Pahokee Peat Humic Acid Research Articles

Electron Transfer from Humic Substances to Biogenic and Abiogenic Fe(III) Oxyhydroxide Minerals

Microbial humic substance (HS) reduction and subsequent abiotic electron transfer from reduced HS to poorly soluble Fe(III) (oxyhydr)oxides, a process named electron shuttling, significantly increases microbial Fe(III) mineral reduction rates. However, the importance of electron shuttling in nature and notably the electron transfer from HS to biogenic Fe(III) (oxyhydr)oxides have thus far not been determined. In this study, we have quantified the rate and extent of electron transfer from reduced and nonreduced Pahokee Peat humic acids (PPHA) and fresh soil organic matter (SOM) extracts to both synthetic and environmentally relevant biogenic Fe(III) (oxyhydr)oxides. We found that biogenic Fe(III) minerals were reduced faster and to an equal or higher degree than their abiogenic counterparts. Differences were attributed to differences in crystallinity and the association of bacterial biomass with biogenic minerals. Compared to purified PPHA, SOM extract transferred fewer electrons per milligram of carbon and electron transfer was observed only to poorly crystalline ferrihydrite but not to more crystalline goethite. This indicates a difference in redox potential distribution of the redox-active functional groups in extracted SOM relative to the purified PPHA. Our results suggest that HS electron shuttling can also contribute to iron redox processes in environments where biogenic Fe(III) minerals are present.

Humic acid complexation of Th, Hf and Zr in ligand competition experiments: Metal loading and pH effects

The mobility of metals in soils and subsurface aquifers is strongly affected by sorption and complexation with dissolved organic matter, oxyhydroxides, clay minerals, and inorganic ligands. Humic substances (HS) are organic macromolecules with functional groups that have a strong affinity for binding metals, such as actinides. Thorium, often studied as an analog for tetravalent actinides, has also been shown to strongly associate with dissolved and colloidal HS in natural waters. The effects of HS on the mobilization dynamics of actinides are of particular interest in risk assessment of nuclear waste repositories.Here, we present conditional equilibrium binding constants (Kc,MHA) of thorium, hafnium, and zirconium–humic acid complexes from ligand competition experiments using capillary electrophoresis coupled with ICP-MS (CE–ICP-MS). Equilibrium dialysis ligand exchange (EDLE) experiments using size exclusion via a 1000Da membrane were also performed to validate the CE–ICP-MS analysis. Experiments were performed at pH3.5–7 with solutions containing one tetravalent metal (Th, Hf, or Zr), Elliot soil humic acid (EHA) or Pahokee peat humic acid (PHA), and EDTA. CE–ICP-MS and EDLE experiments yielded nearly identical binding constants for the metal–humic acid complexes, indicating that both methods are appropriate for examining metal speciation at conditions lower than neutral pH. We find that tetravalent metals form strong complexes with humic acids, with Kc,MHA several orders of magnitude above REE–humic complexes. Experiments were conducted at a range of dissolved HA concentrations to examine the effect of [HA]/[Th] molar ratio on Kc,MHA. At low metal loading conditions (i.e. elevated [HA]/[Th] ratios) the ThHA binding constant reached values that were not affected by the relative abundance of humic acid and thorium. The importance of [HA]/[Th] molar ratios on constraining the equilibrium of MHA complexation is apparent when our estimated Kc,MHA values attained at very low metal loading conditions are compared to existing literature data. Overall, experimental data suggest that the tetravalent transition metal/-actinide–humic acid complexation is important over a wide range of pH values, including mildly acidic conditions, and thus, these complexes should be included in speciation models.

Complexes of the antimicrobial ciprofloxacin with soil, peat, and aquatic humic substances

Natural organic matter (NOM) is implicated in the binding of antibiotics by particles in soils and waters. The authors' previous computational study revealed structural rearrangement of both hydrophilic and hydrophobic moieties of NOM to favor H-bonding and other intermolecular interactions, as well as both competition with ion-exchange reactions and bridging interactions by NOM-bound divalent cations. The importance of these interactions was investigated using fluorescence-quenching spectroscopy to study the adsorption of ciprofloxacin (Cipro), a fluoroquinolone antibiotic, on 4 reference humic substances (HSs): Elliott soil humic acid (HA), Pahokee peat HA, and Suwannee river HA and fulvic acid. A simple affinity spectrum HS model was developed to characterize the cation-exchange capacity and the amount of H-bond donor moieties as a function of pH. The adsorption results stress the influence of both pH conditions and the type of HS: both soil HA and peat HA exhibited up to 3 times higher sorption capacity than the aquatic HS at pH ≥ 6, normalizing to the aromatic C content accounted for the differences among the terrestrial HS, and increasing the concentration of divalent cations led to a decrease in adsorption on aquatic HA but not on soil HA. In addition, the pH-dependent speciation models of the Cipro-HS complexes illustrate an increase in complexation due to an increase in deprotonation of HS ligands with increasing pH and, at circumneutral and alkaline pH, enhanced complexation of zwitterionic Cipro only in the presence of soil HA and peat HA. The findings of the present study imply that, in addition to electrostatic interactions, van der Waals interactions as facilitated by aromatic structures and H-bond donating moieties in terrestrial HS may facilitate a favorable binding environment. Environ Toxicol Chem 2013;32:1467-1478. © 2013 SETAC.

Humic acid addition lowers methane release in peats of the Mer Bleue bog, Canada

The effects of humic substances on methanogenesis in peats are poorly understood. In incubation experiments we added Pahokee Peat Humic Acid (DOM), a suppressor for methanogenesis (2-bromoethane sulfonic acid, BES), and BES + DOM to bog peat and studied CO2 and CH4 release. Addition of DOM lowered CH4 (p < 0.05) but not CO2 release compared to controls. BES addition suppressed CH4 release compared to controls (p < 0.01). DOM/BES addition lowered the release even further (p = 0.06) but did not induce net CH4 loss. DOM was thus utilized as electron acceptor and lowered CH4 release; anaerobic CH4 oxidation may have contributed to this effect.

Influence of humic acid imposed changes of ferrihydrite aggregation on microbial Fe(III) reduction

Microbial reduction of Fe(III) minerals at neutral pH is faced by the problem of electron transfer from the cells to the solid-phase electron acceptor and is thought to require either direct cell-mineral contact, the presence of Fe(III)-chelators or the presence of electron shuttles, e.g. dissolved or solid-phase humic substances (HS). In this study we investigated to which extent the ratio of Pahokee Peat Humic Acids (HA) to ferrihydrite in the presence and absence of phosphate influences rates of Fe(III) reduction by Shewanella oneidensis MR-1 and the identity of the minerals formed. We found that phosphate generally decreased reduction rates by sorption to the ferrihydrite and surface site blocking. In the presence of low ferrihydrite concentrations (5mM), the addition of HA helped to overcome this inhibiting effect by functioning as electron shuttle between cells and the ferrihydrite. In contrast, at high ferrihydrite concentrations (30mM), the addition of HA did not lead to an increase but rather to a decrease in reduction rates. Confocal laser scanning microscopy images and ferrihydrite sedimentation behaviour suggest that the extent of ferrihydrite surface coating by HA influences the aggregation of the ferrihydrite particles and thereby their accessibility for Fe(III)-reducing bacteria. We further conclude that in presence of dissolved HA, iron reduction is stimulated through electron shuttling while in the presence of only sorbed HA, no stimulation by electron shuttling takes place. In presence of phosphate the stimulation effect did not occur until a minimum concentration of 10mg/l of dissolved HA was reached followed by increasing Fe(III) reduction rates up to dissolved HA concentrations of approximately 240mg/l above which the electron shuttling effect ceased. Not only Fe(III) reduction rates but also the mineral products changed in the presence of HA. Sequential extraction, XRD and 57Fe-Mössbauer spectroscopy showed that crystallinity and grain size of the magnetite produced by Fe(III) reduction in the presence of HA is lower than the magnetite produced in the absence of HA. In summary, this study shows that both the concentration of HA and Fe(III) minerals strongly influence microbial Fe(III) reduction rates and the mineralogy of the reduction products. Thus, deviations in iron (hydr)oxide reactivity with changes in aggregation state, such as HA induced ferrihydrite aggregation, need to be considered within natural environments.

Cd and Pb removal from contaminated environment by metal resistant bacterium Cupriavidus metallidurans CH34: importance of the complexation and competition effects

Environmental contextLive bacteria are widely used to remove toxic metals from contaminated environments. We use the metal-resistant bacterium Cupriavidus metallidurans, in both model solutions and aqueous extracts of soils, to investigate the complexation and competition effects on Cd and Pb uptake. Accumulation of Cd was more affected by competition with Ca, Mg and Zn, whereas Pb accumulation was more influenced by complexation with humic acids. The study highlights the need to consider chemical site-specificity in the removal of metals from contaminated environments. AbstractThe present study aims to improve the understanding of the role of complexation and competition effects on Cd and Pb accumulation by the metal resistant bacterium Cupriavidus metallidurans largely used in bioremediation. Adsorbed and intracellular metal content in bacteria were determined in model exposure medium within a concentration range spanning from 10–9 to 5 × 10–5 M of Cd or Pb and water extracts from soils. In parallel, the free metal ion concentrations ([M2+]) were measured by an ion exchange technique. Obtained results demonstrated that Cd and Pb accumulation by C. metallidurans was related to [M2+] in the solution. The adsorbed and intracellular M fractions were significantly reduced by nitrilotriacetic acid, Elliot or Pahokee Peat humic acids, as well as by a large excess of Ca, Mg and Zn. No effect on Cd and Pb bioaccumulation was observed in the presence of Mn, Cu or Co at a 10-fold excess for bacteria exposed to 10–6 M of Cd or Pb. Adsorbed and intracellular metal determined when bacteria were exposed to water extracts of soil were in the same order as expected from the model experiments when complexation and competition effects are considered. The study emphasises the necessity of taking into account chemical site-specificity of soil solutions and water, including dissolved organic ligands, pH and the presence of other metals when developing metal removal technologies by living bacteria.

Mobilization and deposition of iron nano and sub-micrometer particles in porous media: A glass micromodel study

Mobilization and deposition of iron nano and sub-micrometer particles (INSMP) in a porous medium were investigated using a water-saturated glass micromodel. The deposition and detachment of INSMP in the micromodel were visualized by taking serial images and experimentally verified by analysis of breakthrough curves. This first visualization study of INSMP fate showed that there were dense aggregations at the pores as the concentration of INSMP increased. The presence of dissolved humic substances (>1 ppm) significantly reduced deposition of suspended particles and enhanced detachment of the deposited particles. The mobility of INSMP in the presence of Pahokee peat fulvic acid standard II (PPFA) was higher than for Pahokee peat humic acid standard I (PPHA) due to the presence of more aromatic groups and the molecular weight in PPFA. Interfacial energy estimation based on the DLVO theory revealed that the adsorption of humic substances onto the INSMP increased the energy barrier and reduced the depth of secondary minimum between particles. The “affinity transition” in the initial deposition of INSMP within the micromodel was observed in the presence of Pahokee peat humic substances.

Organic ligands influence leaching kinetics of fixated FGD material

The presence of organic ligands, including oxalate, citrate, maleate and Pahokee peat humic acid (PPHA), influenced the leaching kinetics of fixated flue gas desulfurization (FGD) material in acidic solution (pH 2.9–5.0). In the presence of oxalate, the leaching was inhibited at all pH values examined. XRD and SEM analyses demonstrated that formation of a calcium oxalate mineral phase on the fixated FGD material surface created a surface layer which reduced the extent of leaching. Maleate and PPHA inhibited leaching at pH 2.9, but either promoted, inhibited, or had no effect on leaching, depending on the particular element, at pH 5.0. ATR-FTIR analysis indicated maleate and PPHA formed both inner- and outer-spherically bound species at pH 2.9, whereas at pH 5.0 only outer-sphere complexes seemed evident. These surface species likely inhibited the leaching process at pH 2.9 through a surface blocking mechanism. At pH 5.0, the ligand surface complexes either promoted or inhibited leaching, depending on the element, through a combination of direct and indirect mechanisms. Citrate significantly promoted the leaching process at all pH values examined.

A capillary electrophoresis-ICP-MS study of rare earth element complexation by humic acids

To predict metal–humic interactions in nature, experiments must be carried out using environmentally relevant concentrations of metal and humic substances (HS). Capillary electrophoresis coupled with ICP-MS (CE-ICP-MS) is a rapid and sensitive method to separate and detect metal species at trace-element concentrations. In this study CE-ICP-MS was used to measure partitioning of the rare earth elements (REE) between HS and a competing ligand (EDTA) under near environmental conditions (pH 6–10, 0.1 mol L − 1 NaNO 3, 100 nmol L − 1 REE, 10 mg L − 1 HS). Conditional binding constants, K c,LnHS, were calculated for REE complexed with Elliot soil humic acid, Pahokee peat humic acid, Summit Hill soil humic acid, and Suwannee River natural organic matter and compared to existing data for REE complexation by Leonardite coal humic acid and Suwannee River fulvic acid. Results of these experiments show an increase in log K c,LnHS with decrease in ionic radius for REE-humic complexes (the lanthanide contraction effect), overall log K c,LnHS values ranging from 9 and 16 and indicating strong REE-HS binding, and a 2.7 log unit variation in K c,LnHS among humic sources. The inter-humic affinity for REE correlates with proton affinity and aromaticity of the HS sources.

Binding Constants of Divalent Mercury (Hg2+) in Soil Humic Acids and Soil Organic Matter

Distribution coefficients (K(OC)) for Hg2+ binding by IHSS Pahokee peat humic acid (PHA) and humic acids separated from O-horizons and peats in a northern temperate forest were determined using a competitive ligand-exchange method. All measurements were made at low ratios of added Hg2+ to reduced S. The commonly used chelating agents, EGTA and DTPA, were found to be ineffective competitive ligands; thus, we used DL-penicillamine, a synthetic amino acid with a thiol group. Calculated free [Hg2+] at equilibrium is very low, ranging from 10(-26.4) at pH 1.9 to 10(-36.9) at pH 5.8. Corresponding log Koc values ranged from 22.6 to 32.8. The slope of the plot of pH versus log K(OC) was 2.68, suggesting that two or more protons are released when each Hg2+ is bound. This is consistent with binding of Hg2+ to bidentate thiol sites with some participation of a third weak-acid group, presumably a thiol. The 1:2 stoichiometry is consistent with X-ray spectroscopy data for Hg2+ bound to HA and with other pH-dependency results showing release of two protons with the binding of each Hg2+. Our K(OC) values are much greater than indicated by the data from most previous studies.

Adsorption of Organic Matter at Mineral/Water Interfaces. IV. Adsorption of Humic Substances at Boehmite/Water Interfaces and Impact on Boehmite Dissolution

The adsorption of Suwannee River fulvic acid (SRFA) and Pahokee peat humic acid (PPHA) at the boehmite (gamma-AlOOH)/water interface and the impact of SRFA on boehmite dissolution have been examined over a wide range of solution pH conditions (pH 2-12), SRFA surface coverages (Gamma(SRFA), total SRFA binding site concentration normalized by the boehmite surface area) of 0.0-5.33 micromol m(-2), and PPHA surface coverages (Gamma(PPHA), PPHA binding site concentration normalized by boehmite surface area) of 0.0-4.0 micromol m(-2), using macroscopic adsorption and in situ attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy. At relatively high SRFA surface coverages (Gamma(SRFA) = 5.33 micromol m(-2)), in situ ATR-FTIR spectral features of adsorbed SRFA are very similar to those measured for SRFA in solution at approximately 1-3 pH units higher. At sub-monolayer surface coverages (Gamma(SRFA) = 1.20 and 2.20 micromol m(-2)), several new peaks and enhancements of the intensities of a number of existing peaks are observed. The latter spectral changes arise from several nonorganic extrinsic species (i.e., adsorbed carbonate and water, for alkaline solution conditions), partially protonated SRFA carboxyl functional groups (near-neutral pH conditions), and small quantities of inner-spherically adsorbed SRFA carboxyl groups and/or Al(III)-SRFA complexes (for acidic conditions). The spectra of PPHA adsorbed at boehmite/water interfaces also showed changes generally consistent with our observations for SRFA sorbed on boehmite. These observations confirm that SRFA and PPHA are predominantly adsorbed at the boehmite/water interface in an outer-sphere fashion, with minor inner-sphere adsorption complexes being formed only under quite acidic conditions. They also suggest that the positively charged boehmite/water interface stabilizes SRFA and PPHA carboxyl functional groups against protonation at lower pH. Measurements of the concentration of dissolved Al(III) ions in the absence and presence of SRFA showed that the boehmite dissolution process is clearly inhibited by the adsorption of SRFA, which is consistent with previous observations that outer-spherically adsorbed organic anions inhibit Al-(oxyhydr)oxide dissolution.

Nitrite Fixation by Humic Substances Nitrogen‐15 Nuclear Magnetic Resonance Evidence for Potential Intermediates in Chemodenitrification

Studies have suggested that NO−2, produced during nitrification and denitrification, can become incorporated into soil organic matter and, in one of the processes associated with chemodenitrification, react with organic matter to form trace N gases, including N2O. To gain an understanding of the nitrosation chemistry on a molecular level, soil and aquatic humic substances were reacted with 15N‐labeled NaNO2, and analyzed by liquid phase 15N and 13C nuclear magnetic resonance (NMR). The International Humic Substances Society (IHSS) Pahokee peat and peat humic acid were also reacted with Na15NO2 and analyzed by solid‐state 15N NMR. In Suwannee River, Armadale, and Laurentian fulvic acids, phenolic rings and activated methylene groups underwent nitrosation to form nitrosophenols (quinone monoximes) and ketoximes, respectively. The oximes underwent Beckmann rearrangements to 2° amides, and Beckmann fragmentations to nitriles. The nitriles in turn underwent hydrolysis to 1° amides. Peaks tentatively identified as imine, indophenol, or azoxybenzene nitrogens were clearly present in spectra of samples nitrosated at pH 6 but diminished at pH 3. The 15N NMR spectrum of the peat humic acid exhibited peaks corresponding with N‐nitroso groups in addition to nitrosophenols, ketoximes, and secondary Beckmann reaction products. Formation of N‐nitroso groups was more significant in the whole peat compared with the peat humic acid. Carbon‐13 NMR analyses also indicated the occurrence of nitrosative demethoxylation in peat and soil humic acids. Reaction of 15N‐NH3 fixated fulvic acid with unlabeled NO−2 resulted in nitrosative deamination of aminohydroquinone N, suggesting a previously unrecognized pathway for production of N2 gas in soils fertilized with NH3