Designed assembly of Ni/MAX (Ti₃AlC₂) and porous graphene-based asymmetric electrodes for capacitive deionization of multivalent ions

Highlights

• The Ni/MAX nanocomposite was synthesized via a simple microwave technique.

• The 3D porous graphene oxide (pRGO) was prepared by using lemon juice and fish-sperm DNA.

• 2D-Ni/MAX and 3D-pRGO were tested as asymmetric CDI electrodes for heavy metals removal.

• The developed electrodes exhibited electrosorption capacity of 76 mg g⁻¹ for Pb²⁺.

Abstract

The contamination of aquatic ecosystems by fluoride and heavy metal ions constitute an environmental hazard and has been proven to be harmful to human health. This study explores the feasibility of using asymmetric capacitive deionization (CDI) electrodes to remove such toxic ions from wastewater. An asymmetric CDI cell was fabricated using 2D Ni/MAX as an anode and 3D porous reduced graphene oxide (pRGO) as a cathode for the electrosorption of F⁻, Pb²⁺, and As(III) ions. A simple microwave process was used for the synthesis of Ni/MAX composite using fish sperm DNA (f-DNA) as a cross-linker between MAX nanosheets (NSs) and the metallic Ni nanoparticles (NPs). Further, pRGO anode was prepared through effective reduction of RGO using lemon juice as green reducing agent with the assist of f-DNA as a structure-directing agent for the formation of 3D network. With this tailored nanoarchitecture, pRGO and Ni/MAX electrodes exhibited a high specific capacitance of 760 and 385 F g⁻¹, respectively. The fabricated Ni/MAX and pRGO based CDI system demonstrated a high electrosorption capacity of 68, 76, and 51 mg g⁻¹ for the monovalent F⁻, divalent Pb²⁺, and trivalent As(III) ions at 1.4 V in neutral pH. Furthermore, Ni/MAX//pRGO system was successfully applied for the removal of total F(T), Pb(T), and As(T) ions from real industrial wastewater and contaminated groundwater. The present findings indicate that the fabricated Ni/MAX//pRGO electrode has excellent electrochemical properties that can be exploited for the removal of anionic and cationic metal ions from aqueous solutions in a CDI based system.

Introduction

The drastic growth in human population, industrialization, and contemporary lifestyle demands a higher quantity of freshwater and/or treated wastewater to maintain the living standards. In general, industrial wastewater usually contains heavy metals, radioactive elements, and other synthetic compounds such as dyes and pharmaceutical ingredients (Selvaraj et al., 2020). Predominantly, the heavy metals (density > 5 g/cm³) severely affects the ecological system due to their toxicity and non-biodegradability, unlike other organic constituents (Sajid et al., 2018). Hazardous components such as fluoride (F⁻), lead (Pb²⁺), cadmium (Cd²⁺), copper (Cu²⁺), zinc (Zn²⁺), mercury (Hg²⁺), chromium (Cr³⁺), and arsenic (As³⁺) present in wastewaters, are of great concern as they explicitly contaminate the freshwater sources, and pose a serious threat to human health. Most of these constituents are carcinogenic in nature and cause many health issues like anemia, lungs/kidneys cancer, internal tumors, and nervous system disorders in living organisms (Ihsanullah et al., 2016) These toxic pollutants have been treated by number of techniques such as adsorption, photocatalytic degradation, coagulation-flocculation, membrane technology, ion-exchange, electrodialysis, and bioelectrochemical systems (Rambabu and & Velu, 2014; Gao et al., 2014; Imran Ahmed et al., 2016). However, all these processes have some limitations and cannot be effectively employed for heavy metals removal from waste streams. For instance, adsorption, membrane technology and ion-exchange processes require frequent regeneration steps, electrodialysis and membrane separations are energy extensive techniques, while coagulation-flocculation and bioelectrochemical methods produce a secondary waste (Huang et al., 2016). Thus, there is a need for the development of reliable methods to treat or remove heavy metals from water and wastewater in an eco-friendly manner.

Capacitive deionization (CDI) is an emerging and robust water treatment technology for heavy metals removal and brackish/seawater desalination applications (Qin et al., 2019; Elimelech and Phillip, 2011). Briefly, in CDI process, the cathode (negatively charged electrode) attracts the metal ions (Na⁺, Ca²⁺, Hg²⁺, Mg²⁺, Pb²⁺, Cd²⁺, Cr³⁺, As³⁺), while the anions (F⁻, Cl⁻, SO₄²⁻, NO³⁻, AsO₄³⁻) are attracted on the positively charged electrode surface under the influence of an applied electric field of 0–1.2 V. Compared to conventional methods, CDI is rapid, cost-effective and environmentally safe water treatment technique without the production of secondary pollutants. CDI technique has effectively been adopted to treat effluents even at very low concentration of heavy metals (Huang et al., 2016). The applied voltage, flow-rate, cell configuration, feed composition and electrode materials are the dominant factors that affect the CDI performance. The adsorption capacity and the electrochemical characteristics of CDI setup specifically depends on the nature and structure of the electrode materials. The synergistic physicochemical characteristics, higher surface area, selective surface chemistry, tunable geometry, superior electrochemical properties, higher sorption capacity and mechanical stability are the desirable features of the CDI electrodes (Oren, 2008). The inadequacy of an ideal electrode material with all these features have attracted the researchers to develop novel, efficient and cost-effective CDI electrodes for heavy metals removal and desalination applications.

Over the past few decades, carbon-based materials have been investigated as efficient and economic electrodes due to their high specific surface area, superior electrical conductivity, and low cost of production (Jayaraman et al., 2018; Theerthagiri et al., 2018, 2019). The activated carbon derived from tea waste, peanut shells, and watermelon, and their associated metallic nanocomposites have been tested for CDI applications (Bharath et al., 2019a, 2020a, 2020b; Rambabu et al., 2020). These carbon composite electrodes exhibited higher electrosorption capacity and cyclic stability than the pristine carbon materials. Zou et al. (Li et al., 2009) synthesized the ordered mesoporous carbon (OMC) decorated with nickel salts through a modified sol-gel process. They reported a specific surface area and adsorption capacity of 1491 m²/g and 15.9 μmol/g, respectively. Further, 2D hexagonal OMC has also been developed and established as CDI electrode materials (Peng et al., 2011). It was reported that the as-developed 2D OMC exhibited higher sorption capacity compared to 3D symmetry cubic and 3D structured mesoporous carbon (Peng et al., 2011). Huang et al. (2016) reported the competitive removal of metal ions such as Pb²⁺, Cd²⁺, and Cr³⁺ through CDI using activated carbon cloth as an electrode material. The maximum ion removal efficiencies of 81%, 78% and 42% were achieved for Pb²⁺, Cr³⁺ and Cd²⁺ with an initial concentration of 0.05 mM, respectively. The activated carbon electrode derived from chicken feathers was also investigated for Pb²⁺ removal through CDI technique, which exhibited a sorption capacity of 4.1 mg g⁻¹ for the initial salt concentration of 100 mg L⁻¹. However, the pure porous carbon revealed lower adsorption capacity and limited desalination enactment due to lower interactions level between the charged ions and the electrode surface.

Recently, 2-dimensional (2D) nanomaterials such as transition metal dichalcogenides (TMDs) and transition metal carbides have attracted the attention of researchers because of their intrinsic characteristics, superior electrical conductivity and higher charge storage capacity (Ihsanullah, 2020; Rasool et al., 2019; Kokulnathan et al., 2020; Suvina et al., 2020). Based on these fascinating features, the 2D materials enhanced the selective removal of ions without using ion-exchange membranes, even at higher molar concentrations (Torkamanzadeh et al., 2020). MXene belongs to the family of transition metal carbides and offer variety of applications because of their remarkable physicochemical, structural, surface and electrochemical characteristics and multifarious chemical composition (Naguib et al., 2014; Jayaraman et al., 2015). Srimuk et al. (2018) reported that the MXene-based CDI electrode exhibited a desalination capacity of 13 mg g⁻¹ for an applied cell voltage of 1.2 V. The developed electrodes showed superior regeneration performance up to 30 operational cycles. The NaCl removal efficiency of MAX, and MXene-based CDI electrodes was further enhanced by incorporation of carbon nanotubes and polymer binders onto the surface of MXenes (Kumar et al., 2020). These 2D materials could also be effectively utilized for heavy metals removal through the CDI technique (Shen et al., 2020).

To improve the electrosorption capacity, charge efficiency, surface wettability, and mechanical stability, MXene-based composite electrodes can be instigated by incorporation of various nanomaterials. In this study, porous reduced graphene oxide (pRGO), Ni, MAX and Ni/MAX nanocomposites were synthesized and analyzed through various analytical techniques to explore their physiochemical features and electrochemical characteristics Sakthinathan et al., 2019; Theerthagiri et al., 2020. Subsequently, the pristine pRGO, and Ni/MAX nanocomposite electrodes were used for the removal of monovalent F⁻, divalent Pb²⁺, and trivalent As(III) ions by CDI technique. The effect of various operational parameters, including cell voltage, pH, and feed concentrations on the sorption capacity was also presented. Interestingly, the real samples of underground water and industrial discharge to river were used to compare the performance of as-developed electrodes. The electrode with the highest ions removal and sorption capacity was further explored to examine its stability and recyclability. The detailed results with necessary discussions and inferences are presented.