The development of sustainable adsorbents from agricultural residues represents a promising approach to combining waste valorisation with the removal of organic contaminants from water. In this study, carbonaceous materials were produced from sunflower husks and cherry stones by gasification at 900 °C under CO2 and CO2–steam atmospheres. The resulting materials were subsequently washed with demineralised water, 1 M HCl, or a mixture of 0.1 M HNO3 and 0.1 M H2SO4 to evaluate the influence of post-treatment on their physicochemical properties and phenol adsorption performance. The obtained carbons were characterised in terms of specific surface area using nitrogen adsorption, ash and moisture contents, and leachates pH. Thermogravimetric analysis demonstrated that both water and acid washing substantially reduced the mineral fraction of most materials. However, the effectiveness of demineralisation depended on the biomass precursor, gasification atmosphere, and washing solution. Phenol adsorption was investigated in batch experiments under controlled conditions. The effect of initial phenol concentration was evaluated, and the final solution pH was monitored. The results showed that adsorption performance was governed by the combined influence of pore structure, surface chemistry, mineral composition, and solution conditions rather than by specific surface area alone. Sunflower-husk-derived materials generally exhibited higher phenol uptake than cherry-stone-derived carbons. Washing modified both the inorganic fraction and surface properties of the adsorbents, thereby affecting their affinity for phenol. In particular, changes in surface pH and mineral content influenced the interactions between phenol molecules and the carbon surface. Overall, the results demonstrate that gasification-derived carbons from agricultural residues can be tailored through simple water or acid washing to obtain materials with reduced ash content and improved properties for phenol removal from aqueous solutions. The study highlights the importance of selecting both appropriate gasification conditions and post-treatment procedures when designing sustainable carbon adsorbents for water treatment applications.