Sorption irreversibility of 1,4‐dichlorobenzene in two natural organic matter–rich geosorbents

Abstract

Hysteresis, a frequently observed phenomenon in sorption studies, is inconsistent with the key assumption of sorption reversibility in most fate and bioavailability models. Therefore, a study of the underlying causes of hysteresis is essential. Carbon-radiolabeled 1,4-dichlorobenzene (DCB) isotope tracer exchange was carried out at select points along the isotherms of DCB in a brown coal and a peat soil, holding total DCB concentration constant. Tracer exchange was performed both in the forward (sorption) and reverse (desorption) directions at the bulk sorption points and in the desorption direction at the corresponding bulk desorption points. Bulk DCB isotherms showed concentration-dependent hysteresis. However, tracer reequilibration in all cases was consistent with free exchange between sorbed and aqueous-phase molecules. These results rule out common experimental artifacts and demonstrate that sorption of bulk DCB is truly hysteretic (i.e., irreversible). The differences in rates between bulk and tracer sorption and desorption are consistent with the coupling of bulk DCB diffusion to other processes that retard equilibration, which we assign to matrix swelling or shrinking. Hysteresis is attributed to matrix deformation--specifically, to inelastic expansion and creation of voids accommodating sorbate molecules in the matrix, which leads to enhanced affinity in the desorption step. Comparing the results to previous results for naphthalene in the coal, we find that irreversible effects are similar for DCB and naphthalene in the coal but differ for DCB between the two sorbents. An explanation based on the different physical properties of these sorbents is provided. Solid-phase extraction of equilibrated DCB with Tenax revealed a highly desorption-resistant fraction. While too small to account for the observed hysteresis, this fraction may represent molecules that become trapped as the matrix collapses and simultaneously stiffens during abrupt desorption.

Biochar Soil Additions Affect Herbicide Fate: Importance of Application Timing and Feedstock Species

“…14 Irreversibility in sorption of organic compounds can involve sorption intraparticle diffusion, pore deformation during sorption, or entrapment into micropores. 43,44 Dissipation of herbicides in the soil amended with biochars. Figure 3 shows the dissipation curves of CMZ and BYP in unamended soil and BC-amended soils.…”

Long-term experiments to investigate irreversibility in sorption of pesticides to soil

“…14 C and 12 C) in the system. This assumption is supported by the research of Sander & Pignatello [17] who showed complete reversibility of sorption at low concentrations in their long-term isotope exchange experiments. In this circumstance and assuming that all sorbed 14 C-pesticide is available for exchange, the K d will be maintained at the pre-exchange value and there will be a net transfer of 12 C-pesticide out of solution into the sorbed phase and a numerically equivalent net transfer of 14 C-pesticide in the opposite direction.…”

Sources, Interactions, and Ecological Impacts of Organic Contaminants in Water, Soil, and Sediment: An Introduction to the Special Series

“…Further research is needed to characterize sorption mechanisms, the infl uence of conditions such as pH, and the contribution of mineral phases and diff erent fractions of natural organic matter (Pan et al, 2009). • Th e last stages of sorption and desorption to soil particles of even simple apolar compounds occur on very long time scales (Pignatello and Xing, 1996), and their isotherms can display true hysteresis (Sander and Pignatello, 2009). Th ese behaviors are poorly predictable at the present time.…”

“…Further research is needed to characterize sorption mechanisms, the influence of conditions such as pH, and the contribution of mineral phases and different fractions of natural organic matter (Pan et al, 2009). The last stages of sorption and desorption to soil particles of even simple apolar compounds occur on very long time scales (Pignatello and Xing, 1996), and their isotherms can display true hysteresis (Sander and Pignatello, 2009). These behaviors are poorly predictable at the present time. Detection and quantification of “emerging” and wastewater‐related contaminants and their metabolites in solid environmental matrices remain a challenge and are central to evaluation of their fate and risk. More studies are needed to assess the impact of waste disposal practices on groundwater recharge, particularly in areas where groundwater is the major source of water supply.…”