Suwannee River Research Articles

Macromolecular Characterization of Compound Selectivity for Oxidation and Oxidative Alterations of Dissolved Organic Matter by Manganese Oxide.

Manganese (Mn) oxides can oxidize dissolved organic matter (DOM) and alter its chemical properties and microbial degradability, but the compound selectivity for oxidation and oxidative alterations remain to be determined. We applied ultrahigh mass spectrometry to catalog the macromolecular composition of Suwannee River fulvic acid (SRFA) before and after oxidation by a Mn oxide (δ-MnO2) at pH 4 or 6. Polycyclic aromatic hydrocarbons, polyphenols, and carbohydrates were more reactive in reducing δ-MnO2 than highly unsaturated and phenolic (HuPh) compounds and aliphatics, but highly abundant HuPh contributed the most (∼50%) to the overall reduction of δ-MnO2. On average, oxidized species had higher molecular weights, aromaticity, carbon unsaturation degree, nominal oxidation state of carbon, and oxygen and nitrogen contents but were lower in hydrogen content compared to unoxidized species. The oxidation decreased these molecular indices and oxygen and nitrogen contents but increased the hydrogen content, with stronger changes at the lower pH. This DOM oxidation on polar mineral surfaces was more selective but shared similar selectivity rules to adsorption. The abiotic oxidation resembles microbial oxidative degradation of organic matter, and Mn oxide-oxidizable carbon may be a useful index for detection and identification of labile organic carbon.

Relating natural organic matter conformation, metal complexation, and photophysics

We investigated the relationship between changes in fluorescence intensity and in fluorescence anisotropy for Suwannee River Natural Organic Matter (SRNOM) due to the formation of NOM-metal complexes with divalent and trivalent metals commonly present in both fresh water and sea water environments. We chose metal ions whose complexes give rise to both fluorescence quenching (Fe3+, Cu2+) and fluorescence enhancement (Al3+, Mg2+). Stern–Volmer type analyses quantified the changes in the SRNOM fluorescence as a function of metal concentration. All metals display strong complexation with SRNOM, associated with their effect on fluorescence. Experiments with Fe3+ further show strong effects due to NOM aggregation at all but the lowest metal concentrations studied here. There was little to no change in the conformation of SRNOM as inferred from fluorescence anisotropy caused by increasing metal concentration. These results suggest that there is no correlation between photophysical changes and conformational changes in NOM associated with complexation by the metal ions.

Effect of Background Water Matrices on Pharmaceutical and Personal Care Product Removal by UV-LED/TiO2

In this study, we evaluated the effectiveness of UV-LED-irradiated TiO2 in removing 24 commonly detected PPCPs in two water matrices (municipal wastewater effluent and Suwannee River NOM–synthetic water) and compared their performance with that of ultrapure water. Relatively fast removal kinetics were observed for 29% and 12% of the PPCPs in ultrapure water and synthetic surface water, respectively (kapp of 1–2 min−1). However, they all remained recalcitrant to photocatalysis when using wastewater effluent as the background matrix (kapp < 0.1 min−1). We also observed that the pH-corrected octanol/water partition coefficient (log Dow) correlated well with PPCP degradation rate constants in ultrapure water, whereas molecular weight was strongly associated with the rate constants in both synthetic surface water and wastewater. The electrical energy per order (EEO) values calculated at the end of the experiments suggest that UV-LED/P25 can be an energy-efficient method for water treatment applications (2.96, 4.77, and 16.36 kW h m−3 in ultrapure water, synthetic surface water, and wastewater effluents, respectively). Although TiO2 photocatalysis is a promising approach in removing PPCPs, our results indicate that additional challenges need to be overcome for PPCPs in more complex water matrices, including an assessment of photocatalytic removal under different background water matrices.

Heteroaggregation of different surface-modified polystyrene nanoparticles with model natural colloids

This study investigated heteroaggregation of three surface-functionalized polystyrene nanoparticles (PSNPs), i.e. negatively charged unfunctionalized nanoparticles (Bare-PS) and carboxylated nanoparticles (COOH-PS), and positively charged amino-functionalized nanoparticles (NH2-PS), with two model natural colloids, positively charged hematite and negatively charged kaolin, respectively. Heteroaggregation was conducted at a constant natural colloid concentration and variable NP/colloid concentration ratios. Electrostatic interaction was the main mechanism driving the formation of heteroaggregates. In binary systems containing hematite and Bare-PS/COOH-PS, a charge neutralization – charge inverse mechanism was observed with the increase of PSNP concentration. At NP/hemetite concentration ratios much smaller or larger than the full charge neutralization point, the primary heteroaggregates were stable, while full charge neutralization induced the formation of large secondary heteroaggregates. Large aggregates were not observed in suspensions containing kaolin and NH2-PS, as highly positively charged NH2-PS reversed surface charges of kaolin at extremely low concentrations. Heteroaggregation between PSNPs and natural colloids with the same charge is unfavorable due to strong electrostatic repulsion. In the presence of electrolytes, homoaggregation and heteroaggregation both occurred, and homoaggregation of hematite played a key role when the concentration of PSNPs was low. The presence of Suwannee River natural organic matter (SRNOM) could modify surface charges of nanoparticles, and thus affect heteroaggregation behaviors of the binary suspension. When SRNOM and electrolytes were both present, whether SRNOM inducing or hindering the stability of the binary system was a combined effect of NP/colloid concentration ratios, SRNOM concentrations, electrolyte types and ionic strength. Mechanisms extensively reported in homoaggregation such as steric hindrance and cation bridging effects between SRNOM and Ca2+ also stand for heteroaggregation. These results highlight the critical role of surface modification on the environmental behaviors of NPs, and will underpin our understanding of the fate and transport of NPs in the aquatic environment.

Photo-oxidation of arsenite in acidic waters containing Suwannee River fulvic acid: roles of 3SRFA* and hydroxyl radical.

The photo-oxidation of arsenite (As(III)) in solution containing Suwannee River fulvic acid (SRFA) under the ultraviolet A (UVA) irradiation (λmax = 365 nm) was studied. In a solution containing 100.0 μg·L-1 As(III) and 10.0 mg·L-1 SRFA at pH 3.0, SRFA induced As(III) photo-oxidation by producing the triplet excited state of SRFA (3SRFA*) and hydroxyl radical(HO˙). Approximately 82% of As(III) oxidation was attributed to HO˙ which depended strongly on HO2˙/O2˙-. The remaining 18% of As(III) oxidation was attributed to the direct reaction between As(III) and 3SRFA*. The photo-oxidation of As(III) was significantly affected by solution pH. Excess SRFA inhibited As(III) photo-oxidation. The addition of a low concentration of ferric ions retarded the photo-oxidation of As(III) due to the poor photo-activity of Fe(III)-SRFA complexes. In contrast, the addition of ferric ions at high concentration greatly accelerated As(III) photo-oxidation because of the high photo-activity of Fe(III)-OH complexes. The fractions of SRFA with different molecular weight showed different oxidizing capacities under UV irradiation which was possibly related to the different contents of phenolic OH groups. The findings have important environmental implications for the photo-transformation behavior of As(III) in natural surface waters containing dissolved organic matter, especially acidic waters.

Exploring the fluorescence quenching interaction of amino acids and protein with natural organic matter by a multi-spectroscopic method

AbstractThe main objective of this research was to explore the fluorescence quenching mechanism of a humic substance (Suwannee River natural organic matter (SWNOM)) to amino acids (tryptophan, tyrosine) and protein (bovine serum albumin, (BSA)) by multi-spectroscopic methods. The locations of the peaks of tryptophan, tyrosine, and BSA from Parallel Factor Analysis were at Ex/Em 280/356 nm, 275/302 nm, and 280/344 nm, respectively. For SWNOM, two peaks appeared at Ex/Em of 240/448 nm, and 350/450 nm. Static quenching was the dominant quenching mechanism between BSA and SWNOM, whereas no quenching was observed between tryptophan or tyrosine and SWNOM. Fourier-transform infrared spectroscopy and thermodynamic calculation demonstrated that hydrogen bonding and van der Waals force are the potential binding forces of the BSA-SWNOM complex, as a result of rearrangement in the secondary polypeptide carbonyl hydrogen bonding network of BSA. This rearrangement led to the conformational change in BSA that induced quenching of BSA fluorescence by SWNOM.

Tracking Br-DBPs and bromine substitution factors by two-stage differential characterization of water matrix and NOM during chlorination

Tracking disinfection byproducts (DBPs) upon chlorination becomes challenging whenever bromides occur because they alter DBP formation from chlorinated to brominated DBPs (Br-DBPs). Although epidemiological studies have emphasized the need to control Br-DBPs because of their notable carcinogen effects, work on Br-DBP monitoring is timely, since limited attention has to date been paid to this. This study proposes a two-stage differential characterization approach to a water matrix and natural organic matter (NOM) surrogates as a more robust way to trace Br-DBP precursors and bromine substitution factors (BSF). This two-stage approach monitors the differential variations (Δ) of both inorganic and organic precursors, e.g., dissolved organic carbon (DOC), bromide levels, Cl2 content, specific ultraviolet absorbance (SUVA), fluorescent spectra, before and after chlorination. To mimic practical conditions, chlorination experiments were conducted with two principle NOMs, including Suwannee River fulvic (SW) and M. aeruginosa organic matter (MA), with Br− levels from 0 to 2000 μg/L and chlorination times from 15 min-24 h. When Br− levels increased, all Br-DBP yields and BSF values correspondingly magnified, accounted for 70–90% of the total DBP yields from chlorinated SW or MA water. At each Br− level, prolonging chlorination time also increased the overall Br-DBPs and varied BSF values, but these differences were of no significance (p > 0.05). Correlation tests highlighted the significant insufficiency of the conventionally-used DBP surrogates (initial DOC, Cl2 dose, and SUVA) for Br-DBPs and BSF monitoring. By contrast, our two-stage characterization parameters are superior, with correlation coefficients ρ > 0.8 (p < 0.01), especially for those parameters including ΔBr as a numerator. Accordingly, regression models of Br-DBPs and BSFs built by our two-stage tertiary parameters exhibited high adjusted R2 values mostly >0.7 and >0.8 (n = 60), indicating a promising application for effective Br-DBPs and BSF monitoring.

Coprecipitation of Fe/Cr Hydroxides with Organics: Roles of Organic Properties in Composition and Stability of the Coprecipitates.

Iron hydroxides are important scavengers for dissolved chromium (Cr) via coprecipitation processes; however, the influences of organic matter (OM) on Cr sequestration in Fe/Cr-OM ternary systems and the stability of the coprecipitates are not well understood. Here, Fe/Cr-OM coprecipitation was conducted at pH 3, and Cr hydroxide was undersaturated. Acetic acid (HAc), poly(acrylic acid) (PAA), and Suwannee River natural organic matter (SRNOM) were selected as model OMs, which showed different complexation capabilities with Fe/Cr ions and Fe/Cr hydroxide particles. HAc had no significant effect on the coprecipitation, as the monodentate carboxyl ligand in HAc did not favor complexation with dissolved Fe/Cr ions or Fe/Cr hydroxide nanoparticles. Contrarily, PAA and SRNOM with polydentate carboxyl ligand had strong complexation with Fe/Cr ions and Fe/Cr hydroxide nanoparticles, leading to significant amounts of PAA/SRNOM sequestered in the coprecipitates, which caused the structural disorder and fast aggregation of the coprecipitates. In comparison with that of PAA, preferential complexation of Cr ions with SRNOM resulted in higher Cr/Fe ratios in the coprecipitates. This study advances the fundamental understanding of Fe/Cr-OM coprecipitation and mechanisms controlling the composition and stability of the coprecipitates, which is essential for successful Cr remediation and removal in both natural and engineered settings.

Investigation of the molecular structure complexity of dissolved organic matter by UPLC-orbitrap MS/MS

The complex natural organic matter of the Suwannee River fulvic acid (SRFA) standard was analyzed by online reversed-phase chromatography with Orbitrap MS/MS using collision-induced dissociation (CID). The number of isobars per nominal mass could be reduced to a single dominantly abundant species in a chromatographic run, sharing some ions with signals having the identical molecular formula in adjacent chromatographic segments and later serving as a precursor ion for fragmentation. A very large proportion of the same fragment ions existed in adjacent chromatographic fractions. The difference in the fragment ions in adjacent chromatographic fractions could be attributed to a gradual change in the formula composition of precursor ions in a chromatographic run. It could be concluded that dissolved organic matter (DOM) molecules with the same elemental composition in different chromatographic fractions may have very similar molecular structures. In addition, we propose a possible DOM model that might greatly deepen our understanding of the behavior of DOM in aquatic matrices.

Removing heavy metals from landfill leachate using electrospun polyelectrolyte fiber mat-laminated ultrafiltration membrane

This study investigated the removal of selected heavy metals from landfill leachate by electrospun polyacrylic acid (PAA)/polyallylamine hydrochloride (PAH)-laminated ultrafiltration (UF) membranes (PAA/PAH-UF). The effect of leachate matrix on metal removal by the PAA/PAH-UF membrane was also examined. In synthetic metal solutions, the PAA/PAH-UF membrane exhibited 38–85% higher removal efficiency when compared to the unmodified membrane. This could be attributed to complexation of the metal ions with the carboxylate ions (–COO-) from the PAA/PAH fiber mats. Heavy metal removal was generally higher (up to approximately 18%) in landfill leachates than the laboratory-prepared solutions, likely resulting from the fouling of PAA/PAH fiber mat and the UF membrane substrate and the metal-organic complexation. The higher metal ion removal efficiencies (up to 20%) by the PAA/PAH-UF membranes in the presence of Suwannee River natural organic matter (SRNOM) were associated with the higher concentrations (50 and 100 mg/L) of SRNOM. When compared to the lower concentration (10 mg/L), metal removal increased (up to 30%) at higher concentrations (50 and 100 mg/L), with the influence of SRNOM being the most pronounced for Cu, followed by Cd and Pb. The enhanced metal removal, in the presence of SRNOM, can be attributed to metal complexation with SRNOM functional groups in addition to the rich functional groups from the PAA/PAH fiber mats. The trivial difference in flux decline of the PAA/PAH-UF membrane compared to the unmodified membrane may be outweighed by the marked enhancement in heavy metal removal when functionalizing the membrane with PAA/PAH fiber mats.

A Novel Method for Unraveling the Black Box of Dissolved Organic Matter in Soils by FT-ICR-MS Coupled with Induction-Based Nanospray Ionization

Dissolved organic matter (DOM) in soils plays important roles in carbon and nutrient cycling. However, our knowledge of the molecular composition of DOM is limited by selective loss during pretreatment [solid-phase extraction (SPE)] or small amounts of pore water. We proposed a novel method for characterizing DOM by coupling a Fourier transform ion cyclotron resonance mass spectrometer and a self-built induction-based nanospray ionizer. This approach reduces the ion suppression effect and avoids the need for SPE by substantially reducing the matrix effects. Moreover, highly diverse compound types were detected in Suwannee River natural organic matter. Cwₜ/Nwₜ was close to the actual C/N ratio. Only 0.5 μL of pore water (5 ng of C) was needed (vs ∼10 mL for the conventional method). This method was validated by analyzing forest and agricultural soil samples. Soil DOM was not dominated by lignin-like compounds, as determined using the conventional method. The novel method revealed more peptide/carbohydrate-like compounds, highly condensed aromatics, and more than double the number of CHNO compounds. A more complete understanding of DOM will allow us to elucidate the biogeochemical processes of nutrient and DOM–pollutant complexes in soils as well as to develop comprehensive policies for soil remediation.

Dissolved black carbon induces fast photo-reduction of silver ions under simulated sunlight

Silver has been associated with significant environmental challenges. This study aimed to investigate the photo-reduction of silver ions (Ag+) mediated by dissolved black carbon (DBC), a vital dissolved organic matter released from black carbon. Suwannee River humic acid (SRHA) was used as a comparison group. DBC was observed to effectively mediate the photo-reduction of Ag+ to the silver nanoparticles (nAg) under simulated sunlight. The initial rate of formation of nAg mediated by DBC was approximately 3.0–9.5 times higher than that of nAg mediated by SRHA. The Ag+ concentration required to form nAg in the DBC solution was observed to be only 0.5 mg L−1, which was one-tenth of the concentration in the SRHA solution. Further, the mediation efficiency of DBC was higher than that of SRHA, which was attributed to the facilitated ligand-to-metal electron transfer and reduced generation of the oxidative phototransients. The Ag+ sorption played a crucial role during the photo-reduction process by altering the reaction pathway and local Ag+ concentration as well as affecting the initial rate of formation of nAg. Furthermore, the silver seeds generated by DBC are able to trigger the formation of nAg by SRHA at low Ag+ concentrations. Overall, the findings from this study highlight the vital role of DBC in mediating the sunlight-induced speciation of the silver ions.

Effect of UV/chlorine treatment on photophysical and photochemical properties of dissolved organic matter

Dissolved organic matter (DOM) is a ubiquitous component in effluents, DOM discharged with an effluent can affect the composition and properties of natural DOM in the receiving waters. As the photophysical and photochemical properties of effluent DOM can be changed by wastewater treatment processes, the effect of UV/chlorine treatment on the photophysical and photochemical properties of DOM was investigated using Suwannee River fulvic acid (SRFA) and Suwannee River natural organic matter (SRNOM) as representatives. Results showed that the absorbance of the two DOM was significantly decreased. The evolution trends of three representative photophysical parameters upon increase of chlorine dosages were observed. Also, a decrease in DOM aromaticity, molecular weight and electron-donating capacity was observed upon increasing chlorine dosage. Quantum yields of excited triplet state of DOM (3DOM*), singlet oxygen (1O2) and hydroxyl radicals (·OH) first decreases and then increased in the UV/chlorine systems upon increasing chlorine dosages due to the different reaction pathways of the two DOM. Moreover, 3DOM* can not only be regarded as a “controller” of other reactive intermediates, but also effectively promote the photodegradation of bezafibrate, which is classified as a persistent organic contaminant. This study gives deep insights into effects of UV/chlorine on the photophysical and photochemical properties of DOM, and is helpful for understanding the dynamic roles of DOM in the photodegradation of micropollutants.

Assessing hydrologic and water quality effects of land use conversion to Brassica carinata as a winter biofuel crop in the southeastern coastal plain of Georgia, USA using the SWAT model

AbstractCarinata (Brassica carinata) is an industrial oilseed feedstock for renewable fuels grown as a winter crop in the southeast US, which may provide a new rotation alternative and benefits for water quality. However, the effects of carinata on water quantity and quality at the watershed and local scales are unknown. In this study, we use the Soil and Water Assessment Tool (SWAT) to assess the potential influence of carinata on water balance components, nutrients and sediment loads under plausible future scenarios of land use change in the upper Suwannee River Basin in the Atlantic Coastal Plain Physiographic Region near Tifton in South‐Central Georgia. Three future scenarios are considered, including planting stand‐alone carinata in winter fallow land every third year, planting stand‐alone winter wheat in winter fallow land every third year, and carinata and winter wheat in rotation, one year of winter carinata followed by two years of winter wheat during simulation periods. The results show that under all three future scenarios, surface runoff, sediment, phosphorus, and nitrogen loadings decrease at both watershed and local scales, with higher average monthly reductions in the stand‐alone carinata scenario versus the stand‐alone winter wheat scenario. When carinata and winter wheat were planted over 36% of the total watershed area, reduction in total sediment, mineral phosphorus, and nitrate loads was ranging from 11.5% to 50.0%. However, when only 12% of the total watershed area was converted to carinata, the simulated reductions ranged from 3.8% to 14.0%. This suggests that the extent of carinata planting is crucial in assessing its hydrologic and water quality benefits. Overall, these results indicate that planting the biofuel carinata as a winter crop can reduce sediment and nutrient loading and provide water quality benefits to downstream waterbodies.

Aggregation and Aggregate Strength of Microscale Plastic Particles in the Presence of Natural Organic Matter: Effects of Ionic Valence

Considering the recent plastic loads in water bodies we studied the aggregation, charging, and aggregate strength of polyethylene microsphere (PEM) particles in the presence of natural organic matter (NOM) as a function of KCl and CaCl2 concentrations. We used the Suwannee river humic acid (SRHA) as NOM and PEM as microplastics. The aggregation was triggered in the presence of CaCl2 solutions more effectively than KCl due to the divalent bridging and strong electrostatic attraction between Ca2+ and SRHA as well as between PEM particles. We found that the maximum aggregate strength around 1.87 nN in the presence of 100 mg/L SRHA at 0.5 M CaCl2 solution. The aggregate strength of PEM particles in the KCl solution was lower than that of CaCl2 solution, manifesting the more effective bridging flocculation and divalent bridging due to the Ca2+ ions.

Empirical equation for the correction of fluorescence quenching of proteinaceous substance by Suwannee river natural organic matter

Abstract Fluorescence quenching of proteinaceous substances by natural organic matter is a well-known phenomenon, but there are no known methods for correcting it. The main objective of this research was to develop an empirical equation to correct the fluorescence quenching of different concentrations of bovine serum albumin (BSA – 0.15, 0.25, 0.5, 0.75, 1, 1.25 μmol/L (μM)) by Suwannee river natural organic matter (SWNOM - 0,2,4,6,8,10 mg-C/L) using the fluorescence titration method. The excitation emission matrix (EEM) data were analyzed by parallel factor analysis with inner filter effect removal. With increasing SWNOM concentration, BSA peak intensity quenching was in the range 29–85%, with a linear relationship for increment of either BSA or SWNOM concentration. A higher ratio of SWNOM to BSA resulted in greater BSA peak intensity quenching. The unquenched BSA peak (BSA (RU)) is given by the empirical equation. The calculated unquenched BSA peak intensities using the empirical equation agreed well with the actual unquenched peak values (R2 = 0.98, mean absolute error = 0.33 RU). The equation is expected to help in rapid estimation of the quenching effect of SWNOM on BSA.

Enhanced formation of bromophenols by anthraquinone-2-sulfonate and benzophenone: implications for photochemical production of organobromine compounds by dissolved organic matter in a marine environment

Environmental context Organobromine compounds are a potential environmental hazard, but there are many uncertainties about their natural sources. This paper investigated the photochemical generation of bromophenols in the presence of dissolved organic matters (DOMs) and proxies, and demonstrated that DOMs enhance the photobromination reaction. The result indicates that the bromination process induced by sunlit DOMs likely contributes to the natural sources of organobromine compounds in the marine environment. Abstract Organobromine compounds are substantial environmental hazards owing to their high toxicity on organisms. Here we study the photochemical formation of bromophenols from phenol in bromide aqueous solutions (0.8–80 mM) in the presence of anthraquinone-2-sulfonate (AQ2S) and benzophenone (BP), which were adopted as the proxies of dissolved organic matter (DOM) having quinones and aromatic ketones structures. The formation of bromophenols increased with the increase of the concentrations of AQ2S and BP, and the promotion effect was in the order AQ2S &gt; BP. Bromide and chloride ions were found to promote the formation of bromophenols. Moreover, natural DOM from Suwannee River was found to enhance this photobromination reaction at a low concentration (1 mg L−1). These results demonstrate the generation of reactive halogen species from sunlit DOM, and such a process could account for the abiotic source of organobromine compounds in a marine environment, as terrestrial DOM distributes universally in estuaries and coastal seawater and could diffuse to the open sea.

Ionization selectivity of electrospray and atmospheric pressure photoionization FT-ICR MS for petroleum refinery wastewater dissolved organic matter.

Dissolved organic matter (DOM) in petroleum refinery wastewater is an extremely complex mixture. A better understanding of chemical compositions of DOM at the molecular level is necessary for the design and optimization of wastewater treatment processes. In this study, two largely different DOM samples, one from a petroleum refinery wastewater and the other from the Suwannee river water, were characterized by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) coupled with positive-/negative-ion electrospray ionization (ESI), and positive-ion atmospheric pressure photoionization (APPI). For wastewater DOM, a total of 6226 molecular formulae were assigned in the three ionization modes. However, only 1182 molecular formulae were common in all three mass spectra, indicating that the techniques were highly complementary in the types of molecules they ionize. Acid Ox (x = 1-9) and basic N1Ox (x = 0-2) classes were dominant in the wastewater DOM detected in negative-ion and positive-ion ESI mode, respectively. And the wastewater DOM contains considerable amounts of polycyclic aromatic hydrocarbons that did not respond to ESI but can be ionized selectively by APPI. Compared with riverine DOM, the refinery wastewater DOM has a higher molecular complexity and is more enriched in hydrocarbon, and nitrogen- and sulfur-containing compounds. The results show that the major components of refinery wastewater DOM were distinctive from those of the natural organic matter. Though not quantitative, the results obtained by various ionization techniques were found to be complementary, and are helpful to our understanding of the selectivity of different ionization techniques as well as the molecular compositions of DOM.

Complexes of humic acid with cationic surfactants support the supramolecular view of extracted humic matter

Environmental context The molecular organisation of humic substances is key to understanding various natural and engineered processes, such as metal speciation and bioavailability. Two main models have been proposed: polymeric organisation and supramolecular organisation. A physicochemical approach, based on the interaction of humic acid with increasing amounts of cationic surfactants, leads to a sequence of molecular structures that provides support to the supramolecular view of humic organisation. Abstract The structural organisation of extracted humic matter, polymeric or supramolecular, has been a long-standing controversial issue. In this paper, we show that the interaction between a reference humic material, the Suwannee River Humic Acid (SRHA), and a homologous series of cationic surfactants (n-alkyl trimethyl ammonium chloride) provides key insights to resolve the controversy. By combining measurements of turbidity, electrophoretic mobility, surface tension and cryogenic transmission electron microscopy, we show that: (i) the binding of cationic surfactant to the anionic humic acid is initially triggered by electrostatic interactions; (ii) the contrasting evolution of turbidity–surfactant concentration curves implies that the surfactant alkyl chain interacts with the hydrophobic moieties of humic matter from very low concentrations of surfactant; (iii) a drastic restructuring of humic matter occurs on surfactant binding, which brings out supplementary anionic humic sites; (iv) in the presence of C12 cationic surfactant, SRHA spontaneously forms stable nanoscale unilamellar vesicles, which, in addition to the high surface activity of complexes, could be of interest in the remediation of oil-contaminated environments; (v) the sequence of molecular structures obtained from SRHA with increasing amounts of C12-cationic surfactant – humic-rich vesicles, aggregates, surfactant-rich vesicles – bears striking similarities with the phase behaviour of aqueous mixtures of anionic and cationic surfactants, thus supporting the supramolecular view of humic structure.

Acute waterborne and chronic sediment toxicity of silver and titanium dioxide nanomaterials towards the oligochaete, Lumbriculus variegatus

The use of silver (Ag) and titanium dioxide (TiO2) nanomaterials (NMs) in industrial processes and consumer products has experienced considerable growth since the late 20th century. Throughout their lifecycle, both Ag NM and TiO2NM are released into the environment, with benthic systems anticipated to be the final sink. Their potential toxicity towards benthic species is therefore of major concern. This study investigated the toxicity of silver (Ag; NM-300 K) and titanium dioxide (TiO2; NM-104) NMs to the freshwater oligochaete, Lumbriculus variegatus in acute (0–96-h) waterborne and chronic (28-d) sediment studies. Toxicity was investigated via assessment of mortality, behaviour, and antioxidant enzyme activity. The 96-h LC50 for Ag NMs in water was 0.51 mg/l (95% CI, 0.45–0.56), with L. variegatus displaying inhibited predation-avoidance behaviour compared to controls (6.66 ± 10%) successful response at 24-h), as well as significant increases (p < 0.05) in catalase (CAT) activity at sub-lethal concentrations at 24-h. Behavioural improvement and the return of antioxidant enzymes to control levels was observed after 48 and 72-h. AgNO3 exposure proved more toxic than Ag NM (96-h LC50 = 0.034 mg/l, 95% CI, 0.031–0.037) but resulted in no changes to antioxidant enzymes following sub-lethal exposure. Furthermore, Ag dissolution from Ag NM (~2–4%) could not account for the full extent of toxicity observed, suggesting a nano-specific effect. Increased environmental relevance via the inclusion of Suwannee River Humic Acid (SRHA, 5 mg/l) alleviated sub-lethal Ag NM toxicity despite a comparable 96-h LC50 (0.54 mg/l, 95% CI, 0.51–0.57). Significant effects of Ag NMs in formulated sediments (mortality, biomass) were only recorded according to OECD 225 at the highest test concentration (1333 mg/kg) for Ag NM indicating a potential attenuating effect of sediments towards toxicity. No toxicity was observed for TiO2 NM in aquatic or sediment exposures up to concentrations of 2000 mg/l and 1333 mg/kg, respectively.