Assessment of simultaneous removal of salt and dye by utilizing capacitive deionization and UV-electro oxidation hybrid process in saline wastewater treatment

In the face of an increasing water crisis, we are developing tools to optimize capacitive deionization (CDI) for brackish water desalination. CDI can be a competitive technology for brackish water treatment due to its higher energy efficiencies compared to reverse osmosis and its more inherent resistance to fouling [1]. However, CDI is still a developing technology where adsorption capacity and salt removal rates into porous, tortuous carbon electrodes is still low. We have designed vertically-aligned carbon nanotube (VA-CNT) electrodes, with minimal tortuosity, to investigate the role of porous geometry on the performance of flow-by CDI devices, specifically examining changes in diffusion resistance, salt adsorption rate and capacity. Previously, a breadth of carbon materials have been investigated for CDI including activated carbon, carbon aerogels, ordered mesoporous carbons, carbide-derived carbons, and carbon nanotubes, among many others [2]. High salt adsorption capacities have been attributed to the presence of micropores (< 2 nm), [3] and optimal materials seem to have a presence of macroporous pathways with microporous structures. [4, 5] However, many carbon materials have tortuous pores making it challenging to decouple the role of pore diameter on salt adsorption rate. VA-CNTs are an exciting material for investigating this issue due to the ability to manipulate the inter-CNT spacing, to study changes in the ion transport rate as a function of geometry, while maintaining mininimal tortuosity and intrinsic capacitance. In this work, we synthesize VA-CNT electrodes that are sparse (inter-CNT spacings upto 100 nm), and more dense CNTs (inter-CNT spacings upto 25 nm). These CNTs are grown using a standard chemical vapor deposition of ethylene on silicon wafers with iron catalyst. By varying the partial pressure of ethylene and the oxygen content in the furnace, we synthesize CNTs of varying densities. The VA-CNT forests is delaminated from the substrate with a high-temperature oxygen etch, seperating the carbon from the corrosive catalyst. The free-standing films are mounted against Ti metal plates serving as the current collectors, and constrained by a PEEK porous mesh (200 μm window, 40 μm wire diameter). We use a flow-by CDI experimental set up to study the role of varying voltages, electrode thicknesses, CNT densities, and chemical functionalization on desalination performance. We find that in a 1mM NaCl solution, CNT electrodes can adsorb from 5-15 mg salt/g carbon, at rates of 0.2-1 mg/g-min. Through densification, we maintain gravimetric performance, but increase our volumetric salt adsorption capacity from 0.2 to 0.3 g/cm3. We achieve charge efficiencies upto 80% for these systems. We combine this experimental investigation with an electric double layer model for macroporous electrodes to inform the design of carbon electrode materials for optimal ion adsorption and throughput in a flow-by CDI device. [1] T. Humplik, J. Lee, S. O'Hern, B. Fellman, M. Baig, S. Hassan, et al., Nanotechnology, 22, 292001 (2011). [2] S. Porada, R. Zhao, A. v. d. Wal, V. Presser, and P. M. Biesheuvel, Progress in Materials Science, 58, 8 (2013). [3] S. Porada, L. Weinstein, R. Dash, A. v. d. Wal, M. Bryjak, Y. Gogotsi, et al., ACS applied materials & interfaces, 4, 3 (2012). [4] M. Suss, T. Baumann, W. Bourcier, C. Spadaccini, K. Rose, et al., Energy & Environmental Science, 5, 11 (2012). [5] C. Tsouris, J. Mayes, K. Sharma, S. Yiacoumi, et al., Environmental Science & Technology, 45, 23 (2011).

Construction and evaluation of a novel three-electrode capacitive deionization system with high desalination performance

Capacitive Deionization (CDI) is one of the most relevant emerging desalination technologies. CDI is based on the electrosorption of salts using a pair of electrically charged porous electrodes. In this way the CDI process consists on charging (ion removal)/discharging (ion desorption) cycles using the same operational principles of the supercapacitors. Therefore, CDI cells might potentially allow to recover part of the energy consumed in the desalination process while delivering drinking water. One of the key elements in this technology are, as in the supercapacitor field, the electrodes. In accordance with this, intensive research has been conducted in order to find the best electrode candidates in terms of specific surface area (SSA), porosity, electrical conductivity, surface chemistry and optimized chemical surface charge of the electrode. The combination of these properties are expected to provide higher values of salt adsorption capacity (SAC) and average salt adsorption rate (ASAR). Moreover, it should be also remarked that one of the critical requirements in order to make CDI competitive with current desalination technologies (such as reverse osmosis or electrodialysis) is the cost of the electrode materials. In order to address this challenge, the preparation of carbon electrodes from abundant biowaste precursors was studied as a low cost and green alternative to more innovative carbon composite complex materials.In this work the authors decided to evaluate different activation treatments of a biowaste precursor in order to obtain suitable activated carbons for capacitive deionization. The different structural properties (BET SSA, pore size distribution, chemical surface groups) of the synthesized AC’s were correlated not only with their electrochemical response (specific capacitance) but also with the CDI performance (SAC, electrosorption kinetics, charge efficiency and the energy consumption).

Metal-organic framework–derived carbon/N-doped three-dimensional reduced graphene oxide composite with high capacitive deionization performance

Pseudocapacitive deionization with polypyrrole grafted CMC carbon aerogel electrodes

Tamarind shell derived N-doped carbon for capacitive deionization (CDI) studies

A direct comparison of flow-by and flow-through capacitive deionization

Brackish water resources may be an attractive option for human consumption, agriculture, and industry if efficient water purification can be implemented. In the past few decades, research and development of various desalination technologies have been carried out, among which distillation, reverse osmosis, and electrodialysis are the most commonly known and widespread.1 Capacitive deionization (CDI) is an alternative, emerging, and energy-efficient technology for water desalination, which employs an electrochemical flow cell configured with polarized porous carbon electrodes to remove ionized salts in a stream with low molar concentration. Briefly, by regulating an external voltage to a CDI cell, ionized salts are electrostatically captured (or released) in the pores of the carbon electrodes, resulting in the stream being deionized (or the electrodes being regenerated).2-4 Recent studies have found that the salt adsorption capacity (SAC) could be substantially improved by using surface modified carbon electrodes resulting from nitric acid and ethylenediamine treatments.5 Combined with the modified Donnan model including a term of chemical surface charge, this improved SAC was accounted for by enhancement of the chemical charges immobilized in the carbon micropores, validating both enhanced CDI (e-CDI) and extended-voltage CDI (eV-CDI) effects in the CDI literature (Fig. 1).6In summary, it is considered that, for the carbon electrodes used in a CDI cell, an increase in the chemical surface charges makes the pores more readily available for salt adsorption under proper applied voltages. In addition to the surface modified carbon electrodes, immobilized chemical charges can be found in ion-exchange materials. For instance, a well-known cation-exchange polymer, Nafion, contains the negative chemical charges, -SO3 -, while an anion-exchange polymer typically holds positive chemical charges, e.g., NR4 + and NR3 +. As a consequence, together with the knowledge gained above, ion-exchange polymers coating were used in our current studies to explore new composite carbon electrodes for CDI cycling tests. As shown in an initial test (Fig. 2), the addition of ion-exchange polymers results in the SAC not only being increased but also being stabilized with operational time when NaCl solution was used. In this presentation, the preparation and characterizations of composite carbon electrodes will be detailed including comparisons to conventional CDI and membrane capacitive deionization cells. Furthermore, these composite electrodes will be configured into a CDI cell to investigate both e-CDI and eV-CDI effects in various salt solutions such as CaCl2, Na2SO4, and NH4NO3. In addition, the relevant charge efficiency and cycling longevity will be reported and discussed. Figure 1. Demonstration of both enhanced CDI (e-CDI) and extended voltage CDI (eV-CDI) effects using the modified Donnan model with the addition of chemical surface charge. The parameters used in the model can be found in ref. (5 and 6). Figure 2. Improved salt adsorption capacity and operational stability using cation- and anion-exchange polymers added to the carbon cathode and anode, respectively, in a CDI cell. The CDI cell was operated using 1 V charging and at 0 V discharging in ~31 L of ~7 mM deaerated NaCl solution.

Defluorination technology is crucial for ensuring the safety of accessible water. The application of capacitive deionization (CDI) technology faces challenges due to competitive adsorption of fluoride ions within complex natural fluoride-rich brackish water matrices, which often contain high levels of dissolved inorganic carbon (DIC) species (mainly HCO3– and CO32–). These DIC species are pH-dependent, playing a significant role in the selective removal of fluoride by the CDI process. Thus, there is a knowledge gap in understanding the effects of membranes in membrane capacitive deionization (MCDI) on fluoride removal. In this study, we examined the key operating parameters in CDI and MCDI, including applied constant voltages and different types of anion-exchange membranes (AEMs), on the desalination performance in F- and dissolved inorganic carbon water matrices. The application of AEMs significantly improve the salt adsorption capacity (SAC) for both F- and DIC species, and reduced energy consumption. However, it simultaneously results in a notable decrease in F- selectivity as membranes control mass transfer. Higher applied voltages enhance the SAC performance for F- and DIC species, but also induce more severe Faradaic reactions, leading to increased energy consumption and lower energy efficiency. Additionally, ion species and pH changes during CDI and MCDI processes are interrelated, indicating that stability tests of CDI electrodes in batch mode are not reliable when using the same testing solution repeatedly. The diverse valence states of ions in the solution impact pH variations under different voltages in the CDI/MCDI process. These findings provide valuable insights into the development of water purification and desalination technology, particularly for the application and further advancement of selective fluoride removal by the CDI process.

Capacitive deionization (CDI) represents one of the most thermodynamically efficient technologies for brackish water desalination, and its performance is highly dependent on the intrinsic properties of carbon materials. Ideally, CDI carbons should have high ion-accessible surface area, high porosity for ion mobility, great hydrophilic properties, excellent corrosion tolerance and good processability. Pyrolysis of precursory zeolitic-imidazolate frameworks (ZIFs) serves as a promising way to produce carbonaceous materials with great compositional and structural tunability.In this work, we systematically prepared an array of ZIFs (Zn(ligand)2) with different side-chain substitutions, which upon pyrolysis gave rise to carbon materials with variable elemental compositions, surface properties, wettability and graphitization levels; all are impacting the CDI performance. Zn(4abIm)2-C afforded the best salt adsorption capacity, while Zn(mIm)2-C showed the best overall salt adsorption capacity and rate; both exceeded the performance of the commercial carbon blacks.After careful correlation between the structures and the electrochemical results, it has been demonstrated that the CDI salt adsorption capacity increases with the carbon’s double layer capacitance. Additionally, graphitization level is significantly correlated with the CDI charge efficiency and energy consumption with more electronic conductive higher charge efficiency and lower energy consumption, which provides new insights to the field.

Analysis of the desalting performance of flow-electrode capacitive deionization under short-circuited closed cycle operation

The novel α-TiO2@g-C3N4 heterostructure for ultra rapid ionic pumping and effective capacitive deionization

The magnitude and distribution of the electric field between two conducting electrodes of a capacitive deionization (CDI) device plays an important role in governing the desalting capacity. A dielectric coating on these electrodes can polarize under an applied potential to modulate the net electric field and hence the salt adsorption capacity of the device. Using finite element models, we show the extent and nature of electric field modulation, associated with changes in the size, thickness, and permittivity of commonly used nanostructured dielectric coatings such as zinc oxide (ZnO) and titanium dioxide (TiO2). Experimental data pertaining to the simulation are obtained by coating activated carbon cloth (ACC) with nanoparticles of ZnO and TiO2 and using them as electrodes in a CDI device. The dielectric-coated electrodes displayed faster desalting kinetics of 1.7 and 1.55 mg g-1 min-1 and higher unsaturated specific salt adsorption capacities of 5.72 and 5.3 mg g-1 for ZnO and TiO2, respectively. In contrast, uncoated ACC had a salt adsorption rate and capacity of 1.05 mg g-1 min-1 and 3.95 mg g-1, respectively. The desalting data is analyzed with respect to the electrical parameters of the electrodes extracted from cyclic voltammetry and impedance measurements. Additionally, the obtained results are correlated with the simulation data to ascertain the governing principles for the changes observed and advances that can be achieved through dielectric-based electrode modifications for enhancing the CDI device performance.

Hybrid capacitive deionization (HCDI) is emerging as an energy-efficient alternative for brackish water desalination with low expenditure cost and favourable ion removal effectiveness by adopting battery-type electrodes in the traditional capacitive deionization (CDI) system. Herein, an unscalable electrospinning method is introduced to contrive cathode material for HCDI system. Co3O4 and nitrogen-doped carbon nanotube decoration are formed successfully and uniformly distributed with the hollow structure of free-standing carbon nanofibers (Co3O4@CNF@CNT). The conductive “bridges” provided by the CNF matrix significantly shorten the diffusion length of Na+ and promote the electrical conductivity of the Co3O4 nanoparticles. Moreover, benefiting from the incorporation of nitrogen-doped CNTs as the “medium”, the contact between electrolyte and electrode material is improved and the electrical conductivity is further enhanced. The as-prepared Co3O4@CNF@CNT cathode shows excellent pseudocapacitive performance of 395 F g-1 at a scan rate of 1 mV s-1 and superior rate performance of 279 F g-1 at 100 mV s-1. The HCDI system delivers an outstanding salt adsorption capacity (SAC) of 58.6 mg g-1 and a highest salt adsorption rate (SAR) of 12.27 mg g-1 min-1 with a potential difference of 1.4 V, indicating the great potential of applying Co3O4@CNF@CNT in the practical HCDI system.

Hierarchical Co3O4/CNT decorated electrospun hollow nanofiber for efficient hybrid capacitive deionization