Chemosphere 287 (2022) 132018

Available online 24 August 20210045-6535/© 2021 Elsevier Ltd. All rights reserved.

Hydrogen peroxide and high-temperature heating differently alter the stability and aggregation of black soil colloids

Guolian Xu a, Chong Chen a,*, Chongyang Shen a, Hu Zhou a, Xiang Wang a, Tao Cheng b, Jianying Shang a,**

a Key Laboratory of Arable Land Conservation (North China), Ministry of Agriculture, College of Land Science and Technology, China Agricultural University, Beijing, 100193, China b Department of Earth Sciences, Memorial University St. John’s, Newfoundland and Labrador, A1B 3X5, Canada

H I G H L I G H T S

• H2O2 treatment decreased colloidal stability of black soil more than heating treatment. • Stability of three colloids in salt solutions was in the order of Na2SO4 > NaCl > CaCl2. • Proposed model could well fit colloidal stability profiles at fast aggregation stage. • Model parameters reflected the stability behaviors of soil colloids in salt solutions.

A R T I C L E I N F O

Handling Editor: T Cutright

Keywords: Colloid properties Colloidal stability profiles Salt concentrations Salt types Model fitting

A B S T R A C T

Chemical oxidation and high-temperature heating have been widely used for the decontamination of soils polluted by hydrocarbons and the removal of soil organic matter. Chemical oxidation and high-temperature heating decreased the stability of soil colloids, but the difference in colloidal stability and aggregation behav-iors of soil after chemical oxidation and high-temperature heating is not clear. In this study, taken black soil as an example, we tested the stability profiles of black soil colloids (BC), hydrogen peroxide (H2O2) treated black soil colloids (BC_H2O2), and 350 ◦C treated black soil colloids (BC_350 ◦C) in three salt solutions (NaCl, CaCl2, and Na2SO4) with different salt concentrations. The stability of soil colloids in salt solutions was in the order of BC >BC_350 ◦C > BC_H2O2. The salt concentrations at which three colloids started to be unstable were much lower for CaCl2 solution than those for NaCl and Na2SO4 solution. Salt concentrations that suspension started to be un-stable were similar in NaCl and Na2SO4 solution for all the three colloids, but the colloidal stability profile in NaCl solution decreased faster than that in Na2SO4 solution when the suspension was unstable. The stability profiles of three colloids at the fast aggregation stage could be well fitted with the proposed exponential model, and model parameters (t0 and Smax) could reflect the stability behaviors of soil colloids in various salt solutions.

1. Introduction

Soil colloids are the most active constituent in the soil due to their small sizes, large surface areas, and high surface charges. The stability and aggregation behaviors of suspended soil colloids are closely related to the formation of surface crusts (Southard et al., 1988), soil water erosion (Kjaergaard et al., 2004a), clay translocation (Huang et al., 2016), and colloid-mediated transport of contaminants (Grolimund and Borkovec, 2005). In terms of classical

Derjaguin− Landau− Verwey− Overbeek (DLVO) theory, the behaviors of soil colloids in aqueous solution are determined by two major forces including van der Waals force attraction (FvdW) and electrical double layer force (FEDL) (Hiemenz, 1986). The FvdW is caused by the electro-magnetic effects of the molecules, and the FEDL is caused by the over-lapping of electric double layers of two charged particles (Liang et al., 2007). Additionally, non-DLVO interactions, such as steric repulsion and molecular bridging, also play an important role in the stability and ag-gregation behaviors of soil colloids when macromolecules such as

* Corresponding author. ** Corresponding author.

E-mail addresses: chenchongsxnd@cau.edu.cn (C. Chen), jyshang@cau.edu.cn (J. Shang).

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https://doi.org/10.1016/j.chemosphere.2021.132018 Received 19 July 2021; Received in revised form 18 August 2021; Accepted 23 August 2021

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dissolved organic carbon (DOC) exist (Philippe and Schaumann, 2014). Solution chemical conditions (such as pH, cation type, salt concentra-tion, the concentration of suspended particles, and DOC) and properties of soil colloids (such as surface charge) affect the interaction forces between two particles, and further influence the behaviors of soil col-loids in aqueous solution (Heil and Sposito, 1993a; Zhang et al., 2009; Sequaris, 2010; Sheng et al., 2016; Zhu et al., 2017; Yan et al., 2019).

Soils are subjected to chemical oxidation or high-temperature heat-ing in some practical situations. For example, various chemical oxidants (such as hydrogen peroxide (H2O2), persulfate, and permanganate) and heating are widely used for the decontamination of soils polluted by hydrocarbons (Ranc et al., 2016; Vidonish et al., 2016; Liao et al., 2019; Zhao et al., 2019). Additionally, soils can be heated by a forest fire or straw burning (Gonzalez-Perez et al., 2004; Romasanta et al., 2017). To examine the effects of soil organic carbon (OC) on soil properties, chemical oxidants (such as Na2S2O8, H2O2, or NaOCl) and heating are widely used to remove soil OC under experimental conditions (Heil and Sposito, 1993a; Sequaris, 2010; Wang et al., 2011; Wang et al., 2019a). These treatments may greatly change soil properties (such as OC con-tent, DOC, surface charge, specific surface area) (Santos et al., 2016; Wang et al., 2019a; Durn et al., 2019), which affect the behaviors of soil colloids in aqueous solution and further in the environment. Previous studies suggested that the H2O2 and 400 ◦C heating treatments decreased the stability of soil colloids through decreasing electrostatic repulsion or steric repulsion (Heil and Sposito, 1993b; Sequaris, 2010). However, the differences in the behaviors of soil colloids treated by chemical oxidation and high-temperature heating in aqueous solutions are currently not clear.

Thus, the main objective of this work is to clarify the differences and reasons in the stability of soil colloids treated by chemical oxidation and high-temperature heating in aqueous solutions. In this study, used black soils as an example, the stability profiles of black soil colloids (BC), black soil colloids treated by H2O2 (BC_H2O2), and black soil colloids treated by 350 ◦C heating (BC_350 ◦C) were determined in NaCl solutions, CaCl2 solutions, and Na2SO4 solutions with different salt concentrations. The properties of soil colloids and aqueous solution and the interactions between particles were conjointly considered to elucidate the reasons for the differences in the behaviors of the three colloids in aqueous solutions.

2. Materials and methods

2.1. Sample collection and treatments

Black soil samples were collected from the surface layer (0–20 cm) at Minzhu town in the Daowai district of Haerbin city, China (E126◦51′05′′, N45◦50′3′′). The soil, developed from loess-like deposits, is classified as a Pachic Udic Argiboroll according to the USDA soil classification system (Soil Survey Staff, 2010). According to USDA classification, this soil is a silty loam soil. Soil pH is 6.23, and soil OC content is 15.8 g kg− 1. Clay mineralogy is dominated by hydromica, kaolinite, and chlorite. After air-dried and passing through a 2-mm sieve, the soils were homogenized and split into three subsamples for the different treatment: one sub-sample was untreated, and the other two sub-samples were treated with H2O2 and 350 ◦C heating, respec-tively (Wang et al., 2011; Lyttle et al., 2015). Specifically, ~300 mL of 30% (mass fraction) H2O2 was added to a 100 g soil sample in a beaker, and the mixture was allowed to react thoroughly at room temperature (~23 ◦C). Finally, the beaker was heated at 80 ◦C to degas the excess H2O2 after a week. For the heating treatment, soil was heated at 350 ◦C for 12 h. The H2O2 and 350 ◦C treated soils were air-dried, grounded, and passed through a 2-mm sieve.

For the three investigated soils, the colloid fractions (particle size <2 μm) were obtained and determined with the pipette method (Gee and Bauder, 1986). The DOCs were extracted using deionized water at a water to soil ratio of 5:1. After shaking the mixture for 4 h at room

temperature with an over-head shaker, soil solution was centrifuged for 20 min at 3500 rpm. The supernatant was filtered through a 0.45-μm membrane. A portion of the supernatant was used to determine DOC concentration with an Elementar vario EL Cube (Germany). The deter-mination of DOC was performed in duplicate. The specific UV absor-bance of the supernatant at 254 nm (SUVA254) was used as a proxy for the aromaticity in DOC. The SUVA254 (L mg C− 1 m− 1) is calculated by normalizing the absorbance at 254 nm (m− 1) by the DOC concentration (mg L− 1) (Helms et al., 2008). As high-molecular-weight molecules absorb light more strongly at longer wavelengths than at shorter wavelengths, the molecular size of DOC was inferred based on the ratio of absorption at 250 nm to 365 nm (E2:E3) (Helms et al., 2008).

2.2. Soil colloid suspensions preparation

To prepare the stock suspensions of BC, BC_H2O2, and BC_350 ◦C, 30 g of soil samples (untreated black soil, H2O2 and 350 ◦C treated black soils) were suspended in 1L DI water, and then the suspensions were shaken and sonicated for 1 h using an ultrasonic machine (Kunshan Ultrasonic Instrument Ltd., China). Following dispersion, the suspension was transferred to a 1 L measuring cylinder and settled for a certain time to recover the stock suspensions of three colloids with particles size < 2 μm. This process was repeated several times until the supernatant became transparent. The concentrations of the stock suspensions were determined using drying-weighing method.

2.3. Characterization of soil colloids and stability test

The OC contents of BC, BC_H2O2, and BC_350 ◦C obtained by freeze- drying suspensions were determined using a FLASH 2000 organic elemental analyzer (Thermo Fisher Scientific). For the investigated three soil colloids, the hydrodynamic diameters (D) at 1 mM NaCl solution and zeta potentials at selected salt solutions (NaCl, CaCl2, and Na2SO4) with different concentrations were determined using a zetasizer Nano-ZS90 (Malvern Instruments Ltd., Malvern, UK). The measurements of D and zeta potentials were performed in triplicate.

The predetermined volumes of colloidal stock suspensions were mixed with electrolyte solutions to achieve 100 mg L− 1 colloidal sus-pensions. Before the stability test, the pH of the suspension was adjusted to 7 using 0.1 M NaOH or 0.1 M HNO3 solution, and the suspension was sonicated for 3 min. The absorbance of the suspension was then moni-tored with a UV–Vis spectrophotometer (TU-1900, Persee, China) at a wavelength of 300 nm every 2 min during 0–120 min. For each soil colloid, the stability profiles were determined in NaCl (10, 30, 50,100, 300, 500, 700 mM), CaCl2 (0.5, 0.75, 1, 3, 5, 10 mM), and Na2SO4 (10, 30, 50, 100, 300, 500, 700 mM) solutions with different salt concentrations.

2.4. Modeling of colloid stability profiles

When salt solution concentration exceeded critical coagulation concentration (CCC), the colloidal stability profile begins to coincide and is at the fast aggregation stage. At this stage, the electrostatic repulsion force between colloids is negligible, and the behavior of col-loids in an aqueous solution was controlled by diffusion-limited aggre-gation (DLA) regime (Lin et al., 1989). The colloidal stability profiles of the investigated three colloids were S-shaped at the fast aggregation stage (Fig. S1), and were fitted using the proposed formula:

A/A0 = 1 −a

1 + e− k(t− t0)(1)

where A0 is the initial absorbance, A is the absorbance measured at different times, A/A0 is the relative absorbance value, t is the time (min), t0 is the time required when the decrease in relative absorbance reaches the fastest (min), a is the difference between the relative absorbance at the end and the beginning of the colloidal stability curve, k is a shape

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factor. Fig. 1 presents an example of the stability profile of BC at the fast aggregation stage in NaCl solution. The slope of colloid stability profile (S) represents the change of suspension concentration with time. The maximum slope of colloid stability profile (Smax) reflects the maximum aggregation rate at the fast aggregation stage. It can be written by:

Smax =

−

kae− k(t− t0)

(1 + e− k(t− t0))2

=

ka4

(2)

2.5. Model evaluation

The model performance was evaluated quantitatively using the co-efficient of determination (R2) and the root mean square error (RMSE).

RMSE=

∑n

i=1

(Pi − Oi)2

n

√

(3)

where Pi and Oi represent the estimated and measured relative absor-bance value, respectively, and n is the number of measurements.

3. Results and discussion

3.1. Characterization of BC, BC_H2O2, and BC_350 ◦C

The colloid fractions (particle size < 2 μm) of untreated and H2O2 treated black soil were similar, but the colloid fraction of 350 ◦C treated black soil was decreased by 63.6% relative to the untreated black soil (Table 1). This might be attributed to the fusion of fine particles into coarse particles during the heating process (Sertsu and Sanchez, 1978). The DOC concentrations of H2O2 and 350 ◦C treated black soils were ~5 times higher than that of the untreated black soil, and the E2:E3 values of H2O2 and 350 ◦C treated black soils were ~2 times higher than that of the untreated black soil (Table 1). These results suggested that the H2O2 treatment and the 350 ◦C heating treatment increased DOC amount but decreased the mean molecular size of DOC. The higher SUVA254 value of 350 ◦C treated black soil than that of H2O2 treated black soil indicated that the DOC of 350 ◦C treated black soil had higher aromaticity. Pre-vious studies also reported that the DOC and aromatic structures in the soluble fraction increased in soils after high-temperature heating (Choromanska and DeLuca, 2002; Guerrero et al., 2005; Santos et al., 2016). Possible explanations included: (i) high temperature promoted the formation of oxygenated- and aliphatic-rich water-extractable

organic matter (Santos et al., 2016); (ii) high temperature resulted in the formation of new aromatic structures and the selective enrichment of aromatic and phenolic components in soil organic matter (Gonzalez-Perez et al., 2004). The aromatic structures can be oxidized by H2O2 treatment (Durn et al., 2019), so the DOC in H2O2 treated black soil had lower aromaticity than that in the 350 ◦C treated black soils. The D value of BC_350 ◦C was significantly smaller than those of BC and BC_H2O2 (Table 1). This might be due to the agglomeration of larger colloid particles during the heating process. Compared with BC, the OC contents of BC_H2O2 and BC_350 ◦C were decreased by 76.9% and 64.7%, respectively (Table 1). This result suggested that the H2O2 treatment and the 350 ◦C heating treatment did not completely remove OC in soil colloids, which is consistent with previous studies (Sequaris, 2010; Durn et al., 2019).

The zeta potentials of BC, BC_H2O2, and BC_350 ◦C in NaCl, CaCl2, and Na2SO4 solutions with different salt concentrations are shown in Fig. 2. The BC, BC_H2O2, and BC_350 ◦C were negatively charged under all tested experimental conditions. The zeta potentials of the three col-loids in NaCl solution were more negative than those in CaCl2 solution. This might be because Ca2+ had stronger compression on the electric double layer relative to Na+ (Saka and Güler, 2006). At the salt con-centration of 10 mM, the average value of zeta potentials for the three colloids in Na2SO4 solution was more negative than that in NaCl solu-tion. One possible reason was that the adsorbed SO4

2− through coordi-nation increased the negative charge on soil colloid surface. In NaCl solution with the salt concentration below 50 mM or Na2SO4 solution with the salt concentration of 10 mM, the zeta potentials of BC_H2O2 and BC_350 ◦C were more negative than that of BC. The explanation might be that organic macromolecules of the BC shielded the surface negative charge. The zeta potentials of the three colloids in CaCl2 solution did not vary much. With increasing solution concentration in three salt solu-tions, the zeta potentials of three colloids became less negative, which was due to the suppression of the electric double layer (Baik and Lee, 2010).

3.2. Stability and aggregation of BC, BC_H2O2, and BC_350 ◦C

The stability and aggregation behaviors of colloid suspensions (BC, BC_H2O2, and BC_350 ◦C) in NaCl, CaCl2 and Na2SO4 solutions with different salt concentrations are shown in Fig. 3. In NaCl and Na2SO4 solutions, the salt concentrations at which BC, BC_H2O2 and BC_350 ◦C started to be unstable were 300 mM, 30 mM, and 100 mM, respectively. In CaCl2 solution, the salt concentrations at which the BC, BC_H2O2 and BC_350 ◦C started to be unstable were 3 mM, 0.75 mM, and 1 mM, respectively. The above results showed that the H2O2 treatment and the 350 ◦C heating treatment reduced the stability of soil colloid in salt solutions, and the stability of BC_H2O2 was decreased more significantly than that of BC_350 ◦C relative to BC. For all of the three colloids, salt

Fig. 1. Modelling of stability profile at fast aggregation process for black soil colloid (BC) in NaCl solution using Equation (1).

Table 1 Colloid fraction, dissolved organic carbon (DOC), properties of DOC for pristine black soil and black soils treated by H2O2 and 350 ◦C heating, hydrodynamic diameter (D) and organic carbon (OC) of black soil colloid (BC), H2O2 and 350 ◦C treated black soil colloid (BC_H2O2 and BC_350 ◦C).

Treatment Colloid Fraction (%)

DOC (mg L− 1)

SUVA254 (L mg− 1 m− 1)

E2: E3

D (nm)

OC (%)

Pristine 11 ± 3 32 1.09 5.2 366 ± 9

2.55

H2O2 11 ± 1 157 0.01 10.0 344 ± 14

0.59

350 ◦C 4 ± 3 160 0.26 11.4 294 ± 13

0.90

SUVA254 is calculated by normalizing the absorbance at 254 nm (m− 1) by DOC concentration (mg L− 1). E2:E3 represents the ratio of absorption at 250 nm–365 nm.

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concentrations at which suspension started to be unstable were similar in NaCl and Na2SO4 solutions, but the colloidal stability profile in NaCl solution decreased faster than that in Na2SO4 solution when the sus-pension was unstable. For example, after 2 h of aggregation at the salt concentration of 300 mM, the A/A0 values of the three colloids in Na2SO4 solution were 25.5%–47.4% higher than that in NaCl solution. Thus, the aggregation behaviors of BC, BC_H2O2, and BC_350 ◦C were different in three different salt solutions.

The stability profiles of the three colloids at the fast aggregation stage could be well fitted with the proposed function (Equation (1)) (R2

> 0.99) (Fig. S1). The values of the parameters (t0 and Smax) for the three

colloids in the three different salt solutions are shown in Fig. 4. In NaCl and Na2SO4 solutions, the t0 value was in the order of BC_H2O2 <

BC_350 ◦C < BC, and the Smax value was in the order of BC_H2O2 >

BC_350 ◦C > BC. In CaCl2 solution, the t0 and Smax values of BC and BC_350 ◦C were similar. However, the t0 values of BC and BC_350 ◦C were ~23% higher than that of BC_H2O2, and the Smax values of BC and BC_350 ◦C were ~26% lower than that of BC_H2O2. For all three col-loids, the t0 value was the largest in Na2SO4 solution and the smallest in CaCl2 solution, and the Smax value was the largest in CaCl2 solution and the smallest in Na2SO4 solution. The Smax values of BC, BC_H2O2, and BC_350 ◦C in CaCl2 solution were increased by 61.2%, 16.0%, and

Fig. 2. The zeta potentials of black soil colloid (BC), H2O2 treated black soil colloid (BC_H2O2), and 350 ◦C treated black soil colloid (BC_350 ◦C) in NaCl solutions, CaCl2 solutions, and Na2SO4 solutions with different salt concentrations.

Fig. 3. The stability behaviors of black soil colloid (BC), H2O2 treated black soil colloid (BC_H2O2), and 350 ◦C treated black soil colloid (BC_350 ◦C) in NaCl so-lutions, CaCl2 solutions, and Na2SO4 solutions with different salt concentrations.

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22.5%, respectively, and that in Na2SO4 solution were decreased by 24.9%, 33.5%, and 29.4%, respectively, relative to that in NaCl solution. Based on the stability behaviors of the three colloids, the suspension with higher stability had higher t0 and lower Smax. The Smax value decreased with increasing t0, and the relationship between Smax and t0 could be well described with a power function (R2 = 0.97) (Fig. 5). Thus, at the fast aggregation stage, the increase of t0 implied the decrease of maximum aggregation rate, and the two parameters (t0 and Smax) could well reflect the stability of soil colloid at the three different salt solutions.

3.3. Mechanisms controlling stability and aggregation of BC, BC_H2O2, and BC_350 ◦C

The H2O2 treatment and the 350 ◦C heating treatment decreased the stability of soil colloid in NaCl, CaCl2, and Na2SO4 solutions. Previous studies also found that chemical oxidation (such as Na2S2O8, H2O2, or NaOCl) or 400 ◦C heating removed most of OC in the soil and decreased the stability of soil colloid in aqueous solution (Heil and Sposito, 1993a; Kretzschmar et al., 1993; Kjaergaard et al., 2004b; Sequaris, 2010). The OC increases the stability of soil colloid suspension mainly through increasing electrostatic repulsion or steric repulsion (Heil and Sposito, 1993b; Sequaris, 2010; Zhu et al., 2017). The measured zeta potentials for BC were not more negative than that for BC_H2O2 and BC_350 ◦C in all investigated salt solutions (Fig. 2). Thus, the effects of OC on the stability of soil colloids were not caused by electrostatic interaction but instead were due to steric interaction.

An interesting phenomenon was that the stability of BC_350 ◦C was higher than that of BC_H2O2 in salt solutions (Fig. 3). From classic DLVO theory, the sum of contributions from electrostatic repulsion and van der Waals attraction control the colloid stability. In some salt solutions (such as 300 mM NaCl, 1 mM CaCl2, or 100 mM Na2SO4 solution), the zeta potentials of BC_350 ◦C and BC_H2O2 were similar, but there was a clear difference in the stability of the two colloids. Thus, electrostatic inter-action was not the main reason for this difference. The smaller particles have higher Debye length, and higher salt concentration is needed to compress completely the electrical double layer, so the smaller particles have higher CCC (Hsu and Liu, 1998; Sheng et al., 2016). The smaller particle diameter and higher stability of BC_350 ◦C than BC_H2O2 sug-gested that particle size might be one reason for the difference in the stability of the two colloids. In addition, both H2O2 treatment and 350 ◦C heating treatment did not completely remove OC from the black soil, with OC of BC_350 ◦C higher than that of BC_H2O2 (Table 1). Thus, steric interaction caused by small amounts of OC might be another reason for the higher stability of BC_350 ◦C than BC_H2O2. In addition to the amount of OC, the property of OC is also an important factor affecting colloidal stability (Philippe and Schaumann, 2014; Zhu et al., 2017). The DOC of 350 ◦C treated soils had higher aromaticity than that of H2O2 treated soils, and kaolinite suspension in the presence of DOC extracted from 350 ◦C treated soils was more stable than that in the presence of DOC extracted from H2O2 treated soils (Fig. S2). Zhu et al. (2017) also found that organic matter with higher aromaticity had a stronger ability to increase the stability of soil nanoparticles. Thus, the difference in stability of BC_350 ◦C and BC_H2O2 might be related to colloid size, OC content, and DOC properties.

Salt concentrations in CaCl2 solution at which the three colloids were unstable were much lower than that in NaCl and Na2SO4 solutions, and the zeta potential of the three colloids in CaCl2 solution were less negative than that in NaCl and Na2SO4 solution under the same con-centration condition (Fig. 2). This result agreed with Schulze–Hardy rule that the CCC is inversely proportional to the sixth power of counter ion valence. At the fast aggregation stage, the electrostatic repulsion force is negligible, so the aggregation behaviors of soil colloids are mainly determined by van der Waals interaction and steric interaction in NaCl solution. However, in the presence of Ca2+, cation bridging can form through the connection of organic functional groups and Ca2+, and further accelerate the aggregation of colloids (Philippe and Schaumann, 2014; Wang et al., 2019b). The Smax value of BC, BC_H2O2 and BC_350 ◦C in CaCl2 solution increased by 61.2%, 16.0%, and 22.5%

Fig. 4. Values of two model parameters (t0 and Smax) obtained by fitting the stability profiles at fast aggregation stage for black soil colloid (BC), H2O2 treated black soil colloid (BC_H2O2), and 350 ◦C treated black soil colloid (BC_350 ◦C) in NaCl solutions, CaCl2 solutions, and Na2SO4 solutions using Equation (1).

Fig. 5. Relationship between two model parameters (t0 and Smax) of colloid stability profiles at fast aggregation stage.

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compared to that in NaCl solution. The higher OC content of BC than BC_H2O2 and BC_350 ◦C suggested that the higher increase of Smax value for BC was caused by Ca2+ bridging. For all the three colloids, salt concentrations at which suspension started to be unstable are similar in NaCl and Na2SO4 solution. Previous studies found that the CCC values of negatively charged colloidal particles had no significant difference in NaCl and Na2SO4 solution (Smith et al., 2009; Yang et al., 2019). This result implied that the intermolecular forces controlling the stability of each colloid in NaCl solution were similar to that in Na2SO4 solution. When colloid suspensions were unstable, the A/A0 values of three col-loids in Na2SO4 solution were higher than that in NaCl solution under the same salt concentration after 2 h of aggregation. All colloids had lower t0 values and higher Smax values in NaCl solution than in Na2SO4 solution. These results suggested that it was easier to aggregate in NaCl solution than that in Na2SO4 solution for a given colloid. Possible rea-sons were that: (i) higher valent anions had higher interaction energies with metal ions and enhanced competition with negatively charged mineral colloids in the regard of capturing cations, and further inhibit the aggregation of colloids (Tian et al., 2015); (ii) the adsorption of SO4

2− on the surface of soil colloids increased surface negative charge, and further decrease the aggregation of colloids in Na2SO4 solution.

3.4. Implication

The treatments of 350 ◦C heating and H2O2 increased the amount of DOC and altered its properties in black soils, and the effects of the two treatments on DOC are different. The DOC had a significant effect on the properties of soil colloids and their environmental behaviors. Addi-tionally, 350 ◦C heating decreased the amount and particle size of soil colloids. As the DOC with high mobility and soil colloids with large surface area play important roles in the migration and transformation of soil nutrients and pollutants in the environment, it is necessary to consider the changes of DOC and soil colloids after soils suffered from high-temperature heating (e.g. soil thermal remediation, wildfire) and chemical oxidation. Some studies reported that the effects of high- temperature heating on DOC and soil colloids were related to soil properties (such as soil texture, organic matter) and high-temperature treatment conditions (such as heating temperature, rate, and time) (Vidonish et al., 2016; Santos et al., 2016; Durn et al., 2019). The per-formance of chemical oxidation on soils is also affected by soil organic matter composition and oxidant types (Liao et al., 2019). Thus, it is difficult to completely predict the effects of high-temperature heating and chemical oxidation on DOC and soil colloid behaviors in the envi-ronment. Further studies are needed to systematically consider the ef-fects of high-temperature heating and chemical oxidation conditions on DOC and soil colloid behaviors in soils of different properties.

4. Conclusion

This study investigated the stability and aggregation behaviors of BC, BC_H2O2, and BC_350 ◦C in various salt solutions. The decreases in the stability of soil colloid after H2O2 and heating treatments were due to the decreased steric interaction. The smaller colloid particle size, more OC, and higher aromaticity of DOC were the reasons for the higher stability of BC_350 ◦C than BC_H2O2. The stronger ability of Ca2+ to compress the electric double layer relative to Na + resulted in the lower stability of the three colloids in CaCl2 solution than in NaCl and Na2SO4 solutions. Additionally, cation bridging was formed through the connection of organic functional groups on BC and Ca2+. The intermo-lecular forces controlling the stability of each colloid in NaCl solution are similar to that in Na2SO4 solution, so salt concentrations that sus-pension started to be unstable were similar in the two salt solutions. All the three colloids were more stable in Na2SO4 solution than in NaCl solution when suspensions were unstable. The proposed exponential model produced satisfactory fits to the stability profiles of the three colloids at the fast aggregation stage, and the two model parameters

could well describe the stability characteristics of colloids in various salt solutions. Our results implied that chemical oxidation and heating treatments would alter the properties of soil colloids, and further affect their transport and fate in the environment.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We gratefully acknowledge Dr. Liang Jin for providing soil samples for this study. This work is supported by the National Natural Science Foundation of China (NO. 42177273 and 41907002).

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi. org/10.1016/j.chemosphere.2021.132018.

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