Ultrasonic Pretreated Sludge Derived Stable Magnetic Active Carbon

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Ultrasonic Pretreated Sludge Derived Stable Magnetic Active Carbon for Cr(VI) Removal from Wastewater Kedong Gong, Qian Hu, Lu Yao, Min Li, Dezhi Sun, Qian Shao, Bin Qiu, and Zhanhu Guo ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04421 • Publication Date (Web): 20 Apr 2018 Downloaded from http://pubs.acs.org on April 20, 2018

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ACS Sustainable Chemistry & Engineering

Ultrasonic Pretreated Sludge Derived Stable Magnetic Active Carbon for Cr(VI) Removal from Wastewater

Kedong Gong1, Qian Hu1,3, Lu Yao1, Min Li1, Dezhi Sun1, Qian Shao2, Bin Qiu1,*, Zhanhu Guo3,* 1

Beijing Key Laboratory for Source Control Technology of Water Pollution, College of Environmental Science

and Engineering, Beijing Forestry University, 35 Qinghua East Road, Haidian District, Beijing, 100083 China 2

College of Chemical and Environmental Engineering,

Shandong University of Science and Technology, 579 Qianwangang Road, Huangdao District，Qingdao 266590, China 3

Department of Chemical and Biomolecular Engineering，

University of Tennessee, 1512 Middle Dr, Knoxville, TN 37996 USA

*: to whom the correspondence should be addressed

[email protected] (B. Qiu) [email protected] (Z. Guo)

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ACS Paragon Plus Environment

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ABSTRACT A stable magnetic carbon was synthesized using activated sludge as the carbon precursor. The ultrasonic pretreatment was used to destroy the cells in the activated sludge and to release the soluble carbon source, which was responsible for the improved stability of the synthesized magnetic carbon. 800 W was demonstrated as the optimized ultrasonication power for the pretreatment of activated sludge. Then the carbonization parameters, such as pyrolysis temperature, heating rate and dwell time were optimized as 800 oC, 10 oC/min and 60 min, respectively. To be more specific, this activated sludge derived magnetic carbon can reduce almost all the hexavalent chromium (Cr(VI)) (2.0 mg/L) in 10 minutes, and has a maximum capacity as high as 203 mg/g. The iron release rate of the synthesized activated sludge derived magnetic carbon was decreased, which improved the electron utilization of zero valent iron (ZVI). This composite was demonstrated to have a good stability and recyclability as well. Finally, the Cr(VI) removal mechanisms were clarified under the acidic and the natural conditions.

KEYWORDS: Magnetic carbon, Ultrasonic pretreatment, Zero valent iron, Activated sludge, Cr(VI) removal

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INTRODUCTION Over the past decades, huge amounts of Cr(VI)-containing wastewater has been generated from numerous industries,1-2 and poses serious threats to the aqueous environment.3 It mainly exists as HCrO4-, CrO42- and Cr2O72- anions in the wastewater, and has high mobility and toxicity in the aqueous environment.4-5 By contrast, Cr(III) ions are much less toxic and immobile.6 Therefore, reducing poisonous Cr(VI) anions to less poisonous Cr(III) ions was regarded as a promising approach for eliminating environmental risk. For this reduction process, the zero valent iron has been applied widely due to its good ability of electron donation. As documented, Cr(VI) reduction prefers in the acidic solution (pH=1~3).7 Thus, the ZVI can be dissolved easily in the acidic solution, generating Fe2+ and electrons at a high rate. The fast generation of electrons leads to the production of hydrogen by the reduction of H+,7 which caused the waste of electrons in ZVI. Therefore, carbon materials were often used to coat on the surface of ZVI to protect it against the oxidation by oxygen and the corrosion by the acid due to the advantages of carbons, such as good resistance to extreme condition, abundant porosity and low cost.8-10 For the synthesis of magnetic carbon composites, various carbon sources were used, including cellulose,7 glucose,11 polymers and waste plastics.12-13 Recently, carbon precursors like the biomass waste attracted increasing attention due to their low-cost.14 Among these precursors, the activated sludge was considered as a cheap one for synthesizing activated carbon. However, activated sludge produced in wastewater treatment plant (WWTP) inevitably contains bacterial cells, and may cause serious pollution if it is disposed improperly.15-16 Therefore, using activated 3



sludge as the carbon source for synthesizing magnetic carbon is an alternative method to avoid its further pollution and to achieve the resource recovery. Sludge derived carbon materials were often synthesized by chemical carbonization,17 pyrolysis and microwave-assisted pyrolysis methods.18-19 More efforts have been focused on the optimizing the synthetic method and conditions of magnetic carbon composites.20 The activated sludge derived magnetic carbon was found to have excellent Cr(VI) removal ability. However, for the synthesis of biomass based magnetic carbon, the iron ions were often mixed with the biomass by an impregnation method,7 and were just attached on the surface of biomass.21 Thus the generated iron particles were mainly distributed on the surface of magnetic carbons.22 The attached irons can be easily dissolved in the acidic solution, leading to the waste of iron. Therefore, a new method is needed urgently to improve the stability of the activated sludge derived magnetic carbon, which can efficiently improve the electron utilization efficiency of the ZVI during treating the Cr(VI) wastewater. The cells in the activated sludge contain a large amount of cytoplasm, which is considered as a good soluble carbon precursor. Moreover, the ultrasonication was reported as a good method to destroy the bacterial cells in the activated sludge, releasing the soluble carbon source, such as the cytoplasm from the inner cell as well as the extracellular polymeric substances.23 The soluble carbon source mainly contains the proteins and polysaccharides and is negatively charged in solution,24 which facilitates the iron ions be surrounded by the soluble proteins and polysaccharides. Thus, a carbon layer would be formed on the surface of the irons when it was calcined, and could act as a protective layer for the irons against the extreme condition. 4


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Moreover, the iron forms in the magnetic carbon are important for Cr(VI) reduction. The iron oxides, e.g. γ-Fe2O3 and Fe3O4 introduced in the magnetic carbon, made the materials possess the magnetic property.25 However, the iron oxides have a low ability to donate electrons.26 Thus ZVI was expected to be formed in the magnetic carbon materials due to its great ability of electrons donation. The ZVI can be obtained by optimizing the carbonization conditions, such as the carbonization temperature,27 heating rate and the retention time.7 In this study, a stable magnetic carbon was prepared using the activated sludge from municipal sewage treatment plant as a green carbon precursor. The ultrasonic-assisted impregnation was used for pretreating the activated sludge before the carbonization process for the first time. The ultra-sonication destroyed the cell wall to release the soluble cytoplasm organics. The iron was supposed to be surrounded well by the soluble carbon source in the ultrasonic-assisted impregnation, which was important to improve the stability of the magnetic carbon. The carbonization conditions, e.g. pyrolysis temperature, heating rate and dwell time, was optimized according to the morphology, iron form, porosity and stability of the synthesized activated sludge derived magnetic carbon. The influence of the environmental factors on Cr(VI) removal by this activated sludge derived magnetic carbon was explored by batch assays. The stability of this magnetic carbon was significantly improved as well as the electron utilization efficiency of the ZVI. Meanwhile, this work provides an effective resource way to use excess activated sludge from municipal sewage treatment plant. MATERIALS AND METHODS

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Materials. The activated sludge was collected from a secondary clarifier of wastewater treatment plant in Beijing, China. Fe(NO3)3·9H2O and K2Cr2O7 were supplied by Beijing Chemical Works, China. Synthesis of magnetic carbon. The activated sludge was pretreated by an ultrasonic-assisted impregnation method before being carbonization. Briefly, 10.0 g Fe(NO3)3·9H2O was added into 900 mL sludge (suspended solid concentration: 15.25 g/L). The agitated mixture (300 rpm, 30 min) was transferred to an Ultrasonic Cell Crusher with different powers (200, 400, 600, 800 and 1000 watt) for 30 min, in order to destroy the cell wall and release the soluble cytoplasm. The released soluble carbon source was extracted by centrifugation at 5000 rpm for 10 min,28 then the concentration of the soluble carbon source was detected by the soluble chemical oxygen demand (SCOD) index using the Hach closed reflux method. The mixture was dried at 105 oC to get the solid samples. Finally, the solid samples were pyrolyzed in tube furnace with desired temperature (400, 600, 800 and 1000 oC), heating rates (5, 10, 15 and 20 oC/min) and dwell time (0, 30, 60 and 90 min) under N2 atmosphere. The magnetic carbons synthesized with and without the ultrasonic pretreatment were named as UMC-x-y-z and MC-x-y-z, where x, y and z represented the pyrolysis temperature, heating rate and dwell time, respectively. Cr(VI) removal. Batch experiments was employed to evaluate the performance of the activated sludge derived UMC. Briefly, 20.0 mg UMC were introduced into Cr(VI)-containing solution (20 mL, initial pH at 3.0, 2.0 mg/L) for 30-minute reaction. To evaluate the pH influence on Cr(VI) reduction, the pH values of solutions (2.0 mg/L) were adjusted ranging from 6


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3 to 10 by NaOH and HCl. The removal capacity of the activated sludge derived UMC was determined with different initial concentration (2 ~ 1000 mg/L). The Cr(VI) anions and iron ions remained in treated wastewater were measured by a UV-Vis spectra method and inductively coupled plasma (ICP) method (Optima 8X00, USA),2 respectively. Characterizations. The morphology was visualized using scanning electron microscope (SEM, JEOL JSM 6301F) and a transmission electron microscopy (TEM, JEOL JEM 2010F). The iron form in obtained magnetic carbons was analyzed by a Bruker AXS D8 Discover diffractometer with GADDS operating with a Cu-Kα radiation source filtered with a graphite monochromator (λ=1.5406 Å). The carbon structure property was detected by a Raman spectra (Horiba Join-Yvon Lab Ram Raman confocal microscope) with 785 nm laser excitation at a 1.5 cm-1 resolution at room temperature.29 The specific surface area (SBET) of magnetic carbons was detected by a Quanta chrome Nova 2200e.30 The chromium form after being treated by the magnetic carbons was determined by an X-ray photoelectron spectroscopy (XPS) (Physical Electronics, Inc., Chanhassen, MN, USA). The magnetic property of magnetic carbons before and after being used was determined by a Lakeshore 7300 quantum design Versalab vibrating sample magnetometer (SQUID-VSM, USA). RESULTS AND DISCUSSION Effect of ultrasonic pretreatment. The ultrasonic pretreatment was employed to destroy the cell and to release the soluble carbon source. The SCOD was used to index the concentration of the soluble carbon in the aqueous solution. As shown in Figure 1 A, ~1000 mg/L SCOD was obtained from the original activated sludge, while it increased obviously with increasing the 7



ultrasonic power in the pretreatment. ~3276 mg/L SCOD was obtained when the ultrasonic power was controlled at 800 W. The proteins were detected as the main composition in the soluble carbon source (Figure S1), and had a concentration of 1297 mg/L. Moreover, 937.18 mg/L polysaccharides were also detected as the other main composition. With increasing the ultrasonic power to 1000 W, no obvious increase in the SCOD was observed. Almost all the cells in the activated sludge can be mechanically destroyed by sonication with a power of 800 W, leading to a full release of the soluble carbon source. As shown in Figure S2, the isoelectric point of the carbon solution extracted from the activated sludge was ~3.3. It was negatively charged when the pH was above 3.3, which facilitated its surrounding distribution on the surface of the positive Fe3+ ions. Figure 1 In this work, the influence of the pretreated ultrasonication power on the stability of the synthesized UMC was investigated. A suitable release rate of iron from the magnetic carbons was important for improving the electron utilization of the magnetic carbon. As shown in Figure 1B, the release rate of Fe3+ from MC synthesized without the ultrasonic pretreatment in the acidic solution was ~0.457 mg/(L·min), which was much higher than the UMCs synthesized with ultrasonic pretreatment. Moreover, the Fe3+ release rate decreased with increasing the ultrasonic power during the pretreatment. The lower iron release rate was detected from the UMC synthesized with a pretreatment under an ultrasonic power of 800 W. This was consistent with the result that more SCOD was released from the activated sludge when the ultrasonication power was controlled at 800 W (Figure 1A). However, no obviously increased SCOD was 8


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observed when the pretreatment power raised at 1000 W, indicating that 800 W was considered as the optimal ultrasonic power for pretreating the activated sludge. The morphology of the magnetic carbon synthesized with and without ultrasonic pretreatment was compared as well. As shown in Figure 2A, the iron crystals were distributed on the surface of the activated sludge derived MC without ultrasonic pretreatment. The MC presented an irregular spherical morphology, and had an average diameter of ~300 nm (Figure 2B&C). Moreover, rough surface was observed on the surface of iron crystals for the activated sludge derived UMC synthesized with the 800 W ultrasonic pretreatment (Figure 2D). It indicated that the ultrasonic pretreatment facilitated the soluble cytoplasm to coat on the iron surface. Then the surrounded cytoplasm formed a carbon layer when it was calcinated at 800 oC, which protected the inner ZVI against the oxidation and corrosion by acid. Figure 2 Effect of carbonization process. Besides the pretreatment, the carbonization conditions, e.g. the pyrolysis temperature,31 heating rate and dwell time have been reported to influence the synthesized magnetic carbons.32 Thus the effect of the carbonization temperature on the iron forms, porosity and the carbon layer was determined. As documented, the diffraction peaks at 30.1°, 35.5°, 43.2°, 53.5° and 56.9° in the XRD patterns was indexed as the (220), (311), (400) , (422) and (511) reflections of Fe3O4,33-34 respectively. The plane of (002), (100), (110) and (200) of the ZVI were presented at diffraction peaks 25.6°, 44.7°, 45.0° and 65.2° in the XRD patterns.35 Moreover, the sharp peaks existing at 39.8°, 40.6° and 48.3° are the signals of (002), (201) and (022) planes of the cubic Fe3C.36 As shown in Figure 3, the Fe3O4 was detected as the 9



main form of the iron in the UMC when the carbonization temperature was controlled lower than 600 oC. Rising pyrolysis temperature increased the intensity ratio of the ZVI/Fe3O4 in the synthesized materials, which implied that Fe3O4 was reduced to ZVI by carbon at high pyrolysis temperatures. Thus, the ZVI and Fe3C were found to be the main forms of iron in the synthesized UMC when the pyrolysis temperature was 1000 oC, indicating that the Fe3O4 was further reduced to Fe3C at 1000 oC. Both ZVI and Fe3C were good electron donors,7 which facilitate the Cr(VI) reduction in the wastewater. The peaks at 26.2° in all XRD patterns with a maximum intensity are attributed to the (002) plane of the graphite structure.37-38 The iron might facilitate the formation of ordered graphitic carbon structure by serving as catalysts during the carbonization of activated sludge.39 Figure 3 The structure features of the carbon layer in the UMC was detected by the Raman spectra. For all the synthesized activated sludge derived UMC, two major broad peaks at 1293 cm-1 (D-band) and 1588 cm-1 (G-band) were detected (Figure 4).40-41 Specifically, the G-band presented a shift to higher frequency accompanied by the increase of pyrolysis temperature, which inferred the interaction between carbon and iron during the pyrolysis process. A higher intensity ratio of G-band to D-band (ID/IG) indicated more ordered carbon generated.42 The band peaks of UMC synthesized at 400 oC are not well-defined, which is due to the incomplete carbonation reaction at 400 oC. The ID/IG increased from 0.988 to 1.006 with increasing the final temperature from 600 to 1000 oC, indicating an increase of sp3 C-C in the activated sludge derived UMC with increasing the carbonization temperature.43 10


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Figure 4

The porosity property was also affected by the pyrolysis temperature. The type-IV behaviour indicative of isotherm with a hysteresis loop (Figure 5A) indicated a mesopores structure of the activated sludge derived UMC. The SBET were calculated and listed in Table S1. The SBET of UMC increased with increasing the pyrolysis temperature, and a higher SBET of 114.24 m2/g was achieved for the UMC synthesized at 800 oC. Rising the temperature led to a more consumption of carbon during the pyrolysis process, which facilitated to form porous structure.44 Moreover, the pore size of the UMC was centered at 3.86 nm when the carbonization temperature was controlled at 800 oC (Figure 5A (insert)), which was the same as the cellulose derived magnetic carbon.7 However, the carbon layer can also give electrons to reduce iron oxides to ZVI or Fe3C when the pyrolysis temperature was further rised to 1000 oC, which lead to the decrease of SBET, as shown in Table S1. Based on the results of the iron form, structure features and porosity of the synthesized UMC, 800 oC was suggested as the optimized pyrolysis temperature for synthesizing the activated sludge based UMC. Figure 5 The effect of heating rate on the UMC was investigated under the condition of the optimized carbonization temperature at 800 oC. Figure 3 showed that much higher intensity ratio of ZVI to Fe3O4 was detected in the UMC-10-800-0 than others. It was inferred that 10 oC/min of heating rates facilitated the reduction of iron oxides and the formation of ZVI. The ID/IG of the UMCs synthesized with different heating rates were 1.009, 1.001, 1.011 and 0.997 (Figure 4), 11



respectively. This indicates that a suitable heating rate (5 ~15 oC/min) facilitated the formation of ordered carbon layer. The more disordered carbon was supposed to be consumed for reducing the iron oxides, which was supported by the XRD results (Figure 3) that higher proportion of ZVI was formed when the heating rate is 10 oC/min. Moreover, a bigger SBET (103.65 m2/g) was achieved for the UMC with this heating rate. A porous carbon layer was proposed to be better on protecting the inner ZVI against acids in the solution. Based on above discussion, the heating rate for synthesizing the activated sludge based UMC was optimized at 10 oC/min.

Finally, the effect of the dwell time on the iron forms and porosity of the UMC was investigated under optimized conditions of the carbonization temperature and heating rate. As shown in Figure 3, higher ZVI proportion was achieved with a longer retention time, which facilitated the reduction of iron oxides to ZVI by the carbon layer. The peaks corresponding to ZVI were obviously observed in the UMC synthesized with a retention time of 60 and 90 min. The observed broad diffraction peaks in the range of 40-50° were attributed to the crystalline iron in the UMCs.45 The peaks of the D-band of the carbon layer were shifted to lower wavenumbers with increasing the retention time as well as the G-band (Figure 4), indicating the consumption of carbon for the reduction of iron oxide during the retention period. A long retention time might promote the bonding of formed ZVI and the carbon layer.40 The ID/IG of the UMC synthesized at 800 oC was 1.001. It was increased to 1.025, 1.009 and 1.035 with increasing the retention time to 30, 60 and 90 min, respectively. The slightly increased IG/ID further demonstrated that some of the disordered carbon was used to reduce the iron oxides. A 12


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bigger SBET of 131.13 m2/g was achieved when the retention time was controlled at 60 min, which was higher than the reported (32 m2/g),5 and was similar to the cellulose derived magnetic carbon.6 Moreover, the SBET of the magnetic carbon depended mainly on the proportion of irons. And it can be inferred that the consumption of disordered carbon layer would be beneficial to form pores, which was attributed to the larger SBET (Figure 5(C) and Table S1). Thus the retention time for synthesizing the activated sludge based UMC was optimized at 60 min.

Figure 6

As discussed above, the synthesis pathway of stable activated sludge based UMC was proposed as shown in Figure 6. The cells in the activated sludge contain a large amount of cytoplasm, which is considered as a soluble carbon source. The cells in the activated sludge can be mechanically destroyed by the force generated by the 800 W sonication, leading to a full release of the soluble carbon source. The sonication also facilitated a good mixture of soluble carbon source with the iron ions. Moreover, the positively charged Fe3+ added can be easily surrounded by the negatively charged cytoplasm. Then the cytoplasm surrounded irons were carbonized and formed the carbon layer surrounded irons. Moreover, more ZVI was obtained due to the reduction of iron oxides by the carbon. The porous property of the carbon layer surrounded the ZVI can somehow protect the ZVI against the H+ and decrease the electron release. This surrounding carbon layer was important for the stability of UMC. Generally, the destroying of cell and the release of soluble carbon source were the key for the stability of UMC.

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Cr(VI) removal investigation. The activated sludge derived UMC prepared under the optimal condition was used to treat the Cr(VI)-containing wastewater. Figure 7A showed that Cr(VI) concentration was an important factors affecting the performance of activated sludge derived UMC. The removal capacity was detected by rising the Cr(VI) concentration. ~203 mg/g of removal capacity was achieved for the UMC with ultrasonic pretreatment, which was much higher than the magnetic carbon synthesized by simple calcination of polymer (38.8 mg/g) and epichlo-rohydrin (55.8 mg/g).46-47 Figure 7 To further illuminate the Cr(VI) removal mechanism, the XPS was used to characterize the synthesized UMC after treating Cr(VI)-containing wastewater. The binding energy peaks at 576.55, 577.2, 577.8, 579.5, 586.85 and 588.6 eV (Figure 8A) was indexed as Cr(III),7 which implied that the Cr(VI) was removed absolutely and adsorbed on the UMC in the form of Cr(III). Moreover, the peaks at 711.1, 712, 713.75, 724.65 and 726.1 eV (Figure 8B) corresponded to Fe3O4 in the UMC treated by the Cr(VI),48 which indicated that parts of ZVI donated its electrons to reduce Cr(VI) and was oxidized to iron oxides. Meanwhile, C-O=O, C-O and C-C groups (peaks at 288.65, 286.2 and 285.2, 284.75 and 284.3 eV) were detected as well for the treated UMC (Figure S3),48 which inferred to that the carbon layer can also give electrons to reduce Cr(VI).

Figure 8

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The reduction kinetics by the UMC was investigated as well. The Cr(VI) reduction by this material was a fast process (Figure 7B). This Cr(VI) removal process fitted better with the second order kinetic model (Figure S4), the reaction constants with initial pH of 1, 3, 7 and 10 were calculated to be 1.1115, 0.9904, 0.0246 and 0.0160 mg-1· L· min-1, indicating that a faster removal was achieved under the acidic condition. Cr(VI) containing wastewater (2.0 mg/L) can be completely reduced by the activated sludge derived UMC within 10 minutes when the initial pH was controlled at 1.0. Moreover, ~95% can be reduced during the beginning 5 minutes. This was much faster than the spherical iron–carbon composites (25 min)26 and nanoscale magnetite (20 min).49 However, the reduction rate was decreased obviously when the solution turned to be alkaline (pH > 7). This was due to the slow electron release from the ZVI in the UMC.

The pH of the reaction system is important for the Cr(VI) reduction reaction by the magnetic nanocomposites.26, 50 Figure 7C showed almost all the Cr(VI) was removed with the initial pH lower than 3. The removal percentage gradually decreases with rising the initial pH value. ZVI in the UMC was corroded by acid in solution, generating the reductive intermediates such as H , hydrogen and Fe2+. These intermediates were the direct electron donors to reduce ●

Cr(VI). Cr(VI) mainly exists as the forms of CrO42-, Cr2O72-, HCrO4- and H2CrO4 in the aqueous solution, and the existing forms depend on the pH of solution.51 When the pH was adjusted below 6.8, most of Cr(VI) exits as the form of HCrO4- and H2CrO4.51 As documented, higher oxidation reduction potential (1.33 eV) of HCrO4- determined its easy reduction by reluctant, which determined a better removal performance in the acidic solution.52 However, the ZVI can’t 15



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be easily oxidized in the basic solution, thus the carbon layer is one of the main electron donors for this reduction reaction. The weak ability of the electron donation of the carbon corresponded to a slow reduction rate and low removal percentage under alkaline condition.

Stability and recyclability. As shown in Figure S5, 10.4 and 10.3 emu/g of the saturation magnetization were detected for the sludge derived UMC prepared with and without ultrasonic pretreatment. However, it was decreased to 7.6 and 5.4 emu/g after treating Cr(VI)-containing wastewater. It was inferred that more iron was dissolved from the MC, indicating that the stability of the magnetic carbon was improved by the ultrasonic pretreatment. Almost the same reduction capacity was achieved for the UMC and MC (203 and 198 mg/g). However, the Cr(VI) reduction consumed different quantities of ZVI in the magnetic carbons. Figure 7D shows the iron concentrations detected in the solutions after being treated by the MC and UMCs. For the MC, 5.121 mM ZVI was consumed to reduce 0.654 mM Cr(VI), which was much higher than 1.103 mM for the UMC. It was inferred that the ultrasonic pretreatment facilitated the stability of the synthesized UMC. The recyclability of synthesized UMC was also investigated. Almost 100% of the removal percentage was reached by the UMC for the initial fourth cycles, and only a little decrease (~5%) was observed at the fifth cycle (Figure S6). This implied that the activated sludge derived UMC has a good recyclability.

CONCLUSION The ultrasonic pretreatment was demonstrated to be an effective method to improve the stability of the synthesized magnetic carbons synthesized by using activated sludge as carbon 16


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source. The power for ultrasonic pretreatment was optimized as 800 W, and the optimal carbonization conditions were 800 oC of carbonization temperature, 10 oC/min of heating rate and 60 min of retention time. The synthesized UMC had a high proportion of ZVI and a SBET of 131.1 m2/g. It was demonstrated to be excellent on treating Cr(VI)-containing wastewater. The capacity reached at as high as 203 mg/g, which was higher than many reported materials. Both the ZVI and carbon in the UMC was disclosed to be the domain electron donors for treating Cr(VI)-containing wastewater. Moreover, the electron utilization efficiency of ZVI in the activated sludge derived magnetic carbon was obviously improved by the ultrasonic pretreatment. ■ SUPPORTING INFORAMTION Structure properties of magnetic carbons, 3D-EEM spectra and Zeta potential of the soluble carbon source, C1s XPS spectra and magnetic property of magnetic carbon, Cr(VI) adsorption kinetics and recyclability of magnetic carbon. These materials are available free of charge via the Internet at http://pubs.acs.org ■ AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (Bin Qiu) *E-mail: [email protected] (Zhanhu Guo) Notes The authors declare no competing financial interest. 17



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Figures and Figure Captions

Figure 1 (A) SCOD released from the activated sludge with different ultrasonic powers; (B) concentrations of iron ions released from the magnetic carbons synthesized with different ultrasonic pretreatments.

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Figure 2 SEM images of the (A) MC, and (B&C) UMC; (D) TEM images of the UMC.

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Figure 3 XRD patterns of the synthesized magnetic carbons.

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Fig. 4 Raman spectra of the synthesized magnetic carbons.

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Figure 5 N2 adsorption-desorption isotherms and pore size distribution (inserted) of magnetic carbons synthesized with different (A) carbonization temperatures, (B) heating rates and (C) retention time.

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Figure 6 Proposed synthesis pathway of the stable active sludge based magnetic carbon.

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Figure 7 (A) Effect of initial Cr(VI) concentration on the Cr(VI) removal efficiency by UMC; (B) Effect of contact time on the Cr(VI) removal by UMCs; (C) Effect of pH on the Cr(VI) removal by UMC; (D) Amount of iron and chromium in solutions after being treated by the MC and UMC.

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Figure 8 (A) Cr2p and (B) Fe2p XPS spectra of UMC after being treated with 2.0 mg/L Cr(VI) solution.

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Table of content

The ultrasonic pretreatment improves the stability of sludge based magnetic carbon, which reduces the iron waste in Cr(VI) reduction.

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Ultrasonic Pretreated Sludge Derived Stable Magnetic Active Carbon

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