Optimizing adsorption of arsenic(III) by NH2-MCM-41 using response surface methodology

https://doi.org/10.1016/j.jiec.2013.11.009Get rights and content

Abstract

Response surface methodology (RSM) was used to optimize process parameters for arsenic (As(III)) removal from aqueous solution using amine-functionalized MCM-41 (NH2-MCM-41). Four independent variables such as pH, initial metal concentration, temperature and adsorbent dosage were investigated. The optimal conditions to remove As(III) by NH2-MCM-41 was found to be pH 5.62, initial As(III) concentration 5.00 mg/L, temperature 20 °C and NH2-MCM-41 dosage 5.00 g/L. XRD, FTIR and SEM analyses testified to the obvious change of the surface morphology and the presence of metal on the sorbent after adsorption.

Introduction

Growing attention is being given to the potential health hazard presented by heavy metals to the environment [1]. Arsenic is known to be one of the most toxic elements and has serious effects on plants, animals and human health [2], [3]. Arsenate (H3AsO4, H2AsO4, HAsO42−) and arsenite (H3AsO3, H2AsO3, HAsO32−) are the two major arsenic species generally found in the groundwater system [4]. Arsenite [As(III)] is generally more toxic and predominant in many affected groundwaters [5], [6]. Various treatment technologies have been used to remove arsenic from water [7]. The common methods adopted including coprecipitation [8], [9], electrocoagulation [10], ion exchange [11], use of adsorbent media [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], zero valent iron [24], [25], membrane [26], bioremediation [27], etc.

As an alternative to these treatment methods, adsorption has been recognized as an effective technique for treatment of water contaminated with arsenic [28]. There are challenging tasks still left for researchers to develop cheaper and effective adsorbents for removal of arsenic and other water pollutants [7]. Ordered mesoporous silica MCM-41 has attracted considerable attention for possible application as an adsorbent, because its high pore volume, large surface area, and ease of functionalization [29]. Pore diameters, pore volumes, particle morphology, and surface modifications are critical textural properties of mesoporous silica that control the metal ions loading and release properties. Their modification by organic functional groups can lead to adsorbents with specific properties [30]. Amine functionalized MCM-41 (NH2-MCM-41) is regularly populated with basic amine sites on the walls of a silica framework that increase metal adsorption as well as elucidate the mechanism [31].

Response surface methodology (RSM) is a collection of statistical and mathematical technique useful for modeling and analysis of problems in which a response of interest is influenced by several variables [32]. In literature, RSM based on Central Composite Design (CCD) has been used in different applications of engineering including heavy metal removal, dye removal, electrocoagulation, ethanol production, etc. [33], [34], [35], [36], [37].

In literature, there is little information available on the optimization of arsenite adsorption on NH2-MCM-41. Therefore, we decided to evaluate the As(III) removal potential of NH2-MCM-41 under various operating conditions such as pH, initial As(III) concentrations, temperature, and adsorbent dosage. The parameters for maximum removal efficiency are optimized by using central composite design (CCD) under RSM. The physicochemical characteristics of NH2-MCM-41 before and after adsorption are examined with Fourier transform infrared (FTIR), X-ray diffraction (XRD) and scanning electron microscopy (SEM). Moreover, the adsorption isotherms, including Langmuir, Freundlich and Dubinin–Radushkevich (D–R) isotherms were applied to the equilibrium data to describe the main interactive mechanisms involved the removal process. Kinetic parameters were also calculated from the pseudo-first-order and pseudo-second-order models for better description of the adsorption mechanism. Thermodynamic parameters were also computed.

Section snippets

Materials

Sodium arsenite (NaAsO2, 99%) was obtained from Sigma. Stock As(III) solution (13.3 mM) was prepared by dissolving 0.866 g NaAsO2 into 500 mL Milli-Q water. The As(III) stock solution was stored in a high density polyethylene bottle, and was kept in darkness at 4 °C [38].

NH2-MCM-41 was prepared according to the method of Ho et al. [39], [40]. Calcined MCM-41 (2.5 g) were refluxed in 50 mL of n-hexane containing 2.5 g of 3-aminopropyltrimethoxisilane for 6 h. The powder of NH2-MCM-41 was filtered,

Characterization of NH2-MCM-41

After the amino functionalization process, NH2-MCM-41 exhibited a high affinity for removing As(III) from liquid solution. The results showed that the removal of As(III) adsorbed on the NH2-MCM-41 was 50.7%, while the value obtained with the pristine MCM-41 was 11.4%.

The XRD patterns of MCM-41 and NH2-MCM-41 in low angles are shown in Fig. 1. A significant difference was observed in the intensity of the peaks in the XRD spectrum before and after amine functionalized. These results indicated

Conclusions

The amine functionalized MCM-41 prepared by synthesis method was found to be an efficient adsorbent for the removal of arsenic (III) ions from aqueous solution. The operating parameters, pH of solution, temperature, initial arsenic (III) concentration, and NH2-MCM-41 dosage are effective on the adsorption efficiency of arsenic (III) ions. The adsorption capacity of arsenic (III) onto NH2-MCM-41 was found to be 5.89 mg/g at pH 5.62. RSM by the CCD model were appropriate for determining the

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