Molecular insight into the interfacial chemical functionalities regulating heterogeneous calcium-arsenate nucleation

Abstract

Heterogeneous nucleation induced by natural organic matter (NOM) can lower the energy barrier for calcium arsenate (Ca-As) precipitation, which aids in immobilizing arsenate (AsⅤ). However, it remains unclear how certain chemical functionalities of NOM affect Ca-As nucleation at the molecular scale. By analyzing changes in the local supersaturation and/or interfacial energy, the present work investigates the Ca-As heterogeneous nucleation kinetics and mechanisms on functional-group–modified model surfaces.Mica surfaces modified by functional groups of amine (NH2), hydroxyl (OH), or carboxyl (COOH) through self-assembled monolayers were used to investigate how chemical functionalities affect the Ca-As heterogeneous nucleation, in which the distributions of formation kinetics and size (as measured by the change in particle height) of nucleated Ca-As particles were measured by using in situ atomic force microscopy. In a parallel analysis, a quartz-crystal microbalance with dissipation was used to detect the buildup of Ca2+ and/or HAsO42− ions at the solid–fluid interface. PeakForce quantitative nanomechanical mapping and dynamic force spectroscopy using functional-group–modified tips made it possible to calculate the binding energies holding functional groups to Ca-As particles. Nucleated Ca-As particles were characterized by using Raman spectroscopy and high-resolution transmission electron microscopy.The results indicate that the height of amorphous Ca-As particles formed on a modified mica surface may be ranked in descending order as NH2 > OH > bare mica > COOH, as determined by the buildup of Ca2+ and HAsO42− ions at the solid–fluid interface and the decrease of interfacial energy due to the functional groups. These nanoscale observations and molecular-scale determinations improve our understanding of the roles played by chemical functionalities on NOM in immobilizing dissolved As through heterogeneous nucleation in soil and water.