Preface: humic substances in the environment

The fate of natural organic matter, and especially humic substances (HSs), has attracted increasing interests of scientists representing various disciplines over recent decades. Among the various organic substances occurring naturally, HSs are the most widespread, being present in soil, water, lake sediments, peat, brown coal, and shales. HSs represent about 25% of the total organic carbon on the earth. These substances represent a class of naturally occurring complex molecular structures, formed by aggregation and assemblage processes through which biomolecules originating from plant and animal residues are progressively transformed via biotic and abiotic pathways. The research of HSs is complex because these organic compounds are bound by, or associated with, soil mineral fractions, and require physical and/or chemical separation from the inorganic components through an extraction procedure, prior to their physico-chemical analysis. The most efficient of these separation procedures implies an extraction with alkali which operatively identifies three HSs fractions,