Differential Complexation between Zn2+ and Cd2+ with Fulvic Acid: A Computational Chemistry Study

A method was developed for the analysis of humic and fulvic acids by size-exclusion chromatography-electrospray ionization-tandem mass spectrometry using a completely volatile eluent. Humic and fulvic acids were separated into three peaks. These fractions occupied different mass ranges and showed differences in the fine structure of their mass spectra. The low-molecular-weight (LMW) fraction of fulvic acids is most sensitively determined by ESI-MS, and it appears that previous results obtained by infusion-ESI-MS were primarily determined by this fulvic acid fraction. The average molecular weight of this fractions turned out to be lower than that reported from infusion-ESI-MS measurements. Its scan spectra and the product ion spectra of some of its molecular anions perfectly match those previously obtained from whole fulvic acid mixtures. Obviously, a class of well-defined polycarboxylated molecules exist that occurs in all fulvic acid fractions thus far investigated. With decreasing elution time and increasing molecular weight, detection by ESI-MS loses sensitivity as compared to the parallel UV recording, and the fine structure of the scan spectra becomes increasingly uniform for both fulvic and humic acids. The average molecular weight of the HMW fraction exceeds those values calculated from infusion experiments. Scan spectra and product ion spectra of the high-molecular-weight (HMW) fraction of both the humic and the fulvic acids suggest that the HMW fraction consists of several subunits that originate from the LMW fraction.

Analysis of the carbohydrates of fulvic and humic acids as their partially methylated alditol acetates

The Fourier transform Surface-enhanced Raman spectra of fulvic and humic acids from sea water adsorbed on an Au electrode were determined. At neutral and basic conditions, the humic acid was hung on the Au surface through its carbonyl groups, and the fulvic acid from the same source was adsorbed in both “end on” and “flat on” configurations. At acidic conditions, both of the fulvic and humic acids were adsorbed on the surfaces through their carboxylate and pyridine-like groups. The SERS behaviours of the humic and fulvic acids at different pH and potentials were from the difference of their chain sizes, charges and flexibility as well as the changes of their conformations. This preliminary study suggested that Fourier transform Surface-enhanced Raman spectroscopy could be used as a powerful probe with which to directly “see” humic substances at low concentrations.

Influence of humic, fulvic and hydrophilic acids on the growth, photosynthesis and respiration of the dinoflagellate Prorocentrum minimum (Pavillard) Schiller

The presence of natural organic matter (NOM) in drinking water can increase the levels of copper released from copper pipes to water and inhibit the formation of protective deposits such as malachite. Since adsorption of NOM on copper pipes surfaces is believed to be one of mechanisms that explains this phenomenom, the objective of this study was to determine kinetics and the adsorption equilibrium of main components of NOM, humic acid (HA) and fulvic acid (FA), onto copper surfaces. The kinetics and equilibrium adsorption of HA and FA on copper foils were examined using batch experiments at 22°C. HA and FA followed pseudo second-order kinetics adsorption. Rate constants measured were 2.59 × 10−1 (mgTOC cm−2 h−1) for HA and 3.13 × 10−1 (mgTOC cm−2 h−1) for FA. The adsorption behavior of HA and FA on the copper surface is in accordance with the Langmuir adsorption isotherm. Langmuir adsorption constants measured were 5.98 × 10−2 L mg−1 for HA and 4.78 × 10−2 L mg−1 for FA. The copper foils exposed during five months to FA formed malachite deposits, whereas those exposed to HA did not and just cuprite was found. The results of this study showed that both HA as well as FA adsorption on copper surfaces is favored and no significant differences were found in the adsorption parameters calculated for both compounds. However, the inhibition of the malachite precipitation could be attributed to the HA adsorption.

The time course photodegradation of the Nordic aquatic fulvic and humic acids and Suwannee River XAD-4 acids subjected to UV irradiation with an unfiltered medium pressure mercury lamp was studied by liquid-state 13C nuclear magnetic resonance. Photodecarboxylation was a significant pathway in all cases. Decreases in ketone, aromatic, and O-alkyl carbons were observed throughout the course of the irradiations, whereas C-alkyl carbons resisted photodegradation. Peaks attributable to the low-molecular-weight photodegradation products bicarbonate, formate, acetate, and succinate grew in intensity with irradiation time. The final products of the irradiations were decarboxylated, hydrophobic, predominantly C-alkyl and O-alkyl materials that were resistant to further photodegradation. The total amount of carbon susceptible to loss appeared to be related mainly to the total concentration of carbonyl and aromatic carbons and partly to the concentration of O-alkyl carbons in the fulvic, humic, and XAD-4 acids. The carbon losses for Nordic fulvic, Nordic Humic, Suwannee fulvic, and Suwannee XAD-4 acids were estimated to be 75, 63, 56, and 17%, respectively. More detailed analyses of the effects of irradiation on the carbonyl functionality in Nordic humic acid and Laurentian soil fulvic acid through reaction with hydroxylamine in conjunction with 15N nuclear magnetic resonance analysis confirmed preferential photodegradation of the quinone/hydroquinone functionality over ketone groups and the loss of ester groups in Laurentian fulvic acid.

The objective of this study was to investigate the effects of fulvic acid (FA) and humic acid (HA), the two main compounds of humic substances (HSs), on copper (Cu) and zinc (Zn) homeostasis. Seventy-two male Wistar rats were randomly divided into nine experimental groups. The control diet (AIN-93G formula) and the diets supplemented with 0.1%, 0.2%, 0.4% and 0.8% FA or HA were fed for 26 days. Cu and Zn concentrations of the large intestinal content (LIC), liver, kidney, femur and hair were determined. FA and HA did not influence significantly the Cu or Zn contents of the experimental diets, the rats' feed intake, weight gain and the feed to gain ratio. Both FA and HA decreased the Cu concentrations of the LIC significantly and in a dose-related manner; however the absorption-stimulating effect of HA was more pronounced. FA increased the Cu content of the liver, but neither FA nor HA had a dose-dependent effect on it. FA or HA supplementations had no significant effect on the Cu concentration of the kidney. At the concentrations used, dietary FA or HA supplementations are not promising growth promoters. FA influences the Cu homeostasis unlike HA, because FA not only stimulates Cu absorption, but the extra quantity of absorbed Cu is retained in the organism. The stimulatory effect of HA on Zn absorption may not be manifested in Cu and Zn homeostasis, because of the tight connection of these microelements to FA and HA, which prevents the transmission of Zn from the ZnHA complex to the organs. As regards the effect of FA and HA on Cu and Zn homeostasis, both FA and HA stimulated the absorption of these microelements, but only FA increased the retention of Cu (in the liver) and Zn (in the kidney).

Inhibition of dicalcium phosphate dihydrate (DCPD) precipitation rate by organic acids may play an important role in controlling phosphate activity and availability in P‐fertilized soils. The precipitation rate of DCPD was measured at pH 5.7 and 25 °C in the absence and presence of organic acids common to soil solutions using a seeded crystal growth method. Forward rate constants ( k f ) were determined from an integrated form of the second‐order rate law expression: rate = k f S [Ca 2+ ][HPO 2‐ 4 ]γ 2 2 , where brackets represent concentrations, S is the surface area (m 2 L −1 ), and γ 2 is the divalent ion activity coefficient calculated from the Davies equation. The average k f (L 2 mol −1 m −2 s −1 ) for DCPD precipitation without organic acids was 9.7 ± 0.5. Additions of 0.26 to 2.06 m M C as humic acid and 1.1 to 3.3 m M C as fulvic acid resulted in induction periods lasting 2.0 to 632 and 1.5 to 10 min, respectively, prior to subsequent precipitation of DCPD. This indicates that DCPD precipitation was able to overgrow adsorbed humic and fulvic acids. Rates of DCPD precipitation were determined in the presence of humic, fulvic, citric, and tannic acids at concentrations ranging from 0.26 to 9.0 m M total soluble C (C TS ). The highest levels of humic, fulvic, citric, and tannic acids added were 2.05, 7.8, 8.0, and 9.0 m M C TS , which resulted in decreased k f values of 1.1, 0.1, 0.6, and 2.1 L 2 mol −1 m −2 s −1 , respectively. Precipitation was inhibited by adsorption of these organic acids onto DCPD surfaces blocking sites acting as nuclei for new crystal growth. Adsorption characteristics of these organic acids were related to their functional‐group content and size. The ability of DCPD to precipitate on seed crystals with adsorbed C may explain the kinetically favored formation of DCPD vs. more thermodynamically stable octacalcium phosphate (OCP) and hydroxyapatite (HAP), where precipitation rates are more strongly inhibited by soluble C.

To gain a better understanding of the role of charred plant materials, which were produced during the burning of vegetation by human activity and wildfires, in the formation of humic and fluvic acids in Japanese volcanic ash soils, the quantitative contribution of charred and buried plant fragments to their acids in whole soils were investigated using three volcanic ash soil samples. Charred fragments were the main components in the fraction of less than specific gravity 1.6 g cm−3 (< 1.6 fraction), which was isolated after HCl–HF treatment of the soil samples. The percentage contribution of organic C content in the < 1.6 fractions to that of the whole soils ranged from 13.9 to 32.0%. All humic acids obtained from the < 1.6 fractions and whole soils were classified into Type A, which is characterized by a high degree of darkening and the presence of a graphite-like structure. However, the color coefficient (ΔlogK) and relative color intensity (RF) values of the humic acids in the < 1.6 fractions were different from those in whole soils. In all soils studied, the amounts of NaOH-extractable humic (OH-HA) and fulvic acids (OH-FA) were much greater than those of Na4P2O7-extractable humic (SPP-HA) and fulvic acids (SPP-FA), respectively, and the amounts of humic acids were substantially greater than those of fulvic acids. The proportion of the quantitative contribution of humic and fulvic acids in the < 1.6 fractions to those in the whole soils ranged from 12.0 to 43.8% for OH-HA, from 3.80 to 9.56% for OH-FA, and from 2.92 to 22.3% for SPP-HA, respectively. The proportion was very small for SPP-FA. It was assumed that in Japanese volcanic ash soils, parts of charred plant materials are subjected to oxidative degradation over a long period of time after burning, and are converted to fulvic acids and, particularly, humic acids.

The compound nitrapyrin is easily adsorbed by soil organic matter in high-organic matter soils, and this results in its effectiveness reducing significantly. In this study, the adsorption characteristics and mechanisms of nitrapyrin as an adsorptive on humic acid (HA) and fulvic acid (FA) as adsorbents were investigated. The results showed that the kinetics of adsorption of nitrapyrin on both HA and FA followed pseudo-second-order kinetic models (R2 ≥ 0.925, P < 0.05) and the adsorption process included an initial fast-adsorption stage and a slow-adsorption stage thereafter. The adsorption efficiencies of nitrapyrin on HA + FA were higher than that on HA or FA alone, and that of HA was higher than that of FA. The adsorption isotherms of nitrapyrin on HA and FA could be optimally fitted with the Langmuir equation (R2 ≥ 0.982, P < 0.05). The maximum adsorption capacities of nitrapyrin on HA, FA and HA + FA were 4896.49, 3173.70 and 4925.56 mg kg−1, respectively. Synergistic adsorption of nitrapyrin in co-existing systems of HA and FA was also observed. The adsorption mechanism of nitrapyrin on both HA and FA involved hydrogen bonding and hydrophobic interaction. Therefore, HA and FA in the soil environment can adsorb a large amount of nitrapyrin and reduce its effectiveness, and they have a positive synergistic effect.

Natural organic polyelectrolytes, such as humic and fulvic acids, were removed by adsorption onto silicate rocks. Tobermorite, zeolite, and molecular sieves 5A were used as the adsorbents. Tobermorite was more efficient by 40−50% in the removal of fulvic acids, and by 30−50% for humic acid than zeolite or molecular sieves, respectively. Humic acid removal from the solution by adsorption onto silicate rocks took place more readily than fulvic acid removal. From the determined heat of adsorption, the adsorption process in the present study may be chemisorption (ligand exchange). Metal/humic acid complexes were effectively removed by adsorption onto tobermorite. Because tobermorite (a silicate rock) can be easily synthesized and obtained commercially, the adsorption method of removal of fulvic and humic acids is superior to their precipitation.

Humic and fulvic acids in environmental water were determined by UV spectrophotometry after concentration with diethylaminoethyl(DEAE)-cellulose and separation at pH 1 by centrifugation. The ranges of fulvic and humic acids in Katsura, Uji, Kidzu and Yodo Rivers were from about 0.50 to 3.2 mg/l and 0.025 to 0.2 mg/l, except for the sampling points, which were located downstream of a sewage-disposal plant. Fulvic acid was predominant in these rivers. The concentrations of humic substances were high in the summer and low in the winter. The concentrations of humic substances in these rivers decreased fairly much from December of 1992 to August of 1993. The annual changes in the values of humic substances during 1993 to 1996 were smaller than the change from 1992 to 1993. The trends for the seasonal and annual changes in the concentration of the trihalomethane formation potential in Yodo rivers were almost consistent with those of humic substances.

The complexation of Cu and Cd by ligands observed in filtered unfractionated lakewaters is compared to the complexation by humic and fulvic acids. Complexation parameters (conditional stability constants and ligand concentrations) of Suwannee River fulvic acids (FA), purified peat humic acids (HA) and of ligands in lakewater samples have been determined using the same methods (ligand-exchange and CSV (cathodic stripping voltammetry) or ASV (anodic stripping voltammetry)), and the same titration ranges of Cu, Cd and organic carbon concentrations. The performance of the used techniques is first evaluated in FA and HA suspensions, and gives comparable results with the literature values for the same materials, according to published models (5-site model, NICA model) and parameters. Model calculations using the WHAM model for FA and HA (Tipping, 1994) are also presented. The comparison of titrations of FA and HA with Cu and Cd with those of lakewater samples indicates that stronger ligands than FA and HA are present at low concentrations in the lakewaters. Specific strong ligands occur in particular in eutrophic lake waters, whereas in a lake with higher metal concentrations and low biological productivity the ligands more closely match the fulvic acid characteristics.

Humic acids (HAs) and fulvic acids (FAs) are naturally occurring compounds that influence the fate and transportation of various compounds in the soil. Although HAs and FAs have multiple uses, the reports about their sorbent potential for environmental pollutants are scanty and sparse. In this study, HA and FA, isolated from lignite samples from two mines in Greece, were studied as sorbent materials for three active compounds of plant protection products, namely glyphosate (herbicide), cypermethrin (pyrethroid insecticide), and azoxystrobin (fungicide). According to the results, both HA and FA are promising sorbent materials for these active compounds, with HA achieving better sorption for cypermethrin and azoxystrobin, while FA was found to be more efficient for glyphosate. Moreover, their performance was not compromised by other components commonly found in commercially available herbicides/insecticides/fungicides. In addition, no significant leaching of the sorbed compounds was recorded. Finally, the two materials achieved similar sorption efficiency of the compounds from lake water.

A better understanding of metal ion binding to soil organic substances is of fundamental importance in geochemical modeling of environments. Fulvic acids (FA) and humic acids (HA) make up an important part of soil organic matter, and their binding capacity affects the fate of metal ions and plays an important role in their mobility. Binding constants of Pb(II) to HA and FA were evaluated by anodic stripping square wave voltammetry (ASSWV) where the binding reaction was performed at pH 4.5 in a medium of 0.020 mol L-1 KNO3. Results showed that ASSWV technique was well suited for the estimation of the binding capacity of a natural organic matter towards heavy metals. Based on the voltammetric titration curves, binding constants of Pb(II) complexes formed with HA and FA were 0.78times106 and 0.15times106, which indicated that complex of Pb2+ with HA was more stable than with FA. The average molecular weight of HA and FA prepared from soil samples were also found to be 1821 g mol.L-1 and 805 g mol.L-1, respectively.