Zinc speciation in highly weathered tropical soils affected by large scale vegetable production.

Agriculture in highly weathered tropical soils often requires considerable application of lime and fertilizers to ensure satisfactory plant nutrient levels. The consequences of these continue long-term applications is not well understood may induce changes in soil chemical properties, the abundance, and speciation of potentially toxic trace element and as well as of micronutrients in agriculture soils. In this study, we evaluated the adsorption (at pH 5) and speciation of Zn in tropical soils (both agricultural and native vegetation) as a function of fertilization and contact time using chemical fractionation analyses and X-ray absorption spectroscopy. The soils overall had high Zn adsorption capacities (∼ 700 mg kg-1), but the agricultural soil was approximately 30 % higher than of the soil under native vegetation, and the proportion of Zn in the mobile fraction was 35 % in native vegetation and 21 % in agricultural soils. Zn speciation via linear combination analysis showed a strong relationship with soil mineralogical composition and reveled that Zn associated with organic matter decreased while Zn associated with P increased after the conversion of soils from native vegetation to highly fertilized soil. Aluminosilicate soil minerals were identified as major sinks of soil Zn, accounting for 34 % of total Zn retention regardless of soil origin and land use. Association of Zn with phosphate (i.e., hopeite) was observed in the agricultural soil samples, which might be an unexpected Zn-bearing mineral in highly weathered tropical soils and could have impacts on Zn plant nutrition.

Interaction of divalent metals with struvite: sorption, reversibility, and implications for mineral recovery from wastes.

Phosphorus (P) recovered from wastewater as struvite (MgNH4PO4·6H2O) can meet high P demands in the agricultural sector by reuse as a P fertiliser. Heavy metals are prevalent in wastewaters and are common fertiliser contaminants, therefore struvite as a sorbent for metals requires evaluation. Struvite sorption experiments were conducted in model solutions with cadmium (Cd), cobalt (Co), copper (Cu), nickel (Ni), lead (Pb), and zinc (Zn) at 1-5 µM concentrations from pH 7-10. The struvite metal loading increased with dissolved metal concentration and pH, ranging from 2 to 493 mg kg-1. Highest loadings were observed for 5 µM Pb, which exceeded the 120 mg kg-1 European Union (EU) struvite fertiliser limit at all pH values. At 5 µM concentrations, Ni and Cd loadings exceeded EU limits of 100 mg kg-1 at pH 10, and 60 mg kg-1 at pH 8-10, respectively. In desorption experiments, 10-85% metal was released after resuspension in metal-free solutions, with a positive correlation between initial loading and amount desorbed. Distortions of the struvite phosphate band, by Fourier transformation infrared (FTIR) spectroscopy, indicated lowered symmetry of phosphate vibrations with metal sorption. X-ray absorption fine structure spectroscopy (XAFS) analysis of pH 9 solids indicated tetrahedral coordination for Cu and Zn, octahedral coordination for Co and Ni, and Pb in 9-fold coordination. Precipitation of Pb-phosphate minerals was a primary mechanism for Pb sorption. The results provide insight into metal contaminant sorption with struvite in wastewaters, and the potential for metal desorption after recovery.

Thermochemical Analysis of Ammonia Gas Sorption by Struvite from Livestock Wastes and Comparison with Biochar and Metal-Organic Framework Sorbents.

Struvite-bearing solids from swine (S) and dairy (D) wastewater, heat-treated to 150-300 °C, were evaluated as ammonia gas (NH3(g)) sorbents and compared to biochar (BC) and a metal-organic framework (MOF). Simultaneous thermal analysis-pulse thermal analysis-Fourier-transform infrared spectroscopy (STA-PTA-FTIR) was used to determine sorption capacity, reversibility, thermodynamics, and kinetics. For wastewater-derived sorbents, S solids heated to 150 °C (S-150) had the highest NH3(g) sorption capacity (47.2-49.9 mg g-1), comparable to BC (50.8 mg g-1). Enthalpies increased with sorption capacity, and the energy released per mole sorbed NH3(g) indicated stronger bonds formed with S sorbents than BC. After desorption, S-150 retained more NH3(g) (48-51%) than BC (39%). The MOF had the highest sorption capacity (289.7 mg g-1) and irreversibly bound NH3(g) (81%) but similar sorption activation energy (Ea) as S-150. The rates (k) of NH3(g) sorption and desorption were fastest for S-150. Overall, S-150 sorbents performed similarly to BC but were less effective than MOF for NH3(g) sequestration. However, advantages of S-150 for NH3(g) mitigation include wastewater valorization, minimal synthesis, low heat treatment, and potential use in agricultural applications. Evaluation of struvite-based wastewater-derived sorbents, comparison with commonly used sorbents, and the implementation of thermochemical analysis for this purpose are all novel aspects of this study.

Effects of humic substances on Fe(II) sorption onto aluminum oxide and clay.

We studied the effects of humic substances (HS) on the sorption of Fe(II) onto Al-oxide and clay sorbents at pH 7.5 with a combination of batch kinetic experiments and synchrotron Fe K-edge EXAFS analyses. Fe(II) sorption was monitored over the course of 4 months in anoxic clay and Al-oxide suspensions amended with variable HS types (humic acid, HA; or fulvic acid, FA) and levels (0, 1, and 4 wt%), and with differing Fe(II) and HS addition sequences (co-sorption and pre-coated experiments, where Fe(II) sorbate was added alongside and after HS addition, respectively). In the Al-oxide suspensions, the presence of HS slowed down the kinetics of Fe(II) sorption, but had limited, if any, effect on the equilibrium aqueous Fe(II) concentrations. EXAFS analyses revealed precipitation of Fe(II)-Al(III)-layered double hydroxide (LDH) phases as the main mode of Fe(II) sorption in both the HA-containing and HA-free systems. These results demonstrate that HS slow down Fe(II) precipitation in the Al-oxide suspensions, but do not affect the composition or stability of the secondary Fe(II)-Al(III)-LDH phases formed. Interference of HS with the precipitation of Fe(II)-Al(III)-LDH was attributed to the formation organo-Al complexes HS limiting the availability of Al for incorporation into secondary layered Fe(II)-hydroxides. In the clay systems, the presence of HA caused a change in the main Fe(II) sorption product from Fe(II)-Al(III)-LDH to a Fe(II)-phyllosilicate containing little structural Al. This was attributed to complexation of Al by HA, in combination with the presence of dissolved Si in the clay suspension enabling phyllosilicate precipitation. The change in Fe(II) precipitation mechanism did not affect the rate of Fe(II) sorption at the lower HA level, suggesting that the inhibition of Fe(II)-Al(III)-LDH formation in this system was countered by enhanced Fe(II)-phyllosilicate precipitation. Reduced rates of Fe(II) sorption at the higher HA level were attributed to surface masking or poisoning by HA of secondary Fe(II) mineral growth at or near the clay surface. Our results suggest that HS play an important role in controlling the kinetics and products of Fe(II) precipitation in reducing soils, with effects modulated by soil mineralogy, HS content, and HS properties. Further work is needed to assess the importance of layered Fe(II) hydroxides in natural reducing environments.

Trace elements in struvite equine enteroliths: Concentration, speciation and influence of diet.

Equine enteroliths ∼1.5cm in diameter were collected from an Arabian horse in Louisville, Kentucky, United States. Scanning electron microscopy (SEM) and light microscope imaging of a sectioned enterolith showed two distinct regions of concentric growth outward from the central nidus, a small pebble. After initial growth, acidic colonic fluids permeated the stone inducing recrystallization and alteration of crystals closest to the nidus. A second growth event, when mineral crystallization was again favorable, produced an outer region of unaltered crystals at the rim. The mineral was identified as struvite (MgNH4PO4∙6H2O) by powder X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy. Elemental analysis confirmed concentrations of P, Mg and N consistent with the struvite composition, and detected trace elements Fe (1050-1860mgkg-1), Mn (262-280mgkg-1) and Zn (197-238mgkg-1). All elements were traced to dietary sources, with the Fe:Mn:Zn ratio of the enterolith consistent with that of the horse feed. X-ray absorption fine structure (XAFS) spectroscopy at the Zn K-edge revealed distorted ZnO4 tetrahedra located between crystallographic planes in the struvite structure forming bidentate linkages to struvite phosphate groups. Emplacement of Zn in structural cavities likely occurs during struvite crystallization. Trace elements and organic impurities increase susceptibility of the enterolith to heat-induced decomposition relative to pure struvite, which could be a consideration for treatment. Results reveal enterolith growth processes, composition and mechanisms of trace metal accumulation that can inform management and prevention of equine enteroliths.

Adsorption of arsenic with struvite and hydroxylapatite in phosphate-bearing solutions.

Arsenic sorption at above neutral pH is relevant when considering contaminant mobility in alkaline, phosphorus-bearing wastewaters, and may be viable in the presence phosphate minerals. Arsenic adsorption on struvite (MgNH4PO4 · 6H2O, MAP) and hydroxylapatite (Ca5(PO4)3OH, HAP) was evaluated at pH 8-11 from solutions with 2.7-0.125 mM phosphate and 0.05 mM As(III) or As(V). Over 7 d, As(III) removal from solution was minimal, but As(V) removal increased with pH, and was higher in the presence of MAP compared to HAP with a maximum of 74% removal in pH 11 MAP-bearing solutions. X-ray absorption fine structure spectroscopy (XAFS) analysis of solids recovered from pH 10-11 solutions revealed different mechanisms of As(V) sorption with MAP and HAP. Arsenic forms monodentate mononuclear surface complexes with MAP through the formation of a Mg-O-As bond, but is incorporated at the near-surface of HAP forming a johnbaumite-like (Ca5(AsO4)3OH) structure. Experiments using radioactive (33)P at pH 10-11 revealed faster exchange of P at the HAP surface, which could promote more facile As incorporation. Near-surface incorporated As in HAP may be less susceptible to remobilization compared to surface adsorbates formed with MAP. Overall, both MAP and HAP may sorb As at high pH in the presence of phosphate. This is relevant to the fate of As in alkaline contaminated waters in contact with phosphate mineral phases.

Zinc interaction with struvite during and after mineral formation.

Sorption of Zn with struvite was assessed both during and after mineral formation at pH 9.0 for 1-100 µM (0.065-6.54 mg L(-1)) aqueous Zn. The Zn loadings of recovered solids were lower when Zn was present during struvite precipitation compared to when Zn was added to struvite-bearing solutions. X-ray absorption fine structure spectroscopy confirmed that Zn added to struvite-bearing solutions at concentrations≤5 µM sorbed as both octahedral and tetrahedral complexes (Zn-O 1.98-2.03 Å), with evidence for bidentate configuration (Zn-P 3.18 Å). Bidentate complexes were incorporated into the near-surface structure, contributing to distortion of the struvite ν3 PO4(3-) band in the Fourier transform infrared spectra. At Zn concentrations>5 µM, tetrahedral monodentate adsorbates (Zn-O 1.98 Å) dominated, transitioning to a Zn-phosphate precipitate at 100 µM. When Zn is present during struvite precipitation, octahedral monodentate sorbates detected at 1 µM (Zn-O 2.08-2.10 Å; Zn-P 3.60-3.64 Å) polymerized at 5-50 µM, ultimately forming a Zn-hydroxide precipitate at 100 µM. The lowest initial Zn concentrations (0.065 mg L(-1)) and resultant solid loadings from precipitation experiments (13 mg kg(-1)) are consistent with those reported for struvite recovered from wastewater, suggesting that similar Zn sorption processes may occur in more complex systems.

Leaching characteristics of vanadium in mine tailings and soils near a vanadium titanomagnetite mining site.

A series of column leaching experiments were performed to understand the leaching behaviour and the potential environmental risk of vanadium in a Panzhihua soil and vanadium titanomagnetite mine tailings. Results from sequential extraction experiments indicated that the mobility of vanadium in both the soil and the mine tailings was low, with <1% of the total vanadium readily mobilised. Column experiments revealed that only <0.1% of vanadium in the soil and mine tailing was leachable. The vanadium concentrations in the soil leachates did not vary considerably, but decreased with the leachate volume in the mine tailing leachates. This suggests that there was a smaller pool of leachable vanadium in the mine tailings compared to that in the soil. Drought and rewetting increased the vanadium concentrations in the soil and mine tailing leachates from 20µgL(-1) to 50-90µgL(-1), indicating the potential for high vanadium release following periods of drought. Experiments with soil columns overlain with 4, 8 and 20% volume mine tailings/volume soil exhibited very similar vanadium leaching behaviour. These results suggest that the transport of vanadium to the subsurface is controlled primarily by the leaching processes occurring in soils.

Heavy metal distribution in an urban wetland impacted by combined sewer overflow.

The heavy metal content and distribution in an urban wetland affected by combined sewer overflow (CSO) discharge during dry conditions was evaluated. Metals identified in the CSO discharge were also measured upstream and downstream of the CSO. Metals were detected in the acid-extractable fraction of the wetland sediments and the roots of Phragmites australis plants. Sediment from the banks of a pool created by the CSO, and from a clay bed upstream were found to be moderately contaminated with Cu, Pb and Zn. Micro X-ray fluorescence (µ-XRF) of Phragmites roots from the CSO banks showed a correlation in the spatial distribution of Fe and Mn, attributed to the formation of mineral plaques on the root surface. Micro X-ray absorption near edge spectroscopy (µ-XANES) revealed that Cu and Zn were complexed with the organic ligands phytate and cysteine. The findings indicated that continuous discharge from the CSO is a source of heavy metals to the wetland. Metals bound to sediments are susceptible to remobilization and subsequent transport, whereas those associated with Phragmites roots may be more effectively sequestered. These observations provide insight into the behavior of heavy metals in urban areas where CSOs discharge into wetlands.

Influence of pH on the reductive transformation of birnessite by aqueous Mn(II).

We investigated the effect of pH (5.5-8.5) on the mineralogical transformation of hexagonal birnessite induced by reaction with aqueous Mn(II) (50-2200 µM), using batch sorption experiments, X-ray diffraction analyses, X-ray absorption and infrared spectroscopic measurements. Samples reacted at pH < 7.0 exhibited disrupted stacking of birnessite sheets, but no mineralogical transformation products were observed. At pH 7.0 and 7.5, reaction with Mn(II) under anoxic conditions caused reductive transformation of birnessite into manganite (γ-MnOOH), whereas at pH 8.0 and 8.5, conversion into hausmannite (Mn3O4) occurred. Feitknechtite (ß-MnOOH) is a major transformation product at low Mn(II) inputs at pH 7.0-8.5, and represents a metastable reaction intermediate that is converted into manganite and possibly hausmannite during further reaction with Mn(II). Thermodynamic calculations suggest that conversion into hausmannite at alkaline pH reflects a kinetic effect where rapid hausmannite precipitation prevents formation of thermodynamically more favorable manganite. In oxic systems, feitknechtite formation due to surface catalyzed oxidation of Mn(II) by O2 increases Mn(II) removal relative to anoxic systems at pH ≥ 7. The results of this study suggest that aqueous Mn(II) is an important control on the mineralogy and reactivity of natural Mn-oxides, particularly in aqueous geochemical environments with neutral to alkaline pH values.

Temperature-dependent phosphorus precipitation and chromium removal from struvite-saturated solutions.

The effect of temperature from 25 to 300°C on the precipitation of phosphorus (P) from struvite-saturated (MgNH(4)PO(4)·6H(2)O) solutions was explored. Scanning electron microscopy (SEM) revealed reduced particle size and a change in morphology from elongated to rhombohedral crystals with temperature. X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR) indicated that at 25°C, the precipitate was struvite, while newberyite (MgHPO(4)·3H(2)O) formed at 100°C, and magnesium pyrophosphate (Mg(2)P(2)O(7)) at 300°C. Increased temperature reduced the association of ammonium and water of crystallization with the solid and increased P polymerization. The behavior of dissolved chromium (Cr) under these conditions was also assessed. Removal of Cr with the solid phase from Cr(III) solutions was observed at all temperatures, whereas removal from Cr(VI) solutions was significant only at 300°C. X-ray absorption fine structure spectroscopy (XAFS) revealed that regardless of initial oxidation state in solution, Cr(III) was associated with the solid, interacting by the adsorption of short-range Cr polymers. Therefore, for struvite-saturated solutions, increasing the temperature changed both the mineralogy of the P phase recovered and enhanced the interaction of otherwise unreactive Cr(VI) with the substrate. These results have implications for the temperature-enhanced recovery of P from wastewater.

Sorption of chromium with struvite during phosphorus recovery.

Struvite (MgNH(4)PO(4)·6H(2)O; MAP) precipitation is a viable means of phosphorus (P) recovery from animal and human wastes. The behavior of metal contaminants such as chromium (Cr) during struvite precipitation, however, requires consideration. Here the influence of both Cr concentration and oxidation state on sorption is assessed. The Cr content of struvite precipitated in the presence of 1-100 µM Cr as Cr(III) (22.3-3030.1 mg/kg) was higher than that of solids from Cr(VI) (4.5-5.1 mg/kg) solutions. For 1-20 µM Cr(III) solids, scanning electron microscopy (SEM) revealed etch pit formation on struvite crystal surfaces, indicative of a surface interaction. The formation of an adsorbate was confirmed by X-ray absorption fine structure spectroscopy (XAFS). At initial concentrations ≥20 µM Cr(III), XAFS confirmed the formation of a Cr(OH)(3)·nH(2)O(am) precipitate. Fourier transform infrared (FT-IR) spectroscopy revealed that both Cr(III) and Cr(VI) sorption resulted in distortion of the PO(4)(3-) tetrahedra in the mineral structure. This, combined with SEM results revealed that even at low sorbed concentrations, the Cr impurity can affect the mineral surface and structure. Thus, the initial Cr concentration and oxidation state in wastes targeted for P recovery will dictate the final Cr content, the mechanism of sorption, and impact on the struvite structure.

Influence of pH and oxidation state on the interaction of arsenic with struvite during mineral formation.

Struvite (MgNH(4)PO(4)·6H(2)O) precipitated from animal and human wastes may be a sustainable source of fertilizer. However, arsenic, present in some wastes, may be removed with struvite. Here the sorption of As with struvite during mineral formation at pH 8-11 was assessed. The yield of struvite increased with pH, and was highest at pH 10. For recovered struvite, XRD indicated reduced crystallinity and particle size, and FT-IR suggested less distortion of phosphate tetrahedra with increased pH. The As impurity did not affect the crystallinity or particle size, but did contribute to phosphate distortion. Sorption of As(V) was observed at all pH values, and was highest at pH 10. As(III) sorption was consistently lower than that of As(V), but increased with pH. XAFS suggested coprecipitation of As(V), and adsorption of As(III) as the potential sorption mechanisms. Solids derived from As(III) solutions exhibited dual mechanisms due to the partial oxidation of As(III) to As(V) in solution prior to sorption. For struvite recovery in the presence of As, optimizing the pH to improve yields may increase the As content. Adsorbed As(III) could be removed prior to fertilizer application, however coprecipitated As(V) will release upon mineral decomposition, linking its cycling to that of phosphorus.

X-ray absorption fine structure study of the effect of protonation on disorder and multiple scattering in phosphate solutions and solids.

Phosphorus K-edge X-ray absorption fine structure (XAFS) was explored as a means to distinguish between aqueous and solid phosphates and to detect changes in phosphate protonation state. Data were collected for H(3)PO(4), KH(2)PO(4), K(2)HPO(4) and K(3)PO(4) solids and solutions and for the more complex phosphates, hydroxylapatite (HAP) and struvite (MAP). The X-ray absorption near-edge structure (XANES) spectra for solid samples are distinguishable from those of solutions by a shoulder at approximately 4.5 eV above the edge, caused by scattering from cation sites. For phosphate species, the intensity of the white line peak increased for solid and decreased for aqueous samples, respectively, with phosphate deprotonation. This was assigned to increasing charge delocalization in solid samples, and the effect of solvating water molecules on charge for aqueous samples. In the extended X-ray absorption fine structure (EXAFS), backscattering from first-shell O atoms dominated the chi(k) spectra. Multiple scattering (MS) via a four-legged P-O(1)-P-O(1)-P collinear path was localized in the lower k region at approximately 3.5 A(-1) and contributed significantly to the beat pattern of the first oscillation. For EXAFS analysis, increasing Debye-Waller factors suggest more disorder in the P-O shell with addition of protons to the crystal structure due to the lengthening effects of P-OH bonds. This disorder produces splitting in the hybridized P 3p-O 2p band in the density of states. For aqueous samples, however, increased protonation reduced the structural disorder within this shell. This was linked to a change from kosmotropic to chaotropic behavior of the phosphate species, with reduced effects of H bonding on structural distortion. The intensity of MS is correlated to the degree of disorder in the P-O shell, with more ordered structures exhibiting enhanced MS. The observed trends in the XAFS data can be used to distinguish between phosphate species in both solid and aqueous samples. This is applicable to many chemical, geochemical and biological systems, and may be an important tool for determining the behavior of phosphate during the hydrothermal gasification of biomass.

The effect of aging and pH on Pb(II) sorption processes at the calcite-water interface.

The effect of aging on Pb(II) retention in 1 microM Pb, calcite suspensions at pH 7.3, 8.2, and 9.4, under room-temperature conditions, was explored via a combination of batch sorption-desorption experiments and X-ray absorption spectroscopy (XAS). Short-term experiments, up to 12 days, reveal the predominance of an adsorption mechanism at pH 8.2, as confirmed by XAS analysis. Linear-combination fitting of XANES spectra indicates a dual sorption mechanism, with approximately 95% adsorbed and appromicately 5% coprecipitated, and approcimately 75% adsorbed and approsimately 25% coprecipitated Pb at pH 7.3 and 9.4, respectively. For long-term sorption, 60-270 days, slow continuous uptake occurs at pH 7.3 and 8.2, determined by EXAFS to be due to an adsorption mechanism. At pH 9.4, no further uptake occurs with aging, and the solid-phase distribution of Pb is commensurate with that for short-term experiments, suggesting that coprecipitated metal may alterthe calcite surface precluding further Pb sorption. Desorption experiments indicate that at pH 7.3 and 8.2 long-term sorption products-constituted primarily of Pb inner-sphere adsorption complexes-are reversibly bound. For aged pH 9.4 samples, significant sorption irreversibility indicates that the coprecipitated component is not readily exchangeable with the aqueous phase, and thus coprecipitation may be effective for long-term metal sequestration.

Divalent Cd and Pb uptake on calcite {1014} cleavage faces: an XPS and AFM study.

The interaction of divalent Cd and Pb with the {101 4} cleavage faces of calcite has been investigated with X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Analysis of the {101 4} cleavage planes of calcite was carried out with X-ray photoelectron spectroscopy (XPS) after exposure to divalent metal-bearing solutions in the 0.1-100 microM concentration range for times ranging from 1 to 24 h. The uptake of Cd2+ by calcite was determined to be greater than that of Pb2+ under similar experimental conditions (1 microM, pH 8.2, 24 h exposure time). In both cases, the majority of the divalent metal was postulated to exist in a surface precipitate. AFM results showed that the exposure of calcite to a 1 microM Pb2+ solution resulted in ellipsoidal surface growths that were attributed to the nucleation of a PbCO3 bulk phase. In the Cd circumstance, AFM showed comparatively flat growth features forming on the calcite surface even at concentrations down to 0.1 microM, where the solution would be expected to be undersaturated with respect to Cd bulk phases. These features were attributed to a (Ca,Cd)CO3 solid solution. The individual exposure of these Cd/CaCO3 and Pb/CaCO3 samples to water pre-equilibrated with calcite (metal free) for 1 h led to the removal of no more than 20% of the divalent metal, suggesting that if there was an adsorbed Pb or Cd complex initially on the calcite surface, it was an minority species compared to the precipitate phase. Exposure of calcite to 100 microM Cd and Pb resulted in the accumulation of precipitate on the calcite surface presumably due to the divalent metal initial solution concentrations exceeding the solubility products of CdCO3 and PbCO3, respectively.

Electrolyte and pH effects on Pb(II)-calcite sorption processes: the role of the PbCO0 (3aq) complex.

Batch sorption experiments were conducted under conditions of ambient temperature and atmospheric PCO2g to determine the effects of electrolyte type, ionic strength, and pH on Pb(II) interactions with calcite. For 0.15 M nitrate and chloride solutions at pH 8.2, no significant effect of electrolyte type on Pb sorption was observed. Varying ionic strength from 0.15 to 0.5 M produced little effect on Pb sorption in nitrate compared to chloride solutions in which Pb uptake decreased with increasing ionic strength. For a pH range of 7.3-9.4 in 0.15-0.2 M nitrate solutions, Pb sorption increased from pH 7.3 to 8.5 with a subsequent decrease in uptake out to pH 9.4. The trends in electrolyte and pH experiments correlate well with those for PbCO0(3aq) speciation, indicating that this metal-ligand complex in solution dictates Pb sorption in the system under investigation.

X-ray absorption spectroscopic evidence for the formation of Pb(II) inner-sphere adsorption complexes and precipitates at the calcite-water interface.

Combined batch sorption and in situ X-ray absorption spectroscopy provide direct assessment of the mechanisms for Pb(II) sorption atthe calcite--water interface under low-temperature conditions. At low metal concentration, 1 microM initial Pb, X-ray absorption fine structure data indicate the formation of Pb mononuclear inner-sphere complexes at the surface. A first-shell Pb-O bond length of 2.34 A is consistent with nearest oxygen neighbors in 3- or 4-fold coordination with a distorted trigonal pyramidal or square pyramidal geometry with a stereochemically active electron lone pair. For high initial Pb concentrations, 20 and 60 microM Pb, precipitation of hydrocerussite and cerussite secondary phases dominates Pb partitioning. At 5 and 10 microM initial Pb, the sorption mechanism is dual in nature with persistence of the mononuclear adsorption complex combined with precipitation of a cerussite phase occurring prior to saturation of theoretically available surface sites. The formation of inner-sphere complexes implies strong metal interactions with the surface-the mechanistic reason for the affinity of Pb for calcite as observed in macroscopic studies. The geometry of the adsorbed complex can influence Pb coprecipitation, as a change to octahedral coordination is required for incorporation into calcite. The results provide the basis for predictions of Pb sequestration by calcite in natural systems.