TOXIC CHEMICALS IN ENVIRONMENT11

TOXIC CHEMICALS IN ENVIRONMENT - Mercury : 

TOXIC CHEMICALS IN ENVIRONMENT - Mercury

INTRODUCTION : 

Mercury - a very dense, silver-colored, liquid metal that is poisonous. It is used in thermometers and still used in some dental amalgams (fillings). It is named after a Roman god known for speed and mobility. Mercury occurs in deposits throughout the world mostly as cinnabar (mercuric sulfide). There are seven stable isotopes of mercury with 202Hg being the most abundant (29.86%). The longest-lived radioisotopes are 194Hg with a half-life of 444 years, and 203Hg with a half-life of 46.612 days. Most of the remaining radioisotopes have half-lives that are less than a day. INTRODUCTION

Sources of mercury in environment : 

Sources of soil contamination with Hg include metal processing industries certain chemical works (especially chloralkali process) Dental amalgam wastes Measuring devices Electrolyte refining waste the use of fungicides containing Hg and Other sources of mercury also include primary dry batteries. Sources of mercury in environment

Slide 4: 

Effluent Limitations for Selected Toxic Pollutants for Discharge to Surface Waters (All Values in mg/L)

Slide 5: 

Industrial Waste Pretreatment Limits 16

Mercury cycle : 

Mercury in the environment is constantly cycled and recycled through a biogeochemical cycle. The cycle has six major steps: Degassing of mercury from rock, soils, and surface waters, or emissions from volcanoes and from human activities. Movement in gaseous form through the atmosphere. Deposition of mercury on land and surface waters. Conversion of the element into insoluble mercury sulfide. Precipitation or bioconversion into more volatile or soluble forms such as methylmercury. Reentry into the atmosphere or bioaccumulation in food chains. Mercury cycle

Slide 7: 

Transformations of mercury in soils and sediments

Detection of genes using reporter genes : 

Biosensors for the detection of Hg(I1) have been constructed and use strain HMS174 as the host to examine bioluminescent responses (SELIFONOVAet al., 1993). Construction of mer-lux fusion plasmids was accomplished by cloning the mercury resistance operon (Tn21 mer) upstream from a promoterless lux operon (1uxCDABE) from I/: fischeri. The plasmid pOS14 (Ro/pTPC), containing the regulatory gene merR, the operator-promoter (o/p), and the merTPC, which mediates active transport of Hg(II), exhibited light emission at 0.5 nM Hg(I1). The sensitivity was reduced in pOS15 (Ro/pTPCAD) by introduction of erA that encodes mercuric reductase. Detection of genes using reporter genes

Removal of mercury from environment : 

Removing mercury from the water can be achieved using four processes: Coagulation/Filtration, Granular Activated Carbon, Lime Softening, and Reverse Osmosis. Removal of mercury from environment

Slide 12: 

Coagulation/filtration is a common treatment which uses AlSO4 that reacts with the mercury to form a solid which can precipitate out of the water. The sludge then must be disposed of in a hazardous waste landfill. Granular activated carbon uses porous carbon media. This media is a very heavy charcoal material. As the water passes through, the dissolved contaminants are absorbed and held on the solid surface. This process has its limitations because the effectiveness depends on the concentration of mercury in the water. Lime Softening uses excess Ca(OH) to raise the pH level and then the heavy metal precipitates out as Hg(OH). In reverse osmosis, water is pushed through a semi permeable membrane. A Common membrane material is a polyamide film. This produces high quality water, but is fairly expensive.

TOXIC CHEMICALS IN ENVIRONMENT – Lead : 

TOXIC CHEMICALS IN ENVIRONMENT – Lead

Slide 14: 

Lead is a soft, malleable poor metal, also considered to be one of the heavy metals. Lead has a bluish-white color when freshly cut, but tarnishes to a dull grayish color when exposed to air. It has a shiny chrome-silver luster when melted into a liquid. Metallic lead does occur in nature, but it is rare. Lead is usually found in ore with zinc, silver and (most abundantly) copper, and is extracted together with these metals. The main lead mineral is galena (PbS), which contains 86.6% lead. Other common varieties are cerussite (PbCO3) and anglesite (PbSO4). Lead has many isotopes but 4 stable ones. The 4 stable isotopes are204Pb, 206Pb, 207Pb and 208Pb with 204Pb regarded as primordial lead and 206, 207, 208 are formed from decay of uranium and thorium. The one common radiogenic isotope, 202Pb, has a half-life of approximately 53,000 years

Sources of lead in environment : 

Major sources of lead are: paints Leaded fuels water distribution systems Lead batteries waste Ore extraction processes Welding Radiation shields waste People who eat animals hunted with lead bullets may be at risk for lead exposure etc Sources of lead in environment

Discharge limitations for toxic compounds : 

Discharge to Surface Waters Discharge limitations for toxic compounds

Treatment : 

Remediation of lead can be achieved in several ways Physical Chemical Biological It has been reported that vetiver (vetiveria zizanioides), water hyacinth and zeolite can remediate contamination of lead and cadmium. Vetiver grasses reduce concentration of lead in the soil by as much as 38-60% Treatment

References : 

Waste water treatment in industries; In-plant management and disposal of industrial hazardous waste; L.K. Wang Fundamentals of site remediation for metals and hydrocarbons in contaminated soil; chemistry of common contaminant elements; John Pichtel Understanding our environment, An introduction to environmental chemistry and pollution; ecological and health effects of chemical pollution; R.M. Harrison Introduction to environmental science; soil pollution; Y Anjaneyulu References