Emerging contaminants antibiotic resistance genes and microplastics in the environment: Introduction to 21 review articles published in CREST during 2018–2022

Abstract

Antibiotic resistance genes (ARGs) and microplastics are two classes of emerging contaminants that are of significant concern worldwide. Due to the potential adverse effects associated with their exposures, these contaminants have been extensively investigated during the past decades. Here, based on 21 review articles published in Critical Reviews in Environmental Science and Technology (CREST) during 2018–2022, current knowledge on ARGs and microplastics are summarized. Topics cover their analysis, occurrence, transfer, transformation, and ecological and human health risks in aquatic, terrestrial and atmospheric environments. Further, strategies to remove ARG-related contaminants from wastewater, manure, and sludge are discussed. Limitations and challenges in investigating their transport, fate, risks and removal techniques are highlighted for future research. In addition, we emphasize the importance of the One Health perspective to study ARGs and microplastics to better manage their environmental behaviors and associated risks.

GRAPHICAL ABSTRACT

Keywords:

• Occurrence
• transport and transformation
• ecological and human risk
• toxicity
• removal
• one health

1. Introduction

Due to overuses of antibiotics during the last few decades, increasing antibiotic resistance in bacteria has been identified as a vital threat to public health. Antibiotic resistance genes (ARGs) are genetic elements carried by antibiotic resistant bacteria. They are the culprit for antibiotic resistance and widely accumulated in human-impacted environments as a class of emerging environmental contaminants (Zhao et al., 2021). Bacteria can evolve to new resistance strains via genetic mutations or horizontal transfer of ARGs. The evolution of antibiotic resistance is one of the greatest threats to human health in this century, endangering the success of antibiotic potency, thereby causing more deaths worldwide (Keenum et al., 2022).

Microplastics refer to plastic debris with diameter <5 mm, with polyethylene, polyethlene terephthalate, polystyrene, polypropylene and polyvinylchloride being typical microplastics (Xu et al., 2020). The ubiquity of microplastics is an environmental and economic issue that has attracted attention worldwide. It has been reported that ∼79% of plastic wastes are mismanaged, with some plastic wastes entering landfills or natural environment (Xu et al., 2021). Given their small size, microplastics may be easily ingested by organisms and cause adverse impacts on ecosystem and human health (Markic et al., 2020). In addition, as organic contaminants, microplastics have high affinity toward organic contaminants, making them transferable. The translocation of microplastics is accompanied by transported toxic contaminants via sorption and desorption. This process is referred to as “Trojan-Horse effect” (Zhang & Xu, 2022), which is known as co-transport in the field of colloid transport. Under Trojan-Horse effect, the ultimate fate and risks of both microplastics and the associated contaminants would be altered during their life cycles in the environment.

Due to the extensive uses of antibiotics and plastics, various environmental matrices, including aquatic environments (Keenum et al., 2022; Xu et al., 2021), soils (Wang et al., 2022; Xu et al., 2020) and ambient air (Jin et al., 2021; Zhang & Xu, 2022), serve as reservoirs and secondary sources for these contaminants. Once in the environment, these contaminants may undergo migration, transfer, transformation, degradation and dissipation processes, and may ultimately threat ecosystem and human health.

Due to their widespread occurrence and deleterious effects, more research on ARGs and microplastics have been conducted. To summarize research advances in these two classes of emerging contaminants, this review covers 21 relevant reviews published in CREST during 2018–2022. A wide range of topics on the source, transport, fate, toxicity, and human health risk of ARGs and microplastics were documented.

One Health is a collaborative, multi-sectoral and transdisciplinary approach focusing on the animal–human-ecosystems interfaces with the goal of attaining optimal health outcomes (Jin et al., 2021; Zhao et al., 2021). Given the dependent relationships between human, animal, and environmental dimensions of contaminants, the importance of One Health concept in studying the environmental effects of ARGs and microplastics has been emphasized. Therefore, in addition to provide a broader understanding of the fate and potential risks of ARGs and microplastics in the environment, we introduced related work under the One Health framework.

2. Measurement of ARGs and microplastics

Quantification of ARGs and microplastics in the environment is critical to better understand their fate, transport and transformation mechanisms to determine their potential risks to ecosystem and human health.

2.1. Antibiotic resistance genes

Method standardization is important to accurately quantify contaminants, which ensures data reliability and comparability across sample matrices. To date, rapid molecular tools including quantitative polymerase chain reaction (qPCR) and metagenome sequencing are promising techniques for ARG analysis. Since the application of qPCR technique was reviewed for ARG analysis among our selected papers, the following contents will be emphasized via qPCR-based monitoring of ARGs.

qPCR provides sensitive and quantitative measures of ARGs in various environmental matrices, including soil, water, sediments, biosolids, manure, food and air/dust. Many studies have used qPCR methods to analyze DNA to inform water treatment removal rates and human health risk assessment (Keenum et al., 2022; Vaz-Moreira et al., 2021). In addition, qPCR has also been applied to monitor mobile genetic elements, which are materials that can move within a genome as vehicles for ARG dissemination. However, a review of qPCR approaches for ARG analysis from 117 papers showed that methodologies from sample collection to data analysis varied greatly, highlighting the need to standardize methods to compare outcomes across different studies, environments and scenarios (Keenum et al., 2022). To standardize qPCR assessment, Keenum et al. (2022) developed a qPCR-based framework to monitor ARGs in aquatic environments, including surface water, recycled water and wastewater. During ARG analysis in water samples, various protocols were assessed, including sample collection and concentration, DNA extraction, primer/probe specificity, amplification conditions, amplicon length, PCR inhibition evaluation and detection limits.

qPCR technique is also a common approach to study the potential transfer of ARGs during and after the formation of viable but nonculturable (VBNC) bacteria (Cai et al., 2022). VBNC cells are dormant bacteria induced by light-based disinfection. The VBNC state is an important strategy for bacteria to respond to environmental stimuli, with the ARG transfer risk being of increasing concern. Despite the availability of some methods, development of in-situ and real-time quantitative detection methods for VBNC bacteria is lacking, especially for assessments during sewage disinfection processes.

2.2. Microplastics

Even though many studies show the presence of microplastics in aquatic, terrestrial and atmospheric environments, their quantification, characterization, and descriptive units for reporting have not been standardized. Besseling et al. (2019) reported significant challenges in quantifying microplastic concentrations in environmental samples, including defining size range, particle shapes and units, maxima calculations, extraction approaches and identification methods.

Based on the literature from 2008 to 2019, Xu et al. (2020) summarized available sampling techniques, extraction methods, and analytical procedures to detect microplastics of different sizes in soils. Although a variety of approaches are available, each exhibiting advantages and disadvantages. As such, identifying nano-plastics in soil environments still faces significant challenges due to method limitation. In addition, Markic et al. (2020) compiled methods regarding sampling, detection, solation and characterization to study the ingestion of microplastics by fish. Given the methodological bias associated with species-specific and location-specific occurrence of fish ingestion, plastic ingestion by some species was underestimated.

Compared to conventional analytical methods to identify plastic debris consisting of diverse polymers, including polyethylene, polypropylene, polystyrene, polyamide, and polyethylene terephthalate, Fourier transform infrared (FTIR) technique is an advanced approach, which determines the chemical structure as well as functional groups of polymer molecules. Bond et al. (2018) compared spectroscopic methods to analyze microplastics in environmental samples and argued that spectroscopic methods may not avoid ambiguous identification of polymer types so more precise criteria to classify polymer types should be developed to standardize characterization.

By screening more than 400 papers, Veerasingam et al. (2021) reviewed analytical methods including FTIR techniques on characterization, identification, weathering, and aging of microplastics to trace their transport and fate in the environment. Data processing methods for microplastics in sediments, water, biota, wastewater treatment plants, air/dust and salt were discussed and related criteria were proposed. They concluded that attenuated total reflectance technique coupled with FTIR spectroscopy are normally applied to analyze large size microplastics, especially in water and sediment, while micro Fourier Transform Interferometer (μFTIR) for smaller microplastics. In addition, μFTIR techniques have been widely used to study microplastics accumulation in biota and the related toxicity effects.

3. Sources and occurrence of ARGs and microplastics

ARGs and microplastics have been detected worldwide, including in geographically-remote regions. Their sources and occurrence in the environment are commonly researched topics since they are prerequisites to evaluate the potential risks to ecosystem and human health.

3.1. ARGs in aquatic environments

The emergence of ARGs in aquatic environments comes from sources with high concentrations of antibiotics, such as human and veterinary clinical settings. However, based on the knowledge of ARGs regarding their existence in aquatic environments, including the sources of antibiotics and ARGs in freshwater, wastewater and marine environments, Amarasiri et al. (2020) proposed that the evolution of antibiotic resistant bacteria in natural aquatic environments at low antibiotics concentrations should not be overlooked. Further, the acquisition of antibiotic resistance by horizontal gene transfer and mutation were discussed. Co-existing contaminants like disinfectants, disinfectant by-products antibiotics and metals are recognized as selective pressure for the development and horizontal gene transfer of ARGs in aquatic environments.

The presence of ARGs in the environment is not only controlled by their sources, but also influenced by other variables including socioeconomic indices and environmental factors, such as climate conditions, environmental compartments and co-existing chemicals. In comparing two different countries Poland and Portugal, which are located at extreme latitude and distant longitude of Europe, Vaz-Moreira et al. (2021) summarized the differences in the occurrence of ARGs and antibiotic residues in wastewater. Distinct patterns and contamination loads are found in these countries, mainly due to differences in socioeconomic (antibiotic consumption by the population), geographic and climate conditions (season differences in temperature and rainfall), and volume of antibiotics used in animal production.

3.2. ARGs in terrestrial environments

The soil environment, especially agricultural soil, is a significant reservoir for ARGs in terrestrial ecosystems. Inputs via application of animal manure and biosolids, and irrigation with reclaimed wastewater are major sources in agricultural soils (Wang et al., 2022). These agricultural practices favor ARG dissemination in soil environments and facilitate ARG migration into the food chain, raising concerns regarding their potential risk to human health through food consumption.

To better understand the sources of soil ARGs, Wang et al. (2022) summarized the correlations between ARGs and soil characteristics under diverse agricultural practices, including heavy metals and organic contaminants. Inconsistent factors including soil environmental factors and co-existing contaminants that drive ARG spread in soils in different studies were discussed. In addition, they addressed the antibiotic resistance dissemination from manure, biosolids, and wastewater to soil. Wu et al. (2022) discussed the distribution, transfer and fate of ARGs in soil environment at macro- and micro-scales. Their distribution and proliferation in soil microorganisms were also summarized.

Livestock systems are a rich reservoir of ARGs. By summarizing the antibiotic uses in animal production and associated ARG risk, Zhao et al. (2021) evidenced that the antibiotic resistance in animals were affected by co-selective agents based on the One Health perspective. They suggested that livestock systems may contribute to the global dissemination of ARGs via diverse transmission routes.

3.3. ARGs in the air

The role of atmospheric transport on the spread of antibiotic resistance is important but it has not been paid enough attention. Jin et al. (2021) summarized the dissemination and transmission of airborne antibiotic resistance under physiochemical influences and the associated health implications. The roles of fine particulate matter in the dissemination of antibiotic resistance were highlighted since the stable condition of bioaerosols may facilitate the occurrence of resistance prevalence in clinical pathogen ARGs. In addition, contributions from natural and anthropogenic sources to airborne ARGs were discussed and compared.

3.4. Microplastics in aquatic environments

Microplastics were first reported as marine contaminants by researchers, with plastic wastes being the major sources of microplastics. The prevalence of plastic wastes in aquatic environments is well-known. However, two recent reviews (Besseling et al., 2019; Bond et al., 2018) highlighted that the microplastic contamination in aquatic ecosystems is highly underestimated. Bond et al. (2018) reviewed the polymer abundance and compositions in marine and freshwater environments by integrating their physicochemical properties to predict the fate of plastic litter in aquatic systems. Their accumulation in sewage treatment works, sediments, along shorelines and deep sea, as well as their smaller particles that cannot be captured and identified by existing experimental methods explain the substantial amounts of missing plastic entering into the ocean. Besseling et al. (2019) summarized the exposure concentrations and distribution of microplastics in aquatic systems based on a field study in the Netherlands. They highlighted that more attentions should be paid to microplastic accumulation in sediments.

Freshwater bodies are hot spots of microplastic contamination because they are normally close to plastic sources and offer an open area for plastic storage. To better understand the behaviors of microplastics in freshwater environments, Junaid and Wang (2021) reviewed the sources, formation and occurrence characteristics of microplastics in typical freshwaters. In addition, they discussed the interactions of microplastics with extracellular (natural organic matter) and intercellular biomolecules (ARGs and mobile genetic elements) in rivers and lakes. They summarized that the interactions of microplastics with biomolecules may affect food chains, toxicities and diseases of organisms.

Although many studies have reported microplastic contamination loads in freshwater environments, their occurrence and hazardous effects in urban areas remain poorly understood. To address this gap, Xu et al. (2021) discussed microplastic contamination in urban freshwater catchments in China based on their abundance, characteristics and sources. They reviewed factors controlling the occurrence and distribution of microplastics in urban freshwater environments. Parameters including microplastic properties, environmental conditions, population size and local land-use functions were recognized, which could control the microplastic contamination in urban water bodies of China. In addition, current legislation and policies regarding plastic contamination, as well as recommendations to control microplastic contamination were discussed.

3.5. Microplastics in terrestrial environments

Although concerns regarding microplastic contamination originate from ocean impacts, the terrestrial environment receives 4–23 times more plastic wastes compared to the marine environment. Wang et al. (2022) summarized the sources, occurrence and characteristics of microplastics in soils under different uses. They found that available studies on the occurrence of soil microplastics mostly focus on agroecosystems and identified that organic fertilizers, residual films from plastic mulching and atmospheric deposition are the main sources.

Xu et al. (2020) summarized possible routes of microplastics entering into soil environments and their distribution and characteristics. The sorption of toxic chemicals onto microplastics makes them a carrier and sink of chemical contamination. They also illustrated the competitive sorption between microplastics and soil organic matter as the main factor to determine the distribution of organic chemicals and heavy metals between microplastics and soils.

4. Transfer and transformation of ARGs and microplastics

Once in the environment, a number of physical and biochemical processes, including transport and transformation, influence the fate of ARGs and microplastics in the environment. Both ARGs and microplastics can transfer between environmental matrices and undergo long-distance migration, resulting in global cycling. Given that the data are emerging regarding global cycling, a greater understanding of their transfer and transformation is important for risk management and environmental remediation.

4.1. Antibiotic resistance genes

Genetic mutation and horizontal transfer via conjugation, transformation, and transduction pathways are dominant driving forces to disseminate antibiotic resistance by microbes in the environment.

Soil biofilms contain a large number of bacteria, so they are the hot spots of horizontal transfer of ARGs. Wu et al. (2022) discussed the key roles of soil biofilms in ARG transfer and related influencing factors, including soil biofilm spatial structure, composition of extracellular polymeric substances and interactions between populations. As the primary horizontal transfer mode, the importance of conjugative transfer of ARGs in the environment was discussed. They also highlighted the application of microfluidic platforms to mimic the complexity of soil environments in studying ARG transfer in soil and biofilms at a micro-scale due to its accurate mastery of structure and fluids. In a review by Wang et al. (2022), the spread mechanisms of ARGs between bacteria were synthesized by discussing the transfer from soils to crops in soil-plant systems and from soils to human between bacteria.

Recently, VBNC (viable but nonculturable) bacteria have attracted attention due to their resistance to traditional disinfection techniques, including chlorine, ultraviolet, and ozone. The horizontal transfer of ARGs can be promoted when VBNC bacteria are resuscitated under favorable external conditions, resulting in potential transfer risk. To provide a general perception of VBNC bacteria and the potential transfer of ARGs, Cai et al. (2022) reviewed the changes, health risks and environmental application of bacteria in VBNC state. They concluded that morphological changes occur in cell wall, cell membrane and cytoplasm after bacteria enter the VBNC state. However, the formation of VBNC bacteria by light-based wastewater disinfection technologies and the horizontal transfer of ARGs by VBNC bacteria require more studies.

To better understand the transformation and underlying mechanisms of ARGs, Yin et al. (2022) summarized their persistence and the subsequent geochemical processes in water environments. The transformation of ARGs in a lighted-semiconductor mineral interface was discussed, which is a special natural mineral interface under light irradiation. They concluded that the interactions with environmental stimulation may affect bacterial stress responses and transformation.

4.2. Microplastics

Microplastics can be transported long distances due to their small size, resulting in wide distribution of these environmental contaminants. While Xu et al. (2021) discussed microplastic dynamics in freshwater environments in China, Zhang and Xu (2022) summarized microplastic transport in water and terrestrial environments. Their transfer into the ocean, between freshwater and land, and by the atmosphere, as well as their transport into rivers and soils were also discussed.

Microplastics act as shuttle Trojan Horses, which carry loaded chemicals and additives distributing in the environment and enter into living organisms. Apart from accumulation, microplastics may also change the risks of sorbed contaminants by changing their bioavailability and fate for living organisms. This effect makes the fate and threats of microplastics and associated contaminants more complicated and uncertain. However, Zhang and Xu (2022) argued that the Trojan-Horse effect of microplastics contributing to the accumulation of organic chemicals in the environment is limited, whereas endogenetic additives or monomers in microplastics may be the main exposure.

Mathematical models are promising tools to forecast the fate of microplastics in the environment. Besseling et al. (2019) discussed the advantages and applications of various models, including river transport, emission-based mass flow, multi-media, estimates based on plastic production and other fate models used in estuarine and marine studies. They proposed that the direct feedback between microplastic fate and effects on organisms, internal exposure to plastic particles, and data used for calculating steady-state concentrations and organism species should be taken into account when using transport models.

Microplastic transport in soil environments includes vertical and horizontal movement, and abiotic and biotic migration. Xu et al. (2020) discussed their transport in porous media and migration in soils, as well as the transfer of microplastic-carried contaminants to soil fauna. In turn, soil fauna also mediates the migration, trophic transfer and fate of microplastics by contributing to their secondary formation and breakdown. Wang et al. (2022) concluded that soil fauna not only accelerate the migration of microplastics in soils and to higher trophic levels, but also facilitate their fragmentation and decomposition with microbes.

Weathering and degradation are important mechanisms to dissipate microplastics in the environment. Bond et al. (2018) reviewed the weathering and degradation of different microplastics in water environments under environmentally-relevant conditions, showing the process is much slower than the disposal rate, leading to microplastic accumulation in the environment.

5. Ecological and human health risks of ARGs and microplastics

Antibiotic resistance and microplastic crises are both worldwide environmental health challenges of the 21^st century. Their wide presence threatens food safety and living environments for ecological and human receptors, hindering the global achievement of sustainable development goals.

5.1. ARGs in aquatic environments

The wide presence of ARGs in aquatic systems has received considerable attention. Besides drinking water, humans are exposed to ARGs through bathing, aquatic sports, occupational exposure during irrigation, and consumption of food products irrigated with reclaimed water. Amarasiri et al. (2020) discussed the human risk assessment of ARGs in aquatic environments. Their occurrence, proliferation and dissemination in aquatic systems are important to better evaluate their risks to human health. The authors also suggested that additional information, such as exposure and dose-response assessment data under different scenarios, unknown hotspots, and suitable antibiotic resistance markers, is needed to quantify the microbial risk to accurately forecast the related risks.

5.2. ARGs in terrestrial environments

After receiving antibiotics, the ARGs in farmed animals may be transmitted to humans via direct contact or indirect animal-environment-human pathways (Zhao et al., 2021). To untangle the complexities of ARGs across animals, environments and humans, they analyzed animal-to-human transmission pathways of ARGs in livestock systems. Direct exposure to ARG-carrying animals, indirect exposure via the food chain and food trade, and contact with farmland soil and aquatic environments, as well as horizontal transfer all promote exposure and human health risk to ARGs. However, a number of mitigation strategies to protect human health with the One Health perspective may be implemented by controlling selective agents, managing manure use, increasing removal efficiency of ARGs in wastewater, and improving management and intervention policies.

5.3. ARGs in the air

Studies have shown that air is a potentially-important pathway for ARG dissemination. By highlighting important puzzles remained for risk assessment of ARGs, Jin et al. (2021) proposed approaches to untangle the airborne transmission chain of antibiotic resistance from environmental sources to ambient air and human airways. The environmental factors in association with this process and the consequent health implications were discussed. A holistic methodological framework to quantify the role of environmental pathways to community-acquired antibiotic resistance was developed under the One Health-based surveillance.

5.4. Microplastics in aquatic environments

Microplastic ingestion by aquatic biota is of increasing concern as accumulated microplastics may transfer to higher trophic levels. Markic et al. (2020) summarized the knowledge on microplastic ingestion by wild marine fish. They compared the geographical and temporal distribution of microplastic ingestion in fish based on fish species and trophic levels, with plastic ingestion being detected in 323 of 494 examined fish species (65%), and 262 of 391 examined commercial fish species (67%). However, methodology needs to be optimized and standardized to draw firm conclusions regarding their observations.

Bivalves are common aquatic organisms in marine and freshwater ecosystems, which serve as an important link in the ecological chain. The extensive microplastic contamination becomes an emerging threat for bivalves as increased microplastics have been detected in their bodies, causing both direct and indirect adverse effects (Zhang et al., 2020). To understand the impacts of microplastics on bivalves and human health, Zhang et al. (2020) reviewed the potential exposure pathways. The type of microplastics accumulated in bivalves, and main factors (abundance and size) influencing their accumulation were summarized. The authors concluded that microplastic accumulation causes neurotoxicity, genotoxicity and reproductive health on bivalves, and affects their habitats and food sources together with the transport of toxic substances and harmful microbes. The human health risks via consumption of microplastic-contaminated seafood products were also elaborated.

The interactions between microplastics and biomolecules including protein, natural organic matter, and antibiotic resistance elements may change the transport, fate and bioavailability of microplastics in the environment, thus altering their adverse effects on ecosystem and human health. Junaid and Wang (2021) provided a full picture on the ecological and human health impacts caused by the interactions between microplastics with biomolecules in freshwater environments. The exposure routes and toxicities of microplastics to fish and various aquatic organisms, invertebrates and mammals were discussed. Since the intake of aquatic products is the major exposure pathway for microplastics to enter into human bodies, the impacts of microplastics on human health cannot be ignored.

Due to their sorption ability for other contaminants, ecological and human health risks posed by microplastics also arise from microplastic-sorbed contaminants. Zhang and Xu (2022) summarized the vector role of microplastics on the bioaccumulation of organic contaminants in ecological receptors. They proposed that the contributions of microplastics to chemical accumulation in organisms may be enhanced for endogenetic additives or monomers, including chemicals to improve plastic performances and unpolymerized precursors or degradation products of plastic.

To develop effective methods for assessing risk assessment from environmental microplastics exposure, Besseling et al. (2019) compared exposure tools and effect assessments within a microplastics risk assessment framework. They reviewed the effect thresholds of the physiological effects of microplastics based on species sensitivity distribution approach. The exposure concentrations of microplastics in aquatic system were further screened using the effect levels as benchmarks. In addition, the roles of microplastics in chemical bioaccumulation and the risk characterization for plastic particle effects were discussed.

5.5. Microplastics in terrestrial environments

The impacts of microplastics on soil ecology may be even greater than that on aquatic ecology due to their greater release into terrestrial ecosystems. The existence of microplastics may deteriorate ecological functions of soil fauna and endanger human health through the food chain. To make a systematic cognition of their nexus, Wang et al. (2022) discussed the nontoxic and toxic effects of microplastics on soil fauna, including ingestion and bioaccumulation, histopathological damage, oxidative stress, DNA damage, genotoxicity and reproductive toxicity, neurotoxicity, metabolic disorders and gut microbiota dysbiosis, as well as their joint toxicity with accompanied contaminants. They suggested that these mechanisms are responsible for the negative impacts of microplastics on the growth, reproduction, and metabolism of soil fauna.

With an overview on microplastics and their migration with toxic chemicals in soils, Xu et al. (2020) also addressed the impacts of soil microplastics on soil properties, microbial communities, enzyme activities, plant growth and soil fauna. They concluded that the contribution of microplastics to the transfer of hydrophobic organic chemicals to soil-dwelling organisms is limited.

6. Removal techniques for ARGs and microplastics

Due to the risks of ARGs and microplastics to the environment and human health, remediation techniques are needed to mitigate their negative effects. However, no review among the 21 papers covers the removal techniques for microplastics, so the following discussion focuses only on ARGs.

Up to date, efficient techniques have been developed to eliminate ARGs during animal waste utilization, wastewater treatment process, and soil-plant systems. Wastewater treatment plant is a large repository of ARGs, so it plays a key role in controlling ARG distribution, which can be used to prevent their spread in the environment. Biological techniques are the most common in wastewater treatment. Xue et al. (2019) discussed research advances on the degradation and reduction of ARGs in biosolids and wastewater in wastewater treatment plants. Techniques, including anaerobic/aerobic digestion, composting, and bio/air-drying, can effectively reduce ARGs during sludge treatment. The underlying mechanisms and influencing factors on ARG removal efficiency were further discussed. Pretreatment techniques like microwave, thermal hydrolysis and ozone can enhance ARG removal from wastewater when coupled with anaerobic digestion. However, ARG rebounding in the following anaerobic digestion processes should not be ignored. In addition, the fates of AGRs during activated sludge processes and wetland were compared.

Amarasiri et al. (2020) compared ARG removal capability in water and wastewater treatment plants by common treatment processes. Wang and Chen (2022) summarized the feasibility and efficiency of ARG removal from wastewater via bio-physicochemical and advanced oxidation treatment, as well as the corresponding advantages and limitations. In both reviews, biological treatments like membrane bioreactor and constructed wetlands are recommended due to their excellent efficiency for ARG removal.

Many factors impact the abundance of ARGs and antibiotics in wastewater before and after treatment. Based on studies of wastewater treatment plants, Vaz-Moreira et al. (2021) discussed how the load of resistance and microbial community composition of the sewage, treatment types, contamination degree, organic matter load, and temperature influenced the treatment efficiency of ARGs in wastewater.

To attenuate the effects of ARGs on terrestrial ecosystems, Wang et al. (2022) discussed several effective techniques, including aerobic composting, and aerobic/anaerobic digestions of manure, sludge and wastewater, to prevent ARGs from entering into soils. They also proposed strategies to alleviate ARGs in soils and outlined the research directions on the mechanisms of ARG decay in soil treatment processes.

It should be noted that natural processes such as photochemical and microbial degradation can also help ARG attenuation in the environment. Attention on these processes should be paid in future studies to develop cost-effective techniques to remove ARGs from the environment.

7. Summary

This article summarized important information based on 21 CREST review articles published during 2018–2022, which focused on two classes of emerging contaminants ARGs and microplastics in the environment. Research on these contaminants has predominantly focused on their analytical techniques, environmental sources and occurrence, fate and transport, ecological and human health risk assessment, and removal techniques. The ubiquitous existence of ARGs and microplastics in the environment makes it necessary to demand more efforts to fill knowledge gaps to manage their adverse effects on ecosystem and human health.

Limitations and challenges facing ARGs and microplastics research include the following four major areas. (1) Analytical techniques to identify and characterize ARGs and microplastics in different sample matrices have been developed to improve analytical quality. However, protocols from sample collection to data analysis should be further standardized to facilitate data comparison from different studies to generate large, consolidated datasets. This helps to solve problems and provide scientific basis for decision-makers to mitigate contamination threat. (2) Studies on the occurrence, fate and transport of ARGs and microplastics have been conducted worldwide, but some important scientific questions remain to be answered. Mechanisms controlling ARG and microplastics transport and transformation are required to help unravel links between the environment and human/ecological impacts. Suitable contamination markers should be used for risk assessment purposes while additional studies are needed on the interactions between ARGs/microplastics and environmental matrices, co-existing contaminants and local organisms. Multi-scale studies including field- and laboratory-scale experiments in multimedia are also needed. (3) Predicting contaminant-induced ecological and human health risk is important but challenging. A limitation is the lack of dose-response assessments under different scenarios under both field-scale and microcosm studies. Together with mathematical models, this dose-response database enables the development of reliable risk-assessment guidelines for both ecological and human receptors. Furthermore, the dual toxicity of ARGs and microplastics with other co-existing chemicals and their trophic transfer deserve more attention. (4) Cost-effective techniques to treat ARGs and microplastics in the environment, especially under specific treatment scenarios, are lacking. Their removal efficiency should be improved not only via existing technologies, but also via innovating technologies. The comprehensive understanding of the associated mechanisms during treatments is also imperative.

The One Health framework can establish an evidence chain of environmental contaminants to health outcomes by connecting all information together. It is a promising concept for studying ARGs and microplastics in the future. This way, a better picture of ARGs and microplastics from environments to humans is formed for effective mitigation measures to benefit ecological and human health.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported in part by Natural Science Foundation of Zhejiang Province (No. LY22B070004).