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1.
《Chemical Geology》2007,236(3-4):199-216
The chemical characteristics of freshwaters draining the silicate rocks in the northern part of Okinawa Island were studied to understand solute generation processes, and to determine rates of chemical weathering and CO2 consumption. It was observed that the water chemistry is highly influenced by marine aerosols, contributing more than 60% of total solute. Significant positive correlations observed for chloride versus dissolved silica and chloride versus bicarbonate suggest a strong influence of evapotranspiration on the seasonality of solute concentration. It was also found that chemical weathering has been highly advanced in which the dominant kaolinite minerals are being gibbsitized. Carbonic acid was found to be the major chemical weathering agent, releasing greater than 80% of weathering-derived dissolved cations and silica while the remaining portion was attributed to weathering by sulfuric acid generated via oxidation of pyrite contained in the rocks. The flux of basic cations, weathering-derived silica and CO2 consumption were relatively high due to favourable climatic condition, topography and high rate of mechanical erosion. Silicate weathering rates for basic cations were estimated to be 6.7–9.7 ton km− 2 y− 1. Carbon dioxide consumed by silicate weathering was 334–471 kmol km− 2 y− 1 which was slightly higher than that consumed by carbonate weathering. In general, divalent cations (Mg and Ca) and bicarbonate alkalinity derived from carbonate dissolution were higher than those from silicate weathering. As a consequence, the evolution of chemical species in the freshwaters of northern area of Okinawa Island to a large extent could be explained by mixing of two components, characterized by waters with Na+ and Cl− as predominant species and waters enriched with Ca2+ and HCO3−. 相似文献
2.
The potential for metal release associated with CO2 leakage from underground storage formations into shallow aquifers is an important consideration in assessment of risk associated with CO2 sequestration. Metal release can be driven by acidification of groundwaters caused by dissolution of CO2 and subsequent dissociation of carbonic acid. Thus, acidity is considered one of the main drivers for water quality degradation when evaluating potential impacts of CO2 leakage. Dissolution of carbonate minerals buffers the increased acidity. Thus, it is generally thought that carbonate aquifers will be less impacted by CO2 leakage than non-carbonate aquifers due to their high buffering potential. However, dissolution of carbonate minerals can also release trace metals, often present as impurities in the carbonate crystal structure, into solution. The impact of the release of trace metals through this mechanism on water quality remains relatively unknown. In a previous study we demonstrated that calcite dissolution contributed more metal release into solution than sulfide dissolution or desorption when limestone samples were dissolved in elevated CO2 conditions. The study presented in this paper expanded our work to dolomite formations and details a thorough investigation on the role of mineral composition and mechanisms on trace element release in the presence of CO2. Detailed characterization of samples from dolomite formations demonstrated stronger associations of metal releases with dissolution of carbonate mineral phases relative to sulfide minerals or surface sorption sites. Aqueous concentrations of Sr2+, CO2+, Mn2+, Ni2+, Tl+, and Zn2+ increased when these dolomite rocks were exposed to elevated concentrations of CO2. The aqueous concentrations of these metals correlate to aqueous concentrations of Ca2+ throughout the experiments. All of the experimental evidence points to carbonate minerals as the dominant source of metals from these dolomite rocks to solution under experimental CO2 leakage conditions. Aqueous concentrations of Ca2+ and Mg2+ predicted from numerical simulation of kinetic dolomite dissolution match those observed in the experiments when the surface area is three to five orders of magnitude lower than the surface area of the samples measured by gas adsorption. 相似文献
3.
The detailed hydro-chemical study of meltwater draining from Khangri glacier Arunachal Pradesh has been carried out to evaluate the major ion chemistry and weathering processes in the drainage basin. The investigative results shows that the meltwater is almost neutral to slightly acidic in nature with Mg–HCO3-dominated hydro-chemical facies. In glacial meltwater, Ca+?2 is the most dominated cation followed by Mg+2, Na+, and K+, while HCO3? is the most dominant anion followed by SO42?, NO3?, and Cl?. The dominant cations such as Ca+2 and Mg+2 show a good relation with the minerals abundance of the rocks. Calcite (CaCO3) and biotite [K(Mg,Fe)3AlSi3O10(F,OH)2] are the most abundant minerals in the deformed carbonate-rich metasedimentary rocks near to the snout with some K feldspar (KAlSi3O8) and quartz (SiO2). This suggests Ca+2 have definitely entered into the water due to the dissolution of calcite and Ca feldspar (CaAl2Si2O8), while one of the source of Mg+2 is biotite. Na feldspar (NaAlSi3O8) has contributed towards the availability of sodium ion, while potassium ion is derived from the chemical weathering of K feldspar and biotite. The chemical weathering is the foremost mechanism controlling the hydro-chemistry of the Khangri glacier because of the least anthropogenic interferences. The mineralogy of surrounding rocks is studied to understand better, the rock–water interaction processes, and their contribution towards ionic concentration of meltwater. The meltwater discharge and individual ion flux of the catchment area have also been calculated, to determine the ionic denudation rate for the ablation season. The high elemental ratio of (Ca?+?Mg)/(Na?+?K) (7.91?±?0.39 mg/l) and low elemental ratio of (Na?+?K)/total cations (0.11?±?0.004) indicate that the chemical composition of meltwater is mainly controlled by carbonate weathering and moderately by silicate weathering. The scatter plot result between (Ca?+?Mg) and total cations confirms that carbonate weathering is a major source of dissolved ions in Khangri glacier meltwater. In addition, the statistical analysis was also used to determine the correlation between physical parameters of glacier meltwater which controlled the solute dynamics. 相似文献
4.
The short term climatic sensitivity of carbonate and silicate weathering fluxes: Insight from seasonal variations in river chemistry 总被引：2，自引：0，他引：2
Edward T. Tipper Mike J. Bickle A. Joshua West Hazel J. Chapman 《Geochimica et cosmochimica acta》2006,70(11):2737-2754
Large seasonal variations in the dissolved load of the headwater tributaries of the Marsyandi river (Nepal Himalaya) for major cations and 87Sr/86Sr ratios are interpreted to result from a greater dissolution of carbonate relative to silicate at high runoff. There is up to a 0.003 decrease in strontium isotope ratios and a factor of 3 reduction in the Si(OH)4/Ca ratio during the monsoon. These variations, in small rivers sampling uniform lithologies, result from a different response of carbonate and silicate mineral dissolution to climatic forcing. Similar trends are observed in compiled literature data, from both Indian and Nepalese Himalayan rivers. Carbonate weathering is more sensitive to monsoonal runoff because of its faster dissolution kinetics. Silicate weathering increases relative to carbonate during the dry season, and may be more predominant in groundwater with longer water-rock interaction times. Despite this kinetic effect, silicate weathering fluxes are dominated by the monsoon flux, when between 50% and 70% of total annual silicate weathering flux occurs. 相似文献
5.
蚯蚓肠道内小分子有机酸与摄入的土壤矿物相互作用,加速矿物溶解。摄入的土壤在蚯蚓肠道内平均停留时间约为12 h,不足以使土壤矿物产生显著的溶解特征,因此这一过程难以在蚯蚓体内进行评估。本研究通过体外实验控制pH值和有机酸浓度,模拟蚯蚓肠道中有机酸对土壤中常见矿物的溶解反应,探讨了方解石和钾长石在蚯蚓肠道环境中的初始溶解动力学。研究发现,矿物在混合有机酸中的溶解速率比在纯水中高一个数量级,说明有机配体和质子促进了矿物溶解。溶解速率及粒度分析表明,方解石(CaCO3)溶解速率不受溶解过程中粒度变化的影响,而钾长石(KAlSi3O8)粒度在溶解期间未出现显著变化。在此基础上,采用初始速率法模拟了钾长石的初始溶解动力学,计算得出的溶解速率表明钾长石在溶解初期主要为表面K~+的释放。使用缩核模型(shrink core model)和Hixson-Crowell模型对方解石溶解过程进行动力学解析,发现方解石的溶解主要受溶液中反应物内扩散的速率影响。这定量描述了两种矿物在有机酸溶液和纯水中的溶解差异。现有研究表明,有机配体和质子协同促... 相似文献
6.
Water and bed sediment samples collected from the Damodar River and its tributaries were analysed to study elemental chemistry
and suspended load characteristics of the river basin. Na and Ca are the dominant cations and HCO3 is the dominant anion. The water chemistry of the Damodar River basin strongly reflects the dominance of continental weathering
aided by atmospheric and anthropogenic activities in the catchment area. High concentrations of SO4 and PO4 at some sites indicate the mining and anthropogenic impact on water quality. The high concentration of dissolved silica,
relatively high (Na+K)/TZ+ ratio (0.2–0.4) and low equivalent ratio of (Ca+Mg)/(Na+K) indicate that dissolved ions contribute significantly to the weathering
of aluminosilicate minerals of crystalline rocks. The seasonal data show a minimum ionic concentration in the monsoon season,
reflecting the influence of atmospheric precipitation on total dissolved solids contents. The suspended sediments show a positive
correlation with discharge and both discharge and suspended load reach their maximum value during the monsoon season. Kaolinite
is the mineral that is possibly in equilibrium with the water. This implies that the chemistry of the Damodar River water
favours kaolinite formation. The concentration of heavy metals in the finer size fraction (<37 μ m) is significantly higher
than the bulk composition. The geoaccumulation index values calculated for Fe, Mn, Zn, Ni and Cr are well below zero, suggesting
that there is no pollution from these metals in Damodar River sediments.
Received: 21 January 1998 · Accepted: 4 May 1998 相似文献
7.
Lead- and Pb-As-jarosites are minerals common to acidic, sulphate-rich environments, including weathering zones of sulphide ore deposits and acid rock or acid mine drainage (ARD/AMD) sites, and often form on or near galena. The structures of these jarosites are based on linear tetrahedral-octahedral-tetrahedral (T-O-T) sheets, comprised of slightly distorted FeO6 octahedra and SO42− (-AsO43− in Pb-As-jarosites) tetrahedra. To better understand the dissolution mechanisms and products of the break down of Pb- and Pb-As-jarosite, preliminary batch dissolution experiments were conducted on synthetic Pb- and Pb-As-jarosite at pH 2 and 20 °C, to mimic environments affected by ARD/AMD, and at pH 8 and 20 °C, to simulate ARD/AMD environments recently remediated with slaked lime (Ca(OH)2). All four dissolutions are incongruent. Dissolution of Pb-jarosite at pH 2 yields aqueous Pb, Fe and SO42−. The pH 8 Pb-jarosite dissolution yields aqueous Pb, SO42− and poorly crystalline Fe(OH)3, which does not appear to resorb Pb or SO42−, possibly due to the low solution pH (3.44-3.54) at the end of the experiment. The pH 2 and 8 dissolutions of Pb-As-jarosite result in the formation of secondary compounds (poorly crystalline PbSO4 for pH 2 dissolution; poorly crystalline PbSO4 and Fe(OH)3 for pH 8 dissolution), which may act as dissolution inhibitors after 250 to 300 h of dissolution. In the pH 2 dissolution, aqueous Fe, SO42− and AsO43− also form, and in the pH 8 dissolution, Fe(OH)3 precipitates then subsequently resorbs aqueous AsO43−. The dissolutions probably proceed by preferred dissolution of the A- and T-sites, which contain Pb, and SO42− and AsO43−, respectively, rather than Fe, which is sterically remote, within the T-O-T Pb- and Pb-As-jarosite structures. These data provide the foundation necessary for further, more detailed investigations into the dissolution of Pb- and Pb-As-jarosites. 相似文献
8.
几种镁铁矿物平衡共生的成分标志 总被引：3，自引：0，他引：3
本文以晋冀内蒙边界地区变质岩中主要镁铁质造岩矿物辉石,闪石为例,阐明镁铁矿物平衡共生的一条普遍规律,即平衡共生的正辉石,钙闪石之间有相同的镁铁比,其分配系数等于或接近于1.引用冀东,吉南,泰山和南极凯西站等地的实例,论证了此规律同时存在于黑云母与钙闪石,黑云母与正辉石之间,也存在于辉长岩和花岗岩等岩浆岩的橄榄石和正辉石,角闪石与黑云母等矿物对之间。这是判断镁铁矿物是否平衡的成分标志。 相似文献
9.
Stefán Arnórsson 《Geochimica et cosmochimica acta》1984,48(12):2489-2502
Germanium concentrations in geothermal waters in Iceland lie mostly in the range 2–30 ppb. There is an overall positive relation between the germanium content of the water and its temperature. Most of the germanium occurs as Ge(OH)?5in solution but Ge(OH)4 may also be present in significant amounts in saline waters when above 200°C. Evidence indicates that aqueous germanium concentrations are controlled by exchange reactions where it substitutes for silica in silicates and iron in sulphides. It is the rate of dissolution and the relative abundance of the alteration minerals which take up germanium to a variable extent that ultimately fix Ge(OH)4 concentrations in the water. This, together with water pH, fixes total dissolved germanium. It is mostly the primary rock composition that dictates the relative abundance of the alteration minerals. Conductive cooling in upflow zones favours removal of germanium from solution. During the initial stages of boiling of rising hot water dissolution is enhanced but precipitation at later stages.Thermodynamic data of various aqueous germanium species and several minerals are summarized and dissociation constants and solubilities estimated at elevated temperatures using available predictive methods. 相似文献
10.
Numerous previous studies indicate that several different authigenic aluminosilicates form in the oceans. In this study we show, using dissolved Al distributions in sediments and waters from the nearshore regions of the East China Sea, that the process of aluminosilicate formation probably begins rapidly upon contact of detrital clays with seawater. Statistical analyses of dissolved Al-Si-H+ relations in surface sediments indicate that the minerals forming in East China Sea sediments low in dissolved Fe are dioctahedral chlorites with an average composition EX0.91Mg0.77Al5.0Si2.7O10(OH)8 (where EX = exchangeable + 1 cation). This composition is also consistent with dissolved Al and Si measurements as a function of salinity in turbid overlying waters. Results suggest a dissolution—reprecipitation mechanism for clay mineral reconstitution. This mechanism can help to explain why different authigenic clays are found in different areas of the oceans. In the East China Sea the total amount of authigenic clays present must constitute a very minor fraction of the bottom sediments. Thus, the formation of these minerals has a relatively small impact upon dissolved Si distributions. Clay mineral reconstitution in nearshore regions may provide a mechanism for buffering sediments and overlying waters with respect to pH, as the composition of minerals formed should be a direct function of the H+ activity in the surrounding environment. 相似文献
11.
Asit Kumar Batabyal 《Environmental Earth Sciences》2017,76(7):285
The hydrogeochemistry of groundwater in rural parts of Birbhum district, West Bengal, India, has been studied to understand the contaminants and prime processes involved in their enrichment with a focus on F? concentration. The lithological units consist of Quaternary alluviums with underlying Rajmahal basaltic rocks of Middle Jurassic age. Groundwater occurs in the alluviums, weathered residuum and fracture zone of Rajmahal rocks. Studies show elevated concentration of Cl?, SiO2, Fe and F?; excess Cl? is attributed to anthropogenic inputs, SiO2 is ascribed to high degree of weathering of silica rich host rocks, and high Fe is due to the interaction of water with Fe-rich sediments under reducing condition. The F? concentration is found high (>1.20 mg/L) mainly in water from Rajmahal rocks revealing a lithological control on F? enrichment. The weathering of silicates and ion exchange are the leading controlling processes for major ions in groundwater. The F? enrichment is due to the dissolution of F?-bearing minerals and perhaps also through anion exchange (OH? for F?) on clay minerals at high alkaline conditions; precipitation of CaCO3 favours CaF2 dissolution leading to elevated F? concentration. CaHCO3, the dominant water type, contains low F? while NaHCO3 and NaCl types exhibit high F? concentrations. Among the three spatial associations, Cluster-1 and Cluster-2 are CaHCO3 type; Cluster-3 shows NaHCO3 and NaCl waters with low Ca2+ and Mg2+ and high Na+ contents. Cluster-1 and Cluster-2 waters are, in general, drinkable barring the elevated Fe content, while Cluster-3 water is unsafe for drinking due to the high F? concentration. 相似文献
12.
Biotite dissolution experiments were carried out to better understand the dissolution kinetics and Fe behavior under low O2 conditions, and to give an insight into the Precambrian weathering. Mineral dissolution with a continuous flow-through reactor was employed at 25 °C for up to 65 days varying partial pressure of atmospheric oxygen (PO2), pH (6.86 and 3.01) and Fe content in mineral (1.06 and 0.11 mol of Fe per O10(OH,F)2 for biotite and phlogopite, respectively) independently for the examination of their effects on biotite dissolution. Low PO2 conditions were achieved in a newly developed glove box (PO2 ? 6 × 10−4 atm; referred to as anoxic conditions), which was compared to the present, ambient air conditions (0.2 atm of PO2; oxic conditions). The biotite dissolution rate was slightly faster under anoxic conditions at pH 6.86 while it was not affected by PO2 at pH 3.01. There was no direct effect of Fe content on dissolution rate at pH 6.86 while there was a small difference in dissolution rate between biotite and phlogopite at pH 3.01. The 1.5 order-of-magnitude faster release rate of Fe under anoxic conditions for biotite dissolution at pH 6.86 resulted from the difference in ratio of Fe3+ precipitates remaining in the reactor to Fe dissolved (about 60% and 100% under anoxic and oxic conditions, respectively), which is caused mainly by the difference in PO2. The results infer that the Fe2+ and Fe3+ contents in the Paleoproterozoic paleosols, fossil weathering profiles, are reflected by atmospheric oxygen levels at the time of weathering. 相似文献
13.
The study presents composition data of 87 surface water samples from high alpine catchments of the Zermatt area (Swiss Alps). The investigated area covers 170 km2. It was found that the surface runoff acquires the dissolved solids mostly by reaction of precipitation water with the minerals of the bedrock. Total dissolved solids (TDS) vary from 6 to 268 mg L?1. All collected water shows a clear chemical signature of the bedrock mineralogy. The contribution of atmospheric input is restricted to small amounts of ammonium nitrate and sodium chloride. NH4 is a transient component and has not been detected in the runoff. Evaporation is not a significant mechanism for TDS increase in the Zermatt area. The chemical composition of the three main types of water can be related to the mineralogy of the dominant bedrock in the catchments. Specifically, Ca-HCO3 (CC) waters develop from metamorphic mafic rocks and from carbonate-bearing schists. Mg-HCO3 water originates from serpentinites and peridotites. Ca-SO4 (CS) waters derive from continental basement rocks such as pyrite-rich granite and gneiss containing oligoclase or andesine. The collected data suggest that, together with reaction time, modal sulfide primarily controls and limits TDS of the waters by providing sulfuric acid for calcite (CC waters) and silicate (CS waters) dissolution. If calcite is present in the bedrock, its dissolution neutralizes the acid produced by sulphide weathering and buffers pH to near neutral to weakly alkaline conditions. If calcite is absent, the process produces low-pH waters in gneiss and granite catchments. The type of bedrock and its mineral assemblage can be recognized in water leaving very small catchments of some km2 area. The large variety of water with a characteristic chemical signature is an impressive consequence of the richly diverse geology and the different rock inventory of the local catchments in the Zermatt area. 相似文献
14.
River water composition (major ion and 87Sr/86Sr ratio) was monitored on a monthly basis over a period of three years from a mountainous river (Nethravati River) of southwestern India. The total dissolved solid (TDS) concentration is relatively low (46 mg L−1) with silica being the dominant contributor. The basin is characterised by lower dissolved Sr concentration (avg. 150 nmol L−1), with radiogenic 87Sr/86Sr isotopic ratios (avg. 0.72041 at outlet). The composition of Sr and 87Sr/86Sr and their correlation with silicate derived cations in the river basin reveal that their dominant source is from the radiogenic silicate rock minerals. Their composition in the stream is controlled by a combination of physical and chemical weathering occurring in the basin. The molar ratio of SiO2/Ca and 87Sr/86Sr isotopic ratio show strong seasonal variation in the river water, i.e., low SiO2/Ca ratio with radiogenic isotopes during non-monsoon and higher SiO2/Ca with less radiogenic isotopes during monsoon season. Whereas, the seasonal variation of Rb/Sr ratio in the stream water is not significant suggesting that change in the mineral phase being involved in the weathering reaction could be unlikely for the observed molar SiO2/Ca and 87Sr/86Sr isotope variation in river water. Therefore, the shift in the stream water chemical composition could be attributed to contribution of ground water which is in contact with the bedrock (weathering front) during non-monsoon and weathering of secondary soil minerals in the regolith layer during monsoon. The secondary soil mineral weathering leads to limited silicate cation and enhanced silica fluxes in the Nethravati river basin. 相似文献
15.
《Applied Geochemistry》2002,17(5):583-603
This study identifies and quantifies the water–rock interactions responsible for the composition of 25 spring waters, and derives the weathering rates of rock-forming minerals in a complex of petrologic units containing ultramafics, amphibolites, augengneisses and micaschists. Bulk chemical analyses were used to calculate the mineralogical composition of these rocks; the composition of the rock-forming minerals were determined by microprobe analyses. The soils developed on augengneisses and micaschists contain predominantly halloysite; on the other units mixtures of halloysite and smectites. The mineralogical and chemical data on rocks and soils are essential for writing the proper weathering reactions and for solving mole balances between the amounts of weathered primary minerals and secondary products formed (soils and solutes in groundwater). Ground waters emanating in springs were collected in 3 consecutive seasons, namely late Summer, Winter and Spring, and analyzed for major components. Using an algorithm based on mole and charge balance equations, the average concentrations of the solutes were linked with a combination of possible weathering reactions. To sort out the best match of weathering reactions and the concomitantly generated water composition, the results were checked against the limiting condition of similarity between the predicted and actual clay mineral abundance in the soils. Having selected the best-fit weathering reactions, the mineral weathering rates could also be calculated by combining the median discharge rates and recharge areas of the springs and normalizing the rates by the mineral abundance. For the one case—plagioclase—for which comparison with published results was possible, the results compare favorably with rates calculated by other groups. For the most abundant primary minerals the following order of decreasing weathering rates was found (in moles/(ha·a·%mineral)): forsterite (485) > clinozoisite (114) > chlorite (49) > plagioclase (45) > amphibole (28). In as far as this order differs from commonly used orders of weatherability, this has to be due to differences in the hydrologic regime within this area and between this and other case studies. As additional objective, the authors wanted to explain the effects of contributions by sources other than water-rock interactions. The latter processes are coupled with acquisition of carbonate alkalinity and dissolved silica. Contributions by sources other than water–rock interactions are manifest by the Cl−, SO2−4 and NO−3 concentrations. It was possible to approximate the contribution of atmospheric deposition. More importantly, knowledge of the application and composition of fertilizers enabled assessment of the effects of farming on the composition of ground waters emanating in the springs. It was also possible to estimate how selective uptake of nutrients and cations by vegetation as well as ion-exchange processes in the soil modified the spring water composition. Using this rather holistic approach, it is possible to satisfactorily explain how spring waters, in this petrologically and agriculturally diverse area, acquired their composition. 相似文献
16.
Laboratory studies of the weathering of sulphide ores have centred around using samples of ore as electrodes and accelerating the weathering processes by passing an electric current. The results of reacting 19 different ore types under varying conditions are compared with data from co-precipitating, Fe and Cu, Fe and Ni, Fe and Zn, Fe and Co, and Fe and Pb, over a pH range from 2 to 11. An electrochemical cell specially designed to fit onto an optical microscope has allowed direct observation of the changes in sulphide mineral grains as they are anodically weathered.These experiments are used to demonstrate that the pH of the environment during the weathering of sulphides to sulphates is the most important parameter in determining the initial gossan minerals that form. Factors that will cause the pH to be high are buffering from gangue and wallrock minerals, low iron content in the sulphide and a high metal to sulphur ratio in the sulphide. A low pH is favoured by the converse, namely a sulphide sufficiently massive to override the buffering effects of the wallrocks and any gangue minerals present, a high iron content in the sulphide and a low metal to sulphur ratio in the sulphide.Two mechanisms of iron hydrolysis dominate the weathering processes where iron is a major metal being released from a sulphide.
- 1. (1) The high pH process. Where there is sufficient buffering for the pH to remain at or above 7, most of the base metals including ferrous iron will be hydrolysed and pyroaurite type of minerals form for Ni, Zn and Co, while mixed Fe-Cu hydroxycarbonates and hydroxysulphates form for Cu, and mixed iron lead hydrocarbonates form for Pb. The iron is located in these initial compounds as a green rust where it is effectively bound as ferrous hydroxide. Subsequent oxidation of this hydroxide produces no further acid. 4Fe(OH)2 + O2 + 2H20 → 4Fe(OH)3
- 2. (2) The low pH process. Where the buffering is insufficient and the pH is below 7, even though some of the ferrous iron will have precipitated as an equivalent to Fe(OH)2, the solubility is such that sufficient Fe2+ will remain in solution so that further oxidation will produce acid. 4Fe2+ + O2 + 1OH2O → Fe(OH)3 + 8H+ This acid will bring more of the Fe2+ into solution to create more acid and the pH will gradually fall even further, so that the gossan forming environment will be at a pH less than 5 and may be as low as 3. At these low pH values, the base metals are soluble and not prone to co-precipitation or adsorption with the gossan minerals. Only elements present in solution as anions, such as Se, As, Mo and Sb, are likely to be bound into gossans forming at low pH.
17.
Geochemistry of soil, soil water, and soil gas was characterized in representative soil profiles of three Michigan watersheds. Because of differences in source regions, parent materials in the Upper Peninsula of Michigan (the Tahquamenon watershed) contain only silicates, while those in the Lower Peninsula (the Cheboygan and the Huron watersheds) have significant mixtures of silicate and carbonate minerals. These differences in soil mineralogy and climate conditions permit us to examine controls on carbonate and silicate mineral weathering rates and to better define the importance of silicate versus carbonate dissolution in the early stage of soil-water cation acquisition.Soil waters of the Tahquamenon watershed are the most dilute; solutes reflect amphibole and plagioclase dissolution along with significant contributions from atmospheric precipitation sources. Soil waters in the Cheboygan and the Huron watersheds begin their evolution as relatively dilute solutions dominated by silicate weathering in shallow carbonate-free soil horizons. Here, silicate dissolution is rapid and reaction rates dominantly are controlled by mineral abundances. In the deeper soil horizons, silicate dissolution slows down and soil-water chemistry is dominated by calcite and dolomite weathering, where solutions reach equilibrium with carbonate minerals within the soil profile. Thus, carbonate weathering intensities are dominantly controlled by annual precipitation, temperature and soil pCO2. Results of a conceptual model support these field observations, implying that dolomite and calcite are dissolving at a similar rate, and further dissolution of more soluble dolomite after calcite equilibrium produces higher dissolved inorganic carbon concentrations and a Mg2+/Ca2+ ratio of 0.4.Mass balance calculations show that overall, silicate minerals and atmospheric inputs generally contribute <10% of Ca2+ and Mg2+ in natural waters. Dolomite dissolution appears to be a major process, rivaling calcite dissolution as a control on divalent cation and inorganic carbon contents of soil waters. Furthermore, the fraction of Mg2+ derived from silicate mineral weathering is much smaller than most of the values previously estimated from riverine chemistry. 相似文献
18.
Yuanzhi Tang 《Geochimica et cosmochimica acta》2011,75(17):4951-4962
Siderite (FeCO3) is an important reduced phase iron mineral and end product of bacteria anaerobic respiration. This study addresses its dissolution behavior in the presence of the oxidant chromate, which is a common environmental contaminant. Macroscopic dissolution experiments combined with microscopic observations by atomic force microscopy show that at pH < 4.5 the dissolution rate with chromate is slower than that in control solution without chromate. Isolated deep dissolution pits and clustered shallow pits occur simultaneously with surface precipitation. The implication is that the surface precipitate inhibits further dissolution. For 5 < pH < 9.5, the slowest dissolution and the fastest precipitation rates are observed, both at edge steps and on terraces. For pH > 10, the dissolution rate in the presence of chromate exceeds that of the control, plausibly due to electron transfer facilitated by [Fe3+(OH)4]-. Dissolution and re-precipitation of round hillocks are observed. X-ray photoelectron spectroscopy indicates the presence of Cr(III) as well as reaction products in a hydroxide form. Based on the redox reaction mechanism, macroscopic dissolution behavior, and previous studies on the reaction products of Fe(II) with Cr(VI), we propose the formation of a low solubility nano-sized Cr(III)-Fe(III)-hydroxide as the surface precipitate. Results from this study provide a basis for understanding and quantifying the interactions between reduced-iron minerals and aqueous-phase oxidants. 相似文献
19.
Iron silicate minerals (bronzite, fayalite), exposed to aqueous dissolution in the laboratory for up to 60 days at room temperature and pH 1, 1.5, and 6, have been studied for evidence of changes in surface composition, using XPS, and these results compared with those obtained from solution chemical analysis. In the absence of dissolved O2 or at low pH (1–1.5) dissolution proceeds congruently after the initial formation of a thin (<10 Å) protonated surface layer depleted in Fe relative to Si. This layer is unstable and does not grow with time as attested to by long term congruent dissolution and by the formation of an amorphous silica surficial breakdown product at pH 1 and 1.5. In bronzite the layer is also slightly depleted in Mg but much less than it is in Fe due to the preferential occupation by Fe+2 of more weakly bonded M2 sites. The behavior of the layer is similar to that found earlier on iron-free pyroxene (Schottet al., 1981); in other words, because of its thinness and instability it is not diffusion-inhibiting or protective toward dissolution.In the presence of dissolved O2, as would be the case in most weathering solutions, dissolution of bronzite and fayalite results in the formation of two surface layers whose compositions were deduced by measurements of XPS binding energies. The outer layer, consisting of hydrous ferric oxide, is readily removed by ultrasonic cleaning and, most likely, is not protective toward dissolution. The inner layer consists of Fe+3 in a protonated or hydroxylated silicate (Mg-silicate in the case of bronzite) matrix. This layer appears to impede dissolution over the time scale of the experiment as attested to by parabolic dissolution rates. However, the layer does not continue to grow on the time scale of weathering because ultrasonically cleaned soil grains (Berner and Schott, 1982) exhibit surface compositions similar to those found in the present month-long laboratory experiments. In other words, a thick, highly altered, diffusion-inhibiting, protective surface layer does not form at the acidic pH of most soils. 相似文献
20.
Chemical weathering of silicate minerals consumes atmospheric CO2 and is a fundamental component of geochemical cycles and of the climate system on long timescales. Artificial acceleration of such weathering (“enhanced weathering”) has recently been proposed as a method of mitigating anthropogenic climate change, by adding fine-grained silicate materials to continental surfaces. The efficacy of such intervention in the carbon cycle strongly depends on the mineral dissolution rates that occur, but these rates remain uncertain. Dissolution rates determined from catchment scale investigations are generally several orders of magnitude slower than those predicted from kinetic information derived from laboratory studies. Here we present results from laboratory flow-through dissolution experiments which seek to bridge this observational discrepancy by using columns of soil returned to the laboratory from a field site. We constrain the dissolution rate of olivine added to the top of one of these columns, while maintaining much of the complexity inherent in the soil environment. Continual addition of water to the top of the soil columns, and analysis of elemental composition of waters exiting at the base was conducted for a period of five months, and the solid and leachable composition of the soils was also assessed before and after the experiments. Chemical results indicate clear release of Mg2+ from the dissolution of olivine and, by comparison with a control case, allow the rate of olivine dissolution to be estimated between 10−16.4 and 10−15.5 moles(Mg) cm−2 s−1. Measurements also allow secondary mineral formation in the soil to be assessed, and suggest that no significant secondary uptake of Mg2+ has occurred. The olivine dissolution rates are intermediate between those of pure laboratory and field studies and provide a useful constraint on weathering processes in natural environments, such as during soil profile deepening or the addition of mineral dust or volcanic ash to soils surfaces. The dissolution rates also provide critical information for the assessment of enhanced weathering including the expected surface-area and energy requirements. 相似文献