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. 2026 Jan 12;11(3):4155–4166. doi: 10.1021/acsomega.5c08272

Characteristics of Dissolved Organic Matter from Feces-Based Biochar: Effects of Pyrolysis Temperature and Extraction Solution pH

Tengjiao Xie †,‡, Luyun Liao †,‡,*, Li Wei †,‡, Wei Zuo †,‡, Jinling An †,‡, Chen Yang †,‡, Jing Li †,‡, Zhen Liu †,‡, Xiaogang Han †,‡
PMCID: PMC12854521  PMID: 41626479

Abstract

Pyrolysis enables resource recovery from human feces, yet the mechanisms of release of dissolved organic matter (DOM) from the resulting biochar remain poorly understood. This study investigated how extraction solution pH influences DOM release from feces-derived biochar prepared at different pyrolysis temperatures (280–580 °C), using multispectral techniques, PARAFAC modeling, and two-dimensional correlation spectroscopy (2D-COS). Results showed that dissolved organic carbon (DOC) concentrations ranged from 6.77 to 36.52 mg·g–1, with higher DOM release under alkaline conditions. PARAFAC identified three humic-like components and one protein-like component. Protein-like substances were preferentially released under acidic conditions, while humic-like components dominated under weakly acidic to alkaline environments. 2D-COS analysis further revealed a sequential release of DOM components at the molecular level, influenced by the pyrolysis temperature through modifications in DOM functional groups. These findings provide a theoretical basis and practical insight for employing feces-derived biochars in environmental remediation.

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1. Introduction

The United Nations Millennium Development Goals and the subsequent Sustainable Development Goals have continuously driven global progress toward universal access to safe sanitation. , However, the reality indicates that achieving universal safe sanitation by 2030 still faces severe challenges. − In this context, resource-oriented sanitation (ROS) concepts are widely regarded as an important pathway to achieving sustainable environmental sanitation. , However, such systems pose major challenges for human feces treatment and utilization. Traditional feces resource utilization technologies face several challenges, such as the low efficiency of composting and the high risk of secondary pollution. Although anaerobic digestion can produce biogas, its process is severely constrained by environmental factors, leading to drawbacks such as slow startup, poor operational stability, and low load adaptability. In contrast, pyrolysis technology offers advantages such as rapid reduction, complete harmlessness, and product stability, making it especially suitable for centralized treatment scenarios in areas without sewage systems. Community-scale pyrolysis systems, represented by the OMNI Processor (OPS), have achieved efficient treatment of feces sludge for populations of up to ten thousand, demonstrating excellent performance in terms of cost and carbon emissions: the average daily cost per person is only $0.05 to $0.09, and greenhouse gas emissions are significantly lower than those of traditional methods such as anaerobic digestion. , Moreover, carbon sequestration and nutrient recovery are realized through biochar, further reducing net treatment costs. , During the pyrolysis of feces, the dissolved organic matter (DOM) generated in the biochar is one of the most active components in its environmental behavior, potentially exerting complex effects on ecosystems. , Therefore, a thorough analysis of the characteristics and evolution mechanisms of DOM in feces-based biochar is crucial for enhancing the environmental sustainability and application safety of the pyrolysis technology.

Previous studies using biochar have shown that the content and characteristics of DOM are related to biomass type and pyrolysis temperature, − but are also determined by the extraction conditions (e.g., pH and temperature). , Zhou et al. reported that rice husk biochar released lower amounts of biodegradable dissolved organic matter (BDOM) under low pH conditions. Additionally, Wu et al. found that the DOM derived from wetland-plant biochar in 0.1 mol·L–1 NaOH could reach as high as 23,500 mg DOC·kg–1. In recent years, UV–visible spectroscopy, three-dimensional fluorescence spectroscopy (EEM), and Fourier transform infrared spectroscopy (FTIR), in conjunction with parallel factor (PARAFAC) analysis and two-dimensional correlation spectroscopy (2D-COS), have been applied to evaluate the properties of biochar-derived DOM. − Using the 2D-FTIR-COS analysis, He et al. reported that changes in the sequence of functional groups in DOM-derived biochar with increasing pyrolysis temperature. He et al. also explored the binding order of biochar-derived DOM with metal ions using spectroscopic techniques and 2D-COS, revealing differences in the binding of DOM with various heavy metal ions. In addition, Liu et al. determined the distribution characteristics of DOM in algal biochar under conditions of 200–500 °C using the EEM-PARAFAC analysis method. The DOM obtained from the extraction solution with a higher pH exhibited a higher fluorescence intensity. Within the given DOM, the proportion of protein-like substances was found to be less than that of humic-like substances. Currently, more research is needed to reveal the complex correlations of biomass type, pyrolysis temperature, and extraction conditions with biochar-derived DOM characteristics. , Feces-derived biochar, derived from human feces, possesses both pollution attributes and resource potential, making it a subject of significant research value. Therefore, investigating the composition and characteristics of DOM released from feces-derived biochar produced at varying pyrolysis temperatures is particularly important. However, the current understanding of how extraction solutions influence the DOM release characteristics of this biochar remains unclear, and related studies are notably scarce. Thus, assessing the effects of extraction solutions is crucial for accurately elucidating the environmental behavior and effects of DOM in feces-derived biochar, providing guidance for promoting its safe and efficient resource utilization.

In this study, human feces were collected to prepare feces-based biochar by pyrolysis at 280–580 °C (the reasons for choosing 280–580 °C are detailed in part I of Supporting Information), and the DOM derived from feces-based biochar was extracted in various extraction solutions. The characteristics and structure of DOM released from feces-based biochar were evaluated using UV–vis spectroscopy, synchronous fluorescence (SF), EEM, and FTIR in combination with PARAFAC analysis and 2D-COS. The main aims of this study are to (1) explore the influences of different extracting solutions on the DOM released from feces-based biochar produced under different pyrolysis temperatures, (2) evaluate the variations in the composition and properties of the DOM derived from feces-based biochar, and (3) determine the sequence of functional groups and component changes in biochar-derived DOM released from different extraction solutions. The research findings will provide data support for assessing the environmental behavior and ecological risks of feces-derived biochar and will promote its safe and efficient resource utilization.

2. Material and Methods

2.1. Source of Materials

In this study, the biochar used was produced from the biomass of human feces. The original human excrement was taken from the toilet of Sichuan Geological Engineering Survey Institute Group Co., Ltd. The collected feces were promptly dried at 105 °C until a constant weight was achieved. Subsequently, they were pulverized, passed through a 2 mm sieve, and stored in a desiccator. We collected dried human feces continuously for 1 month and mixed the dried feces prior to pyrolysis. Dry feces samples were placed in a tubular furnace (TF1200–60, ACX·CN, China) and heated at a rate of 10 °C·min–1 in a nitrogen atmosphere (100 mL·min–1) to reach four target temperatures (280 °C, 380 °C, 480 °C, and 580 °C), where they were maintained for 80 min to ensure complete carbonization. , After cooling to 20 °C–25 °C, the obtained biochar was then ground and passed through a 75-μm sieve. Finally, the biochar samples were stored in a dry, sealed glass vial prior to extraction. The feces-based biochar produced at 280–580 °C was labeled as FB280, FB380, FB480, and FB580.

2.2. Biochar-Derived DOM Extraction and Analysis

The DOM derived from feces-based biochar was extracted. The biochar and different pH extraction solutions (1.5, 6.5, and 13; 0.1 mol·L–1 NaOH and 0.1 mol·L–1 H2SO4 to adjust the pH of the solution) were mixed according to a solid/liquid ratio of 1:100 (w/v) and placed in centrifuge tubes, shaken in the dark for 24 h using a thermostatic shaker (200 rpm, 25 ± 1 °C). The biochar extract was filtered through a 0.45 μm hydrophilic poly­(ether sulfone) membrane. The filtrate was stored in brown glass vials and refrigerated at 4 °C. The DOC contents of the filtrate were analyzed using a total organic carbon (TOC) analyzer (TOC-LCPH/CPN, Shimadzu). All filtrate samples were analyzed in triplicate. The concentration (mg·g–1) of WSOC was calculated using the following formula:

WSOC=(V×C)/W

where V, C, and W represent the volume of DOM extract (L), DOC concentration (mg·L–1) of DOM extract, and the mass of biochar (g), respectively.

2.3. The UV–Vis Spectroscopy and FTIR Measurement

The absorbance values of the released DOM samples were measured using a UV–vis spectrophotometer (SPECORD plus200) with ultrapure water used to establish the baseline, and scanning between 190 and 600 nm at a wavelength step of 1 nm. The specific UV absorbance at 250, 254, and 365 nm was measured. The SUVA254 was calculated by dividing the absorbance at 254 nm by the DOC concentration, and the A 250/A 365 was calculated using the ratio of the absorbance at 250 and 365 nm, as detailed in Table S1. The FTIR spectra of DOM samples were obtained using a Fourier transform infrared spectrometer (Nicolet iS 50, Thermo Scientific). Specifically, the FTIR range of scanning was 400–4000 cm–1, with 65 scans and the scan resolution of 4 cm–1. The functional groups corresponding to the FTIR spectral peaks are summarized in Table S3.

2.4. Synchronous Fluorescence and EEM Measurement

The SF and EEM were recorded on a three-dimensional (3D) fluorescence spectrophotometer (F-7000, Hitachi, Japan). The scanning range of the synchronous fluorescence was set from 250 to 500 nm for the excitation wavelengths and 280 nm for the emission wavelength. For the EEM, the excitation wavelength range was 200–550 nm (scan interval at 5 nm), the emission wavelength range was 250–600 nm (scan interval at 5 nm), and the scan speed was 2400 nm·min–1. The photomultiplier detector voltage was set at 700 V. The pH value and DOC concentrations of all DOM samples were adjusted to 7 and 10 mg·L–1, respectively. The fluorescence parameters described the DOM quality indices, including the biological index (BIX), humification index (HIX), and fluorescence index (FI), which are described in detail in Table S1.

2.5. Data Processing

In this study, all the values obtained were the mean value of three replicate samples, including statistical analyses (mean values and standard deviations) and the ANOVA were performed via SPSS 22 software (SPSS Inc., Chicago, USA). The PARAFAC fitting was conducted using MATLAB 2018a (MathWorks, USA) and the DOMFluor v.1.7 toolbox, following the procedure provided by Stedmon and Bro. Prior to 2D-COS analysis, all infrared, UV–vis, and SF spectral data were normalized, and the normalized spectral data were analyzed using 2D-shige software (Kwansei-Gakuin University, Japan). All the graphs were drawn by using Origin 2018 and MATLAB 2018a.

3. Results and Discussion

3.1. Variations of DOM Released from Feces-Based Biochar

Figure b presents the DOC concentration of feces-based biochar (280 °C, 380 °C, 480 °C, and 580 °C) in various extract solutions (pH = 1.5, 6.5, and 13). The concentration of DOC from feces-based biochar-derived DOM changed from 8.70 to 36.52 mg·g–1, 6.95 to 15.98 mg·g–1 and 6.77 to 27.23 mg·g–1 for pH extract solutions of 1.5, 6.5, and 13, respectively. With the increase of production temperature, the DOM content of feces-based biochar in the different values of pH extract solutions showed a similar variation trend, gradually declining from 280 to 580 °C. According to the feces-based biochar yield (Figure a) and TG and DTG curves of feces (Figure S1), the weight loss occurred mainly within the range of 280 °C–480 °C, with two major peaks (280 °C–380 °C, 380 °C–480 °C) during the pyrolysis of dried human feces. The weight loss of dried feces observed at 280 °C–480 °C may be due to the pyrolysis of substances such as cellulose and fat. , When the production temperature was over 480 °C, all of the volatile matter almost escaped, the biochar yield slowly declined, and the rate of weight loss tended to level off. At 280 °C, the initial stage of the active pyrolysis of feces begins, marking the onset of thermal decomposition of the primary organic substances. However, this temperature is insufficient for the complete decomposition of biopolymers. ,, Instead, it produces a substantial amount of soluble intermediates, such as amines, phenols, aldehydes, and ketones, which are retained in the biochar, leading to an increase in the DOC content in FB280. As the temperature rises to the later stages of the active zone, these intermediate compounds undergo deep cracking and volatilization, thereby reducing the sources and concentrations of DOC.

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Biochar yield at different pyrolysis temperatures (280 °C, 380 °C, 480 °C, and 580 °C) (a) and the DOC concentrations from the feces-based biochar at different pyrolysis temperatures in various extracts (pH = 1.5, 6.5, and 13) (b).

The DOC concentration of different extract solutions followed the order: pH = 13 > pH = 1.5 > pH = 6.5 when the production temperature was less than 380 °C, while the order was pH = 13 > pH = 6.5 > pH = 1.5 when it was over 380 °C. The DOM released from biochar produced under different temperatures (280 °C, 380 °C, 480 °C, and 580 °C) was highest in the alkaline solution (pH = 13). In addition, when the production temperature of biochar was less than 380 °C, the DOM released in pH = 1.5 solutions was higher than that in the pH = 6.5 solutions. However, the opposite phenomenon occurred when the production temperature was greater than 380 °C. This phenomenon indicates that the release of DOM in feces-based biochar is influenced by the interaction between its physicochemical properties and external environmental conditions. First, the abundant acidic oxygen-containing functional groups in biochar are core factors driving the release of DOM. In alkaline extraction solutions (pH = 13), these functional groups undergo deprotonation, significantly increasing the charge density and hydrophilicity of both the biochar matrix and the DOM molecules. , This process greatly facilitates the dissolution and desorption of organic components, explaining why the concentration of DOC remains the highest under alkaline conditions. In acidic environments (pH = 1.5), certain components produced in biochar at temperatures below 380 °C, which can be dissolved by strong acids (such as specific sugars and proteins), as well as acidic organic substances that are easily leached from micropores, may be released, leading to higher DOC release compared to neutral conditions. , However, when the pyrolysis temperature exceeds 380 °C, the carbon framework of biochar undergoes aromatic rearrangement, and volatile substances, along with a large number of unstable aromatic functional groups, decompose at high temperatures, resulting in a decrease in overall acidity. At this point, in a strong acid environment (pH = 1.5), the available acidic components for dissolution decrease, hydrophobicity increases, and the solubility of DOM is reduced, resulting in DOC release amounts that are even lower than those observed under neutral conditions.

3.2. Characterizing the Composition of DOM Released from Feces-Based Biochar

The four DOM components from feces-based biochar in different extract solutions were identified according to the EEM-PARAFAC analysis (Figure ), including three humic-like components (C1, C2, and C3) and one protein-like component (C4). As shown in Figure , the C1 (emission: 380, excitation: 224/335) component was categorized as a humic-like substance, generally expressed as fulvic acid and humic acid. The C2 (emission: 439, excitation: 258/357) was defined as UVC humic-like and UVA humic-like (A + C) with large molecular components, and it originated from terrestrial and microbial input. The C3 (emission: 390, excitation: 229/307) component was classified as fulvic acid and UVA marine humic-like (M) with a smaller molecular size than C2. The C4 was assigned to a protein-like component, including tyrosine and tryptophan, which represented aromatic protein as described in previous reports.

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EEM contours and spectral loadings of four components were identified by the EEM-PARAFAC analysis.

In order to assess the difference and variability of DOM released from feces-based biochar (280 °C, 380 °C, 480 °C, and 580 °C) in various extracts, the fluorescence intensity and relative distribution of four components are shown in Figures and S2. As shown in Figure S2, the fluorescence intensity and relative distribution of C1, C3, and C4 in different pH solutions showed different trends with the increase of pyrolysis temperature. Moreover, we also found that the relative abundance of protein-like substances released from high-temperature biochar was highest in different extraction solutions. As shown in Figure a, the fluorescence intensity of the four components released from FB280 in the different extraction solutions followed the order of pH = 6.5 > pH = 13 > pH = 1.5. For the FB380, the fluorescence intensity of C1 and C2 was highest when FB380 was extracted in the pH = 6.5 solution. The fluorescence intensity of C3 and C4 in the pH = 1.5 solution was higher than that in other extract solutions. For the FB480 and FB580, the fluorescence intensity of C4 was highest in the pH = 1.5 solution, while the fluorescence intensity of humic-like components (C1 + C2 +C3) in the pH = 6.5 solution was highest. These results indicated that the extraction solution affected the release of humic acid-like, fulvic acid-like, and protein-like substances. In addition, the study also found that acidic conditions tend to release protein-like substances.

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Fluorescence intensity (a) and relative distribution (b) of the EEM-PARAFAC in DOM released from feces-based biochar (280 °C, 380 °C, 480 °C, and 580 °C) in different values of pH extract solutions.

Figure b shows the relative abundance of four EEM components in DOM released from biochar (280 °C, 380 °C, 480 °C, and 580 °C) in different values of pH extracting solutions. For the FB280, the relative abundance of the four components in the pH = 1.5 extracting solution was 28%, 31%, 21%, and 20%, which suggests a major presence of C3 (fulvic acid and UVA marine humic-like). Similarly, the DOM components in the pH = 13 extractant were also dominated by C3. For the FB380, the relative content of the C4 component is high in acidic solutions (pH = 1.5), while the C1 component was dominant in weak acidic (pH = 6.5) and alkaline solutions (pH = 13). For the FB480 and FB580, the relative abundance of the C4 component was larger in different pH extracts due to the higher relative content of protein-like substances at high temperatures (Figure S2). Moreover, the C4 relative content was the highest in the acidic solution. In summary, the pH of the extraction solution and pyrolysis temperature jointly determine the compositional characteristics of DOM in biochar. Acidic conditions promote the release of protein-like substances, while neutral and alkaline environments favor the dissolution of humic-like substances. This release behavior directly influences the environmental effects of DOM due to significant differences in binding mechanisms and affinities toward pollutants, such as heavy metals and antibiotics, among various DOM components. , Therefore, assessing the ecological risks of biochar necessitates integrating field environmental conditions and studying the interactions between the DOM components released from biochar and pollutants.

3.3. Spectral Properties of DOM Released from Feces-Based Biochar

The UV–vis spectral analysis of DOM released from different extract solutions is shown in part II of Supporting Information. The spectral parameters (UV–vis spectral parameters and fluorescence parameters) of biochar-derived DOM are commonly used to analyze its basic characteristics and properties. The spectral parameters of DOM released from the feces-based biochar produced at different temperatures in various extracts are presented in Figure .

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Spectral characteristic parameters of DOM derived from feces-based biochar (280 °C, 380 °C, 480 °C, and 580 °C) in various pH extract solutions (pH = 1.5, 6.5, and 13). The parameters include: (a) SUVA254; (b) A 250/A 365; (c) FI; (d) BIX; and (e) HIX.

The lower SUVA254, the lower the aromaticity. Figure a shows the values of SUVA254 for all DOM samples. The SUVA254 values of DOM released from biochar in pH = 13 and pH = 6.5 solutions showed similar changes, i.e., gradually decreasing from 280 to 580 °C. However, the SUVA254 values of DOM released from biochar in pH = 1.5 solutions showed a trend that decreased and then increased. The SUVA254 values of DOM released from low-temperature biochar (280 and 380 °C) in different extract solutions followed the order: pH = 6.5 > pH = 1.5 > pH = 13, while the order was pH = 1.5 > pH = 13 > pH = 6.5 at higher production temperatures (480 and 580 °C). The above results indicated that the aromaticity of DOM released from low-temperature biochar in the weak acidic extraction solution was higher, and the DOM released from high-temperature biochar in the acidic extraction solution had more aromaticity. In addition, the A 250/A 365 was used to represent the DOM molecular weight, and high A 250/A 365 values reflected low molecular weight DOM. In this study, the A 250/A 365 ratio of DOM released in the acidic extraction solution was always higher than that of DOM released in the weak acidic and alkaline extraction solutions (Figure b), which indicated that acidic conditions promoted the release of low-molecular-weight DOM. At low production temperatures, the molecular weight of DOM released in the weak acidic extraction solution was higher than that of DOM in the alkaline extraction solution. However, the molecular weight of DOM released in the alkaline extraction solution was larger when the production temperature was higher. Moreover, we also found that low-temperature biochar-derived DOM released in different extract solutions had a larger molecular weight.

As shown in Figure c, the FI values from the pH = 13 solutions were higher than those from the pH = 6.5 solutions (except for 580 °C), followed by the pH = 1.5 solution. In addition, the FI values for all DOM samples were always greater than 1.8. These results suggested that the feces-based biochar (low-temperature biochar and high-temperature biochar)-derived DOM released from different extract solutions was mainly autogenous, and that alkaline environments favor the release of autochthonous DOM. Figure d presents the HIX values of DOM derived from feces-based biochar (low-temperature biochar and high-temperature biochar) in various pH extract solutions. The HIX of DOM released from various extract solutions showed irregular patterns of variation. The DOM (280 °C) released under acidic conditions had a high humification degree compared to alkaline and weak acidic conditions. The weakly acidic environment was conducive to the extraction of highly humified DOM when the production temperature was 380 and 580 °C, while highly humified DOM was significantly released in alkaline conditions when the production temperature was 480 °C. With the increase of production temperature, the BIX of DOM in the different extract solutions showed a similar variation, i.e., increased and then decreased (Figure e). In addition, the BIX for DOM samples was more than 1 (except for DOM 580 °C) in samples extracted at pH = 6.5 and 13 solutions, indicating that these DOM extracts had high bioavailability. The BIX of DOM in different extract solutions followed the order: pH = 1.5 > pH = 6.5 > pH = 13 when the production temperature was greater than 380 °C, while the order was pH = 6.5 > pH = 1.5 > pH = 13 when it was 280 °C. Spectral parameter analysis further revealed differences in the composition characteristics of DOM under varying pH conditions and their environmental significance. Particularly in acidic environments, DOM released from high-temperature biochar exhibits high aromaticity, low molecular weight, and elevated BIX values. These characteristics may influence its environmental behavior: high aromaticity enhances its binding capacity with pollutants, significantly impacting their migration and fate in the environment, which presents both dispersion risks and potential remediation benefits. Low-molecular-weight and highly bioactive components enhance microbial activity and soil vitality; however, over the long term, they diminish the carbon sequestration potential of biochar.

3.4. Variation in Feces-Based Biochar-Derived DOM Release Characteristics Using 2D-COS Analysis

The SF spectra were used to understand the changes in substances in DOM. The protein-like peak, fulvic acid, and humic acid were located at 250–300 nm, 300–380 nm, and 380–500 nm. With the increase of pyrolysis temperature, the peak intensity of humic acid in the different extract solutions gradually declined, while the peak intensity of protein-like ions became larger (Figure S5). Moreover, the humic acid-like (fulvic acid and humic acid) substances were prone to release from feces-based biochar (280 °C, 380 °C, 480 °C, and 580 °C) under weak acidic and alkaline conditions (Figure S6). However, acidic conditions promoted the solubilization of protein-like substances (Figure S6). This finding is consistent with the findings of EEM. In order to reveal the release properties of feces-based biochar-derived DOM affected by extracting solutions, the 2D-SFS-COS is shown in Figure (A). For the FB280, three major autocorrelated peaks of DOM were observed at 282 nm (protein-like), 322 nm (fulvic acid), and 370 nm (humic acid) (Figure A­[a]). Based on the rules of two-dimensional correlation analysis shown in Table S2, the release order of the three components significantly changed with the increase of extract pH. Specifically, the order was humic acid > fulvic acid > protein-like. There were three main autocorrelated peaks for FB380, located at 280 nm (protein-like), 313 nm (fulvic acid), and 390 nm (humic acid) (Figure A­[b]). The order of the change of the peaks was humic acid > fulvic acid = protein-like. For the FB480 and FB580, one autocorrelated peak of DOM was at 282 nm (protein-like). In addition, there were no cross peaks in the synchronous spectra (Figure A­[c,d]), while cross peaks were present in the asynchronous spectra (Figure A­[c-1,d-1]). According to Table S2, the order of release for the protein-like and fulvic acid components was uncertain. This phenomena might be attributed to the small amounts of fulvic acid in high-temperature biochar-derived DOM not being observed as release trends. In conclusion, the humic acid-like substances were released first as the pH of the extract increased.

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(A) 2D-SF-COS and (B) 2D-FTIR-COS analysis of DOM released from feces-based biochar (280 °C, 380 °C, 480 °C, and 580 °C) in different pH extract solutions (a–d: synchronous map of FB280, FB380, FB480, and FB580; a-1, b-1, c-1, and d-1: asynchronous map of FB280, FB380, FB480, and FB580).

The FTIR spectra are shown in Figure S7. The spectral peaks representing the functional groups are presented in Table S3. The variations in the functional groups in the different extract solutions were significantly different. The peak intensity of DOM is the highest in the acidic extracts, while it is the lowest in the weak acidic extracts. In order to better analyze how the functional groups of DOM are affected by extracting solutions, the 2D-COS method evaluates the FTIR spectral data, as shown in Figure B. According to Table S3, there are five main functional groups for all DOM extraction samples: −OH in alcohols, phenols, or soluble proteins; CO in carboxylic acids or C–N and CN vibrations in amides; vibrations of CH2 and CH3 or vibration of benzene ring CC; C–O or C–O–P stretching vibrations in polysaccharides, alcohols, carboxylic acids, and lipids; and C–H, N–H. Based on the rules shown in Table S2, the order of functional group changes in FB280 is CH2 and CH3 or CC > C–O or C–O–P > C–H, N–H C–N and CN or CO > −OH. For the FB380, the order for different peaks is C–O or C–O–P > C–H, N–HC–N and CN or CO > −OH > CH2 and CH3 or CC. For the FB480 and FB580, the order for peak changes is CH2 and CH3 or CC > C–O or C–O–PC–H, N–HC–N and CN or CO > −OH. These findings indicate that the order of release for functional groups is similar in FB280, FB480, and FB580, while the fastest release in FB380 is C–O or C–O–P. The DOM derived from FB380 undergoes recombination reactions due to the decomposition of organic matter such as fat and fiber (Figure S1). This may explain the phenomenon of the DOM release discrepancy in FB380.

3.5. Relationship between Components and Functional Groups in DOM

To further investigate the variations of functional groups in each DOM component, the heterospectral 2D-COS between the SF and FTIR spectrum was applied. In the synchronous map of FB280 (Figure a), several positive peaks at 3425, 1624, 1000–1300, and 500–1000 cm–1 were observed at the fluorescence wavelength of 280 nm, corresponding to −OH, C–N, CN or CO, C–O or C–O–P, and C–H, N–H in protein-like substances. Two positive peaks at (390 nm, 1423 cm–1) and (312 nm, 1423 cm–1) were observed, suggesting that the CH2 and CH3 or C–C originate from humic acid and fulvic acid. For the synchronous map of FB380 (Figure b), a few positive peaks located at 3423, 1627, 1000–1300, and 500–1000 cm–1 were shown at fluorescence wavelengths of 280 and 317 nm, indicating that −OH, C–N, CN or CO, C–O or C–O–P and C–H, N–H were present in protein-like and fulvic acid components. One positive peak at (390 nm, 1434 cm–1) was shown, meaning that the CH2 and CH3 or C C originate from humic acid. As shown in Figure c,d, the −OH, C–N, CN or CO, C–O or C–O–P, and C–H, N–H originate from the protein-like component for FB480 and FB580. These results suggest that the protein-like components contain abundant functional groups for all DOM samples. Combined with the asynchronous map (Figure (a-1–d-1)), the −OH, C–N, CN or CO, CH2 and CH3 or CC, C–O or C–O–P, and C–H, N–H were precedent to changes in protein-like substances for DOM released from FB280. Moreover, the variations in C–N, CN or CO, CH2 and CH3 or C C, C–O or C–O–P, and C–H, N–H in fulvic acid and humic acid substances varied slowly, while the −OH in fulvic-like and humic-like substances likely changed first. For the FB380 DOM, −OH, C–N, CN or CO, and C–H, N–H were precedents for protein-like, fulvic acid, and humic acid substance changes. The CH2 and CH3 or CC and C–O or C–O–P groups changed more slowly in protein-like, fulvic acid, and humic acid substances. The −OH, C–H, and N–H in protein-like substances of DOM from FB480 changed first, while the C–N; CN or CO; CH2 and CH3; or CC and C–O or C–O–P in protein-like substances likely changed more slowly. Compared to the protein-like substances of DOM from FB480, the variations in C–N and CN or CO varied first for the FB580 DOM. These results indicate that the pyrolysis temperature significantly influences the release behavior of DOM in biochar by regulating the diversity of its chemical composition and the characteristics of its functional groups. The DOM released from biochar produced at medium to low temperatures exhibits complex behavioral characteristics, primarily driven by the preferential response of easily ionizable functional groups within the humic components. In contrast, the DOM release behavior from high-temperature biochar is relatively straightforward and is predominantly controlled by protein-like substances.

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Heterospectral 2D-COS of SF and FTIR spectroscopy (a, b, c, and d: synchronous map of FB280, FB380, FB480, and FB580; a-1, b-1, c-1, and d-1: asynchronous map of FB280, FB380, FB480, and FB580).

3.6. Environmental Implications

This study systematically reveals the release patterns of DOM from feces-based biochar under different environmental pH levels and pyrolysis temperatures. The research indicates that alkaline conditions (pH = 13) significantly promote DOM release, particularly at low-temperature pyrolysis (280–380 °C), where the concentration of DOC reaches its highest levels (27.23–36.52 mg·g–1). In contrast, acidic environments (pH 1.5) tend to release protein-like components (C4), while neutral and alkaline environments facilitate the leaching of humic components (C1–C3) with higher aromaticity and larger molecular weights. This release characteristic renders its environmental effects significantly condition-dependent. Previous studies have indicated that the interaction between DOM derived from biochar and copper (Cu) arises from the complexation between copper and carboxyl groups. Huang et al. revealed that the binding of cadmium (Cd) with DOM primarily occurs in proteins. Therefore, the humic DOM released in neutral/alkaline environments may complex with heavy metals such as Cu due to its abundant carboxyl groups, thereby reducing their bioavailability, whereas the proteinaceous DOM (C4) released in acidic environments may preferentially bind with heavy metals like Cd, and the soluble complexes formed may increase the migration risk of heavy metals. This study found that low-temperature biochar releases the highest amount of DOM under alkaline conditions, which suggests that substances such as polycyclic aromatic hydrocarbons contained within may pose potential toxicity risks to aquatic organisms. , Furthermore, the large release of humic DOM under neutral/alkaline conditions is known to be a strong precursor to disinfection byproducts, significantly increasing the risk of generating carcinogens in subsequent water treatment processes. , In summary, the environmental impact of feces-based biochar is not fixed but rather depends on its application context. Therefore, in practical environmental applications, it is essential to guide the release behavior of DOM by regulating pyrolysis temperature and release conditions based on the objectives (e.g., heavy metal remediation or risk avoidance) and to implement corresponding pretreatment measures to maximize its environmental benefits while controlling potential risks.

4. Conclusions

This study investigates the effects of pyrolysis temperature and pH on the release and characteristics of DOM from feces-based biochar. The analysis is conducted using techniques such as EEM-PARAFAC, UV–vis, and 2D-COS. The results demonstrate that alkaline conditions (pH = 13) significantly enhance the release of DOM. The highest concentrations of DOC (27.23–36.52 mg·g–1) were observed at lower pyrolysis temperatures (280–380 °C). Acidic conditions (pH 1.5) specifically induced the release of protein-like components, while neutral and alkaline environments favored the dissolution of larger humic-like components. The chemical properties and release behavior of DOM are influenced by the preparation temperature and environmental conditions. This finding indicates that the environmental impact of biochar is not static but is highly contingent upon specific application scenarios. Therefore, for practical applications such as heavy metal remediation or carbon sequestration, it is essential to select pyrolysis temperature and environmental conditions to optimize DOM release toward the desired outcomes. Implementing appropriate pretreatment measures can maximize environmental benefits while mitigating potential risks. This study provides new insights into the safe and targeted utilization of feces-based biochar; however, future research is necessary to verify the long-term behavior and ecological effects of the released DOM in complex real-world environments.

Supplementary Material

ao5c08272_si_001.pdf (1,004KB, pdf)

Acknowledgments

The authors acknowledge the support provided by Sichuan Institute of Geological Engineering Investigation Group Co. We are grateful for the human feces samples and the necessary experimental and sampling conditions provided for this study. We also extend our thanks to all coauthors for their valuable contributions to this research. Their collective efforts in method design, validation analysis, data organization, and manuscript preparation were crucial to the completion of this study.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.5c08272.

  • Thermogravimetric analysis (DTG and TG curves) of human feces for pyrolysis temperature selection (280–580 °C); UV–vis spectra of DOM from feces-based biochar at different pyrolysis temperatures in varying pH extract solutions (pH = 1.5, 6.5, 13); SF spectra of DOM released under different pyrolysis temperatures and pH conditions; FTIR spectra of DOM from biochar produced at different temperatures and extracted at different pH values; EEM-PARAFAC results for DOM from feces-based biochar under varying temperature and pH conditions; spectral parameter definitions for UV–visible and fluorescence analysis (SUVA254, A 250/A 365, HIX, BIX, FI); rules for interpreting 2D-COS; FTIR band assignments for functional group identification (PDF)

The manuscript was mainly contributed by the first author (T.X.) under the supervision of the corresponding author (L.L.). All other authors contributed partially. All authors have approved the final version of the manuscript.

The authors declare no competing financial interest.

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