Mitigation of arsenic and zinc toxicity in municipal sewage sludge through co-pyrolysis with zero-valent iron: A promising approach for toxicity reduction of sewage sludge

: The co-pyrolysis (at 300°C and 600°C) of municipal sewage sludge (SS) with zero-valent iron (Fe 0 : 1.5% and 3%) was investigated to reduce the toxicity of arsenic (As) and zinc (Zn) in SS. The BCR sequential extraction method, desorption kinetic analysis, and material characterization techniques (Fourier transform infrared spectroscopy, ﬁ eld-emission scanning electron microscopy, X-ray di ﬀ raction) were used to evaluate the e ﬀ ects of the treatments on Zn and As behavior. The results showed that co-pyrolysis signi ﬁ cantly reduced the acid-soluble fraction (18 – 43% for Zn; 83 – 95% for As) and mobility factor (45 – 85% for Zn; 86 – 96% for As) of Zn and As compared to untreated SS. Desorption experiments indicated a signi ﬁ cant reduction in Zn and As release in treated samples, particularly in the co-pyrolysis sample at 600°C and Fe 0 3% (67% for Zn; 88% for As) in comparison with untreated SS. Co-pyrolysis of Fe 0 and SS led to the formation of new functional groups (Si – O, aromatic), a more porous surface morphology, and highly stable chemical crystals (ferric arsenate, zinc arsenide), which played a crucial role in Zn and As stabilization. The ﬁ ndings of this study suggest that co-pyrolysis is a promising approach for mitigating As and Zn toxicity in SS. However, additional ﬁ eld testing with plant-based systems is necessary for con ﬁ rmation.


Introduction
Sewage sludge (SS) is a by-product generated by wastewater treatment plants.Although SS carries the potential advantages of serving as a supplier of crucial elements and organic materials for plant development, its application introduces notable risks owing to the presence of harmful pathogens and organic-inorganic pollutants [1].The carcinogenic and mutagenic effects of pathogens and toxic pollutants in SS on human health are well-documented [2,3].Consequently, researchers are exploring solutions to mitigate the adverse effects of SS in the environment by focusing on the destruction of organic pollutants, stabilization of heavy metals, and reduction of pathogens.Usual techniques for SS management encompass stabilization, composting, anaerobic digestion, thermal treatment, and thermochemical processes [4].Currently, thermochemical methods are recognized as viable approaches for SS management and valuable nutrient recovery [5].Among these methods, hydrothermal treatment, gasification, and pyrolysis have emerged as particularly promising options [5].Pyrolysis methods offer a dual advantage by reducing both the volume and environmental risks associated with SS.Simultaneously, they convert SS into valuable compounds, including solid carbon such as biochar and hydrochar, as well as biofuels [6].Biochar, a carbon-rich solid substance, is produced during the pyrolysis process of biomass in the absence or partial presence of oxygen [7]).Previous reports indicate that pyrolysis, especially at higher temperatures, can decompose organic pollutants, eliminate pathogens, and stabilize heavy metals in SS [8,9].
However, studies have also reported a high risk of heavy metal leaching in soil after SS biochar application due to changes in the soil environment [10,11].Researchers believe that co-pyrolysis processes involving multiple feedstocks may be more effective than singular pyrolysis processes using only one feedstock for stabilizing heavy metals in SS [12,13].Numerous studies have demonstrated the reduction of heavy metal mobility and toxicity in SS through co-pyrolysis with organic wastes such as rice husks, walnut shells [12,13], and mineral compounds such as ZnCl 2 , K 2 CO 3 , Ca(H 2 PO 4 ) 2 , and CaSO 4 [11,[14][15][16].Li et al. [17] discovered that the addition of kaolin or zeolite to SS during co-pyrolysis can effectively stabilize heavy metals.This process involves the reaction between oxygen ions from the aluminosilicate additives, leading to the formation of stable compounds like silicates (e.g., PbSiO 3 ) and aluminates (e.g., CuAl 2 O 4 ).As a result, heavy metals are converted into more stable fractions, reducing their potential environmental impact.Interestingly, kaolin exhibits a notable advantage due to its dehydroxylation tendency at high temperatures.This characteristic makes kaolin particularly suitable for interacting with heavy metal ions and may explain its superior performance compared to zeolite in terms of heavy metal stabilization.Gu et al. [14] aimed to investigate the effects of co-pyrolyzing SS with different ratios of Ca(H 2 PO 4 ) 2 (0%, 15%, and 30 wt%) on the immobilization of heavy metals and the reduction of metal leachability.The study proposed several potential mechanisms responsible for heavy metal stabilization.These mechanisms included the formation of chelating agents such as metaphosphates and polyphosphates through the condensation dehydration reactions of Ca(H 2 PO 4 ) 2 , the formation of highly aromatic metallic compounds, and physical adsorption.Phosphates were also found to facilitate the formation of stable forms of lead (Pb 2 P 2 O 7 ) and pyromorphite-like minerals.
Zero-valent iron (Fe 0 ) has shown significant potential for stabilizing various toxic heavy metals in solid environments like soil and SS [18,19].Hence, there is a hypothesis that the inclusion of Fe 0 as a feedstock (or as a catalyst) in the co-pyrolysis process with SS may lead to improved outcomes in terms of stabilizing heavy metals, surpassing the results achieved by individual pyrolysis processes.
In a previous study conducted by the author, the characteristics of SS from the Kerman sewage treatment plant were analyzed, revealing higher levels of arsenic (As) and zinc (Zn) than the allowable limits for agricultural purposes [20].To address this concern, composting was implemented as a method to reduce the mobility of heavy metals.However, contrary to expectations, the results indicated that composting had the unintended effect of increasing the mobility of Zn [20].Given the unexpected increase in Zn mobility during composting, this project aims to comprehensively investigate the application of the co-pyrolysis method as a promising solution to effectively decrease the availability of As and Zn elements in SS.Currently, there is no published research on the co-pyrolysis of Fe 0 and SS for heavy metal immobilization.
The objective of this study is to examine and compare the impacts of co-pyrolysis of SS with Fe 0 , as well as the effects of using Fe 0 in SS without the pyrolysis process, on the behavior of Zn and As.The investigation involves analyzing the chemical forms and desorption kinetics of these elements.To gain a deeper understanding of the underlying processes influencing the behavior of these elements, the study employs various techniques such as FTIR, SEM, and XRD.

Characterization of treated samples
Figure 1 illustrates the changes in FTIR spectra between the RSS sample and the treated samples (with 3% Fe 0 ).In the RSS sample, a prominent FTIR absorption band was observed in the range of 1,080-1,030 cm −1 (1,065 cm −1 ), indicating the presence of C-O groups associated with polysaccharides or polysaccharide-like materials, Si-O bonds from silicate, and clay minerals complexed with humic acids [21,22].The application of the pyrolysis process in the presence of Fe 0 at 600°C and 300°C resulted in contrasting effects on the intensity of the mentioned peak compared to the RSS sample.At 600°C, the intensity of the peak at 1,038 cm −1 increased, indicating an enrichment and higher intensity of Si-O functional groups, which can be attributed to the non-degradable portion of SS present at high temperatures.In contrast, at 300°C, the intensity of the peak at 1,034 cm −1 decreased, suggesting a reduction in the Si-O functional groups.The non-pyrolyzed sample also exhibited a decrease in the intensity of the peak at 1,038 cm −1 .It appears that the application of a high pyrolysis temperature to the samples, despite causing the degradation of polysaccharides, led to the enrichment and increased intensity of Si-O functional groups originating from the non-degradable fraction of SS at high temperatures.
Conversely, at a low pyrolysis temperature, the decomposition of the polysaccharide component was minimal and did not contribute to the enrichment of the Si-O functional group.Additionally, the use of Fe 0 likely contributed to the decrease in the intensity of this peak, although further investigation is required to determine the exact mechanism involved.The existence of three spectra of 1,460, 1,540, and 1,660 cm −1 in the RSS sample indicates the presence of water (1,540 cm −1 ), protein (amide: 1,540 and 1,660 cm −1 ) [23], and CH 2 rocking functional groups (1,460 cm −1 ) [24].The application of Fe 0 treatment, with and without the pyrolysis process, resulted in a reduction of these spectra.The absorption spectra at 2,920 and 2,850 cm −1 in the RSS sample can be attributed to methane, aliphatic methylene, aliphatic CH 2 groups [25,26], fats and lipids groups [23], and C-H stretching of alkyl structures [27].The application of Fe 0 treatment, with or without the pyrolysis process, has resulted in a reduction and shift in the absorption of these spectra.This can be attributed to the decomposition of organic fatty hydrocarbons into CH 4 , CO 2 , and smaller aromatic structures.During pyrolysis, the shift in the absorption spectra, especially in the amid functional group, has been reported by Lu et al. [28] to be caused by the interaction between amide groups and heavy metals, leading to the formation of complexes.The spectrum at 3,290 cm −1 (3,300-3,500 cm −1 ) in the RSS sample indicates the presence of N-H groups.Upon applying or not applying the pyrolysis process at 300°C, a significant decrease in the intensity of this spectrum was observed.However, in the case of B600 (pyrolysis at 600°C), there was a shift in the peak position from 3,294 to 3,421 cm −1 .In general, the FTIR results demonstrate that the utilization of Fe 0 and pyrolysis processes have induced specific changes in the functional groups of RSS (increase of Si-O and aromatic functional groups), which can potentially influence the behavior of As and Zn.
The surface morphology (FE-SEM) of RSS affected by Fe 0 application and the pyrolysis process is shown in Figure 2. In addition, to confirm the presence of Fe 0 and also to determine the quantitative changes of some elements affected by pyrolysis, an energy-dispersive X-ray spectroscopy (EDX) test was also done (Figure 2).According to the FE-SEM results, the surface of the RSS sample has a homogeneous surface, and after pyrolysis, the heterogeneous surfaces (relatively higher porosity) are clearly visible.Both B300 and B600 samples exhibit a mixture of mineral and organic components, which stabilized on their surface.Despite Mitigating As and Zn toxicity in SS through co-pyrolysis with Fe 0  3 the heterogeneous structure of samples produced by pyrolysis, the EDX results show zero amounts of Fe 0 in the RSS sample and the presence of significant amounts of Fe 0 in the pyrolyzed samples.The presence of higher amounts of cationic metals such as Ca during the pyrolysis process indicates the enrichment of pyrolysis-resistant elements in the produced samples.
The crystal structure and mineral composition of the studied sample are shown in Figure 3.The aim was to investigate the influence of the co-pyrolysis process with Fe 0 on the behavior of Zn and As elements by analyzing the X-ray diffraction patterns of the samples.The analysis of the RSS sample uncovered the presence of calcite, quartz, microcline, almandine, bianchite, and copper arsenate.Through the implementation of the co-pyrolysis process, changes in the structure of the samples were observed.In the B300 sample, a distinct peak at 2θ = 15.12°indicated the presence of ferric arsenate and quartz compounds.Additionally, at 2θ = 26.6°, the presence of graphite, microcline, and coesite was observed in this sample.Other peaks investigated in the B300 sample exhibited the presence of ferric arsenate, coesite, and microcline.In the B600 sample, along with ferric arsenate, graphite, coesite, and copper arsenate compounds were also detected.Notably, the absence of peaks associated with calcite, quartz, microcline, almandine, and bianchite structures, coupled with the presence of coesite, were significant findings of this study.Coesite possesses a denser crystal structure compared to quartz and may contribute to the formation of a more robust complex with the existing silicate structure.The presence of ferric arsenate in both the B300 and B600 samples, known to be highly immobile according to previous reports [29], could potentially explain the reduced availability of As.Furthermore, the occurrence of zinc arsenide in the B600 sample may account for the diminished availability of Zn in this particular sample.

Chemical forms of Zn and As in treated samples
Analysis of variance (ANOVA) revealed that all chemical forms of Zn and As were significantly influenced by the treatments investigated (Tables 1 and 2).Tables 3 and 4 Figure 3: XRD pattern of the treated samples and RSS.
show the comparison of the average chemical forms of Zn and As affected by different treatments.In addition, Figures 4 and 5 also display the relative percentage of each chemical form of Zn and As.In the RSS sample, the highest chemical form of Zn was found in the chemical form F4 (223.2 mg•kg −1 ; 33.27%), followed by chemical forms F1 (173.3 mg•kg −1 ; 25.83%), F2 (171.2 mg•kg −1 ; 25.52%), and F3 (103 mg•kg −1 ; 15.36%), respectively.Previous studies by Ščančar et al. [30] on chemical forms of Zn in the SS samples of Slovenia showed that the Zn chemical forms of F1, F2, F3, and F4 were 46%, 36.2%,11.3%, and 6.5% of the total Zn, respectively.Wang et al. [31] also studied the chemical forms of different elements in different SS and showed that despite the differences in the amounts of Zn chemical forms in different SS, the chemical forms of F1 and F3 had the highest amount of Zn chemical forms.Similarly, Karwowska and Dąbrowska [32] reported that F3 (45.9-46.3%)had the highest amount of Zn chemical forms in two Polish SS samples investigated, followed by F2 (34.8-37.8%),F1 (12.9-13.3%),and F4 (3-6%).Application of Fe 0 , without pyrolysis, significantly reduced the F1 form and insignificantly decreased F4, while significantly increasing F2 and F3 forms.Fe 0 application at 1.5% and 3% resulted in an 18% and 24% reduction in F1 and 19.7% and 0.4% reduction in F4, respectively.In contrast, Fe 0 use at the same levels caused an increase of 35% and 10% in F2 and 17% and 26% in F3 forms.Pyrolysis at 300 and 600°C increased the total Zn content compared to the RSS sample, which was significant in   pyrolysis at 600°C.The pyrolysis process at 300°C and 600°C increased the total Zn by 2.3-3.5% and 23.6-24.1%,respectively, compared to the RSS sample.Pyrolysis at 600°C resulted in a 21.2-21.4% increase in total Zn compared to pyrolysis at 300°C.However, pyrolysis at both temperatures significantly decreased the F1, F2, and F3 forms while increasing F4 form.In the pyrolysis process at 300°C, in the presence of 1.5% and 3% Fe 0 , F1, F2, and F3 showed a reduction of 43-57%, 63-75%, and 6-13%, respectively.Conversely, F4 increased by 95-115%.In the pyrolysis process at 600°C, in the presence of 1.5% and 3% Fe 0 , F1, F2, and F3 showed a reduction of 70-82%, 63-67%, and 40-47%, respectively, while F4 increased by 196% and 206%.The MF of each metal is a useful indicator of its availability in SS.In Figure 6, the highest MF was found in the RSS sample (25.8%).However, the application of Fe 0 , with and without pyrolysis, caused a reduction in the MF of Zn.The most significant decreases were observed in samples of B600 (6.3-3.7%),B300 (14.2-0.8%), and SS (21.1-19.4%).
According to the study results, the most prevalent form of As in RSS is F1, accounting for 58.72% of the total As.It is followed by F4 at 33%, with F2 and F3 forms contributing 5.7% and 2.8%, respectively.In the study of chemical forms and risk assessment of heavy metals in biochars obtained from SS and anaerobically digested sludge, Zhao et al. [33] showed that F4 (54%), F3 (18%), F1 (17.8%), and F2 (10.2%), respectively, have the highest to lowest percentage of As in raw SS, which the application of pyrolysis process caused an increase in F4 (75%) and a decrease in F1 (0.92%).When Fe 0 was used in RSS (without pyrolysis), it caused a significant decrease in the F1 (8-11%) form and an increase in the F2 (7.7-11.7%)and F4 (74.2-84.2%)forms of As.This effect was more evident at the 3% level.The use of 1.5% and 3% Fe 0 without pyrolysis resulted in a substantial decrease of 81% and 85% of the F1 form, respectively, compared to the RSS sample.It also caused an increase of 100% and 37.5% in the F2 form and 119% and 160% in the F4 form, respectively, compared with RSS.Pyrolysis at 300 and 600°C resulted in an increase of 23-19% and 32-34% in total As, respectively.At a pyrolysis temperature of 300°C, the presence of 1.5% and 3% Fe 0 resulted in significant changes in the distribution of chemical forms.Specifically, the F1 form decreased by 83% and 85%, respectively, while the F2 form increased by 100% and 550%, and the F4 form increased by 198% and 133%, respectively, in comparison to the RSS.Similarly, pyrolysis at 600°C in the presence of 1.5% and 3% Fe 0 resulted in a decrease of 91% and 95% in the F1 form, respectively, in contrast to the RSS.There was also an increase of 637% and 487% in the F2 form and 155% and 188% in the F4 form, respectively, compared to the RSS.The study found that treating the samples with Fe 0 , either with or without pyrolysis, significantly reduced the MF of As (Figure 7).Fe 0 treatment without pyrolysis at 1.5% and 3% levels reduced the MF by 81% and 86%, respectively, compared to the RSS sample.Fe 0 treatment with pyrolysis at 1.5% and 3% for 300 and 600°C resulted in even greater reductions of 86-88% and 93-96%, respectively.

Desorption kinetics of Aa and Zn in treated samples
The release kinetics of As over time in the studied samples is shown in Figure 8.According to the results, a biphasic Mitigating As and Zn toxicity in SS through co-pyrolysis with Fe 0  7 trend of As release is observed, with higher release in the initial stages and lower release and reaching equilibrium later in the process in all samples.The high and low releases of As in the early and intermediate stages, respectively, are associated with more mobile and less mobile fractions of As.The RSS sample exhibited the highest As cumulative released amount (21.6 mg•kg −1 ) after 1,440 min.In contrast, samples treated with F 0 pyrolysis at 600°C showed the lowest As release (2.73-4.55mg•kg −1 ) after 1,440 min.The samples treated with B300 and SS showed lower amounts of As release compared to the RSS sample, with cumulative releases of 6.37-8.32 and 11.06-13.72mg•kg −1 after 1,440 min, respectively.In the RSS sample, more than 39% of the total As was released after 1,440 min of contact with the extractant, while in the SS, B300, and B600 samples, these amounts were 19.4-25.2%,9.2-12.4%,and 3.6-6.1%,respectively, indicating significant effects of Fe 0 applications with and without pyrolysis on As stabilization.
The Zn release kinetics in the studied samples, like the As desorption showed the of a biphasic process of releasing high amounts in the early times and releasing to a lesser extent during the desorption process in all samples (Figure 9).Similar to the results of As desorption, the RSS sample showed the highest cumulative Zn desorption (243 mg•kg −1 ) after 1,440 min.On the other hand, B600, B300, and SS samples showed the lowest and highest Zn release in the treated samples with values of 86-93, 110-87, and 182-148 mg•kg −1 , respectively.In the RSS sample, at the end of the desorption time, more than 36% of total Zn was released, while in the SS, B300, and B600 samples, this amount was 22-27%, 12.7-16%, 11.3-10.3%,respectively.
The fitted desorption data for As and Zn using the two first-order reaction models are presented in Table 5. Q1, representing the accessible and easily extractable fraction, displayed the highest amount for both As and Zn in the RSS sample, which serves as the control sample.Different treatments applied to the RSS sample resulted in a noticeable decrease in the Q1 parameter.Among the treated samples, B600 exhibited the lowest Q1 parameter values for both As and Zn, followed by B300 and SS.Comparing the Q2 parameter values alone may not be a reliable criterion for comparison, given the different total values observed for both elements.In contrast, the evaluation of the Q3 parameter, representing the non-extractable fraction, revealed that the RSS sample had the lowest value, while the treated  samples showed higher values.The highest Q3 parameter value was found in B600, followed by B300 and SS.
A correlation analysis (Pearson, 95% confidence level) was conducted to examine the relationship between parameters derived from the two first-order reaction models and the chemical forms of As and Zn elements (Tables 6  and 7).In terms of As, the correlation analysis revealed the following associations between the components of the two first-order reactions model and the chemical forms: Q1 (accessible part) exhibited a statistically significant positive correlation with Q2, F1, and MF, and a statistically significant negative correlation with Q3, F2, F4, and total As.Q2 (the less extractable part) displayed a statistically significant positive correlation with F1 and a statistically significant negative correlation with Q3, F4, and MF.Q3 (nonextractable part) showed a significant positive correlation with F2 and total As.The correlation analysis between the parameters obtained from the first-order reaction model and the chemical forms of Zn exhibited a similar trend to that of As.The accessible fraction (Q1) in Zn showed a significant positive correlation with Q2, F1, F2, and MF, while having a significant negative correlation with Q3 and F4.The less-extractable fraction (Q2) exhibited a significant positive correlation with F1, F2, and MF; and a significant negative correlation with Q3 and F4.Finally, the non-extractable fraction (Q3) showed a significant positive correlation with F4 and total Zn, while having significant negative correlations with F2, F3, and MF.

Possible mechanisms of Zn and As stabilization in treated samples
According to the results, pyrolysis caused a noticeable increase in the total amount of Zn and As elements.The increased abundance of Zn and As in biochars may be attributed to their greater stability in comparison to other components found in RSS [34].Furthermore, as the temperature increases, the concentration of Zn within the biochar increases due to a higher rate of mass loss during co-pyrolysis [35].
The current study proposes the hypothesis that the pyrolysis of SS enhances the stabilization of Zn and As through two primary mechanisms.First, during the pyrolysis process, the sludge undergoes decomposition, resulting in the generation of carbon, gases, and new organic compounds.This decomposition process creates pores that effectively immobilize Zn and As.Second, the inorganic components found in the RSS, such as silicates, phosphates, and iron oxides, undergo reactions with Zn and As during pyrolysis, leading to their conversion into more stable forms, including silicate minerals, carbonates, organometallic compounds, and alkali compounds.These findings are supported by research conducted by Sun et al. [36], Liang et al. [37], and Liu et al. [11].Kończak and Oleszczuk [35] conducted a study on the mechanisms involved in stabilizing heavy metals in biochars produced from the co-pyrolysis of SS and willow.Their findings revealed several factors that can impact the stabilization of heavy metals, including their occlusion within minerals, entrapment within biochar pores, and the formation of π-bonds with electron-rich domains on biochar's aromatic groups.Yang et al. [38] performed a Pearson correlation analysis to investigate the underlying mechanism of heavy metal transformation and assess the relationship between biochar characteristics and the leachable fractions of heavy metals.Their findings indicated that changes in heavy metal availability were primarily influenced by the chemical forms of Cd and Pb in the biochars, the alkalinity of biochar, and the formation of aromatic structures.
Previous research has demonstrated that Fe 0 exhibits diverse primary interactions with metals, including reduction, adsorption, oxidation/re-oxidation, co-precipitation, and precipitation [39].Fe 0 is known for its electron-donating properties, which enable it to modify the reduction potential (E 0 ) of contaminants that exceed −0.447 V [40].Typically, metals with a similar or more negative E 0 compared to Fe 0 , such as Zn with (E 0 : −0.76), can be effectively treated through processes such as adsorption onto the iron (hydr)oxide shell [41].Conversely, metals with E 0 values higher than Fe 0 , such as As (E 0 : +0.56), are preferably stabilized through reduction and/or precipitation [41].Therefore, it can be concluded that the stabilization of Zn and As in RSS occurs through absorption processes onto iron (hydr)oxide shells and through reduction and/or precipitation in the presence of Fe 0 , respectively.These mechanisms of stabilization also take place during the co-pyrolysis process.
In addition, there is a hypothesis that the use of Fe 0 , with its catalytic properties, results in increased decomposition of the organic components of RSS and prevents the formation of acid-soluble fraction (F1) as well as the oxidizable fraction (F3), leading to a relative increase in the formation of F4 in Zn.Additionally, as a result of the reaction between Fe 0 and the accessible Zn element (Zn 2+ ) in SS, Zn 2+ can be regenerated and form a less accessible Zn precipitate.
During the co-pyrolysis of Fe 0 and SS, interactions between the solid and liquid phases occur, resulting in the formation of chemically stable and strong crystals.These crystals play a crucial role in immobilizing Zn and As.Interestingly, Liu et al. [11] obtained similar findings when studying the co-pyrolysis of SS with CaSO 4 .They demonstrated that CaSO 4 can react with organic intermediate products, leading to the formation of efficient immobilizing agents such as CaCO 3 , Ca(OH) 2 , CaO, and CaS.Moreover, their research indicated that high pyrolysis temperatures promote the conversion of Cr, Pb, and Zn into silicates, iron oxides, and manganese oxides.Wang et al. [16] investigated the impact of carbonates on the stabilization of heavy metals, specifically through the activation of K 2 CO 3 during the co-pyrolysis of SS and cotton stalks.The addition of K 2 CO 3 resulted in the conversion of heavy metals into more stable compounds, including the oxidizable fraction (F3) and residual fraction (F4).This activation process increased the alkalinity of the resulting biochar and facilitated the formation of CaO, CaCO 3 , and aluminosilicates, which effectively immobilized the heavy metals.Min et al. [15] conducted a study to examine the impact of ZnCl 2 utilization for stabilizing heavy metals in biochars derived from camellia oleifera shells and SS copyrolysis.ZnCl 2 incorporation led to a decrease in the overall concentration of heavy metals through chlorination and the formation of CdCl 2 and PbCl 2 .Notably, there was a substantial increase in the fraction of oxidizable heavy metals (F3), with Ni and Cu demonstrating the most pronounced effects.

Conclusions
In this study, the co-pyrolysis process, in combination with Fe 0 , was employed to stabilize As and Zn in SS.The findings indicate that pyrolysis with F 0 significantly reduced the acid-soluble fraction and increased the residual fraction of As and Zn.Additionally, the co-pyrolysis process exhibited noticeable effects in stabilizing these elements, as evident from the desorption of As and Zn.During the co-pyrolysis process, the primary mechanisms for Zn and As stabilization by Fe 0 were surface adsorption and reduction/precipitation processes, respectively.Furthermore, the treated samples displayed enhanced stabilization of As and Zn compared to the RSS sample, attributed to the development of functional groups containing Si-O, a more porous surface morphology, and a crystalline structure comprising ferric arsenate and zinc arsenide compounds.Although the underlying reasons for the chemical stabilization of these elements through the co-pyrolysis process remain somewhat obscure, it is apparent that the utilization of co-pyrolysis of SS and Fe 0 holds promise in stabilizing various heavy elements.Further research is warranted to explore this potential more comprehensively.

Experimental
To produce SS biochar samples, dried SS samples were collected from sludge-drying lagoons at the Kerman wastewater treatment plant in Kerman, Iran.The SS samples were dried at 70°C, and the concentrations of heavy metals were measured using the ICP-MS method.The contents of heavy metals in dry SS were as follows: Zn 659 mg•kg −1 , As 48 mg•kg −1 , Cd 0.37 mg•kg −1 , Pb 36 mg•kg −1 , Ni 23 mg•kg −1 , Cu 162 mg•kg −1 , and Cr 74 mg•kg −1 .The organic carbon, pH, electrical conductivity, and cation exchange capacity values of the investigated SS were found to be 28.3%, 6.8, 7.2 ds•m −1 , and 48.6 cmol (+) kg −1 , respectively.The dried SS (RSS) samples were transferred to specific reactors and then mixed with Fe 0 (micro-scale) at concentrations of 1.5% and 3% (weight/weight).The co-pyrolysis process was carried out at two temperatures, 300°C (B300) and 600°C (B600), under anaerobic conditions using N 2 gas (0.5 L•min −1 ), with a residence time of 4 h.The heating rate was 8°C•min −1 .
To better compare the effects of using Fe 0 in SS on the behavior of As and Zn without the pyrolysis process, Fe 0 (micro-scale) at concentrations of 1.5% and 3% were added to RSS sample and incubated at 25°C for 90 days under field capacity (FC) humidity (SS).The BCR (Bureau Communautaire de Référence) method was employed as a sequential extraction technique to examine the behavior of As and Zn in the treated SS.This method enables the identification of four distinct chemical forms in which As and Zn may exist, namely: acid-soluble fraction (F1), reducible fraction (F2), oxidizable fraction (F3), and residual fraction (F4).The summary of the methods used in the present study is presented in Table 8.The mobility factor (MF) of these metals was calculated based on Eq. 1.
The kinetics desorption of As and Zn from treated SS was investigated by extracting the treated samples with 0.5 M NaHCO 3 (pH = 8.5) and 0.01 M EDTA at different time intervals, respectively.To conduct the experiment, 5 g of each treated sample was transferred to a centrifuge tube, and 25 mL of extractant (NaHCO 3 or EDTA) was added.The samples were shaken back and forth (150 RPM) for different time intervals (30,60,90,180,360, 720, 1,440 min), and at the end, the solutions were separated from the solid part by centrifugation.The amounts of As and Zn in the solutions were measured using an atomic absorption spectrophotometer (AAS: Varian Spectra AA-10).A multiple first-order reaction model was used to predict the desorption changes, and its parameters were used as indicators of accessible, less accessible, and not-accessible parts.Eq. 2 shows the multiple two first-order reactions model and its components: where q is the amount of metal released at time t, Q 1 (mg•kg −1 ) is the "labile" fraction, readily extractable, associated with the rate constant k 1 , Q 2 (mg•kg −1 ) is the "moderately labile" fraction, less extractable, associated with the rate constant k 2 , Q 3 (mg•kg −1 ) is the form of the element which is not extractable, and q total is the total concentration of As or Zn in the sample.Several analytical techniques were utilized to determine the potential mechanisms of As and Zn stabilization in the studied samples.Fourier Transform Infrared Spectroscopy (FT-IR) was employed to identify changes in functional groups Mitigating As and Zn toxicity in SS through co-pyrolysis with Fe 0  11 produced as compared to the RSS.To confirm the presence of Fe 0 in the produced biochars, the surface morphology was assessed using Field-Emission Scanning Electron Microscopy (Fe-SEM; Zeiss Sigma 500 VP), and quantitative determination of elements was carried out using EDX.Additionally, the X-ray Diffraction (XRD: Bruker D8 Advance X-ray diffractometer) technique was utilized to identify the compounds formed during the co-pyrolysis process.
The laboratory research was conducted using a completely randomized design.Data analysis was performed using SAS statistical software (SAS 9.4 M7) through one-way ANOVA.Each mean value represents the average of three replicates.To determine significant differences between treatment means, the Duncan's multiple range test was applied at a significance level of P < 0.05.Graphs were generated using Microsoft Excel 2010, and data correlation was assessed using SPSS V19 software.

Figure 1 :
Figure 1: FTIR spectra of the treated samples and RSS.

Figure 2 :
Figure 2: FE-SEM images and EDX spectrum of the treated samples and RSS.

Figure 4 :Figure 5 :
Figure 4: The relative distribution of Zn chemical fractions in the treated samples.

Figure 6 : 7 :Figure 8 :
Figure 6: Mobility factor of Zn in the treated samples (mean in each column by the same letters are not significantly different at 5% probability level using Duncan's multiple range test).

Figure 9 :
Figure 9: Cumulative Zn released with time in the treated samples.

Table 1 :
ANOVA of Zn fractionation as affected by different treatments

Table 2 :
ANOVA of As fractionation as affected by different treatments

Table 3 :
Mean comparison of Zn chemical forms (mg•kg −1 ) in the treated samples (values are means of triplicate observations) aDifferent letters in columns indicate a significant difference at the level of 5% based on the Duncan test.

Table 4 :
Mean comparison of As chemical forms (mg•kg −1 ) in the treated samples (values are means of triplicate observations) a Different letters in columns indicate a significant difference at the level of 5% based on the Duncan test.ND: not detected by AAS.

Table 5 :
Values of coefficient of determination (R 2 ), standard error of the estimate (SE), and rate constants of the two first-order reaction model fitted on As and Zn desorption data in treated samples

Table 6 :
Simple correlation coefficient (r) between the parameters of two first-order reactions and chemical forms of As in treated samples Correlation is significant at the 5% level; ** correlation is significant at the 1% level. *

Table 7 :
Simple correlation coefficient (r) between the parameters of two first-order reactions and chemical forms of Zn in the treated samples