Professor of Research Emeritus
Martin Reinhard is Professor for Research emeritus at the Department of Civil and Environmental Engineering, Stanford University and recently Visiting Professor at the National University of Singapore. There he studied the occurrence, fate and transport of emerging contaminants in urban water bodies that serve as drinking water supplies.
Sorption of emerging contaminants including PFASs to sediments of an urban surface water body: the influence of sorbate structure on hysteresis
The sorption of emerging contaminants [ECs] including pharmaceuticals, personal care products, plasticizers, and polyfluoroalkyl substances [PFAS] to sediments of an urban freshwater body was studied concerning isotherm linearity, hysteresis, solution matrix, sediment organic content, (fOC), and compound properties. The ECs included caffeine [CF], salicylic acid [SA], acetaminophen [ACT], bisphenol A [BPA], N, N-diethyl-m-toluamide [DEET], triclosan [TCS], benzophenone-3 [BP-3]), and the three ubiquitous PFASs perfluorooctane sulfonate [PFOS], perfluorononanoic acid [PFNA], and perfluorodecanoic acid [PFDA]. The isotherms of organic compounds absorbing to soils and sediments are typically hysteretic, depend on solution conditions and exposure history. Fate and transport models often approximate contaminant sorption with a single KD value, which applies to linear and reversible, i.e., partitioning-like sorption but fails to reflect hysteretic sorption behavior. The latter has been documented in numerous studies, e.g., for PFASs and hydrocarbon compounds, and could be one of the reasons for the high KD of aged sediments. Depending on the compound structure, desorption can be highly hysteretic even though the initial absorption isotherm is practically linear. This behavior has been attributed to contaminant uptake by organic carbon domains with different properties: one which is soft and reversibly sorbing and one which is glassy and irreversibly sorbing. This study was designed to evaluate this concept by conducting consecutive adsorption-desorption cycles. The observed isotherms ranged from linear and reversible to completely hysteretic. DEET and PFOA sorbed reversibly (thermodynamic irreversibility index (TII) = 0). SA, BPA, CF and ACT, PFOS and PFNA showed a mixed behavior exhibiting partial hysteresis with TII ranging from 0.25 for SA to 0.98 for PFNA while TCS and BP-3 sorbed completely irreversibly (TII = 1). The initial absorption isotherms were highly linear (R2 > 0.95) in all cases except for PFOA (R2 = 0.73). The log KOCs correlated with the pH-dependent KOW, Log DOW (R2 = 0.87). Exceptions were the SA and CF, which sorbed more strongly than predicted based on the correlation, likely due to specific interactions of SA and CF with sediment components. SA forms strong surface complexes with iron oxides and CF forms charge-transfer complexes with aromatic compounds. The high concentration of divalent cations promoted the sorption of anionic compounds (SA), apparently by forming cation bridges. After the reversible fraction was removed by repeated desorption experiments, KDs were determined in a second absorption experiment. Except for DEET, which adsorbed reversibly, the KDs determined in the second absorption experiment increased significantly. The KDs increased with increasing TII and nearly doubled for BP-3 and TCS. The increase was attributed to the increased contaminant uptake by the glassy domain. The capacity of sediment to act as sinks as evaluated by extrapolating the remaining mass after indefinite desorption steps. Results are consistent with field data of aged sediment that exhibit significantly higher KDs (or KOC) than KD values determined in laboratory experiments with fresh sediments. KD values determined from single absorption experiments are therefore not adequate to evaluate sediments as sinks for organic contaminants.