Application of Nanomaterials in Wastewater Treatment

. Water scarcity is one of the current problems that people need to solve, and wastewater treatment is one of the effective ways to solve it. However, traditional wastewater treatment methods are inefficient, and pollutants cannot be removed entirely. Nanotechnology can effectively address the above issues and its application in wastewater treatment has been researched extensively. This paper introduces nano-adsorbents, nano-photocatalysts and nanomembranes, which are all considered to have a promising prospect in wastewater treatment. Nano-adsorbents are classified as carbon-based nanomaterials, metal-based nanomaterials, nano-polymer and nano-zeolite adsorbents depending on the type of materials, mainly for adsorbing heavy metal ions. Nano-photocatalysts are good at removing bacteria and dyes from water and titanium dioxide (TiO2) is the commonly used material. Nanomembranes are divided into nanofiltration membranes, nanocomposite membranes, nanofiber membranes and aquaporin biomimetic membranes. Metal ions and organic pollutants can be separated by adding different materials into the membrane. The addition of molecular receptors to nanofiber membranes could use in the selective separation of some ions. The recovery of used nano-adsorbents and fouling resistance of nanomembranes are currently the main challenges for nanotechnology in wastewater treatment


Introduction
The growth and continued existence of humans are both dependent on water as an important resource.Water occupies about 70% of area on earth and almost all of it (97.5%) is saline water, and it is difficult to extract clean water resources from saline water.Only 0.3% of the remaining 2.5% is readily accessible; the remainder is typically glaciers and groundwater.The people in some countries still have problems assessing clean water resources.Part of the reason for this is that in developing countries, the technical limitations of wastewater treatment do not allow for the complete removal of impurities, resulting in severe water pollution and threatening the environment and human health.Finding a low-cost and efficient wastewater treatment method is critical for developing countries.
Primary wastewater sources are domestic, agricultural, industrial, and commercial waste, and wastewater generally contains 99.09% water and 0.10% contaminants.Impurities commonly include nutrients, heavy metals, inorganic particles, various microbes, suspended solids, and organic compounds.For instance, heavy metals are present in the wastewater that is discharged by the electroplating industry.These heavy metals include ions of chromium, cadmium, copper, and zinc.Public health and the environment could be threatened if untreated [1,2].
Traditional wastewater treatment is generally divided into three main processes.Screening is used in the first treatment process in order to remove more extensive floating waste, and grit removal is utilised in order to remove inorganic particles.The primary process is responsible for the removal of around 55% of the faecal coliforms, approximately 80%-90% of the suspended particles, and approximately 40% of the biological oxygen demand (BOD) [1].During the secondary treatment, oxidation ditches are used to remove activated sludge from the wastewater that has already been treated in the initial treatment process.The tertiary treatment removes the remaining organic, inorganic and microbial matter from the secondary treatment and disinfects the treated wastewater.Chlorine, sodium hypochlorite and chloramine are usually used.
General wastewater treatment methods are usually done by adsorption, flocculation, oxidation, membrane, filtration, and biological treatments.However, these methods are only the primary wastewater treatment and are ineffective enough to remove contaminants from wastewater entirely [1,3].Matter can be manipulated at the atomic levels via the use of nanotechnology.Nanomaterials have been shown to be effective in removing a variety of inorganic and organic pollutants, biological toxins, and some pathogens that may lead to diseases such as cholera and typhoid.Adsorption, nanomembranes, and photocatalysis are three main categories that include nanotechnology used in wastewater treatment [1].This review described the nanotechnology used in wastewater treatment and some nanomaterials that can be used for wastewater treatment.
Adsorption is the process of molecules or ions from a gas or liquid being attracted by chemical or physical interactions to the surface of an adsorbent.Common nano-adsorbents can be grouped into carbon-based nanoadsorbents, metal-based nano-adsorbents, polymeric nano-adsorbents and zeolites.
Carbon nanotubes (CNTs) are carbon-based nanoadsorbents that are commonly used to investigate new types of nano-adsorbents in wastewater treatment.Carbon nanotubes are giant molecules with a cylindrical structure consisting of hexagonal, hybridised carbon atoms.The high porosity and specific surface area of this structure make carbon nanotubes an excellent choice for wastewater treatment.Depending on whether CNTs are single or multiple sheets, CNTs may be classified as either single-walled CNTs (SWCNTs) or multiplewalled CNTs (MWCNTs).The literature written by Yin et al. indicates that multi-walled CNTs form a high affinity for adsorption with aniline and phenol function groups [4].In contrast, single-walled carbon nanotubes are unable to produce high adsorption affinities.Binti Meera Mydin et al. investigated chromium adsorption using MWCNTs.A chromium removal rate of 99.97% was reported at a pH of 7, using 0.04 g of MWCNTs [5].The significant amount of chromium removal rate suggests that MWCNTs are an effective adsorbent for chromium removal from aqueous solutions [5].Nevertheless, the high cost of CNTs prevents them from being used for large-scale wastewater treatment.Another common nanomaterial, graphene, has a honeycomb arrangement of carbon atoms held together by π -bonds and σ -bonds.The improved Hummer's method is currently the best method for synthesising graphene oxide (GO).The presence of oxygenated functional groups that can be complexed with metal ions.Katubi et al. prepared MnFe2O4/GO nanocomposites by the in situ method and found that this composite showed good adsorption of Pb2+ at pH 6 and a removal rate of 98.8% [6].
The metal-based adsorbent materials are mostly nanoscale metal oxides.The interaction between the metal ions in the wastewater and the metal oxides is the main reason for adsorbing on the metal surface.For instance, nanoscale Fe3O4 is used in the treatment of wastewater to remove metal ions.He et al. combined the nanoscale Fe3O4 with graphene oxide (GO).GO-COOH structures obtained by carboxylation of graphene oxide, and Fe3O4/GO-COOH nano-adsorbent can remove approximately 78.4% of calcium ions and 51% of copper ions in one hour, which can be used to treat oilfield wastewater [7].In the current research, carbon-based and metal-based nano-adsorbents are often combined to form nanocomposite adsorbent materials.Sun et al. used the hydrothermal synthesis method to create carbon cloth covered by porous Li2Si2O5.On the carbon fibres, the Li2Si2O5 is synthesised in the form of nano-brushes.Surface complexation and ion exchanges have been confirmed as the two main adsorption mechanisms.This material exhibited exceptional absorption properties in experiments involving wastewater treatment [8].The carbon cloth covered by porous Li2Si2O5 composite material displayed good adsorption of metal ions in wastewater, with removal rates above 88% and have highest removal rate (98.05%) for Al3+ [8].
Polymeric nano-adsorbents can remove organic pollutants from wastewater and are mostly made of polystyrene or polyacrylate.The large surface area and regenerative properties improve the adsorption efficiency.Dendrimers are a type of artificial polymer commonly used in water treatment.Panahi et al. synthesised carboxyl-terminated hyper branched Polyamidoamine dendrimer grafted superparamagnetic nanoparticles (CT-HPMNPs) as mercury ion adsorbents.The synthesised adsorbents demonstrated a high adsorption capacity of mercury ions, with an adsorption capacity 72.3 mg/g at pH 5 [9].From this investigation, CT-HPMNPs could efficiently remove mercury ions from wastewater.Another biopolymer, chitosan, can be used for wastewater remediation.The low surface area and porosity limit the adsorption ability of chitosan, synthesising chitosan with other carbonaceous materials to form composite nanomaterials could improve the adsorption.Subedi et al. created magnetic chitosan nanocomposites (CS@Fe3O4) and demonstrated efficient adsorption of Cr(VI) with an adsorption capacity was 143.32 mg/g.CS@Fe3O4 has the potential to be used in the process of removing harmful Cr(VI) ions from wastewater, which is one of its possible applications [10].
Due to the low cost and excellent adsorption capacity, many industries use zeolites as molecular sieves, ion exchangers, and catalysts.Natural zeolite is an excellent heavy-metal ion adsorbent.10 g of natural zeolite could remove 99% of copper ions in experiments.Zeolite nanoparticles can absorb heavy metals in wastewater treatment.Isawi et al. produced zeolite / polyvinyl alcohol / sodium alginate (Zeo/PVA/SA) nanocomposite beads by combining zeolite nanoparticles (Zeo) with polyvinyl alcohol (PVA) and sodium alginate (SA).Zeo/PVA/SA has a high rate of removal for numerous heavy metal ions, effectively adsorbing transition metal ions at pH 6 with removal rates above 92%.Fe3+ and Al3+ have removal rates of 96.5% and 94.5% respectively at pH 5 [11].After ten replicate experiments, the adsorption capacity of the material declined and exerted a favourable potential in wastewater treatment [11].

Photocatalysis
Photocatalysis is used in wastewater treatment for the oxidative elimination of micropollutants and microbial pathogens [12].Photocatalysis takes place on the surface of the photocatalyst.TiO2 is a common photocatalyst.When the surface of TiO2 is irradiated with 200-400 nm wavelength light, electrons are photoexcited and move to the conduction band, creating electron-hole pairs.Water reacts with the hole to form hydroxyl radicals and hydroxyl radicals could oxidise organic compounds and dyes.Figure 1 displays the mechanism of this process.Since UV-A radiation is only about 5% of sunlight and causes a very low photon efficiency, activation of TiO2 is usually caused by UV lamps.Patidar et al. put out a proposal that extracts of Moringa oleifera leaves can be used in an environmentally friendly way to produce titanium dioxide nanoparticles [13].They ground the dried leaves into a powder and extracted the leaf extract with ethanol.The leaf extracts were mixed with titanium tetraisopropoxide at 50 °C to produce TiO2 nanoparticles [13,14].This method can reduce the cost and is also an environmentally friendly synthetic method.The literature published by Vanlalhmingmawia et al. mentioned that silver nanoparticles facilitated the electronic excitation of TiO2 and can effectively degrade antibiotics.For example, 63.48% degradation efficiency of amoxicillin was observed by using Ag (3.0 wt%)doped TiO2 [15].In addition, Ag-doped TiO2 had a 53.28% improvement over TiO2 nanoparticles in the degradation of nitrobenzene [15].

Nanomembranes
The use of membrane separation techniques is becoming increasingly popular in the wastewater treatment industry.The primary mechanism is the separation of pollutants in water using different-sized pores in a membrane.Improved water quality, environmental friendliness and low cost make membranes commonly used in wastewater treatment [16].However, the main issue with membranes is the need to consider the balance between membrane permeability and selectivity.Membranes can be generally classified as nanofiltration membranes, nanocomposite membranes, nanofiber membranes and aquaporin biomimetic membranes (ABM).
The primary mechanism of a nanofiltration membrane is nanofiltration.Nanofiltration is a pressuredriven filtration and is a process of membranes between ultrafiltration and reverse osmosis.The pore size of nanofiltration is typically smaller than 1 nanometre and has a high rejection rate for multivalent ions.Carbonbased membranes are considered particularly suitable for desalination because of their high flexibility and electrical conductivity [17].A recent research was written by Xie et al.They prepared a graphene oxide/MIL-88A(Fe) (GO/M88A) membrane by vacuum filtration.GO/M88A exhibited a high separation efficiency for contaminants and a 98.81% degradation efficiency for methylene blue [18].The stable GO membrane demonstrated that incorporating a Fe(III)-based metal-organic framework into GO nanosheets is an effective way to stabilise GO-based membranes.
Membranes known as thin film composites (TFC) are made up of three layers: a polyamide layer, a porous polymer layer that is used for filtering, and a non-woven layer that can be utilised as a support layer.Nanocomposite membranes are formed by incorporating nanoparticles into TFC membranes.Nanoparticles are usually incorporated by coating the membranes with multiple layers of nanoparticles or by blending nanoparticles into a polymer layer.Metal oxide nanoparticles are commonly incorporated into TFC to enhance the hydrophilic properties of the membranes.As polymers are organic and metal oxides are inorganic, this leads to issues with the colloidal aggregation of metal oxide nanoparticles at high concentrations, which can block the pores and reduce the performance of the membrane [19].This is also a concern for metal oxide nanocomposite membranes.In Sahu et al. ' s literature, they state that the incorporation of silver nanoparticles and zinc oxide nanoparticles into membranes could improve the antibacterial characteristics of the membranes, effectively inhibiting the activity of Escherichia coli and Staphylococcus aureus [19].Heavy metal ions are efficiently adsorbable by polyvinylidene difluoride membranes when silicon dioxide nanoparticles are incorporated [19].
Nanofibers are generated by an electric field and electrospinning methods.These fibres are usually incorporated into polymeric membranes to form nanofiber membranes.The high porosity and the ability to adjust the interconnected open pore structure allow nanofiber membranes to separate and filter impurities from wastewater.Recent research by Jo et al. demonstrated a new type of nanofiber membranes in wastewater remediation.Electrospinning was used to produce crown ether/graphene-in-polyethersulfone nanofiber (CGPNF) membranes.The primary mechanism involved the selective adsorption of lithium ions by 12-crown-4 as a molecular receptor and polyethersulfone nanofibers combined with graphene nanosheets.After ten regeneration cycles, the CGPNF membrane still preserved 93% of its original adsorption capacity and the adsorption capacity of the first regeneration cycle was 86.3 mg/g [20].The recovery and treatment of lithium ions from lithium battery waste solutions can be done using CGPNF membrane.They also proposed the integration of molecular receptor/graphene nanosheets into polymer fibres for the recovery and separation of other metal ions [20].
Aquaporin biomimetic membranes (ABM) are biomimetic membranes consisting of aquaporin (AQP) and membranes or vesicles.The main production steps of ABM use a bacterial host to synthesise the AQP, and vesicles are used to reconstruct the AQP and finally the fixation of the protein membrane [21].AQP repels most molecules and ions, and the high water permeability allows ABM to separate water and impurities.ABM is commonly studied in seawater desalination.ABM membranes can also be used in wastewater treatment to remove trace organic pollutants due to their properties, and the high permeability of ABM membranes at low pressures makes them much cheaper to use.In a review by Giwa et al., it was mentioned that ABM membranes achieved 97% removal of trace organic pollutants in a forward osmosis experiment [22].

Conclusion
Nanotechnology is used extensively in wastewater treatment.Adsorption is the treatment of wastewater using the adsorption of pollutants.Depending on the material, they can be divided into carbon-based adsorbents, metal-based adsorbents, polymer adsorbents and zeolites.Depending on the material, they can adsorb different pollutants, and the adsorbent can effectively remove most metal ions.Photocatalysts are used for the oxidative degradation of dyes and bacterial DNA in wastewater by radicalising titanium dioxide, which could produce hydroxyl radicals.Nanomembranes can separate some pollutants by nanofiltration, and hydrophilicity affects their separation efficiency.There are challenges with nanotechnology in wastewater treatment.Nanoscale adsorbents can cause secondary contamination when entering organisms and the environment through air and water.Development for safely recycling used nanoadsorbents or creating biodegradable adsorbents is a significant challenge for nano-adsorbents in wastewater treatment.Fouling on the nanomembranes has always been a challenge for reliable nanomembranes.The fouling may prevent the nanomembranes' pores from functioning properly, which would reduce their ability to perform nanofiltration.Therefore, the impact of fouling and fouling resistance of nanomembranes should be taken into consideration while researching the nanomembranes.