Marc F. Benedetti

Ion binding to natural organic matter : competition, heterogeneity, stoichiometry and thermodynamic consistency

Abstract The general principles of cation binding to humic materials are discussed. Important aspects that need to be included in general purpose speciation models are: the extreme binding heterogeneity of natural humic materials, the variable stoichiometry of binding (monodentate, bidentate and tridentate), the competition between specifically-bound ions, especially protons and metal ions, and electrostatic effects which give rise to ionic strength effects and the non-specific binding of counterions. The NICCA–Donnan model is a semi-empirical model that addresses these issues. It is similar to the previously published NICA–Donnan model except that it introduces an additional degree of scaling that ensures thermodynamic consistency and allows for variable stoichiometry of binding. It implicitly accounts for the large degree of chemical heterogeneity of humic particles. The NICCA (consistent NICA) model also recognizes that the affinity distributions are ion specific and are not fully correlated. The model requires no assumptions to be made about the geometry of the humic particles, but the Donnan submodel does allow for shrinking and swelling. Important model parameters such as the site density and median binding constants ( log K ) are not dependent on pH, metal ion concentration, ionic strength, etc. Data are analysed for H+, Ca2+, Cd2+, Cu2+, Pb2+ and Al3+ binding to a single purified peat humic acid. The NICCA–Donnan model captures the non-linearity of the observed isotherms even at very low free metal ion concentrations. After fitting the model to datasets containing only the proton and one metal ion, the model was able to predict Cd2+–Ca2+, Cu2+–Ca2+ and Pb2+–Al3+ competition reasonably well. It also gave satisfactory predictions of the H+/Mz+ molar exchange ratios. These ratios varied strongly with metal ion: Ca2+ (0.2–0.5); Cd2+ (0.5–1.0); Pb2+ (1.1–1.2); Cu2+ (1.2–1.7) and Al3+ (2.1–2.7), and also to a varying degree with pH and free metal ion concentration.

Metal ion binding to humic substances: application of the non-ideal competitive adsorption model.

The application of a new model to describe metal ion binding by humic acids is discussed. Metal ion binding is always of a competitive nature since the proton is always present. Although of great practical importance, the combination of a chemically heterogeneous system with competitive binding poses difficult problems from both experimental and theoretical points of view. The new Non-Ideal Competitive Adsorption model (NICA model) used here is able to account for the non-ideal binding to heterogeneous ligands. A good description of the binding of H, Ca, Cd, and Cu to a purified peat humic acid is achieved over a wide range of free metal ion concentrations (-2 > log Me 2+ > -14) and pH (2 < pH < 10). The results show that binding of metal ions to humic acid is strongly influenced by the intrinsic chemical heterogeneity of the humic material itself as well as by ion-specific non-ideality. The results indicate that copper competes much more efficiently with protons bound to the phenolic type groups than calcium and cadmium.

Metal ion binding by natural organic matter: from the model to the field.

With the newly developed NICCA-Donnan model, we estimate the activity of toxic metal ions from simple measurements like total metal concentration and organic matter content. The model evaluates Cu and Cd binding from three field systems, a mountain lake and two sandy soils, using model parameters calibrated for natural organic matter analogues with laboratory measurements. This is possible because the model includes site binding heterogeneity, electrostatic effects, competitive binding, and ion specific nonideality. The predictions derived closely matched the field observations when site binding densities are adjusted. The partition coefficients between soil and soil solution were also predicted for Cd and Cu under conditions where the organic matter controls the metal binding in both soil and soil solution. The model calculations show that in soil solutions 50% of the Cd and 99.99% of the Cu is bound to the dissolved organic matter. The model can be used to evaluate the effects of variations in the chemical conditions (e.g., acidification or total metal loading) on the free metal ion concentration in solution.

Metal ions speciation in a soil and its solution: experimental data and model results

Abstract The bioavailability and the potential toxicity of heavy metals in the environment depend on their speciation in the soil and the soil solution. The analytical task is rendered difficult because the individual chemical species are often present at nano- and picomolar concentration in natural systems. To determine the free metal ion (FMI) concentration, both analytical and computational approaches were used. The recently developed Donnan Membrane Technique (DMT) succeeded in measuring the free metal ion concentration. In a first analytical step, some Zn smelter impacted topsoil (0–5 cm) columns were linked to the DMT cell and metal concentrations were measured at different pHs. The measured free metal concentrations in the soil solution were compared to those obtained from model calculations. In the second part, metal concentrations in the soil solution are predicted with the soil major constituents characteristics. These combined approaches allow a good description of the soil solution chemistry. Cd and Zn are mainly in solution as free aquo-ion that will migrate easily. The speciation of Pb and Cu is regulated by the dissolved organic matter. Their transport will be controlled by organic matter mobility. Cd, Zn and Pb speciation in the soil is controlled by organic matter, metallic oxides (Fe, Mn) and newly formed Zn-bearing minerals. All these calculated speciation and distribution are in reasonable agreement with EXAFS data previously published for the same soil sample.

Amazon River carbon dioxide outgassing fuelled by wetlands

River systems connect the terrestrial biosphere, the atmosphere and the ocean in the global carbon cycle. A recent estimate suggests that up to 3 petagrams of carbon per year could be emitted as carbon dioxide (CO2) from global inland waters, offsetting the carbon uptake by terrestrial ecosystems. It is generally assumed that inland waters emit carbon that has been previously fixed upstream by land plant photosynthesis, then transferred to soils, and subsequently transported downstream in run-off. But at the scale of entire drainage basins, the lateral carbon fluxes carried by small rivers upstream do not account for all of the CO2 emitted from inundated areas downstream. Three-quarters of the world’s flooded land consists of temporary wetlands, but the contribution of these productive ecosystems to the inland water carbon budget has been largely overlooked. Here we show that wetlands pump large amounts of atmospheric CO2 into river waters in the floodplains of the central Amazon. Flooded forests and floating vegetation export large amounts of carbon to river waters and the dissolved CO2 can be transported dozens to hundreds of kilometres downstream before being emitted. We estimate that Amazonian wetlands export half of their gross primary production to river waters as dissolved CO2 and organic carbon, compared with only a few per cent of gross primary production exported in upland (not flooded) ecosystems. Moreover, we suggest that wetland carbon export is potentially large enough to account for at least the 0.21 petagrams of carbon emitted per year as CO2 from the central Amazon River and its floodplains. Global carbon budgets should explicitly address temporary or vegetated flooded areas, because these ecosystems combine high aerial primary production with large, fast carbon export, potentially supporting a substantial fraction of CO2 evasion from inland waters.

Plant-induced weathering of a basaltic rock: Experimental evidence

Abstract The active role of higher plants in the weathering of silicate minerals and rocks is still a question for debate. The present work aimed at providing experimental evidence of the important role of a range of crop plants in such processes. In order to quantitatively assess the possible effect of these diverse plant species on the weathering of a basaltic rock, two laboratory experiments were carried out at room temperature. These compared the amounts of elements released from basalt when leached with a dilute salt solution in the presence or absence of crop plants grown for up to 36 days. For Si, Ca, Mg, and Na, plants resulted in an increase in the release rate by a factor ranging from 1 to 5 in most cases. Ca and Na seemed to be preferentially released relative to other elements, suggesting that plagioclase dissolved faster than the other constituents of the studied basalt. Negligible amounts of Fe were released in the absence of plants as a consequence of the neutral pH and atmospheric pO2 that were maintained in the leaching solution. However, the amounts of Fe released from basalt in the presence of plants were up to 100- to 500-fold larger than in the absence of plants, for banana and maize. The kinetics of dissolution of basalt in the absence of plants showed a constantly decreasing release rate over the whole duration of the experiment (36 days). No steady state value was reached both in the absence and presence of banana plants. However, in the latter case, the rates remained at a high initial level over a longer period of time (up to 15 days) before starting to decrease. For Fe, the maximum rate of release was reached beyond 4 days and this rate remained high up to 22 days of growth of banana. The possible mechanisms responsible for this enhanced release of elements from basalt in the presence of plants are discussed. Although these mechanisms need to be elucidated, the present results clearly show that higher plants can considerably affect the kinetics of dissolution of basalt rock. Therefore, they need to be taken into account when assessing the biogeochemical cycles of elements that are major nutrients for plants, such as Ca, Mg, and K, but also micronutrients such as Fe and ‘nonessential’ elements such as Si and Na.

Remobilization of arsenic from buried wastes at an industrial site : mineralogical and geochemical control

An industrial area contaminated by As was studied to determine the source of this element and its speciation in As-bearing solids and in run-off waters. Mineral precipitates and water samples were collected and analyzed to assess processes controlling As mobility at this site. The integrated study of a contaminated industrial area allowed identification of the source of the As and of the nature of secondary As-bearing phases. The results obtained both on solid and water samples were used to model As behavior during waste leaching on carbonate rocks. At the upper end of a topographic transect across the site, run-off waters (pH=7.9) interact with surficial waste piles (containing arsenolite, arsenopyrite and pyrite), becoming acidic (pH=2.2) and concentrated in dissolved arsenate species (As5+) (ΣAs ranging from 0.961 to 3.149·10−3 mol/l). Those acidic waters interact with the limestone substratum, providing dissolved Ca which reacts with As to precipitate 1:1 Ca arsenates (weilite CaHAsO4, haidingerite CaHAsO4.H2O and pharmacolite CaHAsO4.2H2O) and, in minor amounts, Ca–Mg arsenates (picropharmacolite (Ca,Mg)3(AsO4)2 6H2O). The 1:1 Ca arsenates identified are known to precipitate at low pH (3–6) and seem to be stable in media with high dissolved CO2, in comparison with other types of Ca arsenates. However, due to their high solubilities, they are not strictly relevant candidates to immobilize As in contaminated surficial environments. Although reported solubilities decrease to values close to the French and US drinking standards in Ca-rich solutions, a thorough examination of the precipitation/dissolution kinetics of Ca arsenates should be undertaken to assess their long-term stability and their efficiency in rapidly immobilizing As in contaminated surficial environments.

Stable isotopes of Cu and Zn in higher plants: evidence for Cu reduction at the root surface and two conceptual models for isotopic fractionation processes

Recent reports suggest that significant fractionation of stable metal isotopes occurs during biogeochemical cycling and that the uptake into higher plants is an important process. To test isotopic fractionation of copper (Cu) and zinc (Zn) during plant uptake and constrain its controls, we grew lettuce, tomato, rice and durum wheat under controlled conditions in nutrient solutions with variable metal speciation and iron (Fe) supply. The results show that the fractionation patterns of these two micronutrients are decoupled during the transport from nutrient solution to root. In roots, we found an enrichment of the heavier isotopes for Zn, in agreement with previous studies, but an enrichment of isotopically light Cu, suggesting a reduction of Cu(II) possibly at the surfaces of the root cell plasma membranes. This observation holds for both graminaceous and nongraminaceaous species and confirms that reduction is a predominant and ubiquitous mechanism for the acquisition of Cu into plants similar to the mechanism for the acquisition of iron (Fe) by the strategy I plant species. We propose two preliminary models of isotope fractionation processes of Cu and Zn in plants with different uptake strategies.

Occurrence of Zn/Al hydrotalcite in smelter-impacted soils from northern France: Evidence from EXAFS spectroscopy and chemical extractions

Abstract Zinc speciation was studied by EXAFS spectroscopy, μ-SXRF elemental mapping, XRD, and chemical extraction methods in two smelter-impacted soils sampled near one of the largest Pb and Zn processing plants in Europe, which is located in northern France about 50 km south of Lille. The tilled and wooded soils chosen for study differ in Zn concentration (≈600 and 1400 mg/kg, respectively), soil pH (7.5 and 5.5, respectively), and organic matter content (1.5 and 6.4 wt% TOC, respectively). In both soils, the occurrence of Fe- and Zn-rich (up to 10 wt% Zn) slag particles ranging in size from a few micrometers to a few millimeters, was shown by m-SXRF elemental mapping of soil thin sections as well as by SEM and chemical analysis of different soil size fractions. For both soils, XRD analysis of the dense coarse fraction, which contains up to 10 wt% Zn, revealed the presence of a minor amount (1-1.5 wt%) of crystalline ZnS (sphalerite and wurtzite). In this fraction, EXAFS data show that Zn is mainly incorporated in the tetrahedral sites of a magnetite- franklinite solid solution. The clay fraction (<2 μm) represents the largest pool of Zn in both soils, with 77 and 62% of the total Zn in the tilled and wooded soils, respectively. However, XRD was not able to detect any Znbearing phases in this fraction. Comparison of Zn K-EXAFS data of untreated and chemically treated samples from the bulk (<2 μm) and the clay (<2 μm) soil fractions with Zn K-EXAFS data from more than 30 model compounds suggests that Zn is present in the following chemical forms: (1) Zn outer-sphere complexes, (2) Zn-organic matter inner-sphere complexes, (3) Zn/Al-hydrotalcite (Zn/ Al-HTLC), (4) phyllosilicates in which Zn is present in the dioctahedral layer at dilute levels, and (5) magnetite-franklinite solid solutions inherited from the smelting process. The presence of exchangeable Zn outer-sphere complexes and of Zn inner-sphere complexes on organic matter is indicated by the relative increase of second-neighbor contributions in the EXAFS RDFs after chemical treatments with 0.01 M CaCl2 and 0.1 M Na4P4O7. The occurrence of Zn/Al-HTLC is demonstrated by the persistence of a Zn-Zn pair correlation at 3.10 ± 0.04 Å (i.e., edge sharing ZnO6 octahedra in the trioctahedral layer structure) in EXAFS data of Na4P2O7 treated soil samples and its disappearance after treatment with 0.45 M HNO3. This latter treatment also revealed the occurrence of Znbearing phyllosilicate minerals, as shown by two Zn-Mg/Al/Si pair correlations at 3.05 ± 0.04 Å and 3.26 ± 0.04 Å, and of magnetite-franklinite solid solutions, as indicated by a Zn-Mn/Fe/Zn pair correlation at 3.50 ± 0.04 Å. Significant changes in the relative proportions of the different forms of Zn between the two soils explain their different responses to chemical treatments and emphasizes the relationships between solid state speciation and mobility of Zn in soils.

Water-rock interactions in tropical catchments: field rates of weathering and biomass impact

The knowledge of dissolution and precipitation rates of rocks and minerals is important to understand the effect of natural as well as human influences on the soil system. The combination of mass-balance calculations and water sample geochemistry allows us the computation of weathering rates for small watersheds. The basaltic area of Ribeirao Preto (Parana, Brazil) provides an opportunity to study solute acquisition by meteoric waters in basaltic terranes, to quantify the water-basalt interaction, to address the impact of biomass on the weathering and to estimate the age of the weathering processes affecting the basalts under humid tropical climate conditions. The mass-balance approach is extended to Si and trace elements as well as neoformed minerals. The computations are showing that the transformation rates calculated with a biomass factor are 1.3–5 times faster than the rates achieved for the model not taking into account the biomass. The rates of release of Si (log k, k in mol m−2s−1 Si) computed for augite and labradorite have the same magnitude: −12.6 and are two orders of magnitude smaller than the experimental rates. The rate of precipitation of kaolinite (log k, k in mol m−2s−1 Si) is − 15.2. The mean rate of lowering of the weathering front has a value of around 40 ± 10 mm kyr−1 under humid tropical climate. A mean age of 700 ± 200 kyr is calculated for the weathering processes affecting the basalts of the Parana basin.