Lead removal from aqueous solutions by a Tunisian smectitic clay

https://doi.org/10.1016/j.jhazmat.2007.12.080Get rights and content

Abstract

The adsorption of Pb2+ ions onto Tunisian smectite-rich clay in aqueous solution was studied in a batch system. Four samples of clay (AYD, AYDh, AYDs, AYDc) were used. The raw AYD clay was sampled in the Coniacian-Early Campanian of Jebel Aïdoudi in El Hamma area (South of Tunisia). AYDh and AYDs corresponds to AYD activated by 2.5 mol/l hydrochloric acid and 2.5 mol/l sulphuric acid, respectively. AYDc corresponds to AYD calcined at different temperatures (100, 200, 300, 400, 500 and 600 °C). The raw AYD clay was characterized by X-ray diffraction, chemical analysis, infrared spectroscopy and coupled DTA–TGA. Specific surface area of all the clay samples was determined from nitrogen adsorption isotherms. Preliminary adsorption tests showed that sulphuric acid and hydrochloric acid activation of raw AYD clay enhanced its adsorption capacity for Pb2+ ions. However, the uptake of Pb2+ by AYDs was very high compared to that by AYDh. This fact was attributed to the greater solubility of clay minerals in sulphuric acid compared to hydrochloric acid. Thermic activation of AYD clay reduced the Pb2+ uptake as soon as calcination temperature reaches 200 °C. All these preliminary results were well correlated to the variation of the specific surface area of the clay samples.

The ability of AYDs sample to remove Pb2+ from aqueous solutions has been studied at different operating conditions: contact time, adsorbent amount, metal ion concentration and pH. Kinetic experiments showed that the sorption of lead ions on AYDs was very fast and the equilibrium was practically reached after only 20 min. The results revealed also that the adsorption of lead increases with an increase in the solution pH from 1 to 4.5 and then decreases, slightly between pH 4.5 and 6, and rapidly at pH 6.5 due to the precipitation of some Pb2+ ions. The equilibrium data were analysed using Langmuir isotherm model. The maximum adsorption capacity (Q0) increased from 25 to 25.44 mg/g with increasing temperature from 25 to 40 °C. Comparative study between sulphuric acid activated clay (AYDs) and powder activated carbon (PAC) for the adsorption of lead was also conducted. The results showed that sulphuric acid activated clay is more efficient than PAC.

Introduction

Lead ions have become one of the major environmental pollutants due to its presence in automobile fuel and subsequent emission into the atmosphere in the exhaust gases [1]. It enters the environment as a result of both natural process and anthropogenic activities [2]. Methods like ion exchange, solvent extraction, reverse osmosis, precipitation and adsorption are available for its removal from water and wastewater. Among all these methods, adsorption is shown to be economically favourable (compared with ion exchange) and technically easy (compared with precipitation or reverse osmosis) [3].

Due to its inherent physical properties, large surface area, microporous structure, high adsorption capacity and surface reactivity, activated carbon have been received recently a considerable attention for the removal of organic and inorganic pollutants from contaminated water [4]. The high cost of the activated carbon limits however its use as an adsorbent [5] and encourage its substitution by clays. These materials, owing to their high cationic exchange capacity and specific surface area and their good chemical and mechanical stability, often used to treat wastewater [6]. It is well known that smectitic clay have very high cation exchange capacities (90–120 meq/100 g) due to substitutions of Mg2+ and Fe2+ in place of Al3+ in the octahedral positions and, to a higher degree, to substitutions of Al3+ in place of Si4+ in the tetrahedral positions. Moreover, they possess higher elasticity and plasticity. These properties make them particularly suitable as low-cost natural sorbents for the treatment of industrial and processing waters and wastewaters and/or as barriers in landfills to avoid pollutant release. During acid activation, exchangeable cations are replaced by protons and a part of octahedral cations dissolve creating new acid sites in the structure. This makes the smectitic clay more porous and acidic. Moreover, activation of smectitic clay by acid treatment is effective in limiting possible decomposition of the crystalline structure and increasing the specific surface area. For this reason, clays modified in various ways, such as treatment by inorganic and organic compounds, acids and bases have higher adsorption capacity [7]. For example, montmorillonite, coated and intercalated by aluminium hydroxides exhibits much higher adsorption capacity for some heavy metal ions, than that of natural montmorillonite [8]. The bleach of vegetal and mineral oils by smectitic clay increases with inorganic acids treatment [9].

The smectitic clay can adsorb heavy metals via two different mechanisms: (1) cation exchange at the planar sites, resulting from the interactions between metal ions and negative permanent charge (outer-sphere complexes) and (2) formation of inner-sphere complexes through Sisingle bondO and Alsingle bondO groups at the clay particle edges [10]. Both mechanisms are pH dependent but the latter is particularly influenced by pH because in acidic conditions (pH < 4) most silanol and aluminol groups on edges are protonated. For this reason, it is necessary to improve the knowledge of the effect of pH on the sorption capacity of smectitic clay in solid–solution system.

The aim of this work is to study the removal of Pb2+ ions from aqueous solution by adsorption on smectite-rich clay (AYD) and on its acid (2.5 mol/l hydrochloric, 2.5 mol/l sulphuric) and thermic activated products. Comparative study between sulphuric activated clay and activated carbon for the adsorption of Pb2+ is also conducted.

Section snippets

Preparation of the adsorbents

The raw clay (AYD) used in the present study was collected from the meridional Atlas of Tunisia. It was sampled in Jebel Aïdoudi in El Hamma area. It is Coniacian – early Campanian in age (Fig. 1). AYD clay was kept in an oven at 70 °C.

For preparing acid activated clays (AYDh and AYDs), a suspension was made by mixing 10 g of AYD sample with 100 ml of 2.5 mol/l H2SO4 or 2.5 mol/l HCl at 60 °C for 0.5, 1, 2, 4 and 6 h. The suspension was then filtered off and the treated material washed several times

Characterization of AYD clays

The X-Ray diffraction analysis indicated that the raw AYD clay is mainly composed of smectite (74%) associated to kaolinite (9%), illite (3%), quartz (6%), calcite (6%), and feldspars (2%) (Table 1). The chemical analysis showed that the main constituents of AYD clay are silica (47.74%), alumina (18.59%), and iron oxides (7.39%) (Table 2). The loss of ignition (LOI) is 15.3% (Table 2). It is mainly attributed to the loss of H2O from clay minerals, especially smectite, and CO2 originated from

Conclusions

Four smectitic clay samples: untreated clay (AYD), hydrochloric activated clay (AYDh), sulphuric activated clay (AYDs) and thermic activated clay (AYDc) were used for the removal of Pb2+ ions from aqueous solutions, the following conclusions have been drawn from this investigation:

  • Raw smectitic clay, sampled in Jebel Aïdoudi in El Hamma area (meridional Atlas of Tunisia) has high surface charges resulting from the spread of isomorphous substitution in tetrahedral and octahedral sheets. This

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