The link between buffer capacity and the ability of clays to retain Pb in interactions between Pb contaminant leachate and the clays, has been studied, using a natural clay from Quebec and laboratoryprepared clays (kaolinite, illite, and smectite). The retention of Pb in the clay suspensions as they received increasing amounts of acid was investigated, and the results examined in terms of the buffer capacity of the clays. As the clays receive increasing amounts of acid (e.g. akin to addition of acid leachate), high amounts of Pb can be retained if the buffer capacity prevents the pH from dropping to values where precipitation mechanisms are not operative. The high carbonate content in the illite gave it a higher buffer capacity than the smectite and natural clay, and permitted it to retain high amounts of Pb. High Pb uptake by the clays can affect the buffer capacity and the Pb retention capacity.
The problem of varying species of heavy metals existing as individual species or in combination with each other in simulated contaminant leachates (solutions) is studied in relation to selectivity and retention in four different kinds of soil. The influence of pH on the retention characteristics is also examined. The results obtained from the heavy metals single-species and composite-species adsorption tests indicate that in evaluating the attenuation potential of the soil substrate (i.e., contaminant attenuation), account needs to be given to the sensitivity of the soil – heavy metals contaminant system to the pH of the leachate and the competitive effect between the heavy metals present in the leachate. A change in the soil solution pH results in a corresponding change of the dominant retention mechanism of heavy metals in the soils. At high pH values, precipitation mechanisms (e.g., precipitating as hydroxides and (or) as carbonates) dominate. As pH decreases, precipitation becomes less important, and cation exchange becomes dominant. High amounts of heavy metals are retained in the soils if the soil buffer capacity remains high enough to resist a change in pH. The selectivity order of heavy-metal retention in soils depends on the pH of soil solution. At soil solution pH values above 4 or 5, when precipitation prevails, the selectivity order obtained is given as Pb > Cu > Zn ≥ Cd, as demonstrated for the illite, montmorillonite, and natural clay soils. At lower soil solution pH values, the selectivity order obtained is given as Pb > Cd > Zn > Cu, as can be seen in the case of kaolinite and montmorillonite. Key words : heavy metals, retention, selectivity, preferential adsorption, precipitation.
The selective sequential extraction method of analysis is used in this study to determine the distribution of heavy metals in some clay soils, to assist in evaluation of the role of the various clay soil solids (clay minerals, organics, amorphous materials, and carbonates) in heavy-metal retention capability as a function of acidity of the leachate. The chemical speciation model MINTEQ (metal speciation equilibrium model for surface and groundwater) is used to calculate the probable percent distribution of different species of heavy metals present in the leachate used. The selective sequential extraction method is based on the fact that different forms of heavy metals that are retained in soil (e.g., as oxides, hydroxides, carbonates, bound with organic matter) can be extracted selectively by using appropriate reagents. The results show that heavy metals can be retained in the four clay soils studied by several soil phases or mechanisms such as exchangeable, carbonate, hydroxide, and organic phases. The retention of heavy metals in any phase depends on soil solution pH, soil constituents, and the type of heavy metal. At high soil solution pH values, retention of heavy metal by precipitation mechanisms prevails, whereas at low soil solution pH, retention by cation exchange mechanisms becomes dominant. The results from the selective sequential extraction analysis support the conclusion of the significance of soil buffer capacity with regard to heavy-metal retention. The capacity of the soils to retain high amounts of heavy metals as they receive increasing amounts of acid (i.e., as the pH is reduced) depends directly on the soil initial pH values and on their buffer capacities.La mCthode d'analyse basCe sur l'extraction ~Cquentielle sClective est utilisCe dans cette Ctude pour determiner la distribution des metaux lourds dans des sols argileux, afin d'aider h 1'Cvaluation du rBle des divers solides des sols argileux (minCraux argileux, matibres organiques, matiriaux amorphes, et carbonates) dans la capacitC de rCtention de mCtaux lourds en fonction de I'aciditC du lixiviant. Le modble de definition des espbces chimiques MINTEQ (Metal Speciation Equilibrium Model for Surface and Groundwater) est utilisC pour calculer la distribution probable du pourcentage des differentes espbces de mCtaux lourds presents dans le lixiviant utilise. La mCthode d'extraction ~Cquentielle selective est basCe sur le fait que diffkrentes formes de mCtaux lourds qui sont retenus dans le sol (par exemple : les oxydes, hydroxydes, carbonates, liCs par des matibres organiques) peuvent Ctre extraites de fac;on sClective en utilisant des rCactifs appropriCs. Les rCsultats montrent que les mCtaux lourds peuvent Ctre retenus dans les quatre sols argileux CtudiCs par plusieurs phases du sol ou mCcanismes comme les phases Cchangeables, de carbonate, d'hydroxyde, et de matibre organique. La retention de mCtaux !ourds dans n'importe quelle phase depend du pH de la solution, des constituants de sol, et du type de metal lourd. A des valeurs ClevCes de ...
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