Sorption and redox reactions of As(III) and As(V) within secondary mineral coatings on aquifer sediment grains

Nano-mineral assemblages in mercury- and silver-contaminated soils: records of sequestration, transformation, and release of mercury- and silver-bearing nanoparticles

Mercury-bearing nano-mineral assemblages (Hg-NMAs) are chemically and mineralogically heterogeneous, micrometer-sized aggregates of nanoparticles (NPs) found in contaminated soils and sediments. Although these NMAs control sequestration and release of Hg that is a global contaminant, our understanding is limited with respect to the conditions of different types of Hg-NMAs, the diversity of its minerals, the size distribution of its NPs and whether mineral replacement and alteration reactions in these NMAs result in the release of Hg-bearing NPs. For this purpose, Hg-NMAs in four sediment samples from the Guanajuato Mining District (GMD) in Mexico, a region that was polluted by Hg and silver (Ag) due to historical mining involving Hg amalgamation, are characterized at the micro- and nanoscale. Microscale examinations with SEM show that the majority of Hg-NMAs occurs in mineral surface coatings (MSC) and fillings in fractures within quartz grains and are enriched in Hg and sulfur (S) relative to Ag, and in S and selenium (Se) relative to chloride (Cl). Examinations at the nanoscale show that Hg-NMAs contain (a) residuals of the patio process such as amalgam phases and elemental Ag; (b) associations of Hg- and Ag-sulfide NPs with pyrite and marcasite; (c) associations of Hg- and Ag-sulfide NPs with goethite and clay minerals along the rims of the MSC. The latter minerals replaced the Fe-Si-rich matrix at high-water rock ratios most likely due to an increase in porosity during flooding of the Pastita River. Consequently, the rims are depleted in Hg-Ag-sulfide NPs relative to the unaltered Fe-Si-rich matrices indicating that changes in the physiochemical conditions of soils and sediments in the GMD can result in the release of Hg-Ag-bearing NPs. In this context, this study discusses whether release and dissolution of Hg-Ag-bearing NPs contribute to the recently observed elevated gaseous elemental Hg concentrations in the soil, interstitial air and ambient air, and to the fate and effects of Hg in local aquatic environments.

Arsenic immobilization by in-situ iron coating for managed aquifer rehabilitation

A long-lasting challenge in eliminating the worldwide impact of geogenic arsenic (As)-contaminated groundwater is the development of efficient, in-situ treatment technologies that are applicable in decentralized and rural areas. Here we present a managed aquifer rehabilitation (MAR) approach based on the in-situ creation of Fe-oxide scavengers for remediating As-contaminated groundwater. The Fe-oxide coatings on sediment surfaces were generated via periodic injection of Fe²⁺ and ClO^- solutions into an As-affected sandy aquifer at the Datong Basin, northern China for 25 days. This treatment prompted the buildup of weakly alkaline/circumneutral and oxidizing conditions to enhance As(III) oxidation in the target aquifer. Dissolved As concentrations decreased from the initial average 78.0 to 9.8 μg/L over the 25-d amendment. Sediment imaging by scanning electron microscope-X-ray energy dispersive spectroscopy confirms the deposition of Fe-rich precipitates on sediment surfaces with the simultaneous retention of As, and high density electrical tomography suggests the occurrence of such a process throughout the target zone. Further X-ray diffraction analysis and sequential chemical extraction reveal that the neo-formed Fe minerals comprised both poorly crystalline (e.g., ferrihydrite) and better crystalline (e.g., goethite) Fe oxides. The process-based reactive-transport modeling for the variations of As species in the treated groundwater supports that the new Fe-oxide minerals, most probably goethite, acted as efficient removers of aqueous As. The low As level of ∼10 μg/L was maintained during the following 215-d monitoring, demonstrating the long effectiveness of the MAR approach. This study highlights the feasibility of As immobilization by manipulating in-situ Fe-oxide coating on sandy sediments at the pilot scale. The MAR technology may be applicable for As-affected aquifers with controlled oxidizing conditions in the Datong Basin and likely other high-As regions with similar hydrogeochemical settings.

Remediation of groundwater contaminated with arsenic through enhanced natural attenuation: Batch and column studies

Batch and column laboratory experiments were conducted on natural sediment and groundwater samples from a contaminated site in Maine, USA with the aim of lowering the dissolved arsenate [As(V)] concentrations through chemical enhancement of natural attenuation capacity. In batch factorial experiments, two levels of treatment for three parameters (pH, Ca, and Fe) were studied at different levels of phosphate to evaluate their impact on As(V) solubility. Results illustrated that lowering pH, adding Ca, and adding Fe significantly increased the sorption capacity of sediments. Overall, Fe amendment had the highest individual impact on As(V) levels. To provide further evidence for the positive impact of Ca on As(V) adsorption, isotherm experiments were conducted at three different levels of Ca concentrations. A consistent increase in adsorption capacity (26-37%) of sediments was observed with the addition of Ca. The observed favorable effect of Ca on As(V) adsorption is likely caused by an increase in the surface positive charges due to surface accumulation of Ca²⁺ ions. Column experiments were conducted by flowing contaminated groundwater with elevated pH, As(V), and phosphate through both uncontaminated and contaminated sediments. Potential in-situ remediation scenarios were simulated by adding a chemical amendment feed to the columns injecting Fe(II) or Ca as well as simultaneous pH adjustment. Results showed a temporary and limited decrease in As(V) concentrations under the Ca treatment (39-41%) and higher levels of attenuation in Fe(II) treated columns (50-91%) but only after a certain number of pore volumes (18-20). This study illustrates the importance of considering geochemical parameters including pH, redox potential, presence of competing ions, and sediment chemical and physical characteristics when considering enhancing the natural attenuation capacity of sediments to mitigate As contamination in natural systems.

Sequestration of Pb-Zn-Sb- and As-bearing incidental nanoparticles by mineral surface coatings and mineralized organic matter in soils

Nanoparticles (NPs) often play significant roles in dictating the transport, distribution, bioavailability and toxicity of contaminants in the environment. Incidental NPs (i.e. NPs of anthropogenic origin but not purposely engineered) are often overlooked in contaminant transport and fate studies; yet in many systems they dominate contaminant transport processes. Using surficial contaminated regosols from Trail, British Columbia, Canada, a metal smelting and refining area along the banks of the Columbia River, we show that sequestration of Pb-, Zn-, Sb-, and As-bearing incidental NPs is strongly influenced by their aggregation, crystal growth, and/or particle attachment to mineral surface coatings (MSC) and in mineralized organic matter (MOM). Transmission electron microscopy shows the occurrence of NPs of anglesite (PbSO₄), Fe-As-phosphate, kintoreite (Pb[(Fe,Al)₃(P(As)O₄)(PO₃(OH))(OH)₆]), and franklinite (ZnFe₂O₄) in matrices of amorphous silica which retain different stages of their agglomeration and aggregation. Other identified nano-size phases in the MSC and MOM indicate a complex and previously unrecognized mineralogy of Pb-, Zn-, Sb-, and As-phase in surficial soils. Mineralogical complexity and the various sequestration processes observed in this study indicate a new dimension of nano-scale processes on mineral surfaces and organic matter that have been previously overlooked when studying the fate of contaminants with bulk-analytical tools such as micro-X-ray diffraction or synchrotron-based spectroscopic methods.

Bioavailability and risk assessment of arsenic in surface sediments of the Yangtze River estuary

The bioavailability and risk assessment of As were studied in sediments of the Yangtze River estuary (YRE). Results showed that residual fractions dominated the As partition (>85%), which attenuated overall bioavailability. After the residual fraction, As mainly partitioned into the Fe-Mn oxides fraction (3.16-4.22%). Arsenic bound to Fe-Mn oxides was higher in wet seasons. The carbonate fraction was minimal, which may result from the negative state presence of As in sediments. According to the risk assessment code, the YRE was classified as low risk. Additionally, the reduction of As(V) to As(III) may occur due to the reducing condition in wet seasons. Considering As(III) is more toxic and mobile, As bound to the exchangeable and Fe-Mn oxides fractions may have more potential ecological risk. Thus, the speciation and fraction should be both considered on the ecological risk of As in sediments of the YRE.

Binding mechanism of arsenate on rutile (110) and (001) planes studied using grazing-incidence EXAFS measurement and DFT calculation

Characterization of contaminant molecules on different exposed crystal planes is required to conclusively describe its behavior on mineral surfaces. Here, the structural properties and relative stability of arsenate adsorbed on rutile TiO2 (110) and (001) surfaces were investigated using grazing-incidence extended X-ray absorption fine structure (GI-EXAFS) spectra and periodic density functional theory (DFT) calculation. The combined results indicated that arsenate mainly formed inner-sphere bidentate binuclear (BB) and monodentate mononuclear (MM) complexes on both surfaces, but the orientational polar angles of arsenate on the (110) surface were commonly smaller than that on the (001) surface for the two adsorption modes. The DFT calculation showed that the (110) plane had a higher affinity toward arsenate than the (001) plane, suggesting that, for a given adsorption mode (i.e., MM or BB structure), a small polar angle was more favorable for arsenate stabilized on the rutile surfaces.