Natural Aquatic Systems Research Articles

Defining the transcription landscape of the Gram-negative marine bacterium Vibrio harveyi

Vibrio harveyi is a Gram-negative pathogenic bacterium ubiquitously present in natural aquatic systems. Although environmental adaptability in V. harveyi may be enabled by profound reprogramming of gene expression previously observed during responses to starvation, suboptimal temperatures and other stress factors, the key characteristics of V. harveyi transcripts and operons, such as their boundaries and size as well as location of small RNA genes, remain largely unknown. To reveal the main features of the V. harveyi transcriptome, total RNA of this organism was analyzed by differential RNA sequencing (dRNA-seq). Analysis of the dRNA-seq data made it possible to define the primary transcriptome of V. harveyi along with cis-acting regulatory elements (riboswitches and leader sequences) and new genes. The latter encode a number of putative polypeptides and new phylogenetically conserved antisense RNAs potentially involved in the post-transcriptional control of gene expression.

Microhabitat selection of axolotls, Ambystoma mexicanum, in artificial and natural aquatic systems

Studies of habitat selection are crucial for the conservation of threatened amphibians. Wild salamanders are often distributed near rocks or vegetation, which provide shelter. However, nothing is known about habitat selection of the Mexican axolotl (Ambystoma mexicanum), an endangered salamander of great cultural and ecological value. This study aims to test the relationship between vegetation presence and the distribution of captive-raised axolotls in two systems: a closed canal in their native ecosystem (n = 10) and an artificial canal within a zoological park (n = 6). We used radio-telemetry to analyse the hourly distribution and movement patterns of axolotls in each study site during 72-h observational periods. We found that movement patterns and microhabitat selection were related to vegetation coverage and diurnal and nocturnal periods. Sex and age had no effect in habitat selection. Axolotls in both study sites preferred vegetated microhabitats, but in Xochimilco this preference was only significant during daytime when they were less active. These habitat-specific patterns of spatial distribution may represent behavioural strategies for reducing predation. The first approach of behavioural insights from this study will inform the construction of refuges to reduce the alarming depletion of axolotls in the wild.

Production of extracellular reactive oxygen species by phytoplankton: past and future directions.

In aquatic environments, phytoplankton represent a major source of reactive oxygen species (ROS) such as superoxide and hydrogen peroxide. Many phytoplankton taxa also produce extracellular ROS under optimal growth conditions in culture. However, the physiological purpose of extracellular ROS production by phytoplankton and its wider significance to ecosystem-scale trophic interactions and biogeochemistry remain unclear. Here, we review the rates, taxonomic diversity, subcellular mechanisms and functions of extracellular superoxide and hydrogen peroxide production by phytoplankton with a view towards future research directions. Model eukaryotic phytoplankton and cyanobacteria produce extracellular superoxide and hydrogen peroxide at cell-normalized rates that span several orders of magnitude, both within and between taxa. The potential ecophysiological roles of extracellular ROS production are versatile and appear to be shared among diverse phytoplankton species, including ichthyotoxicity, allelopathy, growth promotion, and iron acquisition. Whereas extracellular hydrogen peroxide likely arises from a combination of intracellular and cell surface production mechanisms, extracellular superoxide is predominantly generated by specialized systems for transplasma membrane electron transport. Future insights into the molecular-level basis of extracellular ROS production, combined with existing high-sensitivity geochemical techniques for the direct quantification of ROS dynamics, will help unveil the ecophysiological and biogeochemical significance of phytoplankton-derived ROS in natural aquatic systems.

Adsorption of Methylmercury ontoGeobacter bemidijensisBem

The anaerobic bacterium Geobacter bemidijensis Bem has the unique ability to both produce and degrade methylmercury (MeHg). While the adsorption of MeHg onto bacterial surfaces can affect the release of MeHg into aquatic environments as well as the uptake of MeHg for demethylation, the binding of MeHg to the bacterial envelope remains poorly understood. In this study, we quantified the adsorption of MeHg onto G. bemidijensis and applied X-ray absorption spectroscopy (XAS) to elucidate the mechanism of MeHg binding. The results showed MeHg adsorption onto G. bemidijensis cell surfaces was rapid and occurred via complexation to sulfhydryl functional groups. Titration experiments yielded cell surface sulfhydryl concentrations of 3.8 ± 0.2 μmol/g (wet cells). A one-site adsorption model with MeHg binding onto sulfhydryl sites provided excellent fits to adsorption isotherms conducted at different cell densities. The log K binding constant of MeHg onto the sulfhydryl sites was determined to be 10.5 ± 0.4. These findings provide a quantitative framework to describe MeHg binding onto bacterial cell surfaces and elucidate the importance of bacterial cells as possible carriers of adsorbed MeHg in natural aquatic systems.

Environmental impacts assessment of industrial wastewater treatment system using electroless nickel plating and life cycle assessment approaches

Wastewater treatment systems have been designed to minimize the environmental impacts of discharging untreated wastewater in natural aquatic systems. Different wastewater treatment options have different performance characteristics and different direct impacts to the environment. The main aim of this study is to assess and quantify the environmental impacts of an electroless nickel plating wastewater treatment plant in Skudai, Johor Bahru, Malaysia. A life cycle assessment method, based on the international organization for standardization 14040 series framework, was used to assess the environmental impacts of the selected wastewater treatment plant. The ReCiPe life cycle impact assessment method attached to GaBi 4 software was used to assess the environmental impact of the selected industrial wastewater treatment plant. The results show that the existing chemical treatment process had the highest environmental impacts in terms of climate change with 1552.89 kg of CO2 and terrestrial acidification of 5.79 kg SO2 equivalent compared to the physical and biological treatment processes. Hence, two different scenarios, one utilizing chitosan and the other wet air oxidation (WAO) process, were formulated to reduce the environmental impacts of the conventional chemical wastewater treatment process. The impacts on the climate change using chitosan and WAO scenarios were both seen to have less than 110 kg CO2-equivalent emissions compared to the existing chemical process.

Characterization of Dissolved Organic Matter Fractions in the Ning-Meng Section of the Yellow River and Relationship with Metal Ions

Dissolved organic matter(DOM)is an important element of natural aquatic systems. Due to differences in their hydrophobic/hydrophilic properties and various functional groups, chemical appearances of DOM fractions also vary. In this study, seven natural waters, extending from Xiaheyan to Toudaoguai along the Ning-Meng section of the Yellow river, were sampled in April 2015. Four DOM fractions were obtained by pumping through XAD-4 and XAD-8 resins, i.e., hydrophobic acid (HOA), hydrophobic base (HOB), weak hydrophobic acid (WHOA), and hydrophilic matter (HYI). Based on detection by three-dimensional excitation-emission matrix fluorescence (EEM) and correlation analysis, relationships with five metal ions (Pb, Zn, Cu, Cr, As) were analyzed. Results show that DOC gradually increased along an upstream to downstream continuum in the Ning-Meng section. HYI (small molecular proteins) was the main DOM fraction present, followed by HOA, suggesting enhanced microbial-sourced impact from industrial sewage discharges. The significant peaks of humic-like (A, C) and protein-like compounds (T1) in the EEM chart further highlight the effect of endogenous pollution caused by wastewater. Furthermore, SPSS fitting results indicate that DOM is correlated with all five metal ions, especially with Cu. In terms of the four DOM fractions, HYI showed the strongest correlation with Cu, illustrating the significant relationship between HYI and Cu during the migration and transformation process. Moreover, the fluorescence intensity of protein-like compounds decreased with increasing Cu concentration, possibly due to fluorescence quenching caused by complexation between Cu and proteins in HYI.

Investigating diversity of pathogenic microbes in commercial bait trade water.

The recreational bait trade is a potential pathway for pathogen introduction and spread when anglers dump bait shop sourced water into aquatic systems. Despite this possibility, and previous recognition of the importance of the bait trade in the spread of aquatic invasive species (AIS), to date there has been no region wide survey documenting pathogens in retail bait shops. In this study, we analyzed 96 environmental DNA samples from retail bait shops around the Great Lakes region to identify pathogens, targeting the V4 hypervariable region of the 16S rRNA gene. Additionally, we used samples from one site in Lake Michigan as a comparison to pathogen diversity and abundance in natural aquatic systems. Our results identified nine different groups of pathogens in the bait shop samples, including those that pose risks to both humans and fish species. Compared to wild sourced samples, the bait shops had higher relative abundance and greater taxonomic diversity. These findings suggest that the bait trade represents a potentially important pathway that could introduce and spread pathogens throughout the Great Lakes region. Improving pathogen screening and angler outreach should be used in combination to aid in preventing the future spread of high risk pathogens.

Metabolite identification of ibuprofen biodegradation by Patulibacter medicamentivorans under aerobic conditions

ABSTRACTIbuprofen (IBU) is a non-steroidal anti-inflammatory drug that is becoming increasingly recognized as an important micropollutant to be monitored in wastewater treatment plants (WWTP), since it has been detected in effluents at the µg L−1 level. The IBU metabolites from biological degradation are not completely understood and can represent a threat to natural aquatic systems. P. medicamentivorans was previously isolated from WWTP sludge and found to be capable of IBU degradation. The aerobic biodegradation of ibuprofen by this organism was investigated in a batch lab-scale reactor for the identification of the metabolites formed. The metabolites were analysed and putatively identified by HPLC-DAD-MS/MS and GC-MS and biodegradation pathways were proposed. The toxicity and the biodegradability potential of the metabolites were also investigated. The results showed that IBU biotransformation was achieved by hydroxylation followed by the formation of a carboxylic acid in the IBU molecule and by the formation of a catechol, allowing the aromatic ring cleavage. Two biodegradation pathways were proposed: in one, the metabolites generated from the enzymatic action correspond to a less biodegradable chemical structure of the intermediate products (isobutylbenzene and 3-isobutylphenol), with comparatively higher toxicity; in the other mechanism, more oxidable chemical structures were formed with less toxicity and higher biodegradability. This suggests that the biodegradation of IBU by P. medicamentivorans can take place by more than one mechanism regarding the enzymes formed by this Gram-positive bacterium, with subsequent oxidation of the parent compound to overall more soluble and less toxic compounds to fish, daphnia and green algae.

Fine particle attachment to quartz sand in the presence of multiple interacting dissolved components

In natural aquatic systems water chemistry is complicated and fine particles encounter multiple water components simultaneously, yet the combined effects of some multiple components on the fate and transport of these particles have not been elucidated. In this study nTiO2 and illite colloid attachment to quartz sand was investigated in 1 mM NaCl and 0.5 mM CaCl2 background solutions using a range of phosphate concentrations (0 to 10 mg/L) at pH 5 and 9. The results obtained from the batch experiments indicated that without using phosphate, nTiO2 aggregation and attachment was strongly influenced by pH and Ca2+, both of which modified nTiO2 surface charges. nTiO2 attachment was high in CaCl2 solution at pH 9 due to attractive forces between nTiO2 and sand, as well as ripening. Furthermore, phosphate adsorption to nTiO2 was higher in CaCl2 solution at pH 9 than that at pH 5 due to attractive forces between nTiO2 and phosphate anions, and also potential surface precipitation of Ca-P minerals at pH 9. Phosphate adsorption to illite was low owing to strong repulsive forces between illite and phosphate. The effect of phosphate on nTiO2 and illite attachment to sand was influenced by pH and cation valency. A decreasing trend in nTiO2 attachment with phosphate addition was observed in NaCl solution at pH 5 and 9, and in CaCl2 solution at pH 5; however, in CaCl2 solution at pH 9, the surface charge of nTiO2 reversed from negative to positive and a substantial amount of nTiO2 attached to sand. Moreover, illite attachment to sand was much lower than that of nTiO2 under all the conditions tested in this study. These findings are important for understanding of the fate and transport of nTiO2 and illite colloids in natural aquatic systems where various anions and cations co-exist.

Different electrically charged proteins result in diverse bacterial transport behaviors in porous media

The influence of proteins on bacterial transport and deposition behaviors in quartz sand was examined in both NaCl (10 and 25 mM) and CaCl2 solutions (1.2 and 5 mM). Bovine Serum Albumin (BSA) and bovine trypsin were used to represent negatively and positively charged proteins in natural aquatic systems, respectively. The presence of negatively charged BSA in suspensions increased the transport and decreased bacterial deposition in quartz sand, regardless of the ionic strength and ion types. Whereas, positively charged trypsin inhibited the transport and enhanced bacterial deposition under all experimental conditions. The potential mechanisms controlling the changes of bacterial transport behaviors varied for different charged proteins. The steric repulsion resulting from BSA adsorption onto both bacteria and quartz sand was found to play a dominant role in the transport and deposition of bacteria in porous media with BSA copresent in suspension. BSA adsorption onto bacterial surfaces and competition for deposition sites onto sand surfaces (adsorption of quartz sand surfaces) contributed to the increased cell transport with BSA in suspension. In contrast, the attractive patch-charged interaction induced by the adsorption of trypsin onto both bacteria and quartz sand had great contribution to the decreased bacterial transport in porous media with trypsin copresent in suspension. The increase in bacteria size, and the adsorption of trypsin onto cell surfaces (resulting in less negative cell surface charge) and quartz sand surfaces (providing extra deposition sites) were found to be the main contributors to the decreased transport and increased deposition of bacteria in quartz sand with trypsin in suspension.

Facile fabrication of ascorbic acid reduced graphene oxide-modified electrodes toward electroanalytical determination of sulfamethoxazole in aqueous environments

In the present work, a novel reduced graphene oxide (rGO)-based electrochemical sensor has been developed for the detection of sulfamethoxazole (SMZ) using differential pulse voltammetry. Ascorbic acid (AA) under acid conditions has proven to be an efficient yet environmentally benign reductant to recover the outstanding electrochemical activity of graphene. Additionally, the ascorbic-acid-reduced graphene oxide exhibits a high total removal rate of epoxide and hydroxyl functional groups, with a rich presence of carbonyl and carboxyl functional groups, allowing an excellent solution processing ability for realizing further functionalization and applications, unlike reduced graphene oxide produced by conventional harmful reductants (e.g., hydrazine). Moreover, excellence in the determination of SMZ is likely due to selective reduction of oxygen functional groups on the graphene basal planes by ascorbic acid, which enhances π–π interactions with SMZ, which is present in anionic forms in natural aqueous systems. The AArGO-modified electrodes achieved a linear range of 0.5–50 μM with a limit of detection of 0.04 μM and satisfied recoveries in different water samples. Additionally, the cationic surfactant (e.g., cetyltrimethylammonium bromide, CTAB) showed a promoting effect toward SMZ determination. Collectively, the AArGO modified electrodes showed great improvement in the anodic oxidation reactivity of SMZ with high selectivity and stability, allowing promising and feasible applications in on-site environmental monitoring.

PH-dependence of production of oxidants (Cu(III) and/or HO•) by copper-catalyzed decomposition of hydrogen peroxide under conditions typical of natural saline waters

The oxidation and reduction kinetics of copper in natural aquatic systems determines its prevailing redox state, with implication to its toxicity, bioavailability and solubility. When reduced copper(I) co-occurs with ubiquitous hydrogen peroxide (H2O2), their reaction forms an oxidizing intermediate able to affect redox transformation of copper, other transition metals and organic carbon. In this work the kinetics and mechanism of reactions of nanomolar concentrations of copper (Cu) with H2O2 in bicarbonate-buffered 0.7 M NaCl solutions over the pH range from 6.0 to 8.0 have been investigated and a kinetic model developed to assist in understanding the results obtained. While the rate of cuprous ion (Cu(I)) peroxidation did not vary markedly with pH, the rates of both cupric ion (Cu(II)) reduction by H2O2 and the production of oxidative intermediates showed strong pH dependence with both rates observed to increase with increasing pH. The increasing proportion of H2O2 present as the hydroperoxyl anion (HO2−) with increasing pH plays an important role in these trends. However, in order to adequately describe the behavior observed, it is also necessary to invoke a pH-dependent intrinsic conditional rate constant for the reaction between Cu(II) and HO2−. The nature of the oxidants formed when Cu(I) is oxidized by H2O2 has been probed by measuring the hydroxylation of phthalhydrazide and the degradation of formate in the absence and presence of probe compounds with well-established reactivity toward hydroxyl radicals (HO•). The results obtained suggest that a higher oxidation state of copper, Cu(III), rather than HO• is the key oxidant formed over the pH range of 6.0–8.0. Simulations under conditions relevant to natural saline environments reveal that increase in pH results in acceleration in the H2O2-driven cycling of Cu between +I and +II oxidation states but does not significantly influence the rate of production of Cu(III) with important implication for the geochemistry of copper, other redox active elements and oxidizable organic compounds.

An epidemiological model of virus transmission in salmonid fishes of the Columbia River Basin

We have developed a dynamic epidemiological model informed by records of viral presence and genotypes to evaluate potential transmission routes maintaining a viral pathogen in economically and culturally important anadromous fish populations. In the Columbia River Basin, infectious hematopoietic necrosis virus (IHNV) causes severe disease, predominantly in juvenile steelhead trout (Oncorhynchus mykiss) and less frequently in Chinook salmon (O. tshawytscha). Mortality events following IHNV infection can be devastating for individual hatchery programs. Despite reports of high local mortality and extensive surveillance efforts, there are questions about how viral transmission is maintained. Modeling this system offers important insights into disease transmission in natural aquatic systems, as well as about the data requirements for generating accurate estimates about transmission routes and infection probabilities. We simulated six scenarios in which testing rates and the relative importance of different transmission routes varied. The simulations demonstrated that the model accurately identified routes of transmission and inferred infection probabilities accurately when there was testing of all cohort-sites. When testing records were incomplete, the model accurately inferred which transmission routes exposed particular cohort-sites but generated biased infection probabilities given exposure. After validating the model and generating guidelines for result interpretation, we applied the model to data from 14 annual cohorts (2000–2013) at 24 focal sites in a sub-region of the Columbia River Basin, the lower Columbia River (LCR), to quantify the relative importance of potential transmission routes in this focal sub-region. We demonstrate that exposure to IHNV via the return migration of adult fish is an important route for maintaining IHNV in the LCR sub-region, and the probability of infection following this exposure was relatively high at 0.16. Although only 1% of cohort-sites experienced self-exposure by infected juvenile fish, this transmission route had the greatest probability of infection (0.22). Increased testing and/or determining whether transmission can occur from cohort-sites without testing records (e.g., determining there was no testing record because there were no fish at the cohort-site) are expected to improve inference about infection probabilities. Increased use of secure water supplies and continued use of biosecurity protocols may reduce IHNV transmission from adult fish and juvenile fish within the site, respectively, to juvenile salmonids at hatcheries. Models and conclusions from this study are potentially relevant to understanding the relative importance of transmission routes for other important aquatic pathogens in salmonids, including the agents of bacterial kidney disease and coldwater disease, and the basic approach may be useful for other pathogens and hosts in other geographic regions.

Nutrient Removal Rates of Permeable Reactive Concrete

AbstractNitrogen and phosphorus contained in stormwater runoff contaminate both surface and groundwaters, causing problems for natural aquatic systems and human health. Pervious concrete specifical...

Hydrolysis, adsorption, and biodegradation of bensulfuron methyl under methanogenic conditions

Bensulfuron methyl (BSM), one of the most widely used herbicides in paddy soils, is frequently detected in natural and artificial aquatic systems. However, BSM transformation under methanogenic conditions has not been given sufficient attention. In this study, BSM elimination and transformation by anaerobic enrichment cultures were investigated. The results showed that BSM can be mineralized to methane through hydrolysis, adsorption, and biodegradation under a methanogenic environment. The adsorption led to protein static quenching in the extracellular polymeric substances (EPSs) of the enrichment cultures. Specifically, BSM mainly reacted with the amine, amide, amino acid, and amino sugar functional groups in proteins. BSM hydrolysis and biodegradation occurred through the breakage of the sulfonylurea bridge and sulfonyl amide linkage. The cleavage of the sulfonylurea bridge occurred in both hydrolysis and biodegradation, while the cleavage of the sulfonyl amide linkage only occurred in hydrolysis. These results elucidated the complex transformation of BSM under methanogenic conditions, which will advance the studies on sulfonylurea herbicide biotransformation and hazard assessment in the environment.

Kinetics of Decomposition of Thiocyanate in Natural Aquatic Systems.

Rates of thiocyanate degradation were measured in waters and sediments of marine and limnic systems under various redox conditions, oxic, anoxic (nonsulfidic, nonferruginous, nonmanganous), ferruginous, sulfidic, and manganous, for up to 200-day period at micromolar concentrations of thiocyanate. The decomposition rates in natural aquatic systems were found to be controlled by microbial processes under both oxic and anoxic conditions. The Michaelis-Menten model was applied for description of the decomposition kinetics. The decomposition rate in the sediments was found to be higher than in the water samples. Under oxic conditions, thiocyanate degradation was faster than under anaerobic conditions. In the presence of hydrogen sulfide, the decomposition rate increased compared to anoxic nonsulfidic conditions, whereas in the presence of iron(II) or manganese(II), the rate decreased. Depending on environmental conditions, half-lives of thiocyanate in sediments and water columns were in the ranges of hours to few dozens of days, and from days to years, respectively. Application of kinetic parameters presented in this research allows estimation of rates of thiocyanate cycling and its concentrations in the Archean ocean.

Tracking of a Fluorescent Dye in a Freshwater Lake with an Unmanned Surface Vehicle and an Unmanned Aircraft System

Recent catastrophic events in our oceans, including the spill of toxic oil from the explosion of the Deepwater Horizon drilling rig and the rapid dispersion of radioactive particulates from the meltdown of the Fukushima Daiichi nuclear plant, underscore the need for new tools and technologies to rapidly respond to hazardous agents. Our understanding of the movement and aerosolization of hazardous agents from natural aquatic systems can be expanded upon and used in prevention and tracking. New technologies with coordinated unmanned robotic systems could lead to faster identification and mitigation of hazardous agents in lakes, rivers, and oceans. In this study, we released a fluorescent dye (fluorescein) into a freshwater lake from an anchored floating platform. A fluorometer (fluorescence sensor) was mounted underneath an unmanned surface vehicle (USV, unmanned boat) and was used to detect and track the released dye in situ in real-time. An unmanned aircraft system (UAS) was used to visualize the dye and direct the USV to sample different areas of the dye plume. Image processing tools were used to map concentration profiles of the dye plume from aerial images acquired from the UAS, and these were associated with concentration measurements collected from the sensors onboard the USV. The results of this project have the potential to transform monitoring strategies for hazardous agents, enabling timely and accurate exposure assessment and response in affected areas. Fast response is essential in reacting to the introduction of hazardous agents, in order to quickly predict and contain their spread.

Density functional theory characterization of the structures of H3AsO3 and H3AsO4 adsorption complexes on ferrihydrite.

Reactions occurring at ferric oxyhydroxide surfaces play an important role in controlling arsenic bioavailability and mobility in natural aqueous systems. However, the mechanism by which arsenite and arsenate complex with ferrihydrite (Fh) surfaces is not fully understood and although there is clear evidence for inner sphere complexation, the nature of the surface complexes is uncertain. In this work, we have used periodic density functional theory calculations to predict the relative energies, geometries and properties of arsenous acid (H3AsO3) and arsenic acid (H3AsO4), the most prevalent form of As(iii) and As(v), respectively, adsorbed on Fh(110) surface at intermediate and high pH conditions. Bidentate binuclear (BB(Fe-O)) corner-sharing complexes are shown to be energetically favoured over monodentate mononuclear complexes (MM(Fe-O)) for both arsenic species. The inclusion of solvation effects by introducing water molecules explicitly near the adsorbing H3AsO3 and H3AsO4 species was found to increase their stability on the Fh surface. The adsorption process is shown to be characterized by hybridization between the interacting surface Fe-d states and the O and As p-states of the adsorbates. Vibrational frequency assignments of the As-O and O-H stretching modes of the adsorbed arsenic species are also presented.

Chromium isotope fractionation during Cr(VI) reduction in a methane-based hollow-fiber membrane biofilm reactor

Chromium (Cr) isotope fractionation analysis is a promising tool for monitoring Cr(VI) reduction in natural aqueous systems. In addition, large amounts of CH4 in natural aqueous sediments are oxidized to CO2 through methanotrophs, thereby mitigating emissions to the atmosphere. However, the investigations on the Cr(VI) reduction process with methanotrophs, and the associated Cr isotope fractionation patterns are scarce. In this study, we have shown that Cr(VI) reduction can occur in the presence of CH4 as the sole electron donor in a hollow-fiber membrane reactor (HfMBR) after direct bacteria enrichment from sediment samples. Products of the methane oxidation by the methanotrophs are used by microbes to reduce Cr(VI) as shown by the progressive increase in δ53Cr with time in the CH4 feed reactor. The isotope fractionation factor (ε) of −2.62 ± 0.20‰ was obtained from the application of the Rayleigh distillation model. The results of Cr isotope fractionation analysis also explained the decrease of Cr(VI) concentration in the N2 feed reactor, where the δ53Cr values remained steady in the first two weeks but significantly increased in the last two weeks, indicating that physical adsorption and subsequent Cr(VI) reduction occurred. This study extended the application of Cr isotope fractionation, showing the suitability of this method for clarifying different Cr(VI) removal processes.

Solution Chemistry of Arsenic Anions in the Presence of Metal Cations

The interaction of arsenic(V) and arsenic(III) oxyanions with metal cations was investigated by potentiometry under temperature and ionic strength conditions approaching those prevailing in natural waters. The selection includes the major metal cations and some other ions of high environmental relevance. Ionic pairs [M(AsVO4)]−, [M(HAsVO4)] and [M(H2AsIIIO3)]+ formation is suggested for all +2 metal cations, based on the potentiometric results. These ion-pairs between arsenic anions and other metal cations are hardly ever mentioned or taken into account when arsenic speciation in natural waters is considered. These results provide the basis for studying arsenic speciation in natural aquatic systems, on which environmental fate, bioavailability and toxicity of the element depend. Some extrapolations to the conditions of the natural waters are presented as well as some insights into the adsorption process onto hydrous oxides.