# Solar-Assisted Approach for the Synthesis of Nanoadsorbents for Biogas Desulfurization Using Wastes

Kostas Simeonidis, C Martinez-Boubeta, I Kellartzis, A Makridis, E Delli, Anita Haeussler, P Kougias, G Vourlias, Ll Balcells

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Kostas Simeonidis, C Martinez-Boubeta, I Kellartzis, A Makridis, E Delli, et al. Solar-Assisted Approach for the Synthesis of Nanoadsorbents for Biogas Desulfurization Using Wastes. SSRN Electronic Journal, 2023, 10.2139/ssrn.4394001. hal-04241246

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# Solar-assisted approach for the synthesis of nanoadsorbents for biogas desulfurization using wastes☆

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Konstantinos Simeonidis<sup>a, *</sup>, Carlos Martinez-Boubeta<sup>b</sup>, Ioannis Kellartzis<sup>a</sup>, Antonios Makridis<sup>c</sup>, Evangelia Delli<sup>c</sup>, Anita Haeussler<sup>d</sup>, Panagiotis G. Kougias<sup>e</sup>, George Vourlias<sup>c</sup>, Lluis Balcells<sup>f, **</sup>

## ARTICLE INFO

**Article history:**
Received 25 April 2023
Received in revised form
16 July 2023
Accepted 21 August 2023
Available online 23 August 2023

**Keywords:**
Biogas
Hydrogen sulfide
Solar energy
Waste valorization
Nanocomposites

## ABSTRACT

Minimizing food wastes and finding a second life use for industrial residuals have become some of the top priorities of modern society. This work considers the use of spent eggs and mussels shells, as well as marble dust, as raw sources to develop nanoparticles involving renewable resources in both their preparation and adoption in technological applications. Specifically, Ca/Mg-based nanoparticles were obtained by evaporating such wastes in a physical vapor deposition system using concentrated solar beam and explored as high capacity H₂S adsorbents for the purification of biogas. The evaluation of their uptake performance in a fixed-bed configuration indicates that the formation of a thick layer of Ca(OH)₂ on very small nanoparticles (<70 nm) inhibits H₂S uptake, whereas the presence of Mg phases (dolomite) favors its potentiation. Importantly, the co-evaporation of iron provides an extra amplification of the absorption capacity due to the synergy of the Ca/Mg neutralizing character and the affinity of Fe for sulfur. In the best case, the nanoparticles obtained from mussels and 10 %wt. Fe reached an uptake capacity of 0.92 mg/g. This high yield is attributed to the formation of oxides, such as Ca₂Fe₂O₅, that allow a sulfide to sulfate oxidation-adsorption mechanism.

© 2023 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license

(http://creativecommons.org/licenses/by/4.0/).

## 1. Introduction

The anaerobic digestion of organic matter is the common process to generate biogas, a prevalent form of renewable energy source [1]. Typically, the treatment involves animal manure, wastewater biomass, sewage sludge, food wastes, and plant residuals broken down by bacteria and archaea into sealed vessels in the absence of oxygen [2–6]. Biogas is composed mainly of methane (CH₄), that constitutes its energy potential, and some inert components, such as carbon dioxide (CO₂), water vapor, and

hydrogen sulfide (H₂S), in proportions that vary depending on the feedstock. When present in high concentrations, hydrogen sulfide can curb the use of biogas in internal combustion engines, turbines, pumps, storage tanks, valves or fuel cells as a consequence of its corrosiveness to the metallic parts [7–10]. Thus, the recommended levels of H₂S in biogas combustion systems vary between 200 and 500 ppm [11]. In addition, H₂S contributes to the appearance of unpleasant odor and the emission of sulfur compounds in the exhaust. One should consider that concentrations above 100 ppm may cause respiratory problems and death to human, whereas dissolved H₂S is able to inhibit the fermentation process in the anaerobic bioreactor [12,13].

This is a crucial issue in the evolution of biogas technology that has motivated a large multidisciplinary research effort to develop efficient H₂S removal methods combining high efficiency with low cost of materials and minimum labor, transport, or disposal requirements [14]. Proposed technologies for post-

<footnote><sup>a</sup> Department of Chemical Engineering, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece</footnote>

<footnote><sup>b</sup> Nanotech Solutions S.L., Ctra. Madrid 23, 40150 Villacastin, Spain</footnote>

<footnote><sup>c</sup> Department of Physics, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece</footnote>

<footnote><sup>d</sup> Processes, Materials and Solar Energy Laboratory, CNRS-PROMES, 7 Rue Du Four Solaire, 66120 Font-Romeu, France</footnote>

<footnote><sup>e</sup> Soil and Water Resources Institute, Hellenic Agricultural Organization DIMITRA, 57001 Thermi, Greece</footnote>

<footnote><sup>f</sup> Institut de Ciencia de Materials de Barcelona, CSIC, Campus Universitat Autònoma de Barcelona, A08193 Bellaterra, Spain</footnote>

<footnote>* Based on experiments carried out at the CNRS-PROMES laboratory, UPR 8521, belonging to the French National Center for Scientific Research (CNRS).</footnote>

<footnote>** Corresponding author.</footnote>

<footnote>** Corresponding author.
E-mail addresses: ksime@physics.auth.gr (K. Simeonidis), balcells@icmab.es (L. Balcells).</footnote>
digestion purification include the use of adsorbents in fixed or fluidized filters, membranes, scrubbing with water or NaOH, and biological uptake. Unfortunately, the methods involving chemical reactions or solvents face strict requirements of environmentally safe handling and disposal of high volumes of residuals [11]. On the other hand, biofilters are a widely studied solution, but its major problem is the frequent occlusion that demands an extra labor cost [15]. Consequently, adsorption on granular solids is referred as the most efficient option to date to tackle this problem and offers a reliable solution for the upgrade of biogas or natural gas supply [16,17]. According to the simple setup, the possibility for automated operation and the low adsorbent cost, especially when solid wastes from other processes are used, efficient adsorption is always the option of preference. Ideally, the biogas comes into contact with the adsorbent and H₂S is captured on its surface through a non-reversible chemical reaction [18–20]. Alkaline and transition metal oxides, hydroxides or carbonates, activated carbon, mesoporous silicas, zeolites, metal-organic frameworks, and resins are now the most commonly reported adsorbents.

In this regard, calcium and magnesium oxides (CaO, MgO), hydroxides (Ca(OH)₂, Mg(OH)₂), and carbonates (CaCO₃, MgCO₃, (Ca,Mg)CO₃) enjoy the advantage of their alkaline character to neutralize acidic H₂S [21–23]. Under alkaline conditions, its transformation into sulfur is spontaneous since the oxidation of S²⁻ becomes favorable.

$$S^{2-} \rightarrow S + 2e^{-} + 0.48 \text{ V}$$

However, the efficiency of such compounds when applied in adsorption beds is not always competitive due to the low rate of neutralization reaction at ordinary temperatures, especially when the specific surface area is kept at low levels in conventional-sized materials signifying a proportional low surface activity of the used solids. Iron-based adsorbents can help overcome most of these drawbacks by providing a much faster kinetic owed to their oxidative nature and larger number of structural defects [24–26]. For instance, the product of H₂S reaction with iron oxides is usually a stable iron sulfide with the possibility of its regeneration after oxygen flow [27,28].

The design of such adsorbents based on the valorization of waste materials can be more profitable since the associated cost can be reduced. In this context, the design of a sustainable manufacturing procedure that allows the reuse of the calcium carbonate (CaCO₃) contained in limestone and other carbonated rocks or waste material from the food industry is a great challenge. For example, marble ranks the biggest market of natural stone in the world and its waste is an environmental nuisance [29]. Similarly, disposal of by-products derived from aquaculture industrial mussel processing can be costly [30]. To comply with these considerations, the aim of this work was to develop novel eco-friendly adsorbents with advanced desulfurization potential. For this reason, nanoparticles based on Ca/Mg were produced by means of a physical vapor deposition (PVD) technique and combined with iron in an attempt to achieve synergistic activity of these compounds toward the capture of H₂S. Such approach was expected to bring upgrade of efficiency in comparison to conventional fine powders by providing higher specific area and chemical activity on its surface. Well-defined nanoparticles were obtained by applying a sustainable PVD technique which uses concentrated solar beam as an energy source. In addition, a waste valorization character is introduced since mineral and food wastes such as eggshells, mussels, and mining dust were used as the raw source for the developed nanoparticles.

## 2. Experimental

### 2.1. Nanoparticles synthesis

Solar energy is the largest renewable resource in nature. From this perspective, large-scale production of adsorbents from solar energy is an attractive route. Our approach is based on the use of concentrated solar radiation for the vacuum evaporation of materials (Scheme 1). The studied nanomaterials were obtained using a simple and low-cost PVD method in a Heliotron glass chamber operated in the 1.5 kW facilities of the CNRS-PROMES laboratory facilities in Odeillo, France, capable of achieving mean solar concentration ratios exceeding 10000 suns. For each sample, the corresponding target was prepared by pressurizing fine powders of by-products containing CaCO₃ or MgCO₃ such as eggshells, bivalve mollusks (mussels) and mining dusts (marble and dolomite). These samples correspond to the separate evaporation of one of the byproducts, hereafter referred as single-phase evaporates. A second series of samples was produced by co-evaporation of the same phases with Fe described as multiphase evaporates. Pure analytical grade CaCO₃ and MgCO₃ powders were evaporated separately to serve as reference samples. Evaporation occurred under a controlled physico-chemical environment at 10 mbar with a continuous flow of Ar gas (base pressure 5·10⁻² mbar, 99.999% purity, <2 ppm of O₂). With respect to the growth mechanism, the gaseous products exiting the irradiation zone underwent rapid cooling and condensation [31]. Then, particles were collected on a nanoporous alumina filter located in the direction of pumping flow. Table 1 summarizes the preparation parameters and the main characteristics for the examined samples.

### 2.2. Characterization

The structural characterization of the obtained samples was performed by X-ray diffraction (XRD) using a Rigaku Ultima + powder diffractometer with Cu-Kα radiation. During measurements, the instrument used a step size of 0.05°, a step time of 2 s, a voltage of 40 kV, and a current of 30 mA. Existing phases were identified after comparison of the obtained diagrams with the powder diffraction files (ICDD-PDF) database [32]. Quantification of determined phases in XRD diagrams was performed following the Rietveld refinement method using the Fullprof software [33]. The following ICDD-PDF database cards were used as input in this analysis: #82–1691 (CaO), #84–1275 (Ca(OH)₂), #87–0651 (MgO), #79–1345 (CaMg(CO₃)₂), #87–0722 (Fe), #75–1372(Fe₃O₄), #88–1941 (MgFe₂O₄), and #71–2264 (Ca₂Fe₂O₅).

The morphology of nanopowders was visualized by field-emission scanning electron microscopy taken in a FEI Quanta 200 ESEM FEG microscope. The field-emission gun operated at 15–30 kV and the elemental analysis of the observed areas was supported by an energy-dispersive X-ray spectroscopy (EDS) analyzer of EDAX company equipped in the microscope. For higher magnifications of the nanoparticles, transmission electron microscopy (TEM) images were taken using a JEM-1210 (JEOL) microscope, operating at 120 kV. Samples for TEM analysis were prepared by the deposition of a drop of the particles dispersion in isopropanol onto a carbon-coated copper grid. For the determination of the hydrodynamic size in the range 1–10000 nm, a Horiba nanoPartica SZ-100V2 nanoparticle analyzer was used and data were fitted by a log-normal distribution function.

The magnetic properties of Fe-loaded adsorbents were evaluated by using a vibrating sample magnetometer Oxford Instruments 1.2H/CF/HT, operating at room temperature. Hysteresis loops were measured with a varying magnetic field in the range ±1 T.
<figure><img src="image_4.png" /><figcaption>**Scheme 1.** Diagram of the solar PVD system for nanoparticles preparation and hydrogen sulfide adsorption setup for the packing of obtained nanoparticles in a fixed-bed reactor. PVD, physical vapor deposition.</figcaption></figure>

<table><caption>**Table 1**

Major parameters of the studied samples prepared by solar-assisted physical vapor deposition.</caption><thead><tr><th rowspan="2">Target</th><th rowspan="2">Product<br>(%wt.)</th><th rowspan="2"></th><th colspan="3">EDS analysis (%wt.)<sup>a</sup></th><th rowspan="2">Mean particles<br>size (nm)</th><th rowspan="2">H<sub>2</sub>S uptake<br>(mg/g)</th></tr><tr><th>Ca</th><th>Mg</th><th>Fe</th></tr></thead><tbody><tr><td rowspan="2">Mussels shells</td><td>CaO</td><td></td><td>Ca(OH)<sub>2</sub></td><td>100</td><td>—</td><td>—</td><td>105</td><td>0.52</td></tr><tr><td>27.9</td><td></td><td>72.1</td><td></td><td>—</td><td>—</td><td></td><td></td></tr><tr><td rowspan="2">Eggshells</td><td>CaO</td><td></td><td>Ca(OH)<sub>2</sub></td><td>100</td><td>—</td><td>—</td><td>65</td><td>0.10</td></tr><tr><td>28.8</td><td></td><td>71.2</td><td></td><td>—</td><td>—</td><td></td><td></td></tr><tr><td rowspan="2">Marble</td><td>CaO</td><td>MgO</td><td>Ca(OH)<sub>2</sub></td><td>47.3</td><td>52.6</td><td>—</td><td>120/210</td><td>0.08</td></tr><tr><td>41.5</td><td>54.4</td><td>4.1</td><td></td><td>—</td><td>—</td><td></td><td></td></tr><tr><td rowspan="2">Dolomite</td><td>CaO</td><td>MgO</td><td>CaMg(CO<sub>3</sub>)<sub>2</sub></td><td>42.6</td><td>57.4</td><td>—</td><td>110</td><td>0.53</td></tr><tr><td>26.8</td><td>48.2</td><td>25.0</td><td></td><td>—</td><td>—</td><td></td><td></td></tr><tr><td rowspan="2">CaCO<sub>3</sub></td><td>CaO</td><td></td><td>Ca(OH)<sub>2</sub></td><td>100</td><td>—</td><td>—</td><td>55</td><td>0.08</td></tr><tr><td>9.8</td><td></td><td>90.2</td><td></td><td>—</td><td>—</td><td></td><td></td></tr><tr><td rowspan="2">MgCO<sub>3</sub></td><td></td><td>MgO</td><td></td><td>—</td><td>100</td><td>—</td><td>75</td><td>0.17</td></tr><tr><td></td><td>100</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="2">Mussels/10% Fe</td><td>CaO</td><td>Ca<sub>2</sub>Fe<sub>2</sub>O<sub>5</sub></td><td>Fe<sub>3</sub>O<sub>4</sub></td><td>33.7</td><td>—</td><td>66.3</td><td>110</td><td>0.92</td></tr><tr><td>14.6</td><td>37.8</td><td>47.6</td><td></td><td>—</td><td>—</td><td></td><td></td></tr><tr><td rowspan="2">Eggshells/40% Fe</td><td>CaO</td><td></td><td>Fe</td><td>30.2</td><td>—</td><td>69.8</td><td>60</td><td>0.38</td></tr><tr><td>41.4</td><td></td><td>58.6</td><td></td><td>—</td><td>—</td><td></td><td></td></tr><tr><td rowspan="2">Dolomite/40% Fe</td><td>MgFe<sub>2</sub>O<sub>4</sub></td><td>MgO</td><td>Fe</td><td>—</td><td>32.8</td><td>67.1</td><td>125</td><td>0.83</td></tr><tr><td>59.6</td><td>26.6</td><td>13.8</td><td></td><td>—</td><td>—</td><td></td><td></td></tr></tbody></table>

Saturated adsorbents were analyzed using X-ray photoelectron spectroscopy (XPS) to identify the occurring adsorption mechanism on the solid's surface. Spectra were acquired in a KRATOS Axis Ultra DLD system with a monochromated Al-Kα X-ray beam employed as the excitation source. The pass energy was 160 eV for survey scans and 40 eV for high resolution spectra. Charging of specimens was avoided via a low-energy electron neutralizer, while calibration was performed by using the C 1s peak of adventitious carbon at 284.6 eV as reference.

### 2.3. H<sub>2</sub>S uptake

To evaluate the uptake efficiency of nanoadsorbents for the H<sub>2</sub>S removal, a quantity of the samples was packed on a porous support and placed in a vertical fixed-bed reactor (Scheme 1). The gas flowed from the top side of the setup and the concentration of H<sub>2</sub>S

was determined in the outflow stream using a continuous gas analyzer ecom J2KNpro Expert. A typical concentration of a contaminated biogas was achieved in the H<sub>2</sub>S stream (1100 ppmv) by properly diluting the stock H<sub>2</sub>S with high purity nitrogen.

## 3. Results and discussion

### 3.1. Single-phase evaporation

Structural characterization, by means of XRD, for the samples prepared by single-phase evaporation indicates that calcium oxide and hydroxide are the main phases in all powders received using CaCO<sub>3</sub> containing targets (Fig. 1a). In particular, the disassociation of carbonates causes the production of CaO in nanosized grains. In fact, the proportional high specific surface area is the reason for the broadened peaks that correspond to the formation of Ca(OH)<sub>2</sub> due

<footnote><sup>a</sup> Excluding oxygen, carbon, and sulfur.</footnote>
<figure><img src="image_5.png" /><figcaption>**Fig. 1.** XRD diagrams of nanoparticles obtained after evaporation of various industrial wastes consisting of calcium and magnesium carbonates (a) and those obtained after evaporation of mussels shells, eggshells and dolomite dust with Fe powder (b). XRD, X-ray diffraction.</figcaption></figure>

to hydration of CaO upon exposure to the atmosphere. Quantification of diagrams by means of Rietveld analysis suggests that the hydroxide formation reaches very high levels for the pure CaCO$_3$ target (~90 %wt.), while for eggs and mussels shells, the percentage just overcomes 70 %wt. (Table 1). On the other hand, the evaporation of targets containing Mg favors the presence of high percentages of MgO compared to the corresponding Ca phases. For instance, when marble and dolomite targets were used, MgO was the major crystal phase (54.4 and 48.2 %wt., respectively), whereas CaO content was found much lower despite its higher percentage in the initial material due to the challenges of thermally evaporating the Ca-O system [34]. In these samples, spontaneous hydration of CaO seems to be inhibited with the percentage of Ca(OH)$_2$ lying under 5 %wt. Another important observation is the preservation of dolomite (~25 %wt.) in the corresponding evaporate, which suggests the possibility that the carbonate (CaMg(CO$_3$)$_2$) is transferred from the target without a complete disassociation of its structure. The reference sample obtained from pure MgCO$_3$ confirms that obtained MgO nanoparticles do not show a great tendency to hydration. It should be pointed out the lower crystallinity of MgO obtained from the mining waste, as revealed by the broadening of diffraction peaks. Certainly, this is evidence of some difference in the condensation mechanism and the growth extent of MgO in the presence of other phases.

Scanning electron microscopy observations for the nano-particles obtained from studied wastes of food and mining industries are summarized in Fig. S1. The evaporation resulted in nearly spherical agglomerations of individual nanoparticles with dimensions that vary depending on the target. For example, CaO/Ca(OH)$_2$ nanoparticles collected from eggshells (Fig. S1a) show the smaller size with an average at around 65 nm, while the corresponding ones from mussels (Fig. S1b) are characterized by a larger particle size (~120 nm) and variability (Fig. S1c). Finally, nanoparticles produced by marble show a bimodal distribution with peaks at 120 and 210 nm (Fig. S1d). EDS analysis on the observed sample regions was carried out to estimate the average Ca and Mg percentages especially for the nanomaterials collected from dolomite and marble targets. As shown in Table 1, Mg has slightly higher presence than Ca in the product of evaporation of these wastes whereas pure Ca was determined in the nanoparticles obtained by mussels and eggshells.

Notwithstanding, TEM images (Fig. 2) show that, according to the low contrast in the projected center, nanoparticles are hollow structures, probably as a consequence of the Kirkendall effect. As previously described in detail [35,36], a central void is formed because an oxidizing element reacts with the forming metal, while vacant sites merge due to a much larger cation outward diffusion rate through the oxide layer than anion inward diffusivity. In our case, it stems from the dissociation of Ca and/or Mg carbonates in the evaporation reactor accompanied by the release of carbon dioxide. Electron diffraction patterns of these areas are shown in Fig. S2 indicating the presence of Ca(OH)$_2$ or MgO as major phases.

Complementary to the electron microscopy observations, estimation of the hydrodynamic diameter of samples in aqueous dispersions provides a better understanding of the actual extent of agglomeration considering their potential application. The results of dynamic light scattering measurements are summarized in Fig. S3. The hydrodynamic size is always many times larger than the dimensions determined using TEM. For samples prepared from eggshell targets, the mean size was determined to be around 950 nm, decreasing to around 700 nm when Fe was included. Evaporation from mussels generates a hydrodynamic diameter greater than 1 µm, but their doping with Fe results in a bimodal distribution with peaks at 300 and 900 nm. Finally, dolomite samples depict the largest hydrodynamic diameter, ranging from 3 µm (pure) to around 2 µm when Fe was added to the target.

## 3.2. Multi-phase evaporation

The solid wastes consisting of Ca or Mg carbonates (eggshells, mussels shells, and dolomite) were co-evaporated with Fe powder under the same conditions to examine nanocomposites with combined alkaline and iron-based compounds. XRD analysis indicates that more complicated products become favorable when the percentage of Fe varies (Fig. 1b). In particular, evaporation of mussels shells with a low weight fraction of Fe (10 %wt.) results in most of the Ca participating in a srebrodolskite ($Ca_2Fe_2O_5$) structure (37.8 %wt.) together with the formation of a small proportion of CaO (14.6 %wt.). A significant part of the nanocomposite consists of a ferrite, probably $Fe_3O_4$ (47.6 %wt.). However, when eggshells were combined in the evaporation target with a higher Fe percentage (40 %wt.), separate phases of Fe and CaO were stabilized. The absence of iron oxides is attributed to the larger nanoparticles size and the possible reducing effect of sulfur also present natively in avian eggshells [37]. The use of a Mg-containing waste such as the dolomite brought a different product composition with a magnetic ferrite $MgFe_2O_4$ being the major phase (59.6 %wt.) in coexistence with MgO (26.6 %wt.) and Fe (13.8 %wt.). The presence of most of these structures was verified also by the electron diffraction
<figure><img src="image_6.png" /><figcaption>**Fig. 2.** TEM images of nanoparticles obtained after evaporation of pure eggshells (a), mussels (b), and dolomite (c), compared to same samples including Fe powder (d–f). While biogenic-derived calcium carbonate forms an almost continuous network of spherical particles, its fossil counterpart is made of truncated cubic shapes with a polycrystalline shell, probably attributed to the coexistence of MgO and CaO regions. TEM, transmission electron microscopy.</figcaption></figure>

analysis of TEM images (Fig. S2), while EDS elemental analysis (Table 1) mostly comply with the total percentages of Ca, Mg, and Fe.

Morphologically, major differences were observed in Fe-doped products (Fig. 2, left column) when compared with the control samples (right column). High magnification images of the nano-composites indicate the presence of a core-shell arrangement, which comes to replace the hollow configuration of single-phase evaporates. It should be pointed that the Fe-based compounds appear in dark contrast located in the core region, while CaO or MgO are distributed in the bright shell. Thus, iron atoms seem to participate in the described Kirkendall effect by providing an additional diffusion component. Such a core-shell arrangement is

more obvious in the case of nanoparticles developed by the co-evaporation of eggshells with 40 %wt. of Fe powder (Fig. 2b). The more complicated composition of nanoparticles derived from mussels shells and iron is also reflected in the TEM image (Fig. 2d), where the higher electron contrast suggests the presence of Ca₂Fe₂O₅ and Fe₃O₄ with a thin CaO coating layer. In the case of dolomite/Fe nanocomposite, the hollow cubic nanostructures are still favorable, but here, the crystal structure is replaced by the Mg-ferrite and Fe (Fig. 2f).

The presence of magnetic phases in the nanocomposites features the corresponding samples with magnetic response that could be considered for potential applications where particles guidance by an external magnetic field would be favorable. Fig. 3
shows the hysteresis loops of these nanoparticles. The saturation magnetization for the sample obtained by eggshells/Fe evaporation is the highest among them approaching 145 emu/g, in accordance with its high Fe content which has a bulk magnetization value of around 220 emu/g. Assuming Fe as the only magnetic phase of the sample, such value implies to a percentage content of around 65 % wt., which is close to the result estimated by XRD quantitative analysis in Table 1. The coercive field for the same sample is 49 mT. The co-existence of Fe with the Mg ferrite in the dolomite/Fe-derived nanoparticles defines a lower magnetization value (87 emu/g) and coercive field (32 mT). Finally, the sole presence of Fe₃O₄ in the mussels/Fe evaporation derivatives explains the significantly low saturation magnetization (46 emu/g) as well as the drop in the coercive field to around 21 mT.

### 3.3. Hydrogen sulfide uptake

Assuming the neutralizing activity of the studied nanoparticles owed to the presence of the alkaline earth metal compounds (CaO, Ca(OH)₂, MgO), their efficiency to capture H₂S by a gas stream was evaluated in a fixed column bed setup. Fig. 4a presents six representative breakthrough curves plotted after the continuous monitoring of the H₂S concentration in the outflow. The adsorption capacity of each sample was calculated by subtracting the reference value for a bed filled with sand. Results are summarized in Fig. 4b. Among the nanoparticles produced by single-phase evaporation, those obtained from mussels shells and dolomite bring the highest efficiency, with capacities overcoming 0.50 mg/g. It is surprising that eggshells-derived nanoparticles with practically the same composition, but different microstructure (Table 1), show a significantly lower uptake response with a maximum sorption capacity of 0.10 mg/g. This observation can be attributed to the smaller size of eggshells nanoparticles than their mussels counterparts, which signifies a thicker layer of Ca(OH)₂ that probably interferes with the capture of H₂S [38]. On the contrary, in the presence of MgO, the formation of Ca(OH)₂ seems to be inhibited and the adsorption activity shows a higher potential although the larger dimensions of the material units in these samples oppose the significant increase of efficiency. The absence of hydrolysis in MgO nanoparticles grown by evaporation of MgCO₃ explains the higher performance of this sample than the one obtained by CaCO₃ (Table 1). This is attributed to the easier approach of acidic H₂S onto the basic MgO surface in

<figure><img src="image_7.png" /><figcaption>Fig. 3. Magnetic hysteresis loops of nanoparticles obtained from evaporation of mussels shells, eggshells and dolomite dust with Fe powder.</figcaption></figure>

comparison to the case of the CaO passivated by Ca(OH)₂. The chemical nature of the weakly ionic Mg–O bond also features oxygen sites with strong alkaline properties enabling electron donation to the H₂S gas [39].

Sulfur capacity increased with the iron loading by a factor of 1.7–3.2. This finding is attributed to the synergistic effect of the high affinity from the Fe-based phases for H₂S in collaboration with the neutralizing mechanism of the alkaline earth metals and the lower percentage of Ca(OH)₂ in those samples. The formation of oxides such as Ca₂Fe₂O₅ and MgFe₂O₄ in mussels/Fe and dolomite/Fe evaporates seems to favor the capture of H₂S in the form of an iron sulfide catalyzed by the presence of Ca or Mg cations. The adsorption capacity of these nanoparticles is estimated to be 0.92 and 0.83 mg/g, respectively. At the same time, the decreased formation of Ca(OH)₂ in the eggshells/Fe sample should be the explanation for the extremely efficiency increase to almost 0.40 mg/g.

It has been shown that Ca/Mg-based compounds interact differently with iron oxides [40]. For example, the charge transfer from the iron atom on Mg-based surfaces was shown to increase the binding interaction with the acidic adsorbate by an order of magnitude, compared to undoped and Ca-based ones [39]. It is also described that the substitution of structural sites by Fe favors adsorption via chemisorption instead of physisorption. Thus, the positive correlation between Mg and Fe concentrations and the H₂S removal efficiency suggests that the sulfur capacity increases by increasing Fe³⁺ concentration [41]. Indeed, the reaction of ferric (hydr)oxides with H₂S is a prominent pathway contributing to nutrients bioavailability in aquatic ecosystems [42], in which a variety of mineral transformations can be induced depending on the initial ratio of S to Fe³⁺ [43].

To assess the effect of the oxidation state of sulfur phases and reveal a possible reaction path, XPS measurements were employed for the saturated mussels/Fe nanocomposite collected at the end of the adsorption experiment focusing on the identification of the chemical states of sulfur. Fig. 5 shows the high resolution spectra and the corresponding fitting analysis for the Ca 2p, Fe 2p, S 2p, and O 1s peaks. The S 2p peak was fitted by two contributions located at relative high binding energies above 168 eV and which, in agreement with the literature [44], have been assigned to the 2p3/2 and 2p1/2 doublet. Such observation makes straightforward the identification of sulfates (SO₄²⁻) or bisulfates (HSO₄⁻) attached to the metal oxides. Therefore, it is concluded that hydrogen sulfide capture involves an oxidation step during which S²⁻ species from H₂S are converted into the S⁶⁺ ones found in the sulfates. This mechanism is promoted by the presence of the Fe³⁺ oxides (Ca₂Fe₂O₅, Fe₃O₄), which act as electron donors to the oxidation of H₂S and its precipitation as sulfates attached to the iron or calcium atoms [26,45,46]. The initiation of such mechanism is triggered by the hydroxyl presence in surface Ca(OH)₂ which contributes to the hydration of H₂S.

$$8\text{Fe}^{3+} + \text{H}_2\text{S} + 2\text{Ca(OH)}_2 \rightarrow 8\text{Fe}^{2+} + 2\text{Ca}^{2+} + \text{SO}_4^{2-} + 6\text{H}^+$$

Alternatively, the proposed mechanism may be presented as an oxidative dissociation procedure in the surface of srebrodolskite wherein the oxidation of sulfur and uptake of sulfates are accompanied by the separation of Ca ions and hydroxyl adsorption (Fig. 6):

$$\text{Ca}_2\text{Fe}_2^{3+}\text{O}_5 + \text{H}_2\text{S} \rightarrow 2\text{Ca}^{2+} + 2\text{Fe}^{2+} + \text{SO}_4^{2-} + \text{H}_2\text{O}$$

The ability of Ca₂Fe₂O₅ nanoparticles to interact in electron exchange processes has been illustrated in their operation as anodes in Li-ion cells with their performance being attributed to the combined crystal matrix of CaO and the high concentration of
<figure><img src="image_8.png" /><figcaption>**Fig. 4.** Breakthrough curves of H₂S uptake from a 1100 ppmv gas stream by the studied nanoparticles packed in a vertical fixed-bed reactor. Dotted line indicates the corresponding data for the empty bed, whereas continuous line represents the reference sand bed (a). Estimated adsorption efficiency of the studied nanoparticles for H₂S (b).</figcaption></figure>

oxygen vacancies [47]. Elsewhere, the Mn-doped Ca₂Fe₂O₅ nano-particles have been successfully tested as oxygen carriers to the catalytic conversion of CO₂ to CO and the production of syngas through chemical looping gasification of biogas residues [48]. Such potential can be further optimized with the synergetic activity of other structures such as polyoxometalates known as efficient catalysts for the oxidative desulfurization of fuels [49] or by their arrangement in 2D porous materials featured with electrochemical activity properties [50].

Quantification of the XPS scan suggests that the Fe/Ca ratio is about 1.41 whereas adsorbed sulfur represents around 2.3% of the Fe content on the nanoparticles near-surface area. The analysis of

the Fe 2p binding spectrum indicates the presence of a multiplet corresponding to three chemical states for Fe, i.e. to the Fe²⁺ located in Fe₃O₄ (710.0 eV) and the Fe³⁺ contributions from Fe₃O₄ (711.5 eV) and Ca₂Fe₂O₅ (713.5 eV). The same states were also used for the interpretation of the Fe 2p₃/₂ for the as-prepared sample before its use in adsorption experiments (Fig. S4). However, in the H₂S-saturated nanoparticles, an extra contribution appears at 714.8 eV which is probably related to the coordination of surface Fe with the generated sulfate ions. On the other hand, Ca 2p spectra peaks are reproduced by a doublet owed to the CaO or its possible surface hydrated component (346.7 eV) and the coordination of Ca in the Ca₂Fe₂O₅ structure (347.3 eV). Finally, O 1s spectrum consists
<figure><img src="image_9.png" /><figcaption>**Fig. 5.** High resolution XPS spectra of Fe 2p, Ca 2p, S 2p, and O 1s orbits for the H₂S-saturated nanoparticles obtained after evaporation of mussel shells and Fe powder. XPS, X-ray photoelectron spectroscopy.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Fig. 6.** Suggested mechanism of H₂S uptake by Ca₂Fe₂O₅ nanoparticles through an oxidative dissociation.</figcaption></figure>

of two regions, each one further analyzed in multiplets. At binding energies above 530.5 eV, the presence of sulfates, hydroxides, and carbon contaminants contributes to the obtained spectrum, whereas the shoulder observed at around 529.5 eV should be

attributed to oxygen participating in the oxides. In principle, the change in the patterns and interpretation of Ca 2p and the O 1s for the as prepared sample, is minor (Fig. S4).

### 3.4. Prospects for scale up

The current study was performed under ideal experimental conditions to succeed the maximum quality of morphology and growth control. Under these conditions, production cost is defined by Ar gas consumption, the electricity for the vacuum pump and the sun tracking system, the cooling water, and the materials to prepare the evaporation targets. Since residual wastes are mostly used in targets, the cost related with materials is insignificant. However, the commonly applied procedure for used facility (Heliotron) to prepare high quality nanoparticles and avoid oxidation effects, i.e. the pump's operation to preliminary decrease the chamber's pressure and the Ar flow to adjust the pressure at 10 mbar, possibly moderate the overall profit. Hopefully, the evaporation of Ca/Mg carbonates is not expected to meet similar restrictions, and therefore, it is very possible that working at higher pressure or replacing Ar flow with nitrogen or air flow would not affect the final product. Accordingly, testing the system with modified vacuum conditions and gas flow is the key to design a feasible and cost-effective large-scale system. In any case, scale up
of the process requires the consideration of a feasibility study and life cycle analysis to include the aspects of carbon dioxide generation and spent water and energy.

## 4. Conclusions

The conversion of food and industry solid wastes into nanosized particles was found to be a profitable option in the effort to minimize the environmental fingerprint of relevant processes and promote the use of renewable energy sources. In particular, added-value nanoparticles consisting of Ca/Mg-based phases were developed by a solar-beam-assisted vapor condensation method introducing residual eggshells, mussels shells, and marble dust as raw materials. This study shows that obtained nanoparticles can be used as potential H₂S adsorbents able to purify biogas or natural gas and extend the operation lifetime of the power generation facilities. The high efficiency of such nanoparticles is attributed to the increase of specific surface area and the neutralizing character of Ca/Mg phases against H₂S decomposition. It was also found that the addition of iron goes one step further to improve the removal capacity by introducing a sulfur oxidation mechanism and its transformation into sulfate ions with affinity for iron species. Importantly, the magnetic response of the participating ferrite phases (Ca₂Fe₂O₅, Fe₃O₄, MgFe₂O₄) features these nanoparticles with potential to engage magnetic handling and separation during application in fluidized column bed configurations.

Dataset related to this study acquired during access in NFFA facilities is available in http://doi.org/10.5281/zenodo.7572430.

## Author contributions statement

K.S., C.M.B., A.M. A.H. and LI.B. prepared the samples in the solar furnace facilities, and carried out the materials characterization. I.K. and P.G.K. designed, performed and evaluated the hydrogen sulfide uptake experiments. E.D. and G.V. carried out the XPS characterization, the analysis and the interpretation of results. K.S. and C.M.B. prepared the manuscript with the supervision of LI.B. All authors contributed in the editing of the final draft.

## Funding

We thank the CNRS-PROMES laboratory, UPR 8521, belonging to the French National Center for Scientific Research (CNRS) for providing access to its installations, the support of its scientific and technical staff, and the financial support of the SFERA-III project (Grant Agreement No 823802). This work was supported by the French "Investments for the future" program managed by the National Agency for Research, under contact ANR-10-EQPX-49-SOCRATE (Equipex SOCRATE). This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 101007417, having benefited from the access provided by CSIC-ICMAB in Barcelona within the framework of the NFFA-Europe Pilot Transnational Access Activity, proposal ID262. C.M. Boubeta is granted financial support through the Torres Quevedo programme (PTQ2021-012178) by the Spanish MCIN/AEI/10.13039/501100011033. LI. Balcells acknowledges the Ministry of Science and Innovation of Spain for the support through the OXISHOT project (PID2021-1284100B-100).

## 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.

## Data availability

Data will be made available on request.

## Acknowledgments

We thank Pablo Gil Collazo, owner of a shellfish farming company in Ría de Vigo basin, for having supplied us with mussels.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.mtener.2023.101395.

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