Nanowaste: Another Future Waste, Its Sources, Release Mechanism, and Removal Strategies in the Environment

Round 1

Reviewer 1 Report

In this manuscript, the researchers have given their overview on the potential of nano wastes, sources, and associated release mechanisms as well as their treatment approach. However, the manuscript lacks in variety of technical aspects. Some key points need special attention including:

1) Is nano fertilizer a source of nano waste or nano pollutant. Researchers have enlisted it as nanoproducts resulting in further problems in environmental compartments during their illustration in Figure 1.

2) In section 2: Once, researchers are talking about sources of nano-pollutants, the explanation itself is too generic. I was expecting that researchers have segregated nanowaste materials and enlist their sources separately. In a paragraph form, whenever you add such critical information regarding nanowaste material, it is always preferable to provide details separately for each category of nanowaste material.

3) In section 3: Life cycle assessment of nanowaste and release mechanisms are two different things. The researchers have merged both concepts in this section. You can at least subcategorize it in tow different schemes. One with release during production phase and second release behavior during consumption phase. Again in the later part, researchers must add up details like release behavior in a simple environment and release behavior in complex environments. Again this section becomes much more significant since depending upon the type or nature of nanowaste being release in environment, the release mechanism also varies.

4) For section 4, again similar comment. Just like researchers indicate impact of individual nanowaste in water, similarly, such key influence of individual nano waste was expected in other environmental compartments like soil and air.

5) Section 5.1. I don't get the exact purpose of this section. since the main heading indicates that researchers are suggesting various ways of treating solid waste from extensive literature review, however in the main body, they are talking about waste handling, capturing of air pollutants using nanomaterials. It's confusing and needs significant workout.

6) Section 5.2. very generic explanation! No solution indication in treatment of wastewater containing nanowaste. Doesn't fullfill the purpose of section.

7) Section 5.3. and 5.4 Just looks like reporting what has been done and again in a generic way. There should be specific way out and indication of gap. It's always that gap analysis is main purpose of review article otherwise there is no need of reporting of any result. 

8) Researchers just talk about recommendations which are mainly based on legislation point of view. There should be some points related to way forward like technical improvement and key approaches and solutions so that apart from funds requirement, there should be some appropriate way out or research direction to the researchers of the field to look on it.

Author Response

After re-reading the manuscript with the reviewer’s comments in mind, we agree with the reviewers that we need to incorporate the suggested information. So, we carefully considered your comments to improve the revised manuscript. Herein, we explain how we revised the paper based on those comments and recommendations. We want to extend our appreciation for taking the time and effort necessary to provide such insightful guidance.

Note: In revised manuscript file all the changes made are in red colored text.

Reviewer(s)' Comments to Author:

Reviewer: 1

In this manuscript, the researchers have given their overview on the potential of nano wastes, sources, and associated release mechanisms as well as their treatment approach. However, the manuscript lacks in variety of technical aspects. Some key points need special attention including:

1. Is nano fertilizer a source of nano waste or nano pollutant. Researchers have enlisted it as nanoproducts resulting in further problems in environmental compartments during their illustration in Figure 1.

 

Nanofertilizer comes under the category of nanoproducts, and they can generate nanowaste if not used in appropriate way or overuse or at their disposal.

 

2. In section 2: Once, researchers are talking about sources of nano-pollutants, the explanation itself is too generic. I was expecting that researchers have segregated nanowaste materials and enlist their sources separately. In a paragraph form, whenever you add such critical information regarding nanowaste material, it is always preferable to provide details separately for each category of nanowaste material.

Thanks for your comment. In revised manuscript we have categorized the main sources of nanowaste in two categories and added their relevant information under their headings. (Line # 78-152).

 

3. In section 3: Life cycle assessment of nanowaste and release mechanisms are two different things. The researchers have merged both concepts in this section. You can at least subcategorize it in two different schemes. One with release during production phase and second release behavior during consumption phase. Again in the later part, researchers must add up details like release behavior in a simple environment and release behavior in complex environments. Again this section becomes much more significant since depending upon the type or nature of nanowaste being release in environment, the release mechanism also varies.

 

As per your suggestion we have revised the section 3 as follows: (Line # 184-268)

 

3.1 Potential release during the production phase

ENM can be released into technical or environmental compartments [30-34]. Examples of technical compartments are manufacturing areas, storehouses, conveyance vehicles, markets, homes, workplaces, swimming pools, garbage disposal areas, and any place where ENM and nanoproducts are utilized, kept, or processed. Environmental compartments include soil, water, and air. However, the release of ENM during production phase depends on the process employed. Higher concentration of smaller size NP is more likely to release during high energy processes including spraying and synthesis. Whereas moderate or trivial release of relatively massive agglomerates occur during low energy processes such as cleaning, handling, and packing of ENM in industries [35]. ENM can contaminate the environment through intrinsic and extrinsic factors. Intrinsic factors are not only associated to the built-in properties of ENM but also to the properties of nano-products in terms of released amount into the environment [36]. Whereas extrinsic factors are linked to the systems where ENM are being processed and utilized including management, consumer choices, technological aspect, and economic development etc. Conclusively, in the production or manufacturing stage, the release of ENM depends on the production method and employed technology, also the environmental policies followed by companies as well as the filtration technique employed to evade its release into the environment [27]. During handling phase, the release of ENM depends on the method of their incorporation into the products (i.e., suspension in solids or liquids, surface-bound or aerosols) and their usage by consumers [37]. System variables that affect its release during manufacturing involve technological as well as economic development such as market penetration, expansion, and advanced innovation of nano-applications.

 

3.2 Release behavior during the consumption phase

Environmental contamination by ENM can be either direct (directly into the nature) or indirect (first into a compartment and then into the nature). Technical facilities may reduce the contamination of the environment due to ENM by applying filtration methods [27]. For example, a swimmer wearing a sunscreen result in the release of nano-TiO₂ into the water and ultimately the sewage, thus causing an indirect transfer. On the other hand, if the swimmer swims in a river or lake, causes a direct transfer. ENM contamination in the environment can be intentional or unintentional. Planned transfer of ENM into the nature, for example to restore water bodies (e.g., nano-scale zerovalent iron are used for water treatment) is an intentional release [38]. Unintentional release of ENM can occur due to wearing and tearing of nanoproducts and during their handling [39], or due to their use in applications like aerosols which result in 100% release into the atmospheric environment.

The release of composite nanoobjects, agglomerates and aggregates (NOAAs) from consumer used materials have been reported in literature while only a limited number of studies reported the release of free ENM. For example, a study reported that free nano- TiO₂ particles were released from nano- TiO₂ containing paints in minute form, however most of them were entrapped by the organic paint binder, resulting in small pieces of paint matrix having nano- TiO₂ [40]. Another study also reported that the release of free TiO₂ can occur due to the wear and tear of photocatalytic nano-coatings used in building materials [17]. It is reported that NOAAs enter the soil or groundwater due to wear and tear, or leaching, or they can be released indirectly by being released into the environment through atmospheric deposition. Either directly or indirectly, all NOAAs released land up in the soil or water bodies where they can cause detrimental effects on human health and environment [41].

 

3.3 Potential Release during the end-of-life phase

The employed technology and waste handling protocols determine the release of ENM during disposal [30,42-44]. The implemented policies affect the techniques employed for nano-products such as landfill, incineration, and recycling, whereas, the characteristics of the processes are defined by available technology [28]. Biodegradable nanowaste such as graphene nanoparticles (GNPs) and multi-wall carbon nanotubes (MWCNTs) can be easily leached out into environment through weathering and partial degradation if disposed-off into landfills. The compost preparing from this kind of nanowaste must have huge number of GNPs as well as the bulk graphene and carbon nanotubes. Once this compost is used for soil improvement might be the potential source of ENM into food chain by making its way from soil to groundwater [45].

Recently waste management strategy in terms of waste to energy production (thermal valorization) has been studied, however, the risk of thermoplastic emissions into environment through incineration must be investigated in this scenario [44]. Furthermore, in underdeveloped areas, any kind of environmental hazards (burning or accidental fire) in landfills might be a potential risk to humans and environment because of this huge explosion. In particular, above 600 ◦C carbon nanotubes (CNTs) and graphene are combustible, and thus can be converted into CO/CO₂ during this process [44]. However, the residues (ashes) formation containing unburned CNTs due to the incineration of polymer composites with CNTs has been reported in very few studies [46-48]. Also, the CNTs might be released in combustion gas phase [44].

During incineration or fire, the air borne particles from nanomaterials can be released. A study reported that GNPs and MWCNTs can either be release into the air or remained in the burned residues during incineration. Complete biodegradation of GNP/MWCNTs results in large amounts of GNPs and MWCNTs particles release. This issue has raised serious concerns about environmental and human health risks related to the composting of nanoproducts waste since such NP become a part of the soil. Incineration of nanowastes at 500 ◦C has the risk of “unacceptable risk/hazard” level due to GNPs and MWCNTs release, but this risk drops down to “high” if the temperature increased up to 850 ◦C. Specifically, incineration of nanoproducts waste in open areas like dumping grounds (a common practice in underdeveloped areas) leads to “high” risk. Thus, these procedures need to be monitored carefully and regulated because they can lay much more detrimental impacts on humans and the environment. Accidental fires can result in “acceptable” risk, but it does result in negative effects, therefore, safety actions are required to be implemented [45].

4. For section 4, again similar comment. Just like researchers indicate impact of individual nanowaste in water, similarly, such key influence of individual nano waste was expected in other environmental compartments like soil and air.

 

Thanks for your suggestion. In updated version, we have added the suggested information (line # 396-430).

4.3 Potential Impacts on Air

Like soil and water, NP are likely to be found in atmosphere in form of fine dust particles for prolonged period [102]. Both natural and synthetic NP are present in atmosphere but synthetic or engineered NP pose serious threat due to their smaller size and larger surface area. Rapid urbanization and industrial development, released waste in form of volatile organic compounds, suspended airborne particles and toxic gases (sulfur oxide, ozone, nitrogen oxides and carbon oxides). Whereas in urban areas, vehicles are deemed as the key contributor of ENM [103]. The problem associated with air in form of pollution is first noticed by people living in urban areas but now it is a matter of global significance due to its widespread evolution in rural areas too [104]. The probable negative impacts of these ENM on human health, global climate and urban visibility compel us to control their emissions. ENM having diverse chemical composition, influence the atmospheric chemistry by opening innovative chemical transformation pathways. Similarly, the smaller particles have higher probability to suspend in atmosphere with higher residence time and can be easily penetrate or deposit in respiratory as well as cardiovascular system [105].

Exposure to aerosolized ENM at consumer level can occur through various pathways including, dermal, ingestion and inhalation. Among these, inhalation is thought to be most probable entry route into human body [106]. The aerodynamic diameter of airborne ENMs is responsible for their deposition in respiratory system [107]. It is a misconception that during production, workers’ exposure to ENM is less likely due to controlled environmental conditions. However, it is stated that maximum exposure to a person occurs in workplace during production, handling, and use of ENM [108]. The synthesis method that requires controlled environment or vacuum are less emissive as compared to the method that occurs in ambient air under atmospheric conditions [109].

The inhalation process is more critical in use of cosmetic powders and sprays as the emissions are closely occur near to breathing zone and predictably lead to the production of aerosol particles [110]. Based on their probable contact, sprays containing NP were labeled as the significant class of consumer products [111]. The urban infrastructures such as paint and tiles of buildings, roadside antireflection mirrors, and concrete pavements are also the source of ENM entering to human body via inhalation. Exposure of ENM is triggered due to various weather conditions such as rain, ice, and wind. Besides this, release of ENM into atmosphere occur through the nanoadditive lubricants in road traffic. Cerium oxide is used to enhance fuel efficiency and control the emission of particulate matters. It is stated that morphology and size of cerium oxide can be easily altered during combustion which would affect its fate into the environment [112]. Tires manufacturing also involves the addition of nanocomposites, which on applying breaks can be released into air through erosion. In addition to above mentioned processes, accidental release of ENM from industries involve explosion, fire, and loss of contaminants [113]. However, an air quality expert group in UK recently reported about ultrafine particles and stated that none of these emission routes capable enough to release significant NP concentration in atmosphere, specifically compared with carbon particles released during combustion [114]. The atmospheric release of ENM in terms of their concentration and size are governed by various environmental factors such as turbulence, relative humidity, and temperature. Likewise, photochemical reactions initiated by UV radiations and free radicals also facilitate the uprising of atmospheric ENM [63].

Likewise sprays, paints, and tiles, various cleaning products also have ENM in their composition in indoor environment. They can be easily ingested if deposit on beverages or food whereas their presence in dust might pose a serious threat to children. Their presence in indoor environment might have direct (skin deposition) or indirect contact (surface deposition like furniture) with skin [107]. We are unable to identify significant concentration of ENM in air due to limited studies on their quantification and spatial analysis. The health issues triggered by these airborne ENM are still ambiguous. Health risks associated with black carbon and diesel exhaust have been reported by World Health Organization (WHO). However, general effects include oxidative stress, coronary heart diseases, lipid peroxidation, blood pressure, respiratory diseases, and cardiovascular diseases. The extent of these effects depends upon various factors duration, frequency, and dose exposure. Besides this, human susceptibility linked to lifestyle, sex, age, genetic background, diet, health condition and family traits also affect the toxic effects of these ENM [104].

Conclusively, existence, detection, and hazardous impacts of these ENM in air are still progressing due to limited studies, standard methods, and regulations. However, few studies addressed workers exposure to ENM but we are unable to find any study regarding secondary human exposure to materials containing nanomaterials from air compartment [113].

5. Section 5.1. I don't get the exact purpose of this section. since the main heading indicates that researchers are suggesting various ways of treating solid waste from extensive literature review, however in the main body, they are talking about waste handling, capturing of air pollutants using nanomaterials. It's confusing and needs significant workout.

 

Thanks for your valuable comment. In updated manuscript, we have added new information and have revised this section as follows: (Line # 462-493)

5.1 Strategies for Solid Nanowastes

Generally, the landfilling and incineration are common practices for solid nanowaste management. However, they can pose environmental risk and can pollute the soil and ground water if the waste is not appropriately managed. Thus, recycling is a much better option compared to landfilling and considered to be a sustainable approach [115]. A recent report stated that the nanowaste (including nano glass waste, nano rice husk ash, nano silica, and nano metakaolin) incorporated in ultra-high-performance concrete enhances both the concrete bulk resistance and the charge transfer resistance which delays the corrosion of high strength steel embedded in concrete [116].

Nanowaste can also be entrapped in a solid matrix binder so that they can be separated easily, or it should be entrapped in an impenetrable vessel to avoid leakage in soil environment. Detoxification of solid nanowastes is an area of study where there is no adequate information available. There is a need for extended research to be done in this area so that appropriate environment friendly techniques can be employed for the removal and decontamination of such nanowastes from the environment. There are some disposal methods suggested to deal with chromium dioxide (Cr(IV))-containing nanowastes, which usually result from industrial waste sludge [117,118]. In the chlor-alkali and chlorate industries, it was found that wastewater had magnesium hydroxide (Mg(OH)₂) NP that were formed as a result of the brine purification step (of sea salt) [119]. The study showed that converting Mg(OH)₂ NP into bulk resulted in the release of Cr(IV) in solution (that was adsorbed before). This CrVI-containing solution can be recycled via the chlorate process whereas, the detoxified solids can be reused as additives in other applications like paints, ceramics, and lubricants. This strategy could pave the way towards solutions for dealing with other nanosized pollutants [118].

Another possible management technique is to change the burning process of the huge amount of solid nanowaste. Incineration is common in various waste treatments facilities, that must be accompanied along the scrubbers for the exhaust gases, to avoid the release of NP in environment. Various other techniques exist to manage solid wastes in such a way that they do not contaminate the waterbodies and the soil. For example, vitrification was used to immobilize wastewater streams, e.g., urban, industrial & nuclear [120]. The success rate of the technique depends on the type and properties of the NP that we want to either dispose of or recycle.

6. Section 5.2. very generic explanation! No solution indication in treatment of wastewater containing nanowaste. Doesn't fulfill the purpose of section.

 

Thanks for your valuable comment. We have revised the said section as follows: (Line # 494- 527).

5.2 Strategies for Liquid Nanowastes

 

Use of nanomaterials for water/wastewater treatment is opening new horizon in the field of water treatment technologies. However, the presence of nanomaterials (as nanowaste) in water either treated or untreated is significant due to challenging process of identification, extraction and removal owing to their physico-chemical properties. Suitable strategies are required to remove nanowaste from aquatic environment if it exceeds permissible limit.

Based on the recent studies, various investigated NP such as zinc oxide, titanium dioxide and silver are eliminated from water up to 90-95% except silica NP due to their surface properties [121]. The NP extracted from water are then become a part of sewage sludge and required separate treatment. Computer-based simulation models represent that treated wastewater is a considerable reason of freshwater contamination with nanomaterials [122].

Coagulation coupled with flocculation and sedimentation is a conventional treatment method to remove nanowatse from water bodies. Few studies have been reported to remove ENM from water using aluminium (Al) and iron (Fe)-based coagulants [123-125]. Al-based coagulants exhibited higher removal efficiency as compared to Fe-based. In these studies, for the removal of TiO₂ NPs, FeSO₄ had an advantage over FeCl₃ [124], whereas, FeCl₃ was more effective for the removal of Ag NPs [123]. It is also stated that a single treatment process is not sufficiently enough to remove ENM from water, however, the combination of two or more techniques can be effective in this regard. A relatively simple approach “hand-shaking” showed 100% efficiency to remove 1D and 2D nanomaterials from water by emulsification of polluted water with oil. Nanowaste encapsulates in oil and can be easily removed once the oil is separated from water [126]. Few studies are conducted for the removal of ENM from natural and synthetic drinking water using filtration technique. From these studies, it is proven that instead of membrane pore size, stability and size of ENM also play important role for their effective removal from membranes [125]. Another study reported the highest removal of TiO₂ NPs (~99%) using microfiltration experiments and more than 95% removal of AgNPs and TiO₂ NPs using ultrafiltration. However, for the ZnO NPs, the lowest removal was observed due to their dissolution [127].

There is still insufficient knowledge about dealing with nanowaste and to avoid their exposure [8]. Thus, there is a dire need to work on research techniques to reduce or minimize the release of nanowastes into the environment.

 

7. Section 5.3. and 5.4 Just looks like reporting what has been done and again in a generic way. There should be specific way out and indication of gap. It's always that gap analysis is main purpose of review article otherwise there is no need of reporting of any result. 

Thanks for your suggestion. In updated version we have removed the unnecessary information and sections.

 

8. Researchers just talk about recommendations which are mainly based on legislation point of view. There should be some points related to way forward like technical improvement and key approaches and solutions so that apart from funds requirement, there should be some appropriate way out or research direction to the researchers of the field to look on it.

 

Thanks for your suggestion. In revised manuscript we have updated the following information (Line # 529-569).

 

Nanotechnology can be employed to cope with various problems, however there is a need for more recognition and consciousness in its applications and use. Campaigns that increase the knowledge and aware people can result in more understanding and ultimately less dangerous situations, in case of any interaction with nanomaterials [6,8]. Research and funds should be invested to assess current methods and come about new methods to discard and recycle nanomaterials, as well as identify the dangers of using such materials. Most funds are utilized for advancements in new nanomaterials, with less attention towards focusing on their discarding methods. There is a dire need to work upon and improve current procedures and regulations of nanowaste since nanotechnology is estimated to grow exponentially [2].

Discarding products containing nanomaterials should be handled with caution. Hazardous, toxic, or highly reactive nanowastes should be treated before or converted to nontoxic form before disposal. These nanowastes are usually formed during various industrial processes, where a large amount is released as a byproduct or direct product. Thus, one technique cannot be used to tackle all the different types of nanowastes. Hence, some other techniques must be executed to neutralize or reduce the toxic impacts of nanowaste such as dissolution of metal oxides in appropriate acid baths for safe disposal and recycling [128]. Keeping in mind, the green nanotechnological era, genetically engineered fungi and plants are required having high resistance to ENM along with capability to immobilize nanowaste in waste streams. However, till date, this field of biotreatment is still challenging for researchers in view of uptake, transfer, and accumulation mechanism.

Before disposal, collection of reusable ENM by aggregation followed by short-circuiting is an efficient waste management strategy. For example, pyrometallurgical and hydrometallurgical pathways can be implemented to retrieve nanomaterials from electronic waste such as batteries [129]. Besides this, ENM a significant portion of sewage sludge can also be acquired after wastewater treatment [63].

Recycling of nanowaste is a possible way to separate ENM from ordinary waste. However, the problem linked to recycling is the differentiation between natural and ENM. Also, the presence of dust waste residual demands precautionary measures to protect human and environment. The most common nanowaste management technique is landfill deposition. But lack of proper landfill lining is a major obstacle to implement this technique. Neutralizing and deactivating nanowaste, especially industrial nanowaste should be done before discarding to make it non-toxic and non-hazardous. This neutralization can be based on the chemical, thermal or physical processing of nanotechnology [130,131].

Nanowaste is also incinerated to destroy the flammable parts and remaining the non-combustible or persistent residuals in the chamber which requires specific attention for their removal. Unfortunately, no specific data is available to investigate the behavior of various nanomaterials in the combustion zone. There is a need to conduct detailed research to find out specific conditions for combustion and leftover residuals to reduce the pollution load. Further research in this area would make incineration an efficient end-of-life technique to constrain the hazardous impacts of ENM on human and environmental compartments [132].

Author Response File: Author Response.docx

Reviewer 2 Report

This review MS tried to describe Nano-waste affects in the environment. As brought in the different part of the MS, environment is referred to three main compartments (i.e., water, soil, and air). I think the authors should include the sub-section entitled’’ 4.3 Potential Impacts on air” in the section 4. The authors have not discussed about its potential impact on air as a part of environment to get findings or conclusions.

Author Response

Response to the reviewers’ comments Manuscript ID: sustainability-1561930

After re-reading the manuscript with the reviewer’s comments in mind, we agree with the reviewers that we need to incorporate the suggested information. So, we carefully considered your comments to improve the revised manuscript. Herein, we explain how we revised the paper based on those comments and recommendations. We want to extend our appreciation for taking the time and effort necessary to provide such insightful guidance.

Note: In revised manuscript file all the changes made are in red colored text.

Reviewer(s)' Comments to Author:

Reviewer: 2

1. This review MS tried to describe Nano-waste affects in the environment. As brought in the different part of the MS, environment is referred to three main compartments (i.e., water, soil, and air). I think the authors should include the sub-section entitled’’ 4.3 Potential Impacts on air” in the section 4. The authors have not discussed about its potential impact on air as a part of environment to get findings or conclusions.

 

Thanks for your valuable comment. We have added the following information in updated version (line # 396-430).

 

4.3 Potential Impacts on Air

Like soil and water, NP are likely to be found in atmosphere in form of fine dust particles for prolonged period [102]. Both natural and synthetic NP are present in atmosphere but synthetic or engineered NP pose serious threat due to their smaller size and larger surface area. Rapid urbanization and industrial development, released waste in form of volatile organic compounds, suspended airborne particles and toxic gases (sulfur oxide, ozone, nitrogen oxides and carbon oxides). Whereas in urban areas, vehicles are deemed as the key contributor of ENM [103]. The problem associated with air in form of pollution is first noticed by people living in urban areas but now it is a matter of global significance due to its widespread evolution in rural areas too [104]. The probable negative impacts of these ENM on human health, global climate and urban visibility compel us to control their emissions. ENM having diverse chemical composition, influence the atmospheric chemistry by opening innovative chemical transformation pathways. Similarly, the smaller particles have higher probability to suspend in atmosphere with higher residence time and can be easily penetrate or deposit in respiratory as well as cardiovascular system [105].

Exposure to aerosolized ENM at consumer level can occur through various pathways including, dermal, ingestion and inhalation. Among these, inhalation is thought to be most probable entry route into human body [106]. The aerodynamic diameter of airborne ENMs is responsible for their deposition in respiratory system [107]. It is a misconception that during production, workers’ exposure to ENM is less likely due to controlled environmental conditions. However, it is stated that maximum exposure to a person occurs in workplace during production, handling, and use of ENM [108]. The synthesis method that requires controlled environment or vacuum are less emissive as compared to the method that occurs in ambient air under atmospheric conditions [109].

The inhalation process is more critical in use of cosmetic powders and sprays as the emissions are closely occur near to breathing zone and predictably lead to the production of aerosol particles [110]. Based on their probable contact, sprays containing NP were labeled as the significant class of consumer products [111]. The urban infrastructures such as paint and tiles of buildings, roadside antireflection mirrors, and concrete pavements are also the source of ENM entering to human body via inhalation. Exposure of ENM is triggered due to various weather conditions such as rain, ice, and wind. Besides this, release of ENM into atmosphere occur through the nanoadditive lubricants in road traffic. Cerium oxide is used to enhance fuel efficiency and control the emission of particulate matters. It is stated that morphology and size of cerium oxide can be easily altered during combustion which would affect its fate into the environment [112]. Tires manufacturing also involves the addition of nanocomposites, which on applying breaks can be released into air through erosion. In addition to above mentioned processes, accidental release of ENM from industries involve explosion, fire, and loss of contaminants [113]. However, an air quality expert group in UK recently reported about ultrafine particles and stated that none of these emission routes capable enough to release significant NP concentration in atmosphere, specifically compared with carbon particles released during combustion [114]. The atmospheric release of ENM in terms of their concentration and size are governed by various environmental factors such as turbulence, relative humidity, and temperature. Likewise, photochemical reactions initiated by UV radiations and free radicals also facilitate the uprising of atmospheric ENM [63].

Likewise sprays, paints, and tiles, various cleaning products also have ENM in their composition in indoor environment. They can be easily ingested if deposit on beverages or food whereas their presence in dust might pose a serious threat to children. Their presence in indoor environment might have direct (skin deposition) or indirect contact (surface deposition like furniture) with skin [107]. We are unable to identify significant concentration of ENM in air due to limited studies on their quantification and spatial analysis. The health issues triggered by these airborne ENM are still ambiguous. Health risks associated with black carbon and diesel exhaust have been reported by World Health Organization (WHO). However, general effects include oxidative stress, coronary heart diseases, lipid peroxidation, blood pressure, respiratory diseases, and cardiovascular diseases. The extent of these effects depends upon various factors duration, frequency, and dose exposure. Besides this, human susceptibility linked to lifestyle, sex, age, genetic background, diet, health condition and family traits also affect the toxic effects of these ENM [104].

Conclusively, the existence, detection, and hazardous impacts of these ENM in air are still progressing due to limited studies, standard methods, and regulations. However, few studies addressed workers exposure to ENM but we are unable to find any study regarding secondary human exposure to materials containing nanomaterials from air compartment [113].

 

Author Response File: Author Response.docx

Reviewer 3 Report

The present review entitled “Nano-waste: Another future waste, its sources, release mechanism, and removal strategies in the environment” authored by Zahra et al. describe the highlighted concerns about nanowaste as future waste, that needs to be addressed. This present review focuses on engineered nanomaterials waste (in the form of nanoparticles, nanotubes, nanowires, quantum dots), generated from the manufacturing of a wide variety of nanoproducts ending up as nanowaste and affecting the environment. Furthermore, the review has considered all types of engineered nanomaterials in waste streams, and the environment is referred to three main compartments such as water, soil, and air. It is a well-organized review article and lacks of major errors. However, some issues are detailed below which need to be addressed.

I advise the authors to take the following points into account while revising their manuscript.

Comment 1:  Firstly, I would like to draw the attention of the authors that I found there are some typographical errors in the manuscript, so authors need to correct them in the revised manuscript.

Comment 2: Add the list of acronyms or abbreviations.

Comment 3: Include the table of content in the revised manuscript.

Comment 4: Abstract is poorly written, should be improved.

Comment 5: Include some more recent literature in the introduction section to strengthen the manuscript.

Comment 6: Reference style is not uniform, in some places journal names are written in full form, and in some places abbreviation form. So, correct it maintain the uniformity.

 

Author Response

Response to the reviewers’ comments Manuscript ID: sustainability-1561930

After re-reading the manuscript with the reviewer’s comments in mind, we agree with the reviewers that we need to incorporate the suggested information. So, we carefully considered your comments to improve the revised manuscript. Herein, we explain how we revised the paper based on those comments and recommendations. We want to extend our appreciation for taking the time and effort necessary to provide such insightful guidance.

Note: In revised manuscript file all the changes made are in red colored text.

Reviewer(s)' Comments to Author:

Reviewer: 3

 

1. Firstly, I would like to draw the attention of the authors that I found there are some typographical errors in the manuscript, so authors need to correct them in the revised manuscript. 

Thanks for your comment. In revised manuscript, we have tried to correct the typographical errors and other mistakes at various points. All those edits are in red color text in the updated manuscript. For example, in line # 25, 28, and 37.

2. Add the list of acronyms or abbreviations.

As per suggestion, in revised manuscript, the following information has been added as follows: (Line # 392-395)

Abbreviations: Au: Gold, Ag: Silver, CdS: Cadmium sulphide, GO: Graphene oxide, rGO: reduced graphene oxide, MG: Multi-layer graphene, GNPs: Graphene nanoparticles, TiO₂: Titanium oxide, ZnO: Zinc oxide, CNTs: Carbon nanotubes, MWCNTs: Multi-walled carbon nanotubes, NOAAs: Nanoobjects, agglomerates and aggregates, Cu: Copper, UV: Ultraviolet, ROS: Reactive Oxygen Species, SiO₂: Silicon dioxide

 

3. Include the table of content in the revised manuscript.

As per your suggestion, we have made the table of content as follows but there is no format given for this in the journal, so we are unable to add it in manuscript now.

 

1. Introduction
2. Sources of nano-waste (point and non-point)

2.1 Point Sources

2.2 Non-point Sources

3. Release mechanism in different environmental compartments and exposure pathways

3.1 Potential release during production phase

3.2 Release behavior during consumption phase

• Potential release during the end-of-life phase

4. Environmental impacts of nano-waste on different environmental compartments

4.1 Potential impacts on soil

• Potential impacts on water

4.3 Potential impacts on air

5. Removal and Management strategies

5.1 Strategies for Solid Nanowastes

5.2 Strategies for Liquid Nanowastes

6. Recommendations

 

• Abstract is poorly written, should be improved.

 

As per your suggestions we have revised the abstract as follows: (Line # 14-22)

Abstract: Nanowaste is defined as waste derived from materials with at least one dimension in the 1-100 nm range, and their containing products considered as “nanoproducts” that led in the development of nanomaterial-containing waste also termed as “nanowaste”. The increased production and consumption of these engineered nanomaterial (ENMs) and nanoproducts generating enormous amount of nano waste have raised serious concerns about their fate, behavior, and ultimate disposal in the environment. It should be of utmost importance that the nanowaste must be disposed of in an appropriate manner to avoid their adverse impacts on human health and environment. As ENM possess unique properties, and there is inadequate understanding to establish the appropriate treatment technique for many forms of nanowaste, making nanowaste disposal complex.

 

• Include some more recent literature in the introduction section to strengthen the manuscript.

 

Thanks for your valuable suggestion. We have added the updated literature in the revised manuscript (Line # 43-49).

Furthermore, they can also occur in the form of organic or inorganic substances (i.e., carbon, metals, and metal oxides) or in combinations to form complex substances [3]. There is need to implement the framework for the effective monitoring and disposal of these nanomaterial-containing waste. In waste management, quantification of waste produced is the first step. Nanomaterials undergo several modifications throughout their stay in the environment and other systems, depending on their properties, making the nanowaste quantification more challenging [4].

 

• Reference style is not uniform, in some places journal names are written in full form, and in some places abbreviation form. So, correct it maintain the uniformity.

Thanks for your comment. We have tried to write the references in uniform manner.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The authors have significantly improved the content and purpose of the manuscript. I suggest accepting manuscript in present form.