Sorption and Mobility of Charged Organic Compounds: How to Confront and Overcome Limitations in Their Assessment

Abstract

Permanently charged and ionizable organic compounds (IOC) are a large and diverse group of compounds belonging to many contaminant classes, including pharmaceuticals, pesticides, industrial chemicals, and natural toxins. Sorption and mobility of IOCs are distinctively different from those of neutral compounds. Due to electrostatic interactions with natural sorbents, existing concepts for describing neutral organic contaminant sorption, and by extension mobility, are inadequate for IOC. Predictive models developed for neutral compounds are based on octanol–water partitioning of compounds ( K ow ) and organic-carbon content of soil/sediment, which is used to normalize sorption measurements ( K OC ). We revisit those concepts and their translation to IOC ( D ow and D OC ) and discuss compound and soil properties determining sorption of IOC under water saturated conditions. Highlighting possible complementary and/or alternative approaches to better assess IOC mobility, we discuss implications on their regulation and risk assessment. The development of better models for IOC mobility needs consistent and reliable sorption measurements at well-defined chemical conditions in natural porewater, better IOC-, as well as sorbent characterization. Such models should be complemented by monitoring data from the natural environment. The state of knowledge presented here may guide urgently needed future investigations in this field for researchers, engineers, and regulators.

Assessing the Persistence and Mobility of Organic Substances to Protect Freshwater Resources

“…41 However, this parameter just considers the solubility of the charged and neutral species at a specific pH, and not the pH dependence of the ionic interactions with the soil, so it is not appropriate as a proxy for log K OC . 61 Nevertheless, it may still play a role as a screening parameter for prioritizing what charged or ionizable substances are potentially mobile. 61 Herein the performance of using various screening parameters for half-lives (e.g., readily biodegradable tests, QSARs) and K OC (i.e., using K OW and D OW values) is investigated empirically to assess their performance as screening parameters to identify PMT/vPvM substances.…”

“…61 Nevertheless, it may still play a role as a screening parameter for prioritizing what charged or ionizable substances are potentially mobile. 61 Herein the performance of using various screening parameters for half-lives (e.g., readily biodegradable tests, QSARs) and K OC (i.e., using K OW and D OW values) is investigated empirically to assess their performance as screening parameters to identify PMT/vPvM substances.…”

Persistent, mobile, and toxic plastic additives in Canada: properties and prioritization

“…[33][34][35] With that said, log K oc can be highly variable, especially for many ionic compounds, based on pH, ion exchange interactions with soil minerals, and weathering effects, to name a few. [36][37][38] While a log K oc value <4.0 over a pH range of 4-9 is the current metric for mobility assessment by UBA, discussions are underway to reassess the mobility criteria, namely reducing this log K oc value from <4 to <3, as a log K oc value <4 can include several substances which can be highly adsorbed, and thus unlikely to be mobile. 32,33 Nevertheless, log K oc serves as the best measure of mobility for a wide range of compounds.…”

“…Although log K ow and log D ow do not account for the soil sorption properties that log K oc does, they do provide a measure of the hydrophilicity of the compound and thereby its sorption properties. 21,37 4.3.2 Limitations. An important limitation of this study was that the substances evaluated were all known plastic additives registered for use on the Canadian Domestic Substance list that have been evaluated as persistent and toxic.…”

Screening of Contaminants of Emerging Concern in Surface Water and Wastewater Effluents, Assisted by the Persistency-Mobility-Toxicity Criteria

“…Concerning the mobility criteria, a substance was considered vM if the lowest partition coefficient (log D) in the pH range 4-9 is less than 3.5 and M when log D is between 3.5 and 4.5 [38]. Although the log K oc has also been suggested as a better estimation, this parameter is anyway derived from log D in QSARs, thereby requiring improved models for ionic substances to be implemented in the future [39]. As set for QSAR Toolbox estimations, a substance was considered vP if the biological oxygen demand (BOD) was equal to or lower than 30% and P if BOD ranged between 30 and 40% and/or biodegradation probability was lower than 0.5 (experimental data and BIOWIN 5 and 6 models) [37,40].…”