Selective Removal of Caffeine and Sulfadimethoxine by Adsorption–Fenton Integrated Treatment Using a Phosphorus-Doped Carbon from Coffee Waste

While aquatic systems frequently contain drug and stimulant compounds, research on their removal under competitive conditions remains limited. In this work, we study the removal of the antibiotic sulfadimethoxine (SDMX) and the stimulant caffeine (CAF) via adsorption and Fenton oxidation in both isolated and simultaneous (one-step) processes. For that purpose, the study has three stages: (I) adsorption, (II) Fenton oxidation, and (III) adsorption + Fenton reaction. First, we evaluate the adsorption capacity of a carbon synthesized from coffee waste, functionalized with P-groups (P-CS). Outcomes showed removal efficiencies of about 90% under isolated conditions, with SDMX exhibiting a higher equilibrium capacity (∼132 mg g–1) relative to CAF (∼42 mg g–1). Kinetic data were well described by a pseudo-first-order model, consistent with a process governed by external film diffusion. The Sips isotherm model provided the best fit for equilibrium data, indicating a heterogeneous adsorption process. Thermodynamic analysis suggests that CAF adsorption was weakly exothermic (ΔH° = −6.78 kJ mol–1), governed by π–π interactions, while SDMX adsorption was endothermic (ΔH° = +23.31 kJ mol–1, ΔS° = +174.77 J mol–1 K–1), with hydrogen bonding interactions with C–P and C–O groups. In the competitive system (SDMX + CAF), SDMX showed better surface affinity, suppressing CAF removal (∼32%) more than SDMX (∼20%). In the Fenton system, a [H2O2]/[Fe2+] ratio of 2 resulted in >96% removal of CAF and >99% of SDMX within 60 min under isolated conditions, and >90% removal of CAF and >98% of SDMX within 60 min under competitive conditions. When adsorption and Fenton oxidation were combined into a single integrated step, both pollutants were reduced below 1 mg L–1 within 60 min, reaching 99.5% (CAF) and 99.9% (SDMX) removal in isolated systems and 94.2% (CAF) and 99.4% (SDMX) removal under competitive conditions. The greater removal of SDMX in the competitive system could be attributed to its stronger surface interaction with the adsorbent. This work demonstrates the efficiency of P-doped carbon in the competitive removal of pollutants via a combined adsorption–Fenton process, highlighting its potential for scalable water treatment.