922
Ind. Eng. Chem. Res. 1994,33,922-928
Removal of Arsenic from Aqueous Solution by Adsorbing Colloid Flotation Felicia F. Peng' and Pingkuan Di The Department of Mineral Processing Engineering, College of Mineral and Energy Resources, West Virginia University, Morgantown, West Virginia 26506-6070
Adsorbing colloid flotation (ACF) with ferric hydroxide as the coprecipitant, anionic surfactant sodium dodecyl sulfate (SDS) as the collector, and nitrogen microbubbles has been shown to be effective in removing arsenic from low concentration of arsenic aqueous solution ( 10 mg/dm3). Experiments were conducted to assess the effects of pH, dosages of SDS and Fe(III), gas flow rate, foreign anions including NO3-, S042-,and Po43-,and Al(II1) addition on the efficiency of the arsenic removal by ACF. When p H is at the range of 4-5,99.5 % arsenic removal efficiency can be achieved. The optimal operating conditions are 35 ppm SDS, 80 ppm Fe(III), 40 cm3/min gas flow rate, 500 rpm rotary speed of stirrer, and 5 min ferric hydroxide floc formation time. The analysis results indicate that foreign anions S042-and PO4%have an inhibiting effect on the removal of arsenic, while Al(II1)additions can compensate for the effect. An actual mine wastewater sample containing 10.4 ppm arsenic(V), 0.76 ppm chromium(VI), and 0.55 ppm lead was tested by adsorbing colloid flotation a t the optimal operation conditions. The residual arsenic, chromium, and lead are 0.07, 0.06, and 0.04 ppm, respectively. The interaction mechanisms between Fe(OH)a-bearing arsenic floc and SDS were analyzed and interpreted by means of (-potential, infrared spectra analysis, and molecular orbital theory. The adsorption of SDS can occur physically and chemically on Fe(OH)3 a t the pH range of 4-5. N
Introduction In the mining industry, some mine water and tailing water from mineral processing plants, particularly from nonferrous metal mines, usually contain arsenic. The discharge of the wastewater containing arsenic to an aquatic system poses a potential threat to the environment, i.e., a deleterious effect on human health, animals, and plant life. Arsenic is a metalloid which forms a number of toxic compounds. Arsenic occurs in both the +I11 and +V oxidation states. Arsenic is directly below phosphorus in the periodic table, is similar to phosphorus in some chemical properties, and is always encountered as an impurity in phosphorus minerals. Because of its toxicity, inorganic compounds of arsenic constituents which include arsenic acid, arsenic(II1) oxide, and arsenic(V) oxide have been specified as hazardous waste by the EPA. Therefore, it is necessary to treat wastewater containing arsenic. Existing methods of arsenic removal include precipitation (Harper and Kingham, 1992; Chow, 1987; Gulledge and O'Connor, 1973;Ramana and Sengupta, 1992),adsorption (Gupa and Chen, 1978; Huang,1984), ion exchange (Clifford and Lin, 1991), ultrafiltration (Bellack, 1971), and reverse osmosis (Clifford and Lin, 1991). Chemical precipitation with lime and ferric salt has proven to be effective (Harper and Kingham, 1992; Chow, 1987; Shen, 1973). However, it yields large quantities of sludge, which is difficult to treat further or dispose of directly. The other methods have their limitations and are expensive. Adsorbing colloid flotation (ACF) is an innovative flotation method which is under development to remove heavy metals. ACF involves the addition of a coagulant such as alum or ferric hydroxide to produce a floc. The dissolved metal is adsorbed on the flocs and is coprecipitated with them. A surfactant acted as a collector is then added, is adsorbed on the flocs, and renders them hydrophobic. Subsequently, the flocs are removed by microbubbles using a flotation technique. This process
* To whom correspondence should be addressed.
has proven to be very effective in removing various heavy metal ions from dilute aqueous solutions (Chaine and Zeitlin, 1974;McInture et al., 1982;Lin and Huang, 1989/ 90; Robertson et al., 1976; Zouboulis et al., 1990). It has several advantages including the ability to treat low residual metal concentration, rapid operation, less space requirements, less need of surfactant, and low cost. However,very little information is available concerning treatment of mine wastewater containing arsenic through the use of adsorbing colloid flotation. De Carlo and Thomas (1985) reported the removal of arsenic from geothermal fluids by adsorptive bubble flotation with colloidal ferric hydroxide. Specific information on the major operating parameters of ACF affecting arsenic removal and specific adsorption mechanisms between coprecipitant and surfactant are not available. The objectives of this work are (1)to investigate various operation parameters affecting the removal of arsenic by adsorbing colloidalflotation, (2) to determine the optimum operating conditions for treatment of mine wastewater containing arsenic using adsorbing colloid flotation, and (3) to analyze the adsorbing mechanism of arsenic species on ferric hydroxide flocs and the interaction between ferric hydroxide and surfactant. Experimental Section Materials. All chemicals used in this study were analytical reagent grade. Sodium dodecyl sulfate (SDS) was used as the surfactant, while ferric hydroxide was used as the coprecipitant. NaN03, Na2S04, and Na3P04 were used as the sources of foreign anions. The arsenicbearing feed water was prepared by dissolving a predetermined amount of Na3AsOg12H20in deionized distilled water. The pH of the solution was adjusted to 2.0-2.5 by adding a dilute nitrate acid solution. All flotation tests were carried out using an initial As concentration of 10 mg/dm3. The 10 mg/dm3 arsenic concentration is usually met in spent mine water or tailings, where such a flotation technique may be applied. 0 1994 American Chemical Society
Ind. Eng. Chem. Res., Vol. 33, No. 4,1994 923 Foam Discharge Port
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H*O Figure 1. Schematic diagram of adsorbing colloid flotation system. Table 1. Operating Conditions of Adsorbing Colloidal Flotation of Arsenic
As concn of feed water Fe(OH)s sodium dodecyl sulfate PH Nz flow rate speed of stirring bar foreign ion concn: NaN03, Na2S04, NasPO4
10 mg/dm3 20-120 ppm 15-100 ppm 2.5-10 20-150 cm3/min 200-1500 rpm 5 X 1-4 X 1W2mol/dm3
Procedures. The removal of As from arsenic-bearing feed water was studied by using a laboratory flotation column. A schematic diagram of the laboratory flotation column and auxiliary units for adsorbing colloid flotation is shown in Figure 1. The flotation column was constructed by a 30-mm4.d. and 35-cm-high Pyrex glass tubing with a fritted glass disk. The fritted glass disk which has 1540-pm nominal porosity was used as a sparger to generate the bubbles for flotation. The feed water for the flotation column was prepared from 200 cm3 of 10 ppm As solution in a beaker. A predetermined concentration of Fe(OH)3 was added and the pH of the solution was adjusted. The mixed solution was stirred for 5 min, then a predetermined amount of SDS (the surfactant) was added, and the solution was stirred again for 1min using a magnetic stirrer. After the pH of the solution was adjusted to a desired value, the solution was transferred to the flotation column. The desired amount of high purity nitrogen gas (purity 99.99%) was adjusted using a bypass line (vent). To start a flotation test, the predetermined nitrogen flow rate was diverted to the flotation column. At the top of the flotation column, a thin layer of foam was formed. The foam containing the precipitates flowed continuously through a foam discharge port and was collected in a beaker. The foam product was generally a small portion of the original solution volume. When the appropriate period of flotation time was given (i.e., 5 min flotation time), the gas was turned off and a small amount of the remaining solution was taken from the lower part of the flotation column to determine the arsenic concentration remaining in the solution. The operating conditions of the flotation tests for arsenic removal are summarized in Table 1. The analysis of arsenic in the residual sample was carried out by using a Hitachi 180-80 polarized Zeeman atomic absorption spectrophotometer. The residual ar-
Figure 2. Effect of pH on As(V)-Fe(OH)s removal by adsorbing colloid flotation and by filtration. SDS = 35 ppm; Fe(II1) = 80 ppm; stirring speed = 500 rpm; floc formation time = 5 min; gas flow rate = 40 cms/min. ( 0 )Arsenic removal by flotation; (A)residual As(V) in filtrate.
senic concentration was expressed as the removal efficiency of arsenic (As removal, %) which can be expressed as follows: arsenic removal, % = (1 - (CA$CA,,o))X 100% (1) where C ~ , and O CA,are the initial and final (residual) concentrations of arsenic in the solution, respectively. {-Potential Measurement. To prepare the samples for {-potential measurement, 38 mg of ferric hydroxide was added to a solution of 100 cm3 containing 10 ppm arsenic. The pH of the solution was adjusted, and the solution was stirred for 10 min. A similar solution was used to prepare an As solution containing the surfactant. After a predetermined amount of the surfactant was added, the solution was stirred for 3 min using a magnetic bar. The {-potentials of colloidalparticles were measured using a DXD-I zeta potential meter, immediately after the pH of the solution had stabilized to the desired value. The reported {-potential represents the average for 10-15 colloidal particles tracked a t each given pH value. Sample Preparation for Infrared Analysis. For infrared analysis, 190 mg of ferric hydroxide was added to a 500-cm3solution containing 10 ppm arsenic. The pH of the solution was adjusted to 4.5, and the solution was stirred for 10 min using a magnetic stirrer. This mixed solution was then divided into two equal portions. One of the samples was mixed with 5 cm3of 1% SDS solution and was stirred for 30 min. Both samples were separately filtered and air dried at room temperature. Subsequently, the dried samples were used to measure infrared spectra, using a Nicolet 170 SX FTIR spectrometer with diffuse reflectance.
Results and Discussion Effect of pH on the Arsenic Removal. The removal efficiency of arsenic as a function of pH is presented in Figure 2. There are three distinct segments in the As removal-pH curve. It includes (1) an increase in As removal efficiency at the pH range of 2-4, (2) a maximum removal of arsenic over 99 5% in the pH range of 4-5, and (3)a decrease in As removal efficiency above the pH range of 5. It is clearly shown that the optimum pH range for arsenic removal using adsorbing colloidal flotation is the pH range of 4-5. The results of atomic absorption spectroscopic (AA) analysis for arsenic residual samples
924 Ind. Eng. Chem. Res., Vol. 33, No. 4, 1994 Table 2. Equilibrium Equations for Major Species in Arrrenic Solution (Smith and Martell, 1976) 1 FeAs04(s)= Fe3+ + h O & 2 Fe(OH)s(s) = Fe3++ 30H3 Fe(OH)s(s) = Fe(OHI2++ 20HFe(OH)s(s) = Fe(OH)z++ OH4 Fe(OH)s(e)+ OH- = Fe(0H)r 5 Fe(OH)*+= Fe3+ + OH6 I FeAsO&) + OH- = Fe(OHP + b04” 8 H&O4 = H h O c + H+ H&04- = HAsOIZ + H+ 9 10 HASO,” = As04a+ H+
50 1
-i
20
m 8 N
0
+j
[As(V)Ibd = [H3AsO41+ A HA SO,^] + [H&30[1+ [AsO,~I (2)
At the pH range of 4-5, ferric hydroxide flocs bear a positively charged surface which may have the forms of Fe(OH)2+,Fe(OH)2+,and Fe3+ by ionization reactions in the solution (Fuerstenau and Palmer, 1976). The total concentration of Fe(II1) may be written as
+ [Fe(OH?+I
30
10
show that the concentration of arsenic in the treated water can be less than 0.1 mg/dm3. In other words, the arsenic removal percent can be as high as 99% or more. At the lower pH, the solution of arsenic may contain various arsenic oxyanions. They may include HAsO4, HAs04s,H2AsO4-, and AsOr3-(Brewster,1992). The total concentration of As(V) in the solution is given by
[Fe(III)lbd = [Fe3+l+ [Fe(OH);I
4
40
(3)
Using the equilibrium equations for various major species in the solution (as shown in Table 21, and combining eq 2 and 3, the total h ( V ) concentration can be derived as
Equation 4 gives the theoretical solubility of As(V) as a function of the pH value in the solution. At pH 4, when an initial h ( V ) concentration is 10 ppm, and Fe(II1) concentration is 80 ppm, the residual concentrations of As(V) and Fe(II1) in the treated water are determined as 0.06 and 0.34ppm (6.07 X 10-8 M)respectively by AA. The theoretical solubility of As(V) at pH 4 is calculated as 2.01 ppm by using eq 4. The results indicate that the mechanism of the formation of iron-arsenate precipitates can not be solely accountable for such a high efficiency of As(V) removal (99.4%). The adsorption may also play a significant role in the As(V) removal mechanism. At the pH range of 4-5, the ferric hydroxide flocs bear positively charged surfaces as shown in Figure 3, which favor the adsorption of arsenic oxyanions. At pH values below 4, the removal efficiency of h ( V ) is low. The low As removal efficiency may be attributed to (1) the poor collectability of the collector SDS and (2) the poor floc formation of Fe(OH)s. This is visually observed with a smaller size and with a fewer number of flocs, thus providing an insufficient surface area for As(V) adsorption. Therefore, the flotation responses of As removal are low. This is evident from the results of filtration experiments also presented in Figure 2. At pH values above 5, the surface charges of ferric hydroxide tend to become less positive due to the increase of Fe(OH14- and the decrease of Fe3+,Fe(OH)2+,and Fe(OH)2+in the solution. Thus, the adsorption of negatively charged As(V) species and SDS on Fe(OH)a(s)decreases, which in turn lowers the As removal.
-10
-20
2
4
6
a
10
PH Figure3. PPotential as a function of pH for iron(II1)floce with and without SDS Addition. SDS = 35 ppm. (0)Without SDS; (A)with SDS.
A t pH values above 7, the flocs bear negatively charged surfaces which are unfavorable for adsorption of arsenic species or SDS on the flocs. Therefore, a very low As(V) removal efficiency is obtained. Electrokinetics of Flocs. It can be seen from Figure 2 that pH is the most important factor to affect the adsorbing colloid flotation of arsenic. The S‘-potentialsof iron(II1) floc containing arsenic with and without the addition of SDS at different pH ranges are given in Figure 3. The results show that the ZPC (zero point of charge) of Fe(OH)3is at 7.0 in the absence of SDS. However, the ZPC of Fe(OH)3shifts to the value of 6.5 in the presence of SDS. At a pH value of 7 or higher, iron(II1) flocs bear negative surface charges, which are unfavorable for electrostatic adsorption of SDS on the surfaces. At a pH value of 7 or lower, iron(II1) flocs appear to have positive surface charges, and the positive charges increase as pH decreases. Sodium dodecylsulfate (SDS) may adsorbmore easily on the surfaces of the flocs. Below a pH level of 4, the ability of flocs to collect SDS decreases drastically. This may be attributed to an increase in the repulsive energy between colloidal flocs at higher positive charges of iron(II1)floc surfaces. This increase in repulsive energy is unfavorable to the growth of the flocs. These observations agree with the experiment results presented in Figure 2. Effects of Gas Flow Rate and Mixing Intensity/ Duration on Arsenichmoval. In the flotation process, the gas flow rate not only affects the characteristics of bubbles, but also interferes with the flow pattern in the flotation column. In Figure 4, the percent of arsenic removal is plotted as a function of the nitrogen gas flow rate. The gas flow rate is varied from 20 to 140 cm3/min, while other operating conditions remain constant. The results show that the percent of arsenic removal remains predominantly constant when the nitrogen gas flow rate is greater than 40 cm3/min. However, if the nitrogen gas flow rate is less than 40 cm3/min,the flotation rate and percentage of arsenic removal decrease. In the process of adding and mixing the coprecipitant in the solution, the mixing intensity and duration subsequently affect the flotation responses. The experiments were carried out by varying the mixing time and the speed of rotation of the stirrer, to examine the mixing intensity and duration effect on the efficiency of As removal. Figure 5 shows that the optimum mixing condition is at the mixing intensity of 500 rpm. The mixing time is also important to provide sufficient time for the formation of the flocs. The optimum mixing time of 5 min (not shown) is found
Ind. Eng. Chem. Res., Vol. 33, No. 4, 1994 925 100
--
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>
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96
01
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.-0C
94
01
2
92
90 0
20 40
60 80 100 120 140 160
Gas FL,ow Rate (cm3/min) Figure 4. Arsenic removal as a function of nitrogen gas flow rate. pH = 4.5; SDS = 35 ppm; Fe(II1) = 80 ppm; stirring speed = 500 rpm; floc formation time = 5 min. I
100 I
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Qd In a dilute aqueous solution of SDS,the adsorption density of SDS can be calculated by the Gibbs equation, I’ = -(c/2RT)(dy/dC), where C is the concentration of SDS and y is the surface tension of the solution. It is possible to substitute a semiempirical equation, suggested by Adamson (19761,dy/dC = -+IC, into the Gibbs equation, to obtain: 2RT
90
’
I
0
400
800
1200
1600
RPM Figure 5. Arsenic removal as a function of rotary speed (rpm). pH = 4.5; SDS = 35 ppm; Fe(II1) = 80 ppm; stirring speed = 500 rpm; floc formation time = 5 min; gas flow rate = 40 cm3/min. 100
98
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re-YOB
a
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efficient foam for complete removal of arsenic. When the concentrations of SDS were very low, a thin and unstable foam layer was generated and readily broke apart. Therefore, removal of the colloidal ferric hydroxide was incomplete. However, the use of excessive amounts of surfactant may impair the performance of flotation. Excessive amount of surfactant causes the formation of hydrophilic micelles around the floc particles, which may render the floc particles unfloatable. The critical micelle concentration (crnc) of SDS during the column flotation can be calculated by determining of the adsorption density of SDS on the surfaces of bubbles and the total surface area of the bubbles. The number of microbubbles (n in 1L of water) can be calculated as n = (6Vt)/(?rQd3), where V is the gas flow rate (cm3/min) and t is the flotation time (min). Q is the volume of the wastewater sample (L),and d is the average diameter of bubbles (cm). The total surface of bubbles in 1 L of water is
94
e,
92
90 0
20
40
60
80
100
120
SDS Concentration (pprn) Figure 6. Arsenic removal as a function of collector (sodium dodecyl sulfate) concentration. pH = 4.5; Fe(II1) = 80 ppm; stirring speed = 500 rpm; floc formation time = 5 min; gas flow rate = 40 cmVmin.
to be adequate for floc formation. Excessive intensive stirring may be destructive to the floc formation. Effect of Anionic Surfactant (Collector) Concentration. Figure 6 shows that the effect of the anionic surfactant, sodium dodecyl sulfate (SDS),concentration on arsenic removal by ACF. A concentration of 35 ppm sodium dodecyl sulfate was found to produce a stable and
The cmc of SDS is given by the product of adsorption density, r, and the total bubble surface area, St:
3r$ v t [cmc] = rs, = -RTdQ
(7)
where yo is the surface tension of pure water a t 20 “C (72.75 erg/cm2). B is an empirical constant,R is a universal constant, and T represents the absolute temperature. For this study, the operation parameters of adsorbing colloid flotation are V = 40 cm3/min, Q = 0.2 L, t = 3 min, d = 65 X 106 dm, and B = 0.2. The calculated cmc value for SDS is 0.165 X 103 mol/dm3 (47.5 ppm). In this flotation system, the presence of anions such as Nos-, SO4%, or PO4” in the solution lowers the cmc of SDS. This occurs because the anions compete with the SDS adsorption on the surface of floc particles, thereby reducing the adsorption density of SDS. Therefore, it is possible that the micelles may form on floc particle surfaces when the SDS concentration exceeds the optimum level. This is possibly the reason for the plateau at the optimum SDS concentration. Effect of CoprecipitantConcentration. In order to investigate the effect of concentrations of the coprecipitant on the removal of arsenic, seta of experimenta were performed with various concentrations of Fe(II1) at a pH value of 4.5. The efficiency of arsenic removal improves significantly as the concentration of ferric hydroxide increases, as shown in Figure 7 . This is because higher concentration of ferric hydroxide provides a larger surface area available for adsorption of arsenic. No significant improvement in the efficiency of arsenic removal is observed when the concentration of Fe(II1) is over 80 ppm. Effect of Foreign Anions. In the mine wastewater, anions such as N03-, S O P , and Pod3- commonly coexist with arsenic oxyanions. Figure 8 shows the effect of these foreign anions on the removal of arsenic by adsorbing
926 Ind. Eng. Chem. Res., Vol. 33, No. 4, 1994 loo
*
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r------
10
5 0
-5 -10’ 0
20
40
60
80
100 120 140
3.0
.
‘ 4.0
Fe(lll) Concentration (ppm)
’
‘
.
‘
’
6.0
5.0
7.0
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Figure 7. Arsenic removal as a function of coprecipitant (Fe(0H)s) concentration. pH = 4.5; SDS = 35 ppm; stirring speed = 500 rpm; floc formation time = 5 min; gas flow rate = 40 cm3/min.
Figure 9. {-Potential as a function of pH for As(V)-Fe(OH)S flocs in the presence of so4” or POI” with or without Al(II1) addition. SDS = 35 ppm; Fe(II1) = 80 ppm; As(V) = 10 ppm. ( 0 )0.01 M NazSO4 without Al(II1);( 0 )0.01 M NazSO4 with 20 ppm Al(II1); (A) 0.01 M Na3P04 without Al(II1); (A)0.01 M Na3P04 with 20 ppm Al(II1). Floc
Floc
Surface
Surface
I
4 -Fe -0 - H
i + SO,‘-
-Fe-0-H
-Fe -0, 40 ___( ,S, -Fe-0 0
+
2OH-
Figure 10. Formation of sulfate complex. 100
0.00
0.01
0.02
0.03
0.04
0.05
Anion Concentration (mol/dm3) Figure 8. Effect of anionic concentrations on arsenic removal. pH = 4.5; SDS = 35 ppm; Fe(II1) = 80 ppm; stirring speed = 500 rpm; floc formation time = 5 min; gas flow rate = 40 cm3/min. ( 0 )NOS-; (m) Sodt; (A)PO4”.
-
8 m >
96
0
5
94
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92
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colloid flotation. These anions are introduced as sodium salts including NaN03, Na2S04, and Na3P04. The effect of nitric anion on the arsenic removal is insignificant even when the concentration of Nos- is 0.04 mol/dm3. The presence of sulfate and phosphate anions in the solution is found to reduce the efficiencyof arsenic removal greatly. Significant reduction in percent arsenic removal by sulfate or phosphate anions may be due to (1)the competitive adsorption of S042-or Pod3- and anionic surfactant (SDS) or arsenic oxyanions on the positively charged ferric hydroxide surfaces or (2) the specificinteraction with ferric hydroxide surfaces. The adsorption of SO4%or PO& anions on the positively charged floc surfaces reduces the {-potentials of the flocs as shown in Figure 9, and thereby reduces their attraction to the negatively charged As(V)species and SDS. Divalent oxyanions (with the exception of tellurate) coordinate two surface iron cations directly (Sigg and Stummn, 19801981; Harrison and Berkheiser, 1982). In the case of divalent oxyanion S042-, the ferric sulfate complex is formed by substitution of hydroxyl group by sulfate anion as shown in Figure 10. The trivalent oxyanion Pod3-is probably combined with Fe3+ on the ferric hydroxide surface to form insoluble F e P 0 ~ 2 H 2 0(pK,, = 361, which is evident from the observation of light lemon yellow precipitates appearing
98
r
a
90 88
‘
0.00
1 0.01
0.02
0.03
0.04
Anion Conc e nt r at io n ( rno I/d m3) Figure 11. Effect of anionic concentrations on arsenic removal in the presence of trivalent ion (Al(II1)).pH = 4.5; Al(II1) = 20 ppm; Fe(II1) = 80 ppm; SDS = 35 ppm; stirring speed = 500 rpm; floc formation time = 5 min; gas flow rate = 40 cm3/min. (A)s04%; (0) PO4”.
in the solution. For monovalent oxyanion, NOa- is mainly adsorbed on the surface of ferric hydroxide by electrostatic force. Effect of Trivalent Aluminum. Figure 8 shows the inhibitory effect of foreign anions (i.e., sulfate, Sod2-,or phosphate, PO4%, ions) on the removal of arsenic by adsorbing colloid flotation. This inhibition effect may be overcomewith the addition of trivalent ion, Al(II1). Upon the addition of 20 ppm Al(III), the removal of arsenic is significantly increased, as shown in Figure 11. Al(II1) is an effective activator or accelerator when Cu(II1) is removed by the process of adsorbing colloid flotation, which uses ferric hydroxide as the coprecipitant and
Ind. Eng. Chem. Res., Vol. 33, No. 4, 1994 927 50
I
I
I
I
I
1
I
I
I
I
(3) Fe(OH)3+ SDS
-30
z 0
' 2
v)
4
0
6
10
2
H
PH
v)
Figure 12. {-Potential as a function of pH for As(V)-Fe(OH)3 flocs with or without 20 ppm Al(II1). SDS = 35 ppm; Fe(II1) = 80 ppm; As(V) = 10 ppm; ( 0 )Without Al(II1); (A)with 20 ppm Al(II1).
sodium lauryl sulfate as the surfactant in the presence of foreign anion (Le., sulfate), according to Choi and Ihm (1988). The improvement of arsenic removal through theuse of an Al(II1) activator may be explained as when Al(II1) is used as the activator, the {-potentials of the floc and the floc-bearing foreign anions increase as shown in Figures 1 2 and 9, respectively. The increase in the affinity between the coprecipitant and the anionic surfactant may result in better arsenic removal efficiency. In Figure 11, it can be seen that the straight-line correlation is obtained when arsenic removal is plotted against anionic concentrations. Addition of trivalent aluminum clearly shows the compensating effect on the arsenic removal. Therefore, the apparent linear dependence of arsenic removal on sulfate or phosphate concentration, in the case of 20 ppm Al(II1) addition, can be expressed as arsenic removal (AR, % ) as a function of anionic concentration x: AR (%) = b k x . By regression analysis, AR (% ) = 99.4 - 196x, and AR (% ) = 99.4 310xph are obtained for sulfate and phosphate ions, respectively. The value of b = 99.4% corresponds to the arsenic removal when there is no anion present in the solution. xBand x p h represent the concentration of sulfate and phosphate anions. The slopes of the lines are the proportional constant k. They have the values of k, = -196 (%/(mol/dm3))for sulfate ion and k p h = -310 ( % / (mol/dm3))for phosphate ions, respectively. These values are not only the characteristics of divalent and trivalent anions, but also depend on the concentration of aluminum in the solution. The ratio of the proportional constants for divalent sulfate anion and trivalent phosphate is k8:kph = 2:3.2, at 20 ppm Al(II1) addition. Interaction Mechanism between SDS and Ferric Hydroxide. From Figure 2 and Figure 3, it can be seen that adsorption of sodium dodecyl sulfate (SDS)on ferric hydroxide may be due to electrostatic adsorption (Le., physical adsorption at the pH range of 4-5). However, the interaction between SDS and Fe(OH)3 floc is very strongly based on the characters of the flotation process. This suggeststhat a chemical adsorption may exist between SDS and Fe(OH)3. An infrared spectra measurement was then commenced to study this mechanism. The infrared spectra of plain ferric hydroxide and ferric hydroxide that has been conditioned with SDS are presented in Figure 13. For infrared spectrum 2 of SDS, the range of 1400-700 cm-l represents the characteristic vibrations of the sulfate group (Mielczarski et al., 1983; Shergold, 1972). Mielczarski and Shergold, along with their co-workers,reported the characteristic sulfate bands
+
z
is
680
F
1220
2920
"0"
v3400
4000
3600
3200
2800
2400
2000
1600
1200
800
400
WAVENUMBER (cm') Figure 13. Infrared spectra for ferric hydroxide, sodium dodecyl sulfate (SDS), and ferric hydroxide conditioned with SDS.
at 1275, 1235, 1220, 1195, 1080, 1060, and 820 cm-' for barium dodecyl sulfate, 1240, 1210, 1105, 970, and 860 cm-1 for calcium dodecyl sulfate, and 1215,1080,990, and respectively. 825 cm-l for sodium dodecyl sulfate (SDS), Upon the adsorption of SDS on calcium fluorite, the 1215cm-l band shifted and became the doublet bands of 1240 and 1206 cm-l while 1080-cm-1bands appeared as 1080and 1065-bands. In this study, by comparing IR spectrum 3 and IR spectrum 2 in Figure 13,there are greater changes at the three adsorption bands, namely, 1220, 1080, and 835 cm-l, in which the bands of 1220 and 1080 cm-' are antisymmetrical stretch vibrations of a sulfate group in the SDS molecule. These two most intense adsorption bands shift to 1205 and 1060 cm-' in spectrum 3, respectively. This may be attributed to DS- in SDS molecules linked to Fe3+on Fe(OH)3surfaces to form iron dodecyl sulfate, which causes the bands to shift to the lower wavenumbers. This indicates that chemical adsorption might take place between SDS and Fe(OH)3. Following molecular orbital theory (Cotton and Wilkinson, 1972), Fe3+ (central ion) on the surface of ferric hydroxide has vacant orbits. The vacant orbits can form six overlapped bondings, six antibonding and three nonbonding molecular orbits with filled atomic orbits of 0- (ligand) in -Sod- radical of adsorbed SDS molecule based on the requirement of similar symmetry. The d-electrons of the central metal ion and a-electrons of the ligand are filled in these orbits according to the Pauli exclusion principle and Hund rules (Berry et al., 1980). The electronic structure of the metal complex can be
928 Ind. Eng. Chem. Res., Vol. 33, No. 4, 1994 Table 3. Analysis of Mine Wastewater Sample
4.7
10.4
0.76
0.55
48
98
34
16
written as 2A1,6TlU4E,6T2,. Hence it forms a stable structure of an octahedron complex. According to the soft and hard acids and bases (SHAB) theory (Pearson, 1963),Fe3+is a hard acid metal ion, and -SO4- is a hard base radical. The hard acid metal ion preferentially binds to the hard base radical; they form a stable metal complex. This agrees with the above drawn conclusion using molecular orbital theory. Removal of As(V) in Mine Wastewater. A wastewater sample from a metal mine site was collected and analyzed for its major constituents. The analysis results are presented in Table 3. The operating conditions for treating the wastewater sample by adsorbing colliod flotation are as follows: 80 ppm Fe(II1) and 20 ppm Al(111) were added to the 200-cm3wastewater sample and agitated for 5 min with a magnetic stirrer at 500 rpm. Thirty-five ppm SDS was added to the coagulated colloid solution, and the agitation was continued for another 1 min. Next, the solution which contained flocs was poured into the flotation column and then the flotation test was carried out. The microbubbles of the flotation column were generated by setting the nitrogen gas flow rate at 40 cm3/min. The flotation test was run for 5 min, and the treated water was analyzed for arsenic, chromium, and lead contents by AA. The average results of 10 repeated tests were the residual 0.07 ppm As(V),0.06 ppm Cd, and 0.04 ppm Pb. The results showed that arsenic, chromium, and lead can be effectively and simultaneously removed from the mine wastewater by using adsorbing colloid flotation.
Conclusions Low concentrations of arsenic in aqueous solutions can be removed by adsorbing colloid flotation (ACF). A complete removal of arsenic can be achieved by using colloidal ferric hydroxide as a coprecipitant and sodium dodecyl sulfate (SDS)as the collector at the pH range of 4-5. The arsenic concentration of the treated water is less than 0.1 mg/dm3. The experimental flotation results and l-potentials of colloidal ferric hydroxide show that the pH of solutions plays an important role in the removal of arsenic by ACF. With an optimum pH range of 4-5, ferric hydroxide flocs bear positive charges and SDS can be attached onto the floc surface by electrostatic force, i.e., physical adsorption. Meanwhile, the infrared spectra show that a chemical adsorption also exists between ferric hydroxide and SDS. The foreign anions Sod2- and Pod3- have an inhibiting effect on the removal of arsenic by ACF, but addition of Al(II1) can compensate for the effect.
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