Potential of zero-valent iron in remediation of Cd(II) contaminated soil: From laboratory experiment, mechanism study to field application

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ScienceDirect Soils and Foundations 59 (2019) 2099–2109 www.elsevier.com/locate/sandf

Potential of zero-valent iron in remediation of Cd(II) contaminated soil: From laboratory experiment, mechanism study to field application Qiang Tang a,c, Peixin Shi a,⇑, Zhao Yuan a, Shenjie Shi a, Xiaojing Xu b, Takeshi Katsumi c,* a

School of Rail Transportation, Soochow University, Xiangcheng District, Suzhou 215131, China b Suzhou Tonghe Environmental Engineering Co. Ltd., Suzhou 215011, China c Graduate School of Global Environmental Studies, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan Received 3 April 2019; received in revised form 9 September 2019; accepted 1 November 2019 Available online 11 December 2019

Abstract This study investigates the potential of ZVI on Cd(II) contaminated soil remediation through laboratory experiment, mechanism study and field application. The results show that the dosage of ZVI, the initial concentration level and the reaction time have significant impacts on Cd(II) adsorption and about 88% of aqueous Cd(II) can be removed from soils. The ZVI is observed to promote the increase of Cd(II) residual fraction in Cd(II) contaminated soils according to the sequential extraction procedure (SEP) results. Regression of the laboratory experimental data based on the Langmuir isotherm equation shows the adsorption capacity can reach 34.7 mg/g. Such a high value is attributed to physical and chemical adsorption, which is proved by X-ray diffraction patterns (XRD), scanning electron microscope images (SEM), and Brunner-Emmet-Teller & Barret–Joyner–Halenda (BET-BJH) analysis. The field application shows that the ZVI can reduce the Cd(II) content in soils and brown rice by 51% and alleviate soil acidification, resulting in a 9.4% increase in rice yields. Ó 2019 Production and hosting by Elsevier B.V. on behalf of The Japanese Geotechnical Society.

Keywords: Cadmium; Zero-valent iron; Adsorption; Passivation; Remediation

1. Introduction With rapid industrialization and urbanization in China, heavy metal contamination in paddy soils has become a serious problem threatening food safety and human health due to its high toxicity. Without appropriate controls in developing countries, heavy metals are emitted into farmlands through such routes as atmospheric deposition, livestock manure, urban sludge, pesticides, fertilizers, and the smelting, mining and chemical industries (Bian et al., 2014, Cai et al., 2015, Lu et al., 2012). Compared to easily Peer review under responsibility of The Japanese Geotechnical Society. ⇑ Corresponding authors. E-mail addresses: [email protected] (P. Shi), katsumi.takeshi. [email protected] (T. Katsumi).

biodegradable organic pollutants, t heavy metals are difficult to degrade and are easily absorbed by the human body through the food chain (Tang et al., 2014, Wu et al., 2015). According to a recent survey, 19.4% of cultivated land in China is contaminated by heavy metals, and cadmium is a major pollutant (Teng et al., 2014, Yang et al., 2009). Cadmium originates mainly from alloy manufacturing, electroplating anticorrosion and rechargeable batteries. The accumulation of cadmium in farmland is associated with such undesirable consequences as a reduction in the ability of crop roots to absorp nutrients and water and the destruction in chlorophyll in leaves (Hussain et al., 2010, Liu et al., 2016). In the human body, exposure to a high concentration of cadmium reduces the body’s ability to synthesize RNA and protein, destroys DNA, and can

https://doi.org/10.1016/j.sandf.2019.11.005 0038-0806/Ó 2019 Production and hosting by Elsevier B.V. on behalf of The Japanese Geotechnical Society.

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causes acute or chronic poisoning, such as the ‘‘Itai-Itai disease” in Japan and the ‘‘Cadmium brown rice” in China (Brus et al., 2009, Nordberg, 2004). The upper limit for Cd (II) in the farmland ecosystem is restricted strictly worldwide. The UK Environment Agency (2002) recommends that the Cd concentration in soils does not exceed 3.5 mg/kg (Davies, 1997). The Chinese Environmental Quality Standard for Soils (1995) proposes a maximum allowable content of cadmium in crops of 0.2 mg/kg (Yang et al., 2014). It was found that Cd(II) leaching from industrial wastewater and the use of phosphate fertilizer with a high Cd content results in cadmium pollution in farmland, with contaminated rice paddies yielding Cd-rich rice grains, and crop safety and human put into jeopardy (Bolan et al., 2013). The geochemical behavior of cadmium in paddy fields can be changed substantially by the flooding and drainage cycles. Cd(II) bioavailability may effectively decrease in flooded soils due to the increase of pH or the decrease of sulfur content. Besides, the reduction of Fe (III) oxides can partly limit the entry of Cd(II) into root by adsorption in the rhizosphere environment. In the risk management of Cd(II) in rice ecosystems, appropriate methods such as decreasing Cd inputs to rice soils, water management, usage of low Cd-accumulating rice cultivar, phytoremediation and soil washing can minimize Cd(II) contamination. Ishikawa et al. (2010) reported a majoreffect QTL on controlling Cd(II) concentration in rice grains, which did not affect the concentrations of essential trace and also had no significant effect on important agronomic traits of rice. Makino et al. (2006, 2007) developed a new soil-washing practice combined with on-site wastewater treatment. Murakami et al. (2007) selected ‘‘Milyang 23” rice as a potential cultivar for the phytoextraction of Cd in low-to-moderate concentration paddy soils. In the end treatment of paddy soil, various geoenvironmental techniques have been utilized to prevent the migration of heavy metals to the surrounding water and soil environment. These include such techniques as thermal desorption, electrokinetic remediation, bioremediation and chemical passivation technology (Acar et al., 1995, Aresta et al., 2008, Gavrilescu, 2010, Yao et al., 2012). Among them, chemical passivation has been widely adopted because it is harmless, inexpensive, and highly effective. The widely used remediation agents are characterized by their adsorption or precipitation ability, and include organic materials, clay minerals and metal oxides (Akar et al., 2005, Kumpiene et al., 2008, Hanauer et al., 2011, Zhu et al., 2012, Gomes et al., 2012, Tang et al., 2017). A by-product of iron-related industry, ZVI, is recognized as a potential agent for remediation because it does not result in secondary environment pollution (Hartley et al., 2004, Lee et al., 2009). Although previous studies have investigated adsorption and redox processes as the removal mechanisms of metals by ZVI, relatively little research has focused on a combination of detailed adsorption characteristics and field applica-

tions of metal removal by ZVI. Therefore, the results of this investigation have potential for use as guidelines for the field application of ZVI in the effective control of Cd (II) pollution. In this study, ZVI was selected as the remediation agent, and its performance in the control of aqueous Cd(II) was investigated. A laboratory experiment was conducted first. The effects of the dosage of ZVI, initial solution concentration, reaction time, and the pH of the initial solution were investigated. The heavy metal fixing mechanism was explored by the sequential extraction procedure (SEP), scanning electron microscope (SEM) analysis, Brunner-Emmet-Teller and Barret-Joyner-Halenda (BET-BJH) results and X-ray diffraction (XRD) spectra. A field experiment was carried out in a paddy field, and the Cd(II) content, soil pH, and rice yield were measured in harvested rice, and the appropriate application ratio of ZVI was determined. 2. Laboratory experimental materials and methods 2.1. Experimental preparations ZVI powders were collected from an iron and steel company and provided by Suzhou Tonghe Corporation. Acidic mine wastewater was mixed with absolute ethanol (volume ratio = 1: 2), and then stirred evenly at 30 °C. An NaBH4 solution was added to the mixed solution: the amount of NaBH4 was more than 3 times the amount of iron in mine wastewater. Under the protection of N2, the reaction was stirred at 20 °C until the iron was completely reduced to zero-valent iron. The black solids were washed with ethanol 3 times and dried in vacuum drying chamber for 500 min at 70 °C to prepare ZVI. The collected ZVI powders were washed with distilled water (DIW), stored in an oven (XMA-600, Yatai Corporation, China) at 105 °C for 12 hrs, cooled down to 25 °C, and then passed through 100-mesh sieve with diameters of 150 ± 15 um. The image of ZVI is shown in Fig. 1. The Cd(NO3)2
4H2O (AR, ZHENXIN, China) was dissolved into de-ionized water (DIW) to prepare 1 mol/L standard stock solution, which then was stored in a refrigerator at a temperature of

Fig. 1. The image of ZVI.

Q. Tang et al. / Soils and Foundations 59 (2019) 2099–2109

5 °C, and diluted to the desired concentrations for use. Polyvinyl chloride (PVC) tubes and flasks were washed by 0.01 mol/L HNO3 (AR, ZHENXIN, China) and DIW f three times (Tang et al., 2012). The pH value of the initial solution was adjusted by diluted either the HNO3 or NaOH (AR, ZHENXIN, China) solution. The soils for laboratory experiment were collected from a paddy site in Guangdong province, China, where the field application was carried out. The experimental soils were air-dried, pulverized and then passed through 50-mesh screen with diameters of 300 ± 15 um. The 50 mL and 100 mL Cd(II) solutions were taken from a 1 mol/L standard Cd(NO3)2 solution and then were both diluted to 1000 mL. The diluted solutions were then mixed with 1 kg soils to prepare for the contaminated soils with concentration of Cd(II) content around 3 and 6 mg/kg. The surface morphology of ZVI and Cd(II)-loaded ZVI was observed by scanning electron microscope (EVO, Zeiss Corporation, Germany). The XRD spectrum of ZVI and Cd(II)-loaded ZVI was gained by D/MAX-RA diffractometer (TF-5500, Tongda Corporation, China). The Cd (II)-loaded ZVI sample was obtained by centrifuging the mixed solution at 3000 rpm for 3 min (ZVI dosage of 10 g/L and Cd(II) concentration of 500 mg/L was blended for 24 h at 25 °C for sampling). The specific surface area (SSA) and pore size distributions of ZVI were measured by the BET-N2 test (3H-2000PM1 apparatus, Beishide Corporation, China) since the parameters are closely related to its adsorption capacity. A pH glass electrode was used to measure the pH value of the reaction solution (pH 211, Gaoss Union Corporation, China). The above test together with laboratory adsorption experiment results were used for a further discussion to determine the heavy metal fixation mechanism (Zhang et al., 2019). 2.2. Lab-scale experimental programs 2.2.1. Adsorption experiments According to the experimental design, the samples were divided into four groups to evaluate the heavy metal fixation effect of four different influence factors: the dosage of ZVI, the initial solution concentration, the reaction time and the pH value of initial solution. In group 1, the dosage of ZVI was increased from 1, 2, 5, 10 to 20 g/L. The ZVI of different amount and two different initial concentration solutions (100, 300 mg/L) were mixed. In group 2, the ZVI (10, 20 g/L) was respectively mixed with six different Cd(II) solutions with increasing initial solution concentration (50, 100, 200, 500, 1000, and 1500 mg/L). In group 3, the ZVI dosage was fixed at 20 g/L and various Cd(II) concentrations of 100, 300 mg/L, while the reaction time was designed from 15, 30, 60, 120, 300, 600, 1440 to 2880 min, respectively. In group 4, both the ZVI dosage and the initial solution concentrations were fixed (20 g/L; 100, 300 mg/L). The solution pH values were regulated ranging from 4 ± 0.2 to 10 ± 0.2 with an increment of 3.

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All samples were transferred into PVC centrifuge tubes and then put into a thermostatic shaking agitator (25 °C) (HY-60F, Well Corporation, China) at 150 rpm. Subsequently, the tubes were centrifuged for 3 min at 3000 rpm (TDZ5-WS, XIANGYI, China). The supernatant was separated, and the Cd(II) concentrations were measured by Atomic Adsorption Spectrophotometer (AAS). 2.2.2. Incubation experiments The experiments were divided into eight groups to investigate the change of heavy metal Cd speciation under different conditions. The cultivated Cd(II) contaminated soils (3, 6 mg/kg) were placed in plastic bottles, and then mixed with ZVI with 0%, 0.1%, 0.2%, and 0.4% dry soil mass, respectively. DIW was added to each sample to achieve mixture with the desired liquid–solid ratio of 1:1 (water: (ZVI + contaminated soils)) At the end, each plastic bottle was sealed and left to stand for 16 days under standard curing conditions (temperature 25 ± 1 °C, humidity 95 ± 1%) (Satapanajaru et al., 2008). After 16 days, the speciation of Cd in the sample soils was determined by the Tessier sequential extraction procedure (SEP). The extraction method involved five steps, and allowed the heavy metals for be divided into five different forms: exchangeables, carbonates, Fe/Mn oxides, organics, and residual fractions (Tessier et al., 1979). The Cd(II) concentrations in the leaching solution were measured by Inductively Coupled Plasma Emission Spectroscopy (ICPE-9000, Shimadzu, Japan). 2.3. Experimental data fitting 2.3.1. Isothermal adsorption models The adsorption isotherm is represented by a curve showing the relationship between equilibrium adsorption amount Qe and adsorption equilibrium concentration Ce at a constant temperature. Three adsorption isotherms, Langmuir, Redlich-Peterson (R-P) and Freundlich model, were used for data analysis. The Langmuir model assumed the occurrence of monolayer adsorption on a homogeneous surface, with no interaction between the absorbed molecules (Tang et al., 2009, Wang et al., 2017) Qe ¼

Qm K L C e 1 þ K LCe

ð1Þ

where Qe represents the unit equilibrium adsorption amount (mg/g), Qm is the adsorption capacity (mg/g), Ce is the equilibrium concentration (mg/L), and KL is the Langmuir characteristic constant (L/mg) corresponding to the affinity between adsorbent and adsorbate. The adsorption enthalpy increases along surface coverage, which is non-uniformly distributed on the surface of the adsorbent according to the Freundlich model (Wang et al., 2017). It is normally used to describe adsorption onto heterogeneous surfaces or surfaces which contain various affinity sites (Artola et al., 2000). It can be expressed as

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Qe ¼ K F C 1=n e

ð2Þ

where the value of KF represents Freundlich characteristic constant (mg/g) associated with adsorption capacity and strength, 1/n indicates the adsorption efficiency, and its value < 1 represents favorable adsorption conditions (Kooh et al., 2016). Redlich-Peterson equation is a versatile model which can be applied in both homogeneous and heterogeneous systems (Redlich and Peterson, 1959), and can be expressed as Qe ¼

K RP C e 1 þ aRP C be

ð3Þ

where KRP (L/g), b and aRP ((L/mg)b) are Redlich-Peterson characteristic constants, and the change of their values can simplify the R-P model to Langmuir or Henry model (Wu et al., 2012). 2.3.2. Adsorption kinetics Three kinetics equations are used to estimate the adsorption rate and contact time required for optimal adsorption, including Pseudo-first order, Pseudo-second order kinetic equations and Intraparticle diffusion model (Tang et al., 2009 and 2012, Tu et al., 2012). The Pseudofirst order kinetic equation can be written as Qt ¼ Qe ð1  ek1 t Þ

ð4Þ

where Qe and Qt is respectively the solute amount adsorbed per unit adsorbent at equilibrium and any time (mg/g), and k1 is adsorption rate constant (min1). The Pseudo-second order kinetic equation can be expressed as Qt ¼

k 2 Q2e t 1 þ k 2 Qe t

ð5Þ

where k2 is the Pseudo-second order rate constant (g/ mg
min). The intraparticle diffusion equation can be written as Qt ¼ k int t

1=2

þC

ð6Þ 1/2

where kint (g/mg
min ) represents the rate constant and C is the intercept in its linear form. 2.4. Field applications The field applications were conducted in Shaoguan and Shantou city, Guangdong province to evaluate the effect of ZVI stabilizing the Cd(II) contaminated soils by measuring the Cd content in soil and Cd accumulation in brown rice. Fig. 2(a) shows the geographical location of Shaoguan and Shantou city which were located close to each other in southeast China. The two application sites were all heavily polluted by Cd from mining activities. The soil Cd content was about 3–6 mg/kg, and the soil pH value was about 5–6. The two sites were shown in Fig. 2(b) and (c), respectively.

Figs (d) and (e) show the measurements of monthly temperature and rainfall in Shaoguan and Shantou city, respectively. The maximum monthly temperature and rainfall occur close to June. The two sites were treated separately by adding different dosages of ZVI. The ratio of added ZVI dosage was taken from 0 to 0.4% of the soil incubation experiments. Four treatments (0, 100 (about 0.06% ZVI per acre), 200 (0.13%), and 400 (0.25%) kg/acre) for contaminated soil of Shaoguan city were carried out and the corresponding group number was A1 (control subject), A2, A3, and A4, respectively. The other one was divided into 5 groups (B1, B2, B3, B4, and B5), including adding the ZVI of 0, 50 (about 0.03% ZVI per acre), 100 (0.06%), 200 (0.13%), and 300 (0.19%) kg/acre. The addition time of ZVI began in July 2015. Each treatment was repeated 3 times and was randomly arranged in 12 experimental plots each with an area of 50 m2, and the irrigation and drainage of each plot were independent. ZVI was added to each plot in the form of fertilizer and mixed with the soils of plough layer (about 20–30 cm). The rest field management methods were the same as that used by the farmers. The paddies (Wufengyou 615, China) were planted in the two experimental sites in April 2016. The remediation time of ZVI for contaminated soil lasted about 9 months, about 15 times that of indoor soil incubation experiments. The soils of taproot system and the samples of brown rice were collected during the rice maturity period in July 2016. Rice yields were calculated through weighing, and the soil pH value was measured by the pH glass electrode. Diethylene triamine pentaacetic acid was used to extract the available Cd in soils and Cd accumulation of brown rice, which concentrations were measured by AAS (Sinegani and Monsef, 2016). 3. Results and discussions 3.1. Characterization of adsorbents The SEM images (magnification = 1000) of the ZVI before and after Cd(II) adsorption are shown in Fig. 3. Fig. 3(a) shows the ZVI particles before adsorption are relatively uniform and distributed in the block form with the average particle size of 22 um. Fig. 3(b) shows a large number of small particles attached to the surface of the ZVI after adsorption, which resulted in an uneven appearance. This occurs due to changes in the intergranular structure of ZVI, with flocculation and crystal precipitation due to surface complexation and electrostatic interaction processes. The BET-BJH measurement results of the ZVI are shown in Fig. 4. The SSA and total pore volume of the ZVI was estimated as 152.6 m2/g and 0.6 cm3/g by N2 adsorption isotherm, respectively. The pore size of the ZVI was mainly in the range of 10 to 15 nm. This indicates that the ZVI sample is an ordered mesoporous material. As a mesoporous material, ZVI readily adsorbs Cd(II). This

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Fig. 2. Basic information of Shaoguan and Shantou city: (a) Geographical location of Shaoguan and Shantou (b) Experimental soil of Shanguan (c) Measured monthly temperature and rainfall of Shaoguan (d) Experimental soil of Shantou (e) Measured monthly temperature and rainfall of Shantou.

4.40E-3

100

Content Pore volume

SSA: 152.6 m2/g Temperature: 22°C R2 = 0.999

80

3.30E-3

60

2.20E-3

40

1.10E-3

20

0.00

0-5

Content (%)

Pore volume (cm3/g)

5.50E-3

0 5-10

10-15 15-20 20-25

Pore radius (nm) Fig. 4. BET-BJH measurement results of the ZVI.

3.2. Lab-scale adsorption experimental results

may be because the diffusion of Cd(II) from the mesopore to the micropore is shorter than the direct migration from the liquid phase to the micropore. The mesopore improves the Cd(II) migration in the micropore and the equilibrium coverage ratio of the microporous surface, which enhances the adsorption capacity (Asuquo et al., 2017). However, macropores may facilitate the fast flow of liquid or gas through ZVI, affecting the adsorption of contaminants.

30

300

Duration: 24 h Temperature: 25°C

240

24 300 mg/L

18

180

12

120

6

60

100 mg/L

0

0 0

5

10

15

20

Dosage (g/L) Fig. 5. Dosage effect on Cd(II) adsorption.

Equilibrium concentration Ce Circle: (mg/L)

Fig. 3. SEM images of the ZVI for Cd(II) adsorption at 1000 magnification: (a) Before reaction (b) After reaction.

Unit adsorption amount Qe Square: (mg/g)

The effect of ZVI dosage on Cd(II) adsorption is displayed in Fig. 5. It is apparent that the equilibrium Cd (II) concentration gradually decreased with increases in the dosage of ZVI. At ZVI dosages of less than 5 g/L, as the ZVI dosage gradually increased, the equilibrium concentration of Cd(II) slowly decreased. This is likely due to the limited number of binding sites available at low

Q. Tang et al. / Soils and Foundations 59 (2019) 2099–2109

24

80

18

60 20 g/L

12

40 20

6 10 g/L

0

320

640

Dosage

Unit

10 g/L

20 g/L

Langmuir isotherm model mg/g Qm KL L/mg R2

34.7 0.002 0.97

22.6 0.006 0.99

Freundlich isotherm model mg/g KF 1/n R2

0.5 0.6 0.97

1.4 0.4 0.96

Redlich-Peterson isotherm model L/g KRP aRP (L/mg)b b R2

0.1 0.05 0.7 0.97

0.2 0.01 0.9 0.98

The fixation mechanism involves selective and irreversible reaction characteristics, including chemisorbed innersphere complexes (Sposito, 1984). The maximum adsorption capacity Qm of ZVI (34.7 and 22.6 mg/g) is several dozen times larger than Freundlich and R-P isotherm models. The KL and KF value increase with the increase of dosage, indicating that a high dosage of adsorbents results in better adsorption, which is accordance with the results of ZVI dosage effect. The 1/n values of the Freundlich isotherm lying between 0.3 and 0.6 confirm the favorable condition for Cd(II) adsorption. The constant values of the R-P isotherm indicate a hybrid chemical reaction-sorption process, and its b value is close to one (0.9), indicating the adsorption is in accordance with the ideal Langmuir condition. The effect of reaction time on Cd(II) adsorption is shown in Fig. 7. The equilibrium concentration decreased with the increase in the reaction time, while the unit adsorption amount increased with the increasing reaction time. After 48 hrs of reaction, the unit adsorption amount was as high as 3.84 mg/g and 8.45 mg/g, and the removal ratio was as high as 76.8% and 56.3%. Compared to some biochemical or natural soil materials, the equilibrium adsorption time of this reaction is longer, at approximately 24 hrs (Tang et al., 2008, Tang et al., 2009 and 2010). This can likely be attributed to slower adsorption, the low occu-

100

Duration: 24 h Temperature: 25°C

0

Table 1 Predicted constants of isotherm models.

960

1280

0 1600

Initial solution concentration C0 (mg/L) Fig. 6. Initial solution concentration effect on Cd(II) adsorption.

Unit adsorption amount Qe Square: (mg/g)

30

Removal ratio Circle: (%)

Unit adsorption amount Qe Square: (mg/g)

adsorbent quantities, effectively restricting the progress of adsorption. With increasing ZVI, the adsorption sites on SSA were gradually occupied by Cd(II), resulted in a significant decrease in the equilibrium Cd(II) concentration. The unit adsorption amount exhibited a negative relationship along ZVI dosage since the availability of the adsorbent for Cd(II) decreased (Nemesß and Bulgariu, 2016). The trend is non-linear. When ZVI was between 0 and 5 g/L, the unit adsorption amount dropped sharply, and then gradually flattened at a high dosages. A high dosage of adsorbent can lead to particle aggregation, which would result in the SSA decrease of the adsorbent and an increase in the length of the effective diffusion path (Bhattacharyya and Gupta, 2008). The effect of initial Cd(II) concentration on removal efficiency is shown in Fig. 6. It can be seen that the unit adsorption amount increased rapidly as the initial solution concentration increased. When the Cd(II) concentration was high, the area of effective contact sites between pollutants and ZVI increased. The collision probability between the two became large, and the binding sites on the surface of the ZVI were fully utilized. Around Cd(II) concentration of 1500 mg/L, the unit adsorption amount reached the highest value at 25.3 (ZVI = 10 g/L) and 19.8 mg/g (ZVI = 20 g/L), respectively. The trend of Cd(II) removal ratio was opposite to the unit adsorption amount along the increase of the initial solute concentration. The continuous increase of Cd(II) concentration led to the maximum adsorption capacity of ZVI to Cd(II) and more and more positive cations compete for adsorption, resulting in the decrease of the adsorption efficiency. In addition, H+ in the solution reacted with ZVI to form Fe(II) or Fe(III), which caused iron oxides or hydroxides to adhere to the surface of ZVI and blocked the adsorption to Cd(II) (Komnitsas et al., 2007, Rangsivek and Jekel, 2005). For further discussion on the adsorption capacity, type as well as mechanism, the test data were fit based on the Langmuir, R-P, and Freundlich isotherm models, and the results are shown in Table 1. The Langmuir model was the best fit for the test data best according to the correlation coefficients (R2 = 0.97 and 0.99), indicating that the monolayer adsorption occurs between the surface of adsorbent and Cd(II) solute interface, i.e. chemical adsorption.

10

300

Dosage: 20 g/L Temperature: 25°C

8

240 300 mg/L

6 4

180 120

100 mg/L

2 0 0

600

1200

1800

2400

60 0 3000

Equilibrium concentration Ce Circle: (mg/L)

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Reaction time t (min) Fig. 7. Reaction time effect on Cd(II) adsorption.

Q. Tang et al. / Soils and Foundations 59 (2019) 2099–2109

pation of sites and the reduced adsorption area at lower ZVI dosages and the temperatures (Devi and Saroha, 2015, Rajput et al., 2016). The experimental data were further analyzed using the above mentioned three adsorption kinetic models, and the calculated parameters are listed in Table 2. According to the correlation coefficient (R2 = 0.97 and 0.99), it can be concluded that the Pseudo-second order kinetic model is suitable to describe the Cd(II) adsorption process. This procedure is more likely to predict the behavior over whole adsorption process and is consistent with chemical adsorption being the rate-controlling ¨ nlu¨ and Ersoz, 2006). Also, the rate constant k2 step (U decreases with the increase of initial solution concentration, indicating that the contaminated solution with the low concentration is likely to equilibrate relatively quickly (see Table 3). Fig. 8 exhibits the effect of initial solution pH environment on Cd(II) adsorption. The dissolution of ZVI is observed at pH = 4, in effect reducing the removal efficiency of Cd(II). When the pH value rises from 4 to 7, Cd(II) removal rate increases gradually. This may be because H+ and cations compete for adsorption sites in acidic solutions, resulting in lower removal efficiency. As pH values continue to increase from 7 to 10, the adsorption surface becomes less active and so electrostatic attraction between Cd(II) and ZVI surface may increase. According to a survey, the pH values of farmland and rainfall in Guangdong Province were around 4 to 8 (Yu et al., 2017, Zhang et al., 2018). Combined with the above key factors, the actual working conditions were all considered completely. The results of the adsorption experiments had reference value, and their conclusions could be used real-scale applications. 3.3. Lab-scale incubation experimental results Fig. 9 shows the effect of ZVI dosage on Cd speciation in Cd(II) contaminated soil of different concentrations. The speciation of Cd was mainly reflected in the form of carbonates, Fe/Mn oxides, and residual fractions, among which the content of Cd carbonate-bound fraction was Table 2 Predicted constants of kinetic models. Initial solution concentration

Unit

100 mg/L

300 mg/L

Pseudo-first order kinetic model Qe mg/g k1 min1 R2

3.7 0.005 0.93

7.9 0.007 0.97

Pseudo-second order kinetic model mg/g Qe k2 g/mg
min R2

4.1 0.002 0.97

8.8 0.001 0.99

Intraparticle diffusion model kint C R2

0.1 0.9 0.83

0.1 2.3 0.76

g/mg
min1/2

2105

the highest. However, due to the different concentrations of the Cd(II) contaminated soil, the variation trend of Cd speciation content differed as the ZVI dosage was increased. In Fig. 9(a), the content of carbonates was 46.0%, and the Fe/Mn oxides fraction was 20.8% in the contaminated soil without added ZVI. The lowest contents were 44.7% and 17.5%, respectively, at a ZVI dosage was 0.1%, tWith increasing doses of ZVI, their content also rose. In contrast, the content of the residual fractions was the largest at a ZVI dosage of 0.1% and decreased with the increasing ZVI dosage. In Fig. 9(b), the content of carbonates, Fe/Mn oxides, and residual fractions was respectively 55.5%, 24.0%, and 17.6% in the contaminated soil without added ZVI. When the ZVI dosage was 0.2%, the content of carbonate and Fe/Mn oxides form fraction was respectively 41.6% and 21.4%, the residual fraction reached a maximum of 33.7%. This leaching risk of residual fraction in all speciations of heavy metals was the smallest among the results published to date (Cuong and Obbard, 2006, Lasheen and Ammar, 2014). The increasing Cd residual fraction with increasing ZVI indicates that the addition of ZVI has a significant effect on the reduction of cadmium activity in soil and stabilizes the contaminated soil. In addition, there is a relationship between soil contaminated concentration and ZVI dosage. As the Cd concentration increases, increases in the ZVI dosage increase its passivation effect on the contaminated soil. 3.4. Heavy metal fixation mechanism Fig. 10 exhibits the XRD patterns of ZVI and Cd(II)loaded ZVI. Characteristic peaks indicating the presence of ZVI appear at 2h = 44.6°, 65°, and 82.3°, as shown in Fig. 10(a). According to a semi-quantitative analysis, no characteristic bands of iron oxides and hydroxides were found, and the mass fraction of ZVI was higher than 99%. In Fig. 10(b), the characteristic peak of ZVI almost disappeared, and some oxide-hydroxides of iron were produced, indicating the ZVI was consumed during the adsorption process. The characteristic peak of Fe3O4 was observed at a diffraction angle 2h = 35.6°, FeOOH (Fe2O3
H2O) peaks appeared at 2h = 14.2°, 27.14°, 36.4°, and 47.0°, and its content was the highest among the newly formed substances. In addition, the heaved baseline strongly indicates the presence of very poorly ordered ferrihydrite. Hence, it was natural to assume that Cd was retained mostly by lepidocrocite and ferrihydrite. The complexation on the surface of ZVI was stable and indicates the effective removal of Cd(II) from contaminated sources under various chemical conditions (Boparai et al., 2013). Because of the strong reducibility of ZVI, Cd(II) in solution readily reacted with ZVI to form lepidocrocite. The newly formed iron minerals from the reaction process of ZVI provided new adsorption sites for Cd(II) adsorption, and Cd(II) may also be combined with the lepidocrocite and ferrihydrite, leading an increase in Cd immobilization. The most probable fixation mechanism is, therefore, sur-

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Table 3 The relationship between ZVI ratio and available Cd in soil, Cd accumulation in brown rice, pH value, rice yield. Test site

ZVI ratio

Available Cd in soil

Cd accumulation in brown rice

pH value

Rice yield

A

0.06% 0.13% 0.25%

30.1% 16.1% 30.1%

12.5% 3.1% 28.1%

+4% +4.2% +5.4%

–

B

0.03% 0.06% 0.13% 0.19%

19% 19% 30.5% 19%

33% 40% 47% 51%

+2.5% +5% +5.9% +8.3%

+5.9% +3.5% 2.8% +9.5%

2

20

Dosage: 20 g/L Duration: 24 h Temperature: 25°C

0 2

4

6

8

0 12

10

FeOOH

FeOOH

ZVI (a) Cd(II) laden ZVI (b)

Step size: 0.2° Time per step: 10 s

Initial solution pH (pHi) Fig. 8. Initial solution pH effect on Cd(II) adsorption.

10

20

30

Fe

40

100 mg/L

(b)

Fe

4

60

X-ray type: Ionizing radiation

Fe

300 mg/L

6

Fe3O4

80

FeOOH

8

FeOOH

100

Intensity (cps)

10

Removal ratio Circle: (%)

Unit adsorption amount Qe Square: (mg/g)

Note: ‘‘” represents decrease; ‘‘+” represents increase.

40

50

60

(a)

70

80

90

2 Theta (°)

(a)

Exchangeable fraction Carbonate-bound fraction Fe/Mn oxides-bound fraction Organic-bound fraction Residual fraction

Fig. 10. XRD patterns of ZVI and Cd(II)-loaded ZVI.

Cd-3 mg/kg-0 Cd-3 mg/kg-0.1% Cd-3 mg/kg-0.2% Cd-3 mg/kg-0.4% 0

20

40

60

80

100

Content (%)

(b)

increase of ZVI, the Cd residual fraction and pH value in soils increased to a certain degree. This residual substance was mainly CdS, which may be due to the reduction reaction of sulfate to sulfide by ZVI in paddy soils: CdS is likely to be stable at higher pH values. Thus, another fixation mechanism may be the promotion of stable CdS formation by ZVI. 2Fe þ 3Cd2 þ þ 4H2 O ! 2FeOOHðsÞ þ 3CdðsÞ þ 6H

Exchangeable fraction Carbonate-bound fraction Fe/Mn oxides-bound fraction Organic-bound fraction Residual fraction

þ

ð7Þ

Cd-6 mg/kg-0

3.5. Field application results Cd-6 mg/kg-0.1% Cd-6 mg/kg-0.2% Cd-6 mg/kg-0.4% 0

20

40

60

80

100

Content (%) Fig. 9. Dosage effect on Cd speciation in different concentration contaminated soil: (a) 3 mg/kg Cd(II) contaminated soil (b) 6 mg/kg Cd(II) contaminated soil.

face complexation by lepidocrocite and ferrihydrite (Sposito, 1984). The major reaction formula is written as (7). In addition, as shown in Figs. 9 and 12, with the

The effects of ZVI on the remediation of cadmium contaminated soil were evaluated through two different experimental farmlands. The results of available Cd in soil and brown rice in different farmlands are displayed in Fig. 11, while Fig. 12 shows the soil pH and rice yield in different farmlands. In Fig. 11(a), when the addition amount of ZVI was 100 (0.06% ZVI per acre) kg/acre (A2), 200 (0.13%) kg/acre (A3), and 400 (0.25%) kg/acre (A4), the remediation effect of ZVI on Cd was significant. Compared with the control subject (A1), the available Cd decreased from 0.64 (about 14.3% of total Cd in soils) mg/kg to 0.45 (10%), 0.54 (12%), and 0.45 (10%) mg/kg, decreasing by approximately 30.1%, 16.1%, and 30.1%, respectively. The content of Cd in brown rice decreased by 12.5%,

Q. Tang et al. / Soils and Foundations 59 (2019) 2099–2109

0.6

0.5 0.4 0.3

0.4

0.2

0.2

0.1

0.0

A1

A3

A2

A4

0.0

0.09 0.06

Cd in brown rice Blank group a 100 kg/acre a 50 kg/acre 300 kg/acre a a 200 kg/acre a a b bc bc c

0.03 0.00

0.12 0.09 0.06 0.03

B1

B2

B3

B4

B5

0.00

Cd in brown rice (mg/kg)

Available Cd in soil (mg/kg)

0.12

0.15

Available Cd in soil

650 600

6.0

550

5.5

500

5.0

450

4.5

A1 A2 A3 A4 B1 B2 B3 B4 B5

400

Fig. 12. Results of soil pH and rice yield in different farmlands: (a) Shaoguan City (b) Shantou City.

(b) 0.15

Soil pH Rice yield

6.5

Soil pH

0.8

Available Cd in soil Cd in brown rice 200 kg/acre Blank group a a ab 100 kg/acre ab 400 kg/acre b c c b

Cd in brown rice (mg/kg)

Available Cd in soil (mg/kg)

1.0

(b)

(a)

Rice yield (kg/acre)

7.0

(a)

2107

Fig. 11. Results of available Cd in soil and Cd in brown rice in different farmlands: (a) Experimental soil of Shaoguan (b) Experimental soil of Shantou (Note: Significant differences are indicated by different letters (P < 0.05)).

3.1%, and 28.1%, from 0.32 mg/kg to 0.28, 0.31, and 0.23 mg/kg, respectively. As shown in Fig. 11(b), as the addition amount of ZVI increased, the available Cd in soil decreased (19%, 19%, 30.5%, 19%, respectively). The Cd in brown rice decreased by different amounts depending on the treatment (B2 (0.03% ZVI per acre), B3 (0.06%), B4 (0.13%), and B5 (0.19%)), at levels of 33%, 40%, 47% and 51%, from 0.1 mg/kg (B1) to 0.067, 0.060, 0.053, and 0.049 mg/kg. In addition, according to Fig. 12(a) and (b), the introduction of ZVI was associated with a clear increase in soil pH from 4.98 to 5.25 in Shaoguan City, and from 6.05 to 6.55 in Shantou City. Such an enhancement in soil pH environment prevented the release of heavy metals, further benefiting heavy metal fixation and alleviating soil acidification, which is important especially in developing countries (Tang et al., 2015 and 2018). Moreover, the rice yield of B2, B3, and B5 group increased by 5.9%, 3.5%, and 9.5% compared with B1, since iron has proven to be a necessary trace element for the growth of brown rice (Khoshgoftarmanesh et al., 2010). The results of this study indicate many factors at play in the field environment. While it was difficult to quantify them one by one, it was possible to determine the appropriate ZVI dosage, concentration level, reaction time, and initial pH. The end result was that the available Cd(II) in soil was well fixed: that is, its migration was largely stopped and its accumulation inside brown rice was obstructed.

Added benefits included significant increases in the soil pH and rice yield. These results clearly indicate that the application of ZVI has great potential in the remediation of Cd(II) contaminated soil. 4. Conclusions ZVI can be used as an effective passivator for the remediation of Cd(II) contaminated soils. According to the laboratory experimental results, the pH environments (pH = 7), ZVI dosage (20 g/L ZVI), reaction time (24 h), and contamination level (50 mg/L Cd(II) concentration) play a significant role in the heavy metal fixation effect, and a Cd(II) removal percentage as high as 88% cabn be achieved. The Langmuir isotherm model fit the test data well and indicated that monolayer adsorption takes place. The maximum adsorption capacity of ZVI was predicted to be approximately 34.6 mg/g. This excellent adsorption performance of ZVI is mainly attributed to both chemical and physical fixation. Based on Tessier sequence extract procedure results, ZVI was found tohave the ability to transform Cd(II) exchangeables and bound fractions into residual fractions. The means of reduction and surface complexation effectively reduce the activity and transfer ability of Cd(II) in contaminated soil. The valid period of the ZVI proved to be much longer in practical applications (The application efficiency of ZVI is up to about one year). The relationship between the ZVI ratio and available Cd in soil, Cd accumulation in brown rice, pH value, and the rice yield was determined. Field studies indicated the content of available Cd(II) in the soil and accumulated in harvested brown rice was inhibited by 30.5% and 51%, respectively, by the addition of 0.13% and 0.19% ZVI. Furthermore, 0.19% ZVI was shown to mitigate soil acidification (pH value increased 8.3%) and also improve rice yields (9.5%). Acknowledgments This study was supported by the National Natural Science Foundation of China (51778386 and 51708377), Natural Science Foundation of Jiangsu Province (BK20170339), Natural Science Fund for Colleges and Universities in Jiangsu Province (17KJB560008) and pro-

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