Weight Fractions Of Humic Substances Research Articles

Interactions of mercury with different molecular weight fractions of humic substances in aquatic systems

Interactions of mercury (Hg) with different molecular weight fractions of humic substances (HS) play an important role in controlling distribution, diffusion, speciation, and bioavailability of Hg in natural systems. This study suggests that Hg prefers to associate with higher molecular weight fractions of HS and this association particularly predominates at low pH and high ionic strengths of the medium. The concentrations of aggregated HS (with higher molecular weight) become high at lower pH (acidic condition) and high ionic strength. Molecular weight of HS gradually decreases with the increasing pH (basic condition) and low ionic strength of the medium. The disaggregation property of HS which involves the release of monomers from the surface of the aggregates produces HSs of different intermediate molecular weight with different Hg complexing capacity. Distribution of Hg in different molecular weight fractions of HS is dependent on aggregational and disaggregational properties of HS in aquatic medium. Association of Hg with high molecular weight fraction of HS may alter distribution and bioavailability of Hg in a system as the bioreactivity of organic matter decrease along a continuum of size in aquatic medium.

Identification of Pt and Pd bound to humic acid species in spiked soils and street dusts by size exclusion chromatography and ICP-MS

The formation of PGE-humic acid (HA) complexes in soil and street dust samples was investigated. In order to assess the distribution of Pt and Pd among molecular weight fractions of humic substances, the HA extracts (extracted by 0.1 mol L−1 sodium pyrophosphate) were analysed by size exclusion chromato-graphy coupled on-line with UV-Vis detection. Similar chromatograms were obtained for soils and street dust samples (254 nm, 280 nm) and two size fractions were operationally defined as high (1600–5000 Da) and low molecular (< 1600 Da) HA fractions. The concentration of Pt and Pd in the separated extracts was determined by ICP-MS. The results indicate that up to 43% of Pt is in the high molecular and up to 52% in the low molecular HA fraction. In both type of soil samples, Pd is preferentially bound to the low molecular HA fraction. Dependence of Pd–HA and Pt–HA formation on the sample type both in the soils and in the street dust sample as well as on Pt oxidation state was established. Metallic Pt shows a tendency for complexation with the fractions of HA higher than 5000 Da. In the street dust samples, the distribution of Pt and Pd is similar and is strongly dependent on the sample type, being bound mainly to the fractions: higher than 5000 Da, 1600–5000 Da and the fraction lower than 1600 Da.

SEC-ICP-MS studies for elements binding to different molecular weight fractions of humic substances in compost extract obtained from urban solid waste.

In this work, the speciation of elements in compost was studied with emphasis on their binding to humic substances. In order to assess the distribution of As, Cd, Co, Cr, Cu, Mn, Mo, Ni, Pb, U, Th and Zn among molecular weight fractions of humic substances, the compost extract (extracted by 0.1 mol l(-1) sodium pyrophosphate) was analyzed by size exclusion chromatography coupled on-line with UV-Vis spectrophotometric and ICP-MS detection. Similar chromatograms were obtained for standard humic acid (Fluka) and for compost extract (254 nm, 400 nm) and three size fractions were operationally defined that corresponded to the apparent molecular weight ranges > 15 kDa, 1-15 kDa and < 1 kDa. The percentage of total element content in compost that was leached to the extract ranged from 30% up to 100% for different elements. The elution profiles of Co, Cr, Cu, Ni and Pb (ICP-MS) followed that of humic substances, while for other elements the bulk elution peak matched the retention time observed for the element in the absence of compost extract. Spiking experiments were carried out to confirm elements' binding and to estimate the affinity of individual elements for humic substances derived from compost. The results obtained indicated the following order of decreasing affinity: Cu > Ni > Co > Pb > Cd > (Cr, U, Th) >> (As, Mn, Mo, Zn). After standard addition, further binding of Cu, Ni and Co with the two molecular weight fractions of humic substances was observed, indicating that humic substances derived from compost were not saturated with these elements.

COMPLEXING CAPACITY OF DIFFERENT MOLECULAR WEIGHT FRACTIONS OF SEDIMENTARY HUMIC SUBSTANCES

In this paper we studied the complexing capacity of different molecular weight fractions of humic substances, subdivided into humic (HA) and fulvic acids (FA), extracted from Arno river sediment. Humic acids are characterised by a high degree of aromaticity and a low nitrogen content whereas fulvic acids display aliphatic features and are characterised by a number of oxygen and nitrogenous functional groups such as carboxylic, alcoholic and/or phenolic and peptide groups. The higher degree of condensation of humic acids than fulvic acids is confirmed by their different molecular weight distribution; HA molecules are characterised by a narrow range of molecular weights while fulvic acids are distributed over a wider range of molecular weights. The complexation capacity for HA and FA increases with increasing pH value and at the same pH value the CC for HA and FA is equal even if structural features and molecular weight distribution are different. Moreover, the calorimetric measurements relative to fulvic acids at different pH values show that by increasing the pH, the quantity of heat involved in the metal ion-fulvate interactions changes from an exothermic response to an endothermic one. This shows that pH value influences the different reactions involved in the binding process, such as coordination reactions, electrostatic interactions, deprotonation due to exchange of copper with proton, interactions between hydrolysis products of copper and fulvic acids, in different ways.

Ultrafiltration processes for removing humic substances: effect of molecular weight fractions and PAC treatment

The role of humic substances in general and different molecular weight fractions of humic substances in particular on the membrane performance in terms of quality (chlorination byproducts) and quantity (permeate flux) is unclear. One of the goals of the present study is to evaluate the effect of fractionated humic substances on the ultrafiltration (UF) performance. A commercial humic solution was subject to gel filtration chromatography, and the resultant four fractions representing the apparent molecular weight (AMW) from 180 to 22 600 Daltons (Da) were filtered through a single module UF system, with a molecular weight cutoff 100 kDa. The fraction with the largest AMW (6.5–22.6 kDa) exhibits the worst flux decline with the best permeate quality, whereas the smallest AMW fraction (160–650 Da) exerts little effect on flux decline. Although the UF system is able to remove a significant portion of THMFP (trihalomethanes formation potential) in larger AMW fractions, the permeate THM in terms of μg THMs/mg carbon is still high. Thus, UF systems are not effective in removing disinfection-byproducts. The use of powdered activated carbon (PAC) for pretreatment of humic substances, or in combination with different UF operating systems, to reduce membrane fouling actually yields adverse effects. The PAC was ineffective in removing those AMW fractions less than 300 or greater than 17 000 Da; the higher the PAC dosage, the more removal in the middle range of thee AMW factions. The flux decline for PAC-treated streams is worse than the original humic substances. Similarly, the combined UF-PAC system significantly decreases the flux.

Studies on marine humic substances I. Isolation of humic substances from seawater with an effective adsorbent, GDX-102 adsorption resin

Apart from about 35 g/1 of inorganic salts seawater also contains a small amount of organic matter ranging 0.5--2.0 mg C/1. Dissolved organic matters (DOM) are the main component in total organic matter, and only I0~o of these DOM including sugars, amino acids, fatty acids etc. are identified~X~L By the estimate of Rashidtn3, however, humic substances constitute over 60---80 ~ of naturally occurring organic matter in seawater with the concentration of only 0.003--0.8 rag/1 11~. The humic substances in the ocean are mainly originated from phytoplankton, but in the coastal water the humie substances from land and benthic brown seaweed exudates are also mixed in them. Marine humic substances are extremely complex amorphous high molecular weight polymers, and are strongly reactive with many different metallic ions, affecting directly their speciation, concentration and equilibrium relation in seawater, as well as their precipitation and transfer in marine environment. In addition, the low molecular weight fractions of humic substances in seawater show a promoting effect on the growth of phytoplankton Es.ga. Thus, humic substances in seawater not only play an important role in the geochemical processes of metallic ions and organic matters, but also have an important ecological significance. The properties of humic substances from soils and coals have been studied for a long time, and a great deal of informations have been published te.ls~. But the investigation on humic substances from seawater, owing to their low concentration, only began in the early fifties by Wilson et al 1203, Jeffrey et al csl and Johnston et al 141 using active carbon as the adsorption medium. Afterwards Moed 171 and Williams et al txs3 recommended the use of inorganic adsorbents, but their adsorption efficiency was low. Since the latter part of the sixties Sieburth et alCXs~, Riley et al 1:23 and Stuermer et al t~6~ have tried to use nylon stocking and powder, and styrene-divinylbenzene polymer Amberlite XAD resins to extract humic substances from seawater. Amberlite XAD-2 was used with some success by Stuermer et al 1163, and recently XAD-7 and _8Ex3 were found to be much better than XAD-2 in isolating humic substances from bog and river waters. A comparative study of the adsorption e~ciency for humic substances from seawater