Biosorption of aquatic cadmium(II) by unmodified rice straw.

Cadmium is the most common toxic metal threatening safe rice supply. Rice straw has the potential to remove Cd from large-scale effluent contaminated by heavy metals since it exhibited a short biosorption equilibrium time of 5 min, high biosorption capacity (13.9 mg g(-1)) and high removal efficiency at a pH range of 2.0-6.0. The main Cd biosorption mechanism was Cd(2+) ion exchange with K(+), Na(+), Mg(2+) and Ca(2+), together with chelation with functional groups such as C=C, CO, OH and carboxylic acids. When 0.5% (w/v) rice straw was exposed to 50 mg mL(-1) CdSO(4) solution with shaking at 150 r min(-1) for 3h, about 80% of the aquatic Cd was absorbed and the Cd content in rice straw reached 8-10 mg g(-1), suggesting that the metal-enriched rice straw could become high quality bio-ore by virtue of the industrial mining grade of its metal content and easy metal recovery.