Wastewater Remediation

The compounds found in industrial wastewater typically show high toxicity, and during this way, they need to become a primary environmental concern. Several techniques are applied in industrial effluent remediation. In spite of the efforts, these techniques are yet to be ineffective to treat oily wastewater before it is often discharged safely to the environment. Bioadhesion-inspired surface engineering constructing robust, hydrophilic membranes is important for highly-efficient wastewater remediation. Biochar may be a novel green technology used as an effective adsorbent as it is obtained from renewable resource like microalgal biomass.

The exploration of novel media for environmental remediation, particularly wastewater treatment, maybe a global imperative. Many methods of water treatment such as physical, chemical, electrochemical and biological are discussed and worked around the world. The introduction of nanotechnology into industry may represent a significant advance and zero-valent iron nanoparticles (INPs) have been intensively studied for potential therapeutic applications. Theoretically, the development of INP-containing nano-composites to overcome these issues provides a logical next step to develop nano-materials that are better suited for wide application in the water industry. Other than that Self-propelled nanomotors hold great promise for the development of innovative environmental applications. Newly introduced micromotors with dual functionality for mixing liquids on a microscopic scale and enhancing chemical reactions for the degradation of organic pollutants vastly broaden the range of applications for the environment.

Introduction

With increasing global population, urbanization, industrialization and global climate change, securing access to clean water is a global grand challenge. Concerns have therefore been raised that multifarious chemical-containing effluents might move up the organic phenomenon entering human consumption without our perceived knowledge. Thus, it's imperative that these diverse chemicals be managed meticulously before discharging into watercourses. a good range of technologies has been acknowledged for the removal of effluent from wastewater.  However, these approaches aren't without their disadvantages such as high cost, being chemically intensive, difficult to use and practically challenging ⁽¹⁴⁾.

The phenomenon has caused severe health issues and reduced the supply of clean water. Numerous technologies like adsorption, biological oxidation and chemical oxidation are used for the removal of all types of organic pollutants. the restrictions of conventional methods and materials used to eliminate unwanted and toxic contaminants for water remediation have prompted the use of more effective and sustainable approaches ⁽¹⁾.

Catalytic reduction may be a new and promising water management technique that has been thoroughly researched both at the laboratory- and field-scale. Hence there's a need to design new materials which could play an important role in the development of modern, quick, environmentally safe and cost-effective methods for the identification, degradation, reduction and removal of organic and inorganic toxic pollutants ⁽¹⁴⁾.

Nowadays, many techniques are being employed in oily wastewater treatment, like ultrafiltration, ultrasonic separation, adsorption, and coagulation/flocculation, among others. However, these techniques present some drawbacks, like the large space requirement, low cost and therefore the generation of secondary pollutants. On the opposite hand, membrane processes are considered an appropriate and low-cost alternative to the conventional techniques for treatment of oilfield wastewater, thanks to unique properties such as the simples process, low energy consumption and no phase transformation ⁽⁹⁾.

Role of Functional Nanomaterials

Nanotechnology has gained prominence as an advanced field of science, particularly its propensity to solve various environmental challenges. The use of nanomaterials is advantageous due to the unprecedented properties of nanomaterials including high surface area, high reactivity and strong mechanical properties that have been shown to be highly efficient and effective characteristics for wastewater treatment. The development of various nanomaterials provides the most exciting and encouraging advances based on their size effects to the targeted particulates. Various forms of nanomaterials, single or hybrid, have been widely used for the removal of heavy metals, microorganisms and organic pollutants from wastewater. Particularly, nanomaterials possess unique characteristics, depending on their classification and dimensions, which are beneficial for wastewater treatment. Nanomaterials include carbon-based nanomaterials, metal and metal oxides, metal–organic frameworks as well as hybrid nanomaterials. As defined by the National Nanotechnology Initiative (NNI), nanomaterial is a material with dimensions between 1 and 100 nm. The size effect of nanomaterials is the main contributor to achieving better performance. The size of nanomaterials is closely related to their exceptionally high surface area and surface reactivity. Many of them have shown other interesting properties including superior surface to volume ratio, photocatalytic properties, improved solubility, large surface charge and abundant reactions sites. In the past two decades, exciting achievements have been made in the development of novel nanomaterials for different aspects of wastewater treatment, exploration of new materials, establishment of simple, green or economic synthesis routes, and fabrication of nanocomposite materials. The size of nanomaterials can be affected by several parameters, like the method used for synthesis, temperature, pressure, time, pH and concentration. On the basis of their function, nanomaterials have been synthesized into various dimensions and shapes, including spheres, fibres, tubes, sheets and interconnected architectures. The zero-dimensional (0D) structure is characterized by spherical shape, fibers and tubes are the common shapes of one-dimensional (1D) structures, two-dimensional (2D) structure presents in the form of sheet-like structures and interconnected architectures are normally characterized as three-dimensional (3D) structures. The construction of nanostructure materials with multi-dimensions offers very interesting morphologies, properties and functions. The surface area, adhesion, adsorption, reflectance, and carrier transportation properties have been harnessed for various applications, including wastewater remediation ⁽¹⁾.

TiO₂, silver (Ag) and zinc oxide (ZnO) with their 0D nanostructures are the most popularly investigated metal and metal-oxide in addressing energy and environmental issues. They are notable for their significant antibacterial, antifungal and antiviral activity and are applied as water disinfectants, hence they have been used for self-cleaning surfaces, air and water purification systems and act as photocatalysts in water treatment. In addition, these oxides and their hybrids can perform simultaneous actions in degrading organic moieties and also killing different organisms in a single wastewater treatment.In the field of water purification, along with the development and improvement, the morphological structures of TiO₂ nanoparticles have been widely explored including spherical titania (TNP), titania nanotube (TNT) and titania nanosheets (TNS). TNP usually exhibits high specific surface area, pore volume and pore size ⁽¹⁾.

The ZnO-NPs were synthesized employing biogenic green reduction and precipitation approach. ZnO NPs were prepared using green method from Eucalyptus leaf extract and applied for dye removal. The characterization of ZnO NPs was done using various techniques such as FESEM, XRD, BET, TGA, HRTEM, EDX, and FTIR. Maximum removal was achieved at pH 6.0 and pH 8.0 for Congo Red (CR) and Malachite Green (MG) dyes respectively. Dye adsorption process showed better fit with Langmuir and Temkin isotherm models for CR dye and MG dye respectively. Maximum adsorption capacity of ZnO NPs was 48.3 mg/g for CR dye and 169.5 mg/g for MG dye. The dye adsorption followed pseudo-second order model and values of thermodynamic parameters confirmed that the adsorption process was spontaneous and favourable. Reusability efficiency of the nanoparticle was explored using ethanol and water and it was inferred that ZnO-NPs can be reused for dye removal. Effect of salinity on the removal of CR and MG dyes was also explored and found that presence of salinity in aqueous medium have adverse impact on the dye removal efficiency of ZnO-NPs ⁽¹⁰⁾.

Due to the unique structural properties of most carbon-based nanomaterials, they normally exhibit high specific surface area, area functionalization and chemical stability. Various carbon-based nanomaterials such as graphene oxide (GO), single-walled carbon nanotubes (SWCNT) and multiwalled carbon nanotubes (MWCNT) have been explored as having high-potential for water purification. These allotropes of carbon of cylindrical nanostructure offer frictionless transport of water molecules to enhance water permeability. Moreover, to balance between the permeation and rejection of ions through the CNT, it is possible to modify the membrane to provide an appropriate pore diameter ⁽¹⁾.

MOF has been applied for the purification and separation of contaminated water. Several strategies have been used to functionalize the MOF through pre- or post-synthesis modification to address some of their limitations including limited activity and structural stability. The functionalization of organic ligands can be performed during the MOF synthesis or during the post-synthesis modification through coordinate bonds by using organic groups that can attach to a metal centre. These approaches could fine-tune the pore dimensions and functional properties of the MOF to improve catalytic activity, leaching of the functional site and its stability. The integration of MOF with zeolite has been developed to provide structural stability by incorporating inorganic zeolites into the nanocomposite material. MIL-101 is an example that has been used in treating wastewater and in photocatalysis due to its high specific surface area, large pore volume and uniform pores. This catalyst also exhibits excellent catalytic recyclability and stability for the treatment of organic pollutants. MIL-101 is stable in polar and nonpolar solvents at high temperatures ⁽¹⁾. 

Nowadays, hybrid nanomaterials are almost everywhere because they have a wide range of applications such as in polymer nanocomposites, health and the environment. The integration of different kinds of materials could result in a new multifunctional hybrid and allow the combination of the advantages of each into one composite material. These materials possess extraordinary physical and chemical properties derived from their size in nanoscale. Hybrid nanoparticles not only have the characteristics of both nanomaterials, but also have unique properties that surpass those for the original components ⁽¹⁾. The incorporation of molybdenum into expanded starch was evidenced initially by the appearance of the bluish colour characteristic of MoO₃ nanoparticles. In the presence of starch, the H⁺ and OH⁻ ions present in the water molecules bind strongly to nuclei (MoO₆, OH⁻ and NH₄⁺) and help enhance the surface coverage. The hydroxyl ions aid the hydrolysis reaction and enhance the dissolution–crystallization process, providing ample time for collision and for the diffusion of ions at the crystal surface. This helps in rearrangement and steric shielding of nucleation, which in turn helps in the growth of nanoparticles ⁽¹⁴⁾. 

Nanomaterials have an important role in photolysis for water and wastewater treatment processes. Photocatalytic degradation is an advanced oxidation process (AOP) using ultraviolet (UV) light as an energy source. Irradiation by UV light generates the highly reactive hydroxyl radical (˙OH) for the degradation of recalcitrant chemicals present in wastewater. Photocatalysis possesses several advantages over other AOPs including Fenton and photon-Fenton catalytic reactions and hydrogen peroxide: it is able to operate under ambient conditions, and uses inexpensive and non-toxic photocatalysts as well as atmospheric air as an oxidant. Photocatalysts used for wastewater treatment can be classified into solubilized/suspended photocatalyst particles and photocatalyst particles immobilized in/on a membrane ⁽¹⁾.

Nano adsorbents exhibit advantages including high adsorption capacity, cost effectiveness, environmental non-toxicity, ease of separation and robustness and reusability. Usually, nanosized particles are more efficient than their larger-sized counterpart. Generally, the mechanism of adsorption includes the diffusion to adsorbent surface, migrate into pores of adsorbent and monolayer build-up of adsorbate. The mechanism of adsorption relies on the surface features, electrochemical potential and the ion-exchange process. For example, the charged contaminants tend to adsorb via electrostatic attraction on adsorbents with an oppositely charged surface. The adsorbents with mesoporous structures and larger surface area show high adsorption capacities and kinetics. Metal and metal-oxide nanoparticles play an important role in the efficiency of contaminant adsorption ⁽¹⁾. 

Disinfectants are the chemical agents that are used for inactivating and destroying microorganisms on inert surfaces, especially in the wastewater purification. Disinfection commonly causes cell wall corrosion, interfering with cell permeability, the protoplasm or enzyme activity, thus disturbing the activity of microorganisms so they can no longer multiply, causing them to die. However, disinfection also faces the issue of the harmful disinfection by-products (DBP) of conventional chemical disinfectants, including chlorine, chloramines and ozone, that can react with various wastewater materials to form DBP. The use of nanomaterials can get rid of this issue as they can act without creating DBP. The most common inorganic antimicrobial nanomaterials are Ag, TiO₂ and ZnO nanoparticles ⁽¹⁾.

Membrane separation processes have become an emerging technology in the field of water and wastewater treatment due to their efficiency in contaminant removal. The structure of the membrane is one of the main factors responsible for determining the separation characteristics and transport mechanisms across the membrane. This technology has been demonstrated to be more effective not only in removal efficiency, but also because of its smaller footprint, and ease of installation, operation and scaling up. Great efforts have been made to develop and enhance membrane filtration through the modification of membrane surface properties. In general, membrane separation depends on absorption, sieving, and electrostatic phenomena. These mechanisms correspond with the solute and membrane hydrophilic/hydrophobic behaviour. The incorporation of functional nanomaterials offers a great diversity of membranes with different levels of rejection, mechanical strength and fouling liability. Various nanomaterials with different dimensions have been considered as modifiers to improve selectivity, strength, antifouling and permeability by enhancing the hydrophilicity of the polymeric membrane ⁽¹⁾.

Dairy wastewater remediation

Metagenomic analyses and microscopic images of the consortium revealed the presence of Chlorella variabilis, Parachlorella kessleri, Thermosynechococcus elongatus, Chlamydomonas, Phaeodactylum tricornutum, Oscillatoriales, Synechocystissp., Microcystis aeruginosa, Nostocales, Naviculales, Stramenopiles, other members of Chlorophyceae, Trebouxiophyceae, and Chroococcales along with implicit bacterial bioremediants.  There was up to 93 % and 87.2 % reduction in chemical oxygen demand (COD) and ammonium concentration, independently. Further, nearly 100 % removal of nitrates and phosphates from the dairy wastewater upon 48 h of treatment with polyculture under ambient temperature (25 ± 2 °C) with 6309 lux illumination and mild aeration, was observed. Interestingly, the nutrient and COD concentrations in the treated water were below the discharge norms as per Central Pollution Control Board (CPCB) standards. In addition, biomass (reported as dry cell weight) was enhanced by 67 % upon treatment with ammonia-rich dairy wastewater laying out 42 % lipid, 55 % carbohydrate, and 18.6 % protein content improvement. The polyculture substantially grown as attached biofilm to the surface offered easy harvesting and separation of grown biomass from the treated wastewater. Overall, dairy wastewater was set up to be an implicit nutrient source for microalgae- bacteria culture thereby making the treatment process sustainable and eco-friendly ⁽²⁾.

Role of Hydrophilic membranes

The membrane separation system can explosively advance the energy-effective water remediation due to its low cost and small footmark. Still, the built-in hydrophobicity of utmost polymeric membrane materials limits the water- treatment effectiveness, which urgently requires to develop surface hydrophilization engineering on hydrophobic polymeric membranes. Inspired by the structure/ function characteristics of robust mussel/ tunicate adhesive proteins, a one-step, eco-friendly, and cost-effective biomimetic coating system was designed and employed to realize membrane surface hydrophilization. The as-sheeted poly (vinylidene fluoride) micro-/ultra-filtration (MF/UF) membranes displayed greatly enhanced wettability, good harsh condition forbearance, high filtration effectiveness, and excellent fouling resistance. The coating procedure is ecofriendly, cost-effective, one- step, and universal. Both of emulsified oleaginous and simulated protein wastewater were remediated consequently. The optimal MF membrane demonstrated oil rejection effectiveness beyond ca. 99.5% during the cycling emulsion filtration tests. Biomimetic hydrophilic membranes show excellent submerged oil- repellent capability. The covered UF membrane held a benign protein interception effectiveness (BSA rejection of ca. 95.4% with water flux of ca. 400.7 L m⁻² h⁻¹) and greatly enhanced fouling resistance (78.0% increase). The new biomimetic synergistic coating strategy can promote the speedy development of advanced membranes via elegant surface manipulation towards water-energy nexus ⁽³⁾.

Role of Microalgal-based biochar

The high output rate of biomasses produced are appealing for conversion into volatile biochar. Torrefaction, pyrolysis and hydrothermal carbonization are the recommended thermochemical conversion methodologies that could produce microalgal- grounded biochar with desirable physiochemical properties similar to high surface area and hole volume, abundant surface functional groups, as well as functionality similar as high adsorption capacity. The characterizations of the biochar significantly impact the mechanisms in the adsorption of impurities from wastewater. Microalgal biomass is suitable for its conversion into biochar with volatile uses, videlicet fertilizer, and adsorbent. Microalgae live in fine particle form owing to their cellular structure, and their biomasses primarily contain carbohydrates, proteins, fats, ash, water, and particularly contain little or no lignin. The low lignin content gives them benefit over lignocellulosic biomasses as they bear simpler pre-treatment compared to the lignocellulosic biomasses with high lignin content and fibrous structure. The general properties of a biochar include insolubility, stability, porosity, aromaticity, and so on that could promote their adsorbability to remove a wide range of adulterants from wastewater. Biochars made from microalgae typically have lower surface area than lignocellulosic biomass, still it could be bettered by adding pyrolytic temperature. Plus, microalgal biochars generally have further nitrogen and oxygen content, as well as lower carbon content compared to lignocellulosic biochars. This gives them high O/C proportion and low C/N proportion, which are associated with surface hydrophilicity and hydrophobicity independently. High O/C proportion contributes to good adsorption of heavy metals, whereas low C/N proportion favours the adsorption of inorganic adulterants. Microalgal- grounded biochars are low in carbon content, surface area, and cation exchange capacity (CEC), but high in pH, nutrient content (e.g. nitrogen), ash, and inorganic elements. The main adsorptive mechanisms of a biochar include physical sorption, electrostatic interaction, surface complexation, co-precipitation, ionic exchange, deterioration or oxidation, π- π interactions and so on. Still, they're dependent on the characteristics of adulterants and surface chemistry of biochar ⁽⁴⁾.

Role of Chemically activated carbon 

Chemical activation is known to induce specific surface features of porosity and functionality which play a definite part in enhancing the adsorptive capability of the developed activated carbons. Different conditions of temperature, time, reagent type and impregnation rate were applied on sawdust precursor and their effect on the physical, surface chemical features and eventually on the adsorption capability of the developed activated carbons were analysed. Under activation conditions of 600 °C, 1 hr, 1: 0.5 rate, ZnCl₂ saturated carbon (CASD_ZnCl₂) resulted in microporosity while KOH impregnation (CASD_KOH) yielded a carbon having a wider pore size distribution. The surface chemistry revealed analogous functionalities. At same pH, temperature and adsorbate concentrations, CASD_KOH demonstrated better adsorption capability (1.06 mmoles/g for Cd²⁺ and 1.61 mmoles/g for Ni²⁺) in comparison to CASD_ZnCl₂ (0.23 mmoles/g and 0.33 mmoles/g for Cd²⁺ and Ni²⁺ independently). Other features were a short equilibrium time of 60 mins and an adsorbent dosage of 0.2 g/L for the CASD_KOH in comparison to CASD_ZnCl₂ (equilibrium time of 150 min and dose of 0.5 g/L). The results reveal the capability of chemical activation so as to achieve the best physico-chemical properties suited for energy effective, economizing and eco-friendly water treatment ⁽⁵⁾.

Macroporous salecan polysaccharide-based adsorbents

A new salecan polysaccharide- based biosorbent was designed for discarding of Cd²⁺ ions from aqueous solutions. Adsorption of Cd²⁺ onto the salecan biosorbent was evaluated taking into account salecan quantity, sorbent dose, solution pH, initial Cd²⁺ concentration and contact time.  ^-SO3H,^- OH and permeable structure eased the Cd(II) discarding. Pseudo-second-order kinetic model and Weber–Morris intra-particle diffusion model well fitted the kinetic results, suggesting chemisorption and intra-particle diffusion as the most probable adsorption technique. Meanwhile, the equilibrium adsorption data was nicely described by Langmuir isotherm model with a maximum adsorption capacity of 170.1 mg Cd²⁺ per gram of sample. The adsorption matched the Freundlich isotherm model. Eventually, the salecan biosorbent displayed an excellent reusability and 89.2% of the original sorption capability remained after 6 cycles. The salecan-based biosorbents have potential for operation as a wastewater remediation device ⁽⁶⁾.

Bacterially-augmented floating treatment

Floating treatment wetlands (FTWs) is an effective and low- cost technology that uses the synergistic relation between plant roots and microbes for in situ remediation of wastewater. Plant species affect the effectiveness of the developed floating treatment wetlands. The bacteria exposed continuity and activity of the wetlands. Hydrocarbon degeneracy was connected with bacterial survival and activity. FTW-based remediation of oil field- produced wastewater using an relation between two plant species, Typha domingensis and Leptochloa fusca, in cooperation with an association of crude oil-degrading bacterial species, Bacillus subtilis LORI66, Klebsiella sp. LCRI87, Acinetobacter junii TYRH47, and Acinetobacter sp. BRSI56 reduced adulterant levels, but T. domingensis, in combination with bacterial inoculation, displayed the upmost reduction in hydrocarbon (95%), COD (90%), and BOD content (93%) as compared to L. fusca. This effect was further signified by the continuity of bacteria (40%) and considerable abundance (27%) and expression (28.5%) of the alkB gene in the rhizoplane of T. domingensis in comparison to that of L. fusca. T. domingensis, in combination with bacterial association, has significant potentiality for treatment of oil field-produced water and can be exploited on large scale in FTWs ⁽⁷⁾.

Micropollutants cometabolism of microalgae

Adding the sole carbon sources in the presence of MPs increased EPS and superoxide dismutase and peroxidase enzyme concentrations from 2 to 100-fold in comparison with only sole carbon sources. This verified that MPs cometabolism had happened. The discarding efficacy of tetracycline, sulfamethoxazole, and bisphenol A ranged from 16-99%, 32–92%, and 58–99%, independently. By raising EPS and enzyme activity, the MPs concentrations accumulated in microalgae cells also fell 400-fold. Metabolites of BPA were detected. A straight chain product ( m/z 253) was found and didn't exist in Eawag database. The cometabolism process resulted in several degeneration products of MPs. This study drew a perceptive understanding of cometabolism for MPs remediation in wastewater. Based on the results, proper carbon sources for microalgae can be opted for practical operations to remediate MPs in wastewater while concurrently recovering biomass for several industries and gaining profit ⁽⁸⁾.

Role of Polymer-Based Membranes

Membrane technology is an appealing approach to treating oily wastewater. This is devoted to the immobilisation of TiO₂ nanoparticles on polyvinylidene fluoride–trifluoro ethylene (PVDF-TrFE) permeable matrix by solvent casting. Membranes with interconnected pores with an average diameter of 60 µm and a contact angle of 97 °, decorated with TiO₂ nanoparticles, are acquired. The degeneration of oily wastewater demonstrated the high photocatalytic effectiveness of the nanocomposite membranes Under sun irradiation for seven hours, colourless water was acquired. moreover, the hydrophobicity of polymeric membranes promotes their impurity in the filtration process, especially in oil/water separation, leading to the reduction of water flux and rejection effectiveness. Membranes based on PVDF have been investigated for the discarding of different adulterants from water, similar as copper ions, natural organic matter, proteins, organic compounds, volatile organic compounds and desalination, among others. The photocatalytic assays allowed complete disposal of the COD (99.6%), total hydrocarbons (99.6%), and phosphates (100%), as well as a significant reduction of the suspended solids (76%), turbidity (97%), TOC (98%), nitrates (69%), and chloride (26%) from oily wastewater. The pH range that allows better disposal efficacy ranges between 4 and 5.5. Of all the PVDF copolymers, poly vinylidene fluoride- trifluoroethylene (PVDF-TrFE) presents suitable physicochemical properties for photocatalytic operations, similar as a high UV resistance, mechanical and chemical resistance and hydrolytic and thermal stability. This polymer also allows for the yield of membranes with controlled porosity and pore size. More recently, a titanium dioxide (TiO₂) / PVDF-TrFE nanocomposite showed a remarkable solar photocatalytic activity in tartrazine degeneration—degeneration of 78% under five hours of sun irradiation. As a catalyst, titanium dioxide (TiO₂) is the most extensively used photocatalyst for degeneration of organic compounds due to its properties, similar as non-toxicity, low cost and plenty, physical and chemical stability, superhydrophilicity, and superior photocatalytic activity under UV radiation (λ<390 nm).

The incorporation of TiO₂ into polymeric membranes solves some disadvantages related to its use in suspension, namely the recovery of the nanoparticles. An effective option to applying photocatalytic membranes are the photocatalytic membrane reactors (PMRs). PMRs have shown a great capability as a “zero” waste process for oily wastewater treatment since they can reduce the loss of catalytic nanoparticles, control the contact time between catalyst and contaminant, and guarantee a nonstop process. PMRs can enhance the process efficacy and stability, and reusability of the nanocomposite to reduce the operating cost. Therefore, TiO₂ nanoparticles immobilised on a PVDF–TrFE permeable membrane is an effective technique for the degeneration of oil in wastewater in a solar photoreactor, making use of the UV radiation (3–5%) present in the solar radiation ⁽⁹⁾.

An environmentally friendly, low cost, self-cleaning, largely stable and multifunctional TiO₂ enhanced Sodium alginate composite (SAT) aerogel for oil- water separation is developed via a simple combined ionic cross-linking and freeze drying technique. The SAT aerogel is of three- dimensional permeable microstructure with uniformly dispersed TiO₂ nanoparticles (NPs) on alginate matrix. The hydrophilic alginate matrix combined with this unique morphostructure will assure the SAT aerogels with underwater oleophobicity and subsequent excellent oil/ water separation efficacy (up to 99.7%). Also, the firmly attached TiO₂ NPs can also endow the aerogel with better UV-aging resistance, photoinduced self-cleaning and environmental remediation performance. The SAT aerogel exhibits better oil/water separation reusability (at least 60 cycles) compared to SA aerogel and excellent photocatalytic performance for methyl orange (MO) degeneration (>85% after 6 times repeated use). The “green” facile fabrication process, excellent oil/water separation, photoinduced self-cleaning and reusable performances make the SAT aerogel promising for practical operation in aquatic surroundings. This may open up new avenues for designing versatile aerogel for wastewater remediation ⁽¹⁵⁾.

Pullulan-derived nanocomposite hydrogels

Overall, raising the chain length resulted in a lower pore size and stronger mechanical strength. Tetramethylene glycol diglycidyl ether (longest chain length)-derived Gel- T, which had the best performance (acceptable permeable structure, good swelling capability and strong rigidness), was used to produce a nanocomposite hydrogel with montmorillonite (MMT). The incorporation of MMT led to a decline in gel swelling and an increase in gel strength. The acquired nanocomposite system displayed excellent adsorption properties (80 mg/g) towards crystal violet, and the adsorption behaviours were well represented by the pseudo-second-order kinetic and Langmuir isotherm models. Altogether, this provides a better understanding of the structure- function connections of pullulan-constructed hydrogel materials and will help to design more practical adsorbents for dye disposal ⁽¹¹⁾.

Human-Hair-Derived N, S-Doped Porous Carbon

Nitrogen and sulfur codoped carbon (NSC) is fabricated with extremely high specific surface areas (BET surface areas of 3015 m²/g) by employing human hair as the precursor. The graphitization degree, N/S doping content, and pore structures of NSC can be readily modulated by the heat treatment conditions. Furthermore, NSC acquired at 800 °C (NSC-800) indeed outperforms maximum traditional metal-based catalysts under the same conditions for BPA degeneration. Influences of various operating parameters (NSC, Oxone, and BPA concentration) and the inorganic salts humic acid (HA) on BPA degeneration are systematically studied. besides, ¹O₂ should be the main active species generated from PMS activation, and the contribution of •OH/ SO4•^– is actually limited. This is hoped to arouse further interests about biomass- derived carbon for PMS activation and environmental remediation ⁽¹²⁾.

Cr (vi) wastewater remediation

Bismuth oxide (Bi_xO_y), an important bismuth-based semiconductor with visible-light response, good structural stability and non-toxicity, has been identified as a potential seeker for chromium (Cr(VI))-containing wastewater remediation. Constructing a heterostructure is demonstrated to be an effective strategy for enhancing the catalytic performance of Bi_xO_y - based photocatalysts due to the enhanced photoresponse and promoted interfacial carrier mobility. Herein, CdS nanoparticles with low work function and good visible-light response were chosen as the cocatalyst to construct the p–n type Bi_xO_y / CdS heterostructure via a solvothermal technique. Due to the formation of typical II-type induced method, the interfacial carrier mobility and broadband light response were effectively enhanced; thereby, remarkably enhanced simulated solar-light (SSL)-induced aqueous Cr (VI) reduction activity (0.1265 min⁻¹) was achieved by the optimum Bi_xO_y / CdS catalyst in the absence of scavengers, which is ∼34.2 - fold higher than pristine Bi_xO_y (3.7 × 10⁻³ min⁻¹) and possesses distinct advantage compared with former reports. This provides a viable strategy for aqueous Cr (VI) pollution remediation ⁽¹³⁾.

Role of a novel molybdenum oxide–Starbon catalyst

In situ green synthesis, characterisation and operation of a new Starbon™ compound comprising molybdenum oxide nanoparticles are reported. Starbons™ are carbonaceous mesoporous materials derived from starch with operations ranging from chromatography to gas captive. The molybdenum (Mo) loading, confirmed by inductively coupled plasma-mass spectrometry (ICP- MS), was 179.337 mg g⁻¹, and the molybdenum oxide nanoparticles were observed. The Mo-containing compound was an effective catalyst for the reduction of 4- nitrophenol (4-NP) to 4- aminophenol (4-AP) in the presence of sodium borohydride, NaBH4 (k = 11.2 × 10⁻² min⁻¹). The Mo-compound showed superior 2,2- diphenyl-1-picrylhydrazyl (DPPH) radical quenching activity with a low inhibitory concentration (IC₅₀ = 1.006 mg ml⁻¹) and ferric reducing power compared with other green synthesised composites and nanoparticles. The new Mo-containing Starbon™ compound has real time operations in water treatment similar as in catalysis, adsorption and filtration. Starbons™ possess high surface area, large accessible space, electrical conductivity, thermal stability, scalability, low density and interlinked hierarchical porosity of different length scales, which are actually favorable for adsorption, photocatalysis, separation, and energy storehouse and conversion. Combining the built-in properties of Starbon™ (mesoporous and high surface area) with the excellent properties of MoO₃ nanoparticles, this new compound is explored as a catalyst for the abatement of 4-nitrophenol (4-AP), a common water contaminant and a known carcinogen, teratogen and mutagen. Additionally, the radical quenching activity of the synthesized compound is assessed via DPPH and FRAP assays. The structure–property relationship, morphology, composition, and surface functionality with respect to catalytic and radical quenching activity are also developed ⁽¹⁴⁾.

Scenedesmus obliquus mediated brewery wastewater remediation

Microalgae can be used for wastewater bioremediation with concurrent CO2 biofixation producing precious biomass. Scenedesmus obliquus is a versatile, truly robust and fast-growing organism that can be well cultivated in different wastewaters and environmental conditions. The biomass of Scenedesmus also proved to be a good feedstock for several of the bioenergy vectors, similar as biodiesel, bioethanol, biohydrogen, biogas and bio-oil. The PBRs were fed with ambient air and the effect of a 10% (v/v) brewery CO2 supplement was studied. The maximum ash-free dry weight (AFDW) biomass productivity was acquired for a HRT of 3.5 days (0.29 d⁻¹ dilution rate; 0.2 g L⁻¹ d⁻¹ (in terms of AFDW). The topmost contaminant discarding efficacy were 92.9% and 88.5% for ammonia and total nitrogen, independently, 40.8% for phosphorus, and 61.9% for COD. Aiming to concurrently maximize biomass volumetric productivity, CO2 biofixation rate and wastewater treatment effectiveness, while minimizing residence time, 0.29 d⁻¹ represents the optimal dilution rate value. The capability of the produced Scenedesmus obliquus biomass was estimated for the generation of biohydrogen through dark fermentation with Enterobacter aerogenes, and of bio-oil, bio-char and bio-gas through a pyrolysis process. The yields acquired were 67.1 mL H₂ g⁻¹ (in terms of volatile solids-VS) for bioH₂ and 64%, 30% and 6% for bio-oil, bio-char and bio-gas, independently (dry mass content (%) calculated over freeze dryer biomass basis) ⁽¹⁶⁾.

 Zero-valent Iron nanoparticles

Zero-valent iron has been reported as a successful remediation agent for environmental issues, being widely used in groundwater treatment. The use of zero-valency nanoparticles has arisen as a highly effective method due to the high specific surface area of zero-valency nanoparticles ⁽¹⁷⁾. Recently, zero-valent metal nanoparticles, such as Fe, Zn, Al, and Ni, are gaining wide research interest in water treatment. Nanozero-valent aluminum is thermodynamically unstable in water due to its high reductive capabilities, which leads to the formation of oxides/hydroxides on the surface, which completely interferes with electron transfer from the metal surface to the contaminants. The standard reduction potential of Ni is less negative than that of Fe, indicating a lower reduction potential, whereas the nano-zero-valent Fe or Zn has a moderate standard reduction potential and an ideal reducing agent relative to most redox-labile contaminants. While Fe has a weak reduction ability, it has many effects on water pollution and is an excellent adsorbent, precipitate and oxidized (if oxygen is present), and is relatively inexpensive. So far, zerovalent iron nanoparticles have been subjected to the most extensive study among zerovalent metal nanoparticles ⁽¹⁸⁾.

Nanotechnology in wastewater treatment

Nanotechnology, the manipulation of matter at the molecular or atomic level to prepare new structures, devices, and systems with improved electronic, optical, magnetic, conductive and mechanical properties, is emerging as a promising technology, leading to a variety of fields. There has been a remarkable development. Did the feat Sectors including waste water treatment ⁽¹⁹⁾. Conventional water treatment is not always very effective in removing contaminants such as metals and micro-organisms. Another problem is the creation of disinfection byproducts (DPBs), which can harm human health. DPBs are formed when chemical disinfectants react with organic matter and inorganic ions in water. Few studies have studied the removal of metals, microbes and oil from contaminated water using nanomaterials. Nanomaterials have been used as a substitute to remove contaminants. The nanometer scale is used to study the phenomenon, in nanoscience. Nanotechnology includes materials with at least one component whose dimensions are less than 100 nm. These materials differ greatly from conventional materials in terms of mechanical, electrical, optical and magnetic properties due to their nanoscale size ⁽¹⁸⁾. Nanotechnology-based routes that are being employed for wastewater treatment are adsorption and biosorption, nanofiltration, photocatalysis, disinfection and disease control, sensing and monitoring, etc ⁽¹⁹⁾.

Adsorption is an exothermic process and a surface phenomenon that involves the transfer of a phase (a molecule or ion present in either a liquid or gaseous bulk), called adsorbate, onto a solid, rarely liquid surface called an absorbant to form a monomolecular layer on the surface through physicochemical or chemical interactions under specific conditions. Biosorption is a type of adsorption in which biological substances such as certain types of bacteria, algae or fungi act as adsorbents, because of their intrinsic properties to bind and cleave heavy metals, even in a very dilute aqueous. via mediated by solution or metabolically (by making use of ATP) or by spontaneous physicochemical pathways of uptake (not at the expense of ATP). The process of bioabsorption mainly involves microprecipitation, ion exchange, and cell-surface complexation ⁽¹⁹⁾.

Wastewater remediation mediated by nanomotors

Recent advances in environmental studies with micromotors that combine motion and wastewater treatment remain the need for reusable and multifunctional micro and nanomotors with extended lifetimes that can efficiently degrade a variety of pollutants. Artificial nanomotors can sense a variety of analyzes – and therefore contaminants, or "chemical hazards" – can be used for testing water quality, selective removal of oil and changes in their speed, which can be used to determine the presence of certain substances in solution in which they swim. The newly introduced micromotors with dual functionality for mixing liquids on a micro-scale and enhancing chemical reactions for the degradation of organic pollutants vastly broaden the range of applications for the environment. The novelty of the work lies in the coordination between the internal and external functionality of the micromotors. Notably, these multifunctional microtubules use hydrogen peroxide as a fuel for motor motion, which is produced by the generation of oxygen bubbles in the inner Pt layer, while serving as an active material for the useful function of the outer Fe wall. The mechanism of degradation is based on the Fenton reaction which relies on spontaneous acidic degradation of the ferrous metal surface of micromotors in the presence of H₂O₂, as a reagent for the Fenton reaction and as a fuel to induce micromotors. acts as. These findings proved that the advantage of the newly designed micromotors is increased catalytic reactions, by virtue of their ability to confer due to their speed. The significance of these findings opens the way to creating autonomous micro-cleaning systems that can operate without external energy input and much faster than their stationary counterparts. Micromotors provide the ability to move catalysts (ions) without external activation or additional catalysts (iron salts) to achieve water treatment, organic dyes removal, etc. Remarkably, even in the absence of a surfactant oxidation is achieved dual functionality of Fe/Pt microtubular motors. Micromotors can still self-propel without surfactant, though at a slower rate. Nevertheless, their active motion enhances the degradation of model pollutants such as rhodamine 6G ⁽²⁰⁾.

Redox processes in wastewater remediation

Reduction also plays an important role in the destruction or stabilization of various water contaminants. The redox conditions of water can be manipulated by adding liquid or gaseous reductants, or reduced colloids. The use of redox processes in water treatment technologies has not been properly reviewed. Many wastewater treatment technologies, such as ultrasonication, bioremediation, electrokinetics and nanotechnology, are closely related to redox processes. Redox processes control the chemical specificity, bioavailability, toxicity, mobility and adsorption of water pollutants in the environment ⁽²¹⁾. Various treatment techniques are used to treat different types of water pollutants, but none of them can effectively treat water with high concentrations of organic contaminants. Furthermore, these existing technologies generate harmful side products that require further treatment ⁽²²⁾. Several soluble reductants such as sulfate, thiosulfate, hydroxylamine and dithionite have been studied at bench scale under anoxic conditions. Dithionite has been found to be most efficient. The gaseous reductant that has been tested is hydrogen sulfide, and the colloidal reductants are clay-supported Fe(0) and Fe(II) ⁽²¹⁾. Catalyzed redox processes can convert water pollutants into environmentally acceptable end-products (ie, CO₂, NH₃ and H₂O) and are also a green alternative to water treatment ⁽²²⁾.

Conclusion

Rapid urbanization, population growth and climate disruption have resulted in increasing demand and a lack of clean water that have become unique global issues. Globally, water purification is a major issue for human use, ecosystem management, agriculture and industry. The water sanitation process using nanoparticles is quite effective ⁽²³⁾. Nanotechnology should be used as a complementary technology among other means for wastewater treatment. The best way to use nanomaterials in this process is by incorporation or coating them with membranes and composites. When comparing nanofilters with conventional systems, nanofilters have the following major advantages: less pressure is required to pass water across the filter, which means that it dramatically reduces operating costs, they are more are efficient, and they have vast surface areas that can be easily cleaned by back flushing compared to traditional methods ⁽¹⁸⁾. Current technologies are insufficient to meet the demand in augmenting processes reported in the treatment of large amounts of polluted water, and much more work needs to be done. Several issues need to be resolved before taking a step towards practical application. For example, the lifetime of multifunctional micromotors is limited by the remaining materials in its body that were involved/consumed in oxidation or locomotion reactions (Fe, Mg). Other drawbacks may be the toxicity of the platinum layer due to the presence of compounds in the wastewater that can bind chemically to the active surface sites of the catalyst, or the high viscosity of the treated wastewater which can impede the movement of micromotors ⁽²⁰⁾. Thus, there is a long way to study and understand the appropriate methods of combining diverse treatment systems to reach several best treatment processes whereby humans can save water on Earth for more years for the next generations ⁽¹⁸⁾.

The use of nanomaterials in addressing issues related to the environment is arising due to their fascinating features that satisfy the desired conditions. The constant fresh water demand has motivated the constant development and advancement of nanotechnology in various forms in wastewater treatment. To date, nanoscience and nanotechnology developments have opened new openings for the advancement of water treatment. Thus, advancements in nanomaterial properties are constantly being made to meet the high demand from experimenters, industry and policy makers. Furthermore, particular emphasis is placed on the construction of nano-structures with versatile dimensions and morphologies, and in particular their yield routes, in order to enrich their properties and functions as well as to assure environmental sustainability. In particular, there are still doubts about the fate of nanomaterials as well as managerial challenges with regard to the environment. Green nanotechnology is generally related to the formation of green nano-products and use of these products to achieve sustainable development ⁽¹⁾.

Biochars have been an option with great adsorption activity that could reduce or replace the use of coal-based carbons in wastewater treatment process. Compared to the non-renewable triggered carbons, biochars are fairly renewable, stable, environmentally sustainable and cost-effective due to lower yield cost. The renewable resources could be readily acquired from a wide range of raw biomasses, similar as agrarian wastes. Biochars also retain parallel or better adsorptive capacity than the triggered carbon in removing of organic and inorganic adulterants from aqueous solution or wastewater effectively. Occasionally, biochar need not to be subjected to additional physical or chemical activation, therefore making it a more accessible choice. Presently, the low- cost, environmentally friendly biochar technology has been intensely explored for its application in wastewater remediation ⁽⁴⁾. Developing effective and “green” approaches to handle aquatic pollution problems is presently one of the most important exploration subjects of environmental remediation ⁽¹⁵⁾.

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Author Details

• Atindra Karar^*, Akanksha Bharati^**.

• ^*Semester I Postgraduate student, Department of Zoology, Ramakrishna Mission Vivekananda Centenary College (Autonomous and Affiliated to West Bengal State University).

• ^** Semester I Postgraduate student, Department of Zoology, Dinabandhu Andrews College (Under University of Calcutta)