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ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERINGAsia-Pac. J. Chem. Eng. 2007; 2: 460–467Published online 21 August 2007 in Wiley InterScience(www.interscience.wiley.com) DOI:10.1002/apj.082

Research ArticleA preliminary study of processing seafood shells foreutrophication control

J. A. Currie,1 N. R. Harrison,1 L. Wang,1 M. I. Jones1* and M. S. Brooks2

1Department of Chemical and Materials Engineering, University of Auckland, Auckland, New Zealand2Department of Process Engineering and Applied Science, Dalhousie University, Halifax NS B3J 2X4, Canada

Received 23 November 2006; Revised 13 February 2007; Accepted 23 February 2007

ABSTRACT: Shells from the seafood processing industry in New Zealand are currently an under-utilized wasteresource. In this study we investigate the processing conditions required for the creation of calcium oxide (lime) fromgreen-lipped mussel (Perna canaliculus) and pacific oyster (Crassostrea gigas) shells. Lime is commonly used inwastewater treatment for the removal of phosphorous compounds from water, thus providing a means of eutrophicationcontrol. Mussel and oyster shells were processed in a horizontal tube furnace at various temperatures (650–800 ◦C)in both air and nitrogen environments. From X-ray diffraction (XRD) and weight-loss measurements, the extent oflimestone calcination was found to increase with increasing furnace temperature for both shell species and furnaceatmospheres. Analysis showed that the lime was present as a layer on the surface of the shell particles. From scanningelectron microscope (SEM) images, significant changes in the surface morphology of the raw shells were observedas a result of heat treatment in both air and nitrogen atmospheres. Preliminary testing of shells heat-treated in bothair and nitrogen atmospheres indicated that both types of shell removed about 90% of phosphates in water within30 min, whereas up to 40% of phosphates were removed with untreated shells. Our results for heat-treated oyster shelldiffer slightly with studies in the literature, which report an absence of compositional and structural changes in an airatmosphere. 2007 Curtin University of Technology and John Wiley & Sons, Ltd.

KEYWORDS: shells; pyrolysis; heat treatment; phosphate; lime

INTRODUCTION

Shellfish farming in New Zealand is a well-establishedindustry; the two commercial species of interest are thegreen lipped mussel (Perna canaliculus) and pacificoyster (Crassostrea gigas). Figures from the Ministryof New Zealand Fisheries indicate that the annual cropof mussels alone is around 67 000 tonnes. Processedshellfish are often sold without their shells, creatinglarge volumes of shell waste.

Shell disposal can be problematic due to currentenvironmental regulations in New Zealand, which limitthe disposal of shells back to the sea. The use oflandfills is limited by the fee that is charged on a pervolume basis and the requirement of some sites thatshells are free of organic matter. Traditional alternativesto landfill include producing low-value products fromshells such as aggregates for driveways and pathways.An alternative solution is to use heat processing toconvert the shells to lime (calcium oxide) for use in

*Correspondence to: M. I. Jones, Department of Chemical andMaterials Engineering, University of Auckland, Private Bag 92019,Auckland, New Zealand. E-mail: [email protected]

water treatment as a phosphate removal agent (Kwonet al ., 2004; Lee et al ., 2005). Waterways with highphosphate levels are a problem in New Zealand: around90% of lakes in the North Island are consideredeutrophic (Taylor and Smith, 1997).

In this study we investigate lime formation frommussel and oyster shells and use the treated shells toperform preliminary phosphate-removal tests on water.

MATERIALS AND METHODS

Materials

Shells from green-lipped mussels (P. canaliculus) andthe pacific oyster (C. gigas) were obtained fromCoromandel-based shellfish processors owned by San-fords, a New Zealand seafood company. These werestored in a freezer. Prior to use, the frozen shells werethawed and scrubbed clean. The shells were then driedin an oven, crushed in a ring mill and sieved. Crushedshell particles with size range of 300–600 µm wereused for experiments.

2007 Curtin University of Technology and John Wiley & Sons, Ltd.

Asia-Pacific Journal of Chemical Engineering PROCESSING SEAFOOD SHELLS FOR EUTROPHICATION CONTROL 461

Heat treatment and pyrolysis

A horizontal tube furnace was used for the treatment ofcrushed shells at 650, 750 and 800 ◦C. In this work, rawshells are those that were not treated in the furnace, heattreatment refers to the situation where heat is applied inan air atmosphere and pyrolysis where heat is appliedin a nitrogen atmosphere. Shell samples (10 g) wereplaced in the cold furnace and the heat source turnedon. Once the desired treatment temperature was reached,samples were left for 1 h in the furnace. Sampleswere then removed from the furnace and allowed tocool. For the pyrolysis treatments the furnace was firstevacuated before allowing nitrogen to return the furnaceto atmospheric pressure. A nitrogen flow of 0.2 l/minwas used to flush the furnace during the pyrolysistreatments.

Characterisation methods

X-ray diffraction (XRD)X-ray diffraction (XRD) analyses were performed usinga Bruker D8 Advance X-ray diffractometer operatingwith a 40-kV Cu Kα X-ray source. Scans were carriedout in the 2θ range 20 to 70◦ with a 0.02◦-step sizeand a 1-s step time. All figures with XRD spectra haveintensity (arbitrary units) plotted against 2θ .

Scanning electron microscopy (SEM)Scanning electron microscope (SEM) studies were per-formed using an FEI Quanta 200 FEG environmen-tal scanning electron microscope (5 kV acceleratingvoltage). Samples were sputter-coated with a plasma-applied platinum coating to prevent charging.

Phosphate removal experimentsPreliminary tests were conducted to assess the phos-phate removal rate from water with added shells. Foreach test, 2 l of phosphate solution (30 mg/l) was dosedwith 10 g of crushed shells. Experiments were car-ried out in a stirred vessel to ensure adequate mixing.Water samples were taken every 20 min and measuredfor phosphate content using a Perkin Elmer Lamda 35UV/Vis Spectrometer at 880 nm (APHA Ascorbic Acidmethod).

RESULTS AND DISCUSSION

The XRD spectra of crushed raw shells (Fig. 1) showthat the crystalline component of the untreated musselshell is aragonite, whereas the crystalline componentof the untreated oyster shell is calcite. Both aragoniteand calcite are different mineral forms of calciumcarbonate. The heat treatment of mussel shells at 650,

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Figure 1. XRD spectra for crushed raw shells: (a) mussel and(b) oyster. XRD spectral peaks corresponding to aragoniteand calcite are labelled with symbols, as indicated.

750 and 800 ◦C results in the formation of lime (calciumoxide); this is shown in the XRD spectra in Fig. 2.In addition, calcite was detected; this is not presentin the raw mussel shell. Pyrolysis of mussel shells at650 and 800 ◦C also results in the formation of calcite,although the formation of lime was not detected at650 ◦C (Fig. 3). The heat treatment of oyster shells at650, 750 and 800 ◦C also resulted in the formation oflime (Fig. 4). Lime was also formed during pyrolysisof the oyster shell (results not shown).

From the XRD results (Figs 2 and 4), an increase inthe furnace temperature results in an increase in limecontent. This is detected by the relative increase in theheight of the diffraction peaks associated with lime.The increase in lime content with furnace temperatureis supported by weight loss measurements, where thesample weight is measured before and after heat treat-ment/pyrolysis. The lime content can be calculated fromEqn (1) with the assumption that all the weight loss isdue to the evolution of carbon dioxide; results are shownin Fig. 5.

CaCO3(s) ⇀↽ CaO(s) + CO2(g) (1)

Shell samples with higher levels of lime also appearvisually lighter than samples treated at lower temper-atures (Fig. 6). It should be noted that the calculated

2007 Curtin University of Technology and John Wiley & Sons, Ltd. Asia-Pac. J. Chem. Eng. 2007; 2: 460–467DOI: 10.1002/apj

462 J. A. CURRIE ET AL. Asia-Pacific Journal of Chemical Engineering

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Figure 2. XRD spectra of mussel shell samples heat-treatedat: (a) 650 ◦C, (b) 750 ◦C and (c) 800 ◦C. XRD spectral peaksfor calcite and lime are labelled with symbols, as indicated.

lime content of the samples in Fig. 5 do not account forthe presence of organic compounds in the raw shells.Therefore these figures may be slightly high due to theadditional weight loss from the burning of the organics.The organic content of the shells was not determinedin this study, but subsequent work has shown that theorganic content of mussel shell can be estimated at6–7 wt%.

The SEM micrographs of the raw shells (Fig. 7) showthat despite sieving, fines adhered to both the musseland oyster particles. Upon pyrolysis at 650 ◦C (Fig. 8),both mussel and oyster shell particles show signs ofhaving undergone sintering, being rounder and havingless defined edges than the raw shell surface particles.

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Figure 3. XRD spectra of mussel shell samples pyrolysed at:(a) 650 ◦C and (b) 800 ◦C. XRD spectral peaks for calcite andlime are labelled with symbols, as indicated.

There are similar morphological changes that occurfor both mussel and oyster shell particles at particularfurnace temperatures during both pyrolysis and heattreatment. For brevity, we will only present SEMmicrographs of heat-treated oyster shell processed atdifferent furnace temperatures (Fig. 9). From Fig. 9 wecan see that sintering of the particles occurs, resultingin the coarsening of particles and densification of thesample. At higher processing temperatures there is anincrease in grain size. Although the SEM images do notgive information about the composition of the samples,they do indicate that there are structural changes thatoccur to the samples during heat treatment.

From the XRD analysis, it is clear that lime has beenproduced from heat-treating and pyrolysing both themussel and oyster shells. To gauge the effectivenessof the treated samples to remove phosphates fromwater, preliminary tests were conducted and the resultscompared to that achieved with raw shells. These resultsare shown in Fig. 10.

From our tests, the raw shells were able to remove upto around 40% of the phosphates from water, whereasboth the pyrolysed and heat-treated shells demonstratearound 90% of phosphate removal. In addition, most ofthe phosphate removal occurred within the first 30 minof the experiments.



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Figure 4. XRD spectra of oyster shell samples heat-treated at: (a) 650 ◦C, (b) 750 ◦C and (c) 800 ◦C. XRDspectral peaks for calcite and lime are labelled withsymbols, as indicated.

Figure 5. Lime content (expressed as a percentage of totalshell weight) vs furnace temperature for mussel and oystershell (heat-treated and pyrolysed). This figure is available incolour online at www.apjChemEng.com.

Given that XRD analysis has shown lime present inthe pyrolysed and heat-treated shells used in the phos-phate removal experiments, the mechanism for phos-phate removal from water can be demonstrated with thefollowing equilibria (Tchobanoglous and Burton, 1991),where the hydroxyapatite, Ca10(PO4)6(OH)2, precipi-tates out the soluble phosphates:

Ca(OH)2 + H2CO3 ⇀↽ CaCO3 + 2H2O (2)

Ca(OH)2 + Ca(HCO3)2 ⇀↽ 2CaCO3 + 2H2O (3)

10Ca2+ + 6PO4−3 + 2OH− ⇀↽ Ca10(PO4)6(OH)2(4)

At the end of the phosphate removal experiments,the formation of a gelatinous-looking ‘sludge’ wasobserved suspended in the phosphate solution anda build-up of ‘scum’ formed on the surface of thesolution. Figure 11 is an SEM micrograph of a driedsludge sample from phosphate removal experiments

Figure 6. Heat-treated shells. Mussel shells (top) and oyster shells (bottom).Treatment temperatures from left to right: 650, 750 and 800 ◦C.



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Figure 7. SEM micrographs of raw shell particlesurface: (a) mussel shell and (b) oyster shell.

using heat-treated oyster shell originally heated to atemperature of 800 ◦C. XRD analysis (Fig. 12) indicatesthat this sludge is comprised of mostly calcite andthat traces of hydroxyapatite may also be present. Aswould be expected, the presence of hydroxyapatiteis difficult to confirm from the XRD analysis owingto the small quantity of hydroxyapatite that wouldform, given the amount of available phosphate insolution. Characterisation of a dried scum sample bySEM and XRD (Figs 13 and 14) shows that the scumcomprises of calcite in the form of distinct crystals,this is different from the calcite morphology presentin the raw oyster shell (Fig. 7(b)) and in the sludge(Fig. 11). It is likely, given the morphology of thecalcite, that the scum results from an equilibriumreaction involving dissolution of carbon dioxide, whichis available from the atmosphere, to form calciumcarbonate. The corresponding mechanism would be as

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Figure 8. SEM micrographs of shells pyrolysedat 650 ◦C: (a) mussel shell and (b) oyster shell.

follows:

CaO + H2O ⇀↽ Ca(OH)2 (5)

Ca(OH)2 + CO2 ⇀↽ CaCO3 + H2O (6)

A sample of ‘used’ oyster shell (previously heat-treated at 800 ◦C) was taken from phosphate removal



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Figure 9. SEM micrographs of oyster shell particles heat-treated at: (a) 650 ◦C, (b) 750 ◦C and(c) 800 ◦C. The images are shown at low and high magnifications.

experiments and analysed by XRD. The XRD result(Fig. 15) shows that calcite and traces of calciumphosphate are present. This indicates that the limeoriginally present as a layer on the surface of theheat-treated shell has eroded, leaving the calcite coreexposed. In future work, we will investigate parametersfor increasing the layer of lime formed on the shellparticles to increase the overall lime production.

Our work shows that shell-lime is effective in phos-phate removal with crushed raw shells also able toremove phosphates, but to a lesser extent. With limenot present in the raw shells, it is likely that adsorption

is an important mechanism for phosphate removal. In astudy of the adsorption kinetics of phosphate removal(Namasivayam et al ., 2005), the authors found thatoyster shell powder (produced from raw shells usinghigh pressure steam at 234 ◦C) removed phosphates toa comparable, though slightly lower, level to the cal-cium carbonate powder. In that study, the initial pHof the solution had no effect on phosphate removalby adsorption; this differs from what we would expectwhen lime is used Eqns (2)–(4). Namasivayam et al .(Namasivayam et al ., 2005) reported that ‘raw oystershells’ did not remove any phosphates, however, from



our current work and the work done by others (Kwonet al ., 2004; Lee et al ., 2005), we see that phosphateremoval can occur. Namasivayam et al . (Namasivayamet al ., 2005) do not specify whether the ‘raw’ shellswere crushed and this may explain the different results,particularly as surface area is an important parameterfor adsorption.

In the work on crushed oyster shell waste by Kwonet al . (Kwon et al ., 2004), the best phosphate removal(98%) was achieved using oyster shell pyrolysed at750 ◦C for 1 h. The authors reported that oyster shellheat-treated in air at 750 ◦C achieved a 68% removalof phosphates and that raw oyster shell achieved onlyaround 10% removal. It is interesting that the authors’

Figure 10. Graph of phosphate removal vs time obtainedwith raw and heat-treated/pyrolysed shells. The raw shellparticles were sieved to be between 38–75 µm and all othersamples to between 300–600 µm. This figure is available incolour online at www.apjChemEng.com.

Figure 11. SEM micrograph of dried suspendedsludge obtained at the end of phosphate removalexperiments using oyster shell (originally heat-treated at 800 ◦C).

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Figure 12. XRD spectrum for dried suspended sludgeobtained at the end of phosphate removal experimentsusing oyster shell (originally heat-treated at 800 ◦C).

Figure 13. SEM micrograph of dried surfacescum obtained at the end of phosphate removalexperiments using oyster shell (originally heat-treated at 800 ◦C).

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Figure 14. XRD spectrum for surface scum collectedduring phosphate removal efficiency of shells. All peaks areassociated with calcite, a mineral form of calcium carbonate.



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Figure 15. XRD spectrum for used oyster shell (originallyheat-treated at 800 ◦C), obtained at the end of phosphateremoval experiments.

XRD results for the heat-treated sample at 750 ◦Cdid not detect the presence of lime – only calciumcarbonate, which was also present in the raw oystershell. In addition, the corresponding SEM micrographshowed no discernable difference between raw oystershell and the sample that was heat-treated at 750 ◦C,unlike our results (Figs 7 and 9). It is possible thatdifferences in the heating regime or the particle size ofthe crushed shells could explain the differing results, asthese are not specified by Kwon et al . (Kwon et al .,2004); further work investigating the effect of theseparameters on lime formation is required to betterunderstand this process.

CONCLUSIONS

We have demonstrated that it is possible to convert bothmussel and oyster shells into lime (calcium oxide) by

crushing the shells and applying heat in either a nitrogenor air atmosphere. Temperatures of 750 and 800 ◦Cwere most successful in producing lime. Preliminarytests indicate that shell-derived lime can remove around90% of phosphate from solution and that raw shells(comprised of calcium carbonate) can achieve up toaround 40% removal.

Acknowledgements

The authors would like to thank the following tech-nicians in the Department of Chemical and Materi-als Engineering, University of Auckland for their helpwith this work: Alec Asadov, Catherine Hobbis, AllanClendinning and Laura Liang.

REFERENCES

Kwon HB, Lee CW, Jun BS, Yun JD, Weon SY, Koopman B.Recycling waste oyster shells for eutrophication control. Resour.Conserv. Recycl. 2004; 41: 75–82.

Lee CW, Kwon HC, Jeon HP, Koopman B. Phosphate recoveryfrom water as hydroxyapatite with activated oystershell. Mater.Sci. Forum 2005; 486–487: 177–180.

Taylor R, Smith I. The State of New Zealand’s Environment. GPPublications: Wellington, 1997.

Tchobanoglous G, Burton FL. Wastewater Engineering: Treatment,Disposal and Reuse. McGraw Hill Book Company: Singapore,1991.

Namasivayam C, Sakoda A, Suzuki M. Removal of phosphate byadsorption onto oyster shell powder. J. Chem. Technol. Biotechnol.2005; 80: 356–358.


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