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Article

Chitosan-Modified Biochar for Improving Water Retention in Karst Quarries: A Potential Solution for Soil Remediation

by
Xiaohua Shu
1,
Shiqing Xiong
1,
Qiulei Wang
1,
Mingyu Yang
1 and
Qian Zhang
2,*
1
Collaborative Innovation Center for Water Pollution Control and Water Safety in Karst Area, Guilin University of Technology, Guilin 541006, China
2
School of Life and Environmental Science, Guilin University of Electronic Technology, Guilin 541000, China
*
Author to whom correspondence should be addressed.
Sustainability 2025, 17(11), 4815; https://doi.org/10.3390/su17114815
Submission received: 12 April 2025 / Revised: 16 May 2025 / Accepted: 22 May 2025 / Published: 23 May 2025
(This article belongs to the Topic The Challenges and Future Trends in Anthropogenic and Natural Pollution Control Engineering)
Browse Figures
Versions Notes

Abstract

:
Biochar has been widely applied in soil remediation. However, few studies have been conducted on its effect on soil water retention in abandoned quarries. Moreover, due to the poor water storage capacity of the quarry, the adhesion and water retention capacity of biochar are limited in its application. Here, we used sugarcane bagasse (SB) and chicken manure (CM) prepared at 300 °C and 500 °C, and modified them with chitosan (CS) to improve the water absorption, and further explored their effects on the soil water retention characteristics in karst, abandoned quarry. The results indicated that the modified biochar significantly improves the hydrophilicity and water absorption capacity of the biochar. The water absorption multiples of 300SBB-CS, 500SBB-CS, 300CMB-CS, and 500CMB-CS were 131.03, 94.47, 86.19, and 114.70 g·g−1. After being applied to the quarry soil, it significantly improved the water retention characteristics. In addition, the application of modified biochar significantly increased the mean weight diameter (MWD), geometric mean diameter (GMD), and cation exchange capacity (CEC) of soil aggregates. Compared with the control, GMD of 300SBB-CS, 500SBB-CS, 300CMB-CS, and 500CMB-CS increased by 24.42%, 32.74%, 8.34%, and 21.20%, respectively. The modified biochar improves the soil’s water retention characteristics by enhancing its water absorption capacity. In addition, the modified biochar improves the stability of soil aggregates by increasing the soil CEC, which indirectly enhances the water retention characteristics of the soil. These findings provide substantial reference information for improving soil conditions in karst regions.

1. Introduction

Karst landscapes provide valuable resources and serve as natural sinks for carbon dioxide, which may contribute to climate change mitigation [1]. Additionally, karst ecosystems are vital for sequestering carbon dioxide from the atmosphere through processes such as limestone dissolution, plant photosynthesis, and the accumulation of soil organic matter (SOM) [2]. Karst areas typically feature soils with low organic matter content and limited thickness [3]. This combination leads to severe soil erosion problems [4]. The unique topographical and geomorphological characteristics of karst determine its ecological vulnerability [5]. The projects of converting farmland back to forests and grasslands have been implemented to promote water resources protection, and the rocky desertification has been comprehensively managed [6]. These projects have improved the situation of soil and water loss, reduced the degree of rocky desertification, and shown overall beneficial effects [7]. However, the abandoned quarry soil is less able to hold water in karst areas. Therefore, it is necessary to improve the water retention and soil structure in quarries of karst areas. The stability of soil aggregates refers to the ability to resist external forces or environmental changes and maintain their original form. Thus, it is also of vital importance to explore the stability of soil aggregates. At present, ecological restoration measures such as afforestation, vegetation restoration, and soil improvement have been applied to prevent and control karst rocky desertification [8].
Biochar has been widely used in soil improvement due to its excellent physical and chemical properties [9]. For example, it can improve soil physical and chemical properties [10,11], reduce the content of heavy metals in soil [12], and enhance soil water-holding capacity [13,14,15]. Currently, research on enhancing soil water retention capacity mainly focuses on sandy loam, agricultural soil, and soil around mining areas. There are few studies on its impact on the soil water retention capacity of abandoned quarries. Moreover, the effect of biochar on the water retention performance of other soil studies may be different due to the uniqueness of soil in karst areas. Therefore, it is necessary to study the influence of biochar on the water retention characteristics of the abandoned quarry soil in karst areas.
The application of biochar is restricted by its limited adhesion and water retention capabilities, which can be attributed to the insufficient water-storage capacity of the quarry. To further improve a certain property of biochar, the modification of biochar has been widely explored. Aborisade et al. applied eggshell activated carbon loaded with nano-zerovalent iron and nanoscale zerovalent iron/activated carbon to soil contaminated by heavy metals, which fixed the heavy metals in the soil and alleviated the pressure caused by toxic metals on vegetable plants [16,17]. Chitosan can be used as a modification material for biochar due to its low cost, non-toxicity, good biodegradability, no secondary pollution, sustainability, and non-toxicity [18]. Moreover, it has a large number of amino (-NH2) groups and hydroxyl (-OH) groups [19] with high water absorption capacity [18]. At present, biochar and chitosan composites are mainly applied in the removal of pollutants in aqueous solutions, the reduction in heavy metals in soil, the lowering of soil toxicity, the promotion of crop growth, and the fixation of cellulase to improve recovery [20]. Additionally, chitosan, as any other biopolymer, can interact with soil components, including adsorbing polymer molecules on the surface of soil components, covering soil particles with thin polymer films to form polymer bonds connecting adjacent particles, through multivalent counterion adhesion, hydrogen bonds, or bridging of soil particles [21]. Therefore, chitosan is used to modify biochar to enhance its water absorption capacity and further verify the effect of the modified biochar on the water retention capacity of the soil in quarries. In addition, previous explorations of the mechanism by which biochar affects soil water retention mainly focused on aspects such as altering soil pores and soil particle size distribution [22]. The mechanism of biochar’s effect on soil water retention still requires further clarification.
Therefore, in this study, bagasse (SBB) and chicken manure (CMB) biochar pyrolyzed at different temperatures (300 °C, 500 °C) were taken as the research objects. They were graft-copolymerized with chitosan and starch. The effects of pyrolysis temperature and raw materials on the water retention characteristics of the modified materials were investigated. We assume that modified biochar can enhance the water retention capacity of the soil by increasing its water absorption capacity, and improve the cation exchange capacity in the soil to enhance the stability of soil aggregates, further strengthening the water retention capacity of the soil. Among the four modified biochar types, 500SBC-CS has a better effect. The major purposes of this study are presented below: (1) to explore the influence of modification on the contact angle and water retention capacity of biochar; (2) to explore the influence of modified biochar on the cation exchange capacity, aggregate stability, and TOC of the soil; and (3) to explore the influence of modified biochar on the soil water retention characteristics (cyclic water absorption capacity, cumulative soil evaporation, soil water characteristic curve). These findings offer significant reference data for enhancing soil conditions in karst regions.

2. Materials and Methods

2.1. Preparation and Modification of Biochar

In this study, chicken manure and bagasse were selected as raw materials for preparing biochar. The biochar is modified by following the previous method [19]. The preparation steps of biochar are as follows: Chicken manure and sugarcane residue were selected and sieved through 100 mesh. According to previous studies [23,24], biochar was put into a muffle furnace for charcoal burning by methods of oxygen restriction and temperature control. We heated them up to 300 °C and 500 °C at a heating rate of 10 °C and maintained them at the set temperatures for 2 h to obtain chicken manure and bagasse biochar at 300 °C and 500 °C. The modification of biochar is specifically as follows: Weigh 0.5 g of chitosan (CS), add it to 30 mL of 2% acetic acid solution, and heat it up to 70 °C in a magnetic stirrer. Then, add 0.08 g of MBA and 12 mL of acrylic acid solution. After reacting for 10 min, add 0.3 g of ammonium persulfate and 0.75 g of biochar. After reacting for 40 min, add 10% NaOH solution to adjust the pH to neutral. After cooling, add absolute ethanol and soak for 24 h to remove by-products. Then, dry and screen to obtain the modified biochar for standby. The synthesis diagram of modified biochar is shown in Figure 1 [19].

2.2. Soil Collection

The sample soil was collected from Zhuge Changzhi lime quarry in Yanshan District, Guilin City, at a depth of 0–10 cm. The collected soil was air-dried at room temperature, and then screened using a 2 mm standard sieve to remove debris, larger particles, and stones. The obtained soil was stored in a self-sealing bag for standby. The soil pH was measured using a soil:water ratio of 1:2.5 (w:v). The soil mechanical composition was determined using a Malvern laser particle size analyzer. The test results showed that the soil particle composition was composed of clay particles (0–2 μm), 2.3%; silt particles (2–20 μm), 12.71%; and sand particles (20–2000 μm), 77.44%. The soil aggregate stability was determined using the dry sieve method. The cation exchange capacity in the soil was determined by extracting with hexamminecobalt (III) chloride. The soil dissolved organic matter was extracted with soil:water ratio of 1:10 (w:v), and its fluorescence excitation-emission matrix (EEM) spectrum was measured.

2.3. Material Characterization

The functional groups on the surface of carbon-based materials were analyzed using Fourier transform infrared spectroscopy (FT-IR, Madison, WI, USA) in the frequency range of 500–4000−1. The specific surface area and pore volume of biochar were determined by using the BET specific surface area analyzer (JW-BK200C, Beijing, China). An organic elemental analyzer (EA, Elementar UNICUBE, Langenselbold, Germany) was used to determine the percentages of carbon (C), nitrogen (N), oxygen (O), and hydrogen (H) in the samples. The contact angle was measured using a static contact angle measuring instrument to determine the hydrophobicity of biochar (Hitachi F-700, Tokyo, Japan).

2.4. Experimental Design

2.4.1. Water Absorption Capacity Test

A total of 0.1 g of the original biochar (BC) and the modified biochar were, respectively, completely immersed in 100 mL of distilled water and soaked at room temperature for 12 h to ensure saturation. Subsequently, the saturated samples were filtered using a 100 mesh sieve to separate the saturated and swollen samples from the aqueous solution. Then, they were allowed to stand for 15 min to remove the excess water, and then the sieve and the swollen samples were weighed simultaneously. The water absorption multiple was determined by the weight difference in the samples before and after the equilibrium water absorption rate [25], and the experiment was repeated 3 times for calculation.
WA = (Ws − Wd)/Wd
where WA is the water absorption multiple per gram of dry biochar, Ws is the weight of the biochar after drying and being saturated with water, and Wd is the weight of the biochar after drying.

2.4.2. Cyclic Water Absorption Capacity Test

We mixed 1.0 g of dry biochar and modified biochar uniformly with 100 g of dry soil, and then put them into a ring knife lined with filter paper at the bottom to prevent the mixture from leaking. The soil sample that was not combined with biochar or carbon-based material was used as the control object (marked as control, CK). Then, we added distilled water to the sample, soaked it, and saturated it for 12 h. After reaching saturation, we dried the moist soil sample at 60 °C until a consistent weight was achieved. For the mixed soil sample, we saturated it again for 12 h before weighing the sample again. This process of swelling, drying, and soaking was repeated five times to determine the reversibility of the carbon-based material and its water reabsorption capacity. Each treatment was carried out in triplicate. Finally, calculate the cyclic water absorption capacity using Equation (1) [19,25].

2.4.3. Water Retention Capacity

Refer to the method to determine the soil water retention curve [26]. After weighing 0.05 g of biochar and modified biochar and mixing them with the soil, we added 1 mL of water dropwise. We weighed it every 1 h and calculated the soil water retention curve using the gravimetric method.

2.4.4. Soil Evaporation Experiment

The soil column used in the natural evaporation experiment is a transparent organic glass column with an inner diameter of 10 cm and a height of 15 cm. When filling the soil column, we calculated and weighed the required biochar and soil sample, respectively. We mixed them thoroughly, compacted them layer by layer, and filled them into the organic glass column in 2 layers (each layer was 5 cm thick). We wet each soil column from the bottom through capillary action (self-absorption method) until the surface of the soil column was wet. To ensure the saturation of the soil column, we measured the water content of the surface soil at that time, and then soaked it for another 12 h. At the same time, we covered the top of the soil column with plastic film to prevent evaporation loss. Then, we let the saturated soil column stand for 12 h to ensure the field water-holding capacity, and then sealed the bottom of the soil column with plastic film. Then, we carried out the evaporation experiment in the laboratory. We weighed the mass of the soil column with an electronic scale at 5 p.m. every day for 30 days. The temperature in the laboratory was measured by a thermometer to be 20 ± 5 °C. We calculated the cumulative evaporation (CE) according to the soil water balance method. The calculation formula is as follows:
CEi = W0 − Wi
where W0 is the initial weight of the soil column at the beginning of the experiment; Wi is the weight of the soil column on the i-th day.

2.4.5. Pot Experiment

We added 750 g of sandy soil (11 cm × 9 cm × 10 cm) to each flower pot and thoroughly mixed the bagasse and chicken manure biochar at 300 °C and 500 °C before and after modification into the sandy soil of the quarry at an addition amount of 2%. We planted one soybean plant in each pot. The greenhouse condition was a constant temperature of 25 °C. The experimental design included a total of 9 treatments, namely control (CK), 300SBC, 500SBC, 300SBC-CS, 500SBC-CS, 300CMB, 500CMB, 300CMB-C, and 500CMB-CS, with three replicate groups. The initial irrigation was set to 70 wt% of the soil and then we added a fixed amount of water to it every two days. After 30 days, we measured the plant height, respectively, air-dried the soil, and measured the soil aggregate stability, dissolved organic matter components, and physical and chemical properties.

3. Results and Discussion

3.1. Characterization and Analysis of Biochar

3.1.1. FT-IR Analysis of Biochar

The composition and structure of the modified materials obtained under different pyrolysis temperatures (300 °C and 500 °C) and different raw materials (bagasse and chicken manure) were studied by FT-IR to determine whether the modified materials were synthesized, and the results are shown in Figure 2. Among them, the characteristic peaks of chitosan are the stretching vibrations of O-H and -NH2 at 3100–3450 cm−1 [27] and C-O-C at 1070 cm−1 [28,29]. The absorption peak at 1050 cm−1 to 1100 cm−1 is attributed to the C-O-C stretching in cellulose and hemicellulose. The absorption peak observed at 1410 cm−1 belongs to the aromatic C=C, while the characteristic peak at 1590–1670 cm−1 is assigned to the aromatic carboxyl/carbonyl (C=O) or quinone group [30]. The absorption peak at 2800–3000 cm−1 is for the aliphatic –CH2 [31], and the characteristic peak at 3370–3400 cm−1 is attributed to –OH [30] (Figure 2). The characteristic peak of bagasse biochar at 1040 cm−1 is mainly the C-O bond, which disappears after modification, indicating that C-OH participated in the chemical reaction during the modification process [32].
The peak near 2920 cm−1 of the modified bagasse biochar may be related to the stretching of –CH2 in acrylic acid [33]. In addition, the unique peak of the modified material near 1600 cm−1 is mainly attributed to the stretching vibration of –COOH, indicating that AA is included in the grafted polymer [33], and a weak new characteristic peak appears near 1600 cm−1, indicating the successful synthesis of the material. The absorption bands of hydroxyl and carboxyl groups overlap in the range of 3400–3600 cm−1 and show broad peaks in the four modified materials (300SBB-CS, 500SBB-CS, 300CMB-CS, and 500CMB-CS). The appearance of the broad peaks in the modified materials further confirms that chitosan was successfully grafted onto the biochar through graft polymerization.

3.1.2. Contact Angle and Elemental Analysis of Biochar

The contact angle can be used to characterize the hydrophilicity of biochar. In this study, the contact angles of the biochar (300SBC, 500SBC, 300CMB, 500CMB) when interacting with water were investigated. The contact angle for 300SBC, 500SBC, 300CMB, and 500CMB were found to be 129.02°, 129.78°, 129.62°, and 70.39°, respectively, while the contact angles for 300SBB-CS, 500SBB-CS, 300CMB-CS, 500CMB-CS were 39.6°, 24.8°, 65.2°, and 50.6°, respectively (Figure 3). Previous studies used cotton straw biochar and chitosan modification to reduce the biochar contact angle by 53.39% [14]. In this paper, 300SBB-CS and 500SBB-CS reduced the biochar contact angle by 69.31% and 80.89%. This is due to the difference in raw materials. In conclusion, the modified biochar has a higher hydrophilicity than the unmodified biochar.
Additionally, it can be seen that bagasse biochar is hydrophobic, while chicken manure biochar at 500 °C is hydrophilic. This may be related to the original materials of biochar. The lignin content in sugarcane bagasse is relatively high and it has strong hydrophobicity [29]. However, chicken manure biochar contains more alkali and alkali metals (K, Ca, Na, Mg), which can promote the formation of oxygen-containing surface functional groups on the surface of biochar [34], making it more hydrophilic at high temperatures. After modification, the more hydrophilic nature of bagasse biochar may be related to the functional groups of the biochar.
The percentages of carbon (C), nitrogen (N), oxygen (O), and hydrogen (H) in biochar were determined by an organic element analyzer to determine the changes in the elemental composition of biochar. After modification with chitosan, the contents of hydrogen (H) and oxygen (O) in both SBB and CMB pyrolyzed at 300 °C and 500 °C increased (Table 1), indicating the successful modification of the biochar. The O/C of 300SBB-CS, 500SBB-CS, 300CMB-CS, and 500CMB-CS were 2.57 times, 4.1 times, 1.43 times, and 2.18 times, respectively, those of 300SBB, 500SBB, 300CMB, and 500CMB. Moreover, the H/C and O + N/C (hydrophilic sites) ratios were also superior to those of the unmodified biochar (Table 1). This indicates that compared with the unmodified biochar, the modified materials possess more oxygen-containing and hydrophilic functional groups, thus significantly improving the hydrophilicity and polarity of the modified materials [19,35]. In addition, modified biochar reduces the ash content of biochar, which can, to a certain extent, alleviate pore clogging and alter porosity [36], thereby further affecting the water retention capacity of biochar.

3.1.3. The Specific Surface Area and Porosity of Biochar

The specific surface area and pore size of biochar can be evaluated using a specific surface area analyzer. The variations in pore size and porosity of bagasse and chicken manure with pyrolysis temperature align with findings from previous studies [37,38]. Following modification, the BET specific surface areas of SBB-CS and CMB-CS were significantly reduced, while the average pore size and porosity volume of both increased (see Table 2). The pore volume of the modified biochar increased, and the specific surface area decreased primarily due to the small, microporous nature of unmodified biochar, compared to the modified biochar where chitosan adhered to its pore structure [20], resulting in a reduction in specific surface area. However, this phenomenon was not observed in the 500CMB sample, which may be attributed to the original chemical composition of the biomass. This composition could influence the thermal decomposition pathway during the pyrolysis process and subsequently affect changes in porosity [39]. In conclusion, the increased porosity and pore size facilitate water storage within the pores, thereby enhancing water retention capacity [40].

3.1.4. Cation Exchange Capacity (CEC) of Biochar

After modification with chitosan, the cation exchange capacity of biochar has been significantly improved. As illustrated in Figure S1, the cation exchange capacities of 300SBB, 500SBB, 300CMB, and 500CMB are 3.08 cmol+/kg, 4.31 cmol+/kg, 2.18 cmol+/kg, and 4.48 cmol+/kg, respectively. After modification, the cation exchange capacities of 300SBB, 500SBB-CS are 89.86 cmol+/kg, 103.54 cmol+/kg. Chitosan has many hydrophilic groups (-NH2, -OH), and chemical modification that changes the semi-crystalline structure of chitosan by introducing hydrophilic cationic groups will result in a membrane with a high ion exchange capacity [41].
Moreover, the nitrogen and oxygen atoms in the functional groups have lone pairs of electrons, which can undergo coordination or electrostatic attraction with cations. As a result, cations are adsorbed onto the chitosan molecules, enhancing the adsorption capacity for cations and thus increasing the cation exchange capacity. In addition, the modified biochar has a highly interconnected and visible pore structure [19]. The pores and channels provide more space and pathways for the diffusion and exchange of cations, enabling cations to more smoothly penetrate into the interior of chitosan and interact with the active groups for exchange, thus increasing the cation exchange capacity.

3.1.5. Water Absorption Capacity of Biochar

Figure S2a shows the influence of the aqueous solution on the water absorption capacity of the biochar before and after modification. Compared with the unmodified biochar, the modified biochar significantly improves the water absorption capacity of the biochar. The water absorption multiples of 300SBC-CS, 500SBC-CS, 300CMB-CS, and 500CMB-CS are 131.03, 94.47, 86.19, and 114.70 g·g−1, respectively. Previous studies have shown that there is a significant positive correlation between the water-holding capacity of biochar and its hydrogen content, oxygen content, H/C ratio, and (O + N)/C ratio [42]. The hydrogen content, oxygen content, H/C ratio, and (O + N)/C ratio of the modified biochar all increase, which may account for the remarkable enhancement of the water absorption capacity of the modified biochar.
The cyclic water absorption capacity of the modified materials in the soil has profound significance for their practical application. Figure S2b shows the cyclic water absorption capacities of 300SBC, 500SBC, 300CMB, 500CMB, and the carbon-based materials for five cycles. The modified biochar materials can maintain five swelling–drying water absorption cycles in sandy soil, indicating that the modified biochar of bagasse and chicken manure can be recycled in such soil. After each drying cycle, the sandy soil improved with the modified materials (SBB-CS, CMB-CS) shows the ability to continuously absorb and retain water. It can be seen from Figure S2b that there is significant difference in the cyclic water absorption capacity between unmodified biochar and modified biochar, and water retention capacity of modified biochar is still strong after five cycles. After the fifth cycle, the water retention capacities of 300SBC, 500SBC, 300CMB, and 500CMB remain at 42.05%, 47.16%, 32.59%, and 37%, respectively. After modification, the water absorption capacities of the biochar are increased by 23.12%, 28.23%, 13.66%, and 18.07%, respectively, which is similar to the previous results [19]. This indicates that the modified biochar can be recycled, which is mainly attributed to the strong water absorption capacity of the modified biochar. The reusability of modified biochar indicates that biochar can retain nutrients in the soil [43]. Over time, biochar slowly releases nutrients, providing a continuous supply of nutrients for plants. This is mainly because chitosan has anti-water solubility and is a promising coating material for soluble fertilizers, which can slow down the release rate and improve nutrient efficiency [44]. This circular water absorption capacity enables plants to make the most of fertilizers and nutrients, and to a certain extent, it can reduce environmental pollution caused by the use of chemical fertilizers.

3.2. Influence of Biochar on Soil Water Retention

3.2.1. Influence of Biochar on Soil Evaporation

As can be seen from Figure 4, this study explores the influence of modified biochar on the short-term changes in soil water loss rate in quarry soil. Compared with the control group and unmodified biochar, the modified biochar can significantly slow down the evaporation of soil moisture within the first 10 h, indicating that both the modified bagasse biochar and chicken manure biochar can slow down the evaporation rate of soil moisture. The addition of the original biochar will darken the color of the soil, and the surface temperature of the soil is relatively high [45]. The viscosity and surface tension of water are reduced, which in turn promotes the evaporation of soil moisture, resulting in a relatively rapid loss of soil moisture. Studies have shown that biochar with a high carbon content indicates high hydrophobicity on the surface of the biochar [46], which increases the repulsive force between the biochar surface and H2O molecules, ultimately facilitating the evaporation of the water bound to the biochar. In addition, the higher hydrophobicity of the biochar surface will increase the repulsive force between the biochar surface and water molecules, ultimately leading to the evaporation of the water bound to the biochar [42]. In this paper, the hydrophobicity of the modified biochar is reduced, which decreases the repulsive force between the surface of the modified biochar and water molecules, thereby slowing down the evaporation of water. In addition, modified biochar increases the porosity of the soil and enhances the water retention capacity of biochar, thereby slowing down evaporation.

3.2.2. Influence of Biochar on Soil Cumulative Evaporation

To further understand the changes in soil moisture, the influence of biochar on the cumulative evaporation of soil in quarries was explored. As can be seen from Figure S3, SBB-CS reduces the cumulative evaporation of soil moisture, while 500CMB-CS increases the cumulative evaporation of soil moisture. The unmodified biochar is dark black, while the modified biochar is grayish black. Adding the unmodified biochar to the soil darkens the soil color, and the surface temperature of the soil is higher. The viscosity and surface tension of water decrease, thus promoting the evaporation of soil moisture [45]. This may be the reason why the modified bagasse biochar reduces the evaporation of soil moisture. The CMB-CS may increase soil cumulative evaporation due to the significantly enhanced water absorption capacity of the modified biochar. In addition, the CMB-CS increases soil porosity and promotes the migration of water in the soil [47], resulting in the CMB-CS enhancing soil cumulative evaporation. The application of synthetic composite hydrogel in sandy soil reported by Zhou et al. tended to stabilize the evaporation on the eighth day [48], while the modified biochar delayed soil evaporation for a longer time.
In addition, after the biochar enters the soil, it fills the gaps between soil aggregates, and water is stored inside the biochar particles [49], resulting in a denser distribution of soil particles and less water evaporation. The reason why 500CMB-CS promotes soil evaporation may be that after it is saturated with water, when the soil absorbs and swells, the pores of the soil are filled, causing the soil to expand excessively, resulting in cracks between the soil particles, exposing them to the air. The cracks become evaporation surfaces, thus promoting evaporation [50]. The above results show that due to different sources, biochar has different influences on the evaporation of soil moisture in quarries, and the modified bagasse has an inhibitory effect on the cumulative evaporation of soil.

3.2.3. Influence of Biochar on Soil Water Characteristic Curve

As shown in Figure 5, the addition of biochar increased the soil water content under the same suction force, that is, the increase in the content of biochar in the soil improved the water-holding characteristics of the soil. This result is consistent with previous study [51]. Among them, the effect of the SBB-CS is better than that of SBB. The order is 500SBB-CS > 300SBB-CS > 500SBB > 300SBB > CK. While for the soil water characteristic curve after the addition of CMB, the order is 300CMB-CS > 500CMB > 300CMB > 500CMB-CS > CK.
The soil water characteristic parameters obtained by different modified biochar are shown in Table 3. Compared with CK, the original biochar and modified biochar significantly increased the FC content of soil, among which 500SBB-CS had the highest FC (31.5%). AWC of soil is the content of water available to plants, which can well reflect the water-holding performance of soil [52], and the addition of biochar increases the AWC of soil. Compared with CK treatment, soil PWP was almost significantly increased after carbon application.
The RETC software (v6.02) was used to fit the water characteristic curve, and the characteristic parameters of soil moisture are shown in Table S1. After adding biochar, the soil showed higher θs and θr. θr reflects the water repellency of the soil. As θr decreases, the water repellency of the soil increases. After adding biochar, the θr of all treatment groups were higher than that of the CK group. Among them, the 500SBB-CS treatment was superior to other treatments and had the best effect, with a θr value of 0.29. For θs, adding unmodified bagasse biochar had no effect on its value, while the treatment groups after adding modified bagasse biochar were all higher than the CK group and the control group. This result is consistent with previous study [51]. An Thuy Ngo et al. also enhanced the volumetric moisture content of the soil by using cellulose nanofibers [53]. After adding chicken manure biochar, almost all treatment groups were higher than the CK. The above results indicate that adding biochar can promote the soil water retention effect.

3.3. Influence of Biochar on Soil Physical and Chemical Properties

3.3.1. Influence of Biochar on Plant Height

Figure S4 shows the influence of four modified materials on the plant height after sowing in sandy soil. As can be seen in the figure, biochar and modified biochar significantly enhance the plant height of soybeans, because the addition of biochar can enhance the nutrients in the soil. Arooj Bashir et al. [54] used biochar and farmyard manure to explore the growth of plants under different moisture conditions and found that it could increase the plant height under both sufficient moisture and drought. The differences in plant height may be attributed to the water absorption capacity of the modified materials. The modified biochar enhances the water-holding capacity of the sandy soil, enabling the soil to retain more water and release it gradually. In addition, and the carbon content of the modified biochar increases, the addition of biochar and modified biochar provides more carbon sources to the soil, thus promoting plant growth, which is consistent with the results of previous study [55].

3.3.2. Influence of Biochar on Soil Aggregate Stability and Cation Exchange Capacity

Soil aggregate stability is one of the main physical indicators of plant health, as it affects soil aeration, water retention, erosion, and nutrient cycling [53]. The stability of soil aggregates is commonly reflected by two indicators, namely the mean weight diameter (MWD) and the geometric mean diameter (GMD). The results of this study show that the application of the unmodified biochar significantly reduced the soil GMD and MWD, while the application of the modified biochar significantly increased the MWD and GMD of the soil aggregates (Figure 6). Compared with CK, the GMDs of 300SBB-CS, 500SBB-CS, 300CMB-CS, and 500CMB-CS increased by 24.42%, 32.74%, 8.34%, and 21.20%, and the MWDs increased by 5.32%, 6.81%, 3.05%, and 8.39%, respectively. The reason might be that chitosan may interact with soil components, including adsorption, multivalent counterion adhesion, hydrogen bonding, or bridging of soil particles, to form polymer bonds connecting adjacent particles [21]. This is consistent with the previous research results [56]. This indicates that adding modified biochar has a positive effect on the soil aggregates in quarry soil.
Previous studies have shown that the application of biochar in soil enhances soil hydraulic properties but does not improve the stability of soil aggregates [13]. However, the application of modified biochar in soil enhances the stability of soil aggregates. This is mainly because modified biochar improves the water-holding capacity of biochar and its application alters the pore distribution of soil [57], thereby improving soil water retention characteristics.
The modified biochar has different trends in the influence on the distribution of soil aggregates in different aggregate fractions (Figure S5a–e). For all aggregate fractions, the aggregates of 0.25–1 mm are dominant, fluctuating around 40%. The aggregates of 0.53−0.25 mm account for the second largest fraction, ranging from 20% to 25%, followed by the aggregates of 1–2 mm (15–20%). The content of macroaggregates (>2 mm) in the soil with the addition of modified biochar increased significantly, and the content of microaggregates (0.053–0.25 mm) decreased significantly. The reason may be that the biochar particles fill the soil pores, and the soil forms aggregates through the surface charge and electrostatic adsorption of the biochar, and further forms large aggregates, resulting in a decrease in the content of microaggregates [58]. The addition of modified biochar may increase the soil organic matter content and enhance the stability of soil aggregates. Moreover, the surface of biochar contains many oxygen-containing functional groups (−OH, -COOH, -C=O). These oxygen-containing functional groups readily undergo complexation reactions with mineral ions like Ca2+ and Fe3+ in the soil to form organic–mineral complexes, which promotes the formation of large aggregates (>2 mm) [59].
CEC is an important indicator characterizing soil fertility and soil environmental quality. Conducting research on soil CEC to grasp the soil fertility level has significant scientific value and guiding significance for improving the soil’s water retention capacity. A higher CEC in the soil indicates that the soil can better retain nutrients and water, thus promoting the formation and stability of aggregates. Compared with CK and unmodified biochar, all the modified biochar significantly increases the content of cation exchange capacity in the soil. The CECs in the soils of 300SBB-CS, 500SBB-CS, 300CMB-CS, and 500CMB-CS are 7.83, 8.96, 9.26, and 8.26 cmol+/kg, respectively (Figure S5f). This is attributed to the high cation exchange capacity of the modified biochar (Figure S1). In addition, the modified biochar has a higher oxygen content (including carbonyl groups, carboxyl groups, and phenolic groups) due to the combination of carboxyl functional groups, and the carboxyl functional groups contribute most of the CEC among the acidic functional groups [60]. CEC in the soil can enhance the stability of soil aggregates through complexation, precipitation, and electrostatic attraction with metal ions in the soil [61]. To a certain extent, the stability of soil aggregates can resist external forces and is less likely to damage the soil structure, thereby enhancing the water retention characteristics of the soil.

3.3.3. Influence of Biochar on Soil Dissolved Organic Matter

Biochar has different effects on soil dissolved organic matter (Figure 7a). It can be seen that biochar and modified biochar significantly increased the total organic carbon (TOC) concentration in the soil. Among them, the TOC concentrations of 300SBB and 500CMB increased significantly after modification. This result may be related to the water absorption capacity of biochar. The water absorption capacities of 300SBB-CS and 500CMB-CS are slightly stronger. After the addition of biochar, the soil structure is improved and the infiltration of soil moisture is enhanced, which may lead to an increase in the dissolution of organic carbon [62]. Some studies have shown that the relatively high pH value of biochar can also promote the release of dissolved organic matter (DOM) and increase the TOC content [63,64].
To further explore the components of soil dissolved organic matter, the fluorescence spectra of DOM in quarry soil were divided into five regions, namely I, II, III, IV, and V, through the fluorescence regional integration (FRI) method (Figure 7b). Regions I and II represent aromatic proteins produced through microbial transformation [65,66]. Regions III and V are considered to contain fulvic-like and humic-like substances affected by human activities [65,67], and Region IV is related to soluble microbial by-product-like substances [66,68]. The regional integration analysis shows that all soil DOM is mainly composed of fulvic-like substances, with a relative abundance ranging from 29% to 46%. The relative abundance of the fluorescence intensity in Region V (humic substances) is between 15% and 25%. Except for the CK group, the fluorescence intensity of aromatic proteins varies from 9% to 33%, while the relative abundances of the remaining regions are relatively small. It can be seen that there are changes to varying degrees after the addition of biochar, among which the aromatic proteins increase significantly, while the content of soluble microbial by-product-like substances decreases.

3.4. Correlation Analysis Between Soil Physical and Chemical Properties and Water Retention Characteristics

To further explore the relationships among the proportion of soil aggregates and each particle size component, soil cation exchange capacity, the fluorescence components of dissolved organic matter in the soil, soil field capacity, plant available water, and wilting coefficient, a correlation analysis was carried out. As shown in Figure 8, there is an extremely significant positive correlation between the cation exchange capacity and the macroaggregates (>2 mm), meso-aggregates (0.25–1 mm), geometric mean diameter (GMD), mean weight diameter (MWD), and field capacity (FC) in the soil (p < 0.01). There is a significant positive correlation (p < 0.05) with the permanent wilting point (PWP), and an extremely significant negative correlation (p < 0.01) with the microaggregates (<0.25 mm). The polyvalent cations calcium (Ca2+), magnesium (Mg2+), and monovalent cations (Na+) in the soil can act as bridges to connect the negatively charged minerals, keeping them electrically neutral, thus maintaining the stability of soil complexes and further promoting the stability of aggregates [69]. A high cation exchange capacity (CEC) can provide more charges. These charges can generate electrostatic attraction with water, thereby enhancing the water absorption capacity of biochar. Additionally, biochar with a high CEC can offer more oxygen-containing functional groups, which can form hydrogen bonds with water molecules, improving the water adsorption capacity [70] and further enhancing the field capacity.
There is an extremely significant positive correlation (p < 0.01) between MWD and GMD, as well as between MWD and the large aggregates (>2 mm), medium aggregates (1–2 mm), and small aggregates (0.25–1 mm) in the soil. There is an extremely significant negative correlation (p < 0.01) between MWD and microaggregates (<0.25 mm). Soil aggregates have the ability to resist external force damage and affect the soil pore structure, water infiltration, and retention. The aggregate structure will seriously affect the stability of the aggregates. Large aggregates have large internal voids and good air permeability, and they can better withstand changes in external forces. In contrast, microaggregates have poor stability, a large specific surface area, high surface energy, and are easily dispersed by external forces, thus reducing the stability of soil aggregates. In addition, there is a significant positive correlation (p < 0.05) between MWD and FC. Aggregates with high stability form a good pore structure, which can increase the soil water-holding capacity. Moreover, after modification, the water absorption capacity of biochar is significantly enhanced. When biochar is mixed with the soil through the void filling effect, changed pore space, it also enhances the soil’s water absorption capacity, thereby increasing the FC of the soil. There is an extremely significant positive correlation (p < 0.01) between GMD and PWP, probably because the application of biochar fills the large voids between coarse particles, resulting in the generation of more small pores, making some of the water in these pores unavailable [71].
There is a significant negative correlation between FC and the proportion of Region V. This indicates that the lower the content of soluble microbial by-product-like substances, the higher the FC, which may be related to the microorganisms in the soil. Soluble microbial by-product-like substances can serve as a substrate for soil microorganisms, promoting microbial activity. Microorganisms attached to the outer layer of the adsorbent matrix can form biofilms, binding larger aggregates together and promoting aggregate stability. However, in this study, no correlation was observed between DOM components and aggregate stability. There is a significant negative correlation (p < 0.05) between GMD, the structure of 1–2 mm aggregates and the proportion in region II (aromatic proteins), indicating that the more aromatic protein substances there are in the soil, the lower the stability of the aggregates and the lower the content of 1–2 mm aggregates will be. Previous studies have shown that the addition of DOM reduces the decomposition of large aggregates and increases the formation of large aggregates, and the cultivation time of the added types of DOM and their interactions do not significantly affect aggregate stability [72]. However, this phenomenon was not observed in this study, which may be related to the soil type and soil disturbance.
Combining the previous water retention indicators and the physical and chemical property indicators of the soil, the application of all modified materials has improved the physical and chemical properties and water retention characteristics of the soil. The effect on 500SBC-CS in the karst quarry is the best. 500CMB-CS has a higher CEC content and may achieve better results in karst soil with a lower CEC content, while 30SBB-CS has the best water absorption capacity, and CMB-CS may be more suitable for karst soil lacking nutrients [73].

4. Conclusions

This study explored the effects of modification with chitosan on the water retention characteristics of two types of biochar (SBB, CMB) at two different temperatures (300 °C, 500 °C). Furthermore, it investigated the impacts of the modified biochar on the water retention characteristics as well as the physical and chemical properties of the soil from quarries. The results showed that the modified biochar enhanced the hydrophilicity of the biochar and improved its water absorption capacity. The water absorption multiples of 300SBC-CS, 500SBC-CS, 300CMB-CS, and 500CMB-CS were 131.03, 94.47, 86.19, and 114.70 g·g−1, respectively. In addition, after the application of the modified biochar, the water retention capacity of the soil was improved and evaporation was reduced. The modified biochar enhanced the cyclic water absorption capacity of the soil in quarries, and the modified sugarcane bagasse biochar reduced the cumulative evaporation of soil moisture. The application of the modified biochar significantly increased the mean weight diameter (MWD), geometric mean diameter (GMD), cation exchange capacity (CEC), and total organic carbon (TOC) concentration of the soil, improved the soil quality, and further enhanced the soil’s water retention capacity. The above results indicate that the modified biochar has great potential in enhancing the water retention characteristics of quarry soil, improves the soil quality, and facilitates restoration work. In subsequent research, it can be applied to soil improvement in arid and semi-arid regions, as well as the reconstruction of abandoned tailings ponds and scenarios requiring ecological restoration such as rocky desertification areas. In future research, the water retention properties of modified biochar should be further verified through long-term continuous field trials. Additionally, the performance of biochar can be enhanced through more raw material options and the modification of slow-release coating materials for application in specific remediation scenarios.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/su17114815/s1. Figure S1: Cation exchange capacity of biochar before and after modification; Figure S2: Water absorption and circulating water absorption capacity of biochar before and after modification. (a) Water absorption; (b) cycle water absorption capacity; Figure S3: Cumulative evaporation of soil; Figure S4: Effect of biochar addition on plant height; Figure S5: Effects of biochar on soil aggregate stability and cation exchange capacity; Table S1: Water parameters fitted by RETC.

Author Contributions

Conceptualization, funding acquisition, project administration, writing—original draft, supervision, X.S.; data curation, formal analysis, writing—original draft, investigation, S.X.; writing—review and editing, Q.W.; writing—review and editing, M.Y.; conceptualization, supervision, writing—review and editing, Q.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by the Guangxi Key Research and Development Program (GuiKeAB21220049), the National Natural Science Foundation of China (No. 42477432).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Synthesis diagram of modified biochar.
Figure 1. Synthesis diagram of modified biochar.
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Figure 2. (a) FT-IR of sugarcane bagasse biochar (SBB) and their modified biochars at 300 and 500 °C; (b) FT-IR of chicken manure biochar (CMB), and their modified biochars at 300 and 500 °C.
Figure 2. (a) FT-IR of sugarcane bagasse biochar (SBB) and their modified biochars at 300 and 500 °C; (b) FT-IR of chicken manure biochar (CMB), and their modified biochars at 300 and 500 °C.
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Figure 3. (a) Contact angle of 300SBB; (b) Contact angle of 500SBB; (c) Contact angle of 300CMB; (d) Contact angle of 500CMB; (e) Contact angle of 300SBB-CS; (f) Contact angle of 500SBB-CS; (g) Contact angle of 300CMB-CS; (h) Contact angle of 500CMB-CS.
Figure 3. (a) Contact angle of 300SBB; (b) Contact angle of 500SBB; (c) Contact angle of 300CMB; (d) Contact angle of 500CMB; (e) Contact angle of 300SBB-CS; (f) Contact angle of 500SBB-CS; (g) Contact angle of 300CMB-CS; (h) Contact angle of 500CMB-CS.
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Figure 4. (a) Water retention curve of SBB and their modified biochars at 300 and 500 °C; (b) Water retention curve of CMB and their modified biochars at 300 and 500 °C.
Figure 4. (a) Water retention curve of SBB and their modified biochars at 300 and 500 °C; (b) Water retention curve of CMB and their modified biochars at 300 and 500 °C.
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Figure 5. (a) Soil water characteristic curve of SBB and their modified biochars at 300 and 500 °C; (b) Soil water characteristic curve of CMB and their modified biochars at 300 and 500 °C.
Figure 5. (a) Soil water characteristic curve of SBB and their modified biochars at 300 and 500 °C; (b) Soil water characteristic curve of CMB and their modified biochars at 300 and 500 °C.
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Figure 6. (a) MWD of the unmodified biochar and the modified biochar; (b) GMD of the unmodified biochar and the modified biochar. Different lowercase letters indicate significant differences under the same treatment.
Figure 6. (a) MWD of the unmodified biochar and the modified biochar; (b) GMD of the unmodified biochar and the modified biochar. Different lowercase letters indicate significant differences under the same treatment.
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Figure 7. (a) Effect of biochar on dissolved organic matter in soil. (b) Influence of biochar on the relative abundance of soil DOM fluorescence. Different lowercase letters indicate significant differences under the same treatment.
Figure 7. (a) Effect of biochar on dissolved organic matter in soil. (b) Influence of biochar on the relative abundance of soil DOM fluorescence. Different lowercase letters indicate significant differences under the same treatment.
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Figure 8. Correlation analysis.
Figure 8. Correlation analysis.
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Table 1. Elemental composition of the unmodified biochar and the modified biochar.
Table 1. Elemental composition of the unmodified biochar and the modified biochar.
Elemental CompositionC (%)H (%)O%H/CAsh (%)O/C(O + N)/C
Biochar
300SBB44.823.6914.750.0833.710.330.37
500SBB43.981.748.960.0443.140.200.24
300SBB-CS41.104.8935.090.1216.960.850.87
500SBB-CS40.415.2933.280.1319.080.820.84
300CMB38.793.2922.620.0831.030.580.66
500CMB34.911.0913.380.0347.510.380.44
300CMB-CS44.825.3937.240.1210.260.830.85
500CMB-CS41.705.4534.660.1316.060.830.85
Table 2. The specific surface area and porosity of biochar.
Table 2. The specific surface area and porosity of biochar.
BET (m2/g)Porosity Volume (m3/g)Pore Size (nm)
300SBB74.3810.1431.881
500SBB14.1000.1826.356
300SBB-CS3.5690.20010.781
500SBB-CS4.4760.25411.549
300CMB47.1480.1353.169
500CMB5.2540.03522.446
300CMB-CS3.5880.2508.452
500CMB-CS3.2590.2838.872
Table 3. Soil field capacity (FC), permanent wilting point (PWP), and plant available water content (AWC).
Table 3. Soil field capacity (FC), permanent wilting point (PWP), and plant available water content (AWC).
FCAWCPWP
CK0.208 ± 0.013 d0.011 ± 0.006 b0.197 ± 0.009 d
300SBB0.259 ± 0.006 bc0.039 ± 0.005 a0.219 ± 0.003 cd
500SBB0.263 ± 0.002 bc0.022 ± 0.012 ab0.240 ± 0.010 bc
300CMB0.243 ± 0.002 c0.018 ± 0.001 ab0.225 ± 0.001 c
500CMB0.268 ± 0.006 b0.034 ± 0.006 a0.233 ± 0.001 bc
300SBB-CS0.272 ± 0.007 b0.029 ± 0.024 ab0.252 ± 0.018 b
500SBB-CS0.315 ± 0.015 a0.025 ± 0.014 ab0.290 ± 0.028 a
300CMB-CS0.259 ± 0.006 bc0.019 ± 0.007 ab0.240 ± 0.007 bc
500CMB-CS0.256 ± 0.003 bc0.019 ± 0.003 ab0.237 ± 0.001 bc
Different lowercase letters indicate significant differences under the same treatment.
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Shu, X.; Xiong, S.; Wang, Q.; Yang, M.; Zhang, Q. Chitosan-Modified Biochar for Improving Water Retention in Karst Quarries: A Potential Solution for Soil Remediation. Sustainability 2025, 17, 4815. https://doi.org/10.3390/su17114815

AMA Style

Shu X, Xiong S, Wang Q, Yang M, Zhang Q. Chitosan-Modified Biochar for Improving Water Retention in Karst Quarries: A Potential Solution for Soil Remediation. Sustainability. 2025; 17(11):4815. https://doi.org/10.3390/su17114815

Chicago/Turabian Style

Shu, Xiaohua, Shiqing Xiong, Qiulei Wang, Mingyu Yang, and Qian Zhang. 2025. "Chitosan-Modified Biochar for Improving Water Retention in Karst Quarries: A Potential Solution for Soil Remediation" Sustainability 17, no. 11: 4815. https://doi.org/10.3390/su17114815

APA Style

Shu, X., Xiong, S., Wang, Q., Yang, M., & Zhang, Q. (2025). Chitosan-Modified Biochar for Improving Water Retention in Karst Quarries: A Potential Solution for Soil Remediation. Sustainability, 17(11), 4815. https://doi.org/10.3390/su17114815

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