Chemistry and Ecology Vol. 24, No. 2, April 2008, 109–117 Heavy metals availability (Fe, Mn, Zn, Cu and Cr) in Aden Gulf sediments under aerobic and anaerobic conditions Mohamed A. Okbah*, Samir M. Nasr and Shaif M. Kasem National Institute of Oceanography & Fisheries, Kayet Bay, Alexandria, Egypt (Received 02 November 2007; final version received 10 January 2008 ) The present study aims to analyse the chemical speciation of heavy metals in relation to aerobic and anaerobic conditions. Two sediment samples (from the Gulf ofAden,Yemen) were incubated under flooded conditions. In particular, the chemical forms of Fe, Mn, Zn, Cu and Cr under the experimental conditions were studied using a sequential chemical extraction method. The pH and Eh of the suspension were measured as critical parameters controlling the fate of the metals in the environment. The results showed that the metals concentration in the different forms varied with time incubation and affected by the variation of redox potential and pH value. Also, the changes in both redox potential (Eh) and pH values had evident effects on heavy metals transformation. It is obvious that the highest redox potential affected the amount of iron and manganese in the oxides form. When the redox potential decreased to −133 and −170 mV, it caused a significant transformation of the Fe-Mn oxide form to the water-soluble and exchangeable fractions. Under anaerobic conditions, the relative percentage of all five metals including the summation of four fractions (the water-soluble and exchangeable, carbonate, oxides and organic fractions) constituted 45–60% of the total amount of iron, 33–50% for manganese, 33–63% for Zn, 63–74% for Cu and 19–43% for Cr. Both zinc and copper among water-soluble and exchangeable fraction were high at the end of incubation period, this accompanied by a significant decrease in the content of the organically bound fraction. In general, the reducing conditions were more favorable for metal bound to water soluble and exchangeable fraction. Keywords: heavy metals; Aden Gulf; sediments; redox potential 1. Introduction The distribution of geochemical forms of a metal into the different sedimentary phases show the important role of these forms in the cycling of metals between the water column and the sediment reservoir. Trace metal fractionation in the sediments is widely used for pollution studies in marine environments [1,2]. The bioavailability, speciation and transportation of heavy metals in the sediment system are very important tools to assess sediment pollution problems and seek remedial measures. Ramirez et al. [3] showed that the sites characterised by the highest metal concentrations in bioavailability phase had the lowest biodiversity.The bioavailability and toxicity can be ascertained by studying the distribution of heavy metals between the different phases. The fractions introduced by man’s activity (including the adsorptive and exchangeable and binding *Corresponding author. Email: mohamed_okbah@yahoo.com ISSN 0275-7540 print/ISSN 1029-0370 online © 2008 Taylor & Francis DOI: 10.1080/02757540801919305 http://www.informaworld.com