Transport mechanisms of the anthropogenic contaminant sulfamethoxazole in volcanic ash soils at equilibrium pH evaluated using the HYDRUS-1D model

The volcanic soils in Chile, where a significant portion of agricultural activities take place, are impacted by the presence of veterinary drugs, including sulfamethoxazole (SMX). The study examines how different soil types influence the movement and retention of sulfamethoxazole (SMX) across four regions of Chile, focusing on conditions at a neutral pH of 7.0. Collipulli's Ultisol soils (CLL), characterized by high clay and sand content but low organic matter (OM), promote low SMX adsorption and rapid transport. In contrast, the volcanic ash-derived Andisols from Frutillar (FRU), Nueva Braunau (NBR), and Osorno (OSR) have high OM and cation exchange capacity (CEC), which enhance their ability to retain SMX and reduce its mobility. Adsorption batch, kinetics, and column breakthrough curve (BTC) experiments were conducted alongside transport modelling. The adsorption kinetics of SMX in CLL soil followed a pseudo-first-order (PFO) model, while FRU, NBR, and OSR soils aligned with a pseudo-second-order (PSO) model. Freundlich isotherms effectively described SMX adsorption in CLL and OSR soils, indicating multilayer adsorption, while Langmuir isotherms fit the FRU and NBR soils, suggesting monolayer adsorption. Using HYDRUS-1D software, we simulated SMX transport in soil columns. BTCs were best modelled using a two-site sorption model with both equilibrium and kinetic adsorption. SMX was more mobile in CLL soil due to its lower organic matter (OM) content and adsorption capacity. In contrast, FRU, NBR, and OSR soils showed slower transport, reflecting higher OM content and greater adsorption capacity, reducing SMX leaching. These findings emphasize the importance of soil properties, such as OM content, in influencing SMX behavior, and are vital for assessing environmental impacts and developing mitigation strategies.