Effect of Initial Concentration on Adsorption-Desorption Characteristics and Desorption Hysteresis of Hexachlorobenzene in Soils

Adsorption and desorption are important processes that influence the transport, transformation, and bioavailability of hexachlorobenzene (HCB) in soils. To examine the adsorption-desorption characteristics of HCB, equilibrium batch experiments were carried out using two soils (red soil and paddy soil) with different initial HCB concentrations (0.25, 0.50, 0.75, 1.00, 1.50, 2.50, 3.50, and 5.00 mg L-1 ) by using 0.01 mol L-1 calcium chloride as the background solution. The successive desorption experiments (48, 96, 144, 192, and 240 h) were conducted after each adsorption equilibrium experiment. The results revealed that adsorption and desorption isotherms of HCB on two soils were nonlinear, which can be best described by the Freundlich equation with the square of the correlation coefficient (r 2 ) ranging from 0.97 to 0.99. Desorption of HCB from the two soils exhibited hysteresis at all HCB concentrations because the Freundlich desorption coefficients were always higher than the Freundlich adsorption coefficients. The hysteretic effect was enhanced with increasing initial HCB concentration, and positive hysteresis was observed at different concentrations.

QSPR Studies on lgK_(ow) and lgK_(oc) of Fluorobenzenes and Property Parameters Based on HF and DFT Calculations

Quantum chemistry parameters of 22 fluorobenzenes were computed at six levels using Hartree-Fock and DFT methods. Based on the experimental data of n-octanol/water partition coefficient （lgKow）, a three-parameter （dipole moments （μ）, zero point energy （ZPE） and free energy （G°）） quantitative correlation equation that can predict IgKow was developed using structural and thermodynamic parameters as theoretical descriptors. Similarly, based on experimental data of soil organic carbon sorption coefficient （lgKoc）, the other three-parameter （the most negative atomic net charge of molecule （q^-）, dipole moments （μ） and molecular volume （Vi）） quantitative correlation equation that can predict lgKoc was given. Quantitative correlation equations based on B3LYP/6- 311 G^＊＊ calculation were validated by VIF （variance inflation factors） and t-test and used to predict IgKow and lgKoc of a series of compounds. The result showed that the correlation and prediction ability of lgKoc equations based on three levels of HF/STO-3G, B3LYP/6-31 G^＊ and B3LYP/6- 311G^＊＊ are all more advantageous than those based on AMI.

Biochar Increases Diuron Sorption and Reduces the Potential Contamination of Subsurface Water with Diuron in a Sandy Soil

The herbicide diuron is widely used in agricultural areas in Brazil, whereas it has high potential for subsurface water contamination due to its physicochemical characteristics. Recent studies have demonstrated the potential of biochar as a sorbent and possible pesticide leaching mitigation. The objective of this study was to investigate the long-term effect of biochar application on the kinetics of sorption and desorption of diuron in a Cerrado Haplic Plinthosol. Samples were collected in an experiment conducted in the field in a randomized block design consisting of the combination of two levels of fertilizer application (0 and 300 kg ha −1 of 5-25-15 formula of NPK fertilizers) and three doses of biochar (0, 16, and 32 Mg ha −1 ). The Freundlich isotherm accurately described the sorption of diuron in all treatments. Biochar application increased the sorption and reduced the desorption of diuron. This effect was attributed to the contribution of biochar to total organic carbon (C) and C in the humin fraction and to the increase in the reactivity of the humic acid and humin fractions, which was significantly highly correlated with the sorption coefficient ( K f ). A positive correlation between the partition coefficient of organic C and K f confirmed the importance of the soil organic compartment for the sorption of diuron. The higher diuron sorption and lower diuron desorption capacities of sandy soils after biochar application could reduce the potential risk of diuron leaching and contamination of subsurface water.