Nanotechnology and Sewage Treatment

INTRODUCTION:

Clean and safe water is a fundamental human need for the development of society and a thriving economy. The enormous rise in population, industrialization, and urbanization have resulted in the generation of polluted wastewater. Millions of people die every year due to diseases communicated through the consumption of contaminated water containing detrimental pathogens. To encounter these issues, several sewage treatment methods such as sedimentation, flocculation, filtration, adsorption, and biological treatment have been applied in the current world scenario. Even though these conventional water treatment processes mainly remove suspended solids, colloidal impurities, and pathogens, it is not ideal for the dissolved organic pollutants in water. Despite the incompatibility of every conventional process in wastewater treatment, the emerging field of Nanotechnology renders a promising technology, which encompasses high reactivity, high adsorption capacity, high surface-to-volume ratio, and ease of functionalization.

CLASSIFICATION OF NANOMATERIALS:

Nanomaterials are classified according to their morphology and physicochemical characteristics. Research analyses classified nanomaterials based on their dimension and composition. The one-dimensional classifications include thin nanofilms, nanolayers, and nanosurfaces. Two-dimensional classifications include nanowires and graphene sheets (rolled into nanotubes) and three-dimensional include fullerenes, dendrimers, and quantum dots. Besides classification, nanomaterials can also be further categorized into single-phase solids (e.g., crystalline, amorphous particles, and layers), multiphase solids (e.g., matrix composites, coated particles), and multiphase systems (e.g., colloids, aerogels, ferrofluids). Further research analyses distinguish nanomaterials into specific material-based categories such as metal-based NMs, metal oxide-based NMs, carbon-based NMs, zeolite and silica-based NMs, ceramic NMs, polymeric NMs and lipid-based NMs.

SYNTHESIS OF NANOMATERIALS:

Nanomaterials are generally synthesized by three distinctive processes namely top-down, bottom-up, and hybrid approaches. The top-down approach preferably describes the synthesis of nanomaterials by destructuring the bulk material over powder to nano-sized shaped materials. This approach is commonly applied to produce structures with long-range orders and make macroscopic connections. Different physical methods such as etching, ball milling, grinding, and Physical Vapor Deposition (PVD) are applicable under this method. The bottom-up approach portrays the self-assembly and molecular patterning by controlling the arrangement atom by atom. This approach is most commonly applied to produce structures with short-range orders. This is mainly based on chemical techniques such as Atomic Layer Deposition (ALD), Molecular Layer Deposition (MLD), sol-gel nanofabrication, and Chemical Vapor Deposition (CVD).

The hybrid approach mainly incorporates both organic and inorganic materials. This approach can rectify technical problems existing with top-down and bottom-up approaches. Additional nanoparticle synthesis techniques include sonochemical processing, thermal plasma processing, cavitation processing, microemulsion processing, and high-energy ball milling.

APPLICATION OF NANOPARTICLES IN WASTE-WATER TREATMENT:

DENDRIMERS IN WATER TREATMENT:

Reverse Osmosis (RO) membranes generally have pore sizes of 0.1-1.0 nm and thus are very effective at retaining dissolved inorganic and organic solutes with molar mass below 1000 Da. However, high pressures are required to operate these membranes. Conversely, Ultrafine (UF) membranes require lower pressure (200-700 kPa) but they are not very effective at removing dissolved organic and inorganic solute with molar mass below 3000 Da. The advent of dendritic polymers in the range of 1-20 nm provides unprecedented opportunities to develop effective ultrafine processes for purification of water contaminated by toxic metal ions, radionuclides, organic and inorganic solutes, bacteria, and viruses. These polymers are symmetrical and spherical macromolecules with hyperbranched polymers which are relatively monodispersed and highly branched consisting of three main components such as core, interior branch cell, terminal branch cell, and tissue silver levels with 10% silver. To obtain a dendrimer structure, several dendrons are reacted with a multifunctional core to yield dendrimers using two key synthetic strategies which include highly functionalized cores and dclickT chemistry. Diallo et al. (2005) tested the feasibility of using dendron-enhanced ultrafiltration (DEUF) and poly (amidoamine) (PAMAM) Dendrimers with Ethylene Diamine (EDA) core and terminal NH2 groups to recover Cu (II) ions from aqueous solutions. The Cu (II) binding capacities of the PAMAM dendrimers showed much larger and more sensitivity to solution pH than those of linear polymers with amine groups. Dendritic polymers have also been successfully used as delivery vehicles for antimicrobial agents such as Ag (I) and quaternary ammonium chlorides. Experiment results reveal the high antimicrobial activity of protected silver and silver compounds without the loss of solubility.

CARBON-BASED NANO-ADSORBENTS:

Despite Activated carbon being used as the most common adsorbent in waste-water treatment due to its high porosity and large surface area, its high-cost confines usage when approached to a larger scale. Therefore different allotropes of carbon such as carbon nanotubes (CNTs), and the C60 family of buckyballs, graphite, and graphene are being examined as nano adsorbents. CNTs are cylindrical, bulky molecules comprised of hybridized carbon atoms in a hexagonal assortment. These CNTs have unique tunable surface chemistry, chemically inert nature, hollow structure, high specific surface area, light mass density, high porosity, and strong interaction when compared to other adsorbents. The investigation of adsorption and desorption of Mn7+ ions revealed that CNTs adsorb Mn7+ efficiently by reducing the concentration from 150 ppm to 3 ppm. Graphene oxide (GO) is a single monomolecular layer of graphite with numerous oxygen-holding functionalities such as hydroxyl, carboxyl, carbonyl, and epoxide groups. GO along with magnetic particles, has gained considerable attention as an adsorbent for wastewater treatment because of its simple design, lower sensitivity, low cost, and ease of operation towards toxic pollutants. Chen et al. envisaged the preparation of aerogel (AG) of GO/Aminated Lignin (GO/AL-AG) for adsorption of malachite green (MG) dye in wastewater. The maximum adsorption capacity and efficiency of prepared GO/AL-AG were found to be 113.5 mg/g and 91.72% under the optimal conditions.

METAL-BASED NANO ADSORBENTS:

Metal-based nano adsorbents including Fe3O4, TiO2, MnO2, MgO, ZnO, and CdO are extensively used to remove heavy metals, radioactive metals, ions, and dyes from wastewater. Their small size and large surface area offer a small intraparticle diffusion distance which can be compressed without altering their surface area. Wastewater coming from oil refineries contains a variety of ions and metals such as calcium (Ca2+) and copper (Cu2+). He et al. prepared reusable nano adsorbents based on Fe3O4/GO-COOH by the magnetization and carboxylation of GO. The nano adsorbents showed 78.4% and 51% per cent removal of Ca2+ and Cu2+ respectively, at 60 min. This system also showed a binding affinity for various cations such as Fe2+, Fe3+, Cd2+, and Pb2+. Jethave and coworkers developed an efficient nano adsorbent that consisted of zinc-aluminium oxide NPs doped with lead (Pb/Zn-AlO/NPs) for adsorption of anionic dyes such as methyl orange (MO). The MO removal efficiency reached 99.60% after 30 min.

NANO FILTERS:

Nanofiltration is a pressure-driven membrane process that operates at a pressure between 5-20 bars with a pore size between 0.5 and 2.0 nm. Nano Filter (NF) lies between ultrafiltration (UF) and Reverse Osmosis (RO) characterized by a high rejection of divalent or higher-valent ions and a low rejection of monovalent ions. Polymeric and ceramic membranes are most commonly employed for these filters. Polymeric membranes end with a short lifetime due to their inferior chemical resistance and a high fouling rate. However, ceramic membranes have higher mechanical, chemical, and thermal stabilities.

PHOTOCATALYSIS:

Photocatalysis generally describes the absorption of light by any photocatalyst (solid material) which induces a chemical reaction. It is one of the Advanced Oxidation Processes (AOPs) involving the production of extremely potent chemical oxidants. The produced hydroxyl radicals are strong enough to oxidize headstrong organic compounds. Photocatalysis has extensively been studied because of its ability to break down an ample range of organic materials, estrogens, dyes, organic acids, pesticides crude oil, microbes, and inorganic molecules.

Photocatalysis is a surface phenomenon and the mechanism undergoes five steps

• Diffusion of pollutants to the surface of the photocatalyst

• Adsorption of pollutants on the surface of the photocatalyst

• Reaction of adsorbed pollutants

• Desorption of products from the surface

• Removal/diffusion of products from the interface

Some most commonly used nanostructured semiconductor photocatalysts are TiO2, Fe2O3, ZnO, zirconium dioxide (ZrO2), zinc sulfide (ZnS), and tungsten trioxide (WO3), and cadmium sulfide (CdS).

MAGNETIC-NANOSPONGES:

Magnetic nanosponges (MNS) derived from advanced scientific studies and macromolecular chemistry renders a crucial role in wastewater treatment. These nanosponges are synthesized by mixing magnetic nanoparticles with natural or synthetic coagulants in an optimal ratio. An external magnetic field applied by these nanosponges removes the turbidity of contaminated water at higher levels. Iron Oxide (Fe3O4) with an average particle size of 300 nm mixed with an alum-based polymeric coagulant shows better results in terms of turbidity removal, Total Organic Carbon content (TOC), Biochemical Oxygen Demand (BOD), and Total Suspended Solids (TSS). Mateus et al. reported that magnetic nanoparticles functionalized with proteins from Moringa oleifera seeds showed 96.8% efficiency (143.33 NTU) in turbidity removal after 10 min of magnetic sedimentation in raw water collected from a Brazilian river basin. Triques et al. performed a similar study wherein they treated effluent from the cleaning-in-place (CIP) of a dairy industry with iron oxide nanoparticles functionalised with the Moringa seed extract and achieved about 90% turbidity removal with a reduced sedimentation time.

- Chriswin Harris