Effect of pH on the biosorption of nickel and other heavy metals by Pseudomonas fluorescens 4F39

Accumulation of heavy metals by Pseudomonas fluorescens 4F39 was rapid and pH-dependent. The affinity series for bacterial accumulation of metal catio ns decreased in the order Ni>>Hg>U>>As>Cu>Cd>Co>Cr>Pb. Metal cations were g rouped into those whose accumulation increased as the pH increased, with a maximum accumulation at the pH before precipitation (Ni, Cu, Pb, Cd, Co), a nd those whose maximum accumulation was not associated with precipitation ( Cr, As, U, Hg). High Ni2+ accumulation was studied. Electron microscopy ind icated that at pH 9, Ni2+ accumulated on the cell surface as needle and hex agon-like precipitates, whose crystalline structure was confirmed by electr on diffraction analysis and corresponded to two different orientations of t he nickel hydroxide crystals. Crystals on cells showed marked anisotropy by X-ray powder diffraction, which differentiated them from crystals observed in nickel solution at pH 10 and 11 and from commercial nickel hydroxide. N ickel biosorption by Pseudomonas fluorescens 4F39 was a microprecipitation consequence of an ion exchange.