Mercury toxic water pollutants

Heavy metal ions are perhaps the most common of all water pollutants. The heavy metals include such frequendy encountered elements as lead and mercury, as well as many less common ones like cadmium, chromium, nickel, and copper. These metals can, at times, be acutely toxic, causing immediate symptoms, but often they are chronically toxic in very small quantities. Chronic toxicity is characterized by nagging symptoms that lessen normal body functions. Inadequate disposal of wastes from mining or industrial activities causes these metals to find their way into water supplies. In addition, some farming activities and the disposal of household wastes can contribute to the presence of heavy metals in our water supplies. 

People usually think of things like lead, mercury, toxic organic compounds, and bacteria as being major pollutants. However, heat can also be a major pollutant. The solubility of a gas in a liquid decreases as the temperature increases (see Chapter 13 for more about the solubility of gases). This means that warm water doesn t contain as much dissolved oxygen as cool water. And how is this related to pollution The amount of oxygen in water has a direct impact on aquatic life. The reduction of the dissolved oxygen content of water caused by heat is called thermal pollution. 

A significant health concern with particles, especially those from combustion sources, is their ability to carry toxic metals. Of these, lead is of the greatest concern because it usually comes closest to being at a toxic level. Problems with particulate lead in the atmosphere have been greatly reduced by the elimination of tetraethyl lead as a gasoline additive, an application that used to spew tons of lead into the atmosphere every day. Another heavy metal that causes considerable concern is mercury, which can enter the atmosphere bound to particles or as vapor-phase atomic mercury. Airborne mercury from coal combustion can become a serious water pollution problem, leading to unhealthy accumulations of this toxic element in some fish. Other metals that can cause health problems in particulate matter are beryllium, cadmium, chromium, vanadium, nickel, and arsenic (a metalloid). 

Combustion of coal produces many of the same ultimate water pollutants as combustion of petroleum does, i.e., PAHs. Coal burning, however, produces greater quantities of metals, sulfur dioxide, and haloacids. Coal combustion stack emissions contain significant quantities of arsenic, mercury, selenium, copper, and tin [25]. Sulfur dioxide is ultimately converted to sulfuric acid in the air. Sulfuric acid and the haloacids (HF, HCl, HBr, and HI) ultimately come down as acid rains and acidify surface waters [26]. Acidified water is by itself toxic to marine life. Its effect is amplified, however, by the solubilization of metals and hydrolysis of other chemical compounds. 

Currently, waterborne toxic chemicals pose the greatest threat to the safety of water supplies in industrialized nations. This is particularly true of groundwater in the U.S., which exceeds in volume the flow of all U.S. rivers, lakes, and streams. In some areas, the quality of groundwater is subject to a number of chemical threats. There are many possible sources of chemical contamination. These include wastes from industrial chemical production, metal-plating operations, and pesticide runoff from agricultural lands. Some specific pollutants include industrial chemicals such as chlorinated hydrocarbons heavy metals, including cadmium, lead, and mercury saline water bacteria, particularly coliforms and general municipal and industrial wastes. 

The toxic nature of mercury and its compounds has caused concern over environmental pollution, and governmental agencies have imposed severe restrictions on release of mercury compounds to waterways and the air (see Mercury). Methods of precipitation and agglomeration of mercurial wastes from process water have been developed. These methods generally depend on the formation of relatively insoluble compounds such as mercury sulfides, oxides, and thiocarbamates. MetaUic mercury is invariably formed as a by-product. The use of coprecipitants, which adsorb mercury on their surfaces facihtating removal, is frequent. 

There is a vast range of aqueous organic pollutants with a wide toxicity profile. Some, e.g. polychlorinated biphenyls, certain herbicides, fungicides and pesticides, and organo-mercury compounds, are persistent and may bioaccumulate in the food chain. Trace contaminants such as sodium chloride, iron and phenols (especially if chlorinated) may also impart a taste to water. Typical consent levels for industrial discharges are provided in Table 13.10. 

Toxic pollutants found in the mercury cell wastewater stream include mercury and some heavy metals like chromium and others stated in Table 22.8, some of them are corrosion products of reactions between chlorine and the plant materials of construction. Virtually, most of these pollutants are generally removed by sulfide precipitation followed by settling or filtration. Prior to treatment, sodium hydrosulfide is used to precipitate mercury sulfide, which is removed through filtration process in the wastewater stream. The tail gas scrubber water is often recycled as brine make-up water. Reduction, adsorption on activated carbon, ion exchange, and some chemical treatments are some of the processes employed in the treatment of wastewater in this cell. Sodium salts such as sodium bisulfite, sodium hydrosulfite, sodium sulfide, and sodium borohydride are also employed in the treatment of the wastewater in this cell28 (Figure 22.5). 

Despite its availability and current use, coal is not as widely used today as the other fossil fuels. Coal s major weakness is that it does not burn cleanly. It often contains trace amounts of other elements, including mercury, arsenic, and sulfur, and when it burns, it releases these toxic substances into the air. Over time, coal pollution builds up in the environment. Mercury released during coal combustion, for example, settles in water and builds up in the bodies of fish and shellfish. When these fish and shellfish are eaten by humans and other animals, harmful amounts of mercury can be ingested. In 2008, bluefm tuna served in expensive New York restaurants was found to contain unacceptably high levels of mercury. These fish eat smaller organisms in the ocean, and when these small organisms contain mercury, the toxic element becomes concentrated in the body of the tuna. 

C. Because they were used in gasoline to boost octane rating, alkyl lead compounds found their way into the environment as toxic pollutants. One way to measure lead compounds in natural waters is by anodic stripping voltammetry (Figure 17-20), in which the lead is first reduced to the element at a mercury electrode and dissolves in the mercury. Reoxidation occurs when the electrode potential is made sufficiently positive, with current proportional to the concentration of dissolved Pb. 

A number of other metals, such as iron and tin. enter into insecticide and peslicide compounds, but as purl of an urganic chemical stniciurc. us exemplified by triphenyltin hydroxide. Such compounds arc sometimes referred lo as organometallics (or, specifically in Ihc case of tin. as orgamilins). Mercury compounds are rapidly being phased out because of their long-term toxic residual effects as pollutants, particularly of fresh and saline waters. Regulations vary I mm one country to another. 

The metals of most concern are the heavy metals, especially cadmium, lead, and mercury. Although it is a metalloid with characteristics of both metals and nonmetals, arsenic is commonly classified as a heavy metal for a discussion of its toxicity. Though not particularly toxic, zinc is abundant and may reach toxic levels in some cases. For example, zinc accumulates in sewage sludge and crop productivity has been lowered on land fertilized with sludge because of zinc accumulation. Copper may be toxic to plants. Aluminum, a natural constituent of soil, may be leached from soil by polluted acidic rainwater and reach levels that are toxic to plants. Other metals that may be of concern because of their toxicides include chromium, cobalt, iron, nickel, and vanadium. Radium, a radioactive alpha particle-emitting metal, can be very toxic at even very low levels in water or food. 

The impact to health has been mostly dependent on the concentration of the candidate metal. Some metals (e.g., mercury, lead, arsenic, cadmium, iron, copper) ultimately find their way into human systems via soil, minerals, and water. Studies have shown the presence of many metals in daily consumable products (e.g., food, fruits, milk, fabric materials, drinking water). Further, heavy metals associated with particle material can be accumulated in areas suitable for sedimentation or particle concentration (e.g., upstream from sills or dams, in estuary sludge clog, etc.). These accumulation areas are creating possible pollution sources, as particles pooled could be resuspended during punctual hydrologic periods (floods, drains). Bioavailability, and therefore toxicity of heavy metals, is strongly bound to the current chemical form. 

Bottom sediments in the coastal zone of the sea may be polluted with copper, zinc, nickel and cadmium. The highest levels of toxic heavy metals are found in the mouths of rivers. The bottom sediments in the Black Sea have a high mercury level—from 0.28 to 0.40 pg/1. In the coastal waters of the Krasnodar Territory the mercury level is 0.15-1.55 pg/1, while its maximum concentrations are registered in the Danube and Dnieper mouth areas. The Danube alone brings annually up to 50-60 tons of mercury, while the Dnieper brings up to 5 tons. The distribution of heavy metals in bottom sediments in the Russian shelf of the Black Sea is not uniform. Their greatest quantities are accumulated in sediments in the deepest part of the shelf where their concentration is 3-5 times higher than in sediments in the shallower part. Toxic metals contained in sea water in the dissolved and suspended forms are ac-... 

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