Carbon nanotube enhanced membrane distillation for simultaneous generation of pure water and concentrating pharmaceutical waste

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Abstract

Carbon nanotube enhanced membrane distillation (MD) is presented as a novel waste concentration method that also generates pure water. In a carbon nanotube immobilized membrane (CNIM), the CNTs serve as sorbent sites, and provide an additional pathway for enhanced water vapor transport. Enrichment effect and mass transfer coefficients using CNIM were as much as 421% and 543% greater than in membranes without the nanotubes. As compared to the original water, the purified water contained less than 10% of residual organics.

Highlights

CNIM is an effective means to remove and concentrate trace pharmaceuticals from water. ► CNIM was significantly more effective than conventional membrane distillation (MD). ► CNIM requires significantly less energy to achieve the same levels of flux. ► MD also generated pure water in the process of concentrating waste.

Introduction

Membrane distillation (MD) is a water removal process that has been used in applications such as desalination [1], [2], [3], [4] and food processing [5], [6]. A hot aqueous solution (60–90 °C) is passed through the lumen of a hydrophobic hollow fiber, which prevents the transport of the liquid phase across the membrane. At these temperatures, the water is partially converted to vapor and MD relies on the net flux of this vapor across the membrane. MD offers the unique opportunity of partial removal of the aqueous matrix for concentration enhancement while generating pure water on the permeate side.

The development of novel membrane architecture is of great importance to enhance the membrane performance in MD. The physico-chemical properties of CNTs are known to play an important role in membrane processes, where the nanotubes have served as channels for mass transport of water vapors and gases [7], [8] and the high flux has been attributed to the atomic-scale smoothness of the CNT walls as well as molecular ordering inside the nanopores [9]. Studies have shown that absorbed water molecules tend to organize themselves into a long lasting hydrogen-bonded network [10]. Since CNTs act as both molecular transporters and sorbents [11] they can increase the permeability of a substance through a membrane as well as increase its selectivity. The CNTs also increase functional surface area in the membrane system due to their high aspect ratio [12]. Furthermore, particularly pertinent to MD is the fact that the high thermal conductivity of the CNTs may reduce the temperature gradient in membrane pores, allowing for reduced liquid condensation. All these mechanisms are expected to play important roles on the molecular transport of water vapors in the presence of the CNTs and lead to enhanced performance in MD.

Recently, our group has reported the development of novel membranes by immobilizing carbon nanotubes (CNTs) into membrane pores [13]. Referred to as the carbon nanotube immobilized membrane (CNIM), here the CNTs serve as a sorbent and provide an additional pathway for solute transport [14]. These have been used in applications such as desalination, pervaporation and solvent extraction [15]. The objective of this work is to implement CNIM as the membrane for MD for concentrating pharmaceutical waste and simultaneously generating pure water. Of particular interest is low level residues in water, which have become important and are classified as emerging contaminants.

Section snippets

Experimental section

The membrane used in this study was Celgard type X-50 hollow fiber membrane from the Membrana-Charlotte division of Celgard, LLC, (Charlotte, NC, USA). Membrane modules were constructed in a shell and tube format using threaded brass pipe fittings. The “shell” portion of the module was ¼ in. ID X 2 in. long, to each end of this was attached a ‘T’ fitting, through which the membrane was introduced. The ends were then sealed with epoxy to prevent leakage into the shell side and the assembled module

Results and discussion

Scanning electron microscopy (SEM) images without and with CNTs are shown Fig. 2A and B. Based on the TGA analysis, it was concluded that the CNIM contained approximately 0.5 wt.% of CNTs. It was also observed that the presence of CNTs enhanced the thermal stability of the membrane by increasing the onset of thermal degradation by as much as 29 °C. This is critical for MD, where relatively high temperatures are used and the higher stability would help slow membrane deterioration. The data is not

Conclusions

MD via CNIM is an excellent waste concentration method that can be used for low level pharmaceutical contamination in waste water. The approach can be applied to other pollutants as well because it relies on the removal of water rather than the selective permeation of the solutes across a membrane. Conventional MD provided a low enrichment, but the introduction of CNTs dramatically increased the performance in terms of concentration enhancement, generation of purified water flux and mass

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