Assisted reverse electrodialysis for CO2 electrochemical conversion and treatment of wastewater: A new approach towards more eco-friendly processes using salinity gradients
Introduction
In the last two decades, a large effort was carried out to increase the utilization of renewable energies. In 2017, 13.5% of the world total primary energy supply was produced from renewable energy sources, which includes hydro, solar, wind, biofuels, waste, geothermal and tidal [1]. In addition, several researchers are trying to exploit other forms of renewable energy including salinity gradient energy (SGE). SGE is available when two solutions with different salinity levels are mixed together. As an example, when rivers discharge into the sea, they release about 1.7 MJ per m3 of river water [2].
Salinity gradients are widely available both in nature (estuaries or coastal areas, mixing of seawater and brackish water) [3] and in industrial plants (as an example, wastewater with different salinities) [4,5] or they can be obtained using thermolytic solutions [6] regenerated with waste heat (> 40 °C) [7,8]. Reverse electrodialysis (RED) is a process for direct electricity production from salinity-gradient energy, based on the use of many pairs of anion and cation exchange membranes situated between two electrodes [9] and on the utilization of proper redox processes [10,11]. Many membrane pairs are needed for the utilization of salinity gradients to produce electricity, resulting in high investment costs for RED systems devoted to the production of electric energy [12]. In this context, in order to improve the economic figures of the overall RED process, several works investigated the main actors of the process to work at the optimal operative conditions, including the optimal load resistance, residence time, feed flow rate, stack design [13,14]. As an alternative, some authors have proposed to use salinity gradients to sustain redox processes for the synthesis of chemical or the treatment of wastewater contaminated by recalcitrant organics adopting RED stacks assembled with relatively few membrane pairs and salinity gradients available in nature or in industrial plants [4,[15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], thus saving the electric energy necessary for conventional electrolysis processes. In particular, some groups have shown that it is possible to use river, seawater and/or brine solutions to treat various kinds of wastewater both in lab-scale devices [[15], [16], [17], [18],21,22] and in a pilot-plant scale [20], while others have shown that RED can be used for the generation of hydrogen using both natural salinity gradients and thermolytic solutions regenerated by waste heat [22], [23], [24], [25], [26], [27]. In this frame, recently, it has been observed that the plants devoted to the treatment of industrial wastewater deal often with waters characterized by different salinity and the relative salinity gradient can be used to depurate part of these wastewaters by RED, allowing potentially a huge saving of energy [4].
However, the development of redox processes driven by salinity gradients is strongly limited, in order to use not too large and too expensively RED stacks, to cases where small cell potentials are required and large salinity gradients are available, thus not allowing to exploit the energy present in waters with limited salinity gradients or to help processes that require high cell potentials. In this context, we want here to propose the utilization of assisted reverse electrodialysis (A-RED) in order to use a larger range of salinity gradients and to sustain electrolysis processes characterized also by high cell potentials, reducing the investment costs for the RED stack.
In A-RED, an external current is applied in the direction of the diffusional transport of ions which follows chemical potential gradients, thus allowing to couple the external electric energy and the energy coming from salinity gradient [30]. A-RED, which was successfully proposed up to now only for desalination process [30], can be potentially used to decrease the required membrane area and/or to use lower salinity gradients and/or to achieve high overall cell potentials for the electrochemical synthesis of a large range of products. Furthermore, from a theoretical point of view, the system is expected to perform better than just the combination of electric and electrochemical driving forces [28]. In particular, in this paper, the utilization of A-RED was proposed and successfully used for the first time for two very different purposes: (i) the treatment of synthetic wastewater contaminated by organics; (ii) the conversion of carbon dioxide to high added value products. These processes were chosen as particularly promising examples, since for both an improvement of economics is necessary for the passage on an applicative scale, which could potentially be achieved using salinity gradients available in industrial plants. It was shown that A-RED allows to accelerate the redox processes, to use smaller salinity gradients with respect to RED and to reduce significantly the energetic consumptions with respect to electrolysis. Furthermore, a simplified economic analysis has shown that A-RED presents better economic data than both RED and electrolysis.
Section snippets
Stack
Experiments were performed in a homemade lab scale stack previously described in detail [11,14] and schematically shown in Fig. 1. Fig. 1 reports the main component of a RED stack showing cationic (CM), anionic (AM) and external membranes, electrodes, electrode chambers and concentrated (HC), diluted (LC) and electrode solutions (ES) flowing in the stack and the ionic flux generated by the salt gradients, as well as the main circuit for the cases-studies: i) anodic oxidation of wastewater
Anodic treatment of synthetic wastewater
The first case-study involved the anodic treatment of a synthetic wastewater with an initial TOC content of 120 mg L−1 with the aim of reducing its TOC content. The fundamentals of anodic oxidation of organics (and, in particular, of carboxylic acids and formic acid) in NaCl solutions at Ir based anodes were widely investigated by various techniques including polarizations, cyclic voltammetry and electrolysis, and clearly discussed in literature [[31], [32], [33], [34],36]. The process is based
Conclusions
Assisted reverse electrodialysis (A-RED) is here proposed for the first time for two different model redox processes, the cathodic conversion of carbon dioxide to formic acid and the anodic treatment of water contaminated by organics, and compared with both electrolysis and RED. In general, the work presents a first proof of concept that A-RED can be potentially used for many purposes, including redox processes that require high cell potentials, using waters with lower salinity gradients with
CRediT authorship contribution statement
Pengfei Ma: Investigation, Methodology, Data curation. Xiaogang Hao: Writing - review & editing. Federica Proietto: Investigation, Methodology, Data curation, Writing - review & editing. Alessandro Galia: Writing - review & editing, Funding acquisition. Onofrio Scialdone: Conceptualization, Methodology, Writing - review & editing, Supervision, Funding acquisition.
Declaration of Competing Interest
None.
Acknowledgments
University of Palermo is gratefully acknowledged for the financial support (FFR Scialdone 2018–2020).
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