The role of gamma-irradiated microplastics in terbuthylazine sorption and desorption processes in contaminated soils

Microplastics (MPs) presence in agricultural soils has raised concerns regarding their interactions with herbicides. This study investigated how gamma-irradiated polyethylene MPs affect the sorption and desorption processes of the herbicide terbuthylazine (TBT) in contaminated soils. Soils were amended with 10 % (w w^-1) MPs irradiated at six gamma doses (0.0, 0.5, 1.0, 2.5, 5.0, and 10 kGy) and treated with five TBT application rates (1.4, 2.8, 4.2, 5.6, and 7.0 kg a.i. ha^-1), in addition to a treatment without MPs (control; unamended soil). Scanning Electron Microscopy (SEM) and Fourier Transform Infrared (FTIR) spectroscopy were used to characterize the MPs before and after irradiation. SEM analyses showed that gamma irradiation progressively enhanced the surface roughness, cracking, and porosity of the MPs. FTIR results confirmed the parallel dose-dependence in carbonyl groups, with the carbonyl index increasing from 0.07 to 0.20 (0.0-10 kGy), demonstrating substantial oxidation of the polymer. Batch equilibrium assays were performed, and TBT concentrations were quantified using high-performance liquid chromatography (HPLC). Sorption and desorption data fitted Linear and Freundlich isotherms, with sorption Linear (K_d) and Freundlich coefficients (K_f) ranging from 0.59 to 1.32 L kg^-1 and 0.43-1.51 mg^(1-1/n) L^1/n Kg^-1, respectively. All irradiation doses increased TBT sorption compared to unamended soil, with MPs exposed to 10 kGy enhancing retention by 13.68 % relative to the control soil and increasing K_d and K_f values. Conversely, desorption remained consistently low (∼5 %) across all treatments, indicating strong retention of TBT in the soil-MPs matrix. These findings highlight the impact of gamma-irradiated MPs on TBT's environmental dynamics, potentially reducing their bioavailability and effectiveness while increasing persistence in the environment.