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Green and efficient removal of cadmium from rice flour using natural deep eutectic solvents
Food Chemistry 244 (2018) 260–265
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Green and efficient removal of cadmium from rice flour using natural deep eutectic solvents ⁎
MARK
⁎
Yao Huanga,1, Fang Fenga,1, Zhi-Gang Chena, , Tao Wub, , Zi-Han Wanga a b
Glycomics and Glycan Bioengineering Research Center, College of Food Science & Technology, Nanjing Agricultural University, Nanjing 210095, PR China The Department of Food Science, The University of Tennessee, 2510 River Drive, Knoxville, TN 37996, United States
A R T I C L E I N F O
A B S T R A C T
Keywords: Natural deep eutectic solvents Cadmium Removal Rice flour
Natural deep eutectic solvents (NADESs) constitute a novel class of biodegradable and inexpensive solvents. In this study, twenty choline chloride- and glycerol-based NADESs were prepared and investigated as washing agents in the removal of cadmium (Cd) from rice flour for the first time. Choline chloride-based NADESs demonstrated good Cd removal (51%–96%). A natural, biodegradable surfactant, saponin, was mixed with the NADESs to enhance their efficiency. No increase in Cd removal was observed when glycerol-based NADESs were combined with 1% saponin; however, synergistic effects between saponin and choline chloride-based NADESs were observed during the washing process and > 99% Cd was removed using NADES-saponin mixtures. Moreover, NADESs washing process did not affect the main chemical components or structure of rice flour. The mechanism of Cd removal by NADESs and regeneration of Cd-contaminated NADESs were also explored. The study presents a green and efficient way of removing Cd from contaminated rice.
1. Introduction Various human activities leaded to Cd pollution of land, ground water and atmosphere such as mining, smelting, fertilization, and so on (Archana & Sharma, 2016; Clemens, Aarts, Thomine, & Verbruggen, 2013; Fu & Wang, 2011). Industrial waste produced by human activities is the main source of Cd pollution of agricultural soils. However, heavy metals which cannot be degraded by microorganisms tend to persist and accumulate in the soils (Archana & Sharma, 2016; Clemens et al., 2013). Rice is easier to accumulate Cd from soil compared with other cereal crops. (Archana & Sharma, 2016; Clemens et al., 2013; Huo, Du, Xue, Niu, & Zhao, 2016; Jorhem et al., 2008; Wu et al., 2016; Zhuang et al., 2016). Rice is an important food commodity in the international market, and is a major staple for about half of the world’s population (Gunduz & Akman, 2013; Zhuang et al., 2016). Cd accumulation in rice is a global issue. The toxicity of Cd is well established, and Cd can accumulate in human kidneys (Archana & Sharma, 2016; Chavez et al., 2015; Clemens et al., 2013). Cd exhibits a long biological half-life of 10–30 years, and is classified as a group 1 carcinogen (Archana & Sharma, 2016; Clemens et al., 2013). Therefore, rice safety has garnered considerable attention in many countries. The Cd concentration in rice should be below accepted limits (0.4 mg/kg as an international standard limit and 0.2 mg/
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1
kg in China) (Huo et al., 2016; Jorhem et al., 2008; Wu et al., 2016). Excess Cd should be removed from Cd-polluted rice grains or their products to reduce the risks to the public. To date, several methods, such as breeding low Cd- accumulating rice cultivars, soil remediation, and phytoremediation have been developed to reduce the accumulation of Cd in rice (Archana & Sharma, 2016; Clemens et al., 2013). However, rice containing a high concentration of Cd is still being produced in Cdcontaminated areas in the world every year. Thus, it is imperative to develop simple and efficient decontamination methods for Cd-polluted rice. Extraction and detection of heavy metals in water, soil, and some agricultural products have been reported by several research groups (Baghban, Shabani, & Dadfarnia, 2012; Behbahani et al., 2013; Behbahani, Abolhasani, et al., 2015; Behbahani, Bagheri, et al., 2015; Moghaddam, Shabani, Dadfarnia, & Baghban, 2014; Mukhopadhyay, Mukherjee, Adnan, et al., 2016; Mukhopadhyay, Mukherjee, Hayyan, et al., 2016; Omidi, Behbahani, Bojdi, & Shahtaheri, 2015; Zarezade, Behbahani, Omidi, Abandansari, & Hesam, 2016). Washing rice grains or soil contaminated with heavy metals and organics is a widely accepted practice (Huo et al., 2016; Mulligan, Yong, Gibbs, James, & Bennett, 1999; Wu et al., 2016), but efficient and environmentally friendly techniques are rare. Recently, natural deep eutectic solvents (NADESs) obtained from natural components have
Corresponding authors. E-mail addresses: [email protected] (Z.-G. Chen), [email protected] (T. Wu). These authors contributed equally to this work.
http://dx.doi.org/10.1016/j.foodchem.2017.10.060 Received 1 May 2017; Received in revised form 17 September 2017; Accepted 9 October 2017 Available online 12 October 2017 0308-8146/ © 2017 Elsevier Ltd. All rights reserved.
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compositions of the NADESs are given in Table 1. NADESs were prepared according to a previously published method (Huang et al., 2017).
emerged as green, versatile solvents for various applications (Huang et al., 2017; Karimi, Dadfarnia, Shabani, Tamaddon, & Azadi, 2015; Paradiso, Clemente, Summo, Pasqualone, & Caponio, 2016; Zhao et al., 2015). Components of NADESs are edible, and NADESs can be classified as “readily biodegradable” solvents (Huang et al., 2017). However, NADESs have not been used as washing agents for contaminant removal from food matrices to date. Therefore, in this study, twenty choline chloride- and glycerol-based NADESs were synthesized and used as washing agents for Cd removal from contaminated rice flour. Moreover, saponin, a natural biodegradable surfactant, is environmentally friendly and has been used in the food and medical industries for many years (Mukhopadhyay, Mukherjee, Hayyan, et al., 2016; Suhagia, Rathod, & Sindhu, 2011). The effect and mechanism of surfactant on heavy metal removal have been reported (Fu & Wang, 2011). So, NADESs were combined with saponin, in order to investigate synergistic effects. The purpose of this study was to develop a novel, inexpensive, and efficient method for removing Cd from rice.
2.3. Sample treatment Cd-contaminated rice flour was dried under vacuum at 80 °C for 4 h. For each experiment, 0.30 g of rice flour was washed with 2.0 mL of the washing solution (NADES, surfactant solutions, and NADES-surfactant mixtures) in a 10.0 mL centrifuge tube. The test tubes were heated to 60 °C and shaken at 100 rpm for 1 h. During this period, ultrasound assisted extraction was performed every 15 min for 5 min using an ultrasonicator (20 kHz, 200 W; Type NP-B-400-15; New Power Co., Ltd., Kunshan, China) equipped with a digital timer and a temperature controller. The water bath temperature was maintained within ± 1 °C. The washing solution was then centrifuged (12,000 rpm, 20 min), and the supernatant was discarded. Ultrapure water (2.0 mL) was added to the tube and it was briefly vortexed, and then centrifuged at 12,000 rpm for 10 min. After centrifugation, the precipitate was transferred to polytetrafluoroethylene (PTFE) digestion tubes (60.0 mL inner volume). The rice flour was further digested using a microwave-assisted digestion system. For comparison, ultrapure water was also used as a washing solvent. The concentrations of surfactant saponin aqueous solutions used in this study were 1% (w/w). NADESs -saponin systems consist of 1% saponin aqueous solution and NADESs by mixing. To minimize the risk of metal contamination, all glass ware was soaked in 5% HNO3 (v/v) for at least 24 h. PTFE digestion tubes were soaked in 20% HNO3 (v/v) overnight prior to digestion. The water used in this study was ultrapure water.
2. Materials and methods 2.1. Reagents and materials Cd-contaminated rice flour sample (Cd: 2.16 mg kg−1) was purchased from National Standard Material Center Network (Beijing, China). Cadmium nitrate was purchased from Macklin Reagent Co., Ltd. (Shanghai, China). Concentrated nitric acid (Guaranteed reagent) was supplied by Nanjing Chemical Reagent Co., Ltd. (Nanjing, China). Compounds for NADESs preparation, including choline chloride (≥98.0%), glycerol (≥99.0%), D-(+)-xylose (≥99.0%), Glucose (≥99.0%), fructose (≥99.0%), L-(−)-sorbose (≥99.0%), mannose (≥99.0%), D-(+)-galactose (≥99.0%), sucrose (≥99.0%), L-(+)- arabinose (≥99.0%), L-(+)- rhamnose (≥99.0%), trehalose (≥99.0%), proline (≥99.0%), L-alanine (≥99.0%), glycine (≥99.0%), L-threonine (≥99.0%), L-histidine (≥99.0%), DL-malic acid (≥99.0%), citric acid (≥99.5%), L-(+)-tartaric acid (≥99.5%), xylitol (≥99.0%), sorbitol (≥99.0%), and saponin (biological reagent) were obtained from Aladdin Chemical Reagent Co., Ltd. (Shanghai, China).
2.4. Sample microwave digestion and determination In the microwave digestion, a conventional acid digestion method was used for the determination of Cd in rice flour. Concentrated nitric acid (8.0 mL) was added to the PTFE digestion tubes containing precipitate. The operating conditions for the MARS6XP1600 microwaveassisted digestion system (USA, CEM) are presented in Table S1. After digestion, a clear solution was obtained. The interior walls of the tube were washed with a minimum amount of ultrapure water. After cooling, the solution was filtered through a 0.45 μm filter and then transferred to a 50.0 mL volumetric flask and diluted with ultrapure water. The concentrations of Cd in rice flour were determined without
2.2. Preparation of NADESs Twenty NADESs were tested in the preliminary study. The Table 1 List of different NADESs prepared from natural products. NADES Abbreviation
Components Component 1
GlyPro GlyAla GlyGly GlyThr GlyHis ChCit ChMal ChTar ChXy ChSo ChXyl ChGlu ChFru ChSor ChMan ChGal ChSur ChAra ChRh ChTre a
Glycerol Glycerol Glycerol Glycerol Glycerol Choline Chloride Choline Chloride Choline Chloride Choline Chloride Choline Chloride Choline Chloride Choline Chloride Choline Chloride Choline Chloride Choline Chloride Choline Chloride Choline Chloride Choline Chloride Choline Chloride Choline Chloride
Component 2
Component 3
Proline
–a – – – – Water Water Water Water Water Water Water Water Water Water Water Water Water Water Water
L-Alanine
Glycine L-Threonine L-Histidine
Citric acid acid L-(+)-Tartaric acid Xylitol Sorbitol D-(+)-Xylose Glucose Fructose L-(−)-Sorbose Mannose D-(+)-Galactose Sucrose L-(+)-Arabinose L-(+)-Rhamnose Trehalose DL-Malic
No water was added.
261
Mole ratio
pH
3:1 3:1 3:1 3:1 3:1 1:1:2 1:1:2 1:1:2 5:2:5 5:2:5 3:1:3 5:2:5 5:2:5 5:2:5 5:2:5 5:2:5 4:1:4 5:2:5 2:1:2 4:1:4
7.74 7.34 7.27 6.48 7.09 0.09 0.23 0.56 6.88 6.33 5.16 4.97 4.36 4.55 5.06 5.04 4.77 4.60 5.09 4.88
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Fig. 1. Effect of different NADESs on the removal of Cd from rice flour. Conditions: 0.30 g rice flour, 2 mL NADESs, 60 °C, 1 h.
100
Cd removal (%)
80
60
40
20
G ly P G ro ly A G la ly G G ly ly Th G r ly H Ch is M a Ch l Ci Ch t Ta r Ch X y Ch So Ch X y Ch l G lu Ch Fr u Ch So Ch r M a Ch n G a Ch l Su Ch c A r Ch a Rh a Ch T Sa re po ni n W at er
0
2.8. Statistical analysis
chemical modifier using a graphite furnace atomic absorption spectrophotometer (GFAAS).
Overall, 3 independent trials were carried out during the study and the results are given as means and standard deviations. Analysis of variance (ANOVA) of the data was carried out using the SAS program (SAS Institute Inc., Carry, N.C., USA) to compare significant differences (p < .05 and p < .01) between the results.
2.5. GFAAS Analysis The concentration of cadmium in the digestion solution was determined by a GFAAS model A3 equipped with a CW-1Y cooling circulator (Beijing Purkinjie General Instrument Co., Ltd., Beijing, China). Argon (99.99%) was used as the purge gas. Sample solutions (10.0 μL) were added to the furnace. The experiments were performed at 228.8 nm. The operating conditions for the graphite furnace are presented in Table S2.
3. Results and discussion 3.1. Effects of NADESs on removal of Cd In rice, Cd primarily associates with organic compounds (mainly proteins) via complexation (Isaure, Fayard, Sarret, Pairis, & Bourguignon, 2006; Kaneta, Hikichi, Endo, & Sugiyama, 1986). Organic matter in rice tends to act as an electron donor and binds Cd2+. The pH of an aqueous suspension of rice flour is 6.35, indicating the weakly acidic nature of the rice flour. Therefore, the successful extraction of Cd depends on the ability of the washing agent to disrupt the interactions between organic compounds and Cd. NADESs are touted as designer solvents, and the primary advantage over conventional solvents is their tunability (Huang et al., 2017; Zhao et al., 2015). The selectivity of NADESs for target compounds can be tailored by changing their components. Twenty different choline chloride- and glycerol-based NADESs (Table 1) were prepared and evaluated in the removal of Cd from rice flour. As shown in Fig. 1, the type of NADES had a considerable effect on the removal efficiency of Cd (for each extraction procedure, RSD ≤ 5%). Five glycerol- based NADESs exhibited low Cd removal efficiencies (≤51%); however, choline chloride-based NADESs demonstrated good Cd removals (51%–96%). For example, the Cd removal efficiency reached 95.9 ± 2.2% and 90.4 ± 1.3% when ChTar and ChXy were used as washing agents, respectively. The removal of Cd using washing agents may involve two processes, namely lysis and complexation (Mukhopadhyay, Mukherjee, Adnan, et al., 2016). The organic material in rice flour acts as an electron donating Lewis base and interacts with Cd2+ ions, which are Lewis acids. The NADESs used in the present study either had acidic pH values (e.g., choline chloride-based NADESs), or weakly alkaline pH values (e.g., glycerol-based NADESs) (Table 1). When acidic NADESs such as ChTar and ChMal were added to the Cd2+ contaminated rice flour, H+ ions
2.6. Characterization of rice flour before and after Cd removal The starch and crude protein contents of the NADES-treated rice flour were determined according to AOAC methods. Changes in the proteins in rice flour due to the Cd removal process were determined by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE, Bio-Rad Laboratories, USA). The rice flour was also evaluated using Fourier Transform infrared spectroscopy (FTIR) (Vector 33 spectrometer, Bruker Company, Germany). X-ray powder diffraction (XRD, Xray diffractometer, Thermo Fisher, USA) was used to inspect the structural changes of the rice flour before and after Cd removal. The thermal stability of the sample was studied via thermogravimetric analysis (TGA) using a HTG-1thermal analyzer (Heng Jiu Science Instrument Factory, Beijing, China). 2.7. Regeneration of Cd-contaminated NADES ChXy ChXy with high Cd removal efficiency (90.35%) and low viscosity was selected in this study. The Cd-contaminated rice flour (0.3 g) was added to ChXy (2 mL) in a 10 mL centrifuge tube. Washing process was then carried out as described in “2.3. Sample treatment”. The supernatant of each centrifugation was collected, and then adsorbed by 0.2 g activated carbon. The tube containing Cd-contaminated ChXy and activated carbon was placed in gas bath thermostatic oscillator (model SHZ-82A, Kexi Laboratory Instruments, Jintan, China) for 2 h at 25 °C with shaking speed of 180 rpm. The mixture was filtered through a 0.45 μm membrane. The filtrate (regenerated ChXy) was reused. Measurements were performed in triplicate. 262
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Table 2 Effect of NADESs-1% saponin mixtures (v:v = 9:1) on removal of Cdfrom rice flour.a Wash solutions
Removal (%)
GlyPro GlyAla GlyGly GlyThr GlyHis ChCit ChSor ChSuc GlyPro+1%Saponin GlyAla+1%Saponin GlyGly+1%Saponin GlyThr+1%Saponin GlyHis+1%Saponin ChCit+1%Saponin ChSor+1%Saponin ChSuc+1%Saponin
1.08 ± 0.23 < 1.00 34.55 ± 3.33 23.41.57 51.29 ± 2.55 73.61 ± 5.99 56.87 ± 1.14 51.29 ± 0.56 2.62 ± 0.17b < 1.00 36.51 ± 2.11 25.72 ± 2.53 42.03 ± 1.61 80.46 ± 1.66 72.27 ± 2.42 66.03 ± 2.66
Fig. 2. Effect of 1% saponin solution content in ChSor on removal of Cd from rice flour. Conditions: 0.30 g rice flour, 2 mL ChSor-1% saponin mixtures, 60 °C, 1 h. Removal efficiencies that were significantly higher than that of ChSor are indicated with *p < .05, ** p < .01, and ***p < .001.
a Conditions: 0.30 g rice flour, 2 mL NADESs or NADESs-1% saponin mixtures (v:v = 9:1), 60 °C, 1 h. b Mean ± SD (n = 3).
percentage of 1% saponin aqueous solution (Table S3). As mentioned above, the physical properties of ChSor can be modified by the addition of small quantities of 1% saponin solution. In this study, ChSor-1% saponin mixtures with saponin fractions ranging from 0% to 50% (v/v) were evaluated in the removal of Cd (Fig. 2). As shown in Fig. 2, the Cd removal improved with increasing proportions of 1% saponin solution, up to 87.7% (with 30% saponin solution, v/v), and remained stable thereafter. Many factors including solubilization, sorption, interfacial tension, and viscosity can affect cadmium removal rate. One of factors contributing to the increase in cadmium removal rate may be attributed to the decrease in washing solvent viscosity. The addition of 1% saponin aqueous solution (< 10%, v/v) led to a decrease in the viscosity of the washing media (Table S3), thus enhancing the mass transfer, and increasing the removal efficiency. There is no significant change in the viscosity of mixtures after adding 20%–30% (v/ v) saponin aqueous solutions (Table S3), but the Cd removal efficiency increased from 70.27% to 87.38%, mainly due to an increase in saponin content. However, high saponin aqueous solution contents (> 30%, v/ v) did not further increase the removal efficiency of Cd.
were introduced. The H+ ions attacked the electron rich sites of the rice flour, and competed with the Cd2+ ions for electrons, thereby pulling out the Cd2+ ions from the rice flour via interactions with the NADES anions present in the washing solution. However, when the slightly alkaline, glycerol-based NADESs were introduced into the system, H+ ions were not supplied. Therefore, the Cd2+ removal was negligible for the glycerol-based NADESs. Water and 1.0% saponin aqueous solutions were used as controls. Cd was strongly bound to the rice flour and water could remove only up to 18% of Cd. A 1% saponin aqueous solution removed up to 68% Cd (Fig. 1). 3.2. Effect of NADES-1% saponin mixture on Cd removal As shown in Fig. 1, saponin could be added to water to improve the C d removal. The pH of the 1% saponin aqueous solution was 2.9, indicating the acidic nature. Therefore, the 1% saponin aqueous solution was mixed with NADESs to enhance the Cd removal efficiency. No increase in the Cd removal was observed when glycerol-based NADESs were combined with 1% saponin aqueous solution (v:v = 9:1) (Table 2). However, a synergistic effect between saponin and choline chloride-based NADESs was observed. For example, NADES ChSor demonstrated a Cd removal of 56%, but up to 72% Cd could be removed with a combination of ChSor and 1% saponin aqueous solution (v:v = 9:1). The synergistic effect between saponin and choline chloride-based NADESs improved the Cd removal. Many factors contributed to the increased Cd removal such as (a) pH, (b) encapsulation of Cd2+ by saponin micelles (c) dilution of NADESs and concomitant facilitation of the removal of loosened Cd from the rice flour surface by water (Mukhopadhyay, Mukherjee, Adnan, et al., 2016). In order to obtain deeper insight into the Cd removal with NADES- saponin mixtures, the ChSor-saponin mixture was selected for subsequent studies.
3.4. Effect of liquid–solid ratio on the removal efficiency of Cd The removal of Cd from rice flour using different liquid (ChSor–saponin mixture)–solid (rice flour) ratios is depicted in Fig. 3.The removal efficiency increased with increasing liquid–solid ratios up to 15.0 mL g−1; no significant improvement was observed with higher liquid–solid ratios (> 15.0 mL g−1). Thus, a liquid–solid ratio of 15.0 mL g−1 was sufficient to obtain a high removal efficiency (> 99%) of Cd. 3.5. Characterization of rice flour before and after washing Next we evaluated the effect of NADES treatment on the quality and structure of rice flour. Starch and protein are the main chemical components of rice flour, and changes in their content affect the physicochemical properties and qualities of rice flour. After the removal of Cd with ChSor and the ChSor-saponin mixture (v:v = 7:3), the rice flour yield was 95% and 91%, respectively. As shown in Table S4, the change in the starch content between the original sample and ChSor or ChSorsaponin mixture (v:v = 7:3) treated samples were not significant. In fact, treatment with ChSor or the ChSor-saponin mixture (v:v = 7:3) can affect the protein content, decreasing by 8.5% and 11.7% respectively after washing. Decrease in protein content of rice flour after washing may be that Cd primarily associates with proteins via
3.3. Effect of 1% saponin aqueous solution content in ChSor on removal of Cd One disadvantage of NADESs is their high viscosity, which not only hinders mass transport but also leads to handling difficulties (Huang et al., 2017). In addition to the molar ratio of the components, the water content also affects the viscosity of eutectic mixtures. In the case of ChSor with different concentrations of the 1% saponin aqueous solution (v/v), its viscosity decreased by∼1/6when diluted with 10% 263
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Georgescu, Mălăeru, & Biriş, 2016; Ignat, Fortuna, Sacarescu, Zaltariov, & Harabagiu, 2015). In this study, NADES ChXy was selected because of high Cd removal efficiency (90.4%). The regeneration of Cdcontaminated ChXy with activated carbon adsorption was evaluated (Table S5, Fig. S5). Cd removal efficiency reached 66.8% and28.6% at first and second activated carbon adsorption, respectively (Table S5). The details of regeneration of Cd-contaminated ChXy were illustrated in Fig. S5. So, activated carbon adsorption was found to be an efficient method for regeneration of Cd-contaminated NADESs. The simplicity and ‘green’ nature of regeneration of Cd-contaminated NADESs renders this washing process highly attractive. In particular, it avoids the use and disposal of environmentally harmful reagents. Moreover, the regeneration and reusability of Cd-contaminated NADESs is also one of the essential factors for cost reduction. There are few studies on the removal of heavy metals from rice. Some studies suggest controlling the contaminated soils to reduce the possible health risk (Archana & Sharma, 2016; Clemens et al., 2013; Fu & Wang, 2011). The measures include bioremediation, soil modifier and mechanical turning. Bioremediation is an efficient and green way to solve this problem. However, it is difficult to screen for the appropriate microbes. Besides, using soil modifier would involve some problems such secondary pollution and high expense (Archana & Sharma, 2016; Clemens et al., 2013). NADESs are easy-to-prepare, non-toxic and biodegradable solvents that have been widely used in different fields (Huang et al., 2017 Karimi et al., 2015; Paradiso et al., 2016). In this study, cadmium was removed by washing process which was a simple, inexpensive and time-saving way. In addition, NADES is easy to dissolve in water and can be separated from rice flour very well.
Fig. 3. Effect of liquid–solid ratio on removal of Cdfrom rice flour. Conditions: 0.30 g rice flour, 1.5–7.5 mL ChSor-1% saponin mixtures (v:v = 7:3), 60 °C, 1 h. Removal efficiencies that were significantly higher than that of liquid–solid ratio = 5 are indicated with ** p < .01.
complexation (Isaure et al., 2006; Kaneta et al., 1986). Changes in the proteins due to the Cd removal process were determined by SDS-PAGE. The original rice flour and rice flour washed with the ChSor–saponin mixture did not show obvious changes in regard to the protein components (Fig. S1). FTIR, XRD, and TGA were also used to evaluate the structure and thermal stability of rice flour before and after Cd removal. However, major structural corrosion (or damage) or changes in thermal stability were not observed (Fig. S2–Fig. S4). In general, the NADES washing process did not affect the main chemical components or structure of the rice flour. Therefore, this technique represents a green, relatively simple, and economic approach to solving the Cd contamination problem in rice.
3.7. Mechanism of Cd removal by NADESs Some mechanisms for heavy metal removal by surfactants have been investigated (Fu & Wang, 2011; Mulligan et al., 1999), but metal removal by NADESs remains to be explored. Previously, ζ-potential was used to determine the mechanism of metal removal (AlOmar et al., 2016; Mulligan et al., 1999). To investigate the interactions between the wash solution and solid material, the ζ-potential of rice flour was measured before and after Cd removal (Table S6). It is obvious that ζpotential of rice flour is affected by saponin. The ζ-potential of rice flour decreased from −22.6 mV to −25.6 mV after washing with the ChSorsaponin mixture. The decrease in the ζ-potential was indicative of adsorption (Mulligan et al., 1999). However, the ζ-potential of rice flour did not significantly change after washing with pure ChXy (−23.0 mV) or GlyHis (−21.5 mV). These results indicate that adsorption is one of
3.6. Regeneration of Cd-contaminated NADES ChXy From both a practical and a theoretical viewpoint, the regeneration of Cd-contaminated NADESs is also major issues to be solved. NADES regeneration after washing has not previously been reported. Exploration of environment-friendly and efficient regeneration methods for the heavy metals-contaminated NADESs is challenging work. Activated carbon is one of most efficient adsorbent applied in wastewaters treatment containing organic and inorganic pollutants due to its high degree of porosity and an extended surface area (Covaliu, Matei,
Fig. 4. Potential mechanism for Cd removal by NADESs or NADESs–saponin mixtures.
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Behbahani, M., Abolhasani, J., Amini, M. M., Sadeghi, O., Omidi, F., Bagheri, A., et al. (2015). A). Application of mercapto ordered carbohydrate-derived porous carbons for trace detection of cadmium and copper ions in agricultural products. Food Chemistry, 173, 1207–1212. Behbahani, M., Bagheri, A., Amini, M. M., Sadeghi, O., Salarian, M., Najafi, F., et al. (2013). Application of multiwalled carbon nanotubes modified by diphenylcarbazide for selective solid phase extraction of ultra traces Cd(II) in water samples and food products. Food chemistry, 141, 48–53. Behbahani, M., Hassanlou, P. G., Amini, M. M., Omidi, F., Esrafili, A., Farzadkia, M., et al. (2015). B). Application of solvent-assisted dispersive solid phase extraction as a new, fast, simple and reliable preconcentration and trace detection of lead and cadmium ions in fruit and water samples. Food Chemistry, 187, 82–88. Chavez, E., He, Z. L., Stoffella, P. J., Mylavarapu, R. S., Li, Y. C., Moyano, B., et al. (2015). Concentration of cadmium in cacao beans and its relationship with soil cadmium in southern Ecuador. Science of the Total Environment, 533, 205–214. Clemens, S., Aarts, M. G. M., Thomine, S., & Verbruggen, N. (2013). Plant science: The key to preventing slow cadmium poisoning. Trends in Plant Science, 18(2), 92–99. Covaliu, C. I., Matei, E., Georgescu, G., Mălăeru, T., & Biriş, S.Ş. (2016). Evaluation of powdered activated carbon performance for wastewater treatment containing organic (C6H6 and C6H5-CH3) and inorganic (Pb+2and Zn+2) pollutions. Environmental Engineering & Management Journal, 15(5), 1003–1008. Fu, F., & Wang, Q. (2011). Removal of heavy metal ions from wastewaters: A review. Journal of Environmental Management, 92, 407–418. Gunduz, S., & Akman, S. (2013). Determination of lead in rice grains by solid sampling HR-CS GFAAS. Food Chemistry, 141, 2634–2638. Huang, Y., Feng, F., Jiang, J., Qiao, Y., Wu, T., Voglmeir, J., et al. (2017). Green and efficient extraction of rutin from tartary buckwheat hull by using natural deep eutectic solvents. Food Chemistry, 221, 1400–1405. Huo, Y., Du, H., Xue, B., Niu, M., & Zhao, S. (2016). Cadmium removal from rice by separating and washing protein isolate. Journal of Food Science, 81(6), 1576–1584. Ignat, M., Fortuna, M. E., Sacarescu, L., Zaltariov, M. F., & Harabagiu, V. (2015). PNiPAM-functionalized mesoporous carbon for the adsorption of vitamin B2. Environmental Engineering and Management Journal, 14(3), 607–613. Isaure, M. P., Fayard, B., Sarret, G., Pairis, S., & Bourguignon, J. (2006). Localization and chemical forms of cadmium in plant samples by combining analytical electron microscopy and X-ray spectromicroscopy. Spectrochimica Acta Part B: Atomic Spectroscopy, 61(12), 1242–1252. Jorhem, L., Åstrand, C., Sundström, B., Baxter, M., Stokes, P., Lewis, J., et al. (2008). Elements in rice from the Swedish market: 1. Cadmium, lead and arsenic (total and inorganic). Food Additives and Contaminants, 25(3), 284–292. Kaneta, M., Hikichi, H., Endo, S., & Sugiyama, N. (1986). Chemical form of cadmium (and other heavy metals) in rice and wheat plants. Environmental Health Perspectives, 65, 33–37. Karimi, M., Dadfarnia, S., Shabani, A. M. H., Tamaddon, F., & Azadi, D. (2015). Deep eutectic liquid organic salt as a new solvent for liquid-phase microextraction and its application in ligandless extraction and preconcentration of lead and cadmium in edible oils. Talanta, 144, 648–654. Moghaddam, R. H., Shabani, A. M. H., Dadfarnia, S., & Baghban, N. (2014). Cold vapor atomic absorption spectrometric determination of cadmium after solid phase extraction on modified TiO2 nanoparticles. Journal of the Brazilian Chemical Society, 25(11), 1975–1983. Mukhopadhyay, S., Mukherjee, S., Adnan, N. F., Hayyan, A., Hayyan, M., Hashim, M. A., et al. (2016). Ammonium-based deep eutectic solvents as novel soil washing agent for lead removal. Chemical Engineering Journal, 294, 316–322. Mukhopadhyay, S., Mukherjee, S., Hayyan, A., Hayyan, M., Hashim, M. A., & Sen Gupta, B. (2016). B). Enhanced removal of lead from contaminated soil by polyol-based deep eutectic solvents and saponin. Journal of Contaminant Hydrology, 194, 17–23. Mulligan, C. N., Yong, R. N., Gibbs, B. F., James, S., & Bennett, H. P. J. (1999). Metal removal from contaminated soil and sediments by the biosurfactant surfactin. Environmental Science & Technology, 33(21), 3812–3820. Omidi, F., Behbahani, M., Bojdi, M. K., & Shahtaheri, S. J. (2015). Solid phase extraction and trace monitoring of cadmium ions in environmental water and food samples based on modified magnetic nanoporous silica. Journal of Magnetism and Magnetic Materials, 395, 213–220. Paradiso, V. M., Clemente, A., Summo, C., Pasqualone, A., & Caponio, F. (2016). Towards green analysis of virgin olive oil phenolic compounds: Extraction by a natural deep eutectic solvent and direct spectrophotometric detection. Food Chemistry, 212, 43–47. Suhagia, B. N., Rathod, I. S., & Sindhu, S. (2011). Sapindusmukorossi (Areetha): An overview. International Journal of Pharmaceutical Sciences and Research, 2, 1905–1913. Wu, Y., He, R., Wang, Z., Yuan, J., Xing, C., Wang, L., et al. (2016). A safe, efficient and simple technique for the removal of cadmium from brown rice flour with citric acid and analyzed by inductively coupled plasma mass spectrometry. Analytical Methods, 8(33), 6313–6322. Zarezade, V., Behbahani, M., Omidi, F., Abandansari, H. S., & Hesam, G. (2016). A new magnetic tailor made polymer for separation and trace determination of cadmium ions by flame atomic absorption spectrophotometry. RSC Advances, 6, 103499–103507. Zhao, B. Y., Xu, P., Yang, F. X., Wu, H., Zong, M. H., & Lou, W. Y. (2015). Biocompatible deep eutectic solvents based on choline chloride: Characterization and application to the extraction of rutin from Sophora japonica. ACS Sustainable Chemistry & Engineering, 3, 2746–2755. Zhuang, P., Zhang, C., Li, Y., Zou, B., Mo, H., Wu, K., et al. (2016). Assessment of influences of cooking on cadmium and arsenic bioaccessibility in rice, using an in vitro physiologically-based extraction test. Food Chemistry, 213, 206–214.
the mechanisms of Cd removal by NADESs in the presence of saponin. The mechanisms for Cd removal by NADESs may be complicated due to the various components of NADESs. Based on the aforementioned results, we postulated that the removal of Cd from rice flour by NADESs occurs via two processes, namely lysis and complexation (Mukhopadhyay, Mukherjee, Adnan, et al., 2016; Mulligan et al., 1999; Wu et al., 2016). Acidic or mildly acidic NADESs (e.g., ChTar, ChXy, and ChSor) acted as Lewis acids, which preferentially interacted with the electron donors in rice flour, thereby pulling the Cd ions out of the rice flour via interactions with NADESs anions in the washing solution. However, when the slightly alkaline glycerol-based NADESs were introduced into the system, H+ ions were not supplied owing to the alkaline nature, and Cd-rice flour complex was not disrupted. Therefore, the Cd2+ removals were negligible for the glycerol-based NADESs. When NADESs were mixed with saponin, which is naturally acidic, more H+ ions were introduced into the system. Saponin also encapsulated Cd2+ via micelle formation (Mukhopadhyay, Mukherjee, Adnan, et al., 2016; Mulligan et al., 1999). The synergistic effect between saponin and NADESs, therefore, improved the Cd removal. A potential mechanism for Cd removal by NADESs and NADES–saponin mixtures is illustrated in Fig. 4. Further studies will be carried out in the future in order to confirm the proposed mechanism. 4. Conclusions Herein, a rapid, green, and efficient method was established for the removal of Cd from rice flour using NADESs and NADESs–saponin mixtures. The optimal conditions for the rice flour washing were established. A Cd removal above 96% was obtained using NADES ChTar. A synergistic effect between saponin and choline chloride-based NADESs was observed and > 99% Cd was removed using a ChSor-saponin mixture. The mechanism for the Cd removal by NADESs was also studied. Moreover, the washing process did not affect the main chemical components or structure of the rice flour. So, it can be used in food industry. Moreover, NADESs are easy to dissolve in water and can be separated from rice flour very well. The content of NADESs in rice flour can be reduced to safety level which will not cause secondary pollution. If further scale-up is possible, the proposed method may have numerous applications in the food industry. Acknowledgements This work was partly supported by Qing Lan Project, PAPD, and the SRF for ROCS. Conflict of interest Authors declare no existing conflict of interest. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.foodchem.2017.10.060. References AlOmar, M. K., Alsaadi, M. A., Hayyan, M., Akib, S., Ibrahim, R. K., & Hashim, M. A. (2016). Lead removal from water by choline chloride based deep eutectic solvents functionalized carbon nanotubes. Journal of Molecular Liquids, 222, 883–894. Archana, G., & Sharma, R. K. (2016). Cadmium minimization in food crops by cadmium resistant plant growth promoting rhizobacteria. Applied Soil Ecology, 107, 66–78. Baghban, N., Shabani, A. M. H., & Dadfarnia, S. (2012). Solid phase extraction of trace amounts of cadmium with cetyltrimethylammonium bromide-coated magnetic nanoparticles prior to its determination by flame atomic absorption spectrometry. Journal of the Chinese Chemical Society, 59, 782–787.
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Green and efficient removal of cadmium from rice flour using natural deep eutectic solvents ⁎
MARK
⁎
Yao Huanga,1, Fang Fenga,1, Zhi-Gang Chena, , Tao Wub, , Zi-Han Wanga a b
Glycomics and Glycan Bioengineering Research Center, College of Food Science & Technology, Nanjing Agricultural University, Nanjing 210095, PR China The Department of Food Science, The University of Tennessee, 2510 River Drive, Knoxville, TN 37996, United States
A R T I C L E I N F O
A B S T R A C T
Keywords: Natural deep eutectic solvents Cadmium Removal Rice flour
Natural deep eutectic solvents (NADESs) constitute a novel class of biodegradable and inexpensive solvents. In this study, twenty choline chloride- and glycerol-based NADESs were prepared and investigated as washing agents in the removal of cadmium (Cd) from rice flour for the first time. Choline chloride-based NADESs demonstrated good Cd removal (51%–96%). A natural, biodegradable surfactant, saponin, was mixed with the NADESs to enhance their efficiency. No increase in Cd removal was observed when glycerol-based NADESs were combined with 1% saponin; however, synergistic effects between saponin and choline chloride-based NADESs were observed during the washing process and > 99% Cd was removed using NADES-saponin mixtures. Moreover, NADESs washing process did not affect the main chemical components or structure of rice flour. The mechanism of Cd removal by NADESs and regeneration of Cd-contaminated NADESs were also explored. The study presents a green and efficient way of removing Cd from contaminated rice.
1. Introduction Various human activities leaded to Cd pollution of land, ground water and atmosphere such as mining, smelting, fertilization, and so on (Archana & Sharma, 2016; Clemens, Aarts, Thomine, & Verbruggen, 2013; Fu & Wang, 2011). Industrial waste produced by human activities is the main source of Cd pollution of agricultural soils. However, heavy metals which cannot be degraded by microorganisms tend to persist and accumulate in the soils (Archana & Sharma, 2016; Clemens et al., 2013). Rice is easier to accumulate Cd from soil compared with other cereal crops. (Archana & Sharma, 2016; Clemens et al., 2013; Huo, Du, Xue, Niu, & Zhao, 2016; Jorhem et al., 2008; Wu et al., 2016; Zhuang et al., 2016). Rice is an important food commodity in the international market, and is a major staple for about half of the world’s population (Gunduz & Akman, 2013; Zhuang et al., 2016). Cd accumulation in rice is a global issue. The toxicity of Cd is well established, and Cd can accumulate in human kidneys (Archana & Sharma, 2016; Chavez et al., 2015; Clemens et al., 2013). Cd exhibits a long biological half-life of 10–30 years, and is classified as a group 1 carcinogen (Archana & Sharma, 2016; Clemens et al., 2013). Therefore, rice safety has garnered considerable attention in many countries. The Cd concentration in rice should be below accepted limits (0.4 mg/kg as an international standard limit and 0.2 mg/
⁎
1
kg in China) (Huo et al., 2016; Jorhem et al., 2008; Wu et al., 2016). Excess Cd should be removed from Cd-polluted rice grains or their products to reduce the risks to the public. To date, several methods, such as breeding low Cd- accumulating rice cultivars, soil remediation, and phytoremediation have been developed to reduce the accumulation of Cd in rice (Archana & Sharma, 2016; Clemens et al., 2013). However, rice containing a high concentration of Cd is still being produced in Cdcontaminated areas in the world every year. Thus, it is imperative to develop simple and efficient decontamination methods for Cd-polluted rice. Extraction and detection of heavy metals in water, soil, and some agricultural products have been reported by several research groups (Baghban, Shabani, & Dadfarnia, 2012; Behbahani et al., 2013; Behbahani, Abolhasani, et al., 2015; Behbahani, Bagheri, et al., 2015; Moghaddam, Shabani, Dadfarnia, & Baghban, 2014; Mukhopadhyay, Mukherjee, Adnan, et al., 2016; Mukhopadhyay, Mukherjee, Hayyan, et al., 2016; Omidi, Behbahani, Bojdi, & Shahtaheri, 2015; Zarezade, Behbahani, Omidi, Abandansari, & Hesam, 2016). Washing rice grains or soil contaminated with heavy metals and organics is a widely accepted practice (Huo et al., 2016; Mulligan, Yong, Gibbs, James, & Bennett, 1999; Wu et al., 2016), but efficient and environmentally friendly techniques are rare. Recently, natural deep eutectic solvents (NADESs) obtained from natural components have
Corresponding authors. E-mail addresses: [email protected] (Z.-G. Chen), [email protected] (T. Wu). These authors contributed equally to this work.
http://dx.doi.org/10.1016/j.foodchem.2017.10.060 Received 1 May 2017; Received in revised form 17 September 2017; Accepted 9 October 2017 Available online 12 October 2017 0308-8146/ © 2017 Elsevier Ltd. All rights reserved.
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compositions of the NADESs are given in Table 1. NADESs were prepared according to a previously published method (Huang et al., 2017).
emerged as green, versatile solvents for various applications (Huang et al., 2017; Karimi, Dadfarnia, Shabani, Tamaddon, & Azadi, 2015; Paradiso, Clemente, Summo, Pasqualone, & Caponio, 2016; Zhao et al., 2015). Components of NADESs are edible, and NADESs can be classified as “readily biodegradable” solvents (Huang et al., 2017). However, NADESs have not been used as washing agents for contaminant removal from food matrices to date. Therefore, in this study, twenty choline chloride- and glycerol-based NADESs were synthesized and used as washing agents for Cd removal from contaminated rice flour. Moreover, saponin, a natural biodegradable surfactant, is environmentally friendly and has been used in the food and medical industries for many years (Mukhopadhyay, Mukherjee, Hayyan, et al., 2016; Suhagia, Rathod, & Sindhu, 2011). The effect and mechanism of surfactant on heavy metal removal have been reported (Fu & Wang, 2011). So, NADESs were combined with saponin, in order to investigate synergistic effects. The purpose of this study was to develop a novel, inexpensive, and efficient method for removing Cd from rice.
2.3. Sample treatment Cd-contaminated rice flour was dried under vacuum at 80 °C for 4 h. For each experiment, 0.30 g of rice flour was washed with 2.0 mL of the washing solution (NADES, surfactant solutions, and NADES-surfactant mixtures) in a 10.0 mL centrifuge tube. The test tubes were heated to 60 °C and shaken at 100 rpm for 1 h. During this period, ultrasound assisted extraction was performed every 15 min for 5 min using an ultrasonicator (20 kHz, 200 W; Type NP-B-400-15; New Power Co., Ltd., Kunshan, China) equipped with a digital timer and a temperature controller. The water bath temperature was maintained within ± 1 °C. The washing solution was then centrifuged (12,000 rpm, 20 min), and the supernatant was discarded. Ultrapure water (2.0 mL) was added to the tube and it was briefly vortexed, and then centrifuged at 12,000 rpm for 10 min. After centrifugation, the precipitate was transferred to polytetrafluoroethylene (PTFE) digestion tubes (60.0 mL inner volume). The rice flour was further digested using a microwave-assisted digestion system. For comparison, ultrapure water was also used as a washing solvent. The concentrations of surfactant saponin aqueous solutions used in this study were 1% (w/w). NADESs -saponin systems consist of 1% saponin aqueous solution and NADESs by mixing. To minimize the risk of metal contamination, all glass ware was soaked in 5% HNO3 (v/v) for at least 24 h. PTFE digestion tubes were soaked in 20% HNO3 (v/v) overnight prior to digestion. The water used in this study was ultrapure water.
2. Materials and methods 2.1. Reagents and materials Cd-contaminated rice flour sample (Cd: 2.16 mg kg−1) was purchased from National Standard Material Center Network (Beijing, China). Cadmium nitrate was purchased from Macklin Reagent Co., Ltd. (Shanghai, China). Concentrated nitric acid (Guaranteed reagent) was supplied by Nanjing Chemical Reagent Co., Ltd. (Nanjing, China). Compounds for NADESs preparation, including choline chloride (≥98.0%), glycerol (≥99.0%), D-(+)-xylose (≥99.0%), Glucose (≥99.0%), fructose (≥99.0%), L-(−)-sorbose (≥99.0%), mannose (≥99.0%), D-(+)-galactose (≥99.0%), sucrose (≥99.0%), L-(+)- arabinose (≥99.0%), L-(+)- rhamnose (≥99.0%), trehalose (≥99.0%), proline (≥99.0%), L-alanine (≥99.0%), glycine (≥99.0%), L-threonine (≥99.0%), L-histidine (≥99.0%), DL-malic acid (≥99.0%), citric acid (≥99.5%), L-(+)-tartaric acid (≥99.5%), xylitol (≥99.0%), sorbitol (≥99.0%), and saponin (biological reagent) were obtained from Aladdin Chemical Reagent Co., Ltd. (Shanghai, China).
2.4. Sample microwave digestion and determination In the microwave digestion, a conventional acid digestion method was used for the determination of Cd in rice flour. Concentrated nitric acid (8.0 mL) was added to the PTFE digestion tubes containing precipitate. The operating conditions for the MARS6XP1600 microwaveassisted digestion system (USA, CEM) are presented in Table S1. After digestion, a clear solution was obtained. The interior walls of the tube were washed with a minimum amount of ultrapure water. After cooling, the solution was filtered through a 0.45 μm filter and then transferred to a 50.0 mL volumetric flask and diluted with ultrapure water. The concentrations of Cd in rice flour were determined without
2.2. Preparation of NADESs Twenty NADESs were tested in the preliminary study. The Table 1 List of different NADESs prepared from natural products. NADES Abbreviation
Components Component 1
GlyPro GlyAla GlyGly GlyThr GlyHis ChCit ChMal ChTar ChXy ChSo ChXyl ChGlu ChFru ChSor ChMan ChGal ChSur ChAra ChRh ChTre a
Glycerol Glycerol Glycerol Glycerol Glycerol Choline Chloride Choline Chloride Choline Chloride Choline Chloride Choline Chloride Choline Chloride Choline Chloride Choline Chloride Choline Chloride Choline Chloride Choline Chloride Choline Chloride Choline Chloride Choline Chloride Choline Chloride
Component 2
Component 3
Proline
–a – – – – Water Water Water Water Water Water Water Water Water Water Water Water Water Water Water
L-Alanine
Glycine L-Threonine L-Histidine
Citric acid acid L-(+)-Tartaric acid Xylitol Sorbitol D-(+)-Xylose Glucose Fructose L-(−)-Sorbose Mannose D-(+)-Galactose Sucrose L-(+)-Arabinose L-(+)-Rhamnose Trehalose DL-Malic
No water was added.
261
Mole ratio
pH
3:1 3:1 3:1 3:1 3:1 1:1:2 1:1:2 1:1:2 5:2:5 5:2:5 3:1:3 5:2:5 5:2:5 5:2:5 5:2:5 5:2:5 4:1:4 5:2:5 2:1:2 4:1:4
7.74 7.34 7.27 6.48 7.09 0.09 0.23 0.56 6.88 6.33 5.16 4.97 4.36 4.55 5.06 5.04 4.77 4.60 5.09 4.88
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Fig. 1. Effect of different NADESs on the removal of Cd from rice flour. Conditions: 0.30 g rice flour, 2 mL NADESs, 60 °C, 1 h.
100
Cd removal (%)
80
60
40
20
G ly P G ro ly A G la ly G G ly ly Th G r ly H Ch is M a Ch l Ci Ch t Ta r Ch X y Ch So Ch X y Ch l G lu Ch Fr u Ch So Ch r M a Ch n G a Ch l Su Ch c A r Ch a Rh a Ch T Sa re po ni n W at er
0
2.8. Statistical analysis
chemical modifier using a graphite furnace atomic absorption spectrophotometer (GFAAS).
Overall, 3 independent trials were carried out during the study and the results are given as means and standard deviations. Analysis of variance (ANOVA) of the data was carried out using the SAS program (SAS Institute Inc., Carry, N.C., USA) to compare significant differences (p < .05 and p < .01) between the results.
2.5. GFAAS Analysis The concentration of cadmium in the digestion solution was determined by a GFAAS model A3 equipped with a CW-1Y cooling circulator (Beijing Purkinjie General Instrument Co., Ltd., Beijing, China). Argon (99.99%) was used as the purge gas. Sample solutions (10.0 μL) were added to the furnace. The experiments were performed at 228.8 nm. The operating conditions for the graphite furnace are presented in Table S2.
3. Results and discussion 3.1. Effects of NADESs on removal of Cd In rice, Cd primarily associates with organic compounds (mainly proteins) via complexation (Isaure, Fayard, Sarret, Pairis, & Bourguignon, 2006; Kaneta, Hikichi, Endo, & Sugiyama, 1986). Organic matter in rice tends to act as an electron donor and binds Cd2+. The pH of an aqueous suspension of rice flour is 6.35, indicating the weakly acidic nature of the rice flour. Therefore, the successful extraction of Cd depends on the ability of the washing agent to disrupt the interactions between organic compounds and Cd. NADESs are touted as designer solvents, and the primary advantage over conventional solvents is their tunability (Huang et al., 2017; Zhao et al., 2015). The selectivity of NADESs for target compounds can be tailored by changing their components. Twenty different choline chloride- and glycerol-based NADESs (Table 1) were prepared and evaluated in the removal of Cd from rice flour. As shown in Fig. 1, the type of NADES had a considerable effect on the removal efficiency of Cd (for each extraction procedure, RSD ≤ 5%). Five glycerol- based NADESs exhibited low Cd removal efficiencies (≤51%); however, choline chloride-based NADESs demonstrated good Cd removals (51%–96%). For example, the Cd removal efficiency reached 95.9 ± 2.2% and 90.4 ± 1.3% when ChTar and ChXy were used as washing agents, respectively. The removal of Cd using washing agents may involve two processes, namely lysis and complexation (Mukhopadhyay, Mukherjee, Adnan, et al., 2016). The organic material in rice flour acts as an electron donating Lewis base and interacts with Cd2+ ions, which are Lewis acids. The NADESs used in the present study either had acidic pH values (e.g., choline chloride-based NADESs), or weakly alkaline pH values (e.g., glycerol-based NADESs) (Table 1). When acidic NADESs such as ChTar and ChMal were added to the Cd2+ contaminated rice flour, H+ ions
2.6. Characterization of rice flour before and after Cd removal The starch and crude protein contents of the NADES-treated rice flour were determined according to AOAC methods. Changes in the proteins in rice flour due to the Cd removal process were determined by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE, Bio-Rad Laboratories, USA). The rice flour was also evaluated using Fourier Transform infrared spectroscopy (FTIR) (Vector 33 spectrometer, Bruker Company, Germany). X-ray powder diffraction (XRD, Xray diffractometer, Thermo Fisher, USA) was used to inspect the structural changes of the rice flour before and after Cd removal. The thermal stability of the sample was studied via thermogravimetric analysis (TGA) using a HTG-1thermal analyzer (Heng Jiu Science Instrument Factory, Beijing, China). 2.7. Regeneration of Cd-contaminated NADES ChXy ChXy with high Cd removal efficiency (90.35%) and low viscosity was selected in this study. The Cd-contaminated rice flour (0.3 g) was added to ChXy (2 mL) in a 10 mL centrifuge tube. Washing process was then carried out as described in “2.3. Sample treatment”. The supernatant of each centrifugation was collected, and then adsorbed by 0.2 g activated carbon. The tube containing Cd-contaminated ChXy and activated carbon was placed in gas bath thermostatic oscillator (model SHZ-82A, Kexi Laboratory Instruments, Jintan, China) for 2 h at 25 °C with shaking speed of 180 rpm. The mixture was filtered through a 0.45 μm membrane. The filtrate (regenerated ChXy) was reused. Measurements were performed in triplicate. 262
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Table 2 Effect of NADESs-1% saponin mixtures (v:v = 9:1) on removal of Cdfrom rice flour.a Wash solutions
Removal (%)
GlyPro GlyAla GlyGly GlyThr GlyHis ChCit ChSor ChSuc GlyPro+1%Saponin GlyAla+1%Saponin GlyGly+1%Saponin GlyThr+1%Saponin GlyHis+1%Saponin ChCit+1%Saponin ChSor+1%Saponin ChSuc+1%Saponin
1.08 ± 0.23 < 1.00 34.55 ± 3.33 23.41.57 51.29 ± 2.55 73.61 ± 5.99 56.87 ± 1.14 51.29 ± 0.56 2.62 ± 0.17b < 1.00 36.51 ± 2.11 25.72 ± 2.53 42.03 ± 1.61 80.46 ± 1.66 72.27 ± 2.42 66.03 ± 2.66
Fig. 2. Effect of 1% saponin solution content in ChSor on removal of Cd from rice flour. Conditions: 0.30 g rice flour, 2 mL ChSor-1% saponin mixtures, 60 °C, 1 h. Removal efficiencies that were significantly higher than that of ChSor are indicated with *p < .05, ** p < .01, and ***p < .001.
a Conditions: 0.30 g rice flour, 2 mL NADESs or NADESs-1% saponin mixtures (v:v = 9:1), 60 °C, 1 h. b Mean ± SD (n = 3).
percentage of 1% saponin aqueous solution (Table S3). As mentioned above, the physical properties of ChSor can be modified by the addition of small quantities of 1% saponin solution. In this study, ChSor-1% saponin mixtures with saponin fractions ranging from 0% to 50% (v/v) were evaluated in the removal of Cd (Fig. 2). As shown in Fig. 2, the Cd removal improved with increasing proportions of 1% saponin solution, up to 87.7% (with 30% saponin solution, v/v), and remained stable thereafter. Many factors including solubilization, sorption, interfacial tension, and viscosity can affect cadmium removal rate. One of factors contributing to the increase in cadmium removal rate may be attributed to the decrease in washing solvent viscosity. The addition of 1% saponin aqueous solution (< 10%, v/v) led to a decrease in the viscosity of the washing media (Table S3), thus enhancing the mass transfer, and increasing the removal efficiency. There is no significant change in the viscosity of mixtures after adding 20%–30% (v/ v) saponin aqueous solutions (Table S3), but the Cd removal efficiency increased from 70.27% to 87.38%, mainly due to an increase in saponin content. However, high saponin aqueous solution contents (> 30%, v/ v) did not further increase the removal efficiency of Cd.
were introduced. The H+ ions attacked the electron rich sites of the rice flour, and competed with the Cd2+ ions for electrons, thereby pulling out the Cd2+ ions from the rice flour via interactions with the NADES anions present in the washing solution. However, when the slightly alkaline, glycerol-based NADESs were introduced into the system, H+ ions were not supplied. Therefore, the Cd2+ removal was negligible for the glycerol-based NADESs. Water and 1.0% saponin aqueous solutions were used as controls. Cd was strongly bound to the rice flour and water could remove only up to 18% of Cd. A 1% saponin aqueous solution removed up to 68% Cd (Fig. 1). 3.2. Effect of NADES-1% saponin mixture on Cd removal As shown in Fig. 1, saponin could be added to water to improve the C d removal. The pH of the 1% saponin aqueous solution was 2.9, indicating the acidic nature. Therefore, the 1% saponin aqueous solution was mixed with NADESs to enhance the Cd removal efficiency. No increase in the Cd removal was observed when glycerol-based NADESs were combined with 1% saponin aqueous solution (v:v = 9:1) (Table 2). However, a synergistic effect between saponin and choline chloride-based NADESs was observed. For example, NADES ChSor demonstrated a Cd removal of 56%, but up to 72% Cd could be removed with a combination of ChSor and 1% saponin aqueous solution (v:v = 9:1). The synergistic effect between saponin and choline chloride-based NADESs improved the Cd removal. Many factors contributed to the increased Cd removal such as (a) pH, (b) encapsulation of Cd2+ by saponin micelles (c) dilution of NADESs and concomitant facilitation of the removal of loosened Cd from the rice flour surface by water (Mukhopadhyay, Mukherjee, Adnan, et al., 2016). In order to obtain deeper insight into the Cd removal with NADES- saponin mixtures, the ChSor-saponin mixture was selected for subsequent studies.
3.4. Effect of liquid–solid ratio on the removal efficiency of Cd The removal of Cd from rice flour using different liquid (ChSor–saponin mixture)–solid (rice flour) ratios is depicted in Fig. 3.The removal efficiency increased with increasing liquid–solid ratios up to 15.0 mL g−1; no significant improvement was observed with higher liquid–solid ratios (> 15.0 mL g−1). Thus, a liquid–solid ratio of 15.0 mL g−1 was sufficient to obtain a high removal efficiency (> 99%) of Cd. 3.5. Characterization of rice flour before and after washing Next we evaluated the effect of NADES treatment on the quality and structure of rice flour. Starch and protein are the main chemical components of rice flour, and changes in their content affect the physicochemical properties and qualities of rice flour. After the removal of Cd with ChSor and the ChSor-saponin mixture (v:v = 7:3), the rice flour yield was 95% and 91%, respectively. As shown in Table S4, the change in the starch content between the original sample and ChSor or ChSorsaponin mixture (v:v = 7:3) treated samples were not significant. In fact, treatment with ChSor or the ChSor-saponin mixture (v:v = 7:3) can affect the protein content, decreasing by 8.5% and 11.7% respectively after washing. Decrease in protein content of rice flour after washing may be that Cd primarily associates with proteins via
3.3. Effect of 1% saponin aqueous solution content in ChSor on removal of Cd One disadvantage of NADESs is their high viscosity, which not only hinders mass transport but also leads to handling difficulties (Huang et al., 2017). In addition to the molar ratio of the components, the water content also affects the viscosity of eutectic mixtures. In the case of ChSor with different concentrations of the 1% saponin aqueous solution (v/v), its viscosity decreased by∼1/6when diluted with 10% 263
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Georgescu, Mălăeru, & Biriş, 2016; Ignat, Fortuna, Sacarescu, Zaltariov, & Harabagiu, 2015). In this study, NADES ChXy was selected because of high Cd removal efficiency (90.4%). The regeneration of Cdcontaminated ChXy with activated carbon adsorption was evaluated (Table S5, Fig. S5). Cd removal efficiency reached 66.8% and28.6% at first and second activated carbon adsorption, respectively (Table S5). The details of regeneration of Cd-contaminated ChXy were illustrated in Fig. S5. So, activated carbon adsorption was found to be an efficient method for regeneration of Cd-contaminated NADESs. The simplicity and ‘green’ nature of regeneration of Cd-contaminated NADESs renders this washing process highly attractive. In particular, it avoids the use and disposal of environmentally harmful reagents. Moreover, the regeneration and reusability of Cd-contaminated NADESs is also one of the essential factors for cost reduction. There are few studies on the removal of heavy metals from rice. Some studies suggest controlling the contaminated soils to reduce the possible health risk (Archana & Sharma, 2016; Clemens et al., 2013; Fu & Wang, 2011). The measures include bioremediation, soil modifier and mechanical turning. Bioremediation is an efficient and green way to solve this problem. However, it is difficult to screen for the appropriate microbes. Besides, using soil modifier would involve some problems such secondary pollution and high expense (Archana & Sharma, 2016; Clemens et al., 2013). NADESs are easy-to-prepare, non-toxic and biodegradable solvents that have been widely used in different fields (Huang et al., 2017 Karimi et al., 2015; Paradiso et al., 2016). In this study, cadmium was removed by washing process which was a simple, inexpensive and time-saving way. In addition, NADES is easy to dissolve in water and can be separated from rice flour very well.
Fig. 3. Effect of liquid–solid ratio on removal of Cdfrom rice flour. Conditions: 0.30 g rice flour, 1.5–7.5 mL ChSor-1% saponin mixtures (v:v = 7:3), 60 °C, 1 h. Removal efficiencies that were significantly higher than that of liquid–solid ratio = 5 are indicated with ** p < .01.
complexation (Isaure et al., 2006; Kaneta et al., 1986). Changes in the proteins due to the Cd removal process were determined by SDS-PAGE. The original rice flour and rice flour washed with the ChSor–saponin mixture did not show obvious changes in regard to the protein components (Fig. S1). FTIR, XRD, and TGA were also used to evaluate the structure and thermal stability of rice flour before and after Cd removal. However, major structural corrosion (or damage) or changes in thermal stability were not observed (Fig. S2–Fig. S4). In general, the NADES washing process did not affect the main chemical components or structure of the rice flour. Therefore, this technique represents a green, relatively simple, and economic approach to solving the Cd contamination problem in rice.
3.7. Mechanism of Cd removal by NADESs Some mechanisms for heavy metal removal by surfactants have been investigated (Fu & Wang, 2011; Mulligan et al., 1999), but metal removal by NADESs remains to be explored. Previously, ζ-potential was used to determine the mechanism of metal removal (AlOmar et al., 2016; Mulligan et al., 1999). To investigate the interactions between the wash solution and solid material, the ζ-potential of rice flour was measured before and after Cd removal (Table S6). It is obvious that ζpotential of rice flour is affected by saponin. The ζ-potential of rice flour decreased from −22.6 mV to −25.6 mV after washing with the ChSorsaponin mixture. The decrease in the ζ-potential was indicative of adsorption (Mulligan et al., 1999). However, the ζ-potential of rice flour did not significantly change after washing with pure ChXy (−23.0 mV) or GlyHis (−21.5 mV). These results indicate that adsorption is one of
3.6. Regeneration of Cd-contaminated NADES ChXy From both a practical and a theoretical viewpoint, the regeneration of Cd-contaminated NADESs is also major issues to be solved. NADES regeneration after washing has not previously been reported. Exploration of environment-friendly and efficient regeneration methods for the heavy metals-contaminated NADESs is challenging work. Activated carbon is one of most efficient adsorbent applied in wastewaters treatment containing organic and inorganic pollutants due to its high degree of porosity and an extended surface area (Covaliu, Matei,
Fig. 4. Potential mechanism for Cd removal by NADESs or NADESs–saponin mixtures.
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Behbahani, M., Abolhasani, J., Amini, M. M., Sadeghi, O., Omidi, F., Bagheri, A., et al. (2015). A). Application of mercapto ordered carbohydrate-derived porous carbons for trace detection of cadmium and copper ions in agricultural products. Food Chemistry, 173, 1207–1212. Behbahani, M., Bagheri, A., Amini, M. M., Sadeghi, O., Salarian, M., Najafi, F., et al. (2013). Application of multiwalled carbon nanotubes modified by diphenylcarbazide for selective solid phase extraction of ultra traces Cd(II) in water samples and food products. Food chemistry, 141, 48–53. Behbahani, M., Hassanlou, P. G., Amini, M. M., Omidi, F., Esrafili, A., Farzadkia, M., et al. (2015). B). Application of solvent-assisted dispersive solid phase extraction as a new, fast, simple and reliable preconcentration and trace detection of lead and cadmium ions in fruit and water samples. Food Chemistry, 187, 82–88. Chavez, E., He, Z. L., Stoffella, P. J., Mylavarapu, R. S., Li, Y. C., Moyano, B., et al. (2015). Concentration of cadmium in cacao beans and its relationship with soil cadmium in southern Ecuador. Science of the Total Environment, 533, 205–214. Clemens, S., Aarts, M. G. M., Thomine, S., & Verbruggen, N. (2013). Plant science: The key to preventing slow cadmium poisoning. Trends in Plant Science, 18(2), 92–99. Covaliu, C. I., Matei, E., Georgescu, G., Mălăeru, T., & Biriş, S.Ş. (2016). Evaluation of powdered activated carbon performance for wastewater treatment containing organic (C6H6 and C6H5-CH3) and inorganic (Pb+2and Zn+2) pollutions. Environmental Engineering & Management Journal, 15(5), 1003–1008. Fu, F., & Wang, Q. (2011). Removal of heavy metal ions from wastewaters: A review. Journal of Environmental Management, 92, 407–418. Gunduz, S., & Akman, S. (2013). Determination of lead in rice grains by solid sampling HR-CS GFAAS. Food Chemistry, 141, 2634–2638. Huang, Y., Feng, F., Jiang, J., Qiao, Y., Wu, T., Voglmeir, J., et al. (2017). Green and efficient extraction of rutin from tartary buckwheat hull by using natural deep eutectic solvents. Food Chemistry, 221, 1400–1405. Huo, Y., Du, H., Xue, B., Niu, M., & Zhao, S. (2016). Cadmium removal from rice by separating and washing protein isolate. Journal of Food Science, 81(6), 1576–1584. Ignat, M., Fortuna, M. E., Sacarescu, L., Zaltariov, M. F., & Harabagiu, V. (2015). PNiPAM-functionalized mesoporous carbon for the adsorption of vitamin B2. Environmental Engineering and Management Journal, 14(3), 607–613. Isaure, M. P., Fayard, B., Sarret, G., Pairis, S., & Bourguignon, J. (2006). Localization and chemical forms of cadmium in plant samples by combining analytical electron microscopy and X-ray spectromicroscopy. Spectrochimica Acta Part B: Atomic Spectroscopy, 61(12), 1242–1252. Jorhem, L., Åstrand, C., Sundström, B., Baxter, M., Stokes, P., Lewis, J., et al. (2008). Elements in rice from the Swedish market: 1. Cadmium, lead and arsenic (total and inorganic). Food Additives and Contaminants, 25(3), 284–292. Kaneta, M., Hikichi, H., Endo, S., & Sugiyama, N. (1986). Chemical form of cadmium (and other heavy metals) in rice and wheat plants. Environmental Health Perspectives, 65, 33–37. Karimi, M., Dadfarnia, S., Shabani, A. M. H., Tamaddon, F., & Azadi, D. (2015). Deep eutectic liquid organic salt as a new solvent for liquid-phase microextraction and its application in ligandless extraction and preconcentration of lead and cadmium in edible oils. Talanta, 144, 648–654. Moghaddam, R. H., Shabani, A. M. H., Dadfarnia, S., & Baghban, N. (2014). Cold vapor atomic absorption spectrometric determination of cadmium after solid phase extraction on modified TiO2 nanoparticles. Journal of the Brazilian Chemical Society, 25(11), 1975–1983. Mukhopadhyay, S., Mukherjee, S., Adnan, N. F., Hayyan, A., Hayyan, M., Hashim, M. A., et al. (2016). Ammonium-based deep eutectic solvents as novel soil washing agent for lead removal. Chemical Engineering Journal, 294, 316–322. Mukhopadhyay, S., Mukherjee, S., Hayyan, A., Hayyan, M., Hashim, M. A., & Sen Gupta, B. (2016). B). Enhanced removal of lead from contaminated soil by polyol-based deep eutectic solvents and saponin. Journal of Contaminant Hydrology, 194, 17–23. Mulligan, C. N., Yong, R. N., Gibbs, B. F., James, S., & Bennett, H. P. J. (1999). Metal removal from contaminated soil and sediments by the biosurfactant surfactin. Environmental Science & Technology, 33(21), 3812–3820. Omidi, F., Behbahani, M., Bojdi, M. K., & Shahtaheri, S. J. (2015). Solid phase extraction and trace monitoring of cadmium ions in environmental water and food samples based on modified magnetic nanoporous silica. Journal of Magnetism and Magnetic Materials, 395, 213–220. Paradiso, V. M., Clemente, A., Summo, C., Pasqualone, A., & Caponio, F. (2016). Towards green analysis of virgin olive oil phenolic compounds: Extraction by a natural deep eutectic solvent and direct spectrophotometric detection. Food Chemistry, 212, 43–47. Suhagia, B. N., Rathod, I. S., & Sindhu, S. (2011). Sapindusmukorossi (Areetha): An overview. International Journal of Pharmaceutical Sciences and Research, 2, 1905–1913. Wu, Y., He, R., Wang, Z., Yuan, J., Xing, C., Wang, L., et al. (2016). A safe, efficient and simple technique for the removal of cadmium from brown rice flour with citric acid and analyzed by inductively coupled plasma mass spectrometry. Analytical Methods, 8(33), 6313–6322. Zarezade, V., Behbahani, M., Omidi, F., Abandansari, H. S., & Hesam, G. (2016). A new magnetic tailor made polymer for separation and trace determination of cadmium ions by flame atomic absorption spectrophotometry. RSC Advances, 6, 103499–103507. Zhao, B. Y., Xu, P., Yang, F. X., Wu, H., Zong, M. H., & Lou, W. Y. (2015). Biocompatible deep eutectic solvents based on choline chloride: Characterization and application to the extraction of rutin from Sophora japonica. ACS Sustainable Chemistry & Engineering, 3, 2746–2755. Zhuang, P., Zhang, C., Li, Y., Zou, B., Mo, H., Wu, K., et al. (2016). Assessment of influences of cooking on cadmium and arsenic bioaccessibility in rice, using an in vitro physiologically-based extraction test. Food Chemistry, 213, 206–214.
the mechanisms of Cd removal by NADESs in the presence of saponin. The mechanisms for Cd removal by NADESs may be complicated due to the various components of NADESs. Based on the aforementioned results, we postulated that the removal of Cd from rice flour by NADESs occurs via two processes, namely lysis and complexation (Mukhopadhyay, Mukherjee, Adnan, et al., 2016; Mulligan et al., 1999; Wu et al., 2016). Acidic or mildly acidic NADESs (e.g., ChTar, ChXy, and ChSor) acted as Lewis acids, which preferentially interacted with the electron donors in rice flour, thereby pulling the Cd ions out of the rice flour via interactions with NADESs anions in the washing solution. However, when the slightly alkaline glycerol-based NADESs were introduced into the system, H+ ions were not supplied owing to the alkaline nature, and Cd-rice flour complex was not disrupted. Therefore, the Cd2+ removals were negligible for the glycerol-based NADESs. When NADESs were mixed with saponin, which is naturally acidic, more H+ ions were introduced into the system. Saponin also encapsulated Cd2+ via micelle formation (Mukhopadhyay, Mukherjee, Adnan, et al., 2016; Mulligan et al., 1999). The synergistic effect between saponin and NADESs, therefore, improved the Cd removal. A potential mechanism for Cd removal by NADESs and NADES–saponin mixtures is illustrated in Fig. 4. Further studies will be carried out in the future in order to confirm the proposed mechanism. 4. Conclusions Herein, a rapid, green, and efficient method was established for the removal of Cd from rice flour using NADESs and NADESs–saponin mixtures. The optimal conditions for the rice flour washing were established. A Cd removal above 96% was obtained using NADES ChTar. A synergistic effect between saponin and choline chloride-based NADESs was observed and > 99% Cd was removed using a ChSor-saponin mixture. The mechanism for the Cd removal by NADESs was also studied. Moreover, the washing process did not affect the main chemical components or structure of the rice flour. So, it can be used in food industry. Moreover, NADESs are easy to dissolve in water and can be separated from rice flour very well. The content of NADESs in rice flour can be reduced to safety level which will not cause secondary pollution. If further scale-up is possible, the proposed method may have numerous applications in the food industry. Acknowledgements This work was partly supported by Qing Lan Project, PAPD, and the SRF for ROCS. Conflict of interest Authors declare no existing conflict of interest. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.foodchem.2017.10.060. References AlOmar, M. K., Alsaadi, M. A., Hayyan, M., Akib, S., Ibrahim, R. K., & Hashim, M. A. (2016). Lead removal from water by choline chloride based deep eutectic solvents functionalized carbon nanotubes. Journal of Molecular Liquids, 222, 883–894. Archana, G., & Sharma, R. K. (2016). Cadmium minimization in food crops by cadmium resistant plant growth promoting rhizobacteria. Applied Soil Ecology, 107, 66–78. Baghban, N., Shabani, A. M. H., & Dadfarnia, S. (2012). Solid phase extraction of trace amounts of cadmium with cetyltrimethylammonium bromide-coated magnetic nanoparticles prior to its determination by flame atomic absorption spectrometry. Journal of the Chinese Chemical Society, 59, 782–787.
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