Valorization of tangerine peels in the preparation of adsorbents for removal of Ni(II) from aqueous solutions

Mestrado de dupla diplomação com a UTFPR - Universidade Tecnológica Federal do Paraná

The discharge of industrial and domestic wastewater is one of the main causes of contamination of water bodies by heavy metals. Among these heavy metals, nickel (Ni) is one of the most used in industrial processes, such as the production of alloys, stainless steel and automotive batteries, as well as electroplating activities, causing high concentrations in the effluents of these industries. Several techniques, such as chemical precipitation and filtration, have been studied in order to promote the removal of these heavy metals from wastewater. However, these techniques are often financially and technically costly. Under this context, adsorption using activated carbons (ACs) appears as an alternative, which is characterized by being an effective and economically viable technique. Citrus fruits, such as tangerine, are of great importance in the Portuguese economic context. In 2017, just over 374.4 thousand tons were produced, with 10% of this volume corresponding to tangerine. This research aimed, therefore, to produce ACs from tangerine peels and use them in the adsorption of nickel from aqueous medium. The ACs were produced in two stages: activation and carbonization. Upon activation, the hydrothermal carbonization process (HTC) was used considering FeCl3 as activating agent under three concentrations (0.5, 1.0 and 2.5 M). Afterwards, the prepared materials were carbonized in a tubular oven at 800 ºC for 4 h. The samples were named as follows: TW-C (pyrolyzed peels without HTC), TW-Fe-0.5-C / TW-Fe-1.0-C / TW-Fe-2.5-C (HTC and pyrolyzed peels) and TW-Fe-2.5 (peels with HTC without pyrolysis). The physico-chemical properties of the ACs, such as elemental analysis, ash content, acidity and basicity, were determined to correlate with their performance. The kinetic and isotherm adsorption of Ni(II) onto the ACs was assessed at the following operating conditions: 2.5 g L-1 of AC, 25 ºC and pH 6 and modelled by the kinetic models of pseudo-first order, pseudo-second order and Elovich, as well as the adsorption isotherms of Langmuir, Freundlich and the General Isotherm Equation (GIE) of Tóth. The materials TW-C showed the best adsorption results and removed 99% of Ni(II) at pH 9. The kinetic models that best described the adsorption process were pseudo-second order (TW-Fe-2.5-C) and Elovich (TW-C). The GIE of Tóth showed the best fit, however, its parameters did not show statistical significance at the 5% confidence level, being rejected. The Freundlich model was able to represent the experimental data with certain precision (R² reached 0.9557 and 0.9785 for TW-C and TW-Fe-2.5-C, respectively) and statistical significance according to the t-test and F-test.

A descarga de águas residuais industriais e domésticas é uma das principais causas de contaminação de corpos d'água por metais pesados. Entre esses metais pesados, o níquel (Ni) é um dos mais utilizados em processos industriais, como na produção de ligas metálicas, aço inoxidável e baterias de automóveis, bem como na galvanoplastia, originando altas concentrações nos efluentes dessas indústrias. Várias técnicas, como precipitação química e filtração, foram estudadas para promover a remoção desses metais pesados de águas residuais. No entanto, essas técnicas são financeiramente e tecnicamente caras. Como alternativa surge a adsorção utilizando carvão ativado (AC), caracterizada por ser uma técnica eficaz e economicamente viável. Os citrinos, como a tangerina, apresentam relevada importância no contexto económico português. No ano de 2017 foram produzidas pouco mais de 374,4 mil toneladas, onde 10% desse volume correspondeu a tangerina. Tendo este enquadramento em consideração, este trabalho objetivou produzir ACs a partir de cascas de tangerina para aplicação na adsorção de níquel presente em meio aquoso. Os ACs foram produzidos em duas etapas: Ativação e Carbonização. Na ativação foi usado o processo de carbonização hidrotérmica (HTC) considerando FeCl3 como agente ativante em três concentrações (0,5; 1,0 e 2,5 M); posteriormente, os materiais preparados foram carbonizados em forno tubular a 800 ºC por 4 h. As amostras foram nomeadas da seguinte forma: TW-C (cascas pirolisadas sem HTC), TW-Fe-0,5-C / TW-Fe-1,0-C / TW-Fe-2,5-C (HTC e cascas pirolisadas) e TW-Fe-2,5 (cascas com HTC sem pirólise). As propriedades físico-químicas dos ACs foram determinadas por análise elementar, teor de cinzas, acidez e basicidade, para correlacionação com o seu desempenho no processo de adsorção. A cinética de adsorção e isotérmicas de adsorção de Ni(II) nos ACs foi avaliada nas seguintes condições operacionais: 2,5 g L-1 de AC, 25 ºC e pH 6, aplicando os modelos cinéticos de pseudo-primeira ordem, pseudo-segunda ordem e Elovich, bem como as isotérmicas de adsorção de Langmuir, Freundlich e a Equação Geral da Isotérmica (GIE) de Tóth. O carvão TW-C apresentou os melhores resultados de adsorção e removeu 99% de Ni(II) a pH 9. Os modelos cinéticos que melhor descreveram o processo de adsorção foram o de pseudo-segunda ordem (TW-Fe-2.5-C) e Elovich (TW-C). O GIE de Tóth apresentou o melhor ajuste, no entanto, os seus parâmetros não apresentaram significância estatística no nível de confiança de 5%, sendo rejeitado. O modelo de Freundlich foi capaz de representar os dados experimentais com certa precisão (R² atingiu 0,9557 e 0,9785 para TW-C e TW-Fe-2,5-C, respectivamente) e significância estatística de acordo com o teste t e o teste F.

First of all, I would like to thank my parents, Edson and Claudete, without whom I would not be given so many opportunities, I thank you for the support and encouragement during all these years of graduation and for sharing with me the challenges and difficulties faced away from home. I thank my brother for the partnership, complicity, for being present even with the distance. I thank my friends Pedro and Larissa, for their support during this stage and for their friendship over the years. To my supervisor, teacher and friend Tatiane, for all the help, advice and opportunities during the undergraduate years and later with the master's degree. I am grateful to my co-supervisor José, who was fundamental in the execution of this work and for whom I learned to cultivate respect and admiration. I thank my supervisor, Professor Helder, for the opportunity and confidence. My laboratory friends Fernanda, Adriano and Gabriel, who shared their knowledge and helped me with the execution of my practical work. I thank Maria João, technician at the Chemical Process Laboratory, for being helpful and patient. I would like to thank Professor Margarida and the Mountain Research Center (CIMO) for their assistance with my analysis. I am grateful to the Foundation for Science and Technology (FCT, Portugal) for financial support by national funds FCT/MCTES to CIMO (UIDB/00690/2020).