The Structure and Dynamics of the NaOH(aq):Gibbsite Interface

Gibbsite (γ-Al(OH)3) is one of the primary mineral structures containing Al, and its interfacial chemistry in NaOH(aq) solutions is relevant to the industrial processing of aluminum, including metal production and the disposition of Al-containing wastes. To date, little is known about the structure, speciation, and dynamic behavior of gibbsite interfaces with NaOH(aq)—particularly as a function of ionic strength. Yet this is critical in order to understand interfacial reactivity, including dissolution mechanisms and rates that underpin industrial Al processing. This work reports equilibrium molecular dynamics simulations of the γ-Al(OH)3:NaOH(aq) interface, revealing the sorption behavior and speciation of ions from 0.5 – 10 M [NaOH]. As inner-sphere complexes, Na+ primarily coordinates to the side of the gibbsite hexagonal cavities, while OH- accepts hydrogen-bonding from the surface -OH groups. The mobility of inner-sphere Na+ and OH- ions is obviously reduced due to the strong surface affinity. At high [NaOH], ion pairing that is observed in the bulk solution is partially disrupted upon sorption to the gibbsite surface by the individual ion-surface interactions. We anticipate that surface ion-pairing is a key element of the dissolution mechanism and that its modulation (by considering concentration and different ion combinations) may be used to tune gibbsite interfacial chemistry.