Alfonso Mucci

A REVIEW OF THE BIOCHEMISTRY OF HEAVY METAL BIOSORPTION BY BROWN ALGAE

The passive removal of toxic heavy metals such as Cd(2+), Cu(2+), Zn(2+), Pb(2+), Cr(3+), and Hg(2+) by inexpensive biomaterials, termed biosorption, requires that the substrate displays high metal uptake and selectivity, as well as suitable mechanical properties for applied remediation scenarios. In recent years, many low-cost sorbents have been investigated, but the brown algae have since proven to be the most effective and promising substrates. It is their basic biochemical constitution that is responsible for this enhanced performance among biomaterials. More specifically, it is the properties of cell wall constituents, such as alginate and fucoidan, which are chiefly responsible for heavy metal chelation. In this comprehensive review, the emphasis is on outlining the biochemical properties of the brown algae that set them apart from other algal biosorbents. A detailed description of the macromolecular conformation of the alginate biopolymer is offered in order to explain the heavy metal selectivity displayed by the brown algae. The role of cellular structure, storage polysaccharides, cell wall and extracellular polysaccharides is evaluated in terms of their potential for metal sequestration. Binding mechanisms are discussed, including the key functional groups involved and the ion-exchange process. Quantification of metal-biomass interactions is fundamental to the evaluation of potential implementation strategies, hence sorption isotherms, ion-exchange constants, as well as models used to characterize algal biosorption are reviewed. The sorption behavior (i.e., capacity, affinity) of brown algae with various heavy metals is summarized and their relative performance is evaluated.

The incorporation of Mg2+ and Sr2+ into calcite overgrowths: influences of growth rate and solution composition

Abstract The concentrations of Mg2+ and Sr2+ incorporated within calcite overgrowths precipitated from seawater and related solutions, determined at 25°C, were independent of the precipitation rate over approximately an order of magnitude. The saturation states used to produce this range of precipitation rates varied from 3 to 17 depending on the composition of the solution. The amount of Mg2+ incorporated in the overgrowths was not directly proportional to Mg 2+ Ca 2+ in solution over the entire range (1–20) of ratios studied. Below a ratio of 7.5, the overgrowth was enriched in MgCO3 relative to what is predicted by the constant distribution coefficient measured above a ratio of 7.5. This increased MgCO3 correlates with the relative enrichment of adsorbed Mg2+. Above a ratio of 7.5 the concentration of MgCO3 in the calcite overgrowths followed a classical thermodynamic behavior characterized by a constant distribution coefficient of 0.0123 (±0.008 std dev). The concentration of SrCO3 incorporated in the overgrowths was linearly related to the MgCO3 content of the overgrowths, and is attributed to increased solubility of SrCO3 in calcite due to the incorporation of the smaller Mg2+ ions. The kinetic data indicate that the growth mechanism involves the adsorption of the cations on the surface of the calcite prior to dehydration and final incorporation. It is suggested that dehydration of cations at the surface is the rate controlling step.

Preservation of organic matter in sediments promoted by iron

The biogeochemical cycles of iron and organic carbon are strongly interlinked. In oceanic waters, organic ligands have been shown to control the concentration of dissolved iron. In soils, solid iron phases shelter and preserve organic carbon, but the role of iron in the preservation of organic matter in sediments has not been clearly established. Here we use an iron reduction method previously applied to soils to determine the amount of organic carbon associated with reactive iron phases in sediments of various mineralogies collected from a wide range of depositional environments. Our findings suggest that 21.5 ± 8.6 per cent of the organic carbon in sediments is directly bound to reactive iron phases. We further estimate that a global mass of (19–45) × 1015 grams of organic carbon is preserved in surface marine sediments as a result of its association with iron. We propose that these associations between organic carbon and iron, which are formed primarily through co-precipitation and/or direct chelation, promote the preservation of organic carbon in sediments. Because reactive iron phases are metastable over geological timescales, we suggest that they serve as an efficient ‘rusty sink’ for organic carbon, acting as a key factor in the long-term storage of organic carbon and thus contributing to the global cycles of carbon, oxygen and sulphur.

Acid base reactions, phosphate and arsenate complexation, and their competitive adsorption at the surface of goethite in 0.7 M NaCl solution

Potentiometric titrations of the goethite-water interface were carried out at 25°C in 0.1, 0.3 and 0.7 M NaCl solutions. The acid/base properties of goethite at pH . 4 in a 0.7 M NaCl solution can be reproduced successfully using either the Constant Capacitance (CCM), the Basic Stern (BSM) or the Triple Layer models (TLM) when two surface acidity constants are considered. Phosphate and arsenate complexation at the surface of goethite was studied in batch adsorption experiments. The experiments were conducted in 0.7 mol/L NaCl solutions at 25°C in the pH range of 3.0 to 10.0. Phosphate shows a strong affinity for the goethite surface and the amount of phosphate adsorbed decreases with increasing pH. Phosphate complexation is described using a model consisting of three monodentate surface complexes. Arsenate shows a similar adsorption pattern on goethite but a higher affinity than phosphate. A model including three surface complexation constants describes the arsenate adsorption at (AsO4)init 5 23 and 34 mmol/L. The model prediction, however, overestimates arsenate adsorption at (AsO4)init 5 8.8 mmol/L. The goethite surface acidity constants as well as the preceding phosphate and arsenate surface complexation constants were evaluated by the CCM and BSM with the aid of the computer program FITEQL, version 2.0. The experimental investigation of phosphate and arsenate competitive adsorption in 0.7 mol/L NaCl was performed at (PO4)/(AsO4) ratios of 1:1, 2.5:1 and 5:1 with (AsO4)init 5 9.0 mmol/L and at a (PO4)/(AsO4) ratio of 1:1 with (AsO4)init 5 22 mmol/L. The surface complexation of arsenate decreases significantly in competitive adsorption experiments and the decrease is proportional to the amount of phosphate present. Phosphate adsorption is also reduced but less drastically in competitive adsorption and is not affected significantly by incremental additions of arsenate at pH . 7. The equilibrium model derived by combining the single oxyanion subsystems predicts the shape of the competitive adsorption data but fails to reproduce it quantitatively. In competitive experiments, phosphate adsorption is underpredicted whereas arsenate adsorption is overpredicted. The inability of the models to accurately predict competitive adsorption may be due to site heterogeneity, adsorption kinetics, as well as the limitations of the present models. Copyright

Partitioning of rare earth elements (REEs) between calcite and seawater solutions at 25°C and 1 atm, and high dissolved REE concentrations

The partitioning of rare earth elements (REEs) in calcite overgrowths precipitated from sea-water solutions under steady-state conditions was investigated experimentally using a constant addition technique. The composition of the overgrowths and their parent solutions was determined by chelation and gradient ion chromatography (CGIC) and described using nonthermodynamic homogeneous partition coefficients. REEs are strongly partitioned into calcite and substitute for Ca2+. Their partition coefficients decrease systematically with atomic number, from ∼103.6 for La3+ to ∼101.9 for Yb3+ when their concentrations in solution are maintained at 70 nM. Under our experimental conditions, the partition coefficient of individual REEs was not affected by the calcite precipitation rate or [CO32−] of the solutions. The partitioning of light REEs, however, was influenced by variations of their absolute solution concentrations. The partitioning behaviour of REEs is correlated to the solubility of their respective carbonate minerals. REE speciation in solution, adsorption on the surface of calcite, and subsequent reactions (e.g., dehydration) affect the partitioning process. Although the dissolved REE concentrations used in these experiments are exceptionally high for natural marine environments, the compatibility of our results with those obtained from field studies suggests that the partition coefficients derived from this study may have some potential for the interpretation of environmental, diagenetic, and paleoceanographic data.

Influence of temperature on the composition of magnesian calcite overgrowths precipitated from seawater

A constant disequilibrium technique was used to determine the composition of magnesian calcite overgrowths precipitated on pure calcite seeds from artificial seawater at 5, 25 and 40°C. The amount of magnesium incorporated in the overgrowths at a given temperature is independent of the precipitation rate over a wide range of saturation states and is believed to correspond to a composition in true equilibrium with seawater. The distribution coefficient of magnesium, DMg2+c, in the magnesian calcite overgrowths increases almost linearly with temperature, being 0.0121 ± 0.0013 at 5°C, 0.0172 ± 0.0022 at 25°C, and 0.0271 ± 0.0013 at 40°C. These values apply only to magnesian calcites precipitated from standard composition seawater, since a previous study has shown DMg2+c to be a function of the [Mg2+][Ca2+] ratio in the parent solution. Results of this study are compared with values reported previously by other workers, and with the compositional distribution of naturally occurring magnesian calcite cements and ooids found in seawater. It appears that variations in temperature are not sufficient to account for the compositional variability of naturally occurring “inorganic” marine magnesian calcite cements.

Calcite and aragonite precipitation from seawater solutions of various salinities: Precipitation rates and overgrowth compositions

Abstract The reaction rate and composition of calcite and aragonite overgrowths precipitated from seawater solutions of various salinities (i.e. S=5, 15, 25, 35, 44) were determined at 25°C and 10−2.5-atm. CO2 partial pressure using a constant disequilibrium seeded technique. The rate data were fitted to an empirical rate law of the form: log R= n (ω c( or a ) -1)+ log k where n is the empirical reaction order; and k is the rate constant. Calcite precipitation rates in seawater solutions do not vary appreciably as a result of salinity variations over the range investigated, while those for aragonite show an increase in going from the higher (i.e. S=35, 44) to the lower (i.e. S=5, 15, 25) salinity range. This study also confirms previously published findings that above a given saturation state (i.e. Ω c >/2.6 ) aragonite precipitates more rapidly than calcite at 25°C. The incorporation of Sr2+ in aragonite and Mg2+ in calcite overgrowths are independent of the precipitation rate. The partition coefficient of Sr2+ in aragonite is approximately equal to unity and is unaffected by salinity variations between 5 and 44. However, the Mg2+ partition coefficient in calcite, increases with decreasing salinity of the parent seawater solutions, possibly as a result of variations in the sulfate content of the solutions and solids. The experimental results were discussed in the context of a number of geological environments.

Behaviour of anthropogenic mercury in coastal marine sediments

Abstract The diagenetic behaviour of anthropogenic mercury accumulated in sediments of the Saguenay Fjord (Canada) was investigated. Box-cores taken along its main axis and in the St. Lawrence estuary were analyzed for bulk sediment and porewater total and methyl-mercury concentrations as well as a number of other chemical variables. Mercury concentrations as high as 10,000 ng g −1 (dry weight) were measured in a core taken at the head of the fjord attesting to the presence of large quantities of mercury discharged by a chlor-alkali plant operated two decades ago along the Saguenay River. Porewater mercury concentrations ranged from 17 to 500 ng 1 −1 but were not correlated to the Hg content of the solid phase. Most of the mercury appears to be bound to organic matter, part of it is recycled with Mn and/or Fe oxides at the redox boundary whereas some may be adsorbed to or coprecipitated with the anomalously abundant acid volatile sulphides. These sulphides are very susceptible to oxidation and provide a more reactive sink to Hg than would pyrite. Despite the closure of the chlor-alkali plant in 1976 and relatively high sedimentation rates, surficial sediment Hg concentrations remain abnormally high. We investigated whether this observation could be explained by the diagenetic remobilization of Hg from the highly contaminated sediments buried below or resulted from other processes. The remobilization of Hg from deeper layers appears to be too slow to account for the high surficial sediment concentrations. Resuspension of older, contaminated sediments upstream during spring runoff or submarine mass flow may explain these observations. Methylation increases the solubility and mobility of Hg in sulphidic sediments but the CH 3 Hg(II) flux (0.07 ng cm −2 yr −1 ) from the contaminated layers to the surface sediment is negligible and accounts for only 0.01% of the present accumulation rate of mercury at the sediment surface. Dissolved and solid CH 3 Hg(II) profiles also indicate that this species may not diffuse through the thin oxic layer at the sediment-water interface. The estimated flux of Hg to the water column (20 ng cm −2 yr −1 ), however, could be underestimated since the activity of burrowing organisms would increase the exchange rate with the water column.

Growth kinetics and composition of magnesian calcite overgrowths precipitated from seawater: Quantitative influence of orthophosphate ions

Abstract A constant disequilibrium seeded-precipitation technique was used to determine the quantitative influence of orthophosphate ions on the growth kinetics and composition of magnesian calcite overgrowths, precipitated from artificial seawater (S = 35 per mil), at 25°C over a wide range of saturation states in the presence of 0 to 500 μM concentrations of Na2HPO4 The amount of Mg incorporated in the magnesian calcite overgrowths is, as previously reported, independent of the precipitation rate and is not influenced by the presence of up to 60 μM concentrations of Na2HPO4 A reevaluation of previous data and results of this study confirm the rate dependence of the Sr distribution coefficient in calcite, DeSr2+, observed by Lorens (1981). The influence of orthophosphate ions on the distribution coefficient of Sr in magnesian calcite overgrowths appears to be restricted to their influence on the precipitation rate. The precipitation rate data were fitted to an empirical kinetic expression R = k(Ω − 1) n where R = precipitation rate, k = rate constant, Ω = saturation state , and n = reaction order. A regression analysis of the data indicates that the precipitation-inhibition effectiveness of orthophosphate is a function of the PO43− concentration and increases as Ω approaches unity. A linear log-log relationship between the rate constant and the PO−34 concentration suggests a strong interaction between the PO43− and the energetically heterogeneous surface of the growing magnesian calcite.

Interactions between metal oxides and species of nitrogen and iodine in bioturbated marine sediments

By using a gold amalgam (Au/Hg) voltammetric microelectrode, we have measured simulta- neously and with millimeter resolution the distributions of O 2, Mn(II), Fe(II), I(2I), and HS(2I) in bioturbated sediment cores from the Laurentian Trough. We also measured nitrate and ammonia in the pore water, total I and ascorbate- and HCl-extractable Fe and Mn in the solid-phase sediment, and fluxes of O 2, NO3 , and NH4 across the sediment-water interface. The concentrations of O2 and Mn(II) were below their respective detection limits of 3 and 5 mM between 4 and 12 mm depth, but a sharp iodide maximum occurred at the depth where upward diffusing Mn(II) was being removed. We propose that the iodide peak is maintained through the reduction of IO3 by Mn(II), reoxidation of I(2I) to IO3 in the oxic zone above the peak and oxidation to I2 below where it is ultimately trapped by reaction with organic matter. The iodide production rate is sufficient to account for the oxidation of all of the upward diffusing Mn(II) by IO3 . Nitrate plus nitrite (SNO3) decreased to a minimum within 10 mm of the sediment-water interface, in agreement with flux measurements which showed SNO3 uptake by the sediment. Below the minimum, SNO3 rebounded, and reached a maximum at 40- to 50-mm depth. This rebound is attributed to the anaerobic oxidation of ammonia by manganese oxides. Fe(II) was always first detected below the anoxic SNO3 maximum, and was accompanied by colloidal or complexed Fe(III). A sharp upward-directed ammonia gradient was recorded near the sediment-water interface, but no ammonia was released during the first 48 h of the incubations. If the ammonia removal were due to coupled bacterial nitrification- denitrification, more than one half of the total measured oxygen uptake (6.7 to 7.3 mmol/m 2 /d) would be required, and more organic carbon would be oxidized by nitrate than by oxygen. This scenario is not supported by nitrate flux calculations. Alternatively, the oxidation of ammonia to N2 by manganese oxides is a potential removal mechanism. It would require one quarter of the total oxygen flux. The high-resolution profiles of redox species support the conceptualization of bioturbated sediments as a spatially and temporally changing mosaic of redox reactions. They show evidence for a multitude of reactions whose relative importance will vary over time, and for reaction pathways complementing those usually considered in diagenetic studies. Copyright