Reduction and Immobilization of Potassium Permanganate on Iron Oxide Catalyst by Fluidized-Bed Crystallization Technology

Abstract

A manganese immobilization technology in a fluidized-bed reactor (FBR) was developed by using a waste iron oxide (i.e., BT-3) as catalyst which is a by-product from the fluidized-bed Fenton reaction (FBR-Fenton). It was found that BT-3 could easily reduce potassium permanganate (KMnO₄) to MnO₂. Furthermore, MnO₂ could accumulate on the surface of BT-3 catalyst to form a new Fe-Mn oxide. Laboratory experiments were carried out to investigate the KMnO₄-reduction mechanism, including the effect of KMnO₄ concentration, BT-3 dosage, and operational solution pH. The results showed that the pH solution was a significant factor in the reduction of KMnO₄. At the optimum level, pH_f 6, KMnO₄ was virtually reduced in 10 min. A pseudo-first order reaction was employed to describe the reduction rate of KMnO₄.

1. Introduction

Recently, numerous researches have focused on adsorptive arsenic removal, in which iron oxide was considered to be an appropriate adsorbent due to its high affinity [1,2,3,4,5,6,7]. However, the As(III) adsorption is less effective than the As(V) adsorption by adsorbents in natural water. Arsenite [As(III)] is much more toxic, soluble and mobile than arsenate [As(V)]. Manganese oxides have been extensively investigated as oxidizing agents for arsenite; the reaction is written as follows [8,9,10]:

MnO₂ + H₃AsO₃ +2H⁺ = Mn²⁺ + H₃AsO₄ + H₂O

Accordingly, a novel binary oxide concept in which the Mn-O catalyzed the As(III) pre-oxidation to As(V) and the Fe-O functioned as adsorbent, was therefore proposed [11,12]. To our knowledge, the Mn-Fe binary oxide materials was easily prepared by the co-precipitation methods, where the Mn reduction by ferrous is dependent on the solution pH [12,13]:

3Fe²⁺ + MnO₄⁻ + 4OH⁻ + 3H₂O → 3Fe (OH) ₃(s) + MnO₂(s) + H⁺

However, the powder product of micrometers in size was difficul for solid-liquid separation. Based on the perspective of Mn-Fe binary oxide for arsenic removal, this study applied a millimeter scale iron oxide (BT-3) as support and aims at the synthesis of manganese oxides on its surface through the redox of potassium permanganate ⁺ in a fluidized bed reactor (FBR). The FBR technology could immobilize potassium permanganate onto BT-3, which was a waste iron oxide from real fluidized bed-Fenton (FBR-Fenton) instruments and was mainly composed of poorly crystallized goethite [14].

Therefore, the employment of BT-3 as the support had the advantages of being cost-efficient and facilitated easier separation and disposal during the synthetic process of binary oxide, and during the application for arsenic removal. Our previous studies have explored the ability of BT waste iron oxide as an excellent adsorbent for F⁻, PO₄³⁻, Cu²⁺, and Pb²⁺ removal because of its high surface area and porosity nature [14,15,16]. In this study, we attempted to investigate the feasibility of Mn immobilization onto BT-3 without ferrous addition, and then to develop a novel Mn-Fe binary oxide using fluidized-bed crystallization technology. The characterization of BT-3 was examined first. Then, the Mn removal efficiency in solution was detected as a function of reaction time. Furthermore, the effect of KMnO₄ concentration, BT-3 dosage, and initial solution pH_i was studied through bench scale studies. The optimized parameters were also proposed. Finally, a possible mechanism to synthesize the novel Mn-Fe was rationally proposed via the kinetic study of Mn removal.

2. Material and Methods

The experimental set-up was composed of a 500 mL FBR and a pump which are shown in Figure 1. 40 g (or 100 g) of the fluidized media (BT-3) was placed into the reactor. The dimensions of the reactor were 480 cm³ at high 80 cm. The input flow rate was 60 mL/min and the reflux flow rate was 150 mL/min. The concentration of the potassium permanganate was around 20 mg/L to 70 mg/L. The potassium permanganate (KMnO₄) in the aqueous solution was fresh prepared from KMnO₄ (Merck), and the solution pH_f was adjusted using NaOH and HClO₄. At the selected time intervals, the samples were withdrawn from the top of the FBR by syringe. Each sample was filtrated immediately with a 0.45 mm membrane and measured the concentration of KMnO₄ with a UV–vis detector. The UV-vis detector was scanning from 200 to 800 nm. The total iron ([Fe]) and manganese ([Mn]) content in samples without filtration were dissolved in concentrated HCl and then were measured with an atomic absorption spectrophotometer. The immobilizations of potassium permanganate defined as ([KMnO₄]_i − [KMnO₄]_a)/ [KMnO₄]_i indicated that the extent of Mn immobilized onto support media rather than through the formation of a precipitate in solution. Where i is the initial concentration, and a is the KMnO₄ concentration after adsorption.

Figure 1. Fluidized bed reactor.

Figure 1. Fluidized bed reactor.

The physic-chemical characteristics of the BT-3 were examined using standard procedures. The morphology of the BT-3 was determined using a JEOL JSM-6700F HR-FESEM. An XRD powder diffraction measurement of BT-3 was performed on a powder diffractometer (Rigaku RX III) using Cu Kα radiation. The accelerating voltage and current were 40 kV and 30 mA. Brunauer-Emmett-Teller (BET) surface area and porosity of the adsorbents were obtained from the isotherms. The surface area of the adsorbents was calculated from the BET equation.

3. Results and Discussion

3.1. Characterization of BT-3

The properties of BT-3 are listed in Table 1. The average particle size is about 0.5–1 mm. Furthermore, the bulk density and true density are 1.56 and 2.38 g cm⁻¹, respectively, which indicates that it is easy to separate from aqueous solutions. The BET specific surface area and pore volume of BT-3 are 174 m²/g-solid and 0.14 m²/g-solid, respectively, which reveals the high surface area of BT-3 adsorbent. Figure 2 displays four different magnifications of the morphology of BT-3. An oval shape with irregular surface morphology reveals its high surface area. Figure 3 shows the XRD patterns of BT-3. According to the diffraction files of the Joint Committee on Powder Diffraction Standards (JCPDS), the main diffraction peaks of BT-3 adsorbent at 2θ = 21.5°, 36.7°and 53.3° adsorbent was identified as the α-FeOOH phase.

Table 1. Physical properties of BT-3.

Properties	BT-3
Color	Black
Material	Iron oxide
Bulk density (g·cm−1)	1.56
True density (g·cm−1)	2.38
Total iron content (g·Kg−1)	649
Specific surface area (BET) (m2/g-solid)	174
Pore vol. (cm3/g)	0.14
Average grain size (mm)	0.5–1

Table 1. Physical properties of BT-3.

Properties	BT-3
Color	Black
Material	Iron oxide
Bulk density (g·cm−1)	1.56
True density (g·cm−1)	2.38
Total iron content (g·Kg−1)	649
Specific surface area (BET) (m2/g-solid)	174
Pore vol. (cm3/g)	0.14
Average grain size (mm)	0.5–1

Figure 2. SEM-EDS images of (a) BT-3,(b) BT-3 covered by MnO₂.

Figure 2. SEM-EDS images of (a) BT-3,(b) BT-3 covered by MnO₂.

Figure 3. X-ray diffraction pattern of the (a) standard of αFeOOH (b) BT-3 adsorbent.

Figure 3. X-ray diffraction pattern of the (a) standard of αFeOOH (b) BT-3 adsorbent.

3.2. Effect of KMnO₄ Concentration

Figure 4 shows the effect of initial concentration of KMnO₄ on its immobilization efficiency onto BT-3 (BT-3 = 100 g, pH_i = 2.5), in which the reduction rate of KMnO₄ decreased with increasing KMnO₄ concentration from 20 to 70 mg/L. The time for complete decomposition of 20 mg/L KMnO₄ is 1.5 h, and that with 70 mg/L KMnO₄ is even longer than 6 h.

Figure 4. The effect of initial concentration of KMnO₄ on the immobilized efficiency onto BT-3.

Figure 4. The effect of initial concentration of KMnO₄ on the immobilized efficiency onto BT-3.

3.3. Effect of BT-3 Dosage

As shown in Figure 5, when the pH_i, and the initial concentration of KMnO₄ was 2.5 and 35 mg/L, the reduction rate increased with increasing the dose of adsorbent. The maximum removal rate was exhibited at a dosage of 100 g adsorbent when the initial concentration of KMnO₄ was 35 mg/L. Reusing BT3 or higher reusing BT-3 dosage may be attributed to the limitation of catalytic activity sites the BT-3 could provide, which results in a decrease in reduction efficiency of KMnO₄.

Figure 5. Effect of dosage of BT-3 on the immobilization of KMnO₄ in binary system.

Figure 5. Effect of dosage of BT-3 on the immobilization of KMnO₄ in binary system.

3.4. Effect of the Operational pH Level

The solution pH was a significant factor on the reduction of KMnO₄. In this study, we applied four different operation pH levels which used the NaOH to adjust the operational pH level. Figure 6 indicated the effect of different PH level on the reduction and immobilization of KMNO₄ in which the concentration of KMnO₄ was 20 mg/L and the BT-3 dosage was 100 g.

Figure 6. Effect of pHf on the reduction and immobilization of KMnO₄.

Figure 6. Effect of pHf on the reduction and immobilization of KMnO₄.

The optimum pH_i was about 6, at which KMnO₄ could be virtually reduced to MnO₂ in 10 min. Chen [17] discussed the reduction of KMnO₄ in H₂SO₄ solution at temperature of 70–95 °C for 30 min, the chemical reaction between potassium permanganate and sulfuric acid can be formulated as:

MnO₄− + 8H⁺ + 5e⁻== Mn²⁺ + 4H₂O

2MnO₄− + 3Mn²⁺ + 2H₂O == 5MnO₂ + 4H⁺

It showed low pH of the reaction system would prohibit the formation of MnO₂. Therefore, a neutral or weak acid system medium is preferable for such reaction.

3.5. The Possible Mechanism of KMnO₄ Immobilization on BT-3

The probable reaction of KMnO₄ reduction in weak acidic condition could be written as follows:

MnO₄− + 4H⁺+ 3e⁻ == MnO₂+ 2H₂O (1)

The MnO₂ formed from the reduction of KMnO₄ could be adsorbed on the BT-3. However, contrary to the reduction of KMnO₄ with homogeneous catalytic oxidation, as shown in Equation (3), this study aimed to investigate the reduction of KMnO₄ with heterogeneous catalysis by iron oxide, which is seldom discussed in the literature.

According to Ma and Lin [18,19], the reduction of KMnO₄ could proceed in two possible stages. In the first stage, the BT-3 surface was positively charged via hydrolysis in acidic condition. MnO₄⁻ ions were absorbed, thereby producing a heterogeneous MnO₄⁻ film on the BT-3.

≡FeOOH + H⁺ ≡FeOOH₂⁺ (2)

≡FeOOH₂⁺ + MnO₄⁻ ≡FeOOH₂-MnO₄ (3)

Moreover, Chou et al. [20], proposed that iron is generated as Fe²⁺ by reductive dissolution of FeOOH. Therefore another possible mechanism is proposed for the acid condition.

FeOOH(s) + 3H⁺ + e⁻ Fe²⁺+ 2H₂O (4)

3Fe²⁺ + MnO⁴⁻ + 4OH⁻ + 3H₂O 3Fe (OH) ₃(s) + MnO₂(s) + H⁺ (5)

3.6. Kinetics of KMnO₄ Decomposition

The results concerning the immobilization of KMnO₄ at pH_i = 2.5 in the presence of 100 g BT-3 particles are presented in Figure 7.

Figure 7. First-order fit of KMnO₄ decomposition for different concentrations of KMnO₄.

Figure 7. First-order fit of KMnO₄ decomposition for different concentrations of KMnO₄.

The logarithmic KMnO₄ concentration is plotted as a function of the reaction time for different initial concentrations of KMnO₄, ranging from 20 to 70 mg/L. The well fitted data to a straight line indicates that the decomposition of KMnO₄ in the presence of BT-3 follows a first-order kinetic rate law:

−d [KMnO₄]/dt = k [KMnO₄]

thus ln[KMnO₄]/[KMnO₄]₀ = −kt

where k is the observed first-order rate constant and [KMnO₄] and [KMnO₄]₀ are the concentrations of KMnO₄ in the solution at any time t and time zero, respectively. The lines were treated by linear regression, which produced correlation coefficients >0.982. Furthermore, the k is 2.34, 1.04 and 0.45 when the concentration of KMnO₄ is 20, 35 and 70 mg/L, respectively.

Obviously, initially the first-order date did not intersect with the origin; this is probably due to the trace amounts of Fe²⁺ on the surface of BT-3 which could accelerate the reduction in the short term. To prove this, Fe²⁺ with a molar ratio Fe:Mn = 2:1, was added in the same experiment but without BT-3 (date not shown), the KMnO₄ was reduced in 10 min, then almost maintained at the same level.

Conclusion

A new and novel Mn-Fe binary oxide was synthesized using a fluidized bed reactor. In the binary system, through in situ monitoring of the KMnO₄ concentration, the decomposition of KMnO₄ in the presence of BT-3 follows a first-order kinetic rate law. Low KMnO₄ concentration is advantageous in increasing the reduction rate, which decreased from 2.34 to 0.45 when the KMnO₄ concentration was 20 and 70 mg/L, respectively. High BT-3 dosage could also be favorable to the reduction rate. Furthermore, the optimum pHi condition for the immobilization of KMnO₄ is 6, and KMnO₄ could be virtually reduced in 10 min. Finally, a possible mechanism of KMnO₄ immobilization, where the BT-3 was found to act as the substrate and reduction agent, was proposed.