Batch Leaching Experiments Research Articles

Comparative assessment of batch and column leaching studies for heavy metals release from Coal Fly Ash Bricks and Clay Bricks

Abstract The present work explores the leaching of 12 heavy metal from Coal Fly Ash Brick (CFAB) and Clay Brick (CB) samples collected from Badarpur and Khaperkheda Thermal Power Plant (TPP) using batch and column leaching tests. The batch leaching test was performed fallowing standard procedure of WET, TCLP, SPLP and DWLP, and the column leaching studies were done at pH 4 and 7. The study also focuses on the comparative assessment of heavy metal leaching from collected samples. Results of both batch and column leaching experiments showed that the concentration of leached heavy metals was within the limit specified by Bureau of Indian Standard IS-10500:2012. It was observed that the heavy metals concentration leached in batch leaching experiments were comparatively more than column experiments. The study supports the use of Coal Fly Ash (CFA) during bricks manufacturing as there were minimum leaching observed from CFAB compared to CB.

Leaching Characteristics of Iron and Manganese from Steel Slag with Repetitive Replenishment of Leachate

Steel slag reuse as a construction material is an attractive solution for the management of the industrial byproduct. However, concern exists on the release of steel slag constituents when a massive amount of the material is reused in the field. This study is aimed to improve the understanding on the leaching characteristics of iron and manganese, two heavy metals that are present in steel slag at relatively high contents. Concentrations of iron and manganese in the batch leaching experiment readily reach close to equilibrium within 1 hour. Batch leaching experiments conducted with exchange of the leaching solution at 1 hour intervals show that the leaching occurs consistently with relatively constant concentration up to 50 events of exchange. The leaching is a strong function of salinity showing factors of 11.6 and 3.8 increase in iron and manganese leachate concentrations, respectively. Despite the wide size ranges (<0.5 to >25 mm) tested, effect of slag grain size is less pronounced compared to the salinity effect. The average concentration in the leachate for 50 leaching events is sub-mg L−1 level for iron at most and an order of magnitude lower for manganese than the value for iron. These results show that steel slag is an almost permanent but a very low-level source of iron and manganese when reused in the environment.

Simulation of one-dimensional column leaching of weathered crust elution-deposited rare earth ore

The ion exchange model of the leaching process was determined via batch leaching experiments using the Kerr model, with the selectivity coefficient experimentally determined to be 12.59×10−10 L2/g2. Solute transport laws of ammonium ions (NH4+) and rare earth ions (RE3+) in column leaching were described by the convection−dispersion equation (CDE). The source and sink in the CDE were determined by the Kerr model. The CDE with strong nonlinearity was solved using the sequential non-iterative method. Compared with the breakthrough curve of RE3+, the correlation coefficient between the simulated and experimental curves reached 0.8724. Therefore, this method can simulate the one-dimensional column leaching of weathered crust elution-deposited rare earth ore. Moreover, the effects of different concentrations of ammonium sulfate ((NH4)2SO4) solution on the leaching rate of rare earth were analyzed. The optimal concentration of the (NH4)2SO4 solution had a linear relationship with the rare earth grade.

Mobilization and isotope fractionation of chromium during water-rock interaction in presence of siderophores

Chromium mobilization and isotope fractionation during water-rock interaction in presence of the biogenic siderophore desferrioxamine B (DFOB) was studied with batch leaching experiments on chromitite and other igneous oxide and silicate rocks. Siderophores are a group of organic ligands synthesized and excreted by bacteria, fungi and plants to enhance the bioavailability of key nutrients like Fe. However, the DFOB siderophore also has a strong affinity for complexation with other metals such as Cr, U and rare earth elements. Here we show that leaching of rocks in the presence of the hydroxamate siderophore DFOB significantly increased the mobilization of Cr from all investigated rocks and caused an enrichment of the heavier 53Cr isotope in leachates from chromitite (δ53Crleach = +0.15 ± 0.087‰ to +2.14 ± 0.042‰) and from altered silicate rock (δ53Crleach = +0.48 ± 0.07‰). In contrast, stable isotope fractionation of Cr was not observed in DFOB leachates of pristine silicate and low-Cr oxide rocks. Leaching in the presence of citric acid significantly enhanced Cr mobility, but did not result in fractionation of Cr isotopes. Chromium isotope fractionation is used in geochemistry as a quantitative proxy for oxidative weathering, because Cr(III) is oxidized to Cr(VI) in presence of MnO2 and the associated Cr isotope fractionation is commonly linked to the presence of oxygen in the atmosphere. Our findings indicate that the presence of specific biogenic ligands with a high affinity for Cr may also cause Cr isotope fractionation. The presence of biomolecules like siderophores during weathering, hydrothermal alteration or during mineral precipitation, therefore, may put constraints on the applicability of certain trace metals and their isotopes as redox proxies in modern and past environments. The results of our study also suggest that siderophores may have a high potential for (bio)remediation of Cr-contaminated sites and detoxification of contaminated natural waters.

Distribution of Zinc, Copper, and Iron in the Tailings Dam of an Abandoned Mine in Shimokawa, Hokkaido, Japan

This paper addresses the mechanism of acid mine drainage generation in tailings from an abandoned mine site and predicts the evolution of zinc (Zn), copper (Cu), and iron (Fe) concentrations. Batch leaching experiments and sequential extractions were conducted to investigate the leaching behavior of these contaminants from the tailings and to understand their solid-phase partitioning. Acid-base accounting and principal component analysis (PCA) were used to confirm factors affecting Zn, Cu, and Fe leaching and acid formation based on the leaching experiments. There were strong positive correlations between Zn, Fe, or EC and SO42−, indicating that pyrite and sphalerite are the major minerals releasing Zn and Fe. This aligns with the PCA results. In the upper part of the tailings, the water-soluble and sulfide fractions of Zn, Cu, and Fe were almost flushed out, whereas they remained high in the deeper tailings. This implies that the tailings will likely continue to release these contaminants (Zn > Cu > Fe) for a long time unless remedial measures are taken.

Microbial removal of sulphur from petroleum coke (petcoke)

Petroleum coke (pet coke) is a carbonised heavy residual product obtained from petroleum processing. Pet coke contains between 1 and 10% w/w sulphur as both inorganic and organic forms. The removal of organic sulphur from pet coke is desirable to reduce the environmental harm of its combustion emissions but the elevated temperatures and pressures needed result in a lower fuel-value. The removal of sulphur by microbial attack involving acidophilic bacteria is an alternative approach and is the subject of this study. Single and mixed cultures of Thiobacillus thiooxidans (isolated from natural hot sulphur spring) and Thiobacillus ferrooxidans (isolated from an existing culture) were investigated for total sulphur removal from pet coke. Batch leaching experiments showed a reduction in total sulphur of 92% over 28 days. A combination of Thiobacillus species performed better than the single bacterial culture.

Alkaline-assisted leaching of iron-cyanide complex from contaminated soils

Soils with persistent cyanide compounds (including ferricyanide and ferrocyanide) pose potential hazards to the environment and require remediation before utilization. This study investigated the alkaline-assisted leaching behavior of iron-cyanide complexes from five cyanide-contaminated soils via batch leaching experiments spanning a wide pH range (10–14). The results showed that the leaching ratio of iron-cyanide complex (R) was highly dependent on pH. Leaching with alkaline solution of pH < 12 resulted in a small desorption ratio (1%–5% for the laboratory-contaminated A type soils and 11% for B type soil), while alkaline solution with elevated pH (>13) significantly enhanced the desorption ratio, reaching 87%–100% within a reaction time of 0.5 h except one soil sample. The desorption kinetics of the cyanide complexes at different pHs indicated that time is another parameter closely related to R. The time-dependent leaching ratio at pH 12 only slightly increased from 0.5 h to 48 h, reaching a maximum desorption ratio of 24% and 31% for ferricyanide and ferrocyanide, respectively. Whereas, the desorption profiles of contaminated soils at pH 13 revealed a rapid desorption of iron-cyanide complexes within the first two hours followed by a plateau that corresponded to a complete desorption. The leaching characteristics of all cyanide contaminated soils revealed the fact that ferricyanide has a stronger affinity with soils than ferrocyanide, which implies a lower mobility of ferricyanide.

Pyrite oxidation in the presence of calcite and dolomite: Alkaline leaching, chemical modeling and surface characterization

Abstract The effect of calcite and dolomite on the oxidation of pyrite was studied using batch-leaching experiments complemented with chemical modeling, solution analysis and solid characterization techniques. Leaching tests conducted at 80 °C and p(O2)=1.013×105 Pa and pH>13 showed that pyrite alkaline oxidation rate decreased in the presence of both calcite and dolomite, while more detrimental effect was observed in the case of calcite. FE–SEM/EDS analysis exhibited a surface layer containing Ca (in the case of calcite) and Mg (in the case of dolomite) on the pyrite surface, which justified the slowdown in pyrite leaching rate. This surface layer was formed due to partial dissolution of carbonate minerals, which was affected dominantly by the pH and temperature of the leaching solution based on the chemical modeling data. The surface layer was characterized as Ca or Mg hydroxide using XRD and FTIR analysis. It was also found that this layer was thin and continuous in the case of calcite while it was thick and discontinuous in the presence of dolomite.

Cadmium isotope fractionation during coal combustion: Insights from two U.S. coal-fired power plants

Coal combustion, one of the principal energy sources of electricity in the United States, produces over 100 million tons of coal combustion products (CCPs) per year in the U.S. The reuse and disposal of CCPs has the potential to release toxic trace elements, including cadmium (Cd), into the environment. In this study, we investigated CCPs, including bottom ash (BA), economizer fly ash (EFA), and fly ash (FA), as well as feed coal (FC) and pulverized coal (PC) collected from two U.S. coal-fired power plants in New Mexico and Ohio with different coal supplies. The New Mexico plant uses high volatile C bituminous, low-sulfur coals mined from the San Juan Basin (Cretaceous Fruitland Formation) and the Ohio plant uses high volatile A bituminous, high-sulfur central Appalachian Basin coals (Upper Pennsylvanian Monongahela Formation). Mineralogical and elemental analysis showed that these CCP samples consist of ∼70% amorphous Al-Si-rich glasses and ∼30% mineral phases of quartz (SiO2) and mullite (Ai6Si2O13). The Cd isotope compositions (δ114Cd, normalized to NIST Cd standard 3108) of FA and EFA samples (ranging from −0.51 to +0.47‰) are distinctively heavier than those of BA samples (−0.75 to −0.52‰) in both power plants. We interpret this Cd isotope difference as a result of Cd condensation from the gas phase during flue gas cooling, instead of evaporation of Cd phase during coal combustion. Cd condensation is the main process to generate the isotopically heavy Cd signatures that preferentially partition on the fine FA particles. We also investigated Cd isotope compositions in different leachate products from a series of batch-leaching experiments with these CCPs, using diluted acetic acid, hydroxyl ammonium chloride, hydrogen peroxide followed by ammonium acetate, and 5% nitric acid, as a possible means to identify CCP-released Cd in the environment. Unusually and significantly heavier Cd isotope compositions were observed in each leachate of FA samples (+1.10 to +7.09‰), which fall far outside from the range of Cd isotope ratios observed in natural soils and rocks, but less so for the EFA samples (−0.43 to +1.18‰). Such an observation is consistent with the interpretation that isotopically heavy Cd preferentially partitions on the fine FA particles after coal combustion and is readily to be released during these leaching experiments. This study demonstrates that high-temperature coal combustion can lead to a very large degree of fractionation of Cd isotopes that can be used as a unique tracer for identifying anthropogenic metal inputs in the environment. The major Cd isotope fractionation process occurs as the Cd gas phase condenses on fine FA particles during the flue gas cooling stage after coal combustion.

REY‐Th‐U Dynamics in the Critical Zone: Combined Influence of Reactive Bedrock Accessory Minerals, Authigenic Phases, and Hydrological Sorting (Mule Hole Watershed, South India)

AbstractThe sources of REY‐Th‐U and long‐term mass balance were assessed in the gneissic tropical forested Critical Zone Observatory of Mule Hole, India. The study relies on the characterization of the solid compartments (bedrock, soils, streambed and suspended sediments), on batch leaching experiments of the parent gneiss and on extractions of cation‐exchangeable and iron‐related pools of selected soil samples. The REY‐Th‐U primary reservoir is controlled by monazite, xenotime, thorite, allanite, bastnesite, titanite, apatite, and, to a lesser extent, by zircon. This accessory mineral assemblage was profoundly modified by episodes of metamorphism and hydrothermal activity. Allanite, bastnesite, titanite, and apatite are prone to break down at incipient weathering stage while monazite, xenotime and zircon are resistant minerals. On a long‐term basis, REY are roughly inert in the immature saprolite and depleted in the soil cover and are strongly redistributed among secondary phases. LREE and HREY are mainly controlled by poorly crystallized secondary iron oxides and oxyhydroxides, and, to a lesser extent, by secondary phosphates (e.g., rhabdophane). Th, inert in the saprolite, is controlled by well‐crystallized secondary iron oxides and oxyhydroxides and likely by secondary thorianite (ThO2). REY and Th are not easily mobilized during surface processes such as chelation by organic matter and uptake by plants. REY and Th are, by far, exported as suspended sediments associated with iron oxides. The U export by groundwater and suspended sediments dominates over those of streambed sediments and stream.

Simultaneous suppression of acid mine drainage formation and arsenic release by Carrier-microencapsulation using aluminum-catecholate complexes

Pyrite (FeS2), the most common sulfide mineral in nature, plays an important role in the formation of acid mine drainage (AMD), one of the most serious environmental problems after the closure of mines and mineral processing operations. Likewise, arsenopyrite (FeAsS) is an important sulfide mineral because its dissolution releases toxic arsenic (As) into the environment. To mitigate the serious environmental problems caused by pyrite and arsenopyrite, this study investigated carrier-microencapsulation (CME) using Al-catecholate complexes, a technique that selectively forms protective coatings on the surfaces of sulfide minerals, by electrochemical techniques and batch leaching experiments coupled with surface sensitive characterization techniques.Cyclic voltammetry (CV) of Al-catecholate complexes (mono-, bis-, tris-catecholate) suggest that these three species could be oxidatively decomposed in this order: [Al(cat)3]3–→[Al(cat)2]–→[Al(cat)]+→Al3+, and these reactions were irreversible. Among these three species, [Al(cat)]+ was the most effective in suppressing pyrite and arsenopyrite oxidations because it requires less steps for complete decomposition than the other two complexes. Analyses of CME treated minerals by scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) indicated that they were covered with Al-oxyhydroxide (γ–AlO(OH)), which became more extensive at higher [Al(cat)]+ concentrations. In addition, this coating was stable even at relatively high applied potentials that simulated surface oxidizing conditions. Based on these results, a detailed mechanism of Al-based CME is proposed: (1) adsorption of [Al(cat)]+ on the surface of mineral, (2) oxidative decomposition of [Al(cat)]+ and release of “free” Al3+, and (3) precipitation and formation of Al-oxyhydroxide coating.

Influence of solid-liquid separation method parameters employed in soil leaching tests on apparent metal concentration

Soil leaching tests are commonly used to evaluate the leachability of hazardous materials, such as heavy metals, from the soil. Batch leaching tests often enhance soil colloidal mobility and may require solid-liquid separation procedures to remove excess soil particles. However, batch leaching test results depend on particles that can pass through a 0.45μm membrane filter and are influenced by test parameters such as centrifugal intensity and filtration volume per filter. To evaluate these parameters, we conducted batch leaching experiments using metal-contaminated soils and focused on the centrifugal intensity and filtration volume per filter used in solid-liquid separation methods currently employed in standard leaching tests. Our experiments showed that both centrifugal intensity and filtration volume per filter affected the reproducibility of batch leaching tests for some soil types. The results demonstrated that metal concentrations in the filtrates significantly differed according to the centrifugal intensity when it was 3000 g for 2h or less. Increased filtration volume per filter led to significant decreases in filtrate metal concentrations when filter cakes formed during filtration. Comparison of the filtration tests using 0.10 and 0.45μm membrane filters showed statistically significant differences in turbidity and metal concentration. These findings suggest that colloidal particles were not adequately removed from the extract and contributed substantially to the apparent metal concentrations in the leaching test of soil containing colloidal metals.

Leaching of TCIPP from furniture foam is rapid and substantial

A series of laboratory experiments were conducted, in which waste furniture polyurethane foam samples containing tris (1-chloro-2-propyl) phosphate (TCIPP) were contacted with a range of leaching fluids, formulated to simulate the composition of landfill leachate. Leaching was examined under a number of different scenarios, such as: dissolved humic matter concentration, pH, and temperature, as well as the effect of agitation, and waste:leaching fluid contact duration. In addition to single batch (no replenishment of leaching fluid), serial batch (draining of leachate and replenishment with fresh leaching fluid at various time intervals) experiments were conducted. Leaching of TCIPP from PUF appears to be a first order process. Concentrations of TCIPP in leachate generated by the experiments in this study ranged from 13 mg L−1 to 130 mg L−1. In serial batch leaching experiments, >95% of TCIPP was depleted from PUF after 168 h total contact with leaching fluid. Our experiments indicate leaching is potentially a very significant pathway of TCIPP emissions to the environment.

Simultaneous leaching of arsenite, arsenate, selenite and selenate, and their migration in tunnel-excavated sedimentary rocks: I. Column experiments under intermittent and unsaturated flow

Rocks excavated in tunnel construction projects for roads and railways throughout Japan often leached out hazardous trace elements like arsenic (As) and selenium (Se) upon their exposure to the environment. In nature, the various oxyanionic species of As and Se not only coexist but also exhibit contrasting adsorption-desorption behaviors, so speciation is a crucial factor in their migration through natural geologic media. In this study, the leaching and transport of arsenite (AsIII), arsenate (AsV), selenite (SeIV) and selenate (SeVI) in four tunnel-excavated rocks from the Cretaceous-Paleocene Yezo forearc basin were investigated using laboratory column experiments supplemented by batch leaching experiments. The single- and consecutive-batch leaching results revealed that AsIII, AsV, SeIV and SeVI were released simultaneously, which could be attributed to the rapid dissolution of trace evaporite salts found in the rocks. Arsenic in the leachates was also predominated by AsV while SeIV and SeVI concentrations were nearly equal, which are both consistent with predictions of equilibrium Eh-pH diagrams. Under intermittent and unsaturated flow, however, periods when AsIII and SeVI predominated in the effluents were observed. Spatial distributions of As and Se species with depth at the end of the column experiments suggest that migrations of AsIII, AsV and SeIV were delayed, the extent of which depended on the rock. These results indicate that migration and speciation of As and Se in the rocks are controlled by preferential adsorption-desorption reactions, the effects of which were most probably magnified by changes in the pH and concentrations of coexisting ions due to intermittent and unsaturated flow.

Optimization of Microbial Leaching of Base Metals from a South African Sulfidic Nickel Ore Concentrate by Acidithiobacillus ferrooxidans

ABSTRACTX-ray diffraction analysis revealed that pentlandite and chalcopyrite were the prominent mineral phases in a South African sulfidic nickel ore concentrate that hosted nickel and copper. Cobalt was found to be closely associated with the nickel-bearing pentlandite phase of the ore sample. Microbial batch leaching experiments designed according to a central composite design model were run for 15 days in a shaking incubator (150 rpm) at a constant temperature (30°C) with variations in experimental parameters like ore pulp density, particle size, bacterial inoculum, pH of the culture medium, and residence time. Quadratic mathematical models were developed to predict the rate of metal extractions. The suitability of the model of the microbial leaching process was confirmed from normal probability curves. An analysis of variance indicated that the residence time, pulp density of the ore, and particle size were the most significant factors. Bacterial inoculum size hardly showed any effect on the total metal extractions. Maximum nickel (82%), cobalt (76%) and copper (25.6%) extractions were achieved under optimum conditions, operated for 15 days at pulp density of 2% and particle size of −75 µm at pH 1.5.

Release behavior of uranium in uranium mill tailings under environmental conditions

Uranium contamination is observed in sedimentary geochemical environments, but the geochemical and mineralogical processes that control uranium release from sediment are not fully appreciated. Identification of how sediments and water influence the release and migration of uranium is critical to improve the prevention of uranium contamination in soil and groundwater. To understand the process of uranium release and migration from uranium mill tailings under water chemistry conditions, uranium mill tailing samples from northwest China were investigated with batch leaching experiments. Results showed that water played an important role in uranium release from the tailing minerals. The uranium release was clearly influenced by contact time, liquid-solid ratio, particle size, and pH under water chemistry conditions. Longer contact time, higher liquid content, and extreme pH were all not conducive to the stabilization of uranium and accelerated the uranium release from the tailing mineral to the solution. The values of pH were found to significantly influence the extent and mechanisms of uranium release from minerals to water. Uranium release was monitored by a number of interactive processes, including dissolution of uranium-bearing minerals, uranium desorption from mineral surfaces, and formation of aqueous uranium complexes. Considering the impact of contact time, liquid-solid ratio, particle size, and pH on uranium release from uranium mill tailings, reducing the water content, decreasing the porosity of tailing dumps and controlling the pH of tailings were the key factors for prevention and management of environmental pollution in areas near uranium mines.

Effects of organic acids on dissolution of Fe and Mn from weathering coal gangue

Understanding the effects of organic acids (OA) on the transformation of Fe and Mn to surface water from the weathering coal gangue is of great benefit to risk assessment and remediation strategies for contaminated water and soil. Based on the investigation on surface water in the central coal districts of the Guizhou Province, 18 water samples were collected for heavy metal analysis. The results indicated that the pH value of surface water is low (3.11–4.92), and Fe concentration (1.31–5.55 mg L−1) and Mn concentration (1.90–5.71 mg L−1) were, on average, 10.86 and 34.33 times the limit of Surface Water Quality Standards, respectively. In order to evaluate the effects of the OA on the dissolution of Fe and Mn from the weathering coal gangue, column elution and batch leaching experiments were conducted. The results show that the low molecular weight of organic acids (LMWOAs, i.e., oxalic, tartaric, malic and citric acids) and fulvic acids significantly accelerated the dissolution of Fe and Mn; in addition, when the concentration of OA reached 25 mmol L−1, the concentrations of Fe, and Mn were 1.14–67.08 and 1.11–2.32 times as high as those in 0.5 mmol L−1 OA, respectively. Furthermore, the migration of Fe and Mn was significantly influenced by the pH and Eh, especially for Fe; the ion Mn was dissolved from the gangue more easily than the ion Fe in the column leaching, which was contrary to the results of batch leaching.

Leaching of pyrite in cupric chloride solution

Pyrite (FeS2 ) is the most abundant sulphide mineral in the world and may contain significant amounts of gold locked in its structure. Traditionally, pyrite has to be oxidized in order to break down the mineral matrix for better gold liberation by e.g. roasting, pressure leaching and bioleaching [1]. Due to environmental and health concerns, as well as public opinion, alternative methods are developed to replace gold cyanide leaching, which has been the preferred gold leaching process. Gold is known to form a stable complex with chloride as [AuCl4 ]- or [AuCl2 ]-. In chloride leaching of pyrite in the presence of [Cu2+ ] which acts as a catalyst/oxidant, the pyrite structure can be simultaneously broken down and the gold dissolves into a solution from which it can be later recovered [2, 3]. In this study, dissolution of the pyrite concentrate in cupric chloride solutions was investigated. The effect of cupric ion concentration (16, 32 g/L), temperature, (60, 75, 90 °C), chloride ion concentration (50, 150 g/L), pH (1, 1.5) and mixed chloride-bromide ion concentration (75+75 g/L) on pyrite dissolution were studied during 6 hour batch leaching experiments. Redox potentials between 0.495-0.511 V vs. Ag/AgCl resulted in pyrite dissolution of 31-34%, whereas redox potentials between 0.523-0.573 V vs. Ag/AgCl resulted in 45-68% pyrite dissolution.

Batch Studies On Arsenic Adsorption Onto Lignite, Bentonite, Shale And Iron Sand: Effects Of Ph, Time, Particle Size And Sulfate Concentration

Arsenic (As) is a toxic element found in both natural and anthropogenic sources. High concentration of this element was recently uncovered in the groundwater of Sumbawa Island, Indonesia. To mitigate this problem, As adsorption potential of natural geological materials like lignite, bentonite, shale, and iron sand obtained in Indonesia were evaluated by batch experiments. Arsenic adsorption onto these materials was investigated as a function of solution pH, particle sizes of adsorbents and coexisting sulfate concentration. In addition, batch leaching experiments were performed to elucidate the stability of geogenic As present in all adsorbents at different pHs. The results showed that among these natural materials tested, lignite was the most effective adsorbent of As(V) followed by bentonite, shale and then iron sand, and that the amounts of As(III) adsorbed onto all adsorbents were lower than those of As(V).This indicates that As(III) is more mobile in comparison to As(V). The adsorption isotherms of As(III) and As(V) conformed to nonlinear types, either Langmuir or Freundlich. It was found that adsorption of As onto these natural adsorbents was pH-dependent. This could be attributed to the changes in the surface charges of the adsorbents with pH. With respect to the adsorbent particle size, the amount adsorbed somewhat increased with decreasing particle size, which could be explained by the larger surface area of the smaller particles. Acidic (pH &lt; 6) and alkaline (pH &gt;10) conditions destabilized the geogenic As content of the adsorbents, indicating that the effectiveness of these natural materials as adsorbents is greatly limited by the pH of the contaminated system. Keywords: Adsorption, arsenic, natural geological materials, particle size, pH

Oxidative mobilization of cerium and uranium and enhanced release of “immobile” high field strength elements from igneous rocks in the presence of the biogenic siderophore desferrioxamine B

Polyvalent trace elements such as the high field strength elements (HFSE) are commonly considered rather immobile during low-temperature water–rock interaction. Hence, they have become diagnostic tools that are widely applied in geochemical studies. We present results of batch leaching experiments focused on the mobilization of certain HFSE (Y, Zr, Hf, Th, U and rare earth elements) from mafic, intermediate and felsic igneous rocks in the presence and absence, respectively, of the siderophore desferrioxamine B (DFOB). Our data show that DFOB strongly enhances the mobility of these trace elements during low-temperature water–rock interaction.The presence of DFOB produces two distinct features in the Rare Earths and Yttrium (REY) patterns of leaching solutions, regardless of the mineralogical and chemical composition or the texture of the rock type studied. Bulk rock-normalized REY patterns of leaching solutions with DFOB show (i) a very distinct positive Ce anomaly and (ii) depletion of La and other light REY relative to the middle REY, with a concave downward pattern between La and Sm. These features are not observed in experiments with hydrochloric acid, acetic acid or deionized water. In DFOB-bearing leaching solutions Ce and U are decoupled from and selectively enriched relative to light REY and Th, respectively, due to oxidation to Ce(IV) and U(VI). Oxidation of Ce3+ and U4+ is promoted by the significantly higher stability of the Ce(IV) and U(VI) DFOB complexes as compared to the Ce(III) and U(IV) DFOB complexes. This is similar to the relationship between the Ce(IV)- and Ce(III)-pentacarbonate complexes that cause positive Ce anomalies in alkaline lakes. However, while formation of Ce(IV) carbonate complexes is confined to alkaline environments, Ce(IV) DFOB complexes may produce positive Ce anomalies even in mildly acidic and near-neutral natural waters. Siderophore-promoted dissolution processes also significantly enhance mobility of other ‘immobile’ HFSE and may not only cause or modify Ce anomalies and Th–U fractionation, but also mobilization of and fractionation between Zr, Hf, Th and redox-insensitive REY during weathering, pedogenesis, diagenesis and incongruent dissolution of particles in seawater and freshwater.Siderophores may significantly affect the use of HFSE as geochemical tools. Concave downward light REY patterns may be used as a biosignature for water–rock interaction in the presence of siderophores. Enhanced and preferential mobilization of U relative to Th in the presence of siderophores may produce Th–U signals comparable to those indicative of weathering under oxidized conditions, which might constrain the use of U concentrations and Th/U ratios as a paleoredox-proxy. The enhanced mobilization of Zr and especially Hf from igneous rocks in the presence of DFOB might have implications for the use of the latter as a tracer for the impact of continental weathering on seawater chemistry. Because siderophore complexes affect the particle-reactivity of Hf and Zr, they may prevent effective removal of terrigenous Hf and Zr during aggregation/coagulation of riverine particles in estuaries. Siderophore-promoted solubilization and stabilization might hence be an additional way to transport continental Hf and Zr to the oceans. Furthermore, siderophore-enhanced mobilization may also have implications for the remediation techniques employed to immobilize HFSE such as U, Th and REY, at nuclear waste and reprocessing sites and at REY ore processing plants, where soils are commonly contaminated with these (sometimes radioactive) heavy metals.