No abstract

Recebido em 2/10/03; aceito em 30/1/04; publicado na web em 17/6/04 HEAVY METALS REMOVAL FROM WASTEWATER BY THE NATURAL ZEOLITE SCOLECITE -TEMPERATURE AND pH INFLUENCE IN SINGLE-METAL SOLUTIONS. Cation exchange capabilities of a Brazilian natural zeolite, identified as scolecite, were evaluated for application in wastewater control. We investigated the process of sorption of chromium(III), nickel(II), cadmium(II) and manganese(II) in synthetic aqueous effluents, including adsorption isotherms of single-metal solutions. The natural zeolite showed the ability to take up the tested heavy metals in the order Cr(III) > Cd(II) > Ni(II) > Mn(II), and this could be related to the valence and the hydration radius of the metal cations. The influence of temperature (25, 40 and 60 o C) and initial pH value (from 4 to 6) was also evaluated. It was found that the adsorption increased substantially when the temperature was raised to 60 o C and that maximum adsorption capacity was observed at pH 6. These results demonstrate that scolecite can be used for removal of heavy metals from aqueous effluents, under optimized conditions. Keywords: cation exchange; heavy metals; scolecite. INTRODUÇÃOA atividade industrial tem contribuído muito para um aumento significativo nas concentrações de íons metálicos em águas, representando uma importante fonte de contaminação dos corpos aquáti-cos, principalmente quando consideramos que tais íons podem ser disseminados via cadeia alimentar. Dos 2,9 milhões de toneladas de resíduos industriais perigosos gerados anualmente no Brasil, somente 850 mil toneladas recebem tratamento adequado, conforme estimativa da Associação Brasileira de Empresas de Tratamento, Recuperação e Disposição de Resíduos Especiais (ABETRE). Os 72% restantes são depositados indevidamente em lixões ou descartados em cursos d'água sem qualquer tipo de tratamento 1 . Metais pesados em excesso podem causar muitas doenças e séri-os problemas fisiológicos, já que são acumulativos no corpo humano. Os resíduos contendo cádmio, cromo, manganês e níquel possuem alto poder de contaminação e, com facilidade, atingem os lençóis freáticos ou mesmo reservatórios e rios, que são as fontes de abastecimento de água das cidades. O contato com a pele pode causar dermatite alérgica e, mais raramente, provocar ulcerações na pele formando cicatrizes, perfurações do septo nasal, câncer, distúrbios afetivos, irritação neuromuscular, cefaléia, náuseas e desmaios. Há também suspeitas de que possam afetar o sistema imunológico de seres humanos 2 . Os efluentes líquidos industriais apresentam composição quími-ca bastante complexa, contendo compostos orgânicos e inorgânicos. O método mais utilizado no tratamento de efluentes líquidos contendo matéria orgânica é o biológico. No entanto, a presença de compostos inorgânicos pode inibir este processo. Neste caso, é necessá-rio realizar uma etapa de tratamento químico antes da realização do tratamento biológico, visando a remoção desta carga inorgânica. O tratamento clássico de efluentes contendo metais pes...

The adsorption capacities of commercial and Brazilian natural clays were evaluated to test their applications in wastewater control. We investigated the process of sorption of manganese(II) and cadmium(II) present in synthetic aqueous effluents, by calculating the adsorption isotherms at 298 K using batch experiments. The influence of temperature and pH on the adsorption process was also studied. Adsorption of metals was best described by a Langmuir isotherm, with values of Q 0 parameter, which is related to the sorption capacity, corresponding to 6.3 mg g −1 for K-10/Cd(II), 4.8 mg g −1 for K-10/Mn(II), 11.2 mg g −1 for NT-25/Cd(II) and 6.0 mg g −1 for NT-25/Mn(II). We observed two distinct adsorption mechanisms that may influence adsorption. At the first 5 min of interaction, a cation exchange mechanism that takes place at exchange sites located on (001) basal planes is predominant. This process is inhibited by low pH values. After this first and fast step, a second sorption mechanism can be related to formation of inner-sphere surface com--970. plexes, which is formed at edges of the clay. The rate constants and the initial sorption rates correlate positively with temperature in all studied systems, denoting the predominance of a physisorption process. The addition of complexing agents that are incorporated within the K10 structure, enhance metal uptake by the adsorbent. The results have shown that both Cd(II) and Mn(II) were totally retained from a 50 mg L −1 solution when K10 grafted with ammonium pyrrolidinedithiocarbamate (APDC) was used as adsorbent.