Photochemical Degradation Of Selected Polycyclic Aromatic Hydrocarbons On Soil Surface

Polycyclic aromatic hydrocarbons(PAHs) are a group of ubiquitous persistent toxic substance which are generated during the incomplete combustion of solid and liquid fuels or derived from industrial activities.PAHs are harmful to environment and health of human being due to their high degree of carcinogenicity,teratogenicity and mutagenicity whne they enter into environment and human body.PAHs have become an issue of increasing concern in recent decades.As one of the typicai traditional industrial base in China,the Northeast area ha s a largh population of people and a large number of industries which highly consumpt fossil fuel.In addition,coal and biomass fuel are the traditional practice in heating and cooking in the rural area.Thus the Northeast area suffers from severe PAHs pollution.PAHs tend to be strongly adsorbed to soil due to their high octonal-water partition coefficient.Soil is considered as a major environmental reservoir for PAHs due to its high storage capacity.Photochemical degradation is an important pathway for the transformation of PAHs in soil.To understand the photochemical transformation of PAHs in soil and futher investigate their transfer and transformation regulations in soil,photochemical reactions of three typical PAHs:Flu,BaA,BaP on soil surface under UV irradiation were studied.The relationship between photodegradation rate constant and the molecular weight,molecular structure was also investigated.In addition,the effect of added HA on the photodegradation of selected PAHs was also studied.Firstly,photoreaction experiments of three selected PAHs on soil surface were performed according to the classical soil thin-layer photochemical experimental approach.Te results showed that the photoreaction of Flu,BaA,BaP occurred on soil surface under UV irradiation.The remaining concentrations of selected PAHs decreased with the increase of irradiation time.The kinetic analysis of Flu,BaA and BaP illustrated that the photoreaction of them in the irradiation period followed the pseudo first-order kinetics well,and the rate constants were 0.0249,0.0788,and 0.0928 h^-1,respectively.Secondly,in the current experimental system where Flu,BaA,BaP mixed-polluted in soil,the order of photodegradation rate is as followed:Flu＜BaA＜BaP.The photodegradation half-lives followed the order:BaP＜BaA＜Flu.The photoreactions of selected PAHs are related to their molecular weight and molecular structure.The results showed that photodegradation rate constants increased as their molecular weight and the number of benzene rings in their molecular increased.The photodegradation half-lives for selected PAHs negatively correlated with semi-empirically calculated energy of the highest occupied molecular orbital(E_HOMO).The photodegradation half-lives decreased as their E_HOMO increased.Thirdly,HA are the most widespread organic substance in nature environment which play important roles in the environmental behaviour of organic pollutants.Previous studies have shown that the humic substance control many photochemical degradation in two ways. Some studies proved that the presence of HA could enhance the photodegradation of organic pollutants while other studies showed that HA had the inhibition effect for photodegradation. Under UV irradiation,photoreaction of Flu,BaA and BaP occurred on soil surface added with HA.Kinetic analysis of photoreaction for Flu,BaA and BaP illustrated that their photoreaction also well followed the pseudo first-order kinetics,and the rate constants were 0.0191,0.0162,and 0.0465 h^-1 respectively.Comparerd to the photodegradation rate constants of Flu,BaA and BaP on soil surface without HA,the presence of HA in soil inhibit the photodegradation in the current experiment system.The reason for this inhibiton effect might be:HA,as an inner filter in soil,competitively absorbed photons radiated to soil surface thus slow down the photodegradation rate;HA reduced photodegradation of organic pollutants by quenching the excited states of PAHs moleculars;HA has a large adsorption capacity for PAHs.They appear to trap or bind PAHs in relative stable complexes within their macromolecular structures which make PAHs moleculars unavailable for photoreaction.