Synthesis of fly ash based zeolite-reduced graphene oxide composite and its evaluation as an adsorbent for arsenic removal
Graphical abstract
Introduction
Arsenic, being one of the toxic pollutants is introduced in the environment through weathering of rocks, discharge of industrial waste, use of fertilizers and pesticides, smelting of metal ores, burning of fossil fuels (Altundoğan et al., 2000, Benner, 2010, Shukla et al., 2010, Nidheesh and Singh, 2017). Arsenic is present in several forms in food and environmental media but it is mainly encountered in drinking water as inorganic arsenic. In this form it is highly toxic and readily bioavailable (Camacho et al., 2011, Dubey et al., 2012). There are two inorganic states of arsenic in natural water viz. Arsenite (As-III) and Arsenate (As-V). Short-term exposure to high levels of arsenic is fatal, and long-term exposure to trace levels of arsenic (i.e., inhalation, ingestion) may lead to several chronic diseases, including skin, cardiovascular, respiratory diseases and cancer (Baskan and Pala, 2011, Polowczyk et al., 2016, Wu et al., 2017). Therefore many countries and organizations including World Health Organization (WHO) adopts the guideline of 10 μg/L as maximum permissible limit (Mohan and Pittman, 2007, Mondal et al., 2013).
In order to safeguard the environment and health problems caused by arsenic in water, various treatment techniques which include coagulation, adsorption, ion exchange, electrochemical process, membrane separation and reverse osmosis have been used for arsenic removal (Kumar et al., 2004, Shevade and Ford, 2004, Kabir and Chowdhury, 2017). Among the mentioned techniques, adsorption is a popular method owing to its advantage of ease of operation, versatility, availability of various adsorbents and potential of regeneration (Simeonidis et al., 2016, Li et al., 2018). Also economic feasibility and simple synthesis adds up to the advantage of using adsorbents for removal (Baskan and Pala, 2011).
Till date, various adsorbents have been used for the removal of arsenic from water. In recent past, zeolites have been explored as adsorbents due to their structural characteristics and valuable properties in heavy metal removal from water (Chunfeng et al., 2009, Merrikhpour and Jalali, 2013). Zeolites are three dimensional micro and mesoporous crystalline solids with well-defined structures that contain aluminum, silicon and oxygen in their regular framework (Tavolaro and Drioli, 1999, Wdowin et al., 2014). In addition, it should be noted that zeolites are compatible with the environment; they are stable at high temperatures, in acidic and corrosive environments, and also have potential selectivity towards some cations (Khatamian et al., 2015). To reduce the cost of zeolite production, efforts have been made to find a material, which has abundant natural availability or is a waste material or an industrial by-product. Fly ash (inexpensive waste material) based zeolites have been employed for heavy metal removal by various researchers (Querol et al., 2006, Chunfeng et al., 2009, Polowczyk et al., 2016). In the present study fly ash is used as a substrate for zeolite synthesis. Valorization of fly ash to form zeolites is of great interest as zeolites have widespread industrial application and their sale can make up for disposal cost as well as it will reduce the environmental liability (Chunfeng et al., 2009, Musyoka et al., 2013).
Recently, graphene based materials have gained tremendous popularity for environmental remediation and energy applications because of their high surface area and functional groups (Wang et al., 2013a, Wang et al., 2013b, Yusuf et al., 2015). But, owing to high cost, large scale and high quality synthesis, the applicability of graphene for commercial scale applications is limited (Khatamian et al., 2015). In this regard, synthesis of graphene oxide (GO) through chemical methods and its subsequent reduction to form reduced graphene oxide (rGO) enhanced the possibility for the application of graphene derivatives as adsorbents in water purification. The insulating/non-conducting characteristics of GO owing to the various hydrophilic oxygen groups (epoxide, hydroxyl, carbonyl, and carboxyl groups) restricts its use for various applications including water treatment (Zhu et al., 2014). Conversely, the tuneable conductivity (Soni et al., 2018) (based on the degree of reduction) of rGO may be more suitable for water treatment. On the other hand, graphene-based composites are emerging as a new class of materials that promises applications in several fields (Luo et al., 2011). It is believed that these composites exhibit modified properties compared with their individual components. It may be proposed that zeolites can be considered as proper candidate for preparation of graphene-based composites due to their valuable properties (Khatamian et al., 2015).
In present work, we proposed to prepare a zeolite-reduced graphene oxide (ZrGO) based composite using a facile, cost effective process thereby integrating the advantageous features of individual adsorbents to evaluate the removal of arsenic (III) from water. As Arsenic (III) is the most toxic and mobile form of Arsenic in the environment (Wu et al., 2017), hence, the present study focusses on removal of Arsenic (III) from water. Zeolite was synthesized using fly ash, thus making the process cost effective and environment friendly. The performance evaluation of the synthesized composite, ZrGO was done and compared with zeolite and rGO for arsenic removal from water. The obtained ZrGO composite shows better adsorption towards arsenic as compared to their individual counterparts. Furthermore, the adsorption kinetics and isotherms were studied for ZrGO to understand the adsorption mechanism.
Section snippets
Chemicals and materials
Graphite (99.99% pure), potassium permanganate (KMnO4), sodium nitrate (NaNO3), hydrochloric acid (HCl), sodium hydroxide (NaOH) and hydrogen peroxide (H2O2) were obtained from Merck. Sulphuric acid (H2SO4), arsenic trioxide (As2O3), sodium hydroxide (NaOH) and hydrochloric acid (HCl) was obtained from Fisher Scientific. N-Methyl-2-pyrrolidone (NMP) from Alfa Aesar. De-ionized (DI) water with a resistivity of 18.2 MΩ cm (from Elga Labwater) was used for cleaning and solution preparation.
Synthesis of zeolite using fly ash
Fly ash
Material characterization
The synthesized materials are characterized using FTIR, RAMAN and SEM-EDAX. The results are discussed below.
Adsorption kinetics and adsorption isotherms
ZrGO gave the best result as an adsorbent in the present study and was able to bring down the residual arsenic concentration well within the WHO standards. Therefore, the adsorption kinetic and adsorption isotherm study was performed for the data obtained for ZrGO.
For the present study, the plots were made for three initial arsenic concentrations (100, 200, 300 μg/L). The experimental data was fitted in the mentioned three kinetic models. The average R2 value for pseudo first order kinetic
Conclusions
ZrGO composite was successfully synthesized by a simple cost-effective method and can be used as an effective adsorbent for arsenic removal in aqueous solutions. Results showed that ZrGO gave the best performance as an adsorbent to remove arsenic when compared with zeolite and rGO individually. ZrGO shows 97% removal efficiency and was able to bring down the arsenic concentration well within WHO limits. The kinetic model fits the pseudo first order kinetics and indicates the adsorption
Acknowledgements
This work was financially supported by Science and Engineering Board (SERB) project no. PDF/2016/000338 and SR/FTP/ES-6/2013.
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