The promotion of systems for production of green and low-carbon energy is becoming increasingly critical for the development of countries. Biomass and waste materials are increasingly being utilized as abundant, zero-carbon renewable resources for the production of various by-products. Among these, the conversion of waste into adsorbents and its subsequent use as a low-cost, efficient adsorbent is a topic of growing research, due to its proven ability to effectively remove both organic and inorganic pollutants. Adsorption is one of the most commonly employed techniques for pollutant removal, with phenolic compounds, particularly phenol, serving as widely studied probe molecule over the years. In this context, by-products obtained from gasification and carbonization processes—derived from mixtures of biomass and refuse-derived fuel—were tested for their efficacy in liquid phase adsorption of phenol. Various activation methods were employed to produce the samples used in this study. The adsorption experiments were conducted using UV-Visible spectroscopy, and the results were analyzed through adsorption isotherms. The experimental data were then fitted to the Langmuir and Freundlich isotherm models to evaluate the adsorption capacity and mechanism of the adsorbent material.