Field of Art

The present invention relates to a novel use of iron nitride (Fe _x N) particles and particles containing Fe _x N (wherein x = 2-4) for the removal of halogenated compounds, especially chlorinated hydrocarbons, from the aqueous environment.

Background Art

Incorporation of nitrogen atoms into metal lattice has been widely used in metallurgy for a long time to improve steel hardness and wear and corrosion resistance (Kunze, J. Nitrogen and Carbon in Iron and Steel Thermodynamics (Physical Research),' Akademie Verlag, Berlin, 1990; Fry, A. Stickstoff in Eisen, Stahl Und Sonderstahl. Ein Neues Oberflachenhartungsverfahren. Stahl undEisen 1923, 43, 1271). Iron nitrides (Fe _x N) have been studied extensively as promising materials in magnetic recording heads and magnetic recording media due to their excellent magnetic properties. Nitrogen-poor iron nitrides (< 25% atomic nitrogen) are stable ferromagnets at ambient conditions (Bhattacharyya, S. Iron Nitride Family at Reduced Dimensions: A Review of Their Synthesis Protocols and Structural and Magnetic Properties. J. Phys. Chem. C 2015, 119 (4)).

Fe _x N have been also studied as (electro)catalysts. In the 1950s, hydrogenation catalysts for the Fischer-Tropsch synthesis based on Fe _x N were developed by Anderson and co-workers (Anderson, R. B. Nitrided Iron Catalysts for the Fischer-Tropsch Synthesis in the Eighties. Catal. Rev. 1980, 21 (1), 53-71. https://doi.org/10.1080/03602458008068060). Since then, Fe _x N materials have been found to catalyze amine synthesis, ammonia and hydrazine decomposition, and oxidative reactions with persulfate. Moreover, Fe _x N were recently investigated as promising electrochemical catalysts for water splitting, oxygen reduction reaction, and CO2 reduction.

Conventionally, remediation of halogenated compounds, in particular, chlorinated hydrocarbons is performed using zero-valent iron (ZVI) particles in the aqueous environment. However, ZVI particles, especially at the nanoscale (nZVI), suffer from low electron efficiency for the contaminants due to their high reactivity with water and other reducible species (Schbftner, P.; Waldner, G.; Lottermoser, W.; Stbger-Pollach, M.; Freitag, P.; Reichenauer, T. G. Electron Efficiency of NZVI Does Not Change with Variation of Environmental Parameters. Sci. Total Environ. 2015, 535, 69-78; Reinsch, B. C.; Forsberg, B.; Penn, R. L.; Kim, C. S.; Lowry, G. V. Chemical Transformations during Aging of Zerovalent Iron Nanoparticles in the Presence of Common Groundwater Dissolved Constituents. Environ. Sci. Technol. 2010, 44 (9), 3455-3461; Liu, H.; Wang, Q.; Wang, C.; Li, X. Electron Efficiency of Zero-Valent Iron for Groundwater Remediation and Wastewater Treatment. Chem. Eng. J. 2013, 215-216, 90- 95). This limitation has a negative impact on particle reactivity, longevity, contaminant removal capacity, and particle mobility in the subsurface (Mueller, N. C.; Braun, J.; Bruns, J.; Cernik, M.; Rissing, P.; Rickerby, D.; Nowack, B. Application of Nanoscale Zero Valent Iron (NZVI) for Groundwater Remediation in Europe. Environ. Sci. Pollut. Res. 2012, 19 (2), 550-558; Keane, E. Fate, Transport and Toxicity of Nanoscale Zero-Valent Iron (NZVI) Used during Superfund Remediation,' Durham, 2009; Guan, X.; Sun, Y.; Qin, H.; Li, J.; Lo, I. M. C.; He, D.; Dong, H. The Limitations of Applying Zero-Valent Iron Technology in Contaminants Sequestration and the Corresponding Countermeasures: The Development in Zero-Valent Iron Technology in the Last Two Decades (1994-2014). Water Res. 2015, 75, 224-248).

Disclosure of the Invention

The present invention aims to provide materials for remediation of halogenated compounds from the aqueous environment such as groundwater or wastewater, wherein the materials would have improved performance over the known remediation materials.

The halogenated compounds are preferably chlorinated hydrocarbons. Chlorinated hydrocarbons include aliphatic chlorinated hydrocarbons and aromatic chlorinated hydrocarbons. Halogenated compounds are typically considered pollutants in aqueous environments, and need to be remedied, i.e., removed.

The present invention provides a method of remediation of halogenated compounds from the aqueous environment by contacting them with particles containing or consisting of Fe _x N, wherein x = 2-4.

The particles containing or consisting of Fe _x N show a remarkable improvement in the reduction rates for halogenated compounds, in particular for chlorinated hydrocarbons. The observed pseudo-first-order removal rate constants of chlorinated hydrocarbons with the particles containing or consisting of Fe _x N were up to 20-fold higher, respectively, than those with conventional ZVI (zero-valent iron particles). Concurrently, the undesired particle reaction with water (i.e., corrosion) was suppressed, resulting in a significantly higher electron selectivity. Fully dechlorinated aliphatic hydrocarbons were detected as dechlorination products when the particles containing or consisting of Fe _x N were used as dechlorination agents for chlorinated hydrocarbons. Complete dechlorination of all tested chlorinated hydrocarbons was corroborated by chlorine balance analysis, which confirmed that all chlorine has been converted to inorganic chloride ions.

The particles containing or consisting of Fe _x N of the present invention contain at least 1% w/w of Fe _x N, wherein x = 2-4, localized on the particle surface. Preferably, the particles contain at least 10 % w/w of Fe _x N, wherein x = 2-4, localized on the particle surface and/or in the bulk of the particles. Even more preferably at least 25 % w/w of Fe _x N, wherein x = 2-4, localized on the particle surface and/or in the bulk of the particles. Yet more preferably the particles contain at least 50 % w/w of Fe _x N, wherein x = 2-4, localized on the particle surface and/or in the bulk of the particles; and even more preferably at least 75 % w/w of Fe _x N, wherein x = 2-4, localized on the particle surface and/or in the bulk of the particles.

The particles containing or consisting of Fe _x N may contain Fe _x N phases distributed homogenously, regularly, or irregularly within the particles. In some embodiments, the particles containing Fe _x N are core-shell particles, wherein the shell is formed by Fe _x N. The core of the particles containing Fe _x N may be formed preferably by zero-valent iron or iron salts and (hydroxides, such as magnetite.

In some embodiments, the particles containing or consisting of Fe _x N contain the Fe _x N in the form of y'-Fe4N and/or s-Fe2-3N phases.

The individual particles containing or consisting of Fe _x N may typically have a size below 1000 nm, preferably within the range of 20 to 250 nm. In some embodiments, primary particles may form agglomerates, the size of the agglomerates being preferably from 0.1 micrometers to 100 micrometers, even more preferably from 0.1 micrometers to 10 micrometers.

The BET surface of the particulate material is preferably within the range 0.05 to 50 m ² /g, even more preferably 10 to 50 m ² /g. The chlorinated hydrocarbons which can be reductively dechlorinated using the particles containing or consisting of Fe _x N are preferably saturated or unsaturated aliphatic chlorinated hydrocarbons, more preferably Cl -CIO chlorinated hydrocarbons, even more preferably Cl- C5 chlorinated hydrocarbons. Especially preferably, chlorinated ethanes, ethenes, and ethynes are reductively dechlorinated using the particles containing or consisting of Fe _x N.

The particles containing or consisting of Fe _x N can be used for water remediation at temperatures from 5 to 95 °C, preferably from 5 to 20 °C.

The pH values and dissolved oxygen levels suitable for remediation with particles containing or consisting of Fe _x N range within 4-11 and 0-20 mg/L, respectively. Even more preferably, pH ranges within 6-9 and dissolved oxygen levels ranges within 0-10 mg/L.

The particles containing or consisting of Fe _x N can be used for remediation without the use of additional additives (plain) or can be modified by colloidal stabilizers, support materials, emulsifying agents, and/or further surface modification.

Remediations can be performed with the plain or modified particles containing or consisting of Fe _x N or with the particles in combination with thermal, electrothermal, and biotic processes. In combined treatments, electrolytes and nutrients may be added as well.

The reductive dechlorination of chlorinated hydrocarbons by the particles containing or consisting of Fe _x N occurs by mechanisms similar to conventional ZVI remediation. These include a-elimination, P-elimination, hydrogenolysis, and hydrogenation (Arnold, W. A.; Roberts, A. L. Pathways and Kinetics of Chlorinated Ethylene and Chlorinated Acetylene Reaction with Fe(0) Particles. Environ. Sci. TechnoL 2000, 34 (9), 1794-1805), with P- elimination being the dominant pathway for chlorinated ethenes. Importantly, no or only negligible amounts of less-chlorinated by-products are being formed in reactions with chlorinated hydrocarbons. The major reaction products are completely dechlorinated hydrocarbons, with all chlorine being completely reduced to inorganic chloride ions.

The particles containing or consisting of Fe _x N hence present very promising materials for the cleanup of contaminated water systems that have the potential to overcome limitations of the current (n)ZVI technologies. Moreover, unlike catalytic metals suggested as dopants for (n)ZVI particles, nitrogen is a light, non-toxic, and naturally abundant element and, therefore, its use is not accompanied by high secondary risks such as heavy metal pollution. The performance of the particles containing or consisting of Fe _x N is higher than that of recently researched sulfidated nZVI (S-nZVI) particles, which exhibited increased removal only for trichloroethylene (TCE), while their removal rate for other chlorinated ethenes, namely perchloroethylene (PCE) and c/.s-di chloroethylene (c/.s-DCE), was not positively affected.

Brief Description of Drawings

Figure 1. X-ray diffraction pattern of freshly prepared y'-Fe4N, 8-Fe2-sN, and nZVI

Figure 2. TCE removal by fresh Fe _x N, nZVI, and S-nZVI particles (A) and corresponding chlorine balance at the end of the three-week dechlorination experiment (B).

Figure 3. PCE removal by fresh Fe _x N, nZVI, and S-nZVI particles (A) and corresponding chlorine balance at the end of the three-week dechlorination experiment (B). Note that the pseudo-first-order fit for S-nZVI does not describe the removal kinetics appropriately and was included only for comparison.

Figure 4. c/.s-DCE removal by fresh Fe _x N, nZVI, and S-nZVI particles (A) and corresponding chlorine balance at the end of three-week dechlorination experiment (B). Note that the pseudo- first-order fit for S-nZVI does not describe the removal kinetics appropriately and was included only for comparison.

Figure 5. Hydrogen formation during dechlorination reaction of TCE (A), PCE (B), and cis- DCE (C) for all types of particles tested.

Examples

Example A: Preparation of FexN nanoparticles

Two types of Fe _x N nanoparticles were prepared according to a published study (Arabczyk, W.; Zamlynny, J.; Moszynski, D. Kinetics of Nanocrystalline Iron Nitriding. Polish J. Chem. Technol. 2010, 12 (1), 38-43. https://doi.org/10.2478/vl0026-010-0008-z.) with minor modifications. Briefly, 50 g of nZVI particles (NANOFER 25P, NANO IRON, s.r.o., Czech Republic) were processed in a fluid laboratory furnace in a mixture of anhydrous NH3/N2 (Messer Technogas, Czech Republic) atmosphere. Details on nitriding experimental conditions for nanoparticles with a low and high degree of nitriding are provided in Table 1. A flow of NH3/N2 mixture was kept up until the furnace temperature decreased below 250 °C to avoid nitride decomposition after the main nitriding step. Afterwards, the furnace atmosphere was maintained inert using a N2 stream until it reached ambient temperature. Finally, Fe _x N nanoparticles were transferred into an air-tight container and stored under an inert atmosphere. The Fe _x N particle types are further referred to as -Fe4N and £-Fe2- ₃ N, based on the predominant phase (>80% content) identified by X-ray Powder Diffraction (XRD) analysis.

Table 1. Nitriding experimental conditions

Composition of

Particle gas used for Nitriding Nitriding Gas Flow _Nitr iding t _vnp nitriding temperature pressure * t ^ype (gradient) (bar) (L-h' ¹ ) time (mm)

(NH ₃ : N2) y’-FeiN 1 :2 500 °C 0.5 30 180 g-Fe^N 2: 1 300 C 0.5 30 330

Example B: Preparation of S-nZVI nanoparticles (comparative material)

The comparative sulfidated nanoscale zero-valent iron (S-nZVI) particles with a nominal S/Fe mass ratio of 0.01 were synthesized according to a published study (Brumovsky, M.; Filip, J.; Malina, O.; Oboma, J.; Sracek, O.; Reichenauer, T. G.; Andryskova, P.; Zbofil, R. Core-Shell Fe/FeS Nanoparticles with Controlled Shell Thickness for Enhanced Trichloroethylene Removal. ACS Appl. Mater. Interfaces 2020, 12 (31), 35424-

35434. https://doi.org/10.1021/acsami.0c08626). Briefly, S-nZVI nanoparticles were prepared from commercially available nZVI (NANOFER 25P, NANO IRON, s.r.o., Czech Republic) and an aqueous solution of sodium sulfide (Na2S-9H2O, Alfa Aesar, United Kingdom). The Na2S solution of relevant concentration was freshly prepared using deionized water, which was deoxygenated for 30-45 min by N2 stripping. The S-nZVI particle preparation was performed under anaerobic conditions (N2-filled glovebox, SylaTech GmbH, Germany) from 1 g nZVI and 4 mL of Na2S solution. 1 g of nZVI and 4 mL of N2S solution was dispersed together using a T10 ULTRA- TURRAX® disperser (IKA, Staufen, Germany) at 11 000 rpm for 2 min in a 13 mL plastic test tube with a round bottom. The plastic test tube with prepared concentrated S-nZVI slurry was sealed and transferred outside the anaerobic glovebox. Subsequently, the S- nZVI slurry placed in the plastic test tube was shaken for 1 hour on a horizontal shaker at 160 rpm. Example C: Characterization of fresh FexN particles

For the characterization of the particles, the following techniques were used:

The phase composition and morphology of the freshly synthesized and aged y -Fe4N, £-Fe2-3N nanoparticles were characterized by X-ray diffraction (XRD) with an Aeris diffractometer (PANalytical, B.V.) operating in Bragg-Brentano geometry. The diffractometer was equipped with a CoKa radiation source, fixed divergence, diffracted beam anti-scatter slits, and PIXcel detector. The patterns were measured in 29 range from 5 to 105° and the data were processed using HighScorePlus software in conjunction with PDF-4+ and ICSD databases. Crystalline phases were quantified using the Rietveld refinement (Rietveld, H. M. A Profile Refinement Method for Nuclear and Magnetic Structures. J. Appl. Crystallogr. 1969, 2 (2), 65-71. https://doi.org/10.1107/S0021889869006558).

Particle size was examined by transmission electron microscopy (TEM) JEOL 2100 instrument equipped with X-MaxN 80T SDD EDS detector (Oxford Instruments) at an electron acceleration voltage of 200 kV. Particle size was evaluated using the Image J software (100 measurements were performed for each distribution).

The Brunauer-Emmett-Teller (BET) surface area was determined on dried samples using a NOVA 2000e Surface Area Analyzer (Quantachrome, U.S.A.) by multipoint BET analysis of the nitrogen adsorption data at the relative pressure of 0.05-0.3 at -196 °C. Prior to the measurements, all samples were outgassed at 25 °C for at least 24 h.

The freshly prepared -Fe4N and £-Fe2-3N particles had a mean particle size of 73±21 nm and 77±21 nm, respectively. The specific surface area (SSA) of -Fe4N and £-Fe2-3N particles was 18.91 m ² /g and 23.15 m ² /g, respectively. The values of average particle sizes and SSA, including precursor nZVI, are shown in Table 2.

Table 2. Mean particle size and specific surface area of particles according to the examples

Mean size particle ± SD (nm) 72 ± 26 73 ± 21 77 ± 21 72.6±25

BET SSA± SD (m ² -g ) 22.7 ± 1.0 18.9 ± 0.3 23.2 ± 0.6 22.7±0.3

The crystalline phase composition of the freshly prepared Fe _x N nanoparticles was determined by X-ray powder diffraction (Fig. 1). The quantified abundance of individual phases is shown in Table 3. The most abundant phases (>85%) in all types of freshly prepared nanoparticles contained iron in a reduced state (i.e., as y’-Fe4N and s-Fe2-3N in case of Fe _x N nanoparticles and as a-Fe° in case of nZVI).

Table 3. Phase composition of freshly prepared nanoparticles according to the examples

Abundance of crystalline phases (wt.%)

Particle type _ _Fe y». _{Fe4N £} -Fe _{2 3} N Fe ₃ O ₄ MgCO ₃ nZVI 96.1 - - 3.9 -Fe _x N - 91.1 8.9 e-Fe _x N - 6.3 82.6 11.1

Example D: Testing of fresh FexN particles reactivity with chlorinated ethenes (ClEs) and hydrogen formation

The following chlorinated ethenes were selected as they most frequently occur in contaminated waters: TCE (tri chloroethene), purity 99.9%, PCE (tetrachloroethene), purity 99.9%, and cis- DCE (c/.s- l ,2-di chloroethene), purity 99.9%. ClEs and other reagent-grade chemicals as sodium sulfide and constituents of synthetic moderately hard water (sodium hydrogen carbonate, calcium sulfate dihydrate, magnesium sulfate anhydrous, potassium chloride) were purchased from Sigma Aldrich (now Merck). Commercial ZVI nanoparticles “NANOFER 25P” were obtained from the NANO IRON company (NANO IRON, s.r.o., Czech Republic).

Batch experiments with fresh plain Fe _x N particles were performed in 42-mL glass vials containing 20 mL of synthetic moderately hard water (US EP A. Methods for Measuring the Acute Toxicity of Effluents and Receiving Waters to Freshwater and Marine Organisms, Fifth Edition; Washington, DC, 2002). The water was deoxygenated by purging with N2 for 45 min. The vials were capped with PTFE-lined Mininert® valves (Sigma-Aldrich). 82 pL of freshly prepared Fe _x N stock suspensions (20% w/w) were introduced to vials to obtain a particle concentration of 1 g L' ¹ . To initiate reaction, 50 pL of appropriate C1E standard stock solution in methanol was injected. The initial concentrations were: TCE 20 mg/L, PCE 3 mg/L, and cis- DCE 20 mg/L. The vials were then placed on a horizontal shaker (125 rpm) at 22 ± 1 °C. An aliquot (25-100 pL) of headspace gas was withdrawn in regular intervals using a gastight syringe and analyzed using a 7890B gas chromatograph (GC, Agilent Technologies, USA) for the amount of ClEs, their C2-degradation products, and hydrogen for three weeks. The GC system was equipped with the following columns and detectors: i) an Rtx-VMS capillary column (0.25 mm ID, 30 m, Restek, USA), followed by an electron capture detector (ECD) for the detection of cis-DCE, TCE, PCE, and di chloroethene isomers, ii) an Rt-Q-BOND PLOT column (0.32 mm ID, 30 m, Restek, USA) connected to a valve coupled with a Carboxen-1010 PLOT column (0.32 mm ID, 30 m, Supelco, USA) coupled with a thermal conductivity detector (TCD) for H2 detection. The injector temperature was set to 150 °C and the temperature of the detectors to 250 °C. An oven temperature program (40 °C for 5 min, ramp 8 °C/min to 200 °C and hold for 10 min) was applied to separate the analyzed species. N2 5.0 (constant pressure 80 kPa) was used as carrier gas. PCE, TCE, and di chloroethene isomers were calibrated using liquid standards. H2, vinyl chloride, ethane, ethene, acetylene, and propane were calibrated using gaseous standards (H2 1% in N2, other compounds 1 000 ppm in N2). Measured amounts of analytes were corrected for overpressure and sampling-induced headspace losses as described elsewhere (Brumovsky, M.; Filip, J.; Malina, O.; Oboma, J.; Sracek, O.; Reichenauer, T. G.; Andryskova, P.; Zbofil, R. Core-Shell Fe/FeS Nanoparticles with Controlled Shell Thickness for Enhanced Trichloroethylene Removal. ACS Appl. Mater. Interfaces 2020, 12 (31), 35424-35434. https://doi.org/10.1021/acsami.0c08626). Total concentrations of all analytes were computed from the detected headspace concentrations after accounting for partitioning between headspace and aqueous phase using the respective Henry’s Law constants. Observed pseudo-first-order reaction rate constants, tabs, were calculated using non-linear regression based on a Levenberg-Marquardt least-squares algorithm. The concentration of dissolved chloride (Cl-) at the end of dechlorination experiments was determined using a 930 IC Flex ion chromatography system (Metrohm, Switzerland). A Metrosep A Supp 5-150/4.0 (Metrohm, Switzerland) was used as a separation column and a Metrosep A Supp 5 Guard/4.0 (Metrohm, Switzerland) as a guard column. The eluent consisted of a NaHCO3/Na2CO3 buffer (1.0/3.2 mM). Control experiments with plain nZVI were performed in parallel.

Reactivity with TCE:

The Fe _x N particles were able to dechlorinate TCE significantly faster than conventional nZVI. The dechlorination rate of 7 -Fe4N was even better than that of sulfidated nZVI particles (S/Fe mass ratio 0.01), which were recently suggested as a promising material for TCE remediation (Figure 2A). Chlorine balance was complete for all tested particles and corresponded to the observed TCE degradation kinetics (Figure 2B). The products of TCE dechlorination were completely dechlorinated aliphatic hydrocarbons such as ethane and ethene for all particle types tested, while no toxic vinyl chloride was observed. Fe _x N particles produced more longer-chain hydrocarbons than nZVI.

Reactivity with PCE:

Both Fe _x N particle types were able to dechlorinate PCE significantly faster than conventional nZVI (Figure 3A). Interestingly, sulfidation of nZVI particles (S/Fe mass ratio 0.01) negatively affected the PCE dechlorination rate. Chlorine balance was complete for samples with Fe _x N, where all PCE has been dechlorinated (Figure 3B). nZVI and S-nZVI reached only 90% and 80% chlorine balance, respectively, likely as a result of sorption of PCE that has not been decomposed onto iron and its corrosion products such as iron (oxyhydr)oxides. Similar to TCE removal experiments, the products of PCE dechlorination were predominantly ethene and ethane for all particle types tested. Neither vinyl chloride nor DCE isomers were detected. Fe _x N particles produced more longer-chain hydrocarbons than nZVI.

Reactivity with c/.s-DCE:

G.s-DCE has been dechlorinated substantially faster with both Fe _x N particle types compared to conventional nZVI. Interestingly, sulfidation of nZVI particles (S/Fe mass ratio 0.01) did not affect the c/.s-DCE removal rate (Figure 4A). Chlorine balance was complete for all tested particles and corresponded to the observed c/.s-DCE degradation kinetics (Figure 4B). Similar to previous examples, the major products of c/.s-DCE dechlorination by all tested particle types were ethene and ethane. Only trace levels of vinyl chloride (~1 % of the initial amount of cis- DCE) were detected for pristine nZVI during the whole experiment duration, while for both Fe _x N particle types trace levels of vinyl chloride were detected only within the first hours of reaction and later dropped below the detection limit. Fe _x N particles produced more longer-chain hydrocarbons than nZVI.

Hydrogen formation:

Hydrogen formation resulting from particle corrosion in the aqueous environment was significantly lower for Fe _x N particles in experiments with different ClEs (Figure 5) compared to conventional nZVI particles. S-nZVI particles also formed lower amounts of hydrogen, which were similar to Fe _x N particles.