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Utilização de resíduos agroindustriais como fonte de adsorventes: materiais à base de carbono e sílica para descontaminação de águas
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Abstract(s)
A crescente contaminação dos recursos hídricos representa uma ameaça tanto para a saúde humana como para os ecossistemas aquáticos. O presente estudo teve como objetivo a produção de adsorventes à base de sílica e carbono
para a remoção de contaminantes em águas residuais, com especial enfoque nos pesticidas. Na primeira etapa do trabalho foram produzidos e caracterizados seis adsorventes de sílica e um adsorvente de carbono. A sílica resultou de um processo de extração que incluiu trituração, secagem, digestão alcalina, neutralização e precipitação, utilizando diferentes matérias-primas (casca de arroz, palha de arroz, palha de trigo, cinzas domésticas e cinzas do Norte e Centro de Portugal). O maior rendimento mássico em sílica foi obtido a partir da casca de arroz (97 %). O adsorvente de carbono foi obtido por hidrocarbonização térmica da casca de arroz, às escalas laboratorial (8,5 h) e piloto (17 h), alcançando rendimentos mássicos ponderados de 5 % e 37 %, respetivamente, correspondendo a um rendimento mássico global de 33 %. A caracterização físico-química dos adsorventes, das matérias-primas e dos subprodutos
residuais foi realizada recorrendo a várias técnicas analíticas: Fluorescência de Raios X, Espetroscopia de Quebra Induzida por Laser, Espetroscopia de Infravermelho com Transformada de Fourier por Reflexão Total Atenuada, Espalhamento Dinâmico da Luz, Análise Termogravimétrica, Microscopia Eletrónica de Varrimento com Espetroscopia de raio-X com dispersão de energia e análise Brunauer-Emmett-Teller. Entre os principais resultados destacam-se:
• a sílica da casca de arroz, com 33 % de silício (XRF), nanopartículas de 43,7 nm e teor semelhante confirmado por SEM-EDS (35 %).
• a presença de sódio, introduzido durante a digestão alcalina (LIBS).
• grupos silanóis livres nas sílicas obtidas da casca de arroz, palha de arroz e palha de trigo (950 cm⁻¹, FTIR-ATR), sendo esta última a mais reativa (31 mmol/g, TGA).
• maior funcionalização do hidrocarbonizado com o aumento do tempo de reação, devido à maior presença de grupos carbonílicos e hidroxila e menor proporção de grupos aromáticos.
• potencial zeta mais negativo na sílica das cinzas do Norte de Portugal (-36,1 mV) e melhor uniformidade de partículas no hidrocarbonizado (Índice de Polidispersividade = 0,312).
• maior área superficial na sílica da palha de trigo (305,96 m²/g, BET).
Na segunda etapa do trabalho foi desenvolvido um método de extração e quantificação do pesticida clorpirifós em água desionizada. A extração líquido-líquido com n-hexano seguida de análise por GC-FPD apresentou boa precisão (CV = 3 %), limites de deteção e quantificação de 34 e 112 μg/L, respetivamente, e uma gama de trabalho entre 150 e 900 μg/L. As eficiências de extração foram consistentes (111 ± 4 %) para concentrações de 150 a 500 μg/L. Por fim, realizaram-se ensaios de adsorção utilizando a sílica da casca de arroz e o hidrocarbonizado. Este último apresentou melhor desempenho, com 80 % de remoção e capacidade de adsorção de 3608 μg/g (razão massa de adsorvente/volume de solução de 0,2 g/L; concentração inicial de 899 μg/L), em contraste com a sílica (57 % de remoção e 5,14 μg/g, para 100 g/L). O tempo de equilíbrio do hidrocarbonizado foi de 27 min. O modelo de Freundlich revelou-se o mais adequado para descrever o processo de equilíbrio de adsorção. Em síntese, os resultados demonstram o potencial de resíduos agroindustriais e domésticos como precursores de adsorventes sustentáveis para o tratamento de águas residuais contaminadas por pesticidas.
The growing contamination of water resources poses a threat to both human health and aquatic ecosystems. This study aimed to produce silica and carbon-based adsorbents for the removal of contaminants in wastewater, with a special focus on pesticides. In the first stage of the work, six silica adsorbents and one carbon-based adsorbent were produced and characterized. The silica was obtained through an extraction process that included grinding, drying, alkaline digestion, neutralization and precipitation, using different raw materials (rice husk, rice straw, wheat straw, domestic ash and ash from northern and central Portugal). The highest silica mass yield was obtained from rice husk (97 %). The carbon adsorbent was obtained by hydrothermal carbonization of rice husk, at laboratory (8,5 h) and pilot (17 h) scales, achieving mass weighted yields of 5 % and 37 %, respectively, corresponding to an overall mass yield of 33 %. The physico-chemical characterization of the adsorbents, raw materials and residual byproducts was performed using various analytical techniques: X-Ray Fluorescence, Laser Induced-Breakdown Spectroscopy, Fourier Transform Infrared Spectroscopy-Attenuated Total Reflectance, Dynamic Light Scattering, Thermogravimetric Analysis, Scanning Electron Microscopy-Energy Dispersive Spectroscopy, and Brunauer-Emmett-Teller analysis. Key results include: • rice husk silica with 33 % silicon (XRF), nanoparticles with size 43,7 nm and a similar content confirmed by SEM-EDS (35 %). • the presence of sodium, introduced during alkaline digestion (LIBS). • free silanol groups in silica from rice husk, rice straw and wheat straw (950 cm⁻¹, FTIRATR), being the latter the most reactive (31 mmol/g, TGA). • greater functionalization of the hydrochar with the increase of reaction time, resulting in more carbonyl and hydroxyl groups and a lower proportion of aromatic groups. • more negative zeta potential in silica from the ashes of Northern Portugal (-36,1 mV) and a better particle uniformity in the hydrochar (Polydispersity Index = 0.312). • largest surface area in wheat straw silica (305,96 m²/g, BET). In the second phase, a method for removing and quantifying the pesticide chlorpyrifos, in deionized water, was developed. Liquid-liquid extraction with n-hexane followed by GC-FPD analysis yielded good results (CV = 3 %), with limits of detection and quantification of 34 and 112 μg/L, respectively, and a range of work between 150 and 900 μg/L. Removal efficiencies were consistent (111 ± 4 %) for concentrations ranging from 150 to 500 μg/L. Finally, adsorption tests were performed using rice husk silica and hydrochar. The latter showed better performance, with 80 % removal yield and an adsorption capacity of 3608 μg/g (adsorbent mass/volume of solution ratio of 0,2 g/L; initial concentration of 899 μg/L), in contrast to silica (57 % removal yield and 5,14 μg/g, for 100 g/L). The hydrochar equilibrium time was 27 min. The Freundlich model proved to be the most appropriate to describe the equilibrium adsorption process. In summary, the results show the potential of agro-industrial and domestic waste as precursors for sustainable adsorbents for the treatment of pesticide-contaminated wastewater.
The growing contamination of water resources poses a threat to both human health and aquatic ecosystems. This study aimed to produce silica and carbon-based adsorbents for the removal of contaminants in wastewater, with a special focus on pesticides. In the first stage of the work, six silica adsorbents and one carbon-based adsorbent were produced and characterized. The silica was obtained through an extraction process that included grinding, drying, alkaline digestion, neutralization and precipitation, using different raw materials (rice husk, rice straw, wheat straw, domestic ash and ash from northern and central Portugal). The highest silica mass yield was obtained from rice husk (97 %). The carbon adsorbent was obtained by hydrothermal carbonization of rice husk, at laboratory (8,5 h) and pilot (17 h) scales, achieving mass weighted yields of 5 % and 37 %, respectively, corresponding to an overall mass yield of 33 %. The physico-chemical characterization of the adsorbents, raw materials and residual byproducts was performed using various analytical techniques: X-Ray Fluorescence, Laser Induced-Breakdown Spectroscopy, Fourier Transform Infrared Spectroscopy-Attenuated Total Reflectance, Dynamic Light Scattering, Thermogravimetric Analysis, Scanning Electron Microscopy-Energy Dispersive Spectroscopy, and Brunauer-Emmett-Teller analysis. Key results include: • rice husk silica with 33 % silicon (XRF), nanoparticles with size 43,7 nm and a similar content confirmed by SEM-EDS (35 %). • the presence of sodium, introduced during alkaline digestion (LIBS). • free silanol groups in silica from rice husk, rice straw and wheat straw (950 cm⁻¹, FTIRATR), being the latter the most reactive (31 mmol/g, TGA). • greater functionalization of the hydrochar with the increase of reaction time, resulting in more carbonyl and hydroxyl groups and a lower proportion of aromatic groups. • more negative zeta potential in silica from the ashes of Northern Portugal (-36,1 mV) and a better particle uniformity in the hydrochar (Polydispersity Index = 0.312). • largest surface area in wheat straw silica (305,96 m²/g, BET). In the second phase, a method for removing and quantifying the pesticide chlorpyrifos, in deionized water, was developed. Liquid-liquid extraction with n-hexane followed by GC-FPD analysis yielded good results (CV = 3 %), with limits of detection and quantification of 34 and 112 μg/L, respectively, and a range of work between 150 and 900 μg/L. Removal efficiencies were consistent (111 ± 4 %) for concentrations ranging from 150 to 500 μg/L. Finally, adsorption tests were performed using rice husk silica and hydrochar. The latter showed better performance, with 80 % removal yield and an adsorption capacity of 3608 μg/g (adsorbent mass/volume of solution ratio of 0,2 g/L; initial concentration of 899 μg/L), in contrast to silica (57 % removal yield and 5,14 μg/g, for 100 g/L). The hydrochar equilibrium time was 27 min. The Freundlich model proved to be the most appropriate to describe the equilibrium adsorption process. In summary, the results show the potential of agro-industrial and domestic waste as precursors for sustainable adsorbents for the treatment of pesticide-contaminated wastewater.
Description
Keywords
adsorbent OPPs silica hydrochar sustainability hidrocarbonizado sílica pesticida cromatografia gasosa adsorção