Systematic life cycle assessment of cobalt-based blue ceramic pigments: evaluating CoAl₂O₄ and lower-cobalt alternatives

Abstract

To enable a sustainable transition of the ceramic industry towards digital decoration via inkjet printing, it is imperative to promote the production of more environmentally friendly pigments. Despite its widespread use as a blue pigment, the environmental profile of cobalt–aluminium spinel (CoAl₂O₄) has never been assessed by Life Cycle Assessment (LCA), therefore potentially overlooking opportunities for less impactful alternatives. This work presents the first comparative environmental LCA of four distinct synthetic strategies for producing CoAl₂O₄ and several lower-cobalt alternatives. The study also accounts for two hybrid pigments, three M²⁺-doped spinels (M_1−xCo_xAl₂O₄, M = Zn²⁺, Mg²⁺), cobalt olivine (Co₂SiO₄) and its lower-cobalt variants, including Co²⁺-doped willemite (Zn₂SiO₄). The results throw light on the main substances and processes contributing to the environmental burdens of cobalt-based blue ceramic pigments and identify strategies to reduce their potential impacts. The study revealed that hybrid pigments are promising candidates with intense blue hues and lower environmental impacts than the traditional CoAl₂O₄, with Co_0.05Zn_1.95SiO₄ delivering the lowest environmental impacts among all the analysed pigments. The reliability of these findings was validated through Monte Carlo (MC) simulations. Potential environmental impacts were plotted against CIELAB colour parameters to identify pigments that exhibit desirable blue hues while being environmentally sustainable, thereby supporting their practical application.

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Green foundation

1. This study applies Life Cycle Assessment (LCA) to evaluate the environmental impacts of various synthetic strategies for cobalt–aluminium spinel (CoAl₂O₄) and a range of lower-cobalt containing blue ceramic pigments. The analysis includes four synthesis routes for CoAl₂O₄, two hybrid pigments, several M²⁺-doped spinels, cobalt olivine, and its low-cobalt variants. Environmental performance was assessed alongside chromatic quality using CIELAB parameters.

2. Among the compounds studied, hybrid pigments (CoAl₂O₄/kaolin and CoAl₂O₄/Al₂O₃) and Co²⁺-doped willemite (Co_0.05Zn_1.95SiO₄) emerged as the most sustainable alternatives to the traditional CoAl₂O₄. These materials demonstrated significantly reduced environmental impacts while maintaining strong blue hues.

3. By integrating environmental and chromatic performance, this work provides a framework for selecting ceramic pigments that meet both technical and sustainability criteria grounded in green chemistry. The approach supports the development of high-quality, low-impact materials and highlights LCA as a strategic tool for advancing greener innovation in pigment design and industrial ceramic applications.

1 Introduction

The introduction of large-scale inkjet printing for ceramic tiles has revolutionized the production of ceramic pigments, introducing new challenges for both the chemical and ceramic industries. While preserving their colouring ability, the technological properties of pigments must be compatible with specific types of inkjet systems, necessitating suitable additives and solvents.¹

Ceramic pigments are inorganic materials that retain their colouring properties at high temperatures. Thus, the colouring powders must be chemically and thermally stable at the firing temperatures of the ceramic matrices in which they are embedded, insoluble in water—as they are applied in suspension form—and inert when mixed with frits, opacifiers, fluxes and additives. While technological properties such as oil absorption or hiding power are relevant for pigments used in paints or coatings,² ceramic pigments are primarily evaluated based on their thermal and chemical stability, low solubility in water, and visual appearance after firing.

Most commercial blue ceramic pigments use cobalt, whose intense coloration is attributed to the d–d electronic transitions of the Co²⁺ ion in tetrahedral coordination, particularly within oxides and silicates.³ The chromatic performance of cobalt-containing compounds in ceramic applications is closely tied to their structural stability under glaze firing conditions. For instance, cobalt olivine (Co₂SiO₄) undergoes a change in Co²⁺ coordination from tetrahedral to octahedral, resulting in a diminished colour intensity, compared to CoAl₂O₄ spinel.⁴ Although cobalt phosphate (Co₃(PO₄)₂) is very effective in ink formulations, it decomposes in glazes at firing temperatures. Similarly, when heated above 800 °C, cobalt oxide, Co₃O₄, decomposes to CoO,⁵ making it ineffective in porcelain stoneware glazing.

Known as Thenard's blue, cobalt(II) aluminate (CoAl₂O₄) is the most widely used blue ceramic pigment, characterized by a normal spinel structure featuring remarkable stability at temperatures exceeding 1380 °C.⁶ This pigment is highly versatile, capable of producing a range of colours from pale pink violet to deep blue, depending on the preparation method and doping with other metals.^7,8 Despite its widespread use, the production and use of CoAl₂O₄ raise significant concerns, primarily due to the environmental impacts associated with cobalt ore extraction.⁹ Furthermore, cobalt is classified as a toxic and hazardous element,⁴ particularly in the form of airborne dust generated during glaze preparation, which can pose risks to human health.¹⁰ Additionally, cobalt's scarcity has led to its rapidly escalating prices.¹¹

Traditionally, Co₂AlO₄ is prepared via solid-state reaction (SSR) which involves calcining the pre-ball milled metal oxides or carbonates at elevated temperatures (900–1600 °C) in a kiln for 3–12 h. However, the high temperatures and prolonged reaction times induce lattice defects, potentially reducing the pigment's chromatic properties.¹² In response to the limitations of the solid-state route, several alternative synthetic strategies—described hereafter—have been developed with the aim of reducing environmental burdens and enhancing the chromatic performance of the resulting pigments.

Recent reviews on ceramic pigments’ production¹³ have highlighted the effect of particle size in enhancing their optical efficiency for digital decoration. Consequently, research has focused on developing novel preparation methods, as alternatives to the solid-state approach, to better control the particle size and thus, enhance the colouring performance of the pigments.

In addition to SSR, Co₂AlO₄ can be prepared via coprecipitation (CPT), hydrothermal (HT), polyol, sol–gel (SG) and sol–gel auto-combustion (SGA) processes. These methods typically rely on complex chemical syntheses wherein the starting materials (besides cobalt precursors) may involve strong inorganic acids (i.e., nitric, sulfuric and hydrofluoric acids), alkalis and, in some cases, organic reagents and solvents. The polyol method consists in heating a solution of metal salts in glycol (e.g., diethylene glycol) for 10–12 h at 180–200 °C, followed by centrifugation and drying.¹⁴ The sol–gel technique begins with the preparation of a homogeneous solution comprising salts and alkoxides dissolved in organic solvents such as ethanol or butanol.¹⁵ To adjust the pH, specific reagents such as ammonia and alkali bases may be introduced in small amounts. Evaporation of this solution results in gel formation, which upon calcination, produces the powder of the targeted pigment. The SGA method is conducted in the presence of an organic reagent, such as urea,¹⁶ glycine¹⁷ or citric acid,¹⁸ which simultaneously acts as complexing agent that stabilizes metal ions in solution, reducing agent that facilitates redox processes, and fuel that provides the combustion energy. The CPT method involves an aqueous solution of two or more soluble salts that precipitate in presence of a base such as ammonia or alkalis. The resulting precipitate is centrifuged, washed and calcined for 1–2 h to form the desired ceramic pigment. The advantage of this method is that it is relatively short and offers a better particle size control over other common strategies.¹³

Efforts to decrease the cobalt content in pigments have been pursued to mitigate costs and environmental risks. This has been achieved by doping the pigments with zinc and magnesium, namely, substituting Co²⁺ with Zn²⁺ and Mg²⁺, to obtain spinels with the formula Co_1−x−yZn_xMg_yAl₂O₄,¹⁹ where 0 ≤ x, y < 1. Alternatively, hybrid pigments with reduced cobalt content of cobalt such as Co₂AlO₄/halloysite,²⁰ Co₂AlO₄/kaolin²¹ and Co₂AlO₄/Al₂O₃^22,23 have been synthesized, demonstrating excellent colouring properties.

Despite the multiple attempts to provide greener synthetic methods for CoAl₂O₄ and some lower cobalt-containing alternatives, to date, the environmental impacts of these strategies have not yet been assessed systematically. To identify the most effective solutions towards improving the environmental performance of these products, the Life Cycle Assessment (LCA) methodology can be effectively applied. Currently, LCA is highly regarded for decision making and it is the most widely employed method to quantify potential environmental impacts, including – but not limited to – climate change caused by GHG emissions. It is defined as “the compilation and evaluation of the inputs, outputs, and potential environmental impacts of a product system throughout its life cycle”.^24,25 LCA is a growing quantitative and qualitative tool for green chemistry, enabling early-stage identification of environmental hotspots, preventing burden shifting, and guiding the design of inherently greener chemical products and processes with demonstrable life-cycle environmental and economic benefits.²⁶

Comparative LCA can be employed to evaluate different products or systems that fulfil the same functions, or alternative pathways and processes to produce the same product.^27–29 This approach enables the identification of environmentally preferable options. For instance, Feijoo et al.³⁰ compared magnetic nanoparticles with different surface coatings for enzyme immobilisation, while Franco et al.³¹ conducted a comparative LCA of lead zirconate titanate (PZT) ceramics and lead-free halide perovskite composites for piezoelectric applications. Dutta et al.³² performed a comparative LCA of ten metal–organic frameworks (MOFs), evaluating different synthetic methods based on scaled-up literature procedures. Despite differences in chemical composition, these materials were assessed based on equivalent functionality.

LCA studies on ceramic pigments are limited in the scientific literature, with a predominant focus on the production of titanium dioxide (TiO₂).³³ Middlemas et al.³⁴ compared the production of TiO₂via alkaline roasting of titania slags with sulfate and chloride methods, finding the former to be more favourable in terms of cumulative energy demand (CED) and CO₂ emissions. Grubb and Bakshi³⁵ conducted an LCA on TiO₂ produced from ilmenite, highlighting significant contributions from raw materials and fossil-source energy across multiple indicators. Dai et al.³⁶ examined the environmental impacts of TiO₂ production in China, comparing the chloride and sulfate routes, and concluded that the chloride route performed better overall. Pini et al.³⁷ assessed the production of a suspension of TiO₂ nanoparticles (NPs) via the hydrolysis of titanium isopropoxide and identified three main drivers of environmental impacts: electricity consumption, production of titanium isopropoxide, and consumption of thermal energy. Caramazana-González et al.³⁸ performed an LCA study on the production of TiO₂ NPs from five different precursors, through continuous-flow hydrothermal synthesis, highlighting the environmental benefits of using titanium oxysulphate as precursor. Wu et al.³⁹ compared the environmental performance of TiO₂ NPs synthesis via chemical, biological and physical routes–identifying the last one as the most energy-intensive and environmentally impactful among three strategies. Rosa et al.⁴⁰ conducted LCA studies demonstrating that solution combustion synthesis of TiO₂ NPs exhibits lower environmental impacts than more conventional methods, such as hydrolytic sol–gel synthesis and non-hydrolytic sol–gel synthesis.

Additionally, a simulation study on the production of Cr₂O₃ green ceramic pigment made it possible to estimate the heat demand and CO₂ emissions, which were used as input data in an LCA.⁴¹ The assessment focused solely on global warming potential (GWP) as an indicator for sustainability. The results showed that the production and transportation of the starting materials caused a major contribution to the CO₂ emissions, whereas the pigment production process (i.e., calcination) was responsible for only 1.3–3.5% of the total GWP.

The aim of this work is to systematically evaluate the potential environmental impacts of CoAl₂O₄ as blue ceramic pigment, synthesized through four different strategies, alongside assessing lower-cobalt alternatives such as hybrid pigments, Mg²⁺- and Zn²⁺-doped CoAl₂O₄ and Co²⁺-doped ZnAl₂O₄. Furthermore, this study extends the analysis to Co₂SiO₄ and some of its lower-cobalt variants (i.e., Co²⁺-doped Zn₂SiO₄) as blue pigments. The quantified environmental performance was also critically examined in relation to their functional properties, particularly their ability to produce desirable blue hues, as assessed through CIELAB⁴² colorimetric parameters.

This comprehensive evaluation seeks to rise a deeper understanding of the environmental burdens associated with cobalt-based blue ceramic pigments, thereby contributing to the development of more sustainable strategies for both the ceramic and chemical industries.

2 Materials and methods

2.1 Experimental section

The synthesis methods selected for this research are described in this section. Technical details are gathered in Table 1,^{19–22,46–51} which specifies the chromatic properties, particle size and cobalt content of each pigment. All syntheses were scaled to 1 kg, and the simulations to a larger scale⁴³ were conducted using similar equipment to minimize errors associated with varying equipment efficiencies. In the abbreviated nomenclature used below, “A” refers to CoAl₂O₄ pigments, “B” and “C” denote hybrid pigments based on CoAl₂O₄/kaolin and CoAl₂O₄/Al₂O₃, respectively; “D” corresponds to Mg²⁺- and Zn²⁺-co-doped CoAl₂O₄, (Co_0.67Mg_0.16Zn_0.16Al₂O₄), “E” to Mg²⁺-doped CoAl₂O₄ (Co_0.25Mg_0.75Al₂O₄), “F” to Co²⁺-doped ZnAl₂O₄ (Zn_0.9Co_0.1Al₂O₄), “G” to cobalt olivine (Co₂SiO₄), “H” and “I” to Co²⁺-doped willemite compositions Co_0.5Zn_1.5 SiO₄ and Co_0.05Zn_1.95SiO₄, respectively. The experimental procedures for each pigment are detailed in the SI, appendix A.

Table 1 Summary of the synthetic pathways, chromatic properties, cobalt content and particle sizes for the ceramic pigments assessed by LCA in this study

Pigment	Formula	Synthesis method	Calcination step	L*	a*	b*	C*	h*	% Co	Particle size	Ref.
Abbreviations: SSR – solid state reaction, CPT – coprecipitation, SGA – sol–gel autoignition, MW-HT – microwave-assisted hydrothermal, HPT – heterogeneous precipitation, Ref – references.a L* and a* values were not reported in the original source; values were estimated using image-based colour extraction tools.
CoAl 2 O 4 (A) synthetic pathways
A1	CoAl2O4	SSR	1500 °C, 3h	26.6	−5.3	−27.5	28.0	259.0	33.3	2 μm	46, 47
A2	CoAl2O4	CPT	1100 °C, 2h	37.4	−0.5	−41.1	41.1	269.3	33.3	10–20 nm	20
A3	CoAl2O4	SGA	750 °C, 4 h	34.8	14.7	−23.9	28.0	301.6	33.3	18.14 nm	48
A4	CoAl2O4	HT-MW	1200 °C, 2 h	31.8	−14.8	−29.1	32.6	243.0	33.3	47 nm	21
CoAl 2 O 4 -derived hybrid pigments
B	CoAl2O4/kaolin	HT-MW	900 °C, 2 h	34.4	16.7	−70.5	72.4	283.3	20.0	100–200 nm	21
C	CoAl2O4/Al2O3	HPT	1100 °C, 3 h	40.7	−7.5	−46.8	47.3	260.9	14.2	—	22
M ^2+ -doped CoAl 2 O 4 and Co ^2+ -doped ZnAl 2 O 4
D	Co0.67Mg0.16Zn0.16Al2O4	SGA	1000 °C, 1h	27.9^a	4.9^a	−29.7	—	—	23.0	26.9 nm	19
E	Co0.25Mg0.75Al2O4	SGA	900 °C, 6 h	62.6	−15.7	−27.8	31.9	240.6	9.8	<5 μm	49
F	Zn0.9Co0.1Al2O4	SGA	—	67.9	−3.7	−39.0	39.1	264.6	3.2	—	50
Cobalt olivine and Co ^2+ -doped willemite
G	Co2SiO4	SSR	1300 °C, 1 h	45.7	5.1	−3.2	6.0	327.9	56.1	—	51
H	Co0.5Zn1.5 SiO4	SSR	1300 °C, 1 h	48.5	7.2	−36.5	37.2	281.1	13.4	—	51
I	Co0.05Zn1.95SiO4	SSR	1300 °C, 1 h	54.1	0.1	−41.1	36.3	270.1	1.3	—	51

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The CIELAB colour space, defined by Commission internationale de l’éclairage (CIE) in 1976 ⁴² and later standardised (ISO/CIE 11664-4:2019⁴⁴), is extensively utilized in research and across various industries. CIELAB expresses colour using three values: L* for lightness, and a* and b* for the colour-opponent dimensions, which correspond to red-green and blue-yellow respectively. CIELAB is mainly employed for colour specification and colour difference evaluation. Its applications include quality control and colouring performance evaluation, using perceptual attributes like lightness, chroma, and hue. Additionally, the CIELAB framework is independent of the specific instrument used, allowing for valid cross-study comparisons of chromatic properties, as performed in this work. For each pigment and synthesis combination analysed in this study, the reported CIELAB parameters from the respective studies were used for comparison of colouring properties as detailed in section 4.

2.2 Life cycle assessment (LCA)

LCA was applied according to ISO 14040 and ISO 14044 guidelines.^24,45

2.2.1 Goal and scope definition. This study aims to objectively quantify and compare the potential environmental impacts of producing various cobalt-based blue ceramic pigments, as described in section 2.1, using the LCA methodology. Multiple preparation strategies for CoAl₂O₄ (A) are explored, including SSR, CPT, SGA and HT-MW methods. Additionally, the synthesis of two hybrid pigments, B and C, and three M²⁺-doped pigments, D, E and F, are included in this work. The study also examines cobalt olivine, Co₂SiO₄ (G), and two Co²⁺-doped Zn₂SiO₄ pigments, H and I.

The primary goal is to identify the processes and substances that significantly contribute to the environmental impacts of the pigments under study. By comparing their environmental footprints, the study aims to determine the most environmentally friendly synthetic methods and alternatives to CoAl₂O₄, which is traditionally prepared through SSR, thereby offering support to both pigment and ceramic industries for a sustainable transition in inkjet printing technologies.

2.2.2 System definition and declared unit. The system encompasses the synthesis of various cobalt-based blue ceramic pigments through distinct pathways. The system boundaries are delineated from cradle-to-gate, spanning from the extraction of raw materials to the production of the finished ceramic pigments. As the study does not include the use phase, a declared unit of 1 kg of blue pigment intended for ceramic applications is adopted. These pigments are characterized by their unique chromatic properties, which render them valuable for industrial applications, as detailed in Table 1. The scale-up from laboratory to 1 kg production scale was conducted following the methodological framework proposed by Piccinno et al.,⁴³ which supports inventory adaptation and energy scaling for chemical processes, and was here applied to ceramic pigment synthesis. Schematic flowcharts for the production of each pigment are depicted in Fig. S1–S10 of the SI.

2.2.3 Life cycle inventory (LCI) and life cycle impact assessment (LCIA). The data utilized for the Life Cycle Inventory (LCI) phase in the synthesis of ceramic pigments were secondary data, derived from previously published procedures as outlined in Table 1 and the Experimental section. Detailed inventories are provided in Tables S1–S13 of the SI. The inventories account for the contributions from all raw materials, energy inputs, equipment used, waste treatment throughout the synthesis, and atmospheric emissions. Fugitive emissions of liquid solvents were estimated at 2% or 1%, depending on their boiling point at atmospheric pressure within the temperature ranges of 20–60 °C and 60–120 °C, respectively.⁵²

The processes were modelled by employing datasets from the ecoinvent database (EID, version 3.10),^53,54 following an attributional approach (i.e., APOS system model).⁵⁵ The inventories were modelled in SimaPro 9.6.0.1 software.⁵⁶

Mass and energy balances were performed for each modelled process by quantifying all material inputs and outputs through stoichiometric and yield-based calculations, and by estimating energy requirements using thermodynamics and equipment power.

The Life Cycle Impact Assessment (LCIA) was performed by using the Environmental Footprint (EF3.1) method,⁵⁷ which represents the Product Environmental Footprint (PEF)⁵⁸-compliant and recommended methodological approach for quantifying environmental performance by the European Commission.

The geographic scope of the study is set in Europe; therefore, datasets for the electricity mix, heat supply, and background processes for chemicals were, where possible, selected from the Rest of Europe (RER) region in the ecoinvent database to ensure consistency with the European context and the application of the EF3.1 method.

3 Life cycle impact assessment results

The potential environmental impacts, calculated at midpoint-level, were assessed for all pigments and are presented as both absolute and relative values. Midpoint environmental single scores (in Pt) were also calculated by normalising and weighting the midpoint results using EF3.1 factors (July 2022).⁵⁷ For pigments B–I, absolute impacts are provided in the SI (Tables S14–S16). Relative impacts, expressed as percent contributions (%), are illustrated in Fig. S11–S22 and highlight the main burdens for each impact category. Detailed analyses of selected syntheses (A1, A4, B, C, F, I) are provided in appendix B of SI.

3.1 CoAl₂O₄ (A): comparison of different synthetic pathways

The potential environmental impacts associated with the production of 1 kg of CoAl₂O₄ (A) through four synthetic strategies (i.e., SSR, CPT, SGA and HT-MW) are detailed in Table 2.

Table 2 Potential environmental impacts, calculated at midpoint level (EF 3.1), for the production of 1 kg of CoAl₂O₄ spinel (A) via SSR (A1), CPT (A2), SGA (A3) and HT-MW (A4)

Impact category	Unit	A1	A2	A3	A4
Acidification	mol H^+ eq	3.15 × 10^−1	2.43 × 10^−1	2.09 × 10^−1	1.86 × 10^−1
Climate change	kg CO2 eq.	4.06 × 10^1	3.78 × 10^1	3.35 × 10^1	2.60 × 10^1
Ecotoxicity, freshwater	CTUe	3.58 × 10^2	3.07 × 10^2	2.56 × 10^2	2.43 × 10^2
Particulate matter	Disease inc.	2.40 × 10^−6	1.76 × 10^−6	1.66 × 10^−6	1.50 × 10^−6
Eutrophication, marine	kg N eq	3.82 × 10^−2	3.58 × 10^−2	2.82 × 10^−2	2.41 × 10^−2
Eutrophication, freshwater	kg P eq	2.39 × 10^−2	2.22 × 10^−2	1.44 × 10^−2	1.21 × 10^−2
Eutrophication, terrestrial	mol N eq	3.32 × 10^−1	3.74 × 10^−1	3.33 × 10^−1	2.80 × 10^−1
Human toxicity, cancer	CTUh	1.98 × 10^−7	1.87 × 10^−7	1.35 × 10^−7	1.13 × 10^−7
Human toxicity, non-cancer	CTUh	1.78 × 10^−6	9.89 × 10^−7	9.24 × 10^−7	8.82 × 10^−7
Ionising radiation	kBq U-235 eq.	1.90 × 10^1	1.44 × 10^1	9.13 × 10^0	7.64 × 10^0
Land use	Pt	1.86 × 10^2	1.61 × 10^2	1.15 × 10^2	9.75 × 10^1
Ozone depletion	kg CFC11 eq.	1.37 × 10^−6	8.54 × 10^−7	7.57 × 10^−7	5.65 × 10^−7
Photochemical ozone formation	kg NMVOC eq	1.49 × 10^−1	1.05 × 10^−1	9.17 × 10^−2	7.45 × 10^−2
Resource use, fossils	MJ	8.67 × 10^2	6.45 × 10^2	5.14 × 10^2	3.85 × 10^2
Resource use, minerals and metals	kg Sb eq.	3.52 × 10^−3	1.73 × 10^−3	1.71 × 10^−3	1.68 × 10^−3
Water use	m3 depriv.	1.59 × 10^2	7.81 × 10^1	8.23 × 10^1	7.32 × 10^1

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Results indicate that the traditional SSR pathway (i.e., A1) exhibits the highest environmental impacts across most categories, whereas the HT-MW (i.e., A4) pathway demonstrates the lowest impacts (see Fig. 1). For all four synthetic methods, the primary contributions to the environmental profile of cobalt–aluminium spinel arise from the lifecycle of the cobalt precursor used in the synthesis (i.e., CoO or Co(NO₃)₂·6H₂O), as depicted in Fig. 2. These upstream processes encompass various operations such as copper–cobalt ore mining, hydrometallurgical ore processing and chemical production. Additional impacts arise from the consumption of electricity and various chemical reagents such as aluminium nitrate and aluminium oxide as sources of aluminium, sodium hydroxide, ammonia, and urea as fuel for the SGA method.

Fig. 1 Relative environmental impacts, calculated at midpoint level (EF 3.1), for the production of 1 kg of CoAl₂O₄ spinel (A) via SSR (A1), CPT (A2), SGA (A3) and HT-MW (A4) paths.

Fig. 2 Single score results (EF 3.1), for the production of 1 kg of CoAl₂O₄ spinel (A) produced via SSR (A1), CPT (A2), SGA (A3) and MW-HT (A4) methods. The highlighted contributions include starting materials (i.e., cobalt oxide, cobalt nitrate, aluminium oxide, aluminium nitrate, ethanol, sodium hydroxide, urea) and energy used in the process. “Other contributions” refers to minor contributors to the environmental footprints of the analyzed pigments such as transports, equipment and waste treatment.

Synthetic pathways utilising cobalt nitrate as a precursor generally exhibit lower environmental impacts compared to the SSR route, which starts with CoO. This difference is attributed to the higher environmental impacts associated with Co₃O₄, which is the precursor for CoO, compared to the cobalt(II) hydroxide (Co(OH)₂), which is the precursor for cobalt nitrate. In the reference EID datasets,^59,60 cobalt metal, Co₃O₄, Co(OH)₂ and CoCO₃ are co-products of nickel and copper production. The chosen allocation method to partition the environmental impacts of the system among them was an economic allocation. Since cobalt hydroxide is an intermediate product, it has a lower allocation coefficient, resulting in lower attributed impacts.

3.2 CoAl₂O₄-derived hybrid pigments

Hybrid pigments of CoAl₂O₄ have garnered significant attention due to their reduced costs and enhanced chromatic properties. Bright blue TiO₂@CoAl₂O₄ pigments⁶¹ have been synthetized via encapsulation of TiO₂ into layered Co–Al hydroxides, with the blue component (b*) being intensified by increasing the content of cobalt. Additionally, various hybrid CoAl₂O₄-clay mineral pigments have been developed using kaolin,^21,62 halloysite²⁰ and sepiolite.⁶³ The colouring performance of these pigments depends on the type of clay and the synthesis method employed. Furthermore, CoAl₂O₄/Al₂O₃ hybrid pigments prepared by SSR or by HPT also can exhibit superior blue hues compared to CoAl₂O₄.^22,23 This study undertakes a comparative analysis of the environmental impacts of two hybrid pigments: B (CoAl₂O₄/kaolin) prepared by the HT-MW method and C (CoAl₂O₄/Al₂O₃) synthesised by heterogeneous precipitation. Both pigments showed excellent colouring properties, as reported in Table 1.

It is observed that, despite comparable electrical energy consumption, the hybrid pigment B results in higher environmental burdens than C for most impact categories (see Fig. 3). This difference is primarily attributed to the higher cobalt content, which is 20.0% for B and 14.2% for C. Additionally, the diminished potential environmental impacts of C are also associated with the use of Al₂O₃ rather than aluminium nitrate as the precursor for aluminium, as observed from the single score results (see Fig. 4).

Fig. 3 Relative environmental impacts, calculated at midpoint level (EF3.1) associated with the preparation of 1 kg of hybrid pigments B (CoAl₂O₄/kaolin) and C (CoAl₂O₄/Al₂O₃).

Fig. 4 Single score results (EF 3.1) for the production of 1 kg of hybrid pigments, B (CoAl₂O₄/kaolin) and C (CoAl₂O₄/Al₂O₃). The highlighted contributions include starting materials (i.e., cobalt nitrate, aluminium oxide, aluminium nitrate, sodium hydroxide, kaolin) and energy used in the process. “Other contributions” refer to minor contributors to the environmental footprints of the analyzed pigments such as transports, equipment and waste treatment.

The higher burdens for aluminium nitrate arise from the employment of nitric acid in the synthesis, as well as the electricity consumption for powering the equipment.

3.3 M²⁺-doped CoAl₂O₄ and Co²⁺-doped ZnAl₂O₄

The reduction of cobalt content in ceramic pigments has been successfully achieved by substituting Co²⁺ ions with Mg²⁺ and Zn²⁺, resulting in M²⁺-doped CoAl₂O₄ or Co²⁺-doped ZnAl₂O₄. The advantage of this method is the ability to easily control the stoichiometry, and to produce a wide range of pigments with the general formula M_1−xCo_xAl₂O₄, potentially having lower cost and enhanced colouring properties.

As described in section 2.1 (Table 1), this study evaluates and compares the environmental impacts of three doped pigments with different contents of cobalt, i.e., D (Co_0.67Mg_0.16Zn_0.16Al₂O₄), E (Co_0.25Mg_0.75Al₂O₄), and F (Zn_0.9Co_0.1Al₂O₄).

Despite having a lower content of cobalt (i.e., 9.76%), E exhibits higher environmental impacts across most categories compared to D, which contains 22.98% of cobalt (see Fig. 5). As observed from single score results (see Fig. 6), the primary environmental contributions for E stem from the electricity required for powering the reactor for 12 hours and from the use of citric acid. Conversely, D's major environmental impacts arise from cobalt nitrate, with minor contributions from glycine and from electricity for the equipment. For F, with a cobalt content of 3.23%, the significant environmental impacts are associated with the use of aluminium nitrate as an aluminium source and the mixture of glycine and urea as fuels.

Fig. 5 Relative environmental impacts, calculated at midpoint level (EF3.1) associated with the preparation of 1 kg of D (Co_0.67Mg_0.16Zn_0.16Al₂O₄), E (Co_0.25Mg_0.75Al₂O₄), and F (Zn_0.9Co_0.1Al₂O₄).

Fig. 6 Single score results (EF3.1) associated with the preparation of 1 kg of D (Co_0.67Mg_0.16Zn_0.16Al₂O₄), E (Co_0.25Mg_0.75Al₂O₄), and F (Zn_0.9Co_0.1Al₂O₄). The highlighted contributions include starting materials (i.e., cobalt nitrate, aluminium nitrate, magnesium nitrate, zinc nitrate, urea, citric acid, ethylene glycol, glycine) and energy used in the process. “Other contributions” refer to minor contributors to the environmental footprints of the analyzed pigments such as transports, equipment and waste treatment.

Notably, the environmental profile of D shows lower contributions from the corresponding fuel (glycine, 25.0% of the total single score) compared to E (citric acid + ethylene glycol, 40.0% of the total single score) and F (glycine + urea, 30.8% of the total single score). These findings demonstrate that reducing cobalt content alone is insufficient for achieving a more environmentally friendly synthesis of cobalt-containing pigments. For example, in the SGA method the type and quantity of fuels used as well as the electricity consumption are also critical factors.

3.4 Cobalt olivine and Co²⁺-doped willemite

CoAl₂O₄ is often preferred to Co₂SiO₄ because of its lower content of cobalt and its enhanced blue colour. However, a different approach to reducing the cobalt content in blue ceramic pigments involves the synthesis of Co²⁺–doped willemite solid solutions. In these materials, Co²⁺ ions substitute for Zn²⁺ in tetrahedral coordination sites within the orthosilicate framework, and this results in more intense blue hues.⁴ Specifically, the compounds H (Co_0.5Zn_1.5 SiO₄) and I (Co_0.05Zn_1.95SiO₄) in the present contribution (see Table 1 for detail) have shown good blue hues (b* = −36.3, −41.1, respectively) when compared to the original purple cobalt olivine G (b* = −3.2).⁵¹

Compared to cobalt olivine G, pigments H and I exhibit much lower impacts across all categories, primarily due to the reduction of cobalt content, and implicitly, cobalt oxide within the starting materials (see Fig. 7). Notably, pigment I (Co_0.05Zn_1.95SiO₄), which contains only 1.32% of cobalt (compared to 56.1% in Co₂SiO₄), shows a 95% reduction in the total single score, relative to G (see Fig. 8).

Fig. 7 Relative environmental impacts, calculated at midpoint level (EF3.1) associated with the preparation of 1 kg of G (Co₂SiO₄), H (Co_0.5Zn_1.5SiO₄) and respectively, I (Co_0.05Zn_1.95SiO₄).

Fig. 8 Midpoint single score results (EF3.1) associated with the preparation of 1 kg of G (Co₂SiO₄), H (Co_0.5Zn_1.5SiO₄) and I (Co_0.05Zn_1.95SiO₄). The highlighted contributions include starting materials (i.e., cobalt oxide, aluminium oxide, zinc oxide and silicone dioxide) and energy used in the process. “Other contributions” refer to minor contributors to the environmental footprints of the analyzed pigments such as transports, equipment and waste treatment.

3.5 Uncertainty analysis

A Monte Carlo (MC) simulation was conducted to assess the reliability and uncertainty of the results obtained, involving 1000 runs within a 95% significance threshold.

Uncertainties for background processes were sourced from the ecoinvent database, which applies the pedigree matrix approach to characterize data quality across multiple dimensions (e.g., reliability, completeness, temporal, geographical, and technological correlation).

This method typically assigns lognormal distributions to input data based on qualitative scores, following the framework described by Weidema et al. (JRC report, 2013).⁵³

The MC simulation was conducted using the implementation available in the LCA software, which propagates these uncertainties throughout the model. The simulation results include the mean, median, standard deviation (SD), and coefficient of variation (CV)—the latter representing the ratio of the standard deviation to the mean, providing insight into the relative magnitude of uncertainty. Additionally, the 2.5th and 97.5th percentiles were reported to define the 95% significance threshold, along with the standard error of the mean (SEM), which reflects the influence of the final iteration on the calculated mean.

The MC simulation results obtained for the synthetic pathways for CoAl₂O₄ spinel are illustrated in Fig. 9 and Tables S17–22 of the SI.

Fig. 9 Monte Carlo simulation, performed at midpoint level (EF.3.1 impact assessment method), with 1000 runs in a 95% significance threshold, for the comparative environmental impacts of (a) A1 vs. A2; (b) A1 vs. A3; (c) A1 vs. A4; (d) A2 vs. A3; (e) A2 vs. A4; (f) A3 vs. A4.

For instance, comparing A1 with A2 at the midpoint level, it can be stated that within the 95% significance threshold, the first one shows higher environmental impacts across 11 impact categories: “acidification”, “climate change”, “particulate matter”, “eutrophication, marine”, “eutrophication, freshwater”, “ionising radiation”, “land use”, “ozone depletion”, “photochemical ozone formation”, “resource use, fossils” and “resource use, minerals and metals” (see Fig. 9a). Conversely, A2 shows higher impacts than A1 for “eutrophication, terrestrial”. The impacts in human toxicity (cancer and non-cancer indicators), “water use” and “ecotoxicity, freshwater” were not significantly different. Within the same significance threshold, when comparing A3 with A4, the results were validated for 12 impact categories (see Fig. 9f).

Similarly, the uncertainty results confirm that for most impact categories, the impacts increase in the order A4 < A3 < A2 < A1. However, in most cases, for ecotoxicity (freshwater), human toxicity (cancer and non-cancer) and water use, the differences were not statistically different.

For hybrid pigments, MC analysis confirms that pigment B exhibits higher impacts than pigment C over 11 impact categories, as depicted in Fig. S23 and Table S23 of the SI. This result aligns with the general observation that a reduced cobalt content usually correlates with lower environmental impacts.

For the comparison of M²⁺-doped CoAl₂O₄ and Co²⁺-doped Zn₂AlO₄, similar results were obtained upon MC simulation, as shown in Fig. S24 and Tables S24–S26 of the SI. For most impact categories, the environmental impacts follow the order F < D < E, highlighting the significant influence of fuel on the environmental profile of the pigments when the SGA method is employed.

For the comparison between cobalt olivine (G) and Co-doped willemite (H and I), the results of MC analysis are reported in Fig. S25 and Tables S27–S29 of the SI. For 12 impact categories, the environmental impacts follow the order I < H < G, whereas for the remaining four categories, the differences are not significant.

4 Balancing environmental sustainability and colouring performance of Co-based ceramic pigments

The comparative analysis of cobalt-containing ceramic pigments using LCA provided a reliable ranking of these products based on their potential environmental impacts. By including the colouring performance of the pigments under examination, product quality is incorporated into the declared unit. This approach enables the determination of trade-offs between environmental sustainability and colouring performance, thereby supporting potential technical viability.

To assess this, the obtained environmental single scores were plotted against selected CIELAB parameters to identify which pigments are environmentally friendly while exhibiting good chromatic performance. Rather than relying on wavelength data, which are merely physical values, CIELAB results were chosen as the key parameters in colour characterization because they are specifically designed to reflect human visual perception. Moreover, they are consistently reported across the literature studies evaluated in this work, thus allowing for a reliable and comparative assessment of colour quality.

In particular, the analysis was focused on the b* component (Fig. 10), where more negative values indicate stronger blue hues; L* (Fig. 11), which represents the perceptual lightness (0 for black and 100 for white), with moderate to low values—typically in the range 30–55—being desirable to maintain the depth of blue tones; chroma (C*) (Fig. 12), which reflects colour saturation, with higher values being preferred for more vivid colours; and the hue angle (h*) (Fig. 13), which indicates the blue shades interval, with an ideal value at 270° corresponding to pure blue.

Fig. 10 The environmental single scores (mPt) obtained for the production of 1 kg of ceramic pigments A–I as a function of blue-yellow component (i.e., the more negative b*, the bluer colour). The red dotted lines outline the threshold region defined by b* < –35 and the midpoint environmental single scores below 3.24 mPt, corresponding to the 33rd percentile. The pigments are represented by symbols as follows: ■ CoAl₂O₄ (A1–4), ▲ hybrid pigments (B and C), ● Mg²⁺- and Zn²⁺-doped CoAl₂O₄ (D and E) and Co²⁺ doped ZnAl₂O₄ (F) and ◆ cobalt olivine (G) and Co²⁺-doped willemite (H and I). Note: marker colours reflect the actual hues of the pigments.

Fig. 11 The environmental single scores (Pt) obtained for the production of 1 kg of ceramic pigments A–I as a function of perceived lightness L*. The red dotted lines outline the threshold region defined by L* values in the range of 30–55 and the midpoint environmental single scores below 3.24 mPt, corresponding to the 33rd percentile. The pigments are represented by symbols as follows: ■ CoAl₂O₄ (A1–4), ▲ hybrid pigments (B and C), ● Mg²⁺- and Zn²⁺-doped CoAl₂O₄ (D and E) and Co²⁺-doped ZnAl₂O₄ (F) and ◆ cobalt olivine (G) and Co²⁺-doped willemite (H and I). Note: marker colours reflect the actual hues of the pigments.

Fig. 12 The environmental single scores (Pt) obtained for the production of 1 kg of ceramic pigments A–I as a function of chroma (C*). Chroma represents the colour saturation, where . The red dotted lines outline a threshold region defined by 35 < C* and the midpoint environmental single scores below 3.24 mPt, corresponding to the 33rd percentile. The pigments are represented by symbols as follows: ■ CoAl₂O₄ (A1–4), ▲ hybrid pigments (B and C), ● Mg²⁺- and Zn²⁺-doped CoAl₂O₄ (D and E) and Co²⁺-doped ZnAl₂O₄ (F) and ◆ cobalt olivine (G) and Co²⁺-doped willemite (H and I). Note: marker colours reflect the actual hues of the pigments.

Fig. 13 The environmental single scores (Pt) obtained for the production of 1 kg of ceramic pigments A–I as a function of hue angle (h*), calculated with the formula h* = arctan(b*/a*). The red dotted lines outline a threshold region defined by h* values in the range of 255–285° and the midpoint environmental single scores below 3.24 mPt, corresponding to the 33rd percentile. The pigments are represented by symbols as follows: ■ CoAl₂O₄ (A1–4), ▲ hybrid pigments (B and C), ● Mg²⁺- and Zn²⁺-doped CoAl₂O₄ (D and E) and Co²⁺ doped ZnAl₂O₄ (F) and ◆ cobalt olivine (G) and Co²⁺-doped willemite (H and I). Note: marker colours reflect the actual hues of the pigments.

For each parameter, a selected region was defined based on either statistical or perceptual criteria: the environmental impact threshold was set at the 33rd percentile (3.24 mPt), while the colorimetric thresholds were established according to perceptual relevance—specifically, b < –35 to reflect increasingly blue hues along the blue-yellow axis, L* values between 30 and 55, C* > 35 to ensure sufficient colour saturation, and hue angles (h*) between 255 and 285° to delimit the blue region. Based on the b* component, the most intense blue hues are expressed by the hybrid pigments B and C, with values of −70.49 and −46.78, respectively (Fig. 10). The same pigments also display the most vivid colours, with C* values of 72.4 and 47.4, respectively (Fig. 12). The L* values ranging from 30 and 40 indicate a medium-dark appearance (Fig. 11). Pigment B has a hue angle of 283°, placing it into the blue-violet quadrant, while pigment C, at 261°, lies into the blue-green quadrant—both within the defined blue region (Fig. 13). In addition to their chromatic performance, B and C have lower potential environmental impacts than CoAl₂O₄ (A1–4) and M²⁺-doped pigments D and E, and show a single score comparable to M²⁺-doped ZnAl₂O₄ (F). Pigment F demonstrates both good environmental and chromatic performance, with a b* value of −39 and a hue angle (h*) of 264.6°, indicating a balanced blue hue. Its colour saturation is moderate (C* = 39.1), but due to its low content of cobalt (i.e., 3.23%), it appears significantly lighter than most studied pigments, as proved by its L* value of 67.9, which falls outside the defined lightness threshold.

Pigment I, having the lowest content of cobalt (i.e., 1.32%), shows the lowest potential environmental impacts among all studied pigments, with its single score being 94% lower than CoAl₂O₄ prepared via the standard SSR method (i.e., A1). It displays a pleasing blue hue, characterized by a b* value of −36.5 and a hue angle (h*) of 270°, placing it at the centre of the blue region. Additionally, it shows moderate colour saturation (C* = 36) and lightness (L* = 54). All colorimetric parameters fall within the defined perceptual threshold regions.

Pigment H lies at the threshold boundary, with an environmental single score slightly exceeding the 33rd percentile cutoff (3.28 vs. 3.24 mPt). Despite this marginal deviation, it exhibits comparable colorimetric performance to Pigment I and falls within all defined perceptual threshold regions for b*, L*, C*, and h* (Fig. 10–13).

Among CoAl₂O₄ synthetic paths, the CPT method (i.e., A2) yields the best colouring performance across all parameters, with h* = 269.3, precisely aligning with the blue region. It has a moderate saturation (C* = 41), and a lightness (L*) of 37.4, which is moderately dark. However, when considering both environmental and colouring parameters, A2 is less favourable than the hybrid pigments B and C.

Hybrid pigments B and C emerge as the most promising candidates, combining vivid blue coloration with relatively low environmental impacts. Pigment I also demonstrates strong potential, meeting all defined thresholds while exhibiting the lowest environmental burdens among the studied pigments.

This study advances the principles of green chemistry by identifying alternative, more sustainable design and synthetic strategies for blue ceramic pigments. By integrating low energy synthesis methods such as the microwave-assisted hydrothermal route and reducing the dependence on critical raw materials like cobalt, the research demonstrates how pigment design can align with environmental and safety goals. A key contribution of this work is the application of LCA to evaluate the environmental footprint of various pigment formulations, enabling a data-driven comparison of their sustainability profiles. The identification of pigments B, C, and I—each combining good chromatic performance with lower environmental impacts—illustrates how material innovation can support greener production practices. These findings provide actionable insights for the chemical and ceramic industries, offering viable pathways toward more sustainable pigment technologies and fostering innovation across the pigment value chain.

5 Conclusions

In this study, various synthetic strategies for the production of CoAl₂O₄ and some lower-cobalt alternatives were assessed by life cycle assessment methodology to determine and compare their potential environmental impacts concurrently with their colouring performance through CIELAB parameters. The analysis encompassed four synthetic strategies for the preparation of CoAl₂O₄ (i.e., solid state reaction, coprecipitation, sol–gel autoignition and microwave assisted hydrothermal). Additionally, two hybrid pigments (CoAl₂O₄/kaolin and CoAl₂O₄/Al₂O₃) and three M²⁺-doped CoAl₂O₄ and Co²⁺-doped ZnAl₂O₄ pigments with the general formula Co_1−x−yZn_xMg_yAl₂O₄ where 0 ≤ x, y < 1 were studied (i.e., Co_0.67Mg_0.16Zn_0.16Al₂O₄, Co_0.25Mg_0.75Al₂O₄ and Zn_0.9Co_0.1Al₂O₄). The comparison also considered cobalt olivine (Co₂SiO₄) and two low-cobalt variants with good chromatic properties (Co_0.5Zn_1.5 SiO₄ and Co_0.05Zn_1.95SiO₄).

For the synthesis of CoAl₂O₄, solid-state reaction (SSR) showed the highest environmental impacts, mainly attributable to the use of CoO as the starting material. In contrast, the microwave-assisted hydrothermal (HT-MW) approach demonstrated the best environmental performance. For the two hybrid pigments, CoAl₂O₄/Al₂O₃ produced by heterogeneous precipitation (HPT) led to lower impacts across most categories, mainly due to its lower cobalt content (i.e., 14.2%) than CoAl₂O₄/kaolin prepared through HT-MW. The results for the M²⁺-doped CoAl₂O₄ prepared through sol-gel autoignition (SGA) indicated that the environmental profiles of the resulting pigments strongly depend on the electricity consumption and the type of fuel used for autoignition, with Co_0.25Mg_0.75Al₂O₄ relying on citric acid that produces the highest impacts. For olivine (Co₂SiO₄) and the lower cobalt variants Co_0.5Zn_1.5 SiO₄ and Co_0.05Zn_1.95SiO₄ (all of which obtained by SSR), the impacts decreased significantly with reduced cobalt content. In particular, Co_0.05Zn_1.95SiO₄, which contains only 1.32% of cobalt, showed a 95% reduction in environmental impact (based on the single score) compared to cobalt olivine (56.1% cobalt).

A Monte Carlo (MC) simulation was conducted to assess the reliability and uncertainty of the obtained results, involving 1000 runs within a 95% threshold significance. The MC simulation confirmed the above findings for most impact categories, allowing for a more reliable ranking of the studied pigments in function of their environmental footprints.

Furthermore, the study also considered the as-calculated environmental impacts in conjunction with the CIELAB parameters to identify pigments that are both environmentally friendly and exhibit good chromatic performance. This approach enabled a comprehensive evaluation of the trade-offs between environmental sustainability and product quality. Hybrid pigments B and C emerged as promising candidates, offering intense blue hues and vivid colours with lower environmental impacts compared to traditional CoAl₂O₄ pigments. Co_0.05Zn_1.95SiO₄ (pigment I, produced via SSR) stood out for its lowest environmental impacts among all studied pigments, while also meeting all defined thresholds for chromatic performance. Zn_0.9Co_0.1Al₂O₄ (pigment F, synthesised via self-propagating combustion) also combined good environmental performance and chromatic characteristics, with a balanced blue hue and moderate saturation. However, its lightness value exceeded the defined threshold, making it less suitable for applications requiring deeper blue tones.

This work emphasises the importance of a life cycle approach to the evaluation of ceramic pigments, considering both environmental impacts and colouring performance. The findings provide a reliable framework for selecting pigments that meet specific application requirements while minimizing environmental footprints. As the demand for sustainable materials continues to grow, LCA studies are valuable tools in guiding research, development and commercialisation of high-quality, environmentally friendly pigments.

Author contributions

A. U.: conceptualization, methods, investigation, software, validation, writing – original draft, writing – review & editing; A. S.: investigation, writing – original draft, writing – review & editing; P. N.: investigation, software; R. R.: supervision, investigation, visualization, writing – original draft, writing – review & editing; A. M. F.: supervision, project administration, resources.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included within the article and as part of the SI.

Supplementary information is available. Flowcharts of the synthetic strategies for A1–A4 and B–I (Fig. S1–S10), potential relative impacts of A1–A4 and B–I (Fig. S11–S22), Monte Carlo simulations for the comparative environmental impacts (Fig. S23–S25), complete inventories used to model each process (Tables S1–S13), potential absolute environmental impacts of B–I (Tables S14–S16), Monte Carlo simulations (Tables S17–S28), experimental procedures for each synthetic strategy assessed by LCA (appendix A), and detailed environmental impact analyses for A1, A4, B, C, F and I (appendix B). See DOI: https://doi.org/10.1039/d5gc02709h.

Acknowledgements

This work was supported by the Programma Operativo Nazionale Ricerca e Innovazione 2014-2020 (CCI 2014IT16M2OP005), under CUP E85F21003350001, for which the authors are sincerely grateful.

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