The application of sorptive flotation for the removal of metal ions

There is a stringent need for the control of toxic metals produced during various production processes and released into the aqueous environment in order to avoid damage relevant to soil or groundwater contamination. Among the available treatment technologies, sorption has been considered of particular importance. In the present study, a synthetic ultrafine zeolite was initially applied as sorbent (in suspension) for the removal of copper and zinc cations, and also chromium(VI) oxyanions — the latter less effectively. The removal of cations by zeolite particles depends mainly on the respective pH value, influencing the aquatic speciation during the process, as well as the zeta-potential values (surface charge) of the system. Flotation was subsequently applied for the efficient (more than 95%) solid/liquid separation of metal-laden sorbents. A suitable surface-active agent (acting as collector) should be added such as sodium dodecyl sulfate, dodecylamine or hexadecyl-trimethyl-ammonium bromide, depending on the specific experimental conditions in order to render hydrophobic and hence, to increase the floatability of solids. As a result, a foam concentrate is produced consisting of the metal-loaded zeolite particles and of purified water as underflow, which could be more easily subjected to further treatment (for instance, by membrane filtration), if necessary. Both the aforementioned technological stages, sorption and flotation, can be operated in the same unit, and the integrated innovative process was termed “sorptive flotation”.

It has been established beyond any doubt that dissolved metals, escaping into the environment from anthropogenic activities, pose a serious health hazard. Due to solubility and mobility properties, heavy metals can cause significant damage to the environment by contaminating the soil or even passing to groundwater resources. A commercial zeolite (type 4A) was initially applied as a sorbent for the removal of copper and zinc cations by ion exchange. The efficiency of this sorbent for the removal of cations depended mainly upon the pH value of solution. The impact of chemical speciation during the process of metals separation was stressed. Flotation was subsequently applied for the solid/liquid separation of metal-loaded sorbents. Both technological processes, sorption and flotation, can operate in the same treatment unit and the combination was termed “sorptive flotation”. As an example, for the removal of 50 mg L!1 zinc at pH 6, the addition of 2 g L!1 of zeolite was capable of separating almost 100% of the metal. The DAF of zinc/zeolite (2 g L!1) at pH 6 was subsequently performed by the addition of a convenient anionic collector (such as SDS), and the separation of both zeolite and toxic metal was on the order of 99% under optimized conditions, i.e., for SDS concentration of 40 mg L!1 (Fig. 9). When the experiments were carried out at pH 9, it was preferable to use a cationic surfactant such Fig. 9. Flotation of zeolite by a SDS collector at pH 6 (following zinc removal).