immediate remediation of heavy metal (cr(vi)) contaminated soil by high energy electron beam...
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Accepted Manuscript
Title: Immediate remediation of heavy metal (Cr(VI))contaminated soil by high energy electron beam irradiation
Author: Jing Zhang Guilong Zhang Dongqing Cai ZhengyanWu
PII: S0304-3894(14)00907-8DOI: http://dx.doi.org/doi:10.1016/j.jhazmat.2014.11.007Reference: HAZMAT 16379
To appear in: Journal of Hazardous Materials
Received date: 29-6-2014Revised date: 15-9-2014Accepted date: 12-11-2014
Please cite this article as: Jing Zhang, Guilong Zhang, Dongqing Cai,Zhengyan Wu, Immediate remediation of heavy metal (Cr(VI)) contaminatedsoil by high energy electron beam irradiation, Journal of Hazardous Materialshttp://dx.doi.org/10.1016/j.jhazmat.2014.11.007
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Immediate remediation of heavy metal (Cr(VI)) contaminated soil by high
energy electron beam irradiation
Jing Zhang,†,‡ Guilong Zhang,†,‡ Dongqing Cai,*,†,‡ and Zhengyan Wu*,†,‡
†Key Laboratory of Ion Beam Bioengineering, Hefei Institutes of Physical Science,
Chinese Academy of Sciences, Hefei 230031, People’s Republic of China
‡Bioenergy Forest Research Center of State Forestry Administration, Hefei 230031,
People’s Republic of China
*Corresponding authors. Tel.: +86-551-65595012; fax: +86-551-65591413.
E-mail addresses: [email protected] (D. Cai), [email protected] (Z. Wu).
Highlights
►
• An immediate remediation method for Cr(VI) contaminated soil (CCS) was
developed.
• High energy electron beam (HEEB) irradiation could reduce Cr(VI) in CCS to
Cr(III).
• This effect was attributed to electrons, hydrated electrons and reductive radicals.
• This remediation method was effective, environmentally friendly and low-cost.
Abstract
This work developed an immediate and high-performance remediation method for
Cr(VI) contaminated soil (CCS) using high energy electron beam (HEEB) irradiation.
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The result indicated that, compared with γ-ray irradiation, HEEB irradiation displayed
a significant reduction efficiency on Cr(VI) in CCS to Cr(III) with substantially lower
toxicity, which was mainly attributed to the reduction effects of electrons, hydrated
electrons and reductive radicals generated in the irradiation process of HEEB. This
work could provide a one-step and effective method for the remediation of heavy
metal contaminated soil (HMCS).
Keywords: High energy electron beam irradiation, Cr(VI) contaminated soil,
Reduction, Reductive radicals
1.
2.
3.
4. Introduction
Discharging industrial waste containing heavy metals into water and soil could
cause severe environment contamination. Moreover, heavy metals could enter the
dining tables of households and restaurants through the crops grown in contaminated
fields. Soil contamination by heavy metal has caused serious public concern and
anxiety on food safety [1-3]. Ensuring the soil security is of crucial importance for a
safe food supply. It is time to develop a high-performance remediation approach for
heavy metal contaminated soil (HMCS) to improve soil quality and maintain soil
health.
Until now, several remediation approaches for HMCS have been reported mainly
through physical (electrical enrichment and washing), chemical (reduction, adsorption
and inactivation) and biological (plant enrichment) methods [4-6]. Although these
methods could resolve the HMCS problem to various degrees, the procedure of the
physical method was usually relatively complex with high cost, the chemical methods
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could introduce secondary pollution, and the biological method needed rather long
time. Therefore, it was necessary to develop a low-cost, high-efficiency and
environmentally-friendly approach for the remediation of HMCS. In this case, a new
technology, high energy electron beam (HEEB) irradiation, was introduced to resolve
this problem.
Because of the high sterilization efficiency, low cost and facile operation, HEEB
irradiation has been applied widely in food fresh keeping [7]. In addition, HEEB
irradiation was also used in the remediation of heavy metal contaminated water [8].
However, HEEB was rarely applied to treat the heavy metals in soil. Hence, we
attempted herein to investigate the remediation performance of HEEB irradiation on
HMCS in consideration of its industrial scale treatment capacity and
environmentally-friendly property.
In this work, chromium was selected as the mode heavy metal contaminant of soil,
because chromium is one of the extremely toxic heavy metals in soil mainly attributed
to the discharge of chromium-containing waste and waste-water from ore refining,
steel and alloys production, metal plating, tannery, wood preservation, pigmentation
and so on [9]. The most stable states of chromium in soil were Cr(III) and Cr(VI),
wherein Cr(VI) displayed higher toxicity, carcinogenicity, solubility and mobility and
thus higher health risk for humans compared with Cr(III) [10-12]. Hence, the
reduction of Cr(VI) to Cr(III) using reduction agents, as a promising approach, is
attracting more and more attention for the remediation of Cr(VI) contaminated soil
(CCS) [13-15]. However, this method required quantities of chemical reagents, which
could probably result in secondary pollution, high cost and thus small application
prospect [13, 14]. Therefore, it is necessary to develop a new Cr (VI) reduction
approach with simple procedure, low cost, high efficiency, environmentally-friendly
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property and industrial-scale treatment capacity.
Here, HEEB irradiation was used as a facile approach for reduction of Cr(VI) to
Cr(III) in soil, and it was found that this method could effectively and immediately
reduce Cr(VI) to Cr(III) in soil. The mechanism was also investigated and the results
indicated that both electrons and radicals played key roles in the reduction process.
Additionally, the influences of the initial Cr(VI) concentration and the HEEB
irradiation fluency on the reduction performance were also studied. This work is
attempting to provide an immediate and promising approach for the reduction of
Cr(VI) in soil, which could be favorable for the treatment of the other heavy metal
contaminants in soil.
5. Materials and methods
5.1. Materials
K2Cr2O7 and other chemicals were of analytical reagent grade and purchased from
Sinopharm Chemical Reagent Company (Shanghai, China). Deionized water was used
throughout this work.
5.2. Cr(VI) contaminated soil sample preparation
Soil (0-20 cm depth) was collected from the Dongpu Island in Anhui province of
China. Then it was ground to 100-150 mesh after air drying. Cr(VI) (actually the
K2Cr2O7) aqueous solution (2 g) with a certain concentration (60, 80, 100 or 120 ppm)
was added to the resulting soil (2 g) in a centrifuge tube (10 mL) to obtain the CCS
sample with Cr(VI) initial concentration of 30, 40, 50, or 60 ppm.
5.3. HEEB or γ-ray irradiation
HEEB irradiation of Cr(VI) aqueous solution (100 ppm) or CCS samples in sealed
centrifuge tubes was carried out using the HEEB accelerator (10 MeV and 10 kW)
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(IHI10, IHI Co., Japan) with absorbed doses of 10, 20, 30 and 40 kGy detected by
dosimeter of silver dichromate at room temperature. Afterwards, the concentration of
Cr(VI) remained in the solution or soil was determined using a UV-Vis
spectrophotometer (UV 2550, Shimadzu Co., Japan) at a wavelength of 520 nm by
diphenylcarbazine (DPC) method [16]. All experiments were performed in triplicate.
In addition, γ-ray irradiation on the CCS sample with an initial concentration of
50 ppm in a sealed centrifuge tube (10 mL) was performed using a cobalt-60 radiation
source with absorbed doses of 10 and 20 kGy.
5.4. Preparation and addition of DMSO
In order to investigate the influence of reductive radicals on the reduction
performance of HEEB, 0.2 mL of dimethylsulfoxide (DMSO) aqueous solution with
concentration of 20, 40, 200, or 700 µL/L, as a reductive radical scavenger, was added
to the CCS samples respectively before HEEB irradiation. After that, their reduction
efficiencies were determined compared with the one without DMSO.
6. Results and discussion
6.1. Reduction performance investigation
To obtain the reduction performance of HEEB irradiation on Cr(VI) in soil, the
reduction performance in aqueous solution was investigated firstly, as Cr(VI) exists in
soil is mainly aqueous Cr(VI). As shown in Fig. 1, with the increasing absorbed dose
of HEEB, the reduction efficiency (RE) of HEEB irradiation on Cr(VI) in aqueous
solution increased initially (<30 kGy), reaching the maximum value (approximately
97%) at 30 kGy, and then decreased (>30 kGy). In other words, 30 kGy is the
optimum absorbed dose of HEEB for the reduction of Cr(VI) in aqueous solution.
This result indicated that HEEB possessed a high reduction ability on Cr(VI) in
aqueous solution, and is feasible to reduce the Cr(VI) in soil.
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The reduction performance of HEEB on Cr(VI) in CCS was further investigated. It
can be seen in Fig. 2, with the increase of absorbed dose of HEEB, the RE on Cr(VI)
in CCS increased initially (<30 kGy) and then decreased (>30 kGy), which was
similar to that in aqueous solution. It is worth noting that the maximum value of RC
(89%) in CCS (Fig. 2A) was lower than that (94%) in aqueous solution with Cr(VI)
initial concentration of 60 ppm (Fig. 2B), which was probably because of the
resistance effect of soil colloidal particles on the HEEB lowering the contact
probability between electrons and Cr(VI). Interestingly, the RE of HEEB generally
increased with the initial concentrations of Cr(VI) in CCS, which was probably
attributed to the cascade effect of HEEB or some secondary particles such as hydrated
electrons (eaq-) (e- + n H2O = eaq
-) [8], and reductive radicals such as hydroxyl radicals
generated from water radiolysis during the irradiation process. Thus higher initial
Cr(VI) concentration was favorable to the cascade effect and resulted in higher RE.
This result demonstrated that HEEB had a significant reduction capacity on the Cr(VI)
in CCS probably with the aid of the reduction effects of electrons and some secondary
particles.
6.2. Mechanism study for the reduction performance of the HEEB irradiation
To obtain the reduction mechanism of HEEB on Cr(VI) in CCS, the reduction
performance of HEEB on Cr(VI) in CCS was investigated compared with γ-ray. It
could be clearly seen in Fig. 3 that the RE of HEEB was rather high, while γ-ray
nearly had no reduction effect on Cr(VI), illustrating that electrons or some secondary
particles other than γ-ray played a key role in the reduction of Cr(VI).
Besides the electrons, reductive radicals (mainly H· and OH·) generated during
the irradiation process could also probably has reduction capacity on Cr(VI) in CCS
[8]. To evaluate the reduction effect of reductive radicals, DMSO as a radical
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scavenger was added to the CCS before irradiation. As shown in Fig. 4, the RE of
Cr(VI) decreased with the increasing DMSO concentration, proving that reductive
radicals could be generated in the irradiation process and possessed a reduction ability
on Cr(VI) to a certain degree.
Based on the preceding analysis, HEEB possessed an immediate and significant
reduction performance on Cr(VI) in CCS through the reduction effects of electrons,
hydrated electrons, and reductive radicals (Fig. 5). To remedy the Cr(VI)
contaminated soil, the soil could be placed on a conveyor and treated by a fixed
HEEB facility as shown in Fig. 5. Of course it is promising to treat a whole field of
Cr(VI) contaminated soil if a mobile HEEB facility is available.
7. Conclusions
In this article, HEEB irradiation, as a remediation approach for HMCS, was used to
treat the Cr(VI) contaminated soil and displayed an immediate and significant
reduction ability from Cr(VI) to Cr(III), so that the toxicity of chromium in soil was
substantially lowered. Electrons, hydrated electrons, and reductive radicals generated
in the irradiation process could probably play key roles in the reduction process. This
work could provide a one-step and effective method for the remediation of heavy
metal contaminated soil.
Acknowledgments
The authors acknowledge financial support from the Key Program of the Chinese
Academy of Sciences (No. KSZD-EW-Z-022-05) and the Scientific and
Technological Project of Anhui Province of China (No. 1206c0805014).
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Figure captions: Fig. 1. Reduction performance of HEEB irradiation on Cr(VI) in aqueous solution
with an initial concentration of 100 ppm.
Fig. 2. (A) Reduction performance of HEEB irradiation on Cr(VI) in CCS with
different initial concentrations; (B) Reduction performance of HEEB irradiation on
Cr(VI) in aqueous solution with an initial concentration of 60 ppm.
Fig. 3. Reduction performance of γ-ray and HEEB irradiation on Cr (VI) in CCS with
an initial concentration of 50 ppm.
Fig. 4. Reduction performance of HEEB irradiation on Cr(VI) in CCS (initial
concentration of 50 ppm) after addition of DMSO aqueous solutions (0.2 mL) with
different concentrations of 0 (a), 20 (b), 40 (c), 200 (d), and 700 µL/L (e).
Fig. 5. Schematic diagram showing the CCS placed on a conveyor system and treated
by a fixed HEEB facility.
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Figures:
Fig. 1
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Fig. 2
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Fig. 3
Fig. 4
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Fig. 5
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