Electro-capture of heavy metal ions with carbon cloth integrated microfluidic devices

https://doi.org/10.1016/j.seppur.2017.10.064Get rights and content

Highlights

  • An electro-dialysis inspired microfluidic device was used to recover metal ions.

  • Valuable metal ions were reduced into metal form on a porous carbon platform.

  • Ability to generate alloys by selective recovery of metal ions.

Abstract

A new multi-compartment microfluidic device was developed to simultaneously desalinate and recover valuable metal ions from aqueous streams mimicking metal plating and mining tailing wastewaters. Heavy and valuable metal ions including copper, zinc, nickel, silver and zinc/copper ionic mixtures were selectively transferred from the feed solution to a reduction chamber through ion-exchange membranes under the influence of an electrical field. A porous and conductive carbon cloth material was used as reduction platform, which led to improved desalination efficiencies, previously unachievable due to inevitable poisoning and degradation of the membranes at high current densities. The use of conductive carbon clothes enabled the recovery of the metal deposits, post reduction, through a simple electro-oxidation process. This novel microfluidic method, using ion-exchange materials as channel walls, has also the potential for the controlled decoration of materials with metal nanoparticle patterns and for the regeneration of rare earth trace contaminants by electro-sorption assisted electro-dialysis.

Introduction

Globally, large amounts of industrial tailing effluents containing toxic and valuable metal ions and metal oxide particles are continuously generated by numerous industrial activities including mining, textile, tanning, electroplating and metal finishing operations [1], [2]. Valuable metals such as copper, zinc, iron, nickel, silver and many more are often discarded and sent to landfill or waterbodies due to the lack of effective, onsite separation and recovery methods [2], [3].

In addition to partial economic loss, the inadequate release of heavy metal ions poses serious environmental and health risks particularly prevalent in developing countries where insufficient water purification systems still remain in operation [4], [5]. Traces of heavy metal elements can now be detected in the vegetal biomass due to the use of contaminated water and polluted soils [2]. However, stricter legislations and environmental awareness are now pushing for the implementation of waste minimization and management solutions [2], [5]. Chemical precipitation, solvent extraction, membrane separation, electro-chemical reduction, ion-exchange separation and adsorption are among the main methods used for the remediation of heavy metal ions from diluted and concentrated solutions [2], [3]. Each technique has specific advantages and limitations related to the durability, energy consumption and selectivity of the process [2], [3]. Particularly, pressure-driven and electro-membrane separation processes such as electro-dialysis (ED), capacitive deionisation, nanofiltration, reverse osmosis and membrane distillation are proposed for the removal of heavy metal ions from liquid streams [6], [7].

The advantages of membrane-based processes over traditional techniques include fewer chemical consumption, reduction of sludge volume, fast and limited materials regeneration as well as simpler and scalable module design [6]. Electro-separation and electro-membrane techniques used in electro-catalytic reactors offer additional advantages due to their ability to provide specific surfaces for the reduction or precipitation of the heavy metals in direct contact or through the membrane material [8], [9]. ED is one of the most mature electro-membrane technologies widely used for the desalination of brackish water or for the pre-concentration of mineral salts [10], [11]. During ED operation, ions and charged species are selectively separated from a feed to a concentrate solution across ion-exchange membranes (IEMs) under the influence of an electric potential gradient generated between two electrodes. Many successful ED applications associated with a variety of IEM brands can be found in the literature for the removal and concentration of heavy metal ions in aqueous solutions such as zinc, copper and silver [12], [13], [14], [15]. However, one of the key limitations of ED process applied for the desalination of heavy metal streams remains the poisoning of the IEMs due to the precipitation of the metal cations across the ion-exchange polymer matrix [16].

In this research, a novel, portable and scalable microfluidic ED process is demonstrated for the one-step concentration of heavy metal ions from model wastewaters and their reduction to solid metal onto a protective, porous and electrically conductive carbon cloth. The use of microfluidic devices allowed to study the electro-diffusion and electro-reduction of ionic species in a controlled and optimized process environment. Real mining, tanning, textile or electro-plating industrial waste effluents often include a variety of organic molecule pollutants such as dyes, pesticides or resins residues [17]. However, due to the selectivity of the IEMs towards dissociated and charged species, model solutions composed of copper, zinc, nickel and silver as well as zinc/copper ionic mixtures were prepared to study the recovery of the metal ions within the microfluidics system and mimic specifically electro-plating and mining effluents. Hydrophobic macro-porous carbon cloths inserted between the cation-exchange membrane (CEM) and the anion-exchange membrane (AEM), which formed the surface of the microfluidic device, were used as reducing platforms. The experimental conditions were systematically varied to optimise the metal ions transfer rates and understand the deposition kinetics of metal ions across the carbon cloths. The metal deposits were then isolated using thermal or electrolytic methods. This novel approach where, for the first time, the microfluidics device is composed of ion exchange materials offered a quick and easy way for the recovery of heavy metals, which may be applied to a very large range of industrial feed sources from which ionic species may be recovered.

Section snippets

Materials and chemicals

Sodium chloride (NaCl, AR grade, >99.7% purity, Mw = 58.44 g·mol−1) and sodium sulphate anhydrous (Na2SO4, analytical grade, >99% purity, Mw = 142.04 g·mol−1) were purchased from ChemSupply, SA, Australia. BioReagent grade copper sulphate pentahydrate (CuSO4·5H2O, > 98% purity, Mw = 159.61 g·mol−1), zinc sulphate heptahydrate (ZnSO4·7H2O, > 99% purity Mw = 287.54 g·mol−1), silver nitrate (AgNO3, > 99%, Mw = 169.87 g·mol−1), nickel sulphate heptahydrate (NiSO4·7H2O, purum grade, >95%, Mw

Carbon cloth properties

The properties and SEM image of the plain carbon cloth materials used as a collection template are shown in Supplementary Materials, Table S1 and Fig. S1(a), respectively. The carbon cloth presented a structure of plain woven carbon fibers with average pore size of 130 μm. The pore size distribution is shown in Fig. S1(b). The macro-porous carbon cloth material provided a continuous medium for the electro-migration of ions and solvent molecules across the microfluidic module. The water contact

Conclusions

The combined desalination and recovery of an aqueous stream containing valuable single and mixed heavy metal ions was performed in a novel ED microfluidics set-up. A carbon cloth element was inserted in the concentrate compartment and acted as an electron sink limiting the adverse consequences of the polarization concentration effect on the IEMs. The heavy metal ions were captured on the carbon cloth surface by a mixed mechanism of electro-chemical reduction and alkaline precipitation. The

Acknowledgements

F.-M. Allioux would like to thank the Institute for Frontier Materials, Deakin University, Victoria, Australia for funding his PhD scholarship and AINSE Ltd for providing financial assistance (PGRA Award - 30290). L.F. DUMEE acknowledges Deakin University for his Alfred Deakin Post-doctoral Fellowship.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

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