Revealing the brick-laying process of foulants layer in membrane distillation for desulfurization wastewater treatment: Insight into the mineral scaling

Mineral scaling is one key obstacle to membrane distillation (MD) in hypersaline wastewater desalination, but the scaling and fouling mechanism are poorly understood. Addressing this challenge required revealing the foulants layer formation process. In this study, MD was applied for flue gas desulfurization wastewater treatment with a plug flow module, aims at unfolding the foulants accumulation process into three dimensions (3D) space as the flow velocity had significant influence on foulants distribution. Hence, validated offline method was used to detect the formation of foulants layer by SEM-EDS-FTIR-XPS. Mineral scaling was the key fouling mechanism, and the fouling tendency increased along the flow pass by brick-laying process. Hexagonal CaSO4, primarily in the form of α‑calcium sulfate hemihydrate, and lamellate Mg-Si complexes are the leading foulants, acting as “bricks” during the layer-by-layer manufacturing process of scalants. The pioneering foulant, structured by organic matters and Mg-Si complexes, was the key incipient foulant and acted as “concrete”. Heterogeneous nucleation is the primary crystallization mechanism for CaSO4, whereas Mg-Si complexes was formed in bulk. The “brick-laying” process sheds light on the distribution and formation of mineral scaling and membrane fouling in MD, which can be extended to other membrane desalination processes.