A hydrophilic and antifouling nanofiltration membrane modified by citric acid functionalized tannic acid (CA-f-TA) nanocomposite for dye removal from biologically treated baker's yeast wastewater

Highlights

• Citric acid factionalized tannic acid nanofiller was successfully synthesized using a green and facile method.

• Noticeable impact on membrane structural properties was found by using the citric acid factionalized tannic acid.

• Pure water flux and antifouling properties of the prepared membranes were significantly improved.

• Appreciable dye rejection from biologically treated baker's yeast wastewater was obtained.

• An excellent stability during filtration operation and immersion in water was achieved.

Abstract

The aim of this study was to remove non-biodegradable colored compounds from biologically treated baker's yeast effluent using an antifouling membrane in order to facilitate reuse of the wastewater. To get this purpose, in a facile effort, a new nanofiller, citric acid functionalized tannic acid (CA-f-TA), was synthesized by employing a simple, economical and green method. The produced nanofiller was used in the fabrication of polyethersulfone (PES) nanofiltration (NF) membranes. The influence of CA-f-TA on the cross section morphology, pore size, porosity, water content, hydrophilic properties, and roughness of the modified PES/NF membrane was evaluated. The performance of the fabricated membranes was analyzed as pure water flux (PWF), antifouling and dye rejection capacity. Based on obtained results, incorporation of the CA-f-TA nanofiller resulted in an increase in the PWF from 23.5 to 23.6 and 34.4 kg m⁻² h⁻¹ for the membranes blended with CA-f-TA-1, CA-f-TA-0.5 and CA-f-TA-0.1, respectively, in contrast to 16.4 kg m⁻² h⁻¹ for the reference membrane. The antifouling characteristics of the prepared membranes dramatically raised owing to improvement in smoothness and hydrophilicity of membrane surface. The synthesized NF membrane with 1 wt% CA-f-TA nanofiller had the highest flux recovery ratio (FRR, 93.2%) and dye rejection (95.7% for Direct Red 16 (DR 16) and 97% for biologically treated baker's yeast wastewater), and the lowest irreversible fouling resistance (6.8%). Furthermore, PWF of 34 kg m⁻² h⁻¹, FRR of 97%, TSS and turbidity removal efficiencies of 100%, and COD removal efficiency of 80% were obtained using the 1 wt% CA-f-TA membrane after filtration of a field sample i.e. biologically treated baker's yeast wastewater over a 1-h filtration.

Introduction

In recent decades, the pollution of water resources as well as their scarcity has become a global problem for both marines’ and humans’ life. Hence, development of various technologies to reuse water is the most sustainable approach to protect the water resources. Among the methods employed, mechanical treatment processes such as biological treatment systems [1], [2], [3], [4], advance oxidation processes using photocatlysts [5], [6], [7], coagulation and electro-coagulation processes [8], [9], [10], adsorption [11] and membrane filtration [12] can be mentioned. Besides, the natural treatment processes have been also used as a treatment approach with low energy consumption. However, the natural treatment processes require large area [13], [14].

Industry of baker's yeast production as one of the most significant industrial sectors among food industries annually discharges a large amount of extremely contaminated wastewater into the environment. The concerning challenge accompanied with such a contaminated wastewater is related to dark brown color that cannot be removed completely by employing the sole biological treatment methods. The main cause of the remained color, COD and nitrogen in biologically treated baker's yeast effluent is melanoidins that highly restrict wastewater recovery [15], [16], [17]. The melanoidins are brown-colored polymers with high molecular weights (ranging from 400 to 1800 Da) produced during fermentation process of molasses due to re-polymerization of pigments [18], [19], [20]. Color is considered as a significant quality parameter, restricting extremely broad applications of the biologically treated baker's yeast effluent. Therefore, it is a must that the biological treatment methods are combined with other advanced technologies in order to benefit from synergetic effects of the combined methods to purify and reuse wastewater for various purposes. In this regards, the use of membrane technology as supplemental treatment to remove these colored compounds from biologically treated baker's yeast wastewater is considered to be impactful. The membrane-based technology is introduced as one of the environmentally friendly strategies commonly used in practical applications. Since the membrane-assisted treatment process privileges fantastic merits of energy saving, not having unwanted products, effective cost, and sustainability. However, there are limited reports dealing with the usage of the membrane technologies to remove the colored compounds from biologically treated baker's yeast effluent [16].

In spite of said advantages, there are still inevitable bottlenecks in the acquisition of the existing membranes such as reduction in flux and performance, and the following pollution in rinsing residuals which all are ascribed to membrane fouling [21]. Therefore, in order to overcome these challenges, a great deal of efforts has been in progress through employing new nanomaterials and methods, especially, over a few recent years. In this line, a large number of nanomaterials, for instance, multiwall carbon nanotubes (MWCNTs), graphene oxide (GO), tannic acid (TA) [22] and chitosan [23], [24] have been used to effectively modify the membrane with the goal of betterment in permeate and the flux recovery ratio (FRR) known as an indicator of the fouling level.

Tannic acid (TA) is of increasing scientific interest, and has attracted intense attention as a feasible contender in the field of membrane modification [25], [26]. Furthermore, tannic acid has obtained a potential application in many industries, for example, leather tanning industry due to having anti-enzymatic, astringent, and anti-bacterial features [22]. TA is explored as a natural polyphenol found in green tea, oak, nettle, wood, and berries [22]. Commercially, tannic acid is available in the form of a light yellowish and amorphous powder. Very recently, TA was utilized to fabricate composite nanofiltration (NF) membrane using interfacial polymerization technique by Zhang et al. [22]. In another study, TA has been employed to functionalize graphene oxide (GO) and then its contribution in fabrication of membrane using cross-linking method [27]. In some studies, tannic acid has been utilized to amend the membranes using coating strategy [26], [28]. Moreover, the surface of polyethersulfone (PES) membrane was amended with the help of tannic acid and zwiterionnic polymer in order to enhance the antifouling properties [29].

It seems that the process of the membrane modification is mainly toxic, time-consuming, complex and tedious and fairly high cost, and it might bring about restriction in further development. Hence, more recently, green chemistry as an environmentally friendly approach has stimulated much attention in various fields [30], [31]. Fan et al. also have prepared NF membrane via green coating approach with the aid of tannic acid and iron ions [26].

Citric acid (CA) is an edible and environmentally friendly chemical substance with a large number of carboxyl groups which make it hydrophilic. This substance is cheap and commercially available. Thus, it has been used in many studies as a pore maker and also modifier due to useful properties [29], [32]. Daraei et al. have utilized CA as a monomer to fabricate polycitrate ( PCA) modified MWCNT dendrimer to fabricate the NF membrane [32].

In line with efforts made to recovery and reuse of wastewater using various advanced technologies, we are intended to decolorize biologically treated baker's yeast effluent and simultaneously develop a new cost effective nanofiltration (NF) membrane with appreciable dye retention capacity. To reach these goals, an intriguing and economic citric acid functionalized tannic acid (CA-f-TA) dendrimer was synthesized using a green and simple strategy under mild conditions, and incorporated inside NF polymeric membrane body. According to the authors' information, this study is the first attempt to functionalize tannic acid using citric acid, and subsequent application in the preparation of the NF membrane. Benefiting from abundant active carboxyl groups of citric acid as well as the numerous hydroxyl groups of tannic acid, the resulting CA-f-TA dendrimer possesses a high degree of hydrophilic functional groups. Generally, this work reports the fabrication of the NF nanocomposite membranes using a new CA-f-TA nanofiller, evaluation of physico-chemical characteristics and performance of the prepared membranes to get optimal casting solution composition for final goal of decolonization of biologically treated baker's yeast effluent.