Applied Geochemistry (v.41, #C)
Pyrrhotite dissolution in acidic media by Paul Chiriţă; J. Donald Rimstidt (1-10).
Display OmittedNon-oxidative dissolution rates for hexagonal pyrrhotite (Fe1− x S) were measured for pH values ranging from 0 to 1.3 and temperatures ranging from 25 to 70 °C. These results showed that pyrrhotite dissolution rate increases with decreasing pH or increasing temperature.The 16 new rate data from this study were combined with 46 data taken from the literature (1 datum was discarded) to develop a rate equation for non-oxidative pyrrhotite dissolution r H + ( mol / m 2 s ) = 1.58 × 10 7 e - 65 , 900 R 1 T M H + 1.46 This fit spans a range of pH values from 0 to 5 and temperatures from 20 to 90 °C. It is most reliable for low pH (⩽2.75), where most rate data were measured. This equation reasonably predicts rates for all pyrrhotite (Fe1− x S) compositions regardless of the value of x, including monoclinic pyrrhotite, hexagonal pyrrhotite and troilite (FeS).In addition, an equation that expresses pyrrhotite rate as a function of temperature and P O 2 was developed using 35 rate data taken from the literature r O 2 ( mol / m 2 s ) = 1.10 × 10 - 2 e - 30 , 200 R 1 T P O 2 0.352 and an equation that expresses the rate of oxidation of pyrrhotite as a function of temperature and Fe(III) concentration was developed using 48 previously published data r Fe ( III ) ( mol / m 2 s ) = 0.516 e - 33 , 600 R 1 T M Fe ( III ) 0.368 The correlation coefficients for these regression models were relatively low due the narrow range of experimental conditions, relatively few data, and possible uncontrolled variables. These equations predict similar rates for all three reactions near pH 2 where many of the experiments were carried out leading to concerns that some of the experiments may have measured combined rates. For natural conditions, the rates of the H+ and Fe(III) reactions are predicted to be relatively fast and similar at pH < 3. The O2 reaction is dominant at higher pH > ∼3.
Review of arsenic contamination, exposure through water and food and low cost mitigation options for rural areas by Anitha K. Sharma; Jens Chr. Tjell; Jens J. Sloth; Peter E. Holm (11-33).
Arsenic (As) is a toxic metalloid found to be an important groundwater contaminant of mainly natural geogenic origin worldwide particularly in large deltas and along major rivers in poor regions of South- and East-Asia. Excessive and long-term human intake of toxic inorganic As with food and water is causing arsenicosis, which is disfiguring, disabling, and leading to potentially fatal diseases like skin- and internal cancers. It is estimated that more than 100 million people mainly in developing countries are at risk. The arsenicosis situation in affected countries has been named the largest chemical threat to public health ever experienced and arsenicosis is spreading to regions where near-sterile well water loaded with As has replaced microbial suspect surface water containing lower As concentrations. This review provides an overview of the state of the art knowledge on the water and food As intake and exposure, and how the As chemistry in water and food may influence chosen mitigation strategies. Although reports on severe health effects from exposure to As in water are abundant there are several weak points in our knowledge on causes and prevalence of arsenicosis in order to devise effective mitigation. The main mitigation strategies focus on drinking water based on exploration of As-free water and As removal from extracted water, whereas mitigation strategies on cooking water and reducing exposure through food are quite often overlooked. The experiences of adopted low cost methods for lowering the human intake of As in rural areas are critically evaluated in terms of public acceptance, sustainability and impact on arsenicosis.
Deciphering the impact of land-uses on terrestrial organic matter and mercury inputs to large boreal lakes of central Québec using lignin biomarkers by Matthieu Moingt; Marc Lucotte; Serge Paquet; Bassam Ghaleb (34-48).
To evaluate watershed impacts of anthropogenic activities on terrestrial organic matter (TOM) and total mercury (THg) dynamics in large boreal lake ecosystems, we studied sediment cores retrieved in eight large lakes of Québec (Canada). Two lakes with pristine watersheds were considered as reference lakes and six lakes with watersheds affected by different types of anthropogenic activities (e.g. logging and/or mining activities) were used to illustrate the influence of land-use on TOM and Hg cycling in lakes. A Geographical Information System (GIS) approach was used to correlate the evolution of anthropogenic land-uses from 1979 to 2010 (e.g. logging and mining activities) to TOM and THg contents measured in sediment cores. In each core, THg concentrations gradually increased over the recent years. Using lignin biomarkers, we noticed that the presence of both intense logging and mining activities in the watershed does not necessarily correspond to noticeable changes in the relative amount of terrestrial organic matter (TOM) exported from the watershed to the sediments and by extension to the level of THg measured in sediments. Apparently large-scale watersheds show some “buffering” capacity to land-use disturbance.
The geochemistry during management of lake acidification caused by the rewetting of sulfuric (pH < 4) acid sulfate soils by Luke M. Mosley; Paul Shand; Peter Self; Rob Fitzpatrick (49-61).
Understanding the geochemistry and kinetics of acidification events arising from acid sulfate soils is important to enable effective management and risk assessment. Large-scale exposure and oxidation of acid sulfate soils occurred during a drought in the Lower Lakes (Murray–Darling Basin) of South Australia. We examined the geochemical changes that occurred in one region (Boggy Lake) that experienced surface water acidification and was subsequently neutralised via aerial limestone (CaCO3) dosing and dilution via natural lake refill. Very low pH (< 3) and high concentrations (≈10–1000 mg/L Fe, Al, Mn) of dissolved metals were initially found in surface water. The water chemistry exhibited pH-dependent enhancement of constituents typically associated with acid sulfate soils (SO4, Al and Fe). Geochemical speciation calculations indicated that most (60–80%) of the acidity was present as dissolved metal-sulfate complexes at low pH. X-ray diffraction (XRD) analyses showed that the orange-brown precipitates present after an initial limestone dosing were secondary oxyhydroxysulfate minerals (schwertmannite, jarosite). Further limestone dosing resulted in neutralisation of the pH, reduction in dissolved metal concentrations, dissolution of jarosite and schwertmannite precipitates, and formation of other metal oxyhydroxide phases. The results were consistent with a pE-pH diagram constructed for metal-sulfur geochemistry. Assessment of the measured and simulated (using PHREEQC) pH and Ca/Cl ratio during limestone dosing indicated that only about 25% of the limestone dissolved. XRD analyses suggested this passivation of the limestone was due to coating with gypsum and schwertmannite.
Seasonal variations of rare earths and yttrium distribution in the lowland Havel River, Germany, by agricultural fertilization and effluents of sewage treatment plants by Peter Möller; Andrea Knappe; Peter Dulski (62-72).
REE and Y (REY) distribution in the lowland Havel River passing the Federal State of Berlin, Germany, depends on contributions of point sources such pharmaceutical and high-tech industries, acid water from the open pit lignite mining, and medical application of very stable organic Gd chelates. Another omnipresent dispersed source of REY are water-soluble Ca-phosphates containing micro-amounts of Eu(II)-bearing barite as components of common agricultural fertilizers. After distribution in the field during the cold season (October through March) these Ca-phosphates dissolve and secondary phosphates and calcite precipitate both being enriched in light REE. Heavy REE are preferably exported by runoff together with part of the micro-contaminant barite leading to high Yb/Nd ratios in the Havel water and REY distribution patterns with only small Eu deficits. During the warm season (April through September) light REE together with phosphate are leached from secondary soil minerals by runoff. The micro-component barite is retained in vegetation-covered soil. Thus, REY patterns of Havel water show significant Eu deficits. The high Gd anomalies result from medical applications of Gd-chelates which after urination pass the sewage treatment plants. The seasonal variations of total Gd in the Havel River are artifacts based on seasonal locally varied discharge of effluents from sewage treatment plants. The natural Gd concentration of 8 pmol/l in the northern Havel is enhanced to 3300 pmol/l, when the Havel River leaves Berlin territory. The elimination of phosphate from Lake Tegel water affects the fractionation of REE but not the concentration of total Gd. Although enhanced in total phosphorus (TP), the REE concentrations in the water from the Spree River and the Teltow Canal are less than in the Havel water before their confluence. Only Yb and Lu do not decrease. The contributors of the Havel River are high in total organic carbon (TOC) and dissolved organic carbon (DOC) compared to the Havel water before their indicating that REY are preferentially sorbed by settling organic matter. Applying PHREEQC and assuming that only 10% of TP is present as ortho-phosphate yields that only carbonate complexes are essential.
Chemical and isotopic characteristics of the warm and cold waters of the Luigiane Spa near Guardia Piemontese (Calabria, Italy) in a complex faulted geological framework by Giovanni Vespasiano; Carmine Apollaro; Francesco Muto; Elissavet Dotsika; Rosanna De Rosa; Luigi Marini (73-88).
Waters discharging at the Luigiane Spa come from two different hydrogeological circuits, which are chiefly hosted in the carbonate rocks and Upper Triassic evaporites of two distinct geological units, known as Verbicaro Unit and Cetraro Unit. The first unit contains a cold and relatively shallow aquifer behaving as a sort of piston-flow circuit with high flow rate, whereas the second unit encloses a warm and comparatively deep aquifer acting as a sort of well-mixed reservoir, where the circulation is slower and the rate is lower.Meteoric waters infiltrating along the Coastal Chain at similar elevations (615–670 m asl on average, in spite of considerable uncertainties) recharge both aquifers and, in the first case, acquire heat from rocks through conductive transfer as a consequence of deepening along a fault system and/or crossing between different systems, as suggested by local structural geology. In particular, the warm deeper reservoir has a temperature of ∼60 °C, as indicated by the chalcedony solubility and the Ca–Mg and SO4–F geothermometers, which were specifically calibrated for the peculiar water–rock-interaction (WRI) processes originating the Na–Cl–SO4 high-salinity warm waters that discharge at the Luigiane Spa. The warm deeper reservoir is probably located at depths close to 1.4 km, assuming a geothermal gradient of 33 °C km−1. The water leaving the deep reservoir discharges at the surface at 40.9 ± 3.3 °C after a relatively fast upflow and limited cooling. The upward part of both hydrogeological circuits is controlled by local low- and high-angle fault systems as well as by the tectonic window of Guardia Piemontese, where the Verbicaro Unit crops out and the Cetraro Unit approaches the surface.The reconstruction of this conceptual model has been made possible thanks to the adoption of a “local” approach integrating previously existing and new geological, hydrogeological and geochemical data and including the use of sulfur isotope data. This last technique has proven most important, as it enabled us to recognize the Upper Triassic fingerprint of dissolved sulfate, once the effects of bacterial sulfate reduction had been properly taken into account.
Investigation of off-site airborne transport of lead from a superfund removal action site using lead isotope ratios and concentrations by Michael J. Pribil; Mark A. Maddaloni; Kimberly Staiger; Eric Wilson; Nick Magriples; Mustafa Ali; Dennis Santella (89-94).
Lead (Pb) concentration and Pb isotopic composition of surface and subsurface soil samples were used to investigate the potential for off-site air transport of Pb from a former white Pb processing facility to neighboring residential homes in a six block area on Staten Island, NY. Surface and subsurface soil samples collected on the Jewett White Pb site were found to range from 1.122 to 1.138 for 206Pb/207Pb and 2.393 to 2.411 for 208Pb/207Pb. The off-site surface soil samples collected from residential backyards, train trestle, near site grass patches and background areas varied from 1.144 to 1.196 for 206Pb/207Pb and 2.427 to 2.464 for 208Pb/207Pb. Two soil samples collected along Richmond Terrace, where Jewett site soils accumulated after major rain events, varied from 1.136 to 1.147 for 206Pb/207Pb and 2.407 to 2.419 for 208Pb/207Pb. Lead concentration for on-site surface soil samples ranged from 450 to 8000 ug/g, on-site subsurface soil samples ranged from 90,000 to 240,000 ug/g and off-site samples varied from 380 to 3500 ug/g. Lead concentration and isotopic composition for the Staten Island off-site samples were similar to previously published data for other northeastern US cities and reflect re-suspension and re-mobilization of local accumulated Pb. The considerable differences in both the Pb isotopic composition and Pb concentration of on-site and off-site samples resulted in the ability to geochemically trace the transport of particulate Pb. Data in this study indicate minimal off-site surface transport of Pb from the Jewett site into the neighboring residential area.
REE potential of the Nordkinn Peninsula, North Norway: A comparison of soil and bedrock composition by Julian Schilling; Clemens Reimann; David Roberts (95-106).
Regional-scale, low-density sampling, geochemical surveys using a variety of different sample materials have repeatedly indicated the Nordkinn Peninsula (northern Norway) as a substantial rare earth element (REE) anomaly. Recently, a more detailed soil geochemical survey, covering about 2000 km2 at a sample density of 1 site per 2 km2, was carried out in the area. The new geochemical survey outlined a large area (several hundred km2) where the soil samples contained several hundred and up to over 2000 mg/kg aqua regia extractable REE. In the surroundings of the highest soil anomalies, bedrock samples were collected for a mineralogical and compositional characterisation of the metasedimentary bedrock with focus on the possible economic potential.The REE concentrations obtained for aliquots of bedrock following aqua regia extraction, 4-Acid digestion and Li-borate fusion/decomposition closely match the results from soil pulps after an aqua regia extraction. Total contents for the REE determined in bedrock using the above methods range between 19 and 429 mg/kg, indicating an overall limited economic REE potential and the predominance of the light REE over the heavy REE. In terms of petrography, essentially all the bedrock samples are characterised by the presence of detrital, altered and locally even decomposed allanite (a LREE-incorporating, epidote-group mineral) and minor xenotime (a HREE-incorporating phosphate) while texturally stable REE phases are scarce. It is the poor condition of the REE minerals that makes them prone towards acidic leaching and, given similar results for all of the above preparation methods, even the relatively weak AR extraction dissolved the REE almost completely from the samples. The REE anomaly observed for the soil samples thus rather reflects the state of alteration of the REE phases than ore-grade concentrations in the underlying bedrock.
Characterization and modeling of the cemented sediment surrounding the Iulia Felix glass by Denis M. Strachan; Jarrod V. Crum; Joseph V. Ryan; Alberta Silvestri (107-114).
About 1800 years ago a Roman Corbita sunk off the coast of Italy carrying a barrel of glass cullet to the floor of the Adriatic Sea. Samples of glass cullet and the cemented surrounding sediment have been characterized and the reaction between the glass and the sea water saturated with respect to calcite and dolomite has been modeled. Results from characterization and modeling show that the phase surrounding and cementing together the sediment grains is a high-Mg calcite. We find that the origin of this cementing phase is likely the reaction between the glass and the sea water to form a Mg–silicate, here modeled as sepiolite [Mg4Si6O15(OH)2⋅6(H2O)].
Modeling of oxygen gas diffusion and consumption during the oxic transient in a disposal cell of radioactive waste by Laurent De Windt; François Marsal; Jérôme Corvisier; Delphine Pellegrini (115-127).
The oxic transient in geological radioactive waste disposals is a key issue for the performance of metallic components that may undergo high corrosion rates under such conditions. A previous study carried out in situ in the argillite formation of Tournemire (France) has suggested that oxic conditions could have lasted several years. In this study, a multiphase reactive transport model is performed with the code HYTEC to analyze the balance between the kinetics of pyrite oxidative dissolution, the kinetics of carbon steel corrosion and oxygen gas diffusion when carbon steel components are emplaced in the geological medium. Two cases were modeled: firstly, the observations made in situ have been reproduced, and the model established was then applied to a disposal cell for high-level waste (HLW) in an argillaceous formation, taking into account carbon steel components and excavated damaged zones (EDZ).In a closed system, modeling leads to a complete and fast consumption of oxygen in both cases. Modeling results are more consistent with the in situ test while considering residual voids between materials and/or a water unsaturated state allowing for oxygen gas diffusion (open conditions). Under similar open conditions and considering ventilation of the handling drifts, a redox contrast occurs between reducing conditions at the back of the disposal cell (with anoxic corrosion of steel and H2 production) and oxidizing conditions at the front of the cell (with oxic corrosion of steel). The extent of the oxidizing/reducing front in the disposal cell is strongly dependent on the gas diffusion coefficient in partially saturated zones.
Dissolution kinetics of Devonian carbonates at circum-neutral pH, 50 bar pCO2, 105 °C, and 0.4 M: The importance of complex brine chemistry on reaction rates by Peter Crockford; Kevin Telmer; Melvin Best (128-134).
The dissolution kinetics of carbonate rocks sampled from the Keg River Formation in Northeast British Columbia were measured at 50 bar pCO2 and 105 °C, in both natural and synthetic brines of 0.4 M ionic strength. Natural brines yielded reaction rates of −12.16 ± 0.11 mol cm−2 s−1 for Log R Ca, and −12.64 ± 0.05 for Log R Mg. Synthetic brine yielded faster rates of reaction than natural brines. Experiments performed on synthetic brines, spiked with 10 mmol of either Sr or Zn, suggest that enhanced reaction rates observed in synthetic brines are due to a lack of trace ion interaction with mineral surfaces. Results were interpreted within the surface complexation model framework, allowing for the discrimination of reactive surface sites, most importantly the hydration of the >MgOH surface site. Dissolution rates extrapolated from experiments predict that CO2 injected into the Keg River Formation will dissolve a very minor portion of rock in contact with affected formation waters.
A unified approach to model uptake kinetics of trace elements in complex aqueous – solid solution systems by Bruno M.J. Thien; Dmitrii A. Kulik; Enzo Curti (135-150).
Assessment of a predictive model to describe the migration of major inorganic cations in a Bt soil horizon by Jun Lu; Emmanuel Tertre; Catherine Beaucaire (151-162).
The aim of this study was to test the ability of a previous published model describing the sorption properties of complex solids (Bt soil horizon, sediment) under static conditions (batch mode) to describe sorption data obtained under dynamic conditions. This model assumes that the sorption properties of the multicomponent solid can be described by those of smectites present in the mineralogical assemblage. In our case, the reference smectite is a Wyoming montmorillonite. To test the model, experimental breakthrough curves of some major cations were obtained using a Bt soil horizon in different physico-chemical conditions. The fairly good agreement between the different experimental data sets and predicted breakthrough curves demonstrates that our proposed model can be used to accurately predict ion exchange reactions occurring under dynamic conditions between Na+, Ca2+, and H+ cations in a complex mineralogical assemblage. In addition, this model is also able to accurately predict previously published experimental data obtained with another B soil horizons and using Na+, Ca2+, and Mg2+ as cations. Other models reported from the literature, based either on sorption properties of pure smectites or of complex assemblages, are not able to accurately interpret experimental data proposed in this study motivating our purpose to propose another model. Therefore, our predicted model represents an alternative to models based on the generalized composite approach, which describes the reactivity of a complex material using generic sorption sites for which reactivity is not explicitly related to the properties of the individual phases of the complex material.
Source of groundwater salinity in coastline aquifers based on environmental isotopes (Portugal): Natural vs. human interference. A review and reinterpretation by Paula M. Carreira; José M. Marques; Dina Nunes (163-175).
Environmental stable (δ18O, δ2H, δ13C) and radioactive (3H and 14C) isotopes, together with geochemical data were used to identify the origin of salinization in different environments. Three case studies from sedimentary basins of continental Portugal are presented: (i) two at the Meso-Cenozoic Portuguese southern border (Algarve basin) and (ii) one at the Lower Tagus–Lower Sado basin (central Portugal), with a new data interpretation. Groundwater salinization occurs in all three cases, and may reach values of several grams of Total Dissolved Solids per liter; above accepted limits for drinking water. The source of this high mineralization could be: (a) seawater intrusion (ancient or modern); (b) dissolution from diapiric structures intruding on the aquifer systems; (c) brine dissolution at depth; and (d) evaporation of irrigation water. The results obtained have provided an effective label for seawater and freshwater, to enable seawater intrusion to be traced, as well as the identification of other processes that might be responsible for groundwater salinization, such as salt minerals dissolution and ion exchange.
Seasonal cycling and mass-loading of dissolved metals and sulfate discharging from an abandoned mine site in northern Canada by Michael C. Moncur; Carol J. Ptacek; Masaki Hayashi; David W. Blowes; S. Jean Birks (176-188).
Drainage from sulfide-rich mine waste can contaminate surface waters directly by surface runoff and indirectly through groundwater discharge. The Camp Lake watershed, located in northern Manitoba, receives both direct and indirect drainage from an abandoned tailings impoundment which has resulted in severely degraded water quality. Sulfide oxidation has been occurring for over 70 years at an abandoned mine site in this watershed, resulting in high concentrations of oxidation products in the surface water and groundwater discharging from the two tailings impoundments, both of which flow into the adjacent Camp Lake. A 5-year hydrological and geochemical sampling program was conducted at this small Precambrian shield lake prior to the initiation of remedial actions, to identify the seasonal distribution and mass-loading of dissolved metals and sulfate discharging from Camp Lake to the downstream Kississing Lake. Weekly and biweekly sampling of outflow from the lake clearly showed a seasonal cycle of dissolved metals, sulfate and pH. During months when the lake was ice-covered, the discharge water had a neutral pH with low concentrations of dissolved metals and sulfate similar to background concentrations. At spring melt the discharge sampled at Camp Weir revealed abrupt increases in dissolved metal and sulfate concentrations and decreases in pH to values that remained relatively constant until fall freeze-up, when dissolved metal concentrations and pH returned to winter values. The annual and interannual variations in loadings measured at Camp Weir are different from those measured at the two streams feeding Camp Lake showing the contribution of groundwater flow through the tailings to SO4 and metal loadings to the lake during dry years and the potential for significant loadings due to flushing of the thick unsaturated zone in the tailings during relatively wet years. The abrupt changes in pH, metal and sulfate concentrations and the timing of these changes with the appearance and disappearance of ice-cover on the lakes suggests a combination of physical and geochemical controls related to shifts in sources of water, mixing and changes in solubility. Despite fairly low average annual metal concentrations measured in Camp Lake discharge, concentrations of Zn and Cu were elevated above background in bottom sediments of Kississing Lake in a zone extending 9.5 km2 from the location of Camp Lake inflow. These results show the importance of the open-water period in controlling metal discharge from mine-impacted areas, and demonstrate that even when average annual concentrations are only slightly elevated, long-term metal loadings can have a significant impact on the receiving aquatic systems.
Mineralogy and geochemistry of alteration induced by hydrocarbon seepage in an evaporite formation; a case study from the Zagros Fold Belt, SW Iran by Majid H. Tangestani; Khadijeh Validabadi (189-195).
Leaking hydrocarbon and associated fluids produce a reduced environment which initiates diagenetic reactions in the rocks and soils overlying hydrocarbon reservoirs. This article introduces mineralogical and geochemical alterations induced by hydrocarbon seepages in the Gachsaran evaporite Formation, Masjed Soleiman, Zagros Folded Belt, SW Iran, using various geochemical methods. X-ray diffraction (XRD) analysis revealed two dominant groups of mineralogical changes in the anomalous zone including: (1) spotty occurrences of jarosite, natroalunite, and sulfur; (2) simultaneous decrease of sulfate and increase of carbonate. The δ13C values of carbonate phases vary between −8.9‰ and −32.3‰ which support a partial contribution of hydrocarbon in these phases. pH measurements of rock samples indicated that hydrocarbon seepage causes decrease of pH, though not very significantly. However, pH in areas that show ongoing sour hydrocarbon seepage among rarely pure gypsum beds decrease to −0.09 which produces acid sulfate soil. Trace element concentrations indicated that Cr, Co, Ni, V, Cd, U, Cu, and Zn increase in various patterns around the hydrocarbon seepages excepting the areas with very low pH.
A review of high arsenic groundwater in Mainland and Taiwan, China: Distribution, characteristics and geochemical processes by Huaming Guo; Dongguang Wen; Zeyun Liu; Yongfeng Jia; Qi Guo (196-217).
China is a typical high-As region, where 20 provinces have high As groundwaters among 34 provinces. These groundwaters usually occur in both arid–semiarid inland basins and river deltas. In the inland basins, mainly distributed in the northwest of China, shallow groundwaters usually have high As concentrations in alluvial lacustrine or lacustrine sediment aquifers, while high As groundwater mainly occurs in fluvial–marine sedimentary aquifers in the river deltas, which have been affected by transgression. In both the inland basins and the river deltas, high As groundwaters, mainly occurring in reducing conditions, are characterized by high Fe and Mn concentrations, high pH and HCO 3 - concentration, and relatively low NO 3 - and SO 4 2 - concentrations. Although As contents are well correlated to Fe/Mn contents in the aquifer sediments, groundwater As concentrations are generally independent of sediment As contents. Redox processes, microbe-related reduction, and desorption processes are the major geochemical processes for As enrichment in groundwaters. In reducing conditions, both reductive dissolution of Fe oxides and reductive desorption of As are believed to result in As mobilization, which would be catalyzed by indigenous microbes. Although decomposition of the low-molecular weight organic matter during microbe metabolization would also release the colloid-bound As into groundwater, the cycling of colloidal As still needs to be further investigated during redox processes. Besides, high pH and high HCO 3 - lead to As desorption from adsorption sites in the aquifer systems. However, the contribution of competitive desorption to high As concentrations is still unknown and remains to be discovered, relative to reductive dissolution of Fe oxides, especially in the inland basins.
Rare earth element mineralogy and geochemistry in a laterite profile from Madagascar by Alfons Berger; Emilie Janots; Edwin Gnos; Robert Frei; Felix Bernier (218-228).
In this study, rare earth element (REE) distribution has been investigated in a weathering profile from central Madagascar. Combination of bulk rock geochemical data (elements and isotopes) with mineral characterization reveals a remarkable evolution of the REE abundances and REE-minerals in the vertical weathering profile. In the fresh tonalite (bedrock), REE + Y concentrations are typical of granitoids (299–363 ppm) and the main REE-minerals are allanite and chevkinite. In the C-horizon (saprolite), primary REE-minerals disappear and REEs are transported via fluid to precipitate rhabdophane group minerals in cracks and pores. The presence of sulfate ligands, produced by sulfide oxidation, may be responsible for the REE speciation, as suggested by the composition of the secondary REE-minerals. Rhabdophane group minerals contain up to 9 wt% SO3 and 7 wt% CaO, indicating a mixture between rhabdophane sensu stricto, (REE)PO4·H2O, and tristamite, (Ca,U,Fe(III))(PO4,SO4)·2H2O. Due to intense Ca-leaching, rhabdophane disappears and Al-phosphates (alunite–jarosite group) are found in the soil. Cerianite (Ce(IV)O2) also precipitates in the B-horizon of the soil.Mass transfer calculations based on immobile Ti indicate significant REE leaching in A-horizon with preferential leaching of the heavy REE. REEs accumulate partly in the B-horizon. The uniform Nd isotope compositions and the constant proportion of immobile elements do not reveal external input. In the B-horizon, total REE + Y reach 2194 ppm with high Ce concentrations (1638 ppm; 9*Cebedrock) compared to other REE (3–4*REEbedrock). Tetravalent Ce state is dominant in the B-horizon and requires oxidizing conditions that likely account for the accumulation of redox-sensitive elements in B-horizon (e.g., Mn, Fe, Co). Under oxidizing conditions, cerianite precipitation causes a Ce fractionation from other trivalent REE. In comparison to the ion adsorption clay of southern China, preferential heavy REE enrichment was not observed in the weathering profile.Another remarkable peculiarity of the studied profile is the occurrence of Gd2SO6 grains. The discovery of this new mineral demonstrates that a natural process exists that that can fractionate REE to such an extent to produce a pure gadolinium end-member mineral. An understanding of such a mechanisms is crucial for the REE geochemistry of low temperature alteration processes as well as for the formation of REE ore deposits or industrial processing.