Insight into citric acid-enhanced abiotic degradation of benzo[a]pyrene: transformation of iron species and hydroxyl radical production

Abstract

Low-molecular-weight organic acids (LMWOAs) are known to influence organic pollutant transformation in soil. However, the abiotic degradation of pollutants in natural soils amended with LMWOAs remains poorly understood. This study evaluated how LMWOAs enhanced the abiotic degradation of benzo[a]pyrene (BaP) in soil. The results showed that citric acid (CA) significantly increased BaP degradation, with a degradation rate that is 2.51-fold higher than that of the control group. Electron paramagnetic resonance (EPR) spectroscopy and quenching experiments demonstrated that CA promoted the generation of hydroxyl radicals (˙OH) and superoxide radicals (O₂˙⁻), with ˙OH production in the CA group being 1.89-fold higher than that in the control group. Mechanism investigation further revealed that CA could decrease the soil pH to enhance BaP degradation in soil. In addition, CA consumed more dissolved Fe(II) (Fe(II)_dis) than adsorbed Fe(II) (Fe(II)_ad) in BaP-contaminated soil. The cyclic voltammetry test indicated that CA could chelate Fe(II)_dis to form Fe(II)_dis–CA and reduce the standard electrode potential of Fe(III)/Fe(II), thereby accelerating the oxidation of Fe(II)_dis. Based on the results of mass spectrometry analysis, two potential degradation pathways of BaP were proposed. The toxicity of BaP degradation intermediates, identified using ECOSAR and TEST software, was lower than that of BaP. This study provides valuable insights for the LMWOA-enhanced abiotic degradation of PAHs in soils.