A DIFFERENTIAL HYSTERESIS COEFFICIENT FOR CHARACTERIZING THE ADSORPTIVE-DESORPTIVE BEHAVIOUR OF CONTAMINANTS IN SEDIMENTS

摘要

In this work, a new hysteresis coefficient is proposed and its usefulness for determining the adsorption-desorption behavior of pollutants on sediments and soils is examined. In sediment pollution and remediation, the adsorption and desorption play a key role on pollutants availability and transport. For several pollutants and solid matrices such as sediments, the desorption pathway is different from that of the adsorption. This phenomenon is known as hysteresis. Here, we define a hysteresis coefficient CH as the ratio of the slope (derivative) of the adsorption curve and the slope of the desorption curve in a given point (C_j, q_j) of interest. We demonstrate that: ⅰ) CH is dimensionless; ⅱ) when hysteresis is not important, CH=1, i.e., the adsorption is reversible; ⅲ) when hysteresis is important, CH>1, i.e., the adsorption is irreversible; ⅳ) the greater the hysteresis, the greater the CH; ⅴ) the CH can be determined at any convenient point (C_j, q_j) of interest of the adsorption curve, peforming a few consecutive desorption steps, and there is no need to determine the full adsorption-desorption cycle; ⅵ) the CH is consistent (i.e. C_H ≥1) for the most common isotherm models (linear, Freundlich and Langmuir); ⅶ) there exist also analytical, particular, simple equations for finding CH for the linear, Freundlich and Langmuir isotherm models; ⅷ) the CH shows several advantages over the well known hysteresis indices defined by Huang and Weber (1997) and Ma et al. (1993). Using experimental data from refereed literature, we also show that the CH provides a quantitative basis for ⅰ) comparing the irreversibility of the adsorption of different individual pollutants on a given soil; ⅱ) comparing the irreversibility of the adsorption of a given pollutant on different soils; ⅲ) determining the effect of aging and weathering on adsorption irreversibility. Overall, the CH allows for the quantitative determination of pollutant availability and this, in turn, could be a valuable tool for estimating the remediation potential of polluted sediments.