- Email: [email protected]
Efficient removal of nitrate using electrochemical-ion exchange method and pretreatment of straw with by-products for biological fermentation
Desalination 278 (2011) 275–280
Contents lists available at ScienceDirect
Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Efficient removal of nitrate using electrochemical-ion exchange method and pretreatment of straw with by-products for biological fermentation Miao Li a,⁎, Chuanping Feng b, Rui Zhao c, d, Zhenya Zhang c, Xiang Liu a, Qiang Xue c, Weifang Ma a, Norio Sugiura c a
School of Environment, Tsinghua University, Beijng 100084, China School of Water Resources and Environment, China University of Geosciences (Beijing), Beijing 100083, China Doctoral Program in Life and Environmental Sciences, University of Tsukuba,Tsukuba 3058572, Japan d National Institute of Crop Science, National Agriculture and Food Research Organization, Tsukuba, 3058518, Japan b c
a r t i c l e
i n f o
Article history: Received 4 November 2010 Received in revised form 15 May 2011 Accepted 16 May 2011 Available online 1 June 2011 Keywords: Nitrate Rice straw Electrochemical reduction Zeolite adsorption Pretreatment
a b s t r a c t An environmentally benign cycle system was tried to be established in this work, in which simultaneous electrochemical reduction of nitrate and adsorption of ammonia by-product with zeolite, and the pretreatment of straw using the ammonia-loaded zeolite were studied. The employment of Ti/IrO2-Pt anode and Cu/Zn alloy cathode for electrolysis, and zeolite for adsorption were efficient for nitrate removal. After treatment, the concentration of nitrate-N in treated water decreased from 100.0 to 3.6 mg L − 1 over 300 min and the ammonia by-product was completely adsorbed from treated water by zeolite, meeting the allowed limit for drinking water. Pretreatment of rice straw with the spent zeolite containing ammonia (ZCA) showed good performance in delignification and increasing the hydrolysis ratio, giving an attractive alternative for both disposal of the used zeolite and pretreatment of straw. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Nitrate contamination of water bodies is a widespread environmental problem [1–3]. Nitrate-contaminated water above the permissible drinking water limit is toxic to human health, specifically to children [4]. A maximum limit of 50 mg L − 1 NO3− (15 mg L − 1 NO3− for infants), 0.5 mg L− 1 NO2− and 0.5 mg L − 1 NH3 in drinking water was permitted [5–7]. Possible technologies for the treatment of nitrate include Extractive methods, heterogeneous catalysis, and biological treatment [8–12]. However, those methods all have their disadvantages such as difficult to maintain, pH control, addition of carbon source, or result in second pollution. Recently, a lot of researchers have focused on the electrochemical reduction of nitrate [13–16] due to its ease of operation, high treatment efficiency, no sludge production, small area occupied by the plant and relatively low investment costs. As the large number of nitrogen compounds, electrochemical reduction of nitrate leads to a relatively broad spectrum of products [16–18]. At the cathode, the nitrates are mainly reduced to nitrites, ammonia and nitrogen, which is further electrochemically inactive [19–21]. Ma′cova′ and Bouzek [22] found that high electrocatalytic activity was obtained using a Cu/Zn cathode and a Pt anode but the
⁎ Corresponding author at: School of Environment, Tsinghua University, 100084, China. Tel.: + 86 10 82327933; fax: + 86 10 64882672. E-mail address: [email protected] (M. Li). 0011-9164/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2011.05.036
main reduction product is ammonia. Although De Vooys et al. [23] reported that Pd–Cu cathode showed a very high activity along with a good N2 selectivity, large amount of ammonia was produced. Dash et al. [24] studied electrochemical reduction of nitrate by aluminum, graphite, iron and titanium electrodes and found that large amount of ammonia was one of the main reduction products, especially by iron electrodes. In general, applications of the electrochemical process for denitrification are limited due to generation of ammonia and nitrite. On the other hand, since the 1970s, zeolites have been found to be very effective at removing ammonia from water due to their excellent ion exchange capacity [25–30]. Rahmani et al. [30] reported good performance for ammonia removal by natural Chinese clinoptilolite. Wang et al. [29] studied a Chinese clinoptilolite, determined its adsorption capacity, and examined the effect of ammonium ions on its adsorption capacity. Moreover, growing concerns over the environmental impact of fossil fuels and their inevitable depletion have led to intense research on the development of alternative energy sources. Biomass, such as wheat straw, corn stover, rice straw, and cotton stalks, is a renewable resource that stores energy from sunlight in its chemical bonds [31]. Therefore, ethanol fermentation from lignocellulosic biomass, such as wood or wheat straw, was proposed. One promising technology is to convert this abundant and renewable lignocellulosic biomass to ethanol through an enzyme-based process [32]. The conversion of lignocellulosic biomass to ethanol is, however, more challenging than corn due to the complex structure of the plant cell wall. Pretreatment
276
M. Li et al. / Desalination 278 (2011) 275–280
is required to alter the structural and chemical composition of lignocellulosic biomass to facilitate rapid and efficient hydrolysis of carbohydrates to fermentable sugars [33]. Alkali pretreatment refers to the application of alkaline solutions to remove lignin and various uronic acid substitutions on hemicellulose that lower the accessibility of enzyme to the hemicellulose and cellulose [33]. Ammonia is known as an effective reagent for delignification of lignocellulosic biomass during pretreatment process. It has been reported that ammonia pretreatment causes an increase of internal surface by swelling, decrease of polymerization degree and crystallinity, destruction of links between lignin and other polymers, and breakdown of lignin [34–36]. The aim of this work is to establish an environmentally benign cycle system that involves electrochemical reduction of nitrate, adsorption of the ammonia by-product with zeolite and the pretreatment of straw with the spent zeolite, which contains large amounts of ammonia. As the Cu/Zn cathode is known as an efficient promoter for nitrate electro-reduction [22] and the Ti/IrO2–Pt anode is efficient and stable [37], the combination of these electrodes was selected for nitrate reduction. Zeolite was used for adsorption of the by-products, especially ammonia, produced during nitrate reduction. The efficiency of electrochemical nitrate reduction combined with zeolite adsorption, and the effect of pretreatment of rice straw with ammonialoaded zeolite were studied. To our knowledge, the use of this method for removal of ammonia and pretreatment of rice straw has not been previously reported. 2. Materials and methods 2.1. Materials The synthetic zeolite used as the ion exchanger and regenerated in the experiments was obtained from Wako Pure Chemical Industries, Ltd. The synthetic zeolite is the crystalline aluminosilicate hydrate containing alkaline metal or alkaline earth metal. The general formula is MeO·Al2O3 mSiO2·nH2O (Me: metal ion). The particle size of the chosen zeolite was a 4–8 mesh, 2.36–4.75 mm; it was washed with distilled water to remove very fine particles and dried in an electric drying oven at 80 °C. Rice straw was harvested from Ibaragi of Japan. The rice straws were ground on mixer minutely and screened to particle size of 30–60 mesh. All of the materials were stored under dry conditions at approximately 80 °C until to be used for pretreatment. 2.2. Batch electrolysis A continuous electrochemical cell was designed in our laboratory with a net working volume of 160 mL (Fig. 1). A Cole Parmer model peristaltic pump was used to circulate the nitrate solution from a 1000-mL adsorption tank to the electrochemical cell at a speed of 10 mL min − 1. 200.0 g zeolite was put into the adsorption tank for ammonia adsorption. Preliminary experiments showed that under
those conditions (flow rate: 10 mL min − 1, 200.0 g zeolite) the nitrate removal was relative efficient. The electrolysis cell was made of acrylic plates with four outer ports for insertion of electrodes. For the cell, Ti/ IrO2–Pt plate (TohoTech Company, Japan) of 75 cm2 (15 cm× 5 cm) was used as the anode and Cu/Zn plate (Cu: 62.2 wt.%; Zn: 37.8 wt.%) with the same area as the cathode with a distance of 10 mm between the two electrodes. The immersed areas of the anode and cathode in the treated solution were same, which was 40 cm2. A DC potentiostat (Takasago, EX1500H2) with a voltage range of 0–240 V and a current range of 0–25 A was employed as power supply for electrochemical reduction of nitrate. 2.3. Treatment process In the present study, synthetic sodium nitrate solutions with concentration of 100.0 mg L − 1 NO3−–N was prepared for the electrolysis experiments. 0.50 g L− 1Na2SO4 was added into all the experiments in order to enhance the conductivity. Preliminary experiments showed that under those conditions (flow rate: 10 mL min− 1, 200.0 g zeolite) the nitrate removal was relative efficient. 400 mL of synthetic nitrate solution prepared as before was poured into the electrochemical cell, the reaction started with the application of specified current density. At various time intervals, 1.5 mL of the sample was drawn from the electrochemical cell by a chromatography syringe for analysis. The electrolysis was ceased when either 90% of the initial nitrate was converted or 5 h elapsed. The electrolysis was carried out at the temperature uncontrolled. After electrolysis, 50.0, 100.0, and 150.0 g of zeolite in the adsorption tank were then taken out for pretreatment of 1.0 g straw samples, respectively. Pretreatments were performed by mixing the straw with zeolite in an autoclave at 121 °C with 15 psi (103.4 kPa) pressure for residence times of 5 h. After pretreatment, the solids were separated by filtering, washed with distilled water, and then placed in desiccator under dry conditions at 80 °C until its use for the solid compositional analyses and enzymatic digestibility tests. Replicate experiments were carried out in all cases in order to confirm the reproducibility. 2.4. Equilibrium experiments Batch adsorption isotherm for the ammonia removal was investigated. A 2,000 mL beaker was used to determine the batch adsorption of ammonia removal, to which 1000.0 mL of (NH4)2SO4 solution with a 100.0 mg L − 1 concentration of NH4+–N at pH values of 2–12 and 100.0 g of zeolites was added. The solutions were adjusted with hydrochloric acid or sodium hydroxide to pH values ranging from 2 to 12. As preliminary tests had confirmed that 5 h of contact time was sufficient to stabilize the ammonium concentration in the solution— assumed to signify equilibrium—the beaker was then shaken at 20 °C for 5 h. At the designated time the two phases were separated and the final ammonia concentration remaining in the solution was analyzed. 2.5. Enzymatic hydrolysis
+
DC power supply
Anode Cell
Adsorption tank
Cathode Zeolite P Pump Fig. 1. Schematic diagram of the apparatus.
The mixture of Commercial cellulase (80 FPU mL − 1) and Novozym 188 (792 CBU mL − 1) was used for cellulose hydrolysis. Hydrolysis by the Cellulase and Novozym 188 mixture was carried out using 2% (w/ v) of substrate in 50 mM (0.05 mol L − 1) citrate buffer solution (pH 4.8) with the enzyme loading of 15 FPU g − 1 substrate. The prepared enzyme solution and certain amount of treated samples were placed in test tubes. And then test tubes were capped and placed in a shaking water bath to be agitated at 150 rpm at a constant temperature of 50 °C ± 0.5 °C for 24 h. After enzymatically hydrolyzed, the test tubes were placed in a bath containing boiling water to inactivate the enzymes. After boiling for 10 min, the samples were cooled for taking to glucose analysis in order to check the Hydrolysis ratio. The total reducing sugar was determined by 3, 5-dinitrosalichlic acid
M. Li et al. / Desalination 278 (2011) 275–280
Delignification ratio ð%Þ =
Total lignin in substrate−residual lignin in pretreated substrate Total lignin in substrate
Hydrolysis ratio ð%Þ =
Sugar yield × ð162a =180b Þ Pretreated biomass used in hydrolysisc −residual lignin in pretreated substrated
A 100.0 Concentration (mg L-1)
colorimetry (DNS Method). The hydrolysis ratio was calculated as follows:
277
TN Nitrate-N Nitrite-N Ammonia-N
80.0 60.0 40.0 20.0 0.0 0
Note:
30
60
90
120 150 180 210 240 270 300
T (min)
a
2.6. Other analysis All analyses were done according to standard methods [38]. Nitrate was determined by standard colorimetric method using spectrophotometer (DR/4000 U Spectrophotometer, USA) and ion chromatography (Yokogawa IC7000, AS9-HC column). Nitrite was analyzed by ion chromatography (Yokogawa IC7000, AS9-HC column). The determination of ammonia was performed by Ion meter (Ti 9001, Toyo Chemical Laboratories Co., Ltd.). Total nitrogen (T-N) was determined with a T-N, T-P auto analyzer (Auto Analyzer 3, Bran + Luebbe). Morphological features of the rice straw before and after pretreatment were analyzed by a scanning electron microscope (SEM, JSM-6700 F, JEOL, Japan). 3. Results and discussion 3.1. Electrochemical reduction of nitrate in the absence and presence of zeolite adsorption Fig. 2 shows the variation of total nitrogen, nitrate-N, nitrite-N, and ammonia-N during electrolysis in the absence and presence of zeolite adsorption at a current density of 40 mA cm − 2. It can be seen from Fig. 2 that the nitrate reduction had similar behavior in the absence and presence of zeolite adsorption but the by-product levels in the treated solutions differed. In the absence of zeolite adsorption, the concentration of nitrate-N decreased from 100.0 to 4.0 mg L − 1 in 300 min and the ammonia-N increased from 0 to 52.5 mg L − 1. It was confirmed in previous studies [23,39] that ammonia is the main byproduct of electrochemical nitrate reduction. The nitrite-N increased to 2.3 mg L − 1 over the first 120 min, then decreased to 0 at 300 min. This is in agreement with previous research [22], in which it was proven that nitrite is an intermediate product during nitrate reduction and is probably further reduced into gaseous products or ammonia. The major electrochemical reactions involved in the electrochemical reduction of nitrate are [14]: − NO3
−
þ H2 O þ 2e
¼
− NO2
þ 2OH
−
−
−
¼ 1=2N2 þ 6OH
−
−
¼ NH3 þ 7OH
NO3 þ 3H2 O þ 5e NO2 þ 5H2 O þ 6e −
−
2NO2 þ 4H2 O þ 6e
−
¼ N2 þ 8OH
−
ð1Þ −
ð2Þ ð3Þ ð4Þ
As the overpotential of the Ti/IrO2–Pt anode [37] is not high enough to extend the lifetime of the hydroxyl radicals, thus the byproduct ammonia would not be removed by anodic oxidization. On
B Concentration (mg L-1)
Mol. weight of cellulose. b Mol. weight of glucose. c The weight of dried pretreated biomass used in hydrolysis at present study is 0.1 g. d The ratio of residual lignin in pretreated sample per 0.1 g.
100.0 80.0
TN Nitrate-N Nitrite-N Ammonia-N
60.0 40.0 20.0 0.0 0
30
60
90
120 150 180 210 240 270 300
T (min) Fig. 2. Variation of nitrate, nitrite, ammonia and total nitrogen during electrolysis by Cu/ Zn cathode and Ti/IrO2–Pt anode in the absence (A) and presence (B) of 200.0 zeolite adsorption, I = 40 mA cm− 2.
the other hand, at the cathode, the formation of H2 is favorable for ammonia production, this led to the high production of ammonia during electrochemical nitrate reduction, which resulted in relatively low total nitrogen removal rate. On the other hand, electrochemical nitrate reduction was not disturbed by the presence of zeolite adsorption. The concentration of nitrate-N decreased from 100.0 to 3.6 mg L − 1 over 300 min. Moreover, the ammonia-N increased from 0 to 23.1 mg L − 1 over the first 60 min and then decreased to 0 at 240 min. The ammonia produced during electrolysis was completely adsorbed by zeolite, and thus meets the allowed limit for drinking water. The nitrite-N increased to 2.5 mg L − 1 over the first 60 min and then decreased to 0 at the 240 min. According to the amount of ammonia produced in the absence of zeolite adsorption, about 21.0 mg of ammonia was assumed to have been adsorbed by the 200.0 g of zeolite. During adsorption, ammonia ions will be adsorbed by the zeolite through exchange with Na +, which can be expressed as follows: þ
þ
þ
þ
Na ·Zeolite þ NH4 →NH4 ·Zeolite þ Na
ð5Þ
Because only nitrate was present in the solution after electrochemical reduction and adsorption and it was below the allowed limit for drinking water, the combination of electrochemical reduction and zeolite adsorption is an attractive method for nitrate removal. To optimize the electrochemical and adsorption processes, the speed of the pump was set at 5, 10 and 20 mL min − 1, whereby in the absence of zeolite adsorption, the concentration of nitrate-N decreased from 100.0 to 5.2, 4.0, and 21.0 mg L − 1, respectively, over 300 min. It was shown that a flow rate of 10 mL min − 1 was relative efficient for nitrate reduction. At a lower speed (5 mL min − 1), the circulation of the nitrate solution was not sufficient during 5 h of electrolysis, while at the higher speed (20 mL min − 1), the adsorption of nitrate ions onto the cathode surface could be disturbed by the flow
M. Li et al. / Desalination 278 (2011) 275–280
of the nitrate solution. On the other hand, experiments with 100.0, 200.0, and 300.0 g zeolite for ammonia adsorption were carried out at a pump speed of 10 mL min − 1. It was found that after 5 h, with 100.0 g zeolite, there was residual ammonia in the treated solution (5 mg L − 1 ammonia-N), while with 200.0 and 300.0 g of zeolite, no ammonia was detected in the treated solution. Therefore, 200.0 g zeolite was relatively efficient for removal of ammonia. The pH changes during nitrate reduction in the absence and presence of zeolite adsorption are shown in Fig. 3. In the absence of zeolite adsorption, it can be seen that the pH of treated solutions increased from 6.5 to 12.1 over 300 min. The pH values of the treated solutions became alkaline, mainly due to the formation of hydroxyl ions and ammonia during the electrochemical reduction of nitrate. The increases in final pH values suggest that the production of H + did not match the production of OH − ions. More alkaline media suppressed hydrogen evolution during nitrate reduction. Cattarin [40] reported that the electrochemical reduction of nitrate in highly alkaline solution with a Cu electrode gave mainly ammonia at very negative potentials, in agreement with our results. In contrast, in the presence of zeolite adsorption, the pH of the solutions increased to 10.2 over the first 60 min but then decreased to 8.9 at 300 min. That was due to neutralization by zeolite adsorption, making the treated water more suitable for use as a source of drinking water. It is well known that pH has an impact on ammonia removal by zeolite because it can influence both the character of the exchanging ions and the character of the zeolite itself. At higher pH, the ammonium ions are transformed to aqueous ammonia, while at lower pH, the ammonium ions have to compete with hydrogen ions for exchange sites. Because the pH of the treated solution increased to alkaline during electrochemical reduction of nitrate, the effect of pH on ammonia adsorption capacity of zeolite was investigated. Fig. 4 shows the change in ammonia adsorption capacity at pH values of 2 to 12 for a 100.0 mg L − 1 NH4+–N solution. At pH values of 4 to 10, the zeolite has good performance for ammonia adsorption and the optimum ammonia adsorption capacity was achieved at a pH value of 6.5. The concentration of NH4+–N in the beaker decreased from 100.0 mg L − 1 to 13.6 mg L − 1 at pH 6.5, meaning that about 86.4 mg ammonia was absorbed into 100.0 g of zeolite, indicating an adsorption capacity of 0.86 mg NH4+–N g − 1 zeolite. Preliminary experiments showed that nitrate-N concentration had little effect on the adsorption capacity of the zeolite (0.84 mg NH4+–N g− 1 zeolite in the presence of 100 mg L − 1 nitrate-N, pH 6.5 and 0.85 mg NH4+–N g− 1 zeolite in the presence of 50 mg L − 1 nitrite-N, pH 6.5). 3.2. Effect of pretreatment on lignin degradation The content of lignin in lignocellulosic biomass is high, which obstructs the saccharification reaction. Therefore, pretreatment by which lignin is divided from cellulose and hemicellulose is necessary. Delignification of rice straw with 50.0, 100.0 g and 150.0 g zeolite
14.0
Adsorption capacity (mg(N in NH4+)/g(dry zeolite))
278
1 0.8 0.6 0.4 0.2 0
0
2
4
6
8
10
12
14
pH Fig. 4. Effect of pH on ammonia adsorption capacity of zeolite.
gave removal ratios of total lignin of 22.1, 37.1 and 40.1%, respectively. With high concentrations of ammonia water pretreatment, the total amount of lignin solubilized would increase. It also indicates that 100.0 g zeolite containing about 10.5 mg ammonia is relatively efficient for the pretreatment of 1.0 g rice straw. As the enzyme adsorbs the lignin making the enzyme ineffective, lignin is believed to be a major hindrance in enzymatic hydrolysis, so the removal of lignin is useful to expose the highly-ordered crystalline structure of cellulose and facilitates substrate access by hydrolytic enzymes. In the present experiment, the lignin removal ratio is relatively high. Xu et al. [41] found that only 12.32% of lignin was removed by pretreatment of pulverized soybean straw for 24 h at room temperature with 10% ammonia. It was also reported that the yield of solid residue and its lignin content was decreased by ammonia pretreatment of pine wood [42]. It is well known that during alkaline pretreatment, solvation and saponification reactions will take place. This causes swelling of the biomass and makes it more accessible to enzymes and bacteria. ‘Peeling’ of end-groups, alkaline hydrolysis and degradation and decomposition of dissolved polysaccharides can take place at relatively strong alkali concentrations, allowing dissolution. Loss of polysaccharides is mainly caused by peeling and hydrolytic reactions [36]. This peeling has an advantage for later conversion because of the formation of lower molecular weight compounds. The hydrolysis ratio percentage with enzyme hydrolysis due to 100.0 g zeolite containing ammonia (ZCA) pretreatment was 39.8%, while without pretreatment the hydrolysis ratio was only 3.0%. These results suggested that the ZCA pretreatment could help for enhancing enzymatic digestibility. It is supposed that with ZCA pretreatment under conditions of higher temperatures and pressures or concentrations of ammonia can further remove lignin and increase the accessible surface area that will further improve the enzymatic digestibility, which will be studied in the future. Above all, it can be concluded that ZCA pretreatment showed good performance for delignification of rice straw and increased the hydrolysis ratio, which has potential as an attractive alternative for disposal of the used zeolite containing ammonia.
12.0
pH
10.0
3.3. SEM
8.0 Absence of adsorption
6.0
Presence of adsorption
4.0 2.0
0
30
60
90
120 150 180 210 240 270 300
T (min) Fig. 3. pH changes in the absence and presence of zeolite I = 40 mA cm− 2, 200.0 g zeolite.
Fig. 5 shows the morphology of treated and untreated straw obtained using SEM analysis. It is noted that the wet treated biomass felt rougher to manual touch than the untreated biomass. The untreated sample exhibited relatively rigid and highly ordered fibrils. The pretreatment altered the biomass structure substantially. The ammonia pretreatment can cause the increasing of internal surface by swelling, decrease of polymerization degree and crystallinity, destruction of links between lignin and other polymers, and breakdown of lignin. The topography of the surface of alkali treated fiber shows
M. Li et al. / Desalination 278 (2011) 275–280
A
279
Fundamental Research Funds for the Central Universities for the financial support of this work.
References
B
Fig. 5. SEM micrographs of A) untreated straw; B) treated straw.
the formation of scratches within it, although the change is not very obvious, where the structure was destroyed due to the removal of lignin.
4. Conclusions Simultaneous electrochemical reduction of nitrate and adsorption of the ammonia by-product with zeolite, and pretreatment of straw with zeolite containing a large amount of ammonia were achieved in this work. After treatment of nitrate contaminated water, the concentration of nitrate-N was decreased from 100.0 to 3.6 mg L − 1 over 300 min, and the by-product ammonia was completely adsorbed from the treated water by zeolite, meeting the allowed limit for drinking water. The zeolite containing ammonia (ZCA) had good performance in the pretreatment of rice straw for delignification and increased the hydrolysis ratio during enzyme hydrolysis. The delignification of rice straw indicated that the portion of total lignin removed was 37.1% and the hydrolysis ratio was 39.8% after a 100.0 g zeolite pretreatment process. This suggests that it is an attractive alternative for disposal of the spent zeolite and the pretreatment of rice straw. In the future, the effect of various factors on nitrate removal and pretreatment of rice straw should be studied to further improve the method. Acknowledgements The authors thank the Water Pollution Control and Management Project of China (2008ZX07313-001, 2009ZX07318-008) and the
[1] S. Ghafari, M. Hasan, M.K. Aroua, Bio-electrochemical removal of nitrate from water and wastewater—a review, Bioresource. Technol. 99 (2008) 3965–3974. [2] T. Parvanova-Mancheva, V. Beschkov, Microbial denitrification by immobilized bacteria Pseudomonas denitrificans stimulated by constant electric field, Biochem. Eng. J. 44 (2009) 208–213. [3] H. Uemoto, M. Suzuki, M. Morita, A. Watanabe, H. Shinozaki, Simplified denitrification system using ethanol released from non-porous polyethylenefilm bag, Biochem. Eng. J. 45 (2009) 35–40. [4] S.K. Gupta, A.B. Gupta, R.C. Gupta, A.K. Seth, J.K. Bassain, Gupta, Recurrent acute respiratory tract infections in areas with high nitrate concentrations in drinking water, Environ. Health Perspect. 108 (4) (2000) 363–365. [5] EEC Council Recommendations (1987). [6] EEC Council Directive 98/83/EC (1998). [7] World Heath Organization (WHO), Guidelines for Drinking Water Quality, third ed WHO, Geneva, 2004. [8] M. Morit, H. Uemoto, A. Watanabe, Nitrogen-removal bioreactor capable of simultaneous nitrification and denitrification for application to industrial wastewater treatment, Biochem. Eng. J. 41 (2008) 59–66. [9] I. Katsounaros, D. Ipsakis, C. Polatides, G. Kyriacou, Efficient electrochemical reduction of nitrate to nitrogen on tin cathode at very high cathodic potentials, Electrochim. Acta 52 (2006) 1329–1338. [10] G. Zhu, Y. Peng, L. Zhai, Y. Wang, S. Wang, Performance and optimization of biological nitrogen removal process enhanced by anoxic/oxic step feeding, Biochem. Eng. J. 43 (2009) 280–287. [11] W.L. Ma, R. Qi, Y. Zhang, J. Wang, C.Z. Liang, M. Yang, Performance of a successive hydrolysis, denitrification and nitrification system for simultaneous removal of COD and nitrogen from terramycin production wastewater, Biochem. Eng. J. 45 (l) (2009) 30–34. [12] J. Virkutyte, V. Jegatheesan, Electro-Fenton, hydrogenotrophic and Fe2+ ions mediated TOC and nitrate removal from aquaculture system: different experimental strategies, Bioresource Technol. 100 (2009) 2189–2197. [13] K. Bouzek, M. Paidar, A. Sadilkova, H. Bergmann, Electrochemical reduction of nitrate in weakly alkaline solutions, J. Appl. Electrochem. 31 (2001) 1185–1193. [14] X. Lei, T. Maekawa, Electrochemical treatment of anaerobic digestion effluent using a Ti/Pt–IrO2 electrode, Bioresource Technol. 98 (2007) 3521–3525. [15] J. Souza-Garcia, E.A. Ticianelli, V. Climent, J.M. Feliu, Nitrate reduction on Pt single crystals with Pd multilayer, Electrochim. Acta 54 (2009) 2094–2101. [16] C.P. Huang, H.W. Wang, P.C. Chiu, Nitrate reduction by metallic iron, Water Res. 32 (1998) 2257–2264. [17] M.S. El-Deab, Electrochemical reduction of nitrate to ammonia at modified gold electrodes, Electrochim. Acta 49 (2004) 1639–1645. [18] S. Taguchi, J.M. Feliu, Kinetic study of nitrate reduction on Pt(110) electrode in perchloric acid solution, Electrochim. Acta 53 (2008) 3626–3634. [19] H.A. Duarte, K. Jha, J.W. Weidner, Electrochemical reduction of nitrates and nitrites in alkaline media in the presence of hexavalent chromium, J. Appl. Electrochem. 28 (1998) 811–817. [20] D. De, J.D. Englehardt, E.E. Kalu, Electroreduction of nitrate and nitrite ion on a platinum-group-metal catalyst-modified carbon fiber electrode-hronoamperometry and mechanism studies, J. Electrochem. Soc. 147 (2000) 4573–4579. [21] Y. Wang, J. Qu, R. Wu, P. Lei, The electrocatalytic reduction of nitrate in water on Pd/Sn-modified activated carbon fiber electrode, Water Res. 40 (2006) 1224–1232. [22] Z. Ma′cova′, K. Bouzek, Electrocatalytic activity of copper alloys for NO3− reduction in a weakly alkalinesolution Part 1: copper–zinc, J. Appl. Electrochem. 35 (2005) 1203–1211. [23] A.C.A. De Vooys, R.A. Santen, J.A.R. Veen, Electrocatalytic reduction of NO3− on palladium/copper electrodes, J. Mol. Catal. A. Chem. 154 (2000) 203–215. [24] B.P. Dash, S. Chaudhari, Electrochemical denitrification of simulated ground water, Water Res. 39 (2005) 4065–4072. [25] S.E. Jorgensen, O. Libor, K. Lea, K. Graber, K. Barkacs, Ammonia removal by use of clinoptilolite, Water Res. 10 (1976) 213–224. [26] J.R. Klieve, M.J. Semmens, An evaluation of pretreated natural zeolites for ammonia removal, Water Res. 14 (1980) 161–168. [27] J. Hlavay, G. Vigh, V. Olaszi, J. Inczedy, Investigations on natural Hungarian zeolite for ammonia removal, Water Res. 16 (1982) 417–420. [28] N.A. Booker, E.L. Cooney, A.J. Priestley, Ammonia removal from sewage using natural Australian zeolite, Water Sci. Technol. 34 (1996) 17–24. [29] Y. Wang, S. Liu, Z. Xu, T. Han, C. Sun, T. Zhu, Ammonia removal from leachate solution using natural Chinese clinoptilolite, J. Haz. Mat. B136 (2006) 735–740. [30] A.R. Rahmani, A.H. Mahvi, A.R. Mesdaghinia, S. Nasseri, Investigation of ammonia removal from polluted waters by Clinoptilolite zeolite, J. Enviro. Sci. & Tech. 1 (2004) 25–133. [31] P. McKendry, Energy production from biomass (part 1): overview of biomass, Bioresource Technol. 83 (2002) 37–46. [32] D.J. Schell, J. Farmer, M. Newman, J.D. McMillan, Dilutesulfuric acid pretreatment of corn stover in pilot-scale reactor-investigation of yields, kinetics, and enzymatic digestibilities of solids, Appl. Biochem. Biotechnol. 105 (2003) 69–85.
280
M. Li et al. / Desalination 278 (2011) 275–280
[33] V. Chang, M. Holtzapple, Fundamental factors affecting biomass enzymatic reactivity, Appl. Biochem. Biotechnol. 86 (2000) 5–37. [34] Y. Sun, J. Cheng, Hydrolysis of lignocellulosic materials for ethanol production: a review, Bioresource Technol. 83 (2002) 1–11. [35] N. Mosier, Ch. Wyman, B.E. Dale, E.R. lander, Y.Y. Lee, M. Holtzapple, M. Ladisch, Features of promising technologies for pretreatment of lignocellulosic biomass, Bioresource Technol. 96 (2005) 673–686. [36] D. Fengel, G. Wegener, Wood: Chemistry, Ultrastructure, Reactions, De Gruyter, Berlin, 1984. [37] M. Li, C.P. Feng, W.W. Hu, Z.Y. Zhang, N. Sugiura, Electrochemical degradation of phenol using electrodes of Ti/RuO2–Pt and Ti/IrO2–Pt, J. Haz. Mat. 162 (2009) 455–462. [38] APHA, AWWA, WPCF, Standard Methods for the Examination of Water and Wastewater, American Public Health Association, Washington, DC, USA, 1998.
[39] C. Polatides, M. Dortsiou, G. Kyriacou, Electrochemical removal of nitrate ion from aqueous solution by pulsing potential electrolysis, Electrochim. Acta 50 (2005) 5237–5241. [40] S. Cattarin, Electrochemical reduction of nitrogen oxyanions in 1 M sodium hydroxide solutions at silver, copper and cuinse electrodes, J. Appl. Electrochem. 22 (1992) 1077–1081. [41] Z. Xu, Q. Wang, Z. Liang, The study of the ammonia pretreatment effect on the enzyme hydrolysis of soybean straw to produce sugar, J. Chem. Eng. Chi. Uni. 18 (6) (2004) 71–75. [42] M.V. Efanov, R.Yu. Aberin, Peroxide-ammonia delignification of pine wood, Chem. Nat.Com. 40 (2) (2004) 172–175.
Contents lists available at ScienceDirect
Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Efficient removal of nitrate using electrochemical-ion exchange method and pretreatment of straw with by-products for biological fermentation Miao Li a,⁎, Chuanping Feng b, Rui Zhao c, d, Zhenya Zhang c, Xiang Liu a, Qiang Xue c, Weifang Ma a, Norio Sugiura c a
School of Environment, Tsinghua University, Beijng 100084, China School of Water Resources and Environment, China University of Geosciences (Beijing), Beijing 100083, China Doctoral Program in Life and Environmental Sciences, University of Tsukuba,Tsukuba 3058572, Japan d National Institute of Crop Science, National Agriculture and Food Research Organization, Tsukuba, 3058518, Japan b c
a r t i c l e
i n f o
Article history: Received 4 November 2010 Received in revised form 15 May 2011 Accepted 16 May 2011 Available online 1 June 2011 Keywords: Nitrate Rice straw Electrochemical reduction Zeolite adsorption Pretreatment
a b s t r a c t An environmentally benign cycle system was tried to be established in this work, in which simultaneous electrochemical reduction of nitrate and adsorption of ammonia by-product with zeolite, and the pretreatment of straw using the ammonia-loaded zeolite were studied. The employment of Ti/IrO2-Pt anode and Cu/Zn alloy cathode for electrolysis, and zeolite for adsorption were efficient for nitrate removal. After treatment, the concentration of nitrate-N in treated water decreased from 100.0 to 3.6 mg L − 1 over 300 min and the ammonia by-product was completely adsorbed from treated water by zeolite, meeting the allowed limit for drinking water. Pretreatment of rice straw with the spent zeolite containing ammonia (ZCA) showed good performance in delignification and increasing the hydrolysis ratio, giving an attractive alternative for both disposal of the used zeolite and pretreatment of straw. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Nitrate contamination of water bodies is a widespread environmental problem [1–3]. Nitrate-contaminated water above the permissible drinking water limit is toxic to human health, specifically to children [4]. A maximum limit of 50 mg L − 1 NO3− (15 mg L − 1 NO3− for infants), 0.5 mg L− 1 NO2− and 0.5 mg L − 1 NH3 in drinking water was permitted [5–7]. Possible technologies for the treatment of nitrate include Extractive methods, heterogeneous catalysis, and biological treatment [8–12]. However, those methods all have their disadvantages such as difficult to maintain, pH control, addition of carbon source, or result in second pollution. Recently, a lot of researchers have focused on the electrochemical reduction of nitrate [13–16] due to its ease of operation, high treatment efficiency, no sludge production, small area occupied by the plant and relatively low investment costs. As the large number of nitrogen compounds, electrochemical reduction of nitrate leads to a relatively broad spectrum of products [16–18]. At the cathode, the nitrates are mainly reduced to nitrites, ammonia and nitrogen, which is further electrochemically inactive [19–21]. Ma′cova′ and Bouzek [22] found that high electrocatalytic activity was obtained using a Cu/Zn cathode and a Pt anode but the
⁎ Corresponding author at: School of Environment, Tsinghua University, 100084, China. Tel.: + 86 10 82327933; fax: + 86 10 64882672. E-mail address: [email protected] (M. Li). 0011-9164/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2011.05.036
main reduction product is ammonia. Although De Vooys et al. [23] reported that Pd–Cu cathode showed a very high activity along with a good N2 selectivity, large amount of ammonia was produced. Dash et al. [24] studied electrochemical reduction of nitrate by aluminum, graphite, iron and titanium electrodes and found that large amount of ammonia was one of the main reduction products, especially by iron electrodes. In general, applications of the electrochemical process for denitrification are limited due to generation of ammonia and nitrite. On the other hand, since the 1970s, zeolites have been found to be very effective at removing ammonia from water due to their excellent ion exchange capacity [25–30]. Rahmani et al. [30] reported good performance for ammonia removal by natural Chinese clinoptilolite. Wang et al. [29] studied a Chinese clinoptilolite, determined its adsorption capacity, and examined the effect of ammonium ions on its adsorption capacity. Moreover, growing concerns over the environmental impact of fossil fuels and their inevitable depletion have led to intense research on the development of alternative energy sources. Biomass, such as wheat straw, corn stover, rice straw, and cotton stalks, is a renewable resource that stores energy from sunlight in its chemical bonds [31]. Therefore, ethanol fermentation from lignocellulosic biomass, such as wood or wheat straw, was proposed. One promising technology is to convert this abundant and renewable lignocellulosic biomass to ethanol through an enzyme-based process [32]. The conversion of lignocellulosic biomass to ethanol is, however, more challenging than corn due to the complex structure of the plant cell wall. Pretreatment
276
M. Li et al. / Desalination 278 (2011) 275–280
is required to alter the structural and chemical composition of lignocellulosic biomass to facilitate rapid and efficient hydrolysis of carbohydrates to fermentable sugars [33]. Alkali pretreatment refers to the application of alkaline solutions to remove lignin and various uronic acid substitutions on hemicellulose that lower the accessibility of enzyme to the hemicellulose and cellulose [33]. Ammonia is known as an effective reagent for delignification of lignocellulosic biomass during pretreatment process. It has been reported that ammonia pretreatment causes an increase of internal surface by swelling, decrease of polymerization degree and crystallinity, destruction of links between lignin and other polymers, and breakdown of lignin [34–36]. The aim of this work is to establish an environmentally benign cycle system that involves electrochemical reduction of nitrate, adsorption of the ammonia by-product with zeolite and the pretreatment of straw with the spent zeolite, which contains large amounts of ammonia. As the Cu/Zn cathode is known as an efficient promoter for nitrate electro-reduction [22] and the Ti/IrO2–Pt anode is efficient and stable [37], the combination of these electrodes was selected for nitrate reduction. Zeolite was used for adsorption of the by-products, especially ammonia, produced during nitrate reduction. The efficiency of electrochemical nitrate reduction combined with zeolite adsorption, and the effect of pretreatment of rice straw with ammonialoaded zeolite were studied. To our knowledge, the use of this method for removal of ammonia and pretreatment of rice straw has not been previously reported. 2. Materials and methods 2.1. Materials The synthetic zeolite used as the ion exchanger and regenerated in the experiments was obtained from Wako Pure Chemical Industries, Ltd. The synthetic zeolite is the crystalline aluminosilicate hydrate containing alkaline metal or alkaline earth metal. The general formula is MeO·Al2O3 mSiO2·nH2O (Me: metal ion). The particle size of the chosen zeolite was a 4–8 mesh, 2.36–4.75 mm; it was washed with distilled water to remove very fine particles and dried in an electric drying oven at 80 °C. Rice straw was harvested from Ibaragi of Japan. The rice straws were ground on mixer minutely and screened to particle size of 30–60 mesh. All of the materials were stored under dry conditions at approximately 80 °C until to be used for pretreatment. 2.2. Batch electrolysis A continuous electrochemical cell was designed in our laboratory with a net working volume of 160 mL (Fig. 1). A Cole Parmer model peristaltic pump was used to circulate the nitrate solution from a 1000-mL adsorption tank to the electrochemical cell at a speed of 10 mL min − 1. 200.0 g zeolite was put into the adsorption tank for ammonia adsorption. Preliminary experiments showed that under
those conditions (flow rate: 10 mL min − 1, 200.0 g zeolite) the nitrate removal was relative efficient. The electrolysis cell was made of acrylic plates with four outer ports for insertion of electrodes. For the cell, Ti/ IrO2–Pt plate (TohoTech Company, Japan) of 75 cm2 (15 cm× 5 cm) was used as the anode and Cu/Zn plate (Cu: 62.2 wt.%; Zn: 37.8 wt.%) with the same area as the cathode with a distance of 10 mm between the two electrodes. The immersed areas of the anode and cathode in the treated solution were same, which was 40 cm2. A DC potentiostat (Takasago, EX1500H2) with a voltage range of 0–240 V and a current range of 0–25 A was employed as power supply for electrochemical reduction of nitrate. 2.3. Treatment process In the present study, synthetic sodium nitrate solutions with concentration of 100.0 mg L − 1 NO3−–N was prepared for the electrolysis experiments. 0.50 g L− 1Na2SO4 was added into all the experiments in order to enhance the conductivity. Preliminary experiments showed that under those conditions (flow rate: 10 mL min− 1, 200.0 g zeolite) the nitrate removal was relative efficient. 400 mL of synthetic nitrate solution prepared as before was poured into the electrochemical cell, the reaction started with the application of specified current density. At various time intervals, 1.5 mL of the sample was drawn from the electrochemical cell by a chromatography syringe for analysis. The electrolysis was ceased when either 90% of the initial nitrate was converted or 5 h elapsed. The electrolysis was carried out at the temperature uncontrolled. After electrolysis, 50.0, 100.0, and 150.0 g of zeolite in the adsorption tank were then taken out for pretreatment of 1.0 g straw samples, respectively. Pretreatments were performed by mixing the straw with zeolite in an autoclave at 121 °C with 15 psi (103.4 kPa) pressure for residence times of 5 h. After pretreatment, the solids were separated by filtering, washed with distilled water, and then placed in desiccator under dry conditions at 80 °C until its use for the solid compositional analyses and enzymatic digestibility tests. Replicate experiments were carried out in all cases in order to confirm the reproducibility. 2.4. Equilibrium experiments Batch adsorption isotherm for the ammonia removal was investigated. A 2,000 mL beaker was used to determine the batch adsorption of ammonia removal, to which 1000.0 mL of (NH4)2SO4 solution with a 100.0 mg L − 1 concentration of NH4+–N at pH values of 2–12 and 100.0 g of zeolites was added. The solutions were adjusted with hydrochloric acid or sodium hydroxide to pH values ranging from 2 to 12. As preliminary tests had confirmed that 5 h of contact time was sufficient to stabilize the ammonium concentration in the solution— assumed to signify equilibrium—the beaker was then shaken at 20 °C for 5 h. At the designated time the two phases were separated and the final ammonia concentration remaining in the solution was analyzed. 2.5. Enzymatic hydrolysis
+
DC power supply
Anode Cell
Adsorption tank
Cathode Zeolite P Pump Fig. 1. Schematic diagram of the apparatus.
The mixture of Commercial cellulase (80 FPU mL − 1) and Novozym 188 (792 CBU mL − 1) was used for cellulose hydrolysis. Hydrolysis by the Cellulase and Novozym 188 mixture was carried out using 2% (w/ v) of substrate in 50 mM (0.05 mol L − 1) citrate buffer solution (pH 4.8) with the enzyme loading of 15 FPU g − 1 substrate. The prepared enzyme solution and certain amount of treated samples were placed in test tubes. And then test tubes were capped and placed in a shaking water bath to be agitated at 150 rpm at a constant temperature of 50 °C ± 0.5 °C for 24 h. After enzymatically hydrolyzed, the test tubes were placed in a bath containing boiling water to inactivate the enzymes. After boiling for 10 min, the samples were cooled for taking to glucose analysis in order to check the Hydrolysis ratio. The total reducing sugar was determined by 3, 5-dinitrosalichlic acid
M. Li et al. / Desalination 278 (2011) 275–280
Delignification ratio ð%Þ =
Total lignin in substrate−residual lignin in pretreated substrate Total lignin in substrate
Hydrolysis ratio ð%Þ =
Sugar yield × ð162a =180b Þ Pretreated biomass used in hydrolysisc −residual lignin in pretreated substrated
A 100.0 Concentration (mg L-1)
colorimetry (DNS Method). The hydrolysis ratio was calculated as follows:
277
TN Nitrate-N Nitrite-N Ammonia-N
80.0 60.0 40.0 20.0 0.0 0
Note:
30
60
90
120 150 180 210 240 270 300
T (min)
a
2.6. Other analysis All analyses were done according to standard methods [38]. Nitrate was determined by standard colorimetric method using spectrophotometer (DR/4000 U Spectrophotometer, USA) and ion chromatography (Yokogawa IC7000, AS9-HC column). Nitrite was analyzed by ion chromatography (Yokogawa IC7000, AS9-HC column). The determination of ammonia was performed by Ion meter (Ti 9001, Toyo Chemical Laboratories Co., Ltd.). Total nitrogen (T-N) was determined with a T-N, T-P auto analyzer (Auto Analyzer 3, Bran + Luebbe). Morphological features of the rice straw before and after pretreatment were analyzed by a scanning electron microscope (SEM, JSM-6700 F, JEOL, Japan). 3. Results and discussion 3.1. Electrochemical reduction of nitrate in the absence and presence of zeolite adsorption Fig. 2 shows the variation of total nitrogen, nitrate-N, nitrite-N, and ammonia-N during electrolysis in the absence and presence of zeolite adsorption at a current density of 40 mA cm − 2. It can be seen from Fig. 2 that the nitrate reduction had similar behavior in the absence and presence of zeolite adsorption but the by-product levels in the treated solutions differed. In the absence of zeolite adsorption, the concentration of nitrate-N decreased from 100.0 to 4.0 mg L − 1 in 300 min and the ammonia-N increased from 0 to 52.5 mg L − 1. It was confirmed in previous studies [23,39] that ammonia is the main byproduct of electrochemical nitrate reduction. The nitrite-N increased to 2.3 mg L − 1 over the first 120 min, then decreased to 0 at 300 min. This is in agreement with previous research [22], in which it was proven that nitrite is an intermediate product during nitrate reduction and is probably further reduced into gaseous products or ammonia. The major electrochemical reactions involved in the electrochemical reduction of nitrate are [14]: − NO3
−
þ H2 O þ 2e
¼
− NO2
þ 2OH
−
−
−
¼ 1=2N2 þ 6OH
−
−
¼ NH3 þ 7OH
NO3 þ 3H2 O þ 5e NO2 þ 5H2 O þ 6e −
−
2NO2 þ 4H2 O þ 6e
−
¼ N2 þ 8OH
−
ð1Þ −
ð2Þ ð3Þ ð4Þ
As the overpotential of the Ti/IrO2–Pt anode [37] is not high enough to extend the lifetime of the hydroxyl radicals, thus the byproduct ammonia would not be removed by anodic oxidization. On
B Concentration (mg L-1)
Mol. weight of cellulose. b Mol. weight of glucose. c The weight of dried pretreated biomass used in hydrolysis at present study is 0.1 g. d The ratio of residual lignin in pretreated sample per 0.1 g.
100.0 80.0
TN Nitrate-N Nitrite-N Ammonia-N
60.0 40.0 20.0 0.0 0
30
60
90
120 150 180 210 240 270 300
T (min) Fig. 2. Variation of nitrate, nitrite, ammonia and total nitrogen during electrolysis by Cu/ Zn cathode and Ti/IrO2–Pt anode in the absence (A) and presence (B) of 200.0 zeolite adsorption, I = 40 mA cm− 2.
the other hand, at the cathode, the formation of H2 is favorable for ammonia production, this led to the high production of ammonia during electrochemical nitrate reduction, which resulted in relatively low total nitrogen removal rate. On the other hand, electrochemical nitrate reduction was not disturbed by the presence of zeolite adsorption. The concentration of nitrate-N decreased from 100.0 to 3.6 mg L − 1 over 300 min. Moreover, the ammonia-N increased from 0 to 23.1 mg L − 1 over the first 60 min and then decreased to 0 at 240 min. The ammonia produced during electrolysis was completely adsorbed by zeolite, and thus meets the allowed limit for drinking water. The nitrite-N increased to 2.5 mg L − 1 over the first 60 min and then decreased to 0 at the 240 min. According to the amount of ammonia produced in the absence of zeolite adsorption, about 21.0 mg of ammonia was assumed to have been adsorbed by the 200.0 g of zeolite. During adsorption, ammonia ions will be adsorbed by the zeolite through exchange with Na +, which can be expressed as follows: þ
þ
þ
þ
Na ·Zeolite þ NH4 →NH4 ·Zeolite þ Na
ð5Þ
Because only nitrate was present in the solution after electrochemical reduction and adsorption and it was below the allowed limit for drinking water, the combination of electrochemical reduction and zeolite adsorption is an attractive method for nitrate removal. To optimize the electrochemical and adsorption processes, the speed of the pump was set at 5, 10 and 20 mL min − 1, whereby in the absence of zeolite adsorption, the concentration of nitrate-N decreased from 100.0 to 5.2, 4.0, and 21.0 mg L − 1, respectively, over 300 min. It was shown that a flow rate of 10 mL min − 1 was relative efficient for nitrate reduction. At a lower speed (5 mL min − 1), the circulation of the nitrate solution was not sufficient during 5 h of electrolysis, while at the higher speed (20 mL min − 1), the adsorption of nitrate ions onto the cathode surface could be disturbed by the flow
M. Li et al. / Desalination 278 (2011) 275–280
of the nitrate solution. On the other hand, experiments with 100.0, 200.0, and 300.0 g zeolite for ammonia adsorption were carried out at a pump speed of 10 mL min − 1. It was found that after 5 h, with 100.0 g zeolite, there was residual ammonia in the treated solution (5 mg L − 1 ammonia-N), while with 200.0 and 300.0 g of zeolite, no ammonia was detected in the treated solution. Therefore, 200.0 g zeolite was relatively efficient for removal of ammonia. The pH changes during nitrate reduction in the absence and presence of zeolite adsorption are shown in Fig. 3. In the absence of zeolite adsorption, it can be seen that the pH of treated solutions increased from 6.5 to 12.1 over 300 min. The pH values of the treated solutions became alkaline, mainly due to the formation of hydroxyl ions and ammonia during the electrochemical reduction of nitrate. The increases in final pH values suggest that the production of H + did not match the production of OH − ions. More alkaline media suppressed hydrogen evolution during nitrate reduction. Cattarin [40] reported that the electrochemical reduction of nitrate in highly alkaline solution with a Cu electrode gave mainly ammonia at very negative potentials, in agreement with our results. In contrast, in the presence of zeolite adsorption, the pH of the solutions increased to 10.2 over the first 60 min but then decreased to 8.9 at 300 min. That was due to neutralization by zeolite adsorption, making the treated water more suitable for use as a source of drinking water. It is well known that pH has an impact on ammonia removal by zeolite because it can influence both the character of the exchanging ions and the character of the zeolite itself. At higher pH, the ammonium ions are transformed to aqueous ammonia, while at lower pH, the ammonium ions have to compete with hydrogen ions for exchange sites. Because the pH of the treated solution increased to alkaline during electrochemical reduction of nitrate, the effect of pH on ammonia adsorption capacity of zeolite was investigated. Fig. 4 shows the change in ammonia adsorption capacity at pH values of 2 to 12 for a 100.0 mg L − 1 NH4+–N solution. At pH values of 4 to 10, the zeolite has good performance for ammonia adsorption and the optimum ammonia adsorption capacity was achieved at a pH value of 6.5. The concentration of NH4+–N in the beaker decreased from 100.0 mg L − 1 to 13.6 mg L − 1 at pH 6.5, meaning that about 86.4 mg ammonia was absorbed into 100.0 g of zeolite, indicating an adsorption capacity of 0.86 mg NH4+–N g − 1 zeolite. Preliminary experiments showed that nitrate-N concentration had little effect on the adsorption capacity of the zeolite (0.84 mg NH4+–N g− 1 zeolite in the presence of 100 mg L − 1 nitrate-N, pH 6.5 and 0.85 mg NH4+–N g− 1 zeolite in the presence of 50 mg L − 1 nitrite-N, pH 6.5). 3.2. Effect of pretreatment on lignin degradation The content of lignin in lignocellulosic biomass is high, which obstructs the saccharification reaction. Therefore, pretreatment by which lignin is divided from cellulose and hemicellulose is necessary. Delignification of rice straw with 50.0, 100.0 g and 150.0 g zeolite
14.0
Adsorption capacity (mg(N in NH4+)/g(dry zeolite))
278
1 0.8 0.6 0.4 0.2 0
0
2
4
6
8
10
12
14
pH Fig. 4. Effect of pH on ammonia adsorption capacity of zeolite.
gave removal ratios of total lignin of 22.1, 37.1 and 40.1%, respectively. With high concentrations of ammonia water pretreatment, the total amount of lignin solubilized would increase. It also indicates that 100.0 g zeolite containing about 10.5 mg ammonia is relatively efficient for the pretreatment of 1.0 g rice straw. As the enzyme adsorbs the lignin making the enzyme ineffective, lignin is believed to be a major hindrance in enzymatic hydrolysis, so the removal of lignin is useful to expose the highly-ordered crystalline structure of cellulose and facilitates substrate access by hydrolytic enzymes. In the present experiment, the lignin removal ratio is relatively high. Xu et al. [41] found that only 12.32% of lignin was removed by pretreatment of pulverized soybean straw for 24 h at room temperature with 10% ammonia. It was also reported that the yield of solid residue and its lignin content was decreased by ammonia pretreatment of pine wood [42]. It is well known that during alkaline pretreatment, solvation and saponification reactions will take place. This causes swelling of the biomass and makes it more accessible to enzymes and bacteria. ‘Peeling’ of end-groups, alkaline hydrolysis and degradation and decomposition of dissolved polysaccharides can take place at relatively strong alkali concentrations, allowing dissolution. Loss of polysaccharides is mainly caused by peeling and hydrolytic reactions [36]. This peeling has an advantage for later conversion because of the formation of lower molecular weight compounds. The hydrolysis ratio percentage with enzyme hydrolysis due to 100.0 g zeolite containing ammonia (ZCA) pretreatment was 39.8%, while without pretreatment the hydrolysis ratio was only 3.0%. These results suggested that the ZCA pretreatment could help for enhancing enzymatic digestibility. It is supposed that with ZCA pretreatment under conditions of higher temperatures and pressures or concentrations of ammonia can further remove lignin and increase the accessible surface area that will further improve the enzymatic digestibility, which will be studied in the future. Above all, it can be concluded that ZCA pretreatment showed good performance for delignification of rice straw and increased the hydrolysis ratio, which has potential as an attractive alternative for disposal of the used zeolite containing ammonia.
12.0
pH
10.0
3.3. SEM
8.0 Absence of adsorption
6.0
Presence of adsorption
4.0 2.0
0
30
60
90
120 150 180 210 240 270 300
T (min) Fig. 3. pH changes in the absence and presence of zeolite I = 40 mA cm− 2, 200.0 g zeolite.
Fig. 5 shows the morphology of treated and untreated straw obtained using SEM analysis. It is noted that the wet treated biomass felt rougher to manual touch than the untreated biomass. The untreated sample exhibited relatively rigid and highly ordered fibrils. The pretreatment altered the biomass structure substantially. The ammonia pretreatment can cause the increasing of internal surface by swelling, decrease of polymerization degree and crystallinity, destruction of links between lignin and other polymers, and breakdown of lignin. The topography of the surface of alkali treated fiber shows
M. Li et al. / Desalination 278 (2011) 275–280
A
279
Fundamental Research Funds for the Central Universities for the financial support of this work.
References
B
Fig. 5. SEM micrographs of A) untreated straw; B) treated straw.
the formation of scratches within it, although the change is not very obvious, where the structure was destroyed due to the removal of lignin.
4. Conclusions Simultaneous electrochemical reduction of nitrate and adsorption of the ammonia by-product with zeolite, and pretreatment of straw with zeolite containing a large amount of ammonia were achieved in this work. After treatment of nitrate contaminated water, the concentration of nitrate-N was decreased from 100.0 to 3.6 mg L − 1 over 300 min, and the by-product ammonia was completely adsorbed from the treated water by zeolite, meeting the allowed limit for drinking water. The zeolite containing ammonia (ZCA) had good performance in the pretreatment of rice straw for delignification and increased the hydrolysis ratio during enzyme hydrolysis. The delignification of rice straw indicated that the portion of total lignin removed was 37.1% and the hydrolysis ratio was 39.8% after a 100.0 g zeolite pretreatment process. This suggests that it is an attractive alternative for disposal of the spent zeolite and the pretreatment of rice straw. In the future, the effect of various factors on nitrate removal and pretreatment of rice straw should be studied to further improve the method. Acknowledgements The authors thank the Water Pollution Control and Management Project of China (2008ZX07313-001, 2009ZX07318-008) and the
[1] S. Ghafari, M. Hasan, M.K. Aroua, Bio-electrochemical removal of nitrate from water and wastewater—a review, Bioresource. Technol. 99 (2008) 3965–3974. [2] T. Parvanova-Mancheva, V. Beschkov, Microbial denitrification by immobilized bacteria Pseudomonas denitrificans stimulated by constant electric field, Biochem. Eng. J. 44 (2009) 208–213. [3] H. Uemoto, M. Suzuki, M. Morita, A. Watanabe, H. Shinozaki, Simplified denitrification system using ethanol released from non-porous polyethylenefilm bag, Biochem. Eng. J. 45 (2009) 35–40. [4] S.K. Gupta, A.B. Gupta, R.C. Gupta, A.K. Seth, J.K. Bassain, Gupta, Recurrent acute respiratory tract infections in areas with high nitrate concentrations in drinking water, Environ. Health Perspect. 108 (4) (2000) 363–365. [5] EEC Council Recommendations (1987). [6] EEC Council Directive 98/83/EC (1998). [7] World Heath Organization (WHO), Guidelines for Drinking Water Quality, third ed WHO, Geneva, 2004. [8] M. Morit, H. Uemoto, A. Watanabe, Nitrogen-removal bioreactor capable of simultaneous nitrification and denitrification for application to industrial wastewater treatment, Biochem. Eng. J. 41 (2008) 59–66. [9] I. Katsounaros, D. Ipsakis, C. Polatides, G. Kyriacou, Efficient electrochemical reduction of nitrate to nitrogen on tin cathode at very high cathodic potentials, Electrochim. Acta 52 (2006) 1329–1338. [10] G. Zhu, Y. Peng, L. Zhai, Y. Wang, S. Wang, Performance and optimization of biological nitrogen removal process enhanced by anoxic/oxic step feeding, Biochem. Eng. J. 43 (2009) 280–287. [11] W.L. Ma, R. Qi, Y. Zhang, J. Wang, C.Z. Liang, M. Yang, Performance of a successive hydrolysis, denitrification and nitrification system for simultaneous removal of COD and nitrogen from terramycin production wastewater, Biochem. Eng. J. 45 (l) (2009) 30–34. [12] J. Virkutyte, V. Jegatheesan, Electro-Fenton, hydrogenotrophic and Fe2+ ions mediated TOC and nitrate removal from aquaculture system: different experimental strategies, Bioresource Technol. 100 (2009) 2189–2197. [13] K. Bouzek, M. Paidar, A. Sadilkova, H. Bergmann, Electrochemical reduction of nitrate in weakly alkaline solutions, J. Appl. Electrochem. 31 (2001) 1185–1193. [14] X. Lei, T. Maekawa, Electrochemical treatment of anaerobic digestion effluent using a Ti/Pt–IrO2 electrode, Bioresource Technol. 98 (2007) 3521–3525. [15] J. Souza-Garcia, E.A. Ticianelli, V. Climent, J.M. Feliu, Nitrate reduction on Pt single crystals with Pd multilayer, Electrochim. Acta 54 (2009) 2094–2101. [16] C.P. Huang, H.W. Wang, P.C. Chiu, Nitrate reduction by metallic iron, Water Res. 32 (1998) 2257–2264. [17] M.S. El-Deab, Electrochemical reduction of nitrate to ammonia at modified gold electrodes, Electrochim. Acta 49 (2004) 1639–1645. [18] S. Taguchi, J.M. Feliu, Kinetic study of nitrate reduction on Pt(110) electrode in perchloric acid solution, Electrochim. Acta 53 (2008) 3626–3634. [19] H.A. Duarte, K. Jha, J.W. Weidner, Electrochemical reduction of nitrates and nitrites in alkaline media in the presence of hexavalent chromium, J. Appl. Electrochem. 28 (1998) 811–817. [20] D. De, J.D. Englehardt, E.E. Kalu, Electroreduction of nitrate and nitrite ion on a platinum-group-metal catalyst-modified carbon fiber electrode-hronoamperometry and mechanism studies, J. Electrochem. Soc. 147 (2000) 4573–4579. [21] Y. Wang, J. Qu, R. Wu, P. Lei, The electrocatalytic reduction of nitrate in water on Pd/Sn-modified activated carbon fiber electrode, Water Res. 40 (2006) 1224–1232. [22] Z. Ma′cova′, K. Bouzek, Electrocatalytic activity of copper alloys for NO3− reduction in a weakly alkalinesolution Part 1: copper–zinc, J. Appl. Electrochem. 35 (2005) 1203–1211. [23] A.C.A. De Vooys, R.A. Santen, J.A.R. Veen, Electrocatalytic reduction of NO3− on palladium/copper electrodes, J. Mol. Catal. A. Chem. 154 (2000) 203–215. [24] B.P. Dash, S. Chaudhari, Electrochemical denitrification of simulated ground water, Water Res. 39 (2005) 4065–4072. [25] S.E. Jorgensen, O. Libor, K. Lea, K. Graber, K. Barkacs, Ammonia removal by use of clinoptilolite, Water Res. 10 (1976) 213–224. [26] J.R. Klieve, M.J. Semmens, An evaluation of pretreated natural zeolites for ammonia removal, Water Res. 14 (1980) 161–168. [27] J. Hlavay, G. Vigh, V. Olaszi, J. Inczedy, Investigations on natural Hungarian zeolite for ammonia removal, Water Res. 16 (1982) 417–420. [28] N.A. Booker, E.L. Cooney, A.J. Priestley, Ammonia removal from sewage using natural Australian zeolite, Water Sci. Technol. 34 (1996) 17–24. [29] Y. Wang, S. Liu, Z. Xu, T. Han, C. Sun, T. Zhu, Ammonia removal from leachate solution using natural Chinese clinoptilolite, J. Haz. Mat. B136 (2006) 735–740. [30] A.R. Rahmani, A.H. Mahvi, A.R. Mesdaghinia, S. Nasseri, Investigation of ammonia removal from polluted waters by Clinoptilolite zeolite, J. Enviro. Sci. & Tech. 1 (2004) 25–133. [31] P. McKendry, Energy production from biomass (part 1): overview of biomass, Bioresource Technol. 83 (2002) 37–46. [32] D.J. Schell, J. Farmer, M. Newman, J.D. McMillan, Dilutesulfuric acid pretreatment of corn stover in pilot-scale reactor-investigation of yields, kinetics, and enzymatic digestibilities of solids, Appl. Biochem. Biotechnol. 105 (2003) 69–85.
280
M. Li et al. / Desalination 278 (2011) 275–280
[33] V. Chang, M. Holtzapple, Fundamental factors affecting biomass enzymatic reactivity, Appl. Biochem. Biotechnol. 86 (2000) 5–37. [34] Y. Sun, J. Cheng, Hydrolysis of lignocellulosic materials for ethanol production: a review, Bioresource Technol. 83 (2002) 1–11. [35] N. Mosier, Ch. Wyman, B.E. Dale, E.R. lander, Y.Y. Lee, M. Holtzapple, M. Ladisch, Features of promising technologies for pretreatment of lignocellulosic biomass, Bioresource Technol. 96 (2005) 673–686. [36] D. Fengel, G. Wegener, Wood: Chemistry, Ultrastructure, Reactions, De Gruyter, Berlin, 1984. [37] M. Li, C.P. Feng, W.W. Hu, Z.Y. Zhang, N. Sugiura, Electrochemical degradation of phenol using electrodes of Ti/RuO2–Pt and Ti/IrO2–Pt, J. Haz. Mat. 162 (2009) 455–462. [38] APHA, AWWA, WPCF, Standard Methods for the Examination of Water and Wastewater, American Public Health Association, Washington, DC, USA, 1998.
[39] C. Polatides, M. Dortsiou, G. Kyriacou, Electrochemical removal of nitrate ion from aqueous solution by pulsing potential electrolysis, Electrochim. Acta 50 (2005) 5237–5241. [40] S. Cattarin, Electrochemical reduction of nitrogen oxyanions in 1 M sodium hydroxide solutions at silver, copper and cuinse electrodes, J. Appl. Electrochem. 22 (1992) 1077–1081. [41] Z. Xu, Q. Wang, Z. Liang, The study of the ammonia pretreatment effect on the enzyme hydrolysis of soybean straw to produce sugar, J. Chem. Eng. Chi. Uni. 18 (6) (2004) 71–75. [42] M.V. Efanov, R.Yu. Aberin, Peroxide-ammonia delignification of pine wood, Chem. Nat.Com. 40 (2) (2004) 172–175.