Surface complexation modelling of Cd(II), Cu(II), Ni(II), Pb(II) and Zn(II) adsorption onto kaolinite

Proton binding constants for the edge and basal surface sites of kaolinite were determined by batch titration experiments at 25 °C in the presence of 0.1 M, 0.01 M and 0.001 M solutions of NaNO₃ and in the pH range 3-9. By optimizing the results of the titration experiments, the ratio of the edge sites to the basal surface sites was found to be 6:1. The adsorption of Cd(II), Cu(II), Ni(II), Zn(II) and Pb(II) onto kaolinite suspensions was investigated using batch adsorption experiments and results suggested that in the lower pH range the metallic cations were bound through non-specific ion exchange reactions on the permanently charged basal surface sites (X⁻). Adsorption on these sites was greatly affected by ionic strength. With increasing pH, the variable charged edge sites (SOH) became the major adsorption sites and inner-sphere specifically adsorbed monodentate complexes were believed to be formed. The effect of ionic strength on the extent of adsorption of the metals on the variable charged edge sites was much less than those on the permanently charged sites. Two binding constants, log K_(X2Me) and log K_(SOMe), were calculated by optimizing these constants in the computer program FITEQL. A model combining non-specific ion exchange reactions and inner-sphere specific surface complexations was developed to predict the adsorption of heavy metals onto kaolinite in the studied pH range. Linear free energy relationships were found between the edge site binding constants and the first hydrolysis constants of the metals.     

Modeling fractional crystallization of group IVB iron meteorites

A ¹⁸⁷Re-¹⁸⁷Os isochron including data for all twelve IVB irons gives an age of 4579 ± 34 Ma with an initial ¹⁸⁷Os/¹⁸⁸Os of 0.09531 ± 0.00022, consistent with early solar system crystallization. This result, along with the chemical systematics of the highly siderophile elements (HSE) are indicative of closed-system behavior for all of the HSE in the IVB system since crystallization.Abundances of HSE measured in different chunks of individual bulk samples, and in spot analyses of different portions of individual chunks, are homogeneous at the ±10% level or better. Modeling of HSE in the IVB system, therefore, is not impacted by sample heterogeneities. Concentrations of some other elements determined by spot analysis, such as P, Cr and Mn, however, vary by as much as two orders of magnitude and reflect the presence of trace phases.Assuming initial S in the range of 0 to 2 wt.%, the abundances of the HSE Re, Os, Ir, Ru, Pt, Rh, Pd and Au in bulk IVB irons are successfully accounted for via a fractional crystallization model. For these elements, all IVB irons can be interpreted as being representative of equilibrium solids, liquids, or mixtures of equilibrium solids and liquids.Our model includes changes in bulk D values (ratio of concentration in the solid to liquid) for each element in response to expected increases in S and P in the evolving liquid. For this system, the relative D values are as follow: Os > Re > Ir > Ru > Pt > Rh > Pd > Au. Osmium, Re, Ir and Ru were compatible elements (favor the solid) throughout the IVB crystallization sequence; Rh, Pd and Au were incompatible (favor the liquid). Extremely limited variation in Pt concentrations throughout the IVB crystallization sequence requires that D(Pt) remained at unity.In general, D values derived from the slopes of logarithmic plots, compared with those calculated from recent parameterizations of D values for metal systems are similar, but not identical. Application of D values obtained by the parameterization method is problematic for comparisons of the compatible elements with similar partitioning characteristics. The slope-based approach works well for these elements. In contrast, the slope-based approach does not provide viable D values for the incompatible elements Pd and Au, whereas the parameterization method appears to work well. Modeling results suggest that initial S for this system may have been closer to 2% than 0, but the elements modeled do not tightly constrain initial S.Consistent with previous studies, our calculated initial concentrations of HSE in the IVB parent body indicate assembly from materials that were fractionated via high temperature condensation processes. As with some previous studies, depletions in redox sensitive elements and corresponding high concentrations of Re, Os and Ir present in all IVB irons are interpreted as meaning that the IVB core formed in an oxidized parent body. The projected initial composition of the IVB system was characterized by sub-chondritic Re/Os and Pt/Os ratios. The cause of this fractionation remains a mystery. Because of the refractory nature of these elements, it is difficult to envision fractionation of these elements (especially Re-Os) resulting from the volatility effects that evidently affected other elements.     

Platinum-group elements (Ir,Pd, Pt and Rh) in road-deposited sediments in two urban watersheds,Hawaii

This study represents the first detailed investigation of platinum-group elements (PGEs) in road-deposited sediment (RDS) in Hawaii, USA. Thirty-three sample locations, in two urban watersheds in Honolulu, Oahu, Hawaii were sampled. The <63 μm fraction of RDS was digested with aqua regia, followed by matrix separation with Dowex AG50-X8 cation exchange resin. PGEs were analyzed with inductively coupled plasma-mass spectrometry (ICP-MS) equipped with a desolvating nebulizer. Concentrations of Rh, Pd and Pt in residential streets reached 64, 105 and 506 ng/g, respectively. Maximum enrichment ratios, computed as RDS concentrations relative to baseline values, exceeded 400, indicating a significant anthropogenic signal with the sequence Rh > Pt > Pd. Iridium concentrations were uniformly low <1 ng/g, and enrichment ratios support a geogenic source. Significant interelement PGE correlations (Pd–Pt–Rh), combined with the magnitude of PGE pair-wise ratios (Pt/Pd, Pt/Rh and Pd/Rh), and relative percentages comparable to European RDS and roadside soil in Indiana, USA all suggest an automobile source. Attrition of PGE-loaded automobile catalytic converters and subsequent loss to the environment by exhaust emissions explains the significant environmental signal of PGEs in road environments of Hawaii. Further PGE work is required to quantify urban transport paths as PGEs are known to bioaccumulate, cause cellular damage and may have detrimental human health effects.     

Fractionation of Cu and Zn isotopes during adsorption onto amorphous Fe(III) oxyhydroxide: Experimental mixing of acid rock drainage and ambient river water

Fractionation of Cu and Zn isotopes during adsorption onto amorphous ferric oxyhydroxide is examined in experimental mixtures of metal-rich acid rock drainage and relatively pure river water and during batch adsorption experiments using synthetic ferrihydrite. A diverse set of Cu- and Zn-bearing solutions was examined, including natural waters, complex synthetic acid rock drainage, and simple NaNO₃ electrolyte. Metal adsorption data are combined with isotopic measurements of dissolved Cu (⁶⁵Cu/⁶³Cu) and Zn (⁶⁶Zn/⁶⁴Zn) in each of the experiments. Fractionation of Cu and Zn isotopes occurs during adsorption of the metal onto amorphous ferric oxyhydroxide. The adsorption data are modeled successfully using the diffuse double layer model in PHREEQC. The isotopic data are best described by a closed system, equilibrium exchange model. The fractionation factors (α_soln-solid) are 0.99927 ± 0.00008 for Cu and 0.99948 ± 0.00004 for Zn or, alternately, the separation factors (Δ_soln-solid) are −0.73 ± 0.08‰ for Cu and −0.52 ± 0.04‰ for Zn. These factors indicate that the heavier isotope preferentially adsorbs onto the oxyhydroxide surface, which is consistent with shorter metal-oxygen bonds and lower coordination number for the metal at the surface relative to the aqueous ion. Fractionation of Cu isotopes also is greater than that for Zn isotopes. Limited isotopic data for adsorption of Cu, Fe(II), and Zn onto amorphous ferric oxyhydroxide suggest that isotopic fractionation is related to the intrinsic equilibrium constants that define aqueous metal interactions with oxyhydroxide surface sites. Greater isotopic fractionation occurs with stronger metal binding by the oxyhydroxide with Cu > Zn > Fe(II).     

Competitive adsorption of Cd,Cu, Pb and Zn by different soils of Eastern China

Simultaneous competitive adsorption behavior of Cd, Cu, Pb and Zn onto nine soils with a wide physical–chemical characteristics from Eastern China was measured in batch experiments to assess the mobility and retention of these metals in soils. In the competitive adsorption system, adsorption isotherms for these metals on the soils exhibited significant differences in shape and in the amount adsorbed. As the applied concentration increased, Cu and Pb adsorption increased, while Cd and Zn adsorption decreased. Competition among heavy metals is very strong in acid soils with lower capacity to adsorb metal cations. Distribution coefficients (K _dmedium) for each metal and soil were calculated. The highest K _dmedium value was found for Pb and followed by Cu. However, low K _dmedium values were shown for Zn and Cd. On the basis of the K _dmedium values, the selectivity sequence of the metal adsorption is Pb > Cu > Zn > Cd and Pb > Cu > Cd > Zn. The adsorption sequence of nine soils was deduced from the joint distribution coefficients (K _dΣmedium). This indicated that acid soils with low pH value had lower adsorption capacity for heavy metals, resulting in much higher risk of heavy metal pollution. The sum of adsorbed heavy metals on the soils could well described using the Langmuir equation. The maximum adsorption capacity (Q _m) of soils ranged from 32.57 to 90.09 mmol kg⁻¹. Highly significant positive correlations were found between the K _dΣmedium and Q _m of the metals and pH value and cation exchange capacity (CEC) of soil, suggesting that soil pH and CEC were key factors controlling the solubility and mobility of the metals in soils.     

Comparison of trace metal adsorption onto different solid materials and their chemical components in a natural aquatic environment

Solid materials such as suspended particulate matter (SPM), deposited sediment (DS) and natural surface coatings (NSC, composed of biofilms and associated minerals) are important sinks and potential sources of pollutants in natural aquatic environments. Although these materials can exist in the same water body, few studies have been conducted to compare their ability to adsorb trace metals. In this study, the adsorption of Pb, Cu and Cd by these solids, collected from an urban lake, was investigated. In addition, the metal adsorption properties of the main components of these solids, namely Mn and Fe oxides and organics, were also investigated using the method of selective extraction followed by metal adsorption. The solids that co-existed in water showed similarities and differences in their compositions. For each metal, adsorption to the solids occurred in the same order: NSC > SPM > DS. For Pb and Cd, Fe and Mn oxides and organics contributed to the adsorption by NSC and SPM, and the adsorption by DS was dominated by Fe oxides. For Cu, the organics were the main adsorptive phase. The specific adsorption capability of these components decreases in the following order: Mn oxides > Fe oxides > organics. Overall, the results presented herein indicate that different solids and their components played important roles in the adsorption of trace metals.     

Behaviour of Ni-PGE-Au-Cu in mafic-ultramafic volcanic suites of the 2.7 Ga Kambalda Sequence, Kalgoorlie Terrane, Yilgarn Craton

The 2.7 Ga Kambalda Sequence comprises a mafic to ultramafic dominated volcanic rock sequence of the Kalgoorlie Terrane, Yilgarn Craton, Western Australia. The Sequence is divided into Lower and Upper Units separated by the Kambalda Komatiite Formation. Five basalt suites of the Lower Unit are tholeiitic where MgO spans 5-10 wt.% MgO, with minor assimilation-fractional crystallization (AFC), whereas six volcanic suites identified in the Upper Unit are tholeiitic to komatiitic-basalts with MgO 24-5 wt.% having generally greater degrees of AFC. Upper suites plot at Al₂O₃/TiO₂ (17-26) close to the primitive mantle ratio of 21, and Pt + Pd (19-31 ppb), whereas the PGE-depleted Lower basalts plot at generally lower Al₂O₃/TiO₂ (<16) and Pt + Pd (<10 ppb). Most suites have an average Pt/Pd ratio of 1.11, despite large variations in MgO contents, broadly consistent with the Pt/Pd ratio in the primitive mantle. On primitive mantle-normalised PGE plots, Upper suites generally display less fractionated patterns of the IPGE (Os, Ir, Ru and Rh) from the PPGE (Pt and Pd) relative to the Lower basalts. Most suites exhibit patterns with positive slopes reflecting relative enrichment of Pd, Pt, Au and Cu relative to Ni and IPGE. In suites of both Units, the concentrations of Ir and Ru fall with decreasing MgO contents, indicating their broadly compatible behaviour during magmatic evolution that involved AFC. Platinum and Pd behave as incompatible elements in the high-MgO suites, whereas Pt and Pd behave compatibly during crystallisation of the Lower basalt magmas, an interpretation consistent with progressively higher Cu/Pt and Cu/Pd ratios at decreasing MgO contents, and with falling Pt/Ti, collectively due to sulphur saturation induced by AFC as recorded in an antivariance of Pd/Ir with Nb/Th, a monitor of AFC.Collectively, the data suggest that several of the Lower Basalt suites crystallised under sulphide-saturated conditions, whereas most of the Upper Basalt Sequences remained sulphur undersaturated during magmatic evolution. Alteration, and fractional crystallisation of silicate and oxide phases, can be ruled out as factors governing PGE distribution in these mafic-ultramafic suites. Instead, the data suggest that discrete PGE-bearing phase (s) fractionated from the magmas. Such phases could be platinum group minerals (PGM; e.g., laurite) and/or alloys, or discrete PGE-rich nuggets or sulphides.     

Rhodium, gold and other highly siderophile element abundances in chondritic meteorites

The abundances of the highly siderophile elements (HSE) Re, Os, Ir, Ru, Pt, Rh, Pd and Au, and ¹⁸⁷Os/¹⁸⁸Os isotope ratios have been determined for a set of carbonaceous, ordinary, enstatite and Rumuruti chondrites, using an analytical technique that permits the precise and accurate measurement of all HSE from the same digestion aliquot. Concentrations of Re, Os, Ir, Ru, Pt and Pd were determined by isotope dilution ICP-MS and N-TIMS analysis. The monoisotopic elements Rh and Au were quantified relative to the abundance of Ir.Differences in HSE abundances and ratios such as Re/Os, ¹⁸⁷Os/¹⁸⁸Os, Pd/Ir and Au/Ir between different chondrite classes are further substantiated with new data, and additional Rh and Au data, including new data for CI chondrites. Systematically different relative abundances of Rh between different chondrite classes are reminiscent of the behaviour of Re. Carbonaceous chondrites are characterized by low average Rh/Ir of 0.27 ± 0.03 (1s) which is about 20% lower than the ratio for ordinary (0.34 ± 0.02) and enstatite chondrites (EH: 0.33 ± 0.01; EL: 0.32 ± 0.01). R chondrites show higher and somewhat variable Rh/Ir of 0.37 ± 0.07.Well-defined linear correlations of HSE, in particular for bulk samples of ordinary and EL chondrites, are explained by binary mixing and/or dilution by silicates. The HSE carriers responsible for these correlations have a uniform chemical composition, indicating efficient homogenization of local nebular heterogeneities during or prior to the formation of the host minerals in chondrite components. Excepting Rumuruti chondrites and Au in carbonaceous chondrites, these correlations also suggest that metamorphism, alteration and igneous processes had negligible influence on the HSE distribution on the bulk sample scale.Depletion patterns for Rh, Pd and Au in carbonaceous chondrites other than CI are smoothly related to condensation temperatures and therefore consistent with the general depletion of moderately volatile elements in carbonaceous chondrites. Fractionated HSE abundance patterns of ordinary, enstatite and Rumuruti chondrites, however, are more difficult to explain. Fractional condensation combined with the removal of metal phases at various times, and later mixing of early and late formed metal phases may provide a viable explanation. Planetary fractionation processes that may have affected precursor material of chondrite components cannot explain the HSE abundance patterns of chondrite groups. HSE abundances of some, but not all Rumuruti chondrites may be consistent with solid sulphide-liquid sulphide fractionation processes during impact induced melting.     

Sorption and distribution of adsorbed metals in three soils of India

The mobility and bioavailability of heavy metals depends on the metal retention capacity of soil and also on the geochemical phases with which metals are associated. Laboratory batch experiments were carried out to study the sorption and distribution of Cd, Ni and Pb in 3 soils differing in their physicochemical properties from India: Oxyaquic Haplustalf (SL1), Typic Haplustalf (SL2) and Typic Haplustert (SL3). The heavy metal adsorption was studied by isotherms and the distribution coefficient (K_D) for each metal was obtained from the linear regressions of the concentration of metal remaining in equilibrium solution and the amount adsorbed. In general, the sorption capacity for all the metals decreased in the order: SL3>SL2>SL1. Among metals, the sorption capacity in all the soils decreased in the order: Pb>>Ni>Cd. Distribution of sorbed metals at various equilibrating concentrations was studied by sequential extraction. Results showed significant differences in the distribution of metals in these soils. At higher additions (such as 200 μM l⁻¹) most of the metals were extracted in their more mobile fractions, exchangeable and/or inorganic in contrast to their original partitioning in soils, where they were preferentially associated with the less mobile residual fraction. Largest percentages of metals extracted in the exchangeable fraction corresponded to those soil–metal systems with smaller K_D values, e.g. Cd, Ni and Pb in SL1 and Cd and Ni in SL2. In neutral and alkaline soils (SL2, pH=7.1, and SL3, pH=8.6) Pb was predominantly extracted from the inorganic fractions and this corresponded to higher K_D values for Pb in these soils. The predominance of metals associated with the exchangeable fraction together with low K_D values indicates higher mobility of metals retained in the acidic soil (SL1, pH=5.2) compared with the others.     

Platinum-group elements in the serpentinite lateritic mantles of the Kongo–Nkamouna ultramafic massif (Lomié region,South-East Cameroon)

The behaviour of PGE in a rainforest ecosystem were investigated in four lateritic profiles (Nkamouna, Napene, West and East Mada) developed on serpentinites in the Kongo–Nkamouna massif (Lomié region, South-East Cameroon). In serpentinites, the total PGE content attains 22 ppb whilst it ranges between 26 and 200 ppb in the weathering blanket. Amongst the analyzed elements (platinum, iridium, ruthenium, rhodium, palladium), platinum and ruthenium contents are high in the saprolite zone and in the hardened materials of some weathering profiles (40–66 ppb for platinum, 50–71 ppb for ruthenium). Apart from the hardened materials, the total PGE content decreases from the coarse saprolite towards the clayey surface soil. The Fe₂O₃-PGE diagram indicates a relatively similar behaviour in these iron-rich samples. The Pt–Ir, Pt–Pd, Pt–Ru, Pt–Rh diagrams portray positive correlations between platinum and other PGE. This fact is supported by the positive correlation noticed between IPGE and PPGE. The Pt/Ir, Pt/Pd, Pt/Ru and Pt/Rh values indicate that iridium, palladium, ruthenium and rhodium are more mobile than platinum. These data confirm the mobility of PGE in laterites and the positive correlation reveals that PGE might be accommodated in the interfaces of iron oxides. The mass balance assessment shows that PGE are strongly leached from the Kongo–Nkamouna weathering blanket except in the coarse saprolite of the Nkamouna profile.     

Exploring sustainability of aquifers based on predictive modeling of sorption characteristics of arsenic enriched Holocene sediments in Bangladesh

The importance of accessing safe aquifers in areas with high As is being increasingly recognized. The present study aims to investigate the sorption and mobility of As at the sediment-groundwater interface to identify a likely safe aquifer in the Holocene deposit in southwestern Bangladesh. The upper, shallow aquifer at around 18 m depth, which is composed mainly of very fine, grey, reduced sand and contains 24.3 μg/g As, was found to produce highly enriched groundwater (190 μg/L As). In contrast, deeper sediments are composed of partly oxidized, brownish, medium sand with natural adsorbents like Fe- and Al-oxides; they contain 0.76 μg/g As and impart low As concentrations to the water (4 μg/L). These observations were supported by spectroscopic studies with SEM, TEM, XRD and XRF, and by adsorption, leaching, column tests and sequential extraction. A relatively high in-situ dissolution rate (R_r) of 1.42 × 10⁻¹⁶ mol/m²/s was derived for the shallower aquifer from the inverse mass-balance model. The high R_r may enhance As release processes in the upper sediment. The field-based reaction rate (K_r) was extrapolated to be roughly 1.23 × 10⁻¹³ s⁻¹ and 6.24 × 10⁻¹⁴ s⁻¹ for the shallower and deeper aquifer, respectively, from the laboratory-obtained adsorption/desorption data. This implies that As is more reactive in the shallower aquifer. The partition coefficient for the distribution of As at the sediment-water interface (K_d-As) was found to range from 5 to 235 L/kg based on in-situ, batch adsorption, and flow-through column techniques. Additionally, a parametric equation for K_d-As (R² = 0.67) was obtained from the groundwater pH and the logarithm of the leachable Fe and Al concentrations in sediment. A one-dimensional finite-difference numerical model incorporating K_d and K_r showed that the shallow, leached As can be immobilized and prevented from reaching the deeper aquifer (∼150 m) after 100 year by a natural filter of oxidizing sand and adsorbent minerals like Fe and Al oxides; in this scenario, 99% of the As in groundwater is reduced. The deeper aquifer appears to be an adequate source of sustainable, safe water.     

Semi-empirical proton binding constants for natural organic matter

Average proton binding constants (K_H,i) for structure models of humic (HA) and fulvic (FA) acids were estimated semi-empirically by breaking down the macromolecules into reactive structural units (RSUs), and calculating K_H,i values of the RSUs using linear free energy relationships (LFER) of Hammett. Predicted log K_H,COOH and log K_H,Ph-OH are 3.73 ± 0.13 and 9.83 ± 0.23 for HA, and 3.80 ± 0.20 and 9.87 ± 0.31 for FA. The predicted constants for phenolic-type sites (Ph-OH) are generally higher than those derived from potentiometric titrations, but the difference may not be significant in view of the considerable uncertainty of the acidity constants determined from acid-base measurements at high pH. The predicted constants for carboxylic-type sites agree well with titration data analyzed with Model VI (4.10 ± 0.16 for HA, 3.20 ± 0.13 for FA; Tipping, 1998), the Impermeable Sphere model (3.50-4.50 for HA; Avena et al., 1999), and the Stockholm Humic Model (4.10 ± 0.20 for HA, 3.50 ± 0.40 for FA; Gustafsson, 2001), but differ by about one log unit from those obtained by Milne et al. (2001) with the NICA-Donnan model (3.09 ± 0.51 for HA, 2.65 ± 0.43 for FA), and used to derive recommended generic values. To clarify this ambiguity, 10 high-quality titration data from Milne et al. (2001) were re-analyzed with the new predicted equilibrium constants. The data are described equally well with the previous and new sets of values (R² ? 0.98), not necessarily because the NICA-Donnan model is overparametrized, but because titration lacks the sensitivity needed to quantify the full binding properties of humic substances. Correlations between NICA-Donnan parameters are discussed, but general progress is impeded by the unknown number of independent parameters that can be varied during regression of a model fit to titration data. The high consistency between predicted and experimental K_H,COOH values, excluding those of Milne et al. (2001), gives faith in the proposed semi-empirical structural approach, and its usefulness to assess the plausibility of proton stability constants derived from simulations of titration data.     

Reactive transport of gentisic acid in a hematite-coated sand column: Experimental study and modeling

The adsorption of gentisic acid (GA) by hematite nano-particles was examined under static and dynamic conditions by conducting batch and column tests. To simulate natural sediments, the iron oxide was deposited on 10 μm quartz particles. The GA adsorption was described by a surface complexation model fitted to pH-adsorption curves with GA concentrations of 0.1-1 mM in a pH range of 3-10. The surface was described with one type of site (FeOH°), while gentisic acid at the surface was described by two surface complexes (FeLH₂°, log K_int = 8.9 and FeLH⁻, log K_int = −8.2). Modeling was conducted with PHREEQC-2 using the MINTEQ database. From a kinetic point of view, the intrinsic chemical reactions were likely to be the rate-limiting step of sorption (∼10⁻³ s⁻¹) while external and internal mass transfer rates (∼10² s⁻¹) were much faster. Under flow through conditions (column), adsorption of GA to hematite-coated sand was about 7-times lower than under turbulent mixing (batch). This difference could not be explained by chemical adsorption kinetics as shown by test calculations run with HYDRUS-1D software. Surface complexation model simulations however successfully described the data when the surface area was adjusted, suggesting that under flow conditions the accessibility to the reactive surface sites was reduced. The exact mechanism responsible for the increased mobility of GA could not be determined but some parameters suggested that decreased external mass transfer between solution and surface may play a significant role under flow through conditions.     

Highly siderophile element composition of the Earth’s primitive upper mantle: Constraints from new data on peridotite massifs and xenoliths

Osmium, Ru, Ir, Pt, Pd and Re abundances and ¹⁸⁷Os/¹⁸⁸Os data on peridotites were determined using improved analytical techniques in order to precisely constrain the highly siderophile element (HSE) composition of fertile lherzolites and to provide an updated estimate of HSE composition of the primitive upper mantle (PUM). The new data are used to better constrain the origin of the HSE excess in Earth’s mantle. Samples include lherzolite and harzburgite xenoliths from Archean and post-Archean continental lithosphere, peridotites from ultramafic massifs, ophiolites and other samples of oceanic mantle such as abyssal peridotites. Osmium, Ru and Ir abundances in the peridotite data set do not correlate with moderately incompatible melt extraction indicators such as Al₂O₃. Os/Ir is chondritic in most samples, while Ru/Ir, with few exceptions, is ca. 30% higher than in chondrites. Both ratios are constant over a wide range of Al₂O₃ contents, but show stronger scatter in depleted harzburgites. Platinum, Pd and Re abundances, their ratios with Ir, Os and Ru, and the ¹⁸⁷Os/¹⁸⁸Os ratio (a proxy for Re/Os) show positive correlations with Al₂O₃, indicating incompatible behavior of Pt, Pd and Re during mantle melting. The empirical sequence of peridotite-melt partition coefficients of Re, Pd and Pt as derived from peridotites () is consistent with previous data on natural samples. Some harzburgites and depleted lherzolites have been affected by secondary igneous processes such as silicate melt percolation, as indicated by U-shaped patterns of incompatible HSE, high ¹⁸⁷Os/¹⁸⁸Os, and scatter off the correlations defined by incompatible HSE and Al₂O₃. The bulk rock HSE content, chondritic Os/Ir, and chondritic to subchondritic Pt/Ir, Re/Os, Pt/Re and Re/Pd of many lherzolites of the present study are consistent with depletion by melting, and possibly solid state mixing processes in the convecting mantle, involving recycled oceanic lithosphere. Based on fertile lherzolite compositions, we infer that PUM is characterized by a mean Ir abundance of 3.5 ± 0.4 ng/g (or 0.0080 ± 0.0009*CI chondrites), chondritic ratios involving Os, Ir, Pt and Re (Os/Ir_PUM of 1.12 ± 0.09, Pt/Ir_PUM = 2.21 ± 0.21, Re/Os_PUM = 0.090 ± 0.002) and suprachondritic ratios involving Ru and Pd (Ru/Ir_PUM = 2.03 ± 0.12, Pd/Ir_PUM = 2.06 ± 0.31, uncertainties 1σ). The combination of chondritic and modestly suprachondritic HSE ratios of PUM cannot be explained by any single planetary fractionation process. Comparison with HSE patterns of chondrites shows that no known chondrite group perfectly matches the PUM composition. Similar HSE patterns, however, were found in Apollo 17 impact melt rocks from the Serenitatis impact basin [Norman M.D., Bennett V.C., Ryder G., 2002. Targeting the impactors: siderophile element signatures of lunar impact melts from Serenitatis. Earth Planet. Sci. Lett, 217-228.], which represent mixtures of chondritic material, and a component that may be either of meteoritic or indigenous origin. The similarities between the HSE composition of PUM and the bulk composition of lunar breccias establish a connection between the late accretion history of the lunar surface and the HSE composition of the Earth’s mantle. Although late accretion following core formation is still the most viable explanation for the HSE abundances in the Earth’s mantle, the “late veneer” hypothesis may require some modification in light of the unique PUM composition.     

Linear Free Energy Relationships for Metal-Ligand Complexation: Bidentate Binding to Negatively-Charged Oxygen Donor Atoms

Stability constants for metal complexation to bidentate ligands containing negatively-charged oxygen donor atoms can be estimated from the following linear free energy relationship (LFER): log K_ML = χ_OO(α_O log K_HL,1 + α_O log K_HL,2) where K_ML is the metal-ligand stability constant for a 1:1 complex, K_HL,1 and K_HL,2 are the proton-ligand stability constants (the ligand pK_a values), and α_O is the Irving-Rossotti slope. The parameter χ_OO is metal specific and has slightly different values for five and six membered chelate rings. LFERs are presented for 21 different metal ions and are accurate to within approximately 0.30 log units in predictions of log K_ML values. Ligands selected for use in LFER development include dicarboxylic acids, carboxyphenols, and ortho-diphenols. For ortho-hydroxybenzaldehydes, α-hydroxycarboxylic acids, and α-ketocarboxylic acids, a modification of the LFER where log K_HL,2 is set equal to zero is required. The chemical interpretation of χ_OO is that it accounts for the extra stability afforded to metal complexes by the chelate effect. Cu-NOM binding constants calculated from the bidentate LFERs are similar in magnitude to those used in WHAM 6. This LFER can be used to make log K_ML predictions for small organic molecules. Since natural organic matter (NOM) contains many of the same functional groups (i.e. carboxylic acids, phenols, alcohols), the LFER log K_ML predictions shed light on the range of appropriate values for use in modeling metal partitioning in natural systems.     

In situ adsorption of mercury,methylmercury and other elements by iron oxyhydroxides and organic matter in lake sediments

Samples of authigenic material, sediment overlying water and oxic surface sediment (0–0.5-cm depth) from a perennially oxygenated lacustrine basin were analysed to investigate which solid phases are important for binding a suite of trace elements (Ag, As, Ca, Cd, Cu, Hg, In, methylmercury (MeHg), Mg, Mo, Pb, Sb and Zn). The authigenic material, which was collected with inert Teflon sheets deployed for several years across the sediment–water interface, contained mainly poorly crystallized Fe oxyhydroxides and natural organic matter, presumably humic substances derived from the watershed. Manganese oxyhydroxides were not present in the collected authigenic material due to the slightly acidic condition (pH = 5.6) of the lake that prevents the formation and recycling of these compounds. Conditional equilibrium constants for the adsorption of cationic (K_Fe–M) and anionic (K_Fe–A) trace elements onto the authigenic Fe oxyhydroxides were estimated from their concentrations in the authigenic material and in bottom water samples. These field-derived values of K_Fe–M and K_Fe–A were compared with those predicted by the surface complexation model, using laboratory-derived intrinsic adsorption constants and the water composition at the study site. Equilibrium constants (K_POM–M) were also calculated for the adsorption of the cationic trace elements onto the humic substances contained in the diagenetic material. The field-derived values of K_POM–M were compared to those predicted by the speciation code WHAM 6 for the complexation of the trace elements by dissolved humic substances in the lake. Combining the results of the present study with those on the distributions of trace elements in the porewater and solid-phase sediments reported in previous studies at the same site, it was determined whether the trace elements bind preferentially to Fe oxyhydroxides or natural organic matter in oxic sediments. The main inferences are that the anionic trace elements As, Mo and Sb, as well as the cationic metal Pb are preferentially bound to the authigenic Fe oxyhydroxides whereas the other trace elements, and especially Hg and MeHg, are preferentially bound to the humic substances.     

Groundwater chemistry of Al under Dutch sandy soils: Effects of land use and depth

Aluminium has received great attention in the second half of the 20th century, mainly in the context of the acid rain problem mostly in forest soils. In this research the effect of land use and depth of the groundwater on Al, pH and DOC concentration in groundwater under Dutch sandy soils has been studied. Both pH and DOC concentration play a major role in the speciation of Al in solution. Furthermore, the equilibrium with mineral phases like gibbsite, amorphous Al(OH)₃ and imogolite, has been considered. Agricultural and natural land use were expected to have different effects on the pH and DOC concentration, which in turn could influence the total Al concentration and the speciation of Al in groundwater at different depths (phreatic, shallow and deep). An extensive dataset (n = 2181) from the national and some provincial monitoring networks on soil and groundwater quality was used. Land use type and groundwater depth did influence the pH, and Al and DOC concentrations in groundwater samples. The Al concentration ranged from <0.4 μmol L⁻¹ at pH > 7 to 1941 μmol L⁻¹ at pH < 4; highest Al concentrations were found for natural-phreatic groundwater. The DOC concentration decreased and the median pH increased with depth of the groundwater. Natural-phreatic groundwater showed lower pH than the agricultural-phreatic groundwater. Highest DOC concentrations were found for the agricultural-phreatic groundwater, induced by the application of organic fertilizers. Besides inorganic complexation, the NICA-Donnan model was used to calculate Al³⁺ concentrations for complexation with DOC. Below pH 4.5 groundwater samples were mainly in disequilibrium with a mineral phase. This disequilibrium is considered to be the result of kinetic constraints or equilibrium with organic matter. Log K values were derived by linear regression and were close to theoretical values for Al(OH)₃ minerals (e.g. gibbsite or amorphous Al(OH)₃), except for natural-phreatic groundwater for which lower log K values were found. Complexation of Al with DOC is shown to be an important factor for the Al concentrations, especially at high DOC concentrations as was found for agricultural-phreatic groundwater.     

The behavior of noble-metal admixtures during fractional crystallization of As- and Co-containing Cu–Fe–Ni sulfide melts

To study the behavior of macrocomponents and admixtures during the fractional crystallization of sulfide melts and the influence of As on noble metals in this process, we performed a quasi-equilibrium directional crystallization of melt of composition (at.%): Fe—35.5, Ni—4.9, Cu—10.4, and S—48.3, with admixtures of Pt, Pd, Rh, Ru, Ir, Au, Ag, As, and Co (each 0.1 at.%), which imitates the average (by Cu contents) compositions of massive ores at the Noril'sk Cu-Ni deposits. The following sequence of phase formation from melt has been established: mss (zone I) / mss + iss (zone II) / iss (zone III) (mss is (Fe_zNi_1–z)S_1+δ, iss is (Fe_xCu_yNi_1–x–y)_zS_1–z); it corresponds to the distribution of main elements along the sample (primary zoning). Distribution curves for macrocomponents in zones I and II of the sample were constructed, as well as the dependencies of their partition coefficients (k) between solid solutions and sulfide melt on the fraction of crystallized melt. The secondary (mineral) zoning resulted from subsolidus phase transformations has been revealed. Five subzones have been recognized: mss + cp (Ia) / mss + cp + pn (Ib) / mss + pc + pn (IIa) / mss + pc + pn + bn (IIb) / pc + bn + pn + unidentified microphases (III). Admixture species in the sample were studied: (1) admixtures dissolved in primary solid solutions and in main minerals resulted from solid-phase transformations and (2) admixtures forming their own mineral phases. The partition coefficients of Co, Rh, and Ru (mss/L), Ru, Ir, and Rh (mss/cp), and Co, Rh, and Pd (mss/pn) were determined. Minerals of noble metals have been recognized: Pt₃Fe, PtFe, Au, (Ag,Pd), (Au,Pt), Ag, Ag₃Cu, Au₃(Cu,Ag,Pd,Pt), etc., and the regularities of their distribution in the sample have been established. It is shown that some noble-metal admixtures are prone to interact with As. Mineral arsenides and sulfoarsenides of noble metals produced during fractional crystallization have been recognized: PtAs₂, Pd₃As, (RhAsS), (IrAsS), and (Ir,Rh)AsS. The discovered drop-like inclusions of noble-metal arsenides suggest the separation of the initial sulfide-arsenide melt into two immiscible liquids. By indirect features, the micromineral inclusions are divided into primary, crystallized from melt, and secondary, produced in solid-phase reactions. The results of study are compared with literature experimental data obtained by the isothermal-annealing method and with the behavior of noble metals and As during the formation of zonal massive orebodies at the Noril'sk- and Sudbury-type deposits.     

The effect of species diversity on metal adsorption onto bacteria

In this study, we measure proton, Pb, and Cd adsorption onto the bacteria Deinococcus radiodurans, Thermus thermophilus, Acidiphlium angustum, Flavobacterium aquatile, and Flavobacterium hibernum, and we calculate the thermodynamic stability constants for the important surface complexes. These bacterial species represent a wide genetic diversity of bacteria, and they occupy a wide range of habitats. All of the species, except for A. angustum, exhibit similar proton and metal uptake. The only species tested that exhibits significantly different protonation behavior is A. angustum, an acidophile that grows at significantly lower pH than the other species of this study. We demonstrate that a single, metal-specific, surface complexation model can be used to reasonably account for the acid/base and metal adsorption behaviors of each species. We use a four discrete site non-electrostatic model to describe the protonation of the bacterial functional groups, with averaged pK_a values of 3.1 ± 0.3, 4.8 ± 0.2, 6.7 ± 0.1, and 9.2 ± 0.3, and site concentrations of (1.0 ± 0.17) × 10⁻⁴, (9.0 ± 3.0) × 10⁻⁵, (4.6 ± 1.8) × 10⁻⁵, and (6.1 ± 2.3) × 10⁻⁵ mol of sites per gram wet mass of bacteria, respectively. Adsorption of Cd and Pb onto the bacteria can be accounted for by the formation of complexes with each of the bacterial surface sites. The average log stability constants for Cd complexes with Sites 1-4 are 2.4 ± 0.4, 3.2 ± 0.1, 4.4 ± 0.1, and 5.3 ± 0.1, respectively. The average log stability constants for Pb complexes with Sites 1-4 are 3.3 ± 0.2, 4.5 ± 0.3, 6.5 ± 0.1, and 7.9 ± 0.5, respectively. This study demonstrates that a wide range of bacteria exhibit similar proton and metal adsorption behaviors, and that a single set of averaged acidity constants, site concentrations, and stability constants for metal-bacterial surface complexes yields a reasonable model for the adsorption behavior of many of these species. The differences in adsorption behavior that we observed for A. angustum demonstrate that genetic differences do exist between the cell wall functional group chemistries of some bacterial species, and that significant exceptions to the typical bacterial adsorption behavior do exist.     

Competitive adsorption,release and speciation of heavy metals in the Yellow River sediments,China

The competitive adsorption and the release of selected heavy metals and their speciation distribution before and after adsorption in the Yellow River sediments are discussed. The adsorption of metals onto sediments increases with increasing pH value and decreases with increasing ionic strength. The competitive coefficient K _c and the distribution coefficient K _d are obtained to analyze the competitive abilities of selected heavy metals, which are ranked as Pb > Cu >> Zn > Cd. The competition among selected heavy metals becomes more impetuous with increasing ion concentration in water. Speciation analysis was done by an improved analytical procedure involving five steps of sequential extraction. Cu, Pb and Zn were mainly transformed into the carbonate-bound form (50.8–87.7%) in adsorption. Most of (60.7–77.3%) Cd was transformed into the exchangeable form, and the percentage of carbonate-bound Cd was 19.7–30.4%. The release reaction was so quick that the release capacity of selected heavy metals from sediments to aqueous solution reached half of the maximum value only in 30 s. As opposed to adsorption, the release capacities of selected heavy metals were ranked as Cd > Zn >> Cu > Pb. In this study, Cd produces the most severe environmental hazards, because its concentration in the release solution is 85.8 times more than the human health criteria of US EPA.