Environmental Science And Pollution Research

Environmental Science And Pollution Research

Environmental Science And Pollution Research

Discuss About The PH And Chlorides On Washing Of Mercury-Contaminated Soil.

Answer

Introduction

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.Environmental Science And Pollution Research

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). Environmental Science And Pollution Research

Conventional Methods For Removal Of Mercy From Water

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).

Removal Through Chemical Precipitation

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.

Sulfide Precipitation

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).

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%.

Toho Process

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.

Removal By Blue PRO Reactive Filtration Process

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).

Removal By Reverse Osmosis

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).

Use Of Some Flocculants In The Removal Of Mercury

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

Description And Principles Of Technology

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. Environmental Science And Pollution Research

Applicability, Benefits, And Disadvantages

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). Environmental Science And Pollution Research

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. Environmental Science And Pollution Research

Factors Influencing Precipitation Performance And Cost

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).

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.