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Revista Brasileira de Ciência do Solo

On-line version ISSN 1806-9657

Rev. Bras. Ciênc. Solo vol.24 no.4 Viçosa Oct./Dec. 2000

https://doi.org/10.1590/S0100-06832000000400008 

SEÇÃO III - BIOLOGIA DO SOLO

 

A natureza fractal de ácidos húmicos

 

Fractal nature of humic acids

 

 

A. C. SilvaI; E. S. MendonçaII; M. L. Martins III; C. ReisIV

IDoutorando em Solos e Nutrição de Plantas, Universidade Federal de Viçosa - UFV. Av. PH Rolfs s/n, 36571-000 Viçosa (MG). E-mail: ale@alunos.ufv.br
IIProfessor de Departamento de Solos - UFV. Bolsista do CNPq. E-mail: esm@alunos.ufv.br
IIIProfessor de Departamento de Física - UFV. E-mail: mmartins@mail.ufv.br
IVProfessor de Departamento de Química - UFV. E-mail: cesar@mail.ufv.br

 

 


RESUMO

Dentre as ferramentas usadas para descrever a estrutura ramificada ou a superfície rugosa e distorcida de ácidos húmicos (AH), a geometria fractal aparece como uma das mais adequadas para explicar a conformação de partículas húmicas (agregados moleculares). Do ponto de vista experimental, a dimensão fractal (D) de sistemas naturais pode ser determinada a partir do monitoramento da luz transmitida, não espalhada e não absorvida (turbidimetria 'τ'). A presença de fractais implica que o sistema pode ser decomposto em partes, em que cada uma, subseqüentemente, é cópia do todo. A determinação do valor 'D' dessas partículas foi conseguida pela utilização de turbidimetria, em que suspensões de AH-comercial e de AH-Espodossolo foram analisadas por espectrofotometria UV-Vis. O fundamento matemático utilizado foi a lei de potência τ ∝ λβ, em que β < 3 indica a presença de fractal de massa (Dm); 3 < β < 4 indica fractal de superfície (Ds), e β ≅ 3 indica não-fractal (NF). A declividade das retas (β) por meio do gráfico (logτ vs logλ) permitiu a obtenção de 'D'. Segundo os resultados, partículas de AH em suspensões aquosas diluídas formam estruturas fractais, cuja geometria pode ser caracterizada por meio de turbidimetria. Entretanto, a faixa de comprimento de onda usada (400 a 550 nm) ainda é pequena para se afirmar sobre a natureza fractal de AH e determinar suas dimensões fractais com precisão.

Termos de indexação: substâncias húmicas, conformação, turbidimetria.


SUMMARY

Among the tools used to describe the ramified structure or the rugged and distorted surface of humic acids (HA), fractal geometry seems to be one of the most adequate to explain the conformation of humic particles (molecular aggregates). From the experimental point of view, fractal dimension (D) of natural systems may be measured through the transmitted light, not scattered or absorved (turbidimetry'τ'). The presence of fractals indicates that the system may be decomposed in parts, each part being a copy of the whole. The determination of the fractal dimension (D) of these particles was achieved by the turbidimetric technique, where diluted solutions of commercial-HA and Espodossolo-HA were analyzed by spectrophotometry UV-Vis. The mathematical base used was the potential law τ ∝ λβ, where β < 3 indicates the presence of mass fractal (Dm); 3 < β < 4 of surface fractal (Ds) and β ≅ 3, of non-fractal (NF). The slope of the lines (β) was taken from the graphics (logτ vs logλ) to obtain D. The results show that the HA particles in diluted aqueous suspension form fractals structures, whose geometry may be characterized by turbidimetry. However, the wavelength range used (400 to 550 nm) is still small to make statements about the fractal nature of HA and determine its fractal dimensions with precision.

Index terms: humic substances, conformation, turbidimetry.


 

 

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Full text available only in PDF format.

 

 

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Recebido para publicação em dezembro de 1999
Aprovado em julho de 2000

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