Minerals soil organic matter, clay

The water-rock/soil and organic matter-minerals (including soil organic matter-clay), interactions can also be studied by infrared spectroscopy (Chapter 2, Section 2.1.2). 

Metal oxides have a significant role in influencing physical, chemical, and biological properties of soils. They may exist as ciystalline minerals, as short-range ordered (SRO) mineral colloids, or as surface coatings on clay minerals and organic matter. Organic compounds influence the formation, transformation, and surface properties of these metal oxides. The SRO A1... 

Carrizosa MJ, Hermosin MC, Koskinen WC, Cornejo J (2004) Interactions of two sulfonylurea herbicides with organoclays. Clays Clay Miner 52 643-649 Celis R, Hermosin MC, Cornejo J (2000) Heavy metal adsorption by functionalized clays. Environ Sci Technol 34 4593-4599 Chappell MA, Laird DA, Thompson ML, Li H, Teppen BJ, Johnston CT, Boyd SA (2005) Influence of smectite hydration and swelling on atrazine sorption behavior. Environ. Sci Technol 39 3150-3156 Chiou CT (1989) Theoretical considerations of the partition uptake of nonionic organic compounds by soil organic matter. In Sawhney BL, Brown K (eds) Reactions and movement of organic chemicals in soils. Soil Science Society of America, Madison, WI, pp 1-29... 

The first of these environmentally-important parameters can be expressed as a partition coefficient. In aqueous solution many, but not all pesticide compounds exhibit strong affinity for soil organic matter or concentrate in the lipid phase of soil organisms. Some, notably the cationic group, also exhibit marked affinity for clay or other mineral surfaces. An overall partition (or distribution) coefficient (kD) can be defined ... 

Although triazines are dominantly sorbed by soil organic matter, the clay minerals also make a substantial contribution to triazine sorption by soils. For instance, the organic and inorganic components comprise 11% and 89%... 

Hance (1967) investigated the rate of sorption and desorption of four pesticides (monuron, linuron, atrazine, and chlorpropham) on two soils, a soil organic matter fraction, and bentonite, a 2 1 smectitic clay mineral. An equilibrium in sorption was reached in 24 h for every system except one (Table 6.1). With eight of the 18 systems equilibrium was reached in less than 4 h, and in five cases equilibrium was established in 1 hr. Equilibrium was attained for most of the systems in 4-24 h. Desorption was slower than sorption. In only eight systems was an equilibrium reached in 24 h. Hance... 

Not reaching an equilibrium between the soil and pesticide before desorption is begun could also cause nonsingularity. Diffusion of pesticides into soil micropore sites associated with clay minerals and organic matter could cause a pseudo-equilibrium (Hance, 1967 Rao et al., 1979). 

Explain the potential reactivity of soil organic matter with (a) cations and (b) clay minerals. Explain the practical significance of this reactivity. 

Name the various functional groups of (a) clay mineral surfaces and (b) soil organic matter. Explain which of these functional groups exhibits constant charge or variable charge behavior and discuss the practical significance of this behavior. 

As mentioned previously, the total surface of the rock and soils is the sum of the external and internal surfaces. Both external and internal surfaces have charges, and so, when the internal surface is significant (such as in the case of humic substances and expandable clay minerals), that extent of the interfacial layer is quite great. All system has external surfaces, while internal surfaces are significant only in the case of certain minerals and organic matter therefore, we will discuss first the properties of external surfaces. 

As mentioned in Sections 1.3.2.1 and 1.3.2.2, the CEC and specific surface area (both internal and external) are higher than those of clay minerals. The functional groups of soil organic matter (Table 1.5) can be deprotonated or protonated, depending on pH. It means that they have pH-dependent charges. The major functional groups can be deprotonated at pH values characteristic of soils (pH = 6-8) so that they can sorb cations. It has been estimated that the CEC of soils comes from the soil organic matter in 20%-70% (Stevenson 1982). 

Soils vary greatly in composition and reactivity. Many complex and dynamic processes occur continuously in most soils composed primarily of mineral and organic matter, water, and air. The soil atmosphere is composed of oxygen, carbon dioxide, nitrogen, and several minor gases whereas the mineral fraction varies in amounts of sand, silt, and clay and in types and amounts of clay minerals. Moreover, hydration and base saturation of the clay minerals also vary considerably. The organic matter and mineral colloids present in the soil contribute directly and indirectly to the extremely active nature of pesticide-soil systems. Since soil water contains many soluble compounds, it serves as an essential medium for many chemical and physical processes. The extreme complexity of these soil systems has been the primary reason that so few fundamental studies have been undertaken involving the ultimate fate of pesticides in soils. 

Clays and particles of soil organic matter have negative charges and high CECs, as confirmed by the following values 150-500 meq/100 g for organic matter and 3-150 meq/100 g for clay minerals (as kaolinite and smectite). On the other hand, aluminum and iron hydroxide colloids tend to be hydrophobic, with surface positive charges, and, consequently, they are predominantly anion adsorbers. 

The soil is a dynamic biotic and abiotic system. Pesticides deposited in or on the soil have varying capacities to be adsorbed to clay minerals and organic matter. Such adsorption reduces both the movement and the biological activity of the pesticide. In addition to soil adsorption, several other factors are known to influence the behavior and fate of pesticides after the chemicals are in contact with soil. 

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