# Environmentally sustainable production of bio-based mineral acids

Claire M Grison, Claude Grison

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Claire M Grison, Claude Grison. Environmentally sustainable production of bio-based mineral acids. Cleaner Engineering and Technology, 2025, 28, pp.101060. 10.1016/j.clet.2025.101060. hal-05192976

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# Environmentally sustainable production of bio-based mineral acids

Claire M. Grison, Claude Grison* ID

Laboratoire de Chimie Bio-inspirée et Innovations Ecologiques, UMR 5021, Centre National de la Recherche Scientifique – Université de Montpellier, Grabels, 34790, France

## ARTICLE INFO

**Keywords:**

Cleaner production

Brønsted acids

Oxalic acid

Bio-based mineral acids

Sustainable processes

Comparative environmental analysis

## ABSTRACT

The production of bio-based mineral acids, using hydrochloric acid as a model, was revisited here. Simple, affordable, renewable and non-hazardous reactants –namely sodium chloride and oxalic acid– enabled the preparation of concentrated solutions of hydrochloric acid (6 N) in good yields and in mild conditions, including at pilot scale. The process was easily implemented by mixing the two reactants in refluxing water, followed by azeotropic distillation of the reaction mixture. This approach was extended to other mineral acids such as HBr, HI and HNO₃, using the corresponding sodium salt. The described process was compared with current industrial methods for HCl production, highlighting its efficiency, improved safety through the elimination of hazardous substances such as chlorine or concentrated sulfuric acid and its precursors, and enhanced sustainability through reduced of energy consumption and the exclusive use of bio-based reactants. This process exemplifies how green chemistry can contribute to cleaner production by addressing both environmental and sustainability challenges.

## 1. Introduction

The emerging concepts of green and sustainable chemistry have led to the development of bio-based catalysts. Bio-based catalysis often refers to biocatalysis, which consists of using enzymes to speed up chemical reactions. Biocatalysis offers many advantages, since enzymes are bio-based, biodegradable and renewable; they require mild reaction conditions (physiological pH, temperature, aqueous solvent), and lead to high chemo-, regio- and stereoselectivities (Sheldon et al., 2007). The use of biocatalysis has enabled the production of various chemicals, including chemical building blocks, detergents, foams, flavours, perfumes, vitamins, chiral pharmaceuticals. Biocatalysis has inspired the concept of asymmetric organocatalysis. Organocatalysis relies on small chiral organic molecules, much simpler than enzymes. For example, (S)-proline, a natural amino acid, is an excellent asymmetric catalyst of intramolecular aldol reactions. Today, asymmetric organocatalysis can be extended to enamine and iminium activation, Brønsted acid/base catalysis, Lewis base catalysis, H-bonding catalysis, and quaternary ammonium salt ion, leading to efficient syntheses of pharmaceuticals, fragrances and terpenoid natural compounds. Lately, organocatalysis has also contributed to the developments of photoredox catalysis and to the concept of organotextile catalysis (García Mancheño and Waser, 2023). In parallel, simple natural extracts have gained attention as cost-effective catalysts. For example, citric, malic, tartaric acids derived

from fruits (Gulati et al., 2020), oleic acid from bamboo, caffeic acid from coffee beans (Murthy et al., 2023, p. 29) have been used as acid catalysts for the eco-friendly preparation of heterocyclic compounds.

Despite growing interest in bio-based catalysis, the preparation of strong bio-based Brønsted acid catalysts remains a significant challenge. In this work, the preparation of bio-based HCl was reported using only natural reactants, i.e. sodium chloride and oxalic acid. The synthesis was optimised to achieve maximum productivity at both laboratory and pilot scales. This methodology was further extended to the preparation of other mineral acids such as HBr, HI and HNO₃. A comparative analysis was performed between this sustainable strategy and conventional industrial methods, focusing on toxicity, ecotoxicity and industrial risks.

## 2. Experimental part

### 2.1. Materials and methods

Oxalic acid dihydrate certified 100 % natural by ECOCERT was purchased from Phenix Nature.

Sodium chloride 100 % sea salt and from organic farming was purchased from Eggnergy.

X-ray powder diffraction (XRPD) data measurements were performed using a Bruker diffractometer (D6 Advance, with a Cu Kα radiation λ = 1.54086 Å) equipped with a LYNXEYE SSD160-2 detector.

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: claude.grison@cnrs.fr (C. Grison).</footnote>
## 2.2. Typical procedure for the preparation of bio-based HCl

NaCl and oxalic acid (0.10 mol, 10 g) were stirred in distilled water under reflux at 100 °C for 1 h. Different quantities of NaCl and water have been tested.

Then, a system of distillation was installed on the flask in order to distillate the azeotropic mixture and isolate the concentrated aqueous solution of hydrochloric acid. The distillation was carried out at atmospheric pressure until the residual reaction mixture was dry.

The normality, or concentration in $[H^+]$, of the fractions could be determined by measuring the pH of the fractions using a pH-meter or titrating the fractions for more accuracy, especially for high concentrations of $[H^+]$, which give negative pH values. Titration of the fractions was conducted using a 2 M solution of NaOH and the volume at the equivalence led to the $[H^+]$ normality of the fractions.

## 2.3. Scale-up procedure for the preparation of bio-based HCl

Into a 2 L two-bottom necked flask, were introduced NaCl (638 g, 11 mol) and oxalic acid (500 g, 5 mol) in distilled water (600 mL) under mechanical stirring. The suspension was refluxed at 100 °C for 2 h. Then a system of distillation was installed on the flask in order to distillate the azeotropic mixture and isolate the concentrated aqueous solution of hydrochloric acid. The distillation was carried out at atmospheric pressure until the residual reaction mixture was dry. 5 fractions were collected when the temperature of the head thermometer significantly changed (f1: 20 mL, 0.6 M; f2: 75 mL, 0.8 M; f3: 160 mL, 1.4 M; f4: 300 mL, 4.6 M; f5: 120 mL, 11.2 M). Titration of the fractions was conducted using a 2 M solution of NaOH and the volume at the equivalence led to the $[H^+]$ normality of the fractions.

## 3. Results

The process for producing environmentally friendly, bio-based strong mineral acids -starting with HCl as model- relies on the use of the strongest natural acid, oxalic acid ($pKa_1 = 1.27$ and $pKa_2 = 4.27$) and a naturally occurring mineral salt, sodium chloride. Bio-based oxalic acid is commercially available in large quantities and is produced via biosynthesis using the fungus *Aspergillus niger*. *A. niger* is easy to cultivate and can ferment a wide range of raw materials, including glucose, lactose, whey, molasses, post-refining fatty acids, lipids, or residual glycerol from biodiesel production (*Betiku et al., 2016*). Both starting materials are natural, non-toxic, inexpensive and readily available.

The synthetic strategy involves the reaction of oxalic acid with sodium chloride in water, producing a mixture of hydrochloric acid and sodium oxalate, followed by the azeotropic distillation of the HCl-H₂O mixture (Scheme 1).

The formation of the HCl-H₂O azeotrope indicates the presence of a strong intramolecular hydrogen bonding between HCl and H₂O, which is stronger than intermolecular interactions of HCl-HCl or H₂O-H₂O.

The distillation of the reaction mixture was divided into two fractions. The first fraction, consisting of the slightly acidic water ($pH = 3$ to 1), was collected at 100 °C and continued until the temperature at the distillation head reached 104 °C. This temperature corresponds to the boiling point of the 6 N HCl-H₂O azeotrope, which defined the beginning of the second fraction. The pH of this second fraction was so low that it

required titration. Titration of this second fraction using a potassium hydroxide solution (2 M) gave the equivalent volume and normality of HCl (Table 1).

The best compromise between high HCl normality and good yield is obtained with 2 equivalents of NaCl and 14 mL H₂O for 0.1 mol of oxalic acid. Under these conditions, a 5.6 M HCl solution was obtained with a satisfying yield of 62%. Increasing the amount of NaCl slightly improves the yield to 74%, but does not allow for a quantitative yield, as excessive salt hinders the agitation of the reaction medium and increasing the water volume decreases the normality of the most concentrated fraction.

The addition of CaCl₂ and especially MgCl₂ significantly improves the reaction yield, respectively 75 and 137% - the yield was calculated based on the reaction of one carboxylic acid function per oxalic acid molecule. Exceeding 100%, the yield should be divided by two, considering that the two carboxylic acid functions have reacted.

The Grison group has previously shown that CaCl₂ and MgCl₂ do not exhibit any Brønsted acidity (Deyris et al., 2019). The authors hypothesize that the addition of CaCl₂ and MgCl₂ increases the acidity of oxalic acid through Lewis acid-base interactions. Indeed, the $Ca^{2+}$ and $Mg^{2+}$ cations are Lewis acids and may interact with the carbonyl group of oxalic acid's carboxylic functions, leading to a stronger ionization of the O-H bond and enabling both carboxylic functions of oxalic acid to react. One can hence hypothesize the formation of calcium or magnesium oxalate.

This hypothesis is corroborated by the pH measurement before and after the addition of CaCl₂ and MgCl₂ in an aqueous solution of oxalic acid: the pH goes from 0.6 to 0.1 and from 0.6 to -0.3 respectively. The higher Lewis acidity of MgCl₂ can explain a greater increase in the acidity of the reaction medium. The reaction is more efficient with MgCl₂, which is in agreement with literature, which established that magnesium oxalate was more stable than calcium oxalate (Riley et al., 2013). This hypothesis is supported by the reaction of oxalic acid and MgCl₂ alone which led to a yield of 84%.

These results are particularly noteworthy given that both MgCl₂ and CaCl₂ occur naturally - especially MgCl₂, which is found in Nigari Sea salt. However, it is important to note that the major industrial source of MgCl₂ is a petroleum-based and requires HCl for its own production (Büchel et al., 2008). This fact puts into perspective the relevance of adding these chlorides in the preparation of bio-based HCl.

These good results encouraged us to scale up the bio-based HCl preparation process. Thus, the reaction was carried out on 5 mol (or 500 g) of oxalic acid, and 11 mol of NaCl, corresponding to a 50-fold scale-up. A volume of 600 mL of water was necessary to ensure good stirring of the heterogeneous medium. The suspension was heated for 2 h at reflux, then the distillation of the medium was carried out and each distilled fraction was titrated with a KOH solution (Table 2).

Although the scale was increased by a factor of 50, the results obtained at the laboratory scale (0.1 mol of oxalic acid) were successfully reproduced without difficulty, yielding a similar amount concentrated HCl (yield of 55%, combination of fractions 4 & 5: [HCl] = 6.5 M, V = 420 mL). Moreover, it is noteworthy that the scale-up enabled the isolation of a fraction with an HCl concentration as high as 11.2 M.

This scale up made it possible to study the workup procedure of the distillation residue. It spontaneously decanted into two layers: a liquid aqueous layer rich in NaCl and a white solid, which was dried and analysed by Infrared and X-Ray Powder Diffraction. XRPD analysis

<figure><img src="image_2.png" /><figcaption>Scheme 1. Preparation of bio-based HCl by reaction of oxalic acid and sodium chloride.</figcaption></figure>
<figure><img src="image_3.png" /><figcaption>**Table 1**
Optimisation of the experimental conditions for the preparation of concentrated solutions of HCl (using 0.1 mol of oxalic acid).</figcaption></figure>

<table><thead><tr><th>NaCl (eq.)</th><th>H<sub>2</sub>O (mL)</th><th>additives</th><th>time (h)</th><th>[HCl] (volume)</th><th>yield<sup>b</sup> (%)</th></tr></thead><tbody><tr><td>1</td><td rowspan="4">14</td><td rowspan="4">/</td><td>1</td><td>5.2 M (10 mL)</td><td>52</td></tr><tr><td>2</td><td>2</td><td>5.6 M (11 mL)</td><td>62</td></tr><tr><td>5</td><td>1</td><td>6.2 M (12 mL)</td><td>74</td></tr><tr><td>8</td><td>1</td><td>0<sup>b</sup></td><td>0</td></tr><tr><td>2</td><td>20</td><td>/</td><td>1</td><td>3.7 M (16 mL)</td><td>59</td></tr><tr><td>2</td><td>14</td><td>CaCl<sub>2</sub></td><td>1</td><td>5.5 M (14 mL)</td><td>77</td></tr><tr><td>2</td><td>14</td><td>MgCl<sub>2</sub></td><td>1</td><td>8.6 M (16 mL)</td><td>137</td></tr><tr><td>/</td><td>14</td><td>MgCl<sub>2</sub></td><td>1</td><td>6.0 M (14 mL)</td><td>84</td></tr></tbody></table>

<table><caption>**Table 2**
Description of the distillation parameters for the preparation of HCl at a 50-fold scale-up (using 5 mol of oxalic acid).</caption><thead><tr><th>distillation fraction</th><th>volume (mL)</th><th>distillation head (T °C)</th><th>[HCl] (M)</th></tr></thead><tbody><tr><td>1</td><td>20</td><td>90–103</td><td>0.6</td></tr><tr><td>2</td><td>75</td><td>102</td><td>0.8</td></tr><tr><td>3</td><td>160</td><td>104</td><td>1.4</td></tr><tr><td>4</td><td>300</td><td>105</td><td>4.6</td></tr><tr><td>5</td><td>120</td><td>104–105</td><td>11.2</td></tr><tr><td>4 + 5</td><td>420</td><td>104–105</td><td>6.5</td></tr></tbody></table>

revealed that the solid residue (325 g) was composed of the monosodium oxalate (NaOOC-COOH, 93 %), NaCl (1.5 %) and oxalic acid (5 %) (Fig. 1). This result is consistent with the acidity of oxalic acid, in which only the highest carboxylic acid function reacts with NaCl to form HCl (in the absence of any additive).

The process was extended to other strong mineral acids: HBr, HI and HNO₃ using the corresponding salts NaBr, NaI and NaNO₃. The same strategy – oxalic acid with 2 equivalents of sodium chloride in refluxed

water for 2 h, followed by an azeotropic distillation – was successfully applied to the preparation of HBr, HI and HNO₃ solutions (Table 3).

The temperature of the azeotropic mixtures remained close to the previous one: 110 °C for HBr, 120 °C for HI and 100 °C for HNO₃. Although the yields were lower than those obtained with HCl, the principle of a fully bio-based synthesis was respected. Indeed, NaBr and NaI are organic salts naturally present in seawater, while NaNO₃ occurs naturally as a saltpetre.

## 4. Discussion

The environmental impact of our method was compared with that of current industrial production processes of hydrogen chloride. Hydrogen chloride can be produced through various routes, three of which are summarized in Table 4 (Büchel et al., 2008).

The historical process for producing hydrogen chloride was based on the reaction of sodium chloride with sulfuric acid at high temperatures. This method was later adapted primarily to produce sodium sulfate, with HCl generated as a by-product. Until the 1970's, approximately

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** XRPD analyses of the solid layer from the distillation base.</figcaption></figure>

<footnote>a The large amount of NaCl does not allow the reaction mixture to be stirred.</footnote>

<footnote>b The yield is established by relating the number of moles of HCl compared to the number of moles of the limiting reactant, oxalic acid (0.1 mol).</footnote>
<caption>**Table 3**</caption>

Optimisation of the experimental conditions for the preparation of concentrated solution of other HX mineral acids.

<figure><img src="image_5.png" /></figure>

<table><caption>**Table 4**</caption><thead><tr><th>Salt - HX</th><th>H<sub>2</sub>O (mL)</th><th>time (h)</th><th>[HCl] (volume)</th><th>Yield (%)</th></tr></thead><tbody><tr><td>NaBr</td><td>14</td><td>2</td><td>3.9 M (7 mL)</td><td>27</td></tr><tr><td>NaI</td><td>14</td><td>2</td><td>3.3 M (12 mL)</td><td>40</td></tr><tr><td>NaNO<sub>3</sub></td><td>14</td><td>2</td><td>0.6 M (12 mL)</td><td>14</td></tr></tbody></table>

Processes for the industrial production of HCl.

<table><thead><tr><th>1. Historic production as a by-product of sodium sulfate preparation</th><th>2. Combustion of pure gases in an “HCl oven”</th><th>3. As a by-product of chlorinated product production</th></tr></thead><tbody><tr><td>2NaCl(s) + H<sub>2</sub>SO<sub>4</sub>(l) → Na<sub>2</sub>SO<sub>4</sub>(s) + 2HCl(g)</td><td>H<sub>2</sub>(g) + Cl<sub>2</sub>(g) → 2HCl(g)</td><td>RH + Cl<sub>2</sub>(g) → RCl + HCl(g)</td></tr></tbody></table>

half of Europe's sodium sulfate production relied on this process, which has since less common (Nield, 1914; Stauffer, 2004).

Today, two industrial processes dominated hydrogen chloride production. The first involves the direct combustion of dihydrogen and chlorine in a flame at nearly 2000 °C. While this reaction yields high-purity HCl, it is also highly exothermic and poses significant industrial risks, requiring specialized equipment such as quartz or graphite burners, synthesis furnaces and steel coolers (Hill, 1948).

The second process generates hydrogen chloride as a by-product of large-scale chlorination or halogen exchange reactions, which are widely used in the manufacture of aliphatic and aromatic chlorinated hydrocarbons (Büchel et al., 2008).

All three industrial methods require a source of sodium chloride and hydrogen. While NaCl is an abundant raw material, readily obtained from seawater, natural deposits, or solution mining, the sourcing of hydrogen poses greater environmental and safety concerns (Scheme 2):

- Process 1 (sulfuric acid route): Sulfuric acid is primarily manufactured by the so-called “contact process”, which involves three stages: combustion of sulfur to produce sulfur dioxide, catalytic oxidation of sulfur dioxide to sulfur trioxide using vanadium pentoxide (V<sub>2</sub>O<sub>5</sub>), and subsequent hydration of sulfur trioxide to form sulfuric acid catalysed by a diluted solution of itself, sulfuric acid probably produced from earlier industrial production (King et al., 2013).

- Process 2 (hydrogen/chlorine combustion): Dihydrogen is typically produced via steam reforming of hydrocarbons, such as methane or biomethane, to yield “grey hydrogen”. This process releases carbon dioxide and requires high temperatures (>800 °C), moderate pressure (20–30 bar), and a catalyst. Although “green hydrogen” can be obtained through water electrolysis, this method remains economically uncompetitive and is reserved for high-purity applications such as electronics (Saha et al., 2024).

- Process 3 (by-product of chlorination): The environmental impact depends on the source of hydrogen. For example, ethylene-derived hydrogen is obtained through steam cracking of hydrocarbons at high temperatures (>800 °C from ethane), which themselves originate from petroleum distillation or natural gas (East et al., 1969; Phillips and Glazer, 1984).

- In addition, both processes 2 and 3 require the use of chlorine. Industrial chlorine is produced almost exclusively (97 %) by the electrolysis of sodium chloride solutions, a process which is highly energy-intensive, consuming approximately 3,491,000 kWh/kg of chlorine. Chlorine production is among the most electricity-

<figure><img src="image_6.png" /><figcaption>**Scheme 2.** Comparison of processes for the industrial production of HCl with the bio-based production described here.</figcaption></figure>
demanding chemical processes, second to steel manufacturing (Société chimique de France, Chlore & dichlore).

All three industrial processes raise several concerns, particularly regarding their high energy demands –requiring temperatures above 800 °C or even 2000 °C for hydrocarbon cracking and sulfuric acid production– as well as the use of hazardous reagents and reactants such as chlorine, sulfuric acid and its precursors (sulfur dioxide, sulfur trioxide and vanadium pentoxide). The classification of chemical hazards –via pictograms, hazard statements and H-phrases– clearly reflects the risks associated with these conventional methods. These examples underscore the urgent need to develop more sustainable, environmentally friendly, and safer alternatives to traditional chemical manufacturing, such as the methodology described here.

## 5. Conclusion

This article presents a novel and practical approach to the preparation of bio-based and widely used strong Brønsted acids (HCl, HBr, HI and HNO₃). It exemplifies a cleaner chemical process carries out under mild conditions and relying exclusively on renewable and non-toxic feedstocks –namely, sodium salts and oxalic acid. Today, bio-based oxalic acid is readily available on a large-scale and accessible to all chemists, making this production process of strong mineral acids applicable in both academic and industrial laboratories.

This transformation demonstrates how green and sustainable chemistry can contribute to human health by reducing the use of hazardous substances at the source (chlorine, sulfuric acid and its precursors), while also benefiting the environment through the use of bio-based inputs and in energy-efficient conditions.

The inclusion of such a process in the certification frameworks for bioactive compounds intended for well-being and cosmetics applications would further support laboratories in transitioning toward cleaner and more sustainable chemistry.

## CRediT authorship contribution statement

**Claire M. Grison:** Validation, Methodology, Investigation, Data curation, Conceptualization. **Claude Grison:** Writing – original draft, Validation, Supervision, Methodology, Conceptualization.

## 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.

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