Res Chem Intermed DOI 10.1007/s11164-016-2494-y

Application of stabilized zero valent iron nanoparticles for immobilization of lead in three contrasting spiked soils Mostafa Emadi1 • Mohaddeseh Savasari1 • Mohammad Ali Bahmanyar1 • Pourya Biparva2

Received: 4 December 2015 / Accepted: 21 February 2016  Springer Science+Business Media Dordrecht 2016

Abstract This study was conducted to synthesize and characterize the ZVINs stabilized with acid ascorbic (AAS-ZVINs) and to assess their ability to immobilize Pb2? in sandy, acidity and calcareous spiked soils. To determine the availability of Pb2?, the DTPA-extractable Pb in three spiked soils was studied in an experiment of a completely randomized design with a factorial arrangement of treatments consisting of the AAS-ZVIN dosage, Pb2? contamination levels and contact times (aging). The distribution of the chemical forms of Pb was also determined using the sequential extraction method. The SEM and XRD analyses indicated that AASZVINs had the mean size of less than 50 nm and the maximum 2h peak at 44.8, respectively, demonstrating the nano-sizes and zero valence of the iron particles. The results indicated that the DTPA-extractable Pb in three spiked soils decreased significantly with increasing the AAS-ZVIN dose at the contamination levels of 50 and 150 mg kg-1. The acidic soil displayed the greatest DTPA extractable Pb reduction cmopared with the other two soils. Continuous reduction of the DTPA extractable Pb in all three spiked soils was observed as the contact time (aging) reached 4 weeks. Sequential extraction procedures showed a significant decrease of soluble, exchangeable and carbonate-bound Pb fractions and a pronounced increase of organic matter-bound, Fe/Mn oxides-bound and residual Pb fraction after the soils were treated with AAS-ZVINs. The results obtained in the present study suggest that the use of AAS-ZVIN to remediate soils polluted with Pb could be a promising in situ strategy.

& Mostafa Emadi [email protected] 1

Department of Soil Science, Sari University of Agricultural Sciences and Natural Resources, Sari, Iran

2

Department of Basic Sciences, Sari University of Agricultural Sciences and Natural Resources, Sari, Iran

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Keywords extraction

Zero valent iron nanoparticles  Lead  Spiked soils  Sequential

Introduction Soil contamination with heavy metals is one of the major environmental concerns in human societies [1]. Heavy metal polluted soil has become a serious concern due to intensive industrialization, urbanization and inadequate waste disposal. The extensive use of lead (Pb) by man over time has led to the extensive pollution of surface soils on a local scale, mainly associated with mining and smelting of Pb [2, 3]. Concerns about health effects due to Pb pollution particularly in urban areas and to Pb uptake in agricultural crops have led to the development of a variety of soil remediation techniques. The in situ stabilization of Pb on soil particles (e.g., immobilization by chemical amendments and phytostabilization) and the ex situ extraction or separation of Pb from contaminated soils (e.g., phytoextraction, washing, flotation and land filling) are two strategies to remedy the Pb-contaminated soils [4, 5]. Although these approaches are effective in some cases, they have been proven to be time consuming, labor intensive and inherent variability [6]. Therefore, the use of alternative and innovative ways to stabilize and reduce the bioavailability of heavy metals especially Pb have been introduced by nanotechnology techniques. The zero valent iron nanoparticles (ZVINs) technology has reached commercial status in many countries worldwide [7] due to its potential for broader application, higher reactivity and cost-effectiveness compared to other iron products. In last decade, ZVINs have been investigated as a new tool for the treatment of contaminated water and soil [8–13]. The ZVINs applications as a solution for some problems caused by pollutants are being used successfully for treating the various heavy metal ions including Cr6? [14], As5? [15, 16] Cd2? [8, 17, 18] and Pb2? [19, 20] from aquatic systems. Nanoparticles due to small size, high surface area, crystal form, unique network order, and highly reactivity can be used for treatment of pollutants to non-hazardous materials in soils [21, 22]. The extraordinary properties and feasible applications made zero-valent iron nanoparticles a promising candidate for soil remediation [23–25]. The main advantage of iron is that it is the most abundant metal on earth and hence the adsorbents prepared from this metal will be very cost-effective [22]. Although most studies have focused on using ZVINs to remove environmental contaminants (chlorinated organic compounds, inorganic compounds, metals and metalloids) from water and groundwater [10, 14, 26–28], soil remediation by using ZVIN has not been paid much attention [11, 29, 30]. It has been suggested that the use of ZVINs to remediate As-polluted soils is a promising in situ strategy [30]. Zhang et al. [13] reported a high reduction of As bioavailability in soils after treatment with ZVINs. Alidokht et al. [31] also found good results for Cr immobilization in a spiked soil following the starch-stabilized ZVINs application. Reyhanitabar et al. [12] indicated that the increasing ZVINs dosage from 0.5 to 3 % w/w caused a 70 % increase in the immobilization efficiency of Cr in spiked soils [32] and concluded that stabilization of a chromated copper arsenate-contaminated

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soil with ZVINs led to a decrease of dissolved forms of As and Cr in soil, and lower concentrations of these elements in leachate, soil pore water and plant shoots. The reductive immobilization of chromate in a sandy loam soil using stabilized ZVI nanoparticles was shown by [9]. Wang et al. [33] reported that the Pb removal efficiencies reached 64 and 83 % for mine and farmland soil by addition of 0.2 M citric acid and 2.0 g/L ZVINs, respectively. The iron nanoparticles in some previous aforementioned research contained the reductive element of iron which is subject to oxidation by air under regular conditions. The oxidation products partly or totally lose their original reactivity, compromising the decontamination effectiveness of these particles [7, 34]. Moreover, a requirement for a strict anaerobic environment often renders the fabricating, transporting or storing of these oxygen-sensitive nanoparticles difficult and costly [34]. Therefore, the synthesis of stable nanoparticles in natural and aerobic environments such as soils have a great importance. Air-stable ZVINs need to be developed for soil remediation. This study tried to synthesize ZVINs stabilized by ascorbic acid for long-term reactivity and durability in soil environments.However, there are no available data regarding the immobilization of Pb in ZVINs contact time (aging) or for different Iranian spiked soils using ZVINs. Therefore, the main objectives of this research were (1) to synthesize and characterize the stabilized ZVINs, (2) to determine the effectiveness of synthesized ZVINs for immobilization of Pb in sandy, acidic and calcareous spiked soils, and (3) to evaluate the stability of immobilized Pb using ZVINs aging in soils by performing a sequential extraction procedure on treated soils samples after 1 day, and 1 and 4 weeks.

Materials and methods Synthesis and characterization of ZVINs One of the novelties of this work is the utilization of a new class of nanoparticles that stabilized with ascorbic acid. The ZVINs was prepared in cold distilled water by reducing Fe3? to Fe0 in the presence of ascorbic acid as a reducing agent to stabilize ZVIN without air evacuation. In this approach, the stabilized ZVIN was synthesized by a NaBH4 reduction method without application of inert gas according to the following reaction: 2FeCl34 þ 18H2 O ! 2Fe0 þ 6NaCl þ 6BðOHÞ3 þ21H2

ð1Þ

The detailed information of synthesis procedure was described in our previous work [8]. The freshly synthesized ZVINs were washed three times and then used for the subsequent analysis. Morphology characterization of the synthesized ZVINs was carried out by scanning electron microscope (SEM; Hitachi S-2600N, at 5.0 kV). X-ray diffraction (XRD) was also performed using a Philips D500 diffractometer ˚ ). Patterns were with Ni-filtered Cu Ka radiation (40 kV, 30 mA, l = 1.5406 A recorded from samples over a 2h range of 20–70.

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The soil sample and preparation of Pb-spiked soils Three contrasting surface soils from Mazandaran province in northern Iran including sandy, acidic and calcareous soils were used in this study. The soil samples were collected from the surface layer (0–30 cm depth). Soil samples were air-dried and sieved (\2 mm) before further analysis. The main soil properties are shown in Table 1. In brief, electrical conductivity and pH were measured in extracts of saturated paste and unsaturated paste, respectively. Organic carbon was determined using the Walkley–Black method [35]. The percentage of calcium carbonate equivalent (CCE) was calculated by titration [23]. Available Pb was extracted with diethylenetriamine pentaacetic acid (DTPA) and assessed using atomic absorption spectrometry (AAS; Varian Spectr AA-10) [36]. Total Pb in the soil samples was determined by acid digestion with a mixture of nitric acid (69 % purity) and of HCl (37 % purity) in a microwave reaction system. The content of sand, silt and clay was determined using the hydrometric method [37]. The cation exchange capacity (CEC) was determined with 1 N NH4OAc buffered at pH 8.2 [38]. Artificial contamination with Pb2? was applied to 80 g of the three contrasting soils. Each soil sample was spiked with an aqueous solution containing the appropriate concentration of lead nitrate in order to obtain two levels of contamination, 50 and 150 mg Pb2? per dry kg of soil. Pb-spiked soils were mixed thoroughly and then underwent three wetting–drying cycles of approximately 1 week at room temperature to mimic field conditions. The total Pb levels in the three spiked soil samples were determined by AAS after acid digestion. For soils with a contamination level of 50 mg kg-1, the total Pb contents of the spiked sandy, acidic and calcareous soils were 71.5, 83.5 and 102.6 mg kg-1, respectively, while for soils with a contamination level of 150 mg kg-1, the total Pb contents of the spiked sandy, acidic and calcareous soils were 170.5, 181.6 and 202.6 mg kg-1, respectively.

Table 1 The main physical and chemical properties of three contrasting soils Properties

Unit

Acidic soil 4.42

7.39

7.1

EC

dS m-1

0.81

1.64

1.01

CEC

cmole(?) kg-1

16.95

22.17

4.78

Organic carbon

%

0.89

1.24

0.2

CCEa

%

1.04

26.71

16

Available Pb

mg kg-1

1.47

1.99

0.89

Total Pb

mg kg-1

36.82

53.5

22.75

pH

Calcareous soil

Sandy soil

Sand

%

58

12

94

Silt

%

21

37

1.5

Clay

%

21

51

4.5

a

Calcium carbonate equivalent

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Soil sequential extractions and amendment with ZVINs To test the ZVINs effectiveness in the immobilization of Pb for the three spiked soils, an experiment with a factorial arrangement in a completely randomized design was performed. Four doses of ZVINs (0, 0.5, 1 and 2 % by weight), two levels of contamination (50 and 150 mg kg-1) and the three spiked soils, sandy, acidic and calcareous, were studied. Upon application of the ZVINs, the soil samples were mixed thoroughly and the available Pb concentrations were determined after 24 h by extraction of the treated soils with 0.005 M DTPA (pH 7.3). Then, 20 mL of DTPA solution was added to 10 g of the treated and untreated soil samples placed in polypropylene bottles. The bottles were shaken on a rotating shaker for 2 h and then centrifuged and filtered [36]. The Pb concentrations present in the DTPA extraction were determined by AAS. All measurements were performed in triplicate. To evaluate the durability effect of ZVINs on the immobilization of Pb for the three spiked soils, the DTPA extractable Pb was also determined at 1 and 4 weeks after application of the synthesized ZVINs. Before and after nanoparticle amendment with 2 % of the application dose, the sequential extraction procedures developed by Tessier et al. [39] and then modified by others [29, 40] were employed to quantify the fraction of various operationally defined Pb forms. Extractions with solutions of increasing strengths were sequentially added to the soil sample. The relative availability is: exchangeable [ carbonate-bound [ Fe/Mn oxides-bound [ organic matter-bound [ residual [29]. The Pb concentrations were measured by AAS in each extracts. Statistical analysis All treatments were replicated three times. The analysis of variance was carried out with Statistix 8 software [41]. When a significant (P \ 0.05 or P \ 0.01) difference was observed between treatments, multiple comparisons were performed using the LSD test.

Results and discussion Characterization of synthesized ZVINs The SEM images of the synthesized ZVINs are shown In Fig. 1. The synthesized ZVINs are comprised of individual and spherical particles with particle sizes ranging from 20 to 75 nm and assembled in chains. It can be observed that the ZVINs were stabilized by ascorbic acid as a reducing agent within the synthesis procedure under aerobic conditions. Chain structure formations have been attributed to the magnetic interactions between the adjacent metal particles [42]. Furthermore, there was an apparent separation between ZVINs with little aggregation. The diffraction peak of the synthesized ZVINs at 2h of 44.8 as shown in Fig. 2 correspond to the formation of iron mainly in its zero valent state. In the XRD

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Fig. 1 SEM image of ZVINs stabilized by ascorbic acid

Fig. 2 XRD pattern of synthesized ZVINs

pattern of freshly synthesized AAS-ZVINs, no special signals for the iron oxides (hematite or magnetite) were observed. Effect of ZVINs application on DTPA extractable Pb Figure 3 shows the effect of the ZVINs application dose on DTPA extractable Pb in three contrasting spiked soils. The results indicated that, by increasing the ZVINs application dose, the DTPA extractable Pb was reduced significantly (P \ 0.05) compared to control in the three spiked soils (Fig. 2). In spiked sandy soils with 50 mg Pb kg-1, the ZVINs application doses of 0.5, 1 and 2 % led to 85.20, 86.75 and 89.51 % reductions in DTPA extractable Pb, respectively. For the contamination level of 150 mg kg-1, the reductions of DTPA

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Fig. 3 Effect of ASS-ZVINs application dose on DTPA extractable Pb (mg kg-1) of sandy (a), acidic (b) and calcareous (c) soils contaminated with 50 and 150 mg kg-1 after 1 day

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extractable Pb were 89.83, 74.31 and 59.58 % for the ZVINs application doses of 0.5, 1 and 2 %, respectively. The relative reduction of DTPA extractable Pb in spiked calcareous soils was lower than for spiked sandy soils. At the contamination level of 50 mg kg-1, the ZVIN application doses of 0.5, 1 and 2 % yeeilded 53.23, 59.39 and 67.19 % decreases of DTPA extractable Pb, respectively. The decrease of DTPA extractable Pb at the contamination level of 150 mg kg-1 for the ZVIN application dosea of 0.5, 1 and 2 % were 17.96, 29.55 and 33.63 %, respectively. The reduction of DTPA extractable Pb in spiked acidic soils was higher than for the sandy and calcareous spiked soils. In the acidic spiked soils, the ZVIN application doses of 0.5, 1 and 2 % led to 78.27, 92.14 and 95.38 % reductions in DTPA extractable Pb, respectively, at the contamination level of 50 mg kg-1. For the contamination level of 150 mg kg-1, the reductions of DTPA extractable Pb were 18.44, 86.77 and 86.91 % for the ZVIN application doses of 0.5, 1 and 2 %, respectively. In all three spiked soils, increasing the AAS-ZVINs application dose resulted in the reduction of DTPA extractable Pb being increased but these reductions were not statistically significant, demonstrating the high reactivity of synthesized AASZVINs even in small application doses. As the dosages of AAS-ZVINs increase, their specific surface area simultaneously increases and the availability of reactive sites for reducing the Pb(II) ions rises [27, 43]. At the high Pb contamination level, a significant small adsorption is possibly due to the saturation of surface active sites with the adsorbate molecules [44]. At higher Pb(II) ion concentrations, the available adsorption sites decreased and thus the sorption of Pb(II) decreased. The reduction of Pb at low Pb contamination of acidic spiked soils was much greater (95 % reduction in 1 day amendment at the application dose of 2 %) than for the sandy (89 % reduction in 1 day at the application dose of 2 %) and calcareous soils (67 % reduction in 1 day at the application dose of 2 %). The main reason for this could be attributed to the soil pH. This observation also suggested that the nanoparticle treatment is likely more suitable for immobilizing Pb 2? sorbed in acidic soils [34]. Effect of ZVINs aging on DTPA extractable Pb The reduction of heavy metals by ZVINs can be slowed or inhibited due to the oxidation of ZVINs. Although ZVINs aging has been identified as a significant issue impacting reactivity, only limited work on this aspect has been carried out. To test the performance of the AAS-ZVINs for Pb2? immobilization, the DTPA extractable Pb in untreated and nanoparticle-treated soils was measured and compared. Figure 4 shows the DTPA extractable Pb for the three untreated soils and the soils treated with different application doses of AAS-ZVINs at 1 and 4 weeks contact times (aging). When soils were amended with the AAS-ZVINs at the dose of 2 % for 1 week, the DTPA extractable Pb in the acidic soil decreased from the original 90.11–11.79 mg kg-1, or a 89.9 % decrease at the contamination of 150 mg kg-1.

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Fig. 4 Effect of AAS-ZVINs aging on soil DTPA extractable Pb. a Sandy, b acidic and c calcareous

The DTPA extractable Pb was lowered to 1.18 mg kg-1, or a 96.3 % reduction after 4 weeks of aging in acidic soils spiked with 50 mg kg-1 at the application dose of 2 %. Continuous reduction of the DTPA extractable Pb in all three spiked soils was observed as the aging reached 4 weeks, as shown in Fig. 4. The final DTPA extractable Pb amounts in the acidic, sandy and calcareous soils after 4 weeks

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amendment were 3.1, 1.18 and 8 mg kg-1, indicating 92.4, 96.4 and 78.3 % reductions for the contamination level of 50 mg kg-1, respectively. For the sandy soils with contamination of 150 mg kg-1, the 1-week treatment lowered the DTPA extractable Pb from 101 to 59 mg kg-1 (i.e. a 41 % reduction). The DTPA extractable Pb was further reduced to 40.2 mg kg-1 after 4 weeks leading to a 61 % reduction of DTPA extractable Pb. Overall, the acidic soil displayed the greatest DTPA extractable Pb reduction compared with the other two soils. It is also remarkable that nano-sized zero valent iron nanoparticles, which have higher reactivity due to their small size and high specific surface area, might also promote the conversion of other solid forms of Pb to more stable ones [45]. Liu and Zhao [45] reported that, after soils were amended with iron phosphate (vivianite) nanoparticles, the reduction of the Pb2? leachability was much more distinctive for the acidic soil (56 % reduction in 1 day amendment) than for neutral (30 % reduction in 1 day) and calcareous soils (26 % reduction in 1 day). Pb2? may be removed by nano-sized zero valent iron via reduction to Pb0 and by adsorption of Pb2? [28, 46, 47]. While reacting with nano-sized zero valent iron, Pb2? also precipitates as Pb(OH)2 and oxidizes as a-PbO2 [21, 46, 48]. Metals with slightly more positive E0 than Fe0, (including Pb) can be removed by both reduction and adsorption. Therefore, the main mechanism of immobilization of Pb in soils can be absorbtion and chemical reduction. Fajardo et al. found no negative effects on physicochemical soil properties after aged ZVINs exposure [49]. Research showed that ZVINs are more effective for Pb immobilization than Zn in contaminated soils [29]. The results reported by Fajardo et al. [49] indicated that the pollutant and its ZVINs interaction should be considered when designing soil nano-remediation strategies for the immobilization of heavy metals. Wang et al. [33] suggested that combining low molecular weight organic acid and ZVINs could be a promising alternative approach for remediation of Pb-contaminated soils. Effect of ZVINs application on Pb12 fractions in spiked soils The main goal of in situ remediation strategies is to reduce the mobile fraction of heavy metals in the soil that could reach the groundwater or be taken up by soil organisms and plants. The fractionation of Pb within the soil solid phase could be used to identify the relative availability/leachability of soil-sorbed Pb by revealing the operationally defined fractionation of Pb in the solid phase [29, 39, 40, 50]. The effects of AAS-ZVINs application on Pb availability in three different soils (calcareous, sandy and acidic) were examined by comparing the distribution of Pb in the different soil fractions obtained by a sequential extraction procedure. The fractions of Pb were determined in treated and AAS-ZVINs-treated soils and are shown in Fig. 5 and Table 2. In Table 2 and Fig. 5, five operationally defined Pb fractions in the three soils untreated or treated with AAS-ZVINs was mentioned. The five Pb fractions are defined as water soluble/exchangeable (EX), carbonate-bound (CB), Fe/Mn oxidebound (OX), organic matter-bound (OM), and residual (RS) Pb. The plant

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Fig. 5 Effect of contact time (aging) of ASS-ZVINs in three contrasting soils spiked with 150 mg kg-1 at 2 % application dose on the relative abundance of various Pb forms. EX soluble/exchangeable, CB carbonate-bound, OX Fe/Mn oxides-bound, OM organic matter-bound, RS residual

availability of different Pb fractions follows the sequence EX [ CB [ OX [ OM [ RS. As Table 2 indicates, the EX and CB fractions in spiked sandy soils decreased significantly about 60 %, while the OX and OM bound fractions increased about 10 and 40 %, respectively, when treated with 2 % of ASS-ZVINs dose. The RS fraction significantly increased by about 4 times more than observed in untreated sandy soils spiked with 50 mg kg-1 contamination. The same trend was observed for acidic soils. However, the OX and RS fractions were increased by about 2.5 and 1.5 times compared with untreated soils at the contamination of 50 mg kg-1. In spiked calcareous soils, the EX and CB fractions were statistically significantly decreased by about 40 and 60 %, respectively, while the OX, OM and RS fractions were increased by about 1.2, 1.2 and 4.5 times compared with untreated calcareous soils at the contamination of 50 mg kg-1. Figure 5 shows the percentage of Pb retained in each soil fraction for the sandy, acidic and calcareous soils spiked with 150 mg kg-1 after treatment with AASZVINs at the application dose of 2 % in 1 day, and 1 and 4 weeks. As shown in Fig. 5, a significant reduction in the Pb contents of the exchangeable fraction (in the range 15–35 %) occurred after treatment of the contaminated soils with AASZVINs. As observed at the contamination of 50 mg kg-1, the soil Pb forms with the more easily available fractions (EX and CB) decreased, whereas the percentages of Pb associated with the less available fractions (OX, OM and RS) increased at the higher contamination level. Figure 5 shows that the distribution of various Pb forms for the calcareous soil lies between those for the sandy and acidic soils.

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M. Emadi et al. Table 2 Effect of 2 % application dose of AAS–ZVINs on various fractions of Pb (mg kg-1) in three different soils spiked with the contamination level of 50 mg kg-1 Soil types

Contact time (aging)

EX-Pb

CB-Pb

OX-Pb

OM-Pb

RS-Pb

Untreated sandy soil

1 day

12.10a

17.54a

11.21b

5.42b

4.42d

1 week

12.90a

17.80a

11.41b

5.49b

4.32d

4 weeks

11.32b

16.59a

11.52b

4.13b

6.15c

1 day

3.82c

6.45b

12.33b

7.66a

19.27b

1 week

3.84c

6.52b

12.42b

7.76a

19.47b

4 weeks

2.98c

5.14b

14.38a

4.15b

23.66a

1 day

20.24a

5.72a

11.80ab

4.23c

8.61c

1 week

20.29a

5.78a

11.83ab

4.33c

8.81c

4 weeks

19.29a

4.82ab

11.45b

5.47c

8.76c

1 day

12.2b

5.19ab

13.44a

9.32a

11.14b

1 week

12.00b

5.09ab

13.44a

9.62a

11.24b

4 weeks

11.35b

3.86b

12.17bc

7.40b

15.72a

1 day

4.22a

22.68a

11.74b

8.52b

3.20c

1 week

4.12a

22.58a

11.74b

8.52b

3.40c

Treated sandy soil

Untreated acidic soil

Treated acidic soil

Untreated calcareous soil

Treated calcareous soil

4 weeks

3.73a

21.13a

13.20b

7.61b

4.45c

1 day

2.65b

7.12b

15.1a

10.32a

15.01b

1 week

2.45b

7.52b

15.2a

10.35a

15.18b

4 weeks

2.59b

6.22b

15.84a

8.66b

17.62a

Different letters in columns indicate a significant difference at the level of 5 % based on the LSD test for each spiked soil EX soluble/exchangeable, CB carbonate-bound, OX Fe/Mn oxides-bound, OM organic matter-bound and RS residual

Although decreases in the EX and CB fractions following the aging of the ASSZVINs application from 1 day to 4 weeks in three treated soils were observed, it was not statistically significant (Fig. 5). Thus, the immobilization of Pb with ASSZVINs application was stable for at least a month. Some researchers [27, 29, 33, 34] also did not find differences in the immobilization processes of As, Pb, Pb and Pb/ Zn. This fact is of great interest because of the need to apply long-term immobilization technologies [29]. These results are in agreement with the findings of Gil-Dı´az et al. [29] who indicated that the Pb/Zn associated with the more easily available fractions (EX and CB) decreased, whereas the percentages of these metals associated with the less available fractions (OX, OM and RS) increased for soils contaminated with commercial ZVINs suspension at a dose of 20 %. The percentages of heavy metals in the less available fractions after application of ZVINs were 65.6 % for Pb and 45.4 % for Zn, confirming the greater effectiveness of ZVINs in the immobilization of Pb compared to that of Zn in soils [11]. As organic matter, the OX and RS fractions are very persistent in the soil solid phase and unlikely to be extracted easily [51], the ASS-ZVINs application led to the transformation of labile fractions EX and CB species) to less available fractions leading to the immobilization of Pb in the three spiked soils.

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Conclusion The major purpose of this paper was to synthesize stable ZVINs and to examine the effectiveness of using these materials to immobilize Pb2? in three contrasting spiked soils. The results presented in this research allow the conclusion that it is possible to produce highly effective and durable ZVINs with sizes ranging from 15 to 45 nm using ascorbic acid as a reducing agent. According to the results, the application of AAS-ZVINs had the highest efficiency for Pb immobilization in acidic soils compared to sandy and calcareous soils at both contamination levels. Results from a sequential extraction procedure showed that the nanoparticle treatment of the soils converted large fractions of water-soluble/exchangeable and carbonate-bound Pb to the most stable form of the residual Pb, resulting in enhanced Pb immobilization. Moreover, the sequential extraction performed after 4 weeks of incubation imply that, after nearly 1 month, AAS-ZVINs were able to decrease the amount of exchangeable and carbonate-bound Pb. Considering the slightly positive standard redox potential (E0) value of Pb compared with ASS-ZVINs, the reaction mechanisms during the decontamination process are reduction and also adsorption on ZVINs. The ZVINs stabilized with ascorbic acid improved the colloidal stability and diffusion of the nanoparticle suspensions and consequently accelerated the Pb adsorption rate from contaminated soils. As nano-scale iron particles remain suspended in their colloidal solution, they can be injected straight into contaminated soils, sediments and aquifers. Therefore, these nanoparticles may potentially be useful for restoring Pb-contaminated soils, but future studies are required to explore the mechanism of Pb removal and the toxicity of ZVINs for plant and soil organisms. Future work and studies are necessary to focus on the stability of the immobilized Pb in order to determine the applicability of this emerging technology for soil remediation, and specific attention should be paid to the potential risks of applying immobilization amendments and its long-term effects on field soils.

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