Heavy metals mobilization

Despite intensive research in the past decade, most of the studies done in the laboratory have not yet been investigated in the field. In fact, many researches show that organic C and contaminants in aquatic ecosystems are partly from terrestrial ecosystems through runoff and percolation. However, it is impossible to predict how much DOM and DOM-facilitated solutes are transferred to aquatic environments without better understanding of the behavior of DOM itself and interaction of DOM and metals in soils. The aim of this chapter, based on a series of trials that we conducted, is to give a brief summary on the behavior of DOM derived from organic wastes in soils and its effect on heavy metal mobility, and to propose areas of future research. 

Heavy metal mobilization is often followed by microorganism and plant uptake, and intracellular accumulation. Filamentous fungi transport heavy metals and radionuclides along their hyphae. This may be a mechanism of mobilization from mycor-rhizal fungi to higher plants. An alternative pathway involves direct root uptake of heavy metals mobilized by microbial acid production or chelation. 

In a UK study, single oral doses of dimercaptopropane sulfonic acid or succimer in different combinations with or without acetylcysteine and potassium citrate were given to 191 patients considered to have mercury toxicity from amalgam dental fillings (2). After a single dose, about 5% of patients complained of mild gastrointestinal discomfort, fatigue, mental fuzziness, headache, and diuresis. These usually cleared within 6 hours of the dose and were considered to be due to heavy metal mobilization. There were no cases of hypersensitivity. 

W. Hartley, R. Edwards, and N.W. Lepp, Arsenic and Heavy Metal Mobility in Iron Oxide-amended Contaminated Soils As Evaluated by Short- and Long-term Leaching Tests, Environ. Pollut. 131(3), 495-504, Oct. (2004). 

Fangueiro, D., Bermond, A., Santos, E., Carapuca, H., and Duarte, A. (2002). Heavy metal mobility assessment in sediments based on a kinetic approach of the EDTA extraction search for optimal experimental conditions. Anal. Chim. Acta 459, 245-256. 

In the case of sediment S, the unenhanced electrokinetic treatment, as expected, was not effective toward heavy metal mobilization, since the process was unable to induce significant pH changes along the whole specimen. The high electric current intensities observed in run Wl-S were caused by ion migration related to the soluble salt content, as confirmed by the drastic reduction in electric conductivity with time as soon as such species were depleted from the material, with a final value very close to that of the prewashed material ( lmS/cm). Small fractions of Cu and Ni... 

In the case of EDTA-enhanced test on prewashed sediment S (run EDTA-S), the results in terms of heavy metal mobilization were modest, with a slight accumulation of Cd and Pb in the central sections of the specimen, probably as a result of electromigration of negatively charged EDTA metal complexes toward the anode and an opposite mass transport caused by the electroosmotic flow toward the cathode (Fig. 7.10) however, the low metal mobilization may also derive from the lower voltage gradient applied (1 V/cm) if compared to tests on sediment V (2.5V/cm). 

Figure 7.17 shows the cumulative mass of Ca, Mg, Pb, Zn, Ni, and Cd over time measured in the cathodic and anodic chambers. As mentioned, Ca and Mg mostly accumulated at the cathode, and their migration toward the electrodes clearly started readily at the beginning of the test conversely, the presence of trace metals in the chambers could only be detected after 5-10 days of treatment. The onset of heavy metal mobilization was found to correspond to -10% electro-removal of Ca from sediment. It should be emphasized that this adds to -30% of Ca removal accomplished through the preliminary mechanical separation of the 500-pm oversize fraction. 

In situ treatment is a perspective of particular interest since it may be applied where decontamination is required but dredging is not and, most of all, would avoid sediment removal and, therefore, the related risk of spreading the contamination. To this regard, innovative approaches involve placing one electrode in the sediment and the other one in water in order to promote heavy metal mobilization/ accumulation, or degradation of organic compounds through electrochemical oxidation. 

Capiat, C., Texier, H. BariUier, D., et al. 2005. Heavy metals mobility in harbour contaminated sediments The case of Port-en-Bessin. Marine Pollution Bulletin 50(5) 504-511. 

Ottosen et al. [16] compiled literature with results obtained on laboratory and pilot scale with electrokinetic removal of heavy metals from industrially polluted soil. The general trend was that for Cd, Cu, and Zn good removal was obtained in most investigated soil types by applying the electric potential to the soil, utilizing the acidic front developing from the anode for heavy metal mobilization, as described in Fig. 1. In many cases, 80-98 % of the heavy metals are removed (both on lab scale and field scale). For... 

The optimum conditions of heavy metals extraction from ordinary chernozem in different solvents are selected both at determination of the mobile forms of elements, and at an estimation of their gross contents. It is established, that the stage of elements extraction in the greater measure depends on time of action and intensity of ultrasonic, nature of selected solvents and determinated elements. It is shown, that for all type of soils the time of low frequency ultrasonic action does not exceed 10 minutes, and the intensity ranges in an interval of 3-4 W/cm. ... 

Accumulation of heavy metals (HM) in different compartments of the biosphere, and their possible mobilization under changing environmental conditions induce a perturbation of the ecosystem and adverse health effects. Fast and correct estimating the environmentally relevant fonus of HM in soils, sediments, and sewage sludge is an urgent need for environmental monitoring and assessment. 

The behavior of elements (toxicity, bioavailability, and distribution) in the environment depends strongly on their chemical forms and type of binding and cannot be reliably predicted on the basis of the total concentration. In order to assess the mobility and reactivity of heavy metal (HM) species in solid samples (soils and sediments), batch sequential extraction procedures are used. HM are fractionated into operationally defined forms under the action of selective leaching reagents. 

Mobilization of inicronutrients such as Zn, Mn, Cu, and Co and of heavy metals (Cd, Ni) in soil extraction experiments with root exudates isolated from various axenically grown plants is well documented (61,204-206) and has been related to the presence of complexing agents. 

F. Awad and V. Rdmheld, Mobilization of heavy metals from contaminated calcareous soils by plant-borne chelators and its uptake by wheat plants. J. Plant Nutr. 23 (2000) in press. 

Species may differ by oxidation state for example, manganese(II) and (IV) iron(II) and (III) and chromium(III) and (VI). Oxidation state is influenced by the redox potential. Mobility is affected because oxidation state influences precipitation-dissolution reactions and also toxicity in the case of heavy metals. 

Adsorption is a physicochemical process whereby ionic and nonionic solutes become concentrated from solution at solid-liquid interfaces.3132 Adsorption and desorption are caused by interactions between and among molecules in solution and those in the structure of solid surfaces. Adsorption is a major mechanism affecting the mobility of heavy metals and toxic organic substances and is thus a major consideration when assessing transport. Because adsorption is usually fully or partly reversible (desorption), only rarely can it be considered a detoxification process for fate-assessment purposes. Although adsorption does not directly affect the toxicity of a substance, the substance may be rendered nontoxic by concurrent transformation processes such as hydrolysis and biodegradation. Many chemical and physical properties of both aqueous and solid phases affect adsorption, and the physical chemistry of the process itself is complex. For example, adsorption of one ion may result in desorption of another ion (known as ion exchange). 

A complex ion is one that contains more than one ion. Because of its effect on mobility, complexation, the process by which complex ions form in solution, is very important for heavy metals and may be significant for organic wastes. Heavy metals are particularly prone to complexation because their atomic structure (specifically the presence of unfilled d-orbitals) favors the formation of strong bonds with polar molecules, such as water and ammonia (NH3), and anions, such as chloride (CO and cyanide (CN ). Depending on the chemistry of an injected waste and existing conditions, complexation can increase or decrease the waste s mobility. 

Strongly influences adsorption, because hydrogen ions play an active role in both chemical and physical bonding processes. Mobility of heavy metals is strongly influenced by pH. Adsorption of some organics is also pH-dependent. 

Oxidation-reduction reactions may affect the mobility of metal ions by changing the oxidation state. The environmental factors of pH and Eh (oxidation-reduction potential) strongly affect all the processes discussed above. For example, the type and number of molecular and ionic species of metals change with a change in pH (see Figures 20.5-20.7). A number of metals and nonmetals (As, Be, Cr, Cu, Fe, Ni, Se, V, Zn) are more mobile under anaerobic conditions than aerobic conditions, all other factors being equal.104 Additionally, the high salinity of deep-well injection zones increases the complexity of the equilibrium chemistry of heavy metals.106... 

Forstner and Wittmann107 reported the following observations about the general mobility of heavy metals in groundwater ... 

Chemical remediation refers to the application of various minerals or chemicals to adsorb, bind, precipitate or co-precipitate trace elements and heavy metals in soils and waters thereby reducing their bioavailability, toxicity, and mobility. In situ immobilization refers to the treatment of contaminants in place without having to excavate the soils or waste, often resulting in substantial cost savings. However, in situ immobilization or extraction by these physicochemical techniques can be expensive and are often only appropriate for small areas where rapid and complete decontamination is required. 

Howari F. Heavy metal speciation and mobility assessment of arid soils in the vicinity of A1 Ain landfill, United Arab Emirates Int J Environ Pollut 2004 22 721-731. 

Zeien H., Brummer G.W. Determination of the mobility and binding of heavy metals in soil by sequential extraction. Mitteilungen der deutschen bodenkundlichen gesellschaft. 1991 66 397-400. 

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