Simple and Rapid Method of Isolating Humic Acids from Tropical Peat Soils (Saprists)
Osumanu Haruna Ahmed,
Nik Muhamad Ab. Majid
Mohamadu Boyie Jalloh
Abstract: Problem Statement: The isolation (extraction, fractionation and purification) of humic acids (HA) from soils is laborious, time consuming and expensive. The extraction, fractionation and purification periods of these substances vary from 12 h-7 days. In order to facilitate production of HA at competitive cost, this study was conducted to investigate whether a simple and rapid procedure could be developed for isolation of HA from well decomposed tropical peat soils (Saprists). Approach: A 0.1 M KOH was used to isolate HA of air dry peat soil at 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, and 24 h extraction periods after which samples (liquid obtained after centrifugation at 16,211 G for 15 min) were fractionated (using 6 M HCl) at 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, and 24 h. Samples were purified by washing them five times using distilled water instead of using HCl, HF, and an expensive process called dialysis that requires 1 to 7 days to purify HA. Each washing time was 10 min. Standard procedures were used to ascertain the purity (Ash, C, E4/E6, carboxylic, phenolic, total acidity, and K, Ca, Mg, and Na) and quantity of HA yield. Statistical Analysis System (SAS) was used for statistical analysis. Results: Although there was a linear relationship between extraction period and HA yield, there was no relationship between fractionation period and yield of HA. Distilled water used in this study was effective in purifying HA of the Saprists within 1 h without altering the true chemical nature of HA as it significantly reduced the mineral content of HA. Besides, C, E4/E6, carboxylic, phenolic, and total acidity of the isolated HA were typical of standard ones. Conclusion: The isolation of HA from peat soils can be reduced to 9 h (4 h extraction period, 4 h fractionation period and 1 h purification period) instead of the existing range of 1-7 days.
Fig. 1) from the tropical peat soil studied. Contrary to period of extraction, no significant relationship was found between the period of fractionation with yield of HA (Fig. 2). There was no significant interaction between extraction period and fractionation period (Fig. 3).
When HA are extracted they are normally not free from mineral matter. As a result, the purity of HA is usually ascertained from ash content (mineral matter), elemental composition, functional groups, spectral characteristics and so forth. The ash content should not be more than 1%. The ash content of the HA was found to range from 0.3-0.83%, a range less than the generally accepted one.
As shown in Table 1, the contents of Na, K, Ca and Mg at first washing were generally high but their contents consistently decreased considerably at the fifth washing (Table 1).
||Relationship between extraction period and humic acids yield
||Relationship between fractionation period and humic acids
of fractionation periods under different Extraction Periods (EP). Note,
no significant relationship p = 0.05 between different fractionation periods
and yield of humic acid for different extraction periods studied
||Ranges of the elemental concentrations in the supernatant
solutions at first and fifth washing of peat soil Humic Acids (HA) during
||Comparison of ranges of carbon, phenolic OH, carboxylic COOH,
total acidity and E4/E6 of Saprists peat Humic Acids (HA) with related reports
The C content, phenolic OH, carboxylic COOH and total acidity ranges of the
HA in this study were found to be within the ranges reported by other authors
(Table 2). Additionally, the E4/E6 values
of the HA which indicate level of humification were comparable with those reported
The linear relationship between extraction period and yield of HA was consistent with the observations of Goh and Reid that the yield of HA increases with increasing extraction period. This finding for Saprists can be explained based on the fact that besides slowing down depolymerisation of high molecular weight complexes, the solubilisation of the peat soil increased with increasing period because the exchange process of Na ions at the exchange sites (carboxylic, phenolic functional groups) of peat progressed with the extraction period until 24 h, a period when most of these sites may have been saturated with Na ion, making the peat soil highly soluble and hence the maximum yield at this extraction period. Another reason could be the difficulty of extracting the humic substances in the initial shorter period (e.g., 4 h) because of the difficulty of wetting the peat soil due to irreversible drying associated with peat soils upon drying. It must, however, be pointed out that where the characterization of the HA is of primary importance instead of the amount of HA isolated, an optimum yield (16.54%) of HA equivalent to optimum extraction period of 4 h or less could be considered, as the longer the extraction period, the greater will be the chemical changes. Optimum yield was calculated as 90% of the maximum yield of HA corresponding to maximum extraction period of 24 h beyond which this yield will not be time wise economical[9,10].
No significant relationship between the period of fractionation with yield of HA suggests that the yield of HA was not affected by fractionation period. Therefore, it is assumed that the minimum fractionation period in this study is 4 h. This is because the fractionation period was sufficient for the H ions (after acidification with 6 M HCl) to displace Na ions and other cations at the exchange complexes of the HA.
Furthermore, the fact that there was no significant interaction between extraction
period and fractionation period (Fig. 3) suggests that the
performances of the different fractionation periods with any of the extraction
periods is the same, therefore it is reasonable to assume that 4 h of equilibration
after acidification is sufficient to displace the sodium ions with hydrogen
ions at the exchange sites of the HA.
The low ash content suggests that the HA in this study contained a relatively low content of inorganic ions. The relatively low ash content of the HA indicates that the purification process was effective in reducing mineral matter.
Apart from removing cations in solution, the excess water used during purification served as Bronsted-Lowry acid thereby donating more hydrogen ions which effectively replaced some of the remaining Na, K, Ca and Mg at the exchange complexes of the HA which were not replaced by hydrogen ions (during acidification with 6 M HCl) during fractionation (precipitation). The consistency of the range of carbon content of the HA (Table 2) with those reported by[13,15]also confirms the purity of the HA, a further indication of the purity of the HA.
The relatively high E4/E6 values indicate prominence of aliphatic components or the HA in this study are of relatively low molecular weights[6,13].
The nature of the relationship between period of extraction and yield of HA from Saprists peat is linear. The optimum yield of HA can be obtained at extraction period of 4 h. There is no relationship between period of fractionation and yield of HA, so from an economic point of view 4 h is sufficient to equilibrate HA immediately after acidification for tropical peat soils. The HA can be purified within 1 h using distilled water. The significance of this study is that HA of tropical peat soils can be isolated within 9 h (i.e., 4 h extraction period, 4 h fractionation period and 1 h purification period) or less instead of the existing range of 1 to 7 days, hence helping in facilitating the idea of producing ammonium and potassium humates from tropical peat soils, a practice that could have less undesirable environmental effects.
The authors acknowledge the financial support of this research by the Ministry of Higher Education Malaysia." class="btn btn-success" target="_blank">View Fulltext