Sediments AND waste disposal

Contents of Part V

Chapter 15 Toxic Heavy Metals Chapter 16 Wastes, Soils, and Sediments

Environmental Instrumental Analysis VI

Inductively Coupled Plasma Determination of Lead

Scientific American Feature Article Mapping Mercury

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In this chapter, the following introductory chemistry topics are used:

■ Redox half-reactions

■ Electrolysis

■ Half-life calculations

■ Solubility product and acid-base equilibrium constant calculations, including manipulations for multiple equilibria

Background from previous chapters used in this chapter:

■ Steady state; UV and visible light wavelengths (Chapter 1)

■ Synergism (Chapter 4)

■ Bioaccumulation; LD50; dose-response curves; maximum contaminant level (Chapter 10)

■ Aerobic, anaerobic, and calcareous waters (Chapter 13) Introduction

In chemistry, heavy metal refers not to a type of rock music but rather to a type of chemical element, many examples of which are poisonous to humans. The five main ones discussed here—mercury (Hg), lead (Pb), cadmium (Cd), chromium (Cr), and arsenic (As)—present the greatest environmental hazard due to their extensive use, their toxicity, and their widespread distribution. None have yet pervaded the environment to such an extent as to constitute a widespread danger. However, each one has been found at toxic levels in certain locales in recent times. Metals differ from the toxic organic compounds


we discussed in Chapters 10-12 in that they are totally nondepadable to nontoxic forms, although they ultimately may be transformed to insoluble forms, which therefore are biologically unavailable unless they are again converted into more soluble substances. The ultimate sinks for heavy metals are soils and sediments.

The heavy metals occur near the middle and bottom of the periodic table. Their densities are high compared to those of other common materials. The densities of the metals of interest here arc collected in Table 15-1, as are values for water and two common "light" metals for contrast.

Although we commonly think of heavy metals as water pollutants, they are for the most part transported from place to place via the air, either as gases or as species adsorbed on, or absorbed in, suspended particulate matter. As we shall see later in the chapter, the deposition of airborne lead into European lake sediments dates back to the time of the ancient Greeks. These days, over half the heavy-metal input into the waters of the Great Lakes, for example, is due to deposition from the air.

Continue reading here: Speciation and the Toxicity of Heavy Metals

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