Pyrite has been used in photo-Fenton reactions for the degradation of pollutants, but the application of photo-Fenton processes with extra H2O2 in real water/wastewater treatment has still been limited by the economic cost of H2O2 and artificial light sources. Herein, citric acid (CA) and simulated/natural sunlight are used to develop a pyrite-based photo-Fenton system (pyrite–CA–light) in situ generating H2O2 through the enhanced activation of molecular oxygen. The degradation of pharmaceuticals and personal care products (PPCPs), especially acetaminophen (APAP) as the main target pollutant, in the pyrite–CA–light system was investigated. The effects of influencing factors such as various organic acids, APAP concentration, pH, pyrite dosage, CA concentration and co-existing anions (HCO3−, Cl−, NO3−, SO42− and H2PO4−) were examined. At a pyrite dosage of 0.1 g L−1, CA concentration of 0.6 mM and an initial pH of 6.0, the degradation efficiency of APAP (30 μM) was 99.1% within 30 min under the irradiation of xenon lamp (70 W, λ ≥ 350 nm). Almost the same high efficiency of APAP degradation (93.9%) in the system was achieved under natural sunlight irradiation (ca. 650 W m−2). The scavenging experiments revealed that the dominant active species for degrading APAP was hydroxyl radical (HO•). Moreover, a quantitative structural–activity relationship (QSAR) model for pseudo-first-order rate constants (kobs) was established with a high significance (R2 = 0.932, p = 0.001) by using three descriptors: octanol–water partition coefficient (logKow), dissociation constant (pKa) and highest occupied molecular orbital (HOMO). This work provides an innovative strategy of the photo-Fenton process for the degradation of PPCPs using natural minerals and ordinary carboxylic acid under sunlight.
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