Enhancement of Cd(II) electrosorption using electrosorption process with manganese oxide nanomaterial electrodeposited
Graphical abstract
Introduction
Water contamination by heavy metals, including cadmium, was a significant problem for humans and the environment [1]. Heavy metal ions were major components in pollutants contained in wastewater [2], accumulate due to their non-biodegradability properties in living organisms [3], and causing high toxicity and carcinogenicity [4]. Exposure to cadmium ions can cause several adverse effects, including nausea, muscle cramps, loss of the calcium element from bones, skeletal deformities, the formation of cadmium rings (yellow coloration of teeth), and vomiting [5]. It is also known for its half-life in the human body, estimated between 4 and 38 years [6]. The principal sources of cadmium release originate from industrial processes involving dyes, metallurgy, battery production [7], plastics, and fuel combustion [8]. The World Health Organization (WHO) recommended a guideline value of 0.005 mg/L as the limit for Cd(II) in drinking water [9]. Consequently, it was crucial to reduce or eliminate as much as possible the excess Cd(II) ions contained in wastewater before it was released into the environment, with the implementation of efficient and inexpensive technologies and processes. Several methods and processes were used to remove heavy metals from aqueous solution, including membrane processes, ion exchange, coagulation-flocculation, flotation, and chemical precipitation…etc. These techniques were inefficient to remove heavy metals and presents operational difficulties. Among these cited methods was the technique of conventional chemical precipitation, and despite its usability and relatively low cost [10], it proved to be ineffective for alkali-soluble metal ions and to respond to increasingly strict environmental regulations. Other processes have advantages such as, membrane separation and ion exchange because of their rapidity and efficiency. The application of these two techniques was limited due to the high cost of resins and membranes. The biosorption and ion exchange methods were used to remove some heavy metals (Cd, Zn, Cu, Cr… etc.) but before these methods, a pretreatment was indispensable to remove the suspended solids [11]. In this context, it was necessary to develop efficient, less expensive, energy-saving and environmentally friendly techniques to reduce or eliminate metal ions definitively. The most promising water treatment techniques that have attracted remarkable interest in recent years were the electrosorption or electrochemical adsorption or even the capacitive deionization. These techniques were very efficient and effective because they combine the advantages of adsorption and electrochemical technology [12]. These processes were characterized by lower energy consumption, operational simplicity, absence of secondary pollution, and respect for the environment [13]. Several electrodes were used in deionization capacitors, the most frequently used were carbon-based materials, including carbon aerogels [14], carbon materials doped with nitrogen [15], graphene [16], and porous carbon [17]. The capacitive deionization technique was also used to remove metal ions in an aqueous medium, the technology performance was affected by the type of materials or electrodes used, and also the potential applied. For example, the conventional carbon-based materials were used to remove As(III) and As(V) ions from an aqueous solution [18].This carbon electrode demonstrated a removal capacity of 24.57 for Cu(II) (Eapplied = 0.8 V) [19], and 2.47 × 10−2 mg/g for As(V) (Eapplied = 1.2 V) [18]. At an applied potential of 1.2 V, the activated carbon revealed an efficiency of 42%, 78% and 81% for Cd(II), Cr(III) and Pb(II) ions respectively.
To improve the electrosorption capacity, other studies were proceeded to use transition metal oxide/carbon nanocomposites in the form of materials such as TiO2/graphene [20], and ZnO/activated carbon cloth [21]. Among the transition materials have attracted a lot of attention is manganese oxide, characterized by a high specific capacity [22], high adsorption capacity [23], abundant resources [24] and environmental friendliness, and low cost [25]. Birnessite is a mineral oxide belongs to the large manganese oxide family (MnO2), characterized by a lamellar structure, consisting of octahedral MnO6 layers, separated by water molecules and alkali metal ions (K+, Na+… etc.) with a spacing of 7 Å [26]. Birnessite-type MnO2 proved in several studies its high specific electrochemical capacity [27], high adsorption capacity and reactivity to metal ions [28], and these properties were principally attributed to the layered nature of the material structure and the Mn(IV) vacancies available in the octahedral layers of MnO6, as well as to the spacing between these layers [29]. The use of chemically synthesized birnessite-type (powder) exhibited a high removal efficiency of heavy metals (Pb(II), Cd(II)) compared to other structures [28]. In fact, the adsorption capacity of Pb(II) and Cd(II) was higher for birnessite in comparison with cryptomelane. This difference was particularly due to the structure, chemical composition, surface properties, and material crystallinity [28]. Interlayer alkali cations such as K+, H+ and Na+ inserted in birnessite-type nanomaterials demonstrated their effect on the structure and morphology of nanomaterials and consequently on their metal ion removal efficiency [30].
Electrochemical methods have also used chemically synthesized birnessite as an electrode to remove various heavy metals such as Cd(II) [31], [32], Zn(II) and Ni(II) [33] through the imposition of a continuous voltage and a galvanostatic charge-discharge. The Birnessite-type MnO2 (chemically synthesized) was a suitable adsorbent for heavy metals removal by electrochemical methods. In spite of the high electrosorption/adsorption capacity recorded for these chemically synthesized materials, they release a significant amount of Mn in solution [32], [34]. Little attention is focused on the use of electrodeposited birnessite and its application for the electrosorption of heavy metals. The manganese oxides elaboration can be carried out by electrochemical route. In fact, the application of this method presents several advantages, the electrodeposited material was realized in soft and simple operating conditions and did not require any specific reagent.
The aim of this work is to remove Cd(II) ions in aqueous medium by using a nanomaterial (named Na-MnO2/SnO2) synthesized by electrochemical process and by using electrosorption as a powerful technique. This approach allows to improve the efficiency of removing metal ions and to avoid using electrodes based on materials synthesized by chemical route. The fabrication of this type of material requires additional steps such as filtration, while respecting the principles and aspects of green and sustainable chemistry to ensure a cleaner and non-toxic treatment process. The electrosynthesis of manganese oxide birnessite-type on a SnO2 electrode was achieved by an electrochemical process. This method does not require filtration steps during the electrosynthesis, even after the metal ions electrosorption. The structure and morphology of the formed layers were determined by X-ray diffraction (XRD) and scanning electron microscopy (SEM) coupled with energy dispersive spectrometry (EDS). The electrochemical performance of the electrodeposited layers was evaluated by cyclic voltammetry (CV). The electrodeposited birnessite application to remove Cd(II) in an aqueous medium by the electrosorption method was investigated. The electrosorption study was performed by applying a constant potential over time, and by studying the effect of applied potential on the electrosorption capacity of Cd(II) by thin films. The effect of the electrodeposited charge, the initial pH and the metal ions concentration on the thin films performance were also evaluated. A comparative study between the two techniques adsorption and electrosorption of Cd(II) by the electrodeposited nanomaterial was achieved. During cadmium ion electrosorption, nanomaterial stability was performed by measuring manganese released in solution. The possibility of the nanomaterial reusability was also discussed. This paper presents a new approach for the metal ions electrosorption from wastewater by electrochemically synthesized birnessite thin films.
Section snippets
Chemical compounds and electrodeposition of Na-MnO2/SnO2
All reagents MnSO4·H2O (98%), Na2SO4 (99%), H2SO4 (95–98%), NaOH (95–98%) and Cd(NO3)2·4H2O (98%), NaNO3 (99%), NaCl (99%), Mohr's salt ((NH4)2Fe(SO4)2·6H2O) and sodium pyrophosphate (Na2H2P2O7) were purchased from Sigma Aldrich and were used without further purification. The Millipore Milli-Q deionized water used for the preparation of the solutions. Electrochemical measurements were performed at room temperature using an AUTOULAB potentiostat, controlled by a computer using the software
Electrodeposition of Na-MnO2/SnO2 thin films
The appropriate potential for the elaborated manganese oxide electrodeposited was determined by cyclic voltammetry (CV) (Fig. S1).
According to the literature, electrodeposition was carried out in 3 oxidation reactions. This electrochemical behavior can be explained by the following global reaction [36]:
Electrodeposition of manganese oxide was performed at room temperature in a classical electrochemical cell by chronoamperometry at E = 1.1 V with a fixed electric charge
Evaluation of the energy consumption
In both processes used adsorption and electrosorption, the total energy consumed to remove 30 mg/L of Cd(II) by the electrodeposited nanomaterial Na-MnO2/SnO2, was calculated according to Eq. (2):with
ET (kWh): total energy consumed.
Eelectrodeposition (kWh): Energy consumed for the electrodeposition of the Na-MnO2/SnO2 nanomaterial.
Einteraction (kWh): Energy consumed during the interaction of the Na-MnO2/SnO2 with Cd(II).
Conclusion
A innovative clean and non-toxic approach without secondary pollution is achieved for the removal of Cd(II) metal ions, using the electrosorption technique and Na-MnO2/SnO2 as an electrodeposited nanomaterial. The birnessite thin layers (Na-MnO2/SnO2) were electrodeposited by a simple, soft and non-toxic method under ambient conditions. The application of the electrosorption technique promotes the Cd(II) ions removal by Na-MnO2/SnO2 nanomaterial. The structural and morphological
Novelty statement
Sorption is an efficient process for transferring heavy metals from polluted water to a solid phase. Birnessite, which is non-toxic layered manganese oxide is particularly interesting for this objective due to its high sorption capacities towards heavy metals. In this present work, we proposed a new approach to reduce the content of Cd(II) ions in an aqueous medium by electrosorption technique using an electrodeposited nanomaterial (birnessite type). Pure and nanostructured birnessite was
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors warmly thank the Office Chérifien des Phosphates (OCP), Research and Development Foundation (R&D) for the financial support of this project National Centre for Scientific and Technical Research (CNRST), Ministry of Higher Education, Scientific Research and Training.
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