Toxicity and Sources of Pb, Cd, 2 Hg, Cr, As, and Radionuclides in the Environment Ghinwa M. Naja and Bohumil Volesky CONTENTS 2.1 Metal Toxicity ..................................................................................................................... 14 2.1.1 Selected Heavy Metals ............................................................................................ 14 2.1.1.1 Lead ......................................................................................................... 14 2.1.1.2 Cadmium .................................................................................................. 16 2.1.1.3 Mercury .................................................................................................... 18 2.1.1.4 Chromium ................................................................................................ 21 2.1.2 Radionuclides ......................................................................................................... 22 2.1.2.1 Uranium ................................................................................................... 22 2.1.2.2 Radon ....................................................................................................... 24 2.1.3 Arsenic Pollution .................................................................................................... 25 2.1.3.1 Arsenic Speciation and Toxicity .............................................................. 25 2.1.3.2 Arsenic-Contaminated Countries ............................................................. 25 2.1.3.3 Clinical Effects ........................................................................................ 26 2.2 Metals in Groundwaters ...................................................................................................... 27 2.2.1 Heavy Metals in Aquifers ....................................................................................... 28 2.2.2 Cases and Remediation ........................................................................................... 33 2.3 Heavy Metal Pollution Sources ........................................................................................... 35 2.3.1 Acid Mine Drainage ................................................................................................ 35 2.3.1.1 Chemistry of Acid Mine Water ................................................................ 35 2.3.1.2 Extent of the Damage .............................................................................. 36 2.3.1.3 Radioactive AMD .................................................................................... 37 2.3.1.4 Treatment of AMD ................................................................................... 38 2.3.2 Metal Finishing and Surface Treatment Operations ............................................... 40 2.3.2.1 A Typical Electroplating Process ............................................................. 43 2.3.2.2 Future Trends in Electroplating ............................................................... 45 2.3.3 Leather Tanning Process ......................................................................................... 45 2.3.3.1 Description of the Chromium Tanning Process ....................................... 46 2.3.3.2 Wastes Generated in the Chromium Tanning Process ............................. 48 2.3.3.3 Effl uent Treatment ................................................................................... 50 2.3.4 Ferrous Metal Industries ......................................................................................... 51 2.3.4.1 Ferrous Metal Processing ........................................................................ 52 13 773168_C002.indd3168_C002.indd 1133 22/7/2009/7/2009 22:43:09:43:09 PPMM 14 Heavy Metals in the Environment 2.3.5 Coal-Fired Power Generation ................................................................................. 53 2.3.5.1 Coal-Fired Station Types ......................................................................... 55 2.3.5.2 Generating Station Water Use .................................................................. 57 2.3.5.3 Conclusions .............................................................................................. 58 References .................................................................................................................................... 58 2.1 METAL TOXICITY Out of 106 identifi ed elements, about 80 of them are called metals. These metallic elements can be divided into two groups: those that are essential for survival, such as iron and calcium, and those that are nonessential or toxic, such as cadmium and lead. These toxic metals, unlike some organic substances, are not metabolically degradable and their accumulation in living tissues can cause death or serious health threats. Furthermore, these metals, dissolved in wastewaters and discharged into surface waters, will be concentrated as they travel up the food chain. Eventually, extremely poisonous levels of toxin can migrate to the immediate environment of the public. Metals that seep into groundwaters will contaminate drinking water wells and harm the consumers of that water. Pollution from man-made sources can easily create local conditions of elevated metal presence, which could lead to disastrous effects on animals and humans. Actually, man’s exploitation of the world’s mineral resources and his technological activities tend to unearth, dislodge, and disperse chemicals and particularly metallic elements, which have recently been brought into the environ- ment in unprecedented quantities and concentrations and at extreme rates. Man’s new technologies involving nuclear fi ssion opened up a whole new area of hope and concern at the same time. Radioactive isotopes of elements and, indeed, new elements have been discovered and handled in historically unprecedented quantities and concentrations. The sneaking and deadly danger of radio- activity associated particularly with long-lived and high-radiation isotopes has cast a shadow over our lives. Actually, the disposal problems concerning radioactive isotopes originating directly or indirectly from the operation of nuclear generating facilities have produced a considerable slow-down in deployment of this technology, which, after all, may only be a transient phase made obsolete by the dangers it generates. 2.1.1 SELECTED HEAVY METALS Heavy metals can be defi ned in several ways. One possible defi nition is the following: Heavy metals form positive ions in solution and they have a density fi ve times greater than that of water. They are of particular toxicological importance. Many metallic elements play an essential role in the function of living organisms; they constitute a nutritional requirement and fulfi ll a physiological role. However, overabundance of the essential trace elements and particularly their substitution by non- essential ones, such as the case may be for cadmium, nickel, or silver, can cause toxicity symptoms or death. Humans receive their allocation of trace elements from food and water, an indispensable link in the food chain being plant life, which also supports animal life. It is a well-established fact that assimilation of metals takes place in the microbial world as well as in plants, and these ele- ments tend to get concentrated as they progress through the food chain. It has been shown that spectacular metal enrichment coeffi cients of the order of 105–107 can occur in cells [1]. Imbalances or excessive amounts of a metal species along this route lead to toxicity symptoms, disorders in the cellular functions, long-term debilitating disabilities in humans, and eventually death. 2.1.1.1 Lead Lead is the most common of the heavy elements. Several stable isotopes exist in nature, 208Pb being the most abundant. The average molecular weight of lead is 207.2. Lead is a soft metal that resists 773168_C002.indd3168_C002.indd 1144 22/7/2009/7/2009 22:43:09:43:09 PPMM Toxicity and Sources of Pb, Cd, Hg, Cr, As, and Radionuclides in the Environment 15 corrosion and has a low melting point (327°C). From a drinking water perspective, the almost uni- versal use of lead compounds in plumbing fi ttings and as a solder in water distribution systems is important. Distribution systems and plumbing installed before 1945 were made from lead pipes [2]. Solid and liquid (sludge) wastes account for more than 50% of the lead discharged into the environ- ment, usually into landfi lls, but lead has been dispersed more widely in the general environment through atmospheric emissions—particularly from car exhausts. With the introduction of unleaded fuel, lead emissions from this source declined. The annual consumption of lead is in the order of 3 million tons, of which 40% is used in the production of electrical accumulators and batteries, 20% is used in gasoline as alkyl additives, 12% in building construction, 6% in cable coatings, 5% in ammunition, and 17% in other usages. It is estimated that approximately 2 million tons are mined yearly. Probably 10% of this total is lost in treatment of the ore to produce the concentrate, and a further 10% is lost in making pig lead. The amount of lead discharged into the environment is equal to the amount weathered from igneous rocks. In global lead level terms, the power storage battery industry may have a relatively low impact on the environment because about 80% of all batteries are recycled. 2.1.1.1.1 Exposure Lead is present in tap water as a result of