1 Introduction

Hydrothermal carbonization (HTC), a thermochemical technology, has gained recognition in recent years as an environmentally friendly and efficient approach for valorizing agroforestry residues (Cavali et al. 2023; Li et al. 2020b; Wu et al. 2021). The preparation of hydrochar is not only simple, but also has the advantages of low exhaust gas emissions and fewer mass transfer constraints (Guo et al. 2016). Hydrochar has been widely used in catalysis (Liu et al. 2016; Zeng et al. 2023), sewage treatment (Hu et al. 2021), soil remediation and improvement (Li et al. 2022b), and other fields (Cavali et al. 2023) because of its special physical–chemical characteristics, such as rich surface functional groups, multi-level pore structure (Cheng et al. 2021; Li et al. 2021; Liu et al. 2023), and a substantial amount of nutritional elements (Song et al. 2018). More importantly, returning the hydrochar to the soil was beneficial for greenhouse gas emissions mitigation and carbon sink (Malghani et al. 2014; Zhu et al. 2019), which is crucial for combating climate change and accomplishing carbon peaking and carbon neutrality targets (Lv et al. 2022; Tipping et al. 2011). When hydrochar enters environmental media (soil and water), its active component of dissolved organic matter (DOM) is released with the flow of water (Cheng et al. 2021; Zhu et al. 2019). DOM plays a crucial role in the environment, which could combine with nutrients or pollutants and undergo synergistic migration due to its water solubility and high mobility. As well as in soil and water environments, DOM can form complexes with heavy metals, including cadmium, copper, lead, etc. (Tipping et al. 2011; Ye et al. 2022). Besides, the DOM also affected the variety of the microbial population and the composition of soil aggregates (Ding et al. 2020; Guo et al. 2022a). Hence, understanding how different hydrothermal temperatures influence the properties of DOM in hydrochar is crucial. In addition, that complexation behavior can also affect the migration and toxicity of heavy metals (Aftabtalab et al. 2022). Their possible complexation behavior with pollutants also needs to be further explored.

Cu(II) is a heavy metal pollutant widely present in the environment, adversely affecting organisms and ecosystems (Rehman et al. 2019). At the same time, Cu(II) is also an effective fluorescence quencher, which can reduce or eliminate its fluorescence intensity by interacting with fluorescent groups (Liu et al. 2017). The adsorption performance and mechanism of hydrothermal carbon can be studied using this fluorescence quenching effect (Guo et al. 2022a). These studies help to understand and control the environmental impacts of Cu(II) and develop hydrothermal carbon's environmental remediation capabilities (Wang et al. 2022). However, the principles and rules of the hydrothermal carbonization temperature and the fluorescence quenching effect of copper still need to be clarified, and more experiments and theories are required to improve hydrothermal carbon's adsorption and selectivity.

Previous studies have shown that the application of sludge hydrochar increases soil DOM content (Zhu et al. 2023) and aging behavior of hydrochar in the environment leads to more DOM release and an increase in its combination with heavy metal elements within the soil (Guan et al. 2021; Xia et al. 2020). The properties of DOM from hydrochar are determined chiefly by the raw materials and the hydrothermal temperature (Sun et al. 2021). For instance, the amounts of dissolved organic carbon (DOC) and hydrophilic functional groups in peanut straw hydrochar were both higher than those in reed and spartina alterniflora hydrochar (Ji et al. 2021). In addition, as the hydrothermal temperature increased, the organic acid and phenol content in hydrochar DOM increased (Soroush et al. 2024), and the structural properties of hydrochar DOM were shown to be significantly connected to hydrothermal temperature, which influences their interaction with heavy metals (Babeker and Chen 2021). The hydrothermal temperature was confirmed to be the most critical factor affecting the properties of DOM at present.

Sawdust materials are one of the preferred raw materials for hydrothermal reactions, not only because they are renewable lignocellulosic biomass resources but also because of their excellent performance in the hydrothermal carbonization process (Cavali et al. 2023; Kabakcı and Baran 2019; Khan et al. 2021; Zhang et al. 2017). Recent studies have shown that under hydrothermal conditions, sawdust can be converted into hydrochar with a high calorific value and can be used as a soil remediation agent (Güler and Aydın Temel 2023; Khosravi et al. 2022). However, current research on the DOM characteristics of saw dust under hydrothermal conditions, especially the spectral characteristics and its ability to bind heavy metal elements, is still relatively limited.

In recent years, spectroscopic analysis technology has been broadly used to explore DOM characteristics and their interaction with pollutants due to its non-destructive, fast, and high-sensitivity nature (Ding et al. 2022; Li et al. 2018; Maqbool et al. 2020; Wang et al. 2019). For instance, the qualitative and semiquantitative analyses of DOM were carried out by using UV–Vis absorption spectroscopy and 3D-EEMs (Xi et al. 2018), and Gui et al. (2020) investigated the substance and structure of DOM in livestock and poultry manure biochar by employing UV–Vis absorption spectrum and 3D-EEMs. Moreover, the properties and mechanism of DOM’s binding to heavy metals were widely used to investigate fluorescence quenching titration and EEM-PARAFAC (Huang et al. 2019; Wang et al. 2019; Wei et al. 2020; Zhu et al. 2021). For instance, Huang et al. (2019) evaluated the properties of Cu(II) binding to DOM derived from biochar using that method. Hence, it is beneficial to use spectroscopic analysis techniques to investigate the DOM characteristics of sawdust-derived hydrochar from different hydrothermal temperatures and its complexation behavior with heavy metals.

In this work, a series of hydrochar was created by hydrothermally carbonizing sawdust, which is a typical waste biomass material, at varying temperatures from 150 and 300 °C, and its DOM was extracted using ultrapure water. The particular objectives of this work were to (1) study the effect of temperature on DOM characteristics by UV–Vis absorption spectra and 3D-EEMs analysis, (2) evaluate the potential ability of DOM complexation with Cu(II), and (3) explore the mechanism of DOM complexation with Cu(II).

2 Materials and methods

2.1 Hydrochar production and DOM extraction

The sawdust was sourced at a wood processing factory in Fengyang County, Anhui Province. The sawdust hydrochar preparation method followed our past research (Li et al. 2020a; Li et al. 2022a; Wei et al. 2022). Briefly, 200 mL super-pure water and 20 g desiccated sawdust were added in a 500 mL hydrothermal reactor (YT-500, China) at a solid-to-liquid ratio of 1:10. Then, the hydrothermal reaction was performed at a set temperature (150–300 °C, intervals of 30 °C) for a duration of 2 h, and cooled down naturally to room temperature. After that, the mixture produced a solid product (hydrochar) by centrifugation, which was dried at 45 °C.

Hydrochar and super-pure water at a ratio of solids to liquids of 1:100 (g mL−1) were added to a bottle, sonicated for 30 min, avoided light and was shaken for 24 h (25 °C, 160 rpm min−1) (Gui et al. 2020; Rajapaksha et al. 2019). After centrifugation, the liquid phase was filtered through a filter head (pore size: 0.45 μm, Polyether sulfone, Jinteng, China) and was labeled as sawdust hydrochar DOM (HDOM). The filtrate (HDOM) was stored in a brown volumetric flask and kept refrigerated at 4 °C for use (Chen et al. 2020). DOM of different hydrochar was marked as hydrochar initials and hydrothermal temperature digital (H150, H180, etc.).

The pH and the DOC content of DOM were determined, respectively, using a pH meter (PHS-3C, China) and a TOC analyzer (Shimadzu TOC-LCPH, Japan).

2.2 Spectral analysis of DOM

The UV–Vis spectrophotometer (Shimadzu, UV-2600, Japan) was used to measure the absorption spectra of DOM at a 1 nm interval from 200 to 600 nm. The DOM stock solution was deliquated to roughly 10 mg L−1 to prevent the inner filter effect of fluorescence spectroscopy (Gui et al. 2020), and the measured UV–Vis spectroscopy was used to normalize the fluorescence inner filter effect caused by concentration. The fluorescence spectrophotometer (Hitachi F-4600, Japan) was used to measure the 3D-EEMs of DOM with the following instrument settings: excitation wavelength (Ex): 200–450 nm, emission wavelength (Em): 250–550 nm, interval: 5 nm, and scanning speed: 12,000 nm min−1. The EEM spectrum was normalized using the pure water Raman peak.

2.3 Fluorescence quenching of Cu(II)

The DOM solution was diluted to 6 mg L−1 before titration to decrease the inner filter effect (Tian et al. 2021). Then 0.1 mol L−1 HCl and 0.1 mol L−1 NaOH were used to bring the pH of the DOM down to 6.0 ± 0.05 in order to avoid precipitation of Cu(II) (Chen et al. 2020). The corresponding DOM diluent (20 mL) was dispensed into a 40 mL sealed vial and 20 μL of the different concentration of Cu(II) solution (0–100 mmol L−1) was added to each sealed vial to get the set concentration of Cu(II) (0, 5, 10, 20, 40, 60, 80 and 100 μmol L−1) in the mixture solution (Zhu et al. 2021). To make sure that the complexation in the solution reached equilibrium, all of the sealed vials were continuously oscillated at 25 °C for 24 h in the dark (160 rpm min−1). All the titration experiments were carried out three times, and 3D-EEM and UV–Vis spectral measurements were measured immediately after the completion of all solution oscillations.

The modified Sterne-Volmer model was used to calculate the stability and complexation parameters between DOM and Cu(II). The calculation formula is as follows (Fan et al. 2021; Hur and Lee 2011):

$$\frac{{{\text{F}}_{0} }}{{{\text{F}}_{0} - {\text{F}}}} = \frac{1}{{{\text{f}}\cdot{\text{K}}_{{\text{M}}} \cdot{\text{C}}_{{\text{M}}} }} + \frac{1}{{\text{f}}}$$

where F0 and F represent the maximum fluorescence intensity (Fmax) in the absence of Cu(II) and the presence of Cu(II) at a concentration of CM, respectively; KM is the conditional stability constant, which is usually analyzed using the logarithmic conditional stability constant log KM. The f represents the proportion of fluorescent groups involved in the binding. KM and f are estimated according to the drawn F0/(F0 − F) and 1/CM linear fitting.

2.4 Data analysis

The fluorescence spectrum was analyzed by using the three-dimensional fluorescence spectrum data processing software platform (EFC) developed by China University of Geosciences and Peking University (He and Hur 2015). The Pearson correlation coefficient method was used to perform the correlation analysis of the interrelated parameters. Origin Pro2022 software was used for graphing. DOM characteristic parameters selected in this paper are shown in Table S1 (in supplementary material).

3 Results and discussion

3.1 The pH and DOC of HDOM

DOM derived from hydrochar which produced under different hydrothermal temperatures was all acidic (pH < 7, Fig. 1) and consistent with previous research indicating that hydrochar exhibits weak acidity (Sun et al. 2014) but contrary to the DOM from biochar. As hydrothermal temperatures increased from 150 °C to 240 °C, the pH value of DOM reduced by 2.2 units from 6.6 to 4.4 (Fig. 1), indicating that the formation of more acidic functional groups on the surface of the sawdust hydrochar might be produced by the production of acidic substances by the decarboxylation of sawdust at high temperatures (Hu et al. 2021; Saha et al. 2019). The research results of Babeker et al. (Babeker and Chen 2021) showed that when wood-based materials are carbonized at low temperatures (< 220 °C), the first components affected are cellulose and hemicellulose, which are converted into simple acidic compounds. This, in turn, leads to a decrease in the pH value of hydrochar-derived DOM, which is consistent with the results of this article. As the temperature further increases (> 220 °C), the structure of lignin begins to be affected, and depolymerization and dehydration reactions occur in a strongly acidic environment (Yu et al. 2018). In this test, the pH value then increased from 4.6 at 270 °C to 4.9 at 300 °C (Fig. 1), probably due to the further breakdown of acidic compounds at temperatures above 240 °C, or/and the creation of alkaline compounds in biomass during the HTC process as the hydrothermal temperature increased (Song et al. 2019a; Xu et al. 2022b).

Fig. 1
figure 1

The pH and content of dissolved organic carbon (DOC) derived from hydrochar produced at different hydrothermal temperatures (150, 180, 210, 240, 270 and 300 °C)

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The change tendency of DOC concentration in HDOM varying in hydrothermal temperature was opposite to that of pH (Fig. 1), and there was a significant negative correlation between pH and DOC (P < 0.01, R2 = − 0.64) due to the reality that most of the organic matter in HDOM is weakly acidic. The DOC concentration steadily increased from 19.30 mg L−1 at 150 °C to 37.29 mg L−1 at 240 °C due to high temperature strengthening the decomposition of hemicellulose or other components in sawdust, which produced more organic matter, including phenol, organic acid, furan, etc. (Dong et al. 2012). Nevertheless, the DOC concentration started to decrease at hydrothermal temperatures above 240 °C; and at 300 °C, it decreased to 8.81 mg L−1, which could be attributed to the degradation of cellulose in biomass, repolymerization of small molecular organic matter into macromolecular organic matter, and furan degradation, causing the release of DOM from hydrochar to be reduced (Nitsos et al. 2013; Saha et al. 2019). The DOC content first increased and then decreased (Fig. 1). This observation was in good agreement with that the higher HTC temperature increased the DOC content of bamboo hydrochar (180–330 °C) (Hao et al. 2018), whereas it was opposite to the finding for pig manure hydrochar (170–190 °C) (Song et al. 2020). In general, higher temperatures enhanced DOM release from hydrochar, which could be attributed to a progressive increase in biomass cracking during the HTC process. As the temperature of hydrochar formation increased, more broken hydrophobic organic molecules were enriched on the hydrochar surface, resulting in increased DOM release (Hao et al. 2018).

3.2 UV–Vis spectra and analysis of UV characteristic parameters

The UV–Vis spectrum exhibited two absorption peaks. One was at about 230 nm (Fig. 2) and corresponded to the π–π* electronic transition of C=C (Hu et al. 2021). The other was near 275 nm, which was connected to the C=C electronic transition at the π–π* level and the C=O electronic transition at the n–π* level in aromatic compounds (Fan et al. 2021). Taking the absorbance at 275 nm as an example, the UV absorbing capacity of HDOM steadily increased with increasing hydrothermal temperature, reached its peak value at 240 °C, and then began to decrease. This indicates that high-temperature sawdust hydrochar contains more monocyclic aromatic compounds (Hu et al. 2021), and affects its DOM. The HDOM extracted at a high hydrothermal temperature showed a big absorbance value (Fig. 2), indicating that it had more aromatic compounds (Hao et al. 2018).

Fig. 2
figure 2

The UV–vis spectra of HDOM derived from hydrochar produced at different hydrothermal temperatures (150, 180, 210, 240, 270 and 300 °C)

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To better examine the characteristic of DOM, UV–Vis spectra parameters were used to clarify the molecular characteristic of DOM. The parameter α(355) was often adopted to estimate the fraction of chromophoric groups present in the DOM or the relative concentration of chromophoric dissolved organic matter (CDOM) (Wang et al. 2021; Zhang et al. 2022). The value of α(355) exhibited an increase with temperature, rising from 2.76 L (mg m)−1 at 150 °C to 10.59 at 240 °C first, and then began to decrease; at 300 °C, it decreased to 5.07. This trend is consistent with the trend of DOC change (Fig. 1), which may be ascribed to the breakdown and decomposition of organic materials as the temperature rises.

SUVA254 is used to evaluate the aromaticity of HDOM. The SUVA260 index, which represents the content of hydrophobic components, can be used to characterize the hydrophobicity of DOM. These two indicators and values generally exhibit a positive correlation (Dilling and Kaiser 2002; He et al. 2021). The values of SUVA254 and SUVA260 of HDOM varying in different temperatures showed a consistent trend in this study (Table S2). In addition, the two HDOM indexes increased progressively with temperature, showing that the aromaticity and hydrophobicity of HDOM increased gradually with hydrothermal temperature. The macromolecular aromatic compounds, as well as highly hydrophobic substances, are part of the HDOM based on the two indexes analysis (Guo et al. 2022b; Huang et al. 2019). All of these showed that temperature greatly influenced the characteristics of HDOM; high temperature enhanced the aromaticity and hydrophobicity of HDOM, thereby enhancing the stability of hydrochar (Cao et al. 2022).

Spectral slope (SR) is an index usually adopted to describe the makeup and construction of DOM, and the calculation formula is presented in supplementary material. The SR value of DOM has an opposite relationship to its molecular weight, which means high SR means low molecular weight (Xiao et al. 2013; Zhang et al. 2022). Table S2 presents that the SR of HDOM increased gradually and finally decreased with the temperature increase, and  it had the highest molecular weight at 240 °C. This supports the following hypothesis that the change in DOC content and UV spectrum at 240 °C, mentioned above, is due to the stage-dependent changes of HTC products of sawdust at increased hydrothermal temperatures.

The DOM composition is usually identified by the UV–Vis ratio at 300 nm to 400 nm (E3/E4), E3/E4 value > 3.5, which means the primary composition of DOM is fulvic acid. Conversely, the value < 3.5, the main composition of DOM is humic acid (Artinger et al. 2000; Kirchman 2003). Table S2 shows the E3/E4 values of HDOM were all above 3.5, indicating that the primary composition of each HDOM was fulvic acid.

3.3 3D-EEM and PARAFAC analysis and fluorescence characteristics parameters

The fluorescence contour plot of the HDOM (Fig. 3) was separated into five sections based on the method of fluorescence regional integration (FRI). The FRI theory divides the fluorescence spectrum into five regions based on the different compositions of fluorescent groups at different positions. The specific partition range and corresponding substances are shown in Table S3 (Peng et al. 2020; Song et al. 2019b), respectively.

Fig. 3
figure 3

The 3D fluorescence spectra of HDOM in different hydrochar (H150, H180, H210, H240, H270 and H300 represent DOM derived from sawdust hydrochar produced at different hydrothermal temperatures, respectively; FRI partition (I: tyrosine, II: tryptophan, III: fulvic acids, IV: soluble microbial by-products, V: humic acids))

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The 3D-EEMs diagram of all the HDOM at different temperatures showed four main peaks (A–D). Peak A: 200–215 nm / 300–325 nm; peak B: 200–215 nm / 360–400 nm; peak D: 250–300 nm / 325–425 nm; peak D: 300–340 nm / 370–430 nm (Ex/Em). As the temperature of hydrothermal processes increased, the intensity of peak A (belongs to Region I) gradually weakened to disappear (Fig. 3), indicating the degradation of tyrosine is correspondingly enhanced. The fluorescence intensity of Peak B (belongs to Region III) increased, indicating an increase in fulvic acid substances. It reached the greatest fluorescence intensity at 270 °C but decreased at 300 °C, signifying the decomposition of fulvic acid at higher temperatures. The intensity of peak C (belongs to Region IV) progressively dropped, indicating a decrease in soluble microbial by-products. Peak D (belongs to Region IV) appeared with increasing temperature, indicating that high temperatures promoted the formation of humic acid organic matter.

Two typical fluorescence indices, Fn(280) and Fn(355), are used to analyze the composition and transformation of DOM (Zhang et al. 2023). The Fn(280) value is the highest fluorescence intensity in the range of Em = 340–360 nm when Ex = 280 nm, representing the proportion of protein-like compounds of DOM. The Fn(355) value is the greatest fluorescence intensity in the range of Em = 440–470 nm when Ex = 355 nm, representing the relative amount of humic-like compounds in DOM, and both of the two indices are positively correlated (Jin et al. 2020). The change of Fn(280) and Fn(355) showed the same trend (Table S4): with temperature, the proportion of compounds with protein-like properties from Fn(280) and humic-like compounds from Fn(355) increased, reaching maximum concentration at 270 °C, and slightly decreased at 300 °C.

The changes in bioavailability (BIX) and humification index (HIX) of HDOM are shown in Table S4. BIX represents the fluorescence intensity ratio at Ex = 310 nm, Em = 380, and Em = 480 nm, which reflects the autochthonous characteristics of DOM. When Ex = 254 nm, the fluorescence intensity integral ratio between Em = 435–480 and Em = 300–345 nm is referred to as HIX, indicating the humification index of DOM (Peng et al. 2020). All of the HDOM had BIX values of more than 1 (1.07–1.49), which is similar to previous research and indicates significant autochthonous characteristics and high bioavailability of HDOM (He et al. 2021). This is likely because the HTC process and the microbial breakdown course are so similar (Pages-Diaz and Huilinir 2020). The humification degree from HIX of HDOM followed the order: H240 > H270 > H300 > H210 > H180 > H150. This trend was consistent with the DOC concentration change and UV spectrum analysis, implying that the humification degree of HDOM is significantly affected by hydrothermal temperature. In addition, that trend was consistent with the change of SR and E3/E4 (in supplementary material for details) ratio values. All the results are consistent with prior research (Yang et al. 2019a), indicating that the hydrothermal process favored the development of humus and increased the level of humification.

3.4 Fluorescent components detection of EEM-PARAFAC modeling

The PARAFAC model effectively detected four fluorescent components of HDOM, containing two microbial humus substances (C1 and C4, Fig. 4), a humic-like material (C2), and a protein-like material (C3). Furthermore, the OpenFluor database was used to compare these four components with published components, using Tucker’s congruence coefficient (TCC) of at least 0.95 for both excitation and emission spectra (Murphy et al. 2014).

Fig. 4
figure 4

EEM contour plots and spectral loadings for four components of HDOM distinguished by EEM-PARAFAC analysis (C1: humic humus component; C2: humic-like component; C3: protein-like component; C4: microbial-derived humic-like component)

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C1 (Ex/Em = 225, 400 nm) has been reported as a microbial humus component (Derrien et al. 2020, Derrien et al. 2019). C2 (Ex/Em = 205/240/300, 450 nm) is a humic-like substance (Chen et al. 2017; Lin and Guo 2020; Yang et al. 2019b). The C3 component is relatively unique due to its two peaks in the Em band, and it is similar to the fluorescence characteristics of protein-like compounds containing tyrosine and tryptophan found in previous studies (Kim et al. 2020; Yamashita et al. 2010). C4 (Ex/Em = 200/225/320, 390 nm) may be related to humic-like compounds generated from microbes (Chen et al. 2018; Gullian-Klanian et al. 2021).

Figure 5 displays the relative content evolution of the four fluorescent components in HDOM. The microbial humus component (C1) increased from 25% for H150 to 50% for H300 with the hydrothermal temperature increased; on the contrary, the protein-like substance (C3) decreased from 61% for H150 to 18% for H270 and H300. In addition, the fraction of humic-like compounds (C2) and microbial-derived humic-like substance (C4) showed a similar change tendency, gradually increasing with the hydrothermal temperature increasing from 150 to 240 °C, and then decreasing to their minimum values at 300 °C after reaching their maximum values, which was consistent with the DOC change (Fig. 1), and there was a significant correlation with between the DOC with C2 and C4, indicating that the C2 and C4 determined the DOC content of HDOM. The protein-like substance (C3) is the dominant fluorescent component in high-temperature DOM (H150), and the microbial humus component (C1) was in the low-temperature (H300). Noticeably, the proportion of the four components in HDOM of H270 and H300 had been fully reacted and stabilized when the temperature reached 270 °C and above (Fig. 5).

Fig. 5
figure 5

Relative abundance evolution of fluorescent components of HDOM derived from hydrochar produced at different hydrothermal temperatures (150, 180, 210, 240, 270 and 300 °C). (C1: humic humus component; C2: humic-like component; C3: protein-like component; C4: microbial-derived humic-like component)

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3.5 Fluorescent quenching

Cu(II) can form complexes with DOM, leading to fluorescence quenching (Mounier et al. 2011). As a fluorescence quencher, copper ions can effectively reduce fluorescence intensity through their unique chemical properties and interaction with fluorescent substances (Guo et al. 2015). This property makes copper ions very important in fluorescence titration analysis, helping to detect and quantify fluorescent substances (Xu et al. 2022a; Zhao et al. 2018). The fluorescence quenching effect is enhanced as the concentration of Cu(II) increased (Fig. 6), indicating that there was a coordination interaction between Cu(II) and HDOM. This result corresponds with prior work by Ren et al. (2020). Figure 6 clearly demonstrates that in the absence of Cu(II), microbial humus substances (C1 and C4) accounted for the bulk of HDOM. However, when Cu(II) was added, both C1 and C4 components exhibited significant fluorescence quenching.

Fig. 6
figure 6

Fluorescence quenching curves of PARAFAC-derived components in HDOM with the addition of Cu(II) (H150, H180, H210, H240, H270 and H300 represent DOM derived from sawdust hydrochar produced at different hydrothermal temperatures, respectively)

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According to previous studies, quenching can be used to determine the binding constant and complexation ratio of heavy metal ions to DOM (Wu et al. 2011, 2012). To investigate differences in the binding characters with various fluorescent compositions and DOM samples, the linear modified Stern–Volmer model, in conjunction with PARAFAC analysis, was used to research the quenching degree for HDOM constituents with Cu(II). And the complexation parameters were determined based on the Fmax numbers for each component exhibiting quenching behavior (Table 1 and Fig. S1).

Table 1 Conditional stability constants (log KM) and proportion of fluorescent groups involved in the binding (f) for Cu(II) complexation to PARAFAC-components from HDOM as calculated using the modified Stern–Volmer equation
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Table 1 displays the effective complexation constant (log KM) and the ratio of binding fluorophore (f) to Cu(II) of the complexation reaction with HDOM, while Fig. S1 illustrates the fluorescence intensity fitting diagram of the combining of Cu(II) with HDOM. The linear association among F0/(F0 − F) and 1/Cu2+ of HDOM constituents at different temperatures was strong (R2 > 0.712), indicating that the equation can be effectively used to describe the fluorescence quenching process.

The data presented in Table 1 reveal that the effective quenching constant of Cu-DOM fell between 4.398 and 5.898. Furthermore, the effective complexation constants (log KM) of the four components with Cu(II) were 4.938–5.404, 5.166–5.898, 4.830–5.292, and 4.882–5.701, respectively. This indicates that the C2 component had the strongest binding ability with Cu(II). This may be due to the stronger affinity of humicoid (C2) to Cu(II) (Xing et al. 2020). This is consistent with the enhanced humification degree of HDOM caused by the increase in hydrothermal temperature. At the same time, the strong correlation between HIX and C2 components in Pearson correlation analysis (R2 = 0.99, P < 0.01) further confirmed this. In contrast, the C1 component had the weakest binding ability. It should be noted that, as judged from the fluorescence quenching curve, the components C1 and C4 in HDOM were affected by the quenching effect of Cu(II) more than the component C2 (Fig. 6). This is likely as a result of the more aromatic and high molecular size compounds in component C1 and C4 (Huang et al. 2021). At different temperatures, the C4 component of H150 and H300 had the highest fluorescence quenching percentage, which may mean that the C4 component interacted with Cu(II) most significantly at these temperatures. At the same time, the log KM of all components of H300 was higher, which indicates that HDOM at 300 °C had the strongest copper ion complexing ability. This may be related to the molecular structure and molecular weight caused by temperature changes (Hao et al. 2018). The quenching curve of protein-like compounds in HDOM (component C3) showed a declining trend with the addition of Cu(II) (Fig. 6). These findings suggest that protein-like components played a role in the binding of Cu(II) and HDOM. Previous investigations also discovered comparable phenomena (Wu et al. 2011; Yamashita and Jaffé 2008). The complexation of HDOM and Cu(II) can reduce the bioavailability and toxicity of Cu(II) in soil, laying the foundation for the environmental remediation application of hydrochar.

3.6 Pearson’s correlation analysis

To explore the effect of hydrothermal temperature on the content and characteristics of HDOM and to reveal the correlation between hydrothermal temperature and various parameters, Pearson's correlation coefficient was used to calculate the correlation coefficients between the pH and DOC values, the UV characteristic parameters (α(355), SUVA254, SUVA260, SR, E3/E4), and the PARAFAC components (C1, C2, C3, C4) and the HDOM fluorescent indices [HIX, BIX, Fn(280), Fn(355)]. The results are displayed in Fig. 7. Their relationship with the correlation coefficients was evaluated using Pearson’s correlation coefficient (R2). As can be seen from the figure, there was a significant correlation between the parameters. pH was negatively correlated with other parameters, except for SR, which was extremely significantly positively correlated (R2 = 0.84, P < 0.01). DOC content had a very significant positive correlation with α(355), E3/E4, SR, C2, C3, and other parameters (P < 0.01), indicating that the content and characteristics of chromophoric dissolved organic matter (CDOM) affected the DOC content of sawdust hydrochar. The positive correlation between DOC and E3/E4, C2, and C3 indicates that HDOM was mainly composed of macromolecular fulvic acid, mainly derived from C2 and C3 components, which is also consistent with the change of DOC content (Fig. 1) and the change of C2 component proportion at different temperatures (Fig. 5). SUVA254 and SUVA260 reflect the aromaticity and hydrophobicity of DOM. They had an extremely significant positive correlation with HIX, BIX, Fn(280), Fn(355), C1, C2, C3, C4, and other parameters (P < 0.01), indicating that HDOM was mainly composed of humified components with high aromaticity and hydrophobicity. HIX reflects the humification degree of DOM, and it also had a very significant positive correlation with BIX, Fn(280), Fn(355), C1, C2, C3, C4 and other parameters (P < 0.01), indicating that HDOM had a high degree of humification and was mainly determined by fulvic acid and humic-like components (C2), which can be seen from the correlation between HIX and BIX (R2 = 0.64, P < 0.01) and C2 (R2 = 0.98, P < 0.01). Fn(280) reflects the content of humic-like substances in CDOM, and it significantly correlated with humic-like components (C3) (R2 = 0.79, P < 0.01). Fn(355) reflects the content of protein-like substances in CDOM, and it had a very significant correlation with protein components (C2) (R2 = 0.99, P < 0.01). C1 and C3 had a very significant positive correlation (R2 = 0.69, P < 0.01), which is consistent with their change trend in the component proportion diagram (Fig. 5).

Fig. 7
figure 7

Pearson correlation analysis between HDOM components derived from sawdust hydrochar and pH, DOC content, UV Characteristic Parameters and fluorescence characteristics parameters (positive correlations are represented by red circles, and negative correlations by blue circles; significance level: P < 0.05 is indicated with “*”, and P < 0.01 is indicated with “**”; numbers display the correlation coefficient)

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4 Conclusion

As the hydrothermal temperature increased, the DOC concentration of hydrochar increased rapidly to reach the maximum value at 240 °C (H240), then gradually dropped. The high temperature increased the aromaticity, hydrophilic, and humification properties of HDOM. These characteristics enhance the stability of hydrochar. The three main fluorescent components of HDOM are proteins, humic substances, and microbial humic substances. The capacity of Cu(II) to complex with HDOM displayed significant variations at the different hydrothermal temperatures, and at 300 °C, the HDOM component had a higher complexing concentration with Cu(II). Microbial humic substances demonstrate greater complexation ability than humic substances and protein-like substances. Spectroscopic techniques offer a quick and convenient approach to assessing the environmental impact of eco-friendly materials. This study demonstrates the potential of spectroscopic techniques to assess the environmental impact of hydrochar and other biochar materials. This study will contribute to the ongoing efforts to evaluate the environmental impact of hydrochar applications.