Impact of metal sorption and internalization on nitrification inhibition

The goal of this study was to explore the relationship between metal extracellular sorption, intracellular accumulation, and nitrification inhibition. Metal sorption on nitrifying biomass was rapid and could be described by linear partitioning with partition coefficients (Kp) of 20.3 +/- 0.1, 0.4 +/- 0.0, 0.1 +/- 0.0, and 0.2 +/- 0.0 L/g biomass chemical oxygen demand for Cu, Zn, Ni, and Cd, respectively. On the other hand, intracellular Zn, Ni, and Cd concentrations continued to increase with time beyond 12 h after metal addition, whereas intracellular Cu attained equilibrium after 4 h. Metal internalization kinetics could be described by an intraparticle diffusion model, with characteristic diffusion time constants (td) of 9.4, 64.6, 80.5, and 66.1 h for Cu, Zn, Ni, and Cd, respectively. Ultimate internalized percentages of the total cell-associated metal were 1.4 +/- 0.0, 4.3 +/- 0.5,7.6 +/- 1.0, and 2.7 +/- 0.2% for Cu, Zn, Ni, and Cd, respectively. Nitrification inhibition was not a function of the sorbed metal fraction but correlated well with intracellular Zn, Ni, or Cd fractions. An intraparticle diffusion model coupled with a saturation-type biological toxicity model fit the inhibition data for varying initial Cd concentrations and exposure periods. In contrast, no relationship between intracellular or sorbed Cu concentrations and nitrification inhibition was observed. In the presence of 1 mM Cu, less than 13.3 +/- 10.5% cells remained viable as compared to 72.8 +/- 7.5,104.8 +/- 1.7, and 84.7 +/- 7.0% (assumed 100% viable cells in metal-free control) in the presence of 1 mM Zn, Ni, and Cd, respectively. Hence, the observations that inhibition by metals such as Zn, Ni, and Cd is related to their intracellular fraction and the slow kinetics of metal internalization indicate that metal inhibition can easily be underpredicted from short-term batch assays. Furthermore, the inhibitory mechanism of Cu was very different from Zn, Ni, and Cd and may involve rapid loss of membrane integrity.     

Slow formation and dissolution of Zn precipitates in soil: a combined column-transport and XAFS study

Recent spectroscopic studies have demonstrated the formation of layered double hydroxides (LDH) and phyllosilicates upon sorption of Zn2+, Ni2+, and Co2+ to clay minerals and aluminum oxides at neutral to alkaline pH and at relatively high initial metal concentrations (>1 mM). The intention of the present study was to investigate whether such phases also form in soil under slightly acidic conditions and at lower metal concentrations. Columns packed with a loamy soil were percolated with aqueous solutions containing 0.1 or 0.2 mM Zn, Ni, Co, and Cd in a 10 mM CaCl2 background at pH 6.5. Metal breakthrough curves indicated a rapid initial sorption step, resulting in retarded breakthrough fronts, followed by further slow metal retention during the entire loading period of 42 days (7000 pore volumes). Total metal sorption and the contribution of slow sorption processes decreased in the order Zn > Ni > Co > Cd. Leaching the reacted soil with 10 mM CaCl2 at pH 6.5 remobilized 8% of the total retained Zn, 15% of Ni, 21% of Co, and 77% of Cd. Subsequent leaching with acidified influent (pH 3.0) remobilized most of the remaining metals. X-ray absorption fine-structure (XAFS) spectroscopy revealed that slow Zn sorption was due to the formation of a Zn-Al LDH precipitate. Although Ni, Co, and Cd concentrations were too low for XAFS analysis, their leaching patterns suggest that part of Ni and Co were also incorporated in solid phases, while most sorbed Cd was still present as exchangeable sorption complex after 42 days. A small but significant percentage of the sorbed metals (2-5%) remained in the soil, even after leaching with more than 3000 pore volumes at pH 3.0, which may suggest micropore diffusion or incorporation into more stable mineral phases.     

Residual alcohol influence on NAPL saturation estimates based on partitioning tracers

The influence of residual cosolvent on the partitioning tracer technique for estimating a nonaqueous phase liquid (NAPL) saturation in porous media was investigated. Batch equilibrium and column miscible displacement tests were used to evaluate the influence of residual alcohol cosolvents in the aqueous phase on partitioning and transport of alcohol tracers through sandy soil columns containing tetrachloroethylene (PCE). As the volume fraction of cosolvent alcohol (f(c)) increased ( f(c) < or = 0.1; 10 vol %), partition coefficients (K(nc)) for the alcohol tracers linearly decreased for residual cosolvent ethanol, linearly increased for residual cosolvent tert-butyl alcohol, and did not exhibit an evident change for residual cosolvent 2-propanol. These observations are consistent with measured changes in solubility (S(c)) of the alcohol tracers over the same range (f(c) < or = 0.1) of these residual cosolvent alcohols. Column miscible displacement tests using ethanol as a residual cosolvent ( f(c) < or = 0.1) exhibited earlier partitioning tracer breakthrough leading to an underestimation of NAPL saturation (S(n)) when constant, cosolvent-free partitioning coefficients were assumed. The underestimation magnitude increased with higher initial residual cosolvent alcohol in the columns. The S(n) underestimates were not significant but were 1-10% lower than the actual S(n) (0.18). The estimated partition coefficients based on column tests with residual cosolvent (K(col)) were consistently less than those based on batch tests. Column tests with low (0.5%) and high (15%) S(n) revealed that the residual cosolvent alcohol effect was different depending on the amount of NAPL in the column. Using ethanol for a cosolvent (10%) and 2,4-dimethyl-3-pentanol as a partitioning tracer, the S(n) values were underestimated by about 17% and 5%, respectively, in the low and high NAPL saturation columns.     

Reaction-based model describing competitive sorption and transport of Cd, Zn, and Ni in an acidic soil

Predicting the mobility of heavy metals in soils requires models that accurately describe metal adsorption in the presence of competing cations. They should also be easily adjustable to specific soil materials and applicable in reactive transport codes. In this study, Cd adsorption to an acidic soil material was investigated over a wide concentration range (10(-8) to 10(-2) M CdCl2) in the presence of different background electrolytes (10(-4) to 10(-2) M CaCl2 or MgCl2 or 0.05 to 0.5 M NaCl). The adsorption experiments were conducted at pH values between 4.6 and 6.5 A reaction-based sorption model was developed using a combination of nonspecific cation exchange reactions and competitive sorption reactions to sites with high affinity for heavy metals. This combined cation exchange/specific sorption (CESS) model accurately described the entire Cd sorption data set. Coupled to a solute transport code, the model accurately predicted Cd breakthrough curves obtained in column transport experiments. The model was further extended to describe competitive sorption and transport of Cd, Zn, and Ni. At pH 4.6, both Zn and Ni exhibited similar sorption and transport behavior as observed for Cd. In all transport experiments conducted under acidic conditions, heavy metal adsorption was shown to be reversible and kinetic effects were negligible within time periods ranging from hours up to four weeks.     

Extracellular polymeric substances from Bacillus subtilis associated with minerals modify the extent and rate of heavy metal sorption

Extracellular polymeric substances (EPS) are an important source of organic matter in soil. Once released by microorganisms, a portion may be sorbed to mineral surfaces, thereby altering the mineral?s ability to immobilize heavy metals. EPS from Bacillus subtilis were reacted with Ca-saturated bentonite and ferrihydrite in 0.01 M KCl at pH 5.0 to follow the preferential uptake of EPS-C, -N, and -P. The sorption kinetics of Pb(2+), Cu(2+), and Zn(2+) to the resulting EPS-mineral composites was studied in single and binary metal batch experiments ([metal](total) = 50 μM, pH 5.0). Bentonite sorbed much more EPS-C (18.5 mg g(-1)) than ferrihydrite (7.9 mg g(-1)). During sorption, EPS were chemically and size fractionated with bentonite favoring the uptake of low-molecular weight components and EPS-N, and ferrihydrite selectively retaining high-molecular weight and P-rich components. Surface area and pore size measurements by N(2) gas adsorption at 77 K indicated that EPS altered the structure of mineral-EPS associations by inducing partial disaggregation of bentonite and aggregation of ferrihydrite. Whereas mineral-bound EPS increased the extent and rate of Pb(2+), Cu(2+), and Zn(2+) sorption for bentonite, either no effect or a decrease in metal uptake was observed for ferrihydrite. The extent of sorption always followed the order Pb(2+) > Cu(2+) > Zn(2+), which also prevailed in binary Pb(2+)/Cu(2+) systems. In consequence, sorption of EPS to different minerals may have contrasting consequences for the immobilization of heavy metals in natural environments by inducing mineral-specific alterations of the pore size distribution and, thus, of available sorption sites.     

Sorption and desorption of Cd2+ and Zn2+ ions in apatite-aqueous systems

As a low-soluble phosphate mineral capable of binding various metal ions, apatite can be used to immobilize toxic metals in soils and waters. In the present research the factors affecting sorption and desorption of Cd2+ and Zn2+ ions on/from apatites are investigated. Batch experiments were carried out using synthetic hydroxy-, fluoride-, and carbonate-substituted apatites having various specific surface area (SSA). Apatite sorption capacity was found to depend mainly on its SSA, ranging from 16 to 78 and from 11 to 79 mmol per 100 g of apatite for Cd2+ and Zn2+, respectively. The solution composition (pH, and presence of Cl- and NO3- ions) had no essential impact on sorption. Desorption of bound cations depended both on the sorption level and solution composition. The amount of desorbed Cd2+ and Zn2+ increased proportionally to the amount of sorbed cations. However, apatites having higher sorption capacity release relatively less sorbed cations. Desorption increases with increasing Ca2+ concentration in the solution, reaching 8-20% of sorbed Cd2+ in 0.002 M, 10-35% in 0.01 M, and 33-45% in 0.05 M Ca(NO3)2 solution. Compared to nitrate solutions, the presence of Cl- ions in the solution promotes the release of bound cations. Desorption of Zn2+ is slightly higher than that of Cd2+. The desorption mechanism was assumed to include both ion-exchange and adsorption of Ca2+ ions on apatite surface.     

Understanding the differences in Cd and Zn bioaccumulation and subcellular storage among different populations of marine clams

The marine clams Mactra veneriformis were collected from three different locations in a contaminated bay in Northern China. Another species of clams Ruditapes philippinarum was collected from the same contaminated bay as well as from a relatively clean site in Hong Kong. The indices of Cd and Zn bioaccumulation (assimilation efficiency, dissolved uptake rate, and efflux rate), tissue concentration, subcellular distribution, metallothionein (MT) content, and clearance rate of the clams were subsequently quantified in these populations in the laboratory. In the two species of clams, the population with a higher Cd tissue concentration assimilated Cd and Zn more efficiently, in correlation with an increase in the Cd associated with the metallothionein-like protein (MTLP) fraction. The subcellular partitioning of Zn was similar among the different populations. The dissolved uptake rates of Cd and Zn were not influenced by the different tissue concentrations of metals in the clams. However, the clam R. philippinarum from the contaminated site reduced their Zn uptake rate constants in response to increasing Zn concentration in the water. Differences in Cd and Zn tissue concentrations had little influence on the metal efflux rate constant and the clams' clearance rate. Our results indicate that the higher Cd and Zn tissue concentrations observed in these two species may be partially caused by the high levels of metal assimilation. Populations living in contaminated environments may be able to modify their physiological and biochemical responses to metal stress, which can subsequently alter trace metal bioaccumulation to aquatic animals. The relative significance of dietary uptake and the potential trophic transfer of metals in the contaminated areas may be substantially different from those in the clean environments.     

Solid-solution partitioning of Cd, Cu, Ni, Pb, and Zn in the organic horizons of a forest soil

We report the solid-liquid partitioning of Cd, Cu, Ni, Pb, and Zn in 60 organic horizon samples of forest soils from the Hermine Watershed (St-Hippolyte, PQ, Canada). The mean Kd values are respectively 1132, 966, 802, 3337 and 561. Comparison of those Kd coefficients to published compilation values show that the Kd values are lower in acidic organic soil horizons relative to the overall mean Kd values compiled for mineral soils. But, once normalized to a mean pH of 4.4, the Kd values in organic soil horizons demonstrate the high sorption affinity of organic matter, which is either as good as or up to 30 times higher than mineral soil materials for sorbing trace metals. Regression analysis shows that, within our data set, pH and total metal contents are not consistent predictors of metal partitioning. Indeed, metal sorption by the solid phase must be studied in relation to complexation by dissolved organic ligands, and both processes may sometime counteract one another.     

Modeling of single and competitive metal adsorption onto a natural polysaccharide

Sugar beet pulp, a common agricultural waste, was studied in the removal of metal ions from aqueous solutions. Potentiometric titrations were used to characterize the surface acidity of the polysaccharide. The acid properties of the material can be described by invoking three distinct types of surface functional groups with the intrinsic acidity constants (pKa(int)) values 3.43+/-0.1, 6.05+/-0.05, and 7.89+/-0.1, respectively. The contents of each functional group (i.e., the carboxyl and phenol moieties) were also determined. Then, a simple surface complexation model with the diffuse layer model successfully described the sorption of several metal ions (Cu2+, Zn2+, Cd2+, and Ni2+) onto the polysaccharide under various experimental conditions: pH ranging from 2 to 5.5, ionic strength from 0.01 to 0.1 M, metal concentration between 10(-4) and 10(-3) M, for a constant sorbent concentration equal to 2.5 g x L(-1). It was observed experimentally that the affinity of the polysaccharide was in the sequence of Cu2+ > Zn2+ > Cd2+ > Ni2+. Predictions of sorption in binary-metal systems based on single-metal data fits represented competitive sorption data reasonably well.     

Geochemistry of Cd,Cr, and Zn in highly contaminated sediments and its influences on assimilation by marine bivalves

We tested the controls of metal geochemistry in sediments collected from an extremely contaminated Chinese bay on metal assimilation by marine mussels and clams. Metal speciation in the contaminated sediments, quantified by the Tessier operational extraction method, was significantly dependent on metal concentrations in the sediments. The fractions of Cd in the easily exchangeable and carbonate phases increased, while the reducible and residue phases decreased with increasing Cd concentration. The majority (72-91%) of Cr was associated with the residue component with the remainder of Cr in the organic matter and reducible phases. Zn in carbonate phase increased, whereas in the organic matter and residue phases it decreased with increasing Zn concentration. The bioavailability of Cd, Cr, and Zn to marine green mussels (Perna viridis) and clams (Ruditapes philippinarum) was quantified using radiotracer spiked technique with concurrent measurements of speciation of spiked metals. There was a significant correlation between the Cd assimilation efficiency (AE) by both mussels and clams and Cd partitioning in the easily exchangeable and reducible phases. In contrast to previous studies, a negative correlation was found between the Cd AE and its total concentration in sediment, likely caused by the saturation of Cd binding sites in the gut or by its antagonistic interaction with a very high Zn concentration in these collected sediments. In contrast, there was no significant correlation between the AEs of Cr or Zn and any of their geochemical phases or their concentrations. The metal AEs were further quantified by experimentally manipulating different concentrations and ratios of acid volatile sulfide (AVS) and simultaneously extractable metals (SEM). There was no statistically significant relationship between the AEs of the three metals and the concentrations of AVS and SEM or [SEM-AVS]. Geochemical controls on metal assimilation from contaminated sediment are therefore only relatively apparent for Cd. The influences of metal speciation on metal bioavailability can be confounded by the degree to which sediments are contaminated with metals.     

Chemical speciation of Zn,Cd, Cu,and Pb in pore waters of agricultural and contaminated soils using Donnan dialysis

Knowledge of trace metal speciation in soil pore waters is important in addressing metal bioavailability and risk assessment of contaminated soils. Numerous analytical methods have been utilized for determining trace metal speciation in aqueous environmental matrixes; however, most of these methods suffer from significant interferences. The Donnan dialysis membrane technique minimizes these interferences and has been used in this study to determine free Zn2+, Cd2+, Cu2+, and Pb2+ activities in pore waters from 15 agricultural and 12 long-term contaminated soils. The soils vary widely in their origin, pH, organic carbon content, and total metal concentrations. Pore water pM2+ activities also covered a wide range and were controlled by soil pH and total metal concentrations. For the agricultural soils, most of the free metal activities were below detection limit, apart from Zn2+ for which the fraction of free Zn2+ in soluble Zn ranged from 2.3 to 87% (mean 43%). Five of the agricultural soils had detectable free Cd2+ with fractions of free metal ranging from 59 to 102% (mean 75%). For the contaminated soils with detectable free metal concentrations, the fraction of free metal as a percentage of soluble metal varied from 9.9 to 97% (mean 50%) for Zn2+, from 22 to 86% (mean 49%) for Cd2+, from 0.4 to 32.1% (mean 5%) for Cu2+, and from 2.9 to 48.8% (mean 20.1%) for Pb2+. For the contaminated soils, the equilibrium speciation programs GEOCHEM and WHAM Model VI provided reasonable estimates of free Zn2+ fractions in comparison to the measured fractions (R2 approximately 0.7), while estimates of free Cd2+ fractions were less agreeable (R2 approximately 0.5). The models generally predicted stronger binding of Cu2+ to DOC and hence lower fractions of free Cu2+ as compared with the observed fractions. The binding of Cu2+ and Pb2+ to DOC predicted by WHAM Model VI was much strongerthan that predicted by GEOCHEM.     

Batch and continuous packed column studies of cadmium biosorption by Hydrilla verticillata biomass

The removal of heavy metal ions by the nonliving biomass of aquatic macrophytes was studied. We investigated Cd biosorption by dry Hydrilla verticillata biomass. Data obtained in batch experiments indicate that H. verticillata is an excellent biosorbent for Cd. Cd was rapidly adsorbed and such adsorption reached equilibrium within 20 min. The initial pH of the solution affected Cd sorption efficiency. Results obtained from the other batch experiments conformed well to those obtained using the Langmuir model. The maximum adsorption capacity q(max) for H. verticillata was 15.0 mg/g for Cd. The breakthrough curve from the continuous flow studies shows that H. verticillata in the fixed-bed column is capable of decreasing Cd concentration from 10 to a value below the detection limit of 0.02 mg/l. The presence of Zn ions affected Cd biosorption. It can be concluded that H. verticillata is a good biosorbent for treating wastewater with a low concentration of Cd contaminants.     

Solubility and sorption by soils of 8:2 fluorotelomer alcohol in water and cosolvent systems

The solubility and sorption by five soils of 8:2 fluorotelomer alcohol (FTOH) were measured from water and cosolvent/ water solutions. Aqueous solubility and soil-water distribution coefficients (Kd,w, L kg(-1)) were extrapolated from cosolvent data using a log-linear cosolvency model and compared to direct aqueous measurements. Liquid chromatography tandem mass spectrometry with electrospray ionization was employed to analyze the 8:2 FTOH in solutions and soil extracts. The cosolvent-extrapolated water solubility is 0.224 mg L(-1), in good agreement with the measured value of 0.194 mg L(-1). All sorption isotherms were generally linear regardless of cosolvent composition or soil organic carbon (OC) content. Kd,w values extrapolated from cosolvent data were similar but consistently higher than those measured in aqueous solutions. The latter was hypothesized to be due to dissolved OC (DOC) in the aqueous slurries. An average log KDOC of 5.30 was estimated and supported by DOC and Kd,w measurements at two soil-water ratios. Sorption appeared to be driven by hydrophobic partitioning with a log KOC value of 4.13 +/- 0.16. Irreversible sorption was also observed and appeared to be related to OC content, with the extraction efficiency reduced from 85% to 45% with increasing contact time from 3 to 72 h for the highest OC soil.     

Characterization of zinc, lead, and cadmium in mine waste: implications for transport, exposure, and bioavailability

We characterized the lability and bioaccessibility of Zn, Pb, and Cd in size-fractionated mine waste at the Tar Creek Superfund Site (Oklahoma) to assess the potential for metal transport, exposure, and subsequent bioavailability. Bulk mine waste samples contained elevated Zn (9100 +/- 2500 ppm), Pb (650 +/- 360 ppm), and Cd (42 +/- 10 ppm), while particles with the greatest potential for windborne transport and inhalation (< 10 microm) contained substantially higher concentrations, up to 220 000 ppm Zn, 16 000 ppm Pb, and 530 ppm Cd in particles < 1 microm. Although the mined ore at Tar Creek primarily consisted of refractory metal sulfides with low bioavailability, sequential extractions and physiologically based extractions indicate that physical and chemical weathering have shifted metals into relatively labile and bioaccessible mineral phases. In < 37 microm mine waste particles, 50-65% of Zn, Pb, and Cd were present in the "exchangeable" and "carbonate" sequential extraction fractions, and 60-80% of Zn, Pb, and Cd were mobilized in synthetic gastric fluid, while ZnS and PbS exhibited minimal solubility in these solutions. Our results demonstrate the importance of site-specific characterization of size-fractionated contemporary mine waste when assessing the lability and bioavailability of metals at mine-waste impacted sites.     

Formation and dissolution of single and mixed Zn and Ni precipitates in soil: evidence from column experiments and extended X-ray absorption fine structure spectroscopy

The stability and the formation and dissolution kinetics of mixed trace metal precipitates in soils are currently unknown. The objective of this study was to investigate slow sorption and release processes of Zn and Ni in a loamy soil using a combination of soil column experiments and extended X-ray absorption fine structure (EXAFS) spectroscopy. To investigate slow sorption processes, the soil material was packed into columns and leached with 5400 pore volumes of 10(-2) M CaCl2 solutions containing either ZnCl2 (5.2 x 10(-5) M) or NiCl2 (5.2 x 10(-5) M) or both ZnCl2 and NiCl2 (5.2 x 10(-5) M each). The Zn and Ni concentrations in the column effluents were monitored. The metal breakthrough curves showed that slow sorption processes lead to metal retention, whereby Zn was more strongly retained than Ni. In the experiment with both Zn and Ni present, amounts of Zn and Ni similar to those in the experiments with either Zn or Ni alone were retained. Analysis of soil samples by EXAFS spectroscopy showed that layered double hydroxide (LDH)-type precipitates had formed in all columns and that a mixed ZnNi-LDH had formed in the presence of both Zn and Ni. The dissolution of those precipitates under acidic conditions was assessed by subsequent leaching of the columns with a 10(-2) M CaCl2 solution at pH 3.0 (approximately 3000 pore volumes). When only Zn was present, 95% of the retained Zn was leached at pH 3. In contrast, only 23% of the retained Ni was leached in experiments with Ni alone. When Zn and Ni were present, 90% of the retained Zn and 87% of the retained Ni were released upon acidification. EXAFS analysis revealed that the LDH phases in the Zn experiment and the Zn-Ni experiment had been completely dissolved, while the LDH phase formed in the Ni experiment was still present. The higher resistance of Ni-LDH against dissolution at low pH could also be shown in dissolution studies with synthetic Zn-LDH, Ni-LDH, and ZnNi-LDH. Our results suggest that the individual rates at which Zn and Ni cations enter into the LDH structure determine the composition of the mixed ZnNi-LDH precipitate, and that the LDH composition determines the rate at which the LDH phase dissolves under acidic conditions.     

Uptake of heavy metals by vegetable plants grown on contaminated soil and their bioavailability in the human gastrointestinal tract.

Lettuce, spinach, radish and carrot were grown on compost that had previously been contaminated at different concentrations of Cd, Cu, Mn, Pb and Zn. Control plants of each vegetable were also grown on unadulterated compost. The experiment was carried out under greenhouse conditions. Mature plants were harvested and their roots and leaves collected. Soil samples from each growing pot and plant materials were acid digested and analysed to determine total metal concentration. Flame-Atomic Absorption Spectroscopy (FAAS) was employed to determine metal concentrations in soil and plant samples (Mn and Zn), while Cd, Cu and Pb in plant materials were analysed by Differential Pulse Anodic Stripping Voltammetry (DP-ASV). Soil (BCR 146R and GBW 07310) and plant (tea leaves, INCT-TL-1) certified reference materials were used to assess accuracy and precision. The edible part of plants, i.e. the leaves of lettuce and spinach and the roots of radish and carrot, were also extracted using an in vitro gastrointestinal (GI) extraction to assess metal bioavailability. The results showed that the uptake of Cd, Cu, Mn and Zn by plants corresponded to the increasing level of soil contamination, while the uptake of Pb was low. Soil-to-plant transfer factor (TF) values decreased from Mn > Zn > Cd > Cu > Pb. Moreover, it was observed from this investigation that individual plant types greatly differ in their metal uptake, e.g. spinach accumulated a high content of Mn and Zn, while relatively lower concentrations were found for Cu and Pb in their tissues. From the in vitro gastrointestinal (GI) study, results indicate that metal bioavailability varied widely from element to element and according to different plant types. The greatest extent of metal releasing was found in lettuce (Mn, 63.7%), radish (Cu, 62.5%), radish (Cd, 54.9%), radish (Mn, 45.8%) and in lettuce (Zn, 45.2%).     

Trace metal levels in uncontaminated groundwater of a coastal watershed: importance of colloidal forms

Groundwater and surface water were collected using trace metal clean techniques from the upper glacial aquifer of West Neck Bay (Shelter Island) in eastern Long Island, NY, during the late spring and summer of 1999. The collection sites on Shelter Island are located in an area that is primarily residential and believed to have uncontaminated groundwater. Ultrafiltration was used to size-fractionate the dissolved (<0.45 microm) fraction into colloidal (1 kDa - 0.45 microm) and low molecular weight (<1 kDa) size pools. These fractions were analyzed for trace metals (Al, Ag, Cd, Cu, Mn, Pb, and Zn), organic carbon, and inorganic nutrients (NH4, NO3, PO4). The levels of metals and organic carbon in the groundwater were as low as those found in the open ocean, far removed from anthropogenic inputs. These findings corroborate the need to apply trace metal clean techniques in the determination of metal levels in uncontaminated groundwater. A significant fraction of dissolved metals (22-96%) and organic carbon (approximately 40%) in the groundwater and in surface waters of the Bay was found to be associated with colloids. The significance of the metal association with the colloidal fraction decreased in the order of Al > Cu > Ag > Zn = Cd = Mn and appeared to be dependent on the affinities of these metals for humic substances. In contrast, NO3 and NH4 were found to be almost entirely (approximately 98-99%) in the low molecular weight size fraction. Metal/aluminum and metal/carbon ratios measured in the colloids were similar to those reported for humic substances and significantly different from those of soils. This suggests that colloidal particles might originate from humic materials as opposed to purely inorganic minerals. These results indicate the need to consider the colloidal fraction in the fate and mobility of metals in groundwater and that, despite the low levels of organic matter (<50 microM of DOC) measured in groundwater, some groundwater colloids appear to be organic in nature.     

Adsorption of several metal ions onto a low-cost biosorbent: kinetic and equilibrium studies

Sugar beet pulp generated by sugar-refining factories has been shown to be an effective adsorbent for the removal of heavy metals from aqueous solutions. The structural components related to the metallic adsorption being determined, batch adsorption studies were performed for several metal ions, namely, Pb2+, Cu2+, Zn2+, Cd2+, and Ni2+ cations. Two simple kinetic models, that is, pseudo-first- and pseudo-second-order, were tested to investigate the adsorption mechanisms. The kinetic parameters of the models were calculated and discussed. For an 8 x 10(-4) M initial metal concentration, the initial sorption rates (v0) ranged from 0.063 mmol x g(-1) x min(-1) for Pb2+ to 0.275 mmol x g(-1) x min(-1) for Ni2+ ions, in the order Ni2+ > Cd2+ > Zn2+ > Cu2+ > Pb2+. The equilibrium data fitted well with the Langmuir and Freundlich models and showed the following affinity order of the material: Pb2+ > Cu2+ > Zn2+ > Cd2+ > Ni2+. The metal removal was strongly dependent on pH and, to a lesser extent, ionic strength. Ion exchange with Ca2+ ions neutralizing the carboxyl groups of the polysaccharide was found to be the predominant mechanism, added with complexation for Pb2+, Cu2+, and Zn2+ metals.     

Modeling of the solid-solution partitioning of heavy metals and arsenic in embanked flood plain soils of the rivers Rhine and Meuse

The aim of this study is to predict the solid-solution partitioning of heavy metals in river flood plain soils. We compared mechanistic geochemical modeling with a statistical approach. To characterize the heavy metal contamination of embanked river flood plain soils in The Netherlands, we collected 194 soil samples at 133 sites distributed in the Dutch part of the Rhine and Meuse river systems. We measured the total amounts of As, Cd, Cr, Cu, Ni, Pb, and Zn in the soil samples and the metal fraction extractable by 2.5 mM CaCl2. We found a strong correlation between heavy metal contamination and organic matter content, which was almost identical for both river systems. Speciation calculations by a fully parametrized model showed the strengths and weaknesses of the mechanistic approach. Cu and Cd concentrations were predicted within one log scale, whereas modeling of Zn and Pb needs adjustment of some model parameters. The statistical fitting approach produced better results but is limited with regard to the understanding it provides. The log RMSE for this approach varied between 0.2 and 0.32 for the different metals. The careful modeling of speciation and adsorption processes is a useful tool for the investigation and understanding of metal availability in river flood plain soils.     

Zinc adsorption on clays inferred from atomistic simulations and EXAFS spectroscopy

Clay minerals are efficient sinks for heavy metals in the geosphere. Knowing the uptake mechanism of these elements on clays can help to protect the natural environment from industrial pollution. In this study ab initio molecular dynamics (MD) calculations were applied to simulate the uptake of Zn on the edge surfaces of montmorillonite, a dioctahedral clay, and to explain the measured K-edge extended X-ray absorption fine structure (EXAFS) spectra of adsorbed Zn. These experiments were carried out using a high ionic strength Na background electrolyte that enables one to block cation exchange processes and to restrict the Zn uptake to the sorption complexation at the edge sites of clay. The analysis of the experimental data and simulation results suggest that structurally incorporated Zn preferentially substitutes for Al(III) in the trans-symmetric sites of the octahedral layer. At low loading, Zn is incorporated into the outermost trans-octahedra on (010) and (110) edges. At medium loading, Zn forms mono- and bidentate inner-sphere surface complexes attached to the octahedral layer of (010) and (110) edge sites. The maximal site density of inner-sphere sorption sites inferred from molecular simulations agrees well with site capacities of surface complexation sites derived from macroscopic studies and modeling.