Structural Match of Heterogeneously Nucleated Mn(OH)₂(s) Nanoparticles on Quartz under Various pH Conditions

The early nucleation stage of Mn (hydr)oxide on mineral surfaces is crucial to understand its occurrence and the cycling of nutrients in environmental systems. However, there are only limited studies on the heterogeneous nucleation of Mn(OH)₂(s) as the initial stage of Mn (hydr)oxide precipitation. Here, we investigated the effect of pH on the initial nucleation of Mn(OH)₂(s) on quartz. Under various pH conditions of 9.8, 9.9, and 10.1, we analyzed the structural matches between quartz and heterogeneously nucleated Mn(OH)₂(s). The structural matches were calculated by measuring the lateral and vertical dimensions using grazing incidence small angle X-ray scattering and atomic force microscopy (AFM), respectively. We found that a poorer structural match occurred at a higher pH than at a lower pH. The faster nucleation under a higher pH condition accounted for the poorer structural match observed. By fitting the structural match using classical nucleation theory, we also calculated the interfacial energy between Mn(OH)₂(s) and water: γ_nf = 71 ± 7 mJ/m². The calculated m values and γ_nf provided the variance of interfacial energy between quartz and Mn(OH)₂(s): γ_sn = 262-272 mJ/m². This study provides new qualitative and quantitative information on heterogeneous nucleation on an environmentally abundant mineral surface, quartz, and it offers important underpinnings for understanding the fate and transport of trace ions in environmental systems.