Important Geochemical Processes Research Articles

Natural hydrogen extracted from ophiolitic rocks: A first dataset

Serpentinization is an important geochemical process producing natural hydrogen (H2), and mafic and ultramafic serpentinized rocks, typically occurring in ophiolites and peridotite massifs, are major targets for H2 exploration. Understanding the volumes of H2 stored in these rocks is a key step towards establishing the potential of sepentinized systems to host H2 sources and reservoirs. Nonetheless, published data on the concentration of H2 directly extracted from serpentinized rocks are extremely scarce. This work provides a first dataset of H2 extracted by planetary ball mill from 58 rock samples from different ophiolites in Greece, peridotites of the Tekirova ophiolite in Turkey, and chromitites and surrounding talc schist from an Archean-Paleoproterozoic greenstone belt in Brazil. The highest H2 concentrations, from 0.1 mL/kg (considered as a threshold above which artificial H2 by milling is negligible) to 16.2 mL/kg, were found in serpentinized harzburgites or dunites, and in chromitites hosted within serpentinized peridotites. The H2 content of other mafic rocks that are not in contact or influenced by a serpentinized system, mostly basalts, is below 0.1 mL/kg. Serpentinized peridotites represent the logical source rocks of H2 (i.e., hydrogen in peridotites is generally autochtonous) and chromitites, lacking significant olivine and serpentinized minerals, may host H2 derived by the surrounding serpentinized rocks (hydrogen in chromitites is mostly allochtonous). The H2 dataset of this work can be used as a reference for further studies on the capacity of ophiolitic rocks to produce and host natural hydrogen.

Fluorescent Visualization of Chemical Profiles across the Air-Water Interface.

Chemical transfer across the air-water interface is one of the most important geochemical processes of global significance. Quantifying such a process has remained extremely challenging due to the lack of suitable technologies to measure chemical diffusion across the air-water microlayer. Herein, we present a fluorescence optical system capable of visualizing the formation of the air-water microlayer with a spatial resolution of 10 μm and quantifying air-water diffusion fluxes using pyrene as a target chemical. We show for the first time that the air-water microlayer is composed of the surface microlayer in water (∼290 ± 40 μm) and a diffusion layer in air (∼350 ± 40 μm) with 1 μg L-1 of pyrene. The diffusion flux of pyrene across the air-water interface is derived from its high-resolution concentration profile without any pre-emptive assumption, which is 2 orders of magnitude lower than those from the conventional method. This system can be expanded to visualize diffusion dynamics of other fluorescent chemicals across the air-water interface and provides a powerful tool for furthering our understanding of air-water mass transfer of organic chemicals related to their global cycling.

Assessment of cation exchange as conditioning processes of water chemistry in freshwater lenses.

Freshwater lenses are groundwater sources of limited dimensions that can be usually found in a variety of climates worldwide. These aquifers' quality is important for socioeconomic development, being cation exchange one of the most important geochemical processes that can change the water geochemistry. This study aims to assess the cation exchange processes which determine the chemistry of freshwater lenses in a multilayer aquifer type, considering the center-east of the Pampean Region (Argentina) as a case study. Water samples were taken from the freshwater lenses at different depths to analyze major ions in the laboratory. In addition, geological profiles were made along with the extraction of sediment samples for XRD and laboratory tests to analyze the cation exchange capacity. The results show that water stored in the lenses has a vertical facies variation from Ca-HCO3 to Na-HCO3 . According to the laboratory results, the change of water facies mainly occurs in the clayey sediments which divide the carbonate bioclastic material above and the loessic sediment below, being cation exchange the most important process.

Carbonate accelerated transformation of ferrihydrite in the presence of phosphate

In soils and rhizospheres, iron (oxyhydr)oxides and oxyanions like carbonate and phosphate occur ubiquitously and their interaction have important implications for nutrients and metals cycling. An elevated activity of carbonate in soils and sediments (e.g., pCO2, ∼2%) above current atmospheric CO2 (∼0.04%) is observed. The level of agronomic soil test phosphorus (P) in intensively managed agricultural soils can be up to several hundred mg kg−1. Although it is known that the transformation of iron (oxyhydr)oxides is suppressed by phosphate adsorption, it is unclear how increasing carbonate activities exert the reaction process. Here, the effects of carbonate on the transformation of ferrihydrite were evaluated at pH 7.5 in the presence of phosphate using experimental geochemistry, Fe K-edge X-ray absorption spectroscopy, transmission electron microscopy-energy dispersive X-ray spectroscopy, and Fourier transform infrared spectroscopy. Our results, for the first time, showed that the inhibitory effect of phosphate on ferrihydrite transformation was retarded by carbonate, and it transformed into hematite. Under pCO2 = 408 ppm (0.213 mM carbonate), an increase in [phosphate]o from 0 to 0.5 mM decreased the transformation rate of ferrihydrite from 0.009 to 0.003 d−1. However, the suppression was drastically perturbed when carbonate was added to the phosphate-ferrihydrite system. When 11.42 mM carbonate (pCO2 = 20000 ppmv) was present, the rate increased from 0.009 to 0.033 d−1 without phosphate and from 0.003 to 0.018 d−1 with 0.5 mM of phosphate. Moreover, carbonate promoted the formation of more crystalline rhombic hematite particles, while phosphate modified the morphology of hematite from rhombic to ellipsoidal-like with rough surfaces. The distinctive difference of the influence of carbonate and phosphate on the transformation kinetics, products distribution, and morphologies come from the interaction between carbonate/phosphate and ferrihydrite. Although phosphate formed strong inner-sphere complexes on the ferrihydrite surface, which inhibited the direct contact and dissolution of ferrihydrite, co-adsorbed carbonate still promoted the olation reaction, facilitating the formation of hematite. These results suggest that as an important geochemical process, the increasing activity of carbonate or pCO2 can override the retarding impact of phosphate on the transformation of ferrihydrite and control the occurrence/crystallization of hematite in the subsurface soils under the scenario of climate change.

Photoreductive Dissolution of Iron (Hydr)oxides and Its Geochemical Significance

Iron (hydr)oxides are the most abundant metal oxides, which are widespread on Earth’s surface in the major form of micro/nanoparticles. Dissolution of iron (hydr)oxides significantly controls their compositions on Earth’s surface and is a critical step for the global Fe cycling. Photoreductive dissolution of iron (hydr)oxides is recognized as one of the most important process for generating Fe2+ in surface water and is also a common pathway for transforming solar energy into chemical energy. This review article dissects the main characteristics of photoreductive dissolution of iron (hydr)oxides and discusses its geochemical and environmental significance. We categorize the mechanisms for photoreduction of iron (hydr)oxides into three types: reduction by intrinsic photogenerated electrons, reduction by ligand to metal electron transfer (LMCT), and reduction by direct injection of exogenous photoelectrons. The efficiency of photoreductive dissolution is constrained by both the structure of iron (hydr)oxides (e.g., crystal structure and particle size) and environmental conditions (e.g., light, pH, and concurrent chemicals). Therefore, different iron (hydr)oxides may exhibit quite distinctive photoreductive dissolution characteristics because of their unique crystal structures and physicochemical properties. Iron (hydr)oxides with low crystallinity (e.g., ferrihydrite, lepidocrocite) are subject to direct photoreductive dissolution, while those with high crystallinity (e.g., goethite, hematite) generally need ligands to proceed with photoreductive dissolution. The photoreductive dissolution of iron (hydr)oxides is involved in many important geochemical and environmental processes, such as the Fe availability to primary producers, the generation of reactive oxygen species, the transportation and fate of contaminants, and the phase transformation of iron (hydr)oxides. Given the ubiquitous occurrence of photoreductive dissolution of iron (hydr)oxides, this review will advance our understanding of the role of this process in Earth’s surface environments.

Hydrogeochemical processes controlling the groundwater salinity in the coastal aquifers of Southern Tamil Nadu, India

The current study identifies groundwater quality issues and investigates the most important geochemical processes that control seawater intrusion using various ionic ratios, hydrochemical facies evolution, and geochemical modelling. Cl-/Br ratio is an important indicator to identify the origin of groundwater salinity in coastal aquifers. Nineteen percent of the groundwater samples with Cl−/Br− ratio similar to that of Standard Mean Ocean Water (SMOW) are affected by seawater intrusion in the study area. Particularly, nine groundwater samples have high chloride values and are similar to SMOW, and it may derived salinity from seawater sources from the Bay of Bengal due to the over-pumping of production wells in the Uvari zone. Five samples are similar to SMOW, which is due to the presence of salt pan activities. The bivariate plots such as Ca2+ + Mg2+ vs Cl−, EC vs Cl−, and Na+/Cl− ratio indicate that seawater intrusion is the primary source for groundwater salinisation. Evaporation is the dominant process controlling groundwater chemistry, rather than rock-water interaction and precipitation, according to mechanisms controlling groundwater chemistry. Direct ion exchange and converse ion exchange are the critical controlling factors for groundwater salinisation, according to the hydrochemical facies evolution diagram (HFED). The water quality index (WQI) shows that most groundwater belongs to the poor to the marginal category. The saturation indices show that the groundwater samples are saturated with minerals such as dolomite, calcite, aragonite and magnesite. Therefore, these minerals are susceptible to precipitation due to the effective leaching of calcareous minerals from the bedrocks. Compiled hydrogeochemical analysis and multivariate statistical analysis revealed that the Tiruchendur and Uvari zone was affected by the seawater intrusion and led to an increase in the salinity of the groundwater.

Recharge sources and hydrogeochemical evolution of groundwater in a heterogeneous karst water system in Hubei Province, Central China

Although numerous studies on hydrogeochemistry in karst catchments have been performed, some challenges in understanding their hydrogeochemical behavior remain due to the unique architecture of karst groundwater systems and the high heterogeneity of the karst medium. Here we used a multicriteria approach combining hydrodynamic, hydrochemical, and stable isotopic tools to investigate the impact of heterogeneity on karst hydrogeochemistry in a mountainous area of Hubei Province, Central China. Our results showed that groundwater was dominated by Ca2+, Mg2+, HCO3−, and SO42−, and typically characterized by HCO3–Ca·Mg type in the shallow aquifer and by SO4–Ca·Mg type in the deep aquifer. With increasing circulation depth, water-rock interactions and the heterogeneity of the karst medium produced water with increased Ca2+, Mg2+, Sr2+, and SO42− concentrations, and also increased the dissolved total solids. Calcite, dolomite, and gypsum dissolution in local and intermediate flows were important geochemical processes in groundwater evolution, and calcite precipitation occurred in the regional flow due to dedolomitization, supported by PHREEQC modeling of minerals. The decreased major ions and increased nitrate with increasing δ18O due to the dilution of shallow groundwater by high δ18O recharge and nitrogen fertilizer required during the summer rainy season were found relying on Plots of δ18O vs major ions, which offer an alternative approach to learn seasonal effects of hydrogeochemistry. All advances presented were integrated into a hydrogeochemical conceptual model, including local, intermediate, and regional flows, hydrogeochemical evolution, horizontal and vertical karst zones, hydrochemistry, and stable isotopes of groundwater, providing a typical model for multiple karst groundwater systems in Central China.

Selenium isotope fractionation during adsorption onto montmorillonite and kaolinite

Adsorption is an important geochemical process constraining the global cycling of selenium (Se) in the environment. However, Se isotope fractionation during adsorption onto clay minerals has been rarely reported. In this study, Se isotope fractionation during adsorption onto montmorillonite and kaolinite was investigated by a combination of adsorption experiments and extended X-ray absorption fine structure (EXAFS) spectroscopy. Results showed that a small adsorption and negligible isotope fractionation were observed during Se(VI) adsorption on montmorillonite and kaolinite at pH 4.5. By contrast, Se isotope fractionations (Δ82/76Sedissolved-adsorbed) during the adsorption of Se(IV) on montmorillonite and kaolinite were ≤ 0.23‰, suggesting that lighter Se(IV) isotopes are preferentially adsorbed onto clay minerals. In addition, different effect of ionic strength on Se(IV) isotope fractionation was found between kaolinite and montmorillonite, which is likely ascribed to distinct surface charge and structure between the two clay minerals. These little or no Se isotope fractionations could be related to the fact that Se oxyanions are mainly adsorbed on montmorillonite and kaolinite via the outer-sphere complexation, as revealed by Se K-edge EXAFS analysis. The findings from this study would help further identify and constrain the key geochemical processes causing Se isotope variations, providing a foundation to develop Se isotope as a proxy for biogeochemical cycling of Se in the natural environment.

Geochemistry of paleokarst-hosted uranium anomalies at Abu Zarab area, southwestern Sinai, Egypt

This paper discusses the behavior of elements in the paleokarst and attempts to establish the relation between uranium and other elements in the paleokarst conditions at Abu Zarab locality. Abu Zarab is a small area within Um Bogma environ located in southwestern Sinai, Egypt, where paleokarsts are widespread, especially in carbonate rocks. One of these paleokarsts at Abu Zarab area was selected to geochemical investigation. The studied paleokarst was dissected by three excavated trenches constructed by Nuclear Materials Authority team to reveal its geologic features. It is filled with lateritic components represented mainly, by intercalation of clay with gibbsite and kaolinite, ferruginous siltstone and clay minerals.Twelve samples were collected from the walls of these trenches and chemically analyzed for this purpose in term of major oxides, traces and rare earth elements. The geochemical data of the major oxides display three important geochemical processes in the lateritic components of the paleokarst: (1) Enrichment of aluminum and iron; (2) depletion of calcium and magnesium (3) silicon experienced both depletion and enrichment. The geochemical data of trace and rare earth elements (REE) display enrichment of U, Cu, Pb, Zn, Ag, As and Cd with obvious enrichment of REE and conversely some elements were depleted such as V, Cr and Ga. It also, noticed that uranium has strong positive correlation with both iron and Aluminum.It is more likely, according to geochemical features of radioactive elements, that the uranium enrichment process was later than the laterite formation and the iron played an important role in capturing and trapping uranium.

Small-catchment perspective on chemical weathering and its controlling factors in the Nam Co basin, central Tibetan Plateau

Very little data is available on chemical weathering and its controlling factors in small catchments of the central Tibetan Plateau (TP) despite being the main part of the plateau. Hydrochemistry of river water in 24 small catchments in the Nam Co basin draining different strata and recharge forms was investigated monthly during the 2018 monsoon season. The results show that carbonate dissolution is the most important geochemical process controlling the evolution of hydrochemistry of most river water in the Nam Co basin (the average contribution to riverine solutes reaches 46%) even though its contribution shows significant spatial distribution: low values occur in some granite regions (e.g. 27% in H10) but is high in catchments where carbonate rocks are prevalent (e.g. 68% in H23). The contribution of silicate weathering is low, with an average of 11% in catchments located on the east and south banks draining sandstone/mudstone, while this value increases to 32% in catchments on the south bank which pass through granite, and on the west bank. The contribution of sulfide oxidation in most rivers on the south bank is unexpectedly high (40%) but is much lower in other rivers (13%). This spatial difference is attributed to the influence of topography, particularly slope. The carbonate weathering rate (CWR) in small catchments of the Nam Co basin shows spatial heterogeneity and is significantly and positively correlated with the lithologic coefficient which proposed to indicate the degree of rock weatherability and negatively correlated with mean slope and mean altitude. The spatial distribution of the silicate weathering rate (SWR) is not as significant as the CWR and is mainly governed by runoff depth. Chemical weathering rates in the glacierised catchments are 1.3 times higher than those in the non-glacierised regions which share similar lithology. Thus, glaciation enhances chemical weathering in the study area. The area-weighted average value of CWR and SWR in the study area is 23.2 t/km2/y and 5.2 t/km2/y, respectively. Chemical weathering rates in the Nam Co basin, central TP are relatively low when compared with those in catchments located on the edge of the TP, indicating the carbon sequestration effect caused by this supergene geological process in this area may be insignificant.

Hydrogeochemical changes during managed aquifer recharge (MAR) in a salinised coastal aquifer

The southern part of the Plana de Castellón aquifer (Spanish Mediterranean coast) has experienced persistent seawater intrusion over the last 50 years. The salinisation process is related to the lateral advance of the saline wedge and saline upconing as a result of intensive local groundwater pumping. In this context, managed aquifer recharge (MAR) was performed to aid in the recovery of this coastal aquifer. The artificial recharge water (ARW) was characterised by very low mineralisation, as evidenced by a mean electrical conductivity of 330 μS/cm, whereas native groundwater (NGW) was brackish, with EC values of approximately 3000–3500 μS/cm.The dynamics of the injected water and physicochemical changes to groundwater during the MAR event were evaluated using a combination of hydrogeochemical methods. EC, chloride ions, nitrate ions, and stable water isotopes (oxygen-18 and deuterium) were used as tracers to quantify the presence of injected water in groundwater (mixing ratios).Hydrochemical analysis (of major cations and anions) was used to determine changes to water characteristics and hydrogeochemical processes associated with the mixing of ARW and NGW and water-rock interactions. The injection significantly reduced groundwater salinity: EC and major ions rapidly and continuously declined after the first days of recharge. Percentage decreases in electrical conductivity of 80–90% and 30% were detected at 80 and 450 m from the recharge wells (values of 350–550 μs/cm), respectively.The principal process occurring in the aquifer during injection was dilution due to the displacement of NGW and mixing of ARW and NGW. Carbonate and sulfate mineral dissolution, cation exchange, and redox reactions were also identified, and their magnitudes were estimated. In quantitative terms, cation exchange was the most important geochemical process occurring during the MAR event.

Removal of Arsenate and Arsenite in Equimolar Ferrous and Ferric Sulfate Solutions through Mineral Coprecipitation: Formation of Sulfate Green Rust, Goethite, and Lepidocrocite.

An improved understanding of in situ mineralization in the presence of dissolved arsenic and both ferrous and ferric iron is necessary because it is an important geochemical process in the fate and transformation of arsenic and iron in groundwater systems. This work aimed at evaluating mineral phases that could form and the related transformation of arsenic species during coprecipitation. We conducted batch tests to precipitate ferrous (133 mM) and ferric (133 mM) ions in sulfate (533 mM) solutions spiked with As (0–100 mM As(V) or As(III)) and titrated with solid NaOH (400 mM). Goethite and lepidocrocite were formed at 0.5–5 mM As(V) or As(III). Only lepidocrocite formed at 10 mM As(III). Only goethite formed in the absence of added As(V) or As(III). Iron (II, III) hydroxysulfate green rust (sulfate green rust or SGR) was formed at 50 mM As(III) at an equilibrium pH of 6.34. X-ray analysis indicated that amorphous solid products were formed at 10–100 mM As(V) or 100 mM As(III). The batch tests showed that As removal ranged from 98.65–100%. Total arsenic concentrations in the formed solid phases increased with the initial solution arsenic concentrations ranging from 1.85–20.7 g kg−1. Substantial oxidation of initially added As(III) to As(V) occurred, whereas As(V) reduction did not occur. This study demonstrates that concentrations and species of arsenic in the parent solution influence the mineralogy of coprecipitated solid phases, which in turn affects As redox transformations.

Geochemical processes influencing stream water chemistry: a case study of Ala River, Akure, Southwestern Nigeria

Ala River is the main river that drains Akure metropolis in southwestern Nigeria. The river takes its sources from deep-seated fractures in granite and charnockite rock bodies at the North and North-western regions of Akure town, flowing through quartzite and granite gneiss exposures. This study was carried out to assess the geochemical processes influencing the quality of Ala river water. The dendritic river system, covering an area of about 427 km2, was studied and water was sampled at thirty-seven (37) points for in-situ determination of some physicochemical parameters and dissolved concentrations of the major ions. pH values obtained varied between 7.16 and 7.75, electrical conductivity ranged from 123 to 520 µs/cm, total dissolved solids varied between 62 and 260 mg/l-CaCO3 while average temperature value was 26.65 °C. Relative concentration of cations and anions were in the order of Ca2+ > Mg2+ > Na+ > K+ and HCO3− > SO42− > Cl− > NO3−. Mean values of dissolved Na+, K+, Ca2+, Mg2+, Fe2+, Cl−, HCO3−, SO42−, NO3−, and PO42− were 34.028, 8.872, 34.175, 18.289, 0.428, 16.543, 56.41, 12.503, 0.495 and 2.029 mg/l, respectively. Two hydochemical facies namely Calcium-Bicarbonate facies and mixed calcium–magnesium–chloride facies dominated the chemistry of the water. CaHCO3 facies revealed the contribution of geogenic processes while the mixed Ca(Mg)-Cl type reflected domestically induced anthropogenic activities influencing chemistry of the water. Gibbs diagrams identified rock–water interaction as an important geochemical process in the study area. Silicate weathering of rocks and carbonate weathering were identified as the important geochemical processes which influence the water chemistry of the study area.

Quantitative determination of redox-active carbonyls of natural dissolved organic matter

Natural dissolved organic matter (DOM) is ubiquitous in environment and plays an important role in numerous environmental processes. Although the molecular basis of the reactivity of DOM remains poorly understood due to its extreme complexity, redox-active carbonyls (aromatic ketones/aldehydes and quinones) within DOM are believed vitally important. Except the rough determination of total carbonyls (including non-redox active –COOR) based on inflexible 13C chemical shift range by expensive and time-consuming solid-state nuclear magnetic resonance (NMR), there is no ready method to quantify redox-active carbonyls in DOM. Here we show that after treatment with sodium borohydride (NaBH4) by selectively eliminating redox-active carbonyls, quenched fluorescence of carbon quantum dots (CD) by DOM recovered dramatically, and displayed a good linear relationship between redox-active carbonyls detected and DOM concentration (R2 ≥ 0.977), thus allowing first quantitative determination of the redox-active carbonyls of DOM. Eight DOM isolates present 0.59%–0.90% redox-active carbonyls by the current method. And this method is robust from coexisting proteins and salts. This method could provide better or equal instructive results compared with solid-state NMR for total carbonyls or electrochemical method for electron-accepting capacities (EAC). Our results provide the underlying structural basis of many important geochemical processes that mediated by DOM. We posit that this method could apply to other complex molecular systems such as the atmospheric aerosols and extracellular polymeric substances (EPS), too.

The formation of •OH with Fe-bearing smectite clays and low-molecular-weight thiols: Implication of As(III) removal

Low-molecular-weight thiols (LMWTs) are widely occurring in waters and soils, which can act as electron shuttles in biogeochemical cycles. It is interesting to study the interactions between LMWTs and clay minerals, which would produce free radicals on clay surfaces and influence As(III) transformation. Batch experiments and spectroscopic analysis in combined with computational modeling were conducted with three Fe-bearing clay minerals (Na–NAu-1, Na–NAu-2 and Na-SAz-2) and four LMWTs (l-cysteine, cysteamine, homocysteine, and glutathione) to investigate the reaction mechanisms of LMWTs with Fe-bearing clay minerals and influences of clay types and LMWT structures on the interactions. The results showed that Fe-bearing clay minerals can improve 2.4–3.7 times of •OH formation in 96-h LMWTs oxidation. Quenching experiments confirmed surface-Fenton-like reactions were the main pathways of •OH formation in the presence of Fe-bearing smectite clay minerals. The most possible hypothesis is that structural Fe (III) can accept electrons from LMWTs through proton-coupled transfer from –SH functional group, which was supported by FTIR, XRD and Mössbauer spectroscopies. The results of DFT calculations suggested that clay surfaces could accelerate RS• formation and stabilize the radicals. The addition of Na–NAu-2 in the cystein solution could increase As(III) oxidation to As(V) from 16.3% to 42.0%. The results imply that in-situ •OH formation in the presence of LMWTs and smectite clays may be an important geochemical process for the transformation of environmental contaminants.

Subsurface Stappia: Success Through Defence, Specialisation and Putative Pressure-Dependent Carbon Fixation.

Diverse microbial communities living in subsurface coal seams are responsible for important geochemical processes including the movement of carbon between the geosphere, biosphere and atmosphere. Microbial conversion of the organic matter in coal to methane involves a complex assemblage of bacteria and archaea working in syntrophic relationships. Despite the importance and value of this microbial process, very few of the microbial taxa have defined metabolic or ecological roles in these environments. Additionally, the genomic features mediating life in this chemically reduced, energy poor, deep subsurface environment are not well characterised. Here we describe the isolation and genomic and catabolic characterisation of three alphaproteobacterial Stappia indica species from three coal basins across Australia. S. indica genomes from coal seams were compared with those from closely related S. indica isolated from diverse surface waters, revealing a coal seam-specific suite of genes associated with life in the subsurface. These genes are linked to processes including viral defence, secondary metabolite production, polyamine metabolism, polypeptide uptake membrane transporters and putative energy neutral pressure-dependent CO2 fixation. This indicates that subsurface Stappia have diverse metabolisms for biomass recycling and pressure-dependent CO2 fixation and require a suite of defensive and competitive strategies relative to their surface-dwelling relatives.

Biological Fe(II) and As(III) oxidation immobilizes arsenic in micro-oxic environments

Fe(III) oxyhydroxides play critical roles in arsenic immobilization due to their strong surface affinity for arsenic. However, the role of bacteria in Fe(II) oxidation and the subsequent immobilization of arsenic has not been thoroughly investigated to date, especially under the micro-oxic conditions present in soils and sediments where these microorganisms thrive. In the present study, we used gel-stabilized gradient systems to investigate arsenic immobilization during microaerophilic microbial Fe(II) oxidation and Fe(III) oxyhydroxide formation. The removal and immobilization of dissolved As(III) and As(V) proceeded via the formation of biogenic Fe(III) oxyhydroxides through microbial Fe(II) oxidation. After 30 days of incubation, the concentration of dissolved arsenic decreased from 600 to 4.8 μg L−1. When an Fe(III) oxyhydroxide formed in the presence of As(III), most of the arsenic ultimately was found as As(V), indicating that As(III) oxidation accompanied arsenic immobilization. The structure of the microbial community in As(III) incubations was highly differentiated with respect to the As(V)-bearing ending incubations. The As(III)-containing incubations contained the arsenite oxidase gene, suggesting the potential for microbially mediated As(III) oxidation. The findings of the present study suggest that As(III) immobilization can occur in micro-oxic environments after microbial Fe(II) oxidation and biogenic Fe(III) oxyhydroxide formation via the direct microbial oxidation of As(III) to As(V). This study demonstrates that microbial Fe(II) and As(III) oxidation are important geochemical processes for arsenic immobilization in micro-oxic soils and sediments.

X-ray Diffraction and X-ray Absorption Near-Edge Structure Spectroscopic Investigation of Hydroxyapatite Formation under Slightly Acidic and Neutral pH Conditions

Precipitation of hydroxyapatite is one of the important geochemical processes controlling the concentration of phosphorus (P) in alkaline soils, though its formation at below neutral pH is not clea...

The kinetics, thermodynamics and mineral crystallography of CaCO3 precipitation by dissolved organic matter and salinity

Calcium carbonate (CaCO3) precipitation is an important geochemical process. In the estuary zone and some arid shallow lakes, DOM (dissolved organic matter) and salinity are two frequent changing factors that may affect CaCO3 precipitation. The joint effect of DOM and salinity on CaCO3 precipitation kinetics and thermodynamics are still unclear. In this study, effects of DOM on CaCO3 precipitation process at 0.5‰ and 70‰ salinity were investigated by QCM (Quartz Crystal Microbalance) technique, real-time pH measurement and single-injection nanoliter ITC (isothermal titration calorimetry). The mineral crystallography was analyzed by SEM-EDS. Both DOM and salinity had inhibitory effect on CaCO3 precipitation. DOM had more pronounced inhibitory effect on CaCO3 precipitation at lower salinity. Regardless of DOM, 70‰ salinity inhibited CaCO3 precipitation >0.5‰ salinity. The CaCO3 precipitation kinetics followed the first-order kinetic model and the adhesion kinetics of the instantaneous nucleation and crystal growth stage could be well described by the exponential function. CaCO3 precipitation was an endothermic process and high salinity strongly hindered CaCO3 precipitation. The effect of DOM on the absorbed heat was significant at 0.5‰ salinity. At 70‰ salinity, regardless of the effect of DOM, CaCO3 precipitation rate was greatly slowed down because it needed much more heat. CaCO3 minerals were dominated by rhombohedral calcite while CaCO3 minerals were mainly shaped as spherical vaterite at 0.5‰ salinity and rhombohedral calcite at 70‰ salinity. The crystal phase changed during CaCO3 precipitation at 0.5‰ salinity. In conclusion, the presence of DOM had substantial impact on the micrograph of the CaCO3 minerals. The percentage of flawed crystals with rough surface increased significantly with increased DOM concentration.

Stabilization of Cd2+/Cr3+ During Aqueous Fe(II)‐Induced Recrystallization of Al‐Substituted Goethite

Core Ideas Recrystallization of Al‐goethites affected mobility of heavy metals in soils. Co‐existing Cd2+/Cr3+ decreased the Fe atom exchange rates between Fe2+aq and Al‐goethites. Fe2+aq‐induced recrystallization of Al‐goethites stabilized Cd2+ and Cr3+. More Cr3+ are stabilized due to its similar ionic radius with Fe3+ and Al3+. Cd2+ ions more active than Cr3+ considering iron mineral activity in soils. The aqueous Fe2+ (Fe2+aq)–induced recrystallization of iron minerals is an important geochemical process with critical effects on the environmental behavior of metal pollutants in soils. However, the effects of different heavy metals with distinctive physicochemical properties on the Fe2+aq–induced recrystallization of iron minerals and the different stabilization behaviors of these heavy metals remain unclear. This study examined the effects of Cd2+ and Cr3+ on the Fe2+aq–induced recrystallization of Al‐substituted goethites and the simultaneous stabilization of Cd2+/Cr3+ ions by the recrystallized Al‐substituted goethites. Results from stable Fe isotopic tracer experiments and Mössbauer characterization show that Fe atoms were exchanged between Fe2+aq and structural Fe(III) in Al‐substituted goethites with coexisting Cd2+/Cr3+, although both Cd2+ and Cr3+ decreased the exchange rates. During the Fe atom exchange, Al‐substituted goethites were recrystallized and a portion of Cd2+/Cr3+ ions were stabilized by the resulting goethite products. Compared with Cd2+, more Cr3+ ions were immobilized by the recrystallized Al‐substituted goethites due to the lower hydrolysis constant of Cr3+ than Cd2+, as well as the closer ionic radius of Cr3+ with Fe3+ and Al3+. Al‐goethites with higher Al contents further decreased the Fe atom exchange rates while increasing the amounts of immobilized metal ions when in the presence of Cd2+ or Cr3+. The findings of the present study suggest that the physicochemical properties of metal ions play critical roles in affecting their environmental behavior and fates during Fe2+aq–induced recrystallization of iron minerals in soils.