How soil organic matter composition controls hexachlorobenzene-soil-interactions: adsorption isotherms and quantum chemical modeling.
Hazardous persistent organic pollutants (POPs) interact in soil with the soil organic matter (SOM) but this interaction is insufficiently understood at the molecular level. We investigated the adsorption of hexachlorobenzene (HCB) on soil samples with systematically modified SOM. These samples included the original soil, the soil modified by adding a hot water extract (HWE) fraction (soil+3 HWE and soil+6 HWE), and the pyrolyzed soil. The SOM contents increased in the order pyrolyzed soil<original soil<soil+3 HWE<soil+6 HWE. For the latter three samples this order was also valid for the HCB adsorption. The pyrolyzed soil adsorbed more HCB than the other samples at low initial concentrations, but at higher concentrations the HCB adsorption became weaker than in the samples with HWE addition. This adsorption combined with the differences in the chemical composition between the soil samples suggested that alkylated aromatic, phenol, and lignin monomer compounds contributed most to the HCB adsorption. To obtain a molecular level understanding, a test set has been developed on the basis of elemental analysis which comprises 32 representative soil constituents. The calculated binding energy for HCB with each representative system shows that HCB binds to SOM stronger than to soil minerals. For SOM, HCB binds to alkylated aromatic, phenols, lignin monomers, and hydrophobic aliphatic compounds stronger than to polar aliphatic compounds confirming the above adsorption isotherms. Moreover, quantitative structure-activity relationship (QSAR) of the binding energy with independent physical properties of the test set systems for the first time indicated that the polarizability, the partial charge on the carbon atoms, and the molar volume are the most important properties controlling HCB-SOM interactions.
University of Rostock, Institute of Physics, D-18051 Rostock, Germany; University of Cairo, Faculty of Science, Department of Chemistry, 12613 Giza, Egypt; University of Rostock, Interdisciplinary Faculty, Department of Life, Light and Matter, D-18051 Rostock, Germany. Electronic address: ashour.ahmed@uni-rostock.de.
Ashour A
AA
Ahmed
University of Rostock, Institute of Physics, D-18051 Rostock, Germany; University of Rostock, Interdisciplinary Faculty, Department of Life, Light and Matter, D-18051 Rostock, Germany. Electronic address: oliver.kuehn@uni-rostock.de.
Oliver
O
Kühn
King Abdulaziz University, Faculty of Science, Chemistry Department, Jeddah 21589, Saudi Arabia. Electronic address: saziz@kau.edu.sa.
Saadullah G
SG
Aziz
University of Cairo, Faculty of Science, Department of Chemistry, 12613 Giza, Egypt; King Abdulaziz University, Faculty of Science, Chemistry Department, Jeddah 21589, Saudi Arabia. Electronic address: rhilal@kau.edu.sa.
Rifaat H
RH
Hilal
University of Rostock, Soil Science, D-18051 Rostock, Germany; University of Rostock, Interdisciplinary Faculty, Department of Life, Light and Matter, D-18051 Rostock, Germany. Electronic address: peter.leinweber@uni-rostock.de.
Peter
P
Leinweber
D000327:Adsorption; D006581:Hexachlorobenzene / Q000737:chemistry*; D008956:Models, Chemical; D021281:Quantitative Structure-Activity Relationship; D012987:Soil / Q000737:chemistry*; D012989:Soil Pollutants / Q000737:chemistry*
D012987:Soil; D012989:Soil Pollutants; D006581:Hexachlorobenzene
Hexachlorobenzene adsorption; Persistent organic pollutants; QSAR; Quantum chemical calculations; SOM modeling; Soil organic matter (SOM)