Mecanismos de remoção de cromo (VI) em solução por zeólita e vermiculita pré-tratadas com ferro (II)
Data
2021-07-08
Autores
Rosa, Maria Isabel Garcia 
Orientadores
Shinzato, Mirian Chieko
Tipo
Dissertação de mestrado
Título da Revista
ISSN da Revista
Título de Volume
Resumo
As atividades industriais podem causar a contaminação de solos e águas ao despejar efluentes contendo metais, como o cromo hexavalente (Cr(VI)), que é altamente tóxico à saúde humana e animal. Nesse contexto, o presente trabalho avaliou a capacidade da zeólita e vermiculita pré-tratadas com Fe(II) em reduzir o Cr(VI) em solução. Os minerais naturais foram pré-tratados com Fe(II) em diferentes condições de temperatura (ambiente e 40 oC), tempo (1 e 24 h) e concentração de Fe(II) (0,5 e 1 mol L-1). Os minerais pré-tratados com Fe(II) selecionados de acordo com sua eficiência de remoção de Cr(VI) foram utilizados em testes de remoção de Cr(VI) em batelada para estudar os efeitos do tempo e da concentração. Em seguida, as concentrações de Cr(VI), Fe(II), K+, e Cr(III) foram analisadas nas soluções e nos adsorventes, a fim de avaliar os principais mecanismos envolvidos no processo de remoção de Cr(VI). Os resultados do pré-tratamento revelaram que a vermiculita adsorveu mais Fe(II) (21,8 mg g-1) do que a zeólita (15,1 mg g-1). Em testes de remoção de Cr(VI) ambos adsorventes pré-tratados liberaram Fe(II) que reduziu 100% de Cr(VI) em menos de 1 minuto, de uma solução contendo 20 mg L-1 Cr(VI). Considerando que 3 mols de Fe(II) são necessários para reduzir 1 mol de Cr(VI) (3Fe+2 + Cr+6 → 3Fe+3 + Cr+3), o teor de ferro liberado por Z-Fe e V-Fe foi suficiente para reduzir 100% de Cr(VI) em solução com até 50 mg L-1 Cr(VI), e cerca de 90% (V-Fe) e 95% (Z-Fe) em solução com 100 mg L-1 Cr(VI). Os mecanismos de remoção de Cr(VI) envolveram a troca catiônica entre Fe+2 de Z-Fe e K+ da solução de dicromato de potássio, e entre Fe+2 de V-Fe e H+ da solução ácida (pH em torno de 5). Em seguida, o Fe+2 liberado pela troca iônica reduziu o Cr(VI) a Cr(III), que assim como o ferro oxidado (Fe(III)), foi removido da solução por precipitação. No entanto, quando o pH da solução ficou mais ácido (< 5), o processo de precipitação diminuiu e, consequentemente, parte do Cr(III) permaneceu em solução. A adsorção de Cr(III) por Z-Fe e V-Fe não foi favorecida devido à preferência de seus sítios de troca por outros íons presentes na solução, como K+ e H+, respectivamente. Dessa forma foi possível observar que tanto a zeólita quanto a vermiculita tratadas com Fe(II) são capazes de reduzir Cr(VI) da solução, cuja eficiência pode ser influenciada pelos cátions presentes no meio.
Industrial activities can cause soil and water contamination by accidentally dumping effluents containing metals, such as hexavalent chromium (Cr(VI)), which is highly toxic to human and animal health. In this context, the present work evaluated the ability of zeolite and vermiculite pretreated with Fe(II) to reduce Cr(VI) in solution. Minerals pretreated with Fe(II) were used in Cr(VI) removal batch tests to study the effects of time and concentration in this process. Then, the concentrations of Cr(VI), Fe(II), K+, and Cr(III) were analyzed in the solutions and in the adsorbents, in order to evaluate the main mechanisms involved in the Cr(VI) removal process. Pretreatment results revealed that vermiculite adsorbed more Fe(II) (21.8 mg g-1) than zeolite (15.1 mg g-1). In Cr(VI) removal tests both pretreated adsorbents released Fe(II) which reduced 100% of Cr(VI) in less than 1 minute from a solution containing 20 mg L-1 Cr(VI). Considering that 3 moles of Fe(II) are needed to reduce 1 mole of Cr(VI) (3Fe+2 + Cr+6 → 3Fe+3 + Cr+3), the iron content released by Z-Fe and V-Fe was enough to reduce 100% of Cr(VI) in solution with up to 50 mg L-1 Cr(VI), and about 90% (V-Fe) and 95% (Z-Fe) in solution with 100 mg L-1 Cr(VI). The Cr(VI) removal mechanisms involved the cation exchange between Fe+2 from Z-Fe and K+ from the potassium dichromate solution, and between Fe+2 from V-Fe and H+ from the acidic solution (pH around 5). Then, the Fe+2 released by ion exchange reduced the Cr(VI) to Cr(III), which, like the oxidized iron (Fe(III)), was removed from the solution by precipitation. However, when the pH of the solution became more acidic (< 5), the precipitation process decreased and, consequently, part of the Cr(III) remained in solution. Adsorption of Cr(III) by Z-Fe and V-Fe was not favored due to the preference of their exchange sites for other ions present in the solution, such as K+ and H+, respectively. To prevent Cr(III) oxidation, it can be removed from the solution by precipitation (correcting pH > 5) or by adsorption using untreated vermiculite. Finally, the ions adsorbed by zeolite and vermiculite can be recovered by cation exchange, enabling the reuse of these adsorbents for subsequent removal of Cr(VI).
Industrial activities can cause soil and water contamination by accidentally dumping effluents containing metals, such as hexavalent chromium (Cr(VI)), which is highly toxic to human and animal health. In this context, the present work evaluated the ability of zeolite and vermiculite pretreated with Fe(II) to reduce Cr(VI) in solution. Minerals pretreated with Fe(II) were used in Cr(VI) removal batch tests to study the effects of time and concentration in this process. Then, the concentrations of Cr(VI), Fe(II), K+, and Cr(III) were analyzed in the solutions and in the adsorbents, in order to evaluate the main mechanisms involved in the Cr(VI) removal process. Pretreatment results revealed that vermiculite adsorbed more Fe(II) (21.8 mg g-1) than zeolite (15.1 mg g-1). In Cr(VI) removal tests both pretreated adsorbents released Fe(II) which reduced 100% of Cr(VI) in less than 1 minute from a solution containing 20 mg L-1 Cr(VI). Considering that 3 moles of Fe(II) are needed to reduce 1 mole of Cr(VI) (3Fe+2 + Cr+6 → 3Fe+3 + Cr+3), the iron content released by Z-Fe and V-Fe was enough to reduce 100% of Cr(VI) in solution with up to 50 mg L-1 Cr(VI), and about 90% (V-Fe) and 95% (Z-Fe) in solution with 100 mg L-1 Cr(VI). The Cr(VI) removal mechanisms involved the cation exchange between Fe+2 from Z-Fe and K+ from the potassium dichromate solution, and between Fe+2 from V-Fe and H+ from the acidic solution (pH around 5). Then, the Fe+2 released by ion exchange reduced the Cr(VI) to Cr(III), which, like the oxidized iron (Fe(III)), was removed from the solution by precipitation. However, when the pH of the solution became more acidic (< 5), the precipitation process decreased and, consequently, part of the Cr(III) remained in solution. Adsorption of Cr(III) by Z-Fe and V-Fe was not favored due to the preference of their exchange sites for other ions present in the solution, such as K+ and H+, respectively. To prevent Cr(III) oxidation, it can be removed from the solution by precipitation (correcting pH > 5) or by adsorption using untreated vermiculite. Finally, the ions adsorbed by zeolite and vermiculite can be recovered by cation exchange, enabling the reuse of these adsorbents for subsequent removal of Cr(VI).