Achieving arsenic concentrations of <1 µg/L by Fe(0) electrolysis: The exceptional performance of magnetite.

Consumption of drinking water containing arsenic at concentrations even below the World Health Organization provisional limit of 10 µg/L can still lead to unacceptable health risks. Consequently, the drinking water sector in the Netherlands has recently agreed to target 1 µg/L of arsenic in treated water. Unfortunately, in many poor, arsenic-affected countries, the costs and complexity of current methods that can achieve <1 µg/L are prohibitive, which highlights the need for innovative methods that can remove arsenic to <1 µg/L without costly support infrastructure and complicated supply chains. In this work, we used Fe(0) electrolysis, a low cost and scalable technology that is also known as Fe(0) electrocoagulation (EC), to achieve <1 µg/L residual dissolved arsenic. We compared the arsenic removal performance of green rust (GR), ferric (oxyhydr)oxides (Fe(III) oxides) and magnetite (Mag) generated by EC at different pH (7.5 and 9) in the presence of As(III) or As(V) (initial concentrations of 200-11,000 µg/L). Although GR and Fe(III) oxides removed up to 99% of initial arsenic, neither Fe phase could reliably meet the 1 µg/L target at both pH values. In contrast, EC-generated Mag consistently achieved <1 µg/L, regardless of the initial As(V) concentration and pH. Only solutions with initial As(III) concentrations ≥2200 µg/L resulted in residual arsenic >1 µg/L. As K-edge X-ray absorption spectroscopy showed that Mag also sorbed arsenic in a unique mode, consistent with partial arsenic incorporation near the particle surface. This sorption mode contrasts with the binuclear, corner sharing surface complex for GR and Fe(III) oxides, which could explain the difference in arsenic removal efficiency among the three Fe phases. Our results suggest that EC-generated Mag is an attractive method for achieving <1 µg/L particularly in decentralized water treatment.

Emerging investigator series: interdependency of green rust transformation and the partitioning and binding mode of arsenic.

We investigated the impact of aging-induced structural modifications of carbonate green rust (GR), a mixed valent Fe(ii,iii) (hydr)oxide with a high oxyanion sorption affinity, on the partitioning and binding mode of arsenic (As). Suspensions of carbonate GR were produced in the presence of As(v) or As(iii) (i.e. co-precipitated with As(iii) or As(v)) and aged in anoxic and oxic conditions for up to a year. We tracked aqueous As over time and characterized the solid phase by X-ray absorption spectroscopy (XAS). In experiments with initial As(v) (4500 µg L-1, As/Fe = 2 mol%), the fresh GR suspension sorbed >99% of the initial As, resulting in approximately 14 ± 8 µg L-1 residual dissolved As. Anoxic aging of the As(v)-laden GR for a month increased aqueous As to >60 µg L-1, which was coupled to an increase in GR structural order revealed by Fe K-edge XAS. Further anoxic aging up to a year transformed As(v)-laden GR into magnetite and decreased significantly the aqueous As to <2 µg L-1. The As binding mode was also modified during GR transformation to magnetite from sorption to GR particle edges to As substitution for tetrahedral Fe in the magnetite structure. These GR structural modifications altered the ratio of As partitioning to the solid (µg As/mg Fe) and liquid (µg As per L) phase from 2.0 to 0.4 to 14 L mg-1 for the fresh, month, and year aged suspensions, respectively. Similar trends in GR transformation and As partitioning during anoxic aging were observed for As(iii)-laden suspensions, but occurred on more rapid timescales: As(iii)-laden GR transformed to magnetite after a day of anoxic aging. In oxic aging experiments, rapid GR oxidation by dissolved oxygen to Fe(iii) precipitates required only an hour for both As(v) and As(iii) experiments, with lepidocrocite favored in As(v) experiments and hydrous ferric oxide favored in As(iii) experiments. Aqueous As during GR oxidation decreased to <10 µg L-1 for both As(v) and As(iii) series. Knowledge of this interdependence between GR aging products and oxyanion fate improves biogeochemical models of contaminant and nutrient dynamics during Fe cycling and can be used to design more effective arsenic remediation strategies that rely on arsenic sorption to GR.

Analysis of cationic surfactants by microbore high-performance liquid chromatography-electrospray mass spectrometry.

In the work reported here, a state-of-the-art analytical method for the quantitative analysis of cationic surfactants in environmental matrixes is described. High-performance liquid chromatography on-line coupled via an electrospray interface to a mass spectrometer (HPLC-ESP-MS) is used for the determination of ditallowdimethylammonium chloride (DTDMAC) and two of its most important substitution products, diethylester dimethylammonium chloride (DEEDMAC) and Diesterquaternary (DEQ). Using the analytical method developed in our laboratory, it is possible to determine single homologues of these surfactants as well as the first hydrolysis products of DEQ and DEEDMAC. In combination with our extraction procedure, which is based on ion-pair extraction, cationic surfactants were determined in environmental samples (sewage influent, sewage effluent, river water); concentrations ranged from 0.4 to 140 µg/L. The linear dynamic range of the HPLC-ESP-MS method, which is an injected amount between 0.4 and 30 ng, is well suitable for the analysis of these samples, as well as the performance of the quantification through external standards.

Is part of the molecular basis of the perineurial barrier function the lack of endogenous carbohydrate-binding proteins?

The sugar part of cellular glycoconjugates and specific endogenous sugar receptors, i.e., lectins, can establish a system of biological recognition based on protein-carbohydrate interactions. An assortment of labelled (neo)glycoproteins, carrying different types of sugar moieties, is synthesized to localize respective sugar receptors. With these tools, the histochemical patterns of endogenous carbohydrate-binding receptors of the epi-, peri-, and endoneurium were analyzed in human sural and accessory nerves and in swine sciatic nerve. This approach is complementary to the application of plant lectins, focusing on endogenous carbohydrate-binding proteins (lectins). In contrast to the epi- and endoneurium, which bound certain types of carbohydrates, such endogenous sugar receptors were histochemically not detectable in the perineurial cells. Moreover, no histochemical reaction was present in the "connective tissue septa" localized in the endoneurium in which the endoneurial vessels were embedded. This common property supplies evidence that these septa are composed of perineurial cells. They may represent a barrier in addition to the capillary endothelium. Our observations suggest histogenetical differences between the cell populations of epi- and endoneurium vs. perineurium. This significant difference in the ability to bind carbohydrate residues, conjugated to a carrier protein, is contradictory to the assumption that perineurial cells and fibroblasts are functional variants of the same cell type. The histochemical patterns of endogenous carbohydrate-binding receptors found in human and swine nerves were similar but not identical, with exception of the perineurium, reflecting phylogenetic differences in the expression of sugar-binding proteins. The absence of specific sugar receptors in perineurial cells, however, seems to be a more general phenomenon.(ABSTRACT TRUNCATED AT 250 WORDS)

Facilitatory interaction between cutaneous afferents from low threshold mechanoreceptors and nociceptors in segmental reflex pathways to alpha-motoneurons.

EPSPs evoked by air puff stimulation of the hairy skin of the hindlimb or by low strength stimulation of the lateral sural nerve were facilitated by radiant heat of 50-53 degrees C but not by radiant heat of 42-43 degrees C. The results indicate that afferents from low threshold mechanoreceptors and from nociceptors of the skin converge onto common interneurons in reflex pathways to alpha-motoneurons. This would allow a functionally useful cooperation of these two types of afferents in spinal motor control.