- Email: [email protected]
Removal of nitrate ions from aqueous solution using zero-valent iron nanoparticles supported on high surface area nanographenes
Journal of Molecular Liquids 212 (2015) 708–715
Contents lists available at ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Removal of nitrate ions from aqueous solution using zero-valent iron nanoparticles supported on high surface area nanographenes Mohamed Abdel Salam a,⁎, Olfat Fageeh a,b, Shaeel A. Al-Thabaiti a, Abdullah Y. Obaid a a b
Chemistry Department, Faculty of Science, King Abdulaziz University, P.O Box 80200, Jeddah 21589, Saudi Arabia Center of Excellence in Environmental Studies, King Abdulaziz University, P.O. Box 80216, Jeddah 21589, Saudi Arabia
a r t i c l e
i n f o
Article history: Received 6 July 2015 Received in revised form 8 September 2015 Accepted 17 September 2015 Available online xxxx Keywords: Catalysis Mechanism Nitrate Nanographene Reduction Zero-valent iron nanoparticles
a b s t r a c t In this manuscript, zero-valent iron nanoparticles (ZVINPs) were synthesized, and then supported on high surface area nanographenes (NGs) to prepare a stable ZVINP/NG nanocomposite. XRD measurements showed the stabilization of the ZVINPs upon their support on NGs, which enhanced their activity. The ZVINP/NG nanocomposite was used for the catalytic reduction of nitrate ions in model solution, and the results showed the dependency of the removal process on the ratio of ZVINPs to NGs in the nanocomposite, time of removal, and solution pH. The effect of ultrasonication was also explored, and the results showed that ultrasonication could significantly decrease the removal time required by the nanocomposite. The mechanism of nitrate reduction by ZVINP/NG nanocomposite was studied, and the results showed the conversion of nitrate to nitrite and/or ammonia, then to nitrogen gas. © 2015 Published by Elsevier B.V.
1. Introduction The increase in demand for water and food supplies is placing increasing stress on ground and surface water quality and quantity. One environmental problem that has become an increasingly important issue in developed and developing countries is nitrate contamination of surface water and groundwater [1,2]. Although groundwater is an important source of water in the Kingdom of Saudi Arabia, nitrate contamination is considered a limitation for using groundwater. Accumulation of high levels of nitrate that reach the water surface is contributing to depleting dissolved oxygen, which kills fish and creates a harmful environment for humans [3–5]. Nitrates possess a high risk to human health and have been listed on the USEPA's Drinking Water Contaminant List with the maximum contaminant level of 10 mg/L as nitrogen [6]. Nitrate is considered relatively nontoxic to the human body, but its toxicity is due to its reduction to nitrite by denitrifying bacteria in the upper gastrointestinal tract, which may cause many diseases, such as blue baby syndrome and cancer [1,2,7–9]. These nitrites transform hemoglobin to methemoglobin by oxidation of ferrous iron (Fe2+) in hemoglobin to ferric form (Fe3 +), preventing or reducing the ability of blood to transport oxygen, which causes cyanosis and anoxemia [10]. Although nitrate could be removed from aquatic systems by different methods such as adsorption [11], catalytic reduction using monometallic
⁎ Corresponding author. E-mail address: [email protected] (M.A. Salam).
http://dx.doi.org/10.1016/j.molliq.2015.09.029 0167-7322/© 2015 Published by Elsevier B.V.
particles [12] or bimetallic particles [13], and photocatalytic reduction [14], the search for a cost-effective method to remove nitrate from groundwater is a crucial issue for the health of the aqueous environment. Chemical methods using nano zero-valent metals, such as Fe, Al, Zn, and Mg, have been frequently used for the reduction of nitrate anions [15]. Among these zero-valent metals, iron possesses a great importance for this purpose because it possesses a high specific surface area, which leads to high surface reactivity. Also, because of its high reduction potential, zero-valent iron (ZVI) is considered to be an important remediation reagent, and its chemical reaction steps occur relatively rapidly. In recent years there has been growing interest in applications of ZVI to groundwater remediation, which has helped to make ZVI the most widely studied reductant chemical for environmental applications [16]. However, one problem associated with the application of ZVI for reduction of nitrate is the low stability of zero-valent iron, especially in its nanoparticle form. Zero-valent iron nanoparticles (ZVINPs) are very reactive and oxidize easily to iron oxides, which decreases their reduction ability. In the present work, the catalytic reduction of nitrate ions using ZVINPs supported on high surface area nanographene (ZVINP/NG) nanocomposite was studied. The effects of different factors that affect the efficiency of nitrate removal were investigated, namely the mass ratios of ZVINPs and NGs, mass of the ZVINP/NG nanocomposite, solution pH, contact time, and effect of ultrasonication on the removal process. In addition, the catalytic reduction mechanism of the nitrate ions using ZVINPs and ZVIPN/NG nanocomposite was explored and verified.
M.A. Salam et al. / Journal of Molecular Liquids 212 (2015) 708–715
where, Cο, Ce, Ms., and V are the initial concentration (mg/L), is the equilibrium concentration (mg/L), the mass of ZVINPs (g), and the volume of solution (L), respectively.
2. Experimental 2.1. Materials High surface area nano graphene (NGs) were obtained from XG Science, USA. xGnP® and were used as received. Ferric chloride (60% w/v, BDH Chemicals Ltd., Poole, England), sodium borohydride (BDH Chemicals Ltd., Poole, England), were used for the preparation of ZVINPs. Nitrate stock solution of 1000 mg L−1 was prepared using sodium nitrate obtained from (Fisher Scientific Company). Sodium hydroxide and hydrochloric acid (BDH Chemicals Ltd., Poole, England) were used to adjust the solution pH and using pH meter Mettler Toledo AG (Switzerland). All solutions were prepared using deionized water and degassed by purging with nitrogen gas. 2.2. Synthesis of ZVINPs The synthesis process was conducted in three necked-flask reactor. One of the necks was attached to nitrogen gas. 120 mL sodium borohydride (NaBH4) of 0.16 M was added drop-wise to 200 mL of 0.1 M ferric chloride solution (FeCl3) with continuous stirring and flowing of nitrogen gas to prevent oxidation of iron. The produced ZVINPs were collected by magnet, and then it was washed 3 times by deionized water and finally dried over night at 110°°C. Reduction reaction of ferric (Fe3+) into zero-valent iron occurred according to the equation [17]. 4Fe3þ þ 3BH4− þ 9H2 O→4Fe0 þ 3H2 BO3− þ 12Hþ þ 6H2
ð1Þ
2.3. Preparation of ZVINP/NG nanocomposite The nanocomposites of ZVINP/NG were prepared by mixing ZVINPs with an appropriate quantity of NGs using a mortar and the mixture was well grinded for 30 min to ensure homogeneity. 2.4. Characterizations TEM (JEOL 2100F) operating at 200 kV with a Field Emission Gun, obtaining a point resolution of 0.19 nm. X-ray diffraction (XRD) patterns were recorded for phase analysis and the measurement of crystallite size on a Philips X pert pro diffractometer, which was operated at 40 mA and 40 kV by using CuKα radiation and a nickel filter in the 2θ range from 2 to 80° in steps of 0.02°, with a sampling time of one second per step. The specific surface area was determined from nitrogen adsorption/desorption isotherms which were measured at 77 K by using a Nova 2000 series Chromatech. Prior to analysis, the samples were outgassed at 250°°C for 4 h. 2.5. Reduction of nitrate ions Reduction of nitrate was studied using ZVINPs and ZVINP/NG nanocomposite. Weighed amounts of the catalyst (0–30 mg), was added into glass vials containing 10 mL of 50 mg/L sodium nitrate at neutral pH, then the solutions were shacked for 24 h at a speed of 150 rpm at room temperature. After desired time passed, the suspensions were filtered through 0.22 μm Millipore syringe filters and the solutions concentration were then measured using ion chromatography (Dionex ICS-2100 system). The percentage nitrate ions removed and the nitrate removal capacity (qe) were calculated according to Eqs. (2) and (3), respectively. %Removal ¼
qe ðmg=g Þ ¼
C 0 −C e 100 C0
C 0 −C e V Ms
709
ð2Þ
ð3Þ
3. Results and discussion 3.1. Characterization of ZVINP/NG nanocomposite The structure and composition of the prepared ZVINPs, NGs, and ZVINP/NG nanocomposite were investigated using TEM, XRD, and surface area analysis. Fig. 1 shows the morphological structures of the prepared ZVINP/NG nanocomposite using TEM at different magnifications. The TEM showed homogenous dispersion of the prepared ZVINPs over the transparent NGs surface. ZVINPs appeared to be spherical in shape with non-uniform size and an average diameter of 50 nm. It is clear from the TEM images that ZVINPs tended to be distributed well over the NG surface and did not tend to agglomerate. The XRD patterns of the ZVINPs, NGs, and ZVINP/NG nanocomposite are shown in Fig. 2a,b, & c. The diffraction peaks of ZVINPs at Fig. 2a revealed the presence of three different iron-based compounds: synthetic zero-valent iron (α-Fe(0), ref. JCPDS 06-0696), maghemite (Fe2O3, ref. JCPDS 13-0458), and synthetic magnetite (Fe3O4, ref. JCPDS 88-0315), with their corresponding peaks. Although it was believed that only ZVINPs were synthesized, but because of the high reactivity of the ZVINPs; in addition to the time to analyze the sample with the XRD, the ZVINPs were oxidized to form different iron oxides: maghemite and synthetic magnetite. Also, the diffraction peaks at 2θ angles of 26.3–54.1° in the XRD pattern of NGs revealed the presence of pristine graphene in the form of nanoplatelets, same as that of native graphite (JCPDS No. 75-1621); as it was presented in Fig. 2b. The pristine graphene nanoplatelets showed a (0 0 2) diffraction peak at 26.3° (2θ), corresponding to a dspacing of 33.8 nm. As it is presented in Fig. 2c; surprisingly, the XRD pattern of ZVINP/NG nanocomposite showed the characteristic peaks of graphene nanoplatelet diffraction at 26.3° (2θ), in addition to the characteristic diffraction peak of the ZVINPs only at 44.4° (2θ), corresponding to a d-spacing of 20.4 nm, without any characteristic peaks of iron oxides diffraction pattern. This is clear evidence that the NGs greatly stabilized the ZVINPs by π-electrons present at the NG surface, which prevented the oxidation of the ZVINPs and facilitated their storage till the application time. The specific surface areas of the NGs, ZVINPs, and prepared ZVINP/NG nanocomposite were measured using the nitrogen adsorption/desorption isotherms at 77 K. The measurements showed that the BET-specific surface areas were 677.5 m2/g, 48.0 m2/g, and 569.2 m2/g for the NGs, ZVINPs, and prepared ZVINP/NG nanocomposite, respectively. It is obvious that the NGs had a high surface area, which was reduced upon mixing them with the ZVINPs. Also as was expected, the surface area of the ZVINPs was greatly enhanced upon mixing them with the NGs, which would enhance the activity and efficiency of the ZVINPs for catalytic reduction of nitrate ions in aqueous solution. 3.2. Reduction of nitrate by ZVINPs and ZVINP/NG nanocomposite 3.2.1. Effect of composite mass Because the removal of nitrate by ZVINPs involves a reaction on the metal surface, the iron-to-nitrate ratio (gram nanoscale iron/milligram nitrate) is a significant variable parameter [18]. As shown in Fig. 3, the percent nitrate removal was greatly enhanced by increasing the ZVINP mass. When 1.0 mg of ZVINPs was used, it was accompanied by percent nitrate removal of 3.8%. This removal percentage was enhanced by increasing the mass of ZVINPs till it reached equilibrium when 20.0 mg ZVINPs were used and 92.0% removal efficiency was achieved. Further increase in the amount of ZVINPs used was associated with a slight enhancement in the percentage of nitrate removed from aqueous solution. This is mainly because increasing the mass of ZVINPs provides more active sites of iron nanoparticles available for nitrate reduction. This
710
M.A. Salam et al. / Journal of Molecular Liquids 212 (2015) 708–715
of ZVINPs and NGs was prepared with a total mass of 30 mg. It was observed that pristine NGs alone could not remove any of the nitrate ions from the solution because of the electrostatic repulsion between the negatively charged nitrate ions and NG surface, which is mainly covered with the negatively charged π electrons. This confirmed that NGs have no role in nitrate removal, as the removal involved the reduction of nitrate ions at the ZVINP surface only. Also, the results revealed the gradual and significant increase in percent removal of nitrate with the increasing mass ratio of ZVINPs in the ZVINP/NG nanocomposite. Meanwhile, using ZVINP/NG nanocomposite with mass ratios of 1:29 mg, 2:28 mg, 3:27 mg, 4:26 mg, and 5:25 mg showed percent removal of 4.5%, 18.6%, 34.7%, 54%, and 71.8%, respectively. A further increase of ZVINP content in ZVINP/NG nanocomposite led to equilibrium; mass ratios of 10:20, 15:15, and 20:10 (as mg) showed percent removal values of 83.5%, 90.5%, and 93.2%, respectively. On the other hand, Fig. 4 shows the amount of nitrate ions removed from aqueous solution based on the amount of ZVINPs in ZVINP/NG nanocomposite. From this result it is found that as ZVINPs content increases in ZVINP/NG nanocomposite, the amount of nitrate removed was enhanced until the ZVINP-to-NG ratio was 5:25 with 71.8 mg/g of nitrate removed. The further increase in ZVINPs content in the ZVINP/NG composite caused a significant decrease in the amount of nitrate removed; 41.8 mg/g, 30.2 mg/g, and 23.3 mg/g with mass ratios of 10:20, 15:15, and 20:10, respectively. This result indicated that the ZVINP/NG nanocomposite with a mass ratio of 5:25 had the highest efficiency for the removal of nitrate based on the amount removed, and this finding agreed well with previous research [20]. It is noteworthy to mention that the percent nitrate removed from solution was enhanced 125.1% upon using NGs as the support for ZVINPs; percent nitrate removed went from 57% to 71.8%. This means that the catalytic reduction of nitrate ions for supported ZVINPs was higher than that of unsupported ZVINPs, and this result is in good agreement with previous research study [21]. This attributed to the enhancement in the specific surface area of the ZVINPs compared with the ZVINP/NG nanocomposite – 200.4 m2/g to 569.0 m2/g, respectively – which greatly enhanced the reactivity of the ZVINPs for nitrate reduction due to the dispersion of the ZVINPs over the NGs plates as was shown in the TEM images [22]. Also, as was confirmed by the XRD measurements that supporting ZVINPs by the NG plates greatly enhanced the stability of the ZVINPs and made them available only for the reduction of nitrate rather than allowing them to be oxidized by atmospheric oxygen. Therefore, it could be concluded that using NGs as the supporting material for ZVINPs could achieve much better nitrate removal efficiency compared with using ZVINPs alone. In addition to the above, NGs stabilize the dispersion of ZVINPs in solution and preventing ZVINPs from being aggregated. All these factors might contribute to the significant enhancement of nitrate reduction by ZVINPs after supporting them on NGs.
Fig. 1. TEM images of the ZVINP/NG nanocomposite at different magnifications.
agreed well with previous studies that reported that increasing the amount of iron particles in solution speeds up the initial reaction because it provides more active sites that can be used for collision of nitrate ions during the reduction [18,19]. Hence, it could be concluded here that the optimum amount of ZVINPs for nitrate removal is 30 mg. As discussed earlier, mixing ZVINPs with NGs was expected to form a more active nanocomposite with high stability and efficiency for the reduction of nitrate ions in aqueous solution. Hence, a different mass ratio
3.2.2. Effect of pH The effect of solution pH ranging from 2 to 12 on the removal of nitrate was explored, and the results were presented in Fig. 5. The results revealed that optimum nitrate removal was obtained at solution pH of 2, and as solution pH increased, there was accompanied by a significant decrease in percent nitrate removal. At pH 2, the percent removal of nitrate reached 89.4%, and it decreased to 79.6%, 74.7%, 70.3%, and 69.9% at pH values of 4, 6, 7, and 8, respectively. A further pH increase was associated with a further decrease in the percent nitrate removal, which reached 53.8% at pH 12. This significant decrease in the percent nitrate removal by increasing the solution's pH could be mainly attributed to the fact that reduction of nitrate by ZVINPs is a very rapid process in acidic solution [23]. Suzuki et al. reported that reduction of nitrate using ZVI involved a direct transport of electrons from ZVI into nitrate [24], as ZVI is an effective electron donor. At low pH, iron oxides present at the surface of ZVINPs are positively charged and attract anions such as nitrate, and simultaneously lowering the solution's pH would dissolve the ferrous hydroxide and other protective layers at the surface of the ZVINPs, yielding more fresh reactive sites for chemical reduction of
M.A. Salam et al. / Journal of Molecular Liquids 212 (2015) 708–715
711
Fig. 2. XRD patterns of the ZVINPs, NGs, and ZVINP/NG nanocomposite.
nitrate. High pH values are not favorable for nitrate reduction because at high pH values iron hydroxide precipitates, leading to formation of a passive layer on the iron particle surface caused by Fe(OH)2 and Fe(OH)3 precipitation, which deactivates the ZVINPs and consequently
decreases the percentage of nitrate removed [25,26]. Therefore, based on the present experimental results, in general, the efficiency of nitrate removal by ZVINPs increased as the solution pH decreased, which agreed well with previous studies [27–29]. Accordingly, it could be
712
M.A. Salam et al. / Journal of Molecular Liquids 212 (2015) 708–715
Fig. 3. Effect of ZVINPs mass on the % nitrate removed from aqueous solution. (Experimental conditions: 10 mL of 50 mg/L NaNO3 solution, 298 K, 24 h and pH 7).
concluded that nitrate reduction by ZVINPs is an acid-driven process [29]. 3.2.3. Effect of time The effect of contact time was studied at 296 K, and the results were presented in Fig. 6a. The figure showed a gradual and significant increase in the removal efficiency of nitrate with increasing contact time till it reached equilibrium after 16 h with removal efficiency of 67.9% and removal capacity of 67.9 mg NO− 3 /g ZVINPs. The gradual increase may be due to the fact that the initial reactive sites for nitrate reduction on the metal surface were vacant and the nitrate concentration gradient was high until all the active sites on the ZVINP/NG nanocomposite were occupied. A further increase in contact time was accompanied by a slight increase in the amount nitrate ions removed by the ZVINP/NG nanocomposite; after 40 h, the percent nitrate removed was 70.4%. This result revealed that reduction of nitrate reached equilibrium within 16 h, which agreed well with one of the previous studies showing that nitrate removal is negligible after 10 h at pH N 4 [30].
Fig. 5. Effect of pH on nitrate removal by ZVINP/NG nanocomposite from aqueous solution. (Experimental conditions: 30 mg of ZVINP/NGs, 10 mL of 50 mg/L NaNO3 solution, 298 K, 24 h and pH 2–12).
result revealed that ultrasonication decreases the time required for the reduction of nitrate with much better removal efficiency compared with the removal process in the absence of ultrasonication. This may be because better mixing and dispersion of ZVINP/NG nanocomposite can be obtained by ultrasonication, which enormously accelerates the heterogeneous reaction between ZVINPs and nitrate ions. In addition, the acoustic cavitation generated by ultrasonication and the implosive collapse of bubbles in liquids played an important role in the
3.2.4. Effect of ultrasonication Ultrasonication has become an important variable that has distinct effects on different aspects of chemistry. Fig. 6b showed the effect of ultrasonication time on nitrate removal by ZVINP/NG nanocomposite from aqueous solution. It is clear that increasing the ultrasonication time was accompanied by a significant increase in the percent nitrate removed from solution, and equilibrium was achieved within 3 h with removal efficiency of 87.9%. A further increase in the ultrasonication time was associated with insignificant enhancement in the percent nitrate removed from solution until it reached 90.8% after 5 h. This
Fig. 4. Efficiency of nitrate removal at different composition ZVINP/NG nanocomposite ratios. (Experimental conditions: 10 mL of 50 mg/L NaNO3 solution, 298 K, 24 h and pH 7).
Fig. 6. Effect of contact time on nitrate removal (%) by ZVINP/NG nanocomposite from aqueous solution (a) at normal condition, (b) with ultrasonication. (Experimental conditions: 30 mg of ZVINP/NG, 10 mL of 50 mg/L NaNO3 solution, 298 K, 0–40 h and pH 7).
M.A. Salam et al. / Journal of Molecular Liquids 212 (2015) 708–715
improvement of nitrate reduction by ZVINPs. The implosive collapse of bubbles creates localized, short-lasting hot spots with high pressure through adiabatic compression or shock-wave formation within the gas phase of the collapsing bubbles; these localized hot spots can improve the mixing and dispersion of ZVINPs and thus enormously accelerate the heterogeneous reaction between ZVINPs and nitrite [31]. This finding is in agreement with Cravotto et al., that ultrasoundaccelerated chemical reactions could be carried out under ultrasound irradiation to give higher results in shorter reaction times and milder conditions [32]. This large removal percentage is attributed to the fact that using ultrasound waves causes a decrease in particle size, which effectively enhances particles' reactivity in addition to increasing surface area. High-intensity ultrasound (100 kHz) is used to induce a chemical or physical change on the target medium [33]. Ultrasonication could provide the solution with a high rate of mixing and mass transfer through particles [34]. Due to decreasing particle size by ultrasonication, the loading of ZVINPs on graphene sheets would be uniform, and further ultrasonication could be responsible for reducing the agglomeration tendency of graphene nanosheets and making the loading of ZVINPs easier [35]. According to the above results, the optimum time required for removal of nitrate without ultrasonication was 16 h with removal percentage of 67.9%. This time was significantly reduced by ultrasonication, and the optimum time required to attain equilibrium was 3.0 h with a removal percentage of 90.8%. Hence, ultrasonication could reduce the time required for nitrate removal to almost one-fifth that of the removal process without ultrasonication. It is noteworthy to mention that the ultrasonication process was carried out mainly at room temperature.
69% after 40 h, as shown in Fig. 7b. This improved result was obtained using graphene as a supporting material, which increased surface area, stability, and dispersion of ZVINPs in the solution, hence improving the reduction reaction between ZVINPs and nitrate ions. According to previous studies, nitrate ions can be reduced into nitrite ions, ammonium ions, and/or nitrogen gas. Nitrite ions were detected with a very low concentration in the first stage of the reaction in Fig. 7a and b; their concentration was less than 0.1% in both cases and then disappeared completely. This may be attributed to the fact that nitrate was reduced into nitrite very quickly, and nitrite is considered an intermediate [35]. Nitrite can also be reduced into ammonium, according to Eq. (7), or into nitrogen gas, according to Eq. (8) [18]. 0 þ 2þ þ NHþ NO− 2 þ 3Fe þ 8H →3Fe 4 þ 2H2 O
ð7Þ
þ 2þ þ 4H2 O þ N2 ðgÞ 3Fe0 þ 2NO− 2 þ 8H →3Fe
ð8Þ
On the other hand, the amount of ammonium ions was identified and monitored as shown in Fig. 7a by using ZVINPs alone; about 20% of ammonium ions were detected in the solution after 24 h, and this amount decreased to 16% after 40 h of the reaction. Ammonium ions can be produced by the reduction of nitrite, as previously discussed, or by direct reduction of nitrate, as shown in Eqs. (9) and (10) [36]. þ − 4Fe0 þ NO− 3 þ 7H2 O→4FeðOHÞ2 þ NH4 þ 2OH
þ − 3Fe0 þ NO− 3 þ H2 O→Fe3 O4 þ NH4 þ 2OH
3.3. Mechanism of nitrate reduction using ZVINP/NG nanocomposite
713
ð9Þ
ð10Þ
The removal of nitrate from water involves three main steps, adsorption of nitrate onto the ZVINP surface, reduction, and finally desorption into the solution. There are two possible mechanisms proposed for nitrate reduction using ZVINPs, either: a) direct transfer of electrons from metallic iron, as presented in Eqs. (4) and (5), or b) indirect electron transfer by hydrogen atoms yielding corrosion, as in Eq. (6) [18]. 3Fe0 →3Fe2þ þ 6e−
ð4Þ
0 þ 2þ þ NO− NO− 3 þ Fe þ 2H →Fe 2 þ H2 O
ð5Þ
þ þ NO− 3 þ 4H2 þ 2H →NH4 þ 3H2 O
ð6Þ
An acidic medium is favorable for nitrate reduction using ZVINPs because an acidic medium usually dissolves the iron oxides and other protective layers, hence the regenerated Fe0 will reduce nitrate throughout the cavities produced on the metal surface where nitrate was adsorbed into these cavities. In contrast, the basic medium is not favorable for nitrate reduction, as Fe2+ will transform into ferrous hydroxide that is thermodynamically unstable and may be further oxidized into magnetite. The accumulation of magnetite on the Fe0 surface may slow down the reaction when pH is higher than 6–7 [21]. Xu et al. reported that at neutral pH, the reduction reaction would be slowed down because of consumption of H+ [23]. In the present work, the various nitrogen species were identified and monitored during the reduction reaction time using ZVINPs and ZVINP/NG nanocomposite, as presented in Fig. 7a and b, respectively. As presented in Fig. 7a, when ZVINPs were used alone, after 24 h of reaction the percent removal of nitrate reached 56.0%, and after 40 h it reached 63%. Meanwhile, when ZVINP/NG nanocomposite was used, the percent removal of nitrate was greatly enhanced, as the percent removal reached 68% after 24 h and reached
Fig. 7. Amount of nitrogen species during nitrate reduction using (a) 5 mg of ZVINPs only, and (b) 30 mg of ZVINP/NG nanocomposite.
714
M.A. Salam et al. / Journal of Molecular Liquids 212 (2015) 708–715
At the same time, the most important phenomenon is that there were no detectable ammonium ions in the solution when ZVINP/NG nanocomposite was used. The amount of ammonium reached 3.0% after 24 h of reaction, while at the end of reaction, after 40 h, it was completely removed. The reason for this phenomenon is the adsorption of positively charged ammonium ions onto the negatively charged nanographene surface. Hence, using ZVINP/NG nanocomposite doesn't require further treatment to remove the by-products of the nitrate reduction by ZVINPs. Romero et al. reported that a small charge transfer occurs because of adsorption of NH3 onto the graphene surface [37], and the same result was confirmed by Schedin et al. [38]. Moreover, it was found that there is a great difference between using ZVINPs alone and using ZVINPs nanocomposite supported onto NGs graphene, at which 90% of the produced ammonium can be removed from solution by adsorption onto graphene surface when ZVINP/NG nanocomposite was used. Meanwhile, by using ZVINPs alone all the produced ammonium was detected in solution and can't be removed. In addition, the total nitrogen balance for all nitrogen species produced from the reduction reaction in the solution doesn't equal the initial concentration, at which from Fig. 7a total nitrogen showed 54% of nitrogen species is found in the solution after equilibrium; 24 h, and there is about 45% couldn't be detected in solution. The same observation was found in the case of using ZVINP/NG nanocomposite; the total nitrogen balance detected was 32% of nitrogen species in the solution, while 68% of nitrogen species was undetectable, as presented in Fig. 7b. From our results, it was found that the amount of undetectable nitrogen species was 45% in the case of using ZVINPs alone, while this amount increased in the case of using ZVINP/NG nanocomposite to 68% because of the adsorption of the produced ammonium ions onto the graphene surface. In these observations, the overall amount of nitrogen species was less than the initial ones, which implies that the missing amount was mainly attributable to generation of nitrogen gas as the final product of the nitrate ions reduction by the ZVINPs. Nitrogen gas may be produced either by direct reduction of nitrate, as shown in Eq. (11) [19], or by reduction of nitrite, as was previously discussed. The conversion of nitrate into nitrogen gas is favorable thermodynamically [39]. Unfortunately, the amount of nitrogen gas produced couldn't be identified or analyzed because of the nature of the open system of the reaction, and
because of a shortage of lab instrumentation, the only way to detect nitrogen gas was by using GC–MS. þ − 5Fe0 þ 2NO− 3 þ 6H2 O→5Fe2 þ N2 ðgÞ þ 12OH
ð11Þ
In general, the reduction mechanism using ZVINP/NG nanocomposite didn't differ significantly from what occurred using ZVINPs alone, except in two important ways: the amount of removed nitrate increased with the presence of NGs as the support catalyst, and the amount of ammonium released differed because of adsorption of ammonium onto the graphene surface in the case of using the ZVINP/NG nanocomposite. In conclusion, Fig. 8 represents the schematic of the possible mechanism of the catalytic reduction of nitrate ions by ZVINP/NG nanocomposite. These great and promising results show the importance of using these NGs as a support for the reduction of nitrate ions in aqueous solution using ZVINPs. The advantage of using graphene is that it not only increases the surface area and prevents aggregation and oxidation of ZVINPs, but it can also solve a major problem that arises from the accumulation of ammonium ions released from the reduction of nitrate using ZVINPs, and as a result further treatment is not required when using ZVINP/NG nanocomposite for the reduction of nitrate.
4. Conclusions ZVINPs were synthesized then supported on high surface area nanographenes (NGs) to obtain ZVINP/NG nanocomposite. The results showed the presence of ZVINPs in a spherical shape with average particle size of 50 nm and homogenous distribution of the ZVINPs on the transparent NG surface. The XRD measurements showed that the ZVINPs alone had low stability but greatly stabilized upon their support on the NG surface, which provided the ZVINPs with electrons and prevented their oxidation. The surface area analysis showed the high surface area of ZVINPs, NGs, and ZVINP/NG nanocomposite; 200.4 m2/g, 677.5 m2/g, and 569.2 m2/g, respectively. The ZVINP/NG nanocomposite was used for the catalytic reduction of nitrate ions in model solution, and the effects of different parameters that affect the removal process were studied. Most of the nitrate ions could be removed
Fig. 8. Schematic representation of the possible mechanism of the catalytic reduction of nitrate ions by ZVINP/NG nanocomposite.
M.A. Salam et al. / Journal of Molecular Liquids 212 (2015) 708–715
from solution using ZVINP/NG nanocomposite in a mass ratio of 5:25, and supporting ZVINPs on the NG surface enhanced the removal process by 125.1% compared with using ZVINPs alone. Also, the results showed that the removal process via catalytic reduction is an acid-driven process, as the optimum removal was obtained at acidic pH values. Most of the nitrate ions could be removed within 16 h at ambient conditions, and this time could be shortened to 3 h by ultrasonication. The mechanism of nitrate reduction by ZVINPs was explored, and the results showed that nitrate ions could be reduced to nitrite and/or ammonium, which then reduced to nitrogen gas. Supporting ZVINPs on NGs greatly enhanced the removal of nitrate because they enhanced their stability and surface area and prevented their aggregation and agglomeration. The NGs were also able to adsorb the ammonium produced from the reduction of nitrate in the solution. Acknowledgments This project was funded by Saudi Basic Industries Corporation (SABIC) and the Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah, Saudi Arabia, under grant no. 15/260/1434/MS. The authors, therefore, acknowledge with thanks SABIC and DSR technical and financial support. References [1] P. Mikuska, Z. Vecera, Simultaneous determination of nitrite and nitrate in water by chemiluminescent flow-injection analysis, Anal. Chim. Acta 495 (2003) 225–232. [2] A. Rezaee, H. Godini, S. Dehestani, A. Khavanin, Application of impregnated almond shell activated carbon by zinc and zinc sulfate for nitrate removal from water, Iran. J. Health Sci. Eng. 5 (2008) 125–130. [3] N.N. Rabalais, R.E. Turner, D. Scavia, Beyond science into policy: Gulf of Mexico hypoxia and the Mississippi River, Bioscience 52 (2002). [4] D. Scavia, K.A. Donnelly, Reassessing hypoxia forecasts for the Gulf of Mexico, Environ. Sci. Technol. 41 (2007) 8111–8117. [5] N.N. Rabalais, R.E. Turner, Hypoxia in the Northern Gulf of Mexico: Coastal Hypoxia: Consequences for Living Resources and Ecosystems, Coastal and Estuarine Studies 58, American Geophysical Union, Washington, D.C., 2001 [6] C.o.T. National Research Council, Nitrate and Nitrite in Drinking Water, National Academies Press, Washington, DC, 1995. [7] R. Grommena, I.V. Hauteghem, M.V. Wambeke, W. Verstraete, An improved nitrifying enrichment to remove ammonium and nitrite from freshwater aquaria systems, Aquaculture 211 (2002) 115–124. [8] W.R. Melchert, C.M.C. Infante, F.R.P. Rocha, Development and critical comparison of greener flow procedures for nitrite determination in natural waters, Microchem. J. 85 (2007) 209–213. [9] A. Afkhami, Adsorption and electrosorption of nitrate and nitrite on high-area carbon cloth: an approach to purification of water and wastewater samples, Letters to the Editor/Carbon, 41 2003, pp. 1309–1328. [10] C.S. Bruning-fann, J.B. Kaneene, The effects of nitrate, nitrite, and N-nitroso compounds on animal health, Vet. Hum. Toxicol. 35 (1993) 237–253. [11] A. Bhatnagar, M. Sillanpaa, Applications of chitin- and chitosan-derivatives for the detoxification of water and wastewater — a short review, Adv. Colloid Interf. Sci. 152 (2009) 26–38. [12] F.S. Fateminiaa, C. Falamakib, Zero valent nano-sized iron/clinoptilolite modified with zero valent copper for reductive nitrate removal, Process. Saf. Environ. Prot. (2013) 304–310.
715
[13] L. Durivault, O. Brylev, D. Reyter, M. Sarrazin, D. Bélanger, L. Roué, Cu–Ni materials prepared by mechanical milling: their properties and electrocatalytic activity towards nitrate reduction in alkaline medium, J. Alloys Compd. 432 (2007) 323–332. [14] D. Kyle, Environmentally Responsible Use of Nanomaterials for the Photocatalytic Reduction of Nitrate in Water, (PhD thesis) of ARIZONA STATE UNIVERSITY, 2013. [15] S.F. Cheng, S.C. Wu, The enhancement methods for the degradation of TCE by zerovalent metals, Chemosphere 41 (2000) 1263–1270. [16] H.J. Sunge, I. Francis, Nano Technology for Environmental Remediation, 5, Springer Science 2006, pp. 5–42. [17] T.C. Zhang, Y.H. Huang, Effects of selected Good's pH buffers on nitrate reduction by iron powder, J. Environ. Eng. 131 (2005) 461–470. [18] J.H. Zhang, Z.W. Hao, Z. Zhang, Y.P. Yang, X.H. Xu, Kinetics of nitrate reductive denitrification by nanoscale zero-valent iron, Process Saf. Environ. 88 (2010) 439–445. [19] F. Xiaomeng, G. Xiaohong, M. Jun, A. Hengyu, Kinetics and corrosion products of aqueous nitrate reduction by iron powder without reaction conditions control, J. Environ. Sci. 21 (2009) 1028–1035. [20] F. Liu, J.H. Yang, J. Zuo, D. Ma, L. Gan, B. Xie, P. Wang, B. Yang, Graphene-supported nanoscale zero-valent iron: removal of phosphorus from aqueous solution and mechanistic study, J. Environ. Sci. 26 (2014) 1751–1762. [21] W.-X. Zhang, D.W. Elliott, Applications of iron nanoparticles for groundwater remediation, Remediat. J. 16 (2006) 7–21. [22] C.B. Wang, W. Zhang, Synthesizing nanoscale iron particles for rapid and complete dechlorination of TCE and PCBs, Environ. Sci. Technol. 31 (1997) 2154–2156. [23] J. Xu, Z.W. Hao, C.S. Xie, X.S. Lv, Y.P. Yang, X.H. Xu, Promotion effect of Fe2+ and Fe3O4 on nitrate reduction using zero-valent iron, Desalination 284 (2012) 9–13. [24] T. Suzuki, M. Moribe, Y. Oyama, M. Niinae, Mechanism of nitrate reduction by zerovalent iron: equilibrium and kinetics studies, Chem. Eng. J. 183 (2012) 271–277. [25] W. Sigmund, Particulate Systems in Nano- and Biotechnologies, CRC Press, New York, 2009. [26] L. Xiao-qin, D.W. Elliott, W.X. Zhang, Zero-valent iron nanoparticles for abatement of environmental pollutants: materials and engineering aspects, Cent. Adv. Mat. Nanotechnol. 31 (2006) 111–122. [27] M.J. Alowitz, M.M. Scherer, Kinetics of nitrate, and Cr (VI) reduction by iron metal, Environ. Sci. Technol. 36 (2002) 299–306. [28] H.-Y. Hu, N. Goto, K. Fugie, Effect of pH on the reduction of nitrite in water by metallic iron, Water Res. 35 (2001) 2789–2793. [29] G.C. Yang, H.L. Lee, Chemical reduction of nitrate by nanosized iron: kinetics and pathways, Water Res. 39 (2005) 884–894. [30] C.P. Huang, H.W. Wang, P.C. Chiu, Nitrate reduction by metallic iron, Water Res. 32 (1998) 2257–2264. [31] L. Feng, F. Jing, G. Yanhui, F. Maohong, W. Jianji, Y. Hongqun, Reduction of nitrite by ultrasound-dispersed nanoscale zero-valent iron particles, Ind. Eng. Chem. Res. 47 (2008) 8550–8554. [32] G. Cravotto, P. Cintas, Power ultrasound in organic synthesis: moving cavitational chemistry from academia to innovative and large-scale applications, Chem. Soc. Rev. 35 (2006) 180–196. [33] G. Kianpoura, M. Salavati-Niasaria, H. Emadi, Ultrason. Sonochem. 20 (2013) 418–424. [34] M.R. Jamei, R.M. Khosravi, B. Anvaripour, Investigation of ultrasonic effect on synthesis of nano zero valent iron particles and comparison with conventional method, J. Chem. Eng. 8 (2013) 767–774. [35] M.P. Deosarkara, S.M. Pawara, B.A. Bhanvaseb, In situ sonochemical synthesis of Fe3O4–graphene nanocomposite for lithium rechargeable batteries, Chem. Eng. Process. 83 (2014) 49–55. [36] A. Ryu, S.W. Jeong, A. Jang, H. Choi, Reduction of highly concentrated nitrate using nanoscale zero-valent iron: effects of aggregation and catalyst on reactivity, Appl. Catal. B Environ. 105 (2011) 128–135. [37] H.E. Romero, P. Joshi, A.K. Gupta, H.R. Gutierrez, M.W. Cole, S.A. Tadigadapa, P.C. Eklund, Adsorption of ammonia on graphene, Nanotechnology 20 (2009) 245501. [38] F. Schedin, A.K. Geim, S.V. Morozov, E.W. Hill, P. Blake, M.I. Katsnelson, K.S. Novoselov, Detection of individual gas molecules adsorbed on graphene, Nat. Mater. 6 (2007) 652–655. [39] C. Seunghee, C. Yoon-Young, H. Kyung-Yub, K. Jeehyeong, Kinetics of reductive denitrification by nanoscale zero-valent iron, Chemosphere 41 (2000) 1307–1311.
Contents lists available at ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Removal of nitrate ions from aqueous solution using zero-valent iron nanoparticles supported on high surface area nanographenes Mohamed Abdel Salam a,⁎, Olfat Fageeh a,b, Shaeel A. Al-Thabaiti a, Abdullah Y. Obaid a a b
Chemistry Department, Faculty of Science, King Abdulaziz University, P.O Box 80200, Jeddah 21589, Saudi Arabia Center of Excellence in Environmental Studies, King Abdulaziz University, P.O. Box 80216, Jeddah 21589, Saudi Arabia
a r t i c l e
i n f o
Article history: Received 6 July 2015 Received in revised form 8 September 2015 Accepted 17 September 2015 Available online xxxx Keywords: Catalysis Mechanism Nitrate Nanographene Reduction Zero-valent iron nanoparticles
a b s t r a c t In this manuscript, zero-valent iron nanoparticles (ZVINPs) were synthesized, and then supported on high surface area nanographenes (NGs) to prepare a stable ZVINP/NG nanocomposite. XRD measurements showed the stabilization of the ZVINPs upon their support on NGs, which enhanced their activity. The ZVINP/NG nanocomposite was used for the catalytic reduction of nitrate ions in model solution, and the results showed the dependency of the removal process on the ratio of ZVINPs to NGs in the nanocomposite, time of removal, and solution pH. The effect of ultrasonication was also explored, and the results showed that ultrasonication could significantly decrease the removal time required by the nanocomposite. The mechanism of nitrate reduction by ZVINP/NG nanocomposite was studied, and the results showed the conversion of nitrate to nitrite and/or ammonia, then to nitrogen gas. © 2015 Published by Elsevier B.V.
1. Introduction The increase in demand for water and food supplies is placing increasing stress on ground and surface water quality and quantity. One environmental problem that has become an increasingly important issue in developed and developing countries is nitrate contamination of surface water and groundwater [1,2]. Although groundwater is an important source of water in the Kingdom of Saudi Arabia, nitrate contamination is considered a limitation for using groundwater. Accumulation of high levels of nitrate that reach the water surface is contributing to depleting dissolved oxygen, which kills fish and creates a harmful environment for humans [3–5]. Nitrates possess a high risk to human health and have been listed on the USEPA's Drinking Water Contaminant List with the maximum contaminant level of 10 mg/L as nitrogen [6]. Nitrate is considered relatively nontoxic to the human body, but its toxicity is due to its reduction to nitrite by denitrifying bacteria in the upper gastrointestinal tract, which may cause many diseases, such as blue baby syndrome and cancer [1,2,7–9]. These nitrites transform hemoglobin to methemoglobin by oxidation of ferrous iron (Fe2+) in hemoglobin to ferric form (Fe3 +), preventing or reducing the ability of blood to transport oxygen, which causes cyanosis and anoxemia [10]. Although nitrate could be removed from aquatic systems by different methods such as adsorption [11], catalytic reduction using monometallic
⁎ Corresponding author. E-mail address: [email protected] (M.A. Salam).
http://dx.doi.org/10.1016/j.molliq.2015.09.029 0167-7322/© 2015 Published by Elsevier B.V.
particles [12] or bimetallic particles [13], and photocatalytic reduction [14], the search for a cost-effective method to remove nitrate from groundwater is a crucial issue for the health of the aqueous environment. Chemical methods using nano zero-valent metals, such as Fe, Al, Zn, and Mg, have been frequently used for the reduction of nitrate anions [15]. Among these zero-valent metals, iron possesses a great importance for this purpose because it possesses a high specific surface area, which leads to high surface reactivity. Also, because of its high reduction potential, zero-valent iron (ZVI) is considered to be an important remediation reagent, and its chemical reaction steps occur relatively rapidly. In recent years there has been growing interest in applications of ZVI to groundwater remediation, which has helped to make ZVI the most widely studied reductant chemical for environmental applications [16]. However, one problem associated with the application of ZVI for reduction of nitrate is the low stability of zero-valent iron, especially in its nanoparticle form. Zero-valent iron nanoparticles (ZVINPs) are very reactive and oxidize easily to iron oxides, which decreases their reduction ability. In the present work, the catalytic reduction of nitrate ions using ZVINPs supported on high surface area nanographene (ZVINP/NG) nanocomposite was studied. The effects of different factors that affect the efficiency of nitrate removal were investigated, namely the mass ratios of ZVINPs and NGs, mass of the ZVINP/NG nanocomposite, solution pH, contact time, and effect of ultrasonication on the removal process. In addition, the catalytic reduction mechanism of the nitrate ions using ZVINPs and ZVIPN/NG nanocomposite was explored and verified.
M.A. Salam et al. / Journal of Molecular Liquids 212 (2015) 708–715
where, Cο, Ce, Ms., and V are the initial concentration (mg/L), is the equilibrium concentration (mg/L), the mass of ZVINPs (g), and the volume of solution (L), respectively.
2. Experimental 2.1. Materials High surface area nano graphene (NGs) were obtained from XG Science, USA. xGnP® and were used as received. Ferric chloride (60% w/v, BDH Chemicals Ltd., Poole, England), sodium borohydride (BDH Chemicals Ltd., Poole, England), were used for the preparation of ZVINPs. Nitrate stock solution of 1000 mg L−1 was prepared using sodium nitrate obtained from (Fisher Scientific Company). Sodium hydroxide and hydrochloric acid (BDH Chemicals Ltd., Poole, England) were used to adjust the solution pH and using pH meter Mettler Toledo AG (Switzerland). All solutions were prepared using deionized water and degassed by purging with nitrogen gas. 2.2. Synthesis of ZVINPs The synthesis process was conducted in three necked-flask reactor. One of the necks was attached to nitrogen gas. 120 mL sodium borohydride (NaBH4) of 0.16 M was added drop-wise to 200 mL of 0.1 M ferric chloride solution (FeCl3) with continuous stirring and flowing of nitrogen gas to prevent oxidation of iron. The produced ZVINPs were collected by magnet, and then it was washed 3 times by deionized water and finally dried over night at 110°°C. Reduction reaction of ferric (Fe3+) into zero-valent iron occurred according to the equation [17]. 4Fe3þ þ 3BH4− þ 9H2 O→4Fe0 þ 3H2 BO3− þ 12Hþ þ 6H2
ð1Þ
2.3. Preparation of ZVINP/NG nanocomposite The nanocomposites of ZVINP/NG were prepared by mixing ZVINPs with an appropriate quantity of NGs using a mortar and the mixture was well grinded for 30 min to ensure homogeneity. 2.4. Characterizations TEM (JEOL 2100F) operating at 200 kV with a Field Emission Gun, obtaining a point resolution of 0.19 nm. X-ray diffraction (XRD) patterns were recorded for phase analysis and the measurement of crystallite size on a Philips X pert pro diffractometer, which was operated at 40 mA and 40 kV by using CuKα radiation and a nickel filter in the 2θ range from 2 to 80° in steps of 0.02°, with a sampling time of one second per step. The specific surface area was determined from nitrogen adsorption/desorption isotherms which were measured at 77 K by using a Nova 2000 series Chromatech. Prior to analysis, the samples were outgassed at 250°°C for 4 h. 2.5. Reduction of nitrate ions Reduction of nitrate was studied using ZVINPs and ZVINP/NG nanocomposite. Weighed amounts of the catalyst (0–30 mg), was added into glass vials containing 10 mL of 50 mg/L sodium nitrate at neutral pH, then the solutions were shacked for 24 h at a speed of 150 rpm at room temperature. After desired time passed, the suspensions were filtered through 0.22 μm Millipore syringe filters and the solutions concentration were then measured using ion chromatography (Dionex ICS-2100 system). The percentage nitrate ions removed and the nitrate removal capacity (qe) were calculated according to Eqs. (2) and (3), respectively. %Removal ¼
qe ðmg=g Þ ¼
C 0 −C e 100 C0
C 0 −C e V Ms
709
ð2Þ
ð3Þ
3. Results and discussion 3.1. Characterization of ZVINP/NG nanocomposite The structure and composition of the prepared ZVINPs, NGs, and ZVINP/NG nanocomposite were investigated using TEM, XRD, and surface area analysis. Fig. 1 shows the morphological structures of the prepared ZVINP/NG nanocomposite using TEM at different magnifications. The TEM showed homogenous dispersion of the prepared ZVINPs over the transparent NGs surface. ZVINPs appeared to be spherical in shape with non-uniform size and an average diameter of 50 nm. It is clear from the TEM images that ZVINPs tended to be distributed well over the NG surface and did not tend to agglomerate. The XRD patterns of the ZVINPs, NGs, and ZVINP/NG nanocomposite are shown in Fig. 2a,b, & c. The diffraction peaks of ZVINPs at Fig. 2a revealed the presence of three different iron-based compounds: synthetic zero-valent iron (α-Fe(0), ref. JCPDS 06-0696), maghemite (Fe2O3, ref. JCPDS 13-0458), and synthetic magnetite (Fe3O4, ref. JCPDS 88-0315), with their corresponding peaks. Although it was believed that only ZVINPs were synthesized, but because of the high reactivity of the ZVINPs; in addition to the time to analyze the sample with the XRD, the ZVINPs were oxidized to form different iron oxides: maghemite and synthetic magnetite. Also, the diffraction peaks at 2θ angles of 26.3–54.1° in the XRD pattern of NGs revealed the presence of pristine graphene in the form of nanoplatelets, same as that of native graphite (JCPDS No. 75-1621); as it was presented in Fig. 2b. The pristine graphene nanoplatelets showed a (0 0 2) diffraction peak at 26.3° (2θ), corresponding to a dspacing of 33.8 nm. As it is presented in Fig. 2c; surprisingly, the XRD pattern of ZVINP/NG nanocomposite showed the characteristic peaks of graphene nanoplatelet diffraction at 26.3° (2θ), in addition to the characteristic diffraction peak of the ZVINPs only at 44.4° (2θ), corresponding to a d-spacing of 20.4 nm, without any characteristic peaks of iron oxides diffraction pattern. This is clear evidence that the NGs greatly stabilized the ZVINPs by π-electrons present at the NG surface, which prevented the oxidation of the ZVINPs and facilitated their storage till the application time. The specific surface areas of the NGs, ZVINPs, and prepared ZVINP/NG nanocomposite were measured using the nitrogen adsorption/desorption isotherms at 77 K. The measurements showed that the BET-specific surface areas were 677.5 m2/g, 48.0 m2/g, and 569.2 m2/g for the NGs, ZVINPs, and prepared ZVINP/NG nanocomposite, respectively. It is obvious that the NGs had a high surface area, which was reduced upon mixing them with the ZVINPs. Also as was expected, the surface area of the ZVINPs was greatly enhanced upon mixing them with the NGs, which would enhance the activity and efficiency of the ZVINPs for catalytic reduction of nitrate ions in aqueous solution. 3.2. Reduction of nitrate by ZVINPs and ZVINP/NG nanocomposite 3.2.1. Effect of composite mass Because the removal of nitrate by ZVINPs involves a reaction on the metal surface, the iron-to-nitrate ratio (gram nanoscale iron/milligram nitrate) is a significant variable parameter [18]. As shown in Fig. 3, the percent nitrate removal was greatly enhanced by increasing the ZVINP mass. When 1.0 mg of ZVINPs was used, it was accompanied by percent nitrate removal of 3.8%. This removal percentage was enhanced by increasing the mass of ZVINPs till it reached equilibrium when 20.0 mg ZVINPs were used and 92.0% removal efficiency was achieved. Further increase in the amount of ZVINPs used was associated with a slight enhancement in the percentage of nitrate removed from aqueous solution. This is mainly because increasing the mass of ZVINPs provides more active sites of iron nanoparticles available for nitrate reduction. This
710
M.A. Salam et al. / Journal of Molecular Liquids 212 (2015) 708–715
of ZVINPs and NGs was prepared with a total mass of 30 mg. It was observed that pristine NGs alone could not remove any of the nitrate ions from the solution because of the electrostatic repulsion between the negatively charged nitrate ions and NG surface, which is mainly covered with the negatively charged π electrons. This confirmed that NGs have no role in nitrate removal, as the removal involved the reduction of nitrate ions at the ZVINP surface only. Also, the results revealed the gradual and significant increase in percent removal of nitrate with the increasing mass ratio of ZVINPs in the ZVINP/NG nanocomposite. Meanwhile, using ZVINP/NG nanocomposite with mass ratios of 1:29 mg, 2:28 mg, 3:27 mg, 4:26 mg, and 5:25 mg showed percent removal of 4.5%, 18.6%, 34.7%, 54%, and 71.8%, respectively. A further increase of ZVINP content in ZVINP/NG nanocomposite led to equilibrium; mass ratios of 10:20, 15:15, and 20:10 (as mg) showed percent removal values of 83.5%, 90.5%, and 93.2%, respectively. On the other hand, Fig. 4 shows the amount of nitrate ions removed from aqueous solution based on the amount of ZVINPs in ZVINP/NG nanocomposite. From this result it is found that as ZVINPs content increases in ZVINP/NG nanocomposite, the amount of nitrate removed was enhanced until the ZVINP-to-NG ratio was 5:25 with 71.8 mg/g of nitrate removed. The further increase in ZVINPs content in the ZVINP/NG composite caused a significant decrease in the amount of nitrate removed; 41.8 mg/g, 30.2 mg/g, and 23.3 mg/g with mass ratios of 10:20, 15:15, and 20:10, respectively. This result indicated that the ZVINP/NG nanocomposite with a mass ratio of 5:25 had the highest efficiency for the removal of nitrate based on the amount removed, and this finding agreed well with previous research [20]. It is noteworthy to mention that the percent nitrate removed from solution was enhanced 125.1% upon using NGs as the support for ZVINPs; percent nitrate removed went from 57% to 71.8%. This means that the catalytic reduction of nitrate ions for supported ZVINPs was higher than that of unsupported ZVINPs, and this result is in good agreement with previous research study [21]. This attributed to the enhancement in the specific surface area of the ZVINPs compared with the ZVINP/NG nanocomposite – 200.4 m2/g to 569.0 m2/g, respectively – which greatly enhanced the reactivity of the ZVINPs for nitrate reduction due to the dispersion of the ZVINPs over the NGs plates as was shown in the TEM images [22]. Also, as was confirmed by the XRD measurements that supporting ZVINPs by the NG plates greatly enhanced the stability of the ZVINPs and made them available only for the reduction of nitrate rather than allowing them to be oxidized by atmospheric oxygen. Therefore, it could be concluded that using NGs as the supporting material for ZVINPs could achieve much better nitrate removal efficiency compared with using ZVINPs alone. In addition to the above, NGs stabilize the dispersion of ZVINPs in solution and preventing ZVINPs from being aggregated. All these factors might contribute to the significant enhancement of nitrate reduction by ZVINPs after supporting them on NGs.
Fig. 1. TEM images of the ZVINP/NG nanocomposite at different magnifications.
agreed well with previous studies that reported that increasing the amount of iron particles in solution speeds up the initial reaction because it provides more active sites that can be used for collision of nitrate ions during the reduction [18,19]. Hence, it could be concluded here that the optimum amount of ZVINPs for nitrate removal is 30 mg. As discussed earlier, mixing ZVINPs with NGs was expected to form a more active nanocomposite with high stability and efficiency for the reduction of nitrate ions in aqueous solution. Hence, a different mass ratio
3.2.2. Effect of pH The effect of solution pH ranging from 2 to 12 on the removal of nitrate was explored, and the results were presented in Fig. 5. The results revealed that optimum nitrate removal was obtained at solution pH of 2, and as solution pH increased, there was accompanied by a significant decrease in percent nitrate removal. At pH 2, the percent removal of nitrate reached 89.4%, and it decreased to 79.6%, 74.7%, 70.3%, and 69.9% at pH values of 4, 6, 7, and 8, respectively. A further pH increase was associated with a further decrease in the percent nitrate removal, which reached 53.8% at pH 12. This significant decrease in the percent nitrate removal by increasing the solution's pH could be mainly attributed to the fact that reduction of nitrate by ZVINPs is a very rapid process in acidic solution [23]. Suzuki et al. reported that reduction of nitrate using ZVI involved a direct transport of electrons from ZVI into nitrate [24], as ZVI is an effective electron donor. At low pH, iron oxides present at the surface of ZVINPs are positively charged and attract anions such as nitrate, and simultaneously lowering the solution's pH would dissolve the ferrous hydroxide and other protective layers at the surface of the ZVINPs, yielding more fresh reactive sites for chemical reduction of
M.A. Salam et al. / Journal of Molecular Liquids 212 (2015) 708–715
711
Fig. 2. XRD patterns of the ZVINPs, NGs, and ZVINP/NG nanocomposite.
nitrate. High pH values are not favorable for nitrate reduction because at high pH values iron hydroxide precipitates, leading to formation of a passive layer on the iron particle surface caused by Fe(OH)2 and Fe(OH)3 precipitation, which deactivates the ZVINPs and consequently
decreases the percentage of nitrate removed [25,26]. Therefore, based on the present experimental results, in general, the efficiency of nitrate removal by ZVINPs increased as the solution pH decreased, which agreed well with previous studies [27–29]. Accordingly, it could be
712
M.A. Salam et al. / Journal of Molecular Liquids 212 (2015) 708–715
Fig. 3. Effect of ZVINPs mass on the % nitrate removed from aqueous solution. (Experimental conditions: 10 mL of 50 mg/L NaNO3 solution, 298 K, 24 h and pH 7).
concluded that nitrate reduction by ZVINPs is an acid-driven process [29]. 3.2.3. Effect of time The effect of contact time was studied at 296 K, and the results were presented in Fig. 6a. The figure showed a gradual and significant increase in the removal efficiency of nitrate with increasing contact time till it reached equilibrium after 16 h with removal efficiency of 67.9% and removal capacity of 67.9 mg NO− 3 /g ZVINPs. The gradual increase may be due to the fact that the initial reactive sites for nitrate reduction on the metal surface were vacant and the nitrate concentration gradient was high until all the active sites on the ZVINP/NG nanocomposite were occupied. A further increase in contact time was accompanied by a slight increase in the amount nitrate ions removed by the ZVINP/NG nanocomposite; after 40 h, the percent nitrate removed was 70.4%. This result revealed that reduction of nitrate reached equilibrium within 16 h, which agreed well with one of the previous studies showing that nitrate removal is negligible after 10 h at pH N 4 [30].
Fig. 5. Effect of pH on nitrate removal by ZVINP/NG nanocomposite from aqueous solution. (Experimental conditions: 30 mg of ZVINP/NGs, 10 mL of 50 mg/L NaNO3 solution, 298 K, 24 h and pH 2–12).
result revealed that ultrasonication decreases the time required for the reduction of nitrate with much better removal efficiency compared with the removal process in the absence of ultrasonication. This may be because better mixing and dispersion of ZVINP/NG nanocomposite can be obtained by ultrasonication, which enormously accelerates the heterogeneous reaction between ZVINPs and nitrate ions. In addition, the acoustic cavitation generated by ultrasonication and the implosive collapse of bubbles in liquids played an important role in the
3.2.4. Effect of ultrasonication Ultrasonication has become an important variable that has distinct effects on different aspects of chemistry. Fig. 6b showed the effect of ultrasonication time on nitrate removal by ZVINP/NG nanocomposite from aqueous solution. It is clear that increasing the ultrasonication time was accompanied by a significant increase in the percent nitrate removed from solution, and equilibrium was achieved within 3 h with removal efficiency of 87.9%. A further increase in the ultrasonication time was associated with insignificant enhancement in the percent nitrate removed from solution until it reached 90.8% after 5 h. This
Fig. 4. Efficiency of nitrate removal at different composition ZVINP/NG nanocomposite ratios. (Experimental conditions: 10 mL of 50 mg/L NaNO3 solution, 298 K, 24 h and pH 7).
Fig. 6. Effect of contact time on nitrate removal (%) by ZVINP/NG nanocomposite from aqueous solution (a) at normal condition, (b) with ultrasonication. (Experimental conditions: 30 mg of ZVINP/NG, 10 mL of 50 mg/L NaNO3 solution, 298 K, 0–40 h and pH 7).
M.A. Salam et al. / Journal of Molecular Liquids 212 (2015) 708–715
improvement of nitrate reduction by ZVINPs. The implosive collapse of bubbles creates localized, short-lasting hot spots with high pressure through adiabatic compression or shock-wave formation within the gas phase of the collapsing bubbles; these localized hot spots can improve the mixing and dispersion of ZVINPs and thus enormously accelerate the heterogeneous reaction between ZVINPs and nitrite [31]. This finding is in agreement with Cravotto et al., that ultrasoundaccelerated chemical reactions could be carried out under ultrasound irradiation to give higher results in shorter reaction times and milder conditions [32]. This large removal percentage is attributed to the fact that using ultrasound waves causes a decrease in particle size, which effectively enhances particles' reactivity in addition to increasing surface area. High-intensity ultrasound (100 kHz) is used to induce a chemical or physical change on the target medium [33]. Ultrasonication could provide the solution with a high rate of mixing and mass transfer through particles [34]. Due to decreasing particle size by ultrasonication, the loading of ZVINPs on graphene sheets would be uniform, and further ultrasonication could be responsible for reducing the agglomeration tendency of graphene nanosheets and making the loading of ZVINPs easier [35]. According to the above results, the optimum time required for removal of nitrate without ultrasonication was 16 h with removal percentage of 67.9%. This time was significantly reduced by ultrasonication, and the optimum time required to attain equilibrium was 3.0 h with a removal percentage of 90.8%. Hence, ultrasonication could reduce the time required for nitrate removal to almost one-fifth that of the removal process without ultrasonication. It is noteworthy to mention that the ultrasonication process was carried out mainly at room temperature.
69% after 40 h, as shown in Fig. 7b. This improved result was obtained using graphene as a supporting material, which increased surface area, stability, and dispersion of ZVINPs in the solution, hence improving the reduction reaction between ZVINPs and nitrate ions. According to previous studies, nitrate ions can be reduced into nitrite ions, ammonium ions, and/or nitrogen gas. Nitrite ions were detected with a very low concentration in the first stage of the reaction in Fig. 7a and b; their concentration was less than 0.1% in both cases and then disappeared completely. This may be attributed to the fact that nitrate was reduced into nitrite very quickly, and nitrite is considered an intermediate [35]. Nitrite can also be reduced into ammonium, according to Eq. (7), or into nitrogen gas, according to Eq. (8) [18]. 0 þ 2þ þ NHþ NO− 2 þ 3Fe þ 8H →3Fe 4 þ 2H2 O
ð7Þ
þ 2þ þ 4H2 O þ N2 ðgÞ 3Fe0 þ 2NO− 2 þ 8H →3Fe
ð8Þ
On the other hand, the amount of ammonium ions was identified and monitored as shown in Fig. 7a by using ZVINPs alone; about 20% of ammonium ions were detected in the solution after 24 h, and this amount decreased to 16% after 40 h of the reaction. Ammonium ions can be produced by the reduction of nitrite, as previously discussed, or by direct reduction of nitrate, as shown in Eqs. (9) and (10) [36]. þ − 4Fe0 þ NO− 3 þ 7H2 O→4FeðOHÞ2 þ NH4 þ 2OH
þ − 3Fe0 þ NO− 3 þ H2 O→Fe3 O4 þ NH4 þ 2OH
3.3. Mechanism of nitrate reduction using ZVINP/NG nanocomposite
713
ð9Þ
ð10Þ
The removal of nitrate from water involves three main steps, adsorption of nitrate onto the ZVINP surface, reduction, and finally desorption into the solution. There are two possible mechanisms proposed for nitrate reduction using ZVINPs, either: a) direct transfer of electrons from metallic iron, as presented in Eqs. (4) and (5), or b) indirect electron transfer by hydrogen atoms yielding corrosion, as in Eq. (6) [18]. 3Fe0 →3Fe2þ þ 6e−
ð4Þ
0 þ 2þ þ NO− NO− 3 þ Fe þ 2H →Fe 2 þ H2 O
ð5Þ
þ þ NO− 3 þ 4H2 þ 2H →NH4 þ 3H2 O
ð6Þ
An acidic medium is favorable for nitrate reduction using ZVINPs because an acidic medium usually dissolves the iron oxides and other protective layers, hence the regenerated Fe0 will reduce nitrate throughout the cavities produced on the metal surface where nitrate was adsorbed into these cavities. In contrast, the basic medium is not favorable for nitrate reduction, as Fe2+ will transform into ferrous hydroxide that is thermodynamically unstable and may be further oxidized into magnetite. The accumulation of magnetite on the Fe0 surface may slow down the reaction when pH is higher than 6–7 [21]. Xu et al. reported that at neutral pH, the reduction reaction would be slowed down because of consumption of H+ [23]. In the present work, the various nitrogen species were identified and monitored during the reduction reaction time using ZVINPs and ZVINP/NG nanocomposite, as presented in Fig. 7a and b, respectively. As presented in Fig. 7a, when ZVINPs were used alone, after 24 h of reaction the percent removal of nitrate reached 56.0%, and after 40 h it reached 63%. Meanwhile, when ZVINP/NG nanocomposite was used, the percent removal of nitrate was greatly enhanced, as the percent removal reached 68% after 24 h and reached
Fig. 7. Amount of nitrogen species during nitrate reduction using (a) 5 mg of ZVINPs only, and (b) 30 mg of ZVINP/NG nanocomposite.
714
M.A. Salam et al. / Journal of Molecular Liquids 212 (2015) 708–715
At the same time, the most important phenomenon is that there were no detectable ammonium ions in the solution when ZVINP/NG nanocomposite was used. The amount of ammonium reached 3.0% after 24 h of reaction, while at the end of reaction, after 40 h, it was completely removed. The reason for this phenomenon is the adsorption of positively charged ammonium ions onto the negatively charged nanographene surface. Hence, using ZVINP/NG nanocomposite doesn't require further treatment to remove the by-products of the nitrate reduction by ZVINPs. Romero et al. reported that a small charge transfer occurs because of adsorption of NH3 onto the graphene surface [37], and the same result was confirmed by Schedin et al. [38]. Moreover, it was found that there is a great difference between using ZVINPs alone and using ZVINPs nanocomposite supported onto NGs graphene, at which 90% of the produced ammonium can be removed from solution by adsorption onto graphene surface when ZVINP/NG nanocomposite was used. Meanwhile, by using ZVINPs alone all the produced ammonium was detected in solution and can't be removed. In addition, the total nitrogen balance for all nitrogen species produced from the reduction reaction in the solution doesn't equal the initial concentration, at which from Fig. 7a total nitrogen showed 54% of nitrogen species is found in the solution after equilibrium; 24 h, and there is about 45% couldn't be detected in solution. The same observation was found in the case of using ZVINP/NG nanocomposite; the total nitrogen balance detected was 32% of nitrogen species in the solution, while 68% of nitrogen species was undetectable, as presented in Fig. 7b. From our results, it was found that the amount of undetectable nitrogen species was 45% in the case of using ZVINPs alone, while this amount increased in the case of using ZVINP/NG nanocomposite to 68% because of the adsorption of the produced ammonium ions onto the graphene surface. In these observations, the overall amount of nitrogen species was less than the initial ones, which implies that the missing amount was mainly attributable to generation of nitrogen gas as the final product of the nitrate ions reduction by the ZVINPs. Nitrogen gas may be produced either by direct reduction of nitrate, as shown in Eq. (11) [19], or by reduction of nitrite, as was previously discussed. The conversion of nitrate into nitrogen gas is favorable thermodynamically [39]. Unfortunately, the amount of nitrogen gas produced couldn't be identified or analyzed because of the nature of the open system of the reaction, and
because of a shortage of lab instrumentation, the only way to detect nitrogen gas was by using GC–MS. þ − 5Fe0 þ 2NO− 3 þ 6H2 O→5Fe2 þ N2 ðgÞ þ 12OH
ð11Þ
In general, the reduction mechanism using ZVINP/NG nanocomposite didn't differ significantly from what occurred using ZVINPs alone, except in two important ways: the amount of removed nitrate increased with the presence of NGs as the support catalyst, and the amount of ammonium released differed because of adsorption of ammonium onto the graphene surface in the case of using the ZVINP/NG nanocomposite. In conclusion, Fig. 8 represents the schematic of the possible mechanism of the catalytic reduction of nitrate ions by ZVINP/NG nanocomposite. These great and promising results show the importance of using these NGs as a support for the reduction of nitrate ions in aqueous solution using ZVINPs. The advantage of using graphene is that it not only increases the surface area and prevents aggregation and oxidation of ZVINPs, but it can also solve a major problem that arises from the accumulation of ammonium ions released from the reduction of nitrate using ZVINPs, and as a result further treatment is not required when using ZVINP/NG nanocomposite for the reduction of nitrate.
4. Conclusions ZVINPs were synthesized then supported on high surface area nanographenes (NGs) to obtain ZVINP/NG nanocomposite. The results showed the presence of ZVINPs in a spherical shape with average particle size of 50 nm and homogenous distribution of the ZVINPs on the transparent NG surface. The XRD measurements showed that the ZVINPs alone had low stability but greatly stabilized upon their support on the NG surface, which provided the ZVINPs with electrons and prevented their oxidation. The surface area analysis showed the high surface area of ZVINPs, NGs, and ZVINP/NG nanocomposite; 200.4 m2/g, 677.5 m2/g, and 569.2 m2/g, respectively. The ZVINP/NG nanocomposite was used for the catalytic reduction of nitrate ions in model solution, and the effects of different parameters that affect the removal process were studied. Most of the nitrate ions could be removed
Fig. 8. Schematic representation of the possible mechanism of the catalytic reduction of nitrate ions by ZVINP/NG nanocomposite.
M.A. Salam et al. / Journal of Molecular Liquids 212 (2015) 708–715
from solution using ZVINP/NG nanocomposite in a mass ratio of 5:25, and supporting ZVINPs on the NG surface enhanced the removal process by 125.1% compared with using ZVINPs alone. Also, the results showed that the removal process via catalytic reduction is an acid-driven process, as the optimum removal was obtained at acidic pH values. Most of the nitrate ions could be removed within 16 h at ambient conditions, and this time could be shortened to 3 h by ultrasonication. The mechanism of nitrate reduction by ZVINPs was explored, and the results showed that nitrate ions could be reduced to nitrite and/or ammonium, which then reduced to nitrogen gas. Supporting ZVINPs on NGs greatly enhanced the removal of nitrate because they enhanced their stability and surface area and prevented their aggregation and agglomeration. The NGs were also able to adsorb the ammonium produced from the reduction of nitrate in the solution. Acknowledgments This project was funded by Saudi Basic Industries Corporation (SABIC) and the Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah, Saudi Arabia, under grant no. 15/260/1434/MS. The authors, therefore, acknowledge with thanks SABIC and DSR technical and financial support. References [1] P. Mikuska, Z. Vecera, Simultaneous determination of nitrite and nitrate in water by chemiluminescent flow-injection analysis, Anal. Chim. Acta 495 (2003) 225–232. [2] A. Rezaee, H. Godini, S. Dehestani, A. Khavanin, Application of impregnated almond shell activated carbon by zinc and zinc sulfate for nitrate removal from water, Iran. J. Health Sci. Eng. 5 (2008) 125–130. [3] N.N. Rabalais, R.E. Turner, D. Scavia, Beyond science into policy: Gulf of Mexico hypoxia and the Mississippi River, Bioscience 52 (2002). [4] D. Scavia, K.A. Donnelly, Reassessing hypoxia forecasts for the Gulf of Mexico, Environ. Sci. Technol. 41 (2007) 8111–8117. [5] N.N. Rabalais, R.E. Turner, Hypoxia in the Northern Gulf of Mexico: Coastal Hypoxia: Consequences for Living Resources and Ecosystems, Coastal and Estuarine Studies 58, American Geophysical Union, Washington, D.C., 2001 [6] C.o.T. National Research Council, Nitrate and Nitrite in Drinking Water, National Academies Press, Washington, DC, 1995. [7] R. Grommena, I.V. Hauteghem, M.V. Wambeke, W. Verstraete, An improved nitrifying enrichment to remove ammonium and nitrite from freshwater aquaria systems, Aquaculture 211 (2002) 115–124. [8] W.R. Melchert, C.M.C. Infante, F.R.P. Rocha, Development and critical comparison of greener flow procedures for nitrite determination in natural waters, Microchem. J. 85 (2007) 209–213. [9] A. Afkhami, Adsorption and electrosorption of nitrate and nitrite on high-area carbon cloth: an approach to purification of water and wastewater samples, Letters to the Editor/Carbon, 41 2003, pp. 1309–1328. [10] C.S. Bruning-fann, J.B. Kaneene, The effects of nitrate, nitrite, and N-nitroso compounds on animal health, Vet. Hum. Toxicol. 35 (1993) 237–253. [11] A. Bhatnagar, M. Sillanpaa, Applications of chitin- and chitosan-derivatives for the detoxification of water and wastewater — a short review, Adv. Colloid Interf. Sci. 152 (2009) 26–38. [12] F.S. Fateminiaa, C. Falamakib, Zero valent nano-sized iron/clinoptilolite modified with zero valent copper for reductive nitrate removal, Process. Saf. Environ. Prot. (2013) 304–310.
715
[13] L. Durivault, O. Brylev, D. Reyter, M. Sarrazin, D. Bélanger, L. Roué, Cu–Ni materials prepared by mechanical milling: their properties and electrocatalytic activity towards nitrate reduction in alkaline medium, J. Alloys Compd. 432 (2007) 323–332. [14] D. Kyle, Environmentally Responsible Use of Nanomaterials for the Photocatalytic Reduction of Nitrate in Water, (PhD thesis) of ARIZONA STATE UNIVERSITY, 2013. [15] S.F. Cheng, S.C. Wu, The enhancement methods for the degradation of TCE by zerovalent metals, Chemosphere 41 (2000) 1263–1270. [16] H.J. Sunge, I. Francis, Nano Technology for Environmental Remediation, 5, Springer Science 2006, pp. 5–42. [17] T.C. Zhang, Y.H. Huang, Effects of selected Good's pH buffers on nitrate reduction by iron powder, J. Environ. Eng. 131 (2005) 461–470. [18] J.H. Zhang, Z.W. Hao, Z. Zhang, Y.P. Yang, X.H. Xu, Kinetics of nitrate reductive denitrification by nanoscale zero-valent iron, Process Saf. Environ. 88 (2010) 439–445. [19] F. Xiaomeng, G. Xiaohong, M. Jun, A. Hengyu, Kinetics and corrosion products of aqueous nitrate reduction by iron powder without reaction conditions control, J. Environ. Sci. 21 (2009) 1028–1035. [20] F. Liu, J.H. Yang, J. Zuo, D. Ma, L. Gan, B. Xie, P. Wang, B. Yang, Graphene-supported nanoscale zero-valent iron: removal of phosphorus from aqueous solution and mechanistic study, J. Environ. Sci. 26 (2014) 1751–1762. [21] W.-X. Zhang, D.W. Elliott, Applications of iron nanoparticles for groundwater remediation, Remediat. J. 16 (2006) 7–21. [22] C.B. Wang, W. Zhang, Synthesizing nanoscale iron particles for rapid and complete dechlorination of TCE and PCBs, Environ. Sci. Technol. 31 (1997) 2154–2156. [23] J. Xu, Z.W. Hao, C.S. Xie, X.S. Lv, Y.P. Yang, X.H. Xu, Promotion effect of Fe2+ and Fe3O4 on nitrate reduction using zero-valent iron, Desalination 284 (2012) 9–13. [24] T. Suzuki, M. Moribe, Y. Oyama, M. Niinae, Mechanism of nitrate reduction by zerovalent iron: equilibrium and kinetics studies, Chem. Eng. J. 183 (2012) 271–277. [25] W. Sigmund, Particulate Systems in Nano- and Biotechnologies, CRC Press, New York, 2009. [26] L. Xiao-qin, D.W. Elliott, W.X. Zhang, Zero-valent iron nanoparticles for abatement of environmental pollutants: materials and engineering aspects, Cent. Adv. Mat. Nanotechnol. 31 (2006) 111–122. [27] M.J. Alowitz, M.M. Scherer, Kinetics of nitrate, and Cr (VI) reduction by iron metal, Environ. Sci. Technol. 36 (2002) 299–306. [28] H.-Y. Hu, N. Goto, K. Fugie, Effect of pH on the reduction of nitrite in water by metallic iron, Water Res. 35 (2001) 2789–2793. [29] G.C. Yang, H.L. Lee, Chemical reduction of nitrate by nanosized iron: kinetics and pathways, Water Res. 39 (2005) 884–894. [30] C.P. Huang, H.W. Wang, P.C. Chiu, Nitrate reduction by metallic iron, Water Res. 32 (1998) 2257–2264. [31] L. Feng, F. Jing, G. Yanhui, F. Maohong, W. Jianji, Y. Hongqun, Reduction of nitrite by ultrasound-dispersed nanoscale zero-valent iron particles, Ind. Eng. Chem. Res. 47 (2008) 8550–8554. [32] G. Cravotto, P. Cintas, Power ultrasound in organic synthesis: moving cavitational chemistry from academia to innovative and large-scale applications, Chem. Soc. Rev. 35 (2006) 180–196. [33] G. Kianpoura, M. Salavati-Niasaria, H. Emadi, Ultrason. Sonochem. 20 (2013) 418–424. [34] M.R. Jamei, R.M. Khosravi, B. Anvaripour, Investigation of ultrasonic effect on synthesis of nano zero valent iron particles and comparison with conventional method, J. Chem. Eng. 8 (2013) 767–774. [35] M.P. Deosarkara, S.M. Pawara, B.A. Bhanvaseb, In situ sonochemical synthesis of Fe3O4–graphene nanocomposite for lithium rechargeable batteries, Chem. Eng. Process. 83 (2014) 49–55. [36] A. Ryu, S.W. Jeong, A. Jang, H. Choi, Reduction of highly concentrated nitrate using nanoscale zero-valent iron: effects of aggregation and catalyst on reactivity, Appl. Catal. B Environ. 105 (2011) 128–135. [37] H.E. Romero, P. Joshi, A.K. Gupta, H.R. Gutierrez, M.W. Cole, S.A. Tadigadapa, P.C. Eklund, Adsorption of ammonia on graphene, Nanotechnology 20 (2009) 245501. [38] F. Schedin, A.K. Geim, S.V. Morozov, E.W. Hill, P. Blake, M.I. Katsnelson, K.S. Novoselov, Detection of individual gas molecules adsorbed on graphene, Nat. Mater. 6 (2007) 652–655. [39] C. Seunghee, C. Yoon-Young, H. Kyung-Yub, K. Jeehyeong, Kinetics of reductive denitrification by nanoscale zero-valent iron, Chemosphere 41 (2000) 1307–1311.