Should Transformation Products Change the Way We Manage Chemicals?

When chemical pollutants enter the environment, they can undergo diverse transformation processes, forming a wide range of transformation products (TPs), some of them benign and others more harmful than their precursors. To date, the majority of TPs remain largely unrecognized and unregulated, particularly as TPs are generally not part of routine chemical risk or hazard assessment. Since many TPs formed from oxidative processes are more polar than their precursors, they may be especially relevant in the context of persistent, mobile, and toxic (PMT) and very persistent and very mobile (vPvM) substances, which are two new hazard classes that have recently been established on a European level. We highlight herein that as a result, TPs deserve more attention in research, chemicals regulation, and chemicals management. This perspective summarizes the main challenges preventing a better integration of TPs in these areas: (1) the lack of reliable high-throughput TP identification methods, (2) uncertainties in TP prediction, (3) inadequately considered TP formation during (advanced) water treatment, and (4) insufficient integration and harmonization of TPs in most regulatory frameworks. A way forward to tackle these challenges and integrate TPs into chemical management is proposed.

A Deep Insight into Perfluorooctanoic Acid Photodegradation Using Metal Ion-Exchanged Zeolites.

Treating perfluorooctanoic acid (PFOA) in an aqueous environment is problematic due to its low concentration and its high resistance to biological and chemical degradation. To tackle this challenge, combinations of pre-enrichment and photodegradation processes are promising solutions. In this work, we investigated metal ion-exchanged zeolites as adsorbents and photocatalysts for PFOA treatment. Among various transition metal ion-exchanged BEA zeolites, Fe-exchanged BEA (Fe-BEA) zeolites showed significant activity for the photodegradation of PFOA. The isolated iron species in Fe-BEA zeolite are responsible for PFOA photodegradation, whereas other iron species present from excess iron loading in the zeolite will lower its photocatalytic activity. Furthermore, it was proved via size exclusion tests using branched PFOA isomers that the photodegradation of PFOA took place on the internal surface rather than the external surface of Fe-BEA zeolite. Photodegradation of PFOA was also tested to be effective with Fe-exchanged BEA-type zeolites having various SiO2/Al2O3 ratios, but ineffective with FAU-type zeolites. The optimal Fe-BEA zeolite showed a sorption coefficient Kd of 6.0 × 105 L kg-1 at an aqueous phase PFOA concentration of 0.7 µg L-1 and a PFOA half-life of 1.8 h under UV-A irradiation. The presented study offers a deeper understanding of the use of metal ion-exchanged zeolites for photodegradation of PFOA.

Fe-zeolites for the adsorption and oxidative degradation of nitroaromatic compounds in water.

Nitroaromatic compounds (NACs) are prominent explosives. In this context, these toxic substances were released into the environment and cause long-lasting groundwater contamination. In preparation of a possible in-situ remediation, colloidal Fe-zeolites were investigated for their capabilities as adsorbents and oxidation catalysts. It was shown that the Fe-zeolites FeBEA35 and FeFAU55 are potent inorganic adsorbents for NACs and simultaneously capable of activating H2O2 as Fenton-like oxidation catalysts. Adsorption isotherms of 15 NACs on both zeolites were measured to evaluate the option of coupling adsorptive contaminant enrichment with oxidative degradation. The faujasite-type zeolite FeFAU55 showed a distinct S-type adsorption behaviour and reached significantly higher NAC loadings of > 20 wt%. For FeBEA35, L-type adsorption isotherms and maximum loadings qmax of about 4 wt% were obtained. Degradation of all NACs, monitored by nitrate formation, was observed. Apparent rate constants of the NACs with hydroxyl radicals in a homogeneous, stoichiometric Fenton reaction were related to the heterogeneous system to examine the role of adsorption on the oxidative degradation. Beneficial influence of the adsorption on the oxidation rates was identified. The results of this work open up promising prospects for future application of Fe-zeolites for the in-situ remediation of NAC-contaminated groundwater.

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.

Bottom-Up Synthesis of De-Functionalized and Dispersible Carbon Spheres as Colloidal Adsorbent.

Recent innovative adsorption technologies for water purification rely on micrometer-sized activated carbon (AC) for ultrafast adsorption or in situ remediation. In this study, the bottom-up synthesis of tailored activated carbon spheres (aCS) from sucrose as renewable feedstock is demonstrated. The synthesis is based on a hydrothermal carbonization step followed by a targeted thermal activation of the raw material. This preserves its excellent colloid properties, i.e., narrow particle size distribution around 1 µm, ideal spherical shape and excellent aqueous dispersibility. We investigated the ageing of the freshly synthesized, highly de-functionalized AC surface in air and aqueous media under conditions relevant to the practice. A slow but significant ageing due to hydrolysis and oxidation reactions was observed for all carbon samples, leading to an increase of the oxygen contents with storage time. In this study, a tailored aCS product was generated within a single pyrolysis step with 3 vol.-% H2O in N2 in order to obtain the desired pore diameters and surface properties. Adsorption characteristics, including sorption isotherms and kinetics, were investigated with monochlorobenzene (MCB) and perfluorooctanoic acid (PFOA) as adsorbates. The product showed high sorption affinities up to log (KD/[L/kg]) of 7.3 ± 0.1 for MCB and 6.2 ± 0.1 for PFOA, respectively.

Uniform and dispersible carbonaceous microspheres as quasi-liquid sorbent.

Functional colloidal carbon materials find various applications, including the remediation of contaminated water and soil in so-called particle-based in-situ remediation processes. In this study, uniform and highly dispersible micro-sized carbonaceous spheres (CS) were generated by hydrothermal carbonization (HTC) of sucrose in the presence of carboxymethyl cellulose (CMC) as environmentally friendly polyelectrolyte stabilizer. In order to ensure their optimal subsurface delivery and formation of a self-contained treatment zone, a narrow size distribution and low agglomeration tendency of the particles is desired. Therefore, the obtained CS were thoroughly characterized and optimized with respect to their colloidal properties which are a crucial factor for their application as quasi-liquid sorbent. The as-prepared uniform CS are readily dispersible into single particles in water as confirmed by digital microscopy and form stable suspensions. Due to their perfectly spherical shape, particle sedimentation in aqueous suspensions is well predicted by Stokes' law. High sorption coefficients on the synthesized CS KD,CS were determined for phenanthrene (up to log (KD,CS/[L kg-1]) = 5) and other hydrophobic groundwater contaminants. This confirms the application potential of the CS, which were prepared by an economic low-temperature process using sucrose as bio-based precursor, for generating in-situ sorption barriers for groundwater and soil remediation.

Efficient removal of trifluoroacetic acid from water using surface-modified activated carbon and electro-assisted desorption.

Trifluoroacetic acid (TFA) is a very persistent, very mobile substance (vPvM) with potential toxicity, and causes increasing environmental concerns worldwide. Conventional wastewater treatment strategies are inefficient for selective TFA removal in the presence of inorganic anions. Here we show that surface defunctionalized activated carbon felt (DeACF) carrying anion exchange sites exhibits an outstanding adsorption efficiency towards TFA thanks to introduced electrostatic attraction and enhanced interactions between hydrophobic carbon surface and CF3 moieties (qmax = 30 mg/g, Kd = (840 ± 80) L/kg at cTFA = 3.4 mg/L in tap water). Flow-cell experiments demonstrated a strongly favored TFA uptake by DeACF from tap water over Cl- and SO42- but a remarkable co-adsorption of the inorganic water contaminant NO3-. Electro-assisted TFA desorption using 10 mM Na2SO4 as electrolyte and oxidized ACF as anode showed high recoveries of ≥ 87% at low cell voltages (< 1.1 V). Despite an initial decrease in TFA adsorption capacity (by 33%) caused by partial surface oxidation of DeACF after the 1st ad-/desorption cycle, the system stability was fully maintained over the next 4 cycles. Such electro-assisted 'trap&release' approach for TFA removal can be exploited for on-site regenerable adsorption units and as a pre-concentration step combined with degradation technologies.

Adsorption of polar and ionic organic compounds on activated carbon: Surface chemistry matters.

Persistent and mobile organic compounds (PMOCs) are often detected micropollutants in the water cycle, thereby challenging the conventional wastewater and drinking water treatment techniques. Carbon-based adsorbents are often less effective or even unable to remove this class of pollutants. Understanding of PMOC adsorption mechanisms is urgently needed for advanced treatment of PMOC-contaminated water. Here, we investigated the effect of surface modifications of activated carbon felts (ACFs) on the adsorption of six selected PMOCs carrying polar or ionic groups. Among three ACFs, defunctionalized ACF bearing net positive surface charge at neutral pH provides the most versatile sorption efficiency for all studied PMOC types representing neutral, anionic and cationic compounds. Ion exchange capacity giving quantitative information of sorbent surface charges at specified pH is recognized as a frequently underestimated key property for evaluating adsorbents aiming at PMOC adsorption. A most recently developed prediction tool for Freundlich parameters in PMOC adsorption was applied and the prediction results are compared to the experimental data. The comparison demonstrates the so far underestimated importance of the sorbent surface chemistry for PMOC adsorption affinity and capacity. PMOC adsorption mechanisms were additionally investigated by adsorption experiments at various temperatures, pH values and electrolyte concentrations. Exothermic sorption was observed for all sorbate-sorbent pairs. Adsorption is improved for ionic PMOCs on AC carrying sites of the same charge (positive or negative) at increased electrolyte concentration, while not affected for neutral PMOCs unless strong electron donor-acceptor yet weak non-Coulombic interactions exist. Our findings will allow for better design and targeted application of activated carbon-based sorbents in water treatment facilities.

Photodegradation of Perfluorooctanesulfonic Acid on Fe-Zeolites in Water.

Perfluorooctanesulfonic acid (PFOS) decomposition in an aqueous environment remains a huge challenge because of its extreme chemical and biological inertness even when compared with other per- and polyfluoroalkyl substances (PFAS). In this work, we demonstrate for the first time a successful photochemical PFOS degradation by irradiation with 254 nm ultraviolet (UV) light after adsorption on µm-sized Fe(III)-loaded zeolites under ambient conditions with oxygen (O2) as the terminal oxidant. Twenty µM PFOS loaded on 0.5 g L-1 Fe-zeolites in aqueous suspension was degraded up to 99% within 96 h under acidic conditions (pH ≤ 5.5) in the presence of oxygen. Besides fluoride and sulfate, short-chain perfluorinated carboxylic acids (PFCAs) were identified and quantified as products. In addition, the effects of initial pH, catalyst dosages, and operation temperature on the degradation of PFOS were investigated. We also successfully applied the system to real groundwater samples where trace PFOS was present. Our results indicate that PFOS degradation is initiated by electron transfer from sulfonate to iron. The presented experimental study offers an option for a novel water remediation technology, comprising first a zeolite-based adsorption step followed by a step for photochemical regeneration of the adsorbent.

Mechanistic insights into fast adsorption of perfluoroalkyl substances on carbonate-layered double hydroxides.

Layered double hydroxide (LDH) with the metal composition of Cu(II)Mg(II)Fe(III) was prepared as an adsorbent for fast adsorption of perfluorooctane sulfonate (PFOS) and perfluorooctanoate (PFOA). 84% of PFOS and 48% of PFOA in relation to the equilibrium state were adsorbed in the first minutes of contact with 0.1 g/L of suspended µm-sized LDH particles. The adsorption mechanisms of PFOS and PFOA on the CuMgFe-LDH were interpreted. Hydrophobic interactions were primarily responsible for the adsorption of these compounds in accordance with the different adsorption affinities of long-chain (C8, Kd = 105 L/kg) and short-chain (C4, Kd = 102 L/kg) perfluorinated carboxylic acids. PFOA adsorption on CuMgFe-LDH was strongly suppressed under alkaline conditions while PFOS uptake was only slightly affected in the pH range from 4.3 to 10.7, indicating a significant role of electrostatic interactions for PFOA adsorption. The adsorption of PFOS and PFOA was rather insensitive to competition by monovalent anions. The previously reported 'memory effect' of calcined CuMgFe-LDH for sorption of organic anions was not confirmed in the present study. Spent CuMgFe-LDH could be easily regenerated by extraction with 50 vol% methanol in water within 1 h and maintained a high PFOS removal in subsequent usage cycles.

What is specific in adsorption of perfluoroalkyl acids on carbon materials?

Various activated carbon products show wide variability in adsorption performance towards perfluoroalkyl acids (PFAAs) and predictive tools are largely missing. In order to gain a better understanding on the adsorption mechanisms of PFAAs, perfluorooctanoic acid (PFOA) was compared with its fluorine-free analogon octanoic acid (OCA) as well as phenanthrene (nonionic) in terms of their response towards changes in carbon surface chemistry. For this approach, a commercial activated carbon felt (ACF) with high content of acidic surface groups was modified by amino-functionalisation as well as thermal defunctionalisation in H2 (yielding DeCACF). While improvement by amino-functionalisation was moderate, defunctionalisation drastically enhanced adsorption of PFOA and other PFAAs. In comparison, OCA and phenanthrene were much less affected. Electrostatic interactions and charge compensation provided by positively charged surface sites (quantified by their anion exchange capacity) are obviously more crucial for PFAAs than for common organic acids (such as the tested OCA). A possible reason is their exceptionally strong acidity with pKa < 1. Nevertheless, at the best modified ACF material (DeCACF) the sorption coefficients (Kd) for PFOA and perfluorooctylsulfonic acid (PFOS) at environmentally relevant concentrations reach the range of 107 L/kg which is outstanding. DeCACF provides a surface with overall low polarity (low O-content), low density of acidic sites causing electrostatic repulsion, but nevertheless a sufficient density of charge-balancing sites for organic anions. The results of the present study contribute to an optimized selection of adsorbents for PFAA adsorption from water considering also various salt matrices and the presence of natural organic matter.

H/D-isotope fractionation due to aqueous phase diffusion - Deuterated hydrocarbons revisited.

Diffusive isotope fractionation of non- and perdeuterated benzenes and toluenes in aqueous solution was investigated. The experimental method was based on a Stokes diaphragm cell. The isotope composition of diffusate and retentate was found to be identical within a range of uncertainty of ±5‰ for benzene and ±10‰ for toluene. These data are consistent with a previous fractionation study using phase-transition kinetics as the potentially fractionating step. The present study contributes to strengthening the data base for diffusive isotope fractionation of organic compounds in aqueous solution. According to the presented data, diffusion of naturally occurring, monodeuterated organic compounds does not significantly affect their hydrogen isotope pattern.

Interaction of zero-valent iron and carbonaceous materials for reduction of DDT.

Dechlorination of dichlorodiphenyltrichloroethane (DDT) as a model compound was performed with zero-valent iron (micro-ZVI and nano-ZVI) as reductant and carbonaceous adsorbents as sink and catalyst in water. DDT is rapidly converted to dichlorodiphenyldichloroethane (DDD) in direct contact with ZVI. However, up to 90% of the DDD is transformed into non-identified, most likely oligomeric products. There is no indication of dechlorination at the aromatic rings. DDT is still rapidly dechlorinated when it is adsorbed on carbonaceous adsorbents, even though ZVI particles have no direct access to the adsorbed DDT. The carbonaceous materials function as adsorbent and catalyst for the dechlorination reaction at once. From electrochemical experiments, we deduced that direct physical contact between ZVI particles and the adsorbent is essential for enabling a chemical reaction. Electron conduction alone does not effect any dechlorination reaction. We hypothesize hydrogen species (H∗) which spill from the ZVI surface to the carbon surface and initiate reductive transformations there. The role of carbonaceous adsorbents is different for different degradation pathways: in contrast to hydrodechlorination (reduction), adsorption protects DDT from dehydrochlorination (hydrolysis).

Sulfidation of ZVI/AC composite leads to highly corrosion-resistant nanoremediation particles with extended life-time.

Nanoscale zero-valent iron (nZVI) is a powerful reductant for many water pollutants. The lifetime of nZVI in aqueous environments is one of its limitations. Sulfidation of the nZVI surface by reduced sulfur species is known to significantly modify the particle properties. In the present study we examined various post-synthesis sulfidation methods applied on Carbo-Iron, a composite material where iron nanostructures are embedded in colloidal activated carbon (AC) particles. In such cases, where ZVI is surrounded by carbon, sulfidation largely inhibits the anaerobic corrosion of ZVI in water whereas its dechlorination activity was slightly increased. Even at a very low molar S/Fe ratio of 0.004 a strong decrease of the corrosion rate by a factor of 65 was achieved, while concurrently dechlorination of tetrachloroethene (PCE) was accelerated by a factor of three compared to the untreated particles. As a consequence, over 98% of the reduction equivalents of the sulfidated ZVI were utilized for the reduction of the target contaminant (33 mg L-1 PCE) under simulated groundwater conditions. In a long-term experiment over 160 days the extended life-time and the preservation of the reduction capacity of the embedded ZVI were confirmed. Reasons for the modified reaction behavior of Carbo-Iron after sulfidation compared to previously studied nZVI are discussed. We hypothesize that the structure of the carbon-embedded iron is decisive for the observed reaction behavior. In addition to reaction rates, the product pattern is vastly different compared to that of sulfidated nZVI. The triple combination of ZVI, AC and sulfur makes the composite particle very suitable for practical in-situ applications.

Comment on "Mistakes and inconsistencies regarding adsorption of contaminants from aqueous solution: A critical review, published by Tran et al. [Water Research 120, 2017, 88-116]".

Correct treatment of adsorption thermodynamics requires consistent handling of adsorption isotherms and the corresponding sorption coefficients. The present comment points to some inconsistencies and misunderstandings in a recent review article.

Isotope fractionation in phase-transfer processes under thermodynamic and kinetic control - Implications for diffusive fractionation in aqueous solution.

Diffusive isotope fractionation of organic compounds in aqueous solution was investigated by means of liquid-liquid and liquid-gas partitioning experiments under kinetic control. The two-film model was used to describe phase-transfer kinetics. It assumes the diffusion of solutes across a stagnant water boundary layer as the rate-controlling step. For all investigated solutes (benzene-D0 and -D6, toluene-D0, -D5, and -D8, cyclohexane-D0 and -D12), there was no significant observable fractionation effect between nondeuterated and perdeuterated isotopologues, resulting in a ratio of diffusion coefficients Dlight: Dheavy=1.00±0.01. In addition, isotope fractionation due to equilibrium partitioning of solutes between water and n-octane or gas phase was measured. The deuterated compounds are more hydrophilic than their light isotopologues in all cases, giving rise to fractionation coefficients αHpart=Koctane/water,H: Koctane/water,D=1.085 to 1.15. Thus, thermodynamic fractionation effects are much larger than diffusion fractionation effects. Methodical and environmental implications of these findings are discussed.

What Controls Selectivity of Hydroxyl Radicals in Aqueous Solution? Indications for a Cage Effect.

The oxidation of three isotopologues of methylcyclohexane (MCH: C7H14, C7D14, c-C6D11-CH3) by OH-radicals (•OH) in aqueous solution was investigated. Intermolecular and intramolecular H/D kinetic isotope effects (KIE = kH:kD) for the abstraction of H and D atoms by •OH were measured. These KIEs reflect inter- and intramolecular selectivities of hydrogen abstraction, i.e., the selection of •OH attack on carbon-hydrogen bonds in different molecules and in different positions of one molecule, respectively. The intermolecular selectivity of •OH attack in aqueous solution is largely discriminated against in comparison with the intramolecular selectivity. The observed extent of discrimination cannot be explained by partial diffusion control of the overall reaction rates. A cage model, where •OH and hydrocarbon molecules are entrapped in a solvent cage, is more appropriate. The much higher intramolecular KIEs compared to the intermolecular KIEs of the same chemical reaction, R-H + •OH → R• + H2O, indicate a high degree of mobility of the two reaction partners inside of the solvent cage. This mobility is sufficient to develop an intramolecular selectivity comparable to that of gas-phase reactions of •OH. Furthermore, literature data on KIEs of H-abstraction by •OH in aqueous and gas phases are discussed. There is a general tendency toward lower selectivities in the aqueous phase.