Molecular Size Of Humic Acids Research Articles

Adsorption of molecular size fractions of humic acid onto anion-doped TiO2 specimens

Humic acids (HA) constitute the major fraction of natural organic matter that should be removed during water treatment. Besides conventional treatment methods, application of advanced oxidation processes more specifically photocatalysis has gained much attention in recent decades. Titanium dioxide (TiO2) is universally recognized as a standard photocatalyst. Since photocatalysis occurs through a surface-oriented mechanism, adsorptive properties of TiO2 specimens deserve special attention. Moreover, nowadays visible light activated TiO2 developed by modifications through the use of various dopants has been the subject of numerous investigations. Understanding of the surface interactions prevailing between the anion-doped oxide surface and humic subfractions is important for the determination of the role of humic substances during photocatalysis. The aim of this study was to investigate surface interactions between different molecular size fractions of HA and TiO2, namely bare TiO2 and anion-doped TiO2 (C-doped, N-doped, S-doped and N–S co-doped) specimens. Therefore, adsorption properties of HA and its molecular size fractions onto bare TiO2 and anion-doped TiO2 specimens were evaluated by dissolved organic carbon, UV–vis spectral properties, and respective specific UV–vis parameters (SCoA, SUVA365, SUVA280, and SUVA254). Furthermore, the data achieved by adsorption experiments were assessed by Freundlich, Langmuir as well as Dubinin–Radushkevich isotherm models. The results based on varying molecular size fractions of HA displayed remarkable differences with respect to the type of the dopant in comparison to the bare TiO2 specimen. Consequently, when different molecular size fractions of HA were compared, Freundlich model displayed lower KF and higher 1/n values as well as Langmuir model exhibited maximum quality adsorbable in the presence of lower molecular size fraction. The reason could be also attributed to the compositional properties of HA subfraction along with the alterations in TiO2 specimens due to doping. From a general perspective, E values in Dubinin–Radushkevich model indicated that the main mechanism for the adsorption of diverse molecular size of HA onto bare and anion-doped TiO2 specimens could be mainly attributes to physical forces. Referring to the fundamental aim of the study indicating that the studied dose range of TiO2 (0.1–1.0 mg mL−1) was selected with respect to the photocatalytically active concentration range, the attained results should be carefully interpreted.

Functional groups and reactivity of size-fractionated Aldrich humic acid

The complexation affinity of the Aldrich Chemicals humic acid with 60Co was examined with respect to molecular size of humic acid. Correlations between the affinity and the structures of humic acid were studied. At low humic acid concentration range, 60Co was interacted with the humic acid of size fraction over 100kDa (HA(100<)). With increasing humic acid concentration, the 60Co was preferentially interacted with the 30k–100kDa of humic acid (HA(30–100)). Fractionated HA(100<) and HA(30–100) were characterized from their FTIR (Fourier Transform Infra-Red), 13C NMR spectral analyses and acid–base titration curves. The HA(10<) having aliphatic branched structure, smaller amount of COOH group and large proton exchangeable capacity, seem to show low covalent bonding nature and high ion exchangeability in the complexation. In addition, steric hindrance may affect on the complexation, by winding up like random coils from the branched structure. The HA(30–100) is dominated with the aromatic COOH group and OH group and it may preferentially coordinate to 60Co by covalent binding. These presumptions were supported by XPS analysis, in which the biding energy of cobalt-humates was discussed.

Humus composition and the structural characteristics of humic substances in soils under different land uses in Leyte, Philippines

The aim of the present study was to describe the effects of land use on humus composition and the structural properties of humic substances (HS) in degraded soil in Leyte, Philippines. Five adjacent land-use types, primary forest (PF), mahogany plantation (MP), rainforestation farming (RF), coffee plantation (CP) and grassland (GR), with comparable geology, parent material, soil type and climate were sampled and examined. The contents of humic acids (HAs), fulvic acids (FAs), polyvinylpyrrolidone (PVP)-adsorbed FAs and PVP-non-adsorbed FAs in the soil, and the elementary composition, infrared (IR) spectra and molecular size distribution of HAs and PVP-adsorbed FAs were compared among the different land uses. Results revealed that conversion of PF into other land uses has led to a decrease in FAs, which were higher under intensive cultivation conditions, such as RF, CP and GR. The amounts of HAs were small and also affected by land use. The amount of PVP-non-adsorbed FAs was more affected by land use than the amount of PVP-adsorbed FAs. Variation in the molecular size of HAs and PVP-adsorbed FAs provided evidence of variations in the structural characteristics of HS as a result of land-use change, although there were no appreciable differences in IR spectra and elementary composition. The relationship between land use and the degree of humification of HAs was poor. The HAs were still in the early stage of humification as indicated by the Type Rp in most soils, the higher contents of elemental H and N, and the high H/C and O/C ratios.

Sorption behavior of plutonium(IV) onto soils in the presence of humic acid

Influence of humic acid on the sorption of 238Pu onto geologic media using a coastal sand, which does not sorb humic acid, and an ando soil, which sorbs humic acid very well, was examined with respect to molecular sizes of humic acid. Apparent sorption affinity of 238Pu for the coastal sand decreased with increasing humic acid concentration. As to the ando soil, the sorption affinity of 238Pu became largest in the presence of several mg/dm3 of humic acid. The humic acid fraction smaller than 100,000 daltons relates to the complexation with 238Pu and the sorption onto the soils, particularly the humic acid fraction of 30,000-100,000 daltons was dominant. Most 238Pu formed humic compounds even in a low humic acid concentration region; resulting plutonium humates are considered to control the apparent sorption affinity of ~38Pu. It is concluded from above results that a specific molecular size of humic acid controls the complexation ability and the apparent sorption affinity of 238Pu.

Degradation characteristics of humic acid during photo-Fenton processes.

Changes in the molecular and structural characteristics of humic acid (HA) during photo-Fenton processes were studied. When aqueous solutions at pH 5.0, which contained HA, Fe(III), and H2O2, were irradiated (lambda > 370 nm), the concentrations of total organic carbon (TOC) decreased with increasing irradiation time, indicating that a portion of the HA was mineralized to CO2 during this process. To investigate the changes in molecular and structural characteristics, the HA was reisolated from the reaction mixtures after each period of irradiation. The increased elution volumes required for isolation by gel permeation chromatography indicated that the molecular size of HA decreased as a result of the irradiation. In the FTIR spectra, ether and epoxide functional groups were identified, after irradiation. These products could be formed via radical coupling and/or via peroxy radical addition reactions to the unsaturated groups in the HA, such as vinyl and aromatic groups. Moreover, an analysis of structural fragments in HA by pyrolysis-GC/MS showed that the cinnamic acid moieties (CA) disappeared, as a result of irradiation. In the molecular weight fractionated HA, the majority of the iron species were complexed with the high molecular weight HA fraction, and the CA levels of the high molecular weight fraction were larger than those in the low molecular weight fraction. These results are consistent with residues, as the reactive sites in the photo-Fenton systems. Therefore, the degradation of these sites in the high molecular weight fraction may serve as a factor in decreasing the molecular size of HA.

Effects of mineral and monocarboxylic acids on the molecular association of dissolved humic substances

SummaryThe conformational structure of dissolved humic substances is an important property that controls the reactivity of humus in the soil solution. High performance size‐exclusion chromatography was used here to study the changes in molecular size of different humic substances brought about by addition of mineral (HCl) and monocarboxylic (formic, acetic, propionic, and butyric) acids. The CPMAS‐NMR spectra showed that humic substances had varying chemical composition and that the ratio of hydrophilic to hydrophobic carbon (HI/HB) was greater for a humic acid from soil than for ones from oxidized coal and lignite. All humic substances showed a decrease in UV absorbance of chromatographic peaks when treated with either HCl or monocarboxylic acids. This was due to the hypochromic effect by which the absorptivity of associated molecules is decreased when they are separated. We attributed the molecular separation upon acid treatment to the formation of intermolecular hydrogen bonding that alters the original conformation stabilized mainly by weaker hydrophobic interactions. Addition of organic acids not only further decreased peak absorbances of humic acids but also caused their shift to larger elution volumes, indicating a larger conformational disruption than with HCl. The extent of the molecular size changes showed a relation to the number of carbons of monocarboxylic acids and to the HI/HB ratios of humic materials. The larger the carbon content of organic acids and the smaller the HI/HB ratio of humic materials, the larger was the decrease of the average molecular size of humic acids. These results suggest that dissolved humic substances associate predominantly by hydrophobic forces and that the apolar components of humic substances largely control their aggregation and reactivity in the environment.

Distribution Coefficient in the Sorption of Radionuclides onto Ando Soil in the Presence of Humic Acid. Influence of the Molecular Size of Humic Acid.

Influence of humic acid (HA) on the sorption of 60 Co, 85 Sr and 241 Am onto Ando soil was examined with respect to HA of various molecular sizes. Distribution coefficient (K d ) in the sorption of 60 Co on this type of soil was only slightly affected by the presence of HA. The K d of 85 Sr as the HA concentration increased. The K d of 241 Am decreased as the HA concentration increased and the K d of HA showed a similar trend. In the solution, 241 Am selectively formed stable compounds with an HA ranging from 30,000 to 100,000 in molecular weight (MW), whereas 60 Co and 85 Sr preferentially but weakly interacted with HA fractions smaller than 100,000 MW. These results suggest that the K d in the case of 60 Co and 85 Sr is mainly controlled by sorption of both cationic species and humic compounds, while the K d of 241 Am is controlled by the sorption of HA.

クロボク土への放射性核種の収着に及ぼす共存フミン酸のサイズの影響

クロボク土への60Co, 85Srおよび241Amの収着実験をバッチ法で実施し、クロボク土とこれら核種間の相互作用に及ぼすフミン酸の影響について，フミン酸の分子サイズに着目して調べた。60Coの分配係数(Kd)はフミン酸の存在によってほとんど影響を受けなかったが，85SrのKdは共存するフミン酸の濃度が高くなるに従って大きくなった。一方、241Amの Kdは，クロボク土に対するフミン酸のKdと同様に，共存するフミン酸の濃度が高くなるに従って小さくなった。水溶液中で、241Amは分画分子量30,000～100,000のサイズのフミン酸と安定な結合体を選択的に形成したが，60Coおよび85Srは100,000以下のサイズのフミン酸と241Amに比べて弱く結合することが分かった。これらの結果から，フミン酸との相互作用が小さな60Coや85SrのKdは陽イオンとフミン酸結合体の両化学種の収着によって主に支配されるが，フミン酸との錯形成能が高い241AmのKdはクロボク土に対するフミン酸の収着特性に依存することが示された。

Molecular Size and Functional Groups of Humic Substance Complexing with 60Co and 241Am.

The effects of molecular size of humic acid (HA) on the interaction affinity with 60Co and 241Am have been examined with respect to different molecular size fractions which were isolated from the Aldrich Chemical's HA into 5000-30000, 30000 -100000 and >100 000 in molecular weight (MW), while the functional groups of the molecular size fractions have been characterized with respect to their FTIR spectral characteristics.The HA of >100000 MW which was dominated with aliphatic functional groups did not take part in the interaction with 60Co and 241Am. The HA of 30000-100000 MW was dominated with both aromatic COOH and phenolic OH, and it perferentially interacted with 60Co and 241Am. However, 241Am did not interact with the HA of 5000-30000 MW which was mainly dominated with aromatic COON, while 60Co interacted. Such effects of molecular size of HA may be caused by more than two different functional groups concerning to the complexation and/or steric hindrance around the functional groups.

Effects of the extraction temperature on the characteristics of a humic fertilizer obtained from lignite

Solutions of 0·1 m and 0·25 m potassium hydroxide were employed to extract a humic fertilizer from lignite at 20, 50 and 80°C. The effects of the temperature on the yields of humic and fulvic acids and on the characteristics of the former were investigated. Higher extraction temperatures did not significantly enhance yield in either case, and neither were the ratios between humic and fulvic acids significantly affected. Humic acids released by hot 0·25 M KOH solutions held less N-rich material and exhibited a lower content of functional groups than those obtained at 20°C. When 0·1 M KOH was employed at 50 and 80°C, changes in the molecular structure of humic acids were less apparent but there were fewer functional groups than in the humic acids obtained at 20°C. Increasing extraction temperatures were matched by the larger molecular size of humic acids extracted with both KOH solutions.

Interactions of atrazine with humic substances of different origins and their hydrolysed products

Three humic acids (HA) extracted, respectively, from a volcanic soil, a North Dakota Leonardite, and an oxidised coal and their hydrolysed products obtained after acidic hydrolysis, were studied for their interaction with atrazine. Adsorption kinetics showed that HA from oxidised coal had the highest capacity of adsorbing atrazine (84%) followed in order by HAs from Leonardite (45.3%) and volcanic soil (31.7%). A reverse order is shown by the hydrolysed products, HA from volcanic soil being the highest adsorber (69.2%) followed by Leonardite HA (45.6%) and by oxidised coal HA (27%). Desorption kinetics, mass balance extractions and adsorption isotherms confirmed these results. Characterization by 13C NMR indicated that humic acid aromaticity decreased and aliphaticity increased in the order: oxidised coal, Leonardite, and volcanic soil. Chemical characterization showed that total, carboxylic, and phenolic acidities increased after hydrolysis for all HAs and failed to explain the observed differences in atrazine adsorption. Molecular weight distribution by gel permeation chromatography showed that the structural modifications induced by hydrolysis are able to explain the different adsorbing capacities of HA samples. Molecular dimension and concentration of chromophore groups seemed to govern atrazine adsorption. These results indicate that (1) atrazine is mainly adsorbed through a charge-transfer mechanism between electron-poor groups of HAs and electron-rich atoms in atrazine and (2) more atrazine is adsorbed the higher the aromaticity, the polycondensation, and the molecular size of HAs.

Formation of humic acids in lignites and subbituminous coals by dry air oxidation

A study of dry oxidation of coals by air has been performed. Four Spanish coals, ranging from lignite to subbituminous, were studied at three moderate temperatures for two different periods of time. The main goal with respect to possible use as fertilizers was to increase the humic acid content of the above-mentioned coals. The best results for humic acid production were obtained at the highest temperatures and after longer times. Lower-rank coals yielded the highest humic acid contentrations, but due to the severe loss of weight of these coals, the nett increase was lower than that obtained from subbituminous coals. A detailed study of the elemental composition of the coals, the generation of oxygenated functional groups and the molecular size of humic acids is reported. The evolution of sulphur forms during the process is also described.

Molecular size distribution of humic acids as affected by the ionic strength and the degree of humification

Molecular size distribution of soil humic acids was investigated by permeation chromatography on porous silica (µBondagel) and porous glass (Controlled Pore Glass, CPG) which enabled rapid analysis. Humic acid molecule was fully expanded at low ionic strength at pH 7.5, but it shrank considerably with increasing ionic strength. This behavior of humic acid molecule was analogous to that of linear polyelectrolyte molecule. Molecular size of humic acids decreased with increasing RF value and carboxyl group content and with decreasing 41og K value (namely with the increasing degree of humification). This phenomenon was attributed to oxidative depolymerization and intramolecular condensation during the process of humification. Based on the fractionation experiments on CPG, the fraction with the highest RF value and the lowest Δlog K value had an intermediate molecular size among the fractions. The excluded large molecular size fraction contained both highly humified components and a large amount of non-colored...