Article pubs.acs.org/est
Efficient Removal of Sulfur Hexafluoride (SF6) Through Reacting with Recycled Electroplating Sludge Jia Zhang,† Ji Zhi Zhou,† Qiang Liu,† Guangren Qian,*,† and Zhi Ping Xu*,‡ †
School of Environmental and Chemical Engineering, Shanghai University, Number 333 Nanchen Road, Shanghai 200444, P. R. China ‡ ARC Centre of Excellence for Functional Nanomaterials, Australian Institute for Bioengineering and Nanotechnology, The University of Queensland, Brisbane, QLD 4072, Australia S Supporting Information *
ABSTRACT: This paper reports that recycled electroplating sludge is able to efficiently remove greenhouse gas sulfur hexafluoride (SF6). The removal process involves various reactions of SF6 with the recycled sludge. Remarkably, the sludge completely removed SF6 at a capacity of 1.10 mmol/g (SF6/sludge) at 600 °C. More importantly, the evolved gases were SO2, SiF4, and a limited amount of HF, with no toxic SOF4, SO2F2, or SF4 being detected. These generated gases can be readily captured and removed by NaOH solution. The reacted solids were further found to be various metal fluorides, thus revealing that SF6 removal takes place by reacting with various metal oxides and silicate in the sludge. Moreover, the kinetic investigation revealed that the SF6 reaction with the sludge is a first-order chemically controlled process. This research thus demonstrates that the waste electroplating sludge can be potentially used as an effective removal agent for one of the notorious greenhouse gases, SF6.
1. INTRODUCTION Sulfur hexafluoride (SF6) is a colorless, inert, and noncombustible greenhouse gas. Because of its inert stability, SF6 is being widely used in various industrial processes, commercial products, and scientific fields as a protection gas.1 Once SF6 is released into the atmosphere, its natural removal is extremely slow, as its atmospheric lifetime is as long as about 3200 years.2 Since 1975, the global SF6 concentration has grown from less than 1 ppt to more than 7 ppt now.3 Moreover, SF6 is one of the six greenhouse gases in Kyoto Protocol with a global warming potential of 23 900 times that of CO2.4 Therefore, it is necessary to mitigate and control the emission of SF6 at the industrial source sites to reduce its contribution to global warming. In general, there are four ways to treat and remove SF6. The first method is decomposition carried out using plasma, electrical discharge, or spark with or without microwave radiation, which normally leads to almost 100% decomposition of SF6.5−7 Unfortunately, many corrosive and toxic gases are produced, including SOF4, SO2F2, SF4, SOF2, S2F10, S2OF10, S2O2F10, and S2O3F6.5−7 The second way is adsorption with inorganic materials, such as silicalite, zeolites, carbon nanotubes, and pillared clays.8−11 These materials normally have a very limited SF6 adsorption amount, resulting in low removal efficiency. The third technology is separation by gas hydrate formation.12 By this novel method, SF6 could be separated, recovered, and reused. However, some additional pressure © XXXX American Chemical Society
equipment is required. The forth one is catalytic decomposition, including photo and thermal degradation.13−15 Some catalysts are able to decompose nearly 100% SF6,13,15 with fewer corrosive and toxic gases generated than the first method. A few materials have been investigated as the catalysts. For example, polyisoprene15 and metal phosphate13 were used as catalysts for photo and thermal degradation of SF6. However, additional equipment, such as for UV radiation15 and an evaporator,13 was required to assist the decomposition. Therefore, searching for cost-effective catalysts/reagents is a key task in removal and control of SF6 emission. On the other hand, there is a huge amount of electroplating sludge produced as a hazardous material in the treatment of electroplating wastewaters.16 The sludge usually contains various metal elements, such as Fe, Cr, Cu, Ni, Zn, Pb, Cd, Hg, Mn, Sn, Au, and/or Ag16,17 in the form of mixed oxide, hydroxide, sulfate, silicate, and/or phosphate. The metal oxides and phosphates in the sludge can be potentially used as catalytic components in many industrial applications. In fact, there are some reports of using the sludge as catalysts to decompose pollutions, such as hydrogen sulfide and phenol.18−20 Thus it is Received: February 4, 2013 Revised: April 29, 2013 Accepted: May 24, 2013
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dx.doi.org/10.1021/es400553e | Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Environmental Science & Technology
Article
The reacted sludge was collected and dissolved in 5% HNO3 solution. Then, this solution and the outlet stream-absorbed basic solution were analyzed to determine concentrations of F−, SO32−, SO42−, and SiO32− by an ion chromatograph (ICS 1100) with the standard calibration method. XRD patterns of the sludge before and after the catalytic reaction were recorded on a Rigaku D/max RBX X-ray diffractometer with Cu Kα radiation (λ = 0.154 nm) at a scanning rate of 4°/min in the 2θ range of 10−75°.
our belief that the electroplating sludge could be a potential SF6 decomposition catalyst/reagent. The present research has thus comprehensively examined the conversion of SF6 over one recycled electroplating sludge under various conditions (temperature, SF6 concentration, and sludge amount), and furthermore identified and analyzed the generated gases and solid components to understand the reaction mechanism. We found that SF6 can be efficiently removed through reacting with various metal oxides and silicate in the sludge, without generating corrosive and toxic gases.
3. RESULTS AND DISCUSSION 3.1. Influence of Temperature and Sludge Diameter. Figure 1A shows the SF6 specific accumulative converted
2. EXPERIMENTAL SECTION 2.1. Sludge Catalyst. The electroplating sludge used in this research was supplied by Shanghai Pudong Xinsheng Electroplating Co., Ltd. Before being used, the sludge was first dried at 100 °C for 24 h. Then the dried sludge was milled to the particle size of 50, 100, or 150 mesh, and stored in a desiccator for further use. The major elements in the dried sludge (w/w %) were determined by XRF and ICP (after digestion), as listed in Supporting Information Table S1. In general, this sludge contained 38.7% O, 26.5% C, 3.1% Si, and 7.3% Fe. 2.2. Experiment Setup. First, a certain amount of the dried sludge (0.5 g in most tests) was filled in a corundum reactor with an inner diameter of 8 mm with a packing density of approximately 0.65 g/mL. Then, SF6 gas (Shanghai Fubang Chemical Industry Co., Ltd.) was mixed with N2 (Shanghai Pujiang Specialty Gases Co., Ltd.) at a designed ratio through a mass flowmeter (D07-7B, D07-19B Beijing Seven-star Electronics Co., Ltd.) to gain a constant flow rate of 13.4 mL/min with the SF6 concentration of 29, 59, 87, 116, 130, or 174 mg/ L. In the case of 0.5 g of sludge used, the gas hourly space velocity (GHSV) was about 1050 h−1 (= 13.4 × 60/(0.5/ 0.65)). The mixed gaseous stream then flowed through the system at room temperature for 30 min to stabilize the gas flow. After the detected SF6 concentration (via GC) became steady, the tube furnace (SK2-2-10L, Shanghai Shiyang Fume Co., LTD) started to increase the temperature to 400, 500, 600, or 700 °C. When the tube furnace reached the preset temperature, the GC started to read the SF6 concentration every 5 min. The outlet gas stream was then absorbed by 500 mL of 0.01 M NaOH solution to collect possible acidic and soluble gases, such as HF SO2 and SiF4. This basic solution was further analyzed to determine concentrations of anions of interest, such as F−, SiO32−, SO32−, and SO42− with an ionic chromatograph (ICS 1100, Dionex). Then, the total amount of these anions (mmol) collected in solution was calculated and transformed into the evolved amount per gram of sludge (mmol/g). The outlet stream and solution-absorbed stream were also analyzed by an online FTIR to identify the main gaseous species. 2.3. Data Acquisition, Treatment, and Presentation. SF6 concentration in the outlet at any time was determined using a gas chromatograph (GC 9800), so the accumulative converted SF6 amount per unit catalyst mass (specific accumulative converted amount, SACA, mmol/g) at any time was estimated and plotted with the reaction time, as explained in Figure S1. 2.4. Characterization. An online FTIR (Nicolet 380) was used as a detector to analyze the species in the outlet stream and solution-absorbed stream. In general, the spectra were recorded every 0.65 min in the range of 400−4000 cm−1 at a resolution of 2.0 cm−1 for 32 scans after the first 200 mL of gases flowed through the FTIR gas cell.
Figure 1. SF6 SACA (mmol/g) under the following conditions: 0.5 g of sludge, flow rate of 13.4 mL/min (GHSV = 1050 h−1). (A) Influence of temperature (
