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Effect of biochar on desiccation cracking characteristics of clayey soils
Geoderma 364 (2020) 114182
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Effect of biochar on desiccation cracking characteristics of clayey soils a
a,b,⁎
Yuping Zhang , Kai Gu a b c
a
a
c
, Jinwen Li , Chaosheng Tang , Zhengtao Shen , Bin Shi
a
T
School of Earth Sciences and Engineering, Nanjing University, Nanjing, Jiangsu 210023, China State Key Laboratory of Geohazard Prevention and Geoenvironmental Protection, Chengdu University of Technology, Chengdu 610059, China Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton T6G 2E3, Canada
ARTICLE INFO
ABSTRACT
Handling Editor: Cristine L.S. Morgan
Biochar is a promising material for soil remediation. However, the influence of biochar on soil cracking has not been clearly understood to date. Soil cracking can significantly change the mobility of contaminants in soil and consequently the remediation performance of biochar. This study investigates the effect of a wood biochar on the desiccation shrinkage characteristics of two clayey soils (PKE and XS). The biochar dosages selected are 0%, 0.5%, 2%, 4%, 6%, and 10% (w/w). The results indicate that biochar affects the desiccation cracking characteristics of clayey soils by changing the evaporation process. For PKE, the evaporation rate decreased by 6.56% and 5.59% with 0.5% and 2% biochar addition, respectively, and increased by 2.11% and 17.20% with 4% and 6% biochar addition. Similarly, with the biochar dosage ranges from 0.5% to 10% for XS soil, the evaporation rate decreased by 8.57%, 8.73%, 4.56%, and increased by 0.96% and 41.06%, respectively. Image processing analysis on the cracks indicates that the addition of biochar decreases the quantitative parameters such as the crack ratio, the number of soil mass and the fractal dimension for both treated soils. The crack ratio, the number of cracks and the number of soil mass were reduced by 16.85%, 32.26% and 39.22% for 4% biochar addition in PKE. The value of XS soil with 6% biochar addition decreased by 30.80%, 8.28%, and 11.61%, respectively. Furthermore, biochar can effectively reduce the width of cracks. The role of biochar in the development of desiccation cracking may include (1) occupying the shrinkage space of soil particles; (2) weakening the bonding between soil particles and (3) providing hydrophobic channels. In general, the application of 4% and 6% biochar in PKE and XS respectively has the best performance in inhibiting soil cracking.
Keywords: Biochar Clayey soils Desiccation cracking Evaporation rate Image processing
1. Introduction There has been a growing interest in biochar in recent years because of two significant challenges in climate change and sustainable environmental management. The characteristics of biochar (i.e., low density, high porosity, high pH, high CEC and high specific surface area, etc.) makes it applicable for contaminated site remediation (Palansooriya et al., 2019; Shaaban et al., 2018; Yuan et al., 2019). In general, the application of biochar in soil remediation could offer multiple benefits including environmental improvement, reuse of waste, enhancement of soil fertility and resilience and carbon storage (Giagnoni et al., 2019; Shen et al., 2018b). Most of the existing studies focused on the influence of biochar on heavy metal mobility and risks in soil (O’Connor et al., 2018; Shen et al., 2019). A few have investigated the influence of biochar on the mechanical properties (Pardo et al., 2019). However, there is a research dearth regarding the effect of biochar on the cracking of soils. Desiccation cracking is a common phenomenon that is caused by the
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shrinkage of cohesive soil with the loss of water content. The occurrence of desiccation cracking may alter the mechanical and volume change behavior of soils (Lozada et al., 2015), and therefore inducing the concern of the long-term performance of biochar treated soils. For instance, the development of cracks may destruct the integrity of soils, leading to the loss of mechanical resistance and providing preferential infiltration channels for chemical pollutants and raising migration (Hewitt and Philip, 1999; Li et al., 2016; Rayhani et al., 2007). The promoted hydraulic conductivity due to cracking may also lead to the enhanced impact of rainfall wash and thereby, the enhanced mobilization of contaminants in soil (Shen et al., 2018a). Hallett and Newson (2005); Reddy et al., 2015b; Vogel et al. (2005) demonstrated that desiccation shrinkage is a complex process, which is affected by clay mineral content, water content, soil structure, and external environmental factors (i.e., temperature, humidity, and pressure). Blanco-Canqui (2017) reported that biochar could effectively change the porosity and physicochemical properties of soils. For coarsetextured soils, biochar may reduce the saturated hydraulic conductivity
Corresponding author at: School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China. E-mail address: [email protected] (K. Gu).
https://doi.org/10.1016/j.geoderma.2020.114182 Received 1 August 2019; Received in revised form 6 January 2020; Accepted 12 January 2020 0016-7061/ © 2020 Elsevier B.V. All rights reserved.
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the soil altered by biochar (Zong et al., 2016, 2014). It is essential to study the effect of biochar on these properties and consequently the cracking dynamics of the soils. Given these considerations, this study investigates the impact of wood biochar on the desiccation cracking characteristics of two clayey soils. By using image processing, microstructural analysis based on scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP), and soil suction measurement, an insight into desiccation cracking characteristics of biochar treated clayey soils was provided, and also a mechanism was proposed. It is suggested that when applying biochar to soil remediation, the influence of soil cracking should be considered. This study aims to provide some valuable insights into this aspect.
Table 1 Basic properties of clayey soils. Soil properties
PKE
XS
Classification
Clay with high plasticity (CH) 2.71 76.0 29.0 47.0 1.67 42
Clay with low plasticity (CL) 2.73 34.5 17.0 17.5 1.70 24
Specific gravity Liquid limit (%) Plastic limit (%) Plastic index Max dry density (g/cm3) Clay content (%)
Table 2 Basic properties of biochar. Physicochemical property
Value
Pyrolysis temperature (℃) Pyrolysis time (h) Ash content (%) pH Specific surface area (m2/g) Bulk density (g/cm3)
500 5 28.26 10.28 51.7 0.47
2. Materials and methods 2.1. Soil samples and biochar used Desiccation cracking mainly occurs for cohesive soils. Two soils, namely Pukou expansive soil (PKE, Clay with high plasticity) and Xiashu soil (XS, Clay with low plasticity) were used in this study. The volume change (e.g. swelling and shrinkage) of PKE is susceptible to the change of water content due to its high content of clay minerals (Reddy et al., 2015b). XS is a typical soil in the middle and lower reaches of the Yangtze River. The basic physical parameters of the two soils can be found from our previous studies (Gu et al., 2014; Zeng et al., 2019b) and are tabulated in Table 1. After air-drying and crushing, soils were sieved through 0.25 mm and then stored for further use. Biochar from Qingdao Biochar Biotechnology Co., Ltd., China, was used. It was produced by the pyrolysis of wood at 500℃ for 5 h, then crushed and passed through 0.25 mm sieves before adding to the soil. Table 2 summarizes the physical and chemical properties of the biochar.
(Note: data of biomass pyrolysis temperature and time were from suppliers; special surface area of biochar was determined by liquid nitrogen static adsorption-BET method; ash content was determined after burning off the organic matter in a sample in a muffle furnace at 700 °C; pH of biochar was measured with a pH meter using 1 g of biochar in 20 mL of distilled water; bulk density was determined by placing and weighing the material in a container with a known volume) Table 3 Basic parameters of the samples. Soil
Sample No
Biochar dosage (%)
Initial water content (%)
PKE
PKE0-1, PKE0-2 PKE0.5–1, PKE0.5–2 PKE2-1, PKE2-2 PKE4-1, PKE4-2 PKE6-1, PKE6-2 XS0-1, XS0-2 XS0.5-1, XS0.5-2 XS2-1, XS2-2 XS4-1, XS4-2 XS6-1, XS6-2 XS10-1, XS10-2
0 0.5 2 4 6 0 0.5 2 4 6 10
100
XS
2.2. Evaporation of biochar treated soil Based on previous studies, the dosage of biochar added to soils was designated at the range of 0–10% (w/w) (Table 3). The samples were named based on the type of the soil and the dosage of biochar. For instance, the biochar was mixed with PKE soil at five application rates (0, 0.5, 2, 4, 6% dry weight), representing the PKE0, PKE0.5, PKE2, PKE4, and PKE6 treatments, respectively. A total of 11 sets were prepared for the test, and each set has duplicate samples for parallel tests. For sample preparation, a certain amount of biochar was thoroughly mixed with the soils (in the dry state), and then a designated amount of water was added. The mixture was further homogenized using a mechanical device for 30 min. The samples were incubated in sealed container for 48 h and homogenized for 10 min before pouring into 20 cm × 20 cm plexiglass container, with the thickness of the slurry being 1 cm. The air bubbles were removed by carefully vibrating the container and the sample surface was flattened. The prepared samples were left to evaporate at constant room temperature and relative humidity. During the evaporation, the weight mass of samples was monitored using an electronic balance with an accuracy of 0.01 g, and the dynamic process of cracks was photographed using a high-definition camera at certain time intervals.
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(Ksat), improve the water holding capacity and ameliorate the stability of soil aggregates, thereby affecting the evaporation characteristics of soils (Günal et al., 2018; Villagra-Mendoza and Horn, 2018; Wang et al., 2018). Reddy et al. (2015a) and Wong et al. (2018) reported that the Ksat increases with the addition of 5% and 20% biochar for clayey soils. On the other hand, Lim et al. (2016) found that the Ksat of a Webster clay loam increased with 1% and 2% biochar addition but higher biochar dosage (5%) provided no alterations. Such difference can be attributed to the nature of the soils. The desiccation cracking of clay is mainly related to its mineralogy, and many studies have shown that the shrinkage more violent with larger clay compositions, especially montmorillonite content (Tay et al., 2001; Vogel et al., 2005). Moreover, water evaporation and the development of cracks were also affected by the pore structure of the soil. Wong et al. (2017) found that biochar particles can fill the inter-pores between kaolin clay aggregates. The bulk density, pore structure, and the coefficient of linear extensibility in
2.3. Image processing and quantitative analysis The desiccation cracking geometry of each sample was quantitatively characterized using NJU-CIAS developed by Nanjing University. To eliminate the container boundary effect, only middle part (18 cm × 18 cm) of the image of the crack was used for analysis. NJU-
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Fig. 1. The schematic diagram of cracks image processing: (a) original color image, (b) gray image, (c) binary image, (d) image de-noising, (e) crack skeletonization, (f) crack statistics.
mixed and saturated slurry samples with 0, 0.5%, 2%, 4%, 6% and 10% biochar for both soils were prepared and left to evaporate at constant room temperature (25℃). During the drying, the suction of soils was measured using a dew-point water potentiometer WP4C (Decagon, USA), then the water content was determined by the oven drying method at 105℃.
CIAS can transfer original color images (Fig. 1a) to grayscale images (Fig. 1b). Images were then binarized by setting grayscale thresholds, and therefore the crack portion and the soil mass portion were separated (Fig. 1c). Subsequently, the threshold denoising, branching (Fig. 1d), skeletonizing (Fig. 1e), and characterizing (Fig. 1f) of the cracks were performed afterward. Quantitative parameters such as the number of cracks, crack ratio, the number of soil mass and fractal dimensions were obtained (Tang et al., 2008). The skeleton line between two adjacent nodes on the skeleton is referred to as a crack. The number of cracks counts the crack quantity in soils. The crack ratio refers to the ratio of the plane cracks area to the total area, presenting the degree of cracking of the sample. The number of soil mass refers to the closed area enclosed by the cracks, a higher value of it means the good connectivity of desiccation cracking. The fractal dimension can comprehensively describe the degree of cracking of the sample, the low value of the fractal dimension represents better integrity of the soil (Zeng et al., 2019a; Zhang et al., 2016).
3. Results 3.1. Evaporation characteristics of biochar-treated soils The change of water content as a function of time for different samples was presented in Figs. 2 and 3. For each sample, there was a linear decline of water content with time in the beginning, then the evaporation gradually slowed down and finally reached a stable stage. It can be seen from the figures that the evaporation rate of the sample first maintains a relatively stable state, and then continuously decreases to zero. An et al. (2018) and Tang et al. (2011a) suggested that the evaporation process of soil can be divided into three stages according to the evaporation rate: the constant rate stage, the deceleration rate stage and the residual stage. As illustrated in Figs. 2 and 3, biochar altered the duration of each stage and the water evaporation rate. It is considered that the water content tends to be stable when soil water evaporation transits from the deceleration rate stage to the residual stage. For PKE with a small dosage of biochar, it takes slightly longer for the sample to reach the residual stage than the reference sample due to the lower evaporation rate, but the time required is shortened with the increment of biochar. With a 4% addition of biochar, there was no significant difference in time due to the similar evaporation rate, the constant rate stage was significantly shorter for the sample with 6% biochar caused by a higher evaporation rate. For XS, biochar ranges from 0.5% to 10%, the time to get to the residual stage increases first and then decreases (Fig. 3). Except for the 10% biochar sample, the time required for other samples to reach the residual stage was longer than that of the reference samples. In comparison to the reference sample, the duration of the constant rate stage was significantly
2.4. Microstructural analysis Scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) measurements were conducted on selected samples at the end of evaporation, to explore the microstructure and porosity size distribution of samples. Prior to testing, the prepared samples (with a size of ~0.5 × 0.5 × 2 cm) were carefully freeze-dried using liquid nitrogen and freeze dryer (SCIENTZ-18N, Xinzhi, China). SU3500 (Hitachi, Japan) and AutoPore IV 9500 (Mike, USA) were used for the SEM and MIP measurements, respectively. 2.5. Suction measurement of biochar treated soil The behavior of soils during the evaporation is greatly determined by the soil suction, which changes significantly with the water content of soils. Soil water characteristic curve (SWCC) was used to describe the relationship between water content and soil suction. Here SWCCs for soils treated by the different dosages of biochar were determined. Fully
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Fig. 2. Evaporation process curve of PKE: (a) reference sample, (b) 0.5% biochar, (c) 2% biochar, (d) 4% biochar, (e) 6% biochar.
abbreviated with a 10% biochar addition. The positions indicated by black and red arrows in Fig. 2 and Fig. 3 represents the time and water content of the sample when cracks occur. It can be seen that the time and the water content were not completely consistent even between the same treated samples, because of the three-phase porous medium characteristics of the soil. The evaporation rate of the PKE and XS sample was relatively constant within the first 100 h and the first 60 h, respectively. The average evaporation rate during this period was calculated (Fig. 4). The evaporation rate became smaller first and then larger following the increment of biochar dosage for both clayey soils. For PKE, the minimum evaporation rate was 1.60 g/h with 0.5% biochar, which decreased by 6.43% than that of the reference sample, and the
evaporation rate of 6% biochar treated sample was maximal. For XS, the minimum and maximum evaporation rates occurred in 0.5% and 10% biochar sample, respectively, the minimum evaporation rate decreased by 8.3%. With the drying of the soil, there is an increase in soil suction. At the combined effect of soil suction and pore water surface tension, soil particles are subjected to tensile stress in different directions (Tang et al., 2018). When the shrinkage was constrained or the induced tensile stress exceeds the tensile strength of the soil, cracks initiate (Nahlawi and Kodikara, 2006; Tang et al., 2011b; Zeng et al., 2019a). For biochar treated soils, the first crack was found to occur in the constant rate stage. This agrees well with the results reported by Peron et al. (2009) and Tang et al. (2011b), in the studies of La Frasse clay and
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Fig. 3. Evaporation process curve of XS: (a) reference sample, (b) 0.5% biochar, (c) 2% biochar, (d) 4% biochar, (e) 6% biochar, (f) 10% biochar.
Romainville clay, respectively. The corresponding water content at the occurrence of the first crack was defined as critical water content (wcri). Fig. 5 shows the effect of biochar on wcri and the time taken for the first crack. For PKE, wcri varies in the range of 40%-60%, which is not significantly correlated to biochar dosage. On the other hand, wcri of XS range from 28% to 38% as the increment of biochar. It seems that biochar will make the sample crack at high water content. The time of surface cracks appeared earlier for 6% and 10% biochar addition in PKE and XS, respectively, due to the accelerated evaporation rate. Apart from this, it can be interpreted as the reduction of tensile strength caused by biochar (Lu et al., 2014; Zong et al., 2016). However, the effect of small dosage biochar on it was not obvious, the time for the crack appearance with 0.5% biochar was
even later than that of the reference sample due to the low evaporation rate. 3.2. Final cracks morphology of biochar-treated soils The desiccation cracking network of the sample is fully developed when the evaporation goes to the residual stage (Tollenaar et al., 2017), Figs. 6 and 7 shows the quantitative characterization results of cracks of biochar treated PKE and XS. For PKE, the crack ratio, the number of soil mass and the fractal dimension of biochar treated samples were smaller than those of untreated. In comparison to the reference sample, the crack ratio and the fractal dimension decreased by 22.64% and 2.52% of 6% biochar
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3.3. Effects of biochar on cracks development and crack width The development of cracks can be divided into three stages: the early stage of crack development, the generation stage of secondary cracks, and the stability stage of cracks network (Tang et al., 2011a). As an example shown in Fig. 8, the first generation cracks developing into the longer and wider ones can be identified as primary cracks. Secondary narrow cracks formed within soil blocks or intersected with primary cracks, which started at existing primary cracks (Tang et al., 2018). For the sample without biochar addition, the number of cracks on the surface and the number of soil mass is larger, and the primary and secondary cracks can be distinguished. Compared with the reference sample, the number of surface cracks and the number of soil mass in the sample with 2% biochar dosage were less. Samples with a biochar dosage of 6% had more cracks, however, most of them were fine cracks. There were no connections between the cracks, so the soil cannot be cut into separate pieces by cracks, and the area of the total cracks was smaller than that of the reference sample. Thus, the number of cracks can not directly indicate the degree of the cracking of the sample, and the parameters such as the crack ratio and the fractal dimension are also related to the crack width. Although the crack ratio of the sample decreases gradually with the addition of biochar, the crack width changes due to the significant change in the number of cracks. Statistical results on the crack width distribution in the various ranges are presented in Fig. 9. For PKE, the crack width is in the range of 0–10 mm, but the distribution of different biochar dosage samples is inconsistent. The crack width is evenly distributed between 0 and 6 mm for the reference sample. For samples containing 2% biochar, the cracks with a width of 3–4 mm are dominated. However, the cracks with a width of 0–3 mm are dominated for the sample with 6% biochar. For XS, the crack width is distributed within 0–2.7 mm. Only the reference sample developed a crack greater than 2 mm, and the samples with biochar dosage of 6% and 10% show a crack width of less than 2 mm (Fig. 9b). The crack width was mainly concentrated within 0.9–1.3 mm for the sample without biochar addition. For the samples containing 6% and 10% biochar, the peak width of the crack is in the range of 0.7–1.1 mm, and the crack width is mostly less than 1.1 mm. Therefore, the addition of biochar will induce more narrow cracks in the soil. Albrecht and Benson (2001) reported that the shrinkage of highly plastic clay is higher than the lower plastic clay, because of the high content of clay minerals. Moreover, the crack ratio and the crack width of high plasticity index soil are larger than those of the low plasticity index (Tang et al., 2007). Therefore, it agrees with the results that the crack width of PKE samples was generally larger than XS samples.
Fig. 4. Average evaporation rate of clayey soils.
Fig. 5. Water content and time when desiccation crack occurred.
treated samples, respectively, and the number of soil mass was smallest with 4% biochar, which decreased by 39.22%. The decreased crack ratio suggests that biochar reduced the shrinkage of the sample, and the reduced number of soil mass and fractal dimension indicates that biochar can improve the integrity of the soil. With the increasing dosage of biochar, the number of cracks shows a process of decreasing first and then increasing. The samples with biochar dosage of 2% showed the largest decline, which decreased by 34.41% than reference. The number of cracks in 6% biochar samples even exceeds that of the reference samples, increasing by 51.61%, but which have the lowest crack ratio. For XS, with the addition of biochar, the quantitative parameters of cracks are smaller than that of the untreated sample. The crack ratio, the number of soil mass and the fractal dimension are the smallest with 6% biochar dosage. These values are reduced by 30.80%, 11.61%, and 3.48% than the reference sample, respectively. With 2% biochar, the number of cracks was the smallest, which decreased by 9.76% than the reference sample. The quantitative parameters of cracks decrease first and then slightly increase when the dosage of biochar increases, but the values are lower than the reference sample. This indicates that the number of cracks, the connectivity degree of cracks, and the number of soil mass are decreasing. Therefore, biochar has a beneficial effect on the morphology of cracks of XS soil.
3.4. Microstructure characteristics of biochar-treated soils Samples at the edge of cracks were chosen for SEM analysis (Fig. 10). For PKE, the reference sample has less pore on the surface (Fig. 10a), but it increases with the addition of biochar. Biochar particles are firmly embedded in soil aggregates, and some fine clay particles can fill in the pores of biochar (Fig. 10b). For the XS sample, it can be seen from SEM images that the particles are coarser and there are more voids on the surface (Fig. 10c). Biochar particles are surrounded by clay particles, yet their contact is not close, and the structure is relatively porous (Fig. 10d). In the case of the XS sample, the blockage of biochar pores was not observed, mainly because the PKE contains more clay minerals and its particles are finer. Fig. 11 shows the results of the pore size distribution (PSD) of biochar treated samples and reference samples. For PKE, PSD's exhibited two distinct peaks in the range of investigated pores (Fig. 11a). Two peaks appeared in the pore size ranges from 0.003 to 0.005 μm
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Fig. 6. Quantitative analysis result of cracks in PKE soil: (a) crack ratio, (b) crack number, (c) mass number, (d) fractal dimension.
(first peak) and 0.009–0.012 μm (second peak) for the samples without biochar, but that of the samples with 6% biochar were 0.01–0.02 μm and 0.07–0.25 μm, respectively. The pore volume of the treated soils was characterized by an increased pore volume in the diameter ranges from 0.018 to 2.0 μm. The XS samples exhibited a single, sharply defined peak in the diameter range of 0.9–3.5 μm (Fig. 11b). Although the peak pore size is almost the same between the reference sample and the sample with 6% biochar, the volume of pore for 6% biochar sample significantly increased in the diameter ranges of 0.9–3.5 μm. The clay mineral content in PKE is higher than that in XS, and the pores of biochar in PKE are more severely blocked (Fig. 10b & d). Therefore, the peaks caused by biochar in the two soils are inconsistent. The addition of biochar leads to a loose structure of the sample and raises the porosity of soil (Li et al., 2018), which facilitates the formation of successive hydrophobic channels.
4. Discussion The generation of desiccation cracking is related to the tensile stress concentration in the soil (Costa et al., 2013; Morris et al., 2009). Tensile stress and tensile strength are the main mechanical cause of cracking. Changes in tensile strength largely depend on clay content, soil porosity, aggregate, microstructure characteristics, and organic composition. In general, biochar can reduce the tensile strength of soil by reducing the density and overall cohesive and weakening the interparticle bonding of the soil (Lu et al., 2014; Zong et al., 2016). The reduction of soil suction (Fig. 12) in biochar treated soil is more conducive to generate cracks, especially with a large dosage of biochar. However, it has little effect on soil suction with a small dosage of biochar in the soil. Thus the time of cracks occurrence was not notably different from that of the reference. Thus, cracks appeared earlier in PKE with 6% biochar and in XS with 10% biochar (Fig. 5), and the water evaporation is the precondition for cracks. The water evaporation rate of the soil is mainly determined by external factors and the internal structure of soil (Khan and Azam, 2017). At the initial stage, the pore structure of soil was a major factor since the external factors were the same, which can affect the water evaporation capacity of the soil. The results show that the presence of biochar in the soil can affect the soil water evaporation process, the effect was more pronounced with a large dosage of biochar. For small biochar dosage samples (Fig. 13b), the inter-aggregate voids were filled by biochar particles and water was stored in the small pores within these biochar particles (Wong et al., 2017). It was difficult to form an
3.5. Evolution process of soil suction during drying The SWCCs of biochar treated soils are given in Fig. 12. The soil suction raises exponentially with the reduction of water content. Except for the PKE sample with 0.5% biochar, the value of soil suction for biochar treated samples are smaller than those without biochar. The soil suction value of the sample decreases gradually with the increase of biochar. In the case of the same water content, the soil suction value of the PKE is greater than that of the XS soil, because the PKE has more clay minerals (Rao and Revanasiddappa, 2003).
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Fig. 7. Quantitative analysis result of cracks in XS soil: (a) crack ratio, (b) crack number, (c) mass number, (d) fractal dimension.
Fig. 8. Cracks development images with no biochar and final images with different biochar dosage in PKE.
effective water migration channel. Consequently, a small dosage of biochar has no obvious effect on water evaporation rate, and even slow it down due to the improved water hold capacity in soil caused by biochar. The increasing presence of porous biochar created more water migration channels (Fig. 13c) and water can migrate along the channels formed by biochar particles. The reduction of the soil suction suggests that less energy is required for water to migrate to the evaporation surface, and the soil loses water more easily. For the samples with a high dosage of biochar, it was more prone to cracking, and the cracks become the evaporating surface to promote the evaporation of water.
Therefore, the evaporation rate of the soil with a large dosage of biochar is higher than that of the reference sample. As shown in Fig. 13, the soil may have both vertical and horizontal shrinkage deformation with drying (Tang et al., 2011a). Biochar is a non-plastic material with high particle strength (Sadasivam and Reddy, 2015), randomly distributed in inter-aggregate voids and intra-aggregate voids. Firstly, they may occupy the shrinkage space of soil, therefore a reduced crack ratio was observed for both soils (Figs. 6a & 7a). Meanwhile, the addition of biochar results in the generation of more narrow cracks in the sample (Fig. 9). Additionally, biochar
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Fig. 9. Crack width distribution of different biochar dosage samples: (a) PKE, (b) XS.
Fig. 10. SEM image of biochar treated samples: (a) PKE-with no biochar, (b) PKE-with 6% biochar, (C) XS-with no biochar, (b) XS-with 10% biochar.
weakens the bonding and electrochemical interaction between soil particles (Zong et al., 2016, 2014), the effect of a small dosage of biochar on soil suction was not noticeable. The studies of Bordoloi et al., (2018a,b) and Wei et al., (2018) demonstrated that biochar can reduce the number of cracks by improving the water holding capacity of the soils. Hence, with a small dosage of biochar, the number of cracks reduced. However, as the dosage of biochar increased, the evaporation rate of the sample increased evidently. The development of cracks was
promoted due to the decreased soil suction, leading to an increase in cracks for the high-dosage biochar sample (Fig. 6b & Fig. 7b). Furthermore, the number of soil mass refers to the number of closed areas surrounded by cracks, which is closely related to the number of cracks. Therefore, the changing trend of the number of soil mass and the number of cracks is consistent. For PKE and XS, the evaporation rate of soil was slightly reduced by biochar, as the addition of biochar was less than 4% and less than 6%,
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Fig. 11. Effect of biochar on the pore size distribution (PSD) of clayey soils: (a) PSD curves of PKE soil, (b) PSD curves of XS soil.
Fig. 12. Soil water characteristic curve of soils: (a) PKE soil, (b) XS soil.
respectively. With the combined effect of water evaporation rate and soil suction, the occurrence time of cracks was later. Moreover, biochar can improve the water hold capacity of the soil (Fischer et al., 2019), therefore inhibits the development of cracks. However, the evaporation rate of soil significantly increases, and the value of soil suction reduces when the dosage of biochar in PKE and XS exceeds 4% and 6%, respectively. The cracking condition of clayey soils can be satisfied in a relatively short time than the reference sample, which promotes the development of cracks. Meanwhile, in the range of 0.5%-10% biochar dosage, the crack ratio became smaller than the reference sample, due to the reduced shrinkage space occupied by the biochar particles.
connection between soil particles, it does not necessarily induce more cracks. Only 6% and 10% dosage of biochar in PKE and XS, respectively, is conducive to the development of cracks. Nevertheless, image processing analysis demonstrated that biochar reduces the crack ratio by occupying shrinkage space, and effectively inhibits the width of the crack. Moreover, biochar reduces the degree of cracking of the soil and keeps the soil relatively intact since the crack ratio, the number of soil mass and the fractal dimension were reduced. From the degree of soil cracking in environmental applications, 4%, and 6% biochar in PKE and XS, respectively, are the optimal dosage. Data availability
5. Conclusions
Some or all data, models, or code generated or used during the study are available from the corresponding author by request.
Soil water evaporation and desiccation cracking behavior can be affected by biochar. On one hand, biochar reduced the evaporation rate of both clayey soils by filling soil voids and storing water in voids, when the dosage of biochar in PKE and XS soils was less than 4% and 6%, respectively. On the other hand, a larger dosage of biochar accelerates the evaporation rate by forming connected water migration channels in soils. It is important to note that although biochar weakens the
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.
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Fig. 13. Dry shrinkage of biochar-treated clayey soils.
Acknowledgments
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We would like to thank Dr. Gangadhara N. Reddy for the language support. We also acknowledge the constructive reviews provided by the editor and three anonymous reviewers. This research was financially supported by the National Natural Science Foundation of China, China (41977217), the Open Foundation of State Key Laboratory of Geohazard Prevention and Geoenvironmental Protection, Chengdu University of Technology, China (SKLGP2016K010), the Fundamental Research Funds for the Central Universities, China (020614380061) and the Key Project of Nanjing University Technology Innovation Fund, China (SC-2019-101). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.geoderma.2020.114182. References Albrecht, B.A., Benson, C.H., 2001. Effect of desiccation on compacted natural clays. J. Geotech. Geoenviron. Eng. 127, 67–75. An, N., Tang, C.S., Xu, S.K., Gong, X.P., Shi, B., Inyang, H.I., 2018. Effects of soil characteristics on moisture evaporation. Eng. Geol. 239, 126–135. Blanco-Canqui, H., 2017. Biochar and soil physical properties. Soil Sci. Soc. Am. J. 81, 687–711. Bordoloi, S., Garg, A., Sreedeep, S., Lin, P., Mei, G., 2018a. Investigation of cracking and water availability of soil-biochar composite synthesized from invasive weed water hyacinth. Bioresour. Technol. 263, 665–677. Bordoloi, S., Gopal, P., Boddu, R., Wang, Q., Cheng, Y.-F., Garg, A.S.S., 2018b. Soil-biochar-water interactions: role of biochar from Eichhornia crassipes in influencing
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Y. Zhang, et al. D., 2018. Biochar application for the remediation of heavy metal polluted land: a review of in situ field trials. Sci. Total Environ. 619–620, 815–826. Palansooriya, K.N., Ok, Y.S., Awad, Y.M., Lee, S.S., Sung, J.K., Koutsospyros, A., Moon, D.H., 2019. Impacts of biochar application on upland agriculture: a review. J. Environ. Manage. 234, 52–64. Pardo, G.S., Sarmah, A.K., Orense, R.P., 2019. Mechanism of improvement of biochar on shear strength and liquefaction resistance of sand. Géotechnique 69, 471–480. Peron, H., Hueckel, T., Laloui, L., Hu, L.B., 2009. Fundamentals of desiccation cracking of fine-grained soils: experimental characterisation and mechanisms identification. Can. Geotech. J. 46, 1177–1201. Rao, S.M., Revanasiddappa, K., 2003. Role of soil structure and matric suction in collapse of a compacted clay soil. Geotech. Test. J. 26, 102–110. Rayhani, M.H., Yanful, E.K., Fakher, A., 2007. Desiccation-induced cracking and its effect on the hydraulic conductivity of clayey soils from Iran. Can. Geotech. J. 44, 276–283. Reddy, K.R., Yaghoubi, P., Yukselen-Aksoy, Y., 2015a. Effects of biochar amendment on geotechnical properties of landfill cover soil. Waste Manag. Res. 33, 524–532. Reddy, N.G., Thasildar, J., Rao, B.H., 2015b. Evaluating the influence of additives on swelling characteristics of expansive soils. Int. J. Geosynth. Gr. Eng. 1, 13. Sadasivam, B.Y., Reddy, K.R., 2015. Engineering properties of waste wood-derived biochars and biochar-amended soils. Int. J. Geotech. Eng. 9, 521–535. Shaaban, M., Van Zwieten, L., Bashir, S., Younas, A., Núñez-Delgado, A., Chhajro, M.A., Kubar, K.A., Ali, U., Rana, M.S., Mehmood, M.A., Hu, R., 2018. A concise review of biochar application to agricultural soils to improve soil conditions and fight pollution. J. Environ. Manage. 228, 429–440. Shen, Z., Hou, D., Xu, W., Zhang, J., Jin, F., Zhao, B., Pan, S., Peng, T., Alessi, D.S., 2018a. Assessing long-term stability of cadmium and lead in a soil washing residue amended with MgO-based binders using quantitative accelerated ageing. Sci. Total Environ. 643, 1571–1578. Shen, Z., Li, Z., Alessi, D.S., 2018b. Stabilization-based soil remediation should consider long-term challenges. Front. Environ. Sci. Eng. 12, 3–5. Shen, Z., Zhang, J., Hou, D., Tsang, D.C.W., Ok, Y.S., Alessi, D.S., 2019. Synthesis of MgOcoated corncob biochar and its application in lead stabilization in a soil washing residue. Environ. Int. 122, 357–362. Tang, C.S., Cui, Y.J., Shi, B., Tang, A.M., Liu, C., 2011a. Desiccation and cracking behaviour of clay layer from slurry state under wetting-drying cycles. Geoderma 166, 111–118. Tang, C.S., Shi, B., Cui, Y.J., 2018. Behaviors and mechanisms of desiccation cracking of soils. Chinese J. Geotech. Eng. 40, 1415–1423. Tang, C.S., Shi, B., Liu, C., Suo, Bin, W., Gao, L., 2011b. Experimental characterization of shrinkage and desiccation cracking in thin clay layer. Appl. Clay Sci. 52, 69–77.
Tang, C.S., Shi, B., Liu, C., Wang, B.J., 2007. Factors affecting the surface cracking in clay due to drying shrinkage. J. Hydraul. Eng. 38, 1186–1193. Tang, C.S., Shi, B., Liu, C., Zhao, L.Z., Wang, B.J., 2008. Influencing factors of geometrical structure of surface shrinkage cracks in clayey soils. Eng. Geol. 101, 204–217. Tay, Y.Y., Stewart, D.I., Cousens, T.W., 2001. Shrinkage and desiccation cracking in bentonite-sand landfill liners. Eng. Geol. 60, 263–274. Tollenaar, R.N., van Paassen, L.A., Jommi, C., 2017. Observations on the desiccation and cracking of clay layers. Eng. Geol. 230, 23–31. Villagra-Mendoza, K., Horn, R., 2018. Effect of biochar addition on hydraulic functions of two textural soils. Geoderma 326, 88–95. Vogel, H.J., Hoffmann, H., Roth, K., 2005. Studies of crack dynamics in clay soil: I. Experimental methods, results, and morphological quantification. Geoderma 125, 203–211. Wang, T., Stewart, C.E., Sun, C., Wang, Y., Zheng, J., 2018. Effects of biochar addition on evaporation in the five typical Loess Plateau soils. Catena 162, 29–39. Wei, C., Gao, W., Whalley, W.R., Li, B., 2018. Shrinkage Characteristics of Lime Concretion Black Soil as Affected by Biochar Amendment. Pedosphere 28, 713–725. Wong, J.T.F., Chen, Z., Chen, X., Ng, C.W.W., Wong, M.H., 2017. Soil-water retention behavior of compacted biochar-amended clay: a novel landfill final cover material. J. Soils Sediments 17, 590–598. Wong, J.T.F., Chen, Z., Wong, A.Y.Y., Ng, C.W.W., Wong, M.H., 2018. Effects of biochar on hydraulic conductivity of compacted kaolin clay. Environ. Pollut. 234, 468–472. Yuan, P., Wang, J., Pan, Y., Shen, B., Wu, C., 2019. Review of biochar for the management of contaminated soil: Preparation, application and prospect. Sci. Total Environ. 659, 473–490. Zeng, H., Tang, C., Cheng, Q., Inyang, H.I., Rong, D., Lin, L., Shi, B., 2019a. Coupling effects of interfacial friction and layer thickness on soil desiccation cracking behavior. Eng. Geol. 260, 105220. Zeng, H., Tang, C., Liu, C., Lin, L., Wang, D., Shi, B., 2019b. Effects of boundary friction and layer thickness on desiccation cracking behaviors of soils. Chinese J. Geotech. Eng. 41, 544–553. Zhang, Z.B., Zhou, H., Lin, H., Peng, X., 2016. Puddling intensity, sesquioxides, and soil organic carbon impacts on crack patterns of two paddy soils. Geoderma 262, 155–164. Zong, Y., Chen, D., Lu, S., 2014. Impact of biochars on swell-shrinkage behavior, mechanical strength, and surface cracking of clayey soil. J. Plant Nutr. Soil Sci. 177, 920–926. Zong, Y., Xiao, Q., Lu, S., 2016. Acidity, water retention, and mechanical physical quality of a strongly acidic Ultisol amended with biochars derived from different feedstocks. J. Soils Sediments 16, 177–190.
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Effect of biochar on desiccation cracking characteristics of clayey soils a
a,b,⁎
Yuping Zhang , Kai Gu a b c
a
a
c
, Jinwen Li , Chaosheng Tang , Zhengtao Shen , Bin Shi
a
T
School of Earth Sciences and Engineering, Nanjing University, Nanjing, Jiangsu 210023, China State Key Laboratory of Geohazard Prevention and Geoenvironmental Protection, Chengdu University of Technology, Chengdu 610059, China Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton T6G 2E3, Canada
ARTICLE INFO
ABSTRACT
Handling Editor: Cristine L.S. Morgan
Biochar is a promising material for soil remediation. However, the influence of biochar on soil cracking has not been clearly understood to date. Soil cracking can significantly change the mobility of contaminants in soil and consequently the remediation performance of biochar. This study investigates the effect of a wood biochar on the desiccation shrinkage characteristics of two clayey soils (PKE and XS). The biochar dosages selected are 0%, 0.5%, 2%, 4%, 6%, and 10% (w/w). The results indicate that biochar affects the desiccation cracking characteristics of clayey soils by changing the evaporation process. For PKE, the evaporation rate decreased by 6.56% and 5.59% with 0.5% and 2% biochar addition, respectively, and increased by 2.11% and 17.20% with 4% and 6% biochar addition. Similarly, with the biochar dosage ranges from 0.5% to 10% for XS soil, the evaporation rate decreased by 8.57%, 8.73%, 4.56%, and increased by 0.96% and 41.06%, respectively. Image processing analysis on the cracks indicates that the addition of biochar decreases the quantitative parameters such as the crack ratio, the number of soil mass and the fractal dimension for both treated soils. The crack ratio, the number of cracks and the number of soil mass were reduced by 16.85%, 32.26% and 39.22% for 4% biochar addition in PKE. The value of XS soil with 6% biochar addition decreased by 30.80%, 8.28%, and 11.61%, respectively. Furthermore, biochar can effectively reduce the width of cracks. The role of biochar in the development of desiccation cracking may include (1) occupying the shrinkage space of soil particles; (2) weakening the bonding between soil particles and (3) providing hydrophobic channels. In general, the application of 4% and 6% biochar in PKE and XS respectively has the best performance in inhibiting soil cracking.
Keywords: Biochar Clayey soils Desiccation cracking Evaporation rate Image processing
1. Introduction There has been a growing interest in biochar in recent years because of two significant challenges in climate change and sustainable environmental management. The characteristics of biochar (i.e., low density, high porosity, high pH, high CEC and high specific surface area, etc.) makes it applicable for contaminated site remediation (Palansooriya et al., 2019; Shaaban et al., 2018; Yuan et al., 2019). In general, the application of biochar in soil remediation could offer multiple benefits including environmental improvement, reuse of waste, enhancement of soil fertility and resilience and carbon storage (Giagnoni et al., 2019; Shen et al., 2018b). Most of the existing studies focused on the influence of biochar on heavy metal mobility and risks in soil (O’Connor et al., 2018; Shen et al., 2019). A few have investigated the influence of biochar on the mechanical properties (Pardo et al., 2019). However, there is a research dearth regarding the effect of biochar on the cracking of soils. Desiccation cracking is a common phenomenon that is caused by the
⁎
shrinkage of cohesive soil with the loss of water content. The occurrence of desiccation cracking may alter the mechanical and volume change behavior of soils (Lozada et al., 2015), and therefore inducing the concern of the long-term performance of biochar treated soils. For instance, the development of cracks may destruct the integrity of soils, leading to the loss of mechanical resistance and providing preferential infiltration channels for chemical pollutants and raising migration (Hewitt and Philip, 1999; Li et al., 2016; Rayhani et al., 2007). The promoted hydraulic conductivity due to cracking may also lead to the enhanced impact of rainfall wash and thereby, the enhanced mobilization of contaminants in soil (Shen et al., 2018a). Hallett and Newson (2005); Reddy et al., 2015b; Vogel et al. (2005) demonstrated that desiccation shrinkage is a complex process, which is affected by clay mineral content, water content, soil structure, and external environmental factors (i.e., temperature, humidity, and pressure). Blanco-Canqui (2017) reported that biochar could effectively change the porosity and physicochemical properties of soils. For coarsetextured soils, biochar may reduce the saturated hydraulic conductivity
Corresponding author at: School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China. E-mail address: [email protected] (K. Gu).
https://doi.org/10.1016/j.geoderma.2020.114182 Received 1 August 2019; Received in revised form 6 January 2020; Accepted 12 January 2020 0016-7061/ © 2020 Elsevier B.V. All rights reserved.
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the soil altered by biochar (Zong et al., 2016, 2014). It is essential to study the effect of biochar on these properties and consequently the cracking dynamics of the soils. Given these considerations, this study investigates the impact of wood biochar on the desiccation cracking characteristics of two clayey soils. By using image processing, microstructural analysis based on scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP), and soil suction measurement, an insight into desiccation cracking characteristics of biochar treated clayey soils was provided, and also a mechanism was proposed. It is suggested that when applying biochar to soil remediation, the influence of soil cracking should be considered. This study aims to provide some valuable insights into this aspect.
Table 1 Basic properties of clayey soils. Soil properties
PKE
XS
Classification
Clay with high plasticity (CH) 2.71 76.0 29.0 47.0 1.67 42
Clay with low plasticity (CL) 2.73 34.5 17.0 17.5 1.70 24
Specific gravity Liquid limit (%) Plastic limit (%) Plastic index Max dry density (g/cm3) Clay content (%)
Table 2 Basic properties of biochar. Physicochemical property
Value
Pyrolysis temperature (℃) Pyrolysis time (h) Ash content (%) pH Specific surface area (m2/g) Bulk density (g/cm3)
500 5 28.26 10.28 51.7 0.47
2. Materials and methods 2.1. Soil samples and biochar used Desiccation cracking mainly occurs for cohesive soils. Two soils, namely Pukou expansive soil (PKE, Clay with high plasticity) and Xiashu soil (XS, Clay with low plasticity) were used in this study. The volume change (e.g. swelling and shrinkage) of PKE is susceptible to the change of water content due to its high content of clay minerals (Reddy et al., 2015b). XS is a typical soil in the middle and lower reaches of the Yangtze River. The basic physical parameters of the two soils can be found from our previous studies (Gu et al., 2014; Zeng et al., 2019b) and are tabulated in Table 1. After air-drying and crushing, soils were sieved through 0.25 mm and then stored for further use. Biochar from Qingdao Biochar Biotechnology Co., Ltd., China, was used. It was produced by the pyrolysis of wood at 500℃ for 5 h, then crushed and passed through 0.25 mm sieves before adding to the soil. Table 2 summarizes the physical and chemical properties of the biochar.
(Note: data of biomass pyrolysis temperature and time were from suppliers; special surface area of biochar was determined by liquid nitrogen static adsorption-BET method; ash content was determined after burning off the organic matter in a sample in a muffle furnace at 700 °C; pH of biochar was measured with a pH meter using 1 g of biochar in 20 mL of distilled water; bulk density was determined by placing and weighing the material in a container with a known volume) Table 3 Basic parameters of the samples. Soil
Sample No
Biochar dosage (%)
Initial water content (%)
PKE
PKE0-1, PKE0-2 PKE0.5–1, PKE0.5–2 PKE2-1, PKE2-2 PKE4-1, PKE4-2 PKE6-1, PKE6-2 XS0-1, XS0-2 XS0.5-1, XS0.5-2 XS2-1, XS2-2 XS4-1, XS4-2 XS6-1, XS6-2 XS10-1, XS10-2
0 0.5 2 4 6 0 0.5 2 4 6 10
100
XS
2.2. Evaporation of biochar treated soil Based on previous studies, the dosage of biochar added to soils was designated at the range of 0–10% (w/w) (Table 3). The samples were named based on the type of the soil and the dosage of biochar. For instance, the biochar was mixed with PKE soil at five application rates (0, 0.5, 2, 4, 6% dry weight), representing the PKE0, PKE0.5, PKE2, PKE4, and PKE6 treatments, respectively. A total of 11 sets were prepared for the test, and each set has duplicate samples for parallel tests. For sample preparation, a certain amount of biochar was thoroughly mixed with the soils (in the dry state), and then a designated amount of water was added. The mixture was further homogenized using a mechanical device for 30 min. The samples were incubated in sealed container for 48 h and homogenized for 10 min before pouring into 20 cm × 20 cm plexiglass container, with the thickness of the slurry being 1 cm. The air bubbles were removed by carefully vibrating the container and the sample surface was flattened. The prepared samples were left to evaporate at constant room temperature and relative humidity. During the evaporation, the weight mass of samples was monitored using an electronic balance with an accuracy of 0.01 g, and the dynamic process of cracks was photographed using a high-definition camera at certain time intervals.
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(Ksat), improve the water holding capacity and ameliorate the stability of soil aggregates, thereby affecting the evaporation characteristics of soils (Günal et al., 2018; Villagra-Mendoza and Horn, 2018; Wang et al., 2018). Reddy et al. (2015a) and Wong et al. (2018) reported that the Ksat increases with the addition of 5% and 20% biochar for clayey soils. On the other hand, Lim et al. (2016) found that the Ksat of a Webster clay loam increased with 1% and 2% biochar addition but higher biochar dosage (5%) provided no alterations. Such difference can be attributed to the nature of the soils. The desiccation cracking of clay is mainly related to its mineralogy, and many studies have shown that the shrinkage more violent with larger clay compositions, especially montmorillonite content (Tay et al., 2001; Vogel et al., 2005). Moreover, water evaporation and the development of cracks were also affected by the pore structure of the soil. Wong et al. (2017) found that biochar particles can fill the inter-pores between kaolin clay aggregates. The bulk density, pore structure, and the coefficient of linear extensibility in
2.3. Image processing and quantitative analysis The desiccation cracking geometry of each sample was quantitatively characterized using NJU-CIAS developed by Nanjing University. To eliminate the container boundary effect, only middle part (18 cm × 18 cm) of the image of the crack was used for analysis. NJU-
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Fig. 1. The schematic diagram of cracks image processing: (a) original color image, (b) gray image, (c) binary image, (d) image de-noising, (e) crack skeletonization, (f) crack statistics.
mixed and saturated slurry samples with 0, 0.5%, 2%, 4%, 6% and 10% biochar for both soils were prepared and left to evaporate at constant room temperature (25℃). During the drying, the suction of soils was measured using a dew-point water potentiometer WP4C (Decagon, USA), then the water content was determined by the oven drying method at 105℃.
CIAS can transfer original color images (Fig. 1a) to grayscale images (Fig. 1b). Images were then binarized by setting grayscale thresholds, and therefore the crack portion and the soil mass portion were separated (Fig. 1c). Subsequently, the threshold denoising, branching (Fig. 1d), skeletonizing (Fig. 1e), and characterizing (Fig. 1f) of the cracks were performed afterward. Quantitative parameters such as the number of cracks, crack ratio, the number of soil mass and fractal dimensions were obtained (Tang et al., 2008). The skeleton line between two adjacent nodes on the skeleton is referred to as a crack. The number of cracks counts the crack quantity in soils. The crack ratio refers to the ratio of the plane cracks area to the total area, presenting the degree of cracking of the sample. The number of soil mass refers to the closed area enclosed by the cracks, a higher value of it means the good connectivity of desiccation cracking. The fractal dimension can comprehensively describe the degree of cracking of the sample, the low value of the fractal dimension represents better integrity of the soil (Zeng et al., 2019a; Zhang et al., 2016).
3. Results 3.1. Evaporation characteristics of biochar-treated soils The change of water content as a function of time for different samples was presented in Figs. 2 and 3. For each sample, there was a linear decline of water content with time in the beginning, then the evaporation gradually slowed down and finally reached a stable stage. It can be seen from the figures that the evaporation rate of the sample first maintains a relatively stable state, and then continuously decreases to zero. An et al. (2018) and Tang et al. (2011a) suggested that the evaporation process of soil can be divided into three stages according to the evaporation rate: the constant rate stage, the deceleration rate stage and the residual stage. As illustrated in Figs. 2 and 3, biochar altered the duration of each stage and the water evaporation rate. It is considered that the water content tends to be stable when soil water evaporation transits from the deceleration rate stage to the residual stage. For PKE with a small dosage of biochar, it takes slightly longer for the sample to reach the residual stage than the reference sample due to the lower evaporation rate, but the time required is shortened with the increment of biochar. With a 4% addition of biochar, there was no significant difference in time due to the similar evaporation rate, the constant rate stage was significantly shorter for the sample with 6% biochar caused by a higher evaporation rate. For XS, biochar ranges from 0.5% to 10%, the time to get to the residual stage increases first and then decreases (Fig. 3). Except for the 10% biochar sample, the time required for other samples to reach the residual stage was longer than that of the reference samples. In comparison to the reference sample, the duration of the constant rate stage was significantly
2.4. Microstructural analysis Scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) measurements were conducted on selected samples at the end of evaporation, to explore the microstructure and porosity size distribution of samples. Prior to testing, the prepared samples (with a size of ~0.5 × 0.5 × 2 cm) were carefully freeze-dried using liquid nitrogen and freeze dryer (SCIENTZ-18N, Xinzhi, China). SU3500 (Hitachi, Japan) and AutoPore IV 9500 (Mike, USA) were used for the SEM and MIP measurements, respectively. 2.5. Suction measurement of biochar treated soil The behavior of soils during the evaporation is greatly determined by the soil suction, which changes significantly with the water content of soils. Soil water characteristic curve (SWCC) was used to describe the relationship between water content and soil suction. Here SWCCs for soils treated by the different dosages of biochar were determined. Fully
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Fig. 2. Evaporation process curve of PKE: (a) reference sample, (b) 0.5% biochar, (c) 2% biochar, (d) 4% biochar, (e) 6% biochar.
abbreviated with a 10% biochar addition. The positions indicated by black and red arrows in Fig. 2 and Fig. 3 represents the time and water content of the sample when cracks occur. It can be seen that the time and the water content were not completely consistent even between the same treated samples, because of the three-phase porous medium characteristics of the soil. The evaporation rate of the PKE and XS sample was relatively constant within the first 100 h and the first 60 h, respectively. The average evaporation rate during this period was calculated (Fig. 4). The evaporation rate became smaller first and then larger following the increment of biochar dosage for both clayey soils. For PKE, the minimum evaporation rate was 1.60 g/h with 0.5% biochar, which decreased by 6.43% than that of the reference sample, and the
evaporation rate of 6% biochar treated sample was maximal. For XS, the minimum and maximum evaporation rates occurred in 0.5% and 10% biochar sample, respectively, the minimum evaporation rate decreased by 8.3%. With the drying of the soil, there is an increase in soil suction. At the combined effect of soil suction and pore water surface tension, soil particles are subjected to tensile stress in different directions (Tang et al., 2018). When the shrinkage was constrained or the induced tensile stress exceeds the tensile strength of the soil, cracks initiate (Nahlawi and Kodikara, 2006; Tang et al., 2011b; Zeng et al., 2019a). For biochar treated soils, the first crack was found to occur in the constant rate stage. This agrees well with the results reported by Peron et al. (2009) and Tang et al. (2011b), in the studies of La Frasse clay and
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Fig. 3. Evaporation process curve of XS: (a) reference sample, (b) 0.5% biochar, (c) 2% biochar, (d) 4% biochar, (e) 6% biochar, (f) 10% biochar.
Romainville clay, respectively. The corresponding water content at the occurrence of the first crack was defined as critical water content (wcri). Fig. 5 shows the effect of biochar on wcri and the time taken for the first crack. For PKE, wcri varies in the range of 40%-60%, which is not significantly correlated to biochar dosage. On the other hand, wcri of XS range from 28% to 38% as the increment of biochar. It seems that biochar will make the sample crack at high water content. The time of surface cracks appeared earlier for 6% and 10% biochar addition in PKE and XS, respectively, due to the accelerated evaporation rate. Apart from this, it can be interpreted as the reduction of tensile strength caused by biochar (Lu et al., 2014; Zong et al., 2016). However, the effect of small dosage biochar on it was not obvious, the time for the crack appearance with 0.5% biochar was
even later than that of the reference sample due to the low evaporation rate. 3.2. Final cracks morphology of biochar-treated soils The desiccation cracking network of the sample is fully developed when the evaporation goes to the residual stage (Tollenaar et al., 2017), Figs. 6 and 7 shows the quantitative characterization results of cracks of biochar treated PKE and XS. For PKE, the crack ratio, the number of soil mass and the fractal dimension of biochar treated samples were smaller than those of untreated. In comparison to the reference sample, the crack ratio and the fractal dimension decreased by 22.64% and 2.52% of 6% biochar
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3.3. Effects of biochar on cracks development and crack width The development of cracks can be divided into three stages: the early stage of crack development, the generation stage of secondary cracks, and the stability stage of cracks network (Tang et al., 2011a). As an example shown in Fig. 8, the first generation cracks developing into the longer and wider ones can be identified as primary cracks. Secondary narrow cracks formed within soil blocks or intersected with primary cracks, which started at existing primary cracks (Tang et al., 2018). For the sample without biochar addition, the number of cracks on the surface and the number of soil mass is larger, and the primary and secondary cracks can be distinguished. Compared with the reference sample, the number of surface cracks and the number of soil mass in the sample with 2% biochar dosage were less. Samples with a biochar dosage of 6% had more cracks, however, most of them were fine cracks. There were no connections between the cracks, so the soil cannot be cut into separate pieces by cracks, and the area of the total cracks was smaller than that of the reference sample. Thus, the number of cracks can not directly indicate the degree of the cracking of the sample, and the parameters such as the crack ratio and the fractal dimension are also related to the crack width. Although the crack ratio of the sample decreases gradually with the addition of biochar, the crack width changes due to the significant change in the number of cracks. Statistical results on the crack width distribution in the various ranges are presented in Fig. 9. For PKE, the crack width is in the range of 0–10 mm, but the distribution of different biochar dosage samples is inconsistent. The crack width is evenly distributed between 0 and 6 mm for the reference sample. For samples containing 2% biochar, the cracks with a width of 3–4 mm are dominated. However, the cracks with a width of 0–3 mm are dominated for the sample with 6% biochar. For XS, the crack width is distributed within 0–2.7 mm. Only the reference sample developed a crack greater than 2 mm, and the samples with biochar dosage of 6% and 10% show a crack width of less than 2 mm (Fig. 9b). The crack width was mainly concentrated within 0.9–1.3 mm for the sample without biochar addition. For the samples containing 6% and 10% biochar, the peak width of the crack is in the range of 0.7–1.1 mm, and the crack width is mostly less than 1.1 mm. Therefore, the addition of biochar will induce more narrow cracks in the soil. Albrecht and Benson (2001) reported that the shrinkage of highly plastic clay is higher than the lower plastic clay, because of the high content of clay minerals. Moreover, the crack ratio and the crack width of high plasticity index soil are larger than those of the low plasticity index (Tang et al., 2007). Therefore, it agrees with the results that the crack width of PKE samples was generally larger than XS samples.
Fig. 4. Average evaporation rate of clayey soils.
Fig. 5. Water content and time when desiccation crack occurred.
treated samples, respectively, and the number of soil mass was smallest with 4% biochar, which decreased by 39.22%. The decreased crack ratio suggests that biochar reduced the shrinkage of the sample, and the reduced number of soil mass and fractal dimension indicates that biochar can improve the integrity of the soil. With the increasing dosage of biochar, the number of cracks shows a process of decreasing first and then increasing. The samples with biochar dosage of 2% showed the largest decline, which decreased by 34.41% than reference. The number of cracks in 6% biochar samples even exceeds that of the reference samples, increasing by 51.61%, but which have the lowest crack ratio. For XS, with the addition of biochar, the quantitative parameters of cracks are smaller than that of the untreated sample. The crack ratio, the number of soil mass and the fractal dimension are the smallest with 6% biochar dosage. These values are reduced by 30.80%, 11.61%, and 3.48% than the reference sample, respectively. With 2% biochar, the number of cracks was the smallest, which decreased by 9.76% than the reference sample. The quantitative parameters of cracks decrease first and then slightly increase when the dosage of biochar increases, but the values are lower than the reference sample. This indicates that the number of cracks, the connectivity degree of cracks, and the number of soil mass are decreasing. Therefore, biochar has a beneficial effect on the morphology of cracks of XS soil.
3.4. Microstructure characteristics of biochar-treated soils Samples at the edge of cracks were chosen for SEM analysis (Fig. 10). For PKE, the reference sample has less pore on the surface (Fig. 10a), but it increases with the addition of biochar. Biochar particles are firmly embedded in soil aggregates, and some fine clay particles can fill in the pores of biochar (Fig. 10b). For the XS sample, it can be seen from SEM images that the particles are coarser and there are more voids on the surface (Fig. 10c). Biochar particles are surrounded by clay particles, yet their contact is not close, and the structure is relatively porous (Fig. 10d). In the case of the XS sample, the blockage of biochar pores was not observed, mainly because the PKE contains more clay minerals and its particles are finer. Fig. 11 shows the results of the pore size distribution (PSD) of biochar treated samples and reference samples. For PKE, PSD's exhibited two distinct peaks in the range of investigated pores (Fig. 11a). Two peaks appeared in the pore size ranges from 0.003 to 0.005 μm
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Fig. 6. Quantitative analysis result of cracks in PKE soil: (a) crack ratio, (b) crack number, (c) mass number, (d) fractal dimension.
(first peak) and 0.009–0.012 μm (second peak) for the samples without biochar, but that of the samples with 6% biochar were 0.01–0.02 μm and 0.07–0.25 μm, respectively. The pore volume of the treated soils was characterized by an increased pore volume in the diameter ranges from 0.018 to 2.0 μm. The XS samples exhibited a single, sharply defined peak in the diameter range of 0.9–3.5 μm (Fig. 11b). Although the peak pore size is almost the same between the reference sample and the sample with 6% biochar, the volume of pore for 6% biochar sample significantly increased in the diameter ranges of 0.9–3.5 μm. The clay mineral content in PKE is higher than that in XS, and the pores of biochar in PKE are more severely blocked (Fig. 10b & d). Therefore, the peaks caused by biochar in the two soils are inconsistent. The addition of biochar leads to a loose structure of the sample and raises the porosity of soil (Li et al., 2018), which facilitates the formation of successive hydrophobic channels.
4. Discussion The generation of desiccation cracking is related to the tensile stress concentration in the soil (Costa et al., 2013; Morris et al., 2009). Tensile stress and tensile strength are the main mechanical cause of cracking. Changes in tensile strength largely depend on clay content, soil porosity, aggregate, microstructure characteristics, and organic composition. In general, biochar can reduce the tensile strength of soil by reducing the density and overall cohesive and weakening the interparticle bonding of the soil (Lu et al., 2014; Zong et al., 2016). The reduction of soil suction (Fig. 12) in biochar treated soil is more conducive to generate cracks, especially with a large dosage of biochar. However, it has little effect on soil suction with a small dosage of biochar in the soil. Thus the time of cracks occurrence was not notably different from that of the reference. Thus, cracks appeared earlier in PKE with 6% biochar and in XS with 10% biochar (Fig. 5), and the water evaporation is the precondition for cracks. The water evaporation rate of the soil is mainly determined by external factors and the internal structure of soil (Khan and Azam, 2017). At the initial stage, the pore structure of soil was a major factor since the external factors were the same, which can affect the water evaporation capacity of the soil. The results show that the presence of biochar in the soil can affect the soil water evaporation process, the effect was more pronounced with a large dosage of biochar. For small biochar dosage samples (Fig. 13b), the inter-aggregate voids were filled by biochar particles and water was stored in the small pores within these biochar particles (Wong et al., 2017). It was difficult to form an
3.5. Evolution process of soil suction during drying The SWCCs of biochar treated soils are given in Fig. 12. The soil suction raises exponentially with the reduction of water content. Except for the PKE sample with 0.5% biochar, the value of soil suction for biochar treated samples are smaller than those without biochar. The soil suction value of the sample decreases gradually with the increase of biochar. In the case of the same water content, the soil suction value of the PKE is greater than that of the XS soil, because the PKE has more clay minerals (Rao and Revanasiddappa, 2003).
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Fig. 7. Quantitative analysis result of cracks in XS soil: (a) crack ratio, (b) crack number, (c) mass number, (d) fractal dimension.
Fig. 8. Cracks development images with no biochar and final images with different biochar dosage in PKE.
effective water migration channel. Consequently, a small dosage of biochar has no obvious effect on water evaporation rate, and even slow it down due to the improved water hold capacity in soil caused by biochar. The increasing presence of porous biochar created more water migration channels (Fig. 13c) and water can migrate along the channels formed by biochar particles. The reduction of the soil suction suggests that less energy is required for water to migrate to the evaporation surface, and the soil loses water more easily. For the samples with a high dosage of biochar, it was more prone to cracking, and the cracks become the evaporating surface to promote the evaporation of water.
Therefore, the evaporation rate of the soil with a large dosage of biochar is higher than that of the reference sample. As shown in Fig. 13, the soil may have both vertical and horizontal shrinkage deformation with drying (Tang et al., 2011a). Biochar is a non-plastic material with high particle strength (Sadasivam and Reddy, 2015), randomly distributed in inter-aggregate voids and intra-aggregate voids. Firstly, they may occupy the shrinkage space of soil, therefore a reduced crack ratio was observed for both soils (Figs. 6a & 7a). Meanwhile, the addition of biochar results in the generation of more narrow cracks in the sample (Fig. 9). Additionally, biochar
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Fig. 9. Crack width distribution of different biochar dosage samples: (a) PKE, (b) XS.
Fig. 10. SEM image of biochar treated samples: (a) PKE-with no biochar, (b) PKE-with 6% biochar, (C) XS-with no biochar, (b) XS-with 10% biochar.
weakens the bonding and electrochemical interaction between soil particles (Zong et al., 2016, 2014), the effect of a small dosage of biochar on soil suction was not noticeable. The studies of Bordoloi et al., (2018a,b) and Wei et al., (2018) demonstrated that biochar can reduce the number of cracks by improving the water holding capacity of the soils. Hence, with a small dosage of biochar, the number of cracks reduced. However, as the dosage of biochar increased, the evaporation rate of the sample increased evidently. The development of cracks was
promoted due to the decreased soil suction, leading to an increase in cracks for the high-dosage biochar sample (Fig. 6b & Fig. 7b). Furthermore, the number of soil mass refers to the number of closed areas surrounded by cracks, which is closely related to the number of cracks. Therefore, the changing trend of the number of soil mass and the number of cracks is consistent. For PKE and XS, the evaporation rate of soil was slightly reduced by biochar, as the addition of biochar was less than 4% and less than 6%,
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Fig. 11. Effect of biochar on the pore size distribution (PSD) of clayey soils: (a) PSD curves of PKE soil, (b) PSD curves of XS soil.
Fig. 12. Soil water characteristic curve of soils: (a) PKE soil, (b) XS soil.
respectively. With the combined effect of water evaporation rate and soil suction, the occurrence time of cracks was later. Moreover, biochar can improve the water hold capacity of the soil (Fischer et al., 2019), therefore inhibits the development of cracks. However, the evaporation rate of soil significantly increases, and the value of soil suction reduces when the dosage of biochar in PKE and XS exceeds 4% and 6%, respectively. The cracking condition of clayey soils can be satisfied in a relatively short time than the reference sample, which promotes the development of cracks. Meanwhile, in the range of 0.5%-10% biochar dosage, the crack ratio became smaller than the reference sample, due to the reduced shrinkage space occupied by the biochar particles.
connection between soil particles, it does not necessarily induce more cracks. Only 6% and 10% dosage of biochar in PKE and XS, respectively, is conducive to the development of cracks. Nevertheless, image processing analysis demonstrated that biochar reduces the crack ratio by occupying shrinkage space, and effectively inhibits the width of the crack. Moreover, biochar reduces the degree of cracking of the soil and keeps the soil relatively intact since the crack ratio, the number of soil mass and the fractal dimension were reduced. From the degree of soil cracking in environmental applications, 4%, and 6% biochar in PKE and XS, respectively, are the optimal dosage. Data availability
5. Conclusions
Some or all data, models, or code generated or used during the study are available from the corresponding author by request.
Soil water evaporation and desiccation cracking behavior can be affected by biochar. On one hand, biochar reduced the evaporation rate of both clayey soils by filling soil voids and storing water in voids, when the dosage of biochar in PKE and XS soils was less than 4% and 6%, respectively. On the other hand, a larger dosage of biochar accelerates the evaporation rate by forming connected water migration channels in soils. It is important to note that although biochar weakens the
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.
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Fig. 13. Dry shrinkage of biochar-treated clayey soils.
Acknowledgments
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