ARTICLE pubs.acs.org/IECR
Adsorption Isotherms of Arsenic on Conditioned Layered Double Hydroxides in the Presence of Various Competing Ions Megha Dadwhal, Muhammad Sahimi, and Theodore T. Tsotsis* Mork Family Department of Chemical Engineering and Materials Science, University of Southern California, University Park, Los Angeles, California 90089-1211, United States ABSTRACT: Analysis of effluents from various power plants indicates that, in addition to arsenic (As), these effluents also contain such ions as fluorides, nitrates, chlorides, carbonates, sulfates, and phosphates. Adsorption is considered the method of choice for the treatment of waste streams containing As. Of concern, therefore, is the impact of such competing anions on the capacity for As adsorption of the various materials utilized. In this work, we systematically investigated the effects of such ions on the As removal capacity of layered double hydroxides (LDHs), a class of materials investigated by our group and others as effective adsorbents for As and other heavy metals. The adsorption of As in the presence of each of these competing ions on LDH follows the extended Sips isotherm. It is concluded from our study that, of all of these ions, fluorides and nitrates have the least effect on As adsorption, followed by chlorides, carbonates, sulfates, and phosphates. Phosphates, in particular, have a very strong impact.
1. INTRODUCTION The accumulation of toxic heavy metals in the environment due to discharges from mining operations and various other industries is presently a serious concern. Among such metals, arsenic (As) is a key priority pollutant, naturally occurring in the environment as a result of the weathering of rocks, but also occurring in the discharge of effluents from various industries and power plants, as well as agricultural effluents because of the use of arsenic-containing pesticides. Arsenic is a metalloid chemical element and is widely known for its high chronic toxicity to humans.1 The maximum concentration limit (MCL) for As in drinking water was recently revised in various countries including the United States, Europe, and Japan2 from 50 to 10 μg/L. In countries such as Bangladesh, more than 60% of the groundwater contains naturally occurring As with a concentration that often significantly exceeds the World Health Organization (WHO) guidelines of 10 ppb. Given how widespread As contamination is and the health threat that it poses, it is important to develop effective techniques for its cleanup. Methods that have been investigated include precipitation-filtration, ion exchange, membrane separation, and adsorption; the latter technique, in particular, has been found to be superior for water reuse applications, in terms of initial capital costs, simplicity in design, and ease of operation. So far, common adsorbents, such as various clays and naturally occurring minerals,3,4 activated5-7 and mesoporous alumina,8 ferric hydroxide,9 and ferrihydrite,10 have all been investigated. Layered double hydroxides (LDHs), which offer a large interlayer surface to host diverse anionic species11-14 with the additional advantage of being potentially easily recyclable, have also received considerable attention in recent years by various groups,15-19 including our own.12-14 LDHs have been shown to be effective adsorbents for As (as well as for other heavy metals such as Se); nevertheless, the performance of these materials in practical situations still needs further investigation. r 2011 American Chemical Society
In the present study, the focus was on the use of an LDH for the removal of As from real power-plant effluents. Such effluents contain, in addition to As, various others ions, so the ability of the LDH to remove As from these effluents still needs to be tested, because these ions can, in principle, strongly compete with As for the available adsorption sites on the LDH. In the past, our group carried out preliminary studies on the effects of some of these competing ions on the As and Se adsorption capacities of uncalcined and calcined LDHs.12 In those experiments, solutions were treated containing 20 ppb of As (or Se) and one additional competing ion (such ions included NO3-, CO32-, SO42-, and HPO42-) with concentrations ranging from 40 ppb to 1000 ppm. It was found that the effect of competing anions on the adsorption of As(V) decreased in the order HPO42- > CO32- > SO42- > NO3- and for Se(IV) in the order HPO42- > SO42- > CO32- > NO3-. The structural characteristics of the LDH and their effect on As adsorption were also investigated in these studies, as were the effects of other process parameters, such as adsorbent dosage.12-14 Other groups have also studied the effects of competing ions on the adsorption of As (and Se) on LDHs. Their efforts were limited, however, and involved experiments with solutions containing a single competing ion and As (or Se), both at fixed concentrations. Based on such experiments, for example, You et al.20 concluded that competing ions such as phosphates, sulfates, carbonates, fluorides, chlorides, bromides, iodides, and nitrates strongly affect As adsorption on LDHs in the order HPO42- > SO42- > CO32- > F- > Cl- > Br- ≈ I- > NO3-. Wang et al.,21 who also studied the effects of such competing ions on As adsorption on LDHs, reported similar effects, with ions affecting Received: June 3, 2010 Accepted: December 15, 2010 Revised: December 8, 2010 Published: January 18, 2011 2220
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Industrial & Engineering Chemistry Research the As adsorption capacity in the order HPO42- > HCO32- > SO42- > F- > Cl-. For selenite (SeO32-) adsorption, You et al.22 concluded that the competing anions also strongly affected its adsorption on LDHs, in increasing order HPO42- < SO42-< CO32- < NO3-. Delorme et al.23 studied the treatment of a mixture of anionic pollutants (F-, HAsO42-, and NO3-) in water by a mixed-oxide adsorbent prepared by the thermal treatment of quintinite (a Mg4Al2 LDH) at moderate temperatures. They observed that the adsorbent had a low potential for removing nitrates (removing only 15% of the ions in the solution); furthermore, these ions were fully released into the solution upon addition of CO32-. On the other hand, the adsorbent was effective for removing As (100% removal), with only 20% of these ions released into the solution upon addition of CO32-. It was also effective for the removal of Fl- (80% removal); however, 90% of the adsorbed ions were released into the solution when CO32- was added. To summarize, although some preliminary investigations have been carried out, to date, no detailed investigation has been published on how competing ions, typically encountered in power-plant effluents, affect As adsorption on LDHs. How each of the ions affects the adsorption and how they compete with As for the available sites on an LDH are, of course, important issues in terms of determining the eventual applicability of these materials in the treatment of real power-plant effluents. As part of our study, in collaboration with the Los Angeles Department of Water Quality (LADWQ), we first collected power-plant effluent data from the greater Los Angeles area to determine the typical concentrations of As and other common ions. This was necessitated by the fact that we could find no prior study of ions that are typically present in power-plant effluents and their concentration levels. We then first carried out batch isotherm experiments of As on an LDH in the presence of each of the individual ions at fixed concentrations typical to those found in power-plant effluents. The experiments were complemented with further binary (As and the individual competing ion) adsorption experiments over a broader range of conditions with the initial concentration ratio of As to the competing ion ranging from 1:1 to 1:32. The experimental results are described below and are instructive in gaining a better understanding of the potential and limitations of LDH adsorbents for the treatment of As in power-plant and other types of effluents.
2. COMPOSITION OF POWER-PLANT EFFLUENTS As noted above, in collaboration with the LADWQ, we collected data on various power-plant effluents in the greater Los Angeles area, with the goal of identifying the various species present and their concentrations. The data collected are reported in Table 1 (for As) and Table 2 (for other common anions). Based on these data, in this study, a simulated power-plant effluent with the following composition was selected for batch adsorption and column (not reported here) flow experiments: As, 20 ppb; F-, 1 ppm; NO3-, 5 ppm; Cl-, 100 ppm; CO32-, 5 ppm; SO42-, 100 ppm; HPO42-, 1 ppm; pH, 8.0. 3. EXPERIMENTAL SECTION 3.1. Preparation of LDH. A Mg-Al-CO3 LDH sorbent with a Mg/Al mole ratio of 2.87 was utilized in the experiments. It was prepared by the coprecipitation method of Roelofs et al.,24 which involves adding under vigorous stirring 140 mL of a
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Table 1. As Concentrations in the Effluents of Various Power Plants station
month (2006-2007)
Haynes
May
arsenic (ppb) 172.00 179.00 174.00 1.10
Harbor
Jan
1.18
AES Alamitos
Oct
1.44
6.35 1.56 0.78 0.92 4.56 Long Beach
Dec
1.40 5.00 4.00 3.50 4.20
Long Beach Range
Jan
2.6 - 4.6
Mandalay
March
5.2-9.8 10.00
Scattergood
May
185.00
6.01 179.00 195.00 159.00 El Segundo
June
7.23
June
0.21 1.58 1.06 1.56 2.15
solution containing 0.7 mol of NaOH and 0.18 mol of Na2CO3 to 180 mL of a second solution containing 0.115 mol of Mg(NO3)2 3 6H2O and 0.04 mol of Al(NO3)3 3 9H2O (corresponding to a Mg/Al ratio of 2.87). The thick gel obtained was aged for 24 h at 333 K, filtered and washed with distilled water, and then dried at 333 K. Inductively coupled plasma mass spectrometry (ICP-MS) analysis of the resulting LDH material indicated that its Mg/Al mole ratio was ∼2.9, which is very close to the Mg/Al ratio of the starting salts. The Mg-Al LDH was heated in an air atmosphere in a muffle furnace from room temperature to 773 K (at a heating rate of 2 K 3 min-1), was allowed to stay at that temperature for 4 h, and was subsequently cooled to room temperature (cooling rate of 2 K 3 min-1). The calcined LDH was conditioned by shaking it in deionized water for 24 h (changing the deionized water every 6 h) and then drying it in air at 353 K for 12 h. The conditioned LDH was then crushed and sieved using standard testing sieves (VWR) to obtain different particle size fractions. The LDH used in these experiments was the particle size fraction in the range of 180-300 μm. 3.2. Batch Sorption Experiments. As(V) was the metal investigated in this study in the presence of various competing ions, such as fluorides (F-), nitrates (NO3-), chlorides (Cl-), carbonates (CO32-), sulfates (SO42-), and phosphates (HPO42-). 2221
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Table 2. Concentrations of Various Ions Present in Power-Plant Effluents and pH Data parameter
units
pH
Long Beach 7.78
temperature
°F
chlorine
ppm
CaCO3
ppm
3.9
ammonia (as N)
ppm
0.9-2.2
organic N
ppm
ND-1.8
AES Alamitos 7.6
Redondo
Mandalay
7-8.3
7.6-7.9
60-85
60-80
0.01-0.10
0.08-0.17
El Segundo refinery 8-8.1
5.98 0.1
nitrite (as N)
ppm
ND-0.17
0.1
0.1
nitrate (as N) nitrate þ nitrite
ppm ppm
3.58-7.89 3.62-7.93
2.5 2.5-4.0
0.1 0.6
total nitrogen
ppm
2.7-10.4
4.5-5.5
3.5
chloride (Cl-)
ppm
107-124 (215)
80-90
75-105
60-80
sulfate (SO42-)
ppm
61-118 (448)
50-85
70-85
45
ortho phosphate P
ppm
ND-2.9
ND
ND
ND
total phosphate
ppm
ND-3.2
ND
ND
ND
total phosphorus
ppm
ND-1.04
ND
ND
ND
sulfide
ppm
The As(V) solutions used for the adsorption experiments were prepared from As2O5 powder purchased from Sigma-Aldrich. The solutions containing various ions were prepared using NaF, NaNO3, NaCl, Na2CO3, Na2SO4, and K2HPO4 salts (HPO42- is the main ion that results, with only minor amounts of H2PO41and PO43- being present12). No effort was made to minimize contact with atmospheric air during the preparation of the various solutions. Prior to the initiation of the adsorption experiments, the solution pH was adjusted to the desired value (∼8.0) using a 0.1 M NaOH solution. The pH was not adjusted during the adsorption experiments, but changes, if any, were recorded using an Acumet Basic AB 15 pH meter. All experiments were performed at 298 K, with an adsorbent loading (dose) of 0.016 g/L. (As noted above, the effect of adsorbent dosage on As adsorption was investigated previously, and further technical details can be found elsewhere.12) The As(V) concentration was determined using an ICP-MS instrument (Perkin-Elmer ELAN-9000). To measure the concentrations of nitrate, fluoride, and chloride ions, we used the corresponding ion-selective electrodes purchased from Daigger. The reproducibility of the measurement using these electrodes was (2%. To measure phosphate and sulfate ions, we carried out lead perchlorate titrations25-27 and used a lead ion-selective electrode. The error involved in the measurement of phosphates and sulfates was determined to be (4%. The concentration of carbonates was measured by titrating with HCl, using phenolphthalein as an indicator, with an experimental measurement error of (2%. 3.2.1. Adsorption Isotherm Experiments 3.2.1.1. Adsorption Isotherms of the Competing Ions. Before studying the effects of competing anions on As adsorption, we investigated the adsorption capacity of the LDH toward these ions without the As being present. These experiments were performed using 500 mL of aqueous solutions of various individual ions contained in 1000-mL high-density polyethylene bottles with predetermined initial concentrations ranging from 20 to 5000 ppb. The bottles containing the ion and the LDH suspension were then placed in a reciprocal shaking water bath (Precision model 25) and shaken at 170 rpm for as long as was needed for the adsorption to reach equilibrium (at least 5 days).
12.5
0.75
The mixture in each bottle was then centrifuged, and the ion concentration in the supernatant solution was determined. 3.2.1.2. Adsorption Isotherms of Binary Mixtures of As and the Individual Competing Ions. The adsorption isotherms of As in the presence of individual anions at initial concentrations equal to the level found in the “typical” power-plant effluent, as noted above, were first investigated. In the experiments, we varied the initial As(V) concentration from 20 to 240 ppb while holding the initial competing-ion concentration constant. A constant mass of 8 mg of LDH was again added to 500 mL of an aqueous solution containing both the As and the competing ion, and the mixtures were placed in 1-L bottles in a reciprocal shaking water bath (Precision model 25) and shaken at 170 rpm for 5 days, a time that was confirmed to be sufficiently long for the adsorption to reach equilibrium. At the end of the experiment, the mixture in each bottle was centrifuged, and the As and competing-ion concentrations in the supernatant solutions were determined. Binary adsorption isotherms of As and the individual competing ions for a broader range of competing-ion concentrations were also investigated. In the experiments, aqueous solutions of As(V) of predetermined concentration ranging from 20 to 5000 ppb were mixed with various concentrations of the individual competing ions, such as fluorides, nitrates, chlorides, carbonates, sulfates, and phosphates (HPO42-). We utilized initial concentration ratios of As to the competing ion equal to 1:1, 1:2, 1:4, 1:8, 1:16, and 1:32. The other experimental conditions were similar to those for the experiments with fixed competing-ion concentrations.
4. RESULTS AND DISCUSSION 4.1. Adsorption Isotherms of the Competing Ions. Adsorption isotherms for the various individual competing ions are shown in Figure 1. We used the Sips isotherm to fit the data, which is given by the equation
q ¼
Kqs Cn 1 þ KCn
ð1Þ
where qs and K are the maximum sorption capacity and the Sips constant, respectively, and n is the parameter characterizing the 2222
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Figure 1. Adsorption isotherms for individual competing ions on conditioned LDH.
Table 3. Sips Isotherm Parameters for the Various Competing Anions on Conditioned LDH
Figure 2. Adsorption isotherms for As(V) on conditioned LDH in the presence of various individual competing ions at a fixed initial concentration, along with Sips fits.
Table 4. Sorption Isotherm Parameters for As(V) Uptake on Conditioned LDH in the Presence of Various Competing Ions
qs (μg/g)
K (L/g)
n
R2
carbonates
46010.4
0.029
0.88
0.99
sulfates
41868.4
0.019
0.89
0.99
arsenic
3619.9
phosphates fluorides
36854.7 34927.2
0.024 0.013
0.90 0.90
0.99 0.99
arsenic (fluorides)
3516.2
arsenic (nitrates)
3479.3
chlorides
32394.7
0.011
0.92
0.99
arsenic (chlorides)
nitrates
30017.3
0.006
0.95
0.99
system heterogeneity. The fitted Sips parameters and the goodness of fit (R2) are reported in Table 3, and the corresponding plots are shown in Figure 1, where the points represent the experimental data and the solid lines are the Sips isotherm fits. Of all ions, the carbonates are adsorbed the most on conditioned LDH, with qs being 46010.4 μg/g. They are followed by the sulfates, phosphates, fluorides, chlorides, and nitrates, the latter getting adsorbed the least, with a qs value of 30017.3 μg/g. The adsorption data in Figure 1 are in agreement with the fact that LDHs are known to have greater affinities for multivalent anions (CO32-, SO42-, and HPO42-) rather than monovalent anions (Fl-, Cl-, and NO3-).28,29 The K values also decrease in the same order as the qs values for the various ions, being the highest for carbonates (0.029 L/g) and the lowest for nitrates (0.006 L/g). All ions have fairly high n values, the highest being ∼0.95 for nitrates, indicative of single-site adsorption. 4.2. Adsorption Isotherms of As in the Presence of Competing Ions. Adsorption isotherms for As in the presence of the competing ions at a fixed initial concentration equal to that of the typical power-plant effluent described above are shown in Figure 2, where again the points represent the experimental data and the solid lines are the Sips fits. The various fitted isotherm parameters (and the goodness of fit, the R2 test) are reported in Table 4. Table 5 also reports the initial and final (after adsorption equilibrium has been established) competing-ion concentrations for various initial As concentrations. (For the chlorides and sulfates, the changes are less than the experimental measurement error.) Table 4 also reports the Sips parameters for As alone (i.e., in the absence of the competing ions). Notably, As adsorption follows the Sips isotherm both in the absence and in the presence of the competing anions. Moreover, the presence of various
n
R2
0.649
0.58
0.91
0.143
0.88
0.94
0.107
0.91
0.98
3015.5
0.043
0.95
0.99
arsenic (carbonates)
2910.9
0.047
0.93
0.98
arsenic (sulfates) arsenic (phosphates)
2386.6 1678.1
0.017 0.007
0.97 0.98
0.99 0.99
qs (μg/g)
K (L/g)
anions significantly affects the As adsorption and the corresponding parameters of the Sips isotherm. The anions impact both the total number of sites available for As adsorption (i.e., qs) and the “affinity” as exemplified by the adsorption equilibrium constant K. The strength of the effect varies with the anion. Fluorides and nitrates, for example, have a relatively minor impact on the maximum adsorption qs, but they greatly impact the adsorption equilibrium constant K. On the other hand, chlorides, carbonates, sulfates, and phosphates (HPO42-) severely impact both the qs and K values. For example, the K value in the presence of phosphates (the anion that impacts As adsorption the most) is about 93 times less than that for the pure As, and the qs value is about 2.2 times lower. Interestingly enough, for all competing anions, the exponent n changes from the value of ∼0.5 (which signifies a dual-site adsorption mechanism) to a value that is much closer to 1 (see further discussion below). 4.3. Binary Adsorption Isotherms of As and Competing Ions. The binary adsorption isotherm data of As(V) and the various anions on conditioned LDH are shown in three-dimensional plots in Figure 3a-f in terms of either qAs (the amount of As adsorbed at equilibrium) or qion (the amount of ion adsorbed) plotted as a simultaneous function of CAs and Cion (the concentrations at equilibrium of As and the ion, correspondingly). As already mentioned, these experiments were run for various initial concentration ratios of As to the individual competing ion in the range from 1:1 to 1:32. As can be observed in Figure 3 (and as expected in view of the results with the individual ions shown in Tables 3 and 4), the 2223
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Table 5. Reduction in the Competing-Ion Concentrations during the Isotherm Experiments as a Function of Arsenic Concentration
fluorides
nitrates
chlorides
carbonates
sulfates
phosphates
arsenic
initial ion
final ion
concentration
concentration
concentration
(ppb)
(ppm)
(ppm)
20
1
0.51
40
1
0.52
80 120
1 1
0.52 0.53
160
1
0.53
200
1
0.54
240
1
0.55
20
5
4.54
40
5
4.55
80
5
4.55
120 160
5 5
4.56 4.56
200
5
4.57
240
5
4.57
20
100
99.51
40
100
99.52
80
100
99.52
120
100
99.52
160 200
100 100
99.53 99.53
240
100
99.54
20
5
4.28
40
5
4.28
80
5
4.29
120
5
4.29
160
5
4.30
200 240
5 5
4.30 4.31
20
100
99.35
40
100
99.35
80
100
99.36
120
100
99.36
160
100
99.36
200
100
99.37
240 20
100 1
99.37 0.42
40
1
0.43
80
1
0.43
120
1
0.44
160
1
0.44
200
1
0.44
240
1
0.44
amounts of the different competing ions absorbed are much higher than the corresponding amounts of As, typically an order of magnitude higher. The much higher adsorption saturation capacities of the competing ions when compared to As are consistent with a mechanism of adsorption whereby not all of the LDH adsorption sites are available to the adsorption of the As ionic species. This can be attributed to the relatively larger molecular size of the latter species30-34 (see Table 6), which
makes it potentially difficult for the As ion to readily accommodate itself within the interlayer space of the LDH (between the brucite layers), which is reported to be ∼2.4 Å35 in size. In view of this experimental observation, we fitted the experimental binary isotherm data using an extended multicomponent Sips isotherm model that was modified to take into account the presence of two different adsorption sites on the adsorbent material. The concentration of sites, assuming that only anions other than As adsorb, is given by the difference between the maximum saturation loading of the competing ion, qs,I, measured in the absence of As (Figure 1 and Table 3) and the maximum saturation loading of As, qs,A, measured in the absence of the competing ion, namely, qs,I - qs,A. This extended Sips isotherm model is described by the following two equations for the adsorptions of As and the competing ion, respectively36 qe, A ¼
qs, A KA Ce, A nA 1 þ KA Ce, A nA þ KI Ce, I nI 0
qe, I ¼
ðqs, I - qs, A ÞKI Ce, I nI 0
1 þ KI Ce, I nI
0
0
þ
ð2Þ
qs, A KI Ce, I nI ð3Þ 1 þ KA Ce, A nA þ KI Ce, I nI
In these equations, qs,A, KA, and nA are, respectively, the maximum sorption capacity, the extended Sips constant, and the extended Sips exponential factor for As, as measured during As sorption experiments in the absence of competing anions (first row in Table 4), and qs,I is the maximum sorption capacity for the individual ion (in the absence of As), as noted above (values reported in Table 3). KI and KI0 are the extended Sips equilibrium constants, and nI and nI0 are the two sets of extended Sips exponential factor for two kind of sites available to the competing ions. These values were determined by first fitting the binary adsorption data for As to determine KI and nI values and then using the ion isotherm data (and the fitted KI and nI values) to determine the KI0 and nI0 values. The fitted parameters are reported in Table 7, and the fitted plots are shown in Figure 3a-f (solid lines), along with the experimental data (points). The fit is generally very good for the As data, but less so for the competing-ion data. For the latter data, the fit is generally very satisfactory for the higher ion/As initial ratios, deteriorating markedly for the lower ratios. Notably, the As binary adsorption data are fitted well using the same Sips exponent parameter as for the pure-As data, which makes more sense that the change in nA value (see Table 4) predicted when fitting the binary isotherm data in Figure 2 using a single-site adsorption model. Interestingly, the nI values for the ions competing for the same sites as As are close to the values for As (compare Tables 4 and 7), but much different than values for the single competing-ion adsorption isotherms (compare Tables 3 and 7), as well as the nI0 values for ion adsorption on the sites that can accommodate only the ions other than As. The affinities of the ions toward both types of sites are very similar, as can be seen by comparing the fitted KI and KI0 values with themselves as well as with the K values for single-ion adsorption. This would tend to signify that the chemical natures of these sites are very similar and that the differences in the fitted nI and nI0 values can be attributed to the position of these sites. The sites on which only the competing ions adsorb must be in a part of the structure where they are not accessible by the larger As ions. We also fitted the As isotherm data reported in section 4.2 using the extended dual-site Sips isotherm (eq 2) with the fitted 2224
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Figure 3. Binary adsorption isotherms for As(V) and the competing ions on conditioned LDH.
Table 6. Ionic Radii of Various Ions ion
Table 8. Fitted Parameters for the Isotherm Data with the Power-Plant Concentration Ratios
radius (Å)
-
KI (L/g)
nI
R2
Fl CO32-
1.33 1.74
fluorides
0.013
0.65
0.95
Cl-
1.81
nitrates
0.006
0.66
0.91
NO3-
2.3
SO42-
2.65
chlorides carbonates
0.011 0.029
0.52 0.62
0.97 0.96
sulfates
0.019
0.59
0.95
phosphates
0.028
0.90
0.96
HAsO42-
5
Table 7. Fitted Parameters for the Binary Isotherm Data 0
0
KI (L/g)
nI
KI (L/g)
nI
fluorides
0.013
0.65
0.013
0.85
nitrates
0.006
0.66
0.006
0.92
chlorides
0.011
0.62
0.011
0.85
carbonates
0.029
0.62
0.029
0.83
sulfates
0.019
0.64
0.019
0.82
phosphates
0.028
0.64
0.024
0.82
parameters reported in Table 8. The parameters for As (i.e., qs,A, KA, and nA) were again taken to be those for As when present by itself, and the parameters KI and nI were fitted by using the As isotherm data. The fitted parameters, reported in Table 8, are generally similar to the values reported in Table 7, other than some differences in the nI values (most notably for phosphates). It should be noted, once more, that the As binary adsorption data are fitted well using the Sips exponent parameter for the pure-As data, which makes more sense than the change in the nA value
(see Table 4) predicted when fitting the binary isotherm data in Figure 2 using a single-site adsorption model.
5. SUMMARY AND CONCLUSIONS It was found that power-plant effluents contain ions such as fluorides, nitrates, chlorides, carbonates, sulfates, and phosphates in addition to As. The effects of these competing ions on As adsorption by conditioned LDH were investigated. The isotherms for the individual competing ions were first measured. It was found that carbonates adsorb the most on the LDH, whereas nitrates adsorb the least among the various ions present in the power-plant effluents. The binary isotherms of As and the individual competing ions on the LDH at concentration levels that are found in typical power-plant effluents were also generated. It was observed that nitrates and fluorides have almost no effect at these concentration levels, whereas phosphates (HPO42-) compete the most with As for the adsorption sites on an LDH. Binary isotherms were also generated by varying the initial concentration ratio of As to the individual competing ions in the 2225
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Industrial & Engineering Chemistry Research range from 1:1 to 1:32, to gain a better understanding of the competition for the available adsorption sites. Consistently with the single-ion adsorption data, it was observed that the amounts of the different competing ions absorbed were much higher than the corresponding amounts of As. This was attributed to the relatively larger molecular size of As when compared to the competing anions. The adsorption isotherm data were fitted using the extended Sips isotherm equation, which takes into account the fact that not all of the LDH adsorption sites are available to the adsorption of the larger As ionic species. The fit obtained was generally very good for the As data, but less so for the competing-ion data. For the latter data, the fit was generally very satisfactory for the higher ion/As initial ratios, deteriorating markedly for the lower ratios. From the isotherm experiments, it can be concluded that the As adsorption capacity decreases in the presence of various individual competing ions in the order HPO42- > SO42- > CO32- > Cl- > NO3- > F-.
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