Electrocoagulation is a promising alternative to the conventional chemical coagulation in water treatment systems. In electrocoagulation, coagulants are generated in situ by anodic dissolution of sacrificial electrodes, usually aluminum or iron electrodes. Anodic dissolution of the sacrificial anodes leads to the formation of hydrolysis products (hydroxo-metal species) that involve the destabilization of suspended, emulsified or dissolved pollutants and/or the formation of insoluble particles that adsorb and enmesh the pollutants. Furthermore, the formation of hydrogen bubbles as a result of water reduction at the cathode surface promotes the flocculation process by the soft turbulence in the system and produces a soft mix. The electrogenerated gaseous bubbles help the destabilized particles to colloid and generate larger particles which facilitate separation of the flocculated pollutants by carrying the particles to the top of the solution where they can be more easily removed by electroflotation. Electrochemical coagulation was successfully applied for turbidity, heavy metals, dyes and phenols removals from synthetic and real wastewaters and for breaking oil/water emulsions at both laboratory scale and pilot plant scale.

In this work, effects of some experimental parameters (supporting electrolyte, current density, and initial pH) on anodic dissolution of aluminum and on electrocoagulation of tannic acid aqueous solutions as well as real industrial wastewaters containing tannic acid were investigated. Experimental results indicated that both chemical and electrochemical dissolution play an important role in the formation of hydroxo-aluminum species. The chemical dissolution of aluminum is strongly influenced by the solution pH.

Electrocoagulation using aluminum electrodes achieved high removal efficiency of chemical oxygen demand from aqueous solutions containing tannic acid. The primary mechanism for removing tannic acid from water by electrocoagulation using Al electrodes involves the adsorption of tannic acid molecules on the aluminum hydroxide surface. Also, results of the treatment of real wastewater obtained from pulp and paper industry with initial COD concentration of 1450 mg/L have shown that more than 60 % of COD can be removed by electrocoagulation using Al electrodes under optimized experimental conditions. The specific energy required for the electrochemical process with Al electrodes was estimated to range from 1 to 2 kWh m-3.


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