Mixed scaling patterns and mechanisms of high-pressure nanofiltration in hypersaline wastewater desalination.

• Different scaling patterns and mechanisms by Mg, Si, and organic matter in NF are disclosed. • Mg-Si hydrate, CaF 2 , and CaCO 3 are incipient scalants in hypersaline desalination. • Bulk crystallization and resultant pore blocking are the main mixed scaling mechanisms. • Humic acid enhanced incipient scaling but showed anti-scaling effects on bulk crystallization. • Mg-Si hydrate is a key scalant in dense scaling layer by high pressure and drag force in NF. Nanofiltration (NF) will play a crucial role in salt fractionation and recovery, but the complicated and severe mixed scaling is not yet fully understood. In this work, the mixed scaling patterns and mechanisms of high-pressure NF in zero-liquid discharge (ZLD) scenarios were investigated by disclosing the role of key foulants. The bulk crystallization of CaSO 4 and Mg-Si complexes and the resultant pore blocking and cake formation under high pressure were the main scaling mechanisms in hypersaline desalination. The incipient scalants were Mg-Si hydrates, CaF 2 , CaCO 3 , and CaMg(CO 3) 2. Si deposited by adsorption and polymerization prior to and impeded Ca scaling when Mg was not added, thus pore blocking was the main mechanism. The amorphous Mg-Si hydrates contribute to dense cake formation under high hydraulic pressure and permeate drag force, causing rapid flux decline as Mg was added. Humic acid has a high affinity to Ca2+by complexation, which enhances incipient scaling by adsorption or lowers the energy barrier of nucleation but improves the interconnectivity of the foulants layer and inhibits bulk crystallization due to the chelation and directional adsorption. Bovine serum albumin promotes cake formation due to the low electrostatic repulsion and acts as a cement to particles by adsorption and bridging in bulk. This work fills the research gaps in mixed scaling of NF, which is believed to support the application of ZLD and shed light on scaling in hypersaline/ultra-hypersaline wastewater desalination applications. [Display omitted] [ABSTRACT FROM AUTHOR]