Hello,Humic acid and fulvic acids differ by molecular weights,number of phenolic OH carboxyl,groups, and the extent of polymerisation,hence they will have different COD values

Dear Oumar,

The percentage of carbon in the humic acids (about 50%) is higher than the percentage of carbon in the fulvic acids. It is for this reason that the COD is greater for humic acids.

With my best regards

Prof. Bachir ACHOUR

Dear Oumar,

The difference in the COD between humic acid and fulvic acid is attributed to the strength of the chemical bonds contained in both species. The chemical bonds in fulvic acids are weaker than that in humic acids. Therefore lesser oxygen is required to oxidize fulvic acid than humic acid, thus the COD of humic acid is higher than fulvic acid.

Please read the following text which gives clearer picture to what I depicted above:

HUMIC ACIDS

Humic acids (HAs) comprise a mixture of weak aliphatic (carbon chains) and

aromatic (carbon rings) organic acids which are not soluble in water under acid conditions but are soluble in water under alkaline conditions. Humic acids consist of that fraction of humic substances that are precipitated from aqueous solution when the pH is decreased below 2. Humic acids (HAs) are termed polydisperse because of their variable chemical features. From a three dimensional aspect these complex carbon containing compounds are considered to be flexible linear polymers that exist as random coils with cross linked bonds. On average 35% of

the humic acid (HA) molecules are aromatic (carbon rings), while the remaining components are in the form of aliphatic (carbon chains) molecules. The molecular size of humic acids (HAs) range from approximately 10,000 to 100,000. Humic acid (HA) polymers readily bind clay minerals to form stable organic clay complexes. Peripheral pores in the polymer are capable of accommodating (binding) natural and synthetic organic chemicals in a lattice (clathrate) type arrangements.

Humic acids (HAs) readily form salts with inorganic trace mineral elements. An analysis of extracts of naturally occurring humic acids (HAs) will reveal the presence of over 60 different mineral elements present. These trace elements are bound to humic add molecules in a form that can be readily utilized by various living organisms. As a result humic acids (HAs) function as important ion exchange and metal complexing (chelating) systems.

FULVIC ACIDS

Fulvic acids (FAs) are a mixture of weak aliphatic and aromatic organic acids

which are soluble in water at all pH conditions (acidic, neutral and alkaline). Their composition and shape is quite variable. The size of fulvic acids (HFs) are smaller than humic adds (HAs), with molecular weights which range from approximately 1,000 to 10,000. Fulvic acids (FAs) have an oxygen content twice that of humic acids (HAs). They have many carboxyl ( COOH) and hydroxyl ( COH) groups, thus fulvic acids (FAs) are much more chemically reactive. The exchange capacity of fulvic acids (FAs) is more than double that of humic acids (HAs). This high exchange capacity is due to the total number of carboxyl ( COOH) groups present. The number of carboxyl groups present in fulvic acids (FAs) ranges from 520 to 1120 cmol (H+)/kg. Fulvic acids collected from many different sources and analyzed, show no evidence of methoxy groups ( CH3) groups, they are low in phenols, and are less aromatic compared to humic acids from the same sources.

For more on this topic, please use the following link:

http://www.humates.com/pdf/ORGANICMATTERPettit.pdf

Regarding the answer of Susan,

It is right that humic acid has a higher molecular weight than fulvic acid. According to your theory if the molecular weight is a factor in determining the COD, you should see lower COD values for humic acid than fulvic acid since for the same mass the number of moles of humic acid is smaller that the number of moles of fulvic acid. Thus the amount of oxygen needed to oxidize humic acid is less than that needed for fulvic acid.

Hoping this will be helpful,

Rafik

Dear Oumar:

the basic reason for different CODs of humic and fulvic acids fraction is, that not only TOC forms COD. Oxygen consumption could be caused by many other species, functional groups etc. present in fractions of fulvic and humic acids.(as Susan Verghese supposed above).

Best regards

Vit

Hello everyone,

Thanks you all for your interest toward my question. I understand more about the subject throughout your interesting explanations. In summary, I understand that the main difference between humic acid and fulvic in term of COD might be explained by the fact that fulvic acids contain more oxygen in its molecular structure (carboxyl and hydroxyl groups) than humic acids. Since these functional groups are chemically very reactive, less oxygen demand will be required to oxidize the fulvic acids molecules. On the other hands, HA contain more carbon in the form of aliphatic chain and aromatic rings which are more challenging to oxidize.

I had the wrong idea to just consider the carbon portion throughout the TOC value.

I gree with Rafik Karaman and Susan Verghese, there are different in molecular weights,number of phenolic OH carboxyl,groups.

i agree with dr.mohamed