Binding mechanisms and QSAR modeling of aromatic pollutant biosorption on Penicillium oxalicum biomass

The biosorption of eight aromatic compounds with different functional groups by Penicillium oxalicum biomass were investigated. The affinity of the biomass for the eight compounds at pH 6.0 follows the following trend: 1-naphthalenamine > naphthol > benzoic acid > p-toluidine > p-cresol > p-toluic acid > phenol > p-toluenesulfonic acid. Biomass surface was characterized, and it was found that four discrete binding sites, corresponding to carboxyl (pKa = 4.0), phosphoric (pKa = 7.0), amine (pKa = 8.8), and hydroxyl groups (pKa = 10.0), were identified on the biomass surface by using the linear programming method (LPM) for the fitting of the titration data and FTIR analysis. The carboxyl and amine groups dominate the biomass surface sites, which might have played an important role in biosorption of organic compounds. Furthermore, the compounds were divided into two groups based on the calculation of ionization degree for toluene derivatives and the comparison on the number of benzene rings for barely ionized compounds. It was found that low ionization degree and high hydrophobicity favor the biosorption for the two groups, respectively. Moreover, a Radj2 of 0.724 between the log of Freundlich coefficient (log Kf) and log Kow indicated that the hydrophobicity plays role in the sorption of eight organic compounds. The QSAR model with one variable was developed for the first time between log Kf and polar surface area (PSA) to predict the biosorption behaviors of organic compounds (Radj2=0.960) except for p-toluenesulfonic acid (with pKa < 0), which also supported the electrostatic attraction and hydrophobicity mechanisms.