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Article

Assessment of the Characters of a Novel Phosphoric Acid and Mineral-Comodified Biochar Composite and Its Potential Application in Saline–Alkali Soil Improvement

by
Hao Dai
1,2,
Zhuangzhuang Liu
2,
Jinping Yu
2,
Xiaoming Teng
3,
Lei Liu
3,
Mingyun Jia
2,* and
Jianhui Xue
1,2,*
1
College of Ecology and Environment, Nanjing Forestry University, Nanjing 210037, China
2
Jiangsu Key Laboratory for the Research and Utilization of Plant Resources, Institute of Botany, Jiangsu Province and Chinese Academy of Sciences (Nanjing Botanical Garden Mem. Sun Yat-Sen), Nanjing 210014, China
3
Lianyungang Golden Coast Development and Construction Co., Ltd., Lianyungang 222042, China
*
Authors to whom correspondence should be addressed.
Agriculture 2025, 15(7), 785; https://doi.org/10.3390/agriculture15070785
Submission received: 12 March 2025 / Revised: 3 April 2025 / Accepted: 4 April 2025 / Published: 5 April 2025
(This article belongs to the Section Agricultural Soils)
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Abstract

:
Amending saline–alkali soils to improve agricultural productivity is critical for addressing global food security challenges. Biochar is a promising soil amendment, and its modified composites offer significant potential for soil remediation. In this study, we developed a novel phosphoric acid–mineral-comodified biochar composite for saline–alkali soil improvement. SEM and XRD analyses indicate that chemical interactions between phosphoric acid, minerals, and biochar result in the formation of distinct mineral phases on the composite surface. Furthermore, FTIR analysis reveals that these interactions give rise to functional groups such as Si-O-Si, and thermogravimetric analysis demonstrates that the modified biochar composite exhibited enhanced stability. Compared with raw biochar, the modified biochar composites exhibited significant decreases in pH, EC, and base cation content (especially Na+), with maximum reductions of 7.26 pH units, 639.5 μS/cm, and 3.69 g/kg, respectively. In contrast, the contents of P, Si, and Ca increased significantly, with maximum increases of 140.04 g/kg, 90.32 g/kg, and 114.27 g/kg, respectively. In addition, the specific surface area and pore volume of the modified biochar composite increased by up to 5.2 and 15 times, respectively. Principal component analysis indicates that mineral type was the primary factor influencing the properties of the composites: hydroxyapatite enhanced porosity and phosphorus levels, whereas kaolinite and montmorillonite increased silicon content. Pot experiments show that the modified biochar composite increased alfalfa plant height by 17.36–20.27% and shoot biomass by 107.32–125.80% in saline–alkali soils. Overall, the newly developed phosphoric acid–mineral–biochar composites were evaluated to have high application potential for saline–alkali soil amendment.

1. Introduction

Saline–alkali soils cover approximately 8.31 × 109 hectares globally and are expanding at an alarming rate of 10% annually, posing a threat to global food security [1,2]. High salinity, alkalinity, and nutrient and organic matter deficiencies restrict crop productivity and quality in saline–alkali soils [3,4,5]. In the context of global climate change and insufficient arable land resources, the rational development and utilization of saline–alkali soil are highly important [6]. Traditional methods, such as salt leaching and topsoil covering, are costly and inefficient [7]. Therefore, novel approaches for the remediation and utilization of saline–alkali soils are urgently needed [8,9].
Biochar—a carbon-rich material derived from organic biomass via pyrolysis—has garnered increasing attention due to its good application potential in wastewater treatment, catalysis, soil remediation, and carbon sequestration [10,11]. Considering its porous structure and abundant surface functional groups, biochar is a promising material for saline–alkali soil amendment [3,12,13]. However, most biochars are alkaline and contain large amounts of base cations, which might limit their application for saline–alkali soil improvement [14,15]. Furthermore, negative effects of direct biochar application (e.g., reduced plant biomass and increased soil salinity and pH) on plant growth in saline–alkali soils have been reported [14,15]. Thus, biochar modification is a beneficial and sometimes necessary step when biochar is used as a saline–alkali soil amendment.
High pH, high soluble salt content, and nutrient deficiencies are the main factors limiting the productivity of saline–alkali soil [3,9]. In recent years, researchers have used various methods to modify biochar for soil remediation, for example, soaking the biochar in metal solutions and grinding the biochar in ball mills [16,17]. However, these studies have mainly focused on increasing the biochar’s surface area, number of pores, and number of oxygen-containing functional groups [18,19,20], while the pH, EC, and nutrient contents of biochar have commonly been overlooked. Consequently, it is unclear whether modified biochars can be used to achieve high amendment efficiency in saline–alkali soils.
Recent studies have shown that minerals can increase the stability of biochar and its soil carbon sequestration-promoting effect [21,22]. In addition, combining biochar with minerals enhances the adsorption capacity of NaCl onto biochar, which may help to alleviate Na+ toxicity to plants in saline–alkali soil [23,24]. However, biochar modification with a single mineral has shown little effect on the pH of biochar [25]. Recent studies have shown that phosphoric acid modification of biochar can significantly reduce the pH of biochar and that the application of this modified biochar to soil results in a reduction in the pH of saline–alkali soil [26]. Moreover, phosphorus is a key growth-limiting nutrient in plants [27]. However, phosphorus modification alone likely cannot result in a decrease in the EC of biochar. In this line, using both phosphoric acid and minerals to comodify biochar could offset the limitations of the individual modifiers and meet the needs for saline–alkali soil amendment. To the best of our knowledge, there is no research focused on the effects of biochar comodification with phosphoric acid and minerals on the properties and stability of the resulting biochar.
In this study, we aimed to investigate the synthesis procedure and properties of phosphoric acid–mineral-modified biochar composites and assess their application potential as saline–alkali soil amendments. We anticipate that our findings will contribute to the development of effective strategies for sustainable soil management and agricultural productivity in saline–alkali areas. The objectives of this study are as follows: (1) To obtain phosphoric acid–mineral-comodified biochar composite materials, (2) to understand how phosphate and minerals alter the properties of biochar, and (3) to assess the potential applications of phosphoric acid–mineral-comodified biochar composites in saline–alkali soils.

2. Materials and Methods

2.1. Materials

Corn straw biochar, purchased from the Qinfeng Zhongcheng Biomass New Materials Co., Ltd. (Nanjing, China) was used as the raw material. The pristine biochar was produced via pyrolysis at 500 °C in a reactor. Kaolinite (CAS: 1332-58-7, chemically pure, particle size < 50 μm, and crystalline phase confirmed by XRD), montmorillonite (CAS: 1318-93-0, K-10 grade, purity > 95%, and in a calcium-saturated form), and hydroxyapatite (CAS: 1306-06-5, purity > 95%, and particle size < 80 μm) were purchased from Macklin Biochemical Technology Co., Ltd. (Shanghai, China). Analytical grade phosphoric acid (CAS: 7664-38-2 and 85% w/w) was purchased from Guanghua Sci-Tech Co., Ltd. (Guangzhou, China).

2.2. Preparation of Modified Biochar Composites

On the basis of preliminary experiments (Table S1), a phosphoric acid/mineral/biochar ratio (v:w:w) of 7.5:2:1 was used for biochar modification. Specifically, 10.00 g of corn straw biochar (BC) and 20.00 g of kaolinite were weighed and placed in a beaker, after which 75 mL of 1.00 mol/L phosphoric acid was added to the beaker. The beaker was transferred to an ultrasonic cleaner (DOVES, Huace Technology Co., Ltd., Shenzhen, China) and sonicated at 40 kHz for 30 min. The resulting mixture was subsequently stirred via a magnetic stirrer (JJ-1A, Surui Instrument Co., Ltd., Changzhou, China) at 500 r/min for 4 h. The mixture was oven-dried at 105 °C for 24 h, transferred to an iron container, and placed in a muffle furnace (volume: 0.027 m3; KSL-1200X, HF-Kejing, Hefei, China) for pyrolysis. The pyrolysis program was set according to Liu et al. [28]. The heating rate was 10 °C/min, the peak temperature was 500 °C, and the retention time was 2 h. The muffle furnace was turned off after the temperature dropped below 300 °C. After cooling, the produced H3PO4–kaolinite–biochar composite was removed from the iron container, crushed, uniformly mixed, and labeled HBCK. The same preparation method was used to produce the H3PO4–montmorillonite–biochar composite (HBCM) and the H3PO4–hydroxyapatite–biochar composite (HBCHAP). In addition, biochars modified with minerals or phosphoric acid alone were prepared, and deionized water-washed biochar was used as the control. The kaolinite, montmorillonite, hydroxyapatite, and phosphoric acid-modified biochar composites are referred to as BCK, BCM, BCHAP, and HBC, respectively. Raw corn biochar and deionized water-washed biochar are referred to as BC and DWBC, respectively. The key differences are that the modified biochar composites (HBCK, HBCM, and HBCHAP) were prepared by mixing phosphoric acid, minerals (kaolinite, montmorillonite, or hydroxyapatite), and corn straw biochar in a 7.5:2:1 ratio, whereas the control biochar (DWBC) was simply washed with deionized water in a 2.5:1 ratio.

2.3. Physicochemical Properties of the Modified Biochar Composites

The physicochemical properties were measured, as described previously [28]. The pH and electrical conductivity (EC) were determined using a pH meter (PHS-3E, INESA, Shanghai, China) and a conductivity meter (DDS-307, INESA, Shanghai, China), respectively. The biochar/deionized water ratio was 1:20 (w/v), and the measurement was conducted after the mixture was shaken for 30 min. The ash content (Ash) and volatile matter content (VM) of the biochar and modified biochar composites were determined according to the Chinese National Standard GB 5009.4-2016. For ash content determination, 1.00 g of biochar or modified biochar composite was placed in a ceramic crucible (without a ceramic lid) and combusted in a muffle furnace at 750 °C for 6 h. For volatile matter content measurement, 1.00 g of biochar or modified biochar composite was placed in a ceramic crucible with a loose ceramic lid and combusted in a muffle furnace at 900 °C for 6 min.
The specific surface area, pore volume, and pore size distribution of the biochar and modified biochar composites were measured via the N2 adsorption method in an automated surface area and porosity analyzer (ASAP 2460, Micromeritics, Norcross, GA, USA).

2.4. Thermal Stability of the Modified Biochar Composites

The thermal stability of the biochar and modified biochar composites was analyzed via a thermal analysis instrument (STA 6000, PerkinElmer, Waltham, MA, USA). The biochar and modified biochar composites were heated from 30 °C to 900 °C at a rate of 10 °C/min in an air atmosphere. Thermogravimetric analysis of the biochars was conducted and corrected according to Harvey et al. [29]; in particular, TGA correction was performed to remove the influence of water and ash content in the biochars on the results. R50 was calculated by dividing the T50 (temperature corresponding to the 50% oxidation/volatilization of biochar) by T50graphite (temperature corresponding to the 50% oxidation/volatilization of graphite, and T50graphite = 886 °C).

2.5. Structural of the Modified Biochar Composites

The surface morphology and elemental composition of the biochar and modified biochar composites were analyzed via an environmental scanning electron microscope equipped with an energy-dispersive X-ray spectrometer (SEM–EDS, QUANTA 200, FEI, Hillsboro, Hillsboro, OR, USA). The surface functional groups of the biochar and modified biochar composites were characterized with a Fourier transform infrared spectrometer (FTIR, VERTEX 80 V, Bruker, Ettlingen, Germany). The crystal morphologies of the biochar and modified biochar composites were detected via X-ray diffraction (XRD, XRD Ultima IV, Rigaku, Tokyo, Japan).

2.6. Elemental Analysis of the Modified Biochar Composites

The contents of carbon (C), hydrogen (H), and nitrogen (N) in the biochar and modified biochar composites were determined via an elemental analyzer (Elemental Analyzer, 2400 II, PerkinElmer, Waltham, MA, USA). The contents of phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sodium (Na), and silicon (Si) in the biochar and modified biochar composites were measured according to previously reported methods with some modifications [28]. A 0.100 g sample was placed in a polytetrafluoroethylene crucible and then digested sequentially with HCl, HNO3, HF, and HClO4. Finally, the solution was transferred to a 50 mL volumetric flask and brought to a volume with 2% HNO3. The solution was filtered through a 0.22 μm filter membrane, and the filtrate was collected for the detection of mineral elements. The contents of P, K, Ca, Mg, Na, and Si were analyzed using a dual-channel inductively coupled plasma optical emission spectrometer (ICP–OES; Thermo Scientific, Waltham, MA, USA).

2.7. Pot Experiments

Saline–alkali soil was collected from the topsoil layer (0–20 cm depth) of a wetland in Lianyun District, Lianyungang, Jiangsu Province, China (34.77168072° N, 119.21959452° E). The collected soil was subjected to air-drying, milling, and sieving (<2 mm) following the removal of plant debris and gravel. The main physicochemical properties of the soil sample were as follows: pH 9.01, EC 917 uS/cm, total soluble salt 0.71%, SAR 28.97, water-soluble Na+ 726.50 mg/kg, water-soluble K+ 53.65 mg/kg, water-soluble Ca2+ 13.45 mg/kg, water-soluble Mg2+ 20.55 mg/kg, water-soluble HCO3- 64.05 mg/kg, water-soluble Cl- 507.62 mg/kg, water-soluble SO42− 736.65 mg/kg, soil OM 15.30 g/kg, total N 1.13 g/kg, and total P 0.72 g/kg, which is indicative of a typical saline–alkali soil.
A pot experiment was conducted with nine biochar-based remediation treatments (BC, DWBC, HBC, BCK, BCM, BCHAP, HBCK, HBCM, and HBCHAP) and an unamended control (CK). Each plastic pot (15 cm height × 10 cm upper diameter) received 600 g of air-dried saline–alkali soil, which was homogenously mixed with either biochar or modified biochar composites at a 5% (w/w) application rate. Control groups comprised soils without biochar amendment. Ten alfalfa (Medicago sativa L.) seeds were sown per pot, followed by thinning to three seedlings per pot after the emergence of the second true leaf. All treatments were maintained in a greenhouse (20–28 °C) under controlled irrigation (soil moisture maintained at 60% of field capacity) for a 30-day cultivation period. Upon harvest, the alfalfa shoot height was measured from the base to the apical meristem. Plants were then separated into shoots and roots, oven-dried at 105 °C for 30 min (enzyme deactivation), and dried at 60 °C to constant weight. The dry biomass of both shoots and roots was measured gravimetrically. Each treatment was conducted in triplicate.

2.8. Statistical Analysis

The measured data were processed using SPSS 20.0 (SPSS Inc., Chicago, IL, USA) for statistical analysis, while Origin 2017 (OriginLab Corp., Northampton, MA, USA) was used for graphical representation. Prior to parametric testing, all datasets were evaluated for normality using Shapiro–Wilk tests and homogeneity of variance using Levene’s tests. For normally distributed data with equal variances (p > 0.05), significant differences among treatments were assessed via one-way analysis of variance (ANOVA) followed by Duncan’s multiple range test (p = 0.05), with variations indicated by different lowercase letters. Non-normal datasets were analyzed using Kruskal–Wallis non-parametric tests. Pearson’s correlation analysis and principal component analysis were conducted using R 4.3.1 software.

3. Results and Discussion

3.1. Structural Analysis of the Modified Biochar Composites

Typical SEM images of the biochars with and without modification are shown in Figure 1. The surfaces of BC, DWBC, and HBC appear smooth with honeycomb-like structures, and C, O, and Si were identified as the main components on the surface. In contrast, a much rougher surface was observed for the mineral-loaded biochars (Figure 1d–i). Globular and granular crystals clearly formed on the surfaces of the mineral–biochar composites. The SEM-EDX results (Figure S2) reveal that these crystals were composed of Ca or Al, which are typical elements in kaolinite, montmorillonite, and hydroxyapatite. In short, SEM-EDS analysis reveals that mineral-modified biochars developed rough surfaces with distinct Ca or Al-rich granular crystals, contrasting with the smooth, honeycomb-like morphology and C-, O-, and Si-dominated composition of raw biochar.
The XRD patterns of the biochar and modified biochar composites are shown in Figure 2. The sharp peaks at 20.79°, 26.57°, and 27.82° in the BC, DWBC, and HBC spectra, respectively, were attributed to SiO2 [20]. However, the diffraction peaks of SiO2 were much weaker in HBC, likely because the crystal structures were destroyed by phosphoric acid. The peak spacings at 6.50°, 19.68°, 21.92°, 29.36°, 30.50°, and 35.36° in BCM, HBCM, BCK, and HBCK were assigned to phyllosilicates (i.e., montmorillonite and kaolinite) [18]. For BCHAP, five typical peaks of hydroxyapatite at 25.84°, 31.94°, 32.92°, 34.12°, and 46.76° were detected. Peak spacings at 26.76°, 29.06°, and 30.56°, representing Ca2P2O7, appeared in HBCHAP, indicating that the phosphoric acid reacted with hydroxyapatite and changed the crystal morphology on the surface of the biochar [30]. Compared with BCM, the peak spacing at 6.50° (assigned to montmorillonite) disappeared in HBCM, confirming that phosphoric acid underwent complex reactions with the minerals and biochar. In short, the mineral modification led to the formation of new mineral phases on the biochar surface, while phosphoric acid modification significantly weakened the SiO2 diffraction peaks, reacted with hydroxyapatite to generate new Ca2P2O7 phases, and completely eliminated the characteristic diffraction peaks of montmorillonite.
The FTIR spectra reveal changes in the functional groups of the biochar and modified biochar composites (Figure 3). The assignments of the different FTIR peaks in raw corn straw biochar are summarized as follows: -OH (3438 cm−1), aromatic C=C (1627 cm−1), -CH3 (2925 cm−1), -CH2 (2854 cm−1), -O- (1066 cm−1), and -CH (761 cm−1). The FTIR spectra remained unchanged after phosphoric acid or deionized water treatment (Figure 3a). In contrast, the FTIR spectra presented different changes after mineral modification and phosphoric acid–mineral comodification. After montmorillonite modification, the FTIR bands assigned to the -O- functional groups (1066 cm−1) in the biochar shifted to Si-O-Si (1031–1100 cm−1, 467 cm−1, and 518 cm−1) and dramatically increased (Figure 3b). Similarly, after kaolinite modification, the FTIR bands assigned to the -O- functional groups (1066 cm−1) in the biochar shifted to Si-O-Si (1100 cm−1 and 472 cm−1) and Si-O (800 cm−1) (Figure 3c). This might have occurred due to reactions between biochar and minerals (kaolinite or montmorillonite) during pyrolysis, which resulted in the formation of new Si–O–Si functional groups on the biochar surface. In the phosphoric acid and mineral-comodified biochar composites, the absorption peaks of the silicon-containing functional groups (1031–1100 cm−1 and 467–518 cm−1) weakened (Figure 3b,c). After hydroxyapatite modification, the FTIR bands assigned to the -O- functional group in the biochar shifted to -PO4 (1100–900 cm−1) and O-P-O (600–540 cm−1), and the intensity of these functional groups decreased when phosphoric acid was present (Figure 3d). These results are consistent with the XRD results (Figure 2) and indicate that the phosphoric acid and mineral modification did not alter the basic C skeleton structure of the biochar; however, some reactions may have occurred, generating new functional groups on the biochar composites, such as Si-O-Si and O-P-O [24].

3.2. Stability Analysis of the Modified Biochar Composites

The content of volatile matter (VM)—which has been reported to be positively correlated with labile carbon but negatively correlated with stable carbon content [31,32]—can be used to assess the stability of biochar. In this study, we found that phosphoric acid and mineral modification decreased the VM content (Table 1). The phosphoric acid and mineral-comodified biochar composites had the lowest VM content, showing a reduction of 63.89–76.12%, potentially indicating that these composites had the highest stability. In addition, kaolinite, montmorillonite, and hydroxyapatite modification dramatically increased the biochar ash content (Table 1), which may have also improved the thermal stability and antioxidant properties of the modified biochar composites [33].
To further assess the long-term oxidative and thermal stability of the biochars, thermogravimetric analysis (TGA) was employed [34,35]. Typical thermograms of the biochar and modified biochar composites are presented in Figure 4a. Mineral modification decreased the weight loss rate from 59.54% to 6.50–32.85%. This result is consistent with that of Wang et al. [36], who reported that the weight loss rates of montmorillonite-modified and kaolinite-modified biochars were 20.9–22.5% lower than those of untreated samples via TGA. The recalcitrance index (R50) serves as a specific indicator of the thermal oxidation stability of biochar, with higher R50 values signifying greater resistance to thermal degradation and oxidation, as described by Harvey et al. [29]. The corrected thermogravimetric curves (i.e., with the influence of water and ash content removed) of the biochar and modified biochar composites are shown in Figure 4b. Both phosphoric acid and mineral modification increased the R50 value of the biochar (Table 1). The R50 values were in the following order: raw biochar < mineral-modified biochar composites < phosphoric acid and mineral-comodified biochar composites < phosphoric acid-modified biochar. Thus, phosphoric acid-modified biochar had the highest thermal oxidation stability, indicating good potential in long-term applications. To our knowledge, this is the first study reporting that phosphoric acid can enhance the thermal oxidation stability of biochar.

3.3. Physicochemical Properties Analysis of the Modified Biochar Composites

Both mineral and phosphoric acid modification significantly decreased the pH of the modified biochar composites, except in the biochar modified with montmorillonite (Table 1). Copyrolysis with kaolinite and hydroxyapatite caused the pH of the modified biochar composite to decrease by 3.6 and 3.2 units, respectively. This result is consistent with that of Jing et al. (2022) [24], who suggested that the positively charged cations of minerals decreased the pH value of biochar. Phosphoric acid modification dramatically reduced the pH value of the modified biochar composites. Among all the studied modified biochar composites, the pH value of HBC was the lowest, followed by that of HBCK. Studies have shown that high pH is one of the main factors limiting the utilization of saline–alkali soil, leading to decreased soil nutrient availability and affecting plant growth [37,38,39]. Pan et al. [26] reported that acid-modified biochar successfully reduced soil pH, alleviated soil alkaline stress, and promoted the growth of strawberry plants in coastal saline–alkali soils. Zhou et al. [2] reported that the application of acidic biochar can reduce soil pH and salinity while increasing soil nutrient content.
Kaolinite and hydroxyapatite modification reduced the EC value of the modified biochar composites (Table 2) from 849 μS/cm to 185.85 μS/cm and 341.50 μS/cm, respectively. Although phosphoric acid modification increased the EC values of the biochar, combined modification with phosphoric acid and minerals significantly decreased the EC values of the biochar (Table 2). Soils with high EC values can affect the availability of nitrogen, phosphorus, potassium, and other nutrients, resulting in a decline in vegetation coverage [40]. Additionally, high EC reduces the activity of soil enzymes such as catalase, urease, acid phosphatase, and alkaline phosphatase [41]. Kaolinite, montmorillonite, and hydroxyapatite modification altered the base cation (Ca, Na, K, and Mg) contents of the biochar, with a notable reduction in Na content for kaolinite and hydroxyapatite modifications (Table 2). Specifically, kaolinite modification significantly reduced the K, Ca, Mg, and Na contents of the modified biochar composites; montmorillonite modification significantly reduced the K, Ca, and Mg contents; and hydroxyapatite modification significantly reduced the K and Na contents. This difference is attributed mainly to the variations in the elemental composition of the minerals; montmorillonite contains Na, whereas hydroxyapatite contains Ca and Mg. Base cations—especially Na—are major factors limiting the productivity of saline–alkali soils. High Na can lead to ion toxicity and imbalance in plants, inhibiting plant growth [38]. A reduction in the base cation contents of the modified biochar composites was beneficial for the use of this material for saline–alkaline soil amendment.
A high specific surface area (SSA) in biochar is an important indicator of its potential for improving the quality of saline–alkali soils, as this type of biochar offers more adsorption sites [4]. The variations in the specific surface areas of the biochar and modified biochar composites are presented in Table 2. Mineral modification increased the surface area and pore volume of the modified biochar composites, particularly in the hydroxyapatite-modified biochar composite, and phosphoric acid modification had little effect on the SSA of the modified biochar composites. The SSA of BCHAP was 38.7 times greater than that of BC. This result aligns with that of Jing et al. [24], who reported that clay minerals could increase the specific surface area and pore volume of clay–biochar composites through catalyzing the pyrolysis of carbon. The SSA of the phosphoric and mineral-comodified biochars was lower than that of the mineral-modified biochar (Table 2), which may have occurred due to the interactions among phosphoric acid, minerals, and biochar. Phosphoric acid might destroy the layered structure of minerals (Figure 2 and Figure 4), leading to the formation of new particles that adhere to the biochar surface and fill pores (Figure 1) [30].
In summary, phosphoric acid and mineral comodification can reduce the pH and EC of biochar, improve the pore size distribution, and increase its specific surface area. Therefore, in theory, phosphoric acid and mineral-comodified biochar composites should be more suitable and applicable for amending saline–alkali soils.

3.4. Nutrient Content of the Modified Biochar Composites

Kaolinite or montmorillonite combined with phosphoric acid dramatically increased the Si content of the biochar, which was increased by 77.37–127.60% compared to raw biochar (Table 2). Previous studies have revealed that Si can protect the carbon fraction of biochar [35,42]. Minerals contain a large amount of Si and can bind with biochar to increase its stability [43,44]. Additionally, Si can promote plant growth [45]. Sattar et al. [46] reported that the simultaneous application of silicon and biochar can increase stress resistance and promote growth in plants. The application of silicon-modified biochar can reduce the salinity of soil by promoting phosphate absorption in plants, reducing the sodium content in both soil and plants, and increasing the soil potassium content, thereby mitigating the effects of salt stress on plants [47].
Phosphoric acid and/or hydroxyapatite modification increased the phosphorus (P) content of the biochar. Compared with kaolinite or montmorillonite modification alone, comodification of the biochar with the individual minerals and phosphoric acid significantly increased the P content of the biochar by approximately 50 g/kg (Table 2). The P content in HBC, HBCM, and HBCK was 58.64, 53.84, and 48.10 g/kg, respectively. Notably, hydroxyapatite modification increased the P content of the biochar by 42.61–57.70 times to yield values of 105.25–142.51 g/kg. Phosphorus is an important factor in saline–alkali soil improvement and plays an important role in soil N, C, and S cycling during the restoration process of saline–alkali soils [27]. Bouras et al. [48] reported that the application of phosphate-containing fertilizer promoted the growth of corn under salt stress.

3.5. Principal Component Analysis

Principal component analysis (PCA) was employed to further explore the correlations between the properties of modified biochar composites and modification methods (Figure 5). The results reveal that BC, DWBC, and HBC were located in quadrant I; BCHAP was located in quadrant II; HBCHAP was located in quadrant III; and BCM, BCK, HBCM, and HBCK were located in quadrant IV. These results indicate that the properties of the modified biochar composites were determined by mineral type. The contents of Na and Si had positive loadings on PC1 but were negatively loaded on PC2. The clay minerals kaolinite and montmorillonite had similar impacts on the properties of the modified biochar composites, mainly affecting the Si contents of the modified biochar composites. The SSA, PV, PD, and P contents presented negative loadings on PC1, suggesting that hydroxyapatite was the primary factor influencing the pore structure of the modified biochar composites. In addition, the influence of hydroxyapatite on the P content of the modified biochar composites exceeded the effect of phosphoric acid, possibly due to the higher P content in hydroxyapatite. PCA reveals that the properties of the modified biochar composites were primarily determined by the mineral type, with clay minerals (kaolinite/montmorillonite) mainly influencing Si content and hydroxyapatite exerting a dominant effect on pore structure and phosphorus levels.

3.6. Enhancement of Plant Growth

As evidenced in Figure 6, phosphoric acid–mineral-modified biochar composites demonstrated marked improvements in the plant height, shoot biomass, and root biomass of alfalfa relative to control (CK) and raw biochars (BC and DWBC). All modified biochar composites significantly increased plant height (by 17.36–20.27%) compared to CK, and there was no significant difference between them. The highest shoot biomass was observed in the HBC treatment (5.25 g), followed closely by HBCK (4.88 g), HBCM (4.83 g), and HBCHAP (4.82 g), representing an increase of 107.32–125.80% over the raw biochar, indicating that these modified biochar composites promote the development of aboveground plants. Similarly, the root biomass results reveal that the modified biochar composites were equally effective, showing notable improvements compared to CK and the raw biochars. These results validate the potential of phosphoric acid–mineral-modified biochars as highly effective amendments for saline–alkali soil remediation.

4. Conclusions

In this study, we successfully developed novel composite materials by modifying raw biochar with phosphoric acid and various minerals (kaolinite, montmorillonite, and hydroxyapatite). The prepared composite materials exhibited improved characteristics, including abundant oxygen-containing functional groups, lower pH (2.32) and EC values (185.85 μS/cm), and higher nutrient contents (e.g., silicon at 171.27 mg/kg and phosphorus at 142.51 mg/kg). In addition, these modifications significantly improved the specific surface area (46.27 m2/g), pore volume (0.3239 cm3/g), and thermal stability of the biochars. When applied to saline–alkali soils, the modified biochar composites effectively enhanced alfalfa growth, increasing plant height by 17.36–20.27% and boosting shoot biomass by 107.32–125.80%. These findings suggest that the modified biochar composites are expected to play important roles in the remediation of saline–alkali soils, though further studies are needed to evaluate their long-term effects and impact mechanisms.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/agriculture15070785/s1, Table S1: pH and EC of modified biochar composites with different mineral addition ratios. Figure S1: Schematic diagram of material synthesis. Figure S2: SEM-EDS of biochar and modified biochar composites.

Author Contributions

Investigation, H.D., Z.L., J.Y., X.T., L.L., M.J. and J.X.; methodology, writing—original draft, data curation, and formal analysis, H.D.; writing—review and editing, M.J. and J.X.; resources and funding acquisition, J.Y., M.J. and J.X. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Special Fund on Technology Innovation of Carbon Dioxide Peaking and Carbon Neutrality of Jiangsu Province (BE2022306) and the Jiangsu Provincial Forestry Science and Technology Innovation and Promotion Project (LYKJ[2022]02).

Institutional Review Board Statement

This study did not require ethical approval.

Data Availability Statement

The data that support the findings of this study are available upon reasonable request from the authors.

Conflicts of Interest

The authors declare no conflicts of interest. The funding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

Abbreviations

The following abbreviations are used in this manuscript:
BCraw corn straw biochar
DWBCdeionized water-washed biochar
HBCH3PO4-modified biochar
BCKkaolinite–biochar composite
BCMmontmorillonite–biochar composite
BCHAPhydroxyapatite–biochar composite
HBCKH3PO4–kaolinite–biochar composite
HBCMH3PO4–montmorillonite–biochar composite
HBCHAPH3PO4–hydroxyapatite–biochar composite

References

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Agriculture 15 00785 g001
Figure 1. Typical SEM images of biochar and modified biochar composites. (a) BC; (b) DWBC; (c) HBC; (d) BCM; (e) HBCM; (f) BCK; (g) HBCK; (h) BCHAP; (i) HBCHAP.
Figure 1. Typical SEM images of biochar and modified biochar composites. (a) BC; (b) DWBC; (c) HBC; (d) BCM; (e) HBCM; (f) BCK; (g) HBCK; (h) BCHAP; (i) HBCHAP.
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Figure 2. XRD spectra of biochar and modified biochar composites.
Figure 2. XRD spectra of biochar and modified biochar composites.
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Figure 3. FTIR analysis of biochar and modified biochar composites. (a) BC; DWBC, HBC (b) BC; BCM, HBCM (c) BC; BCK, HBCK (d) BC, BCHAP, HBCHAP.
Figure 3. FTIR analysis of biochar and modified biochar composites. (a) BC; DWBC, HBC (b) BC; BCM, HBCM (c) BC; BCK, HBCK (d) BC, BCHAP, HBCHAP.
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Figure 4. Original thermogravimetric patterns (a) and corrected thermogravimetric patterns (b) of the biochar and modified biochar composites.
Figure 4. Original thermogravimetric patterns (a) and corrected thermogravimetric patterns (b) of the biochar and modified biochar composites.
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Figure 5. Principal component analysis (PCA) of the physicochemical parameters of the biochar and modified biochar composites.
Figure 5. Principal component analysis (PCA) of the physicochemical parameters of the biochar and modified biochar composites.
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Figure 6. Effect of the biochar and modified biochar composites on plant growth in saline–alkali soil: (a) plant height, (b) shoot biomass, and (c) root biomass. Different small letters in each column indicate significant difference (p < 0.05) among biochar and modified biochar composites.
Figure 6. Effect of the biochar and modified biochar composites on plant growth in saline–alkali soil: (a) plant height, (b) shoot biomass, and (c) root biomass. Different small letters in each column indicate significant difference (p < 0.05) among biochar and modified biochar composites.
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Table 1. Selected physicochemical properties of biochar and modified biochar composites.
Table 1. Selected physicochemical properties of biochar and modified biochar composites.
Ash [%]Volatile Matter [%]T50R50pHEC [μs/cm]Specific Surface Area [m2/g]Pore Volume [cm3/g]Pore Diameter [nm]
BC34.89 ± 2.49 f14.07 ± 0.07 a462.9352.2510.17 ± 0.24 a849.00 ± 31.04 b1.260.002880.19
DWBC38.69 ± 2.65 f8.99 ± 0.38 b471.2653.199.74 ± 0.08 b789.50 ± 19.19 b8.910.011550.47
HBC46.76 ± 3.44 e9.18 ± 1.12 b534.4360.322.32 ± 0.01 h2700.00 ± 89.81 a1.220.0035122.01
BCK88.34 ± 3.90 ab5.15 ± 1.33 c480.3554.226.55 ± 0.01 d185.85 ± 8.53 e11.130.0531186.26
BCM66.70 ± 0.19 d8.29 ± 0.45 b479.7254.1410.18 ± 0.01 a837.00 ± 14.70 b16.850.0568132.90
BCHAP83.29 ± 0.52 bc4.86 ± 0.18 cd483.5354.576.98 ± 0.04 c341.50 ± 9.39 d46.270.3239277.27
HBCK78.77 ± 0.70 c4.40 ± 0.33 cd524.1259.162.91 ± 0.00 g638.00 ± 2.45 c4.370.0136121.91
HBCM82.51 ± 2.91 bc5.08 ± 0.46 cd511.7157.765.41 ± 0.01 f401.00 ± 16.33 d5.950.0316207.66
HBCHAP89.58 ± 0.59 a3.36 ± 0.87 d499.8556.426.13 ± 0.05 e209.50 ± 2.86 e6.540.0420252.93
Different small letters in each column indicate significant difference (p < 0.05) among biochar and modified biochar composites.
Table 2. Elemental composition of biochar and modified biochar composites.
Table 2. Elemental composition of biochar and modified biochar composites.
P [g/kg]Si [g/kg]K [g/kg]Ca [g/kg]Mg [g/kg]Na [g/kg]Al [g/kg]
BC2.47 ± 0.12 e75.25 ± 6.69 c16.00 ± 0.89 a6.18 ± 0.80 c0.60 ± 0.25 c4.92 ± 1.42 c2.26 ± 0.43 bc
DWBC2.42 ± 0.11 e83.82 ± 6.33 c15.71 ± 0.55 a5.70 ± 1.17 cd0.59 ± 0.21 c5.02 ± 1.21 c2.55 ± 0.48 b
HBC58.64 ± 5.10 c75.19 ± 6.45 c13.95 ± 0.71 b4.73 ± 0.83 cd0.49 ± 0.25 c4.32 ± 1.11 cd2.43 ± 0.69 bc
BCK2.19 ± 0.11 e164.66 ± 2.71 a6.37 ± 0.58 de2.50 ± 0.30 cd0.18 ± 0.06 c2.77 ± 0.20 cde1.26 ± 0.22 d
BCM1.38 ± 0.08 e171.27 ± 7.99 a8.30 ± 0.40 c3.57 ± 0.98 cd0.33 ± 0.31 c17.96 ± 1.23 a2.59 ± 0.13 b
BCHAP105.25 ± 1.16 b21.21 ± 0.87 d4.78 ± 0.26 f134.40 ± 3.23 a4.09 ± 0.09 a1.46 ± 0.20 e4.43 ± 0.60 a
HBCK48.10 ± 1.83 d133.47 ± 3.94 b5.66 ± 0.23 ef2.35 ± 0.44 d0.60 ± 0.64 c2.43 ± 0.14 de1.56 ± 0.14 cd
HBCM53.84 ± 1.89 c165.57 ± 3.40 a7.11 ± 0.34 cd3.51 ± 1.12 cd0.30 ± 0.27 c14.39 ± 1.09 b2.62 ± 0.26 b
HBCHAP142.51 ± 1.80 a15.31 ± 0.70 d3.09 ± 0.27 g120.45 ± 2.05 b2.28 ± 0.21 b1.23 ± 0.14 e2.71 ± 0.14 b
Different small letters in each column indicate significant difference (p < 0.05) among biochar and modified biochar composites.
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Dai, H.; Liu, Z.; Yu, J.; Teng, X.; Liu, L.; Jia, M.; Xue, J. Assessment of the Characters of a Novel Phosphoric Acid and Mineral-Comodified Biochar Composite and Its Potential Application in Saline–Alkali Soil Improvement. Agriculture 2025, 15, 785. https://doi.org/10.3390/agriculture15070785

AMA Style

Dai H, Liu Z, Yu J, Teng X, Liu L, Jia M, Xue J. Assessment of the Characters of a Novel Phosphoric Acid and Mineral-Comodified Biochar Composite and Its Potential Application in Saline–Alkali Soil Improvement. Agriculture. 2025; 15(7):785. https://doi.org/10.3390/agriculture15070785

Chicago/Turabian Style

Dai, Hao, Zhuangzhuang Liu, Jinping Yu, Xiaoming Teng, Lei Liu, Mingyun Jia, and Jianhui Xue. 2025. "Assessment of the Characters of a Novel Phosphoric Acid and Mineral-Comodified Biochar Composite and Its Potential Application in Saline–Alkali Soil Improvement" Agriculture 15, no. 7: 785. https://doi.org/10.3390/agriculture15070785

APA Style

Dai, H., Liu, Z., Yu, J., Teng, X., Liu, L., Jia, M., & Xue, J. (2025). Assessment of the Characters of a Novel Phosphoric Acid and Mineral-Comodified Biochar Composite and Its Potential Application in Saline–Alkali Soil Improvement. Agriculture, 15(7), 785. https://doi.org/10.3390/agriculture15070785

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