5. Discussion of Results
5.3. Hematite in the Presence of Organic Solutes
5.3.2. Effect of Polymeric Organic Matter
(b), the species distribution is modeled by an equilibrium calculation. Comparing (a) and (b), the adsorption ceases to be substantial at pH>8.0. If it is assumed that the surface is fully covered at pH<8.0, the ^-potential should be similar to the surface potential. At pH>8.0, the ^-potential should follow the reduced potential calculated at Z=4.0 nm. In panel (c) the stability ratio is plotted, and shows agreement with the mobility data. At pH around 6.5 the mobility is approximately zero as a result of adsorption of phthalate, and the corresponding stability is also a minimum. At pH 4, although the adsorption is substantial the particles still carry a net positive charge, which in turn results in relative stability. The adsorption is not substantial at pH 10. The surface charge and potential are determined by surface hydroxyl groups.
Corresponding to the high mobility at pH 10, a high stability ratio is predicted.
Oxalate is expected to have a similar effect to that of phthalate on hematite coagulation. Comparing the acidity constants for the two acids, oxalic acid dissociates more strongly, which indicates a weaker complex with H+ or other Lewis acids, such as Fe3+. Consequently, we expect phthalate ions to form stronger surface complexes than oxalate ions. The coagulation data in Fig. 4.12 support this hypothesis. At a total concentration equal to 10-5M, a considerably smaller stability ratio in the presence of phthalate is observed compared with that for oxalate. This is due to the stronger adsorption of phthalate, which results in a reduced surface potential.
The adsorption of oxalate ions is less extensive, hence particles have relatively high potential and stability. Oxalate acts more like a non-specifically adsorbed ion, since there is no clear indication of an increased stability ratio when the oxalate exceeds the critical coagulation concentration. Compared with the sulfate coagulation data, oxalate seems to complex with hematite surfaces more strongly than does sulfate .
Figure 5.10: Comparison of hematite surface potential, Φ, surface species distribu
tion, Cj, and stability ratio, Wexp, as a function of pH in the presence of 0.2 millimolar phthalate ions.
various types of naturally occurring particles. The presence of natural organic matter changes the surface potential and charge, and consequently the electrostatic interac
tion between particles is altered. Hence, the chemical interaction between natural organic matter and particle surfaces plays a key role in determining the observed co
agulation rate. The results in electrophoretic mobility and coagulation kinetic studies presented in this research show the influence of adsorption on coagulation.
Carboxylic groups have been shown to have a strong affinity for hematite sur
faces (Sec. 5.3.1). A polymer molecule with a large number of carboxylic groups, such as polyaspartic acid, is expected to exert a stronger effect than simple organic molecules on hematite stability, as is explained by Lyklema (1985). Lyklema pointed out that polymer adsorption is driven by the free energy of bonding polymer seg
ments to the adsorbent. As a result, adsorption of polymers is widely observed since only modest segment adsorption free energy is needed. Regarding the conformation of poly electrolytes, Lyklema pointed out that they lay flat on surfaces. This feature was modeled as patch formation by Kasper (1971). During particle encounter, the particles orient themselves in an energetically favorable position in which the reduced local potential shielded by the patches favors the aggregation. The effect of polyas
partic acid (PAA) on the coagulation rate of hematite is shown in Fig. 4.14. The plot resembles Fig. 4.12(b) in which the stability ratio is plotted as a function of phthalate concentration. However, the concentration corresponding to the minimum stability is much lower for PAA (~ 2 × 10-5g∕l, or 6 x 10-7 M of -COOH) than for phthalate (~5x 10~2g∕l, or 6×10~4 of -COOH). At the pH of the study, the side chain car
boxylic groups in PAA are fully dissociated (p∕<=3.9), and the polymer structure is shown overleaf.
As the PAA concentration increases from zero to 2 × 10-5g∕l, the binding of carboxylic groups to the hematite surface effectively reduces the positive surface po
tential, hence a decreased stability ratio is observed. As the PAA concentration is
CHCO — (NHCHCO) n -NHC— COO^
ch 2 ch 2 ch 2
c=o c==o
1 c=o
1
0^ I
O“ O" 1
increased beyond 2 × 10 5g∕l the large number of negative carboxylic groups is able to reverse the surface potential, resulting in an increase in the stability ratio.
The coagulation of hematite in the presence of fulvic (FA) and humic acid (HA) follows the same trend as in the presence of polyaspartic acid. Since carboxylic groups are dominant in natural organics (Thurman and Malcolm, 1983) the organic substances are essentially negatively charged polyelectrolytes. Table 5.2 lists the properties of Suwannee River fulvic acid (after Thurman and Malcolm, 1983). Fulvic acid contains 6 millimoles/g of the functional group -COOH. The critical coagulation concentration of fulvic acid is 10-4 g/1 (see Fig. 4.14). This corresponds to 6×10-7M of -COOH. Thus the critical coagulation concentration of fulvic acid and PAA is the same in terms of -COOH. The molecular weight of Suwannee River humic acid is 3000-5000, and is larger than that of fulvic acid (1000-1700). However, both contain approximately the same number and type of functional groups. Humic acid and fulvic acid therefore have a similar effect on hematite stability, as may be seen in Fig. 4.14.
Preliminary adsorption experiments have revealed that the adsorption of FA on hematite surfaces is of the high affinity type. This agrees with the observations of other researchers (Lyklema, 1985). The electrophoretic mobility data (Fig. 4.22) show that the adsorption of fulvic acid on hematite is due not only to the electrostatic interaction of polyanions and positive surface species. If electrostatic interaction were the sole driving force for adsorption, the mobility would be expected to be reduced to zero as the FA concentration increases, and would remain at zero as
Table 5.2: Properties of Suwannee River fulvic acid (Thurman and Malcolm, 1983).
1. Elemental analysis
C = 51.3 N = 0.56 ash = <0.05
H = 4.32 S = <0.2
O = 42.9 P = <0.1
2. Functional groups (mmol∕g)
-CO2H (titration) = 6.0 phenolic -OH =2.1
-CO2H (NMR-solid) = 6.2 = 1.7
-CO2H (NMR-liquid) = 6.0 = 3.6
hydroxyl (NMR solid) = 8.6 carbonyl (NMR) solid =1.7 hydroxyl (NMR) liquid = 5.4
3. Carbon distribution(%)
aromatic 20.8
aliphatic 36.7
C-OH 20.1
co2h 14.6
phenolic 3.9
carbonyl 4.0
4. Aminoacids = 36 nmol/mg
% N (AA) = 10%
5. % carbohydrate = 4%
6. Molecular weight= 1200 ± 200 7. 14C age = less than 30 yr
the concentration of FA is further increased. The substantial reversal in mobility arises from the specific chemical interaction in addition to the electrostatic force.
Consequently, the mobility data account for the observed stability ratio (Fig. 4.14), where at (FA)< 1 × 10-4g∕l the stability is obtained through net repulsions of positive surfaces, and at (FA)> 1 × 10~4g∕l the stability arises from the repulsion due to the negative potentials.