Environ. Sei. Technol. 1994, 28, 216-221
Dissolved Oxygen Effects on Equilibrium and Kinetics of Phenolics Adsorption by Activated Carbon Nab11 S. Abuzald and Glrgls F. Nakhla' Department of Civil Engineering, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudia Arabia
Phenol and o-cresol have been shown to undergo oxygeninduced polymerization reactions on activated carbon that enhance their adsorption. Four different levels of dissolved oxygen (DO)were involved in the equilibrium and kinetics studies undertaken. I t was found that for both phenol and o-cresol, the adsorptive capacities increase with the increase in DO concentration. The quantities of dimers and trimers formed on the carbon surface were a function of the DO level. Phenol recovery efficiencies around 70% and 25% were found for anoxic and oxic isotherms, respectively. The additional capacity attained under oxic conditions was limited by the masses of DO and granular activated carbon (GAC) in the test environment. Two empirical relationships relating the oxic capacity to the ratio of DO to GAC mass and the anoxic capacities were developed. Batch studies have shown that the apparent surface diffusivity coefficientfor phenol in GAC decreased with increasing DO levels in the sorbate solution. Introduction
Granular activated carbon (GAC) has been successfully used in water and wastewater treatment for the removal of a wide variety of natural and synthetic organic compounds. The relatively high cost of activated carbon (AC) has motivated researchers to investigate and attempt to maximize the adsorptive capacity of AC for hazardous organic compounds (1,2).Factors affecting the adsorptive capacity of such compounds were also investigated in order to fully utilize activated carbon under operational conditions. Traditionally, researchers used to study the effects of temperature (3,4)and pH variations (5-7). The effect of competition from other sorbates and/or organic matter on adsorption was investigated by many researchers (811). In many instances, equilibrium capacities were a function of the test conditions (12,13). A major variable that exerts appreciable impact on adsorption but has been much overlooked is dissolved oxygen (DO). Differences in sorbate solution DO between the various equilibrium test methods have been implied (14) as a potential reason for the discrepancies in adsorption capacities of AC for phenolics reported in the literature. Prober et al. (15) found that molecular oxygen increases base sorption capacity due to the formation of acidic surface oxides. The same phenomenon was confirmed by Coughlin and Ezra (16),who observed reduction in adsorption capacity for phenol and nitrobenzene, and Snoeyink et al. (17), who reported a 50% reduction in adsorptive capacity of phenol and nitrophenol due to the formation of acidic surfaces oxides. Recently, however, Vidic et al. (14), Nakhla et al. (181, and Grant and King (19) have found that the existence of dissolved oxygen increases the adsorptive capacity of AC for phenolic compounds. Vidic et al. (20) found that while the
* To whom all correspondence should be addressed. 216
Environ. Sci. Technol., Vol. 28, No. 2, 1994
enhancement phenomenon was valid for phenolics and natural organic matter, it was not applicable to aliphatics. The causes of this enhancement were investigated by Grant and King (19) and Vidic et al. (20). Both studies showed that dissolved oxygen promotes polymerization of phenolics on the carbon surface, thus increasing irreversible adsorption and hence reducing the regeneration efficiency of spent AC. The aforementioned recent work has either focused on AC adsorption equilibrium of phenolics at two levels of DO that are unlikely to prevail in practical applications of GAC, Le., zero and saturation with pure oxygen (14,18, 20),or emphasized irreversibility at extreme temperatures and pHs prevailing in the industrial regeneration process (19). The objectives of this study are to provide further insight into the effect of DO on the kinetics and equilibrium of phenols adsorption onto AC and to present a mathematical model of such effects that could be used to describe adsorption capacities at various DO concentrations. Isotherm studies at four levels of DO were conducted, namely, 0, 4, 9, and 30 mg/L. The DO levels will be denoted hereafter as DO levels 1, 2, 3, and 4, respectively. Furthermore, closed batch kinetic studies were performed on the adsorption of phenol on GAC under the aforementioned four levels of dissolved oxygen. Materials and Methods
The coal-based Filtra-sorb 400 GAC used in this study was supplied by Fisher Scientific in 10 X 40 US.mesh sizes. The physical properties of F-400GAC are presented elsewhere (21). The 10 X 16 fraction with a geometric mean diameter of 0.156 cm was washed several times with deionized water to remove all fines, subsequently dried in an oven at 110 "C for 1 day, allowed to cool at room temperature for about 10 min, and finally stored in a desiccator until use. Isotherm Tests. Single-solute stock solutions (1000 mg/L each) of phenol and o-cresol(BDH Chemicals,U.K.) were prepared and subsequently buffered with KHzP04 to maintain neutral pH. For each compound, four sets of 160-mL bottles containing varying amounts of 10 X 16 U.S. mesh size activated carbon were prepared and subsequently filled with 100 mL of adsorbate solution. One set was purged with nitrogen until oxygen was completely stripped, and the bottles were quickly closed with a rubber stopper. Nitrogen was slightly purged in the second set until a DO of 3-4 mg/L was achieved. Bottles of the third set were purged with air so that saturation with air can be achieved (around 9.0 mg/L DO). The last set of bottles was purged with pure oxygen until saturation was achieved as evidenced by a DO concentration around 30 mg/L. Each set of bottles included two bottles without activated carbon to serve as blanks to check for sorbate volatilization and adsorption of sorbate onto the walls of the container. All bottles were placed on a rotary shaker at a room temperature of about 21 f 2 "C for a period of 0013-936X/94/0928-0216$04.50/0
0 1994 American Chemical Society
Figure 1. Reactor setup for the kinetic experiments.
14days. Measurementof residualsorbate concentrations for up to 23 days and evidence from literature (20) ensured the attainment of equilibrium after 2 weeks. At the end of the equilibration period, sampleswere withdrawn from each bottle, filtered thrugh0.45-pmMillipore filter paper, and analyzed for sorbate residual concentrations using Spectronic 21 spectrophotometer (Bausch and Lomb Model, UV-D) at a wavelength of 270 and 271 nm for phenol and o-cresol, respectively. Extraction Experiments. GAC samples used in the anoxic and 'oxygen-purged" phenol isotherms and those used in the anoxic, 'air-purged", and oxygen-purged o-cresol isotherms were extracted in a Soxhlet extraction apparatus. GAC samples were first extracted for 1 day with methanol and then with methylene chloride for 3 days following the procedure of Vidic et al. (20). Subsequently, the extracts were analyzed with GC-MS. Virgin GAC samples and the pure chemicals used in the preparation of the sorbate solutions were also treated and analyzed similarly. Kinetic Experiments. The rate experiments were conducted in 5-L completely mixed finite-hath reactorsof the type shown in Figure 1 (22) with a liquid volume of 4.5 L. The objective of trapping the GAC particles in a basket rather than freely mixing them with the solution was to increase external mass transfer by maximizing the fluid velocity relative to that of GAC. Four identical reaetorswiththe samemixingconditions andinitialsorbate and GAC concentrations, but different DO concentrations, were run simultaneouslyat neutral pH and 21 "C. The four different DO levels were achieved by a purging proceduresimilar tothat used in theisothermexperiments. Samples were analyzed at predetermined time intervals until equilibrium, indicated by the constant concentration for three consecutive samples, was attained. The cumulative volume of samples constituted
