ZHOU Wen, XU Ying*, WANG Dongfeng, and ZHOU Shilu

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Chitosan Removes Toxic Heavy Metal Ions from Cigarette Mainstream Smoke

ZHOU Wen, XU Ying, WANG Dongfeng, and ZHOU Shilu

College of Food Science and Engineering, Ocean University of China, Qingdao 266003, P. R. China

This study investigated the removal of heavy metal ions from cigarette mainstream smoke using chitosan. Chitosan of various deacetylation degrees and molecular weights were manually added to cigarette filters in different dosages. The mainstream smoke particulate matter was collected by a Cambridge filter pad, digested by a microwave digestor, and then analyzed for contents of heavy metal ions, including As(III/V), Pb(II), Cd(II), Cr(III/VI) and Ni(II), by graphite furnace atomic absorption spectrometry (GFAAS). The results showed that chitosan had a removal effect on Pb(II), Cd(II), Cr(III/VI) and Ni(II). Of these, the percent removal of Ni(II) was elevated with an increasing dosage of chitosan. Chitosan of a high deace tylation degree exhibited good binding performance toward Cd(II), Cr(III/VI) and Ni(II), though with poor efficiency for Pb(II). Except As(III/V), all the tested metal ions showed similar tendencies in the growing contents with an increasing chitosan molecular weight. Nonetheless, the percent removal of Cr(III/VI) peaked with a chitosan molecular weight of 200 kDa, followed by a dramatic decrease with an increasing chitosan molecular weight. Generally, chitosan had different removal effects on four out of five tested metal ions, and the percent removal of Cd(II), Pb(II), Cr(III/VI) and Ni(II) was approximately 55%, 45%, 50%, and 16%, respectively. In a word, chitosan used in cigarette filter can remove toxic heavy metal ions in the mainstream smoke, improve cigarette safety, and reduce the harm to smokers.

chitosan; heavy metal ions; cigarette mainstream smoke; percent removal

1 Introduction

Tobacco smoking is a worldwide problem with 1.3 billion people smoking cigarettes and one person losing life every 6s due to cigarette-related illnesses (WHO, 2008). The prevalence of numerous health hazards associated with cigarette smoking has been well documented (Mello, 2010), and one of the most serious diseases caused by smoking is cancer (Chen., 2011). In addition, people around smokers are exposed to side-stream smoke, which contains even higher concentrations of hazard components than the mainstream smoke (Wong., 2004). There are about 600000 people losing their lives each year because of passive smoking (Hammer., 2011).

Tobacco smoke is a complex mixture of thousands of chemicals, including a number of serious carcinogens (Shah and Karnes, 2010). It is a rich source of toxic heavy metals, which are preferentially enriched in tobacco leaves during the plant growth (Golia., 2007). Heavy metal ions such as As(III/V), Pb(II), Cd(II), Cr(III/VI) and Ni(II) are seriously toxic substances in cigarette smoke. Research has shown that the smoking history or exposure to secondhand smoke is correlated with elevated Cd(II) levels in many human organs and tissues such as lung, liver, and kidney tissue (Gairola and Wagner, 1991). In addition, smoking has been found associated with elevated Pb(II) levels in the blood and amniotic fluid (Milnerowicz., 2000).

Elevated exposure to heavy metals from smoking can increase the incidence of diseases such as cancer (Chen., 2011) and/or complications of pregnancy (Milner- owicz., 2000). Direct Cd(II) exposure may cause many diseases, probably due to endothelial dysfunction (Gallagher and Meliker, 2010). Chronic exposure to Cd(II) can also lead to emphysema, deregulated blood pressure, bone disorders and immune-suppression (Jarup., 1998). Certain metals such as Pb(II) and Ni(II) have been declared probable human carcinogens by International Agency for Research on Cancer (IARC, 1993). Despite the significant hazardous effects of cigarette smoking on human health, it is hard for the majority of smokers to quit smoking and cigarettes are unlikely to be completely banned (Coral and Wayne, 2010). Therefore, it is urgent to remove heavy metals from cigarette smoke for reducing the harm to human health.

Chitosan (poly (β-1,4)2-amino-2-deoxy-D-glucopyra- nose) is a natural biopolymer extracted from crustacean shells by partial deacetylation of its acetamido groups using strong alkaline solutions (Synowiecki and Al-Kha- teeb, 2003). Chitosan has unique biological and chemical functions regarding biodegradability, biocompatibility and bioactivity (Guibal, 2004), with various potential applications in many fields such as biomedical production and wastewater treatment (Ravi Kumar., 2004; Verma., 2012). A previous study has shown that chitosan has a capacity to fix a large variety of heavy metals due to the relatively high proportion of active nitrogen sites (Guibal, 2004). So far, chitosan has commonly been used as an absorbent to remove heavy metal ions from wastewater (Benavente., 2011; Amit and Mika, 2009). Few reports have documented the removal of heavy metal ions from cigarette mainstream smoke using chitosan. In this study, we attempted to evaluate the removal effect of chitosan added to cigarette filter on toxic heavy metal ions in the mainstream smoke.

2 Materials and Methods

2.1 Materials and Reagents

Cigarette samples were provided by China Tobacco Shandong Industrial Co., Ltd. Chitosan powder of different molecular weights and deacetylation degrees were purchased from Qingdao Haihui Biochemical Pharmaceutical Co., Ltd, and used without further purification. Concentrated nitric acid (HNO), concentrated hydrochloric acid (HCl), hydrogen peroxide (HO), hydrofluoric acid (HF) and acetic acid were purchased from Merck. All reagents, unless indicated, were analytical grade and purchased from local commercial sources.

2.2 Cigarette Filter Preparation and Chitosan Treatment

Cigarette filters with chitosan (α-chitosan) were made on the basis of a patent (Wang., 2011). Chitosan powder of various deacetylation degrees and molecular weights was dissolved in 1mL of acetic acid (2%) solution in different dosages. The cigarette filter was removed from the tipping paper, and immersed into the pre-made chitosan solution until it was completely absorbed. The filter was air-dried in shade and then put back onto the tipping paper. Cigarette filter without chitosan treatment was used as blank.

2.3 Cigarette Preparation

Prior to smoking, all cigarettes were conditioned in house for at least 48h at 22℃ with 60% relative humidity. Thereafter, smoking was performed on an automated linear 10-port ASM500 smoking machine (Cerulean, Milton Keynes, UK). All the cigarettes were smoked to a butt length of the filter tipping paper plus 3mm. One cigarette was smoked per Cambridge filter. Triplicate samples were collected from mainstream smoke particulate trapped on the Cambridge filter for heavy metal analysis. The cigarette filters after smoking were also collected for analysis.

2.4 Sample Preparation

Microwave digestion (Anton Paar, Perkin Elmer) was used to prepare cigarette samples and the blank. Cambridge filters that trapped mainstream smoke particulate matter and the cigarette filters were treated according to Pappas. (2006).

2.5 Heavy Metal Analysis by Graphite Furnace Atomic Absorption (GFAAS)

The contents of Ni(II), Pb(II), Cd(II), Cr(III/VI), and As(III/V) in the mainstream smoke particulate matter were determined following the method provided by Torrence(2002) with slight modifications.

2.6 Determination of Percent Removal

The percent removal of heavy metal ions was calculated as follows:

,

,

whereis percent removal (%);is the content of heavy metal ion in untreated cigarette mainstream smoke trapped by the Combrige filter (μg);is the content of heavy metal ion in the blank Combrige filter (μg);is the cigarette per Cambridge filter (=1 in this study);is the content of heavy metal ion in treated cigarette mainstream smoke trapped by the Combrige filter (μg);Xis the content of heavy metal ion in one untreated cigarette mainstream smoke (μgcig) andXis the content of heavy metal ion in one treated cigarette mainstream smoke (μgcig).

2.7 Statistical Analysis

All assays were performed in triplicate. The data are expressed as the mean values ± standard deviation.

3 Results and Discussion

3.1 Influence of Chitosan Dosage

As shown in Table 1, chitosan dosage had a strong influence on the content of heavy metal ions trapped in the filter and the mainstream smoke (chitosan deacetylation degree 93% and molecular weight 500kDa). Of the five tested heavy metal ions, Cr (III/VI) content was the highest whereas Cd(II) and As(III/V) contents were the lowest in the cigarettes. For untreated sample, the contents of As(III/V), Pb(II), Cd(II) and Cr(III/VI) were lower in the filter than those in the mainstream smoke, while the content of Ni(II) was much higher in the filter than in the mainstream smoke. Except for As(III/V), the contents of all tested heavy metal ions in the mainstream smoke were decreased with an increasing chitosan dosage, suggesting a lack of chitosan removal effect on As(III/V) in the cigarette mainstream smoke. In contrast, the amount of Pb(II), Cd(II) and Cr(III/VI) trapped by the cigarette filter was increased with chitosan dosage. Of note, the majority of all tested heavy metal ions were fixed in treated or untreated cigarette filters.

Table 1 The contents of heavy metal ions in cigarette filter and mainstream smoke associated with different chitosan dosages

Note: Values are given as means ± standard deviation (=3).

As shown in Fig.1, the percent removal of heavy metal ions, except As(III/V), grew with the increase in chitosan dosage. The percent removal of Cr(III/VI) was the highest, followed by those of Cd(II), Pb(II) and Ni(II). However, the percent removal of Cd(II) was higher than that of Cr(III/VI) when the chitosan dosage reached 30 mg. There was no apparent effect on Ni(II) removal until the dosage of chitosan was added to 30mg.

Fig.1 Effects of chitosan dosage on the percent removal of heavy metal ions in the mainstream smoke.

Our results indicated that the addition of chitosan to cigarette filters affected heavy metal distribution in the cigarette mainstream smoke. This could be attributed to the unique properties of chitosan such as bioactivity (Guibal, 2004; Muzzarelli, 2011). Due to the sticky chitosan solution and the hydrogen bonds between chitosan and acetate fiber, chitosan was well distributed in the acetate fiber filter and its net structure could amplify the adsorption surface area of the filter. In addition, the introduction of active groups of chitosan could increase the adsorption efficiency of the cigarette filter (Wang., 2004).

The removal effects of chitosan on tested heavy metals were found substantially different. The maximum percent removal of Cd(II), Cr(III/VI), Pb(II), Ni(II) was 33%, 31%, 30% and 9%, respectively. The interactions between different metal ions and amine groups of chitosan were not the same. For example, the adsorption of Ni(II) can be improved when β-chitosan is applied, as the formation of complex β-chitosan-Ni(II) is suitable for its molecular arrangement. In β-chitosan, the NHgroups are packed in a parallel molecular arrangement throughout its polymer chains, which favors the formation of strong interactions between chitosan and Ni(II). However, this work used α-chitosan, the most commonly used form of chitosan (Paulino., 2006), which presents NHgroups packed in an anti-parallel molecular arrangement through- out its polymeric chains (Synowiecki and Al-Khateeb, 2003).

3.2 Influence of Chitosan Deacetylation Degree

The chitosan with various deacetylation degrees had different removal effects on the five heavy metal ions in the cigarette filter and mainstream smoke (Table 2, chitosan dosage 25mg and molecular weight 200kDa). There was almost no change in As(III/V) content with different chitosan deacetylation degrees, demonstrating a lack of chitosan effect. The contents of Cd(II), Cr(III/VI) and Ni(II) increased while Pb(II) decreased in the filter with an increasing degree of chitosan deacetylation. In addition, the percent removal of Cd (II), Cr (III/VI) and Ni (II) positively increased with the degree of chitosan deacetylation (Fig.2). The maximum percent removal of Cd(II), Cr(III/VI), Pb(II), and Ni(II) was 40%, 49%, 41% and 13%, respectively. These illustrated that the chitosan, which contains active sites such as amino and hydroxyl groups, has great potential as an adsorbent for removal of heavy metal ions. It has been shown that the adsorption ability of chitosan is improved by an increasing degree of deacetylation due to a growing number of amino and hydroxyl groups (Wu., 2001).

Differently, the percent removal of Pb(II) decreased with an increasing chitosan deacetylation degree, that is, high Pb(II) adsorption was observed with a deacetylation degree of 75%, and further elevated deacetylation degree did not lead to an improvement of Pb (II) binding (Fig.2), consistent with previous findings by Paulino. (2006). It was suggested that the long deacetylation reaction time induced a high deacetylation degree, which also contributed to polymer degradation, resulting in chain structure of low average molecular weight. The polymer chains with either low molecular weight and/or its fragments were released from the coiled structure of chitosan to the surrounding liquid, affecting the adsorption capacity of chitosan dramatically (Paulino., 2007).

Table 2 The relationship between chitosan deacetylation degree and heavy metal ion content

Note: Values are given as means ± standard deviation (=3).

Fig.2 Effects of chitosan deacetylation degree on the percent removal of heavy metal ions in the mainstream smoke.

3.3 Influence of Chitosan Molecular Weight

The relationship between chitosan molecular weight and heavy metal content in cigarette and the mainstream smoke is shown in Table 3 (chitosan dosage 25mg and deacetylation degree 90%). It was obvious that the contents of Pb(II), Cd(II), Cr(III/VI) and Ni(II) trapped by the filter increased with the mounting molecular weight of chitosan, while those of heavy metal ions in the mainstream smoke showed a decreasing tendency. One exception was Cr(III/VI), which showed a significant decrease in content in the filter when the chitosan molecular weight reached 200kDa. Also, the total contents of the tested metal ions were almost the same as the results we achieved in Table 3.

In this study, the changes in percent removal of five tested heavy metal ions were evaluated (Fig.3). The maximum percent removal of heavy metal ions followed Cr(III/VI)>Pb(II)>Cd(II)>Ni(II), and associated values were 55%, 49%, 45% and 16% respectively. However, when the chitosan molecular weight was 500kDa, the percent removal of Cd(II) became the largest while that of Cr(III/VI) decreased significantly. Our results regarding metal absorption of chitosan were slightly different from those in several other studies (Becker., 2000; Yang and Shao, 2000). This might be caused by the formation of chitosan as well as the environment metal ions existing. That is, different formation of chitosan and different environment metal ions existing could cause different interaction between metal ions and chitosan.

Some researchers argue that it is easier for low-mo- lecular-weight chitosan to bind toxic materials than for high-molecular-weight chitosan (Park., 2004). The chains of high-molecular-weight chitosan are considered to wind up more tightly than those of low-molecular- weight chitosan, and the latter allow the exposure of active sites to heavy metal ions. On the other hand, there are hydrogen bonds in the high-molecular-weight chitosan (Park., 2004; Kamil., 2002). Results from the present study suggested the interaction was more likely to occur between heavy metal ions and high-molecular- weight chitosan than between heavy metal ions and low- molecular-weight chitosan. Hence, the chitosan with higher- molecular-weight is more suitable for use in cigarette filters for high removal efficiency of heavy metal ions.

Table 3 The relationship between chitosan molecular weight and heavy metal ion contents

Note: Values are given as means ± standard deviation (=3).

Fig.3 Effects of chitosan molecular weight on the percent removal of heavy metal ions in the mainstream smoke.

4 Conclusions

This study investigated the removal effects of chitosan regarding dosage, deacetylation degree and molecular weight on five major heavy metal ions in the cigarette mainstream smoke. Overall, chitosan had no significant removal effect on the As(III/V) in the mainstream smoke. Together our results demonstrated that chitosan is a promising bio-material that can be added into cigarette filters to trap toxic heavy metal ions. Generally, the percent removal of Pb(II), Cd(II), Cr(III/VI) and Ni(II) increased with an increasing chitosan dosage. For chitosan of a high deacetylation degree, the percent removal of Cd(II), Cr(III/VI) and Ni(II) was relatively high, with an exception of Pb(II) showing a lower percent removal. All the tested metal ions appeared to increase with an increasing chitosan molecular weight. For Cr(III/VI), the percent removal decreased dramatically when the molecular weight of chitosan reached 200kDa. The percent removal of Cd(II), Pb(II), Cr(III/VI) and Ni(II) were approximately 55%, 45%, 50%, and 16%, respectively. Despite extensive previous studies regarding the removal effects of chitosan dosage, degree of deacetylation and molecular weight on the heavy metal ions in the cigarette mainstream smoke, further investigation is needed for testing the adsorption mechanism of chitosan.

Acknowledgements

This research was supported by the Natural Science Foundation of China (No.30972289) and Ocean Public Welfare Scientific Research Special Appropriation Project (No.201005020).

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(Edited by Qiu Yantao)

10.1007/s11802-013-2032-0

ISSN 1672-5182, 2013 12(3): 509-514

. Tel: 0086-532-82031575 E-mail: xuy@ouc.edu.cn

(May 11, 2012; revised June 19, 2012; accepted September 3, 2012)

? Ocean University of China, Science Press and Spring-Verlag Berlin Heidelberg 2013