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Ind. Eng. Chem. Res. 2003, 42, 1076-1086
Removal of the Herbicide MCPA by Commercial Activated Carbons: Equilibrium, Kinetics, and Reversibility Olga Gimeno, Pawel Plucinski, and Stan T. Kolaczkowski Department of Chemical Engineering, University of Bath, Bath BA2 7AY, U.K.
Francisco J. Rivas* and Pedro M. Alvarez Departamento de Ingenierı´a Quı´mica y Energe´ tica, Facultad de Ciencias, Universidad de Extremadura, 06071 Badajoz, Spain
The adsorption of the herbicide 4-chloro-2-methylphenoxyacetic acid (CAS 94-74-6) has been studied using four commercial activated carbons (Norit 0.8, Aquacarb 207C, Aquacarb 208A, and Aquacarb 208EA). Adsorption equilibrium isotherms were obtained in the 293-358 K temperature range. The trend of adsorption for MCPA onto the four activated carbon was in the order Norit 0.8 > Aquacarb 207C > Aquacarb 208A > Aquacarb 208EA. Among various adsorption isotherm models, the Freundlich equation best fit the experimental data. Experiments conducted at different temperatures allowed for the calculation of the isosteric heat of adsorption, revealing that the adsorption process was exothermic for three of the four adsorbents studied. The distribution of the herbicide into the pores of activated carbon Norit 0.8 was studied by considering the Dubinin-Radushkevich isotherm. Also, the adsorption kinetics were assessed by means of a simplistic mechanism based on the shrinking-core mass-transfer model. Finally, as a preliminary step in the study of the potential use of wet air oxidation (WAO) for the regeneration of exhausted activated carbon, the reversibility of the adsorption process was tested. Only partial desorption of MCPA was achieved under WAO conditions, which involves a likely double route of activated carbon regeneration, i.e., liquid and surface contaminant oxidation. Introduction The lack of clean water has always been an issue of major environmental concern all over the world. The main sources of water pollution are industrial (chemical, organic, and thermal wastes), municipal (largely sewage consisting of human wastes, other organic wastes, and detergents), and agricultural (herbicides, pesticides, and fertilizers). The wide utilization of herbicides for agriculture practices has already contributed to the increasing contamination of surface and underground waters. For this reason, in recent years, emerging concerns about the quality of water intended for human consumption have greatly increased. The toxicity of herbicides and their degradation products makes the contamination caused by these chemical substances a potential environmental hazard.1 MCPA (4-chloro-2-methylphenoxyacetic acid, CAS number 94-74-6) is a post-emergence phenoxy herbicide extensively used in agriculture to control annual and perennial weeds in cereals, grasslands, trees, and turf. Similarly to other phenoxy herbicides, MCPA is an acid, but it is often formulated as a salt. It is very soluble (825 mg L-1 in water at 293 K), highly mobile, and can leach from the soil. This compound has been found in well water in some countries and is classified by the U.S. Environmental Protection Agency (EPA) as a potential groundwater contaminant.2 The major metabolites of these phenoxy acid herbicides are phenols. The resulting * To whom correspondence should be addressed. Telephone: 34 924 289385. Fax: 34 924 271304. E-mail: fjrivas@ unex.es.
chlorophenols might pose an additional risk for groundwater pollution. Several treatment processes are available for the removal of such compounds from waters and wastewaters. Chemical oxidation with ozone;3,4 photodegradation;5-8 combined ozone and UV irradiation;9,10 Fenton degradation,11 biological degradation;12 coagulation;13 and adsorption onto porous solids such as zeolites, clays,14,15 and fly ash16 have been investigated with varying success. Among the available adsorbents for the removal of herbicides from water, activated carbon (AC) is the preferred solid because of its high effectiveness. Activated carbon is a meso-microporous adsorbent that can be manufactured from a variety of carbonaceous materials, including wood, coal, lignin, coconut shells, and sugar. Its unique adsorption properties result from its high surface area, meso- and micropore structure, and broad range of surface functional groups, which influences its adsorption properties and reactivity. Adsorption on activated carbon is the most widespread technology used to deal with pesticides and other hazardous chemicals in drinking-water plants. Thus, a number of studies have been reported on the use of this method for the purification of water contaminated by herbicides.17-21 Additionally, adsorption on activated carbon is also an expensive technology because once AC is exhausted with the herbicide, a regeneration step is needed that is able to eliminate the herbicide from the surface without significant loss in adsorption capacity. An emerging technology that uses milder conditions than thermal regeneration, is wet air regeneration,22 (i.e., high temperature and pressure oxidation in aqueous media).
10.1021/ie020424x CCC: $25.00 © 2003 American Chemical Society Published on Web 01/31/2003
Ind. Eng. Chem. Res., Vol. 42, No. 5, 2003 1077 Table 1. General Characteristics of Activated Carbonsa property cm-3)
bulk density (g moisture content (wt %) specific surface area (m2 g-1) ash content (wt %) shape particle size (mm) pH a
Norit 0.8
Aquacarb 207C
Aquacarb 208A
Aquacarb 208EA
0.39 2 1150 7 pellets
