Suwannee River Research Articles

Molecular Signatures of Three Fulvic Acid Standard Samples as Revealed by Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

AbstractIt is necessary to understand the molecular composition of fulvic acid (FA), which plays a significant role in the transport and bioavailability of pollutants in the environment. FA collected from Suwannee River in South Georgia (SRFAriver) and Elliott soil in fertile prairie of Illinois (ESFAsoil) are two standard fulvic acid samples of International Humic Substances Society (IHSS). FA collected from Beijing Jiufeng forest soils in China (JFFAsoil) was extracted according to the standard method recommended by IHSS. In this study, the compositions of three FA from two origins (i. e. river and soil) are characterized using negative ion mode electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI FT‐ICR MS). Compounds possessing higher aromaticity usually contain more nitrogen or sulfur atoms. The aromaticity and humification degree of soil‐derived FA are higher. Soil‐derived FA contains more nitrogen and sulfur elements because compounds containing nitrogen and sulfur atoms are easily absorbed by clay minerals and preserved in aromatic molecules. Comparing the two soil‐derived FA, they differed little in formula and component compositions, but JFFAsoil was slightly more aromatic and less labile than ESFAsoil. These are useful for understanding the difference between the carbon, nitrogen and sulfur element enrichment process in the formation of fulvic acid in river and soil environment at the molecular level.

Formation of assimilable organic carbon (AOC) during drinking water disinfection: A microbiological prospect of disinfection byproducts

Disinfection processes might alter the chemical structure of biological recalcitrant natural organic matter (NOM) in source water to form assimilable organic carbon (AOC), which can be readily utilized by microbes for growth. However, AOC has not been classified as disinfection byproducts (DBPs) before and little is known about the chemical and structural nature of AOC. This study, for the first time, considers the disinfection-induced AOC as DBPs from a microbiological perspective. The AOC formation by three types of disinfection processes, i.e., chlorination, UVC irradiation (254 nm) and photocatalysis represented by TiO2-UVA in drinking water containing two reference NOM materials of Suwannee River and Nordic Reservoir (SRNOM and NRNOM, respectively) were comparatively benchmarked using Pseudomonas aeruginosa as inoculum. Results showed that chlorination caused a substantial increase in AOC content, whereas TiO2-UVA led to a moderate increase in AOC content and UVC rendered the AOC content unchanged, independent of the types of NOM. Molecular weight indicated by spectral slope ratio and fluorescence fingerprint were found to not provide critical information about the AOC formation potential. FTIR and FT-ICR-MS results indicated that the AOC formation by chlorination was attributed to the oxidation and chlorine substitution on aromatic molecules to form molecules with carboxylic- and alcohol- functionalities, as well as chlorinated aromatics. These molecules could be metabolized and assimilated by Pseudomonas species by a catechol pathway. The results obtained in this study can provide valuable insight regarding the selection of proper technologies for disinfection to prevent microbial growth/regrowth in the distributing system and is intended to encourage more thinking and research on AOC as a new prospect of DBPs during disinfection of drinking water.

Molecular traits of phenolic moieties in dissolved organic matter: Linkages with membrane fouling development

Phenolic moieties are important constituents in dissolved organic matter (DOM) in natural and engineered systems. However, their roles in membrane fouling mechanism during drinking water treatment by ultrafiltration (UF) have remained elusive. Herein, by using water insoluble polyvinylpolypyrrolidone (PVPP) resins, we sequestered the phenolic moieties from a model DOM (Suwannee River DOM, SRDOM) and characterized their molecular profiles using electrospray ionization coupled with Fourier transform ion cyclotron resonance mass spectrometry (ESI-FT-ICR-MS). Subsequently, their roles in UF membrane fouling propensity were investigated using reconstituted DOM solutions with various concentrations of phenolic moieties. The results showed that the phenolic moieties were of higher molecular weight and rich in unsaturation cyclic structures and oxygen-rich groups. Van Krevelen diagrams revealed that the sequestered sample was rich in aromatics structures and tannins-like compounds while contained less alicyclic organic acids in comparison with the original SRDOM, which was consistent with the aromaticity index (AI) analysis. UF experiments showed that the more phenolic moieties in DOM solution, the severer decline of flux was observed. The phenolic moieties played a significant role in membrane irremovable fouling due to the hydrophobic interactions and their higher molecular weight as evidenced by membrane cleaning tests. By surface characterization, the SRDOM fouled membrane was identified to have a higher water contact angle value and abundant C–O groups, likely due to the adsorption of more hydrophobic phenolic moieties. Overall, these findings highlighted links between phenolic moieties and membrane fouling development, and implied that membrane performance could be improved by pre-removal of phenolic moieties in DOM.

Dissolved Organic Matter Affects Arsenic Mobility and Iron(III) (hydr)oxide Formation: Implications for Managed Aquifer Recharge.

During managed aquifer recharge (MAR), injected water significantly alters water chemistry in an aquifer, affecting arsenic mobility. To elucidate the effects of dissolved organic matter (DOM) on arsenic mobilization during MAR, this bench-scale study examined arsenic mobilization from arsenopyrite (FeAsS, an arsenic-containing sulfide) in the presence of Suwannee River natural organic matter, humic acid, and fulvic acid (SRNOM, SRHA, and SRFA), alginate (Alg), polyaspartate (PA), and glutamate (Glu). Suwannee River DOM (SRDOM) decreased arsenic mobility in the short term (<6 h) via inhibiting arsenopyrite oxidative dissolution, but increased arsenic mobility over a longer experimental time (∼7 days) via inhibiting secondary iron(III) (hydr)oxide precipitation and decreasing arsenic adsorption onto iron(III) (hydr)oxide. In situ grazing incidence small-angle X-ray scattering measurements indicated that SRDOM decreased iron(III) (hydr)oxide nucleus sizes and growth rates. A combined analysis of SRDOM and other proteinaceous or labile DOM (Alg, PA, and Glu) revealed that DOM with higher molecular weights would cause more increased arsenic mobility. These new observations advance our understanding of the impacts of DOM in injected water on arsenic mobility and secondary precipitate formation during MAR, and in other systems where interactions between DOM, arsenic, and iron(III) (hydr)oxides take place.

Is Superoxide-Mediated Fe(III) Reduction Important in Sunlit Surface Waters?

Two major pathways are reported to account for photochemical reduction of Fe(III) in sunlit surface waters, namely, ligand-to-metal charge transfer (LMCT) and superoxide-mediated iron reduction (SMIR). In this study, we investigate the impact of Fe(III) speciation (organically complexed (Fe(III)L versus iron oxyhydroxide (AFO)) on Fe(III) reducibility by photogenerated superoxide (O2•-) and LMCT. To simulate conditions typical of fresh, estuarine, and coastal waters, we have used Suwannee River Fulvic Acid (SRFA) as a representative of the natural organic matter likely to associate with Fe(III). Our results show that the photolabile Fe(III)SRFA complex is reduced rapidly by LMCT, while O2•- does not play a role in reduction of these entities. In contrast, the relatively less photolabile AFO is reduced by both O2•- and LMCT. The reduction of AFO by O2•- occurs following the dissolution of AFO, and hence, the contribution of O2•- to reductive dissolution of AFO is dependent on conditions such as the age of the AFO and initial AFO concentration affecting the rate of dissolution of AFO. Our results further show that while colloidal Fe(III) (in this work, particles >0.025 μm) is reduced by O2•-, there is no involvement of O2•- in dissolved Fe(III) reduction. Overall, our results show that superoxide-mediated iron reduction will be important only in natural waters containing limited concentrations of Fe binding ligands.

13C and 15N NMR identification of product compound classes from aqueous and solid phase photodegradation of 2,4,6-trinitrotoluene.

Photolysis is one of the main transformation pathways for 2,4,6-trinitrotoluene (TNT) released into the environment. Upon exposure to sunlight, TNT is known to undergo both oxidation and reduction reactions with release of nitrite, nitrate, and ammonium ions, followed by condensation reactions of the oxidation and reduction products. In this study, compound classes of transformation products from the aqueous and solid phase photodegradation of 2,4,6-trinitrotoluene (TNT) have been identified by liquid and solid state 13C and 15N NMR. Aqueous phase experiments were performed on saturated solutions of T15NT in deionized water, natural pond water (pH = 8.3, DOC = 3.0 mg/L), pH 8.0 buffer solution, and in the presence of Suwannee River Natural Organic Matter (SRNOM; pH = 3.7), using a Pyrex-filtered medium pressure mercury lamp. Natural sunlight irradiations were performed on TNT in the solid phase and dissolved in the pond water. In deionized water, carboxylic acid, aldehyde, aromatic amine, primary amide, azoxy, nitrosophenol, and azo compounds were formed. 15N NMR spectra exhibited major peaks centered at 128 to 138 ppm, which are in the range of phenylhydroxylamine and secondary amide nitrogens. The secondary amides are proposed to represent benzanilides, which would arise from photochemical rearrangement of nitrones formed from the condensation of benzaldehyde and phenylhydroxylamine derivatives of TNT. The same compound classes were formed from sunlight irradiation of TNT in the solid phase. Whereas carboxylic acids, aldehydes, aromatic amines, phenylhydroxylamines, and amides were also formed from irradiation of TNT in pond water and in pH 8 buffer solution, azoxy and azo compound formation was inhibited. Solid state 15N NMR spectra of photolysates from the lamp irradiation of unlabeled 2,6-dinitrotoluene in deionized water also demonstrated the formation of aromatic amine, phenylhydroxylamine/ 2° amide, azoxy, and azo nitrogens.

Effect of organic C on stable Fe isotope fractionation and isotope exchange kinetics between aqueous Fe(II) and ferrihydrite at neutral pH

Low-temperature geochemical cycling of Fe, C, nutrients, and toxic metals in nature are largely regulated by the stability and reactivity of fine-grained ferrihydrite. The presence of impurities such as C and Si in ferrihydrite structure are found to inhibit the rapid transformation of ferrihydrite to more stable iron oxides upon interaction with aqueous Fe(II). Therefore, understanding the factors controlling the reactivity of ferrihydrite, especially in the presence of C, is critical to evaluate the role of ferrihydrite in geochemical cycles. Equilibrium stable isotope fractionations are fundamental thermodynamic properties and therefore equilibrium fractionation in 56Fe/54Fe ratios reflects the nature of Fe bonding in ferrihydrite and its reactivity. In this study, we investigated stable Fe isotope fractionation between aqueous Fe(II) and ferrihydrite-organic matter coprecipitates to evaluate whether previously documented inhibition of mineralogical transformation by organic C is accompanied by changes in Fe isotope fractionation. Experiments conducted using Suwannee River natural organic matter (SRNOM) that was coprecipitated with ferrihydrite (molar C:Fe = 1.2) produced an equilibrium 56Fe/54Fe fractionation of −2.36 ± 0.26‰ between aqueous Fe(II) and ferrihydrite-SRNOM coprecipitates. This fractionation factor significantly differs from that previously determined between Fe(II)aq and pure ferrihydrite (−3.20‰) but is similar to that measured for Fe(II)aq-Si-ferrihydrite (−2.58 ± 0.14‰, molar Fe:Si = 1). Furthermore, the addition of C in the ferrihydrite structure at molar C:Fe ∼1.2 markedly increased the extent of Fe isotope exchange to a similar degree as observed with Si-ferrihydrite with molar Fe:Si ∼1. These observations suggest similar effects on both bonding and reactivity of ferrihydrite upon addition of equimolar C and Si. Due to the coexistence of ferrihydrite with organic matter in nature (e.g., wetlands), these results are important for understanding Fe isotope fractionation and exchange kinetics during mineral-fluid interactions in natural ferrihydrite-bearing systems.

A characterization of HULIS-C and the oxidative potential of HULIS and HULIS-Fe(II) mixture in PM2.5 during hazy and non-hazy days in Shanghai

Humic-like substances (HULIS) were isolated from PM2.5 in Shanghai. The oxidative potential and the cytotoxicity of the isolated HULIS and its Fe(II) binary mixtures were evaluated by using cell-free Dithiothreitol (DTT) and cellular 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays. Our data showed that HULIS-C contents in spring during hazy and non-hazy days were relatively higher than in winter. Cell-free DTT and bioassay (reactive oxygen species (ROS) and MTT) results showed that higher ROS values were released from individual HULIS or SRFA (Suwannee River Fulvic Acid) compounds than HULIS-Fe(II) complexes and Fe(II) alone. In addition, DTT and ROS correlation results were in line with each other and the oxidative potentials of HULIS and its binary mixtures were strongly inter-correlated with carbonaceous components (WSOC, OC) in PM2.5. Dose-dependent MTT results revealed that cell mortality positively correlated with the mass concentrations of HULIS (10–50 μg/ml). In the HULIS-Fe(II) system, the ROS generation ability depended on the mixing time of HULIS-C with Fe(II), thus suggesting that the organic-metal complexation time and the Fenton reaction induced by Fe (II) play a role in the cell mortality rate.

Aggregation kinetics of different surface-modified polystyrene nanoparticles in monovalent and divalent electrolytes

The intentional production and degradation of plastic debris may result in the formation of nanoplastics. Currently, the scarce information on the environmental behaviors of nanoplastics hinders accurate assessment of their potential risks. Herein, the aggregation kinetics of different surface-modified polystyrene nanoparticles in monovalent and divalent electrolytes was investigated to shed some light on the fate of nanoplastics in the aquatic environment. Three monodisperse nanoparticles including unmodified nanoparticles (PS-Bare), carboxylated nanoparticles (PS–COOH) and amino modified nanoparticles (PS–NH2), as well as one polydisperse nanoparticles that formed by laser ablation of polystyrene films (PS-Laser) were used as models to understand the effects of surface groups and morphology. Results showed that aggregation kinetics of negatively charged PS-Bare and PS-COOH obeyed the DLVO theory in NaCl and CaCl2 solutions. The presence of Suwannee river natural organic matters (SRNOM) suppressed the aggregation of PS-Bare and PS-COOH in monovalent electrolytes by steric hindrance. However, in divalent electrolytes, their stability was enhanced at low concentrations of SRNOM (below 5 mg C L−1), while became worse at high concentrations of SRNOM (above 5 mg C L−1) due to the interparticle bridging effect caused by Ca2+ and carboxyl groups of SRNOM. The cation bridging effect was also observed for PS-laser in the presence of high concentrations of divalent electrolytes and SRNOM. The adsorption of SRNOM could neutralize or even reverse surface charges of positively charged PS-NH2 at high concentrations, thus enhanced or inhibited the aggregation of PS-NH2. No synergistic effect of Ca2+ and SRNOM was observed on the aggregation of PS-NH2, probably due to the steric repulsion imparted by the surface modification. Our results highlight that surface charge and surface modification significantly influence aggregation behaviors of nanoplastics in aquatic systems.

Coupled retention of bovine serum albumin and NOM by porous membranes

Coupled retention of natural organic matter (NOM) is an important process in membrane water treatment. A mechanistic understanding is lacking due to the complex nature of NOM. In this study, filtration experiments were conducted with the mixture of bovine serum albumin (BSA) and Suwannee River NOM (SRNOM) through different pore-sized membranes. A macroscopic depth filtration theory was utilized for analyzing the underlying mechanisms. The results showed the importance of BSA-SRNOM interactions on NOM retention. At a neutral pH of 7.0, the co-existing SRNOM reduced BSA retention by up to 24% by mitigating concentration polarization and inner-pore BSA deposition. Meanwhile, the presence of BSA elevated SRNOM retention by up to 23% because of SRNOM adsorption onto the deposited BSA, as well as enhanced steric hindrance and concentration polarization effect. Furthermore, the reduction of solution pH to 3.7 enhanced BSA-SRNOM interactions that favored multilayer BSA/SRNOM deposition. As a result, BSA and SRNOM retention increased by 54% and 79%, 78% and 37%, and 48% and 10% for membranes having pore radii of 7.5 nm, 15 nm and 25 nm, respectively. This study demonstrated that the interplay of concentration polarization, steric hindrance, and inner-pore deposition governed NOM retention by porous membranes.

Transformation of silver ions to silver nanoparticles mediated by humic acid under dark conditions at ambient temperature

The conversion of silver materials in environments would impact their toxicity and risk. Previous studies have reported that silver ions (Ag+) could be reduced to silver nanoparticles (AgNPs) by natural organic matters (NOM) under sunlight or heating conditions. However, whether such reaction could occur in darkness at ambient temperature and the transformation mechanism were unclear. This study found that Ag+ at environmentally relevant concentrations (as low as 1 μg/L) could be reduced to AgNPs by Suwannee River humic acid (SRHA) in darkness at 30 °C. The reaction mechanism probed by X-ray absorption fine structure spectroscopy revealed that Ag+ was first bound to the carboxylic groups of SRHA to form Ag+-SRHA ligands, which were then reduced to metallic Ag. The increase of pH (6–9) and the coexistence of formate, acetate, carbonate, and sulfate promoted the formation of AgNPs. Besides, the formed AgNPs would coalesce to large aggregates under acidic conditions or in the presence of sulfate. These results suggest that the dark transformation of Ag+ to AgNPs mediated by NOM could occur in environments and are important for the better understanding of the natural origin of AgNPs.

Role of surface functionalities of nanoplastics on their transport in seawater-saturated sea sand

The transport and retention of nanoplastics (NP, 200 nm nanopolystyrene) functionalized with surface carboxyl (NPC), sulfonic (NPS), low-density amino (negatively charged, NPA−), and high-density amino (positively charged, NPA+) groups in seawater-saturated sand with/without humic acid were examined to explore the role of NP surface functionalities. The mass percentages of NP recovered from the effluent (Meff) with a salinity of 35 practical salinity units (PSU) were ranked as follows: NPC (19.69%) > NPS (16.37%) > NPA+ (13.33%) > NPA− (9.78%). The homoaggregation of NPS and NPA− was observed in seawater. The transport of NPA− exhibited a ripening phenomenon (i.e., a decrease in the transport rate with time) due to the high attraction of NP with previously deposited NP, whereas monodispersed NPA+ presented a low Meff value because of the electrostatic attraction between NPA+ and negatively charged sand. Retention experiments showed that the majority of NPC, NPS and NPA+ accumulated in a monolayer on the sand surface, whereas NPA− accumulated in multiple layers. Suwannee River humic acid (SRHA) could remarkably improve the transportability of NPC, NPS, and NPA− by increasing steric repulsion. The strong attraction between NPA+ and the deposited NPA+ in the presence of SRHA triggered the weak ripening phenomenon. As seawater salinity decreased from 35 PSU to 3.5 PSU, the increase in electrostatic repulsion of NP-NP and NP-sand enhanced the transport of NPC, NPS, and NPA−, and the ripening of NPA− breakthrough curves disappeared. In deionized water, NPC, NPS, and NPA− achieved complete column breakthrough because the electrostatic repulsion between NP and sand intensified. However, the Meff values of NPA+ in 3.5 PSU seawater and deionized water presented limited increments of 15.49% and 23.67%, respectively. These results indicated that the fate of NP in sandy marine environments were strongly affected by NP surface functionalities, seawater salinity, and coexisting SRHA.

Specific ion effects on the aggregation behavior of aquatic natural organic matter

Specific ion effects on the aggregation behavior of a reference aquatic natural organic matter (NOM), Suwannee River NOM (SRNOM), were investigated using kinetic, titration, calorimetric, and surface tension methods. Monovalent cations induced structural compacting of SRNOM, but not its aggregation. Their ability to induce structural compacting follows the order: Cs+ > Rb+ > K+ > Na+ > Li+. Divalent cations except Mg2+ can readily induce SRNOM aggregation. Their critical coagulation concentrations (CCC) follow the order: CCCSr > CCCCa > CCCBa. Electrokinetic, titration, and calorimetric data suggest that monovalent cations have weak interactions with SRNOM, while divalent cations strongly interact with SRNOM. Overall, the cation specificity in aggregation is determined by cation-NOM interactions and their ability to modulate surface tension. Specific ion effects of monovalent cations correlate to their hydration free energy, while that of divalent cations correlate to the ratio of the hydration entropy of cation to the enthalpy change of cation-NOM interactions. The cation specificity is consistent with the extended Derjaguin-Landau-Verwey-Overbeek theory, and the intermolecular interaction energy is dominated by the Lewis acid-base interactions. Our results suggest that specific cations should be targeted to predict or manipulate intermolecular interactions of aquatic NOM in natural and engineered settings.

Analysis of fluorophores of organic substances dissolved in Suvani river water using reversed-phase liquid chromatography

Reversed-phase high-efficiency liquid chromatography was used in combination with detection by multiwave fluorescence for analysis of organic substances dissolved in natural water of the Suwannee River. Also analyzed were the stable electrophoretic fractions А, В, and C+D, obtained by a combination of preparative size-exclusion chromatography and analytical electrophoresis in a polyacrylamide gel. Fraction А has the largest molecular size, and fraction C+D, the smallest. Using 3D fluorescent analysis, humic-like fluorescence was detected both in the original sample and in all fractions; protein-like fluorescence is almost fully localized in fractions А and В of the largest and middle molecular sizes. The wide peak of humic-like fluorescence is split into several groups of fluorophores with different emissions maxima (435, 455, 460, and 465 nm) and degrees of hydrophobicity. The obtained results were analyzed in relation to contemporary theories of formation of humic-like fluorescence of dissolved organic substances. The low-molecular free aromatic amino acids tyrosine and tryptophan were identified in fractions А and В of the highest molecular size and constitute &gt;50% of the protein-like fluorescence of the organic substances dissolved in the Suwannee River water. The data obtained ensure better understanding of the molecular nature of protein-like and humic-like fluorescence of organic substance dissolved in natural water.

Photocatalytic degradation of cephalexin by ZnO nanowires under simulated sunlight: Kinetics, influencing factors, and mechanisms

Increasing concentrations of anthropogenic antibiotics and their metabolites in aqueous environments has caused growing concerns over the proliferation of antibiotic resistance and potential adverse impacts to agro-environmental quality and human health. Photocatalysis using novel engineered nanomaterials such as ZnO nanowires may be promising for removing antibiotics from waters. However, much remains to be learned about efficiency and mechanism for photocatalytic degradation of antibiotics by ZnO nanowires. This study systematically investigated photodegradation of cephalexin using ZnO nanowires under simulated sunlight. The degradation efficiency of cephalexin was substantially increased in the presence of ZnO nanowires especially at circumneutral and alkaline condition (solution pH of 7.2–9.2). The photodegradation followed the first-order kinetics with degradation rate constants (k) ranging between 1.19 × 10−1 and 2.52 × 10−1 min−1 at 20–80 mg L−1 ZnO nanowires. Radical trapping experiments demonstrated that hydroxyl radicals (OH) and superoxide radicals (O2−) predominantly contributed to the removal of cephalexin. With the addition of HCO3− (1–5 mM) or Suwannee River natural organic matter (SRNOM, 2–10 mg L−1), the k values were substantially decreased by a factor of 1.8–70 to 1.69 × 10−3–6.67 × 10−2 min−1, probably due to screening effect of HCO3− or SRNOM sorbed on ZnO nanowires and scavenging of free radicals by free HCO3− or SRNOM in solution. Combining product identification by mass spectrometry and molecular computation, cephalexin photodegradation pathways were identified, including hydroxylation, demethylation, decarboxylation, and dealkylation. Overall, the novel ZnO nanowires have the potential to be used for removing antibiotics from contaminated waters.

Sunlight-induced phototransformation of transphilic and hydrophobic fractions of Suwannee River dissolved organic matter

Sunlight-induced chemical changes of both transphilic (SWR-TPH) and hydrophobic (SWR-HPO) fractions of Suwannee River dissolved organic matter (DOM) were followed by ultrahigh resolution Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). Irradiated SWR-TPH exhibited increase of chemodiversity, loss of some aromatic compounds, and almost no change in terms of average values of m/z, O/C and double bond equivalents (DBE). Irradiated SWR-HPO showed decrease of chemodiversity, average values of m/z, O/C and DBE. Irradiation of SWR-HPO produced oxygenated (O/C > 0.7) and aliphatic new compounds and removed some aromatics and carboxyl-rich alicyclic molecules (CRAM). Comparatively, CHO-compounds of SWR-TPH were relatively stable with a minor class of aromatic compounds disappeared under sunlight irradiation. Photochemical processing of SWR-HPO generated highly oxygenated new compounds that were readily present in SWR-TPH, implying that sunlight changes the hydrophobicity of DOM and that SWR-HPO is a photochemical precursor for SWR-TPH. This study contributed to the developing knowledge on organic matter phototransformation, particularly the transformation pattern of SWR-TPH that was never described previously; it also demonstrated the role of sunlight in producing SWR-TPH compounds from SWR-HPO and consequently driving the transformation of organic matter.

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.

Towards early-warning gene signature of Chlamydomonas reinhardtii exposed to Hg-containing complex media

The potential of using gene expression signature as a biomarker of toxicants exposure was explored in the microalga Chlamydomonas reinhardtii exposed 2 h to mercury (Hg) as inorganic mercury (IHg) and methyl mercury (MeHg) in presence of copper (Cu) and Suwannee River Humic Acid (SRHA). Total cellular Hg (THg = IHg + MeHg) decreased in presence of SRHA for 0.7 nM IHg and 0.4 nM MeHg, but increased for 70 nM IHg exposure. In mixtures of IHg + MeHg and (IHg or MeHg) + Cu, SRHA decreased THg uptake, except for 0.7 nM IHg + 0.4 nM MeHg which was unchanged (p-value>0.05). In the absence of SRHA, 0.5 μM Cu strongly decreased intracellular THg concentration for 70 nM IHg, while it had no effect for 0.7 nM IHg and 0.4 nM MeHg. The expression of single transcripts was not correlated with measured THg uptake, but a subset of 60 transcripts showed signatures specific to the exposed metal(s) and was congruent with exposure concentration. Notably, the range of fold change values of this subset correlated with THg bioaccumulation with a two-slope pattern in line with [THg]intra/[THg]med ratios. Gene expression signature seems a promising approach to complement chemical analyses to assess bioavailability of toxicants in presence of other metals and organic matter.

Microcystin-LR removal by ion exchange: Investigating multicomponent interactions in natural waters

Microcystin-LR (MCLR) is the most commonly encountered toxic microcystin variant. MCLR is usually present along with common surface water constituents such as inorganic ions and natural organic matter (NOM) which compete with MCLR for active sites during ion exchange (IX) process. Consequently, development of a multicomponent competitive model is essential for practical IX applications. This is critically important given that the NOM characteristics (charge density and molecular weight distribution) and inorganic ions concentrations are spatially variable and can change seasonally. In the present study, a systematic study was carried out into the multicomponent interactions of IX resin with inorganic ions and NOM during the MCLR removal process. This involved evaluation of MCLR removal in a single component system (i.e., MCLR only), a dual component system (MCLR and one other contaminant such as NOM), and a multiple component system (MCLR with NOM and different inorganic ions present in natural waters). A comprehensive understanding of the dynamic adsorption behavior showed that the experimental data for single component systems agree well with a Freundlich isotherm. For multicomponent interactions, the Equivalent Background Concentration (EBC) model which is derived from the Ideal Adsorption Solution Theory (IAST) provided the best correlation with the experimental data in natural waters. The concentrations of competing NOM and inorganic ions estimated by the EBC model were <10% of their initial concentrations. Sulphates are the most competitive inorganic ions followed by nitrates and bicarbonates and the multicomponent interactions could be well predicted by using the IAST-EBC model. However, the EBC model failed in the presence of higher molecular weight Suwannee River Humic Acid (SRHA) molecules due to neglecting of the pore blocking phenomenon. In the presence of higher molecular weight SRHA molecules, the Redlich-Peterson Isotherm (RP) model exhibited a better performance than the Sheindorf–Rebuhn–Sheintuch (SRS) and the EBC models.

Interactions between natural organic matter (NOM) and the cationic dye toluidine blue at varying pHs and ionic strengths: Effects of NOM charges and Donnan gel potentials

This study examined effects of charges of molecules of natural organic matter (NOM) on its interactions with organic cations represented by toluidine blue (TB). NOM was exemplified in this study by Suwannee River (SR) and South Platte (SP) NOM. TB/NOM interactions were probed in the range of pH 3 to 11, DOC concentrations 0–10.6 mg L−1 and ionic strengths 0.001–0.1 mol L−1. Changes of TB absorbance at indicative wavelengths of 640 and 550 nm located in the characteristic absorbance band of TB and their ratio R640/550 were used to quantify TB/NOM binding and the formation of TB dimers and possibly higher order aggregates in the Donnan volume of NOM molecules. These effects were interpreted based on the modeling of NOM charges and their Donnan gel potentials (ψD). The spectroscopic and modeling data showed that TB/NOM binding can be quantified via a complexation model that reflects the extent of electrostatic attraction between TB cations and anionic NOM molecules. The dependence of the apparent formation constant vs. ionic strength was shown to be qualitatively correlated with the ψD potentials of NOM molecules.