Chromium Determination in Leather and Other Matrices

Keywords: trivalent chromium, hexavalent chromium, chrome tanning, speciation analysis, isotope dilution

A Review

Leather industry plays an essential role in the world's economy; however, it also has a negative environmental impact due to the generation of significant quantities of wastes, some of which are classified as hazardous chemicals.

Chrome tanning, the most popular tanning process, employs chromium salts, acids, and some other chemicals. Some dyes can be also a source of chromium.

As a result, hexavalent chromium, a known carcinogenic and mutagenic, can be found in leather products and cause allergic dermatitis or trigger other diseases.

For this reason, it is important to quantify the total amount of chromium in final leather goods, as well as the oxidation state in which this element is found. This paper aims to summarize chromium contamination due to the leather production processes, and to review the analytical methods that have been used to determine chromium's most abundant species: Cr(III) and Cr(VI) in leather and other matrices (foodstuffs, cosmetic products, environmental, and pharmaceutical samples). The international and European regulations are presented as well as the last academic developments to extract and quantify chromium species. The future outlook of pre-treatment and quantification techniques are also discussed in this work, with a special focus on chromium interconversions.

Introduction

Leather industry plays an essential role in world's economy. Leather and leather products are among the most extensively traded commodities globally. However, negative environmental impacts arise due to the generation of relevant quantities of wastes in the leather industry, and the use of hazardous chemicals in tanneries, such as chromium, synthetic tannins, oils, resins, biocides and detergents [START_REF] Dixit | Toxic Hazards of Leather Industry and Technologies to Combat Threat: A Review[END_REF].

In average, 3.5 million tons of different chemicals are used for leather processing. Some of the chemicals used are lime, sodium sulfide, chromium salts, dyes, and acids (Figure 1). A huge fraction of these chemicals are discharged into the effluent.

Approximately, around 50 m 3 of liquid wastes are discharged per ton of raw hide treated [START_REF] Kanagaraj | Solid Wastes Generation in the Leather Industry and Its Utilization for Cleaner Environment[END_REF]. Figure 1. Major polluting chemicals used in leather processing. Obtained with permission from [START_REF] Kanagaraj | Solid Wastes Generation in the Leather Industry and Its Utilization for Cleaner Environment[END_REF].

In the tanning process, trivalent chromium salts such as basic chromium sulfate (Cr 2 (OH(SO 4 ) 3 ) are widely used to transform raw skin into leather [START_REF] Ertani | Chromium in Agricultural Soils and Crops: A Review[END_REF]. For chrome tanning, chromium waste emissions amount from 3.0 to 7.5 kg per ton of processed hide (Table 1) [START_REF] Kanagaraj | Solid Wastes Generation in the Leather Industry and Its Utilization for Cleaner Environment[END_REF]. About 90% of all hides are treated with trivalent chromium salts [START_REF] Dias Da Silva | Evaluation of the Chromium Bioavailability in Tanned Leather Shavings Using the SM&T Sequential Extractions Scheme[END_REF].

Table 1. Waste emissions per leather manufacturing process. Modified from [START_REF] Kanagaraj | Solid Wastes Generation in the Leather Industry and Its Utilization for Cleaner Environment[END_REF]. Nevertheless, even if Cr(III) is not considered as toxic, a fraction of this chromium can be oxidized into Cr(VI), a hazardous species. This unintentional oxidation of Cr(III) into Cr(VI) can be attributed to a high temperature, a dry atmosphere, and exposure to UV light [START_REF] Fontaine | Hexavalent Chromium Release from Leather over Time Natural Ageing vs Accelerated Ageing According to a Multivariate Approach[END_REF]. Consequently, Cr(VI) can still be found in leather products and cause allergic dermatitis or trigger more serious illnesses. As many leather products are directly in contact with the skin, it is important to limit as much as possible the use of chromium salts as tanning agents.

Soaking

It is important to remark that even if chromium tanning effluents contribute to a high concentration of chromium in the soils, this represents a minimal risk to crops. To delve more deeply into this subject, both chromium species can be absorbed by plants, yet, Cr(VI) is preferred due to sulfate carriers. In the roots, Cr(VI) is reduced to Cr(III) by the action of Fe(III) reductase enzymes [START_REF] Hamilton | Chromium Speciation in Foodstuffs: A Review[END_REF]. Therefore, even if crops are irrigated with contaminated effluents, Cr(VI) is not likely to be transferred to foodstuffs.

Antioxidants also contribute to limit the presence of Cr(VI) species in foodstuffs, which is the case of tea leaves and infusions, since they have a high content of flavonoid compounds [START_REF] Pyrzynska | Determination of Cr (VI) in Tea[END_REF]. Additionally, in the acidic conditions of the stomach, hexavalent chromium is likely to be reduced to trivalent chromium, representing no major risk to human health [START_REF] Štupar | Simultaneous Determination of Chromium(III) Complexes and Chromium(VI) by Fast Protein Anion-Exchange Liquid Chromatography-Atomic Absorption Spectrometry[END_REF].

However, due to the dangerous effects of hexavalent chromium in leather,

European and international agencies have adopted directives that limit its content in final products. Standard protocols have been developed in order to regulate the extraction and determination of Cr(VI). This review provides a summary of the pretreatment techniques and the analytical methods that have been used for chromium speciation in leather, as well as in other matrices such as foodstuffs, cosmetic products, environmental, and pharmaceutical samples. The latest developments and the future outlooks on chromium speciation are also discussed herein. Abbreviations used in this article are given in Table 2.

Leather production processes

The leather manufacturing process can be divided into four main steps: 1) preservation of hides and skins storage, 2) beam house operations, 3) tanning, and 4) post-tanning and finishing operations. A process diagram is shown in Figure 2.

Preservation of hides and skins storage

In this step, skins and hides are sorted and trimmed. A curing process is then implemented in order to prevent their decomposition, usually by salting and drying the raw material. This allows the skins and hides to be stored in ventilated or cooled areas for further processing [START_REF]Technical EIA Guidance Manual for Leather/Skin/Hide Processing Industry Prepared for The Ministry of Environment and Forest[END_REF].

Beam house operations

In the first instance, hides are soaked to get rid of salts and other solids, as well as to rehydrate. Subsequently, a liming process (using lime liquors along with sodium sulfide) is performed to facilitate the removal of hair, flesh, fat, and inter-fibrillary protein.

Hides are then fleshed to remove excess tissues, muscles and fat. In a de-liming procedure, pH is adjusted around 8.0 in order to convert some proteins to soluble forms.

Pickling is required to correct the pH suitable to the tanning procedure (pH 2.8-3.0)

with the use of a brine solution and sulfuric acid [START_REF] Dixit | Toxic Hazards of Leather Industry and Technologies to Combat Threat: A Review[END_REF][START_REF]Technical EIA Guidance Manual for Leather/Skin/Hide Processing Industry Prepared for The Ministry of Environment and Forest[END_REF][START_REF] Sundar | Cleaner Chrome Tanning -Emerging Options[END_REF].

Tanning

The purpose of tanning is to transform raw hide into leather. There are several tanning methods, among which are chrome tanning, vegetable tanning, aldehyde tanning, synthetic tanning, and alum-tawing; being chrome and vegetable tanning the most widespread techniques [START_REF] Dixit | Toxic Hazards of Leather Industry and Technologies to Combat Threat: A Review[END_REF].

The electronic structure of chromium ([Ar]3d 5 4s 1 ) makes it a perfect tanning agent due to its incomplete 3d orbitals that can host additional electrons to form coordination complexes, which is one of the reasons why chrome tanning is the most common tanning system [START_REF] Fontaine | Étude de l'évolution de la teneur en chrome hexavalent dans le cuir, influence des conditions fabrication et importance des facteurs environnementaux[END_REF][START_REF] Onem | Comparison of Different Tanning Agents on the Stabilization of Collagen via Differential Scanning Calorimetry[END_REF]. Chrome tanning is performed with trivalent chromium salts, which ensures an excellent quality of the final product in terms of mechanical and hydrothermal resistance, attributable to the high rate of infiltration of Cr(III) salts into the interfibrillar spaces of the skins [START_REF] Dias Da Silva | Evaluation of the Chromium Bioavailability in Tanned Leather Shavings Using the SM&T Sequential Extractions Scheme[END_REF].

Collagen, the most abundant protein in the skins, consists of three polypeptide chains in the form of a helix. Aspartic and glutamic acid, some of the principal amino acids of collagen, provide amine (R-NH 2 ) and carboxyl (R-COOH) groups that are essential for chrome tanning [START_REF] Fontaine | Étude de l'évolution de la teneur en chrome hexavalent dans le cuir, influence des conditions fabrication et importance des facteurs environnementaux[END_REF]. During this process, pickled skins are treated with basic chromium sulfate, (Cr 2 (OH(SO 4 ) 3 ), (6-8%) and water as the medium. The first step is carried out in acidic medium (part of the pickling bath) permitting carboxyl groups to protonate. The objective of this step is to allow the chromium salts to penetrate homogeneously in the skin. Subsequently, the pH of the bath is raised to 3.8-4.0 with 10% alkali solution. In this step, carboxylate ions (R-COO -) are liberated from collagen, and chromium creates coordinate covalent bonds with the radicals containing the ionized carboxyl groups. After a self-polymerization reaction, this results in the formation of chromium-collagen complexes [START_REF] Sundar | Cleaner Chrome Tanning -Emerging Options[END_REF][START_REF] Fontaine | Étude de l'évolution de la teneur en chrome hexavalent dans le cuir, influence des conditions fabrication et importance des facteurs environnementaux[END_REF][START_REF] Onem | Comparison of Different Tanning Agents on the Stabilization of Collagen via Differential Scanning Calorimetry[END_REF]. An example of a coordinate complex between collagen and chromium is shown in Figure 3.

Successful crosslinking of collagen in hides results in outstanding properties of the leather [START_REF] Brown | A Conformational Study of Collagen as Affected by Tanning Procedures[END_REF]. The duration of chrome tanning is around 6 hours. After this process, leather is called wet blue [START_REF] Sundar | Cleaner Chrome Tanning -Emerging Options[END_REF].

Figure 3. Interaction of chromium with collagen.

On the other hand, vegetable tanning, one of the most ancient tanning methods, works through the action of water infusions of tannins or tannic acid [START_REF]Leather Processing & Tanning Technology Handbook[END_REF]. Vegetable tanning agents are leached from wood, bark, leaves, roots, etc. [START_REF] Kanth | Cleaner Tanning Practices for Tannery Pollution Abatement: Role of Enzymes in Eco-Friendly Vegetable Tanning[END_REF] They may be described as a mixture of polyphenols which bind to collagen in order to confer resistance to the hides. Vegetable tanning materials can be divided into two groups [START_REF]Leather Processing & Tanning Technology Handbook[END_REF]:

(1) Pyrogallol group. They provide a yellow to light brown deposit (ellagic acid) to the leather and improve wearing properties and water resistance. Examples: sumach, chestnut, myrabolams, divi-divi, oak-wood, oak galls valonia, oak-barks.

(2) Catechol group. They provide a pink to red sediment (phlobaphenes) to the leather and the resultant leather is more solid. Examples: gambier, hemlock, quebracho, mimosa, mangrove, pine, oak-bark.

Although vegetable tanning is a greener technology, chrome tanning reaction is faster (6 hours instead of 3 weeks), confers high hydrothermal stability, alters the structure of collagen in only a small way, makes collagen more hydrophobic (hence, water-resistant) and allows the retention of colorants since Cr(III) can act as a dye fixing agent [START_REF] Covington | Tanning Chemistry: The Science of Leather[END_REF].

Post-tanning and finishing operations

Post-tanning operations include the removal of excess moisture (sammying), splitting the material into the desired thickness, mechanical shaving and trimming, and rechroming or semi-chroming to improve the chromium content in leather. Subsequently, pH is neutralized and leather is dyed. Finally, the leather is fat-liquored and finished to obtain softness and fullness [START_REF]Technical EIA Guidance Manual for Leather/Skin/Hide Processing Industry Prepared for The Ministry of Environment and Forest[END_REF].

During the dyeing process, the leather is colored using: acid dyes, basic dyes, direct dyes, mordant dyes, premetallized dyes, reactive dyes or sulfur dyes. Azo dyes are organic colorants containing the azo group (N=N), usually bonded with chromophores. The azo group contributes to the delocalization of electrons in the chromophore, therefore, to the generation of color [START_REF] Covington | Tanning Chemistry: The Science of Leather[END_REF]. Orange II, shown in Figure 4, is an example of an azo dye widely used in the leather industry. These synthetic colorants are very popular in leather processing, though, its toxicity has been extensively investigated. Several azo dyes, specially benzidine-based colorants, have been reported to have a negative effect on human health, causing allergies or even triggering cancer [START_REF] Golka | Carcinogenicity of Azo Colorants: Influence of Solubility and Bioavailability[END_REF][START_REF] Chung | Azo Dyes and Human Health: A Review[END_REF]. Apart from the toxicity of some colorants, another inconvenient can emerge when hexavalent or trivalent chromium is determined through colorimetric methods.

Some colorants (examples shown in Table 3) absorb in the same wavelength as some complexes used for chromium determination. Hence, for chromatographic applications, the efficiency of the pre-column or guard column is of great importance. into Cr(VI) [START_REF]:2018 Leather -Chemical Determination of Chromium(VI) Content in Leather -Thermal Pre-Ageing of Leather and Determination of Hexavalent Chromium[END_REF].

In order to study the different storage conditions that could influence the evolution of hexavalent chromium content in leather, Fontaine et al. [START_REF] Fontaine | Hexavalent Chromium Release from Leather over Time Natural Ageing vs Accelerated Ageing According to a Multivariate Approach[END_REF]

Regulations on hexavalent chromium

Because of the dangerous effects of hexavalent chromium in human health, its content in leather products should be closely monitored.

As a result of an increment in chromium allergy, 16 228 patients with dermatitis patch tested between 1985 and 2007 at the Department of Dermato-Allergology, Getofte Hospital (Denmark), were studied to determinate the possible causes of this allergy. In particular, this study showed that around 55% of 197 patients with chromium allergy tested between 1995 and 2007 had been exposed to leather products (shoes, gloves and other sources) [START_REF] Thyssen | The Prevalence of Chromium Allergy in Denmark Is Currently Increasing as a Result of Leather Exposure: The Development of Chromium Allergy[END_REF] .

Further determinations of hexavalent chromium in leather goods performed in Germany and Denmark between 2007 and 2008 showed that more than 30% of these leather articles contained above 3 mg/kg of hexavalent chromium [23].

In 2012, the Danish government submitted a report in which they proposed to the European Chemicals Agency a maximum hexavalent chromium content of 3 ppm in leather products sold in the European Union [23,[START_REF] Thyssen | Hexavalent Chromium in Leather Is Now Regulated in European Union Member States to Limit Chromium Allergy and Dermatitis[END_REF].

Consequently, in 2014 the European Union adopted the restriction proposed by the Danish government, limiting the content of hexavalent chromium to 3 mg/kg of the total dry weight in leather articles coming into contact with the skin; otherwise these products could not be commercialized [START_REF]Evaluation, Authorisation and Restriction of Chemicals (REACH) as Regards Chromium VI Compounds[END_REF].

Currently, the ISO 17075 standard is the only internationally recognized procedure to determine the content of hexavalent chromium in leather via a colorimetric (part 1) [START_REF]IULTCS/IUC 18-1] Leather -Chemical Determination of Chromium(VI) Content in Leather -Part 1: Colorimetric Method[END_REF] and a chromatographic (part 2) analysis [START_REF]IULTCS/IUC 18-2] Leather -Chemical Determination of Chromium(VI) Content in Leather -Part 2: Chromatographic Method[END_REF]. This standard requires a previous selective extraction of Cr(VI) with K 2 HPO 4 •3H 2 O at pH 8.0, and it is suitable for concentrations above 3 mg/kg. The colorimetric analysis requires a reaction between Cr(VI) and DPC, and the detection of the resulting complex by UV-Vis spectroscopy at 540 nm. Concerning the chromatographic analysis, it can be performed by direct detection of the chromate peak at 372 nm, or through a post-column reaction with DPC.

Post-column reaction offers more sensitivity and should be preferred in case the sample may contain reducing species that interfere with the analyte [START_REF] Fontaine | Ion Chromatography with Post Column Derivatization for the Determination of Hexavalent Chromium in Dyed Leather. Influence of the Preparation Method and of the Sampling Location[END_REF][START_REF]Methods of Testing Cement. Part 10, Part 10, Determination of the Water-Soluble Chromium (VI) Content of Cement[END_REF]. Though, an inconvenient can surge when using post-column reaction due to the interference of colorants that may absorb at the same wavelength as the complex analyzed.

Chemistry of chromium

Chromium is a transition metal that, after being processed, occurs in several chemical species: metallic chromium, trivalent chromium and hexavalent chromium [START_REF] Gómez | Chromium Determination and Speciation since 2000[END_REF].

Metallic chromium, chromium 0, can be found in alloys and coatings. As a coating, chromium exhibits anti-wear and anticorrosion properties, and can also be used as decoration. It is obtained as a result of an electrochemical reduction of chromium salts [START_REF] Weng | Direct Electro-Deposition of Metallic Chromium from K2CrO4 in the Equimolar CaCl2-KCl Molten Salt and Its Reduction Mechanism[END_REF][START_REF] Ferreira | Electrochemical Studies of Metallic Chromium Electrodeposition from a Cr(III) Bath[END_REF].

Trivalent chromium, Cr(III), exists as cationic species (Cr 3+ , Cr(OH) 2+ , Cr(OH) 2 + ) [START_REF] Hamilton | Chromium Speciation in Foodstuffs: A Review[END_REF] and has been considered as an essential nutrient involved in the regulation of glucose [START_REF] Sharma | Beneficial Effect of Chromium Supplementation on Glucose, HbA1C and Lipid Variables in Individuals with Newly Onset Type-2 Diabetes[END_REF][START_REF] Paiva | Beneficial Effects of Oral Chromium Picolinate Supplementation on Glycemic Control in Patients with Type 2 Diabetes: A Randomized Clinical Study[END_REF], due to its participation in the activation of an insulin receptor [START_REF] Vincent | The Biochemistry of Chromium[END_REF][START_REF] Chen | Simultaneous Analysis of Cr(III), Cr(VI), and Chromium Picolinate in Foods Using Capillary Electrophoresis-Inductively Coupled Plasma Mass Spectrometry: CE and CEC[END_REF]. It can be frequently found in the form of chromium picolinate in dietary supplements. Nonetheless, some studies suggest no relevant benefits in certain populations [START_REF] Bailey | Improved Meta-Analytic Methods Show No Effect of Chromium Supplements on Fasting Glucose[END_REF]. Though, more studies on proper dosage and length of time of supplementation should be performed in order to accurately define the advantages of chromium picolinate supplementation [START_REF] Marmett | Effects of Chromium Picolinate Supplementation on Control of Metabolic Variables: A Systematic Review[END_REF].

By the other hand, hexavalent chromium, Cr(VI), exists as anionic species

(HCrO 4 - , CrO 4 2- , HCr 2 O 7 - , Cr 2 O 7 2-
), and it is a known mutagenic, classified by the International Agency for Research on Cancer (IARC) as carcinogenic to humans [START_REF] Nascimento Monteiro | Mutagenic and Histopathological Effects of Hexavalent Chromium in Tadpoles of Lithobates Catesbeianus (Shaw, 1802) (Anura, Ranidae)[END_REF].

For this reason, it is of great importance the selective determination of Cr(VI).

As shown in the Eh-pH diagram from Figure 6, in aqueous systems, the oxidation state of chromium is determined by redox potential and pH [START_REF] Jin | Electrochemical Processes for the Environmental Remediation of Toxic Cr(VI): A Review[END_REF]. The reduction process in acidic solution can be expressed as [START_REF] Saha | Sources and Toxicity of Hexavalent Chromium[END_REF]:

(

and in alkaline solution:

(2)

As we can see, interconversions can take place between the oxidation state of chromium. Cr(VI) can be reduced to Cr(III) under acidic conditions; by the other hand, Cr(III) can be oxidized to Cr(VI) in the presence of oxygen under alkaline conditions [START_REF] Ertani | Chromium in Agricultural Soils and Crops: A Review[END_REF].

Dias da Silva et al. [START_REF] Dias Da Silva | Evaluation of the Chromium Bioavailability in Tanned Leather Shavings Using the SM&T Sequential Extractions Scheme[END_REF], after performing an extraction scheme that will be further described, noticed that in one of the steps (oxidizing conditions) about 48%, 5%, and 0.9% of chromium (percentages from three different samples) were converted into Cr(VI). Martone et al. [START_REF] Martone | Determination of Chromium Species in Dietary Supplements Using Speciated Isotope Dilution Mass Spectrometry with Mass Balance[END_REF], knowing the possibility of this type of transformations, employed isotopic dilution techniques to follow species interconversions during the analysis of Cr(VI) in dietary supplements. The results showed that 11 samples contained a highly reductive matrix that transformed all the spiked 53 Cr(VI) into 53 Cr(III). For 14 samples, the phosphate buffer extraction (basic media) caused an oxidation of Cr(III) to Cr(VI) that ranged from 0.3 to 47.9%.

This poses a difficulty in the speciation of chromium, as the sample preparation protocols can lead to an interconversion between Cr(III) and Cr(VI), thus distorting the results [START_REF] Novotnik | Chromate in Food Samples: An Artefact of Wrongly Applied Analytical Methodology?[END_REF][START_REF] Pastore | Evidence of Cr(VI) Formation during Analysis of Leather[END_REF]. Therefore, the real challenge is to preserve, as much as possible, the initial distribution of both chromium species throughout the pre-treatment and analysis stages. Protocols should guarantee that no Cr(VI) is generated from the oxidation of Cr(III). In the same way, no Cr(VI) should be masked by its reduction to Cr(III) during its extraction and determination.

Complexing agents

The use of complexing agents can be a useful tool to achieve trivalent and hexavalent chromium separation. Some of the most popular chelating agents are 1,5diphenylcarbazide (DPC), ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), 2,6-pyridinedicarboxylic acid (PDCA), 1,2-cyclohexanediaminetetraacetic acid (DCTA), and 2-hydroxybenzaldiminoglycine (HBIG). Some of them can form a complex with Cr(III), or with Cr(VI) after oxidation of the organic agent. The main examples are presented below.

DPC

In acidic medium, Cr(VI) oxidizes DPC to 1,5-diphenylcarbazone (DPCO), resulting in a violet Cr(III)-diphenylcarbazone complex that can be detected by UV-Vis spectroscopy at 540 nm [START_REF]IULTCS/IUC 18-1] Leather -Chemical Determination of Chromium(VI) Content in Leather -Part 1: Colorimetric Method[END_REF][START_REF] Marczenko | Preconcentration and Spectrophotometry in Inorganic Analysis[END_REF][START_REF] Duffy | ChromiSense: A Colourimetric Lab-on-a-Disc Sensor for Chromium Speciation in Water[END_REF]. The reaction is shown in Figure 7.

EDTA

EDTA can form colored complexes with some cations. In slightly acidic medium, EDTA can react with Cr(III) to form a violet Cr(III)-EDTA -complex (anionic species)

that absorbs strongly at 540 nm [START_REF] Marczenko | Preconcentration and Spectrophotometry in Inorganic Analysis[END_REF]. Due to the slow rate of complexation at room temperature, solutions are frequently heated to accelerate the reaction [START_REF] Byrdy | Chromium Speciation by Anion-Exchange High-Performance Liquid Chromatography with Both Inductively Coupled Plasma Atomic Emission Spectroscopic and Inductively Coupled Plasma Mass Spectrometric Detection[END_REF][START_REF] Gürleyük | Determination of Chromium(III) and Chromium(VI) Using Suppressed Ion Chromatography Inductively Coupled Plasma Mass Spectrometry[END_REF].

DTPA

Similarly to EDTA, DTPA can react with trivalent chromium to produce the [Cr(DTPA)] 2-complex. Yet, poorer stability has been reported for [Cr(DTPA)] 2- [START_REF] Chen | Speciation of Chromium in Waste Water Using Ion Chromatography Inductively Coupled Plasma Mass Spectrometry[END_REF].

PDCA PDCA is also used to complex Cr(III) due to its high selectivity compared to other ligands, resulting in a [Cr(PDCA) 2 ] -complex [START_REF] Chen | Separation of Chromium (III) and Chromium (VI) by Capillary Electrophoresis Using 2,6-Pyridinedicarboxylic Acid as a Pre-Column Complexation Agent[END_REF]. The reaction between Cr(III) and 2 molecules of PDCA is described in Figure 8. [Cr(PDCA) 2 ] -can be quantified through direct UV detection at 335 nm and 535 nm [START_REF] Duffy | ChromiSense: A Colourimetric Lab-on-a-Disc Sensor for Chromium Speciation in Water[END_REF].

DCTA

In a slightly acidic medium (pH 6.5 -7.0), Cr(III) can be chelated with DCTA to produce a single charged Cr-DCTA -complex [START_REF] Chen | Simultaneous Analysis of Cr(III), Cr(VI), and Chromium Picolinate in Foods Using Capillary Electrophoresis-Inductively Coupled Plasma Mass Spectrometry: CE and CEC[END_REF]. This reaction occurs slowly at room temperature but can be warmed up to 60 ºC to speed up the rate of reaction. The Cr-DCTA -complex can be measured through UV detection at 210 nm [START_REF] Padarauskas | Speciation of Cr(III) and Cr(VI) and Separation of Common Anions by Ion Pair Chromatography with Trans-1,2-Diaminecyclohexane-N,N,N′,N′-Tetraacetic Acid[END_REF].

HBIG HBIG has also showed potential to chelate Cr(III). Complexation occurs instantaneously at pH 7.5, and requires the use of a buffer solution: disodium hydrogen orthophosphate (0.01 M) and sodium hydrogen orthophosphate (0.01 M). The chromium(III)-HBIG complex has a maximum absorbance a 565 nm [START_REF] Kumar | Spectrophotometric Determination of Chromium(III) with 2-Hydroxybenzaldiminoglycine[END_REF]. This method is selective and sensitive.

Nevertheless, UV detection is not a very selective method and some other compounds, especially when working on complexes matrices, can be extracted and absorb at the same wavelength than the complex formed between chromium and the ligand.

Pre-treatment and determination methods

Due to the nature of the samples, chromium speciation in leather requires a treatment before approaching its determination. Concerning leather products, only few pretreatment and analytical methods have been reported. Therefore, this section presents a review of pre-treatment and determination methods for chromium species in leather, but also includes chromium speciation studies in environmental and pharmaceutical samples, foodstuffs, and cosmetic products. In this way, we can see what is being done with other matrices, and what can be implemented when working with leather. Table 4 provides a summary of the reported literature of chromium speciation. It is arranged by analytical technique, and includes an exhaustive revision of pre-treatment and analytical methods. The way in which the different analytical figures of merit were calculated is specified in Table 5 (NA whenever the author did not specified the calculation).

Sample pre-treatment

Depending on the extraction conditions (i.e. pH, concentration, etc.), chromium can be encountered in different forms as specified in Table 6. For this fraction, they observed that a part of the released chromium was oxidized to from Cr(III) to Cr(VI), which exposes the danger of the disposal of this type of waste under oxidative conditions.

For soils, sludges, sediments, and similar waste materials, the SW-846

Environmental Protection Agency (EPA) 3060A method defines an extraction protocol for hexavalent chromium using 0.28M Na 2 CO 3 /0.5M NaOH, a phosphate buffer (0.5M K 2 HPO 4 /0.5M KH 2 PO 4 ) and MgCl 2 [START_REF]SW-846 Test Method 3060A: Alkaline Digestion for Hexavalent Chromium[END_REF]. Mg 2+ serves to suppress oxidation of Cr(III) into Cr(VI). Petrucci and Senofonte [START_REF] Petrucci | Determination of Cr(VI) in Cosmetic Products Using Ion Chromatography with Dynamic Reaction Cell-Inductively Coupled Plasma-Mass Spectrometry (DRC-ICP-MS)[END_REF] adapted the EPA 3060A method for cosmetic products pre-treatment. This alkaline digestion has been employed for traditional Chinese medicines as well [START_REF] Li | Determination of Hexavalent Chromium in Traditional Chinese Medicines by High-Performance Liquid Chromatography with Inductively Coupled Plasma Mass Spectrometry: Liquid Chromatography[END_REF].

An alkaline extraction of hexavalent chromium with NH 4 OH is frequently used when working with foodstuffs (milk, cheese, yoghurt, cereal products, chocolate, vegetables, fruits, meat, fish, eggs, and beverages) [START_REF] Hernandez | Optimisation of Selective Alkaline Extraction for Cr(VI) Determination in Dairy and Cereal Products by HPIC-ICPMS Using an Experimental Design[END_REF][START_REF] Hernandez | Development and Application of a Method for Cr(III) Determination in Dairy Products by HPLC-ICP-MS[END_REF][START_REF] Vacchina | Cr(VI) Speciation in Foods by HPLC-ICP-MS: Investigation of Cr(VI)/Food Interactions by Size Exclusion and Cr(VI) Determination and Stability by Ion-Exchange on-Line Separations[END_REF]. Some other selective alkaline extractions of Cr(VI) require the use of 0.1M Na 2 CO 3 [START_REF] Mathebula | Speciation of Chromium in Bread and Breakfast Cereals[END_REF][START_REF] Mandiwana | Determination of Chromium(VI) in Black, Green and Herbal Teas[END_REF] or 0.01M NaOH / 1M NH 4 NO 3 [START_REF] Soares | Chromium Speciation Analysis in Bread Samples[END_REF].

Cloud Point Extraction (CPE) has been used for separation and preconcentration of metallic ions, among them chromium [START_REF] Bosch Ojeda | Separation and Preconcentration by a Cloud Point Extraction Procedure for Determination of Metals: An Overview[END_REF][START_REF] Pytlakowska | Complex-Forming Organic Ligands in Cloud-Point Extraction of Metal Ions: A Review[END_REF][START_REF] Mortada | Recent Developments and Applications of Cloud Point Extraction: A Critical Review[END_REF]. Kiran et al. [START_REF] Kiran | Speciation Determination of Chromium(III) and (VI) Using Preconcentration Cloud Point Extraction with Flame Atomic Absorption Spectrometry (FAAS)[END_REF] reported the determination of Cr(III) in industrial wastewater, natural water, and spiked water samples after its complexation with bis-[2-hydroxynaphthaldehyde] thiourea using CPE.

For the determination of Cr(VI) in turmeric samples, Tiwari et al. [START_REF] Tiwari | Cloud Point Extraction and Diffuse Reflectance-Fourier Transform Infrared Spectroscopic Determination of Chromium(VI): A Probe to Adulteration in Food Stuffs[END_REF] complexed Cr(VI) with DPC and then used the non-ionic surfactant Triton X-100 for the determination of Cr(VI) by CPE. Likewise, Solid Phase Extraction (SPE) has also permitted the preconcentration of Cr(VI) on Amberlite XAD-1180 resin after its reaction with DPC, as reported by Narin et al. [START_REF] Narin | A Novel Solid Phase Extraction Procedure on Amberlite XAD-1180 for Speciation of Cr(III), Cr(VI) and Total Chromium in Environmental and Pharmaceutical Samples[END_REF] For total chromium determination in siliceous matrices, organic matrices, and other complex matrices, the SW-846 EPA 3052 method proposes a microwave digestion with HNO 3 and HF. HCl and H 2 O 2 can be alternatively used [START_REF]SW-846 Test Method 3052: Microwave Assisted Acid Digestion of Siliceous and Organically Based Matrices[END_REF].

Determination methods

UV-Visible detection

Regarding hexavalent chromium determination in leather, ISO proposes a colorimetric analysis by UV-Vis spectroscopy at 540 nm after the reaction between Cr(VI) and DPC, as well as a chromatographic analysis (using an anion exchange column and a precolumn) by direct detection of the chromate peak at 372 nm, or after a post-column reaction with DPC by measuring the absorption peak at 540 nm [START_REF]IULTCS/IUC 18-1] Leather -Chemical Determination of Chromium(VI) Content in Leather -Part 1: Colorimetric Method[END_REF][START_REF]IULTCS/IUC 18-2] Leather -Chemical Determination of Chromium(VI) Content in Leather -Part 2: Chromatographic Method[END_REF]. Similarly, the SW-846 EPA 7196A method describes a UV-Visible determination of hexavalent chromium in groundwaters based on the same reaction [START_REF]SW-846 Test Method 7196A: Chromium, Hexavalent (Colorimetric)[END_REF]. In order to evaluate the ISO standardized protocol, Pastore et al. [START_REF] Pastore | Evidence of Cr(VI) Formation during Analysis of Leather[END_REF] performed Ion Chromatography and postcolumn reaction with DPC, and obtained a limit of quantification (LOQ) of 0.

Off-line analytical techniques

Certain off-line techniques that have been used for chromium speciation in various matrices are Flame Atomic Absorption Spectrometry (FAAS) [START_REF] Kiran | Speciation Determination of Chromium(III) and (VI) Using Preconcentration Cloud Point Extraction with Flame Atomic Absorption Spectrometry (FAAS)[END_REF][START_REF] Narin | A Novel Solid Phase Extraction Procedure on Amberlite XAD-1180 for Speciation of Cr(III), Cr(VI) and Total Chromium in Environmental and Pharmaceutical Samples[END_REF], Electrothermal Atomic Absorption Spectroscopy (ETAAS) [START_REF] Soares | Chromium Speciation Analysis in Bread Samples[END_REF], Graphite Furnace Atomic Absorption Spectrometry (GFAAS) [START_REF] Mandiwana | Determination of Chromium(VI) in Black, Green and Herbal Teas[END_REF] and High-Resolution Continuum Source Atomic Absorption Spectrometer (HR-CS AAS) [START_REF] Mathebula | Speciation of Chromium in Bread and Breakfast Cereals[END_REF]. Nevertheless, these techniques do not allow the separation of species and require a selective extraction of either one of the two species. Generally, the other species is calculated by subtraction of the extracted species from the total content of chromium.

Capillary electrophoresis

A different approach was reported by Chen et al. [START_REF] Chen | Simultaneous Analysis of Cr(III), Cr(VI), and Chromium Picolinate in Foods Using Capillary Electrophoresis-Inductively Coupled Plasma Mass Spectrometry: CE and CEC[END_REF] via Capillary Electrophoresis -

Inductively

Chromatographic methods coupled to Mass Spectrometry

Coupling chromatographic methods with Inductively Coupled Plasma -Mass Spectrometry (ICP-MS) has facilitated and increased speciation studies, allowing the separation and posterior quantification of trivalent and hexavalent chromium. HPLC-ICP-MS was also implemented for the analysis of Cr(III) and Cr(VI) in environmental samples. Jabłońska-Czapla et al. [START_REF] Czapla | Development and Validation of HPLC-ICP-MS Method for the Determination Inorganic Cr, As and Sb Speciation Forms and Its Application for Pławniowice Reservoir (Poland) Water and Bottom Sediments Variability Study[END_REF] accomplished chromium speciation in water and bottom sediments using an anion exchange Dionex IonPac AG7 guard column and 0.1M NH 4 NO 3 / 0.8M HNO 3 gradient elution after buffer extraction.

According to Séby et al. [START_REF] Séby | Chromium Speciation by Hyphenation of High-Performance Liquid Chromatography to Inductively Coupled Plasma-Mass Spectrometry-Study of the Influence of Interfering Ions[END_REF], who worked with an ion exchange Dionex IonPac CS5A column and a CG5A guard column, they were not able to quantify Cr(III) in natural water samples due to its possible precipitation in alkaline medium. To avoid this problem, Hu and his colleagues [START_REF] Hu | Occurrence and Speciation of Polymeric Chromium(III), Monomeric Chromium(III) and Chromium(VI) in Environmental Samples[END_REF] stabilized Cr(III) with EDTA, and were able to separate it from hexavalent chromium and polymeric Cr(III) using a reversed phase Spheri-5 C18 column, obtaining a LOD of 0.9 µg/L for both chromium species in wood leachates, surface waters, tap water, and wastewater. Cr(III)-EDTA -, and confirmed the kinetical stability of the chromium-chelate complex.

Direct injection of the samples was possible and no Cl -interferences were found.

Hagendorfer and Goessler [START_REF] Hagendorfer | Separation of Chromium(III) and Chromium(VI) by Ion Chromatography and an Inductively Coupled Plasma Mass Spectrometer as Element-Selective Detector[END_REF] studied chromium speciation in cement and homeopathic drugs by IC-ICP-MS without derivatization. This was attained by taking into account the distribution of Cr species in water and working in the window of pH where Cr(VI)

is not reduced and Cr(III) does not precipitate, which can be described as the gray shading in Figure 9. For this reason, they used a mobile phase of pH 3.5 (90 mM (NH 4 ) 2 SO 4 and 10 mM NH 4 NO 3 ) and a metal-free Rspak NN-814 PEEK column. When working with ICP-MS for Cr determination, it is important to precise that polyatomic interferences can arise with the presence of carbon or chlorine. For 52 Cr + the interferences could be 35 column and an AG20 guard column. 50 Cr(VI) and 53 Cr(III) tracers were used by Drinčić et al. [START_REF] Drinčić | Determination of Hexavalent Cr in River Sediments by Speciated Isotope Dilution Inductively Coupled Plasma Mass Spectrometry[END_REF] to follow possible interconversions in the determination of hexavalent chromium by HPLC-ICP-MS using an anion exchange Mono Q HR 5/5 column.

However, Cr(VI) was not detected in river sediments. SIDMS has also been reported for teas, bread samples [START_REF] Novotnik | Chromate in Food Samples: An Artefact of Wrongly Applied Analytical Methodology?[END_REF], and dietary supplements [START_REF] Martone | Determination of Chromium Species in Dietary Supplements Using Speciated Isotope Dilution Mass Spectrometry with Mass Balance[END_REF].

IDMS was studied by Fabregat-Cabello et al. [START_REF] Fabregat-Cabello | Fast and Accurate Procedure for the Determination of Cr(VI) in Solid Samples by Isotope Dilution Mass Spectrometry[END_REF], who quantified Cr(VI) in several Certified Reference Materials (CRMs) after microwave assisted extraction with 5mM EDTA (5 min at 90 ºC and 5 min at 110 ºC) by IC-ICP-MS using an anion exchange Dionex IonPac AS9-HC column. The CRMs analyzed in this work were: soil containing chromite ore processing residue, welding dust loaded on a filter, and sandy clay soil. In this work, only 50 Cr(VI) was added to the samples as Cr(III) was found to be stable in the extracting conditions. Guidotti et al. [START_REF] Guidotti | Quantification of Cr(VI) in Soil Samples from a Contaminated Area in Northern Italy by Isotope Dilution Mass Spectrometry[END_REF] modified these extraction conditions (only one cycle of 5 min at 80 ºC) to minimize Cr(VI) reduction during the analysis of soil samples from a contaminated area. 53 Cr(VI) was traced to attain accurate quantification by Liquid Chromatography -ICP-MS (LC-ICP-MS) using the same column as Fabregat-Cabello et al. [START_REF] Fabregat-Cabello | Fast and Accurate Procedure for the Determination of Cr(VI) in Solid Samples by Isotope Dilution Mass Spectrometry[END_REF]. The application of both IDMS and SIDMS was reported by Wolle et al [START_REF] Wolle | Optimization and Validation of Strategies for Quantifying Chromium Species in Soil Based on Speciated Isotope Dilution Mass Spectrometry with Mass Balance[END_REF] in soil samples.

In the absence of CRMs for chromium speciation in leather samples, only spiking and isotope dilution techniques can help to improve the accuracy of the measurements. Therefore, these techniques that had been employed in other matrices should set the tone for what should be considered when working with leather. Cr(III) was chelated with DCTA to form a single charged Cr-DCTA - complex.

Chen et al. [START_REF] Chen | Simultaneous Analysis of Cr(III), Cr(VI), and Chromium Picolinate in Foods Using Capillary Electrophoresis-Inductively Coupled Plasma Mass Spectrometry: CE and CEC[END_REF] Turmeric samples

The Cr(VI)-DPC complex was extracted of using Triton X-100 as non-ionic surfactant. CPE was used as preconcentration process. [START_REF] Rezić | Determination of Extractable Chromium from Leather[END_REF] NA

DRS-FTIR

Future outlooks

As a result of the environmental concerns regarding leather industry, researchers have been continuously trying to find ways to reduce wastes, as well as alternatives to conventional chrome tanning.

Recently, Md.A. Moktadir et al. [START_REF] Moktadir | Circular Economy Practices in the Leather Industry: A Practical Step towards Sustainable Development[END_REF] analyzed the challenges of implementing Circular Economy (CE) practices in leather industry. CE is a strategy that aims to limit the input of virgin materials and the production of wastes, creating a closed-loop system. This work revealed that the major challenge is the lack of financial support from the authorities, followed by the lack of technological advancement. According to them, financial support could help implement CE policies for environmental protection.

Meanwhile, technological advancement could improve tannery effluent treatment, which would reduce wastes and protect the environment. The analysis of wastewater, solid waste, sulfide, Chemical Oxygen Demand (COD), ammonium salt, chloride and chromium with CE practices is essential to accomplish sustainable development of leather industry [START_REF] Hu | Ecological Utilization of Leather Tannery Waste with Circular Economy Model[END_REF].

According to Abdel-Shafy et al. [START_REF] Abdel-Shafy | Treatment of Leather Industrial Wastewater via Combined Advanced Oxidation and Membrane Filtration[END_REF], the liming/unhairing operation produces highly polluted effluents (high levels of COD, BOD, TDS and sulfides) that should be treated separately before its discharge. They were able to develop a wastewater treatment based on the Fenton technique for the oxidation of organic matter in water employing H 2 O 2 and Fe 2+ (Fenton reagent). This method was followed by membrane filtration and showed to be effective for the non-biodegradable portions of wastewater.

The final effluent could meet the permissible limits of the unrestricted water reuse according to the Egyptian Environmental Association Affair (EEAA). El-Khateeb et al. [START_REF] El-Khateeb | Environmental Impact Elimination of Chrome Tanning Effluent Using Electrocoagulation Process Assisted by Chemical Oxidation[END_REF] employed the electrocoagulation process assisted by chemical oxidation to treat chrome tanning effluents. The electrocoagulation treatment consists in forming an agglomeration (that can be subsequently separated by filtration) of the suspended matter by means of an electrical current. H 2 O 2 was used to support the oxidation process. This methodology permitted a 99.99% of chromium removal and 99.97% of sulfides removal. In further studies, El-Khateeb et al. [START_REF] El-Khateeb | Combining Chemical Coagulation Process and Innovative Aerobic Reactor for the Treatment of De-Hairing Wastewater[END_REF] combined chemical coagulation process (using FeCl 3 and lime) and an aerobic reactor to treat effluents generated from beam house operations. The developed system efficiently reduced the COD, BOD and sulfides levels: 96.1%, 85.2% and 99.3% removal, respectively. The final treated effluent could be used for irrigation of trees or reused in the tanning industry.

Another option to minimize the environmental problems caused by chrome tanning is the extraction of chromium from leather scrap. Ferreira et al [START_REF] Ferreira | Finished Leather Waste Chromium Acid Extraction and Anaerobic Biodegradation of the Products[END_REF]. developed a method for the acidic extraction of chromium, using concentrated H 2 SO 4 , recovering 30-60% of chromium and rendering the residue suitable for anaerobic biodegradation.

Similarly, Popiolsky and her coworkers [START_REF] Popiolski | Ultrasound-Assisted Extraction of Cr from Residual Tannery Leather: Feasibility of Ethylenediaminetetraacetic Acid as the Extraction Solution[END_REF] achieved 98% of chromium removal from residual leather tannery via an EDTA ultrasound-assisted extraction. Additionally, they proposed EDTA's recovery by adding H 2 SO 4 to precipitate EDTA-H 4 .

On the other hand, alternatives to conventional chrome tanning have also been explored. Some of these alternatives include: high exhaustion chrome tanning using exhaust aids or high exhaust tanning salts [START_REF] Sundar | Cleaner Chrome Tanning -Emerging Options[END_REF][START_REF] Zhang | An Integrated Technology to Minimize the Pollution of Chromium in Wet-End Process of Leather Manufacture[END_REF], ultrasound assisted chrome tanning which improves chromium requirement and reduces shrinkage [START_REF] Mengistie | Ultrasound Assisted Chrome Tanning: Towards a Clean Leather Production Technology[END_REF],

microwave/ultrasound assisted chrome tanning which additionally decreases tanning time [START_REF] Zhang | A Rapid and Cleaner Chrome Tanning Technology Based on Ultrasound and Microwave[END_REF], salt-free pickling chrome tanning which diminishes the discharge of COD and Total Dissolved Solids (TDS) [START_REF] Jia | A Salt-Free Pickling Chrome Tanning Technology: Pretreatment with the Collective Polyoxyethylene Diepoxy Ether and Urotropine[END_REF], and enzyme assisted vegetable tanning which enhances the diffusion of vegetable tannins into the leather fibers [START_REF] Kanth | Cleaner Tanning Practices for Tannery Pollution Abatement: Role of Enzymes in Eco-Friendly Vegetable Tanning[END_REF].

Still, chrome tanning remains the preferred alternative due to the versatility of the resulting leather. Ultrasound assisted [START_REF] Mengistie | Ultrasound Assisted Chrome Tanning: Towards a Clean Leather Production Technology[END_REF] and salt-free pickling chrome tanning [START_REF] Jia | A Salt-Free Pickling Chrome Tanning Technology: Pretreatment with the Collective Polyoxyethylene Diepoxy Ether and Urotropine[END_REF] seem to be promising alternatives as they maintain the good quality of the leather.

On the other hand, high exhaustion and microwave/ultrasound assisted chrome tanning resulted in some properties that were inferior than the conventional technique, such as thermal stability and non-uniform distribution of chromium [START_REF] Sundar | Cleaner Chrome Tanning -Emerging Options[END_REF][START_REF] Zhang | A Rapid and Cleaner Chrome Tanning Technology Based on Ultrasound and Microwave[END_REF].

Eco-friendly chrome-free tanning is another alternative to conventional chrome tanning. This approach has been attempted with different tanning agents such as titanium salts [START_REF] Zuriaga-Agustí | Pollution Reduction in an Eco-Friendly Chrome-Free Tanning and Evaluation of the Biodegradation by Composting of the Tanned Leather Wastes[END_REF], natural and unnatural amino acids [START_REF] Krishnamoorthy | Green Chemistry Approaches to Leather Tanning Process for Making Chrome-Free Leather by Unnatural Amino Acids[END_REF][START_REF] Krishnamoorthy | Greener Approach to Leather Tanning Process: D-Lysine Aldehyde as Novel Tanning Agent for Chrome-Free Tanning[END_REF][START_REF] Wu | An Eco-Friendly Tanning Process to Wet-White Leather Based on Amino Acids[END_REF], and an aluminum sulfate -composite material [START_REF] Gao | An Eco-Friendly Approach for Leather Manufacture Based on P(POSS-MAA)-Aluminum Tanning Agent Combination Tannage[END_REF]. Leathers obtained by way of these tanning agents have shown good qualities, comparable or superior to the properties of chrome-tanned leather. The exploitation of these promising approaches could help reduce the environmental impacts of tanning industry.

Regarding chromium determination, there is no current standard describing a technique to extract and quantify Cr(III) and Cr(VI) simultaneously in leather products coming from chrome tanning. Hence, it is of particular importance to develop a protocol that takes into account the possible interconversion processes, eventually through the use of SIDMS. The production of CRMs for speciation of Cr in leather articles would also represent a huge step on the mitigation of these obstacles.

Conclusion

In the leather manufacturing process, chrome tanning is the most common tanning system due to the quality of the final products. However, the environmental impacts of this process have encouraged researchers to find cleaner alternatives to conventional chrome tanning. Even if only trivalent chromium salts are used during chrome tanning, hexavalent chromium can still be found in leather, which represents a risk to human health. Additionally, natural ageing of the leather could also favor its formation. The European regulation restricts Cr(VI) content to 3 mg/kg in leather articles coming into contact with the skin. As the oxidation state of chromium is determined by redox potential and pH, the current international standard for hexavalent chromium determination in leather goods should consider any possible interconversion between Cr(III) and Cr(VI) during the extraction and analysis procedure. The introduction of isotope dilution techniques and the production of CRMs could help mitigate this struggle. The review of the analytical methods for chromium extraction and quantification showed that most of the studies have been performed in foodstuffs and environmental samples, which is comprehensible. However, more attention should be given to leather articles in order to develop more reliable analytical protocols that monitor possible interconversions and reveal the real health risk consumers are exposed to. Further regulations must also contemplate the evolution of hexavalent chromium over time.

8 (

 8 28.0 1.5-3.0 1.5-3.8 23.0-38.0 0.8-1All values are expressed in kg/ton of hide processed). * BOD: Biochemical Oxygen Demand. **COD: Chemical Oxygen Demand

Figure 2 .

 2 Figure 2. Leather manufacturing process.

Figure 4 .

 4 Figure 4. Molecular structure of Acid Orange 7 (Orange II).

  penetration and levelness of coloring, high fixation, etc.[START_REF] Covington | Tanning Chemistry: The Science of Leather[END_REF] An example of a premetallized dye (Acid Blue 158) is shown in Figure5. Evidently, the use of these type of colorants contributes to the total content of chromium, which could pose a problem in case of an unintentional oxidation of Cr(III) into Cr(VI).

Figure 5 .

 5 Figure 5. Molecular structure of a premetallized dye: Acid Blue 158.

Figure 6 .

 6 Figure 6. Eh-pH diagram of chromium. Reproduced of [40] with permission from

Figure 7 .

 7 Figure 7. Complexation of Cr(VI) with DPC.

Figure 8 .

 8 Figure 8. Complexation of Cr(III) with PDCA.

  Coupled Plasma -Mass Spectrometry (CE-ICP-MS), achieving the separation of Cr(III), Cr(VI) and Chromium Picolinate (CrPic) in nutritional food samples. This was accomplished by chelating Cr(III) with DCTA, allowing the separation of the single-charged Cr-DCTA -complex from the double-charged molecule CrO 4 2-(Cr(VI)), and CrPic (no charge).

Hernandez and her team[ 58 ]

 58 analyzed Cr(VI) in dairy (milk, cheese, yoghurt) and cereal products by High-Performance Ionic Chromatography -ICP-MS (HPIC-ICP-MS) with an ion exchange Dionex IonPac CS5A column and a CG5A guard column, attaining the optimization of a sensitive and specific on-line method where no pre-concentration was required. In a further study, they determined Cr(III) in dairy products by in-situ complexation of EDTA and the determination of the Cr(III)-EDTA -complex by High-Performance Liquid Chromatography -ICP-MS (HPLC-ICP-MS) (using the same columns as in the previous study)[START_REF] Hernandez | Development and Application of a Method for Cr(III) Determination in Dairy Products by HPLC-ICP-MS[END_REF]. Vacchina et al.[START_REF] Vacchina | Cr(VI) Speciation in Foods by HPLC-ICP-MS: Investigation of Cr(VI)/Food Interactions by Size Exclusion and Cr(VI) Determination and Stability by Ion-Exchange on-Line Separations[END_REF] also worked with HPLC-ICP-MS (ion exchange Dionex IonPac CS5A column) to accomplish hexavalent chromium determination in dairy products, flour, chocolate, vegetables, fruits, meat, fish, eggs, and beverages. To study the interactions between Cr(VI) and the matrices, they employed size-exclusion HPLC-ICP-MS (gel filtration Superdex-200 HR 10/30 column). However, no Cr(VI) was found in any of the samples, attributing these results to the instability of Cr(VI) in food matrices.

Figure 9 .

 9 Figure 9. Distribution of Cr species in water. Reproduced of [82] with permission from Elsevier.

Table 2 .

 2 Full names and abbreviations.

	IUPAC	International Union of Pure and Applied Chemistry
	KED	Kinetic Energy Discrimination
	LC-ICP-MS	Liquid Chromatography -Inductively Coupled Plasma -Mass Spectrometry
	LOD	Limit of Detection
	LOQ	Limit of Quantification
	MDL	Method Detection Limit
	PDCA	2,6-pyridinedicarboxylic acid
	PQL	Practical Quantification Limit
	SEC	Size-Exclusion Chromatography
	SIDMS	Speciated Isotope Dilution Mass Spectrometry
	SM&T	Standard, Measurement and Testing Programme
	SPE	Solid Phase Extraction
	TBAH	Tetrabutylammonium Hydroxide
	TDS	Total Dissolved Solids
	Tris	Tris(hydroxymethyl)aminomethane
	UV-Vis	UV-Visible Spectroscopy

  The European Union, after consulting the Scientific Committee on Toxicity, Ecotoxicity and the Environment (CSTEE) concerning the carcinogenicity of some azo dyes, provided a list of 22 prohibited aromatic amines. Azo dyes which may release

	these toxic aromatic amines in detectable concentrations, above 30 ppm, should not be
	used in textile and leather articles coming into contact with the human skin. This list
	includes the following aromatic amines: biphenyl-4-ylamine; benzidine; 4-chloro-o-
	toluidine; 2-naphthylamine; o-aminoazotoluene; 5-nitro-o-toluidine; 4-chloroaniline; 4-
	methoxy-m-phenylenediamine; 4,4'-methylenedianiline; 3,3'-dichlorobenzidine; 3,3'-
	dimethoxybenzidine; 3,3'-dimethylbenzidine; 4,4'-methylenedi-o-toluidine; 6-methoxy-
	m-toluidine; 4,4'-methylene-bis-(2-chloroaniline); 4,4'-oxydianiline; 4,4'-thiodianiline;
	o-toluidine; 4-methyl-m-phenylenediamine; 2,4,5-trimethylaniline; o-anisidine; 4-
	aminoazobenzene [19].
	Besides azo dyes, premetallized dyes are another class of colorants that are
	widely used in the leather industry. Its concept relies on avoiding the two-step
	mordanting-dyeing process. Consequently, premetallized dyes are beforehand prepared
	by complexing a metal atom, usually chromium, with the molecules of the dye. These
	premetallized dyes exhibit interesting properties such as: increased fastness, good

Table 3 .

 3 Wavelengths of maximum absorbance of different colorants used in leather industry. Modified from[START_REF] Fontaine | Étude de l'évolution de la teneur en chrome hexavalent dans le cuir, influence des conditions fabrication et importance des facteurs environnementaux[END_REF].

	Description	Acid Red 97	Acid Red 131	Brown HH 469
	CAS Number 10169-02-5	70210-37-6	94933-05-8
		530 nm	550 nm	540 nm
	Structure

Evolution of chromium content during leather products' life-cycle

	As we mentioned before, chrome-tanned leather can represent a risk to human health
	due to the unintentional oxidation of Cr(III) into Cr(VI).
	In 2014, Hedberg et al[20] characterized four differently tanned leather samples
	and exposed them to artificial sweat and phosphate buffer for chromium extraction,
	observing that co-released leather-specific species can reduce released hexavalent
	chromium. Therefore, no Cr(VI) was found in artificial sweat or phosphate buffer.
	However, leather samples with no reduction capacity and fewer aromatic groups,
	identified by Attenuated Total Reflectance -Fourier Transform Infrared Spectroscopy
	(ATR-FTIR), revealed Cr(VI) at the surface.
	Some years later, in 2018, the International Organization for Standardization
	(ISO) published a thermal pre-ageing procedure that permits to predict the evolution of
	hexavalent chromium content in tanned leather. The ISO 10195 standard requires the
	sample to be heated in an oven for a specific period of time (60º and relative humidity
	<20%, or 80º and relative humidity <10%, 24 hours). Nevertheless, this method does
	not simulate real conditions and only provides a tendency for the oxidation of Cr(III)

Table 6 .

 6 Dominant Cr species in different aqueous conditions.

	Cr oxidation state	Aqueous conditions	Dominant aqueous Cr species
		pH < 3.5	Cr +
		pH > 3.5	CrOH 2+
		Increased pH	Cr(OH) 2	+
	Cr(III) Cr(VI)	Increased pH Increased pH Higher Cr concentrations Higher Cr concentrations pH < 6.5 & conc. < 30 mM pH < 6.5 & conc. > 30 mM	o (Aq) -Cr(OH) 4 Cr(OH) 3 4+ Cr 2 (OH) 2 5+ Cr 3 (OH) 4 -HCrO 4 Cr 2 O 7 2-
		pH > 6.5	CrO 4	2-

Costa et al.

[START_REF] Costa | Chromium Speciation in Leather Samples: An Experiment Using Digital Images, Mobile Phones and Environmental Concepts[END_REF]

tried an alkaline extraction of Cr(VI) in leather using a NaOH 0.1M solution. A different approach was studied by Dias da Silva et al

[START_REF] Dias Da Silva | Evaluation of the Chromium Bioavailability in Tanned Leather Shavings Using the SM&T Sequential Extractions Scheme[END_REF]

on which tanned leather shavings were pre-treated using the SM&T (Standard, Measurement and Testing Programme) sequential extractions scheme. Considering Cr concentrations, the extracted fractions can be ordered as follows: oxidizable > exchangeable > reducible > residual. The oxidizable fraction was tested to determine the concentration of Cr(VI).

  Bothcomplexes were analyzed using an ion exchange Dionex IonPac CS5A column and a CG5A guard column. The detection was set at 335 nm from 0.0 to 4.7 min for Cr(III), and at 530 nm from 4.7 to 8.0 min for Cr(VI). Tiwari et al.[START_REF] Tiwari | Cloud Point Extraction and Diffuse Reflectance-Fourier Transform Infrared Spectroscopic Determination of Chromium(VI): A Probe to Adulteration in Food Stuffs[END_REF] detected the same reddish-violet complex at 540 nm by Diffuse Reflectance -Fourier Transform Infrared Spectroscopy (DRS-FTIR) with a LOD of 1.22 µg/mL for Cr(VI) in turmeric samples.

	15 mg/kg
	after changing to a more suitable extraction solution to avoid Cr(III) oxidation into
	Cr(VI) (0.2 g of leather and 100 mL of 0.1M phosphate solution at pH 4.4, and 5%
	NaCl). For this study, they used an anion exchange Dionex IonPac AG7 guard column.
	To eliminate any interference of colorants, they employed SPE C18 cartridges that were
	activated before injection.
	For drinking water, groundwater, and industrial wastewater effluents, Ion
	Chromatography (anion exchange Dionex IonPac AS7 column and NG1 guard column
	capable of removing strongly absorbing organics) with UV-Vis detection after post-
	column reaction with DPC has been reported in the SW-846 EPA 7199 method with a
	method detection limit (MDL) of 0.3-0.4 µg/L [72] and by the Environmental
	Monitoring Systems Laboratory (EMSL) obtaining a MDL of 1 µg/L [73]. For soil and

UV-Visible determination through Cr(VI) and DPC complexation has been extensively executed by several authors. Rezić and Zeiner

[START_REF] Rezić | Determination of Extractable Chromium from Leather[END_REF]

analyzed leather products from different chromium tanning processes and obtained a limit of detection (LOD) of 15 µg/L for Cr(VI). They also quantified total Cr after oxidation of Cr(III). Costa et al.

[START_REF] Costa | Chromium Speciation in Leather Samples: An Experiment Using Digital Images, Mobile Phones and Environmental Concepts[END_REF]

evaluated leather samples using this complexation together with a colorimetric analysis of digital images by means of a mobile phone and PhotoMetrix App, achieving a LOD of 0.6 mg/kg. Dias da Silva et al.

[START_REF] Dias Da Silva | Evaluation of the Chromium Bioavailability in Tanned Leather Shavings Using the SM&T Sequential Extractions Scheme[END_REF]

determined extractable hexavalent chromium in leather samples, in the oxidizable fraction of the SM&T extractions scheme, by UV-Visible detection through the DPC reaction.

wastewater samples, Phesatcha et al.

[START_REF] Phesatcha | Application Update 165: Separation of Chromium (III) and Chromium (VI) by Ion Chromatography[END_REF]

isolated Cr(III) by derivatization with PDCA on a pre-column and complexed Cr(VI) by post-column reaction with DPC.

  Li et al.[57] attained a good recovery of Cr(VI) in traditional Chinese medicines by HPLC-ICP-MS using a weak anion exchange Agilent Bio WAX column and NH 4 NO 3 as mobile phase. Evaluating

	iron supplements, Araujo-Barbosa et al.[78] reported a fast chromatographic run (5
	minutes) using 1.0mM tetrabutylammonium hydroxide (TBAH), 0.7mM EDTA and 5%
	methanol at pH 7.2. as mobile phase and a Phenomenex Kinetex reversed phase C18
	column, allowing the separation of Cr(III) and Cr(VI).
	Speciation in drinking water has been addressed by Ion Chromatography -ICP-
	MS (IC-ICP-MS) with minimal sample preparation using nitric acid as mobile phase
	and an anion exchange Dionex IonPac AG7 guard column for separation of both

species, attaining low LOD (ng/g, pg/L)

[START_REF] Kutscher | Speciation Analysis of Cr (III) and Cr (VI) in Drinking Waters Using Anion Exchange Chromatography Coupled to the Thermo Scientific ICAP Q ICP-MS[END_REF][START_REF] Schmelzel | Determination of Chromium Species Using Ion Chromatography Coupled to Inductively Coupled Plasma Mass Spectrometry[END_REF]

. Similar conditions were used by Kutscher et al.

[START_REF] Kutscher | The Migration of Elements from Toys and Speciation of Chromium (VI) in Toy Material Using a Low Cost IC-ICP-MS Solution[END_REF]

for Cr(VI) quantification in toys and toy material with the aim of studying the migration of elements in these samples. IC-ICP-MS with an anion exchange Dionex IonPac AS7 column and an AG7 guard column was also employed by Petrucci and Senofonte

[START_REF] Petrucci | Determination of Cr(VI) in Cosmetic Products Using Ion Chromatography with Dynamic Reaction Cell-Inductively Coupled Plasma-Mass Spectrometry (DRC-ICP-MS)[END_REF]

, where they found that one of the 4 cosmetic products evaluated contained more than 5 mg/L of Cr(VI). Chen at al.

[START_REF] Chen | Speciation of Chromium in Waste Water Using Ion Chromatography Inductively Coupled Plasma Mass Spectrometry[END_REF]

investigated wastewater by IC-ICP-MS using 20mM NH 4 NO 3 and 10mM NH 4 H 2 PO 4 at pH 7.0 as mobile phase, an anion exchange Agilent G3154A/101 column, and a G3154A/102 guard column. Subsequently, they evaluated the stability of Cr(III) complexes with three aminopolycarboxylic acids (EDTA, DTPA and PDCA) by Electrospray Ionization -Mass Spectrometry (ESI-MS). They attained the separation of Cr(VI) and Cr(III) as

  Cl 16 O 1 H + , 35 Cl 17 O + and 40 Ar 12 C + ; and for 53 Cr + the interferences could be 37 Cl 16 O + , 40 Ar 13 C + and 40 Ar 12 C 1 H + . These interferences can be reduced by

	To improve the accuracy of chromium speciation, several authors have implemented
	isotope dilution techniques. The two different approaches of isotope dilution are Isotope
	Dilution Mass Spectrometry (IDMS) and Speciated Isotope Dilution Mass Spectrometry
	(SIDMS). While IDMS serves for total element determination through the addition of a
	known quantity of an enriched isotope, SIDMS is capable of tracing species
	interconversions by labelling each species with enriched isotopes.
	The SW-846 EPA 6800 method describes a procedure for the implementation of
	IDMS and SIDMS, for the speciation of different elements, among which chromium is
	using collision/reaction cell and kinetic energy discrimination (KED) with He as
	collision gas[58, 83].
	Isotopic dilution techniques

part of the list, in a variety of samples such as waters, solids, blood, foods, extracts or digests

[START_REF]Test Method 6800: Elemental and Molecular Speciated Isotope Dilution Mass Spectrometry[END_REF]

. Mädler et al. implemented SIDMS by adapting the EPA 6800 method for trace (MDL of 0.007 µg/L)

[START_REF] Mädler | Trace-Level Analysis of Hexavalent Chromium in Lake Sediment Samples Using Ion Chromatography Tandem Mass Spectrometry[END_REF]

and ultra-trace level analysis (MDL of 2 ng/L)

[START_REF] Mädler | Ultra-Trace Level Speciated Isotope Dilution Measurement of Cr(VI) Using Ion Chromatography Tandem Mass Spectrometry in Environmental Waters[END_REF]

of Cr(VI) in environmental water and sediment samples by Ion Chromatography Tandem Mass Spectrometry (IC-MS/MS) through an anion exchange Dionex IonPac AS20

Table 4 .

 4 Pre-treatment and determination methods for chromium speciation in different matrices.

	Matrix	Sample pre-treatment	Analytical technique	Analyte	Analytical Conditions	Analytical Figures of Merit	Important Remarks	Reference
						IDL: 0.10 ng/mL for Cr(VI),
	Nutritional food samples	Extraction with 25% methanol solution under ultrasonic agitation.	CE-ICP-MS	Cr(III), Cr(VI), CrPic	CE voltage: 13 kV. 50 cm x 75 µm i.d. x 375 µm o.d. fused silica capillary	0.18 ng/mL for Cr(III), 0.20 ng/mL for CrPic. MDL: 1.0 ng/g for Cr(VI), 1.8 ng/g for
						Cr(III), 2.0 ng/g for CrPic.

Table 5 .

 5 Description of the calculation of LOD, LOQ, IDL, MDL, and/or PQL according to the different authors cited in Table4.

	Reference	LOD	LOQ	IDL	MDL	PQL
	Chen et al.[36]			Concentration necessary to yield a net signal equal to 3 times the standard	Calculated from IDL and mass ratio of final solution used for CE-ICP-MS

Acknowledgments

The authors gratefully acknowledge the support of CONACYT's grant (CVU 707668).

This work was also supported by the European Regional Development Fund (ERDF) N° HN0001343, the Région Normandie, and the Laboratoire d'Excellence (LabEx) SynOrg (ANR-11-LABX-0029).

Abbreviations

Conflict of interest

The authors declare no conflict of interest.