Use of bentonite to control the release of copper from contaminated soils.
A decrease in release and availability of heavy metals in soil has
been of worldwide interest in recent years. Bentonite is a type of
expandable montmorillonite clay, and has strong sorption for heavy
metals. In this work, the control of amended bentonite on the release of
copper ([Cu.sup.2+]) from spiked soils was investigated using a batch
equilibrium technique. Sorption of Cu by bentonite was pH-dependent, and
could be well described using the Langmiur model. Maximum sorption
capacity of the bentonite used in this study was 5.4 mg/g, which was
much greater than soils reported in the literature. The extent of
[Cu.sup.2+] release from spiked soils was correlated with slurry
concentrations, pH, and soil ageing process. In all cases, the amendment
of bentonite was observed to effectively decrease the release of
[Cu.sup.2+] from soils. The apparent aqueous concentrations of
[Cu.sup.2+] released from soils devoid of bentonite treatment were
113-1160% higher than those from the soils amended with bentonite.
Moreover, the magnitude of [Cu.sup.2+] release decreased with increasing
amount of bentonite added to soils. The bentonite added was more
effective in retaining [Cu.sup.2+] in sorbents for aged contaminated
soils. Such enhanced retention resulting from the presence of bentonite
was observed within a wide pH range from 2.5 to 7.0. Bentonite, as one
of the most abundant minerals in soils, is regarded to improve the soil
overall quality. The results obtained from this work provide useful
information on utilisation of bentonite to control the release of heavy
metals from contaminated soils.
Additional keywords: heavy metals, soil, bentonite, clay, sorption, desorption, availability, remediation.
Copper (Chemical properties)
Soil pollution (Research)
|Publication:||Name: Australian Journal of Soil Research Publisher: CSIRO Publishing Audience: Academic Format: Magazine/Journal Subject: Agricultural industry; Earth sciences Copyright: COPYRIGHT 2007 CSIRO Publishing ISSN: 0004-9573|
|Issue:||Date: Dec, 2007 Source Volume: 45 Source Issue: 8|
|Topic:||Event Code: 690 Goods & services distribution; 310 Science & research Advertising Code: 59 Channels of Distribution Computer Subject: Company distribution practices|
|Product:||Product Code: 1452000 Bentonite; 4950000 Pollution Control; 9106400 Pollution Control & Abatement NAICS Code: 212325 Clay and Ceramic and Refractory Minerals Mining; 562 Waste Management and Remediation Services; 92411 Administration of Air and Water Resource and Solid Waste Management Programs SIC Code: 3331 Primary copper; 3341 Secondary nonferrous metals; 1459 Clay and related minerals, not elsewhere classified; 4950 Sanitary Services|
|Geographic:||Geographic Scope: China Geographic Code: 9CHIN China|
Soil contamination by heavy metals has been a long-term and worldwide environmental problem generated by anthropogenic activities of the past several decades. Natural and xenobiotic heavy metals present in soils could find their way into human and animal populations through direct exposure or food chain/web, posing a serious risk to human health (Garcia-Sanchez et al. 1999; Gao et al. 2003). Because of the limited land resource available for agriculture, particularly in those countries with great population density, remediation of contaminated lands needs immediate action to make land available for agricultural production.
Many physiochemical and biological processes, such as soil washing, thermal desorption, electrochemical remediation, supercritical fluid extraction, bioremediation, and phytoremediation, have been used to clean up heavy metal contaminated soils (Jang et al. 1998; Khan et al. 2000; Yoon et al. 2006). However, these remediation techniques are generally time-consuming and expensive. To date there are still scarce cost-effective and reliable remediation technologies available on a large scale (Gao et al. 2003). On the other hand, since these heavy metals are nondegradable and thereby persistent in soils, understanding of their release into the water environment is paramount to determining their transport in soil plant systems and risks to food security and human health (Gao et al. 2006). As such, decreasing the release and hence mobility of heavy metals in soils has been of worldwide interest for many years (Phillips 1998; Garcia-Sanchez et al. 1999; Gao et al. 2003).
Because of large specific surface area, low cost, and ubiquitous presence in soils, clay is usually selected to decrease the release of heavy metals from soils into the environment (Mellah and Chegrouche 1997; Liu et al. 2006). Among these clay minerals, bentonite is of most interest because it is present on most continents and has high surface area and cation exchange capacity (Khan et al. 1995; Kaya and Oren 2005; Lacin et al. 2005). Bentonite is a type of expandable montmorillonite clay; the unit layer structures consist of 1 layer of [Al.sup.3+] octahedral structure sandwiched by 2 layers of [Si.sup.4+] tetrahedral sheet. The isomorphous substitution of [Mg.sup.2+]/[Zn.sup.2+] for [Al.sup.3+] in the octahedral layer and [Al.sup.3+] for [Si.sup.4+] in the tetrahedral layer results in net negative charges on clay surfaces (Zhu et al. 2000; Naseem and Tahir 2001; Lacin et al. 2005). The charge imbalance is offset by exchangeable cations such as [Na.sup.+] or [Ca.sup.2+]. It has been well documented that bentonite is a very efficient adsorbent for heavy metals such as lead, copper, cadmium, and zinc (Li et al. 2002; Al-Qunaibit et al. 2005; Donat et al. 2005; Kaya and Oren 2005; Andini et al. 2006). The high sorption ability of bentonite provides a possibility to reduce the release, mobility, and therefore availability of heavy metal contaminants in soils through amendment. In addition, bentonite has been shown to improve the overall soil quality (Phillips 1998; Sheta et al. 2003). However, limited information is available on the release of heavy metals from soils in the presence of bentonite.
Copper and many other heavy metals (e.g. [Zn.sup.2+], [Co.sup.2+]) are plant micronutrients needed in a very small amount to maintain the normal growth. However, they become toxic at higher concentrations (Abollino et al. 2003). Copper is widespread with high concentrations in soils in China and elsewhere (Gao et al. 2003, 2006), posing threats to ecosystems and human health. In the present study, we seek to evaluate the effects of bentonite on the release of copper from contaminated soils. Results obtained from this work are hoped to provide useful information on reducing metal toxicity and availability in contaminated soils. The results may also benefit the evaluation of these marginally contaminated sites for agricultural production.
Materials and methods
The bentonite used in this study was collected from Inner Mongolia, China. The air-dried bentonite sample was grounded and passed through a 60-mesh sieve. It has been reported that the untreated clays including bentonite have higher affinities to metals than the treated ones (Al-Qunaibit et al. 2005). Therefore, the bentonite used in this study was in the natural state without any further treatments. The chemical compositions and several selected properties of bentonite are listed in Table 1. The BET-[N.sub.2] surface area and cation exchange capacity (CEC) of bentonite were 60.9 [m.sup.2]/g, and 108.4 mmol/kg, respectively. The BET-[N.sub.2] surface area was analysed using a Coulter-100CX surface area analyser. The CEC was determined using an ion-exchange method (Zhu et al. 2000).
A yellow-brown soil, representative from the east of China, was used in this study. Soil was taken from A horizon (0-0.20 m), air-dried, and sieved through 60 mesh. The soil organic carbon content ([f.sub.oc]) was 1.28%.
To prepare [Cu.sup.2+]-contaminated soil, the soil sample was spiked with [Cu.sup.2+] solution (Gao et al. 2003, 2006; Saison et al. 2004). Copper chloride (Cu[Cl.sub.2].2[H.sub.2]O) was dissolved in water, then 10 mL of the prepared solution was mixed with 100 g of soil. The soil-sorbed concentration was 50 times greater than the final concentration in the soil used in later studies. Then certain amounts of the dried spiked sample were thoroughly mixed with the soil without [Cu.sup.2+] spiking, and passed through a 60-mesh sieve. The final [Cu.sup.2+] concentration was 800 mg/kg. A set of the contaminated soils was then packed, sealed, and kept in dark for 30 and 50 days after a certain amount of water was spiked to maintain soil moisture at 20%.
Sorption of [Cu.sup.2+] by bentonite
The batch sorption method was used to determine [Cu.sup.2+] sorption and release in soil (Ling et al. 2005; Gao et al. 2006). A volume of 20 mL of [Cu.sup.2+] solution with 0.005 mol/L of KN[O.sub.3] background was added to 25-mL glass centrifuge tubes sealed with screw caps containing 0.2 g of bentonite. The tubes were shaken in the dark for 24 h at 250 r.p.m, and 28[degrees]C on a gyratory shaker. The solution and soil were separated by centrifugation at 3000g for 20 min. An aliquot of supernatant was removed and measured for pH values and [Cu.sup.2+] concentrations by an atomic absorption spectrophotometry (AAS). Control samples were prepared in the absence of bentonite to evaluate sorption of [Cu.sup.2+] by glass tubes, which was found to be negligible.
Release of [Cu.sup.2+] from contaminated soils in the presence of bentonite
A series of 20mL of 0.005 mol KN[O.sup.3]/L solutions at several pH values were placed into glass tubes each containing 0-1.0 g of bentonite and 0-2.0 g of [Cu.sup.2+]-contaminated soils. The mixtures were shaken for 24 h at 250 r.p.m, and 28[degrees]C, and then centrifuged at 3000g for 20 min. The supernatant was removed and measured for pH and [Cu.sup.2+] concentration using AAS.
Results and discussion
Sorption of Cu by bentonite
The sorption isotherm of [Cu.sup.2+] by bentonite is shown in Fig. 1. The sorption isotherm is nonlinear, and could be described using the Langmuir model (Al-Qunaibit et al. 2005; Kaya and Oren 2005; Liu et al. 2006):
Q = K[Q.sub.max][C.sub.e]/1 + K[C.sub.e] (1)
where Q is the amount of [Cu.sup.2+] sorbed by sorbent (mg/g), [C.sub.e] is the equilibrium concentration of [Cu.sup.2+] in solution (mg/L), [Q.sub.max] is the maximum sorption capacity of the sorbent (mg/g), and K is the affinity constant (L/mg). The Langmuir equation could be equally expressed in a linear form (Eqn 2) from which we obtained K and [Q.sub.max] :
[C.sub.e]/Q = [C.sub.e]/[Q.sub.max] + 1/K[Q.sub.max] (2)
The sorption of [Cu.sup.2+] by bentonite fitted well to the linear form of the Langmuir equation with a correlation constant (R) of 0.96. The estimated [Q.sub.max] for [Cu.sup.2+] was 5.4 mg/g. [Q.sub.max] could be used to evaluate the adsorption capacity of different adsorbents. Andini et al. (2006) reported that the maximum adsorption capacity of [Cd.sup.2+] by a bentonite was 2.8 mg/g. Garcia-Sanchez et al. (1999) found that the maximum sorption capacity of clays including sepiolite, palygorskite, and bentonite for [Cd.sup.2+], [Cu.sup.2+], and [Zn.sup.2+] were generally <10mg/g. The [Q.sub.max] of bentonite for [Cu.sup.2+] in this work was of about the same magnitude as those values reported in literature (Garcia-Sanchez et al. 1999; Abollino et al. 2003; Andini et al. 2006). Hu et al. (2005) reported that the [Q.sub.max] values of 4 typical soils from China including yellow-cinnamon soil, yellow-brown soil, red soil, and brick-red soil were 5.88, 2.92, 0.67, and 1.11 mg Cu/kg of soil, respectively. It was noted that the [Q.sub.max] values for clays (i.e. bentonite) for [Cu.sup.2+] were generally 2-3 times sorption by soils. This suggests the possibility of using bentonite as soil amendment to reduce the release and mobility of metals in soil.
[FIGURE 1 OMITTED]
Soil pH is known to be a determinant of heavy metal sorption by clays and soils. The influence of pH on adsorption of [Cu.sup.2+] by bentonite is shown in Fig. 2; 20 mL of the initial [Cu.sup.2+] concentration (50 mg/L) was mixed with 0.2 g of bentonite. [Cu.sup.2+] adsorption by bentonite increased with increasing pH from 3.8 to 7.8, then reached a plateau at pH > 7.8. This phenomenon was consistent with previous studies (Kaya and Oren 2005; Liu et al. 2006; Oyanedel-Craver and Smith 2006). It has been accepted that surface complexation occurs between the cations and negatively charged groups on bentonite surfaces through electrostatic attraction. At a low pH, some groups on bentonite surfaces are protonated and become positively charged, and [H.sup.+] competes with metal ions for adsorptive sites resulting in reduced sorption for [Cu.sup.2+]. In contrast, at high pH, more negatively charged surfaces are created and available, thus facilitating greater metal adsorption (Abollino et al. 2003). However, at a certain higher pH (e.g. pH > 7.8), [Cu.sup.2+] could precipitate, and the aqueous concentrations were observed below detection limit. At high pH, the loss of [Cu.sup.2+] from the aqueous phase could be contributed from metal precipitation and adsorption by minerals.
[FIGURE 2 OMITTED]
Release of [Cu.sup.2+] from soils
To assess the release of Cu from soils, a given amount of spiked soil samples (0-2.0g) was mixed with 20mL of 0.005 mol KN[O.sub.3]/L solution, and then [Cu.sup.2+] aqueous concentrations were measured using AAS. The trend is that the more spiked soil samples are added, the greater the release of [Cu.sup.2+] from soils (Table 2). In contrast, the desorption ratio (the ratio of the released Cu to the total added amounts) decreased from 13.4% to 5.78% with increasing soil samples. This suggests that the greater amount of soil present reduced the release of [Cu.sup.2+] from soil. Kaya and Oren (2005) reported that the sorption of zinc by Na-enriched bentonite and natural bentonite was reduced 64% and 52% when the sorbent to solution ratios increased from 1 to 10 g/L. In general, higher solid/liquid ratio might lead to the formation of aggregates of particles. By contrast, sorbent particles are relatively dispersed in diluted suspensions; therefore, more active sites on sorbent surfaces are available to the solvent, resulting in a higher desorption ratio (Bordas and Bourg 2001; Kaya and Oren 2005).
The release of [Cu.sup.2+] from contaminated soil was pH-dependent. To evaluate the influence of pH on [Cu.sup.2+] release, soil samples (1.0 g) were placed into a series of glass tubes each containing 20mL of 0.005 mol KN[O.sub.3]/L solution of a certain initial pH. [Cu.sup.2+] concentrations in supernatant were measured. As shown in Fig. 3, [Cu.sup.2+] concentrations in solution sharply decreased with the increase of the pH from 2.59 to 6.88. At pH 2.59, Cu concentration in solution was up to 37.2mg/L, and approximately 93% of soil-retained [Cu.sup.2+] was released into water. At pH > 5.0, [Cu.sup.2+] aqueous concentrations were <3.0 mg/L, and <7.5% of soil-retained [Cu.sup.2+] was released. At pH 6.88, [Cu.sup.2+] aqueous concentration was undetectable. These observations indicate that at lower soil pH, the metals in soils are more available to plant uptake, posing more serious risks to agricultural food products and human health. The influence of pH values on Cu release depends on the protonation of the solid surface at low pH values, [H.sup.+] competition, and metal precipitation at high pHs.
Effects of bentonite on the release of [Cu.sup.2+] from soils
Heavy metals present in soils are toxic and persistent. They can be taken up by plants when releasing into the soil water, and therefore enter into the food chains, posing threats to human health. Reduction of availability and mobility of these contaminants in soil is needed. Bentonite, with great sorption capacity, can be used as a soil amendment to reduce the releases of heavy metals from soils. The release of [Cu.sup.2+] from soil in the presence of bentonite is shown in Fig. 4. The amount of bentonite added was 0.4 g for all treatments. The presence of bentonite significantly inhibited the release of [Cu.sup.2+] from soil. The apparent [Cu.sup.2+] aqueous concentrations released from soil without the addition of bentonite were 113-1160% greater than those samples amended with bentonite. For instance, for the systems containing 1.0 and 2.0g of soil, the [Cu.sup.2+] aqueous concentrations in the presence of bentonite were 0.49 and 2.17mg/L, which were 85.1% and 53.1% less than their corresponding bentonite-free controls (3.27 and 4.62 mg/L).
[FIGURE 3 OMITTED]
The amount of amended clay also influences the release of [Cu.sup.2+] from soil. The presence of clay could effectively inhibit the release of [Cu.sup.2+] from soil (Fig. 5); even a very small amount of bentonite (e.g. 0.1 g) resulted in >29% more [Cu.sup.2+] retained by solids compared with the control. As the added bentonite exceeded 0.5 g, the [Cu.sup.2+] aqueous concentrations were <0.16mg/L, which was <5.2% of the control (3.06mg/L). This means >94.8% of [Cu.sup.2+] potentially released from soil was adsorbed by the added bentonite. As the added bentonite exceeded 1.0g, the [Cu.sup.2+] concentrations in solution were undetectable.
[FIGURE 4 OMITTED]
The high efficiencies of retaining [Cu.sup.2+] with amended bentonite were manifest at different pH conditions. In Fig. 3, the presence of bentonite dramatically decreased the release of [Cu.sup.2+] from soil relative to the controls without bentonite amendment at pH ranging from 2.68 to 6.86. Compared with the bentonite-free controls, at pH 2.68 the added bentonite inhibited ~49% of [Cu.sup.2+] released from soil; at higher pH, bentonite was also effective at reducing the release of [Cu.sup.2+] from soil (Fig. 3). The influence of pH on the reduced release of metals in the presence of bentonite could be ascribed to protonation of sorbent surfaces, [H.sup.+] competition, and metal precipitation as described above.
Sorption of heavy metals by solids commonly includes 2 steps: an initial fast adsorption followed by a much slower diffusion. The former could be described as a rapid adsorption of ions from the aqueous solution to external sorbent surfaces, and the latter to the diffusion of ions into inner sorptive surfaces (Al-Qunaibit et al. 2005). As such, the ageing process could lead to stronger sorption and less availability (Gao et al. 2003; Serrano et al. 2005; Mustafa et al. 2006). The soil was spiked with [Cu.sup.2+] solution and aged for 0, 30, and 50 days. The effect of the ageing process on the release of [Cu.sup.2+] from soil is presented in Fig. 6. For the soil without amendment of bentonite, the aqueous [Cu.sup.2+] concentrations were 3.06, 1.66, and 1.57 mg/L for the soil aged with spiked [Cu.sup.2+] for 0, 30, and 50 days. This indicated that [Cu.sup.2+] could diffuse into the soil inner adsorptive sites, which is believed to be irreversibly sequestrated (Boonfueng et al. 2006). The presence of bentonite obviously inhibited the release of [Cu.sup.2+] from solids for all aged samples with the least release for the soils amended with the greatest amount of bentonite. Compared with the freshly spiked soil, the aged soils needed less bentonite amendment to reach similar levels of [Cu.sup.2+] aqueous concentrations (Fig. 6). This suggests that bentonite might be more effective to control the release of heavy metals for aged soils.
We define a parameter, inhibition ratio (I), to evaluate the effectiveness of bentonite on [Cu.sup.2+] release. Inhibition ratio refers to I=([C.sub.e-ck]-[C.sub.e-b]) x 100/[C.sub.e-ck] in which [C.sub.e-ck] is the Ce for bentonite-free controls, and [C.sub.e-b] is the corresponding Ce in the presence of bentonite. The greater I value means the stronger retention of [Cu.sup.2+] in soils. The calculated I values are presented as a function of the amended bentonite in Fig. 7. The I values increased with increase of the added bentonite. More interestingly, the same amount of bentonite caused a greater I value for longer aged soils. For instance, the I values were 36.8%, 62.6%, and 77.8% with amendment of 0.2g of bentonite for 0-, 30-, and 50-day aged soils, respectively.
[FIGURE 6 OMITTED]
Bentonite is an expandable montmorillonite with large specific surface areas and high cation exchange capacity. This clay is low cost, and ubiquitous in soils. Natural bentonite manifested strong adsorption for [Cu.sup.2+]. The sorption was observed to be pH-dependent, and could be well described using the Langmiur model. The maximum capacity of the bentonite was approximately 5.4 mg/g, which was 2-3 times the sorption by soils reported in literatures. This provides a possibility to use bentonite to reduce the release of heavy metals from soil.
It was observed that soil aging process reduced the release of [Cu.sup.2+] from soils into solution. Soil pH played a major role in [Cu.sup.2+] adsorption in which sorption increased with increasing pH. This result suggests that [Cu.sup.2+] in the contaminated soil would be more available to release in acidic conditions. Amendment of bentonite is an effective approach to reduce the release of [Cu.sup.2+] from soil into the surrounding aqueous phase. The larger amount of bentonite present in soil results in an enhanced inhibition of [Cu.sup.2+] release from soil. This amendment protocol is more effective to retain heavy metals for aged soils contaminated by heavy metals. Bentonite has been accepted to improve the soil overall quality (Phillips 1998). This study provides useful information on utilisation of bentonite for controlling the release of heavy metals from contaminated soils.
[FIGURE 7 OMITTED]
This work was financially supported by the National Natural Science Foundation of China (20777036, 40701073), the National Natural Science Foundation of Jiangsu Province (BK2007580, BK2006518), the Program for New Century Excellent Talents in University (NCET T-06-0491), and the Foundation of Ministry of Education Key Laboratory of Environment Remediation and Ecological Health (050302).
Manuscript received 6 June 2007, accepted 22 October 2007
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Wanting Ling (A,B), Qing Shen (C), Yanzheng Gao (A,D), Xiaohong Gu (A), and Zhipeng Yang (A)
(A) College of Resource and Environmental Sciences, Nanjing Agricultural University, Nanjing 210095, China.
(B) Ministry of Education Key Laboratory of Environmental Remediation and Ecological Health, College of Natural Resources and Environmental Science, Zhejiang University, 310029 Hangzhou, China.
(C) Department of Food Sciences, the University of Reading, Whiteknights, Reading, RG6 6DW, UK.
(D) Corresponding author. Email: firstname.lastname@example.org
Table 1. Selected properties of the tested bentonite Chemical composition (wt %) Si[O.sub.2] [Al.sub.2][O.sub.3] CaO MgO 58.8 16.4 2.72 5.47 Chemical composition (wt %) [Fe.sub.2][O.sub.3] [K.sub.2]O [Na.sub.2]O LOI (A) 4.45 0.12 0.07 11.3 BET-[N.sub.2] surface areas CEC (B) ([m.sub.2]/g) (mmol/kg) 60.9 108.4 (A) Loss on ignition. (B) Cation exchange capacity. Table 2. Release of Cu from contaminated soils Data in parentheses were standard deviations Amount of Initial amount [Cu.sup.2+] Spiked soil of [Cu.sup.2+] released Desorption sample (g) in soil (mg) from soil (mg) ratio (%) 0.1 0.08 0.011 (0.0006) 13.4 (0.72) 0.2 0.16 0.020 (0.0014) 12.8 (0.90) 0.4 0.32 0.035 (0.0016) 10.9 (0.49) 0.6 0.48 0.049 (0.0030) 10.1 (0.63) 0.8 0.64 0.057 (0.0010) 8.89 (0.15) 1.0 0.80 0.065 (0.0035) 8.18 (0.44) 1.2 0.96 0.070 (0.0007) 7.25 (0.07) 1.4 1.12 0.074 (0.0035) 6.63 (0.31) 1.6 1.28 0.080 (0.0028) 6.23 (0.22) 1.8 1.44 0.087 (0.0040) 6.07 (0.28) 2.0 1.60 0.092 (0.0024) 5.78 (0.15) Fig. 5. Relation of [Cu.sup.2+] release from soil to the amounts of added bentonite, R is the ratio of [Cu.sup.2+] concentrations in the presence of bentonite to the bentonite-free control (ck) ck 100 0.1 70.8 0.2 63.2 0.4 52.1 0.5 25.6 0.6 5.14 0.7 5.17 0.8 1.96 0.9 1.31 1 0 Note: Table made from bar graph.
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