Role of solute chemistry on membrane crystallisation of inorganic salts

Membrane distillation crystallisation (MDCr) is a process developed to recover valuable salts from salt-rich solutions like sea water, brackish water, waste and others for the promotion of zero liquid discharge (ZLD). The chemistry of the crystallised salts is characterised by their different crystal-liquid interfacial energy derived from their solubility. Various studies have evidenced the importance of the interfacial energy in the nucleation process through Gibbs energy but it has never previously used to determine the scaling, nucleation and crystal growth kinetics of membrane distillation crystallisation. Using a combination of a backscatter technique and digital microscopy to detect bulk and scaling induction, this work provides a precise determination of scaling and bulk induction and metastable zone width (MSZW), resulting into a critical differentiation of heterogeneous adhesive growth for low interfacial energy salt and homogeneous bulk deposition for high interfacial energy growth. After understanding that the membrane is not playing as much role as previously thought in MDCr for high interfacial energy salts, another crucial component of the chemistry of the salt and its solubility, is the solubility–temperature dependence. It was found that even in low interfacial energy salts, the dominant factor of scaling is temperature polarisation and not heterogeneous nucleation as previously assumed. To confirm the less critical role of the membrane in MDCr, three membranes comprised of distinctive properties were compare through the use of a neutral solubility–temperature dependent salt. The results demonstrate how both scaling and crystal growth are kinetically controlled rather than thermodynamically dependent upon the material properties to initiate nucleation. To enhance the dependency of the membrane for nucleation within a kinetically controlled environment, K₂SO₄ was studied which exhibits a sharp positive solubility–temperature dependency. The modification of the solubility limit due to temperature polarisation, modifies the thermodynamic barrier of the MSZW which increase scaling, the extent of which is evidenced to directly inform the nucleation kinetics within the bulk solution. This thesis collectively describes and relates solute chemistry to both scaling and nucleation, enabling enhanced crystallisation strategies to be developed for MDC.