• Ab initio calculations reveal that illite(001) exhibits significantly stronger adsorption affinity for As atoms and H3AsO3 molecules than Na-MMT(001), with adsorption energies of −1.94 eV and −1.40 eV, respectively.
• Electron transfer analysis confirms that electrons are transferred from clay mineral surfaces to adsorbed arsenic species, indicating chemisorption and providing mechanistic insights into arsenic retention.
• Arsenic adsorption on both illite and Na-MMT surfaces is energetically more favorable than that of Hg, Cd, and Cr, highlighting the high affinity of these clay minerals for arsenic.
• Illite-rich soils are more susceptible to arsenic contamination than Na-MMT-dominated soils, informing the selection of clay minerals for effective soil remediation strategies.