Adsorption des métaux lourds (Cu, Zn, Cd et Pb) par les sédiments superficiels d'un cours d'eau: rôle du pH, de la température et de la composition du sédiment

Une étude expérimentale concernant l'adsorption des métaux lourds Cu, Cd, Zn et Pb par des sédiments d'un cours d'eau pollué par des rejets industriels a été entreprise pour mettre en évidence l'importance de certains paramètres expérimentaux, notamment le pH, la masse de sédiments et la température. Elle a permis également d'analyser la nature des liens qui participent à la fixation des cations métalliques sur les différentes fractions sédimentaires déterminées selon la méthode de TESSIER et al. (1979) et d'interpréter les capacités d'adsorption relativement variables suivant la nature du métal. Les résultats confirment le rôle particulier des fractions réductible (oxydes de fer et de manganèse) et organique (substances humiques en particulier), dont les propriétés respectives d'échange d'ions et de complexation ont été souvent vérifiées. La capacité d'adsorption de ce sédiment a notamment pu être interprétée en termes d'isothermes d'adsorption en exploitant les modèles de Langmuir et de Freundlich et leurs équations linéarisées.In an experimental study of the adsorption of Cu, Cd, Zn and Pb by surface sediment in a small stream polluted by the industrial drain of electro-refinery, we have demonstrated the irnportance of some such experimental parameters as pH, sediment concentration and temperature. These experiments were conducted in batch systems at constant temperature with continuous agitation, using a mixture of sediment and metals at an adjusted pH; the quantity of metals remaining in solution was determined by a polarographic method. Adsorption percentages for the concentratons of sediment (200 and 1000 mg/L) and metals (1 mg/L) reached the following maximum values: Pb (99-l00%o), Zn (80-90 %), Cd (75-85 %) and Cu (70-80%). These variations in metal soprtion are attributed to differences in binding energy between the metallic cation and the sediment sites, when all other parameters are fixed. Based on metal partitioning among the different sedimentary fractions, as determined according to the sequential extraction method of TESSIER et al. (1979), it was possible to attribute metal adsorption to complexation, coprecipitation and complexation reactions respectively with organic matter, carbonates and Fe-Mn oxides or alumino-silicates. Our results highlight the specific roles played by the reducible Fe-Mn oxides and by humic substances. Fulvic and humic acids, which are considered as the stable fraction of sedimentary organic matter, can form complexes and participate in the fixation of metals on the sediments (GODFRIN and BLADEL, 1990; WILLIAM and HANSON, 1979; FITCH et al., 1968; BIZRI et al., 1985). The stability of these complexes depends on the variety of reaction sites in these macromolecules, which in turn determines the degree of fixation of the cations (specific adsorption). Concerning the iron-manganese oxides and the alumino-silicate compounds, their surface sites are engaged insurface complex formation by a mechanism of proton exchange in which humic substances can be also involved (BELZILE et al., l989a; BELZILE and TESSIER, 1900; BELZILE et al., 1989b).Adsorption kinetics for Cu, Zn and Cd were relatively fast with more than 50 % of the metal adsorbed in a few hours, followed by a partially reversible stage over the next few days leading to an equilibrium state. Reversibilty of Pb binding was not signifrcant (attributed to the chemical precipitation of Pb3(PO4)2). An enhancement of adsorption with increasing pH between 5 and 8 was noted for Cu, Zn and Cd. Several factors may contribute to this increase in the quantity of absorbed metals:-the M+ and MOH+ species berome more competitivethan H+ fortte adsorption sites on the sediment;- the number of adsorption sites increases;- the change in conformation of the humic substances, from aggregated to stretched forms, may render the metal complexing sites more accessible;- the rate of Fe(III) and Mn(IV) formation is more significant at higher pH values;- at higher pH, the precipitation of oxides, hydroxides and hydroxycarbonates becomes important as well as the adsorption on the suspended phases;- the degree of oxide crystallization is influenced by the pH value and hence the adsorption capacity of reducible phases is also affected.A decrease in metal adsorption was also observed as the temperature increased between 10°C and 40°C. The adsorption of metals was described using FREUNDLICH and LANGMUIR equations in their linear form