ark flour from ponderosa pine (Pinus ponderosa) was consolidated into pellets using citric acid as cross-linking agent. The pellets were evaluated for removal of toxic heavy metals from synthetic aqueous solutions. When soaked in water, pellets did not leach tannins, and they showed high adsorption capacity for Cu(II), Zn(II), Cd(II), and Ni(II) under both equilibrium and dynamic adsorption conditions. The experimental data for Cd(II) and Zn(II) showed a better fit to the Langmuir than to the Freundlich isotherm. The Cu(II) data best fit the Freundlich isotherm, and the Ni(II) data fitted both Freundlich and Langmuir isotherms equally. According to the Freundlich constant KF, adsorption capacity of pelletized bark for the metal ions in aqueous solution, pH 5.1 ± 0.2, followed the order Cd(II) > Cu(II) > Zn(II) >> Ni(II); according to the Langmuir constant b, adsorption affinity followed the order Cd(II) >> Cu(II) ≈ Zn(II) >> Ni(II). Although data from dynamic column adsorption experiments did not show a good fit to the Thomas kinetic adsorption model, estimates of sorption affinity series of the metal ions on pelletized bark derived from this model were not consistent with the series derived from the Langmuir or Freundlich isotherms and followed the order Cu(II) > Zn(II) ≈ Cd(II) > Ni(II). According to the Thomas kinetic model, the theoretical maximum amounts of metal that can be sorbed on the pelletized bark in a column at influent concentration of ≈10 mg/L and flow rate = 5 mL/min were estimated to be 57, 53, 50, and 27 mg/g for copper, zinc, cadmium, and nickel, respectively. This study demonstrated the potential for converting low-cost bark residues to value-added sorbents using starting materials and chemicals derived from renewable resources. These sorbents can be applied in the removal of toxic heavy metals from waste streams with heavy metal ion concentrations of up to 100 mg/L in the case of Cu(II).
Pelletized ponderosa pine bark for adsorption of toxic heavy metals from water
