Sorption and lability of heavy metals in latosols in Minas Gerais

Com o presente trabalho, objetivou-se estudar a sorção e a labilidade de Cd, Cu, Ni, Pb e Zn em 12 amostras (6 superficiais e 6 subsuperficiais) de Latossolos de Minas Gerais, a fim de verificar quais atributos dos solos estariam regulando tais fenômenos. Foram conduzidos dois experimentos de laboratório: i) um estudo de adsorção, utilizando dez concentrações crescentes dos metais, ajuste dos dados às isotermas de Langmuir e de Freundlich e correlação dos coeficientes com das isotermas as principais características físicas e químicas, mineralógicas das amostras de solo - pH, CTC efetiva, teores orgânico, de carbono argila, gibbsita, caulinita, goethita e hematita; ii) extração de formas lábeis de metais pesados por meio de resina de troca catiônica, a partir da incubação de uma dose dos metais com as amostras de solo e extração em três épocas, aos 15, 30 e 45 dias de incubação. Neste experimento, os tratamentos se originaram de um arranjo fatorial 12x5, sendo 12 amostras de solo e adição de 5 metais. foram Os tratamentos distribuídos delineamento em inteiramente casualizado com três repetições, totalizando 180 experimentais. Os unidades dados de adsorção ajustaram - se adequadamente isotermas, não às havendo diferença entre os graus de ajuste. Os fatores que mais influenciaram a capacidade máxima de adsorção foram pH (Cd e Cu), CTC efetiva (Cu e Ni), teor de hematita goethita (Pb) e de e teor de carbono orgânico (Zn). Os valores do coeficiente a, relacionado ligação, à energia de correlacionaram-s e com os valores de pH e de CTC efetiva para os metais Cd, Cu, Ni e Zn. O coeficiente k da isoterma de Freundlich correlacionou-s e positivamente com os valores de pH e de CTC efetiva (Cd, Cu, Ni e Zn) e com os teores de hematita e de goethita e, negativamente, com os teores de argila e de gibbsita (Pb). O coeficiente n da isoterma de Freundlich correlacionou-s e positivamente com os valores de pH e de CTC efetiva (Cd, Ni, Pb e Zn) e com os teores de hematita e goethita e, negativamente, com os teores de caulinita, gibbsita e argila (Cu). teores Foram de obtidos formas baixos lábeis de metais, o que foi atribuído à intensidade adsorção das dos reações metais de nos colóides do solo. A partir da variação dos teores dasformas lábeis de metais, foi possível separar os solos estudados em dois grupos: i) solos com maior labilidade de metais – LB, LAd, LVwf (2) e ii) solos com menor labilidade de metais – LVe, LVj e LVwf. A maior labilidade foi atribuída à presença de gibbsita e de caulinita, enquanto que a menor labilidade foi atribuída à presença de hematita e de goethita. tempo A de verificada influência do incubação foi de forma mais nítida somente para o Ni, o que foi atribuído rapidez com processam as à grande que se reações de adsorção e à tendência mais pronunciada desse elemento de manter-se retido no solo de forma não específica. Houve diferenças dos teores de formas lábeis de metais entre os horizontes A e B das amostras estudadas. diferenças foram aos teores de Tais atribuídas argila e de carbono orgânico presentes.The objective of this work was to study the sorption and lability of Cd, Cu, Ni, Pb and Zn in 12 samples (six superficial ones and six subsuperficial) of Latosols in Minas Gerais, Brazil, to verify which soil attributes would be regulating such phenomena. Two laboratory experiments were conducted: (i) an adsorption study, using ten increasing concentrations of metals, fitting of the data to the Langmuir and Freundlich isotherms and their coefficient correlation with the main chemical, physical and mineralogical characteristics of soil samples - pH, Effective CEC, contents of organic carbon, clay, gibbsite, kaolinite, goethite and hematite; (ii) extraction of labile forms of heavy metals by means of cation- exchange resin, from incubation of a metal dose with the soil samples and extraction at three different time periods, 15, 30 and 45 incubation days. The experimental treatments originated from a 12 x 5 factorial arrangement, with 12 soil samples and 5- metal addition, arranged in a randomized complete design with three repetitions, totaling 180 experimental units. The adsorption data fit adequately to the isotherms, with no difference among the fitting degrees. The factors which most influenced the maxim um adsorption capacity were pH (Cd and Cu), Effective CEC (Cu and Ni), goethite and hematite contents (Pb) and organic carbon content (Zn). The a coefficient values, related to the bonding energy, were correlated with the pH and Effective CEC values for th e metals Cd, Cu, Ni and Zn. The k coefficient of the Freundlich isotherm was positively correlated with the pH and Effective CEC values (Cd, Cu, Ni and Zn) and with the hematite and goethite contents and, negatively, with the clay and gibbsite contents (Pb). The n coefficient of the Freundlich isotherm was positively correlated with the pH and Effective CEC values (Cd, Ni, Pb and Zn) as well as with hematite and goethite contents and, negatively, with the kaolinite, gibbsite and clay (Cu) contents. Low contents of labile metal forms were obtained, what was attributed to the intensity of the metals adsorption reactions in the soil colloids. From the variation content of the metal labile forms, it was possible to group the soils into two types: (i) soils with greater metal lability - LB, LAd, LVwf (2) - and (ii) soils with smaller metal lability - LVe, LVj and LVwf. Greater lability was attributed to the presence of gibbsite and kaolinite, whereas smaller hematite and lability goethite. was attributed Incubation time to the influence presence was of more clearly felt only for Ni, what was attributed to the great speed the adsorption reactions are processed and this element's most pronounced tendency to be retained in the soil in a non-specific form. There were differences in the contents of labile metal forms between the A and B horizons samples. Such differences were attributed to the clay and organic carbon contents present.Conselho Nacional de Desenvolvimento Científico e Tecnológic

Lability and sorption of heavy metals as related to chemical, physical, and mineralogical characteristics of highly weathered soils

Heavy metal lability, probably, is the most important isolated factor to cause toxicity in plants and organisms in soils. Sorption of heavy metals, in turn, affects directly the amount of their labile forms in soils. Therefore, to assess sorption and quantify labile forms of Cd, Cu, Ni, Pb, and Zn, adsorption and incubation studies were carried out.The adsorption experiment consisted of a 12×5×10 factorial design with 12 samples, five metals, and ten doses. An incubation experiment and metal extraction by cationic exchange resin in membrane form, with capacity to exchange 2.80 molc kg−1 dry resin, were conducted in samples from A and B horizons of six highly weathered soils from Minas Gerais, Brazil.The data from the adsorption experiment was fitted to nonlinear Langmuir and Freündlich models to give the b and k constants, which were used in a correlation study with soil properties. Cationic resin was used to extract the labile metal forms of heavy metals, and low concentrations of the labile forms were determined for all metals and soils in all incubation times.Factors that influenced Langmuir maximum adsorption and Freündlich buffer capacity were pH and CEC, as chemical characteristics, and goethite or hematite contents, as mineralogical attributes. Negative charge density and hydrolyzed species formation may have been main factors related to maximum adsorption and buffer capacity for most heavy metals. Low contents of labile metal forms obtained by cationic resin extraction were attributed to high intensity of metal adsorption reaction onto soil colloids. Greatest lability was found in soils with mineralogy dominated by gibbsite and kaolinite, while lowest metal lability was determined in soils with higher hematite and goethite contents. Due to their specificities, tropical soils should have more research toward the understanding of the relationship soil vulnerability to heavy metals pollution and availability/lability of these important toxic substances in the environment