Geochemical predictive models of manganese, zinc, nickel, copper and cadmium in Nile shelf sediments

Abstract The shelf sediments off the Nile delta are of heterogeneous origin and include sediments of marine, fluviomarine and fluvial origin. The heterogeneity of these sediments is reflected in their mineralogical and hence geochemical composition. The mineral association of the Nile sediments consists of quartz, feldspars, epidote, monoclinic pyroxenes, opaques, mica and hornblende, with minor occurrences of zircon, apatite, rutile, sphene and garnets. The clay minerals are predominantly represented by the smectite group. The partition of manganese, zinc, nickel, copper and cadmium were studied in an operationally defined selective extraction scheme to develop predictive geochemical models. The models were developed to predict the mode of occurrence and behaviour of these elements in the marine environment. The models showed that under the prevailing oxidising conditions carbonates, hydroxides, oxides and silicate solids control the partition and behaviour of metals. Although the terrestrial flux (biotite, chlorite and hornblende) contributes significant amounts of manganese, the metal principally exists as the amorphous oxide by precipitation from seawater. Zinc, nickel and copper are influenced by absorption and complexation of the dissolved forms by organic matter and organic detritus transported from land-based sources. Cadmium is only enriched in the carbonate fraction of the sediment principally as cadmium carbonate by solid solution reaction with calcite. Its origin is mainly anthropogenic. The deficiency of cadmium in the adsorbed form is the result of its great affinity to desorb from the sediment.