Molecular Weight Humic Substances Research Articles

Characterization of humic substances obtained from calcareous soils from Greece with various extractants. II. Physical characterization

AbstractHumic substances from the Ah horizons of five differently calcareous soils from Greece were successively extracted with organic solvents and/or 0.1 N NaOH and they were characterized by UV/VIS/IR spectroscopy and differential thermal analysis (DTA).Diethylether, acetone and dioxane extracts contained non‐humic substances such as aliphatic and aromatic ethers, esters, alcoholes and long aliphatic chains, while low molecular weight humic substances were found in the ethanol extracts. All the extracts were found to contain phenolic and ϵ‐acceptor compounds.Some differences in the physical characteristics of the humic substances were associated with the kinds of vegetation. The IR‐spectra showed no essential differences between HA's and FA's extracted directly with NaOH or after previous soil extraction with organic solvents. HA's extracted directly by NaOH from soil samples under grass had a higher degree of condensation and thermal stability and generally higher carbon contents than those from soils under pine, cultivation and sedges.

Studies of molecular weight distributions of fulvic and humic acids by gel permeation chromatography. Examination of the solute molecular composition using RI, UV, fluorescence and weight measurement as detection techniques

The system Sephadex LH 60/dimethylformamide/acetic acid was found to be useful for the determination of the molecular weights of humic substances. Fulvic and humic acids concentrated from natural waters, sediments and soil have number- and weight-average molecular weights less than 2000 and 4000, respectively. A theoretical model for the dependence of an experimental GPC curve on the relation between the measured property and the structure of heterogeneous polymers, is presented. The results from the examination of the molecular composition, obtained by using RI, UV, fluorescence and weight measurement as detection techniques, show that the apparent distributions of aromatic moities and fluorescent groups in the humic substances were similar to those of repeating structural subunits and end groups respectively, in synthetic polymers.

Calculation of molecular weights of humic substances from colligative data: Application to aquatic humus and its molecular size fractions

A rigorous mathematical expression for the dependence of colligative properties on acid dissociation of water soluble humic substances is presented. New data for number average molecular weights of a river derived humic material and its gel permeation Chromatographic fractions are compared with M ̄ n values obtained by a reevaluation of previously published experimental observations on soil and water fulvic acids. The results reveal a remarkable similarity of fulvic acids from widely different sources with respect to number-average molecular weight.

Correction of osmometric number-average molecular weights of humic substances for dissociation

It is necessary to correct the number-average molecular-weight ( Mn ) values of organic compounds obtained by colligative methods such as vapour-pressure osmometry (VPO) for dissociation of any acidic functional groups present. A novel correction method for the determination of Mn corr-values is described which does not require prior knowledge of the concentrations or dissociation constants of the acidic functional groups present. The procedure has been applied to the evaluation of Mn corr-values of a range of freshwater and marine humic substances and gave results in the range 500–1000. A correlation between salinity and Mn corr has been found for a range of marine fulvic acids, providing further evidence for the removal of the higher-molecular-weight fractions of dissolved organic matter (DOM) with increasing salinity.

Kinetics of formation of humic substances in activated sludge systems and their effect on flocculation

AbstractHumic substances were isolated during batch aeration studies with activated sludge and a complex waste source, by using concentration and separation techniques that employ reverse osmosis, ultrafitration, and gel permeation chromatography. The study suggests that the formation of high molecular weight humic substances may occur after the removal of the readily available carbon source. The amount of refractory material finally present in the solution will depend on its adsorptive properties toward bacterial cells. The adsorptive characteristics may be determined by the magnitude of the carbohydrate fraction present in the humic substances. If the carbohydrate content decreases, adsorption onto the cells may decrease resulting in an impairment of both the floe formation and settleability of the sludge floes. Decreased adsorption will result in a higher total organic carbon content and an increase in color bearing materials in the effluent.

Use of “Sephadex” Gel for the Estimation of Molecular Weight of Humic Substances in Natural Water

THOUGH little is actually known about the shape of humus molecules, and only a few experiments have been described1,2 on the fractionation of humus on ‘Sephadex’ gel (Pharmacia, Uppsala, Sweden), there is strong evidence for the existence of a relationship between the elution rate through the ‘Sephadex’ column and the molecular weight3,4.