Comparison of electrochemical reduction and chemical reduction for chromium(VI) removal

• Main Authors: Chiao-Lin Feng, 馮喬麟
• Other Authors: Chi-Wang Li
• Format: Others
• Language: en_US
• Published: 2019
• Online Access: http://ndltd.ncl.edu.tw/handle/22gs86

碩士 === 淡江大學 === 水資源及環境工程學系碩士班 === 107 === Electrochemical reduction (ECR) and chemical reduction (CR) processes were employed to treat Cr(VI)-containing wastewater. The Cr(VI) reduction was extremely fast with reaction kinetics limited by electro-generation of Fe(II) and chemical dosage of Fe in ECR and CR processes, respectively. In ECR process, the Cr(VI) reduction increased with decreasing pH at the initial stage of reaction, but the time to reach complete Cr(VI) reduction is pH independent. The amount of Fe(II) required was 10% and 13% higher than the stoichiometric value to remove Cr(VI) completely for initial pH of 7 and 9, respectively. For pH 3 and 5, the values were around 32.0 % less than the stoichiometric value. XPS results proved that the adsorption of Cr(VI) onto the produced flocs was the reason. In contrast, the time to reach 100% removal was pH dependent if the system pH was controlled throughout the reaction. Indirect reduction was the main mechanism for Cr(VI) removal under controlling pH of 3, 7, and 9. At controlled pH of 5, both indirect reduction and adsorption were responsible for the removal of Cr(VI). Various mixing mechanism including aeration, nitrogen purging, and mechanic mixing were investigated to study the effects of DO. Fe(II) needed for one mole of Cr(VI) reduced was 3.67% higher the stoichiometric value of 3 under nitrogen purging. The values were 12.79% and 15.82% higher the stoichiometric value of 3 moles of Fe(II) needed for one mole of Cr(VI) for mechanical mixing and aeration conditions, respectively. Therefore, the content of DO did affect the reduction of Cr(VI) using ECR. In CR process, iron is dosed dropwise every 4 min according to the dosage of Fe(II) generated from electrochemical reduction process. The reaction was time and pH-independent. Under controlled solution pH, the values were the same as the stoichiometric value of 3 moles of Fe(II) needed for one mole of Cr(VI) being reduced for pH 3, 7 and 9, indicating that indirect reduction was the main removal mechanism. The value of 0.73 was two times higher than the stoichiometry value of 0.33 under controlled pH of 5, indicating that adsorption was around 54% after subtracting the stoichiometric value of 3. The operation cost of ECR process was higher than CR process at current intensities of 0.75 and 1 A. Meanwhile, the cost was lower than that of CR process at low current intensities. Energy consumption was the main cost for ECR process. Thus, the operation cost of ECR process depends the current intensity applied.