In this research, I focused on elucidating the crystallisation kinetics and formation mechanism of calcium sulfate phases, specifically gypsum, in the presence and absence of additives as inhibitors. In many industries such as oil production and water desalination, calcium sulfate formation is a problem that causes pipeline and membrane clogging and reduces system efficiency. Thus, different types of additives are added to these systems as inhibitors to tackle the scaling problem. However, their efficiency or effectiveness in terms of calcium sulfate inhibition has not been fully tested and the processes are not well-understood at the mechanistic level. In this thesis, therefore, I investigated the effects of various carboxylic acids, alkali / alkaline earth metal cations, polycarboxylic acids and phosphonates as gypsum inhibitors to fill this knowledge gap.
My results revealed that all additives delayed the crystallisation of gypsum to various degrees and by various pathways. I monitored the change in the time needed for turbidity in a reacting solution to start (induction time) and develop (crystallisation kinetics). I analysed the changes in both solution chemistry and solid characteristics, including surface properties, morphologies as well as composition, to derive a mechanistic understanding of how these additives affect gypsum formation.
The results illustrated that among the tested carboxylic acids (tartaric, maleic and citric), citric acid performed far better than tartaric and maleic (at equivalent concentrations) and using citric acid dramatically increased the induction time (~ 4 fold, to ~ 25 minutes).
Among the tested alkali / alkaline earth metal cations (Li+, K+, Na+ and Mg2+), Mg2+ decreased the nucleation and growth kinetics ~ 5 to ~ 10 fold more than Li+, Na+ and K+ even at low concentrations. Mg2+, Li+ and K+ only adsorbed to the gypsum crystals surfaces, while ~ 25% of associated Na+ became incorporated into the synthesised crystals and Li+ and Mg2+ also acted as shape and size modifiers.
When I tested the effects of biodegradable polycarboxylic additives (polyaspartic acid; PASP and polyepoxysuccinic acid; PESA) and compared their efficiency with a traditionally used non-biodegradable (polyacrylic acid; PAA) antiscalant, I showed that PASP and PESA had a profound effect on gypsum crystallisation, with PASP having the highest inhibition efficiency. The PAA conformation and molecular weight both played important roles in affecting the crystallisation kinetics because of changes in surface adsorption mechanisms.
Finally, I tested some industrial phosphonate inhibitors and demonstrated that they are indeed strong gypsum inhibitors, but I also showed for the first time that an increase in the number of functional groups affected the efficiency of the additive. Among the tested phosphonates with five phosphonate functional groups, the one with longer molecular chains was the better inhibitor. It is unclear how these additives interacted with the growing gypsum crystals (surface adsorption and / or structural incorporation), but I clearly showed that they affected gypsum growth kinetics and morphologies.
