Abiotic ammonium formation in the presence of Ni-Fe metals and alloys and its implications for the Hadean nitrogen cycle

Abstract

Experiments with dinitrogen-, nitrite-, nitrate-containing solutions were conducted without headspace in Ti reactors (200°C), borosilicate septum bottles (70°C) and HDPE tubes (22°C) in the presence of Fe and Ni metal, awaruite (Ni₈₀Fe₂₀) and tetrataenite (Ni₅₀Fe₅₀). In general, metals used in this investigation were more reactive than alloys toward all investigated nitrogen species. Nitrite and nitrate were converted to ammonium more rapidly than dinitrogen, and the reduction process had a strong temperature dependence. We concluded from our experimental observations that Hadean submarine hydrothermal systems could have supplied significant quantities of ammonium for reactions that are generally associated with prebiotic synthesis, especially in localized environments. Several natural meteorites (octahedrites) were found to contain up to 22 ppm N_tot. While the oxidation state of N in the octahedrites was not determined, XPS analysis of metals and alloys used in the study shows that N is likely present as nitride (N^3-). This observation may have implications toward the Hadean environment, since, terrestrial (e.g., oceanic) ammonium production may have been supplemented by reduced nitrogen delivered by metal-rich meteorites. This notion is based on the fact that nitrogen dissolves into metallic melts.

Introduction

Ammonia (NH₃) or ammonium (NH₄⁺), henceforth NH₃/NH₄⁺, are necessary precursors for reactions associated with prebiotic syntheses, such as the Strecker synthesis. It has been experimentally shown that NH₃/NH₄⁺ environments are more efficient in organic synthesis than those dominated by dinitrogen (henceforth N₂) in both aqueous and gaseous environments [e.g., [1]] [2,3]. This notion is not unexpected, considering that, the strong triple bond (948 kJ.mol^-1) of the N₂ would presumably result in large reaction activation barriers (i.e., low conversion rates), even if the overall reaction is thermodynamically favored.

Several possible pathways to abiotic NH₃/NH₄⁺ on early the Earth have been proposed: reduction of NO₂^-/NO₃^- by Fe⁺⁺/FeS in the ocean [e.g., [4]] [5,6]; atmospheric production from N₂ and HCN [e.g., [7]] [8]; release from rocks and minerals [e.g., [9]]; photoreduction on mineral surfaces [e.g., [10]] [11,12]; and hydrothermal aqueous reduction from N₂ in the presence of minerals under conditions typical of submarine hydrothermal systems [e.g., [13]] [14,15]. Each of the mechanisms relies on a different set of assumptions and none of the proposed mechanisms has, in our opinion, gained universal acceptance in the scientific community as the predominant source of abiotic NH₃/NH₄⁺.

In this scientific contribution we focus on the catalytic properties of Ni and Fe metals and their alloys which can form in submarine hydrothermal systems (SHS), especially those driven by exothermic hydration reactions (e.g., serpentinization) in an off-axis tectonic setting. Upon dissolution of Ni-containing rock-forming minerals (e.g., olivine, pyroxene, amphibole), released Ni and Fe can react to form metals and alloys under extreme reducing conditions imposed on the system by the serpentinization processes [16-18]. The conditions are commonly reducing enough to stabilize Ni-Fe alloys (e.g., awaruite – Ni₃Fe), metallic nickel (Ni⁰) and even iron (Fe⁰). These minerals occur regularly, albeit in small quantities in both ancient and modern serpentinites [19-26]. A compilation of representative chemical analyses of metals and alloys found in serpentinites is presented in Fig. 1. The observations from natural systems have been corroborated by laboratory experiments [27,28].

Figure 1.

A ternary diagram of naturally occurring Ni-Fe-M (M = Cr, Sb, Pb, Cu, Co, As, Ag, PGE) alloys and their comparison to synthetic alloys used in this study. Analyses of natural samples were adapted from: [19] [22] [109-115]. Each data point represents an electron microprobe analysis expressed in weight percent.

An active global tectonic cycle is not required for the formation and operation of serpentinization-driven SHS and hence we assume that these environments were commonly present on the Hadean Earth. Moreover, the lack of oxygen and the possible presence of significant amounts of hydrogen gas in the Hadean atmosphere (and consequently in the ocean water) may have further enhanced the stability of base metals and their alloys [29,30].

The most abundant reactant for abiotic NH₃/NH₄+ formation in the Hadean was N₂ dissolved in the seawater from the N₂-rich atmosphere. NO₂^- and NO₃^- are also thought to have been available, although likely in low concentrations. These oxidized N species could have formed in high energy events such as lightning, corona discharge and/or impacts and subsequently rain out into the ocean [31-34].

In this contribution we report the results of an experimental study undertaken to evaluate the hypothesis that abiotic NH₄⁺ formation from dissolved N₂, NO₂^- and NO₃^- in the presence of Ni₃Fe, NiFe, Ni⁰ and Fe⁰ was an operative synthetic route at anaerobic conditions potentially present in the Hadean Ocean. Furthermore, we attempt to quantify global NH₄⁺ yields in the Hadean Ocean produced by investigated mechanisms.

Methods and materials

Reduction experiments

Three sets of experiments were conducted at three different temperatures: 200°C (runs 1–36), 70°C (runs 37–59) and 22°C (runs 60–83). The choice of reactors was based on experimental temperature: 15 mL passivated HIP^® Titanium 64 tube reactors (200°C); 20 mL I-Chem^® borosilicate vials with PTFE/Si septum caps (70°C) and 15 mL BD HDPE Falcon^® tubes (22°C). Reactors were kept at constant temperature in a heated water bath (20, 70°C) or an Isotemp^® oven (200°C). All experiments lasted 24 hours and were conducted in the absence of headspace (e.g., no gas phase). No additional pressure other than that of expanding liquid was imposed on the vessels (~400 psi/27 bars with Ni⁰ to ~800 psi/55 bars with Fe⁰ at 200°C).

All reacting solutions were prepared from freshly drawn UV/UF deionized water (henceforth DI) either by purging with a UHP gas of interest (e.g., Ar, N₂, H₂/N₂) and/or by dissolving appropriate amounts of reagents. NH₄⁺ content of all unreacted reagent solutions (e.g., blanks) was below the detection limit of ion chromatography (<0.1 μmol.kg^-1, reported by the manufacturer). DI purged with N₂ gas under ambient conditions results in equilibrium N₂(aq) concentration of 0.59 mmol.kg^-1. The effect of O₂ was, however, also determined (runs 39, 41, 45, 47, 50, 52, 56, 58, 62, 64, 68, 70, 74, 76, 80, 82) in experiments with DI equilibrated with present-day atmosphere (e.g., no N₂ purging).

To ensure clean and fresh mineral surfaces (e.g., free of oxidation products and/or atmospheric sorbed gases), all metal/alloys were ultrasonically cleaned for 1 hour in 0.06 M HCl immediately preceding the experiments. Subsequently they were washed three times with the designated reacting solution and loaded into reactors in the form of slurry. This "wet loading" procedure eliminated sorption of gases from the atmosphere onto freshly cleaned metal surfaces which was especially important in blank experiments.

Background NH₄⁺ production (e.g., release from reactants, reaction vessels, etc.) was assessed in blank experiments with Ar-saturated solutions and no added N source (runs 1, 9, 17, 25, 37, 43, 49, 54, 60, 66, 72, 78). NO₃^- and NO₂^- solutions were prepared by dissolution of ACS reagent grade KNO₃ and KNO₂, respectively. The pH was not buffered and was allowed to change as a result of solution-metal/alloy interactions and was recorded before and after the experiment. After pH measurements, all samples were acidified with 0.2 M HCl to ensure the conversion of NH₃ to NH₄⁺ and to prevent the formation of Fe precipitates. The samples were stored at 1°C and analyzed within 24 – 48 hours. The summary of all experimental conditions is presented in Tab. 1.

Table 1.

Physical and chemical characteristics of alloys/metals used in study. Formula and the "a" parameter determined from EPMA analyses and from LeBail refinement of XRD data, respectively. The chemical composition of Ni-Fe alloys was confirmed by EPMA as: Ni_{49.78(± 0.49)}Fe_{50.22(± 0.49)} and Ni_{79.14(± 0.43)}Fe_{20.86(± 0.43)}.

Metal/Alloy	Formula	Manufacturer	Surface Area [m2]	Space Group	Unit cell [Å]	Natural analog
Iron	Fe	Alfa Aesar	0.6536 ± 0.0776	Im-3m	2.866(1)	Iron
Nickel	Ni	Alfa Aesar	0.5044 ± 0.0309	Fm-3m	3.523(1)	Nickel
Ni50Fe50	NiFe	Goodfellow	0.3685 ± 0.0094	Fm-3m	3.586(3)	Tetrataenite
Ni81Fe19	Ni3.7Fe	Goodfellow	0.1877 ± 0.0131	Fm-3m	3.547(6)	Awaruite

Analysis of solids, their surfaces, and reaction products

Metals and alloys representing an fcc solid solution of Ni in Fe were purchased from Alfa Aesar^® and Goodfellow. All starting and selected reacted solids were characterized by X-Ray Diffractometry, X-Ray Photoelectron Spectroscopy, Scanning Electron Microscopy, B.E.T. surface analysis, and Electron Microprobe. The results of metal/alloy characterization are summarized in Tab. 2 and Fig. 2, 3.

Table 2.

Conditions and results of reduction experiments conducted in this study. All concentrations are in μmol.kg^-1. Values and their errors bigger than 10 are rounded to the nearest whole number, those smaller than 10 are rounded to the nearest tenth.

Metal/Alloy	Run	Solution	T°C	pHpre	pHpost	NH4+	%	NH4+norm*	NO2-**	NO3-**
Fe	1	Ar	200	5.7	9.9	171 ± 1	-	340 ± 21	n.a.	n.a.
Fe	2	650 N2	200	5.8	8.9	187 ± 1	2.5	371 ± 23	n.a.	n.a.
Fe	3	39/617 H2/N2	200	5.8	8.4	230 ± 1	10	455 ± 28	n.a.	n.a.
Fe	4	459 KCl in N2	200	6.2	8.9	267 ± 2	15	529 ± 32	n.a.	n.a.
Fe	5	484 KNO2 in N2	200	6.0	9.8	768 ± 1	100	1522 ± 93	0	0
Fe	6	465 KNO3 in N2	200	6.2	9.8	760 ± 8	100	1507 ± 92	0	0
Fe	7	485 KNO2 in Ar	200	6.0	10.2	749	100	1486 ± 91	0	0
Fe	8	477 KNO3 in Ar	200	6.2	10.3	752 ± 3	100	1491 ± 91	0	0
Ni	9	Ar	200	5.8	8.7	18	-	27 ± 3	n.a.	n.a.
Ni	10	650 N2	200	5.7	8.3	28	1.5	43 ± 5	n.a.	n.a.
Ni	11	39/617 H2/N2	200	5.8	6.3	30	1.8	45 ± 5	n.a.	n.a.
Ni	12	459 KCl in N2	200	6.2	6.7	20	0.3	30 ± 4	n.a.	n.a.
Ni	13	484 KNO2 in N2	200	6.0	9.1	559 ± 8	100	855 ± 102	0	0
Ni	14	465 KNO3 in N2	200	6.2	9.0	538 ± 7	100	823 ± 98	0	0
Ni	15	485 KNO2 in Ar	200	6.0	9.3	544 ± 7	100	833 ± 99	0	0
Ni	16	477 KNO3 in Ar	200	6.2	9.3	543 ± 4	100	830 ± 99	0	0
Ni50Fe50	17	Ar	200	5.8	8.7	15	-	40 ± 1	n.a.	n.a.
Ni50Fe50	18	650 N2	200	5.7	7.9	17	0.3	47 ± 1	n.a.	n.a.
Ni50Fe50	19	39/617 H2/N2	200	5.6	8	17	0.3	45 ± 1	n.a.	n.a.
Ni50Fe50	20	476 KCl in N2	200	6.3	7.1	14	0	39 ± 1	n.a.	n.a.
Ni50Fe50	21	480 KNO2 in N2	200	5.9	9.8	418 ± 3	100	1133 ± 29	0	0
Ni50Fe50	22	463 KNO3 in N2	200	6.0	9.7	490 ± 2	100	1330 ± 34	0	0
Ni50Fe50	23	474 KNO2 in Ar	200	6.1	10.3	475 ± 5	100	1289 ± 33	0	0
Ni50Fe50	24	487 KNO3 in Ar	200	6.0	9.9	476 ± 6	100	1293 ± 33	0	0
Ni81Fe19	25	Ar	200	5.8	8.6	18	-	98 ± 7	n.a.	n.a.
Ni81Fe19	26	650 N2	200	5.7	7.9	19	0.2	99 ± 7	n.a.	n.a.
Ni81Fe19	27	39/617 H2/N2	200	5.6	8.3	19	0.2	103 ± 7	n.a.	n.a.
Ni81Fe19	28	476 KCl in N2	200	6.3	6.2	18 ± 1	0	96 ± 7	n.a.	n.a.
Ni81Fe19	29	480 KNO2 in N2	200	5.9	9.9	410 ± 6	100	2186 ± 153	0	0
Ni81Fe19	30	463 KNO3 in N2	200	6.0	9.8	483 ± 4	100	2576 ± 180	0	0
Ni81Fe19	31	474 KNO2 in Ar	200	6.1	10.3	485 ± 4	100	2583 ± 180	0	0
Ni81Fe19	32	487 KNO3 in Ar	200	6.0	9.9	493 ± 3	100	2626 ± 183	0	0
-	33	497 KNO2	200	6.0	9.4	45	9.1	-	288 ± 2	73 ± 1
-	34	444 KNO3	200	6.1	6.4	3.6	0.8	-	273 ± 1	129 ± 1
-	35	506 FeCl2 in N2	200	4.7	3.3	8.1 ± 0.1	1.2	-	n.a.	n.a.
-	36	492 NiCl2 in N2	200	5.3	5.7	8.0 ± 0.3	1.2	-	n.a.	n.a.
Fe	37	Ar	70	5.7	7.6	3.8	-	7.4 ± 0.5	n.a.	n.a.
Fe	38	650 N2	70	5.8	8.5	3.6	0	7.2 ± 0.4	n.a.	n.a.
Fe	39	492 KNO2 in O2	70	6.9	10.1	559 ± 3	100	1107 ± 68	0	0
Fe	40	502 KNO2 in N2	70	6.1	9.8	550 ± 3	100	1091 ± 67	0	0
Fe	41	511 KNO3 in O2	70	6.7	9.9	558 ± 2	98	1106 ± 68	0	8.8 ± 0.2
Fe	42	486 KNO3 in N2	70	6.3	10.1	512 ± 6	98	1015 ± 62	0	9.6 ± 0.2
Ni	43	Ar	70	5.7	7.6	0.9	-	1.4 ± 0.2	n.a.	n.a.
Ni	44	650 N2	70	5.8	6.9	0.9	0	1.4 ± 0.2	n.a.	n.a.
Ni	45	492 KNO2 in O2	70	6.9	10.1	528 ± 2	100	807 ± 96	0	0
Ni	46	502 KNO2 in N2	70	6.1	9.9	534 ± 3	100	818 ± 97	0	0
Ni	47	511 KNO3 in O2	70	6.7	10.2	521 ± 5	100	797 ± 95	0	0
Ni	48	486 KNO3 in N2	70	6.3	10.2	534 ± 5	100	817 ± 97	0	0
Ni50Fe50	49	Ar	70	5.7	8.2	0.4	-	1.2	n.a.	n.a.
Ni50Fe50	50	650 N2	70	5.8	8.3	0.4	0	1.2	n.a.	n.a.
Ni50Fe50	50	492 KNO2 in O2	70	6.9	8.4	22 ± 1	3.9	61 ± 2	473 ± 3	0
Ni50Fe50	51	502 KNO2 in N2	70	6.1	8.1	23 ± 1	2.8	62 ± 2	488 ± 2	0
Ni50Fe50	52	511 KNO3 in O2	70	6.7	8.0	19 ± 1	8	53 ± 1	0	470 ± 2
Ni50Fe50	53	486 KNO3 in N2	70	6.3	8.4	28 ± 1	6.2	77 ± 2	0	456 ± 2
Ni81Fe19	54	Ar	70	5.7	8.2	0.5	-	2.9 ± 0.2	n.a.	n.a.
Ni81Fe19	55	650 N2	70	5.8	8.1	0.5	0	2.7 ± 0.2	n.a.	n.a.
Ni81Fe19	56	492 KNO2 in O2	70	6.9	9.2	23 ± 1	4.7	125 ± 9	410 ± 1	0
Ni81Fe19	57	502 KNO2 in N2	70	6.1	9.2	71 ± 2	14	379.27	406 ± 1	0
Ni81Fe19	58	511 KNO3 in O2	70	6.7	8.6	35	6.8	186 ± 13	13 ± 1	425 ± 2
Ni81Fe19	59	486 KNO3 in N2	70	6.3	8.7	69 ± 2	14.2	370 ± 26	9.1 ± 0.7	424 ± 3
Fe	60	Ar	22	5.7	6.4	0.7	-	1.4	n.a.	n.a.
Fe	61	650 N2	22	5.8	6.6	0.7	0	1.4	n.a.	n.a.
Fe	62	541KNO2 in O2	22	6.2	10.8	544 ± 6	100	1078 ± 66	0	0
Fe	63	549 KNO2 in N2	22	6.0	10.6	542 ± 5	99	1075 ± 66	0	0
Fe	64	529 KNO3 in O2	22	6.4	10.5	527 ± 4	100	1044 ± 64	0	0
Fe	65	521 KNO3 in N2	22	5.9	10.8	486 ± 5	93	963 ± 59	0	0
Ni	66	Ar	22	5.7	6.9	0.4	-	0.6	n.a.	n.a.
Ni	67	650 N2	22	5.8	5.6	0.4	0	0.6	n.a.	n.a.
Ni	68	541KNO2 in O2	22	6.2	10.6	422 ± 3	78	646 ± 77	0	0
Ni	69	549 KNO2 in N2	22	6.0	10.7	451 ± 5	82	690 ± 82	0	0
Ni	70	529 KNO3 in O2	22	6.4	10.2	139	26	213 ± 25	43	36
Ni	71	521 KNO3 in N2	22	5.9	10.1	364 ± 1	70	557 ± 66	1.2	22
Ni50Fe50	72	Ar	22	5.7	6.3	0	-	0	n.a.	n.a.
Ni50Fe50	73	650 N2	22	5.8	6.3	0	0	0	n.a.	n.a.
Ni50Fe50	74	541 KNO2 in O2	22	6.2	6.8	1.4	0.3	3.9 ± 0.1	553 ± 2	0
Ni50Fe50	75	549 KNO2 in N2	22	6.0	7.7	1.1 ± 0.1	0.2	3.1 ± 0.1	556	0
Ni50Fe50	76	529 KNO3 in O2	22	6.4	7.2	0.8	0.2	2.1 ± 0.1	0	526 ± 1
Ni50Fe50	77	521 KNO3 in N2	22	5.9	7.3	1	0.2	2.7 ± 0.1	0	523 ± 2
Ni81Fe19	78	Ar	22	5.7	6.9	0	-	0	n.a.	n.a.
Ni81Fe19	79	650 N2	22	5.8	6.6	0	0	0	n.a.	n.a.
Ni81Fe19	80	541 KNO2 in O2	22	6.2	9.4	3 ± 0.1	0.6	16 ± 1	515 ± 2	0
Ni81Fe19	81	549 KNO2 in N2	22	6.0	9.3	1.9 ± 0.1	0.4	10 ± 1	539 ± 3	0
Ni81Fe19	82	529 KNO3 in O2	22	6.4	8.9	0	0	0	0	493 ± 1
Ni81Fe19	83	521 KNO3 in N2	22	5.9	9.0	1.9 ± 0.1	0.4	10 ± 1	0	482

"n.a." denotes "not analyzed, "-" not applicable. * denotes NH₄⁺ concentration formed in the experiment normalized to 1 m² of surface area of the metal/alloy. ** denote residual concentration after the experiment was completed.

Figure 2.

XRD patterns of metals/alloys used in this study. Note the similarity in patterns of Ni, Ni₅₀Fe₅₀ and Ni₈₁Fe₁₉ stemming from the same space group (Fm-3m). Fe⁰ possesses Im-3m space group.

Figure 3.

SEM photographs of unreacted alloys/metals used in this study. A) Ni metal, b) Ni₈₁Fe₁₉, c) Ni₅₀Fe₅₀ and d) Fe metal.

X-Ray Diffraction (XRD) data were collected using a Scintag PAD X diffractometer under the following conditions: CuK_α1, 40 kV, 25 mA, 5° – 90° 2Θ, step 0.02° 2Θ and variable scan rates.

Scanning Electron Analysis (SEM) was performed on the LEO 1550 SFEG scanning electron microscope equipped with an EDAX energy dispersive X-ray spectrometer (EDS) using an accelerating voltage of 15 kV and a 30 μm aperture.

Oxidation state of Ni, Fe and the presence of N in alloys were determined by X-Ray Photoelectron Spectroscopy (XPS). The data were acquired with unmonochromatized Mg_Kα and Al_Kα radiation at 1253.6 eV and 1486.7 eV using a Physical Electronics source controller in a vacuum chamber with a base pressure of 1 × 10^-9 Torr. A VG Microtech hemispherical analyzer was used to obtain the energy distribution of the photoemitted electrons at pass energies of 50 and 75 eV. The binding energies were calibrated by fixing the Au 4p3/2 and 4f7/2 peaks (546.3 eV, 87.5 eV) from a gold standard, and the metallic Fe 2p3/2 and Ni 2p3/2 cores (707.0 eV, 852.3 eV). Selected particles were sputtered with Ar⁺ accelerated to 2 kV to expose their interior and check for the presence of nitrogen using a Physical Electronics ion gun controller.

B.E.T. surface analysis (BET) was performed using a Micromeritics ASAP 2010 analyzer with a 10-mm Hg transducer using UHP N₂ gas. The surface area was calculated from measurements at 5 different N₂(g) pressures (42.58196, 85.30770, 132.82384, 180.25029 and 227.66101 Torr).

Chemical composition of alloys/metals was determined by a Cameca Camebax Micro electron microprobe (EPMA) equipped with four wavelength dispersive spectrometers and a Kevex Analyst 8000 energy dispersive detector. During all analyses, the accelerating voltage and beam current used were 15 kV and 10 nA (nominal), respectively.

Molecular hydrogen was analyzed on a SRI 8610C single column gas chromatograph (GC) with a TCD detector, 6' Hayesep D column and N₂ carrier gas. The sample (~0.2 mL) was withdrawn from the reaction vessel into a gastight^® Hamilton™ syringe with a Mininert™ valve and immediately analyzed using a 4-point calibration curve. Gas mixtures (Matheson™) of known composition were used as calibration standards.

Total nitrogen content of metals/alloys was analyzed by IMR Test Labs (Lansing, NY) by inert gas fusion [35]. During the analysis, N is released from the metal at 1900°C into the stream of He gas and analyzed in a thermal conductivity cell.

Experimental solutions were analyzed for NH₄⁺ using a Dionex DX-500 ion chromatograph (IC) with a 100 μL sample loop. NH₄⁺ and NO₃^-/NO₂^- were analyzed using a 4 mm Dionex IonPac^® CS-16 (40°C, 34 mN H₂SO₄ eluent) and IonPac^® AS4A-SC (22°C, 5 mmol.kg^-1 Na₂B₄O₇ eluent), respectively. Concentrations were calculated from a 4-point calibration curve with R² values above 0.99.

No other compounds were analyzed. It is expected, however, that other reaction products and/or intermediates (e.g., NO) may have formed during a complex sequence of electron transfer reactions.

Geochemical equilibrium modeling was performed with the Geochemist's Workbench 5 [36] software package with the thermo.com.V8.R6.full thermodynamic database complemented with data for Ni₃Fe and NiFe [37].

Results

Dinitrogen reduction

The results of NH₄⁺ formation from N₂ and the effect of added H₂ and KCl at 200°C (normalized to 1 m² surface area) are shown in Fig. 4 and summarized in Tab. 1 (runs 1–32). All results are compared with respect to blank experiments conducted with Ar and no N₂ added. The blank experiments thus represent the background NH₄⁺ production from the metal/alloy involved and reactor catalysis. Error bars were calculated by propagating errors from solution dilutions and B.E.T. and IC analyses. Due to relatively large error bars, only results differing from the blank (or each other) by more than the calculated error will be discussed.

Figure 4.

NH₄⁺ formation from N₂ in the presence of Ni, Fe metals and alloys at 200°C. The dashed lines correspond to the Ar blank. The concentrations of KCl in the experiments were 459 μmol.kg^-1 in A, B and 476 μmol.kg^-1 in C, D. Results are normalized to 1 m² of surface area.

In the presence of N₂, only Ni₈₁Fe₁₉ did not show appreciable activity toward NH₄⁺ formation (Fig. 4d). Within our experimental certainty, Fe⁰ was the material associated with the most NH₄⁺ production (31 μmol.kg^-1.m², Fig. 4a), followed by Ni⁰ (16 μmol.kg^-1.m², Fig. 4b) and Ni₅₀Fe₅₀ (7 μmol.kg^-1.m², Fig. 4c). H₂(aq) was observed to form in the presence of all studied metals and alloys. Representative concentrations of H₂(aq) at 200°C (measured in one experiment with N₂-saturated, O₂-free DI per metal/alloy only) were 0.38 mmol.kg^-1 with Ni⁰, 0.28 mmol.kg^-1 with Ni₈₁Fe₁₉, 0.7 mmol.kg^-1 with Ni₅₀Fe₅₀ and 21 mmol.kg^-1 with Fe⁰. We point out that this H₂ production is relatively large compared to the background H₂ production of the Ti reaction vessels that was determined to be 12 μmol.kg^-1 in an experiment with DI saturated with Ar at 200°C. An interesting observation during our experiments was that the reactivity of Fe⁰ towards O₂-free DI was so rapid that gas bubbles were forming on its surface after just a few hours of exposure at 22°C (by analogy with 200°C experiments we assume it is H₂).

Our results showed that the addition of H₂ (5%/95% H₂/N₂) into the reaction mixture only resulted in change in the Fe⁰ circumstance where 158 μmol.kg^-1.m² NH₄⁺ was produced (Fig. 4a). Fe⁰ was also the material most affected by the addition of K⁺ (KCl). In this case, 189 μmol.kg^-1.m² of NH₄⁺ was produced, 158 μmol.kg^-1.m² more than with N₂ alone (Fig. 4a). In general, our experiments showed that Ni₈₁Fe₁₉ was the least, and Fe⁰, the most, affected by the addition of H₂ or KCl into the reacting solution (Fig. 4d, a).

Aqueous Fe and Ni cations (NiCl₂, FeCl₂) only had a small effect on N₂ reduction chemistry, converting about 1% of the total available N into NH₄⁺ at 200°C (Tab. 1). A post-reaction visual inspection of the FeCl₂ solution showed a fine-grained colloid of reddish color that, in the presence of atmospheric O₂ changed color to light brown. The amount of recovered solids was insufficient for analysis by XRD.

At temperatures of 70 and 22°C no NH₄⁺ formation from N₂ was observed (Tab 1, runs 38, 44, 50, 55, 61, 67, 73, 79). Use of these lower temperatures also resulted in lower background NH₄⁺ production in solutions containing the metals/alloys. The yield of NH₄⁺ ranged from 7.4 (Fe⁰) to 1.2 μmol.kg^-1.m² (Ni₅₀Fe₅₀) at 70°C (Tab. 1, runs 37, 43, 49, 54) and 1.4 (Fe⁰) to 0 μmol.kg^-1.m² (Ni₅₀Fe₅₀, Ni₈₁Fe₁₉) at 22°C (Tab. 1, runs 60, 66, 72, 78).

Nitrite and nitrate reduction

The results of our NO₂^- and NO₃^- reduction experiments are shown in Fig. 5. At 200°C all tested metals/alloys were found to be very effective in converting NO₂^-/NO₃^- into NH₄⁺. To assess if the presence of NO₂^- or NO₃^- in the solution had any effect on N₂ reduction, each experiment was conducted in duplicate with Ar (Tab. 1, runs 7, 8, 15, 16, 23, 24, 31, 32) and N₂ (Tab. 1, runs 5, 6, 13, 14, 21, 22, 29, 30) saturated solutions (e.g., Fe⁰ with NO₂^-/N₂ and NO₂^-/Ar). These experiments taken in sum showed that there was no difference between the N₂ and Ar runs (Fig. 5a).

Figure 5.

NH₄⁺ formation from nitrite and nitrate, expressed in terms of % conversion. Panels A, B and C correspond to sets of experiments at 200, 70 and 22°C, respectively.

The results collected at 70°C reveal prominent differences between alloys and metals (Fig. 5b). While Ni and Fe achieved almost 100% conversions of NO₂^-/NO₃^- to NH₄⁺ (Tab. 1, runs 40, 42, 46, 48), interestingly, no- or insignificant reduction (less than 20%) was observed in solutions reacted with alloys (Tab. 1, runs 51, 53, 57, 59). The presence of O₂ in the solution appears to have an inhibitive effect on the reduction process, especially in the presence of Ni₈₁Fe₁₉ alloy (Tab. 1, runs 56, 58; Fig. 5b).

Low temperature experiments (22°C) further confirmed the temperature dependence of NO₂^-/NO₃^- reduction in the presence of alloys (Fig. 5c). Compared to 200°C and even at 70°C, NH₄⁺ formation was negligible (less than 1% conversion) (runs 63, 65, 69, 71, 75, 77, 81, 83). The effect of O₂ in the reacted solution was most pronounced with NO₃^- in the presence of Ni⁰ (run 70). In general, at 22°C Fe⁰ was the most efficient material in converting NO₂^-/NO₃^- to NH₄⁺, regardless of the O₂ content (Tab. 1, runs 62–65; Fig. 5c).

It is important to note that at 200°C both NO₂^- and NO₃^- decomposed in the absence of metal/alloys as well (Tab. 1, runs 34, 34). 42% of the initial 497 μmol.kg^-1 KNO₂ solution was converted into NO₃^- (~15%), NH₄⁺ (~9%) and other N compounds (~18%) that were not analyzed. Of the initial 444 μmol.kg^-1 KNO₃ solution, 71% was converted into NO₂^- (~61%), NH₄⁺ (~1%) and about 9% corresponds to other unanalyzed N compounds. It is not clear if this is a result of thermally induced decomposition, catalysis or reaction by/with the titanium reaction vessel, or a combination of all; nevertheless, NH₄⁺ was not the dominant reaction product. At 70 and 22°C, both NO₂^- and NO₃^- solutions were found to be stable in experiments without metals or alloys during the 24-hour reaction period.

Metal/alloy alteration

Generally, the extent of alteration of Fe-containing metal/alloy increased with temperature, as demonstrated by the presence of secondary minerals (Fig. 6). Magnetite (Fe₃O₄) was the most abundant alteration product, predominantly forming euhedral to subhedral crystals up to several μm in size (Fig. 6a, b, d, e). Pseudomorphoses of magnetite after reacted spherical Fe⁰ particles were common (Fig. 6d). The second most common alteration phase were Fe-(oxy)hydroxides (e.g., lepidocrocite) usually of amorphous appearance or forming needle-like (Fig. 6c) and platy crystals several tens of nm thin and several μm long. Both magnetite and Fe-(oxy)hydroxides commonly occur simultaneously in all reacted samples (SEM) (Fig. 6a,b); however, only magnetite is identified by the XRD method (Fig. 7). This suggests that the Fe-(oxy)hydroxides either lack long range order (e.g., "X-ray amorphous") and/or their abundance is less than 5%, the approximate detection limit of XRD. In general, the lower the Fe content, the lower the extent of alteration (Fig. 6e). In contrast, reacted Ni⁰ exhibited no microscale (SEM) evidence of reaction (Fig. 6f, compare with Fig. 3a), as corroborated by the XPS spectra documenting the presence of residual zero-valent Ni species on the surface (Fig. 8). As a result of solution interactions with metals/alloys, the resulting pH in most experiments was higher than the starting value (see discussion) (Tab. 1).

Figure 6.

SEM microphotographs of reacted metals and alloys. A) Fe⁰ reacted in N₂(aq) solution depicting coexisting magnetite and Fe-(oxy)hydroxides; B) Detail of magnetite single crystals formed on Fe⁰; C) clusters of needle-like Fe-(oxy)hydroxide crystals formed on Fe⁰; D) pseudomorphoses of magnetite after Fe⁰ in the H₂/N₂ solution; E) magnetite crystals formed on Ni₅₀Fe₅₀ in the KNO₃ solution; F) Ni⁰ skeletal crystal revealing no change after reaction.

Figure 7.

Normalized XRD patterns of unreacted (lower) reacted (upper) Fe⁰ showing the presence of magnetite (Mt).

Figure 8.

Ni 2p3/2 peaks showing the speciation of Ni on the surface Ni metal powder at various stages of the experiment. Note the presence of residual zerovalent Ni on the surface after 24 hour reaction at 200°C.

Discussion

Nitrogen in alloys and metals

All the metals and alloys investigated in this work were found to contain N, which resulted in a background production of NH₄⁺. The presence of atomic N was based on two lines of evidence: 1) The presence of the N1s peak in the XPS spectra of starting metals and alloys even after "cleaning" the surface by sputtering with Ar⁺ ions (Fig. 9) and; 2) quantitative analysis of N_tot content of the starting materials by inert gas fusion with a thermal conductivity detection (Tab. 3).

Figure 9.

N 1 s peaks centered around 400 eV documenting the presence of N in the tested alloys and metals after sputtering with Ar⁺ (Ni⁰ 400.1 eV, Ni₅₀Fe₅₀ 399.7 eV, Fe⁰ 398.9 eV).

Table 3.

Total N content of metals and alloys used in this study as determined by inert gas fusion – thermal conductivity method (IMR Test Labs).

Sample	NTOTAL [wt.%]
Fe0	0.0124 ± 0.001
Ni0	0.0012 ± 0.0005
Ni50Fe50	0.0030 ± 0.0005
Ni81Fe19	0.0009 ± 0.0005

Commercially available Ni, Fe metals/alloys may contain N due to the manufacturing process which employs either inert (N₂, Ar) or reducing (NH₃, H₂) atmospheres to prevent oxidation [38,39] [Alfa Aesar, Goodfellow – pers.comm]. For example, during the synthesis, N₂ chemisorbs on the surface of molten/hot metal and dissociates (Equation 1).

N₂ (gas) → 2N (in metal) (1)

In the subsequent step N enters the structure via diffusion or convection to form primarily monoatomic interstitial and to lesser extent substitutional solid solutions. This process is governed by the Sievert's law (Equation 2), which predicts that diatomic gases such as N₂ dissolve in metals (c_N) proportionally to the square root of the partial pressure (p_N2) in the coexisting gas phase [40,41].

$$K=\frac{c_N^2}{p_{N_2}}$$ (2)

Assuming homolytic N₂ bond cleavage, each N atom would have three unpaired electrons available for bonding with the surrounding metal atoms. XPS spectra collected from unreacted metals/alloys in our experiments point to nitride (N^3-, 398.6 eV) as the likely N species (Fig. 9) [42]. Consequently, upon release into the solution, N^3- is expected to react with available protons to form NH₃/NH₄⁺ (Reaction 3, 4) and contribute to their high background productivity.

N^3- + 3H⁺ → NH₃ (3)

N^3- + 4H⁺ → NH₄⁺ (4)

However, undissociated N₂ gas may get trapped in the molten metal (e.g., in inclusions) as well.

Dinitrogen reduction

Although batch-type experiments, such as the ones described above, provide little insight into the kinetics of a reaction and much less the reaction mechanism, the results do allow one to compare the amount of NH₃/NH₄⁺ formed under different conditions. In addition the results can be placed in the context of previously published research related to the reduction of N-species to NH₃/NH₄⁺. Due to the immense importance of NH₃/NH₄⁺ in industry and agriculture, several decades of research exist on its synthesis and production from N₂ gas [e.g., [43]]. The industrial Haber-Bosch process utilizes Fe⁰ catalyst at high temperatures and pressures (~500°C, ~100 bars) to synthesize NH₃ from H₂ and N₂ gas (Reaction 5).

N₂(g) + 3H₂(g) → 2NH₃(g) (5)

In brief, the reaction proceeds as follows: sorption of H₂ and N₂ gases on the surface is followed by the formation of atomic H_ads and N_ads (dissociative sorption). Fe⁰ then facilitates electron transfer from H_ads to N_ads (e.g., N reduction), followed by the formation of NH₃ gas on the surface and subsequent desorption. The dissociative chemisorption of N₂ is generally taken to be the rate-limiting step [44]. For comparison, modern life overcomes the reaction's activation barrier using the enzyme nitrogenase composed of dinitrogenase (MoFe center) and dinitrogenase reductase (Fe center) proteins. In Reaction 6, Fd stands for ferredoxin, the electron-transfer protein [45,46].

N₂(g) + 8Fd⁰_red + 10H⁺ → 2NH₄⁺ + 8Fd⁺_ox + H₂(g) (6)

Analogous overall reactions of abiotic N₂ reduction can be written for aqueous solutions (Reaction 7, 8), although it is important to note that since this reaction requires a transfer of multiple electrons, several reaction intermediates must be involved. Once in solution, NH₃ and NH₄⁺ exist in a pH dependent equilibrium (Equation 9; valid for 25°C).

N₂(aq) + 2H⁺ + 3H₂(aq) → 2NH₄⁺ (7)

N₂(aq) + 3H₂(aq) → 2NH₃(aq) (8)

$$pH=9.25+\log \left\{\frac{[N{H}_3]}{[N{H}_4^{+}]}\right\}$$ (9)

We hypothesize that reactions between H₂, N₂ and the metal/alloy surface are taking place in our experiments; however, they are orders of magnitude slower than those described above in the gas phase. Undoubtedly, this is due to the fact that aqueous reactions occurring in our experiments are not only more complex but also take place at much lower T, P conditions and H₂/N₂ concentrations than those typical for Haber-Bosch synthesis. Such kinetic constraints could explain low NH₄⁺ yields, even though the thermodynamic equilibrium models predict NH₄⁺ to be the dominant N species (Tab. 4).

Table 4.

Predicted equilibrium pH and nitrogen speciation in the N-H₂O-Fe-Ni system (only species with molalities above 1·10^-8 shown). Concentrations and fugacities of dominant N species are in bold. The last column shows the predicted final mineral assemblage.

Mineral	T [°C]	pH	mNH3	mNH4+	mN2	mH2	fNH3	fN2	fH2	Assemblage
Nickel	200	6.95	6.4·10-4	4.63·10-5	2.47·10-4	3.73·10-5	1.24·10-3	0.2769	0.24	BN, NI
Nickel	70	8.92	1.00·10-3	1.34·10-4	2.2·10-5	2.68·10-7	9.98·10-5	0.0477	3.68·10-4	BN, NI
Nickel	22	10.2	1.03·10-3	1.34·10-4	8.12·10-6	2.18·10-8	1.41·10-5	0.0118	2.71·10-5	BN, NI
Awaruite	200	7.07	1.12·10-3	6.14·10-5	<1·10-8	0.0597	2.16·10-3	<1·10-8	38.54	AW, MT, NI
Awaruite	70	8.93	1.04·10-3	1.37·10-4	<1·10-8	0.0152	1.04·10-4	<1·10-8	20.85	AW, MT, NI
Awaruite	22	10.2	1.05·10-3	1.35·10-4	<1·10-8	0.0103	1.44·10-5	<1·10-8	12.82	AW, MT, NI
Tetrataenite	200	8.7	5.92·10-4	7.96·10-7	<1·10-8	0.0865	1.14·10-3	<1·10-8	55.85	AW, MT, TT
Tetrataenite	70	8.93	1.04·10-3	1.37·10-4	<1·10-8	0.0222	1.04·10-4	<1·10-8	30.51	AW, MT, TT
Tetrataenite	22	10.2	1.05·10-3	1.35·10-4	<1·10-8	0.0159	1.44·10-5	<1·10-8	19.73	AW, MT, TT
Iron	200	7.07	1.15·10-3	6.22·10-5	<1·10-8	1.532	2.22·10-3	<1·10-8	989.1	MT, FE
Iron	70	8.93	1.06·10-3	1.38·10-4	<1·10-8	0.785	1.05·10-4	<1·10-8	1077	MT, FE
Iron	22	10.2	1.06·10-3	1.36·10-4	<1·10-8	0.623	1.45·10-5	<1·10-8	771.5	MT, FE
Goethite	200	5.59	<1·10-8	<1·10-8	5.86·10-4	<1·10-8	<1·10-8	0.6574	<1·10-8	HM
Goethite	70	6.1	<1·10-8	<1·10-8	5.86·10-4	<1·10-8	<1·10-8	1.273	<1·10-8	HM
Goethite	22	6.22	<1·10-8	<1·10-8	5.86·10-4	<1·10-8	<1·10-8	0.8475	<1·10-8	HM
Magnetite	200	6.08	1.04·10-5	5.42·10-6	5.82·10-4	1.79·10-6	2.0·10-5	0.653	1.16·10-3	HM, MT
Magnetite	70	8.47	1.26·10-4	4.73·10-5	5.04·10-4	2.36·10-8	1.25·10-5	1.094	3.24·10-5	HM, MT
Magnetite	22	9.97	3.1·10-4	7.35·10-5	3.98·10-4	<1·10-8	4.26·10-6	0.5759	3.32·10-6	HM, MT
Ferrihydrite	200	5.59	<1·10-8	<1·10-8	5.81·10-4	<1·10-8	<1·10-8	0.6512	<1·10-8	HM
Ferrihydrite	70	6.1	<1·10-8	<1·10-8	5.81·10-4	<1·10-8	<1·10-8	1.261	<1·10-8	HM
Ferrihydrite	22	6.22	<1·10-8	<1·10-8	5.81·10-4	<1·10-8	<1·10-8	0.8396	<1·10-8	HM

Abbreviations: BN = bunsenite, NI = Ni metal, AW = awaruite, MT = magnetite, TT = tetrataenite, FE = Fe metal, HM = hematite.

We speculate that in the presence of Fe⁰ most of the NH₄⁺ was rapidly formed in the first few hours of the experiment when unreacted surface was still available for reaction. In this scenario (e.g., run 2) most (and possibly all) H₂ is formed in situ as a result of interactions between the pristine Fe⁰ surface and H₂O (Reaction 10) (e.g., mH₂ = 0 at t₀); however, the simultaneous Fe oxidation passivates the surface, reduces the availability of suitable H₂/N₂ sorption sites and the overall yield of the N₂-reduction reaction.

Fe + 2H₂O → H₂ + Fe²⁺ + 2OH^- (10)

This notion is corroborated by experiments carried out with conditions in which H₂ was present in the system from the start of the reaction (e.g., run 3), as a result of purging the solution with a H₂/N₂ mixture prior to loading (e.g., mH₂ > 0 at t₀). Abundant H₂ in this run correlates with greater NH₄⁺ production as the overall N₂ conversion rate increases from 2.5 to 10% (Tab. 2). Assuming that in both cases Fe⁰ surface passivates at the same rate, then the H₂ purged system produces more NH₄⁺ per unit of time because it does not depend on the Fe⁰ alteration process (Reaction 10) to provide H₂. This circumstance may be more typical of natural serpentinization-driven SHS where H₂ can be provided by a number of processes, especially by Fe²⁺ oxidation during alteration of rock-forming silicates [17,47].

Similar assumptions can be made about Ni₅₀Fe₅₀ and Ni₈₁Fe₁₉ assuming that Fe atoms exposed on the surface played a role in the reduction process. Due to good corrosion resistance, Ni⁰ reacted to a much lesser extent and consistent with this lower activity is our experimental observation that the Ni⁰ surface was not significantly altered (e.g., by precipitation of neoformed phases) throughout the experiment. Different modes of metal/alloy participation in studied reactions are discussed below.

The addition of KCl into the Fe⁰-H₂O-N₂ system in our experiments resulted in higher NH₄⁺ yield (Fig. 4a). While it may be intriguing to draw parallels with the Haber-Bosh process, where K is added to improve sticking coefficients and to help stabilize sorbed species [44], the apparent promoting effect of KCl may be partially or entirely caused by the presence of chloride ion (Cl^-) in the solution. Cl^- can react with dissolved iron in the solution (Reaction 11) and remove products from the Fe⁰ dissolution reaction (Reaction 10).

Fe²⁺ + 2Cl^- → FeCl₂ (11)

Such a complexation reaction would result in an equilibrium shift towards the product side and further drive the dissolution process and release of structurally bound reduced N species into the solution (Reactions 3, 4). This is in agreement with the results of Reardon [48] who observed an increase in Fe⁰ corrosion rates in low ionic strength (~0.02 m) anaerobic NaCl, NaHCO₃ and Na₂SO₄ solutions compared to DI water. The NH₄⁺ content of the KCl reagent solution at concentrations used in our experiments was found to be below the detection limit of ion chromatography.

Nitrite and Nitrate Reduction

Due to their status as environmental contaminants, NO₂^-/NO₃^- reduction has been extensively studied, especially focused on the reduction of NO₃^- by Fe⁰. Most of the published results concur that the reduction reaction (Reaction 12) at anaerobic ambient conditions exhibits the following set of features: 1) reaction rates decrease with increasing pH; 2) pH in unbuffered solutions becomes more alkaline as the reaction progresses; 3) NH₄⁺ and Fe₃O₄ are the dominant reaction products; 4) NO₃^- reduction slows down once Fe⁰ is coated with Fe₃O₄ unless Fe²⁺, Cu²⁺, Al³⁺ or Fe³⁺ are present; 5) SO₄^2- inhibits the reaction; and 6) the molar N₂:NH₄⁺ ratio in reaction products increases with pH [49-57].

4Fe⁰ + 10H⁺ + NO₃^- → NH₄⁺ + 4Fe²⁺ + 3H₂O (12)

3Ni₃Fe + 4NO₂^- + 24H₂O → 32OH^- + 4NH₄⁺ + 3Fe²⁺ + 9Ni²⁺ (13)

Analogous reactions can be written for other metals/alloys as well as NO₂^- (Reaction 13), although they have been by comparison less studied. The absence of resonance structures in the NO₂^- molecule makes it easier to reduce than NO₃^- [58], which is reflected in reduction reaction rates. For example, NO₃^- reduction to NH₄⁺ in the presence of Fe²⁺ was found to be a factor of 8 slower than that of NO₂^- [5]. Because the reduction from NO₃^- to NH₄⁺ requires transfer of at least 8 electrons, several intermediates must be formed in the process. Moreover, the formation of any N-N bonds must be avoided because it is effectively inert under all but the highest temperatures investigated here.

It is widely recognized, however, that NO₂^- is a reaction intermediate in NO₃^- reduction [e.g., [51]] [57,59]. For example, Wärna et al [59] proposed a reaction sequence from NO₃^- and NO₂^- through nitric oxide (NO), imidogen (HN˙) and aminyl radical (H₂N¨) to NH₃/NH₄⁺ on the surface of Fe⁰. Several studies with NO₂^-/NO₃^- as well as some organic compounds suggest that Fe²⁺, Fe⁰, and Fe²⁺ sorbed on neoformed Fe minerals (e.g., magnetite) are likely electron donors for the reduction reactions [e.g., [60]] [61,62]. The intriguing consequence of such a reaction mechanism in natural systems is that precipitation of secondary (neoformed) Fe minerals further along the flow path followed by surface sorption of Fe²⁺ would provide additional reaction sites for the reduction of NO₂^-/NO₃^- [e.g., [63]].

The role of alloys/metals

Based on the XPS (oxidation state of Ni) and SEM (abundance of Fe- and the absence of Ni alteration phases) results it is possible to construct an order of apparent stability of studied alloys and metals

Ni⁰ > Ni₈₁Fe₁₉ > Ni₅₀Fe₅₀ > Fe⁰

where Ni⁰ is most- and Fe⁰ is least stable under the studied experimental conditions. This enables us to generalize that the higher the Fe content, the higher the reactivity towards potential oxidizing agents (e.g., H₂O, NO₂^-, NO₃^-) and thus the higher extent of alteration. Metals and alloys typically undergo reductive dissolution (e.g., Reaction 10); however alloys frequently dissolve the less noble metal preferentially, leaving the surface enriched in the more noble metal [e.g., [64]]. For example, the reductive dissolution of Ni₅₀Fe₅₀ alloys is expected to result in preferential release of Fe and a concomitant increase in the Ni:Fe ratio of the residual alloy.

Our findings are in agreement with metallurgical studies in which it has been demonstrated that Ni⁰ is more corrosion resistant than Fe⁰, a notion that serves as a basis for their frequent alloying [38,65]. Unlike Fe⁰, Ni⁰ reacts to a lesser degree in aqueous environments (reaction produces H₂ and Ni²⁺), especially under reducing conditions. The presence of an oxidizing agent is usually required for significant corrosion; however, a protective oxide film may develop and impede further reactions [38,66]. Ni⁰ with a combination of catalytic properties and corrosion resistance (e.g., slow dissolution kinetics) may serve as a basis for a unique mechanism of N₂ reduction, where Ni acts both as a reactant and a catalyst. We hypothesize that Ni⁰ reacts with H₂O to produce H₂, a portion of which may stay adsorbed on the surface in its atomic form (H_ads) (Reaction 14). If N₂ is also dissociatively chemisorbed (Reaction 15), surface-mediated reduction reactions may proceed (Reaction 16).

Ni + 2H₂O → 2H_ads + Ni²⁺ + 2OH^- (14)

N₂(aq) → 2N_ads (15)

2N_ads + 2H⁺ + 6H_ads → 2NH₄⁺ (16)

This set of reactions may operate until all plausible sorption sites are exhausted and/or deactivated. By analogy, we argue that if the experimental conditions were approaching the stability field of Fe metal (e.g., at sufficiently high fH₂) it could behave in a similar manner.

There exist; however, "true" catalytic systems for NO₂^-/NO₃^- reduction, such as bimetallic Cu-Pt and Cu-Pd, Ag-Pd, Ag-Pt, which couple a noble metal and an oxidizable promoter. The reactions take place on the surface of Cu⁰ which acts as an electron donor for the reduction of N species and as an acceptor of electrons from dissociative sorption of H₂ on the surface of Pt [67,68]. Even though natural alloys of platinum group elements (Pt, Pd, Ir, Os, Rh, Ru) are scarce on modern Earth and are almost exclusively limited to magmatic segregation deposits, placers, and meteorites [e.g., [69]] [70,71], their significance for prebiotic synthesis should not be overlooked [72].

The predominantly alkaline pH in reacted samples (Tab. 2) is likely a result of several pH controlling reactions such as reductive dissolution of metals (e.g., Reaction 10), mineral formation (e.g., magnetite), and the buffering reactions involving charged species including, but not limited to NO₃^-, NO₂^-, NH₃, or NH₄⁺. The fate of Fe²⁺ in the experiments reported here is difficult to constrain. Assuming completely anoxic conditions, temperatures below 85°C and a negligible pCO₂ in our experiments, Fe(OH)₂ (white rust) could precipitate (Reaction 17) and due to its instability serve as a precursor to other Fe oxides and hydroxides, most notably Fe₃O₄ (Reaction 18).

Fe⁺⁺ + 2OH^- → Fe(OH)₂ (17)

3Fe(OH)₂ → Fe₃O₄ + 2H₂O + H₂ (18)

At higher temperatures and/or in the presence of trace levels of O₂ or other oxidizing agents (e.g., NO₃^-), mineral intermediates such as green rust (mixed-valence hydroxide) may have been involved [e.g., [73]] [74,75].

The conversion of the original metal/alloy into a new mineral phase (e.g., coatings) may not necessarily negatively affect the NH₃/NH₄⁺ production. For example, wüstite (FeO) and green rust – both possible reaction products/intermediates during anaerobic Fe⁰ oxidation, have been shown to reduce NO₂^-/NO₃^- [e.g., [50]] [76-78]. Magnetite and even goethite can also act as reductants for NO₂^-/NO₃^-, provided cations such as Fe²⁺, Cu²⁺, Fe³⁺, Al³⁺ are present in the system [e.g., [53]] [56,79]. Equilibrium thermodynamic calculations, however, predict very little reactivity of magnetite, goethite, or ferrihydrite alone towards N₂ and anaerobic experiments in the presence of green rust at ambient T, p conditions corroborate these predictions [80].

Implications for the Hadean Earth

Different modes of metal/alloy participation have different implications for natural systems, especially in terms of the amount of metal/alloy required to achieve the same NH₃/NH₄⁺ production. A catalyst remains stable during the reaction and therefore a small amount can, in theory, catalyze the formation of large amounts of NH₃/NH₄⁺. Conversely, a reactant would have to be present in sufficient amounts and/or would have to be continually formed in order to achieve comparable NH₃/NH₄⁺ production. While both mechanisms are plausible on the Hadean Earth, it is hard to assess which of the two would be prevalent. Based of equilibrium geochemical modeling, Smirnov [28] concluded that at 200°C Ni metal is stable at fH₂ orders of magnitude lower than Fe metal and even Ni₅₀Fe₅₀ (tetrataenite) and Ni₈₁Fe₁₉ (awaruite). Combined with results acquired from this study, it would appear that Ni metal is the most suitable candidate for a sustained long-term NH₃/NH₄⁺ formation. Moreover, if we consider that the Hadean atmosphere may have had up to 30% H₂ [29], the primordial ocean would contain significantly higher concentrations of dissolved H₂ than today. In SHS, H₂ from advected seawater combined with H₂ formed by serpentinization could create conditions sufficiently reducing for stabilization of Fe containing alloys (e.g., awaruite, tetrataenite) and possibly even Fe⁰. Furthermore, as shown by Schoonen et al [72], seawater trapped in closed SHS (i.e., not open to seawater circulation) evolves to become extremely reduced as the partial pressure of hydrogen builds up.

The possibility of hydrothermal reduction of N₂ to NH₄⁺ permits us to attempt to constrain the total NH₄⁺ flux from Hadean off-axis SHS. The following set of assumptions and variables were taken into account:

1) The total heat production of the Hadean Earth was several times higher than today [81,82]. Because the exact value is unknown, we calculated scenarios for 2- to 8-times the present day heat flow (PDHF = 4.3 × 10¹³ W) [83] (Fig. 10); however, only values between 4 and 8 times PDHF are reported.

Figure 10.

Hadean hydrothermal flow as a function of total Earth's heat flow (expressed as multiplicities of present day heat flow – PDHF). Each data line thus represents percentage of the total heat flow released through hydrothermal systems at a given value of Hadean heat flow. The shaded area represents assumed realistic scenarios for the Hadean.

2) Because it is unclear if a global tectonic cycle was operational during the Hadean, we are unable to comment on the dissipation of Earth's internal heat, especially on the percentage of heat released through SHS. Therefore, two endmember scenarios are considered: a) heat is dissipated predominantly via volcanism (possibly through several supervolcanoes) and only 5% is released through hydrothermal activity; and b) 80% of heat is dissipated predominantly through hydrothermal activity (Fig. 10). For comparison, presently about 20% of PDHF is released through hydrothermal activity [83].

3) Due to the increased heat flow and possibly due to the blanketing effect of the atmosphere [e.g., [84]], we assume the mean ocean water temperature to be 70°C. Although modern serpentinization-driven SHS commonly vent fluids below 100°C [85-87], we assume that the higher overall heat flow in the Hadean would also increase the fluid temperature of hydrothermal vents [e.g., [88]]. The temperature of the discharging fluid is thus assumed to be 200°C for the purpose of this calculation. Although the temperature of ambient seawater and of discharging fluid directly influences the total hydrothermal fluid mass flux (equation 19), their variations (± 20°C) only produced small changes in the final NH₄⁺ fluxes (usually within the same order of magnitude; data not shown). The heat capacity (c_p) of hydrothermal seawater at 200°C and P ~100–600 bar is 4.1 J.g^-1K^-1[89].

4) Ocean water is assumed to be in equilibrium with 1 bar of N₂, resulting in a dissolved N₂(aq) concentration of 0.481 mmol.kg^-1 at 70°C [90]. For simplicity, no other gases and/or aqueous ions were taken into consideration.

5) Even though experimental results reported in this study suggest a conversion of N₂-to-NH₄⁺ 0.2 to 2.5% we calculate a variety of scenarios ranging from 0.1% to 10%. Although the 10% conversion may appear overly optimistic, our experiments suggest that the presence of advected H₂ and or K⁺ may significantly improve the NH₄⁺ production (Tab. 2; Fig. 4a). Metals/alloys may act as either catalysts or reactants, however, if metals/alloys do react, it must be assumed that the rate of their destruction (e.g., passivation, poisoning) is equal to their rate of formation (e.g., via serpentinization). It is important to point out that for simplicity, we do not distinguish between respective metals/alloys and we are only concerned with their capability to facilitate the conversion of N₂ to NH₄⁺ (in %).

The mass flux of seawater through hydrothermal systems (F) can be estimated from heat flux (H) in Watts, heat capacity of seawater (c_p at 200°C) in J.g^-1K^-1 and temperature anomaly ΔT in Kelvin [83]:

$$F=\frac{H}{c_p\Delta T}.$$ (19)

Using hydrothermal heat fluxes from Fig. 10 we can calculate annual seawater mass fluxes from SHS. Subsequently, using various N₂-to-NH₄⁺ conversion percentages (0.1 to 10%), annual NH₄⁺ production of Hadean SHS is calculated (Fig. 11). Assuming the most conservative scenario with 0.1% conversion of N₂ to NH₄⁺, the annual NH₄⁺ production would be between 5.9 × 10⁸ mol (4 × PDTH) and 1.2 × 10⁹ mol (8 × PDTH) if 5% of Earth's heat is removed via SHS and between 9.4 × 10⁹ mol (4 × PDTH) and 1.9 × 10¹⁰ mol (8 × PDTH) if 80% of heat is removed via SHS. Conversely, with a 10% N₂ conversion efficiency, the annual NH₄⁺ production would be between 5.9 × 10¹⁰ mol (4 × PDTH) and 1.2 × 10¹¹ mol (8 × PDTH) if 5% of heat is removed via SHS and between 9.4 × 10¹¹ mol (4 × PDTH) and 1.9 × 10¹² mol (8 × PDTH) if 80% of heat is removed by SHS (Tab. 5). To place these modeled fluxes in context we can compare their magnitude to those of other proposed NH₃/NH₄⁺ formation mechanisms (Fig. 11). An annual NH₄⁺ flux at 1% conversion efficiency, for example, would be comparable to that based on a homogeneous reaction (Reaction 20) of Summers and Chang [4] or to the flux calculated by Brandes et al [13], which was based on NH₃ formation in the presence of various minerals between 300 and 800°C.

Figure 11.

NH₄⁺ formation from N₂ in Hadean hydrothermal systems. Fluxes are calculated as a function of N₂ conversion between 1 and 10%. NH₄⁺ formation from NO₂^-/NO₃^- is not included in these calculations.

Table 5.

Total N content of four octahedrites analyzed by inert gas fusion (IMR Test Labs).

Sample	NTOTAL [wt.%]
Bogou Meteorite	0.0032 ± 0.0005
N'Goureyma Meteorite	0.0022 ± 0.0005
Sikhote Alin Meteorite	0.0032 ± 0.0005
Canyon Diablo Meteorite	0.0022 ± 0.0005

6Fe²⁺ + 7H⁺ + NO₂^- → 6Fe(III) + 2H₂O + NH₃ (20)

Although we cannot comment on the total NH₄⁺ content of the Hadean Ocean, we can estimate the contribution of hydrothermal N₂ reduction per unit of time. For timescales longer than 1 year, the following equation may be used (21):

$$N{H}_4^{+}additiontotheocean=\frac{annualN{H}_4^{+}production}{V_{HadeanOcean}}•t.$$ (21)

NH₄⁺ production in mol.yr^-1 can be taken from Fig. 11 or Tab. 6 (or supplied from reader's own sources), t denotes the time period in years and V_ocean is the total volume of Hadean Ocean in liters. We have calculated a scenario for one million years using the present-day global ocean volume (1.37 × 10²¹ L) but alternative calculations can be quickly performed for different volumes (e.g., if the Hadean ocean had twice the volume of the present-day ocean, the NH₄⁺ concentrations in Fig. 12 and Tab 7 would be 50% smaller). For example, the conversion of 0.5% of N₂ entrained in the advecting seawater would raise the NH₄⁺ content of the (completely homogenized) Hadean ocean by ~8 to 17 μmol.kg^-1 (4× to 8× PDHF) every 1 Ma (Tab. 7). These results (Fig. 12, Tab. 7) represent an upper contribution limit of this reaction, because no sinks (ion exchange, photooxidation, loss to the gas phase, formation of organic molecules, etc) were taken into account. In the absence of a comprehensive Hadean Nitrogen Cycle model, it is difficult to quantitatively assess the annual loss of NH₃/NH₄⁺ from the ocean; however, the numbers in Fig. 12, Tab. 7 can be simply amended by assumption of loss expressed in %.

Table 6.

NH₄⁺ formation from N₂ in Hadean hydrothermal systems. Fluxes (mol.kg^-1.yr^-1) are calculated as a function of N₂ conversion between 1 and 10%. Note that NH₄⁺ formation from NO₂^-/NO₃^- is not included in these calculations.

5% of Earth's heat flow is released via hydrothermal activity
N2 conversion	Hadean heat flow (× PDHF)
%	4	5	6	7	8
0.1	5.9 × 108	7.3 × 108	8.8 × 108	1.0 × 109	1.2 × 109
0.5	2.9 × 109	3.7 × 109	4.4 × 109	5.1 × 109	5.9 × 109
1	5.94 × 109	7.3 × 109	8.8 × 109	1.0 × 1010	1.2 × 1010
2	1.2 × 1010	1.5 × 1010	1.8 × 1010	2.1 × 1010	2.4 × 1010
5	2.9 × 1010	3.7 × 1010	4.4 × 1010	5.1 × 1010	5.9 × 1010
10	5.9 × 1010	7.3 × 1010	8.8 × 1010	1.0 × 1011	1.2 × 1011
20% of Earth's heat flow is released via hydrothermal activity
N2 conversion	Hadean heat flow (× PDHF)
%	4	5	6	7	8
0.1	9.4 × 109	1.2 × 1010	1.4 × 1010	1.7 × 1010	1.9 × 1010
0.5	4.7 × 1010	5.9 × 1010	7.1 × 1010	8.2 × 1010	9.4 × 1010
1	9.5 × 1010	1.2 × 1011	1.4 × 1011	1.7 × 1011	1.9 × 1011
2	1.9 × 1011	2.4 × 1011	2.8 × 1011	3.3 × 1011	3.8 × 1011
5	4.7 × 1011	5.9 × 1011	7.1 × 1011	8.2 × 1011	9.4 × 1011
10	9.4 × 1011	1.2 × 1012	1.4 × 1012	1.7 × 1012	1.9 × 1012
80% of Earth's heat flow is released via hydrothermal activity
N2 conversion	Hadean heat flow (× PDHF)
%	4	5	6	7	8
0.1	9.4 × 109	1.2 × 1010	1.4 × 1010	1.7 × 1010	1.9 × 1010
0.5	4.7 × 1010	5.9 × 1010	7.1 × 1010	8.2 × 1010	9.4 × 1010
1	9.5 × 1010	1.2 × 1011	1.4 × 1011	1.7 × 1011	1.9 × 1011
2	1.9 × 1011	2.4 × 1011	2.8 × 1011	3.3 × 1011	3.8 × 1011
5	4.7 × 1011	5.9 × 1011	7.1 × 1011	8.2 × 1011	9.4 × 1011
10	9.4 × 1011	1.2 × 1012	1.4 × 1012	1.7 × 1012	1.9 × 1012

Figure 12.

Estimated increase in NH₄⁺ concentration of the Hadean Ocean (in μmol.L^-1) from the hydrothermal N₂ reduction reaction per 1 Ma as a function of N₂-to-NH₄⁺ conversion percentages, heat flow and percentage of heat released via hydrothermal systems (5, 20 and 80%).

Table 7.

Estimated increase in NH₄⁺ concentration of the Hadean Ocean (in μmol.L^-1) from the hydrothermal N₂ reduction reaction per 1 Ma as a function of N₂-to-NH₄⁺ conversion percentages, heat flow and percentage of heat released via hydrothermal systems.

5% of Earth's heat flow is released via hydrothermal activity
N2 conversion	Hadean heat flow (× PDHF)
%	4	5	6	7	8
0.1	0.4	0.5	0.6	0.8	0.9
0.5	2.1	2.7	3.2	3.8	4.3
1	4.3	5.4	6.4	7.5	8.6
2	8.6	10.7	12.9	15.0	17.2
5	21.4	26.8	32.2	37.5	42.9
10	42.9	53.6	64.3	75.0	85.8
20% of Earth's heat flow is released via hydrothermal activity
N2 conversion	Hadean heat flow (× PDHF)
%	4	5	6	7	8
0.1	1.7	2.1	2.6	3.0	3.4
0.5	8.6	10.7	12.9	15.0	17.2
1	17.2	21.4	25.7	30.0	34.3
2	34.3	42.9	51.5	60.0	68.6
5	85.8	107	129	150	172
10	172	214	257	300	343
80% of Earth's heat flow is released via hydrothermal activity
N2 conversion	Hadean heat flow (× PDHF)
%	4	5	6	7	8
0.1	6.9	8.6	10.3	12.0	13.7
0.5	34.3	42.9	51.5	60.0	68.6
1	68.6	85.8	103	120	137
2	137	172	206	240	274
5	343	429	515	600	686
10	686	858	1030	1200	1370

It is imperative to note that due to a large number of unknown and/or poorly constrained variables, these calculations should only be regarded as a first order approximation. However, it is clear that N₂ reduction, albeit very inefficient, could have been a significant source of NH₄⁺, especially in localized environments.

Because NO₂^-/NO₃^- are inherently easier to reduce than N₂, its presence in advected seawater could have significantly increased the annual NH₄⁺ hydrothermal flux. It is unclear; however, how much NO₂^-/NO₃^- would be advected into the crust, especially in the presence of such significant sinks as the reduction by Fe(II) [5]. While this process is a viable pathway to abiotic NH₃/NH₄⁺, its operation is dependent on atmospherically-driven processes of NO₂^-/NO₃^- formation as well as chemical composition of the Hadean Ocean, especially pH and mFe²⁺. A change in one of the parameters (e.g., a shift in oceanic pH) may have negatively affected or completely halted this pathway. We assume that serpentinization-driven SHS would have been less affected by changes in ocean water chemistry because their physico-chemical conditions (e.g., pH, fH₂) are determined by fluid-rock interactions (e.g., availability of fresh rock) and possibly magmatic input rather than ocean composition. Moreover, the high temperature and pressure conditions combined with accumulations of suitable minerals would make these environments well suited for a long term, sustained NH₃/NH₄⁺ production on the Hadean Earth.

Besides facilitating the production of NH₃/NH₄⁺, metals and alloys in SHS may have been involved in other reactions potentially important for prebiotic synthesis. In the context of environmental science, for example, Fe⁰ was found to reduce nitrobenzene [91] or to facilitate reductive dehalogenation of carbon tetrachloride and chloroform [92-94]; Fe²⁺ sorbed on Fe(III) minerals decomposes nitrobenzene [60]. Our future research will also assess SHS as potential sinks of prebiotic molecules during the late Hadean/early Archaean.

The notion that N is commonly present in Fe⁰ in its reduced form presents a possibility of meteoritic delivery of reduced N species to Earth, especially during the periods of heavy bombardment. Fe⁰ and its alloys (e.g., tetrataenite, awaruite, kamacite) are among the dominant mineral phases in iron meteorites and to a lesser extent in stony-iron meteorites [e.g., [95]] [96-99].

To assess the possible importance of meteoric delivery of reduced N to Earth, we submitted four octahedrites for inert gas fusion analyses (IMR Test Labs, Lansing, NY). The meteorites – Bogou (IAB), Sikhote Alin (IIAB), Canyon Diablo (IAB), N'Goureyma (Ungrouped) [100-103] (Stony Brook University's meteorite collection) contained 22 to 32 ppm of N_TOT (Tab. 5). Although these analyses provided no insight into the oxidation state or speciation of nitrogen in these meteorites, it is likely to be present predominantly in the form of nitride (N^3-) as is the case in similar commercially available metals, alloys and known meteorite minerals (e.g., roaldite, carlsbergite). Hence in the following calculation we assume that all meteorite-associated nitrogen is present as nitride. Nitride would be readily released from meteorites after falling into the Hadean Ocean as a result of the rapid and complete dissolution due to inherent instability of Fe⁰ in aqueous solutions (even in O₂-free solutions). Aqueous nitride is expected to react quickly with protons to form NH₃/NH₄⁺. Similar scenario for meteoritic delivery of phosphorus has already been proposed by Pasek and collaborators [104,105].

To constrain the influx of meteoritic N to the Hadean Ocean we have adapted the meteorite flux values used by Pasek et al [104,105]: 2 × 10⁵ kg.year^-1 (current flux of iron meteorites to Earth; 50% of total meteoritic flux by weight) and meteoritic flux 10⁵-10⁶ times the present-day value during the Late Heavy Bombardment Period. Using these values we have created models for varying total reduced N content of iron meteorites: 5, 10, 15, 20, and 30 ppm. For comparison, average and median values for N_TOT from 91 published analyses [106,107] and four analyses acquired in this study were 20.1 and 12.3 ppm respectively.

The results presented in Fig. 13 show that during the Late Heavy Bombardment Period, iron meteorites could have delivered ~10³ to 10⁵ mol.yr^-1 of N_TOT to Earth which is approximately six to nine orders of magnitude less than our estimates for hydrothermal production (Fig. 11). Although the N influx was likely negligible on the global scale it is possible that crater lakes associated with iron meteorite impacts [108] may have contained significant concentrations of NH₃/NH₄⁺. Environments containing NH₃/NH₄⁺ and organic phosphorous compounds (e.g., phosphonates, organophosphates) from corroding iron meteorites ((Fe,Ni₃)P) [104,105] thus could have created very favorable, spatially-restricted conditions for prebiotic synthesis, perhaps unparalleled on the prebiotic Earth.

Figure 13.

Estimated annual flux of nitrogen from iron meteorites. Different line styles correspond to different average N content of iron meteorites. Shaded areas represent iron meteorite flux during the period of Late Heavy Bombardment.

Conclusion

1) N₂ reduction to NH₄⁺ was found to be limited (up to 2.5% at 200°C) compared to NO₂^-/NO₃^- (100% at 200°C)

2) Metals are more effective at reducing NO₂^-/NO₃^- than alloys; NH₄⁺ is the dominant reaction product.

3) The reduction process exhibits a strong temperature dependence.

4) Fe⁰ and Ni⁰ were found to be least- and most resistant to alteration, respectively.

5) Ni⁰, Fe⁰, Ni₅₀Fe₅₀, Ni₈₁Fe₁₉ were found to contain up to 124 ppm of nitrogen in their structures, some of which is released upon dissolution and reacts to form NH₄⁺.

6) Serpentinization-driven SHS were likely important sources of abiotic NH₃/NH₄⁺ in the Hadean Ocean.

Competing interests

The authors declare that they have no competing interests.