Sorption Mechanism and Performance of Peat Soil Humin for Methylene Blue and p-Nitrophenol

The responsible mechanism and performance of peat soil humin for the sorption of methylene blue (MB) and p-nitrophenol (p-NP) have been investigated. Humin was obtained from peat soil of Siantan, West Kalimantan, Indonesia, after removing the content of humic and fulvic acids into a NaOH solution using the recommended procedure of International Humic Substances Society (IHSS). The obtained humin was then purified by rigorous stirring in a mixed solution of HCl 0.1 M and HF 0.3 M. Ash content in humin after the purification abruptly decreased from 36.84 to 1.26 wt.% indicating that minerals and other inorganic impurities were mostly removed. Phenolic -OH and carboxyl (-COOH) functional groups contributing to the acidity of humin were in the level of 3.44 and 2.10 mmol/g, respectively. At optimum medium pH of 6.20 for MB and 7.00 for p-NP, -COO- as the deprotonated product of -COOH was the most responsible active site in sorbing MB and p-NP through electrostatic interaction and hydrogen bonding, respectively. The homogeneity of -COO- as the active site for the sorption of MB and p-NP implied that the surface of humin sorbent was energetically uniform and thereby the sorption of both MB and p-NP followed better the Langmuir than the Freundlich isotherm model with sorption capacity of 0.19 and 0.26 mmol/g and sorption energy of 32.92 and 27.27 kJ/mol, respectively.