Removal of microplastics from the environment. A review

TLDR: In this article, a review of microplastics occurrence, transport, raw polymers and additives, toxicity and methods of removal is presented, including physical sorption and filtration, biological removal and ingestion, and chemical treatments.

Abstract: The production of fossil fuel-derived, synthetic plastics is continually increasing, while poor plastic waste management has recently induced severe pollution issues. Microplastics are plastic particles smaller than 5 mm. Microplastics are ubiquitous and slowly-degrading contaminants in waters and soils. Microplastics have long residence time, high stability, high potential of being fragmented and can adsorb other contaminants. Many aquatic species contain microplastics, which are in particular easily accumulated by planktonic and invertebrate organisms. Then, microplastics are transferred along food chains, leading to physical damages, decrease in nutritional diet value and exposure of the living organism to pathogens. Raw plastics contain chemical additives such as phthalates, bisphenol A and polybrominated diphenyl ethers that may induce toxic effects after ingestion by living organisms. Furthermore, the adsorption capability of microplastics makes them prone to carry several contaminants. Methods to remove microplastics from water and other media are actually needed. Here, we review microplastics occurrence, transport, raw polymers and additives, toxicity and methods of removal. Removal methods include physical sorption and filtration, biological removal and ingestion, and chemical treatments. Mechanisms, efficiency, advantages, and drawbacks of various removal methods are discussed.

Figures

Table 1 Persistency criteria for contaminants in different media according to the REACH Annex XIII (Verschoor 2015)

Fig. 6 Bench-scale reactor setup for the removal of microplastics using electrocoagulation method in which Al3+ acts as coagulation agent (Perren et al. 2018)

Fig. 2 Dynamic membrane experimental setup and graph showing the decrease in turbidity with time when microplastics are removed (Li et al. 2018b). TMP transmembrane pressure, NTU nephelometric turbidity unit

Fig. 5 Removal of polyethylene microplastics from the wastewater using Fe3+ coagulation, sedimentation and ultrafiltration processes (Ma et  al. 2019). The effect of polyacrylamide (PAM), pH and the formation of Fe-based flocs on the removal efficiency is well represented. MPs microplastics, UF ultrafiltration

Fig. 10 Photocatalytic removal of polystyrene microplastics in water by a Au–Ni–TiO2 micromotor (Wang et al. 2019). The microscopy images represent the efficient interactions leading to photodegradation reaction of the microplastics on the surface of the micromotor

Fig. 1 Microplastics sources, transformation and transport

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Removal of microplastics from the environment. A
review
Mohsen Padervand, Eric Lichtfouse, Didier Robert, Chuanyi Wang
To cite this version:
Mohsen Padervand, Eric Lichtfouse, Didier Robert, Chuanyi Wang. Removal of microplastics from the
environment. A review. Environmental Chemistry Letters, Springer Verlag, 2020, 18 (3), pp.807-828.
�10.1007/s10311-020-00983-1�. �hal-02562545�

Environmental Chemistry Letters (2020) 18:807–828
https://doi.org/10.1007/s10311-020-00983-1
Removal ofmicroplastics fromtheenvironment. Areview
MohsenPadervand
1,2
· EricLichtfouse
3
· DidierRobert
4
· ChuanyiWang
1
Abstract
The production of fossil fuel-derived, synthetic plastics is continually increasing, while poor plastic waste management
has recently induced severe pollution issues. Microplastics are plastic particles smaller than 5mm. Microplastics are ubiq-
uitous and slowly-degrading contaminants in waters and soils. Microplastics have long residence time, high stability, high
potential of being fragmented and can adsorb other contaminants. Many aquatic species contain microplastics, which are
in particulareasily accumulated by planktonic and invertebrate organisms. Then, microplastics are transferred along food
chains, leading to physical damages, decrease in nutritional diet value and exposure of the living organism to pathogens.
Rawplastics contain chemical additives such as phthalates, bisphenol A and polybrominated diphenyl ethers that may induce
toxic effects after ingestion by living organisms. Furthermore, the adsorption capability of microplastics makes them prone
to carry several contaminants. Methods to remove microplastics from water and other media are actually needed. Here, we
review microplastics occurrence, transport, raw polymers and additives, toxicity and methods of removal. Removal methods
include physical sorption and filtration, biological removal and ingestion, and chemical treatments. Mechanisms, efficiency,
advantages, and drawbacks of variousremoval methods are discussed.
Keywords Microplastic pollution· Environment· Removal method
Introduction
The global production of plastics has highly increased since
1950 to improve human life and reached almost 381 million
tons in 2015 (Ritchie and Roser 2018). This increase has,
however, induced global plastic pollution, making plastics
pollutants of concern (MacArthur etal. 2016). Microplastics
are plastics with size lower than 5mm, originating from
the exfoliation and degradation of many types of plastic-
based products released into ecosystems (Zhang etal. 2018).
Microplastics has been reported in ocean sediments (Van
Cauwenberghe etal. 2013), urban and rural areas (Hirai
etal. 2011), freshwaters (Faure etal. 2015) and seawaters
(Law etal. 2014). Most reports suggest an accumulation
of microplastics in aquatic environments, and, as a conse-
quence, a higher exposure of living organisms to microplas-
tics and their degradation by-products (Andrady 2011; Sun
etal. 2019).
Microplastics are categorized as primary microplastics,
which are raw materials used in domestic and personal
care products, and secondary microplastics arising from
thedegradation of raw plastic particles by physical, chemi-
cal, and biological processes in the environment (Galgani
etal. 2013). Long-term durability due to theirpolymeric
structure and easy transport between different habitats make
microplastics of high concern for biologists and environmen-
talists (Fig.1). Major raw polymers include polyethylene
terephthalate(PET), polyurethane(PU), polystyrene(PS),
polyvinylchloride(PVC), polypropylene(PP), polyesters,
* Mohsen Padervand
padervand@maragheh.ac.ir; mohsenpadervand@gmail.com
* Chuanyi Wang
wangchuanyi@sust.edu.cn
Eric Lichtfouse
eric.lichtfouse@gmail.com
Didier Robert
didier.robert@univ-lorraine.fr
1
School ofEnvironmental Science andEngineering, Shaanxi
University ofScience andTechnology, Xi’an710021, China
2
Department ofChemistry, Faculty ofScience, University
ofMaragheh, Maragheh, Iran
3
Aix-Marseille Univ, CNRS, IRD, INRAE, Coll France,
CEREGE, 13100Aix-en-Provence, France
4
ICPEES, Université de Lorraine, 12 rue Victor Demange,
57500Saint-Avold, France

polyethylene(PE) and polyamide (PA,nylon). Poor plastic
waste management has resulted in ubiquitous microplastics
occurrence (Gilani etal. 2019; Thompson 2015). Several
reports show that long-term exposure to microplastics causes
chronic toxicity, yet there is no evidence on their acute fatal
effects (Li etal. 2018a; Sussarellu etal. 2016). Microplas-
tic toxicity is controlled by different routes depending upon
theirchemical structure and additives used as linkage dur-
ing polymerization (Meeker etal. 2009; Sussarellu etal.
2016). As an example, polystyrene microplastics are able to
betransferred in blood, causing reproductive disruption in
marine filter feeders (Law etal. 2014).
To our best knowledge, this is the first review on micro-
plastics removal. We discuss microplastic additives, occur-
rence, transport and toxicity, then we review removal
methods. Removal methods include sorption and filtration,
removal based on chemical phenomena, and biological
ingestion treatments. Advantages, disadvantages and effi-
ciency of different methods are compared at the end.
Microplastic sources, transport, polymers
andadditives
Microplastic sources andoccurrence
Microplastics can be found worldwide in coastal regions
and aquatic ecosystems in various size fractions due to
the transport phenomena including wind and ocean cur-
rents. Primary sources are household sewage discharge
including polymeric particles from cosmetic and cleaning
products, feedstocks used to manufacture plastic products,
and plastic pellets or powders used for air blasting (Jiang
2018). Progressive fragmentation of larger plastic items
under the atmospheric conditions, e.g., by mechanical
degradation and UV light exposure, thuscontributing to
theentrance ofconsiderable amounts of microplastics to
the environment, is the secondary source of microplastics
(Eriksen etal. 2014). This increases plastic debris availabil-
ity for being ingested by a large variety of organisms and
highlights the appearance of further environmental hazards
(Thompson etal. 2009).
Wastewater treatment plants are alsoa major source of
microplastics release (Browne etal. 2011; Long etal. 2019).
Whereas largeplastic particles are efficiently removed dur-
ing wastewater treatment, microplastics often bypass the
treatment units, thusentering and accumulating in the
aquatic environment (Murphy etal. 2016). Noteworthy,
a large number of water treatment plants are located near
the ocean and seawater, thus inducing a highmicroplastic
release source. For instance, in mainlandChina, about 1873
wastewater plants (56%), out of3340, with 78 × 10
6
m
3
/day
of treatment capacity, are located in coastal regions where
their effluents can be directly or indirectly discharged into
aquatic ecosystems (Jin etal. 2014). To address this issue,
many researchers are investigating the fate, occurrence,
detection and removal of microplastics in the water treat-
ment plants (Beljanski 2016; Carr etal. 2016; Sun etal.
2019).
Microplastic transport
Sea and ocean are viewed as themajor sinks for microplas-
tics, whereasfreshwaters and terrestrial environments are
the main sources. Indeed, early research found thatmicro-
plastic litter reaching oceans by rivers contains particles
found in soils (Horton etal. 2017a). This implies that
freshwaters and soils are also sinks of microplastics, as
evidenced byhigh concentrations of microplastics in some
terrestrial and freshwater areas (Nizzetto etal. 2016).
Thelong-term durability of microplastic fibers found in
deeper layers (~ 25cm) of agricultural soils treated by
sewage sludge as fertilizer (Zubris and Richards 2005),
suggests agradual transport in solid media, then further
Fig. 1 Microplastics sources, transformation and transport

accumulation at depth;thus making agricultural and forest
soils more likely to retain microplastics compared tourban
areas (Lwanga etal. 2017).
As rivers carry a huge volume of plastic particles over
the large distances, microplastics probably settle out along
with sinking sediments, particularly where flow energy
drops, for instance in retard-moving riverbeds. Accord-
ingly, futhersediment deposition of microplastics in lakes
where water flow is the lowest and sedimentation rate is
high, should induce high accumulation (Corcoran etal.
2015).
The shape diversity, small size, lightweight and low
density of microplastics contribute to their widespread
transport and facile dispersal across large distances on
land and within aquatic systems by storm sewers, wind and
other natural currents (Horton and Dixon 2018). Larger
size and higher density result in facile sinking and sedi-
ment deposition of the microplastics (Horton etal. 2017b).
Furthermore, irregularly shaped microplastics with jagged
geometry and sharp ends are more likely retained under-
water, rather than returning to the surface, whereas spheri-
cal particles show ahigher tendency to stay at the surface
(Ballent etal. 2012; Lagarde etal. 2016).
Microplastics transport pathways in the air are not fully
understood (Horton and Dixon 2018). Noteworthy, in the
air, there are few dispersal boundaries, compared to water
systems. Nonetheless,microplastics transport within the
atmosphere is not totallyindependent of aquatic and ter-
restrial pollutions, and herefurther investigations are
needed toelucidate the mechanisms (Dris etal. 2016).
As another majorconcern, due to theirhydrophobic-
ity and high surface area/volume ratio, microplastics are
highly susceptible to sorb and carry persistent organic
pollutants such as polychlorinated biphenyls(PCB),
dichlorodiphenyltrichloroethane(DDT) and polyaromatic
polycyclic aromatichydrocarbons (PAH),which can be
subsequently transferred to coastal regions and bedes-
orbed inside livingorganisms (Browne etal. 2013). Conse-
quently, the concentration of organic pollutants in coastal
areas is expected to increase several orders of magnitude
as a result of pollutanttransport by microplastics. Micro-
plastic morphology and transport are thus major charac-
teristics controlling theother waterborne pollutants (Cole
etal. 2011).
Microplastic raw polymers andadditives
Polymeric ingredients of primary microplastics mainly
include polyethylene, polypropylene and polystyrene,
depending upon the type of the products manufactured by the
factory; while secondary microplastics are predominantly
made of polyester, acrylic and polyamide, forming fibers in
the environment (Jiang 2018). The microplasticnumber in
the inland freshwaters of Wuhan in China rangedbetween
1660.0 ± 639.1 and 8925 ± 1591 numbers/m
3
; herethe major
types were polyethylene terephthalate and polypropylene
(Wang etal. 2017). Low-density polyethylene has been also
identified as the dominant type of microplastics.
Microplastics containa large variety of chemical addi-
tives such as bisphenol A, phthalates and polybrominated
diphenyl ethers, which are used during raw plastic synthesis
to improve plasticity (Besseling etal. 2014, Murphy 2001).
These additives are endocrine disruptors, and thus may
exhibit toxic effects upon release.The concentration of such
plasticizers in plastic debris of remote and urban beaches
is up to 35ng/g in remote beaches andup to 700ng/g in
urban beachesfor bisphenol A; between 0.1 and 400ng/g
in remote beaches andup to 9900ng/g in urban beachesfor
polybrominated diphenyl ethers; and up to 3940 ng/g for
phthalates (Hirai etal. 2011). These plastic additives have
been detected in most microplastic polymers (Jiang 2018).
Researchers also reported the leaching of bisphenol A and
nonylphenol from silicone and polycarbonate microplas-
tics (Fasano etal. 2012). Accumulation of such chemicals
in human bodies through biological phenomena is also
reported (Talsness etal. 2009). The most alarming exposure
route to microplastics for human is food, where the adverse
effects of the chemical additives and mechanism of entrance
to the organs are still unexplored (Wright and Kelly 2017).
Accordingly, many efforts must be devoted to finding effi-
cient strategies to abate the presence of microplastics in the
environment. While there have been published reports on
characterizing sources, occurrence, fate, methods for detec-
tion, and environmental effects; to date, few research and
review papers have discussed removal processes of micro-
plastics from contaminated systems.
Toxicity ofmicroplastics
Toxicity fromthechemical structure
The potential toxicity of microplastics arises from unre-
acted monomers, oligomers and chemical additives leaked
from the plastic in the long rub (Thompson etal. 2004).
Monomers and oligomers areboth able to migrate from
food packaging materials (Piringer and Baner 2008). As the
concentration of the residuals reaches specific limits, they
can be potentially absorbed by human bodies via different
pathways. For instance, the presence of polystyrene residuals
in food materials is reported to cause serious health issues,
while epoxy resins made of bisphenol A are absorbed by
living tissues, then interfer with the rate of cell division
(Lau and Wong 2000).
Chemical additivesare used during polymers manufac-
turing for improving the products performance. Additives

include functional additives such as plasticizers, heat stabi-
lizers, flame retardants, antioxidants, colorants, e.g. soluble
azo-colorants and pigments, fillers such as kaolin and clay,
and reinforcements, e.g. carbon and glass fibers. These addi-
tives are another source of toxicity.For example, researchers
found that the release level of some phthalates from baby
bottles was in the range of 50–150μg/kg of food content
after the contact time of 120min at 70°C (Simoneau etal.
2012). The release level of bisphenol A from food packag-
ing items was estimated to be in the range of 100–800ng/L,
while the values were in the range of μg/L for some phtha-
lates under the same conditions (Fasano etal. 2012). Most of
these additives are not chemically bound to the bulk plastic
structures, implying easier release.
Nobre etal. (2015) studied the toxicity of raw and beach-
stranded microplastics on the development of embryos of
Lytechinus variegatus, simulating leaching of the chemi-
cal additives into thewater column and interstitial water by
assays of elutriate and pellet–water interface, respectively.
They found thatraw microplastics induced more toxicity,
enhancing anomalous embryonic development by 58.1%
and 66.5% for the former and latter evaluation method,
respectively. Their results also implied that the leaching
of chemical compounds strongly depends upon the media
compartment in which microplastics accumulate, and upon
theexposure pathway. Hahladakis etal. (2018) reviewed
migration and release rate, fate, and potential toxicity effects
of additives on organisms and environment. The release of
volatilecompounds, e.g., benzene, toluene, ethylbenzene,
styrene and methylene chloride, from plastics can also con-
tribute to chronic health effects (Andrady 2017; Huff etal.
2010; Wexler and Gad 1998).
Toxicity fromphysical properties
Microplastics exert damage through theeffect of arelatively
large surface area/volume ratio. They absorb hydrophobic
pollution from water, then carry this pollution to other habi-
tats (Setälä etal. 2014). A study of theeffect of phenan-
threne-loaded low-density polyethylene glycol microplastics
on biomarker responses in juvenile African catfish revealed
significant tissue changes in theliver and brain of the organ-
ism (Karami etal. 2016).
The ingestion of microplastics by biota is a common way
to inducetoxicological effects (Hämer etal. 2014). Polysty-
rene microplastics enhance the bioavailability of fluoran-
thene compounds to marine mussels (Mytilus spp.) after
7days of exposure under controlled experimental condi-
tions (Paul-Pont etal. 2016). These results mean that ther
is a higher fluoranthene concentration in mussels exposed
to fluoranthene-loaded microplastics than those exposed to
pure fluoranthene. Highest levels of antioxidant markers
and histopathological damages were also observed for the
former case. They explained the mechanismbyinteractions
between the cell wallcomponents of the marine mussels,
e.g. p-glycoprotein, involved in pollutant excretion, and
themicroplastics surface.
Zhang etal. (2017) investigated the adverse effects of
microplastics on thephotosynthesis of themarine microal-
gae Skeletonema costatum. Theyfound that the maximum
growth inhibition ratio reached up to 39.7% after 96h of
exposure to microplastics with average diameter of 1μm.
Their results show a drastic decrease in chlorophyll content
(20%) and photosynthetic efficiency (32%) after exposure
to high concentration of microplastic (50mg/L), leading
to negative effects on microalgae growth. According to
the results of morphological studies and scanning electron
microscopy (SEM), they proposed both adsorption and
aggregation of microplastics on the outer surface of micro-
algae as the most probable mechanism of toxicity.
Size dependency of microplastics toxicity was also con-
firmed by Lu etal. (2016) who investigated the exposure
effects of polystyrene microplastics to zebra fish. They stated
that a7-day exposure resulted in accumulating the micro-
plastics with size of 5μm in liver, gill and gut, while those
with size of 20μm were just found in fish gill and gut. More-
over, lipid accumulation and inflammation of liver, oxidative
stress, and adverse alterations in themetabolism profile of
the fish liver were the main toxicity outcomes.
The shape and texture of the ingested microplastics also
influence their toxicity and absorption capability. According
to Au etal. (2015), polypropylene microplastic fibers were
more toxic than polyethylene microplastic spherical particles
to the freshwater amphipod, Hyalella azteca. They attrib-
uted this to the longer residence time of the fibers in gut,
whichmodifies the ability of food processing, thus leading
to serious changes in sublethal endpoints.
Toxicity frommicroorganisms carried
bymicroplastics
The potential of microplastics to carry pathogenic bacteria
has been explored by Kirstein etal. (2016). They observed
Vibrio parahaemolyticus bacterial strains on some polyeth-
ylene, polypropylene and polystyrene marine microplas-
tic particles gathered from North Sea. They highlighted
the need for further consideration of health impacts of
microbial assemblagesin microplastics. A10-day expo-
sure to five types of ~ 70 μmmicroplastics led to intesti-
nal damage including splitting of enterocytes and cracking
of villi in zebrafish Danio rerio and nematode C. elegans,
as modelorganisms of freshwater (Lei etal. 2018). They
also demonstrated that 2-day exposure of 5.0mg/m
2
of
microplastics considerably reduced calcium levels and sur-
vival rates, and inhibited body length and reproduction of

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "Removal of microplastics from the environment. a review" ?

Here, the authors review microplastics occurrence, transport, raw polymers and additives, toxicity and methods of removal. Mechanisms, efficiency, advantages, and drawbacks of various removal methods are discussed. 

Q2. What is the common process for the breaking of polymeric chains?

Photooxidation degradation, that is oxidation in the presence of light source andair, is the mostly common process for the breaking of polymeric chains (Yousif and Haddad 2013). 

Q3. How much of the microplastics population was removed during the primary step of aerated?

58.84% of the microplastics population in influents was removed during the primary step ofaerated grit treatment, while the removal efficiency reached to 71.67% following the advance treatment processes. 

Q4. What are the main sources of microplastics in the ocean?

Microplastic sources, transport, polymers and additivesMicroplastics can be found worldwide in coastal regions and aquatic ecosystems in various size fractions due to the transport phenomena including wind and ocean currents. 

Q5. What is the main reason for the high concentration of microplastics in the aquatic environment?

Whereas large plastic particles are efficiently removed during wastewater treatment, microplastics often bypass the treatment units, thus  entering and accumulating in the aquatic environment (Murphy et al. 2016). 

Q6. What is the effect of membrane technology on the removal of microplastics from polluted aquatic?

The removal efficiency over the membranes particularly depends on its durability, influent flux, size, and concentration of the microplastics. 

Q7. What are the main reasons for the differences in removal efficiency?

The most important reasons that cause variation in removal efficienciesmight be daily processing volume, different raw water and type of treatment processes. 

Q8. How many steps were used to remove microplastics from the wastewater?

All plants that used several treatment steps such as subsequent tanks for floating and sedimentation, and filtration processes, eliminated more than 90% of microplastics from the influents, with a final removal efficiency reaching 97.15%. 

Q9. What did the researchers find in the SEM images?

SEM images also revealed the effect of salt on microplastic surface morphology, in which observable crack lines were clearly detected. 

Q10. What is the effect of the fragmentation of polyethylene microplastics on the marine ecosystem?

Although these microorganism communities can probably catalyze the metabolic reactions of microplastics resulting in their further biodegradation, the transport of nonnative organisms, which does not naturally occur in such environments, can induce negative effects on marine ecosystems variety (Urbanek et al. 2018). 

Q11. What is the probable mechanism of toxicity of polystyrene microplastics?

According to the results of morphological studies and scanning electron microscopy (SEM), they proposed both adsorption and aggregation of microplastics on the outer surface of microalgae as the most probable mechanism of toxicity. 

Q12. What is the sorption behavior of polystyrene microplastics on seaweed?

The diameter size of the polystyrene microplastics was ~ 20 μm, while the plant cells of the sorbent contained very narrow microchannels to restrict the translocation of polystyrene microplastics into the tissues. 

Q13. What is the way to remove polyethylene microplastics?

Researchers have also used the robust and environmentally compatible electrocoagulation technique (Perren et al. 2018), which allows sludge minimization, energy efficiency, cost-effectiveness, and flexibility to automation, to remove the polyethylene microplastics in a stirred-tank batch reactor (Fig. 6). 

Q14. What is the effect of pH on the removal of microplastics?

the microplastic removal efficiency was scarcely modified by the pH of the solution at low concentration of Al coagulant source, 0.5 mM, whereas removal efficiency decreased by increasing the pH, particularly for small-sized microplastics, of diameter lower than 0.5 mm. 

Q15. Why is alginate able to improve the adherence of polystyrene?

Indeed, because of the gelatinous characteristics of this anionic polysaccharide substance, alginate is able to improve the adherence of polystyrene particles on the seaweed’s surface (Martins et al. 2013). 

Q16. How did Auta and others analyze the morphological and structural changes of the polyethylene?

In addition to scanning the morphological and structural changes using electron microscopy and FTIR analyses, the rate of biodegradation was assessed via measuring the microplastics weight loss. 

Q17. What is the likely mechanism of toxicity of polystyrene microplastics?

According to Au et al. (2015), polypropylene microplastic fibers were more toxic than polyethylene microplastic spherical particles to the freshwater amphipod, Hyalella azteca. 

Q18. How did they remove the microplastics from the wastewater?

Their results revealed the key role of clam’s shells to remove the microplastics by sorption on the surface, as they resulted in 66.03% of removal from the wastewater. 

Q19. What is the alarming exposure route to microplastics for human?

The most alarming exposure route to microplastics for human is food, where the adverse effects of the chemical additives and mechanism of entrance to the organs are still unexplored (Wright and Kelly 2017). 

Q20. What is the growth rate of small microplastics?

The growth rate was highly enhanced when anionic polyacrylamide was used for the removal efficiency for the smaller microplastics (d < 0.5 mm): here the removal was raised from 25.83% without polyacrylamide to 61.19% with 15 mg/L polyacrylamide, while it just increased from 4.27% to 18.34% for larger microplastics, of 2–5 mm diameter.