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

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 (Kow) and organic-carbon content of soil/sediment, which is used to normalize sorption measurements (KOC). We revisit those concepts and their translation to IOC (Dow and DOC) 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.

Aqueous-Phase Temperature Swing Adsorption for Pesticide Removal.

Recently, activated carbon adsorption for water treatment regained substantial attention due to the emerging task to remove trace organic compounds such as pesticides. In many applications, especially in decentralized water treatment, one major drawback of adsorbents is their limited recyclability due to inadequate logistics or uneconomical reactivation. In this lab-scale study, we present the temperature swing adsorption in the aqueous phase that allows the in situ regeneration of fixed-bed adsorbers, and prove its technical feasibility. Complying with circular water economy principles, we eliminated the pivotal need for regular replacement and consumables by employing only clean water instead of dedicated regeneration solutions. Adsorption of the herbicide amitrole in aqueous solution on granular activated carbon was exothermic (Δ H = -14.4 ± 3.2 kJ mol-1 for T = 20-94 °C) and followed the Freundlich model. The proposed method consisting of a short counterflow flush with liquid water at 125 °C effectively regenerated the adsorbent. Hence, we obtained a cyclic steady state operation with breakthrough after 122 ± 14 bed volumes (at cout/ cin = 0.2), cycle-average rejection of 90 ± 1%, and water recovery of up to 78 ± 4%. No thermal aging of adsorbent was observed over the investigated 17 cycles.

Is sorption technology fit for the removal of persistent and mobile organic contaminants from water?

Persistent, Mobile, and Toxic (PMT) and very persistent and very mobile (vPvM) substances are a growing threat to water security and safety. Many of these substances are distinctively different from other more traditional contaminants in terms of their charge, polarity, and aromaticity. This results in distinctively different sorption affinities towards traditional sorbents such as activated carbon. Additionally, an increasing awareness on the environmental impact and carbon footprint of sorption technologies puts some of the more energy-intensive practices in water treatment into question. Commonly used approaches may thus need to be readjusted to become fit for purpose to remove some of the more challenging PMT and vPvM substances, including for example short chained per- and polyfluoroalkyl substances (PFAS). We here critically review the interactions that drive sorption of organic compounds to activated carbon and related sorbent materials and identify opportunities and limitations of tailoring activated carbon for PMT and vPvM removal. Other less traditional sorbent materials, including ion exchange resins, modified cyclodextrins, zeolites and metal-organic frameworks are then discussed for potential alternative or complementary use in water treatment scenarios. Sorbent regeneration approaches are evaluated in terms of their potential, considering reusability, potential for on-site regeneration, and potential for local production. In this context, we also discuss the benefits of coupling sorption to destructive technologies or to other separation technologies. Finally, we sketch out possible future trends in the evolution of sorption technologies for PMT and vPvM removal from water.

Linking the effect of temperature on adsorption from aqueous solution with solute dissociation.

Imperative decarbonization of water purification processes entails alternative regeneration methods for activated carbon. Regeneration based on changing dissociation equilibria, i.e. a major influencing factor on adsorption, usually requires the addition of acids/bases, but may also be triggered by temperature swing. Although adsorption and dissociation are both temperature-dependent phenomena, their conjunction has received little attention regarding trace organic compounds (TrOCs) and large temperature intervals, in particular above ΔT ≥ 50 ∘C. Therefore, we studied the adsorption equilibria of 16 TrOCs onto one granular activated carbon at temperatures ranging from 20 to 95 ∘C. The majority of compounds (12/16) exhibited an exothermic apparent adsorption enthalpy, while 3 out of 16 exhibited an endothermic apparent enthalpy. The range spanned from - 46 to + 50 kJ mol-1 (median at - 17 kJ mol-1). The possible origins of endothermic adsorption were discussed. A rationale of shifting pKa and thus changing dissociation of TrOCs was introduced and traded off against existing rationales, i.e. changing solute solubility, changing adsorption heat capacity, and saturation effects of the adsorbates. This knowledge may allow designing temperature swing adsorption processes that unlock the dissociation switch. The augmented process efficiency can thus provide the foundation for low-carbon emission, circular water purification processes.

The hydrothermal solution for self-sustaining drinking water purification at point of use.

Decentralized drinking water purification complements water supply in areas with unreliable or absent infrastructure. The exacerbating consequences of climate change in form of droughts and floods force remote households to tap various water sources. Hence, household-based processes must be versatile to cope with e.g. contaminated ground water and turbid surface waters. Purification at household level must be self-sustaining in order to enable independence from continuous supply of power and consumables. In this study, we design a process accordingly and we prove its technical feasibility on pilot scale. The two-step process utilizes gravity-driven ultrafiltration and activated carbon adsorption to purify water, whereas the process regeneration is accomplished by combining Temperature Enhanced Backwash and Temperature Swing Adsorption to clean the membrane and adsorber, respectively. We obtained stable operation over >40 days with a sustained flowrate of ∼5 Lh-1 and consistent product quality (turbidity ≤0.2 NTU) for all relevant water matrices: synthetic ground water, river water and even secondary effluent. We achieved a high removal of the spiked model micropollutant amitrole, environmental endocrine disruptors and bulk dissolved organics of ∼93%, >65% and ∼69%, respectively, at the optimal water recovery for river water of ∼80%. In-situ regeneration promises long-term, self-sufficient operation without exhaustion.

Temperature Enhanced Backwash.

Decentralized drinking water treatment is limited by supply of service, consumables, spare parts and in particular, power. Therefore, gravity-driven dead-end ultrafiltration is applied to purify surface water with high suspended solid loading. To obtain high flux in the long term, an effective membrane backwash is mandatory. Also, disinfection and cleaning is required regularly. Here we propose a new process coping with these particular challenges in decentralized water production: Temperature Enhanced Backwash. Herein, the membrane is backwashed at elevated temperature and corresponding steam pressure. A mathematical description of the Temperature Enhanced Backwash reveals that membrane pores are filled predominantly with liquid phase, irrespectively of whether membranes are charged with saturated steam or boiling liquid. A steam - water mixture is discharged at the module outlet suggesting evaporation at the end of the pores. This evaporation at membrane - fluid interface supposedly creates high volume fluxes shearing off potential fouling layers. Combined with gravity-driven filtration, the overall process potentially can cope with highly intermittent electrical power supply or even its absence. The methodology shows competitive cleaning efficacy compared to mechanical backwashing as demonstrated experimentally using silica nanoparticles, humic acid and river water.