Infiltration behavior of heavy metals in runoff through soil amended with biochar as bulking agent

Environmental Pollution 254 (2019) 113114

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Infiltration behavior of heavy metals in runoff through soil amended with biochar as bulking agent* Ling Zhao a, Hongyan Nan a, Yue Kan b, Xiaoyun Xu a, Hao Qiu a, Xinde Cao a, c, * a

School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China Civil and Environmental Engineering Department, Stanford University, Stanford CA 94305, USA c Institute of Pollution Control and Ecological Security of Shanghai, Shanghai 200040, China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 6 July 2019 Received in revised form 20 August 2019 Accepted 24 August 2019 Available online 28 August 2019

Biochar as a porous carbon material could be used for improving soil physical and chemical properties, while insufficient attention has been paid to potential risks induced by infiltration of heavy metals in the runoff water flowing through biochar-amended soil. Four different soil-biochar matrices with same volumes were constructed including soil alone (M1), biochar alone (M2), soil-biochar layering (M3) and soil-biochar mixing (M4). Leaching experiments were conducted with Pb, Cu, and Zn contaminated runoff water. Results showed that biochar amendment greatly improved the water permeation, and the infiltration rates in M2, M3, and M4 were 2.85e23.0 mm min
1, being much higher than those in M1 (1.33e4.05 mm min
1), though the rates decreased as the leaching volumes increased. However, biochar induced more Pb, Cu, and Zn infiltrated through soil-biochar matrix. After 350-L leaching, M1 retained about 95% Pb, 90% Cu, and 36% Zn, while M2 only retained 4.80% Pb, 17.4% Cu, and 4.01% Zn; about 30% Pb, 80% Cu, and 15% Zn were retained in M3 and M4. Notably, Zn was trapped first and then re-leached into the filtrate, which resulted in a much higher effluent Zn than the influent Zn at the later stage. However, the unit weight of biochar showed a higher capacity for retaining heavy metals compared to per unit of soil. Under the dynamic water flow, all benefits and disadvantages induced by biochar were weakened with its physical disintegration. Biochar as soil amendment can enhance plant growth via ameliorating soil structure, while it would pose risks to environment because of large penetration of heavy metals. If biochar was compacted to form a denser physical structure, perhaps more heavy metals could be retained. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Barley grass biochar Soil-biochar structure Water infiltration rate Heavy metals leaching Plant growth

1. Introduction Biochar is the solid by-product from pyrolysis of biomass residues from agriculture or forestry production, which is primarily produced for sequestrating carbon (Cao et al., 2009; Xiao et al., 2018). Recently, biochar gains large attention in that it brings many additional advantages besides carbon sequestration when amended into soil. For instance, biochar has a higher cation exchange capacity (CEC) than soil, which benefits more nutrients retention (Xu, 2012; Yang et al., 2019); due to their high porosity, high alkalinity, various minerals, and rich organic functional groups, biochar can stabilize heavy metals in contaminated soil

* This paper has been recommended for acceptance by Dr. Yong Sik Ok. * Corresponding author. School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China. E-mail address: [email protected] (X. Cao).

https://doi.org/10.1016/j.envpol.2019.113114 0269-7491/© 2019 Elsevier Ltd. All rights reserved.

(Ippolito et al., 2012). To our knowledge, most researches focused on the chemical influence of biochar on soil, while physical alteration of soil bulk induced by biochar and the potential influence on water infiltration was less investigated, especially the infiltration behavior of contaminants in rainfall runoff through a soil matrix (Zhao et al., 2016; Yang et al., 2019). A few studies showed that biochar can increase water infiltration through a compacted subsoil layer as a soil-bulking agent (Karolina and Rainer, 2018; Novak et al., 2016). More precisely, researchers reported that impact of biochar amendment on soil hydraulic properties (Ksat) depends on soil texture and biochar particle size. The Ksat decreased when biochar was added to coarse and fine sands (Jtf et al., 2018). Biochar with larger particles sizes (60%; >1 mm) decreased Ksat to a larger degree than the smaller particle size biochar (60%; <1 mm) in the two sandy textured soils (Lim et al., 2016). Biochar amendment could increase water retention in the coarse textured sandy soil by forming an internal

2

L. Zhao et al. / Environmental Pollution 254 (2019) 113114

pore structure which is able to resist the hydraulic deformation induced by drying, thus biochar addition benefited the soil rigidity. (Ajayi & Horn, 2017; Villagra-Mendoza & Horn, 2018). Jeffery also reported that biochar enhances soil productivity by lowering the density of soil, and therewith influencing the plant roots growth and microbiologic population in soil (Jeffery et al., 2011; Major et al., 2010). The chemical and physical alteration of biochar to soil would impact the infiltration behavior of contaminations in the runoff derived from rainwater or irrigation, which have been increasingly serious globally due to the transport and accumulation of environmental contaminants from agriculture, industrial and municipal waste emissions (Zobrist et al., 2000). Heavy metals, especially Pb, Cu, and Zn, are of particularly concerns in such runoff and irrigation water. Many studies have been conducted to elucidate the interacting mechanisms of these elements with biochar (Beesley et al., 2011), and it could be concluded that Pb is readily to precipitate with minerals such as phosphorus (P) in biochar (Cao et al., 2011) and Cu tends to form stable complex with the functional groups on biochar-surface (Jiang & Xu, 2013). Adsorption of Zn by biochar is related to negative charges depending on pH (Jiang et al., 2016), while Zn is too active being difficult to be stabilized. Despite of the above knowledge, there are still lack of studies on the changes in soil physical structure and hydraulic conditions caused by biochar, and the effects of combined chemical reactions on the infiltration behavior of contaminants. Particularly, the effects of different modes of biochar addition in soil as bulking agent are worth exploring. This study aims at investigating the infiltration of Pb, Cu, and Zn with runoff passing through the soil-biochar matrix. Different biochar amendment modes were constructed: 1) soil alone, 2) biochar alone, 3) soil-biochar layering, and 4) soil and biochar were mixed completely. The heavy metals migrating flux and retention as well as mechanisms in this system were elucidated. 2. Materials and methods 2.1. Biomass collection and biochar production A common biomass, barley grass, collected from Yancheng city, southeastern China, was used as the feedstock for biochar production. The biomass was chosen because of its large production and difficulties in disposal. After being air-dried and broken into 2.0-cm size, barley grass was put into a furnace (SX2-12-10, China) of slow pyrolysis system, and converted into biochar at the highest temperature of 500  C for 2 h with a N2 flow rate of 15 mL min
1 (Nan et al., 2019). 500  C was a moderate temperature for biochar production balanced the heat consumption and the carbonization level. The obtained biochar was crushed and sieved yielding a 0.5mm size sample. Properties of the biochar were analyzed. Element contents including C, H, O, S and N were measured with elemental analyzer (Vario Macro Cube, German); Total metal (Zn, Cu and Pb) concentrations were determined using Atomic Absorption Spectroscopy (AAS) (contrAA, German) after microwave assisted acid digestion and being filtered through a 0.45-mm filtration membrane. Biochar pH was measured with pH Meter (EUTECH pH510, USA) in 1: 20 (w/ w) biochar/Milli-Q water, and soil pH in 1: 2.5 (w/w) soil/Milli-Q water for 16 h in a shaker at room temperature (Chen et al., 2018). Biochar pore structure characteristics were determined using a BET-N2 superficial area (SSA) analyzer (BK122T-B, JWGB, China) according to the standard of the International Union of Pure and Applied Chemistry (IUPAC). The biochar samples were pretreated in vacuum for 2 h under temperature of 50  C lower than the biochar production temperature. Specific surface area is based on multi-

point BET adsorption. The analysis of mesopore and macropore was based on BJH method (Barrett-Joyner-Halenda). Micropore calculation was based on HK method (Horv ath-Kawazoe) (Zhao et al., 2014).

2.2. Soil-biochar matrix construction A common sandy-clay soil was collected from Shanghai, China. After being air-dried, soil was crushed and sieved to obtain 2-mm soil particles. Properties of the soil are presented in Table 1. Four lab-scale simulated soil-biochar matrices were constructed, in which vertically oriented columns made of cylindrical transparent PVC pipe with an inner diameter of 13 cm was used (Fig. 1). Each column was designed to have multiple layers, and the total depth of the column was about 20 cm. Each layer of the column can be separated into two half-circle parts for extracting biochar and soil samples from the containers. Four columns were run simultaneously (Fig. 1), which represented the soil alone (Matrix 1), biochar alone (Matrix 2), soilbiochar layering (Matrix 3), and soil-biochar mixing (Matrix 4). Each column consisted of three layers: in the top layer, a common herbaceous plant named “Zoysia matrella” was planted; in the sub layer, biochar was amended with three different structures, which consisted of 100% biochar for Matrix 2, 6% (w/w) biochar addition to soil for Matrix 3 and 4; cobblestones and quartz sands were mixed and laid at the bottom layer in order to hold the above matrix and form a homogeneous leachate. The constitutions of the four devices were presented in Table 2. The four matrices constructed in this study had the same volume (13 cm diameter ⅹ10 cm height). The bulk density of the matrices was calculated based on the weight of soil and biochar mixture and the matrix volume. The M1 and M2 were constituted by 1.70 kg soil alone and 193.5 g biochar alone, with the bulk density being 1.281 g cm
3 and 0.146 g cm
3, respectively; the M3 consisted of the upper soil (6.5 cm, 1.10 kg) and the lower biochar (3.5 cm, 72.5 kg), and their bulk densities were 1.275 g cm
3 and 0.156 g cm
3; the M4 had a compromise density of 0.882 g cm
3 after the mixing of 1.10 kg soil and 70.5 kg biochar (Table 2).

2.3. Leaching experiment of simulated water runoff Simulated contaminated water runoff was prepared according to the reported values (Zobrist et al., 2000; Gardner & Carey, 2004). Contaminants including Zn, Cu, and Pb were prepared at the concentrations of 30.0 mg L
1, 20.0 mg L
1, and 20.0 mg L
1 using Zn(NO3)2, Cu(NO3)2, and Pb(NO3)2, respectively. The pH of the simulated water runoff was 4.0e5.0, which was close to the typical acid rain pH (Davis et al., 2001; Gan et al., 2008; Daniel et al., 2012). The volume of water runoff was identified based on the maximum rainfall volume in Shanghai (100 mm h
1): in one hour, the rainfall volume on the column with inner diameter of 13 cm was 1.33 L (eq. (1)) (Anna et al., 2015). The infiltration rate of water was measured periodically during the leaching experiment. At different volume leaching water, the time for 1-L water flowing through the matrix was recorded, and the infiltration rate was calculated as eq. (2) shown, which represented the average value at that period.

Leaching volume ¼ 100 mm$h
1  1 h 

p 4

 2  13  103 mm

¼ 1:33 L (1)

L. Zhao et al. / Environmental Pollution 254 (2019) 113114

3

Table 1 Properties of biochar, soil and plant. Property

pH

C

H

O

S

N

P

Ca

Biochara Soil

b

Plant a b c d e f

c

Mg

Fe

Al

Mn

g$kg
1

% 9.97 60.9 2.42 13.3 SSAd: 35.4 m2 g
1; PVe: 0.049 cm3 g
1; 7.44 2.04 0.375 3.67 Sand: 68.5%; Silt: 15.2%; Clay: 16.3% e 44.1 5.56 38.2

Zn

Cu

Pb

Cd

mg$kg
1

0.547 1.07 APSf: 8.20 nm 0.005 0.080

0.276

9.54

2.97

940

450

80

116

24.7

0.125

0.062

0.045

21.8

12.6

18.2

24.1

25.3

140

42.1

13.7

3.20

0.025

0.021

3.56

1.32

543

364

64

82.7

27.4

0.010

0.005

0.010

Biochar was produced using barley grass under the pyrolysis temperature of 500  C. Soil was a sand soil collected from Shenyang, China. Plant was a common herbaceous plant named “Zoysia matrella”. SSA is specific surface area determined by BET-N2 surface area (m2$g
1). PV is pore volume (cm3$g
1). APS is average pore size (nm).

concentrations of Zn, Cu, and Pb were measured using atomic absorption spectrometry after 0.45-mm filtration. Phosphorus was determined using the colorimetric method. Solid samples were collected when the experiments finished and dried for characterization.

Top Middle

3. Results

Bottom

3.1. Biochar as soil bulking agent promoted water infiltration and plant growth

Drainage Matrix 1: Soil

Matrix 2: Biochar

Matrix 3: Soilbiochar layering

Matrix 4: Soilbiochar mixing

Infiltration is a dynamic process that water on the surface of the ground enters into soil. It is one of the most important factors in the soil phase of the hydrological cycle, since infiltration determines the amount of runoff as well as the supply of water to the soil profile (Lado & Ben-Hur, 2009). Biochar amendment is likely to change soil physical structure and influence the water infiltration rate, which enables biochar to act as soil bulking agent. As shown in Table 2, biochar amendment as 6% (w/w) significantly decreased the density of soil bulk. The bulk density of soil alone was 1.281 g cm
3 and that of the biochar alone was 0.146 g cm
3. The matrix of soil-biochar layering has an upper bulk density of 1.275 g cm
3 and a lower bulk density of 0.156 g cm
3. When soil and biochar was mixed, a compromised bulk density of 0.882 g cm
3 was obtained. The water infiltration rates in the soil-biochar matrices can be seen in Fig. 2. With more water leaching through the soil-biochar matrix, the infiltration rate of water decreased gradually due to the collapse of pores and shrink of materials. In this study, after 340-L runoff water leached, the water infiltration rate decreased by approximately 45e65% for these matrices. As expected, the infiltration rate of water in biochar alone matrix was much higher (12.8e23.0 mm min
1) than the other three ones (1.33e9.05 mm min
1). The matrix of soil alone had a very low water infiltration rate from 1.33 to 3.97 mm min
1. The values of the layering and mixing structures laid in 2.60e8.93 mm min
1 and 4.02e9.17 mm min
1, respectively. This indicated that biochar can

Vegetable layer (5 cm) Soil-biochar matrix (10 cm) Drainage layer (5 cm)

Fig. 1. Schematic diagram and lab-scale simulated soil-biochar matrix.

Infiltration rate ¼ ¼

Rainfall volume through the column Cross senctional area of the column  t0

1L

(2)

p  ð13mmÞ2  t ðsÞ 0 4

Therefore, in each single leaching experiment, 1.5 L contaminated water was leached within one hour. About 20 mL of the leachate was collected using a syringe from the three layers. The

Table 2 The constitution of experimental devices with four biochar-soil structures. Matrix

Structure

Biochar addition rate

Constitution

Hight Biochar

Soil

Biochar

Bulk density

0 cm 10 cm 3.5 cm

1.281 g cm
3 0.146 g cm
3 1.275 g cm
3 0.156 g cm
3 0.882 g cm
3

M1 M2 M3

Soil alone Biochar alone Soil-Biochar layering

0% 100% 6%

1.70 kg 0 kg 1.10 kg

0g 193.5 g 72.5 g

10 cm 0 cm 6.5 cm

M4

Soil-Biochar Mixing

6%

1.10 kg

70.5 g

10 cm

Note: The matrixes were structured with same volume, while different weights.

Density

Soil

L. Zhao et al. / Environmental Pollution 254 (2019) 113114

-1

Infiltration rate of water (mm min )

4

24 21 18

Matrix 1: Soil Matrix 2: Biochar Matrix 3: Soil-Biochar layering Matrix 4: Soil-Biochar mixing

15 12 9 6 3 0

0

50

100

150

200

250

300

350

Volume of leaching water (L) Fig. 2. Infiltration rate of water in the soil-biochar matrixes.

alter the density of soil matrix and improve the water infiltration conditions. As water flowing through the soil matrix continually, influence of physical conditions will be weakened. Other factors such as hydrophobicity/hydrophilicity and preferential flow in soil also affected water infiltration rate (D'Angelo et al., 2014; Cipolla et al., 2016). The plant growth was enhanced significantly by biochar amendment, which can be seen from Fig. 1. The plants in M2 and M4 were much healthier than those in M1 and M3 marked by the greener color and the more biomass. The average heights of the plants in M2 and M4 were 17.2 cm and 16.9 cm, respectively, being much higher than those in M1 and M3, which were only 12.5 cm and 8.51 cm, respectively. The root elongation also corresponded to the height, which were 5.32 cm and 4.85 cm for M2 and M4, 3.60 cm and 4.03 cm for M1 and M3, respectively. Compared with M1 and M3, the roots of the plants could contact with biochar directly in M2 and M4. We inferred that biochar amendment to soil created a loose structure, which facilitated the development of the plant root. Moreover, nutrients in biochar such as phosphorus, minerals and soluble organic carbon could promote plant growth from a large extent. Despite of the obvious benefits brought by biochar amendment into soil, their alteration to soil structure might pose potential risk with regards to heavy metals infiltration, which is the focus of this study. 3.2. Biochar induced more Pb, Cu, and Zn infiltrated through soilbiochar matrix In this study, the transport of Pb, Cu, and Zn in the soil-biochar matrix was observed via dynamic evolution of metals concentration in leachate from each layer, which was shown as a% of the initial concentration of the water (Fig. 3). The metals concentration from the top of the column was almost equal to the concentration of inflow, indicating that the root of plants had negligible influence on the metal retention. At the beginning of the leaching, the matrix retained all metals, thus almost no metals were detected in the leachate from the middle and bottom of the columns. With continuous water runoff passed through the soil-biochar layers underneath the plant, metals concentration in the leachate increased gradually. It presented a similar, while different leaching behavior for the three metals and different matrices. As one of the most stable heavy metals, Pb is difficult to

transform in soil structure. It can be seen from Fig. 3 that even after 300 L water leaching, no Pb was detected from the bottom effluent in the system of soil alone, while in the biochar structure, Pb reached penetration after only 80-L water leaching. The soilbiochar mixing mode presented a later but faster Pb penetration at the middle layer, compared to the soil-biochar layering mode (Fig. 3). It indicated that the upper layer of soil could retard the transformation of Pb penetrated from the lower biochar layer effectively. Perhaps the interface of soil and biochar could also slow down the metal infiltration. Infiltration of Cu showed distinctive behaviors among the four different matrices. In the system with biochar alone, only 42-L and 90-L water infiltration resulted to the penetration of Cu, and the sudden rising of its concentration in the leachate from the middle and bottom layers of the matrix, respectively. Presence of soil showed a much higher retaining-ability to Cu, especially in the soil alone, in which Cu could be detected in the middle and bottom leachate only after 110-L and 300-L water infiltration, respectively. Amendment of biochar into the soil with layering and mixing modes resulted to a Cu penetration after about 80-L and 180-L water leaching. Zn was the most mobile metal among the three metals and reached the saturation point the earliest (<100 L leaching water). In the biochar matrix, Zn concentration at the middle and bottom suddenly rose at the water volume of 24 L and 48 L, respectively. At about 90 L, the effluent Zn was equal to the influent Zn, indicating that the matrix had been saturated, and it cannot retain Zn any more. It was notable that in the soil systems, Zn was trapped and then re-leached into the filtrate, especially in the soil control, which resulted to at the later stage the effluent Zn was much higher than the influent Zn (Fig. 3). Results warned us that biochar as a bulking agent of soil has tendency to induce a potential risk of heavy metals when it loosens the physical structure of soil, despite biochar has been proven to bind heavy metals via various mechanisms (He et al., 2019). The interaction of biochar and heavy metals in this dynamic water flowing system will be discussed in the following sections. 3.3. Retention and distribution of Pb, Cu and Zn in soil-biochar bulk After successive leaching, the cumulated retained metals in the whole matrix were measured. The total leached amount of the water was 350 L and the total Pb, Cu and Zn amount were 2936 mg, 6574 mg and 5640 mg, respectively for inputting into each column. The soil matrix retained about 95% of Pb, 90% of Cu and 36% of Zn, while the biochar matrix only retained 4.80% of Pb, 17.4% of Cu and 4.01% of Zn; retention of Pb, Cu and Zn in the matrix of soil-biochar layering and soil-biochar mixing presented a similar tendency and the amount was between soil and biochar, i.e., total of 32.1% Pb, 82.3% Cu, and 16.8% Zn were retained in M3, and 27.9% Pb, 75.9% Cu and 13.3% Zn were immobilized in the M4. Biochar amendment led to a much less retaining of heavy metals in the soil-biochar matrices. Although presence of biochar weakened the retaining capacity of soil matrix to the heavy metals, the unit weight of biochar showed a strong ability to adsorb contaminants, as indicated by the higher concentration of Pb, Cu and Zn in the system with biochar added, which was calculated and shown in Fig. 4. After 350-L water leaching, the distribution of metals concentration along the matrix depth was measured based on the dry solid of the soil-biochar mixture (Fig. 4). The concentrations of Pb, Cu and Zn (mg$kg
1) were much higher in the biochar alone matrix (M2) than in the soil alone matrix (M1), and in the soil-biochar structures (M3 and M4) by about 25e40% and 35e50%, respectively. Since Cu is a metal most readily binds to organic substances, plant roots presented the

L. Zhao et al. / Environmental Pollution 254 (2019) 113114

120

)

Middle  Bottom 

20 0 0

50

100

150

200

250

60 40 20 0

300

0

50

Volume of leaching water (L)

Matrix 1: Soil

120

80 60 40 20 0 50

100

150

200

250

80 60 40 20 0 200

250

120

40 20 0 50

100

150

200

250

50

300

120 80 60 40 20 0 150

200

250

250

40 20 0

300

0

50

120

60 40 20 0 50

100

150

200

250

300

Volume of leaching water (L)

100

150

200

250

300

Volume of leaching water (L)

80

Matrix 4: Soil-Bichar mixing

100 80 60 40 20 0

300

0

50

100

150

200

250

300

Volume of leaching water (L)

160

160

100

100

200

60

Matrix 3: Soil-Biochar layering

0

Matrix 2: Biochar

50

150

80

Volume of leaching water (L)

140

0

100

100

300

Relative concentration of Zn (%)

Relative concentration of Zn (%)

Relative concentration of Zn (%)

120

150

0

Volume of leaching water (L)

100

100

0

Matrix 4: Soil-Bichar mixing

Volume of leaching water (L)

60

160

Volume of leaching water (L)

20

Matrix 2: Biochar

0

Matrix 1: Soil

50

40

300

80

300

140

0

250

100

Volume of leaching water (L)

160

200

60

Relative concentration of Cu (%)

100

0

150

80

Volume of leaching water (L)

Relative concentration of Cu (%)

Relative concentration of Cu (%)

120

100

120 100

Relative concentration of Cu (%)

40

80

Matrix 3: Soil-Biochar layering

100

Matrix 3: Soil-Biochar layering

Relative concentration of Zn (%)

Top (

60

Relative concentration of Pb (%)

80

120

Matrix 2: Biochar

100

Relative concentration of Pb (%)

Matrix 1: Soil

Relative concentration of Pb (%)

Relative concentration of Pb (%)

120 100

5

140 120 100 80 60 40 20 0 0

50

100

150

200

250

300

140 120 100 80 60 40 Matrix 4: Soil-Bichar mixing

20 0 0

Volume of leaching water (L)

50

100

150

200

250

300

Volume of leaching water (L)

Fig. 3. Concentrations of Pb, Cu and Zn in leachate from the top, middle and bottom of the soil-biochar matrix.

greatest ability of binding with Cu. With continuous water leaching, metals transformed from the top to the bottom gradually, while not all the retained metals distributed averagely along the depth (Fig. 4). In the biochar matrix, metals concentration in the bottom was higher than those in the middle and top, indicating that metals were easily transformed down in biochar structure, while in the soil structure, the tendency was opposite. It could be summarized that unit weight of biochar exhibited a higher capacity to immobilize heavy metals than unit weight of soil, while due to its loose physical structure, when it was amended into soil, heavy metals have a greater tendency to migrate through the soil with water flow. Therefore, chemical interaction and physical structure together determine the infiltration behavior of heavy metals, and the latter dominated their behaviors when considered in a dynamic water flowing system.

3.4. Conditions influencing the heavy metals behavior: phosphorus and pH Biochar and soil contained considerable amounts of inherent P, which were 2600 mg kg
1 in the biochar and 3900 mg kg
1 in the soil (Song et al., 2019). Therefore, at the beginning of the leaching, a relative high concentration of P (2.5e3.5 mg L
1) was detected in the leachate from each matrix, and the concentration in the bottom of the column was obviously higher than those in the middle layer (Fig. 5). With increasing water leached through the soil, P-concentration decreased sharply in the M2 (Fig. 5 b) and M4 (Fig. 5 d), while decreased slowly in the M1 (Fig. 5 a) and M3 (Fig. 5 c). This exactly reflected the ability of the dense soil structure to retain phosphorus (M1), even only by the upper layer of soil (M3), compared with the matrix loosen by biochar (M2 and M4).

Presence of P contributed a lot in retaining heavy metals, especially for Pb, due to the potential formation of insoluble minerals such as Pb3(PO4)2, Pb5(O4)3Cl, etc (Inyang et al., 2012; Cui et al., 2016). Besides, the improvement of plant growth was also partly attributed to the P in biochar. Many studies have proven that applying Pcontaining biochar can increase soil available P, and significantly decreased soil labile heavy metals (Mohamed et al., 2017; Ahmad et al., 2018). Biochar can be used as a soil amendment for another reason that it is alkaline (pH ¼ 9.0e10.0) because of minerals and negative charges (Yuan et al., 2011). This enables biochar to be used as a neutralizer to alleviate the potential harm of acid rain. In this study, pH of the inflow water was adjusted to a range of 4.0e5.0 to simulate acid rain (Whittinghill et al., 2016). After the water passed through the matrix with biochar, pH of the leachate increased to 9.0e10.0, while it declined with continuous leaching. At the middle stage of the leaching with about 90-L water infiltrated, almost no difference of pH could be detected in the all four systems being 5.14e5.34, 5.5e5.95 and 5.89e6.41 at the top, middle and the bottom, respectively. After 300-L leaching, the final pH of the systems further decreased to a range of 4.50e6.0 (Table 3). Results indicated that biochar has a better pH buffering ability than soil, though neither of them can resist the prolonged acid rain erosion regardless of their structural patterns, thus the liming effect for promoting the immobilization of the heavy metals by biochar was limited (Jiang et al., 2016). However, Rinklebe et al. (2016) reported that high pH (>8.0) might enhance hydrolysis of Cu2þ to form Cu(OH)þ, Cu(OH)2, or even precipitation n of Cu as hydroxides (Cu2þ þ 2OH ¼ Cu(OH)2). With pH increasing, more Cu2þ was adsorbed by variable-charge soil components, which had a higher sorption affinity for CueOH than Cu2þ. Therefore, a positive effect

6

L. Zhao et al. / Environmental Pollution 254 (2019) 113114

Pb

Pb

Cu

Zn

Zn

Pb

Pb

Cu

Zn

3000

Cu

Zn

Pb Cu Zn

Pb

Matrix 1: Soil

Cu

Zn 6000

9000

12000 15000

0

Matrix 2: Biochar

-1

Heavy metals retained in the matrix (mg·kg )

3000

9000

Pb

Cu

Zn

6000

Cu

Zn

Pb

Pb

Cu

Zn

Cu

Zn

Pb

Cu

Zn

Pb

Matrix 3 Soil-Biochar layering

Pb

Cu

Zn 3000

6000

Cu

Zn

Pb

Matrix 4 Soil-Biochar mixing

Cu

Zn 9000

Matrix 4: SoilBiochar mixing

12000 15000

Heavy metals retained in the matrix (mg·kg-1)

0

3000

6000

9000

Top Middle Bottom

3.0 2.5 2.0 1.5 1.0 0.5 0.0 0

Phosphorus concentration (mg·L-1)

(a)

20 40 60 80 100 Volume of leaching water (L)

Matrix 3: Soil-Biochar layering

3.5

(c)

2.5 2.0 1.5 1.0 0.5 0.0 20 40 60 80 100 Volume of leaching water (L)

4.0

120

Matrix 2: Biochar

3.5

(b)

3.0 2.5 2.0 1.5 1.0 0.5 0.0 0

120

3.0

0

Phosphorus concentration (mg·L-1)

Matrix 1: Soil

3.5

Phosphorus concentration (mg·L-1)

Phosphorus concentration (mg·L-1)

4.0

4.0

20 40 60 80 100 Volume of leaching water (L)

120

Matrix 4: Soil-Biochar mixing

3.5

(d)

3.0 2.5 2.0 1.5 1.0 0.5 0.0 0

12000 15000

Heavy metals retained in the matrix (mg·kg-1)

Fig. 4. Accumulated retention of Pb, Cu and Zn in the soil-biochar matrix along the depth.

4.0

12000 15000

Heavy metals retained in the matrix (mg·kg-1)

Pb

0

Cu

Pb

Cu

Zn

Matrix 3: SoilBiochar layering

Matrix 2: Biochar

Zn Pb

Matrix 1: Soil 0

Cu

20 40 60 80 100 Volume of leaching water (L)

Fig. 5. Concentration of P in the leachate from the top, middle and bottom of the matrix.

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L. Zhao et al. / Environmental Pollution 254 (2019) 113114

7

Table 3 pH of the leachate during the leaching. Matrix

M1 M2 M3 M4

Structure

Soil Biochar Soil-Biochar layering Soil-Biochar mixing

Middle stage e pH (90 L)

Initial pH (6 L)

Final pH (300 L)

Inflow

top

middle

bottom

Inflow

top

middle

bottom

Inflow

top

middle

bottom

4.49 4.49 4.49 4.49

6.43 8.47 8.14 7.41

7.41 9.2 8.3 8.62

7.31 9.67 9.18 8.49

4.48 4.48 4.48 4.48

5.19 5.25 5.34 5.14

5.88 5.56 5.95 5.5

6.20 5.89 6.41 6.24

4.79 4.79 4.79 4.79

5.49 6.02 4.31 4.46

4.83 6.04 4.51 5.17

5.79 5.99 5.68 5.13

of biochar can be anticipated in terms of shifting the soil pH with initial low acid neutralization capacity value to higher pH value in which the extractable fraction of metals could be decreased (Venegas et al., 2016).

4. Discussions

thus the compactness of soil-biochar bulk should be an important factor to retain Pb (Marc et al., 2013). Considering that retention of pollutants depends on the biochar composition, feedstock type, etc., while this study only investigated the effect of the most common plant-based biochar, more different biochars such as cow manure biochar, wood biochar, sewage sludge biochar, etc. should be studied in the future.

4.1. Dynamic interaction of heavy metals and soil-biochar matrix Many previous studies have explored the affinity of heavy metals to biochar theoretically, and insights into the various sorption mechanisms have been approached. Pb mainly is immobilized via formation of precipitates with anions such as phosphate, which have been revealed by our previous study (Liang et al., 2014; Cao & Harris, 2010); Cu is easy to form stable inner-ring complex with organic matter in soil, or with organic functional groups on biocharsurface, which can act as complexation sites via p electrons (C]C) (Jiang & Xu, 2013; Li et al., 2013); Compared with soil itself, biochar was characterized with abundant organic groups such as eOH, eCOOH, eCHO, eNH2, etc., which are mainly hydrophilic groups (Gusiatin & Kulikowska, 2016). Biochar immobilizes Zn mostly via electrostatic adsorption due to the negative charges on biochar, while this combination is not stable (Jiang & Xu, 2013). However, all these affinities of biochar to heavy metals perform effectively only when biochar and heavy metals coexist in the system over time. Therefore, biochar is once assumed as a remediation agent for soil heavy metals. In this study, heavy metals passed through the fixed matrix with water-flow, rather than mixed together with biochar and soil. Physical attachment was the first step, and then metals interacted with the matrix via chemical processes. Therefore, the hydraulic retention time of water-flow determined by the physical structure posed important influence on the transport and retention of heavy metals. Different properties of the three metals determined their distinctive behaviors in the matrix with the activity being: Zn > Cu > Pb. The detachment of Zn from the matrix after an attachment, while unbinding with the biochars occurred easily, which resulted to the re-release of Zn in a relatively large extent. Copper was likely to be retained on biochar surfaces through complexation, and amendment of biochar could convert a portion of Cu from available pool to more stable forms; thus, resulting in decreased activities of free Cu2þ and increased activity of organic Cu complexes in leachate (Bakshi et al., 2014). The higher retention rate of Cu might be explained by its binding both to the outer surfaces of soil/biochar and, when those retention sites were effectively saturated, further adsorbed to the network of pores and fissures that form soil/biochar's complex inner micro-structure. In this study, the specific surface area of this biochar was relatively low (35.4 m2 g
1) with a moderate pore volume of 0.049 cm3 g
1. As for the metal Pb, the slight solubility enabled it to stay in the solid matrix rather than escape with water flow (Forj an et al., 2016). Although the organic functional groups on biochar-surface could act as a “stumbling block” to hinder the movement of Pb, it could not balance the Pb loss through the loose structure induced by biochar amendment,

4.2. Time-dependent change of soil-biochar structure physically and chemically After multiple leaching, the infiltration rate in all these systems declined with time, which was accordant with previous studies (Githinji, 2013). Novak et al. (2016) also reported that biochar enhancing the water infiltration was transient, and it collectively declined between 30% and 50% after each additional water leaching event. This can be explained from the change of physical structure, i.e., reduction of particle viscidity, porosity and permeability (Bedbabis et al., 2014). Declines in water infiltration was related to biochar's pores essentially filling with water or their physical disintegration (Spokas et al., 2014). If the micro-pores and fissures of biochar become water-filled, its impact on water infiltration potentials would essentially be nil, leaving water infiltration as a function of gravity (Novak et al., 2016). Given the size of the primary biochar particles and the jagged-edge morphology of these biochar particles, it is conceivable that soil micro-pores could eventually become physically clogged, thus reducing water infiltration (Bohara et al., 2019). The decline extent of infiltration rate for biochar matrix was larger than the other three matrices, even though, the permeability of biochar matrix was still significantly higher than the other ones (Yuan et al., 2014). Novak et al. (2016) also pointed out that this enhancement of water hydrodynamic condition dissipates with continuous water leaching as a function of both feedstock and soil texture. In the matrix of soil-biochar mixing, soil compaction is possibly enhanced by biochar's structurally degrading as results of water flushing, potential heavy traffic during application, and soil tillage after application. These dislodged fragments were hypothesized to clog soil pores. Micrometer to nano-meter size of biochar fragments with jagged edges could be suspended in percolating water and eventually move downward through the soil profile. It is conceivable that soil micropores might eventually be physically clogged, thus further benefiting heavy metals retention. Therefore, biochar's structural stability in soil is a topic worthy of scientific exploration. Despite of the above physical disintegration of biochar, its amendment in the subsoil brought significant benefits. It enriched the elements in soil, including basic elements (C, H, O), minerals (K, Na, Ca, Mg, etc.), and nutrients (P and N). These depended on the type of source biomass for biochar production (Teutscherova et al., 2018). Addition of biochar with plenty of alkaline minerals can alleviate the acidic soil, makes the sandy soil have better water retention, and make the clay soil have better permeability. However, all these benefits seemed transient, and would be weakened gradually with water runoff infiltrating events. In this study, a

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L. Zhao et al. / Environmental Pollution 254 (2019) 113114

notable phenomenon was that biochar amendment promoted plant growth obviously. Compared with the soil alone matrix and soilbiochar layering one, biochar alone matrix and soil-biochar mixing matrix enabled plant grow more vigorously (Fig. 1). This suggested that although biochar amendment into soil might cause risk of heavy metal infiltration, it altered the soil structure physically and facilitated growth of plant roots. Considering the higher capacity of the unit weight of biochar for retaining heavy metals compared to per unit of soil, it was assumed that the more biochar was compacted into the matrix and the denser a physical structure was formed, the larger amounts of heavy metals would be retained. 5. Conclusions This study investigated the infiltration behavior of heavy metals including Pb, Cu, and Zn in the runoff water flowing through the soil matrix with biochar amendment as bulking agent. Addition of biochars improved the water infiltration rate significantly, especially for the matrix only constituted by biochar alone. Contacting of plant roots and biochar could promote plant growth remarkably, which has been observed both in the matrix of biochar alone and in the matrix of biochar mixed with soil. This notable phenomenon was because that biochar altered the soil structure physically facilitating plant roots developing; biochar also provided nutrients to plant such as phosphorus, minerals, or soluble organic carbon. Despite these benefits, biochar amendment resulted in a sharp increase of heavy metals permeation, which is a potential risk to the environment. It was demonstrated that in a dynamic water leaching system, the potential advantages of biochar such as immobilizing Pb, Cu and Zn could not be realized. With multiple leaching, their capacity of releasing phosphorus and buffering pH weakened quickly. However, the unit weight of biochar showed a higher capacity for retaining heavy metals compared to per unit of soil, it was assumed that the more biochar was compacted into the matrix and the denser a physical structure was formed, the larger amounts of heavy metals would be retained. Therefore, further studies should be performed focusing on using the highly compacted biochar structure as soil amendment. Acknowledgments This work was supported in part by National Natural Science Foundation of China (Nos. 21537002, 21577087, 41877110), and National Key R&D Program of China (Nos. 2018YFC1800600, 2018YFC1802701). References Ahmad, M., Usman, A.R.A., Al-Faraj, A.S., Ahmad, M., Sallam, A., Al-Wabel, M.I., 2018. Phosphorus-loaded biochar changes soil heavy metals availability and uptake potential of maize (Zea mays L.) plants. Chemosphere 194, 327e339. Ajayi, A.E., Horn, R., 2017. Biochar-induced changes in soil resilience: effects of soil texture and biochar dosage. Pedosphere 27 (2), 236e247. Anna, P.B., Eva, S.I., Avelina, R.G.,L., Barbassa, A.P., Josa, A., Rieradevall, J., Gabarrell, X., 2015. Environmental and economic assessment of a pilot stormwater infiltration system for flood prevention in Brazil. Ecol. Eng. 84, 194e201. Bakshi, S., He, Z.L., Harris, W.G., 2014. Biochar amendment affects leaching potential of copper and nutrient release behavior in contaminated sandy soils. J. Environ. Qual. 43 (6), 1894. Bedbabis, S., Rouina, B.B., Boukhris, M., Ferrara, G., 2014. Effect of irrigation with treated wastewater on soil chemical properties and infiltration rate. J. Environ. Manag. 133, 45e50. Beesley, L., Moreno-Jimenez, E., Gomez-Eyles, J.L., Harris, E., Robinson, B., Sizmur, T., 2011. A review of biochars' potential role in the remediation, revegetation and restoration of contaminated soils. Environ. Pollut. 159, 3269e3282. Bohara, H., Dodla, S., Wang, J.J., Darapuneni, M., Acharya, B.S., Magdi, S., Pavuluri, K., 2019. Influence of poultry litter and biochar on soil water dynamics and nutrient leaching from a very fine sandy loam soil. Soil Tillage Res. 189, 44e51. Cao, X., Harris, W., 2010. Properties of dairy-manure-derived biochar pertinent to its potential use in remediation. Bioresour. Technol. 101 (14), 5222e5228.

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