Copper-Modified Polymeric Membranes for Water Treatment: A Comprehensive Review

Abstract

In the last decades, the incorporation of copper in polymeric membranes for water treatment has received greater attention, as an innovative potential solution against biofouling formation on membranes, as well as, by its ability to improve other relevant membrane properties. Copper has attractive characteristics: excellent antimicrobial activity, high natural abundance, low cost and the existence of multiple cost-effective synthesis routes for obtaining copper-based materials with tunable characteristics, which favor their incorporation into polymeric membranes. This study presents a comprehensive analysis of the progress made in the area regarding modified membranes for water treatment when incorporating copper. The notable use of copper materials (metallic and oxide nanoparticles, salts, composites, metal-polymer complexes, coordination polymers) for modifying microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), forward osmosis (FO) and reverse osmosis (RO) membranes have been identified. Antibacterial and anti-fouling effect, hydrophilicity increase, improvements of the water flux, the rejection of compounds capacity and structural membrane parameters and the reduction of concentration polarization phenomena are some outstanding properties that improved. Moreover, the study acknowledges different membrane modification approaches to incorporate copper, such as, the incorporation during the membrane synthesis process (immobilization in polymer and phase inversion) or its surface modification using physical (coating, layer by layer assembly and electrospinning) and chemical (grafting, one-pot chelating, co-deposition and mussel-inspired PDA) surface modification techniques. Thus, the advantages and limitations of these modifications and their methods with insights towards a possible industrial applicability are presented. Furthermore, when copper was incorporated into membrane matrices, the study identified relevant detrimental consequences with potential to be solved, such as formation of defects, pore block, and nanoparticles agglomeration during their fabrication. Among others, the low modification stability, the uncontrolled copper ion releasing or leaching of incorporated copper material are also identified concerns. Thus, this article offers modification strategies that allow an effective copper incorporation on these polymeric membranes and solve these hinders. The article finishes with some claims about scaling up the implementation process, including long-term performance under real conditions, feasibility of production at large scale, and assessment of environmental impact.

Keywords: biofouling; copper nanomaterials; nanocomposites; polymeric membranes; water treatment.

Figure 1

Schematic representation of membranes-based separation process.

Figure 2

Distribution of the different techniques used for modified membranes with copper. The diagrams are plotted using the data presented in articles [14,27,28,32,36,40,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,81,82,83,84,85,86,87].

Figure 3

Results of the functionalization of TFC-RO membranes with Cu-NPs reported by Ben-Sasson et al. [32]. (a) Schematic of the electrostatic binding between the Cu-NPs (positively charged) and the carboxyl groups (negatively charged) on the active layer of the pristine membrane. (b) Biocide capacity comparison between the pristine membrane (black) and modified with the capping agent (PEI) alone (green) and Cu-NPs (red) over E. coli, P. aeruginosa (gram negative bacteria) and S. Aureus (gram positive bacteria). Asterisks (*) indicate a statistically significant difference between the functionalized and pristine membranes (p < 0.05). Adapted from [32].

Figure 4

(a) Schematic of copper-modified RO membranes implemented by Ma et al. [79]. From left to right: Coating of membrane surface by in situ Cu-NPs reduction (RO-Cu), grafting of RO membrane with cysteamine linker and Cu-NPs (RO-Cys-Cu) and with graphene oxide linker and Cu-NPs (RO-GO-Cu); (b) Number of viable cells attached (CFU) in modified membranes compared to pristine membrane; (c) Quantity of NPs in each membrane after the release for a period of 7 days and after regeneration with Cu-NPs (note that the amount of NPs after regeneration is higher than the original amount of copper). Asterisks (*) indicate a statistically significant difference between the pristine and modified mem-branes (p < 0.05). All images and graphs are extracted from [79].

Figure 5

(a) Schematic of CuO-NPs addition to TFC-RO membranes during IPP and (b) of a TFC-RO membrane modified by formation of copper-oligomer complex (Cu-mPD) in situ. Extracted and adapted from [24]; (c) Bactericidal capacity (quantified by CFU) of copper modified membrane by addition in IPP of Cu-NPs, CuO-NPs and Cu-MPD compared to a pristine membrane (PA/PS). Extracted from [85].

Figure 6

(a) Schematic representation of the IPP and mineralization of TFC-RO membrane with formation of Cu(OH)₂ surface modification; (b) Mechanism of electrostatic repulsion between BSA and membrane surface. Extracted and adapted from [141].

Figure 7

FO water flux of the membranes in atomic layer deposition (ALDS) mode and using different DS concentrations (T ¼ 25 C, feed ¼ DI water), error bars represent standard deviation over runs. Extracted from [73].

Figure 8

Schematic of the one-pot chelating copper ions modification on FO membrane. Copper ion release and interactions between bacteria and the copper-containing surface results add biocide properties. Extracted from [75].

Figure 9

Schematic diagram of the surface modifications of the HPAN membrane via two-step deposition and co-deposition using PDA and CuNPs. Extracted from [64].

Figure 10

Diagrammatic sketch of membrane fabrication process of CSPM-Cu(II)/mPAN: (a) formation of CSPs, (b) deposition of CSP suspension onto mPAN to form CSPM/mPAN, (c) Cu(II) chelation of CSPM/mPAN to form CSPM-Cu(II)/mPAN. Extracted from [71].

Figure 11

PES UF membranes modified by the SPAES and Cu-NPs incorporation. SEM images of PES/SPAES (a) and PES/SPAES/Cu-NPs membranes (b). PWF and BSA rejection of the PES/SPAES/Cu NPs membrane (c). FRR of the PES/SPAES/Cu-NPs membranes to humic acid (HA), sodium alginate (SA) and BSA. Adapted from Zhang et al. (2018) [55].

Figure 12

PES UF Membranes performance. PWF of neat membrane with respect to modified membranes given their incorporation into Cu₂O-NPs, the inset figure exhibits a TEM image of Cu₂O-NPs (a) and rejection of BSA, HA and Oil-Water (b). Modified from Pravallika et al. (2016) [50].