Os efeitos do pH e da força iônica (I) sobre a adsorção de cobre (Cu) e a força direcional da reação em solos com cargas variáveis foram avaliados a partir de experimentos tipo "batch". Os resultados foram ajustados pelo modelo de Langmuir e, de acordo com as isotermas de adsorção, a afinidade de Cu (K L ) foi maior nas amostras do subsolo (0,061-0,468 L kg -1 ) do que na camada superficial (0,169-0,359 L kg -1 ). A adsorção máxima variou de 1114-2422 mg kg -1 (superfície) a 1002-1334 mg kg -1 (subsolo). Foi observada forte dependência da adsorção de Cu em relação ao pH nas amostras do subsolo, com nítido aumento da retenção de Cu (20-90%) no intervalo de pH do solo entre 4,0-5,0. A adsorção de Cu foi alterada pelo aumento da I e indicou diferentes mecanismos (esferas interna e externa) de retenção do metal. A reação foi favorável e espontânea, conforme indicado pelos valores negativos da variação da energia livre ( G) e pelo fator de separação K R < 1. A interface solo-solução e a adsorção de Cu foram termodinamicamente descritas por uma abordagem teórica. Adsorption reaction was favorable and spontaneous, as indicated by negative values of the free energy variation ( G) and the separation factor K R < 1. Soil-solution interface and Cu adsorption were also thermodynamically described by a theoretical approach. Effects of pH and ionic strength (I) on copper (Cu
-The objective of this work was to assess the effects of pH and ionic strength upon zinc adsorption, in three highly weathered variable charge soils. Adsorption isotherms were elaborated from batch adsorption experiments, with increasing Zn concentrations (0-80 mg L -1 ), and adsorption envelopes were constructed through soil samples reactions with 0.01, 0.1 and 1 mol L -1 Ca(NO 3 ) 2 solutions containing 5 mg L -1 of Zn, with an increasing pH value from 3 to 8. Driving force of reaction was quantified by Gibbs free energy and separation factor. Isotherms were C-, H-and L-type and experimental results were fitted to nonlinear Langmuir model. Maximum adsorption ranged from 59-810 mg kg ). Zinc adsorption was favorable and spontaneous, and showed sharply increase (20-90%) in the 4-6 pH range. No effect of ionic strength was observed at pH values below 5, because specific adsorption mechanisms predominated in the 3-5 pH range. Above pH 5, and in subsoil samples, Zn was adsorbed by electrostatic mechanisms, since ionic strength effect was observed. Despite depth and ionic strength effects, Zn adsorption depends mainly on the pH.Index terms: pH, adsorption envelope, adsorption isotherm, ionic strength, trace element. Adsorção de zinco em solos altamente intemperizadosResumo -O objetivo deste trabalho foi avaliar o efeito do pH e da força iônica sobre a adsorção de zinco, em três solos altamente intemperizados, com predomínio de cargas variáveis. A partir de experimentos tipo "batch", foram elaboradas isotermas de adsorção, com quantidades crescentes de Zn (0-80 mg L ).A adsorção de Zn foi favorável e espontânea, e mostrou nítido aumento (20-90%) no intervalo de pH entre 4 e 6. Não houve efeito da força iônica nos valores de pH abaixo de 5, o que indica o predomínio de mecanismos de adsorção específica na faixa de pH 3-5. Acima de pH 5 e no subsolo, o Zn foi adsorvido por mecanismos eletrostáticos, já que foi observado efeito da força iônica. Apesar dos efeitos da profundidade e da força iônica, a adsorção de Zn depende principalmente do pH.Termos para indexação: pH, envelope de adsorção, isoterma de adsorção, força iônica, elemento-traço.
The effects of pH and ionic strength (I)
Batch adsorption experiments were conducted to assess the effects of pH and ionic strength (I) on cadmium (Cd) adsorption by two Brazilian Oxisols. Adsorption envelopes were constructed through soil sample reactions with 0.01, 0.1, and 1 mol L 21 calcium nitrate [Ca(NO 3 ) 2 ] solutions containing 5 mg L 21 of Cd, with an increasing pH value from 3 to 8. The adsorption increased drastically with increasing pH, varying from 20 to 90% in a narrow pH range (4-6 in topsoil and 5-6 in subsoil). Gibbs energy (DG) for Cd adsorption was negative, and the phenomenon became more thermodynamically spontaneous with an increase in pH. Under the standard 0.01 mol L 21 I and at pH close to natural, the DG values ranged from 2796 to 23427 J mol 21 . No effect of I was observed on the DG values for Cd adsorption at pH values less than 6. At values greater than pH 6, sharp changes in the Cd adsorption pattern were observed on subsoil samples. The only soil attribute significantly correlated with the spontaneity of Cd adsorption was the effective cation exchange capacity, ECEC (r 5 0.97; p , 0.1).
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