Sorption Of PAHs To Soybean And Wheat Roots And The Bioavailability Of Sorbed PAHs

The investigation regarding polycyclic aromatic hydrocarbons(PAHs)sorbed onto crop roots and the bioavailability of sorbed PAHs is beneficial to regulate the translocation of PAHs from soil to plants and remediate PAH-contaminated soils by plants.In this paper,the experiments were conducted on the sorption/desorption of phenanthrene on soybean and wheat roots under different sorption times and different phenanthrene concentrations;the sorption/desorption of phenanthrene on roots,cell walls,and the main components of cell walls under different phenanthrene concentrations;the kinetics and thermodynamics of phenanthrene sorbed by soybean and wheat roots;and the sorption of naphthalene,phenanthrene and pyrene on soybean and wheat roots.The aim of all experiments was to provide theoretical basis for revealing the process and mechanism of root sorbed PAHs and the bioavailability of sorbed PAHs.The main results are as follows:(1)The sorption of phenanthrene under different times and concentrations,desorption and bioavailability of sorbed phenanthrene in soybean and wheat roots were studied by isothermal adsorption test.The results showed that the trend of phenanthrene sorption in vivo was first increased and then decreased and then to be balance,which was related to the transport delay in vivo;the trend of dead and dried roots was first increased and then balanced.The greater specific surface area and the higher fat content,the faster the balance was.Freundlich isotherm was fitted better than Henry isotherm for dead and dried roots,Langmuir isotherm fit in wheat roots was best.All of the fitted isotherms indicate that the distribution and the surface adsorption could control the phenanthrene sorption.Because of the special binding between living roots and phenanthrene,the fit effect is poor.The phenanthrene sorption capacity of soybean roots was higher than that of wheat,which was related to the high water content,fat content and membrane permeability.The phenanthrene sorbed on the roots was hard to desorb,the desorption trend was wheat roots>soybean roots;living roots>dried roots>dead roots.The bioavailability of root-sorbed phenanthrene was consistent with the desorption results.(2)The sorption amount of phenanthrene on soybean and wheat roots,cell wall and its main components increased with the increase of phenanthrene concentration.The sorption amount of phenanthrene was lignin>soybean roots>wheat roots>soybean cell wall>wheat cell wall>pectin>cellulose.Freundlich and Langmuir isothermal equations were fitted better than Henry isothermal equation.The desorption trend of sorbed phenanthrene was lignin<soybean roots<wheat roots<soybean cell wall<wheat cell wall<pectin<cellulose.The bioavailability of roots was consistent with the desorption results.(3)With the increase of time,sorption of phenanthrene on soybean and wheat roots increased firstly and then reached equilibrium.Compared with soybean roots,the wheat roots could reach the equilibrium of sorption faster.The sorption kinetics of soybean and wheat roots conformed to the pseudo two order kinetic equation.The sorption capacity decreased with increasing temperature.The sorption of phenanthrene on soybean and wheat roots was a spontaneous exothermic process.The sorption of phenanthrene on soybean roots was more stable than that of wheat roots at the same temperature.(4)The sorption amount of naphthalene phenanthrene and pyrene on soybean and wheat roots increased with the increase of PAH concentrations.The sorption capacity was naphthalene<phenanthrene<pyrene,and soybean roots>wheat roots.Freundlich and Langmuir isothermal equation were fitted better than Henry isothermal equation.The absorption ability of PAHs on roots was highly correlated with its hydrophobicity.The high content of cellulose and hemicellulose,the low content of fat and lignin in wheat roots could influence the sorption of PAHs,which made the PAHs absorption capacity lower than that of soybean roots.Therefore,it is concluded that the sorption ability of PAHs on roots is related to the content of lignin,fat,cellulose,hemicelluloses,PAH hydrophobicity and temperature.The absorption capacity is positively correlated with the partition coefficient(Kd),and the desorption capacity is negatively correlated with Kd.The bioavailability of root-sorbed phenanthrene is consistent with the desorption results.The desorption rate of PAHs could be used to characterize their bioavailability.