Inhibition of silica scaling with functional polymers: Role of ionic strength, divalent ions, and temperature.

High concentrations of dissolved silica in saline industrial wastewaters and brines cause silica scale formation, significantly hampering the efficacy of diverse engineered systems. Applying functional polymers as scale inhibitors in process feedwater is a common strategy to mitigate silica scaling. However, feedwater characteristics often vary widely, depending on the specific processes, making the inhibition of silica scaling challenging and complex. In this study, we systematically investigate the role of ionic composition, specifically ionic strength and divalent ions, and solution temperature, in inhibiting silica scaling using molecularly designed amine/amide polymers. The inhibitor demonstrates effective stabilization of silicic acid, with inhibition efficiency of 74 and 55 % in the absence and presence of 20,000 ppm NaCl, respectively. However, further increasing the ionic strength of oversaturated silicic acid solutions significantly diminishes inhibition performance, rendering it ineffective at 180,000 ppm NaCl. Divalent inorganic cations exhibit a stronger impact on reducing inhibition efficiency compared to sodium ions. Molecular dynamics simulations reveal a competition mechanism between anionic silicic acid reactants (i.e., H3SiO4-) and chlorides for binding to ammonium groups within the polymeric inhibitor. Additionally, cations form clusters with H3SiO4- ions, hindering their stabilization with polymeric inhibitor. Notably, at elevated temperatures, the inhibitor achieves near-perfect inhibition for 500 ppm silicic acid solutions. This comprehensive assessment provides important insights into the effectiveness of silica scaling inhibitors under solution conditions relevant to real-world applications, addressing the challenges posed by varying solution parameters in diverse industrial processes.

Molecular Design of Functional Polymers for Silica Scale Inhibition.

Silica polymerization, which involves the condensation reaction of silicic acid, is a fundamental process with wide-ranging implications in biological systems, material synthesis, and scale formation. The formation of a silica-based scale poses significant technological challenges to energy-efficient operations in various industrial processes, including heat exchangers and water treatment membranes. Despite the common strategy of applying functional polymers for inhibiting silica polymerization, the underlying mechanisms of inhibition remain elusive. In this study, we synthesized a series of nitrogen-containing polymers as silica inhibitors and elucidated the role of their molecular structures in stabilizing silicic acids. Polymers with both charged amine and uncharged amide groups in their backbones exhibit superior inhibition performance, retaining up to 430 ppm of reactive silica intact for 8 h under neutral pH conditions. In contrast, monomers of these amine/amide-containing polymers as well as polymers containing only amine or amide functionalities present insignificant inhibition. Molecular dynamics simulations reveal strong binding between the deprotonated silicic acid and a polymer when the amine groups in the polymer are protonated. Notably, an extended chain conformation of the polymer is crucial to prevent proximity between the interacting monomeric silica species, thereby facilitating effective silica inhibition. Furthermore, the hydrophobic nature of alkyl segments in polymer chains disrupts the hydration shell around the polymer, resulting in enhanced binding with ionized silicic acid precursors compared to monomers. Our findings provide novel mechanistic insights into the stabilization of silicic acids with functional polymers, highlighting the molecular design principles of effective inhibitors for silica polymerization.

Significance of Co-ion Partitioning in Salt Transport through Polyamide Reverse Osmosis Membranes.

Salt permeability of polyamide reverse osmosis (RO) membranes has been shown to increase with increasing feed salt concentration. The dependence of salt permeability on salt concentration has been attributed to the variation of salt partitioning with feed salt concentration. However, studies using various analytical techniques revealed that the salt (total ion) partitioning coefficient decreases with increasing salt concentration, in marked contrast to the observed increase in salt permeability. Herein, we thoroughly investigate the dependence of total ion and co-ion partitioning coefficients on salt concentration and solution pH. The salt partitioning is measured using a quartz crystal microbalance (QCM), while the co-ion partitioning is calculated from the measured salt partitioning using a modified Donnan theory. Our results demonstrate that the co-ion and total ion partitioning behave entirely differently with increasing salt concentrations. Specifically, the co-ion partitioning increased fourfold, while total ion partitioning decreased by 60% as the salt (NaCl) concentration increased from 100 to 800 mM. The increase in co-ion partitioning with increasing salt concentration is in accordance with the increasing trend of salt permeability in RO experiments. We further show that the dependence of salt and co-ion partitioning on salt concentration is much more pronounced at a higher solution pH. The good co-ion exclusion (GCE) model─derived from the solution-friction model─is used to calculate the salt permeability based on the co-ion partitioning coefficients. Our results show that the GCE model predicts the salt permeabilities in RO experiments relatively well, indicating that co-ion partitioning, not salt partitioning, governs salt transport through RO membranes. Our study provides an in-depth understanding of ion partitioning in polyamide RO membranes and its relationship with salt transport.

Relating Selectivity and Separation Performance of Lamellar Two-Dimensional Molybdenum Disulfide (MoS2) Membranes to Nanosheet Stacking Behavior.

Increased demand for highly selective and energy-efficient separations processes has stimulated substantial interest in emerging two-dimensional (2D) nanomaterials as a potential platform for next-generation membranes. However, persistently poor separation performance continues to hinder the viability of many novel 2D-nanosheet membranes in desalination applications. In this study, we examine the role of the lamellar structure of 2D membranes on their performance. Using self-fabricated molybdenum disulfide (MoS2) membranes as a platform, we show that the separation layer of 2D nanosheet frameworks not only fails to demonstrate water-salt selectivity but also exhibits low rejection toward dye molecules. Moreover, the MoS2 membranes possess a molecular weight cutoff comparable to its underlying porous support, implying negligible selectivity of the MoS2 layer. By tuning the nanochannel size through intercalation with amphiphilic molecules and analyzing mass transport in the lamellar structure using Monte Carlo simulations, we reveal that small imperfections in the stacking of MoS2 nanosheets result in the formation of catastrophic microporous defects. These defects lead to a precipitous reduction in the selectivity of the lamellar structure by negating the interlayer sieving mechanism that prevents the passage of large penetrants. Notably, the imperfect stacking of nanosheets in the MoS2 membrane was further verified using 2D X-ray diffraction measurements. We conclude that developing a well-controlled fabrication process, in which the lamellar structure can be carefully tuned, is critical to achieving defect-free and highly selective 2D desalination membranes.

High-flux operation of MBRs with ceramic flat-sheet membranes made possible by intensive membrane cleaning: Tests with real domestic wastewater under low-temperature conditions.

This study investigated the efficiency of intensive membrane cleaning for membrane bioreactors (MBRs) using a combination of mechanical scouring with granules and chemically enhanced backwashing (CEB). The implementation of such intensive cleaning was possible with ceramic flat-sheet membranes. Experiments were carried out using bench-scale MBRs at an existing wastewater treatment plant. First, CEB with NaClO was investigated in terms of the CEB frequency, duration, and concentration of the chemical reagent. CEB carried out for 60 min every 6 h, with 50 ppm of NaClO, was found to be effective, and it enabled an MBR to operate at 50 LMH, two to three times higher than the flux of full-scale MBRs. However, these CEB conditions were insufficient when the temperature was low (i.e. in winter), when an adhesive gel layer formed on the membrane surface. Its high resistance to cleaning might be explained by the increased levels of soluble microbial products and/or the presence of algal cells. Alkaline-assisted CEB, with NaClO (pH 12) and an increase in the volume of granules in the membrane tank, solved this problem. With the modified cleaning method, the fouling could be almost perfectly controlled at low-temperature conditions, such as 13 °C. MBRs may be regarded as fouling-free MBRs when the proposed cleaning method is used with ceramic flat-sheet membranes. Most real-world MBR operations operate with lower fluxes than the flux examined in this study, and at higher temperatures.

Graphene Oxide-Functionalized Membranes: The Importance of Nanosheet Surface Exposure for Biofouling Resistance.

Surface functionalization using two-dimensional (2D) graphene oxide (GO) materials is a promising technique to enhance the biofouling resistance of membranes used in water purification and reuse. However, the role of GO exposure, which is crucial for the contact-mediated toxicity mechanism, has not been well evaluated or elucidated in previous studies. Herein, we employ bioinspired polydopamine chemistry to fabricate GO-functionalized membranes through two strategies: coating and blending. The two types of GO-functionalized membranes displayed comparable roughness, hydrophilicity, water permeability, and solute retention properties but different degrees of GO nanosheet exposure on the membrane surface. When in contact with the model bacterium, Escherichia coli, the GO-coated membrane exhibited enhanced biofouling resistance compared to that of the GO-blended membrane, as evidenced by lower viable cells in static adsorption experiments, and lower water flux decline and higher flux recovery in dynamic biofouling experiments. Moreover, the development of biofilm on the GO-coated membrane was also inhibited to a greater extent than on the GO-blended membrane. Taken together, our findings indicate the paramount importance of GO exposure on the membrane surface in conferring antibacterial activity and biofouling resistance, which should be considered in the future design of antibiofouling membranes using 2D nanomaterials.