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Traces of arsenic can be selectively removed from drinking water by adsorption processes using natural manganese oxides. Monocomponent adsorption of phosphate and arsenate has been studied by means of column experiments with two natural manganese oxides. A simple technique was proposed to deduce the concentration variations at the column outlet from the conductivity and pH measurements. These macroscopic-scale data enabled us to obtain phenomenological information on the type of surface reactions involved. Two behaviors were encountered with the oxides:  (i) only surface complexation was involved, the developed technique could be applied, and adsorption isotherms could be rapidly and accurately measured by a series of column experiments with very few analyses; or (ii) surface complexation was coupled with anion exchange. The latter behavior was far more complex, and the direct deduction of arsenate breakthrough from the conductivity and pH signals was no longer possible.

Adsorption processes are extensively used for the selective removal of trace elements from drinking water. They must be designed not only with equilibrium data but also with masstransfer data. Indeed, this paper investigates the transport of arsenate in natural manganese oxide columns. Column experiments were run at different conditions of particle sizes and flow rates, including an interrupted-flow experiment. The breakthrough curve analysis showed that transport was affected by nonlinear adsorption and intraparticle diffusion. Nonconventional features were observed, such as the variation of the total adsorption capacity with the flow rate and with the particle size. They were attributed to the complex porous structure of the grains. Results were interpreted by means of a transport model including Langmuir adsorption and mass transfer, with a single adjustable parameter: the effective diffusivity of arsenate in the grain. This parameter included both adsorption and diffusion. In a second step, diffusivities including intraparticle diffusion alone were calculated. Values between 0.6 and 7.0 × 10 -11 m 2 s -1 were found; they were close to published data of the pore diffusivity of arsenate in activated alumina grains. Moreover, the model, despite its simplicity, succeeded in predicting the breakthrough points of arsenate at different flow rates with a single value of the effective diffusivity. This modeling approach is of great importance for the design of adsorption processes for arsenate removal from drinking water.

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The efficiency of arsenic removal from drinking water in adsorption processes using natural oxides may be influenced by the presence of other adsorbable anions. The present paper focuses on the study of arsenate adsorption by a natural manganese oxide. The objective is to determine which of the anions usually present in drinking water may be adsorbed: hydrogen carbonate, sulfate, chloride, nitrate, phosphate and arsenate. A kinetic batch experiment was conducted with a natural drinking water, leading to a first qualitative selection: nitrate and chloride have little interaction with the adsorbent, sulfate and hydrogen carbonate are adsorbed while phosphate and arsenate are strongly adsorbed. Then column experiments were run with aqueous solutions containing either chloride, sulfate, etc. The previous trends were confirmed and the equilibrium isotherms of the adsorbable anions were built by integration of the breakthrough curves. The isotherms fitted with a Langmuir model showed that the capacitieswere low (a few μmol.g-1). The affinity order was determined from the isotherm initial slopes: arsenate ≫ phosphate > hydrogen carbonate ≌ sulfate. Given the strong affinity of the adsorbent for arsenate and the low arsenate concentration in drinking water, the process selectivity for As traces from drinking water is ensured.

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