Uranium and other heavy metal sorption from Moroccan phosphoric acid with argan nutshell sawdust

Abstract

Argan nutshells, available in Morocco as a low-cost by-product from Argan oil production, were used to produce a sawdust biosorbent for the extraction of U and other heavy metals (Cd, As, Zn, Cu, Ni and Cr) from merchant-grade phosphoric acid (PA). Packed column (D = 20 mm, L = 250 mm, flowrate 1 mL/min) lab-scale experiments were carried out with three different solutions: (1) a synthetic heavy metals solution, (2) pure PA at different concentrations (0.85%, 8.5% and 85%) in water, and (3) merchant-grade PA from an industrial fertilizer plant in Morocco that was diluted with water to analyze different P2O5 contents (5%, 30% and 54%) and a U concentration of up to 157 ppm. The sawdust successfully adsorbed 99% of the U from (1) the synthetic solution and (2) the pure PA independent of the acid concentration. It further managed to adsorb more than half (54%) of U, 75% of Cd, 74% of As, 84% of Zn, 86% of Cu, 83% of Cr of the merchant-grade PA at 54% P2O5 content and a flow-rate of 1 mL/min. The sawdust was characterized by SEM and FTIR to understand the adsorption of metal ions. Column studies were carried out to know the breakthrough point. At breakthrough, the effluent volume was found to be 50 mL and the maximum adsorption capacity for U was found to be 0.93 mg/g. Heavy metal recovery using ANS does not seem to be promising as a result of the low (>10%) desorption rates if citric- or oxalic acid is used as was done in this study. Heavy metal extraction without direct recovery using ANS is promising and should further be investigated though.