Abstract
Bioavailability of heavy metals at contaminated sites is largely controlled by the physicochemical properties of the environmental media such as dissolved organic matter, hydroxides and clay colloids, pH, soil cation exchange capacity and oxidation-reduction potential. The aim of this study was to investigate soil pH and heavy metal solubility effect by levels of humic and fulvic acids applied in soil samples with different levels of contamination by heavy metals. The soil samples used in this study were collected in a known metal-contaminated site. Humic acid (HA) and fulvic acid (FA) were purchased as a commercially available liquid material extracted from Leonardite. The experiment was carried out in a factorial scheme of 4 × (4 + 1), with four contaminated soil samples and four treatments, comprised of two levels of HA, two levels of FA and a control. The HA treatments increased the solubility of Cu, Zn, Ni, Cr, Cd, Pb, As and Ba from soils, while FA treatments decreased, thus raising or not their availability and mobility in soil. Humic acid concentration did not influence soil pH and FA decreased soil pH until 0.7 units. The initial heavy metal concentration in soil affects the magnitude of the processes involving humic substances. The lower releases of heavy metals by FA verified the importance of the complexation properties of organic compounds. These results appear to encourage the use of HA for increased plant-availability of heavy metals in remediation projects and the use of FA for decreased plant-availability of heavy metals at contaminated sites with a risk of introducing metals into the food chain.
Highlights
Heavy metals are problematic because, unlike most organic contaminants, they are non-biodegradable and can accumulate in living tissues, posing a great threat to both human health and for ecological environment (Lesmana et al, 2009)
Bioavailability of heavy metals at contaminated sites is largely controlled by the physicochemical properties of the environmental media such as dissolved organic matter, hydroxides and clay colloids, pH, soil cation exchange capacity and oxidation-reduction potential
In the treatments with fulvic acid (FA), the soil pH decreased to 0.7 units, so this substance had an acidifying effect on the soil (Table 3)
Summary
Heavy metals are problematic because, unlike most organic contaminants, they are non-biodegradable and can accumulate in living tissues, posing a great threat to both human health and for ecological environment (Lesmana et al, 2009). Metals present in soils can be associated with several reactive components such as clay minerals, insoluble soil organic matter, oxides, or be taken up by plants and soil organisms mainly through the soil solution. The bioavailability of heavy metals at contaminated sites is largely controlled by the physicochemical properties of the environmental media such as dissolved organic matter, hydroxides and clay colloids, pH, soil cation exchange capacity and oxidation-reduction potential (Sparks, 2003; Violante et al, 2010; An et al, 2015). The functional groups identified in humic acids (HA) are identified in fulvic acids (FA), but FA contain considerably more groups of an acidic nature, -COOH (Jackson et al, 1978; Pandey et al, 2000)
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