Distribution coefficient (KD) of heavy metals in soils of the State of São Paulo. / Coeficiente de distribuição (Kd) de metais pesados em solos do estado de São Paulo.

AUTOR(ES)
DATA DE PUBLICAÇÃO

2005

RESUMO

Disposal of solid residues and application of pesticides and fertilizers can lead to an increase in the concentration of heavy metals in soils and groundwater. The legislation on heavy metals nearly always refers to the total contents of these elements; however, an evaluation of their hazard potential and toxicity requires an assessment of the metal fraction that is mobile and possibly bioavailable. A few numerical parameters have been used as a foundation to make decisions and to focus on prevention or remediation strategies in contaminated areas. The solid-solution distribution coefficient (Kd), defined as the relation between metal concentrations that are adsorbed and those present in the solution, allows a comparison between the behavior of elements in different systems. Environmental protection agencies use generic Kd values found in bibliographic references; these values are often obtained under distinct conditions from those found in the tropical region, and may lead to an erroneous hazard estimates. Thus, it becomes clearly necessary to obtain Kd values that will validate these contamination estimates or provide guidance for intervention strategies in already contaminated areas. The objectives of this work were to evaluate heavy metal retention in 30 representative soils of the State of São Paulo, based on a quantification of distribution coefficient values (Kd) and their relation with soil attributes, such as pH, CEC and clay contents, organic matter, and various forms of Fe, Al, and Mn oxides. The distribution coefficients were obtained from linear adsorption isotherms, constructed after the addition of 0.1; 0.5; 1.0; 2.5; and 5.0 mg L-1 Cd, Co, Cr, Cu, Ni, Pb, and Zn, in batch-type experiments. The smallest variations in Kd value were recorded for Pb (one order of magnitude – 121 to 7,020 L kg-1) and Ni (two orders of magnitude - 6 to 998 L kg-1), while variations indicating four orders of magnitude were observed for Cd (7-14,339 L kg-1), Co (2 - 34,473 L kg-1), and Cr (1 - 21,267 L kg-1). Zinc distribution coefficients were between 5 and 123,849 L kg-1, with a variation of five orders of magnitude. The values herein obtained were similar to those that have been used by CETESB, except for Cr and Ni. The following order of affinity was obtained: Pb>>>Cu>>Cd>Zn≅Ni≅Cr>Co. More than 55% of the variation in distribution coefficients for metallic cations of the groups IIB (Cd and Zn) and VIIIB (Co and Ni) were explained by pH. In the joint analysis, CEC and pH explained about 80% of the variation in Kd values for Cd, Co, and Ni, and showed that nonspecific adsorption mechanisms are involved in the retention of these elements. Regarding the distribution coefficients for Cu and Pb, pH and clay content accounted for about 63% of the variation. Kd values for Cr showed an inverse correlation with pH, especially in the joint analysis with clay contents, and 61% of the variation were explained by these variables.

ASSUNTO(S)

metal pesado do solo propriedades físico-químicas tropical soils distribution coefficient solos heavy metals

ACESSO AO ARTIGO

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