Evaluating Scatchard and Differential Equilibrium Functions to study the binding properties of Cu(II) to the surface of mixed species of lyophilized Spirulina (Cyanobacteria)

The binding of Cu(II) to the surface of mixed species of Spirulina was studied at pH 6.0 by potentiometric titration monitored with a copper ion selective electrode. Three materials were studied: the total suspension of alga, the washing water that resulted from centrifugation of the total suspension, and the water-washed cells. The Scatchard method and the Differential Equilibrium Functions (DEF) were used for the treatment of the titration data. The Scatchard plots determined two classes of binding sites, with log K' values for stronger sites varying between 7.3 and 7.9. For the weaker sites the log K' values ranged between 3.5 and 3.9. The total concentration of binding sites were 1.6±0.1, 1.5±0.5 and 0.92±0.08 mmol g-1 for the total suspension, washing water, and washed cells, respectively. The DEF approach revealed a linear variation of log K DEF as a function of log theta (theta = degree of site occupation), with log K DEF decreasing from 9 to 4 as a consequence of increasing log theta from -2.5 to -0.25. The degree of site heterogeneity determined by the DEF approach was between 0.4 and 0.5 for the three materials studied.