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Microplastic (MP) pollution has been rising as a threatening risk and recently has appealed to the attention of more researchers. In this study, influential parameters affecting the removal rate of polyethylene microplastics (PEMPs) were optimized through response surface methodology (RSM). In Box Behnken Design (BBD), independent parameters were pH, PEMP size, coagulant dosage and polyacrylamide dosage. Two experimental sets were conducted, one with ferric chloride and the second with poly aluminum chloride as two commonly applied coagulants in drinking water treatment plants (DWTPs). Comparing the results of optimized parameters, PAC was a better coagulant with the predicted removal rate of 58.19%, while the removal rate with ferric chloride as a coagulant was predicted to be 56.37%. Moreover, some experiments were conducted to analyze the effect of ozone gas and sodium hypochlorite as disinfectants on removal rate. The highest removal rate was observed when 2 ppm of O3 was added to the solution coagulated with optimal dosage of PAC, reaching the removal rate of 76.8%.

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  • Use of RSM for MPs removal optimization in coagulation process.

  • Investigation of disinfectant effects on MPs sinking behavior.

  • Comparison of two commonly used coagulants in optimization.

  • Comparison of ozone and chlorine as disinfectants in MPs removal behavior.

coagulation, disinfection, microplastics, polyethylene, RSM
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Recently, microplastics (plastic particles smaller than 5 mm in size; MPs) pollution has been considered a major problem worldwide due to its ubiquity, persistence and potential threat to living organisms (Schmidt et al. 2020; Shruti et al. 2020; Qiang & Cheng 2021). Among the polymer types of plastics, polypropylene (PP; 19.7%), low-density polyethylene (LDPE; 17.4%) and high-density polyethylene (HDPE; 12.9%) comprise the most produced plastic material (PlasticEurope 2021). There are two types of MPs in terms of origination: Primary MPs that are directly manufactured for consumer or industrial purposes including personal care products (Jaikumar et al. 2019), exfoliating products (Adib et al. 2021) and air-blasting technology (Wu et al. 2016) and secondary MPs that originate from the fragmentation of larger plastic materials through weathering (Anderson et al. 2017), laundering (Raju et al. 2018), photodegradation (Hebner & Maurer-Jones 2020) and biological degradation (Sighicelli et al. 2018). Since these particles are not completely removed in wastewater treatment plants (Kay et al. 2018; Prata 2018; Magni et al. 2019) (WWTPs), they end up in freshwater resources through effluent release to the environment along with deposition of airborne MPs (Klein & Fischer 2019; Wright et al. 2020) and people activities (Schmidt et al. 2020). Hence, a significant amount of MPs has been detected in rivers (Lin et al. 2018; Crew et al. 2020) and lakes (Grbić et al. 2020; Mao et al. 2020a).

Generally, drinking water treatment plants (DWTPs) feed from freshwater resources to provide drinking water to people. However, these facilities are not also able to completely remove MPs, thereby a significant number of MPs have been observed in treated water (Pivokonsky et al. 2018; Pivokonský et al. 2020; Wang et al. 2020b; Adib et al. 2021). For example, Tong et al. (2020), according to their results from MP pollution in tap water in China, reported that adult individuals are prone to intake 660 MPs per day (Tong et al. 2020). Not only do MPs cause adverse effects on humans when ingested (Hwang et al. 2020; Çobanoğlu et al. 2021; Zhang et al. 2022), they are able to adsorb other pollutants in the natural environment and desorb them in the body (Llorca et al. 2020; Zhou et al. 2020; You et al. 2021). Based on the identification of MPs in treated water in DWTPs, multiple shapes include fibers, fragments, spheres and films, but in terms of MP size, particles down to the size of one μm have been investigated (Pivokonsky et al. 2018; Pivokonský et al. 2020; Wang et al. 2020b; Adib et al. 2021). In DWTPs, the coagulation/flocculation process has a great role in the removal of MPs (Tang & Hadibarata 2021), but the removal rate in this process differs based on multiple factors including coagulant type and dosage. In some studies, it is observed that the removal rate is as low as 48.4% (Adib et al. 2021) to as high as 88.6% (Wang et al. 2020b). Therefore, removal characteristics of MPs in DWTPs should be further investigated.

Iron and aluminum salts are commonly used as coagulants in the coagulation/flocculation process (Jiang 2015). Therefore, these materials were mostly investigated in MP removal characterization by the coagulation process (Ma et al. 2019a, 2019b; Lu et al. 2021; Xue et al. 2021). For example, Ma et al. (2019a) investigated the removal characteristics of pristine PE MPs by ferric chloride and they observed that 90.91 ± 1.01% of PE MPs were removed by 2 mM (540 mg/L) ferric chloride coupled with 15 mg/L polyacrylamide (Sarmah & Rout 2018) as coagulants. Moreover, Zhang et al. (2021b) reached a removal rate of 91.45% for Polyethylene Terephthalate (PET) MPs with 200 mg/L polyaluminum chloride (Paço et al. 2017) and 100 mg/L PAM. According to the typical coagulant dosage in DWTPs, the amount of coagulants for MP removal in the mentioned studies is very high. However, in some other studies, lower amounts of coagulants with desirable results have been observed (Li et al. 2021; Xue et al. 2021). It is worth mentioning that the removal characterization of pristine and weathered MPs differ due to variation in density, changes in chemical bonds and adsorption of different materials on the surface of MPs (Monira et al. 2021; Nakazawa et al. 2021). Since MPs in nature are weathered, it is not suggested to utilize pristine MPs in removal characterization. In addition, it has been observed that chlorination and ozonation can affect the chemical properties of MPs (Kelkar et al. 2019; Li et al. 2022) which can influence the sinking speed of these particles (Lin et al. 2022). However, to the best knowledge of the authors, no studies have investigated the effect of both coagulation and disinfection on the MP removal rate. Therefore, this study aims to compare the use of ferric chloride and PAC as coagulants to remove MPs through Response Surface Methodology (RSM). In this regard, four variables including pH, PEMPs size, coagulant and PAM concentration will be evaluated. Afterward, the influence of chlorination and ozonation process on MP removal after the coagulation will also be investigated.

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Materials

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All the chemicals used in this study were analytical grade from Merck, Germany, unless stated, including sodium hydroxide (NaOH), hydrochloric acid (HCl), kaolin, sodium chloride (NaCl), humic acid (HA; Sigma Aldrich, USA), ferric chloride (FeCl3). PAC was purchased from Tianshi (Jiangsu) Fine Chemicals Co. (Changzhou, China) and sodium hypochlorite (NaOCl) was provided from Neutron Chemical Company (Tehran, Iran). All the samples were conducted with prefiltered deionized water. Polyethylene microplastics (PEMPs) were provided by milling PE pellets (0.92 g·cm−3, LL 0209 KJ, Shazand Petrochemical company, Iran) by an ultra-centrifugal mill (ZM 200, Retsch®, Germany) until they were micronized. The composition of purchased pellets was characterized by Fourier Transform Infrared Spectrophotometer (FT-IR; Avatar 380, Thermo Scientific, USA). Data acquisition was conducted in the transmission mode of 2 cm−1 resolution, and collection time of 3 s, wavenumber ranging from 400 to 4000 cm−1. Spectra were compared with a database provided by Omnic software (Thermo Phisher Scientific, USA). Before milling, the pellets were immersed in liquid nitrogen until they reached −196 °C. Finally, PEMPs were sieved into two different sizes, including 40 < d < 70 μm and 70 < d < 100 μm. Moreover, clear PE microspheres with the size of 10 < d < 45 μm were provided from Cospheric, CA, USA.

Coagulation experiment

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First, to remove residuals, PEMPs were immersed in 1 M HCl and were placed in oven at 70 °C for 24 h until they were fully dried. As described by Ma et al. (2019a, 2019b) and Adib et al. (2022), 0.5 L beakers were used to carry out the experiments (Ma et al. 2019b; Adib et al. 2022). To simulate turbidity, kaolin was added to DI water until it reached to 5 NTU by a turbidity meter (AL450T-IR, Aqualytic, Germany). Five NTU is the average turbidity of DWTPs of Iran (Hashempour & Mortezazadeh 2020). Moreover, background ionic strength was created by adding 0.1 M NaCl to the solution (Wang et al. 2013) and natural organic matter (NOM) was simulated by adding 1 mg/L HA to the solution. Then, to adjust pH, a prepared 1 M HCl and 1 M NaOH solution was used and the pH of stock solution was measured by a pH meter (3510, Jenway, UK). Afterward, 0.100 g of PEMPs were added to the solution. All the chemicals used in the solution were balanced by precision balance with a minimum range of 1.0 × 10−3 g (LST-JM-102, CGOLDENWALL, China). Finally, a predetermined amount of coagulant (whether PAC or ferric chloride) and anionic PAM were added to the solution 30 s after the start of mixing by a Jar test apparatus (Tak Azama, Iran). The solution was mixed for 1 min at 300 rpm, following a slow stirring of 14 min at 100 rpm (Ma et al. 2019a, 2019b; Monira et al. 2021; Zhang et al. 2021b; Adib et al. 2022). After the stirring, samples were left for 30 min for sedimentation (Sillanpää et al. 2018). In this study, to measure the removed PEMPs, a weighing method was used (Ma et al. 2019b; Adib et al. 2022). In this method, supernatants were carefully removed from the beaker and filtered through a 0.45 μm membrane filter. To digest the flocs, supernatants on the filter were immersed in 1 M HCl for 1 h and treated in an ultrasonic bath for 5 min. Then, the supernatants were filtered again and dried in an oven at 70 °C for 12 h. After drying, according to Equation (1), the weight of the removed PEMPs based on percentage was calculated (Wdried):
(1)

Aside from the characterization of PEMPs, a mixture of these particles trapped in the flocs made by coagulants, with or without PAM was also characterized by FT-IR analysis. Moreover, electro-kinetic potential of the solution in three different pH was also measured by a zeta potential analyzer with a measurement range of ±200 mV (SZ-100, Horiba Scientific, Japan). Furthermore, the dynamic size of the formed flocs (d50) in three pH ranges were measured every 30 s by a static light scattering (SLS) particle size analyzer (PSA) with particle size range of 0.02–2000 mm (SLS-PSA; Mastersizer 2000, Malvern panalytical, UK). Finally, morphology of the PEMPs was analyzed using a scanning electron microscope (Quanta 200, FEI ESEM, USA) equipped with Energy Dispersive X-ray Spectroscopy (EDX). The acceleration of SEM images was 25 kV and working distance (WD) of 9.6 mm. Before imaging, a gold layer was sputtered onto the samples to create conductivity. To prepare samples for SEM analysis, PAC or ferric chloride flocs trapping PEMPs, with or without PAM, were extracted from the bottom of the beaker and filtered through a membrane filter and dried for 12 h at 70 °C in an oven.

Experimental design

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To optimize the parameters and to reach the highest rate of removal, RSM with Box Behnken Design (BBD) was utilized in this study. pH, coagulant dosage, PAM dosage and PEMPs size were chosen as independent variables and the removal rate was chosen as a dependent variable. As per the BBD model, three levels for each parameter were chosen in Design-Expert® software (Version 11, Stat-Ease Inc., USA); 5, 7 and 9 for pH, 6, 12 and 8 ppm for PAM dosage, 50, 200 and 350 ppm for coagulant dosage and 10–45, 40–70 and 70–100 μm for PEMP size (see Table 1; the data of this table were used for both PAC and ferric chloride coagulants).

Table 1

Three levels of chosen independent variables designed by BBD model

FactorUnitLowMiddleHigh
PEMP size μm 10 (10–45) 40 (40–70) 70 (70–100) 
pH – 5 7 9 
Coagulant dosage ppm 50 200 350 
PAM dosage ppm 6 12 18 
FactorUnitLowMiddleHigh
PEMP size μm 10 (10–45) 40 (40–70) 70 (70–100) 
pH – 5 7 9 
Coagulant dosage ppm 50 200 350 
PAM dosage ppm 6 12 18 
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According to Equation (2) (Parsa et al. 2020), 29 experiments were provided by Design-Expert including five replicated center points:
(2)
where N is the number of total experiments, k is the number of independent variables and N0 is the replicated center points. In this equation, 2k and 2k represent factorial points and axial points, respectively. Table 2 represents the 29 experiments with PAC and ferric chloride in different conditions. In total, 58 experiments were conducted with both coagulants.
Table 2

Actual and predicted values of removal rate with ferric chloride and PAC at different conditions

StdA:pHB:PEMP sizeC:Coagulant concentrationD:PAM concentrationMP removal (PAC
MP removal (ferric chloride)
ActualPredictedActualPredicted
–mmppmppm%
1 5 10 200 12 0.5 0.39 18.7 15.79 
2 9 10 200 12 24.7 22.72 55.3 47.59 
3 5 70 200 12 14.7 16.68 16.2 15.54 
4 9 70 200 12 43.9 39.02 32.8 27.34 
5 7 40 50 6 38.2 40.46 36.1 29.09 
6 7 40 350 6 7.8 8.76 12.8 11.64 
7 7 40 50 18 10.9 12.73 18.7 26.39 
8 7 40 350 18 36.5 37.03 11.5 8.94 
9 5 40 200 6 2.3 6.75 8.5 9.47 
10 9 40 200 6 59.1 52.78 25.9 31.27 
11 5 40 200 18 20.8 30.71 6.8 6.77 
12 9 40 200 18 30.2 29.35 24.9 28.57 
13 7 10 50 12 13.5 8.25 34.7 40.41 
14 7 70 50 12 23.9 24.55 28.2 30.16 
15 7 10 350 12 4.7 4.55 22.8 22.96 
16 7 70 350 12 19.5 20.85 12.2 12.71 
17 5 40 50 12 10.9 3.98 26.6 24.39 
18 9 40 50 12 18.9 26.32 44.5 46.19 
19 5 40 350 12 9.6 0.29 0.3 6.94 
20 9 40 350 12 16 22.62 24.5 28.74 
21 7 10 200 6 19.2 22.86 22.5 25.49 
22 7 70 200 6 33.6 39.16 16.4 15.24 
23 7 10 200 18 19.3 23.13 28.7 22.79 
24 7 70 200 18 44.1 39.43 15.4 12.54 
25 7 40 200 12 45.2 41.34 29.8 26.56 
26 7 40 200 12 41.6 41.34 30.7 26.56 
27 7 40 200 12 44.8 41.34 25.3 26.56 
28 7 40 200 12 45.8 41.34 26.2 26.56 
29 7 40 200 12 39.9 41.34 22.8 26.56 
StdA:pHB:PEMP sizeC:Coagulant concentrationD:PAM concentrationMP removal (PAC
MP removal (ferric chloride)
ActualPredictedActualPredicted
–mmppmppm%
1 5 10 200 12 0.5 0.39 18.7 15.79 
2 9 10 200 12 24.7 22.72 55.3 47.59 
3 5 70 200 12 14.7 16.68 16.2 15.54 
4 9 70 200 12 43.9 39.02 32.8 27.34 
5 7 40 50 6 38.2 40.46 36.1 29.09 
6 7 40 350 6 7.8 8.76 12.8 11.64 
7 7 40 50 18 10.9 12.73 18.7 26.39 
8 7 40 350 18 36.5 37.03 11.5 8.94 
9 5 40 200 6 2.3 6.75 8.5 9.47 
10 9 40 200 6 59.1 52.78 25.9 31.27 
11 5 40 200 18 20.8 30.71 6.8 6.77 
12 9 40 200 18 30.2 29.35 24.9 28.57 
13 7 10 50 12 13.5 8.25 34.7 40.41 
14 7 70 50 12 23.9 24.55 28.2 30.16 
15 7 10 350 12 4.7 4.55 22.8 22.96 
16 7 70 350 12 19.5 20.85 12.2 12.71 
17 5 40 50 12 10.9 3.98 26.6 24.39 
18 9 40 50 12 18.9 26.32 44.5 46.19 
19 5 40 350 12 9.6 0.29 0.3 6.94 
20 9 40 350 12 16 22.62 24.5 28.74 
21 7 10 200 6 19.2 22.86 22.5 25.49 
22 7 70 200 6 33.6 39.16 16.4 15.24 
23 7 10 200 18 19.3 23.13 28.7 22.79 
24 7 70 200 18 44.1 39.43 15.4 12.54 
25 7 40 200 12 45.2 41.34 29.8 26.56 
26 7 40 200 12 41.6 41.34 30.7 26.56 
27 7 40 200 12 44.8 41.34 25.3 26.56 
28 7 40 200 12 45.8 41.34 26.2 26.56 
29 7 40 200 12 39.9 41.34 22.8 26.56 

In total, 29 experiments with ferric chloride and 29 experiments with PAC were conducted.

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Based on the fit summary, a quadratic model (p-value < 0.00001) was suggested. In this model, AB, AC, BC, BD and D2 for PAC experiments and AC, AD, BC, BD, CD, A2, B2 and C2 were omitted from analysis of variance (ANOVA), since they were not significant (p-value > 0.1). Moreover, Equation (3) (Parsa et al. 2020) was used as a relationship between independent and dependent variables:
(3)
where Y is predicted response (removal rate), β0 is the constant regression coefficient for the intercept, βi is the linear coefficient, βii is the quadratic coefficient, βij is the interaction coefficient, xi and xj are coded values for independent variables. Using Design-Expert® coefficient (Radj2), standard software, analysis of variance (ANOVA) and regression coefficients were applied to analyze data at the 90% confidence level. Multiple factors of practical data sets were analyzed to test the model fitness, including F-value, p-value, degree of freedom (DF), mean square (MS), sum of squares (SS), correlation coefficient (R), determination coefficient (R2), adjusted R2, standard deviation and coefficient of variance (CV).

Disinfection experiments

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After achieving the optimum points in all four parameters, other experiments were conducted by adding ozone gas (O3) or sodium hypochlorite (NaOCl, 6–14% active chlorine) as disinfectants to simulate disinfection unit of a DWTP. When stirring with the Jar test was finished, the solution was instantly decanted into an Imhoff cone equipped with a glass faucet. Three levels of concentration (1, 2 and 3 mg/L) for O3 or NaOCl (with PAC or ferric chloride) were considered to evaluate the rate of removal at disinfection unit. The O3 gas was generated by an ozone generator with a capacity of 3 g/h. O3 gas was transferred through a plastic hose capped with a rectangular ozone diffuser. Thirty min after sedimentation, O3 or NaOCl were added to the solution and after another 30 min, the faucet was opened to release the settled floc until supernatants remained. Finally, supernatants were collected and immersed in 1 M HCl for flocs containing PEMPs to be digested. Then, PEMPs were filtered and dried in an oven at 70 °C for 12 h.

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Statistical analysis

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To predict the removal rate (response value) in different situations, independent variables (A: pH, B: PEMP size, C: coagulant concentration and D: PAM concentration) were applied in Equations (4) and (5) based on ferric chloride and PAC, respectively:
(4)
(5)
where Y is the response value (removal rate), AB, AD and CD are interaction effects and A2, B2, C2 and D2 are two second-order effects for A, B, C and D, respectively. Table 3 represents analysis of variance (ANOVA) for both responses. All the coefficient variables were evaluated at 95% confidence level. In this table, F-value shows the effectiveness of the variable which was 25.27 and 22.61 for ferric chloride and PAC, respectively. Other evaluated data sets are DF, MS, p-value, coefficient of variance (CV) for checking fitness of the model, standard deviation, R2 and adjusted R2 (Singh & Kumar 2020).
Table 3

Table of ANOVA for the quadratic model for PEMPs removal with ferric chloride and PAC

SourceSum of squaresdfMean squareF-valuep-value
Ferric Chloride 
Model 3177.06 6 529.51 25.27 <0.0001 
A-pH 1425.72 1 1425.72 68.04 <0.0001 
B-MP Size 315.19 1 315.19 15.04 0.0008 
C-Coagulant Concentration 913.51 1 913.51 43.60 <0.0001 
D-PAM Concentration 21.87 1 21.87 1.04 0.3181 
AB 100.00 1 100.00 4.77 0.0399 
D² 400.77 1 400.77 19.13 0.0002 
Residual 460.97 22 20.95   
Lack of Fit 418.28 18 23.24 2.18 0.2354 
Pure Error 42.69 4 10.67   
Cor Total 3638.03 28    
SD = 4.58, C.V. = 19.53%, R2 = 0.8733, R2adj = 0.8387 
PAC 
Model 6351.80 9 705.76 22.61 <0.0001 
A-pH 1496.33 1 1496.33 47.94 <0.0001 
B-MP Size 797.07 1 797.07 25.54 <0.0001 
C-Coagulant Concentration 41.07 1 41.07 1.32 0.2656 
D-PAM Concentration 0.2133 1 0.2133 0.0068 0.9350 
AD 561.69 1 561.69 17.99 0.0004 
CD 784.00 1 784.00 25.12 <0.0001 
A² 881.34 1 881.34 28.24 <0.0001 
B² 699.35 1 699.35 22.41 0.0001 
C² 1852.86 1 1852.86 59.36 <0.0001 
Residual 593.07 19 31.21   
Lack of Fit 566.63 15 37.78 5.72 0.0522 
Pure Error 26.43 4 6.61   
Cor Total 6944.87 28    
SD = 5.59, C.V. = 21.89, R2 = 0.9146, R2adj = 0.8742 
SourceSum of squaresdfMean squareF-valuep-value
Ferric Chloride 
Model 3177.06 6 529.51 25.27 <0.0001 
A-pH 1425.72 1 1425.72 68.04 <0.0001 
B-MP Size 315.19 1 315.19 15.04 0.0008 
C-Coagulant Concentration 913.51 1 913.51 43.60 <0.0001 
D-PAM Concentration 21.87 1 21.87 1.04 0.3181 
AB 100.00 1 100.00 4.77 0.0399 
D² 400.77 1 400.77 19.13 0.0002 
Residual 460.97 22 20.95   
Lack of Fit 418.28 18 23.24 2.18 0.2354 
Pure Error 42.69 4 10.67   
Cor Total 3638.03 28    
SD = 4.58, C.V. = 19.53%, R2 = 0.8733, R2adj = 0.8387 
PAC 
Model 6351.80 9 705.76 22.61 <0.0001 
A-pH 1496.33 1 1496.33 47.94 <0.0001 
B-MP Size 797.07 1 797.07 25.54 <0.0001 
C-Coagulant Concentration 41.07 1 41.07 1.32 0.2656 
D-PAM Concentration 0.2133 1 0.2133 0.0068 0.9350 
AD 561.69 1 561.69 17.99 0.0004 
CD 784.00 1 784.00 25.12 <0.0001 
A² 881.34 1 881.34 28.24 <0.0001 
B² 699.35 1 699.35 22.41 0.0001 
C² 1852.86 1 1852.86 59.36 <0.0001 
Residual 593.07 19 31.21   
Lack of Fit 566.63 15 37.78 5.72 0.0522 
Pure Error 26.43 4 6.61   
Cor Total 6944.87 28    
SD = 5.59, C.V. = 21.89, R2 = 0.9146, R2adj = 0.8742 
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In the chosen model, based on ANOVA, R2 and the adjusted R2 are 0.87 and 0.84 for ferric chloride and 0.92 and 0.87 for PAC, respectively. Generally, higher R2 represents a close relation between actual and predicted value. Moreover, a higher F-value indicates that the variable has more impact on the coagulation process.

Effect of coagulant concentration on PEMP removal

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The concentration of ferric chloride and PAC (50, 200 and 350 ppm) were investigated along with different conditions of other variables. It is clear that by the increase of coagulant dosage, the removal rate did not increase, rather it was observed that nearly no flocs were created in most of the experiments with 350 ppm ferric chloride or PAC and the turbidity of water increased due to the presence of dissolved coagulants, because in lower turbidity, a higher amount of coagulant is not effective to create flocs (Rawi et al. 2014). Therefore, lower amounts of coagulants (less than 200 ppm) were more effective in removal rate. However, based on the BBD model, the optimum concentration of ferric chloride was chosen at the minimum value (50 ppm) and that of PAC was chosen at 128 ppm. This suggests that the optimum concentration of ferric chloride can be found even lower than 50 ppm, but due to the range of investigated concentration, the minimum amount was suggested by the model. Moreover, the effectiveness of the variable in removal rate (F-value) for ferric chloride was 43.6 and for PAC was 1.32. This indicates that ferric chloride was the second most influential variable among other variables, while the effectiveness of PAC along with the other two variables (pH and PEMP size) was less significant. In other similar studies, it has been observed that usage of 2 mM (324 ppm) ferric chloride coupled with PAM removed more than 90% of PEMP of smaller than 500 μm (Ma et al. 2019a). Unlike the present study, they did not face any decrease in removal rate by increasing the coagulant dosage from 1 to 5 mM (270 to 1351 ppm). This can be due to the fact that turbidity was constant in this study. Moreover, Xue et al. (2021), investigated the removal of carboxylated polystyrene microplastics (PSMPs) by alum-based coagulant (10–50 ppm). They observed that by increasing the coagulant dosage, the removal rate of larger particles improved, while similar to this study, the removal rate of smaller particles did not improve when the dosage of coagulant was more than 30 ppm. They also speculated that turbidity of water influenced removal rate (Xue et al. 2021). Furthermore, Zhou et al. (2021) investigated the removal rate of PSMPs and PEMPs by ferric chloride and PAC and, similar to this study, they observed that PAC caused better results in removal rate (Zhou et al. 2021). In their study, different concentrations of coagulants, from 0 to 180 ppm, were examined. They observed that when PAC was 90 ppm, the removal efficiency of PEMPs was 29.7%, while with the increase in coagulant dosage, the removal rate either decreased or did not change. In a recent study, Adib et al. (2022) studied the removal of PPMPs in the coagulation process by PAC and anionic PAM. Due to the low density of PEMPs, they reached only 18.75% removal with PAC concentration of 200 ppm. This amount of PAC was at the minimum of the chosen range, which indicates that the chosen range in this study (50–350 ppm) was properly chosen. Figure 1 shows the morphology of PEMPs and PEMPs trapped in flocs (circled in yellow). EDX results of the following SEM images are also provided. From the surface of PEMPs, it is clear that the surface of PEMPs is rough and wrinkled which indicates that they were fairly aged (Mao et al. 2020b). Since PEMPs were kept in the laboratory for almost a year under room temperature and limited source of lamp light and the fact that they were able to adsorb HA (as a model of NOM), PEMPs are not considered virgin in this study.
Figure 1
SEM images: (a) PEMPs trapped in ferric chloride flocs, (b) flocs of ferric chloride without PEMPs, (c) PEMPs trapped in PAC flocs, (d) PAC flocs without PEMPs, (e) and (f) illustrate morphology of PEMPs before coagulation, (g) EDX result of ferric chloride flocs, (h) EDX result of PAC flocs and (i) EDX result of PEMPs.
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SEM images: (a) PEMPs trapped in ferric chloride flocs, (b) flocs of ferric chloride without PEMPs, (c) PEMPs trapped in PAC flocs, (d) PAC flocs without PEMPs, (e) and (f) illustrate morphology of PEMPs before coagulation, (g) EDX result of ferric chloride flocs, (h) EDX result of PAC flocs and (i) EDX result of PEMPs.

Figure 1
SEM images: (a) PEMPs trapped in ferric chloride flocs, (b) flocs of ferric chloride without PEMPs, (c) PEMPs trapped in PAC flocs, (d) PAC flocs without PEMPs, (e) and (f) illustrate morphology of PEMPs before coagulation, (g) EDX result of ferric chloride flocs, (h) EDX result of PAC flocs and (i) EDX result of PEMPs.
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SEM images: (a) PEMPs trapped in ferric chloride flocs, (b) flocs of ferric chloride without PEMPs, (c) PEMPs trapped in PAC flocs, (d) PAC flocs without PEMPs, (e) and (f) illustrate morphology of PEMPs before coagulation, (g) EDX result of ferric chloride flocs, (h) EDX result of PAC flocs and (i) EDX result of PEMPs.

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Effect of pH on PEMP removal

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Based on the BBD model, pH 5, 7 and 9 were studied in this process. In all the experiments, both in ferric chloride and PAC experiments, it has been observed that with the increase of pH, the removal rate of PEMPs increased. The effectiveness of this variable was highest among other variables in both PAC (F-value = 47.94) and ferric chloride (F-value = 68.04) experiments. According to the PSA results (Figure 2), with the increase in pH, the size of created flocs increased. Therefore, more PEMPs can be trapped into the flocs and settle due to the heavier agglomerated mass. Accordingly, the optimum level of pH was detected at the highest level of analyzed range (pH = 9). Based on Figure 2, mean floc size at pH 9 reached over 800 μm after 5 min, while it remained under 600 and 400 μm at pH 7 and 5, respectively. Moreover, it was clear that mean floc size did not change dramatically after 5 min, so the analysis of PAS stopped at 10 min. In a similar study, Ma et al. (2019a), by analyzing PSA results of floc size at same three pHs, observed that ferric chloride floc sizes were approximately 400, 600 and near 800 μm at pH 6, 7 and 8 (Ma et al. 2019a, 2019b). However, they continued the experiment with PSA until 15 min, but the floc size remained almost constant. In contrary to this study, Esfandiari & Mowla (2021) observed that a better removal rate of PEMPs happens at lower pH (Esfandiari & Mowla 2021). They analyzed the removal rate of PEMPs by coagulation and dissolved air flotation (DAF) at three pHs of (6, 7 and 8) through RSM. Although similar to this study, the removal rate at pH 6 with AlCl36H2O was 59.5% which is very close to the result of the present study.
Figure 2
Created floc size (d50) based on time detected by PSA at three different pHs.
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Created floc size (d50) based on time detected by PSA at three different pHs.

Figure 2
Created floc size (d50) based on time detected by PSA at three different pHs.
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Created floc size (d50) based on time detected by PSA at three different pHs.

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According to the zeta potential analysis (Figure 3), by the increase of pH, zeta potential decreased which means that particles tend to be less stable in comparison with lower pH. This supports the prediction of RSM in the BBD model that higher pH causes more removal of PEMPs. In this analysis, the zeta potential at pH 5 was 11.2 ± 1.3 mV, but the increase of pH to 7 and 9, the zeta potential reduced to 8.1 ± 1.2 and 4.6 ± 0.9 mV, respectively. However, at pH 10, the zeta potential faced a negligible change which indicated that the removal rate does not appear to increase above pH 10.
Figure 3
Zeta potential of flocs created by ferric chloride (in red) and PAC (in blue) at different pH.
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Zeta potential of flocs created by ferric chloride (in red) and PAC (in blue) at different pH.

Figure 3
Zeta potential of flocs created by ferric chloride (in red) and PAC (in blue) at different pH.
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Zeta potential of flocs created by ferric chloride (in red) and PAC (in blue) at different pH.

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In a study by Zhang et al. (2021a), coagulation removal of PEMPs in wastewater were analyzed via magnetic magnesium hydroxide coagulant (MMHC) and PAM as coagulants. In zeta potential analysis, they observed that zeta potential increased from −18 mV to −10 mV when pH increased from 7 to 9 which indicates that coagulant and kaolin interact with each other to form flocs (Zhang et al. 2021a). Similarly, Adib et al. (2022) analyzed zeta potential results in different pH. They similarly found out that by the increase of pH from 5 to 9, zeta potential decreased from 11.35 ± 0.15 to 3.85 ± 0.15%, approaching zero (Adib et al. 2022). This is in line with the results of this study, since zeta potential near zero means instability of particles and their inclination toward settling (Selvamani 2019). Moreover, Lu et al. (2021) faced a different result. They tested zeta potential at different concentrations of Al3+ as coagulant under pH 6, 7 and 8. They reported that dosage of coagulant was significant in the removal of PET-MPs, while the solution pH was not (Lu et al. 2021). They reckoned that discordant results in zeta potential is due to the various types of MPs analyzed and different conditions of mixing solution in the Jar test.

Effect of PEMP size on PEMP removal

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Three size ranges of PEMPs including 10–45, 40–70 and 70–100 μm were studied in this process. This variable was the third influential parameter in the ferric chloride experiment (F-value = 15.04), but with F-value = 25.54, the effectiveness was the second among the other three variables. In the experiment with ferric chloride, the optimum particle size was detected at the minimum of the analyzed range (10 μm: 10–45 μm), while the optimum size in PAC experiments was 52 μm: 40–70 μm. This can be due to the smaller dynamic size of the flocs created by ferric chloride, since larger flocs are capable of trapping larger MPs. Figure 4 is the result of characterization of PEMPs with coagulants by FTIR. In combination with PEMPs + ferric chloride and PEMPs + ferric chloride + PAM, Alcohol/Phenol O-H Stretch (3200–3550 cm−1) is more pronounced than other spectra. Moreover, Alkyl C-H Stretch (2850–2950 cm−1) is stronger in PEMPs spectrum than its combination with coagulants. Furthermore, Aromatic C = C Bending (1500–1700 cm−1) became stronger in PEMPs and coagulants than PEMPs alone. However, 721 cm−1 (rocking deformation) and 1468 cm−1 (bending deformation) in PEMPs decreased in strength in combined coagulant spectra.
Figure 4
FTIR analysis of PEMPs and PEMPs trapped in flocs of PAC or ferric chloride with the presence or absence of PAM.
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FTIR analysis of PEMPs and PEMPs trapped in flocs of PAC or ferric chloride with the presence or absence of PAM.

Figure 4
FTIR analysis of PEMPs and PEMPs trapped in flocs of PAC or ferric chloride with the presence or absence of PAM.
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FTIR analysis of PEMPs and PEMPs trapped in flocs of PAC or ferric chloride with the presence or absence of PAM.

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Figure 5 illustrates diagrams related to the result of optimum condition with ferric chloride. Figure 5(a) and 5(b) show 3D and 2D diagrams of pH and PEMPs size interaction. It is clear that with the decrease in PEMPs size, the removal rate increases. Figure 5(d) shows that PEMPs size and pH influence on removal rate is linear. In different studies that investigated coagulation removal of MPs, the size of these particles has been influential in the sinking behavior of flocs. For example, Xue et al. (2021) used 6, 25, 45 and 90 μm PS microspheres in coagulation-flocculation sedimentation (CFS) treatment. They reported that the removal of MPs decreased with an increase in their size (Xue et al. 2021). Similarly, Adib et al. (2022) and Ma et al. (2019a) reported that smaller PEMPs can be easily removed in comparison with larger particles (Ma et al. 2019b; Adib et al. 2022). However, Zhang et al. (2021b) investigated the coagulation of coagulation characteristics of PET-MPs and observed that larger MPs with the size of 400–500 μm were removed better than smaller PET-MPs with the size of 100–400 and smaller than 100 μm (Zhang et al. 2021b). This difference in results can be ascribed to the polymer type of MPs.
Figure 5
Diagrams of optimum condition with ferric chloride in which (a) shows 3-D diagram of interaction between pH and PEMP size, (b) illustrates 2-D diagram of pH and PEMP size interaction, (c) illustrates comparison of 29 actual and predicted experiments and (d) shows perturbation plot of four analyzed variables.
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Diagrams of optimum condition with ferric chloride in which (a) shows 3-D diagram of interaction between pH and PEMP size, (b) illustrates 2-D diagram of pH and PEMP size interaction, (c) illustrates comparison of 29 actual and predicted experiments and (d) shows perturbation plot of four analyzed variables.

Figure 5
Diagrams of optimum condition with ferric chloride in which (a) shows 3-D diagram of interaction between pH and PEMP size, (b) illustrates 2-D diagram of pH and PEMP size interaction, (c) illustrates comparison of 29 actual and predicted experiments and (d) shows perturbation plot of four analyzed variables.
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Diagrams of optimum condition with ferric chloride in which (a) shows 3-D diagram of interaction between pH and PEMP size, (b) illustrates 2-D diagram of pH and PEMP size interaction, (c) illustrates comparison of 29 actual and predicted experiments and (d) shows perturbation plot of four analyzed variables.

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Effect of PAM concentration on PEMP removal

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PAM is a coagulant-aid that is usually added to water coupled with coagulants to boost the sedimentation rate of suspended particles (Xiong et al. 2018). In this study, this variable had the least efficacy in both ferric chloride (F-value = 1.04) and PAC (F-value = 0.007) experiments. The optimum dosage of PAM in ferric chloride experiments was detected at 11.4 ppm, but with PAC experiments it was the minimum of the analyzed range (6 ppm). Figure 5 represents diagrams of coagulation experiments with PAC. Figure 5(a) shows a 3D diagram of interaction between PAM dosage and pH. With the decrease in PAM dosage, it is clear that removal rate decreases too. However, excess in using PAM did not have a direct relationship with removing PEMPs and the removal rate remained almost constant. This effect has been observed in other studies too (Ahmad et al. 2008; Kim 2016; Liu et al. 2017). For example, Ma et al. (2019b) observed that usage of PAM increased the removal rate of small PEMPs (smaller than 0.5 mm), but the highest removal rate with anionic PAM was at 3 ppm and for cationic PAM was 6 ppm. More usage of PAM decreased the removal rate (Ma et al. 2019b). However, Zhang et al. (2021a) changed the value of PAM in the presence of a constant amount of mg(OH)2 and F3O4 to remove virgin PEMPs smaller than 270 μm. They reported that with the increase in PAM dosage, removal rate increased, but they stopped the test at 5 ppm of PAM (Zhang et al. 2021a).

Optimization

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Based on the BBD model in Design-Expert software, variables were optimized to reach the maximum removal rate of PEMPs. In this regard, based on Figure 6, in the experiments with ferric chloride as a coagulant, the optimum condition of variables were pH 9, PEMP size 10 μm (10–45 μm), ferric chloride dosage of 50 ppm and PAM dosage of 11.4 ppm. With the mentioned conditions, maximum removal rate was predicted to be 56.4% which was acquired by setting the variables ‘In range’ and the response in ‘Maximum’ (Figure 7). The maximum predicted removal rate was chosen among 100 optimum conditions with the desirability of 0.562. Furthermore, in experiments with PAC as a coagulant, the predicted optimum condition was pH 9, PEMP size 52 μm (40–70 μm), PAC dosage of 128 ppm and PAM dosage of 6 ppm. With the mentioned conditions, the maximum removal rate of PEMPs was predicted to be 58.2%. This removal rate was also based on setting independent variables in ‘In range’ and the dependent variable (removal rate) in ‘Maximum’ and the desirability of 0.580. Finally, to test the fitness of the model with actual conditions, three experiments with PAC and three experiments with ferric chloride under the suggested conditions were conducted. In actual experiments, removal rate with PAC and ferric chloride was 51.3 ± 2.33 and 49.1 ± 2.10%, respectively. This result shows that the chosen model fairly fits the actual results. In another study, Zhang et al. (2021a) observed the PEMPs removal rate of 87.1% with MMHC and 92% with MMHC + Fe3O4 + Mg(OH)2. These other coagulants that are not necessarily common can be more efficient in removing PEMPs (Zhang et al. 2021a). Moreover, Adib et al. (2022) in an optimized coagulation process via RSM observed the removal rate of 18.00 ± 1.43% of PPMPs with PAC as coagulant (Adib et al. 2022). The reason of weak removal rate can be attributed to low density of PPMPs.
Figure 6
Diagrams of optimum condition with PAC in which (a) shows 3-D diagram of interaction between pH and PAM concentration, (b) illustrates 2-D diagram of pH and PEMP size interaction, (c) illustrates comparison of 29 actual and predicted experiments and (d) shows perturbation plot of four analyzed variables.
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Diagrams of optimum condition with PAC in which (a) shows 3-D diagram of interaction between pH and PAM concentration, (b) illustrates 2-D diagram of pH and PEMP size interaction, (c) illustrates comparison of 29 actual and predicted experiments and (d) shows perturbation plot of four analyzed variables.

Figure 6
Diagrams of optimum condition with PAC in which (a) shows 3-D diagram of interaction between pH and PAM concentration, (b) illustrates 2-D diagram of pH and PEMP size interaction, (c) illustrates comparison of 29 actual and predicted experiments and (d) shows perturbation plot of four analyzed variables.
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Diagrams of optimum condition with PAC in which (a) shows 3-D diagram of interaction between pH and PAM concentration, (b) illustrates 2-D diagram of pH and PEMP size interaction, (c) illustrates comparison of 29 actual and predicted experiments and (d) shows perturbation plot of four analyzed variables.

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Effect of disinfectants on removal rate

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In this process, the effect of two common disinfectants, including O3 and NaOCl, on removal rate of PEMPs was investigated. Common concentration of both the disinfectants used in DWTPs is between 1 and 2 ppm. In this regard, three concentrations of 1, 2 and 3 ppm were analyzed. All the experiments were conducted in suggested optimum conditions, while after 30 min of sedimentation, disinfectant was added to know if more PEMP settle. Figure 8 shows the removal rate of PEMPs with suggested concentrations of PAC or ferric chloride when 1–3 ppm O3 or NaOCl was added.
Figure 7
Predicted optimum condition for reaching the maximum removal rate in which (a) represent the conditions with PAC as coagulant and (b) represents ferric chloride as coagulant.
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Predicted optimum condition for reaching the maximum removal rate in which (a) represent the conditions with PAC as coagulant and (b) represents ferric chloride as coagulant.

Figure 7
Predicted optimum condition for reaching the maximum removal rate in which (a) represent the conditions with PAC as coagulant and (b) represents ferric chloride as coagulant.
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Predicted optimum condition for reaching the maximum removal rate in which (a) represent the conditions with PAC as coagulant and (b) represents ferric chloride as coagulant.

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Figure 8
Removal rate of PEMPs with suggested optimal condition and different concentrations of disinfectants. All the experiments in this section were conducted in triplicate.
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Removal rate of PEMPs with suggested optimal condition and different concentrations of disinfectants. All the experiments in this section were conducted in triplicate.

Figure 8
Removal rate of PEMPs with suggested optimal condition and different concentrations of disinfectants. All the experiments in this section were conducted in triplicate.
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Removal rate of PEMPs with suggested optimal condition and different concentrations of disinfectants. All the experiments in this section were conducted in triplicate.

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According to Figure 8, O3 and NaOCl had an influence on the removal rate of PEMPs after the coagulation process. One ppm of O3 or NaOCl had a better result with ferric chloride than 2 or 3 ppm, increasing the removal rate to 58.9 and 71.5%, respectively, whereas 2 ppm of O3 and NaOCl decreased the removal rate to 48.9 and 39.5%, respectively. On the other hand, 2 ppm of O3 or NaOCl had a better result in removing PEMPs than 1 or 3 ppm in experiments with PAC. It has been observed that disinfecting 2 ppm of solution with O3 or NaOCl can increase the removal rate to 76.8 and 69.1%, respectively. This difference in results with these variations in disinfectant concentration entails that more studies need to be conducted to better investigate the effect of disinfectants on MPs removal behavior. A recent study has investigated the sinking behavior of PSMPs after disinfection. Lin et al. (2022) observed that in O3 treated MPs, the sinking ratio was increased to 62.3%, while chlorinated MPs decreased to a 20% sinking ratio (Lin et al. 2022). This difference in MPs sedimentation in contact with disinfectants can be due to degradation and mineralization of MPs surface (Li et al. 2022).

Further study

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This study indicated that the coagulation process is not capable of removing a considerable amount of PEMPs, so other studies need to be conducted to elucidate the role of other treatment sections including granular activated carbon (GAC) or sand filtration on the removal of specific polymers of MPs. Moreover, this study showed that disinfection systems influence the removal rate. However, there usually are two steps of disinfection processes in DWTPs (primary and secondary) where the second step is right before the distribution system (Godo-Pla et al. 2021). Therefore, only the primary step can influence the removal rate of PEMPs in the coagulation process. Moreover, MPs are susceptible to adsorbing chemicals and undergo weathering in nature (Li et al. 2018; Wang et al. 2020a; Abdurahman et al. 2020; Llorca et al. 2020). Since this study did not control the aging of PEMPs, more studies need to be conducted to analyze the influence the rate of weathering on removal rate. According to the results of this study, PAC was a better coagulant than ferric chloride in coagulation process. Therefore, other studies are needed to analyze different types of coagulants in PEMP removal.

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This study investigated the removal characteristics of PEMPs through RSM with two commonly used coagulants and the effect of common disinfectants in PEMPs removal after coagulation. Four parameters were investigated and optimized in the coagulation process including, pH, PEMP size, coagulant and PAM concentration. In experiments with PAC as a coagulant, optimum conditions for PAM and PAC concentrations were 6 and 128 ppm, respectively, while in experiments with ferric chloride as a coagulant, the mentioned parameters were optimized at 11.4 and 50 ppm, respectively. In addition, PEMP size was optimized at 52 μm when PAC was coagulant, but when the ferric chloride was the coagulant, PEMP size was optimized at the minimum of the analyzed range. In addition, the optimum pH condition was 9 for both experiment sets with ferric chloride and PAC. According to the results, PAC was a better coagulant than ferric chloride in removing PEMPs in the coagulation process, removing 51.3 ± 2.33% in optimum conditions. Moreover, it was observed that O3 and NaOCl as disinfectants had a significant influence on PEMPs removal after the coagulation process. Both the disinfectants increased the removal rate when PAC was coagulant in comparison with the optimum condition in the coagulation process without disinfectant (51.3 ± 2.33 and 49.1 ± 2.10% with PAC and ferric chloride, respectively).

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All the expenses of this study were supported by Fatemeh Tabatabaei. We are grateful to the Islamic Azad University, West Tehran Branch for providing equipment to conduct this study.

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

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The authors declare there is no conflict.

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