Trace element particle association

Coal ash is an aluminosilicate glass, consisting of the oxides of Si, Al, Fe and Ca, with minor amounts of Mg, Ti, Na, K and S, and various trace elements. The trace-element concentrations associated with the ash may be either adsorbed onto the surface of the particle or incorporated into the matrix (Kaakinen et al., 1975 Cambell et al., 1978). Collin (1974) has proposed a model whereby acidic, neutral and basic layers are formed on the surface of the fly ash particle as it is carried through the gas stream. 

One of the important features of Figure 1 is that the concentration, even for trace elements, varies fairly smoothly and continuously with depth. This casts doubt on some erratic and highly discontinuous values unless there are obvious reasons for them, such as hydrothermal influence or difference in the water masses. The data shown in Figure 1 are largely based on filtered samples and therefore, can be referred as dissolved concentration. For conservative elements, it does not matter whether the water sample is filtered or not, since there is virtually no difference in the analytical results. For most nutrient-type elements, particle association in the open ocean is generally small (< 5%) and therefore, the gross features of unfiltered samples remains the same as... 

More recently, Wells and Goldberg (1991) report that very small marine colloids (d < 120 nm) are, by at least three order of magnitudes, more abundant. A vertical stratification of these particles was found (very high concentrations in the thermodine and season-dependent in the bottom-waters). This stratification indicates that these very small colloidal particles are reactive. The apparent close association of metals with these colloids suggests that they may play an important part in the transport and fate of trace elements in seawater (Wells and Goldberg,... 

As was previously mentioned, trace elements that sublime at temperatures below those attained during coal combustion (e.g., As, Se, Hg, Zn), and are associated with thermally unstable solid phases (in particular organic matter and sulphide minerals), are subject to vaporization into furnace gases. Once these gases, and fly ash particles entrained in the gases, are vented from the combustion furnace they quickly cool, leading to the condensation of volatilized elements onto the... 

In conclusion, although some trace elements are concentrated in minerals such as magnetite, it is the association with the surfaces of the ash particles that dominates. 

As discussed in the previous section, trace elements are essentially retained in the solid combustion products and, because many are present on the surfaces of the particles, they are potentially leachable. Our data show the elements Mo, As, Cu, Zn, Pb, U, Tl, and Se will be readily accessible for leaching. A significant fraction of the V, Cr, and Ni, and a minor proportion of the Ba and Sr will also be potentially leachable because of the surface association, but most of these elements appear to be located in particles and will be released more slowly as the dissolution of the glass and other phases takes place. Rubidium, Y, Zr, Mn, and Nb are contained almost entirely within the particles and dissolution is potentially slower. The extent to which elements are leached also depends on their speciation and solubility in the porewaters, and the pH exerts a major control. In oxidizing solutions, elements such as, Cd, Cu, Mn, Ni, Pb, and Zn form hydrated cations that adsorb onto mineral surfaces at higher pH values and desorb at lower pH values. In contrast, the elements As, U, Mo, Se, and V, under similar Eh conditions, form oxyanions that adsorb onto mineral surfaces at low pH values and desorb at higher values (Jones 1995). 

Studies of fresh ash produced by coal combustion have shown that many trace elements (As, B, Bi, Cd, Cr, Cu, Ge, Hg, Mo, Pb, Ni, Se, Sr, Tl, V, W, Zn) are enriched in the fly ash compared to the bottom ash (Hansen Fisher 1980 Eary et al. 1990 Mukhopadhyay et al. 1996 Karayigit et al. 2001). For example, Mukhopadhyay et al. (1996) reported 10-20 times enrichment of most trace elements in the fly ash compared to the feed coal and association of As with crystalline Fe-0 and Fe-S phases in the bottom ash from a power plant in Nova Scotia fed by eastern Canadian coal. Elements enriched in fly ash are typically those more easily volatilized. Because fly ash particles also have smaller sizes and therefore greater reactivity than bottom ash, the probability of metal leaching is correspondingly greater. Ainsworth Rai (1987) and Rai et al. (1988) found that most of the Cu, Mo, Se, Sr, and V in fly ash was readily soluble. 

Our results indicated that coals ground to —60 mesh failed to yield a consistently acceptable precision for most trace element determinations. Therefore, it was necessary to evaluate the errors associated with trace element determinations in coals ground to various particle sizes. 

Similarly, different results have been found for the impact of brake wear particles. While unequivocally a strong enrichment of brake-related chemical trace elements (Sb, Ba, Cu) is found at trafficked sites [63], the quantification of overall PM contribution from brake wear is associated with uncertainties. An even wider range of emission factors was found for re-suspension of road dust (cf. Table 4). It should be noted that re-suspension may be a strong source of PM during wintertime when de-icing salt is spread out. For a traffic site in southern Germany exceedance of the daily PM limit value could be tracked back to road salting in 12 of 43 cases [66]. 

Physical cleaning of various coals by oil agglomeration reduced levels of As, Cr, Pb, Mn, Mo, Ni, and V by 50-80%, while levels of some other trace elements were reduced by lesser amounts (20). Oil agglomeration appeared to be more effective at removing trace elements than the wet concentrating table or float/sink density separations. This may be related to an increase in the liberation of mineral matter associated with grinding to produce the relatively fine particle sizes required in the oil agglomeration technique. 

Assuming that particle-reactive radionuclides and many other trace elements are scavenged by particles settling through the water column, and that these fluxes of sediment and the associated radionuclide have remained constant over time, an accumulation rate (A) can be calculated using the following equation ... 

G. A. Knauer, J. H. Martin and others, VERtical Transport and Exchange (VERTEX) program Pioneering research on the relationships between particulate matter production, export and remineralization tracer studies particle-associated nitrification trace element (Fe) controls on primary production establishment of an 18-month ocean time-series at 33°N, 139°W... 

The aim of sample treatment is frequently the differentiation of water components on the basis of their physical-chemical properties. More separation techniques can be applied to divide trace elements in sea water in fraction (55). The major assumption made by applying separation techniques is that removal of one or more components from a sample does not disturb the solution equilibria, but frequently there is evidence that this is not true e.g., it was observed that after removal of particles from water there was regrowth of filterable particles. However, operationally defined separation techniques are a useful means of comparing the characteristics of different samples. The problems and the advantages associated with specific separation methods for trace elements speciation has heen reviewed (55). 

Speciation studies have been carried out for methylated species of As, Sb, Ge, and Sn and on different redox states of these elements—As(III-V), Sb(III-V), or Cr(III-VI)— (Andreae and Froehlich, 1984), as shown in Fig. 13.2. In oxic Baltic waters the pentavalent species of As and Sb predominate, while in the anoxic basins the distribution shifts to the trivalent species and possibly also to polysulfide complexes. The methylated species of As, Sb, and Ge are detectable throughout the water column. Since the mid-1980s, an improvement in the understanding of the behavior of trace elements in the Baltic Sea has been achieved. First investigations into the speciation of mercury have been realized by Brugmann (1979) and Brugmann et al. (1991). A special emphasis has been made on the different physicochemical forms, such as dissolved Hg, weakly associated Hg with particles, and total Hg. 

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