2.5. Description of the biosorption process and sorption isotherms
The amount of ions adsorbed per unit of biomass (qe, in moles of metal ions per g of root powder) was evaluated using the following equation: qe= ((C0–Ce)/m) × V 1 where C0 is the initial metal ions concentration (mol L− ), Ce the
equilibrium metal ions concentration (molL−1), V is the volume
of aqueous phase (L) and m is the amount of the powder used (g). Sorption efficiency () is considered as a percentage according to Taty-Costodes et al. (2003):
= 100–100 × Ce/C0
The Langmuir model was applied to the sorption equilibrium in order to establish the maximal sorption capacity. The equilibrium removal of the Cd and Zn ions can be mathematically expressed by the adsorption isotherms:
qe= qm× (bCe/(1 + bCe))
or its linearized form:
Ce/qe= Ce/qm+ 1/bqm
qm and b are the Langmuir constants related to the maximum adsorption capacity and energy of adsorption, respectively. The values of b and qm can be determined from the linear plot of Ce/qe versus Ce.
2.6. Regeneration of heavy metal contaminated powder
Regeneration attempts were performed in order to remove adsorbed heavy metals from contaminated powder. This procedure is expected to allow re-utilization of the same powder for several subsequent cleaning treatments. For this purpose, powder obtained from control plants and used for biosorption was shaken for 10 min in an acidic solution (HCl, concentrations ranging from 0 to 0.5 M). After the filtration, as previously described, the heavy metal concentration was determined in the filtrate, after acid neutralization with an appropriate amount of NaOH. After such an acid treatment, powder was rinsed with deionized water, dried in an oven at 70◦C for 48 h and used for ion quantification, as stated above.
2.7. Statistical treatment of the data
The statistical analysis was performed using SAS software (SAS System for Windows, version 9.1). The normal distributions of the data were analysed using a Shapiro-Wilk test. When needed, the data were transformed to have a normal distribution. Percentage data were transformed to arcsine values before statistical analysis. Data were then subjected to an analysis of variance (ANOVA I and ANOVA II, depending on the considered experiment and tested parameter). The statistical significance of the results was analysed by the Student-Newman-Keuls test at the 5% level.