Chemical Engineering Journal 244 (2014) 343–349
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XANES evidence of molybdenum adsorption onto novel fabricated nano-magnetic CuFe2O4 Yao-Jen Tu a,⇑, Chen-Feng You a,b,⇑, Chien-Kuei Chang c, Ting-Shan Chan d, Sheng-Hsien Li b a
Earth Dynamic System Research Center, National Cheng-Kung University, No 1, University Road, Tainan City 701, Taiwan, ROC Department of Earth Sciences, National Cheng-Kung University, No 1, University Road, Tainan City 701, Taiwan, ROC c Department of Chemical and Materials Engineering, National Kaohsiung University of Applied Science, No 415, Chien Kung Road, Kaohsiung 807, Taiwan, ROC d National Synchrotron Radiation Research Center (NSRRC), No 101, Hsin-Ann Road, Hsincho 30076, Taiwan, ROC b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A technology for preparation of
a r t i c l e
i n f o
Article history: Received 2 December 2013 Received in revised form 24 January 2014 Accepted 25 January 2014 Available online 2 February 2014 Keywords: Molybdenum Mo K-edge XANES Adsorption Industrial sludge Nano-magnetic CuFe2O4
Acid leaching reactor
Chemical exchange reactor
Industrial Cu Sludge
1.25
Magnet
Mo K-edge
Adsorbent recovery
1.00
4+
MoO2(Mo ) 6+
MoO3(Mo )
0.75
CuFe2O4-Mo (pH=2.75)
0.50
0.25
0.00 19980
20000
20020
20040
20060
Energy (eV)
a b s t r a c t An efficient Molybdenum (Mo) removal technology in aqueous solutions was developed for the first time using nano-magnetic CuFe2O4 manufactured from printed circuit board (PCB) industrial sludge. This nano-magnetic CuFe2O4 adsorbent displayed a nonlinear L-type isotherm that fitted well with the Langmuir isotherm, suggested limited adsorption sites and monolayer sorption on surface. The K-edge X-ray absorption near-edge structure (XANES) spectra demonstrated that Mo(VI) was the predominant oxidation species on nano-magnetic CuFe2O4 and the maximum adsorption capacity was found to be 30.58 mg g1 at pH 2.75. When pH became higher, more negative charges would occur at the surface of adsorbent and lead to more electric repulsion. Consequently, Mo adsorption was sharply reduced in alkaline condition. Importantly, these adsorbed Mo anions were replaced easily by OH ions in NaOH solution and showed huge potential for removal/concentration of Mo in industrial wastewater, groundwater, and tap water. This unique Mo separation technique can also be potentially applied for geochemical investigation in various natural aqueous solutions. Ó 2014 Elsevier B.V. All rights reserved.
⇑ Corresponding authors at: Earth Dynamic System Research Center, National Cheng-Kung University, No 1, University Road, Tainan City 701, Taiwan, ROC. Tel./fax: +886 6 2758682 (Y.-J. Tu). E-mail addresses:
[email protected] (Y.-J. Tu),
[email protected] (C.-F. You). http://dx.doi.org/10.1016/j.cej.2014.01.084 1385-8947/Ó 2014 Elsevier B.V. All rights reserved.
Ferrite process reactor
Nano-magnetic CuFe2O4
Normalized absorption
CuFe2O4 is developed from industrial sludge. The CuFe2O4 is effective in removing Mo from wastewater, groundwater and tap water. The K-edge XANES spectra show that Mo(VI) was the dominant species on CuFe2O4. The data implies that 0.001 N NaOH solution is sufficient for Mo desorption. The CuFe2O4 could be rapidly separated and recycled by a magnet in 20 s.
1. Introduction Molybdenum (Mo) is a trace element which presents widely in nature and is required by the human body for many important biological and physiological processes [1–3]. The estimated daily
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demand of Mo in adults and older children is 75–250 lg per day [4,5]. However, high concentration of Mo uptake may cause some health problems such as anemia, gout, digestive problems, growth retardation, bone deformities, and sterility [5]. Mo also plays an important role in industrial society and is economically important as a component of metal alloys, additive in stainless-steel, leather, anti-corrosive agent, rubber, catalyst, and fertilizer [5–9]. The growing production and usage of Mo represents a hazardous potential for increased release and distribution in the natural environments. It was reported that Mo concentrations in surface waters are normally less than 5 lg L1 [2]. However, much higher concentrations (several tens lg L1 to mg L1 level) could be found in aquatic systems due to anthropogenic activities. For example, according to the effluent database of Environmental Protection Administration in Taiwan, the effluent Mo concentration in photo-electric industries range between 66 and 260 lg Mo L1. If there is no suitable method for treating the Mo anthropogenic contamination, human health and aquatic ecosystems will be threatened. An effective and economic technology for Mo removal from water systems has consequently become an important issue. Adsorption could be considered as a fast, efficient, and economical method for removing trace metals from water. Many literatures have been focused on the removal of most toxic heavy metals such as Cd, Cr, Cu, Pb, Hg [10–14]. However, the investigation of Mo removal from water is still paucity. It should be mentioned that Mo is the most concentrated trace metal in seawater, in part owing to its stability and weak adsorption behavior [15]. Thus, to find a fast and efficient adsorbent for removing Mo from water is a critical issue. Several adsorbents have been studied for their removal feasibility of Mo from water. For instance, EI-Moselhy et al. investigated the removal of Mo(VI) from wastewater using carminic acid modified anion exchanger. The result showed that the maximum Langmuir adsorption capacity was found to be 13.5 mg Mo(VI) g1 of the adsorbent [15]. Lian et al. reported that the sulfuric acid modified cinder can remove Mo(VI) with a maximum adsorption capacity of 10.8 mg Mo(VI) g1 adsorbent at pH between 4.0 and 6.0 [16]. However, the mentioned adsorbents showed only low Mo adsorption capacity in water system. Besides, the high price of these adsorbents may obstruct their development when performing in the actual industrial plants. Nano-magnetic CuFe2O4, with the spinel structure, has a cubic close-packed arrangement of the oxygen ions with Cu2+ and Fe3+ ions at two different crystallographic sites [17]. It has been reported that CuFe2O4 has the potential to remove some hazardous materials (As, Cd, acid orange II) from water [18–20]. Nevertheless, the cost of CuFe2O4 synthesized from sol–gel method [21], autocombustion [22], or co-precipitation [23] is still high because the raw materials used to produce CuFe2O4 are adopted from the pure chemicals. If the raw materials used to produce CuFe2O4 can be replaced by industrial sludge, the cost of CuFe2O4 would be reduced dramatically. This study aims to investigate the Mo removal process using the nano-magnetic CuFe2O4, which is recycled from the sludge in printed circuit board (PCB) industry. Our previous study has successfully recycled copper powder from PCB sludge by combination of acid leaching and chemical exchange [24]. After these two combinations of technologies, ferrite process (FP) has been conducted not only to make sure the supernatant but also the sludge can meet the environmental rules. It should be noticed that FP is used to treat wastewater containing heavy metals for many years [25–29]. The product generated from FP such as Fe3O4, is a magnetic iron oxide containing Fe2+ and Fe3+ in the spinel structure. It can be synthesized through the reaction expressed by Eq. (1) [29,30].
3Fe2þ þ 6OH þ 1=2O2 ! Fe3 O4 þ 3H2 O
ð1Þ 2+
When heavy metal ions coexist with Fe , they can be incorporated into the structure through co-precipitation [29,30]. The principle of FP to catch heavy metals is presented in Eq. (2).
xM2þ þ ð3 xÞFe2þ þ 6OH þ 1=2O2 ! Mx Feð3xÞ O4 þ 3H2 O
ð2Þ
The sludge (MxFe(3x)O4) generated from ferrite process thus is regarded as a novel adsorbent and is used for testing its capability for Mo removal efficiency. A series of systematic experiments were designed to evaluate the feasibility of Mo removal from aqueous solutions by nanomagnetic CuFe2O4 under various conditions. The basic physical/ chemical properties inclusive of adsorbent crystalline phase, density, saturation magnetization, point of zero charge, specific surface area, and primary particle size were carefully examined. The adsorption kinetics and isotherms of Mo on nano-magnetic CuFe2O4 were also calculated and discussed. Furthermore, Mo K-edge X-ray absorption near edge spectra (XANES) were used to realize the oxidation state of Mo after the adsorption of nanomagnetic CuFe2O4. The information gained here demonstrates the great potential for developing an effective adsorbent for uptake Mo using nano-magnetic CuFe2O4. 2. Materials and methods 2.1. Preparation of nano-magnetic CuFe2O4 The preparation of nano-magnetic CuFe2O4 was followed as our earlier study by a combination of acid leaching, chemical exchange and ferrite process [24]. Briefly, acid leaching was conducted using 500 g of the industrial sludge as 10 L diluted sulfuric acid was added for extracting Cu from solids. Fe powder was used as sacrificed metal to substitute Cu2+ in the liquids during chemical exchange reaction. To ensure the supernatant fulfill the effluent standards, ferrite process was performed after the chemical exchange. A novel low-cost adsorbent, nano-magnetic CuFe2O4, was hence manufactured after the ferrite process. Eqs. (3)–(5) display the corresponding reaction of acid leaching, chemical exchange, and ferrite process, respectively. The more detailed experimental information could be found in our previous study [24].
Cu-sludge þ H2 SO4 ! Cu2þ þ sludge
ð3Þ
Fe0 þ Cu2þ ! Fe2þ þ Cu0
ð4Þ
Cu2þ þ 2Fe2þ þ 6OH þ 1=2O2 ! CuFe2 O4 þ 3H2 O
ð5Þ
The prepared adsorbent was collected using a magnetic separation method by taking advantage of its magnetism. The adsorbent was then washed with Milli-Q water many times until the pH in solution reached about 7. The solids were then dried at 323 K for 24 h in an oven and stored for further investigation. 2.2. Batch adsorption procedure To evaluate the equilibrium states of Mo during adsorption process, a series of batch experiments were carried out. Stock solutions of Mo (1000 mg L1) were prepared by dissolving sodium molybdate 2-hydrate (Na2MoO42H2O) in Milli-Q water. Working solutions for experiments were freshly prepared from the stock solution. The detailed experimental procedures were described as followed. 10 mL Mo solution and fixed amount of nano-magnetic CuFe2O4 were poured into 15 mL centrifuge tubes. The centrifuge tubes were then put on the shaft of a rotary shaker after tightening the caps.
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All equilibrium adsorption experiments were individually conducted at 298 ± 1 K for Mo, by shaking 0.02 g of the nano-magnetic CuFe2O4 with 100 mg L1 metal solution using a thermostated shaker at a speed of 30 rpm for 2 h. Adsorption kinetics for Mo were measured by taking 100 mg L1 Mo solution with 0.02 g of the nano-magnetic CuFe2O4 and shaking the mixture at pH 2.75 and 298 ± 1 K. To investigate the effect of pH, the pHs were set at 2.75 ± 0.01, 3.39 ± 0.01, 4.10 ± 0.01, 5.06 ± 0.01, 7.04 ± 0.01, 9.14 ± 0.01, and 11.02 ± 0.01 by adding 0.1 N NaOH or HNO3 solutions under the conditions of Mo 100 mg L1 and nano-magnetic CuFe2O4 0.02 g. The metal uptake qt (mg g1) was determined by Eq. (6):
qt ¼
ðC o C t Þ V m
ð6Þ
where Co and Ct are the metal concentration in liquid phase at the initial and at time t (mg L1), respectively; m is the adsorbent amount (g); V is the volume used in the adsorption process (L). The solid and liquid phases were magnetically separated using a magnet with 4000 Gauss. The Mo concentrations in the filtrate were determined by ICP-OES (iCAP 6500, Germany). The adsorbed amount of Mo on the nano-magnetic CuFe2O4 was determined using the differences between the initial and equilibrium concentrations. 2.3. Mo desorption Desorption experiments were conducted at room temperature using five different concentrations of NaOH solutions (0.4 N, 0.2 N, 0.1 N, 0.01 N, 0.001 N). Nano-magnetic CuFe2O4 were first reacted with 10 mg L1 Mo solution at pH 2.75. Subsequently, the ferrite samples were washed with Milli-Q water several times to remove excessive salts and the NaOH solution was added into the sample to initiate the desorption process. The suspensions were shaken for 30 min, and the ferrite solids were then separated from the solutions using a magnet with 4000 Gauss. The desorption efficiency was calculated from the amount of Mo released into the solutions. 2.4. Mo K-edge XANES analysis The Mo K-edge XANES spectra measurements were conducted at the Beamline 01C1 of the National Synchrotron Radiation Research Center in Hsinchu, Taiwan. The Mo K-edge XANES spectra of the samples were obtained on fluorescent mode using a Lytle detector, and at least three scans were obtained for each sample. All spectra were calibrated to the edge of Mo foil at 20,000 eV. The scans for each sample were averaged, followed by background removal and normalization. The spectra of pure MoO2 and MoO3 chemicals were also obtained to serve as the reference standards for the Mo(IV) and Mo(VI) oxidation states.
Table 1 Basic properties of the manufactured nano-magnetic CuFe2O4 used in this study. Parameters
Value
Crystalline pattern Density (g cm3) Point of zero charge (PZC) Saturation magnetization (emu g1) Specific surface area (m2 g1) Primary particle size (nm)
CuFe2O4 5.2 7.3 61.41 48.3 20–120
determined to be 48.3 m2 g1 and 61.41 emu g1 (Fig. S1c), respectively. No remnant was detected in the adsorbent, showing that the prepared adsorbent is super-paramagnetic. Additionally, the XRD pattern shows the crystalline phase of this adsorbent matched well with the copper ferrite (CuFe2O4) standard (JCPDS file number 00-025-0283) (Fig. S1d), confirming that the main phase of our synthesized adsorbent is CuFe2O4.
3.2. Influence of pH on the adsorption In general, the removal of metal ions from aqueous solutions by adsorption is highly depended on the solution pHs. The solution pH thus has been considered as the most important parameter controlling metal adsorption on sorbents. In this study, experiments were carried out in pH 2.75–11.02 to verify its effect on Mo adsorption over nano-magnetic CuFe2O4. Figs. 1 and 2 display the tendency of Mo adsorbed over nano-magnetic CuFe2O4 at different pHs. The results reveal clearly that the pH dependency of the Mo removal was found to be critical at the investigated pH values. It is noted that the removal efficiency decreased with increasing pH and showed a maximum removal value at pH 2.75 (Fig. 1). This is possible due to the fact that at higher pH, the negatively charged surface sites do not favor for the adsorption of Mo oxyanion due to electrostatic repulsion [15]. A similar Mo adsorption trend was observed in several different adsorbents [15,16,31–34]. Besides, all Mo removal efficiencies investigated in this study could reach more than 88% when adsorption conditions were set at pH 2.75– 4.10. However, almost no Mo removal ( 10) (Fig. 2). The speciation in solution and the surface charge of the nanomagnetic CuFe2O4 play an important role in the Mo adsorption process. Guibal et al. reported that the distribution of molybdate species in solutions could be summarized as MoO2 4 , HMoO4 ,
3. Results and discussion 3.1. Basic properties of the adsorbent The physical/chemical properties of the adsorbent were examined using standard procedures and the results were displayed in Table 1 and Fig. S1. Briefly, SEM image of the synthesized adsorbent presents numerous particles are in almost spherical shape and the primary particle size ranged from 20 to 120 nm, with a mean particle size of 60 nm (Fig. S1a). The density and the point of zero charge (PZC) of the manufactured adsorbent were found to be around 5.2 g cm3 and 7.3 (Fig. S1b). The BET surface area and the saturation magnetization of the adsorbent were
Fig. 1. The Mo adsorption tendency on nano-magnetic CuFe2O4 under various pHs. Condition: T 298 K, solution volume 10 mL, nano-magnetic CuFe2O4 0.02 g, initial Mo concentration 100 mg L1.
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(Ce) at the end of 120 min was found to be 30.12 (mg g1) and 42.17 (mg L1), respectively. Fig. S3a presents a plot of ln (qe qt) versus t. It shows a good linearity as R2 = 0.9807. A plot of t/qt versus t is given in Fig. S3b and it could be observed that the pseudo-second-order model fits the data well with R2 of 0.9998. These results demonstrate that the pseudo-second-order kinetic model yields a better fit. Similar phenomenon were observed in better fitting of pseudo-second-order model as compared to the first-order-model for Mo adsorption onto sulfuric acid-modified cinder [16], ZnCl2 activated coir pith carbon [31], and waste Fe(III)/Cr(III) hydroxide [32]. 3.4. Adsorption isotherms
Fig. 2. pH effect of Mo adsorption on nano-magnetic CuFe2O4 as a function of reaction time at 298 K. Condition: solution volume10 mL, nano-magnetic CuFe2O4 0.02 g, initial Mo concentration 100 mg L1.
H2MoO4, Mo7 O21 ðOHÞ3 Mo7 O21 ðOHÞ4 Mo7 O23 ðOHÞ and 3 , 2 , 5, Mo7 O6 [34]. Obviously, all Mo species are anions except H2MoO4. 24 It should been noticed that anions are preferably adsorbed at low pH [35]. The effect of pH on Mo adsorption arose apparently from the charge properties of both Mo and nano-magnetic CuFe2O4. The surface of the adsorbent subjected to protonation/ deprotonation is depending on the solution pH. As indicated by the PZC of the synthesized CuFe2O4 (i.e. 7.3), the net surface charge at pH < 7.3 is positive, which is beneficial for adsorbing the anionic Mo species. This explains the high Mo uptake in acidic conditions (pH 2.75–4.10). An increase in solution pH resulted in a buildup of negative charges on both adsorbent and adsorbate, leading to an enhanced electric repulsion between them. Consequently, a dramatic decreasing Mo adsorption was observed at high pH. Very similar results were reported by EI-Moselhy et al. [15], Lian et al. [16], and Namasivayam and Sangeetha [31]. Furthermore, the PZC of the synthesized CuFe2O4 is at pH 7.3, while the drastic drop in Mo adsorption happens between pH 4 and 5 which is explainable by the weak acids adsorption around their dissociation constants [36]. 3.3. Adsorption kinetic models Adsorption kinetics are of great significance to evaluate the performance of an adsorbent and gain insight of the underlying mechanisms. Two well-known kinetic models, pseudo-first-order [37] and pseudo-second-order [38] were used to fit the data shown in Fig. S2. Eqs. (7) and (8) present the linear forms of pseudo-first-order and pseudo-second-order kinetics respectively,
ln ðqe qt Þ ¼ ln qe k1 t
ð7Þ
t 1 t ¼ þ qt k2 q2e qe
ð8Þ
where t is the contact time (min), qe (mg g1) and qt (mg g1) is the amount of Mo adsorbed at equilibrium and at any time t respectively, k1 (1/min) and k2 (g/mg min) is the rate constant of pseudo first-order and pseudo-second-order kinetics respectively. Fig. S2 reveals the time evolution of Mo adsorption from a solution containing 100 mg L1 of Mo. It shows that Mo was adsorbed rapidly in the first 20 min. After 120 min, the amount of Mo adsorption kept rather constant. Therefore, a 120 min contact time is sufficient to achieve equilibrium. The equilibrium values of Mo uptake (qe) on the nano-magnetic CuFe2O4, and Mo left in solution
The equilibrium data, commonly known as the adsorption isotherms, is a basic requirement for establishing the adsorption system and provides information on the capacity of the adsorbent or the amount required to remove a unit mass of pollutant. Two popularly used adsorption isotherms, Freundlich [39] and Langmuir models [40], were hence commissioned to fit the experimental data. The batch experimental data were fitted to the isotherm models of the Langmuir and Freundlich using the least squares method. The isotherms are mathematically expressed as follows. The Langmuir isotherm model is obtained by combination of the adsorption and desorption rate equations [41],
dht ¼ kads C t Nð1 ht Þ kd Nht dt
ð9Þ
where N is the maximum number of adsorption sites occupied by the Mo and ht is the dimensionless surface coverage ratio. When the adsorption process reaches an equilibrium state, Eq. (9) yields
qe ¼
K L qm C e 1 þ K LCe
ð10Þ
where qm is the maximum adsorption capacity (mg g1); qe is the amount of Mo sorbed at equilibrium (mg g1); and KL is the Langmuir constant. Eq. (11) shows the other re-arranging form yielded from Eq. (10).
Ce 1 Ce ¼ þ qe K L qm qm
ð11Þ
The Freundlich isotherm model shown in Eq. (12) displays the relationship between the amount of Mo adsorbed by the nanomagnetic CuFe2O4 (qe, mg g1) and the equilibrium concentration of Mo (Ce, mg L1) in solution:
Fig. 3. Mo adsorption isotherm on nano-magnetic CuFe2O4 at T 298 K. Conditions: Mo initial concentration 10–200 mg L1, pH 2.75, solution volume 10 mL, nanomagnetic CuFe2O4 0.02 g.
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Y.-J. Tu et al. / Chemical Engineering Journal 244 (2014) 343–349 Table 2 Comparison of Mo adsorption by the nano-magnetic CuFe2O4 and other adsorbents at 298 K. Adsorbents
pH
Mo adsorption capacity (mg g1)
References
Chitosan beads APF gel Maghemite Nano-magnetic CuFe2O4 Modified mesoporous zirconium silicates Goethite ZnCl2 activated coir pith carbon Carminic acid modified anion exchanger Waste Fe(III)/Cr(III) hydroxide Sulfuric acid-modified cinder Iron-based adsorbents Pyrite
3.0–3.5 1 M HCl 4.0–6.0 2.8 2.0–3.0 4.0 4.0 4.0–6.0 4.0 4.0–6.0 3 M HNO3 4.0
184.5 100.7 33.4 30.6 22.8 15.5 14.2 13.5 12.3 10.8 10.4 1.5
[34] [44] [3] This study [42] [33] [31] [15] [32] [16] [43] [33]
qe ¼ K F C 1=n e
ð12Þ
where KF and n are Freundlich constants that are related to the adsorption capacity and adsorption intensity, respectively. Fig. S4a and b shows linear plots of ln qe versus ln Ce and Ce/qe versus Ce. In Fig. S4a, linear regression analysis was used for treatment of isotherm data (KF and n). A similar analysis method was used for the values of qm and KL according to Fig. S4b. Table S1 presents the results along with associated correlation coefficients (R2). It reveals that the Langmuir model yielded a better fit than the Freundlich model (The R2 for Langmuir isotherm and Freundlich
isotherm is 0.9863 and 0.9762, respectively) for Mo adsorption on nano-magnetic CuFe2O4 at 298 K. It should be mentioned that the value of the maximum adsorption capacity (qm) calculated from Langmuir isotherm model was found to be 30.58 mg g1. This value is very close to our experimental maximum adsorption capacity 26.0 mg g1 (Fig. 3), confirming again that Mo adsorption onto nano-magnetic CuFe2O4 followed well with the Langmuir isotherm. Additional comparison of the synthesized nano-magnetic CuFe2O4 with other adsorbents for Mo adsorption at 298 K was summarized in Table 2. It is apparent that nano-magnetic CuFe2O4 uptake efficiently of Mo from aqueous solutions. This adsorption capacity (30.58 mg g1) is much higher than the other adsorbents investigated, modified mesoporous zirconium silicates (22.8 mg g1) [42], goethite (15.5 mg g1) [33], ZnCl2 activated coir pith carbon (14.2 mg g1) [31], carminic acid modified anion exchanger (13.5 mg g1) [15], waste Fe(III)/Cr(III) hydroxide (12.3 mg g1) [32], sulfuric acid-modified cinder (10.8 mg g1) [16], iron-based adsorbents (10.4 mg g1) [43], and pyrite (1.5 mg g1) [33]. Although some adsorbents show very high adsorption capacity of Mo (184.5 mg g1 and 100.7 mg g1) [34,44], however, the complex preparation methods or high cost of adsorbents may decrease their competition in the market. 3.5. Mo desorption and regeneration of nano-magnetic CuFe2O4 To make the adsorption process more economical, the desorption experiments were conducted to investigate the potential of nano-magnetic CuFe2O4 regeneration. Fig. 4a shows the Mo desorption efficiency of five NaOH solutions (0.4 N, 0.2 N, 0.1 N, 0.01 N, 0.001 N) at 30 min. These results display about 75% Mo 1.25
Normalized absorption
Mo K-edge 1.00
MoO 2(Mo4+ ) MoO3(Mo6+ ) CuFe 2O4-Mo(pH=2.75)
0.75
0.50
0.25
0.00 19980
Fig. 4. (a) The Mo desorption rates from nano-magnetic CuFe2O4 in 0.4–0.001 N NaOH solutions at 298 K for 30 min; (b) reusability of nano-magnetic CuFe2O4 to desorb in 0.001 N NaOH solutions at 298 K for 30 min. The relative standard deviation (RSD) of three replicated analyses was lower than 3%.
20000
20020
20040
20060
Energy (eV) Fig. 5. The K-edge XANES spectra of Mo(IV), Mo(VI) standards, and Mo adsorbed on nano-magnetic CuFe2O4.
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Table 3 The Mo removal efficiency in various water matrixes using nano-magnetic CuFe2O4. Water type
Adsorption pH
Mo (mg L1)
Fe (mg L1) 1
Before adsorption (mg L
)
Cu (mg L1)
Removal (%)
After adsorption
Industrial wastewater Groundwater
2.93 (initial pH) 6.33 (initial pH) 2.97 (adjusted pH)
10.41 10.17 10.17
99.1 19.9 99.3
0.18 b.d.a 0.09
0.08 b.d. 0.06
Tap water
7.03 (initial pH) 3.04 (adjusted pH)
10.03 10.03
9.1 99.2
b.d. b.d.
b.d. b.d.
Note: Amount of adsorbent = 0.02 g, Volume = 10 mL, Temperature = 298 K, Time = 30 min. a b.d.: Below detection limit (For Mo: 0.012 mg L1, Fe: 0.023 mg L1, Cu: 0.031 mg L1).
desorption under investigated NaOH concentrations. It is clear that OH- ions could replace Mo anion from the adsorbent sites on nanomagnetic CuFe2O4. This phenomenon was consistent with the results found by Nakashima et al. [6], EI-Moselhy et al. [15], Lian et al. [16], and Namasivayam and Sangeetha [31], indicating the strong affinity between OH ions and nano-magnetic CuFe2O4. In addition, our results imply that 0.001 N NaOH is sufficient for Mo desorption. In real industrial application, three times of desorption could reach 98% of Mo desorption. This high desorption of Mo from the adsorbents makes the removal process economical since both adsorbent and Mo are regenerated and can be recycled efficiently. The three cycles adsorption–desorption experiments were carried out to investigate the regeneration of the nano-magnetic CuFe2O4 for Mo removal efficiency. The results show that 99.2% Mo can be removed at the first adsorption, and followed by 96.5%, and 93.2%, respectively, in the subsequent cycles (Fig. 4b). The main reasons for this decreased removal may be due to the adsorption sites occupied by the un-desorbed Mo or collection loss of the synthesized nano-particles. However, these results confirm that our synthesized nano-magnetic CuFe2O4 has high potential to be re-used for at least three cycles. 3.6. Mo K-edge XANES spectra Mo K-edge XANES spectra was performed to clarify the oxidation state of Mo after adsorption on nano-magnetic CuFe2O4. The XANES of pure MoO2 and MoO3 used as the standards of Mo(IV) and Mo(VI), respectively, are also shown for comparison. The XANES spectrum of MoO3 exhibited a strong pre-edge feature at E = 20,007 eV, which is dipole allowed for tetrahedral symmetry [45]; it can be used to identify Mo(VI) in a sample because, comparatively, the pre-edge region of Mo(IV) in MoO2 exhibits very weak and broadening pre-edge peak (Fig. 5). Because we saw apparent presence of the pre-edge feature at E = 20,007 eV of CuFe2O4-Mo synthesis sample at pH 2.75, so we conclude that Mo(VI) was the predominant oxidation state of Mo sorbed on nano-magnetic CuFe2O4. Because Mo(VI) was originally present in the system, the predominant occurrence of Mo(VI) on the nano-magnetic CuFe2O4 surface indicated that Mo(VI) was not reduced to Mo(IV) even Mo(0) on nano-magnetic CuFe2O4. 3.7. Mo removal from industrial wastewater, groundwater, and tap water In order to evaluate the feasibility of the synthesized nano-magnetic CuFe2O4 in real waters, 10 mg L1 Mo was added into the actual photoelectric industrial wastewater (initial pH 2.93), groundwater (initial pH 6.33), and tap water (initial pH 7.03) to test Mo removal performance in various water matrixes. The results showed that Mo removal efficiency could reach 99.1%, 19.9%, and 9.1% in actual photoelectric industrial wastewater,
groundwater, and tap water, respectively (Table 3). Interestingly, the Mo removal percentage can reach 99.3% and 99.2% from groundwater and tap water after the pH was adjusted to 2.97 and 3.04, respectively. It confirmed again that Mo could be easily absorbed in acidic condition due to the electric attraction mentioned in Section 3.2. The residual Mo in each sample was lower than 0.09 mg L1, much lower than the maximum level of effluent (0.6 mg L1) regulated by Environmental Protection Administration, Taiwan. The results obtained here implied that the removal/ concentration of Mo using the synthesized nano-magnetic CuFe2O4 from groundwater and tap water can be significantly enhanced via pre-acidification of the waters. Additionally, it should be mentioned that the concentrations of Fe and Cu were below detection limit before adsorption. Almost no Fe and Cu were detected in solutions after the adsorption (pH 2.93–7.03), supporting the high stability of our nano-magnetic CuFe2O4. 4. Conclusion The evaluated nano-magnetic CuFe2O4 manufactured from industrial sludge is fast and effective for Mo removal in solutions. The optimum pH for maximum removal is 2.75 at the pH range of 2.75–11.02. These nano-particles were identified as CuFe2O4 in spinel structure and their SEM morphology showed that the primary particle size fall in a range between 20 and 120 nm. Their adsorption behavior followed Langmuir and Freundlich isotherm models and the Langmuir adsorption capacity was found to be 30.58 mg g1. The pseudo-second-order model accurately describes the adsorption kinetics. Desorption studies demonstrate that the Mo adsorption could easily be replaced in NaOH solution. Eventually, the nano-magnetic CuFe2O4 could be rapidly recovered by a magnet from the solutions. These findings are valuable to design a new technology for treating Mo bearing industrial wastewater or natural water using nano-magnetic CuFe2O4. Acknowledgments This research is financially supported by MOE and NCKU to CFY. The authors would like to thank Prof. Jiang Wei-Teh and Mr. Lee Po-Shu for their support on XRD analysis under the project of NSC1002116M006002. We also thank NSRRC staff for useful discussions and experimental support. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2014.01.084. References [1] J.L. Johnson, H.P. Jones, K.V. Rajagopalan, In vitro reconstitution of demolybdosulfite oxidase by a molybdenum cofactor from rat liver and other sources, J. Biol. Chem. 252 (1977) 4994–5003.
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