Mechanisms of selenium removal by partially oxidized magnetite nanoparticles for wastewater remediation

Magnetite nanoparticles are a promising cost-effective material for the remediation of polluted wastewaters. Due to their magnetic properties and their high adsorption and reduction potential, they are particularly suitable for the decontamination of oxyanion-forming contaminants, including the highly mobile selenium oxyanions sele-nite and selenate. However, little is known how the remediation efficiency of magnetite nanoparticles in field applications is affected by partial oxidation and the formation of magnetite/maghemite phases. Here we char-acterize the retention mechanisms and capacity of partially oxidized nanoparticulate magnetite for selenite and selenate in an oxic system at different pH conditions and ionic strengths. Data from adsorption experiments showed that retention of selenate is extremely limited except for acidic conditions and strongly influenced by competing chloride anions, indicating outer-sphere adsorption. By contrast, although selenite adsorption ca-pacity of oxidized magnetite is also adversely affected by increasing pH, considerable selenite quantities are retained even at alkaline conditions. Using spectroscopic analyses (XPS, XAFS), both mononuclear edge-sharing (2E) and binuclear corner-sharing (2C) inner-sphere selenite surface complexes were detected, while reduction to Se(0) or Se(–II) species could be excluded. Under favourable adsorption conditions, up to ~pH 8, the affinity of selenite to form 2C surface complexes is higher, whereas at alkaline pH values and less favourable adsorption conditions 2E complexes become more dominant. Our results demonstrate that magnetite can be used as a suitable reactant for the immobilization of selenite in remediation applications, even under (sub)oxic conditions and without the involvement of reduction processes

Insights into Phytoplankton Dynamics and Water Quality Monitoring with the BIOFISH at the Elbe River, Germany

Understanding the key factors influencing the water quality of large river systems forms an important basis for the assessment and protection of cross-regional ecosystems and the implementation of adapted water management concepts. However, identifying these factors requires in-depth comprehension of the unique environmental systems, which can only be achieved by detailed water quality monitoring. Within the scope of the joint science and sports event “Elbschwimmstaffel” (swimming relay on the river Elbe) in June/July 2017 organized by the German Ministry of Education and Research, water quality data were acquired along a 550 km long stretch of the Elbe River in Germany. During the survey, eight physiochemical water quality parameters were recorded in high spatial and temporal resolution with the BIOFISH multisensor system. Multivariate statistical methods were applied to identify and delineate processes influencing the water quality. The BIOFISH dataset revealed that phytoplankton activity has a major impact on the water quality of the Elbe River in the summer months. The results suggest that phytoplankton biomass constitutes a substantial proportion of the suspended particles and that photosynthetic activity of phytoplankton is closely related to significant temporal changes in pH and oxygen saturation. An evaluation of the BIOFISH data based on the combination of statistical analysis with weather and discharge data shows that the hydrological and meteorological history of the sampled water body was the main driver of phytoplankton dynamics. This study demonstrates the capacity of longitudinal river surveys with the BIOFISH or similar systems for water quality assessment, the identification of pollution sources and their utilization for online in situ monitoring of rivers

Geochemistry and evolution of groundwater resources in the context of salinization and freshening in the southernmost Mekong Delta, Vietnam

Study region Ca Mau Province (CMP), Mekong Delta (MD), Vietnam. Study focus Groundwater from deep aquifers is the most reliable source of freshwater in the MD but extensive overexploitation in the last decades led to the drop of hydraulic heads and negative environmental impacts. Therefore, a comprehensive groundwater investigation was conducted to evaluate its composition in the context of Quaternary marine transgression and regression cycles, geochemical processes as well as groundwater extraction. New hydrological insights for the region The abundance of groundwater of Na-HCO$_{3}$ type and distinct ion ratios, such as Na$^{+}$/Cl$^{-}$, indicate extensive freshwater intrusion in an initially saline hydrogeological system, with decreasing intensity from upper Pleistocene to deeper Miocene aquifers, most likely during the last marine regression phase 60–12 ka BP. Deviations from the conservative mixing line between the two endmembers seawater and freshwater are attributed to ion-exchange processes on mineral surfaces, making ion ratios in combination with a customized water type analysis a useful tool to distinguish between salinization and freshening processes. Elevated salinity in some areas is attributed to HCO$_{3}$$^{-}$ generation by organic matter decomposition in marine sediments rather than to seawater intrusion. Nevertheless, a few randomly distributed locations show strong evidence of recent salinization in an early stage, which may be caused by the downwards migration of saline Holocene groundwater through natural and anthropogenic pathways into deep aquifers

An Improved Groundwater Model Framework for Aquifer Structures of the Quaternary-Formed Sediment Body in the Southernmost Parts of the Mekong Delta, Vietnam

The Ca Mau peninsula (CMP) is a key economic region in southern Vietnam. In recent decades, the high demand for water has increased the exploitation of groundwater, thus lowering the groundwater level and leading to risks of degradation, depletion, and land subsidence, as well as salinity intrusion in the groundwater of the whole Mekong Delta region. By using a finite element groundwater model with boundary expansion to the sea, we updated the latest data on hydrogeological profiles, groundwater levels, and exploitation. The basic model setup covers seven aquifers and seven aquitards. It is determined that the inflow along the coastline to the mainland is 39% of the total inflow. The exploitation of the study area in 2019 was 567,364 m3/day. The most exploited aquifers are the upper-middle Pleistocene (qp2–3) and the middle Pliocene (n22), accounting for 63.7% and 24.6%, respectively; the least exploited aquifers are the upper Pleistocene and the upper Miocene, accounting for 0.35% and 0.02%, respectively. In the deeper aquifers, qp2–3 and n22, the change in storage is negative due to the high exploitation rate, leading to a decline in the reserves of these aquifers. These groundwater model results are the calculations of groundwater reserves from the coast to the mainland in the entire system of aquifers in the CMP. This makes groundwater decision managers, stakeholders, and others more efficient in sustainable water resources planning in the CMP and Mekong Delta (MKD)

Groundwater Use Habits and Environmental Awareness in Ca Mau Province, Vietnam: Implications for Sustainable Water Resource Management

The Vietnamese Mekong Delta including Ca Mau province (CMP) is seriously affected by land subsidence. Groundwater over-extraction is considered to be a major driver for this process. To address the reduction of groundwater (GW) extraction as a potential counter measure for further subsidence, this study focuses on understanding the importance of GW in people\u27s life and water using habits as well as their awareness with current environmental problems in Ca Mau. Therefore, GW sampling campaigns and surveys were conducted in all 9 districts of Ca Mau province in 2019 and 2020. The analyzed water samples showed a connection with information from questionnaires and created a general picture of water using habits. GW plays an important role in people\u27s lives, it is used for washing, cooking, drinking and other activities. People use GW for different purposes depending on their perception of water quality. For important and direct health related purposes, such as cooking or drinking, people prepare to treat water more carefully or choose another alternative water resource. The analytical approach to evaluation results based on viewpoints from general to detail helped to dig deeper into people\u27s stories to explain research results with their behavior in each situation. When people are dependent on GW and have no option to use alternative water resources, the importance level of GW in their life increases and their awareness of GW over-extraction becomes less. If people have another water source to use such as tap water (TW), habits of using GW change. This opens up the idea that a potential alternative water will reduce the dependence of people on GW and protect GW from over-exploitation. Besides, people in Ca Mau do not have much awareness of land subsidence or the reason leading to environmental problems. Therefore, raising the awareness of people by well-design education campaigns should be strongly considered

Immobilisierung gelöster Selen-Oxyanionen während der Bildung der Eisenoxide Hämatit und Magnetit

Aufgrund der geringen Konzentrationsspanne zwischen seiner Bedeutung als essentieller Nährstoff und der Bedeutung als toxisches Spurenelement spielt das Verhalten von Selen (Se) in vielen Umweltbereichen eine wichtige Rolle. Darüber hinaus tritt Se in radioaktivem Abfall als Radionuklid 79Se auf, welches wegen seiner langen Halbwertzeit und potentiell hohen Mobilität zur Gruppe der langzeitsicherheitsrelevanten Radionuklide gehört. Innerhalb eines aquatischen Systems wird die Se-Mobilität vorwiegend von der Gegenwart der leicht löslichen Se-Oxyanionen Selenit [Se(IV)] und Selenat [Se(VI)] sowie deren Interaktion mit Mineralphasen bestimmt. Da Eisenoxide in der Umwelt weitverbreitet und im Gegensatz zu anderen Materialien zudem in der Lage sind, über einen weiten pH-Bereich mit anionischen Spezies zu interagieren, wurde die Retention von gelösten Se-Spezies infolge einer Adsorption an Eisenoxid-Mineralen bereits in vergangenen Studien detailliert untersucht. Kaum erforscht wurde bislang hingegen die Immobilisierung von gelösten Se-Oxyanionen durch Eisenoxide während des Mineralbildungsprozesses, obwohl eine Interaktion in dieser Phase durchaus realistisch ist. Dies gilt nicht nur für die Bildung von Eisenoxiden aufgrund von Alterationsprozessen, sondern auch für deren Entstehung bedingt durch eine Korrosion von elementarem Eisen, die insbesondere im Umfeld eines Endlagers von Bedeutung ist. In dieser Arbeit wurde deshalb mit Hilfe von Kopräzipitationsstudien die Immobilisierung gelöster Se-Oxyanionen während der Bildung und Kristallisation der Eisenoxide Hämatit und Magnetit untersucht. Um die dabei wirksamen Retentionsmechanismen sowie deren Retentionspotential und Stabilität im Detail zu charakterisieren, wurden die erzielten Resultate mit äquivalenten Adsorptionsstudien verglichen, die unter identischen hydrochemischen Bedingungen aber nach einer beendeten Mineralbildung stattfanden. Im Falle der Se-Hämatit-Kopräzipitationsstudien erfolgte die Laborsynthese von Hämatit durch die Präzipitation und die anschließende Alterung von Ferrihydrit in einem oxischen Se(IV)- bzw. Se(VI)-System (pH 7.5). Die hydrochemischen Daten dieser Batchversuche ließen eine vollständige Aufnahme der gesamten verfügbaren Se(IV)-Menge bis hin zu einer anfänglichen Se-Konzentration von c(Se)0 = 10-3 mol/L (m/V = 9.0 g/L) erkennen, während die Retention von Se(VI) deutlich geringer ausfiel (max. 15 % von c(Se)0). Bei sehr hohen initialen Se(IV)-Konzentrationen zeigten die Ergebnisse, dass die Interaktion von gelösten Se-Spezies mit Ferrihydrit die Art des Transformationsprodukts beeinflusst und es nicht mehr zur Entstehung einer reinen Hämatitphase kommt. Vergleichende Se-Adsorptionsstudien ermöglichten ferner die Differenzierung zwischen Adsorption und Kopräzipitation und konnten im Falle der Se-Kopräzipitation ein signifikant höheres Retentionspotential nachweisen. Zudem wies das Desorptionsverhalten von Se-haltigen Hämatitproben aus Adsorptions- bzw. Kopräzipitationsexperimenten darauf hin, dass eine Se-Kopräzipitation die Entstehung einer widerstandsfähigen nicht-desorbierbaren Se-Fraktion zur Folge hat. Gemäß der zeitlichen Entwicklung der Se(IV)- bzw. Se(VI)-Retention sowie der detaillierten spektroskopischen Analyse (XPS, XAS) der entsprechenden Syntheseprodukte ist diese nicht-reversible Se-Fraktion das Ergebnis eines Se-Einbauprozesses. Hierbei handelt es sich jedoch nicht um einen strukturellen Se-Einbau, sondern um einen okklusionsähnlichen Einbaumechanismus, der darauf zurückzuführen ist, dass der Großteil des ehemals an Ferrihydrit adsorbierten Se(IV) oder Se(VI) im Zuge der Ferrihydrit-Hämatit-Rekristallisation in die neu entstandene Hämatitphase eingeschlossen wurde. Aus diesem Grund gleicht der Bindungscharakter des eingebauten Se auch dem von adsorbiertem Se an der Mineraloberfläche von Hämatit, welches durch außersphärische Se(VI)-Komplexe oder innersphärische bidentat-mononukleare kantengebundene (2E) und bidentat-binukleare eckengebundene (2C) Se(IV)-Adsorptionskomplexe charakterisiert ist (BÖRSIG et al., 2017). Kopräzipitationsstudien von Se-Oxyanionen und Magnetit basierten dagegen auf der progressiven Oxidation eines alkalischen, anoxischen Fe2+-Systems (pH 9.2). Die Auswertung der hydrochemischen Daten zeigte, dass bei diesem Prozess anwesendes Se(VI) oder Se(VI) vollständig aus der Lösung entfernt wird, solange die initiale Se-Konzentration Werte von c(Se)0 = 10-3 mol/L (m/V = 3.4 g/L) nicht überschreitet. Zurückzuführen ist diese Se-Retention auf die Reduktion von Se(IV) bzw. Se(VI) und eine daraus resultierende Präzipitation schwerlöslicher Se-Verbindungen. In den Syntheseprodukten konnten diese Se-Ausfällungen in Form von elementarem Se(0) eindeutig nachgewiesen werden. Wie die Analyse der zeitlichen Entwicklung der Se-Retention und die Charakterisierung der beteiligen Festphasen erkennen ließ, erfolgt die Se-Reduktion unter den noch anoxischen Bedingungen im Frühstadium der Kopräzipitation aufgrund einer Wechselwirkung mit Eisen(II)-hydroxid und Green Rust, die die Vorläuferphasen des späteren Magnetits darstellen. Mit Hilfe spektroskopischer sowie elektronenmikroskopischer Analysen konnte nachgewiesen werden, dass diese frühe Se-Interaktion zur Bildung einer nanopartikulären Eisenselenid-Phase [FeSe] führt, welche im Laufe der fortschreitenden Oxidation des aquatischen Systems und der damit einhergehenden Transformation der nun instabilen Eisenoxide ebenfalls oxidiert und in trigonales Se(0) umgewandelt wird. Für das Retentionsverhalten von Se ist es hierbei unerheblich, ob am Ende des Oxidationsprozesses neben Magnetit auch andere Eisenoxid-Minerale entstehen. Desorptionsstudien offenbarten zudem, dass die gebildete Se(0)-Phase auch unter oxischen sowie stark alkalischen Bedingungen stabil ist und keine Mineralauflösung erfolgt. Demgegenüber ließen Se-Magnetit-Adsorptionsstudien keine Se-Reduktion und nur ein limitiertes Adsorptionspotential erkennen. Dies gilt speziell für Se(VI), bei pH 9.2 jedoch auch für Se(IV), gleichwohl Se(IV) innersphärische 2E- und/oder 2C-Adsorptionskomplexe ausbildet. Die Ergebnisse dieser Arbeit zeigen, dass Prozesse wie der Einbau von Se-Oxyanionen in Hämatit oder die durch die Interaktion mit Fe(II)-Mineralen verursachte reduktive Se-Fällung relevante Retentionsmechanismen zusätzlich zur reinen Se-Adsorption darstellen, welche die Mobilität von Se-Oxyanionen in natürlichen Systemen oder Se-belasteten Gebieten beeinflussen können

Insights into Phytoplankton Dynamics and Water Quality Monitoring with the BIOFISH at the Elbe River, Germany

Understanding the key factors influencing the water quality of large river systems forms an important basis for the assessment and protection of cross-regional ecosystems and the implementation of adapted water management concepts. However, identifying these factors requires in-depth comprehension of the unique environmental systems, which can only be achieved by detailed water quality monitoring. Within the scope of the joint science and sports event “Elbschwimmstaffel” (swimming relay on the river Elbe) in June/July 2017 organized by the German Ministry of Education and Research, water quality data were acquired along a 550 km long stretch of the Elbe River in Germany. During the survey, eight physiochemical water quality parameters were recorded in high spatial and temporal resolution with the BIOFISH multisensor system. Multivariate statistical methods were applied to identify and delineate processes influencing the water quality. The BIOFISH dataset revealed that phytoplankton activity has a major impact on the water quality of the Elbe River in the summer months. The results suggest that phytoplankton biomass constitutes a substantial proportion of the suspended particles and that photosynthetic activity of phytoplankton is closely related to significant temporal changes in pH and oxygen saturation. An evaluation of the BIOFISH data based on the combination of statistical analysis with weather and discharge data shows that the hydrological and meteorological history of the sampled water body was the main driver of phytoplankton dynamics. This study demonstrates the capacity of longitudinal river surveys with the BIOFISH or similar systems for water quality assessment, the identification of pollution sources and their utilization for online in situ monitoring of rivers

An Improved Groundwater Model Framework for Aquifer Structures of the Quaternary-Formed Sediment Body in the Southernmost Parts of the Mekong Delta, Vietnam

The Ca Mau peninsula (CMP) is a key economic region in southern Vietnam. In recent decades, the high demand for water has increased the exploitation of groundwater, thus lowering the groundwater level and leading to risks of degradation, depletion, and land subsidence, as well as salinity intrusion in the groundwater of the whole Mekong Delta region. By using a finite element groundwater model with boundary expansion to the sea, we updated the latest data on hydrogeological profiles, groundwater levels, and exploitation. The basic model setup covers seven aquifers and seven aquitards. It is determined that the inflow along the coastline to the mainland is 39% of the total inflow. The exploitation of the study area in 2019 was 567,364 m3/day. The most exploited aquifers are the upper-middle Pleistocene (qp2–3) and the middle Pliocene (n22), accounting for 63.7% and 24.6%, respectively; the least exploited aquifers are the upper Pleistocene and the upper Miocene, accounting for 0.35% and 0.02%, respectively. In the deeper aquifers, qp2–3 and n22, the change in storage is negative due to the high exploitation rate, leading to a decline in the reserves of these aquifers. These groundwater model results are the calculations of groundwater reserves from the coast to the mainland in the entire system of aquifers in the CMP. This makes groundwater decision managers, stakeholders, and others more efficient in sustainable water resources planning in the CMP and Mekong Delta (MKD)