| a brown, melanin-tinted mixture of polymers, found in lignite, peat, and soils, where it acts as a cation exchange agent: used in drilling fluids and inks. |
humic acid
one of two classes of natural acidic organic polymer that can be extracted from humus found in soil, sediment, or aquatic environments. The process by which humic acid forms in humus is not well understood, but the consensus is that it accumulates gradually as a residue from the metabolism of microorganisms. Its structure is unlike that of proteins or carbohydrates, the two most common organic polymers found in biological material; instead, humic acid can be characterized as a loose assembly of aromatic polymers of varying acidity and reactivity
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