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Exploring the functional properties and utilisation potential of mollusca shell by-products through an interdisciplinary approach

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Molluscan shellfish aquaculture contributes to 42.6% of global aquaculture production. With a continued increase in shellfish production, disposal of shell waste during processing is emerging as an environmental and financial concern. Whilst major commercial species such as Crassostrea spp. has been extensively investigated on usage of its shell, with information that are crucial for valorisation, e.g. safety and crystal polymorphs, evaluated. There is currently little understanding of utilisation opportunities of shell in several uprising Australian commercially harvested species including Akoya Oyster (Pinctada fucata), Roe’s Abalone (Haliotis roei) and Greenlip Abalone (Haliotis levigata), making it challenging to identify ideal usages based on evidence-based information. Therefore, in this study, an interdisciplinary approach was employed to characterise the shells, and thereafter suggest some potential utilisation opportunities. This characterisation included crude mineral content, elemental profiling and food safety evaluation. As well, physical, chemical, and thermal stability of the shell products was assessed. TGA result suggests that all shells investigated have high thermal stability, suggesting the possibility of utilisation as a functional filler in engineering applications. Subsequent FTIR, SEM and XRD analyses identified that CaCO3 was the main compositions with up to 77.6% of it found to be aragonite. The spectacular high aragonite content compared to well-investigated Crassostrea spp. suggested an opportunity for the utilisation of refined abalone shell as a source of biomedical engineering due to its potent biocompatibility. Additionally, safety evaluations on whole shell also outlined that all investigated samples were safe when utilised as a crude calcium supplement for populations > 11 years old, which could be another viable options of utilisation. This article could underpin abalone and akoya industries actions to fully utilise existing waste streams to achieve a more sustainable future.
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Exploring the functional properties
and utilisation potential of
mollusca shell by-products through
an interdisciplinary approach
Wing H. Chung2,4, Nicholas Sheng Loong Tan1, Manjin Kim3, Thunyaluk Pojtanabuntoeng1
& Janet Howieson2
Molluscan shellsh aquaculture contributes to 42.6% of global aquaculture production. With a
continued increase in shellsh production, disposal of shell waste during processing is emerging as an
environmental and nancial concern. Whilst major commercial species such as Crassostrea spp. has
been extensively investigated on usage of its shell, with information that are crucial for valorisation,
e.g. safety and crystal polymorphs, evaluated. There is currently little understanding of utilisation
opportunities of shell in several uprising Australian commercially harvested species including Akoya
Oyster (Pinctada fucata), Roe’s Abalone (Haliotis roei) and Greenlip Abalone (Haliotis levigata), making
it challenging to identify ideal usages based on evidence-based information. Therefore, in this study,
an interdisciplinary approach was employed to characterise the shells, and thereafter suggest some
potential utilisation opportunities. This characterisation included crude mineral content, elemental
proling and food safety evaluation. As well, physical, chemical, and thermal stability of the shell
products was assessed. TGA result suggests that all shells investigated have high thermal stability,
suggesting the possibility of utilisation as a functional ller in engineering applications. Subsequent
FTIR, SEM and XRD analyses identied that CaCO3 was the main compositions with up to 77.6%
of it found to be aragonite. The spectacular high aragonite content compared to well-investigated
Crassostrea spp. suggested an opportunity for the utilisation of rened abalone shell as a source of
biomedical engineering due to its potent biocompatibility. Additionally, safety evaluations on whole
shell also outlined that all investigated samples were safe when utilised as a crude calcium supplement
for populations > 11 years old, which could be another viable options of utilisation. This article could
underpin abalone and akoya industries actions to fully utilise existing waste streams to achieve a more
sustainable future.
Keywords Shell waste, Blue economy, Seafood, Full utilisation, Upcycling, Supplement safety
Mollusca is a species-rich1 phylum, including well-known subgroups such as gastropods, bivalves, and
cephalopods. ese animals contribute a signicant portion of human seafood consumption2. According to
2024 statistics, 26% of global seafood consumed were shellsh species including mussels, oysters, clams, scallops,
abalone, sea snail, cockle and whelks36.
e majority of these consumption species are characterised by external hard shells. Whilst generally
consumers are interested in consuming minimally processed shellsh, there is also considerable processing of
molluscan shellsh into ready-to-eat items, with Loo7 evidenced that rarely investigated species, such as abalone
oen produce piles of shell by-product in abalone processing3,8. is processing includes “shucking” to separate
the shell from the edible meat, a process which generates a large quantity of shell waste, as the shell accounts
for 35–90% of the animal weight912. Unregulated disposal of these shells is reported to create strong noxious
smells and cause environmental pollution by having shells piling on shoreline, subsequently obstruct the usage
of coastal area which ultimately require dredging to remove, further damaging existing marine habitat4,1215.
1Curtin Corrosion Centre, Curtin University, Bentley, WA 6102, Australia. 2School of Molecular and Life Sciences,
Faculty of Science and Engineering, Curtin University, Bentley 6102, Australia. 3X-Ray Diraction and Scattering
Facility, John de Laeter Centre, Curtin University, Bentley, WA 6102, Australia. 4End Food Waste Cooperative
Research Centre, Wine Innovation Central Building, Level 1, Waite Campus, Urrbrae 5064, Australia. email:
winghuen.chung@curtin.edu.au
OPEN
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Regulated disposal does occur but may incur a sizable operational fee that may be burdensome for small to
medium enterprises12. Although in the past and present, there has been cases the shell waste were being utilised
in Australia, such as utilised as decorative7 and materials for reef restoration16. Existing strategies either have
minimal impacts on reducing vast amount of shell waste produced or shown to have counter eective impact
towards the environment. With the production of shellsh increasing, these existing shell disposal issues, are
also expected to increase. Hence identifying potential strategies in utilising shell waste is important in improving
the sustainability of current shellsh industries12.
In systematic reviews conducted by Zhan et al.4 and Morris et al.12, the major valuable component in shell
waste were found to be calcium carbonate (CaCO3) which consist of three dierent polymorphs—aragonite,
calcite and vaterite. ese polymorphs occur in various ratios, depending on the location and species. Currently,
the majority of molluscan shell valorisation activities are based on the overarching CaCO3 composition
and involve minimal processing of the shell. Such activities include use in poultry feed, animal nutritional
supplementation, bioller, soil liming, concentre aggregate and human supplements (details referred to Morris
et al.12 and Chang et al.17).
Further characterisation of the shell CaCO3, can be undertaken to understand the relative percentages of
the dierent polymorphs (aragonite, calcite and vaterite), which vary according to species and location. Such
characterisation can potentially lead to more specialised valorisation applications. As an example, aragonite
is oen considered an optimal polymorph in the eld of biomedical science due to high biocompatibility and
promising osteo-bioactivity18,19.
Additionally, despite the fact that various human calcium supplements derived from molluscan shell are
currently on the market globally, branded as “Os-Cal” (US), “Hi-Calcium” (US), “Oyster Ca” (Netherlands),
and “Nature Made Oyster Shell Calcium” (Japan)17, understanding of safety regarding calcium supplements
from multiple seafood species is mostly unexplored. Due to chemical similarity of calcium and some heavy
metal, deposition of toxic heavy metal could occur during the biological formation process of shell, therefore,
if shells were harvested from location with great anthropogenic activities, this could lead to potential adverse
health eects if shell-based calcium supplements are to be consumed orally17,2023. Hence, safety characterisation
should be explored to understand the viability of utilising shell waste as a human supplement to ll the existing
knowledge gap.
Although characterisation studies on shell have been conducted on some of the more commonly consumed
shellsh, for instance, oyster, clam and mussel of various species4, characterisation on uprising commercial
edible species such as Akoya oyster (Pinctada fucata), Roe’s abalone (Haliotis roei) and Greenlip abalone (Haliotis
levigata) has not been conducted, which has been evidenced in Loo7 study that has led to various pressing
sustainability concern due to lacking in know-how and fundamental understanding of shell characteristics.
is study was therefore initiated to investigate the compositional, stability and safety aspects of shell from
these species, which aim to explore suitable valorisation formats through interdisciplinary techniques that have
been applied in other shellsh by-product valorisation study. e ndings of this study would be valuable to
identify usage of shell waste from uprising commercial species, as well, providing a systematic methodological
framework to be utilised in the investigation of other shell waste.
Methodology
Raw materials
Shells of Akoya oyster (Pinctada fucata) was provided by Harvest Road Group (Albany, WA, AU), Roe’s Abalone
(Haliotis roei) and Greenlip Abalone (Haliotis levigata) were provided by Rare Foods Australia (Augusta, WA,
AU). For each sample, more than 10kg of pooled samples were collected from the end of processing line. ree
shells from each species were randomly selected using a quartering technique. Shell samples were soaked in
double deionized water overnight to desalinize. Shells were then sundried and stored in ambient environment
until analysis. Chemicals used in this study were sourced from Sigma-Aldrich Ltd. (Macquarie Park, NSW, AU).
Ash content
AOAC24 standard method was utilised to determine the ash content of the samples. In brief, samples were heated
at 550°C in ermolyne mue furnace model 48000 (ermoFisher Scientic Inc, IA, USA) until complete
removal of organic matter as indicated with white ash.
Elemental proling
Elemental quantication via microwave plasma atomic emission spectroscopy (MP-AES)
Individual whole shell was coarsely grounded with 150g of subsample pulverised using TissueLyser II (Qiagen,
Hilden, Germany) to obtain powder with ne our-like consistency and oven dried at 105°C for 48h. 0.5g
of homogenized sub-sample was then transferred to a 50mL Pyrex screw cap culture tube. Due to potential
reactivity, digestion was conducted in an ice slurry bath with 2mL concentrated nitric acid (70%) and 0.5mL
hydrogen peroxide (30%). Digestion was deemed complete when digestate was transparent, solution was then
standardized to 20mL in 1% nitric acid. K, Na, P, Mg, Ca, Fe, Zn, Cu, Cd, Cr, Pb and Ni were determined using
Microwave plasma atomic emission spectroscopy 4100 (MP-AES) (Agilent Technologies, SC, US). At least 7
Multi-element calibration solutions were prepared in the range of 0–20mg/L in 1% nitric acid. All glassware
used in this experiment was cleaned twice with 2% RBS-25 solution to prevent chemical retention and improve
accuracy of elemental determination. Analyses were conducted on three shell per species.
Total and inorganic arsenic quantication via inductively coupled plasma mass spectroscopy (ICP-MS)
ICP-MS was used to determine total (tAs) and inorganic arsenic (iAs) content of shell due to its sensitivity
towards elements with high ionization potential. Digestates were prepared using the method described in
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the previous MP-AES section and analysed via inductively coupled plasma mass spectroscopy ELAN DRC II
(ICP-MS) (PerkinElmer, MA, US). Details of the method is listed in AOAC24 Method 986.15 and Matsumoto-
Tanibuchi et al.25. All samples were measured in duplicate.
Mercury quantication via cold vapor atomic uorescence spectroscopy (CV-AFS)
Cold vapour technique was used to assess mercury (Hg) content due to the potential of mercury being in free
atomic state at room temperature and expectation of detecting it at a low level. Pulverised sample was digested
according to AOAC 974.1424. e quantity of mercury was then determined using cold vapor atomic uorescence
spectroscopy PSA Millennium (CV-AFS) (PS Analytical, LDN, UK).
Safety evaluation
Risk assessment of the shells was conducted to investigate the risk of utilising seashell as an oral calcium
supplement. e method used was the Hazard Index (HI) equation developed by USEPA26 and reproduced
below.
HI
=
n
i=1
(mi·DC)/BW
RfDi
i
(1)
With mi being ith metal in mg/kg Ca, dry basis. DC being recommended dietary allowances (RDAs) of Ca
for dedicated age and gender group, due to low percentage of population consumed Ca supplement dosage
exceeding level of recommended intake (< 1%)27,28. BWrepresent body weight (kg)of adult male and female
based on ABS29 or body weight (kg) of children based on ABS30. RfDi utilised data based on the assumption that
existing metals including Cr31,32, Cd33,34, Hg35,36, Cu37,38 and iAs39,40 are in the most toxic forms.
Thermogravimetric analysis (TGA)
e thermal stability of cured samples was studied from ambient to 800°C using a temperature ramp of 10°C
per min under a nitrogen atmosphere owing at 50 mL/min on SDT 2960 simultaneous DTA-TGA (TA
Instruments, DE, US). Approximately 10–20mg of solid sample was weighed accurately into a 110µL platinum
crucible with a matched empty crucible as a reference. e temperature scale of the instrument was calibrated
using the melting points of 99.999% indium (156.5985°C), 99.99+% tin (231.93°C), 99.99+% zinc (419.53°C),
and 99.99% silver (961.78°C). e balance was calibrated over the temperature range used with alumina mass
standards provided by the instrument manufacturer.
X-ray crystallography (XRD)
e shell waste from Pinctada fucata, Haliotis laevigata, and Haliotis roei were roughly ground using TissueLyser
III (Qiagen, HI, DE) to obtain coarse shell powder. e three coarse powdered samples were then further
pulverized in a micronizing container with ethanol using a McCrone Micronizing Mill (e McCrone Group,
WE, IL) with agate pellets. Sand was used to wash out the container and pellets between the micronization
process for each sample. Approximately 10 weight percent (wt. %) of corundum (as an internal standard) powder
was added to each sample before micronization. e micronized samples were dried for 24h in a fume hood,
followed by being loaded onto PMMA specimen holders. Diraction data were collected using a D8 Advance
diractometer (Bruker Corporation, MA, US) with Cu Kα radiation over a range of 5–80°. Phase identication
was carried out in Bruker EVA 6.0 (Bruker Advanced X-ray Solutions) (Bruker Corporation, MA, US) using the
PDF4+ ICDD database. Crystalline phases were quantied by the Rietveld renement using Bruker Topas 6.0
(Bruker Corporation, MA, US). An assessment of the amorphous fraction of each specimen was made using the
known addition of corundum via the internal standard method.
Scanning electron microscope (SEM)
For morphological investigation, SEM imaging was performed on a Zeiss Neon 40EsB (Zeiss, BW, GE) operating
in a dual-beam eld emission conguration. e images were taken using an accelerating voltage of 10kV with
an aperture size of 30µm. e crushed shell powders were mounted on carbon tape that was adhered to SEM
pin stubs. e samples were then coated with platinum with a thickness of 7nm to prevent charging on non-
conductive samples.
Fourier-transform infrared spectroscopy (FTIR)
e FT-MIR spectrum was also recorded for each shell powder in the range of 4000–450cm−1 on a Nicolet™ iS50
FTIR spectrometer (ermoFisher Scientic, MA, US) operating in ATR sampling mode with a diamond ATR
crystal. e ngerprint region (1200–450cm−1) was analysed and the calcium carbonate phases was identied.
Statistical analysis
Graphical components and statistical analyses in this manuscript were generated using Origin version 2023b
(OriginLab Corporation, Northampton, MA, US) and IBM SPSS version 28 (IBM Australia Ltd, St Leonards,
NSW, AU). In terms of statistical analysis for mineral composition and ash content, Levene’s tests were rst
performed to determine whether the data set fullled the assumption of classic ANOVA. In the case of equal
variance, classic one-way ANOVA was then carried out with Tukey’s post-hoc test. When unequal variance was
noted, to reduce likelihood of Type I error, Welch’s ANOVA with Games-Howell post hoc were carried out to
increase robustness of statistical analysis. Independent t-test was carried out to compare iAs content due to < 3
interventions. To investigate similarity between seashell samples, a principal component analysis (PCA) was
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subsequently conducted with a targeted model of 60% cumulative scores and inclusion of all components with
eigenvalue 1. Signicance value of statistical analysis was set at P 0.05 and quantitative data was presented as
mean ± standard deviation.
Result and discussion
Ash content
Ash contents of investigated molluscan shell in this study were found to be 93.71 ± 0.37 (Pinctada fucata),
95.88 ± 0.38 (Haliotis roei), and 96.40 ± 0.25 (Haliotis laevigata). Similar values were identied in studies
conducted by Elegbede et al.41 where ash content of bloody cockle (Anadara senilis), mangrove oyster (Crassostrea
gasar), and blue mussel (Mytilus edulis) were reported at the range of 92–98%.
In the eld of food compositional analysis, ash content is oen use as an indicative matrix to estimate the
quantity of inorganic matter, the majority of which is usually presumed to be mineral41. For molluscan shells,
95–99% of the mass is reported to be CaCO3 with as low as 1–5% organic matter42,43. When combining results
from the ash analyses with the TGA result reported in a later section (Fig.6), it can be determined that CaCO3
is the major component in ash content for the shells from all three species.
Whilst abalone shell is a valuable commodity that is sometimes marketed and utilized for decorative purposes,
substantial portions may be discarded to landll due to the inability of individual processor to accumulate
sucient volume for ecient commercial outcomes such as exporting and renery7. e current ndings in this
study identifying that abalone shells have similar major component—CaCO3 compared to the other molluscan
shells, this could indicate that there could be possibility of a including investigated shell to existing CaCO3
renery and diverted from entering landll.
In fact, in Galicia, Korea, US and Peru, there are already existing industrial or government-initiated facilities
that rene seashell into lime (CaO derived from CaCO3) or CaCO3 for uses such as soil conditioning, liming44
and prevention of river eutrophication42,45,46. Shell-derived CaCO3 has also been shown to be a possible
replacement for cement or ller in construction4749. is product can also act as a natural-sourced food additive
that could replace synthetic phosphate in meat products, to oer crucial functions such as pH adjustment,
buering, improved production yield and improved sensory characteristics50,51. CaO transformed from shell’s
CaCO3 through simple reactions has also been found to exhibit strong antibacterial activity that can extend
shelf-life of food such as tofu52 and kimchi53.
Elemental prole
Results of mineral analysis are shown in Table 1. All samples had calcium as the highest level components,
reaching 37.84–38.53%. Similar results were also found in a previous study on Black-lip Pearl Oyster (Pinctada
margaritifera) (39.7%)17, Akoya Oyster (35.6%)54, Pacic Oyster (Crassostrea gigas) (41.55–48.27%)55, Bloody
Cockle, Mangrove Oyster, and Blue Mussel (12.44–51.00%)41. e high Ca result suggests that all samples
investigated could be utilised as an human calcium supplement that are comparable to existing oyster shell-based
supplements produced in the US, Netherland and Japan (summarised in17). In Australia, ABS28 identied that
21% of females and 15% of males above 2 years old consume calcium supplements, product thereby reaching 3.8
million population in Australia alone. With the increasing demand of “natural” human product due to consumer
Greenlip Abalone (Haliotis laevigata) Roe’s Abalone (Haliotis roei) Akoya Oyster (Pinctada f ucata)
Macro-minerals (mg/kg dry basis)
K 409.47 ± 110.62a314.85 ± 27.66a237.01 ± 7.45b
Na 1097.50 ± 168.82b1230.76 ± 7.83a1239.20 ± 121.90ab
P 22.56 ± 8.46b29.15 ± 3.09b35.79 ± 3.90a
Mg 35.34 ± 0.75c90.60 ± 0.86b191.20 ± 14.20a
Ca 385,307.10 ± 67.6a382,781.63 ± 15.69b378,400.79 ± 58.05c
Trace elements (mg/kg dry basis)
Fe N.D 25.06 ± 3.10 N.D
Zn N.D N.D 5.77 ± 0.80
Cu 1.29 ± 0.42 1.64 ± 0.35 2.06 ± 0.78
Cd 13.21 ± 0.65a11.51 ± 1.68b11.80 ± 0.86a
Cr 1.98 ± 0.07b2.33 ± 0.13a1.87 ± 0.11b
Pb N.D N.D N.D
tAs 0.08 ± 0.01b1.10 ± 0.00a0.06 ± 0.01c
iAs N.D 0.85 ± 0.11a0.04 ± 0.06b
Hg 0.01 ± 0.01 N.D N.D
Ni N.D N.D N.D
Tab le 1. Mineral content of Greenlip abalone, Roe’s abalone and Akoya oyster shell. Welch’s ANOVA was
conducted on K, Na, P, Mg, Ca, Cd, tAs and independent t-test was carried out on iAs. Values are expressed in
mean ± standard deviation from three individual seashell with triplicate analytical measurement. Signicant
dierences (P 0.05) are indicated with dierent uppercase letters.
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perception of enhanced health benets and safety, the current ndings could be used to propose a potential
business opportunity in valorisation of such shells into high value nutraceuticals56,57.
e high Ca content in the tested samples also highlighted the potential of utilisation as a supplement for
animals, especially egg-laying poultry. Previous studies by Ahmad and Balander58 and Wang et al.59 found that
by supplementing poultry with seashell, eggshell quality improved, which reduce food loss due to breaking of
egg shell during post-harvest handling.
In considering the use of the Ca of the shell as nutraceutical, it is noteworthy that all samples investigated
were found to have relatively high Cd content (11.51–13.21 mg/kg, dry basis) in comparison to previous studies
(< 6.21mg/kg) on Pacic Oyster, Black-lip Pearl Oyster and Hairy Cockle (Anadara antiquata), Eared Horse
Mussel (Modiolus auriculatus), Pacic Jewel-box (Chama pacica), Fluted Giant Clam (Tridacna squamosa),
and Cockle (Periglypta reticulata)17,22,55. In comparison, in the studies of Ha et al.55 and Nour22, other heavy
metals such as Ni, Pb or Cu (up to 87.19, 67.41, 67.74mg/kg, respectively) were found to be more concerning
in molluscan shell in comparison to the Cd levels. e variation in heavy metal ndings was not surprising as
marine invertebrate shells have previously been suggested to be used as marine pollution indicators due to their
ability to accumulate environmental pollution, especially heavy metals60,61.
Heavy metal content in seashell has previously been hypothesised to be incorporated from the surrounding
environment during shell formation62. However, a recent study conducted to understand Cd appearance on
oyster shell has identied that Cd can be attached to shell via precipitation and chemisorption outside of shell
formation63. Although this ability seems to be undesirable as it could impact heavy metal safety if shells were to
be utilised as nutraceuticals, Lee et al.63 has suggested that this mechanism is the principle behind the reported
strong ability of seashells as a biosorbent in removing heavy metals (see also review by Tamjidi and Ameri64).
When statistically comparing the trace element content across samples, there is seemingly a lack of distinctive
genus (Haliotis spp.) similarity between Greenlip and Roe’s Abalone. In comparison, the trace elements prole of
Roe’s Abalone shell appeared to be closer to the Akoya Oyster, the same phylum but a dierent genus. Hence, a
PCA was conducted and the distances across all three species was shown to be evenly spaced (Fig.1), conrming
the earlier observation. Previously, taxonomic dierences has been hypothesized to play an important role in
trace elements content in seashell17,21. However, in this case, perhaps other factors such as variation in ontogenetic
stages, uctuation in salinity and temperature of surrounding water, seasonal factors, similarity of sediments
and proximity of growing locations have overridden the previously observed taxonomical similarity17,21,22. e
ndings also suggest that, since trace element content seems to be unpredictable, if such shells are to be utilised
as a human supplement, trace elements content should be inspected regularly to ensure product safety.
Safety evaluation
As suggested in previous sections on trace-elements results, investigated samples could pose safety concerns
if the shell is to be consumed as a nutraceutical or calcium supplements. Hence, safety evaluations of the
Fig. 1. Principal component analysis (PCA) biplot generated via elemental composition of Greenlip abalone,
Roe’s abalone and Akoya oyster shell.
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dierent shells using the USEPA26 HI equation were conducted to evaluate safety holistically, in considering the
appropriate dosage aligned with all investigated toxic trace elements.
Result of HI analysis was presented in Fig.2, with all investigated seashells being unsuitable and potentially
harmful to populations below 11 years of age (HI > 1). is result is likely a consequences of low body mass
and high calcium needs for growth in this age group30,65. Cd was found to contribute to the majority of the HI
score. Cd exposure to children at this < 11 development stage is particularly detrimental, with the potential to
cause chronic and severe adverse health outcomes66. However, it should be noted that the current investigation
was conducted with a relatively small sample size and number of tests species. As mentioned previously, the
trace element composition of seashell can vary depending on multiple factors. It is therefore recommended to
undertake further investigation to contributory factors for Cd accumulation in molluscan shell.
However, using the cumulative HI, a “worse-case scenario” hazard analysis as described by Bandara et al.67,
the results showed that the shell products posed no concern for any age groups > 12. ese results suggest that
the investigated shells could be safely valorised for calcium supplements for adolescents and adults. Yet, as
stated previous, it is recommended that ongoing monitoring of heavy metal contamination and investigation of
potential contributing factors using a larger sample cohort be completed to allow a more rigorous examination
of the valorisation of seashell, for safe supplement utilisation.
Scanning electron microscopy (SEM)
e shell from the three species of molluscs namely Greenlip abalone (Fig.3a, Haliotis laevigata), Roe’s abalone
(Fig.3b, Haliotis roei) and Akoya oyster (Fig.3c,d, Pinctada fucata) were prepared for SEM characterisation.
SEM imaging of all three pulverised powder samples show similar and distinctive features of molluscan shell
morphology. Based on Fig.3a–d, it was apparent that the investigated shell powders are a mixture of large
lamellar structures (b1) and column-like pieces (d1); the origins of which are aragonite platelets originating from
nacre68 and calcite columnar prismatic crystal arising from the prismatic layer of the shell, respectively69. Layers
of nacre was observed and measured with a mean thickness of about 500nm as shown in Fig.3e. is result was
consistent with those previously reported70. e particle size distribution estimated from the SEM images aer
crushing the shell was broad with particle size ranging from < 1µm to 1mm.
e morphological and particle size distribution results can be an important consideration for potential
utilisation of the shell as a ller in polymer composites. Both morphological characteristics and particle size
distribution can inuence the particle packing behaviour, and consequently, the loading capacity71. Additionally,
morphological dierences such as spherical, lamellar (at-shaped) or irregular shapes can have a signicant
impact on the polymer composite properties such water vapor transmission rate. In particular, lamellar
morphological shapes can act as a physical barrier, and increase tortuosity for various permeants, such as water
Fig. 2. Safety evaluation of greenlip abalone (le), Roe’s abalone (right) and akoya oyster (top) shell as a
human calcium supplement.
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Fig. 3. SEM micrograph of shell powder mixture obtained from three pulverised shell per sample. (a) Greenlip
abalone; (b) Roe’s abalone; (c) Akoya oyster which originates from the nacre layer of the shell due to its plate-
like morphology. (d) Akoya shell powder shows an intact columnar prismatic calcite crystal originating from
the prismatic layer of the molluscs’ shell aer crushing. (e) Nacre layer with a thickness between 400–500 nm.
(b1) indicate lamellar structure of aragonite platelets and (d1) highlight column-like morphology of calcite
columnar prismatic crystal. All SEM images were taken with a Zeiss Neon 40EsB (Zeiss, BW, GE) operating in
a dual-beam eld emission conguration using a 10 kV accelerating voltage and an aperture size of 30µm.
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or oxygen molecules, thereby, reducing permeability72. is strategy is commonly employed in the formulation
of anti-corrosion protective coatings in which ceramic or mineral ller such as glass akes73 and micaceous iron
oxide74 both of which have plate-like morphology are oen used to decrease the permeability of the protective
polymer lm. A similar strategy was implemented in food packaging technology using hexagonal boron nitride
(BN) llers as reported in previous literature75. Based on the shell morphology, evident in the SEM images,
it may also be worthwhile to consider identication of a commercially viable method to isolate homogenous
lamellar shape aragonite crystal which can serve as viable alternative to the more exotic hexagonal BN llers.
For instance, a mechanical, chemical and heating method has been previously shown to be useful in separating
calcite and aragonite crystals76,77.
Powder X-ray diraction (XRD)
In the scientic literature, there are multiple qualitative investigations into identifying crystalline phases of
various molluscan shells, however, there is a paucity of studies on the exact composition of crystalline phases
in the shells of Akoya oyster (Pinctada fucata) and Australian abalone species including Greenlip (Haliotis
laevigata) and Roe’s abalone (Haliotis roei) species. As mentioned previously, composition of the inorganic
phases in molluscan shell is generally comprised of two crystalline phases—calcite and aragonite78 which are the
commonly observed polymorphs of calcium carbonate.
In this investigation, a comprehensive and novel approach was adopted in the quantication of polymorphic
phases of CaCO3 in the tested shell powders to obtain the relative and absolute quantities of the dierent phases.
is was achieved by powdered X-ray diraction (XRD) which was used to identify the CaCO3 polymorphs and
within the same characterisation technique, the Rietveld renement was employed for the quantication of the
crystalline and non-crystalline (amorphous) phases.
From the powdered XRD, two main polymorphs of calcium carbonate (CaCO3), calcite and aragonite, were
identied in the abalone shell powder from both the Roe’s Abalone, Greenlip Abalone and Akoya Oyster. e
most abundant polymorph in both the abalone and oyster shell were the aragonite phases (49.8–77.7%) which
was followed by calcite (10.3–32.9%). e remaining phases were attributed to the amorphous content (1–
15.5%). In part, this amorphous content can be attributed to amorphous calcium carbonate (ACC) as evident in
the IR spectrum (Fig.4) and other miscellaneous organic compounds such as polysaccharide like chitin, protein
and others79. A small amount of contaminant i.e., quartz was observed in all powder XRD diraction pattern
which comes from the cleaning of the container in the micronizer with sand. e summary of the phase analysis
via powder XRD is presented in Table 2.
Based on the composition obtained from three dierent species of molluscs commonly harvested in
Australia, the shell powder obtained from Haliotis roei and Haliotis laevigata have one of the highest aragonite
contents of about 75–77 w/w% whilst the Pinctada fucata shell has about 50 w/w% of aragonite. Drawing on the
Fig. 4. Powder X-ray diraction pattern of Haliotis roei, Haliotis laevigata and Pinctada fucata shell
powder obtained from three homogenized seashell. All samples were spiked with corundum (10% w/w) for
quantication purposes. Quantication was performed via Rietveld renement with all tting having a Rwp
value less than 10.
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current ndings, the data suggests that shell mineralogy is dependent, to a certain extent, on the mollusc’s genus.
However, a previous report, based on historical and resample data, established a relationship between climate
change and its profound impact on biomineralization in Mytilus californianus (common name, California
mussel) leading to a decrease in aragonite content due to anthropogenic ocean acidication80. Previous reports
on shell mineralogy in other cultivated molluscs species such as Blood cockles (Anadara granosa) reveal that its
shell composition is mostly comprised of aragonite, although quantitative contribution was not determined81.
Whilst in other molluscan species such as California mussel (Mytilus californianus), the aragonite content can
be up to 48% by weight80.
In terms of various CaCO3 polymorph, calcite is the most stable in nature which has been applied in
bone regenerative application such as bone gras previously82,83, however, in previous study, when compare
to aragonite and vaterite, it is not as readily able to convert to hydroxyapatite under physiological condition
due to its lower solubility, suggesting that those less stable polymorphs are perhaps superior in biomedical
engineering84. Moreover, natural biogenic aragonite, i.e. nacre from shell, as well has additional favourable
characteristics demonstrated19, for instance, high porosity85, solubility86 and unique topographical features87
which enables an environment conducive for bone tissue regeneration88. In comparison to another polymorph,
vaterite, it has as well been utilised previously in hydrogel for bone regenerative medicine89 due to it being
metastable polymorph which makes it highly soluble under physiological condition providing calcium ions for
conversion into hydroxyapatite90. However, while its metastability improve biocompatibility of craed materials,
since it is relatively unstable, it is diculty to identify natural source with abundant vaterite, hence, limited
opportunities for valorisation due to its low abundance.
e current nding of spectacularly high aragonite content in abalone could indicate a promising, high value
utilisation of the investigated shells, particularly, for bone tissue engineering. Previous research has already
demonstrated the osteogenic, osteoinductive and osteoconductive property of nacre in various in vitro and in
vivo human and animal models9194. Especially, favourable outcomes have been identied in human clinical
trials. For instance, Atlan et al.95 have identied that direct usage of powdered aragonite-rich shell powder from
Pinctada maxima could reconstruct damaged bone in human patients. As well, puried aragonite obtained from
corals has also been used to construct commercial medical implants such as Agili-c scaold (CartiHeal). ese
implants have been shown to be a viable option for use in patients with mild to moderate osteoarthritis, and
aligned, reported to improve patient outcomes94,96.
Since the osteoactivity of nacre are mostly attributed to aragonite92,93, the high aragonite content in Haliotis
roei and Haliotis laevigata could be used to establish a more ecient raw material conversion rate for aragonite
renement. Additionally, in comparison to corals which are used to construct Agili-C, abalone shells are more
sustainable since it is currently being disposed of as a by-product. Renery of aragonite from existing shell
powder is thus recommended as the next step of research. Alternatively, direct utilisation of whole abalone shell
for veterinary purposes could also be investigated.
Fourier transform infrared (FTIR) spectroscopy
To further support the crystalline phase analysis obtained via powdered XRD, Fourier transform infrared (FTIR)
spectroscopy was performed on the shell powder derived from the three dierent mollusc species. While powder
XRD was instrumental in the identication and quantication of crystalline phases, FTIR spectroscopy was
appropriate for the elucidation of the amorphous phases. From the FTIR spectrum presented in Fig.5, it is
evident that CaCO3 polymorphs are present. e key signals characteristic for aragonite can be observed at
around 1 083cm−1, 713cm−1 and 700cm−1 whereas the vibrational band associated with calcite97 are 871cm−1
and 713cm−1. is reinforces the analysis performed for the identication of the crystalline phases of CaCO3
via powder XRD.
As shown in Table 2, powder XRD analysis indicates that shell powder derived from these molluscs have
certain percentage of amorphous phase with Akoya oysters at ~ 15.5% w/w (Table 2) having the highest
amorphous content. An attempt was made to correlate the amorphous content to amorphous calcium carbonate
(ACC) which has been reported in other molluscs such as Mercenaria mercenaria, Crassostrea gigas and
Hyriopsis cumingii and Diplodon chilensis patagonicus 98,99. Based on the FTIR spectrum presented in Fig.5,
the broad band from 1500–1400cm−1 shows an apparent splitting, typically associated with the presence of
ACC100102. e function of ACC in molluscs has been proposed by Weiss, Tuross, Addadi and Weiner (2002) to
Phases
Greenlip Abalone
(Haliotis laevigata)Roe’s Abalone
(Haliotis roei)Akoya Oyster
(Pinctada fucata)
Relative Absolute Relative Absolute Relative Absolute
Corundum* (internal reference) 10.00% 10.00% 10.00%
Aragonite 68.15% 75.72% 69.89% 77.66% 44.84% 49.82%
Calcite 9.26% 10.29% 16.63% 18.48% 29.61% 32.90%
Quartz 2.58% 2.87% 1.91% 2.12% 1.56% 1.73%
Amorphous content 10.01% 11.12% 1.57% 1.74% 13.99% 15.54%
Tab le 2. Quantitative powder X-ray diraction analysis using Rietveld renement showing the relative and
absolute phase content. *Corundum was added as internal standard for quantication purposes and quartz
is a residual contamination from the mechanical cleaning of the micronizer container with silica in between
sample preparation. Values were obtained from three micronized seashell.
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be a precursor phase for the biomineralization of aragonite, usually found during the larval stage of a mollusc’s
life cycle.
Thermogravimetric analysis (TGA)
As highlighted in previous sections, the mineralogy of molluscan shell powder is mainly inorganic CaCO3. e
chemical decomposition of pure CaCO3 in response to heat is as shown in Eq.(2):
CaCO3(s)CaO (s)+ CO2(g)
(2)
CaCO3 found in biological systems is oen embedded among organic molecules such as proteins, glycoproteins
and polysaccharides43,103 which make up a small percentage of the weight of the shell and are not stable to high
temperature. In thermogravimetric analysis (TGA), any degradation of the organic material will culminate in
a weight loss as a function of temperature. e TGA of shell powder demonstrated a small weight loss for both
Roe’s ( 3.0%), Greenlip abalone ( 2.9%) and Akoya shell powder ( 4.1%) starting at 240°C, which is the
temperature associated with the decomposition of organic matter or moisture lost. A maximum weight loss of
42% was observed for all shell powder starting from 650°C onwards which is attributed to the decomposition
of CaCO3 into calcium oxide (CaO) and carbon dioxide (CO2). Overall, the thermal degradation behaviour
based on the shape of the TGA curve presented in Fig.6 of Pinctada fucata is consistent with previous report104
while the TGA curves for Haliotis laevigata and Haliotis roei presents new TGA data for the thermal degradation
behaviour of shell obtained from these Australian species.
e high thermal stability of the shell from all three species suggest that it could possibly be utilised to
improve thermal stability of composite product. For instance, shell powder could be utilised as a functional
bio-ller in biodegradable plastic to overcome the caveat of poor thermal stability in poly (lactic acid) (PLA)
and polyhydroxyalkanoates (PHAs)105. As well the literature suggests that infusing shell powder which is mostly
CaCO3, with biodegradable plastic sources, can result in improvement on mechanical properties106 and increase
biodegradability107. Moreover, modied CaCO3 has also been shown to potentially be able to slowly release
bioactive substances into food product, consequentially, increasing shelf-life108. erefore, future study of
infusing shell by-products with biodegradable plastic could be valuable in reducing food waste, food loss and
improving the current plastic pollution issues simultaneously.
Fig. 5. Normalised transmission FTIR spectra of Haliotis roei, Haliotis laevigata and Pinctada fucata shell
powder. Qualitative data on the presence of calcium carbonate polymorphs such as aragonite and calcite are
apparent in the nger-print region in the FTIR spectrum. Amorphous calcium carbonate (ACC) has also been
identied at around 1425cm−1 and a line for illustration purposes was drawn to show the splitting in the signal
which is characteristic when ACC is present. Micronized samples from XRD analysis were utilised.
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Conclusions
In the rst half of the study, ash content of shell powder from three dierent species of molluscs—Haliotis
laevigata, Haliotis roei and Pinctada fucata was rst determined, followed by characterisation of elemental
proling via MP-AES, CV-AFS and ICP-MS, and subsequently algorithmic safety evaluation was conducted
using HI. e ndings suggested that all investigated samples have similar CaCO3 and Ca content which could
allow centralized multi-species processing that might facilitate better use of shell, by overcoming commercial
viability issues associated with disparate locations of low volumes of molluscan processing in Australia. Whilst
Cd levels were demonstrated to be concerning in elemental analysis, HI analysis identied that shell waste could
be safely utilised as Ca supplement for population > 12 years of age. However, since one of the limitations in the
current study is small sample size, the authors were unable to determine factors contributing to Cd level in shells,
as well as the potential uctuation in the Cd level. us, if shell were to be valorised as a human nutraceutical, or
for food applications, the authors urge further studies to investigate raw material for heavy metal contaminants
in details.
In the second part of the study, physical and chemical properties of the shell powders were investigated. To
the best of our knowledge, this is the rst reported complete characterisation and quantication of the CaCO3
polymorphs performed via powder XRD on powdered shell derived from Haliotis laevigata and Haliotis roei.
FTIR spectroscopy was then completed as a secondary analysis to support the data obtained and TGA conducted
to determine the thermal stability of the shell powder. e various potential applications of the tested shell waste
were highlighted with a primary focus on bone regenerative medicine, as well as potential usage as a functional
ller in food packaging materials, but also consideration as a biosorbent for oil spill remediation or wastewater
treatment.
In conclusion, waste valorisation, is oen a complex topic that requires collaborative eorts to fully understand
the opportunities and possibilities of a commercially viable outcome. In this study, multiple authors with diverse
knowledge and background have enabled exploration of undervalued resources—seashell, allowing derivation
of multiple solutions that could possibly convert this product from exisiting linear economy to a circular
economy. By integrating shell waste in any of the applications proposed earlier, it oers a pathway towards a
more sustainable future, bridging the gap between sustainability and responsible resource management.
Data availability
e datasets used and/or analysed during the current study available from the corresponding author on reason-
able request.
Received: 29 July 2024; Accepted: 11 November 2024
Fig. 6. ermogravimetric analysis of Haliotis roei, Haliotis laevigata and Pinctada fucata oyster shell powder
in nitrogen atmosphere from 25–800°C. Micronized samples from XRD analysis were utilised for this analysis.
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Acknowledgements
e authors thank Abalone Council Australia Ltd. (ACA) and End Food Waste Australia (EFWA) for provid-
ing nancial support; Australian Federal Government for research support; Rare Foods Australia and Harvest
Road Group for generous donation of investigated seashell; Curtin University TrEnD laboratory for equipment
landing.
Author contributions
Wing H. Chung: conceptualization, methodology, soware, formal analysis, investigation, resources, writing—
original dra, writing—review and editing, visualization; Nicholas Sheng Loong Tan: conceptualization, meth-
odology, soware, formal analysis, investigation, resources, writing—original dra, visualization; Manjin Kim:
methodology, soware, formal analysis, investigation, resources, writing—original dra, visualization; Kod Po-
jtanabuntoeng: writing—review and editing, supervision, project administration; Janet Howieson: writing—re-
view and editing, supervision, project administration, funding acquisition.
Funding
e work has been supported by the End Food Waste Cooperative Research Centre whose activities are fund-
ed by the Australian Government’s Cooperative Research Centre Program. Abalone Council Australia (ACA),
Fisheries Research and Development Corporation (FRDC). Curtin University as well provided funding support
for this study.
Declarations
Competing interests
e authors declare no competing interests.
Additional information
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