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Remoção de fosfatos em águas com recurso a conchas de bivalves
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Abstract(s)
A eutrofização resultante da presença excessiva de fosfatos em meios aquáticos constitui um dos principais desafios ambientais atuais, tornando essencial o desenvolvimento de soluções de tratamento sustentáveis e de baixo custo. Neste contexto, o presente trabalho avaliou o potencial de utilização de resíduos de conchas de bivalves, nomeadamente de ostra (Crassostrea spp.) e de mexilhão (Mytilus spp.), como adsorventes alternativos para a remoção de fosfatos em solução aquosa, que trazem a possibilidade de uma reutilização como fertilizantes para a agricultura. A metodologia integrou ensaios cinéticos de deposição química, caracterização morfológica e
composicional por microscopia eletrónica de varrimento com análise de dispersão de energia (SEM-EDS) e modelação matemática do processo. Os ensaios cinéticos evidenciaram a influência significativa da granulometria e do tipo de concha, revelando que a ostra fragmentada em partículas de 1–2 mm apresentou maior capacidade de remoção e taxas de deposição química mais rápidas. A caracterização SEM-EDS corroborou estes resultados, demonstrando que a ostra possui superfícies mais rugosas, heterogéneas e ricas em carbonato de cálcio, em contraste com as superfícies mais homogéneas e compactas observadas no mexilhão. O modelo matemático aplicado mostrou boa concordância com os dados experimentais (R² > 0,98), permitindo a extração de parâmetros cinéticos relevantes, área descoberta inicial, A₀, coeficiente global de transferência de massa, kₘₐₛₛₐ, e capacidade de deposição, γ, ainda que se reconheça o seu carácter simplista. De forma integrada, os resultados estabeleceram um padrão de desempenho claro, possível comprovar com as taxas de remoção, em que a concentração inicial para as conchas de mexilhão era de 47,64 mg/L, para a granulometria mais fina, e 47,77 mg/L para a granulometria maior, e para as conchas era de 46,34 mg/L, para a granulometria mais fina, e 50,07 mg/L para a granulometria maior: ostras 1–2 mm, com 90% de remoção, seguidas das ostras 4–8 mm, 75% de remoção, as quais apresentam resultados próximos às conchas de mexilhão 1–2 mm, 66% de remoção, enquanto os mexilhões de 4–8 mm, 57% de remoção, acabam por apresentar os resultados mais baixos, confirmando a ostra fragmentada em granulometria fina como o material mais promissor. Estes resultados validam a utilização de
resíduos de conchas como materiais adsorventes de baixo custo, contribuindo para a valorização de subprodutos da indústria aquícola e para o desenvolvimento de soluções sustentáveis no tratamento de águas contaminadas por fosfatos.
Eutrophication resulting from the excessive presence of phosphates in aquatic environments is one of today’s major environmental challenges, making the development of sustainable and low-cost treatment solutions essential. In this context, the present study evaluated the potential use of bivalve shell waste—specifically oyster (Crassostrea spp.) and mussel (Mytilus spp.) shells—as alternative adsorbent materials for the removal of phosphates from aqueous solutions, offering the added possibility of reuse as agricultural fertilizers. The methodology comprised kinetic tests of chemical deposition, morphological and compositional characterization by scanning electron microscopy coupled with energydispersive spectroscopy (SEM-EDS), and mathematical modeling of the process. The kinetic assays revealed a significant influence of both particle size and shell type, showing that oyster shells fragmented into 1–2 mm particles exhibited higher removal capacity and faster chemical deposition rates. SEM-EDS characterization corroborated these findings, showing that oyster shells have rougher, more heterogeneous surfaces rich in calcium carbonate, in contrast to the smoother and more compact surfaces observed in mussel shells. The applied mathematical model showed good agreement with the experimental data (R² > 0.98), allowing the extraction of relevant kinetic parameters—initial uncovered area (A₀), global mass transfer coefficient (kₘₐₛₛₐ), and deposition capacity (γ)—while acknowledging its simplified nature. Overall, the results established a clear performance pattern, as confirmed by the removal rates: the initial phosphate concentration for mussel shells was 47.64 mg/L for the finer granulometry and 47.77 mg/L for the coarser one, while for oyster shells it was 46.34 mg/L (fine) and 50.07 mg/L (coarse). The 1–2 mm oyster shells achieved 90% removal, followed by the 4–8 mm oyster shells with 75%, results close to those of the 1–2 mm mussel shells (66%), whereas the 4–8 mm mussel shells showed the lowest performance (57%). These findings confirm finely ground oyster shells as the most promising material. The results validate the use of shell waste as a low-cost adsorbent, contributing to the valorization of aquaculture byproducts and the development of sustainable solutions for the treatment of phosphatecontaminated waters.
Eutrophication resulting from the excessive presence of phosphates in aquatic environments is one of today’s major environmental challenges, making the development of sustainable and low-cost treatment solutions essential. In this context, the present study evaluated the potential use of bivalve shell waste—specifically oyster (Crassostrea spp.) and mussel (Mytilus spp.) shells—as alternative adsorbent materials for the removal of phosphates from aqueous solutions, offering the added possibility of reuse as agricultural fertilizers. The methodology comprised kinetic tests of chemical deposition, morphological and compositional characterization by scanning electron microscopy coupled with energydispersive spectroscopy (SEM-EDS), and mathematical modeling of the process. The kinetic assays revealed a significant influence of both particle size and shell type, showing that oyster shells fragmented into 1–2 mm particles exhibited higher removal capacity and faster chemical deposition rates. SEM-EDS characterization corroborated these findings, showing that oyster shells have rougher, more heterogeneous surfaces rich in calcium carbonate, in contrast to the smoother and more compact surfaces observed in mussel shells. The applied mathematical model showed good agreement with the experimental data (R² > 0.98), allowing the extraction of relevant kinetic parameters—initial uncovered area (A₀), global mass transfer coefficient (kₘₐₛₛₐ), and deposition capacity (γ)—while acknowledging its simplified nature. Overall, the results established a clear performance pattern, as confirmed by the removal rates: the initial phosphate concentration for mussel shells was 47.64 mg/L for the finer granulometry and 47.77 mg/L for the coarser one, while for oyster shells it was 46.34 mg/L (fine) and 50.07 mg/L (coarse). The 1–2 mm oyster shells achieved 90% removal, followed by the 4–8 mm oyster shells with 75%, results close to those of the 1–2 mm mussel shells (66%), whereas the 4–8 mm mussel shells showed the lowest performance (57%). These findings confirm finely ground oyster shells as the most promising material. The results validate the use of shell waste as a low-cost adsorbent, contributing to the valorization of aquaculture byproducts and the development of sustainable solutions for the treatment of phosphatecontaminated waters.
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Keywords
Deposição química Conchas de bivalves Fosfatos Mexilhão Ostra Tratamento de águas Chemical deposition Bivalve shells Phosphates Mussel Oyster Water treatment