ABSTRACT
ABSTRACT Although nickel (Ni) has both important potential benefits and toxic effects in the environment, its behavior in tropical soils has not been well studied. Nickel adsorption-desorption in topsoil and subsoil samples of an acric Oxisol was studied at three pH values (from 3.0 to 8.0). Adsorption-desorption isotherms were elaborated from experiments with increasing Ni concentration (5 to 100 mg L-1), during 0, 4, and 12 weeks, using CaCl2 0.01 and 0.1 M as electrolytic support in order to also verify the effect of Ni-soil time contact and of ionic strength on the reaction. Experimental results of Ni adsorption fitted Langmuir model, which indicated that maximum Ni adsorption (71,440 mg kg-1) occurred at subsoil, after 12 weeks. Nickel affinity (KL) was also greater at subsoil (1.0 L kg-1). The Ni adsorption in the topsoil samples was higher, due to its lower point of zero salt effect (PZSE) and higher organic matter content. The increase in soil pH resulted in the increase of Ni adsorption. Nickel desorbed less from soil samples incubated for 4 or 12 weeks, suggesting that Ni interactions with colloidal particles increase over time. The amount of Ni desorbed increased with increasing ionic strength in both the topsoil and subsoil soil samples. Finally, adsorption-desorption hysteresis was clearly observed. Soil pH, ionic strength of soil solution and the Ni-soil contact time should be considered as criteria for selecting the areas for disposal of residues containing Ni or to compose remediation strategies for acric soils contaminated with Ni.
RESUMO Apesar de o níquel (Ni) ser um elemento importante no ambiente, tanto pelo seu potencial benéfico, como tóxico, informações sobre o seu comportamento em solos tropicais são escassas. Reações de adsorção-dessorção de Ni em amostras superficias e subsuperficias de um Latossolo ácrico foram estudadas em três valores de pH do solo, variando de 3,0 a 8,0. Isotermas de adsorção-dessorção foram elaboradas a partir de experimentos com concentrações crescentes de Ni (5 a 100 mg L-1), durante 0, 4 e 12 semanas, usando CaCl2 0.01 e 0.1 M como eletrólito suporte para verificar o efeito do tempo de contato e da força iônica sobre a reação. Resultados experimentais foram adequadamente ajustados pelo modelo de Langmuir, o qual apontou que a máxima adsorção de Ni (71.440 mg kg-1) ocorreu após 12 semanas de contato do Ni com amostras do subsolo. A afinidade do Ni também foi maior pelas amostras de subsolo (1,0 L kg-1). A adsorção de Ni foi maior nas amostras superficiais, devido ao menor ponto de efeito salino nulo e ao maior tero de matéria orgânica. A elevação do pH do solo aumentou a adsorção de Ni. Houve menos dessorção de Ni das amostras de solo incubadas por 4 e 12 semanas, sugerindo que as interações entre o Ni e as partículas do solo aumentam ao longo do tempo. A quantidade de Ni dessorvido aumentou com o aumento da força iônica, independentemente da profundidade de amostragem do solo. Finalmente, a histerese da reação de adsorção-dessorção de Ni foi claramente observada. O pH do solo, a força iônica da solução e o tempo de contato do Ni com o solo devem ser considerados critérios para a seleção de áreas para a disposição de resíduos contendo Ni e para compor estratégias de remediação para solos ácricos contaminados por Ni.
ABSTRACT
The effects of pH and ionic strength (I) on Ni adsorption in variable charge soils were evaluated by laboratory batch experiments. Experimental results fitted the Langmuir model. Maximum adsorption (Ads max) ranged from 260-2818 mg kg-1 (topsoil) to 532-1541 mg kg-1 (subsoil). Nickel affinity (K L) was higher in the subsoil samples (0.022-0.236 L kg-1) than in topsoil (0.003-0.049 L kg-1). Adsorption envelopes showed sharp increase in Ni adsorption (20-90 percent) in the 4.0-6.0 pH range. Nickel adsorption was affected by I and specific adsorption predominated in the 3.0-5.0 pH range, while above pH 5.0, it was adsorbed by electrostatic mechanisms. Negative values of free energy variation (ΔG) and the separation factor K R<1 indicated that Ni adsorption reaction was favorable and occurred spontaneously, especially when pH increase. Chemical and mineralogical soil attributes should be considered as criteria for selecting the areas for disposal of residues containing Ni to minimize the impact on the environment.