An analytical survey on the role of nanotechnology in groundwater remediation Open Access

The significance of groundwater for the existence of human society cannot be overstated on account of its usability for human consumption and in all sectors of economy. Being a significant part of the hydrological cycle, its accessibility relies upon the precipitation and recharge conditions. Worldwide scientific research has demonstrated that both the quality and quantity of groundwater has become worse to the point whereby immediate remedial measures are required. Groundwater, under most conditions, is more secure and more dependable for use than surface water because the latter is more promptly presented to toxins from processing plants than groundwater. This in no way implies that groundwater is resistant to pollution and contaminants; in fact, toxins may still get to family units through wells. This may present numerous health hazards to the populace. Contamination from agriculture and industrial sources is normal and these can prompt the development of poisonous microorganisms. The agricultural pollutants include pesticides, herbicides, and fertilizers whereas solvents, gasoline and other hydrocarbons, paints, and heavy metals make up the industrial pollutants, most of which are carcinogenic. Human waste is another potential source of contamination which prompts a wide range of effects on wellbeing right from gastrointestinal ailment to, in serious cases, cholera, typhoid, amoebiasis, giardiasis, and E. coli. Chemicals which are effectively solvent and infiltrate the soil are prime contenders for groundwater toxins. Once groundwater is sullied, it is a very exorbitant activity to eliminate pollutants from it. Therefore, the need of groundwater remediation is of utmost priority. Nanoscience and technology engrossing processes such as adsorption, absorption, chemical reactions, photo catalysis, and filtration are advancing fields of research and development in the context of environmental purification. Numerous studies considered them as one of the significant groundwater remediation techniques. Realizing the need, this article is designed to conduct a critical review on the role of nanotechnology in remediating groundwater resource. Analysis and remediation of the environmental danger posed by impaired groundwater quality is a global concern (Macé et al. 2006).

The present research aims to provide an insight into the current state of nanotechnology applications for groundwater sustainability. This work is characterizing the impact of nanotechnology including inorganic nanomaterials (TiO₂, nanoscale zero valent iron (nZVI)), organic, carbon, polymeric and composite based materials for facing the challenges in the changing world by providing remediation to groundwater.

Pure metals like iron, cobalt, silver, and zinc, as well as compounds like iron oxide, have been utilized as a basis for the fabrication of nanomaterials and nanoscale particles. They are multitudinous materials because of large surface to volume ratio and particular size-dependent characteristics. Distinct bases are used to give these materials different characteristics. They are suited for a variety of therapeutic applications due to their outstanding biocompatibility. These nanomaterials and nanoparticles have antitumor characteristics, anti-cancerous qualities, toxicity to many disease-causing organisms, and pollutant removal properties, to name a few. The use of metallic compounds, like elemental iron, in treating chlorinated solvent plumes, has added another measurement for environmental remediation since the mid-1990s. From that point forward the nanomaterials are investigated and applied as in situ (Otto et al. 2008; Zhang 2009) and ex-situ contamination reduction technology. As a result, pathogens, herbicides, dyes, heavy metals, chlorinated organic compounds, and polycyclic aromatic hydrocarbons (Yarima et al. 2020), organophosphorus compounds, volatile organic compounds, and halogenated herbicides (Guerra et al. 2018; Chaturvedi & Dave 2019; Yarima et al. 2020) are all treated with nanomaterials and nanoparticles in the environment. Particle stabilization with nanoparticles is also utilized to decompose organic pollutants in groundwater (Jiang et al. 2020). Also, nanoscale materials like sorbents, catalysts, nanostructured catalytic membranes, and bioactive nanoparticles have high potential in treating surface, ground, and industrial wastewater impacted with bacteria, viruses, organic and inorganic solutes, hazardous metal ions, and radionuclides, bacteria, and viruses (Savage & Diallo 2005; Theron et al. 2008).

Scientific work around the world shows that nanotechnology is one of the significant groundwater remediation techniques (Liang et al. 1996; Savage & Diallo 2005; Otto et al. 2008; Theron et al. 2008; Agarwal & Joshi 2010; Rajan 2011; Li et al. 2014; Thomé et al. 2015; Mosmeri et al. 2017; Corsi et al. 2018; Hu & Xia 2018; Jia et al. 2019; Newsome et al. 2019; Jiang et al. 2020; Qian et al. 2020). Thomé et al. (2015) and Agarwal & Joshi (2010), in particular, offered scientific evidence of effective nanotechnology for groundwater remediation in the US, Europe, and in India, respectively. It has shown promise in humanizing numerous aspects of life. Within the broader field of nanotechnology, nanoparticles of zero valent iron (ZVI) and nanomaterials based on carbon are at the forefront of scientific interest as remediation tools, offering a potential answer in the field of groundwater treatment (Rajan 2011; Matlochova et al. 2013). The technique of injection, spacing and distribution of injection points are influenced by the geology of the treatment zone, type and distribution of pollutants, and the form of nanoscale materials to be injected. The in situ application of nanoscale material is site specific and can be done in numerous ways: (A) The nanoscale iron can be injected directly under pressure or gravity feed. This is achieved through direct push technology or via tube wells. The direct push method entails pushing direct-push rods into the ground, which are similar in size to small drilling augers. This process allows for the infusion of materials without the use of permanent wells (Butler et al. 2000); (B) Recirculation is another way in which nanoscale material is introduced into groundwater before it is extracted and reinjected into the treatment zone (Figure 2).

Well known pulse technology, pneumatic and hydraulic fracturing and liquid atomization injection are other in situ treatment methods and processes. Nanoscale materials are also being investigated for strategies of infusion for improved remediation of groundwater contaminated with different toxins.

Inorganic nanomaterials include metals (such as Pt, Rh, Pd, Ir, Ag, Au, Cu, Co, Ni, FeNi, Cu₃Au, CoNi, CdTe, CdSe, and ZnS), their oxides (such as ZnO, Fe₂O₃, Fe₃O₄, MgO, BaCO₃, BaSO₄, and TiO₂) and semiconductors such as silicon and ceramics (Rizwan et al. 2014; Nnaji et al. 2018).

Iron based remediation is commonly used in different types of industries and in polluted groundwater and land. These include iron based curium (Cm), permeable reactive bathers (PRBs), and engineered nanoparticles. The former is used as precipitant or sorbent for both at and off site application, PRBs are skilled ‘new’ iron-based treatment techniques, but due to their complex chemistry, forecasting their long-term performance is difficult. The latter are used directly to treat source zone as a non-invasive and at-site tool, but there are still some questions about the health effects and environmental fate of these particles.

Emulsified zero valent iron (EZVI) is made up of nano-microscale ZVI encased in an emulsion membrane that allows chlorinated solvents like hydrocarbons to be treated. nZVI, BNPs, and EZVI are nanoscale materials that are used to chemically degrade various pollutants (EPA 2008; Tarr 2013).

The two main reductive dichlorination reaction mechanisms are hydrogenolysis and beta elimination. When a pollutant comes in contact with elemental iron (Fe⁰), it undergoes beta elimination, which follows the steps below:

(EPA 2008)

While hydrogenolysis follows given degradation pathway for reductive dichlorination of PCE:

(Tarr 2003)

Fe⁰ interactions with pollutants are likely to be very complex, involving numerous competing routes.

For remediating groundwater, TiO₂ is investigated for ex situ treatment under a pump and treat system. Photocatalysis has been proven to mineralize and transform various pollutants into less hazardous mixtures (Konstantinou & Albanis 2003).

BNPs, which are made up of ZVI or various metals with a metal catalyst, such as Au, Pt, Ni, or Pd, have been proven to be effective for removing pollutants from groundwater and soil. The mix of metals expands the energy of the oxidation-decrease (redox) response, catalyzing the response. The investigations (Nutt et al. 2005; Zhang & Elliot 2006) proposed that Palladium joined with iron BNPs outflanks the capacities of microscale iron particles. Aside from nZVI, mZVI, EZVI and nano scale TiO₂, specialists across the world are dealing with other nanoscale materials for use as in and ex situ groundwater remediation measures. These particles include dendrimers, swellable naturally altered silica (SOMS), nanotubes, metalloporphyrinogens, SAMMSTM, and ferritin as they have great chemical and thermal stability (Fryxell et al. 2007) to clear radionuclides, pertechnetate, selenite, Hg, Cr, and Ar (Mattigod et al. 2003; Roy et al. 2021).

They are nanosized polymers consisting of branched units and their interior cavities can be used for drug delivery (Roy et al. 2021). Dendrimers are used to synthesize Fe(0)/FeS Nano composites which are then used to build permeable reactive barriers (PRBs) to treat groundwater in and ex situ. Dendrimers have multiple chain ends on their surface which are modified to execute various chemical functions, making them suitable as catalysts (RK et al. 2012). Dendrimers are macromolecules with a central core and two or more repeating branching units (Triano et al. 2015), they are characterized by their sphere-like bifurcated, three-dimensional structure (Harth et al. 2002) and are mono disperse polymers with specific size, solubility, porosity, high degree of molecular uniformity and highly functional terminal groups on their surface (Newsome & Sholl 2006). Sun et al. (2011) investigated the use of nanomaterials and metal-reducing bacteria to treat arsenic-contaminated groundwater. A combination of Pd-akageneite and bacteria eliminated 9 of the arsenic from the contaminated groundwater in a batch experiment. The findings suggested that nanotechnology and biotechnology have the potential to develop unique and effective arsenic-contaminated groundwater treatment solutions. Mosmeri et al. (2017) used calcium peroxide nanoparticles to synthesize CaO₂ to remediate benzene contaminated groundwater. The synthesis process was modeled and optimized by two methods, namely central composite rotatable design (CCRD) and response surface methodology (RSM). Results showed that application of 400 mg/L of CaO₂ in biotic conditions fully dislodged benzene from contaminated groundwater in just 60 days. Further, comparison between biotic and abiotic trials revealed that microbial stimulation with CaO₂ nanoparticles has significant potential for benzene cleanup from groundwater.

SOMS (Osorb) is a type of chemically modified silica that swells and traps small molecules of organic substances after coming into contact with them. According to Edmiston (2009), for organic materials, SOMS can seize up to eight times their quantity, and cannot hold water because of their hydrophobic nature. Fe-Osorb transforms nZVI into Osorb, forming a substance capable of dechlorinating collected organics and being used as an in-situ technique, while Pd-Osorb, metal-glass hybrid material, is used to clean chlorinated volatile organic compounds (VOCs) ex situ groundwater treatment systems (Edmiston 2010). To eliminate halogenated VOCs, palladium nanoparticles are consolidated into the Osorb. Nanotechnology is also being used to develop water treatment membranes, water desalination, and reclamation. A wide range of nanoscale materials, (like alumina, zero-valent iron, and gold), are consolidated in these layers (Theron et al. 2008). To filter organic pollutants from groundwater, carbon nanotubes can be modified to produce layers with nanoscale holes (Meridian Institute 2006; Mauter & Elimelech 2008).

The use of nanotubes as a photocatalytic degrader of chlorinated mixtures has huge promise (Chen et al. 2005). TiO₂ nanotubes are especially viable at elevated temperatures and are equipped for cutting down contaminant concentration by 50% contaminant in just 3 hrs (Xu et al. 2005). This is an out of the site method as powerful enlightenment typically necessitates that cleanup occurs in a reactor, intended for the reason (Tratnyek & Johnson 2006). Bench scale investigations with ferritin reduces the waters’ toxicity, and it is currently being studied for in situ application (Temple University 2004).

Technetium-99/pertechnetate can be successfully separated and recovered from contaminated groundwater using activated carbon. During the pump and treat processes, the technology is low-cost and requires little installation and maintenance (Liang et al. 1996).

Micro-nano-bubbles (MNBs) can boost water oxygen levels. Increased surfactant content in water reduces bubble size, slows dissolution, and raises the zeta potential (zP). Furthermore, MNBs with a higher zP values are more stable. The low rising velocity of MNBs add to protracted water retention. Micronano bubble aeration has been suggested to improve the bioremediation impact (Li et al. 2014). Because of their high oxidation ability, ozone Micro-Nano bubbles (MNBs) can boost cleanup efficiency. It is a novel method for remediating organics-contaminated groundwater in situ (Hu & Xia 2018).

Qian et al. (2020) summarized the application environmental implications and corresponding mechanisms of several types of engineered nanomaterials (ENMs) for groundwater remediation. However, Chaturvedi & Dave (2019) have shown that engineered nanomaterials have high toxic potential during transportation in water and wastewater for human health and the environment. Thus, ethical ENM and nanotechnology advancements should be fostered while minimizing their negative consequences on the ecological system. The goal is to develop long-term remediation strategies that provide a healthy and safe environment while posing no additional risk. Eco safety should be a priority feature of ENMs intended for nano remediation, and according to Corsi et al. (2018); ENMs that comply with the standards of environmental safety will endorse industrial competitiveness, innovation, and sustainability. Predictive safety assessment of ENMs for remediation is required for which greener, sustainable, and innovative nano-structured materials should be supported further.

These are materials that combine nanoparticles with other nanoparticles or other materials. They are composite during which a minimum of one among the phases shows dimensions within the nanometers ranging between 1 and 100 nm (Consoli et al. 2019). Nanocomposite could also be polymer based or non-polymer based nano materials (Pandya et al. 2015), the incorporation of nanomaterials into composites enhances mechanical, thermal and barrier properties of a cloth (Roy et al. 2021). The improvements in these properties have resulted in an interest in nanocomposite been utilized in various industrial applications as found in environmental cleanup.

Faisal et al. (2018) in their paper ‘A Review of Permeable Reactive Barrier as Passive Sustainable Technology for Groundwater Remediation’ explains that the technology of PRBs was created as a substitute to typical pump-and-treat methods for remediating contaminated groundwater. An introduction of this methodology is provided, as well as the promising frontiers for scientific study that combines it with sustainability and green technology practices.

Under anoxic conditions, metalloporphyrinogens are suitable for reducing organic chlorine pesticides and CCl₄. Dror et al. (2005) have examined the potentiality of metalloporphyrinogens for in situ remediation.

Nanoscience and technology are advancing fields of research and development in the context of environmental purification. These have a wide application in groundwater remediation with a continual rise in related research and development. The technology has revolutionized the many sectors that include biomedical, chemical, and electronic and biotechnology. Nanoparticles (NP) have a wide range of uses, including agriculture, food technology, textiles, wastewater treatment, paints, optics, cosmetics, sports goods, tyres, purification procedures, and medications. Most of the research work in the subject of remediation nanotechnology has concentrated on zerovalent iron and its products, but other materials, for example titanium dioxide, are now being developed. Rajan (2011) describes the usage of nanomaterials to purify the environment, such as groundwater remediation for drinking and reuse but the negative consequences of employing these nanoparticles were also highlighted. This study found that nano remediation offers a significant potential for reducing contaminant concentrations to near zero by cleaning vast, contaminated locations in situ, saving time and eliminating the need for contaminants to be removed. Matlochova et al. (2013) carried out another investigation that suggested nanoparticles had a lot of potential in groundwater cleanup. However, the associated environmental risk must not be overlooked. The current state of nanotechnology in contaminated site cleanup was examined in this paper. The authors determined that the carbon-based nanomaterial (CNM) and nZVI outperform other forms of nanomaterials and nanoparticles reported so far. CNM sorbents are also used to remove organic (aliphatic, monocyclic, and polycyclic aromatic hydrocarbons) and inorganic contaminants like divalent metal ions (Cd²⁺, Pb²⁺, Zn²⁺, Ni²⁺, Cu²⁺). TCE, VOC, nitrates, and uranium have all been removed using zero-valent iron nanoparticles.

The present study suggests that nano remediation has high potential to remediate huge sites in-place, cutting cleanup time and removing the requirement for contaminant removal, lowering contamination concentrations to near zero. These are used to clean up the environment (Rajan 2011). It is thus a rapidly growing field in recent years. The relevance of nanoparticles to our well-being is undeniable, but their potential negative health effects must be fully investigated. The size, shape, and surface reactivity of nanoparticles with surrounding tissues will determine their behaviour, which must be understood.

Nanoparticles are used in various aspects of life, yet there are safety concerns about their disposal and long-term effects on the environment (Matlochova et al. 2013). Green nanoparticles (GNP) are an alternative that are made with environmentally friendly procedures and materials, resulting in less waste. They also have the advantage of being environmentally friendly and sustainable. The GNP's distinctiveness is enhanced by its altered physical, chemical, structural, thermal, and magnetic properties as compared to bulk. Recognizing the enormous potential of GNP, scientists are working feverishly to produce new materials with improved qualities. While Green Nanomaterials are a viable alternative, it is critical to comprehend the health, safety, and toxicity of these materials.

The current study received no funding from any organisation or corporation.

There were no humans or animals in the study.

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

The authors declare there is no conflict.