Removal of nickel from electroplating rinse waters using electrostatic shielding electrodialysis/electrodeionization
Introduction
Nickel is a toxic heavy metal and nickel compounds, for instance nickel sulphide, are suspected to cause cancer [1]. Waste streams from nickel electroplating industries, textile industries or washing effluents for remediation of soil contaminated with nickel may contain up to 1000 mg L−1 nickel, which, according to environmental regulations worldwide must be controlled to an acceptable level before being discharged to the environment. Only 30–40% of all metals used in plating processes are effectively utilized i.e. plated on the articles while the rest contaminates the rinse waters during the plating process when the plated objects are rinsed upon removal from the plating bath.
Nickel removal is known to be difficult and consequently a scientific challenge. Several treatment processes have been suggested for the removal of nickel from aqueous waste streams, such as adsorption on activated carbon [2], or on red mud [3], ion exchange on zeolites [4], ion exchange on chelating resins [5], [6], microfiltration [7] and chemical precipitation [8], [9], [10].
Although chemical precipitation is the most economic and the most commonly utilized procedure for the treatment of heavy metal-bearing industrial effluents, it can become ineffective in the presence of strong complexing agents. In addition, the high buffer capacity provided by complexing agents requires excessive amounts of chemicals to neutralize alkalinity. The precipitated sludge containing the concentrated Ni(OH)2 is an extremely hazardous waste and must be disposed of using special facilities at great expense to industry.
From the viewpoint of environmental protection and resource saving, effective recycling and reusing of the metal wastewater is strongly expected. Closed-loop system or so-called effluent-free technology should be developed.
Electrochemical methods such as cathodic reduction and electrowinning [11], [12], [13], [14], electrodialysis [15], [16] and electrodeionization [17], [18], [19], [20], [21] have been recently developed to contribute to the solution of this serious environmental problem of toxic heavy metal removal.
Electrodeionization is the removal of ions and ionizable species from water or organic liquids. It uses electrically active media and an electrical potential to cause ion transport and may be operated batch wise, or continuously.
Continuous processes such as electrodialysis, and filled cell electrodialysis or otherwise called continuous electrodeionization comprise alternating permselective cation exchange membranes and anion exchange membranes, which under the influence of the electric field allow only cations or only anions respectively to permeate their mass and simultaneously retain coions so that diluate and concentrate compartments are created and deionization occurs. Membrane electrodeionization processes exhibit the known limitations associated with membranes, such as membrane fouling, scaling and concentration polarization. Furthermore, these processes cannot avoid the precipitation of bivalent metal hydroxide occurring at the resins interface and the anion exchange membranes [13], [14], [15], [16], [17]. For this reason the application of electrodeionization has been limited in wastewater treatment. Improvements with new configuration of the electrodeionization membrane stack to overcome the undesirable hydroxide precipitation are reported in Refs. [18], [19].
Batch processes such as capacitive deionization [22] or membrane capacitive deionization [23] are collection/discharge processes which rely on the formation of double-layer supercapacitor at the solution/electrode interface and need electrodes with very large specific areas, such as nano-structured activated carbon aerogels.
In our previous works [24], [25], [26], [27] we have shown that electrostatic shielding electrodialysis/electrodeionization can be realized by means of electrostatic shielding zones-ionic current sinks (ESZs-ICSs) instead of permselective ion exchange membranes. The ESZs-ICSs are formed by electronically and ionically conducting media e.g. graphite powder packed beds interposed between the anode and cathode which cause electric field discontinuity inside the electrolytic setup resulting in ion diluting and ion concentrating compartments.
The present paper offers a new alternative way of a membrane-less process of electrodialysis/electrodeionization for removal of Ni2+ ions from water solutions or industrial effluents such as nickel electroplating waste waters.
The proposed new process differs from classical electrodialysis–continuous electrodeionization processes in that it does not use any permselective ion exchange membranes and therefore it does not exhibit the above mentioned membrane associated limitations. It also differs from classical batch wise operated capacitive deionization in that it is a continuous process i.e. diluate and concentrate are received from separate and unchanged compartments without any removal of diluate and concentrate or any down time for electrode saturation, regeneration and rinsing steps.
Section snippets
Electrodes
Platinized titanium grids were used as end-electrodes in all experiments.
The intermediate electrodes ICSs must be electronically and ionically conducting. They are packed beds of graphite powder (Merck, particle size <50 μm, electrical conductivity 2 × 104 S m−2) or electrode graphite powder (Fig. 1) which is used as anode by the electrolytic production of aluminium (Aluminium of Greece, particle size <1 mm, electrical conductivity 3.3 × 104 S m−2). Anode graphite is preferable because of its better
Electrostatically shielded ion concentrating compartments—ESZs, ICSs
Current sinks and sources are local currents from a location where they can be detected into a location they cannot be detected (current sink) or vice versa (current source). Current sinks and sources have particular relevance in current across biological membranes (neurobiology) and have proved to be valuable in the study of brain function [28]. Furthermore, current sinks are used in several electronic applications [29].
However, all known current sinks are related to electronic current sinks
Conclusions
Based on the experiments conducted in this study, the following conclusions can be drawn:
- (1)
Electrostatically shielded zones-ionic current sinks made of graphite powder eliminate the applied electric field inside their mass and can therefore serve as ion traps and ion concentrating compartments in membrane-less electrodialysis and electrodeionization applications.
- (2)
The current density can be enhanced by using thin electrostatic shielding zones-ionic current sinks and therefore avoiding the electron
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