- Email: [email protected]
Removal of humic acids by flotation
Colloids and Surfaces A: Physicochem. Eng. Aspects 231 (2003) 181–193
Removal of humic acids by flotation Anastasios I. Zouboulis∗ , Wu Jun, Ioannis A. Katsoyiannis Laboratory of Chemical Technology, Department of Chemistry, Aristotle University Thessaloniki, P.O. Box 116, Thessaloniki 54124, Greece Received 26 August 2002; accepted 2 September 2003
Abstract The application of flotation for the removal of humic acids was investigated in the present study, as a possible post-treatment stage of simulated landfill leachates, i.e., after biological treatment. Several parameters were examined towards the optimization of humic acids removal; the dosage of collector was found to be the major one, controlling the overall efficiency of the process. However, the type and dosage of frother, the solution pH, ionic strength and flotation time were found to be also significant factors, affecting the removal of humic acids. Under optimized conditions, which are mainly determined by initial concentration of humic acids, the treatment performance was found to be very efficient, reaching almost 99%, indicating that flotation can serve as a possible alternative technology for the removal of humic acids. © 2003 Elsevier B.V. All rights reserved. Keywords: Humic acids; Flotation; Landfill leachates; Post-treatment
1. Introduction Organic matter in the environment can be divided in two main classes of compounds, non-humic material, such as proteins, polysaccharides, nucleic acids, etc. and humic substances. Humic substances are structurally complex large macromolecules, presenting a dark yellow to black appearance. They contain a core structure of phenols and phenolic acids, such as hydrobenzoicacids, vanillic acid, etc. These aromatic groups are linked together by short saturated aliphatic chains, possibly on three or four positions on the aromatic ring. The alkali soluble but acid insoluble fraction of humic substances is called humic acids [1,2]. ∗ Corresponding author. Tel.: +30-231-0997794; fax: +30-231-0997794. E-mail address: [email protected] (A.I. Zouboulis).
These compounds are dark colored, presenting anionic character over the usual pH values encountered in water and wastewater sources. They are capable of complexing with metal ions and are believed to play an important role in metal transport and release in soil and waters, whereas they reduce the removal efficiency of target compounds (metals) due to competition for the available sorption sites of adsorbents [3]. In addition, humic acids are considered as disinfection by-products precursors. When chlorination is used for disinfection of drinking water, they can react and form trihalomethanes and halogenic acids, which are potential carcinogens [4]. Moreover, when humic acids penetrate in groundwater sources, because of the much smaller solubility in water of O2 than CO2 , the water will become depleted in oxygen and low redox values are expected. Under these conditions Fe and Mn may become soluble as Fe2+ and Mn2+ [5]. Iron and
0927-7757/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2003.09.004
182
A.I. Zouboulis et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 231 (2003) 181–193
manganese dissolution may also occur through chemical reactions between humic acids and iron and manganese oxides [1,6], whereas they also contribute to the acidity of natural waters [7]. Furthermore, humic acids have been proved to be resistant to biological treatment, as they are non-biodegradable (refractory) compounds. This has direct consequence on the quality of landfill leachates, usually treated by biological methods [8]. In general, the leachate generated from recently filled domestic waste has high BOD5 values (Biological Oxygen Demand), of several thousands milligram of O2 per liter and even higher COD content (Chemical Oxygen Demand) [9]. Several biological methods (aerobic/anaerobic), as well as combinations of them have been applied for the treatment of leachates [10]. However, significant amount of organic, non-biodegradable, substances remain relatively constant, resulting in residual COD value of around 1000 mg O2 /l. The major fraction of DOC (Dissolved Organic Carbon) in biologically pretreated landfill leachates consists of humic substances, mainly in the form of humic and fulvic acids [11,12]. The ratio of humic/fulvic acids is not constant and changes over time in landfill leachates. Therefore, in the present article, the term “humic acids” will be used as a general term, including all the humic-like material, which is usually present in a biologically pretreated landfill leachate. Due to aforementioned problems, caused by the presence of humic acids in aquatic systems, their removal from water or wastewater is considered of great environmental concern. This is usually accomplished by coagulation and precipitation, i.e., adding salts of hydrolysed metals, such as aluminium sulfate and organic polymers, followed by gravity sedimentation or filtration [13,14]. Other treatment techniques, which have been examined for the removal of humic acids are ion exchange, sorption on iron-coated sand, membrane processes, such as reverse osmosis and chemical oxidation [15–21]. The search for a method, which can be equally efficient in removing humic acids, but meanwhile would be more rapid than gravity sedimentation and less expensive than membrane processes or ion-exchange have resulted in an increasing interest in the application of flotation [22]. For many years, flotation has been extensively used in the mineral industry to selec-
tively separate solids at high rates. Regarding the application of flotation in wastewater treatment, this has been focused on the removal of colloids, ions, macromolecules, microorganisms and fibers [23,24]. However, until to date, the application of flotation for the removal of humic acids from aqueous solutions has been the subject of very few studies [13,25,26]. In the present study, the use of flotation in column has been employed, as a possible post-treatment stage for removing residual humic acids from simulated landfill leachates, pretreated by biological oxidation. Air was introduced through a porous frit in the presence of small amounts of ethanol, which acts as the frother. This technique is not limited by the solubility of air in water (as in dissolved air-flotation) and it can generate much greater bubble densities, than in other flotation techniques [27]. Column flotation has been a subject of great interest in mineral processing and recently, has found applications in wastewater treatment for the removal of toxic metal ions [24], but not for the removal of humic acids. In the present work, the examined parameters were those affecting the efficiency of flotation towards the removal of humic acids, such as initial concentration of humic acids, the dosage of the collector, the type and the dosage of the frother, as well as those affecting the colloid chemistry of humic acids, i.e., the solution pH and the ionic strength, comprising an integrated study of humic acids removal by flotation. The aforementioned studies, were directed towards the examination of humic acids removal from natural waters, thus the respective examined concentrations were substantially lower than the concentration level studied in the present paper. Furthermore, the examination of several parameters such as ionic strength, pH, zeta potential, etc. may provide useful information for the application of flotation as a potential post-treatment method for the removal of humic acids from biologically pretreated landfill leachates.
2. Experimental 2.1. Materials All chemicals were reagent-grade and all glassware was acid-washed and rinsed with deionized water. Humic acids were supplied by Aldrich. Stock solutions
A.I. Zouboulis et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 231 (2003) 181–193
of humic acids were prepared by dissolution of 5 g of humic acids in 1 l of 0.1 M NaOH solution, stirring for 48 h and then filtered through appropriate filter papers (Whatman no. 1). Working solutions were prepared by dilution of stock solution in de-ionized water and final concentrations ranged from 50 to 300 mg/l. It was found that humic acids solution of 50 mg/l corresponds to 67 mg/l COD or 20.8 mg/l TOC. The landfill leachate, after biological treatment typically contains COD values in the range of 500–1000 mg/l, from which around 30% accounts to humic acids [28]. Therefore, humic acids concentrations in the range between 50 and 300 mg/l should be representative of pretreated leachates. The applied surfactant (collector) for the flotation experiments was N-cetyl-N,N,N-trimethylammonium -bromide (CTAB). Stock solutions were prepared by dissolving 0.5 g/l in 250 ml of warm de-ionized water, in order to obtain final concentration 2 g/l. Working solutions were also prepared by further dissolution in de-ionized water, until final concentrations in the range 10–500 mg/l. Ethanol served as convenient frother in most of experiments. However, several other frothers were also examined and compared with ethanol, such as pine oil (in 10% ethanol), methyl–isobutyl–ketone (MIBK) and amylalcohol. Sodium dodecyl sulfate (SDS) was employed for the measurement of the residual concentration of CTAB in the solution, after flotation [29]. SDS solution (0.002 M) was prepared by dissolving 0.2884 g of SDS in 500 ml of deionized water. In addition, methylene blue was used as indicator in the titration measurements and was prepared by dissolving 50 mg of methylene blue, 50 g of sodium sulfate and 12.5 g of concentrated sulfuric acid in 1 l of de-ionized water. 2.2. Experimental setup Column flotation was the experimental technique applied in the removal of humic acids from the simulated landfill leachate. The experimental setup is presented in Fig. 1. A plexi-glass column (50 mm i.d., height 60 cm) was used for the batch flotation experiments. The top of the column was connected with a funnel, in order to collect and remove the foam, containing the floated humic acids. Air was injected from the bottom of column by a microporous frit, in order to create the necessary bubbles, at a flow-rate of
183
Fig. 1. Schematic representation of experimental setup for flotation experiments; column height 60 cm, inner diameter 50 mm.
200 cm3 /min, measured by a rotameter, based on previous experience. Humic acids were mixed with the respective frother and surfactant in a 500 ml beaker (total volume of solution 400 ml), under gentle stirring for 1 min. Then the solution was inserted in the flotation column; after the flotation experiment, samples were collected from the bottom of the column for analytical determinations. 2.3. Analytical determinations Humic acids were determined by spectrophotometer (HITACHI U-2000) at 254 nm, using quartz vesicles. It has been reported that DOC concentration and UV absorbance cam be connected with a strong linear relationship, which was also confirmed by this study, as the calibration curve showed a correlation factor almost equal to one. The residual concentration of CTAB after flotation was measured by titration with sodium dodecyl sulfate solution and methylene blue as indicator [29]. Electrokinetic measurements were also performed, using a zeta-potential analyzer (Rank Brothers Ltd., UK). All mobility measurements were carried out at constant temperature (i.e., 22 ◦ C) and using a constant salt concentration of 0.001N NaCl. The solution pH was adjusted by adding various amounts of either 0.1 M NaOH or 0.1 M HCl. Twenty independent measurements were performed for each experimental condition, without preliminary filtration, as no problems of interference by particulates were observed.
184
A.I. Zouboulis et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 231 (2003) 181–193
The zeta-potential values were determined using the smoluchowski equation.
Table 1 Screening test results of different frothers on the removal of humic acids by flotation Frothers
Concentration (%)
Humic acid removal (%)
MIBK Amylalcohol Pine oil Ethanol
0.4 0.25 0.1 0.4 0.25 0.1
23.7 56.3 33.2 59.8 58 43.7
3. Results and discussion The flotation process involves three main steps: (a) selective chemical modification of specific specie surface, i.e., hydrophilicity/hydrophobicity; (b) contact between and adherence of hydrophobic species to air bubbles; and (c) separation of floatable and non-floatable species [23]. The first step is mainly controlled by the type and dosage of added collector (surfactant). In the second and third steps, the major parameter involved is the type and dosage of the frother [30]. The frother aids flotation by adsorbing onto the gas–liquid interface, thereby reducing its surface tension, which results in the formation of more stabilized bubbles. The creation of finer bubble distribution through changes in dynamic surface tension is also another important result of the frother addition [31]. The selection of the appropriate frother is mainly based upon the consideration of two aspects, when designing flotation experiments. Firstly, the frother has to control efficiently the bubble generation and secondly, it has to combine effectively with the collector, which could lead to synergistic effects on the overall flotation performance [30]. In the present study, several frothers have been examined, based on the specific collector usage (CTAB) and the most appropriate for the removal of humic acids has been selected on the basis of trial and error methodology. The examined frothers were ethanol, amylalcohol, pine oil, and MIBK. These frothers belong to the general category of neutral or non-ionic frothers, which present rather negligible collecting properties; screening results are presented in Table 1. The results indicated that the frother, which produced the best results, in relation to humic acids removal, was ethanol. The removal of humic acids was less efficient, when applying the pine oil or MIBK frothers, whereas the results obtained by the use of amylalcohol were equally efficient; however the use of amylalcohol was considered as less attractive due to its higher cost. Indeed, ethanol has been reported to be a very effective and convenient frother, when applying microflota-
Conditions: initial humic acids concentration 100 mg/l; initial collector concentration 50 mg/l; pH 8; floating time 10 min.
tion. It was found to promote multiple bubble attachment to the colloidal aggregates and to increase significantly the bubble frequency. Furthermore, as ethanol is a good solvent for most organic compounds, the introduction of the collector in an ethanol solution is desirable, because alcohol inhibits micelle formation, which can be detrimental to flotation [13,27]. Therefore, the rest of the experiments were performed by the use of ethanol as the frother. However, the optimum dosage of ethanol in the removal of humic acids by flotation had to be determined. It has been reported that excessive amounts of ethanol could be detrimental to flotation experiments [32]. This detrimental action of higher ethanol concentrations could be attributed to the fact that the respective complexes between collector and species are stabilized in such a degree, which renders the adsorption at the air-interface difficult. Moreover, at higher alcohol concentrations, the number of sites on the liquid–gas interface, available for particle–collector complex will be reduced due to adsorption of alcohol molecules and this might also reduce humic acids removal. Therefore, experiments have been performed using different alcohol concentrations, at constant collector and initial humic acids concentration (Fig. 2). It can be noticed that the concentration of ethanol was increased, humic acids removal was also increased, up to the value of 1% (v/v) ethanol. After this concentration, the removal of humic acid did not change significantly, although it showed a slight decrease, possible due to aforementioned effects. The increase on humic acids removal, by increasing ethanol has been previously reported by other researchers and was mainly attributed to the decrease of bubble size in the presence of small amounts of ethanol [27].
A.I. Zouboulis et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 231 (2003) 181–193
185
80
Humic acids removal (%)
60
40
20
0 0
1
2
Ethanol (%)
Fig. 2. Influence of ethanol concentration on humic acid removal; conditions: humic acid concentration 100 mg/l; collector concentration (CTAB) 50 mg/l; pH 8; floating time 10 min.
The decrease of bubble size by addition of small amounts of ethanol has been attributed to the fact that ethanol lowers the solubility of gas in water. Consequently, it is probable that fine gas bubbles can be formed, which enhance the efficiency of flotation [23]. Smaller bubbles enhance the efficiency of microflotation, because the use of large bubbles can break the formed flocs and produce turbulence, which limits the efficiency of flotation [33]. Nevertheless, during these experiments, the highest humic acids removal, observed at the optimum ethanol concentration (1%), was not found to exceed 70%. These results were considered as not satisfactory and indicated the major role of collector in the overall flotation efficiency. The collector interacts with certain species in solution and as a result converts them from hydrophilic to hydrophobic, enabling their attachment to the generated air bubbles. The selection of the appropriate collector depends on the nature of the target species to be removed. Humic acids are known to be fully protonated, i.e., they are not ionized at pH value 3, thus are negatively charged above this pH value [34]. Since humic acids present an anionic character over the usual pH values encountered in landfill leachates, they are expected to interact strongly with cationic collectors. Therefore, the collector (surfactant) used in this study (CTAB) was a common cationic one. The effect of collector
dosage in relation to the initial humic acids concentration is presented in Fig. 3. It can be noticed that the optimum dosage of collector, in order to achieve efficient humic acids removal varies with initial humic acids concentration and the higher the humic acids concentration, the higher the required dosage of collector. This can be attributed to the fact the required dosage of flotation reagent is closely related to the overall charge carried by the organic impurities. In order to achieve efficient removal of humic acids, their charge has to be neutralized [27]. Therefore, as the amount of humic acids was increased, the charge that had to be neutralized was also increased and this required higher dosages of collector. The results presented in Fig. 3 showed that the optimum collector dosages were 80–100 mg/l for [HA]0 100 mg/l, 170–200 mg/l for [HA]0 200 mg/l, and 250–300 mg/l for [HA]o 300 mg/l. Lower dosages than those aforementioned resulted in decreased humic acids removal, because the concentrations of the collector were not enough for neutralizing the overall (negative) charge of humic acids [22]. The neutralized species have become hydrophobic and floated with the air bubbles. However, the species, which did not react with the collector, remained hydrophilic, of anionic nature and could not float with the bubbles. The addition of collector
186
A.I. Zouboulis et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 231 (2003) 181–193
100
Initial HA 100 mg/L 200 mg/L 300 mg/L
Humic acids removal (%)
80
60
40
20
0 0
100
200
300
400
500
Collector dosage (mg/L) Fig. 3. Influence of collector dosage on humic acids removal by flotation, applying different initial concentrations of humic acids; conditions: pH 8; ethanol 1%; floating time 10 min.
dosages higher than the optimum ones resulted also in decreased humic acids removal and at very high collector concentrations the removal efficiency was substantially decreased. This can be attributed to the fact that by increasing the collector dosage much higher than the required for efficient flotation, the overall charge of humic acids may become cationic [32]. Nevertheless, excessive amounts of collector should be avoided, not only due to higher cost induced, but also because of other negative effects, such as larger foam losses, possibility for micelle or hemi-micelle formation and the potential toxicity of residual amounts of collector in the treated effluent. The adverse effect of excessive collector dosages can be also explained, through electrokinetic measurements carried out at constant pH value and initial humic acids concentration (100 mg/l) but for different initial collector concentrations (Fig. 4). As can be noticed, zeta-potential values were increased by increasing the collector concentration, and at concentration of 85 mg/l, the system humic acid/collector has become
almost neutral. This increase of zeta-potential values by the addition of collector has been reported to be a characteristic behavior of specific chemisorption phenomenon and the hydrophobicity of humic acids has resulted through interaction of collector with humic acids [35]. However, excessive amounts of collector caused further increase of zeta potential of this system and at concentrations over 85 mg/l, a cationic character was observed. Comparing these results with those presented in Fig. 3, it can be noticed that for initial humic acids concentration of 100 mg/l, the removal of humic acids by flotation started to decrease over the collector dosage of 85 mg/l, which is the value when this system showed zeta-potential value around zero, indicating the consistency between zeta-potential measurements and humic acids removal efficiency. The effect of the collector dosage was also investigated in relation with the variation of pH value, by keeping constant the initial humic acid concentration at 100 mg/l (Fig. 5). The pronounced effect of optimum collector dosage was also confirmed, almost
A.I. Zouboulis et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 231 (2003) 181–193
187
Z-potential (mV)
15
0
-15
-30
0
20
40
60
80
100
120
Collector dosage (mg/L) Fig. 4. Electrokinetic measurements of humic acid solution at different collector concentrations—effect on z-potential values; conditions: initial humic acids concentration 100 mg/l; ethanol 1%; floating time 10 min; pH 8.
independently of the solution pH. The difference in pH had a direct effect on the required collector dosage for efficient humic acids removal. When the solution pH was adjusted to 4, efficient removal was achieved at collector concentration of 30 mg/l and was decreased for values above 120 mg/l. On the contrary, when the pH of solution was increased to pH value of 10, the sufficient dosage of collector, in order to achieve efficient removal of the same concentration of humic acids was found to be 80 mg/l, whereas above the concentration value of 100 mg/l, the removal efficiency was found also to be decreased. The effect of collector dosage has been discussed previously. However, the results in Fig. 5 indicated also the significant effect of pH on the effectiveness of flotation as a treatment technology for removing humic acids. At lower pH values, the required dosage of collector for achieving sufficient removal of this humic acids concentration was lower than the amount required at higher (alkaline) pH values.
These results were confirmed by Fig. 6, which illustrates the removal of humic acids, as affected by initial pH values, for constant initial humic acids concentration (100 mg/l) and for three different initial concentrations of collector. As the pH value was increased the removal efficiency of humic acids was decreased, and only when the concentration of collector was high enough (in this case 70 mg/l), the removal efficiency remained unaffected by pH variation. The aforementioned observations can be attributed to the fact that at low pH values (acidic), organic protonation was increased, thus reducing the negative charge density of colloidal organic matter and subsequently, the demand of cationic collector for neutralizing the anionic humic acids [14,34]. Therefore, in higher pH values, the demand for collector was increased. These results were in accordance with the electrokinetic studies, performed for humic acids solution as well as for the system humic acids/collector at different pH values (Fig. 7).
188
A.I. Zouboulis et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 231 (2003) 181–193
Humic acids removal (%)
100
80
60
40 pH=10 pH=8
20
pH=4
0
0
20
40
60
80
100
120
140
160
180
Collector concentration (CTAB, mg/L) Fig. 5. Influence of collector concentration on humic acids removal by flotation at different pH values; conditions: initial humic acid concentration 100 mg/l; ethanol 1%; floating time 10 min.
100
Humic acids removal (%)
80
60 Collector dosage
40
70 mg/L 50 mg/L 35 mg/L
20
0
3
5
7
9
11
pH Fig. 6. Effect of solution pH on the removal of humic acids by flotation at three different initial collector concentrations; conditions: initial humic acids concentration 100 mg/l; ethanol 1%; floating time 10 min.
A.I. Zouboulis et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 231 (2003) 181–193
189
10
0 0
2
4
6
8
10
12
Z-potential (mV)
humic acids (100mg/L)
-10
humic acids (100 mg/L)+ collector (50 mg/L) and ethanol (1%)
-20
-30
-40 pH
Fig. 7. Electrokinetic measurements of humic acids aqueous solution and humic acids/collector system at different pH values—effect on z-potential values; conditions: initial humic acid concentration 100 mg/l; ethanol 1%.
The zeta-potential values of humic acids solution were found to increase by decreasing the pH, indicating the increase of humic acid protonation. The addition of collector, caused an increase of zeta-potential values, as compared with values obtained without the addition of collector. The isoelectric point of the system humic acid/collector was found to be at pH values around 5, indicating the specific interaction (chemisorption) of collector with humic acids. The ionic strength of solution is another important parameter that influences the efficiency of flotation, because the higher the ionic strength, the higher is the amount of other ions in solution, which might interact or compete with humic acids or the collector and affect the removal efficiency. Two different salts were examined in different concentrations and the respective results are presented in Fig. 8. These results indicated that at values of salt concentration lower than 0.01 M, the removal efficiency was only slightly affected by the presence of the other ions introduced in the solution. However, as salt concen-
trations were further increased, the removal of humic acids was found to be decreased, which was apparent for salt concentrations over 0.01 M. However, when sodium chloride was used, the effect on the removal of humic acids was not significant and the decrease in the overall removal efficiency accounted only for 10%. Although, during the application of other treatment techniques (i.e., coagulation), the increase of ionic strength was found to affect positively the removal of humic acids [36], when flotation techniques were employed, ionic strength was found to be a limiting factor, regarding the treatment efficiency. The negative effect of ionic strength, during ion flotation has been also reported by other researchers [37]. As aforementioned, the increase in ionic strength can adversely affect flotation in two directions. Firstly, ions with the same charge of target species (in this case, anionic) may compete or interact with the cationic collector, hence reducing its availability to float the target compounds. In this case, in order to maintain efficient hu-
190
A.I. Zouboulis et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 231 (2003) 181–193 100
Humic acids removal %
90
80
70
Na2SO4 NaCl
60
50 0,00
0,04
0,08
0,12
Salt concentration (mol/L) Fig. 8. Effect of salt concentration on the removal of humic acids by flotation; conditions: initial humic acids concentration 100 mg/l; pH 8; initial collector concentration 70 mg/l; ethanol 1%.
100
Humic acids removal(%)
80
60
Collector dosage 40
50 mg/L 70 mg/L
20
0
0
4
8
12
16
20
24
28
Floating time (min) Fig. 9. Effect of floating time on the removal of humic acids by flotation; conditions: initial humic acids concentration 100 mg/l; ethanol 1%; pH 8.
A.I. Zouboulis et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 231 (2003) 181–193
191
12
Residual collector CTAB (mg/L)
10
8
6
4
2
0 0
20
40
60
80
100
120
140
Initial collector CTAB (mg/L) Fig. 10. Effect of initial concentration of collector (CTAB) on the residual concentration of collector after flotation; conditions: initial humic acids concentration 100 mg/l; pH 8; ethanol 1%; flotation time 20 min.
mic acids removal, the increase of collector dosage is required. However, it is possible that ions with opposite charge with the target species (i.e., cationic) can interact with humic acids, neutralizing the molecules, which cannot further interact with the cationic collector and therefore, cannot float. In summarizing the previous investigations, it was found that flotation using CTAB as the collector and ethanol as the frother can be an efficient treatment method for removing humic acids; if operational parameters would be adjusted properly, removal efficiencies up to 99% can be obtained, for a wide range of initial humic acids concentrations. However, in the design of a process, kinetic investigations have to be also performed, especially when economic considerations are involved. Therefore, experiments were performed by varying the time of flotation and keeping constant the other parameters. The experiments have been performed for two different collector concentrations, 50 and 70 mg/l, respectively. Previous findings have shown that for initial humic acids concentration of 100 mg/l, the optimum concentration of collector was 70 mg/l, whereas a 50 mg/l
collector dosage resulted in humic acids removal of around 65%, at pH 8 and for floating time 10 min. During these experimental series (Fig. 9), it can be noticed that by increasing the flotation time from 10 to 20 min, the removal efficiency was further increased and reached 80%. This can be attributed to the fact that by increasing the flotation time, the collector can be utilized more efficiently and humic acids can float with air bubbles in the froth phase [22]. However, when the optimum collector dosage was employed (70 mg/l), it was noticed that the reaction was very fast and a 5 min flotation time was found to be sufficient for achieving more than 95% removal of humic acid. Therefore, from these experiments, the pronounced effect of collector dosage was also evident. When the collector concentration was less than the optimum dosage, the expansion of the flotation time to 20 min did not produce such efficient results as those obtained for the optimum collector dosage, under which, even for short retention (flotation) time, almost complete humic acids removal could be achieved. Another parameter, which is strongly connected with flotation time, is the residual concentration of
192
A.I. Zouboulis et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 231 (2003) 181–193
Table 2 Residual collector concentration as influenced by flotation time for two different initial collector concentrations Flotation time (min)
Initial collector concentration (mg/l)
Residual collector concentration (mg/l)
10
50 70 50 70
10.2 10.8 8.02 8.31
20
Conditions: initial humic acids concentration 100 mg/l; ethanol 1%; pH 8.
collector (after flotation). As it was previously reported, the collector can interact with the target species and co-float in the froth phase. However, when the floating time was not sufficient for efficient humic acids removal, this has a direct result also in the residual concentration of collector, remaining in the treated solution (Table 2). Thus, when the flotation time was adjusted to 10 min, the residual quantity of collector was greater than when the flotation duration was 20 min. Further investigations related with the residual quantity of collector in the effluent involved the effect of initial collector concentration. During the previous investigations (see Fig. 2), it was noticed that over an optimum collector concentration, the removal of humic acids was decreased. In Fig. 10, it can be observed that over this collector concentration, which for initial humic acids concentration of 100 mg/l was estimated to be around 70 mg/l, the residual quantity of collector was increased, because a significant amount of it cannot react with humic acids and therefore, remain in the treated solution. The increase in flotation time can further decrease the residual concentration of collector. Therefore, when designing a flotation process, apart from other parameters, the residual collector concentration has also to be taken under consideration, because when removing contaminants from water, it is not desirable to contaminate them by the additives used for the accomplishment of treatment process.
of humic acids was found to be very efficient, for a wide range of initial humic acids concentration (50–200 mg/l). The parameters, which control the efficiency of this treatment technique, were mainly the type and dosage of frother, the dosage of collector, solution pH, ionic strength (concentration of other salts) and the flotation time. Based on screening tests, the most efficient frother was found to be ethanol, in concentrations 1% (v/v). The dosage of collector was the main factor controlling the removal of humic acids and the optimum concentration was greatly dependent on the initial humic acid concentration and on the solution pH; higher humic acids concentrations required higher dosages of collector. However, collector dosages higher than those required may produce opposite results and decrease the removal of humic acids. The obtained results were compared by respective z-potential measurements, which demonstrated that at high collector dosages the humic acids/collector system had become cationic, and unable to float. The pH of the solution also affected the removal efficiency, as by varying the pH value, the protonation of humic acids varied. At lower (acidic) pH values, the degree of humic acids protonation was increased. As a result, at these pH values lower dosages of collector were required for efficient humic acids removal. The presence of salts in humic acids solution had an adverse effect on flotation performance, and the extent of this effect depended on the specific salt. The flotation time affected also the efficiency of flotation, because by increasing it, the collector interacted with humic acids in higher extent, while the residual collector quantity in the treated solution was also found to be decreased. In summary, if flotation process would be designed properly and operational parameters can be adjusted accordingly, then flotation can be very efficient towards the removal of humic acids, providing a cost competitive alternative for the treatment of humic acids from contaminated aqueous streams, such as landfill leachates.
4. Conclusions Acknowledgements Flotation was found to be an efficient treatment option as a post-treatment stage for removing humic acids from simulated landfill leachates. In the present study, and under optimized conditions, the removal
Thanks are due to General Secretariat of Science and Technology (Greece) for the funding of specific project, under the framework of common
A.I. Zouboulis et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 231 (2003) 181–193
Greece–China Scientific Technological Cooperation Programme. References [1] A.T. Stone, J.J. Morgan, Environ. Sci. Technol. 18 (1984) 617. [2] M.N. Jones, N.D. Bryan, Adv. Colloid. Interface Sci. 78 (1998) 1. [3] N. Calace, A. Liberatori, B.M. Petronio, M. Pietroletti, Environ. Pollut. 113 (2001) 331. [4] H.U. Galland, U. von Gunten, Water Res. 36 (2002) 65. [5] W. Stumm, J.J. Morgan, Aquatic Chemistry: Chemical Equilibria and Rates in Natural Waters, Wiley, New York, 1996. [6] S.A. Banwart, Geochim. Cosmochim. Acta 63 (1999) 2919. [7] B.G. Oliver, E.M. Thurman, R.L. Malcolm, Geochim. Cosmochim. Acta 47 (1983) 2031. [8] A.A. Tatsi, A.I. Zouboulis, Production, composition and temporal variation of pollution parameters for sanitary landfill leachates, Adv. Environ. Res. 6 (2002) 207–219. [9] M. El-Fadel, E. Bou-Zeid, W. Chachine, B. Alayli, Waste Manage. 22 (2002) 269. [10] E. Chian, F. DeWalle, J. Environ. Eng. Div. (Proc. Am. Soc. Civil Eng.) 102 (1976) 411. [11] J.B. Christensen, D.L. Jensen, C. Gron, Z. Filip, T.H. Christensen, Water Res. 32 (1998) 125. [12] M.A. Nanny, N. Ratasuk, Water Res. 36 (2002) 1577. [13] F.J. Mangravite, T.D. Buzzell, E.A. Cassell, E. Matijevic, G.B. Saxton, J. Am. Water Works Assoc. 67 (1975) 89. [14] S.K. Kam, J. Gregory, Water Res. 35 (2001) 3557. [15] M. Alborfzar, G. Jonsson, C. Gron, Water Res. 32 (1998) 2983. [16] N.D. Graham, Water Sci. Technol. 40 (1999) 141. [17] J. Fettig, Water Sci. Technol. 40 (1999) 173. [18] Y. Seida, Y. Nakano, Water Res. 34 (2000) 1487.
193
[19] C.H. Lai, C.Y. Chen, Chemosphere 44 (2001) 1177. [20] S.G. Yiantsios, A.J. Karabelas, Desalination 140 (2001) 195. [21] P.C. Chiang, E.E. Chang, C.H. Liang, Chemosphere 46 (2002) 929. [22] L. Zhang, P. Smasundaram, V. Ososkov, C.C. Chou, Colloids Surf. A 177 (2001) 235. [23] A.I. Zouboulis, K.A. Matis, G.A. Stalidis, Flotation techniques in water treatment, in: P. Mavros, K.A. Matis (Eds.), Innovations in Flotation Technology, Kluwer Academic Publishers, Durdrecht, MA, 1992. [24] J. Rubio, M.L. Souza, R.W. Smith, Miner. Eng. 15 (2002) 139. [25] E.A. Cassell, E. Matijevic, J. Francis, J.R. Mangravite, T.M. Buzzell, S.B. Blabac, J. Am. Inst. Chem. Eng. 17 (1976) 1486. [26] M. Hiraide, F. Ren, R. Tamura, A. Mizuike, Microchim. Acta 11 (1987) 137. [27] E.A. Cassell, K.K. Kaufman, E. Matijevic, Water Res. 9 (1975) 1017. [28] S.R. Qasim, W. Chiang, Sanitary Landfill Leachate. Generation, Control and Treatment, Technomic, Lancaster, UK, 1994. [29] B.M. Mildiwski, D.M. Gabriel, Detergent analysis: A Handbook of Cost Effective Quality Control, Micelle press, London, 1982. [30] G.H. Harris, R. Jia, Int. J. Miner. Process. 58 (2000) 35. [31] H. El-Shall, N.A. Abdel-Jhalek, S. Svoronos, Int. J. Miner. Process. 58 (2000) 187. [32] L.B. Scorzelli, A.L. Fragomeni, M.L. Torem, Miner. Eng. 12 (1999) 905. [33] S. Ata, N. Ahmed, G.J. Jameson, Int. J. Miner. Process. 64 (2002) 101. [34] M. Bob, H.W. Walker, Colloids Surf. A 191 (2001) 17. [35] A. Martinez-Lueranos, A. Uribe-Salas, A. Lopez-Valdivieso, Miner. Eng. 12 (1999) 919. [36] E.K. Kim, H.W. Walker, Colloids Surf. A 194 (2001) 123. [37] A.I. Zouboulis, K.A. Matis, Water Sci. Technol. 31 (1995) 315.
Removal of humic acids by flotation Anastasios I. Zouboulis∗ , Wu Jun, Ioannis A. Katsoyiannis Laboratory of Chemical Technology, Department of Chemistry, Aristotle University Thessaloniki, P.O. Box 116, Thessaloniki 54124, Greece Received 26 August 2002; accepted 2 September 2003
Abstract The application of flotation for the removal of humic acids was investigated in the present study, as a possible post-treatment stage of simulated landfill leachates, i.e., after biological treatment. Several parameters were examined towards the optimization of humic acids removal; the dosage of collector was found to be the major one, controlling the overall efficiency of the process. However, the type and dosage of frother, the solution pH, ionic strength and flotation time were found to be also significant factors, affecting the removal of humic acids. Under optimized conditions, which are mainly determined by initial concentration of humic acids, the treatment performance was found to be very efficient, reaching almost 99%, indicating that flotation can serve as a possible alternative technology for the removal of humic acids. © 2003 Elsevier B.V. All rights reserved. Keywords: Humic acids; Flotation; Landfill leachates; Post-treatment
1. Introduction Organic matter in the environment can be divided in two main classes of compounds, non-humic material, such as proteins, polysaccharides, nucleic acids, etc. and humic substances. Humic substances are structurally complex large macromolecules, presenting a dark yellow to black appearance. They contain a core structure of phenols and phenolic acids, such as hydrobenzoicacids, vanillic acid, etc. These aromatic groups are linked together by short saturated aliphatic chains, possibly on three or four positions on the aromatic ring. The alkali soluble but acid insoluble fraction of humic substances is called humic acids [1,2]. ∗ Corresponding author. Tel.: +30-231-0997794; fax: +30-231-0997794. E-mail address: [email protected] (A.I. Zouboulis).
These compounds are dark colored, presenting anionic character over the usual pH values encountered in water and wastewater sources. They are capable of complexing with metal ions and are believed to play an important role in metal transport and release in soil and waters, whereas they reduce the removal efficiency of target compounds (metals) due to competition for the available sorption sites of adsorbents [3]. In addition, humic acids are considered as disinfection by-products precursors. When chlorination is used for disinfection of drinking water, they can react and form trihalomethanes and halogenic acids, which are potential carcinogens [4]. Moreover, when humic acids penetrate in groundwater sources, because of the much smaller solubility in water of O2 than CO2 , the water will become depleted in oxygen and low redox values are expected. Under these conditions Fe and Mn may become soluble as Fe2+ and Mn2+ [5]. Iron and
0927-7757/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2003.09.004
182
A.I. Zouboulis et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 231 (2003) 181–193
manganese dissolution may also occur through chemical reactions between humic acids and iron and manganese oxides [1,6], whereas they also contribute to the acidity of natural waters [7]. Furthermore, humic acids have been proved to be resistant to biological treatment, as they are non-biodegradable (refractory) compounds. This has direct consequence on the quality of landfill leachates, usually treated by biological methods [8]. In general, the leachate generated from recently filled domestic waste has high BOD5 values (Biological Oxygen Demand), of several thousands milligram of O2 per liter and even higher COD content (Chemical Oxygen Demand) [9]. Several biological methods (aerobic/anaerobic), as well as combinations of them have been applied for the treatment of leachates [10]. However, significant amount of organic, non-biodegradable, substances remain relatively constant, resulting in residual COD value of around 1000 mg O2 /l. The major fraction of DOC (Dissolved Organic Carbon) in biologically pretreated landfill leachates consists of humic substances, mainly in the form of humic and fulvic acids [11,12]. The ratio of humic/fulvic acids is not constant and changes over time in landfill leachates. Therefore, in the present article, the term “humic acids” will be used as a general term, including all the humic-like material, which is usually present in a biologically pretreated landfill leachate. Due to aforementioned problems, caused by the presence of humic acids in aquatic systems, their removal from water or wastewater is considered of great environmental concern. This is usually accomplished by coagulation and precipitation, i.e., adding salts of hydrolysed metals, such as aluminium sulfate and organic polymers, followed by gravity sedimentation or filtration [13,14]. Other treatment techniques, which have been examined for the removal of humic acids are ion exchange, sorption on iron-coated sand, membrane processes, such as reverse osmosis and chemical oxidation [15–21]. The search for a method, which can be equally efficient in removing humic acids, but meanwhile would be more rapid than gravity sedimentation and less expensive than membrane processes or ion-exchange have resulted in an increasing interest in the application of flotation [22]. For many years, flotation has been extensively used in the mineral industry to selec-
tively separate solids at high rates. Regarding the application of flotation in wastewater treatment, this has been focused on the removal of colloids, ions, macromolecules, microorganisms and fibers [23,24]. However, until to date, the application of flotation for the removal of humic acids from aqueous solutions has been the subject of very few studies [13,25,26]. In the present study, the use of flotation in column has been employed, as a possible post-treatment stage for removing residual humic acids from simulated landfill leachates, pretreated by biological oxidation. Air was introduced through a porous frit in the presence of small amounts of ethanol, which acts as the frother. This technique is not limited by the solubility of air in water (as in dissolved air-flotation) and it can generate much greater bubble densities, than in other flotation techniques [27]. Column flotation has been a subject of great interest in mineral processing and recently, has found applications in wastewater treatment for the removal of toxic metal ions [24], but not for the removal of humic acids. In the present work, the examined parameters were those affecting the efficiency of flotation towards the removal of humic acids, such as initial concentration of humic acids, the dosage of the collector, the type and the dosage of the frother, as well as those affecting the colloid chemistry of humic acids, i.e., the solution pH and the ionic strength, comprising an integrated study of humic acids removal by flotation. The aforementioned studies, were directed towards the examination of humic acids removal from natural waters, thus the respective examined concentrations were substantially lower than the concentration level studied in the present paper. Furthermore, the examination of several parameters such as ionic strength, pH, zeta potential, etc. may provide useful information for the application of flotation as a potential post-treatment method for the removal of humic acids from biologically pretreated landfill leachates.
2. Experimental 2.1. Materials All chemicals were reagent-grade and all glassware was acid-washed and rinsed with deionized water. Humic acids were supplied by Aldrich. Stock solutions
A.I. Zouboulis et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 231 (2003) 181–193
of humic acids were prepared by dissolution of 5 g of humic acids in 1 l of 0.1 M NaOH solution, stirring for 48 h and then filtered through appropriate filter papers (Whatman no. 1). Working solutions were prepared by dilution of stock solution in de-ionized water and final concentrations ranged from 50 to 300 mg/l. It was found that humic acids solution of 50 mg/l corresponds to 67 mg/l COD or 20.8 mg/l TOC. The landfill leachate, after biological treatment typically contains COD values in the range of 500–1000 mg/l, from which around 30% accounts to humic acids [28]. Therefore, humic acids concentrations in the range between 50 and 300 mg/l should be representative of pretreated leachates. The applied surfactant (collector) for the flotation experiments was N-cetyl-N,N,N-trimethylammonium -bromide (CTAB). Stock solutions were prepared by dissolving 0.5 g/l in 250 ml of warm de-ionized water, in order to obtain final concentration 2 g/l. Working solutions were also prepared by further dissolution in de-ionized water, until final concentrations in the range 10–500 mg/l. Ethanol served as convenient frother in most of experiments. However, several other frothers were also examined and compared with ethanol, such as pine oil (in 10% ethanol), methyl–isobutyl–ketone (MIBK) and amylalcohol. Sodium dodecyl sulfate (SDS) was employed for the measurement of the residual concentration of CTAB in the solution, after flotation [29]. SDS solution (0.002 M) was prepared by dissolving 0.2884 g of SDS in 500 ml of deionized water. In addition, methylene blue was used as indicator in the titration measurements and was prepared by dissolving 50 mg of methylene blue, 50 g of sodium sulfate and 12.5 g of concentrated sulfuric acid in 1 l of de-ionized water. 2.2. Experimental setup Column flotation was the experimental technique applied in the removal of humic acids from the simulated landfill leachate. The experimental setup is presented in Fig. 1. A plexi-glass column (50 mm i.d., height 60 cm) was used for the batch flotation experiments. The top of the column was connected with a funnel, in order to collect and remove the foam, containing the floated humic acids. Air was injected from the bottom of column by a microporous frit, in order to create the necessary bubbles, at a flow-rate of
183
Fig. 1. Schematic representation of experimental setup for flotation experiments; column height 60 cm, inner diameter 50 mm.
200 cm3 /min, measured by a rotameter, based on previous experience. Humic acids were mixed with the respective frother and surfactant in a 500 ml beaker (total volume of solution 400 ml), under gentle stirring for 1 min. Then the solution was inserted in the flotation column; after the flotation experiment, samples were collected from the bottom of the column for analytical determinations. 2.3. Analytical determinations Humic acids were determined by spectrophotometer (HITACHI U-2000) at 254 nm, using quartz vesicles. It has been reported that DOC concentration and UV absorbance cam be connected with a strong linear relationship, which was also confirmed by this study, as the calibration curve showed a correlation factor almost equal to one. The residual concentration of CTAB after flotation was measured by titration with sodium dodecyl sulfate solution and methylene blue as indicator [29]. Electrokinetic measurements were also performed, using a zeta-potential analyzer (Rank Brothers Ltd., UK). All mobility measurements were carried out at constant temperature (i.e., 22 ◦ C) and using a constant salt concentration of 0.001N NaCl. The solution pH was adjusted by adding various amounts of either 0.1 M NaOH or 0.1 M HCl. Twenty independent measurements were performed for each experimental condition, without preliminary filtration, as no problems of interference by particulates were observed.
184
A.I. Zouboulis et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 231 (2003) 181–193
The zeta-potential values were determined using the smoluchowski equation.
Table 1 Screening test results of different frothers on the removal of humic acids by flotation Frothers
Concentration (%)
Humic acid removal (%)
MIBK Amylalcohol Pine oil Ethanol
0.4 0.25 0.1 0.4 0.25 0.1
23.7 56.3 33.2 59.8 58 43.7
3. Results and discussion The flotation process involves three main steps: (a) selective chemical modification of specific specie surface, i.e., hydrophilicity/hydrophobicity; (b) contact between and adherence of hydrophobic species to air bubbles; and (c) separation of floatable and non-floatable species [23]. The first step is mainly controlled by the type and dosage of added collector (surfactant). In the second and third steps, the major parameter involved is the type and dosage of the frother [30]. The frother aids flotation by adsorbing onto the gas–liquid interface, thereby reducing its surface tension, which results in the formation of more stabilized bubbles. The creation of finer bubble distribution through changes in dynamic surface tension is also another important result of the frother addition [31]. The selection of the appropriate frother is mainly based upon the consideration of two aspects, when designing flotation experiments. Firstly, the frother has to control efficiently the bubble generation and secondly, it has to combine effectively with the collector, which could lead to synergistic effects on the overall flotation performance [30]. In the present study, several frothers have been examined, based on the specific collector usage (CTAB) and the most appropriate for the removal of humic acids has been selected on the basis of trial and error methodology. The examined frothers were ethanol, amylalcohol, pine oil, and MIBK. These frothers belong to the general category of neutral or non-ionic frothers, which present rather negligible collecting properties; screening results are presented in Table 1. The results indicated that the frother, which produced the best results, in relation to humic acids removal, was ethanol. The removal of humic acids was less efficient, when applying the pine oil or MIBK frothers, whereas the results obtained by the use of amylalcohol were equally efficient; however the use of amylalcohol was considered as less attractive due to its higher cost. Indeed, ethanol has been reported to be a very effective and convenient frother, when applying microflota-
Conditions: initial humic acids concentration 100 mg/l; initial collector concentration 50 mg/l; pH 8; floating time 10 min.
tion. It was found to promote multiple bubble attachment to the colloidal aggregates and to increase significantly the bubble frequency. Furthermore, as ethanol is a good solvent for most organic compounds, the introduction of the collector in an ethanol solution is desirable, because alcohol inhibits micelle formation, which can be detrimental to flotation [13,27]. Therefore, the rest of the experiments were performed by the use of ethanol as the frother. However, the optimum dosage of ethanol in the removal of humic acids by flotation had to be determined. It has been reported that excessive amounts of ethanol could be detrimental to flotation experiments [32]. This detrimental action of higher ethanol concentrations could be attributed to the fact that the respective complexes between collector and species are stabilized in such a degree, which renders the adsorption at the air-interface difficult. Moreover, at higher alcohol concentrations, the number of sites on the liquid–gas interface, available for particle–collector complex will be reduced due to adsorption of alcohol molecules and this might also reduce humic acids removal. Therefore, experiments have been performed using different alcohol concentrations, at constant collector and initial humic acids concentration (Fig. 2). It can be noticed that the concentration of ethanol was increased, humic acids removal was also increased, up to the value of 1% (v/v) ethanol. After this concentration, the removal of humic acid did not change significantly, although it showed a slight decrease, possible due to aforementioned effects. The increase on humic acids removal, by increasing ethanol has been previously reported by other researchers and was mainly attributed to the decrease of bubble size in the presence of small amounts of ethanol [27].
A.I. Zouboulis et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 231 (2003) 181–193
185
80
Humic acids removal (%)
60
40
20
0 0
1
2
Ethanol (%)
Fig. 2. Influence of ethanol concentration on humic acid removal; conditions: humic acid concentration 100 mg/l; collector concentration (CTAB) 50 mg/l; pH 8; floating time 10 min.
The decrease of bubble size by addition of small amounts of ethanol has been attributed to the fact that ethanol lowers the solubility of gas in water. Consequently, it is probable that fine gas bubbles can be formed, which enhance the efficiency of flotation [23]. Smaller bubbles enhance the efficiency of microflotation, because the use of large bubbles can break the formed flocs and produce turbulence, which limits the efficiency of flotation [33]. Nevertheless, during these experiments, the highest humic acids removal, observed at the optimum ethanol concentration (1%), was not found to exceed 70%. These results were considered as not satisfactory and indicated the major role of collector in the overall flotation efficiency. The collector interacts with certain species in solution and as a result converts them from hydrophilic to hydrophobic, enabling their attachment to the generated air bubbles. The selection of the appropriate collector depends on the nature of the target species to be removed. Humic acids are known to be fully protonated, i.e., they are not ionized at pH value 3, thus are negatively charged above this pH value [34]. Since humic acids present an anionic character over the usual pH values encountered in landfill leachates, they are expected to interact strongly with cationic collectors. Therefore, the collector (surfactant) used in this study (CTAB) was a common cationic one. The effect of collector
dosage in relation to the initial humic acids concentration is presented in Fig. 3. It can be noticed that the optimum dosage of collector, in order to achieve efficient humic acids removal varies with initial humic acids concentration and the higher the humic acids concentration, the higher the required dosage of collector. This can be attributed to the fact the required dosage of flotation reagent is closely related to the overall charge carried by the organic impurities. In order to achieve efficient removal of humic acids, their charge has to be neutralized [27]. Therefore, as the amount of humic acids was increased, the charge that had to be neutralized was also increased and this required higher dosages of collector. The results presented in Fig. 3 showed that the optimum collector dosages were 80–100 mg/l for [HA]0 100 mg/l, 170–200 mg/l for [HA]0 200 mg/l, and 250–300 mg/l for [HA]o 300 mg/l. Lower dosages than those aforementioned resulted in decreased humic acids removal, because the concentrations of the collector were not enough for neutralizing the overall (negative) charge of humic acids [22]. The neutralized species have become hydrophobic and floated with the air bubbles. However, the species, which did not react with the collector, remained hydrophilic, of anionic nature and could not float with the bubbles. The addition of collector
186
A.I. Zouboulis et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 231 (2003) 181–193
100
Initial HA 100 mg/L 200 mg/L 300 mg/L
Humic acids removal (%)
80
60
40
20
0 0
100
200
300
400
500
Collector dosage (mg/L) Fig. 3. Influence of collector dosage on humic acids removal by flotation, applying different initial concentrations of humic acids; conditions: pH 8; ethanol 1%; floating time 10 min.
dosages higher than the optimum ones resulted also in decreased humic acids removal and at very high collector concentrations the removal efficiency was substantially decreased. This can be attributed to the fact that by increasing the collector dosage much higher than the required for efficient flotation, the overall charge of humic acids may become cationic [32]. Nevertheless, excessive amounts of collector should be avoided, not only due to higher cost induced, but also because of other negative effects, such as larger foam losses, possibility for micelle or hemi-micelle formation and the potential toxicity of residual amounts of collector in the treated effluent. The adverse effect of excessive collector dosages can be also explained, through electrokinetic measurements carried out at constant pH value and initial humic acids concentration (100 mg/l) but for different initial collector concentrations (Fig. 4). As can be noticed, zeta-potential values were increased by increasing the collector concentration, and at concentration of 85 mg/l, the system humic acid/collector has become
almost neutral. This increase of zeta-potential values by the addition of collector has been reported to be a characteristic behavior of specific chemisorption phenomenon and the hydrophobicity of humic acids has resulted through interaction of collector with humic acids [35]. However, excessive amounts of collector caused further increase of zeta potential of this system and at concentrations over 85 mg/l, a cationic character was observed. Comparing these results with those presented in Fig. 3, it can be noticed that for initial humic acids concentration of 100 mg/l, the removal of humic acids by flotation started to decrease over the collector dosage of 85 mg/l, which is the value when this system showed zeta-potential value around zero, indicating the consistency between zeta-potential measurements and humic acids removal efficiency. The effect of the collector dosage was also investigated in relation with the variation of pH value, by keeping constant the initial humic acid concentration at 100 mg/l (Fig. 5). The pronounced effect of optimum collector dosage was also confirmed, almost
A.I. Zouboulis et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 231 (2003) 181–193
187
Z-potential (mV)
15
0
-15
-30
0
20
40
60
80
100
120
Collector dosage (mg/L) Fig. 4. Electrokinetic measurements of humic acid solution at different collector concentrations—effect on z-potential values; conditions: initial humic acids concentration 100 mg/l; ethanol 1%; floating time 10 min; pH 8.
independently of the solution pH. The difference in pH had a direct effect on the required collector dosage for efficient humic acids removal. When the solution pH was adjusted to 4, efficient removal was achieved at collector concentration of 30 mg/l and was decreased for values above 120 mg/l. On the contrary, when the pH of solution was increased to pH value of 10, the sufficient dosage of collector, in order to achieve efficient removal of the same concentration of humic acids was found to be 80 mg/l, whereas above the concentration value of 100 mg/l, the removal efficiency was found also to be decreased. The effect of collector dosage has been discussed previously. However, the results in Fig. 5 indicated also the significant effect of pH on the effectiveness of flotation as a treatment technology for removing humic acids. At lower pH values, the required dosage of collector for achieving sufficient removal of this humic acids concentration was lower than the amount required at higher (alkaline) pH values.
These results were confirmed by Fig. 6, which illustrates the removal of humic acids, as affected by initial pH values, for constant initial humic acids concentration (100 mg/l) and for three different initial concentrations of collector. As the pH value was increased the removal efficiency of humic acids was decreased, and only when the concentration of collector was high enough (in this case 70 mg/l), the removal efficiency remained unaffected by pH variation. The aforementioned observations can be attributed to the fact that at low pH values (acidic), organic protonation was increased, thus reducing the negative charge density of colloidal organic matter and subsequently, the demand of cationic collector for neutralizing the anionic humic acids [14,34]. Therefore, in higher pH values, the demand for collector was increased. These results were in accordance with the electrokinetic studies, performed for humic acids solution as well as for the system humic acids/collector at different pH values (Fig. 7).
188
A.I. Zouboulis et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 231 (2003) 181–193
Humic acids removal (%)
100
80
60
40 pH=10 pH=8
20
pH=4
0
0
20
40
60
80
100
120
140
160
180
Collector concentration (CTAB, mg/L) Fig. 5. Influence of collector concentration on humic acids removal by flotation at different pH values; conditions: initial humic acid concentration 100 mg/l; ethanol 1%; floating time 10 min.
100
Humic acids removal (%)
80
60 Collector dosage
40
70 mg/L 50 mg/L 35 mg/L
20
0
3
5
7
9
11
pH Fig. 6. Effect of solution pH on the removal of humic acids by flotation at three different initial collector concentrations; conditions: initial humic acids concentration 100 mg/l; ethanol 1%; floating time 10 min.
A.I. Zouboulis et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 231 (2003) 181–193
189
10
0 0
2
4
6
8
10
12
Z-potential (mV)
humic acids (100mg/L)
-10
humic acids (100 mg/L)+ collector (50 mg/L) and ethanol (1%)
-20
-30
-40 pH
Fig. 7. Electrokinetic measurements of humic acids aqueous solution and humic acids/collector system at different pH values—effect on z-potential values; conditions: initial humic acid concentration 100 mg/l; ethanol 1%.
The zeta-potential values of humic acids solution were found to increase by decreasing the pH, indicating the increase of humic acid protonation. The addition of collector, caused an increase of zeta-potential values, as compared with values obtained without the addition of collector. The isoelectric point of the system humic acid/collector was found to be at pH values around 5, indicating the specific interaction (chemisorption) of collector with humic acids. The ionic strength of solution is another important parameter that influences the efficiency of flotation, because the higher the ionic strength, the higher is the amount of other ions in solution, which might interact or compete with humic acids or the collector and affect the removal efficiency. Two different salts were examined in different concentrations and the respective results are presented in Fig. 8. These results indicated that at values of salt concentration lower than 0.01 M, the removal efficiency was only slightly affected by the presence of the other ions introduced in the solution. However, as salt concen-
trations were further increased, the removal of humic acids was found to be decreased, which was apparent for salt concentrations over 0.01 M. However, when sodium chloride was used, the effect on the removal of humic acids was not significant and the decrease in the overall removal efficiency accounted only for 10%. Although, during the application of other treatment techniques (i.e., coagulation), the increase of ionic strength was found to affect positively the removal of humic acids [36], when flotation techniques were employed, ionic strength was found to be a limiting factor, regarding the treatment efficiency. The negative effect of ionic strength, during ion flotation has been also reported by other researchers [37]. As aforementioned, the increase in ionic strength can adversely affect flotation in two directions. Firstly, ions with the same charge of target species (in this case, anionic) may compete or interact with the cationic collector, hence reducing its availability to float the target compounds. In this case, in order to maintain efficient hu-
190
A.I. Zouboulis et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 231 (2003) 181–193 100
Humic acids removal %
90
80
70
Na2SO4 NaCl
60
50 0,00
0,04
0,08
0,12
Salt concentration (mol/L) Fig. 8. Effect of salt concentration on the removal of humic acids by flotation; conditions: initial humic acids concentration 100 mg/l; pH 8; initial collector concentration 70 mg/l; ethanol 1%.
100
Humic acids removal(%)
80
60
Collector dosage 40
50 mg/L 70 mg/L
20
0
0
4
8
12
16
20
24
28
Floating time (min) Fig. 9. Effect of floating time on the removal of humic acids by flotation; conditions: initial humic acids concentration 100 mg/l; ethanol 1%; pH 8.
A.I. Zouboulis et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 231 (2003) 181–193
191
12
Residual collector CTAB (mg/L)
10
8
6
4
2
0 0
20
40
60
80
100
120
140
Initial collector CTAB (mg/L) Fig. 10. Effect of initial concentration of collector (CTAB) on the residual concentration of collector after flotation; conditions: initial humic acids concentration 100 mg/l; pH 8; ethanol 1%; flotation time 20 min.
mic acids removal, the increase of collector dosage is required. However, it is possible that ions with opposite charge with the target species (i.e., cationic) can interact with humic acids, neutralizing the molecules, which cannot further interact with the cationic collector and therefore, cannot float. In summarizing the previous investigations, it was found that flotation using CTAB as the collector and ethanol as the frother can be an efficient treatment method for removing humic acids; if operational parameters would be adjusted properly, removal efficiencies up to 99% can be obtained, for a wide range of initial humic acids concentrations. However, in the design of a process, kinetic investigations have to be also performed, especially when economic considerations are involved. Therefore, experiments were performed by varying the time of flotation and keeping constant the other parameters. The experiments have been performed for two different collector concentrations, 50 and 70 mg/l, respectively. Previous findings have shown that for initial humic acids concentration of 100 mg/l, the optimum concentration of collector was 70 mg/l, whereas a 50 mg/l
collector dosage resulted in humic acids removal of around 65%, at pH 8 and for floating time 10 min. During these experimental series (Fig. 9), it can be noticed that by increasing the flotation time from 10 to 20 min, the removal efficiency was further increased and reached 80%. This can be attributed to the fact that by increasing the flotation time, the collector can be utilized more efficiently and humic acids can float with air bubbles in the froth phase [22]. However, when the optimum collector dosage was employed (70 mg/l), it was noticed that the reaction was very fast and a 5 min flotation time was found to be sufficient for achieving more than 95% removal of humic acid. Therefore, from these experiments, the pronounced effect of collector dosage was also evident. When the collector concentration was less than the optimum dosage, the expansion of the flotation time to 20 min did not produce such efficient results as those obtained for the optimum collector dosage, under which, even for short retention (flotation) time, almost complete humic acids removal could be achieved. Another parameter, which is strongly connected with flotation time, is the residual concentration of
192
A.I. Zouboulis et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 231 (2003) 181–193
Table 2 Residual collector concentration as influenced by flotation time for two different initial collector concentrations Flotation time (min)
Initial collector concentration (mg/l)
Residual collector concentration (mg/l)
10
50 70 50 70
10.2 10.8 8.02 8.31
20
Conditions: initial humic acids concentration 100 mg/l; ethanol 1%; pH 8.
collector (after flotation). As it was previously reported, the collector can interact with the target species and co-float in the froth phase. However, when the floating time was not sufficient for efficient humic acids removal, this has a direct result also in the residual concentration of collector, remaining in the treated solution (Table 2). Thus, when the flotation time was adjusted to 10 min, the residual quantity of collector was greater than when the flotation duration was 20 min. Further investigations related with the residual quantity of collector in the effluent involved the effect of initial collector concentration. During the previous investigations (see Fig. 2), it was noticed that over an optimum collector concentration, the removal of humic acids was decreased. In Fig. 10, it can be observed that over this collector concentration, which for initial humic acids concentration of 100 mg/l was estimated to be around 70 mg/l, the residual quantity of collector was increased, because a significant amount of it cannot react with humic acids and therefore, remain in the treated solution. The increase in flotation time can further decrease the residual concentration of collector. Therefore, when designing a flotation process, apart from other parameters, the residual collector concentration has also to be taken under consideration, because when removing contaminants from water, it is not desirable to contaminate them by the additives used for the accomplishment of treatment process.
of humic acids was found to be very efficient, for a wide range of initial humic acids concentration (50–200 mg/l). The parameters, which control the efficiency of this treatment technique, were mainly the type and dosage of frother, the dosage of collector, solution pH, ionic strength (concentration of other salts) and the flotation time. Based on screening tests, the most efficient frother was found to be ethanol, in concentrations 1% (v/v). The dosage of collector was the main factor controlling the removal of humic acids and the optimum concentration was greatly dependent on the initial humic acid concentration and on the solution pH; higher humic acids concentrations required higher dosages of collector. However, collector dosages higher than those required may produce opposite results and decrease the removal of humic acids. The obtained results were compared by respective z-potential measurements, which demonstrated that at high collector dosages the humic acids/collector system had become cationic, and unable to float. The pH of the solution also affected the removal efficiency, as by varying the pH value, the protonation of humic acids varied. At lower (acidic) pH values, the degree of humic acids protonation was increased. As a result, at these pH values lower dosages of collector were required for efficient humic acids removal. The presence of salts in humic acids solution had an adverse effect on flotation performance, and the extent of this effect depended on the specific salt. The flotation time affected also the efficiency of flotation, because by increasing it, the collector interacted with humic acids in higher extent, while the residual collector quantity in the treated solution was also found to be decreased. In summary, if flotation process would be designed properly and operational parameters can be adjusted accordingly, then flotation can be very efficient towards the removal of humic acids, providing a cost competitive alternative for the treatment of humic acids from contaminated aqueous streams, such as landfill leachates.
4. Conclusions Acknowledgements Flotation was found to be an efficient treatment option as a post-treatment stage for removing humic acids from simulated landfill leachates. In the present study, and under optimized conditions, the removal
Thanks are due to General Secretariat of Science and Technology (Greece) for the funding of specific project, under the framework of common
A.I. Zouboulis et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 231 (2003) 181–193
Greece–China Scientific Technological Cooperation Programme. References [1] A.T. Stone, J.J. Morgan, Environ. Sci. Technol. 18 (1984) 617. [2] M.N. Jones, N.D. Bryan, Adv. Colloid. Interface Sci. 78 (1998) 1. [3] N. Calace, A. Liberatori, B.M. Petronio, M. Pietroletti, Environ. Pollut. 113 (2001) 331. [4] H.U. Galland, U. von Gunten, Water Res. 36 (2002) 65. [5] W. Stumm, J.J. Morgan, Aquatic Chemistry: Chemical Equilibria and Rates in Natural Waters, Wiley, New York, 1996. [6] S.A. Banwart, Geochim. Cosmochim. Acta 63 (1999) 2919. [7] B.G. Oliver, E.M. Thurman, R.L. Malcolm, Geochim. Cosmochim. Acta 47 (1983) 2031. [8] A.A. Tatsi, A.I. Zouboulis, Production, composition and temporal variation of pollution parameters for sanitary landfill leachates, Adv. Environ. Res. 6 (2002) 207–219. [9] M. El-Fadel, E. Bou-Zeid, W. Chachine, B. Alayli, Waste Manage. 22 (2002) 269. [10] E. Chian, F. DeWalle, J. Environ. Eng. Div. (Proc. Am. Soc. Civil Eng.) 102 (1976) 411. [11] J.B. Christensen, D.L. Jensen, C. Gron, Z. Filip, T.H. Christensen, Water Res. 32 (1998) 125. [12] M.A. Nanny, N. Ratasuk, Water Res. 36 (2002) 1577. [13] F.J. Mangravite, T.D. Buzzell, E.A. Cassell, E. Matijevic, G.B. Saxton, J. Am. Water Works Assoc. 67 (1975) 89. [14] S.K. Kam, J. Gregory, Water Res. 35 (2001) 3557. [15] M. Alborfzar, G. Jonsson, C. Gron, Water Res. 32 (1998) 2983. [16] N.D. Graham, Water Sci. Technol. 40 (1999) 141. [17] J. Fettig, Water Sci. Technol. 40 (1999) 173. [18] Y. Seida, Y. Nakano, Water Res. 34 (2000) 1487.
193
[19] C.H. Lai, C.Y. Chen, Chemosphere 44 (2001) 1177. [20] S.G. Yiantsios, A.J. Karabelas, Desalination 140 (2001) 195. [21] P.C. Chiang, E.E. Chang, C.H. Liang, Chemosphere 46 (2002) 929. [22] L. Zhang, P. Smasundaram, V. Ososkov, C.C. Chou, Colloids Surf. A 177 (2001) 235. [23] A.I. Zouboulis, K.A. Matis, G.A. Stalidis, Flotation techniques in water treatment, in: P. Mavros, K.A. Matis (Eds.), Innovations in Flotation Technology, Kluwer Academic Publishers, Durdrecht, MA, 1992. [24] J. Rubio, M.L. Souza, R.W. Smith, Miner. Eng. 15 (2002) 139. [25] E.A. Cassell, E. Matijevic, J. Francis, J.R. Mangravite, T.M. Buzzell, S.B. Blabac, J. Am. Inst. Chem. Eng. 17 (1976) 1486. [26] M. Hiraide, F. Ren, R. Tamura, A. Mizuike, Microchim. Acta 11 (1987) 137. [27] E.A. Cassell, K.K. Kaufman, E. Matijevic, Water Res. 9 (1975) 1017. [28] S.R. Qasim, W. Chiang, Sanitary Landfill Leachate. Generation, Control and Treatment, Technomic, Lancaster, UK, 1994. [29] B.M. Mildiwski, D.M. Gabriel, Detergent analysis: A Handbook of Cost Effective Quality Control, Micelle press, London, 1982. [30] G.H. Harris, R. Jia, Int. J. Miner. Process. 58 (2000) 35. [31] H. El-Shall, N.A. Abdel-Jhalek, S. Svoronos, Int. J. Miner. Process. 58 (2000) 187. [32] L.B. Scorzelli, A.L. Fragomeni, M.L. Torem, Miner. Eng. 12 (1999) 905. [33] S. Ata, N. Ahmed, G.J. Jameson, Int. J. Miner. Process. 64 (2002) 101. [34] M. Bob, H.W. Walker, Colloids Surf. A 191 (2001) 17. [35] A. Martinez-Lueranos, A. Uribe-Salas, A. Lopez-Valdivieso, Miner. Eng. 12 (1999) 919. [36] E.K. Kim, H.W. Walker, Colloids Surf. A 194 (2001) 123. [37] A.I. Zouboulis, K.A. Matis, Water Sci. Technol. 31 (1995) 315.