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Figure 21 Major metal–HA and –FA reaction mechanisms. From Schnitzer (1986a), with permission of the publisher.
B. MIXED LIGAND COMPLEXES
The formation of metal–FA–phosphate complexes was ﬁrst described by
Lévesque and Schnitzer (1967). It is likely that in soils an appreciable portion of
the total P exists in the form of such complexes, but it is difﬁcult to demonstrate
this because of the low P content of soils.
The formation and stability of mixed ligand complexes of the type Cu2+ –FA–
secondary ligand (Y) have been studied by Manning and Ramamoorthy (1973).
Secondary ligands (Y) investigated were citrate, tartrate, salicylate, phosphate, nitrilotriacetate (NTA), aspartate, and glycinate. In neutral to weakly acid solutions,
mixed complexes predominated over simple complexes. Values of equilibrium
constants for mixed complexes with citrate, phosphate, and NTA were particularly high compared to simple complexes. If phosphate functions in the same way as
other oxyanions, the relatively high concentrations of HCOϪ
3 and HSO4 in some
A LIFETIME PERSPECTIVE
soil solutions should lead to the formation of mixed Cu2+ –FA–HCO3 and Cu2+ –
FA–HSO4 complexes. The formation of mixed complexes will prevent the precipitation of metal ions by hydrolysis at elevated pHs and will also interfere with
the precipitation of insoluble metal phosphates, sulfates, and carbonates.
C. ADSORPTION AND DESORPTION
One of the major characteristics of HAs is their ability to adsorb organic and inorganic substances. Kerndorff and Schnitzer (1980) examined the interaction of
HA with a solution containing equimolar concentrations of 11 different metal ions.
They report that the efﬁciency of adsorption of metal ions on HA increases with
rises in pH and HA concentration and decreases in metal concentrations.
At pH 2.4, the order of adsorption is Hg2+>Fe3+>Pb>CuϭAl>Ni>CrϭZnϭ
CdϭCoϭMn. At pH 3.7, the order is Hg2+>Fe3+>Al>Pb>Cu>Cr>CdϭZnϭNiϭ
CoϭMn. At pH 4.7, the order is Hg2+ϭFe3+ϭPbϭAlϭCr>Cd>NiϭZn>Co>Mn.
At pH 5.8, the order is Hg2+ϭFe3+ϭPbϭAlϭCrϭCu>Cd>Zu.Ni>Co>Mn.
Hg2+ and Fe3+ are always adsorbed most strongly by HA, whereas Co and Mn
are adsorbed most weakly. The different metal ions compete for active sites (CO2H
and phenolic OH groups) on the HA. Not only do the 11 metal ions plus H+ ions
(a total of 12 ions) interact with the HA, but they also interact with each other by
ion exchange, coprecipitation, and the formation of inner sphere and outer sphere
complexes. Afﬁnities of the 12 ions for sorption on the HA do not correlate with
their atomic weights, atomic numbers, and crystal and hydrated ionic radii (Kerndorff and Schnitzer, 1980).
The metal ions adsorbed on HA can subsequently be desorbed by a dilute aqueous FA solution. Similarly, FA can desorb metal ions sorbed on clays and hydrous
oxides so that FA can change the sorption, desorption, and precipitation characteristics of metals.
D. DISSOLUTION OF MINERALS
Due to their ability to complex di-, tri-, and tetravalent metal ions, dilute aqueous FA solutions at any pH and aqueous HA solutions at pH Ͼ 6.5 can attack and
degrade minerals to form water-soluble and water-insoluble metal complexes.
Thus, the weathering of minerals in soils and sediments is often enhanced by the
action of naturally occurring humic substances, especially FA. Because of its abundance in soils, its solubility in water, and its ability to complex with metal ions and
interact with silica, the latter may increase the concentrations of these soil constituents in aqueous solutions to levels that far exceed their normal solubilities. In
this manner, aqueous FA solutions may not only bring about the dissolution and
degradation of existing minerals, but also lead to the synthesis of new minerals by
permitting the complexed and dissolved metals and silica to form new combinations. Conversely, active surfaces of inorganic soil constituents may catalyze either the degradation or the synthesis of HAs and FAs.
E. ADSORPTION ON EXTERNAL MINERAL SURFACES
The extent of adsorption of humic materials on mineral surfaces depends on the
characteristic of the surface, the pH of the system, and its water content (Schnitzer,
1978). Kodama and Schnitzer (1974) report high adsorption of FA on sepiolite surfaces. Sepiolite has a channel-like surface formed by the joining of edges of long
and slender talc structures. In untreated sepiolite the channels are occupied by
bound and/or zeolitic water, which can be displaced by undissociated FA in aqueous solution at pH 3.0.
F. ADSORPTION IN CLAY INTERLAYERS
Schnitzer and Kodama (1966) demonstrated that the interlayer adsorption of
FA by expanding clay minerals was pH dependent, being greatest on low pH, and
no longer occurring at pH 5.0. Adsorbed FA could not be displaced from clay interlayers by leaching with 1 M NaCl; an inflection was observed in the adsorption–pH curve near the pH corresponding to the pK of the acid species of FA so
that the adsorption could be classified as a ligand-exchange reaction (Greenland,
1971). In this type of reaction the anion (FA) is thought to penetrate the coordination shell of the dominant cation in the clay and displace water coordinated to
be the dominant cation in the clay interlayer. The ease with which water can be
displaced will depend on the affinity for water of the dominant cation with which
the clay is saturated and also on the degree of dissociation of the FA. Because
the latter is very low at low pH, interlayer adsorption of FA is greatest at low pH
IX. INTERACTIONS OF PESTICIDES AND HERBICIDES
WITH HUMIC SUBSTANCES
The persistence, degradation, bioavailability, leachability, and volatility of pesticides are directly related to the nature and concentration of humic substances in
a particular soil (Khan, 1980). Pesticides may sorb on humic substances and be retained by Van der Waal’s forces, hydrophobic bonding, hydrogen bonding, charge
A LIFETIME PERSPECTIVE
transfer, ion exchange, and ligand exchange. From a study on the chemical mechanisms governing the aciﬂuorfen (a diphenyl ether herbicide)–HA interaction, Celi
et al. (1997a) concluded that the herbicide appeared to be adsorbed unchanged on
external HA surfaces and in internal voids of the model HA structure proposed by
Schulten and Schnitzer (1997). The rate at which a pesticide should be applied to
a soil may vary widely, depending to a large extent on the nature of the SOM (humic substances) the soil contains. Humic substances can promote the nonbiological degradation of pesticides and can also form strong linkages with residues arising from the partial and chemical degradation of the pesticides (Stevenson, 1994).
These processes may play important roles in the detoxiﬁcation, protection, and
preservation of the environment. Soils tend to accumulate increasing amounts of
pesticide residues that are capable of passing into air and water, into plants and microbes, or being degraded to other products (Khan, 1980). These bound residues
may also contain intact pesticide molecules that, when released, could exert deleterious biological effects on the environment. However, ﬁrm binding of pesticides
to humic substances may be a method of decontamination. Information on how
pesticides interact with humic substances provides a rational basis for their effective use and for minimizing undesirable side effects.
X. FUNCTIONS AND USES OF HUMIC SUBSTANCES
A. FUNCTIONS IN SOILS
1. Humic substances exert physical, chemical, and biological effects on soil
quality by serving as soil conditioners, nutrient sources, and substrates for microorganisms.
2. They contribute to the maintenance of an adequate and stable soil structure
by acting as binding agents in the formation of soil aggregates, thus ensuring satisfactory drainage and aeration, and providing protection against erosion, enhancing mechanical soil properties, and playing a major role in water retention.
3. They act as sources and storehouses of N, P, and S and of micronutrients essential for plant growth. They form complexes with many metals and make these
available to plant roots and microorganisms. They buffer soil against drastic
changes in pH and also interact with herbicides and pesticides and assist in their
degradation and detoxiﬁcation.
4. They serve as substrates for macro- and microorganisms in the soil. Soil microorganisms play a major role in the synthesis and degradation of humic substances. Humic substances can also exert direct physiological effects on plants.
5. All of these affect the impact on soil quality and point to a vital role for humic substances in soil fertility.