<p>Owing to the high solubility of boron, it is prone to&#xa0;migration in aqueous environments, which may consequently affect its geochemical processes in soil systems. To better understand the role of clay minerals and coexisting cations in the adsorption and isotopic fractionation of boron, montmorillonite (Mmt) was selected as a model mineral, and batch adsorption experiments were conducted in aqueous solution to explore the kinetic and equilibrium adsorption behavior of boron in the presence of Ca²⁺. The results showed that the dynamic adsorption process could be well described by the Freundlich isotherm model (<i>R</i>² &gt; 0.8). The isotopic composition of adsorbed boron on Mmt decreased with increasing pH when boron existed predominantly as B(OH)₃, which indicated the preferential enrichment of ¹⁰B on the surface of Mmt. Moreover, the presence of Ca²⁺ enhanced the adsorption of boron and led to more negative δ¹¹B values on Mmt, which suggested the formation of a surface-associated Ca¹⁰B(OH)₄⁺ complex and was further supported by negative zeta potentials measured. These findings provide fundamental insights&#xa0;into the adsorption of boron on clay minerals in aqueous systems and offer a basis for future investigations into the regulation of boron-containing fertilizers and the leaching behavior of boron in different types of soils.</p>