Abstract
The application of sewage sludge or biosolids on soils has been widespread in agricultural areas. However, depending on their characteristics, they may cause increase in heavy metal concentration of treated soils. In general, domestic biosolids have lower heavy metal contents than industrial ones. Origin and treatment method of biosolids may markedly influence their characteristics. The legislation that controls the levels of heavy metal contents in biosolids and the maximum concentrations in soils is still controversial. In the long-term, heavy metal behavior after the and of biosolid application is still unknown. In soils, heavy metals may be adsorbed via specific or non-specific adsorption reactions. Iron oxides and organic matter are the most important soil constituents retaining heavy metals. The pH, CEC and the presence of competing ions also affect heavy metal adsorption and speciation in soils. In solution, heavy metals can be present either as free-ions or complexed with organic and inorganic ligands. Generally, free-ions are more relevant in environmental pollution studies since they are readily bioavailable. Some computer models can estimate heavy metal activity in solution and their ionic speciation. Thermodynamic data (thermodynamic stability constant), total metal and ligand concentrations are used by the GEOCHEM-PC program. This program allows studying heavy metal behavior in solution and the effect of changes in the conditions, such as pH and ionic strength and the application of organic and inorganic ligands caused by soil fertilization.
Highlights
Industrialized societies produce large amounts of waste and one of the options for its disposal is through application on agricultural land
The metal concentrations in plant tissue will be the same as those of the plateau (Chang et al, 1997). These studies on the “plateau effect” reject the hypothesis of the “time bomb”, which states that the slow mineralization of the organic matter of the sludge could release the metals in more soluble forms (McBride, 1995)
When heavy metals were applied as biosolids, certain components such as ferric hydrous oxides, organic matter and phosphates, bind metals to the residue, making them less available to plants, animals and humans
Summary
RESUMO: A aplicação agrícola de lodos de esgoto ou biossólidos tem se tornado prática comum, mas pode causar o acúmulo de metais pesados nos solos, dependendo das características desses resíduos. Os metais pesados podem ser adsorvidos por meio de reações de adsorção específica e/ ou não específica. O pH, a CTC e a presença de cátions afetam a adsorção e a especiação iônica de metais pesados nos solos. Os metais encontram-se como íons livres e/ou formam complexos com ligantes orgânicos e inorgânicos. Alguns programas computacionais calculam a atividade dos metais em solução e sua especiação iônica. Isso permite observar o comportamento dos metais em solução e a resposta dos mesmos às mudanças nas condições do meio, tais como a variação do pH e da força iônica e a adição de ligantes orgânicos e inorgânicos via adubação. Palavras-chave: lodo de esgoto, adsorção, especiação, pH, matéria orgânica
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