Conventional Methods for Removal of Mercy from Water
The US Government Agency for Toxic Substances and Disease Registry has ranked mercy in the third position in descending order of the most toxic elements or substances on the surface of the plant and comes immediately after arsenic and lead which are continuously dumped into the waterways and the soils, consumed in the water and food and spilled into the atmosphere. The amount of mercury that is available in the atmosphere has almost been tripped by the human activities and the atmospheric burden has been on the increase by 1.5 per cent per year (Adeleye et al., 2016).
The soils that have undergone contamination through mercury or contaminated water that has been redistributed have the potential to find their way into the food chain via living orgnisms. Upon finding their ways into the food chain, mercury has the potential to bio accumulate resulting into devastating effects to the human health. The strategies and approaches hat mercury find its ways into the food chains have remained widely unclear and likely varied among the various ecosystems (Anishinabek et al., 2016). The numerous and various routes that human beings are exposed to mercy are illustrated in the figure below.
The presence of mercury metal in the environment is highly toxic be it the form of a naturally existing or an induced contaminant to the specific environment. In as much as the potential of its toxicity in heavily contaminated areas has accurately been documented, studies have established that mercury and its presence can be a potential threat to the human health as well as the health of the wildlife in numerous environments that are obviously least or not all polluted (Awad et al., 2017). Such a risk is estimated by the chances of exposure, the form of the mercury that is available (since some forms are more toxic than others), as well as the ecological and geochemical factors which determine the behavior of moves of mercy as it changes from an environment to another.
Owing to the toxic nature and the diverse impacts of the presence of mercury in the water bodies and the environment at large, it attracts special interest that is worth further and in-depth study and analysis. The toxic effects of mercy rely on the chemical form as well as the exposure route. The effects van is on the immune systems, damages to the nervous systems as well as alteration of the enzyme and genetic systems (Awual et al., 2016). This is specifically for the case of methylmercury which is the most toxic form of mercy. This mercury form can as well have damaging offers to a developing embryo and the damage effects are estimated to be five to ten time more sensitive as compared to adults.
Removal through Chemical Precipitation
The concentration of mercury in wildlife and fish has also been established to be high enough to pose a risk to the wildlife. These concentrations of the toxic substance have instigated fish advisories by the various states against the consumption of fish owing to mercury contamination. It is for these devastating implications of mercury present in water bodies and the environment at large that make the chemical a concern and worth studying for this research (Chapman et al., 2016).
In the adsorption process, absorption of the contaminated molecules of gas occurs on activated carbon surface. Carbon is often preferred as the adsorbent owing to its properties of the wide surface area. The various carbon sources that may be used in this process include coconut shells, coal, and wood. The process of activation is conducted at relatively high temperatures under-regulated oxidation process (Dixit et al., 2015). The efficiency of the adsorption relies on the features of the contaminant, the temperature as well as the concentration of the contaminant. The capacity of adsorption of ant of the contaminant is a factor of the quantity of the pollutant that can be adsorbed or a single activated carbon weight. On an average, the adsorption capacity of adsorbed comprises often range between 5 to 30 per cent of the mass of the carbon that has been used (García-Sánchez & Száková 2015).
In this process, mercury is trapped using HgSO4 which is being produced as a by-product in the chemical combination of mercury with sulphuric acid. This process begins with an acid having a concentration of 80% of sulphuric acid at a temperature that is slightly below 50?C. The second phase is conducted in a conventional tower that operates at 93% of sulphuric acid (Gilmour et al., 2018). At later stages, the reaction between mercy and the acid lead to the formation of mercurous sulfate as illustrated in the equation below
HgSO4+Hg Hg2SO4
This is a very common method and often does not need the use of relatively expensive chemicals to achieve the removal of mercury.
The addition of sodium thiosulfate in sulphuric acid can lead to the production of colloidal sulfur as shown in the equation below:
H2SO4+Na2S2O3 S+Na2SO4+H2O+SO2
The initial stage of this process involves the reaction of sulfur with mercury which leads to the formation of crystalline mercury sulfide (He et al., 2015). The sulfur as well reacts with the other available metal contaminates inside the acid leading to the production of insoluble metal sappires. This method works well on acid concentrations that are lower than 85% sulphuric acid. Acid concentrations higher than this percentage may lead to the oxidation of sulfur those results in sulfur dioxide. The acid product is composed of sodium sulfate which is never desirable in the acid product (Henriques et al., 2015).
Sulfide Precipitation
The dosage of sodium thiosulfate has been to be regulated in the process to ensure it generates mercury sulfide that does not easily filter. By using this method, the concentration of mercury can be lowered from 15 ppm to 0.5 ppm within one hour. One of the significant sources of sulfide that is used in the precipitation of mercury besides other metals is hydrogen sulfide. This strategy is often preferred in cases where sodium sulphate is not desirable as one of the end products. When the pH rises above 9, the efficiency of the process begins to decrease. This process serves as a better option in the chlor alkali plant which often has an efficiency ranging between 95 and 99%.
This process is composed of the addition of mercury and potassium iodide which is then precipitated as mercuric iodide as illustrated in the table below
This process involves the addition of cuprous iodide is added besides potassium iodide so as to attain a precipitate of Cu2HgI4 that is more stable. Filtration is used in the separation of the precipitated mercury.
The Blue PRO reactive filtration process may reduce the particulates besides the dissolved species of mercury with the aid of numerous mechanisms of removal. Blue PRO is a tertiary process of water treatment and has the ability to conduct co-precipitation and adsorption, surpassing the limitations of diffusion inside a backwash filter that is continuous and the filtration of particles. An adsorptive media of hydrous ferric oxide is used in the regeneration in the filter, eliminating mercury to very minimal levels (Kabiri et al., 2015). Blue PRO process has been established to be cost-effective in comparison with the other processes of tertiary treatment of wastewater for the elimination of mercury to reduced standards. It is linked with less capital and costs of operations relative to the membrane, granular activated carbon, reverse osmosis as well as coagulation systems (Kanchi, Singh & Bisetty 2014).
Reverse osmosis system is composed of pre-filters of granular activated carbon, a tank for storage, a Reverse Osmosis membrane and a faucet that is used for the purposes of delivery of the water stream that is of low concentration. Cellulose Triacetate and Thin Film Composite are the most commonly used Reverse Osmosis membranes where Thin Film Composites have been found to exhibit higher efficiency than Cellulose Triacetate membranes. Both membranes exhibit very high rates of rejection for mercury types and its contaminants. These membranes are often very cheap and may go for approximately 5 cents for every gallon of pure water (Kumar, Smita & Flores 2017).
Toho Process
Flocculants define chemicals that are used in triggering the flocculation of minute particulates which have been suspended in the liquids to collect and form flocs. Flocculants are mostly applicable in the processes of the treatment of waste products for the general purposes of enhancing the filterability or sedimentation of small particles. Flocculants may be used in the swimming pools or even in water meant for drinking to aid in the elimination of the mercury metal which may be the initiator of the turbidity of water (Lewis et al., 2016).
A significant proportion of flocculants are metal cations of magnesium, calcium, iron or aluminium. The interaction between these cations and anions enhance the aggregation process. Some of the chemicals have suitable pH and the required temperature for reaction with water to generate insoluble hydroxide. When such hydroxides precipitate, long chains are formed and are able to capture the small particles in the nature of larger flocs (Liu et al., 2018).
Precipitation/ Coprecipitation
Technologies are perceived to be precipitation/precipitation if the following steps are involved in them:
- Separation of the solid matrix from the available contaminated water
- Mixing of the treatment chemicals into the water
- Production of a solid matrix via coprecipitation, precipitation or even a combination of the two processes (Ma et al., 2015)
Precipitation often encompasses an adjustment of the pH as well as the addition of a coagulant to chemical precipitants in a bid to change a soluble metal or even an inorganic contaminant into an inorganic salt to insoluble metal. The elimination of mercury encompasses an alteration of the pH of water that is to undergo treatment since the removal undergoes is often maximized at such a pH that enables the least solubility of the precipitated species. The type of waste that is to be treated as well as the specific process of treatment determines or influences the optimal pH that is needed for precipitation. Filtration of clarification is used in the removal of the precipitated or precipitated solid (Mohmood et al., 2016).
Sulfide precipitation has remained to be the leading cost common method of precipitation that is deployed in the elimination of inorganic mercury from wastewater. During this process, the changed pH range is often between 7 and 9 and a precipitant of sulphide, for example, sodium sulfide is then introduced into the stream of wastewater. The precipitant of sulfide changes dissolved mercury to significantly insoluble mercury form. One of the precipitation processes used in precipitating mercury utilizes derivatives of lignin to come up with lignin-mercury colloid (Naik & Dubey 2017). The precipitated solid can thereafter be eliminated through gravity settling which is conducted in a clarifier. This process can be improved through the addition of a chemical coagulant or even the use of a settling aid, for example, ferric chloride and thereafter flocculation and settling.
Removal by Blue PRO reactive filtration process
Precipitation is an active ex situ technology for treatment that is made to function through the routine addition of chemicals and removal of sludge. It often produces a residue of sludge which ideally calls for such treatments as subsequent disposal and dewatering. Some of the sludge derived from the precipitation of mercury can be in the form of relatively dangerous waste and call for extra treatment for example stabilization or solidification in order to be safely disposed as a solid waste or may call for disposal in the form of a hazardous waste (Peterson et al., 2018).
Too much use of the precipitates of chemical sulfide has the ability to form species of soluble mercury sulfide. Mercy is able to resolubilize from the sludge of sulfide under the various conditions that are found in the landfills. and this may result in contamination of the leachate by mercury as well as a likely pollution of groundwater (Rosestolato, Bagatin & Ferro 2015).
The effluents that are obtained from the precipitation of mercury may also call for further treatment, for example, making adjustments on the pH before it can be reused or discharged. The precipitation of mercury using sulfide may produce residual sulfide as part of the effluents. This may as well for the treatment that would be used in the elimination of the residual sulfide.
Chemical dosage: the cost of precipitation often increased with an increase in the quantity of chemical addition. Large quantities of chemicals that are added to the mixture culminate into a large quantity of sludge that would in turn call for more disposal or treatment. The use of too much sulfide precipitants can lead to the formation of mercury sulfide species (Sierra et al., 2016).
pH: Generally, the elimination of mercury is always maximized at a pH value in which there is the least solubility df the precipitated species. The specific process of treatment and the waste that is to be treated are the main determinants of the optimal range of pH for precipitation.
The goal of treatment: In some applications, a single step of precipitation or utilizing precipitation may not manage to achieve the goals of the treatment. Numerous types of precipitation or the utilization of the additional techniques could be found to be of the essence so as to attain the stringent goals of clean-up, standards of disposal or even effluent guidelines (Taylor &Gulf, 2017).
Removal by Reverse Osmosis
Presences of other compounds: The effectiveness of the precipitation to co-precipitation may be influenced by the presence of other contaminants or metals in the wastewater that is to be treated.
Disposal of sludge: The sludge that is generated from the precipitation or precipitation process may be treated as a dangerous waste and would call for the need of extra additional treatment before it is disposed of as a solid waste even as a dangerous waste.
This section explores the chosen benchmark projects which encompass the examination of the innovative technologies and their abilities to aid in the effective treatment of mercy. Among the technologies discussed in this part include nanotechnology, air stripping as well as phytoremediation. Studies are in progress about reactive capping materials including bauxite that is used in mercury for sediments.
These innovative technologies have the ability to provide a more reliable and cost-effective alternative for the treatment of mercury (Terán-Baamonde et al., 2018). From the information that is available from the small fraction of applications of such technologies which have been highlighted, they are usable in the treatment of mercury at a relatively greater frequency in the future. Nonetheless, there is a need for additional information and data to aid in attaining an elaborate and exhaustive understanding of their effectiveness and applicability.
Phytoremediation is one of the technologies which are under evaluation for the efficiency in the elimination of mercury from particles as well as other sources. Phytoremediation makes use of plants for the elimination, transfer, stabilization or even complete destruction of the contaminants that can be found in the soils or groundwater (Wang et al., 2017).
The technique is applicable to every chemical, physical and biological processes which may be in a way or another be influenced by the plants including rhizosphere and that help in attaining a clean-up of the contaminated things. Plants are usable in site remediation, both via accumulation and levels of the heavy metals as well as other inorganic compounds from the soil surface into the shoots above the ground and through the mineralization of the organic compounds (Xu et al., 2015). Phytoremediation is applicable in ex situ or in situ to groundwater, soils, sediments, sludge, and any other solids.
The plants can be engineered genetically in such a way that they are able to promote their ability in promoting the achievement of detoxification of mercury. An example of such is the development of a transgenic plant through modifications done to the rice plant to eliminate mercury from the sediments that are found in water (Terán-Baamonde et al., 2018).
The plant is modified such that it is injected using a gene which generates an enzyme mercuric reductase that has the ability to change ionic mercury into an elemental mercury which thereafter volatizes from the sediments. Numerous studies have been carried out on a study to as well as other plants with the aim of establishing their ability when it comes to remediating mercury (Xu et al., 2014).
One of the classic examples of the illustration of the utilization of nanotechnology for mercury in an aqueous stream could be the Thiol Self-Assembled Monolayers on Mesoporous Silica. Such a novel adsorbent is traced back from the works of the Pacific Northwest National Laboratory staff and is made up of a nonporous ceramic substrate that has high surface areas which have been made functional through a monolayer of the groups of thiol (Xu et al., 2014).
The substrate has been synthesized through a process which is described as self-assembly which adopts sol-gel precursors and surfactant molecules. Preceding calcination to 500?C eliminates the surfactant templates, leaving behind monoporous ceramic substrate which can then be used in the self-assembled monolayers of the adsorptive functional groups. The thiol functional groups have been found to be of very high affinity to the heavy metals, mercury included. These thiol functional groups are sandwiched in the ceramic substrate on a single end and bind on the other end with mercury.
Thiol Self-Assembled Monolayers on Mesoporous Silica has the ability to binding mercury selectively as well as achieving a loading capacity of mercury that is as high as about 635 milligrams per gram. Besides, indications from test data illustrate that the presence of absences of cations and sophisticated anions do not have a significant impact on the adsorption of Thiol Self-Assembled Monolayers on Mesoporous Silica when it comes to attaining waste solutions.
A pilot-scale trialability study was carried out to establish the Thiol Self-Assembled Monolayers on Mesoporous Silica’s ability in the elimination of soluble mercury present in an aqueous meter condensate waste stream. The study was done by doing the treatment on 160 litters of waste using a module that was composed of a filtration unit, pump, and batch reactor with a mixer and a holding drum all of which were to be used in the treatment of the effluent (Terán-Baamonde et al., 2018).
The findings from the initial treatment eliminated about 97.4% of the dissolved mercury from the wastewater and a residual concentration of 0.28 mg/L was obtained. The second treatment recorded a lowered concentration of the mercury to 0.18 mg/L while the last treatment eliminated 99.4% of the dissolved mercury that was originally present inside the untreated waste, yielding a 0.06 mg/L residual concentration. These findings illustrated that Thiol Self-Assembled Monolayers on Mesoporous Silica could effectively be used in scavenging dissolved mercury in the melted waste stream.
This is yet another technology under evaluation of its ability in the elimination of mercury from water. This technique, in general terms, has not been adopted in the elimination of inorganic compounds including mercury. Nonetheless, a bench-scale investigation carried out at the Savannah River Site aimed at examination if the reduction in the chemicals fooled by gathering of elemental mercury from the air in the headspace is able to eliminate low mercury levels from the groundwater (Terán-Baamonde et al., 2018).
The technology under test made use of stannous chloride in the reaction of Hg+2 to Hg0 which is a volatile substance, and thereafter elemental mercury was collected from the headspace air to eliminate the elemental mercury that is found in the water. The concentration of mercury in the underground water extracted was ranging between 120 and 150 nanograms per litter (ng/L) with at least 95% of the mercury being Hg+2(Wang et al., 2017). The results of the study were such that stannous chloride doses which had concentrations of more than 0.011 mg/L generated more than 94% of removal of mercury and the total residual mercury levels being reduced to less than 10 ng/L.
Nonetheless, very low doses of stannous chloride illustrated little elimination of mercury. This investigation illustrated that chemical reduction embedded with air stripping could be an effective strategy in the treatment of mercury. The technology does not generate any solid or liquid secondary waste and it may not need off-gas treatment for the anticipated concentrations of air and mass release.
In Situ Thermal Desorption is under evaluation for its ability to treat mercury. This is a soil remediation process which makes use of both heat and a vacuum to a surface for the attainment of the extraction and degradation of contaminants. Numerous laboratory soil column experiments have been carried out to illustrate the capability of In Situ Thermal Desorption in the treatment of soils that have been contaminated with mercury. One of such experiments involved the injection of about 15.03 g of mercury into a column that was packed with Ottawa sand (Wang et al., 2017). An analysis of the soil at the end of the remediation illustrated that just about 11.1 mg of mercury was left as a residue in the soil by the end of the experiment, which corresponded to a 99.9% efficacy. Such a study illustrated the In Situ Thermal Desorption can be used in the elimination of mercury from contaminated soils.
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