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Iron and trace metal geochemistry in mangrove soils

Abstract:

The widespread use of a large range of chemical substances is contributing to the considerable increase in the concentration of trace elements in soils and sediments. Furthermore, the accumulation of trace elements in estuarine systems is strongly influenced by the geochemical conditions of the soil, and therefore spatial or temporal changes in these conditions may affect the ability of soils to retain toxic metals. The objectives of this study were to establish the geochemical behaviour of trace elements related to Fe and S geochemistry in mangrove soils affected to different degrees and by different types of contamination. The study was carried out in 15 soil profiles from different mangrove systems in the State of São Paulo (Brazil). Eleven of the mangrove soils were affected by some type of human disturbance, whereas the other four were free from any human interference. Redox potential (Eh), pH, total organic carbon content (TOC), total S, total N, and total concentration and partitioning of trace elements were analysed. The pH values of the mangrove soils were generally close to neutral, and anoxic conditions predominated. Under these geochemical conditions, iron was mainly found in the pyrite fraction, and trace metals were significantly correlated with pyritic-Fe, indicating an important degree of trace metal pyritization. However, the degree of pyritization (DTMP) of the trace metals was very different. Incorporation of trace elements in the pyritic fraction followed the order Cu, Hg>> Ni> Pb,Zn,Cr. The concentration of Zn and Pb in pyritic fraction may have been underestimated since ZnS and PbS are soluble in acid media, and may have been extracted during the steps prior to pyrite extraction. We also highlight the need for further studies on the geochemical behavior of Hg and the importance of considering elemental Hg in order to obtain more reliable results. © 2011 Nova Science Publishers, Inc. All rights reserved.