Introduction

A significant shift from the past to in modern times occurred with the invention of synthetic dyes, marking a crucial change in textiles. Approximately 10 million tons of these synthetic dyes are consumed annually1,2. The characteristics of these dyes are considered cheaper to produce, brighter, offering more colors, and are fast and easy to apply to fabric3. However, all of the synthetic dyes create massive amounts of hazardous waste that are deposited into waterways, contributing to serious water pollution, destroying our ecosystem and posing risk to human health4,5. As awareness of environmental issues, scientists and researchers are increasingly seeking sustainable alternatives, notably natural dyes. Consequently, natural dyes are becoming increasingly attractive for textile dyeing because of their environment friendly, biodegradable and low toxicity, along with their reduced allergenic effects on human health6,7,8. Nevertheless, natural dyes face significant challenges such as poor dye uptake, limited color strength, and inadequate color fastness when applied to textiles5,9.

To address these challenges, natural mordants are introduced. Natural mordants enhance the color properties of dyes by interacting with fibers such as cotton, wool, and silk. Silk contain abundant functional groups, such as carboxyl, amino, hydroxyl, and amide groups, which allow it to form strong chemical interactions with these mordants10. Unlike metal mordants such as aluminum, iron, copper, tin, and chrome, which contain heavy metal ions like iron, chromium, tin, nickel, copper, and cobalt, and pose significant environmental risks11,12, bio-mordants emerge as sustainable alternatives. Bio-mordants offer lower toxicity and reduced environmental impact13,14,15. These natural mordants can form hydrogen bonds with the functional groups present in silk fibroin, such as hydroxyl (-OH) and amine (-NH2) groups, improving dye uptake and color fastness.

Previous studies have investigated natural mordants such as pomegranate peel16,17, Banana peel18,19, orange peel20,21, tea leaves22,23 and coffee1. These natural mordants typically contain multiple hydroxyl (-OH) groups that can bond with the hydroxyl and amine groups in silk fibroin, as well as with the functional groups of colorants, thereby enhancing dye uptake and color fastness24,25,26. Utilization of agricultural and food waste for textile dyeing has become increasingly interesting because of its affordability, biodegradability and sustainability. One promising example is the use of waste oyster shells (WOS) as a bio-mordant. Waste oyster shells are a renewable and cost-effective alternative, capable of reducing both textile manufacturing costs and environmental impact27. Annually, the global seafood industry generates millions of tonnes of waste oyster shells through seafood consumption and other related activities, creating significant challenges for disposal and environmental concerns27. For example, the worldwide production of seashell amounted to approximately 16 million tonnes in 2014 28, with China only disposing of 10 million tonnes in landfills29. Similarly, France produced about 2.1 million tonnes of shells each year from aquaculture30. In recent years, Thailand has experienced an increase in disposed oyster shells due to rising domestic demand for oyster consumption and the expansion of the catering industry31. These waste oyster shells contribute to land and air pollution because of the emission of intense odours during microbial decomposition as well as weathering and leaching of heavy metals from the heaps27. Recycling these shells into bio-mordants offers a zero-waste solution that benefits both the economy and the environment.

Oyster shells normally consist of 95 wt% of calcium carbonate (CaCO3) and less than 5% of proteins32,33. CaCO3 exists in three various polymorphs: aragonite, calcite, and vaterite in nature. Heat treatment of the shells leads to an alteration of phase. CaCO3 extracted from shells transforms into CaO at temperatures above 700 °C and has been used in many applications, such as a renewable catalysts for biodiesel production, a material to absorb CO2 for air-cleaning applications and an initial substrate to produce hydroxyapatite ceramic biomaterials33. To the best of our knowledge, while several studies have focused on seashells, little research has investigated the use of calcium derived from waste oyster shells as a mordant and its effect on color properties.

Sappan wood (Caesalpinia sappan L.), also known as Indian Redwood, is a natural dye source commonly found in Southeast Asia. It contains active groups such as hydroxyl and amino groups, which enable it to produce vibrant colors on textile fibers34. Brazilian is the primary coloring component of the sappan wood and shows various medicinal properties such as anti-acne, anti-allergic, antibacterial, antioxidant, anti-inflammatory, cytotoxic, anti-thirst, anti-tumor, anti-photoaging, hypoglycemic, hepatoprotective, analgesic effects34,35.

This research explores the potential of waste oyster shell (WOS) as a sustainable bio-mordant in natural dyeing processes, utilizing sappan wood extract for silk fabrics. The crystal structure of WOS as a bio-mordant is analyzed through X-ray diffraction (XRD) and Rietveld refinement. The bio-mordant is applied using pre-mordanting, meta-mordanting, and post-mordanting methods. Additionally, various characteristics of the dyed silk fabrics are tested, with color fastness evaluated according to ISO standard methods.

Materials and methods

Bio-mordant extraction from the waste oyster shell

Scoured silk fabric and sappan wood were sourced from a local supplier in Thailand, while waste oyster shells (WOS) were collected from a local market. Sodium hydroxide (NaOH, 97%) was procured from RCI Labscan. Alum and ash, used as mordants, were obtained from a fresh-food market and burnt charcoal, respectively, in Thailand. All other chemicals were of laboratory grade and were used without further purification.

The waste oyster shells (WOS) were cleaned using a cellulose sponge and boiled with 5% (w/v) sodium hydroxide (NaOH) for one hour to remove any connective tissue and contaminants. Following this treatment, the cleaned oyster shells were dried in a hot air oven (Binder FED 400, GmbH Co, Germany) for 3–5 days at 80 °C. Afterward that the shells were crushed using a pestle and mortar (90 mm diameter Agate Top) and ground into powder. Next, the shell powder was subsequently sieved using a stainless-steel sieve with a micro-mesh (100 μm Retsch test sieve, Germany) and dried for 15 h at 80 °C in the oven. The dried shell powder was then subjected to calcination at temperatures ranging from 700 °C to 950 °C in an air atmosphere, with a heating rate of 5 °C/min for 3 h. To prevent reaction with carbon dioxide (CO2) and humidity in the air, the calcined powder was stored in an airtight polyethylene plastic bag (JP Packaging) for later use. Both non-calcined and calcined powders were characterized using XRD technique. The extraction of bio-mordant from oyster shell powder was done using the aqueous method. A bio-mordant stock solution was prepared by heating the mixture of WOS powder (5 g) in 100 ml deionized water (DI water) at 95 oC for 1 h under continuous stirring. The extracted solution was then filtered using a Whatman filter No. 42, using DI water and subsequently used for mordanting purposes as (Fig. 1).

Fig. 1
figure 1

Schematic diagram of Bio-mordant extraction from the waste oyster shell.

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Extraction of natural dye from Sappan wood

To extract colorant from the Sappan wood, the wood was dried and then cut into small chips. Fifty grams of dried Sappan wood chips were extracted with 500 mL of distilled water and boiled at 95 °C for 1 h. The resulting solution was filtered through a Whatman filter No. 42, and the filtrate was used as the sappan wood dyeing solution.

Dyeing and mordanting process

Three main mordanting methods, including the pre-, meta-, and post-mordanting were utilized for dyeing. Before the dyeing and mordanting process, the scoured fabrics were soaked in clean water for 30 min. Silk fabric and liquor ratio were 1:20 for both the dyeing and mordanting process. During pre-mordant, the silk fabrics were immersed in the bio-mordant solution for 30 min at a temperature of 80 °C before dyeing whereas for post-mordanting the fabrics were dyed before immersion in bio-mordant solution for 30 min at a temperature of 80 °C. In case of meta-mordant, the 5% of mordant solution was added during the natural dyeing process. During the natural dyeing process, silk fabrics were dyed by utilizing the exhaustion method with a 10% of colorants for one hour at 80 °C. Finally, the dyed fabrics were thoroughly washed with 2 g/L of soap solution at 50 °C for 20 min, rinsed with water, then squeezed and dried at room temperature. Further details of the mordanting and dyeing procedures are provided in Fig. 2. To assess the effectiveness of color fastness compared to other metal and natural mordants, ash and alum mordants were included for comparison.

Fig. 2
figure 2

Schematic diagram of dyeing and mordanting procedure (a) Pre-mordanting, (b) Meta-mordanting, and (c) Post-mordanting.

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Characterizations

The X-ray diffraction (XRD) and Rietveld refinement characterization

The crystal structure of the calcined powders was analyzed using X-ray diffraction (XRD) within the 2θ range of 10°–90° (Philip PW 3040/60 X’Pert Pro) and fitted with the Rietveld refinement procedure using the FullProf program.

The surface morphology

The surface morphology of dyed silk with bio-mordant were observed using a field emission scanning electron microscope (FESEM) (FESEM Apreo S, Thermo Fisher Scientific Inc., Waltham, MA, USA). The microscope operated at 10 kV in high-vacuum (HV) mode with an ETD detector beam current set to 0.05 nA. The FESEM morphological examination was conducted at a magnification of 800x.

UV-visible (UV-vis) absorption measurements

UV-visible (UV-vis) absorption measurements were performed using a UV-vis spectrophotometer (Jusco, V-530, Jasco International Co.,) UV-vis spectroscopy is a good alternative for the identification of the specific dye molecules present in a solution.

Fourier transform infrared spectroscopy (FTIR) measurement

To understand the chemical composition and interaction among the dye, bio-mordant, and fabrics, a Fourier Transform Infrared (FTIR) spectrophotometer (Nicolet™ iS™5 FT-IR spectrometer, Thermo Scientific, USA.) was employed. The FTIR spectrophotometer covered a scanning range from 4000 to 400 cm− 1 with a spectral resolution of 1 cm− 1.

Color strength (K/S) measurement

To analyze color characteristics and absorption behavior of the dyed fabrics, with or without using bio-mordant, the reflectance values and corresponding CIELAB coordinates were measured under illuminated daylight D65, with a 10° standard observer. The color information is represented in CIELAB coordinates: L* corresponds to brightness (100 = white, 0 = black), a* (+/−) indicates the red/green ratio, and b* (+/−) indicates the yellow/blue ratio36.

The Kubelka Munk’s equation colour strength (K/S) values were determined as following:

$${\text{Color}}\;{\text{strength}}\;\left( {{\text{K}}/{\text{S}}} \right)=\frac{{{{\left( {1 - R} \right)}^2}}}{{2R}}$$
(1)

where, R = Reflectance of incident light from the Dyed Material. K = Absorption. S = Scattering coefficient factor of the dyed fabric.

Color fastness measurement

Washing has a noticeable impact on the color stability of the dyed fabrics. Sappan wood dyed silk fabric, with or without using bio-mordant derived from WOS were tested for the color fastness to washing according to ISO 105-C010:2010 standard method. Ratings were assigned on a scale ranging from 1 to 5, where 5 is the best and a grade of 1 is poor. A color fastness rating of less than grade 3 indicates a considerable alteration in color after washing37,38.

Results and discussion

Characterizations of bio-mordant derived from WOS

The phase formation of WOS powder un-calcined and calcined at different of calcination temperature from 700 to 950 °C is presented in Fig. 3(a). Calcium carbonate typically exists in three forms: stable trigonal calcite, less stable hexagonal vaterite, and orthorhombic aragonite39, as illustrated in Fig. 3(b). Upon heating, calcium carbonate undergoes thermal decomposition, producing calcium oxide (CaO) and carbon dioxide (CO2), as shown in Eq. 2.

$$CaC{O_3}(s) \to CaO(s)+C{O_2}(g)$$
(2)
$$Ca{(OH)_2}+C{O_2} \to CaC{O_3}+{H_2}O$$
(3)

When calcium oxide reacts with water, it transforms into calcium hydroxide Ca(OH)2 and the back to CaCO3 when reacts with carbon dioxide gas as Eq. 3 40. Figure 3(c) summarizes the calcium carbonate cycle, highlighting the phase transformations between CaCO3, CaO, and Ca(OH)2.

Fig. 3
figure 3

(a) X-ray diffraction pattern of oyster shell, (b) Three types of calcium carbonate, (c) Calcium carbonate cycle, (d) Rietveld refinement fitting of un-calcined powder and (e) Rietveld refinement fitting of calcined powder at 750 °C.

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In this work, the structure of un-calcined WOS powder shows the CaCO3 in calcite form matching with PDF file 01-071-3699 and previous work41. The powder calcined at 700 °C demonstrates the main peak of CaCO3 in calcite form and a small peak of Ca(OH)2 at 2θ range of 33.8°. The incomplete decomposition at this temperature results in the coexistence of CaCO₃ and Ca(OH)₂ phases. At 750 °C, the diffraction pattern reveals the main peak of CaCO3 with calcite form and also small peaks of Ca(OH)2 and CaO. At this state, the decomposition of CaCO₃ becomes more pronounced, leading to an increased presence of CaO. However, at this stage, there is still some residual CaCO₃, as indicated by the XRD peaks. The formation of Ca(OH)₂ is more evident due to the further reaction of CaO with atmospheric moisture. The presence of CaO and Ca(OH)₂ indicates that the decomposition process is progressing, but not yet complete. When the calcination temperature was raised from 800 °C to 950 °C, the samples showed a main peak of CaO and a small peak of Ca(OH)2. In this temperature range, the decomposition of CaCO₃ is nearly complete, resulting in a predominant CaO phase. However, the peaks of Ca(OH)2 appear in the pattern due to the interaction between CaO with water vapor in the air after sample decomposition. Many works have also reported on this phenomenon33,42. There are many open pores inside the CaO sample aggregates, each particle undergoes a CaO to Ca(OH)2 reaction, and water vapor can freely move through the porosity, all favorable to the conversion of CaO into Ca(OH)2. Thus, the results illustrate that the decomposition of CaCO₃ is an endothermic process, requiring significant energy input. At higher temperatures, the reaction rate increases, leading to a more complete transformation of WOS powder from CaCO₃ to CaO.

The percentage of phases was studied using the Rietveld refinement technique which uses data from X-ray diffraction patterns. The refinement utilized initial models with the following space groups: CaCO₃ (calcite) with R3c, Ca(OH)₂ with P3m1, and CaO with Fm3m. Initial values for cell parameters, space group, and atom coordinates were derived from reference patterns available in the Crystallography Open Database (COD)43. The goodness of fit was evaluated by assessing the profile R-factors included the profile factor (Rp), weighted profile factor (Rwp), expected weighted profile factor (Rexp) and chi-square (χ² ) as showed in Eqs. 4–7, respectively43,44.

Profile R-factor (Rp):

$${R_p}=100\frac{{\sum {\left| {{y_{oi}} - {y_{ci}}} \right|} }}{{\sum {{y_{oi}}} }}$$
(4)

where yoi and yci are observed and calculated background intensities, respectively.

Weighted pattern R-factor (Rwp):

$${R_{wp}}=100{\left\{ {\frac{{\sum {wi{{\left( {{y_{oi}} - {y_{ci}}} \right)}^2}} }}{{\sum {wi{{\left( {{y_{oi}}} \right)}^2}} }}} \right\}^{1/2}}$$
(5)

Rexp is the expected residual:

$${R_{\exp }}=100{\left\{ {\frac{{\left( {N - P+C} \right)}}{{\sum {wi{{\left( {{y_{oi}}} \right)}^2}} }}} \right\}^{1/2}}$$
(6)

where N is the number of observations, P is the number of refined parameters and C is the number of constraints used.

Reduced chi-square:

$${\chi ^2}={\left[ {\frac{{{R_{wp}}}}{{{R_{\exp }}}}} \right]^2}$$
(7)

The low values of χ² (1.41 and 2.37) indicate a high degree of agreement between the calculated and observed XRD patterns. This demonstrats that the refinement models accurately represent the experimental data, as seen in Table 1; Fig. 3(d) and (e). The phase percentage for the un-calcined sample is 100 for CaCO3 (calcite). For sample calcined at 700 °C, the phase percentages of CaCO3 and Ca(OH)2 are 94 and 6, respectively. At 750 °C, the phase percentages of CaCO3, Ca(OH)2 and CaO are 88, 8 and 4, respectively. At calcination temperature between 800 °C and 950 °C, the phase percentage of Ca(OH)2 continuously decreases from 72 to 26 while CaO increases from 28 to 74, as seen in Table 1. Moreover, these results are consistent with the findings of Laonapakul et al.45. , which demonstrated that the elemental composition of oyster shell powder exhibits an increase in CaO content as the calcination temperature rises. This suggests that un-calcined WOS contains a highest phase percentage of CaCO3, whereas calcined WOS at 800 °C and 950 °C produces Ca(OH)2 and CaO, respectively. Therefore, uncalcined WOS and WOS calcined at 800 °C and 950 °C were selected as bio-mordants to investigate their impact on color properties due to the phase transformation of calcium carbonate

Table 1 Goodness of fit, lattice parameters a and c, and the phase percentage of oyster shell powder un-calcined and calcined at different temperatures.
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Extraction of colorant from sappan wood

C. sappan (Fig. 4(a)) is extensively employed for dyeing natural textile fibers containing active groups such as hydroxyl and amino groups, and it can achieve excellent color characteristics46. Within C. sappan, both brazilin and brazilein naturally occur naturally, as shown in Fig. 4(b)-(c), respectively, contributing to a red-orange color. As shown in Fig. 4(d), UV-vis absorption spectroscopy provides information on dye molecules in sappan wood. The color outcomes were related to the molecular structure of brazilein and brazilin, observed from the UV–VIS spectra at the highest peak at 540 nm.

Fig. 4
figure 4

(a) Sappan wood (b) Sappan dye structure of brazilin, (c) Sappan dye structure of brazilein, (d) UV-vis absorption spectroscopy of C. sappan extract and (e) Fourier-transform infrared spectra of C. sappan extract.

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The FT-IR spectrum of the sappan wood extract, as shown in Fig. 4(e), presents a similar trend to previous reports47,48. Brazilein and brazilin have very similar molecular structures because they have similar vibrational spectra. Numerous peaks were evident, especially a broad peak at around 3428 cm− 1 attributable to the -OH functional group (H-bond stretching). Moreover, peak at 1632 cm− 1 was observed, corresponding to C = C stretching vibrations in the aromatic ring. Other peaks at 1270 cm− 1 and 1035 cm− 1 were also detected, which seem to correspond to C-O stretching, may indicate a small presence of phenolic groups.

Color properties

Color properties are described using the CIELAB coordinates: L* corresponds to the brightness (100 = white, 0 = black), a* (+/−) is a red/green ratio and b* (+/−) is yellow/blue ratio. Color strength (K/S) is a value used to determine the intensity of color on a fabric substrate. Using bio-mordanting is an eco-friendly process and has become significantly important in producing colorful fabrics using natural dyes. In our dyeing experiments, silk fabrics were dyed with sappan wood extract both without and with mordants derived from waste oyster shell materials (WOS) with un-calcined, WOS calcined at 800 °C, and WOS calcined at 950 °C. These were compared with alum and ash, selected as common metal and natural mordants, respectively. Pre-, meta-, and post-mordanting methods were employed in our study. The digital images of the sappan wood dyed silk fabrics, the colorimetric data (L*, a*, b*), and color strength (K/S) for the dyed samples are presented in Table 2.

Table 2 Colorimetric properties of silk fabrics dyed with sappan wood extract, comparing the effects of mordants derived from waste oyster shell materials (WOS) in their uncalcined form, as well as calcined at 800 °C and 950 °C, using pre-, meta-, and post-mordanting methods.
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Digital images of dyed fabrics reveal that the use of sappan wood colorants can create different shades of light pink to deep red and brown in the presence of various mordants. This variation is attributed to brazilin, a major compound in sappan wood which provides a red pigment49. The FTIR analysis of Brazilin as Fig. 4(e) exhibited a peak corresponding to hydroxyl (-OH) groups, which can form hydrogen bonds with functional groups on the silk fabric. These hydrogen bonding can promote the adhesion of Brazilin molecules to the silk fabrics, leading to the coloration of the silk fabrics.

Different mordants used in natural dyeing processes can generate variations in the shade and depth of the red color produced by brazilin. The a*-b* plot (Fig. 5(a)-(c)) illustrates that the dyed silk fabrics without and with mordants (uncalcined WOS, WOS at 800 oC, WOS at 950 oC, alum and ash) and through all mordanting process (pre-, meta- and post-mordanting) are located in the red-yellow colorant zone. Experimental observation shows that the color strength (K/S) of mordanted dyed samples is higher than the un-mordanted dyed sample. A higher color strength (K/S) value reflects strong bonds between the applied dyeing chemicals and fabric, and a low color strength may be due to the aggregation of dye molecules and mordant onto fabric50,51. Among the different methods used, pre-mordanting produced the darkest shades and the highest color strength, as shown in Fig. 5(d). The overall K/S value of the bio-mordant derived from waste oyster shell (WOS) calcined at 800 °C in the pre-mordanting process reached 18.2. This value is significantly higher than the un-mordanted sample (K/S = 7.0), being 2.6 times greater. It also surpasses the meta-mordanted sample (K/S = 8.8) by 2.1 times and the post-mordanted sample (K/S = 7.3) by 2.5 times. This increased color strength during pre-mordanting can be attributed to the bonding process, where the mordant first interacts with the amino and carboxyl groups of the silk, forming a bridge that effectively fixes the dye onto the silk fibers52. While meta-mordanting is the simplest method, involving only a single step, it often resulted in less intense colors compared to pre-mordanting. This may be due to the rapid combination of dye and mordant in a single dye bath, leaving a significant amount of dye-mordant complexes in the solution53. In post-mordanting, the dye first bonds with the silk’s amino and carboxyl groups before interacting with the mordant. However, this sequence may limit the mordant’s access to the fiber’s functional groups, leading to weaker interactions and less stable color complexes52.

The color strength (K/S) value of bio-mordanted samples with calcium hydroxide during pre-mordanting (K/S = 18.2) is higher than those with calcium carbonate (K/S = 16.1) and calcium oxide (K/S = 13.2). It is ascribed that Ca(OH)2 in WOS calcined at 800 oC gradually releases hydroxide ions in dye bath, leading to the alkaline environment (pH = 7.9). This alkaline environment can enhance penetration of the dye onto silk fabrics54,55. In contrast, CaCO3 creates a mild alkaline environment (pH = 7.7) leading to the less effective interaction of dye molecules compared to calcium hydroxide. In addition, CaO behave strongly alkaline (pH = 11.5), influencing degradation dye uptake because of less interaction with functional group.

Fig. 5
figure 5

The a*-b* plot of dyed silk fabric (a) pre-mordanting (b) meta-mordanting (c) post-mordanting and (d) K/S of dyed silk fabric using pre-, meta- and post-mordanting method.

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The investigation of FTIR spectra of mordants derived from un-calcined waste oyster shell (WOS) and WOS calcined.

at 800 °C and WOS calcined at 950 °C reveals significant results, as illustrated in Fig. 6(a). A broad peak at 1417 cm− 1 indicates the asymmetric stretching vibration, while another peak at 870 cm− 1 presents its bending vibration of CO3 − 2. Moreover, a peak at 2348 cm− 1 corresponds to the stretching vibration of the C = O bond. Conspicuously, the calcination temperature increases with decrease of intensity of these bands.

Fig. 6
figure 6

The FTIR spectra of (a) mordants, (b) dyed silk fabrics with and without mordant in pre-mordant processing, (c) in meta-processing, and (d) in post-processing.

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Figure 6(b)-(d) illustrate the absorption spectra of silk fabrics with and without mordants (un-calcined WOS, WOS calcined at 800 oC, WOS calcined at 950 oC, alum and ash) during pre-, meta-, and post-mordanting processes. A noticeable difference in these spectra is the presence of a hydroxyl peak in both un-mordanted and mordanted samples. The absorption hydroxyl peak of un-mordanted and mordanted silk fabric was observed around 3270 cm− 1 9 and bending vibration of H-O-H at peak around 1620 cm− 1.

Additionally, interactions among the dye molecules, silk fabric, and the mordant are evidenced by a shift peak at 1515 cm− 1. Since the mordant from WOS contains calcium ions, it may form coordination bonds with dye molecules or other chemical reactions, resulting in a shift peak in the FTIR spectrum compared to the spectrum of the sappan wood dye alone (Fig. 4). The interaction among bio-mordant (such as calcium carbonate, calcium hydroxide), silk fabric and sappan wood dye is illustrated in Fig. 7. In Fig. 7, the functional active molecules of the bio-mordant interacted with the -OH groups of the dye molecules and the protein-based structure of the silk fabrics. Finally, the hydroxyl groups facilitate bounding between the mordant and sappan wood dye.

Fig. 7
figure 7

The interaction among bio-mordant, silk fabric and sappan wood dye.

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The surface morphologies of silk fabrics treated with bio-mordant derived from WOS are illustrated in Fig. 8. Significant changes in the morphology of silk fabrics were observed. The dyed fiber without the bio-mordant shows a smooth fiber surface with no particle deposition (Fig. 8 (a)), indicating minimal surface modification between the dye and the fiber in the absence of a mordant. In contrast, the dyed fiber with the bio-mordant shows the surface roughness with an aggregation of particles derived from waste oyster shells (Fig. 8 (b)-(d)). This roughness surface suggests enhanced interaction between the dye molecule and the silk fabrics. To further verify the presence of elements on the treated silk fabrics, Energy Dispersive X-ray (EDX) mapping was conducted, as shown in Fig. 9(a)–(f). The main elements detected on the silk surface were carbon (C), oxygen (O), nitrogen (N) and calcium (Ca) on the silk surface. The presence of these elements, especially calcium, which originates from the WOS, supports the observation that the bio-mordant modifies the silk surface, contributing to the increased roughness and potential for enhanced dye uptake. These elements are consistent with findings from previous studies, where energy-dispersive spectroscopy (EDS) microanalysis of oyster shell powder revealed elemental composition, with C (9.2 wt%), O (42.4 wt%), and Ca (48.4 wt%) being the primary constituents56. The combined results of FTIR and FE-SEM analysis reveal that the increased surface roughness and the introduction of functional groups such as hydroxyl (-OH), carboxyl (-COOH) or amide group (-NH2) can facilitate both physical adsorption and chemical bonding with dye molecules57.

Fig. 8
figure 8

FE-SEM images of (a) silk fabrics without mordant (b) dyed silk fabrics with un-calcined WOS (c) dyed silk fabrics with WOS calcined at 800 °C and (d) dyed silk fabrics with WOS calcined at 950 oC.

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Fig. 9
figure 9

Energy dispersive X-ray (EDX) mapping analysis of dyed silk fabrics with WOS calcined at 800 °C.

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Color fastness

Color fastness refers to the ability of a dyed sample to retain its color properties under various conditions. Among the various tests, wash is the most important and commonly used color fastness tests. Color fastness to wash is graded in two steps: (1) color staining to the cotton fabric and (2) color change of the sample. Two individual respective grey scales rated the degree of change from 1 to 5 are used for color stain and color change, where 1 is considered very poor and 5 is excellent quality37,38. The results of these tests are presented in Table 3.

Table 3 Color fastness to wash results of dyed samples.
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As Table 3, the results show using a mordant improves the color fastness compared to with un-mordanted sample due to the formation of stronger chemical interactions9. FTIR spectra provides valuable insights into the molecular interactions occurring during dyeing process involving hydroxyl groups, as evidenced by absorption peaks around 3270 cm⁻¹ (OH stretching) and 1620 cm⁻¹ (H-O-H bending). These hydroxyl groups, present in both the dye molecules and the silk fabric, form hydrogen bonds with the bio-mordant24. The formation of these bonds enhances the attachment of the dye to the fabric, thereby improving its resistance to washing. Surface morphological analysis further confirms this by showing increased surface roughness, which enhances the efficiency of dye-mordant bonding and further improves color fastness. Our findings are consistent with previous reports, demonstrating the effectiveness of bio-mordants in enhancing color fastness. Tehrani and co-workers (2023) showed that silk fibers dyed with an extract from spent coffee grounds (SCGs) using different metallic and natural mordants exhibited varying levels of washing fastness. Notably, they found that the washing fastness of silk fibers treated with a natural mordant (pineapple) in the pre-mordanting method was superior to that achieved with tin and copper mordants in the meta- and post-mordanting methods1. Similarly, Rasool et al. (2022) reported that the use of a bio-mordant in dyeing silk and cotton with Bougainvillea extract provided excellent color fastness to washing, light, and rubbing51. Additionally, Chakraborty et al. employed Acacia as a bio-mordant for silk dyeing, resulting in a red-yellow hue. The silk fibers treated with this bio-mordant demonstrated outstanding washing fastness58.

The color fastness to washing shows excellent grading for color staining with rating between 4/5. For color change grade, the grading ranges between 3 and 4 and for un-calcined WOS and 3–4, 4 for WOS at 950 oC. The highest grading 4–5 and 4 is found for WOS at 800 oC as seen in Fig. 10. Bio-mordant derived from WOS significantly enhances colorfastness rating, against washing. The results of this study show that the fastness of bio-mordants is very close to that of metal mordants like alum, giving hope for cleaner dyeing processes. This improvement in fastness may be due to the formation of strong hydrogen bonds between the bio-mordant and both the dye components and the fabric, making them more resistant washing51,59.

Fig. 10
figure 10

Color fastness to washing for silk fabrics treated with WOS calcined at 800 °C: (a) Color change (b) Color stain on multifiber.

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Conclusion

This study investigated the potential of bio-mordants derived from waste oyster shells (WOS), specifically calcium carbonate (CaCO₃), calcium hydroxide (Ca(OH)₂), and calcium oxide (CaO), in enhancing the environmental sustainability and performance of silk fabric dyeing. Our results show that the bio-mordants from WOS generate variations in shade and depth of the red color produced by brazilin with pre-mordanting resulting in a darker shade and higher color strength compared to meta- and post-mordanting methods. This indicates that pre-mordanting enhances the ability of dye bounding and color intensity. The analysis of FTIR spectra and surface morphology confirms that mordants derived from WOS interact effectively with both the dye and silk fabrics.

The study also highlights the influence of different phases of CaCO3 derived from WOS on color strength. Calcium hydroxide from WOS calcined at 800 °C provides the most effective dye uptake by creating a more alkaline environment compared to other phases of WOS. This finding emphasizes the significance of chemical properties and composition of mordant from WOS in optimizing dye performance. Moreover, bio-mordants from WOS significantly improve color fastness ratings against washing, making them comparable to metal mordants like alum. This improvement in color fastness shows the efficacy of WOS as an eco-friendly alternative to conventional mordants.

In conclusion, this research provides valuable insights into the application of bio-mordants from waste materials in textile dyeing as a sustainable and environmentally friendly method. This way not only utilizes waste but also provides a useful solution to the environmental challenges caused by synthetic dyes and heavy metal mordants. Employing green technology in dyeing process can be a sustainable practice in the textile industry that supports the target of environmental stewardship.

Limitations

This study primarily focuses on wash fastness as a key indicator of color retention in dyed fabrics. However, to comprehensively assess the durability of the dyed fabric, particularly in environments where exposure to sunlight or other environmental factors is significant, additional tests such as light fastness and other forms of color fastness should be conducted. These evaluations would provide a more understanding of the silk fabric’s performance after dyeing and mordanting.