Study of Oxalate Mineralization Using Electrochemical Oxidation Technology

Oxalic acid is commonly used in industry. It is also found to be the main intermediate in many advanced oxidation processes for organic removal, especially in the Fenton process. In this study, we used an electrochemical oxidation process to mineralize oxalic acid. Effects of different cathodes, anions and concentration of ferric ion on current efficiency were discussed. Better current efficiency was observed when stainless steel net and chloride ion were utilized. We also investigated the effects of cathode material and anion ions on the mineralization of oxalate. It was found that a Ti-DSA cathode and ferric chloride solution showed the better results than cathodes made of stainless steel and ferric sulfate and ferric nitrate solutions. In this study, chemical oxygen demand (COD) could be completely removed in 3 h with a Ti-DSA cathode and 30 mM ferric chloride solution used to treat 20 mM oxalate. On the other hand, only 91% COD could be removed when using a stainless steel net cathode, and only 55% COD removal was obtained when the ferric sulfate was used. We also found that the removal of TOC was only slightly different when concentration of ferric chloride was reduced from 30 to 6.67 mM; thus the cost of reagents could be reduced. *Corresponding author Email: [email protected] INTRODUCTION Oxalate is a major intermediate in many advanced oxidation processes (AOPs) for treating organics [1-4]. For example, about 50-70% of oxalate is generated after the degradation of citric acidcontaining wastewater by various AOPs [5]. On the other hand, oxalate is also observed to exist in water drops from the air caused by incomplete combustion, oxidation decomposition of ozone, and photooxidation of hydrocarbon in both the gas and liquid phase in the air [8]. Oxalate will be decomposed by sunlight in the presence of ferric ion. Then it is converted into O2 • as shown in the following equations [6,7]: Fe(C2O4)3 3→ Fe(II) + 2C2O4 + C2O4• k = 0.04 s (1) C2O4 • + Fe(C2O4) 3 3→ Fe(II) + 3C2O4 2+ 2CO2 (2) C2O4 •→ CO2 + CO2• k = 2 × 10 s (3) C2O4 •/CO• + O2 → 2CO2/CO2 + O2• k = 2.4 × 10 M s (4) HO2• ↔ O2•+ H K = 1.58 × 10 M (5) O2 • generated from Eq. 4 will be protonated into HOO•. At the same time, it will oxidize some organic compounds in natural surroundings and will initiate a self-cleaning process. In addition to the photooxidation process, there is a novel method, a type of electrochemical oxidation reaction called the electroFenton method, to mineralize oxalate. There are something interesting results which will be discussed later in this document.