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Mechanochemical immobilization of lead contaminated soil by ball milling with the additive of Ca(H2PO4)2
Chemosphere 247 (2020) 125963
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Mechanochemical immobilization of lead contaminated soil by ball milling with the additive of Ca(H2PO4)2 Ziwei Zhang a, Wenyi Yuan b, *, Peizhong Li c, Qingbin Song d, **, Xiaoyan Wang a, Weitong Xu a, Xuefeng Zhu a, Qiwu Zhang e, Jianwei Yue f, Jianfeng Bai b, Jingwei Wang b a
School of Environmental and Materials Engineering, Shanghai Polytechnic University, Shanghai, 201209, China Shanghai Collaborative Innovation Centre for WEEE Recycling, Shanghai Polytechnic University, Shanghai, 201209, China Beijing Key Laboratory of Industrial Land Contamination and Remediation, Environmental Protection Research Institute of Light Industry, Beijing, 100089, China d Macau Environmental Research Institute, Macau University of Science and Technology, Macau, Macao e School of Resources and Environmental Engineering, Wuhan University of Technology, Wuhan, 430070, China f Shanxi Unisdom Testing Technology Co., Ltd. Shanxi, 030006, China b c
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Mechanochemical immobilization for remediation of Pb contaminated soil. Ca(H2PO4)2 applied as stabilizing agent during ball milling. Pb in contaminated soil was transferred as lead precipitate through ball milling. After treatment, leachable Pb concentrations reduced to be lower than 5 mg/L.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 November 2019 Received in revised form 14 January 2020 Accepted 18 January 2020 Available online 20 January 2020
Lead (Pb) pollution in the soil is becoming more and more serious, and lead poisoning incidents also constantly occur. Therefore, the remediation of lead pollution in the soil has attracted widespread attention. In this study, heavy metal lead in soil was remediated by mechanochemical methods. The effects of different ball milling conditions on the toxic leaching concentration and morphological distribution (BCR sequential extraction procedure) of lead in contaminated soil were analyzed, including the addition of calcium dihydrogen phosphate (Ca(H2PO4)2), ball milling time, and ball milling speed. The reaction mechanism was analyzed by X-ray diffractometry (XRD), scanning electron microscopy (SEM), and a laser particle size analyzer. The results show that the optimal conditions for mechanochemical immobilization were 10% additive (Ca(H2PO4)2), milling speed of 550 rpm, and ball milling time for 2 h. Under this condition, the toxic leaching concentration of lead from contaminated soil was 4.36 mg L1,
Handling Editor: X. Cao Keywords: Lead contaminated soil
* Corresponding author. Tel.: þ86 21 5021 8010 ** Corresponding author. E-mail addresses: [email protected] (Z. Zhang), [email protected] (W. Yuan), [email protected] (P. Li), [email protected] (Q. Song), wangxy@ sspu.edu.cn (X. Wang), [email protected] (W. Xu), [email protected] (X. Zhu), [email protected] (Q. Zhang), [email protected] (J. Yue), jfbai@sspu. edu.cn (J. Bai), [email protected] (J. Wang). https://doi.org/10.1016/j.chemosphere.2020.125963 0045-6535/© 2020 Elsevier Ltd. All rights reserved.
2 Mechanochemical ball milling Immobilization
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and in the BCR sequential extraction procedure, Pb was mainly present in the residual fraction (54.96%). The mechanism of mechanochemical solidification of heavy metal lead in soil is that, during the ball milling process, the lead precipitates with Ca(H2PO4)2 to produce dense agglomerates (Pb3(PO4)2 and PbxCa10-x(PO4)6(OH)2), which fixes the lead in the soil and hampers its leaching. © 2020 Elsevier Ltd. All rights reserved.
1. Introduction Lead is a common heavy metal element involved in soil pollution (Peng et al., 2019) and it is a widely distributed and highly enriched environmental pollutant. Lead in soil derives from natural sources and anthropogenic pollution, where natural sources are represented by minerals in weathered rocks. Anthropogenic pollution comes from metal smelting, mining, sewage irrigation, and gasoline processing (Gao and Chen, 2011). Lead can reduce the body’s immunity and reproductive function, leading to various physiological diseases, especially to serious damage to children’s intellectual development (Wang et al., 2005a, 2005b). Many scholars have studied the pollution status of lead and have estimated its average annual emission in the world to be as high as 5 million tons (Peng et al., 2019). There are different levels of lead pollution in various regions of China (Xiong et al., 2019; Wu et al., 2004; Huang et al., 2007), and its poisoning incidents also constantly occur, so the remediation of lead contaminated soil is an urgent need. The existing lead-contaminated soil remediation technology can be divided into three techniques: chemical, physical, and biological (Liu et al., 2015; Li and Liu, 2013). Chemical remediation (Chen et al., 2015; Wang et al., 2018) can be divided into the amendment and the flushing method. In the amendment method, chemical reagents are added to lead-contaminated soil (Yuan et al., 2019a, 2019b) to reduce the mobility or effectiveness of lead in the soil. The US Environmental Protection Agency (EPA) uses phosphoruscontaining substances for this purpose. The amendment method is listed as one of the best lead-contaminated soil management measures. However, this method forms a stable lead phosphate mineral with a slow rate. In the flushing method, chemical reagents are used to form complexes with lead or lead ions in the soil, which are finally rinsed off with clean water to recover the lead and to achieve remediation (Basta et al., 2001). The flushing method is rapid and efficient, but it easily causes secondary pollution and high costs. Physical remediation methods can be divided into the electrodynamic remediation method, the isolation embedding method, and the soil modification method. The electrodynamic remediation method is to insert an electrode into the water saturated, contaminated soil and then pass a direct current through the soil to force the lead ions to the vicinity of the electrode, and concentrate the concentrated lead by engineering measures to achieve remediation (Sawada et al., 2004; Xi et al., 2009; Fang et al., 2008). Bioremediation includes microbial remediation and phytoremediation. Microbial remediation is the removal or stabilization of heavy metal lead by biochemical reactions of microorganisms, which can be divided into microbial transformation and microbial immobilization. Microbial transformation is the use of microorganisms to convert lead and reduce precipitation by biomethylation, which finally reduces lead toxicity. Microorganisms have three ways to immobilize lead (Pan et al., 2017): intracellular accumulation, extracellular precipitation, and extracellular complexation. The phytoremediation method involves the planting of super-enriched plants (Herb of Centipedal Brake) with strong
adsorption capacity for heavy metal elements in soil (Yang et al., 2015]. Harvesting plants and treating them can remove heavy metals and achieve pollution control purposes. Although bioremediation does not cause secondary pollution and completely remediates the soil, the organism requires suitable growth conditions and a long remediation duration. Because the above methods still have many drawbacks, this experiment uses the mechanochemical curing stabilization technology (Montinaro et al., 2007, 2009; Yuan et al., 2018; Zhang et al., 2014; Frost et al., 2001) to deal with contaminated soil samples. This technique can better achieve the remediation effect than those mentioned before. Mechanochemical remediation treatment can achieve immobilization/stabilization of a variety of heavy metals in one step without secondary pollution and is fast and efficient. Mechanochemical immobilization prevents Pb(II) from migrating into the environment due to leaching by limiting the toxicity or solubility of harmful compounds (Wang et al., 2017; Yuan et al., 2019a, 2019b; Montinaro et al., 2008), or by altering the physical properties of the soil and reducing the surface area that heavy metals can attach to. The principal technique involved in this remediation strategy is ball milling with the supplement of additives. One of the most important factors in the ball milling process is the production of significant free radical mechanical forces and chemical activation (Gilman, 1996). Mechanical processing is often used to promote specific transformations, such as progressive or flammable reactions, activation, amorphization, comminution, microstructure refinement, alloying, and cold welding (Wang et al., 2005a, 2005b; Suryanarayana, 2006). In this study, lead-contaminated soil were mechanochemically treated with calcium dihydrogen phosphate (Ca(H2PO4)2). We investigated the ball milling process parameters, including the amount of additive added, the time of ball milling, and the effect of ball milling speed on the immobilization/stabilization of lead in soil. The treatment effect is evaluated by the leached portion of Pb obtained by the toxic leaching procedure (EPA TCLP). In addition, the content and proportion of each form of Pb in the soil sample before and after ball milling were analyzed through sequential extraction procedure by Community Bureau of Reference (BCR). In order to clarify the mechanism of mechanochemical remediation, XRD, particle size analysis, and SEM analysis were carried out in this research. This study aims to find a fast and effective method for immobilizing metal Pb in contaminated soil without secondary pollution. 2. Materials and methods 2.1. Experimental soil and reagents The soil sample of this experiment is from the surface of a farmer’s vegetable garden 0e20 cm soil. The soil was air-dried at room temperature for 30 days, and then sieved through a 0.18 mm stainless standard mesh. The soil pH value (CHN, 2007) was determined to be 8.03. Total soil digestion was carried out (CHN, 2017) and the total Pb content in the soil was determined to be 22.87 mg kg1. Next, the heavy metal contamination of soil with
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heavy metal-Pb was simulated. A certain amount of Pb(NO3)2 was added to the soil in the form of an aqueous solution with concentration of 0.50 mol/L. The soil was mixed uniformly with Pb(NO3)2 solution, and after stabilization for 20 days, the soil was air-dried, and sieved through a 0.18 mm standard mesh. The soil pH in the artificially contaminated soils was again determined and found to be 7.87. Then, the soil was completely digested. After filtering through a 0.4 mm nylon syringe filter, Pb concentration in the filtrate was analyzed, which contained a total Pb content of 10439.35 mg kg1. The reagents used in this research were lead nitrate (Pb(NO3)2), calcium dihydrogen phosphate (Ca(H2PO4)2), acetic acid (CH3COOH), hydroxylamine hydrochloride (NH2OH$HCl), ammonium acetate (CH3COO(NH4)2), which were of analytical grade. Nitric acid (HNO3), hydrofluoric acid (HF), hydrogen peroxide (H2O2), perchloric acid (HClO4) and hydrochloric acid (HCl) were of excellent grade and were purchased from “Sinopharm Chemical Reagent Co., Ltd”. 2.2. Experimental procedures Pb contaminated soil was mixed with (Ca(H2PO4)2) and then placed in a 50 mL zircon vial with 70 g of a 20.0 mm diameter zirconium dioxide ball. The vial was sealed under air atmosphere and fixed on a planetary ball mill (FRITSCH P7, Germany). The mechanochemical ball milling reaction was carried out under conditions of additive ratio, milling time, and milling speed. The operating conditions of the ball mill test are summarized in Table 1. In addition, according to “EPA TCLP” (Method 1311) (US EPA, 1992), to each treatment 3 g of sample and 60 mL of leaching solution were added, i.e., corresponding to a liquid-solid ratio of 20, mixed in a covered polypropylene wide-mouth bottle. These were then rotated on a rotary shaker at 30 ± 2 rpm for 18±2 h (23 ± 2 C). After leaching, the Pb concentration in the leachate was measured by inductively coupled plasma atomic emission spectrometry (ICPOES). The leaching concentration of the original contaminated soil was 214.13 mg L1, which exceeded the EPA regulatory limit (TCLP Pb 5 mg/L) 42 times. Finally, the content and proportion of different forms of Pb in the soil samples before and after ball milling were analyzed by the BCR sequential extraction procedure (CHN, 2016). All sample analyzed in this study were performed in triplicate and the mean values were reported. 2.3. Analysis methods The pH of the soil was measured using a pH meter (ST3100, OHAUS, China), the total digestion of the soil sample was carried out by a microwave digestion apparatus (Multiwave PRO, Anton paar, Austria), and the Pb content in the sample was measured using an inductively coupled plasma optical emission spectrometer (ICP-OES, iCAP 7000, Thermo Scientific, America). The morphology of the soil samples was characterized by X-ray diffraction (D8
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ADVANCE, BRUKER, Germany), and the particle size distribution was measured using a laser particle size analyzer Mastersizer 2000 (MS2000, Malvern, Britain). The particle size ranged from 0.01 to 10,000 mm. The morphology of the soil samples before and after treatment was investigated by scanning electron microscopy (SEM, Phenom Prox, Netherlands). 3. Results and discussion The fixed ball milling conditions were as follows: the ball milling mediumzircon pellets, Ca(H2PO4)2 dosage 10 wt%, the ball-tomaterial mass ratio 14, the ball milling speed 550 rpm, and the ball milling time 4 h. We changed one of the conditions at a time and examined their effects on the fixation of lead in the soil. 3.1. Effects of the mechanochemical parameters on the Pb leaching concentration In this study, EPA TCLP was used to evaluate the effect of mechanochemical immobilizing stabilization. As shown in Fig. 1, the effects of different parameters, including additive ratio, milling time, and milliaddng speed on the stabilization of mechanochemical immobilizing were studied by single factor experiments. Taking Ca(H2PO4)2 as a stabilizing agent, the effect of the added amount on the immobilization effect of lead in soil is shown in Fig. 1a. The toxic leaching concentration of the original soil was 214.13 mg L1. When no additive was added, the leaching concentration of Pb in the soil was 26.85 mg L1 after ball milling at 550 rpm for 4 h. Although it did not reach the EPA regulatory limit value of 5 mg L1, the amount of leaching was greatly reduced, indicating that the mechanochemical ball milling has a certain effect on fixing heavy metal Pb in soil. Mechanical treatment could induce the change on the properties of contaminated soil. Stable aggregate with compact structures could be informed through ball milling process, which could entrap heavy metal Pb tightly within the soil particle. And Pb could have a higher irreversible adsorption on the fresh soil breakage surfaces after ball milling to decrease the leaching of lead from contaminated soil. As the amount of additive added increased, the leaching concentration of Pb decreased. When the additive ratio was 10 wt%, the leaching concentration of Pb was 3.16 mg L1, which meets the EPA regulatory limit (TCLP Pb 5 mg L1). This indicates that the amount of the additive has a significant effect on the immobilization of the heavy metal Pb. The immobilization rate of Pb can, thus, be increased by appropriately changing the amount of the additive. Fig. 1b shows the effect of different milling times on the leaching concentration of Pb in the soil. The toxic leaching concentration of the original soil was 214.13 mg L1, with the increase of ball milling time, the leaching concentration of Pb gradually decreased. When the additive was added, the leaching concentration of Pb decreased to 7.16 mg L1 after ball milling for 1 h, which was close to the regulatory limit value of leaching of 5 mg L1. Under these conditions, the leaching concentration of Pb in the pure ball milled soil
Table 1 Operating conditions for ball milling trials. Experiment 1
Experiment 2
Experiment 3
Ca(H2PO4)2/wt%
Milling time/h
Milling speed/(rpm)
Milling time/h
Ca(H2PO4)/wt%
Milling speed/(rpm)
Milling speed/(rpm)
Milling time/h
Ca(H2PO4)2/wt%
4 6 8 10 12
4 4 4 4 4
550 550 550 550 550
0.5 1 2 4 6
10 10 10 10 10
550 550 550 550 550
250 350 450 550 650
4 4 4 4 4
10 10 10 10 10
4
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Fig. 1. The effects of mechanochemical parameters on the Pb leaching concentration: (a) the effect of different additive ratios at 550 rpm for 4 h; (b) the effect of different milling time with additive of 10 wt% at 550 rpm; (c) the effect of different milling speed for 4 h with additive of 10 wt%.
was 57.37 mg L1, indicating that the addition of additive helped to immobilize the heavy metal Pb in the soil. When the ball milling time was more than 2 h, the leaching concentration of Pb in the soil with added stabilizing agent reached the EPA regulatory limit (TCLP Pb 5 mg L1). Rosenkranz et al. (2011) used a discrete element model (DEM) to analyze the grinding motion during the pulverization process, the results show that the energy transfer during the ball milling process is related to the milling time. Properly increasing of the ball milling time makes the relative impact between the balls and the material stronger, so that more energy is transferred between each other. In this way, the additive and Pb in contaminated soil can fully react and the immobilizing effect is more effective. In Fig. 1c, the effect of different ball milling speed on the leaching concentration of Pb in soil is shown. The toxic leaching concentration of the original soil was 214.13 mg L1. With an increase in the ball milling speed, the leaching concentration of Pb gradually decreased. Under the lower ball milling speed (250 rpm), the leaching concentration of Pb was 94.19 mg L1 when no additive was added, and the leaching concentration of Pb was
12.42 mg L1 when the additive was added. When the ball milling speed increased from 250 rpm to 550 rpm, the leaching concentration of Pb in the treated contaminated soil was lower than the EPA regulatory limit (TCLP Pb 5 mg L1). When the ball milling speed reached 650 rpm, the leaching concentration of Pb in the soil without added stabilizing agent did not reach the specified value, indicating that pure ball milling can only immobilize the heavy metal Pb to a certain extent. To reach the EPA regulatory limit, proper adjustment of the ball milling speed and the supplement of additives is crucial. 3.2. Morphological distribution of Pb in contaminated soil Different morphological distribution of Pb will cause different environmental effects, which directly affect the migration of Pb (Shangguan et al., 2015). The chemical fractions of Pb were classified into an exchangeable/acid-extractable fraction, a reducible fraction, an oxidizable fraction, and a residual fraction by the BCR sequential extraction procedure. In the exchangeable/acidextractable fraction, Pb2þ is precipitated or adsorbed on the
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surface of calcium carbonate to form a carbonate, which is easily released under acidic conditions; the reducible form is a combination with an amorphous iron-manganese oxide and a hydrated oxide. This form is much easy to be released under reducing conditions; the oxidizable fraction is a mainly organically bound fraction, representing a water-insoluble substance formed by combining a heavy metal ion as a central ion and an organic active group as a ligand; the residual fraction is generally called noneffective fraction, where heavy metal elements mainly exist in the mineral lattice and can only be released during the weathering process. This residue state is basically not used by organisms and is a relatively stable fraction (Jiang et al., 2012). Fig. 2 shows the change in the morphological distribution of Pb in the original contaminated soil, pure ball mill, and mechanochemically ball-milling at 550 rpm for 4 h with 10 wt% additive of Ca(H2PO4)2. Pb in the exchangeable/acid-extractable fraction of contaminated original soil accounted for 39.76%, the reducible fraction accounted for 58.55%, the oxidizable fraction accounted for 0.53%, and the residual fraction accounted for 1.16% (Fig. 2a). The morphology of Pb in the contaminated raw soil is mainly related to the exchangeable/acid-extractable and reducible fractions, while the residual fraction is very small. When the contaminated soil was treated under different conditions, the exchangeable/acidextractable fraction reduced greatly, and the contents of the residual and oxidizable fractions increased. At the same time, the content of the residual Pb fraction in the soil with additive was higher than that of the residual Pb fraction in the soil without additive. The residual fraction is a relatively stable fraction, indicating that the addition of additive is beneficial to the stabilization of Pb in contaminated soil. The similar conclusion was also obtained in the previous research (Gao et al., 2017). Fig. 2b shows the change of the morphological distribution of lead in the soil after ball milling at 10 wt% additive (Ca(H2PO4)2) and 550 rpm with different ball milling time. When the ball milling time increased, the residual Pb increased from 1.16% to 61.27%, where the change of lead in the oxidizable fraction was consistent with that of lead in the residual fraction. The exchangeable/acidextractable fraction and the reducible lead fraction tended to decrease. This indicates that a better immobilizing effect can be achieved by appropriately extending the ball milling time.
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3.3. Mechanism analysis The mechanical ball milling process will cause changes in soil topography. Fig. 3 depicts SEM images showing the soil morphology of the original sample, the pure ball milling samples the ball milling sample with additive and the ball milling sample with different ball milling speeds. The original soil had a smooth morphology and a small amount of flaky structure (Fig. 3a). The soil after pure ball milling had a lumpy structure shown in Fig. 3b. After adding the additive, the soil particles became dense and the lead was immobilized (Fig. 3c). At higher ball milling speed, the lead leaching concentration was reduced. There were many layers and flaky structures on the surface of the soil under lower ball milling speed (Fig. 3d). As the ball milling speed increased, the agglomerates formed by soil particles becoming denser, which was caused by mechanical ball milling (Fig. 3e and f). This change explains the decrease in the lead leaching concentration. Fig. 4 shows the particle size change measured by a particle size analyzer with an additive (Ca(H2PO4)2) of 10 wt%, a ball milling time of 4 h, and different ball milling speeds. The D50 of the original soil was 11.15 mm, while of the treated soil samples under different ball milling speeds (250e650 rpm) was 12.55, 11.45, 15.76, 18.75, and 16.41, respectively. After mechanochemical ball milling, the particle size of the soil was larger than that of the original soil, rather than reduced. A possible reason for this could be that the soil particles agglomerate during the reaction, which would also
Fig. 3. SEM micrographs for (a) original contaminated soil magnified at12,000; (b) milled without additive magnified at12,000 at 500 rpm for 4 h; (c) milled with additive 10 wt% magnified at12,000 at 500 rpm for 4 h; (d) milled with 250 rpm magnified at12,000 with additive 10 wt% for 4 h; (e) milled with 550 rpm magnified at 12,000 with additive 10 wt% for 4 h and (f) milled with 650 rpm magnified at 12,000 with additive 10 wt% for 4 h.
Fig. 2. Morphological distribution of Pb in soil: (a) Morphological distribution of Pb in different soils; (b) Effects of different ball milling time on the morphological distribution of Pb in soil.
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Fig. 4. Particle size distribution of soil particles under different ball milling speed.
explain the increasing fixing effect of Pb as the ball milling speed increased. Fig. 5 shows the XRD pattern of the ball milling reaction of pure Ca(H2PO4)2 and Pb(NO3)2 at different times. The mineral structure
of the ball milling reaction did not change significantly at different times, and a new characteristic peak appeared at 2q ¼ 23.50 and 2q ¼ 30.90 compared with the sample of the control group (without ball milling; Fig. 5). The phase identification results show
Fig. 5. XRD patterns before and after ball milling of Ca(H2PO4)2 and Pb(NO3)2.
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Acknowledgements The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 21876106), Gaoyuan Discipline of Shanghai-Environmental Science and Engineering (Resource Recycling Science and Engineering), The Science and Technology Development Fund, Macau SAR (0027/2018/A) and SSPU Foundation (A01GY18EX04, A01GY18F022-d02).
References
Fig. 6. A conceptual model for mechanism of Pb in contaminated soil by mechanochemical immobilization.
that the new characteristic peaks formed were mainly Pb3(PO4)2 and PbxCa10-x(PO4)6(OH)2. These peaks indicate that PO3 4 , released after the addition of Ca(H2PO4)2, reacts with Pb in the soil to form precipitates. These precipitates have low solubility and bioavailability and are stable in the environment. Therefore, the content of soluble Pb in the soil was greatly reduced and the Pb content in the residual fraction was greatly increased. Furthermore, Chen (Chen et al., 2006) and others have shown that the formation of coprecipitation of lead in contaminated soil on the surface of soil granular minerals is also one of the main mechanisms for phosphorus to reduce the migration and transformation of lead. Fig. 6 illustrates a conceptual model for mechanism of Pb in contaminated soil by mechanochemical immobilization. The soil particles and Ca(H2PO4)2 were mixed and mechanically ball milled under certain conditions. Mechanochemical reaction occurred between Pb and Ca(H2PO4)2 in the soil during ball milling process, and Pb3(PO4)2 and PbxCa10-x(PO4)6(OH)2 were formed. 4. Concluding remarks This work provides a means of immobilizing Pb in contaminated soils and shows that mechanochemical methods can effectively reduce the leaching concentration of Pb. With the enhancement of the ball milling process conditions, Pb and Ca(H2PO4)2 produced a precipitated product under solid phase conditions, acting as a solidification. Single factor experiments showed that the leaching concentration of lead decreased from 214.13 mg L1 to the regulatory limit of 5 mg L1 at 550 rpm, 2 h, and 10 wt% additive (Ca(H2PO4)2). The BCR sequential extraction procedure indicated that proper ball milling conditions increase the content of residual Pb, which also indicates that mechanochemical ball milling is beneficial to the solidification of heavy metal Pb. In addition, it is worth noting that, from the perspective of economic cost and environmental protection, the method of mechanochemical ball milling to immobilize heavy metals has the advantages of a simple equipment and no waste liquid. Thus, it is a promising method for remediating heavy metal contaminated soil. CRediT authorship contribution statement Ziwei Zhang: Methodology, Investigation, Writing - original draft. Wenyi Yuan: Resources, Writing - review & editing, Supervision, Data curation. Peizhong Li: Methodology, Software. Qingbin Song: Methodology, Software. Xiaoyan Wang: Formal analysis, Investigation. Weitong Xu: Formal analysis, Investigation. Xuefeng Zhu: Methodology, Software. Qiwu Zhang: Writing - review & editing. Jianwei Yue: Formal analysis. Jianfeng Bai: Writing - review & editing. Jingwei Wang: Writing - review & editing.
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Wang, J.H., Zhang, F.Q., Dong, L.F., 2018. Stabilization effect of curing agent on heavy metals in soil. J. Jiangsu Agric. Sci. 46 (06), 235e238. Wu, X.M., Pan, G.X., Li, L.Q., 2004. Soil pollution of lead in different city zones of Nanjing. J. Environ. Health 21 (5), 291e293. Xi, Y.H., Zhou, G.H., Yang, Y., 2009. Electrokinetic remediation of Pb-polluted soil. J. Guizhou Agric. Sci. 36 (01), 165e168. Xiong, H., Fan, H., Gao, J.F., Liu, L., 2019. Lead morphological evolution and leaching characteristics of solidified/stabilized lead contaminated soil containing phosphorous substrate. Environ. Eng. 37 (05), 39e44. Yang, Q.L., Wu, Z.Z., Chen, J.L., Liu, X.G., Wang, W.H., Liu, Y.W., 2015. Research status of phytoremediation of heavy metal contaminated soil and prospects of water and fertilizer control technology. J. Eco-Environ. 24 (06), 1075e1084. Yuan, W.Y., Xu, W.T., Wu, Z.B., Zhang, Z.W., Wang, L.C., Bai, J.F., Wang, X.Y., Zhang, Q.W., Zhu, X.F., Zhang, C.L., Wang, J.W., 2018. Mechanochemical
treatment of Cr(VI) contaminated soil using a sodium sulfide coupled solidification/stabilization process. Chemosphere 212, 540e547. Yuan, X.C., Li, B., Zhu, R.F., Yao, D., Zhan, F.D., Chen, J.J., Zu, Y.Q., He, Y.M., Li, Y., 2019a. Study on passivation and repair of cadmium and lead pollution in farmland around lead-zinc mining area with different passivating agents. J. Agro-Environ. Sci. 38 (04), 807e817. Yuan, W.Y., Xu, W.T., Zhang, Z.W., Wang, X.Y., Zhang, Q.W., Bai, J.F., Wang, J.W., 2019b. Rapid Cr(VI) reduction and immobilization in contaminated soil by mechanochemical treatment with calcium polysulfide. Chemosphere 227, 657e661. Zhang, K.L., Huang, J., Wang, H.Z., Liu, k., Yu, G., Deng, S.B., Wang, B., 2014. Mechanochemical degradation of hexabromocyclododecane and approaches for the remediation of its contaminated soil. Chemosphere 116 (15), 40e45.
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Mechanochemical immobilization of lead contaminated soil by ball milling with the additive of Ca(H2PO4)2 Ziwei Zhang a, Wenyi Yuan b, *, Peizhong Li c, Qingbin Song d, **, Xiaoyan Wang a, Weitong Xu a, Xuefeng Zhu a, Qiwu Zhang e, Jianwei Yue f, Jianfeng Bai b, Jingwei Wang b a
School of Environmental and Materials Engineering, Shanghai Polytechnic University, Shanghai, 201209, China Shanghai Collaborative Innovation Centre for WEEE Recycling, Shanghai Polytechnic University, Shanghai, 201209, China Beijing Key Laboratory of Industrial Land Contamination and Remediation, Environmental Protection Research Institute of Light Industry, Beijing, 100089, China d Macau Environmental Research Institute, Macau University of Science and Technology, Macau, Macao e School of Resources and Environmental Engineering, Wuhan University of Technology, Wuhan, 430070, China f Shanxi Unisdom Testing Technology Co., Ltd. Shanxi, 030006, China b c
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Mechanochemical immobilization for remediation of Pb contaminated soil. Ca(H2PO4)2 applied as stabilizing agent during ball milling. Pb in contaminated soil was transferred as lead precipitate through ball milling. After treatment, leachable Pb concentrations reduced to be lower than 5 mg/L.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 November 2019 Received in revised form 14 January 2020 Accepted 18 January 2020 Available online 20 January 2020
Lead (Pb) pollution in the soil is becoming more and more serious, and lead poisoning incidents also constantly occur. Therefore, the remediation of lead pollution in the soil has attracted widespread attention. In this study, heavy metal lead in soil was remediated by mechanochemical methods. The effects of different ball milling conditions on the toxic leaching concentration and morphological distribution (BCR sequential extraction procedure) of lead in contaminated soil were analyzed, including the addition of calcium dihydrogen phosphate (Ca(H2PO4)2), ball milling time, and ball milling speed. The reaction mechanism was analyzed by X-ray diffractometry (XRD), scanning electron microscopy (SEM), and a laser particle size analyzer. The results show that the optimal conditions for mechanochemical immobilization were 10% additive (Ca(H2PO4)2), milling speed of 550 rpm, and ball milling time for 2 h. Under this condition, the toxic leaching concentration of lead from contaminated soil was 4.36 mg L1,
Handling Editor: X. Cao Keywords: Lead contaminated soil
* Corresponding author. Tel.: þ86 21 5021 8010 ** Corresponding author. E-mail addresses: [email protected] (Z. Zhang), [email protected] (W. Yuan), [email protected] (P. Li), [email protected] (Q. Song), wangxy@ sspu.edu.cn (X. Wang), [email protected] (W. Xu), [email protected] (X. Zhu), [email protected] (Q. Zhang), [email protected] (J. Yue), jfbai@sspu. edu.cn (J. Bai), [email protected] (J. Wang). https://doi.org/10.1016/j.chemosphere.2020.125963 0045-6535/© 2020 Elsevier Ltd. All rights reserved.
2 Mechanochemical ball milling Immobilization
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and in the BCR sequential extraction procedure, Pb was mainly present in the residual fraction (54.96%). The mechanism of mechanochemical solidification of heavy metal lead in soil is that, during the ball milling process, the lead precipitates with Ca(H2PO4)2 to produce dense agglomerates (Pb3(PO4)2 and PbxCa10-x(PO4)6(OH)2), which fixes the lead in the soil and hampers its leaching. © 2020 Elsevier Ltd. All rights reserved.
1. Introduction Lead is a common heavy metal element involved in soil pollution (Peng et al., 2019) and it is a widely distributed and highly enriched environmental pollutant. Lead in soil derives from natural sources and anthropogenic pollution, where natural sources are represented by minerals in weathered rocks. Anthropogenic pollution comes from metal smelting, mining, sewage irrigation, and gasoline processing (Gao and Chen, 2011). Lead can reduce the body’s immunity and reproductive function, leading to various physiological diseases, especially to serious damage to children’s intellectual development (Wang et al., 2005a, 2005b). Many scholars have studied the pollution status of lead and have estimated its average annual emission in the world to be as high as 5 million tons (Peng et al., 2019). There are different levels of lead pollution in various regions of China (Xiong et al., 2019; Wu et al., 2004; Huang et al., 2007), and its poisoning incidents also constantly occur, so the remediation of lead contaminated soil is an urgent need. The existing lead-contaminated soil remediation technology can be divided into three techniques: chemical, physical, and biological (Liu et al., 2015; Li and Liu, 2013). Chemical remediation (Chen et al., 2015; Wang et al., 2018) can be divided into the amendment and the flushing method. In the amendment method, chemical reagents are added to lead-contaminated soil (Yuan et al., 2019a, 2019b) to reduce the mobility or effectiveness of lead in the soil. The US Environmental Protection Agency (EPA) uses phosphoruscontaining substances for this purpose. The amendment method is listed as one of the best lead-contaminated soil management measures. However, this method forms a stable lead phosphate mineral with a slow rate. In the flushing method, chemical reagents are used to form complexes with lead or lead ions in the soil, which are finally rinsed off with clean water to recover the lead and to achieve remediation (Basta et al., 2001). The flushing method is rapid and efficient, but it easily causes secondary pollution and high costs. Physical remediation methods can be divided into the electrodynamic remediation method, the isolation embedding method, and the soil modification method. The electrodynamic remediation method is to insert an electrode into the water saturated, contaminated soil and then pass a direct current through the soil to force the lead ions to the vicinity of the electrode, and concentrate the concentrated lead by engineering measures to achieve remediation (Sawada et al., 2004; Xi et al., 2009; Fang et al., 2008). Bioremediation includes microbial remediation and phytoremediation. Microbial remediation is the removal or stabilization of heavy metal lead by biochemical reactions of microorganisms, which can be divided into microbial transformation and microbial immobilization. Microbial transformation is the use of microorganisms to convert lead and reduce precipitation by biomethylation, which finally reduces lead toxicity. Microorganisms have three ways to immobilize lead (Pan et al., 2017): intracellular accumulation, extracellular precipitation, and extracellular complexation. The phytoremediation method involves the planting of super-enriched plants (Herb of Centipedal Brake) with strong
adsorption capacity for heavy metal elements in soil (Yang et al., 2015]. Harvesting plants and treating them can remove heavy metals and achieve pollution control purposes. Although bioremediation does not cause secondary pollution and completely remediates the soil, the organism requires suitable growth conditions and a long remediation duration. Because the above methods still have many drawbacks, this experiment uses the mechanochemical curing stabilization technology (Montinaro et al., 2007, 2009; Yuan et al., 2018; Zhang et al., 2014; Frost et al., 2001) to deal with contaminated soil samples. This technique can better achieve the remediation effect than those mentioned before. Mechanochemical remediation treatment can achieve immobilization/stabilization of a variety of heavy metals in one step without secondary pollution and is fast and efficient. Mechanochemical immobilization prevents Pb(II) from migrating into the environment due to leaching by limiting the toxicity or solubility of harmful compounds (Wang et al., 2017; Yuan et al., 2019a, 2019b; Montinaro et al., 2008), or by altering the physical properties of the soil and reducing the surface area that heavy metals can attach to. The principal technique involved in this remediation strategy is ball milling with the supplement of additives. One of the most important factors in the ball milling process is the production of significant free radical mechanical forces and chemical activation (Gilman, 1996). Mechanical processing is often used to promote specific transformations, such as progressive or flammable reactions, activation, amorphization, comminution, microstructure refinement, alloying, and cold welding (Wang et al., 2005a, 2005b; Suryanarayana, 2006). In this study, lead-contaminated soil were mechanochemically treated with calcium dihydrogen phosphate (Ca(H2PO4)2). We investigated the ball milling process parameters, including the amount of additive added, the time of ball milling, and the effect of ball milling speed on the immobilization/stabilization of lead in soil. The treatment effect is evaluated by the leached portion of Pb obtained by the toxic leaching procedure (EPA TCLP). In addition, the content and proportion of each form of Pb in the soil sample before and after ball milling were analyzed through sequential extraction procedure by Community Bureau of Reference (BCR). In order to clarify the mechanism of mechanochemical remediation, XRD, particle size analysis, and SEM analysis were carried out in this research. This study aims to find a fast and effective method for immobilizing metal Pb in contaminated soil without secondary pollution. 2. Materials and methods 2.1. Experimental soil and reagents The soil sample of this experiment is from the surface of a farmer’s vegetable garden 0e20 cm soil. The soil was air-dried at room temperature for 30 days, and then sieved through a 0.18 mm stainless standard mesh. The soil pH value (CHN, 2007) was determined to be 8.03. Total soil digestion was carried out (CHN, 2017) and the total Pb content in the soil was determined to be 22.87 mg kg1. Next, the heavy metal contamination of soil with
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heavy metal-Pb was simulated. A certain amount of Pb(NO3)2 was added to the soil in the form of an aqueous solution with concentration of 0.50 mol/L. The soil was mixed uniformly with Pb(NO3)2 solution, and after stabilization for 20 days, the soil was air-dried, and sieved through a 0.18 mm standard mesh. The soil pH in the artificially contaminated soils was again determined and found to be 7.87. Then, the soil was completely digested. After filtering through a 0.4 mm nylon syringe filter, Pb concentration in the filtrate was analyzed, which contained a total Pb content of 10439.35 mg kg1. The reagents used in this research were lead nitrate (Pb(NO3)2), calcium dihydrogen phosphate (Ca(H2PO4)2), acetic acid (CH3COOH), hydroxylamine hydrochloride (NH2OH$HCl), ammonium acetate (CH3COO(NH4)2), which were of analytical grade. Nitric acid (HNO3), hydrofluoric acid (HF), hydrogen peroxide (H2O2), perchloric acid (HClO4) and hydrochloric acid (HCl) were of excellent grade and were purchased from “Sinopharm Chemical Reagent Co., Ltd”. 2.2. Experimental procedures Pb contaminated soil was mixed with (Ca(H2PO4)2) and then placed in a 50 mL zircon vial with 70 g of a 20.0 mm diameter zirconium dioxide ball. The vial was sealed under air atmosphere and fixed on a planetary ball mill (FRITSCH P7, Germany). The mechanochemical ball milling reaction was carried out under conditions of additive ratio, milling time, and milling speed. The operating conditions of the ball mill test are summarized in Table 1. In addition, according to “EPA TCLP” (Method 1311) (US EPA, 1992), to each treatment 3 g of sample and 60 mL of leaching solution were added, i.e., corresponding to a liquid-solid ratio of 20, mixed in a covered polypropylene wide-mouth bottle. These were then rotated on a rotary shaker at 30 ± 2 rpm for 18±2 h (23 ± 2 C). After leaching, the Pb concentration in the leachate was measured by inductively coupled plasma atomic emission spectrometry (ICPOES). The leaching concentration of the original contaminated soil was 214.13 mg L1, which exceeded the EPA regulatory limit (TCLP Pb 5 mg/L) 42 times. Finally, the content and proportion of different forms of Pb in the soil samples before and after ball milling were analyzed by the BCR sequential extraction procedure (CHN, 2016). All sample analyzed in this study were performed in triplicate and the mean values were reported. 2.3. Analysis methods The pH of the soil was measured using a pH meter (ST3100, OHAUS, China), the total digestion of the soil sample was carried out by a microwave digestion apparatus (Multiwave PRO, Anton paar, Austria), and the Pb content in the sample was measured using an inductively coupled plasma optical emission spectrometer (ICP-OES, iCAP 7000, Thermo Scientific, America). The morphology of the soil samples was characterized by X-ray diffraction (D8
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ADVANCE, BRUKER, Germany), and the particle size distribution was measured using a laser particle size analyzer Mastersizer 2000 (MS2000, Malvern, Britain). The particle size ranged from 0.01 to 10,000 mm. The morphology of the soil samples before and after treatment was investigated by scanning electron microscopy (SEM, Phenom Prox, Netherlands). 3. Results and discussion The fixed ball milling conditions were as follows: the ball milling mediumzircon pellets, Ca(H2PO4)2 dosage 10 wt%, the ball-tomaterial mass ratio 14, the ball milling speed 550 rpm, and the ball milling time 4 h. We changed one of the conditions at a time and examined their effects on the fixation of lead in the soil. 3.1. Effects of the mechanochemical parameters on the Pb leaching concentration In this study, EPA TCLP was used to evaluate the effect of mechanochemical immobilizing stabilization. As shown in Fig. 1, the effects of different parameters, including additive ratio, milling time, and milliaddng speed on the stabilization of mechanochemical immobilizing were studied by single factor experiments. Taking Ca(H2PO4)2 as a stabilizing agent, the effect of the added amount on the immobilization effect of lead in soil is shown in Fig. 1a. The toxic leaching concentration of the original soil was 214.13 mg L1. When no additive was added, the leaching concentration of Pb in the soil was 26.85 mg L1 after ball milling at 550 rpm for 4 h. Although it did not reach the EPA regulatory limit value of 5 mg L1, the amount of leaching was greatly reduced, indicating that the mechanochemical ball milling has a certain effect on fixing heavy metal Pb in soil. Mechanical treatment could induce the change on the properties of contaminated soil. Stable aggregate with compact structures could be informed through ball milling process, which could entrap heavy metal Pb tightly within the soil particle. And Pb could have a higher irreversible adsorption on the fresh soil breakage surfaces after ball milling to decrease the leaching of lead from contaminated soil. As the amount of additive added increased, the leaching concentration of Pb decreased. When the additive ratio was 10 wt%, the leaching concentration of Pb was 3.16 mg L1, which meets the EPA regulatory limit (TCLP Pb 5 mg L1). This indicates that the amount of the additive has a significant effect on the immobilization of the heavy metal Pb. The immobilization rate of Pb can, thus, be increased by appropriately changing the amount of the additive. Fig. 1b shows the effect of different milling times on the leaching concentration of Pb in the soil. The toxic leaching concentration of the original soil was 214.13 mg L1, with the increase of ball milling time, the leaching concentration of Pb gradually decreased. When the additive was added, the leaching concentration of Pb decreased to 7.16 mg L1 after ball milling for 1 h, which was close to the regulatory limit value of leaching of 5 mg L1. Under these conditions, the leaching concentration of Pb in the pure ball milled soil
Table 1 Operating conditions for ball milling trials. Experiment 1
Experiment 2
Experiment 3
Ca(H2PO4)2/wt%
Milling time/h
Milling speed/(rpm)
Milling time/h
Ca(H2PO4)/wt%
Milling speed/(rpm)
Milling speed/(rpm)
Milling time/h
Ca(H2PO4)2/wt%
4 6 8 10 12
4 4 4 4 4
550 550 550 550 550
0.5 1 2 4 6
10 10 10 10 10
550 550 550 550 550
250 350 450 550 650
4 4 4 4 4
10 10 10 10 10
4
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Fig. 1. The effects of mechanochemical parameters on the Pb leaching concentration: (a) the effect of different additive ratios at 550 rpm for 4 h; (b) the effect of different milling time with additive of 10 wt% at 550 rpm; (c) the effect of different milling speed for 4 h with additive of 10 wt%.
was 57.37 mg L1, indicating that the addition of additive helped to immobilize the heavy metal Pb in the soil. When the ball milling time was more than 2 h, the leaching concentration of Pb in the soil with added stabilizing agent reached the EPA regulatory limit (TCLP Pb 5 mg L1). Rosenkranz et al. (2011) used a discrete element model (DEM) to analyze the grinding motion during the pulverization process, the results show that the energy transfer during the ball milling process is related to the milling time. Properly increasing of the ball milling time makes the relative impact between the balls and the material stronger, so that more energy is transferred between each other. In this way, the additive and Pb in contaminated soil can fully react and the immobilizing effect is more effective. In Fig. 1c, the effect of different ball milling speed on the leaching concentration of Pb in soil is shown. The toxic leaching concentration of the original soil was 214.13 mg L1. With an increase in the ball milling speed, the leaching concentration of Pb gradually decreased. Under the lower ball milling speed (250 rpm), the leaching concentration of Pb was 94.19 mg L1 when no additive was added, and the leaching concentration of Pb was
12.42 mg L1 when the additive was added. When the ball milling speed increased from 250 rpm to 550 rpm, the leaching concentration of Pb in the treated contaminated soil was lower than the EPA regulatory limit (TCLP Pb 5 mg L1). When the ball milling speed reached 650 rpm, the leaching concentration of Pb in the soil without added stabilizing agent did not reach the specified value, indicating that pure ball milling can only immobilize the heavy metal Pb to a certain extent. To reach the EPA regulatory limit, proper adjustment of the ball milling speed and the supplement of additives is crucial. 3.2. Morphological distribution of Pb in contaminated soil Different morphological distribution of Pb will cause different environmental effects, which directly affect the migration of Pb (Shangguan et al., 2015). The chemical fractions of Pb were classified into an exchangeable/acid-extractable fraction, a reducible fraction, an oxidizable fraction, and a residual fraction by the BCR sequential extraction procedure. In the exchangeable/acidextractable fraction, Pb2þ is precipitated or adsorbed on the
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surface of calcium carbonate to form a carbonate, which is easily released under acidic conditions; the reducible form is a combination with an amorphous iron-manganese oxide and a hydrated oxide. This form is much easy to be released under reducing conditions; the oxidizable fraction is a mainly organically bound fraction, representing a water-insoluble substance formed by combining a heavy metal ion as a central ion and an organic active group as a ligand; the residual fraction is generally called noneffective fraction, where heavy metal elements mainly exist in the mineral lattice and can only be released during the weathering process. This residue state is basically not used by organisms and is a relatively stable fraction (Jiang et al., 2012). Fig. 2 shows the change in the morphological distribution of Pb in the original contaminated soil, pure ball mill, and mechanochemically ball-milling at 550 rpm for 4 h with 10 wt% additive of Ca(H2PO4)2. Pb in the exchangeable/acid-extractable fraction of contaminated original soil accounted for 39.76%, the reducible fraction accounted for 58.55%, the oxidizable fraction accounted for 0.53%, and the residual fraction accounted for 1.16% (Fig. 2a). The morphology of Pb in the contaminated raw soil is mainly related to the exchangeable/acid-extractable and reducible fractions, while the residual fraction is very small. When the contaminated soil was treated under different conditions, the exchangeable/acidextractable fraction reduced greatly, and the contents of the residual and oxidizable fractions increased. At the same time, the content of the residual Pb fraction in the soil with additive was higher than that of the residual Pb fraction in the soil without additive. The residual fraction is a relatively stable fraction, indicating that the addition of additive is beneficial to the stabilization of Pb in contaminated soil. The similar conclusion was also obtained in the previous research (Gao et al., 2017). Fig. 2b shows the change of the morphological distribution of lead in the soil after ball milling at 10 wt% additive (Ca(H2PO4)2) and 550 rpm with different ball milling time. When the ball milling time increased, the residual Pb increased from 1.16% to 61.27%, where the change of lead in the oxidizable fraction was consistent with that of lead in the residual fraction. The exchangeable/acidextractable fraction and the reducible lead fraction tended to decrease. This indicates that a better immobilizing effect can be achieved by appropriately extending the ball milling time.
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3.3. Mechanism analysis The mechanical ball milling process will cause changes in soil topography. Fig. 3 depicts SEM images showing the soil morphology of the original sample, the pure ball milling samples the ball milling sample with additive and the ball milling sample with different ball milling speeds. The original soil had a smooth morphology and a small amount of flaky structure (Fig. 3a). The soil after pure ball milling had a lumpy structure shown in Fig. 3b. After adding the additive, the soil particles became dense and the lead was immobilized (Fig. 3c). At higher ball milling speed, the lead leaching concentration was reduced. There were many layers and flaky structures on the surface of the soil under lower ball milling speed (Fig. 3d). As the ball milling speed increased, the agglomerates formed by soil particles becoming denser, which was caused by mechanical ball milling (Fig. 3e and f). This change explains the decrease in the lead leaching concentration. Fig. 4 shows the particle size change measured by a particle size analyzer with an additive (Ca(H2PO4)2) of 10 wt%, a ball milling time of 4 h, and different ball milling speeds. The D50 of the original soil was 11.15 mm, while of the treated soil samples under different ball milling speeds (250e650 rpm) was 12.55, 11.45, 15.76, 18.75, and 16.41, respectively. After mechanochemical ball milling, the particle size of the soil was larger than that of the original soil, rather than reduced. A possible reason for this could be that the soil particles agglomerate during the reaction, which would also
Fig. 3. SEM micrographs for (a) original contaminated soil magnified at12,000; (b) milled without additive magnified at12,000 at 500 rpm for 4 h; (c) milled with additive 10 wt% magnified at12,000 at 500 rpm for 4 h; (d) milled with 250 rpm magnified at12,000 with additive 10 wt% for 4 h; (e) milled with 550 rpm magnified at 12,000 with additive 10 wt% for 4 h and (f) milled with 650 rpm magnified at 12,000 with additive 10 wt% for 4 h.
Fig. 2. Morphological distribution of Pb in soil: (a) Morphological distribution of Pb in different soils; (b) Effects of different ball milling time on the morphological distribution of Pb in soil.
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Fig. 4. Particle size distribution of soil particles under different ball milling speed.
explain the increasing fixing effect of Pb as the ball milling speed increased. Fig. 5 shows the XRD pattern of the ball milling reaction of pure Ca(H2PO4)2 and Pb(NO3)2 at different times. The mineral structure
of the ball milling reaction did not change significantly at different times, and a new characteristic peak appeared at 2q ¼ 23.50 and 2q ¼ 30.90 compared with the sample of the control group (without ball milling; Fig. 5). The phase identification results show
Fig. 5. XRD patterns before and after ball milling of Ca(H2PO4)2 and Pb(NO3)2.
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Acknowledgements The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 21876106), Gaoyuan Discipline of Shanghai-Environmental Science and Engineering (Resource Recycling Science and Engineering), The Science and Technology Development Fund, Macau SAR (0027/2018/A) and SSPU Foundation (A01GY18EX04, A01GY18F022-d02).
References
Fig. 6. A conceptual model for mechanism of Pb in contaminated soil by mechanochemical immobilization.
that the new characteristic peaks formed were mainly Pb3(PO4)2 and PbxCa10-x(PO4)6(OH)2. These peaks indicate that PO3 4 , released after the addition of Ca(H2PO4)2, reacts with Pb in the soil to form precipitates. These precipitates have low solubility and bioavailability and are stable in the environment. Therefore, the content of soluble Pb in the soil was greatly reduced and the Pb content in the residual fraction was greatly increased. Furthermore, Chen (Chen et al., 2006) and others have shown that the formation of coprecipitation of lead in contaminated soil on the surface of soil granular minerals is also one of the main mechanisms for phosphorus to reduce the migration and transformation of lead. Fig. 6 illustrates a conceptual model for mechanism of Pb in contaminated soil by mechanochemical immobilization. The soil particles and Ca(H2PO4)2 were mixed and mechanically ball milled under certain conditions. Mechanochemical reaction occurred between Pb and Ca(H2PO4)2 in the soil during ball milling process, and Pb3(PO4)2 and PbxCa10-x(PO4)6(OH)2 were formed. 4. Concluding remarks This work provides a means of immobilizing Pb in contaminated soils and shows that mechanochemical methods can effectively reduce the leaching concentration of Pb. With the enhancement of the ball milling process conditions, Pb and Ca(H2PO4)2 produced a precipitated product under solid phase conditions, acting as a solidification. Single factor experiments showed that the leaching concentration of lead decreased from 214.13 mg L1 to the regulatory limit of 5 mg L1 at 550 rpm, 2 h, and 10 wt% additive (Ca(H2PO4)2). The BCR sequential extraction procedure indicated that proper ball milling conditions increase the content of residual Pb, which also indicates that mechanochemical ball milling is beneficial to the solidification of heavy metal Pb. In addition, it is worth noting that, from the perspective of economic cost and environmental protection, the method of mechanochemical ball milling to immobilize heavy metals has the advantages of a simple equipment and no waste liquid. Thus, it is a promising method for remediating heavy metal contaminated soil. CRediT authorship contribution statement Ziwei Zhang: Methodology, Investigation, Writing - original draft. Wenyi Yuan: Resources, Writing - review & editing, Supervision, Data curation. Peizhong Li: Methodology, Software. Qingbin Song: Methodology, Software. Xiaoyan Wang: Formal analysis, Investigation. Weitong Xu: Formal analysis, Investigation. Xuefeng Zhu: Methodology, Software. Qiwu Zhang: Writing - review & editing. Jianwei Yue: Formal analysis. Jianfeng Bai: Writing - review & editing. Jingwei Wang: Writing - review & editing.
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