Membrane distillation of a silver leaching solution: Role of the coexisting aluminum ions on silica scaling

Advanced membrane-based high-value metal recovery from wastewater

Considering the circular economy and environmental protection, sustainable recovery of high-value metals from wastewater has become a prominent concern. Unlike conventional methods featuring extensive chemicals or energy consumption, membrane separation technology plays a crucial role in facilitating the sustainable and efficient recovery of valuable metals from wastewater due to its attractive features. In this review, we first briefly summarize the sustainable supply chain and significance of sustainable recovery of aqueous high-value metals. Then, we review the most recent advances and application potential in promising state-of-the-art membrane-based technologies for recovery of high-value metals (silver, gold, rhenium, platinum, ruthenium, palladium, iridium, osmium, and rhodium) from wastewater effluents. In particular, pressure-based membranes, liquid membranes, membrane distillation, forward osmosis, electrodialysis and membrane-based hybrid technologies and their mechanism of high-value metal recovery is thoroughly discussed. Then, engineering application and economic sustainability are also discussed for membrane-based high-value metal recovery. The review finally concludes with a critical and insightful overview of the techno-economic viability and future research direction of membrane technologies for efficient high-value metal recovery from wastewater.

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

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.

Membrane fouling behaviors in a full-scale zero liquid discharge system for cold-rolling wastewater brine treatment: A comprehensive analysis on multiple membrane processes

The challenge of water scarcity drives zero liquid discharge (ZLD) treatment to maximize reuse of industrial wastewater. Deciphering the characteristics and mechanisms of membrane fouling in the membrane-based ZLD system is crucial for the development of effective fouling control strategies. However, current studies only focused on the membrane fouling of single step, lacking in-depth understanding on the ZLD systems using multiple membrane processes. Herein, membrane fouling characteristics and mechanisms in a full-scale ZLD system for cold-rolling wastewater brine treatment were investigated via a comprehensive analysis on multiple nanofiltration (NF) and reverse osmosis (RO) membrane processes. The membrane fouling behaviors showed distinct characteristics along the wastewater flow direction in the ZLD system. Increasing amounts of foulants were deposited on the membrane surfaces with the sequence of the 1st pass RO, 1st stage NF, and 2nd stage NF processes. The organic fouling and silica scaling were more intensive in the 1st stage NF and 2nd stage NF for treating the brine of the 1st pass RO, as the foulants were rejected and concentrated by previous membrane processes. Severe inorganic fouling, containing amorphous SiO₂, Al₂O₃, and Al₂SiO₅, occurred on the membrane surface of the 2nd pass RO membrane, due to the recirculated high-concentration silica, high water recovery, and concentration polarization. For the 3rd pass RO process, both the amounts of organic and inorganic foulants decreased dramatically, due to the low foulant concentration in its influent. This work provides a comprehensive understanding of membrane fouling in a membrane-based ZLD system, facilitating the development of membrane fouling control strategies for multiple membrane processes.

Joule-Heated Layered Double Hydroxide Sponge for Rapid Removal of Silica from Water

Dissolved silica is a major concern for a variety of industrial processes owing to its tendency to form complex scales that severely deteriorate system performance. In this work, we present a pretreatment technology using a Joule-heated sponge to rapidly remove silica from saline waters through adsorption, thereby effectively mitigating silica scaling in subsequent membrane desalination processes. The adsorbent sponge is fabricated by functionalizing two-dimensional layered double hydroxide (LDH) nanosheets on a porous, conductive stainless-steel sponge. With the application of an external voltage of 4 V, the Joule-heated sponge achieves 85% silica removal and 95% sponge regeneration within 15 min, which is much more efficient than its counterpart without Joule-heating (360 min for silica adsorption and 90 min for sponge regeneration). Material characterization and reaction kinetics analysis reveal that electrostatic interactions and "memory effect"-induced intercalation are the primary mechanisms for silica removal by the LDH nanosheets. Moreover, Joule-heating reduces the boundary layer resistance on nanosheets and facilitates intraparticle diffusion of dissolved silica, thereby increasing silica removal kinetics. Joule-heating also enhances the release of silicate ions during the regeneration stage through exchange with the surrounding anions (OH^- or CO₃^2-), resulting in a more efficient sponge regeneration. Pretreatment of silica-rich feedwaters by the Joule-heated sponge effectively reduces reverse osmosis membrane scaling by amorphous silica scale, demonstrating great potential for silica scaling control in a broad range of engineered processes.