Transformation from PAHs to oxygenated PAHs: progressive redistribution and accumulation of pollutants in the soil–rice system

Purpose: Polycyclic aromatic hydrocarbons (PAHs) and their derivatives, oxidized polycyclic aromatic hydrocarbons (OPAHs), migrate and transform within the soil–rice system, posing risks to food safety and human health. This study investigated the variations in PAH and OPAH concentrations in paddy soil and rice tissues at different growth stages, to provide insights into mitigating the ecological risks of organic pollutants in the soil–rice system. Materials and methods: In this study, a rice pot experiment was conducted, and the concentrations of 16 PAHs and 11 OPAHs in soil and rice tissues were quantified using gas chromatography–mass spectrometry. Soil enzyme activities were measured to explore their response to and influence on PAHs and OPAHs. The concentration factor and transfer factor were used to measure the ability of rice to absorb pollutants. A correlation matrix analysis was performed to investigate the correlations between PAHs, OPAHs, and soil enzyme activities. Results: Rice cultivation significantly increased the degradation of PAHs and promoted their transformation into OPAHs in soil. In rice tissues, PAH and OPAH concentrations decreased sequentially from the roots to the stems, leaves, and grains. Low-molecular-weight PAHs and OPAHs were dominant in rice tissues. The addition of PAHs increased soil enzyme activities. Conclusions: Our results demonstrated that rice had significant effects on the migration and transformation of PAHs and OPAHs in soil, including a synergistic effect with soil enzymes to transform soil PAHs into OPAHs and to absorb and enrich both PAHs and OPAHs. The effects of soil enzymes on different PAH and OPAH congeners were variable. The environmental risks of OPAHs should not be ignored when plants are used for in situ remediation of PAH-contaminated soil.