C. Faur-Brasquet

REMOVAL OF METAL IONS FROM AQUEOUS SOLUTION BY ADSORPTION ONTO ACTIVATED CARBON CLOTHS: ADSORPTION COMPETITION WITH ORGANIC MATTER

Abstract Activated carbon cloths are recent adsorbents whose adsorption properties are well known for monocomponent solutions of organics or metal ions. However, to treat wastewaters with these materials, their performance has to be determined in multicomponent solution. This work studies adsorption competition between metal ions (Cu 2+ , Pb 2+ ) and organic matter (benzoic acid). The first part investigates adsorption equilibrium of monocomponent metal ions solutions and shows the dependence of adsorption capacities on adsorbent porosity and metal ions chemical properties (molecular weight, ionic radius and electronegativity). The influence of pH is also demonstrated. The second part focuses on adsorption competition: (1) between both metal ions (a decrease of adsorption capacities is observed, whose value is related to adsorption kinetics of metal ions); (2) between metal ions and organic matter, in solution or adsorbed onto the activated carbon cloth (a strong influence of pH is shown: when benzoic acid is under benzoate form, in both cases adsorption is increased due to the formation of ligands between adsorbed benzoate ions and metals).

Modeling the adsorption of metal ions (Cu2+, Ni2+, Pb2+) onto ACCs using surface complexation models

Activated carbon cloths (ACCs), whose efficiency has been demonstrated for microorganics adsorption from water, were here studied in the removal of metal ions from aqueous solution. Two ACCs are investigated, they are characterized in terms of porosity parameters (BET specific surface area, percentage of microporosity) and chemical characteristics (acidic surface groups, acidity constants, point of zero charge). A first part consists in the experimental study of three metal ions removal (Cu2+, Ni2+ and Pb2+) in a batch reactor. Isotherms modeling by Freundlich and Brunauer–Emmett–Teller (BET) equations enables the following adsorption order: Cu2+>Ni2+>Pb2+ to be determined for adsorption capacities on a molar basis. It may be related to adsorbates characteristics in terms of electronegativity and ionic radius. The influence of adsorbent’s microporosity is also shown. Adsorption experiments carried out for pH values ranging from 2 to 10 demonstrate: (i) an adsorption occurring below the precipitation pH; (ii) the strong influence of pH, with a decrease of electrostatic repulsion due to the formation of less charged hydrolyzed species coupled with a decrease of activated carbon surface charge as pH increases. The second part focuses on the modeling of adsorption versus the pH experimental data by the diffuse layer model (DLM) using Fiteql software. The model is efficient to describe the system behavior in the pH range considered. Regarding complexation constants, they show the following affinity for ACC: Pb2+>Cu2+>Ni2+. They are related to initial concentrations used for the three metal ions.

Adsorption of dyes onto activated carbon cloths: approach of adsorption mechanisms and coupling of ACC with ultrafiltration to treat coloured wastewaters

Recent absorbents, activated carbon cloths (ACCs) are used to absorb dyes. First, adsorption is carried out in batch reactors; initial kinetic coefficients and adsorption capacities are determined, thanks to adsorption kinetics and isotherms. Twenty-two dyes are tested and two ACCs are used, one exclusively microporous, the other being partially mesoporous. With the view to understanding the absorption process of dyes onto ACCs, quantitative structure–activity relationships are developed using molecular connectivity indices as dyes descriptions. The statistical tool introduced is the multiple linear regression. Then, the ability of ACC to treat coloured wastewater is assessed by coupling adsorption with ultrafiltration. First, both processes are operated step after step. The membrane filtration step (3000 Da molecular weight cut-off) allows a great removal of turbidity (>98%), whereas adsorption onto ACC provides the decolourization of the stream with an adsorption capacity in continuous flow reactor of 180 mg g−1 of the acid orange 7. Secondly, ultrafiltration and adsorption onto ACC are operated continuously. When the breakthrough is reached, a total volume of 101 l is successfully discoloured, with a permeate flow rate higher than 20 l m−2 h−1.

Fixed-bed study for lanthanide (La, Eu, Yb) ions removal from aqueous solutions by immobilized Pseudomonas aeruginosa: experimental data and modelization

A fixed-bed study was carried out by using cells of Pseudomonas aeruginosa immobilized in polyacrylamide gel as a biosorbent for the removal of lanthanide (La, Eu, Yb) ions from aqueous solutions. The effects of superficial liquid velocity based on empty column, particle size, influent concentration and bed depth on the lanthanum breakthrough curves were investigated. Immobilized biomass effectively removed lanthanum from a 6 mM solution with a maximum adsorption capacity of 342 micromolg(-1) (+/-10%) corresponding closely to that observed in earlier batch studies with free bacterial cells. The Bohart and Adams sorption model was employed to determine characteristic parameters useful for process design. Results indicated that the immobilized cells of P. aeruginosa enable removal of lanthanum, europium and ytterbium ions from aqueous effluents with significant and similar maximum adsorption capacities. Experiments with a mixed cation solution showed that the sequence of preferential biosorption was Eu3+ > or = Yb3+ > La3+. Around 96+/-4% of the bound lanthanum was desorbed from the column and concentrated by eluting with a 0.1 M EDTA solution. The feasibility of regenerating and reusing the biomass through three adsorption/desorption cycles was suggested. Neural networks were used to model breakthrough curves performed in the dynamic process. The ability of this statistical tool to predict the breakthrough times was discussed.

Quantitative structure–activity relationships for the prediction of VOCs adsorption and desorption energies onto activated carbon

The aim of the study is to investigate quantitative relationships to predict the energetic interactions resulting from either adsorption or desorption of volatile organic compounds (VOCs) onto granular activated carbon. For that purpose, an experimental database was first built. Heats of adsorption and desorption were determined onto one activated carbon material for a 40 VOCs panel. The measurements were performed using differential scanning calorimetry coupled to thermogravimetry analysis. Adsorption energies were found to range between 40 and 80 kJ mol−1, whereas the desorption energies appear to be about 16% higher. Multiple linear regressions were afterwards tested in order to relate energies data with VOCs molecular properties. In a first approach, physical and chemical properties of the organic compounds were selected to investigate the best correlations. From the results obtained, the main influence of the ionization potential and of the polarisability were enlightened. In a similar way, connectivity molecular indexes were used. Some additional information were thus provided, which demonstrated the influence of the molecular shape, its branching and the steric hindrance.

Engineered Biofilms for Metal Ion Removal

Firstly, biofilm and biosorbents are defined. Mechanisms of interactions between metal ions and biofilm are discussed in terms of diffusion, mass transfer and sorption. In a second step, different processes using biofilm to remove heavy metal in aqueous solutions are presented. The continuously stirred processes are described for metal ion removal in wastewater by biofilm coating particles. In this case, the equilibrium data obtained with isotherm curves show a good adsorption of several metal ions onto biofilm. Examples of adsorption capacities for a large number of microorganisms and heavy metal ions are presented. The fixed bed reactors packed with grains coated with a biofilm are efficient to get a sorption (adsorption or ion exchange) of cations. The pressure drop is calculated with classical equations. Some values such as adsorption capacities and breakthrough times are got from the breakthrough curves. Several models (Adams-Bohart, mass transfer, and homogeneous surface diffusion models) are applied to get design data. A new approach using neural network to model breakthrough curves is proposed and discussed.

Coupling ultrafiltration with an activated carbon cloth for the treatment of highly coloured wastewaters : A techno‐economic study

Abstract This work investigates the coupling of a membrane technique, ultrafiltration, with a recent adsorbent, activated carbon cloth, for the treatment of industrial highly coloured wastewaters. A first experimental part shows the high treatment ability of this process for foutain‐pen inks effluents arising from the rinsing of vats in which inks were produced. Whereas ultrafiltration enables more than 97% of colour removal, COD and DOC are not completely retained and a residual value of 1,700 mg l−1 of DOC is obtained in the permeate. The second step of the process, activated carbon cloth, allows residual organic matter to be removed and a complete discolouring of the permeate. Adsorption capacities of COD and DOC are high, equal to 500 and 250 mg g−1 respectively. Furthermore, this adsorbent induces a complete removal of glycol compounds (acting as antifreeze) which were not retained by a nanofiltration technique. A second part is an evaluation of the economic feasibility of such an integrated process. Only direct costs are considered at this phase of the study, and are divided into fixed costs (equipment, depreciation, maintenance), variable costs (electricity and chemicals consumption) and labour costs. The technical‐economic study is carried out for two configurations : a low capacity unit (the UF membrane area is 2.4 m2) and an industrial capacity unit (with a 100 m2 UF membrane). Costs per treated m3 are respectively 111 and 32 euros, with costs partitioning which are dependent on the unit capacity.

Modelling of the flow behavior of activated carbon cloths using a neural network approach

This work investigates the hydrodynamic and aerodynamic behaviors of recent adsorbents, activated carbon cloths (ACC). A first part presents their characteristics, a particular attention being given to the properties related to their woven structure. The influence of these characteristics on air and water pressure drops through ACC is shown by experimental measurements. It is established that a classic model set up for particular media, the Ergun model, does not enable a satisfying modelling of experimental data. An artificial neural network (ANN) is then used in order to include as explicative factors the cloths properties. The optimization of the ANN architecture is carried out, in terms of selection of the input neurons and number of hidden neurons. The generalization ability of the ANN is evaluated using a test dataset distinct from the training set. The influence of specific characteristics of cloths on their flow behavior is confirmed by an analysis of inputs sensibility, and the determination of their predictive influence.

Treatment of Complex Aqueous Solutions by the Coupling of Ultrafiltration and Adsorption onto Activated Carbon Cloth

The aim of this work was to estimate the efficiency of coupling the processes of ultrafiltration and adsorption onto an activated carbon cloth, in order to treat complex aqueous solutions containing both organic micropollutants and macromolecules. First, the effectiveness of each treatment process was studied separately in batch reactors for microorganics, like phenol or atrazine, and macromolecules like humic substances. The activated carbon cloth displayed higher adsorption capacities for microorganics than granular activated carbon, from 45 mg g−1 for phenol up to 370 mg g−1 for atrazine, whereas they were not efficient at adsorbing humic substances. Conversely, as many as 80% of these macromolecules were removed by an ultrafiltration membrane with a 10 000 D molecular weight cut-off. Secondly, the coupling was tested directly using a surface water with an initial total organic carbon of 20 mg l−1 and around 30 mg l−1 of suspended solids. At first, batch experiments showed that the ultrafiltration step increased the activated carbon cloth performance, and the comparison of several activated carbon cloths of different properties showed that the total organic carbon removal was dependent on the adsorbent pore-size distribution. Then, continuous flow reactor experiments were carried out using the surface water loaded with atrazine. A coagulation-flocculation step was tested as a pre-treatment. The specific adsorption of micropollutants onto the activated carbon cloth was displayed: the pre-treatment improved the permeate quality (lower values of total organic carbon), whereas it had a negligible impact on atrazine adsorption.

Adsorption of Inorganic Species from Aqueous Solutions

Water and wastewater can be considered as complex mixtures of suspended solids, colloids, and dissolved organic or inorganic pollutants due to natural discharges or human activities. The contaminant levels are quite low in drinking water sources compared to pollutant concentrations found in industrial wastewater. However, to obtain clean water, several physicochemical or biological processes are available and commonly carried out, such as sedimentation, coagulation, flocculation, filtration, adsorption, oxidation, and free or fixed microorganisms [1]. To control and limit the impact of inorganic species on human health and the environment, treatment processes have to be defined and proposed. The methods for the removal of cations or anions from water are precipitation, membrane processes (nanofiltration or reverse osmosis), oxidation, biotreatments, ion exchange, and adsorption [2]. Activated carbon in the form of powder, grains and, more recently, fibers (cloth or felt) [3] is a universal adsorbent and, in particular, some interactions occur with inorganic species present in water.