Photodegradation behavior, toxicity assessment, and machine learning model of liquid crystal monomers on soil...

Liquid crystal monomers (LCMs) are proposed as persistent, bioaccumulative, and toxic emerging contaminants of global concern; however, their transformation behavior and associated risk in soil have rarely been reported. This study investigates the photodegradation kinetics, mechanisms, and toxicity evolution of a representative terphenyl LCM (4''-ethyl-2',3,4,5-tetrafluoro-1,1':4',1''-terphenyl, 2teFT) on soil minerals (SiO₂ and goethite), and 8 biphenyl/triphenyl LCMs on soil. The photolysis rate constants (k_obs) of 2teFT in various systems followed the order k_obs,SiO2 > k_obs,goethite > k_obs,soils, and triphenyl LCMs possessed larger k_obs than biphenyl LCMs. The reduced photo-degradability in soil mainly stems from lower light transmittance and organic matter's light-shielding effects. A machine learning model integrating the soil composition and molecular parameters was further developed to predict k_obs of pollutants including LCMs on soil. The model exhibits satisfactory goodness-of-fit (R²_tr = 0.945, RMSE_tr = 0.277, MAE_tr = 0.206), predictive performance (R²_test = 0.786, RMSE_test = 0.442, MAE_test = 0.353), robustness and generalization ability (Q²_cv = 0.701). Dipole moment, carbon atomic charge, and light sources are the important factors on k_obs of pollutant on soil. Mechanistic insights demonstrated the reactive oxygen species (∙OH, ∙O₂^- and ¹O₂) were responsible for the degradation of diphenyl/terphenyl LCMs, with the main photodegradation pathways of 2teFT being defluorination, oxidation, and ring cleavage. Critically, some degradation products retaining diphenyl/terphenyl structures exhibit toxicity comparable to parent compound 2teFT, highlighting their ongoing environmental risks. These results are helpful for understanding the fate and risk of LCMs in soil.