17 Search Results

In surface and near-surface weathering environments, the mobilization and partial loss of palladium (Pd) under oxidizing and weakly acidic conditions has been attributed to aqueous chloride complexation. However, prior work has also observed that a portion of Pd is retained by iron (oxyhydr)oxides in the weathering zone. The effect chloride has on the relative amount of Pd mobilization versus retention by iron (oxyhydr)oxides is currently unclear. We studied the effect of chloride complexation on Pd(II) adsorption to two iron (oxyhydr)oxides, hematite and 2-line ferrihydrite, at pH 4. Increasing chloride concentration suppresses Pd adsorption for both hematite and ferrihydrite, which displaymore » similar binding affinities under the conditions studied. Thermodynamic modeling of aqueous Pd speciation indicates that greater suppression of binding to iron (oxyhydr)oxides should occur than is observed because of the strength of Pd-Cl complexation, implying that additional interactions at the mineral surface are counteracting this effect. While increasing dissolved chloride concentration does not measurably impact mineral surface charging, extended X-ray absorption fine structure (EXAFS) spectra indicate that ternary Pd-Cl surface complexes form on both hematite and ferrihydrite. The number of Cl ligands in the surface species increase at greater chloride concentration. A mixture of bidentate and monodentate surface species are indicated by the EXAFS spectra, although the fitting uncertainties precludes determining whether these vary in relative abundance with chloride concentration. In order to offset the effect of strong aqueous Pd-Cl complexation and align with our EXAFS results, a surface complexation model developed for Pd adsorption to hematite involves a mixture of three ternary surface complexes containing 1, 2, and 3 chloride ligands. Our results show that Pd is mobilized as a chloride complex in platinum group element-rich weathering zones. As a result, porewater chloride concentrations are thus a dominant control on Pd retention by iron (oxyhydr)oxides in these weakly acidic environments.« less

The adsorption of rare earth elements (REEs) to iron oxides can regulate the mobility of REEs in the environment and is heavily influenced by water chemistry. This study utilized batch experiments to examine the adsorption of Nd, Dy, and Yb to goethite under varying pH, electrolyte (type and concentration), and concentrations of dissolved inorganic carbon and citrate. REE adsorption was strongly influenced by pH, with an increase from essentially no adsorption at pH 3.0 to nearly complete adsorption at pH 6.5 and higher. Citrate enhanced the adsorption of REEs at low pH (<5.0), likely by forming goethite-REE-citrate ternary surface complexes.more » However, citrate inhibited the adsorption of REEs at higher pH (>5.0) by forming aqueous REE-citrate complexes. Ionic strength had a small influence on REE adsorption, and the presence of dissolved inorganic carbon had no discernible effect. Equilibrium adsorption was interpreted with a triple-layer surface complexation model (SCM). The selection of surface complexation reactions was guided by extended X-ray absorption fine structure spectra. An SCM with a single bidentate inner-sphere surface complexation reaction for Nd and two inner-sphere surface complexation reactions (one monodentate and one bidentate reaction) for Dy and Yb effectively simulated adsorption across a broad range of conditions in the absence of citrate. Accounting for the effects of citrate on REE adsorption required the addition of up to two ternary REE-citrate-goethite surface complexes. The SCM can enable predictions of REE transport in subsurface environments that have goethite as an important adsorbent mineral. Furthermore, this predictive capability could contribute to identifying potential REE sources and facilitating efficient extraction of REEs.« less

Denitrification is microbially-mediated through enzymes containing metal cofactors. Laboratory studies of pure cultures have highlighted that the availability of Cu, required for the multicopper enzyme nitrous oxide reductase, can limit N2O reduction. However, in natural aquatic systems, such as wetlands and hyporheic zones in stream beds, the role of Cu in controlling denitrification remains incompletely understood. In this study, we collected soils and sediments from three natural environments -- riparian wetlands, marsh wetlands, and a stream -- to investigate their nitrogen species transformation activity at background Cu levels and different supplemented Cu loadings. All of the systems contained solid-phase associatedmore » Cu below or around geological levels (40 - 280 nmol g-1) and exhibited low dissolved Cu (3-50 nM), which made them appropriate sites for evaluating the effect of limited Cu availability on denitrification. In laboratory incubation experiments, high concentrations of N2O accumulated in all microcosms lacking Cu amendment except for one stream sediment sample. With Cu added to provide dissolved concentrations at trace levels (10-300 nM), reduction rate of N2O to N₂ in the wetland soils and stream sediments was enhanced. A kinetic model could account for the trends in nitrogen species by combining the reactions for microbial reduction of NO₃- to NO₂-/N₂O/N₂ and abiotic reduction of NO₂- to N₂. The model revealed that the rate of N₂O to N₂ conversion increased significantly in the presence of Cu. For riparian wetland soils and stream sediments, the kinetic model also suggested that overall denitrification is driven by abiotic reduction of NO2- in the presence of inorganic electron donors. This study demonstrated that natural aquatic systems containing Cu at concentrations less than or equal to crustal abundances may display incomplete reduction of N₂O to N₂ that would cause N₂O accumulation and release to the atmosphere.« less

Natural aquatic systems undergo fluctuating redox conditions due to microbial activity, varying water saturation levels, and nutrients dynamics. With fluctuating redox conditions, trace metals can mobilize or sequester in response to changes in iron and sulfur speciation and the concentrations and lability of organic carbon. This study examined the effect of redox fluctuations on trace metal mobility in samples collected from two different natural aquatic systems: riparian wetlands and a stream. The wetland soils contained low sulfur and total Fe contents as compared to stream sediments. The mineral composition at both sites was dominated by quartz. We incubated water-saturated soilsmore » under three cycles of anoxic-oxic conditions (τanoxic:τoxic = 3) spanning 24 days and monitored the change in dissolved and bioavailable metal concentrations. For both natural systems, reduction of iron oxides under anoxic conditions caused Co and Zn release. In contrast, oxidation of sulfides mobilized Cu under oxic conditions in both sites. In wetland soils, dissolution of Fe (hydr)oxides increased Ni solubility; however, in stream sediments, Ni release occurred when sulfides or organic matter were oxidized. For stream sediments, each subsequent redox cycle increased the bioavailability of trace metals. Redox fluctuations in wetland soils increased bioavailable Zn and Cu and decreased bioavailable Ni and Co. This study illustrates that different trace metals display distinct bioavailability patterns during redox fluctuations in natural environments. The biogeochemical cycling of nutrients in systems with redox fluctuations may be influenced by these trace metal availability patterns in addition to the availability of electron donors and acceptors.« less

Trace metal are essential for microbially-mediated biogeochemical processes occurring in anoxic wetland soils and stream bed sediments, but low availability of these elements may inhibit anaerobic element cycling and transformations. Solid-phase speciation is likely a critical control on trace metal availability but has seen limited study in anoxic systems having concentrations similar to geological background levels, where metal limitations may be most prevalent. We have investigated trace metal concentrations and solid-phase speciation in three freshwater subsurface aquatic systems: marsh wetland soils, riparian wetland soils, and the sediments of a streambed. These systems displayed low solid-phase trace metal concentrations, generally atmore » or below geological background levels, which generally followed the trend Zn > Cu ≈ Ni > Co and showed no correlation with major element compositions. All soils and sediments were dominated by quartz but varied in clay mineralogy as well as the organic matter, total sulfur, and total iron contents. X-ray absorption near-edge structure (XANES) spectroscopy shows that sulfur speciation in both wetlands is dominated by organic sulfur. Elemental sulfur and iron sulfides together made up <25% of the sulfur in the wetland soils, but the distribution between inorganic and organic forms was reversed in the stream sediments. Ferrous and ferric iron in clay minerals were common species identified by both XANES and extended X-ray absorption fine structure (EXAFS) spectroscopies at all sites. Iron(III) oxides were substantial components in all but the marsh wetland soils. Quantitative analysis of copper, nickel, and zinc XANES spectra revealed similar metal speciation across all sites. Copper speciation was dominated by sulfides, adsorbed species, and minor amounts of copper bound to organic matter; no metallic copper was detected. Nickel speciation also varied little and was dominated by nickel in clay mineral octahedral sheets and nickel sulfide, with adsorbed species also present. Zinc speciation was slightly more varied, with the marsh wetland soils and stream bed sediments containing adsorbed species, zinc associated with clay mineral structures, and zinc bound to reduced sulfur groups on organic matter, whereas the riparian wetland soils lacked clay-associated zinc but contained zinc sulfide. Trace metals bound to reduced sulfur occurred at every site, with a greater sulfur-bound fraction for copper. The fractional abundance of sulfur-bound species showed no relationship with soil or sediment total sulfur content, which varied by two orders of magnitude. More broadly, the observations in this study suggest that trace metal speciation in freshwater wetland soils and stream sediments is consistently dominated by a small set of recurring components which are distinct for each metal. Furthermore, this may represent a general geochemical phenomenon in anoxic soils and sediments containing trace metals at background concentrations (as low as 3 µg g^-1) that was not predicted from systems that are contaminated with or naturally-enriched in copper, nickel, or zinc.« less

Municipal water reuse can contribute to a circular water economy in different contexts and with various treatment trains. This study synthesized information regarding the current technological and regulatory statuses of municipal reuse. It provides process-level information on cost and energy metrics for three potable reuse and one nonpotable reuse case studies using the new Water Techno-economic Assessment Pipe-Parity Platform (WaterTAP3). WaterTAP3 enabled comparisons of cost and energy metrics for different treatment trains and for different alternative water sources consistently with a common platform. A carbon-based treatment train has both a lower calculated levelized cost of water (LCOW) ($0.40/m3) and electricitymore » intensity (0.30 kWh/m3) than a reverse osmosis (RO)-based treatment train ($0.54/m3 and 0.84 kWh/m3). In comparing LCOW and energy intensity for water production from municipal reuse, brackish water, and seawater based on the largest facilities of each type in the United States, municipal reuse had a lower LCOW and electricity than seawater but higher values than for production from brackish water. For a small (2.0 million gallon per day) inland RO-based municipal reuse facility, WaterTAP3 evaluated different deep well injection and zero liquid discharge (ZLD) scenarios for management of RO concentrate. Adding ZLD to a facility that currently allows surface discharge of concentrate would approximately double the LCOW. For all four case studies, LCOW is most sensitive to changes in weighted average cost of capital, on-stream capacity, and plant life. Baseline assessments, pipe parity metrics, and scenario analyses can inform greater observability and understanding of reuse adoption and the potential for cost-effective and energy-efficient reuse.« less

Phosphate is commonly added to drinking water to inhibit lead release from lead service lines and lead-containing materials in premise plumbing. Phosphate addition promotes the formation of lead phosphate particles, and their aggregation behaviors may affect their transport in pipes. In this work, lead phosphate formation and aggregation were studied under varied aqueous conditions typical of water supply systems. Under high aqueous PO₄/Pb molar ratios (>1), phosphate adsorption made the particles more negatively charged. Therefore, enhanced stability of lead phosphate particles was observed, suggesting that although addition of excess phosphate can lower the dissolved lead concentrations in tap water, itmore » may increase concentrations of particulate lead. Adsorption of divalent cations (Ca²⁺ and Mg²⁺) onto lead phosphate particles neutralized their negative surface charges and promoted their aggregation at pH 7, indicating that phosphate addition for lead immobilization may be more efficient in harder waters. The presence of natural organic matter (NOM, ≥ 0.05 mg C/L humic acid and ≥ 0.5 mg C/L fulvic acid) retarded particle aggregation at pH 7. Consequently, removal of organic carbon during water treatment to lower the formation of disinfection-byproducts (DBPs) may have the additional benefit of minimizing the mobility of lead-containing particles. This study provided insight into fundamental mechanisms controlling lead phosphate aggregation. Such understanding is helpful to understand the observed trends of total lead in water after phosphate addition in both field and pilot-scale lead pipe studies. Also, it can help optimize lead immobilization by better controlling the water chemistry during phosphate addition.« less

Phosphate is added to Pb-contaminated soils to induce lead immobilization through lead phosphate precipitation. Organic coatings on soils, which may affect heterogeneous lead phosphate nucleation, can impact the effectiveness of lead immobilization. SiO₂ surfaces were coated with silanol self-assembled thin films terminated with -COOH and -OH functional groups to act as model organic coatings on soil particles. Using grazing incidence small-angle X-ray scattering (GISAXS), heterogeneous lead phosphate nucleation on coatings was measured from mixed Pb(NO₃)₂ and Na₂HPO₄/NaH₂PO₄ solutions at pH 7 with varied ionic strengths (IS = 0.58, 4, and 11 mM). Raman spectroscopy identified the homogeneous precipitates in solutionmore » as hydroxylpyromorphite (Pb₅(PO₄)₃OH). The smallest lead phosphate nuclei (4.5 ± 0.5 nm) were observed on -COOH coatings, which resulted from the highest level of lead and phosphate ion adsorption on -COOH coatings. The IS of the solution also affected the sizes of the heterogeneous precipitates on -COOH coating, with smaller nuclei (1.3 ± 0.4 nm) forming under higher IS (4 and 11 mM). Finally, this study provided new findings that can improve our understanding of lead immobilization in contaminated soil environments.« less

Iron-based electrocoagulation can be highly effective for Cr(VI) removal from water supplies. However, the presence of humic acid (HA) inhibited the rate of Cr(VI) removal in electrocoagulation, with the greatest decreases in Cr(VI) removal rate at higher pH. This inhibition was probably due to the formation of Fe(II) complexes with HA that are more rapidly oxidized than uncomplexed Fe(II) by dissolved oxygen, making less Fe(II) available for reduction of Cr(VI). Close association of Fe(III), Cr(III), and HA in the solid products formed during electrocoagulation influenced the fate of both Cr(III) and HA. At pH 8, the solid products were colloidsmore » (1–200 nm) with Cr(III) and HA concentrations in the filtered fraction being quite high, while at pH 6 these concentrations were low due to aggregation of small particles. In our work, X-ray diffraction and X-ray absorption fine structure spectroscopy indicated that the iron oxides produced were a mixture of lepidocrocite and ferrihydrite, with the proportion of ferrihydrite increasing in the presence of HA. Cr(VI) was completely reduced to Cr(III) in electrocoagulation, and the coordination environment of the Cr(III) in the solids was similar regardless of the humic acid loading, pH, and dissolved oxygen level.« less

Fractures in basalt can provide substantial surface area for reactions, and limited mass transfer in fractures can allow accumulation of cations to form carbonate minerals in geologic carbon sequestration. In this study, flood basalt and serpentinized basalt with engineered fractures were reacted in water equilibrated with 10 MPa CO₂ at 100 °C or 150 °C for up to 40 weeks. Carbonation in basalt fractures was observed as early as 6 weeks, with Mg- and Ca-bearing siderite formed in both basalts reacted at 100 °C and Mg-Fe-Ca carbonate minerals formed in the flood basalt reacted at 150 °C. X-ray μCT segmentationmore » revealed that precipitates filled 5.4% and 15% (by volume) of the flood basalt fracture after 40 weeks of reaction at 100 °C and 150 °C, respectively. Zones of elevated carbonate abundance did not completely seal the fracture. Limited siderite clusters (<1% volume fraction) were found in localized areas in the serpentinized basalt fracture. A 1-dimensional reactive transport model developed in CrunchTope examined how geochemical gradients drive silicate mineral dissolution and carbonate precipitation in the fracture. The model predicts that siderite will form as early as 1 day after the addition of CO₂. In conclusion, the predicted location of maximum siderite abundance is consistent with experimental observations, and the predicted total carbonate volumes are comparable to estimates derived from CT segmentation.« less