Let's deal with tannins and tanic acid first. Humic substances often called humins or tannic acids,Its weak acidity (pKa around 6) is due to the numerous phenol groups in the structure. The chemical formula for commercial tannic acid is often given as C76H52O46, when they are solutes, are a generic catch-all designation for a complex range of dark-colored, variously soluble aromatic organic molecules that result from the incomplete decomposition of plant materials, notably celluloses and lignin. Humic are almost 50% carbon by weight. The source of most of these particulates and humic acids in water is in terrestrial leaf litter through which rainwater percolates; some of this leaf litter gets washed into streams, where it continues its breakdown into humus. Humic substances are only very slowly broken down in water by a community of various bacteria and fungi. The end product will be CO2.
Humic macromolecules are as various as snowflakes: perhaps no two molecules are precisely alike. The two dominant functional groups in these massive molecules are carboxyl groups and phenolic ones. These sections of the complex structures have differing qualities that render the molecules polyelectrolytes; that is to say they are highly polarized, with surfaces that are positive and negative at different sites, though they have an overall negative charge at the pH values of natural freshwaters. Consequently, humic substances are chemically highly reactive: their negative-sum surface electrical charges "pull" ionic heavy metals from the water in the process of chelation and are also sorbed to metallic oxides.
In the aquarium humic substances contribute a desirable or undesirable golden tint to water, which becomes acidic blackwater when humins are sufficiently concentrated. They affect the pH, tending to lower it, and have some additional softening effect on water that is not highly buffered; humins affect the cycling and bioavailability of chemical elements; they repress many bacterial populations and affect the zooplankton: acidic blackwaters rich in humic acids have characteristically low populations of bacteria.
Humic substances interact with over 50 of the elements in the periodic table. These include plant nutrients, heavy metals and the halogens. All humic substances tint the water yellow to rusty brown, and all are able to chelate positively-charged multivalent ions. They can do this because the negative charged groups present in all humic substances attract cations that have a multiple positive charge, such as iron (Fe++ and Fe+++), calcium (Ca++) and magnesium (Mg++). Cations with a single charge, like sodium (Na+), aren't affected; that figures: you know that a peat filter won't "de-salinate" water. Once these divalent cations are bound to a big humic molecule, they have been taken out of circulation, as it were. If the humic molecule itself were adsorbed to granular activated carbon and then removed from the filter, some permanent water softening would be achieved.Tannins that bind iron frustrate water treatment engineers, who can't filter out chelated heavy metals
Do I have everyone totally confused now