4374
Ind. Eng. Chem. Res. 1997, 36, 4374-4380
Abatement of Pollutants by Adsorption and Oxidative Catalytic Regeneration Yurii I. Matatov-Meytal and Moshe Sheintuch* Department of Chemical Engineering, Technion-Israel Institute of Technology, Haifa, Israel 32000
The adsorption of phenol, and halogenated phenols on granular activated carbon (AC) modified with metal oxide catalyst, followed by catalytic oxidative regeneration, was studied as an efficient technology for the treatment of dilute wastewaters. The advantages of the combination of these technologies are as follows: (1) the process will be accelerated by the high concentrations of pollutants eluted from the adsorbent; (2) a large number of adsorption-regeneration cycles are expected without loss in capacity; and (3) the low-temperature regeneration will be conducted in situ, even in small units, thus improving the economy of the process. Oxidative catalytic regeneration of spent carbons, performed at 240-300 °C with air, completely restored the adsorption capacity of phenol on the ACs modified with catalyst, even after 10 cycles of regeneration. Under similar conditions, only partial recovery of the adsorption capacity was obtained for carbons loaded with p-chlorophenol and p-bromophenol. The adsorption capacity and the surface area of the AC (Filtrasorb-400) diminished somewhat with the impregnation of oxides (Fe2O3, CuO, and additivities of Cr2O3 or inert silica), but that did not affect the shape of the adsorption isotherms. Introduction The adsorption of organic pollutants by activated carbon (AC) is a well-established technology, yet its cost is still a prohibitive factor. For economic and environmental reasons, spent AC is not disposed of but undergoes several cycles of regeneration. Thermal regeneration of AC is the most common process, but it requires high temperatures (800-850 °C) and, consequently, is usually not conducted in situ, requiring shipment of the spent AC to special regeneration units and contributing significantly to its cost. Moreover, high-temperature regeneration is economically feasible only for large systems that use more than 500 000 lb of granular AC per year (Sonyheimer et al., 1988). We suggest here to combine an adsorption process of pollutants on well-established adsorbents like AC with periodic low-temperature catalytic regeneration of the adsorbent as an efficient technology for treatment of dilute wastewaters. The suggested advantages of the combination of these technologies are as follows: (1) the process will be accelerated by the high concentrations of pollutants eluted from the adsorbent; (2) a large number of adsorption-regeneration cycles are expected without loss in capacity; and (3) the low-temperature regeneration will be conducted in situ, even in small units, thus improving the economy of the process. The technological and scientific problems addressed here are as follows: (1) the strength, capacity, and reversibility of adsorption on AC impregnated with catalyst; (2) the choice of a catalyst that operates at relatively low temperatures but well below the ignition temperature of AC; (3) the engineering of contacting the pollutants with the AC and the catalyst; and, (4) the efficiency of regeneration in batch and fixed-bed studies. We review below the relevant information on AC structure, strength of adsorption, catalysts used in catalytic regeneration of AC, adsorbate-catalyst contacting modes, and alternative regeneration procedures. * To whom correspondence should be addressed. Telephone: 972 4 8292823. Fax: 972 4 8230476. E-mail: cermsll@ techunix.technion.ac.il. S0888-5885(96)00809-3 CCC: $14.00
Adsorption on and Thermal Regeneration of AC. The adsorption of phenols on ACs is normally characterized as physisorption at low temperatures and chemisorption at high temperatures. At relatively high adsorption temperatures, long adsorption contact times, and high oxygen content, phenols tend to irreversibly adsorb on the carbon surface (Grant and King, 1990; Vidic et al., 1993). Single-component adsorption equilibria of phenols on ACs are usually described by Langmuir (Harriott and Cheng, 1988; Koganovskii and Prodan, 1988; Nelson and Yang, 1995) or Freundlich isotherms (Peel et al., 1981; Yong et al., 1985). This suggests reversible adsorption, but actual desorption experiments have been performed only in a limited number of studies. Desorption can be induced by displacement with a compound of a higher affinity to AC and/or by increasing the temperature. Suzuki et al. (1978) classified the adsorbed organics into three groups according to the characteristics of thermal regeneration of carbon: volatile compounds (type I), unstable compounds and refractory adsorbates (type II), and constituted aromatic compounds with side chains and adjacent OH groups (type III). Activated carbon with sorbed organics undergoes the following scenario with increasing temperature: drying and loss of highly volatile compounds occurs at temperatures below 200 °C, vaporization and decomposition of unstable compounds take place at 200 < T < 500 °C, and pyrolysis of nonvolatile adsorbates to form char occurs at 500 < T < 700 °C followed by oxidation of the residue at higher temperatures. Exposure to temperatures of 750-980 °C lead to oxidation of the residual material as well as that of the carbon itself. The latter step includes oxygen attack on the AC itself, which may alter the pore structures where small pores ( CoO > Cr2O3 > NiO > MnO2 > Fe2O3 > YO2 > Cd2O3 > ZnO > TiO2 > Bi2O3. In gas-phase oxidation the following order of activity was determined within the temperature range 225-440 °C (Simonov, 1990); Pt > Cu > (Cu + Cr) > CuO > (CuO + Cr2O3) > V2O3 > (Cr2O3) > Co3O4 > Fe2O3 > MnO2 > ZnO.
Experimental Section Materials. The granular AC employed in this study (Calgon Filtrasorb-400 supplied by Chemviron Carbon) is derived from bituminous coal and has been widely used in the chemical and food industries for adsorption, decolorization, and pollution abatement purposes. This AC had a specific surface area of 1200 m2/g and a broad pore-size distribution (
