Removal efficiency of anions and total nitrogen

As it was mentioned earlier, electrocoagulation treatment of the prepared solution was performed under the following parameters: volume of the solution, V, was 70 L, current density, J, was 166 A/m², stirring speed was 150 rpm, pH of the solution at the beginning of treatment was adjusted to 3. The treatment time was 5 hours, but the samples were taken at 30 min, 1, 2, 3, and 5 hours to estimate the evolution of the parameters such as removal efficiency, pH, RedOx, conductivity and PSD. The results of the measurements of total nitrogen and ion concentrations in the initial and treated solution are presented in table and graphs in Appendix I. The removal efficiency was calculated according to Eq. 10.1:

D1 =E_%− E₃

E_% ∙ 100 % (10.1)

Where: RE – the removal efficiency, E_% – the initial concentration of the solution, E₃ – concentration of a sample taken at the i-time.

The removal efficiencies for ammonium, nitrate, sulphate, chloride and total nitrogen at different treatment time for iron, aluminum and mixed Al/Fe slurries are listed in Table 10.1. A comparison of the removal efficiency for each of the treated species depending of the electrode materials is illustrated in Appendix I.

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Table 10.1. The removal efficiencies of anions and total nitrogen in the treated solution. Variable parameters were operating time and electrode material; Current density, J = 166 A/m²; Volume of the solution, V = 70 L.

Iron electrodes

Time, h Nitrate, % Sulphate, % Chloride, % Ammonium, % Total nitrogen, %

0.5 13.71 6.50 2.58 2.73 4.44

1.0 40.05 7.25 3.34 -3.08 3.62

2.0 71.51 10.34 1.75 -4.40 7.40

3.0 90.05 11.38 2.88 1.43 15.20

5.0 100.00 10.88 6.75 -0.89 14.79

Aluminum electrodes

Time, h Nitrate, % Sulphate, % Chloride, % Ammonium, % Total nitrogen, %

0.5 12.65 7.75 2.43 -0.33 2.08

1.0 25.55 11.33 3.86 4.43 8.34

2.0 54.01 15.48 4.36 1.82 11.49

3.0 73.24 10.67 5.72 -4.85 9.62

5.0 100.00 13.15 7.43 -11.28 9.34

Aluminum cathode and iron anode

Time, h Nitrate, % Sulphate, % Chloride, % Ammonium, % Total nitrogen, %

0.5 10.85 16.73 4.69 7.65 8.16

1.0 39.15 30.49 13.86 10.40 15.04

2.0 64.55 48.74 21.55 7.99 17.12

3.0 93.92 71.65 11.88 23.52 34.88

5.0 100.00 87.48 14.44 8.52 23.28

It can be seen from the data in Appendix I that the measured initial concentrations of the anions in the solution were closed to the targeted values with the percentage error in the range of 2 – 7%, thus, it can be concluded that the solution was prepared correctly. The only contradiction was the measured concentrations of chloride anions which exceeded the target value by 65 – 75 %. It can be explained by the fact that additional amount of chloride ions entered the solution when pH was adjusted by 1 M HCl before the initial sample was taken.

According to the values presented in Table 10.1 and in Figure 10.1, the best removal efficiency with all types of the used electrodes was obtained for nitrate. The concentration of nitrate gradually decreased during the EC treatment and achieved zero value after 5 hours. Although all electrodes were effective in removal of nitrate, the Al/Fe combination has shown the highest efficiency of 93.92 % after three hours of treatment followed by the iron electrodes with a 90.05 % of amount

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of removed nitrate at three hours. Also, it can be assumed that time required for nitrate concentration to reach zero was less than 5 hours and, perhaps, 4 hours of treatment is enough to fully purify the water from these ionic species. Nitrate is proposed to be removed via reductive mechanism: nitrate -> nitrite -> ammonium -> nitrogen gas (Sillanpää and Shestakova, 2017).

Regarding to this, incomplete reduction of nitrate may occur and some amount of nitrogen can be retained in the solution in the form of nitrite or ammonium.

Figure 10.1. The removal efficiency of nitrate from the treated solution. Variable parameters: operating time and electrode material; Current density, J = 166 A/m²; volume of the solution, V = 70 L.

As for the efficiency of sulphate removal (Figure 10.2), high values were achieved only with Al/Fe electrodes with maximum efficiency of 87.48 % within 5 hours of treatment. Use of iron or aluminum electrodes for sulphate treatment would be an ineffective solution since only one tenth of sulphate can be removed in the process. Since the major mechanism of the sulphate removal is their adsorption onto the surface of the formed metal oxides and hydroxides (Kolics et al., 1998;

Murugananthan et al. 2004), it might happen that iron and/or aluminum hydroxides were generated in insufficient quantity or had the specific surface area that was not large enough to adsorb all the sulphate. Obtained values seem to be more or less stable and the slightly decreased removal efficiency observed can be explained by fluctuations occurred during the process or possible mistakes during the sample collection (e.g. taking samples from a stagnation zone of the reactor where the mixing was not intensive) since it was difficult to control the sampling depth.

0 10 20 30 40 50 60 70 80 90 100

0 1 2 3 4 5

Removal efficiency, %

Time, h Fe/Fe Al/Al Al/Fe

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Figure 10.2. The removal efficiency of sulphate from the treated solution. Variable parameters: operating time and electrode material; Current density, J = 166 A/m²; volume of the solution, V = 70 L.

Even lower efficiency was observed in the case of chloride (Figure 10.3). According to different papers (Kabdasli et al., 2013; Kuokkanen et al., 2013; Sahu et al., 2014; Sillanpää and Shestakova, 2017), chlorides are usually considered in terms of the effect on conductivity or passivation of electrodes. Namely, chlorides are added as NaCl to increase conductivity of the treated solution thus reducing the amount of consumed energy (Sahu et al., 2014). As for the question of passivation, Cl^-–ions inhibit the oxide film formation thus alleviating the passivation of electrodes (Cheng, 1985; Zuo et al., 2008) and enhanced the corrosion of Al-electrodes. At the same time, the presence of Cl^-–ions can slow down the dissolution of Fe-electrodes (Sillanpää and Shestakova, 2017). In the paper of Kolics et al. (1998), the adsorption of chloride on aluminium was studied. It was determined that the chloride adsorption may be not effective due to the competitive effect which occurs in the presence of other anions. However, no articles devoted to removal of chloride by EC were found, perhaps, because of its obvious inefficiency. In the experiments reported in this thesis, the most effective electrodes were coupled Al/Fe where the highest removal efficiency of 21.5 % for chloride was achieved within 2 hours and after this, the chloride concentration started to increase again.

The best results for removal of ammonium were also achieved with Al/Fe electrodes (Figure 10.4).

Similarly to chloride, the highest effect was observed after 3 hours of EC. For two other electrode couples, the effect of treatment was almost negligible and even negative values of the removal efficiency were obtained. One possible explanation for this is that concentration of ammonium

0 10 20 30 40 50 60 70 80 90 100

0 1 2 3 4 5

Removal efficiency, %

Time, h

Fe/Fe Al/Al Al/Fe

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species in solution is interconnected with nitrate concentrations as ammonium can be oxidized to nitrites and then to nitrates and vice versa. Reduction of nitrate goes through the nitrite species to ammonium and then to nitrogen gas. During EC nitrates do reduce, and in cases where reduction was incomplete, some amount of nitrate was not converted to N2 but remained in the solution as ammonia species (Duarte et al., 1998). Thus, the total concentration of ammonium in the solution increases and negative removal efficiency can be observed. As for the ammonium species itself, they can be removed due to volatilization (pH > 9.5 is required) or by their decomposition to N2

gas (Feng et al., 2007). However, overall results have shown poor efficiency when dealing with ammonium under the selected process conditions.

Figure 10.3. The removal efficiency of chloride in the treated solution. Variable parameters: operating time and electrode material; Current density, J = 166 A/m²; volume of the solution, V = 70 L.

Figure 10.4. The removal efficiency of ammonium in the treated solution. Variable parameters: operating time and electrode material; Current density, J = 166 A/m²; volume of the solution, V = 70 L.

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Total nitrogen concentration (Figure 10.5) depends on the concentrations of all nitrogen containing species: nitrate, nitrite and ammonium. As it was observed earlier, the most effective were Al/Fe electrodes whereas Al/Al shows the poorest results. Using total nitrogen concentration, it is possible to determine the extent of reduction of nitrate. The amount of nitrate that is not completely reduced can be calculated as a difference between total nitrogen concentration and initial concentrations of ammonium (and nitrites if they were presented in the solution). It should be noticed that some of the initially contained ammonium (and nitrite) species may also be removed during the treatment and then the amount of nitrate is underestimated.

Figure 10.5. The removal efficiency of total nitrogen in the treated solution. Variable parameters: operating time and electrode material; Current density, J = 166 A/m²; volume of the solution, V = 70 L.

Iron and aluminum are the two most popular materials for EC water treatment (Sahu, 2014), the choice of which depends of the species to be removed and the preferable process conditions, especially, pH. For example, aluminum electrodes have shown slightly higher efficiency in the treatment of pulp and paper industry effluents reducing the content of COD, colour or polyphenol index (Zaied and Bellakhal, 2009; Zodi et al., 2011). When the treatment is made to remove nitrogen (Kushwaha et al., 2010) or such metals as copper, chromium or nickel (Akbal and Camsi, 2011) iron electrodes are more favourable. In the experiments reported in this thesis, the best results for removal of nitrates, sulphates and chlorides were obtained with Al cathode/Fe anode pair. One of the possible reasons for it could be the potential difference between aluminum and iron electrodes. Comparing this result with other studies, the article of Akbal and Camsi (2011) that focused on removal of metals, can be mentioned. In this paper Al/Al, Fe/Fe, Al/Fe (cathode/anode) electrode combinations were tested. It was reported that under the current density

-12 -7 -2 3 8 13 18 23 28

0 1 2 3 4 5

Removal efficiency, %

Time, h Fe/Fe Al/Al Al/Fe

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of 10 mA/cm² (pH 3, conductivity 2 mS/cm) the lowest removal efficiency of Cu, Cr and Ni was observed with Al/Al electrodes whereas Al/Fe pair gave the best results.

Overall, it can be concluded that Al/Fe electrodes were the best in the removal of every monitored compound. The possible reason of it is the amount of aluminum and iron species released from the electrodes. For aluminum electrodes this amount was the least, whereas the dissolution of mixed Al/Fe pair gave the highest material release. It is especially important for the treatment o of sulphates that are removed via adsorption onto the formed metal hydroxide solids. The low release of ions in the Al/Al and Fe/Fe treatment may be explained by the effect of electrode passivation which does not occur in the treatment with electrodes made of different materials.