Electrochemical technologies in wastewater treatment
Introduction
Using electricity to treat water was first proposed in UK in 1889 [1]. The application of electrolysis in mineral beneficiation was patented by Elmore in 1904 [2]. Electrocoagulation (EC) with aluminum and iron electrodes was patented in the US in 1909. The electrocoagulation of drinking water was first applied on a large scale in the US in 1946 [3], [4]. Because of the relatively large capital investment and the expensive electricity supply, electrochemical water or wastewater technologies did not find wide application worldwide then. Extensive research, however, in the US and the former USSR during the following half century has accumulated abundant amount of knowledge. With the ever increasing standard of drinking water supply and the stringent environmental regulations regarding the wastewater discharge, electrochemical technologies have regained their importance worldwide during the past two decades. There are companies supplying facilities for metal recoveries, for treating drinking water or process water, treating various wastewaters resulting from tannery, electroplating, diary, textile processing, oil and oil-in-water emulsion, etc. Nowadays, electrochemical technologies have reached such a state that they are not only comparable with other technologies in terms of cost but also are more efficient and more compact. For some situations, electrochemical technologies may be the indispensable step in treating wastewaters containing refractory pollutants. In this paper, I shall examine the established technologies such as electrochemical reactors for metal recovery, electrocoagulation, electroflotation and electrooxidation. The emerging technologies such as electrophotooxidation, electrodisinfection will not be discussed. In addition, I shall focus more on the technologies rather than analyzing the sciences or mechanisms behind them. For books dealing with environmentally related electrochemistry, the readers are referred to other publications [5], [6], [7], [8].
Before introducing the specific technologies, let us review a few terminologies that are concerned by electrochemical process engineers. The most frequently referred terminology besides potential and current may be the current density, i, the current per area of electrode. It determines the rate of a process. The next parameter is current efficiency, CE, the ratio of current consumed in producing a target product to that of total consumption. Current efficiency indicates both the specificity of a process and also the performance of the electrocatalysis involving surface reaction as well as mass transfer. The space–time yield, YST, of a reactor is defined as the mass of product produced by the reactor volume in unit time withThe space–time yield gives an overall index of a reactor performance, especially the influence of the specific electrode area, a.
Section snippets
Electrochemical reactors for metal recovery
The electrochemical recovery of metals has been practiced in the form of electrometallurgy since long time ago [9]. The earliest reported application of electrochemical phenomena in chemical subjects was supposed to be carried out by Pliny in protecting iron with lead electroplating [10]. The first recorded example of electrometallurgy was in mid-17th century in Europe [11]. It involved the recovery of copper from cupriferous mine water electrochemically. During the past two and half centuries,
Electrocoagulation
Electrocoagulation involves the generation of coagulants in situ by dissolving electrically either aluminum or iron ions from respectively aluminum or iron electrodes. The metal ions generation takes place at the anode, hydrogen gas is released from the cathode. The hydrogen gas would also help to float the flocculated particles out of the water. This process sometimes is called electrofloculation. It is schematically shown in Fig. 10. The electrodes can be arranged in a mono-polar or bi-polar
Electroflotation
Electroflotation is a simple process that floats pollutants to the surface of a water body by tiny bubbles of hydrogen and oxygen gases generated from water electrolysis [53]. Therefore, the electrochemical reactions at the cathode and anode are hydrogen evolution and oxygen evolution reactions, respectively. EF was first proposed by Elmore in 1904 for flotation of valuable minerals from ores [2].
Electrooxidation (EO)
Study on electrooxidation for wastewater treatment goes back to the 19th century, when electrochemical decomposition of cyanide was investigated [105]. Extensive investigation of this technology commenced since the late 1970s. During the last two decades, research works have been focused on the efficiency in oxidizing various pollutants on different electrodes, improvement of the electrocatalytic activity and electrochemical stability of electrode materials, investigation of factors affecting
Summary
Electrochemical technologies have been investigated as the effluent treatment processes for over a century. Fundamental as well as engineering researches have established the electrochemical deposition technology in metal recovery or heavy metal-effluent treatment. Electrocoagulation has been used industrially and demonstrated its superior performances in treating effluents containing suspended solids, oil and grease, and even organic or inorganic pollutants that can be flocculated.
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