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Removal of humic acid from water by precipitate flotation using cationic surfactants
Minerals Engineering 20 (2007) 945–949 This article is also available online at: www.elsevier.com/locate/mineng
Removal of humic acid from water by precipitate flotation using cationic surfactants Mariana Coutinho Brum, Jose´ Farias Oliveira
*
Department of Metallurgical and Materials Engineering, COPPE/UFRJ, Federal University of Rio de Janeiro, P.O. Box 68505, Rio de Janeiro-RJ, Brazil Received 14 December 2006; accepted 16 March 2007 Available online 10 May 2007
Abstract Humic acid (HA) is an ubiquitous substance in the earth environment and is very important regarding soil fertility. It has been shown that HA can be used as a depressant for hematite in iron ore flotation. However, its removal from water is important in many circumstances, because the treatment of the water with chlorine results in the formation of trihalomethanes that are carcinogenic products. In this work the precipitate flotation of HA was studied. The experiments were carried out with the use of cetyl trimethyl ammonium bromide (CTAB) and dodecylamine (DDA) as precipitant collectors. The mean diameter of the particles were determined by light scattering for both precipitates (HA/CTAB and HA/DDA). The electrophoretic mobility studies showed that the interaction HA/CTAB was much stronger than HA/DDA. The precipitate flotation experiments carried out in a column cell showed that over 90% of the HA can be removed from water without the additional use of any other reagents. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Environmental; Fine particles processing; Flotation reagents
1. Introduction Humic substances (HS) are present in natural waters in concentrations ranging from 20 lg/l in groundwaters up to 30 mg/l in surface waters. Humic acid (HA) is a subclass of humic substances that is soluble in water at pH > 2. This macromolecule has a complex structure containing phenolic and carboxylic groups thus carrying negative charges in natural waters (Jones and Bryan, 1998; Suffet and Maccarthy, 1989). Despite the importance of HA regarding soil fertility its presence in the environment can be a problem due to the formation of the complex HA-metal ions which leads to metal transportation and release in soil (Stumm and Morgan, 1981). However, one of the major concerns is the environmental problem caused by the presence of HA in waters *
Corresponding author. Tel.: +55 21 25628532; fax: +55 21 22901615. E-mail address: [email protected] (J.F. Oliveira).
0892-6875/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2007.03.004
that will undergo chlorine treatment for human population consumption. This may result in the formation of trihalomethanes that are carcinogenic products and have their presence limited to 100 ppb in potable water by the World Health Organization (Baird, 2002). Due to the aforementioned problems the removal of HA from water is necessary and some techniques have been used successfully. The coagulation of HA by aluminum sulphate has been studied by Cassel et al. (1975) and Duan et al. (2002), and it is also used in treatment plants to remove humic substances from water (Odegaard et al., 1999). Other techniques were used to remove HA such as natural adsorbents (Daifullah et al., 2004; Zhang and Bai, 2003) and cationic polymers (Kim and Walker, 2001). However, only few studies have dealt with the removal of HA using the cationic quaternary ammonium compound cetyl trimethyl ammonium bromide (CTAB). Adou et al. (2001) proposed a technique to remove humic substances from water that combined the use of CTAB and
946
M.C. Brum, J.F. Oliveira / Minerals Engineering 20 (2007) 945–949
a polymeric synthetic adsorbent in a column. Zouboulis et al. (2003) proposed flotation of HA with CTAB and ethanol as a frother to remove high concentration of HA from landfill leachates. In the present study two different cationic surfactants were used to precipitate low dosage (20 ppm) of HA and their effect has been investigated. A column flotation was designed to generate small bubbles and remove efficiently HA from water.
scattering in a Malvern Mastersizer Miro ‘‘Plus’’, MAF 5001 equipment. A schematic representation of the experimental set up used for flotation experiments is shown in Fig. 1. The total volume of suspension used was 120 ml, and after injection of the gas the froth containing HA was collected from the overflow. After 5 min of flotation the treated suspension remaining in the cell was sampled for analysis. 3. Results and discussion
2. Experimental The humic acid (HA) used in the present work was purchased from ALDRICH and the 5000 ppm HA stock solution was prepared with NaOH 0.1 M (Zouboulis et al., 2003). The surfactant cetyl trimethyl ammonium bromide (CTAB) was supplied by MERCK and the dodecylamine by FLUKA. The zeta potential studies were conducted in a Rank Brothers Mark II equipment with a flat cell, attached to a video camera and a rotating prism that allows the measurement of the particles electrophoretic mobility. A constant ionic strength based on NaCl concentration of 10 3 M was used in all these tests. The humic acid precipitation tests were conducted in a 50 ml glass beaker by contacting HA with the surfactant for 10 min under agitation with a magnetic stirrer. To quantify the precipitation of HA by the surfactants CTAB and DDA the precipitates were centrifuged in a Thermo IEC, Centra-CL3 equipment at 4000 rpm during 20 min. The HA concentration in the supernatant was then analyzed by using a UV–vis 1601 PC (Shimadzu Corporation) spectrophotometer at a wavelength of 254 nm. The choice of wavelength was based on Standard Methods for the Examination of Water and Wastewater, 1998. The mean diameter of the precipitated particles was measured by light
Humic acid molecules generally present an excess negative surface charge as the pH is increased due to the dissociation of carboxylic and phenolic groups (Daifullah et al., 2004). The higher negative values of zeta potential of the HA/DDA precipitates compared to those of HA/CTAB, shown in Fig. 2, indicate a lower neutralization of the HA negative charges when DDA is used. This is probably due to a higher adsorption of CTAB on HA as compared to DDA. The precipitates obtained with CTAB change their charge from negative to positive (Fig. 3) when the surfactant concentration is increased. However, it can be seen also from Fig. 3 that when DDA is used, the precipitate formed does not reverse its charge. The initial precipitation tests were performed in a HA concentration range from 10 ppm up to 70 ppm. The surfactants CTAB and DDA were studied in order to search the influence of different surfactant structures on the precipitation of HA. The minimum and maximum concentration of CTAB needed to precipitate HA at varying concentration at pH 6.0 are shown in Fig. 4 while Fig. 5 presents the results of similar experiments carried out with DDA. The region of precipitation is indicated. Additional precipitation tests were preformed with less than 10 ppm of HA in solution. In this case precipitates could no longer be observed but just a slight turbidity of the solution. This 0
Zeta potential (mV)
-20
-40
-60
-80 DDA (50 ppm) -100
CTAB (50 ppm)
-120 0
2
4
6
8
10
pH Fig. 1. Schematic representation of the flotation column (Column height: 15 cm, inner diameter: 5 cm).
Fig. 2. The effect of pH on the zeta potential of HA (100 ppm) precipitated with CTAB (50 ppm) and DDA (50 ppm).
M.C. Brum, J.F. Oliveira / Minerals Engineering 20 (2007) 945–949
80 CTAB
Zeta potential (mV)
60
DDA
40 20 0 -20 -40 -60 -80 0
20
40
60
80
100
120
Concentration (ppm) Fig. 3. The effect of the type and collector’s concentration on the zeta potential of the precipitates formed with HA (20 ppm).
947
was the main reason for choosing a concentration of HA in solution higher than 10 ppm to perform subsequent tests. The main difference between the precipitation of HA/ CTAB (Fig. 4) and HA/DDA (Fig. 5) is that a much lower concentration of CTAB is required to start the precipitation as compared to DDA. This phenomenon might be explained as being due to the length of non-polar portion of the surfactants molecule. In the case of CTAB a higher insolubility leads to a wider region of precipitation regarding the results presented in Figs. 4 and 5. The detailed results of the precipitation experiments using CTAB and DDA are presented in Figs. 6 and 7, respectively. They show the influence of the surfactant itself and of the surfactant’s concentration and pH on the precipitation of humic acid. The same HA starting concentration was used in both cases (20 ppm). 100
Humic acid precipitation (%)
Concentration, CTAB (ppm)
70 60 Precipitation region
50 40 30 20
80
60
40 pH=4.0 pH=6.0 pH=8.0
20
10
0
0
0 0
10
20 30 40 50 Concentration, Humic acid (ppm)
60
20
40
70
Fig. 4. Minimum and maximum concentration of CTAB required to precipitate humic acid as a function of HA concentration at pH 6.0.
60
80
100
CTAB, ppm Fig. 6. The influence of the concentration of CTAB and pH on the precipitation of HA (20 ppm) in 10 min.
100 Precipitation region
60
Humic acid precipitation (%)
Concentration, DDA (ppm)
70
50 40 30 20 10 0
80
60
40
pH=4.0 pH=6.0 pH=8.0
20
0
0
10
20
30
40
50
60
70
Concentration, Humic acid (ppm) Fig. 5. Minimum concentration of DDA required to precipitate humic acid as a function of HA concentration at pH 6.0.
20
40
60
80
100
120
DDA, ppm
Fig. 7. The influence of the concentration of DDA and pH on the precipitation of HA (20 ppm) in 10 min.
M.C. Brum, J.F. Oliveira / Minerals Engineering 20 (2007) 945–949
Fig. 6 shows that the CTAB concentration necessary to precipitate approximately 80% of HA at pH 4.0 is about 20 ppm. On the other hand, for pH 6.0 and pH 8.0 the maximum is attained only with concentration around 40 ppm. The results also show that as the CTAB concentration is increased the precipitation reaches a maximum and decreases subsequently (Fig. 6). In this system, the precipitation probably decreases due to the zeta potential of the precipitate changing its charge from negative to positive (Fig. 3). Zouboulis et al. (2003) have also observed a maximum removal of HA when using CTAB near the isoelectric point of the precipitate. However, as can be seen in Fig. 3, the increase in DDA concentration does not reverse the charge of the precipitate and this fits the trend of the precipitation curves, obtained with DDA, presented in Fig. 7. The effect of pH is significant in both systems. When the pH is increased a higher concentration of precipitant collector is needed. This is probably due to the increased number of dissociated carboxylic groups at the HA molecule as the pH is increased from 4.0 to 8.0. In Table 1 are shown the mean diameter of the agglomerated particles obtained for the two systems. The analytical concentration of the components was the same in both cases for three different agitation speeds. The reduction of the particles mean diameter in both systems is an expected result when higher agitation is used. The particles size determination, however, was very important to interpret the flotation results, because flotation efficiency is influenced by the particle size of the precipitates. The fragility of the flocs can also be observed from Table 1 because when the agitation speed increases the particle size decreases. The particle size is not much different when using CTAB or DDA, when low agitation speed is used. Because of the much lower consumption to precipitate the HA, CTAB was selected as the most promising one. The flotation experiments were performed under mild agitation to avoid the flocs breakage, since they were too fragile as observed above. The HA flotation as a function of the CTAB collector concentration is shown in Fig. 8. A reduction of flotation occurs for higher collector concentration due to zeta potential effect on the precipitation of HA. It can also be seen that the maximum removal of HA is achieved at a concentration coincident with the CTAB con-
100 80 Flotation (%)
948
60 40 20 0 0
10
20
30
40
50
60
Collector concentration, CTAB (ppm) Fig. 8. Flotation of HA as a function of the precipitant collector concentration at pH 6.0. (HA starting concentration: 20 ppm, flotation time: 5 min).
Fig. 9. Schematic representation of the system HA/surfactant due to the increase of the surfactant’s concentration.
centration for maximum precipitation (Fig. 6). Adou et al. (2001) have studied the precipitation of HA with CTAB aiming at the adsorption onto polypropylene. They also observed that CTAB concentration is a critical parameter. In the present study, the maximum removal of HA (20 ppm) at pH 6.0 is achieved at 30 ppm CTAB concentration (Fig. 8). In the present study the surfactants had, in fact, a triple function. They are at the same time precipitant agents, collectors and frothers. The cationic polar portion of the surfactant bound electrostatically to the dissociated carboxylic groups present in HA molecule forming precipitates. However, as the surfactant concentration is increased the nonpolar portions of the surfactant molecules interact through van der Waals forces forming hemi-micelles as shown in Fig. 9. This affects the hydrophobicity of the precipitate which shows a decrease in floatability for higher concentration of the surfactant (Fig. 8).
Table 1 Influence of stirring speed on the mean diameter of the particles
4. Conclusions
System
Rotation (rpm)
Particles mean diameter (lm)
HA-CTAB (30–50 ppm)
600 1000 2000
43.35 39.00 11.43
HA-DDA (30–50 ppm)
600 1000 2000
56.99 44.08 29.67
The studies showed the difference in the adsorption of the surfactants used to precipitate HA and revealed a higher adsorption of CTAB compared to that of DDA. The concentration of precipitant collector required to achieve maximum removal increases with the pH. However after exceeding the precipitant collectors optimum concentration, the removal tends to zero in the system HA/CTAB.
M.C. Brum, J.F. Oliveira / Minerals Engineering 20 (2007) 945–949
The column projected with a glass frit was effective in generating tiny bubbles to remove by flotation 95% of HA (20 ppm) in the HA/CTAB (30 ppm) system. The results show that if all the parameters are well adjusted flotation can be a useful technique to remove humic acid from water even when present at apparently low concentrations but that are high enough to cause problems in aquatic systems. Acknowledgement The authors acknowledge the Conselho Nacional de Pesquisas – CNPq, Brazil, for its financial support. References Adou, A.F.Y., Muhandiki, V.S., Shimizu, Y., Matsui, S., 2001. A new economical method to remove humic substances in water: adsorption onto a recycled polymeric material with surfactant addition. Water Science and Technology 43 (11), 1–7. Baird, C., 2002. Quı´mica Ambiental, second ed. Bookman, Porto Alegre. Cassel, E.A., Kaufman, K.M., Matijevic, E., 1975. The Effects of Bubble Size on Microflotation, Water Research. The Journal of the International Association on Water Pollution Research 9 (12), 1017–1024. Daifullah, A.A.M., Girgis, B.S., Gada, H.M.H., 2004. A study of the factors affecting the removal of humic acid by activated carbon
949
prepared from biomass material. Colloids and Surfaces A: Physicochemical and Engineering Aspects 235, 1–10. Duan, J., Wang, J., Graham, N., Wilson, F., 2002. Coagulation of humic acid by aluminium sulphate in saline water conditions. Desalination 150, 1–14. Jones, M.N., Bryan, N.D., 1998. Colloidal properties of humic substance. Advances in Colloid and Interface Science 78, 1–48. Kim, E.K., Walker, H.W., 2001. Effect of cationic polymer additives on the adsorption of humic acid onto iron oxide particles. Colloids and Surfaces A: Physicochemical and Engineering Aspects 194, 123–131. Odegaard, H., Eikebrokk, B., Storhaug, R., 1999. Processes for the removal of humic substances from water: an overview based on Norwegian experiences. Water Science and Technology 40 (9), 37–46. Standard Methods for the Examination of Water and Wastewater, 1998. 20th ed. American Public Health Association, American Water Works Association and Water Environment Federation, Washington, DC. Stumm, W., Morgan, J.J., 1981. Aquatic Chemistry: An Introduction Emphasizing Chemical Equilibria in Natural Waters. John Wiley, New York. Suffet, I.H., Maccarthy, P., 1989. Aquatic Humic Substances: Influence on Fate and Treatment of Pollutants. American Chemical Society, Washington, DC. Zhang, X., Bai, R., 2003. Mechanisms and kinetics of humic acid adsorption onto chitosan – coated granules. Journal of Colloid and Interface Science 264, 30–38. Zouboulis, A.I., Jun, W., Katsoyiannis, I.A., 2003. Removal of humic acids by flotation. Colloids and Surfaces A: Physicochemical and Engineering Aspects 231, 181–193.
Removal of humic acid from water by precipitate flotation using cationic surfactants Mariana Coutinho Brum, Jose´ Farias Oliveira
*
Department of Metallurgical and Materials Engineering, COPPE/UFRJ, Federal University of Rio de Janeiro, P.O. Box 68505, Rio de Janeiro-RJ, Brazil Received 14 December 2006; accepted 16 March 2007 Available online 10 May 2007
Abstract Humic acid (HA) is an ubiquitous substance in the earth environment and is very important regarding soil fertility. It has been shown that HA can be used as a depressant for hematite in iron ore flotation. However, its removal from water is important in many circumstances, because the treatment of the water with chlorine results in the formation of trihalomethanes that are carcinogenic products. In this work the precipitate flotation of HA was studied. The experiments were carried out with the use of cetyl trimethyl ammonium bromide (CTAB) and dodecylamine (DDA) as precipitant collectors. The mean diameter of the particles were determined by light scattering for both precipitates (HA/CTAB and HA/DDA). The electrophoretic mobility studies showed that the interaction HA/CTAB was much stronger than HA/DDA. The precipitate flotation experiments carried out in a column cell showed that over 90% of the HA can be removed from water without the additional use of any other reagents. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Environmental; Fine particles processing; Flotation reagents
1. Introduction Humic substances (HS) are present in natural waters in concentrations ranging from 20 lg/l in groundwaters up to 30 mg/l in surface waters. Humic acid (HA) is a subclass of humic substances that is soluble in water at pH > 2. This macromolecule has a complex structure containing phenolic and carboxylic groups thus carrying negative charges in natural waters (Jones and Bryan, 1998; Suffet and Maccarthy, 1989). Despite the importance of HA regarding soil fertility its presence in the environment can be a problem due to the formation of the complex HA-metal ions which leads to metal transportation and release in soil (Stumm and Morgan, 1981). However, one of the major concerns is the environmental problem caused by the presence of HA in waters *
Corresponding author. Tel.: +55 21 25628532; fax: +55 21 22901615. E-mail address: [email protected] (J.F. Oliveira).
0892-6875/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2007.03.004
that will undergo chlorine treatment for human population consumption. This may result in the formation of trihalomethanes that are carcinogenic products and have their presence limited to 100 ppb in potable water by the World Health Organization (Baird, 2002). Due to the aforementioned problems the removal of HA from water is necessary and some techniques have been used successfully. The coagulation of HA by aluminum sulphate has been studied by Cassel et al. (1975) and Duan et al. (2002), and it is also used in treatment plants to remove humic substances from water (Odegaard et al., 1999). Other techniques were used to remove HA such as natural adsorbents (Daifullah et al., 2004; Zhang and Bai, 2003) and cationic polymers (Kim and Walker, 2001). However, only few studies have dealt with the removal of HA using the cationic quaternary ammonium compound cetyl trimethyl ammonium bromide (CTAB). Adou et al. (2001) proposed a technique to remove humic substances from water that combined the use of CTAB and
946
M.C. Brum, J.F. Oliveira / Minerals Engineering 20 (2007) 945–949
a polymeric synthetic adsorbent in a column. Zouboulis et al. (2003) proposed flotation of HA with CTAB and ethanol as a frother to remove high concentration of HA from landfill leachates. In the present study two different cationic surfactants were used to precipitate low dosage (20 ppm) of HA and their effect has been investigated. A column flotation was designed to generate small bubbles and remove efficiently HA from water.
scattering in a Malvern Mastersizer Miro ‘‘Plus’’, MAF 5001 equipment. A schematic representation of the experimental set up used for flotation experiments is shown in Fig. 1. The total volume of suspension used was 120 ml, and after injection of the gas the froth containing HA was collected from the overflow. After 5 min of flotation the treated suspension remaining in the cell was sampled for analysis. 3. Results and discussion
2. Experimental The humic acid (HA) used in the present work was purchased from ALDRICH and the 5000 ppm HA stock solution was prepared with NaOH 0.1 M (Zouboulis et al., 2003). The surfactant cetyl trimethyl ammonium bromide (CTAB) was supplied by MERCK and the dodecylamine by FLUKA. The zeta potential studies were conducted in a Rank Brothers Mark II equipment with a flat cell, attached to a video camera and a rotating prism that allows the measurement of the particles electrophoretic mobility. A constant ionic strength based on NaCl concentration of 10 3 M was used in all these tests. The humic acid precipitation tests were conducted in a 50 ml glass beaker by contacting HA with the surfactant for 10 min under agitation with a magnetic stirrer. To quantify the precipitation of HA by the surfactants CTAB and DDA the precipitates were centrifuged in a Thermo IEC, Centra-CL3 equipment at 4000 rpm during 20 min. The HA concentration in the supernatant was then analyzed by using a UV–vis 1601 PC (Shimadzu Corporation) spectrophotometer at a wavelength of 254 nm. The choice of wavelength was based on Standard Methods for the Examination of Water and Wastewater, 1998. The mean diameter of the precipitated particles was measured by light
Humic acid molecules generally present an excess negative surface charge as the pH is increased due to the dissociation of carboxylic and phenolic groups (Daifullah et al., 2004). The higher negative values of zeta potential of the HA/DDA precipitates compared to those of HA/CTAB, shown in Fig. 2, indicate a lower neutralization of the HA negative charges when DDA is used. This is probably due to a higher adsorption of CTAB on HA as compared to DDA. The precipitates obtained with CTAB change their charge from negative to positive (Fig. 3) when the surfactant concentration is increased. However, it can be seen also from Fig. 3 that when DDA is used, the precipitate formed does not reverse its charge. The initial precipitation tests were performed in a HA concentration range from 10 ppm up to 70 ppm. The surfactants CTAB and DDA were studied in order to search the influence of different surfactant structures on the precipitation of HA. The minimum and maximum concentration of CTAB needed to precipitate HA at varying concentration at pH 6.0 are shown in Fig. 4 while Fig. 5 presents the results of similar experiments carried out with DDA. The region of precipitation is indicated. Additional precipitation tests were preformed with less than 10 ppm of HA in solution. In this case precipitates could no longer be observed but just a slight turbidity of the solution. This 0
Zeta potential (mV)
-20
-40
-60
-80 DDA (50 ppm) -100
CTAB (50 ppm)
-120 0
2
4
6
8
10
pH Fig. 1. Schematic representation of the flotation column (Column height: 15 cm, inner diameter: 5 cm).
Fig. 2. The effect of pH on the zeta potential of HA (100 ppm) precipitated with CTAB (50 ppm) and DDA (50 ppm).
M.C. Brum, J.F. Oliveira / Minerals Engineering 20 (2007) 945–949
80 CTAB
Zeta potential (mV)
60
DDA
40 20 0 -20 -40 -60 -80 0
20
40
60
80
100
120
Concentration (ppm) Fig. 3. The effect of the type and collector’s concentration on the zeta potential of the precipitates formed with HA (20 ppm).
947
was the main reason for choosing a concentration of HA in solution higher than 10 ppm to perform subsequent tests. The main difference between the precipitation of HA/ CTAB (Fig. 4) and HA/DDA (Fig. 5) is that a much lower concentration of CTAB is required to start the precipitation as compared to DDA. This phenomenon might be explained as being due to the length of non-polar portion of the surfactants molecule. In the case of CTAB a higher insolubility leads to a wider region of precipitation regarding the results presented in Figs. 4 and 5. The detailed results of the precipitation experiments using CTAB and DDA are presented in Figs. 6 and 7, respectively. They show the influence of the surfactant itself and of the surfactant’s concentration and pH on the precipitation of humic acid. The same HA starting concentration was used in both cases (20 ppm). 100
Humic acid precipitation (%)
Concentration, CTAB (ppm)
70 60 Precipitation region
50 40 30 20
80
60
40 pH=4.0 pH=6.0 pH=8.0
20
10
0
0
0 0
10
20 30 40 50 Concentration, Humic acid (ppm)
60
20
40
70
Fig. 4. Minimum and maximum concentration of CTAB required to precipitate humic acid as a function of HA concentration at pH 6.0.
60
80
100
CTAB, ppm Fig. 6. The influence of the concentration of CTAB and pH on the precipitation of HA (20 ppm) in 10 min.
100 Precipitation region
60
Humic acid precipitation (%)
Concentration, DDA (ppm)
70
50 40 30 20 10 0
80
60
40
pH=4.0 pH=6.0 pH=8.0
20
0
0
10
20
30
40
50
60
70
Concentration, Humic acid (ppm) Fig. 5. Minimum concentration of DDA required to precipitate humic acid as a function of HA concentration at pH 6.0.
20
40
60
80
100
120
DDA, ppm
Fig. 7. The influence of the concentration of DDA and pH on the precipitation of HA (20 ppm) in 10 min.
M.C. Brum, J.F. Oliveira / Minerals Engineering 20 (2007) 945–949
Fig. 6 shows that the CTAB concentration necessary to precipitate approximately 80% of HA at pH 4.0 is about 20 ppm. On the other hand, for pH 6.0 and pH 8.0 the maximum is attained only with concentration around 40 ppm. The results also show that as the CTAB concentration is increased the precipitation reaches a maximum and decreases subsequently (Fig. 6). In this system, the precipitation probably decreases due to the zeta potential of the precipitate changing its charge from negative to positive (Fig. 3). Zouboulis et al. (2003) have also observed a maximum removal of HA when using CTAB near the isoelectric point of the precipitate. However, as can be seen in Fig. 3, the increase in DDA concentration does not reverse the charge of the precipitate and this fits the trend of the precipitation curves, obtained with DDA, presented in Fig. 7. The effect of pH is significant in both systems. When the pH is increased a higher concentration of precipitant collector is needed. This is probably due to the increased number of dissociated carboxylic groups at the HA molecule as the pH is increased from 4.0 to 8.0. In Table 1 are shown the mean diameter of the agglomerated particles obtained for the two systems. The analytical concentration of the components was the same in both cases for three different agitation speeds. The reduction of the particles mean diameter in both systems is an expected result when higher agitation is used. The particles size determination, however, was very important to interpret the flotation results, because flotation efficiency is influenced by the particle size of the precipitates. The fragility of the flocs can also be observed from Table 1 because when the agitation speed increases the particle size decreases. The particle size is not much different when using CTAB or DDA, when low agitation speed is used. Because of the much lower consumption to precipitate the HA, CTAB was selected as the most promising one. The flotation experiments were performed under mild agitation to avoid the flocs breakage, since they were too fragile as observed above. The HA flotation as a function of the CTAB collector concentration is shown in Fig. 8. A reduction of flotation occurs for higher collector concentration due to zeta potential effect on the precipitation of HA. It can also be seen that the maximum removal of HA is achieved at a concentration coincident with the CTAB con-
100 80 Flotation (%)
948
60 40 20 0 0
10
20
30
40
50
60
Collector concentration, CTAB (ppm) Fig. 8. Flotation of HA as a function of the precipitant collector concentration at pH 6.0. (HA starting concentration: 20 ppm, flotation time: 5 min).
Fig. 9. Schematic representation of the system HA/surfactant due to the increase of the surfactant’s concentration.
centration for maximum precipitation (Fig. 6). Adou et al. (2001) have studied the precipitation of HA with CTAB aiming at the adsorption onto polypropylene. They also observed that CTAB concentration is a critical parameter. In the present study, the maximum removal of HA (20 ppm) at pH 6.0 is achieved at 30 ppm CTAB concentration (Fig. 8). In the present study the surfactants had, in fact, a triple function. They are at the same time precipitant agents, collectors and frothers. The cationic polar portion of the surfactant bound electrostatically to the dissociated carboxylic groups present in HA molecule forming precipitates. However, as the surfactant concentration is increased the nonpolar portions of the surfactant molecules interact through van der Waals forces forming hemi-micelles as shown in Fig. 9. This affects the hydrophobicity of the precipitate which shows a decrease in floatability for higher concentration of the surfactant (Fig. 8).
Table 1 Influence of stirring speed on the mean diameter of the particles
4. Conclusions
System
Rotation (rpm)
Particles mean diameter (lm)
HA-CTAB (30–50 ppm)
600 1000 2000
43.35 39.00 11.43
HA-DDA (30–50 ppm)
600 1000 2000
56.99 44.08 29.67
The studies showed the difference in the adsorption of the surfactants used to precipitate HA and revealed a higher adsorption of CTAB compared to that of DDA. The concentration of precipitant collector required to achieve maximum removal increases with the pH. However after exceeding the precipitant collectors optimum concentration, the removal tends to zero in the system HA/CTAB.
M.C. Brum, J.F. Oliveira / Minerals Engineering 20 (2007) 945–949
The column projected with a glass frit was effective in generating tiny bubbles to remove by flotation 95% of HA (20 ppm) in the HA/CTAB (30 ppm) system. The results show that if all the parameters are well adjusted flotation can be a useful technique to remove humic acid from water even when present at apparently low concentrations but that are high enough to cause problems in aquatic systems. Acknowledgement The authors acknowledge the Conselho Nacional de Pesquisas – CNPq, Brazil, for its financial support. References Adou, A.F.Y., Muhandiki, V.S., Shimizu, Y., Matsui, S., 2001. A new economical method to remove humic substances in water: adsorption onto a recycled polymeric material with surfactant addition. Water Science and Technology 43 (11), 1–7. Baird, C., 2002. Quı´mica Ambiental, second ed. Bookman, Porto Alegre. Cassel, E.A., Kaufman, K.M., Matijevic, E., 1975. The Effects of Bubble Size on Microflotation, Water Research. The Journal of the International Association on Water Pollution Research 9 (12), 1017–1024. Daifullah, A.A.M., Girgis, B.S., Gada, H.M.H., 2004. A study of the factors affecting the removal of humic acid by activated carbon
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