191 research outputs found

    Characterization and pH Dependent Leaching Behavior of Tunisian Phosphogypsum

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    The current study aims to characterize and as well as to investigate the leaching behavior of Tunisian Phosphogypsum (PG). The results of the physical characterization studies showed that, as worldwide Phosphogypsum, the Tunisian PG behaves like fine silty sand with important initial water content. Relevant attention was given to the leaching behavior of the sample which was subjected to two leaching tests according EN 12457-2 (2002)  at liquid to solid ratios (L/S) of 10 and 100 as well as pH stat leaching test according to the CEN/TS 14997 (2006). The progressive release of major elements as well as the metals indicates high mobility of the most analysis elements. Calcium, sulfate and phosphorus were the major elements having the highest leaching concentrations. Broadly, mobility of trace metals in PG was classified into three degrees: elements with high mobility were Sr, Zn, those with moderate mobility were As, Ba, Cd and Cr and those with low mobility were Cu, Ni, Pb, Se, V, Y and Zr. The highest concentrations of the most part of the metals were obtained from L/S 100. Based on the pH dependent leaching experiments, results show that the PG has a maximum susceptibility to leaching out metals when exposed to a strongly acidic condition (2-4). While, alkaline condition appears to be the most stable for the analyzed material. This paper offers a data base which is useful when later recycling actions are taken. Indeed, acidic conditions should be avoided in order to prevent metal leaching from PG.

    Evolution of iron speciation during hydration of C4 AF

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    International audienceIt is now well accepted and demonstrated that calcium silicate, calcium aluminate and calcium sulfo aluminate (ettringite, AFm) phases exhibit a good capability to fix metals and metalloids. Unfortunately the role of minor phases and especially calcium-ferric aluminate phase, shorthand C(4)AF is not well defined. In other systems like in soils or sediments iron phases play a key role in the fixation of pollutant. In cement sorption isotherms, indicated that various metals can be retained by the C(4)AF hydrated products. Therefore the capabilities of those phase to retain heavy metal should not be neglected. Previous investigations have shown that the minerals formed during the hydration of C(4)AF are similar to those formed from C3A (pure tri-calcium aluminate) under comparable conditions. Nevertheless no investigation was conducted at the molecular level and there is still a controversy whether Fe substitutes for Al in the hydrated minerals in whole or in part, or if it forms FeOOH clusters scattered throughout the matrix. In this context we have conducted XAS experiments using synchrotron radiation. It was found that the hydration of C(4)AF forms C(3)AH(6) (hydrogarnet) in which Fe randomly substitutes for Al as well as an amorphous FeOOH phase. Intermediate products like AFm (i.e., an ill organized lamellar phase) are also formed but rapidly evolve to C(3)AH(6); iron does not seem to be incorporated in the AFm structure
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