Influence of bulk concentration on the organisation of molecules at a membrane surface and flux decline during reverse osmosis of an anionic surfactant

Reverse osmosis membrane fouling caused by typical surfactants in the integrated circuit industry: Fouling mechanism and control strategies

In the integrated circuit manufacturing process, reverse osmosis (RO) membranes are widely used for wastewater reclamation. However, fouling by typical surfactants significantly reduces membrane efficiency and lifespan. This study investigates the fouling mechanisms of typical surfactants-cetyl trimethyl ammonium bromide (CTAB, cationic), sodium dodecyl sulfate (SDS, anionic), and polyoxyethylene octyl phenyl ether (TX, nonionic)-on RO membranes. Quartz crystal microbalance analysis results show that CTAB and TX exhibit significantly stronger adhesion to RO membranes than SDS. The order of adsorption mass on the membrane surface is CTAB > TX > SDS, with CTAB causing the most severe fouling. Molecular dynamics (MD) simulations indicate that unclustered CTAB molecules contribute to severe fouling by inserting into the membrane surface. As surfactant concentration increases, clustered CTAB is less likely to enter the membrane's surface layer. A comparison of oxidative technologies-continuous dual-wavelength ultraviolet (VUV/UV), intermittent VUV/UV, and intermittent VUV/UV with chlorine, ozone alone, chlorine alone, and ozone combined with chlorine (ozone/chlorine)- reveals that pre-treating surfactants with ozone/chlorine (simultaneous dosing at 10 mg/L each) before membrane filtration effectively controls fouling. After 30 min of treatment, 29 % of CTAB and 86 % of TX were degraded, respectively. Ozone/chlorine oxidation significantly alleviates membrane fouling, increasing the normalized steady-state permeate flux (Jpss) of CTAB and TX by 245 % and 151 %, respectively. The extended Derjaguin-Landau-Verwey-Overbeek theory calculations and MD simulations show that oxidation weakens the adhesion of CTAB and TX to RO membranes, reducing fouling. Ozone/chlorine treatment also effectively mitigates membrane fouling in actual wastewater from the electronics industry. Post-oxidation, the flux ratio (J/J0) increased from 0.28 to 0.52, resulting in a 116.7 % improvement in the Jpss. This study combines experimental data, theoretical calculations, and MD simulations, highlighting the significance of molecular clustering in surfactant-induced fouling before and after oxidation.

Mechanistic Insights into Micelle-Enhanced Nanofiltration for Heavy Metal Removal: Transformation of Ion Transport and Fouling Phenomena

Toxic heavy metals are widely present in typical scenarios, such as mines and electroplating wastewater, presenting significant risks to biological and environmental safety. Membrane processes encounter a challenge in effectively intercepting heavy metals due to their small hydration radius. This research showcases the high efficiency of micelle-enhanced nanofiltration (MENF) in removing heavy metals. At the critical micelle concentration, sodium dodecyl sulfate demonstrated a high removal of Cu2+, Ni2+, Zn2+, and Cd2+ while maintaining substantial potential for complexation of heavy metals. The formation of micelles and the bonding of heavy metals with surfactants bolstered the resistance of heavy metal ions to transmembrane transport. The presence of heavy metals in ionic form in wastewater facilitated their complexation with surfactants or micelles. Notably, the valence state and concentration of interfering ions in the environment could slightly influence the removal of heavy metals by MENF. Additionally, MENF displayed remarkable antifouling properties. The loose gel layer created by surfactant molecules and the micelle enhanced the membrane permeability and reduced the scaling tendency of heavy metals. This study contributes to an improved understanding of the mechanisms involved in heavy metal rejection by using MENF.

Ionic Liquid-Mediated Interfacial Polymerization for Fabrication of Reverse Osmosis Membranes

This study revealed the effects of incorporating ionic liquid (IL) molecules: 1-ethyl, 1-butyl, and 1-octyl-3-methyl-imidazolium chlorides with different alkyl chain lengths, in interfacial polymerization (IP) on the structure and property (i.e., permeate-flux and salt rejection ratio) relationships of resulting RO membranes. The IL additive was added in the aqueous meta-phenylene diamine (MPD; 0.1% w/v) phase, which was subsequently reacted with trimesoyl chloride (TMC; 0.004% w/v) in the hexane phase to produce polyamide (PA) barrier layer. The structure of resulting free-standing PA thin films was characterized by grazing incidence wide-angle X-rays scattering (GIWAXS), which results were correlated with the performance of thin-film composite RO membranes having PA barrier layers prepared under the same IP conditions. Additionally, the membrane surface properties were characterized by zeta potential and water contact angle measurements. It was found that the membrane prepared by the longer chain IL molecule generally showed lower salt rejection ratio and higher permeation flux, possibly due to the inclusion of IL molecules in the PA scaffold. This hypothesis was supported by the GIWAXS results, where a self-assembled surfactant-like structure formed by IL with the longest aliphatic chain length was detected.

Surface characterization of thin-film composite membranes using contact angle technique: Review of quantification strategies and applications

Thin-film composite (TFC) membranes are the most widely used membranes for low-cost and energy-efficient water desalination processes. Proper control over the three influential surface parameters, namely wettability, roughness, and surface charge, is vital in optimizing the TFC membrane surface and permeation properties. More specifically, the surface properties of TFC membranes are often tailored by incorporating novel special wettability materials to increase hydrophilicity and tune surface physicochemical heterogeneity. These essential parameters affect the membrane permeability and antifouling properties. The membrane surface characterization protocols employed to date are rather controversial, and there is no general agreement about the metrics used to evaluate the surface hydrophilicity and physicochemical heterogeneity. In this review, we surveyed and critically evaluated the process that emerged for understanding the membrane surface properties using the simple and economical contact angle analysis technique. Contact angle analysis allows the estimation of surface wettability, surface free energy, surface charge, oleophobicity, contact angle hysteresis, and free energy of interaction; all coordinatively influence the membrane permeation and fouling properties. This review will provide insights into simplifying the evaluation of membrane properties by contact angle analysis that will ultimately expedite the membrane development process by reducing the time and expenses required for the characterization to confirm the success and the impact of any modification.

Effects of sodium dodecyl sulfate on forward osmosis membrane fouling and its cleaning

Effect of sodium dodecyl sulfate (SDS) on the fouling of a commercial aquaporin based biomimetic forward osmosis (FO) membrane was investigated. Increasing draw solution (DS) concentration and decreasing the cross-flow velocity could aggravate the membrane fouling, and the effect of the latter was greater than the former. SDS as a surfactant could wash away some sodium alginate (SA) and calcium chloride (CaCl₂) which were adsorbed on the surface of the membrane. However, SA and CaCl₂ tended to form irreversible fouling when SDS had already been on the membrane. When SDS + SA + CaCl₂ was used as the feed solution (FS), SDS was first adsorbed on the membrane surface and then SA and CaCl₂ interact with SDS; irreversible fouling was formed when the hydrophobic tail of the SDS was adsorbed to the SA, and reversible fouling was formed while Ca²⁺ (bridged with SA) was bound with the hydrophilic head of the SDS. Afterwards, the cleaning effects of HCl and NaOH solutions on the membrane fouling caused by SDS were studied. The initial normalized flux could be recovered to 0.88 using both methods. Cleaning with HCl solution could slow down the formation of membrane fouling, while cleaning with NaOH solution could damage the aquaporin in the active layer of the membrane.

Transport Models of Ammonium Nitrogen in Wastewater from Rare Earth Smelteries by Reverse Osmosis Membranes

Wastewater from rare earth smelteries contains large amounts of ammonium nitrogen (NH4+-N), which causes severe environmental problems. In this contribution, the desalination efficiency of reverse osmosis (RO) was investigated in the treatment of NH4Cl or NaCl solutions from 0.1 to 40 g/L under different operating pressures with a commercial RO membrane. Experimental results showed that when an operating pressure above 30 bar is applied to the 5 g/L NH4Cl solution, the permeate was found to meet the discharge standards of NH4+-N. Compared to NH4Cl, the permeate fluxes of NaCl solutions were higher due to the higher net driving force and lower propensity to membrane fouling. Theoretical models indicate a linear relationship between water flux and the net driving force for both NH4Cl and NaCl solutions. On the contrary, a power function between the salt flux and concentration difference correlated well with the experimental data for salt transport. The equations for water and salt transport obtained by this work would provide a facile and practical means for predicting the membrane performance in design and optimization of RO processes for the treatment of wastewater from the rare earth industry.

Role of concentration polarization in cross flow micellar enhanced ultrafiltration of cadmium with low surfactant concentration

Concentration polarization is an important issue in micellar enhanced ultrafiltration (MEUF) of wastewater containing heavy metal ions at low surfactant concentrations. In this paper, we studied removal of Cd(Ⅱ) by cross flow MEUF at low sodium dodecyl sulfate (SDS) concentration levels, and the role of concentration polarization in flux decline and Cd(Ⅱ) rejection was emphasized. Concentration polarization resistance and SDS concentration near membrane were calculated to characterize concentration polarization. The results showed that SDS concentration near membrane was 13 mM when feed concentration was merely 0.8 mM. By combining phase diagram of SDS, structures of SDS micelles in concentration polarization layer were deduced and thin layer structure transformed to porous structure formed by accumulated globular micelles when SDS concentration increased. Although micelles formed in concentration polarization layer was responsible for flux decline, they also provided adsorption sites for Cd(Ⅱ).

Roles of surfactants in pressure-driven membrane separation processes: a review

Surfactants widely exist in various kinds of wastewaters which could be treated by pressure-driven membrane separation (PDMS) techniques. Due to the special characteristics of surfactants, they may affect the performance of membrane filtration. Over the last two decades, there are a number of studies on treating wastewaters containing surfactants by PDMS. The current paper gives a review of the roles of surfactants in PDMS processes. The effects of surfactants on membrane performance were discussed via two aspects: influence of surfactants on membrane fouling and enhanced removal of pollutants by surfactants. The characteristics of surfactants in solution and at solid-liquid interface were summarized. Surfactants in membrane filtration processes cause membrane fouling mainly through adsorption, concentration polarization, pore blocking, and cake formation, and fouling degree may be influenced by various factors (feed water composition, membrane properties, and operation conditions). Furthermore, surfactants may also have a positive effect on membrane performance. Enhanced removal of various kinds of pollutants by PDMS in the presence of surfactants has been summarized, and the removal mechanism has been revealed. Based on the current reports, further studies on membrane fouling caused by surfactants and enhanced removal of pollutants by surfactant-aided membrane filtration were also proposed.

Optimization of sulfate removal from wastewater using magnetic multi-walled carbon nanotubes by response surface methodology

This paper reports a facile method for removal of sulfate from wastewater by magnetic multi-walled carbon nanotubes (MMWCNTs). Multi-walled carbon nanotubes and MMWCNTs were characterized by X-ray diffraction, Raman, transmission electron microscopy, Fourier transform infrared spectroscopy, and vibrating sample magnetometry. The results of the analysis indicated that MMWCNTs were synthesized successfully. The MMWCNTs can be easily manipulated in a magnetic field for the desired separation, leading to the removal of sulfate from wastewater. Response surface methodology (RSM) coupled with central composite design was applied to evaluate the effects of D/C (adsorbent dosage per initial concentration of pollutant (mg_adsorbent/(mg/l)_initial)) and pH on sulfate removal (%). Using RSM methodology, a quadratic polynomial equation was obtained, for removal of sulfate, by multiple regression analysis. The optimum combination for maximum sulfate removal of 93.28% was pH = 5.96 and D/C = 24.35. The experimental data were evaluated by the Langmuir and Freundlich adsorption models. The adsorption capacity of sulfate in the studied concentration range was 56.94 (mg/g). It was found out that the MMWCNTs could be considered as a promising adsorbent for the removal of sulfate from wastewater.