Membrane distillation operated at high seawater concentration factors: Role of the membrane on CaCO3 scaling in presence of humic acid

This study aims at investigating the kinetics of calcium carbonate precipitation (scaling), that occurs in the form of vaterite, when treating seawater by direct contact membrane distillation (DCMD) operated at high concentration factors (from 4 to 6). Induction time measurements carried out by dynamic light scattering (DLS) allowed to identify the shifting between homogeneous and heterogeneous nucleation mechanisms as a function of supersaturation. CaCO3 interfacial energy, evaluated for concentrated seawater solutions as 45 mJ/m2, increased by 7% as a consequence of the inhibition effect of humic acid, and it was reduced to 32 mJ/m2 in correspondence of heterogeneous nucleation occurring on microporous polypropylene membranes. Gibbs free energy barrier to the formation of critical nuclei was predicted with good accuracy as a function of physico-chemical properties of the membrane (porosity: 0.70, contact angle: 115 ± 2°). DCMD tests, carried out under a temperature gradient of ∼20 °C (feed temperature: 40 °C), showed a progressive deterioration of the system performance, and the initial distillate flux (2.05 ± 0.05 l/m2h) was reduced by 45% after 35 h of uninterrupted operation. Two-step cleaning with citric acid aqueous solution (20 min)/NaOH aqueous solution (20 min) allowed to completely restore the transmembrane flux and the hydrophobicity of the membrane.