Braida Washington J, White Jason C, Zhao Dongye, Ferrandino Francis J, Pignatello Joseph J
The Connecticut Agricultural Experiment Station, 123 Huntington Street, P.O. Box 1106, New Haven, Connecticut 06504-1106, USA.
Environ Toxicol Chem. 2002 Dec;21(12):2573-80.
Sorption-desorption kinetics play a major role in transport and bioavailability of pollutants in soils. Contaminant concentration is a potentially important factor controlling kinetics. A previous paper dealt with the effect of solute concentration on fractional uptake rates of phenanthrene and pyrene from a finite aqueous source. In this study we determined the effect of initial phenanthrene sorbed concentration (q(0)) on the fractional mass desorption rates from each of six soils to a zero-concentration solution, approximated by including a polymer adsorbent (Tenax) as a third-phase sink. The soils were preequilibrated with phenanthrene for 180 d. Consistent with theory, the fractional desorption rates determined by empirical curve fitting increased with q(0) provided the isotherm was nonlinear. After 500 to 600 d of desorption at the steepest possible concentration gradient, all soils retained a highly resistant fraction, which ranged from 4 to 31% of q(0), except for one soil at a high q(0). The highly resistant fraction decreased with increasing q(0), for nonlinear isotherm cases, but increased with q(0) for linear or nearly linear isotherm cases. Application of a nonlinear diffusion model, the dual-mode diffusion model (DMDM), to the nonresistant fraction gave reasonably good fits. The DMDM attributes the increase with concentration of the apparent diffusivity to a decrease in the proportion of sorbate occupying immobile sites (holes) in soil organic matter. The concentration-dependent term in the expression for the apparent diffusivity correlated with either of two indices that reflect the linearity of the sorption isotherm. Bunker C oil present in one soil acted as a partition domain. The findings of this study are consistent with heterogeneous models of soil organic matter, and indicate that concentration effects should be taken into account whenever desorption rate is important.
吸附-解吸动力学在土壤中污染物的迁移和生物有效性方面起着重要作用。污染物浓度是控制动力学的一个潜在重要因素。之前的一篇论文探讨了溶质浓度对菲和芘从有限水源中分数吸收速率的影响。在本研究中，我们确定了初始菲吸附浓度（q(0)）对六种土壤中每种土壤向零浓度溶液的分数质量解吸速率的影响，通过加入聚合物吸附剂（Tenax）作为第三相汇来近似模拟。土壤用菲预平衡180天。与理论一致，只要等温线是非线性的，通过经验曲线拟合确定的分数解吸速率随q(0)增加。在尽可能陡峭的浓度梯度下解吸500至600天后，除一种高q(0)的土壤外，所有土壤都保留了一个高抗性组分，其范围为q(0)的4%至31%。对于非线性等温线情况，高抗性组分随q(0)增加而降低，但对于线性或接近线性等温线情况，其随q(0)增加。将非线性扩散模型，即双模式扩散模型（DMDM）应用于非抗性组分，拟合效果相当好。DMDM将表观扩散系数随浓度的增加归因于占据土壤有机质中固定位点（孔）的吸附质比例的降低。表观扩散系数表达式中的浓度相关项与反映吸附等温线线性的两个指标中的任何一个相关。一种土壤中存在的邦克C油起到了分配域的作用。本研究的结果与土壤有机质的非均相模型一致，并表明在解吸速率很重要时应考虑浓度效应。
