Sorption Of Trace Elements Research Articles

Factors influencing sorption of trace elements in contaminated waters onto ground nut shells

Biosorbent materials such as nut shells has been considered a promising alternative to the classic methods to remove potentially toxic elements from contaminated waters, improving water quality for other uses. The present study evaluated the sorption capacity of almond, hazelnut, pistachio and walnut shells for the single and simultaneous removal of As, Cd, Co, Hg, Ni, Pb and Zn from contaminated waters. The influence of key parameters such as sorbent dose, ionic strength (ultrapure water vs. mineral water), element competition (mono-vs. multi-element spiked solution) and initial element concentration (maximum allowed value in wastewater discharges and ten times lower) was assessed. Hazelnut shells stood out as the material with higher potential to remove Cd, Co, Hg, Ni, Pb and Zn in the tested conditions, achieving up to 99% of removal (Cd and Pb). Arsenic was not removed by none of the studied materials and the removal of Cr was only obtained in a simple matrix and in mono-element solution (41% of removal). An increase of matrix complexity negatively affected the sorption capacity of all the biosorbents in the removal of all the elements tested. The same behavior was obtained when the elements were simultaneously present in solution, with Pb, Cd and Hg being highly influenced by the presence of other elements. The increase in sorbent dose from 1 to 5 g/L allowed to at least duplicate the removal of Cd, Co, Ni and Zn by all the biosorbents. When decreasing the initial element concentration ten times the maximum allowed value in wastewater discharges, a significant increase of elements removal was attained, more pronounced for Hg and Pb. The possibility to change initial conditions towards high efficiencies validate the high potential of those biosorbents to be used in water remediation, with special focus on hazelnut shells, although other sorbents such as walnut or almond shells can be considered efficient with appropriate initial conditions and depending on the target elements to be removed.

The influence of the physicochemical properties of sediment on the content and ecotoxicity of trace elements in bottom sediments

The sorption mechanisms of the trace elements in bottom sediment are not fully understood. The study aim was to analyse the effect sorption capacity of bottom sediments on the content of trace elements and the bottom sediment ecotoxicity. The study found higher content of trace elements caused higher potential toxicity of bottom sediments. However, the PCA analysis indicated that ecotoxicity to Heterocypris incongruens was not related to the trace element content in the sediments. It was found that some of the physicochemical properties of bottom sediments determine the behavior of the test organism. The study revealed a strong relationships between the properties of bottom sediments and trace element sorption, which results from significant differences in the carbonate and organic matter contents. The obtained relationships showed a significant role in trace element sorption of negatively dissociating functional groups (carboxyl, phenolic, alcohol, and carbonyl groups) of Cha, Cfa, Cnh and DOC fraction and of clay minerals. Positive correlation between the metal content and the volume of ultramicropores, cryptopores and residual pores suggest that these pore groups contain reactive sites capable of effective element sorption. In contrast, larger pores turned out to be of marginal importance in trace elements sorption, probably participating only in their migration within the bottom sediment structure. An understanding of the above factors will provide comprehensive information on the fate of trace elements in aquatic systems.

Trace elements sorption by bentonite in the presence of bacteria

Sorption of lead and copper by calcium bentonite, living and destroyed cells of gram-positive bacteria Bacillus subtilis, as well as in systems including the clay mineral and one of the biotic components in the concentration range of the elements from 25 to 250 μM was studied. The effect of acidity on the biosorption of trace elements was shown. The maximum biosorption of lead and copper was observed at pH 6 and reached 0.72 and 0.52 mM/g of dry matter, respectively. At pH 6 the maximum biosorption by the destroyed cells was also observed — 0.81 mM/g of lead and 0.71 mM/g of copper. Accumulation of trace elements by living and destroyed cells significantly exceeded their sorption by calcium bentonite. In the ternary systems, including bentonite and bacterial cells or bentonite and cell debris, there was an increase in the sorption of lead and copper compared to bentonite alone. At the same time, the sorption of trace elements by a mixture of bentonite and cell fragments was lower than the additively calculated sorption by the individual components for both lead and copper at all concentrations studied. A similar pattern was observed for the system of living cells and bentonite for copper and living cells and bentonite for lead at low metal concentrations.

地球表層物質による微量元素とり込み過程とそのモデル化

Clay minerals, metal oxyhydroxides, and carbonates are important earth surface materials, which are secondary minerals formed from primary minerals with water and atmosphere under earth surface conditions. These minerals usually possess a high surface area and are highly reactive to foreign dissolved species; therefore, they play important roles as sorbents for trace elements in natural water. Predicting distributions of trace elements between solution and mineral phases is important for understanding migration and accumulation behaviors of toxic and useful elements on the earth surface. To predict distributions, it is required to elucidate the chemical processes governing the sorption of trace elements on or in minerals, and to model chemical processes quantitatively. The important chemical processes governing the sorption of trace elements by earth surface materials are adsorption, surface precipitation, ion exchange, mineralization, and coprecipitation. The current understanding of these chemical processes as revealed from laboratory experiments is reviewed, as well as current approaches to modeling.

The potential role of aluminium hydroxysulphates in the removal of contaminants in acid mine drainage

Sorption of trace elements onto poorly-crystalline Al-hydroxysulphate minerals from acid mine drainage (AMD) has received less attention compared to similar Fe(III)-phases because the former are less abundant in many mine drainage environments. In addition, Al-hydroxysulphates precipitate at a higher pH, so their sorption characteristics could be masked or less significant after sorption of trace elements to Fe minerals, which form at lower pH. In this study, oxidation and titration experiments were conducted with Fe(II)-rich AMD solutions under atmospheric and anoxic conditions to elucidate, individually, the sorption capacity of trace elements in solution by Fe and Al-hydroxysulphates. Under atmospheric conditions, precipitation of Fe(III) as schwertmannite, led to total removal of Fe, As, Cr and Pb in solution and 50% of Al. Subsequently, contaminant-depleted solution began to be controlled by precipitation of basaluminite, which acted as an effective sink for the remaining Al, Cu and Si. On the contrary, under anoxic conditions, neutralisation of Fe(II)-rich solutions led first to the basaluminite precipitation, keeping all Fe available in solution and unveiling a heretofore unknown affinity for As and Cr. Basaluminite retains 60% of As in solution compared to 100% removal of As by schwertmannite. However, the sorption capacity of basaluminite is even more significant than that of Fe-phases, considering that Al concentrations in solution were much lower than those of Fe. These findings give new insights into the processes controlling contaminant mobility in anoxic environments (e.g., the bottom of AMD-affected water reservoirs) and pose new opportunities for treatment strategies.

Sorption of trace elements on xylain: An experimental study

Sorption and desorption of Sr, Mn, Co, Ni, Cu, and Zn on xylain from lignites of the Mariza-East coal basin (Bulgaria) were studied. Sorption behaviour for all the studied elements was found to be of similar pH dependence for all elements. The maximum quantities bound vary from 0.104meq (Sr) to 0.167meq for Cu. On treatment with a complex-forming agent such as tartaric acid, a high degree of extraction was achieved. However, appreciable amounts of elements remain tightly bound on the sorbent. When present simultaneously in solution, the quantity of every element sorbed, excluding Cu, is lower as compared with the quantities from the experiments for single cations. The cation-exchange mechanism of the sorption and formation of covalent links was determined. Such processes may be responsible for part of the organically-associated elements in coals in case they are influenced by solutions enriched in some trace elements.

Limited Zn and Ni mobility during simulated iron formation diagenesis

Iron formations (IF) are iron- and silica-rich chemical precipitates that were deposited during the Precambrian. Several recent studies have demonstrated how the trace metal abundances in IF can be used as proxies for the bioavailability of trace metals in ancient seawater; with the ultimate goal being to understand first-order controls on the composition of the ancient biosphere. However, the utility of IF as proxies depends on the immobilization of trace metals during diagenesis. Here, we assess the mobility of Zn and Ni from ferric oxyhydroxides (ferrihydrite) in the absence and presence of organic matter (glucose) during simulated diagenesis (170°C, 1.2kbar); similar to what some Precambrian IF experienced. Quantitative concentration data, coupled with X-ray diffraction analysis and electron microprobe element mapping, demonstrate that both metals are relatively immobile during simulated diagenesis. Additionally, the mechanism for initial Ni sorption is examined using X-ray adsorption spectroscopy. For the initial sorption of trace elements in abiotic ferrihydrite experiments, 93.38% Zn and 65.95% Ni were initially sorbed. In experiments utilizing biogenic ferrihydrite, 97.03% of Zn and 93.38% of Ni were initially sorbed. Following the diagenetic capsule treatments, more than 99% of Zn and more than 91.9% of Ni were retained under the varied conditions considered here. Capsule experiments suggest the strong retention of Zn and Ni following the diagenesis of either abiotic or biogenic ferrihydrite. Overall, our results indicate that paleomarine Zn and Ni concentrations are likely to be faithfully recorded in well-preserved IF deposits.

Gypsum addition to soils contaminated by red mud: implications for aluminium, arsenic, molybdenum and vanadium solubility

Red mud is highly alkaline (pH 13), saline and can contain elevated concentrations of several potentially toxic elements (e.g. Al, As, Mo and V). Release of up to 1 million m(3) of bauxite residue (red mud) suspension from the Ajka repository, western Hungary, caused large-scale contamination of downstream rivers and floodplains. There is now concern about the potential leaching of toxic metal(loid)s from the red mud as some have enhanced solubility at high pH. This study investigated the impact of red mud addition to three different Hungarian soils with respect to trace element solubility and soil geochemistry. The effectiveness of gypsum amendment for the rehabilitation of red mud-contaminated soils was also examined. Red mud addition to soils caused a pH increase, proportional to red mud addition, of up to 4 pH units (e.g. pH 7→11). Increasing red mud addition also led to significant increases in salinity, dissolved organic carbon and aqueous trace element concentrations. However, the response was highly soil specific and one of the soils tested buffered pH to around pH 8.5 even with the highest red mud loading tested (33% w/w); experiments using this soil also had much lower aqueous Al, As and V concentrations. Gypsum addition to soil/red mud mixtures, even at relatively low concentrations (1% w/w), was sufficient to buffer experimental pH to 7.5-8.5. This effect was attributed to the reaction of Ca(2+) supplied by the gypsum with OH(-) and carbonate from the red mud to precipitate calcite. The lowered pH enhanced trace element sorption and largely inhibited the release of Al, As and V. Mo concentrations, however, were largely unaffected by gypsum induced pH buffering due to the greater solubility of Mo (as molybdate) at circumneutral pH. Gypsum addition also leads to significantly higher porewater salinities, and column experiments demonstrated that this increase in total dissolved solids persisted even after 25 pore volume replacements. Gypsum addition could therefore provide a cheaper alternative to recovery (dig and dump) for the treatment of red mud-affected soils. The observed inhibition of trace metal release within red mud-affected soils was relatively insensitive to either the percentage of red mud or gypsum present, making the treatment easy to apply. However, there is risk that over-application of gypsum could lead to detrimental long-term increases in soil salinity.

Pre-Concentration Procedure Based on Chitosan Combined with Ionic Liquid for the Determination of Cobalt, Nickel, and Copper in Water Samples

An environmentally friendly analytical procedure of pre-concentration of cobalt, nickel, and copper according to the rules of green chemistry has been developed. The proposed method is based on using chitosan for sorption of trace elements from water samples. The novel modification of the sorption process is the combination of an ionic liquid (1-butyl-3-methylimidazolium hexafluorophosphate [BMIM][PF6]) with chitosan. Ionic liquid partly decreases the crystallinity of the chitosan. The crystalline regions of the polymer are not accessible to metal ions. Although the ionic liquid cannot completely disrupt the crystalline domains of chitosan, it may gain in the reactive groups of the chitosan, even at the center of the particle. Consequently, the sorption of metal ions by chitosan is significantly improved. In this paper, adsorption characteristics of cobalt, nickel, and copper using newly developed sorption are studied. The effect of pH and time of chitosan activation, as well as sorption, salt concentration, some metals ion concentrations, and the amount of adsorbent on the extent of adsorption, are investigated. Chitosan with adsorbed metal ions was dissolved in acetic acid. After evaporation a solvent film formed and was then analyzed using X-ray fluorescence spectrometry (XRF). As it meets the criterion of thin samples for XRF analysis, the matrix effects can be neglected. With the proposed procedure we obtained detection limits of 7 ng mL(-1) for cobalt, 5 ng mL(-1) for nickel, and 4 ng mL(-1) for copper.

Carbon nanotubes as a solid sorbent for the preconcentration of Cr, Mn, Fe, Co, Ni, Cu, Zn and Pb prior to wavelength-dispersive X-ray fluorescence spectrometry

The preconcentration of trace elements on multiwalled carbon nanotubes (MWCNTs) followed by a wavelength-dispersive X-ray fluorescence analysis (WDXRF) has been investigated. The proposed preconcentration procedure is based on the sorption of trace elements on MWCNTs dispersed in analyzed solution. After sorption, the MWCNTs with the metal ions were collected onto the filter, and then the preconcentrated elements were determined directly by WDXRF. The preconcentration method was optimized, and in consequence, in order to obtain satisfactory recoveries using 100mL of samples, the sorption process was performed with 1mg of MWCNTs within 5min. Some conditions of the preconcentration process such as the pH of analyte solution, amounts of MWCNTs, the volume of the sample, the contact time between analytes and MWCNTs (stirring time), and the effects of foreign metals are discussed in detail in the paper. Adsorption onto raw and oxidized MWCNTs was also studied. The proposed procedure allows obtaining the detection limits of 0.6, 0.6, 1.0, 0.7, 0.6, 0.5, 0.9 and 1.9ngmL−1 for Cr(III), Mn(II), Fe(III), Co(II), Ni(II), Cu(II), Zn(II) and Pb(II), respectively. The recoveries of determined elements were about 100%. Because the analytes are not eluted from the sorbent before WDXRF analysis, the risk of contamination and loss of analytes is reduced to minimum. Moreover, because the samples are analyzed as a thin layer, the matrix effects can be neglected. The proposed preconcentration method using MWCNTs coupled with WDXRF spectrometry was successfully applied to determine trace elements in natural water samples.

Influence of the ionic strength and solid/solution ratio on Ca(II)-for-Na+ exchange on montmorillonite. Part 1: Chemical measurements, thermodynamic modeling and potential implications for trace elements geochemistry

Na/Ca ion-exchange isotherms were performed on a Na-saturated montmorillonite for different constant normalities of the aqueous chloride solution and at two solid/solution ratios. The experimental data suggest that the affinity of Na(+) for the solid increases with total normality and m/V ratio and that a significant proportion of CaCl(+) is also sorbed. A thermodynamic modeling procedure with one sorption site and three sorbed cations (Na(+), Ca(2+) and CaCl(+)) was applied to interpret the data. We show that by accounting for the activities of aqueous species, the experimental data obtained at different total normalities for a given solid/solution ratio can be fitted using a unique set of selectivity coefficient values. However, when the m/V ratio is decreased from 25 to 2.5 g/L, an increase in the log Kc(Na(+)/Ca(2+)) of up to 0.4 ± 0.05 was required to interpret the data with a constant log Kc(Na(+)/CaCl(+)) value. The same behavior concerning the increase in the log Kc(Na(+)/Ca(2+)) with a m/V ratio decrease was observed when using a multi-site model taking into account the specific sorption of H(+). The results clearly indicate that the Na(+)/Ca(2+) selectivity coefficients strongly depend on the solid/solution ratio but are independent of the exchanger composition and the total normality of the solution. Such findings provide possible pitfalls when using selectivity coefficients obtained in diluted suspension to assess the sorption in compacted clayrocks and allow a better prediction of the sorption of trace elements in competition with major cations.

Influence of biochar application to soil on the availability of As, Cd, Cu, Pb, and Zn to maize (Zea mays L.)

Biochar has gained importance due to its ability to increase the long-term soil carbon pool and improve crop productivity. However, little research has been done to evaluate the influence of biochar application to soil on the bioavailability of trace elements to plants. A pot experiment was conducted to investigate the influence of biochar on the availability of As, Cd, Cu, Pb, and Zn to maize (Zea mays L.). An activated wood biochar, pyrolysed at 550°C, was applied at 3 rates (0, 5, and 15 g/kg) in factorial combinations with 3 rates (0, 10, and 50 mg/kg) each of As, Cd, Cu, Pb, and Zn separately to a sandy soil. After 10 weeks of growth, plants were harvested, shoot dry matter yield was measured, and concentration of trace elements in shoots was analysed. The soil in pots was analysed for extractable trace elements. The results showed that the addition of wood biochar to soil did not have any significant effect on the dry matter yield of maize plants, even at the highest rate of application. However, trace element application significantly reduced the dry matter yield from 10 to 93% depending on the type of trace element. Biochar application decreased the concentration of As, Cd, and Cu in maize shoots, especially at the highest rate of trace element application, whereas the effects were inconsistent on Pb and Zn concentrations in the shoots. The concentrations of extractable As and Zn in soil increased with biochar application, whereas extractable Cu did not change, Pb decreased, and Cd showed an inconsistent trend. Sorption of trace elements on biochar with initial loadings up to 200 µmol at pH 7 occurred in the order: Pb > Cu > Cd > Zn > As. The results show that biochar application can significantly reduce the availability of trace elements to plants and suggest that biochar application may have potential for the management of soils contaminated by trace elements.

Polycarboxylic microsphere polymer gel for solid phase extraction of trace elements

A simple and sensitive method has been developed for the determination of trace elements (Cd, Co, Cu, Ni, Fe and Pb) in sea water and natural waters by electrothermal atomic absorption spectrometry. The method is based on column solid phase extraction of trace elements on a newly synthesized polycarboxylic gel. The sorbent was prepared by dispersion copolymerization of methacrylic acid (as a monomer) and trimethylolpropane trimethacrylate (as a crosslinking agent) in the form of monodisperse microspheres. It exhibits high regeneration ability, chemical and mechanical resistivity. The influence of pH, flow rates and concentration of the eluent on the degree of sorption of trace elements have been evaluated. The maximum static adsorption capacities of the sorbent at the optimal conditions for lead, nickel, cobalt, iron, cadmium and copper are determined. Detection limits achieved for the elements studied for 100 mL water sample volume were between 0.005 and 0.05 μg L−1. The relative standard deviation varied in the range 5–13% for all elements studied. The validity of the method was checked by an analysis of standard reference material SRLS-3 Riverine Water. Very good agreement between the analytical results and the certified values (t-test at 95% confidence level) was observed. The new polycarboxylic gel was applied to passive sampling procedures due to its high chemical and mechanical stability.

Sorption-Desorption Processes of Metals and Metalloids in Soil Environments

Trace elements may be present in solution with positive or negative charges and in different redox state. They occur predominantly in cationic form (Pb, Cu, Zn, Ni, Cd, Hg, Cr(III), Co) but some trace elements are present in anionic form (As, Se, Cr(VI), Mo and B). In soil environments, sorption/desorption reactions on/from inorganic and organic sorbents control the concentration of metals and metalloids in solution and affect their bioavailability, leaching and toxicity. Sorption-desorption processes of elements in cationic form differ greatly from those in anionic form. These reactions are affected by many factors, such as pH, nature of the sorbents, presence and concentration of organic and inorganic ligands, including humic and fulvic acids, root exudates and nutrients. Furthermore, redox reactions, both biotic and abiotic, are of great importance in controlling the oxidation state and thus, the mobility, the phytoavailability and the toxicity of many elements, such as Cr, Se, Co, Pb, As, Ni and Cu (Kabata-Pendias, 2001; Huang and Germida, 2002; Violante et al., 2002; Sparks, 2003; Huang and Gobran, 2005; Violante et al., 2007). The aim of this presentation is to provide information on the factors which affect the mobility of trace elements. Special attention is devoted to the influence of inorganic and organic ligands, including nutrients and root exudates, on the sorption/desorption processes of trace elements in cationic and anionic forms on/from soil components, soils and selected synthetic minerals (double layered hydroxides, the so called “anionic clays”). The soil components responsible for trace element sorption include, soil humic substances, phyllosilicates, carbonates and variable charge minerals (constituents such as Fe, Al, Mn and Ti oxides, short-range ordered aluminosilicates as well as phyllosilicates coated by OH-Al or OH-Fe species whose charge varies with the pH of the soil solution). Recently, evidence on the sorption of heavy metals on microorganisms have been reported (Jackson, 1998). Soil components differ greatly in their sorption capacities, their cation and anion exchange capacities, and the binding energies of their sorption sites. Trace element sorption kinetics depend on the type of surface and trace element, but generally are rapid. Half times for bivalent cations, such as Pb, Zn, Cu and Cd, sorption on peat range from 5 to 15 seconds. Except for some noncrystalline minerals that have very high specific surface charge density with highly reactive sites, humic substances appear to have the greatest capacity for sorption of trace elements in cationic form. A body of evidence has demonstrated that humic matter and metal oxides are much more effective scavengers of trace elements in cationic form, than even the most efficient sorbent among phyllosilicates, indicating that specific sorption and other complexation processes are the dominant binding mechanisms (Jackson, 1998; Huang and Germida, 2002; Spark, 2003). Complexation reactions have the following effects: i) metal ions are prevented from being precipitated; ii) complexing agents can act as carriers for trace elements in soil solution; iii) their toxicity is often reduced by complexation.

Study of trace metal partitioning between soil–EDTA extracts and Chelex-100 resin

The partitioning of Cu, Cd and Pb in EDTA leachates from a variety of soils, to Chelex-100 resin was investigated. A series of experiments was conducted to optimise method for the trace element sorption on Chelex-100 in 13 unpolluted soils from Burgundy, France. The extraction efficiency of Chelex-100 varied depending on soil carbonate contents. The largest percentage of trace metals was transferred to the resin in non-calcareous samples, while the resin was used in its Ca form. This technique was able to trap up to 98% of Cu, 72% of Cd and 28% of Pb in non-calcareous soil leachates. The fraction of metal transferred to the Chelex-100 in calcareous soil extracts was 40% less than those in non-calcareous ones. Besides, a kinetic procedure was developed to study the kinetic of cation exchange from soil–EDTA leachates onto solid-phase resin. The soil calcium content was found to be a controlling factor in the Cu sorption on the Ca–Chelex. Two different kinetic behaviours were observed in calcareous and non-calcareous soils. The dissociation rate of Cu in non-calcareous soil extracts was 10 times higher than that in calcareous ones. Application of kinetic model indicated the presence of two components associated to Cu in non-calcareous samples. This was discussed in terms of metal sorption mechanism from soil solution onto chelating resin.

Trace Element Chemistry in Residual‐Treated Soil: Key Concepts and Metal Bioavailability

Trace element solubility and availability in land-applied residuals is governed by fundamental chemical reactions between metal constituents, soil, and residual components. Iron, aluminum, and manganese oxides; organic matter; and phosphates, carbonates, and sulfides are important sinks for trace elements in soil-residual systems. The pH of the soil-residual system is often the most important chemical property governing trace element sorption, precipitation, solubility, and availability. Trace element phytoavailability in residual-treated soils is often estimated using soil extraction methods. However, spectroscopic studies show that sequential extraction methods may not be accurate in perturbed soil-residual systems. Plant bioassay is the best method to measure the effect of residuals on phytoavailability. Key concepts used to describe phytoavailability are (i) the salt effect, (ii) the plateau effect, and (iii) the soil-plant barrier. Metal availability in soil from metal-salt addition is greater than availability in soil from addition of metal-containing residuals. Plant metal content displays plateaus at high residual loadings corresponding to the residual's metal concentration and sorption capacity. The soil-plant barrier limits transmission of many trace elements through the food chain, although Cd (an important human health concern) can bypass the soil-plant barrier. Results from many studies that support these key concepts provide a basis of our understanding of the relationship between trace element chemistry and phytoavailability in residual-treated soils. Research is needed to (i) determine mechanisms for trace element retention of soil-residual systems, (ii) determine the effect of residuals on ecological receptors and the ability of residuals to reduce ecotoxicity in metal-contaminated soil, and (iii) predict the long-term bioavailability of trace elements in soil-residual systems.

Microorganisms and Their Influence on Radionuclide Migration in Igneous Rock Environments

Microorganisms interact with their surroundings and in some cases they greatly modify the characteristics of their environment. Several such interactions may have a significant influence on the behaviour of radionuclides possibly escaping from underground radioactive waste repositories. Microbes can mobilise trace elements. Unattached microbes may act as large colloids, transporting radionuclides on their cell surfaces with the groundwater flow. Many microbes produce ligands that can mobilise trace elements from solid phases and that can inhibit trace element sorption to solid phases. Bacterial species from the deep subsurface have demonstrated a significant effect on the mobilization of 59Fe(III), 147Pm(III), 234Th(IV) and 241Am(III) under varying redox conditions. The extent of bacterial immobilisation of radionuclides has been investigated under in situ conditions. Experiments have demonstrated this effect with 60Co, 147Pm, 234Th, 237Np, and 232U. A large group of microbes catalyse the formation of iron oxides from dissolved ferrous iron in groundwater that reaches an oxidising environment. Such biological iron oxide systems (BIOS) will have a retardation effect on many radionuclides. Microorganisms execute an important influence on the chemical situation in groundwater. Especially, they may catalyse reactions that stabilise the redox potential in groundwater at a low and, therefore, beneficial level for a radioactive waste repository.

Sorption of Trace Elements on Mineral Surfaces: Modern Perspectives from Spectroscopic Studies, and Comments on Sorption in the Marine Environment

The partitioning (or sorption) of trace elements from aqueous solutions onto mineral surfaces and natural organic matter (NOM) has played a major role in determining the trace element content of natural waters. This review examines sorption processes on mineral surfaces for nine trace elements (Cr, Co, Ni, Cu, Zn, Sr, Cd, Hg, Pb), focusing on the results of modern x-ray spectroscopic studies. Such studies provide unique information on the structure and composition of sorption products, including their mode of attachment to mineral surfaces or functional groups in NOM under in situ conditions (i.e., with aqueous solution present at 25°C). The types of chemical reactions (acid-base, ligand exchange, redox, dissolution/reprecipitation) that can occur at mineral-aqueous solution interfaces are also reviewed, and some of the factors that affect the reactivity of mineral surfaces are discussed, including changes in the geometric and electronic structures of mineral surfaces when they first react with aqueous solutions and constraints on the bonding of adions to surface functional groups imposed by Pauling bond valence sums. A summary of electrical double layer (EDL) theory is presented, including the results of several recent x-ray spectroscopic and parameter regression studies of the EDL for metal-(oxyhydr) oxide-aqueous solution interfaces. The effects of common inorganic and organic complexants on the sorption of trace metal cations at mineral-solution interfaces are considered, in the context of spectroscopic studies where possible. The results of sorption studies of trace metal cations on NOM, common bacteria, and marine biomass are reviewed, and the effects of coatings of NOM and microbial biofilms on cation uptake on mineral surfaces are discussed, based on macroscopic and spectroscopic data. The objective here is to assess the relative importance of inorganic versus organic sorption processes in aquatic systems. The paper concludes with a discussion of the effects of water composition on trace element removal mechanisms, with the aim of providing an understanding of the effects of the high salinity of seawater on trace element sorption processes. The information presented in this review indicates that sorption processes on mineral, NOM, and microbial and algal surfaces, including true adsorption and precipitation, are highly effective at removing trace elements from natural waters and generally supports Krauskopf's (1956) conclusion that such processes are likely responsible for the present trace element concentrations in seawater.

KINETICS OF LEAD ADSORPTION/DESORPTION ON GOETHITE: RESIDENCE TIME EFFECT

Sorption/desorption reactions at the mineral/water interface will control solution concentrations of trace elements and their bioavailability in natural environments. While there has been a great deal of research examining trace element sorption reactions on soil and soil constituent surfaces, there is relatively little research examining desorption reactions and the effect of residence time. In this study, we examined the influence of sorption density and residence time on the sorption/desorption kinetics of Pb on goethite. Lead sorption was rapid and nearly complete in less than 1 h, with essentially no change in the quantity of Pb sorbed over the 12-week sorption period. Desorption of Pb was slow and was modeled best by the parabolic diffusion equation. At all sorption densities investigated, desorption rate coefficients and the quantity of Pb desorbed were greater for the short-term experiments (5 days). However, statistical analyses indicated that these differences were not statistically significant. It is suggested that an insufficient desorption period caused by the high affinity of Pb for the goethite surface or the slow diffusion of Pb (OH)+ into crystal defects may be responsible for the absence of a residence time effect. Further studies are necessary to determine if the observed trends are real or are artifacts of the experiment.

Sorption of PAR–metal complexes on activated carbon as a rapid preconcentration method for the determination of Cu, Co, Cd, Cr, Ni, Pb and V in ground water

A simple, rapid and field-oriented preconcentration method is described for the determination of Cu, Co, Cd, Cr, Ni, Pb and V in ground water samples by flame AAS. The method is based on sorption of trace elements as metal-pyridyl azo resorcinol (M-PAR) complexes on activated carbon (AC) at pH 5.5±0.5. The effects of various parameters for quantitative sorption of M-PAR chelates on AC are discussed. The trace elements are desorbed from AC simply by treating AC with 1.6 M nitric acid, thus avoiding the tedious and time consuming operation of ashing AC. An enrichment factor of 200 or greater is easily achievable. The trace elements could be determined down to 1 ppb (V at 5 ppb) with ±15% relative error. The RSD of the method is ±15% at the practical determination limit of 1 ppb with an enrichment factor of 200. The major advantage of the method is the possibility of performing the preconcentration at the sampling site and hence a small preconcentrated volume could be sent for instrumental analysis, thus preventing sorption on a container during storage. The method has been applied to numerous ground water samples for the determination of these trace analytes at ppb levels in connection with the hydrogeochemical exploration for uranium.