Abstract
Electrocoagulation (EC) using Fe(0) electrodes is a low cost water treatment technology that relies on efficient production of Fe(II) from the electrolytic dissolution of Fe(0) electrodes (i.e. a high Faradaic efficiency). However, the (electro)chemical factors that favor Fe(0) oxidation rather than O2 evolution during Fe(0) EC have not been identified. In this study, we combined electrochemical methods, electron microscopy and Fe measurements to systematically examine the interdependent effects of current density (i), anodic interface potential (EA) and solution chemistry on the Faradaic efficiency. We found that Fe(0) oxidation was favored (Faradaic efficiency >0.85) in chloride and bromide solutions at all i, whereas carbonate, phosphate, citrate, and nitrate solutions lead to Faradaic efficiencies <0.1. The anodic reaction (i.e. Fe(0) oxidation or O2 evolution) only depended on i in the sulfate and formate solutions. Experiments in binary-anion solutions revealed that molar ratios of [HCO3−]/[Cl−] near 100 and [NO3−]/[Cl−] near 20 separated the electrochemical domains of Fe(0) oxidation and O2 evolution in the EC system. These molar ratios were supported by experiments in synthetic groundwater solutions. We also found that the EA vs i curves for solutions with poor Faradaic efficiency overlapped but were situated 2–4V vs Ag/AgCl higher than those of solutions with high Faradaic efficiency. Therefore, the position of the EA vs i curve, rather than the EA alone, can be used to determine unambiguously the reaction occurring on the Fe(0) anode during EC treatment.
Highlights
Electrocoagulation (EC) using Fe(0) electrodes is a promising, low cost treatment technology for drinking water and wastewater
A subtle trend in the production of Fe with time is observed for these two electrolytes, with slightly decreased experimental Fe concentration (Feexp) values relative to follows closely the theoretical value (Feth) assuming the constant production of Fe(II) (Eq (1))
The ionic composition and pH of the electrolyte solution largely determined the Faradaic efficiency of Fe(0) EC, with changes in i influencing the domains of Fe(II) production and O2 evolution only in certain electrolyte solutions
Summary
Electrocoagulation (EC) using Fe(0) electrodes is a promising, low cost treatment technology for drinking water and wastewater. This technology is applicable in decentralized and rural areas because of its low infrastructure requirements, ease of use and potential for scalability [1,2]. While EC systems are often accompanied by supplementary treatment steps, including settling tanks and filters [3], the technology centers on applying an electric current to Fe(0) electrodes in contact with an electrolyte solution to promote the oxidative release of soluble Fe(II) ions [4,5]. The Fe(II) ions migrate from the Fe(0) anode surface to the bulk solution, where contaminant removal occurs by several possible pathways. Because Fe(0) EC treatment relies on the production of Fe(II) from the Fe(0) anode [26,27], it is essential to understand the factors that promote efficient Fe(II) generation
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