# The contribution of mechanoradical reactions to crustal hydrogen generation

Nicolas Lefeuvre, Laurent Truche, Frédéric-Victor Donzé, Johan Vandenborre,
Eric Gaucher, Valérie Magnin

► To cite this version:

Nicolas Lefeuvre, Laurent Truche, Frédéric-Victor Donzé, Johan Vandenborre, Eric Gaucher, et al. The contribution of mechanoradical reactions to crustal hydrogen generation. Earth and Planetary Science Letters, 2025, 660, pp.119363. 10.1016/j.epsl.2025.119363. hal-05071256

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Submitted on 17 May 2025

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# The contribution of mechanoradical reactions to crustal hydrogen generation

Nicolas Lefeuvre<sup>a,b,*</sup>, Laurent Truche<sup>a</sup>, Frédéric-Victor Donzé<sup>a</sup>, Johan Vandenborre<sup>c</sup>, Eric C. Gaucher<sup>d,†</sup>, Valérie Magnin<sup>a</sup>

<figure><img src="image_3.png" /></figure>

## ARTICLE INFO

Editor: J.P. Avouac

**Keywords:**

H₂ generation

H₂ fluxes

Serpentinization

Radiolysis

Faults

Earthquakes

## ABSTRACT

Hydrogen (H₂) generation within the Earth's crust contributes significantly to abiotic organic synthesis and the sustenance of deep microbial ecosystems. While natural H₂ is recognized as a potential primary energy resource, current exploration models focus solely on two sources, serpentinization and water radiolysis, leaving other H₂-generating processes largely unexplored. This study investigates mechanoradical reactions specifically, H₂ production resulting from mineral grinding in the presence of water, a process analogous to rock abrasion along fault planes during earthquakes. We performed laboratory experiments simulating these conditions by grinding quartz under controlled conditions of fluid pH, ionic strength, water/rock (W/R) ratio, and grinding energy. Our results show that H₂ production is significantly affected by these parameters. Notably, grinding in acidic environments (6< pH < 4) tripled H₂ production compared to alkaline conditions (pH > 8). Increasing the W/R ratio from 0.1 to 1 resulted in an 18-fold enhancement of H₂ production. Additionally, a linear relationship was observed between H₂ production and the grinding energy applied. Extrapolating our findings to natural fault movements, we estimate that mechanoradical reactions during earthquakes of magnitude greater than 4 generate approximately 1.45 × 10¹³ mol yr⁻¹ of H₂. Although this production rate involved significant uncertainties related to the model assumptions and may not be directly compared to serpentinization and radiolysis - since these H₂-producing processes operate on different time and space scales - our study underscores the importance of including mechanoradical processes in models of crustal H₂ fluxes. Recognizing these reactions expands our understanding of subsurface H₂ generation and its contributions to geochemical and microbial processes in the Earth's crust.

## 1. Introduction

Hydrogen (H₂) anomalies in soils above seismically active faults have been documented since the 1980s (Kita et al., 1980; Sato et al., 1985; Wakita et al., 1980). These anomalies are significant because H₂ can serve both as a precursor indicating seismic activity and as a proxy for mapping surface faults (Kita et al., 1982; Li et al., 2013; Xiang et al., 2020). The movement of faults during earthquakes can release trapped gases, including H₂, after the seismic event. Moreover, H₂ can be generated directly through the crushing of host rocks in fault zones. This process exposes freshly fractured mineral surfaces to aqueous fluids, leading to the formation of highly reactive radical species (Arends et al., 1963; Hasegawa et al., 1995; Walters and Estle, 1961). The

recombination of these radicals results in the rapid production of H₂ and H₂O₂ (Kameda and Okamoto, 2021; Narayanasamy and Kubicki, 2006). The H₂ produced may act as a reducing agent, thereby modifying the redox state of fluids circulating along fault planes. This redox variability has been documented along the Nobeoka Thrust, where Yamaguchi et al. (2011) observed fluctuations in fluid redox conditions before and after seismic rupture. Fluids are generally more oxidizing prior to rupture, while stress release promotes mechanoradical H₂ production and drives a shift toward more reducing conditions. However, the specific role of redox-sensitive species in modulating H₂ generation during mechano-chemical reactions remains to be fully investigated. Consequently, faults act not only as mechanically dynamic zones but also as significant sites of mechano-chemical activity (Kita et al., 1982).

<footnote><sup>a</sup> ISTerre, CNRS, Université Grenoble Alpes, Grenoble, France</footnote>

<footnote><sup>b</sup> LFCR, University of Pau and Pays de l'Adour, 64000, Pau, France</footnote>

<footnote><sup>c</sup> Nantes Université, CNRS, Institut des Matériaux de Nantes Jean Rouxel, IMN, F-44000 Nantes, France</footnote>

<footnote><sup>d</sup> Lavoisier H₂ Geoconsult, 74400, Chamonix, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: nicolas.lefeuvre@univ-pau.fr (N. Lefeuvre).</footnote>
Hydrogen anomalies associated with active faults have been observed worldwide, including in Japan (Dogan et al., 2007; Kita et al., 1980; Satake et al., 1985; Sugisaki et al., 1983), China (Fang et al., 2018; Gong et al., 2017, 2015; Li et al., 2013; Xiang et al., 2020; Zhou et al., 2010), France (Deronzier and Giouse, 2020; Lefeuvre et al., 2022) and the United States (Jones and Pirkle, 1981; Ware et al., 1985; Wiersberg and Erzinger, 2008). Beyond fault zones, H₂ production via mechanor-adical processes occurs in other natural contexts, such as during the abrasion of bedrock by glacier movement (Telling et al., 2015). Additionally, artificial activities like drilling into rock or certain soils can induce H₂ generation, with quantities depending significantly on the organic matter and silica content of the material, potentially exceeding 1 vol. % H₂ (Halas et al., 2021; Lefeuvre et al., 2021; Wenger et al., 2009).

Hydrogen generation through mechanochemical reactions in the crust could therefore be a widespread process that should be considered in global estimates of H₂ fluxes (Klein et al., 2020). In particular, H₂ production during quartz grinding in the presence of water is well documented (Delogu, 2011; Kameda et al., 2003; Kita et al., 1982; Narayanasamy and Kubicki, 2006; Saruwatari et al., 2004). The mechanical breaking of Si–O bonds in quartz generates reactive surface sites that interact with water, leading to the formation of silanol groups and the release of H₂ (Fubini, 1998). The newly formed surface radicals recombine or react with water, producing hydrogen radicals that can combine to form molecular hydrogen (Narayansaly and Kubicki, 2006). The efficiency of these reactions is temperature-dependent; higher temperatures (>200 °C) reduce H₂ production due to changes in reaction kinetics (Kita et al., 1982).

Understanding H₂ generation via mechanochemical reactions is important not only for earthquake prediction but also for evaluating its role in deep microbial ecosystems, abiotic organic synthesis, and as a potential energy resource. However, comparing the relative significance of mechanochemical H₂ production with other crustal H₂-generating processes, such as serpentinization (or more broadly, the oxidation of Fe²⁺-bearing minerals), water radiolysis, and organic matter decomposition, remains challenging due to the lack of standardized metrics and unified experimental frameworks. Previous rock-crushing experiments often report results based on milling rates (e.g., rotations per minute), limiting the ability to generalize findings (Hirose et al., 2011). Moreover, the influence of factors such as fluid composition and water/rock ratio on H₂ production remains inadequately quantified.

This experimental study aims to address these gaps by systematically documenting the effect of grinding energy on H₂ production during quartz crushing as a function of solution composition. We conduct a parametric study to evaluate the impacts of pH, solution chemistry (salinity, alkalinity), water/rock ratio, and initial quartz granulometry before crushing. Furthermore, we estimate H₂ production associated with stress release during earthquakes of various magnitudes and quantify the resulting fluxes to enable comparison with other H₂-producing processes.

## 2. Materials and methods

### 2.1. Gridding mill

The grinding conditions applied in this study were selected to simulate the mechanical environment encountered within natural fault zones. The ball milling technique used here is commonly applied to produce ultrafine powders and has been shown to replicate key characteristics of natural fault gouges, including mechanical spraying and mechanochemical transformations (Kim et al., 2021; Togo and Shima-moto, 2012). In this context, the energy delivered to minerals during milling is analogous to the energy input experienced during seismic activity. While this energy can be described as stress (i.e., force per unit area), it should be noted that strain rates are not explicitly accounted for in this setup.

Grinding experiments were conducted using a Pulverisette 7 (P7)

planetary micro mill (Fritsch®) equipped with two gas-tight tungsten carbide (WC) grinding bowls and 15 mm diameter WC grinding balls (seven balls per bowl). Each bowl has a total volume of 45 mL, including the balls. To enable gas sampling during milling, the bowl lids were modified by adding two miniature quick-connect stems without valves (SS-QM2, Swagelok®; Fig. 1). Temperature was measured in situ during the experiments using a dedicated measurement cell (EasyGTM™; Fritsch®), with recorded values reaching up to 65 °C depending on the grinding speed (rpm).

The energy input during the milling process was calculated based on models of the impact velocity of the balls and the energy transferred to the sample (Burgio et al., 1991; Kho et al., 2014). We adapted the approach described by Kho et al. (2014) for the Pulverisette P6 mill to our P7 mill, considering its specific geometry and velocity parameters.

The total energy ($E_{tot}$) transferred during milling was calculated in three steps:

#### Step1. Energy per Collision (ΔE):

$$ \Delta E = k_c m_b W_p^2 R_p^2 \quad (1) $$

where $k_c = \frac{1}{2}k_a k_b^2$, $k_a$ is the collision constant (approximated as 1 if all energy is transferred), $k_b$ is a geometric coefficient depending on the mill type (typically 0.90 for planetary mills), $m_b$ is the mass of a grinding ball (0.0258 kg), $\omega_p$ is the angular velocity of the milling jar (rad s⁻¹), and $R_p$ is the radius of a grinding ball (0.015 m). At a rotation speed of 300 rpm, the energy per collision in our setup was approximately 0.05 J, consistent with literature values (Gotor et al., 2002; Kho et al., 2014).

#### Step 2. Collision Frequency (f):

$$ f = K k W_p \quad (2) $$

where K is a constant proportional to the diameter of the balls, k is the kinetic factor corresponding to the ratio $\omega_v/\omega_p$ (the ratio between the angular velocities of the jar and the platform), and $W_p$ is the angular velocity of the jar (rad s⁻¹).

#### Step3. Total Energy ($E_{tot}$):

$$ E_{tot} = \Delta E f N_b t \quad (3) $$

where $N_b$ is the total number of balls in the jar, and t is the milling time (s).

### 2.2. Materials

A quartz single crystal (10 cm by 5 cm), synthesized under hydrothermal conditions, was used for the experiments. The crystal was ground and sieved into four grain size fractions: <100 µm, 100–200 µm, 200–300 µm, and 300–600 µm. The quartz fractions were washed with Milli-Q water (conductivity: 18.2 MΩ cm), ultrasonicated, and dried in a glove box under an anoxic Ar atmosphere ([O₂] < 2 ppmv). The aqueous solutions used were prepared from Milli-Q water and analytical-grade salts (>99.9 % purity) and are listed in Table 1. Solutions were deoxygenated by Ar sparging and stored in the glove box prior to the experiments.

### 2.3. Experimental procedure

All reagents (quartz and solutions) were loaded into the grinding
<figure><img src="image_4.png" /><figcaption>Fig. 1. Schematic top and side views of the milling bowl designed to investigate H₂ generation during quartz grinding.</figcaption></figure>

bowls under an Ar atmosphere in a glove box. Conditioning under an inert atmosphere is crucial because radicals generated during grinding (e.g., $\Xi$Si- and H-) can react with O₂, forming less reactive species like $\Xi$Si–O₂⁻ and H-O–O⁻. (Damm and Peukert, 2009). Experiments were conducted under oxygen-free conditions to replicate the anoxic environment typical at depths where earthquakes occur (Hirose et al., 2011).

A total of 43 experiments were conducted under identical grinding conditions: 300 rpm for 10 minutes at room temperature (22 ± 2 °C) and 1 bar Ar pressure. Additionally, seven experiments were performed at varying grinding speeds (100, 200, 400, 600, and 800 rpm) and duration (75 s to 310 s) to assess the effect of grinding energy, on H₂ production. The experimental parameters tested are summarized in Table 1.

## 2.4. Gas sampling and analysis

Gas sampling was performed through one of the SS-QM2 fittings on the modified bowl lid, to which a septum was connected. A 250 µL gas-tight syringe was used to transfer gas samples to the gas chromatograph (GC). Gas analyses were conducted using a PerkinElmer® Clarus 500 GC equipped with a thermal conductivity detector (TCD) and a 2-meter RESTEK® ShinCarbon ST 80/100 column. Argon (grade 5.0) was used as the carrier gas. The GC was calibrated with several gas mixtures of Ar, H₂, CH₄, CO₂, CO, N₂ and O₂ injected with a calibrated-volume syringe. The analytical error was ±3 % at the 95 % confidence level.

## 2.5. Surface texture of quartz

The specific surface area of the quartz samples after grinding was measured using the Brunauer–Emmett–Teller (BET) method. Measurements were performed with a BELSORP-max (Microtrac©) instrument. Samples were dried and degassed overnight at 150 °C under vacuum (3.0 × 10⁻⁵ Pa) before analysis. Nitrogen adsorption measurements were carried out at 77 K.

Representative quartz grains were selected before and after grinding and a detailed textural study was carried out using a Tescan® Vega 3 scanning electron microscope (SEM). The accelerating voltage was set between 14 and 18 kV to get the best contrast and resolution.

# 3. Results

## 3.1. Background production of H₂ by corrosion of the grinding bowl and by grinding quartz without water (blanks)

To quantify the background H₂ production due to corrosion of the grinding equipment and dry grinding of quartz, six blank experiments were conducted (Table 1).

### 3.1.1. H₂ production from dry grinding of quartz

When pure quartz (10.6 g, grain size 500–600 µm) was ground without water, a small but measurable amount of H₂ was produced (3.21 × 10⁻⁷ mol). This production is minimal compared to experiments involving quartz grinding in the presence of aqueous solutions (discussed below). The observed H₂ may result from trace amounts of water adsorbed on quartz surfaces or from fluid inclusions within the quartz crystal, which are sufficient to induce mechanoradical H₂ generation.

### 3.1.2. H₂ production from corrosion of grinding equipment

To assess H₂ production from anoxic corrosion of the tungsten carbide (WC) grinding bowls and balls, blank experiments were performed without quartz but with solutions of varying pH (1 ≤ pH ≤ 13). The solutions were contacted with the sealed bowls under an argon atmosphere for 120 to 240 minutes at room temperature using a mechanical stirrer (without milling to prevent equipment damage in the absence of solids). In highly acidic conditions (pH = 1, 0.1 M HCl solution), significant H₂ production was observed, reaching up to 1.07 × 10⁻⁴ mol after 240 minutes of contact time. In contrast, under neutral to slightly acidic pH (pH = 6, Milli-Q water; pH = 4, citrate buffer $CeH_8O_7$), H₂ production was minimal, measured at 4.06 × 10⁻⁷ mol and 1.49 × 10⁻⁶ mol, respectively. Under alkaline conditions (pH = 13, 0.1 M NaOH solution), H₂ production was below the quantification limit (1.7 × 10⁻⁷ mol). These results indicate that tungsten carbide corrosion under anoxic conditions can produce significant H₂ only in very acidic media. Therefore, all subsequent quartz grinding experiments were conducted at pH levels above 4 to minimize background H₂ from equipment corrosion.
<caption>**Table 1**</caption>

List of experiments conducted in this study, applied parameters and amount of H₂ produced. Grinding conditions are the same for all experiments (300 rpm for 10 min), except when expressly mentioned. The unreported granulometry values in the table correspond to 500–600 µm.

<table><thead><tr><th>Samples names</th><th>m<sub>Quartz</sub> (g)</th><th>m<sub>Solution</sub> (g)</th><th>Granulometry (µm)</th><th>Water:rock ratio (W: R)</th><th>pH</th><th>Composition of solution</th><th>n<sub>H₂</sub> (10<sup>-6</sup> mol)</th><th>H₂ concentration (ppmm)</th></tr></thead><tbody><tr><td>BAQC1</td><td>7,2</td><td>3,6</td><td></td><td>0,5</td><td>4</td><td>C<sub>6</sub>H<sub>8</sub>O<sub>7</sub> Buffer 4</td><td>12,6</td><td>3,52</td></tr><tr><td>B2</td><td>9</td><td>1</td><td></td><td>0,1</td><td>6</td><td>H<sub>2</sub>O</td><td>0,77</td><td>0,17</td></tr><tr><td>B3</td><td>7,3</td><td>3,6</td><td></td><td>0,5</td><td>6</td><td>H<sub>2</sub>O</td><td>7,29</td><td>2,01</td></tr><tr><td>B5</td><td>8</td><td>2,4</td><td></td><td>0,3</td><td>6</td><td>H<sub>2</sub>O</td><td>4,66</td><td>1,17</td></tr><tr><td>B6</td><td>6</td><td>4,2</td><td></td><td>0,7</td><td>6</td><td>H<sub>2</sub>O</td><td>7,35</td><td>2,47</td></tr><tr><td>B7</td><td>5</td><td>5</td><td></td><td>1</td><td>6</td><td>H<sub>2</sub>O</td><td>8,6</td><td>3,47</td></tr><tr><td>B9</td><td>4</td><td>6</td><td></td><td>1,5</td><td>6</td><td>H<sub>2</sub>O</td><td>8,53</td><td>4,21</td></tr><tr><td>BQC1</td><td>3</td><td>1,5</td><td></td><td>0,5</td><td>6</td><td>H<sub>2</sub>O</td><td>4,77</td><td>3,2</td></tr><tr><td>BQC2</td><td>6</td><td>3</td><td></td><td>0,5</td><td>6</td><td>H<sub>2</sub>O</td><td>7,82</td><td>2,63</td></tr><tr><td>BQC4</td><td>5</td><td>2,5</td><td></td><td>0,5</td><td>6</td><td>H<sub>2</sub>O</td><td>6,04</td><td>2,44</td></tr><tr><td>BQC5</td><td>4</td><td>2</td><td></td><td>0,5</td><td>6</td><td>H<sub>2</sub>O</td><td>5,9</td><td>2,98</td></tr><tr><td>BQE100-rpm</td><td>7,2</td><td>3,6</td><td></td><td>0,5</td><td>6</td><td>H<sub>2</sub>O</td><td>0</td><td>0</td></tr><tr><td>BQE200-rpm</td><td>7,2</td><td>3,6</td><td></td><td>0,5</td><td>6</td><td>H<sub>2</sub>O</td><td>0</td><td>0</td></tr><tr><td>BQE400-rpm</td><td>7,2</td><td>3,6</td><td></td><td>0,5</td><td>6</td><td>H<sub>2</sub>O</td><td>13,68</td><td>3,83</td></tr><tr><td>BQE600-rpm</td><td>7,2</td><td>3,6</td><td></td><td>0,5</td><td>6</td><td>H<sub>2</sub>O</td><td>44,38</td><td>12,43</td></tr><tr><td>BQE800-rpm-3min</td><td>7,2</td><td>3,6</td><td></td><td>0,5</td><td>6</td><td>H<sub>2</sub>O</td><td>33</td><td>9,24</td></tr><tr><td>BQE500-rpm-310s</td><td>7,2</td><td>3,6</td><td></td><td>0,5</td><td>6</td><td>H<sub>2</sub>O</td><td>12,4</td><td>3,47</td></tr><tr><td>BQE800-rpm-75s</td><td>7,2</td><td>3,6</td><td></td><td>0,5</td><td>6</td><td>H<sub>2</sub>O</td><td>15,09</td><td>4,23</td></tr><tr><td>BGQ1</td><td>7,2</td><td>3,6</td><td>&lt; 100</td><td>0,5</td><td>6</td><td>H<sub>2</sub>O</td><td>5,66</td><td>1,58</td></tr><tr><td>BGQ2</td><td>7,3</td><td>3,6</td><td>100 to 200</td><td>0,5</td><td>6</td><td>H<sub>2</sub>O</td><td>4,73</td><td>1,31</td></tr><tr><td>BGQ3</td><td>7,2</td><td>3,6</td><td>200 to 300</td><td>0,5</td><td>6</td><td>H<sub>2</sub>O</td><td>5,66</td><td>1,58</td></tr><tr><td>BGQ4</td><td>7,3</td><td>3,6</td><td>300 to 600</td><td>0,5</td><td>6</td><td>H<sub>2</sub>O</td><td>5,13</td><td>1,42</td></tr><tr><td>BAQC4</td><td>7,2</td><td>3,6</td><td></td><td>0,5</td><td>7</td><td>KH<sub>2</sub>PO<sub>4</sub> Buffer</td><td>9,7</td><td>2,72</td></tr><tr><td>BBQ9</td><td>7,3</td><td>3,6</td><td></td><td>0,5</td><td>8,6</td><td>1 nM NaOH</td><td>4,48</td><td>1,24</td></tr><tr><td>BAQ2</td><td>7,2</td><td>3,6</td><td></td><td>0,5</td><td>9</td><td>H<sub>3</sub>BO<sub>3</sub> Buffer 9</td><td>4,65</td><td>1,3</td></tr><tr><td>BBQ7</td><td>7,3</td><td>3,6</td><td></td><td>0,5</td><td>9,4</td><td>10 nM NaOH</td><td>5,74</td><td>1,59</td></tr><tr><td>BBQ6</td><td>7,2</td><td>3,6</td><td></td><td>0,5</td><td>10,3</td><td>0.1 mM NaOH</td><td>6,07</td><td>1,7</td></tr><tr><td>BBQ5</td><td>7,3</td><td>3,6</td><td></td><td>0,5</td><td>11,5</td><td>1 mM NaOH</td><td>6,26</td><td>1,73</td></tr><tr><td>BBQ4</td><td>7,2</td><td>3,6</td><td></td><td>0,5</td><td>12,4</td><td>0.01 M NaOH</td><td>5,43</td><td>1,52</td></tr><tr><td>BBQ2</td><td>7,2</td><td>3,6</td><td></td><td>0,5</td><td>13,4</td><td>0.1 M NaOH</td><td>3,97</td><td>1,11</td></tr><tr><td colspan="9" style="text-align: center; font-weight: bold;">Blank</td></tr><tr><td>BA6</td><td>0</td><td>3,63</td><td></td><td>0</td><td>1</td><td>0.1 M HCl</td><td>10,74</td><td></td></tr><tr><td>BAQ0</td><td>0</td><td>3,63</td><td></td><td>0</td><td>4</td><td>C<sub>6</sub>H<sub>8</sub>O<sub>7</sub> Buffer 4</td><td>1,49</td><td></td></tr><tr><td>BA7</td><td>0</td><td>3,63</td><td></td><td>0</td><td>6</td><td>H<sub>2</sub>O</td><td>0,41</td><td></td></tr><tr><td>BBQ1</td><td>0</td><td>3,63</td><td></td><td>0</td><td>13</td><td>0.1 M NaOH</td><td>0,24</td><td></td></tr><tr><td>B4</td><td>10,6</td><td>0</td><td>Quartz only</td><td>0</td><td>0</td><td>Quartz only</td><td>0,46</td><td></td></tr><tr><td>BA8</td><td>0</td><td>0</td><td>empty</td><td>0</td><td>0</td><td>Empty</td><td>0017</td><td></td></tr></tbody></table>

## 3.2. Effect of quartz mass on H₂ production

The influence of the mass of quartz ground on H₂ production was investigated by varying the quartz mass from 3.04 to 7.30 g (grain size 500–600 µm) while maintaining a constant water/rock (W/R) ratio of

0.5 (Fig. 2). In the absence of quartz (only Milli-Q water), negligible H₂ production was detected (4.06 × 10⁻⁷ mol). As the mass of quartz increased, H₂ production rise from 4.77 × 10⁻⁶ mol to 7.29 × 10⁻⁶ mol. Under the experimental conditions, H₂ production (n<sub>H₂</sub>, in mol) was found to correlate linearly with the mass of quartz ground (m<sub>Qtz</sub>, in g):

$$n_{H_2} = 6.0 \times 10^{-7} \times m_{Qtz} + 3.0 \times 10^{-6} \quad (R^2 = 0.79) \quad (4)$$

Notably, the correlation line does not pass through the origin, suggesting that for quartz masses below 3 g, the linear relationship does not hold, and proportionally less H₂ is produced. Therefore, the linear dependence of H₂ production on the mass of quartz is valid only within the tested mass range and may not apply to lower quartz masses under the given experimental conditions.

<figure><img src="image_5.png" /><figcaption>**Fig. 2.** H₂ production as a function of the mass of ground quartz (at 300 rpm for 10 min) for a fixed W/R ratio of 0.5 in the presence of pure water (initial pH of approximately 6). Two experiments conducted at pH = 4 and pH = 13.4 under the same operating conditions are also presented for comparison. The blank corresponds to H₂ production from water alone, without quartz. Measurement uncertainties are within the size of the symbols.</figcaption></figure>

## 3.3. Effect of water/rock ratio on H₂ production

The impact of the water/rock (W/R) ratio on H₂ production was examined over a range from 0 (no water, 10.06 g of quartz) to 1.5 (6 g of water, 4 g of quartz) (Fig. 3). As the W/R ratio increased from 0.1 to 0.7, H₂ production increased by an order of magnitude, from 7.65 × 10⁻⁷ mol to 7.35 × 10⁻⁶ mol. Beyond a W/R ratio of 1, H₂ production reached a plateau around 8.47 × 10⁻⁶ mol.

The specific surface area of quartz after grinding also correlated with both the W/R ratio and H₂ production. It increased from 0.32 m² g⁻¹ at a W/R ratio of 0.1 to 3.57 m² g⁻¹ at a ratio of 1.5. This suggests that higher water content enhances grinding efficiency, leading to increased surface
<figure><img src="image_6.png" /><figcaption>**Fig. 3.** H₂ production (black circles) and specific surface area of quartz after grinding (red crosses) as a function of the W/R ratio. The grinding experiments were conducted in the presence of pure water and quartz with a particle size of 500 to 600 µm. Measurement uncertainties are within the size of the symbols. Grinding conditions are the same for all displayed experiments: 300 rpm for 10 min.</figcaption></figure>

area and, consequently, greater H₂ production.

### 3.4. Effect of pH on H₂ production

The effect of solution pH on H₂ production during quartz grinding was studied over a pH range from 4 to 13, with a constant W/R ratio of 0.5 and grain size of 500–600 µm (Fig. 4). Hydrogen production decreased by a factor of three as pH increased from 4 to 8.6, dropping from $12.6 \times 10^{-6}$ mol down to $4.5 \times 10^{-6}$ mol. Beyond pH = 8.6, H₂ production remained relatively constant up to pH 13. A slight increase in H₂ production was observed between pH 8.6 and pH 11.5 (from $4.5 \times 10^{-6}$ mol to $6.3 \times 10^{-6}$ mol), followed by a decrease to $4.0 \times 10^{-6}$ mol at pH > 13.

It is also important to note that the pH of the unbuffered solution slightly increase after the grinding attributed to cation exchange between cations and the fresh surface (Kameda et al., 2003).

The ionic strengths of the solutions used was varied, with higher ionic strengths corresponding to buffer solutions at pH= 4 (citrate buffer, C₆H₆O₇, I = 0.6 M) and pH= 7 (KH₂PO₄, I = 0.35 M), while both the H₃BO₃ buffer at pH =9, and the 0.1 M NaOH solutions had ionic strengths of 0.1 M. The lowest ionic strengths ($1.0 \times 10^{-6}$ to $1.0 \times 10^{-2}$ M) corresponded to the least concentrated NaOH solutions. Fig. 4 indicates that ionic strength had minimal impact on H₂ production compared to the effect of pH. The decrease in H₂ production with increasing pH suggests that acidic conditions favor mechanoradical H₂ generation during quartz grinding.

<figure><img src="image_7.png" /><figcaption>**Fig. 4.** H₂ production (circles) as a function of the pH of the solution. The lateral barre indicates a slight increase of the pH after the grinding (Kameda et al., 2003). The W/R ratio is 0.5, and the initial quartz particle size is 500 to 600 µm. The gray dotted lines represent the speciation of silanol groups on the quartz surface as a function of pH (Healy, 1974). Measurement uncertainties on H₂ amounts are within the size of the symbols.</figcaption></figure>

### 3.5. Effect of grinding energy on H₂ production

The relationship between grinding energy (E<sub>g</sub>) and H₂ production was investigated by varying milling rates (100 to 800 rpm) and durations (1 to 10 minutes), while keeping the W/R ratio at 0.5 and the quartz grain size at 500–600 µm (Fig. 5).

At low milling rates (100 to 200 rpm), corresponding to energy inputs of 0.4 to 3.3 kJ, no measurable H₂ production was detected, and the quartz grains remained largely undamaged. Above 300 rpm, H₂ production increased linearly with milling rate. The energy input ranged from 11 to 88 kJ, resulting in H₂ production from $3.6 \times 10^{-6}$ mol to $4.4 \times 10^{-5}$ mol. Under these conditions, approximately 0.5 µmol of H₂ was produced per kJ of energy delivered during grinding. Two additional experiments were performed at milling rates of 500 and 800 rpm, with durations of 310 and 75 s, respectively. This approach was adopted to evaluate the relevance of using grinding energy as a reference unit for H₂ production. An energy input of 26 kJ, consistent across three experiments conducted at 400, 500, and 800 rpm for durations of 10 min, 310 s, and 75 s, respectively, resulted in similar hydrogen productions of, $1.40 \times 10^{-5}$, $1.24 \times 10^{-5}$ and $1.51 \times 10^{-5}$ mol H₂, respectively.

## 4. Discussion

### 4.1. Impact of grinding parameters on H₂ production

Our experimental data are compared with literature values in Table 2, with all measurements normalized to milligrams of H₂ per kilogram of quartz (ppm) for consistency. Under similar experimental conditions, our results closely align with those of Saruwatari et al. (2004), confirming the high reproducibility of H₂ production measurements during quartz grinding.

Experimental conditions vary significantly across studies, particularly in grinding speeds (300 to 800 rpm) and durations (10 to 100 minutes). As shown in Fig. 5, both the speed and duration of grinding have a notable impact on H₂ production. Specifically, increasing the grinding speed from 300 to 600 rpm and tripling the grinding duration from 10 to 30 minutes results increase in H₂ production, from about 1.43 to 52 ppm. This enhancement is attributed to the increased specific surface area generated during grinding (Kameda et al., 2003). Temperature increase due to heat dissipation and pH change induced by surface radical recombinations must not be neglected too, but the exact effect of these parameters remains to be evaluated.

<figure><img src="image_8.png" /><figcaption>**Fig. 5.** H₂ production as a function of the Grinding Energy (E<sub>g</sub>). The grinding experiments were conducted in the presence of pure water and quartz with a particle size of 500 to 600 µm (black circles). Two experiments conducted at pH = 4 and pH = 13.4 under the same operating conditions are also presented for comparison (colored symbols). Measurement uncertainties are within the size of the symbols.</figcaption></figure>
<table><caption>**Table 2**
Comparison of different quartz grinding experiments in pure water. The H₂ concentration is expressed in ppm (mg of H₂ per kg of quartz).</caption><thead><tr><th>References</th><th>Crushing parameter - Speed (rpm)</th><th>Granulometry (µm)</th><th>Duration (min)</th><th>W:R ratio</th><th>m<sub>ech</sub> (g)</th><th>H<sub>2</sub> (ppm)</th></tr></thead><tbody><tr><td>Kameda et al., 2003</td><td>600</td><td>150 - 500</td><td>100</td><td>2</td><td>5</td><td>52</td></tr><tr><td rowspan="3">Saruwatari et al., 2004</td><td rowspan="3">280</td><td rowspan="3">150 - 500</td><td rowspan="3">30</td><td>3</td><td>5</td><td>3.76</td></tr><tr><td>1</td><td>5</td><td>3.00</td></tr><tr><td>0.6</td><td>5</td><td>1.43</td></tr><tr><td rowspan="10">This study</td><td rowspan="5">300</td><td rowspan="5">500 - 600</td><td rowspan="5">10</td><td>0.5</td><td>5</td><td>2.01</td></tr><tr><td>0.7</td><td>6</td><td>2.47</td></tr><tr><td>1</td><td>5</td><td>3.47</td></tr><tr><td>1.5</td><td>4</td><td>4.21</td></tr><tr><td>0.5</td><td>7.3</td><td>0</td></tr><tr><td>100</td><td rowspan="4">500 - 600</td><td rowspan="4">3</td><td>0.5</td><td>7.3</td><td>3.83</td></tr><tr><td>400</td><td>0.5</td><td>7.3</td><td>12.43</td></tr><tr><td>600</td><td>0.5</td><td>7.3</td><td>9.23</td></tr><tr><td>800</td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>

## 4.2. Grinding efficiency, resulting specific surfaces, and H₂ production

Fig. 2 illustrates a strong correlation between the mass of quartz ground and H₂ production at a constant water/rock (W/R) ratio. This relationship is consistent with the dependence of free radical (≡Si-) concentration on the quartz surface area exposed to the solution, which in turn depends on the initial mass of quartz when grain size is constant (Saruwatari et al., 2004). Combined with blank experiments, these results confirm that the observed H₂ production is indeed due to quartz grinding and not an artifact of equipment corrosion.

The amount of H₂ produced also shows a strong correlation with the W/R ratio. As the W/R ratio increases, so does the specific surface area resulting from grinding (Fig. 3, 6). The W/R ratio significantly affects the grinding mechanism: low water content favors abrasion, while higher water content promotes fracturing (Zhao et al., 2012). Enhanced grinding efficiency at higher W/R ratios leads to increased specific surface areas, resulting in more ≡Si- radicals that react with water to produce H₂ via reaction (eq. (5)).

Delegu et al. (2011) describe two types of ≡Si- radicals formed during grinding: those generated by fracturing and those by abrasion. Radicals formed by fracturing are more reactive and have a surface

density up to 30 times higher than those associated with abrasion. Freshly fractured crystalline surfaces undergo relaxation and reconstruction processes, forming unstable transient species that readily react with H₂O. The repeated formation of new surfaces during grinding increases the number of H- radicals, explaining the differences in H₂ production.

## 4.3. Influence of quartz surface site speciation on H₂ production

Quartz surface site speciation changes with pH: ≡SiOH sites dominate at pH < 7, while ≡SiO⁻ sites prevail at pH > 7. As shown in Fig. 4, H₂ production follows the relative abundance of these surface sites as a function of pH. The speciation of surface sites appears to control the radical recombination pathways leading to H₂ production, with ≡SiOH sites promoting mechanoradical H₂ generation under acidic conditions. This phenomenon can be understood by considering the following sequence of reactions:

### 1. Formation of Silanol Groups During Grinding:

Breaking an SiO₂ bond by grinding induces the formation of two types of surface sites (Fubini, 1998): homolytic sites, where bonding

<figure><img src="image_9.png" /><figcaption>**Fig. 6.** Scanning Electron Microscope (SEM) images of quartz grains after grinding at two different water-rock ratio, with all other grinding parameters being identical: a) W/R = 0.5, experiment #BQC3, and b) W/R = 0.1, experiment #B4. Images are taken at the same magnification. A clear difference in grain size is observed between the two experiments, thus demonstrating the impact of W/R ratio on grinding efficiency, and by way of consequences on H₂ production by mechanoradical reactions.</figcaption></figure>
electrons are evenly distributed around the radical atom (e.g., ≡Si and ≡SiO·), and heterolytic sites, where one electron is captured by an atom to form an ion (e.g., ≡Si⁺ and ≡O⁻·Si⁻). These surface sites are considered highly reactive surface defects in the presence of water. These surface sites can recombine to form siloxane bonds (≡Si⁻O⁻·Si⁻·) (Antonini and Hochstrasser, 1972), which further react in the presence of H₂O to form silanol groups (≡Si⁻OH) (D'Souza and Pantano, 2004; Levine and Garofalini, 1985). Two other processes (eq. (5), 6) also result in the formation of silanol groups in the presence of water (Kita et al., 1982; Narayansaly and Kubicki, 2006; Saruwatari et al., 2004). The rate of reaction (5), is higher than that of reaction (6) leading to the net production of H· radicals and thus H₂ (Narayanasamy and Kubicki, 2006; Saruwatari et al., 2004;)

$$≡ Si· + H₂O→ ≡ SiOH + H· \quad (5)$$

$$≡ SiO· + H·→ ≡ SiOH \quad (6)$$

## 2. Surface Site Speciation Changes with pH:

Acid-base reactions, although slower than radical reactions, affect surface site speciation:

$$≡ SiOH→ ≡ SiO⁻· + H⁺ \quad (7)$$

In alkaline solutions, grinding of SiO₂ preferentially produces heterolytic bonds corresponding to charged sites ≡Si⁺ and ≡SiO⁻, which primarily lead to the formation of H⁺ and OH⁻ ions (Delogu et al., 2011; Narayanasamy and Kubicki, 2006; Saruwatari et al., 2004):

$$≡ Si·+ ≡ SiO⁻· + H₂O→2 ≡ SiOH \quad (8)$$

$$≡ SiO⁻· + H₂O→ ≡ SiOH + OH⁻ \quad (9)$$

Under acidic conditions, the formed silanol groups can react with homolytic ≡Si· bonds to form siloxanes and H· radicals (Kameda et al., 2003):

$$Si·+ ≡ SiOH→ ≡ Si⁻O⁻·Si⁻· + H· \quad (10)$$

The increased concentration of H· radicals leads to greater H₂ production via reaction (11) as observed in Fig. 4:

$$H· + H·→ ≡ H₂(g) \quad (11)$$

Analyses of H₂ concentrations near hydrothermal sources associated with active faults have shown a relationship between water pH and H₂ levels. A decrease in pH correlates with increased H₂ concentration (Kameda et al., 2003), consistent with our findings. However, it is important to note that hydrothermal fluid pH can be influenced by reactions other than mechanoradical processes.

## 4.4. Upscaling and comparing mechanoradical H₂ production with other natural H₂-Producing processes

### 4.4.1. Mechanoradical H₂ production during earthquakes

Mechanoradical processes producing H₂ can occur in various geological settings, notably along fault planes during seismic stress release. The energy released during an earthquake is typically partitioned between radiated seismic energy and dissipated energy. The dissipated energy during an earthquake includes several components: fracture energy (Gf in J m⁻²), frictional energy, and other inelastic deformation energy (Venkataraman and Kanamori, 2004). The dissipated energy represents the portion of the total seismic energy that does not contribute to the propagation of seismic waves. Among these, Gf is particularly significant as it quantifies the energy required to propagate the rupture along the fault. In this study, it is especially relevant for

understanding the production of H₂ through mechanochemical processes during earthquakes.

Gf can be estimated using the following equation (Abercrombie and Rice, 2005; Nielsen et al., 2016):

$$Gf = 5.25 \times 10^6 \times AD^{1.28} \quad (12)$$

where AD is the average displacement along the fault (in meters). The average displacement can be calculated as a function of magnitude (M) using (Kim and Sanderson, 2005; Wells and Coppersmith, 1994):

$$M = 6.93 + 0.82 \times \log(AD) \quad (13)$$

By combining these equations, we can calculate the quantity of H₂ produced per square meter during an earthquake of magnitude M, assuming an H₂ production rate of 0.5 µmol kJ⁻¹ as estimated in this study:

$$H₂(M) = Gf \times N(H₂) \quad (14)$$

where N(H₂) is the amount of H₂ produced per unit energy (mol kJ⁻¹).

Using the Gutenberg-Richter relationship, we can estimate the global annual flux of H₂ generated by earthquakes (Gutenberg and Richter, 1944; Hirose et al., 2011):

$$\log_{10}(N) = a - b \times M \quad (15)$$

where N is the earthquake frequency, and a and b are empirically determined parameters (a = 7.47, b = 1).

By integrating over a magnitude range of 4 to 9 and assuming the rupture area consists entirely of quartz, we calculate both the localized H₂ production rate per square meter per year (mol m⁻² yr⁻¹) and the total H₂ production from mechanoradical processes globally (mol yr⁻¹). This provides a comprehensive view of the annual H₂ output from significant earthquakes globally (Fig. 7a).

Our calculations estimate a cumulative H₂ production of approximately 1.82 × 10² mol m⁻² yr⁻¹ for earthquakes with magnitudes between 4 and 9. Considering the rupture surface area proportional to magnitude (Eq. (13)), the total H₂ production amounts equates to approximately 1.45 × 10¹³ molyr⁻¹ for all earthquakes within this magnitude range. Although earthquakes with magnitudes of 8 or 9 have a much lower probability of occurrence than those of lower magnitudes, it is striking that they exert a disproportionately large influence on global H₂ production from earthquakes (Fig. 7b).

However, this estimation involves significant simplifications that do not fully capture the inherent complexity of rupture areas. Key factors such as variations in porosity and permeability, the distinction between core zone and damage zone properties, the fracture energy released during earthquakes, fluctuations in the water/rock ratio, and the heterogeneity of lithologies intersected by the fault are not explicitly addressed in this model. Neglecting these critical parameters may lead to an overestimation of the calculated H₂ fluxes.

Despite these limitations, our method for calculating grinding energy is robust and follows well-established procedures, yielding results consistent with previous studies (Burgio et al., 1991; Gotor et al., 2002; Kho et al., 2014). In particular, our approach is holistic in that it relies on fracture energy derived from fault displacement, allowing a direct link between earthquake magnitude and the energy available for mechanochemical H₂ production.

### 4.4.2. Comparing mechanoradical reactions other H₂-producing natural processes

Hydrogen production by quartz grinding can also be compared with other natural processes such as serpentinization of ultramafic rocks, water radiolysis (Klein et al., 2020; Lin et al., 2005; McColлом, 2016;). Comparisons can be made both as a function of the mass of rock involved and at global scales. It turns out that earthquakes may be major contributors to the global H₂ production on Earth, with a total output
<figure><img src="image_10.png" /><figcaption>**Fig. 7.** a) Amount of H₂ produced as a function of earthquake magnitude. The black dots represent the annual production of H₂ per m² along the fault plane. b) Cumulative amount of H₂ produced as a function of earthquake magnitude. Black line represent the cumulative annual production of H₂ along the fault plane. The blue bars represent the calculated earthquake frequency per year as a function of their magnitude and the associated fracture energy.</figcaption></figure>

possibly higher than that of serpentinization of ultramafic rocks. However, significant differences and uncertainties exist for all these mechanisms. Firstly, the timescales differ substantially. Radical recombinations occur in nanoseconds, while serpeninization and radiolysis occur over geological time scale. Secondly, the efficiency of these processes is uncertain and the complexity of their reaction mechanism is poorly accounted for in global numerical models.

### 4.4.3. Laboratory experiments limitation

Assuming complete serpentinization of a given ultramafic domain or directly extending laboratory grinding results to fault zones necessarily involves strong approximations. Upscaling experimental findings, whether from serpentinization, water radiolysis, or mechanoradical processes, remains challenging due to differences in scale, rock texture, and reaction dynamics.

The main objective of this study is to quantify H₂ production through controlled experimental approaches, using a methodology that can be scaled up to the geological level to better assess the contribution of these processes to the global H₂ budget. While these mechanisms are now widely recognized, significant uncertainties remain when extrapolating laboratory results to natural systems. Both grinding and serpentinization

experiments are typically performed on fine powders, which do not reflect the textures and structures of natural rocks.

In our case, the experimental setup imposes additional constraints, as quartz masses above 10 g could not be tested (see Fig. 2). However, in natural fault zones, the key parameter is not the total mass of quartz, since a fault is primarily a surface, but rather the total surface area of the crushed material. The importance of surface area, as well as the water-to-rock (W/R) ratio, is clearly illustrated in Fig. 3, where a saturation effect or threshold in H₂ production is observed.

Moreover, integrating key reaction parameters such as temperature, pressure, fluid composition, water-to-rock ratio, and mineral assemblages into large-scale reactive transport models remains a major challenge. Addressing these gaps will be essential for improving estimates of natural H₂ production at the global scale.

### 4.5. Evidence of natural hydrogen seepages related to faults ?

Hydrogen has been considered a promising candidate for earthquake prediction in the past (Dogan et al., 2007; Ito et al., 1999; Sato et al., 1985; Sugisaki et al., 1983; Zhou et al., 2021;). An H₂ monitoring system was deployed in Nagashima, near the Yoro fault zone, for over a year.
During this period, four earthquakes ($1.5 < M < 5.4$) occurred and showed a strong correlation with detected hydrogen anomalies, supporting the hypothesis that $H_2$ is produced through mechanoradical processes. Additionally, several punctual soil gas surveys were conducted across various fault types (normal, reverse, and strike-slip), revealing high hydrogen concentrations and well correlated with the fault location (up to several percent; Dogan et al., 2007; Li et al., 2013). However, no $H_2$ flux measurements have yet been conducted in the immediate vicinity of these faults. Addressing this gap would require the development and deployment of long-term soil gas monitoring systems, as illustrated by recent work in Brazil (Moretti et al., 2021).

## 5. Conclusions

Our laboratory experiments on quartz grinding have demonstrated that the water-to-rock (W/R) ratio and pH significantly influence $H_2$ production. The amount of $H_2$ generated is linked to grinding efficiency, the resulting specific surface area, and the speciation of quartz surface sites, which is controlled by pH. Under acidic conditions, where $\equiv SiOH$ surface sites dominate, $H_2$ production is up to three times higher than in alkaline environments dominated by $\equiv SiO^{-}$ species.

Additionally, $H_2$ production is strongly correlated with the grinding energy applied in the system, with an observed yield of approximately 0.5 µmol of $H_2$ per kJ of energy input. By extrapolating our laboratory findings to natural settings, we estimate that the cumulative $H_2$ production from all earthquakes with magnitudes between 4 and 9 is about $1.82 \times 10^2 \text{ mol m}^{-2} \text{ yr}^{-1}$. This $H_2$ flux generated by faults appears to be less significant than previously thought, especially when compared to the $2.3 \times 10^5 \text{ mol m}^{-2} \text{ yr}^{-1}$ estimated by Hirose et al. (2011).

By applying the empirical relationships described by Wells and Coppersmith (1994), which relate earthquake magnitude to average displacement and fault surface area, and taking into account our assumption, we estimate the annual global $H_2$ production through mechanoradical processes to be approximately $1.45 \times 10^{13} \text{ mol yr}^{-1}$. This estimation provides a foundation for comparing mechanoradical $H_2$ production with other geological $H_2$-generating processes, enhancing our understanding of $H_2$ fluxes in the Earth's crust. However, this scale-up comes with some limitations and uncertainties, as we did not account several critical factors, such as permeability and porosity zoning around faults, or variation in lithology. Nonetheless, our work paves the way for future experiments to investigate the $H_2$ production through complexe paragenesis and rock textures. Soil gas measurements will be carried out in the vicinity of faults to constrain the hydrogen fluxes associated with the earthquake. Finnaly, mechanoradical processes are major contributors to the global $H_2$ production on Earth, and their importance in prebiotic chemistry, abiotic organic synthesis, deep microbial ecosystems development, and $H_2$ exploration must not be neglected.

### CRediT authorship contribution statement

**Nicolas Lefeuvre:** Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Resources, Project administration, Methodology, Investigation, Formal analysis, Data curation, Conceptualization.
**Laurent Truche:** Writing – review & editing, Validation, Supervision, Project administration, Funding acquisition.
**Frédéric-Victor Donzé:** Writing – review & editing.
**Johan Vandemborre:** Writing – review & editing.
**Eric C. Gaucher:** Writing – review & editing.
**Valérie Magnin:** Methodology, Formal analysis.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgments

Grateful thanks to editor Pr Jean-Philippe Avouac. The authors warmly thank the anonymous reviewers for their very constructive reviews. This work was conducted in the framework of the Convergence project (https://convergent-margins.com/), funded by TotalEnergies. Additional financial support from the French National Research Agency (grant agreement ANR-20-CE01-0020) is acknowledged. Laurent Truche acknowledges support from the Institut Universitaire de France.

## Data availability

Data will be made available on request.

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