Ball-Milling-Modified Biochar with Additives Enhances Soil Cd Passivation, Increases Plant Growth and Restrains Cd Uptake by Chinese Cabbage

Abstract

Biochar is a popular amendment in Cd polluted soil. However, the performance of bulk biochar is still less than satisfactory, so effective modification is very important to improve its capacity to adsorb Cd. In the present study, biochar derived from reed straw was modified by ball milling with the addition of either potassium hydroxide (KOH) alone (QK) or combined with attapulgite (QKA). Both batch experiments and pot cultivation were conducted to elucidate the adsorption mechanisms of Cd by modified biochar and their effects on Cd passivation and plant uptake in Cd polluted soil. The results showed that QK and QKA could provide higher pH values, and more oxygen-containing functional groups and minerals compared with bulk biochar (YC), promoting the complexation, ion exchange and precipitation of biochar to cadmium (Cd). The modified biochar was more inclined to multi-layer, non-ideal surface and chemical adsorption, which was an endothermic process. Compared to non-biochar addition (CK), the application of QK or QKA significantly promoted the values of pH, EC, CEC, available potassium and organic matter in soil. The addition of QK, QKA and YC decreased the availability of Cd by 22.61%, 22.32% and 14.16%, accompanied by the increase of residual Cd by 47.96%, 47.60% and 37.27%, respectively, indicating the more effective passivation of the modified biochar (QK and QKA). Compared to CK, biochar applications could significantly improve Chinese cabbage growth, and decrease Cd content in the aerial/edible part of plants by 42.97, 18.16 and 7.29%, respectively, for QK, QKA and YC. With the application of QK, Cd concentrations in the aerial/edible part of Chinese cabbage were reduced to 0.15 mg kg⁻¹ (lower than 0.2 mg/kg, the leafy vegetables national safety standard). Generally, the performance of QK on the remediation effects and vegetable production was better than that of QKA, indicating the potential of QK for the remediation of Cd-contaminated soil and the safe production of vegetables.

1. Introduction

Heavy metals exceeding the standard of farmland pollution are mainly Cd, As, Hg, Pb and Cr, which will harm agricultural products and food safety [1]. Among them, Cd is one of the most toxic heavy metal elements, with more serious pollution exceeding the standard and higher hazard index [2]. Excessive heavy metals in soil brings serious harm to crop physiology and products, and consequently to human health and ecosystem [3,4]. Therefore, studies on the remediation of heavy metals, especially Cd, in farmland soil have become a hotspot. Commonly used remediation methods can be classified into physics [5], chemistry [6] and bioremediation [7]. Among them, the in situ chemical remediation method using biochar as a passivator has attracted much attention because of its variety of raw materials and wide sources, as well as its carbon fixation ability and convenient operation.

Most biochars possess alkaline pH values, large specific surface areas, porous structures and generous functional groups, which make it have good adsorption and fixation effects on heavy metal [8]. In addition, biochar contains abundant organic and inorganic components such as carbon, nitrogen, potassium, etc. Applying biochar to soil could not only enhance nutrient contents and stimulate crop growth, but could also improve the physical, chemical and biological properties of soil [9]. More importantly, biochar application is expected to decrease the content of bioavailable heavy metals in polluted soil and reduce the transport of heavy metals from soil to plant, perhaps allowing and even promoting the production of valuable products by the plant [10]. However, the performance of bulk biochar is still less than satisfactory, and there is still plenty of room for improvement.

Chemical and physical modification has been proven to be effective for improving specific properties and the utilization rate of biochar [11]. Modifications give biochar new structure and surface properties, which can produce significant benefits for improving soil quality and health [12,13,14]. The common chemical modification is usually prepared by sequentially adding various liquid modifiers to the bulk biochar, followed by modifier removal, washing and drying [15]. Despite superior performance, chemical modification has the disadvantages of high production cost, complicated procedures and secondary pollution risk [15]. Among diverse physical modification, ball milling, a solvent-free technology, is produced by milling biochar alone or mixing with different solid modifiers and mechanically grinding it to improve the specific surface area, particle size, surface charge and functional groups, which has been regarded as a green, cost-effective, reproducible and convenient modification method [15,16]. Ball-milling-modified biochar combined with modifiers (such as alkaline substances, metal oxides, clay minerals, etc.) has been proven to effectively remove pollutants from polluted water [17] and can also be used to immobilize heavy metals in polluted soils [16,18]. However, studies on ball-milling modification have not been conducted systematically, and there are only a few studies concerned with ball milling with additives.

The properties of modified biochar are highly dependent on the parameters of the ball milling and the modifier species [19]. Potassium hydroxide (KOH) is a common modifier that is rich in potassium, oxygen-containing groups and can increase the pH value of biochar. Attapulgite, a layered and chain-structured clay mineral rich in magnesium aluminum silicate, possesses excellent adsorption and ion exchange properties. In this study, a novel integration of mechanical ball milling combined with either KOH alone or with attapulgite as additives for biochar modification was investigated, and is expected to provide advanced materials for eco-friendly applications. The objectives of this study were to investigate (1) the characteristics of ball-milling-modified biochar with the addition of either KOH alone or combined with attapulgite, (2) the adsorption mechanisms of Cd by ball-milling-modified biochar and (3) their performance on soil Cd passivation, plant growth promotion and Cd uptake reduction by Chinese cabbage in the Cd polluted soil.

2. Materials and Methods

2.1. Modified Biochar Preparation

Bulk biochar (YC) preparation referred to the optimal preparation parameters of biochar found in our previous studies [20]. Briefly, the reed straw was cut into 2–3 cm pieces after air drying and anaerobically fired in a high temperature tubular furnace (OTF-1200X, Hefei Kejing Material Technology Co., Ltd., Hefei, China), then filled with inert nitrogen by heating at the rate of 20 °C per minute, remaining at 600 °C for 30 min and cooling for 2 h. After firing, it was cooled, ground and evenly mixed, and passed through a 100-mesh sieve. After mixing the YC with potassium hydroxide (KOH, purchased from Xilong Chemical Company, Shantou, China) according to the mass ratio of 1:10 or KOH + attapulgite (purchased from Xuyi Xinyuan Company, Huaian, China) according to the mass ratio of 0.5:0.5:10, they were placed in a ball mill (Type BM40, Beijing Gredeman Instrument Equipment Co., Ltd., Beijing, China), ground at 300 rpm for 4 h, and put in a sealed bag for later use. The biochar modified by ball milling with either the addition of KOH alone (QK) or combined with attapulgite (QKA) were prepared. The preparation flow chart of modified biochar can be seen in Figure 1.

2.2. Characterization and Analysis of Modified Biochar

The pore sizes and specific surface areas of the modified biochar were measured by a porous physical adsorption instrument (BET, equipment model: QUANTACHROME/Kontha EVO, Boynton Beach, FL, USA). Polycrystalline diffraction (XRD, equipment model smartlab 9, Rigaku, Tokyo, Japan) was used to identify the image composition. A field scanning electron microscope (SEM, equipment model: JEOL, JSM-7800F Prime, EDS: Thermoscientific NORANTM System7, Thermal Fisher, Waltham, MA, USA) was employed to inspect the modified biochar’s surface morphology. Fourier transform infrared spectroscopy (FTIR, Thermal Fisher: Nicolet 670, Waltham, MA, USA) was used to identify the surface functional groups. The contents of C, H, O, N, S in biochar were determined by an element analyzer (Vario macro cube, Elementar, Langenselbold, Hanau, Germany).

2.3. Batch Experiments

A total of 1000 mg·L⁻¹ Cd stock solution was prepared by CdCl₂·2.5H₂O (Shanghai Aladdin reagent company, Shanghai, China) and stored in a refrigerator at 0–4 °C before dilution according to the required concentration. A total of 2.5 g of different modified biochar was measured out into a 500 mL plastic bottle, 250 mL 50 mg·L⁻¹ Cd solution was added, and then the bottle was placed in a shaking table at 200 rpm and 25 °C, with each treatment conducted in triplicate. After the timing started, 2 mL was taken out at 5, 20, 60, 120, 480, 720 and 1440 min, respectively, and filtered through a ø 0.45 μm microporous membrane. Cd concentration in the filtrate was immediately determined. According to the difference between the Cd concentration in the initial solution and the filtrate sampling at the different adsorption times, the adsorption amount and corresponding time were calculated, and a Langmuir kinetic curve (quasi-first order and quasi-second order kinetic equation) was drawn to compare the adsorb capability of biochar before and after modification.

$$\mathrm{Quasi-First} \mathrm{Order} \mathrm{Dynamics} \ln ({\mathrm{q}}_{\mathrm{e}} - {\mathrm{q}}_{\mathrm{t}}) = {\mathrm{lnq}}_{\mathrm{e}} - {\mathrm{K}}_1\mathrm{t}$$

(1)

$$\begin{array}{l}{\displaystyle \mathrm{Quasi-Second-Order} \mathrm{Dynamics} \frac{\mathrm{t}}{{\mathrm{q}}_{\mathrm{t}}}} = {\displaystyle \frac{1}{{\mathrm{K}}_2 {{\mathrm{q}}_{\mathrm{e}}}^2}} + {\displaystyle \frac{\mathrm{t}}{{\mathrm{q}}_{\mathrm{e}}}}\\ \end{array}$$

(2)

Among them, q_t and q_e are the adsorption amount of biochar for heavy metals (mg·L⁻¹) at time “t” and adsorption equilibrium (mg·g⁻¹), respectively; t is the adsorption time point (min); K₁, K₂ are the adsorption constants for quasi-first-order (min⁻¹) and quasi-second-order kinetic equations (g·mg⁻¹·min⁻¹), respectively. Initial Cd solution concentrations were prepared at 25, 50, 75, 125, 200, 250 and 500 mg·L⁻¹, respectively. A total of 20 mL of Cd solutions at the different concentrations above were added into a 50 mL plastic bottle, followed by 0.02 g YC, QK and QKA, respectively. The plastic bottle tube was put in a shaking table and shaken at a constant temperature (25 °C) for 24 h, with a rotating speed of 200 rpm. Each treatment was repeated three times. After 24 h, a certain amount of supernatant was taken from each centrifuge tube, filtered through a ø 0.45 μm microporous membrane and the Cd concentration in the filtrate was determined. The adsorb capability of bulk/modified biochar YC, QK and QKA to different initial Cd concentrations was calculated. The Langmuir model and Freundlich model were employed to predict the adsorption isotherms of Cd by bulk/modified biochar materials.

$$\mathrm{Langmuir}: {\displaystyle \frac{1}{{\mathrm{q}}_{\mathrm{e}}}} = {\displaystyle \frac{1}{{\mathrm{K}}_{\mathrm{I}}{\mathrm{Q}}_{\mathrm{m}}{\mathrm{C}}_{\mathrm{e}}}} + {\displaystyle \frac{1}{{\mathrm{Q}}_{\mathrm{m}}}}$$

(3)

$$\mathrm{Freundlich}: {\mathrm{lnq}}_{\mathrm{e}} = \ln {\mathrm{K}}_{\mathrm{f}} + {\displaystyle \frac{1}{\mathrm{n}}}\ln {\mathrm{C}}_{\mathrm{e}}$$

(4)

where q_e represents the adsorption amount at equilibrium (mg·g⁻¹₎; C_e represents the solute concentration at equilibrium (mg·L⁻¹); K_I is the parameter of adsorption capacity (mg·g⁻¹); n represents the Freundlich constant, indicating the adsorption strength of biochar to Cd; Q_m is the maximum adsorption capacity of biochar, mg·g⁻¹; and K_f is the parameter directly proportional to the affinity of the adsorption site for the adsorbed heavy metal ions (L·mg⁻¹).

The influence of the initial solution pH on bulk/modified biochar adsorption capacity was observed. A total of 50 mg·L⁻¹ Cd solution was prepared, with initial pH values adjusted to 5, 6, 7, 8, 9 and 10 by 1 mol·L⁻¹ NaOH or 1 mol·L⁻¹ HNO₃, respectively. We accurately measured 20 mL of the above solutions and added them into 50 mL plastic bottles, then accurately measured out 0.02 g YC, QK and QKA, respectively, and added them into the plastic bottles. The plastic bottle caps were placed tightly into a shaker and oscillated at 25 ± 1 °C with a rotating speed of 200 rpm. After 24 h, the bottles were removed, filtered with a ø 0.45 μm microporous membrane, and the residual Cd concentration was immediately measured.

To elucidate the influence of temperature on biochar adsorption capacity, 20 mL 50 mg·L⁻¹ Cd solution was added to a 50 mL plastic bottle, followed by 0.02 g YC, QK and QKA, respectively. The plastic bottle caps were tightened and then shaken under a constant temperature of 25 °C, 200 rpm, with the temperature setting at 10, 20 and 30, respectively. After 24 h, it was taken out and filtered through a ø 0.45 μm microporous membrane, and the Cd concentration in the filtrate was measured immediately.

2.4. Pot Experiment

The surface soil (0–20 cm) was taken from the Cd-contaminated land in Shang Hu Town, Changshu (120.626535 E, 31.717041 N). The soil was air-dried, ground to remove impurities and then passed through a 2 mm sieve and mixed evenly.

Circular plastic pots (150 mm in diameter and 120 mm high) were used in the experiment. A total of 1.5 kg of soil was weighed and added to each pot to reach a soil layer of approximately 10 cm. A total of 2% of the modified biochar QK, QKA and unmodified biochar (YC) were added to the tested soil (weight percentage) based on previous studies [21,22], then stirred and mixed with the soil sample evenly. The basic properties of the tested soil and bulk/modified biochar are listed in

Table 1. Basic properties of soil and biochar tested in this study.

Sample	pH	EC (µS cm−1)	Organic Matter (g kg−1)	Nitrogen (N) (g kg−1)	Available Potassium (g kg−1)	Available Phosphorus (mg·kg−1)	Cadmium Content (mg·kg−1)
Soil	6.63 ± 0.03 d	163.56 ± 2.28 d	23.95 ± 1.5 c	1.18 ± 0.09 c	0.24 ± 0.06 d	21.65 ± 1.41 b	2.16 ± 0.09 a
QK	10.54 ± 0.05 b	6120.4 ± 18.5 a	1054.05 ± 40.7 b	5.14 ± 0.27 b	43.07 ± 1.96 a	29.87 ± 2.75 a	0.042 ± 0.00 b
QKA	11.21 ± 0.08 a	2689.7 ± 17.9 b	1067.67 ± 54.2 b	5.22 ± 0.19 ab	21.89 ± 1.18 b	30.16 ± 2.59 a	0.071 ± 0.01 b
YC	9.46 ± 0.03 c	1196.5 ± 13.4 c	1219.21 ± 58.6 a	5.94 ± 0.32 a	10.44 ± 0.67 c	28.14 ± 2.04 ab	0.046 ± 0.01 b

Note: Values with the different letters in the same columns are significantly different from each other (p < 0.05). Bulk biochar (YC); biochar modified by ball milling with addition of KOH alone (QK) or combined with attapulgite (QKA).

. Soil without biochar addition was used as the control (CK), and each treatment was repeated 3 times. The soil prepared above was watered according to 70% of the field capacity, while stirring evenly, and stood for 2 weeks. Two weeks later, 30 carefully selected Chinese cabbage (Brassica chinensis Linn.) seeds were sown, and 10 plants of approximately uniform size with equal spacing were reserved in each pot after the seeds germinated. During planting, the pots were watered quantitatively to ensure the soil moisture was kept at 70% of the field capacity.

Chinese cabbages were harvested after 21 days of growth. The plants were first rinsed with tap water, then washed by deionized water. After that, the Chinese cabbages were divided into two parts: aerial parts and roots. The Cd content in the aerial parts and roots was determined by ICP-MS (NexION 2000G, Perkin-Elmer, Hopkinton, MA, USA) after being digested in HNO₃ in a microwave digestion system (MILESTONE ETHOS ONE, Milan, Italy) according to the method described by [13]. The detection and quantification limits of Cd in this method were 0.002 and 0.005 mg/kg, respectively.

2.5. Soil Analysis

The pH and EC values of soil were measured by a pH meter (METTLER, FE28, Changzhou, China) and conductivity meter (Shanghai Leici, DDS-307, Shanghai, China) after shaking 10 g soil with 25 mL deionized water and 20 g soil with 100 mL deionized water, respectively. The total Cd concentrations of the soil were measured by ICP-MS (NexION 2000 G, Perkin-Elmer, Massachusetts, USA) after 0.1 g of soil was digested using 9 mL of HNO₃ and 3 mL of HCl in a microwave digester (MILESTONE ETHOS ONE, Milan, Italy). The available Cd concentrations of the soil were extracted according to a diethylenetriamine pentaacetic acid (DTPA) method and measured by atomic absorption spectrophotometry (Agilent 200 Series AA, Santa Clara, CA, USA), as described in GB/T 23739-2009 [23]. The modified Community Bureau of Reference (BCR) procedure was employed to sequentially extract four Cd fractions from the soil: exchangeable fraction (EXC), extracted by 0.11 mol L⁻¹ acetic acid; reducible fraction (RED), extracted by 0.5 mol L⁻¹ hydroxylamine hydrochloride; oxidizable fraction (OXD), first oxidated by 8.8 mol L⁻¹ H₂O₂ and then extracted by 1.0 mol L⁻¹ ammonium acetate; and residual fraction (RES), digested according to the total Cd after drying and grinding to pass through 100 mesh sieve [24,25]. The concentrations of different Cd fractions were measured by ICP-MS (NexION 2000G, Perkin-Elmer, Cambridge, MA, USA). Soil organic matter (potassium dichromate—volumetric method), total nitrogen (Kjeldahl method), available phosphorus (NaHCO₃ extraction- molybdenum blue colorimetry), available potassium (ammonium acetate extraction-flame photometry) and CEC (ammonium acetate exchange method) measurements were taken as described by Sparks, 1996 [26]. The determination of biochar relevant properties refers to the above method.

2.6. Data Statistical Analysis

The data were calculated and plotted using Microsoft Excel 2017 and origin 2021. A one-way analysis of variance (ANOVA) was conducted by SPSS.19. The Duncan method was used to statistically analyze the significant differences among treatments (p < 0.05).

3. Results and Discussions

3.1. Characteristics of Modified Biochar

3.1.1. Elemental Analysis and BET Nitrogen Adsorption

Various modification methods could give biochar different physical properties, for instance pore diameter on the surface and pore volume alters its absorption performance. As shown in

Table 2. The physical/chemical properties of modified biochar.

Treatment	pH	O (%)	N (%)	C (%)	H (%)	S (%)	Specific Surface Area (m2·g−1)	Roe Volume (cc·g−1)	Pore Diameter (nm)
QK	10.54 ± 0.05 b	19.27 ± 0.12 a	0.51 ± 0.03 b	61.14 ± 1.02 b	2.04 ± 0.14 a	0.43 ± 0.02 a	21.454 ± 0.97 b	0.014 ± 0.002 b	1.351 ± 0.095 a
QKA	11.21 ± 0.08 a	18.89 ± 0.09 b	0.52 ± 0.03 b	61.93 ± 1.36 b	1.94 ± 0.09 a	0.25 ± 0.03 b	30.802 ± 1.01 a	0.018 ± 0.001 a	1.351 ± 0.078 a
YC	9.46 ± 0.03 c	16.32 ± 0.10 c	0.59 ± 0.02 a	70.72 ± 1.11 a	1.99 ± 0.09 a	0.25 ± 0.01 b	31.120 ± 1.04 a	0.017 ± 0.001 a	1.348 ± 0.052 a

Note: Values with the different letters in the same columns are significantly different from each other (p < 0.05). Bulk biochar (YC); biochar modified by ball milling with the addition of KOH alone (QK) or combined with attapulgite (QKA).

, the major elements contained in the biochar were carbon (C), hydrogen (H), oxygen (O), nitrogen (N) and sulfur (S). Carbon had the highest concentration, while the others were relatively low. It is remarkable that ball milling with either KOH (QK) or KOH + attapulgite (QKA) could enhance H and S contents, while decreasing those of C and N. The elevation of H and S concentrations could be due to the QK and QKA treatments with extra modifiers, which led to more N, S and H accumulated than bulk biochar (YC). Meanwhile, as shown in Table 2, the surface area in QK biochar shrunk significantly, while the value in QKA was comparable to the value in YC. The decrease in the specific surface area in QK could be due to the fact that the modifier occupied the pore channels of biochar and weakened the physical adsorption capacity of biochar [27].

3.1.2. Analysis of Surface Morphology

As shown in Figure 2, the SEM images revealed distinct morphological differences between bulk biochar (YC) and modified biochar. Many white flocs can be seen on the surface of QK and QKA, while YC displayed a smooth and flat surface. The surface of QK loaded with KOH showed a rougher micro/nano multistage surface structure compared with QKA, which may contribute to the availability of active sites for binding heavy metal ions [11]. The co-pyrolysis of reed straw biochar and modifier KOH destroys the smooth surface of bulk biochar [28]. With the improvement of the adsorbent properties, the composite material presented a denser structure. After pyrolysis and grinding, it was mostly stacked and had more adsorption sites than bulk biochar [29]. The uneven surface morphology indicated that the KOH has been successfully fixed on the biochar surface; similar results have been reported in previous modification studies [30].

3.1.3. Analysis of Infrared Spectrum

Infrared spectrum analysis proved the profile of the functional group on the biochar surfaces. As seen from Figure 3a, the surfaces of QK, QKA and YC were all rich in some of the same functional groups. The characteristic peak at 3500 cm⁻¹ was attributed to the stretching vibration of hydroxyl (-OH), which was significantly enhanced after biochar modification. The peak at 2900 cm⁻¹ was generated by the methylene (CH₂) vibration. It was speculated that the characteristic peaks observed at 1590 and 1388 cm⁻¹ were caused by the stretching vibration of functional group carbonyl (C=O) and carboxyl (COOH), respectively. In the area of the bending vibration of methylene (-CH) at 1100 cm⁻¹, it could be seen that the vibration intensity of COOH was amplified, which indicated that the modified biochar contained an increasing number of carboxyl groups [31]. Different chemicals milled together with biochar such as carboxylic acid, ammonium bicarbonate, alkali, etc., could endow specific properties or load functional groups onto biochar [32]. During the ball-milling process, the transfer of mass and energy and the generation of mechanical stress led to the breaking of the lattice structure of the biochar. The generated energies can aggravate the fracture of bonds and produce oxygen-containing functional groups in diverse ways [15]. These functional groups could complex with Cd, thus reducing its bioavailability [33].

3.1.4. XRD Pattern Analysis

The XRD patterns of the biochar (Figure 3b) showed significant differences in the mineral compositions of YC, QK, and QKA—QK and QKA treatments had more characteristic diffraction peaks compared to YC. All samples had diffraction peaks of SiO₂ at 26° and 31°, and both QK and QKA had diffraction peaks of K at 20.2°, 42°, 28° and 30°, indicating the successful introduction of K into biochar by the modification. Different from QK, the characteristic diffraction peaks of CaO and FeO at 29° were observed on the XRD pattern of QKA, suggesting the successful preparation of the KA–biochar composite and more minerals introduced in QKA [34]. The pH and EC values, as well as the bulk oxygen content, in QK and QKA were higher than in YC, reflecting the induction of potassium hydroxide and/or attapulgite, which inevitably altered the chemical structure of the biochar [11].

3.2. Kinetics and Isotherms of Cd

3.2.1. Adsorption Kinetics

The Cd adsorption kinetic curves of the different biochar treatments are shown in Figure 4. The Cd adsorption capacity of bulk biochar (YC) reached about 4.00 mg·g⁻¹ within 5 min, and it increased to 22.70 mg g⁻¹ within 24 h. By comparison, the Cd adsorption capacity of QK biochar reached about 36.81 mg·g⁻¹ within 5 min, and it increased to 44.79 mg g⁻¹ within 24 h. The removal rate reached 97.84% in 360 min, and the adsorption capacity was 44.1 mg·g⁻¹. The adsorption capacity of QKA biochar reached about 22.67 mg·g⁻¹ in the fifth minute, and 37.21 mg g⁻¹ in the 24th hour. The removal rate reached 65.91% in 360 min, and the adsorption capacity was 30.64 mg·g⁻¹, which was lower than that of QK modification. The results imply that the KOH treatment provided additional affinity and more available binding sites for modified biochar and accelerated the uptake of Cd. According to the results, the combination of KOH and attapulgite accelerated the reaction rate and shortened the time for adsorption equilibrium. Compared to YC, the modified biochar was a fast adsorption process in the first 15 min, and then the adsorption speed slowed down. It could be due to that there were enough Cd adsorption sites on the biochar surface at the beginning of experiment, so the adsorption was faster. As time passed, the adsorption sites were gradually occupied, so the kinetic curves went down with the adsorption speed slowing down [22].

The quasi-first-order and quasi-second-order dynamic models were applied to fitting the adsorption kinetics of Cd by biochar, with the results illustrated in

Table 3. Kinetics fitting Cd adsorption by QK, QKA and YC treatments.

		Quasi-First-Order Dynamics Equation			Quasi-Second-Order Dynamics Equation
Treatment	qe exp	qe	Kl	R2	qe	K2	R2
QK	44.10	42.78	0.392	0.978	44.80	1.38 × 103	0.995
QKA	30.64	30.63	0.251	0.976	36.43	2.75 × 104	0.996
YC	22.70	21.15	0.002	0.986	22.59	6.5 × 10	0.961

Note: Bulk biochar (YC); biochar modified by ball milling with addition of KOH alone (QK) or combined with attapulgite (QKA).

. The adsorption of Cd by QK and QKA was better fitted by the quasi-second-order model, with correlation coefficients of 0.995 and 0.996, respectively, which surpassed those of the quasi-first-order kinetic model at 0.978 and 0.976, respectively. In contrast, Cd adsorption by YC was better predicted by quasi-first-order kinetic equation, and its fitting correlation coefficient reached 0.986, which was higher than the corresponding quasi-second-order kinetic value of 0.961. The q_e values obtained by quasi-first-order and quasi-second-order kinetic models are shown in Table 3, and the actual measured q_{e exp} values are shown in Figure 5. These values indicate the effective fitting of Cd adsorption process by modified biochar in solution. The fitting results show that the quasi-second-order kinetic model was superior in the prediction of Cd adsorption by modified biochar in solution. Pseudo-second-order kinetics have been well employed in the studies concerning Cd adsorption by modified biochar in water or solution [22,35]. The application results show that chemical adsorption, such as complexation and precipitation, mainly contribute to the adsorption process. On the other hand, according to the results shown in Table 3, YC biochar mainly had physical adsorption; and this result was consistent with the electron microscope analysis results, which demonstrated the advantages of YC biochar in specific surface area, pore diameter and volume.

3.2.2. Adsorption Isotherm

The Langmuir and Freundlich adsorption isotherm models are widely applied to express the adsorption of heavy metals by different adsorbents [35,36]. The isothermal adsorption curves and fitting parameters are shown in Figure 5 and

Table 4. Isotherm fitting of Cd adsorption by QK, QKA and YC.

Treatment	Langmuir			Freundlich
	Qm	KI	R2	Kf	1/n	R2
QK	240.67	3.9 × 10−3	0.978	3.88	0.612	0.983
QKA	339.97	1.7 × 10−3	0.976	3.03	0.624	0.984
YC	160.41	2.6 × 10−3	0.989	1.09	0.084	0.981

Note: Bulk biochar (YC); biochar modified by ball milling with addition of KOH alone (QK) or combined with attapulgite (QKA).

, respectively. As shown in Table 4, the fitting coefficient R² of the Freundlich model for bulk biochar (YC) and two kinds of modified biochar (QK and QKA) was greater than R² > 0.98, especially for the two kinds of modified biochar, for which the fitting coefficient R² was higher than YC. The results indicate that the established model was in good agreement with the adsorption test results of Cd. Compared to the Freundlich model, the Langmuir model was more suitable to fit the adsorption data of bulk biochar (YC), R² > 0.98, while the fitting coefficient R² of the two modified biochars (QK and QKA) was approximately 0.97, slightly lower than that of the Freundlich model. QK and QKA exhibited highly efficient adsorption capacities for Cd, and its corresponding K_f values were 3.88 and 3.03 for Cd, respectively, which were both larger than those of YC (1.09). This result demonstrates that ball milling and KOH and/or attapulgite loading markedly improved the biochar’s adsorption capacities for Cd [37].

In the Freundlich isothermal adsorption model, the parameter “1/n” represents the linear relationship of the adsorption process. The expression 1/n > 1 represents that the adsorption is potentially physical adsorption; and 1/n < 1 represents that the adsorption is chemisorption. When n equals to 1, the adsorption capacity is linear. As seen in Table 4, 1/n < 1 in this experiment indicated that the adsorption of Cd by QKA, QK and YC was mainly chemical adsorption. The 1/n value of the Freundlich adsorption isotherm reflects the non-uniformity of adsorption surface; while the higher 1/n value indicates that the non-uniform surface had a wide distribution of adsorption sites. Generally, it was considered that adsorption was easy when 0.1 < 1/n < 0.5, but hard when 1/n > 2 [38]. In the present study, 1/n ranged from 0.084 to 0.624, indicating the relatively easy adsorption capacity of biochar. R² indicated the suitability of the model. In our research, the Freundlich isothermal adsorption model fit the experimental data of modified biochar well, with an R² higher than 0.98, indicating the dominance of multi-layer adsorption or non-ideal surface for Cd adsorption by modified biochar. In addition, the value of parameter K_I obtained by the Langmuir isothermal adsorption model was between 0 and 1, but closer to 0. This result further indicates that adsorption was favorable, with a strong adsorption capacity and irreversible adsorption. The Langmuir isotherm model fitting R² of YC was higher than the Freundlich isotherm model, which indicates that YC displayed monolayer adsorption, with Cd adsorption taking place on the homogeneous surface of the biochar. Summarily, the modified biochar had a multi-layer structure and uneven binding sites; each adsorption site had the same affinity with adsorbate, and each site could only accommodate one adsorbent species. Cd adsorption by modified biochar in water was a multi-layer and uneven reaction.

3.3. Factors Influencing Cd Adsorption in the Solution

3.3.1. Influence of Temperature on the Adsorption Capacity of Modified Biochar

Figure 6a shows the effect of QK, QKA and YC on Cd adsorption at 10–40 °C. It demonstrates that the Cd removal rate by QK and QKA was always higher than that of YC at any temperature. The adsorption capacity of Cd by QK and QKA was promoted in parallel with the increasing temperature; with the highest values of 36.76 and 26.59 mg·kg⁻¹, respectively; while that of YC fluctuated slightly with the increase in temperature, but also showed an overall upward trend. This result indicates that the Cd adsorption process by biochar was endothermic, and higher temperatures were more conducive to the removal of Cd [34].

3.3.2. Effect of pH on Cd Adsorption Capacity by Modified Biochar

As shown in Figure 6b, the adsorption capacity of Cd by YC was first reduced and then increased with the increase in pH value in the solution. However, the adsorption capacity of Cd by QK and QKA increased steadily with the elevated pH value in the solution. Compared to YC, QK and QKA had significantly better adsorption effects on Cd in the acidity range of this study. The hypothesis could be that abundant hydrogen ions in solutions with a low pH value could occupy the active centers of the functional groups, which resulted in the low adsorption to Cd cation. With the increase in pH value, the surface functional groups of carbon would produce a large number of hydrogen ions, which would expose its negatively charged active center, thus increasing its adsorption rate [38].

3.4. Effects of Modified Biochar on Soil Properties

3.4.1. Changes in the Soil pH, EC and CEC

The pH values of both the bulk and modified biochar were alkaline (Table 2). After the addition of the biochar, the pH value in soil was significantly increased (Figure 7a); similar results were found in previous studies [39,40]. Compared to CK, the soil pH values in the QK, QKA and YC treatments increased by 0.46, 0.23 and 0.13, respectively, among which QK had the most evident effect on soil pH. The reason might be that biochar, attapulgite and KOH contain alkaline components and salt-based ions (e.g., K⁺, Ca²⁺ and Na⁺, etc.), which could be released into the soil and contribute to the increase of pH in soil [41]. The abundant negative charge on the surface of biochar particles is of importance to reduce soil pH value due to the binding of H⁺ [13,42]. The elevated soil pH value would enhance the adsorption strength of Cd ions by clay and colloid, reduce the extractable content of Cd in soil, and increase the opportunity of bonding heavy metal cation, thus reducing their availability [43]. Therefore, the soil pH value elevation induced by the application of modified biochar could achieve part of the remediation effect on heavy metals pollution.

Figure 7b shows the change in soil EC value when biochar was applied. Compared to CK, YC had a tendency to increase the EC value of soil, though the difference was not significant. In contrast, after QK and QKA were applied to the soil, the soil EC value increased from 163.56 us·cm⁻¹ to 254.5 and 232.2 us·cm⁻¹, respectively, with an increase of 55.6 and 42.0%. Similar to previous studies, the application of both bulk and modified biochar could effectively increase soil EC, which may be due to the large amounts of soluble ions and ionizable groups in these materials [13,44].

As shows in Figure 7c, the addition of modified biochar could significantly enhance the CEC contents in soil. Compared to YC, the CEC of soil in the QK and QKA treatments also significantly increased by 15.34% and 14.77%, respectively. This could be ascribed to the large number of oxygen-containing functional groups such as –OH and –COOH, which had strong cation adsorption capacities and thus increased the cation exchange capacity [16,37]. The application of QK and QKA changed the physical characteristics, chemical properties and structure of soil itself. It made cation exchange activities more active, thus increasing the amount of cation exchange in soil and promoting the ion exchange between biochar and Cd [45], which was helpful for promoting the fertility of the soil and the growth and yield of crops [46,47].

3.4.2. Change in Soil Fertility

The contents of organic matter in soil represent the strength of soil fertility, which plays an important role in stabilizing soil quality, controlling the distribution of heavy metals in soil particles and reducing the bioavailability of heavy metals in soil [46]. Compared to CK, the application of biochar increased soil organic matter (OM) significantly (Figure 8a), with the same result reported by Jamieson et al. [48]. It could be due to the high content of organic carbon in the modified biochar. Simultaneously, on the contact surface of biochar and soil, soil organic molecules are catalyzed to form organic matter [12].

Total nitrogen in the soil with the addition of biochar increased by 0.2 g·kg⁻¹ compared to CK (Figure 8b), which indicates that biochar effectively increased the total nitrogen content in the soil. However, no significant difference was found between bulk biochar and modified biochar, because the modifiers did not bring more nitrogen into the soil. The available phosphorus content in the soil of the YC treatment was 21.21 mg·kg⁻¹, while those of QK and QKA were 23.46 and 23.01 mg·kg⁻¹, respectively, which was not significantly different from the CK treatment (Figure 8c). The data depicted above also shows that phosphorus was not introduced in YC or the modified biochar.

As shown in Figure 8d, the available potassium contents (AK) in soil were increased by 1~7.3 folders in the different modified biochar treatments compared to CK. AK in CK soil was 0.13 g/kg and QK reached 1.08 g·kg⁻¹, followed by QKA and YC, which were 0.7 and 0.24 g·kg⁻¹, respectively. This result indicates that the K-rich modifier (KOH) could significantly promote the available potassium content in soil, which contributes to improving potassium-deficient soil while repairing heavy metals. The increase in the availability of K in soil produces a positive effect on plant physiological processes, which will promote plant growth and yield, nutritional quality, and relieve stress resistance [47,49].

3.5. Influence of Modified Biochar on Cd Bioavailability in Soils

The environmental risk of heavy metals depends on their bioavailability rather than their total concentration, which is primarily influenced by their chemical form. Therefore, the speciation of heavy metals is a crucial factor affecting their environmental risk [50]. As shown in Figure 9a, DTPA-Cd in the soil was 0.99, 1.00, 1.10 and 1.29 for the QK, QKA, YC and CK treatments, respectively. Biochar addition significantly reduced the content of DTPA-Cd. The reduction of DTPA-Cd was 22.61%, 22.32% and 14.16%, respectively, for the QK, QKA and YC treatments compared to CK. The results suggest that QK and QKA resulted in a more significant reduction in Cd availability in soil compared to that in bulk biochar treatment. By comparison, the immobilization and capture of Cd by QK and QKA were more evident than those by YC.

The content of different Cd fractions in the soil was influenced by the addition of biochar (Figure 9b). The proportions of EXC-Cd, RED-Cd, OXD-Cd and RES-Cd in CK were 29.26%, 68.76%, 0.21% and 1.77%, respectively. Compared to CK, the application of QK, QKA and YC lowered the percentage of EXC-Cd and RED-Cd by 4.24–26.04% and 37.27–47.96%, respectively. The OXD-Cd fraction was relatively low and stable (0.2–0.3%), which was not susceptible to the utilization of biochar with or without modification.

The exchangeable/acid-soluble fractions of heavy metals are the most bioavailable and toxic, being directly bioavailable to plants and soil organisms. The reducible and oxidizable fractions are also bioavailable under certain conditions. These fractions, known as bioavailable states, increase the likelihood of heavy metal release and secondary contamination [51]. In contrast, the residual fractions are not readily taken up biologically, have the weakest toxicity, and are considered stable and non-toxic [52]. The contents of RES-Cd in the soil of the QK, QKA and YC treatments were 42.13%, 42.18% and 28.60%, respectively, significantly higher than that in CK (1.77%). In general, the application of biochar contributed to the reduction of the proportions of EXC-Cd and RED-Cd, and the enhancement of the residual Cd (Res-Cd) percentage (Figure 9b), known as the immobilization or passivation of heavy metals. The above results indicate that the biochar, particularly modified by QK and QKA, converted Cd from the available to the residual fraction, and, therefore, reduced their mobilization in the biochar-amended soil as compared to that of CK. The results were similar to those reported in previous studies [53,54].

In this study, the passivation effects of QK and QKA were quite similar, due to their similar properties after modification. The passivation of Cd by biochar was governed by complicated mechanisms. First, the application of biochar could enhance the organic carbon content, mineral fraction and CEC in soil, which promotes the immobilization of Cd by forming mineral phases and complexation, and thus facilitate Cd in the soil to stabilization fraction [55]. Second, the increase in soil pH following biochar amendment facilitates the deprotonation of functional groups (–OH, C–O–C and –COOH) in biochar, prevents the competitive binding of H⁺ [56], and accelerates hydrolysis of Cd, thus contributing to the immobilization of Cd [57]. Moreover, the higher pH in the QK and QKA treatments than YC facilitated the formation of alkaline precipitation of Cd strongly, contributing to the significant increase in the residual fraction [58]. In general, mechanisms involved in soil Cd immobilization by modified biochar include more intense physical absorption, ion exchange, complexation, precipitation, π-interaction and electrostatic interaction [59,60].

3.6. Influence of Modified Biochar on Plant Growth and Cd Accumulation in Plants

3.6.1. Influence of Modified Biochar on Plant Height, Root Length and Biomass of Chinese Cabbage

Table 5. Effect of modified biochar on the growth and biomass of Chinese cabbage.

Treatments	Plant Height (cm)	Root Length (cm)	Arial Part Fresh Weight (g pot−1)	Root Fresh Weight (g pot−1)
QKA	20.89 ± 0.18 ab	9.75 ± 0.73 ab	107.13 ± 3.14 a	12.81 ± 0.33 ab
QK	21.20 ± 0.43 a	10.16 ± 1.01 a	108.26 ± 5.54 a	13.23 ± 0.29 a
YC	21.28 ± 1.50 a	8.48 ± 1.00 ab	106.53 ± 4.17 ab	12.73 ± 0.29 ab
CK	19.70 ± 0.35 b	7.96 ± 0.49 b	96.70 ± 3.79 b	11.76 ± 0.33 b

Note: Values with the different letters in the same columns are significantly different from each other (p < 0.05). Bulk biochar (YC); biochar modified by ball milling with addition of KOH alone (QK) or combined with attapulgite (QKA).

shows the response of Chinese cabbage to modified biochar in terms of growth and biomass. The plant height significantly increased in the QKA, QK and YC treatments compared to CK. The root length significantly increased by 27.6% after application of QK compared to CK. Due to its porous structure and high surface area, biochar is able to absorb nutrients and retain water, thus promoting the development of roots [61]. The fresh weight of aerial part and plant roots in the QK treatment were significantly higher than those in CK, followed by QKA and YC. QK modification showed a tendency to promote plant growth (plant height, root height, fresh weight of aerial part and fresh weight of plant roots) compared with bulk biochar (YC). However, no significant differences were observed between the QA/QKA and YC treatments. In general, the application of biochar significantly improved plant growth compared to CK, which was in line with previous studies that have been conducted [62,63]. The biochar might supply an alternative source of macro- and micronutrients, such as nitrogen, phosphorus and potassium, all of which improve plant growth and are useful as soil fertilizer when applied to the soil [64]. Additionally, biochar could modulate soil microbial communities to improve plant stress resistance, and increase biotic interaction and water-holding capacity, thereby increasing crop yields [65]. Moreover, modified biochar may also contribute to the slow release of macro- and micronutrients into the soil for crops, reducing nutritional losses and promoting fertilizer utilization efficiency that are vital for sustaining long-term agricultural production and environment health [66].

3.6.2. Influence of Modified Biochar on Cd Accumulation by Chinese Cabbage

Figure 10 shows the Cd contents in different parts of Chinese cabbage (Brassica chinensis Linn.) in the soil of different treatments. It can be seen that Cd was more likely to accumulate in the roots than in the aerial parts of Chinese cabbage, which was consistent with most study results [58,67]. The application of biochar could effectively reduce Cd accumulation in Chinese cabbage. The Cd content in Chinese cabbage roots treated with YC was lower than that in CK by 15.68%. Cd accumulation in the root of Chinese cabbage treated with QK and QKA were decreased by 42.49% and 34.20%, respectively, compared to CK, and by 31.72% and 21.88%, respectively, compared to YC. That is, the Cd content in the roots of Chinese cabbage was significantly reduced by adding modified biochar compared with unmodified biochar.

Compared to CK, Cd contents in the aerial parts of Chinese cabbage treated with bulk biochar (YC) decreased by 7.29%. Modified biochar application could reduce Cd contents in the aerial parts of Chinese cabbage significantly, and reduced Cd content in the QK and QKA treatments by 42.97% and 18.16%, respectively, compared to CK; and 38.48% and 11.72%, respectively, compared to YC. With the application of QK, Cd concentrations in the aerial/edible part of Chinese cabbage was reduced to 0.15 mg kg⁻¹, which is below the 0.2 mg/kg of the leafy vegetables’ national safety standard for Cd (GB2762-2022) [68]. These results illustrate that modified biochar could effectively reduce Cd accumulation in both the aerial parts and roots of Chinese cabbage compared to unmodified biochar. Due to modified biochar application, the immobilization and capture of Cd in the rhizosphere soil and pore water resulted in the reduction of Cd transport from soil to Chinese cabbage plants. In addition, modified biochar can absorb more available Cd from the soil solution and fix them onto solid particles than unmodified biochar, thereby reducing Cd uptake by plants [69]. It should be noted that the performance of QK and QKA on the remediation effects and vegetable production may not be consistent across all soils, as they were highly dependent on the physicochemical characteristics of the biochar and soil properties [70], especially the granulomeric composition of the soil and the organic matter composition (in particular, the content of humic acid).

4. Conclusions

In the present study, the pH and EC of reed biochar were significantly improved by ball milling modified with the addition of either KOH alone or combined with attapulgite. Compared with the bulk biochar (YC), modified biochar provided more oxygen-containing functional groups, minerals and higher pH values, which could promote the adsorption, complexation and precipitation reaction of biochar to Cd. The modified biochar was more inclined to chemical adsorption, which was an endothermic process. In addition, Cd adsorption by modified biochar underwent multi-layer adsorption and non-ideal surface adsorption. In general, the addition of the modified biochar exhibited good performance in immobilizing Cd in soil and limiting Cd translocation to the plants. The application of both bulk and modified biochar (QK and QKA) significantly improved plant growth compared to CK. QK produced a better effect than QKA in reducing Cd availability in soil and Cd accumulation in Chinese cabbages. With the amendment of QK, the Cd content in the aerial (edible) parts of Chinese cabbage was below 0.2 mg/kg, the leafy vegetables’ national safety standard for Cd (GB2762-2022, China) [68].

Further studies are needed to validate the feasibility and stability of modified biochar in different types of soil under different Cd pollution conditions, and to optimize the operational parameters for application-oriented and economical large-scale production. Furthermore, field experiments should be conducted to evaluate the actual application potential of modified biochar for a range of vegetable types over various durations. In addition, long-term research on the environmental functions of modified biochar is needed.