Photochemical Behaviour Of Selected Phenolic Pollutants In Soil

Photoreactions of two phenolic compounds, p-nitrophenol (PNP) and pentachlorophenol(PCP), were studied in different soil reaction systems to understand phototransformation ofphenolic pollutants in soil, and to explore new approaches for improving the photoreactionrate of organic pollutants in soil and promoting the application of photochemical technologiesin soil remediation. The influence of PCP transport on its photoreaction in soil was alsoinvestigated.1. Photoreaction experiments of PNP and PCP on the soil surface were performedaccording to the classical experimental approach. The results showed that photoreactions ofPNP and PCP occurred on the soil surfaces under UV irradiation, and additional TiO₂ couldimprove the processes.The effect of soil water, soil pH, and humic acid (HA) on photoreaction of PNP and PCPwas investigated. No matter whether TiO₂ was added, increase in soil water improved thephotoreaction significantly, whereas humic acid reduced the photoreaction rate. Changes insoil pH resulted in different reaction rates. The highest reaction rate was observed underalkaline condition, and the lowest was observed under acidic condition.Kinetic analysis of PNP and PCP photoreaction illustrated that PNP photoreaction in theinitial 12 h followed the pseudo first-order kinetics well, and the rate contants were 0.064,0.099,0.112,0.121,0.131 h^-1, respectively, when no TiO₂, 0.50ï¼…, 1.0ï¼…, 2.0ï¼…, 3.0ï¼…TiO₂were added. However, for PCP, the pseudo first-order rate constants were different in theinitial and subsequent photoreaction stages. When no TiO₂, 0.50ï¼…, 1.0ï¼…, 2.0ï¼…, 3.0ï¼…TiO₂were added, the rate constants for PCP photoreaction in the initial 3 h were 0.141,0.178,0.198,0.271,0.340 h^-1, respectively; and that in the 3-12 h were 0.059,0.098,0.118,0.102,0.099 h^-1, respectively.2. The photoreaction experiments of PNP and PCP with and without addition of TiO₂were performed in a designed rotary photoreactor, in which the soil particles were stiredcontinually around the lamp and then PNP or PCP in soil accepted uniform UV irradiationreadily.PNP photoreaction curve in the rotary reactor was similar to that in soil layers withoutaddition of TiO₂. When 1.0ï¼…TiO₂ was added, the rate constant of PNP photoreaction in therotary reactor was two times of that on the soil layers, which was attributed to the higherfrequency of contact between PNP on soil particles and TiO₂.Different from PNP, PCP photoreaction in the rotary reactor was much faster than thaton the soil layers, with an increase of 3.0 times in the reaction rate no matter whether TiO₂ was added. This result indicated that transport of PCP in the irradiated soil was slow ascompared to its photoreaction, hence inhibited its photoreaction on the soil layer. In the rotaryreactor, PCP on the soil particles could obtain sufficient irradiation, so the photorcaction wasebhanced dramatically.These results showed that stirring the soil during irradiation was an effective method toimprove the photoreaction of the pollutants in soil.3. Effect of PCP transport on its photoreaction under UV irradiation was studied in the10 mm-thick soil columns which were sliced at different soil depth after irradiation. Theresults are as followed.In the air-dried soil, PCP on the soil surface was phototransformed, but no obviouschange in PCP concentrations occurred in deeper soil. Therefore, the averagephototransformation ratio in the whole soil column was almost zero even after 48 h.When the soil was moist, PCP in the deeper sandy loam where the linear sorptioncoefficient of PCP was 0.22 mL/g transported to the soil surface along with the evaporation ofthe soil water during irradiation and then enhanced PCP photoreaction in the whole soilcolumn. When the initial water contents were 9.3ï¼…and 19.2ï¼…, the average PCPphototransformation ratio in the whole soil column reached 20.9ï¼…and 39.9ï¼…respectively in48 h UV irradiation. These results indicated that the application of photochemicaltechnologies in soil remediation could be promoted by accelerating the transport of organicpollutants in soil.PCP photoreaction in another two soils where the linear sorption coefficients of PCPwere 2.7 and 12 mL/g respectively showed that soil properties affected PCP transportseriously. No obvious PCT transports were observed in these two moist soil columns duringirradiation, and the average phototransformation ratio in the whole soil column was almostzero in 48 h. In the soil which is rich in humic substance (18ï¼…), PCP photoreaction on the soilsurface was also inhibited significantly.In summary, photoreaction of PNP and PCP in the soil were investigated in differentexperiment systems, and the enhancement of TiO₂ for this process was also evaluated. Theresults revealed the important factors in the photoreaction of organic pollutants in the soil.This study put forward new thoughts and approaches to improve the photoreaction rate of theorganic pollutants in soil and provided academic foundation for remediation of contaminatedsoil by photochemical technologies.