Catalytic ozonation of pentachlorophenol in aqueous solutions using granular activated carbon
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The efficiency of granular activated carbon (GAC) was investigated in this study as a catalyst for the elimination of pentachlorophenol (PCP) from contaminated streams in a laboratory-scale semi-batch reactor. The influence of important parameters including solution pH (2–10), radical scavenger (tert-butanol, 0.04 mol/L), catalyst dosage (0.416–8.33 g/L), initial PCP concentration (100–1000 mg/L) and ozone flow rate (2.3–12 mg/min) was examined on the efficiency of the catalytic ozonation process (COP) in degradation and mineralization of PCP in aqueous solution. The experimental results showed that catalytic ozonation with GAC was most effective at pH of 8 with ozone flow rate of 12 mg/min and a GAC dosage of 2 g. Compared to the sole ozonation process (SOP), the removal levels of PCP and COP were, 98, and 79 %, respectively. The degradation rate of kinetics was also investigated. The results showed that using a GAC catalyst in the ozonation of PCP produced an 8.33-fold increase in rate kinetic compared to the SOP under optimum conditions. Tert-butanol alcohol (TBA) was used as a radical scavenger. The results demonstrated that COP was affected less by TBA than by SOP. These findings suggested that GAC acts as a suitable catalyst in COP to remove refractory pollutants from aqueous solution.
KeywordsCatalytic ozonation Pentachlorophenol Activated carbon Ozone
Pentachlorophenol (PCP) is one of the hazardous pollutants extensively used in pesticides, herbicides and wood preservatives. This widespread application could lead to release of PCP into water sources (Öberg et al. 1990). According to US EPA, PCP is one of the prior pollutants whose maximum recommended concentration level in drinking water is 0.001 mg/L. 2, 3, 4, 5, 6 PCP is a white solid which is soluble in water in the range of 10–20 mg/L at room temperature. Even at concentrations less than 0.1 mg/L, this compound is toxic for plants, animals and human beings (Asgari et al. 2014). High levels of PCPs in aqueous environments may be toxic for, and it can also reduce their removal efficiency by the biological processes. Therefore, degradation or removal of PCP contamination often needs chemical and physical techniques. Due to various adverse effects of this compound, several technologies have been used to remove it from aqueous solutions. Advanced oxidation process (AOP) is a new technology used to treat industrial wastewater containing high concentrations of refractory compounds based on generation of very reactive groups, particularly hydroxyl radicals, these processes are used to degrade various organic compounds (Andreozzi et al. 1999). Ozone is a very strong oxidant, but single ozonation process (SOP) performance is not strong enough to remove the organic matter by mineralization because some oxidative reactions are relatively slow and selective. Nowadays, AOPs have been investigated to provide greater ozonation efficiency, the catalytic ozonation is found to be more effective for the removal of several pollutants from aqueous solution. Catalytic ozonation processes (COPs) are recent attractive kinds of AOPs in which various substances are used, as catalysts with the ozone, to generate very strong oxidative radical. (Jung et al. 2007; Moussavi et al. 2009). Based on type of the catalyst, the COPs were classified into homogeneous and heterogeneous processes, with the heterogeneous process having a higher degradation efficiency. (Kasprzyk-Hordern et al. 2003). Nowadays, the capabilities of several materials such as activated carbon, metal ions, metal oxides, natural zeolites and impregnated materials have been investigated as catalyst in COPs (Liotta et al. 2009). In recent years, activated carbon has been taken into huge consideration to be an efficient ozonation catalyst. This adsorbent is added to the ozonation process for the decomposition of O3 and thereby generates reactive radicals such as hydroxyl radicals (Faria et al. 2008). Hydroxyl radicals have high oxidative power in mineralizing refractory organic compounds (Sánchez-Polo et al. 2005).
In this research, GAC was used as a catalyst in catalytic ozonation of PCP from aqueous solutions. In addition, the main objective of this study was to compare and investigate the removal efficiency of PCP by SOP and COP. The catalytic effect of GAC on ozone decomposition and the important parameters (e.g., solution pH, radical scavenger effect, GAC dosage, initial PCP concentration and ozone flow rate) were studied in the COP.
Materials and methods
The GAC used in this study as catalyst was prepared from German Merck Co. It had a particle size 58°, BET surface area of 155 m2/g. Sodium salt PCP (Na–PCP, purity >98 %) was purchased from Aldrich Co. Concentrated stock solutions of PCP were made by dissolving the PCP powder in sodium hydroxide solutions (0.1 mol/L). Subsequent PCP solutions were prepared at different concentrations by dissolving stock solution in certain amounts of distilled water. All the other reagents such as tert-butanol, 4-aminoantipyrine and potassium-ferro-cyanide were analytical grades without further purification.
The catalytic ozonation experiments were carried out as a pilot scale in a semi-batch flow reactor. A glass sparger with volume of 1000 mL was used as ozonation, equipped with a sintered-glass diffuser at bottom, an ozone generator and an oxygen generator. Ozone was made using an ozone-making machine, ARDA model COG-OM. Oxygen required for the ozone generator was supplied by oxygen-making machine, model PORSA VF-3 with high purity level and the ability to adjust oxygen injection level. The effect of parameters such as pH, tert-butyl alcohol radical scavenger, catalyst dosage, initial PCP concentration and ozone flow rate was tested in different contact time (10–30 min). In this study, the effect of pH in range of 2–10 was studied on the both SOP and COP (under the selected condition; PCP concentration = 100 mg/L; catalyst dosage = 8.333 g/L; dissolved ozone dosage = 0.49 g/min). In the next step, after the optimum pH was identified, the effect of tert-butanol alcohol was tested as a radical scavenger on the SOP and COP efficiency in removal of PCP. Moreover, to determine optimum concentration of GAC as catalyst in the COP, various concentrations of GAC in range of 0.416–8.33 g/L were tested under selected condition (pH = 8; PCP concentration = 100 mg/L, contact time = 15 min). Also the effects of various concentration of PCP (from 100 to 1000 mg/L) were evaluated on COP efficiency. In the next step, to determine optimum ozone flow rate, changes of ozone flow were studied in range of 2.3–12 mg/min.
Kobs represents the pseudo-first-order rate constants (min−1). The data from both COP and SOP fitted well to the pseudo-first-order kinetics. Ksop was calculated via method reported by Asgari et al. (2013). Finally, the COP efficiency under optimum condition in removal of COD was investigated.
PCP concentrations in solution were measured by colorimetric method using UV–Vis spectrophotometer (DR 5000) in 500 nm wavelength (Asgari et al. 2014). The mass velocity of ozone was measured by standard iodometry; also the values of COD were determined with a standard potassium dichromate oxidation method (Rand et al. 1976). Point zero charge (pHpzc) of activated carbon was determined using the pH drift method. To determine pHzpc, of activated carbon, NaCl solution (0.01 M) was used as an inert electrolyte. HCL and NaOH (0.1 M) were used and the initial pH of solution was adjusted from 2 to 12. The quantities (0.5 g) of GAC were added to each flask. Flasks were placed on the shaker (Model GFL 3017) with 120 rpm for 48 h. Then, the contents of each flask were filtered using 0.45 µm and final pH was measured using pH meter (Hatch Sinsion1). The pHpzc was determined by drawing initial pH versus final pH (Asgari et al. 2014, 2013).
Results and discussion
Influence of initial pH
Influence of radical scavenger
Influence of catalyst dosage on degradation
Influence of initial PCP concentration
Influence of ozone flow rate
Evaluation the role of GAC in the COP
COD removal in COP under optimum conditions
Kinetics of PCP removal for different pH values in COP and SOP
kobs (min −1)
The present study has explained the role of GAC as a catalyst in the catalytic ozonation of PCP in aqueous solution. The optimum pH, GAC dosage and ozone flow rate in the COP were determined to be 8, 8.33 g L−1 and 12 mg/min, respectively. This work also demonstrated that the COP was less affected than that of SOP by tert-butanol as radical scavengers. After reusing a used GAC in ozonation process the removal efficiency of PCP did not decrease obviously compared to that of fresh GAC under the same experimental conditions, indicating a stable activity in ozonation process. Moreover, the removal efficiency of PCP in the COP was about 1.2 and 6.7 times higher than that of SOP and GAC adsorption process. In addition, the kinetic models investigated in this study indicated that pseudo-second-order kinetic model gave better results than other models. Therefore, these findings suggested that GAC acts as a suitable catalyst in COPs for removal of refractory pollutants from aqueous solution.
The authors would like to thank Hamadan University of Medical Science for its financial support.
Conflict of interest
The authors have declared no conflict of interest.
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