Elsevier

Environmental Research

Volume 188, September 2020, 109831
Environmental Research

The role of seashell wastes in TiO2/Seashell composites: Photocatalytic degradation of methylene blue dye under sunlight

https://doi.org/10.1016/j.envres.2020.109831Get rights and content

Highlights

  • A facile approach was proposed to use shell waste for photocatalyst synthesis.

  • Calcined abalone shell supported TiO2 exhibited highest photocatalytic performance.

  • 100% degradation efficiency was attained under natural sunlight.

  • No obvious byproducts were observed with abalone shell as support.

Abstract

This paper proposes a sustainable and facile approach for the synthesis of photocatalysts in which shell waste is used as support material. The synthesized photocatalysts exhibited a significant performance in the mineralization of organic substances under solar irradiation or artificial lighting. Calcined abalone shell with a TiO2 loading of 23.4% led to a significant improvement in optical absorption: the degradation efficiencies of methylene blue (MB) after 140 min under UV light, vis light, UV–vis light, and natural sunlight were 93%, 96%, 100%, and 100%, respectively. Notably, the byproducts obtained after the degradation by commercial P25 TiO2 disappeared with the utilization of shell waste as support material. The Na, Sr, S present in the calcined abalone shell were doped into the substitutional sites of TiO2 and were indispensable to achieve the desired band-gap narrowing and photocatalytic performance; moreover, the Ti and Zn oxides in the calcined abalone shell acted as semiconductors and improved the charge separation efficiency of TiO2. Above all, this paper describes a green synthesis based on the use of waste seashell. This material acts as an excellent photocatalyst support for environmental pollution treatments, leading to the ‘control of waste by waste’ and opening up new possibilities for shell waste reutilization and sustainable chemistry.

Introduction

Rapid economic growth and quality of life improvement are causing serious environmental problems. In this context, a big problem is represented by solid waste: it presently occupies an area of ~ 800,000 acres and always associates with water issues (Guo et al., 2017). Shell waste has attracted increasing attention in recent years because of its high output. China is the world's largest aquaculture country, with an annual total output of >10 million tons since 2002, accounting for ~ 54% of the global shellfish production (Yao et al., 2014). The major marine shellfish species included in the production are clams, oysters, mussels, scallops, and abalones (Eziefula et al., 2018). A large amount of shell waste is disposed in landfills or seawater. Its decomposition products can cause foul smell and water and soil pollution, especially in coastal areas. In a circular economy framework, seashell waste is a potential resource for the synthesis of value-added products, due to its intrinsic advantages. Although shell waste is partly recycled as food additive and paving material, such products have a low added value and little commercial appeal. The use of byproducts or waste streams in another processes for the production of commercial additives or catalysts with similar chemical make-up is a well-known sustainable option. Shell waste is represented by biomineralized material composed of highly ordered (columnar) tablets and displaying remarkable mechanical properties (Auzoux-Bordenave et al., 2015). The main component of shell waste is calcium carbonate containing traces of Sr, Fe, Mg, Mn and other elements. Several studies have described the utilization of shell waste as a synthesized material for catalysts. Narges et al. calcined oyster shell powder at 900 °C for 6 h and used it as catalyst for one-pot rapid preparation of 1,8-dioxo-octahydroxanthenes (Mohammadian and Akhlaghinia, 2018). Chen et al. synthesized a highly efficient CaO-based catalyst for biodiesel production using abalone shell calcined at 800 °C (Chen et al., 2016). Those studies provided new insights for the circulating utilization of shell waste; however, the associated high energy consumption and complex manufacturing processes have significantly limited their large-scare production and application.

Concerning water pollution, the contents of harmful water-soluble organic pollutants in industrial wastewater is increasing rapidly. Traditional treatment technologies include adsorption (Ghorbani et al., 2020), coagulation (Cañizares et al., 2006), ozonation (Tehrani-Bagha et al., 2010; Wang et al., 2019), the electrochemical method (Brillas and Martínez-Huitle, 2015), and photocatalytic degradation (Liu et al., 2020). Compared to conventional wastewater treatment technologies, which involve a costly disposal process (Anwer et al., 2019), photocatalytic degradation has been considered an innovative and effective alternative for water remediation: photocatalysis can directly transform pollutants into CO2 and H2O by utilizing inexhaustible sunlight (Raza et al., 2020). Therefore, compared with other purification processes, photocatalysis has been proven to be a sustainable and energy-efficient method for water treatment and its applications have greatly progressed in the field of environmental science. Photocatalysis is based mostly on the use TiO2, which is considered an excellent semiconductor. TiO2 possesses strong oxidizing power and redox selectivity, which improve the mineralization efficiency of organic pollutants and even of some refractory organic pollutants (representing a crucial ecological problem) (Anwer and Park, 2018). These properties, coupled with its cheapness, accessibility, chemical stability, and non-toxicity, have led to a widespread use of TiO2 in hydrogen fuel production (Chiarello et al., 2010; Ouyang et al., 2018), air cleaning (Zeng et al., 2019), metal anti-corrosion (Yang et al., 2015), self-cleaning (Wang et al., 2009), wastewater treatment (Buyukada, 2019), and antibacterial activity (C. C. Liu et al., 2016a, 2016b). However, some of the characteristics of TiO2 (e.g., low quantum yields (QYs), wide band gap, and limited reaction rate) hinder its practical applications (Anwer and Park, 2019). Therefore, various solutions are continually being proposed in order to improve the photocatalytic performance of TiO2. These include doping, metal deposition, and its coupling with different semiconductors (MiarAlipour et al., 2018). However, as previously mentioned, no previous studies have considered the use of the elements contained in seashell waste as modifiers for enhancing photocatalyst activeness. The sol–gel technique is the most versatile for the synthesis of TiO2 nanoparticles and has great prospects and many advantages compared to other synthesis methods (e.g., the solvothermal and precipitation methods): (1) a low synthesizing temperature, (2) the possibility of producing highly stable TiO2 in complicated shapes, and (3) the possibility of uniformly mixing raw materials at the molecular level (Gupta and Tripathi, 2011; Noman et al., 2019). In this paper, we synthesized high-performance photocatalysts by the sol–gel method and using shell waste. The photocatalytic performance of the wastewater treatment was systematically investigated by using shell waste-supported TiO2. Several critical issues were researched in detail by: (1) screening for the optimal shell waste (among those of six economic shellfishes characterized by a mature cultivation technology and abundant resources) to be used as TiO2 support and modifier; (2) verifying the photodegradation efficiency and stability of the photocatalyst under different levels of light irradiation; (3) judging whether the prepared photocatalyst can substitute commercial P25 TiO2 in the dye wastewater treatment; (4) investigating the photocatalytic degradation mechanism. This paper is the first to describe a synthesized core–shell structure TiO2–shell composite produced by simple grinding and calcination, followed by the sol–gel process. The ‘soft low-temperature procedure’ was followed throughout the calcination process. Compared with earlier synthesizing procedures, this method avoids high-temperature calcination (≥800 °C), decreasing energy consumption and production costs. This whole process is hence economic, nontoxic, environmentally friendly, easy to operate, applicable, and generalized. Calcined abalone shell, used as support of TiO2, can lead to a high concentration of O vacancies in TiO2 lattice and increase the charge separation efficiency of the TiO2 shell (greatly increasing the photocatalytic activity of TiO2). Therefore, calcined abalone shell can be considered an excellent support for TiO2 and can be extensively applied to the photocatalytic degradation of organic pollutants in wastewater, adding value to shell waste and allowing its recycling.

Section snippets

Experimental materials

Abalone, scallop, oyster, clam, mussel, and razor clam shells dumped from food factories located in Qingdao (Shandong, China) were used as raw materials. All the chemical reagents, including tetrabutyl titanate (TBT), ethanol, and methylene blue (MB) were purchased from Sinopharm Chemical Reagent Co., Ltd. (China).

Photocatalysts preparation

The process followed for the photocatalysts preparation is described in Fig. S1. The obtained materials were washed several times with deionized water to remove impurities and dried

Crystallographic structures

The multilayer structure of mollusk shells can be divided into three primary sections: the periostracum, the prismatic layer, and the nacreous layer (Yao et al., 2014). These layers are mainly composed of multi-scale structures of calcite and aragonite, while a small quantity of organic matrix is distributed along the grain boundary. The XRD patterns in Fig. 1(a) show that the oyster and scallop shells were composed of calcite, while the clam and razor clam shells were composed of aragonite;

Conclusions

In this study, six types of waste shells were tested as supports for TiO2-photocatalysts; moreover, the MB degradation performance was investigated to evaluate their photocatalytic activities. Among scallop, oyster, clam, mussel, razor clam, and abalone shells, the calcined abalone shells were found to function as the best support for TiO2. The MB degradation efficiency was strongly correlated with the amount of TiO2 loading. The catalyst characterized by a 23.4% TiO2 loading exhibited the

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.

Acknowledgment

This work was supported by the National Natural Science Foundation of China (51906175), Tianjin Science and Technology Project (18YFJLCG00090), and Independent Innovation Fund of Tianjin University (2019XYF-0088). I want to thank Prof. L.N for his encouragement in my lowest moments. Finally, I want to thank, in particular, the selfless support from my husband Zhiyuan Lin.

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