The potential capability of Arundo donax stems, Brazil nutshells, sugarcane bagasse, and sawdust from a native wood species (Prosopis ruscifolia) to sequester trace metals from wastewater was comparatively examined using dilute aqueous solutions of Cd(II) or Ni(II) ions as models. Brazil nutshells showed the best effectiveness (>90%) for the uptake of both metals from solutions of 20 mg/L initial concentration for dosages larger than 0.2-0.4 mg/L, even superior to those obtained for a commercial activated carbon and/or red marine algae (Corallinales) used for comparison under identical conditions. Equilibrium isotherms of cadmium on the lignocellulosic and algae samples and of nickel on the nutshells were determined and properly described by the Langmuir model. The highest maximum sorption capacity of Cd(II) ions was obtained for the nutshells (X m ) 19.4 mg/g) among the lignocellulosic samples. The trend in the estimated X m values was found to be consistent with their contents of lignin and total surface acidic functional groups. Nevertheless, X m for the nutshells was lower than that for the algae (X m ) 29.7 mg/g). The nutshells were also found to be less effective at removing Ni(II) ions compared to Cd(II) ions.

Canes from Arundo donax, a rapid-growing plant, were converted to activated carbons by phosphoric acid activation under four different activation atmospheres, to develop carbons with substantial capability to adsorb Cd(II) and Ni(II) ions from dilute aqueous solutions. The carbons showed surface areas and total pore volumes of around 1100 m2/g and 1 cm3/g, respectively. The content of carbons' polar or acidic surface oxygen functional groups, with their development depending on the atmosphere used, influenced predominantly metal adsorption. Carbons derived under flowing air, possessing the largest total content of these groups (3.3 mequiv/g), showed the best adsorption effectiveness (>90%) for both ions, even superior to that determined for a commercial sample used as a reference. A pseudo-second-order rate model properly described adsorption kinetic data obtained for this sample. Equilibrium isotherms using the same carbon were also determined and modeled by the Langmuir isotherm. The influence of the solutions' pH on metal uptake, adsorption competitive effects between Cd(II) and Ni(II) ions, and desorption from the selected metal-loaded carbon for recovery purposes were additionally investigated.

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