Adsorption/Desorption Behaviour of the Fungicide Cymoxanil in Acidic Agricultural Soils

This study investigates the adsorption/desorption behaviour of the widely used fungicide cymoxanil in twelve acidic agricultural soils, providing the first comprehensive assessment of its retention dynamics. Cymoxanil exhibited low adsorption, with Kd(ads) values ranging from 0.57 to 4.40 L kg−1 and adsorption percentages between 18.7 and 65.9% at the highest tested concentration, suggesting high mobility and bioavailability in soils and, consequently, a potential environmental and human health risk. Hysteresis was observed, with desorption percentages for the highest initial concentration ranging from 2.4% to 32.6%, indicating that part of the adsorbed compound remained relatively strongly retained. Adsorption was positively correlated with desorption parameters, reflecting a statistical association whereby soils with higher adsorption tended to exhibit lower desorption. Among soil physicochemical properties, pH appeared to play a key role in controlling cymoxanil retention, as higher pH was associated with greater adsorption and lower desorption in the studied soils. Organic matter, clay content, and exchangeable base cations also appeared to influence cymoxanil behaviour, although to a lesser extent than pH. In this regard, soils richer in organic matter and clay, and with higher effective cation exchange capacity (eCEC), tended to display greater retention. Overall, cymoxanil adsorption appears to be largely governed by physisorption mechanisms—electrostatic interactions, cation exchange, and hydrophobic partitioning—while the observed hysteresis suggests that specific interactions, such as hydrogen bonding and π-π interactions, may also contribute to retention without implying irreversible chemisorption.