Removal of aqueous lead ions by hydroxyapatites: Equilibria and kinetic processes

Issu de : WasteEng05 Conference 1st international conference on engineering for waste treatment, Albi, FRANCE, 17-19 May 2005International audienceThe capacity of hydroxyapatite (HAp) to remove lead from aqueous solution was investigated under different conditions, namely initial metal ion concentration and reaction time. The sorption of lead from solutions containing initial concentrations from 0 to 8000 mg/L was studied for three different HAp powders. Soluble Pb and Ca monitoring during the experiment allows characterizing the mechanism of lead uptake. Dissolution of calcium is followed by the formation of a solid solution, PbxCa10−x(PO4)6(OH)2, with a Ca/P ratio decreasing continuously. Langmuir–Freundlich classical adsorption isotherms modeled adsorption data. The adsorption capacities calculated from this equation vary from 330 to 450 mg Pb/g HAp for the different solids. Modeling of the sorption process allows to determine theoretical saturation times and residual lead concentrations at equilibrium

Vaporisation et/ou stabilisation de métaux lourds au cours du traitement thermique de déchet : cas des déchets ménagers solides et des boues de dragage

International audienceThe fate of heavy metals is one of the main problems that operators of solid waste incineration plants have to deal with to reduce the overall environmental impact of the plant and of the ultimate residue. The question is : should the process conditions avoid toxic metal vaporisation, thus increasing their concentration in bottom ash ? Or, on the contrary, should they favour metal release in combustion gases, thus requiring the highest performance air pollution control (APC) systems and stabilising materials? In order to examine the industrial impact of these two possibilities, we are currently developing two complementary approaches.The first one is concerned with the fate of heavy metals in incineration of municipal wastes and sludge, and coal combustion. We characterised physico-chemically the ultimate residues, and we developed a complete thermodynamic and experimental study of the behaviour of heavy metals in waste incinerators and coal combustors. We developed a general method (inverse method) to identify the kinetics of vaporisation of heavy metals from the on-line analysis of the exhaust gas of solid waste incineration. The second approach deals with the transformation of heavy metals contained in fly ash and dredged materials into phosphate components, using orthophosphate as stabilising agent.The technique involves a chemical treatment followed by a thermal treatment (sintering), that reduce the leachability of heavy metals. Better knowledge of thermal treatment of polluted wastes should lead to safer processes and residue reuse opportunities.La maîtrise des émissions de métaux lourds (ML) est un des problèmes majeurs qui se posent aux opérateurs des UIOM pour réduire l'impact environnemental des incinérateurs puis des déchets ultimes. Nos équipes développent deux approches complémentaires autour de la vaporisation des métaux et de leur stabilisation. Dans la première approche, après avoir caractérisé les résidus ultimes, nous avons développé une étude thermodynamique et expérimentale du comportement des ML dans les procédés de combustion et incinération. Nous avons développé une méthode générale (méthode inverse) permettant d'identifier la cinétique de vaporisation des métaux à partir de l'analyse en ligne des fumées d'incinérateur. La deuxième approche s'intéresse à la stabilisation par les orthophosphates des ML contenus dans les cendres volantes et dans les matériaux de dragage. La technique consiste en un traitement chimique suivi d'un frittage, qui limitent la vaporisation et réduisent la lixiviabilité des ML. Ces connaissances scientifiques doivent aider à une meilleure maîtrise des traitements thermiques de déchets pollués et une meilleure valorisation matière des résidus

Mechanisms of sintering of macroporous hydroxyapatite adsorbents

International audienceThe behaviour during calcination of two hydroxyapatites (HAP), one calcium deficient and the second with calcium excess, was investigated. A structural change in terms of the crystallization, combined with significant decreases of the specific surface area by sintering process, was observed. This process begins near 600degreesC and reaches a maximum near 900degreesC. The decay in specific surface area observed is described accurately by an empirical equation of the form dS/dt = -B'(T)S-h where S is the specific surface area, t is the time, b and B'(T) are respectively the reaction order and the constant rate which depends on the calcination temperature. These constants vary swiftly with changing temperature, suggesting that the mechanism of the sintering may change with the temperature. The results for the two macroporous HAPs are compared and discussed in terms of various possible mechanisms of materials transport involved in the sintering process

The grain model applied to solids with significant varying density and surface area

International audienc

Mechanisms of sintering of macroporous hydroxyapatite adsorbents

International audienceThe behaviour during calcination of two hydroxyapatites (HAP), one calcium deficient and the second with calcium excess, was investigated. A structural change in terms of the crystallization, combined with significant decreases of the specific surface area by sintering process, was observed. This process begins near 600degreesC and reaches a maximum near 900degreesC. The decay in specific surface area observed is described accurately by an empirical equation of the form dS/dt = -B'(T)S-h where S is the specific surface area, t is the time, b and B'(T) are respectively the reaction order and the constant rate which depends on the calcination temperature. These constants vary swiftly with changing temperature, suggesting that the mechanism of the sintering may change with the temperature. The results for the two macroporous HAPs are compared and discussed in terms of various possible mechanisms of materials transport involved in the sintering process

Adsorption du plomb sur des hydroxyapatites et frittage thermique (processus cinétiques et transfert thermique)

Des hydroxyapatites (HA) de porosité, de surface et de rapport molaire Ca/P différents ont été utilisées pour piéger du plomb. Le procédé utilisé a permis une stabilisation chimique et thermique du plomb. Le procédé utilisé a permis une stabilisation chimique et thermqieu du plomb. Ce procédé consiste en une réaction solide - liquide entre le plomb et l'HA suivie du frittage à haute température. Le processus de piégeage est une complexation en surface du plomb suivie d'un échange équipolaire entre Ca2+ et Pb2+ par un mécanisme de dissolution - précipitation. L'évolution des paramètres physiques lors de la calcination des particules entre 400 et 1200ʿC a été aussi étudiée. La modélisation paramétrique des données de surface spécifique a permis de montrer que les mécanismes de transport de matière à l'origine du frittage dépendent de la température. Pour les calcinations de l'HA dopée en plomb à des températures supérieurs ) 800ʿC, le taux de lixiviation du plomb est considérablement réduit. Un modèle thermique tenant compte des évolutions physiques a été développé.Hydroxylapatites (HA) with different porosities, specific surface areas and molar Ca/P ratios were used to remove lead from effluent. The process developed leads to chemical and thermal stabilization of lead. The steps of the process are liquid solid reaction between HA and lead (adsorption) followed by a solid gas process (sintering). Adsorption kinetics show that the lead removal mechanism can be described as a lead surface complexation followed by equimolar ion exchange between Ca2+ and Pb2+ (dissolution precipitation mechanism). The texture and structure of the Has during calcination between 400 and 1200C were investigated. The modeling of experiment datas of specific surface area held to show taht transport mechanism controlling the sintering are function of calcination temperature. For calcination of lead spiked HA particles over 800C, leaching of lead is significantly reduced. A head transfer model taking into account physical properties evolution was also developed.VILLEURBANNE-DOC'INSA LYON (692662301) / SudocALBI-ENSTIMAC (810042301) / SudocSudocFranceF

Réaction solide-liquide et frittage par calcination d'hydroxyapatites chargées en métaux lourds : cinétique et mécanisme de frittage

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Réaction solide-liquide et frittage par calcination d'hydroxyapatites chargées en métaux lourds : cinétique et mécanisme de frittage

National audienc

Removal of lead (Pb) by hydroxyapatite sorbent

International audienceThe capacity of hydroxyapatite (HA) for removal of lead from aqueous solution was investigated under different conditions, namely initial metal ion concentration and reaction time. The sorption of lead solution by synthetic hydroxyapatite was investigated with initial concentration in the solution containing Pb2+ varying from 1000 to 8000mgl(-1). Pb and Ca analysis in the solutions, X-ray diffractometry associated with SEM-WDS and XPS analysis characterized the mechanisms of lead uptake. It was shown that the dissolution of hydroxyapatite is followed by the formation of hydroxypyromorphite, a solid solution of Pb10-xCax(PO4)(6)(OH)2 formula, with Pb ions mostly occupying Ca(II) sites, and that the Ca/P molar ratio of this solid solution decreases continuously. Classical adsorption isotherms were used to describe adsorption data. The adsorption capacity, k(LF), of 320 mg Pb g(-1) HA and the adsorption intensity, n(LF), of 2.5 were calculated from Langmuir-Freundlich. This isotherm was at an initial pH 5 and 25degreesC for 16 mun mean particle size. This shows significant affinity of HA to Pb2+ according to literature data

Kinetics of lead (Pb) adsorption by hydroxyapatite sorbent

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