Keywords

1 Introduction

1.1 Effect of Manganese to the Water

Manganese naturally exists in earth’s crust and it aggregates to water due to the dissolution of Manganese bearing minerals and rocks such as Manganosite, Manganite, Bixbyte when there is weathering. Until Manganese is in groundwater, Manganese exists as a stable form, but as soon as it is exposed to the outside air it oxidizes and precipitates out naturally (Salem et al. 2012).

Manganese generally does not affected human health but it may cause undesirable taste, appearance and straining (Rozainy et al. 2015). Presence of Manganese bearing bacteria in water distribution systems causes bacterial growth in pipes. From the consumer’s view, washing machines and boilers can be damaged due to oxidization and precipitation under conditions of excess Manganese (Barlokova and Ilavský 2009).

1.2 Manganese Phase Diagram

The Manganese phase diagram, (Fig. 1) shows the relationship of the redox potential and hydrogen-ion concentration that are relevant with specific dissolved ions and solids at chemical equilibrium in the presence of water. Phase boundary lines are called Stability field boundaries and in their activities, each adjacent species is nearly equal.

Fig. 1.
figure 1

Manganese phase diagram

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pe is accessible as tendency of solution to donate or accept protons while Eh is known as the potential of the solution relative to the Standard Hydrogen Electrode [SHE]. In here SHE is the reference point and when all conditions are constant no reactions occur and the Eh value can be obtained from connecting the electrode containing another solution.

When Eh is higher, the solution is desirable to accept electrons from another species and when Eh is lower, the solution is desirable to donate electrons to the species. Therefore solution is reducing and oxidizing respectively.

1.3 Redox Reactions

Redox reactions are typically happens in a solution by oxidizing or reducing an electron. Hence, these reactions of oxidation or reduction do not exist in nature individually because free electrons never exist in a solution. Releasing of any electron from one species and its gain by another species happens simultaneously (Appelo and Pstma 2005).

$$ Ox_{1} + Red_{2} \to Red_{1} + Ox_{2} $$

Due to this, pe of the solution cannot be measured practically. Eh can be discovered not only by measuring but also by calculations of the Nernst equation. Therefore, Eh is the integration of both the calculated Eh and the measured Eh (Christensen et al. 2000).

$$ {\text{Eh}} = {\text{E}}_{\text{measured}} + {\text{E}}_{\text{reference}} $$
(1)

Both pe and Eh measure essentially the same and therefore the relationship between them can be written as follows;

$$ Eh = \frac{2.303\;RT}{F} pe $$
(2)

1.4 Behavior of Manganese in Water

Manganese is the 25th element of the periodic table and the most stable oxidation state is +2. When Manganese reacts with water, Mn(OH)2 is obtained but it can change with the condition of the water.

$$ Mn^{2 + } + 2H_{2} O \to Mn\left( {OH} \right)_{2} + H_{2} $$

1.5 Behavior of the Calcium Hypochlorite in Water

Calcium hypochlorite, which has the commercial name of bleaching powder, is an inorganic compound. It is mostly used in water treatments because of the high efficiency and low cost. It exists as a powder form and it should be dissolved with water to acquire Calcium hypochlorite solution.

$$ Ca\left( {OCl} \right)_{2} + 2H_{2} O \to Ca\left( {OH} \right)_{2} + 2HClO $$

1.6 Reaction of Manganese and Calcium Hypochlorite

While Ca(OH)2 is a precipitate Mn(OH)2 is react with HClO.

$$ Mn\left( {OH} \right)_{2 } + 2HClO \to MnO_{2 } + CaCl_{2} + 2H_{2} O $$

1.7 Objective of the Study

To investigate the behavior of pH and Eh of the solution while changing the different concentration of Calcium hypochlorite, in certain concentrations of the Manganese rich solution by studying the Manganese phase diagram.

To discover the particulate form of the Manganese species which can be removed by filtration.

2 Materials and Methodology

2.1 Analytical Techniques

2.1.1 Determination of pH Value

pH values of the water samples were directly measured by the pH meter. Before that, the pH electrode was removed from the storage solution, rinse with distilled water and dried with tissue paper. Then, the water sample was taken to the beaker and the pH electrode was placed in it and the value on the digital screen was recorded. This pH value is at fixed temperature.

2.1.2 Determination of Concentration of Manganese

UV spectrophotometer was used to measure the dissolved Manganese concentration of the sample. A program was selected to measure the concentration of Manganese and the wavelength was kept as 525 nm. Water sample was filled to the two 10 ml cells and one is kept as reference. A Manganese citrate buffer powder pillow was added to the other cell and shaken until it dissolved well. Then, Sodium periade reagent was added and kept for two minutes to complete the reaction. After two minutes, the reference sample was placed inside the hole of the UV spectrophotometer and calibrated to zero. After it became zero, the working cell was placed and the reading on the digital screen of UV spectrophotometer was recorded.

2.1.3 Determination of Oxidation Reduction Potential (ORP) Value

Same pH meter was used to measure the ORP value and it demonstrated the reference Eh value. To get the ORP value, the program was changed by pressing the ‘Option’ button. ORP value is record as millivolt.

2.1.4 Determination of Residual Chlorine (RCl) Value

Residual chlorine value gives the remaining free chlorine amount of the sample. It can be measured directly by using RCl meter. For that, the sample is filled into two cells and one was kept as reference. DPD free chlorine reagent was added to the other sample and shaken well until it fully dissolved. If Chlorine is present in the sample, light pink color appears. The reference sample was placed in the hole inside the RCl meter and calibrated to Zero and then the working sample was placed and the value given in the digital screen was recorded.

2.2 Preparation of Manganese Solution

5 L of 0.9 ppm Mn2+ solution was freshly prepared by dissolving 0.0124 g of MnSO4 in to raw water sample collected from Maduru Oya. A raw water sample was used instead of tap water or distilled water, in order to obtain the natural environmental condition.

2.3 Preparation of Calcium Hypochlorite Solution Series

1L of 10 ppm Calcium hypochlorite solution was made by dissolving 0.01 g of Bleaching powder in to distilled water. Bleaching solution series was selected with respect to the Residual Chlorine value given from the stock sample.

2.4 Method

Initial concentration of the Mn2+ measured by using UV spectrophotometer and Bleaching solution series was added to the 4 beakers which were filled with 500 ml of Mn2+ sample. Each sample was mixed continuously using a shaker for almost 30 min. Finally pH, RCl, ORP and final concentration of Manganese was measured.

3 Result and Discussion

3.1 Calculating the Standard Potential

When considering the redox reactions of Mn2+ and ClO−

Oxidation reaction of Mn2+

$$ Mn^{2 + } + 2H_{2} O \to MnO_{2 } + 4H^{ + } + 2e^{ - } $$

Reduction reaction of ClO−

$$ ClO^{ - } + 2e^{ - } + H_{2} O \to Cl^{ - } + 2OH^{ - } $$

Final reaction of Mn2+ and ClO−

$$ Mn^{2 + } + ClO^{ - } + H_{2} O \to MnO_{2} + 2H^{ + } + Cl^{ - } $$

According to Nernst equation

$$ E = E^{0} + \frac{\text{RT}}{2F}{ \ln }( \frac{{\left( {H^{ + } } \right)^{2} \left( {Cl^{ - } } \right)}}{{\left( {Mn^{2 + } } \right) \left( {ClO^{ - } } \right) }}) $$

The standard potential at 1 atm and 25 °C for oxidation reaction is +1.23 V, while for reduction reaction is +0.9 V.

To obtain \( E^{0} \) for the final reaction, Standard potential of the reduction reaction should be subtracted from standard potential of the oxidation reaction. It is expressed as potential per electron.

Therefore standard potential of the final reaction is 1.23 V − 0.9 V = 0.33 V.

3.2 Removal Efficiency

Table 1 represents the removal efficiency of Manganese after adding the bleaching powder solution. Initial concentration of Manganese is 0.9 ppm

Table 1. Final concentration of Mn with respect to different dosages of ClO− after 30 min of reaction
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After 30 min, in each trial, Residual Chlorine value was zero. It indicated that at that moment, all of the bleaching powder was almost finished.

The variations of removal efficiency and the concentration of ClO−] are shown in Fig. 2. It indicated that increasing concentration of ClO− reduces the Manganese removal efficiency.

Fig. 2.
figure 2

Graph of removal efficiency vs concentration of ClO−

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Since this is the result of synthetic Manganese contained in raw water, it did not give the natural condition of raw water and it also influences this circumstance because in nature, Manganese mostly coexists with Ferrous (Kasim et al. 2017).

3.3 Relationship Between pH and ORP

Figure 3 presented the relation between pH and ORP showed in Table 2. Since the concentration of ClO− is zero, only an oxidation reaction is occurring. According to that, the reaction produces H+ and due to that, pH reduces. Therefore, a higher concentration of final Manganese has a lower pH.

Fig. 3.
figure 3

Graph of ORP value and pH

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Table 2. pH and ORP value of the each sample
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Figure 3 indicates that the ORP value is decreasing with increasing pH. For the most part, ORP increased with increasing oxidant dosage (James et al. 2004). In this experiment, ClO− is oxidant and from trial 1 to trial 4, concentration of the concentration of ClO− is increased. Therefore, ORP value is increasing from trial 1 to trial 4.

3.4 Calculating Ehreference

Ehreference value depends on the concentrations of Mn2+, ClO−, H+, and Cl−. Final concentration of Cl− is zero, and under that condition Ehreference value of the each trial is equal to the standard potential of the reaction as shown in Table 3.

Table 3. Ehreference values calculated from Nernst equation for each trial
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Since the Eh value is the integration of both ORP value and the calculated Eh value from Nernst equation, Table 4 illustrates the final Eh value and pe value, which is acquired from the relationship of Eh and pe (Appelo and Pstma 2005).

Table 4. Indicate that Ereference values influence to the Eh more than Emeasured values.
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Since the higher pe value has higher tendency to accept protons, according to the obtained pe, propensity to accepting protons is developed from trial 1 to trial 4 respectively as shown in Table 5. Therefore, the solution of trial 1 is more oxidized than the trial 2 solution.

Table 5. pH values and Eh values of each trial
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Figure 4 shows that Eh is reduce with increasing pH of the solution.

Fig. 4.
figure 4

Graph of Eh vs pH

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In this experiment, pH of the samples exist between 6.8 and 7.8. According to the Manganese phase diagram shown in Fig. 4, at that level, Manganese can exist as a dissolved species (Mn2+) or as a precipitate such as Mn3O4, MnO2, Mn2O3, and MnO4 (Bruins et al. 2014).

Here, Eh value exists between 0.3 and 0.4. Type of the Manganese species to be obtained varies with the pH and the Eh as shown in Fig. 5. From the above result of Mn3O4, Hausmannite was obtained after adding bleaching powder solution between 0.7 ppm and 6 ppm to 0. 9 ppm Mn2+ solution.

Fig. 5.
figure 5

Phase diagram with analyzed data

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After obtaining the particles, they may be removed by using a filtration process (Isaeva and Castro 2011).

If water contains higher concentrations of Manganese as a dissolved form, before removal it should be converted to its particular form by changing the pH or Eh values. pH can be changed by adding diluted acid or base and Eh can be amended by changing the concentration of the initial concentration of the Manganese solution and the bleaching solution. Particulate form can be selected according to the Manganese phase diagram considering the adequate pH and Eh values.

4 Conclusion

Mn3O4, Hausmannite occurs as a final product when 0.9 ppm synthetically prepared Manganese solution reacts with different concentration of Calcium hypochlorite solutions. Final product can be observed after some time as a brown colour precipitate. This precipitate can be removed by using an adequate filtration method.