catalytic air oxidation of manganese in synthetic.nihankaya.2005

8/18/2019 Catalytic Air Oxidation of Manganese in Synthetic.nihankaya.2005

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CEJC 3(3) 2005 511–519

Catalytic air oxidation of manganese in syntheticwaters

Nihan Kaya∗, Erdal Karadurmus†, Ahmet Alicilar‡§

Chemical Engineering Department,Corum Engineering Faculty, Gazi University,

Corum, Turkey

Received 28 January 2005; accepted 5 May 2005

Abstract: An attempt was made to study the oxidation of manganese by air in synthetic waters.

A series of batch experiments were performed at different values of concentration, temperature

and pH. Unoxidized manganese in the solution was determined by formaldoxime spectrometric

method. Results of these studies indicated that the air oxidation of manganese soluble in water

can be effectively performed in basic media and that oxidation yield increasedwith an increase in

pH and concentration. The yield was very high in the presence of manganese dioxide, sepiolite

or clinoptilolite in solution and, the oxidation was almost completed especially at high values

of pH and concentration. The reaction was found to be first order with respect to Mn2+ with

a very low activation energy. A yield of 62 % was obtained for the air oxidation of wastewater

taken from the treatment plant of Corum Municipality.c Central European Science Journals. All rights reserved.

Keywords: Water pollution, manganese, aeration, oxidation, catalyst

1 Introduction

Manganese is a natural constituent of some soils. It is concentrated in water by contact

with rocks and minerals. Manganese usually does not present a health hazard in the

household water supply. It can, however, affect the flavour and colour of water. It will

typically cause brownish-black staining of laundry, dishes and glassware [1-3].

The water containing high concentrations of manganese is unsuitable for use as drink-

∗ E-mail: [email protected]† E-mail: [email protected]‡ E-mail: [email protected]§ Corresponding author

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512 N. Kaya et al. / Central European Journal of Chemistry 3(3) 2005 511–519

ing water without appropriate treatment. Thus, a simple treatment technique which can

be used widely and for large quantities is needed [4]. Water utilities often remove it by

oxidation using an oxidant such as chlorine dioxide [5]. However, aeration remains an

useful option to oxidize manganese in reservoirs. Aeration induces MnO2 to precipitatewhich is then removed by filtration [1,6]:

Mn2+ + O2 + 2e− → MnO2 (1)

In this paper, the air oxidation of manganese in synthetic waters was studied. A

series of batch experiments were performed at various concentration, temperature and pH

values. The activation energy and the order of reaction were determined. The study was

conducted with the wastewater taken from the treatment plant of Corum Municipality.

2 Experimental

It is known that the rate of air oxidation of manganese in water changes depending on

the pH, amount of manganese and oxygen dissolved in the solution [7]. In the light of

this, the experiments were carried out separately in basic, acidic and neutral media. The

pH was adjusted by adding H2SO4 or Na2CO3 to the solution. Low conversions were

observed in acidic and neutral ones (pH 3 and 7). Therefore, the study was conducted at

four different basic pHs in the range of 8 to 11.

Reagents used were of analytical grade. The manganese solutions of three differ-

ent concentrations (25, 50 and 75 mg Mn2+

/L) were prepared by dissolving appropriateamounts of MnSO4·H2O in water. Air was bubbled through the solution at about 50

mL/s to achieve the oxidation . The reaction temperature was maintained at 20 ◦C. In

order to study kinetics of oxidation, the experiments were repeated at temperatures of

35 and 45 ◦C. The bath temperatures were kept constant by a thermostat.

In order to increase the contact between gas and liquid phases, glass beads with

a diameter of 10 mm were added to the solution. Lastly, the oxidation was tried on

manganese dioxide, sepiolite or clinoptilolite supports. 0.5 g of support was used in 250

mL of solution. The study for manganese dioxide was repeated with an amount of 0.2 g.

The solutions were analyzed by formaldoxime spectrometric method [8]. The oxidationyields were calculated over a time period of 25 minutes, as conversion percentages.

3 Discussion

At first, the experiments were performed in basic, acidic and neutral media. However,

as expected the yields obtained in acidic and neutral ones (pH 3 and 7) were very low

[9-11]. Therefore, the study was conducted at four different basic pHs in the range of 8

to 11. It is apparent from Figure 1 that the conversion increases with an increase in pH

and concentration. This result agreed with those in the literature [7,12,13]. The yield

was above 90 % at high values of concentration (75 mg/L) for a pH of 11 while it was

below 50 % at lower values (25 mg/L and 8).

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Fig. 1 Oxidation yields as a function of pH and concentration (20 ◦C, solution without packing).

An increase in yield with increasing concentration of manganese suggests that dis-

solved oxygen is sufficient for air oxidation of manganese under these experimental condi-

tions. Increase in yield with pH may be explained as a result of formation of manganese

(III) hydroxide. Mn (III) hydroxide is known to be a coagulant and helps accelerate

precipitation leading to a higher level of conversion [14].The second set of experiments were carried out in solution with a packing of glass

beads. As seen from Figure 2, effects of pH and concentration are similar to the previous

ones. However, the yields are higher than those in the solution without packing. This

result may be explained by the fact that the contact efficiency between the gas and liquid

phases increases in the presence of packing [15].

Figure 3 shows the results of experiments where MnO2 was used in addition to glass

beads. In this case, the percent conversion is increased as compared to that in the absence

of MnO2. However, higher yields have also been obtained when the amount of MnO2 was

increased (Figure 4) and is in agreement with the findings of Berbenni et al. [11] thatmanganese oxides catalyze manganese oxidation as efficiently as a chemical catalyst.

High yields have also been achieved when sepiolite or clinoptilolite were used instead

of MnO2 (Figures 5 and 6, respectively). It must be noted that with each of these,

especially at high values of pH and concentration, an almost complete oxidation took

place. However, it must also be stressed that when a larger amount of MnO2 (0.5 g) was

used, the oxidation required shorter time as compared to the other cases.

High yields obtained with all the four different packings may be attributed to the

increase of gas-liquid contact efficiency. However, other effects associated with these

packings such as ion exchange, adsorption and catalysis must also be taken into account.

White and Siddique studied the synthetic manganese dioxide as a material for the re-

moval of Mn(II) from drinking water and observed the ion exchange effect [16]. A filter

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Fig. 2 Oxidation yields as a function of pH and concentration (20 ◦C, solution with glass bead).

Fig. 3 Oxidation yields as a function of pH and concentration (20 ◦C, solution with glass beadsand 0.2 g MnO2).

made of natural clinoptilolitic zeolite covered with manganese oxide was used to remove

manganese from water and the contaminants were removed in the filter bed by adsorption-

oxidation reactions [17]. It was suggested by Simeonova et al. that zeolite covered with

manganese precipitates was highly selective for manganese and this made the treatment of

groundwater with high manganese concentration possible [18]. Berbenni et al. suggested

that manganese oxidation was catalyzed by manganese oxides [11]. Murathan obtained

high efficiency for adsorption of aqueous manganese on sepiolite [19].

It is known that the first order dependence on Mn(II) is a reasonable assumption for

oxidation [7,20]. Therefore, the semi-logarithmic plots of C0/C versus time were plotted

to study the kinetics of oxidation (e.g. Figure 7) and it was demonstrated that the rate of

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Fig. 4 Oxidation yields as a function of pH and concentration (20 ◦C, solution with glass beadsand 0.5 g MnO2).

Fig. 5 Oxidation yields as a function of pH and concentration (20 ◦

C, solution with glass beadsand 0.5 g sepiolite).

removal of manganese is consistent with this assumption. The rate constants calculated

from the slopes of lines drawn are given in Table 1. These values have been calculated

as an average of three different values determined at temperatures of 20, 35 and 45 ◦C

(Table 2).

Thus, the activation energy of reaction can also be calculated by using the Arrhenius

equation [6]. For this purpose, the values of ln k versus 1/T were plotted(e.g. Figure

8) and the calculation of activation energy from the slopes of lines were attempted. As

seen from the figure, the slope of line is close to zero. It may be concluded that the

activation energy of reaction is very low. It would be more realistic to explain the observed

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Fig. 6 Oxidation yields as a function of pH and concentration (20 ◦C, solution with glass beadsand 0.5 g clinoptilolite).

Fig. 7 An example for semi-logarithmic plots of C0

/C versus time (75 mg/L, pH=9, 45

◦

C,solution without packing).

pH k, L/min

8 0.0455

9 0.0518

10 0.0624

11 0.0916

Table 1 Rate constants for 75 mg/L manganese solution without packing.

negligible activation energy as an apparent one and occuring due to a compensation effect

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Temperature, ◦C pH C0 = 25 mg/L C0 = 50 mg/L C0 = 75 mg/L

20 8 47.2 50.0 66.1

9 55.2 61.4 74.1

10 61.2 69.0 79.1

11 70.4 78.2 92.0

35 8 46.4 49.8 65.7

9 54.4 61.0 73.7

10 60.4 68.4 78.7

11 69.6 77.4 91.6

45 8 46.0 49.4 65.5

9 54.0 60.8 73.3

10 60.0 68.0 78.4

11 69.2 77.0 91.1

Table 2 Oxidation yields (%) obtained at various conditions in the solution without packing.

caused by the concentration of dissolved oxygen which strongly decreases with increasing

temperature.

Fig. 8 Dependency on temperature of rate constant (pH=8).

Therefore, a second order kinetic model considering the reaction rate r to be propor-

tional to both C and Sox

, where Sox

is the solubility of oxygen at the given pressure and

temperature, certainly would result in a more realistic determination of activation energy.

Lastly, a study was performed at the wastewater taken from the treatment plant of

Corum Municipality and a yield of 62 % was obtained. This result was an indicator

that the air oxidation can be used to remove manganese from wastewaters; however, the

oxidation must be supported by a catalyst and strong oxidants to achieve higher yields

especially in a reducing media.

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Nomenclature

C Concentration of Mn(II) ions in the solution

C0 Initial concentration of Mn(II) ions in the solutionk Rate constant of reaction

r Reaction rate

Sox Solubility of oxygen in the solution

T Solution temperature

References

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[5] D. Gregory and K. Carlson: “Effect of soluble Mn concentration on oxidationkinetics”, Journal AWWA, Vol. 95(1), (2003), pp. 98–108.

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[12] A. Graveland and P.M. Heertjes: “Removal of manganese from ground water byheterogeneous autocatalytic oxidation”, Trans. Ins. Chem. Eng., Vol. 53, (1975), pp.154–164.

[13] S.H.R. Davies and J.J. Morgan: “Manganese (II) oxidation kinetics on metal oxidesurfaces”, J. Colloid and Interface Sci., Vol. 129, (1989), pp. 63–77.

[14] H. Yalcin and M. Guru: Su Teknolojisi , Palme Yayincilik, Ankara, 2002.

[15] A. Alicilar, A. Bicer and A. Murathan: “The relation between wetting efficiency andliquid holdup in packed columns”, Chem. Eng. Com., Vol. 128, (1994), pp. 95–107.

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[16] D.A. White and A.A. Siddique: “Removal of manganese and iron from drinking waterusing hydrous manganese dioxide”, Solvent Extrac. Ion Change , Vol. 15(6), (1997),pp. 1133–1145.

[17] V. Petkova: “Natural zeolites in manganese removal”, Ingeneria Hidraulica en Mexico,Vol. 12(3), (1997), pp. 41–49.

[18] V.P. Simeonova, M.I. Mintchev and M.L. Rivera: “Pilot study of iron and manganeseremoval from Mexican drinking water supply sources”, Ingeneria Hidraulica en Mexico, Vol. 14(2), (1999), pp. 27–38.

[19] A.S. Murathan: “Removal of strontium, aluminium, manganese and iron ions fromaqueous solutions in packed beds”, Fresenius Env. Bulletin , Vol. 13(6), (2004), pp.481–484.

[20] D. Johnson, B. Chiswell and K. O’halloran: “Micro-organisms and manganese cyclingin a seasonally stratified freshwater dam”, Water Res., Vol. 29, (1995), pp. 2739–2745.

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catalytic air oxidation of manganese in synthetic.nihankaya.2005

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