficiency. Materials derived from sunflower husks exhibited enhanced adsorption performance compared to those obtained from cherry stones, which is attributed to differences in pore structure and surface chemistry rather than surface area alone. The adsorption process was found to be governed primarily by physical interactions, with additional contributions from surface functional groups depending on solution conditions. The presented results demonstrate that thermochemically converted biowaste can serve as a promising source of efficient and sustainable adsorbents for water treatment applications, aligning with current challenges in energy and environmental engineering.