Description: In most case scenarios, individual metals exist as components in mixtures with organic and inorganic substances and/or particulate matter. While the concepts encompassing mixture toxicity and modeling have been around for decades, only recently have new approaches (dynamic speciation techniques and fate and bioavailability models) been expanded to consider metal mixture scenarios. For example, the kinetic features of humic substances and inorganic colloids on the complexation of metals are generally considered. Although current environmental regulations rarely require an assessment of chemicals mixtures, research on these mixtures in the environment is essential for future regulatory demands and is vital for ensuring adequate environmental protection. Interpretation of speciation and bioavailability data from metal mixtures can be very complex and demanding, due to the existence of kinetic physicochemical transformations of the dynamic components. This kinetic effect largely affects metals’ dynamic speciation, culminating in different transformed metal-containing products with different contributions for the metal uptake by a consuming interface. This manuscript is focused on the environmental fate of metal mixtures, which determines how the mixture is biogeochemically processed and which receptors are most exposed (organisms and exposure route), with a special focus on their dynamic speciation, including a critical evaluation of the current challenges and available dynamic speciation techniques as well as computer codes and models.

Description: We study the deprotonation of MAl 12 Keggin ions in monovalent electrolyte solutions of varying composition and concentration by potentiometric titration. The structures exhibit very steep deprotonation, where the singly coordinated aquo groups lose protons within a narrow pH range. Once the deprotonation is substantial, the Keggin ions start to aggregate by dehydration and linkage of terminal functional groups into hydroxo-bridges. In the present study, we address three aspects with our experiments. We test the cation-specificity, the anion-specificity and the overall effect of electrolyte concentration with respect to the deprotonation behavior. Our results show that the cation series in chloride systems does not show any ion-specificity and all the curves coincide for the 100 mM solutions, whereas the anion series in sodium systems does. The most structure-making anion (bromate) used in this study causes aggregation of the Keggins prior to the onset of aggregation in the presence of border-line and structure-breaking anions (chloride, nitrate and perchlorate). For the latter, no significant difference is observed. The fluoride ion causes a completely different behavior. No significant deprotonation pH effect is observed where deprotonation occurs in the absence of fluoride. At even higher pH, massive consumption of hydroxide occurs. Some possible scenarios for the behavior of the fluoride-containing systems are discussed.

Description: In the absence of added electrolyte, at ionic strengths below 0.5 mM, maximum extents on adsorption of 100 μM ligand onto 1.06 g L −1 FeOOH(goethite) increased in the order (2-phenylglycine) 〈 phenylacetate 〈 5-methyl-2-anthranilate 〈 anthranilate 〈 4-aminobenzoate ~  N -phenylglycine 〈 benzoate 〈 4-hydroxybenzoate 〈 4-nitrobenzoate ~ salicylate. With salicylate, the maximum extent of adsorption occurred at a pH 0.60 log units higher than pK a (0/−), which corresponds to conversion of the monoanion to the protonated, neutral species. With the other eight ligands, maximum extents of adsorption occurred within 0.40 log units of pK a (0/−). Capillary electrophoresis enabled us to quantify extents of adsorption and confirm that added ligand had not undergone chemical alteration. The computer program DLIM performed the usual tasks of computer equilibrium speciation models, but also made it possible to integrate concentrations of ionic species from the surface outward toward bulk solution, yielding the nonspecific contribution to adsorption. DLIM enabled us to track changes to ionic strength as a function of reagents added and measured pH. In the low ionic strength regime, the adsorption stoichiometry \({〉}{\text{SOH}} + {\text{L}}^{-} = {〉}{\text{S}}\left( {\text{OH}} \right){\text{L}}^{ - }\) provided a better fit to experimental data for most ligands than the conventional stoichiometry \({〉}{\text{SOH}} + {\text{H}}^{ + } + {\text{L}}^{ - } = {〉}{\text{SL}}^{0} + {\text{H}}_{2} {\text{O}}\) . Partial loss of waters of adsorption accompanying adsorption impedes the adsorption of strongly hydrated anions over less strongly hydrated ones.

Description: Trace metal limitation not only affects the biological function of organisms, but also the health of ecosystems and the global cycling of elements. The enzymatic machinery of microbes helps to drive critical biogeochemical cycles at the macroscale, and in many cases, the function of metalloenzyme-mediated processes may be limited by the scarcity of essential trace metals. In response to these nutrient limitations, some organisms employ a strategy of exuding metallophores, biogenic ligands that facilitate the uptake of metal ions. For example, bacterial, fungal, and graminaceous plant species are known to use Fe(III)-binding siderophores for nutrient acquisition, providing the best known and most thoroughly studied example of metallophores. However, recent breakthroughs have suggested or established the role of metallophores in the uptake of several other metallic nutrients. Furthermore, these metallophores may influence environmental trace metal fate and transport beyond nutrient acquisition. These discoveries have resulted in a deeper understanding of trace metal geochemistry and its relationship to the cycling of carbon and nitrogen in natural systems. In this review, we provide an overview of the current state of knowledge on the biogeochemistry of metallophores in trace metal acquisition, and explore established and potential metallophore systems.

Description: X-ray absorption spectroscopy (XAS) was used to quantify the various proportions of the different ligation spheres of zinc in the sediments of Lake DePue (Illinois, USA), an environment contaminated by smelting operations. Samples were collected with a hand-held piston coring device, sub-cored, sectioned in thin, ~1-mm-thick sections, and kept under cryogenic conditions (liquid N 2 ) to prevent any chemical change. X-ray absorption spectra were recorded in the continuous XAS scanning mode, and the experimental errors of the data collection were directly measured. The extended X-ray absorption fine structure (EXAFS) was decomposed on a basis set of references’ EXAFS spectra. These reference standards were selected relying on geochemical analyses and analytical electron microscopy data. The propagation of experimental errors through the entire process allowed the assessment of the quality of the resulting Zn speciation. The results show that Zn is coordinated with a wide variety of ligands: carbonate, phosphate, sulfide, and water. The proportions of these various coordinative environments vary with both location and depth in the core. Close to the source of contamination, a significant fraction of Zn is present as an aqua-complex, suggesting that Zn is under a rather labile chemical form. Away from the source of contamination, Zn is predominantly bound to sulfide, which is rather inert under the reducing conditions prevailing in these sediments.

Description: The present study explores the effect of salinity and dissolved organic carbon (DOC) gradients on the stability and reactivity of titanium dioxide nanoparticle (TiO 2 -NP) agglomerates in ambient water from the Lagoon of Venice and their possible effect on nauplii sampled at the same locations. In all ambient water samples, TiO 2 -NPs formed rapidly micrometre-sized agglomerates. The increase in the salinity and concomitant decrease in DOC content induced the formation of larger agglomerates, with z-average hydrodynamic diameter increasing with TiO 2 -NP concentration and exposure duration. Under the studied conditions, ζ-potential exhibited negative values. In line with agglomeration results, enhancement of the salinity and lower DOC resulted in less negative ζ-potential with close to 0 values in the dispersions of 100 mg L −1 TiO 2 -NPs in sea water. Two-hour exposure to micrometre-sized agglomerates of TiO 2 -NPs resulted in an increase in the fluorescence of propidium iodide (PI) stained nauplii in comparison with unexposed controls, but had no effect at 24-h exposure. The increase in nauplii -associated PI fluorescence was more noticeable in dispersions containing 100 mg L −1 than those containing 10 mg L −1 TiO 2 -NPs, suggesting membrane permeability alteration in a concentration-dependent manner. However, the PI staining results have to be interpreted with caution because of the possible dye binding to the nauplii surface without penetration of cellular membrane. The effect of TiO 2 -NPs on nauplii was more pronounced at higher salinity and decreased with increasing DOC concentrations at 2 h, while no trends were found at 24-h exposure, as well as exposure to 100 mg L −1 TiO 2 -NPs.

Description: Wetlands are specific areas able to regulate metals mobility in the environment. Among metals, rare earth elements (REE) appear to be particularly interesting because of the information that could be provided by the REE patterns. Moreover, as REE are becoming a matter of great economic interest, their significant release into the environment may be expected over the next few decades. Wetlands would then play a key role in the regulation of their concentration in the environment. This review demonstrated that REE are released in wetland bound to colloidal organic matter. During the flood season, the released REE concentrations are largely higher than those released during the wet period. This solubilization is related to the organic matter desorption caused by the pH rise imposed by the reducing reactions. The resulting REE patterns depend on the heterogeneity of the humic acid (HA) binding sites and the presence of potential competitive cations, such as Fe(III) and Al(III). At high REE loading, REE are bound to HA carboxylic groups and the pattern exhibit a MREE downward concavity. At low loading, REE are bound to phenolic and chelate groups and the pattern exhibits a lanthanide contraction. At low loading, REE seem to act as cationic bridges between two organic molecules, whereas at high loading they seem to be engaged in strong multidentate bonding. Moreover, the REE patterns can be modified with the competitive cations amount and speciation. The prime factor governing all these processes is pH, which drives the organic colloid production, REE loading and solubility of competitive cations.

Description: The tremendous generation of solid waste material is a worldwide problem. Apart from avoidance of waste materials, their reuse is a main goal of modern waste management. Reuse of waste materials demands adequate risk assessment and reliable quantification methods considering the release potential of harmful compounds. In this context, due to its cancerogenity, hexavalent chromium (Cr(VI)) is of high ecotoxicological, human toxicological and environmental relevance. In this contribution, we focus on the speciation of chromium in two representative solid waste materials (demolition waste material, DWM; municipal waste incineration product, MWIP) and eluates obtained after leaching. For chromium speciation, X-ray absorption spectroscopy (XAS) and ion chromatography coupled with inductively coupled plasma optical emission spectrometry (IC/ICP-OES) were applied. It could be observed that in spite of relatively low contents of Cr(VI) in both solid materials (DWM: 16 ± 1 mg/kg, MWIP: 67 ± 5; aqua regia digestion), environmentally relevant concentrations of Cr(VI) were released from the materials as revealed by batch and column leaching experiments. In particular, threshold values for Cr(VI) of the German Federal Soil Protection and Contaminated Sites Ordinance were significantly exceeded. Application of XAS allowed both the determination of mobilizable Cr(VI) directly in the solid materials and in the alkaline eluates, even at concentrations below 500 µg/L. The results obtained were in agreement with data of leaching tests and with IC/ICP-OES measurements. IC/ICP-OES enabled a rapid, well-reproducible and sensitive quantification of Cr(VI) in the eluates in the lower µg/L range. The presented combination of independent analytical methods and mutual validation is highly powerful and is a valuable contribution for risk assessment of waste material. The principle of the approach can be used for the investigation of other solid materials and elements and their assessment.

Description: The technique of diffusive gradients in thin films (DGT) has emerged as a powerful technique for obtaining information on the dynamics of chemical speciation and partitioning of a diverse range of solutes in natural waters. However, there have been concerns that adsorption of dissolved organic matter (DOM) to the diffusive hydrogel could affect measurements, particularly of trace metals. Diffusive gels were exposed to solutions containing DOM from eight different sources and the adsorption of organic matter measured spectrophotometrically, both by its removal from solution and its accumulation on the gel. Both fulvic and humic acid extracted from water adsorbed slightly, but there was much more marked adsorption of humic acid extracted from a peaty soil. There was virtually no evidence of adsorption of DOM in filtered natural waters of various sources, including a lake, river and the porewaters of soils, even when the latter had high concentrations of humic substances. It appears that humic substances in solution, rather than those extractable from the solid phase of soils, do not adsorb markedly to the DGT diffusive hydrogel. DGT measurements of Cd were unaffected if their diffusive gels were pre-exposed to DOM in soil and surface waters, but DGT measurements of Cu were suppressed, especially if the pre-exposure had been to a water containing high concentrations of DOM. This effect on the DGT measurement is likely to be minimised by use of longer deployment times, but further work is required to establish the extent of such effects for a range of realistic conditions.

Description: Polyphosphate is a quantitatively important and dynamic component of the sedimentary phosphorus in many lakes. Kenney et al. correctly note that we misrepresented their data on polyphosphate in Lake Apopka in our article on the phosphorus composition of subtropical lake sediments, and we regret this error. However, we reiterate that their operationally defined heat extraction procedure overestimates polyphosphate in lake sediments because it includes phosphorus from a number of non-polyphosphate sources. In contrast, our measurements by solution 31 P NMR spectroscopy provide direct quantification of polyphosphate in Lake Apopka sediments and are therefore closer to the true values. Future studies addressing the origins and dynamics of polyphosphate in the environment should employ analytical procedures that unequivocally identify and quantify polyphosphate.