Mixed scaling deconstruction in vacuum membrane distillation for desulfurization wastewater treatment by a cascade strategy

Abstract

Mineral scaling is one key obstacle to membrane distillation in hypersaline wastewater desalination, but the scaling or fouling mechanism is poorly understood. Addressing this challenge required revealing the foulants layer formation process. In this work, the scaling process was deconstructed with a cascade strategy by stepwise changing the composition of the synthetic desulfurization wastewater. The flux decline curves presented a 3-stage mode in vacuum membrane distillation (VMD). Heterogeneous nucleation of CaMg(CO₃)₂, CaF₂, and CaCO₃ was the main incipient scaling mechanism. Mg-Si complex was the leading foulant in 2nd-stage, during which the scaling mechanism shifted from surface to bulk crystallization. The flux decreased sharply for the formation of a thick and compacted scaling layer by the bricklaying of CaSO₄ and Mg-Si-BSA complexes in the 3rd-stage. Bulk crystallization was identified as the key scaling mechanism in VMD for the high salinity and concentration multiple. The organic matter had an anti-scaling effect by changing the bulk crystallization. Humic acids (HA) and colloidal silica also contributed to incipient scaling for the high affinity to membrane, bovine serum albumin (BSA) acting as the cement of Mg-Si complexes. Mg altered the Si scaling from polymerization to Mg-Si complex formation, which significantly influence the mixed scaling mechanism. This work deconstructed the mixed scaling process and illuminated the role of main foulants, filling in the knowledge gap on the mixed scaling mechanism in VMD for hypersaline wastewater treatment and recovery.

Keywords: Bricklaying; Bulk crystallization; Desulfurization wastewater; Hypersaline wastewater; Membrane distillation; Mixed scaling.