Chemical Engineering Journal 244 (2014) 19–26

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Chemical Engineering Journal

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Removal of co-contaminants Cu (II) and nitrate from aqueous solutionusing kaolin-Fe/Ni nanoparticles

http://dx.doi.org/10.1016/j.cej.2014.01.0401385-8947/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author at: Cooperative Research Centre for ContaminationAssessment and Remediation of Environments, Mawson Lakes, SA 5095, Australia.Tel.: +61 08 83025057.

E-mail address: zuliang.chen@unisa.edu.au (Z. Chen).

Xiang Cai a, Ying Gao a, Qian Sun b, Zuliang Chen a,c,d,⇑, Mallavarapu Megharaj c,d, Ravendra Naidu c,d

a School of Environmental Science and Engineering, Fujian Normal University, Fuzhou 350007, Fujian Province, Chinab Key Laboratory of Urban Environment and Health, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, Chinac Centre for Environmental Risk Assessment and Remediation, University of South Australia, Mawson Lakes, SA 5095, Australiad Cooperative Research Centre for Contamination Assessment and Remediation of Environments, Mawson Lakes, SA 5095, Australia

h i g h l i g h t s

� K-Fe/Ni used to simultaneously remove Cu (II) and nitrate.� The increase in the degradation of nitrate in the presence of Cu (II).� Characterization by SEM, EDS, XRD and XPS.� Formation of trimetallic K-Fe/Ni/Cu nanoparticles.

a r t i c l e i n f o

Article history:Received 31 October 2013Received in revised form 13 January 2014Accepted 15 January 2014Available online 24 January 2014

Keywords:K-Fe/Ni nanoparticlesCo-contaminantsCu (II) and nitrateNanoremediation

a b s t r a c t

Treatment of wastewater containing co-contaminants poses a significant challenge because heavy metaland inorganic anion contaminants in wastewater have different fates and transport mechanisms. In thispaper, bimetallic Fe/Ni nanoparticles supported by kaolinite (K-Fe/Ni) were used to simultaneouslyremove Cu (II) and nitrate. Results show that the removal of either Cu (II) or nitrate using K-Fe/Ni wasmutually affected. Specifically, 42.5% of nitrate was degraded in the presence of 200 mg/L Cu (II), whileonly 26.9% of nitrate was reduced when Cu (II) was absent. Similar results were also obtained for theremoval of Cu (II) in the absence or presence of nitrate. However, the effect of nitrate concentrationson the removal of Cu (II) was less than that for Cu (II) concentrations regarding the degradation of nitrate.To better understand this process, the K-Fe/Ni before and after reacting with Cu (II) and nitrate was char-acterized with specific surface area, X-ray diffraction (XRD), scanning electron microscope (SEM), X-rayenergy-dispersive spectrometer (EDS), and X-ray photoelectron spectroscopy (XPS). These results showthat there were the formation of iron oxide, metallic Cu and Ni, after reacting with Cu (II) and nitrate. Thisindicates that Cu (II) was reduced to Cu0 onto the surface of K-Fe/Ni, and then a novel catalyst such asK-Fe/Ni/Cu formed. Finally, a possible mechanism for the simultaneous removal of Cu (II) and nitrateis proposed, based on Fe0 as the reductive agent, whilst both Ni and Cu act as the catalysts for thehydrogen generated from water, and kaolin serves as a support.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction excessive Cu (II) in certain organisms can result in diseases [2], and

Heavy metals such as chromium, lead and copper exposed in anaquatic environment have serious impacts on natural bodies ofwater and public health [1–4]. Cu (II) is often distributed in thenatural environment due to its widespread use in various indus-tries. To date, several studies published on toxicology suggest that

even potentially kill flora and fauna in aquifers. Currently, the con-ventional processes for treating wastewater containing Cu (II), in-clude chemical precipitation [3], redox approach, ion exchange[5], electrolysis [6], member separation and adsorption [7]. Thesetraditional methods are useful in removing Cu (II), but they do havedrawbacks such as high cost, poor efficiency, high energy con-sumption and operational limitations [3].

Despite the pollution that heavy metals cause, nitrate is notori-ous for being discharged into aquatic environments from manysources, for example, agricultural fertilization [8], industrial efflu-ents [9] and domestic sewage [10]. This situation is the fact that

20 X. Cai et al. / Chemical Engineering Journal 244 (2014) 19–26

excess nitrate can overflow into aquifers, which is one explanationfor the increase in eutrophication [8–11]. In recent decades,although the natural transition from nutrient-poor to nutrient-richhas been very slow, short-term eutrophication outbreak has occa-sionally occurred due to effluent containing anthropogenic nitrate.Conventional removal of nitrate can be done using a variety oftechniques such as chemical reduction [4], ion exchange, reverseosmosis [12], and biological denitrification [11]. Instead, neitherreverse osmosis nor ion exchange is cost-effective, furthermorechemical reduction and biological denitrification always accom-pany energy-high consumption and low efficiency, respectively[10,12].

To overcome these disadvantages of the conventional treatmentsof Cu (II) or nitrate, iron-based nanoparticles may be emerging ashaving the potential to remove both of them. The removal of heavymetals using iron-based nanoparticles has been reported [4,13].They demonstrated that iron-based nanoparticles can effectively re-move a variety of metallic and metalloid contaminants including Pb(II), Cu (II), Co (II), As (III) and Cr (III) from aqueous solution [1].Meanwhile the iron-based nanoparticles are often used for the deg-radation of nitrate [10]. However, many studies have only focusedon the degradation of single contaminant in aqueous solution ratherthan that of co-contaminants. For instance, either Cu (II) [3] or ni-trate [14] in aqueous solution can be removed by the iron-basednanoparticles, but co-contaminants can be removed simultaneouslyusing iron-based nanoparticles or not is still unclear since there isfew report on removing co-contaminants such as Cu (II) and nitrate.This is due to the fact that wastewater contains many dissolved elec-tron acceptors (e.g. nitrate or sulfate etc.) that could react with nano-scale zero valent iron (nZVI) but led to its surface passivation [13,15].Hence, it is necessary to develop potential techniques to remove theco-contaminants. Furthermore, it is unclear that the removal mightbe enhanced or hindered in the redox reaction by nZVI due to eithercompeting for electrons or passivating surface in the form of metal(hydr)oxides on nZVI [13].

For these reasons, the aim of this study is to develop a novelbimetallic Fe/Ni nanoparticles supported by kaolinite (K-Fe/Ni)for simultaneous removal of Cu (II) and nitrate. Since they have dif-ferent fates in aqueous solution, understanding of removal of Cu(II)–nitrate provides a new insight of nanoremediation for the co-contaminants. To achieve the aim, the mixed Cu (II)–nitrate solu-tion was treated using K-Fe/Ni and the investigation was examinedthe following aspects: removal efficiency, pH change, kinetics,characterization and the potential pathways.

2. Materials and methods

2.1. Chemicals

All chemicals purchased were analytic grade reagents withoutfurther purification, including iron (III) chloride hexahydrate(FeCl3�6H2O) and copper (II) chloride hydrate (CuCl2�2H2O) (TheChinese Medicine Group Chemical Co., Ltd., China); nickel sulfatehexahydrate (NiSO4�6H2O) (Shantou West Long Chemical Co.,Ltd., China); potassium nitrate (KNO3) (Tianjin Bo Di ChemicalCo., Ltd., China); and sodium borohydride (NaBH4, Tianjin StandardScience and Technology Instruments Co., Ltd., China). Kaoliniteserved as the carrier and was provided by Longyan Kaolin Co.,Ltd., in Fujian, China. It was ground through a 200 mesh sieve priorto being used.

2.2. Preparation of K-Ni/Fe

Kaolin-supported bimetallic Fe/Ni (K-Fe/Ni) nanoparticleswere synthesized using sodium borohydride-based liquid-phase

chemical reduction in the laboratory [16]. It was dynamicallystirred and the kaolin served as a carrier that supported nanopar-ticles. Firstly, 2 g of kaolin was added into a three-neck flask, thenstirred immediately with ferric chloride solution containing 1 g Feand nickel sulfate solution containing 0.1 g Ni, where the ratio ofK:Fe:Ni was 20:10:1. Secondly, under a N2 atmosphere, 1.524 gsodium borohydride dissolved in 100 mL solution was droppedstep-wise into the three-neck flask at the rate of 20–30 drops perminute with an electric rod and vigorously stirred.

After an excessive amount of sodium borohydride solution wasadded, the mixture was been stirring until another 20 min hadpassed. The jade-black heterogenous liquid was rapidly filtratedby vacuum filter and irrigated with fine ethanol three times. Afterplaced in the oven overnight for drying at 50 �C, the K-Fe/Ni wasrestored in the drier for sequential use.

2.3. Characterizations and measurements

Specific surface areas of all samples were measured using anASAP 2020 surface analyzer. Specifically, the BET-N2 adsorptionmethod (Brunauere Emmette Teller adsorption isothermal) wasemployed.

X-ray diffraction (XRD) patterns of K-Fe/Ni (20:10:1) before andafter reaction with Cu (II) and nitrate were measured by a PhilipsX’Pert Pro MPD (Philips Electronics Co., Eindhoven, Netherlands)with a high-power Cu Ka radioactive source (k = 0.154 nm) at40 kV/40 mA. The samples were then scanned at a rate of 3� 2hper min from 5� to 90� 2h. Scanning electron microscopy combinedwith energy dispersive spectrometer (SEM-EDS) was performedusing a Philips-FEI XL30 ESEM-TMP (Philips Electronics Co., Eind-hoven, Netherlands). Images and spectra were obtained at differentmagnifications using an operating voltage of 30 kV.

X-ray photoelectron spectroscopy (XPS) was carried out using aPhysical Electronics Quantum (VG ESCALAB 250, X-ray photoelec-tron spectrometer) to determine the surface elemental valence ofspent K-Fe/Ni within a depth of less than 10 nm. The adventitiousaliphatic hydrocarbons C 1s peak at 284.6 eV was employed to cal-ibrate the spectral binding energy.

The concentration of Cu (II) in aqueous solution was measuredvia Atomic Absorbance Spectrometer (VARIAN AAS 240FS, USA).The concentration of nitrate in each tube was detected using a722 N ultraviolet–visible spectrometer (Shanghai Precision & Sci-entific Instrument Co., Ltd., China). Wavelength was set to275 nm to measure the residues of nitrate in accordance with Eq.(1) in the accepted national standard [16] as below:

Rð%Þ ¼ C0 � Ct

C0� 100% ð1Þ

where R (%) represented the nitrate removal efficiency, C0 (mg/L) was the initial concentration of nitrate in the solution and Ct

(mg/L) stood for the concentration of nitrate at t min.

2.4. Batch experiments

Batch experiments were undertaken in 50 mL plastic centrifugetubes containing 0.062 g Fe of K-Fe/Ni and 25 mL solution thatcomprised various concentrations of Cu (II) and nitrate with aheadspace. The tubes were capped with plastic covers, and placedon a rotary shaker at 25 �C and 250 r/min. Afterwards the mixturewas isolated through a 0.45 lm membrane at 5 min intervals andthe filtrate was transported into other tube so that the residue insolution of Cu (II) was measured by AAS and the nitrate was de-tected using an ultraviolet–visible spectrometer. All experimentswere conducted in triplicate.

X. Cai et al. / Chemical Engineering Journal 244 (2014) 19–26 21

3. Results and discussion

3.1. Characterizations

3.1.1. Measurement of BETThe BET specific surface area (SSA) of synthesized K-Fe/Ni was

18.25 m2/g, which was smaller than that without supports(22.5 m2/g). The difference is highlighted by the addition of kaolin-ite, which is a support with large diameter and hence reduces theSSA. Additionally, it was observed in previous report as well [4].

3.1.2. Characterization of SEM-EDSThe microscopic morphology of K-Fe/Ni before and after reac-

tion with Cu (II), nitrate and their mixture with a magnificationof 20,000 are shown in Fig. 1, respectively. The freshly quasi-spher-ical nanoparticles linked together as chain-like beads ranging inthe size from 1 to 100 nm were properly scattered by schistosekaolinite as shown in Fig. 1(a) [17]. However, the Fig. 1(b)–(d)shows that the original necklace-like structure tended to be col-lapsed as discrete aggregates dispersing on the kaolinite surface[18]. Particularly, transformation of Cu0 nanoparticles occurredfrom Cu (II) via a reduction using K-Fe/Ni and settled the surfacedown. Thus, the Cu0 thickened the formation of aggregation, andthis is clearly observed in Fig. 1(c) and (d).

Fig. 2 shows the results obtained from EDS analysis of K-Fe/Nibefore and after reaction with Cu (II) and nitrate. The K-Fe/Ni be-fore reaction contained mainly Fe and O with their weight percent-ages being 39.83% and 33.99%, respectively, while the elementssuch as Ni (4.61%), Si (9.69%), C (7.78%) and Al (4.10%) were alsoobserved, and Al and C were mostly derived from the inorganicconstituents of kaolinite [4]. Furthermore, this was also detectedby XRD patterns. As it can be seen in Fig. 2(b)–(d), the content ofFe0 in K-Fe/Ni after reaction was less than that in K-Fe/Ni beforereaction due to Fe0 having partially oxidized and dissolved intothe aqueous solution. Additionally, a little difference in Ni contentoccurred between the K-Fe/Ni before and after reaction, suggesting

Fig. 1. SEM images of K-Ni/Fe before and after reaction with Cu (II) and/or nitrate. (a) K-(d) K-Ni/Fe after reacting with Cu (II)–nitrate mixture.

that Ni served as a catalyst to accelerate the removal of both Cu (II)and nitrate in aqueous solution. It is evident in Fig. 2(c) and (d) thatthe surface of K-Fe/Ni after reaction contained 3.66% and 1.67% ofCu, respectively. This is due to the fact that Cu (II) was reduced toCu0 by K-Fe/Ni, and therefore the formation of Cu0 on the surface ofK-Fe/Ni acted as the catalyst to enhance the degradation of nitrate[19]. However, N content was not detectable (Fig. 4(b) and (d)).This may be part of the reason why nitrate and its by-productsran off the surface of sample into aqueous solution or air whencentrifuging or drying them.

3.1.3. Analysis of XRDThe XRD was conducted to confirm the surface composition of

the K-Fe/Ni before and after reacting with Cu (II) and nitrate asshown in Fig. 3. The diffraction peak at 2h = 44.9� proved to be con-sistent with a-Fe0 [18], as shown on the XRD patterns for K-Fe/Nibefore and after reacting with Cu (II) and nitrate (Fig. 3(a)–(d)). Anumber of small peaks can be observed at 2h = 21�, 35� or 50�, indi-cating the formation of Fe3O4 or c-Fe2O3 [4]. More loss in intensityof a-Fe0 occurred in the K-Fe/Ni after reaction than that occurredin its fresh one, which suggests that some a-Fe0 were oxidizedinto iron oxides coating on the surface and some became theferric/ferrous ions dissolving into solution. The explanation isthat the outer shell of iron oxides was accumulated step-wiseand coated the inner core of Fe0, which seemed like shrinking.The Fig. 3(b) and (d) peaks at approximately 2h = 44� reveal thatCu0 was present in K-Fe/Ni after it reacted with the solutioncontaining Cu (II) [20]. The apparent S, Q and OCT peaks on thesurface of K-Fe/Ni after reaction in Fig. 3(b)–(d), signified silicadioxide, quartz and opal-CT minerals, respectively, as similar asprevious report [5]. However, these peaks were undetectablefrom the K-Fe/Ni before reaction because those nanoparticlescovered the kaolinite surface uniformly. After reacting withthe Cu (II)–nitrate mixture, the Fe/Ni nanoparticles tended toagglomerate, which resulted kaolinite surface in exposing morethan that reacted with nothing [21].

Ni/Fe; (b) K-Ni/Fe after reacting with nitrate; (c) K-Ni/Fe after reacting with Cu (II);

Fig. 2. EDS analysis of K-Ni/Fe before and after reaction with Cu (II) and/or nitrate. (a) K-Ni/Fe; (b) K-Ni/Fe after reacting with nitrate; (c) K-Ni/Fe after reacting with Cu (II);(d) K-Ni/Fe after reacting with Cu (II)–nitrate mixture.

10 20 30 40 50 60 70 80

d

b

cCu

0

OCTQ

S

2-theta degree

Rel

ativ

e In

tens

ity

(a.u

.)

a

Fe3O

4 / -Fe

2O

3

-Fe0

Cu0

α

Fig. 3. XRD patterns of before and after reaction with Cu (II) and/or nitrate.(a) K-Ni/Fe; (b) K-Ni/Fe after reacting with Cu (II); (c) K-Ni/Fe after reacting withnitrate; (d) K-Ni/Fe after reacting with Cu (II)–nitrate mixture.

22 X. Cai et al. / Chemical Engineering Journal 244 (2014) 19–26

3.1.4. Analysis of XPSFig. 4 shows the XPS surveys on K-Fe/Ni after it reacted with the

Cu (II)–nitrate mixture. The elements including Al (2p 74 eV and 2s121 eV), Si (2p 103 eV and 2s 154 eV) and O (1s 533 eV), weremostly derived from the kaolinite ingredients [4]. Fig. 4(b) indi-cates the XPS survey details concerning the Fe 2p3/2 and Fe 2p1/2

regions. Here a couple of highly weak shoulder peaks at706.10 eV and 720.50 eV implied that a trace amount of Fe0 inthe K-Fe/Ni after reaction and the main peaks at bonding energiesof 711.08 eV and 724.50 eV, revealed the formation of iron oxidespecies (e.g., iron oxide and oxyhydroxide) [4]. As depicted inFig. 4(c), the binding energies at 932.70 eV and 952.90 eV, repre-senting Cu regions of 2p3/2 and 2p1/2, suggested there Cu0 waspresent in the K-Fe/Ni after reaction [22]. These results were

consistent with those from the XRD. With reference to the Ni2p3/2 spectrum (Fig. 4(d)), the bonding energy of the main peakat 852.3 eV indicated that the Ni0 still existed on the surface ofthe K-Fe/Ni after reaction. The reason that it worked as a catalystand did not suffer its loss is explicated by Eq. (10) [23]. TogetherXRD with XPS analysis, both of them (Figs. 3 and 4) confirm thatthe K-Fe/Ni after reaction contained Fe0, Cu0 and Ni0. It thereforedemonstrates that the Fe0, Cu0 and Ni0 nanoparticles were presenton the surface of K-Fe/Ni when K-Fe/Ni was used for simultaneousremoval of Cu (II) and nitrate. Based on the XPS results, it can beconcluded that a novel trimetallic Fe/Ni/Cu was formed. This isthe reason why a higher degradation efficiency of nitrate wasobtained when Cu (II) was more in solution.

3.2. The removal rate of coexisting Cu (II) and nitrate

The removal rate of Cu (II) from aqueous solution at an initialconcentration of 200 mg/L was achieved using K-Fe/Ni under vari-ous initial concentrations of nitrate as shown in Fig. 5(a). It was ob-served that approximately 99.7% of Cu (II) was removed from thenitrate-free solution after 30 min. Nevertheless, 95.0% of Cu (II)was removed in the presence of 100 mg/L nitrate, which meansthe co-existence of nitrate in solution has a weakly inhibitory ef-fect on removal of Cu (II). This is based on the fact that the lossin reductive capacity of Fe0 was caused by nitrate via occupyingthe active sites of Fe0 surface as shown in Eq. (2) [19].

Fe0 þ NO�3 þ 2Hþ ! Fe2þ þH2Oþ NO�2 E0 ¼ 1:37 V ð2Þ

Although the redox potential between copper and iron(E0 = 0.74 V) was less than that between nitrate and iron(E0 = 1.37 V), the removal rate of Cu (II) in nitrate-free solutionwas better than nitrate removed from Cu (II)-free solution, this isdue to the fact that the Cu (II) might be more easily adsorbed bykaolinite than nitrate, and Cu0 originating from Cu (II) could berather easily deposited onto the surface of K-Fe/Ni than nitrateagain. This prevented the nitrate from making direct contact with

1200 1000 800 600 400 200 00

100000

200000

300000

400000

735 730 725 720 715 710 705 70020000

30000

40000

50000

960 955 950 945 940 935 930 925

55000

60000

65000

70000

75000

880 875 870 865 860 855 85048000

50000

52000

54000

O

Fe

ClSiC

Al

Cu

Ni

a

Fe0

Fe2O

3

Fe 2p3/2

Fe 2p1/2

b

Cou

nts

/ s

Cu0

Cu 2p3/2

(932.70)

Cu 2p1/2

(952.90)

c

Bonding Energy (eV)

Ni 2p3/2

(852.3)Ni0

d

Fig. 4. XPS spectra obtained from K-Ni/Fe after reaction with Cu (II)–nitrate mixture.

0 5 10 15 20 25 30

0.0

0.2

0.4

0.6

0.8

1.0

0 5 10 15 20 25 300.5

0.6

0.7

0.8

0.9

1.0

1.1

b

Ct/C

0(Cu(

II))

Time (min)

0 mg/L nitrate40 mg/L nitrate

100 mg/L nitrate

a

Ct/C

0(nitr

ate)

Time (min)

0 mg/L Cu(II)20 mg/L Cu(II)

200 mg/L Cu(II)

Fig. 5. Removal rate of Cu (II) and nitrate using K-Fe/Ni under coexisting at various concentrations. (a) Effect of initial concentrations of nitrate on the removal of Cu (II). InitialCu (II): 200 mg/L; nitrate concentrations: 0, 40, 100 mg/L; (b) Effect of initial concentrations of Cu (II) on the removal of nitrate. Initial nitrate: 40 mg/L; Cu (II) concentrations:0, 20, 200 mg/L.

X. Cai et al. / Chemical Engineering Journal 244 (2014) 19–26 23

the Fe0 [11]. It explains why nitrate has only a slightly inhibitoryeffect on removal of Cu (II), even though the initial concentrationof nitrate increased. The pH values of Cu (II) solution during thereaction with K-Fe/Ni without nitrate maintained at the range from3.3 to 4.72 due to the proton generation in the Fe0 corrosion asshown in Eq. (3).

2Fe0 þ 2H2Oþ O2 ! 2FeOOHþ 2Hþ ð3Þ

Such as shown in Fig. 5(b), the increasing initial Cu (II) concen-tration (from 0 to 200 mg/L) did enhance the degradation effi-ciency of nitrate, causing it rise from 26.9% to 40.4% within30 min when using K-Fe/Ni. The reason might be that the forma-tion of Cu0 on all the surface area was another catalyst other thanNi0 [19], which did increase the number of active sites and finallyenhance the degradation rate of nitrate to nitrite [21], as given inEqs. (4) and (5).

In addition, the maximum pH level of Cu (II) solution was 4.69and 4.50 after 30 min, respectively, when mixed with 40 mg/L and100 mg/L nitrate (Fig. 6(a)). It showed that the solution pH changewas rarely impacted by the increasing concentrations of nitratelike as Eq. (2). This is due to the fact that the reduction of nitratecaused by nZVI did not form proton [20]. Nevertheless, the pH ofnitrate solution and the mixture solution containing nitrate andCu (II) (20 mg/L), increased sharply from 5.86 to 9.92 and from6.06 to 9.86 respectively, as shown in Fig. 6(b). These results canbe explained by the increasing amount of OH-, which was formedfrom nZVI corrosion in aqueous solution as Eq. (6) described.

Iron oxides formed (Eq. (6)) on the surface of nZVI in K-Fe/Ni inalkaline solution were regarded as a layer of obstacle that blockedthe surface-mediated reactions, which reduced the contact be-tween Fe0 and nitrate regardless of whether there was no or lowconcentration of Cu (II) (e.g., 20 mg/L) existing in solution [11].However, the partially acidic trends in pH observed in 200 mg/L

0 5 10 15 20 25 303.0

3.5

4.0

4.5

5.0

5.5

6.0

0 5 10 15 20 25 302

4

6

8

10

12

14

16

a

pH o

f N

itra

te S

olut

ion

pH o

f C

u (I

I) S

olut

ion

Time (min)

0 mg/L NO3

-

40 mg/L NO3

-

100 mg/L NO3

-

b

Time (min)

0 mg/L Cu2+

20 mg/L Cu2+

200 mg/L Cu2+

Fig. 6. Changes in pH values for removal of Cu (II) and nitrate under coexisting at various concentrations. (a) The pH for removal of Cu (II) in the presence of various nitrateconcentrations; (b) The pH for removal of nitrate in the presence of various Cu (II) concentrations.

24 X. Cai et al. / Chemical Engineering Journal 244 (2014) 19–26

concentration of Cu (II) (Fig. 6(b)) did help to degrade nitrate. Thiscan be attributed to two new findings. One is that the more Cu0

produced from a higher Cu (II) concentration coated the surfaceof Fe0, which enhanced the degradation efficiency of nitrate andpumped the electrons from the Fe0 corrosion since both Cu0 andNi0 as the catalysts [10]. Another is that the fresh formation ofCu0 would directly react with nitrate under acidic condition asshown in Eq. (5).

Cu2þ þ Fe0 ! Cu0 þ Fe2þ E0 ¼ 0:74 V ð4Þ

NO�3 þ Cu0 þ 2Hþ ! NO�2 þ Cu2þ þH2O E0 ¼ 0:59 V ð5Þ

5Fe0 þH2Oþ 4O2 ! c-Fe2O3 þ Fe3O4 þ 2OH� ð6Þ

3.3. The mechanism for the removal of Cu (II) and nitrate using K-Fe/Ni

Establishing a kinetic model may be one of the best approachesto understand the surface-mediated reaction rate throughout theCu (II)–nitrate mixture being simultaneously reduced by K-Fe/Ninanoparticles [11]. In this study, data obtained from the batchexperiments suggest that the removal of Cu (II) and nitrate fittedwell the pseudo first-order kinetics, as formulated in Eqs. (7) and(8) below:

�d½Cu2þ�dt

¼ kobs½Cu2þ� )Z Ct

C0

�d½Cu2þ�½Cu2þ�

¼ kobs

Z t

0dt

) lnðC0=CtÞ ¼ kobst þ A1 ð7Þ

�d½NO�3 �dt

¼ kobs½NO�3 � )Z Ct

C0

�d½NO�3 �½NO�3 �

¼ kobs

Z t

0dt

) lnðC0=CtÞ ¼ kobst þ A2 ð8Þ

where kobs is the observed rate constant of pseudo first-order reac-tion (min�1) and can be obtained from the slope of the linear regres-sion plotted as ln(ct/c0) versus contact time with a high correlative

Table 1The parameters on removal kinetics of Cu (II).

No. Results

Initial concentration (mg/L) kobs (min�1) R2

Cu (II) Nitrate

1 200 0 0.3132 0.98792 200 40 0.1808 0.97363 200 100 0.1338 0.9822

coefficient (R2 > 0.95). The data of kinetics on Cu (II) and nitrate re-moval are listed in Tables 1 and 2. It is evident that the higher initialnitrate concentration has the more reduction-inhibitory effect onthe removal rate of Cu (II) in solution according to the kobs valuesof 0.3132, 0.1808 and 0.1338 listed in Table 1. This is, because ni-trate involved in the redox with K-Fe/Ni, resulted in the removalefficiency of Cu (II) decreasing slightly, likewise in Fig. 5(a). Equally,the greater concentration of Cu (II) reacted with K-Fe/Ni, the moreCu0 active sites could be gained. Consequently, the Cu0 active sitescan effectively promote the degradation rate of nitrate as shown inFig. 5(b), and the value of kobs illustrated in Table 2 are 0.0087,0.0105 and 0.0141.

Based on the results discussed previously, the mechanism forsimultaneous removal of Cu (II) and nitrate from aqueous solutionusing K-Fe/Ni involves three strategies. Firstly, Cu (II) in aqueoussolution is adsorbed onto the surface of K-Fe/Ni according to Eqs.(12) and (13), since kaolinite and iron oxide derived from Fe0 cor-rosion can adsorb this metal ion due to its ion exchange or chemi-sorption [5,24]. Secondly, Cu (II) adsorbed on surface of K-Fe/Ni canbe reduced by K-Fe/Ni to Cu0 nanoparticles, which loaded onto thesurface of Fe/Ni nanoparticles to form the trimetallic Fe/Ni/Cunanoparticles catalyst (Eq. (14)). Thirdly, the novel catalyst canfacilitate the degradation of nitrate as indicated in Eq. (15). Conse-quently, a schematic pathway can be used to explain the simulta-neous removal of Cu (II) and nitrate as shown in Fig. 7. Thefollowing equations can explain how this process occurs.

Fe0—Niþ 2H2O! FeOOHþ NiþH2 þHþ ðin acid solutionÞ ð9Þ

5Fe0—NiþH2Oþ 4O2 ! c-Fe2O3 þ Fe3O4 þ 5Ni

þ 2OH� ðin alkaline solutionÞ ð10Þ

Fe0—Niþ 2Hþ ! Fe2þ—NiþH2 ðredoxÞ ð11Þ

Cu2þ þ kaolinite! Cu2þ-kaolinite ðadsorptionÞ ð12Þ

Cu2þ þ c-Fe2O3=Fe3O4 ! Cu2þ-c-Fe2O3=Fe3O4 ðadsorptionÞ ð13Þ

Table 2The parameters on degradation kinetics of nitrate.

No. Results

Initial concentration (mg/L) kobs (min�1) R2

Cu (II) Nitrate

1 0 40 0.0087 0.97392 20 40 0.0105 0.98733 200 40 0.0141 0.9689

Fig. 7. The possible mechanism for simultaneous removal of Cu (II)–nitrate mixture using K-Fe/Ni.

X. Cai et al. / Chemical Engineering Journal 244 (2014) 19–26 25

Cu2þ-adsorbentsþ Fe0—NiþH2

! Fe0—Ni0-Cu0-adsorbents ðredoxÞ ð14Þ

ðð5=3Þx� ð2=3Þy=xÞFe0—Ni0—Cu0-adsorbentsþ xNO�3þ ð3x� yÞH2 ! NxOy þ ð3x� yÞH2O ðredoxÞþ ðð5=3Þx� ð2=3Þy=xÞFe3þ—Ni0—Cu0-adsorbents ð15Þ

4. Conclusions

The synthesized K-Fe/Ni can be successfully used to simulta-neously remove co-contaminants such as Cu (II) and nitrate, whichprovides key insight of removing co-contaminants from aqueoussolution. In addition, the concentrations of nitrate in solution didnot affect the degradation efficiency of Cu (II). For example, a re-moval efficiency of 200 mg/L Cu (II) decreased from 99.7% to96.5% when the concentrations of nitrate ranged from 0 to100 mg/L. In contrast, the degradation of nitrate in the solution in-creased from 26.9% to 40.4% with the concentrations of Cu (II) inaqueous solution ranged from 0 to 200 mg/L. This new findingwas due to the formation of Cu0 dispersing onto the surface ofK-Fe/Ni to form trimetallic K-Fe/Ni/Cu, and it enhanced the denitri-fication of nitrate. It was also confirmed by the characterizationssuch as SEM, EDS, XRD and XPS. Based on these results, a possiblemechanism for the simultaneous removal of Cu (II) and nitrate insolution included: (i) Cu (II) and nitrate were previously absorbedon the kaolinite or iron oxides that produced from the Fe0 corro-sion; (ii) Fe/Ni preferred Cu (II) to be transformed into Cu0 ratherthan driving nitrate away from aqueous solution over the initialperiod of reaction; (iii) After novel catalyst of trimetallic Fe/Ni/Cunanoparticles yielded, the denitrification of nitrate was enhanced.

Acknowledgements

This study was provided by the Foundation of Fujian ‘‘MinjiangFellowship’’ from Fujian Normal University and this work was alsosupported by the Natural Science Foundation of Fujian Province,China (2011J05035).

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