Page 115 - Partition & Adsorption of Organic Contaminants in Environmental Systems

Partition and Adsorption of Organic Contaminants in Environmental Systems. Cary T. Chiou
Copyright ¶ 2002 John Wiley & Sons, Inc.
ISBN: 0-471-23325-0

7 Contaminant Sorption to

Soils and Natural Solids

7.1 INTRODUCTION

Soil may be defined as a collection of natural bodies synthesized in profile
form from a variable mixture of broken and weathered minerals and decay-
ing organic matter that covers the earth surface (Brady, 1974). Discounting
the water content in soil, which varies geographically with the soil and with
the depth in a vertical soil profile, the principal soil components are mineral
matter and organic matter. Minerals are composed of aluminosilicates and
oxides in various crystalline and amorphous forms and vary considerably in
individual quantity and physical size, the latter ranging from very small col-
loidal clay particles (<2mm) to relatively large sand particles (>50 to 60mm).
The soil organic matter, which originates primarily from biologically degraded
plant tissue and becomes part of the underlying horizons by infiltration or
physical incorporation, consists of a heterogeneous makeup of organic con-
stituents, such as lignins, carbohydrates, protein, fats, and waxes. It contains
a large fraction of operationally defined humic substance, which is yellow
to brown in color and moderately refractory to biological degradation
(Stevenson, 1985). Although the molecular structures of humic substances
have not been well characterized, they are known to be high-molecular-weight
amorphous materials (frequently referred to as humic polymers) with signifi-
cant polar-group contents. In ordinary soils, more than 90% of the dry organic
matter is made up of carbon, hydrogen, and oxygen, with minor amounts of
nitrogen, sulfur, and phosphorus. Except for relatively rare organic-rich soils,
which are termed organic soils, ordinary soils are rich in minerals and are
referred to as mineral soils. The organic matter content for most of these
mineral soils falls between 0.5 and 3.0% by weight.
Studies of the sorption of organic compounds to soil began with the advent
of pesticides for pest and weed controls in the 1940s, which called for an under-
standing of their interaction with and persistence in agricultural soils. In
the 1970s, research in this field became more active and intense after wide
varieties and large quantities of pesticides and industrial organic wastes were
found throughout the environment, raising public concerns as to their long-
term environmental impacts. Over the years, research in this field has shed
important light on the characteristics of chemical–soil interactions in terms of
soil organic and mineral components and of the properties of organic com-
pounds. Understanding of the mechanistic roles of organic and mineral matter
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