Selective molecular separation of lignin model compounds by reduced graphene oxide membranes from solvent-water mixture.

Selective separation of lignin depolymerization products is key to fractionating and isolating high-value aromatic compounds from the depolymerization process. The primary aim of this study was to synthesis graphene oxide (GO) membranes for selective separations of lignin oligomeric units from polar organic solvent-water media. GO membranes were synthesized on a polymeric substrate by a shear assisted casting of aqueous GO dispersion using a wire-wound rod. Deposited GO was then reduced to different extents by controlled thermal incubation, and the impact on membrane performance was investigated. The extent of reduction of GO was established by extensive characterization with FTIR, XPS, Raman Spectroscopy, XRD, and contact angle measurements. Impressive performance with the rejection of over 70% for the model compound trimer BMP (2,6-bis[(2-hydroxy-5-methyl phenyl) methyl]-4-methylphenol) was achieved compared to only 20% rejection for the dimer GGE (guaiacylglycerol-ß-guaiacylether) with isopropanol-water (90-10% by volume) as a solvent. This corresponds to an encouraging selective separation with selective permeation of dimer (GGE) 3.5 times higher compared to trimer (BMP). rGO membranes exhibited a stable performance over 84 h of operation at a shear rate of 1.1 Pa in a cross-flow mode of operation. Selective separation of GO can be effectively modulated by controlling the O/C ratio by the extent of reduction of GO; indeed, the retention of trimeric compounds increased with increasing GO reduction. The remarkable performance of GO membranes could enable energy-efficient fractionation of lignin oligomeric compounds from polar organic solvents.

Ion and organic transport in Graphene oxide membranes: Model development to difficult water remediation applications.

The role of steric hindrance and charge interactions in governing ionic transport through reduced graphene oxide (rGO) and commercial (DOW-Filmtec NF270) membranes was elucidated by a comprehensive study of experimental and established mathematical analysis based on Nernst-Planck equation. A charge-dominated salt exclusion mechanism was observed for the rGO membranes, which exhibited retention from low (7%) to moderate (70%) extent depending on the nature of ions (5 mM). Swelling of GO (1.2 nm interlayer distance) in water beyond the hydrated diameter of ions was attributed as a primary cause for lowering steric hindrance effects. The influence of parameters affecting charge interactions, such as pH and ionic strength, on the extent of salt rejection was modelled. The potential impact of the membrane's charge density, GO loading and interlayer spacing on salt retention was quantified by performing sensitivity analyses. For a high TDS produced water sample, the rGO membranes partially retained divalent cations (Ca:13%) and exhibited high dissolved oil rejection. The membranes were found to be suitable for the treatment of high TDS water with the goal of selectively removing organic impurities, and thus minimizing the impact of osmotic pressure effect. Performance of the membranes was also investigated for retention of water remediation related organic anions, using perfluoro octanoic (PFOA) acid as a model compound. rGO membranes exhibited a charge-dominated exclusion mechanism for retention (90%) of PFOA (1 ppm).

Reduced graphene oxide-metal nanoparticle composite membranes for environmental separation and chloro-organic remediation.

This study explores the integration of separation performance of rGO membrane with heterogeneous oxidation reactions for remediation of organic contaminants from water. Herein, an approach was introduced based on layer-by-layer assembly for functionalizing rGO membranes with polyacrylic acid and then by in situ synthesis of Fe based reactive nanoparticles. TEM characterization of the cross-section lamella of the membranes showed a high density of nanoparticles (12% Fe) in the functionalized domain, signifying the importance of polyacrylic acid for in situ synthesis of nanoparticles. The membranes exhibited a pure water permeability of 1.9 LMH bar-1. The membranes had low to moderate salt retention, and more than 90% neutral red retention (organic probe molecule, size: 1.2 nm). The membranes also exhibited high retention of humic acids (80%), preventing these organics from entering the reactive domain, and thus potentially reducing the formation of undesired by-products. A persulfate mediated oxidative pathway was employed to demonstrate the reactive removal of organic contaminants. The membranes achieved >95% conversion by convectively passing 2 mM persulfate feed at a transmembrane pressure of 0.4 bar. Successful degradation of TCE (up to 61%) was achieved in a single pass by convective flowing of the feed solution through the membrane, generating up to 80% of the theoretical maximum chloride as one of the byproducts. Elevated temperatures significantly enhanced persulfate mediated TCE oxidation extent from 24% at 23 oC to 54% at 40 o C under batch operating conditions.

Synthesis of graphene oxide membranes and their behavior in water and isopropanol.

Graphene oxide (GO) membrane has been synthesized on commercial polysulfone ultrafiltration membranes (Pore size: 17 nm) using the drop casting method followed by baking at 90 C for 24 h. Baking resulted in the reduction of GO and removal of bulk water intercalated in the GO sheets. Deposited GO film showed high stability under shear stress variation. This work shows that water adsorption on the GO membrane determines its permeation performance. Despite the higher viscosity of isopropyl alcohol (IPA), its permeability was 7 times higher than water through the baked ("dry") GO membranes, which were never contacted with water. However, IPA permeability of GO membranes dropped to 44% (of deionized water) when contacted with water ("hydrated" or "wet" GO membranes). Extensive size exclusion (rejection) studies with various dye and dendrimer molecules showed pore size reduced from 3.3 nm in the "dry" state to 1.3 nm in the "wet" state of GO membranes. FT-IR characterization of GO membrane suggested adsorption of water on the nanochannels of the active layer. Also, significant decay in flux was observed for water (82% of its initial flux) as compared to IPA (38% of its initial flux) for initially dry GO membranes.

Naphthenic acids removal from high TDS produced water by persulfate mediated iron oxide functionalized catalytic membrane, and by nanofiltration.

Oil industries generate large amounts of produced water containing organic contaminants, such as naphthenic acids (NA) and very high concentrations of inorganic salts. Recovery of potable water from produced water can be highly energy intensive is some cases due to its high salt concentration, and safe discharge is more suitable. Here, we explored catalytic properties of iron oxide (FexOy nanoparticles) functionalized membranes in oxidizing NA from water containing high concentrations of total dissolved solids (TDS) using persulfate as an oxidizing agent. Catalytic decomposition of persulfate by FexOy functionalized membranes followed pseudo-first order kinetics with an apparent activation energy of 18 Kcal/mol. FexOy functionalized membranes were capable of lowering the NA concentrations to less than discharge limits of 10 ppm at 40 °C. Oxidation state of iron during reaction was quantified. Membrane performance was investigated for extended period of time. A coupled process of advanced oxidation catalyzed by membrane and nanofiltration was also evaluated. Commercially available nanofiltration membranes were found capable of retaining NA from water containing high concentrations of dissolved salts. Commercial NF membranes, Dow NF270 (Dow), and NF8 (Nanostone) had NA rejection of 79% and 82%, respectively. Retentate for the nanofiltration was further treated with advanced oxidation catalyzed by FexOy functionalized membrane for removal of NA.