The selective heavy metal ions adsorption of zinc oxide nanoparticles from dental wastewater
Introduction
Despite the increasing popularity of ceramic prosthetics, metallic alloys remain one of the most widely used and oldest materials for the restoration of defects and deletions in dentistry. Most of the dental heavy metal ions waste (e.g. Ni2+, Pb2+, Cu2+, and Cr3+ compounds), including the materials of grinding during the try-in and removing the old restoration, have been discharged through the water source, which not only polluted the surroundings of the clinic, but also threatened the human health on account of the magnifying nature of the food chain [1], [2]. For example, Cr3+ ions, a cancerous and mutagen material, is easily absorbed by any live organism. Furthermore, excessive consumption of Cr3+ suppresses the immune system and may lead to skin allergy and carcinogenesis [3]. The World Health Organization has prescribed allowable limits of heavy metal ions in drinking water. Hence, development of new technologies with cost-effective and environmentally friendly is significant to remove heavy metal ions from dental wastewater.
Currently, there are several different methods to remove toxic ions from the contaminated water, including chemical precipitation [4], bio-sorption [5], adsorption [6], flotation [7], ion exchange [8] and so on. Adsorption, a process in terms of its simple operation, recyclability, environmental and higher adsorbent efficiency, holds immense potential to treat the wastewater efficaciously [9], [10]. A good deal of material oxides are applied in the form of nanoparticles that have a relatively advantageous affinity of heavy metals uptake on account of the key characteristic features like small size and giant specific surface. However, many adsorbents have some weaknesses such as high cost, the production of by-products, low stability and unable to recycle [11]. Recently, ZnO-NPs has been widely used in dental filling materials, gas sensors, catalyst industry and so on [12], [13]. Also, ZnO-NPs received much attention due to its biocompatibility, acceptable cost and long-term stability [14], [15]. Besides, it has been reported that ZnO-NPs can adsorb heavy metals using their hydroxyl groups [16], indicating that it has more significant potential in removing heavy metal ions from the dental wastewater. Many scholars have investigated the optimized parameters for uptake of heavy metal ions onto ZnO-NPs and draw different conclusions. Some studies found that adsorption capacity at equilibrium was obtained at pH around 3 [17], [18], [19], while others stated that of natural pH [15], [20]. Though the application of ZnO-NPs has been extensively investigated for the uptake of numerous industrial effluents, the removal of specifically the dental wastewater using ZnO-NPs has not yet been reported.
The main aims of this study were to assess the feasibility of ZnO-NPs for selective removal process of dental wastewater and evaluate its maximum adsorption conditions, so as to develop a new method for reduce the heavy metal ions from dental wastewater. Therefore, in this work, ZnO-NPs was prepared via the traditional hydrothermal method and characterized through XRD, FT-IR, SEM-EDX and N2 adsorption-desorption measurements to elucidate morphology and the adsorption mechanism. Furthermore, Ni2+, Pb2+, Cu2+ and Cr3+ cations were chosen as typical heavy metal ions existed in the polluted wastewater to investigate the selective adsorption property. Moreover, the effect of varying pH, ionic strength, contact time, initial concentration and adsorbent dose were examined in order to gain insight into the optimal adsorption conditions. The research hypotheses were that ZnO-NPs would show higher affinity towards Cr3+ ions and have significant potential for removing heavy metal ions from the dental wastewater.
Section snippets
Materials
The chemical reagents including zinc acetate dehydrate Zn(CH3COO)2 2H2O, sodium hydroxide NaOH, nickel nitrate Ni(NO3)2, lead nitrate Pb(NO3)2, copper nitrate Cu(NO3)2 and chromic nitrate Cr(NO3)3 were applied in this study. The names, batch numbers, and manufacturers of these materials are listed in Table 1. All the other chemicals were in high purity grade and used as received.
Preparation of ZnO-NPs
ZnO-NPs powders were synthesized via the traditional hydrothermal method as previous reported [21]. First, Zn(CH3COO)2
Results and discussion
It was hypothesized that ZnO-NPs would have selective absorption of Cr3+ ions from polluted wastewater and hold a certain prospect as environmental friendly material candidate sorbent for dental wastewater purification treatment. The hypotheses were verified by the results of this research.
Conclusion
In this study, the ZnO-NPs was synthesized via the traditional hydrothermal method and characterized by XRD, FTIR, SEM-EDX and BET-specific surface area techniques. The adsorption experiments were conducted under varying influencing factors like co-existing heavy metal ions, pH, ionic strength, the dose of adsorbents, contact time, initial aqueous concentration. Our results revealed that ZnO-NPs showed higher affinity towards Cr3+ ions and the optimal condition for Cr3+ ions adsorption onto
CRediT authorship contribution statement
Mengqin Gu: Conceptualization, Writing - original draft. Liying Hao: Methodology, Software. Yigan Wang: Supervision, Formal analysis. Xiyu Li: Methodology, Software. Yifan Chen: Writing - review & editing, Funding acquisition. Wei Li: Project administration, Writing - review & editing. Li Jiang: Project administration, Funding acquisition.
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.
Acknowledgements
This research was funded by Sichuan Science and Technology Program (2016FZ0069) and Stomatological Hospital of Southern Medical University (PY2018003).
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