Photodegradation Of Hexachlorobenzene In Ice By UV-light And The Oxidation Of Hexachlorobenzene By Fenton Reagent In The Freezing Process

In recent years, along with the need of human life and production in a variety ofchemical material and organic fertilizer utilization increase day by day, these organicpollution to the environment are also significantly increasing. Hexachlorobenzene（HCB） is an organic chlorine pesticide and organic pollutant. Although in recent years,organic chlorine pesticides in agricultural production are banned overall, the industrialproduction and other industrial activities still have many HCB. At the same time，there are some reports that HCB has been detected in polar regions, in the wintersnowfall area and perennial frozen area; in addition, the microbial action andmigration for the organic pollutants in snow and ice are relatively weak, so that thephotochemical effect are particularly important. Therefore, this paper carried out theresearch on the transformation of photochemical effect of HCB in ice phase andFenton effect during the freezing of HCB is meaningful. Although some research onthe photochemical reaction has been carried out domestic and overseas, the presentresearch mainly concentrated on the organic pollutants by direct photolysis andindirect photolysis of H₂O₂, N_O^2-and NO₃^-. But in the actual ice environment stillexist many other components may take part in the photochemical actions, such as Fe²⁺,Fe³⁺, SO₄^2-, PO₄^3-and Fenton reagent. The effects of these fractions on organicpollutants in ice have not been attracted.This experiment, took HCB as the research object, and carried out the researchon the photodegradation of HCB in ice phase under the UV irradiation, inspected the effect of two kinds of UV light sources condition for HCB photodegradation; italso research on the influence of different pH values and different concentrations ofinorganic ions (including SO42-, PO43-, N_O^2-, NO₃^-HCO₃^-, Fe²⁺and Fe³⁺), H₂O₂andorganic reagents; and the HCB degradation products were analyzed and determined,inferred the ice phase HCB degradation pathway; in the oxidation of HCB by Fentonreagent in the freezing process experiment, it also carried out preliminary research oneffect of different time, temperature and icing-thawing cycles during freezing ofHCB; it also determined the intermediate products of HCB in icing conditions byFenton reagent, and inferred the effect of water-ice phase interface in actionmechanism of degradation process.In HCB photodegradation experiment, there has no degradation for HCB in icephase without any light; and degradation rate reached60%after8h in254nm light;while there has no degradation in340nm lamp. In254nm lamp irradiation,photodegradation rate would be different when changing the ice phase pH; it hasfound that photodegradation rate were maximum in strong acid and strong alkalicondition. When adding SO₄^2-, PO₄^3-, H₂O₂, N_O^2-, NO₃^-, HCO₃^-and Fe³⁺, it found thatdifferent inorganic ions and H₂O₂in ice phase HCB photodegradation have differenteffects. SO₄^2-, PO₄^3-showed stimulative effect for photodegradation of HCB in icephase; while H₂O₂, N_O^2-, NO₃^-HCO₃^-and Fe³⁺exhibit inhibition effect forphotodegradation of HCB in ice phase. Four different kinds of organic reagents effecton photodegradation of HCB in ice phase are: methanol> acetone≈acetonitrile>dimethyl sulfoxide. Actually, direct photolysis mechanism of HCB in ice phase is aseries of dechlorination process. There four different products after1h light irradiation:benzene, benzene, dichlorobenzene （1,2-dichlorobenzene,1,3-dichlorobenzene,1,4-dichlorobenzene）, trichlorobenzene （1,2,4-trichlorobenzene）, and after48h reachedanother intermediate products-pentachlorobenzene.In the study of oxidation of HCB by Fenton reagent in freezing processexperiments, adding Fenton reagent HCB solution at different temperature anddifferent freezing time, would have different effect on degradation. Fenton reagentwas added to the HCB solution and carried out icing-thawing cycle experiment. With the cycle times increasing, HCB concentration was decreased. It may be due to thecyclic process made Fenton reagents continuously generated·OH radicals, andwould be distributed in ice-water interface continuously, and HCB degraded. Inicing-thawing circulation experiments, HCB degradation products contained PCP, itmight be the result of Fenton reagent addition·OH radical produced various activegroups, such as·OOH and H₂O· etc, these groups take part in the comprehensivereaction and generated PCP.Above all, experimental results revealed the photodegradation mechanism ofHCB in ice phase, expounds HCB transformation by Fenton reagents in freezingprocess, which will provide a theoretical basis for the prediction and assessment forHCB in ice environment.