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Published online by Cambridge University Press: 15 July 2005
Introduction. The isolation of humic acid (Ha) from decomposed organic substances such as compost is time-consuming, as it takes not less than 24 h. We investigated whether the isolation time of Ha produced from composted pineapple leaves could be reduced. Materials and methods. Ha from 10 g of pineapple compost was extracted with 100 mL of 0.10 M NaOH. Different extraction times [(4, 8, 12, 16, 20 and 24) h] were tested. At the end of each time, the samples were centrifuged. The dark-colored supernatant liquor containing Ha was decanted and filtered; the pH of the solution was adjusted to 1.0. After acidification, the fractionation times tested were (3, 6, 9, 12, 15, 18, 21 and 24) h. At the end of each fractionation time, the supernatant liquor (fulvic acid) was siphoned off from the acidified extract. The remainder of the suspension was transferred to a polyethylene bottle. Ha was purified by suspending it in 50 mL distilled water, then centrifuged, and the supernatant was decanted. After repeating this procedure five times, the supernatant and the fulvic acid were analyzed for Ca, K, Mg, Mn, Na, Cu, Fe and Zn. The washed Ha was oven dried at 30 °C to a constant weight and the Ha samples were analyzed for ash and mineral matter. Results and discussion. The ash content was not more than 1%, the generally acceptable maximum value. The mineral contents decreased in Ha samples with washing. The yield of Ha significantly increased with increasing extraction time, except for the (20 and 24) h extraction times, which did not show statistically different Ha yield, meaning that the exchange sites of Ha of the pineapple compost might have been saturated with Na ions from 20 h. There was no significant difference among the fractionation times, indicating that Ha of the pineapple compost requires about 3 h to equilibrate or stand immediately after acidification for the H ions to replace the Na and the other cations. There was no significant interaction between the factors extraction and fractionation. Conclusion. The isolation of humic acids from composted pineapple leaves can be achieved with an extraction time of 20 h and a fractionation time of 3 h.