Mechanism And Model Study On Benzene Sorption And Desorption By Organic Matter In Soil

The effects of organic matter（OM）on the environmental behavior and fate of hydrophobic organic contaminants（HOCs）in soil and sediment have always been a research hotspot in the field of environmental science and engineering.The sorption and desorption processes are the key factors to control the environmental behavior and fate of HOCs.The quantitative characterization of the sorption and desorption processes of HOCs in soil and sediment can provide theoretical support for predicting the environmental behavior and fate of HOCs,evaluating the potential environmental risks,and planning the risk control and remediation strategies.The nonlinear sorption and desorption hysteresis of HOCs in soil and sediment has been confirmed by a large number of laboratory researches,site investigations,and remediation projects.Common nonlinear isothermal sorption models,such as the Langmuir model and the Freundlich model,could fit the experimental data well,but the parameters of these models need to be obtained through a large number of time-consuming batch experiments,which makes the model difficult to be widely used.On the other hand,the simple and convenient linear distribution model cannot predict the nonlinear sorption behavior and desorption hysteresis of HOCs in soil and sediment,which would lead to large errors.To solve this problem,humic acid（HA）and black carbon（BC）,which are ubiquitous in soil and sediment,were used as representative OM to systematically study the sorption and desorption characteristics of benzene,as well as the effects of environmental conditions and coexisting substances.The sorption and desorption mechanism of benzene on OM was reasonably inferred by the thermodynamic and kinetic characteristics,the energy changes of sorption sites and the characterization methods such as elemental analysis,Fourier transform infrared spectroscopy（FTIR）,nuclear magnetic resonance（NMR）,scanning electron microscopy（SEM）,N₂ adsorption-desorption,particle size analysis.The sorbed benzene was divided into linear and non-linear sorption parts,and a two-compartment sorption prediction model for benzene sorption on OM in soil and sediment was constructed.The simple calculation method of the model parameters was given to facilitate the application of the model.The effects of OM on benzene transport were also studied by column experiments.The calculation formula of retardation factor R_d was derived through the two-compartment sorption prediction model,and the transport model of benzene in the organic medium was also established.The main results in this paper are as follows:（1）The main structure of HAs and BCs are aromatic rings,but the surface morphology and physicochemical properties are quite different.The BCs have significantly higher carbon content and aromaticity,lower polarity,and less acidic functional groups such as carboxyl group and hydroxyl groups than HAs.Moreover,the BCs have rough surfaces,well-developed pore structures,large specific surface area and pore volume.（2）The sorption isotherms of benzene on HAs and BCs showed obvious nonlinearity,but they were approximately linear when benzene concentration was low.The sorption capacities and nonlinear degree of two BCs are higher than that of two HAs.The changes in environmental conditions such as temperature,p H and ionic strength would affect the sorption processes.The sorption of benzene on OM is a spontaneous exothermic process,and the higher temperature would suppress the sorption process.According to the thermodynamic characteristics of the sorption process,the sorption mechanism of benzene on OM may belong to physical sorption,and the sorption force may be hydrogen bond or dipole-dipole interaction.The increasing p H will reduce the particle sizes of OM,thus destroy the favorable physical conformation for benzene sorption,and ultimately reduce the sorption capacities of benzene on OM.The change of ionic strength has no significant effects on the sorption process of benzene on HAs,but it will reduce the sorption capacities of BCs,which may be related to the pore blocking.The sorption process of benzene on OM can be divided into fast sorption stage and slow sorption stage.The sorption rate of two HAs for benzene is controlled by the fast sorption stage,while that of BCs is controlled by both two stages.The desorption hysteresis of benzene on OM is obvious.The desorption hysteresis weakens with the increase of benzene concentration.However,some benzene molecules will enter the interior and adsorb on the high-energy sorption sites of OM when the benzene concentration is high,thus the desorption hysteresis would be enhanced.（3）The coexisting substances in soil and sediment significantly affected the sorption and desorption processes of benzene on OM in soil.The coexistence of Cu²⁺and Pb²⁺could significantly suppress the sorption process,reduce the diffusion rate,and promote the desorption process of benzene on OM.Combined with FTIR and the site energy distribution model,it is speculated that the suppression mechanisms of coexisting Cu²⁺and Pb²⁺on the benzene sorption by OM are as follows:（i）Cu²⁺and Pb²⁺would be adsorbed by the acidic function groups of OM,reduce the particle size and pore size of OM through the bridging effect,thus suppress the sorption process of benzene.（ii）the hydrated layer of adsorbed Cu²⁺and Pb²⁺would occupy the surface of OM and shield the sorption sites for benzene,thus inhibiting the interaction between benzene molecules and OM.（iii）the adsorbed Cu²⁺and Pb²⁺would block the pores of OM,suppressing the pore filling process.The effects of different dissolved organic matter（DOM）on benzene sorption by OM are different.The citric acid（CA）can promote the sorption processes and enhance the nonlinearity degree of benzene sorption on OM,while fulvic acid（FA）would reduce the sorption capacities and nonlinearity degree of benzene sorption.CA and FA both decrease the diffusion rate of benzene on OM.CA inhibited the desorption process of benzene on OM,while FA promoted the desorption process.The influence mechanisms of CA and FA on benzene sorption by OM are speculated by the characterization methods.The sorbed CA could increase the electron donor density of OM and improve the energy of sorption sites,thus enhance theπ-πelectron donor-acceptor interaction between OM and benzene.The coexistence of FA in the solution would increase the solubility of benzene,and compete with benzene for the sorption sites of OM,thus inhibiting the sorption process and promoting the desorption process of benzene on OM.（4）Based on the thermodynamic and kinetic characteristics and mechanism of benzene sorption and desorption processes on OM,a two-compartment sorption prediction model was constructed,and the calculation methods of related parameters were given to facilitate the application of the model.The sorption process of benzene on OM can be divided into linear sorption and nonlinear sorption parts.The two-compartment sorption prediction model could fit the experimental data of benzene sorption on OM and the previous experimental data in the literature well.Meanwhile,the two-compartment sorption prediction model has simple parameters including the linear partition coefficient K_1st and organic carbon content f_oc,which could be obtained by simple experiments.The two-compartment sorption prediction model could solve the problem that the parameters of nonlinear sorption models,such as the Freundlich model,need to be fitted by a large number of batch experimental data.The results of parameter sensitivity analysis of the two-compartment sorption prediction model show that the organic-carbon normalized maximum nonlinear sorption capacity K_2nd and nonlinear sorption affinity parameter K’are the key parameters at the lower equilibrium concentration of benzene,while K_2nd is the only key parameter at the higher equilibrium concentration.（5）Based on the two-compartment sorption prediction model,the calculation formula of retardation factor R_d for benzene transport in the organic medium was derived,then the transport model of benzene in the organic medium was also constructed.Combined with the column experiments,the tracer experiments of inert chlorine ion and the breakthrough experiments of benzene were carried out.According to the tracer experiments,the dispersion coefficient and the flow velocity of the experimental media were obtained.The results of breakthrough experiments showed that the transport processes of benzene were slowed by the presence of OM,and the retention effect enhanced with the higher content of OM.The transport model could simulate the transport processes of Cl^-and benzene in the experimental columns and the data of previous research well.The transport model could provide theoretical and technical support for further study on the environmental behavior and fate of HOCs,evaluating the potential environmental risks,and planning the risk control and remediation strategies.