Tire pyrolysis wastewater treatment by a combined process of coagulation detoxification and biodegradation.

Recycling waste tires through pyrolysis technology generates refractory wastewater, which is harmful to the environment if not disposed properly. In this study, a combined process of coagulation detoxification and biodegradation was used to treat tire pyrolysis wastewater. Organics removal characteristics at the molecular level were investigated using electrospray ionization (ESI) coupled with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The results showed that nearly 90% of the organic matter from the wastewater was removed through the process. Preference of the two coagulants for different classes of organics in tire pyrolysis wastewater was observed. The covalently bound inorganic-organic hybrid coagulant (CBHyC) used in this work had a complementary relationship with biodegradation for the organics removal: this coagulant reduced toxicity and enhanced the biodegradation by preferentially removing refractory substances such as lignin with a high degree of oxidation (O/C > 0.3). This study provides molecular insight into the organics of tire pyrolysis wastewater removed by a combined treatment process, supporting the advancement and application of waste rubber recycling technology. It also contributes to the possible development of an effective treatment process for refractory wastewater.

Introduction

With the growing number of vehicles, tire disposal has become a worldwide problem [1]. Tires need to be properly disposed of and recycled to reduce their impact on the environment. At present, the main ways to recycle waste tires include retreading, incineration, gasification, and pyrolysis [2]. Pyrolysis is an attractive solution for reducing waste volume and conserving landfill space while allowing energy recovery [[3], [4], [5]] [[3], [4], [5]] [[3], [4], [5]]. However, the volatile substances in the tires are volatilized and expelled during the pyrolysis process [6,7] and condensed to the liquid product in cooled gas-liquid separators [8]. The liquid product is separated to form two different phases: an organic phase with high oil content and an aqueous phase consisting of aldehydes, ketones and phenolic compounds. The former can serve as fuel and industrial raw material [9,10], while the latter remains an urgent problem as a kind of wastewater. This kind of wastewater from pyrolysis has a high organic concentration and complex composition, which is generally difficult to degrade, and its COD concentration is commonly between 100 and 250 g/L [11]. In literature, very few works are presented on the pyrolysis wastewater treatments, mainly including biochar adsorption [11], heterogeneous advanced oxidation processes [12], and electrochemical treatment [13]. These methods have some limitations in practical industrial applications because of their high cost or unsatisfactory removal efficiency.

The key to treating this type of refractory and hazardous wastewater is the removal of high concentrations of organic matters. As a mature technology, biodegradation is currently the most cost-effective process for wastewater treatment, with the advantages of high pollutant removal efficiency. However, tire pyrolysis wastewater and similar refractory wastewater (landfill leachate, pharmaceutical wastewater, papermaking wastewater, etc.) have a high fraction of non-biodegradable organic matters, which limit the biodegradation efficiency [14]. Treatment of this wastewater generally requires pretreatment using physical-chemical processes [15], such as coagulation [16], adsorption [17,18], and advanced oxidation technologies [[19], [20], [21]]. Although coagulation is commonly used to remove the non-biodegradable organic matters because the process is simple to implement [15], refractory organic matters like low molecular weight hydrophilic compounds are not amenable to be removed by conventional coagulant [22,23], and the enhancement of wastewater biodegradation is limited.

A covalently bound inorganic-organic hybrid coagulant (CBHyC) had been developed, with strong adsorption bridging ability and a wide coagulation pH range. CBHyC can remove low molecular weight compounds better than polyaluminum chloride (PACl) or metal-based coagulants [24,25]. In this study, CBHyC was used to pre-treat tire pyrolysis for detoxification, followed by the biological treatment process. CBHyC had a complementary relationship with biodegradation for removing organic matters and dramatically enhanced the biodegradation of wastewater. In addition, the molecular transformations of organic matters during coagulation and biodegradation processes were investigated using electrospray ionization (ESI) coupled with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The results can guide the optimization of the effective refractory wastewater treatment process and contribute to the application of waste rubber recycling technology.

Materials and methods

Sample collection

Tire pyrolysis wastewater samples were obtained from a resource regeneration company in Chenzhou City, Hunan Province, China. All collected samples were filtered through 0.45-μm filter membranes and were stored in the dark at 4 °C. The water quality of samples is shown in Table S1.

Coagulation and biodegradation experiments

Coagulation experiments were performed using a program-controlled MY3000-6F (Beijing, China) jar test apparatus with six paddles. Conventional coagulant polyferric silicate sulfate (PFSS) was purchased (Shangshan, Langfang, China), and CBHyC was prepared by a slow alkalinity titration method as presented in the Supporting Information (SI).

Batch experiments for the biodegradation of tire pyrolysis wastewater were conducted in a laboratory-scale anaerobic/oxic (A/O) system, which consisted of an anaerobic reactor (2.0 L) and an aerobic reactor (2.0 L). The activated sludge was domesticated using a mixture of the effluent from coagulation and domestic wastewater to adapt to the high concentration of organic matter.

All samples, including raw tire pyrolysis wastewater and effluents from the coagulation unit, anaerobic bioreactor, and aerobic bioreactor, were collected immediately after experiments, filtered with 0.45-μm filters, and kept in the dark at 4 °C before analysis. The detailed procedures of coagulation and A/O biodegradation experiments are included in the SI.

GC-MS analysis

Samples were concentrated by liquid-liquid extraction prior to Gas chromatography-mass spectrometry (GC-MS) analysis (Details are provided in the SI). GC-MS (Shimadzu GCMS-QP2010 SE, Japan) analysis for the organic matters was performed with HP5-MS column (30 m × 0.25 mm × 0.25 μm). The GC oven temperature was programmed initially 50 °C for 5 min, then increased to 300 °C at 5 °C/min and maintained for 10 min The mass spectrometer was operated in the electron impact (EI) ionization mode with an ion source temperature of 230 °C. The ionizing voltage was 70 eV, and the mass range was 45–500.

Negative ion ESI FT-IC MS analysis

All samples were extracted by a Sep-pak C18 solid phase extraction cartridge (1 g, 6 mL, Waters, USA) to remove salt and were analyzed using a Bruker Apex ultra FT-ICR MS equipped with a 9.4 T superconducting magnet.

The procedures for detailed solid phase extraction and FT-ICR MS mass calibration, data acquisition, and processing are provided in the SI. The majority of peaks in this study contained C, H, O, N, and S elements. The compounds containing P and Cl were excluded because of the low levels found in our samples (<0.5%).

Results and discussion

Molecular characterization of organics in wastewater

Over 60 types of compounds were detected by GC-MS in the samples pre-treated by liquid-liquid extraction (Table S2). The organic matters extracted by liquid-liquid extraction are generally hydrophobic organics [26,27], mainly alkanes, olefins, phenols, alcohols, ketones, ethers, pyridines, amides, and heteroaromatic compounds. Most of these compounds are produced by complex isomerization, dehydrogenation, aromatization, and condensation or other reactions in tire pyrolysis process [28]. Among them, alcohols, ketones, and ethers are more abundant. The relative abundances of many aromatic compounds like phenols and N-containing organics like amides are also high. Fig. S2 shows the negative ion mass spectra of tire pyrolysis wastewater. Our samples were analyzed under a very wide m/z range (100–2000 Da), but the peaks are only distributed over the mass range of 150–500 m/z, which is similar to the results of other studies using FT-ICR MS [26,29]. The loss of low m/Z peaks(<150) is due to the very high excitation frequencies, the excitation required to increase the ion radius to a sufficient amplitude to be detected by the detection plates is more difficult to generate [30]. And the loss of peaks above 500 m/z can be attributed to space charge effects within the ICR cell that can decrease high molecular weight signals [31].

The molecular composition of organic matters was visualized in a van Krevelen diagram for further investigation. As shown in Fig. 1(a), the van Krevelen diagrams are divided into several regions corresponding to the seven classes of compounds, including lipids, aliphatic/proteins, lignins/carboxylic-rich alicyclic molecules (CRAM)-like structures, carbohydrates, unsaturated hydrocarbons, aromatic structures, and tannins. Although these classifications are not definitive (i.e., a protein-like formula is not necessarily derived from protein), they provide insights into the general compound classes, and this is sufficient to allow us to characterize and analyze the transformation of contaminants in wastewater [[32], [33], [34], [35]]. The specific classification methods are provided in the SI. The points representing the organic matters in wastewater are distributed centrally in the lipids, lignins/CRAM-like structures, and aliphatic/proteins regions. The distribution of organic matters in the tire pyrolysis wastewater is presented in Fig. 1(b): the most fraction of organic matters is lignins/CRAM-like structures (48.9%), followed by lipids and aliphatic/proteins substances (37.3%) in the high H/C region (H/C > 1.5), unsaturated hydrocarbons (11.5%) in the low O/C region (O/C < 0.1) and other fractions (2.3%). Based on the previous study [24,36], contaminants are divided into C, H, O-containing substance (CHO), C, H, O, N-containing substance (CHON), C, H, O, S-containing substance (CHOS), and C, H, O, N, S-containing substance (CHONS). According to Fig. 1(c), the percentage of CHO substances was around 30%, and the other 70% was the components belonging to CHON (43.9%), CHOS (10.1%), and CHONS (14.9%) subcategories, indicating that N- and S-containing compounds were the major subcategories in the tire pyrolysis wastewater. Fig. S3 provides a visual comparison of CHO, CHON, CHOS, and CHONS classes of organic matters. The distribution of CHO substances is concentrated, mainly lignin/CRAM-like structures, aliphatic/protein substances, and lipids. The proportion of CHON substances is the highest, mainly lignin/CRAM-like structures. The CHOS and CHONS substances account for a smaller proportion of the tire pyrolysis wastewater and are more dispersed in the Van Krevelen diagram. CHOS species are mainly distributed in the region of unsaturated hydrocarbons and CHONS mainly in lipids.

Fig. 1

Characterization of organics in wastewater by FT-ICR MS. (a)Van Krevelen diagram of CHO, CHON, CHOS, CHONS of tire pyrolysis wastewater. Bar diagrams show the contribution of the major subcategories (b) and major classes (c) in the tire pyrolysis wastewater.

Overall removal efficiency

To evaluate the biodegradability of wastewater after PFSS and CBHyC coagulations, raw water, PFSS coagulation effluent, and CBHyC coagulation effluent were diluted to similar COD concentrations and conducted biodegradation processes. The results show that the diluted raw water and PFSS coagulation effluent were resistant to biodegradation (Fig. 2a). On the contrary, organic matters in the CBHyC coagulation effluent can be more easily removed. It seems that the CBHyC coagulation process removed and transformed part of the organic matters in the wastewater and improved the biodegradation performance. A similar result was not observed in PFSS coagulation effluent.

Fig. 2

Wastewater treatment performance of coagulation and biodegradation. (a) COD removal of diluted samples in biodegradation. The three samples were raw water, PFSS coagulation effluent and CBHyC coagulation effluent, which were all diluted to similar concentrations. (b) The removal efficiency of COD, DOC, TN, TP, and NH4+-N during the treatment processes.

Fig. 2(b) shows the high overall removal efficiency for conventional water quality parameters (COD, DOC, TN, NH4+-N, and TP) by the combined process. Removal efficiencies of COD and DOC after coagulation were 75.0% and 62.4%, respectively. Removal efficiencies of COD and DOC after A/O biodegradation were 88.5% and 81.3%, respectively. The combined process also had an excellent removal effect on TN, NH4+-N, and TP, with the removal efficiency of 60.7%, 75.7%, and 81.1%, respectively. The specific water quality parameters of samples in different stages are shown in Table S1.

Characterization of molecular transformation by ESI FT-ICS MS

Compared to raw water, the mass spectra of effluents at different stages had almost the same peak distribution in the mass range of 150–500 m/z, but the relative peak intensities decreased. Due to the complexity of mass spectra, all identified peaks (S/N ≥ 6, excluding the isotopic peaks) at the nominal mass of 296 were expanded in Fig. 3. The compounds corresponding to the peaks were sorted by apparent molecular series according to the literature [37,38], which is related to the replacement of CH4 by oxygen in Table S3 in the SI. From the composition of molecules and relative peak intensities, the CHON and CHONS substances are more abundant. Under the same analysis instrumental conditions, comparison of relative peak intensities of molecules in ESI FTICR MS spectra can be used for semiquantitative analysis of similar types of compounds, which has been demonstrated by many studies [29,[38], [39], [40], [41]]. After coagulation by PFSS, the relative peak intensities of compounds were decreased slightly. In contrast, the relative peak intensities of all compounds remarkably decreased, especially series 1 CHON compounds, and the peaks of all CHONS compounds disappeared after coagulation by CBHyC. It is indicated CBHyC excellently removed organic matters that cannot be removed by conventional coagulants. However, the N-containing compounds were resistant to biological treatment and were poorly removed. The relative intensity of the compounds in biodegradation effluent slightly increased, which may be caused by the further ionization and detection of weakly polar substances as the strongly polar substances are removed [42].

Fig. 3

Negative ion mass spectra expanded (S/N > 6, excluding the isotopic peaks) at nominal mass of 296 of raw water (a), PFSS coagulation effluent (b), CBHyC coagulation effluent (c), and biodegradation effluent (d).

To further illustrate the specific changes in organic matters during the treatment, the van Krevelen diagram was used in the full mass range to visualize the compound distribution. The removed (peak lost), resistant (peak retained), and produced (a new peak) compounds after coagulation (by PFSS and CBHyC) and biodegradation were in Fig. 4 (CHO and CHOS) and Fig. S4 (CHON and CHONS).

Fig. 4

Van Krevelen diagrams of CHO and CHOS of organics in wastewater after coagulation by PFSS (a, d), coagulation by CBHyC (b, e) and A/O biodegradation (c, f). Points in green represent raw wastewater peaks that disappeared after coagulation or biodegradation (consumed), points in red represent raw wastewater peaks that were unchanged (resistant), and points in blue present new peaks that appeared during coagulation and biodegradation (produced).

The results show that the green dots corresponding to the removed compounds during PFSS coagulation were mainly distributed in the relatively low O/C region (O/C ＜ 0.3; mainly lipids, unsaturated hydrocarbons, and CRAM-like structures). The red and blue dots corresponding to resistant and newly formed compounds were distributed in the region where the value of O/C was from 0.3 to 0.6 (mainly aliphatic/proteins and CRAM-like structures). In contrast, the newly formed compounds were concentrated in the region with O/C ＜ 0.3 during CBHyC coagulation process, while the consumed compounds were mainly distributed in the region with O/C ＞ 0.3. Significant differences in organics removal preferences between PFSS and CBHyC coagulation processes were observed. As for biodegradation, the removed compounds are more likely scattered in the low O/C region (O/C ＜ 0.3). The resistant and newly formed compounds were mainly distributed in the high O/C region (O/C ＞ 0.3). This trend was more obvious for CHO and CHOS compounds (Fig. 4).

This result indicates that biodegradation affected the O/C ratios of wastewater by consuming the oxygen-deficient compounds and producing oxygen-rich compounds. The more biodegradable substances are mainly lipid substances and unsaturated hydrocarbons substances with low O/C ratios (O/C ＜ 0.3). In Table S4, the O/C ratio of the wastewater was reduced from 0.387 to 0.264 after coagulation by CBHyC, while it was raised from 0.264 to 0.347 after biodegradation. Fig. S5 clearly illustrates the trend of relative abundance of organic matters with different ratios of O/C, which shows that the relative abundance of substances with a high O/C ratio (O/C ＞ 0.3) decreased during CBHyC coagulation, but increased obviously after biodegradation. This result is consistent with previous studies that the less oxidized organics are more likely to be biodegradable [36,[43], [44], [45], [46]]. In conclusion, the ability of CBHyC to remarkably enhanced biodegradation (Fig. 2a) can be attributed to its efficient removal of oxygen-rich compounds from wastewater, which increased the proportion of oxygen-deficient compounds.

Remaining organics in the final effluent

Although the combined process had a high removal efficiency of all pollutants, it cannot completely remove the organic matters. Fig. 5(a) shows the relative abundance of major classes (unsaturated hydrocarbons, lignin/CRAM-like structures, aliphatic/proteins, lipid, and other substances) in organics of raw wastewater and treatment effluents. The result shows that aliphatic/proteins were easily removed by CBHyC coagulation compared with other classes. Lipids and unsaturated hydrocarbons were readily removed by biodegradation. Aliphatic/proteins were resistant during the process, and the proportion of these substances increased from 25.7% to 69.6%. Fig. 5(b) compares the relative abundance distribution of CHO, CHON, CHOS, and CHONS in the samples, which shows that CHO compounds can be more easily removed. The N-containing compounds (CHON, CHONS) seem to be resistant to biodegradation.

Fig. 5

Contribution of organics in different treatment stages. Bar diagrams show the contribution of the major classes (a) and major subcategories (b) of raw wastewater and treatment effluents (coagulation and biodegradation).

We regarded the remaining organic matters in the final effluent as recalcitrant substances. As shown in Fig. 6(a), CHON and CHONS substances accounted for a large proportion of the recalcitrant substances, and the relative abundance of these substances increased significantly from 28% to 56.6% after biodegradation (Fig. 5(b)). These substances were mainly lignin/CRAM-like structures and aliphatic/proteins substances in the van Krevelen diagram, which are distributed in the region of O/C = 0.3–0.5 and H/C = 1.0–1.75. This result indicates that the N-containing substances in tire pyrolysis wastewater were resistant to the treatment process, and remained in the final effluent. As shown in Fig. 6(b), relative abundances of N1O3–N1O6, N2O3–N2O6, and N1S1O3–N1S1O7 classed of species were higher than other N-containing classes. There were also some CHO and CHOS substances remaining in the effluent, mainly O3–O7 and S1O0–S1O2 classed of species.

Fig. 6

Characterization of organics in the final effluent. (a) Van Krevelen diagram of the biodegradation effluent. (b) Relative abundance of identified classes of remaining organic matters in biodegradation effluent.

Conclusion

This study used the combined process of coagulation detoxification and biodegradation to treat tire pyrolysis wastewater. The combined process removed 88.5% of COD and 81.3% of DOC, respectively. Total nitrogen (TN)、ammonia nitrogen (NH3–N), and total phosphorus (TP) were removed with the removal efficiency of 60.7%, 75.7%, and 81.1%, respectively. Biodegradation was effective in removing the oxygen-deficient compounds (O/C ＜ 0.3). CBHyC coagulation had a complementary relationship with biodegradation, which increased the biodegradability of wastewater due to its preferential removal of the oxygen-rich substances.

This study revealed the removal characteristics of organics in tire pyrolysis wastewater during the selected process at the molecular level using ESI FT-ICR MS for the first time. For tire pyrolysis wastewater or other refractory wastewater, using coagulants with preferential removal effect on recalcitrant substances is ideal for improving the biodegradation process. It implies the possibility of treating wastewater in any case by using economical biological processes combined with a complementary coagulation process, instead of high-cost or hard-to-implement technologies, even for refractory wastewater. However, the lignins/CRAM-like structures and aliphatic/proteins substances containing N are resistant in the treatment processes. Therefore, efforts should be made to develop new coagulants or microorganisms that have an optimized removal effect against these substances.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.