C60-DOM interactions and effects on C60 apparent solubility: a molecular mechanics and density functional theory study

Dissolved organic matter (DOM) plays a critical role in the transport of carbon nano-particles (e.g. C(60)) in the aquatic environment. However, the mechanism for C(60)-DOM interactions and its environmental implications needs further investigations. In this study, the interaction of C(60) with relevant reference compounds of DOM (DOM(R)) is computationally simulated by molecular mechanics and density functional theory (DFT). All the C(60)-DOM(R) complexes are firstly optimized by classical annealing, and then DFT using the Dmol(3) code. The adsorption energies of C(60) on DOM(R) were computed. The computed electrostatic potential indicates that DOM(R) are electron acceptors in the C(60)-DOM(R) complexes, and the thermodynamic calculations indicate that electrostatic interaction is the dominant driving force for the C(60)-gallic acid complexation process in water. The presence of DOM(R) increases the apparent water solubility of C(60). It is also observed that the C(60) apparent water solubility decrease with the increase of the energy gaps of frontier molecular orbitals (E(LUMO)-E(HOMO)) for each C(60)-DOM(R) complex. These findings indicate that computational simulation is an important tool for predicting the behavior and fate of carbon nano-particles in the aquatic environment.     

Environmental risk assessment of metals: tools for incorporating bioavailability

In this paper, some of the main processes and parameters which affect metal bioavailability and toxicity in the aquatic environment and its implications for metal risk assessment procedures will be discussed. It has become clear that, besides chemical processes (speciation, complexation), attention should also be given to physiological aspects for predicting metal toxicity. The development of biotic ligand models (BLMs), which combine speciation models with more biologically oriented models (e.g. GSIM), has offered an answer to this need. The various BLMs which have been developed and/or refined for a number of metals (e.g. Cu, Ag, Zn) and species (algae, crustaceans, fish) are discussed here. Finally, the potential of the BLM approach is illustrated through a theoretical exercise in which chronic zinc toxicity to Daphnia magna is predicted in three regions, taking the physico-chemical characteristics of these areas into account.     

Association of organic matter, iron and inorganic phosphorus in lake waters

Gel permeation chromatography was used to fractionate ³²PO₄³⁻ labelled components by molecular size in filtered, destabilized lake foam, concentrated lake water, and lake sediment extracts. Evidence is presented for the abiotic formation of organic matter iron inorganic P complexes in lake foam, water and sediments. ³²PO₄^staggered was found to excahnge with low molecular weight, dissolved P fraction, probably an organic matter -Fe- inorganic P complex, but not with a high molecular weight, non-inorganic dissolved reactive P pool, hypothesized to be similar to the dimer. Our experiments suggest that inorganic P binding by high molecular weight organic matter may play a significant role in the P cycle. If the bound P is unavailable to algae and bacteria, the complexes could explain Rigler's (1968) hypothesis that the molybdenum blue technique may overestimate the free orthophosphate concentration by 10–100 times.     

Changes in physicochemical forms of lead and cadmium added to freshwater

Transformation of physicochemical forms of cadmium and lead added at 1 μ/L to Lake Michigan water have been studied by anodic stripping voltammetry. Unfiltered, 0.45 μ membrane filtered, ultrafiltered, and UV irradiated ultrafiltered samples were used to differentiate metal binding by particulate, colloidal, and dissolved organic matter. The reaction of added metal with colloids and particles is rapid. Their reaction with soluble organic matter is slow, requiring 1 to 2 days for complexation of one-half the added metal.     

Metal bioremediation through growing cells

Heavy-metal pollution represents an important environmental problem due to the toxic effects of metals, and their accumulation throughout the food chain leads to serious ecological and health problems. Metal remediation through common physico-chemical techniques is expensive and unsuitable in case of voluminous effluents containing complexing organic matter and low metal contamination. Biotechnological approaches that are designed to cover such niches have, therefore, received great deal of attention in the recent years. Biosorption studies involving low-cost and often dead/pretreated biomass have dominated the literature and, subsequently, extensive reviews focusing on equilibrium and kinetics of metal biosorption have also come up. However, the low binding capacity of biomass for certain recalcitrant metals such as Ni and failure to effectively remove metals from real industrial effluents due to presence of organic or inorganic ligands limit this approach. At times, when pure biosorptive metal removal is not feasible, application of a judicious consortium of growing metal-resistant cells can ensure better removal through a combination of bioprecipitation, biosorption and continuous metabolic uptake of metals after physical adsorption. Such approach may lead to simultaneous removal of toxic metals, organic loads and other inorganic impurities, as well as allow optimization through development of resistant species. However, sensitivity of living cells to extremes of pH or high metal concentration and need to furnish metabolic energy are some of the major constraints of employing growing cells for bioremediation. The efforts to meet such challenges via isolation of metal-resistant bacterial/fungal strains and exploitation of organic wastes as carbon substrates have began. Recent studies show that the strains (bacteria, yeast and fungi) isolated from contaminated sites possess excellent capability of metal scavenging. Some bacterial strains possess high tolerance to various metals and may be potential candidates for their simultaneous removal from wastes. Evidently, the stage has already been set for the application of metal-resistant growing microbial cells for metal harvesting. This review focuses on the applicability of growing bacterial/fungal/algal cells for metal removal and the efforts directed towards cell/process development to make this option technically/economically viable for the comprehensive treatment of metal-rich effluents.     

Validating riverine transport and speciation models using nuclear reactor-derived radiocobalt

Risk assessment of intentional or accidental discharges of toxic substances into river systems requires combined hydraulic and chemical modeling. Periodic discharges of known volumes with low radioactivity by the Beznau nuclear reactor (Switzerland) serve as validation tracers for both river flow and chemical speciation simulation. Validation of the former has been achieved by comparison of modeled and measured arrival times of radiocobalt along a 65 km transect with a maximum reaction period of 24 hours. Modeled breakthrough curves coincide well with measurements collected during three field campaigns, in spite of the fact that sorption and sedimentation processes were not activated during simulation. This gives indirect evidence of inefficient cobalt sorption. Particle/solution distribution measurements carried out during breakthrough allow further validation of our speciation approach, which is based on Tableau setup of inorganic reactions combined with sorption and organic complexation. Modeled and measured speciation results confirm recent observations of enhanced complexation of cobalt with dissolved organic substances, which significantly reduces particle sorption. The large variability of conditional stability constants for sorption and complexation reactions, for sorption site densities, and for organic ligand concentrations explains the variability of published particle-solution distribution coefficients.     

Removal of nitrogen oxides with organic amines and metal complexes

Aqueous solutions containing organic amines and metal complexes were applied for the removal of nitrogen oxides. Organic amines were effective for the removal of the mixture composed of nitric oxide and nitrogen dioxide. In particular, methyl and ethyl amines removed these species with 1 : 1 ratio from the mixture at fairly rapid rates. Dinitrogen trioxide (N₂O₃) was suggested to be involved in the course of the removal.Metal complexes removed nitrogen dioxide efficiently. As has been previously reported for a number of inorganic or organic redox systems, it was concluded that the removal of nitrogen dioxide by metal complexes proceeded also by the redox mechanism.Iron (ii) chelate complexes were highly effective for the removal of nitric oxide and this was attributed to a reversible coordination to the complex.     

The influence of ventilation on indoor/outdoor air contaminants in an office building

The air quality in a newly built preschool was investigated in a longitudinal study. Typical air contaminants emanating from building materials were determined, their variation over time (0–18 months) was measured, and the influence of the ventilation system (81%–91% recirculation of return air) on contaminant concentrations was studied. Volatile organic compounds were sampled by adsorption on porous polymer, analysed by a GC/FID system, and identified by MS. A spatial build-up in concentration (ppb or μg/m³ levels) is evident for all the organic compounds, as well as for CO₂, from the outdoor air, through the ventilation system, and through the rooms to the exhaust air. The longitudinal comparison over time shows that all the organic compounds decline in concentration mainly within the first 6 months of occupancy: 1-butanol 4–14 times, toluene and pentanal + hexanal 2–4 times, while formaldehyde remained at a constant low level of 90 ppb (110 μg/m³). It is difficult to believe that the problems of poor air quality in 100 preschools in Stockholm are caused by the organic compounds alone unless interactions occur. A preschool building needs to be gassed off during the first 6 months after its construction with no recirculation of return air allowed (outdoor air rate approx 4–5 ach). During at least 1–2 additional years, it is desired that the recirculation rate of return air is restricted, perhaps to 50%.     

Removal of uranium(VI) from aqueous solutions and nuclear industry effluents using humic acid-immobilized zirconium-pillared clay

Removal of uranium [U(VI)] from aqueous solutions with humic acid-immobilized zirconium-pillared clay (HA-Zr-PILC) was investigated using a batch adsorption technique. The adsorbent was characterized using XRD, FTIR, SEM, TG/DTG, surface area analyzer and potentiometric titration. The effects of pH, contact time, initial concentration, adsorbent dose, and adsorption isotherm on the removal process were evaluated. A maximum removal of 97.6 ± 2.1 and 94.7 ± 3.3% was observed for an initial concentration of 50 and 100 mg L⁻¹, respectively at pH 6.0 and an adsorbent dose of 2.0 g L⁻¹. Equilibrium was achieved in approximately 180 min. The mechanism for the removal of U(VI) ions by HA-Zr-PILC was based on an ion exchange reaction. The experimental kinetic and isotherm data were analyzed using a second-order kinetic equation and Langmuir isotherm model, respectively. The monolayer adsorption capacity for U(VI) removal was found to be 132.68 ± 5.04 mg g⁻¹. An increase of temperature of the medium caused an increase in metal adsorption. Complete removal (≅100%) of U(VI) from 1.0 L of a simulated nuclear industry effluent sample containing 10.0 mg U(VI) ions was possible with 1.5 g of HA-Zr-PILC. The adsorbent was suitable for repeated use (over 4 cycles) without any noticeable loss of capacity.     

Adsorption and desorption of 85Sr and 137Cs on reference minerals, with and without inorganic and organic surface coatings

The adsorption properties of reference minerals may be considerably modified by the presence of the inorganic and organic coatings that are ubiquitous in soils. It is therefore important to assess the effect of such coatings to evaluate the relevance of adsorption studies on pure minerals. The adsorption of trace amounts of (85)Sr and (137)Cs has been studied in dilute suspensions for various minerals that are common components of soils: quartz, calcium carbonate, kaolinite, montmorillonite and illite. We studied the effect of coatings with either Fe or Al oxide with varying additions of soil-extracted humic or fulvic acid. Both adsorption and desorption were measured and data presented as distribution coefficients, Kd. No adsorption was detected on quartz and it was not possible to coat this surface. Adsorption on calcium carbonate was small and not influenced by coatings. Adsorption of Sr on the three clay minerals was very similar, enhanced by the Al-coating, but not affected by Fe and organic coatings. The presence of organic coatings decreased Cs adsorption on illite. Similar but smaller effects were seen on montmorillonite and kaolinite. Aluminum coating enhanced Cs adsorption on illite, whereas both inorganic coatings caused decreases in adsorption on montmorillonite, and there was no effect on kaolinite. Effects were not additive with mixed, organic-inorganic coatings. Adsorption of both Cs and Sr on all minerals was strongly irreversible, with Kd (desorption) being up to four-times greater than adsorption Kd. The ratio of desorption and adsorption Cs Kd values (an assessment of irreversibility) was inversely related to adsorption Kd. This is consistent with a decreasing contribution of high-affinity adsorption as adsorption increases, but may also reflect the partial loss of organic coatings during desorption.     

Combating dye pollution using cocoa pod husks: a sustainable approach

The adsorptive potential of activated carbon prepared by chemical activation of Cocoa pod husk (CPHAA) to remove Congo red (CR) dye from its aqueous solution was investigated in this study. CPHAA was characterised using Energy Dispersive X-Ray, Scanning Electron Micrograph and Fourier Transform Infrared Spectroscopy techniques. The effects of contact time, initial dye concentration, pH and solution temperature were studied. Equilibrium data were fitted to Langmuir, Freundlich, Temkin and Dubinin–Radushkevich isotherm models. The equilibrium data were best represented by Langmuir isotherm model, with maximum monolayer adsorption capacity of 43.67 mg/g. The kinetic data were fitted to Pseudo-first-order, Pseudo-second-order, Elovich and Intraparticle diffusion models; the pseudo-second-order kinetic model provided the best correlation. Thermodynamic parameters such as standard enthalpy (ΔH^o), standard entropy (ΔS^o) and standard free energy (ΔG^o) were evaluated. The thermodynamic study showed that the process is endothermic, spontaneous and feasible. The mean free energy of adsorption shows that the mechanism is by physisorption. CPHAA was found to be an effective adsorbent for the removal of CR dye from aqueous solution.     

Speciation and hydrological transport of metals in non-acidic river systems of the Lake Baikal basin: Field data and model predictions

The speciation of metals in aqueous systems is central to understanding their mobility, bioavailability, toxicity and fate. Although several geochemical speciation models exist for metals, the equilibrium conditions assumed by many of them may not prevail in field-scale hydrological systems with flowing water. Furthermore, the dominant processes and/or process rates in non-acidic systems might differ from well-studied acidic systems. We here aim to increase knowledge on geochemical processes controlling speciation and transport of metals under non-acidic river conditions. Specifically, we evaluate the predictive capacity of a speciation model to novel measurements of multiple metals and their partitioning, under high-pH conditions in mining zones within the Lake Baikal basin. The mining zones are potential hotspots for increasing metal loads to downstream river systems. Metals released from such upstream regions may be transported all the way to Lake Baikal, where increasing metal contamination of sediments and biota has been reported. Our results show clear agreement between speciation predictions and field measurements of Fe, V, Pb and Zn, suggesting that the partitioning of these metals mainly was governed by equilibrium geochemistry under the studied conditions. Systematic over-predictions of dissolved Cr, Cu and Mo by the model were observed, which might be corrected by improving the adsorption database for hydroxyapatite because that mineral likely controls the solubility of these metals. Additionally, metal complexation by dissolved organic matter is a key parameter that needs continued monitoring in the Lake Baikal basin because dependable predictions could not be made without considering its variability. Finally, our investigation indicates that further model development is needed for accurate As speciation predictions under non-acidic conditions, which is crucial for improved health risk assessments on this contaminant.

     

Sources and sinks of carbon dioxide in terrestrial ecosystems: Is the land's carbon budget balanced under the influence of man?

This contribution deals with the controversy between certain scientists on the role of terrestrial vegetation and soils in the global carbon cycle. The hypothesis of a significant net release from the vegetation, is rejected by geochemists because of the limited capacity of the ocean to take up this excess carbon dioxide. As for the man-influenced tropics, a comparison of the figures for the potential and the current phytomass, as well as plausible demographic arguments, support the assertion put forward by ecologists that the carbon budget of this zone cannot be balanced. The tropics lose about 1.7-3.9 × 10¹⁵ g/yr of carbon to the atmosphere; however, for several reasons, 0.5-2.8 × 10¹⁵ g/yr may be returned to land ecosystem, mostly in other climatic zones. Thus, a balance is achieved on combining low estimates for the losses with high estimates for the gains. From an ecological perspective, this solution is not a very probable one; nevertheless, it cannot conclusively be eliminated.     

Variations in bottom sediment nutrient and metal concentrations in the Genesee River watershed,New York

Variations in nutrient and metal concentrations of fluvial sediment may be due to varying combinations of natural and man-made factors: basin geology, surface erosion, riverbank erosion, industrial or other cultural contamination, the presence of minerals rich in trace elements (e.g. chromite), sediment ion-exchange capacity, sediment organic content, and the presence of metallic oxides. The data reported here were obtained in a study in New York State of sediment transport from the Genesee River watershed to Lake Ontario (6,500 km²; 2,400 sq.mi.). One hundred bottom sediment samples collected over a period of a year were chemically analyzed for aluminium, chromium, copper, iron, manganese, nickel, lead, zinc, total carbon, total organic carbon, total nitrogen, and phosphorus. The metal concentrations (arithmetic means ±S.D., in μg/g) were: Al, 6,660±2,620; Cr, 14±9; Cu, 18±7; Fe, 15,060±7,312; Mn, 424±212; Ni, 23±13; Pb, 40±67; and Zn, 69±37. For the major nutrients the results (mean ±1 S.D. in %) were: total carbon, 2.06 ± 1.68; total organic carbon, 1.37 ± 1.28; total nitrogen, 0.105 ± 0.098; and phosphorus, 0.0560 ± 0.014.     

Behavior of heavy metals in model three-component soil systems

Model three-component soil systems including goethite (a mineral component) and a suspension of microorganisms (a biological component) have been used to study the interaction of Pb and Zn with the mineral surface. The results show that live microorganisms markedly reduced the adsorption of these metals, especially Pb, on the goethite shell surface. In the case of Pb, this is due to a decrease in the content of its nonexchangeable form; in the case of Zn, to a decrease in the content of its exchangeably bound compounds. An organic matter preparation obtained by autoclave treatment of microorganisms has a markedly weaker effect on metal adsorption, compared to the suspension of live microbial cells.     

Effect of several organic acids on phosphate adsorption by variable charge soils of central China

The effect of several organic acids on phosphate adsorption by acidic soils in subtropical zone of central China was studied. Results showed: (1) citrate and oxalate remarkably reduced the amount of phosphate adsorption, but tartrate, benzoate and acetate had only a very slight influence on phosphate adsorption; (2) the ability of citrate in reducing phosphate adsorption was greater than that of oxalate, moreover, the reduction percentage was dependent on the concentration of organic ligands in the solution; (3) the effect of organic acids on phosphate adsorption was related to the pH value of organic acid solution. The minimum reduction in adsorption of phosphate was present at a specific pH value of organic acid solution which ranged from 2 to 10; (4) a minimum reduction of phosphate adsorption occurred when phosphate was added to the soils before organic acid, whereas a maximum occurred when organic acid was introduced before the addition of phosphate. Meanwhile, the treatment for the mixture of two organic acids resulted in more reduction in phosphate adsorption than each of the organic acids and less than the total of them. Based on these observations, we suggested that the competition between phosphate and organic acids relied on their relatively affinity to soil mineral surface at different conditions.     

Physiological effects of nanoparticles on fish: a comparison of nanometals versus metal ions

The use of nanoscale materials is growing exponentially, but there are also concerns about the environmental hazard to aquatic biota. Metal-containing engineered nanoparticles (NPs) are an important group of these new materials, and are often made of one metal (e.g., Cu-NPs and Ag-NPs), metal oxides (e.g., ZnO and TiO(2) NPs), or composite of several metals. The physiological effects and toxicity of trace metals in the traditional dissolved form are relatively well known and the overall aim of this review was to use our existing conceptual framework of metal toxicity in fish to compare and contrast the effects of nanometals. Conceptually, there are some fundamental differences that relate to bioavailability and uptake. The chemistry and behaviour of nanometals involves dynamic aspects of aggregation theory, rather than the equilibrium models traditionally used for free metal ions. Some NPs, such as Cu-NPs, may also release free metal ions from the surface of the particle. Biological uptake of NPs is not likely via ion transporters, but endocytosis is a possible uptake mechanism. The body distribution, metabolism, and excretion of nanometals is poorly understood and hampered by a lack of methods for measuring NPs in tissues. Although data sets are still limited, emerging studies on the acute toxicity of nanometals have so far shown that these materials can be lethal to fish in the mg-μgl(-1) range, depending on the type of material. Evidence suggests that some nanometals can be more acutely toxic to some fish than dissolved forms. For example, juvenile zebrafish have a 48-h LC(50) of about 0.71 and 1.78mgl(-1) for nano- and dissolved forms of Cu respectively. The acute toxicity of metal NPs is not always explained, or only partly explained, by the presence of free metal ions; suggesting that other novel mechanisms may be involved in bioavailability. Evidence suggests that nanometals can cause a range of sublethal effects in fish including respiratory toxicity, disturbances to trace elements in tissues, inhibition of Na(+)K(+)-ATPase, and oxidative stress. Organ pathologies from nanometals can be found in a range of organs including the gill, liver, intestine, and brain. These sublethal effects suggest some common features in the sublethal responses to nanometals compared to metal salts. Effects on early life stages of fish are also emerging, with reports of nanometals crossing the chorion (e.g., Ag-NPs), and suggestions that the nano-forms of some metals (Cu-NPs and ZnO NPs) may be more toxic to embryos or juveniles, than the equivalent metal salt. It remains possible that nanometals could interfere with, and/or stimulate stress responses in fish; but data has yet to be collected on this aspect. We conclude that nanometals do have adverse physiological effects on fish, and the hazard for some metal NPs will be different to the traditional dissolved forms of metals.     

Effects of grazing and fencing on carbon and nitrogen reserves in plants and soils of alpine meadow in the three headwater resource regions

Knowledge about carbon and nitrogen in plants and soils and response to fence and graze in alpine ecosystems is still rudimentary because of extremely geographic situation. The purpose of this study was to compare the difference among carbon, nitrogen concentration, and content of unit area and dynamics of above- and below-ground biomass, soil organic carbon and total nitrogen between fencing and grazing alpine meadow. The results showed that total carbon and C: N radio in the aboveground tissue were significantly higher in fenced and ungrazing grassland (FU) than those in free grazing grassland (FG). In addition, the order of total carbon and nitrogen concentration of aboveground tissue of different function groups were not identical between them; The total carbon storage (TCS) per unit of aboveground tissue, roots and 0–30 cm soil layer increased after being fenced for 5 years from free grazing grassland (9255.17 g/m²) to fenced and ungrazing grassland (12637.10 g/m²) by 26.79%. The corresponding total nitrogen storage (TNS) increased by 751.42 g/m². Furthermore over 95% TCS (TNS) come from 0–30 cm soil layer. However there were no significant differences between fenced and ungrazing grasslands of 10 years and 5 years. Therefore fenced to exclude grazing by Tibetan sheep and yaks was an alternative approach to sequester C to the soil in alpine meadow systems.     

Investigating soils retention ratios and modelling geochemical factors affecting heavy metals retention in soils in a tropical urban watershed

This study aimed at investigating the retention of Pb and Cd in soils and the geochemical factors influencing the adsorption of these pollutants. Soil samples were air-dried and ground to pass through a 2-mm sieve, and different soil extracts were prepared for chemical analysis (organic matter, cation exchange capacity and pH). Total Pb and Cd were extracted with diacid using digestion method and determined by atomic adsorption spectrophotometer (AAS) after filtration. Results revealed that the heavy metals retention ratio (RR) of the Rhodic ferralsol, Xanthic ferralsol and Mollic gleysol (2) were very high for Cd (>80 %) and was relatively low (generally < 60 %) for Pb. In contrast, RRs for the Plinthic gleysol and the Mollic gleysol (1) were relatively low (<60 %), regardless of the heavy metal concerned. Multiple regression equations indicated for Pb and Cd concentrations different linear relationships over simple linear regression, when pH, organic matter, clay percentage and cation exchange capacity (CEC) were used as independent variables. Results indicate that organic matter exerts major influences on the retention of Pb and Cd in soils, while CEC, clay content and pH have a minor influence in this process in the Ntem watershed. From these observations, the application of soil organic matter could be a solution in protecting shallow aquifers from heavy metal pollution and thus insuring that they are not a hazard to public health.     

Behavior and analysis of Cesium adsorption on montmorillonite mineral

The adsorption of Cs by montmorillonite and the effects of experimental conditions on adsorption were investigated by using ¹³⁴Cs as a radioactive tracer. Additionally, the Cs-adsorbed and the modified montmorillonite were analyzed by X-ray Diffractometer System (XRD) and Scanning Electron Microscopy (SEM). The results showed that the adsorption of Cs by montmorillonite was efficient in the initial concentration (C₀) of 30 μg/L Cs nitrate solution with 20 g/L montmorillonite at room temperature. In this condition, more than 98% Cs⁺ ions could be adsorbed at pH 8. The adsorption equilibrium was achieved within 5 min and the relationship between the concentration of Cs⁺ in aqueous solutions and adsorption capacities of Cs⁺ can be described by the Langmuir adsorption isotherm. The adsorption rate would decrease when temperature increase from 0 °C to 50 °C or in presence of coexistent K⁺, Na⁺ and Ca²⁺, while modification by (NH₄)₂SO₄, [Ag(NH₃)₂]⁺, [Cu(NH₃)₄]²⁺ or 450 °C could improve the adsorption abilities of montmorillonite for Cs⁺. However, more than 89% of adsorbed Cs⁺ on montmorillonite could be desorbed by 2 mol/L HNO₃ solutions. The XRD and SEM analysis further showed that the structure of the Cs-adsorbed or modified montmorillonite were different from that of the original one.