Shungite application for treatment of drinking water – is it the right choice?

Shungite is a natural carbon containing material that is widely used in water treatment. Scienti ﬁ c research shows that shungite has good adsorption properties towards various organic compounds and heavy metals, as well as exhibiting antibacterial properties. Unfortunately, at the same time shungite releases various chemical elements into the water, including heavy metals. In this study changes in concentration of various heavy metals during drinking water treatment with one commercial and one non-commercial shungite sample were determined. Also sorption of Cu(II) with initial concentration of 2,500 μ g/L onto shungite was investigated. The results showed that various heavy metals like nickel, copper, lead, cadmium, zinc, chromium and arsenic are leaching from shungite into water. Lead and cadmium exceeded the maximum acceptable concentration in drinking water for a few days, but nickel exceeded for up to 2 weeks. At the same time shungite showed good adsorption properties towards copper. Nevertheless, before using shungite in drinking water treatment, it would be advisable to assess the necessity and/or wash shungite with larger volumes of water for a longer period of time than is written in the instructions.


INTRODUCTION
Shungite is a natural mineraloid that contains noncrystalline carbon. There are five types of shungite, classified by carbon content. Type I shungite contains more than 98 mass% of glass-like carbon, type II contains 35-80 mass%, type III 20-35 mass%, type IV 10-20 mass%, but type V contains <10 mass% of carbon. Type III shungite is the most widespread and the largest shungite deposits are located in Karelia region, Russia (Melezhika et al. ; Mosin & Ignatov ; Sineva ). In addition to carbon, shungite usually contains quartz, aluminosilicates, feldspars and carbonates. Also, various micro impurities can be found in shungite: Fe, Ni, Cu Zn and V, mainly as sulphides, sulphates and oxides (Charykova et al. ; Rafienko & Belimenko ). Charykova et al. () showed that in addition to these micro impurities type III shungite also contained Cr, Co, Pb and Mn.
There are a large number of patents on shungite application for drinking water treatment (more than 100 results revealed in the patent search engine https://worldwide.espacenet.com with search keywords 'shungite' and 'drinking water') and a wide range of commercial shungite products for water treatment at home are also available (Karelian Heritage ). Information on most of these products claims that shungite removes bad taste and odour, organic compounds, heavy metals and bacteria and enriches water with microelements. Indeed, studies show that shungite has good adsorption properties towards various organic compounds (Kalsina & Berjoza ; Sineva et al. ; Skorobogatov et al. ) and also antibacterial properties (Charykova et al. ). Fischer et al. () concluded that low-carbon shungite (total carbon content 5.4%) could be used as an alternative adsorbent for Zn(II) removal from water. Efremova () showed that porous sorbent prepared from shungite rock can adsorb Cd(II), Pb(II), Zn(II) and Mn(II) in dynamic conditions. Nevertheless, Charykova et al. () showed that a large number of chemical elements are leaching from shungite into water, including several heavy metals like Cd, Cr, Cu, Ni, Pb and Zn. After 3 days of shungite contact with tap water, many elements exceeded the maximum acceptable concentration in drinking water. The authors suggested that the increased concentrations of some heavy metals could be the reason for the antibacterial properties of 'shungite water' (Charykova et al. ). Some of the elements in large quantities are toxic for humans, therefore in the few websites about shungite it is written that it is advisable to drink just one or two glasses of 'shungite water' due to the presence of heavy metals.
The aim of this study was to determine the changes in concentration of various heavy metals (Ni, Pb, Zn, Cd, Cu, Cr, As, Al) during the application process of drinking water treatment with one commercial and one non-commercial shungite sample. As Cu(II) can still be found in drinking water due to corrosion of copper pipes, the sorption of artificially increased concentrations of copper ions in drinking water was also investigated.

Materials
Two shungite samples with particle sizes in the range of 1-3 mm were used. Sample Com is a commercial product

Preparation of 'shungite water'
The procedure was based on the application instruction inside of the commercial shungite package. The first step written in the instruction is to wash shungite several times with water. Therefore, 10 g of shungite was poured in a beaker, agitated with 200 mL of water for 2 min and then decanted. This procedure was repeated five times. The last decanted water was free from shungite dust particles. The first decanted water was filtered and used for chemical analysis (sample SH-1x and Com-1x). The procedure for sample preparation is shown in Figure 1. The washed shungite samples were mixed with 100 mL of water (shungite: water mass ratio was 1:10) and left still. According to the instructions, after 2-3 days the water is ready for use and every time a certain amount of water is removed for application, the same amount of untreated water is added to shungite. Therefore, on the third day 50 mL of water was removed from the container and the same amount (50 mL) of fresh water was poured back on the shungite. This procedure was repeated every day for the next 14 days. The container with shungite and water was stirred for a few seconds once every day. The chemical analysis was performed for samples removed on the 3rd, 5th, 7th, 11th and 14th day of the experiment.

Adsorption of copper
The initial concentration of Cu(II) ions in water was 2,500 μg/ L, which is slightly higher than the maximum acceptable concentration in drinking water (2,000 μg/L) according to the Council Directive 98/83EC. The procedure for the adsorption experiment was equivalent to the preparation of 'shungite water' described above and shown in Figure 1. Chemical analysis was performed for samples removed on the 3rd, 5th, 7th, 11th, 14th, 17th and 21st day of the experiment.

Analysis of water and 'shungite water'
The chemical analysis of samples was conducted at the Lat-

Release of heavy metals
Shungite SH has slightly higher carbon content but six times lower SSA than shungite Com (Table 1) which can lead to lower sorption properties. According to the shungite classification in Melezhika et al. (), SH belongs to type-II but Com to type-III shungite.
The results in Table 2 show that after the first washing (2 min of contact with shungite) the concentration of heavy metals like Ni, Cu, Zn and Cd is significantly increased. The highest increase can be observed for nickel from both samples.
From these results we can conclude that these heavy metals are released in high concentrations in water and the washing procedure is mandatory not just to remove the small particles (dust), but also to get rid of heavy metals to avoid contamination of the drinking water intended for consumption.   Table 3 shows that after 3 days in contact with both washed shungite samples, the water contained increased levels of Ni, Cu, Pb, Cd, Zn and As, compared to pure water (Table 2). However, only nickel, cadmium and lead (for sample SH) exceeded the maximum acceptable concentration (MAC) in drinking water (Table 4). On the 5th day of exposure, the concentrations of released heavy metals had decreased rapidly and were below MAC, except for nickel.
Similar results were obtained for water with increased copper concentration after exposure to shungite (Table 5).
Here, after 3 days the water also contained increased levels of several heavy metals (Ni, Cu, Pb, Cd, Zn and Cr), where only cadmium and nickel exceeded MAC, but on the 5th day of exposure only nickel exceeded MAC. Compared with pure water, the increase of Ca, Mg, Na, K and As is negligible and does not exceed MAC.    Based on the experimental procedure (Figure 1), every day (starting from the 3rd day) 50 mL of the water exposed to shungite was replaced with 50 mL of fresh water, therefore the concentration of heavy metals was diluted twice each day. For example, in Table 3, on the 3rd day the nickel concentration was 880 μg/L and if we assume that during the next 2 days shungite was not releasing nickel, the concentration on the fifth day should be 220 μg/L, but the analysis showed four times lower concentration (58 μg/L). The same observation was found for copper, lead, zinc and cadmium. We believe that most likely this could be explained by precipitation of salts due to various anions released from shungite in the water, such as sul-

phates, sulphides and carbonates (Turkayeva et al. ;
Rafienko & Belimenko ). Another possible reason could be shungite adsorbing back some part of the released metals due to the fact that shungite contains and releases organic matter that was measured as TOC and DOC (Table 6). Organic matter and DOC forms complexes with metal ions, thereby affecting the adsorption/desorption process (Khokhotva & Waara ). The pH of Evian water was 7.5 and after exposure to shungite the pH of water samples was in the range of 7.1-7.6, therefore the changes in nickel, copper, lead, zinc and cadmium concentration cannot be connected to precipitation due to pH. In Table 5  On the other hand, it could be connected to the presence of additional copper (Figure 2(b)) that caused a more gradual nickel release from shungite. According to the application instructions of Com, shungite should be changed after six months of application, meaning that every six months for a week or two the consumer of 'shungite water' will be exposed to increased levels of nickel.
In Figure 2(a) and 2(b) the concentration of released nickel from shungite Com is significantly higher than from shungite SH after 3 days of exposure. On the 5th day the difference is negligible, but in the following days the released nickel from shungite Com is lower than from shungite SH.
This could be explained by the fact that shungite Com has higher SSA than shungite SH, therefore Com releases nickel much faster.
The recommended tolerable daily intake (TDI) of nickel has changed over the years. In 2005, the European Food

Adsorption of copper
Despite the fact that shungite releases various heavy metals, at the same time it adsorbs copper ( Figure 3) from water.
The results in Figure 3 show that the remaining copper con-

CONCLUSIONS
In the current study, drinking water treatment with shungite was investigated. The results show that shungite samples release various heavy metals into the waternickel, copper, lead, cadmium, zinc, chromium and arsenic. Lead and cadmium is released for a short time and exceeded MAC only after the first 3 days of exposure, but nickel is released for a much longer time and can exceed MAC up to 2 weeks. Increased specific surface area probably accelerates the rate of nickel release from shungite but carbon content in shungite promotes sorption properties.
Based on the obtained data, it would be advisable to give careful consideration to the use of shungite for drinking water treatment. To avoid heavy metal contamination from shungite, prior to application and additionally to the washing procedure written in the instructions, shungite should be washed with a large volume of water for several days (for example, for 5 days with shungite:water mass ratio of 1:10 and by changing the water once a day). Also, after the washing procedure, chemical analysis of the last washing water should be carried out.

ACKNOWLEDGEMENT
This work was supported by the European Regional Development Fund within the Activity 1.

DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.