Catalytic Wet Oxidation of Phenolic Compounds at Moderate

Temperature and Pressure Conditions

Rajendra Mohite and Anurag Garg*

Centre for Environmental Science and Engineering, Indian Institute of Technology Bombay,
Mumbai, 400076, INDIA
*Corresponding author: a.garg@iitb.ac.in

Abstract

Highly concentrated phenolic streams originated from refineries are difficult to treat by conventional biological processes. Wet oxidation (WO) process is suggested as an option for the treatment of high strength wastewaters which have low biodegradability. The process typically requires elevated temperature and pressures for the complete mineralization of the persistent organic or inorganic compounds. The severe oxidation conditions are required to generate free radicals having high oxidizing potential (such as hydroxyl radical). The introduction of a suitable catalyst reduces the energy requirement for the initiation reaction to produce free radicals. Therefore, the present experimental work was conducted to find the performance of activated carbon (AC) supported catalyst (5% Cu/AC) for the removal of phenolic compounds (phenol, o-cresol and 2,5-dimethylphenol) generally found in refinery wastewater.
The catalyst was prepared by impregnating 5% (by weight) copper on the AC. The catalyst was characterized for X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), and scanning electron microscope (SEM) techniques. The catalyst particle size distribution was determined by sieve analysis. The synthetic wastewater was prepared by dissolving equal mass concentrations of phenol, o-cresol and 2,5-dimethylphenol in the distilled water (total phenolics concentration = 10 g/L). In the wastewater, NaOH was also added to raise its pH to
12.6 which is typical for the refinery effluent. The chemical oxygen demand (COD), total organic carbon (TOC) and pH of the synthetic wastewater were ~25000 mg/L, ~7800 mg/L and 12.8, respectively. The laboratory scale batch reactor (total volume = 0.7 L) was used for the non-catalytic and catalytic wet oxidation (CWO) runs. The reactor was charged with synthetic phenolic wastewater (0.25 L) and heated to the desired temperature (130 - 160ºC). After attaining the reaction temperature, the oxygen was supplied and the initial oxygen pressure was kept at 0.8 MPa. A heterogeneous catalyst concentration of 3 g/L was used for the catalytic runs. The samples collected during and after the 4 h reaction were subjected to pH, COD and TOC analyses.
The XRD spectra of Cu/AC and AC show the presence of broad peaks indicating the amorphous structure of the two materials. The location of the peaks for CuO phase on AC could also be observed. BET surface area of the catalyst (924 m2/g) was higher than the support, i.e., AC (874 m2/g). This can be attributed to the further opening of pores and generation of new surfaces during calcination process. A significant difference between particle size obtained from BET (dBET = 3.2 nm) and Scherrer equation (dXRD = 11.1 nm) was noted. This variation may be due to the fact that dBET is surface weighted particle property
while dXRD is mass weighted particle property. The comparison of SEM images for the untreated AC and Cu/AC catalyst also confirmed the deposition of copper on the support surface.
The results from the WO and CWO runs are summarized in Figure 1. The results show that the increase in temperature from 130°C to 160°C had little impact on the TOC reduction (~
44 – 50% for catalytic and ~ 35 – 47% for non-catalytic runs) from the wastewater. After
CWO at 160°C temperature, the TOC reduction was slightly lower than that obtained at
140°C and almost equal to the TOC removal during non-catalytic run. The two possible reasons for the results may be: (i) insufficient catalyst concentration and (ii) the reduced quantity of polymerized products at elevated temperature during CWO which means that higher carbon content should be dissolved in treated water rather than deposited on the catalyst or present in particulate form. The change in initial wastewater pH from 12.6 to 9.0 caused the significant increase in TOC reduction from 44% to 70% (for CWO) at 130°C temperature. Similar result was also observed for non-catalytic WO run. This may be due to reduction of the scavenging of the active free radical species in highly alkaline solution. The extent of phenol dissociation may also have affected the WO chemistry (pKa of phenol ~ 9.9 at 25°C temperature) at two different starting pH values.
The first order kinetic model was used to observe the reaction rate with respect to TOC during WO and CWO runs. The rate constant (k) was in the range of 0.0021–0.0055 min-1. The lowest value was obtained for non-catalytic WO performed at 130°C temperature while the highest was found for CWO performed at 130°C temperature and initial pH of 9.0.
Further studies need to be carried out to estimate the extent of polymerization at various temperatures and the nature of the polymerized products.

100
80
60
Noncatalytic
Catalytic
40
20
0
130 C 130 C, pH=9 140 C 160 C

Temperature

Figure 1. Effect of temperature and pH on TOC reduction (PO2 = 0.8 MPa, catalyst concentration = 3 g/L, reaction time = 4 h, initial pH = 12.6)
Keywords: PHENOLIC COMPOUNDS; CATALYTIC WET OXIDATION; TRANSITION- METAL OXIDE CATALYST