Environ. Sci. Technol. 2006, 40, 3267-3272
Competitive Sorption of Pyrene on Wood Chars X I L O N G W A N G , * ,†,‡ T . S A T O , ‡ A N D B A O S H A N X I N G * ,† Department of Plant, Soil and Insect Sciences, Stockbridge Hall, University of Massachusetts, Amherst, Massachusetts 01003, and Institute of Nature and Environmental Technology, Kanazawa University, Kakuma, Kanazawa, Ishikawa 920-1192, Japan
Sorption isotherms of pyrene on original and heat-treated wood chars were examined to understand its sorption behavior. Pyrene in single-solute systems had nonlinear isotherms. Polanyi-based dual-domain model fit sorption data well, and the model results showed that the adsorption component dominated pyrene sorption by original char at all aqueous concentrations. In contrast, this adsorption component contributed a much lower fraction to the total sorption by the heat-treated char, and dominated only at low solute concentrations; with increasing concentration, partitioning became a predominant contributor to the total sorption. Competitive effect of four cosolutes, phenanthrene (Phen), benzo[a]anthracene (BaA), 2,2methylene-bis (4-methyl-6-tert-butylphenol) (MMBP), and phenol on pyrene sorption by original and treated chars was examined to understand the underlying mechanism of competition. Hydrophobicity (adsorbability) and molecular size of competitors played an important role in competition with pyrene by both chars, suggesting the direct competition for sorption sites and pore blockage mechanism. Competitive sorption results indicated that the fate and transport of hydrophobic organic chemicals (e.g., pyrene) could be strongly affected in the presence of coexisting organic contaminants with high hydrophobicity and large molecular size, thereby, enhancing the mobility and leachability of these chemicals.
Introduction The partial alteration and blackening of plant-derived materials by fires are generally referred to as charring (1). Black carbon (BC) is produced by biomass burning (charring) and includes char and soot. Both char and soot have high organic carbon (OC) content and are chemically heterogeneous (2). As reported, BC can accumulate in soils or ecosystems where fires are frequent, accounting for 10-45% of total organic carbon (3). Although some studies showed that BC can be degraded by microorganisms (4), it was estimated that on a global scale, combustion currently produces more than 5.0 × 107 t chars per year, and the largest fraction (>80%) is from burning vegetation (5). Black carbon attracts great interest for environmental studies because it is viewed as a super-sorbent, with a sorption * Address correspondence to either author. Phone: (413) 5452860 (X.W.); (413) 545-5212 (B.X.). Fax: (413) 545-3958 (X.W.); (413) 545-3958 (B.X.). E-mail:
[email protected] (X.W.);
[email protected](B.X.). † University of Massachusetts. ‡ Kanazawa University. 10.1021/es0521977 CCC: $33.50 Published on Web 04/08/2006
2006 American Chemical Society
capacity much higher than that of humic acids (6-7). Since BC is chemically and structurally different from humic acids, its interactions with hydrophobic organic contaminants (HOCs) may be mechanistically different from the classic sorption theories used for soil/sediment organic matter (SOM) (8). Previous sorption studies on BC primarily focused on single-solute systems, information on competition from cosolute introduction however is scant (7). In addition, char has a microporous network-like structure, consisting primarily of graphene (polycyclic aromatic) sheets arranged in a highly disordered fashion (9-10). As such, accessibility to sorption sites in pore network is likely dependent on molecular size of contaminants. Sander and Pignatello (7) elegantly showed competitive sorption for relatively small molecules such as nitrobenzene and toluene on a wood char; they attributed the strong competition between nitrobenzene and toluene to π-π interactions between nitrobenzene and graphene units in the char. But, competition, as influenced by steric effect of competitors, has not been systematically studied (10). Because of the ubiquitous presence of wood chars in the environment and recurrent forest fires (prescribed and accidental), the existing chars would inevitably be further heated, modified, or even burned again under oxygendeficient conditions without any temperature control. Moreover, fires are now commonly employed to prevent woody plant encroachment into grasslands and reforestation (11). However, the properties of charred materials generated by forest fires, especially the modified chars by further forest fires, and their influence on SOM and sorption affinity for organic contaminants have received little attention until recently (10-11). A recent study examined the single-solute and bi-solute competitive sorption of aromatic compounds by a wood char (7), but the chemical composition and functionality of the char used were not detail-characterized. The primary objectives of this study are to (1) examine changes of wood char (e.g., porosity and functional groups) after heat-treatment and their impacts on pyrene sorption behavior; (2) determine the roles of molecular size and hydrophobicity of competitors on pyrene sorption by original and heat-treated chars; and (3) elucidate mechanisms of competition resulting from competitor introduction against pyrene sorption by the chars. To achieve our goals, heat treatment of a wood char under laboratory conditions was conducted to yield an artificially generated material, attempting to examine the alteration of wood char induced by recurrent forest fires.
Materials and Methods Sorbents and Sorbates. A natural wood char was purchased from a supermarket. It was put in a ceramic cup, covered with a lid of appropriate size, then the cup was placed over a flame (fire) under oxygen-limited condition for 3 h, following the method by Chun et al. (12) for further charring. Such a treatment would partially remove surface O and/or O functionality of char (10), possibly changing its polarity and sorption affinity. Both the original and treated chars were air-dried, ground, and homogenized to pass through a 250 µm sieve. BET-based porosity of the samples was determined by a surface area analyzer (Coulter SA-3100, Coulter Co.) using N2 as sorbate at 77 K following the method of Wang et al. (13). Micropore (
