A comparison of organic wastes as bioadsorbents of heavy metal cations in aqueous solution and their capacity for desorption and regeneration

The adsorption capacity of seven organic wastes/by-products (slash pine, red gum and western cypress bark, composted green waste, prawn exoskeletons, spent brewery yeast and mill mud from a sugar mill) for transition metals were determined at two metal concentrations (10 and 100 mg L⁻¹) and three equilibrium pH values (4.0, 6.0 and 8.0) in batch adsorption experiments. All tested materials indicate a positive affinity to adsorb metal cations from aqueous solution and spent yeast was the least effective. Adsorption generally increased with increasing pH and the order of selectivity of metals was: Cr³⁺ > Cu²⁺ > Pb²⁺ > Zn²⁺ ≥ Cd²⁺. For pine bark, compost, spent yeast and prawn shell, quantities of previously adsorbed Pb and Cd desorbed in 0.01 M NaNO₃ electrolyte were negligible. However, 0.01 M HNO₃, and more particularly 0.10 and 0.50 M HNO₃ were effective at removing both adsorbed Pb and Cd. Using 0.10 M HNO₃ as the regenerating agent, pine bark and compost maintained their Pb and Cd adsorption capacity over eight successive adsorption/regeneration cycles. For mill mud and prawn shell, there was a pronounced decrease in adsorption capacity after only one regeneration cycle. A subsidiary experiment confirmed that acid pre-treatment of the latter two materials appreciably reduced their Pb and Cd adsorption capacity. This was ascribed to the metal adsorption capacity of prawn shell and mill mud being partially attributable to their significant CaCO₃ content and acid treatment induces dissolution of the CaCO₃. It was shown that in relation to both adsorption capacity and desorption/regeneration capability, composted green waste showed the greatest potential.