Humus was extracted from highly weathered soils differed in mineralogical composition according to the parent material type (Table 1). The soils are Aeric Epiaquipts and Typic Haplusteps have developed on residuam derived from Deltaic Alluvium and Alluvium/ Colluvium. The some physical and chemical properties of these soils are listed in Table 2.
Elemental analysis and metal contents of humic acids is listed in table 3. All the treated samples contain certain amount of Si4+, Al3+ and Fe3+/2+ even after treatments to reduce metal ions. In the ‘treated’ HA the contents of all the ions were reduced by reduction in Si4+ level was less than Al3+ or Fe3+/2+.All treated samples had comparable levels of metal ions.
The elemental analysis of treated HAs was unnecessary because on dry, ash-free basis, it will be the same with the untreated ones (Table 3). On treatment to reduce the metal ions, (i) Si4+ content reduces by about 80% in case of Jalpaiguri HA while for Baruipur HA, such reduction is around 92% (ii) Al3+ content reduces by about 97% for both Jalpaiguri and Baruipur HA samples, and (iii) Fe3+/2+ content is lowered by 97% and 84% for Jalpaiguri and Baruipur HA sample, respectively (Table 3).
IR spectra (Figure 1) showed the following changes on reducing metal ions: (i) In the treated HAs the absorption of the 1630 cm-1 decreases whereas that of the 1720 cm-1 increases compared to the untreated HAs. (ii) Simultaneously, the 1400-1390 cm-1 band diminishes in intensity in the treated HAs, compared to that in the untreated HAs.
(iii) Similarly, the 1050 cm-1 is more prominent in the untreated samples whereas it is much less intense in the treated samples. All these indicate that as more polyvalent, metal ions are removed as in treated samples, more carboxylic groups are rendered free. Carboxylic groups react with metal ions to form metal carboxylate bonds. At the same time, hydrogen bonding of C=O of carboxyl or quinines are destroyed. Positions of carboxyl C=O and OH vibrations are, therefore, deformed from their normal positions.
(iv) The sharp decrease in the 1040 cm-1 in the treated samples compared to untreated ones is probably due to reduction in the level of Si-O bonded to HA following treatment to reduce metal ions. This infers that a strong Si-O linkage existed in the HA which can only be partially removed by chemical treatment.
(v) On the other hand, in the treated HA samples, the 3400 cm-1absorption decreases. Assuming that some OH groups are involved in the metal-humic bonds in the untreated HA samples, the removal of metal ions should cause an increase in OH absorption. The contrary behavior suggests that during the removal of metal ions from HA by the methods employed, M-OH bonds (M=metal) are also abstracted. These probably contributed to a significant extent in the 3400 cm-1 absorption and may be the reason for the decrease in intensity of the 3400 cm-1 band in the treated samples. At this stage, it may be conjectured that during the process of humus formation, Fe3+/2+, A13+, Si4+ etc. ions do not incorporate into the humic substances in purely ionic forms, rather, they may react as some partially hydroxylated forms like Fe(OH)2+, Fe(OH)2+, Al(OH)2+, Al(OH)2+, Si(OH)3+, Si(OH)22+, Si(OH)3+, etc. This explains why untreated HA contains some M-OH bonds together with M-HA bonds, (vi) There is also a sharpening of phenoxy C-O (1250 cm-1) in the treated samples. This may be caused by the release of some phenoxy C-O groups, from bonding with metals, (vii) The 1200 cm-1 is absent in the untreated samples whereas the treated samples show this band due to opening of some oxygen containing functional groups.
Although Si-O absorption around 1000 cm-1 reduces considerably on treatment, this was still quite strong in the treated samples indicating a strong Si-O linkage with humus. A reduction in the absorption around 3400cm-1 was also noted on treatment, which suggest the contribution of metal-OH absorptions in the untreated samples.
The X-ray diffractograms are shown in figs. 1 and the data derived therefrom are presented in Table 5. The noteworthy information in this context are as follows: (i) A strong reflection at 9.60 A0 (at 9.82 A0 with Jalpaiguri HA only) is observed with all the samples. This is due to a silicate as humic molecules produce only weak reflections, even a very small quantity of silicate present can yield relatively stronger diffraction bands. (ii) The band around 7.5 A0 may also be due to silicate organic band is extremely strong and this indirectly suggests that the tropical humic substances will be richer in metal ions than the temperate ones. (iii) All the samples show the well-known γ-band, between 4.03 and 4.12 A0. (iv)The other reflections shown by the samples are of not much significance.
On treatment to reduce metal ions, the significant changes in XRD (Figure -1) is that the γ-band (4.03 to 4.12A) becomes more prominent and reflections due to silicate (9.60 - 9.82 A) weaken. The bands like one at 7.5 A assigned to silicate phase and the well known 002 band, due to ordering of condensed aromatic layers normal to their planes (Kodama & Schnitzer ,1967), are exhibited by both 'treated' and 'untreated' samples.
Treated HAs show exotherms at much higher temperatures (Table 4), viz. with Jalpaiguri HA at 605°C instead of 540°C and with Baruipur HA at 600°C instead of 440°C. Ghosh and Schnitzer (1982) observed that the thermal stability of humic substances is decreased as they are complexed with metals. They inferred that complexing by metals exert strains in the humic structure thereby lowering its resistance to thermal stabilities. On treatment, an additional endotherm at 435°C for Jalpaiguri HA and 480°C for Baruipur HA are observed. Strain got free by removal of metals is likely to be responsible for this.
There is substantial increase in total acidity values on treatment to reduce metal ions. With Jalpaiguri HA, such increase is from 475 to 860 cmol (+) kg-1 (about 80%) while for Baruipur HA, the change is from 660 to 860 cmol (+) kg-1 (about 30%). One can conclude that the groups which are responsible for total acidity is humic molecules; which includes weakly acidic functional groups apart from carboxyl and phenolic hydroxyl, are rendered free on removal of metal ions.
On treatments, the E4/E6 ratio (Table 4) of Jalpaiguri HA reduces from 4.95 to 2.46 while such lowering in the case of Baruipur HA, is from 6.11 to 2.74. As E4/E6 ratio is an index of molecular size and weight (Chen et al., 1977) this substantial change indicates that metal ions which were removed, do not act as a bonding between humic molecules. On the contrary, on removal of these metal ions, groups which can form linkages between humic molecules become free and make bonds as H-bonds or homolytic bonds through free radicals, which is quite likely at pH 8.3.
With Jalpaiguri HA, on treatment to reduce metal ions, the viscosity average molecular weight (Mv), is increased from 4,715 to 8,030 an increase of about 70% (Table 4). Such increase is around 20% with Baruipur HA where in the absolute increase in Mv values are from 14,830 to 17,705. This data provide more conclusive evidence of the observation that metal ions do not act as a bridge between humic molecules; only intermolecular bonds are formed. On removal of the metal ions, the acid groups become free and promote intermolecular bonding, thereby increasing the molecular weight. It may be noted that the increase in total acidity value, on treatment to reduce metal ions for Jalpaiguri HA is about 80% while it is 30% for Baruipur HA. This molecular weight increase can be explained by mechanism discussed above with CEC and E4 / E6 values.
Treated HAs, from which the metal ions had been reduced, were interacted with Si4+ ions and in order to observed the reverse behavior and to understand how these metal ions interacted with HA. With Si 4+, usually, desorption occurred at pH 2.0, 6.0 and 8.5 (Table 7). In order to prove the binding of Si 4+ to HA, both Jalpaiguri HA and Baruipur HA was extracted with NaOH at various strengths. At the highest concentration of NaOH only 36.5-38.5% of remaining Si4+ could be dissolved (Table 7). The HAwas then subjected to Hashimoto and Jackson Method (1960) for the removal of Si4+. The sample was subsequently studied for Si4+ adsorption. Even with this sample, only a small adsorption occurred (Table- 6). It was observed that even after treatment with HAs with strong alkali solution at boiling condition only 40.50- 42.5 % of remaining Si4+ could be dissolved under such forcing condition. Thus, it is seen that although humus in nature is strongly bound with fairly large amount of Si4+, after removal of this Si4+, it cannot be re-incorporated into the humus structure.
In humic molecule there is a strong silicon-humus bond like pure organo- metallic compounds . It appears from this studies that the association of metalloid ions, with humus in soils is quantatively and qualitatively different from that of other metal ions. The unique interaction of HA with Si4+ could throw new light on nature of humic substances in soils.