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Colloid-chemical properties of chitosan
Studies in Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) c; 2001 Elsevier Science B.V. All rights reserved.
221
Colloid-chemical properties of chitosan Bratskaya S.Yu., Shamov M.V., Avramenko V.A., Chervonetskiy D.V. Institute of Chemistry, Far East Department of the Russian Academy of Sciences, 159, Prosp. 100-letya Vladivostoka, Vladivostok 690022, Russia Flocculation properties of chitosan in solutions of humic substances were investigated at various pH and ratios chitosan to humic acid. Effect of additions of Fe^"^ on the flocculation effectiveness was also studied. Experimental results on chitosan interaction with carbonic acids of low molecular weight have shown that hydrophobic internal domains of chitosan helices along with chitosan amine groups play an important role in chitosan interaction with organic substances. INTRODUCTION In the recent years, chitosan has been attracting the researches interest as a very promising material with widely ranging applications in water treatment, food and pharmaceutical industries, cosmetology, and biotechnology. Chitosan is a basic polymer of helix structure having reactive amine groups that gives a lot of possibilities of modification and ionic interactions. Combination of chitosan reactive amine groups and helix structure with internal hydrophobic domains determines such colloid-chemical properties of chitosan as organic substances sorption, flocculation, and stabilization of colloid systems that are of great interest both in theoretical terms and in industrial application possibilities. In this paper we discuss use of chitosan for humic substances flocculation. Some results on carbonic acids sorption on chitosan are also presented to illustrate the mechanism of chitosan interaction with organic substances. EXPERIMENTAL Chitosan flakes (dacetylation degree=75%) obtained from crab shells were used in all experiments. Humic acids were isolated from peat [1], amide of humic acids was obtained by heating humic acid solution with NH3 at 140 ° C and high pressure for 5h. For testing chitosan flocculation properties, 1 g of chitosan flakes were dissolved in 100 ml of O.IN HCl solution. Fixed amount of prepared chitosan solution was added to solutions of humic substances with concentration of 50-500 g/L and pH was adjusted to 7. After 24h solutions were filtered and optical density was measured at 413nm. Effectiveness of flocculation was calculated as a percentage of color removal. Interaction of chitosan with carbonic acids was studied as follows: 40mg of dry chitosan flakes and 20 ml of acidic solutions with concentrations from 1 up to 100 ^mol/L were shaken for 24h at controlled temperature 20''C. When equilibrium was established, the equilibrium acid concentration was determined by potentiometric titration with "Radelkis 0P211/1" pH-meter and "ORION 96-21" combination pH-electrode as described in [2].
222 The same method of titration was used to determine pK-distribution in humic acids and their amide. RESULTS AND DISCUSSION Flocculation of humic substances is of great interest for decontamination of natural and waste water from humic substances and metals bound to them as well as for recovery of valuable metals from technological solutions. Conventional method of humic acids removal from solutions is flocculation at pH=l-2 but it does not allow obtaining color removal higher than 80-90%. Our experimental results have shown that flocculation effectiveness of 96-100% can be achieved at pH=6.5-7.5 when chitosan is used as a flocculant in humic acid solutions Fig.l. This figure also illustrates that too high content of chitosan is resulted in the flocculation effectiveness reduction because of the stabilization of humic colloids by chitosan. Effect of chitosan concentration on effectiveness of humic acids and their amides flocculation is shovm in Fig.2.
30
40
50
60
Cchitosan'lO^.^
Fig. 1. Effectiveness of humic acids flocculation at ratios chitosan to humic: • -1:5, •-1:2, A- 1:1, T.2:1,•-4:1
Fig.2. Effect of chitosan concentration on flocculation of humic acids: •-lOOmg/1,. •-500mg/l; amide of humic acids: A-lOOmg/1, T-500mg/l.
It is obvious that chitosan concentration required for effective flocculation depends on humic acid concentration in solution and the nature of their functional groups. As a result of modification, amide of humic acids does not contain functional groups with pK less than 6.8 while original humic acids have carboxylic groups with average pK equal to 4 and 5.5 - Fig.3. Thus, amide of humic acids has a lower charge density in comparison vsdth original humic acids, and, therefore, less amount of chitosan is required to satisfy "cationic demand" of negatively charged colloid particles of humic acid amide. Fig.4 illustrates flocculation of humic acids by chitosan when Fe3+ is added. It is seen that in this case effective flocculation was obtained at significantly lower concentrations of chitosan. It should also be mentioned that even very high concentrations of Fe3-»- used as a coagulant without chitosan addition do not provide effective flocculation of humic acids. Thus, charge neutralization is not the only reason of chitosan effective work as a flocculant.
223 100
0.020
I-
0.015 i
humic acids humic acids amide
0.010
0.005 i
0.000 0
1
2
3
4
5
6
Cchitosan-103.%
Fig.3. pK-distribution o f functional groups of humic acids and their amide
Fig.4. Effect of Fe^* on humic acids (HA) and their amide (AHA) flocculation: • -HA(100mg/l)+ Fe^*(50mg/1), • - H A ( 5 0 0 m g / l ) +Fe^*n00mg/1), A- AHA(100mg/l)+ Fe^*(50mg/I),
W e assume that chitosan internal hydrophobic domains aside from its amine groups play a n important role i n interaction o f chitosan with organic substances including humic acids. T o confirm this assumption w e have studied interaction o f chitosan with homologous series o f carbonic acids - acetic, propionic, butyric, n-valeric and isovaleric acids in aqueous and water-ethanol solutions. Strong correlation w a s found b e t w e e n m a x i m u m sorption o f acid on chitosan and its p K value in aqueous solution with the only exception - isovaleric acid Fig.5. 2.2
2.2
2.0
2.0
y^valericadd aceticacid
propionic add butyric add
I 1.8 \ acetic aa
r
J
V""^* \
•
1
•
c 1.6 o
butyric a d d ^ \ ^
isQK/afericacid
\
]
vatericadd \ ^
/ \\
1.2 propionic add
1.0 4.74
\ \
CO
1.2
isovaleric add •
butyric add
\
I 1.4
I
//
\
\ valeric add
propionic add 1.0
4.76
4.78
4.80
4.82
4.84
4.86
pK Fig. 5. Correlation between maximum sorption on chitosan and pK of carbonic acids: • - in aqueous solution
4.8
i
2
3
4
5
the number of carbon atonns Fig.6. Correlation between hydrocarbon radical length of carbonic acids and their maximum sorption on chitosan: • - in water/edianol (2:1) solution
6
224
Taking into account that this acid has a branched hydrocarbon radical we suggested that structure and length of the radical can also effect carbonic acids sorption on chitosan. This effect should be more explicit in solvent of polarity less than polarity of water, where hydrophobic interactions play more important role, for instance, in water/ethanol solutions. Our experimental results have shown that there is no strong correlation between pK and maximum sorption of carbonic acids on chitosan in water/ethanol solutions. But if we plot maximum sorption versus hydrocarbon radical length, we observe increase of sorption with increase of radical length (Fig.6) that was not observed for aqueous solutions. In water/ethanol solutions internal hydrophobic domains of chitosan helices become more accessible for interaction with hydrocarbon radical of carbonic acids that can result in acid trapping inside chitosan helices. It is obvious that hydrophobic interaction between chitosan and carbonic acids will be stronger with increase of the acid hydrocarbon radical length. In our experiments valeric acid has shown the highest values of sorption in water/ethanol solutions. Nevertheless, isovaleric acid with the same number of carbon atoms has the smallest value of sorption. This effect can be explained by space hindrances during penetration of a branched hydrocarbon radical inside chitosan helices. CONCLUSION We conclude that chitosan can be used as a very effective and biodegradable [3] flocculant of humic substances. Amount of chitosan required for effective flocculation depends on humic acid origin and concentration as well as on content of metal ions, especially Fe"'"*". Chitosan shows the best flocculation properties in pH range from 6.5 up to 7.5, where the highest values (up to 100%) of color removal were obtained. Summarizing the results obtained on sorption of low molecular weight carbonic acids on chitosan, we can conclude that the structure of hydrocarbon radical of the acid effect its sorption on chitosan due to possibility of hydrophobic interaction with internal domains of macromolecular helices. This effect becomes stronger with increase of the hydrocarbon radical length. Thus, we can suggest that for high molecular weight humic acids containing fragments with aromatic and long aliphatic radicals [4] hydrophobic interaction with chitosan should play a very important role. Most likely this type of interaction, aside from ionic interaction with chitosan amine groups, determines high effectiveness of chitosan as a flocculant in humic acids solutions. REFERENCES 1. 2. 3. 4.
Kemdorff H., Schnitzer M., Geochim. et Cosmochim. Acta, Xe 11 (1980) 1701. Bratskaya S.Yu., Golikov A.P., J.Anal.Chem., XeS (1998) 234. Boryniec S., Ratajska M., Fibers and Text. East.Eur. Xo4 (1995) 60. Orlov D.S. Soil Chemistry, Moscow State University, Moscow, 1992.
221
Colloid-chemical properties of chitosan Bratskaya S.Yu., Shamov M.V., Avramenko V.A., Chervonetskiy D.V. Institute of Chemistry, Far East Department of the Russian Academy of Sciences, 159, Prosp. 100-letya Vladivostoka, Vladivostok 690022, Russia Flocculation properties of chitosan in solutions of humic substances were investigated at various pH and ratios chitosan to humic acid. Effect of additions of Fe^"^ on the flocculation effectiveness was also studied. Experimental results on chitosan interaction with carbonic acids of low molecular weight have shown that hydrophobic internal domains of chitosan helices along with chitosan amine groups play an important role in chitosan interaction with organic substances. INTRODUCTION In the recent years, chitosan has been attracting the researches interest as a very promising material with widely ranging applications in water treatment, food and pharmaceutical industries, cosmetology, and biotechnology. Chitosan is a basic polymer of helix structure having reactive amine groups that gives a lot of possibilities of modification and ionic interactions. Combination of chitosan reactive amine groups and helix structure with internal hydrophobic domains determines such colloid-chemical properties of chitosan as organic substances sorption, flocculation, and stabilization of colloid systems that are of great interest both in theoretical terms and in industrial application possibilities. In this paper we discuss use of chitosan for humic substances flocculation. Some results on carbonic acids sorption on chitosan are also presented to illustrate the mechanism of chitosan interaction with organic substances. EXPERIMENTAL Chitosan flakes (dacetylation degree=75%) obtained from crab shells were used in all experiments. Humic acids were isolated from peat [1], amide of humic acids was obtained by heating humic acid solution with NH3 at 140 ° C and high pressure for 5h. For testing chitosan flocculation properties, 1 g of chitosan flakes were dissolved in 100 ml of O.IN HCl solution. Fixed amount of prepared chitosan solution was added to solutions of humic substances with concentration of 50-500 g/L and pH was adjusted to 7. After 24h solutions were filtered and optical density was measured at 413nm. Effectiveness of flocculation was calculated as a percentage of color removal. Interaction of chitosan with carbonic acids was studied as follows: 40mg of dry chitosan flakes and 20 ml of acidic solutions with concentrations from 1 up to 100 ^mol/L were shaken for 24h at controlled temperature 20''C. When equilibrium was established, the equilibrium acid concentration was determined by potentiometric titration with "Radelkis 0P211/1" pH-meter and "ORION 96-21" combination pH-electrode as described in [2].
222 The same method of titration was used to determine pK-distribution in humic acids and their amide. RESULTS AND DISCUSSION Flocculation of humic substances is of great interest for decontamination of natural and waste water from humic substances and metals bound to them as well as for recovery of valuable metals from technological solutions. Conventional method of humic acids removal from solutions is flocculation at pH=l-2 but it does not allow obtaining color removal higher than 80-90%. Our experimental results have shown that flocculation effectiveness of 96-100% can be achieved at pH=6.5-7.5 when chitosan is used as a flocculant in humic acid solutions Fig.l. This figure also illustrates that too high content of chitosan is resulted in the flocculation effectiveness reduction because of the stabilization of humic colloids by chitosan. Effect of chitosan concentration on effectiveness of humic acids and their amides flocculation is shovm in Fig.2.
30
40
50
60
Cchitosan'lO^.^
Fig. 1. Effectiveness of humic acids flocculation at ratios chitosan to humic: • -1:5, •-1:2, A- 1:1, T.2:1,•-4:1
Fig.2. Effect of chitosan concentration on flocculation of humic acids: •-lOOmg/1,. •-500mg/l; amide of humic acids: A-lOOmg/1, T-500mg/l.
It is obvious that chitosan concentration required for effective flocculation depends on humic acid concentration in solution and the nature of their functional groups. As a result of modification, amide of humic acids does not contain functional groups with pK less than 6.8 while original humic acids have carboxylic groups with average pK equal to 4 and 5.5 - Fig.3. Thus, amide of humic acids has a lower charge density in comparison vsdth original humic acids, and, therefore, less amount of chitosan is required to satisfy "cationic demand" of negatively charged colloid particles of humic acid amide. Fig.4 illustrates flocculation of humic acids by chitosan when Fe3+ is added. It is seen that in this case effective flocculation was obtained at significantly lower concentrations of chitosan. It should also be mentioned that even very high concentrations of Fe3-»- used as a coagulant without chitosan addition do not provide effective flocculation of humic acids. Thus, charge neutralization is not the only reason of chitosan effective work as a flocculant.
223 100
0.020
I-
0.015 i
humic acids humic acids amide
0.010
0.005 i
0.000 0
1
2
3
4
5
6
Cchitosan-103.%
Fig.3. pK-distribution o f functional groups of humic acids and their amide
Fig.4. Effect of Fe^* on humic acids (HA) and their amide (AHA) flocculation: • -HA(100mg/l)+ Fe^*(50mg/1), • - H A ( 5 0 0 m g / l ) +Fe^*n00mg/1), A- AHA(100mg/l)+ Fe^*(50mg/I),
W e assume that chitosan internal hydrophobic domains aside from its amine groups play a n important role i n interaction o f chitosan with organic substances including humic acids. T o confirm this assumption w e have studied interaction o f chitosan with homologous series o f carbonic acids - acetic, propionic, butyric, n-valeric and isovaleric acids in aqueous and water-ethanol solutions. Strong correlation w a s found b e t w e e n m a x i m u m sorption o f acid on chitosan and its p K value in aqueous solution with the only exception - isovaleric acid Fig.5. 2.2
2.2
2.0
2.0
y^valericadd aceticacid
propionic add butyric add
I 1.8 \ acetic aa
r
J
V""^* \
•
1
•
c 1.6 o
butyric a d d ^ \ ^
isQK/afericacid
\
]
vatericadd \ ^
/ \\
1.2 propionic add
1.0 4.74
\ \
CO
1.2
isovaleric add •
butyric add
\
I 1.4
I
//
\
\ valeric add
propionic add 1.0
4.76
4.78
4.80
4.82
4.84
4.86
pK Fig. 5. Correlation between maximum sorption on chitosan and pK of carbonic acids: • - in aqueous solution
4.8
i
2
3
4
5
the number of carbon atonns Fig.6. Correlation between hydrocarbon radical length of carbonic acids and their maximum sorption on chitosan: • - in water/edianol (2:1) solution
6
224
Taking into account that this acid has a branched hydrocarbon radical we suggested that structure and length of the radical can also effect carbonic acids sorption on chitosan. This effect should be more explicit in solvent of polarity less than polarity of water, where hydrophobic interactions play more important role, for instance, in water/ethanol solutions. Our experimental results have shown that there is no strong correlation between pK and maximum sorption of carbonic acids on chitosan in water/ethanol solutions. But if we plot maximum sorption versus hydrocarbon radical length, we observe increase of sorption with increase of radical length (Fig.6) that was not observed for aqueous solutions. In water/ethanol solutions internal hydrophobic domains of chitosan helices become more accessible for interaction with hydrocarbon radical of carbonic acids that can result in acid trapping inside chitosan helices. It is obvious that hydrophobic interaction between chitosan and carbonic acids will be stronger with increase of the acid hydrocarbon radical length. In our experiments valeric acid has shown the highest values of sorption in water/ethanol solutions. Nevertheless, isovaleric acid with the same number of carbon atoms has the smallest value of sorption. This effect can be explained by space hindrances during penetration of a branched hydrocarbon radical inside chitosan helices. CONCLUSION We conclude that chitosan can be used as a very effective and biodegradable [3] flocculant of humic substances. Amount of chitosan required for effective flocculation depends on humic acid origin and concentration as well as on content of metal ions, especially Fe"'"*". Chitosan shows the best flocculation properties in pH range from 6.5 up to 7.5, where the highest values (up to 100%) of color removal were obtained. Summarizing the results obtained on sorption of low molecular weight carbonic acids on chitosan, we can conclude that the structure of hydrocarbon radical of the acid effect its sorption on chitosan due to possibility of hydrophobic interaction with internal domains of macromolecular helices. This effect becomes stronger with increase of the hydrocarbon radical length. Thus, we can suggest that for high molecular weight humic acids containing fragments with aromatic and long aliphatic radicals [4] hydrophobic interaction with chitosan should play a very important role. Most likely this type of interaction, aside from ionic interaction with chitosan amine groups, determines high effectiveness of chitosan as a flocculant in humic acids solutions. REFERENCES 1. 2. 3. 4.
Kemdorff H., Schnitzer M., Geochim. et Cosmochim. Acta, Xe 11 (1980) 1701. Bratskaya S.Yu., Golikov A.P., J.Anal.Chem., XeS (1998) 234. Boryniec S., Ratajska M., Fibers and Text. East.Eur. Xo4 (1995) 60. Orlov D.S. Soil Chemistry, Moscow State University, Moscow, 1992.