Ferrihydrite coating reduces microplastic induced soil water repellency.

Addition of microplastics (MP) to soil has the potential to increase soil water repellency. However, coating of MP with soil abundant substances e.g., iron compounds, can reduce this effect. Here, we tested if pre-coating or in situ coating of MP with ferrihydrite (Fh) reduces soil water repellency. We applied hotspots of pristine and coated MP (20-75 µm, PS and PET) to sand and imaged capillary rise via neutron radiography. Capillary rise experiments in wetting-drying cycles were conducted using water and Fh suspension. Pristine MP hotspots were not wettable. Capillary rise of water into coated MP hotspots differed in wettability depending on polymer type. While coated PS was still non-wettable, water imbibed into the coated PET hotspot. Capillary rise of Fh suspensions in wetting and drying cycles also showed varying results depending on polymer type. MP hotspots were still non-wettable and local water content increased only marginally. Our results indicate that Fh coating of MP changes MP surface wettability depending on polymer type and therefore counteracts the hydrophobic properties of pristine MP. However, MP coating is likely to be slowed down by the initial hydrophobicity of pristine MP. Dynamics of MP coating and increasing wettability are key factors for biotic and abiotic degradation processes.

Transformations of Ferrihydrite-Extracellular Polymeric Substance Coprecipitates Driven by Dissolved Sulfide: Interrelated Effects of Carbon and Sulfur Loadings.

The association of poorly crystalline iron (hydr)oxides with organic matter (OM), such as extracellular polymeric substances (EPS), exerts a profound effect on Fe and C cycles in soils and sediments, and their behaviors under sulfate-reducing conditions involve complicated mineralogical transformations. However, how different loadings and types of EPS and water chemistry conditions affect the sulfidation still lacks quantitative and systematic investigation. We here synthesized a set of ferrihydrite-organic matter (Fh-OM) coprecipitates with various model compounds for plant and microbial exopolysaccharides (polygalacturonic acids, alginic acid, and xanthan gum) and bacteriogenic EPS (extracted from Bacillus subtilis). Combining wet chemical analysis, X-ray diffraction, and X-ray absorption spectroscopic techniques, we systematically studied the impacts of C and S loadings by tracing the temporal evolution of Fe mineralogy and speciation in aqueous and solid phases. Our results showed that the effect of added OM on sulfidation of Fh-OM coprecipitates is interrelated with the amount of loaded sulfide. Under low sulfide loadings (S(-II)/Fe < 0.5), transformation to goethite and lepidocrocite was the main pathway of ferrihydrite sulfidation, which occurs more strongly at pH 6 compared to that at pH 7.5, and it was promoted and inhibited at low and high C/Fe ratios, respectively. While under high sulfide loadings (S(-II)/Fe > 0.5), the formation of secondary Fe-S minerals such as mackinawite and pyrite dominated ferrihydrite sulfidation, and it was inhibited with increasing C/Fe ratios. Furthermore, all three synthetic EPS proxies unanimously inhibited mineral transformation, while the microbiogenic EPS has a more potent inhibitory effect than synthetic EPS proxies compared at equivalent C/Fe loadings. Collectively, our results suggest that the quantity and chemical characteristics of the associated OM have a strong and nonlinear influence on the extent and pathways of mineralogical transformations of Fh-OM sulfidation.

Development of a novel sizing approach for passive mine water treatment systems based on ferric iron sedimentation kinetics.

The removal of dissolved and particulate iron (Fe) from contaminated mine drainage is an omnipresent challenge in, and legacy of, the mining industry worldwide. The sizing of settling ponds and surface-flow wetlands for passive Fe removal from circumneutral, ferruginous mine water is based either on a linear (concentration-independent) area-adjusted removal rate or flat assignment of an experience-based retention time, neither of which reflects the underlying Fe removal kinetics. In this study, we evaluated the Fe removal performance of a pilot-scale passive system operating in three identical, parallel lines for treatment of mining-influenced, ferruginous seepage water to determine and parameterise a robust, application-orientated model approach for sizing of settling ponds and surface-flow wetlands, each. By systematically varying flow rates (and thus residence time), we were able to demonstrate that the sedimentation-driven removal of particulate hydrous ferric oxides in settling ponds may be approximated by a simplified first-order approach at low to moderate Fe levels. The first-order coefficient was found in the order of 2.1(±0.7) × 10-2 h-1, which corresponds well with previous laboratory studies. The sedimentation kinetics may be combined with the preceding Fe(II) oxidation kinetics to estimate the required residence time for pre-treatment of ferruginous mine water in settling ponds. In contrast, Fe removal in surface-flow wetlands is more complex due to the phytologic component, which is why we advanced the established area-adjusted Fe removal approach by parameterising the underlying concentration-dependency for polishing of pre-treated mine water. The quantitative results of this study provide a novel, conservative approach for customised sizing of settling ponds and wetlands in integrated passive mine water treatment systems.

Heteroaggregation of PS microplastic with ferrihydrite leads to rapid removal of microplastic particles from the water column.

Microplastic (MP) particles are ubiquitous in aquatic environments. Therefore, understanding the processes that affect their removal from the water column, such as sedimentation, is critical for evaluating the risk they pose to aquatic ecosystems. We performed sedimentation experiments in which polystyrene (PS) and PS + ferrihydrite, a short-range ordered ferric (oxy)hydroxide, were analyzed in settling columns after 1 day and 1 week of settling time. The presence of ferrihydrite increased sedimentation rates of PS at all pH values studied (pH 3-11). At pH 6 we found that almost all PS particles were removed from the water column after only one day of exposure time. SEM/EDS imaging confirmed heteroaggregation between the PS particles and ferrihydrite. Zeta potential measurements indicated that at acidic pH values the negatively charged PS surface was coated with positively charged ferrihydrite particles leading to charge reversal. Our results demonstrate for the first time that ferric (oxy)hydroxides drive heteroaggregation and subsequent removal of MP from the water column, especially at typical pH values found in natural lake environments. Given their abundance in aquatic systems ferric (oxy)hydroxides need to be regarded as key scavengers of MP.

Sedimentation Kinetics of Hydrous Ferric Oxides in Ferruginous, Circumneutral Mine Water.

Transport, transformation, and removal of iron in aqueous environments is primarily controlled by ferrous iron oxidation followed by aggregation and sedimentation of the resultant hydrous ferric oxides. The latter mechanisms are particularly important for passive iron removal in mine water treatment systems, yet the interrelation and underlying kinetics are poorly understood. In this study, the sedimentation behavior of natural hydrous ferric oxides was systematically investigated under different hydrogeochemical conditions via laboratory-based column experiments. The objective was to determine a robust model approach for the approximation of aggregation/sedimentation kinetics in engineered systems. The results showed that sedimentation is governed by two interrelated regimes, a rapid second-order aggregation-driven step (r1) at high iron levels followed by a slower first-order settling step (r2) at lower iron levels. A mixed first-/second-order model was found to adequately describe the process: -d[Fe]dt=kr2[Fe]+kr1[Fe]2 with kr1 = 9.4 × 10-3 m3/g/h and kr2 = 5.4 × 10-3 h-1. Moreover, we were able to demonstrate that the removal of particulate hydrous ferric oxides at low particulate iron levels (<10 mg/L) may be reasonably well approximated by a simplified first-order relationship: -d[Fe]dt=ksed[Fe] with ksed = 2.4 (±0.4) × 10-2 h-1, which agrees well with incipient literature estimates. Only minor effects of pH, salinity, and mineralogy on kinetic parameters were observed. Hence, the results of this study may be broadly transferrable among different mine sites.

Sulfidation of ferric (hydr)oxides and its implication on contaminants transformation: a review.

Rapid industrialization and urbanization have resulted in elevated concentrations of contaminants in the groundwaters and subsurface soils, posing a growing hazard to humans and ecosystems. The transformation of most contaminants is closely linked to the mineralogy of ferric (hydr)oxides. Sulfidation of ferric (hydr)oxides is one of the most significant biogeochemical reactions in the anoxic environments, causing reductive dissolution and recrystallization of ferric (hydr)oxides and further affecting the transformation of iron-associated contaminants. This paper provides a comprehensive review on the sulfidation process of ferric (hydr)oxides and the transformation of relevant contaminants. This review presents detailed reaction mechanisms between ferric (hydr)oxides and dissolved sulfide, and elucidates the factors (e.g. crystallinity of ferric (hydr)oxides, the ratio of sulfide concentration to the surface area concentration of ferric (hydr)oxides) that control the formation of surface associated Fe(II), iron sulfide minerals, as well as transformation of secondary minerals. Then, we summarized the transformation mechanisms of a variety of typical environmentally relevant contaminants existing in groundwater and subsurface soils, including heavy metals, metal(loid) oxyanions (arsenic, antimony, chromium), radionuclides (uranium, technetium), organic contaminants and phosphate/nitrate species. The general mechanisms of contaminant transformation involve a combination of release, reduction and re-adsorption/incorporation processes, the specific pathway of which is highly dependent on the properties of the contaminant itself and the extent of sulfidation. Moreover, the challenge of extending our knowledge towards in situ remediation, as well as further research needs are identified.

Silicon increases the phosphorus availability of Arctic soils.

Phosphorus availability in soils is an important parameter influencing primary production in terrestrial ecosystems. Phosphorus limitation exists in many soils since a high proportion of soil phosphorus is stored in unavailable forms for plants, such as bound to iron minerals or stabilized organic matter. This is in spite of soils having a high amount of total soil phosphorus. The feasibility of silicon to mobilize phosphorus from strong binding sites of iron minerals has been shown for marine sediments but is less well studied in soils. Here we tested the effect of silicon on phosphorus mobilization for 143 Artic soils (representing contrasting soil characteristics), which have not been affected by agriculture or other anthropogenic management practices. In agreement with marine studies, silicon availabilities were significantly positive correlated to phosphorus mobilization in these soils. Laboratory experiments confirmed that silicon addition significantly increases phosphorus mobilization, by mobilizing Fe(II)-P phases from mineral surfaces. Silicon addition increased also soil respiration in phosphorus deficient soils. We conclude that silicon is a key component regulating mobilization of phosphorous in Arctic soils, suggesting that this may also be important for sustainable management of phosphorus availability in soils in general.

Controls on iron(II) fluxes into waterways impacted by acid mine drainage: A Damköhler analysis of groundwater seepage and iron kinetics.

When acidic groundwater flows into an aquatic system the sediment water interface (SWI) acts as a transition zone between the groundwater and lake water, and often exhibits strong physical and biogeochemical gradients. The fate of groundwater-borne solutes, such as Fe(II), is determined by the balance between the exposure time during transport across the SWI and the reaction time within the SWI, however the relative role of groundwater seepage rates and iron kinetics on acidity generation in lakes is unknown. Porewater seepage velocities, porewater chemical profiles, and limnological data were collected across multiple field campaigns over the last two decades, in acid Mine Lake 77, in Lusatia, Germany. This rare data set was analyzed using a Damköhler approach that compares exposure and reactions timescales, to determine that Fe(II) would typically be transported with little reaction across the SWI, spatially separating it from sediment-processes that produce alkalinity and providing a source of acidity to the lake. This Damköhler analysis further showed that remediation should be focused on reducing groundwater seepage velocities and enhancing exposure times. Strategic planting of submerged benthic macroalgae would slow groundwater inflows, as well as oxygenating overlying waters and supplying organic matter to the sediments. A similar Damköhler analysis could be used to assess the fate of any groundwater-borne reactive chemicals (e.g. phosphorus) into lakes and streams.

Stratification of reactivity determines nitrate removal in groundwater.

Biogeochemical reactions occur unevenly in space and time, but this heterogeneity is often simplified as a linear average due to sparse data, especially in subsurface environments where access is limited. For example, little is known about the spatial variability of groundwater denitrification, an important process in removing nitrate originating from agriculture and land use conversion. Information about the rate, arrangement, and extent of denitrification is needed to determine sustainable limits of human activity and to predict recovery time frames. Here, we developed and validated a method for inferring the spatial organization of sequential biogeochemical reactions in an aquifer in France. We applied it to five other aquifers in different geological settings located in the United States and compared results among 44 locations across the six aquifers to assess the generality of reactivity trends. Of the sampling locations, 79% showed pronounced increases of reactivity with depth. This suggests that previous estimates of denitrification have underestimated the capacity of deep aquifers to remove nitrate, while overestimating nitrate removal in shallow flow paths. Oxygen and nitrate reduction likely increases with depth because there is relatively little organic carbon in agricultural soils and because excess nitrate input has depleted solid phase electron donors near the surface. Our findings explain the long-standing conundrum of why apparent reaction rates of oxygen in aquifers are typically smaller than those of nitrate, which is energetically less favorable. This stratified reactivity framework is promising for mapping vertical reactivity trends in aquifers, generating new understanding of subsurface ecosystems and their capacity to remove contaminants.

Origin and fate of nitrate runoff in an agricultural catchment: Haean, South Korea - Comparison of two extremely different monsoon seasons.

The monsoon season in South Korea has great influences on biogeochemical and hydrological processes in the entire country, but is specifically of concern in the Soyang lake watershed, the main drinking water reservoir for the 20-million-people metropolis Seoul. Therefore, water quality and nitrate concentration control in Lake Soyang is of high public priority. The Haean catchment is the most prominent agriculture-dominated sub-catchment of the Soyang lake watershed. It is a complex terrain influenced by extreme rain events and non-point nitrate sources. In this investigation we used input-output calculations and a stable isotope approach to quantify and determinate the origin of nitrate inputs into the rivers that later flow into the lake. During pre-monsoon and monsoon seasons in 2013 and 2014 we measured daily rainfall and river water discharge within the Haean catchment and collected rain, river water and groundwater samples in order to analyze nitrate concentrations and nitrate nitrogen and oxygen isotope abundances. Furthermore, we collected nitrogen fertilizers as applied in the catchment. Heavy monsoon events, as in 2013, were the most pronounced drivers of nitrate leaching being responsible for >80% of the nitrate output in the river runoff. On the other hand, an almost missing summer monsoon in 2014 drove the nitrate runoff in a different manner, being responsible for only 0.4% of the total nitrate nitrogen river discharge in the previous year. Results of nitrate nitrogen and oxygen isotope abundance analyses suggest soil microbial nitrification as the most important contributor to the nitrate in the river runoff. In addition, nitrate from groundwater partially affected by microbial denitrification contributed to the nitrate in the runoff due to river-aquifer exchange fluxes during the monsoon season. Direct leaching of nitrate from mineral fertilizers and atmospheric nitrate deposition were obviously only minor contributors to the nitrate in the runoff.

Extremophile microbiomes in acidic and hypersaline river sediments of Western Australia.

We investigated the microbial community compositions in two sediment samples from the acidic (pH ∼3) and hypersaline (>4.5% NaCl) surface waters, which are widespread in Western Australia. In West Dalyup River, large amounts of NaCl, Fe(II) and sulfate are brought by the groundwater into the surface run-off. The presence of K-jarosite and schwertmannite minerals in the river sediments suggested the occurrence of microbial Fe(II) oxidation because chemical oxidation is greatly reduced at low pH. 16S rRNA gene diversity analyses revealed that sequences affiliated with an uncultured archaeal lineage named Aplasma, which has the genomic potential for Fe(II) oxidation, were dominant in both sediment samples. The acidophilic heterotrophs Acidiphilium and Acidocella were identified as the dominant bacterial groups. Acidiphilium strain AusYE3-1 obtained from the river sediment tolerated up to 6% NaCl at pH 3 under oxic conditions and cells of strain AusYE3-1 reduced the effects of high salt content by forming filamentous structure clumping as aggregates. Neither growth nor Fe(III) reduction by strain AusYE3-1 was observed in anoxic salt-containing medium. The detection of Aplasma group as potential Fe(II) oxidizers and the inhibited Fe(III)-reducing capacity of Acidiphilium contributes to our understanding of the microbial ecology of acidic hypersaline environments.

Interference of Nitrite with Pyrite under Acidic Conditions: Implications for Studies of Chemolithotrophic Denitrification.

Chemolithotrophic denitrification coupled to pyrite oxidation is regarded a key process in the removal of nitrate in aquifers. A common product is nitrite, which is a strong oxidant under acidic conditions. Nitrite may thus interfere with Fe(II) during acidic extraction, a procedure typically used to quantify microbial pyrite oxidation, in overestimating Fe(III) production. We studied the reaction between pyrite (5-125 mM) and nitrite (40-2000 µM) at pH 0, 5.5, and 6.8 in the absence and presence of oxygen. Significant oxidation of pyrite was measured at pH 0 with a yield of 100 µM Fe(III) after 5 mM pyrite was incubated with 2000 µM nitrite for 24 h. Dissolved oxygen increased the rate at pH 0. No oxidation of pyrite was observed at pH 5.5 and 6.8. Our data imply a cyclic model for pyrite oxidation by Fe(III) on the basis of the oxidation of residual Fe(II) by NO and NO2. Interference by nitrite could be avoided if nitrite was removed from the pyrite suspensions through a washing procedure prior to acidic extraction. We conclude that such interferences should be considered in studies on microbially mediated pyrite oxidation with nitrate.

Electron transfer budgets and kinetics of abiotic oxidation and incorporation of aqueous sulfide by dissolved organic matter.

The reactivity of natural dissolved organic matter toward sulfide and has not been well studied with regard to electron transfer, product formation, and kinetics. We thus investigated the abiotic transformation of sulfide upon reaction with reduced and nonreduced Sigma-Aldrich humic acid (HA), at pH 6 under anoxic conditions. Sulfide reacted with nonreduced HA at conditional rate constants of 0.227-0.325 h(-1). The main transformation products were elemental S (S0) and thiosulfate (S2O3(2-)), yielding electron accepting capacities of 2.82-1.75 µmol e- (mg C)(-1). Native iron contents in the HA could account for only 6-9% of this electron transfer. About 22-37% of S reacted with the HA to form organic S (Sorg). Formation of Sorg was observed and no inorganic transformation products occurred for reduced HA. X-ray absorption near edge structure spectroscopy supported Sorg to be mainly zerovalent, such as thiols, organic di- and polysulfides, or heterocycles. In conclusion, our results demonstrate that HA can abiotically reoxidize sulfide in anoxic environments at rates competitive to sulfide oxidation by molecular oxygen or iron oxides.

Occurrence of surface polysulfides during the interaction between ferric (hydr)oxides and aqueous sulfide.

Polysulfides are often referred to as key reactants in the sulfur cycle, especially during the interaction of ferric (hydr)oxides and sulfide, forming ferrous-sulphide minerals. Despite their potential relevance, the extent of polysulfide formation and its relevance for product formation pathways remains enigmatic. We applied cryogenic X-ray Photoelectron Spectroscopy and wet chemical analysis to study sulfur oxidation products during the reaction of goethite and lepidocrocite with aqueous sulfide at different initial Fe/S molar ratios under anoxic conditions at neutral pH. The higher reactivity of lepidocrocite leads to faster and higher electron turnover compared to goethite. We were able to demonstrate for the first time the occurrence of surface-associated polysulfides being the main oxidation products in the presence of both minerals, with a predominance of disulfide (S2(2-)(surf)), and elemental sulfur. Concentrations of aqueous polysulfide species were negligible (<1%). With prior sulfide fixation by zinc acetate, the surface-associated polysulfides could be precipitated as zerovalent sulfur (S°), which was extracted by methanol thereafter. Of the generated S°, 20-34% were associated with S2(2-)(surf). Varying the Fe/S ratio revealed that surface polysulfide formation only becomes dominant when the remaining aqueous sulfide concentration is low (<0.03 mmol L(-1)). We hypothesize these novel surface sulfur species, particularly surface disulfide, to act as pyrite precursors. We further propose that these species play an overlooked role in the sulfur cycle.

Redox stability of As(III) on schwertmannite surfaces.

As(III)-enriched mine discharge often drains through Fe(III)-mineral abundant land covers which makes the understanding of its fate and redox behaviour extremely important. We therefore conducted batch kinetic and equilibrium studies at pH 3.0±0.05 in anoxic media coupled with spectroscopic and microscopic examinations at variable conditions to understand possible As(III) binding mechanisms and the redox stability of As(III) on schwertmannite, a prominent ferric mineral in acid mine drainage environments. Schwertmannite acted as an efficient scavenger for As(III) compared to goethite at identical sorbent:solute ratios. As K-edge X-ray absorption near-edge structure (XANES) demonstrated partial oxidation of sorbed As(III) to As(V) on both the minerals depending on the Fe(III)/As(III) ratios (goethite acted as a better oxidant than schwertmannite). Sorbed As(III) and As(V) coordinated in a bidentate binuclear binding mechanism with As(III)/As(V)-O and As(III)/As(V)-Fe interatomic distances as 1.78/1.69 and 3.37/3.31Å, respectively. Scanning (SEM-EDX) and transmission (TEM) electron microscopic, and IR spectroscopic measurements revealed the formation of As-containing surface coatings by sorbed As on schwertmannite.

As(III) retention kinetics, equilibrium and redox stability on biosynthesized schwertmannite and its fate and control on schwertmannite stability on acidic (pH 3.0) aqueous exposure.

High As(III) enrichment in schwertmannite precipitated acid mine impacted areas is a major concern considering its acute toxicity and mobility where the current knowledge of their interaction, redox conditions and schwertmannite metastability is inadequate. In this study we have investigated such aspects through batch isotherm, microscopic and spectroscopic techniques. Schwertmannite produced by biotic process with 14.7 m(2)g(-1) surface area demonstrated a rapid As(III) uptake followed by slow retention possibly into the internal absorbing sites through multilayer and heterogeneous sorption processes. Chemical, X-ray diffraction, infrared and microscopic examinations revealed that ionic exchange between schwertmannite SO(4)(2-) and As(III) and surface precipitation governed the total As(III) uptake where lower dissolved SO(4)(2-) and high sorbent mass enhanced As(III) retention. Redox instability of sorbed As(III) was evidenced from the near-edge spectroscopic analysis at extremely high Fe(III):As(III) ratio (5.5×10(5)) leading to surface oxidation to As(V), while As(III) was found as the predominant redox species at high As(III):Fe(III) ratios. Only 0.83% of sorbed As(III) was released which was subsequently re-adsorbed into schwertmannite during 4 months stabilization without any evidence of mineralogical transformation.

Does iron cycling trigger generation of acidity in groundwaters of Western Australia?

In large areas of Western Australia, acidic groundwaters occur with pH values distinctly lower than 3, generation of which has been attributed to the oxidation of Fe(II). Incubation experiments performed with sediments from playas receiving acid groundwater demonstrated occurrence of reductive dissolution of ferric iron minerals at rates [670 nmol (g reactive iron)(-1) h(-1)] similar to those observed in sediments of acidic mining lakes (AML), indicating thatthe pH was established through an acidity-driven iron cycle in analogy to processes occurring in AML systems. The low pH values observed in acidic groundwaters and AML, however, can only be achieved if the anion corresponding to Fe(III) is that of a strong acid. In AML, sulfate is derived from pyrite oxidation. Because this process is reported not to occur in the acidic groundwater systems of Western Australia, we have derived a conceptual model according to which sulfate is generated upon reaction of weathering-derived alkalinity with gypsum to form calcite, which is abundant in these areas. The model proposes that part of the alkalinity generated during weathering is stared as calcite in the landscape, which leads to spatial separation of acidity and alkalinity.

Investigation of factors influencing biogas production in a large-scale thermophilic municipal biogas plant.

A continuously operated, thermophilic, municipal biogas plant was observed over 26 months (sampling twice per month) in regard to a number of physicochemical parameters and the biogas production. Biogas yields were put in correlation to parameters such as the volatile fatty acid concentration, the pH and the ammonium concentration. When the residing microbiota was classified via analysis of the 16S rRNA genes, most bacterial sequences matched with unidentified or uncultured bacteria from similar habitats. Of the archaeal sequences, 78.4% were identified as belonging to the genus Methanoculleus, which has not previously been reported for biogas plants, but is known to efficiently use H(2) and CO(2) produced by the degradation of fatty acids by syntrophic microorganisms. In order to further investigate the influence of varied amounts of ammonia (2-8 g/L) and volatile fatty acids on biogas production and composition (methane/CO(2)), laboratory scale satellite experiments were performed in parallel to the technical plant. Finally, ammonia stripping of the process water of the technical plant was accomplished, a measure through which the ammonia entering the biogas reactor via the mash could be nearly halved, which increased the energy output of the biogas plant by almost 20%.

Arsenic and chromium partitioning in a podzolic soil contaminated by chromated copper arsenate.

This research combined the use of selective extractions and X-ray spectroscopyto examine the fate of As and Cr in a podzolic soil contaminated by chromated copper arsenate (CCA). Iron was enriched in the upper 30 cm due to a previous one-time treatment of the soil with Fe(II). High oxalate-soluble Al concentrations in the Bs horizon of the soil and micro-XRD data indicated the presence of short-range ordered aluminosilicates (i.e., proto-imogolite allophane, PIA). In the surface layers, Cr, as Cr(III), was partitioned between a mixed Fe(III)/ Cr(III) solid phase that formed upon the Fe(II) application (25-50%) and a recalcitrant phase (50-75%) likely consisting of organic material such as residual CCA-treated wood. Deeper in the profile Cr appeared to be largely in the form of extractable (hydr)oxides. Throughout the soil, As was present as As(V). In the surface layers a considerable fraction of As was also associated with a recalcitrant phase, probably CCA-treated woody debris, and the remainder was associated with (hydr)oxide-like solid phases. In the Bs horizon, however, XAS and XRF findings strongly pointed to the presence of PIA acting as an effective adsorbent for As. This research shows for the first time the relevance of PIA for the adsorption of As in natural soils.

Reactivity of alkaline lignite fly ashes towards CO2 in water.

The reaction kinetics between alkaline lignite fly ashes and CO2 (pCO2 = 0.01--0.03 MPa)were studied in a laboratory CO2 flow-through reactor at 25--75 degrees C. The reaction is characterized by three phases that can be separated according to the predominating buffering systems and the rates of CO2 uptake. Phase I (pH > 12, < 30 min) is characterized by the dissolution of lime, the onset of calcite precipitation and a maximum uptake, the rate of which seems to be limited by dissolution of CO2. Phase II (pH < 10.5, 10--60 min) is dominated by the carbonation reaction. CO2 uptake in phase III (pH < 8.3) is controlled by the dissolution of periclase (MgO) leading to the formation of dissolved magnesium-bicarbonate. Phase I could be significantly extended by increasing the solid-liquid ratios and temperature, respectively. At 75 degrees C the rate of calcite precipitation was doubled leading to the neutralization of approximately 0.23 kg CO2 per kg fly ash within 4.5 h, which corresponds to nearly 90% of the total acid neutralizing capacity.