Study of Humic Acid Adsorption Character on Natural Maifan Stone: Characterization, Kinetics, Adsorption Isotherm, and Thermodynamics.

In this paper, the adsorption of humic acid (HA) on natural maifan stone (MS) in aqueous medium was investigated. The changes in MS after adsorption have been characterized explicitly. The adsorption behavior was studied by varying the factors of pH (5-10), reaction time (10-180 min), initial HA concentration (5-50 mg/L), adsorbent dosage (0.1-1.2 g), and temperature (25-45 °C). The kinetics of the adsorption process of HA was fitted well with the pseudo-second-order model ( = 0.99). The isothermal results revealed that the adsorption process is favorable, and highly fitting Langmuir models ( > 0.99) were used. Additionally, the obtained maximum adsorption capacity of MS for HA was approximately 1 mg/g. The adsorption process of HA onto MS was endothermic according to the thermodynamic study. The changes in the excitation-emission-matrix of HA and the X-ray diffraction of MS after adsorption indicate the interaction of HA and MS. However, the reason for these changes is still unclear. Thus, the results show that the natural MS exhibited a certain adsorption capacity for HA. It is promising to develop novel natural MS-based materials for adsorption of HA.