Environ. Sci. Technol. 2000, 34, 4363-4369
Sorption of Hydrophobic Organic Compounds by Soil Materials: Application of Unit Equivalent Freundlich Coefficients ANA M. CARMO, LAKHWINDER S. HUNDAL, AND MICHAEL L. THOMPSON* Agronomy Department, Iowa State University, Ames, Iowa 50011-1010
Prediction of the fate of hydrophobic organic compounds (HOCs) in the soil environment is difficult due to the highly complex interaction between the organic molecules and the soil components. The Freundlich isotherm is the most commonly used model to describe retention of HOCs by heterogeneous sorbents. Because of inconsistent units of constants calculated from the Freundlich model, it is difficult to strictly compare the sorption of organic molecules by different sorbents. In this study, we modified the Freundlich equation to better assess the effects of organic carbon content and physical heterogeneity on the sorption of HOCs by soils and microaggregate fractions, using naphthalene and phenanthrene as probes. Isotherms for naphthalene and phenanthrene sorption were nonlinear and well described by the Freundlich equation. Unitequivalent Freundlich coefficients (K′f) were calculated by normalizing the liquid-phase equilibrium concentrations (Ce) to the supercooled liquid-state solubility (Sscl) of naphthalene and phenanthrene. The K′f values for both naphthalene and phenanthrene sorption correlated well with organic carbon (OC) content, but they did not show a systematic trend with N2 specific surface area (SSA). Isotherm deviation from linearity, as expressed by the Freundlich exponent (n), was larger for phenanthrene than for naphthalene, indicating that another sorption mechanism in addition to partitioning into soil organic matter was responsible for retention of phenanthrene. Despite the difference in n values, the K′f values (µg g-1) for naphthalene and phenanthrene sorption were nearly equal, indicating that a given sorbent has same capacity to sorb phenanthrene and naphthalene. Since sorption was nonlinear, this type of comparison could only have been made by using the modified Freundlich equation.
Introduction The mobility and bioavailability of hydrophobic organic compounds (HOCs) in soil/water systems is greatly affected by sorption to soil materials. Because clay minerals in soils are normally hydrated, sorption of HOCs by soil organic matter generally predominates over sorption to minerals (1). In the laboratory, the sorption behavior of soils and sediments under equilibrium conditions is often studied using sorption * Corresponding author fax: (515)294-3163; e-mail: mlthomps@ iastate.edu. 10.1021/es000968v CCC: $19.00 Published on Web 09/19/2000
2000 American Chemical Society
isotherms, which are graphic representations of the distribution of a given compound between a liquid phase and a solid phase at a constant temperature. Both linear (1-5) and nonlinear (6-14) sorption isotherms have been reported for sorption of HOCs to soils and sediments in aqueous systems. Linear isotherms are considered to primarily result from partitioning (dissolution) of HOCs into the three-dimensional matrix of soil organic matter (1, 4). There are two schools of thought regarding the cause of isotherm nonlinearity: (i) retention by heterogeneous soil organic matter that contains both “rubbery” (or soft) and “glassy” (or hard) polymer-like sorption domains (6-12) and (ii) the presence of small quantities of high-surface-area carbonaceous materials such as soot (13-15). Sorption capacity and/or nonlinearity have been shown to be a function of certain polarity indices of organic matter, e.g., elemental ratios (O/C) and aromatic or aliphatic character (8, 12, 16-19). However, the actual cause of isotherm nonlinearity is still not well understood (20, 21). Interpreting the shape of sorption isotherms is a commonly used macroscopic approach to assess the role of soil organic matter in the sorption of HOCs. But sorption behavior is further complicated by the physical complexity and heterogeneity of soil materials (6, 10). Soil components are not simply mixtures of discrete organic and mineral phases, but they are organized into aggregates of organomineral complexes. Finer particle-size fractions (silt and clay) tend to have a higher capacity to sorb HOC than does the whole soil due to their higher organic carbon (OC) content and higher surface area. They tend to be easily transported by water and accumulate down slope, thus having a large potential environmental impact by contributing to the mobility and toxicity of sorbed pollutants. The chemical nature of soil organic matter varies with aggregate size. For example, the total OC; nitrogen content; degree of aromaticity (22); distribution of alkyl, O-alkyl, aromatic, and carbonyl groups (23); polydispersivity; and thermal stability of organic matter (24) have been reported to vary among sand, silt, and clay fractions of soils. Thus, the mechanisms of HOC sorption by soil components could vary as a function of aggregate size due to differences in content as well as chemical composition of organic matter in the aggregates. Conceptual models that depict soil aggregates having multiple intraaggregate sorption domains have been introduced to explain the variety of sorption behaviors encountered in the laboratory studies (6, 10). Sorption of HOCs by heterogeneous sorbents such as soils and soil components is often described by the Freundlich equation. The Freundlich equation can be written as
Cs ) KfCen
(1)
where Cs is the amount of HOCs sorbed per unit mass of sorbent, Ce is the aqueous concentration of HOC in the equilibrium solution, the Freundlich coefficient (Kf) is an index of the sorption capacity of the sorbent, and the Freundlich exponent (n) denotes the degree of deviation from isotherm linearity. The Freundlich isotherm (eq 1) often fits data for HOCs sorption to soil materials and reflects heterogeneous sorption sites with a distribution of sorption energies (25). However, characterization of the sorption behavior of soil materials using the Freundlich isotherm is problematic. A dimensional analysis of eq 1 shows that the units of Kf vary nonlinearly with n (26-28). For example, if Ce is given in µg mL-1 and Cs is given in µg g-1, then Kf will be expressed in µg(1-n) g-1 mLn. Therefore, Kf values derived from sorption isotherms with different n values cannot be VOL. 34, NO. 20, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Particle Size, Chemical, and Mineral Characterization of the Bulk Soil Samples soil
clay (%)
silt (%)
sand (%)
CEC (cmol kg-1)
pH
dominant clay mineral
Zook Sparta Paleosol
43 15 34
42 32 50
15 53 15
38 9.8 32
5.9 6.2 7.3
smectite smectite smectite
directly used to compare sorption behavior of different sorbents. Although the absolute value of Kf equals the value of Cs (i.e., sorbent concentration) at Ce ) 1, the units of Kf differ significantly from those of Cs. In addition, the absolute value of Ce depends on choice of units so that Ce ) 1 could refer to a concentration outside the solubility limits of the solute. Despite the complex nature of its dimensions, Kf has been normalized to OC (Kfoc ) 100 × Kf /%OC) by various workers and has been used inappropriately to compare HOC sorption data from different sorbents (29-32). Such comparisons may lead to incorrect interpretations of the data or to erroneous predictions of HOC fate in the environment. In this paper, we provide a method for comparing the sorption characteristics of heterogeneous sorbents using unitequivalent Freundlich coefficients. We do this in two steps. First, we show how the Freundlich equation can be modified using the concept of reduced concentration (Cr). We show that by replacing Ce with Cr in eq 1, the modified Freundlich coefficient (K′f) has units that are independent of the values of n, thus allowing direct comparison of sorption data for different sorbents. Second, we use the modified Freundlich equation to compare the sorption of naphthalene and phenanthrene by two soil materials and their aggregate size fractions. Because soil organic matter characteristics are a function of aggregate size, we hypothesized that sorption of HOCs would also vary with aggregate size. This study is unique in its application of the modified Freundlich equation to compare the sorption of HOCs by heterogeneous soil components.
Experimental Section Sorbent Characteristics. The sorbents used in this study were samples from the A horizons of a Sparta soil (sandy, mixed, mesic Entic Hapludoll) and a Zook soil (fine, smectitic, mesic Cumulic Vertic Endoaquoll). The clay fraction of a Yarmouth-Sangamon Paleosol (YSP) (33) was also included because it had similar mineralogy to the clay fractions of the Sparta and Zook soils but very little organic matter. The particle-size distribution, cation exchange capacity (CEC), pH, and mineralogy of the soil materials were determined using standard procedures (34-37) and are reported in Table 1. The soil materials were sieved in their natural field-moist state to remove particles >2 mm. To collect microaggregates of various sizes, the samples were gently dispersed by either shaking overnight in distilled water (Zook and YSP) or by a 5-min sonification (Sparta) and were separated into effectivesettling diameter fractions of clay (
