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.
Physicochemical properties of ball milled plastic char derived from PET water bottle at different temperatures
Product yield (%)
BET Surface Area(m²/g)
Average Pore size (nm)
Average Particle Size (µm)
AI [mol DBEAI/(mol CAI)]
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 ID/IG 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℃ ID/IG 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. KL, 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 KL 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. KF 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 KF 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).