Electrochemical removal of hexavalent chromium using electrically conducting carbon nanotube/polymer composite ultrafiltration membranes

Abstract

Hexavalent chromium (Cr(VI)) contamination in drinking water resources remains a challenge in many parts of the United States, as well as in regions affected by industrial pollution. In this study, we demonstrated how electrically conducting carbon nanotube (CNT)polyvinyl alcohol (PVA) composite ultrafiltration (UF) membranes can be used to remove Cr(VI) from water through a combined process of electrostatic repulsion, electrochemical reduction, and precipitation. The impact of different operational (flux, contact time, applied electrical potential) and environmental (pH and salinity) conditions on Cr(VI) removal were evaluated. Due to the native electrical potential of the CNT/PVA UF membrane material, approximately 45% removal of 1 ppm Cr(VI) solution was detected under neutral pH conditions in deionized water. Increased Cr(VI) removal was observed with increasing membrane surface charge density, which was accomplished through the application of an external potential (3V, 5V and 7V, membrane as cathode) to the electrically conductive membrane surface. The solution ionic strength showed a significant impact on Cr(VI) removal. By increasing the ionic strength without applying external potential on the membrane, the electrostatic repulsive force between the charged membrane surface and the CrO 4 2ion was eliminated, and Cr(VI) removal dropped to zero. The highest removal (95%) was achieved when 7V was applied to the membrane/counter electrode with a 6 µm-thick membrane. Here, Cr(VI) was electrochemically reduced to Cr(III) on the membrane surface, followed by Cr(III) precipitation as chromium hydroxide Cr(OH) 3(s) , which occurred by Cr(III) reacting with hydroxide ions generated via water splitting on the CNT network. Precipitated Cr(OH) 3 was then removed by the UF membrane. In addition, CNT-PVA UF membranes were used to treat tap water spiked with Cr(VI); under these conditions, 99% Cr(VI) removal was observed when 7V were applied to the membrane/counter electrode. Furthermore, we demonstrate that other trace inorganic contaminants, such as uranium, were effectively removed as well.

Single and binary protein electroultrafiltration using poly(vinyl-alcohol)-carbon nanotube (PVA-CNT) composite membranes

“…From the cross-sectional SEM image of the PVA-CNT/PS-35 composite membrane (S3 Fig in S1 File), the thickness of the PVA-CNT thin film was determined to be approximately 6 μm compared to the 165 μm thickness of the commercial polysulfone membrane which agrees with previously reported measurements [5,11]. With an approximate 10 μm thickness of the selective layer [5], the length fraction of the support layer, X, was determined to be 0.91.…”

“…Jassby et al [5][6][7][9][10][11] developed methods of coating commercial polymeric UF membranes with a thin layer of a poly(vinyl-alcohol) (PVA) polymer cross-linked with multiwalled carbon nanotubes to form highly electrically conductive PVA-CNT composite membranes. In a study of the effect of moderate applied electric potentials (-1.5 and -3.4 V vs Ag/AgCl references) on fouling of high concentrations (3-5 g/L) of synthetic wastewater containing negatively charged alginic acid during EUF using PVA-CNT composite membranes, Dudchenko and Jassby et al observed substantial fouling inhibition and reduction of operating pressure in a constant flux system [6].…”

A Comprehensive Review on Polymeric Nano-Composite Membranes for Water Treatment

“…They are either single walled carbon nanotubes (CNTs) or multiwalled carbon nanotubes (CNTs) possessing hexagonal carbon lattice well known for modification of membrane materials due to their low production cost, high purity and easy scaling up characteristics [140]. Wenyan et al [141] reported the fabrication of carbon nanotube (CNT) -Polyvinyl Alcohol (PVA) composite polysulfone Ultrafiltration (UF) membrane for chromium ion removal. Increase in surface charge density of membrane surface results in increased chromium ion removal.…”

Single and binary protein electroultrafiltration using poly(vinyl-alcohol)-carbon nanotube (PVA-CNT) composite membranes

“…Two-way analysis of variance (ANOVA) followed by post hoc Tukey's multiple comparisons tests were performed using Graphpad Prism version 6.01 (Graphpad Software, La Jolla, CA, USA) to assess the statistical differences among the weighted factor means at a significance level of p < 0.05. reported measurements [5,11]. With an approximate 10 µm thickness of the selective layer [5], the length fraction of the support layer, X, was determined to be 0.91.…”

“…As can be seen, unlike electroultrafiltration processes with electrodes on each side of the membrane, the external electric field terminates with the poly(vinyl-alcohol)carbon nanotube layer, which is upstream of the semipermeable, polymeric membrane. Jassby et al [5][6][7][9][10][11] developed methods of coating commercial polymeric UF membranes with a thin layer of a poly(vinyl-alcohol) (PVA) polymer cross-linked with multiwalled carbon nanotubes to form highly electrically conductive PVA-CNT composite membranes. In a study of the effect of moderate applied electric potentials (-1.5 and -3.4 V vs Ag/AgCl references) on fouling of high concentrations (3-5 g/L) of synthetic wastewater containing negatively charged alginic acid during EUF using PVA-CNT composite membranes, Dudchenko and Jassby et al observed substantial fouling inhibition and reduction of operating pressure in a constant flux system [6].…”