Photocatalytic and excimer laser photolytic studies of the environmentally hazardous compound 4-chlorophenol.

摘要

One of the toxic products generated during the chlorine bleaching of wood pulp is 4-chlorophenol (4-CP). Detoxification of such chlorinated organic compounds usually requires their dechlorination. The study presented here includes a fairly detailed comparison of the attempts and successes of the photocatalytic and photolytic dechlorination of 4-CP in aqueous solutions.; In the direct photolysis experiments, ArF* (193 nm) and KrF* (248 nm) excimer lasers were used. Concentration effects were studied by using a higher {dollar}(1.1times10sp{lcub}-2{rcub}{dollar} M) and a lower {dollar}(4.5times10sp{lcub}-4{rcub}{dollar} M) starting substrate concentration. Also, quantum yields, product distributions, and the effects of the presence or absence of dissolved O{dollar}sb2{dollar} are compared for the degradation of 4-CP. The conditions leading to the highest quantum yields include the use of (i) 193 nm radiation, (ii) O{dollar}sb2{dollar}-saturation, and (iii) dilute solutions {dollar}(4.5times10sp{lcub}-4{rcub}{dollar} M). However this most efficient process leads to the formation of insoluble oligomers as major products, which are not biodegradable, and thus are not acceptable products for discharge into the environment. Therefore, the overall best process is found to include (i) 248 nm radiation, (ii) O{dollar}sb2{dollar}-saturation, and (iii) dilute solutions {dollar}(4.5times10sp{lcub}-4{rcub}{dollar} M), because oligomers do not form under these conditions.; In the photocatalysis studies, quantum yields at 366 nm for the disappearance of 4-CP {dollar}(rmPhisb{lcub}4-CP{rcub}){dollar} and chloride ion generation {dollar}rm(Phisb{lcub}Cl-{rcub}){dollar} were determined for three particulate semiconductors (SnO{dollar}sb2,{dollar} TiO{dollar}sb2{dollar} and {dollar}beta{dollar}-SiC) suspended in aqueous solutions of 4-CP {dollar}(1.0times10sp{lcub}-4{rcub}{dollar} M). Particulate SnO{dollar}sb2{dollar} showed no detectable photocatalytic activity for the dechlorination of 4-CP after either two or four hours of irradiation (313 nm). For TiO{dollar}sb2{dollar} the average quantum yields were {dollar}Phisb{lcub}4-{lcub}rm CP{rcub}{rcub}=0.0084pm0.0005{dollar} and {dollar}rmPhisb{lcub}Cl-{rcub}=0.0084pm0.0005{dollar} (366 nm; 1 g TiO{dollar}sb2{dollar} per liter of solution).; The semiconductor, {dollar}beta{dollar}-SiC was also investigated as a potential photocatalyst for the dechlorination of 4-CP {dollar}(1.0times10sp{lcub}-4{rcub}{dollar} M) with 366 nm radiation. This is the first reported use of {dollar}beta{dollar}-SiC as a photocatalyst in the dechlorination of chlorinated organic compounds. The average quantum yields with {dollar}beta{dollar}-SiC were {dollar}rmPhisb{lcub}4-CP{rcub}= 0.0012pm0.0007{dollar} and {dollar}rmPhisb{lcub}Cl-{rcub}=0.0014pm0.0004{dollar} (1 g {dollar}beta{dollar}-SiC per liter of solution). However, a more effective comparison of the efficiencies of {dollar}beta{dollar}-SiC and TiO{dollar}sb2{dollar} can be made by comparing their specific quantum yields ({dollar}Phispprime),{dollar} that is, quantum yield per unit surface area. For TiO{dollar}sb2{dollar} the specific quantum yields (m{dollar}sp{lcub}-2{rcub}){dollar} were {dollar}rmPhispprimesb{lcub}4-CP{rcub}=0.016pm0.001{dollar} and {dollar}rmPhispprimesb{lcub}Cl-{rcub}=0.016pm0.001,{dollar} whereas the average specific quantum yields for {dollar}beta{dollar}-SiC were {dollar}rmPhisb{lcub}4-CP{rcub}=0.011pm0.006{dollar} and {dollar}rmPhispprimesb{lcub}Cl-{rcub}=0.013pm0.004,{dollar} comparing favorably with TiO{dollar}sb2.{dollar}; Finally, a mixed semiconductor system (1 g {dollar}beta{dollar}-SiC + 1 g TiO{dollar}sb2{dollar} per liter of {dollar}1.0times10sp{lcub}-4{rcub}{dollar} M solution) was studied to determine whether the efficiency of dechlorinating 4-CP is higher as a result of interparticle charge transfer, which usually reduces the rate of electron/hole