Adding value to waste from the aquaculture industry: the development of green processing technologies, characterization, and applications of waste blue mussel shells

Murphy, Jennifer Nicole (2019) Adding value to waste from the aquaculture industry: the development of green processing technologies, characterization, and applications of waste blue mussel shells. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Mussels (Mytilus edulis) are a high protein food grown using aquacultural methods around the world. By-product streams from this industry include raw discards (small or damaged product) and cooked shells that retain their posterior adductor muscle. The shell by-product streams from aquaculture could be used in a range of applications but shell storage without protein removal is problematic. An environmentally friendly shell cleaning protocol was developed using two industrial available food grade enzymes (Multifect PR 6L and PR 7L) to remove protein from mussel shells and is described in Chapter 2. This process was optimized using a Design of Experiments approach and could be performed in seawater or tap water using raw or cooked mussels. This method provides two product streams: a biorenewable calcium carbonate and protein hydrolysate. Characterization of the shells, including by ¹H MAS NMR spectroscopy, is described in Chapter 3. Heat treatment of blue mussel shells yields four CaCO₃ materials with differing ratios of aragonite and calcite. Heat treatment (220 °C, 48 h) of the shells caused a large decrease in organic matrix levels as shown by ¹H MAS NMR spectroscopy and this reduction in matrix content leads to a simple way to separate the prismatic calcite layer from the nacreous layer, allowing easy isolation of natural platelets of nacre. Using mussel protein hydrolysate from the shell cleaning protocol as an additive, synthetic nacre was prepared using a CaCl₂ and Na₂CO₃ mixing method. The preparation of calcium acetate from mussel shell materials and acetic acid is described in Chapter 4. An exploratory central composite design compared the yield of Ca(CH₃COO)₂ with respect to shell material used, concentration of CH₃COOH, and time. The yield of Ca(CH₃COO)₂·H₂O was optimized further using heated, crushed shells using a custom optimal design. A maximum yield of 93% was reached after 32 h using 9% CH₃COOH and food-grade white vinegar gave an 85% yield of Ca(CH₃COO)₂·H₂O after 24 h. De-icing experiments showed that the Ca(CH₃COO)₂·H₂O produced from waste blue mussel shells melted 10-13 wt.% of ice in 15 min at concentrations of 20-30% (m/v) at –16 °C. During the synthesis of calcium acetate, a new material was discovered (Chapter 5), which was formed by the reassembly of calcite prisms held together by the organic matrix ‘glue’ as evidenced by NMR spectroscopy. This self-assembled calcite (SAC) material has a nest-like morphology and can absorb 10 times its mass in water. This inorganic sponge was used to adsorb dyes from aqueous solution (24 wt% of fabric dye) and absorb crude oil. SAC had an average crude oil absorption capacity of 978% +/- 84.3% with no consequence on crude oil absorption over 10 cycles of re-use. The SAC material was also used as an inorganic filler in ionic liquid gel polymer electrolytes (Chapter 6). The addition of SAC resulted in mechanically and thermally stable films with good conductivity and large electrochemical window. Cell capacitors fabricated using this material had very good capacitance, 110 F/g at a current density of 2 A/g. The maximum energy density and power density of the device containing SAC was 28 Wh/kg and 8.1 kW/kg at 0.5 and 10 A/g, respectively.

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