Ya-Qi
Zhang
ab,
Stefan
Stolte
ab,
Gizem
Alptekin
a,
Alica
Rother
a,
Michael
Diedenhofen
c,
Juliane
Filser
d and
Marta
Markiewicz
*ab
aUFT – Center for Environmental Research and Sustainable Technology, Department of Sustainable Chemistry, University of Bremen, Leobener Str. 6, D-28359 Bremen, Germany. E-mail: marta.markiewicz@tu-dresden.de
bInstitute of Water Chemistry, Technische Universität Dresden, Berg Str. 66, D-01069 Dresden, Germany
cBIOVIA, Dassault Systèmes Deutschland GmbH, D-51379 Leverkusen, Germany
dUFT – Center for Environmental Research and Sustainable Technology, Department of General and Theoretical Ecology, University of Bremen, Leobener Str. 6, D-28359 Bremen, Germany
First published on 17th September 2020
Liquid organic hydrogen carriers (LOHCs) are an energy system that can be used to store and transport hydrogen under standard temperature and pressure chemically bound to a carrier. The LOHC systems show advantages over conventional energy systems (recyclability, higher sustainability and lower emissions) and other hydrogen-based systems (lower loses, ease of handling and higher safety), and are applied in stationary and mobile applications worldwide. The scale and type of use indicate that the release of LOHCs to the environment can be expected. Yet, their behaviour and fate have not been investigated especially with regard to assessment of exposure, mobility and possibility to reach surface water, groundwater or drinking water sources. To investigate that we studied the mobility of thirteen technologically promising LOHC candidates including indole, quinaldine, carbazole derivatives, benzyltoluene and dibenzyltoluene, and their (partially) saturated forms in soil, for the first time. The substances were classified into mobility classes based on their organic carbon–water partition coefficients (Koc) determined via in silico models and HPLC screening. The logKoc values increased in the order indoles < quinaldines < carbazole derivatives < benzyltoluenes < dibenzyltoluenes covering a full spectrum of mobility scale (from highly mobile to immobile). The behaviour of exemplary LOHC system – quinaldine including H2-unsaturated, partially and fully saturated forms – was further assessed by investigating the soil-water partition coefficients (Kd) via adsorption batch equilibrium and column leaching test. The study showed that some LOHCs can be expected to be very mobile in soils and have the potential to reach groundwater.
LOHC systems usually consist of a tandem of organic molecules, one of which is saturated (H2-rich, loaded), and the other is unsaturated (H2-lean, unloaded or spent). The compounds forming the LOHC tandem can be reversibly converted into each other by catalytic hydrogenation (e.g., catalysts Ru/Al2O3, 10–50 bar,8 150 °C (ref. 10)) of the unsaturated form and dehydrogenation (e.g., catalysts Pt/Al2O3, 1–5 bar,8 130–300 °C (ref. 8, 10 and 11)) of the saturated form (Fig. 1).9 The LOHC chemicals are not consumed in the process of energy generation and can be subject to multiple hydrogenation and dehydrogenation cycles, which is very different from fossil fuels.2 Due to the similarities in the physicochemical properties of LOHCs to those of fossil fuels,2,3 they can be implemented using the existing infrastructure, such as ships, ports,2 oil tanks, pipelines, and fuelling stations,1,12 that were developed for the transport, distribution and processing of fossil fuels. Use of existing infrastructure is economically attractive due to the lower investment costs3,4 and a smooth transition between conventional and LOHC energy systems.1,3
Indole,3,8 quinaldine,3,8 and carbazole derivatives,2,3,8,12 as well as isomeric mixtures of benzyltoluene or dibenzyltoluene13,14 with hydrogen storage capacities ranging from 5 to 7 wt%,3,4 have been recently proposed as the most promising potential candidates. The incorporation of nitrogen into the cyclic ring of LOHCs reduces the dehydrogenation enthalpy3 and improves the thermodynamics and kinetics of dehydrogenation.4,8 This renders nitrogen-substituted heterocycles particularly promising candidates.3 The technological advantages and disadvantages related to particular LOHC candidates, especially in automotive applications, have recently been reported in detail,3,12–16 showing that different LOHC systems can be chosen to fulfil requirements of different applications.
The considerable application potential of LOHC systems opens the global markets to this technology and relevant projects and product developments have been initiated in 2011, 2013, and 2014 by Chiyoda Corporation (Japan), Hydrogenious LOHC Technologies (Germany), and Hynertech (China), respectively.17 Accordingly, several large-scale pilot applications were launched around the world recently, for instance in Japan (global hydrogen supply chain),18 Germany (off-grid power supply8,19 and railway transport powered by LOHC20), and China (fuel cell bus fuelled by LOHC21 and mobility infrastructure22). In addition, the prospect of using LOHC technology in underground mining equipment and stationary power applications in South Africa has been recently announced.10 Moreover, some LOHC chemicals like benzyltoluene and dibenzyltoluene are already used, although on smaller scale, as heat transfer liquids23 while others are components of fossil fuels (e.g., quinaldine or unsubstituted carbazole).24
Usually a tank containing approximately 80 kg of LOHCs is required to achieve a driving range of approx. 700 km.8 The full-scale application of this technology and the complete replacement of fossil fuels would thus require approx. 2 × 1010–3 × 1010 tons of LOHCs to be handled, processed, stored and transported to satisfy the current energy demand worldwide.4,11 Due to such large volume in application and transport, the consequence of LOHC release (e.g., through leaks and accidental spills during production, transport and fuelling) into the environment should be considered. More specifically, the environmental hazard potential of these chemicals should ideally be known before the technology enters widespread commercial use to assure that the LOHC systems of both the best technological performance and least harm to the environment are chosen. However, experimental data regarding the environmental behaviour (in terms of mobility, adsorption and fate) of the LOHCs are still too scarce to make a reliable assessment.
There are several reasons that make the assessment of environmental impacts of the LOHC chemicals particularly interesting besides the large application scale. Firstly, many LOHC chemicals are organic bases with (predicted) pKb (base dissociation constant) values within environmentally relevant range, meaning they will be at least partially positively charged. Ionic and ionisable chemicals are outside the applicability domain of most models designed to predict affinity to soils.25,26 Consequently, these models deliver inadequate predictions, sometimes several orders of magnitude different from the measured value.27 Secondly, the lack of experimental data for Kow (n-octanol/water partition coefficient), Dow (pH-dependent distribution coefficient) or pKb that are needed for the models means that these values have to be estimated as well, which additionally increases the uncertainty of prediction. Lastly, our recent studies have indicated that some of the LOHC chemicals investigated here are poorly biodegradable (i.e. potentially persistent) or show considerable acute aquatic toxicity11 and chronic soil toxicity.28 Therefore, their continued release to the environment would lead to increasing concentrations which might adversely affect the biota.
Recently, in addition to Persistent, Bioaccumulative and Toxic (PBT) substances, a new class of substances of high concern was identified by regulatory agencies (German Environment Agency, UBA – Das Umweltbundesamt) namely Persistent, Mobile and Toxic (PMT) substances.29 These chemicals are polar and exhibit lower affinities to organic matter and solids in soils and sediments, as a result they might move through aquifers and various natural (e.g., riverbank and soil layers) or artificial (wastewater treatment plant29,30) barriers,29 implying increased probability to enter the water cycle and perhaps even to reach drinking water sources.29 Therefore, unlike PBT substances to which biota and humans are primarily exposed via diet, PMT chemicals are likely to exert negative effects via water sources.29 The UBA has recently proposed to qualify PMTs as Substances of Very High Concern (SVHC),29,31 which need to be treated with “equivalent level of concern” to those listed by REACH (e.g., PBTs) in terms of the direct impacts to human health or the environment29via e.g., drinking water supplies.
Due to the physicochemical properties of LOHCs as well as the types of use, all environmental compartments, i.e. soil, water and air, are likely to be affected. Among these compartments, soil is of particular importance since it usually acts as a barrier and sink for anthropogenically produced organic chemicals. Once in the soil, organic contaminants may undergo various processes, among which adsorption is particularly important in defining their mobility, i.e. how far they can spread from the point of release and what concentrations can be expected in adjacent ground and surface waters. The assessment of the chemicals’ affinity to soils is therefore crucial in determining their probability to reach the groundwater and drinking water sources. Adsorption also defines the extent of exposure of soil-dwelling organisms in which uptake is not based on the consumption of soil particles (i.e. porewater is the dominant route of exposure).
The present study therefore aimed to characterise the behaviour of several promising LOHC systems based on: quinaldine, indole, carbazole derivatives (ethylcarbazole, butylcarbazole, propylcarbazole), benzyltoluene and dibenzyltoluene, by investigating their adsorption to and mobility in standard soil. To this end, organic carbon–water partition coefficients (Koc) were measured using HPLC, and based on these values LOHC chemicals were assigned to mobility classes. Additionally, the soil-water partition coefficients (Kd) were measured using batch equilibrium and soil column leaching methods.
LOHC name | Abbreviation | Formula | Chemical structure | MW [g mol−1] | Log![]() ![]() |
Log![]() ![]() |
Log![]() ![]() |
P.![]() |
---|---|---|---|---|---|---|---|---|
“P.” indicates percentage of protonation; “n.a.” indicates values are out of the environmentally relevant range.a Sw: water solubility at 25 °C, log![]() ![]() |
||||||||
2-Methyl-quinoline | Quin-2Me | C10H9N |
![]() |
143.2 | 3.95 | 2.45 | 2.18 | 40 or 36 |
Tetrahydro-2-methyl-quinoline | Quin-2Me-ph | C10H13N |
![]() |
147.2 | 2.66 | 3.04 | 2.48 | 26 or 23 |
Decahydro-2-methyl-quinolineb | Quin-2Me-H10 | C10H19N |
![]() |
153.3 | 3.86 | 3.25 | 2.53 | 100 |
Indole | Indole | C8H7N |
![]() |
117.2 | 4.05 | 2.32 | 2.21 | n.a. |
Indoline | Indoline | C8H9N |
![]() |
119.2 | 3.71 | 2.04 | 1.94 | 16 or 14 |
9-Ethyl-9H-carbazole | Car-2 | C14H13N |
![]() |
195.3 | 1.09 | 4.42 | 3.36 | n.a. |
9-Ethyl-hexahydro-carbazoleb | Car-2-ph | C14H19N |
![]() |
201.3 | 0.97 | 4.60 | 3.08 | 20 or 17 |
9-Ethyl-octahydro-carbazole | C14H21N |
![]() |
203.3 | 0.36 | 5.19 | 3.26 | n.a. | |
9-Ethyl-dodecahydro-carbazole | Car-2-H12 | C14H25N |
![]() |
207.4 | 1.91 | n.a. | n.a. | 100 |
9-Propyl-9H-carbazole | Car-3 | C15H15N |
![]() |
209.3 | 0.54 | 4.96 | 3.61 | n.a. |
9-Propyl-hexahydro-carbazoleb | Car-3-ph | C15H21N |
![]() |
215.3 | 0.41 | 5.16 | 3.36 | 27 or 23 |
9-Propyl-octahydro-carbazole | C15H23N |
![]() |
217.4 | −0.19 | 5.72 | 3.50 | n.a. | |
9-Propyl-dodecahydro-carbazole | Car-3-H12 | C15H27N |
![]() |
221.4 | 1.40 | n.a. | n.a. | 100 |
9-Butyl-9H-carbazole | Car-4 | C16H17N |
![]() |
223.3 | −0.001 | 5.50 | 3.89 | n.a. |
9-Butyl-hexahydro-carbazoleb | Car-4-ph | C16H23N |
![]() |
229.4 | −0.04 | 5.58 | 3.62 | 27 or 24 |
9-Butyl-octahydro-carbazole | C16H25N |
![]() |
231.4 | −0.77 | 6.27 | 3.79 | n.a. | |
9-Butyl-dodecahydro-carbazole | Car-4-H12 | C16H29N |
![]() |
235.4 | 0.89 | n.a. | n.a. | 100 |
Benzyltoluene | MLH | C14H14 |
![]() |
182.3 | 0.69 | 4.78 | 3.34 | n.a. |
Benzyltoluene (partially hydrogenated)b | MLH-ph | C14H20 |
![]() |
188.3 | 0.02 | 5.46 | 3.65 | n.a. |
Dibenzyltolueneb | MSH | C21H20 |
![]() |
272.4 | −1.37 | 6.84 | 4.52 | n.a. |
The quinaldines were extracted using a liquid–liquid extraction and the amount was measured using gas chromatography mass spectrometry (GC/MS) analysis.
The amount of quinaldine adsorbed to the soil was calculated by subtracting the amount remaining in the aqueous phase at equilibration from the amount initially added. The concentrations of the test compounds in the soil phase (qe) were plotted against the concentrations in the liquid phase (Ce), and Kd (batch-Kd) was thus calculated as the slope of the linear portion of the adsorption curve. Freundlich model (eqn (1)) was used to fit the experimental data:
log![]() ![]() ![]() ![]() | (1) |
Another set of Kd values (column-Kd) was thus determined from the column leaching experiments according to the method proposed by the Environmental Protection Agency (EPA).36 Here, the Kd values were directly calculated from the retardation factor (Rf) and factors related to the soil properties (n, total porosity; ρb, bulk density) using the equation (eqn (2)):
Kd = [(Rf − 1) × n]/ρb | (2) |
The details on the determination of the parameters for the calculation of Kd are available in ESI-S5.†
a Mean value ± SD of the components (include cis and trans isomers) in mixtures. |
---|
![]() |
Quinaldine | Quin-2Me | Quin-2Me-ph | Quin-2Me-H10 | |
---|---|---|---|---|
Parameter | ||||
a Determined in the adsorption batch equilibrium experiment in soil I (n = 6, ±SD). b Determined in the soil column leaching test in soil II (n = 4, ±SD). c Predicted using COSMO-RS. d Estimated using MarvinSketch.35 e Could not be calculated due to strong retention of the substance in soil column. f Not available in HPLC due to the lack of UV activity of the substance. | ||||
Batch-Kd![]() |
Average | 2.03 ± 0.12 | 6.57 ± 0.39 | 2.42 ± 0.43 |
R 2 | 0.99 | 0.99 | 0.91 | |
Column-Kd![]() |
1.00 ± 0.40 | n.a.e | 0.23 ± 0.03 | |
Log![]() |
2.19 | 2.36 | n.a.f | |
Log![]() |
2.23 | 2.73 | 2.30 | |
Log![]() |
2.10 | n.a.e | 1.46 | |
Log![]() ![]() |
2.18 | 2.48 | 2.53 | |
Log![]() |
2.07 | 2.23 | -0.88 | |
pKb (25 °C)d | 5.15 (5.7–5.8)40 | 4.88 | 10.75 |
Furthermore, adsorption isotherms were fitted using Freundlich model (R2 = 0.95–0.97) (Table 4). The highest adsorption coefficient (Kf) estimated by the model was found for Quin-2Me-ph (9.20 mL g−1), which was followed by Quin-2Me-H10 (5.72 mL g−1) and Quin-2Me (4.19 mL g−1). Comparatively, low values of Freundlich parameter (1/n) were observed for Quin-2Me and Quin-2Me-H10 while higher 1/n approaching unity was found for Quin-2Me-ph (0.96, Table 4). This signifies that while the adsorption of the two former compounds is decreasing with concentration, the adsorption of Quin-2Me-ph is increasing almost linearly with concentration showing nearly no sign of approaching saturation. This is concurrent with the shape of the isotherms themselves (Fig. 2).
Quinaldine | Model | K f [mL g−1] | 1/n | R 2 |
---|---|---|---|---|
Quin-2Me | log![]() ![]() ![]() |
4.19 | 0.79 | 0.97 |
Quin-2Me-ph | log![]() ![]() ![]() |
9.20 | 0.96 | 0.96 |
Quin-2Me-H10 | log![]() ![]() ![]() |
5.72 | 0.83 | 0.95 |
Since Koc is the soil-water partition coefficient normalised to the amount of soil organic matter according to the empirical equation Koc = Kd/foc, Koc can be easily calculated from the measured batch-Kd values in soil I, giving logKoc, batch values of 2.23, 2.73, and 2.30 for Quin-2Me, Quin-2Me-ph, and Quin-2Me-H10, respectively (Table 3).
![]() | ||
Fig. 3 Breakthrough curves for Quin-2Me (A), Quin-2Me-ph (B) and Quin-2Me-H10 (C) in two independent experiments (grey and black), each with two columns packed with soil II. “PV”: pore volume. |
The column-Kd values calculated according to the EPA36 yielded values of 1.00 mL g−1 and 0.23 mL g−1 for Quin-2Me and Quin-2Me-H10, respectively (Table 3). The Kd obtained for Quin-2Me in the column experiment was within a factor of 2 compared to the value measured in the batch experiment. Quin-2Me-ph was strongly retained in the column, therefore no column-Kd value was calculated. This suggested however much higher affinity to soil than could have been expected based on batch-Kd and as compared to other two compounds. On the contrary, the column-Kd for Quin-2Me-H10 was an order of magnitude lower than that obtained in the batch experiment. Additionally, using the empirical equation for Koc, logKoc, column values yielded 1.76–2.24 (avg. 2.01, Table S4†) were calculated for atrazine which were very close to literature values of 1.92. The log
Koc, column values for Quin-2Me and Quin-2Me-H10 was calculated to be 2.10 and 1.46, respectively (Table 3).
It is often assumed that the hydrophobic interactions with soil organic matter are the main forces driving adsorption of organic compounds to soils. For that reason, if experimentally determined Koc values are not available, Kow is often used to predict parameters defining affinity to soils e.g., Koc. The logKoc values obtained using HPLC showed a good correlation with the COSMO-RS-predicted log
Kow values (R2 = 0.94, Fig. S5†). Yet the Kow seems to underestimate the Koc value obtained by HPLC, especially for more hydrophobic compounds with a log
Kow > 3.0 (Fig. S5†).
The logKoc values for the LOHC candidates and some of their partially hydrogenated forms measured using HPLC spanned five orders of magnitude (1.71–5.41) (Table 2). The potential mobility of these compounds in soil, ranged from highly mobile to immobile, proving that they have vastly different potentials to contaminate drinking water sources. Partitioning into soil organic matter plays an important role in the retention of many chemicals in soil41–43 and the mobility decreases with increasing Koc.44,45 The compounds with the highest Koc values are more likely to be retained and accumulated at the soil surface where they were released. Therefore, it seems that indoline, Quin-2Me, Quin-2Me-ph and indole would likely be transported further and deeper crossing natural barriers such as soil layers and aquifers than the carbazole derivatives, MLHs and MSH.
In spite of the role of organic matter in controlling chemicals’ adsorption in soils, among the test set are compounds that can be charged to a different extent depending on the pH of the solution (see Table 1 for potential protonation and Table 3 for pKb values). Hydrophobic interactions with soil organic carbon dominating the sorption is often true for neutral organic compounds but might not be the case for ionisable compounds like quinaldine derivatives.46 Ionisation influences not only the compounds’ hydrophobicity but also the ability to interact with soil via electrostatic interactions. This type of interaction is not present in a cyanopropyl HPLC column that is recommended for estimation of Koc according to OECD guideline.32 It was also reported that for permanently charged or ionisable compounds their affinity to soils may be largely underestimated by Kow.25 Additionally, the degree of saturation of cyclic rings also influences the ability to interact through pi–pi interactions47,48 between aromatic rings present in the solute and those present in soil organic matter. We therefore chose a quinaldine-based system to investigate in more detail on how these structural differences will influence the affinity to model soils. To that end the obtained Kd values in the batch equilibrium and column leaching tests were recalculated into Koc to provide basis for comparisons based on the amount of organic matter present in model soils (Table 3). As we progressed from the least (in silico model) to the more complex and realistic scenario (soil columns) for assessing soil affinity a markable difference in the behaviour of the three members of quinaldine-based LOHC systems became apparent.
For Quin-2Me, the Koc obtained using modelling (COSMO-RS) and different experimental methods (HPLC, batch and column) were similar (Table 3), with the logKoc values ranging from 2.10 (log
Koc, column) to 2.23 (log
Koc, batch) thus Quin-2Me can be classified as “highly mobile” (column leaching) to “moderately mobile” (batch test).38 For the saturated form, Quin-2Me-H10, more significant difference in Koc values (of nearly an order of magnitude) was observed with the log
Koc, column apparently lower than the log
Koc, batch (i.e. 1.46 vs. 2.30) implying that the compound's mobility ranges from “very high” (log
Koc < 1.7) to “moderate”.38 In other words, the fully saturated quinaldine (Quin-2Me-H10) might be more mobile than could have been expected based on Koc values predicted by COSMO-RS or obtained experimentally in the batch test system. Quin-2Me-ph was retained strongly in the soil column with less than 1% of the spiked mass being leached out. Quin-2Me-ph thus seems to be rather “immobile” in soil which was quite unexpected taking into account its rather low predicted and measured Koc values obtained in HPLC and batch systems.
It is noteworthy that soil in the environment is highly heterogeneous. Large variation of soil composition, from texture to minerals to varied quantity and quality of organic carbon, has to be taken into consideration. In other words, adsorption and mobility of the LOHCs and their accessibility to organisms can vary according to local soil49,50 and climatic conditions.51
Although the column leaching experiment was not performed for the other potentially ionisable LOHC compounds in this study, i.e. indoline and alkylcarbazoles, it is probable that their mobility in soils would also deviate from the Koc obtained using HPLC. Further confirmation of the affinities of these compounds by batch equilibrium and leaching experiments is recommended if these compounds were to be implemented as LOHCs on larger scale.
Both column leaching and batch equilibrium tests are commonly conducted to evaluate sorption. The duration of the batch equilibrium test is much shorter than that of the column test, especially when investigating strongly adsorbing substrates (such as Quin-2Me-ph in this study).54 Moreover, due to efficient mixing, the adsorption equilibrium is generally achieved faster in the batch equilibrium test because macroscopic mass transfer is not hindered.54 Therefore, the batch equilibrium test is often treated as the “worst case scenario”.54 However, the mobility (leachability) in soils can be underestimated by the batch equilibrium test,55 as what has been shown here particularly for Quin-2Me-H10. Column leaching represents a dynamic system that is supposed to simulate the downward movement of chemicals through soil.54 Although the column experiment is more time-consuming, it provides better simulation of the water flow through the porous soil profile due to a more realistic solid-to-liquid ratio.54 Processes that occur in nature, such as particle-associated transport,44 are also accounted for in column leaching tests.54 In such a test, the adsorption equilibrium is not necessarily achieved in the dynamic process, even at the point of breakthrough,54 due to insufficient mixing and the adsorption/desorption processes of various, often competing, ions.54,56 In addition, column leaching test could provide more information for the assessment of the potential for groundwater contamination, which can eventually be correlated to the risk of human exposure via drinking water or contaminated crops.44
The concentration used to spike the soil columns in the study was 100 mg kg−1 being the worst-case scenario, which might occur in the environment only through heavy contamination resulting from accidental spills. However, the localised leakage or spillage of these compounds is possible since they may eventually be produced and transported on a large scale and handled by citizens in the same way fossil fuels currently are.
a Determined using ready biodegradability test according to OECD 301F.11
b Unpublished results.
c Mobility classified based on log![]() ![]() |
---|
![]() |
In our previous studies we have shown that Quin-2Me-H10 and Quin-2Me-ph are resistant to biodegradation in ready biodegradability test whereas Quin-2Me is biodegradable to high extent (i.e. not persistent)11 (Table 5). This warrants classification of the two former substances as ‘P’ and the latter as ‘not P’. Additionally, all H2-lean and H2-rich carbazole derivatives showed no degradation in ready biodegradability test (partially hydrogenated forms were not tested) and were therefore classified as ‘P’ (Table 5).11 On the contrary, H2-lean forms of MLH and MSH were significantly degraded in that test system and classified as ‘not P’ (unpublished results). Indole was shown to be susceptible to biodegradation59 and was assessed as readily biodegradable.
Furthermore, low to moderate toxicity of the three quinaldines as well as H2-rich forms of alkylcarbazoles was observed, showing the effects categorised as “acute 2” or “acute 3” in aquatic11 and “harmful” in terrestrial28 organisms – none of which fulfils the ‘T’ criterion in current ‘PMT’ assessment approach (Table 5).29,57,58 Based on this screening-level data Quin-2Me-H10 would be considered potentially “PM” (persistent and mobile) substance.29 Quin-2Me does not raise concerns at the moment due to its biodegradability, while the risk of Quin-2Me-ph to contaminate drinking water would be low because of the immobility. No experimental data are currently available for H2-lean or intermediate forms of alkylcarbazoles, MLH- or MSH-based LOHC as well as indoline that could be used for preliminary hazard classification. In order to fill that gap we used a mathematical model (ECOSAR part of US EPA EPI Suite package63) to predict acute toxicity towards algae, invertebrates and fish. The exact values are presented in Table S5.† Based on these predicted values all alkylcarbazole- as well as MLH- and MSH-derivatives except ethylcarbazole (Car-2) would be classified as aquatic acute toxicity category 1 and therefore would fulfil the ‘T’ criterion (Table 5). Based on model values Car-2 belongs to acute category 2 and indoline to acute category 3, meaning none of them fulfils the ‘T’ criterion. So far, the accuracy of that predictions cannot be verified. Nevertheless, based on results obtained in vitro tests (in mammalian cell line) partially hydrogenated alkylcarbazoles seem to be approximately two orders of magnitude more toxic than H2-rich/lean quinaldine suggesting generally high toxicity.11
The components of each LOHC system tested present different characteristics in terms of persistence, mobility and toxicity (Table 5). More data are needed for a full-scale evaluation of the environmental and human risk, and for an overall hazard assessment at least two compound forms (H2-rich and H2-lean) of each LOHC system should be taken into account, since they can differ considerably as was shown here for members of quinaldine-based LOHC system. So far based on available data the LOHC systems do not seem to be clearly less hazardous than fossil fuels such as diesels regarding the “P, M, T” criterion (Table 5). However, the fact that LOHCs contribute to safer, high-capacity and long-term storage and transport of renewable energy, make them superior to fossil fuels. The less complex composition than that of fossil fuels11 would be another benefit for the hazard monitoring and management of these compounds in the environment.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0gc02603d |
This journal is © The Royal Society of Chemistry 2020 |