Catalytic Ozonation Of Phenolic Compounds In Water By Activated Carbon Fiber

Phenolic compounds are common used raw materials for manufacturing pesticides, wood preservation agents, herbicides, paint, dyes and phenolic resins,and could be found in the wastewater discharged from petrochemical plants, coking plants and oil refineries, etc. More and more attention was paid to the research on the treatment of phenolic wastewater. The heterogeneous catalytic ozonation using solid as catalyst can efficiently degrade the organic pollutants in water and is thought to be a promising advanced oxidation technology for water treatment. The process of GAC (granular activated carbon) adsorption has been widely used in the water treatment, however, the GAC can easily get saturated, which requires regeneration or even replacement, cause the cost of adsorption process increased and limit its application. The combination of both adsorption and ozonation into a single process was found to be able to offer strong synergetic effects on the removal of many contaminants in water, thereby remedy the shortcomings of single adsorption and ozonation. ACF (activated carbon fiber) is abundant in the micropore open on the surface of the fiber, it thus has higher adsorption rate and possess excellent adsorption and desorption properties. Phenol and 2,4-dichlorophenol (2,4-DCP) were selected as target pollutants in this work, the affecting factors in the ozonation catalyzed by GAC and ACF were studied and compared. The effect of ozonation on the surface carbon properties was also investigated; and finally, the ozonation of refinery wastewater containing mainly alcohol ethoxylates (AEs) in the presence of GAC and ACF was carried out.The effect of some operating variables, such as the pore structure parameters and amount of GAC and ACF, initial pH and phenol concentration, and the inlet ozonized-air flowrate, on the ozonation of phenol and 2,4-DCP were investigated. Although the specific surface area and the total pore volume of GAC and ACF increased significantly after secondary activation, leading to the increase of adsorption to the phenolic compounds in water, the ozonation efficiency of GAC before and after activation had hardly any changes, and the activation can only cause a slight improvement for ACF. These indicated that the ozonation rate on the surface of carbon is much higher. Only those pores open on the external surface of the carbon particles with a faster adsorption rate could exhibit the catalysis. Therefore, the pores formed in the internal of GAC during the secondary activation did not almost contribute to the ozonation, whereas most of the new formed pores of ACF were also open on the surface of fiber, thus leading the improvement of ozonation. An increase of carbon amount and inlet air flowrate could cause an increase of catalytic ozonation efficiency, but it is more significant for the first variable. The catalytic ozonation efficiency of ACF is times of GAC due to the pores of ACF open on the surface of fiber. Because of the disassociation of phenol and neglected adsorption in the basic pH, the degradation rate is lower than that in the acidic pH, and decreased with the increase of basic pH. However, the strength of acidic pH had almost no influence on the ozonation process. The increase of initial phenol concentration only caused the efficiency declined slightly, but the total removed phenol increased significantly. Because the small-molecule organics generated in the phenol decomposition were more resistant to ozone oxidation than their precursors, the COD (chemical oxygen demand) removal was therefore lower than phenol by ozonation. The repeated uses of these two carbons showed that they all could be regenerated in situ in the ozonation, the catalytic activity changed little.Attacked by the active species formed on the carbon surface due to the ozone decomposition, the ozonation process in water can significantly change the composition of acidic surface oxygen-containing groups of ACF, leading to the increase of carboxylic, hydroxylic and carbonylic groups, and the slight decrease of the lactonic groups. Consequently, the surface of the exhausted ACF became more hydrophilic and the pHPZC decreased. Furthermore, this process can also increase the surface area and total pore volume of ACF. Due to the new micropore formation and some pore enlargement, the micropore became smaller, and the mesopore and macropore got bigger.The presence of Ca2+ (temporary hardness) can effectively enhance the phenol and COD removal rate, because Ca2+ can bind with some of the intermediate products of phenol to form insoluble precipitates during the ozonation. However, the specific surface area and total pore volume decreased slightly due to the block of particles. On the contrary, Ca2+ (permanent hardness) could cause the phenol and COD removal decrease due to the effect of the binding of Ca2+ with carboxyl on the carbon surface formed during the ozonation on the adsorption. The complexation of Fe3+ and phenol caused the decrease of adsorption and reaction probability, therefore, Fe3+ in the system could cause the fall of ozonation efficiency. But for Mn2+, due to the coagulation and catalysis of colloid MnO2 formed in the ozonation, a slight higher efficiency at the initial stage was observed, but the final result had almost no change. The specific surface area and total pore volume for the ACF after ozonation in the presence of Ca2+ (permanent hardness), Fe3+ and Mn2+. The most increase was obtained for Ca2+ (permanent hardness), and the least for Mn2+. The characteristic of kinetics for the ozonation of the wastewater containing AEs in the presence of GAC and ACF was different from that of phenolic compounds. GAC was more efficient than ACF because the molecular size of AEs is more than 2nm, which could not adsorbed easily by ACF. Its adsorption independent of pH on GAC led to higher catalytic ozonation efficiency in the basic condition because of the formation of active species from the catalytic decomposition of ozone by OH?. Below 35°C, the rise of temperature could speed up the molecule movement and lower the activation energy, thus improve the ozonation. When the temperature is over 40°C, the decrease of ozone solubility in water and the rapid increase of ozone decomposition rate cause the decrease of degradation rate contrarily.