Adsorption Performance Of Activated Carbon And Degradation Of Organic Substances By Activated Carbon Supported Photocatalyst

Removal of dissolved organic matters is a general problem in drinking water and industrial water treatment process. The humic substances and their degradation products with large molecular weight are the major compositions of natural organic matters(NOM). With the serious water environmental contamination, such artificial organic matters as estrogens and pesticides have become the major source of low molecular organic matters in water. The applicable treatment technologies for DOM include adsorption and advanced oxidation process(AOP). The adsorption technology by using activated carbon(AC) has been extensively applied in drinking water and industrial water treatment; however, AC adsorption is a process with saturated problem and of pollutants transfer rather than completely degradation. Among various AOPs, photocatalytic oxidation technology is a hot topic in advanced water treatment owing to its high-efficiency, energy-saving and enviromental friendly characteristics. Nevertheless, the low adsorption performance and separation problems are the bottleneck limiting practical applications of photocatalysts.Based on the introduction of AC adsorption and AOPs, influential factors on AC adsorption capacity and advances on adsorption isotherm models were summarized for NOM removal, and applications of AOPs on NOM degradation were analyzed. A novel adsorption index, caramel decolorization ratio(CDR), and a general adsorption isotherm model were introduced to evaluate AC adsorption capacity and analyze relationship with structural parameters. According to CDR, AC with optimal adsorption performance was chosen as a carrier to prepare AC composite photocatalyst for the degradation of NOM, and the change of organic molecular structure in the process of degradation was analyzed. The major research contents and results are listed below.(1) A novel adsorption index of AC and its correlation with structural parametersFifteen types of AC used for water treatment of coal-fired power plants with different physic-chemical properties were investigated to analyze the correlation between adsorption performances and pore structural characteristics, and to construct the correlation model between CDR and pore structural parameters. There were significant correlation of CDR with average pore diameter, total pore volume, mesoporous pore volume and mesoporous pore area at α=0.01 level, with correlation coefficient of 0.931, 0.860, 0.970 and 0.939, respectively. The results suggested that CDR depended on pore structural parameters of AC. A multiple linear regression model was constructed with CDR correlated to average pore diameter, total pore volume and mesoporous pore volume with determination coefficient of 0.9574. The adsorption experiment results indicated that CDR of AC was significantly correlated with average adsorption rate of four typical organic matters(hunic acid, fulvic acid, lignin and tannins) in the natural water. The column test of AC also showed that periodical water production was correlated with CDR for the treatment of water from Huangpu River. The results suggested that the novel adsorption performance index, CDR, evaluated the adsorption performance of AC for NOM scientifically and objectively.(2) Study on a general isotherm model for activated carbon adsorption of typical natural organic mattersThe AC adsorption curves of organic matters in natural water were type II and III isotherms, which cannot fitted accurately by the Langmuir and Freundlich models. The AC samples with different CDRs(CDR of 4 kinds of AC greater than 50%, the other 4 less than 50%) were chosen to determine their adsorption isotherm for four typical NOMs. The results indicated that the adsorption curves of AC with different adsorption performances for lignin were all type III isotherms, and those of humic acid, fulvic acid and tannins were almost type II isotherms. A modified SR model was adopted for fitting of these adsorption isotherms, and results showed that modified SR model has a high degree of fit to both type II and III isotherms(determination coefficient >0.93) compared with Freundlich and Langmuir model. The type of isotherm depends on model parameter 1/n with type II of 1/n<0.5 and type III of 1/n ≥ 0.5. In order to make the predicted value of the modified SR model having practical significance, the limiting adsorption capacity Clim, which is related to the pore structure of AC and organic molecular size, is introduced. The modified SR model could calculated the predicted qe at any equilibrium concentration and the minimum AC dosage when the equilibrium concentration below Clim. The fitted parameter Kn and predicted adsorption capacity qe characterized adsorption performance of AC, and their values were both positively correlated with CDR at α=0.01 level, which further to illustrate CDR of AC was evaluated accutately the adsorption capacity of it.(3) Preparation of photocatalytic materials and comparison on photocatalytic activityThe hydrothermal preparation process of N-doped Ti O2(N-Ti O2) was optimized to enhance photocatalytic activity under visible irradiation by the response surface methodology. Effects of p H, calcinations time and calcination temperature on photocatalytic activity of N-Ti O2 photocatalyst were investigated. The quadratic regression model was set up according to the experimental data, methylene blue(MB) with the initial concentration of 10 mg/L as objets, the dosage of N-Ti O2 with 200 mg/L, degadation rate of MB under the visible light for 4h as response value. And response surfaces were delineated on the basis of best-fit mathmatic models for optimal preparation conditions. The results indicated that calcination temperature was the main factor for N-Ti O2 preparation. The maximum methylene blue(MB) removal efficiency of 95.1% was achieved at p H 6.05, calcination time 6.87 h and calcination temperature 403 oC.The Ag3PO4 sample with visible photocatalytic activity was prepared by precipitation method, and achieved MB(10 mg/L) removal above 90% with the dose of 200 mg/L after 40 min visible illumination. The photocatalyst Ag/Ag3PO4 with higher photocatalytic activity and stability was prepared by both microwave method and photoreduction method, and degraded MB completely after 30 min visible illumination at the same dose and initial concentration. Photoreduction time, microwave treatment time and pyridine content all had significant effects on photocatalytic activity of the prepared Ag/Ag3PO4. The bisphenol A(BPA), a common environmental estrogens in the natural waters, were degraded by Ag3PO4 photocatalyst(400 mg/L) under visible light radiation. Experimental results demonstrated that BPA(10 mg/L) was effectively degraded by Ag/Ag3PO4 with removal efficiency of 80% in 30 min. The photocatalytic degradation process of BPA can be divided into two stages, higher degradation rate in the initial stage of radiation(5 min), and then degradation followed the first-order kinetics in 6~30 min. The degradation rate depended on radiation intensity, initial concentration and dosage of photocatalyst.In oder to further increase the catalytic activity, Ag3PO4 was modified by Ag/Ag Cl, and the Central Composite Design method was adopted for RSM to optimize the preparation of Ag/Ag Cl/Ag3PO4 photocatalyst. The optimized results was the mole ratio with Na Cl and Na2HPO4(Cl/P) of 0.56, photoreduction time of 20.25 min and no reductant addition, and the error between the predicted and measured optimal MB removal(99.7%) was less than 0.5%. The kinetics of MB degradation process applied to pseudo-first order kinetics with determination coefficient higher than 0.98. The degradation rate of MB by Ag/Ag Cl/Ag3PO4 was 4.8 times of Ag3PO4 and 42.4 times of N-Ti O2. The herbicides were degraded by Ag3PO4 photocatalyst under visible light radiation. The photocatalytic degradation of metribuzin by Ag3PO4 and Ag/Ag Cl/Ag3PO4 also complied with first-order kinetics. The apparent degradation rate constant of metribuzin by Ag3PO4 was enhanced from 0.086 to 0.161 min-1 after modified by Ag/Ag Cl. The photocatalyst Ag/Ag Cl/Ag3PO4 showed higher stability according to the results of repeated use experiment.(4) Preparation and characterization of AC supported photocatalystExperimental results indicated that AC supported Ag/Ag Cl/Ag3PO4 showed greatly higher photocatlytic reactivity than the AC supported N-Ti O2. The effects of ratios between AC and Ag/Ag Cl/Ag3PO4(2:1, 1:1 and 1:2) on photocatalytic activity were also investigated. The prepared photocatalysts were characterized by X-ray diffractometer(XRD), UV-Vis diffuse reflectance spectrum(UV-vis) and scanning electron microscope(SEM). The results indicated that characteristic peaks of Ag3PO4, Ag Cl and Ag° were appeared on XRD spectrum. For supported photocatalyst with different coating ratio, the peak intensity of AC-Ag/Ag Cl/Ag3PO4(2:1) was higher than photocatalyst prepared by the other two ratios, indicating the best crystallinity of the material Ag/Ag Cl/Ag3PO4. The UV-vis spectra demonstrated that all the photocatalysts showed absorption peak in visible light region. The AC showed the highest adsorption intensity, followed by AC-Ag/Ag Cl/Ag3PO4, and Ag/Ag Cl/Ag3PO4 had lowest adsorption intensity. The SEM results indicated that Ag3PO4 particles showed irregular spherical shape with uneven diameter, and the spherical Ag Cl particles were attached on the surface of Ag3PO4 particles. The small Ag nanoparticles with diameter of 10~50 nm were attached on the surface of Ag Cl/Ag3PO4 particles. The Ag/Ag Cl/Ag3PO4 was precipitated on the surface of AC and formed clusters to block part of macropores of AC. Compared to the pure Ag/Ag Cl/Ag3PO4, the Ag Cl/Ag3PO4 particles on the surface of AC still retained irregular spherical shape with smaller diameter and more Ag nanoparticles on their surface. Under the same conditions, the MB removal was 65.53% by AC adsorption, while the MB degradation efficiency was 84.17% by Ag/Ag Cl/Ag3PO4 photocatalytic oxidation. Under the synergistic effect of adsorption and photocatalytic degradation, the highest MB removal efficiency of 98.54% was achieved with composite material AC-Ag/Ag Cl/ Ag3PO4(2:1).(5) Adsorption and degradation of natural humus by AC supported photocatalystThe humus extracted from natural waters was utilized as degradation target with total organic carbon(TOC) of 6.52 mg/L. The results of photocatalytic degradation by using visible light showed that humus had no self-sensitizable degradation effect by visible light. The TOC removal was 38.52% and 68.08% for the degradation of humus by Ag/Ag Cl/Ag3PO4 and AC-Ag/Ag Cl/Ag3PO4 under photocatalyst dosage of 0.5 g/L and radiation time of 240 min. The TOC removal was 53.12% for humus by only AC adsorption. The adsorption and degradation of photocatalyst AC-Ag/Ag Cl/Ag3PO4 was dominantly achieved by ·O2- generated from oxygen molecules, and followed by electron. Comparatively speaking, there are two routes for the degradation of organic matters by cavity:(1) direct surface oxidation of humic acid;(2) indirect oxidation by ·Cl generated from the oxidation of chloride. The oxidation effects of ·OH was the lowest. The photocatalytic degradation results indicated that high-intensive optical radiation was required for such macromolecular organic matters as humic acid.The photocatalytic degradation process of humic acid by AC-Ag/Ag Cl/Ag3PO4 followed the pseudo-first order kinetics under visible light radiation. The reaction rate enhanced with increasing dosage of photocatalyst at low dosage. The degradation rate was raised by 1.38 times with dosage increased from 0.2 to 1.0 g/L and TOC of 6.52 mg/L, and then remained constant with further increase of photocatalyst dosage. With photocatalyst dosage of 1.0 g/L, the photocatalytic degradation efficiency firstly increased and then decreased with initial TOC increasing from 3 to 12 mg/L after 4 h radiation under visible light. The concentration of residual humus was maintained almost constant with TOC of 1.28~1.80 mg/L in a certain range of initial concentrations. The effect of adsorption and degadation were enhanced with the lower p H(p H>2), but the photocatalytic degradation ability of Ag3PO4 was eliminated due to the dissolution of it at p H≤2.The organic matters in the photocatalytic degradation process were characterized by such fingerprints as UV-vis spectrum and three-dimensional fluorescence. The results indicated that macromolecular humus was degraded into small molecules, and the florescent peak intensity was cut down. The parallel factor analysis on three-dimensional fluorescent spectrum showed that the photocatalytic degradation of humic-like florescent substances complied with first-order kinetics, and the first-order kinetic constant was 1.47 and 0.91 h-1 under p H 6 and 8, respectively.