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chitin mixture
Waler Research Vol. 14. pp. 1307 to 1311 Pergamon Press Ltd 1980 Printed in Great Britain
REMOVAL OF ARSENIC FROM CONTAMINATED DRINKING WATER BY A CHITOSAN/CHITIN MIXTURE CLIVE M. ELSON, t DONALD H. DAVIES1 a n d ERNEST R. HAYES2 Department of Chemistry, Saint Mary's University, Halifax, N.S., BH3 3C3, and 2Department of Chemistry, Acadia University, Wolfville, BOP 1XO, Canada
(Received 23 January 1980) Abstraet--A chitosan/chitin mixture was studied as a potential agent for the removal of arsenic (V) from ground waters. The arsenic concentration of contaminated waters was lowered to levels accepted by the Canadian Department of Health and Welfare and the World Health Organization upon treatment with the mixture and, furthermore, the copper and sulfate levels were also reduced. The capacity of the mixture at pH 7 was found to be 0.13/~-equiv As g- t mixture with a distribution coefficient of 65. The interaction between arsenic and these polymers is also discussed.
INTRODUCTION
Model HGA 2100), deuterium arc background corrector, timebase recorder (Omniscribe, Houston Instruments) and Recently, it has been reported that chitosan, poly-Nthe appropriate lamps. The arsenic lamp was an electrodeacetyl-D-glucosamine with 70-80~o of the glucosamine less discharge lamp. The temperature programs for arsenic units deacetylated, is an effective anion-exchanger for and copper determinations were: dry at 150°C for 20 s, metal oxyanions such as metavanadate, molybdate, char at 400°C for 20 s (As), 800°C for 20 s (Cu) and atomize at 2300°C for 6 s (As), 2500°C for 7 s (Cu). The arsenic c h r o m a t e (Muzzarelli, 1977a) and, in one case limited standard solutions were prepared from dried As203 and data has been presented for the collection of arsenic the copper standards were prepared from the metal. I0~1 from acidic solutions (Muzzarelli & Tubertini, 1972). aliqu0ts were injected into the furnace using Eppendorf Since in certain regions of C a n a d a there is, at present, pipets. The mass spectra were obtained using a Dupont (model concern over the c o n t a m i n a t i o n of domestic water 21-491) double-focusing mass spectrometer having an supplies by arsenic, this study was designed to deterionizing voltage of 82 V and a heated direct inlet probe. mine the efficacy of a chitosan/chitin mixture in ' Determination of water characteristics was made photoremoving arsenic from natural waters (chitin is polymetrically using the Hach system (DR/2 SpectrophoN-acetyl-D-glucosamine which is 10-15~o deacetytometer). lated). T h e treatment will be considered effective if the Preparations arsenic content of the water is lowered to less than Washedchitosan. 2 g chitosan was homogenized in 80 ml 0.05 p p m while the p H is m a i n t a i n e d in the p r o b a b l e 50% acetic acid until dissolved. The solution was filtered range (Canadian D r i n k i n g W a t e r Standards, 1968), through gauze and then mixed with 40 ml of 20~ NaCI solution. The white precipitate of chitosan hydrochloride a n d if the capacity of the chitosan/chitin mixture is was collected and alcohol washed. The precipitate was then c o m p a r a b l e to commercial resins, redissolved in 100 ml H 2 0 and NaOH (10~) was added until the chitosan precipitated. The solid was washed until EXPERIMENTAL chloride could not be detected in the filtrate with AgNO3. The washed chitosan was dried in an oven at 50°C overSamples night and stored in a desiccator. Water samples refer to domestic water samples taken Chitosan-arsenicderivative. 10g As20 s was dissolved in from drilled wells near Waverley, Nova Scotia, which con50 ml of 5 N NaOH and then HNO a was added to adjust rained c. 0.03 ppm As and which were spiked to between the pH to 6. 17 g of wet chitosan hydrochloride, as pre0.1--0.3 ppm As for preliminary tests. The "contaminated" pared above, were dissolved in 50 ml of 50% acetic acid water sample contained 0.3 ppm As, had a pH 7.5 and was and this solution was then added to the arsenic solution. obtained from a drilled well in Waverley, Nova Scotia. Within 30 min a light brown coloured precipitate formed which was collected, washed with alcohol, dried at 50°C Reacjents overnight and stored in a desiccator. All chemicals were of reagent grade except for acids and Cross-linkedchitosan. Cross-linked chitosan was prebases which were of ultra-pure grade (Ultrex, J. T. Baker & pared following the method of Masri & Randall (1978). To Co.). Distilled water was obtained from an all glass still 9.0 g of 60 mesh chitosan was added a mixture of 4.5 ml of and contained less than 0.005 ppm As. All glassware was 25~oaqueous glutaraldehyde plus 36 mt distilled water. The washed with hot 1 : 1 HNO3 before use. Chitin and chitomixture was kept at 20°C and stirred occasionally for l h. san were purchased from the Kypro Corp. The solid material was collected by filtration, washed with water and methanol, and air dried.
Equipment
Arsenic and copper were determined using an atomic absorption spectrophotometer (Perkin-Elmer Corp. Model 403) equipped with a graphite furnace (Perkin-Elmer Corp. 1307
Procedures Preliminary batch experiments involved stirring l g of chitosan flakes or chitin flakes or a mixture of both with
t308
(:'LIVE M ELS(}N. DONALD H. DAVIES and ER,~ESr R. HA',~S
approximately 50 ml of sample and monitoring the pH and As level, Columns (1.I cm i.d.) were usually packed with either 2.3 or 4.6 g of 1:1 chitosan to chitin mixture to give column heights of 11 or 22 cm respectively. The chitosan and chitin had been passed through a 60 mesh sieve. The flow rate of effluent was adjusted to c. 3 ml r a i n - ' 20 ml portions of effluent were collected and analyzed except for the column regeneration experiments in which 5 ml portions were collected. All column work was performed at least in duplicate. The capacity of a column was based on the point on an elution curve at which the arsenic concentration of the effluent increased by 0.01 ppm above the level found in the first 20ml portion Of effluent. Arsenic was eluted from the chitosan/chitin mixture by washing with a buffer solution containing 0.1M NH3-0.1M NH4C1 (pH95) at room temperature, The distribution coefficient, Dg, defined as the ratio of amount of As in 1 g of polymer to the amount of arsenic in 1 ml of solution at equilibrium was determined by stirring 1.00 g of mixture with 30.0 ml of 0.066 pro. As at various pHs (6.5, 7.5 and 8.5) at 25 + I°C and monitoring the arsenic concentration of the solution. Equilibrium was considered to be established when the concentration of As remained constant (c. 7 h). The loading of the resin was always less than 10~o under these conditions; hence, the Dg values should be independent of concentration, The oxidation state of the arsenic species was determined by solvent extraction following the method of Reinke et al. (1975) where 4 ml of sample was mixed with 6 ml of conc HC1. Half of the mixture was retained and the other half was extracted twice with 5 ml portions of benzene. The As concentration of the two aqueous portions was subsequently determined. Appropriate blanks were also run. The percentage of As(Ill) in the sample was obtained by compairing the arsenic levels in the two aqueous portions. The arsenic content of washed chitosan and the chitosan-arsenic derivative was measured in dupfieate by digesting known amounts of each material in cone HNO3
until a clear solution resulted. The digest wa~ d~iu{e,i. small amount of NH4OH was added to raise ,he pH r~ and the arsenic concentration was determined.
RESULTS
The arsenic concentration of the sample v, as catculated by comparison with a calibration curve which was prepared daily. The method of standard additions produced the same results. Moreover, the addition of nickel nitrate to the furnace was found to be unnecessary since the premature volatilization of arsenic was not observed. This was possibly due to the type of sample a n d the low charring temperature employed. W h e n flakes of chitosan were stirred with a water sample, the pH of the mixture increased from 6.7 to over 8 a n d the arsenic level in the water remained constant at 0.1 ppm. However. when the pH of a sample was held between 5.5 and 6.5 by the addition of acid, the arsenic concentration of a sample was lowered by 800/o after 150 rains of stirring. By comparison, when flakes of chitin were stirred with a sample, the p H decreased by 2 units a n d 25% of the arsenic was removed. The effluent from 11-cm columns containing the chitosan a n d chitin flakes had a p H of less than 7 a n d an initial arsenic concentration of 0 . 0 4 p p m (sample originally contained 0.1 p p m As). The arsenic uptake by these columns was 0.03 ttrnol As. This column also removed other metal ions such as copper. Passing a 0 . 1 0 p p m Cu(II) solution t h r o u g h the mixture yielded a n effluent containing <0.005 p p m Cu even after 250 ml of effluent had
o 3ol-
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~o,5. g o
1
005~ '
././ •--.-:-"
Volume
of
eluent (m~)
Fig. 1, Elution curves from the treatment of arsenic-containing water: (A) 4.6 g of 1 : 1 chitosan: chitin resin washed with contaminated well water (0.3 ppm As) (B) 4.6 g of 1 : 1 resin washed with 0.78 ppm As standard solution (C) 4.4g of 1:1 cross-linked chitosan:chitin resin washed with contaminated well water (0.3 ppm As).
Removal of arsenic from contaminated drinking water o~ o~
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i
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i
70
(ml)
Fig. 2. Elution curve for the regeneration of an arseniccontaining column: 4.6g of 1:1 chitosan: chitin used to treat contaminated well water (0.3 ppm As) washed with 0.1 M NH3-0.1MNH4CI(pH9.5). been collected. Doubling the length of the column doubled the amount of arsenic removed but left the pH and the minimum As concentration of the effluent unaffected. Mixing chitosan and chitin flakes in the ratio of 1:2 by weight produced a column which had a slightly more acidic effluent but greatly reduced efficiency, To increase the capacity of the columns, sieve chitosan and chitin were used as packing. The elution curve obtained by treating a sample of contaminated water (0.3 ppm As) with a 22 cm column is shown in Fig. 1. The pH of the effluent was 6.7 and the first 150 ml of effluent met the Canadian drinking water requirements of less than 0.05 ppm As. The column capacity was 0.13/amol As g-1 mixture; the capacity of an identical column used to treat a 0.78 ppm As standard solution at pH 6.0 was 0.40/maol As g-1 (Fig. 1). Repeating these experiments produced results that agreed to within + 10~o. Both the contaminated water sample and the first 50 ml of effluent were also analyzed for several major constituents and the results appear in Table 1. Arsenic in the contaminated sample and in the effluent from the treatment column was in the 5+ oxidation state (>95% of As present) as demonstrated by the failure to extract any arsenic(Ill) into benzene from acidified samples, Washing the columns that had been employed to remove arsenic from the contaminated water samples with an alkaline buffer released approximately 20% of the total bound arsenic (Fig. 2). A second series of 22 cm treatment columns were prepared using a 1 :I mixture of 60 mesh cross-linked
1309
chitosan and chitin. Passing the contaminated water sample through these columns produced the elution curve shown in Fig. 1. The effluent had a pH comparable to that of the simple columns but the capacity was only 61% of the corresponding chitosan/chitin columns. Analysis of samples of washed chitosan and the chitosan-arsenic derivative by chemical and mass spectral methods demonstrated two major differenties. Firstly, the arsenic content of the derivative prepared in acid medium was approximately 3 x 105 times that of the washed chitosan: the chitosanarsenic derivative contained 0.23 + 0.01 g As g-1 or 23~ As and the washed chitosan contained 0.70 + 0.20#g As g - i or 0.7ppm As. It must be stressed that the high level of arsenic in the derivative (3 mmol g - 1) resulted from the presence of acid during the preparation. The influence of pH on the uptake of arsenic by the chitosan/chitin mixture was further demonstrated by measuring the distribution coefficient (Dg) as a function of hydrogen ion concentration: Dg = 58 + 6 at pH 6.5; 65 + 7 at pH 7.5; and 1.4 + 0.5 at pH 8.5. The second difference, although only a qualitative one, appeared in the mass spectra of the two polymers. Both compounds exhibited sizable peaks for ions of mass less than 50 m/e up to 250°C, the decomposition temperature. On average, signals in the spectrum of washed chitosan between 31 and 45 m/e were at least three times as large as the corresponding peaks of the same m/e ratio for the chitosan-arsenic derivative recorded under identical conditions.
DISCUSSION
Chitosan is a basic polymer (Muzzarelli, 1977b) which reacts with water according to equation (1). Chitosan-NH2 + H 2 0 ~ c h i t o s a n - N H ~ + O H - . (I) Hence, when chitosan is slurried with water, the pH increases slightly. The amount of hydroxide ion and chitosan cation produced is small, since the pKb for chitosan is 7.7 (as calculated from the pK, of 6.3, Nud'ga et al., 1970). The chitosan cation in solution associates with an anion to form an ion-exchange site according to equation (2). Chitosan-NH~" X - + Y chitosan-NH~Y- + Y- + X-
(2)
Table 1. Characteristics of Waverley well water (ppm) Sample Contaminated well water Et~uent from chitin/chito~an column • ND
Not determined.
Total hardness
Chloride
Total iron
Copper
Nitrate
Phosphate
115+10 (as CaCO3) 97 + 10 (as CaCO 3}
12+_2
0
0.04+-0.005
2+1
44___5
70 + 5
ND*
< 0.005
3+ 1
45 + 5
Sulphate 23+-3 1+ 2
!310
(LIVE M, ELSON. DONALD H. DAVIESand ERNESTR H",~!S
In acidic solution, the number of cationic sites is large, and chitosan has been reported to be an effectire anion-exchanger under these conditions (Muzzarelli, 1977a). In the present work. 3 mmol of arsenic were collected per gram of chitosan from acidic solutions, based on the arsenic content of the chitosanarsenic derivative. Such a capacity was comparable to that of typical anion-exchangers. However, when the pH of a water sample in contact with the chitosan; chitin mixture was raised, the capacity of the column used to treat the contaminated sample was 0.13 ~mol g-~ mixture at pH 7. For samples with pH greater than 8, the arsenic level was not lowered as corroborated by the distribution coefficients (Dg/ which decreased by a factor of 50 when the pH changed from 7.5 to 8.5. The variations in the capacity and Dg values with pH were presumably due to changes in the number of protonated, amine exchange sites. The number of sites was further reduced due to the collection of ions other than arsenic, such as copper (see Table 1 and Kurita et aL, 1979). This was confirmed by comparing the removal efficiencies of identical columns used to treat the contaminated water and a pure arsenic standard; the capacities were 0.13 gmol g - ~ and 0.50,umol g- ~ respectively, One other factor that was affected by pH was the form of the arsenic, We have established that the species present in the contaminated water was arsenic (V) but whether the simple arsenates, HzAsO2 and HAsOT, ~ (pK2 6.8) existed is not known but, according to Andreae (1977), the simple inorganic arsenates are the prodominant species, It has tgeen assumea above mat cnttosan ~s a more active ion-exchanger than chitin since the number of deacetylated nitrogen centres in chitosan is 5-6 times that of chitin. Furthermore, chitin demonstrated only 30'},~ of the capacity of chitosan for arsenic collection and when the proportion of chitin in the treatment mixture was increased the column efficiency decreased. Chitin did, however, neutralize the hydroxide produced by the reaction of chitosan with water (equation 1) and maintained the pH of the effluent near 7. Chitin probably contained hydrochloric acid bound as the hydrochloride salt of the deacetylated amines since HCI is used in the preparation of chitin (Muzzarelli, 1973) and since the effluent from the columns contained 5-45 times more chloride than the original samples (Table i), Treatment of the contaminated Waverley well water by the chitosan/chitin mixture altered the levels of several other ions. Of the polyoxy anions studied in addition to the arsenates, sulfate was removed from the sample while nitrate and phosphate were not In addition, the mixture effectively bound copper but not calcium, thus confirming earlier reports (Muzzarelli, 1977c). The increase in chloride concentration of the effluent has already been discussed. The method of preparation of the cross-hnked chitosan was expected to introduce c. 1.4 mmol of glutaraldehyde g - ~ of chitosan. Our sample of chitosan h ~
been reported to contain 5 mmol g ~ of amine (Haw-., & Davies, 1978): Hence. cross-linking the chm)sa,~ reduced the free amine content and potential c~exchange sites by approximately 35°;. The capacit) ~ a column employing the cross-linked chitosan,'chitJn mixture was 39°,~ less than the normal mixture. Th~ agreement between these two figures may ~.ell be ,¢('~ tuitous; however, it does confirm the importance ,,~ the number of ion-exchange sites to the collection el]iciency of the mixture. When a column that had been used ! , collect arsenic was washed with an ammonium chloride buffer, approximately 20°~o of the arsenic wa~ r~, covered, The mass spectral evidence, although not definitive, suggested that the chitosan-arsenic derivative was less volatile and more stable than the washed chitosan. These results indicate that the bonding between arsenic and the polymers involved more than simple ion-exchange. In the case of sulphate. Muzzar elli & Rochetti (1974) hypothesized that the structure of chitosan changes to accomodate two linkages between two amine sites and sulphate. However, the sulphate was still exchangeable whereas in the present case only 20°~, of the arsenic was. It is suggested, therefore, that chelation of the arsenic anion ~)ccurs in addition to electrostatic, ion-exchange. The importance of the ion-exchange process cannot be ignored as evidenced by the variation in capacity of the mixture from acidic to neutral solutions and the change in the distribution coefficients with pH. Furthermore, chitosan has been reported to form coordinate linkages to cations (Muzzarelli, 1977c) In conclusion, a chitosan/chitin mixture has beet: demonstrated to be capable of removing certain polyoxy-anions including arsenates from water samples of neutral p H However, the capacity of the mixture was approximately 0.1 ,umolg-~ which is l04 times smaller than commercial ion-exchangers. The arsenic was bound to the polymers by a combmatlon of electrostatic, ion-exchange attraction and chelation. REFERENCES
Andrear M. O. 11977) Determination of arsenic species m natural waters. Analyt Chem. 49, 820-823. Canadian Drinking Water Standards and Objectives (1968! Department of National Health and Welfare, Canada Hayes E. R. & Davies D H. (1978) Characterization of chitosan--II. The determination of the degree of acetylation of chitosan and chitin. Proc. O[ First International Conf. on Chitin/Chitosan~ (Edited by Mussarelli R A A & Pariser E. R.), pp. a06-420. Massachusetts Institute oJ Technology,MA. Kurita K., Sannan T & lwakura "v 11979) S(ud~e., :~ chitin. VI. Bonding of metal cations J 4ppl Potvm..%~ 23, 511-515 MasriM. A. & Randall "~ G. (19781Chitosan and cmtosat~ derivitiesfor removal of toxic metallic ions from manufacturing plant waste streams. Pro~. oIFirst lnter~mtionat Co,V on Chitm:Chito~w~ Edited b? Muzzarelli R A. ~, & Pariser E RI, pp. 2"7 2R7. Ma,;sachusetts Institute o{ Technology,MA. MuzzarelliR .~ ~ {19"r?a~ ('hittn, p t45 Pergam~m Pr~s,. (3"~for,:]
Removal of arsenic from contaminated drinking water Muzzarelli R. A. A. (1977b) Chitin p. 183. Pergamon Press, Oxford. Muzzarelli R. A. A. (1977c) Chitin, p. 140. Pergamon Press, Oxford. Muzzarelli. R. A. A. (1973) Natural Chelating Polymers, p. 96. Pergamon Press, Oxford. Muzzarelli R. A. A. & Rochetti R. (1974) Enhanced capacity of chitosan for transition-metal ions in sulphatesulphuric and solutions. Talanta 11, 1137-1143. Muzzarelli R. A. A. & Tubertini O. (1972) Radiation resist-
1311
anee of chitin and chitosan applied in the chromatography of radioactive substances. J. Radioanalyt. Chem. 12, 431--440. Nud'ga L. A., Plisko E. A. & Danilou S. M. (1970) Zh. obshch.Khim. 41, 2550--2560. Reinke J., Uthe J. F., Freeman H. C. & Johnston F. R. (1975) The determination of arsenite and arsenate ions in fish and shellfish by selective extraction and polarography. Environ. Lett. 8, 371-379.
REMOVAL OF ARSENIC FROM CONTAMINATED DRINKING WATER BY A CHITOSAN/CHITIN MIXTURE CLIVE M. ELSON, t DONALD H. DAVIES1 a n d ERNEST R. HAYES2 Department of Chemistry, Saint Mary's University, Halifax, N.S., BH3 3C3, and 2Department of Chemistry, Acadia University, Wolfville, BOP 1XO, Canada
(Received 23 January 1980) Abstraet--A chitosan/chitin mixture was studied as a potential agent for the removal of arsenic (V) from ground waters. The arsenic concentration of contaminated waters was lowered to levels accepted by the Canadian Department of Health and Welfare and the World Health Organization upon treatment with the mixture and, furthermore, the copper and sulfate levels were also reduced. The capacity of the mixture at pH 7 was found to be 0.13/~-equiv As g- t mixture with a distribution coefficient of 65. The interaction between arsenic and these polymers is also discussed.
INTRODUCTION
Model HGA 2100), deuterium arc background corrector, timebase recorder (Omniscribe, Houston Instruments) and Recently, it has been reported that chitosan, poly-Nthe appropriate lamps. The arsenic lamp was an electrodeacetyl-D-glucosamine with 70-80~o of the glucosamine less discharge lamp. The temperature programs for arsenic units deacetylated, is an effective anion-exchanger for and copper determinations were: dry at 150°C for 20 s, metal oxyanions such as metavanadate, molybdate, char at 400°C for 20 s (As), 800°C for 20 s (Cu) and atomize at 2300°C for 6 s (As), 2500°C for 7 s (Cu). The arsenic c h r o m a t e (Muzzarelli, 1977a) and, in one case limited standard solutions were prepared from dried As203 and data has been presented for the collection of arsenic the copper standards were prepared from the metal. I0~1 from acidic solutions (Muzzarelli & Tubertini, 1972). aliqu0ts were injected into the furnace using Eppendorf Since in certain regions of C a n a d a there is, at present, pipets. The mass spectra were obtained using a Dupont (model concern over the c o n t a m i n a t i o n of domestic water 21-491) double-focusing mass spectrometer having an supplies by arsenic, this study was designed to deterionizing voltage of 82 V and a heated direct inlet probe. mine the efficacy of a chitosan/chitin mixture in ' Determination of water characteristics was made photoremoving arsenic from natural waters (chitin is polymetrically using the Hach system (DR/2 SpectrophoN-acetyl-D-glucosamine which is 10-15~o deacetytometer). lated). T h e treatment will be considered effective if the Preparations arsenic content of the water is lowered to less than Washedchitosan. 2 g chitosan was homogenized in 80 ml 0.05 p p m while the p H is m a i n t a i n e d in the p r o b a b l e 50% acetic acid until dissolved. The solution was filtered range (Canadian D r i n k i n g W a t e r Standards, 1968), through gauze and then mixed with 40 ml of 20~ NaCI solution. The white precipitate of chitosan hydrochloride a n d if the capacity of the chitosan/chitin mixture is was collected and alcohol washed. The precipitate was then c o m p a r a b l e to commercial resins, redissolved in 100 ml H 2 0 and NaOH (10~) was added until the chitosan precipitated. The solid was washed until EXPERIMENTAL chloride could not be detected in the filtrate with AgNO3. The washed chitosan was dried in an oven at 50°C overSamples night and stored in a desiccator. Water samples refer to domestic water samples taken Chitosan-arsenicderivative. 10g As20 s was dissolved in from drilled wells near Waverley, Nova Scotia, which con50 ml of 5 N NaOH and then HNO a was added to adjust rained c. 0.03 ppm As and which were spiked to between the pH to 6. 17 g of wet chitosan hydrochloride, as pre0.1--0.3 ppm As for preliminary tests. The "contaminated" pared above, were dissolved in 50 ml of 50% acetic acid water sample contained 0.3 ppm As, had a pH 7.5 and was and this solution was then added to the arsenic solution. obtained from a drilled well in Waverley, Nova Scotia. Within 30 min a light brown coloured precipitate formed which was collected, washed with alcohol, dried at 50°C Reacjents overnight and stored in a desiccator. All chemicals were of reagent grade except for acids and Cross-linkedchitosan. Cross-linked chitosan was prebases which were of ultra-pure grade (Ultrex, J. T. Baker & pared following the method of Masri & Randall (1978). To Co.). Distilled water was obtained from an all glass still 9.0 g of 60 mesh chitosan was added a mixture of 4.5 ml of and contained less than 0.005 ppm As. All glassware was 25~oaqueous glutaraldehyde plus 36 mt distilled water. The washed with hot 1 : 1 HNO3 before use. Chitin and chitomixture was kept at 20°C and stirred occasionally for l h. san were purchased from the Kypro Corp. The solid material was collected by filtration, washed with water and methanol, and air dried.
Equipment
Arsenic and copper were determined using an atomic absorption spectrophotometer (Perkin-Elmer Corp. Model 403) equipped with a graphite furnace (Perkin-Elmer Corp. 1307
Procedures Preliminary batch experiments involved stirring l g of chitosan flakes or chitin flakes or a mixture of both with
t308
(:'LIVE M ELS(}N. DONALD H. DAVIES and ER,~ESr R. HA',~S
approximately 50 ml of sample and monitoring the pH and As level, Columns (1.I cm i.d.) were usually packed with either 2.3 or 4.6 g of 1:1 chitosan to chitin mixture to give column heights of 11 or 22 cm respectively. The chitosan and chitin had been passed through a 60 mesh sieve. The flow rate of effluent was adjusted to c. 3 ml r a i n - ' 20 ml portions of effluent were collected and analyzed except for the column regeneration experiments in which 5 ml portions were collected. All column work was performed at least in duplicate. The capacity of a column was based on the point on an elution curve at which the arsenic concentration of the effluent increased by 0.01 ppm above the level found in the first 20ml portion Of effluent. Arsenic was eluted from the chitosan/chitin mixture by washing with a buffer solution containing 0.1M NH3-0.1M NH4C1 (pH95) at room temperature, The distribution coefficient, Dg, defined as the ratio of amount of As in 1 g of polymer to the amount of arsenic in 1 ml of solution at equilibrium was determined by stirring 1.00 g of mixture with 30.0 ml of 0.066 pro. As at various pHs (6.5, 7.5 and 8.5) at 25 + I°C and monitoring the arsenic concentration of the solution. Equilibrium was considered to be established when the concentration of As remained constant (c. 7 h). The loading of the resin was always less than 10~o under these conditions; hence, the Dg values should be independent of concentration, The oxidation state of the arsenic species was determined by solvent extraction following the method of Reinke et al. (1975) where 4 ml of sample was mixed with 6 ml of conc HC1. Half of the mixture was retained and the other half was extracted twice with 5 ml portions of benzene. The As concentration of the two aqueous portions was subsequently determined. Appropriate blanks were also run. The percentage of As(Ill) in the sample was obtained by compairing the arsenic levels in the two aqueous portions. The arsenic content of washed chitosan and the chitosan-arsenic derivative was measured in dupfieate by digesting known amounts of each material in cone HNO3
until a clear solution resulted. The digest wa~ d~iu{e,i. small amount of NH4OH was added to raise ,he pH r~ and the arsenic concentration was determined.
RESULTS
The arsenic concentration of the sample v, as catculated by comparison with a calibration curve which was prepared daily. The method of standard additions produced the same results. Moreover, the addition of nickel nitrate to the furnace was found to be unnecessary since the premature volatilization of arsenic was not observed. This was possibly due to the type of sample a n d the low charring temperature employed. W h e n flakes of chitosan were stirred with a water sample, the pH of the mixture increased from 6.7 to over 8 a n d the arsenic level in the water remained constant at 0.1 ppm. However. when the pH of a sample was held between 5.5 and 6.5 by the addition of acid, the arsenic concentration of a sample was lowered by 800/o after 150 rains of stirring. By comparison, when flakes of chitin were stirred with a sample, the p H decreased by 2 units a n d 25% of the arsenic was removed. The effluent from 11-cm columns containing the chitosan a n d chitin flakes had a p H of less than 7 a n d an initial arsenic concentration of 0 . 0 4 p p m (sample originally contained 0.1 p p m As). The arsenic uptake by these columns was 0.03 ttrnol As. This column also removed other metal ions such as copper. Passing a 0 . 1 0 p p m Cu(II) solution t h r o u g h the mixture yielded a n effluent containing <0.005 p p m Cu even after 250 ml of effluent had
o 3ol-
oz~-
~o,5. g o
1
005~ '
././ •--.-:-"
Volume
of
eluent (m~)
Fig. 1, Elution curves from the treatment of arsenic-containing water: (A) 4.6 g of 1 : 1 chitosan: chitin resin washed with contaminated well water (0.3 ppm As) (B) 4.6 g of 1 : 1 resin washed with 0.78 ppm As standard solution (C) 4.4g of 1:1 cross-linked chitosan:chitin resin washed with contaminated well water (0.3 ppm As).
Removal of arsenic from contaminated drinking water o~ o~
I ~
"~¢~o4 o: i~
t 2
~
• '~
~ "i ' ' - "
i
0
i'O
20
30 Volume
of
.L . . i .
i
40
50 at~nl
60
i
70
(ml)
Fig. 2. Elution curve for the regeneration of an arseniccontaining column: 4.6g of 1:1 chitosan: chitin used to treat contaminated well water (0.3 ppm As) washed with 0.1 M NH3-0.1MNH4CI(pH9.5). been collected. Doubling the length of the column doubled the amount of arsenic removed but left the pH and the minimum As concentration of the effluent unaffected. Mixing chitosan and chitin flakes in the ratio of 1:2 by weight produced a column which had a slightly more acidic effluent but greatly reduced efficiency, To increase the capacity of the columns, sieve chitosan and chitin were used as packing. The elution curve obtained by treating a sample of contaminated water (0.3 ppm As) with a 22 cm column is shown in Fig. 1. The pH of the effluent was 6.7 and the first 150 ml of effluent met the Canadian drinking water requirements of less than 0.05 ppm As. The column capacity was 0.13/amol As g-1 mixture; the capacity of an identical column used to treat a 0.78 ppm As standard solution at pH 6.0 was 0.40/maol As g-1 (Fig. 1). Repeating these experiments produced results that agreed to within + 10~o. Both the contaminated water sample and the first 50 ml of effluent were also analyzed for several major constituents and the results appear in Table 1. Arsenic in the contaminated sample and in the effluent from the treatment column was in the 5+ oxidation state (>95% of As present) as demonstrated by the failure to extract any arsenic(Ill) into benzene from acidified samples, Washing the columns that had been employed to remove arsenic from the contaminated water samples with an alkaline buffer released approximately 20% of the total bound arsenic (Fig. 2). A second series of 22 cm treatment columns were prepared using a 1 :I mixture of 60 mesh cross-linked
1309
chitosan and chitin. Passing the contaminated water sample through these columns produced the elution curve shown in Fig. 1. The effluent had a pH comparable to that of the simple columns but the capacity was only 61% of the corresponding chitosan/chitin columns. Analysis of samples of washed chitosan and the chitosan-arsenic derivative by chemical and mass spectral methods demonstrated two major differenties. Firstly, the arsenic content of the derivative prepared in acid medium was approximately 3 x 105 times that of the washed chitosan: the chitosanarsenic derivative contained 0.23 + 0.01 g As g-1 or 23~ As and the washed chitosan contained 0.70 + 0.20#g As g - i or 0.7ppm As. It must be stressed that the high level of arsenic in the derivative (3 mmol g - 1) resulted from the presence of acid during the preparation. The influence of pH on the uptake of arsenic by the chitosan/chitin mixture was further demonstrated by measuring the distribution coefficient (Dg) as a function of hydrogen ion concentration: Dg = 58 + 6 at pH 6.5; 65 + 7 at pH 7.5; and 1.4 + 0.5 at pH 8.5. The second difference, although only a qualitative one, appeared in the mass spectra of the two polymers. Both compounds exhibited sizable peaks for ions of mass less than 50 m/e up to 250°C, the decomposition temperature. On average, signals in the spectrum of washed chitosan between 31 and 45 m/e were at least three times as large as the corresponding peaks of the same m/e ratio for the chitosan-arsenic derivative recorded under identical conditions.
DISCUSSION
Chitosan is a basic polymer (Muzzarelli, 1977b) which reacts with water according to equation (1). Chitosan-NH2 + H 2 0 ~ c h i t o s a n - N H ~ + O H - . (I) Hence, when chitosan is slurried with water, the pH increases slightly. The amount of hydroxide ion and chitosan cation produced is small, since the pKb for chitosan is 7.7 (as calculated from the pK, of 6.3, Nud'ga et al., 1970). The chitosan cation in solution associates with an anion to form an ion-exchange site according to equation (2). Chitosan-NH~" X - + Y chitosan-NH~Y- + Y- + X-
(2)
Table 1. Characteristics of Waverley well water (ppm) Sample Contaminated well water Et~uent from chitin/chito~an column • ND
Not determined.
Total hardness
Chloride
Total iron
Copper
Nitrate
Phosphate
115+10 (as CaCO3) 97 + 10 (as CaCO 3}
12+_2
0
0.04+-0.005
2+1
44___5
70 + 5
ND*
< 0.005
3+ 1
45 + 5
Sulphate 23+-3 1+ 2
!310
(LIVE M, ELSON. DONALD H. DAVIESand ERNESTR H",~!S
In acidic solution, the number of cationic sites is large, and chitosan has been reported to be an effectire anion-exchanger under these conditions (Muzzarelli, 1977a). In the present work. 3 mmol of arsenic were collected per gram of chitosan from acidic solutions, based on the arsenic content of the chitosanarsenic derivative. Such a capacity was comparable to that of typical anion-exchangers. However, when the pH of a water sample in contact with the chitosan; chitin mixture was raised, the capacity of the column used to treat the contaminated sample was 0.13 ~mol g-~ mixture at pH 7. For samples with pH greater than 8, the arsenic level was not lowered as corroborated by the distribution coefficients (Dg/ which decreased by a factor of 50 when the pH changed from 7.5 to 8.5. The variations in the capacity and Dg values with pH were presumably due to changes in the number of protonated, amine exchange sites. The number of sites was further reduced due to the collection of ions other than arsenic, such as copper (see Table 1 and Kurita et aL, 1979). This was confirmed by comparing the removal efficiencies of identical columns used to treat the contaminated water and a pure arsenic standard; the capacities were 0.13 gmol g - ~ and 0.50,umol g- ~ respectively, One other factor that was affected by pH was the form of the arsenic, We have established that the species present in the contaminated water was arsenic (V) but whether the simple arsenates, HzAsO2 and HAsOT, ~ (pK2 6.8) existed is not known but, according to Andreae (1977), the simple inorganic arsenates are the prodominant species, It has tgeen assumea above mat cnttosan ~s a more active ion-exchanger than chitin since the number of deacetylated nitrogen centres in chitosan is 5-6 times that of chitin. Furthermore, chitin demonstrated only 30'},~ of the capacity of chitosan for arsenic collection and when the proportion of chitin in the treatment mixture was increased the column efficiency decreased. Chitin did, however, neutralize the hydroxide produced by the reaction of chitosan with water (equation 1) and maintained the pH of the effluent near 7. Chitin probably contained hydrochloric acid bound as the hydrochloride salt of the deacetylated amines since HCI is used in the preparation of chitin (Muzzarelli, 1973) and since the effluent from the columns contained 5-45 times more chloride than the original samples (Table i), Treatment of the contaminated Waverley well water by the chitosan/chitin mixture altered the levels of several other ions. Of the polyoxy anions studied in addition to the arsenates, sulfate was removed from the sample while nitrate and phosphate were not In addition, the mixture effectively bound copper but not calcium, thus confirming earlier reports (Muzzarelli, 1977c). The increase in chloride concentration of the effluent has already been discussed. The method of preparation of the cross-hnked chitosan was expected to introduce c. 1.4 mmol of glutaraldehyde g - ~ of chitosan. Our sample of chitosan h ~
been reported to contain 5 mmol g ~ of amine (Haw-., & Davies, 1978): Hence. cross-linking the chm)sa,~ reduced the free amine content and potential c~exchange sites by approximately 35°;. The capacit) ~ a column employing the cross-linked chitosan,'chitJn mixture was 39°,~ less than the normal mixture. Th~ agreement between these two figures may ~.ell be ,¢('~ tuitous; however, it does confirm the importance ,,~ the number of ion-exchange sites to the collection el]iciency of the mixture. When a column that had been used ! , collect arsenic was washed with an ammonium chloride buffer, approximately 20°~o of the arsenic wa~ r~, covered, The mass spectral evidence, although not definitive, suggested that the chitosan-arsenic derivative was less volatile and more stable than the washed chitosan. These results indicate that the bonding between arsenic and the polymers involved more than simple ion-exchange. In the case of sulphate. Muzzar elli & Rochetti (1974) hypothesized that the structure of chitosan changes to accomodate two linkages between two amine sites and sulphate. However, the sulphate was still exchangeable whereas in the present case only 20°~, of the arsenic was. It is suggested, therefore, that chelation of the arsenic anion ~)ccurs in addition to electrostatic, ion-exchange. The importance of the ion-exchange process cannot be ignored as evidenced by the variation in capacity of the mixture from acidic to neutral solutions and the change in the distribution coefficients with pH. Furthermore, chitosan has been reported to form coordinate linkages to cations (Muzzarelli, 1977c) In conclusion, a chitosan/chitin mixture has beet: demonstrated to be capable of removing certain polyoxy-anions including arsenates from water samples of neutral p H However, the capacity of the mixture was approximately 0.1 ,umolg-~ which is l04 times smaller than commercial ion-exchangers. The arsenic was bound to the polymers by a combmatlon of electrostatic, ion-exchange attraction and chelation. REFERENCES
Andrear M. O. 11977) Determination of arsenic species m natural waters. Analyt Chem. 49, 820-823. Canadian Drinking Water Standards and Objectives (1968! Department of National Health and Welfare, Canada Hayes E. R. & Davies D H. (1978) Characterization of chitosan--II. The determination of the degree of acetylation of chitosan and chitin. Proc. O[ First International Conf. on Chitin/Chitosan~ (Edited by Mussarelli R A A & Pariser E. R.), pp. a06-420. Massachusetts Institute oJ Technology,MA. Kurita K., Sannan T & lwakura "v 11979) S(ud~e., :~ chitin. VI. Bonding of metal cations J 4ppl Potvm..%~ 23, 511-515 MasriM. A. & Randall "~ G. (19781Chitosan and cmtosat~ derivitiesfor removal of toxic metallic ions from manufacturing plant waste streams. Pro~. oIFirst lnter~mtionat Co,V on Chitm:Chito~w~ Edited b? Muzzarelli R A. ~, & Pariser E RI, pp. 2"7 2R7. Ma,;sachusetts Institute o{ Technology,MA. MuzzarelliR .~ ~ {19"r?a~ ('hittn, p t45 Pergam~m Pr~s,. (3"~for,:]
Removal of arsenic from contaminated drinking water Muzzarelli R. A. A. (1977b) Chitin p. 183. Pergamon Press, Oxford. Muzzarelli R. A. A. (1977c) Chitin, p. 140. Pergamon Press, Oxford. Muzzarelli. R. A. A. (1973) Natural Chelating Polymers, p. 96. Pergamon Press, Oxford. Muzzarelli R. A. A. & Rochetti R. (1974) Enhanced capacity of chitosan for transition-metal ions in sulphatesulphuric and solutions. Talanta 11, 1137-1143. Muzzarelli R. A. A. & Tubertini O. (1972) Radiation resist-
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anee of chitin and chitosan applied in the chromatography of radioactive substances. J. Radioanalyt. Chem. 12, 431--440. Nud'ga L. A., Plisko E. A. & Danilou S. M. (1970) Zh. obshch.Khim. 41, 2550--2560. Reinke J., Uthe J. F., Freeman H. C. & Johnston F. R. (1975) The determination of arsenite and arsenate ions in fish and shellfish by selective extraction and polarography. Environ. Lett. 8, 371-379.