Peat soil humin, obtained after the removal of fulvic and humic acid fractions according to the recommended procedure of the International Humic Substances Society (IHSS), has the ability to remove AuCl-4 ions from aqueous solution. The removal was greatest at medium acidity, which was equivalent to pH 2.0, and it was governed by two different processes, i.e. attachment onto un-ionized-COOH groups through hydrogen bonding and reduction to Au metal by the action of the-OH group. Purification of humin through repeated immersion and shaking in a mixed solution consisting of 0.1 M HCl and 0.05 M HF led to the abrupt increase in the-COOH contents from 386 mmol/kg to 2510 mmol/kg, while the content of total-OH functional groups remained relatively constant at values slightly higher than 3000 mmol/kg. The increase in-COOH content improved the removal of AuCl-4 ions from 23 mg/g to 27 mg/g, but it was unfortunately accompanied by a diminished reduction ability to Au metal. Hence, the role of the-COOH functional group in removing AuCl-4 ions through hydrogen bonding was more significant for the purified humin, while that of the-OH group in removing AuCl-4 ions through reduction to Au metal was better for the crude (unpurified) humin.
Synthesis of a new type of adsorbent has been conducted by immobilizing peat soil humic acid (HA) on chitin isolated from crab shell waste. The adsorbent was then applied to adsorb Cu(II) in aqueous medium. The HA was extracted from peat soil of Gambut District, South Kalimantan, Indonesia; while the chitin was isolated from marine crab shell waste of seafood restaurants. The extraction of HA was performed by the commonly used alkaline extraction in NaOH 0.1 M solution, and the isolation of chitin was conducted through deproteination using NaOH 3.5%(w/v) and followed by removal of inorganic impurities using HCl 1 M. The extracted HA and the isolated chitin were characterized by Fourier Transform Infra Red (FT-IR) spectroscopy. Parameters investigated in this study consisted of method of HA immobilization on chitin, stability test of the immobilized HA, as well as rate, capacity, and energy of adsorption. The FT-IR spectra revealed the presence of main functional groups of COOH and OH (phenolic-and alcoholic-OH) in HA, and those of NH, C=O, CH3, and OH groups in chitin. The immobilization of HA on chitin was successfully done by reacting gelatinous chitin (40 g) in 250 mL of HCl 0.5 M and solution of HA (4 g) in 500 mL of NaOH 0.5 M. This immobilization produced an adsorbent of humic acid immobilized on chitin (Chitin-HA) with the content of HA was 1.98%(w/w). The HA immobilized was stable in the acidity range of pH 3.0 to 11.0. At the acidity giving maximum adsorption, i.e. pH 4, the first order rate constant, capacity, and energy of adsorption of Cu(II) on the HAC were 4.6 × 10 −4 min-1, 0.282 mmol/g, and 20.35 kJ/mol, respectively. Compared to the adsorption of Cu(II) on chitin, the first order rate constant and capacity of adsorption of Cu(II) on the Chitin-HA were, respectively, 1.24 and 1.42 time higher; while the adsorption energy was 0.90 time smaller.
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