Arsenic removal from groundwaters using modified double potential step chronoamperometry (DPSC)

Abstract

Due to the high toxicity and ubiquitous presence of inorganic arsenic (As) in groundwaters, there have been a number of studies investigating the removal of inorganic arsenic though most of the existing technologies are constrained by low energy efficiency and ineffectiveness in As(III) removal under circumneutral pH conditions. In this work we present proof-of-concept of a modified double potential step chronoamperometry (DPSC) method which achieves in-situ As(III) oxidation and concomitant electro-sorption of As(V) onto the electrode. The use of both carbon cloth and redox active polyvinylferrocene (PVF) functionalized carbon nanotube electrodes are investigated in this work. Our work with the carbon cloth electrode showed that in-situ anodic As(III) oxidation and concomitant sorption of As(V) formed was achieved at an applied voltage of 1.2 V. Our results further showed that the sorbed As(V) was effectively electro-desorbed by reversing the polarity thereby regenerating the electrode. The in-situ anodic As(III) oxidation, sorption of As(V) and desorption of As(V) are affected by aqueous pH with high oxidation and sorption/desorption rates observed at elevated pH. The increase in As(III) oxidation and concomitant As(V) adsorption at higher pHs is related to (i) the rapid oxidation of the deprotonated species compared to the protonated species and (ii) stronger electrochemical interaction between the multi-charged As(V) species and the electrodes. At 1.2 V and an electrical 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 cm × 8 cm and electro-sorption time of 120-min. Based on the experimental results, we have developed a mathematical model to describe the kinetics and mechanism of arsenic removal by the modified DPSC method using the carbon cloth electrode with this model of use in predicting, and potentially optimising, process performance under various conditions. While the carbon cloth electrode was effective in As removal in the absence of competing anions, in the presence of anions such as Cl−, SO42− and NO3−, the As(III) removal efficiency by the carbon cloth electrode was very low. In order to improve the selectivity of As over competing anions that are typically present in groundwaters, we investigated the sorption of arsenic from simulated groundwaters by a redox active PVF functionalized electrode. Our results show that effective and sustainable As(III) removal was achieved even at 0 V once the electrode is activated via anodic polarization. During activation, ferrocene (Fc) in PVF is oxidized to the ferrocenium ion (Fc+) with the latter facilitating As(III) sorption and subsequent oxidation as well as As(V) sorption. The high affinity of Fc+ to As and weak attraction to competing anions at 0 V ensure high selectivity of As over Cl−, SO42− and NO3− at concentrations typical of groundwaters. Following the removal process, efficient regeneration of the electrode is achieved at −1.2 V wherein Fc+ is reduced to Fc thereby facilitating As desorption from the electrode surface. Our results further show that O2 and associated generation of hydrogen peroxide (H2O2) during the electrode regeneration step drives the oxidation of Fc to Fc+, thereby maintaining the constant generation of Fc+ required to achieve As(III) removal in subsequent cycles. Our results show that 91.8%  0.6% of As(III) could be selectively removed from a simulated groundwater over 10 cycles at an ultralow energy consumption of 0.12 kWh m−3. We also investigated the influence of divalent cations (i.e. Ca2+and Mg2+) on the removal of As(III) by a PVF functionalized electrode. Our results show that in the absence of divalent cations, nearly 90% ± 0.9% of As(III) removal is achieved over ten continuous cycles by single-pass DPSC, even in the presence of competing anions (5 HCO3−, 3 mM Cl−, 0.5 mM NO3−, 2 mM SO42−) however the presence of divalent cations (Ca2+ and Mg2+) significantly inhibits electrode regeneration. While the presence of Ca2+ and Mg2+ facilitates As(III) removal in the 1st step of the DPSC, the regeneration of the electrode in the 2nd step of DPSC is significantly inhibited, thereby decreasing the As(III) removal in the subsequent cycles. Based on the results from various control experiments and surface characterization, it appears that Ca2+/Mg2+ either acts as a bridge between the electrode surface and As anions or the sorption of Ca2+/Mg2+ increases the positive charge on the electrode surface thereby facilitating the sorption of As. The Ca2+/Mg2+ assisted sorption occurs even during the electrode regeneration process, thereby decreasing the electrode regeneration efficiency and As(III) removal in subsequent cycles. Overall, the results show that the DPSC method with either carbon cloth or PVF functionalized electrodes achieves in-situ As(III) oxidation, concomitant sorption of As(V) and desorption of As(V). However, the adverse impact of the presence of divalent cations (Ca2+ and Mg2+) on electrode regeneration and thus the efficacy of the As removal process must be taken into account. Future work should focus on i) optimization of the configuration of the system in order to minimize the influence of divalent cations on regeneration of the electrodes and ii) scale up of the unit to a size that is meaningful for practical application. Additionally, development of more stable PVF electrodes and/or alternate electrode materials for better removal efficiency is also required.