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Abstract: Divalent cations have been reported to develop bridges between anionic polyelectrolytes and negatively-charged colloidal particles, thereby enhancing particle flocculation. However, results from this study of kaolinite suspensions dosed with various anionic polyacrylamides (PAMs) reveal that Ca2+ and Mg2+ can lead to colloid stabilization under some conditions. To explain the opposite but coexisting processes of flocculation and stabilization with divalent cations, a conceptual flocculation model with (1) particle-binding divalent cationic bridges between PAM molecules and kaolinite particles and (2) polymer-binding divalent cationic bridges between PAM molecules is proposed. The particle-binding bridges enhanced flocculation and aggregated kaolinite particles in large, easily-settleable flocs whereas the polymer-binding bridges increased steric stabilization by developing polymer layers covering the kaolinite surface. Both the particle-binding and polymer-binding divalent cationic bridges coexist in anionic PAM- and kaolinite-containing suspensions and thus induce the counteracting processes of particle flocculation and stabilization. Therefore, anionic polyelectrolytes in divalent cation-enriched aqueous solutions can sometimes lead to the stabilization of colloidal particles due to the polymer-binding divalent cationic bridges. [Copyright &y& Elsevier]

Reductive dechlorination of chlorinated organic contaminants is an effective approach to treat this widespread group of environmentally hazardous substances. Metalloporphyrins can be used to catalyze reduction reactions by shuttling electrons from a reducing agent (electron donor) to chlorinated organic contaminants, thus rendering them to non-chlorinated acetylene, ethylene or ethane as major products. Iron, nickel and vanadium oxide tetraphenyl porphyrins (TPPs) were used as models of non-soluble metalloporphyrins that are common in subsurface environments, and hence may inflect on the ability to use natural ones. The effect of cosolvents on metalloporphyrins is demonstrated to switch the reduction of tetrachlorethylene (PCE) from no reaction to complete PCE transformation within 24h and the production of final non-chlorinated compounds. Variations in product distributions for the different metalloporphyrins indicate that changes in the core metal can influence reaction rates and effective pathways. Furthermore, different cosolvents can generate varied product distributions, again suggesting that different pathways and/or rates are operative in the reduction reactions. Comparison of different cosolvent effects on PCE reduction using vitamin B12—a soluble natural metalloporphyrinogen—as the catalyst shows less pronounced differences between reactions in various cosolvent solutions versus only aqueous solution. [Copyright &y& Elsevier]

Molar-based Setschenow constants (Ks) for six alkali and alkaline earth metal-based inorganic salts were determined at 20°C to evaluate their influence on the solubilities, and thus the aqueous activity coefficients, of three polycyclic aromatic hydrocarbons (PAHs). The six salts tested exhibited a wide range of Ks values, varying from 0.105±0.009 M-1 (for NaClO4 and pyrene) to 1.29±0.17 M-1 (for K2SO4 and perylene). In general, salting out effects with these electrolytes were observed in the order Ca2+>Na+>K+ and SO42->Cl->ClO4-, consistent with previous reports. However, the expected salting out trend of perylene > pyrene > naphthalene was only observed with K2SO4. In CaCl2 solutions, the Ks value of pyrene was significantly lower than that of naphthalene. For NaCl, KCl and NaClO4, pyrene Ks values were found to be lower than, but not significantly different from, those of naphthalene. Setschenow constants for all six salts were predicted using a semi-empirical, thermodynamically-based equation that relates the standard free energy change associated with transferring solutes from water to a salt solution to the difference in surface tensions between the two solutions. With this equation, predicted Ks values were in reasonable agreement with observed Ks values (generally within±50%). Lack of better agreement between predicted and observed values likely reflects the inability of the simple surface tension model to account for all interactions among the cations, anions, PAH molecules and water molecules in the respective systems. [Copyright &y& Elsevier]

Interactions of Ag+ with naphthalene and pyrene in aqueous solution were investigated using ultraviolet (UV) absorption and steady-state and time-resolved fluorescence spectroscopies. Small red-shifts in the two primary absorption bands of naphthalene and pyrene were observed in the presence of high concentrations of Ag+, indicating that ground-state cation–aromatic π electron interactions occurred. Ag+ complexation constants (K1 and K2) for naphthalene were determined directly from steady-state and time-resolved fluorescence data, whereas the formation of a pyrene–Ag+ exciplex required an additional correction to remove its interference on apparent pyrene complexation constants. The correction utilized ratios of the exciplex and monomer preexponential factors obtained from pyrene fluorescence decay curves measured at several emission wavelengths that were impacted to different degrees by the exciplex emission. The novel approach developed here to quantify ground-state complexes between Ag+ and polycyclic aromatic hydrocarbons (PAHs) offers new opportunities to investigate weak metal–organic complexes such as those resulting from cation–π interactions. [Copyright &y& Elsevier]

• Traditional site index (tree height at base age) may not be suitable for urban trees. • Rapid and non-destructive tree age evaluation methods need to be developed. • Loblolly pine tree age was determined using a minimally-invasive recording resistance drill based technique. • The method allowed rapid evaluation of tree age across an urban forest. Conventional site index (tree height at base age) may not be suitable for urban trees, therefore it is important to assess its applicability in urban environments. In addition, rapid and non-destructive tree age evaluation methods need to be developed. The objectives of this study were to: (1) develop a rapid and non-destructive method to determine tree age in urban environments, (2) examine the relationship between tree height, age, location, and soil type, and (3) compare the measured site indices with traditional site indices reported in Soil Survey Geographic Database (SSURGO). The study was conducted at the Clemson Experimental Forest (CEF) using conventionally-managed loblolly pine (Pinus taeda) stands, and loblolly pines in the urban environment of the Clemson University campus (CUC). Tree age obtained with a recording resistance drilling method and stump tree ring counting were comparable (R2 = 0.98, n = 14). Tree age estimates from recording resistance drilling and from increment boring were also similar (R2 = 0.93, n = 25). Replicate drilling samples had a low standard deviation, which indicated the high precision of the methodology. An additional benefit of the recording resistance drilling method is the ability to enable rapid evaluation of tree age across an urban forest. There was no discernible relationship between tree height and recording resistance drill age estimate for loblolly pines on the CUC (n = 25). In contrast, there was a relationship between measured diameter at breast height (DBH, cm) and recording resistance drill age estimates. A portion of the loblolly pine trees in the urban environment on the CUC appeared to have reduced productivity, as measured by the height/age relationship when predominantly surrounded by pavement or grass areas. [ABSTRACT FROM AUTHOR]

Dynamic exchanges between dissolved organic matter (DOM) and particulate organic matter (POM) plays a critical role in organic carbon cycling in coastal and inland aquatic ecosystems, interactions with aquatic organisms, mobility and bioavailability of pollutants, among many other ecological and geochemical phenomena. Although DOM-POM exchange processes have been widely studied from different aspects, little to no effort has been made to date to provide a comprehensive, mechanistic, and micro-spatial schema for understanding various exchange processes occurring in different aquatic ecosystems in a unified way. The phenomena occurring between DOM and POM were explained here with the homogeneous and heterogeneous mechanisms. In the homogeneous mechanism, the participating components are only organic matter (OM) constituents themselves with aggregation and dissolution involved, whereas OM is associated with other components such as minerals and particulate colloids in the heterogeneous counterpart. Besides the generally concerned processes of aggregation/dissolution and adsorption/desorption, other ecological factors such as sunlight and organisms can also participate in DOM-POM exchanges through altering the chemical nature of OM. Despite the limitation of current analytical technologies, many unknown and/or unquantified processes need to be identified to unravel the complicated exchanges of OM between its dissolved and particulate states. Based on the review of several previous mathematical models, we proposed a unified conceptual model to describe all major dynamic exchange mechanisms on the basis of exergy theory. More knowledge of dynamic DOM-POM exchanges is warranted to overcome the potential problems arising from a simple division of OM into dissolved versus particulate states and to further develop more sophisticated mathematic models. [ABSTRACT FROM AUTHOR]

Abstract: Changes in the characteristics of a leaf litter-derived humic substance (LLHS) that resulted from its adsorption onto kaolinite or exposure to simulated solar irradiation were tracked using selected spectroscopic descriptors, apparent weight-average molecular weight (MWw) and pyrene binding. Heterogeneity within the original bulk LLHS was confirmed by a range of different characteristics obtained from ultrafiltration-based size fractions. In general, trends of some changing LLHS characteristics were similar for the adsorption and irradiation processes when tracked against percent carbon removal. For example, the overall values of specific ultraviolet absorbance (SUVA), MWw, and humification index (HIX) all decreased with increasing irradiation time and with increasing concentration of mineral adsorbent in the respective experiments, indicating that both processes resulted in less aromatic and smaller-sized LLHS components remaining in solution. In addition, both the adsorption and irradiation experiments resulted in enrichment of the relative distribution of protein-like fluorescence (PLF), implying the PLF-related components had low affinities for phototransformation and mineral surface adsorption. Despite these apparently similar overall trends in LLHS characteristics caused by the adsorption and irradiation processes, closer examination revealed considerable differences in how the two processes altered the original material. Net production of intermediate-sized constituents was observed only with the irradiation experiments. In addition, residual LLHS resulting from the adsorptive fractionation experiments exhibited consistently higher pyrene binding versus the irradiated LLHS despite having comparable MWw values. Changes in LLHS characteristics due to adsorption by kaolinite were likely caused by physical mechanisms (primarily hydrophobic interactions between LLHS components and the kaolinite surface) whereas the irradiation-induced changes appear to have been governed by the combined effects of several alteration mechanisms, including the transformation of more condensed aromatic structures to less aromatic constituents, conformational changes resulting from selective photooxidation, and the photochemical disruption of intramolecular charge-transfer interactions. [Copyright &y& Elsevier]

Abstract: Real-time or near real-time in-situ monitoring of dissolved organic matter (DOM) composition in natural waters and engineered treatment systems provides critical information to water quality scientists and engineers, particularly when the monitoring techniques can provide some information about the chemical nature of DOM. The efficacy of various indices derived from rapid, low-cost spectroscopic and chromatographic techniques to discriminate DOM composition was tested for samples prepared from well-defined mixtures of purified Aldrich humic acid (PAHA) and Suwannee River fulvic acid (SRFA). Sensitivities of the discrimination indices were examined by comparing (1) the differences between measured values and those predicted based from mass balance and the end member characteristics, and (2) the linear correlations between index values and mass ratios of the DOM mixtures. Size exclusion chromatography (SEC) results revealed that the weight-average molecular weight (MWw) may be a useful approach for tracking DOM mixing processes, although the number-average molecular weight (MWn) may be better for distinguishing different DOM compositions. Specific ultraviolet absorbance measured at 254nm (SUVA254) performed better as a discrimination index than did two previously recommended absorbance ratios, both in terms of making better predictions of intermediate compositions and in exhibiting a more linear correlation with PAHA mass ratio. Several well-defined peaks in the derivative absorption spectra (301 and 314nm for the first derivative, 217nm for the third derivative, and 211 and 224nm for the fourth derivative) also were found to be promising potential DOM discrimination indices. Finally, a fluorescence ratio based on humic- versus fulvic-like fluorescence proved to be a superior DOM discrimination index for the two DOM end members studied here. In general, this study illustrates the evaluation process that should be followed to develop rapid, low-cost discrimination indices to monitor DOM compositions based on end member mixing analyses. [Copyright &y& Elsevier]

The primary mechanism responsible for the dynamic fluorescence quenching of polycyclic aromatic compounds (PACs) by Ag+ ions is enhanced intersystem crossing from the lowest singlet excited state to the lowest triplet excited state. For some PACs, however, exciplex formation with Ag+ in polar organic solvents has also been reported. Quenching of pyrene fluorescence by Ag+ in two polar solvent systems (aqueous and ethanolic solutions) was examined here using steady-state and time-resolved fluorescence techniques. In both solvents, quenching led to the formation of a pyrene–Ag+ exciplex, (1Py⋯Ag+)*, which rapidly equilibrated with excited singlet pyrene molecules (1Py*) and Ag+ ions. The exciplex and pyrene monomer emission spectra strongly overlapped, with the dominant exciplex peaks in water and ethanol red shifted from the 0–0 transition of pyrene by 5.08 and 2.56 kJ/mol, respectively. Rate constants for the formation and dissociation of the exciplex were much larger than the radiative and nonradiative decay rate constants for both the excited state monomer and exciplex. The short exciplex fluorescence lifetimes and very low quantum yields observed in the two solvents can be attributed to enhanced nonradiative decay processes for the exciplex. The emission quantum yield of the exciplex formed in aqueous solution was approximately an order of magnitude larger than that in ethanolic solution, which is likely to be attributable to the higher polarity of water versus ethanol. [Copyright &y& Elsevier]

Summary: Changes in disinfection byproduct (DBP) precursors during storm events have not been intensively studied to date despite their dramatic impact on downstream drinking water supplies. For this study, variations in dissolved organic matter (DOM) spectroscopic properties and DBP formation potentials (DBPFPs), and the correlations among these various parameters, were investigated in river water samples collected under two contrasting storm event conditions (a strong summer storm versus a weak fall event). Fluorescence excitation–emission matrices (EEMs) combined with parallel factor analysis revealed that a combination of two humic-like components (C1 and C2) dominated the EEM data of the storm samples. Measured DOM characteristics and DBPFPs varied over a wider range during the more intense summer storm, exhibiting higher values of specific ultraviolet absorbance and C1/C2 ratio at high flow conditions. Variations in the fluorescence index were less pronounced during the two storm events. The spectroscopic changes were well explained by the input of terrestrial DOM sources from the surrounding catchments during the storms. Different trends of the formation potentials for trihalomethanes and haloacetic acids on the recession limb of the hydrographs suggest that the structures responsible for DBP precursors during storm events may not be the same for the two classes of DBPs. The ultraviolet light-absorbing moieties in DOM appear to play major roles in generating DBPs, as evidenced by their strong positive correlations with DBPFPs. Higher correlations with DBPFP for C1 versus C2 suggest that humic-like substances associated with more aromatic and condensed structures have a greater proclivity to generate DBP upon chlorination. [Copyright &y& Elsevier]

Abstract: Emission from two different peroxyoxalate chemiluminescence (PO-CL) reaction pathways (i.e., 1,1′-oxalyldiimidazole (ODI) and bis(2,4,6-trichlorophenyl)oxalate (TCPO) CL reactions) is significantly reduced in the presence of increasing Cu2+ concentrations. The CL intensity decrease in the ODI-CL reaction results from deactivation of the high-energy intermediate(s) by Cu2+, whereas the apparent quenching in the TCPO-CL reaction results from both this effect and formation of Cu2+–imidazole (ImH) complexes [Cu(ImH) n=1–4]2+. Because the reaction between Cu2+ and ImH occurs competitively with the formation of a high-energy intermediate (X) in TCPO-CL reactions, accurately determining Cu2+ concentrations with a TCPO-CL-based system is more difficult versus an ODI-CL-based system. Using ODI-CL, the detection limit of Cu2+ dissolved in water was determined to be 5.0×10−8 M (signal:noise=3) in a 1cm fluorescence flow micro-cell. ODI-CL-based systems are expected to be useful for the rapid quantification of many quenchers in environmental and biological samples because the interactions between the high-energy intermediate(s) and quencher are very rapid relative to other competitive reactions. [Copyright &y& Elsevier]

Development of low-cost strategies enabling phosphorus (P) use, recovery, and reuse across multiple economic sectors is needed to ensure its long-term availability for those industries that require P to enhance plant productivity. Irrigation return water from conventional agriculture and specialty crop production remains a source of excess P loading to aquatic ecosystems. Low-cost and technology treatment options are critical for the long-term economic and environmental sustainability of agricultural producers. To address P removal with a sustainable lens, we identified iron oxides (FeOx, a waste product from treating coal mine drainage) as a potential substrate for P sorption, and further evaluated the potential for reuse of P-saturated FeOx as a fertilizer source. The overall objectives of this study were to quantify P sorption, desorption, and plant availability of P adsorbed to FeOx. Batch sorption experiments were conducted to measure the effect of mixing FeOx with silica sand (1:1; FeOx:sand, by mass) on P sorption capacity. The sorption capacity of the FeOx:sand mixture was 21.5 mg P/g FeOx using deionized water. Less than 1% of initially adsorbed P desorbed over 96 h in treatments using irrigation reservoir water adjusted to either pH 6.0 or pH 10.0, demonstrating the stability of the sorption complexes against pH changes expected in irrigation return water systems. Plant available P, which we define as P that desorbed after contact with a CaCl 2 solution, ranged from 0.10–0.12 mg/L P; this range is within nutrient recommendations for most plants. Phosphorus-saturated FeOx may be suitable as a fertilizer amendment, as well as a sustainable option for P removal from water. This research provides the foundation for a circular P economy, with viable P treatment and reuse paired with managing a waste product from another widespread industry. Unlabelled Image [ABSTRACT FROM AUTHOR]