Summary Understanding attenuation processes is important not only for predicting the behavior of contaminants in soil and formulating remediation strategies, but also for mitigating and enhancing the availability of micronutrients in soil for agricultural applications. Natural Attenuation of Trace Element Availability in Soils brings together pioneering researchers who discuss their cutting-edge work in this area.
The first chapters focus on practical analytical techniques for the measurement and the biological assessment of natural attenuation of trace elements. The following chapters analyze the processes that occur in the natural attenuation of contaminants and nutrients, covering the structural dynamics of mineral surfaces, partitioning, diffusion, fixation, biological and redox processes, and the reversibility of these processes. The remaining chapters consider the impact and implications of natural attenuation in terms of risk assessment, remediation of inorganic contaminants, and bioavailability of essential nutrients.
Offering a concise, well-rounded perspective, Natural Attenuation of Trace Element Availability in Soils demonstrates how attenuation processes can significantly impact strategies for soil remediation and serve as a basis for environmental regulations. Isotopic Dilution Methods; S. Implications for Risk Assessment; E. Degryse Introduction Zinc Natural Attenuation: Sustainability, Side Effects, and Failures; M.
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The open symbols are equivalent data using radiolabile adsorbed Cd. The solid line is the fit of a Freundlich equation to all the radiolabile points treated as a single dataset. This was despite the marked hysteresis shown by sorbed Cd when subject to desorption. Both E- and L-values represent the labile metal pool in soils and therefore might be expected to be equal. As described previously, specific assumptions regarding the extent of mixing of the isotope with indigenous soil metal underpin this expectation Stanhope et al.
In addition, identical Eand L-values will only be found if the presence of the plant has no effect on transfers between labile and nonlabile forms of metal. Therefore, a comparison of E- and L-values may be a useful tool with which to investigate possible chemical or biological mobilization of fixed forms of metal in the soil Smolders et al. Dissolution of nonlabile forms of metal may occur in the rhizosphere through processes such as alteration of pH or the production of metal-solubilizing root exudates or microbial metabolites Grinsted et al.
Isotopic Dilution Methods 35 in the apparent L-value over the E-value. Thus, differences between the L- and E-values may provide quantitative estimates of the mobilization of nonlabile metal within the rhizosphere. Reports of such differences vary depending upon methodology and choice of plant and soil. One of the largest reported differences in E- and L-values was found for Fe by Dyanand and Sinha They measured FeL using Sorghum vulgare with 59Fe and used the electrolyte mix 0.
The ratio of L: E varied from 1. It was postulated that when Fe is in short supply, the plant roots reduce pH locally and release citric acid to promote nonlabile Fe dissolution. They found that L-values slightly exceeded the corresponding E-values, by a factor of between 1. They found very little difference between E- and L-values for Cd in most cases e. However, for 1 population of Thlaspi caerulescens grown on a soil contaminated with minespoil, the ratios of CdL: The solid lines represent total and radiolabile Cd measured in vitro as the E-value.
Reproduced with permission from New Phytologist , , — Similarly, Gerard et al. There were small differences which may have been due to inadequate penetration of the isotope in the potted soils CdL CdE. There was no evidence of phytomobilization of fixed forms of Cd as E- and L-values were broadly similar, and no trend in the ratio CdL: They found that CdE was essentially unaffected, except where pH was changed by the organic acids, and concluded that it is unlikely that nonlabile Cd could be solubilized at organic ligand concentrations normally found in the rhizosphere.
Isotopic Dilution Methods 37 pared specific activities in the Zn hyperaccumulator Thlaspi caerulescens and wheat, planted sequentially in soil spiked with 65Zn and Cd. For both metals, there were no significant differences in L-values between the cereal and hyperaccumulator plants. This is partly a reflection of the limitations inherent in ID methods when these are applied in isolation to soil chemical studies.
Nevertheless, it is clear that progress is being made and ID methods have already contributed substantially to our understanding of metal attenuation by soils. Recent work has explored some long-standing deficiencies in methodology, and it seems likely that more standardized procedures will emerge shortly. For obvious reasons, there is a long-standing preference in soil chemistry for simple metal-extraction schemes. Thus, future work will undoubtedly continue the search for extractants which can serve as analogues for ID-based estimates of labile metal pools.
The justification for this effort is clear, provided the degree of compromise required to accept chemical extraction methods is sufficiently small. For more mechanistic studies, exploration of the use of stable isotopes appears particularly attractive in view of the technological improvements in mass spectroscopy. It is likely that ID methods will be increasingly used as a complementary tool, alongside more precise speciation techniques e.
It is also expected that the ongoing development of mechanistic models of metal solubility and speciation may take greater advantage of the kinetic information which can be gained from ID methods. The full understanding of metal dynamics in soils is a clear objective, but realization of such a goal will rely upon the marriage of a number of complementary approaches. Cadmium reactivity in metal contaminated soils using a coupled stable isotope dilution-sequential extraction procedure. Changes with time in the availability of soil applied zinc to navy beans and in the chemical extraction of zinc from soils.
Aust J Soil Res Partitioning and reaction kinetics of Cd and Zn in an alum shale soil as influenced by organic matter at different temperatures. Trace elements in soil: Testing a mechanistic model. The effects of time and temperature on the reaction of zinc with a soil. Reactions with variable-charge soils. Developments in plant and soil sciences. Comparison of the chemical changes in the rhizosphere of the nickel hyperaccumulator Alyssum murale with the nonaccumulator Raphanus sativus.
Effect of nitrogen carrier, nitrogen rate, zinc rate, and soil pH on zinc uptake by sorghum, potatoes, and sugar beets. Reaction of zinc with soil affecting its availability to subterranean clover. Effect of soil properties on the relative effectiveness of applied zinc. The relationship between critical concentrations of extractable zinc and properties of Australian soils responsive to zinc. Reaction kinetics of the adsorption and desorption of nickel, zinc and cadmium by goethite. Adsorption and diffusion of metals.
Effect of organic ligands on isotopically exchangeable Cd E-value in and desorption from two heavy metal contaminated soils. Organic ligand and pH effects on isotopically exchangeable cadmium in polluted soils. Deist J, Talibudeen O. Dyanand S, Sinha MK. Labile pool and selective distribution of iron in calcareous and sodic soils.
Assessment of phytoavailability of nickel in soils. General purpose Freundlich isotherms for cadmium, copper and zinc in soils. Eur J Soil Sci Cadmium sorption and desorption in limed topsoils as influenced by pH: Isotherms and simulated leaching. Fujii R, Corey RB. Estimation of isotopically exchangeable cadmium and zinc in soils. Isotopic exchange kinetics method for assessing cadmium availability in soils. Cadmium availability to three plant species varying in cadmium accumulation pattern.
J Envirol Qual Field-based partition coefficients for trace elements in soil solutions. Selective distribution and labile pools of micronutrient elements as factors affecting plant uptake. Plant induced changes in the rhizosphere of rape Brassica napus var. Availability of zinc and cadmium to different plant species. Long-term changes in cadmium bioavailability in soil. Use of the hyperaccumulator Thlaspi caerulescens for bioavailable contaminant stripping. Interferences in the determination of isotopically exchangeable P in soils and a method to minimise them. Aust J Soil Res 8: Mechanisms of attenuation of metal availability in in situ remediation treatments.
Kinetic-radioactive investigations on the active surface of crystalline powders. Trans Farad Soc Kinetic-radioactive investigations on the active surface of crystalline powders, II. Trans Farad Soc 35, — Isotopic exchange in soil fertility studies. The use of P32 in studies on the uptake of phosphorus by plants.
Larsen S, Cooke IJ. The influence of radioactive phosphate level on the adsorption of phosphate by plants and on the determination of labile soil phosphate. Modelling pH buffering and aluminium solubility in European forest soils. Lofts S, Tipping E.
An assemblage model for cation binding by natural particulate matter. Geochem Cosmochim Acta Lability of Cd, Cu and Zn in polluted soils treated with lime, beringite and red mud and identification of non-labile colloidal fraction of metals. Labile pool and plant uptake of micronutrients: Dehydration, diffusion and entrapment of zinc in bentonite.
Clays Clay Min Ageing of metals in soils changes bioavailability. Fact sheet on environmental risk assessment. Factors affecting the lability of cadmium in soil [PhD thesis]. University of Nottingham, UK. The significance of radio labile Cadmium pools in soil. Labile pool of cadmium in sludge-treated soils. Papadopoulos P, Rowell DL. The reactions of cadmium with calcium carbonate surfaces. J Phys Radium 8: Isotopic equilibria between phosphates in soil and their significance in the assessment of fertility by tracer methods. J Soil Sci 5: Assessment of isotopically exchangeable zinc in polluted and nonpolluted soils.
Development of zinc bioavailability and toxicity for the springtail Folsomia candida in an experimentally contaminated field plot. Cadmium fixation in soils measured by isotopic dilution. The fate of Zn from tyre debris in soil. Use of isotopic dilution techniques to assess the mobilization of non-labile Cd by chelating agents in phytoremediation. Availability and fixation of Zn and Cd in soils amended with metal sulphate.
The behaviour of mercury in contaminated soils [Masters Thesis]. Soil zinc and its uptake by plants. Isotopic exchange equilibria and the application of tracer techniques. Soil chemistry in relation to prediction of availability. Development of critical level methodologies for toxic metals in soils and surface waters-dynamic modelling. Trivedi P, Axe L. Modeling Cd and Zn sorption to hydrous metal oxides. Predicting arsenic solubility in contaminated soils using isotopic dilution techniques. Methods for determining labile cadmium and zinc in soil.
Rates of metal ion fixation in soils determined by isotopic dilution. These processes are important in terms of both risk assessment and crop production because of their effects on metal toxicity and micronutrient availability, respectively. However, most of the research related to natural attenuation of metals in soil has been conducted using a chemocentric approach. Adsorption and desorption methods were used over 30 years ago by Tiller et al. Single or sequential extractions have also been widely used to assess aging of a variety of metals and metalloids in soil see Chapter 1.
More recently, isotopic dilution techniques have been employed to assess changes in metal lability over time see Chapter 2. These chemical techniques provide information regarding changes in soil-solution partitioning, extractability, and lability of metals in soil, but do not directly measure changes in biological availability or toxicity of metals Stevens and McLaughlin Therefore, biological and chemical assessment of aging must be integrated to adequately understand both the mechanisms and the effects of natural attenuation of metals in soil.
In this chapter, we will also provide a biological viewpoint of natural attenuation. Growth response curves can be used to examine attenuation from the perspective of micronutrient deficiency. In this case, the relationship between yield, or any other plant growth parameter y , and the amount of micronutrient metal applied to the soil x is generally fitted using a Mitscherlich equation Barrow and Mendoza Using this equation, a set of curves outlined in Figure 3.
It should be noted that the slope of the curves decreases with time, indicating that the effectiveness of the micronutrient metal in terms of plant growth declines over time Figure 3. Using the parameters of the Mitscherlich equation 3.
RE is calculated by dividing the c parameter of Equation 3. The biological approaches to estimating the natural attenuation rates of metals, when present in the toxicity range in soils, are quite different from those used in agronomic studies on micronutrient metals. One way to characterize the reversion of metals to nontoxic forms is to determine toxicity-evaluated abatement rates.
A toxic response to a metal can be defined see Figure 3. This toxic response curve can be defined for any soil using a variety of biological end points. Following a certain period of incubation of metal with soil t1 , the relevant end point e. The ratio of end points measured as EC50 values at different times can be used to calculate the rate of abatement of toxicity for each soil. A number of biological end points can be used to assess the natural attenuation of metals.
Plants, invertebrates, and the soil microbial community represent important classes of soil-dwelling organisms and differ in terms of sensitivity, pathways of exposure, and trophic levels. The different slopes are representative of curves expected for either different metals with different attenuation rates in the same soil or, for the same metal, differences in aging rate in different soils. Furthermore, their relative limitations and advantages will also be discussed.
Such interferences are particularly pertinent to aging studies, in which the time variable must also be considered. The optimum method to account for the variability of environmental conditions affecting plant responses is to add the metal to be investigated to soils at different times creating different ages or times of incubation , maintain the soils at a controlled moisture and temperature, and then perform the bioassay at the same time for all of the treated soils.
However, for practical or logistic reasons, this is not always possible. For instance, in field agronomical trials, micronutrient metals are often added at the same time, and the attenuation monitored by progressive bioassays e. The problem with this type of assessment is that variability in environmental factors such as temperature, humidity, and soil moisture will generally give rise to variability in the biotic response, thereby potentially confounding the assessment of metal availability. In this case, increasing the number of replicates and, especially, the number of time points, may help to improve the clarity of relationships between environmental influences and attenuation processes on biological responses.
For field trials, comparing the effectiveness of a micronutrient metal added in the past with a fresh metal addition is a preferable option, because the plants in the assay are grown at the same time and hence experience the same growth conditions. Obviously, pot trials under controlled conditions offer the possibility to minimize any confounding environmental factors. However, it can be argued that this type of standardized experiment is less environmentally relevant than field trials. Various plant tests can be used to assess natural attenuation.
In agronomic trials investigating changes in micronutrient availability over time, yield of crops or uptake of micronutrients is usually measured. Toxicity studies may focus on chronic or acute plant responses. In chronic toxicity tests, plant parameters such as biomass are generally measured after a few weeks or months of growth in contaminated soils. On the other hand, acute toxicity can be measured using much shorter tests such as root elongation or germination assays, which can be completed in a few days.
This implies that the time frame for investigation of attenuation will be different depending on whether chronic or acute tests are conducted. Obviously, chronic tests cannot be used to investigate attenuation processes over a few days. Also, when plants are grown for several months in a soil, the difference in metal bioavailability between the beginning and the end of the period of plant growth could be significant because of attenuation processes.
Therefore, the use of chronic tests is limited to investigation of natural attenuation over a long period of time, in the order of years. Residual value of manganese fertilizer for lupin grain production. Aust J Exp Agr Some examples in which biological end points agronomic trials have been used to provide experimental evidence of changes in micronutrient availability over time are reported in the following paragraphs.
A more comprehensive review of these experiments is given in Chapter The results indicated that the rate of Mn aging depended on soil properties Figure 3. The decline in effectiveness was larger in a gravel-sandy soil than in a sandy soil. The gravel sand had a greater content of clay, silt, and organic C and a larger cation exchange capacity in comparison to the sandy soil.
However, the study conducted by Brennan et al. This kind of information is only attainable if a large number of soils, differing in chemical and physical characteristics, are used. This result is similar to that reported by Young et al. Chapter 2 , who showed that soil pH enhances natural attenuation of metals and therefore decreases their effectiveness. Data from Brennan, RF. Reaction of zinc with soil affecting its availability to subterranean clover: Effect of soil properties on the relative effectiveness of applied Zn. The last example of changes in micronutrient effectiveness over time relates to the aging comparison between 2 micronutrients, Mo and Zn, in the same soil Brennan , RE of Mo appears to decline much more rapidly than that of Zn Figure 3.
The following examples describe studies in which plant bioassays have been used to indicate natural attenuation of metals from a toxicological point of view. Bruus Pedersen et al. In their study, Cu was added to an uncontaminated soil and dose-response curves were reported between root or shoot biomass and total, DTPA- or CaCl2-extractable soil Cu. As suggested by the authors, the time span used in this study may have been too narrow to reveal any clear effects of Cu aging.
This is corroborated by the fact that concentrations of extractable metals did not change between 1 and 12 weeks of incubation. Interestingly, Bruus Pedersen et al. Residual value of zinc fertilizer for production of wheat. Residual value of molybdenum trioxide for clover production on an acidic sandy podsol. Lock and Janssen studied the influence of aging on Cu bioavailability and toxicity to red clover Trifolium pratense.
In this study, the bioavailability of Cu in 25 contaminated soils collected by Cu runoff from bronze statues was compared to Cu bioavailability in control soils freshly spiked to the same Cu concentration of the corresponding field soils. Their results indicated that growth of clover was less in the freshly spiked soils and that this was related to the Cu concentration in soil-pore water. However, the pH of the soil after spiking decreased in comparison to the historically contaminated soils.
Therefore, it was not possible to unequivocally relate the decrease in Cu bioavailability to attenuation of Cu by the soil. This method, as well as other isotopic dilution techniques used to investigate natural attenuation of metals in soil, are reviewed in Chapter 2. Therefore, invertebrates are an important class of organism for soil toxicity testing and could be used to assess natural attenuation of metals in soil.
Similarly to plants, a variety of invertebrate tests are available. Short-term survival tests can provide a snapshot of acute toxicity. As discussed for plants, the time frame for aging investigations will vary with the test chosen. For instance, survival studies that can be completed in a few days may provide a higher time resolution than reproduction tests that require several weeks to be completed.
One aspect that needs to be considered when soil invertebrates are used to assess attenuation processes relates to the pathways of exposure to metals. For invertebrates, the major exposure routes can include both the soil-pore water as well as direct ingestion of soil particles. In the case of soil ingestion, metal speciation may be changed in the gut of the organism. Therefore, it is possible that the attenuated metal could be remobilized in the gut environment. If this occurs, use of invertebrates to assess metal attenuation may provide different results in comparison to other biological or chemical assessments.
However, recent results using earthworms and isotopic dilution techniques suggest that Zn uptake by Eisenia andrei is predominantly via the exchangeable pools possibly the soil-pore water rather than by any gut-induced dissolution of Zn held in nonexchangeable pools in the soil ScottFordsmand et al. A limited number of studies have attempted to assess natural attenuation of metals using soil invertebrates. Smit and Van Gestel investigated the effect of aging on bioaccumulation and toxicity of Zn for the springtail Folsomia candida.
Using the weight of the springtail as the biological end point, they determined that the effect of Zn was larger by a factor of 5 to 8 in freshly spiked soils in comparison to test soils subjected to aging under field conditions for 1. However, these results were complicated by a significant difference in pH between freshly spiked and field-aged soils, which has a strong effect in terms of Zn solubility and availability. Also, Smit and Van Gestel suggested that the presence of relatively large amounts of Cl introduced with the spiking of ZnCl2 may have resulted in an enhanced toxicity in freshly contaminated soils through a synergistic toxic effect of Zn and Cl, or through Cl altering the bioavailability of Zn.
In a similar experiment, Smit et al. In this case, the end point investigated was mortality and reproduction of F. Although a relationship between mortality and Zn concentration was not observed, production of juveniles appeared to be a sensitive and reproducible parameter to assess Zn toxicity and to study Zn aging. Similar to other studies, comparison between freshly spiked soils and soils incubated in the field for several months was difficult because of a lower pH of the freshly spiked soils.
However, after a few months, the pH of treatments stabilized, and a comparison between EC50s at different times was possible. This data indicated that an increase in Zn sorption and a decrease in bioavailability over time lead to reduced toxicity. Smit and Van Gestel also compared the difference in Zn toxicity between soil collected around a Zn smelter and control soil spiked with Zn salts. In this case, the reproduction of F. In the freshly spiked soils, EC50 for reproduction was mg Zn kg—1.
Lock and Janssen used a central composite experimental design to develop a model to predict Zn toxicity to F. This model was able to predict chronic Zn toxicity to the springtail in freshly spiked soils as a function of soil pH, cation exchange capacity, and total Zn in soil. However, when applied to 19 soils historically contaminated by previous smelting activities or application of dredgecontaminated sediments, the model failed to predict Zn toxicity.
Lock and Janssen suggested that this was probably due to changes in Zn availability caused by aging processes. In subsequent work, the same authors investigated the influence of aging on acute Cu toxicity to the potworm Enchytraeus albidus and on chronic toxicity to F. The toxicity of historically contaminated soils collected around bronze statues and of freshly spiked control soils was compared.
Toxicity in the historically contaminated soils was lower than in the laboratory-spiked soils. However, pH differences between the 2 sets of soil treatments made it impossible to ascertain whether aging, or simply a difference in Cu partitioning due to pH, was responsible for the decrease in Cu toxicity observed in the field-contaminated soils. A large number of tests are available, and most relate to some function or characteristic of the indigenous soil microbial community or of a specific group of this community. This is in contrast to laboratory tests conducted with invertebrates or plants in which these organisms are usually placed in the soil at the time of testing.
Consequently, in the case of microbial tests, aging generally occurs in the presence of the organisms used in the test. As a result of this, and because of the rapid life cycle of a number of microorganisms, there is a factor in addition to attenuation that may influence the toxic response. This factor is adaptation. Adaptation of microorganisms to metals can be achieved by means of intrinsic properties tolerance or by developing specific mechanisms of detoxification resistance as a result of exposure to metals.
Adaptation is therefore a factor that, similar to aging, will develop with time after the application of metals to soil.
Also, adaptation, as with aging, results in a decrease of the detrimental effect when assessed with microbial toxicity tests. As discussed in Chapter 2, there is generally a very rapid decrease in availability immediately after the addition of a metal to a soil, but this decrease in availability slows down with time.
Similarly, adaptation will proceed more rapidly immediately after metals are added to the soil because their availability, and therefore the selective pressure to which the microbial community is exposed, is larger immediately after metal addition and decreases with time. Such large additions of metals with a single application are very rare in the environment and would be restricted to accidental spills. Various studies have demonstrated induction of microbial community tolerance to metals when soils have been contaminated.
Other methods, such as 3H-thymidine incorporation and phospholipid-ester-linked fatty acid PLFA analyses have also measured increases in the tolerance of soil microorganisms upon exposure to metal Baath ; Frostegard et al. Recently, Hamon et al. This method can be used for the investigation of metal aging avoiding the problem of tolerance.
In this way, the microbial community is not exposed to metals during the aging process, thus avoiding the confounding factor of adaptation excluding adaptation processes taking place during the testing itself. However, care would need to be taken to account for any effects that the sterilization process may have on influencing metal availability and toxicity. Another way to avoid the problem of metal adaptation of the indigenous microbial community is based on the use of added, or nonindigenous, microorganisms.
An example is the use of microbial biosensors to test metal bioavailability in soil-pore water Paton et al. However, so far these methods have not been employed in natural attenuation studies, and the interpretation of published results must take into account the possibility of microbial adaptation as a confounding factor. Significant changes in the community structure were observed immediately after spiking 15 d but decreased with time d incubation.
However, the effect of changes in Zn availability over time was not separated from adaptation of the microbial community. Doelman and Haanstra studied the changes in basal soil respiration in 5 soils spiked with Zn and incubated for up to 43 weeks. A decline of Zn toxicity with time was found in some soils but not in all. Nitrification, respiration, and N-mineralization rates were significantly reduced in the laboratory-spiked soils in comparison to the control but were unaffected, or even increased with Zn concentration, in the field transect soils Figure 3.
Even though adaptation of the soil microbial population may have been a reason for the difference in Zn toxicity between field- and laboratory-contaminated soils, attenuation of Zn bioavailability has probably been a contributing factor. In fact, large differences in Zn solubility were observed between aged and freshly spiked soils.
The highest soil-solution Zn concentrations in the field soils were always lower than the soil-solution EC50s of spiked soils. Not all the evidence obtained with microbial tests suggests that natural attenuation of metals in soil occurs. For instance, Chaudri et al.
No significant effects were reported up to 12 months. However, after 18 months, the population decreased at higher Zn concentrations. This is opposite to what should be expected if bioavailability of metals was the only factor controlling the survival of rhizobium in the soil. These results indicate that the effect of time on metal aging must be related to the life cycle and any lag-time response at community or population levels.
Comparison of toxicity of zinc for soil microbial processes between laboratory-contaminated and polluted field soils. Environ Toxicol Chem For instance, adaptation is a confounding factor, which is important to consider when indigenous microbial assays are used, but is not applicable for short-term tests in which organisms such as plants or invertebrates are added to the soil.
However, other limitations are more generic in character and have implications for every biological approach, independent of the test organism considered. It is widely accepted that biological variability, and consequently measurement of biological parameters, is larger than analytical errors in chemical tests. This must be taken into account when biological methods are used to assess natural attenuation. In fact, natural attenuation usually produces small changes in bioavailability over periods that can be considerably long. If these changes are smaller than the error associated with the measurement of the biological parameters, aging responses cannot be revealed using these techniques.
Variability in biological assays is caused by the intrinsic biological variability in the bioassay used, the response of the bioassay to environmental conditions, and the analytical or physical measurement of the particular biological parameter used. Each of these sources of variability can be minimized by using, for instance, clonal biological material, conducting the testing under controlled environmental conditions, and following standard analytical procedures supported by adequate quality controls.
For instance, results from a laboratory assessment using a specific microbial biosensor, under very defined environmental conditions, is probably less environmentally relevant than field measurements of changes in soil microbial functions or community structure over time. A large number of toxicological studies reviewed in the previous sections compared freshly spiked control soils with field-contaminated soils in which metals were aged.
This is a useful shortcut because it allows the assessment of natural attenuation in a short time avoiding the need to age soils for long periods of time. However, it is not always easy to find field soils contaminated by a single metal and with a known history of contamination. Moreover, spiking of uncontaminated soils with large amounts of metal salts can cause several artifacts that confound the interpretation of the results. For instance, soil pH in the spiked soils can significantly decrease in comparison to field soils that have been contaminated slowly with a metal and have undergone leaching through natural rainfall events.
The decrease in soil pH after addition of metal salts is due to metal hydrolysis and metal adsorption by soil, and consequent protons released in the soil solution Basta and Tabatabai Furthermore, metal spiking can significantly increase the salinity in the soil-pore water with consequences for the solid-solution partitioning of metals. Biological assessment of natural attenuation using the RE approach does not suffer the same confounding factors discussed for toxicological studies.
In fact, micronutrient metals are usually added at small rates that do not cause changes in pH or salinity. However, RE approaches are limited to deficiency situations and to elements for which there is a positive response in the organism tested in comparison to an untreated control. Consequently, a biological assessment of aging processes appears to be extremely appropriate. On the other hand, because of the natural variability of biological systems and a series of confounding factors previously discussed, at present it is difficult to assess natural attenuation of metals, at least when the interest is focused on the toxicity range.
Attenuation processes taking place at low metal loading may be quite different from those occurring under toxic conditions. More research is needed to understand the implications of natural attenuation for metals in the toxic range. It should be emphasized that a biological approach to the assessment of natural attenuation of metal toxicity is quite recent, and most of the methods employed are the same as those used in ecotoxicological studies. More work is therefore needed to fine-tune these methods to obtain reliable information regarding processes resulting in attenuation of trace element bioavailability.
Measurement of heavy metal tolerance of soil bacteria using thymidine incorporationn into bacteria extracted after homogenisation-centrifugation. Soil Biol Biochem Equations for describing sigmoid yield responses and their application to some phosphate responses by lupins and by subterranean clover. Effect of cropping systems on adsorption of metals by soils.
Residual value of zinc fertiliser for production of wheat. Residual value of manganese fertiliser for lupin grain production. Toxicity and bioaccumulation of copper to black bindweed Fallopian convolvolus in relation to bioavailability and age of soil contamination. Arch Environ Contam Toxicol Survival of the indigenous population of Rhizobium leguminosarum biovar trifolii in soil spiked with Cd, Zn, Cu and Ni salts.
Multiple heavy metal tolerance of soil bacterial communities and its measurement by a thymidine incorporation technique. Appl Environ Microbiol Doelman P, Haanstra L. Short-term and long-term effects of cadmium, chromium, copper, nickel, lead and zinc on soil microbial respiration in relation to abiotic soil factors. Shifts in the structure of soil microbial communities in limed forests as revealed by phospholipid fatty-acid analysis. Metals and soil microbes: Changes in soil microbial communities over time resulting from one time application of zinc: Effects of soil copper on the black bindweed Fallopian convolvolus in the laboratory and in the field.
Lock K, Janssen CR. Ecotoxicity of zinc in spiked artificial soils versus contaminated field soils. Influence of aging on copper bioavailability in soils. Environ Tox Chem Test methods to determine hazards of sparingly soluble metal compounds in soils.
Zeit Pflanzen Bodenk Development of an acute and chronic ecotoxicity assay using lux-marked Rhizobium leguminosarum biovar trifolii. Lett App Microbiol Rapid method for assessing pollution-induced community tolerance in contaminated soil. Do earthworms mobilize fixed zinc from ingested soil? Desorption of native and added zinc from a range of New Zealand soils in relation to soil properties. Comparison of the toxicity of zinc for the springtail Folsomia candida in artificially contaminated and polluted field soils.
Appl Soil Ecol 3: Effects of soil type, prepercolation, and ageing on bioaccumulation and toxicity of zinc for the springtail Folsomia candida. Bioindicators for assessing natural attenuation of metals in soil. Determining toxicity of runoff lead and zinc in soils-salinity effects on metal partitioning and phytotoxicity. These oxides occur as discrete particles or as coatings on other mineral surfaces; they have large surface areas, microporous structures, and high affinities for metal ions Jenne ; Sparks The sorption of contaminants to these microporous oxides is recognized as a 2-step process: Many mechanistic models have been invoked to explain transient sorption, including diffusion into micropores Hodges and Johnson ; Barrow et al.
In modeling intraparticle diffusion, adsorbent characteristics that significantly affect the intra-aggregate mass transport of the contaminants include particle size and geometry, porosity, and pore size distribution Papelis et al. For example, Papelis et al. Surface area also serves as another tool in assessing the physical properties of an adsorbent Gregg and Sing Such structural differences significantly affect sorption behavior.
In addition, Papelis et al. Recently, Trivedi demonstrated the role of porosity in assessing the longterm sorption of contaminants.
Likewise, Cole et al. These results emphasize the need to account for the microporosity of the different components of soils and sediments to appraise their role in the long-term fate of the contaminants. Furthermore, to identify the type of diffusion mechanism, it is important to evaluate the pore size distribution in porous sorbents.
If the effective radius of the pore is much greater than the radius of the diffusing species, then bulk diffusion is expected. Methods of preparation and characterization have been detailed elsewhere Axe and Anderson ; Trivedi and Axe , Diffusion kinetics can take years to reach equilibrium, hence methods are needed to understand the sorption in a convenient time frame.
In traditional sorption studies, the total amount of the adsorbate is fixed, and, consequently, the driving force for diffusion decreases with time. Intraparticle diffusion of metal contaminants along the micropore walls of amorphous oxides. The grey spherical area represents the amorphous oxide particle with micropores and dark points represent the metal adsorbate.
Reproduced from Trivedi P. The relative importance of external mass transfer vs.
Cadmium fixation in soils measured by isotopic dilution. This book covers 3 main areas of research. The presence of the mineral surface may also enhance the formation of a new precipitate phase by lowering the energy barrier to nucleation of a new solid phase Stumm The relative importance of external mass transfer vs. It should be emphasized that a biological approach to the assessment of natural attenuation of metal toxicity is quite recent, and most of the methods employed are the same as those used in ecotoxicological studies. Larsen S, Cooke IJ.
Therefore, for experimental studies, the system must be maintained in a turbulent hydraulic regime to minimize the external mass transfer resistance Fogler Although they have been employed to model the sorption kinetics, they have serious limitations where their use in fitting often does not provide any mechanistic insight Sparks In contrast, using a mass balance with welldefined boundary conditions and sorbent characteristics has resulted in 1 fitting parameter, surface diffusivity Axe and Anderson , ; Trivedi and Axe , , a.
With negligible external mass transfer resistance and dilute adsorbate concentrations, and assuming the internal sites are similar to the external ones, the mass balance for spherical aggregates yields the following partial differential equation Crank Given the boundary conditions, including a constant surface concentration, integration of the analytical solution to Equation 4.
All the parameters, except the surface diffusivity, are determined from the characterization and the isotherm studies. By minimizing the variance, the only fitting parameter — surface diffusivity — is obtained. Given the particle size distribution of each oxide, sorption reaches equilibrium in approximately 3 to 4 months for Sr, 2 to 5 years for Cd, and as much as 5 to 8 years for Zn Figure 4. They attributed this slow sorption to intraparticle diffusion. Liang and Tsai estimated the diffusivity of Cs in natural modernite in the order of 10—14 cm2 s—1.
Papelis, Roberts et al. All these studies indicate that in soils and aquatic environments where microporous oxides and clay minerals are present, these sorbents potentially act as sinks for metal contaminants. This potential can be described by a sinusoidal function whose minima represent the sorption sites and maxima signify the energy barrier or the activation energy, EA. The random walk model has also been applied, for example, in predicting the diffusivities of volatile organic compounds in adsorbents such as activated carbon Lordgooei et al.
In amorphous oxide systems, the activation energy was found to be unique for a specific metal ion Trivedi ; Trivedi and Axe a. Furthermore, this activation energy, which is simply the energy barrier between sites, was correlated to the hydrated radius RH of the cation with charge Z and its primary hydration number N Figure 4. Evangelou noted that reactions with activation energies less than 42 kJ mol—1 are indicative of diffusion-controlled processes, where as those with greater activation energies are surface controlled.
The diffusivities predicted using Equation 4. Solid lines represent the best fit of the experimental diffusivity. Long dashed lines represent predictions of theoretical diffusivities based on site activation theory. Short dashed lines are modeling errors for the experimental diffusivity.
Offering a concise, well-rounded perspective, Natural Attenuation of Trace Element Availability in Soils demonstrates how attenuation processes can. Natural attenuation of trace element availability in soils, by Rebecca Hamon, Mike McLaughlin, and Enzo Lombi. Hunter Anderson.
Solid symbols with error bars represent the theoretical Ds, and open symbols represent experimental Ds for Sr, Cd, Ni, and Zn. A number of researchers have employed XAS to elucidate the sorption mechanism of environmentally relevant metal contaminants to important soil components Charlet and Manceau ; Bargar et al.
However, the application of XAS to elucidate diffusion kinetics is limited.
They found that Pb sorbed to aluminum oxide via an inner sphere bonding mechanism and that, as sorption increased as a function of time, the coordination environment was invariant, thus suggesting diffusion of Pb along the micropores of the oxide. In the long-term samples aged for as long as d, metal ions were found to have similar local structure to that of their respective short-term 4 h samples. These results support the conclusion that the slow uptake of metal ions in HMO results from intraparticle diffusion wherein the sorption sites located along the micropore walls are similar to those available on the external surfaces and macropores.
Likewise, Scheinost et al. Interestingly, the local structure of Pb and Cu sorbed to these iron oxides was invariant with time up to 8 weeks in the absence or presence of competing ions or fulvic acid; thus, they attributed the slow uptake of metal ions by ferrihydrite to intraparticle diffusion. The spectroscopic evidence asserts the need to include contributions from diffusion for modeling the long-term fate and bioavailability of metal pollutants in soil and aquatic environments. Limited research has been conducted to address this issue. For example, Dong et al.
Anderson and Benjamin , as well as Meng and Letterman , have developed models of adsorption at multicomponent oxide surfaces. These studies also indicate that coatings change the surface chemistry of the substrate, and thus significantly influence the mobility of the metal contaminants. Most of the studies conducted on oxide coatings are based on short-term macroscopic experiments, suggesting the need for long-term experiments on kinetics of metal sorption onto the coatings.
In light of the development of composite adsorbents for wastewater treatment, a number of studies have demonstrated intraparticle diffusion of aqueous metal contaminants onto composite adsorbents as the predominant mode of sorption. These adsorbents range from cemented goethite Theis et al. Thus, there is a strong need to understand metal sorption onto oxide-coated mineral and organic surfaces to effectively predict the fate of metals in soils and sediments.
Macroscopic modeling efforts provide the basis for evaluating diffusivities. XAS studies have further supported the rate-limiting mechanism, intraparticle diffusion. Time-resolved in situ spectroscopic studies need to be extensively applied for better understanding of diffusion-related processes in microporous components of soils, such as hydrated metal oxides. The challenge now lies in assessing these diffusion-limited processes for multicomponent systems and incorporating them in transport models and waste management programs.
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