LONG-TERM ENVIRONMENTAL STABILITY OF HYDROTALCITE PRECIPITATED IN INDUSTRIAL SYSTEMS – A MOLECULAR MODELLING APPROACH

Chi, L., Murthy, V. and Smith, H.D.

Hydrotalcite is a layered double hydroxide that precipitates spontaneously when seawater is used to neutralise wastewater from the Bayer process prior to disposal; however, ability to retain intercalated anions in the marine environment remains largely unstudied. This paper reports preliminary results obtained from novel computational molecular models being developed to examine hydrogen bonding and electrostatic forces within the molecular matrix of hydrotalcite; and give insight into its long-term environmental stability.

During in-situ precipitation, anionic species that may be harmful to or accumulate in biota become entrained in hydrotalcite via a number of bonding mechanisms, the most common of which is held to be intercalation. Modeling of intercalation suggests that a structured environment within the interlamellar space that hold anions principally by hydrogen bonding with either water molecules or terminal –OH groups of the Mg/Al lamellae forms. Electrostatic forces between anions and the metal hydroxide layers also exist, but the strength of these bonds is pH dependent and anion specific.

Results show that while the most of anions are strongly held by hydrotalcite, leaching and ion-exchange of less strongly bonded anions may still occur. This suggests that the long-term environmental stability of hydrotalcite may not be as great as previously thought. The models show potential as useful predictive tools for other hydrotalcite bonded anionic species not investigated here, although some refinements may be required to provide closer correlation with results measured for precipitates prepared under industrial conditions.