Remediation of metal contaminated soil by aluminium ...

Applied Clay Science 137 (2017) 115–122

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Research Paper

Remediation of metal contaminated soil by aluminium pillared bentonite: Synthesis, characterisation, equilibrium study and plant growth experiment P. Kumararaja a,b, K.M. Manjaiah a,⁎, S.C. Datta a, Binoy Sarkar c,1 a b c

Division of Soil Science and Agricultural Chemistry, ICAR-Indian Agricultural Research Institute, New Delhi, India Central Institute of Brackishwater Aquaculture, Indian Council of Agricultural Research, Chennai, Tamil Nadu, India Environmental Science and Engineering Strand, Future Industries Institute, University of South Australia, Mawson Lakes campus, Mawson Lakes, SA 5095, Australia

a r t i c l e

i n f o

Article history: Received 27 November 2015 Received in revised form 9 December 2016 Accepted 12 December 2016 Available online xxxx Keywords: Pillared bentonite Heavy metals Adsorption Immobilisation Hazard quotient

a b s t r a c t In order to enhance the efficiency of metal immobilisation, bentonite clay was pillared with polyhydroxy aluminium complexes. The pillared bentonite was systematically characterised by multiple techniques including x-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM). The clay product was assessed for its metal removal efficiency from aqueous systems through batch experiments with variables in pH, amount of adsorbent and initial metal concentration. The adsorption data were fitted with Langmuir and Freundlich isotherm models. The maximum monolayer adsorption capacity of pillared bentonite was 61.4, 32.3 and 50.3 mg g−1 for Cu (II), Zn (II) and Ni (II), respectively. The immobilisation efficiency of pillared bentonite was assessed by greenhouse pot culture experiment with amaranth as the test crop. Amendment of soil with pillared bentonite at 2.5% significantly improved the plant growth as well as reduced the bioavailable metals in the metal spiked soils. The study demonstrated that pillared bentonite could potentially be used for addressing heavy metal pollutions by immobilising the metals in the contaminated soil. © 2016 Published by Elsevier B.V.

1. Introduction Heavy metals contamination of the environment has emerged as a serious problem due to the rapid industrialisation and urbanisation of the society. The wastes or by-products generated through these activities are discharged into the environment, which directly contaminates the water bodies and soils. Inorganic wastes containing heavy metals pose a great threat to human being because of their non-degradability and biomagnifications (Mapanda et al., 2005; Kurniawan et al., 2006; Singh et al., 2010). Additionally, due to the paucity of good quality waters, industrial and/or municipal wastewaters are commonly used for irrigation purposes in peri-urban areas across the developing countries. The excessive accumulation of heavy metals in agricultural soils by repeated applications of polluted waters may cause soil contamination, and seriously impact the food quality and safety by imposing the risk of bioaccumulation in the food chain (Pescod, 1992; Rattan et al., 2009). For example, in the peri-urban Keshopur area near New Delhi in India, the concentrations of heavy metals, specially Zn, Cu and Ni, were respectively 208, 170 and 83% greater in soils irrigated with ⁎ Corresponding author. E-mail address: [email protected] (K.M. Manjaiah). 1 Current address: Department of Geological Sciences, Indiana University, Bloomington, IN 47405, USA.

http://dx.doi.org/10.1016/j.clay.2016.12.017 0169-1317/© 2016 Published by Elsevier B.V.

sewage treatment water over tube well water for 20 years (Datta et al., 2000; Rattan et al., 2005). Since heavy metals are non biodegradable, an ecofriendly, economically viable and comprehensive remediation technique is the need of the hour. Among many remediation approaches, the risk-based land management aims to manage the contaminated soils through reduction of the risks to an acceptable level instead of complete removal of the contaminants from the system which otherwise incurs prohibitively high costs of ex-situ and in-situ clean up (Naidu, 2013). Stabilisation of metals in soil by amendments is a rational method which can effectively reduce the bioavailability of contaminants and thus the risks below the level of concern (Hettiarachchi and Pierzynsk, 2002; Bolan et al., 2014; Shaheen et al., 2015). For stabilising metals in the soil, the quantity and quality of colloid- forming adsorptive complexes is the most important factor which determines the bioavailability of the metals. The high metal adsorption capacity of clay minerals (because of their large specific surface area and reactive functional groups) makes them one of the most common amendments for metal adsorption/stabilisation (Lothenbach et al., 1999; Prost and Yaron, 2001; Saha et al., 2002; Kim et al., 2005; Fu and Wang, 2011). Furthermore, the metal loading capacity of clay minerals can be improved significantly through various modifications (Cooper et al., 2002; War et al., 2006; Sarkar et al., 2012). One such modification is the pillaring process where a guest metal species is inserted without changing the layered

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structure of the clay mineral (Schoonheydt et al., 1999). Few studies previously investigated the adsorption of heavy metals by pillared clay minerals mainly from aqueous solutions (Bhattacharya and Gupta, 2006; Manohar et al., 2006; Perelomov et al., 2015). Nevertheless, additional information are needed on the use of functionalised clays for the immobilisation of heavy metals and their effect on the health risk assessment of vegetable crops grown in metal contaminated soils. Therefore, in the present study, an aluminium-pillared bentonite was evaluated as the immobilising agent for the remediation of a metal spiked soil. Bioconcentration factor and hazard quotient were calculated to assess the efficiency of the pillared bentonite in reducing the metal uptake by a leafy vegetable crop (Amaranthus viridis) which was grown in the contaminated soil. 2. Materials and methods 2.1. Materials A bentonite sample was procured from Minerals Limited, New Delhi, India, and the clay size (b 2 μm) bentonite particles were isolated by employing the sedimentation procedure. All chemicals supplied by Sisco Research Laboratories Pvt. Ltd. (SRL), India, were of analytical grade and used without further purification. Bentonite was pillared with AlCl3.6H2O. Stock solutions of metals (Zn, Cu and Ni) containing 1000 mg L−1 metal ion was separately prepared by dissolving appropriate amount of analytical reagent grade nitrate [Ni(NO3)2.6H2O] and sulphate [Cu(SO4)2.5H2O; Zn(SO4)2.7H2O] salts of the heavy metals in double distilled water. The working standard solutions of the metals were prepared from the stock solution by appropriate dilution in double distilled water. The soil for the greenhouse pot culture experiment was collected from the top 10 cm of the agricultural farm of Indian Agricultural Research Institute (IARI), New Delhi. Selected physico-chemical properties of the experimental soil are listed in Table 1. The soil was spiked with 250 mg kg−1 of Zn and 100 mg kg−1 each of Cu and Ni by addition of dissolved metal solution and mixing it thoroughly with the soil. Among other heavy metals, Zn, Cu and Ni concentrations were alarmingly high in the sewage irrigated soils in the peri-urban area of New Delhi (Datta et al., 2000; Rattan et al., 2005). For this reason, these three heavy metals were selected in the current study. 2.2. Synthesis of pillared bentonite Bentonite (b 2 μm) was pillared with aluminium according to a previously published method (Lothenbach et al., 1999) with some minor Table 1 Selected physico-chemical properties of the experimental soil. Soil properties Mechanical composition Sand % Silt % Clay % Soil texture pH (1:2 Soil:water) EC (1:2) (dS m−1) CEC (cmol (p+)) kg−1 Soil organic carbon (g kg−1) Available soil N (mg kg−1) Available soil P (mg kg−1) Available soil K (mg kg−1) DTPA extractable metals Zn (mg kg−1) Cu (mg kg−1) Ni (mg kg−1) Total metal content Zn (mg kg−1) Cu (mg kg−1) Ni (mg kg−1)

modifications. Sodium saturated bentonite was first prepared by adding 10 mL of sodium chloride solution (0.25 M) drop-wise to a 10% (w/v) bentonite suspension in double distilled water. The mixture was continuously stirred on a magnetic stirrer for 16 h. Following separation through centrifugation (5000 rpm for 10 min), the sodium saturated bentonite particles were washed with double distilled water until the supernatant became chloride free. The product was then dried in an oven for 2 h and grinded to powder. The pillaring solution was prepared by drop-wise addition of 0.2 M sodium hydroxide to 0.2 M aluminium chloride solution under continuous stirring to reach the OH/Al ratio of 2. The pillaring solution was prepared and kept at room temperature (25 °C) for 5 days for the purpose of aging. The pillaring solution was added at a rate of 3–4 mL min−1 to 10% (w/v) sodium saturated bentonite suspension under continuous stirring on a magnetic stirrer and the reaction mixture was aged at 25 °C for 16 h after the pillaring process. The final product was washed with distilled water to remove the excess precipitate, dried at 80 °C, calcined at 300 °C for 2 h in a hot-air oven, grinded, sieved through 100 mesh, and kept in a desiccator until further use. 2.3. Characterisation of pillared bentonite In order to determine the changes in crystallinity, X-ray diffraction (XRD) patterns of the pillared bentonite was recorded with a Philips model PW1710 diffractometer, fitted with a Cu tube (λ = 1.5418 Å, 40 kV and 20 mA, scanning from 3° to 15° 2θ at a step angle of 0.1°, 5 s/step). The XRD was performed at room temperature at 21–26% relative humidity without adding any saturating cation to the samples. Fourier transform infrared (FTIR) analyses were performed at room temperature in the spectral range of 4000–600 cm−1 using a FTIR spectrometer (model SPECTRUM-1000, Perkin Elmer). The pillared bentonite along with dehydrated KBr (0.02% (w/w)) was ground and mixed thoroughly, and the mixture was made into pellet by a hydraulic press. The spectra were collected with 64 scans of accumulation at a resolution of 4 cm−1. The surface morphology of the pillared clay was observed under a Scanning Electron Microscope (Zeiss Evoma10) at up to 20 keV primary electron beam energy. Finely powdered sample was mounted on a double-sided tape placed on an aluminium stub with industrial glue and coated by 20 nm thick palladium layers in vaccum prior to analysis. Images were acquired using the Secondary Electron (SE) signals (EverhartThornley detector). Specific surface area (SSA) and cation exchange capacity (CEC) of the pillared bentonite were determined by EGME (ethylene glycol monoethyl ether) method (Carter et al., 1965) and Ca\\Mg exchange method (Jackson, 1973), respectively. 2.4. Adsorption experiments

Values 76.3 11.6 12.1 Sandy loam 8.32 0.45 11.4 5.42 156 9.1 218 1.12 1.11 nd 85 28 2

All the adsorption equilibrium experiments were conducted by batch method in triplicate. Our previous reports already demonstrated that the Al-bentonite adsorbed significantly greater quantity of heavy metals than the raw bentonite (Kumararaja et al., 2014; Kumararaja and Manjaiah, 2015). The current study therefore focussed on some specific reaction parameters for the adsorption of multiple heavy metals by the pillared clay and its ability to immobilise heavy metals in contaminated soils. To examine the effect of adsorbent dose on metal adsorption, 50 mL of 25 mg L−1 metal solution with different concentration of pillared bentonite (0.01, 0.025, 0.05, 0.1, and 0.2 g) in polypropylene bottles was shaken for 24 h on an end to end shaker. Preliminary experiments showed that a 24 h agitation was sufficient to reach the adsorption equilibrium. The pH of the metal solution was adjusted with 0.01 M HCl or NaOH to 6, 7 and 8 for Cu, Zn and Ni, respectively. Following agitation the solution was filtered through Whatman No. 42 filter paper. The initial and equilibrium concentrations of metal in the aqueous solutions were analysed by Atomic Absorption


Q ¼ ½ðC0 −Ct Þ=m V

ð1Þ

where, Q is the amount of metal ions adsorbed onto unit amount of the adsorbent (mg g−1), C0 and Ct are the initial and final concentrations of metal in solution (mg L−1), V is the volume of solution (L) and m is the mass of the adsorbent (g), respectively. 2.5. Pot culture experiment A pot culture experiment was conducted at the Division of Soil Science and Agricultural Chemistry, ICAR-Indian Agricultural Research Institute, New Delhi, India, by growing amaranth (Amaranthus viridis) as the test crop. Amaranth was chosen in this study because it is a staple vegetable crop grown throughout the year all over India including in marginal lands. Air dried, well ground, 2 mm sieved soil (4 kg) was used in each pot. In triplicate, the pillared bentonite was added at 3 application rates (0.5%, 1.5% and 2.5%) along with a control without the amendment. The recommended fertiliser dose of 180:90:45 mg pot−1 NPK for amaranth was added to each pot. Half the dose of N and the full dose of P and K were applied as the basal dose and remaining half of N was applied 30 days after sowing of the crop. The seeds were washed with distilled water and directly sown in the individual pot. The plants were thinned to 5 seedlings per pot after one week of sowing. Two cuttings of the crop were taken at 30 days interval, and the roots were also collected for analysis at the time of the second harvest. Soil and plant samples were analysed for heavy metal concentrations following aqua regia digestion by employing standard protocols (Jackson, 1973). 2.6. Risk assessment The non-carcinogenic risk of consumption of amaranth grown on the metal spiked soils amended with aluminium-pillared bentonite (Al-bentonite) was assessed by calculating the hazard quotient (HQ) which is the ratio of the average daily dose (ADD; mg kg−1 day−1) of metals to their reference dose (RfD; mg kg−1 day−1) (Eq. (2)). The reference dose (RfD) is defined as the maximum tolerable daily intake of the specific metal that does not result in any deleterious health effects. HQ ¼ ADD=RfD

ð2Þ

The values of RfD for Zn and Ni were 0.3 and 0.02 mg kg−1 day−1, respectively (IRIS, 2014). For Cu, a provisional maximum tolerance daily intake (PMTDI) of 0.5 mg kg−1 day−1 was used in place of RfD (WHO, 1982; Alam et al., 2003). Daily intake of green vegetable was assumed to be 0.2 kg day−1 which is the recommended amount from a nutritional point of view. A factor of 0.082 was used for amaranth to represent on dry weight basis (Ray et al., 2013). Average body weight for an adult was assumed to be 70 kg. Thus, the HQ for an adult was calculated

as (Eq. (3)): HQ ¼ ðMPlant W FÞ=ðRfD 70Þ

ð3Þ

where, MPlant is the metal content (mg kg−1) of plant, W is the daily intake of green vegetable (kg kg−1 body weight) and F is the factor of conversion of fresh to dry weight. The means and standard deviations of data were calculated using Microsoft Office Excel 2010. Analysis of variance and comparison between means were carried out by Duncan's test using SPSS 16 statistical packages. 3. Results and discussion 3.1. Characterisation of pillared bentonite The XRD patterns of the randomly oriented samples (Fig. 1) showed a clear shift of the reflection to the left from 2θ = 6.8° to 2θ = 5.45° and the corresponding increase of d-value from 12.2 Å to 16.2 Å due to the pillaring process (Karamanis et al., 1997; Tomul and Balci, 2007; Basogul and Balci, 2010; El Miz et al., 2014). The FTIR spectra of Al-bentonite differed from that of the raw bentonite (Fig. 2). The bands between 3550 cm− 1 and 4000 cm−1 corresponded to the structural hydroxyl groups in addition to the water molecules in the interlayer space of the raw bentonite. The band at 3620 cm−1 was due to O\\H stretching, and a broad band centred on 3442 cm−1 was due to the interlayer and intralayer H-bonded O\\H stretching. The band at 1637 cm− 1 represented the H\\O\\H bending vibration of water, while the band at 1032 cm− 1 might be attributed to the siloxane (\\Si\\O\\Si\\) group stretching. The band at 913 cm− 1 was due to Al-OH, and the band at 791 cm−1 might correspond to the Mg O\\H vibration. A substantial reduction in the intensity of H-bonded-OH stretching at 3442 cm−1 was observed in Al-bentonite as a result of dehydration of the interlayer water species (Chae et al., 2001; Jiang and Zeng, 2003; Perelomov et al., 2015). The broadening of band at 3442 cm− 1 might also be attributed to the existence of Keggin-OH and Keggin-H2O stretches, which confirmed the presence of Keggin ions in the Al-pillared bentonite (Acemana et al., 1999). The change in the intensity of Si-O-Si band at 1032 cm−1 could be ascribed to the formation of protonic sites during calcination of the intercalated cation (Kloprogge et al., 2002; Xue et al., 2007; Okoye and Obi, 2011). The bands at 540–470 cm−1 in the pillared bentonite represented the Si\\O bending and Al\\O stretching, and a slight increase in the intensity of Al\\O stretching occurred by the pillaring process. The scanning electron micrograph of Al-bentonite revealed a more open porous and fluffy surface than that of the bentonite (Fig. 3) due to a reduction in its amorphous nature and a probable change of surface charge (Ding et al., 2001; Altunlu and Yapar, 2007). The difference in the surface morphology was also an indication of the successful pillaring process (Salerno and Mendioroz, 2002; War et al., 2006; Zeng et al., 2013). 16.2 Å

Intensity (counts)

Spectroscopy (AAS) using a spectrometer (ZEEnit 700, Analytik, Jena, Germany) equipped with an air–acetylene flame atomiser and a hollow cathode lamp specific for each metal ion. The effect of pH on metal adsorption was assessed by shaking 0.1 g of the adsorbent with 50 mL of 25 mg L−1 metal solutions at different pH values (1, 2, 3, 4, 5, 6, 7 and 8). The mixtures were shaken for 24 h in polypropylene bottles. The filtrate was then analysed for metal concentrations as stated earlier. To obtain adsorption isotherm, a known volume (50 mL) of metal solution of varying initial concentrations (0 to 100 mg L−1) in polypropylene bottles was shaken with a desired dose (0.1 g) of adsorbent for 24 h on an end to end shaker at room temperature. The initial and equilibrium concentrations of metal in the clear solutions were then analysed by AAS. The amount of metal adsorbed (Q in mg g−1) was determined as follows (Eq. (1)):

117

2000 1800 1600 1400 1200 1000 800 600 400 200 0

0

2

4

12.2Å

6 8 10 Degrees 2

Bent

Al-Bent

12

14

Fig. 1. X-ray diffractograms of bentonite and aluminium pillared bentonite.

16

118


Fig. 2. FTIR spectra of bentonite (a) and Al-bentonite (b).

The pillaring process also resulted in an increase of the specific surface area of the bentonite from 399 to 678 m2 g−1 due to the increase in interlayer spacing. Contrarily, the modification resulted in a decrease of the CEC from 83.3 to 45.4 cmol (p+) kg−1. The CEC value was reduced by the pillaring process because most of the adsorption sites were occupied by the polymeric hydroxy aluminium ions (Karamanis et al., 1997). 3.2. Adsorption study 3.2.1. Effect of adsorbent dose The amount of adsorbent is an important parameter which determines the total adsorption capacity. The percentage removal of metals increased from 67 to 99%, 45 to 99% and 49 to 99% in case of Cu (II), Zn (II) and Ni (II), respectively (Fig. 4a). The increase in the percentage of metal removal consistent with an increase in the adsorbent dosage could be attributed to the rise in the effective surface area of the adsorbent. A higher adsorbent dosage also reflected a greater number of available adsorption sites which facilitated a greater metal ion adsorption. However, a reduction in the adsorption capacity of the pillared clay was observed for all the metals with an increase in the adsorbent dose. With the increase in clay amount at a fixed concentration of metal ions, the ratio of number of adsorption sites to the number of metal ions would increase. This would result in a greater number of unabsorbed reaction sites. Additionally, at a higher adsorbent dose, the

particle aggregation might decrease the total effective surface area of the adsorbent, and thus led to a smaller efficiency of the adsorbent. 3.2.2. Effect of pH The pH of the suspension strongly affected the adsorption capacity of the pillared bentonite. The adsorption of metals increased with an increasing pH of the suspension (Fig. 4b). The results indicated that the maximum adsorption of Cu (II), Zn (II) and Ni (II) occurred at pH 6, 7 and 8, respectively. A smaller adsorption of metals by the pillared clay at an acidic pH was attributed to a higher concentration of H+ and hydronium (H3O+) ions, and their competition with the metal ions for the adsorption sites. As indicated by the following reactions (Eqs. (4) and (5)), the development of positive charge at the edges of Al-bentonite could also contribute to the reduction of metal adsorption at a lower pH value (Avena et al., 1990): SO− þ Hþ →SOH

ð4Þ

SOH þ Hþ →SOHþ

ð5Þ

where, S denotes any surface binding site. Due to the reduced inhibitory effect of H+ at a higher pH value, the metal adsorption increased (Karamanis et al., 1997). Cu (II) and Cd (II)

Fig. 3. SEM images of bentonite (a) and Al-bentonite (b).

(a)

4

85 75 65

3

55

2

45

1

35

0

25 0.01

0.025

Zn

Cu

0.05 0.1 Amount of clay (g) Ni

Zn %

0.2

Percentage removal

5

(b)

18

100 80

14

60

10

40 6

20 0

Amount adsorbed

120

95 Percentage Removal

Amount adsorbed (mg g-1)

6

119

(mg g-1)


2 2

3

4

5

6

7

8

9

pH Cu %

Ni %

Zn

Cu

Ni

Zn %

Cu %

Ni %

Fig. 4. Effect of adsorbent dose (a) and pH (b) on the percentage removal and amount (mg g−1) of Cu, Zn and Ni adsorbed onto Al-bentonite.

could get precipitated at pH N 6, and resulted in a lower adsorption (Gustafsson, 2003). 3.2.3. Adsorption isotherms The Langmuir and Freundlich models are often used to describe metal adsorption isotherms. The Langmuir equation is expressed as (Eq. (6)): Q e ¼ Q o bCe =ð1 þ bCe Þ

ð6Þ

The linear form of the Langmuir equation is (Eq. (7)): Ce =qe ¼ 1=Q o b þ Ce =Q o þ 1

ð7Þ

where, Qo is the maximum adsorption at monolayer (mg g−1), Ce is the equilibrium concentration of metal (mg L−1), qe is the amount of metal adsorbed per unit weight of the adsorbent, b is the Langmuir constant related to affinity of binding site (L mg−1) and also denotes a measure of energy of adsorption. A linearised plot of Ce/qe against Ce gives the Qo and KL values. The Freundlich isotherm is an empirical equation and mathematically expressed as (Eq. (8)): qe ¼ K f C1=n

ð8Þ

where, qe is the amount of metal ion adsorbed (mg g−1) onto the adsorbent. Kf and n are the Freundlich parameters, indicating the adsorption capacity and intensity, respectively. The linear plots of log qe versus log Ce (log qe = log Kf + 1/n log Ce) for the different initial metal ion concentrations gives the Kf and n values. As seen from the correlation coefficient (R2) values, the adsorption data fitted slightly better to the Freundlich model than the Langmuir model (Table 2). The Langmuir constant Qo of Al-bentonite, representing the maximum monolayer adsorption capacity, were 61.35, 32.26 and 50.25 mg g−1 for Cu (II), Zn (II) and Ni (II), respectively, and the sequence of metal adsorption was Cu (II) N Ni (II) N Zn (II). A similar sequence of heavy metal adsorption onto aluminium oxides was reported by various researchers (Saha et al., 2002; Vidal et al., 2009). Copper adsorbed as the Table 2 Langmuir and Freundlich adsorption isotherm parameters for Cu, Zn and Ni on Al-pillared bentonite.

Langmuir parameters Qo (mg g−1) KL (L mg−1) R2 Freundlich parameters Kf 1/n R2

Cu

Zn

Ni

61.35 0.40 0.93

32.26 0.24 0.96

50.25 1.4 0.97

15.80 0.62 0.98

7.48 0.44 0.99

14.2 0.50 0.97

square planar species and irreversibly bonded while nickel and zinc preferred to remain in the solution phase where their hydration requirement was better satisfied. The higher adsorption of Cu (II) by Al-bentonite could be due to its paragmagneticity, higher electronegativity, coordination geometry and Jahn-Teller distortion (Sparks, 2003). The sequence of Qo was consistent with the first hydrolysis constants (Kh) of the metals. Metals with a larger Kh (lower pKh values) form surface hydroxo species within a given pH range and are chemisorbed to the surface in a greater extent. The greater adsorption of Cu than that of other metals might also be due to the precipitate phase formation by Cu on the clay surfaces. The sequence of Qo was positively correlated with the hardness of metals. Higher the hardness, more was the affinity to the surface functional groups (e.g.,\\OH groups), which resulted in a higher adsorption of Cu (Gomes et al., 2001; Violante et al., 2010). Similar to the Qo values, Cu (II) had the highest Kf and n values over Zn (II) and Ni (II) (Table 2). The highest n value for Cu (II) meant it had the highest affinity towards the functional groups of the adsorbent, and a stronger bond formation occurred in the case of a higher Kf value. According to the Freundlich adsorption theory, the n values between 1 and 10 would indicate a beneficial adsorption process, and in this study the n values were more than unity for all the three metals (Table 2), which demonstrated the beneficial adsorption. It was evident that the chemical modification of bentonite significantly enhanced the heavy metal adsorption capacities. The metal cations could be incorporated into the aluminium hydroxide layer (Lothenbach et al., 1997). Surface induced precipitation of Zn in the hydroxide layer of montmorillonite and its incorporation into the aluminium lattice could be the mechanism of zinc adsorption by the poly hydroxyl aluminium treated bentonite (Furrer et al., 1994; Badora et al., 1998; Lothenbach et al., 1999). Similarly copper incorporated in the aluminium oxide precipitates preferentially by occupying the positions at or near the oxide surfaces. In Al-bentonite, the cation was adsorbed specifically on the surface sites of aluminium hydroxide polymers (Harsh and Donner, 1984; Kukkadapu and Kevan, 1988; Cooper et al., 2002; War et al., 2006). Nickel was included slightly above and below the vacant octahedral sites of the postulated interstitial gibbsite monolayer and the metal was included in a monolayer gibbsite or gibbsite “islands” formed in the interstitial space of the clay mineral, which potentially led to a permanent sequestration of the metal from the aqueous solution (Furrer et al., 2001; Nachtegaal et al., 2005). The binding of metals to oxides was based on the softness of cations, i.e., the tendency to form covalent bonds with the ligands. Copper adsorption on the pillared clay might involve both a cation exchange process in the surface as well as the interlamellar clay sites and a complexation reaction with the pillared oxides (Harsh and Donner, 1984; Karamanis et al., 1997; Bhattacharyya and Gupta, 2008) 3.3. Pot culture experiment 3.3.1. Effect of Al-bentonite amendment on biomass production The biomass dry weight of shoots usually reflects the tolerance capability of plants to an adverse environment. The added amount of pillared


bentonite had a significant effect on the biomass production by amaranth during both the harvest occassions (Fig. 5). For example, the application of Al-bentonite at 2.5% loading (w/w) improved the growth of amaranth by 77 and 80% at the first and second harvest, respectively, compared to the unamended soil. Such an improvement in plant growth was achieved by alleviating the heavy metal stress to the plants through Al-bentonite amendment (Miao et al., 2012; Sun et al., 2012). 3.3.2. Effect of pillared bentonite on metal uptake by amaranth Heavy metals accumulation in plants depends on their bioavailability, i.e., the chemical forms which can be taken up by the plants. The concentration of heavy metals in amaranth shoots was significantly reduced by the application of Al-bentonite (Fig. 6). During the first harvest, copper content of amaranth was reduced from 48.5 in the control to 40.8, 33.8 and 33.5 mg kg−1 by amending the soil with 0.5, 1.5 and 2.5% Al-bentonite, respectively. Similarly, during the second harvest, the copper content was reduced to 26.8 from the control of 40.2 mg kg−1 as a result of Al-bentonite application at 2.5% loading (w/w). Amendments of soil with Al-bentonite at 0.5, 1.5 and 2.5% loading reduced the plant zinc content by 3, 14 and 24% during the first harvest, and 10, 19 and 25% during the second harvest of amaranth, respectively. Similarly, treatment of soil with the pillared bentonite at 2.5% reduced the plant nickel content by 53 and 46% over the control during the first and second harvest, respectively. The bioavailability of heavy metals in soil depends on their concentration in the solution phase and on the release of the ions from the solid phase. The application of clay product could reduce the bioavailability of heavy metals due to the larger surface area as well as the stronger adsorptive capacity of the pillared bentonite which decreased the concentration of cations in the soil solution and thereby reduced their uptake by plants (Furrer et al., 2001; Fawzy, 2008; Malandrino et al., 2011; Koptsik and Zakharenko, 2014). 3.3.3. Effect of pillared bentonite on labile fraction of heavy metals The addition of Al-bentonite at all levels caused a reduction in the diethylenetriaminepentaacetic acid (DTPA) and ethylenediaminetetraacetic acid (EDTA) extractable heavy metals compared to the control (Fig. 7). These two chemically extractable fractions represent the plant bioavailable heavy metals in the soils (Lindsay and Norvell, 1978; Quevauviller, 1998). The DTPA extraction is the best method for getting the plant bioavailable fraction of metals in contaminated soils which are neutral to alkaline in reaction. Numerous previous studies employed the chelant (DTPA and EDTA) based extraction procedures for determining the plant bioavailable fraction of heavy metals (Ghosh et al., 2012; Ray et al., 2013; Deshmukh et al., 2015). As the amount of amendment increased, the bioavailable heavy metal decreased. The DTPA extractable Zn reduced from 103 in control to 92.7, 88.8 and 84.4 mg kg−1 in 0.5, 1.5 and 2.5% Al-bentonite amended soils, respectively. Similarly, the

Harvest II

300 Zn I Zn II

200

Cu I Cu II

100

Ni I Ni II

0

0

0.5 1.5 Amount of clay (%)

2.5

Fig. 6. Effect of Al-bentonite on metal (Zn, Cu and Ni) content (mg kg−1 dry weight) in amaranth. Metal I and II represent metal concentrations during the first and second occasion of harvesting. Bars represent standard deviations at p b 0.05.

DTPA extractable Cu and Ni were reduced to 26.2, 16.5 in 2.5% Al-bentonite amended soil from 36.6 and 25.3 mg kg−1 in control, respectively. Amending the soil with Al-bentonite at 2.5% resulted in 16, 36 and 16% reduction in EDTA extractable Zn, Cu and Ni, respectively. Generally it is advocated that high surface area materials are better adsorbents due to the availability of more surface active sites (Usman et al., 2006; Fawzy, 2008; Varrault and Bermond, 2011; Koptsik and Zakharenko, 2014). 3.3.4. Effect of clay amendment on bioconcentration factor (BCF) and hazard quotient (HQ) The BCF can assess the efficiency of an amendment in immobilising heavy metals in the soil. The BCF of Zn varied from 1.70 and 0.89 in control to 0.94 and 0.59 in the soil amended with 2.5% pillared bentonite at the first and second harvest, respectively (Table 3). Similarly, the BCF of Cu at the first and second harvest varied between 0.37 and 0.30 and 0.19 to 0.08, respectively. The BCF decreased in the order of Zn N Cu N Ni during both the harvests. The addition of pillared bentonite significantly reduced the translocation of metals into the plants as indicated by the BCF values. To assess the efficiency of Al-bentonite on metal immobilisation, the health risk assessment of vegetable consumption from the clay amended and control soils was conducted by calculating the hazard quotient using the US EPA protocol (IRIS, 2014). The HQ of Zn reduced to 0.29 and 0.30 by the application of Al-bentonite at 2.5% from 0.37 and 0.45 in the unamended soil during the first and second harvest, respectively. A similar trend was observed for Cu and Ni as a result of the pillared bentonite application (Table 3). The reduction in hazard quotient was apparently due to the reduced metal uptake by the plants as a result of their immobilisation in the soil. 4. Conclusions Heavy metals are retained in the soil through chemical, physical and biological adsorption processes. The addition of aluminium pillared bentonite increased the chemical adsorption of heavy metals and reduced their mobility under environmental conditions as a consequence

12 Zn

a

10

A

8 B

6 4

D

d

C

b

(mg Kg-1 soil)

Plant biomass yield (g pot-1)

Harvest I

Metal content (mg Kg-1)

120

c

2 0

0

0.5 1.5 Amount of clay (%)

2.5

Fig. 5. Effect of Al-bentonite on biomass yield (g pot−1) of amaranth. Bars represent standard deviations at p b 0.05. Different letters above the bars represent a significantly different mean at p b 0.05.

160 140 120 100 80 60 40 20 0

Cu

Ni EDTA

DTPA

0

0.5

1.5

2.5 0 0.5 Amount of Clay (%)

1.5

2.5

Fig. 7. Effect of Al-bentonite on DTPA and EDTA extractable metal (Zn, Cu and Ni) contents (mg kg−1 dry weight) of soil. Bars represent standard deviations at p b 0.05.


121

Table 3 Effect of Al-bentonite application on the bioconcentration factor and hazard quotient of Zn, Cu and Ni in amaranth at first (I) and second (II) harvest. Different letters represent significantly different mean values at p b 0.05. Bioconcentration factor (BCF)

Hazard quotient (HQ)

Amount of Al-bentonite (%)

Zn I

Zn II

Cu I

Cu II

Ni I

Ni II

Zn I

Zn II

Cu I

Cu II

Ni I

Ni II

0 0.5 1.5 2.5

1.70d 1.19c 1.15b 0.94a

0.89d 0.83c 0.67b 0.59a

0.37d 0.36c 0.33b 0.30a

0.19d 0.10c 0.09b 0.08a

0.60d 0.29c 0.28b 0.22a

0.31d 0.25c 0.24b 0.22a

0.37c 0.35b 0.29a 0.29a

0.45d 0.42c 0.34b 0.30a

0.22c 0.22c 0.20b 0.17a

0.03c 0.02b 0.02a 0.02a

0.69c 0.35b 0.33b 0.28a

0.16d 0.14c 0.13b 0.11a

of complex formation. The results of this study showed that the pillared bentonite at 2.5% application rate demonstrated the best effectiveness towards the immobilisation of heavy metals (Cu, Zn and Ni). The method described in this paper enabled the application of modified bentonite to soil for reducing the mobility and availability of heavy metals to plants and thereby reducing the health risk of consumption of vegetables grown on the metal contaminated soils.

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