Phosphonium-defoliated GO nanosheets as micropore booster of carbon membrane to improve separation of tetracycline from isopropanol

Abstract

This study developed a supported carbon membrane (CM) to conduct organic solvent nanofiltration (OSN). The OSN performance was tuned up by dispersing phosphonium-defoliated graphene oxide (PGO) as filler (0.3 wt%) in the precursor (bitumen) coating. PGO was synthesized through anchoring phosphonium (Ph3PBu+) ions to the oxygenated groups dangled on individual GO sheets that take up 49 wt% of PGO. As PGO experiences elimination of the pendant organic functional groups to form a graphene-like sheet through the course of pyrolysis and carbonization that converts the precursor coating to CM, the graphene-like sheets thus act to induce in-situ face-to-face stacking of the planar polyaromatic hydrocarbons (PAHs) generated from the pyrolysis of bitumen in nanoscale. PGO achieved a superior efficacy of induction than GO because of the defoliating role of phosphonium ion that assures a higher dispersity of graphene-like sheets in the precursor coating. This mechanism in the end casts a higher pore volume in membrane CMPGO than in CMGO (600 vs. 334 μl/g) primarily associating with the micro and meso pores inside the carbonaceous grains (~15–20 nm) that constitute the membrane. These tiny pores connect with the grain boundaries (3–5 nm) to form a porous network in the membrane. Moreover, the resultant membrane operates through a dual interaction model of adsorption and size screening. Membrane CMPGO manifests 99.2~94.2% retention of tetracycline (TC) from the permeate (isopropanol) with an average permeance of 32 L/m2·h·bar over 30 h, whereas the control, CMGO, shows progressive declining from 82% to 30% in 10 h. CMPGO was also evaluated by using the oppositely-charged dyes of different sizes to examine the effect of solute identity on percolation.