Influence des ions metalliques liberes par un charbon actif sur l'adsorption de micropolluants organiques. Le role des ions calcium

Abstract

Recent studies concerning the removal, by activated carbon adsorption, of natural organics or micropollutants found in water supplies, have focused attention on the ionic composition of the treated water. Many workers have demonstrated that both the ionic composition and the salt concentration affect the ability of activated carbon to remove humic substances (Randtke and Jepsen, 1982; Weber et al., 1983; Lafrance and Mazet, 1985) or organic micropollutants (Lafrance et al., 1983; Souabi et al., 1986) from solution. In particular, calcium ions have been shown to significantly increase the adsorption of organics onto activated carbon (Lafrance et al., 1983; Weber et al., 1983). Although calcium salt residues have been found on the surface of commercial activated carbon (Lafrance and Mazet, 1986), no studies have considered the effects of this cation on the adsorption of organics in batch equilibrium experiments. The purpose of the present study is to quantify the effect of calcium ion concentration on the adsorptive capacity of a powdered activated carbon (PAC) for an anionic tensio-active agent: sodium lauryl sulfate. In one series of experiments, measurements of sorption kinetics of lauryl sulfate were made using a PAC treated in three ways: (1) untreated PAC; (2) PAC washed with distilled water before use; and (3) PAC washed with HCl 1N before use. Our results (Fig. 3) show that washing the PAC before conducting an adsorption study diminishes the effectiveness of the carbon in removing lauryl sulfate. These results may be related to the surface ionic content of the PAC, especially the calcium content: the untreated PAC may contain 50 μmol Ca 2+ g −1 of carbon (Fig. 4). The PAC washed with distilled water releases 42 μmol Ca 2+ g −1 of carbon, and the PAC washed with HCl 1N releases 75 μmol Ca 2+ g −1 of carbon (Table 1). In a second series of batch experiments, the effect of micromolar concentrations of Ca 2+ on the adsorptive capacities of the PAC (treated with HCl) for lauryl sulfate was studied. Decreasing the molar ratio [lauryl sulfate]/[Ca 2+] causes an increase in the removal of the lauryl sulfate (isotherms on Figs 5 and 7, and Langmuir equilibrium parameters on Fig. 8). The influence of the order of mixing, of the adsorbates lauryl sulfate and Ca 2+ with the PAC (Table 3), is shown by the sorption kinetic curves in Fig. 6. Results indicate that the residual concentration of lauryl sulfate in solution is not affected by the order of mixing adsorbates and PAC when the molar ratio [lauryl sulfate]/[Ca 2+] is 1.0. Such results support the assumption that calciums ions specifically interact with the carbon surface in order to increase the adsorption of lauryl sulfate. The electrostatic interactions between PAC and Ca 2+, which are probably responsible for an increase in adsorption of lauryl sulfate, are examined by zeta potential measurements of the PAC. Whereas micromolar concentrations of lauryl sulfate causes a marked increase in the negative surface charges of the PAC (Figs 9 and 10), calcium ion induces a rapid neutralization of this zeta potential (Figs 10 and 11). Such neutralization of the negative functional groups of both lauryl sulfate molecules and the PAC surface is strongly related to the increase of the maximum adsorption capacity (Γ ∞) of the PAC for lauryl sulfate (Fig. 11). Batch adsorption experiments using this PAC can lead to different measures of the efficiency of the carbon for the adsorption of some organics, depending on the treatment of the PAC before and use on the release of the ionic content of the PAC. The particular role of calcium ion in the adsorption of lauryl sulfate indicates, that the carbon efficiency in removing anionic compounds can be significantly improved by the addition of micromolar quantities of salt in solution.