Effect of Temperature and pH on the Sorption of Dibutyl Phthalate on Humic Acid

Abstract

Dibutyl phthalate (DBP) is widely used as a plasticizer and softener, and has been found in a variety of environmental media, owing to widespread applications in the industry, medicine, agriculture, and domestication. The toxic effects of DBP in the environment may be mitigated by removing it via adsorption onto humic acid (HA). This study is aimed at investigating the sorption mechanism of DBP on two kinds of HA samples (HA1 and HA2) at different temperatures and pH. The sorption of DBP onto HA was observed to occur in two stages with an equilibration time of 24 h. All the sorption isotherms fit well to the linear model and linearized form of the Freundlich model, with R 2 values between 0.984 and 0.999. Further, the results showed that the sorption capacity decreased with increase in temperature and pH on HA1 and HA2. Thermodynamic analyses demonstrated that the sorption of DBP onto HA was a spontaneous and exothermic process, with physical sorption as the predominant mechanism. Furthermore, thermodynamic and infrared spectral analyses suggested that the main sorption forces involved were hydrogen bonding and dipole forces. 13C NMR spectral analyses demonstrated that the alkyl carbon of DBP played an important role in its sorption on HA1, while the alkyl and aromatic carbons were vital for sorption on HA2. Further, the HA1-DBP NMR spectrum contained a new signal at 220 ppm that was not observed for free HA1. This signal could originate from the -RC = OR group or from the DBP molecule combined with free HA1.