Stephan J. Hug

Arsenic Removal with Composite Iron Matrix Filters in Bangladesh: A Field and Laboratory Study

The main arsenic mitigation measures in Bangladesh, well-switching and deep tube wells, have reduced As exposure, but water treatment is important where As-free water is not available. Zero-valent iron (ZVI) based SONO household filters, developed in Bangladesh, remove As by corrosion of locally available inexpensive surplus iron and sand filtration in two buckets. We investigated As removal in SONO filters in the field and laboratory, covering a range of typical groundwater concentrations (in mg/L) of As (0.14-0.96), Fe (0-17), P (0-4.4), Ca (45-162), and Mn (0-2.8). Depending on influent Fe(II) concentrations, 20-80% As was removed in the top sand layer, but As removal to safe levels occurred in the ZVI-layer of the first bucket. Residual As, Fe, and Mn were removed after re-aeration in the sand of the second bucket. New and over 8-year-old filters removed As to <50 μg/L and mostly to <10 μg/L and Mn to <0.2 mg/L. Vertical concentration profiles revealed formation of Fe(II) by corrosion of Fe(0) with O2 and incorporation of As into forming amorphous Fe phases in the composite iron matrix (CIM) of newer filters and predominantly magnetite in older filters. As mass balances indicated that users filtered less than reported volumes of water, pointing to the need for more educational efforts. All tested SONO filters provided safe drinking water without replacement for up to over 8 years of use.

Surface Complexation of Sulfate by Hematite Surfaces: FTIR and STM Observations

Abstract Sulfate adsorbed to hematite surfaces from aqueous solution is examined using Fourier transform infrared (FTIR) spectroscopy and scanning tunneling microscopy (STM). FTIR was carried out using an Attenuated Total Reflectance (ATR) element coated with a fixed layer of hematite particles; this configuration allowed in situ variation of pH and sulfate concentration. The FTIR results are consistent with an inner-sphere monodentate surface complex. On dried samples, sulfate may form bidentate or possibly monodentate bisulfate complexes. STM was applied to samples that were removed from solution and imaged in air, conditions corresponding to those of the dried samples in FTIR. The images show mobile adsorbates whose lifetimes were greater than 5 ms and less than 240 ms, times that bracket the average lifetimes of aqueous FeSO4+ complexes (∼50 ms). In addition, the images show pairs of bumps, in agreement with STM images of bisulfate adsorbed on Pt(111) electrode surfaces ( Funtikov et al 1995 ). Although the STM images do not provide chemical identification, they are consistent with imaging of adsorbed inner-sphere sulfate (STM is incapable of seeing outer-sphere adsorbates). Our results suggest that categorization of adsorbates into inner-sphere and outer-sphere on the basis of macroscopic adsorption information is perhaps oversimplistic. Instead, a spectrum of intermediate behaviors is likely. Adsorbates classed (macroscopically) as outer-sphere may be those for which a relatively small proportion of adsorbates are in inner-sphere complexes at any given time.

Determination of growth rates of (100) and (110) faces of synthetic goethite by scanning force microscopy

Abstract For the first time, the growth rate of the (100) and (110) faces of goethite has been measured in situ with scanning force microscopy. Submicron sized goethite particles were immersed in aerated aqueous Fe(II)solutions, whereby Fe(III) was formed by oxidation of Fe(II) by oxygen. Oxidation of Fe(II) is an important and ubiquitous geochemical process in soils and sediments exposed to changing redox conditions. The SFM measurements confirmed that Fe(II) oxidation is catalyzed by goethite and showed that Fe(III) is incorporated on the existing crystal faces. The growth velocity of the (100) face exceeded the one of the (110) faces by about a factor of 1.5 at the experimental conditions of this study (10 mM FeSO4 and KCl, 5mM acetate, pH 5). The different growth rates result in a predominance of (110) faces, which is also observed when goethite is formed in oversaturated Fe(III) solutions at pH 9, and explains the generally observed morphology of goethite particles. The growth behavior appears to be reaction controlled rather than transport controlled. The preferential growth on the (100) faces could be driven by steric factors, in that the grooves on the (100) faces formed by rows of missing oxygens provide preferred sites for Fe incorporation. The surface properties on the different crystal faces are discussed in the frame of CD-MUSIC model.

Arsenic Accumulation in a Paddy Field in Bangladesh: Seasonal Dynamics and Trends over a Three-Year Monitoring Period

Shallow groundwater, often rich in arsenic (As), is widely used for irrigation of dry season boro rice in Bangladesh. In the long term, this may lead to increasing As contents in rice paddy soils, which threatens rice yields, food quality, and human health. The objective of this study was to quantify gains and losses of soil As in a rice paddy field during irrigation and monsoon flooding over a three-year period. Samples were collected twice a year on a 3D-sampling grid to account for the spatially heterogeneous As distribution within the soil. Gains and losses of soil As in different depth segments were calculated using a mass-balance approach. Annual As input with irrigation water was estimated as 4.4 +/- 0.4 kg ha(-1) a(-1). Within the top 40 cm of soil, the mean As accumulation over three years amounted to 2.4 +/- 0.4 kg ha(-1) a(-1), implying that on average 2.0 kg ha(-1) a(-1) were lost from the soil. Seasonal changes of soil As showed that 1.05 to 2.1 kg ha(-1) a(-1) were lost during monsoon flooding. The remaining As-loss (up to 0.95 kg ha(-1) a(-1)) was attributed to downward flow with percolating irrigation water. Despite these losses, we estimate that total As within the top 40 cm of soil at our field site would further increase by a factor of 1.5 to 2 by the year 2050 under current cultivation practices.

Arsenic Dynamics in Porewater of an Intermittently Irrigated Paddy Field in Bangladesh

In Bangladesh, irrigation of dry season rice (boro) with arsenic-contaminated groundwater is leading to increased As levels in soils and rice, and to concerns about As-induced yield reduction. Arsenic concentrations and speciation in soil porewater are strongly influenced by redox conditions, and thus by water management during rice growth. We studied the dynamics of As, Fe, P, Si, and other elements in porewater of a paddy field near Sreenagar (Munshiganj), irrigated according to local practice, in which flooding was intermittent. During early rice growth, As porewater concentrations reached up to 500 μg L(-1) and were dominated by As(III), but As release was constrained to the lower portion of the soil above the plow pan. In the later part of the season, soil conditions were oxic throughout the depth range relevant to rice roots and porewater concentrations only intermittently increased to ∼150 μg L(-1) As(V) following irrigation events. Our findings suggest that intermittent irrigation, currently advocated in Bangladesh for water-saving purposes, may be a promising means of reducing As input to paddy soils and rice plant exposure to As.

Arsenic in Soil and Irrigation Water Affects Arsenic Uptake by Rice: Complementary Insights from Field and Pot Studies

Groundwater rich in arsenic (As) is extensively used for dry season boro rice cultivation in Bangladesh, leading to long-term As accumulation in soils. This may result in increasing levels of As in rice straw and grain, and eventually, in decreasing rice yields due to As phytotoxicity. In this study, we investigated the As contents of rice straw and grain over three consecutive harvest seasons (2005-2007) in a paddy field in Munshiganj, Bangladesh, which exhibits a documented gradient in soil As caused by annual irrigation with As-rich groundwater since the early 1990s. The field data revealed that straw and grain As concentrations were elevated in the field and highest near the irrigation water inlet, where As concentrations in both soil and irrigation water were highest. Additionally, a pot experiment with soils and rice seeds from the field site was carried out in which soil and irrigation water As were varied in a full factorial design. The results suggested that both soil As accumulated in previous years and As freshly introduced with irrigation water influence As uptake during rice growth. At similar soil As contents, plants grown in pots exhibited similar grain and straw As contents as plants grown in the field. This suggested that the results from pot experiments performed at higher soil As levels can be used to assess the effect of continuing soil As accumulation on As content and yield of rice. On the basis of a recently published scenario of long-term As accumulation at the study site, we estimate that, under unchanged irrigation practice, average grain As concentrations will increase from currently ∼0.15 mg As kg(-1) to 0.25-0.58 mg As kg(-1) by the year 2050. This translates to a 1.5-3.8 times higher As intake by the local population via rice, possibly exceeding the provisional tolerable As intake value defined by FAO/WHO.

Self-sensitization of photo-oxygen evolution in Ag+ zeolites: computer-controlled experiments

Abstract For several years we have known that aqueous dispersions of Ag+ zeolites are able to produce oxygen under illumination. Recently we have been successful in measuring the self-sensitization of photo-oxygen evolution in these systems. When we tried to study this phenomenon in more detail by applying conventional photochemical methods, we ran into severe experimental problems because series of complex experiments had to be carried out for several days without interruption. To overcome these problems we then decided to build a computer-controlled experiment. In the present paper we describe our initial experimental results. Because our experimental design was found to be very flexible and easily adaptable to different problems, we explain it in detail.

Arsenic accumulation in irrigated agricultural soils in Northern Greece.

The accumulation of arsenic in soils and food crops due to the use of arsenic contaminated groundwater for irrigation has created worldwide concern. In the Chalkidiki prefecture in Northern Greece, groundwater As reach levels above 1000μg/L within the Nea Triglia geothermal area. While this groundwater is no longer used for drinking, it represents the sole source for irrigation. This paper provides a first assessment of the spatial extent of As accumulation and of As mobility during rainfall and irrigation periods. Arsenic content in sampled soils ranged from 20 to 513mg/kg inside to 5-66mg/kg outside the geothermal area. Around irrigation sprinklers, high As concentrations extended horizontally to distances of at least 1.5m, and to 50cm in depth. During simulated rain events in soil columns (pH=5, 0μg As/L), accumulated As was quite mobile, resulting in porewater As concentrations of 500-1500μg/L and exposing plant roots to high As(V) concentrations. In experiments with irrigation water (pH=7.5, 1500μg As/L), As was strongly retained (50.5-99.5%) by the majority of the soils. Uncontaminated soils (<30mg As/kg) kept soil porewater As concentrations to below 50μg/L. An estimated retardation factor R(f)=434 for weakly contaminated soil (<100mg/kg) indicates good ability to reduce As mobility. Highly contaminated soils (>500mg/kg) could not retain any of the added As. Invoked mechanisms affecting As mobility in those soils were adsorption on solid phases such as Fe/Mn-phases and As co-precipitation with Ca. Low As accumulation was found in collected olives (0.3-25μg/kg in flesh and 0.3-5.6μg/kg in pits). However, soil arsenic concentrations are frequently elevated to far above recommended levels and arsenic uptake in faster growing plants has to be assessed.

Cationically Charged MnIIAlIII LDH Nanosheets by Chemical Exfoliation and Their Use As Building Blocks in Graphene Oxide-Based Materials

We report on the synthesis and exfoliation of Mn(II)Al(III) sulfonate and sulfate layered double hydroxides (LDHs) and their combination with graphene oxide by charge-directed self-assembly. The synthesis of the LDH compounds has been accomplished either directly by coprecipitation of the respective hydroxides with sulfonate anions or by ion-exchange of the chloride-containing LDH with sodium dodecylsulfate. Exfoliation of the bulk material in formamide yields colloidal suspensions of positively charged nanosheets with lateral dimensions of tens to hundreds of nanometers and thicknesses down to 1.3 nm, ascertained by TEM and AFM. Flocculation of the LDH nanosheets with an aqueous graphene oxide suspension yields a hybrid material that can be converted to a reduced graphene oxide/LDH composite by hydrazine reduction. The hybrid materials were tested for pseudocapacitive electrochemical storage capacity and electrocatalytic oxygen evolution reactions and showed significant increases compared to the pristine materials.

Photodegradation of EDTA in the presence of lepidocrocite

Based on laboratory experiments combined with kinetic modeling, we propose a conceptual model for the photodegradation of initially uncomplexed EDTA in the presence of γ-FeOOH (lepidocrocite), as follows : Free EDTA becomes adsorbed at the surface of γ-FeOOH and is initially photooxidized as a surface species. Thereby, γ-FeOOH is reductively dissolved. Our results suggest that photooxidation of adsorbed EDTA, coupled to reductive dissolution of γ-FeOOH, occurs through photolysis of the Fe III EDTA surface complex. The photochemically formed Fe(ll) then catalyzes the thermal dissolution of the solid phase in the presence of EDTA. This process results in production of dissolved Fe III EDTA, which is subsequently photolyzed. Hence, in these heterogeneous systems, initially uncomplexed EDTA is photooxidized via two pathways : (i) photooxidation at the surface of γ-FeOOH and (ii) photolysis of dissolved Fe III EDTA that is formed in the Fe(ll)-catalyzed dissolution of γ-FeOOH. Which pathway predominates depends on the relative rates of Fe(II) oxidation and of Fe(II)-catalyzed formation of dissolved Fe III EDTA. At pH 3, photooxidation of EDTA occurred predominantly through photolysis of dissolved Fe III EDTA, whereas at pH 7, photooxidation of adsorbed EDTA was more important in our aerated heterogeneous systems, because of the faster Fe(II) oxidation at pH 7, compared to pH 3. Our results indicate that not only dissolved Fe III EDTA but also Fe III EDTA surface complexes are efficiently photolyzed.