RESUMEN
The immobilization of lead ions by a vermicompost with calcite added was evaluated by adsorption isotherms and the results were explained on basis of the pH dependent surface charge and by IR spectroscopy. The results showed maximum adsorption values between 113.6 mg g(-1) (33 degrees C) and 123.5mg g(-1) (50 degrees C). The point of zero net charge (PZC) was 7.5+/-0.1, indicating the presence of a positive surface charge at the pH of batch experiments. The differences in the IR spectra at pH 3.8 and 7.0 in the region from 1800 to 1300 cm(-1), were interpreted on the basis of the carboxyl acid ionization, that reduced the band intensity around 1725 cm(-1), producing signals at 1550 cm(-1) and 1390 cm(-1) of carboxylate groups. Similar changes were detected at pH 3.8 when Pb2+ was present suggesting that the ion complexation takes place by a cationic exchange equilibrium, between the protons and Pb2+ ions.
Asunto(s)
Plomo/química , Oligoquetos/metabolismo , Suelo , Temperatura , Adsorción , Animales , Concentración de Iones de Hidrógeno , Modelos Químicos , Espectroscopía Infrarroja por Transformada de Fourier , Electricidad Estática , Propiedades de SuperficieRESUMEN
La adsorción de plomo por un vermicompost comercial fue estudiada a 284, 306 y 323 K aplicando los modelos de Langmuir y Freundlich, los cuales describen adecuadamente el proceso de adsorción. Las capacidades de adsorción máximas de 116,3; 113,6 y 123,5 mg.g-¹. De acuerdo a las funciones termodinámicas, la retención de plomo involucra una reacción endotérmica (ΔHadsorción = 4,25 ± 0,92 kCal.mol-¹) con un aumento de la entropía del sistema (ΔS = 7,55 a 8,62 kCal.K-1mol-¹) asociado a la ionización de los grupos carboxílicos. La baja espontaneidad de la reacción fue determinada por los valores positivos de la Energía Libre de Gibbs (ΔG = 2,11 a 1,49 kCal.mol-¹). Las diferencias observadas en los espectros FTIR del compost en la región de 1800 a 1300 cm-¹ a diferentes condiciones de pH fueron interpretadas como el resultado de la ionización de los grupos carboxílicos, reduciendo la intensidad de las bandas alrededor de 1725 cm-¹, generándose las señales a 1550 cm-¹ y 1390 cm-¹ de los grupos carboxilato. Los mismos cambios fueron observados a pH 3,8 en presencia del ión Pb²+, lo que sugiere que el acomplejamiento con el metal pesado se produce por medio de una reacción de intercambio catiónico.
The adsorption of Pb2+ by a comercial vermicompost was studied at 284, 306 and 323 K using Langmuir and Freunldich isotherms. The maximum adsorption capacities were 116.3; 113.6 and 123.5 mg.g-¹. Thermodinamic functions showed that the retention takes place by an endothermic reaction (ΔHadsorción = 4,25 ± 0,92 KCal.mol-¹) in which entropy increases (ΔS = 7,55 to 8,62 kCal.K-1mol-¹) along with the acidity of the solutions due to the ionization of carboxylic groups. The low spontaneity was reflected by positive values of the Gibbs free energy (ΔG = 2,11 a 1,49 kCal.mol-¹). The differences in FTIR spectra of vermicompost at pH 3.8 and 7.0 in the region between 1800 to 1300 cm-¹ were interpreted on the basis of caboxylic acids ionization thet reduced the band intensity around 1725 cm-¹, producing signals at 1550 and 1390 cm-1of the carboxylate groups. Similar changes were detected at pH 3.8 when ionic lead was present which suggested that heavy metal complexation by a cationic exchange reaction.