Efficient Removal of Cr(VI) with Fe/Mn Mixed Metal Oxide Nanocomposites Synthesized by a Grinding Method

Fe/Mn mixed metal oxides were synthesized facilely by a grinding method and were characterized by TEM, XRD, XPS, and BET. The characterization results revealed that mixed metal oxides were mainly composed of not highly crystallized Fe 2 O 3 and Mn 3 O 4 nanoparticles with a diameter about 3–5 nm. The specific BET surface areas of the composite were affected by the amounts of KCl diluent in the preparation process and about 268m/g of the composite can be achieved. Compared with metal oxide adsorbents existent, the composites showed good adsorption capacity, stability, and regeneration activity for Cr(VI) removal. The enhanced adsorption capacity was speculated to be ascribed to the synergistic effect of the mixed metal oxides. By monitoring the valence change in the adsorption process using XPS characterization, the mechanism for Cr(VI) removal on the composites was found to be a combination of electrostatic attraction and ion exchange. The above results demonstrated that the synthesized metal oxides nanocomposite is of great potential for Cr(VI) removal in the fields of remediation of environmental problems.


Introduction
In the past years, the removal of heavy metals from industrial waste water streams has been widely studied due to the high threat of such pollutants to public health and environment [1][2][3][4]. The contamination of hexavalent chromium species is a major problem in many industrial areas, such as electroplating and metal finishing industries [5][6][7]. With the development of the stringent ecological standards, it is urgent for these industries to reduce the chromium in their effluents to an acceptable level before discharging into municipal sewers. Various methods have been developed to remove the chromium from industrial waste water, such as chemical redox followed by precipitation, ion exchange, and reverse osmosis. However, these methods have many drawbacks [8].
Currently, for large-scale used chemical redox method, a major drawback related to precipitation is slurry production. Both ion exchange and reverse osmosis methods are not economically attractive as a result of their high operating costs [8]. Recently, adsorption has been recognized as one of the most promising techniques for removal of chromium and its effectiveness has been demonstrated [5][6][7]9].
In the whole adsorption process, the adsorbents play an important role. How to choose a suitable adsorbent is still a challenge. Several efforts have been devoted to the synthesis of a series of adsorbents. In general, a good adsorbent should not only process high adsorption efficiency but also be regenerated. Activated carbon, processing high BET surface areas, was adopted as an adsorbent to remove the heavy metals and better adsorption efficiency was achieved [10]. However, activated carbon which was commonly used by commercial regenerators was regenerated at a rather high temperature. Ho et al. [11] found that Sphagnum peat moss is an effective adsorbent for hexavalent chromium and 50% of chromium recovery was obtained. Srivastava [12] discovered that sawdust is effective for removal of hexavalent chromium and other heavy metals. These adsorbents also have a disadvantage which is that they could not be regenerated. Therefore, the search of new adsorbents for the removal of hexavalent chromium has directed attention to synthesize new materials with high adsorption efficiency and good regeneration ability. Meanwhile, the development of nanoscience and nanotechnology is expected to provide a solution for the remediation of environmental problems. Thus, the design and application of novel nanomaterials for environmental protection have been receiving more and more attention in recent years. Now, several researchers have focused on metal oxide nanoparticles and their composites for their high BET surface areas and fast adsorption kinetics when subjecting to the treatment of chromium from waste water. Zhong et al. [13,14] [17]. After that, they coated the -Fe 2 O 3 nanoparticles with a layer of FeOOH and the adsorption capacity for Cr(VI) was improved (about 25.6 mg/g) [18]. These results demonstrate that mixed metal oxide nanocomposites possess high adsorption capacity for removal of Cr(VI). The development of metal oxide nanocomposites, which can be used as adsorbent for Cr(VI), is of both economic and environmental significance. A growing number of methods, such as coprecipitation, sol-gel, hydrothermal methods, were involved to solve the problem. As some of mechanochemical syntheses show great potential with regards to not only energy saving and reducing cost but also improved safety, this technology should prove to be highly attractive when substituting methods of preparing conventional adsorbent [19].
In this paper, we use a grinding method to synthesize the Fe/Mn mixed oxides nanocomposites with high BET surface areas. The synthesized composites were applied in the removal of Cr(VI) and the high adsorption capacity, stability, and regeneration ability of the product were achieved. It is expected that the insight provided by this study will contribute to the Cr(VI) removal from the waste water.

Materials Preparation.
For the synthesis of mixed metal oxide nanocomposites, 6.986 g Fe(NO 3 ) 3 ⋅6H 2 O and 1.979 g MnCl 2 ⋅4H 2 O and KCl powders (with different ratio of MnCl 2 to KCl) were mixed together and ground to a very fine powder using an automatic electrical mortar at room temperature for 10 min. The mixture obtained from the grinding was a brown paste. To the mortar was added KOH powder, followed by grinding for another 30 min at room temperature. A significant amount of heat and some vapor was given off in the first few minutes of the KOH addition. After the reaction, the resultant composite was diluted by DI water, then filtered and washed with DI water repeatedly until no Cl − could be detected. The precipitate was collected and dried in an oven at 70 ∘ C overnight. A comparison experiment was conducted by using Fe(NO 3 ) 3 ⋅6H 2 O or MnCl 2 ⋅4H 2 O as only the starting material.

Materials Characterization.
The resultant composites were characterized by powder X-ray diffraction (XRD), BET, X-ray photoelectron spectroscopy (XPS), and high resolution transmission electron microscopy (HRTEM). XRD measurements were conducted on a PAnalytical X'Pert Pro X-ray Diffractometer equipped with Cu-K radiation ( = 1.54178Å), applying 40 kV voltage and 40 mA current. The BET specific surface area was determined by N 2 adsorption at 77 K using a Micromeritics ASAP 2000 system after the sample was degassed in vacuum at 130 ∘ C overnight. XPS spectrum was obtained in a Physical Electronics Corporation PHI 5600 machine with Al K radiation (350 W) under 10 −7 Pa and calibrated internally by carbon deposit C 1s (285.0 eV). TEM observation was carried out on a JEOL JEM-2010F electron microscope equipped with a Bruker Energy Dispersive Spectrometer (EDS). The amount of iron and manganese leaching from the composites was measured using ICP-AES (Perkin Elmer Optima-3000XL). The concentration of the Cr(VI) was measured using an UV-Vis spectrometer (Shimadzu UV-Vis 5000) according to the diphenyl carbazide method [20].

Adsorption Capacity
Test. The experimental procedures were according to the processes reported by Hu et al. [16][17][18]. The adsorption studies were performed by mixing 0.2 g nanocomposites with 40 mL K 2 CrO 4 solution of varying concentration in a 100 mL stopper conical flask. The pH of the suspensions was adjusted using 0.1 M HNO 3 and 0.1 M NaOH solutions. All of the Cr(VI) solutions have a matrix of 0.1 M NaNO 3 to keep the ion strength relatively constant. All the adsorption experiments were carried out at 25 ∘ C, pH 2, and shaking rate of 200 rpm unless stated otherwise. Adsorption equilibrium studies were conducted by shaking 20 mL of Cr(VI) solution at initial concentrations varying from 10 to 200 mg/L and 0.1 g of adsorbent nanoparticles. To study the stability of the adsorbent, 0.01 M NaOH solution was used as eluant and five cycles of adsorption-desorption processes were carried out. For each cycle, 40 mL of 100 mg/L Cr(VI) solution was adsorbed on 0.2 g nanocomposites for 30 min and then desorbed with 20 mL of 0.01 M NaOH solution for 1 h. After each cycle of adsorption-desorption, the nanocomposites were washed thoroughly with ultrapure water to neutrality and reconditioned for adsorption in the succeeding five cycles.  Figure 1(c) showed the obtained Mn 3 O 4 nanoparticles. The picture presented that the Mn 3 O 4 nanoparticles were highly crystalline and the diameter of the particles is about 10-50 nm, which is larger than that of mixed Fe/Mn metal oxides. Meanwhile, when using Fe(NO 3 ) 3 ⋅6H 2 O as the only starting material, the Fe 2 O 3 nanoparticles with a diameter of about 3-7 nm were obtained and these nanoparticles were also aggregated, as shown in Figure 1(d). By comparison of these four pictures, it was shown that the morphology of the mixed metal oxides was similar to that of Fe 2 O 3 nanoparticles. Furthermore, we did not see any particles with size of more than 10 nm. This observation implied that the size of Mn 3 O 4 nanoparticles was reduced and the crystallinity was affected when using both Fe(NO 3 ) 3 ⋅6H 2 O and MnCl 2 ⋅4H 2 O as the starting material.

Results and Discussion
XRD characterization was performed to determine the crystalline structure of these composites and the results were shown in Figure 2. For the sample prepared using MnCl 2 ⋅4H 2 O as the starting material, the diffraction pattern in Figure 2(a) agreed well with those for standard Mn 3 O 4 (JCPDS 18-0803). For the sample prepared using Fe(NO 3 ) 3 ⋅6H 2 O as the starting material, the diffraction peaks of as-synthesized products match well with those of Fe 2 O 3 (JCPDS 25-1402), although the nanoparticles are not wellcrystallized. However, when using both Fe(NO 3 ) 3 ⋅6H 2 O and MnCl 2 ⋅4H 2 O as the starting material, only a weak diffraction peak was detected for the prepared composites, as shown in Figure 2(c). This observation indicated that Fe/Mn mixed metal oxide nanocomposite was not well-crystallized, which was consistent with the TEM observations. Moreover, we found that adding of Fe(NO 3 ) 3 ⋅6H 2 O led to the amorphous structure of the composites. Thus, owing to the not wellcrystallized structure and small size of the sample, it was difficult to determine the exact structure of the obtained composite. In such a process, there were two possible products. One is to form MnFe 2 O 4 nanoparticles. The other is mixed metal oxide composites. Due to the thermodynamic behavior of the MnFe 2 O 4 nanoparticles [21] and the synthetic condition in our experiment, the obtained composites can only be made of mixed metal oxides. From the TEM and XRD analysis above, we speculated that the resultant composites may be made up of not well-crystallized Fe 2 O 3 and Mn 3 O 4 nanoparticles. However, this speculation has to be evidenced.
XPS characterization was involved to determine the structure. Figure 3 shows the XPS overall spectrum and the Mn 2p and Fe 2p spectra of the synthesized composites. The surface analysis indicated that only Fe, Mn, O, and C existed on the sample surface, implying that no other impurities existed in the product. The peak at 285.0 eV comes from the deposited carbon. From the Fe 2p spectra, the XPS peaks have a binding energy of 711.3 eV and a satellite feature centered at 719.9 eV, which is indicative of Fe(III) [3]. It can be concluded that only fully oxidized iron was on the sample. For the Mn species, the Mn 2p 3/2 binding energy is 641.9 eV and Mn 2p 1/2 binding energy is 653.6 eV, coming from Mn 2p 3/2 and Mn 2p 1/2 of Mn 3 O 4 , which was in accordance with [21]. This result indicates that the MnCl 2 ⋅4H 2 O species were fully oxidized into Mn 3 O 4 . Based on the aforementioned characterizations, it could be suggested that the synthesized composites were mixed metal oxides composed of Fe 2 O 3 and Mn 3 O 4 . These results also demonstrated the efficiency of the grinding method for preparation of the mixed metal oxide composites.

Adsorption Kinetic Study.
The as-synthesized composites were used as adsorbents for Cr(VI) removal. Figure 4 showed the effect of contact time on the adsorption of Cr(VI). It can be seen that the uptake of Cr(VI) was finished within 10 min with 93% of the Cr(VI) removed during the first minute of the reaction. The equilibrium time is independent of the initial concentration of Cr(VI). The rapid adsorption of Cr(VI) by the composite is different from the microporous adsorption process. It is perhaps caused by external surface adsorption which is easy to access, while adsorption sites occurred in the exterior of porous adsorbents during the microporous adsorption process. At equilibrium, the adsorption capacities of Cr(VI) at initial concentrations of 50 and 150 mg/L were found to be 9.62 and 26.30 mg/g, respectively.
Compared with another oxide and their composites reported, the Fe/Mn mixed oxides nanocomposites were found to show higher adsorption capacity, as shown in Table 1. At the same time, the adsorption capacity and BET surface areas of the Fe/Mn mixed oxides nanocomposites were strongly dependent on the ratio of MnCl 2 ⋅4H 2 O to KCl. For a grinding process, it is generally accepted that the necessary condition taking place in solid state reaction at room temperature is that one of the reactants should have crystalline waters or a low melting point on one hand, whilst on the other hand sufficient grinding is also required to provide the reactant molecules with more contact opportunities [22]. When the salts with crystalline water and other reactants were reacted, a layer of liquid film over particles was considered to be formed to construct a micro-aqueousenvironment. Hereby, KCl and free water are expected to give a saturated solution of KCl. Then KCl was released to produce a shell surrounding the nanoparticles to prevent them from aggregating into larger particles [23]. Thus, the higher the MnCl 2 ⋅4H 2 O to KCl ratio, the lower the crystal size and the higher BET surface areas. Secondly, the adsorption capacity of Fe/Mn mixed oxides composites was higher compared with separated -Fe 2 O 3 and Mn 3 O 4 . At the same time, we mixed the -Fe 2 O 3 and Mn 3 O 4 metal oxides together with Fe : Mn mole ratio equal to 2. The mixed adsorption capacity for Cr(VI) removal is lower than the prepared Fe/Mn mixed metal oxides. We speculated that the enhanced adsorption capacity may be caused by the synergistic effect of the mixed metal oxides.

Adsorption Isotherm.
The adsorption isotherm experimental results for Cr(VI) on composites at pH 2, 7, and 10 and different initial Cr(VI) concentrations were shown in Figure 5. It was revealed that Cr(VI) uptake increased with the initial Cr(VI) concentrations for the composite. The equilibrium adsorption capacity decreased with an increase in pH, which indicates that the lower pH favors adsorption. The results were accordant with the results reported by Chen and his group [16,17]. It appears that adsorption isotherms on the composites can be well described by the Langmuir equation.

Adsorbent Stability.
To test the stability of the resultant composites, desorption and regeneration experiments were carried out. Figure 6 shows the reusability of the composites for Cr(VI) removal. For the desorption process, 0.01 M NaOH eluant was used and more than 95% of Cr(VI) could be removed from the adsorbent of nanocomposites within 1 h. The adsorbent was recovered by filtration and drying. Recovered nanocomposites were reused again for the adsorption Journal of Nanomaterials of Cr(VI) and the adsorption capacity was tested. After 5 runs, the adsorption capacity tended to become steady. In the fifth cycle, the adsorption capacity was 25.3 mg/g which was 92% of the first cycle. The results indicated that the composites showed good stability and regeneration ability. In addition to chromium, iron and manganese were measured using ICP after each adsorption/desorption process, and the concentrations of these two metals were found to be nearly zero, indicating that the dissolution of the nanoparticles under the stated experimental conditions can be neglected.

Adsorbent Mechanism.
For the removal of Cr(VI) from metal oxides, two commonly used mechanisms were reported by other researchers [16,18]. One is the electrostatic attraction; the other is ion exchange. To find out any possible mechanism for Cr(VI) removal from the nanocomposites, the XPS characterization for the adsorption process was conducted. The first problem needed to be excluded is the valence change of the chromium during the adsorption process. If the valence of the chromium changed from hexavalent to trivalent, it means that a chemical redox reaction happened and the whole process was not an adsorption process. Figure 7 showed the XPS spectra of Cr, Fe, and Mn species before adsorption and after adsorption at pH equal to 2. The peak for Cr 2p 3/2 was centered at 579.8 eV and the peak for Cr 2p 1/2 was centered at 587.2 eV, and both were entirely from Cr(VI) [24]. The Fe 2p 3/2 spectrum registered at 711.5 eV, and the Fe 2p 1/2 spectrum registered at 724.3 eV in the XPS spectra, which indicated fully oxidized iron on the surface. Mn 2p 3/2 and Mn 2p 1/2 peaks centered at 641.9 eV and 653.6 eV, respectively, which came from Mn 3 O 4 . However, the valence of the adsorbed chromium was not changed and the valence of manganese should not vary, because iron is fully oxidative. Thus, it can be suggested that there is no chemical redox reaction occurring in the adsorption process. But this does not mean that the electrostatic attraction is the only mechanism in the adsorption process. When pH is greater than pH zpc (=6.5), no Cr(VI) should be adsorbed on  the composites owing to the electrostatic repulsion. However, a certain amount of Cr(VI) ions was still adsorbed on the composites when the pH equal to 10. Therefore, there should be another mechanism involved in the chemical process. At pH greater than pH zpc (=6.5), it is reported that the chromate can replace hydroxide from the surface of the hydrolyzed metal oxides as a result of the higher affinity of chromate ions with metal oxides than that of hydroxides with metal oxide [25]. Thus, it is concluded that the adsorption mechanism on the composite is the combination of electrostatic attraction and ion exchange.

Conclusion
In conclusion, Fe/Mn mixed metal oxide nanocomposites were successfully prepared using a grinding method at room temperature. The characterization results revealed that  the as-synthesized composites were comprised of not wellcrystallized Fe 2 O 3 and Mn 3 O 4 nanoparticles with a diameter about 3-5 nm. Compared with other oxides and their composites, the obtained composite was a very attractive adsorbent for the removal of the Cr(VI). The enhancement of the adsorption capacity was attributed to its higher BET surfaces and the synergistic effect coming from the mixed metal oxides. The resultant composite also showed good stability and regeneration activity. Combining the XPS results in the adsorption process, the mechanism for Cr(VI) removals on the composites was found to be a combination of electrostatic attraction and ion exchange. The new Fe/Mn mixed metal oxide composites synthesized in this work demonstrated a great potential application in the Cr(VI) removal.