Surface complexation modeling of carbonate effects on the adsorption of Cr(VI), Pb(II), and U(VI) on goethite

Prediction of Cr(VI) and As(V) adsorption on goethite using hybrid surface complexation-machine learning model

This study aimed to develop surface complexation modeling-machine learning (SCM-ML) hybrid model for chromate and arsenate adsorption on goethite. The feasibility of two SCM-ML hybrid modeling approaches was investigated. Firstly, we attempted to utilize ML algorithms and establish the parameter model, to link factors influencing the adsorption amount of oxyanions with optimized surface complexation constants. However, the results revealed the optimized chromate or arsenate surface complexation constants might fall into local extrema, making it unable to establish a reasonable mapping relationship between adsorption conditions and surface complexation constants by ML algorithms. In contrast, species-informed models were successfully obtained, by incorporating the surface species information calculated from the unoptimized SCM with the adsorption condition as input features. Compared with the optimized SCM, the species-informed model could make more accurate predictions on pH edges, isotherms, and kinetic data for various input conditions (for chromate: root mean square error (RMSE) on test set = 5.90 %; for arsenate: RMSE on test set = 4.84 %). Furthermore, the utilization of the interpretable formula based on Local Interpretable Model-Agnostic Explanations (LIME) enabled the species-informed model to provide surface species information like SCM. The species-informed SCM-ML hybrid modeling method proposed in this study has great practicality and application potential, and is expected to become a new paradigm in surface adsorption model.

Adsorption of Cr(VI) and Cr(III) on layered pipe scales and the effects of disinfectants in drinking water distribution systems

Pipe scales in drinking water distribution systems (DWDS) potentially adsorb chromium (Cr). Meanwhile, the fate of Cr in pipe scales and water could be influenced by the disinfectants used in DWDS since they might influence the valence state of Cr. Therefore, the adsorption of Cr (Cr(VI) and Cr(III)) on pipe scales, the transformation between different valence states, and the effects of disinfectants present in DWDS are important research topics for improving tap water quality but have not yet been sufficiently investigated. This study investigated the properties of layered pipe scales and conducted adsorption kinetic experiments in single and binary Cr(VI) and Cr(III) systems, as well as experiments related to the oxidation and adsorption of Cr(III) under the influence of decaying disinfectants. According to the results, pipe scales exhibited distinct layered structures with varying mechanisms for the adsorption of Cr(VI) and Cr(III). Cr(VI) was adsorbed through surface complexation on the surface and porous core layers, while redox reactions predominantly occurred on the shell-like layer. Furthermore, Cr(III) was adsorbed via surface precipitation on the three-layer pipe scales. Importantly, disinfectants promoted the transformation of Cr(III) to the less readily released Cr(VI) in pipe scales, reducing the Cr exposure risk from the pipe scale phase. Pipe scales also decreased the Cr(VI) concentration in water (almost 0 mg/L), enhancing the safety of DWDS. This study provides theoretical guidance on the safe operation of DWDS.

Effects of carbonate on ferrihydrite transformation in alkaline media

Alkaline media widely exist in natural and engineered systems such as semiarid/arid areas, radioactive waste sites, and mine tailings. In these settings, the commonly occurring iron (oxyhydr)oxides differed in their ability to influence the fate of nutrients and contaminants. Due to the substantially increased atmospheric carbon dioxide (CO2) concentration, carbonate stands to increase in these media. However, how increasing carbonate affects the transformation of poorly crystalline iron (oxyhydr)oxides (e.g., two-line ferrihydrite) under alkaline conditions still remains unclear. Here, kinetics of ferrihydrite transformation were evaluated at pH ∼10 as a function of [carbonate] = 0-286 mM using synchrotron-based X-ray and vibrational spectroscopic techniques. The results showed that carbonate slowed down ferrihydrite transformation slightly and suppressed goethite formation, but promoted hematite formation regardless of its concentration. At low carbonate concentration (11.42 mM), the effect of carbonate on product formation was obvious due to the weak inner-sphere complex; however, at high carbonate concentration (80-286 mM), the effect was retarded because of the adsorption equilibrium of carbonate as well as the initial carbonate adsorption followed by desorption. Moreover, carbonate modified the morphology of hematite from rhombic to ellipsoidal to honeycomb and goethite from rod-like to needle-like to spindle-like due to the inner-sphere adsorption-desorption of carbonate and adsorption of hydroxyl ions on reactive sites of iron (oxyhydr)oxides in alkaline media. The results suggest that the concurrently increasing carbonate with enhanced atmospheric CO2 could control the transformation and occurrence of iron (oxyhydr)oxides in natural and engineered environments and have important implications for the biogeochemical cycles of iron and carbon.

Using machine learning to explore oxyanion adsorption ability of goethite with different specific surface area

In this study, we developed prediction models for the adsorption of divalent and trivalent oxyanions on goethite based on machine learning algorithms. After verifying the reliability of the models, the importance of goethite specific surface area (SSA) and the average oxyanion adsorption capacities of goethite with different SSAs were calculated by shapley additive explanations (SHAP) importance analysis and partial dependence (PD) analysis. Despite there were differences in the feature importance of divalent and trivalent oxyanions, the contribution of goethite's SSA to the adsorption amount ranked the fourth based on SHAP importance, indicating SSA played the important role in oxyanion adsorption. Meanwhile, the PD values of SSA and the optimized complexation constants from surface complexation modeling (SCM) both indicated a non-monotonic relationship between the goethite with different SSA and its oxyanions binding capacity. When the total site concentration and crystal face composition were used as the machine learning model input features, the SHAP importance values of crystal faces and the PD decomposition results indicated that the (001) face showed the crucial influence on oxyanions adsorption amount. These findings demonstrated the important role of crystal face composition in goethite's adsorption ability, and provided a theoretical explanation for the variations of oxyanions adsorption amount on different SSA goethite.

Emerging iron-based mesoporous materials for adsorptive removal of pollutants: Mechanism, optimization, challenges, and future perspective

Iron-based materials (IBMs) have shown promise as adsorbents due to their unique physicochemical properties. This review provides an overview of the different types of IBMs, their synthesis methods, and their properties. Results found in the adsorption of emerging contaminants to a wide range of IBMs are discussed. The IBMs used were evaluated in terms of their maximum uptake capacity, with special consideration given to environmental conditions such as contact time, solution pH, initial pollutant concentration, etc. The adsorption mechanisms of pollutants are discussed taking into account the results of kinetic, isotherm, thermodynamic studies, surface complexation modelling (SCM), and available spectroscopic data. A current overview of molecular modeling and simulation studies related to density functional theory (DFT), surface response methodology (RSM), and artificial neural network (ANN) is presented. In addition, the reusability and suitability of IBMs in real wastewater treatment is shown. The review concludes with the strengths and weaknesses of current research and suggests ideas for future research that will improve our ability to remove contaminants from real wastewater streams.

Enhanced remediation of surface-bound hexavalent chromium in soils using the acidic and alkaline fronts of electrokinetic technology

In the subsurface environment, highly toxic hexavalent chromium (Cr(VI)) control and remediation are essential to avoid further ecological impacts and reduce environmental risks. This paper investigated the enhanced Cr(VI) electrokinetic removal in the soil through the approaching cathode method. Besides, a novel four-step sequential fractionation method was used to reflect the strength of Cr(VI) binding to the soil. The approaching cathode enhanced the electrokinetic delivery of surface-bound Cr(VI) by advancing the alkaline front for Cr(VI) desorption and improving the electric potential flattening of the soil layers. Desorption of Cr(VI) by the alkaline front involved converting the inner-sphere complexes form of Cr(VI) to a weakly adsorbed form susceptible to ionic strength. In addition, the acidic front provided a favorable environment for the photochemical reduction of Cr(VI) by soil species or the added citrate as the electron donors. Improving the potential distribution could regulate the energy consumption of individual soil layers and efficiently operate the electrokinetic transfer of pollutants. The work results have significant scientific and practical significance for applying the in-situ electrokinetic technique in subsurface pollution control.

A critical review on the occurrence and distribution of the uranium- and thorium-decay nuclides and their effect on the quality of groundwater

This critical review presents the key factors that control the occurrence of natural elements from the uranium- and thorium-decay series, also known as naturally occurring radioactive materials (NORM), including uranium, radium, radon, lead, polonium, and their isotopes in groundwater resources. Given their toxicity and radiation, elevated levels of these nuclides in drinking water pose human health risks, and therefore understanding the occurrence, sources, and factors that control the mobilization of these nuclides from aquifer rocks is critical for better groundwater management and human health protection. The concentrations of these nuclides in groundwater are a function of the groundwater residence time relative to the decay rates of the nuclides, as well as the net balance between nuclides mobilization (dissolution, desorption, recoil) and retention (adsorption, precipitation). This paper explores the factors that control this balance, including the relationships between the elemental chemistry (e.g., solubility and speciation), lithological and hydrogeological factors, groundwater geochemistry (e.g., redox state, pH, ionic strength, ion-pairs availability), and their combined effects and interactions. The various chemical properties of each of the nuclides results in different likelihoods for co-occurrence. For example, the primordial ²³⁸U, ²²²Rn, and, in cases of high colloid concentrations also ²¹⁰Po, are all more likely to be found in oxic groundwater. In contrast, in reducing aquifers, Ra nuclides, ²¹⁰Pb, and in absence of high colloid concentrations, ²¹⁰Po, are more mobile and frequently occur in groundwater. In highly permeable sandstone aquifers that lack sufficient adsorption sites, Ra is often enriched, even in low salinity and oxic groundwater. This paper also highlights the isotope distributions, including those of relatively long-lived nuclides (²³⁸U/²³⁵U) with abundances that depend on geochemical conditions (e.g., fractionation induced from redox processes), as well as shorter-lived nuclides (²³⁴U/²³⁸U, ²²⁸Ra/²²⁶Ra, ²²⁴Ra/²²⁸Ra, ²¹⁰Pb/²²²Rn, ²¹⁰Po/²¹⁰Pb) that are strongly influenced by physical (recoil), lithological, and geochemical factors. Special attention is paid in evaluating the ability to use these isotope variations to elucidate the sources of these nuclides in groundwater, mechanisms of their mobilization from the rock matrix (e.g., recoil, ion-exchange), and retention into secondary mineral phases and ion-exchange sites.

Co-sorption of metal ions and inorganic anions/organic ligands on environmental minerals: A review

Co-sorption of metal ions and anions/ligands at the mineral-water interface plays a critical role in regulating the mobility, transport, fate, and bioavailability of these components in natural environments. This review focuses on co-sorption of metal ions and naturally occurring anions/ligands on environmentally relevant minerals. The underlying mechanisms for their interfacial reactions are summarized and the environmental impacts are discussed. Co-sorption mechanisms of these components depend on a variety of factors, such as the identity and properties of minerals, pH, species and concentration of metal ions and anions/ligands, addition sequence of co-sorbed ions, and reaction time. The simultaneous presence of metal ions and anions/ligands alters the initial sorption behaviors with promotive or competitive effects. Promotive effects are mainly attributed to surface electrostatic interactions, ternary surface complexation, and surface precipitation, especially for the co-sorption systems of metal ions and inorganic anions on minerals. Competitive effects involve potential complexation of metal-anions/ligands in solution or their competition for surface adsorption sites. Organic ligands usually increase metal ion sorption on minerals at low pH via forming ternary surface complexes or surface precipitates, but inhibit metal ion sorption via the formation of aqueous complexes at high pH. The different mechanisms may act simultaneously during metal ion and anion/ligand co-sorption on minerals. Finally, the potential application for remediation of metal-contaminated sites is discussed based on the different co-sorption behaviors. Future challenges and topics are raised for metal-anion/ligand co-sorption research.

Selective copper recovery from ammoniacal waste streams using a systematic biosorption process

Cu-NH₃ bearing effluents arise from electroplating and metal extraction industries, requiring innovative and sustainable Cu recovery technologies to reduce their adverse environmental impact. CO₃^2- and Zn are often co-occurring, and thus, selective Cu recovery from these complex liquid streams is required for economic viability. This study assessed 23 sustainable biosorbents classified as tannin-rich, lignin-rich, chitosan/chitin, dead biomass, macroalgae or biochar for their Cu adsorption capacity and selectivity in a complex NH₃-bearing bioleachate. Under a preliminary screen with 12 mM Cu in 1 M ammoniacal solution, most biosorbents showed optimal Cu adsorption at pH 11, with pinecone remarkably showing high removal efficiencies (up to 68%) at all tested pH values. Further refinements on select biosorbents with pH, contact time, and presence of NH₃, Zn and CO₃^2- showed again that pinecone has a high maximum adsorption capacity (1.07 mmol g^-1), worked over pH 5-12 and was Cu-selective with 3.97 selectivity quotient (K_Cu/Zn). Importantly, pinecone performance was maintained in a real Cu/NH₃/Zn/CO₃^2- bioleachate, with 69.4% Cu removal efficiency. Unlike synthetic adsorbents, pinecones require no pre-treatment, which together with its abundance, selectivity, and efficiency without the need for prior NH₃ removal, makes it a competitive and sustainable Cu biosorbent for complex Cu-NH₃ bearing streams. Overall, this study demonstrated the potential of integrating bioleaching and biosorption as a clean Cu recovery technology utilizing only sustainable resources (i.e., bio-lixiviant and biosorbents). This presents a closed-loop approach to Cu extraction and recovery from wastes, thus effectively addressing elemental sustainability.

Intercomparison and Refinement of Surface Complexation Models for U(VI) Adsorption onto Goethite Based on a Metadata Analysis

Adsorption of uranium onto goethite is an important partitioning process that controls uranium mobility in subsurface environments, for which many different surface complexation models (SCMs) have been developed. While individual models can fit the data for which they are parameterized, many perform poorly when compared with experimental data covering a broader range of conditions. There is an imperative need to quantitatively evaluate the variations in the models and to develop a more robust model that can be used with more confidence across the wide range of conditions. We conducted an intercomparison and refinement of the SCMs based on a metadata analysis. By seeking the globally best fit to a composite dataset with wide ranges of pH, solid/sorbate ratios, and carbonate concentrations, we developed a series of models with different levels of complexity following a systematic roadmap. The goethite-uranyl-carbonate ternary surface complexes were required in every model. For the spectroscopically informed models, a triple-plane model was found to provide the best fit, but the performance of the double-layer model with bidentate goethite-uranyl and goethite-uranyl-carbonate complexes was also comparable. Nevertheless, the models that ignore the bidentate feature of uranyl surface complexation consistently performed poorly. The goodness of fitting for the models that ignore adsorption of carbonate and the charge distributions was not significantly compromised compared with that of their counterparts that considered those. This approach of model development for a large and varied dataset improved our understanding of U(VI)-goethite surface reactions and can lead to a path for generating a single set of reactions and equilibrium constants for including U(VI) adsorption onto goethite in reactive transport models.

Application of surface complexation modeling on adsorption of uranium at water-solid interface: A review

Precise prediction of uranium adsorption at water-mineral interface is of great significance for the safe disposal of radionuclides in geologic environments. Surface complexation modeling (SCM) as a very useful tool has been extensively investigated for simulating adsorption behavior of metals/metalloids at water-mineral interface. Numerous studies concerning the fitting of uranium adsorption on various adsorbents using SCM are well documented, but the systematic and comprehensive review of uranium adsorption using various SCM is not available. In this review, we briefly summarized the rationale of SCM, including constant-capacitance-model (CCM), diffuse-layer-model (DLM), triple-layer-model (TLM); The recent progress in the application of SCM on the fitting of uranium adsorption towards metal (hydr)oxides, clay minerals and soil/sediments was reviewed in details. This review hopefully provides the beneficial guidelines for predicting the transport and fate of uranium in geologic environments beyond laboratory timescales.

Ground water copper levels in the seawater intrusion area and the possible physical and chemical dynamics

Seawater intrusion, a common geological process along the coastal zones, changes the groundwater properties, which are potentially associated with the groundwater copper (Cu) levels. However, there are no studies on the details of groundwater Cu levels affected by seawater intrusion. The groundwater in the seawater intrusion area of Buzhuang Town was sampled to detect the effect of seawater intrusion on groundwater Cu levels. The Cu levels in the local groundwater range between 0.92 and 4.99 μg L-1, which averages about 5 times than those in the non-intrusion area. The Cu deviations (ΔCu) are positive, and increase with more intrusion of seawater. Simulation experiments also confirm that more Cu leaches from sediments when more seawater or brine water is mixed in. The groundwater Cu levels are positively correlated with TDS, Cl-, Br-, SO42-, HCO3-, Na+, K+ and Mg2+. The Cu-bearing minerals in the local groundwater are under-saturated. The CEC of the sediment for the simulated experiments decreases with more mixture of seawater or brine water. CuCO03, Cu(OH)02, CuHCO3+, Cu(CO3)22-, CuCl2-, Cu2+ species in the local groundwater are obviously higher than those in the non-intrusion area, and the levels of CuCl2-, Cu+, CuCO3, Cu2+, CuSO4, CuOH+, CuCl+, Cu2(OH)22+ are positively correlated with the degree of seawater intrusion, indicating the important role of Cl-, HCO3-, OH- complexation on groundwater Cu levels. Thus, ion competition and complexation are the important dynamics of groundwater Cu enrichment along the coastal zones. A new enrichment model of groundwater Cu in the seawater intrusion area is presented. Seawater intrusion should be taken into consideration when the enrichment mechanisms of groundwater Cu are discussed.

Emerging investigator series: entrapment of uranium-phosphorus nanocrystals inside root cells of Tamarix plants from a mine waste site

We investigated the mechanisms of uranium (U) uptake by Tamarix (salt cedars) growing along the Rio Paguate, which flows throughout the Jackpile mine near Pueblo de Laguna, New Mexico. Tamarix were selected for this study due to the detection of U in the roots and shoots of field collected plants (0.6-58.9 mg kg-1), presenting an average bioconcentration factor greater than 1. Synchrotron-based micro X-ray fluorescence analyses of plant roots collected from the field indicate that the accumulation of U occurs in the cortex of the root. The mechanisms for U accumulation in the roots of Tamarix were further investigated in controlled-laboratory experiments where living roots of field plants were macerated for 24 h or 2 weeks in a solution containing 100 μM U. The U concentration in the solution decreased 36-59% after 24 h, and 49-65% in two weeks. Microscopic and spectroscopic analyses detected U precipitation in the root cell walls near the xylems of the roots, confirming the initial results from the field samples. High-resolution TEM was used to study the U fate inside the root cells, and needle-like U-P nanocrystals, with diameter <7 nm, were found entrapped inside vacuoles in cells. EXAFS shell-by-shell fitting suggest that U is associated with carbon functional groups. The preferable binding of U to the root cell walls may explain the U retention in the roots of Tamarix, followed by U-P crystal precipitation, and pinocytotic active transport and cellular entrapment. This process resulted in a limited translocation of U to the shoots in Tamarix plants. This study contributes to better understanding of the physicochemical mechanisms affecting the U uptake and accumulation by plants growing near contaminated sites.

Fluoride and arsenic contamination in drinking water due to mining activities and its impact on local area population

Contamination of arsenic and fluoride in drinking water reservoirs is a serious health issue in the Sibi district, Balochistan, Pakistan. The contamination has already been affecting a large population of the district. Dental fluorosis and dermatitis are the most common reported illnesses in the area. This study focused on the evaluation of the root causes and pathway by which it reached to the body. Questioner analysis, simple examination, and pictorial representation were used to study the prevalence of diseases caused by As and F. People of the Sibi district were found to consume both surface and groundwater, which were highly contaminated with arsenic and fluoride. The saturation index, ranging from 1 to 7, showed high enrichment of contaminants in both types of water. Geochemical calculations and the sodium absorption ratio were evaluated. High values of the saturation index of different salts showed high saturation of salts in water. The principal component analysis grouped the data into three clusters, showing that the surface water has no resemblance with the control water. High degree of contamination was observed for most of the samples, whereas, some samples of ground water were closed to the control group; a group of samples within WHO limits. The correlation studies and other calculations also revealed that the F and As reached lethal limits in the drinking water and thus caused severe health damage to the local area population. The diseases found in the area are fluorosis, keratosis, dermatitis, and melanosis.

Role of Carbonate in Thermodynamic Relationships Describing Pollutant Reduction Kinetics by Iron Oxide-Bound Fe2

The reduction of environmental pollutants by Fe²⁺ bound to iron oxides is an important process that determines pollutant toxicities and mobilities. Recently, we showed that pollutant reduction rates depend on the thermodynamic driving force of the reaction in a linear free energy relationship that was a function of the solution pH value and the reduction potential, E_H, of the interfacial Fe³⁺/Fe²⁺ redox couple. In this work, we studied how carbonate affected the free energy relationship by examining the effect that carbonate has on nitrobenzene reduction rates by Fe²⁺ bound to goethite (α-FeOOH). Carbonate slowed nitrobenzene reduction rates by inducing goethite particle aggregation, as evidenced by surface charge and particle size measurements. We observed no evidence for carbonate affecting Fe³⁺/Fe²⁺ reduction potentials or the mechanism of nitrobenzene reduction. The linear free energy relationship accurately described the data collected in the presence of carbonate when we accounted for the effect it had on the reactive surface area of goethite. The findings from this work provide a framework for determining why common groundwater constituents affect the E_H-dependence of reaction rates involving oxide-bound Fe²⁺ as a reductant.

Lead immobilization by phosphate in the presence of iron oxides: Adsorption versus precipitation

As a commonly used corrosion inhibitor, phosphate (PO₄) has a complicated effect on the fate and transport of lead (Pb) in drinking water systems. While the formation of pyromorphite has been recognized to be the major driving force of the Pb immobilization mechanism, the role of adsorption on iron oxides is still not clear. This study aims to clarify the contributions of adsorption and precipitation to Pb removal in a system containing both iron oxides and PO₄. A combination of batch experiments, X-ray absorption spectroscopy, infrared spectroscopy, and electron spectroscopy was employed to distinguish the adsorbed and precipitated Pb species. The results indicated that the adsorption of Pb on iron oxides still occurred even when the solution was supersaturated to pyromorphite (i.e., 5 mg/L P with 0.1-30 mg/L Pb in 0.01 M NaCl solution at neutral pH). In the tap water containing 0.92 mg/L P and 1 mg/L Pb, adsorption on iron oxides contributed more (62-67%) than precipitation (33-38%) in terms of Pb removal. Surprisingly, the pre-formed pyromorphite is transformed to adsorbed species after mixing with iron oxides in water for 24 h. The illustration of this transformation is important to understand the immobilization mechanisms and transport behaviors of Pb in drinking water systems after the utilization of PO₄.

Mn-substituted goethite for uranium immobilization: A study of adsorption behavior and mechanisms

Goethite is a common iron hydroxide, which can be substituted by manganese (Mn) in the goethite structure. It is important to investigate the immobilization of uranium(VI) on Mn-substituted goethite (Mn-Goe) to understand the fate and migration of uranium in soils and sediments. In this study, the sorption of uranium(VI) by Mn-Goe was investigated as a function of pH, adsorbent dosage, contact time, and initial uranium concentration in batch experiments. Several material analysis techniques were used to characterize manganese substituted materials. Results indicated that Mn was successfully introduced into the goethite structure, the length of particles increased gradually, the surface clearly exhibited higher roughness with increasing Mn content, and that uranium(VI) sorption of synthetic Mn-Goe appeared to be higher than that of goethite. The sorption kinetics supported the results presented by the pseudo-second-order model. The sorption capacity of uranium on Mn-Goe was circa 77 mg g^-1 at pH = 4.0 and 25 °C. Fourier transform-infrared spectroscopy (FT-IR) analyses revealed that uranium ions were adsorbed through functional groups containing oxygen on the Mn-Goe structure. The enhancement of Mn-substitution for the uranium(VI) sorption capacity of goethite was revealed. This study suggests that goethite and Mn-Goe can both play a significant role in controlling the mobility and transport of uranium(VI) in the subsurface environment, which is helpful for material development in environmental remediation.

Selective Adsorption of Pb(II) on an Annealed Hematite (1102) Surface: Evidence from Crystal Truncation Rod X-ray Diffraction and Density Functional Theory

The Pb(II)-binding mechanism on an annealed hematite (1102) surface was studied using crystal truncation rod (CTR) X-ray diffraction coupled with density functional theory (DFT) calculations. The best fit CTR model suggested that Pb(II) sorbed selectively to one type of edge-sharing surface site (ES₂) over two other potential surface sites. From the best fit model structure, it was found that the Pb surface complex species forms a trigonal pyramid geometry. The base consists of three oxygen groups, two of which are associated with the substrate surface (^IO and ^IIIO) and one that is a distal O extending toward solution. The trigonal pyramid geometry is slightly distorted with Pb-O bond lengths ranging from 2.21 to 2.31 Å and O-Pb-O bond angles ranging from 72° to 75°. Under this structural distortion, the nearest distance between Pb and Fe is found to be 3.39(1) Å. Consistent with the CTR results, DFT calculations indicate the Pb binding energy at the ES₂ site is at least 0.16 eV more favorable than that at the other two potential binding sites considered. Using bond-valence rules we propose a stoichiometry of Pb(II) binding on the hematite (1102) surface which indicates proton release through the deprotonation of all oxygen groups bonding to Pb.

Factors Controlling the Risks of Co-occurrence of the Redox-Sensitive Elements of Arsenic, Chromium, Vanadium, and Uranium in Groundwater from the Eastern United States

The co-occurrence of contaminants in drinking water may pose enhanced risks to health beyond the effects of single contaminants. Here, we investigated the co-occurrence of four health-relevant redox-sensitive elements (U, As, V, and Cr) in 1494 groundwater wells across North Carolina. The highest concentrations of these elements were measured mostly in groundwater from fractured igneous and metamorphic formations throughout the Piedmont region. Arsenic occurred most frequently in suboxic to mixed redox character groundwater, whereas U, V, and Cr occurred mostly in oxic groundwater. Occurrences of As, and to a lesser extent U, increased with pH, likely reflecting desorption, while higher Cr and V levels were measured in near-neutral pH and oxic groundwater, reflecting greater sensitivity to redox conditions. Due to similar geochemistry, V and Cr co-occurred most frequently. Concentrations of V and Cr(VI) co-exceeded health recommendations from the NC Department of Health and Human Services in up to 84% of wells from the King's Mountain Belt and the Charlotte and Milton Belts of the Piedmont region. This study highlights the large gap between health recommendations and enforceable regulations and demonstrates a degree of co-occurrence between redox-sensitive elements, which may pose additional risks to groundwater-reliant individuals.

Predicting Cr(vi) adsorption on soils: the role of the competition of soil organic matter

Cr(vi) has posed a serious risk for the environment and human beings because of its pollution and toxicity. It is essential to understand the equilibrium behavior of Cr(vi) in soils. In this study, the adsorption of Cr(vi) on fourteen soils was studied with batch experiments and quantitative modeling. The batch experiments included the adsorption edge and adsorption isotherm experiments, investigating the adsorption of Cr(vi) with varying soil properties, solution pH, and initial Cr(vi) concentrations. The experimental data were then modeled using the surface complexation models in Visual MINTEQ of CD-MUSIC by considering the adsorption of Cr(vi) and ions onto Fe (hydr)oxides and Al (hydr)oxides, and the Stockholm Humic Model and the fixed charge site model by accounting for the adsorption of the cations to soil organic matter and clay, respectively. Particularly, the modeling method of this study introduced an important parameter RO^- to account for the amount of soil organic matter irreversibly adsorbed on soil minerals. Overall, the model predicted reasonably well for the equilibrium partition of Cr(vi) under various conditions with a root-mean-square-error of 0.35 for the adsorption edge data and 0.19 for the adsorption isotherm data. According to the model calculations, ferrihydrite dominated the binding of Cr(vi) at pH of 3.0-7.0. The content of ferrihydrite and reactive soil organic matter was found to be the main factor influencing RO^-. The modeling results help to understand and predict Cr(vi) adsorption on different soils and are beneficial to environmental risk assessment and pollution remediation.

Climatic and soil-mineralogical controls on the mobility of trace metal contamination released by informal electronic waste (e-waste) processing

Informal e-waste processing is a growing global problem. Local climate and mineralogical factors strongly control the chemical lability and dispersal of trace metals from informal e-waste processing. Previous work on e-waste contamination primarily focused on well-known sites in similar climates. Our exploratory analysis of a long-term (since 2008) e-waste incineration site in East Jerusalem demonstrated the ways in which local factors combined to uniquely control trace metal contaminant mobility. Our results suggest that the combination of e-waste processing methods, climate, and mineralogy at this site generated a geopolymer-like material combining ash from e-waste incineration and mountain rendzina soil. This material strongly sorbs trace metal contaminants. We measured the concentrations of: Cu, Fe, Mn, Pb, and Zn at 29 locations around and within the burn site. Samples collected less than 10 m from the edge of the incineration area had trace metal concentrations below the United States Environmental Protection Agency (U.S. E.P.A.) screening levels for residential soil. Sequential extraction showed that ∼50-80% of the total mobilized Pb was released from the residual solid fraction, suggesting strong sorption or incorporation into soil components. Large differences in the measured average specific surface areas (SSA) of uncontaminated (26.18 m²/g) and contaminated (4.48 m²/g) samples, despite comparable mineralogy by XRD, suggested the production of a geopolymer-like material. This was supported by close similarities between the SSA values of contaminated samples and those measured for geopolymer materials synthesized in the lab using kaolinite clay and fly ash (e.g., 4.9 m²/g).

Selective adsorption of uranyl and potentially toxic metal ions at the core-shell MFe2O4-TiO2 (M=Mn, Fe, Zn, Co, or Ni) nanoparticles

Potentially toxic metal ions (Xⁿ⁺: Rb⁺, Sr²⁺, Cr³⁺, Mn²⁺, Ni²⁺, Zn²⁺, Cd²⁺) usually coexist with uranyl (UO₂⁺), which will have a great influence on the selective adsorption process. Here, the core-shell MFe₂O₄-TiO₂ (M = Mn, Fe, Zn, Co, or Ni) nanoparticles were synthesized and assessed as new selective adsorbents. The results reveal that TiO₂(101) preferentially grows along the MFe₂O₄(311)/(111) orientation. The M²⁺ ions as the mediators transfer the holes from MFe₂O₄ to TiO₂, at the conduction bands. On the TiO₂(101) surfaces and TiO₂(101)-TiO₂(101) gaps, the paired active electrons mainly complex with water molecules as hydroxyl radicals to capture Xⁿ⁺ ions, forming an ion layer to block UO₂²⁺ from being adsorbed. Simultaneously, it should be noted that an interesting adsorption pathway was UO₂²⁺ being horizontally and irreversibly adsorbed in the MFe₂O₄(311)/(111)-TiO₂(101) interface, and therein, the stable adsorption capacity was found to be 66.78 mg g^-1 in the MnFe₂O₄(311)/(111)-TiO₂(101) interface. Finally, a mechanism of hybrid orbitals between MnFe₂O₄-TiO₂ and UO₂⁺-Xⁿ⁺ was proposed.

Evaluation of metal oxides and activated carbon for lead removal: Kinetics, isotherms, column tests, and the role of co-existing ions

Activated carbon (AC) is commonly used in faucet and pitcher filters for lead (Pb(II)) removal in homes. This study evaluated the Pb(II) removal performance of AC and metal oxides (e.g. Fe(OH)₃ and TiO₂), as well as the co-existing ions' effect on Pb(II) removal. Results showed that metal oxides had higher adsorption capacity (28.9-51.5 mg/g) than AC (21.2 mg/g). Pb(II) was inner-spherically adsorbed onto both AC and metal oxides surfaces. Among various metal ions, calcium (Ca(II)) demonstrated dramatic effects on Pb(II) removal ability of AC, while it had no effect on Pb(II) adsorption by metal oxides. This difference resulted from the inner- and outer-sphere adsorption of Ca(II) on AC and metal oxides, respectively. The presence of orthophosphate (orth-P) and sulfate enhanced Pb(II) removal by those three adsorbents, whereas carbonate and silicate had negligible effect on Pb(II) adsorption. Interestingly, while the orth-P was usually used as corrosion inhibitor because of the formation of lead-phosphate coprecipitate, we found that the enhanced effect of orth-P on Pb(II) removal was mainly due to the synergistic adsorption. This study provides valuable information for the selection of effective adsorbents for Pb(II) removal and is helpful for understanding the roles of co-existing ions on it.

Novel synthesis of a clay supported amorphous aluminum nanocomposite and its application in removal of hexavalent chromium from aqueous solutions

Synthesis and application of bentonite supported amorphous aluminum nanocomposite as promising material for the removal of Cr(vi) from aqueous solutions.

A study on the role of hydrogeology on the distribution of uranium in alluvial aquifers of northwest India

A study was undertaken to decipher the uranium distribution in relation to a number of hydrogeological factors in groundwater of southwest Punjab. Existing geological information for the region suggests that the shallow alluvial aquifer extends up to 50-70 m below ground level (bgl) and is in turn underlain by a deeper aquifer which extends to a depth of 250 m bgl. The presence of clayey units limits the vertical mixing of groundwater between the shallow and deep aquifers. Water level data (averaged over 5 years period) indicates that the south and southwestern regions of the study area have shallow water levels (3-5 m bgl) while the north and northeast regions have deep water levels (20-28 m bgl). This difference in water levels is found to be increasing with time. Higher concentrations of uranium occur in the central, southern, and southwestern parts of the study area where the water table occurs at shallow depth. Groundwater in the northern and northeastern parts of the study area shows U concentration within permissible levels for potable use (< 30 μg/L) while the highest concentration of U (341 μg/L) was found in the central part of the study area. Seasonal variation in dissolved U concentration is found to be statistically significant. The observed increases in U concentrations during the post-monsoon season are due to the addition of bicarbonate from the root zone as well as increased dissolved oxygen, nitrate, and sulphate concentration (oxic condition) in the groundwater while the decrease in U concentration is attributed to quick recharge by precipitation through sand dunes and contribution of surface water. Deeper groundwater does not show much seasonal variation in dissolved U concentration. Correlation between U and other hydrochemical parameters was evaluated. Cluster analysis of the data also indicates the oxidative mobilization of U from the sediments. Based on the lithological, hydrogeological, and dissolved U data, a schematic map is prepared depicting the various factors affecting the U distribution in alluvial aquifers, which can also be applied to other regions of similar hydrogeological setup. Graphical abstract ᅟ.

In situ structural study of the surface complexation of lead(II) on the chemically mechanically polished hematite (11¯02) surface

A structural study of the surface complexation of Pb(II) on the (11¯02) surface of hematite was undertaken using crystal truncation rod (CTR) X-ray diffraction measurements under in situ conditions. The sorbed Pb was found to form inner sphere (IS) complexes at two types of edge-sharing sites on the half layer termination of the hematite (11¯02) surface. The best fit model contains Pb in distorted trigonal pyramids with an average PbO bond length of 2.27(4) Å and two characteristic Pb-Fe distances of 3.19(1) Å and 3.59(1) Å. In addition, a site coverage model was developed to simulate coverage as a function of sorbate-sorbate distance. The simulation results suggest a plausible Pb-Pb distance of 5.42 Å, which is slightly larger than the diameter of Pb's first hydration shell. This relates the best fit surface coverage of 0.59(4) Pb per unit cell at monolayer saturation to steric constraints as well as electrostatic repulsion imposed by the hydrated Pb complex. Based on the structural results we propose a stoichiometry of the surface complexation reaction of Pb(II) on the hematite (11¯02) surface and use bond valence analysis to assign the protonation schemes of surface oxygens. Surface reaction stoichiometry suggests that the proton release in the course of surface complexation occurs from the Pb-bound surface O atoms at pH 5.5.

Lead and other heavy metals in soils impacted by exterior legacy paint in residential areas of south west England

Legacy paint on publicly-accessible structures in residential areas of Plymouth, UK (a bridge parapet, hospital railings, a goal frame, urban street paving and a telephone kiosk) and local paint-contaminated soils have been analysed for lead and other heavy metals (chromium, zinc and barium) by X-ray fluorescence spectrometry. Lead was detected in all paints analysed (n=56) apart from two fragments of yellow road markings, with maximum concentrations exceeding 300gkg^-1. Soils were contaminated by Pb to varying degrees that depended on the condition and Pb content of the paint applications and the nature and vegetation of the soil, with a maximum concentration of 27gkg^-1 and a maximum enrichment factor normalised to grain size and regional baseline soil of 270. While Cr showed no clear contamination in soils that could be attributed to paint, contamination from this source was evident for Zn in soil by the goal frame and for Ba and Zn in soil by the bridge parapet. Application of a physiologically-based extraction test to the soils revealed stomach bioaccessibilities that were variable among the samples and between the metals but that were greatest for Zn and lowest for Cr. With the exception of Cr, bioaccessibility generally decreased in the intestine, with mean intestinal bioaccessibilities relative to total metal of about 6% for Pb and Ba, 0.9% for Cr and 1.6% for Zn. From both a health and environmental perspective, Pb is the heavy metal of greatest concern because of its common occurrence at high concentrations in legacy paints, coupled with a relatively high bioaccessibility and well-documented chronic neurotoxicity. Public exposure to Pb in residential areas may arise through direct contact with paint or soil or via the intrusion of contaminated geosolids to the household on shoes or as airborne dust.

Recycling Spent Cr Adsorbents as Catalyst for Eliminating Methylmercaptan

Waste adsorbents generated from treating Cr(VI)-containing wastewater are hazardous materials and generally landfilled or treated by acid or base desorption, with concomitant high cost and toxic effects. The present work shows that these Cr adsorbents can be reused as highly efficient catalysts for treating sulfur-containing VOCs (CH₃SH), not only avoiding the economic and environmental impact from the conventional approaches, but also achieving the efficient treatment of sulfur-containing waste gas. Importantly, these reused Cr adsorbents exhibit enhanced activity and stability compared with the catalysts reported elsewhere, indicating a new avenue of green chemistry. The highly toxic adsorbed Cr(VI) species are reduced to a Cr₂O₃ crystalline phase by calcination and finally immobilized as a Cr₂S₃ solid phase while converting and eliminating CH₃SH. Still, the presence of Cr(VI) species on the reused Cr adsorbent provides enough reactive sites for reaction, but high concentration of Cr(VI) species causes serious accumulation of coke deposit on the catalyst, leading to fast deactivation of the catalyst.

Microwave-Assisted Synthesis of Goethite Nanoparticles Used for Removal of Cr(VI) from Aqueous Solution

The microwave-assisted synthesis of goethite nanoparticles has been studied. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), differential thermal analysis (DTA) and Brunauer-Emmett-Teller (BET) method. Goethite rod-like nanoparticles have been successfully synthesized in 10 min of microwave treating at 100 °C. Particle size is in the range from 30 to 60 nm in width and from 200 to 350 nm in length. BET analysis indicated that the surface area of the product is 158.31 m²g^-1. The feasibility of Cr(VI) removal fromaqueous solution depends on the pH of the solution and contact time. The maximum adsorptionis reached at pH 4.0 and 540 min of contact time. The adsorption kinetics was analyzedby the pseudo-first- and second-order models and the results reveal that the adsorption process obeys the pseudo-second-order model. The adsorption data were fitted well with the Langmuir adsorption isotherm.

Health assessment using aqua-quality indicators of alpine streams (Khunjerab National Park), Gilgit, Pakistan

This preliminary research was conducted to evaluate the alpine stream health by using water quality as an indicator in Khunjerab National park of the Karakoram ranges located in Pak-China boarder Pakistan having altitude of 3660 m. This study investigated the stream health in the context of the presence or absence of sensitive species, their diversity, and their taxa richness. The water and macroinvertebrate samples were collected from 17 different locations from upstream and downstream of the river by using random sampling method. Macroinvertebrate samples were obtained using kick net (500-μm mesh size) and hand-picking method (NYSDEC). A total of 710 counts including 41 families of macroinvertebrates were recorded comprising of 7 orders including: Ephemeroptera (46%) being the most dominant group, Plecoptera (33%), Trichoptera (5%), Chironomidae (Diptera) (14%), Heteroptera (1%), and Coleoptera (1%). Ephemeroptera, Trichoptera, and Plecoptera (EPT) were found in abundance at the main source, Qarchanai, Dhee, and Tourqeen Nullah, as compared to the other locations of the stream. The most dominant macroinvertebrate was Ephemeroptera whose relative abundance is Pi = 0.49 by using the Shannon index. However, different statistical tools, including principal component analysis (PCA), cluster analysis (CA), ANOVA, and linear regression model, show a strong correlation between water quality and macroinvertebrates. The overall results of the biological indicators showed better ecological health at downstream compared to upstream. This study will provide basic information and understanding about the macroinvertebrates for future researchers, and the data will be helpful for upcoming research programs on alpine streams for the discovery and occurrences of macroinvertebrates and associated fauna.

Quantifying Cr(VI) Production and Export from Serpentine Soil of the California Coast Range

Hexavalent chromium (Cr(VI)) is generated in serpentine soils and exported to surface and groundwaters at levels above health-based drinking water standards. Although Cr(VI) concentrations are elevated in serpentine soil pore water, few studies have reported field evidence documenting Cr(VI) production rates and fluxes that govern Cr(VI) transport from soil to water sources. We report Cr speciation (i) in four serpentine soil depth profiles derived from the California Coast Range serpentinite belt and (ii) in local surface waters. Within soils, we detected Cr(VI) in the same horizons where Cr(III)-minerals are colocated with biogenic Mn(III/IV)-oxides, suggesting Cr(VI) generation through oxidation by Mn-oxides. Water-extractable Cr(VI) concentrations increase with depth constituting a 7.8 to 12 kg/km² reservoir of Cr(VI) in soil. Here, Cr(VI) is produced at a rate of 0.3 to 4.8 kg Cr(VI)/km²/yr and subsequently flushed from soil during water infiltration, exporting 0.01 to 3.9 kg Cr(VI)/km²/yr at concentrations ranging from 25 to 172 μg/L. Although soil-derived Cr(VI) is leached from soil at concentrations exceeding 10 μg/L, due to reduction and dilution during transport to streams, Cr(VI) levels measured in local surface waters largely remain below California's drinking water limit.

Click functionalization of poly(glycidyl methacrylate) microspheres with triazole-4-carboxylic acid for the effective adsorption of Pb(ii) ions

The triazole-4-carboxylic acid-modified PGMA resins exhibited high adsorption capacity towards Pb(ii) ions by electrostatic interactions and chelation or complexation.

Analysis of the improvement of selenite retention in smectite by adding alumina nanoparticles

Smectite clay is used as barrier for hazardous waste retention and confinement. It is a powerful material to retain cations, but less effective for retaining anionic species like selenite. This study shows that the addition of a small percentage of γ-Al₂O₃ nanoparticles to smectite significantly improves selenite sorption. γ-Al₂O₃ nanoparticles provide high surface area and positively charged surface sites within a wide range of pH, since their point of zero charge is at pH8-9. An addition of 20wt% of γ-Al₂O₃ to smectite is sufficient to approach the sorption capacity of pure alumina. To analyze the sorption behavior of the smectite/oxide mixtures, a nonelectrostatic surface complexation model was considered, accounting for the surface complexation of HSeO₃^- and SeO₃^2-, the anion competition, and the formation of surface ternary complexes with major cations present in the solution. Selenite sorption in mixtures was satisfactorily described with the surface parameters and complexation constants defined for the pure systems, accounting only for the mixture weight fractions. Sorption in mixtures was additive despite the particle heteroaggregation observed in previous stability studies carried out on smectite/γ-Al₂O₃ mixtures.

Chromate adsorption on selected soil minerals: Surface complexation modeling coupled with spectroscopic investigation

This study investigates the mechanisms of Cr(VI) adsorption on natural clay (illite and kaolinite) and synthetic (birnessite and ferrihydrite) minerals, including its speciation changes, and combining quantitative thermodynamically based mechanistic surface complexation models (SCMs) with spectroscopic measurements. Series of adsorption experiments have been performed at different pH values (3-10), ionic strengths (0.001-0.1M KNO3), sorbate concentrations (10(-4), 10(-5), and 10(-6)M Cr(VI)), and sorbate/sorbent ratios (50-500). Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy were used to determine the surface complexes, including surface reactions. Adsorption of Cr(VI) is strongly ionic strength dependent. For ferrihydrite at pH <7, a simple diffuse-layer model provides a reasonable prediction of adsorption. For birnessite, bidentate inner-sphere complexes of chromate and dichromate resulted in a better diffuse-layer model fit. For kaolinite, outer-sphere complexation prevails mainly at lower Cr(VI) loadings. Dissolution of solid phases needs to be considered for better SCMs fits. The coupled SCM and spectroscopic approach is thus useful for investigating individual minerals responsible for Cr(VI) retention in soils, and improving the handling and remediation processes.

Molecular-Scale Study of Aspartate Adsorption on Goethite and Competition with Phosphate

Knowledge of the interfacial interactions between aspartate and minerals, especially its competition with phosphate, is critical to understanding the fate and transport of amino acids in the environment. Adsorption reactions play important roles in the mobility, bioavailability, and degradation of aspartate and phosphate. Attenuated total reflectance Fourier-transform infrared (ATR-FTIR) measurements and density functional theory (DFT) calculations were used to investigate the interfacial structures and their relative contributions in single-adsorbate and competition systems. Our results suggest three dominant mechanisms for aspartate: bidentate inner-sphere coordination involving both α- and γ-COO(-), outer-sphere complexation via electrostatic attraction and H-bonding between aspartate NH2 and goethite surface hydroxyls. The interfacial aspartate is mainly governed by pH and is less sensitive to changes of ionic strength and aspartate concentration. The phosphate competition significantly reduces the adsorption capacity of aspartate on goethite. Whereas phosphate adsorption is less affected by the presence of aspartate, including the relative contributions of diprotonated monodentate, monoprotonated bidentate, and nonprotonated bidentate structures. The adsorption process facilitates the removal of bioavailable aspartate and phosphate from the soil solution as well as from the sediment pore water and the overlying water.

Chemical stabilization of chromate in blast furnace slag mixed cementitious materials

Cement waste form (CWF) technology is among the leading approaches to disposing of metals and liquid low-level nuclear waste in the United States. One such material, saltstone, includes slag, fly ash and Portland cement to enhance the immobilization of contaminants (e.g., Cr, (99)Tc) in alkaline liquid wastes. To evaluate the stability of such redox sensitive contaminants in saltstone, the effects of slag as a source of reductant on Cr immobilization was evaluated in aged (<300 d) saltstone monoliths. Specifically, we investigated the effects of artificial cement pore waters on the Cr release and the spatially resolved Cr chemical state analysis using synchrotron based microfocused X-ray microprobe analysis. The microprobe analysis indicated the heterogeneous distribution of insoluble Cr(III)-species in saltstone. Although at most of 20% Crtotal was leached at the top few (2-3) millimeter depth, the release of Cr(VI) was small (<5%) at 5-30 mm with slight changes, indirectly suggesting that Cr is likely present as insoluble Cr(III) species throughout the depths. The study suggests that this saltstone formulation can effectively retain/immobilize Cr under the oxic field condition after ⩽300 d of aging time.

Field methods for rapidly characterizing paint waste during bridge rehabilitation

For Department of Transportation (DOT) agencies, bridge rehabilitation involving paint removal results in waste that is often managed as hazardous. Hence, an approach that provides field characterization of the waste classification would be beneficial. In this study, an analysis of variables critical to the leaching process was conducted to develop a predictive tool for waste classification. This approach first involved identifying mechanistic processes that control leaching. Because steel grit is used to remove paint, elevated iron concentrations remain in the paint waste. As such, iron oxide coatings provide an important surface for metal adsorption. The diffuse layer model was invoked (logKMe=4.65 for Pb and logKMe=2.11 for Cr), where 90% of the data were captured within the 95% confidence level. Based on an understanding of mechanistic processes along with principal component analysis (PCA) of data obtained from field-portable X-ray fluorescence (FP-XRF), statistically-based models for leaching from paint waste were developed. Modeling resulted in 96% of the data falling within the 95% confidence level for Pb (R(2) 0.6-0.9, p ⩽ 0.04), Ba (R(2) 0.5-0.7, p ⩽ 0.1), and Zn (R(2) 0.6-0.7, p ⩽ 0.08). However, the regression model obtained for Cr leaching was not significant (R(2) 0.3-0.5, p ⩽ 0.75). The results of this work may assist DOT agencies with applying a predictive tool in the field that addresses the mobility of trace metals as well as disposal and management of paint waste during bridge rehabilitation.

Origin and concentration profile of chromium in a Greek aquifer

In this paper the origin and concentration of chromium (Cr) in an ophiolitic aquifer in Vergina, northern Greece were investigated. The study area has only agricultural activity so that industrial Cr contamination was precluded. Soil sampling included topsoil and drillcore samples collected down to 98 m depth. Groundwater samples were collected from three existing wells and a spring at the area and from different depths of the soil boring using the discrete sampling method. Mineralogical analysis of soils confirmed the presence of ultramafic minerals, including chrysotile and chromite. Soil elemental analysis showed significant concentration of total chromium (Crtot; max 12,000 mg/kg) and hexavalent chromium (Cr(VI); max 7.5mg/kg). Significant Crtot (91 μg/L) and Cr(VI) (64 μg/L) concentrations exceeding the drinking water limit of 50 μg/L were also detected in groundwater. In both the discrete soil and groundwater samples a decreasing trend of Cr(VI) concentration was observed with increasing depth, while Crtot increased. The increasing trend in Crtot is attributed to the increasing contribution of unweathered ultramafic minerals with depth, while the decreasing Cr(VI) may be related to the increasing soil pH that does not favor Cr(III) oxidation by Mn-oxides.

Mechanisms of chromate adsorption on boehmite

Adsorption reactions play an important role in the transport behavior of groundwater contaminants. Molecular-scale information is needed to elucidate the mechanisms by which ions coordinate to soil mineral surfaces. In this study, we characterized the mechanisms of chromate adsorption on boehmite (γ-AlOOH) using a combination of extended X-ray absorption fine structure (EXAFS) measurements, in situ attenuated total reflectance Fourier transform infrared spectroscopy, and quantum chemical calculations. The effects of pH, ionic strength, and aqueous chromate concentration were investigated. Our overall findings were that chromate primarily forms outer-sphere complexes on boehmite over a broad range of pH and aqueous concentrations. Additionally, a small fraction of monodentate and bidentate inner-sphere complexes are present under acidic conditions, as evidenced by two sets of chromate stretching vibrations at approximately 915, 870, and 780cm(-1), and 940, 890, 850, and 780cm(-1), respectively. The bidentate complex is supported by a best-fit CrAl distance in the EXAFS of 3.2Å. Results from DFT also support the formation of monodentate and bidentate complexes, which are predicted to results in Gibbs energy changes of -140.4 and -62.5kJmol(-1), respectively. These findings are consistent with the intermediate binding strength of chromate with respect to similar oxyanions such as sulfate and selenite. Overall, the surface species identified in this work can be used to develop a more accurate stoichiometric framework in mechanistic adsorption models.

Facile synthesis of porous single crystalline ZnO nanoplates and their application in photocatalytic reduction of Cr(VI) in the presence of phenol

Porous single crystalline ZnO nanoplates were successfully synthesized through a facile and cost-effective hydrothermal process at low temperature condition, followed by annealing of the zinc carbonate hydroxide hydrate precursors. The as-prepared products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Brunauer-Emmett-Teller (BET) measurements. The porous single crystalline ZnO nanoplates are with 12nm thickness and pore ranging from 10nm to several tens of nanometers. The porous structure of the ZnO nanoplates caused large amount of surface defects which worked as photogenerated holes' shallow trappers and largely restrained the recombination of photogenerated electrons and holes, resulting in a significantly high photocatalytic activity and durability toward the photoreduction of Cr(VI) under UV irradiation. Moreover, a synergistic effect, that is, increased photocatalytic reduction of Cr(VI) and degradation of phenol, can be observed. Furthermore, the synergistic photocatalytic mechanism has also been discussed. Those results present an enlightenment to employ porous single crystalline nanomaterials to remove Cr(VI) and organic pollutants simultaneously.

An overview of the role of goethite surfaces in the environment

Goethite, one of the most thermodynamically stable iron oxides, has been extensively researched especially the structure (including surface structure), the adsorption capacity to anions, organic/organic acid (especially for the soil organic carbon) and cations in the natural environment and its potential application in environmental protection. For example, the adsorption of heavy metals by goethite can decrease the concentration of heavy metals in aqueous solution and immobilize; the adsorption to soil organic carbon can decrease the release of carbon and fix carbon. In this present overview, the possible physicochemical properties of the goethite surface contributing to the strong affinity of goethite to nutrients and contaminants in natural environment are reported. Moreover, these chemicals adsorbed by goethite were also summarized and the suggested adsorption mechanism for these adsorbates was elucidated, which will help us understand the role of goethite in natural environment and provide some information about goethite as an absorbent. In addition, the feasibility of goethite used as catalyst carrier and the precursor of NZVI was proposed for removal of environmental pollution.

Contamination and risk assessment (based on bioaccessibility via ingestion and inhalation) of metal(loid)s in outdoor and indoor particles from urban centers of Guangzhou, China

Road dust, household air-conditioning (AC) filter dust and PM2.5 were collected to investigate the contamination of metal(loid)s (Cr, Mn, Ni, Cu, Zn, As, Cd, Sn, Sb, Hg and Pb) in outdoor and indoor urban environments of Guangzhou. Zinc was found to be the most abundant element in road dust and household PM2.5, while the concentration of Pb was the highest in AC filter dust. Enrichment factor (EF) was used to assess the influence of human activity on the contamination of these metal(loid)s. Ingestion and inhalation were the two exposure pathways applied for risk assessment. Physiologically based extraction test (PBET) was used to estimate the oral bioaccessibilities of metal(loid)s in road dust and AC filter dust. Respiratory bioaccessible fraction of metal(loid)s via household PM2.5 was extracted with lung simulating solution. Household AC filter dust was more hazardous to human health than road dust, especially to children. Arsenic was found to be the most risky element based on the risk assessment.

Occurrence, behavior and distribution of high levels of uranium in shallow groundwater at Datong basin, northern China

Geochemical investigations of uranium (U) occurrence in the environments were conducted at Datong basin of northern China. The results suggest that U contents were generally <1mg/kg for the igneous and metamorphic rocks, typically 2-5mg/kg for the Carboniferous and Permian sedimentary rocks and around 3mg/kg for sediments and topsoil, respectively. U in the Quaternary aquifer sediments may be primarily associated with carnotite from the Carboniferous and Permian coal-bearing clastic rocks around the basin. Shallow groundwater had U concentrations of <0.02-288 μg/L (average 24 μg/L), with 24% of the investigated boreholes above the WHO provisional guideline of 30 μg/L for U in drinking water. Average U concentration for surface water was 5.8 μg/L. In oxidizing waters, uranyl (UO2(2+)) species is dominant and strongly adsorbed onto iron (hydro)xides, while it would be preferentially complexed with carbonate in the alkaline groundwater, forming highly soluble uranyl-carbonate complexes at Datong. Under reducing conditions, uranous (U(IV)) species is ready to precipitate or bind to organic matter, therefore having a low mobility. At the study area, high U groundwater (>30 μg/L) occurs at the alluvial plains due to intermediate redox and enhanced alkaline conditions. The abnormally high levels of U in groundwater (>100 μg/L) are locally found at the west alluvial plains. By contrast, U co-precipitation with secondary carbonate minerals like Ca2UO2(CO3)3 in the dominant Ca-Mg-Na-HCO3 type groundwater may prevail at the east alluvial plains. Besides, bedrocks such as Carboniferous and Permian sedimentary rocks, especially the coal-bearing strata which have higher U contents at the west mountain areas may also account for the abnormally high levels of U in groundwater.