Stabilization and Ecological Risk Evaluation of Heavy Metals in Farmland Soils by Addition of Attapulgite Modified with Phosphates

Duan, Cuiqing; Ren, Jun; Tao, Ling

doi:https://doi.org/10.1155/2022/7533124

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Volume 2022 | Article ID 7533124 | https://doi.org/10.1155/2022/7533124

Stabilization and Ecological Risk Evaluation of Heavy Metals in Farmland Soils by Addition of Attapulgite Modified with Phosphates

Cuiqing Duan,^1,2,3Jun Ren ,^1,2,4and Ling Tao^1,2,4

Academic Editor: Yaxiang Fan

Received13 Jun 2022

Accepted04 Jul 2022

Published28 Jul 2022

Abstract

Purpose. Attapulgite was modified by sodium dihydrogen phosphate, oxalic acid-activated phosphate rock powder, potassium dihydrogen phosphate, calcium superphosphate, ammonium dihydrogen phosphate, and fused calcium-magnesium phosphate and used in the remediation of Cd, Zn, and Ni. Materials and Methods. Attapulgite was modified by six kinds of phosphate (ratio: 1 : 2), and the improvement effect of passivation material on soil polluted by cadmium, zinc, and nickel was determined. CaCl₂-extractable and toxicity characteristic leaching procedure- (TCLP-) extractable Cd, Zn, and Ni were measured in order to estimate the bioavailability and the stabilization efficiency. Pot experiment was conducted to study the enrichment and transport ability of Cd, Zn, and Ni in corn. The ecological risk and ecological toxicity of soil environment were evaluated by calculating SE_m, ERI_m, CRI_m, and BUF. Results and Discussion. Compared with ATP, passivation materials AAPR, AMRP, ASSP, AMAP, and AFMP can improve the stability of CD, Zn, and Ni in soil, and AAPR has the best effect. Compared with CK treatments and ATP treatments, the concentrations of TCLP-extractable Cd decreased by 30.80% and 24.72%, respectively, the concentrations of TCLP-extractable Zn decreased by 15.50% and 11.18%, respectively, and the concentrations of TCLP-extractable Ni decreased by 31.34% and 23.20%, respectively. Compared with ATP treatments, CRI, BUF-_Cd, BUF-_Ni, and BUF-_Zn decreased by 24.67%, 52.88%, 78.73%, and 41.18%, respectively, in the AAPR treatments. Conclusions. Phosphate-modified attapulgite can effectively improve the stability of heavy metals in soil and reduce the migration of heavy metals. In the soil polluted by Cd, Zn, and Ni, the passivation effect of AAPR is the best. Therefore, AAPR can be used as an economical, safe, and effective passivation material to improve Cd-, Zn-, and Ni-contaminated soil, which would have a high utilization value in field applications.

1. Introduction

In recent years, with the continuous emergence of heavy metal pollution in farmland soil and the aggravation of pollution degree [1], the fact that how to reduce the transfer of heavy metals from soil to crops without affecting farmland crop cultivation has become an urgent problem to be solved. Therefore, heavy metal chemical passivation technology becomes the main research method [2]. At present, passivation materials mainly enhance the adsorption capacity through exploring and modifying all kinds of clay minerals, biological state, industrial by-products, phosphate minerals, etc. [3] and transforming the reduced, oxidized, and extractable heavy metals in the soil into residue, which are fixed in the improver to reduce the migration in the soil [4–6].

As a kind of clay mineral, attapulgite has porous crystal structure and contains tetrahedral layer alloyed by longitudinal chain [7]. This kind of hydrated aluminosilicate mineral is composed of parallel zone of 2 : 1 layer [8]. It has moderately high structural charge and medium specific surface area and can adsorb heavy metals in the crystal structure, thus playing a good role in passivating heavy metals [9]. Studies have shown that adding 4% attapulgite can reduce 92% lead, 77% copper, and 76% cadmium in sandy soil [8]. Tan et al. have shown that by adding appropriate amount of attapulgite clay minerals, the average repair rates of Zn and Cd can reach 26.15% and 34.92%, respectively [10]. After studying the remediation effect of attapulgite on soil and rice and analyzing its potential immobilization mechanism, Liang et al. concluded that the concentration of cadmium extracted by hydrochloric acid, TCLP, calcium chloride, alkali hydrolyzable nitrogen, and hydrobromate could be significantly reduced by adding different amounts of attapulgite, resulting in a significant decrease in the concentration of cadmium in brown rice [11]. The results showed that attapulgite has a good passivation effect on heavy metals.

As a widely used passivator of heavy metals in soil, phosphate can effectively fix cadmium and zinc in soil by coprecipitation, ion exchange, and complexation, thus reducing the transfer of heavy metals to plants [12]. Zhang et al. showed that the leaching concentration of zinc and cadmium in soil with 2-10% APR, decreased by 15-52% and 12-62%, respectively, which had a good passivation effect on cadmium and zinc in soil [13]. The use of hydroxyapatite by Yu et al. could make the curing efficiency of cadmium reach 52.7% [14]. Xiuli et al. found that the application of potassium dihydrogen phosphate, diammonium hydrogen phosphate and zeolite and the combined application of zeolite and dipotassium hydrogen phosphate, zeolite, and diammonium hydrogen phosphate reduced the soil available Cd content by 25.2%, 51.7%, 21.6%, 46.8%, 38.6%, 61.4%, and 34.1%, respectively [15].

To sum up, attapulgite and phosphate can passivate cadmium, zinc, and nickel in soil, but the extensive use of phosphate will cause soil eutrophication and the imbalance of nutrient elements in soil and cause secondary pollution of the environment; at the same time, the cost is more higher. As a natural clay mineral, attapulgite has the advantages of high output, low price, and no secondary pollution. Therefore, it is an effective way to combine phosphate with attapulgite to improve the efficiency of attapulgite passivation of heavy metals. Based on aforesaid view, in the experiment, a kind of phosphate which has the best passivation effect on heavy metals in soil was selected and loaded attapulgite with a variety of phosphates, thus providing a theoretical basis for attapulgite to be put into production as a passivator in the later stage.

2. Materials and Methods

2.1. Soil

Farmland soil sample was collected at 0-20 cm depths from arable land in Silong Town (36° 29 26.2 N, 104° 16 57.4 E), Gansu province, northwest China. The main crop in the area is maize, and the main source of pollution is the erosion of waste water from a nearby chemical factory into the soil. Extracted sample was air-dried and then ground to pass through a 2 mm sieve for the pot trial. The physical and chemical properties of the soil are shown in Table 1.

2.2. Attapulgite-Modified Phosphate

The attapulgite used in the experiment came from Gansu Hanxing Environmental Protection Co., Ltd. It has a fiber bundle structure and is mainly composed of quartz, dolomite, muscovite, chlorite, feldspar, and other minerals [16, 17]. Before using attapulgite, it needs to be further purified and acidified to achieve a better passivation effect. The specific methods are as follows: put a certain amount of attapulgite into 1 L beaker and stir with deionized water. After stirring evenly, the attapulgite in the beaker was placed for 24 hours, after the attapulgite in the beaker was obviously delaminated, removed the impurities such as quartz at the bottom of the beaker, and thus obtained the purified attapulgite. Added 3 mol/L hydrochloric acid to the purified attapulgite for 30 min was oscillated with 200 r/min, then filtered, then washed repeatedly to neutral with deionized water, dried in a drying oven at 105°C, ground, and passed through 100 mesh sieve to obtain acid-activated attapulgite (ATP). Sodium dihydrogen phosphate, oxalic acid, ground phosphate rock, potassium dihydrogen phosphate, calcium superphosphate, ammonium dihydrogen phosphate, and fused calcium-magnesium phosphate were purchased from Guangdong Guanghua Science and Technology Co., Ltd., all of which were analytically pure. The preparation method of oxalic acid-activated phosphate rock powder is as follows: the oxalic acid and phosphate rock powder of 0.5 mol/L were mixed at the ratio of 10 : 1, cultured at 28°C for 6 days, dried at 60°C [18], ground and sifted through 100-mesh sieve, and set aside for further usage.

In this experiment, sodium dihydrogen phosphate, oxalic acid activated phosphate rock powder, potassium dihydrogen phosphate, calcium superphosphate, ammonium dihydrogen phosphate, and fused calcium-magnesium phosphate and attapulgite were mixed into beakers according to the proportion of 1 : 2, and then adding a certain amount of deionized water (solid-liquid ratio: 1 : 10), stirring for 2 hours, then ultrasonic 30 mins, drying at 80°C pyrolysis of the dried material in muffle furnace (KSW-6-12A) at 400°C for 2 hours, and then reduced to room temperature, dried, and sealed.

The attapulgite was mixed with sodium dihydrogen phosphate, oxalic acid-activated phosphate rock powder, potassium dihydrogen phosphate, calcium superphosphate, ammonium dihydrogen phosphate, and fused calcium-magnesium phosphate in a mass ratio of 2 : 1. The modification was carried out for 2 hours under the condition of solid-liquid ratio of 1 : 10 and stirring speed of 500 r·min⁻¹. Then, six kinds of mixed materials were put into ultrasonic cleaner (KH-500DV). Under the condition of 60°C and ultrasonic frequency 40 KHz, ultrasonic 30 min was put into a constant temperature blast drying box, dried at 70°C, then cooled to room temperature and ground through a 100-mesh sieve. Finally, six kinds of dried mixed materials were put into muffle furnace (KSW-6-12A), pyrolyzed at 400°C for 2 hours, and then, reduced to room temperature, that is, different phosphate-modified attapulgite passivation materials were prepared, which were marked AMSP, AAPR, AMRP, ASSP, AMAP, and AFMP, respectively.

2.3. Incubation Experiment

1.5 kg of soil samples was placed in a plastic basin, and the well-prepared passivating materials were evenly added into the basin according to 4% of the soil weight. In this experiment, there were 8 groups of treatments: (1) control group CK, (2) attapulgite treatment group ATP, (3) attapulgite treatment group modified with sodium dihydrogen phosphate AMSP, (4) attapulgite treatment group modified with oxalic acid activated phosphate powder AAPR, (5) attapulgite treatment group modified with potassium hydrogen phosphate ASSP, (6) attapulgite treatment group modified with calcium superphosphate ASSP, (7) attapulgite treatment group modified with ammonium dihydrogen phosphate AMAP, and (8) attapulgite treatment group modified with fused calcium-magnesium phosphate AFMP. Deionized water was added to each treatment, and the field capacity was kept at 70%. The soil was passivated at room temperature for 30 days, and three replicates were set for each treatment. The physical and chemical properties of soil and the content of bioavailable Cd, Zn, and Ni were determined.

2.4. Pot Experiment

Ten corn seeds were evenly scattered in the passivated soil of each pot and randomly placed in the artificial climate incubator to grow. After the corn germinated, the seedlings reached 5 per pot and continued to grow for 60 days. During the growth of corn, the same quality of deionized water was added to each pot of soil by weighing every other day. The aboveground and underground parts of the harvested corn samples were separated, and the impurities left on the plant surface were washed with deionized water, and the root length and stem length of each maize seedling were measured by micrometer. The fresh weight of the above ground and underground parts of the corn samples was determined. After the determination of fresh weight, put the sample in the oven, first kill 30 min at 105°C, and then dry at 70°C to constant weight. After determining the dry matter mass of the above ground and underground parts, respectively, the dry matter is ground and sifted through 100 mesh and then stored in a sealed bag.

2.5. Analytical Methods

Soil pH was measured with the soil/water ratio of 1 : 2.5 () using a pH meter (Lei ci pH3C,Shanghai); soil electric conductivity was measured with the soil/water ratio of 1 : 5 () using a conductivity meter (Lei ci pH3C, Shanghai); soil cation exchange capacity was determined by barium chloride-sulfuric acid forced exchange method, and soil organic matter was determined by potassium dichromate volumetric method. Available N, available K, and available P were determined by alkali hydrolysis diffusion method, NH₄OAC extraction flame photometer method, and sodium bicarbonate extraction-molybdenum-antimony resistance colorimetric method, respectively [19]. The total amount of heavy metals in soil was digested by HCl-HNO₃-HF-HClO₄ () method and determined by atomic absorption spectrophotometer (AAS, Persee TAS-990). The contents of heavy metals in the aboveground and underground parts of maize were determined by HNO₃--HClO₄ digestion, and the contents of cadmium and zinc in the samples were determined by atomic absorption spectrophotometer (AAS, PerseeTAS-990).

2.6. Measurement of Bioavailability of Heavy Metals

The bioavailable Cd, Zn, and Ni in soil were determined by atomic absorption spectrophotometer after extracted with CaCl₂ extract and TCLP extract [20]. The specific methods are as follows: put 1 g of dried soil sample into a centrifuge tube, add CaCl₂ solution of 25 mL0.1 mol/L or 20 mL TCLP extract, shake for 2 hours or 18 hours, respectively, at room temperature, centrifuge 15 min at 4000 rpm/min, extract the supernatant through 0.45um filter paper, and determine the concentration of Cd, Zn, and Ni in the filtrate by atomic absorption spectrophotometer (AAS, PerseeTAS-990).

2.7. Evaluation of Ecological Risk and Ecotoxicity

Stabilization efficiency (SE_m) of heavy metal (m) was calculated by Equation (1), where is the extractable concentration of heavy metal in the control soil (CK), and is the extractable concentration of heavy metal in the amended soil [21, 22].

Ecological risk index (ERI_m) of heavy metal (m) can be calculated by Equation (2) and was usually employed to assess the ecological risk of heavy metals in soil, sludge, biochar, and other materials, where is the total concentration of heavy metal from the soil, and is the extractable concentration of heavy metal from the treatments using kind extractable method. ERI_m had been widely used in the environment science for heavy metal toxicity assessment. ERI_m assesses the availability of heavy metals by applying the percentage of heavy metals present in bioavailability content. Five classification of ERI_m were showed as no risk, ERI_m lower than 1%; low risk, ERI_m in the range of 1~10%; medium risk, ERI_m in the range of 10~30%; high risk, ERI_m in the range of 30~50%; and very high risk, ERI_m higher than 50% [23].

The comprehensive risk index (CRI_m) for individual metal (m) may be calculated by Equation (3) and used to evaluate the ecological risk based on kind extractable methods. was the toxic coefficient of the individual heavy metal, the values of for Zn, Cr, Cu, Ni, Pb, and Cd were in the order of 1, 2, 5, 6, 5, and 30 [24], and the comprehensive risk index (CRI) for some kind of stabilizing material or method can be gotten by Equation (4) based on kind heavy metals:

Bioconcentration factor (BCF) and transfer factor (TF) was used to evaluate the transfer characteristics of heavy metals from the soil to the plant [25–27] as

The ability of plant to translocate and take up heavy metals was assessed using the biological uptaking factor (BUF) which was calculated by Equation (7) [28], where was the biomass of root and was the biomass of shoot.

2.8. Statistical Analysis

All results in this study were presented as the mean values of three replicates and standard deviations, and the data were analyzed using one-way analysis of variance (ANOVA) and the least significant difference (LSD) test with SPSS 22.0 statistics software, taking as the significance level. The graphs and the tables were drawn by Origin 2019 and Microsoft Excel 2016, respectively.

3. Results and Discussion

3.1. Physiochemical Traits

The effects of attapulgite modified with different phosphates on soil physical and chemical properties were shown in Table 2. The soil pH of 8 treatments were 7.41, 7.50, 7.26, 7.60, 6.81, 6.45, 6.28, and 7.00, respectively. Compared with CK treatments, the addition of ATP treatments and AAPR treatments increased soil pH by 0.09 and 0.19, respectively. AMRP treatments, ASSP treatments, AMAP treatments, and AFMP treatments decreased soil pH by 0.15, 0.6, 0.96, 1.13, and 0.41, respectively. The addition of passivation materials also changed the soil EC in the range of 2.28-1.90 ms/cm^-1, in which the AAPR treatments increased the soil EC by 0.12 ms/cm^-1 to 2.28 ms/cm^-1, and the soil EC value decreased slightly compared with CK treatments with the addition of other passivators, which decreased by 0.15 ms/cm^-1, 0.26 ms/cm^-1, 0.07 ms/cm^-1, 0.08 ms/cm^-1, 0.11 ms/cm^-1, and 0.014 ms/cm^-1, respectively. However, there was no significant difference between 8 treatments (). With the addition of passivation materials, soil CEC in different treatments showed an increasing trend compared with CK treatments, with an increase range of 1.15 cmol/kg⁻¹ to 91.75 cmol/kg⁻¹. Especially in the AAPR treatments, the soil CEC value increased by 91.75 cmol/kg^-1 compared with CK treatments, reaching 196.67 cmol/kg^-1. There was significant difference between AAPR treatments and other treatments (), and AMSP treatments, AMRP treatments, AMAP treatments, and AFMP treatments also showed significant differences compared with CK treatments ().

N, P, and K are necessary nutrient elements for plant growth. The content of N, P, and K in soil directly determines the biomass of soil plants. Phosphate, as a kind of fertilizer containing phosphorus, will greatly change the content of N, P, and K in soil. It can be seen from Table 2 that the contents of available P and K in soil increased in varying degrees after the addition of phosphate-modified attapulgite. Among them, the content of soil available P in AMSP treatments, AAPR treatments, AMRP treatments, ASSP treatments, AMAP treatments, and AFMP treatments increased by 152.91 mg/kg^-1, 211.20 mg/kg^-1, 184.87 mg/kg^-1, 147.84 mg/kg^-1, 183.66 mg/kg^-1, and 180.09 mg/kg^-1 compared with CK treatments, respectively. Compared with CK treatments, the content of soil available K increased by 45.84 mg/kg^-1, 130.98 mg/kg^-1, 517.43 mg/kg^-1, 117.85 mg/kg^-1, 78.55 mg/kg^-1, and 75.29 mg/kg^-1, respectively. Compared with CK treatments, the content of soil available N in AAPR treatments, ASSP treatments, AMAP treatments, and AFMP treatments increased by 6.33 mg/kg^-1, 5.94 mg/kg^-1, 44.34 mg/kg^-1, and 22.66 mg/kg^-1, while that in AMSP treatments and AMRP treatments decreased by 0.66 mg/kg^-1 and 0.65 mg/kg^-1, respectively. In the ATP treatments, the contents of available P and available K decreased by 2.96 mg/kg^-1 and 42.58 mg/kg^-1, respectively, compared with CK treatments, and the content of available N increased by 1.06 mg/kg^-1 compared with CK treatments. The contents of soil available P, available K, and available N in AMSP treatments, AAPR treatments, AMRP treatments, ASSP treatments, AMAP treatments, and AFMP treatments were significantly higher than those in CK treatments and ATP treatments, and the contents of soil available N in AAPR treatments, ASSP treatments, AMAP treatments, and AFMP treatments were significantly higher than those in CK treatments and ATP treatments (). According to the analysis of variance of soil available P, available K, and available N in soil, the contents of soil available P and K in AAPR treatments, ASSP treatments, AMAP treatments, and AFMP treatments were significantly higher than those in CK treatments and ATP treatments ().

3.2. Bioavailability of Heavy Metals

Figure 1 indicated that the concentrations of CaCl₂-extractable Cd, Zn, and Ni in the soil decreased to varying degrees. Compared with CK treatments, concentrations of CaCl₂-extractable Cd in the ATP treatments, AMSP treatments, AAPR treatments, AMRP treatments, ASSP treatments, AMAP treatments, and AFMP treatments decreased by 0.72 mg/kg⁻¹, 0.4 mg/kg⁻¹, 1.71 mg/kg⁻¹, 1.18 mg/kg⁻¹, 0.99 mg/kg⁻¹, 0.8 mg/kg⁻¹, and 1.15 mg/kg⁻¹, respectively. The concentrations of CaCl₂-extractable Cd in the soil treated with passivator were significantly different from that of CK (). Compared with CK treatments, when ATP, AMSP, AAPR, AMRP, ASSP, AMAP, and AFMP were applied into the soil, concentrations of CaCl₂-extractable Zn decreased by 0.27 mg/kg⁻¹, 0.31 mg/kg⁻¹, 1.49 mg/kg⁻¹, 0.97 mg/kg⁻¹, 0.42 mg/kg⁻¹, 0.43 mg/kg⁻¹, and 0.6 mg/kg⁻¹, respectively. Except for ATP treatments, the concentrations of soil available Zn in the other six treatments were significantly different from that in CK treatments (). Compared with CK treatments, concentrations of CaCl₂-extractable Ni in the ATP treatments, AMSP treatments, AAPR treatments, AMRP treatments, ASSP treatments, AMAP treatments, and AFMP treatments decreased by 0.83 mg/kg^-1, 1.22 mg/kg^-1, 2.10 mg/kg^-1, 1.18 mg/kg^-1, 1.48 mg/kg^-1, 1.42 mg/kg-¹, and 1.81 mg/kg^-1, respectively. After the attapulgite was modified by various phosphates, except for AMSP treatments, the concentrations CaCl₂-extractable Cd, Zn, and Ni in attapulgite modified by phosphate was lower than that of attapulgite. Among them, the effect of AAPR treatments was the best, and the contents of Cd, Zn, and Ni decreased by 0.99 mg/kg^-1, 1.22 mg/kg^-1, and 1.27 mg/kg^-1, respectively, compared with ATP treatments, and reached a significant difference compared with other treatments ().

Figure 1 indicated that the concentrations of TCLP-extractable Cd, Zn, and Ni in soil CK treatments were 8.67 mg/kg^-1, 60.06 mg/kg^-1, and 7.78 mg/kg^-1, respectively. When ATP, AMSP, AAPR, AMRP, ASSP, AMAP, and AFMP were applied into the soil, the concentrations of TCLP-extractable Cd, Zn, and Ni decreased in varying degrees. Compared with CK treatments, the concentrations of TCLP-extractable Cd decreased by 0.70 mg/kg⁻¹, 1.11 mg/kg⁻¹, 2.67 mg/kg⁻¹, 1.76 mg/kg⁻¹, 1.44 mg/kg⁻¹, 1.28 mg/kg⁻¹, and 1.26 mg/kg⁻¹, respectively. The concentrations of TCLP-extractable Zn decreased by 2.92 mg/kg⁻¹, 4.22 mg/kg⁻¹, 9.31 mg/kg⁻¹, 5.97 mg/kg⁻¹, 4.63 mg/kg⁻¹, 3.46 mg/kg⁻¹, and 2.92 mg/kg⁻¹, respectively. The concentrations of TCLP-extractable Ni decreased by 0.83 mg/kg⁻¹, 1.05 mg/kg⁻¹, 2.44 mg/kg⁻¹, 2.12 mg/kg⁻¹, 1.06 mg/kg⁻¹, 0.99 mg/kg⁻¹, and 1.27 mg/kg⁻¹, respectively. Compared with ATP treatments, the concentrations of TCLP-extractable Cd, Zn, and Ni under the influence of AMSP treatments, AAPR treatments, AMRP treatments, ASSP treatments, AMAP treatments, and AFMP treatments decreased by 5.14%-24.72%, 2.28%-11.18%, and 2.42%-23.20%, respectively. Among them, the effect of AAPR treatments were the best. Compared with CK treatments and ATP treatments, the concentrations of TCLP-extractable Cd decreased by 30.80% and 24.72%, respectively, the concentrations of TCLP-extractable Zn decreased by 15.50% and 11.18%, respectively, and the concentrations of TCLP-extractable Ni decreased by 31.34% and 23.20%, respectively. There was significant difference between AAPR treatments and other treatments ().

3.3. Evaluation of Ecological Risk

Figure 2 shows that under the condition of CaCl₂ extraction, the stabilization efficiency of Cd, Zn, and Ni in the untreated soil were 76.28%, 98.75%, and 91.41%, respectively. When ATP, AMSP, AAPR, AMRP, ASSP, AMAP, and AFMP were applied into the soil, the stabilization efficiency of Cd, Zn, and Ni were gradually enhanced. Compared with CK treatments, the stabilization efficiency of Cd, Zn, and Ni in the ATP treatments, AMSP treatments, AAPR treatments, AMRP treatments, ASSP treatments, AMAP treatments, and AFMP treatments were increased 3.70, 2.05, 8.78, 6.06, 5.08, 4.11, and 5.90 percentage points, respectively. The stabilization efficiency of Zn were increased 0.07, 0.08, 0.39, 0.25, 0.11, 0.11, and 0.16 percentage points, respectively. The stabilization efficiency of Ni were increased 1.0, 1.48, 2.55, 1.43, 1.80, 1.72, and 2.20 percentage points, respectively.

Under the condition of TCLP extraction, the stabilization efficiency of Cd, Zn, and Ni in the untreated soil were 55.49%, 84.34%, and 90.56%, respectively. Compared with CK treatments, the stabilization efficiency of Cd in the ATP treatments, AMSP treatments, AAPR treatments, AMRP treatments, ASSP treatments, AMAP treatments, and AFMP treatments were increased 3.59, 5.70, 13.71, 9.04, 7.39, 6.57, and 6.47 percentage points, respectively. The stabilization efficiency of Zn were increased 0.76, 1.10, 2.43, 1.56, 1.21, 0.90, and 0.76 percentage points, respectively. The stabilization efficiency of Ni were increased 1.00, 1.27, 2.96, 2.58, 1.28, 1.21, and 1.55 percentage points, respectively. In general, passivation materials play a certain role in the stabilization efficiency of Cd, Zn, and Ni in soil, especially AAPR treatments can greatly improve the stabilization efficiency of Cd, Zn, and Ni in soil.

Table 3 shows that in different treatments, the ERIm-CaCl₂ of Cd, Zn, and Ni in the soil is 23.72%-14.94%, 1.25%-0.86%, and 8.60%-6.04%, respectively. Among them, the ecological risk index of Cd is in the range of 10-30%, which belongs to medium risk, and the ecological risk index of Zn and Ni is in the range of 1-10%, which belongs to low risk. The ERIm-TCLP of Cd, Zn, and Ni in the soil is 44.53%-30.79%, 15.66%-13.23%, and 9.44%-6.49%, respectively. Among them, the ecological risk index of Cd is in the range of 30-50%, which belongs to high risk, the ecological risk index of Zn is in the range of 10-30%, which belongs to medium risk, and the ecological risk index of Ni is in the range of 1-10%, which belongs to low risk. The ecological risk degree of the three heavy metals is Cd > Ni > Zn, respectively. Among the different treatments, the effect of AAPR treatments is the best, and the ecological risk index is the lowest.

3.4. Evaluation of Ecotoxicity

Figure 3 shows that the concentrations of Cd and Zn in corn vary greatly under different treatments. The highest contents of Cd and Zn in underground parts were CK treatments, which were 19.21 mg/kg^-1 and 355.66 mg/kg^-1, and the lowest was AAPR treatments, which is 8.37 mg/kg^-1 and 217.95 mg/kg^-1, respectively. The highest concentrations of Cd and Zn in aboveground parts was CK treatments, which were 11.06 mg/kg^-1 and 245.62 mg/kg^-1, and the lowest was AAPR treatments, which was 2.25 mg/kg^-1 and 62.34 mg/kg^-1, respectively. Compared with CK treatments, the enrichment of Cd and Zn in different parts of corn could be reduced by adding passivator. Under the ATP treatments, AMSP treatments, AAPR treatments, AMRP treatments, ASSP treatments, AMAP treatments, and AFMP treatments, the concentrations of Cd in the underground part of corn were decreased by 15.24%, 17.01%, 56.40%, 46.36%, 36.67%, 41.46%, and 42.59%, respectively. The concentrations of Cd in the aboveground part of maize decreased by 43.67%, 45.72%, 79.69%, 64.41%, 63.05%, 68.60%, and 66.49% compared with CK treatments, respectively. Compared with CK treatments, the concentrations of Zn in underground parts of corn decreased by 14.31%, 15.58%, 38.72%, 29.62%, 20.74%, 27.41%, and 29.79%, respectively. The concentrations of Zn in aboveground parts of corn decreased by 36.57%, 43.05%, 74.62%, 64.67%, 53.15%, 60.19%, and 63.21% compared with CK treatments, respectively. In terms of biological concentration factor and translocation factor, the biological concentration factors of Cd and Zn in CK treatments were 0.99 and 0.93, respectively, and the translocation factors were 0.58 and 0.69, respectively. The biological concentration factor and translocation factor of corn Cd and Zn decreased after adding passivation materials. In the ATP treatments, AMSP treatments, AAPR treatments, AMRP treatments, ASSP treatments, AMAP treatments, and AFMP treatments, the biological concentration factor of Cd decreased by 0.15, 0.17, 0.56, 0.46, 0.36, 0.41, and 0.42, respectively. The translocation factor of Cd decreased by 0.19, 0.20, 0.31, 0.19, 0.24, 0.27, and 0.24, respectively. The biological concentration factor of Zn decreased by 0.13, 0.15, 0.36, 0.28, 0.20, 0.26, and 0.28, respectively. The translocation factor of Zn decreased by 0.18, 0.23, 0.41, 0.35, 0.28, 0.31, and 0.33, respectively.

The accumulation of Ni in maize is different from that of Cd and Zn. Figure 3 shows that the concentrations of Ni in the underground and aboveground part of corn are 32.24 mg/kg^-1 and 15.69 mg/kg^-1. When ATP, AMSP, AAPR, AMRP, ASSP, AMAP, and AFMP were applied into the soil, the concentrations of Ni in the underground part of corn decreased by 38.15%, 57.79%, 80.86%, 65.07%, 52.70%, 71.50%, and 58.56%, respectively. The concentrations of Ni in the aboveground part of corn decreased by 33.78%, 64.75%, 93.37%, 64.82%, 52.90%, 76.93%, and 59.08%, respectively. The biological concentration factor and the translocation factor of Ni in CK treatments were 0.39 and 0.49, respectively. In the ATP treatments, AMSP treatments, AAPR treatments, AMRP treatments, ASSP treatments, AMAP treatments, and AFMP treatments, the biological concentration factor of Ni decreased by 0.15, 0.23, 0.32, 0.25, 0.21, 0.28, and 0.23, respectively. In the AMSP treatments, AAPR treatments, AMAP treatments, and AFMP treatments, the translocation factor of Ni decreased by 0.08, 0.32, 0.10, and 0.01, respectively. The translocation factor of Ni increased 0.03 in the ATP treatments. In the AMRP treatments and ASSP treatments, the translocation factor of Ni did not change compared with CK treatments.

The whole biological uptaking factor of corn under 8 different treatments was calculated as shown in Figure 4. In the untreated soil, the BUF-Cd, BUF-Zn, and BUF-Ni were the highest, reaching 45.72, 944.48, and 69.85, respectively. In AAPR treatments, the BUF-Cd and BUF-Ni were the lowest, which decreased to 13.23 and 8.42, respectively. With the addition of passivation materials, the BUF-Cd, BUF-Zn, and BUF-Ni decreased in varying degrees compared with untreated soils, and the reduction ranges were 33.04% to 71.07%, 32.01% to 64.85%, and 43.29% to 87.94%, respectively. Among them, in the AAPR treatments, the BUF-Cd and BUF-Ni decreased by 71.07% and 87.94%, respectively, compared with the CK treatments, and decreased by 52.88% and 78.73% compared with the ATP treatments, respectively. In AMRP treatments, the BUF-Zn was the highest, which decreased by 64.85% and 44.64%, respectively, compared with CK treatments and ATP treatments. According to the significance analysis, AAPR treatments and AMRP treatments are significantly different from other treatments. ().

3.5. Correlation Analysis

The correlation analysis of the bioavailability of Cd, Zn, and Ni in soils, accumulation, and translocation of Cd, Zn, and Ni; the stabilization efficiency (SEm) of Cd, Zn, and Ni in soil; and the ecological risk index (ERIm) of Cd, Zn, and Ni in soil are shown in Table 4. The concentrations of CaCl₂-extractable Cd, Zn, and Ni and the concentrations of TCLP-extractable Cd, Zn, and Ni were positively correlated with accumulation of Cd, Zn, and Ni in corn, BCF, TF, ERIm-CaCl₂, ERIm-TCLP. SEm-CaCl₂ and SEm-TCLP were negatively correlated with the concentrations of CaCl₂-extractable Cd, Zn, and Ni and the concentrations of TCLP-extractable Cd, Zn, and Ni and the accumulation of Cd, Zn, and Ni in mazie and BCF and TF and ERIm-CaCl₂ and ERIm-TCLP.

4. Discussion

The result shows that the contents of soil physical and chemical properties such as pH, EC, and CEC will change after the application of modifiers, which will affect the forms of heavy metals and further affect their bioavailability [15, 29]. Soil pH value can affect the existing forms of heavy metals and the progress of various chemical reactions [30]. Generally speaking, there is a negative correlation between soil EC and CEC and the bioavailability of heavy metals in soil. The higher the soil EC and CEC values, the easier it is for heavy metals to be absorbed, thus reducing the migration capacity [31]. The result also shows that attapulgite modified by phosphate as passivation material has a great influence on the physical and chemical properties of soil. Compared with the addition of attapulgite alone, the soil pH, soil EC, and soil CEC of attapulgite modified by all kinds of phosphate are significantly different from those of CK treatments. Among them, the soil pH, soil EC, and soil CEC values of AAPR treatments were the highest among the 8 treatments, which increased by 2.47%, 5.72%, and 66.31% compared with CK treatments, respectively. The soil pH, soil EC, and soil CEC of AAPR treatments increased by 1.42%, 13.62%, and 53% compared with ATP treatments, respectively. The result also shows that attapulgite modified by oxalic acid-activated phosphate powder can significantly improve the physical and chemical properties of soil, for the reason that phosphate rock powder is a kind of semialkaline substance; at the same time, Ca²⁺ and Mg²⁺ are dissolved in phosphate rock powder after oxalic acid activation, thus improving soil pH and soil CEC. This is consistent with the research results of Huang et al. [32, 33]. In addition, the modification of attapulgite with sodium dihydrogen phosphate, potassium dihydrogen phosphate, ammonium dihydrogen phosphate, and fused calcium-magnesium phosphate can also improve the soil CEC, by 36.95%, 7.61%, 25.69%, and 30.52%, respectively, compared with ATP treatments, thus achieving a better effect of passivating heavy metals. However, after the attapulgite is modified with sodium dihydrogen phosphate, potassium dihydrogen phosphate, calcium superphosphate, ammonium dihydrogen phosphate, and fused calcium-magnesium phosphate, the soil pH value decreases, which is related to the physical and chemical properties of sodium dihydrogen phosphate, potassium dihydrogen phosphate, calcium superphosphate, ammonium dihydrogen phosphate, and fused calcium-magnesium phosphate. The pH values of these phosphates are all less than 5.0. After the attapulgite is modified with these five phosphates, the soil pH value decreases somewhat, but the decrease is not significant, and after the addition of attapulgite, the pH value of these phosphates is less than 5.0. After the addition of attapulgite modified by phosphate, the soil pH value decreases slightly. The pH of soil is above 6. 0. Attapulgite modified with sodium dihydrogen phosphate, oxalic acid activated phosphate rock powder, potassium dihydrogen phosphate, calcium superphosphate, ammonium dihydrogen phosphate, and fused calcium-magnesium phosphate significantly increased the content of available phosphorus in soil, because these phosphates themselves are water-soluble phosphates, and the available phosphorus content of phosphate rock powder significantly increase after oxalic acid activation, so that the content of available phosphorus in soil will significantly increase [34].

The result shows that the interior of attapulgite single crystal is honeycomb inner channel, the single crystal fiber exists in the form of parallel arrangement, and there are many voids between the single crystal fiber, which is rich in metal cations such as Si, Al, and Mg, which can adsorb and exchange with heavy metal ions in soil, thus reducing the bioavailability of heavy metals in soil [35]. On the other hand, phosphate can react with heavy metals such as coprecipitation and chelation, so that more heavy metals in soil can be transformed into residual state, which has a good passivation effect [36]. The result shows that after attapulgite supports oxalic acid activated phosphate rock powder, potassium dihydrogen phosphate, calcium superphosphate, ammonium dihydrogen phosphate, and fused calcium-magnesium phosphate, the concentrations of CaCl₂-extractable Cd in the soil decreased by 25.38%, 11.79%, 6.92%, 2.05%, and 11.03% compared with ATP treatments, respectively. The concentrations of CaCl₂-extractable Zn in the AAPR treatments, AMRP treatments, ASSP treatments, AMAP treatments, and AFMP treatments decreased by 26.99%, 15.49%, 3.32%, 3.54%, and 7.30% compared with ATP treatments, respectively. This is consistent with the research results of Wang et al. [15], Zhang et al. [13], Zhao-bing et al. [37], and others. It shows that the loading of phosphate can effectively increase the coordination exchange and chelating ability of attapulgite, enrich the reaction mechanism between attapulgite and heavy metals, and improve the passivation effect.

By adding AMSP, AAPR, AMRP, ASSP, AMAP, and AFMP, the stabilization efficiency of heavy metals in soil was effectively improved and the ecological risk index of soil was reduced. This is mainly because the passivation materials effectively reduce the content of available heavy metals in the soil and then reduce the ecological risk index of the soil [38]. After attapulgite was modified by phosphate, through the enhancement of complexation and precipitation with heavy metal ions such as Cd, Zn, and Ni, stable phosphate precipitation was formed [39], which reduced the enrichment and transport ability of Cd, Zn, and Ni in corn, thus reducing the ecological toxicity of soil [40].

5. Conclusion

The present study indicated that phosphate-modified attapulgite can effectively change the physical and chemical properties of soil, improve the stability of heavy metals in soil, and reduce the migration of heavy metals. Compared with the single application of attapulgite, the modification of attapulgite with oxalic acid-activated phosphate rock powder reduced concentrations of CaCl₂-extractable Cd and Zn and Ni by 25.38%, 26.99%, and 20.32%, respectively, and TCLP-extractable Zn and Ni decreased by 24.72%, 11.18%, and 23.20%, respectively, which effectively improved the remediation efficiency of soil heavy metals and reduced the ecological risk index of soil environment. In order to achieve the purpose of stabilizing heavy metals. In addition, AAPR can also increase soil pH and CEC, increase the content of available P, available K, and available N, reduce the content of Cd, Zn and Ni in the aboveground part of corn, reduce the accumulation of Cd, Zn, and Ni in the underground part, and transfer to the aboveground part, thus effectively reducing the ecological toxicity of heavy metals.

Data Availability

The dataset can be accessed upon request.

Conflicts of Interest

All authors disclosed no relevant relationships.

Acknowledgments

The current research was financially supported by the National Natural Science Foundation of China (no. 51668034).

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