<p>Zinc chloride-modified (PP-Z), potassium hydroxide-modified (PP-K), and zinc chloride/potassium hydroxide composite-modified (PP-ZK) biochars were prepared from pomelo peels for the adsorption of Congo red (CR). The pyrolytic properties, specific surface area, surface morphology, and atomic ratio of the biochar were obtained via characterization techniques including TGA, EA, SEM, BET, XRD, and FT-IR, and the effects of pH and adsorption time on the adsorption of CR were investigated by adsorption tests. The results showed that the specific surface area of biochar PP-ZK reached 1393.29 m²/g, with good porosity and rich surface functional groups (-COOH, -OH, etc.). The adsorption of CR by PP-ZK under the same conditions reached 71.6&#xa0;mg/g (2.93 times higher than that of the unactivated biochar), and the removal rate of CR was 94.7% at an initial CR concentration of 300&#xa0;mg/L, pH 6, and an adsorption dosage of 2&#xa0;g/L. The adsorption of CR by PP-ZK was more in line with the Langmuir isotherm model and the pseudo-second-order kinetic model, and the adsorption of CR was more in line with the Langmuir isotherm model. The adsorption process of PP-ZK on CR was more in line with the Langmuir isotherm model and quasi-secondary kinetic model, which was monomolecular layer adsorption, and subject to the combined effect of physical adsorption and chemisorption. Moreover, the adsorption of CR was a reversible and spontaneous heat absorption process, and the warming up could help the adsorption. These results confirm that the composite-modified biochar PP-ZK is an effective adsorbent for CR removal from aqueous solutions.</p> Graphical abstract <p></p>