Characteristics of ball-milled PET plastic char for adsorption of different types of aromatic organic pollutants

doi:10.21203/rs.3.rs-1310008/v1

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Characteristics of ball-milled PET plastic char for adsorption of different types of aromatic organic pollutants

https://doi.org/10.21203/rs.3.rs-1310008/v1

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Ball-milled plastic char (BMPC) was manufactured by ball-milling of native plastic char (PC) that was synthesized via slow pyrolysis of PET water bottle waste, by which the adsorption characteristics of aqueous phenanthrene, phenol and 2,4,6-trichlorophenol, and its possible mechanisms were investigated. The special surface area of BMPCs increased obviously as the pyrolysis temperature increased, forming large functional groups compared to PCs. Boehm titration showed that total acidic groups of BMPCs decreased significantly with the increase of pyrolysis temperature. The sorption kinetics of three adsorbates was adequately simulated by Pseudo second-order model (R²＞0.99). Langmuir model fitted well the adsorption isotherms of phenanthrene and phenol, while Freundlich model simulated the adsorption isotherm of 2,4,6-trichlorophenol. The adsorption amount of phenanthrene, phenol and 2,4,6-trichlorophenol increased significantly as the pyrolysis temperature increased. The maximum BMPCs adsorption amount reached 21.9mg/g (for phenanthrene), 106mg/g (for phenol) and 303mg/g (for 2,4,6-trichlorophenol) at 25℃ in aqueous solution. FTIR analysis suggested that surface sorption based π-π interaction was dominant mechanism of phenanthrene adsorption, meanwhile, H-bonding between O-containing groups on BMPCs and hydroxyl groups of absorbates was responsible for phenol and 2,4,6-TCP removal. This paper shows that BMPCs can be used as adsorbent for treating aromatic compounds in aqueous environment, and has an economic worth of application.

Ball-milled plastic char

PET

adsorption

phenanthrene

phenol

6-trichlorophenol

Polyethylene terephthalate (PET), as most important engineering polymer, is harmless to human body, and widely used in all fields of life and industry, such as water bottle, packaging foods, liquid soap and cooking oil, manufacturing films and fibers etc. Enormous consumption of PET plastic leads to abundant accumulation of PET wastes (Ansari et al. 2021), that is, PET waste generation increased from 2015 (13 million tons) to 2019 at a rate of 5.2% annually (Kim et al. 2020). According to official statistics, nearly 20, 000 waste plastic bottles have been thrown away every second, and the recycling rate for these PET bottles is probably 72.1% in Japan, but only 48.3% in Europe and 31% in the United States. Therefore, any new methodologies for the recycling these plastics have been proposed even though its applicability is not so plausible at larger scale (Fuks et al. 2021). Pyrolysis based thermochemical principle is effective technique to convert solid wastes into useable gas, liquid, and solid products in the condition of oxygen-free, and the technology is mature and widely used to treat large amounts of municipal solid wastes (Al-Salem et al. 2017; Dhahak et al. 2020; Muhammad et al. 2015). Recently, it has been proved that the solid by-product (plastic coke) pyrolyzed at temperatures typically between 500℃ and 780℃ (Muhammad et al. 2015) can be efficient for the removal of heavy metals (Fuks et al. 2021; Singh, E. et al. 2020; Singh et al. 2021). It has been improved that the pyrolysis under the inert atmosphere made PET as a highly porous material (Bratek et al. 2013) with pores in the micro-/meso- sizes, which is applicable for different adsorption processes (Parra et al. 2006). Thermal decomposition of PET is initially due to the random disconnection of chains on ester bonds to produce carboxyl and vinyl esters. The thermal decomposition of PET results in the product with high content of aldehides at low temperature treatments, and one with a high content of aromatics at high temperature treatments (Tang et al. 2015). The specific surface area gives also a significant effect on the adsorption of various organic pollutants (Guo et al. 2017). Ball-milling method, as one of the methods to improve the absorbent properties, is a cost-effective way to increase the specific surface area of carbon materials, therefore, ball-milled biochar has a good adsorption effect on heavy metals, dyes and organic pollutants in water and wastewater (Amusat et al. 2021; Xiang et al. 2020).

The organic pollutants studied in here, such as phenanthrene (PHE), phenol, 2,4,6-trichlorophenol (2,4,6-TCP) are aromatic hydrocarbons, which are all toxic and recalcitrant, and their physiochemical properties are different. PHE is a typical aromatic (3-ring) carbides of polycyclic aromatic hydrocarbons, due to its low solubility to water (1.2mg/L at 25℃) (Zindler et al. 2016), it has high ecotoxicity and long-term persistence, and is considered as an environmental pollutant that can cause acute and chronic diseases to the human health (Kumar et al. 2021). On the other hand, phenol as the basic raw materials of industry, is weak acid and has high toxicity and high solubility to water (85g/L at 25℃) (Guo et al. 2021). It seriously polluted many kinds of industry wastewater, e.g., 6-500 mg/L in refinery wastewater, 28-3, 900 mg/L in coking wastewater, and 2.8-1, 220 mg/L in petrochemical industry wastewater (Mei et al. 2020), and undoubtedly its direct discharge to river will bring serious environmental problems (Yan et al. 2018). 2,4,6-TCP has been reported to cause many health diseases, such as respiration, cardiovascular effects, gastrointestinal effects, and cancer except for nervous toxicity to human (Fan et al. 2011). In addition, the high solubility of 2,4,6-TCP at room temperature (434mg/L in water) (Pei et al. 2013) as well as the stable C-Cl bonds and the positions of chlorine atoms in phenol ring lead to higher toxicity and endurance in the environment comparing with phenol (Tan et al. 2009).

Many methods have been proposed to remove organic pollutants, especially with the development of adsorption method, adsorption has been considered to be the most effective and economic approach to remove different organic pollutants from aqueous environments. For example, adsorption capacities of PHE and phenol were 15.8mg/g( for PHE), 166mg/g (for phenol) with biochar based on Bamboo (Mohammed et al. 2018; Tang et al. 2015) and 367mg/g-2,4,6-TCP with Loosestrife-based activated carbon at 25℃, respectively(Fan et al. 2011). PET chemical structure was consisted of the ring structure, so the thermal decomposition of PET-plastic char will also contain many ring-structures, which can effectively adsorb hydrophobic organic materials. In the previous study, the plastic char mainly consisted of mesoporous and macroporous material with adsorption capacities being 3.59–22.2mg/g for methylene blue dye. This indicates that the plastic char can also have good adsorption effect on organic molecules (Bernardo et al. 2012; Sharuddin et al. 2016).

In the study of PET pyrolysis, it is mainly concentrated on the gas and liquid production with the change of thermal decomposition temperature (Dimitrov et al. 2013), however, there were few studies on the characteristics of solid products (Fuks et al. 2021; Parra et al. 2006; Sogancioglu et al. 2017), especially there has been no study on the adsorption and desorption characteristics of organic pollutants by PET plastic char produced from different pyrolysis temperatures. The purpose of this paper is, first, to analyze the physiochemical characteristics of ball-milled plastic chars (BMPCs) produced at different pyrolysis temperatures through the adsorption capacities of PHE, phenol and 2,4,6-TCP in aqueous environment; second, to investigate the adsorption mechanism of BMPCs and relevant factors.

Experimental materials

In this study, Wahaha water bottle was used as raw material for the plastic char. PHE, phenol and 2,4,6-TCP (MERYER 99%) as pollutants, calcium chloride anhydrous (CaCl₂) as regulator of ionic strength and sodium azide (NaN₃) as an inhibitor of microbial growth were purchased from Tianjin Huaxun Medical Technology Co., Ltd (China).

The production of BMPCs

Wahaha water bottle was washed and air-dried, and crushed into pieces with the size below 0.5cm×0.5cm. Firstly, 6g PET pieces was pyrolyzed under various pyrolysis temperatures in N₂ gas condition using a tubular furnace (TDRG, Tengda Thermal Technology Co., Ltd., Yixing, China). The pyrolysis experiment was carried out at different temperature (500, 600, 700, and 800℃) for 30 minutes (See detail in Text S1). Secondly, the four kinds of plastic char samples were ball-milled (F-P2000, Focucy, Hunan, China) for 24 hours (See detail in Text S2).

Characterization

A multiple-point Brunauer Emmett Teller(BET) method was applied to measure the surface area, pore size and pore volume of ball-milled plastic char (BMPC) (ASAP2460, Micromeritics, Atlanta, USA), and element analysis was determined using the CHN/O Analyzer (EA3000, Italy). The pH value was measured by a pH meter (PB-10, Sartorius, Goettingen, Germany). SEM analysis (JSM-7800F, JEOL, Japan) were performed to monitor the shapes and surface morphologies of the samples. Raman spectroscopy (SR-500I-A, TEO, USA) was used to characterize the aromatic structure. The functional groups of BMPCs were investigated by Boehm titration method (See detail in Text S3) and Fourier transform infrared spectroscopy (FTIR) measurements (TENSOR 37, BRUKER, Germany). Zeta potential was determined with a Zetasizer Nano ZS90 (Malvern Instruments, Malvern, UK).

Adsorption experiment

Effects of pyrolysis temperature on PHE, phenol and 2,4,6-TCP sorption capacity

PHE, phenol and 2,4,6-TCP were adopted as experimental organic pollutants, respectively. The adsorption characteristics of BMPCs prepared at different pyrolysis temperatures were studied in batches. For PHE adsorption experiment, test solution of PHE at various concentration was prepared with the PHE stock solution diluted with HPLC-grade methanol containing 0.01mol/L CaCl₂ (ionic strength adjuster) and 200 mg/L NaN₃ (biocide) solutions. The concentration of methanol in the final solutions was always controlled below 0.1% (v/v) (S. 2001). 2mg of BMPCs were respectively put into 50mL brown vials and then a 40mL aliquot of PHE solution (0.6 mg/L) was poured into the vials. The vials were then tightly capped with Teflon-lined screw caps and vibrated at 150r/min on a reciprocating shaker (HNY-2102C, Honour Instrument Co., Ltd., Tianjin, China) under the condition of the dark at 25℃ for 3 days (Jin et al. 2018; Zhao et al. 2014). As for phenol and 2,4,6-TCP adsorption, samples (10mg) of BMPCs were put into 50mL brown vials and then a 40mL aliquot of phenol and 2,4,6-TCP solution (the initial phenol and 2,4,6-TCP concentration were 10 mg/Land 50 mg/L, respectively) was poured into every vial, and carried out for one day. Other operational condition was same as above. Controls were studied in absence of BMPCs dosage, and controls and samples were repeated least triplicate in every experiment.

Influences of BMPCs dosage on tree absorbates removal efficiency

In order to determine the effective dosage for PHE, phenol and 2,4,6-TCP removal, experiments were carried out according to change of BMPCs dosage (0.025-0.15g/L for 0.6mg/L PHE, 0.125-0.375g/L for 10 mg/L phenol, and 0.125-0.5g/L for 50 mg/L 2,4,6-TCP).

Adsorption kinetics of three absorbates

Initial solution concentration of 0.6mg/L PHE, 10mg/L phenol and 50mg/L 2,4,6-TCP was individually used to describe sorption kinetic of PHE, phenol and 2,4,6-TCP, while BMPCs dosage was 0.05g/L for PHE solution and 0.25g/L for phenol and 2,4,6-TCP solution.

Adsorption isotherm of three absorbates

For the sorption isotherm study, experiments were carried out according to initial concentration change of PHE, phenol and 2,4,6-TCP (0.6-1mg/Lfor PHE, 10-50mg/Lfor phenol and 50-100 mg/L for 2,4,6-TCP), in here, dosage in these experiments were 0.05g/L BMPCs per 40mL of PHE solution and 0.25g/L BMPCs per 40mL of phenol and 2,4,6-TCP solution, and experiment was carried out with 72 hr for PHE adsorption and with 24 hr for phenol and 2,4,6-TCP adsorption test.

Analytical methods

The determination of PHE is as follows. 0.5mL of each supernatant that was centrifuged at 4000 r/min for 10min was diluted with 0.5mL of HPLC grade methanol, then followed by filtration using a 0.22 nm PTEF filter membrane. Agilent HPLC 1260 (USA) equipped with a Thermo Scientific C18 column (25cm×4.6 mm) was used for PHE quantification (Shi et al. 2018). A mixture of acetonitrile and Milipore (80:20) water was eluted at a flow rate of 1 mL/min and system temperature of 30℃, and detected at wavelength of 254nm with a UV detector for PHE determination.

Whilst, the following manipulation as performed for phenol and 2,4,6-TCP. 1.5mL of supernatants that were centrifuged at 4000 r/min for 5min were filtered using a 0.22 nm PTEF filter membrane, and then was measured by HPLC. The method is the same as above, but the wavelength (280nm) and the system temperature (25℃) were different to the above.

Physicochemical characterization of BMPCs

The product of PET (C₁₂H₈O₄)_n pyrolysis were gas, waxy and carbonaceous residue (Dhahak et al. 2019), as pyrolysis temperature increases, production yields of solid residues (carbonaceous residue) decreased slightly from 18.9 ± 0.2wt% at 500°C to 13.6 ± 0.2wt% at 800°C. Solid residues from PET pyrolysis were initially lumped like slag, of which, the specific surface area of native plastic chars (PCs) was relatively small, and size of them was arranged from hundreds to thousands of microns (See detail in Table S1). In general, specific surface area is considered to be an important index of the adsorbent sorption ability, therefore, in order to increase specific surface area, produced carbonaceous residues were ball milled for 24 hr. PCs were initially looked like black flakes, couldn`t be distinguished each other, however, after ball-milling, BMPCs could be identified with different pyrolysis temperature. Color of BMPCs was changed from black-brown to strong black as pyrolysis temperature raised (See detail in Figure S1). Specific surface area of BMPCs was increased from 361m²/g to 583m²/g as the pyrolysis temperature raised. pH value is another index to explain adsorbent sorption too (Mohammed et al. 2018; Ri et al. 2022). BMPCs and PCs were weak acid with pH values of 6.41-6.75 and 6.83-6.91, respectively (See detail in Text S1) with the increase of pyrolysis temperature from 500℃ to 800℃. Ball milling made pH values of BMPCs reduced comparing to PCs. The carbon contents of BMPCs increased from 86.4–91.236% when pyrolysis temperature increase from 500℃ to 800℃ (Table 1), whereas hydrogen and oxygen contents were decreased from 4.59–2.48% and from 8.98–6.28%, respectively. According to previous reports, the gas produced during the pyrolysis of plastics such as HDPE, LDPE, PP, PS, PET and PVC contains hydrogen, methane, ethane, ethene, propane, propen, butane and butane (Martı´n-Gullo´n 2001; Sharuddin et al. 2016; Singh, R.K. et al. 2020). In the range of 500-700℃, the content of H₂ gradually increased accompanied by the decrease of CO content with temperature increase, and the contents of CH₄ and CO₂ kept constant. On contrary, the increased of H₂ and CH₄ while the decrease of CO and CO₂ occurred at the stage of 700-800℃. It is worthwhile that the gas fraction of CH₄ increased at higher temperatures during PET pyrolysis (Bai et al. 2020), therefore, BMPC-700 had a high oxygen content more than the others. On the other hand, the ratios of H/C varied from 0.637 to 0.326 concurrently with the aromaticity index (See detail in Text S4) from 0.805 to 0.965, as the charring temperature raised, from which, evidently implying the evolution of conjugated structure.

Table 1

Physicochemical properties of ball milled plastic char derived from PET water bottle at different temperatures
Sample	BMPC-500	BMPC-600	BMPC-700	BMPC-800
Product yield (%) BET Surface Area(m²/g) Pore Volume(cm³/g) Average Pore size (nm) Average Particle Size (µm) pH C (%) H (%) O (%) O/C ^a H/C ^b AI [mol DBE_AI/(mol C_AI)] I_D/I_G Zeta potential(mv)	18.9 361 0.213 2.361 0.934 6.41 86.432 4.589 8.979 0.0779 0.637 0.805 0.920 -33.8	17.3 439 0.258 2.347 0.828 6.59 89.115 4.318 6.567 0.0553 0.581 0.834 0.934 -34.3	15.2 539 0.308 2.288 0.756 6.61 89.225 3.6 7.175 0.0804 0.484 0.885 1.007 -34.5	13.6 583 0.318 2.177 0.524 6.75 91.236 2.48 6.284 0.0689 0.326 0.965 1.004 -35.2
Note: a; O/C; Atomic mole ratio of oxygen to carbon, b; H/C; Atomic mole ratio of hydrogen to carbon, AI; aromaticity index

The ratio of O/C can reflect the changes in O-containing functional groups of BMPCs with different pyrolysis temperatures (Xiao et al. 2016). The ratios of O/C showed a decreasing trend at 500-600℃ (from 0.0779 to 0.0553) but at 600-700℃ increased again from 0.0553 to 0.0804, and at 800℃ decreased again to 0.0689 (Table 1). BMPCs sorption can be interpreted by the structure-activity relationships between the pyrolysis temperature, H/C atomic ratio and aromatic cluster formation, and sorption capacity. Decreasing the ratio of H/C atomic, the sorption capacity of organic pollutant by char increased linearly with different pyrolysis temperatures. As shown in Table 1, not only specific surface area and surface hydrophobicity of BMPCs but also the aromatization degree was gradually increased with increase in pyrolysis temperature. Zeta potential is an important indicator to characterize the behavior of nanoparticles in colloidal solutions (Biriukov et al. 2020). Zeta potential values of all BMPCs were getting more and more negatively charged as charring temperature raised.

Raman measurement showed that BMPCs like biochar represent two significant peaks at 1350cm⁻¹and 1595cm⁻¹, and the I_D/I_G ratio increased with the increase of pyrolysis temperature (500-800℃), explaining the higher the temperature, the more defects, which may be related to the condensation of small amorphous defects, the removal of some O-containing groups and the succession of aromatic clusters (Guizani et al. 2017). But at 800℃ I_D/I_G ratio was a little bit less than at 700℃ (Fig. 1).

Raman spectroscopy can be used to analyze the structural similarity between the produced materials and the perfect graphite. This analysis is usually used to evaluate the aromatic cluster structure of carbonaceous materials. The D-peak (1350~1400 cm-1) represents the in-plane vibrations of sp3 –bonded carbon atoms that reflects structural defects of graphene structure, while the G-peak (1550~1600cm-1) corresponds to the in-plane stretching vibration of SP2 graphic carbon structures. ID/IG indicates the total number and/or size of graphitic micro domains, and the proportion of the sp3 to sp2 bonding, which can be used to evaluate the degree of disordering/ordering structure (Tang et al. 2015; Xiong et al. 2021). Previous study demonstrated that there exists a well-linear relationship between the intensity ratio of Raman band and atom ratio of O, H to C (Veiga et al. 2021; Xu et al. 2020). The Boehm titration result were summarized in Table S2. For all BMPCs and PCs, surface acidity was dominated by phenolic hydroxyl groups, and lactonic and carboxyl groups on the all BMPCs were hold in less 50% of total acidic functional groups. The total acidic functional group content of BMPCs decreased significantly from 0.0651mol/kg to 0.0501mol/kg, while that of PCs changed between 0.0252mol/kg to 0.0285mol/kg, respectively, as pyrolysis temperature raised. Boehm titration experiment showed that these findings were rather consistent with Raman measurement, ball milling enhanced the surface acidity of adsorbents, and the change of O/C ratio and the pH value of BMPCs were depended on total acidic functional groups (Tang et al. 2015; Xu et al. 2020; Yang et al. 2017). The morphology of all BMPCs was also illuminated by SEM analysis (See detail in Figure S2). All BMPCs particles were mostly less than 2µm of diameter in size, which caused the surface to enlarge and form many pores, and especially, BMPC-800 particles were mostly less than 1µm of diameter in size.

FTIR helped to measure the characteristic stretching frequencies of all BMPCs and elucidate the adsorption mechanism of PHE, phenol and 2,4,6-TCP. FTIR spectra of different BMPCs were compared with previous studies. All BMPCs had peaks of many functional groups, including adsorption band positions at wavelengths of ~3600cm⁻¹ (free OH groups representing alcoholic and phenolic), ~3020cm⁻¹ (aromatic hydrogen), ~1710cm⁻¹(-COOH), ~1601cm⁻¹(aromatic C= C/C = O), ~1443cm⁻¹ (=CH₂ bend; aromatic ring), ~1245cm⁻¹ (C-O stretching vibration representing aromatic ethers), and 600~900cm⁻¹(aromatic C-H in plane bending) (Meng et al. 2014; Ri et al. 2022; Singh, R.K. et al. 2020; Tang et al. 2015; Zuo et al. 2021). As shown in Fig. 1, with increasing of thermal decomposition temperature, the richness of functional groups also decreases, especially, the intensity of –OH (3649cm⁻¹), C-H bond (3057cm⁻¹) and =CH₂ bend(-1443 cm⁻¹) were decreased until negligible at 800℃, respectively (Meng et al. 2014). The peak positions of most functional groups shifted from 3637cm⁻¹ to 3649cm⁻¹ and 1443 cm⁻¹ to 1454 cm⁻¹ for =CH₂ stretching band, from 3028cm⁻¹ to 3057cm⁻¹ for C-H stretching band, from 1708 cm⁻¹ to 1716 cm⁻¹ for –COOH stretching band, and from 1245cm⁻¹ to 1265cm⁻¹ for C-O stretching band, whereas negligible for C=C/C=O stretching band. The test of FTIR spectra and Boehm titration explained that BMPCs contained the C atoms bonded to different oxygen-containing groups, such as aromatic C=C/C=O and carboxylic –COOH groups (Amusat et al. 2021; Hu et al. 2014; Parra et al. 2006), and these hydrogen bonds were looser than the tightly bound cyclic OH isomer and OH-ether with increasing of charring temperature. Through the adsorption process of PHE, phenol and 2,4,6-TCP, the characteristics of BMPCs were further explained.

Effects of pyrolysis temperature on PHE, phenol and 2,4,6-TCP sorption capacity

Effects of adsorbents with different pyrolysis temperature on three adsorbates removal efficiency were investigated (Fig. 2). With the increasing pyrolysis temperature, the removal efficiency by using PCs (from 35–68% for PHE, from 28–53% for phenol and from 13–35% for 2,4,6-TCP) and BMPCs (from 72–99% for PHE, from 61–80% for phenol and from 79–90% for 2,4,6-TCP) increased significantly, in particular, 2,4,6-TCP adsorption rate of PCs was more than twice as low as that of BMPCs.

However, 2,4,6-TCP removal efficiency increased from 79–90% at 500-700℃ of pyrolysis temperature, and removal efficiency of BMPC-800 reduced by 82%. The test explained that the removal efficiency of three adsorbates onto BMPCs and PCs was mainly dominated by surface area, and was affected indirectly by O-containing functional groups (Inyang et al. 2014; Lyu, H. et al. 2018). PHE, as a nonionic organic compound, can be hardly ionized, thus its morphological structure may not be so respective to change of the solution pH (Hu et al. 2014), and the adsorption efficiency of phenol is best in the solution pH 6.5 (Lawal et al. 2021), and the TCP removal decreases significantly with initial pH of the solution changed from pH 2 to 12 (Pei et al. 2013; Tan et al. 2009).

The effect of adsorbent dosage on PHE, phenol and 2,4,6-TCP removal

Effects of BMPCs dosage on PHE, phenol and 2,4,6-TCP removal efficiency were shown as Fig. 3. PHE removal efficiency enhanced about from 53–99%, when four kinds of BMPCs dosage was increased from 0.025g/L to 0.15g/L at 0.6 mg/L PHE, phenol removal efficiency enhanced about from 35–99% with increasing BMPCs dosage from 0.05g/L to 0.375g/L at 10 mg/L phenol, and 2,4,6-TCP removal efficiency increased from about 40–99% with BMPCs dosage from 0.125g/L to 0.5g/L at 50mg/L 2,4,6-TCP. PHE and phenol removal efficiency of BMPCs significantly increased with pyrolysis temperature under equal dosage condition, especially, the removal effect of BMPC-800 was better than BMPC-500 and BMPC-600, whilst the adsorption effect was almost no difference compared with BMPC-700, there was rather a slight tendency to shrink. In case of 2,4,6-TCP removal efficiency, BMPC-600 and BMPC-700 had a good removal rate, however, the removal rate of BMPC-800 decreased with the increase of pyrolysis temperature. From the test, in order to demonstrate the characteristics of each BMPCs, the optimal dosage of BMPCs used in subsequent trials was determined to be 0.05g/L for PHE adsorption study and 0.25g/L for phenol and 2,4,6-TCP adsorption study.

Sorption kinetic of PHE, phenol and 2,4,6-TCP on BMPCs

The adsorption kinetics of PHE, phenol and 2,4,6-TCP on BMPCs, were considered via pseudo-first order kinetic model and pseudo-second order model in this work (SM 1) (Mohammed et al. 2018; Wang et al. 2020; Zaghouane-Boudiaf and Boutahala 2011). Sorption kinetic of different pollutants on BMPCs was shown in Fig. 4. Most of PHE, phenol and 2,4,6-TCP were rapidly adsorbed to BMPCs in less than one hour (over 95%), and the apparent adsorption equilibrium was achieved within 72 hours for PHE adsorption (8.6, 9.5, 11.5 and 11.7mg/g), 24 hours for phenol adsorption (25, 27.5, 30 and 31mg/g) and 24 hours for 2,4,6-TCP adsorption (153.8, 178.6, 181.8 and 156.3mg/g). Adsorption capacity of BMPCs increased significantly with increasing pyrolysis temperature in the both (PHE and phenol) cases (Fig. 4A and 4D), whereas adsorption capacity of BMPC-800 was not significantly different comparing to BMPC-700. In addition, 2,4,6-TCP adsorption capacity of BMPC-800 decreased significantly comparing to others (Fig. 4G). Pseudo-first order kinetic model (Fig. 4B, E and H) is the well-known and reliable model for the initial rapid response, and as for the pseudo-second order model (Fig. 4C, F and I), it is assumed that the adsorption rate relies on the number of active sites on the adsorbent surface as a rate constraint step (Hu et al. 2014; Tang et al. 2015).

As expected, pseudo second-order kinetic model fits the best for three kinds of organic pollutants sorption study. In sorption kinetics, R² of BMPCs for the pseudo first order model (0.799-0.821 for PHE, 0.72-0.8 for phenol, and 0.73-0.77 for 2,4,6-TCP) were smaller than that of pseudo-second order model (R²>0.99), which showed that pseudo-second order models described the kinetic adsorption process well. Table S3 shows that the adsorption kinetic parameters, in which the adsorption process of PHE, phenol and 2,4,6-TCP on BMPCs accords with pseudo-second-order kinetics, and reasonably the adsorption rate relates to the number of active sites on the adsorbent surface and the sorption of these pollutants predominantly occurred by chemisorption mechanism (Inyang et al. 2014).

Isotherm models of PHE, phenol and 2,4,6-TCP sorption

By establishing the adsorption isotherm model, the adsorption capacity and equilibrium constant of PHE, phenol and 2,4,6-TCP on BMPCs were derived, and Langmuir and Freundlich model equations were applied to describe the adsorption isotherm in detail (SM 2) (Fan et al. 2011; Godlewska et al. 2019; Hu et al. 2014; Lv and Li 2020; Mohammed et al. 2018).

Figure 5 was shown adsorption isotherm of PHE, phenol and 2,4,6-TCP on BMPCs, and for three absorbates, the sorption data was fitted using Langmuir and Freundlich models (See detail in Table S4). The result showed that two models illustrated the sorption isotherm data of three absorbates well ( R² > 0.92), wherein, for PHE sorption isotherm, the correlation coefficients R² of two models decreased from 0.969 to 0.951 (Langmuir model) and from 0.951 to 0.921 (Freundlich model), and, for phenol sorption isotherm, R² values of Langmuir model decreased from 0.998 to 0.994 and R² values of Freundlich model increased from 0.991 to 0.995, meanwhile, for 2,4,6-TCP sorption isotherm, R² values of two models increased from 0.993 to 0.998 (Langmuir model) and from 0.996 to 0.998 (Freundlich model) with the pyrolysis temperature increase. Adsorption isotherm data explained that for PHE and phenol adsorption on BMPCs Langmuir model gave better fit than Freundlich model, however, for 2,4,6-TCP adsorption on BMPCs Freundlich model fit better than Langmuir model, and Langmuir model fit well in 2,4,6-TCP adsorption on BMPC-800. K_L, Langmuir constant as a measure index of affinity between adsorbent and adsorbate, increased significantly with pyrolysis temperature increase for PHE and phenol sorption isotherm, suggesting that the adsorption capacities increased from 16.69mg/g to 21.91mg/g (for PHE) and from 90.9mg/g to 106.4mg/g (for phenol), however, for 2,4,6-TCP adsorption K_L values were non-linear with pyrolysis temperature, resulting that the adsorption capacities increased from 263mg/g to 303mg/g (500-700℃ of pyrolysis temperature), and at 800℃ adsorption capacity was 270mg/g. K_F and 1/n, the Freundlich constants corresponding to adsorption capacity and adsorption intensity, represented that adsorption was favorable, in here, all the 1/n values were below 1 and decreased non-linearly, and K_F values changed linearly with pyrolysis temperature. Based on the hypothesis of Langmuir isotherm, adsorption occurs at a specific-uniform surface position within the adsorbents, and is considered as single-layer adsorption, meanwhile, Freundlich isotherm is an empirical equation, describing the adsorption on heterogeneous surfaces (Lawal et al. 2021; Wang et al. 2020). In previous studies, surface adsorption seems likely the dominant mechanism at low concentrations (especially, below 1% of its solubility) (Kong et al. 2011). The adsorption capacities for phenol and 2,4,6-TCP are function of molecular weight and cross-sectional area, and are in direct proportion to the hydrophobicity of adsorbate. In addition, with the increase of chlorination degree, the adsorption capacity also increases. Due to chlorine group as an electron-withdrawing group, the increase of chlorine base makes the electron density in the aromatic ring lower (Denizli et al. 2004; Hamdaoui and Naffrechoux 2007), so leading to enhanced adsorption.

Sorption mechanism of PHE, phenol and 2,4,6-TCP onto BMPCs

In order to clarify the adsorption mechanism of three adsorbates on BMPCs, FTIR spectra before and after sorption was performed. As shown in Fig. 6, the peaks position and intensity of FTIR after adsorption shift significantly.

For the adsorption of three adsorbates on BMPC-500 (Fig. 6(a)), the peak corresponding to vibration of free –OH bond (3649 cm^-1) was disappeared after adsorption, such shifts were represented in every FTIR study of BMPCs adsorption. The band at 3028 cm^-1 (aromatic H₂) was shifted to 3024 cm^-1 for PHE and phenol adsorption, and the intensity of the peak was increased significantly. After adsorption, the peaks corresponding to –COOH and aromatic C = C/C = O were shifted from 1708 cm^-1 and 1601 cm^-1 to 1712 cm^-1 and 1597 cm^-1 after phenol and PHE adsorption, respectively, and the intensity of two peaks was increased after adsorption of tree adsorbates. The peaks at 1443 cm^-1 and 1245 cm^-1 assigned to =CH₂ bend (aromatic ring) and C-O stretching band was shifted to 1467cm^-1 (for 2,4,6-TCP), 1469 cm^-1 (for phenol), 1226 cm^-1 (for phenol and 2,4,6-TCP), and 1243 cm^-1 (for PHE), respectively, in here, not only the position of the peaks were shifted, but also was increased significantly in the intensity of the vibration (Pei et al. 2013; Wo et al. 2018; Zhao et al. 2018). This study suggested that three adsorbates attended on the adsorption of BMPC-500 through π-π interaction and O-containing functional groups played in sorption. With the pyrolysis temperature increase, even though the positions of the BMPCs functional groups shifted in a little range, FTIR study of BMPCs was similar to that of BMPC-500 after adsorption of three adsorbates (Fig. 6(b), (c) and (d)). Furthermore, the peaks intensity between 1000 cm^-1 and 1800 cm^-1 corresponding aromatic ring got stronger after 2,4,6-TCP and phenol adsorption than after PHE adsorption. This test explained that specific surface area was the main factor of adsorbents adsorption in this study, and π-π interaction and functional groups such as carboxylic –COOH, aromatic C=O/C=C and so on enhanced the characteristics of adsorbents, possibly due to O-containing groups’ exposure after ball-milling (Lyu, H.H. et al. 2018), and hydroxyl groups of phenol and 2,4,6-TCP could interact with O-containing groups on BMPCs through H-bonding (Pei et al. 2013).

Native plastic char from PET water bottles (PCs) with pyrolysis temperature between 500℃ and 800℃ could adsorb aromatic compounds such as PHE, phenol and 2,4,6-TCP, and special surface area of PCs increased significantly with raising pyrolysis temperature, however, the adsorption efficiency was not good. The ball-milling method increased the area of PCs by more than two times, and also enhanced the adsorption ability of PHE, phenol and 2,4,6 -TCP in aqueous solutions due to exposure functional groups of adsorbents surface. Specific surfaces of BMPCs and adsorption amount of PHE, phenol and 2,4,6-TCP on BMPCs also increased as pyrolysis temperature raised. Recently, the amount of plastic waste in the world has been explosively increasing, and how treating plastic waste controllably, efficiently as well as future-oriented is the great challenge facing to human community. The method proposed in this paper has certain guiding significance for the pyrolysis treatment of PET plastic wastes and its application, and for the adsorption removal effect of organic pollutants in water environment.

Supplementary Information

The online version contains supplementary material available at xxx

Author contribution

All authors contributed to the study conception and design. Conceptualization, methodology, formal analysis, investigation, and writing-original draft were performed by Hyokchol Mun. Jingchun Tang participated in writing–review & editing, Resources, Supervision and Project administration. Formal analysis and editing were performed by Cholnam Ri, and Resources and editing were performed by Qinglong Liu. All authors read and approved the final manuscript.

Funding

This research was financially supported by the National Natural Science Foundation of China (41807363, U1806216), the National Key R&D Program of China (2018YFC182002), and the 111program, Ministry of Education, China (T2017002).

Data availability

Not applicable

Ethics approval and consent to participate Not applicable.

Consent for publication Not applicable.

Competing interests The authors declare no competing interests.

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Characteristics of ball-milled PET plastic char for adsorption of different types of aromatic organic pollutants

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Abstract

Figures

Introduction

Materials And Methods

Experimental materials

The production of BMPCs

Characterization

Adsorption experiment

Effects of pyrolysis temperature on PHE, phenol and 2,4,6-TCP sorption capacity

Influences of BMPCs dosage on tree absorbates removal efficiency

Adsorption kinetics of three absorbates

Adsorption isotherm of three absorbates

Analytical methods

Results And Discussion

Physicochemical characterization of BMPCs

Sorption kinetic of PHE, phenol and 2,4,6-TCP on BMPCs

Isotherm models of PHE, phenol and 2,4,6-TCP sorption

Sorption mechanism of PHE, phenol and 2,4,6-TCP onto BMPCs

Conclusions

Declarations

Supplementary Information

Author contribution

Funding

Data availability

References

Supplementary Files

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