ELECTROCHEMICAL TREATMENT (ECT)
ECT process can be another alternative process for treating various wastewaters. The major methods for ECT are: electro-coagulation (EC), electro-flotation (EF) and electro-oxidation (EO). An ECT unit consists of anodes and cathodes in parallel mode. When electric power is applied from a power source, the anode material gets oxidized and the cathode is subjected to reduction of elemental metals and due to further reactions depending on conditions applied, removal of various pollutants takes place by EC and/or EF and/or EO mechanisms.
EF is a simple process in which buoyant gases bubbles generated during electrolysis take along with them the pollutant materials to the surface of liquid body. The bubbles of hydrogen and oxygen which are generated from water electrolysis move upwards in the liquid phase. A layer of foam, containing gas bubbles and floated particles is formed at the surface of water. The rate of flotation depends on several parameters such as surface tension between the water particles and gas bubbles; the bubble size distribution and bubble density; size distribution of the particles; the residence time of the solution/liquid in the EC cell and the flotation tank; the particle and gas bubble zeta potentials; and the temperature, pH of the solution .
Decomposition of organic materials through EO treatment means the oxidation of organics present in wastewater to carbon dioxide and water or other oxides. The electrochemical oxidation of wastewater is achieved in two ways. First, by direct anodic oxidation, in which organics are adsorbed at the electrode and oxidized at the surface of the electrode and second, by indirect oxidation in which some oxidizing agents are generated electrochemically which are responsible for oxidation of organics present in the solution.
Organic pollutants are adsorbed on the anode surface in direct anodic oxidation process, where active oxygen (adsorbed hydroxyl radicals) or chemisorbed “active oxygen” is accountable for the oxidation of adsorbed Organics pollutants. The mechanism of oxidation of organic matter on oxide anode (MOX) was suggested by Comninellis . The reactions involve are as follows:
The adsorbed hydroxyl radicals may form chemisorbed active oxygen
The liberated chemisorbed active oxygen is responsible for the oxidation. During the EO treatment process, two types, of oxidation is possible. In one way, toxic and non-biocompatible pollutants are converted into bio-degradable organics, so that further biological treatment can be initiated. In contrast, in other way, pollutants are oxidized to water and CO2 and no further purification is necessary. In an indirect oxidation process, strong oxidant such as hypochlorite/chlorine, ozone, and hydrogen peroxide  are regenerated during electrolysis. Following reaction shows the formation of hypochlorite:
High voltage can led to formation of hydrogen peroxide and other molecules as follows:
These oxidants oxidize many inorganic and organic pollutants in the bulk solution.
EC, like coagulation, is the process of destabilization of colloidal particles present in wastewater and can be achieved by two mechanisms: one in which an increase in ionic concentration, reduce the zeta potential, and adsorption of counter-ions on colloidal particles neutralises the particle charge; and other by well known mechanism of sweep flocculation [6,7]. Various reactions take place in the EC reactor during its operation. As the current is applied, the anode material undergoes oxidation and cathode gets reduced. If iron or Al electrodes are used, Fe2+ and Al3+ ion generation takes place at anode by the following reaction [8,9]
In addition, oxygen evolution can compete with iron or aluminum dissolution at the anode via the following reaction:
At the cathode, hydrogen evolution takes place via the following reaction:
Liberated Fe2+/Al3+ and OH– ions react to form various monomeric and polymeric hydrolyzed species. The concentration of the hydrolyzed metal species depends on the metal concentration, and the solution pH. These metal hydrolysed products are responsible for the coagulation of pollutants from solution .
FACTORS AFFECTING ECT PROCESS
Current density (J), electrolysis time (t) and anodic dissolution: Faraday’s law describes the relationship between current density (J) and the amount of anode material that dissolves in the solution. It is given as :
Where, is the theoretical amount of ion produced per unit surface area by current density J passed for duration of time, t. Z is the number of electrons involved in the oxidation/reduction reaction, M is the atomic weight of anode material and F is the Faraday’s constant (96486 C/mol).
The pollutants removal efficiency depends directly on the concentration of aluminum ions produced by the metal electrodes, which in turn as per Faradays law depends upon the t and J. When the value of t and J increases, an increase occurs in the concentration of metal ions and their hydroxide flocs. Consequently, an increase in t and J increases the removal efficiency.
Theoretically, according to the Faraday’s law when 1 F of charge passes through the circuit, 28 g of iron is dissolved at each electrode individually connected to the positive node of the power supply unit. During the coagulation process with iron electrodes, the valency of the coagulant increases, with Fe3+ being much more effective than the Fe2+.
pH: The initial pH (pHi) of the wastewater will have a significant impact on the efficiency of the ECT. The effects of pHi on the ECT of wastewater can be reflected by the solubility of metal hydroxides. The effluent pH after ECT would increase. The incremental increase in pH with an incremental increase in the amount of current applied tends to decrease at higher current . The general cause of the pH increase can be explained from the following equation:
At the cathode, generated hydrogen gas (which attaches to the flocculated agglomerates, resulting in flocs floation to the surface of the water) and this causes the pH to increase as the hydroxide-ion concentration in the water increases. This reaction is one of the dominant reactions that occur in the electro-flocculation system [3, 12]. Also, due to the following reaction, pH is affected:
These two reactions tend to neutralize pH. This is the reason, which, however, prevents larger pH increases due to larger hydroxides formations at higher current densities.
Conductivity and the effect of salts: Feed conductivity is an important parameter in ECT, since it directly affects the energy consumed per unit mass of pollutants removed. If conductivity is low, higher amount of energy is consumed per unit of mass of pollutants removed and vice versa. Due to this, some salts (commonly NaCl) are added to increase the conductivity of feed. When, salt is added to the solution, it reduces the solution resistance and hence, voltage distribution between the electrodes reduces. However, a too high conductivity may lead to secondary parasite reactions, diminishing the main reaction of the electrolytic decomposition. Additionally, the presence of chlorides can enhance the degradation of organic pollutants in the wastewaters due to the formation of various species (Cl2, HOCl and ClO─) formed as function of the pH. ClO─, which is dominating at higher pH, has been reported as better oxidant among all chlorine species . Moreover, the type and concentration of salt also influences the effectiveness of the treatment. Salts of bi- and tri-valent metals are more effective than monovalent salts because of their high ionic strengths. Cl2 and OH- ions are generated on the surface of the anode and the cathode, respectively, when NaCl is used as an electrolyte in ECT. The organics are destroyed in the bulk solution by oxidation reaction of the regenerated oxidant. In an ECT cell, Cl2/hypochlorite formation may take place because chloride is widely presented in many wastewaters .