Removal and concentration of uranium by seaweed biosorbent

Abstract

While only weakly radioactive, traces of natural uranium in surface waters represent a danger because of high toxicity. Mine drainage brings enough uranium into the environment as to cause concern. Acid washed, protonated, non-living biomass of Sargassum seaweed sequestered uranyl ions from solution extremely effectively. At pH 4.0, pH 3.2 and pH 2.6, the maximum uranium uptake values were 560 mg/g, 330 mg/g and 150 mg/g, respectively. The uranium biosorption mechanism was affected by the solution pH through the hydrolysis of uranyl ions in aqueous solution. At low pH value, the uranium is present in the solution mainly in the form of free UO22+ ions, and it was competing with protons for the binding sites on the biomass. The high uranium biosorption at higher pH was attributed to the ion exchange between the hydrolyzed uranyl ions, UO2OH+, (UO2)3(OH)5+, (UO2)2(OH)22+ and protons. Experimental sorption isotherms could be reliably fitted by conventional Langmuir and/or Freundlich models. A flow-through biosorption column demonstrated a high overall column uranium sorption capacity of 105 mgU/g at pH 2.5. About 36 bed volumes of 238 mg/L uranium solution was purified before the breakthrough (at 1mgU/L in the column effluent). The regeneration of the column by elution with 0.1 N HCl resulted in a very narrow peak of the elution curve reflecting a high efficiency in uranium recovery.