Michel Sardin

Selective removal of fluoride ions by a two-way ion-exchange cyclic process

Abstract The demand for the new development of specific separation techniques for the treatment of drinking water leads to re-examining the use of cyclic operations in ion exchange processes. Establishing stable limit fronts inside the column is the required condition to allow the use of two-way chromatography methods and insure the process efficiency. This paper presents numerical simulations and experimental validation at a laboratory pilot scale showing the development of a set of stationary composition profiles during a cyclic operation of a chromatographic ion-exchange cyclic process. It has been shown numerically that stable cycles are obtained in the case where the cut points are chosen following a criterion based on the breakthrough volumes. The properties displayed by means of numerical simulations were experimentally validated. Reconstituted waters were treated by the cyclic process in order to selectively eliminate fluoride ion. The process uses two anion-exchange columns pre-equilibrated with the hydrogen carbonate anion in excess in the feed solution. After some cycles, steady concentrations of the major species (chloride and sulphate ions) are measured: a stable limit cycle is reached and the condition to (i) eliminate fluoride anion, (ii) restore the matrix of the other anions is fulfilled.

Settling velocity model of activated sludge

Abstract The determination of an appropriate settling velocity model is indispensable to calculate secondary settlers of an activated sludge system by using the solid flux theory. Starting from the Carman-Kozeny equation, new settling models were developed by adding the slurry viscosity term. To correlate the settling velocities with the corresponding solid concentrations, Talmage and Fitchs method was utilized. The initial settling period was corrected by Hultman and Hultgrens method. Sedimentation experiments were carried out with a glass column (1 m height, 0.1 m diameter). To observe the variability of the settling characteristics of a flocculated slurry such as activated sludge caused by repetitions of sedimentation and homogenization, clay slurries flocculated with polyacrylamide were used. To verify the settling models, activated sludge slurries from a wastewater treatment plant were used. Six models with two parameters were compared. A newly deduced model v = k exp(-nx)/x fitted the experimental data best, and another new model v = k(1 - nx)4/x gave a better fit than v = k(1 - nx)3/x.

Transport of clay particles and radioelements in a salinity gradient: experiments and simulations

The problem of the transport of radioelements sorbed on colloids often occurs when assessing the safety of radioactive waste disposal. A methodology for studying the influence of particles on the transport of solute is proposed. A salinity gradient of NaCl is flowed through a chromatographic column filled with a mixture of sand and bentonite clay (5% by weight). As long as the NaCl concentration stays above a threshold value equal to 0.16 M, no particle migration out of the column is detected. A dramatic variation of the hydrodynamic properties of the column occurs just before the output of the clay particles: preferential pathways and dead water volumes are formed. The clay migration is first detected when NaCl concentration is <0.16 M, and it is then controlled by varying NaCl concentration. For a given length and composition of the porous medium, the amount of that migrates depends only on the NaCl concentration of the feed solution. An empirical function is proposed to account for this. This function is included in a transport model to account for kinetic mass transfer between immobile and mobile water zones, and adsorption of trace solute such as cesium. The adsorption is assumed to be governed by cation exchange with sodium. This built model is in good agreement with the experimental results.

Hydrodynamic behaviour of full scale trickling filters

In the trickling filter process, knowledge of the hydrodynamic behaviour is essential for accurate mechanistic modelling. Numerous measurements including the mean residence time, the free draining volume and the residence time distribution (RTD) have been performed in other studies. However, these results have often been obtained at pilot scale, and there is still a lack of data for the full scale process. In the present work, eight full scale plastic-packed or stone-packed trickling filters receiving urban wastewater are studied. The free draining volume and the mean residence time obtained are used together with the literature data to find correlations. Despite the variety of operating conditions of the filters, correlations are established for the stone-packed filters as well as for the vertical and random plastic-packed filters. The RTDs were assessed from tracer experiments using injections of lithium chloride. In order to explain the observed RTDs, an hydrodynamic model is developed assuming that the tracer is transported by an axially dispersed plug flow and by diffusion in and out of the biomass. Model fitting permits to identify the volume of biomass and circulating liquid in the filter.

Migration of strontium in clayey and calcareous sandy soil: Precipitation and ion exchange

Abstract An equilibrium model for describing strontium transport through a clayey and calcareous sandy soil is presented. The speciation model includes calco-carbonic equilibria, water dissociation, Ca/Sr ion exchange, and precipitation/dissolution of strontianite and calcite. This speciation mechanism is included in a flow model taking into account dispersion and advection. The transport model is validated by column experiments performed with sand from Gue, France. The model allows the prediction of strontium transport in a wide concentration range. Strontium retardation is mainly governed by strontianite precipitation and Ca/Sr exchange. The ability of the driving water to dissolve the precipitated strontianite can be responsible for an overconcentrated strontium peak, and determines its retardation.

Migration of 45Ca and 90Sr in a clayey and calcareous sand: Calculation of distribution coefficients by ion exchange theory and validation by column experiments

The transient transport of trace radiocalcium and radiostrontium in a clayey and calcareous sandy soil is mainly governed by: (1) cation exchange with the major cations on clay minerals; (2) calcocarbonic equilibria which control the concentration of calcium in the leaching solution; and (3) the initial composition of the leaching solution. If there are no changes in the concentration of the major cations and pH and ionic composition are stable, it is possible to calculate a distribution coefficient (Kd) as a function of the major cations. In a constant physicochemical environment, this coefficient can be introduced in a linear transport model. Chromatographic experiments have been performed in which pulse injections of radiostrontium or radiocalcium are made into sand columns fed with various leaching solutions. The results are compared: (1) with the simulations based on a complete speciation model; and (2) with predictions from a linear model including the previously calculated Kd. The shape and the location of radionuclide peaks are well simulated for a large range of ionic composition (10−3−10−1 M for a mixture of CaCl2 and NaCl). The distribution of clay minerals in the porous medium is of great importance. The local equilibrium assumption only holds when clay minerals particles are uniformly distributed on the sand grains. Kinetic effects occur when clays are in the form of aggregates. These effects are evaluated by means of a first-order transfer model and by comparison with tracer tests at different flow rates.

Nonlinear sorption of naphtalene and phenanthrene during saturated transport in natural porous media

Abstract The chromatographic method has been used to determine the sorption properties of two PAHs (naphtalene and phenanthrene) in a wide concentration range. Two sands were used, a natural organic sand and a sand impregnated with heptamethylnonane (HMN) were used. The experimental results showed nonlinear sorption. The weakness of PAH sorption on mineral material was confirmed. In the presence of organic matter, a strong sorption was observed. At low concentrations ( 1).

Sorption of selenium oxyanions on TiO2 (rutile) studied by batch or column experiments and spectroscopic methods

Selenium is a known toxic element released in the environment by anthropogenic activities. The present study is devoted to the aqueous sorption behaviour of selenium oxyanions (selenate and selenite) on a reference oxide surface, namely rutile TiO(2). Batch sorption kinetics and isotherms have been studied using different physico-chemical conditions of the solution (changes of pH and ionic strength). The sorption was favoured for both anions in acidic conditions, in agreement with a surface complexation mechanism and CD-MUSIC predictions. Spectroscopic investigations of the sorbed rutile powder were also consistent with such a mechanism. EXAFS spectra confirmed that for selenite anions, an inner-sphere mechanism was the most probable process observed. Dynamic sorption experiments using a column filled with rutile powder also substantiated that a part of the surface complexes follows the inner-sphere mechanism, but also evidenced that an outer-sphere mechanism cannot be excluded, especially for selenate anions.

Transport and capture of submicron particles in a natural sand : short column experiments and a linear model

Abstract The transport and capture of latex particles, considered as model colloids, in a column packed with a well-defined natural sand was studied. The shape of the breakthrough curves is analysed in terms of the effect of different operating conditions: flow rate, ionic strength, colloid particle size, and particle concentration. The results confirm the strong influence of flow rate and ionic strength on the retention of particles of a given diameter (0.21 μm). At a very low velocity and for an ionic strength superior to 10−3 M, the breakthrough curves of the particles have a characteristic shape with a time delay followed by a long tail. The retention as a function of colloid particle size is also pointed out. This leads to the establishment of a linear two-site adsorption model to simulate the observed phenomena. Two transfer times were calculated to characterise the capture phenomena associated with each site. One distinguishes fast trapping sites for which the transfer time is independent of flow rate, and the other, relatively slow sites for which the transfer time is inversely proportional to the flow. These sites are tentatively identified by observation of the deposit structure in the packed bed after an elution experiment.

Release of clay particles from an unconsolidated clay-sand core: experiments and modelling

This work identifies the main phenomena that control the peptisation and transport of clay particles in a sand core. Clay can be dispersed into small particles in an aqueous solution of low ionic strength. This property is used to generate clay particles with NaCl concentration varying from 0.5 M to 0.015 M. For this purpose, a chromatographic column is initially packed with a 5% clay-sand mixture. The monitored decrease of the NaCl concentration of the feed solution allows the control of transport of the particles without plugging the porous medium. In this condition it is shown that in a column of a given length, the amount of clay particles released into solution and available to transport, depend only on NaCl concentration. Some clay particles are available to migration when the NaCl concentration of the feed concentration is between 0.16 M and 0.05 M (first domain) or between 0.035 M and 0.019 M (second domain). An empirical function, Pd([NaCl]), accounts for this particle generation. Transport is mainly dependent on the hydrodynamic characteristics of the porous medium that vary during the elution, probably owing to the particle motion inside the column. A phenomenological modelling is derived from these results, coupling the particle generation term, Pd([NaCl]), with an adapted nonequilibrium transport solute model. Similarly to the solute, particles were attributed a characteristic time of mass transfer between mobile and immobile water zones. This is sufficient to take into account the kinetic limitations of particles transport. The values of the parameters are determined by independent experiments. Finally, breakthrough curves of clay particles are predicted when a column of a given length, is flushed by a salinity gradient of NaCl in various conditions.