Elliot Soil Humic Acid Research Articles

Spectroscopic studies of the role of dissolved organic matter in acenaphthene photodegradation in liquid water and ice

The effect of concentration and origin of dissolved organic matter (DOM) on acenaphthene (Ace) photodegradation in liquid water and ice was investigated, and the components in DOM which were involved in Ace photodegradation were identified. The DOM samples included Suwannee River fulvic acid (SRFA), Elliott soil humic acid (ESHA), and an effluent organic matter (EfOM) sample. Due to the production of hydroxyl radical (•OH) and triplet excited-state DOM (3DOM*) which react with Ace, DOM had promotion effects on Ace photodegradation. However, the promotion effects of DOM were prevailed over by their suppressing effect of DOM including screening light effect, intermediates reducing effect and RS quenching effect, and thus, the photodegradation rates of Ace decreased in the presence of the three DOM with concentrations of 0.5–7.5 mg C/L in liquid water and ice. ESHA had higher light absorption and thus had higher screening light effect on Ace photodegradation in liquid water than SRFA and EfOM. At each DOM concentration, ESHA exhibited higher promotion effect on Ace photodegradation than SRFA and EfOM, in liquid water and ice. The binding of Ace with DOM was indicated by decreases in fluorescence intensity of Ace when coexisted with DOM. However, the binding of Ace to DOM played an unimportant role in suppressing Ace photodegradation. The photodegradation behavior of fluorophores in Ace with DOM present in ice was not similar to that in liquid water. C–O, C═O, carboxyl groups O–H and aliphatic C–H functional groups in DOM were involved in the interaction of DOM with Ace. The presence of Ace seemed to have no influence on the photodegradation behavior of functional groups in DOM.

Elucidating the conformation effects within adsorption of natural organic matter on mesoporous graphitic carbon

The conformation effects were frequently reported to affect the adsorption of natural organic matter (NOM), but such a specific mechanism is poorly understood. To address this knowledge gap, based on the adsorption of NOM on mesoporous graphitic carbon nitride (MCN) prepared via a novel pseudo-templating method, the conformation effects were elucidated. The results showed that MCNs nano-architectures with uniform mesopores were successfully fabricated and showed prominent adsorption capacity. The maximum adsorption amounts of MCN-0.5 for fulvic acid (FA) and leonardite humic acid (HA) were 73.61 mgC g−1 and 135.57 mgC g−1, respectively. The adsorptive affinities of components for HA and Elliott soil HA (ESHA) were related to structural condensation; the components with the more coiled conformation exhibited higher affinities because of occupying less active sites and weakening the electrostatic repulsion. Such conformation effects were further established with respect to NOM types and components, as well as solution chemistry. HA and ESHA suffered from variations from the coiled to linear conformations as pH increased, but not the other four types of NOM tested, and particularly the humic-like and territorial humic-like components were critical in such conformational variations. The adsorption of HA and ESHA was affected by pH via both the conformation effects and the interaction mechanism. By contrast, the conformation of six types of NOM was not affected by the common cations (Na+, K+, Mg2+, Ca2+), and these cations impacted NOM adsorption only through the conventional interactions.

Effect of dissolved organic matter on sulfachloropyridazine photolysis in liquid water and ice

Dissolved organic matter (DOM) is an ubiquitous component of environmental snow and ice, which can absorb light and produce reactive species (RS) and thus is of importance in ice photochemistry. The photodegradation of sulfachloropyridazine (SCP) without and with DOM present in liquid water and ice were investigated in this study. The photodegradation rate constants for SCP without DOM present was enhanced by 52.5 % in ice relative to liquid water, likely due to the enhanced role of SCP self-sensitized RS in ice. DOM significantly promoted SCP photolysis in both liquid water and ice, which was mainly attributed to roles of singlet oxygen (1O2) and triplet excited-state DOM (3DOM*) generated from DOM. 1O2 production from DOM was significantly enhanced in ice relative to liquid water. Hydroxyl radical (•OH) production from DOM in ice was similar to those in liquid water. Enhancement in 3DOM* production in ice was observed at low DOM concentrations. Suwannee River Fulvic Acid (SRFA) and Elliott Soil Humic Acid (ESHA) exhibited differences in RS production in liquid water and ice, as well as in enhancement of 1O2 and 3DOM* produced in ice relative to liquid water. DOM induced reaction pathways of SCP different from those without DOM present, and therefore affected toxicity of SCP photoproducts. There were differences in photodegradation pathways of SCP as well as in toxicity of SCP photoproducts between liquid water and ice.

Diffusion of sulphonamide antibiotics in agarose hydrogels enriched by humic acids

Many different pharmaceuticals can evince potentially dangerous behaviour in soil and aqueous environments. Their migration ability and toxicity can be significantly affected by interactions with organic matter. Sulfamethoxazole and sulphapyridine (used as representative commonly used sulphonamide antibiotics) are studied from the point of view of their transport in model hydrogels containing humic substances. Agarose hydrogel was used as a model medium for the transport experiments. Standard Elliot soil humic acid (4S102H) purchased from International Humic Substances Society was incorporated into the hydrogel as an active substance (and representant of organic matter) for interactions with antibiotics and potential suppression of their migration and toxicity in soil systems. Two different experimental arrangements were used: the diffusion from constant source, and the diffusion couple. Diffusion coefficients were determined for pure agarose hydrogels as well as for hydrogels containing humic acids and compared. It was found that the addition of humic acids into hydrogel resulted in the decrease in the effective diffusion coefficient of sulphapyridine and the increase in the effective diffusion coefficient of sulfamethoxazole. Humic acids can suppress the mobility of sulphapyridine and enhance the diffusion of sulfamethoxazole. Batch sorption experiments provided similar adsorption capacities of humic acids for both studied antibiotics. The ratio between adsorption and desorption rate constants were higher than 1 for sulphapyridine and lower than 1 for sulfamethoxazole. It indicated that adsorption predominated for sulphapyridine which resulted in observed lower diffusivity. Desorption experiments showed that a majority of sulphapyridine can be immobilized by interaction with humic acids whereas much more free movable sulfamethoxazole was determined in the comparison with immobilized one.

Release of adsorbed copper and carbon during Fe(Ⅱ) catalytic conversion of ferrihydrite-humic acid coprecipitation under acidic condition: Mechanism and properties

The mobility of copper (Cu) may be significantly impacted during the conversion of ferrihydrite-humic acid (Fh-HA) coprecipitates. However, the environmental behavior of the adsorbed Cu in Fh-HA coprecipitation and the kinetic process of organic carbon (C) during the Fe(Ⅱ)-catalyzed Fh-HA coprecipitates conversion remains unclear. In this study, a series of Fh-HA coprecipitates with various C/Fe molar ratios were synthesized. Furthermore, the effects of the Fe(Ⅱ) mediated Fh and Fh-HA conversion processes on Cu mobility and carbon release characteristics under acidic condition were investigated. The results show that the specific surface area of Fh-HA is reduced as compared to pure Fh, which impedes the conversion of Fe minerals and increases the release of Cu to the solution. The rate of conversion can be accelerated by increasing the Fe(II) concentration, resulting in the retention of Cu in the mineral structure. As compared to artificial HA, Elliott soil humic acid (ESHA) coprecipitated with Fh resulted in a reduced Cu release. According to results of the fluorescence excitation emission matrix-parallel factor analysis, the "release-resorption" process of carbon occurred during conversion and low molecular weight HA related to the biological activity was the main source of the released organic carbon. The major functional groups were C-O functional groups and phenolic hydroxyl groups, and Cu was involved in the formation of Fe-O-Cu after conversion. The findings of this study provide crucial information for comprehending the biogeochemical process of Cu associated with Fh and Fh-HA coprecipitates.

Application of carbon nitride nanosheets for adsorption of various humic substances from aqueous solutions

Humic substances (HSs) cause serious problems such as membrane fouling and disinfection by-products. Particularly, HSs as the main UV-quenching substances in landfill leachate pose great challenges. In this study, we evaluated the application of carbon nitride nanosheets (CN-Ns) for adsorption of various HSs, as well as leachate nanofiltration concentrates (LNCs). The characterization revealed that as increasing thermal oxidation temperature, the properties of CN-Ns were affected, such as morphology, porosity, and crystallinity. The surface area of CN-N at 575 °C (CN-N-575) was 145.3 m2·g−1, resulting in an enhanced adsorption performance. The adsorption capacity of CN-N-575 for lignite humic acid (LHA) and soil fulvic acid (SFA) was considerably greater than that of bulk CN; the maximum adsorption amounts of LHA and SFA were 143.11 and 63.27 mg C·g−1, respectively. Upon adsorption, the removal rate of LHA and Elliott soil humic acid was about 90 %. Among HSs and LNCs, the humic-like components showed superior affinity; in some cases, the removal rate reached 100 %. The adsorption preference of aromatic compounds in HSs and LNCs was ascribed to π-electron interactions. The adsorption of LHA was more pH-dependent than that of SFA, attributed to the interactions (electrostatic, hydrogen bonding, and complexation) along with a specific conformation effect. The findings proved the great potential of CN-Ns in HSs adsorption, especially in the treatment of landfill leachate.

Enhanced adsorption of humic/fulvic acids onto urea-derived graphitic carbon nitride

Since humic substances (HSs) can cause environmental problems, their elimination has been attracting more and more concerns. In this study, we investigated HSs adsorption onto urea-derived graphitic carbon nitride (CNU) and elucidated adsorption mechanisms (i.e. heterogeneity, interface rearrangement, and multiple interactions). The adsorption capacity of CNUs was enhanced as increasing calcination temperature and time. Among CNUs, CNU-575–3 showed the highest adsorption capacity; the maximum adsorption capacities for humic acid (HA) and fulvic acid (FA) were 164.06 mg C/g, 14.61 L/cm·g, 91.12 mg C/g, and 5.34 L/cm·g, respectively. The adsorption affinity of CNUs mainly correlated with the amount of amino groups, and that of HSs components was dependent on aromaticity due to π-π interactions. More specifically, terrestrial humic-like and fulvic-like components within HA and FA showed the greatest adsorption affinity, respectively. HSs adsorption was remarkably affected by pH, alkali metals, and alkali earth metals via electrostatic interactions, H-bonding, cation bridge, and configurational effect. In addition, the adsorption of Elliott soil HA (ESHA) and the landfill leachate concentrate by CNUs was also highly efficient. This study shows the great promise of CNUs for HSs adsorption in waters and wastewaters.

Insights on Dissolved Organic Matter Production Revealed by Removal of Charge-Transfer Interactions in Senescent Leaf Leachates

Dissolved organic matter (DOM) is a critical part of the global carbon cycle. Currently, it is understood that at least a portion of the chromophoric DOM (CDOM) character can be described through an electronic interaction of charge transfer (CT) complexes. While much work has been done to understand the influence of CT on soil and aquatic reference standard DOM, little is known about the influence of CT in fresh terrestrially derived DOM. In this study, leaf litter leachates from three tree species were treated (reduced) with sodium borohydride to determine the contribution of CT on a source of fresh terrestrial DOM. Leaf litter was sampled four times through decomposition under natural (field) conditions to determine the influence of degradation on response to borohydride treatment. Leaf litter CDOM displayed a unique loss of UVB absorption following borohydride treatment, as well as a homogenizing effect on fluorescence emission character. Humification index (HIX) differentiated Elliot Soil Humic Acid and Suwannee River Fulvic Acid from leaf litter leachates. However, biological index (BIX), and spectral slope metrics were not able to differentiate leaf leachates from these reference standards. Apparent quantum yields were similar in magnitude between leaf leachates and reference standards, although leaf leachate spectra displayed features not evident in reference standards. These results help understand the origins of DOM optical properties and associated quantitative indices in freshly sourced terrestrial material. Overall, these results suggest that even at the initial stages of decomposition, terrestrial CDOM exhibits optical characteristics and responses to removal of electron accepting ketones and aldehydes, through borohydride treatment, similar to more processed CDOM.

Adsorption, aggregation and sedimentation of titanium dioxide nanoparticles and nanotubes in the presence of different sources of humic acids.

Environmental behavior, bioavailability and risks posed by TiO2, nanomaterials (TiO2 NMs) in surface waters are affected by morphologies of the particles and geochemistry, including pH, inorganic and organic matter. Here, the adsorption, aggregation and sedimentation of TiO2 nanoparticles (TiO2 NPs) and nanotubes (TiO2 NTs) were investigated in the presence of Elliott Soil humic acid (HAE) and Suwannee River humic acids (HAS). The adsorption amount of HA on TiO2 NMs was inversely proportional to pH of solution. Maximum adsorption amount of HA on the surface of TiO2 NMs follows the order TiO2 NPs + HAE (236.05 mg/g) > TiO2 NTs + HAE (146.05 mg/g) > TiO2 NTs + HAS (70.66 mg/g) > TiO2 NPs + HAS (37.48 mg/g). Stability of TiO2 NPs and TiO2 NTs largely depended on their isoelectric point, morphology and solution pH in the absence of HA. Dispersion of TiO2 NMs was enhanced with solution pH deviated from the isoelectric point of nanomaterials due to electrostatic repulsion. Moreover, tubular structures of TiO2 NTs with higher length-diameter ratio seem to aggregate more easily than dose sphere-like TiO2 NPs. This might be due to their spherical structure enhancing steric repulsion. Notably, the adsorption of HA led to disagglomeration and significant stability of TiO2 NPs and TiO2 NTs due to steric hindrance under varying solution pH. In addition, adsorption time, concentration and sources of HA also influenced suspension/sedimentation behavior of TiO2 NPs and TiO2 NTs, and aromatic-rich HAE stabilized TiO2 NMs suspension more aggressively than aliphatic-rich HAS.

Is the adsorption of soil organic matter to haematite (α‐Fe2O3) temperature dependent?

SummarySoil organic matter interactions with minerals are a key determinant in the long‐term protection and sequestration of carbon in soils and sediments. Climate warming could potentially alter the dynamics of the Earth's largest carbon pools through processes controlling the formation of mineral–organic associations (MOAs). We studied the adsorption of Elliott soil humic acid (HA) and fulvic acid (FA) to the iron oxide mineral haematite (α‐Fe2O3) at temperatures from 15 to 35°C through batch sorption experiments. The ΔG°, ΔH° and ΔS° of sorption were derived for HA and FA and suggested that formation of MOAs was spontaneous and exothermic. The Langmuir sorption coefficients for HA and FA decreased with warming, but the maximum sorption capacities were not temperature dependent. The decrease in sorption coefficient was less when the temperature was increased from 30 to 35°C than from 15 to 30°C. Our infrared, ultraviolet and visible spectroscopy analysis of the bound and unbound HA and FA revealed preferential adsorption of oxygen‐ and hydrogen‐substituted aromatic and aliphatic carbon, which was independent of temperature. The adsorption of a hydrophobic probe molecule, Rose Bengal, to haematite increased with warming. Our results suggest that (i) MOAs in warmer climates might be less sensitive to temperature changes, (ii) the amount of iron oxide‐associated organic matter might remain unchanged with warming and (iii) hydrophobic interactions with iron oxides may be temperature dependent.Highlights How does temperature affect the formation of soil organic matter–iron oxide associations? Warming can change the sorption of organic matter on to minerals thus altering carbon dynamics Warming decreases sorption coefficients, but does not affect maximum sorption capacities The amount of iron oxide-associated organic matter might remain unchanged with warming

Stimulatory effect of magnetite on the syntrophic metabolism of Geobacter co-cultures: Influences of surface coating

Magnetite-mediated direct interspecies electron transfer (DIET) can facilitate syntrophic metabolism in natural microbial communities and also promote the performance of the engineered systems based on syntrophic interactions. In this study, the stimulatory effect of bare synthetic magnetite (Mt), humic acid coated magnetite, and SiO 2 coated magnetite (Mt-SiO 2 ) on DIET in defined co-cultures of Geobacter metallireducens / Geobacter sulfurreducens were studied. Magnetite coated with Aldrich humic acid (HA) and Elliott Soil humic acid (HA ES ), respectively, were prepared, and the two kinds of humic acid influenced the ability of Mt to promote syntrophic metabolism of the co-cultures in a similar way. When weight concentration was the same, pure humic acid presented the stimulatory effect on DIET similar to bare magnetite. However, the presence of HA coating on magnetite surface caused 50% and 61%, respectively, decrease in the rates of ethanol consumption ( R e ) and succinate production ( R s ) in DIET processes. Pure HA in the same weight concentration as the HA coating in Mt-HA induced the similar metabolism rates as Mt-HA. In the Mt-HA mediated DIET, most electrons from ethanol metabolism were transferred to G. sulfurreducens selectively through the HA coating, and magnetite core hardly contributed to DIET processes. The SiO 2 coating on magnetite resulted in 81% and 89%, respectively, decreases in R e and R s , mainly because the non-conductive SiO 2 layer hindered electron transfer between magnetite core and bacteria. After eight-day incubation with the co-cultures, bare magnetite nanoparticles formed relatively larger and more compact aggregates with cells than Mt-HA and Mt-SiO 2 , due to the different surface charge between bare and coated Mt. The generation of dissolved Fe(II) and HCl-extractable Fe(II) due to microbial reduction of magnetite by G. metallireducens and vivianite formation were observed along with DIET processes in all DIET experiments. Based on these results, different pathways of electron transfer in defined co-cultures of Geobacters with bare and coated magnetite nanoparticles were proposed. The findings in this study demonstrate the significant effects of surface properties on the ability of magnetite to stimulate DIET, which needs to be considered in order to comprehensively understand the role and mechanisms of mineral-mediated DIET in natural and engineered systems.

The enhancement and inhibition of mercury reduction by natural organic matter in the presence of Shewanella oneidensis MR-1

Reduction-oxidation (Redox) processes of mercury (Hg) are of significant importance in influencing Hg speciation, bioavailability, and fate in anoxic environments where natural organic matter (NOM) and dissimilatory metal reducing bacteria (DMRB) are widely observed. However, the redox reaction between Hg and NOM, has not yet been studied in the presence of S. oneidensis MR-1 in anoxic environments. We have found that the reduction rate of mercuric mercury [Hg(II)] in the presence of Elliott soil humic acid (ESHA) was 0.02 h−1. It was faster than the rate (0.01 h−1) in the direct microbial Hg(II) reduction, suggesting that ESHA acts as an electron transfer mediator between cells and Hg, which enhances Hg(II) reduction under anoxic conditions. The overall rate of Hg(II) reduction in the presence of ESHA is determined by the rate of electron transfer from S. oneidensis MR-1 to ESHA (rate-limiting step) since the rate of electron transfer from reduced ESHA to Hg(II) was so rapid. In the reaction between S. oneidensis MR-1 and a variety of NOM analogs, the production rate of elemental mercury [Hg (0)] was linearly correlated with the free radical concentrations and aromaticities in reduced NOM analogs. However, at the high ESHA concentrations or cell contents, Hg(II) reduction might be inhibited by thiol functional groups in reduced ESHA and on cells. We suggest that the presence of NOM, cell concentration and NOM source can significantly affect the redox processes of Hg and therefore, have important implications for elucidating Hg redox processes under environmentally relevant complex conditions.

Partitioning of uranyl between ferrihydrite and humic substances at acidic and circum-neutral pH

As part of a larger study of the reactivity and mobility of uranyl (U(VI)O22+) cations in subsurface environments containing natural organic matter (NOM) and hydrous ferric oxides, we have examined the effect of reference humic and fulvic substances on the sorption of uranyl on 2-line ferrihydrite (Fh), a common, naturally occurring nano-Fe(III)-hydroxide. Uranyl was reacted with Fh at pH 4.6 and 7.0 in the presence and absence of Elliott Soil Humic Acid (ESHA) (0–835ppm) or Suwanee River Fulvic Acid (SRFA) (0–955ppm). No evidence was found for reduction of uranyl by either form of NOM after 24h of exposure. The following three size fractions were considered in this study: (1) ≥0.2μm (Fh–NOM aggregates), (2) 0.02–0.2μm (dispersed Fh nanoparticles and NOM macro-molecules), and (3) <0.02μm (dissolved). The extent to which U(VI) is sorbed in aggregates or dispersed as colloids was assessed by comparing U, Fe, and NOM concentrations in these three size fractions. Partitioning of uranyl between Fh and NOM was determined in size fraction (1) using X-ray absorption spectroscopy (XAS). Uranyl sorption on Fh–NOM aggregates was affected by the presence of NOM in different ways depending on pH and type of NOM (ESHA vs. SRFA). The presence of ESHA in the uranyl–Fh–NOM ternary system at pH 4.6 enhanced uranyl uptake more than the presence of SRFA. In contrast, neither form of NOM affected uranyl sorption at pH 7.0 over most of the NOM concentration range examined (0–500ppm); at the highest NOM concentrations (500–955ppm) uranyl uptake in the aggregates was slightly inhibited at pH 7.0, which is interpreted as being due to the dispersion of Fh aggregates. XAS at the U LIII–edge was used to characterize molecular-level changes in uranyl complexation as a result of sorption to the Fh–NOM aggregates. In the absence of NOM, uranyl formed dominantly inner–sphere, mononuclear, bidentate sorption complexes on Fh. However, when NOM concentration was increased at pH 4.6, the proportion of uranyl–Fh inner-sphere sorption complexes decreased relative to uranyl–ESHA or uranyl–SRFA complexes, which comprised up to ∼60% of the total uranyl in the systems studied. At pH 7.0, uranyl–NOM complexes were also present in the Fh–NOM aggregates in the concentration ranges of ESHA or SRFA considered; however, the proportion of these complexes was smaller at pH 7.0 than at pH 4.6 and did not increase significantly with increasing NOM concentration.

Using two-dimensional correlation size exclusion chromatography (2D-CoSEC) to explore the size-dependent heterogeneity of humic substances for copper binding

Knowledge of the heterogeneous distribution of humic substances (HS) reactivities along a continuum of molecular weight (MW) is crucial for the systems where the HS MW is subject to change. In this study, two dimensional correlation spectroscopy combined with size exclusion chromatography (2D-CoSEC) was first utilized to obtain a continuous and heterogeneous presence of copper binding characteristics within bulk HS with respect to MW. HS solutions with varying copper concentrations were directly injected into a size exclusion chromatography (SEC) system with Tris-HCl buffer as a mobile phase. Several validation tests confirmed neither structural disruption of HS nor competition effect of the mobile phase used. Similar to batch systems, fluorescence quenching was observed in the chromatograms over a wide range of HS MW. 2D-CoSEC maps of a soil-derived HS (Elliot soil humic acid) showed the greater fluorescence quenching degrees with respect to the apparent MW on the order of 12500 Da > 10600 Da > 7000 Da > 15800 Da. The binding constants calculated based on modified Stern-Volmer equation were consistent with the 2D-CoSEC results. More heterogeneity of copper binding affinities within bulk HS was found for the soil-derived HS versus an aquatic HS. The traditional fluorescence quenching titration method using ultrafiltered HS size fractions failed to delineate detailed distribution of the copper binding characteristics, exhibiting a much shorter range of the binding constants than those obtained from the 2D-CoSEC. Our proposed technique demonstrated a great potential to describe metal binding characteristics of HS at high MW resolution, providing a clear picture of the size-dependent metal-HS interactions.

Assessing redox properties of standard humic substances

Redox properties of humic substances (HS) control important biogeochemical processes. Thus, accurate estimation of redox properties of HS is essential. However, there is no general consensus regarding the best available measurement method of HS redox properties. In this study, we compared several common HS redox property measurement methods using anthraquinone-2,6-disulfonate (AQDS) as model compound, and standard Elliot soil humic acid (1S102H, ESHA), reference Pahokee peat (1R103H, PPHA), and Suwannee River natural organic matter (1R101N, SRNOM) as representative HS. We found that the H2/Pd reduction method followed by incubation with ferric citrate (FeCit) reagent was incomplete, and the H2/Pd reduction method followed by incubation with potassium ferricyanide (K3Fe(CN)6) was insensitive. Stannous chloride (SnCl2) reduction followed by titration of excess stannous (Sn2+) by potassium dichromate (K2Cr2O7) was found to be most accurate. These findings will help in future investigations on detailed characterizations of functional groups of HS responsible for oxidation/reduction reactions.

Pb, Cu, and Zn distributions at humic acid-coated metal-oxide surfaces

Mineral surfaces are often coated by natural organic matter (NOM), which has a major influence on metal-ion sorption and sequestration because of the abundance of binding sites in such coatings and the changes they cause in local nanoscale environments. The effects of NOM coatings on mineral surfaces are, however, still poorly understood at the molecular level due to the complexity of these systems. In this study, we have applied long-period X-ray standing wave-fluorescence yield (LP-XSW-FY) spectroscopy to measure the partitioning of naturally present Cu(II) (0.0226%), Zn(II) (0.009%), and Pb(II) (∼0.0004%) between Elliott Soil Humic Acid (ESHA) coatings and three model single-crystal metal-oxide substrates: α-Al2O3 (0001), α-Al2O3 (1−102), and α-Fe2O3 (0001). The competitive sorption effects among these metal ions for binding sites in the ESHA coatings and on the metal-oxide surfaces were investigated as a function of reaction time, calcium content, and solution pH. Pb(II) ions present in the ESHA coatings were found to redistribute to reactive α-Al2O3 (1−102) and α-Fe2O3 (0001) surfaces after 3h of reaction (pH=6.0, [Ca(II)]=2mM). Pb(II) partitioning onto these reactive metal-oxide surfaces increased with increasing reaction time (up to 7d). In addition, the partitioning of Cu(II) and Zn(II) from the ESHA coating to the α-Fe2O3 (0001) substrate increased slightly with reaction time (2.4% and 3.7% for Cu(II) and Zn(II), respectively, after 3h and 6.4% and 7.7% for Cu(II) and Zn(II), respectively, after 72h of reaction time). However, no changes in the partitioning of Cu(II) and Zn(II) onto the α-Al2O3 (1−102) surface were observed with increasing reaction time, suggesting that these ions strongly complex with functional groups in the ESHA coatings. Similar results were obtained for Cu(II) and Zn(II) on the ESHA-coated α-Al2O3 (1−102) surfaces in samples without the addition of calcium. However, the amounts of Pb(II) mobilized from the ESHA coatings onto the α-Al2O3 (1−102) surfaces increased from 40% (no added Ca) to 58% (with 2mM Ca) after 72h of reaction time, possibly due to displacement of Pb(II) by Ca(II) from binding sites in the ESHA coatings. In contrast, Pb(II), Cu(II), and Zn(II) present in the ESHA coatings were found to be unreactive with the α-Al2O3 (0001) surface. The observed reactivities of the three ESHA-coated metal-oxide surfaces with respect to metal-ion sorption are consistent with the trend observed for the uncoated metal-oxide surfaces: α-Fe2O3 (0001)>α-Al2O3 (1−102)>α-Al2O3 (0001). In addition, Pb(II) partitioning onto α-Al2O3 (1−102) surfaces increased with increasing pH from 4.0 to 9.0 as a result of the increasingly negative surface charge. These results show that intrinsic properties (nature of binding sites, binding affinities, and surface charge) of the ESHA coatings and metal-oxide surfaces, as well as external parameters such as pH and competing ions, are key factors governing the distribution and speciation of metal ions at complex NOM/mineral interfaces.

Character of Humic Substances as a Predictor for Goethite Nanoparticle Reactivity and Aggregation.

Natural organic matter (NOM) is ubiquitous in surface water and groundwater and interacts strongly with mineral surfaces. The details of these interactions, as well as their impacts on mineral surface reactivity, are not well understood. In this work, both the reactivity and aggregation of goethite (α-FeOOH) nanoparticles were quantified in the presence of well-characterized humic substances. Results from monitoring the kinetics of reductive degradation of 4-chloronitrobenzene (4-ClNB) by Fe(II) adsorbed onto the goethite nanoparticles with and without added humic substances demonstrates that, in all cases, humic substances suppressed Fe(II)-goethite reactivity. The ranking of the standards from the least to most inhibitive was Pahokee Peat humic acid, Elliot Soil humic acid, Suwannee River humic acid, Suwannee River NOM, Suwannee River fulvic acid I, Suwannee River fulvic acid II, and Pahokee Peat fulvic acid. Correlations between eight characteristics (molecular weight, carboxyl concentration, and carbon, oxygen, nitrogen, aliphatic, heteroaliphatic, and aromatic content) and 4-ClNB degradation rate constants were observed. Faster kinetic rates of reductive degradation were observed with increased molecular weight and nitrogen, carbon, and aromatic content, and slower rates were observed with increased carboxyl concentration and oxygen, heteroaliphatic, and aliphatic content. With these correlations, improved predictions of the reactivity of Fe(II)-goethite with pollutants based on properties of the humic substances are possible.

Investigation of adsorptive fractionation of humic acid on graphene oxide using fluorescence EEM-PARAFAC

In this study, the adsorptive fractionation of a humic acid (HA, Elliott soil humic acid) on graphene oxide (GO) was examined at pH 4 and 6 using absorption spectroscopy and fluorescence excitation-emission matrix (EEM)-parallel factor analysis (PARAFAC). The extent of the adsorption was greater at pH 4.0 than at pH 6.0. Aromatic molecules within the HA were preferentially adsorbed onto the GO surface, and the preferential adsorption was more pronounced at pH 6, which is above the zero point of charge of GO. A relative ratio of two PARAFAC humic-like components (ex/em maxima at 270/510 nm and at (250, 265)/440 nm) presented an increasing trend with larger sizes of ultrafiltered humic acid fractions, suggesting the potential for using fluorescence EEM-PARAFAC for tracking the changes in molecular sizes of aromatic HA molecules. The individual adsorption behaviors of the two humic-like components revealed that larger sized aromatic components within HA had a higher adsorption affinity and more nonlinear isotherms compared to smaller sized fractions. Our results demonstrated that adsorptive fractionation of HA occurred on the GO surface with respect to their aromaticity and the sizes, but the degree was highly dependent on solution pH as well as the amount of adsorbed HS (or available surface sites). The observed adsorption behaviors were reasonably explained by a combination of different mechanisms previously suggested.

Photo-transformation of pedogenic humic acid and consequences for Cd(II), Cu(II) and Pb(II) speciation and bioavailability to green microalga

Humic substances (HS) play key role in toxic metal binding and protecting aquatic microorganisms from metal-induced stress. Any environmental changes that could alter HS concentration and reactivity can be expected to modify metal complexation and thus affect metal speciation and bioavailability to microalgae. The present study explores the influence of increased solar irradiance on the chemical structures and molecular weight of Elliott soil humic acid (EHA) and the associated consequences for Cd(II), Cu(II) and Pb(II) complexation and intracellular metal content in microalga. The results demonstrate that high radiance doses induce an oxidation of EHA with a formation of low molecular weight acids, an increase of –OH and –COOH group abundance, and a drop in EHA hydrodynamic size and molecular weight. The photo-induced structural changes are accompanied with a release of metal from M–EHA complexes and narrowing their size distribution, which in turn results in an increase of the intracellular Cd, Cu and Pb contents in microalga Chlamydomonas reinhardtii in agreement with the measured free metal ions concentrations.

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.