Synthesis of nano-zeolite from coal fly ash and its potential for nutrient sequestration from anaerobically digested swine wastewater

Bioresource Technology 110 (2012) 79–85

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Synthesis of nano-zeolite from coal fly ash and its potential for nutrient sequestration from anaerobically digested swine wastewater Chen XiaoYan a, Khunjar Wendell b, Jun Zhu c, Li JiangLi a, Yu Xianxian a, Zhang Zhijian a,b,⇑ a

Institute of Environmental Science, Center of Water Ecosystem and Watershed Management, ZheJiang University, YuangHangTang Avenue 688, HangZhou, ZheJiang Province 310058, China b Department of Earth and Environmental Engineering, Henry Krumb School of Mines, Columbia University, 500 West 120th Street, New York, NY 10027, USA c University of Minnesota, Department of Biosystems and Agricultural Engineering, Southern Research and Outreach Center, 35838 120th Street, Waseca, MN 56093, USA

a r t i c l e

i n f o

Article history: Received 28 October 2011 Received in revised form 15 January 2012 Accepted 19 January 2012 Available online 28 January 2012 Keywords: Nano-zeolite Coal fly ash Anaerobically digested swine wastewater Ammonium Phosphate

a b s t r a c t The treatment of anaerobically digested swine wastewater (ADSW) is problematic due to its high nutrient concentration. This study investigated the simultaneous sequestration of ammonium (N) and phosphate (P) from ADSW using nano-zeolites synthesized from fly ash (ZFA). The nanometer-scale crystalline structures plentiful of zeolite-NaP1 coating on ZFA particle increased the levels of specific surface area and cation exchange capacity at times of 40 and 104, compared to raw fly ash. Kinetic N and P sorption experiments with ZFA were well described by both the Langmuir and Freundlich models, suggesting the co-existence of homogeneous and heterogeneous sorption mechanisms. N and P removal efficiencies ranged from 41% to 95% and 75% to 98%, respectively, across a range of ZFA doses (from 0.25 to 8 g/ 100 ml). Collectively, application of the laboratory-synthesized ZFA can alleviate the nutrient loads in ADSW and therefore modify the ratio of N:P in wastewater beneficial for subsequent biological treatment. Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved.

1. Introduction Concentrated swine operations throughout the world presently produce a considerable amount of manure containing abundant carbon (C), nitrogen (N) and phosphorus (P) (Westerman and Bicudo, 2005). Anaerobic treatment of this waste can result in methane generation, destruction of pathogenic and parasitic organisms, low biomass production, better process stability and lower treatment cost (Park et al., 2010; Rao et al., 2011). However, the effluent from these anaerobic bioreactors, designated as anaerobically digested swine wastewater (ADSW), is still rich in nutrients and requires additional storage and subsequent treatment (Harrington and Scholz, 2010). The nutrients from ADSW can be applied to plants to ensure maximal productivity. However, continuing land application as a means of manure disposal could result in excessive nutrient loss from soil to water, which would significantly degrade the water quality (e.g., eutrophication) and eventually pose direct or indirect impairment on human health (Westerman and Bicudo, 2005).

⇑ Corresponding author at: Institute of Environmental Science, Center of Water Ecosystem and Watershed Management, ZheJiang University, YuangHangTang Avenue 688, HangZhou, ZheJiang Province 310058, China. Tel.: +86 571 8697 1854; fax: +86 571 8697 1719. E-mail addresses: [email protected] (K. Wendell), [email protected] (J. Zhu), [email protected], [email protected] (Z. Zhang).

Therefore, effective and economical removal and/or recovery of nutrients from the digested effluents is necessary for pollution control. A number of technical alternatives of nutrients management on ADSW have been reported in the past 10 years, including calcium phosphate precipitation and struvite recovery (Vanotti et al., 2007; Uludag-Demirer and Othman, 2009), air stripping (Gustin and Marinsek-Logar, 2010), integrated constructed wetland systems (Harrington and Scholz, 2010) and natural mine sorption of zeolite and/or clinoptilolite (Guo et al., 2008; Wang et al., 2010, 2011). Zeolites, aluminosilicate molecular sieves with tri-dimensional networks of well-defined micropores, high surface areas and ionic exchange capacity (Ahmaruzzaman, 2010; Li et al., 2011), are a promising material that can be used for nutrient recovery (Larsen, 2007). Natural or synthesized zeolites have wide application to ion exchange and separation technology but are typically manufactured with micron-sized crystals or crystal aggregates. Recent efforts related to the synthesis of nanocrystalline zeolites, namely zeolites with crystal sizes less than 100 nm, such as zeolite L and/or ZSM-5 (Ivanova et al., 2007), faujasite (Gross-Lorgouilloux et al., 2010) and zeolite beta (Modhera et al., 2009), have resulted in increased surface area and decreased diffusion path lengths, enhancing ion exchange properties. These nanocrystalline zeolites exhibit high potential for environmental catalysis, environmental remediation, decontamination and drug delivery (Larsen, 2007; Modhera et al., 2009; Ahmaruzzaman, 2010).

0960-8524/$ - see front matter Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2012.01.096

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X. Chen et al. / Bioresource Technology 110 (2012) 79–85

Synthesis of nanocrystalline zeolites (coating on the particle surface) from coal fly ash (FA), a by-product of coal combustion in power plants, offers a novel, cost-effective way for manufacturing zeolites without exploiting non-renewable mineral deposits. Based on the features of mineralogical composition and microstructure of coal FA (Querol et al., 2002; Ahmaruzzaman, 2010; Ugurlu and Karaoglu, 2011), two synthesis technologies namely alkaline hydrothermal (Querol et al., 2002; Chen et al., 2006; Gross-Lorgouilloux et al., 2010; Ahmaruzzaman, 2010) and fusion method (Zhang et al., 2011), have been commonly employed to synthesize zeolites from fly ash (ZFA). Previous investigations verified that ZFA possess high-efficient adsorption capacity of simultaneous removal of ammonium and phosphate for aquatic purification and wastewater treatment (Murayama et al., 2003; Chen et al., 2006; Larsen, 2007; Zhang et al., 2011). It is hypothesized that further application of these ZFAs for treatment of ADSW may allow simultaneous removal and recovery of nutrients from ADSW, allowing for the secondary utilization of nutrient resources. This approach will potentially reduce nutrient loads, making the ADSW more amenable to further biological treatment. In this study, the alkaline hydrothermal technique was used to synthesize nanocrystalline zeolites by using coal fly ash in the laboratory. The feasibility of using these laboratory-synthesized ZFA for treating ADSW was then tested by evaluating the nutrient status of ADSW and the corresponding physico-chemical characteristics prior to and after incubations with ZFA. 2. Methods 2.1. Samplings of fly ash and wastewater Coal fly ash was obtained from four thermoelectric power plants located in HangZhou, NingBo, ChangXing and KanFong, ZheJiang province, China, indicating the average particle size diameter of 4.4 lm (Fig. 1). The FA sampling in ChangXing thermoelectric power plant was selected for further tests in this study, because the scale of daily generation capacity in the plant (1.2 million KWh) is the most common operation scale among Chinese counties, and the circulated fluidized bed combustion technique in this plant is also the most popular combustion technique in China. ADSW was obtained from three methane-generating digestion tanks operated in TongXiang (ADSW-1), XiaoShan (ADSW-2) and YuHang (ADSW-3) in Northern ZheJiang Province (China). 2.2. Synthesis of zeolite ZFA was synthesized via a modified alkaline hydrothermal treatment described elsewhere (Querol et al., 2002; Murayama

FA-ChangXing

4.0

FA-HangZhou

3.5

Volume( %)

3.0 2.5 2.0

FA-KanFong

FA-NingBo

1.5 1.0 0.5 0.0

0.0625 0.125 0.25

0.5

1

2

4

8

16

32

Fig. 1. Particle size distribution of raw fly ash (FA).

64

128

et al., 2003). In this study, 30 g of FA and 180 ml of 2.0 mol/L NaOH solution were added into a 500 ml nickel autoclave at the solid– liquid ratio of 1 g/6 ml. This mixture was agitated at 300 rpm for 48 h at the constant temperature of 95 °C. The mixture was then rinsed with deionized water over three times until no NaOH was detected, further washed with 0.5 mol/L CaCl2 solution and again rinsed with distilled water until free from chlorine ion (checked by 1 mol/L AgNO3 solution). The ZFA product was dried in an oven at 90 °C for 24 h. 2.3. Batch immobilization studies Batch studies were used to study the effect of sorption time and dosage of FA/ZFA on ammonium and phosphate removal efficiency. To determine the optimum sorption time, 40 ml of ADSW and 0.4 g of ZFA particle were added into centrifuge tubes, i.e., at the ZFA dosage of 1 g/100 ml and these slurries were shaken for 1–48 h at a fixed temperature (20 °C) with continuous stirring at 150 rpm. At specific sorption times, the suspensions were centrifuged at 6000 rpm for 5 min and the P and N remaining in the supernatants were directly determined by the molybdenum-blue ascorbic acid method and by the Nessler method (APHA, 1998), respectively. To determine the effect of ZFA dosage on the P or N removal efficiency, ZFA and ADSW were combined over the range of 0.25–6.0 mg ZFA/100 ml ADSW at the fixed working conditions (20 °C; 150 rpm). The amounts of ammonium and phosphate immobilized by ZFA from the solution were then calculated from the difference between initial and final concentrations. All experiments were performed in triplicate. The phosphorous sorption index (PSI) of solids (FA or ZFA) was also determined as follows: 1 g of FA/ZFA was added to 50 ml of 1000 mg/L solution prepared by dissolving KH2PO4 in 0.01 M KCl, and then equilibrated for 24 h. The difference in the concentrations of dissolved P in solution before and after equilibrium was considered as adsorbed by the solids. For the sorption studies, 1 g of FA or ZFA was equilibrated with 100 ml of N or P series solution: P stock solutions ranging in concentration from 0 to 600 mg/L were prepared using KH2PO while N stock solutions ranging 0–1000 mg/L were prepared from NH4Cl. Both N and P sorption tests were incubated at 20 °C for 24 h and all tests were performed in triplicate. 2.4. Analytical measurements The chemical composition of the FA and ZFA samples was determined by ICP-AES analysis, whereas the size distribution was measured by laser granulometry (LS-230). Specific surface area (SSA) of the samples was also analyzed by a surface area analyzer (ASIC-2) using Brunauer–Emmet–Teller (BET) method; cation exchange capacity (CEC) of the samples was measured using neutral NH4OAc and 10% NaCl as saturating and replacing solutions, respectively (Chen et al., 2006; Larsen, 2007). The raw ADSW samples were analyzed for total solids (TS), total volatile solids (TVS), total Kjeldahl nitrogen (TKN), total phosphorus (TP) and chemical oxygen demand (COD). Centrifuged supernatant (6000 rpm for 5 min at 20 °C) were tested for pH, dissolved reactive phosphorus (DRP), ammonium nitrogen (NH4–N) and nitrate (NO3–N) using standard methods recommended by the American Public Health Association (APHA, 1998). The physicochemical composition and properties of the ADSW wastewater is shown in Table 1. Samples that were not immediately analyzed after collection were stored in a freezer at 20 °C and thawed prior to laboratory analysis. Langmuir equation and Freundlich equation isotherm models (tested at 20 °C) were used to determine the P/N sorption maximum and the associated retention parameters for FA and ZFA (details shown in the Supplemental information).

X. Chen et al. / Bioresource Technology 110 (2012) 79–85 Table 1 Properties of anaerobically digested swine wastewater (ADSW) from three sampling sites. Sampling sites

pH TS (%) TVS (%) TKN (mg/L) NH4+–N (mg/L) NO3 –N (mg/L) TP (mg/L) DRP (mg/L) COD (mg/L) COD:TKN:TP

ADSW-1

ADSW-2

ADSW-3

8.21 0.584 0.359 739 652 0.0 37.1 31.3 1336 43:24:1

8.47 1.05 0.645 3977 3711 0.0 272 105 6880 25:14:1

7.51 0.753 0.550 1406 1294 0.0 130 58.4 3820 29:10:1

The amount of N/P adsorbed to the FA/ZFA was calculated by the N/P concentration difference in the liquid before and after experiments. The adsorbed N/P per ZFA (in unit of weight) was expressed as the amount of N/P adsorbed from liquid to ZFA divided by the mass of ZFA dosage used in each experiment. Microsoft Excel (2007) was used for curve fitting and for the associated regression analysis under the statistical difference at the p < 0.05 level. 3. Results and discussion 3.1. Characterization of fly ash and its synthesized zeolite – chemical, mineralogical composition and microstructure Table 2 shows the changes of chemical composition of FA prior to and ZFA after hydrothermal treatment. The alkaline hydrothermal technique mainly contributed to the dissolution of Al–Si bearing phases of FA and the subsequent precipitation of the zeolitic materials, which caused reduction in percentage of SiO2 and the ratio of Si/Al. The change of chemical composition of the tested FA caused differentiation in the distribution of the electric charge between the Al–O and Si–O bonds that led to the formation of more complex products, such as, zeolites (Querol et al., 2002; Larsen, 2007; Gross-Lorgouilloux et al., 2010). The X-ray diffraction (XRD) patterns of FA (Supplemental Fig. 1S) indicate that the main mineralogical components were quartz and mullite, suggesting the existence of amorphous alumina and silica with higher activity (Lin et al., 2003). In the XRD pattern of ZFA, a large amount of NaP1 (Na6Al6Si10O32
12H2O) was found. Mullite was present at low abundance relative to NaP1 in ZFA. Field emission scanning electron microscope (SEM) observations of FA particle showed the presence of micro-particles in the shape of smooth balls (Supplemental Fig. 2S), as well as hollow cenospheres, irregularly shaped unburned carbon particles, mineral aggregates and agglomerated particles. After alkaline

Table 2 Chemical composition, surface area and cation exchange capacity of raw fly ash (FA) and zeolite synthesized from fly ash (ZFA). Constituent

FA

ZFA

SiO2 (%) Al2O3 (%) Na2O (%) K2O (%) CaO (%) MgO (%) Fe2O3 (%) n(Si)/n(Al) Specific surface area (m2/g) CEC (cmol/kg) P sorption index (PSI) (g/kg)

36.5 20.5 3.50 5.61 4.41 0.42 12.5 1.52 1.40 2.45 4.06

33.0 21.5 2.56 4.63 10.0 0.85 14.9 1.31 56.9 256 20.8

81

hydrothermal activation, the spheres were deformed and the smooth surfaces changed into various crystal shapes (Supplemental Fig. 2S), which were basically identical to the zeolites synthesized from FA by other similar studies (Querol et al., 1997; Moreno et al., 2001) and by a fusion method (Zhang et al., 2011). High resolution SEM images verified that after treatment, the surface of the parent FA was extensively coated with nanometer-scale crystals (diameter less than 100 nm; Supplemental Fig. 2S). This change of microscopic structure is anticipated to enhance SSA, CEC and PSI of the ZFA versus the untreated FA. Additionally, the surface properties of nanocrystalline zeolite structures were tailored through functionalization of surface silanol groups, by depositing alkaline-induced intermediate gel-like glassy particles (Querol et al., 2002; Larsen, 2007; Gross-Lorgouilloux et al., 2010). From these surface modifications, it can be inferred that the adhesion force between newly-synthesized zeolite and the parent FA during the zeolitation period could be increased by alkine hydrothermal process which might improve the thermal stability of ZFA. 3.2. Adsorption capability of fly ash and its synthesized zeolite The CEC and SSA of the ZFA was more than 104 and 40 times the values obtained for the raw fly ash (Table 2). In addition, the PSI of ZFA also increased from 4.06 to 20.8 g/kg which was five times greater than the PSI for raw FA. These data confirm that the synthesis method utilized in this study could successfully produce ZFA with high CEC, SSA and PSI capacity that exceed the properties of ZFA produced from the fusion method (Zhang et al., 2011). Compared with the raw fly ash, these particles coated with nanometer-scale zeolite are suitable candidates for nutrient sequestration from anaerobically digested wastewater. The Langmuir and Freundlich parameters for the sorption of N and P in the ADSW to FA and ZFA (Table 3) indicate that both Langmuir and Freundlich isotherm models sufficiently describe the sorption process for both FA and ZFA (R2 > 0.893). Parameters from the Langmuir isotherm regression indicate that ZFA has enhanced capacity for N (15 times higher removal than raw FA) and P (six times higher removal than raw FA) removal (Table 3). That both the Langmuir and Freundlich isotherms described the data indicates that homogeneous sorption mechanisms as well as heterogeneous sorption were responsible for N and P removal. Pengthamkeerati et al. (2008) also observed that kinetic equation of P adsorption in the alkinine-chemical modified fly ash both fit well to the Langmuir and Freundlich isotherm. The equilibrium isotherm data for ammonium by ZFA were fitted to the Langmuir, Freundlich, Koble-Corrigan, Tempkin and Dubinin-Radushkevich models (Zhang et al., 2011). Although processed by alkine hydrothermal reaction, the ZFA-coated product is still a complex filled with various minerals, leading to rich diversity on N and P adsorption mechanisms. 3.3. Feasibility of nutrients removal by synthesized zeolite for anaerobically digested swine wastewater treatment 3.3.1. The effect of reaction time Fig. 2 shows removal efficiencies of N and P in ADSW over time at the ZFA dosage of 1 g/100 ml. ADSW-1, both N and P levels decreased rapidly during the first 5 h due to the sorption. Between the 5th and 12th hour, corresponding N and P removal efficiencies increased to 51% and 90%. Within 24 h, the concentration of NH4–N decreased from 652 to 336 mg/L while the concentration of P decreased from 31 to 2.85 mg/L (91% removal). Thereby, it was inferred that more than one-step may be involved in the sharing/ exchange of electrons between sorbent and sorbate. The increasing trend on nutrient sequestration over time (mainly within the first

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X. Chen et al. / Bioresource Technology 110 (2012) 79–85

Table 3 Parameters of adsorption isotherm of raw fly ash (FA) and zeolite synthesized from fly ashes (ZFA). Langmuir model

Freundlich model Smax(g/kg)

KL(L/g)

Kf(L/g)

n

R2

Equation

0.996 0.999

2.23 34.5

55.4 0.656

0.011 0.075

0.848 0.827

0.998 0.967

y = 0.848x 1.95 y = 0.827x 1.13

0.943 0.962

4.1 26.8

0.176 4.15

2.19 0.004

0.106 1.21

0.890 0.929

y = 0.106x + 0.34 y = 1.206x 2.39

Equation

R

FA ZFA

NH4–N y = 120.71x + 0.4583 y = 22.625x + 0.0290

FA ZFA

PO4–P y = 0.7299x + 0.2412 y = 111.33x + 0.0373

2

Note. ‘‘y’’ and ‘‘x’’ represent C/S and C for Langmuir model, and log S and log C for Freundlich model, respectively.

6

70

350

60

300

50

250

40

200

Phosphorus level (mg P/L)

400

N removal

Nitrogen removal (%)

Nitrogen level (mg N/L)

80

N level

5

P removal

95 4 3

90

2 85 1 0

30 0

100 P level

5

80 0

10 15 20 25 30 35 40 45 50

Phosphorus removal (%)

ADSW-1

450

5

10 15 20

Reaction time (h)

25 30 35 40 45 50

Reaction time (h)

ADSW-2 34 30

3300

26 2900 22 2500

18

2100

14 0

3

6

9

12

15

18

21

24

70

80

Phosphorus level (mg P/L)

N removal

Nitrogen removal (%)

Nitrogen level (mg N/L)

N level

P level

P removal

70

60

60 50 50 40

40 30

27

Phosphorus removal (%)

3700

30 0

3

6

Reaction time (h)

9

12

15

18

21

24

27

Reaction time (h)

ADSW-3 40 35

1200

30 1100 25 1000 20 900

15

800

10 0

3

6

9

12

15

18

21

24

27

Reaction time (h)

Phosphorus level (mg P/L)

30

N removal

Nitrogen removal (%)

Nitrogen level (mg N/L)

N level

100 P level

25

P removal

90

20 80 15 70 10 60

5 0

Phosphorus removal (%)

1300

50 0

3

6

9

12

15

18

21

24

27

Reaction time (h)

Fig. 2. Effect of reaction time on removal efficiency of ammonium and phosphate from anaerobically digested swine wastewater (ADSW) by zeolite synthesized from fly ashes (ZFA) (1 g/100 ml).

12 h) is mainly related to the combined effect of dissociated Fe2O3 and exchangeable ion on the zeolite surface (Murayama et al., 2003; Chen et al., 2006), as well as the increased specific surface area (Table 2). Prior kinetic studies showed that the rate-limiting step of N sorption by ZFA follows the pseudo-second-order (the rate-limiting step related to chemical sorption or chemisorption involving valence forces through sharing or exchange of electrons between sorbent and sorbate) model and/or intra-particle diffusion (external diffusion and the diffusion of inter-particle) model

(Ugurlu and Karaoglu, 2011; Zhang et al., 2011). Integrated with points discussed in Section 3.2, the co-existence of homogeneous sorption and heterogeneous sorption mechanisms on N and P isotherm models in this tested ZFA may result in non-linear patterns for nutrient sequestration responding to reaction time in ADSW. At the same time, the terminal group such as „SiAOH, „SiAONa, „SiAOA, („SiAO)3AlAOA in the zeolites (Querol et al., 1997) were gradually saturated which may impair the subsequent P adsorption. The bivalent cation (e.g., Ca2+ and Mg2+) ions in ZFA

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X. Chen et al. / Bioresource Technology 110 (2012) 79–85

competed with NH4+ in liquid and reduced the amount of NH4+ adsorption (Huang et al., 2010). These factors restrained the successive ammoniums exchange between ADSW and ZFA, and even caused somewhat N desorption from ZFA to liquid during the period of 25–50 h for ADSW-1 (Fig. 2). Trends from ADSW-2 and ADSW-3 incubations (higher N and P concentration) were similar to results from ADSW-1 experiments; however, the high concentrations of other cations in mixtures with elevated nutrient composition (ADSW-2 and ADSW-3), considerably reduced the nutrient absorption efficiencies due to ion competition (Querol et al., 2002). In the case of ADSW-2 (the highest levels of nutrient among the three samples), removal efficiencies reached 19% and 50% for N and P, respectively, within 24 h of incubation. Therefore, it was inferred that the kinetic mechanism of N and P sorption by ZFA from ADSW could be adequately described using a pseudo-second-order sorption model. In addition, equilibrium appears to be reached within 12 h, although this value needs to be verified in future pilot-scale studies to ensure that efficient removal of N and P can be achieved on a site specific basis. 3.3.2. The effect of ZFA dose The results obtained from batch experiments evaluating N and P uptake by ZFA 24 h are shown in Fig. 3. The removal rates of N

in ADSW-1 increased from 41% to 95% (N reduced from 382 to 31.8 mg/L) with the increase of ZFA doses from 0.25 to 8.0 g/ 100 ml. The P removal rates also increased from 82% to 99% (P reduction from 5.41 to 0.440 mg/L) in experiments with ZFA doses ranging from 0.25 to 8.0 g/100 ml. Similarly, the removals rates of N and P in ADSW-2 and ADSW-3 were ranged from 17% to 65% and 32% to 89% for ADSW-2 and 24% to 88% and 75% to 98% for ADSW-3 (Fig. 3), indicating that the enhanced nutrient removal was obtained at higher ZFA doses. Non-linear regression analysis confirmed that nutrient removal rate was enhanced as the ZFA dose increased (R2 > 0.948) for all experiments except for N in the case of ADSW-2 (Fig. 3). However, the high nutrient removal rate slowed when the ZFA dose increased above 4 g/100 ml (R2 > 0.737) (Table 4). This finding indicates that although the absolute removal rate increased as the ZFA dose increased, the optimum ZFA dose needs to be identified through site specific pilot-scale investigations that examine nutrient removal efficiency and economic cost. Interestingly, N removal exceeded the maximum adsorption capacity (Smax) (Table 3) for tests with low ZFA dosage. This high degree of N removal could result from volatilization, an artifact of the experimental setup. In these experiments, continuous stirring (24 h stirring at 150 rpm) could act to ‘break’ the liquid biocar-

ADSW-1

90

300

75

200

60

100

45

0

30

105 P level

5

P removal

100

4

95

3 90

2

85

1 0

2

4

6

8

2

4

10

ADSW-2

70

100

3500

50

3000

40 2500 30 2000

20

1500

10

1000

Phosphorus level (mg P/L)

P level

60

2

4

6

8

80

60 60 40 40

20 0

20 0

10

2

ADSW-3

100 80

800 60 600 40 400 20

200 0

0 2

4

8

18

6

8

ZFA doses (g/100mL) Added zeolite (g/100mL)

10

Phosphorus level (mg P/L)

1000

0

6

10

105

N removal

Nitrogen removal (%)

Nitrogen level (mg N/L)

N level

4

ZFAAdded doses zeolite(g/mL) (g/100mL)

Added zeolite(g/100mL) ZFA doses (g/100mL)

1200

P removal

80

0 0

100

N removal

Nitrogen removal (%)

Nitrogen level (mg N/L)

N level

80 10

8

ZFA addition (g/100 mL) ZFA doses (g/100mL)

ZFA (g/100 mL) ZFAaddition doses (g/100mL)

4000

6

15 95 12 P level

P removal

9

85

6 75 3 0 0

2

4

6

8

Phosphorus removal (%)

0

0

Nitrogen removal (%)

400

6

105

Phosphorus removal (%)

N removal

Phosphorus level (mg P/L)

N level

Nitrogen removal (%)

Nitrogen level (mg N/L)

500

65 10

Added ZFA doseszeolite(g/mL) (g/100mL)

Fig. 3. Removal efficiencies of ammonium and phosphate responding to the doses of zeolite synthesized from fly ashes (ZFA), contact time 24 h.

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Table 4 The effect of zeolite synthesized from fly ashes (ZFA) dosage on phosphate and ammonium immobilization, contact time 24 h. Wastewater

ZFA dosage (x: g/100 ml)

Adsorbed N per ZFA (y: g/kg)

Adsorbed P per ZFA (y: g/kg)

Ratio of N:P in the remaining liquid

ADSW-1

0.25 0.50 1.0 2.0 4.0 8.0

98 ± 12 63.5 ± 3.9 35.6 ± 4.6 23.5 ± 5.9 15.1 ± 3.1 7.67 ± 0.45

10.1 ± 1.4 5.61 ± 0.76 2.78 ± 0.17 1.47 ± 0.07 0.78 ± 0.03 0.41 ± 0.11

70.9 98.1 89.0 138 181 169

y=

y=

Regression equation ADSW-2

0.25 0.50 1.0 2.0 4.0 8.0

Regression equation

25.1ln(x) + 49.3

R2 = 0.8474 13.2 ± 0.78 7.76 ± 0.32 5.12 ± 0.33 2.88 ± 0.07 2.02 ± 0.11 1.21 ± 0.04

y=

y=

40.5ln(x) + 88.1

2

ADSW-3

0.25 0.50 1.0 2.0 4.0 8.0

Regression equation

2.70ln(x) + 4.50

R2 = 0.8707 193 ± 10.7 80.7 ± 10.1 65.1 ± 3.9 43.4 ± 3.7 33.0 ± 2.1 29.0 ± 1.2

3.27ln(x) + 6.50

2

R = 0.7320 96.9 ± 10.8 54.8 ± 8.3 40.7 ± 0.76 30.2 ± 1.3 21.1 ± 2.4 14.6 ± 0.8

R = 0.8848 17.9 ± 1.2 9.89 ± 0.32 4.70 ± 0.07 2.56 ± 0.10 1.48 ± 0.07 0.75 ± 0.01

y = 21.5ln(x) + 50.5 R2 = 0.8687

y = 4.667ln(x) + 7.83 R2 = 0.8386

bonate equilibrium (HCO3 + H+ M H2CO3 M H2O + CO2), shifting toward CO2 evolution, which consumes a proton and gives rise an increase in pH (Gernaey et al., 2002). This increase in pH would also enhance struvite crystallization (Song et al., 2011) due to the increase of MgO composition in ZFA (Table 2), resulting in increased sequestration of N as well as P. Alternately, under the non-sterile condition of these tests, the 24-h stirring with no airproof measure easily might accelerate oxygen dissolution into the liquid, which could facilitate nitrification (Kim et al., 2008; Qureshi et al., 2008). Regardless of these findings at low ZFA doses, the primary mechanism of nutrient removal was ZFA sorption. 3.4. Implication on wastewater treatment Nitrogen removal from ADSW can be accomplished using conventional nitrification/denitrification or the energy-saving anaerobic ammonium oxidation (Anammox) processes (Wang et al., 2010; Yamamoto et al., 2011). However, the phosphorous content often exceeds that needed for biomass growth. Consequently, P removal is dependent on the use of enhanced P removal processes (EBPR) (Grady et al., 1999; Zhang et al., 2006) or chemical precipitation (Uludag-Demirer and Othman, 2009; Song et al., 2011). EBPR operation requires sophisticated process control (Zhang et al., 2006; Qureshi et al., 2008; Kim et al., 2008), which is a technical challenge for animal farmers. Lessening P load and therefore increasing the N:P ratio would help to alleviate this challenge by allowing farmers to utilize conventional processes without requiring EBPR. The results from current experiments show that treatment with ZFA concomitantly decreased the absolute nutrient concentrations but also relatively increased the N:P ratio in ADSW compared with the raw wastewater (Table 1). This type of ZFA can help to optimize the N:P of wastewater for subsequent bioprocesses like microalgal reactors. Since it is also expected that synthesized ZFA with no surfactant modification would not preferentially sorb organic carbon (Kamble et al., 2008; Li et al., 2011), ZFA application could also act to further optimize C:N:P ratios. Therefore, the significance of ZFA application lies in not only reduction in nutrient levels of ADSW, but also potential optimization of C:N:P ratio in wastewater for the following bioreactor (e.g.,

45.8 50.0 57.6 60.7 92.3 108

70.8 104 76.9 106 441 158

anoxic/oxic activated sludge process), contributing to the regional pollution reduction and water quality improvement. In addition, the cost of material consumption (chemicals, water and electricity) for ZFA synthesis was estimated to be 0.17 dollars/kg product in this bench-scale study, which is nearly 55–75% of the price of commercial zeolite in China. 4. Conclusion The laboratory-synthesized ZFA resulted in the production of nanometer-scale zeolite crystalline structures. The produced ZFA possessed improved nutrient removal properties with SSA, CEC and PSI of ZFA increased over 40, 104 and five times, while the maximum sorption capacities for N and P increased over 15 and six times as compared with raw FA. N and P sequestration sorption by ZFAs adequately described by both the Langmuir and Freundlich models. Removal efficiencies of N and P ranged between 51% and 98% for all experiments. Future work will be investigated the pilot-scale feasibility of using ZFA for treatment ADSW. Acknowledgements The authors thank Zhejiang and National Natural Science Foundation of China (Y506215/40701162), public sector projects in the State Department of Environmental Protection China (2010467014) and Zhejiang Provincial and Hangzhou Municipal Science & Technology Program (2009C33060/20091633F06). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.biortech.2012.01.096. References Ahmaruzzaman, M., 2010. A review on the utilization of fly ash. Prog. Energy Combust. Sci. 36, 327–363. APHA, 1998. Standard methods for the examination of water and wastewater. In: Lenore, S.C., Greenberg. A.E., Eaton, A.D. (Eds.), American Public Health Association, 20th ed. 1015 Fifteenth Street, NW, Washington, DC, 20005.

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