l-Histidine doped-TiO2-CdS nanocomposite blended UF membranes with photocatalytic and self-cleaning properties for remediation of effluent from a local waste stabilization pond (WSP) under visible light
Introduction
Biological treatment processes have been applied to treat various wastewaters with different compositions (Pirsaheb et al., 2015; Amini et al., 2013; Asadi et al., 2012; Pirsaheb et al., 2009; Zinatizadeh, 2006). Waste stabilization pond (WSP) as a natural treatment system is one of the oldest and most common types of technologies employed to remediate domestic wastewater all over the world (Verbyla and Mihelcic, 2015). This technology is considered for small cities and rural regions, especially once the effluent is land-applied (Oakley, 2005). In these natural treatment systems, balanced coexistence of microalgae and bacteria as well as sunlight irradiation are responsible for assimilation of nutrients and degradation of organic matters. Such systems don’t need aeration since microalgaes themselves provide the required oxygen via photosynthesis reactions. Totally, the natural treatment systems of wastewater are cost effective. However, sometimes the seasonal changes disrupt the balance between population of algaes and bacteria. So that appearance of green color in the WSP effluents originated from high population of algaes can restrict their reuse, specially once a high quality effluent is needed (Torrens et al., 2009). It is noted that one key feature of the WSPs is their relatively long retention time. Therefore, the hydrodynamic behavior and velocity profile in different parts of the ponds should be considered as an effective factor on the system performance (Mansouri et al., 2012; Bonakdari and Zinatizadeh, 2011). Mentioned shortcomings have persuaded many communities to either switch to other treatment systems or upgrade WSPs via post treatment systems in order to enhance the quality of the WSP effluent.
Currently, a lot of strategies have been employed to decrease the production of wastewater and also upgrade treated wastewater quality (Rahimi et al., 2016; Ghasemi et al., 2016a, b; Sharafi et al., 2015; Birjandi et al., 2013). The membrane based processes present significant features such as simplicity in operation, reduction in energy consumption, and high separation efficacy compared to other conventional separation processes (Cassano et al., 2015; Guo et al., 2012).
Among the membrane processes, ultrafiltration (UF) membrane is frequently used for water purification due to its superior advantages such as good stability, high separation efficiency, low operating pressure and temperature (Aliane et al., 2001). It is well documented that UF membranes with high efficiency must have had high flux, high rejection rate, and excellent antifouling properties, which are mainly reflected by pore structure and surface characteristics of the respective membranes (Zhang et al., 2013). Therefore, to raise broad applications of the membranes, it is essential to promote membranes efficiency with purpose of high permeation and antifouling capability. Blending approach with nanofillers has been proved to be a promising, simple, and nondestructive method to prepare nanocomposite UF membranes in comparison to chemical modification (Ma et al., 2017a,b).
Titanium dioxide (TiO2) photocatalyst known as an environmentally friendly and efficient photocatalyst is widely employed to remediate various organic contaminants from water and wastewater (Damodar et al., 2012; Ghasemi et al., 2016a; Zangeneh et al., 2016). Nevertheless, there are two serious limitations for further application of this type of photocatalyst ascribed to rapid recombination of electron (e−) and hole (h+), which subsequently gives rise to low quantum efficiency (Ma et al., 2017a,b; Zhang et al., 2013). Furthermore, TiO2 photocatalyst is able to degrade the potential contaminants under UVA light irradiation due to high band gap energy which makes photocatalysis processes costly (Zangeneh et al., 2015). In light of current studies, some efforts have been directed towards modifying TiO2 photocatalyst by doping method with different metals and nonmetals to shorten the band gap energy from UV light to the visible light region (Zangeneh et al., 2019b). The doping method with nonmetals can be more effective compared to the use of metals. As nonmetals introduce additional energy states into semiconductor photocatalyst of TiO2, thereby, reduce the recombination phenomena (Taufik et al., 2017). Moreover, it is established the use of multiple nonmetals as dopant has indicated much more efficient effect on the photocatalytic capacity than that of single dopant (Jabbari et al., 2016). Despite appreciable advantages of multiple nonmetals in improvement of photocatalytic properties, few studies have been reported in this regards. For example, some researchers improved TiO2 photocatalyst using doping of triple nonmetals of C, N and S with the aid of l-cysteine (Jabbari et al., 2016). Based on the literature review, the combination of TiO2 and CdS photocatalysts doped by C nonmetal is recognized as a potential strategy to utilize visible light in photocatalytic activity via reduction of recombination phenomenon (Lavand et al., 2015). Lavand and co-workers utilized the referenced-above photocatalyst to degrade methylene blue (MB) (Lavand et al., 2015). In this line of studies, most recently, research group of Zangeneh et al. synthesized TiO2-CdS photocatalyst doped by C, N nonmetals using l-Histidine as doping agent (Zangeneh et al., 2019b). The authors employed the synthesized nanocomposite to remediate methyl orange (MO) and biologically treated palm oil mill effluent (POME) under visible light irradiation. Also, in another study, the authors synthesized dual TiO2-ZnO photocatalyst doped by triple nonmetals of C, N and S with the aid of l-Histidine as dopant agent (Zangeneh et al., 2019a). The resultant nanocomposite has then been incorporated into the polyethersulphone (PES) matrix to fabricate photocatalytic and self-antifouling nanofiltration (NF) membrane. The fabricated membranes were applied to purify POME under continuous visible light irradiation. Ma et al. fabricated self-cleaning and UV shielding nanofibrous membranes successfully (Ma et al., 2019a). The synthesized membranes showed the great performance in separation of oil and water mixture. In a study conducted by Lv et al., nanofiber membranes prepared based on ZnO loaded poly(vinyl alcohol) (PVA) and konjac glucomannan (KGM) were employed to decolorize methyl orange under solar irradiation (Lv et al., 2019).
To the best of our knowledge, this is the first report regarding the contribution of C, N doped TiO2-CdS photocatalyst into matrix of the polyethersulphone (PES) polymer to fabricate the ultrafiltration (UF) mixed matrix membranes (MMMs) with photocatalytic and self-antifouling properties. In present study, the mixed matrix membranes were prepared with the aim of remediation of effluent from a local waste stabilization pond (WSP). Firstly, the prepared membranes were characterized through cross sectional morphology (SEM), atomic force analysis (AFM) images, and water contact angle (WCA) measurements. Then, the membranes performance was investigated via the pure water flux (PWF) measurement and antifouling tests in dynamic mode using a dead end set up to determine optimum membrane.
Finally, photocatalytic performance of the optimum membrane with the relation to PWF, flux recovery ratio (FRR), dye rejection capacity and remediation of organic matter stated as chemical oxygen demand (COD) index was pursued and compared to the control membrane during a long term operation in a cross flow set up. The experiments were carried out at three different levels of two variables i.e. working pressure (P) and cross flow rate (Q). The experiments design was done through a central composite design (CCD) of response methodology surface (RSM) using Design Expert software.
Section snippets
Materials and reagents
Tetra-n-butylorthotitanate (TBOT, C16H36O4Ti, 98.0 %), ethanol (C2H5OH, 99.0 %), acetic acid (CH3COOH, 99.8 %), N, N-dimethyl acetamide (DMAc) and all chemicals required for the measure of chemical oxygen demand (COD) (i.e. H2SO4, 98 %; Cr2O7, HgSO4, and AgSO4) were provided from Merck Co., Germany. Cadmium acetate dihydrate (Cd(CH3COO)2.2H2O, 99.9 %), sodium sulfide (Na2S, 99.9 %), l-Histidine (C6H9N3O2, 99 % and PolyN-vinylpyrrolidone PVP, MW of 29,000 g/mol were supplied by Sigma-Alderich
Characterization of the prepared nanocomposite
From FT-IR spectrum of l-Histidine doped TiO2-CdS presented in Fig. 1a, the peaks of 400−800 cm−1 are corresponded to the vibration of TiO and TiOTi bonds. While, the observed peaks at 1630 and 3100–3500 cm−1 are assigned to the stretching and bending vibration of hydroxyl groups of the water molecules adsorbed on the relevant photocatalyst surface (Rafiee et al., 2016; Raizada et al., 2014). Also, the peak appeared in 1154 cm−1 implies to band of Cd-S (Kim et al., 2011). The XRD pattern of C, N
Conclusions
In this research, C, N doped TiO2-CdS photocatalytic nanocomposite was successfully fabricated and employed to prepare PES mixed matrix UF membranes by conventional wet method. The modified membranes revealed the higher pure water flux and better antifouling characteristics than those of the control PES membrane owing to the increase in hydrophilicity and smoothness of the membrane surface. According to the results, the M-0.5 membrane with higher hydrophilicity indicated the highest pure water
Declaration of Competing Interest
None.
Acknowledgments
The authors would like to acknowledge Kermanshah’s Water and Wastewater Company, Iran for the financial support provided for this research work. The authors would like to appreciate Razi University, Kermanshah, Iran, to provide the required equipment.
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