Adsorption and uptake of cadmium by Azolla pinnata: kinetics of inhibition by cations.

A. pinnata showed a remarkable ability of taking up Cd from the external medium. Of the total Cd taken up by the test plant, surface adsorption was about 90%. Cd adsorption did not occur at a constant rate, however, an equilibrium was reached in 2 h. The uptake of Cd occurred at a constant rate. Test cations, including heavy metals (Ca, Mg, K, Na, Ni, Fe, Cu and Zn), inhibited adsorption as well as uptake of Cd. Cd adsorption was competitively inhibited by Cu, Fe and Zn, whereas Ca, Mg, K, Na and Ni caused non-competitive inhibition. Similarly, Cd uptake was competitively (Ca, Mg, Fe) or non-competitively (Na, K, Ni, Zn) inhibited by cations. Inhibition of Cd uptake by Cu was not wholly competitive.