Modified Double Potential Step Chronoamperometry (DPSC) Method for As(III) Electro-oxidation and Concomitant As(V) Adsorption from Groundwaters

Abstract

Constrained by low energy efficiency and ineffectiveness in As­(III) removal under circumneutral pH conditions by many exsiting technologies, As­(III) removal has become an issue. In this work we present proof of concept of a modified double potential step chronoamperometry (DPSC) method of As­(III) oxidation and concomitant As­(V) electro-sorption from aqueous solution. Results show that in situ anodic As­(III) oxidation, As­(V) electro-sorption, and As­(V) electro-desorption are affected by aqueous pH with high oxidation and sorption/desorption rates observed at the elevated pH. We particularly show that effective As­(III) oxidation and concomitant As­(V) adsorption are related to (i) the rapid oxidation of the deprotonated species compared to the protonated species and (ii) stronger electrochemical interaction between the multicharged As­(V) species and the electrodes. At 1.2 V and an electric energy consumption of 0.06 kWh m–3, the total As concentration can be reduced from 150 to 15 μg L–1 using an electrochemical cell with electrode area of 10 × 8 cm2 and electro-sorption time of 120 min. On the basis of the experimental results, we have developed a mathematical model to describe the kinetics and mechanism of arsenic removal by the modified DPSC method with this model of use in predicting, and potentially optimizing, process performance under various conditions.

Selective Arsenic Removal from Groundwaters Using Redox-Active Polyvinylferrocene-Functionalized Electrodes: Role of Oxygen

“…No attempt was made to control the pH during the experiments, although the pH was monitored continuously. The stock solutions of N , N -diethyl-phenylenediamine (DPD, Fluka Analytical), horseradish peroxidase (HRP), and phosphate buffer for hydrogen peroxide (H 2 O 2 ) measurement were prepared as described in our earlier work …”

“…Each electrochemical cell consisted of two endplates (McMasterCarr, Elmhurst IL), one PVF-CNT electrode (10 cm × 8 cm) serving as the working electrode and one carbon cloth electrode (Nantong Senyou Carbon Fiber Cop., China) serving as the counter electrode (10 cm × 8 cm). A cation-exchange membrane (CEM-Type II FUJIFILM, Netherlands) was placed against the counter electrode (as shown in the Supporting Information, Figure S1) to avoid readsorption of As during the regeneration step …”

“…To prevent adverse health effects, the World Health Organization established the threshold concentration of As in drinking water as 10 μg L –1 . However, it is reported that the concentration of arsenic in groundwaters in many locations around the world exceeds the safety level with concentrations often in the range of hundreds of micrograms per liter (μg L –1 ) to several milligrams per liter (mg L –1 ). − Hence, treatment technologies for the removal of arsenic from groundwaters have garnered significant interest in recent years. − Methods to remove As­(III) and As­(V) from solution include membrane separation, − bioremediation, − coagulation and electrocoagulation, − adsorption, − and electrosorption. − Membrane separation (especially nanofiltration (NF) and reverse osmosis (RO)) is highly efficient for As­(V) removal since anionic As­(V) species are readily rejected by negatively charged membranes with low-molecular-weight cut-off. , However, even RO is not very effective for the removal of neutral As­(III) species with a threshold limit of 10 μg L –1 As only achieved at a maximum feedwater As concentration of 350 μg L –1 and at the high energy cost. − Bioremediation is relatively effective for As removal and also results in decrease in the toxicity of As by transforming inorganic As­(III) and As­(V) into organic forms, which have lower toxicities . However, difficulties in the isolation of arsenic resistant bacteria and low removal efficiency for low concentrations of As limit application of this technology. , Coagulation and electrocoagulation (EC) result in precipitation of soluble arsenic species by employing chemical coagulants or in situ generation of a coagulant via electrolytic oxidation of metal electrodes. , While EC is more effective than conventional coagulation, high energy cost, difficulty in the EC reactor design, and low removal efficiency for low As concentrations limit its application. − Adsorption has also been extensively utilized due to its simple operation, high removal efficiency, and low cost .…”

“…Recent studies have shown that the use of electrosorption processes such as capacitive deionization (CDI) , and double potential step chronoamperometry (DPSC) methods at low applied voltage overcomes these disadvantages; that is, effective As­(III) removal is achievable via in-situ As­(III) oxidation under anodic polarization followed by temporal immobilization of As­(V). − These electrochemical processes result in not only removal of As­(III) and effective regeneration of the electrodes through the application of reverse cell voltage but also transformation of toxic As­(III) into less harmful As­(V) anions, thereby reducing the by-product toxicity. However, low arsenic selectivity in the presence of competing ions such as Cl – and SO 4 2– limits the total As uptake capacity of carbon-based electrodes since most arsenic-contaminated groundwaters contain these anions in large excess of arsenic.…”

Asymmetric Redox‐Polymer Interfaces for Electrochemical Reactive Separations: Synergistic Capture and Conversion of Arsenic

“…Electrosorption systems benefit from inherent modularity and scalability, which opens the door to point of source remediation systems. Electrochemical conversion of As(III) to As(V) on carbon electrode has been investigated previously for CDI‐based arsenic remediation 5l,6. However, low arsenic selectivity in the presence of competing ions has limited the total uptake capacity of carbon‐based CDI,5c,h–l as most arsenic contaminated water sources are composed of 10 to 1000‐fold excess salts .…”