VII. Water Retention by Humic Substances

A LIFETIME PERSPECTIVE



39



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.



40



M. SCHNITZER

Table V

BET Parameters for a Haploboroll HA and a Spodosol FAa



Type of

material

HA

HA

FA

FA



Temperature

(°C)



Xmb

(mg H2O/g)



Cc



E1 Ϫ E Ld

(kcal/mol)



Surface area (SBET)

(m2/g)



25

40

25

40



58

59

60

61



3.58

2.77

4.88

4.06



0.688

0.635

0.855

0.871



205.6

209.2

212.7

216.3



aFrom



Chen and Schnitzer (1976a).

of water adsorbed by 1.0 g of HA or FA to form a monolayer.

cexp (E Ϫ E )/RT.

1

L

dHeat of adsorption.

bWeight



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



Table VI

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

RH (%)



HA



FA



35

60

90



58

110

225



60

120

508



aFrom



Chen and Schnitzer (1976a).



A LIFETIME PERSPECTIVE



41



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.



42



M. SCHNITZER



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

Ϫ

other oxyanions, the relatively high concentrations of HCOϪ

3 and HSO4 in some