Many nano-materials are described and have repeatedly demonstrated laboratory-scale efficiency and their usability in water security and water reusability applications.
Advancements in water and wastewater treatment technologies is mainly driven by efficiency to achieve desired result coupled with economy and maintenance cost. At one side complexity in water and wastewater increased due to various anthropogenic activity and process innovations for manufacturing and parallel treatment technologies also improved. In some sectors like sewage treatment and lean pollution potential effluent, membrane technology is widely recognised.
Many types of membrane techniques are used such as Microfiltration, Ultrafiltration, Nanofiltration, Reverse Osmosis, etc. Membrane filtration and its advancements based on application of physical forces like pressure, osmosis, heat, electricity and magnetism has been successful in wastewater reclamation and resource recovery throughout the world. At many places conventional techniques is either supplemented with membrane technology or replaced by membrane based techniques due to better energy and output efficiency. Microfiltration, ultra-filtration and reverse osmosis are more mature methods, but the recent development in the area is inclination towards nano-filtration system for water treatment. Nano-filter has a pore size range of 0.001-0.01μm.
Nano-filter membranes can filter some salts, synthetic dyes and sugars but, unable to remove most aqueous salts and metallic ions hence, the wide acceptance is limited by customised need-based usability of nano-filtration system. Apart from nano-filters, nanoparticles also has potential to contribute for wastewater treatment and resource recovery thus, application of nano-technology for the water and wastewater treatment and resource recovery may be the new direction for customised need based solutions.
“Nano” is typically defined as materials one billionth of a meter (10-9) and even smaller. Nano-particles are those particles having size of 1nm to 100nm.
Various laboratories world over have synthesized nanomaterials in last 40 years for various applications. During the previous decade (2010-2020), the application of nano technology touched several areas like cosmetics, medical science & biologicals, electronic and magnetic materials, material processing, light emitting techniques, etc. Various laboratories synthesized several nanomaterials with known mechanical, electrical, optical, and magnetic properties available, which are different from conventional materials and has potential for various usability. A wide range of nanomaterials have the characteristics of catalysis, adsorption and high reactivity.
This property of nano-materials is used in the field of water and wastewater treatment through various research. Potential capabilities of nanoparticles, nanomembrane and nano-powder are successfully demonstrated for detection and removal of chemical and biological substances including metals (e.g. Cadmium, copper, lead, mercury, nickel, zinc), nutrients (e.g. Phosphate, ammonia, nitrate and nitrite), cyanide, organics, algae (e.g. cyanobacterial toxins) viruses, bacteria, parasites and antibiotics.
At least four classes of nano-materials are being evaluated as functional materials for water purification e.g. carbonaceous nanomaterials, metal-containing nanoparticles, zeolites and dendrimers. Carbon nanotubes and nanofibers have also shown positive results. Nanomaterials reveal good result than other techniques used in water treatment because of its high surface area (surface/volume ratio). Nanotechnology-based methods are generally believed to be more expensive, we found research papers demonstrating it as cheaper and more effective alternatives to conventional techniques. In addition, nano-based techniques may become extremely important in meeting increasingly stringent water quality standards, especially for removal of emerging pollutants and low levels of contaminants. The present article highlights some of the application of nanomaterials for wastewater treatment.
2.0 Nano-particles for Wastewater Treatment
A number of nano-particles are synthesized throughout the world and successfully applied for water and wastewater treatment in customised ways. Few of the synthesized nano-particles and targeted pollutants retrieved from several research publications are compiled and presented in Table-1.
Figure-1. Schematic representation of process involved in nano-material
based water and wastewater treatment
A number of mechanisms is involved for application of nanomaterials in water and wastewater treatment using nano-particles. Figure-1 briefly describes them graphically. Many nano-materials are available which has demonstrated their capability of water and wastewater treatment such as graphene composites, silver nano-particles, etc. The following paragraphs briefly describes repeatedly demonstrated laboratory-scale efficiency of nanomaterials and their usability in water security and water reusability applications.
2.1 Carbon Based Nano Materials: Activated carbon is tried and tested material for adsorption of impurities. Porosity, covalent bonding, electrostatic interactions, hydrogen bonding, pi-pi interactions, etc. are the features of carbon which works for trapping of organic components present in water and wastewater. The scientific understanding demonstrated that carbon nano sheets and carbon nanotubes has high adsorption potential than activated carbons due to higher adsorption efficiency of numerous organic chemicals. More specifically carbon nano tubes demonstrated high strength, high surface area, high adsorption efficiency, chemical resistance and mechanical strength. Further the carbon nano tubes can be modified on its surface for suitability by a process called grafting.
Techniques are also existing for regeneration of carbon nanotubes (CNT) hence, reuse is also possible. Therefore, the heavy metals can be effectively separated by carbon based nanomaterials. In this regard, it has been demonstrated that multiwall carbon nanotubes can remove heavy metals in wastewater like zinc, nickel, manganese, lead, copper, etc. The carbon nano tubes can successfully remove bulky organic molecules in wastewater due to large pores owing to high adsorption capacity of CNTs more efficiently on compounds having OH, NH2 and COOH, functional groups; polar aromatic compounds and polycyclic aromatic hydrocarbons present in wastewater.
2.2 Metal Based Nano-Materials: Metal-based nano absorbents such as metal oxides (MO) are lower in cost and efficient adsorbents for removing heavy metals and radionuclides. In particular, MO-based nano materials have exhibited superior performance over the traditional activated carbon.
The adsorption capacity of arsenic increases over 100 times if nano-magnetite particle size decreases from 300 to 11nm. Currently, various disinfection processes involve the comprehensive application of TiO2 as they can activate as photocatalysis even during visible sunlight. Oxides of magnesium, cerium, and manganese are also very capable for removing heavy metals. Nickel oxide nanostructures also indicated its potential as a photocatalyst in effluent treatment. UV irradiation time, quantity of catalyst, pH and dye concentration were investigated by degrading Rhodamine B dye and other dyes.
These factors indicated that the NiO nanostructures are an effective photocatalyst. Kinetic investigations of photodegradation revealed that the reactions followed the improved Langmuir–Hinshelwood model hence, clearly demonstrated photocatalytic property. MnO2 nano-sheets, graphene composites, metal oxides, antimicrobial nanomaterials, and photocatalysts are employed in the field of wastewater treatment.
Besides standalone nano metal oxides, some composite nanoparticles are also useful. These nanoparticles have material (nickel, iron, cobalt) and a chemical component for removal of heavy metals.
The benefit of these composite compounds are more compared to standalone nano metal oxides as these composite particle can act as magnetic material and super-magnetic materials thereby increasing the adsorption/ filtration efficiency.
2.3 Polymer Based Nano-Particles: These nano absorbents solve dual purpose like multiwall carbon nanotubes. This can remove organics load as well as heavy metals due to the presence of hydroxyl at the external surface (adsorption), presence of hydrophobic behaviour on inner surface (sorption). A polymeric nano absorbent such as dendrimer-ultrafiltration system has efficiently removed metal ions from aqueous solutions. Synthetic polymers such as molecularly imprinted polymers (MIPs) are gaining a reputation for wastewater treatment by mimicking natural detection and identification, demonstrating high affinity and selectivity for its structural analogy as reported by various scientists. The choice of using these nano-materials are guided by customised needs due to high efficiencies.
Polymeric nanoparticles obtained through synthesis of Fe- and Al-doped, activated micron (B0.8 mm), and nano (B100 nm) sized porous adsorbents has successfully removed arsenic (V) (B40 mg/g) and fluoride (B100 mg/g) ions from effluents and wastewater. Adhesive biocatalytic coatings having nano-porous microstructure created by partly-combined latex polymer particles capture highly concentrated microbial cells (harmful pathogenic bacteria) in a dry state stabilized by carbohydrate osmo-protectants during wastewater treatment by Researchers like S. Cortez and others (Figure-2). Thus, biotechnology and nano-particles can also be combined for treating water and wastewater. Depending on features, nanocomposite membranes are generally classified based on membrane structure and positioning of nanomaterials as conventional nanocomposite, thin-film nanocomposite (TFN), thin-film composite (TFC) with nanocomposite substrate and surface located nanocomposite.
It is observed from various researches throughout the world that several nanomaterials are synthesised in last two decades and successfully applied for water and wastewater treatment. There is huge potential of scaling up the nanomaterial use in industry either standalone for water treatment or at secondary and tertiary level in wastewater treatment for reuse. The use of silver nanoparticle is widely accepted for water treatment and disinfection but, the new materials described in this article are based on specific pollutant removal and has potential for use at industrial scale. The regeneration of nano-materials is also possible hence, scaling up in cost-effective way will not hinder the application. However, care must be taken in their use as most of the materials are targeting specific materials as despite various applications and advantages of polymer nanocomposites in water remediationseveral issues related to their use still remain to be addressed.
Selection of suitable standalone wastewater treatment technology or combination of treatment technologies to deal with well characterized targeted wastewater stream is of prime importance to achieve desired level of treatment, particularly when the critical contaminants present are not amenable to conventional biological treatment. The economic model of adoption of such emerging techniques is definitely a problem if it is adopted without understanding the complex mixture targeted to be treated. There is also a need to internalize the cost of managing effluent in the business model. The conventional methods of water/wastewater treatment may also work as pre-requisite for nanotechnology based methods to enhance their efficiency or may be used subsequent to them to achieve better results. (Disclaimer: The views expressed in this article are the understanding of authors only and not of organization they represent)