Original sulfur-doped carbon materials synthesized by coffee grounds for activating persulfate to BPA degradation: The key role of electron transfer

In this work, sulfur-doped (S-doped) carbon materials (SCs) were successfully synthesized by using coffee grounds which are relatively cheap and easily available. Results show that SCs exhibit excellent catalytic activity for persulfate (PS) activation and a more than 99% bisphenol A (BPA) degradation rate was achieved. Meanwhile, 82.3% of total organic carbon (TOC) can be removed in 40 min at 25 °C when applying a dosage of 0.15 g/L SCs and 1 mM PS under an initial pH of about 4.0. SEM and BET characterization methods were used to reveal the surface characteristics of the material. It is found that SCs had a porous structure and a specific surface area of 159.319 m2/g, which conferred to SCs superior adsorption (17.3 mg/g). Additionally, kinetic and adsorption isotherm models for adsorption by SCs were obtained via linear fitting, which proved that the process was better described by the pseudo-second-order model (R2 = 0.9989) and Langmuir model (R2 = 0.9890). The free radical quenching experiments, electron paramagnetic resonance (EPR), and open circuit potential (OCP) tests together demonstrated that the system achieved BPA degradation via non-radical pathways (singlet oxygen and electron transfer). It is speculated that S-doping is able to effectively promote electron transfer given the XPS results. Finally, batch control experiments were run to investigate the influence of PS concentration (0.2 – 2.0 mM), BPA concentration (0.005 – 0.03 mM), SCs dose (0.05 – 0.25 g/L), pH (4.0 – 10.0), and Cl− and HCO3− (1 or 5 mM) on the degradation of BPA. Evidently, the catalysts could be used for a wide range of BPA removal scenarios. In a word, S-doped carbon materials prepared from coffee grounds can improve the degradation of BPA and activation of PS under a wide range of operating conditions. Simultaneously it was strongly associated with electron transfer in non-radical pathways. This research lays the foundation for the rational design of a persulfate-based system for use as an actual water purification catalyst.