Abstract
This paper is devoted to the use of the principles of green chemistry in the search for technologies to reduce the chemical footprints of areas. The chemical footprint for mercury and its compounds was taken as an example to study. These chemicals belong to priority pollutants and their ever-increasing amounts in the environment have caused concern around the world, which is reflected in the adoption of the Minamata Convention. The Minamata Convention aims to protect human health and the environment from anthropogenic releases of mercury and mercury compounds. This Convention is an important component of efforts to achieve sustainable, inclusive and resilient human development through SDGs, which were adopted in September 2015 and especially SDG Goal 12: Ensure sustainable consumption and production patterns. Relevancy of this work is due to the need for the adopting of a series of measures to withdraw some mercury-containing goods from the production cycle. Also, one of the most important statements of the Convention is in reference to the issue of mercury contamination when recycling mercury. An important aspect of the work described in this paper is the reduction of mercury pollution from mercury-containing waste products by the development of technology in accordance with the principles of green chemistry. These are energy-efficient and without waste -water discharge technology. The main result of this work is the fundamental research for a transformation of elemental mercury and its compounds into less dangerous forms for the human body and the environment, providing a guaranteed absence of mercury-containing waste in the atmosphere and water systems. Various conditions for reaction of the immobilization of metallic mercury in mercury-containing wastes were investigated and it was established that it proceeded best under the following conditions:
Reaction of metallic mercury with elementary sulfur;
A ball mill is used as a reactor, which ensures constant updating of the contact area of the phases;
For a good dispersion of mercury and for a relatively quick and complete reaction a large excess of sulfur up to 6500 % by stoichiometry (e.g. ratio of mercury:sulfur = 1:1.5 by weight) is necessary;
The addition of a very small amount of water also has a positive effect (hydromodulus of Solid:Liquid = 3:1 by weight).
Introduction
At present, the concept of a chemical footprint [1] is being substantially developed and is actively used to assess the complex chemical load in various fields, including chemical synthesis [2], consumer goods [3], textiles and apparel [4], etc. Moreover, an increasing number of researchers are considering the chemical footprint as one of the indicators for classifying processes and products to green chemistry [5]. Thus, there is a great need for methods that can be used to calculate indicators of the chemical footprint [6]. However, until recently, there were no universally accepted methods for assessing the chemical footprint [7]. In addition, the complexity of weighing a chemical’s diverse environmental health impacts and combining them into one unit is inherently more difficult than e.g. accounting for carbon molecules [3]. Nevertheless, certain approaches to the assessment of the chemical footprint were developed [8] and tested by a number of scientists [9]. One of the most elaborated and often used approaches for assessing the chemical footprint at the global and/or regional level is the approach based on the USEtox model [10]. It should be noted that the USEtox model does not take into account the spatial distribution of chemicals with water or air flows, which, according to some researchers, can significantly affect the accuracy and adequacy of estimates [11]. To take into account the spatial differentiation of chemicals, approaches based on a combination of the USEtox model with geographic information systems (GIS) data are used [12].
This work is devoted to the development of a model for assessing the chemical footprint and predicting the effect of technological solutions on its value using mercury and its compounds as an example. Mercury and its compounds are selected as chemicals that pose a significant danger to humans and the environment.
It should be noted that the chemical footprint of mercury and its compounds is influenced by substances that enter the environment from a wide variety of sources (Fig. 1). Moreover, one of the most significant sources of mercury releases to the environment, which have undergone minor changes since 2005 [13], is products containing mercury (batteries, dental applications, measuring and control devices, lamps, electrical and electronic devices). The percentage of mercury contained in the waste and released into the environment during its processing is large enough for a number of countries [13], including the Russian Federation [5] and, in accordance with previous assessments; it has a significant effect on the size of the chemical footprint.
![Fig. 1: Evolving mercury demand by sector, including uncertainties [14].](/document/doi/10.1515/pac-2019-0813/asset/graphic/j_pac-2019-0813_fig_001.jpg)
Evolving mercury demand by sector, including uncertainties [14].
To reduce this effect, the authors studied the technology of immobilization of mercury in waste and assessed the impact of the use of these technologies on the chemical footprint. One of the criteria when choosing technologies was their compliance with the principles of green chemistry [15], namely:
Principle # 1 “Prevention”. It is better to prevent waste than to treat or clean up waste after it is formed;
Principle # 6 “Design for Energy Efficiency”. Energy requirement of chemical processes should be recognized for their environmental and economic impacts and should be minimized. If possible, synthetic methods should be conducted at ambient temperature and pressure.
Therefore, preferences were given to drainless and energy-efficient methods. One of the widely used methods for immobilizing mercury and/or its compounds is its conversion to sulphides [16]. Mercury sulphide (HgS) is the most stable compound formed between mercury and sulphur [17]. The first one is a black cubic tetrahedral form (metacinnabar) and the other stable form is the red hexagonal form, found in nature as cinnabar. Both forms are insoluble in water (0.017 mg/L at 18°C) [18] and in acidic solutions. In alkaline solutions with excess of sulfur anions HgS is solubilized by the reaction (1):
In addition, as HgS emitted no mercury vapor [19], sulfide should be an effective agent for stabilizing mercury in mercury contamination wastes.
The solid-phase reaction between mercury and sulphur and/or sulphur-containing compounds, such as iron sulphide or pyrite, can proceed quite quickly under normal conditions with constant renewal of phases, which corresponds to the principles of green chemistry indicated above. The direct reaction between elemental mercury and elemental sulphur to form HgS is widely known. The disadvantage of that method is that the reaction between the elemental S and the elemental Hg is not total, i.e. it is possible that after the reaction a part of the elemental Hg remains as such, without becoming HgS. According to the available data, during the reaction under normal conditions for 24 h, it is not possible to reach a concentration of less than 2 mg/m3 [20]. In addition, several researchers believe that sufficient heat must be available to overcome the energy of activation for the reaction to occur [21]. Thus, the aim of this work was to select parameters for the most complete immobilization of metallic mercury in wastes and evaluate the effectiveness of this method to reduce the complex environmental impact using the chemical footprint methodology as exemplified by the Russian Federation.
A chemical footprint assessment for mercury and its compounds
The USEtox model was used to calculate the chemical footprint for mercury and its compounds [22]. However, we modified this model and combined spatial peculiarities of the fate of the chemicals with the structure of the USEtox model [23].
Using this model, the chemical footprint was assessed for some federal districts [5] and regions of the Russian Federation [24].
The analysis of the result for the RF allowed identifying regions with high-risk. The compartments of the environment with most high-risk of negative effects of mercury and its compounds are water bodies (fresh and sea) and soil.
When analyzing the results obtained, it was determined the main sources of mercury released into the environment, which were of the greatest concern. The sources of anthropogenic mobilization of mercury in Russia are:
the presence of a certain amount of mercury in raw materials and fossil fuels and its emission into the environment during processing and burning.
the usage of mercury catalysts in a variety of production processes.
the presence of mercury in a number of goods.
The dynamics of the contribution of various sources to the level of mercury mobilization, as well as the level of mercury presence/issues in various compartments of the environment was studied/considered [5], and, according to that work, mainly the contribution of such sources as measuring and control devices and electrical and electronic devices was of very significant volumes.
In order to reduce these volumes, a technology was developed to immobilize mercury in the environment. A distinctive feature of this technology is the selection of conditions for carrying out a solid-phase reaction under normal conditions and in the absence of waste waters.
Immobilization of mercury in waste: development of the technology
In our study, mercury thermometers containing up to about 10% metallic mercury were taken as a prototype of mercury-containing wastes. Therefore, the metal mercury – glass system containing approximately 90% of ground glass (a fraction of 70 microns) was taken as the system under study. It should be noted that a number of researchers noted that the bulking agent (any microporous granular material such as clay, cement, soil, etc.) was of sufficient volume for efficient mixing to take place [17].
Since a sulfide film is formed rather quickly on the surface of the mercury during the reaction of mercury with sulfur, that prevents the further course of the reaction, it is necessary to ensure permanent destruction of this film and updating the contact area of the phases. In our study, this was achieved either by grinding the reaction mass in a mortar or by using a ball mill, although it should be noted that there are works when special devices such as a “round-bottomed” reactor (“ball”) were used for such reactions [17], [21].
A significant excess of sulfur in comparison with mercury was found to have a positive effect on the reaction time in the mercury-glass-sulfur system (Fig. 2). All experiments presented in Fig. 2 were carried out during manual grinding of the mixture in mortar. Black mercuric sulphide (metacinnabar) was obtained because of the reaction, which was characteristic of such processes with a low mixing rate [17]. The reaction proceeded most quickly and completely when the mercury to sulfur ratio was 1:1.5 by weight, which significantly exceeded the amount of sulfur required by reaction stoichiometry. Apparently, this was because a significant excess of sulfur in the system contributed to a better dispersion of mercury throughout the entire volume of the mercury-containing waste (MCW) and, accordingly, significantly simplified this reaction. Poor dispersion of mercury was evidenced by significant jumps in the determined amount of mercury immobilized into sulfide in the structure of the MCW and by even the presence of negative values [e.g. curves characteristic of the ratio Hg:S=3:1 by weight (200% by stoichiometry)]. In the case of a significant excess of sulfur, when the ratio Hg:S=1:1 or 1:1.5 (by weight), the obtained curves of the dependences were smoother and closer to the straight line, and the degree of immobilization of mercury with the formation of mercury sulfide for the same period was higher. The best results, when it is possible to convert about 98% of the mercury contained in the MCW into sulfide in 2 h, were observed at a ratio of Hg:S=1:1.5. Other authors [17] obtained similar data on the positive effect of excess sulfur on the reaction between elemental mercury and sulfur. They indicated that it is recommended to use Hg:S ratios of 1:1–1:3 for the reaction of elemental mercury with sulfur by weight.
The effect of excess sulfur on the completeness of the reaction.
Also, one of the experiments (the ratio Hg:S=3:1 by weight or 200% by stoichiometry) was carried out to study the interaction of mercury with sulfur without glass. Moreover, the results showed that the presence of glass in the system promoted renewal of phase contact and a more complete reaction during the same period. As well as negative concentration values during the first hour of the reaction, indicated an uneven distribution of mercury in the mercury-glass-sulfur system, which led to the fact that mercury appeared more in the selected sample than the average for the studied system, the result of which was the negative concentration shown in the graph (Fig. 2).
To assess the reliability of the results obtained and evaluate the error, the experiment on the interaction of MCW with sulfur at a ratio of Hg:S=1:1.5 by weight was performed three times (Table 1).
The data the experiment on the interaction of MCW with sulfur at a ratio of Hg:S=1:1.5 by eight.
| Time, min | Percentage of reacted Hg, % |
Average value, % | ||
|---|---|---|---|---|
| №1 | №2 | №3 | ||
| 60 | 65.4 | 66.5 | 68.4 | 66.8±1.5 |
| 90 | 84.3 | 80.3 | 84.0 | 82.9±2.2 |
| 120 | 98.5 | 98.2 | 98.0 | 98.2±0.3 |
The results obtained were compared with the results obtained using a ball mill (Fig. 3).
Ball mill: (a) the general view, (b) the drum and the metal balls.
The results obtained using a ball mill were comparable with the results obtained during manual grinding (Fig. 4).
The results of measurements during manual grinding of the mixture in mortar and with using a ball mill.
The effect of water on the course of the reaction was also studied, and similar reactions were carried out when iron sulfide or pyrite was used instead of sulfur. The reaction proceeded best in the presence of a small amount of water (Fig. 5). The hydromodulus must be such that the reaction mass had a pasty shape. Since the presence of a larger amount of liquid in the system had an evident negative effect on dispersion and it negatively affected the percentage of sulfide formed.
The results of reactions for the immobilization of mercury-containing waste, conducted under various conditions.
Thus, it was found that to immobilize the metallic mercury contained in the waste, using the method that meets the principles of green chemistry, it is advisable to carry out the reaction of mercury with elemental sulfur, with a constant updating of the phase contact surface, for which it is proposed to use a ball mill as a reactor. The reaction should proceed in excess sulfur and in the presence of a small amount of water.
Assessment of the potential impact of the use of technologies for the immobilization of mercury in wastes on the value of its chemical footprint in the Russian Federation
To assess how our method can affect the chemical footprint value, we calculated separately the contribution of mercury from the devices. Moreover, for this value, we considered separately the chemical value for metallic mercury and mercury entering in the form of sulfide.
To carry out these calculations, the data on metallic mercury and mercury sulfide were added to the USEtox model (Table 2).
Parameters for calculating the chemical footprint.
| Henry’s low constant (at 25 °C), Pa*m3/mol | Vapor pressure (at 25 °C), Pa | Solubility (at 25 °C), mg/L | Partitioning coefficient between |
||||
|---|---|---|---|---|---|---|---|
| n-Octanol/water, L/L | Organic carbon/water, L/kg | Suspended solids/water, L/kg | Marine sediments/water, L/kg | ||||
| Hg0 | 0.32 [27] | 0.3 [32] | 6.0*10−5 [28] | 4.17 [25] | 3.449 [28] | 400 [31] | 1.4*10−2 [33] |
| HgS | 1.0*10−20 [30] | N/a | 2.0*10−27 | 25 [26] | N/a | 6000 [31] | − |
Among mercury species, elemental mercury, being the predominant species in ambient air, has residence time of 0.5–2 years due to its low solubility in water and chemical inertness [34]. Hg0 can be globally cycled by long-range transport [35]. Oxidation to water-soluble Hg2+ plays a major role in Hg0 deposition to ecosystems. The tropospheric lifetime of Hg0 against oxidation is 2.7 months. Observations of the atmospheric variability of total gaseous mercury (Hg0+Hg2+) suggest an atmospheric lifetime against deposition of about 6 months [36]. In the fresh water, the spiked Hg0 rapidly photooxidized with an estimated first-order rate constant of 0.61 h−1 [37].
In addition, since the proposed method of immobilizing mercury can be implemented in sealed apparatus and it does not form waste waters, we suggested that mercury releases into the atmosphere and water during its implementation are practically absent. Thus, the mercury sulfide formed is to be disposed of at landfills.
The assessment’s preliminary results of the reduction of the impact on them and their immobilization (transfer in sulfide assessment) in waste are presented in Fig. 6. The figure clearly shows a decrease in the content in the air and in the agricultural soil (especially if there is no irrigation). There is also a decrease in marine and, in some cases, in freshwater bodies, however, at the same time, all this is not enough to reduce the risk of negative effects on the human body and the environment to an acceptable level.
Radar charts showing estimated mercury impact in regions of Russia in case (a) metallic mercury (b) mercury entering in the form of sulfide. 1-Republic of Bashkortostan; 2-Republic of Mari El; 3-Republic of Mordovia; 4-Republic of Tatarstan; 5-Republic Udmurt; 6-Republic of Chuvash; 7-Perm Region; 8-Kirov Region; 9-Nizhny Novgorod Region; 10-Orenburg Region; 11-Penza Region; 12-Samara Region; 13-Saratov Region; 14-Ulyanovsk Region; 15-Belgorod Region; 16-Bryansk Region; 17-Vladimir Region; 18-Voronezh Region; 19-Ivanovo Region; 20-Kaluga Region; 21-Kostroma Region; 22-Kursk Region; 23-Lipetsk Region; 24-Moscow Region; 25-Oryol Region; 26-Ryazan Region; 27-Smolensk Region; 28-Tambov Region; 29-Tver Region; 30-Tula Region; 31-Yaroslavl Region; 32-Republic of Karelia; 33-Republic of Komi; 34-Arhangelsk Region; 35-Vologda Region; 36-Kaliningrad Region; 37-Leningrad Region; 38-Murmansk Region; 39-Novgorod Region; 40-Pskov Region; 41-Republic of Adygea; 42-Republic of Kalmykia; 43-Krasnodar Region; 44-Astrakhan Region; 45-Volgograd Region; 46-Rostov Region; 47-Republic of Dagestan; 48-Republic of Ingushetia; 49-Republic of Kabardino-Balkaria; 50-Republic of Karachay-Cherkess; 51-Republic of North Ossetia-Alania; 52-Republic of Chechen; 53-Stavropol Region; 54-Kurgan Region; 55-Sverdlovsk Region; 56-Tyumen Region; 57-Chelyabinsk Region; 58-Republic of Altai; 59-Republic of Buryatia; 60-Republic of Tyva; 61-Republic of Khakassia; 62-Altai Region; 63-Transbaikal Region; 64-Krasnoyarsk Region; 65-Irkutsk Region; 66-Kemerovo Region; 67-Novosibirsk Region; 68-Omsk Region; 69-Tomsk Region; 70-Republic of Sakha (Yakutia); 71-Kamchatka Krai; 72-Primorsky Krai; 73-Khabarovsk Region; 74-Amur Region; 75-Magadan Region; 76-Sakhalin Oblast; 77-Jewish Autonomous Region; 78-Chukotka Autonomous District.
As a result, it was found that the application of the developed technology for the immobilization of mercury will reduce the mercury intake in fresh and marine water bodies and into the atmosphere, which, according to our calculations and research data [29], represent the most vulnerable components of the environment.
It should be noted that, although in this paper we give separate estimates for metallic mercury and mercury sulfide, the model requires refinement, first of all, refinement of information on the ratio of various compounds between environmental components. Other uncertainties may be data on entry into various environmental components. This paper presents data that complies with the recommendations of the United Nations Environment and indirect information on the number of mercury devices in circulation that can make significant changes to the final results.
Conclusion
As a result of the studies, it was determined that mercury entering the environment from devices has a significant effect on the size of the chemical footprint. To reduce this effect, in this work, we studied the technologies of immobilization of metallic mercury in waste. Various conditions for reaction of the immobilization of metallic mercury in mercury-containing wastes were investigated and it was established that it proceeded best under the following conditions:
Reaction of metallic mercury with elementary sulfur;
A ball mill is used as a reactor, which ensures constant updating of the contact area of the phases;
For a good dispersion of mercury and for a relatively quick and complete reaction a large excess of sulfur up to 6500% by stoichiometry (e.g. ratio of mercury:sulfur=1:1.5 by weight) is necessary;
The addition of a very small amount of water also has a positive effect (hydromodulus of Solid:Liquid=3:1 by weight).
It was also modeled how the introduction of this technology can affect the situation with mercury pollution in general. The assessments conducted showed that the use of this technology can have a positive effect on the content of mercury in water bodies of the environment, both fresh and marine, and practically eliminate the potential threat of exceeding the established limits.
Article note
A collection of invited papers based on presentations at the 8th IUPAC International Conference on Green Chemistry (ICGC-8), Bangkok, Thailand, 9–14 September 2018.
Funding source: RFBR
Award Identifier / Grant number: 18-29-24212
Funding statement: The reported study was funded by Funder Id: http://dx.doi.org/10.13039/501100002261, RFBR according to the research project № 18-29-24212.
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