1. Introduction

The prompt industrial development and ongoing urbanization have induced growing problems relevant to waste output. Wastes are the generator of hazardous substances which migrate unimpeded through the ecosystem posing severe jeopardy to living organisms. Heavy metals are deemed as a standout among the most toxic groups of inorganic pollutants. The principal origin sources of these heavy metals are usually from electroplating, plastic manufacturing, mining, fertilizers, etc. [1]. The excessive magnitude of these contaminants in industrial wastewaters will severely devastate the ecosystem and human health via accumulation and spreading in the environment and food chain [2]. Faced with the strict environmental enactment, the treatment of wastewater before being discharged into the environment becomes paramount topics worldwide. Diverse treatment processes, for instance, coagulation, precipitation, ion-exchange, reverse osmosis, and

electrolysis, are applied for the remediation of heavy metal ions from aqueous solutions [3–5]. Notwithstanding, by far, the majority of these procedures are restrained by both ecological and economic constraints. In contrast, adsorption has been qualified to be the most relevant and promising method, on account of its low cost, flexible operation, and reversibility. A wide variety of low-cost adsorbents, for example, agricultural by-products, industrial wastes, and natural mineral materials, were found to have good adsorption capacity [6]. However, these materials possess weak resistance to abrasive forces in column apply and leaching of few organics throughout retention process.

of the stability of the developed adsorbents. Therefore, great endeavors are required for the synthesis of hybrid silicate adsorbents which meet the listed

Bi-Functionalized Hybrid Materials as Novel Adsorbents for Heavy Metal Removal from…

Firstly, to synthesize and characterize new functionalized hybrid materials and secondly, to assess their ability to remove Zn (II), Cd(II), and Cu(II) ions from aqueous solution. To achieve these goals, experimental parameters affecting the adsorption such as pH solution and contact time were determined. Kinetic data were expressed by pseudo-first-order, pseudo-second-order, and intra-particle diffusion models. Adsorption equilibrium isotherms were evaluated according to Langmuir, Freundlich, and Dubinin-Radushkevich (D-R) models. In addition, the thermodynamic parameters, selectivity, and the reusability have been determined and extensively discussed. To the best of our knowledge, there are scant studies dealing with the applicability of silica hybrid materials for a continuous process in a fixed-bed column. Herein, we have successfully proven the feasibility of the

All reagents were of analytical grade and used as received without further purification. Hydrophobic tetraethyl orthosilicate (TEOS, 99%) was utilized as a silica precursor, while ethanol was a bridging medium. Cd(NO3)2.4H2O, Cu(NO3) 2.3H2O, and Zn(NO3)2.6H2O were employed as metal sources for batch and column adsorption experiments. These reagents were supplied by Sigma-Aldrich, USA. The organic precursors 1,3,4-thiadiazole-2,5-diamine and 1,3,4-thiadiazole-

Pursuant to our foregoing studies, xerogels were synthesized using the following process [Helali 15 et 16]: 10 ml of deionized water, 20ml of ethanol, and 22.8 mL of TEOS were mixed under vigorous magnetic stirring. To the as-prepared mixture was added a necessary amount of organic precursor (10<sup>1</sup> M, 11.6 g of 1,3,4 thiadiazole-2,5-diamineor or 15 g of 1,3,4-thiadiazole-2,5-dithiol). Thereafter, the reactant mixture was stirred for 6 h at 78°C and at the last ripened for 48 h at 100°C; the resulting xerogels were labeled M1 and M2, and the synthesis mechanism is

Xerogel morphology was carried out using a scanning electronic microscope (Cambridge Instruments Stereoscan 120) operating at 15 kV. The textural properties of hybrid materials were determined from the N2 adsorption-desorption isotherms recorded at 77 K with a Micrometrics ASAP-2000 volumetric apparatus. The specific surface areas were computed by the multi-point analysis (BET) (Brunauer

distribution was acquired from the adsorption-desorption branches of the isotherm

˂0.3. Howbeit pore size

The main aims of the present study were the following:

DOI: http://dx.doi.org/10.5772/intechopen.86802

synthesized adsorbents for large volumes of discharges.

2,5-dithiol were prepared according to the literature [21, 22].

et al., 1938) in the relative pressure interval of 0.03˂P/P°

2. Materials and methods

2.1 Chemical reagents

2.2 Methods

2.2.1 Adsorbent synthesis

represented in Figure 1.

2.2.2 Characterization

129

challenges.

It is well known that the substantial factors affecting the adsorption process are pore size distribution, specific surface areas, and pore surface chemistry. In this endeavor, it is imperative to search materials with enormous porous structures, high specific surface areas, and low density. In the last few decades, ordered mesoporous silica has triggered a growing interest in the field of water treatment, owing to its diverse outstanding properties. These enclose tunable pore-size, high specific surface area, large pore volume, chemical inertness, thermal stability, and the ability to attach a plethora of different functional groups [7–9].

One of the basic techniques applied for fabricating organic-inorganic hybrid materials is the sol-gel method. Tetraalkoxysilanes (Si(OR)4) like tetraethyl orthosilicate (TEOS) or tetramethyl orthosilicate (TMOS) are widely employed as a precursor for preparing monolithic silica, owing to their hydrophobicity and strong covalent Si-O bonding [10]. It is worthy to state that the as-prepared hybrid materials by the sol-gel route are divided into two classes on account of the interaction type between organic and inorganic components. In class I, organic and inorganic components are strongly attached by covalent or dative covalent bonds, while in the second, these two components are weakly bonded through hydrogen or Van der Waals bonds. Indeed, these materials have generated considerable interest for their potential application in multiple fields such as: adsorption [11], drug delivery systems [12, 13], biosensors [14], nanotechnology, and nanomedicine applications [15, 16] and catalysis [17]. Organic-inorganic hybrid materials displayed high efficiency and outstanding selectivity towards target pollutant than the other silica gels. Nitrogen/thiol and magnetic functionalized mesoporous silica have been at the foreground of these composites. In the same vein, Benhamou et al. have shown that amine functionalized MCM-41 and MCM-48 exhibited a higher adsorption capacity than pristine. They also evinced that both hybrid materials have a higher affinity for Cu(II) and Pb(II) than for Cd(II) and Co(II) in single and mixed cation solutions [18]. Shahbazi and co-workers grafted aminopropyl (NH2) and melamine-based dendrimer amines (MDA) to SBA-15 mesoporous silica. They observed that NH2–SBA-15 and MDA–SBA-15 were over ten-fold better than the pristine SBA-15 in the adsorption of Pb(II), Cu (II), and Cd(II) [19]. Interestingly, they also showed that in column studies, the adsorption yield was swayed by the flow rate. Apart from the fact that magnetic silica-based materials exhibited excellent adsorption affinity towards heavy metal ions, such adsorbents compared to the nitrogen and thiol designer silicates can be easily removed from aqueous solution after adsorption. In the same context, Wang et al. [20] synthesized an amino-functionalized core-shell magnetic mesoporous SBA-15 silica composite which displayed a high adsorption capacity for Pb(II) ions. This adsorbent can be readily removed and regenerated. Despite the high adsorption capacity and the extra-ordinary selectivity towards target metal, as well as the capacity for simultaneous removal of aqueous pollutants, organic-inorganic hybrid materials are still not applied for a continuous process in a fixed-bed column. Another challenge to overcome is the difficulty in their largescale production because of the complexity of synthesis methods and the control

Bi-Functionalized Hybrid Materials as Novel Adsorbents for Heavy Metal Removal from… DOI: http://dx.doi.org/10.5772/intechopen.86802

of the stability of the developed adsorbents. Therefore, great endeavors are required for the synthesis of hybrid silicate adsorbents which meet the listed challenges.

The main aims of the present study were the following:

Firstly, to synthesize and characterize new functionalized hybrid materials and secondly, to assess their ability to remove Zn (II), Cd(II), and Cu(II) ions from aqueous solution. To achieve these goals, experimental parameters affecting the adsorption such as pH solution and contact time were determined. Kinetic data were expressed by pseudo-first-order, pseudo-second-order, and intra-particle diffusion models. Adsorption equilibrium isotherms were evaluated according to Langmuir, Freundlich, and Dubinin-Radushkevich (D-R) models. In addition, the thermodynamic parameters, selectivity, and the reusability have been determined and extensively discussed. To the best of our knowledge, there are scant studies dealing with the applicability of silica hybrid materials for a continuous process in a fixed-bed column. Herein, we have successfully proven the feasibility of the synthesized adsorbents for large volumes of discharges.
