Distribuição de metais pesados em frações granulométricas distintas de sedimentos da Lagoa de Extremoz - RN

Abstract

The distribution of heavy metals in the sediment is important to evaluate the welfare of aquatic environments. This study discusses the distribution of Cd, Cr, Cu, Ni, Pb and Zn by grain size fraction and the composition of four sediment samples from Extremoz Lake. The sediment sample collection took place in March, April, August and September of 2014. The granulometric fractions studied were gravel (+2mm), very coarse to coarse sand (-2mm +0.6mm), medium sand (-0.6mm +0.3mm), fine to very fine sand (-0.3mm +0.063mm) and silt + clay (-0.063mm). The study of sample composition included observations with a stereo microscope; XRD analyzes on aliquots of light minerals, non-magnetic heavy minerals and magnetic heavy minerals of the sandy fraction of some samples and; measurement of the organic matter content in each fraction. The distribution of a metal by grain size fraction corresponds to the percentage of metal associated with a given fraction, taking into account the metal concentration in this fraction and its respective mass in relation to the concentrations and relative masses of all the fractions added together. Therefore, the study of the distribution considered the contents of metals obtained by extraction with aqua regia in each granulometric fraction, as well as the granulometric distribution of samples. Sequential extractions in aliquots of the silt + clay fraction of two samples were also performed in order to understand the metal distribution by chemical phase. The samples used in this study have previous results of ecotoxicological tests with the organism Hyalella azteca, in which the samples of March and April were classified as toxic and those of August and September as non-toxic. The distributions of Cr, Cu and Ni showed a negative correlation with the survival of H. azteca in the ecotoxicological tests. These metals occur associated with organic matter in the toxic samples and with Fe hydroxides in the non-toxic samples. Thus, the Fe hydroxides have the ability to immobilize metals when present, but in their absence, heavy metals tend to associate with organic matter. In the samples from Extremoz Lake, the granulometric distribution controls the distribution of metals and their availability by determining the distribution of the phases with higher capacity to accumulate metals: organic matter and Fe hydroxides.