Evaluation of Current Techniques for Isolation of Chars as Natural Adsorbents YUAN CHUN, †,‡ GUANGYAO SHENG,* ,† AND CARY T. CHIOU § Department of Crop, Soil, and Environmental Sciences, University of Arkansas, Fayetteville, Arkansas 72701, Department of Chemistry, Nanjing University, Nanjing 210093, People’s Republic of China, and U.S. Geological Survey, Box 25046, MS 408, Denver Federal Center, Denver, Colorado 80225 Chars in soils or sediments may potentially influence the soil/sediment sorption behavior. Current techniques for the isolation of black carbon including chars rely often on acid demineralization,base extraction,and chemical oxidation to remove salts and minerals, humic acid, and refractory kerogen, respectively. Little is known about the potential effects of these chemical processes on the char surface and adsorptive properties. This study examined the effects of acid demineralization, base extraction, and acidic Cr 2 O 7 2- oxidation on the surface areas, surface acidity, and benzene adsorption characteristics of laboratory- produced pinewood and wheat-residue chars, pure or mixed with soils, and a commercial activated carbon. Deminer- alization resulted in a small reduction in the char surface area, whereas base extraction showed no obvious effect. Neither demineralization nor base extraction caused an appreciable variation in benzene adsorption and presumably the char surface properties. By contrast, the Cr 2 O 7 2- oxidation caused a >31% reduction in char surface area. The Boehm titration, supplemented by FTIR spectra, indicated that the surface acidity of oxidized chars increased by a factor between 2.3 and 12 compared to non- oxidized chars. Benzene adsorption with the oxidized chars was lower than that with the non-oxidized chars by a factor of >8.9; both the decrease in char surface area and the increase in char surface acidity contributed to the reduction in char adsorptive power. Although the Cr 2 O 7 2- oxidation effectively removes resistant kerogen, it is not well suited for the isolation of chars as contaminant adsorbents because of its destructive nature. Alternative nondestructive techniques that preserve the char surface properties and effectively remove kerogen must be sought. Introduction The soiland sediment organic matter (SOM)has been known to act as the principalfraction for the soil/sediment sorption of nonpolar organic compounds (NOCs) from water (ref 1 and references therein). The NOC sorption to SOM occurs primarily by partition, characterized by a high degree of isotherm linearity and the lack of sorptive competition between coexisting solutes (contaminants) (2-4). Recent studies reveal, however, that the NOC sorption to natural solidscontaininga high-surface-area carbonaceousmaterial, often referred to asblackcarbon (BC),mayexhibit a nonlinear and competitive effect, which occurs usually at low relative concentrations(5-8).The BCresults primarilyfrom the field biomass burning that becomes embedded into soils and sediments. By this consideration, the enhanced isotherm nonlinearity of NOCs with the humin fraction of the Florida peat, as compared to the original peat and its humic acid fraction, was ascribed to the enriched BC in the humin fraction when fractionated with a density-fractionation procedure (9). Using the organic petrographic method, the BC particles have been visually identified and characterized (10-14).Theoreticalmodels that combine adsorption byBC and partition into SOM have now been used to assess the contribution of BC in soils and sediments to NOC sorption (15-19). Confirmation of the adsorptive role of chars requires isolation, quantification, and characterization of the char properties. Geologists and limnologists have developed various techniques, including some destructive chemical treatment methods, for BC isolation and quantification on the basis that the BCs themselves are usually relatively inert to chemical treatment (e.g., refs 20 and 21). Common chemical treatments include acid demineralization, base extraction, thermal treatment, alternating base and organic solventextraction,alkalineoxidation,and/oracidicoxidation (22-33). Demineralization using HCl and mixed HF-HCl solutions removes most carbonates and minerals with little formation of undesired precipitates (e.g., refs 22 and 28). Base extraction dissolves humic acid in high pH solutions, such as 0.1 M NaOH (e.g., ref 25). Thermal treatment burns off the organic matter insoluble in basic solution (e.g., kerogen)(15, 29-31),although complete removaloforganic matter is not often achieved due to polymerization of the organic matter when heated in the presence of oxygen (27). Alternating base and organic solvent extraction removes much of kerogen but is too erratic to quantify BCs (25). Oxidation of kerogen with alkaline H2O2 has been used for the isolation of the aeolian BCs from marine sediments (22, 23, 32), but the treatment may not be generally effective due to the resistance ofsome kerogen to peroxide oxidation (33). Alternative to all these techniques, the acidic dichromate oxidation appears to be an effective means for removing kerogen on the recognition that kerogen is rapidly oxidized by dichromate while BCs are only slowly oxidized (26, 27). In 0.1 M Cr 2O7 2- and 2 M H2SO4 at 50 °C, the observed half- life of kerogen removal is 6-180 h compared to 600-2000 h for BC removal (26). By carefully controlling reaction conditions and applying a correction for the loss of BCs, the dichromate oxidation technique appearsadequate to quantify the BC content in soils and sediments. This technique now has been applied to BC quantification for various samples (34, 35). Whereas geologists and limnologists focus attention on BC quantification and age dating, environmental scientists are more interested in BC adsorptive properties. In a recent study, Song et al. (36) adapted a three-step procedure from the literature (i.e., demineralization, base extraction, and acidic dichromate oxidation) to obtain various particle fractions from soils, including BC, with the goal to charac- terize the adsorptive power of the indigenous BC for NOCs. *Correspondingauthor phone: (479)575-6752;fax: (479)575-3975; e-mail: gsheng@uark.edu. † University of Arkansas. ‡ Nanjing University. § U.S. Geological Survey. Environ. Sci. Technol. 2004, 38, 4227-4232 10.1021/es034893h CCC: $27.50  2004 American Chemical Society VOL. 38, NO. 15, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 4227 Published on Web 06/29/2004