Original Research Paper
Silica nanoparticles loaded on activated carbon for simultaneous removal of dichloromethane, trichloromethane, and carbon tetrachloride

https://doi.org/10.1016/j.apt.2016.06.003Get rights and content

Highlights

Abstract

The low efficiency of carbon derived from low-cost materials limits its industrial application as a low-cost adsorbent for the removal of contaminants from wastewaters. In this work, silica nanoparticles were loaded on activated carbon to produce an activated carbon/silica (AC/SiO2) composite. The product was characterized by means of thermogravimetric analyzer, scanning electron spectroscope, energy dispersive X-ray spectroscope, FTIR spectrophotometer, and X-ray diffraction. The AC/SiO2 was found to have a higher thermal stability than the AC. The XRD pattern of AC/SiO2 demonstrated that the AC was loaded with crystalline SiO2 as a tetragonal phase. For the removal of dichloromethane, chloroform and carbon tetrachloride, the operational parameters of contact time, initial concentrations and adsorbent dosage were optimized. The adsorption capacity increased with a decrease in the initial hydrocarbon concentrations. The experimental data were fitted to Langmuir, Freundlich, and Temkin isotherms, and pseudo-kinetic models. The Langmuir isotherm provided a better fit to the experimental data, assuming that adsorption takes place at specific homogenous sites within the adsorbent. The adsorption kinetics better fitted the pseudo-second order. The results of recycling tests demonstrate the excellent potential of adsorbents for pollutant removal.

Introduction

Chlorinated volatile organic compounds (Cl-VOCs) are considered as hazardous waste materials produced from different sources [1]. The proper disposal and control of these compounds have significant concern about the environment because these compounds have chemical and thermal stability, and ability to accumulate in the environment, also contribute to the contamination of groundwater, which poses a threat to living beings and can be considered as dangerous and toxic organic pollutants [2], [3]. The presence of these compounds in the atmosphere leads to the depletion of the ozone layer, the formation of photochemical smog and global warming [4]. Adsorption technology is one of the most commonly used methods for the removal of Cl-VOCs. Activated carbon is frequently used as an adsorbent due to its high surface area, unique microporosity, and low cost [5], [6], [7]. Different methods for removal of Cl-VOCs have been reported such as modified activated carbon [8], biofiltration [9], wet absorption methods [10] as well as catalytic oxidation [11] and thermal oxidation connected with recuperation [12]. There are different porous materials can be used as adsorbents like zeolites [13], clays [14], microporous polymers [15], silicated MCM-41 and spherical ordered silica [16], organ modified silica materials or organic–inorganic composites and activated carbon from different precursors [17]. Commercial activated carbon (CAC) has limitation for using as material for removal of environmental contaminants due to its high cost. Therefore, researchers have investigated different raw starting materials for activated carbon synthesis such as rubbers [18], starch xanthenes [19], sawdust of Pinus sylvestris [20], chitosan [21], bentonite [22], and discarded automobile tires [23]. Tire rubber is one of the promising precursors for the preparation of activated carbon. It is composed of a mixture of different elastomers such as natural rubber, butadiene rubber, and styrene butadiene rubber, plus other additives like carbon black, sulfur, and zinc oxide [24]. Approximately 32% by weight of the waste tire is mainly constituted of carbon black in which the carbon content is as high as 70–75 wt% [25], [26].

Silica is considered as one of the interesting adsorbents due to its unique properties such as stability, possible to be regenerated, equilibrium can be achieved very fast, high mechanical resistance, and high surface area [27]. It can also be modified by different surface groups [28]. Silica nanoparticles are commonly used in the industrial manufacturing, packaging, high-molecule composite materials and ceramics synthesis, disease labeling, drug delivery, cancer therapy and biosensor field [29]. It can be used as a hardening agent in the synthesis of polymer composites and in different medical applications because it is biocompatible and non-toxic [30]. Many studies have been conducted using functionalized silica materials as an adsorbent for pollutant removal [31], [32], [33], [34].

The combination of AC and silica in a composite of activated carbon/silica will yield a composite that combines the properties of both materials. In this work, the silica nanoparticles loaded on activated carbon (AC/SiO2) was synthesized. The product was evaluated for the removal of halomethanes (dichloromethane, chloroform, and carbon tetrachloride). The introduction of silica nanoparticles is expected to enhance the adsorption capacity via providing highly dispersed active sites in the adsorption process.

Section snippets

Materials

The chemical compounds dichloromethane, chloroform and carbon tetrachloride (purity ⩾99%), ethanol, and acetone were obtained from Sigma–Aldrich. Activated carbon used in this work was derived from waste rubber tires based on the process published earlier [23]. Glycol, ethanol, sodium meta silicate; Na2SiO3·9H2O (284.2 g/mol), sodium per-carbonate were used without any further purification. All solutions were prepared with deionized water of resistivity not less than 18.2  cm−1. Safety

Characterization of AC/SiO2

The SEM image of AC/SiO2 composite is presented in Fig. 1. As shown, the surface of the composite is rough and has a large amount of nanoparticles, which represent additional active sites for surface adsorption on the activated carbon surface. Using the scale of the SEM image, one may estimate the average diameter of the silica nanoparticles as approximately 50 nm. This value has been confirmed by taking more images and taken the average. EDX spectrum depicted in Fig. 2 indicates the presence of

Conclusions

This work reports on the synthesis of adsorbent consists of silica nanoparticles loaded on activated carbon for efficient halomethanes (dichloromethane, chloroform, and carbon tetrachloride) remediation of contaminated water. The correlation coefficients for the second-order kinetic model are greater than 0.99 indicating the applicability of this kinetic equation and the second-order nature of the adsorption process of dichloromethane, chloroform and carbon tetrachloride on AC/SiO2. The

Acknowledgement

Support from the Chemistry Department and King Fahd University of Petroleum and Minerals is gratefully acknowledged. The authors would like to acknowledge the support provided by the Deanship of Scientific Research (DSR) at King Fahd University of Petroleum & Minerals (KFUPM) for funding this work through the KFUPM−NUS Collaborative Fund support (NUS15105).

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