The results of Cr (III) adsorbed on activated carbons were quantified by mass balance. To test the system at equilibrium, the following parameters were used: adsorption capacity of the carbon (*qeql*) expressed in terms of metal amount adsorbed on the unitary sorbent mass (mmol/g), i.e. ([*Cr III*]*uptake*); and sorption efficiency of the system (*R%*) indicated from the percentage of removed metal ions relative to the initial amount, i.e. [*CrRem*], %. These

$$\boldsymbol{q}\_{eq} = \frac{\{\mathbf{C}\_{init} - \mathbf{C}\_{eq}\}}{m} \tag{6}$$

$$R \,\%= \frac{(\mathbf{C}\_{\text{init}} - \mathbf{C}\_{eq})}{\mathbf{C}\_{eq}} \,\mathbf{100} \tag{7}$$

where *Cinit* and *Ceql* are, respectively, the initial and equilibrium concentrations of metal ions in solution (mmol/l) and *m* is the carbon dosage (g/l).

The data for the uptake of Cr (III) at different temperatures has been processed in accordance with the linearised form of the Freundlich [8], Langmuir [9] and BET [10] isotherm equations.

For the Freundlich isotherm the log-log version was used [8]:

parameters have been calculated as indicated below [6, 7]:

$$
\log q\_{\rm eyl} = \log K\_{\rm F} + 1/n \text{ log } \mathbb{C}\_{\rm eyl} \tag{8}
$$

The Langmuir model linearization (a plot of *1/qeql* vs *1/Ceql* ) was expected to give a straight line with intercept of *1/qmax* [9]:

$$\frac{1}{q\_{\text{eq}}} = \frac{1}{\mathcal{K}\_{\text{l}} q\_{\text{max}}} \frac{1}{\mathcal{C}\_{\text{eq}}} + \frac{1}{q\_{\text{max}}} \tag{9}$$

The BET model linearization equation [10] was used:

$$\frac{\mathbf{C}\_{eq}}{(\mathbf{C}\_{init} - \mathbf{C}\_{eq})q\_{eq}} = \frac{\mathbf{K}\_{B\overline{t}\overline{t}} - \mathbf{1}}{\mathbf{K}\_{B\overline{t}\overline{t}}q\_{\max}} \frac{\mathbf{C}\_{eq}}{\mathbf{C}\_{init}} + \frac{\mathbf{1}}{\mathbf{K}\_{B\overline{t}\overline{t}}q\_{\max}}\tag{10}$$

For a successful determination of a BET model the limiting case of *KBET* >> 1 is required. In this case, a plot of ( ) *eql eql init eql eql init C C vs C Cq C* yields a straight line with positive slope and intercept from which the constant (*KBET*) and the monolayer sorption capacity (*qmax*) can be obtain.
