Prepared PANI@nano hollow carbon sphere adsorbents with lappaceum shell like structure for high efficiency removal of hexavalent chromium
Graphical abstract
Introduction
Nowadays, the deterioration of water quality caused by toxic heavy metal ions in the drinking water has attracted more and more attention. As we all known, chromium (Cr) as the most common heavy metal pollutants in industrial waste water. Chromate contamination in waste water mainly comes from metal alloys, leather tanning, electroplating and chromate production, and Cr mainly presents in compounds in trivalent and hexavalent. However, hexavalent chromium (Cr(VI)) is much more toxic than trivalent chromium (Xie et al., 2017). Unfortunately, the Cr(VI) have serious effects on the human body due to its mutagenic, teratogenic and carcinogenic (Huang et al., 2018, 2019a, 2019b). The chromate (CrO42−), dichromate (Cr2O72−) and hydrogen chromate (HCrO4− and H2CrO4) are primary forms of Cr(VI) in waste water. And these forms depending on the pH value and the total Cr concentration of the solution. The HCrO4− is the predominant form when pH value is below 6.8, and CrO42− takes over the central position with pH value is above 6.8 (Gupta et al., 2011; Chen et al., 2018). The World Health Organization (WHO) has issued a maximum acceptable Cr(VI) concentration of 0.05 mg/L in drinking water. Consequently, it is necessary to exploiting efficient methods for Cr(VI) removal for human health and sustainable development.
According to the previous literature, many methods have been applied to degradation Cr(VI) in waste water, such as physical or chemical adsorption (Zhang et al., 2016), electrochemical or chemical precipitation (Jagtap et al., 2012; Yáñez-Sedeño et al., 2017), reverse osmosis membrane (Samani et al., 2010) and bio-degradation (Guo and Tian, 2013). Among these techniques, adsorption is one of the most widely used and effective method at present because of its low operation cost, high performance and stability. Up to now, a series of adsorbents have been used to remove heavy mental contamination, such as molecular sieve, carbon based materials and transition metal oxides (Lv et al., 2014; Chen et al., 2019a, 2019b; Huang et al., 2019a, Huang et al., 2019b). Recently, carbon based materials mainly including carbon nanotubes and nano hollow carbon sphere (NHCS) have attracted more and more attention due to their high specific surface area and excellent chemical stability, which can be applied for heavy mental ions removal (Zhang et al., 2017). The NHCS as a new type of carbon nano-materials with an well-defined pore structure is prepared by templating. Dong et al. synthesized porous carbon materials using the sugar beet tailing biochar, which used for adsorption Cr(VI), and the adsorption capacity reached 123 mg/g at acidic condition (Dong et al., 2011). Chen et al. prepared porous hollow carbon sphere for Pb2+ adsorption with excellent reusability (Chen et al., 2019a, 2019b). Moreovre, the cage-like oxidized hollow carbon spheres were fabricated by Lin research group and as a adsorbent for Pb2+ removal (Zhang et al., 2019). However, its remains a great challenge for developing a cost-effective materials with high adsorption and reduction performance for heavy metal ions remove.
Polymers as the function materials for Cr(VI) removal have been studied and attracted more and more attention due to their excellent properties, including good electrochemical properties, environmental friendly and low cost (Pang et al., 2011a, Pang et al., 2011b). Polyaniline (PANI), as one of the most widely used polymers in the environment protection. It is well established that PANI has three idealized oxidation states, which is LB (leucoemeraldine), EB (emeraldine) and PB (pernigraniline), corresponding to the reduced, doped and oxidized states of PANI, respectively. When the state of PANI transitions from the LB or EB state to the PB state, the electrons can be donated. This powerful electron-delivery capability is the most important reason for its application on Cr(VI) remediation (Farrell and Breslin, 2004). The research group of Zhang has prepared sulfuric acid doped PANI and the maximum capacity of Cr(VI) can get up to 95.8 mg/g (Zhang et al., 2010). The PANI/silica gel composite is prepared by Karthic research group, which used for remove Cr(VI) and the maximum capacity is 63.4 mg/g at 303 K (Karthik and Meenakshi, 2014a, 2014b). Furthermore, PANI nanocomposites based on chitosan (Yavuz et al., 2011), sawdust (Mansour et al., 2011) and jute fibers (Kumar et al., 2008) were also investigated by other research group. All these research results shown that PANI has a great potential application for removing heavy metal contaminants.
In this study, the novel polyaniline@nano hollow carbon sphere (PANI@NHCS) with lappaceum shell like structure was fabricated, which used for removing Cr(VI) from waste water. Owing to the unique hollow structure of PANI@NHCS, the Cr(VI) was adsorbed and then reduced to Cr(III) under the optimal condition. The obtained PANI@NHCS was characterized by many technologies including X-ray powder diffraction (XRD), Fourier transformed infrared spectroscopy (FTIR), transmission electron microscopy (TEM), Brunauer-Emmett-Teller (BET), X-ray photoelectron spectroscopy (XPS) and so on. Moreover, the effect of several parameters for Cr(VI) adsorption were investigated in batch adsorption experiments, such as pH, dosage, co-existing ions and Cr(VI) concentration. The synergistic reaction of adsorption and reduction on Cr(VI) was also particularly explained. The adsorption and reduction mechanism, kinetic and adsorption isotherm of Cr(VI) were discussed in detail. Overall, we believe that this research will offer a new adsorbents for removing Cr(VI) from waste water.
Section snippets
Materials
Tetraethyl orthosilicate (TEOS), dopamine hydrochloride (DA), hydrofluoric acid (HF), hydrochloric acid (HCl), ammonia, sodium hydroxide (NaOH), anhydrous ethanol, aniline, ammonium persulfate (APS), Potassium dichromatem (K2Cr2O7) 1,5-diphenylcarbazide (C13H14N4O) and other reagent were purchased from Sinopharm Chemical Reagent Company, China. All other chemicals were of laboratory reagent grade and had not been further purified prior to use.
Synthesis of PANI@NHCS
The prepare processes of NHCS was provided in the
Characterization of PANI@NHCS
Fig. 2a–h shows the SEM images of NHCS and different types PANI@NHCS materials at different magnifications. From Fig. 2a and b, it is obviously observed that the carbon sphere evenly distributed and most of the sphere possessed hollow structure. Furthermore, the average diameter of NHCS is about 330 nm and the surface of NHCS was very smooth. In Fig. 2c, the NHCS was covered by a large amount of PANI and the hollow structure disappeared. Unfortunately, the hollow structure of PANI@NHCS-20 was
Conclusion
In this research, the polyaniline@nano hollow carbon sphere (PANI@NHCS) with lappaceum shell like structure was successfully prepared. The obtained PANI@NHCS-30 exhibited highly performance adsorption capacities for Cr(VI) mainly through hollow structure and redox reaction. The maximum adsorption capacity of PANI@NHCS-30 for Cr(VI) reached 250.0 mg/g (removal efficiency 100%) under the optimal condition. Moreover, the PANI@NHCS-30 had good regenerability and the Cr(VI) removal rate could
Author contributions
Fei Wang: Experiment, Data analysis, Writing Original Draft. Yimei Zhang: Review and Editing, Supervision, Data Curation. Qinglu Fang: Experiment, Data analysis. Zhiying Li: Experiment, Data analysis. Yuxian Lai: Experiment, Data analysis. Yong Wang: Review & Editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This work was financially supported by the National Natural Science Foundation of China (Grant No. 51878272).
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