Article pubs.acs.org/IECR
Production of Red Mud Particle Electrodes (RMPEs) and Its Performance Investigation in a Biological Aerated Filter Coupled with a Three-Dimensional Particle Electrode Reactor (BAF-TDE) Yan Feng,†,‡ Xinwei Wang,∥ Jingyao Qi,*,† Zhaoyang Wang,† and Xin Li§ †
School of Municipal & Environmental Engineering, Harbin Institute of Technology, Harbin 150090, China School of Civil Engineering and Architecture, University of Jinan, Jinan 250022, China § Department of Chemistry, Harbin Institute of Technology, Harbin 150090, China ∥ China Urban Construction Design & Research Institute Co., Ltd. (Shang Dong), Jinan 250022, China ‡
S Supporting Information *
ABSTRACT: Novel particle electrodesred mud particle electrodes (RMPEs)were prepared using waste material (red mud, zeolite slag, and ferric oxide powder with a mass ratio of 5:3:2 and used in a biological aerated filter coupled with a threedimensional particle electrode reactor (BAF-TDE)) for use in municipal wastewater treatment. The physical and chemical characteristics of RMPEs were measured. The surface morphology of the RMPEs was also characterized by SEM. The results showed that (1) under the optimal sintering conditions, red mud could be used to manufacture RMPEs; (2) BAF-TDE had an advantage over the single biological aerated filter (BAF), with regard to the removal of organic matter and ammonia nitrogen; and (3) in the BAF-TDE system, BAF contributed more to the removal of organic matter and ammonia nitrogen than TDE did, while TDE played a crucial important role in BAF-TDE, which can be improved by introducing an electric field.
1. INTRODUCTION Widely distributed emerging organic contaminants, such as endocrine disrupting chemicals (EDCs), pharmaceuticals and personal care products (PPCPs), and other nonbiodegradable contaminants from some wastewaters are deemed to be disadvantageous to the environment.1 Under these circumstances, routine biological treatment techniques do not always reach satisfactory treatment effect;2 also, traditional physical and chemical methods are relatively costly, inoperative, or may bring about secondary pollution. Consequently, how to remove these substances economically and efficaciously is a pressing challenge. The biological aerated filte (BAF) is one of the main biological treatment technology. It was designed with the principal objective of removing easily or moderately biodegradable compounds. A great deal of research has conducted many studies to enhance the efficiency of biological treatment in the past few years. Advanced oxidation processes (AOPs), which are mostly dependent on the generation of hydroxyl radicals (•OH), including ozonation, Fenton reaction, photolysis, and sonolysis, have been studied as possible alternatives to destroy PPCPs in water. Another way to eliminate persistent pollutants is through the use of the electrochemical method. The three-dimensional particle electrode (TDE) has many merits over the other conventional electrochemical oxidation reactors, and has attracted more and more attention since it has been invented. This method has been efficaciously used to purify many types of wastewater. Its applications range from the removal of metal ions3−5 to different types of wastewater such as phenolic compounds and derivatives,6,7 oil refinery wastewater,8 PPCPs,9 and some other emerging organic contaminants.10 © 2015 American Chemical Society
It is well-known that selecting appropriate particle electrodes is crucial to the design and efficient operation of TDE. So far, the most frequently particle electrodes of TDE are metal particles, granular-activated carbon (GAC), carbon aerogel (CA), and modified kaolinite.11,12 Overall, these materials generally have good electrical conductivity and large specific surface area. Synchronously, the particle electrodes supported appropriate catalysts has been developed in recent years. Red mud is the insoluble residue remaining after the caustic digestion of bauxite, which is the ore used in the production of alumina by the Bayer process.13 Red muds are composed of a mixture of caustic insoluble minerals, a caustic aluminate solution reaction product, and various calcium compounds. The caustic insoluble minerals are typically Fe2O3 and (Fe,Al)OOH, along with TiO2. At present, red muds are dumped out as waste materials in many plants.14,15 This not only uses a large amount of arable land, but also pollutes the environment. The development and utilization of red muds are still in their infancy. Thus, the search for a sustainable way to coordinate the economy and environment is a challenging topic. In terms of utilizing red muds, a majority of active ingredients currently are not being fully applied. Choosing red muds as one of raw materials to manufacture particle electrodes may be an excellent choice. As is known to all, particle electrodes can be produced using metal oxides, nonmetal oxides, or supported metal oxides on packing, and red muds are composed of metal oxide and nonmetal oxides such as Fe2O3,CaO, Al2O3, and SiO2. The Received: Revised: Accepted: Published: 6641
February 3, 2015 May 27, 2015 June 9, 2015 June 9, 2015 DOI: 10.1021/acs.iecr.5b01290 Ind. Eng. Chem. Res. 2015, 54, 6641−6648
Article
Industrial & Engineering Chemistry Research
Figure 1. Experimental schematic of the laboratory-scale BAF-TAE and BAF system.
crammed with RMPEs (0.20 m height), and well joined to a power source. The BAF section was filled with zeolite (1.20 m height). First of all, municipal wastewater was poured into the coupled reactor from the bottom, and then most of effluent outflowed from the top. Moreover, the mixed solution between a proportion of effluent and municipal wastewater reflowed to BAF-TDE by pump, which accomplished the coupling between BAF and TDE. The air was injected into the apparatus through an aerometer by an air compressor. The two reactors were backwashed at regular intervals in order to eliminate the accumulated suspended substance, the superfluous biomembrane produced. As far as BAF-TDE is concerned, the periodic backwash also can recover the catalytic activity and adsorption capacity of particle electrode, because of the accumulation of contaminants on particle electrode surface in continuous operation. In the experiments, the backwash was executed every 48 h. 2.2.2. Wastewater Characteristics. In the entire experiment process, the BAF-TDE and BAF were supplied with synthetic municipal wastewater with pH 6.8−7.5, sulfate ion concentration ranging from 80 mg L−1 to 120 mg L−1, chemical oxygen demand (CODcr) ranging from 101.12 mg L−1 to 114.21 mg L−1 and the NH3−N concentration ranging from 19.14 mg L−1 to 26.25 mg L−1. Synthetic concentrated municipal water involved bicarbonate and starch, ammonium chloride, inorganic and organic carbon, as well as other nutrients. The concentrated substrate solution was deliquated by distilled water to reach the requisite concentration before being injected into the reactors. 2.2.3. Startup of BAF-TDE and Single BAF. In order to improve the inoculation rate, activated sludge was introduced to two reactors from wastewater treatment plant. The experimental parameters were the same for them. They were backwashed every 48 h. The two reactors achieved stable running conditions after 7 weeks. 2.3. Analytical Methods. The bulk density and apparent density were tested in accordance with the means of Kent et al.16 Total porosity and total surface area was analyzed by NOVA Touch-LX. The biofilm mass was detected in accordance with the mean reported by Qiu et al.17 The crystal structures were detected by X-ray diffraction (XRD). The surface and cross-section morphology were examined by SEM (Hitachi, Model S-2500). The heavy metal elements in leachate
latent applications of red mud would provide us a prospective method to produce particle electrodes for TDE. Zeolite is natural porous mineral described as crystalline hydrated aluminosilicates. Inside the framework structure of zeolite, alkali or alkaline-earth cations are reversibly fixed in the cavities and can easily be exchanged by surrounding positive ions. Zeolite tailing is a byproduct of zeolite produced in the zeolite mining process. The few reuses and large stockpiles of red mud led to a series of social and environmental problems. This study investigated the possibility of using the red muds as one of principal raw materials to a new red mud particle electrodes (RMPEs) by a high-temperature sintering process. The properties of RMPEs such as apparent and bulk density, total porosity, pore size distribution, total surface area, and heavy metal contents in lixivium and crystal structures were detected and analyzed to research the feasibility of RMPEs for municipal wastewater treatment. Moreover, separated BAF treatment and TDE were coupled into a reactor for sewage treatment by internal mixed liquid recycling. External electric field was used to improve removal efficiency of BAF in the coupled process. The performance of BAF coupled with TDE was researched through comparison with single BAF treatment.
2. MATERIALS AND METHODS 2.1. Materials. RMPEs were composed of red mud, zeolite slag, ferric oxide powder, and organic carbon. Red mud was obtained from Jinan Mining Development Corporation, and zeolite slags were obtained from Weifang Trading Company, Ltd. (Shandong Province, China). The chemical composition of red mud, zeolite slag, iron powder, and organic carbon is shown in Table S1 in the Supporting Information. Red mud was used as the main material; zeolite slag, ferric oxide powder, and organic carbon (as pore-forming material) served as additives and were mixed with red mud to produce the novel particle electrodes of RMPE. 2.2. Reactor Description and Operation. 2.2.1. Reactor Description. Experimental equipment is exhibited in Figure 1, including BAF-TDE and single BAF reactors. In terms of BAFTDE reactor, BAF and TDE were divided by a stainless steel porous cathode plate and enclosed by polyvinyl chloride (PVC) column. The reactors had an upflow configuration with 2.30 m in height and 0.10 m in inner diameter. The TDE section was 6642
DOI: 10.1021/acs.iecr.5b01290 Ind. Eng. Chem. Res. 2015, 54, 6641−6648
Article
Industrial & Engineering Chemistry Research
Figure 2. Illustration of the RMPE preparation process.
electrocatalytic properties of RMPE. There was 81.114% unburned carbon (UC) in the organic carbon, which was propitious to manufacturing lightweight porous RMPE, because of the decomposition of UC and organic matter during calcination. 3.1.2. Preparation of RMPE. Red mud was used as the main material; zeolite slag, iron powder, and carbon (as pore-forming material) served as additives and were mixed with red mud to produce the novel particle electrodes of RMPE. Four factors were taken into account, with regard to the RMPE preparation: the weight ratio of red mud, zeolite slag, and ferric oxide powder; the organic carbon dosage; the calcination temperature; and the soaking time. The influences of these four factors on the properties of RMPE were discussed in an orthogonal experiment. The experimental parameters for producing RMPE are shown Table S2 in the Supporting Information. In order to determine the optimum parameters of preparation RMPE, an orthogonal L9 43 test was designed. The evaluation indexes, such as specific surface area and bulk density, were analyzed statistically. Table 3 (presented later in this work) shows the orthogonal test results. A further orthogonal analysis was approved, because we could not obtain optimum parameters of preparation RMPE only on the basis of the outcomes in Table 3. Consequently, the K, k, and R values were also calculated and are shown in Table S3 in the Supporting Information. The results indicated that the influence of these factors on the comprehensive index of RMPE ranked in the following descending order (according to the R values): red mud:zeolite slag:iron powder weight ratio > organic carbon dosage > soaking time > calcination temperature. The most important decisive factor of preparation was the red mud:zeolite slag:iron powder weight ratio. The best preparation parameters of RMPE were red mud, zeolite slag and ferric oxide powder with a mass ratio of 5:3:2, an organic carbon dosage of 5%, a soaking time of 10 min, and sintering at 1000 °C. The preparation process of the RMPE is shown in Figure 2. RMPEs were manufactured as follows: initially, red mud, zeolite slag, ferric oxide powder, and organic carbon were dried at 110 ± 1 °C for 2 h and then were ground to a powder by a ball crusher. The powder was screened by the 60-mesh sieve. Then, raw materials were blended in a commingler and then diverted to a stirring plate. Simultaneously, tap water was added in order to make mixed materials become globules of 3−5 mm in diameter. Subsequently, semifinished RMPEs were transferred to a revolving furnace to accomplish heating and sintering
of RMPEs were also gauged with inductively coupled plasma (ICP). During the entire set of experiments, influent and effluent samples were taken at regular intervals and CODcr and NH3− N were tested according to conventional national standard methods.18 The temperature, pH, and dissolved oxygen (DO) were analyzed by probes. Overall, influent and effluent samples were taken at the end of the filter run, but occasionally samples were also drawn immediately after backwashing to inspect the recovery of effluent quality. Because of some limitations, we paid close attention to study the removal capability of CODcr and NH3−N, but some other quality parameters such as biochemical oxygen demand (BOD5), as well as P, PPCPs, and nitrate and nitrite (NO2− and NO3−) concentrations were not taken into account in this paper.
3. RESULTS AND DISCUSSION 3.1. Preparation of RMPE. 3.1.1. Chemical Composition of Raw Materials. The chemical analyses of red mud, zeolite slag, ferric oxide powder, and organic carbon are presented in Table S1 in the Supporting Information. The majority of red mud consisted of Fe2O3 (37.024%) and CaO (25.641%), followed by SiO2 (14.082%), MgO (7. 684%), and Al2O3 (6.258%). Besides a few transition-metal oxides (such as TiO2, V2O5, and MnO), a small amount of alkali-metal and alkaline-earth metal oxides involving Na2O, K2O, BaO, and SrO (0.272% in total) were also coexisting. Zeolite slag and iron powder were sieved as supplements to regulate the chemical components of red mud. As exhibited in Table S1 in the Supporting Information, the concentrations of SiO2 (72.272%) and Al2O3 (16.565%) in zeolite slag are significantly higher than that in red mud. SiO2 and Al2O3 are the main ingredients of ceramics, constituting the ceramic glassy structure materials. Therefore, the addition of zeolite slag could simultaneously increase the concentration of SiO2 and Al2O3 in the mixture of red mud, zeolite slag, ferric oxide powder, and organic carbon, so that the essential components of ceramic balls were within Riley’s foaming composition range.19 The content of Fe2O3 (98.425%) in ferric oxide powder is significantly higher than that in red mud. Some transition-metal oxides (e.g., MnO, Fe2O3, V2O5, TiO2) and less-common metal oxides (such as SnO2, RuO2, Sb2O3, and IrO2) were also very suitable as catalysts to degrade organic contaminants. Therefore, the addition of ferric oxide powder could increased the concentration of Fe2O3 in the mixture of red mud, zeolite slag, ferric oxide powder, and organic carbon in order to improve the 6643
DOI: 10.1021/acs.iecr.5b01290 Ind. Eng. Chem. Res. 2015, 54, 6641−6648
Article
Industrial & Engineering Chemistry Research Table 1. Comparison between the Elemental Properties of RMPE and Haydite filter material
particle diameter (mm)
apparent density (kg m−3)
bulk density (kg m−3)
total porosity (%)
biofilm (g g−1)
total surface area (m2 g−1)
RMPE haydite
3−5 3−5
1058.92 2262.40
621.67 1524.93
38.17 29.59
0.032 0.021
21.17 0.73
procedures. Dried globules were heated at 300 °C for 10 min, and then calcined at 1000 °C for 20 min. The heating rate from 300 °C to 1000 °C was set at 70 °C min−1. The sintered globules was gradually cooled to ∼200 °C within 3 h by natural cooling, and then removed from the furnace and cooled to ambient temperature. The RMPEs were finally acquired. 3.2. The Characteristics of RMPE. Table 1 shows the elemental properties of RMPE and haydite. The distinct differences between them cannot be observed in pore size distribution and particle diameter. Nevertheless, higher total surface area and total porosity are observed for RMPE than haydite. Besides, lower apparent and bulk density for RMPE were also attained. All these properties are associated with their chemical composition. Based on the data in Table S1 in the Supporting Information, the organic carbon consisted of 81.114% UC, which was propitious to manufacturing lightweight porous RMPE, because of the decomposition of UC and organic matter during calcination. Consequently, RMPE had a lower apparent and bulk density, because of the addition of organic carbon. In addition, gas-forming components from the raw materials made the surface of RMPE rougher than that of haydite, bringing about a larger surface area. Furthermore, the biofilm mass showed much more on the granular RMPE, because of its rough surface, than haydite, in accordance with the investigation of Kent et al.16 Figure 3 indicates the microstructure surface properties of RMPE and haydite, as determined via SEM. It can be seen that a rougher surface of RMPE distributed with large pores was clearly presented, compared with that of haydite. For RMPE, the total surface area reached over 21.17 m2 g−1 (as demonstrated in Table 1). The rough surface distributed with large pores was beneficial for the biofilms to grow onto the RMPE. These special surface morphology properties also demonstrated that RMPE was fit for particle electrode and filter material.12 Several types of metal elements contained in RMPEs could be shown on the English Dialect Society (EDS) inset in Figure 4. It can be deduced that RMPEs are mainly composed of some metal oxides such as SiO2, Fe2O3, FeO, etc. Figure 5 displays the XRD patterns of particle electrodes. As shown in Figure 5, RMPE had the complicated crystallographic structures that were principally ascribed to chemical composition of raw materials. The predominant crystal was SiO2 in the form of quartz (JCPDS File Card No. 46-1045). The particle electrodes also contain three types of iron oxides. A minor amount of FeO with the crystal phase of wustite (JCPDS File Card No. 461312), Fe2O3 with the crystal phase of hematite (JCPDS File Card No. 33-0664) and maghemite-C (JCPDS File Card No. 39-1346), and Fe3O4 with cubic crystal phase (JCPDS File Card No. 65-3107) can be detected in the particles by means of XRD. Furthemore, some low-concentration heavy metal compounds were also obtained, such as CuSn, massicot, nichrosi, todorokite, and illium. The result obtained from EDS and XRD further demonstrates that the particle electrodes are mainly composed of metals, metallic oxides, and silicon oxides. 3.3. Leachability of Heavy Metals in the RMPE. In order to detect the lixiviation of heavy metal in the water, the
Figure 3. SEM photographs on (a) the haydite surface and (b) the RMPE surface.
detections of heavy-metal concentrations in lixivium of RMPE were performed. Table 2 indicated that the filtrate contents of Cd, Cu, Ba, Hg, Cr, Pb, and As were lower than their detection limits, and the leachable Zn concentration was only 0.028 mg L−1. All the measured heavy metals were less than their thresholds determined by GB 5085.3-2007, China.20 They were almost wholly immobilized in RMPE during the calcining procedure. Consequently, this novel particle electrode had a stable structure, which would not bring about secondary contamination and was safe for municipal wastewater treatment. 3.4. Comparison between BAF-TDE and BAF Reactors. Many operational parameters affected the removal efficiency of biological reactors. In order to investigate the removal characteristics at (a) different organic loading rates (OLR) and (b) different ammonia nitrogen loading rates (ANLR) 6644
DOI: 10.1021/acs.iecr.5b01290 Ind. Eng. Chem. Res. 2015, 54, 6641−6648
Article
Industrial & Engineering Chemistry Research
Table 2. Contents of Heavy Metal Elements in Lixivium of RMPE heavy metal
content in lixivium (mg L−1)
threshold (mg L−1)
heavy metals
Cu Zn Cd Ba
0.012 0.028 0.006 0.044
100 100 1 100
Pb Cr Hg As
content in lixivium (mg L−1) 0.045 0.021 0.011
threshold (mg L−1) 5 5 0.1 5
3.4.1. Organics Reduction in Two Reactors. The influent and effluent CODcr in BAF-TDE and BAF at different OLR are shown in Figure 6a. This figure indicates that both reactors possessed the eximous CODcr removal capability. However, BAF-TDE had a higher removal rate of CODcr, in comparison with BAF at OLR, from 0.39 kg (m3 d)−1 to 1.57 kg (m3 d)−1. The average CODcr removals of BAF-TDE and BAF reactors were 83.76% and 75.42%, respectively. The effluent CODcr concentrations in the BAF-TDE and BAF reactors were in the ranges of 6.00−27.00 mg L−1 (17.43 mg L−1, on average) and 18.00−34.00 mg L−1 (26.38 mg L−1, on average), respectively. All results demonstrated that TDE could dramatically improve the CODcr removal efficiency of BAF. Meanwhile, BAF played an important role in removing CODcr than TDE. In the TDE part, RMPEs are mainly composed of metals, metallic oxides, and silicon oxides. Hardjono et al.25 and Kong et al.26 proved that iron, other transition metals (e.g., manganese, copper, and cobalt), and some less-common metal oxides (such as RuO2, SnO2, IrO2, and Sb2O3) were also very suitable as catalysts to degrade organic contaminants. Zhang claimed that H2O2 could generate hydroxyl radicals in the presence of various catalysts and hydroxyl radicals were beneficial to degrade the organic contaminants.12 Therefore, TDE played a crucial role in degrading the organic contaminants, which might be one of the important reasons why BAF-TDE showed a higher removal rate of CODcr, in comparison with single BAF. In the experiment performed with the OLR at 0.39 kg (m3 −1 d) , the average CODcr effluent concentrations of BAF-TDE and BAF were ∼8.00 mg L−1 and 20.90 mg L−1, respectively, and average CODcr removal rates were ∼92.47% and ∼80.43%, respectively. In terms of the OLR at 0.79 kg (m3 d)−1, the average CODcr effluent concentrations increased to 13.70 mg L−1 and 25.50 mg L−1, respectively, and the average removal
Figure 4. SEM images and EDS of RMPEs.
between BAF and BAF-TDE reactors, parameters such as temperature, DO, and voltage were kept constantly throughout the experimental process. Temperature is one of the parameters affecting the removal efficiency.21 The investigate maintained the temperature at 20 ± 1 °C. DO plays a important role on nitrification. If it is
