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

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Abstract

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.

Introduction

Environmental pollution due to heavy metals is caused by various industries, namely metal plating, mining, painting, or agricultural sources like fertilizers or fungicidal sprays [1]. The presence of the above metals in the environment is of major concern because of their toxicity and threat for human life and for the environment, especially when tolerance levels are exceeded [2]. In application of the “principle of precaution”, relayed at the Rio Conference in 1992, a decrease of water standards relating to metal pollution is provided for. For example, French reglementations for drinking water set the effluent quality standard for lead from 50 to 10 μg l−1 in 2003.

In this context, the search for new technologies to remove metals from wastewaters has become a major topic of research. Among the methods commonly used for this purpose, adsorption was shown to be economically favorable (compared with ion exchange, liquid extraction or electrodialysis) and technically easy (compared with precipitation or reverse osmosis) [3].

Activated carbon cloths (ACCs) belong to new technologies appeared for the last decade. Their interesting properties for microorganic pollutants removal from water were demonstrated by previous researches [4], [5], [6], [7]. On one hand, adsorption velocities were 2–20 times higher than those obtained with a granular activated carbon due to their high external surface area. On the other hand, adsorption capacities ranged between 150 and 400 mg g−1 for more than 60 microorganics. However, their adsorption performance for inorganics treatment has not yet been studied.

This study focuses on their ability to remove metal ions from water, and it is divided into three parts. The first part consists in a characterization of physico-chemical properties of activated carbons used in this work. The second part is an experimental study of three metal ions (Cu2+, Ni2+, Pb2+) adsorption by ACCs. Common models (Freundlich, Brunauer–Emmett–Teller (BET)) are applied to isotherm curves and the influence of pH is investigated. Then, the third part investigates the ability of a surface complexation model (SCM), the diffuse double layer model, to describe metals removal by ACC as a function of pH. This kind of models was originally proposed to describe the interactions of metal ions with natural oxides by explicitly incorporating solution speciation and reaction stoechiometry for the formation of surface complexes [8]. It was successfully applied by several authors to describe the metals sorption phenomena onto various media, namely metal hydroxides [9], [10], soils [11], peat [12], wheat bran [13] or activated carbon in the form of powder or granules [14], [15], [16]. However, it was never applied to the woven form of activated carbon.

Section snippets

Activated carbon cloths

Two ex-rayon ACCs, CS-1501 and RS-1301 (Actitex company, Levallois, France), were used in this study. In order to explain the adsorption mechanism of metal ions, and to enable a modeling of adsorption data versus pH using SCMs, physico-chemical properties of the adsorbents were determined.

Porosity properties were assessed by N2 adsorption at 77 K using a Coulter SA 3100 apparatus. Concerning chemical properties, Boehm’s titration method [17] was used to determine the acidic surface groups

Surface acidity of activated carbons

The amphoteric character of activated carbons was shown by previous researches on GAC [14], [15], [16] and on the ACC of this study [18], and it may be described by the following ionization reactions of surface sites SOH:SOH2+↔SOH+H+SOH↔SO+H+whose intrinsic surface acidity constants, taking into account an electrostatic correction term, are defined respectively byKa1={SOH}{H+}{SOH2+}exp0RTKa2=SOH+SOHexp0RTwhere R is the molar gas constant (8.314 J mol−1 K−1), F the Faraday

Characterisation of activated carbons surface properties

Porous characteristics assessed by N2 adsorption at 77 K and acidic surface groups determined by the Boehm method are given in Table 2. Both adsorbents have high specific surface areas. Whereas, CS-1501 is mainly microporous (more than 96% of micropores), RS-1301 contains about 32% of mesopores. These properties had been observed by scanning electron and atomic force microscopy in a previous study [26]. The total number of surface sites and the specific surface area allow the surface site

Conclusion

This study has shown the ability of ACC to remove metal ions from aqueous solutions. The influence on maximum adsorption capacities of adsorbent porosity and adsorbate characteristics (electronegativity, ionic radius) was pointed out. All the experiments were carried out at pH 5, below the precipitation pH. When adsorption was studied as a function of pH, short adsorption edges showed a strong dependence of adsorption on pH solution because of the decrease of electrostatic interactions due to

Acknowledgements

The authors are grateful to Region Pays de la Loire, France, for partial financial support of this work via a post-doc grant.

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