Keywords

1 Introduction

In the mining and processing industry, during open-pit mining through drilling and blasting operations, quarry and drainage waters are generated. The waters are polluted with inorganic nitrogen compounds (ammonium ions, nitrite and nitrate ions) that are products of decomposition and incomplete use of ammonium nitrate-based explosives [1, 2]. By some estimates, the average amount of NOx released into the environment when using ammonium nitrate-based explosives is 5 kg per 1 tonne of explosive [3]. Multiple exceedance of maximum allowable concentrations of nitrates, nitrites and ammonium ions in drainage water and effluent from mining and processing facilities is one of the challenging environmental and technological problems to solve [4,5,6,7,8].

Low content of organic impurities and bacterial microflora, which does not allow using traditional treatment technologies based on biological methods, are characteristic of quarry and drainage waters. When developing treatment technologies, it is necessary to consider that quarry and drainage waters generated volumes amount to millions of tonnes a year, and for their treatment it is economically expedient to apply cost efficient and effective methods.

Analysis of scientific and technical information showed that for removing nitrogen compounds from quarry water it is proposed to use bioengineered systems, in particular bio-plateau containing hydrophytes as well as microalgae or duckweeds [9, 10]. High activity of nitrate reductase, the enzyme that provides nitrate reduction in plant cells, is conditioned by high photosynthetic activity of plants [11].

The use of wetlands allows combining nitrate assimilation by plants and anaerobic denitrification in bottom layers [12]. However, the method of microbial denitrification depends significantly on transfer of electrons, which are donated by organic compounds in particular [13]. The deficit of electron donors makes the method of microbial denitrification inefficient in the treatment of wastewater with low content of organic substances, in particular quarry wastewater from mining enterprises.

Physico-chemical methods of quarry water treatment are based on creation of filtering sorption dams, masses, trenches with the use of natural materials as sorbents [14].

Currently, the possibility of using artificial chemical barriers containing redox systems for treatment of quarry waters is being investigated. For example, there are known studies of removing a wide range of pollutants (nitrate ions, chromate ions, etc.) from groundwater using zero-valent iron as a reducing agent, which acts as an electron donor [15, 16].

The mechanism of wastewater treatment using iron consists of direct reduction of nitrate and nitrite ions by metallic iron in a weakly acidic environment. Permeable reactive barrier is put into effect in the form of a subsurface placement of chemically active raw materials. The pollutant flow passes under the natural gradient through the barrier material. The PRB system is implemented in situ, completely passively, which allows it to be operated for a long time with minimal maintenance [17, 18].

Known as the galvanochemical method (galvanocoagulation), it is used for wastewater treatment to remove fine particles, colloidal impurities, heavy metal ions, oxidant ions, such as chromate ions. The galvanocoagulation method is based on the principle of a short-circuited galvanic element, in which the water is treated with a mixture of conducting materials, one of which has a high reducing capacity. The current density in galvanic couple field will depend on the materials of the cathode and anode sections [19, 20].

The purpose of this work was to evaluate the effectiveness of zero-valent iron as well as galvanic system, containing iron scraps and carbon materials, in reducing the content of nitrate ions in quarry wastewater of mining and processing enterprises.

2 Materials and Methods

The subject of the study was the quarry wastewater sampled from the quarry water settling tank of a large mining and processing enterprise in Russia.

In accredited laboratory, the chemical composition of the examined wastewater was analyzed using standard methods (the content of nitrate ions was determined photometrically in accordance with PND F 14.1:2:4.4-95, the content of ammonium ions was determined by capillary electrophoresis according to GOST 31869-2012).

We studied the process of nitrate ions reduction by zero-valent iron, for which iron scraps were used, and made a model of nitrate ions reduction by a redox system consisting of galvanic couple “iron scraps-active carbon production waste”. The influence of Fe:C mass ratio and treatment time on the efficiency of removing nitrate ions from the water was examined.

The process was controlled by the residual content of nitrate ions in the studied water.

In order to activate the iron oxidation processes, it is advisable to conduct the process in a weakly acidic environment. For this purpose, we treated current-generating materials with an acid solution for a day at pH = 4.0–4.5. After that, we added the water to the container and carried out the purification process while stirring the suspension.

The effect of pH on the rate and completeness of nitrate ions reduction was studied. The initial pH value of wastewater was set using HCl and NaOH solutions, the pH value was determined potentiometrically. Upon completing the denitrification process, the pH was measured again.

It is known that quarry waters also contain ammonium ions in amounts significantly exceeding the maximum allowable concentrations. The paper presents the research results of the extraction of ammonium ions by lowland peat samples. To determine the sorption capacity of peat by ammonium ions in static conditions, we treated dry peat sub-samples weighing 5 g with 50 ml of wastewater while stirring until equilibration. Studies were conducted at pH 6 and 8. To achieve an alkaline reaction of the medium, CaO was added to the sample.

3 Results and Discussion

3.1 Chemical Composition of Quarry Waters

The composition of the studied wastewater sampled from the quarry water settling tank of the mining and processing enterprise is presented in Table 1.

Table 1. Chemical composition of quarry water
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It was found that the examined quarry wastewater had high content of nitrate ions (204 mg/dm3), ammonium ions (9.7 mg/dm3) and exceeded amount of manganese ions. The obtained results of the quarry water analysis are in agreement with the literature data [21,22,23].

3.2 Use of Redox Systems for Removing Nitrate Ions from Quarry Water

Figures 1 and 2 show the obtained research results of removing nitrate ions from quarry water using zero-valent iron and galvanic couple at the initial value of the water pH 6 (Fig. 1) and at pH 4 (Fig. 2).

Fig. 1.
figure 1

(Source: Compiled by the authors)

Efficiency of removing nitrate ions from wastewater using zero-valent iron at pH 6 and galvanic couple at different ratios of components (iron-carbon)

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Fig. 2.
figure 2

(Source: Compiled by the authors)

Efficiency of removing nitrate ions from wastewater using zero-valent iron at pH 4 and galvanic couple at different ratios of components (iron-carbon)

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It was found that the effectiveness of removing nitrate ions from the water is significantly affected by pH value. When treating the water in acidic environment, the purification efficiency significantly increases and treatment time reduces. Purification efficiency with the use of galvanic couple, when the Fe:C ratio is 3:1, reaches 90% with a contact time of 3 h.

When using zero-valent iron under these conditions, the purification efficiency is 50%. The lower effect can be explained by different mechanisms of nitrate ions reduction by the studied systems.

When using zero-valent iron, the process consists of direct reduction of nitrate ions by iron [24, 25]:

$$ {\text{NO}}_{3}^{ - } + {\text{Fe}}^{0} + 2{\text{H}}_{3} {\text{O}}^{ + } \to {\text{Fe}}^{2 + } + {\text{NO}}_{2}^{ - } + 3{\text{H}}_{2} {\text{O}} $$
(1)
$$ {\text{NO}}_{3}^{ - } + 4{\text{Fe}}^{0} + 10{\text{H}}^{ + } \to 4{\text{Fe}}^{2 + } + {\text{NH}}_{4}^{ + } + 3{\text{H}}_{2} {\text{O}} $$
(2)
$$ {\text{NO}}_{2}^{ - } + {\text{Fe}}^{0} \to {\text{FeOOH}} + 0,5{\text{N}}_{2} $$

To increase the integrity of the reactions, it is necessary to maintain acidity of the medium.

Denitrification of quarry waters using redox system consisting of iron scraps and carbon-containing materials is studied for the first time.

The mechanism of galvanochemical water purification (galvanocoagulation) is based on electrochemical processes. Due to the difference of electrochemical potentials of current-conducting elements, on the contact Fe0 - carbon-containing material many galvanic couples appear, which causes intensive oxidation and dissolution of metal, electrolysis of water, pH shift, hydrolysis and other physical and chemical processes.

At the anode areas of galvanic couples, Fe0 is oxidized and the following reactions are possible:

Anode Area – Fe

$$ {\text{Fe}}-2{\text{e}} = {\text{Fe}}^{2 + } $$
(3)

The resulting ions are able to undergo hydrolysis according to the following scheme:

$$ {\text{Fe}}^{2 + } + {\text{H}}_{2} {\text{O}} = {\text{FeOH}}^{ + } + {\text{H}}^{ + } $$
(4)
$$ {\text{FeOH}}^{ + } {\text{H}}_{2} {\text{O}} = {\text{Fe}}\left( {{\text{OH}}} \right)_{2} + {\text{H}}^{ + } $$
(5)

The resulting Fe2+ ions and iron (II) hydroxide can be oxidized by water-dissolved oxygen or nitrate ions:

$$ 4{\text{Fe}}^{2 + } + {\text{O}}_{2} + 2{\text{H}}_{2} {\text{O}} = 4{\text{FeOH}}^{2 + } $$
(6)
$$ 4{\text{Fe}}^{2 + } + {\text{NO}}_{3}^{ - } + 2{\text{H}}_{2} {\text{O}} = 4{\text{FeOH}}^{2 + } + {\text{NO}} $$
(7)
$$ {\text{Fe}}^{2 + } + {\text{NO}}_{3}^{ - } + 7{\text{H}}^{ + } = {\text{Fe}}^{3 + } + {\text{NH}}_{4}^{ + } + 3{\text{H}}_{2} {\text{O}} $$
(8)
$$ 2{\text{Fe}}^{2 + } + 3/2{\text{O}}_{2} + 3{\text{H}}_{2} {\text{O}} = 2{\text{Fe}}({\text{OH}})_{3} + 3{\text{H}}^{ + } $$
(9)
$$ 4{\text{Fe}}\left( {{\text{OH}}} \right)_{2} + {\text{O}}_{2} = 4{\text{FeOOH}} + 2{\text{H}}_{2} {\text{O}} $$
(10)

The cathode areas of galvanic couples (carbon-containing material) are cathodically polarized, and the following reactions are possible in the presence of oxygen:

$$ 2{\text{H}}_{2} {\text{O}} + {\text{O}}_{2} + 4{\text{e}} = 4{\text{OH}} $$
(11)

In an acidic environment:

$$ 2{\text{H}}^{ + } + 2{\text{e}} = {\text{H}}_{2} $$
(12)

Hydrogen released at the cathode areas can also take part in the nitrate ion reduction:

$$ {\text{NO}}_{3}^{ - } + 2{\text{H}}_{2} + 4{\text{H}}^{ + } \to {\text{NH}}_{4}^{ + } + 3{\text{H}}_{2} {\text{O}} $$
(13)
$$ {\text{NO}}_{3}^{ - } + {\text{H}}_{2} + 2{\text{H}}^{ + } \to {\text{NO}} + 2{\text{H}}_{2} {\text{O}} $$
(14)
$$ 2{\text{NO}}_{3}^{ - } + 5{\text{H}}_{2} + 2{\text{H}}^{ + } \to {\text{N}}_{2} + 6{\text{H}}_{2} {\text{O}} $$
(15)

Hydrolysis of Fe2+ and Fe3+ ions comes with a decrease in the pH value; at the same time, the hydroxide ion will be released at the cathode areas in weakly acidic and neutral environments, which allows maintaining the pH value at 6–7.

Thus, as a result of electrochemical processes, strong reducing agents Fe0, Fe2+, H2 as well as atomic hydrogen are formed in the redox system “iron scraps- carbon”, which leads to significant increase in the efficiency of nitrate ions removal from the water compared with the use of zero-valent iron for this purpose.

3.3 Deammonification of Quarry Wastewater

In order to study the possibility of deammonification of effluents, the use of peat was studied. It is known that peat can reduce the concentration of ammonium ions due to both the porous structure and the interaction of humic acids in its composition.

We used samples of peat with natural pH value and samples pre-treated with calcium oxide. A number of authors [26] cite data on the effect of pH on the binding of heavy metal ions by humic acids of peats. The authors point out that the sorption of heavy metals increases with increasing pH. In such a case, not only hydrogen of carboxyl groups is replaced by metal ions located on the surface of the humic substances molecule, but also the sorption and sedimentation of metal ions on already formed insoluble humate (secondary sorption) takes place [26].

It was found that at pH 6 the purification efficiency of the studied ammonium ions solution was 93%; at pH 8 the purification efficiency reached 98%. Thus, the experimental data obtained also agree with the aforementioned studies and show that ammonium ions formed as a result of nitrate ions reduction can be removed from wastewater using natural material - peat.

4 Conclusion

The main source of nitrate ions and ammonium ions pollution of drainage and quarry waters of mining enterprises during mining by open-pit drilling and blasting method is the use of explosives containing ammonium nitrate. Typical features of quarry wastewaters are their high volumes and low content of organic impurities, which complicates natural mechanisms of denitrification and determines the complexity of the choice of methods for nitrogen compounds removal from mining wastewater. Significant volumes of wastewater make it economically feasible to use fairly inexpensive but effective methods of denitrification.

The obtained experimental data indicate the possibility to apply redox systems containing the galvanic couple “iron scraps - active carbon production wastes” for treating high volumes of wastewater from mining enterprises. To reduce the concentration of ammonium ions in wastewater after its treatment using galvanochemical method (galvanocoagulation), lowland peat, which has high sorption and chemisorption activity against ammonium ions in slightly alkaline environment, can be used.

Implementation of the studied method of removing nitrate ions and ammonium ions from the water is possible by creating a geochemical permeable barrier containing galvanic couple, sand and peat.