Humic Substance and its formation
Humic substances are widely distributed in nature and represent a significant contribution to the global geochemical cycle of carbon .They cnsititute a major fraction of soil organic matter(up to 80% y weight),and represent the largest fraction of natural organic matter in aquatic systems.Foe example,HS account for 55-70% of the total dissolved organic matter in Amazon river.They are also found in terrestrial,lakeand marine sediments,where they may consititute potential precursors to the formation of kerogen,the most abundant form of organic carbon on the earth and the arent material petroleum accumulations.
HS in soil in formed through four different pathways.
Two of these include degradation of lignin by microorganisms,which produce either directly HS or phenolic aldehydes and acids.The later compounds are then enzymatically oxidized to quinones,which subsequently polymerize to give HS.A third pathway involves the microbial degradation of cellulose to polyphenols.These polyphenls are again oxidized to quinones ,the polymerization of which yield HS.The fourth way,corresponds to the condensation of sugars and amino acids (respectively derived from cellulose and polypeptides)to form HS.
Humic substances indeniably play an important role in a number of low temperature geochmical proess,including among others soil pH regulation through the dissociation of pollutants in the environment. Although the acidic and complexation properties of HS are of promary interest to geochemist,soil scientisits,and environmental chemists,these properties have rarely been adequately described due to the complex nature of HS.