Adsorption behavior of some aromatic compounds on hydrophobic magnetite for magnetic separation

Highlights: ► Two kinds of modified hydrophobic magnetites were prepared as an adsorbent. ► The modifications on magnetite were carried out using alkyl chain and phenyl group. ► The aromatic compounds (2 < log Pow < 4) were adsorbed by hydrophobic interaction. ► The compounds (1 < log Pow < 2) were adsorbed by π-electron interaction. ► Adsorption behaviors of compounds (2 < log Pow < 4) were multi-layer adsorption. - Abstract: In this study, a hydrophobic magnetite coated with an alkyl chain or a phenyl group on the surface was prepared and used as an adsorbent to investigate the adsorption behavior of aromatic compounds having various values of log Pow (phenol 1.46, benzonitrile 1.56, nitrobenzene 1.86, benzene 2.13, toluene 2.73, chlorobenzene 2.84 and o-dichlorobenzene 3.38) onto hydrophobic magnetite. The hydrophobic magnetites were modified with stearic acid and phenyltrimethoxysilane, and the modification amounts were 9.84 × 10−3 and 4.17 × 10−2 mmol/g, respectively. The aromatic compounds used in this study were divided into 3 groups depending on the log Pow: 1 < log Pow < 2, 2 < log Pow < 3 and 3 < log Pow. The adsorption amounts of above each group on the magnetite at an initial concentration of 100 ppm were 3.62 × 10−3 (nitrobenzene), 1.92 × 10−2 (phenol), 1.13 × 10−1 (chlorobenzene), 2.42 × 10−1 (benzene), and 3.10 × 10−1 mmol/g (dichlorobenzene), respectively. This indicates that the adsorption behaviors depend on the strength of hydrophobicity of aromatic compounds. The adsorption mechanism for 2 < log Pow < 3 and 3 < log Pow is hydrophobic interaction and that for 1 < log Pow < 2 is π-electron interaction. The quantitative relationship between the amount of adsorbed compounds and modified functional groups and the fitting for adsorption isotherm models suggested that this adsorption might form a multi-layer adsorption in the most cases.