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Article

Inhibition of Polycyclic Aromatic Hydrocarbons Formation During Supercritical Water Gasification of Sewage Sludge by H2O2 Combined with Catalyst

by
Qiao Xu
1,
Fenfen Yan
2,
Yujie Fan
3 and
Miao Gong
1,*
1
School of Civil Engineering, Hefei University of Technology, Hefei 230009, China
2
Human Resources Department, Anhui Xinhua University, Hefei 230601, China
3
Nanyang Institute of Technology, School of Civil Engineering, Nanyang 473004, China
*
Author to whom correspondence should be addressed.
Water 2024, 16(22), 3235; https://doi.org/10.3390/w16223235
Submission received: 10 October 2024 / Revised: 25 October 2024 / Accepted: 7 November 2024 / Published: 11 November 2024
(This article belongs to the Section Wastewater Treatment and Reuse)
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Abstract

:
This work evaluated the alterations in the levels and types of polycyclic aromatic hydrocarbons (PAHs) within both liquid and solid products throughout the process of the catalytic supercritical water gasification of dewatered sewage sludge to examine the catalytic effect of various catalysts and the inhibit reaction pathways. The addition of Ni, NaOH, Na2CO3, H2O2, and KMnO4 reduced the concentrations of PAHs, with Ni and H2O2 showing the best performance. The concentrations of PAHs, especially higher-molecular-weight compounds in the residues, decreased sharply as the H2O2 amount increased. At a 10 wt% H2O2 addition, the levels of PAHs in the liquid and solid products were reduced by 91% and 88%, respectively. High-ring PAHs were not detected in the residues as the H2O2 amount increased to an 8 wt%. H2O2 addition evidently inhibits PAH formation by promoting the ring-opening reactions of initial aromatic compounds in raw sludge and inhibiting the polymerization of open-chain intermediate products. The addition of NaOH + H2O2 or Ni + H2O2 as combined catalysts significantly lowered PAH concentrations while increasing the H2 yield. The addition of 5 wt% Ni + H2O2 reduced PAH concentrations in the liquid and solid residues by 70% and 44%, respectively, while the H2 yield escalated from 0.13 mol/kg OM to 3.88 mol/kg OM. Possible mechanisms associated with the reaction pathways of these combined catalysts are proposed.

1. Introduction

Sewage sludge is a waste by-product generated during the biological treatment of wastewater. Typically, it consists of a high percentage of organic matter (OM), ranging from 40% to 60% on a dry mass basis, making it a viable candidate for bio-energy production [1,2,3]. The supercritical water gasification (SCWG) of sewage sludge is deemed to be both environmentally and economically viable because it is capable of converting the organics in sewage sludge into renewable energy sources such as H2 without costly pre-drying [4,5,6]. However, in addition to potential OM, sewage sludge also contains various heavy metals and organic pollutants, including heterocyclic aromatic compounds and halogenated hydrocarbons [7,8,9,10]. Therefore, the environmental safety and biosafety of the gasification products obtained from this material should be carefully considered.
In previous studies from our group, polycyclic aromatic hydrocarbons (PAHs), phenols, and other unwanted organic pollutants were found to be generated during the SCWG of sewage sludge [11,12,13]. Higher reaction temperatures, longer reaction times, and lower dry matter contents accelerated the generation of PAHs, mainly involving high-ringed PAHs [12,14]. Zhang et al. found large amounts of phenols and phenolic compounds, as well as anthracene and phenanthrene, in heavy oil products during the SCWG of secondary pulp/paper-mill sludge [15], highlighting the need for effective PAH control measures in such processes. Qian et al. detected a high level of refractory intermediates, such as pyridines and phenols, in liquid products resulting from the SCWG of sewage sludge [16]. Li et al. also reported that high concentrations of PAHs remained in byproducts obtained from the hydrothermal treatment of sewage sludge [17]. Therefore, the production of persistent organic pollutants is a common problem associated with the SCWG of waste biomass that already contains organic pollutants. Further investigation is deeply needed to demonstrate feasible approaches to inhibit the formation of refractory pollutants during the SCWG process.
To date, there has been scant research into the suppression of PAH formation during the hydrothermal gasification of sewage sludge. Few studies found in the literature typically applied catalysts and oxidants to inhibit the generation of PAHs throughout the SCWG process. According to Ren et al. [18], the catalysts most widely used during the gasification process to minimize tar (comprising primarily aromatics and PAHs) are dolomite, iron-based, nickel-supported, and carbon-supported materials. Xu et al. [19] reported that alkali salts including KOH, K2CO3, NaOH, and Na2CO3 can promote H2 production while also suppressing tar/char formation from the SCWG of sewage sludge. Yeletsky et al. [20,21] showed that NiMo/SiO2 catalysts can promote the degradation of phenanthrene and anthracene in supercritical water (SCW), and the conversion rate depends on both the amount of Ni as well as the ratio of Ni to Mo within the catalyst. Guan et al. [22] indicated that oxygen serves as a potent reactant in the partial oxidative gasification of phenol in SCW, and it also enhances the ring-opening reactions of aromatic compounds, including PAHs. H2O2 can also serve as an oxidant to promote PAH degradation, and the PAH removal rate is correlated with H2O2 concentration [23]. Onwudili et al. [24] determined that the incorporation of H2O2 effectively promotes the breakdown of naphthalene, such that the degradation percentage increases from 24.4% to 98.4% as the H2O2 concentration increases from 0 to 6.0 vol%. However, a large excess of H2O2 can drive the conversion of H2, CO, and CH4 into CO2 [25,26], thus significantly reducing the H2 yield and affecting the energy conversion efficiency. Therefore, suppressing the formation of PAHs and simultaneously boosting hydrogen production presents a complex challenge in the SCWG of sewage sludge. Zhang et al. [27] explored the formation and inhibition of PAHs from the SCWG of cyanobacterial biomass and found that a 1.0% H2O2 addition strikes a balance between inhibiting PAHs and enhancing hydrogen production. Zhong et al. [28] also found that low additions of H2O2 and KOH can achieve the integrated effect of H2 production and PAH inhibition. Some researchers also have reported that a combination of two additives can achieve a synergistic effect. Muangrat et al. [29] found that the use of both NaOH and H2O2 together yielded a higher hydrogen production compared to using either catalyst individually. The H2O2 evidently functioned to partially oxidize the samples while the NaOH considerably boosted the hydrogen production. Wang et al. [30] established that the combined use of H2O2 and Ni both inhibits the formation of PAHs and accelerates hydrogen production.
Here, we delve into how varying catalysts and their addition amounts influences PAH distribution in residues derived from the SCWG of sewage sludge, aiming to uncover optimal conditions for minimizing PAH formation. Based on the qualitative analysis of solid products during the heating stage of this process by gas chromatography–mass spectrometry (GC/MS), the mechanism by which H2O2 prevents PAH formation and the possible reaction pathways associated with the combined catalysts related to PAH formation and gas production are also examined.

2. Materials and Methods

2.1. Materials

Samples of dewatered sewage sludge (DSS) were collected from the Jiangxinzhou wastewater treatment facility located in Nanjing, China, and were kept in a refrigerator preservation box at a temperature below 4 °C. The proximate analysis and ultimate analysis, as well as the content of PAHs of the DSS used in this study, are detailed in Table 1. The Ni was purchased from the Aladdin Chemistry Co., Ltd. (Shanghai, China), and the Na2CO3, NaNO3, NaOH, 30% H2O2, and KMnO4 were purchased from the Sinopharm Group Chemical Reagent Co., Ltd. (Shanghai, China). All the used reagents were an analytical-grade purity.

2.2. Experimental Apparatus and Procedure

The SCWG experiments were conducted in a 100 mL 316L stainless steel batch reactor, sourced from the Songling Chemical Instrument Co., Yantai, Shandong, China. The schematic of the reaction apparatus has been described in detail previously [31]. In a standard procedure, 43 g of wet sludge and 0–10 wt% of catalyst was loaded into the reactor. The reactor was sealed and positioned in a preheated salt bath furnace kept at 400 °C for a set residence time of 10 min. Once the process was complete, the reactor was removed from the salt bath and quickly cooled to an ambient temperature using a combination of fans and cooling water. After the cooling, the sample collection and separation procedures were conducted, as also described in detail in previous paper [31], and gas, liquid, and solid products were obtained after the separation.
In addition, in order to understand the degradation of PAHs, the variations in PAH concentrations and forms in both liquid and solid products throughout the heating stage was also assessed. The heating process was segmented into five distinct temperature stages: 200, 275, 325, 375, and 400 °C, as described in detail in previous paper [12].

2.3. Products Analysis

The procedures of the extraction, concentration, purification, and analysis of the 16 target PAHs have been fully outlined in a previous study [11]. Each experiment was conducted twice to ensure the reproducibility and comparability of the results. The 16 target PAHs were categorized into higher-molecular-weight (HMW) PAHs (including 4-ring, 5-ring, and 6-ring PAHs) and lower-molecular-weight (LMW) PAHs (including 2-ring and 3-ring PAHs) based on their ring numbers.

2.4. Data Interpretation

The total average relative molecular weight (RMW) was calculated as the sum of the product of the relative proportion by area of each compound (i) and its relative molecular mass, with i ranging from 1 to m, where m represents the total number of peaks detected in the GC/MS total ion chromatogram. Gasification efficiency (GE), carbon gasification efficiency (CE), hydrogen gasification efficiency (HE), and energy recovery (ER) were defined as follows [12,32]:
GE   ( % ) = the   total   mass   of   gaseous   products the   total   mass   of   OM   in   DSS   ( dry   basis ) × 100 %
CE   ( % ) = t h e   t o t a l   c a r b o n   o f   g a s e o u s   p r o d u c t s t h e   t o t a l   c a r b o n   i n   D S S   ( d r y   b a s i s ) × 100 %
HE   ( % ) = t h e   t o t a l   h y d r o g e n   o f   g a s e o u s   p r o d u c t s t h e   t o t a l   h y d r o g e n   i n   D S S   ( d r y   b a s i s ) × 100 %
ER   ( % ) = H H V   o f   c o m b u s t i b l e   g a s × m a s s   o f   c o m b u s t i b l e   g a s H H V   o f   D S S × m a s s   o f   D S S   ( d r y   b a s i s ) × 100 %

3. Results and Discussion

3.1. Effects of Catalysts on the Distribution of PAHs in the SCWG Products

As the conventional catalysts, Ni, NaOH, Na2CO3, NaNO3, H2O2, and KMnO4 were employed in the present work, and their effects on the concentrations and forms of PAHs in the resulting residues are shown in Figure 1. The selection of catalyst was based on the screening of commercial homogeneous catalysts and oxidants commonly used in the SCWG of sewage sludge.
Compared with non-catalytic SCWG (0.39 PAHs μg/mL in the liquid residue and 12.5 μg/g in the solid residue), the distribution of PAH content in the products with the different catalysts varied greatly. These concentrations were in the approximate range of 0.26–0.40 μg/mL in the liquid residue and 6.3–14.4 μg/g in the solid residue. Unlike the other catalysts, the addition of NaNO3 inversely increased the PAH concentrations. This may be due to the lower pH (approximately 7) obtained in the presence of NaNO3 compared to the higher values (pH > 7) produced with the addition of NaOH and Na2CO3. Higher pH values, namely, the strong alkaline condition, have been confirmed to suppress the formation of PAHs [11]. No 6-ring PAHs were detected in the residues, indicating that both the NaOH and Na2CO3 inhibited the increases in the PAH ring numbers. Guan et al. [33] suggested that compounds such as NaOH and KOH produce OH radicals that add to aromatic rings to promote ring-opening, which in turn lowers both PAH concentrations and ring numbers. Jin et al. [34] also established that the presence of K2CO3 increases the gasification of benzene and naphthalene.
Compared to the other catalysts, the addition of individual Ni and H2O2 resulted in the most significant decreases in the PAH concentrations. The PAH concentrations in the liquid product were reduced from 0.39 μg/mL to 0.29 and 0.26 μg/mL, respectively, while those in the solid product dropped from 12.5 μg/g to 7.1 and 6.3 μg/g, respectively. However, higher concentrations of 6-ring PAHs (0.30 μg/g) were still detected in the solid residues when using the Ni catalyst, while the levels of 2-ring to 4-ring PAHs were reduced. The addition of H2O2 had a greater effect in terms of reducing both the PAH concentrations and ring numbers, especially the levels of 5-ring and 6-ring PAHs. Onwudili et al. [24] also discovered that the addition of H2O2 effectively promotes the decomposition of PAHs. Wang et al. [25] explored the decomposition of a phenol, acetic acid, and naphthalene blend in SCW with partial oxidation and discovered that including minor quantities of H2O2 enhanced the ring-opening and suppressed the polymerization reactions.

3.2. Effects of the H2O2 Amount on Alterations in PAHs Distribution in the SCWG Products

Figure 2 illustrates the effects of varying the amount of H2O2 on the PAHs distribution in the SCWG products. It is evident that the addition of H2O2 significantly reduced the PAH concentrations, especially for HMW PAHs, and this effect became more pronounced with increases in the H2O2 loading. As the H2O2 amount was increased from 0 to 10 wt%, the PAH concentration in the liquid product decreased from 0.39 to 0.03 μg/mL (a 91% reduction) while that in the solid residue decreased from 12.5 to 1.5 μg/g (an 88% reduction). The number of rings in the HMW PAHs also decreased gradually with the increase in the H2O2 amount. The level of 5-ring PAHs in the liquid residue was reduced to nil when 4 wt% H2O2 was added, and the concentration of 4-ring PAHs was only 0.002 μg/mL with a 10 wt% H2O2 loading. The level of 6-ring PAHs in the solid residue could not be detected when the addition of H2O2 was above 8 wt%. These results demonstrate that H2O2 promotes the decomposition of PAHs and reduces the formation of HMW PAHs. Wang et al. [30] also found that a total PAH content was decreased gradually as a H2O2 amount increased, with a decrease of 64% obtained at a H2O2 concentration of 10 wt%.
To further assess the impact of the H2O2 amount on the SCWG reaction, the solid residues from the SCWG of DSS with various H2O2 amounts were analyzed by GC/MS. The categorization method has been reported in a previously published paper [12]. Briefly, the compounds were divided into six groups, including non-nitrogen open-chain (NNO) compounds, nitrogen open-chain (NO) compounds, non-nitrogen aromatic (NNA) compounds, nitrogen aromatic (NA) compounds, alicyclic (AC) compounds and other heterocyclic compounds (OHC). The relative proportions of these compounds calculated by area, along with the total average RMW values, are depicted in Figure 3. As the amount of H2O2 was increased, the total average RMW gradually decreased, indicating that the introduction of H2O2 facilitated the decomposition of macromolecular compounds to form smaller molecules. Only NNO, NNA, and AC compounds were detected in the solid residues. As the H2O2 amount was increased from 0 to 10 wt%, the proportion of NNA compounds decreased from 4.13 to 0.88%, which is in agreement with the data presented in Figure 2 and again suggests that H2O2 inhibits the formation of PAHs. The proportion of AC compounds gradually decreased from 20.25 to 4.02%, while the proportion of NNO compounds increased from 75.61 to 95.09%, with increases in the H2O2 amount. It is evident that H2O2 promotes the decomposition of cyclic compounds and contributes to the formation of small open-chain compounds.
Figure 4 demonstrates the impact of the H2O2 addition on the gas composition and the concentration of total phenols in the liquid products after the SCWG of the DSS. As shown in Figure 4a, with the increase in the H2O2 addition, the H2 yield initially rose modestly, from 0.13 mol/kg OM in the absence of a catalyst to a peak of 0.25 mol/kg OM, and then dropped sharply to 0.02 mol/kg OM. When the amount of H2O2 added was greater than 8 wt%, almost no H2 was observed. This finding aligns with Wang et al.’s conclusion that the addition of a small amount of H2O2 can slightly enhance the production of H2 [30]. The addition of H2O2 effectively reduced the concentration of total phenols in the liquid residue (as shown in Figure 4b), which are known to be the primary precursors to PAHs. With the increment in the H2O2 addition from 0 to 10 wt%, the total phenol concentration dropped from 77.5 mg/L to 57.5 mg/L.

3.3. Inhibition Mechanism of H2O2 on PAHs Formation During Heating Stage of SCWG Process

Figure 5 summarizes the variations in the PAHs distribution in the products during the heating stage of the SCWG process with and without 6 wt% H2O2. The presence of H2O2 led to a marked decrease in the PAH concentrations in these residues, suggesting that H2O2 slows the formation of PAHs during this stage. As the reaction temperature rose in the heating stage, the PAH concentrations in the residues were found to increase in the absence of the catalyst but to greatly decrease with the H2O2 addition. As the temperature escalated from 200 to 400 °C, the concentration of PAHs in the liquid residue dropped from 0.17 to 0.10 μg/mL (a reduction of 38%) while that in the solid residue decreased from 6.8 to 4.7 μg/g (a reduction of 31%). Moreover, compared to non-catalytic SCWG, the PAH concentrations in the liquid and solid residues were reduced by 78% and 61%, respectively, at 400 °C. During the heating stage without the catalyst, 6-ring PAHs emerged when reaction temperature surpassed 300 °C, and the concentration of both 5-ring and 6-ring PAHs gradually rose. Following the addition of H2O2, 6-ring PAHs were not detected in the liquid and 5-ring PAHs disappeared above 300 °C, which effectively inhibited any increase in the PAHs ring numbers. The 6-ring PAHs were present above 300 °C in the solid residue, but the concentrations of 6-ring and 5-ring compounds decreased with the increasing temperature and were both much lower than those without a H2O2 addition. It is noteworthy that the concentrations of 2-ring PAHs in the residues with a H2O2 addition were higher than those without H2O2, possibly because the H2O2 enhanced the degradation of HMW PAHs to generate more LMW PAHs. In summary, the introduction of H2O2 significantly curbed the formation of PAHs and the escalation in the number of benzene rings throughout the heating stage.
To gain a more comprehensive insight into the SCWG of the DSS in the presence of H2O2 during the heating stage, solid residues were collected and characterized at five different temperatures with and without H2O2. The relative proportions of the six types of compounds and the total average RMW values as determined by peak areas are presented in Figure 6. The total average RMW values were lower than that of the raw sludge during the heating process, indicating that macromolecular compounds were decomposed to smaller molecules via bond cleavage [16]. The raw sewage sludge was dominated by NNO and AC compounds, accounting for 35.76% and 56.18% based on the peak areas, respectively. This result demonstrates that crude fats and hydrocarbons constituted the predominant organic substances in the raw sludge. The organics in the solid residue primarily comprised open-chain and alicyclic compounds in the absence of H2O2. In contrast, during the heating process, the proportions of aromatic and open-chain compounds rose as the reaction temperature increased. The share of AC compounds was notably reduced and a minor presence of OHC emerged as well. With the addition of H2O2, the organics in the solid residue primarily consisted of open-chain compounds, accounting for approximately 80–90%. During the heating stage in the trials with a H2O2 addition, the proportion of open-chain compounds initially decreased as the temperature was raised and then gradually increased above 325 °C. Accordingly, the proportions of aromatic and alicyclic compounds initially rose and subsequently fell, while the proportion of all cyclic compounds was much lower than that without a H2O2 addition at 400 °C. These findings indicate that H2O2 promotes the ring-opening reactions of various cyclic compounds.
The open-chain species consisted of NNO and NO compounds, which together made up 35.76% of the area in the raw sewage sludge. In the absence of a catalyst, the total open-chain compounds declined sharply to 19.35% at 200 °C then increased progressively to 40.11% at 400 °C. However, when H2O2 was added, the proportion of open-chain compounds rose dramatically to 94.21% at 200 °C, decreased gradually to 77.51% at 325 °C, then increased slowly to 94.16% at 400 °C. The observed increase in open-chain compounds may reflect the appearance of the ring-opening products of various cyclic compounds, because the addition of H2O2 significantly promoted ring-opening reactions.
The aromatics consisted of NNA and NA compounds. The total aromatic compounds in the raw sewage sludge only accounted for 8.05% of the area. When no catalyst was added, this proportion increased slowly when temperature was below 300 °C then reached 24.68% at 400 °C. In the presence of H2O2, the proportion of aromatic compounds initially decreased to 3.76% at 200 °C then slightly rose to 5.31% at 325 °C and finally reduced gradually to 2.45% at 400 °C (i.e., it had a value much lower than the 8.05% in the raw sludge and 24.68% without the catalyst). It is evident that the addition of H2O2 inhibits the formation of PAHs and promotes the degradation of existing aromatic compounds. In addition, NO and NA compounds were not detected during the heating stage with H2O2 (similar to Figure 3), confirming that H2O2 stimulates the deamination and denitrification of N-containing compounds [35,36].
AC compounds in the raw sewage sludge constituted 56.18% of the area, predominantly consisting of steroids, with a minor presence of cycloalkanes, cyclic ketones, and cyclic olefins. When no catalyst was present, this proportion increased dramatically to 71.40% at 200 °C then reduced slowly to 33.33% at 400 °C. With a H2O2 addition, this proportion declined abruptly to 2.04% at 200 °C, rose progressively to 17.18% at 325 °C, then reduced gradually to 3.39% at 400 °C (that is, below the value without a H2O2 addition). These results suggest that H2O2 promotes the decomposition of cyclic compounds.
Overall, it appears that the addition of H2O2 inhibits the polymerization of open-chain intermediate products to form aromatic compounds such as benzene while reducing the levels of reactants required for the production of LMW PAHs, thereby reducing the proportion of aromatic and alicyclic compounds formed during the heating period. This oxidizer also promotes the ring-opening reaction of existing aromatic compounds in raw sludge to form open-chain compounds (accounting for more than 94% of the area) and further reduces the proportions of aromatic and alicyclic compounds.

3.4. Mechanism of Combined Catalysts on PAHs Formation and Gas Production

The addition of a suitable amount of H2O2 can evidently reduce PAH concentrations and ring numbers, but an excess appears to oxidize H2, CO, and CH4 to CO2 [25,26], thus greatly lowering the hydrogen yield (as shown in Figure 4a). Some researchers have reported that a combination of two catalysts can achieve synergistic effects. Based on this observation, NaOH + H2O2 and Ni + H2O2 were selected as combined catalysts in this study to determine the impact of such combinations on PAH formation and gas production. Figure 7 summarizes the PAH concentrations in the residues and the gas compositions obtained from the SCWG of the DSS with or without catalysts. The gasified experiments were conducted at 400 °C and 24 MPa, with a 30 min retention time. The total catalyst loading amount was 5 wt%, with a mixture ratio of 1:1.
As shown in Figure 7a, the addition of the combined catalysts significantly inhibited PAH formation, such that the PAH content in the residues were remarkably reduced. The 5 wt% loading of NaOH + H2O2 and Ni + H2O2 lowered the PAH concentrations in the liquid residue from 0.42 μg/mL to 0.14 and 0.13 μg/mL, respectively (reductions of 66% and 70%), and in the solid residue from 11.04 μg/g to 7.99 and 6.20 μg/g, respectively (reductions of 28% and 44%). The concentration of HMW PAHs were also decreased to a greater extent than the LMW PAHs. In addition, 6-ring PAHs were still detected in the solid residue, possibly due to the lower concentration of H2O2 in these trials (2.5 wt% loading). Increasing the amount of H2O2 would theoretically further reduce the concentration of PAHs but may also lower hydrogen production. Guo et al. [26] reported that the amount of H2O2 loaded is the paramount factor influencing hydrogen production from the partial oxidation of sewage sludge in SCW. Wang et al. [30] assessed the impact of the Ni:H2O2 ratio on H2 yield and the suppression of PAH formation and showed that decreasing the Ni:H2O2 ratio reduced the concentration of HMW PAHs and also lowered hydrogen production.
As shown in Figure 7b, the addition of the combined catalysts had a significant effect on the gasification reaction, such that the total gas and H2 yields were greatly increased. With a 5 wt% loading of the NaOH + H2O2 and Ni + H2O2, the total gas yields rose from 4.47 mol/kg OM to 4.68 and 7.39 mol/kg OM, while the hydrogen yields climbed from 0.13 mol/kg OM to 1.75 and 3.88 mol/kg OM, respectively. Muangrat et al. [29] found that the combined use of NaOH and H2O2 resulted in greater H2 yields than that of NaOH or H2O2 alone, indicating a synergistic effect between the two catalysts. Accordingly, the HE and ER both increased significantly with the addition of the combined catalysts. The HE increased from 0.45% to 4.7% and 11.6%, while the ER increased from 0.46% to 2.9% and 7.3%, respectively. Table 2 summarizes the catalysts’ performance in promoting hydrogen production and inhibiting PAHs simultaneously. Compared to the study of Wang et al. [30], this study has a higher H2 yield and similar percentage of PAHs reduction. The study of Zhang et al. [27] shows that H2O2 alone has a better PAHs inhibition effect but has a limited effect on promoting H2 production. Compared with the study of Zhong et al. [28], our study also demonstrates that the combined catalyst had a better comprehensive catalytic effect than that of KOH or H2O2 alone. In summary, both NaOH + H2O2 and Ni + H2O2 combinations can lower PAH concentrations and increase hydrogen yields during the SCWG of DSS.
Drawing from the aforementioned findings, reaction pathways explaining the effects of the combined catalysts on PAH formation and gas production during the SCWG of DSS are proposed, as shown in Figure 8. During the SCWG process, organic compounds are simultaneously decomposed and polymerized, such that some are gasified and some carbonized [12]. The addition of the combined catalysts primarily promotes decomposition while inhibiting polymerization. The Ni and NaOH in these catalytic systems promote the decomposition and gasification of intermediate products. Ni has been shown to enhance steam reforming and methanation reactions [37], while NaOH is recognized for facilitating the water–gas shift reaction to produce syngas [38]. The H2O2 in the combined catalysts promotes the ring-opening reaction of aromatic compounds to produce ring-opening intermediate products, after which the Ni and NaOH are responsible for the rapid decomposition of the intermediate products to produce H2 and CH4. In this case, the polymerization of intermediate products to form monocyclic aromatic hydrocarbons such as benzenes and phenols is decreased (as shown in Figure 4b). Muangrat et al. [39] explored the partial oxidative gasification of food waste in SCW with H2O2 as an oxidant together with NaOH to aid the samples’ decomposition, promote the water–gas shift reaction, and reduce char/tar formation. Their results also showed a synergy between NaOH and H2O2. Similarly, Xu et al. [40] conducted a study on the partial oxidative gasification of phenol in SCW with Na2CO3 as an additive to obtain higher hydrogen production. In this system, the oxidant facilitated the decomposition of phenol to intermediates while Na2CO3 promoted the degradation of these products by increasing the rate of the water–gas shift reaction to improve the H2 yield. The Na2CO3 also hydrolyzed the hydroxyl groups of the phenol to generate phenolate ions, thus promoting the degradation of the phenol. Therefore, when NaOH/Ni + H2O2 combined catalysts are added, H2O2 acts to enhance the ring-opening reaction of aromatic compounds to form ring-opening intermediates. The NaOH and Ni act to facilitate the gasification reaction of these products. Thereby, the concentrations of polymers such as PAHs in the residues are reduced and the yield of hydrogen in the gas phase is elevated.

4. Conclusions

The effects of catalysts on PAH distribution in products from the SCWG of DSS were investigated. The inhibition effects of H2O2 and NaOH/Ni + H2O2 as combined catalysts were investigated. Drawing from the qualitative GC/MS analysis of solid residues throughout the heating process, potential reaction pathways of the combined catalysts that affect PAH formation and gas production during the SCWG of DSS were also suggested. The key findings from this study can be encapsulated as follows:
  • Except for the NaNO3 catalyst, the addition of the other five catalysts (Ni, NaOH, Na2CO3, H2O2, and KMnO4) reduced PAH concentrations in the liquid and solid residues. In particular, the addition of Ni and H2O2 significantly lowered the PAH concentrations. The PAH concentrations in the liquid residue dropped from 0.39 μg/mL to 0.29 and 0.26 μg/mL while that in the solid residue was reduced from 12.5 μg/g to 7.1 and 6.3 μg/g, respectively.
  • The PAH concentrations in both the liquid and solid residues decreased sharply with increases in the amount of H2O2, leading to decreases of 91% and 88%, respectively, with a loading of 10 wt%. As the H2O2 amount increased to 8 wt%, 5-ring and 6-ring PAHs were not detected in the residues. The H2O2 addition inhibited PAH formation by promoting the ring-opening reaction of existing aromatic compounds in the raw sludge and limiting the polymerization of open-chain intermediates.
  • Both NaOH + H2O2 and Ni + H2O2, acting as combined catalysts, greatly decreased the PAH concentrations while raising the hydrogen yield. The H2O2 generated ring-opening intermediates that were further gasified by the NaOH or Ni. Thus, these combinations exhibited a synergistic effect.

Author Contributions

Conceptualization, M.G. and Q.X.; investigation, M.G., Q.X. and F.Y.; writing—original draft preparation, Q.X., F.Y. and M.G.; writing—review and editing, M.G. and Y.F.; visualization, Q.X. and Y.F.; supervision, M.G.; funding acquisition, M.G. and Y.F. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of PR China (Grant No. 52000056), the Natural Science Foundation of Anhui Province (Grant No. 2408085ME118), the International Science and Technology Cooperation Program of Henan Province (Grant No. 242102521049), and the Fundamental Research Funds for the Central Universities (Grant No. JZ2021HGTB0104).

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

The authors gratefully acknowledge the support given by the Anhui Provincial Engineering Laboratory for Rural Water Environment. The authors also thank Hohai University for providing facilities and technical support.

Conflicts of Interest

The authors declare no conflicts of interest.

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Water 16 03235 g001
Figure 1. Effects of different types of catalysts (5 wt% loading) on PAHs distribution in (a) liquid and (b) solid residues at 400 °C, 24 Mpa, and 10 min.
Figure 1. Effects of different types of catalysts (5 wt% loading) on PAHs distribution in (a) liquid and (b) solid residues at 400 °C, 24 Mpa, and 10 min.
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Water 16 03235 g002
Figure 2. Effects of the H2O2 amount on changes in the PAHs distribution in the (a) liquid and (b) solid residues at 400 °C, 24 Mpa, and 10 min.
Figure 2. Effects of the H2O2 amount on changes in the PAHs distribution in the (a) liquid and (b) solid residues at 400 °C, 24 Mpa, and 10 min.
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Water 16 03235 g003
Figure 3. Qualitative GC/MS analysis of solid residues obtained from SCWG of DSS with different H2O2 amounts.
Figure 3. Qualitative GC/MS analysis of solid residues obtained from SCWG of DSS with different H2O2 amounts.
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Figure 4. Effects of H2O2 amount on (a) gas compositions and (b) total phenol concentration in liquid residues.
Figure 4. Effects of H2O2 amount on (a) gas compositions and (b) total phenol concentration in liquid residues.
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Water 16 03235 g005
Figure 5. Effects of H2O2 addition (6 wt% loading) on changes in the PAH distribution in the (a) liquid and (b) solid residues during the heating stage of the SCWG process.
Figure 5. Effects of H2O2 addition (6 wt% loading) on changes in the PAH distribution in the (a) liquid and (b) solid residues during the heating stage of the SCWG process.
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Figure 6. Qualitative GC/MS analysis of solid residues with and without a 6 wt% H2O2 addition during the heating stage of the SCWG process.
Figure 6. Qualitative GC/MS analysis of solid residues with and without a 6 wt% H2O2 addition during the heating stage of the SCWG process.
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Water 16 03235 g007
Figure 7. Effects of combined catalysts on (a) changes in the PAH distribution in the residues; (b) gas compositions; and CE, HE, GE and ER.
Figure 7. Effects of combined catalysts on (a) changes in the PAH distribution in the residues; (b) gas compositions; and CE, HE, GE and ER.
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Figure 8. Schematic of proposed pathways of combined catalysts on PAHs formation and gas production of SCWG of DSS.
Figure 8. Schematic of proposed pathways of combined catalysts on PAHs formation and gas production of SCWG of DSS.
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Table 1. Properties of the DSS samples used in the experiments.
Table 1. Properties of the DSS samples used in the experiments.
Moisture Content
(wt.%)		pHProximate Analysis
(wt%) 1		Ultimate Analysis
(wt%) 1		HHV (MJ/kg) 3
VMFCAshCHNSO 2
77.057.5027.842.1969.9712.112.081.821.0912.934.91
Heavy Metal Content
(mg/kg) 1		FeNiCuZnCrPbAs
23,20127.6128125445.844.7-
PAHs Content
(μg/g) 1,4		2-Ring3-Ring4-Ring5-Ring6-Ring
0.352.803.380.71N.D
Notes: 1 On an air-dried basis. 2 Calculated by subtraction (O% = 100% − C% − H% − N% − S% − ash%). 3 The higher heating value (HHV) is determined using the Dulong formula: HHV (MJ/kg) = 0.3393C + 1.443 (H − O/8) + 0.0927S + 0.01494N. 4 N.D.: indicates no detectable peak; 2-ring: includes naphthalene, acenaphthene, acenaphthylene, and fluorene; 3-ring: includes anthracene, phenanthrene, and fluoranthene; 4-ring: includes pyrene, chrysene, benz[a]anthracene, benzo(b)fluoranthene, and benzo(k)fluoranthene; 5-ring: includes indeno(1,2,3-cd)pyrene, benzo(a)pyrene, and dibenzo(a,h)anthracene; 6-ring: includes benzo(g,h,i)perylene.
Table 2. Summary of catalyst performance for hydrogen production and PAHs inhibition.
Table 2. Summary of catalyst performance for hydrogen production and PAHs inhibition.
FeedstockExperimental ConditionsCatalystEffects on H2 YieldEffects on PAH Concentrations in Solid ResiduesReference
Without Catalyst
(mol/kg OM)		With Catalyst
(mol/kg OM)		Increase
Multiple		Without Catalyst
(μg/g)		With Catalyst
(μg/g)		Decrease Percentage
DSS400 °C, 10min2.5 wt% Ni + 2.5 wt% H2O20.133.8827.911.046.2044%This work
2.5 wt% NaOH + 2.5 wt% H2O20.131.7512.011.047.9928%
DSS400 °C, 60min2.5 wt% Ni + 2.5 wt% H2O20.290.831.861.270.6648%[30]
Cyanobacterial biomass400 °C, 10min1 wt% H2O20.170.501.9410.543.2669%[27]
Coking sludge400 °C, 30min8 mmol KOH
(about 1.2 wt%)		0.0360.389.56263.1164.6637%[28]
2 mmol H2O2
(about 0.4 wt%)		0.0360.0450.25263.1224.1915%
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Xu, Q.; Yan, F.; Fan, Y.; Gong, M. Inhibition of Polycyclic Aromatic Hydrocarbons Formation During Supercritical Water Gasification of Sewage Sludge by H2O2 Combined with Catalyst. Water 2024, 16, 3235. https://doi.org/10.3390/w16223235

AMA Style

Xu Q, Yan F, Fan Y, Gong M. Inhibition of Polycyclic Aromatic Hydrocarbons Formation During Supercritical Water Gasification of Sewage Sludge by H2O2 Combined with Catalyst. Water. 2024; 16(22):3235. https://doi.org/10.3390/w16223235

Chicago/Turabian Style

Xu, Qiao, Fenfen Yan, Yujie Fan, and Miao Gong. 2024. "Inhibition of Polycyclic Aromatic Hydrocarbons Formation During Supercritical Water Gasification of Sewage Sludge by H2O2 Combined with Catalyst" Water 16, no. 22: 3235. https://doi.org/10.3390/w16223235

APA Style

Xu, Q., Yan, F., Fan, Y., & Gong, M. (2024). Inhibition of Polycyclic Aromatic Hydrocarbons Formation During Supercritical Water Gasification of Sewage Sludge by H2O2 Combined with Catalyst. Water, 16(22), 3235. https://doi.org/10.3390/w16223235

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