<p>Water contamination by dyes and anionic pollutants poses a significant global challenge. Among various organic dyes, azo dyes with azo groups are particularly hazardous. The elimination of methyl orange (MO) dye, nitrate, and phosphate by formaldehyde-modified chitosan is examined in this work, and the adsorbent was characterized by FTIR, SEM, TGA, and XRD spectroscopy. The effects of pH, amount of adsorbent, adsorption time, initial&#xa0;dye or anion concentration, and temperature were assessed on adsorption by batch studies. Under optimal conditions (pH 7 and 30 °C), modified chitosan achieved maximum removal efficiencies of 90.12% for MO, 76.19% for NO₃⁻¹, and 94.49% for PO₄⁻³. Results demonstrated rapid adsorption by the modified chitosan, which was 68.39, 54.83, and 50.44% for MO, NO₃⁻¹, and PO₄⁻³, respectively, within the first 10 min only at room temperature. As the temperature rose, the adsorbent’s capacity decreased. According to kinetic investigations, the pseudo-second-order model best describes the adsorption processes. The values of the isotherm parameters were&#xa0;determined using the Freundlich, Dubinin–Radushkevich, and Langmuir isotherms. Equilibrium data were well-fitted by the Langmuir isotherm. The low mean values of adsorption energy (0.909 to 4.463 kJ/mol) demonstrated the physisorption technique. Thermodynamic analysis revealed the adsorption to be exothermic (-∆<i>H</i>°), spontaneous (-∆<i>G</i>°), highly ordered (-∆<i>S</i>°), and feasible under the investigated conditions. Furthermore, the modified chitosan exhibited good reusability with adsorption–desorption capabilities over four cycles. This paper discusses the demonstration of modified chitosan as a profitable and efficient adsorbent for eliminating MO dye, nitrate, and phosphate from wastewater.</p> Graphical abstract <p></p>