Deactivation Of TiO₂ Photocatalyst Induced By Humic Acids During Photocatalytic Water Treatment:Mechanisms And Regeneration Strategy

Photocatalysis has been intensively investigated for the removal of pollutants,but there is a problem of catalyst deactivation in practical applications,and little attention was paid to the deactivation of photocatalysts during the long-term operation.Natural organic matter（NOM）is ubiquitous in the natural aqueous environment,where it is easily adsorbed on the catalyst surface,thus affecting its surface properties and environmental behavior,and the long-term cumulative effect of NOM is of more practical importance than the short-term effect of NOM on photocatalysts.Herein we used titanium dioxide（Ti O₂）as a model photocatalyst,chlorophenols（TCP）as a model target pollutant,and humic acid（HA）as a representative substance of NOM to investigate the deactivation of Ti O₂ during long period.The cyclic batch processing experiment was carried out to simulate the real photocatalytic water treatment processes in the presence of HA,and the durability of Ti O₂were studied by comparing the photocatalytic degradation rates of TCP.The mechanism of HA-induced deactivation of Ti O₂were analyzed by various techniques.Finally,the deactivated Ti O₂ was regenerated using different methods and the optimal conditions were obtained in a single regeneration method.This study provides basic information for understanding the complex fouling phenomena that may occur severely in water bodies containing natural organic matter during photocatalytic treatment.The main findings of this work are as follows:（1）Single-run photocatalytic experiments:HA affected the efficiency of Ti O₂photocatalytic degradation of TCP in many ways.On the one hand,HA can promote light absorption and the degradation of TCP through photosensitivity,on the other hand,HA can compete active species produced in photocatalytic systems with target pollutants,thereby inhibiting the degradation of TCP.As a result,low concentration HA（5 mg/L）promoted the photocatalytic degradation of TCP,while high concentration HA（20 mg/L or above）showed partial inhibition of TCP degradation.Under dark condition,HA will quickly adsorb on the surface of Ti O₂,and may subsequently cause the accumulation of HA and its transformation byproducts on the Ti O₂ surface,resulting in the inactivation of photocatalyst.（2）Recycled photocatalytic experiments:Without HA,Ti O₂ inactivation was not observed after 60 cycles of TCP photocatalytic degradation experiments,indicating that TCP and its degradation byproducts would not cause the inactivation of Ti O₂;Whereas in the presence of HA,significant inactivation Ti O₂ were observed,and the extent of deactivation of Ti O₂was found to be positively related with the concentration of HA.For instance,20 mg/L HA caused the decrease of TCP degradation rate constant from the initial 0.0525 min^-1 to 0.0195 and 0.0052 min^-1after 10 and 30 cycle reactions,respectively,i.e.,a decrease of 63%and 90%,respectively.The control experiments showed that the loss of catalyst caused by recycling is not the cause of the decrease of TCP degradation efficiency.XRD tests showed that the crystal phase of the inactivated catalyst did not change significantly,indicating that the catalyst inactivation was mainly caused by changes in surface properties.HA adsorption experiments showed that Ti O₂ pre-absorbed with HA caused the decayed TCP degradation rates,but the extent was much lower than those after the recycled photocatalytic reactions,indicating that HA adsorption on Ti O₂surface is not the main cause of catalyst inactivation.The results of XPS showed that the content of such oxygen-containing groups as C-O（286.2 e V）and O-C-OH/O-C-OR（289.0 e V）on the surface of the inactivated photocatalyst increased significantly,indicating that a large number of HA oxidized byproducts were strongly adsorbed on the surface of Ti O₂,resulting in Ti O₂ inactivation.The results of FTIR spectrum,3D fluorescence spectrum and others confirmed this conclusion.The nitrogen adsorption-desorption experiments showed that the adsorption of HA byproducts clogged some micropores of Ti O₂,which also contributed to the inactivation of Ti O₂.In addition,the photocurrent test showed that the adsorbed HA byproducts can reduce the photo-response efficiency of the catalyst by shielding the incident light and consuming the photogenerated charge carriers.（3）The regeneration of the inactivated Ti O₂ was systematically studied.Three regeneration methods were evaluated for their efficiency to recover the photocatalytic degradation of TCP,namely alkaline washing with ultrasound,ultraviolet irradiation and high temperature calcination,and the operating conditions of the three regeneration procedures were optimized.The results showed that the regeneration performance of alkaline washing with ultrasound was the best,and 84%of the photocatalytic activity could be restored under the optimal conditions,i.e.,ultrasonic treatment for 50 min using Na OH solution of p H 10.5-11.On the contrast,after 6hours of UV irradiation,the photocatalytic activity of the deactivated Ti O₂ only restored to 45%of fresh Ti O₂.Besides,aerobic calcination could also effectively regenerate the deactivated Ti O_2,with the photocatalytic activity of Ti O₂ recovered to78%of fresh Ti O₂ after calcination at 500℃for 30 min under air atmosphere.These results showed that the alkaline solution ultrasonic treatment and high temperature heat treatment procedures could effectively remove the oxidized HA layer adsorbed on the surface of the inactivated catalyst,thus regenerating the photocatalyst.