Article pubs.acs.org/IECR
Passivation of Sponge Iron and GAC in Fe0/GAC Mixed-Potential Corrosion Reactor Bo Lai,†,‡,* Yuexi Zhou,‡ and Ping Yang† †
Department of Environmental Science and Engineering, School of Architecture and Environment, Sichuan University, Chengdu 610065, China ‡ Research Center of Water Pollution Control Technology, Chinese Research Academy of Environmental Sciences, Beijing 100012, China S Supporting Information *
ABSTRACT: The Fe0/GAC mixed-potential corrosion reactor was used to treat the complicated, toxic, and refractory ABS resin wastewater. In the 100 days continuous run, the effect of the packing particles passivation on the treatment efficiency of the Fe0/ GAC mixed-potential corrosion reactor was investigated seriously. The formation mechanism of the compounds in passive film was investigated first by SEM, EDS, and X-ray dot-mapping, which was the precondition for the control of the passivation of packing particles. The results show that the passive film consisted of five kinds of compounds such as Fe3(PO)2·8H2O, FePO4 ·3H2O, Fe2O3, Fe3O4, and FeS, which obstructed the formation of macroscopic galvanic cells between sponge iron (Fe0) and GAC and decreased the COD treatment efficiency of the Fe0/GAC mixed-potential corrosion reactor from 45 to 55% to 0%. The formation of passive film mainly resulted from the elements of S and P, which were from the SO42− and PO43− in ABS resin wastewater. Therefore, the inorganic ions in wastewater, especially for SO42− and PO43−, should be removed first before the treatment of the Fe0/GAC mixed-potential corrosion reactor.
1. INTRODUCTION In China, most of the acrylonitrile−butadiene−styrene (ABS) resin manufacturers use emulsion grafting-blend production technology to produce various types of ABS resin.1 With this production technology, acrylonitrile, butadiene, styrene, and hundreds of auxiliary agents are needed in the whole production process, which leads to complicated, toxic, and refractory ABS resin wastewater.2 Thus, the ABS resin wastewater is one of the typical high-strength petrochemical wastewaters. If the untreated wastewater is discharged into the receiving water directly, it will cause profound damage to the environment. The conventional treatment methods including ozonation,3−6 electro-Fenton,7−9 and three-dimensional electrodes10 can be used to treat this wastewater. Nevertheless, all of these treatment technologies suffer the limitations of high costs. Therefore, it is necessary to develop an effective, robust, and economically feasible treatment technology for the ABS resin wastewater. In recent years, zerovalent iron (ZVI) has attracted increasing interest for the treatment of toxic refractory wastewater such as bromoamine acid wastewater,11 liquid crystal display (LCD) manufacturing wastewater,12 nitrobenzene wastewater,13,14 olive mill wastewater,15 and coking wastewater.16 The ZVI has been proven to be a cost-effective treatment approach for toxic refractory wastewater. The pollutant degradation efficiency of the ZVI would be influenced by the type and number of cathode. Granular activated carbon (GAC) is added as cathode into the ZVI system, which changes the ZVI system into a Fe0/GAC mixed-potential corrosion reactor. Then a large number of macroscopic galvanic cells are formed by the contact of Fe0 and GAC, which can improve the current efficiency of the internal electrolysis significantly.17,18 The half-cell reactions of the Fe0/GAC mixed-potential corrosion reactor can be represented as follows: © 2012 American Chemical Society
Cathode (reduction) Acidic:
2H+ + 2e− → 2[H] → H 2↑ Eθ(H+/H 2) = 0.00V
(1)
Acid with oxygen:
O2 + 4H+ + 4e− → 2H 2O Eθ(O2 /OH−) = 1.23V
(2)
Neutral to alkaline:
O2 + 2H 2O + 4e− → 4OH− Eθ(O2 /OH−) = 0.40V
(3)
Anode (oxidation) Fe − 2e− → Fe 2 + Eθ (Fe2 +/Fe) = −0.44V Received: Revised: Accepted: Published: 7777
(4)
December 23, 2011 April 18, 2012 May 3, 2012 May 3, 2012 dx.doi.org/10.1021/ie203019t | Ind. Eng. Chem. Res. 2012, 51, 7777−7785
Industrial & Engineering Chemistry Research
Article
It is clear that organic pollutants can be reduced by [H] produced from electrode action in Fe0/GAC mixed-potential corrosion reactor.19 Furthermore, organic pollutants can also be removed by adsorption, enmeshment, and coprecipitation17 by the ferric and ferrous hydroxides formed from oxidation and precipitation of Fe2+. Zee et al.20 discovered that GAC could improve the reduction by accepting electrons and transferring the electrons to pollutants. Meanwhile, GAC was chosen as cathode because of its strong physical adsorption, and activated carbon can adsorb some of the organic pollutants, especially for the hydrophobic ones.17 In other words, the GAC could enhance the resistance to the shock loadings of toxic refractory wastewater by its strong high physical adsorption. Both ZVI and Fe0/GAC systems are the cost-effective treatment approaches for toxic and refractory wastewater, but they both will loss their reactivity over time which results from the corrosion products or other precipitates on the surface of the Fe0 and GAC.21 In other words, the operating life of the ZVI and Fe0/GAC systems is not long enough for the treatment of wastewater. Liu et al.21 used ultrasound to prolong the operating life of a Fe0/GAC mixed-potential corrosion reactor, but the reactor could not overcome the passivation of Fe0 and GAC completely. Moreover, it would consume energy and increase the operating cost. Therefore, it is necessary to develop a cost-effective and robust Fe0/GAC mixed-potential corrosion reactor for the treatment of toxic and refractory wastewater. In this study, ABS resin wastewater was treated by the Fe0/ GAC mixed-potential corrosion reactor, and the formation mechanism of passive film on the surface of Fe0 and GAC in the Fe0/GAC mixed-potential corrosion reactor after the longterm run was investigated in detail. Effects of PO43− and SO42− on the formation of passive film on the surface of Fe0 and GAC were studied in the treatment process of the Fe0/GAC mixedpotential corrosion reactor.
according to the BET method, a total pore volume of 0.481 mL·g−1, and a bulk density of 0.493 g·cm−3. There are some cracks and big pores on the surface of GAC (see Figure S3cd, Supporting Information). 2.3. Experimental Methods. The pH of the ABS resin wastewater was adjusted to 4.0 using diluted sulfuric acid (10%) or sodium hydroxide solutions (5 mol L−1). The hydraulic retention time (HRT) of the Fe0/GAC mixed-potential corrosion reactor was 4 h, and the temperature of the batch was kept at about 25 °C. The reactors were operated continuously 100 days at a constant hydraulic retention time (HRT) of 4 h, and the COD, PO43−, and total iron of the effluent were determined each day. After 100 days run, the characteristics of passive Fe0 and GAC were analyzed by scanning electron microscope (SEM), energy dispersive spectrometer (EDS), X-ray diffraction (XRD), and specific surface area measuring instrument, respectively. 2.4. Sample Preparation and Analytical Methods. Energy dispersive spectrometry (EDS) analysis was performed by a permanent thin film window link (Oxford Instruments) detector and WinEDS software in a Hitachi S-3500N scanning electron microscope (SEM) using an electron beam operating voltage of 25 kV and emission current of 60−70 μA. The EDS was used to analyze elements on the surface of the sponge iron and GAC. The Hitachi S-3500N SEM was used to observe the morphologies of the sponge iron and GAC. To investigate the element distribution on the cross-section of sponge iron and GAC, the sponge iron and GAC was fixed by epoxy resins, then the fixed sponge iron and GAC was split from the middle by the cutting machine, and their cross-section should be polished before detection. At last, the X-ray dot-mapping was used to characterize the element distribution at the cross-section of the sponge iron and GAC. A Phillips Xpert Pro diffractometer with a Cu Kα radiation source (λ = 1.5406 Å) was used for XRD analysis (generator voltage of 40 keV; tube current of 30 mA). XRD spectra were acquired between 2θ of 4−85, with a step size of 0.05 and a 2 s dwell time. Analysis of the surface physical properties of the sponge iron and GAC includes determination of the pore size distribution, total surface area, and total pore volume. Pore size distribution, total surface area, and total pore volume of the sponge iron and GAC were determined from N2 adsorption and desorption isotherms using a specific surface area measuring instrument (Nova 4200, Quantachrome Instruments, UK). Total surface area was calculated using a BET mathematical model, and the total pore volume and pore size distribution were identified using the t-plot and BJH method. Before the adsorption process commences, all the samples were outgassed for 5 h at 300 °C, in order to remove previously adsorbed gases from the surface. The chemical oxygen demand (COD) and biochemical oxygen demand (BOD5) were determined using a COD analyzer (Hach, USA) and respirometer (OxiTop IS12, WTW, Germany), respectively. PO43− and SO42− were determined by ion chromatography (ICS-1000, Dionex, USA). Total iron of the effluent was determined by atomic absorption spectrometry (AA-6300, Shimadzu, Japan). The pH was measured by a pHS-3C pH Meter (Rex, China).
2. MATERIALS AND METHODS 2.1. Raw Wastewater. The wastewater used in this study was obtained from a petrochemical industry in north China. The main physicochemical characteristics of ABS resin wastewater are listed in Table S1 (Supporting Information). 2.2. Fe0/GAC Mixed-Potential Corrosion. The experimental apparatus was a cylindrical mixed-potential corrosion reactor (Φ10 cm × 50 cm) (see Figure S1, Supporting Information). The reactor was made of transparent synthetic glass columns. The granular activated carbon (GAC) and sponge iron (Fe0) were mixed together with a volumetric ratio of 1:1, and then packed in the reactor as a fixed bed with a bed height of 40 cm. A commercial sponge iron was obtained from Beijing MingJian Technology Company. It has a mean particle size of approximately 3−5 mm and a bulk densities of 3 g·cm−3. The main element of the sponge iron is Fe (>98%), and the residual elements in the sponge iron are Si, Mn, Ca, C, Mg, and Al (
