Acetaminophen-Induced Changes in Soil–Plant Nutrient Dynamics: A Radish System Study

Pharmaceutical residues in agricultural substrates may modify contaminant retention and nutrient dynamics, potentially affecting plant growth and nutrient allocation. This study investigated the adsorption of acetaminophen (ACE) onto washed coarse river sand and its effects on substrate physicochemical properties, elemental partitioning, root retention, plant growth, and metal–ACE interactions within the soil–plant continuum. The river sand used as substrate was characterized by a pH of 8.35, electrical conductivity of 0.117 dS m−1, and cation exchange capacity of 20.4 meq 100 g−1. Adsorption experiments were conducted using ACE concentrations from 5 to 180 mg L−1, and kinetic behavior was evaluated using pseudo-first-order, pseudo-second-order, Elovich, and intraparticle-diffusion models. At an initial concentration of 150 mg L−1, approximately 70% of ACE was removed after 60 min, with the pseudo-first-order model providing the best fit (R2 = 0.936; qe = 69.32 mg g−1). Radish plants were subsequently exposed to 32.6 μM ACE, and elemental distributions in the substrate, roots, stems, and leaves were evaluated by XRF. Following ACE exposure, the substrate showed a decrease in pH (−6.2%) and cation exchange capacity (−21.1%), together with marked increases in electrical conductivity (+685.5%), total P2O5 (+42.9%), and K2O (+304.1%). ACE exposure also altered elemental partitioning within the plant: root-to-leaf translocation factors decreased below 1.0 for Fe, K, Ca, Mn, and Zn, while the Root Retention Index increased from 0.18–0.48 under control conditions to 0.65–0.86 following exposure. The highest retention was observed for Fe (0.86) and K (0.82). ACE exposure was also associated with approximately 50% lower shoot and leaf growth over 15 days. Residual ACE analysis quantified the decrease in ACE concentration in the exposure solution during plant exposure. UV–Vis spectroscopy demonstrated interactions between ACE and Fe2+ and Cu2+, while Job’s method established a 2:1 stoichiometric ratio (ACE) for both systems, with no change in stoichiometry between pH 7.83 and 8.35. Overall, the results indicate that ACE exposure was associated with modifications of the chemical environment of the river-sand substrate, altered elemental partitioning and enhanced root retention, and reduced vegetative growth. These findings support a potential role of ACE–metal interactions and nutrient redistribution in the observed plant response, while recognizing that direct phytotoxicity and other physiological or rhizosphere-mediated mechanisms cannot be excluded.