Metals bioavailability in surface sediments off Nile delta, Egypt: Application of acid leachable metals and sequential extraction techniques.

Surface sediments from Nile Delta coast were analyzed for texture, CaCO3, organic matter, fractionation, and acid leachable metals (Cr, Fe, Mn, Ni, Pb, and Zn). The distribution pattern of acid leachable heavy metals in the sediment follows the sequence: Fe>Mn>Pb>Zn>Ni>Cr. All the acid leachable metals didn't exceed the sediment quality guidelines values (effects range low (ERL) and effects range medium (ERM)) and therefore doesn't represent a danger to marine organisms. The correlation of acid leachable Fe, Ni, and Mn indicates a similarity in the association of metals of similar origin. The negative correlation of sand with acid leachable Cr, Fe, Ni, Pb, and Zn indicates that these elements can be easily released by ion exchange processes due to the electrostatic interaction of trace metals as they are weakly bound and is bioavailable to the liquid phase. The acid leachable Cr, Pb, and Zn indicate their association with the CaCO3, while acid leachable Fe, Mn, and Ni are hardly combined with carbonates. All the contents of acid leachable metals are negatively correlated or uncorrelated with OM, which indicates that the studied heavy metals are hardly combined with OM. The results of the partitioning study showed that the residual form was the dominant fraction of the Cr, Fe, and Ni among most of the studied locations. Among the non-lithogenic fractions, the Fe-Mn oxy-hydroxide is the main scavenger for all metals. In terms of risk assessment code (RAC) value, a decrease order in environmental risk by heavy metals was Pb>Mn>Zn>Ni>Cr>Fe. Although the results of the two techniques were not consistent with each other in terms of predicting the metals bioavailability, a combination of total metal concentrations, acid leachable metals, and sequential extraction analysis is necessary to acquire the comprehensive information on the baseline, anthropogenic input, and bioavailability of heavy metals.