Lead removal from aqueous solutions by a Tunisian smectitic clay
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 SiO− and AlO− 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:
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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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