Molecular Weight Fractions Of Humic Acids Research Articles

Insight to Microbial Fe(III) Reduction Mediated by Redox-Active Humic Acids with Varied Redox Potentials.

Redox-active humic acids (HA) are ubiquitous in terrestrial and aquatic systems and are involved in numerous electron transfer reactions affecting biogeochemical processes and fates of pollutants in soil environments. Redox-active contaminants are trapped in soil micropores (<2 nm) that have limited access to microbes and HA. Therefore, the contaminants whose molecular structure and properties are not damaged accumulate in the soil micropores and become potential pollution sources. Electron transfer capacities (ETC) of HA reflecting redox activities of low molecular weight fraction (LMWF, <2.5) HA can be detected by an electrochemical method, which is related to redox potentials (Eh) in soil and aquatic environments. Nevertheless, electron accepting capacities (EAC) and electron donating capacities (EDC) of these LMWF HA at different Eh are still unknown. EDC and EAC of different molecular weight HA at different Eh were analyzed using electrochemical methods. EAC of LMWF at −0.59 V was 12 times higher than that at −0.49 V, while EAC increased to 2.6 times when the Eh decreased from −0.59 V to −0.69 V. Afterward, LMWF can act as a shuttle to stimulate microbial Fe(III) reduction processes in microbial reduction experiments. Additionally, EAC by electrochemical analysis at a range of −0.49–−0.59 V was comparable to total calculated ETC of different molecular weight fractions of HA by microbial reduction. Therefore, it is indicated that redox-active functional groups that can be reduced at Eh range of −0.49–−0.59 are available to microbial reduction. This finding contributes to a novel perspective in the protection and remediation of the groundwater environment in the biogeochemistry process.

Gamma-irradiation-induced molecular-weight distribution and complexation affinity of humic acid with Cs+, Sr2+, and Eu3+

Solutions of humic acid (HA) were irradiated with 0, 1, 5, 10, 50, and 100 kGy of gamma irradiation using a 60Co source. The non-irradiated and irradiated HA molecules were fractionated by ultrafiltration into four categories: > 100, 50–100, 10–50, and < 10 kDa. Total organic carbon measurements and potentiometric titration analysis suggested that (1) some gamma-irradiated HA molecules were degraded into smaller molecules and (2) radiolytic degradation caused phenolic –OH became the predominant functional group in the small molecular-weight fractions of HA. The effect of absorbed dose of gamma rays on the distributions of Cs+, Sr2+, and Eu3+ ions in the molecular-weight fractions of the metal–HA systems was examined to discuss the complexation affinity. The metal ions were distributed in the smaller molecular-weight fractions at different doses, which corresponded to the degradation of HA molecules. For a predetermined absorbed dose, Cs+ ions did not change the molecular-weight distribution of the total organic carbon content of the degraded HA molecules. Conversely, the Sr2+ and Eu3+ ions redistributed organic carbon toward the larger molecular-weight fractions.

Influence of humic acid and its different molecular weight fractions on sedimentation of nanoscale zero-valent iron.

The effects of humic acid (HA) and its different molecular weight (MW) fractions on the sedimentation of nanoscale zero-valent iron (NZVI) in the absence and presence of cations (i.e., Na+/Mg2+/Ca2+) were investigated. Ultrafiltration (UF) was used as the method of fractionation to obtain four different MW fractions (separated by ultrafiltration membranes of 10kDa, 50kDa, and 100kDa). Differing sedimentation behavior was observed for NZVI with different MW fractions of HA. Generally, the degree of settling of NZVI particles in the presence of high MW fractions of HA was lower than that of low MW fractions of HA and that without HA. The results were mainly attributed to the steric stabilization provided by the high MW fractions of HA. The presence of Na+/Mg2+/Ca2+ alone had insignificant influence on the settling of NZVI, but both Mg2+ and Ca2+ exerted an obvious influence on the settling of NZVI in the co-presence of HA. The settling behavior of NZVI was further examined in the co-presence of different MW fractions of HA and Ca2+. The co-presence of low MW HA fractions and Ca2+ led to a lower settling of NZVI. This might be due to the formation of a layer of HA-Ca2+ complex on the particle surface, providing stronger steric stabilization. Nevertheless, in the co-presence of high MW HA fractions and Ca2+, the settling of NZVI was initially reduced but accelerated with time, which might be due to the gradual aggregation of NZVI with time resulted from the bridging effect of HA-Ca2+ complex.

Adsorption of Dissolved Organic Matter with Different Relative Molecular Masses on Inorganic Minerals and Its Influence on Carbamazepine Adsorption Behavior

In this work, sorption experiments were conducted to understand the adsorption characteristics of dissolved organic matter (DOM) of different relative molecular masses on minerals and the effects of their interaction with PPCPs on the adsorption process. This study chose carbamazepine (CBZ) as the target pollutant, quartz sand as the inorganic mineral, and commercial humic acid (HA) as the DOM. We studied the structural characteristics of HA with different relative molecular masses and the impact of their interaction with CBZ on their adsorption based on dialysis experiments and infrared spectroscopy. It was found that large molecular weight fractions of HA, which contain more hydroxyl, aromatic hydrocarbons, and aliphatic hydrocarbons, were mainly hydrophobic substances, while the smaller fractions, containing more alcohols and carboxyl groups, were mainly hydrophilic substances. The combination of HA and CBZ had two major mechanisms, the combination of amino groups of CBZ and polar functional groups in small molecular weight fractions of HA and hydrophobic interactions between hydrophobic groups and aliphatic or/aromatic compounds in large molecular weight HA. The interaction between CBZ and HA resulted in obvious differences in the adsorption of HA or CBZ. When CBZ does not exist, hydrophobic combinations occurred between HA through the hydrophobic components and quartz sand, which also experienced the exchange coordination reaction with the hydroxyl or carboxyl group of HA. In addition, HA adsorbed the aliphatic and hydrophobic macromolecular components. Quartz sand was used to adsorb the large hydrophobic molecules. When CBZ existed, it adsorbed the small molecular weight fractions of HA. The maximum HA adsorption capacity of quartz sand decreased. CBZ could be adsorbed on quartz sand by hydrophobic interaction, van Edward forces, and polarity interactions in the absence of HA. When adding HA, the adsorption increment of carbamazepine on quartz sand was due to the co-adsorption or accumulation of HA and CBZ after their combination.

Adsorption of humic acid from aqueous solution by hematite: effects of pH and ionic strength

Batch experiments were conducted to investigate adsorption of humic acid (HA) on a commercial hematite. The point of zero charge (PZC) and specific surface area of hematite were 9.17 ± 0.25 and 5.22 ± 0.04 m2 g−1, respectively. The concentrations of HA in solutions were quantified by the ultraviolet–visible spectrophotometer, and molecular weight (MW) was determined by E 2/E 3 (the ratio of absorbance at 250 nm (E 2) and 365 nm (E 3)) measurements of HA in solutions. The adsorption of HA on hematite was strongly dependent on solution pH. More HA was adsorbed at acidic conditions, which was also confirmed by Fourier transform infrared spectroscopy (FTIR) measurements. At pH <9, the E 2/E 3 of the HA remaining in solution after adsorption was higher than its initial value indicating that higher MW fractions of HA were preferentially adsorbed on hematite. At pH >9, no significant change in the E 2/E 3 of the residual HA was observed, so other interactions were involved besides electrostatic interactions. The presence of salt promoted HA adsorption on hematite due to electrostatic interactions. The fractionation of HA during adsorption by hematite would affect the fate of other organic contaminants and heavy metals in natural systems.

Distribution of heavy metals by the molecular-weight fractions of humic acids in the soils of long-term field experiments

The influence of different agricultural treatments on the contents of Cd, Zn, and Co in the organic matter and humic acids of soddy-podzolic and dark gray forest soils was studied in long-term field experiments. The use of gel chromatography proved the complicated molecular-weight composition of the humic acids in all the studied soils. The contents of heavy metals and their distribution by the molecular-weight fractions of humic acids were determined in the experimental soils and in the virgin podzolic soils. The complex nature of organomineral compounds of Cd and Zn with humic acids was shown by means of gel filtration. The thermodynamic stability of the organomineral complexes increased with the increasing weight of the initial molecular-weight fractions of humic acids.