1 Introduction

Low molecular weight organic acids (LMWOAs, < 900 Da) are one of the most active organic components in soil solutions, mainly from plant residues, root exudates and microbial metabolites (Zhang et al. 2020a). The life cycle of LMWOAs in the soil is normally short (several hours), the concentration is relatively low (ranging from a few to tens of millimoles), and the concentration may even decrease by 1–2 orders of magnitude due to the process of biodegradation, migration, rainfall, and irrigation. However, it can be continuously input into the soil solution with the life activities of the soil rhizosphere (Sokolova 2020). Therefore, LMWOAs remain active in soil with considerable quantities. The formation, migration, transformation, adsorption, and biodegradation of LMWOAs play an important role in the global carbon cycle. In addition, LMWOAs are involved in mineral transformation, dissolution, and the fixation and migration of metal elements such as iron, aluminum, cadmium, arsenic, and so on (Qin et al. 2020; Wang et al. 2014), the biodegradation of organic contaminants such as polycyclic aromatic hydrocarbons and petroleum hydrocarbons (Sun et al. 2016; Martin et al. 2014), phosphorus, potassium and other nutrient elements release (Zhang et al. 2020a). The LMWOAs influence the large geological cycle of chemical elements, the immobilization of nutrients and pollutants in the soil, stimulating or inhibiting the growth and development of plants.

In recent years, biochar has attracted the attention of scientists, farmers, enterprises, and governments due to its inherent properties. Biochar pyrolyzed under low temperatures (< 700 °C) with oxygen-limited conditions plays a positive role in sequestering carbon (Paustian et al. 2016), mitigating climate change (Lehmann et al. 2021), improving soil properties, affecting microbial activity (Dai et al. 2021), removing contaminants (Hu et al. 2020; Lian et al. 2021; Safari et al. 2019), and increasing crop yields (Jeffery et al. 2017; Liao et al. 2019; Xua et al. 2021). While improving the utilization efficiency of solid waste (straw, agricultural waste, livestock and poultry manure, and municipal waste), as a cheap and efficient adsorbent, biochar also participates in the remediation of many contaminated sites, the passivation of heavy metals, and the treatment of wastewater and sewage (Lian and Xing 2017). Biochar can exist in the soil for thousands of years (Mia et al. 2017). With the long-term application in soil, biochar particles are broken by soil tillage, crop growth, soil weathering, biological, physical, and chemical effects (Wang et al. 2019; Yang et al. 2019a). Once soil’s physical and chemical condition (irrigation, farming, rainfall, fertilization, etc.) changes, the soil environment is disturbed and undergoes transient changes. The biochar colloids retained on the soil surface are continuously released along with the transient process (Wang et al. 2020). A large number of biochar colloids are inescapably produced from bulk biochar particles. About 3–5% of the biochar particles are colloidal, but the amount of biochar applied is relatively high. Although the fraction of biochar colloid is small, the amounts are huge and easily accumulated with the high concertation (Yang et al. 2019a). Biochar colloids, which have strong surface activity and mobility, will inevitably interact with LMWOAs in the rhizosphere. It has been reported that LMWOAs influenced the stability and mobility of nanomaterials (Li et al. 2019, 2021; Shen et al. 2019), and these effects would be pronounced in lower pH conditions (Tian et al. 2015). The PH values has been established as an important factor influencing biochar mobility (Zhang et al. 2010). Biochar is mainly used in acidic soils due to its inherent alkalinity. The pH at the soil rhizosphere tends to differ by 1–2 units from the pH a few millimeters away (Nye 1981). However, under different pH environments, the effect of LMWOAs on the stability of biochar colloids has not been thoroughly studied, which is an important knowledge gap for evaluating the environmental risks of biochar around the soil rhizosphere.

Critical coagulation concentration (CCC) is often used as an important indicator of the aggregation kinetics for colloids in solution systems (Yang et al. 2019b). The CCCs of biochar colloids were closely related their physical and chemical properties (raw materials, pyrolysis temperature, particle size, and aging) (Leng et al. 2019; Song et al. 2019; Wang et al. 2019; Yang et al. 2019b), and soil solution chemical properties (pH, ionic strength, and natural organic matter) (Bai et al. 2019; Brusseau et al. 2019; Qiao et al. 2021). A previous study showed that compared with high-temperature pyrolysis biochar (600 °C, CCC = 181 mM), low-temperature pyrolysis biochar had a larger CCC value (300 °C, CCC = 300 mM) (Wang et al. 2019). The stability of biochar colloids would be significantly decreased through increased ionic strength (Yang et al. 2019b) and decreased pH. The humic acid adsorbed on the biochar surface significantly improved the CCC value of the biochar colloid from 183 to 806 mM due to increasing the repulsive force and steric force on the biochar surface. Some studies have shown that LMWOAs could affect the physical and chemical properties of biochar (e.g., porosity, functional properties, and inorganic minerals) (Liu et al. 2017), the release of nutrients such as phosphorus and potassium in biochar (Zhang et al. 2020a), and the adsorption capacity of biochar for heavy metals or organic contaminations (Sun et al. 2016; Qin et al. 2020; Wang et al. 2014). However, most of these studies focus on the effect of LMWOAs on bulk biochar particles. The impact of LMWOAs on the aggregation kinetics of biochar colloids under acidic or neutral soil environments has still been unknown.

To investigate the effect of LMWOAs (1 mM) on the aggregation behavior of biochar colloids with the typical acidic soil pH values (pH 4 and 6), the citric acid, oxalic acid, and malic acid, the three main forms of upper soil solution of forest ecosystems (Sokolova 2020), were selected as the typical LMWOAs in our study. The pine-wood and wheat-straw biochars (named PB and WB, respectively) were obtained by pyrolysis at 600 °C. The obtained PB and WB colloids were used as model biochar colloids. Our study evaluated the effects of LMWOAs in different pH conditions on the aggregation behavior of biochar colloids, and provided new evidence for the stability of biochar colloids in the soil solutions, especially the soil rhizosphere.

2 Materials and methods

2.1 Chemical reagent

Oxalic acid (OA, CAS: 144-62-7), malic acid (MA, CAS: 97-67-6), and citric acid (CA, CAS: 77-92-9) were purchased from Sinopharm Chemical Reagent Co. LTD, and the  structure and properties of these three LMWOAs are showed in Table S1. These three LMWOAs contain one, two, and three carboxyl groups, respectively, which were selected in this study as a model LMWOAs because they were common components of root exudates and found everywhere in the rhizosphere soil (Wang et al. 2014). A certain amount of LMWOAs was weighed and prepared in a 10 mM solution, and then stored in brown bottles at 4 °C. NaCl, HCl, and NaOH were obtained from Sinopharm Chemical Reagent Co. LTD. Ultrapure water (18.2 MΩ cm, 25 °C) was used in the experiments.

2.2 Biochar

The biochar particles in this experiment were derived from pine-wood (Harbin, Heilongjiang Province, China) and wheat-straw (Zhengzhou, Henan province, China), which were pyrolyzed at the rate 20 °C min−1 and kept the highest temperature at 600 °C for 1 h (Wang et al. 2020). To obtain the fine biochar particles (0.3–0.7 μm), the produced biochar particles were fully grounded through a ball mill (MM 400, Retsch, Germany). Briefly, the ball mill was used for 4 min with the frequency 25 s−1, and this procedure was repeated more than 10 times and then these fine particles were stored in the dark.

2.3 Biochar characterization

The contents of C, N, O, and H of biochar particles were characterized using an elemental analyzer (Flash 2000, Thermo Scientific, USA). The surface functional groups of the biochars were analyzed through Fourier transformation infrared (FTIR, Spectrum Spotlight 200, PerkinElmer, USA) absorbance spectra in the 400–4000 cm−1 range with the resolution at 2 cm−1. The density of oxygen-containing functional groups (carboxyl, lactone, and phenol) on the biochar surface was determined by Boehm titration (Boehm 1994, 2002). The ash content of the biochar particles was measured by the weight before and after biochar particles burning in a muffle furnace at 750 °C for 6 h. The pH and electrical conductivity (EC) were measured by pH meter (FE22 -Standard, Mettler Toledo, Zurich, Switzerland) and conductivity meter (DDS-307, Leici, Shanghai, China) with a certain ratio of biochar (1 g) and ultrapure water (10 mL).

2.4 Biochar colloids

The grounded biochar particles (1 g) were added into ultrapure water (500 mL), then thoroughly shaken and ultrasonically dispersed for 30 min. To obtain the biochar colloids (< 0.45 μm), the biochar suspension was through a 0.45 μm nitrate cellulose filter (KMCE04550100, Kenker, USA). The PB and WB colloids were obtained after filtration to remove big particles. PB and WB colloids were stored at 4 °C in the dark and preserved for 3 days.

The size distribution of obtained PB and WB colloids in the ultrapure water was analyzed by dynamic light scattering (DLS) with Zetasizer ZS90 (Malvern, UK), and the morphology was shown through a transmission electron microscope (TEM, JEM-1230, JEOL, Japan). The EPMs of PB and WB colloids were analyzed by Zetasizer ZS90 operated with the He–Ne laser in the selected NaCl (1–200 mM) solutions with three types of LMWOAs (1 mM OA, CA, and MA) at room temperature (25 ℃) (pH 4 and 6). The measurement was repeated five times with 10 runs per sample. The zeta potentials of PB (ζPB) and WB (ζWB) colloids were calculated with the values of EPM through the Smoluchowski equation (Hiemenz and Rajagopalan 1997) in 1 mM and pH 7 NaCl solution at room temperature (25 ℃). The static contact angles of PB and WB with water, glycerol, and n-decane were determined by the sessile drop method (Shang et al. 2008) using Contact Angle Meter (JC2000D2, Powereach, China).

2.5 Biochar aggregation experiments

Time-resolved dynamic light scattering was used to investigate the effect of LMWOAs on the aggregation kinetics of biochar colloids in pH 4 and 6 conditions. The set concentrations of LMWOAs (1 mM) in our experiment commonly exist in root exudates and rhizosphere (Sokolova 2020). The intensity weighted hydrodynamic diameter (Dh) of PB and WB was recorded with DLS (scattering angle = 90°, ZetasizerZS90) over time. The colloidal pine-wood particles with oxalic, citric, and malic acids were named PB_OA, PB_CA, and PB_MA, respectively. The wheat-straw derived biochar colloids with oxalic, citric, and malic acids were named as WB_OA, WB_CA, and WB_MA.

Before the kinetic aggregation experiment, a preliminary experiment was conducted to determine the amount of HCl and NaOH required for biochar (200 mg L−1) with LMWOAs (1 mM) at pH 4 and 6 in a series of salt solutions with different concentrations. The biochar suspension was ultrasonic for 15 min, and then a predetermined amount of biochar was added into a DLS polystyrene cuvette, followed by the addition of a predetermined volume of NaCl, LMWOAs, and HCl/NaOH to obtain a 1 mL suspension with a biochar concentration of 200 mg L−1 at a solution pH 4 and 6 with various of ionic strengths (NaCl, 10–200 mM). A vortex mixer (Lab dancer S25, IKA, Germany) was used immediately to uniform the suspension in the cuvette for 10 s, which was then quickly put into the Zetasizer ZS90. The intensity-weighted hydrodynamic diameter (Dh) was recorded continuously for 30 min with 30 s for each run. The pH values of the suspension were adjusted before and after the aggregation kinetic experiment, and the values had no significant changes (± 0.2) during the experiment. Duplicate measurements were conducted for the aggregation kinetics experiment.

The aggregation kinetics of biochar colloids was shown with Dh changes as a function of time (t). The initial aggregation constant (k) and attachment efficiency (α) of biochar colloids were calculated as follows (Holthoff et al. 1996; Huynh and Chen 2011):

$$k\propto \frac{1}{{N}_{0}}{\left(\frac{d{D}_{h}\left(t\right)}{dt}\right)}_{t\to 0}$$
(1)
$$\alpha =\frac{1}{W}=\frac{k}{{k}_{fast}}=\frac{\frac{1}{{N}_{0}}{\left(\frac{d{D}_{h}\left(t\right)}{dt}\right)}_{t\to 0}}{ \frac{1}{{N}_{0,fast}}{\left(\frac{d{D}_{h}\left(t\right)}{dt}\right)}_{t\to 0,fast}}$$
(2)

The k means the initial aggregation rate constant, which is determined by the slope of change of the Dh0 to 1.5 Dh0, where Dh0 represents the initial Dh. N0 is the initial concentration (200 mg L−1) of PB and WB colloids, and α is the attachment efficiency [0–1], representing the inverse stability rate of the colloid (1/W). When α is less than 1, the aggregation behavior stays in the regimes of reaction-limited aggregation (RLA), where the colloidal aggregation is controlled by the interaction energy and energy barrier between colloids, and the collision is only caused to partial aggregation. When α is equal to (or greater than) 1, which was the regime (kfast) of diffusion-limited aggregation (DLA), the aggregation is controlled by the random collision rate, the interaction between particles is attractive, and there is no energy barrier.

The critical coagulation concentrations (CCCs) of biochar colloids with/without LMWOAs were calculated by extrapolating the linear regression between α value in the RLA regimes and electrolyte concentration in logarithmic coordinates, where α value is the average or initial value in the DLA regimes (Shen et al. 2015; Wang et al. 2019; Yang et al. 2019b; Yin et al. 2015).

2.6 Interaction energy

Extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory was used to calculate the total interaction energy between biochar colloids (particle–particle interaction). Details are shown in S1 section of Supplementary Material.

2.7 Statistical analysis

Principal component analysis (PCA) was used to analyze the effect of environmental factors (pH, EPM values, the structure, and molecular weight of LMWOAs) on the value of CCCs changes (∆CCC) using CANOCO 5. Prior to PCA analysis, min–max normalization of the original data was performed to the range of [0,1].

3 Results and discussion

3.1 Biochar property

The physical–chemical properties of the pine-wood and wheat-straw biochars are shown in Table S2. These two biochars had the same pyrolysis process and similar yields. The pH and EC of these two biochars were 9.9 and 1.0 mS cm−1 for PB, and 9.7 and 10.6 mS cm−1 for WB, respectively. The EC value of PB was lower and had a strong relationship with the small ash content. These two biochar samples had the typical surface functional groups of biochar characteristics (Yang et al. 2019a), such as aliphatic C–O–C (1030 cm−1), phenolic–OH (1250 cm−1), carboxyl (1710 cm−1), and hydroxyl groups (3380 cm−1) (Fig. S1). In addition, the stretching vibration peaks of PB in the aliphatic C–O–C peaks (1100–1030 cm−1) in the corresponding cellulose (Cheng et al. 2008) were lower than those of WB. The peak intensities of hydroxy–OH (3380 cm−1), carboxyl C=O (1710 cm−1), and phenol group–OH (1250 cm−1) were higher than those of wheat-straw biochar. The FTIR spectral (Fig. S1) indicated that –OH and –C=O were mainly functional groups in the pine-wood biochar, and the aliphatic C–O–C was the dominant functional group in wheat-straw biochar (Cao et al. 2018; Havers et al. 1998; Wang et al. 2019). The oxygen-contained functional groups were 0.20 mmol g−1 and 0.12 mmol g−1 for the pine-wood and wheat-straw biochars, respectively (Table S2). Compared to WB, PB had a higher carbon content, lower oxygen content, smaller H/C, O/C, and (O + N)/C, which indicated that the PB had the higher aromaticity, lower polarity, and hydrophilicity (Qian and Chen 2014).

3.2 Chemi-physical property of biochar colloids

The morphology and size distributions of PB and WB colloids are exhibited in Fig. 1. The TEM images of PB and WB colloids showed an irregular shape, and the diameters of both colloidal biochars were about 200 nm (Fig. 1a and b). The average hydrodynamic particle sizes of PB and WB colloids were 231 ± 53 nm and 212 ± 58 nm, respectively (Fig. 1c and d), consistent with the particle size range in TEM images. The pH values of PB and WB colloidal suspensions were 6.8 and 7.8, respectively. The lower pH value of PB colloidal suspension might be caused by both the dissociation of carboxyl groups attached to the carbon shelf during the pyrolysis process (Fidel et al. 2017) and the lower ash content of PB (Song et al. 2019). The zeta potential values of PB and WB colloids were − 44.3 ± 1.2 mV and − 47.6 ± 3.6 mV, respectively, in 1 mM NaCl solution with pH 7 at room temperature (25 ℃). Although the high density of oxygen-contained functional groups (like carboxyl groups) was shown in PB (Table S2), more negative charges were found in the WB colloids. The surfaces of PB and WB colloids were hydrophobic, which was determined by the contact angles of biochar colloids with water, glycerol, and n-decane through the sessile drop method (Wang et al. 2019; Yang et al. 2019b).

Fig. 1
figure 1

TEM images (a, b) and hydrodynamic size distributions (c, d) of the biochar colloids derived from pine-wood and wheat-straw. PB pine-wood biochar; WB wheat-straw biochar

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3.3 Electrophoretic mobilities of biochar colloids w/wo LMWOAs

The EPMs of PB and WB colloids were both negative at pH 4 and 6 (Fig. 2). At the acid pH 4 solutions, the EPMs of PB and WB colloids were both decreased with adding three LWOMAs (Fig. 2a and b). The EPM values of PB colloids were reduced from − 2.04 × 10–8 m2 V−1 s−1 (PB) to − 2.03 × 10–8 m2 V−1 s−1 (PB_OA), − 1.90 × 10–8 m2 V−1 s−1 (PB_CA) and − 1.50 × 10–8 m2 V−1 s−1 (PB_MA) in 10 mM NaCl solution (Fig. 2a). The EPMs values of WB colloids were reduced from − 2.67 × 10–8 m2 V−1 s−1 (WB) to − 2.36 × 10–8 m2 V−1 s−1, − 2.28 × 10–8 m2 V−1 s−1 and − 1.84 × 10–8 m2 V−1 s−1 in the presence of OA, CA, and MA, respectively (Fig. 2b). Both the EPM values of PB and WB colloids were decreased with the increase of electrolyte concentration due to the compression of the double electric layer. The absolute values of biochar EPM values were increased with solution pH, indicating further deprotonation of the biochar surfaces. The EPM values of PB colloids were more significantly influenced by pH changes because the PB colloids had more carboxyl and phenolic functional groups (Table S2), and thus, more likely to gain or lose hydrogen ions with the change of pH (Song et al. 2019).

Fig. 2
figure 2

The effect of the low molecular weight organic acids (LMWOAs, 1 mM) on the values of electrophoretic mobility (EPM) for PB and WB colloids as a function of electrolyte NaCl concentrations at pH 4 (a, b) and 6 (c, d). The PB_ OA, PB_ CA, and PB_ MA denote the pine-wood biochar colloids in the presence of the oxalic, citric, and malic acids, respectively. The WB with the LMWOAs had the same named rule. Error bar is standard deviation (n = 3)

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In the natural pH 6 solutions, the EPMs of biochar colloids were also decreased with the addition of LWOMAs (Fig. 2c and d). Li et al. (2019) showed there was almost no effect on the change for zeta potential of graphene oxide in 10 mM NaCl solutions at pH ranging from 4 to 7 when the low concertation (0.1 mM) of LWOMAs, such as acetic, glycolic, malonic, and tartaric acid in the graphene oxide, were added. At the same time, other results showed that the high concentration (10 and 25 mM) of LWOMAs significantly decreased the zeta potential of graphene oxide (Li et al. 2021). The concentrations of LWOMAs were the key factors to assess the effect of LWOMAs on colloid stability. In addition, the impact of LWOMAs on reducing the negative surface charge of colloids was related to the chemical structure (number of carboxyl groups) and the related charged property of LWOMAs (Li et al. 2021). The most significant influence occurred in the MA, which had two carboxyl groups, with positive charge at pH 4 and the minimum negative charge at pH 6, followed by CA (three carboxyl groups, medium charge), and the OA had the little effect (only one carboxyl group, and the maximum charge).

3.4 Aggregation kinetics of biochar colloids with/without LMWOAs

The aggregation kinetics of colloidal particles included two regimes like reaction-limited aggregation (RLA) and diffusion-limited aggregation (DLA) (Figs. S2–S5). Under the same ionic strength, the higher aggregation rates were shown in the biochar colloids with the LMWOAs, whether at the acid or neutral pH conditions. The attachment efficiencies (α) of biochar colloids with the three typical LMWOAs (OA, CA, and MA) in the acid (pH 4) and natural (pH 6) solutions are exhibited in Fig. 3. The α values of both PB and WB colloids were increased with the ionic strength until they reached the CCC value (α = 1) in the absence/presence of LMWOAs, which was controlled by the XDLVO interactions (Zhu et al. 2014). The particle–particle repulsive energy was decreased with the ionic strength increasing (Yang et al. 2019b). When the aggregation rate constant was not changed with the increase of ionic strength, it meant that the repulsive energy between biochar colloids was disappeared and the reaction came to the DLA regimes (Lin et al. 1990).

Fig. 3
figure 3

The effect of the low molecular weight organic acids (LMWOAs, 1 mM) on attachment efficiencies (α) for PB and WB colloids as a function of electrolyte NaCl concentrations at pH 4 (a, b) and 6 (c, d)

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The CCC values of PB and WB colloids were 75 mM and 88 mM, respectively, in the absence of LWMOAs in the acid solution (pH 4), and the values were decreased when the LWMOAs were present (Table 1). The corresponding CCC values of PB colloids were 56 mM, 52 mM, and 47 mM after the OA, CA, and MA were added, respectively. And the CCC values of WB colloids were 80 mM (WB_OA), 69 mM (WB_CA), and 67 mM (WB_MA), respectively. Similar to pH 4, the CCC values of PB and WB colloids at pH 6 were significantly decreased when the LWMOAs were present. Compared with the CCC values of biochar colloids (157 mM for PB colloids and 160 mM for WB colloids), the CCC values in the presence of OA, CA, and MA were decreased by 5.7%, 22.3%, and 24.2% for the PB colloids, and 5.0%, 21.3%, and 25.6% for the WB colloids, respectively. The reduction of these results in the presence of CA and MA was significant because of the inherent carboxyl numbers and charge density in such LMWOAs. XDLVO energy profiles well explained the aggregation kinetics with the LMWOAs in a range of NaCl concentrations at pH 4 and 6 (Fig. 4). The maximum repulsion barrier of PB and WB colloids was reduced with the three types of LMWOAs. PB and WB colloids showed the higher repulsion barrier, and thus, had the higher CCCs when the LMWOAs were absent (Table 1 and Fig. 4). The lower maximum repulsion barrier and EPMs both occurred in the biochar colloids in the presence of MA, which was well explained by the smaller CCCs when the MA was present in the PB and WB suspensions.

Table 1 Critical coagulation concentrations (CCCs, mM) of pine-wood biochar (PB) and wheat-straw biochar (WB) colloids with LMWOAs under the NaCl solutions with various concentrations at the acid and neutral pH
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Fig. 4
figure 4

XDLVO particle–particle interaction energy profiles for PB and WB colloids with/without LWMOAs in NaCl solutions at pH 4 (a, b) and pH 6 (c, d). The concentrations of NaCl were selected at the low ionic strength (10 mM), and high ionic strength (80 mM for pH 4 and 200 mM for pH 6)

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3.5 Factors controlling the changes of CCCs

Changes in the values of CCC (∆CCC) were established through principal component analysis (PCA) of environmental factors (the structure and molecular weight of LMWOAs, solution pH, and the absolute EPM values of biochar colloids with/without LMWOAs), which explained 71.6% (PC1) and 24.1% (PC2) of data variance (Fig. 5). The samples clustered according to solution pH. These results collectively suggested ∆CCC was strongly positively related with the structure and molecular weight of LMWOAs, and negatively correlated with solution pH and absolute EPM values.

Fig. 5
figure 5

Principal component analysis (PCA) of the changes of CCC values (∆CCC), the structure and molecular weight of LMWOAs, solution pH, and the absolute EPM values of biochar colloids w/wo LMWOAs. The x- and y- axis numbers in parentheses (71.6% and 24.1%) represent data variations explained by the first two principal components (PC1 and PC2). Light-blue and light-yellow circles cluster the samples under pH 4 and 6 conditions, respectively

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The complex order of chemical structure (functional group) and molecular weight were as follows: CA > MA > OA (Table S1). It has been pointed out that the chemical structure (functional group) and the molecular weight of LMWOAs could strongly change the interaction force between particles (Chen and Elimelech 2007; Li et al. 2019; Shen et al. 2019; Zhang et al. 2020b), thus significantly influencing the colloidal aggregation kinetics (Fig. 4). The solution pH was another important factor influencing biochar colloids' aggregation by changing the sorption amounts of LWMOAs on the surface of biochar colloids. The adsorption mechanism between LWMOAs and biochar colloids relied on the solution pH, the molecular structure of LMWOA, and contained formation of inner- and outer-sphere surface complexes, ligand exchange, and cation bridging bonds (Sokolova 2020). The larger amounts of LWMOAs would be adsorbed on the biochar colloids in the acid pH environment, thus causing the lower CCC values at pH 4 than pH 6. Similar to the study by Gao et al. (2017), the negatively charged colloidal particles were easier to aggregate at the lower pH (pH 4) due to the protonation of carboxyl groups from the biochar colloids. The aggregation kinetics of colloids with LMWOAs is mainly controlled by the electrostatic repulsion, which was influenced by the EPM values (Li et al. 2021). Although the chemical structure (functional group numbers) and the molecular weight of MA were less than those of CA, a significant decrease of the CCCs for biochar colloids was found in biochar colloids with MA. The reason might be that the MA had a positive charge at pH 4 and a less negative charge at pH 6 (Table S1), which were the favorable conditions for the formation of biochar colloids aggregates. In addition, LMWOAs were also reported to accelerate colloidal aggregation through the hydrogen bonding of surface bridging bonds between colloidal particles (Albert et al. 2021; Cerqueira et al. 2015).

4 Conclusions

This study put an insight into the effect of LWMOAs (OA, CA, and MA) on biochar colloids (PB and WB colloids) aggregation kinetics in different NaCl concentrations at pH 4 and 6. Experiments and modeling both demonstrated that LWMOAs decreased the CCCs of biochar colloids through decreasing the repulsive interaction and promoting the surface bridging bonds between colloidal particles. The significant phenomena were shown at the acid pH (pH 4). Our results showed that the stability of biochar colloids decreased with the chemical structure and the increase of the molecular weight of LWMOAs (Fig. 5). The stability of biochar colloid was reduced to different degrees at different chemical structures, and molecule weights of LWMOAs (OA, CA, and MA), and the MA caused the most significant effect on it (Fig. 6). The more significant CCCs decreased for PB and WB colloids at PH 4 due to the fact that LWMOAs could be more adsorbed on the surface of the colloid in this pH. Similar effects also are shown for the influence of LWMOAs on biochar colloid (PB and WB) from different biomass sources. There were a few different CCC values for the biochar colloids derived from the different biomass.

Fig. 6
figure 6

The schematic diagram of the LWMOAs effect on the aggregation of biochar colloids under pH 4 and 6 conditions

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LMWOAs should be considered as an important factor in environmental monitoring and evaluation before biochar is applied to a wide range of soils. It is reasonable to expect that the effects of LMWOAs on the aggregation kinetics of biochar colloids may also be various due to different biochar pyrolysis temperatures, soil electrolyte solution, soil organic matter type, and the presence of contaminants such as polycyclic aromatic hydrocarbons. Therefore, more research is needed to better understand the aggregation behavior of biochar colloids in the presence of LMWOAs in the actual soil environment. Future work should be focused on the potential effect of the molecular structure, molecular weight, and the concentration of organic acids on the stability and mobility of biochar colloid with/without environmental contaminants in soil solutions.