Fractions collected from humic acid (HA) under acidic conditions and used as adsorbents for various agricultural organophosphate pesticides in hexane are immobilized on silica gel. For most organophosphate analytes examined in this study under the same conditions, the percentage of adsorption achieved nearly 100 % in 1 h and was found to be highly relevant to the structure of the analyte and the type of interaction that occurred between the functional groups attached to it and HA. The interaction leading to adsorption between the functional moieties of the analyte and HA (e.g., P−O or S bond of analyte vs carboxyl group of HA) is believed to be reversible and dipole−dipole oriented and is significantly enhanced in hexane. The enhancement of π−π interaction, even hydrogen bonding in some cases, was also observed in hexane and contributed to the percentage of adsorption to a certain degree. However, the interaction is subject to the steric hindrance effect caused by the bulky group or element surrounding the phosphorus element. Considering the nature of the analyte, the time required to reach the maximum percentage of adsorption is decreased as the amount of adsorbent is increased. Furthermore, the adsorption process is surface oriented because the longer the time that is elapsed, the higher the percent of the analyte that is adsorbed. Factors such as the type of liquid phase or the acidic or basic origin of the additive in the liquid phase of the matrix also affect the adsorption percentage of analyte.

Humic fraction (HF) collected under acidic conditions and used as an adsorbent for various phosphate-based plasticizers in hexane is immobilized on silica gel. Most plasticizer analytes examined in this study under the same conditions achieved adsorption percentages above 90% in 1 h based on the difference in peak area. The Fourier transform infrared (FTIR) spectroscopy results indicate that the interaction leading to the adsorptions between the functional moieties of the analyte and HF (e.g., the carboxylate group of analyte against the carboxyl group of HF) is specific, reversible, and dipole-dipole-oriented. Moreover, it is significantly enhanced by hexane. However, the π−π interaction (even hydrogen bonding in all cases) was either not as significant or absent in hexane and, therefore, contributed little or nothing to the percentage of adsorption. The interaction is highly affected by the acidic or basic origin of the additive introduced to the liquid phase of the matrix, and it is subject to the steric hindrance effect caused by the bulky alkyl groups attached to ether linkages and the relative position of the two ether bonds on the aromatic moiety of the analyte. The pre-concentration of the analyte and, thus, the recycle of the adsorbent can be achieved by adsorbing and, subsequently, desorbing it in a different solvent, such as acetonitrile. Furthermore, the adsorption process is surface-oriented because of its dependence upon both time and the amount of adsorbent.