Predicting the rate and extent of cadmium and copper desorption from soils in the presence of bacterial extracellular polymer

The movement of cationic transition metals through the subsurface is strongly retarded by sorption to the porous media. However, dissolved organic ligands can compete with soil surfaces by providing binding sites for metals in solution. An extracellular polymer produced by a bacterium isolated from soil was used in this study to observe and model the influence of a naturally occurring ligand on the release of adsorbed metals from two test soils. Experimental results show that the presence of dissolved extracellular polymer enhanced the rate and extent of desorptive release of soil-bound cadmium and copper. A kinetic model that uses a gamma distribution of rate constants to account for the physical and chemical heterogeneity of the soil matrix was employed to describe the release of cadmium and copper in batch experiments. Model parameters describing soil, metal and extracellular polymer interactions were obtained through separate experiments. With these parameters the model successfully predicted the influence of dissolved polymer on the rate and extent of release of cadmium and copper from soil in independent batch experiments. These results suggest that the presence of natural metal-binding ligands such as bacterial extracellular polymers can act to increase the driving force for desorption by lowering the aqueous concentration of free unbound metals in solution.     

Metal removal in activated sludge: the role of bacterial extracellular polymers

A combination of flocculation and settling is the mechanism by which metal removal is achieved in activated sludge. Bacterial extracellular polymers appear to play an important role in flocculation; metal cations may also be involved in this process. Extracellular polymers in activated sludge are mainly of a polysaccharide nature, although protein and nucleic acid from autolysis are constituents of the polymer matrix. Precipitated metals may be removed either by independent settling or by physical trapping in the sludge floc matrix. Metals present in the ionic form may be removed from solution by adsorption to sites on bacterial extracellular polymers. Metal ions may also be accumulated in the cytoplasm of a bacterial cell, or adsorbed on to the cell wall.If activated sludge plants are overloaded with metals, toxic effects on bacteria and other microorganisms may severely inhibit the treatment process, resulting in poor quality effluents. Acclimated bacterial cultures can tolerate much higher metal concentrations than non-acclimated cultures; these are advantageous in the treatment of metal-laden wastes.     

Role of bacterial extracellular polymers in metal uptake in pure bacterial culture and activated sludge—I. Effects of metal concentration

A gel filtration technique afforded a good separation between metal complexed with bacterial extracellular polymers and free metal ions. The complexation of polymers extracted from cultures of Klehsiella aerogenes and activated sludge with cadmium, nickel, manganese and cobalt was demonstrated. The extraction of extracellular polymers from cultures of K. aerogenes and activated sludge reduced the capacity of the cells and flocs to adsorb metal. Adsorption and complexation of metals by cells of K. aerogenes and extracellular polymers extracted from activated sludge were fitted to Freundlich equilibrium isotherms. Saturation of activated sludge polymer binding sites occurred at 10 mg 1^?1 metal additions for all the metals studied except manganese which was complexed to a very limited extent. Cells of K. aerogenes exhibited no saturation effects in the range of metal concentrations studied.Precipitation of metals below a concentration of 1 mg 1^?1 was minimal, with the exception of cadmium precipitation. At a concentration of 10 mg 1^?1, precipitation of cadmium, cobalt and manganese may have been the major mechanism of metal removal. The more soluble metals generally displayed the lowest removals. Concentrations of extracellular polymers and soluble chelating agents may be important in controlling removals of metals which are largely soluble in activated sludge.     

Role of bacterial extracellular polymers in metal uptake in pure bacterial culture and activated sludge—II Effects of mean cell retention time

A gel filtration technique afforded a good separation between metal complexed with bacterial extracellular polymers and free metal ions. The complexation of polymers extracted from cultures of Klehsiella aerogenes and activated sludge with cadmium, nickel, manganese and cobalt was demonstrated. The extraction of extracellular polymers from cultures of K. aerogenes and activated sludge reduced the capacity of the cells and flocs to adsorb metal. Adsorption and complexation of metals by cells of K. aerogenes and extracellular polymers extracted from activated sludge were fitted to Freundlich equilibrium isotherms. Saturation of activated sludge polymer binding sites occurred at 10 mg 1⁻¹ metal additions for all the metals studied except manganese which was complexed to a very limited extent. Cells of K. aerogenes exhibited no saturation effects in the range of metal concentrations studied.Precipitation of metals below a concentration of 1 mg 1⁻¹ was minimal, with the exception of cadmium precipitation. At a concentration of 10 mg 1⁻¹, precipitation of cadmium, cobalt and manganese may have been the major mechanism of metal removal. The more soluble metals generally displayed the lowest removals. Concentrations of extracellular polymers and soluble chelating agents may be important in controlling removals of metals which are largely soluble in activated sludge.     

Microbial controls on metal mobility under the low nutrient fluxes found throughout the subsurface

Laboratory simulations and field studies of the shallow subsurface have shown that microbes and their extracellular products can influence the mobility of toxic metals from waste disposal sites. Modelling the transport of contaminants in groundwater may, therefore, require the input of microbial ecology data in addition to geochemical data, thus increasing the costs and the uncertainty of predictions. However, whether microbial effects on contaminant mobility occur extensively in the natural subsurface is unknown because the conditions under which they have been observed hitherto are generally unrepresentative of the average subsurface environment. Here, we show that microbial activity affects the mobility of a toxic trace metal (Cu) under the relatively low nutrient fluxes that dominate subsurface systems. More particularly, we show that under these low nutrient conditions, microbes and microbial products can immobilize metal but may themselves be subject to subsequent mobilization, thus complicating the pattern of metal storage and release. Our results show that the capability of microbes in the subsurface to change both the capacity of porous media to store metal, and the behaviour of metal that is released, is not restricted to the well researched environments close to sites of waste disposal. We anticipate our simulations will be a starting point for generating input data for transport models, and specifying the mechanism of metal remobilisation in environments more representative of the subsurface generally.     

Metals in soils of children's urban environments in the small northern European city of Uppsala

Metals occur naturally in soil, but contents are generally increased in the urban environment due to anthropogenic activities. The presence of elevated metals in soils of the urban environment has been recognized as an important source of metal intake in children and is linked to elevated metal levels in children's blood. Several metals have undesirable health effects, especially on children due to their still developing nervous system and small body volumes. Playgrounds are where urban children spend most of their time outdoors and are also where children most frequently come in contact with soil. Elevated contents of metals in playgrounds are therefore of great concern for children's wellbeing. This study investigates the soil metal content of 25 playgrounds located in different land use areas in urban Uppsala, Sweden's fourth largest city. Uppsala covers an area of approximately 100 km2 and has a population of 136,000. The soil samples were analysed for 12 metals (Al, As, Cd, Cr, Cu, Fe, Hg, Mn, Ni, Pb, W, Zn) using aqua regia. Median metal contents were found to be 1.8, 3.4, 0.21, 32, 25, 2.5, 0.14, 494, 19, 26, 0.35 and 84 mg kg(-1) soil for each of the above metals, respectively. The median clay content was around 20% while the organic matter content was measured by loss on ignition at a median of 8%. The land use areas included industrial land, the city center, road verges, natural land and former industrial land. The results showed that land use did not have the expected large influence on the total metal contents of the soils tested. The clay content together with the age of the site proved to be a more important factor. Sites with elevated clay contents had in general elevated metal contents, which were explained by the relatively high adsorption capacity of clay particles. The soils at sites where land use had not been altered since the 1800s had increased metal contents compared to playgrounds constructed in the late 1900s. The immobility of metals once they had entered the soil system was the reason for increased metal content in soils of old playgrounds. It was concluded that in cities with few internal pollution sources, the soil characteristics of the site and the time the soil has been on-site to accumulate metal residues become important factors in determining the soil metal content.     

Formation and conditional stability constants of complexes formed between heavy metals and bacterial extracellular polymers

The strength and nature of the binding of heavy metal ions to bacterial extracellular polymers has been investigated. The conditional stability constants (K_t) for complexes formed between extracted Klebsiella aerogenes polymer and copper, cadmium, cobalt and nickel were determined, using a gel chromatographic technique, and log K_t values of 7.69, 5.16, 5.48 and 5.49 respectively were obtained. Adsorption isotherms constructed for copper, cadmium and cobalt indicated that metal uptake occurred after the initial complexation capacity had been exceeded, suggesting the presence of more than one binding site, but nickel adsorption ceased when the complexation capacity was reached. Nickel was found to be associated predominantly with the soluble form of polymer, and copper and cadmium with the colloidal fraction when metals were added to the polymer simultaneously, rather than individually. The overall specific metal uptake by polymers extracted from activated sludge was approximately ten times higher than that by K. aerogenes polymer.     

Elimination of cadmium trace contaminations from drinking water

Raw waters polluted with trace heavy metals present serious problems to the part of the Chinese water supply. One of the important contaminants is cadmium. Removal of trace amounts of heavy metals can be achieved by means of selective sorption processes. One of the possibilities is the application of weak base anion exchangers. LEWIS-base/acid interactions lead to an exclusive sorption of heavy metal cations and an equivalent amount of anions of strong acids. The respective elimination of cadmium from pure solutions and spiked natural water and the regeneration of the exhausted exchanger has been investigated. The results demonstrate a very efficient elimination. The standards for drinking water are met for a very large relative volume of treated water. In addition, even a considerable share of dissolved organic matter is adsorbed. Regeneration requires a first step with sulfuric acid to remove the metals and a second one with sodium hydroxide to neutralize the exchanger and to displace the DOC adsorbed. The heavy metals can be concentrated in a small volume which facilitates the discharge of the waste.     

Environmental impacts and metal exposure of aquatic ecosystems in rivers contaminated by small scale gold mining: the Puyango River basin, southern Ecuador.

Gold mining in the Portovelo-Zaruma district in southern Ecuador is causing considerable environmental impacts; the most important ones are related to the discharge of cyanide, mercury and metal rich tailings into rivers of the Puyango catchment area. Cyanide and metal levels in rivers regularly exceed environmental quality criteria. The contamination impacts biodiversity, with cyanide causing a direct lethal effect on biota close to source and metal contaminants considerably reducing aquatic biodiversity further downstream. It is shown that the prevailing neutral or slightly alkaline conditions of the rivers ensure that metals are mainly associated with sediment. However, elevated metal levels in bottom living larvae collected from contaminated sites suggest that these sediment bound metals are readily bioavailable. Leaching experiments indicate that the relative ease by which metals are taken up by larvae is related to the speciation of sediment associated metals. It is further shown that large amounts of metals, which are bound to suspended sediment under ambient pH conditions, enter the dissolved and directly bioavailable state in more acidic conditions. Metal levels in carnivorous fish were found to be modestly elevated only, with the exception of mercury. Mercury levels exceeded 0.5 mg/kg in fish from both contaminated and uncontaminated sites, showing that both methylation and bioaccumulation of mercury are occurring in the Puyango river basin.     

Phosphate sources and their suitability for remediation of contaminated soils

Phosphate minerals and specifically apatite show promise for environmental cleanup because they can form stable compounds with a wide range of cationic contaminants. However, phosphate minerals naturally accumulate some heavy metals that may cause additional contamination of the environment if used improperly. Nine commercially available phosphate materials were evaluated for remediation of contaminated soil based on solubility, concentration of metal/metalloid impurities, and leachability of impurity metal/metalloids. The phosphate materials consisted of three groups: processed (i.e., fertilizers), mined (rock phosphates from different formations), and biogenic (ground fish bone). Processed and mined rock phosphates contained relatively high total concentrations of As, Co, Cr, and Cu but did not exceed the RCRA toxicity characteristic leaching procedure (TCLP) limits. Biogenic apatite contained much lower metal concentrations than processed and mined rock phosphate and was appreciably more soluble. By combining biogenic and mined phosphate it is possible to obtain a wide range of phosphate release rates, permitting rapid immobilization of contaminants while providing a slow release of phosphate for continued long-term treatment.     

Association of cadmium with MnO2 particles generated during permanganate oxidation

Particle filtration and cadmium sorption studies were performed at selected time points during reaction of potassium permanganate with trichloroethylene under varied reaction matrix conditions. The purpose of the studies was to determine the potential impact of manganese oxides particle generation, a by-product of the permanganate reaction, on subsurface metal mobility, with cadmium serving as a representative metal of interest in the environment. Results of the studies indicate that the association of cadmium with the manganese oxides is a function of (1) particle concentration, (2) pH, (3) the presence of calcium in the reaction matrix, and (4) the rate of particle generation and agglomeration. Based on these findings, it is important to give careful consideration to subsurface conditions that can potentially impact the mobility of metals present naturally or as co-contaminants. If subsurface conditions are not appropriately characterized and planned for, deleterious effects could result, including long-term release of metals initially sorbed onto generated particles. Alternatively, the generated manganese oxides may serve as a long-term means of immobilizing metals within the subsurface.     

Contaminants removal by bentonite amended slow sand filter

Earlier studies have indicated that variability in size, surface texture and charge greatly influence the contaminant removal process in granular media. Based on surface characteristics of montmorillonite, it is anticipated that small addition of this clay would increase adhesion sites for bacterial growth and extracellular polymer production in the slow sand filter and thereby enhance its contaminant removal ability. Experiments were performed by permeating groundwater contaminated with pathogens (total coliform and E. Coli) and inorganic contaminants through the bentonite amended slow sand filter (BASSF). Surprisingly, the BASSF retained inorganic contaminants besides pathogens. Water-leach tests (pH of water leachate ranged from 2 to 9) with spent BASSF specimen indicated that the inorganic contaminants are irreversibly adsorbed to a large extent. It is considered that the combined effects of enhanced-organic matter mediated adhesion sites and increased hydraulic retention time enables the BASSF specimen to retain inorganic contaminants. It is envisaged that BASSF filters could find use in treating contaminated groundwater for potable needs at household and community level.     

Dredging-related mobilisation of trace metals: a case study in The Netherlands

Mobilisation of contaminants is an important issue in environmental risk assessment of dredging projects. This study has aimed at identifying the effects of dredging on mobilisation of trace metals (Zn, Cu, Cd and Pb). The intensities and time scales of trace metal mobilisation were investigated during an experimental dredging project conducted under field conditions. The loss of contaminated dredge spoil is mainly reflected by increasing levels of trace metals in the suspended matter, dissolved trace metal concentrations in the water column are not significantly influenced by the dredging activities. This indicates a strong binding mechanism of trace metals to the solid phase or a fast redistribution over sorptive phases in response to oxidation of e.g. trace metal sulphides. Given the differences in levels of reactive phases (Mn, Fe, sulphides and organic matter) between the riverine suspended matter and the sediments, changes in the levels of these parameters in the suspended matter upon dredging may give information on the processes influencing the behaviour of trace metals and on the potential loss of sediment during dredging operations. Therefore, we recommend to routinely measure these parameters in studies on contaminant behaviour related to dredging activities.     

Organic chemicals in sewage sludges

Sewage sludges are residues resulting from the treatment of wastewater released from various sources including homes, industries, medical facilities, street runoff and businesses. Sewage sludges contain nutrients and organic matter that can provide soil benefits and are widely used as soil amendments. They also, however, contain contaminants including metals, pathogens, and organic pollutants. Although current regulations require pathogen reduction and periodic monitoring for some metals prior to land application, there is no requirement to test sewage sludges for the presence of organic chemicals in the U. S. To help fill the gaps in knowledge regarding the presence and concentration of organic chemicals in sewage sludges, the peer-reviewed literature and official governmental reports were examined. Data were found for 516 organic compounds which were grouped into 15 classes. Concentrations were compared to EPA risk-based soil screening limits (SSLs) where available. For 6 of the 15 classes of chemicals identified, there were no SSLs. For the 79 reported chemicals which had SSLs, the maximum reported concentration of 86% exceeded at least one SSL. Eighty-three percent of the 516 chemicals were not on the EPA established list of priority pollutants and 80% were not on the EPA's list of target compounds. Thus analyses targeting these lists will detect only a small fraction of the organic chemicals in sludges. Analysis of the reported data shows that more data has been collected for certain chemical classes such as pesticides, PAHs and PCBs than for others that may pose greater risk such as nitrosamines. The concentration in soil resulting from land application of sludge will be a function of initial concentration in the sludge and soil, the rate of application, management practices and losses. Even for chemicals that degrade readily, if present in high concentrations and applied repeatedly, the soil concentrations may be significantly elevated. The results of this work reinforce the need for a survey of organic chemical contaminants in sewage sludges and for further assessment of the risks they pose.     

Significance and behaviour of heavy metals in waste water treatment processes I. Sewage treatment and effluent discharge

The literature concerning the significance and behaviour of heavy metals in waste waters is reviewed. Factors influencing the partition of heavy metals between the solid and liquid phases in both primary and secondary (biological) treatment are considered in detail. It is evident that during primary treatment only metals present in an insoluble form will be removed, but the removal of such forms is strongly influenced by the factors controlling sedimentation. During secondary treatment metals initially present as soluble forms may be removed by association with the settleable biomass: further removal of insoluble metals also occurs by association with this biomass. Efficient flocculation and settling is critical to the removal of all metal forms during secondary treatment. Extracellular material, principally bacterial extracellular polymers, but also proteins and nucleic acids released as the result of cell lysis, appear to play a very significant role in the removal of soluble metals during secondary treatment.If biological treatment plants are overloaded with metals, toxic effects on bacteria and other micro-organisms may result in poor quality effluents; although the micro-organisms responsible for waste water treatment are able, within limits, to acclimatise to elevated metal concentrations.The impact of heavy metal contaminated effluents on receiving waters has also been reviewed and it is concluded that the most serious problem posed is drinking water production in areas where water re-use is practised.     

Use of spider webs as indicators of air quality assessment of Lahore City

High level of heavy metal contaminants in air is known to cause serious health problems if remained unchecked for a long period of time. Anthropogenic activities such as vehicular emissions and number of industries are responsible for increasing the level of heavy metals. The aim of the present study was to biologically monitor the level of heavy metal contamination by spider webs. These sites were selected based on variable traffic volume and number of industries. Atomic absorption spectrometry (AAS) was used for the analysis of metal pollution content. The study determined that the level of heavy metals at the selected sampling sites were several folds higher than that at the control site and in previously reported literature. Pb was determined to be the most abundant heavy metal, while Thokar Niaz Baig was reported as the most contaminated site. These elevated levels of metal contaminants are associated with heavy traffic flow, number of industries and constructional activities in the respective areas.     

Removal and accumulation of Cu, Ni and Zn in horizontal subsurface flow constructed wetlands: Contribution of vegetation and filling medium

This study investigated the accumulation and removal of Cu, Ni and Zn in two horizontal subsurface flow constructed wetlands for domestic wastewater treatment, which differ by shape, presence of macrophytes and water depth. Between March and December 2007, the three metals were measured in the influent and effluents of the two systems. Average percentage removal rates were extremely low for Cu (3% and 9% in the two beds) and higher for Zn and Ni (between 25 and 35%). Under higher Zn influent concentrations, it was found to be between 78-87%, which is in agreement with other literature data.During the peak standing crop season (August), biomasses of the different parts of Phragmites australis (stems, leaves and flowers, roots and rhizomes) were analysed in terms of weight and heavy metal concentration in order to assess heavy metal distribution among the tissues. It was found that the plants contribute to total heavy metal removal to a lesser extent than the filling medium. Aboveground tissues remove 34% of Cu, 1.8% of Ni and 6.2% of Zn % and, once harvested, their disposal does not appear to pose a problem for the environment. If heavy metals are present at high concentrations in the horizontal subsurface flow bed influent, over time, their accumulation in the filling medium could necessitate special care in the bed's management to avoid release into the surrounding environment.     

Binding of organic solutes to dissolved humic substances and its effects on adsorption and transport in the aquatic environment

Humic substances constitute a major fraction of dissolved organic matter in natural water and effluents. Their effect on the adsorption of organic contaminants to aquifer material was elucidated, and a model was proposed for the adsorption of organic solutes to aquifer solids in the presence of dissolved humic substances. The model is based on the assumption that organic solute binds to dissolved humic substances in a reversible manner to form a solute-humate complex. Following binding, both free and bound fractions of the organic solute are independently adsorbed onto the solid phase. In order to evaluate the validity of the model, the following parameters were determined: (1) the adsorption coefficient of the organic solute to clay; (2) the binding constant of the solute-humate complex; and (3) the adsorption of humic acid (HA) to clay, assuming that the solute-humate complex is adsorbed similarly to humic acid itself. Using these parameters in the model enabled the effect of dissolved humic substances on adsorption to be evaluated. Experimental results obtained for the adsorption of fluoranthene (a model compound of the PAH group) to clay in the presence of dissolved HA were compared with calculated values derived from the model described above. The sensitivity of the model to various parameters was evaluated and a prediction was made with respect to the effect of dissolved humic substances on the adsorption of a variety of organic solutes. It appears that dissolved humic substances solubilize organic solutes which have higher adsorption coefficients to clay than humic substances, but increase the adsorption of solutes having lower adsorption coefficients relative to humic substances.     

Do high levels of diffuse and chronic metal pollution in sediments of Rhine and Meuse floodplains affect structure and functioning of terrestrial ecosystems?

This paper (re)considers the question if chronic and diffuse heavy metal pollution (cadmium, copper, lead and zinc) affects the structure and functioning of terrestrial ecosystems of Biesbosch National Park, the floodplain area of rivers Meuse and Rhine. To reach this aim, we integrated the results of three projects on: 1. the origin, transfer and effects of heavy metals in a soil-plant-snail food chain; 2. the impact of bioavailability on effects of heavy metals on the structure and functioning of detritivorous communities; 3. the risk assessment of heavy metals for an herbivorous and a carnivorous small mammal food chain. Metal pollution levels of the Biesbosch floodplain soils are high. The bioavailability of metals in the soils is low, causing low metal levels in plant leaves. Despite this, metal concentrations in soil dwelling detritivores and in land snails at polluted locations are elevated in comparison to animals from 'non-polluted' reference sites. However, no adverse effects on ecosystem structure (species richness, density, biomass) and functioning (litter decomposition, leaf consumption, reproduction) have been found. Sediment metal pollution may pose a risk to the carnivorous small mammal food chain, in which earthworms with elevated metal concentrations are eaten by the common shrew. Additional measurements near an active metal smelter, however, show reduced leaf consumption rates and reduced reproduction by terrestrial snails, reflecting elevated metal bioavailability at this site. Since future management may also comprise reintroduction of tidal action in the Biesbosch area, changes in metal bioavailability, and as a consequence future ecosystem effects, cannot be excluded.     

Modeling metal bioaccumulation in a deposit-feeding polychaete from labile sediment fractions and from pore water

Estuarine sediments are often highly enriched in particle-reactive metal contaminants and because aquatic animals have often been shown to acquire metals predominantly from their diet, benthic animals feeding on deposited or resuspended sediments may also accumulate metals through this uptake pathway. Laboratory experiments were performed in which the surface deposit-feeding polychaete, Nereis succinea, was exposed to As(+ 5), Cd, and Cr(+ 3) in pore water or in estuarine sediments with and without enrichment with algal debris. These experiments generated metal uptake parameters (assimilation efficiency of ingested metal [AE], uptake rate constant of dissolved metal, efflux rate constants following dietary or aqueous metal exposures) used in a kinetic model of metal bioaccumulation. The model showed that > 97% of the body burden of these metals is accumulated through ingested sediment. The kinetic model was further modified to consider the geochemical fractionation of the metals in the sediments because metals bound to some fractions were shown to be unavailable to these polychaetes. The modified model substituted the AE term for each metal by the percentage of metal extracted in neutral and weak acid exchangeable fractions (termed “carbonex” fraction) multiplied by the slope of the regression between the metal AE and its fractionation in carbonex. The modified model generated predictions of As, Cd, and Cr body burdens in polychaetes at three different estuarine sites that matched independent field observations at these sites (r² = 0.84 for sediments without organic enrichment, r² = 0.87 with organic enrichment). Model predictions that relied on total metal concentrations showed weaker relationships (r² = 0.11-0.50). This study adds to the evidence for the dominance of dietary uptake of metals in aquatic animals and identifies a key sedimentary fraction of metals that can account for bioavailability of sediment-bound metals.