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VII. Water Retention by Humic Substances
A LIFETIME PERSPECTIVE
are shown in Fig. 20. The plots are linear in the 0 –55% RH range for both materials. At higher RHs, slopes of the BET plots rise sharply and become nonlinear.
From the slopes and intercepts of the linear sections of the BET plots in Fig. 20,
the weight of water adsorbed by 1.0 g of HA or FA to form a monolayer (Xm) and
the heat of adsorption (E1-EL) are caculated (see Table V). Very similar weights of
water (58–61 mg) are required to form monomolecular layers on the two humic
surfaces at 35% RH, but weights of water adsorbed at 60% RH, where the plots
become nonlinear, are 110.0 mg for HA and 120 mg for FA, suggesting the adsorption of a second layer of water (Table VI). At 90% RH, 1.0 g of HA adsorbs
225.0 mg of water (four layers), whereas 1.0 g of FA adsorbs 508.0 mg of water
(eight layers). Heats of adsorption (E1-EL in Table V) are less than 1 kcal/mol for
both materials. Increasing the temperature has little effect on these parameters. The
surface areas, measured by the BET method, are very similar for the two materials (Table V).
Figure 20 BET plots of the adsorption of water vapor by HA and FA at 40ЊC. From Chen and
Schnitzer (1976a) with permission of the publisher.
BET Parameters for a Haploboroll HA and a Spodosol FAa
E1 Ϫ E Ld
Surface area (SBET)
Chen and Schnitzer (1976a).
of water adsorbed by 1.0 g of HA or FA to form a monolayer.
cexp (E Ϫ E )/RT.
dHeat of adsorption.
The BET plots have shapes that are similar to “type III” isotherms (Gregg and
Singh, 1967). These isotherms are characteristics of systems in which the adsorption is cooperative, i.e., the more H2O molecules already adsorbed, the easier it is
for additional H2O molecules to become adsorbed. Interactions between adsorbates are enhanced if adsorbate molecules are capable of strong hydrogen bonding, which occurs notably with water (Gregg and Singh, 1967). Thus, the sharp increases in slopes at RHs Ͼ55% can be interpreted as being due to the adsorption
of fresh H2O molecules, hydrogen bonded to H2O molecules already adsorbed so
that HA can adsorb four layers of H2O and FA twice that number under the same
experimental conditions (see Table VI).
As shown in Table V, the net heat of adsorption is low, which means that the attraction of adsorbate (H2O) molecules for each other exceeds their attraction for
the adsorbent (HA or FA). One question that still remains to be answered is why
does FA adsorb eight H2O layers, and HA only four H2O layers? As shown in Table
Water Adsorbed by 1.0 g of HA and 1.0 g of FA
at Different Relative Humidities (RHs)a
H2O (mg) adsorbed by 1.0 g
Chen and Schnitzer (1976a).
A LIFETIME PERSPECTIVE
I, the CO2H content of FA is considerably higher than that of HA. It appears therefore that H2O molecules tend to cluster around the CO2H groups.
For a more detailed discussion of HA and FA surface area measurements and
amounts of H2O required to form monomolecular layers on these materials, see
Schnitzer (1986c), who used ﬁve different methods (water vapor sorption, surface
pressure and surface tension measurements, N2ϩHe adsorption, and electron microscopy) to determine these parameters.
VIII. REACTIONS OF HUMIC SUBSTANCES
WITH METALS AND MINERALS
Humic substances can react with metals and minerals by several mechanisms.
These include (1) formation of water-soluble metal complexes, (2) formation of
water-soluble mixed ligand complexes, (3) sorption on and desorption from water-insoluble HAs and metal–humate complexes, (4) dissolution of minerals, (5)
adsorption on mineral surfaces, and (6) adsorption in clay interlayers.
A. FORMATION OF WATER-SOLUBLE COMPLEXES
Reactions in water near pH 7 between di- and trivalent metal ions and HAs
and FAs are likely to proceed by either one or more of the mechanisms shown
in Fig. 21, taking divalent metal ion M2+ as an example. According to Eq. (4),
one CO2H group reacts with one metal ion to form an organic salt or monodentate complex. Equation (5) describes a reaction in which one CO2H and one adjacent OH group react simultaneously with the metal ion to form a bidentate
complex or chelate. According to Eq. (6), two adjacent CO2H groups interact simultaneously with the metal ion to also form a bidentate chelate. Equation (7)
shows the metal ion Mn+ linked to FA not only by electrostatic bonding but also
through a water molecule in its primary hydration shell to a CuO group. The
complexes described by Eqs. (5) and (6) are stronger than those described by
Eqs. (4) and (7).
Stability complexes of water-soluble metal–HA and –FA complexes have been
determined by several workers (Stevenson, 1994). A number of major problems
have been encountered in the analysis and interpretation of data. One serious obstacle to progress in this area is our lack of adequate knowledge of the chemical
structures of the ligands, i.e., HA and FA. It is to be hoped that the structural models for HA proposed by Schulten and Schnitzer (1997) will provide much needed
background information to those studying the formation and characteristics of
metal–HA and metal–FA complexes.
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