Electrochemical Methods for Water Purification, Ion Separations, and Energy Conversion

Agricultural development, extensive industrialization, and rapid growth of the global population have inadvertently been accompanied by environmental pollution. Water pollution is exacerbated by the decreasing ability of traditional treatment methods to comply with tightening environmental standards. This review provides a comprehensive description of the principles and applications of electrochemical methods for water purification, ion separations, and energy conversion. Electrochemical methods have attractive features such as compact size, chemical selectivity, broad applicability, and reduced generation of secondary waste. Perhaps the greatest advantage of electrochemical methods, however, is that they remove contaminants directly from the water, while other technologies extract the water from the contaminants, which enables efficient removal of trace pollutants. The review begins with an overview of conventional electrochemical methods, which drive chemical or physical transformations via Faradaic reactions at electrodes, and proceeds to a detailed examination of the two primary mechanisms by which contaminants are separated in nondestructive electrochemical processes, namely electrokinetics and electrosorption. In these sections, special attention is given to emerging methods, such as shock electrodialysis and Faradaic electrosorption. Given the importance of generating clean, renewable energy, which may sometimes be combined with water purification, the review also discusses inverse methods of electrochemical energy conversion based on reverse electrosorption, electrowetting, and electrokinetic phenomena. The review concludes with a discussion of technology comparisons, remaining challenges, and potential innovations for the field such as process intensification and technoeconomic optimization.

Emerging organic contaminants in wastewater: Understanding electrochemical reactors for triclosan and its by-products degradation

Degradation technologies applied to emerging organic contaminants from human activities are one of the major water challenges in the contamination legacy. Triclosan is an emerging contaminant, commonly used as antibacterial agent in personal care products. Triclosan is stable, lipophilic and it is proved to have ecotoxicologic effects in organics. This induces great concern since its elimination in wastewater treatment plants is not efficient and its by-products (e.g. methyl-triclosan, 2,4-dichlorophenol or 2,4,6-trichlorophenol) are even more hazardous to several environmental compartments. This work provides understanding of two different electrochemical reactors for the degradation of triclosan and its derivative by-products in effluent. A batch reactor and a flow reactor (mimicking a secondary settling tank in a wastewater treatment plant) were tested with two different working anodes: Ti/MMO and Nb/BDD. The degradation efficiency and kinetics were evaluated to find the best combination of current density, electrodes and set-up design. For both reactors the best electrode combination was achieved with Ti/MMO as anode. The batch reactor at 7 mA/cm² during 4 h attained degradation rates below the detection limit for triclosan and 2,4,6-trichlorophenol and, 94% and 43% for 2,4-dichlorophenol and methyl triclosan, respectively. The flow reactor obtained, in approximately 1 h, degradation efficiencies between 41% and 87% for the four contaminants. This study suggests an alternative technology for emerging organic contaminants degradation, since the combination of a low current density with the flow and matrix induced disturbance increases and speeds up the compounds' elimination in a real environmental matrix.

Self-doped TiO2 nanotube arrays for electrochemical mineralization of phenols

Self-doped TiO₂ nanotube arrays (DNTA) were prepared for the electrooxidation of resistant organics. The anatase TiO₂ NTAs had an improved carrier density and conductivity from Ti³⁺ doping, and the oxygen-evolution potential remained at a high value of 2.48 V versus the standard hydrogen electrode, and thus, achieved a highly enhanced removal efficiency of phenol. The second anodization could stabilize Ti³⁺ and improve the performance by removing surface TiO₂ particles. Improper preparation parameters (i.e., a short anodization time, a high calcination temperature and cathodization current density) harmed the electrooxidation activity. Although boron-doped diamond (BDD) anodes performed best in removing phenol, DNTA exhibited a higher mineralization of phenol than Pt/Ti and BDD at 120 min because intermediates were oxidized once they are produced with DNTA. Mechanism investigations using reagents such as tert-butanol, oxalic acid, terephthalic acid, and coumarin showed that the DNTA mineralization resulted mainly from surface-bound OH, and the DNTA produced more than twice the amount of OH compared with BDD. The free OH on the BDD electrode was more conducive to initial substrate oxidation, whereas the adsorbed OH on the DNTA electrode mineralized the organics in situ. The preferential removal of p-substituted phenols on DNTA was attributed mainly to their electromigration and the aromatic intermediates that are hydrophobic were beneficial to mineralization.

Electrochemical Removal of Ammonium Nitrogen and COD of Domestic Wastewater using Platinum Coated Titanium as an Anode Electrode

Biological treatment systems face many challenges in winter to reduce the level of nitrogen due to low temperatures. The present work aimed to study an electrochemical treatment to investigate the effect of applying an electric voltage to wastewater to reduce the ammonium nitrogen and COD (chemical oxygen demand) in domestic wastewater. This was done by using an electrochemical process in which a platinum-coated titanium material was used as an anode and stainless steel was used as a cathode (25 cm2 electrode area/500 mL). Our results indicated that the removal of ammonium nitrogen (NH4+–N) and the lowering of COD was directly proportional to the amount of electric voltage applied between the electrodes. Our seven hour experiment showed that 97.6% of NH4+–N was removed at an electric voltage of 5 V, whereas only 68% was removed with 3 V, 20% with 1.2 V, and 10% with 0.6 V. Similarly, at 5 V, the removal of COD was around 97.5%. Over the seven hours of the experiment, the pH of wastewater increased from pH 7.12 to pH 8.15 when 5 V was applied to the wastewater. Therefore, electric voltage is effective in the oxidation of ammonium nitrogen and the reduction in COD in wastewater.