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Generally, more than one metal was in excess, but not necessarily. Similarly, the concentrations of As, Hg, Mo and Se were also below, but there were few values determined, making this finding unrepresentative. However, the high number of cases in which these elements were below the limit of detection LOD of the analytical method suggests low concentrations, probably far from reaching the maximum allowable concentrations.
Although they are not legally restrictive, Co, Fe and Mn were in relatively high concentrations in the sludges analyzed. The finding that the Cd did not exceed the limit established by CONAMA is the most positive result of this survey of sewage sludge used in experiments conducted in Brazil. This result indicates that the sludge has low potential for soil contamination with this heavy metal.
Soil contaminated with Cd is a constant concern throughout the world, because this metal can easily enter the food chain and impose risks to human health [ 10 ]. On the other hand, the sludge seems to be a considerable source of other heavy metals such as Cr and Pb. Considering that the sludge can introduce these metals in excess, soils amended with sewage sludge should be carefully evaluated for accumulation and availability of Cu, Cr, Mn, Pb and Zn. Additionally, the Cd should also be monitored because it is highly dangerous.
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The determination of the total concentration of heavy metals in soils has been performed with previous extraction of metals from samples using different mixtures of acids. In Brazil, the methods of extraction most widely used are presented in Table 3. Because of the difference in the chemical composition, some of them may produce quite contrasting results. For Cd, however, the results were not different between the two methods, suggesting that the difference in values between them may depend on the type of metal in question.
The contrasting results between these methods are due to extraction differential capacity of the reagent mixtures employed in each of them. On the other hand, hydrofluoric acid contained in the HClO 4 —HF mixture is able to dissolve silicates and extract metals eventually present in this mineral class.
Thus, it is necessary to standardize the method of extraction of heavy metals to assess the contamination of soil amended with sewage sludge. Cd, Cr, Pb and Zn extracted by two different methods from a Brazilian Oxisol amended with sewage sludge. Values are means of four sewage sludge rates.
Source: Adapted from Nogueira et al. Such methods are not designed to extract fully the metals from the soil, since they normally do not dissolve elements bound to silicates, which are not generally available in the environment, thus without implications for environmental contamination. They extract only elements which could become environmentally available, consequently harmful to living organisms. As the extraction is not fully, the terms pseudototal concentration and total recoverable concentration have recently been employed to designate the concentrations of metals extracted by these official methods, although the use of the term total concentration is still very common.
A summary of these experiments is presented below. The data in Tables 5 and 6 show that there was at least one rate above the annual rate allowed for Cd, Cu, Ni, Pb and Zn, but there was no rate higher than the maximum cumulative rate. Ni was detected only in the second year and at the two highest rates of sludge. In addition to the possibility of rapidly contaminating the soil with heavy metals, high rates of sewage sludge cause prolonged effect on contamination.
Martins et al. Four years later, increases were still linear and the concentrations were similar to the first year, indicating that these elements persist in the soil for a long time. In fact, high persistence is a characteristic of heavy metals added to soils [ 87 ]. On the other hand, low rates of sewage sludge applied to land do not increase excessively the total concentrations of heavy metals in soils.
Oliveira et al. In the study conducted by Silva et al. Thus, application of low rates of sludge with low concentrations of heavy metals in their composition seems to be a strategy to minimize the excessive accumulation of heavy metals in soils. However, this strategy may be insufficient for Cd.
Nogueira et al. Although the concentration of Cd has more than double to reach this limit, the fact that the limit is too low cause concern in relation to any increase in Cd concentration due to new applications of sewage sludge. In the work of these authors has been reported for the first time in Brazil the effect of sewage sludge on the accumulation of Se in soil. Se concentration increased from 0.
Rate and total concentration of heavy metals in a Brazilian Oxisol amended with sewage sludge and cultivated with sugarcane. The concentrations of heavy metals presented above refer only to the topsoil However, deeper and stratified sampling can give an idea of how these metals are distributed among soil layers. Merlino et al.
The concentrations of Cr and Pb increased only in the There was no effect on the concentration of Cd. For Cu and Cr, there was no evidence of leaching. Besides increasing or not be changed, the concentrations of heavy metals can also decrease in response to the application of sewage sludge, as shown in the work of Macedo et al. In general, the application of sewage sludge reduced the concentrations of heavy metals in the surface layers, with decreasing quite evident for Cd in layer of This reduction was not expected since metals were added to the soil.
Probably, the heavy metals added and part of those natives may have combined with components of sewage sludge forming highly stable compounds resistant to the attack of the extractant used. Availability refers to the amount of a chemical e. The availability to plants is called phytoavailability, which is a specific term for this class of organisms.miiquitreceq.tk
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In recent years, the availability has also been called bioavailability. Although bioavailability not yet has a clear and accepted definition [ 93 ], it has been used to characterize the availability of heavy metals in the environment [ 94 ]. However, the uncertainty about the meaning of bioavailability restricts the expansion of its use and thus contributes to the maintenance of the term availability.
The availability of a heavy metal is given by its available concentration. Quantification of the available concentration is performed by extracting the fraction of the metal that is sufficiently soluble to be absorbed by plants. The extraction of this fraction is performed using chemical extractants that try to simulate the potential for uptake of an element of the soil by the roots of plants. There are several extractants which can be used to extract heavy metals from the soil. The extractants used to assess the availability of heavy metals added to Brazilian soils by the application of sewage sludge to land are usually the same ones used in the evaluation of cationic micronutrients, especially Mehlich 1 and DTPA as the most common.
Other extractants, as 0. The concentration of a metal in the soil can only be considered available if it is closely and positively correlated with the concentration of the metal in plant tissues. In terms of assessing the contamination, the available concentration can be correlated to the concentration of the metal in raw or processed agricultural product. When there is no correlation or the correlation is poor, it is more appropriate to use the term extractable concentration. Knowledge of available concentrations is essential for assessing the environmental impact of sewage sludge application to land, since such concentrations represent the amount of heavy metals that could be leached to reach groundwater or could be absorbed by plants and transmitted to the food chain levels until reaching the man.
Assessment of the availability of heavy metals in soils amended with sewage sludge was carried out in some field experiments conducted in Brazil. The main results are summarized below. Oliveira and Mattiazzo [ 68 ] found different effects of sewage sludge rates Table 5 on the availability of some heavy metals evaluated. The sludge was applied for two consecutive years to an Oxisol cultivated with sugarcane, and concentrations of available Cd, Cr, Cu, Ni, Pb and Zn by extractants 0.
Increase in rates of sewage sludge consistently increased concentrations of Cu and Zn extracted with the three extractants in both evaluations. There was increase in concentration of available Ni only at the second assessment and for sewage sludge treatments. However, the authors did not report any negative consequence of these high concentrations to the crop.
However, when included only the sludge treatments, there were few significant correlations, indicating generally low efficiency of extractants to assess the availability of Cu and Zn in soil amended with sewage sludge. The exceptions were significant correlations between 0.
Source: Adapted from Abreu et al. In another study with sugarcane, Nogueira et al. Nevertheless, there was no toxicity in the plant.
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The extractant used was efficient only for Cd and Zn, since their available concentrations in the soil were highly correlated with the concentrations in leaf with top visible dewlap, stalk and juice. For concentrations of As, Cr and Se, there was no effect of sewage sludge application. All metals intentionally evaluated in this study were detected, probably by the use of inductively coupled plasma mass spectrometry ICP-MS , which had LODs very low. The availability of heavy metals in soils amended with sewage sludge has also been evaluated for maize crop.
However, the effect of liming on extractability of these metals was different between the two extractants. In the case of Mehlich 3, concentrations of extractable Cu and Zn increased with liming, which was unexpected, because it is well known the fact that the increase in soil pH due to liming generally decreases the availability of cationic micronutrients.