Recent progress and future directions of membranes green polymers for oily wastewater treatment

Ag NPs

Silver nanoparticles

Alg

Alginate

ALG

Sodium alginate

CA

Cellulose acetate

CaCO₃

Calcium carbonate

CM

Cellulose membrane

CMC

Carboxymethylcellulose

CNF

Cellulose nanofiber

CNTs

Carbon nanotubes

CS

Chitosan

CST

Chitosan–TiO₂

CT

Chitin

DMAc

Dimethylacetamide

DPA

Dopamine

DSD

Droplet size distribution

ER

Epoxy resin

Fe₃O₄

Iron oxide

FeCl₃

Ferric chloride

FP

Filter paper

GA

Glutaraldehyde

GO

Graphene oxide

HDTMS

Hexadecyltrimethoxysilane

LDH

Layered double hydroxides

MCA

Modified cellulose acetate

MCE

Mixed cellulose membrane

MCS

Modified chitosan

MF

Microfiltration

MTS

Methyltrichlorosilane

NaCl

Sodium chloride

NaClO₂

Sodium chlorite

NaOH

Sodium hydroxide

NBKP

Cellulose needle-leaf bleached kraft pulp

N-CQDs

Amino-functionalized carbon quantum dots

NF

Nanofiltration

OCA

Oil contact angle

PAN

Polyacrylonitrile

PBE

Propylene-based elastomer

PBS

Poly (butylene succinate)

PCL

Poly (ε-caprolactone)

PDA

Polydopamine

PDLA

Poly (D-lactic acid)

PEG

Poly (ethylene glycol)

PET

Polyethylene terephthalate

PHAs

Polyhydroxyalkanoates

PHB

Polyhydroxybutyrate

PLA

Poly lactic acid

PLGA

Poly (lactide-co-glycolide)

PPC

Polypropylene carbonate

PPF

Poly (propylene fumarate)

PPS

Polyphenylene sulfide

PPy

Polypyrrole

PS

Polystyrene

PSf

Polysulfone

PVA

Polyvinylpyrrolidone

PVDF

Polyvinylidene fluoride

PVDF-HFP

Poly (vinylidene fluoride-co-hexafluoropro-pylene)

RHs

Relative humidity

RO

Reverse osmosis

SeP

Sepiolite

SiO₂

Silicon dioxide

SMPLA

Superhydrophobic and magnetic poly (lactic acid)

SNPs

Silica nanoparticles

SOCM

Superhydrophobic cellulose membrane

TA

Tannic acid

TEOS

Tetraethyl orthosilicate

TiO₂

Titanium dioxide

UF

Ultrafiltration

UOCA

Underwater oil contact angle

USEPA

United States Environmental Protection Agency

WCA

Water contact angle

WO3

Tungsten oxide

XPS

X-ray photoelectron spectrometer

ZIF-8

Zeolite imidazole framework

Oily wastewater refers to wastewater that contains fats, greases, and oils in high concentrations, together with a variety of dissolved components (organic and/or inorganic) (Adetunji & Olaniran 2018; Wei et al. 2020). There are four types of oil-contaminated water (also known as ‘oily wastewater’): free oil, dispersed oil, emulsified oil, and dissolved oil (Saththasivam et al. 2016). These types are based on physical and chemical properties, with the droplet size distribution (DSD) being a major parameter (El-Naas et al. 2014). Free oil separates from water with ease and floats to the surface in calm conditions due to its lower specific gravity than water. The size of oil droplets in wastewater ranges between micron-sized particles and larger droplets (Al-Ani et al. 2020). The DSD of dispersed oil is normally in the range of a hundred to several hundred microns, and it is characterized such that it takes the form of droplets of oil in water (Li et al. 2015). While oil–water mixtures move through pipes or pumps, oiled wastewater or natural occurrences like agitation and wave action or operations which break up free oil droplets and distribute them generate dispersed oils. (Abdulredha et al. 2020). Emulsions can be formed by stabilizing the oil droplets in the dispersion or by coalescing them to produce free oil over time. Either mechanical or chemical stabilized emulsions are oil emulsions generated by dispersing oil droplets in water (Nikovska 2012; Loh et al. 2020). Conditions and relative proportions of oil and water components can produce various emulsions, including simple emulsions like o/w and w/o, as well as more complex emulsions like o/w/o. Simple emulsifiers, like oil–water and water–oil, are more widely used in petroleum, drug, and food industries than complex emulsions (Haghighat et al. 2020). There are no droplets of oil in the water when the oil dissolves in the water. Hydrocarbons that have one or more rings (benzene, for example), as well as aromatic hydrocarbons (naphthalene and organic acids) are the most common components of dissolved oil (Fakhru'l-Razi et al. 2009).

The United States Environmental Protection Agency (USEPA) classifies (oil-contaminated) wastewater as among the most toxic and harmful wastewaters in the environment. When oily wastewater is incorrectly released, it pollutes the air and contaminates groundwater and surface waters, posing a serious threat to the surrounding environment and human health (Poulton et al. 2002; Robertson et al. 2007; Hejnfelt & Angelidaki 2009; Ibrahim et al. 2016; Mearns et al. 2020; Yazdan et al. 2020).

Though it's possible to completely clean out the oil from the water, this is made more difficult by oil–water emulsions, which are more stable because of various components in the oil or the water that act as emulsion binders such as asphalting and resin, as well as clay particles, mineral salts and so on (Victorovna 2001). To address this issue, a variety of ways have been developed to break down oil–water emulsions: biological and chemical as well as mechanical (settling tanks, cyclones, centrifuges, and separators), thermal, and electrical, microwave, ultrasonic, and membrane separation techniques (Padaki et al. 2015). To separate oil droplets smaller than 10 m, membrane filtration is a promising mechanism (Tanudjaja et al. 2019). As a result, ultrafiltration (UF) and tighter microfiltration (MF) are the better options for separating oil droplets in the 1–10 μm size range from water (Ghadhban et al. 2020; Lee & Patel 2020; Yahya et al. 2021).

Because of ongoing research and development in both academia and the private sector, the membrane technology market has experienced probably much higher growth. It is also becoming increasingly popular as a separation and purification technique because of the advantages of membrane technology, including its low energy consumption, high efficiency, simplicity of continuous operation, and little environmental impact (Lee & Darling 2016). Wastewater treatment, desalination, food processing, chemicals, and pharmaceuticals have all used membrane separation technology effectively for several decades, showing both its technical and economic virtues (Fane et al. 2015; Abbas et al. 2022).

Polymer membranes are semi-permeable polymer filter media being used pressure-driven treated water. Polyvinylidene fluoride, polysulfone, polycarbonate, polyethersulfone, cellulose acetate, and polyamide are all examples of polymeric materials that can be used in the manufacturing of plastic products. Additionally, in polymeric membranes, the pore size of the membrane can be easily controlled during the manufacturing of the membrane. The installation necessitates a high degree of flexibility and a smaller area (Alsalhy et al. 2018). The membrane separation industry prefers polymeric membranes because of their low cost and wide range of potential uses. Polymer membranes, which are also known as organic membranes, are commonly referred to as such. Polymer membranes are part of a larger family of membranes. Polymer membranes can be replaced by ceramic membranes, which are also called inorganic membranes (Chen et al. 2016; Zhu et al. 2016; Chen et al. 2019; Zhong et al. 2021a).

Enhancing the sustainability of membrane production is an important goal for the field of membrane technology. Despite the many potential uses for membrane technology, our focus was on polymeric membranes and their effect on solvents over the long term. Numerous of the research included in this research concentrated on identifying greener alternatives. The term ‘green’ membrane encompasses two distinct but related concepts. However, the use of renewable resources in place of oil-derived plastics is a good option. On the other side, other chemicals like modifiers, solvents, and additives used in the membrane manufacturing process should present the least degree of hazard as feasible and should be sourced from renewable sources, non-toxic to the environment, and reusable from waste (Capello et al. 2007; Figoli et al. 2014; Hadi et al. 2020).

However, the manufacturing of the membrane itself should not include the use of hazardous chemicals. The benefits of a greener and environmental preparation method are immeasurable; therefore, an excellent toxicity profile and specific physical properties, like solubility in water, a low molecular weight, and a high boiling point make this alternative to hazardous solvents the best solution for replacing them without affecting membrane performance (Al-Ani et al. 2019; Kadhim et al. 2020).

Previous reviews highlighted a general topic on membrane synthesis, preparation, and modification, as they looked at membrane formation, installation techniques, fouling membranes, or manufacturing procedures (Jiang et al. 2021; Mamah et al. 2021; Shi et al. 2021; Singh et al. 2021). Without the specialization and focus in studying of the green membranes and their positive influence on the environment on the one hand, and the possibility of their recycling and the economic value of preparation and applications on the other hand (Wang et al. 2017; Xiong et al. 2019; Bandehali et al. 2021). The aim of this review is to illustrate researchers' and scientists' efforts to develop and apply greener synthesizing pathways, as well as chemical processes which progressively have been more sustainable and greener in order to avoid ecological impacts. In addition, give some suggestions, prior strategies, and future studies of prospects for achieving significant enhancements in membrane development. There is also research on the use of membranes in wastewater treatment and oil–water emulsion separation and a variety of membrane-modification techniques are investigated.

Synthesis of a green biodegradable membrane for use in a variety of separation processes is an eco-friendly approach that shows a new method of thinking in chemistry that aims to get rid of toxic waste, decrease energy consumption, and make use of environmental solvents. Effective separations of emulsions are a growing issue due to the increasing amount of oily wastewater and the oil spills caused by the sinking of oil tankers, industrial accidents and other ships. Many studies in recent years have concentrated on creating various membranes to improve the mechanical separation of emulsions. Several polymers were used to prepare membranes for o/w separation. In response to the growing demand for sustainable and cost-effective membrane production methods, many studies have focused on creating and synthesizing a membrane from green basic material (Figoli et al. 2014). As a consequence, bioplastics, such as bio-based and petroleum-based biodegradable polymers, gained a lot of interest in recent years (Hamad et al. 2014). Bio-based and petroleum-based polymers refer to bioplastics derived from both biological and fossil sources. There are two types of biopolymers: those that degrade and those that don't degrade. Biodegradable bioplastics are those that can be broken down by microorganisms in the natural environment. Polymers that degrade in the environment come in three varieties: plants, animals, and synthetically created. Polylactic acid (PLA), poly (lactide-co-glycolide) (PLGA), cellulose acetate (CA), polyhydroxyalkanoates (PHAs), collagen and sericin, chitosan/chitin (CS/CT), poly (vinyl alcohol) (PVA), lignin, alginate (Alg), starch, poly (butylene succinate) (PBS), poly (propylene fumarate) (PPF), poly (ε-caprolactone) (PCL), and other infrequently utilized polymeric materials polymers that degrade in the environment. PHAs, PLA, PLGA, lignin, starch, Alg, and CA acetate are all polymers derived from plants or vegetables. Collagen and sericin are both derived from animals. Chitosan/chitin may be acquired from both plant and animal sources. PBS, PPF, and PCL are synthetic biodegradable polymers. Table 1 provides some advantages and disadvantages of the investigated green material with other options. It has been shown that the use of these substances is increasing at a rate of roughly 10% to 20% every year (Kumar & Maiti 2016; Chang et al. 2021). Since their first use in the packaging sector, the uses of biodegradable polymers have been expanded to include a variety of industries, including membrane separation technologies (Safi et al. 2020; Rashid et al. 2022). In nature, polymers can be either biodegradable or not; this implies that several natural polymers might not have been biodegradable. It is critical that plastic supply be renewable, both environmentally and economically, as well as that its disposal in the natural environment be done without concern or made of recycled (Credou & Berthelot 2014; Lee et al. 2021). Lastly, a great number of used membranes ought to be recycled and reused again. This review therefore investigates the membrane industry's transformation requirements in the context of recycling and reuse.

PLA is a biodegradable polymer generated from renewable sources such as sugarcane, corn, and cellulose. It has biodegradable thermoplastic with good biocompatibility and absorbability. PLA has been used mostly in clinical uses such as implant devices, tissues scaffold, and internal sutures up until the last decade due to its high cost, scarce availability, and restricted molecular weight (Galiano et al. 2019). New methods for producing high-molecular-weight PLA polymers at a lower cost have emerged, expanding the material's potential applications (Huang et al. 2016). As a biodegradable and renewable resource, PLA has been proposed as a reasonable solution to society's mounting solid waste disposal issues (Ummartyotin & Pechyen 2016). PLA is a member of a class of aliphatic polyesters that are generated from hydroxy acids. Lactic acid (2-hydroxy propionic acid), a PLA building block, can be found in both optically activated- and l-enantiomers. PLA with varying material qualities can be made based on the number of enantiomers. This enables the manufacture of a broad range of PLA polymers to meet a variety of performance needs. In comparison to existing petroleum-based polymers, PLA possesses reasonable physical, optical, barrier and mechanical properties (Naffakh et al. 2014).

A biodegradable PLA-based hierarchical functionalized electrospun nanofibrous membrane was successfully fabricated. Nanoscale Ag-NPs on the PLA nanofibers were used to create the hierarchical rough structure by adhering them to the PDA layer by (Liu & Yuan 2018). Fluorinated thiols were then added to the surface to create the super hydrophobic/superoleophilic properties. The super hydrophobicity (θ water = 158.6°) of the produced nanofibrous membrane was found to be stable in a variety of solutions, including alkali, salt, and acid. As a result, the membranes were shown to have a high penetration flux (2,664 Lm⁻²h⁻¹) for a wide range of oils and an acceptable separation performance (more than 95%) separation efficiency for both various oil–water solutions and surfactant stabilized water–oil emulsions). This nanofiber's hydrophobicity (θ water = 142.3°) and hierarchical rough structure were able to hold up after 20 times of repeated use due to the durability of the Ag NPs fixed on the nanofibers and show better recyclability.

Table 2 summarizes the evolution of modern PLA membrane types, where a good rejection rate of more than 90% can be achieved due to an improved membrane structure. Such as improving contact angle, porosity, and pore size, this is done by improving the membrane characteristics by adding the appropriate additives to the polymer casting solution through the membrane preparation.

As the second most prevalent biopolymer in nature, chitosan (CS) can be observed in shells of crustaceans and insect pests, molluscan organs, and fungal structures. In most cases, it's produced by chitin deacetylation (CT) (Anitha et al. 2014; Antony et al. 2019; Liu et al. 2019b). Chitosan is a biopolymer with strong hydrophilicity and antibacterial characteristics that is affordable and harmless. (Abu-Saied et al. 2017). Co-polymerization of β (1, 4)-linked 2-amino-deoxy-D-glucopyranose and N acetylglucosamine results in its chemical structure. The hydroxyl and amino groups (which provide electrons) are both present. Despite this, it is soluble in dilute organic acids such as formic, acetic, and lactic acids and insoluble in a variety of organic solvents, water, and alkalis. With only a modest amount of acid, chitosan can achieve high levels of water solubility because of the presence of the acid-soluble polar ether (COC), hydroxyl (OH), and amino (NH₂) groups (Ma & Sahai 2013; Cao et al. 2018). Because of the availability of amino groups and hydroxyl groups, CS-based materials have been extensively explored for decontamination of minerals from effluents, and they are widely employed in the pharmaceutical, chemical, and paper industries, and protection of the environment sectors. Many chemical alterations have been made to chitosan in order to increase its adsorption capacity (Losev et al. 2017; Jiang et al. 2018; Behr & Ganesan 2022).

Below we will highlight the different types of chitosan membrane fabrication methods used. Some typical examples of chitosan membranes for water–oil separation applications are summarized in Table 3. With a detailed illustration of the prepared membrane characteristics and the membrane performance.

CA is a polymer that is often used in membrane production and has been the subject of much research. CA can be used to create membranes for microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO), and it is typically employed in dialysis applications (Sivakumar et al. 2006; Iwata 2015; Kargarzadeh et al. 2018). CA is biodegradable and sustainable because it is made from cellulose, a naturally occurring substance. Because of its insolubility, cellulose must be treated with acetic anhydride and acetic acid to produce CA (Gopinath et al. 2018). However, it has many disadvantages, including low mechanical strength, thermal resistance, and chemical resistance. As a result, the addition of additives or surface modifications is frequently required to enhance the properties of CA membranes (Alcheikhhamdon & Hoorfar 2017; Mohamed et al. 2017).

What's more, for different purposes, cellulose-based materials (such as filter paper and membranes) can also be chemically changed. Hydrophobic nanoparticles can be applied to filter paper by spin-coating, dipping, or spraying the paper with a solution, emulsion, or suspension (Li et al. 2008; Wang et al. 2008; Huang et al. 2012; Yetisen et al. 2013).

Table 4 summarizes information on the cellulose acetate membrane preparation. It is illustrating the recent development of the performance of the cellulose acetate membranes for water/oil emulsion separation via improving the performance of synthesized membranes, which can be achieved by improving the characteristics of the prepared membranes by using some modern techniques to improve the polymer casting solution.

Green membranes were harnessed to separate oil–water emulsion mixtures, selectively and efficiently, based on their unique structural properties, distinct production methods, and the materials used to enhance their defining characteristics are reviewed in this article. Because membrane oil–water separation/rejection performance is strongly influenced by membrane morphology, several eco-friendly preparations and design options were explored in depth. According to a comprehensive evaluation of membrane preparation processes, several synthesizing active membrane capabilities can be identified. The ecological environment faces a serious threat from oil spills and industrial contamination. Filtration improvements for oily wastewater have been given more attention due to safety concerns. Membrane separation technology, with its low energy consumption, provides a promising and effective choice for treating wastewater. However, finding low-cost, strong, and environmentally friendly composite membranes capable of providing a high level of separation remains a challenge. Most authors agree that there is an urgent, ongoing need to prioritize the use of green composite membrane for oil–water emulsion separation for the reasons stated above. Oily wastewater is a byproduct of several industrial processes and is becoming a growing environmental issue. As a result, researchers are looking for easy, cheap, environmentally friendly, and easily scalable methods of manufacturing novel materials that can efficiently separate oil–water at low cost.

The support of Department of Chemical Engineering, University of Technology Technology Baghdad, Iraq is gratefully acknowledged.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.