Multifunctional hybrid membranes for photocatalytic and adsorptive removal of water contaminants of emerging concern

Highlights

• Synthesis of Au–TiO₂ nanocomposites with ability to absorb visible radiation.

• Production of porous multifunctional Au–TiO₂/Yttrium/PVDF-HFP membranes.

• Photocatalytic and adsorptive efficiency tested in norfloxacin and arsenic removal.

• Multifunctional tests performed for simultaneous photocatalysis and adsorption.

Abstract

This work focuses on the combination of multifunctional photocatalytic and adsorbent materials in a unique polymeric membrane. For this purpose, Au/TiO₂ and Y₂(CO₃)₃ nanoparticles were immobilised onto a poly (vinylidene fluoride-hexafluoropropylene), (PVDF-HFP) membrane, and the physical-chemical characterisation of these materials was performed, as well as pollutant removal efficiency. An efficient TiO₂ functionalisation with gold nanoparticles was achieved, endowing these particles with the capability to absorb visible radiation absorption. A favourable porous structure was obtained for the membranes, with an average pore size of 4 μm, and the nanoparticles immobilisation did not alter the chemical properties of the polymeric membrane. The produced hybrid materials, including both the Au/TiO₂ and Y₂(CO₃)₃ nanoparticles, presented an efficiency of 57% in the degradation of norfloxacin (5 mg/L) under ultraviolet radiation for 120 min, 80% under visible radiation for 300 min, and 58% in arsenic adsorption for 240 min. These membranes represent a new multifunctional platform for removing several pollutants, which may allow their incorporation in more efficient and less energy-consuming water treatment processes favouring its application, even in low energy resources countries.

Introduction

Human health and well-being strongly depend on the water quality available for consumption (Organisation, 2017). Deterioration of water quality has become one of the most worrying and urgent problems since about 1.6 million people die annually from illness related to unsafe water consumption, responsible for the deaths of at least 3900 children a day in developing countries (Shannon et al., 2008).

Among the causing agents are the so-called contaminants of emerging concern (CEC), released daily into wastewaters in an unmonitored form (Schwarzenbach et al., 2006). Emerging contaminants involve pharmaceuticals (Quesada et al., 2019), personal care products (Ebele et al., 2017), agrochemicals (Evans et al., 2019), industrial products (Deblonde et al., 2011), and many others. Antibiotics are abundantly used in modern medicine to fight bacterial infections and are excreted in an unmetabolised way in 95% of the cases, ending in wastewater (Elmund et al., 1971). Due to rising antibiotics consumption, a gradual increase of bacterial resistance is predicted until 2050, which would cause 10 million deaths per year, reducing the world domestic product by approximately 100 trillion dollars (O'Neill, 2018).

Among CEC, Norfloxacin (NOR) is an antibiotic from the fluoroquinolone group that challenges conventional water treatment systems due to its complexity and chemical stability (de Souza et al., 2018). Its inefficient removal from effluents represents a risk for human health and the environment since this antibiotic can be toxic to plants and aquatic organisms (Shah et al., 2018) and responsible for bacterial genotoxicity (Chen and Chu, 2016).

Further, inorganic pollutants such as heavy metals can be found in abundance in various natural systems due to their excessive use in anthropogenic activities and provenance from natural sources (Siegel, 2002). Among the most dangerous heavy metals for the environment and human health, arsenic is one of the ten pollutants of most significant concern for public health, according to the World Health Organization (WHO) (Organisation, 2018). Arsenic exposure is related to several diseases, including respiratory, neurologic, and mutagenic diseases, bladder, viscus, and kidney cancers, and even death (Qiu et al., 2020). It was estimated that arsenic-contaminated water consumption causes more than 150 million deaths in more than 70 countries (Luan et al., 2019). Due to these severe effects, the WHO has defined 10 μg/L as the maximum concentration level (MCL) for arsenic in drinking water (Qiu et al., 2020).

Various studies on removing pollutants through conventional water treatment techniques reveal their inefficiency in removing the pollutants mentioned above, even responsible for forming several harmful by-products (Quintana et al., 2010). Biological processes (Joss et al., 2006), sand filtration (Stackelberg et al., 2007), coagulation/flocculation (Vieno et al., 2007), sedimentation (Arikan, 2008), and chlorine disinfection (Acero et al., 2010) are some of these techniques. The need for further efficient solutions leads to innovative approaches, including photocatalysis, adsorption and membrane-based technologies (Homem and Santos, 2011).

Photocatalysis has been distinguished by its high degradation efficiencies, non-selectivity, photochemical stability and low implementation costs (Prihod'ko and Soboleva, 2013). Due to its properties, photocatalysis is nowadays the most used technique for the degradation of a wide range of contaminants of emerging concern, including pharmaceuticals (Anucha et al., 2021), organic dyes (Zhao et al., 2022), pesticides (Bose et al., 2021), and personal health care products (Venkatesan Savunthari et al., 2021). To an efficient photocatalytic process, only a radiation source and a photocatalyst are necessary. The search for efficient and cost-effective photocatalysts resulted in a wide range of materials: titanium dioxide (TiO₂) (Mousa et al., 2021), zinc oxide (ZnO) (Leng et al., 2021), tungsten trioxide (WO₃) (Mehta et al., 2021), cadmium sulfide (CdS) and graphitic carbon nitrides (g-C₃N₄) (Mehta et al., 2021), amongst others. TiO₂ is the most used photocatalyst due to its high photocatalytic activity, chemical stability, reduced toxicity and relatively low cost (Zhao et al., 2018; Zheng et al., 2018). TiO₂-based photocatalysts have been widely used for photocatalysis in the removal of bisphenol A (Gao et al., 2010), pesticides (Kadam et al., 2014), pharmaceuticals (Anucha et al., 2021; Aoudjit et al., 2021), including antibiotics (Zhao et al., 2020), and particularly norfloxacin (Katzenberg et al., 2020), presenting efficiencies always close to 100% under UV radiation. Disadvantages such as low photocatalytic activity under visible radiation can be overcome with TiO₂ doping (Nguyen et al., 2018; Garzon-Roman et al., 2019), functionalisation with noble metals (Martins et al., 2020b; Salazar et al., 2020a), and the development of hybrid materials (Cheng et al., 2016; Chen et al., 2019).

The arsenic species removal from natural and drinking water is an environmental remediation issue of high importance, being oxidation (Leupin and Hug, 2005), co-precipitation (Wang et al., 2011), ion exchange (He et al., 2012), membrane filtration (Sabbatini et al., 2010; Yu et al., 2013) and adsorption (Mohan and Pittman, 2007; Zhao et al., 2019) the most commonly used methods. Among them, adsorption arises as an exciting alternative for the remediation of these contaminants due to its high efficiencies, straightforward operation, cost-effectiveness, and lack of sludge production. Its application towards heavy metals (Ren et al., 2021) such as arsenic (Xu et al., 2022), chromium (Ntuli et al., 2021) or lead (Hong et al., 2022) has been the focus of intense research in the scope of inorganic contaminants. Concerning arsenic remediation, adsorption using several materials has been reported (Gupta et al., 2021). Special attention has been devoted to adsorbents synthesised through rare metals such as zirconium, lanthanum and cerium, which have been successful in arsenic removal (Mohan and Pittman, 2007). Among these rare metals, yttrium (Y) has shown great potential in the adsorption of arsenic in suspension (Yu et al., 2019), as demonstrated by the use of different Y-based materials for arsenic removal, for instance, Y/PVA (Yu et al., 2018b), Fe–Y binary oxide (Huang et al., 2020), and Co₃O₄/Y₂O₃ (Pei et al., 2021), achieving adsorption efficiencies higher than 90% under specific experimental conditions.

The combination of photocatalytic and adsorption processes may be achieved by immobilising a photocatalyst and an adsorbent material into a unique matrix such as a polymeric membrane, addressing the removal of CECs and heavy metals simultaneously (Du et al., 2019) or to improve the removal of a specific compound by the combined adsorption/degradation process (Song et al., 2021). The production of polymer-based multifunctional materials for adsorptive and photocatalytic effects on the photoreduction and adsorption of heavy metals (Mulungulungu et al., 2021), as well as adsorption and photocatalytic degradation of organic compounds (Pascariu et al., 2021; Zhang et al., 2021). Alternatively, the application of multifunctional materials for simultaneous organic and inorganic contaminants remediation is still poorly reported (Doagoo et al., 2021). The combination of adsorption, photocatalysis, and membrane filtration is often used to remove inorganic and organic species of a specific heavy metal, such as simultaneous removing As(V) and p- Arsanilic acid (Liu et al., 2020). In the last decades, the use of polymeric substrates such as polyester (Yu et al., 2019), polyamide (Cossich et al., 2015), cellulose (Ortelli et al., 2014), chitosan (Jbeli et al., 2018), and polyvinylidene fluoride, to immobilise active nanoparticles as arisen as a strong possibility.

PVDF and its copolymers are widely used to develop membranes due to their mechanical, chemical, and thermal stability (Liu et al., 2011). PVDF-HFP is a PVDF copolymer, and alongside other PVDF copolymers, it has been used in water remediation for the removal of heavy metals (Salazar et al., 2016), organic matter (Zioui et al., 2019), organic compounds (Zioui et al., 2019), and desalination (Zuo et al., 2013) among others.

This work used the PVDF-HFP as a porous substrate to immobilise photocatalytic and adsorptive nanoparticles that endow this membrane with multifunctional properties. This hybrid material will address the removal of norfloxacin and arsenic contaminants simultaneously.