ARTICLE pubs.acs.org/IECR
Simple Technical Approach for Perpetual Use of Electrogenerated Ag(II) at Semipilot Scale: Removal of NO and SO2 as a Model System Muthuraman Govindan, Sang-Joon Chung, and Il-Shik Moon* Department of Chemical Engineering, Sunchon National University, 315 Maegok Dong, Suncheon 540-742, Chonnam, Republic of Korea
bS Supporting Information ABSTRACT: The sustainable applicability of Ag(II)/Ag(I) redox mediator was studied in a semipilot-scale system for treatment of model artificial flue gases containing NO and SO2. Various discontinuous current supplies were tested for their effect on sustainable use of the electrogenerated mediator. Current density, cell volume, number of flow-through cell, and feed flow rates were varied in electrochemical thin layer cells to improve the applicability of the proposed current supply method. Discontinuing current flow every (5/5) min onoff (iDC5) resulted in improved regeneration capacity of Ag(II) after 15 h. After extended operation (24 h), removal efficiencies of 62% NO and 100% SO2 were achieved by mediated electrochemical oxidation both individually and simultaneously using the proposed discontinuous current supply, demonstrating the sustainable use of the mediator. This study will be taken as a pretest for long-term sustainability testing of the mediator, Ag(II), during industrial scaling up.
1. INTRODUCTION Environmentally friendly catalyst ‘electrons’ and ambient temperature reactions are increasingly important as the engineering of electrochemical techniques is required to make environmental considerations. Mediated electrochemical oxidation (MEO) integrated in wet scrubber columns is applied at ambient temperature in environmental processes such as effluent treatment and toxic gas degradation,1,2 as described by the United Nations Environmental Program.3 Novel mediators of electrocatalytic oxidation can improve pollutant elimination. Farmer et al.4 introduced Ag(II) for oxidation of ethylene glycol and benzene in hazardous waste. Ag(II), Co(III), and Ce(IV) have been reported to remove completely organic wastes.5 Commonly used mediators in the mineralization of organic pollutants are Ag(II)/Ag(I), Co(III)/Co(II), Ce(IV)/Ce(III), and Mn(III)/ Mn(II).6 Many novel mediators in MEO-based treatments have been extended to air pollution control, particularly flue gas degradation.710 Electrogenerated Ce(IV) has been used in sulfuric acid for individual wet scrubbing of NOx and SO27,8 and also for simultaneous removal of NOx and SO2 from simulated flue gas. Hoffmann et al.10 reported that electrogenerated Ce(IV) is a good catalyst for complete removal of SO2 and removal of NOx at below 45% efficiency. In addition to finding new mediators, modifying or optimizing existing configurations or processes can improve flue air quality. For example, two scrubber columns were used in a bench-scale system for effective removal of NOx and SO2;11 they could be reduced to one after the effective setting of operational conditions, without compromising the removal efficiency of NOx or SO2.12 In a wet scrubbing unit with integrated MEO, the electrochemical cell’s main function is to maintain the initial concentration of electrogenerated mediators using an internal recycling system to react with target pollutants, e.g., artificial NOx and SO2 flue gas air mixtures.11 However, maintaining the initial concentration of mediator for sustained treatment of pollutants is r 2011 American Chemical Society
difficult. Current supply is considered a key operational parameter, controlling the mediator oxidation/reduction process, electrolyte solution, and electrode/solution interface phenomena.13 Constant current electrolysis is often used in MEO systems; it maintains the higher oxidation state of the mediator during degradation.14,15 In general, continuous constant current supplies are used to generate the reactive mediators in MEO-integrated wet scrubber column systems.1618 Matheswaran et al.1720 studied phenol oxidation using Ag(II) and Ce(IV) mediators with continuous constant flows of various currents. Many research works were employed and reported using continuous constant current flows.2124 Variation of current density has been found to influence mediator regeneration during degradation.25,26 The above studies show that the electrochemical regeneration capacity of the mediator can be maintained for a maximum of 5 h, insufficient for industrial effluent treatment. There is also no report of electrochemical regeneration for long-term degradation processes. Previous problems explored at the bench scale have aided the development of a semipilot-scale process with longer lasting mediator generation that employed various patterns of constant current supply, which is the focus of this report. Operational costs and power consumption are expected to be reduced as the method is optimized. In this present study, NO and SO2 were taken as model air pollutants. They are the most common air contaminants from power plants, incinerators, and boilers and are treated by several removal methods.11,2730 This work’s main objective was to study the feasibility of a modified constant current supply to enhance generation of Ag(II) in a semipilot-scale MEO-integrated wet scrubber system. The examined current supply patterns were (i) (5/5) min alternating onoff periods applied Received: July 21, 2011 Accepted: December 20, 2011 Revised: December 12, 2011 Published: December 20, 2011 2697
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Industrial & Engineering Chemistry Research after 1 h continuous constant current supply, (ii) only (5/5) min alternating onoff periods, (iii) (7/3) min alternating onoff periods, and (iv) continuous constant current supply. To assess the sustainability of each current supply method, current density, anolyte and gas flow rates, and the volume of the electrochemical cell (350 L) and arrays of multiple thin layer electrochemical cells (13 set) were studied. The reactivity of NO and SO2 with Ag(II) and regeneration of Ag(II) were monitored during the gases’ simultaneous degradation. This work demonstrates the extended durability of the Ag(II) mediator in a semipilot-scale integrated wet scrubberelectrochemical reactor and its reactivity behavior when removing NO and SO2 air pollutants individually and simultaneously.
2. EXPERIMENTAL SECTION 2.1. Materials. Silver(I) nitrate (99.8%) (Junsei Chemical Co. Ltd., Japan), nitric acid (60%, Sam Chun Chemicals, Korea), and sulfuric acid (95%, DC Chemicals Co Ltd., Korea) were used as received. All aqueous solutions contained water that had been purified by reverse osmosis (Human Power III plus, Korea). NO (concentration 99.5%) and SO2 (concentration 9980 μmol mol1) gas cylinders were supplied by Inter Gas, Korea. Meshtype Pt-coatedTi (Pt/Ti) and Ti electrodes were purchased from Wesco, Korea. Nafion 324 type membrane was obtained from DuPont, USA. PVDF Raschig rings of 1 or 2 cm were made in the laboratory for scrubber column I. Commercially available packing, B-GON Kimre from Korea, was purchased and used as received in column II. 2.2. Scrubbing Unit and Its Working Conditions. The semipilot-scale apparatus was developed for simulated flue gas removal; it included a combination of a wet scrubber column coupled with an electrochemical cell (see Supporting Information Figure SM-1 for a more detailed description). The artificial flue gasair mixtures were supplied by high-pressure cylinders of NO and SO2 and air blown by an oil-free air-compressor (model AC-B15PA2, Kyungwon Airboy Co. Ltd., Korea). Gaseous mixtures of NO and SO2 were obtained by controlled mixing of air, NO, and SO2 using mass flow controllers (MFC, model 1179A13CS1BK-S, MKS Co. Ltd., USA). Homogenized gas mixture was introduced at the bottom of the scrubber at flow rates ranging from 15 to 60 N m3 h1. The scrubbing solution of 6 M HNO3 containing Ag(II)/Ag(I) redox ions was pumped and distributed at the top of the scrubber bed through a conical tube. In scrubber column II, counter current flow was adopted for effective absorption of escaped untreated flue gases from scrubber column II using an absorption solution (Na2S) at a flow rate of 40 L min1. 2.3. Electrochemical Cell and Its Working Conditions. The electrochemical cell employed was a thin layer flow through a multiarray divided cell configuration (for a detailed description, see Supporting Information Figure SM-2). Different anolyte volumes (3, 5, 15, 30, and 50 L) of 6 M HNO3 containing Ag(I) nitrate from a 60 L PVC liquid storage tank and catholyte solution volumes (3 and 30 L) of 3 M H2SO4 from a 40 L PVC liquid storage tank were employed separately. Both anolyte and catholyte were continuously circulated by magnetic pumps (Pan World Co., Ltd., Taiwan) through the anode and cathode compartments of the electrochemical cell at the range between 4 and 25 L min1, respectively. The electrode area of 140 and 440 cm2 were used for lab-scale and semipilot-scale experiments, respectively. In order to check the electrochemical cell
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performance, the electrode to anolyte volume ratio was changed using up to 3 sets of anodes and cathodes, each electrode containing an area of 440 cm2, in series connection for semipilot-scale gas removal studies. A current source (Korea Switching Instrument) was used to generate Ag(II) by electrolysis. 2.4. Procedure and Analysis. All experimental data were collected using a constant concentration of Ag(I) (0.1 M). The silver ion concentration in the scrubber solution was taken as the initial Ag(I) concentration, i.e., 0.1 M. Ag(II) concentration was determined using ORP (oxidation reduction potential) changes upon titration against FeSO4 (0.1 M) solution using EMC 133 (Pt (6 mm) gel electrolyte) electrode from Germany. An NO and SO2-Fuji ZSU gas analyzer was used to measure the concentrations of NO and SO2. NO2 concentration was measured by an NO2-Teledyne model 9110 gas analyzer. Gas removal efficiencies were calculated from differences of the inlet and outlet concentrations of NO and SO2. Cyclic voltammetry was carried out using a BAS-Epsilon EC instrument (USA). Platinum (2 mm) and Pt wire served as the working and counter electrodes, and Ag/AgCl served as the reference electrode. All experiments were conducted at room temperature (25 °C ( 2) and atmospheric pressure.
3. RESULTS AND DISCUSSION 3.1. Effects of Discontinuous Current Supply and Current Density on Sustainability at Semipilot Scale. Finding an
effective method of maintaining Ag(II) is important for industrial application of this technique. For this, different constant current supplies were compared using a continuous flow of anolyte (5 L/min), 140 cm2 electrode area, and constant current density (71.4 mA cm 2 ) as a first step. Because of our previous experiences, we doubted the constant continuous current supply. In order to confirm this, four different current supplies were compared: (a) continuous constant current supply (iCC), (b) (5/5) min alternating onoff periods of constant current supply after 1 h continuous constant current supply (iCC‑DC5), (c) only (5/5) min alternating onoff periods of constant current supply (iDC5), and (d) (7/3) min alternating onoff periods of constant current supply (iDC7). A 6 M HNO3 anolyte containing 0.1 M Ag(I) was circulated at a flow rate of 4 L min1. Figure 1 shows the oxidation efficiency of Ag(I) in the four current supply systems over 24 h. Oxidation efficiency by iCC shows an initial sharp increase, peaking at 22% Ag(II), before decreasing to zero after 12 h. The efficiencies observed in the other current supply systems, iCC‑DC5, iDC5, and iDC7, support the systems’ abilities to maintain Ag(II): initial increases were later reduced, but almost constant levels were then observed for the remainder of the 24 h. iDC5 showed the highest (25%) generation of Ag(II) under the experimental conditions studied. All previous studies examined a maximum 5 h duration.22 The causes of complete loss of Ag(II) generation after 12 h were analyzed. After 12 h electrolysis, the anolyte was analyzed by the Volhard method using potassium thiocyanate (KCSN) to assess the presence of remaining Ag ions.31 Over 94% of the Ag(I) ion remained in the anolyte, even after 12 h electrolysis. Water splitting by electrogenerated Ag(II) can reduce the Ag(II) concentration.19 Other possible reasons are slow electron transfer or electrode fouling.32,33 Electrode fouling was likely one reason because cell voltage is decreased for iCC supply mode from 2.5 to 2.1 V with the electrolysis time, as shown in the insert of Figure 1, providing indirect evidence of the electrode’s surface partly fouled (platinized Ti). At the same time 2698
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Figure 1. Ag(I) oxidation efficiency with respect to electrolysis time when a constant current was varyingly supplied: (i) continuous constant current (iCC); (ii) (5/5) min onoff after 1 h continuous constant current supply (iCC‑DC5); (iii) (5/5) min onoff (iDC5); (iv) (7/3) min onoff (iDC7). Experimental conditions: electrode area = 140 cm2, Ag(I) = 0.1 M, HNO3 = 6 M, current =10 A, anolyte volume = 5 L.
discontinuous current supplies iCC‑DC5, iDC5, and iDC7 show almost constant cell voltage throughout the electrolysis time, evidence free from electrode fouling. Other possible questions on the Ag(II) diminution are a combination of slow electron transfer and water oxidation by Ag(II). In general, H+ ions can penetrate Nafion membranes during electrolysis from the anodic compartment to the cathodic compartment.34 A continuous current supply over long times favors greater penetration of H+ to the cathodic compartment, which will create excess NO3 salvation to Ag(II)NO3+ complex in the anodic compartment. If nitrate complexes (Ag(II)NO3+) salvation with excess NO3 forms then impede electron transfer. Noteworthy is that excess SO42 has been reported to form complexes easily with Ce(IV), which then suffers reduced electron transfer that leads to peak potential variation and a reduction of peak current and oxidation efficiency of Ce(III) reduced twice.15,35 Cyclic voltammetry (CV) corroborated this assumption. Figure 2a shows the CV responses of the anolyte solution before and after different constant current electrolyses. The diminished redox peak current and positive peak potential shift, as observed by Chen et al.35 for Ce(IV) complexes with excess SO42, was registered for 10 h electrolyzed sample, clearly indicating the salvation of NO3 around Ag(II)NO3+ complex during iCC (Figure 2a, curve b). Under discontinuous current supply (iDC5, Figure 2a curve c) the CV response shows no current diminution or potential variation for 24 h electrolyzed sample. However, peak potential can also vary because of pH differences in the working solution. However, no pH change was observed in the anolyte either before or after electrolysis, suggesting that the redox peak diminution and positive shift in potential were not caused by pH change. Therefore, the CV peak diminution and peak potential variation arose from reduced electron transfer due to excess NO3. In other words, or another cause for the decrease of Ag(II) concentration, Ag(II)NO3+ started oxidizing the water after
Figure 2. (a) CV response of anolyte at different conditions: (a) before electrolysis; (b) electrolysis by iCC (after 10 h); (c) electrolysis by iDC5 (after 24 h). Working electrode: Pt. Scan rate: 50 mV s1. Silver(I) concentration: 0.1 M. (b) Ag(I) oxidation efficiency with respect to HNO3 concentration (M) at iCC current supply. Electrode area = 140 cm2, Ag(I) = 0.1 M, current = 10 A, anolyte volume = 3 L.
attaining its maximum in complex formation (25% in 6 M HNO3). It is well documented that Ag(II) reacts with water in different concentrations of HNO3 as follows.36 The overall reaction at >1 104 M AgðIIÞ þ H2 O f AgðIÞ þ Hþ þ O2
ð1Þ
The overall reaction at
