Arsenic Removal Using Iron Oxide Loaded Alginate Beads | Industrial

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Ind. Eng. Chem. Res. 2002, 41, 6149-6155

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Arsenic Removal Using Iron Oxide Loaded Alginate Beads Anastasios I. Zouboulis* and Ioannis A. Katsoyiannis Division of Chemical Technology, Department of Chemistry, Aristotle University, Thessaloniki GR-54124, Greece

The application of biopolymers (alginate), as sorbent supports, for the removal of arsenic from contaminated water has been investigated in the present study. Calcium alginate beads were placed in a column to form a fixed bed and treated (doped/coated) with hydrous ferric oxides. Three different types of modified alginate beads were examined for the removal of arsenic; the most efficient type was found to be doped with alginate and subsequently coated with iron oxides, whereas the other two types were calcium alginate beads doped or coated with iron oxides. The total amount of iron loaded on this material was found to be 3.9 mg of Fe/g of wet alginate bead. Approximately 230 bed volumes of a 50 µg/L As(V) solution were treated before the breakthrough point was reached, whereas the removal of As(III) was not as efficient, reaching the breakthrough point after the treatment of only 45 bed volumes. The results were modeled using the bed depth service time and empty bed residence time models. Introduction Arsenic is considered as a naturally occurring contaminant of drinking water supplies, causing adverse effects on human health. Chronic exposure to arsenic concentrations above 100 µg/L can cause vascular disorders, such as dermal pigments (blackfoot disease) and skin and lung cancer.1,2 Since 1993, the World Health Organization has recommended a maximum contaminant level (MCL) for arsenic in drinking water of 10 µg/L.3 In 1998, under EC directive 98/83 related to water intended for human consumption, the European Commission adopted the same value as the MCL, and by 2003, all European countries will have to comply with this limit.4 Recently, the same limit has been adopted by the United States, and the date by which water treatment systems must comply with the new standard is January 2006.5 Therefore, the arsenic problem is of primary concern in many countries and research into the development of cost-competitive technologies for arsenic removal from water sources is urgent. Arsenic is present in natural waters mainly in its inorganic oxyanionic forms of arsenate (HAsO42-) and arsenite (H3AsO3). As(V) predominates in oxygenated waters (e.g., surface water), whereas under anoxic conditions (e.g., anaerobic groundwaters), As(III) comprises the main form.6 The main treatment methods used for arsenic removal from contaminated waters fall into the following categories: (a) precipitation-coagulation processes, such as coagulation with iron or aluminum salts or lime softening; (b) membrane processes, such as reverse osmosis, nanofiltration, or electrodialysis; and (c) adsorption processes, such as adsorption on activated alumina or on iron oxides.7-11 A modification of the last category is “adsorptive filtration”. This technique is based on the coating of the filter medium with adsorbents, resulting in modified media that can act simultaneously as a filter and as an adsorbent. The most widely used filter medium in adsorbing filtration technique is iron oxide coated * Corresponding author. Tel./fax: +30 310-997794. Email: [email protected].

sand,12-14 but other materials have been also examined, such as coated catalysts15 and coated olivine.16 In the present study, alginate gel beads have been applied as a support matrix for the immobilization of hydrous ferric oxides. Alginate is a biopolymer, extracted mainly from brown seaweed. It is a linear polysaccharide of (1 f 4)linked R-L-guluronate (G) and β-D-mannuronate (M) residues arranged in a nonregular, block-wise pattern along the linear chain.17,18 Monovalent salts of alginic acid have generally been considered as being the watersoluble species of alginate. With divalent cations, alginate acid produces thermally irreversible gels that are insoluble in water. Calcium is the cation mostly used in the preparation of alginate beads.19 Alginate is one of the most widely used carriers for the immobilization of enzymes and proteins, as well as for the controlled release of drugs. Furthermore, alginate has been applied as a biosorbent material for the removal of cationic metals, such as Pb2+, Zn2+, Cd2+, and Ni2+ from aqueous solutions.18 Regarding the removal of oxyanionic compounds, modified alginate beads have found application in the removal of chromium and selenium,20 whereas for the removal of arsenic, only one citation can be found in the literature.21 In the latter study (batch experiments), promising results were reported regarding the sorption of pentavalent arsenic on modified alginate beads. In the present study, the modified medium was examined under continuous-operation (fixed-bed) mode. The objective of this research was to examine the efficiency of alginate beads as possible supports for hydrous ferric oxides and to apply the modified filter medium in the adsorbing filtration processes for the removal of arsenic from contaminated water sources, to achieve a residual arsenic concentration below the concentration limit of 10 µg/L. The aim is not for this particular modified medium to replace other adsorptive or ion-exchange materials already applied in arsenic removal processes, such as activated alumina or granular ferric hydroxide (GFH), but rather for more information to be obtained regarding the application of this material in the treatment of arsenic-contaminated

10.1021/ie0203835 CCC: $22.00 © 2002 American Chemical Society Published on Web 10/29/2002

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groundwaters where soluble iron might also coexist. Iron is usually removed from groundwaters by oxidation and subsequent filtration. The oxidation of iron and subsequent filtration result in a natural coating of the filter medium. Iron-oxidizing bacteria can then attach and grow on the medium surface, accelerating the overall process. Subsequently, arsenic can be removed by sorption on the naturally produced iron oxides, and possible As(III) oxidation by bacteria might enhance the overall treatment efficiency.22 The obtained results were modeled by the application of the bed depth service time (BDST) and empty bed residence time (EBRT) models.23,24 Materials and Methods Reagents and Stock Solutions. All chemicals used were reagent-grade, and all glassware was acid-washed and rinsed with deionized water. Alginate beads were prepared from sodium alginate (Parneac), which was used without further purification. Arsenate and arsenite stock solutions were prepared from sodium arsenate heptahydrate (Na2HAsO4‚7H2O, Parneak) and arsenic trioxide (As2O3, AnalaR), respectively, dissolved in deionized water. Working solutions were prepared by dilution of certain amounts of stock solutions with tap water. The final concentration of the working solutions for both arsenate and arsenite experiments was 50 µg/ L, i.e., the previous limit for arsenic in drinking water. Calcium solutions, used in the gel synthesis, were prepared from CaCl2‚2H2O (Merck) dissolved in deionized water. Hydrous ferric oxide (HFO) suspensions used for gel activation were prepared from Fe(NO3)3‚ 9H2O (Merck) dissolved in deionized water with adjustment of the pH to 5.0 using NaOH solution. Alginate Bead Synthesis. The biopolymer powder (2 g) was dispersed in 100 mL of deionized water to give a 2% w/v alginate solution; this solution was mixed with a mechanic stirrer until a transparent, viscous solution was obtained. Calcium alginate beads were prepared by dropping sodium alginate aqueous solution into a continuously stirred calcium chloride solution. This technique enabled the formation of spherical droplets, which after being dispersed in CaCl2 solution, would result in the formation of spherical particles. Approximately one drop of polymer solution was dispensed per second into 500 mL of CaCl2 solution under gentle stirring (60 rpm) during formation of the gel beads, because the beads were reported to be susceptible to hydrodynamic forces.18 A glass tube was used to dispense the polymer solution. A pipet tip (3-mm diameter) was positioned approximately 6 cm above the surface of the calcium solution used in the gel formation. It has been reported that this is the optimum distance, as smaller distances lead to the formation of beads with short tails whereas longer distances result in deformation of the obtained particles.18 The beads were allowed to cure in Ca2+ solution and then removed and rinsed twice with deionized water. The concentration of the CaCl2 solution and the contact time of the alginate beads within this solution were selected after preliminary experiments. For the preparation of Ca-Fe-doped beads, biopolymer powder (2 g of sodium alginate) was dispersed in 100 mL of various concentrations of HFO suspensions at pH 5.0.; then, the aforementioned procedure was followed.

Experimental Setup and Conditions for Adsorption Experiments. A glass column (40-cm height, 2.86cm inner diameter) was filled with calcium alginate or Ca-Fe-immobilized alginate beads (of mean diameter 3.6 mm, with approximate bed porosity 0.33). For the preparation of Fe-coated beads, alginate beads were treated with an Fe(NO3)3 suspension (of various initial concentrations) at pH 5.0, where ferric oxides are practically insoluble. The preformed iron oxide suspension was passed through the column in upflow mode under recirculation to coat the surface of filtration media. The bed was subsequently washed with deionized water to remove iron oxides residuals that were not coated on the surface of filter media. This washing action was finished when no iron could be detected in the outlet stream. It should be noted that, in the present research, the artificial coating of beads by ferric oxides has been examined, whereas natural coating might also find application under certain conditions (i.e., treatment of iron-rich groundwaters). Following bed modification, aqueous solutions spiked with arsenic entered the treatment column in upflow mode and passed once through the modified medium, and samples were collected from the exit of the column. After the treatment of several bed volumes, the breakthrough point was reached. Breakthrough was defined as the point at which the effluent arsenic concentration was over the permissible maximum concentration limit of 10 µg/L. Analytical Determinations. The determination of total arsenic in the collected samples was performed by hydride-generation atomic absorption spectrophotometry (HG-AAS, Perkin-Elmer MHS 10, Perkin-Elmer 2380). The method is based on the conversion of soluble arsenic in the respective volatile arsines, which are subsequently determined by atomic flame absorption. The reduction of arsenic was performed by KI addition (10% w/v), and the pretreatment of the samples also involved the addition of HCl (32% v/v). Following this procedure, the samples were reacted with Na(BH4), and the produced arsines were determined by means of the respective calibration curve. The calibration curve enables the determination of arsenic in the concentration range from 1 to 20 µg/L. To measure separately the trivalent arsenic (speciation), samples were pretreated only with acetic acid (0.05 M), to avoid the reduction of pentavalent arsenic. Following pretreatment, samples were also analyzed by hydride generation atomic absorption spectrophotometry.25 The samples were also analyzed for iron by flame atomic absorption spectrometry. To determine the amount of iron loaded on the calcium alginate beads, 10 g of modified beads was treated with (1 M) HCl acid. In this way, iron was dissolved in the acid solution. Results and Discussion Alginate Bead Characterization. The concentration of cationic solution (CaCl2), as well as the duration of incubation (curing) in this solution, was determined on the basis of preliminary experiments. Alginate bead characterization was performed by scanning electron microscopy (SEM JSM-840A, JEOL scanning microscope). It was found that the contact time in CaCl2 solution, as well as the CaCl2 concentration, affects the structure and the mechanical properties of the beads. The most appropriate beads for sorption experiments in column operation were those formed by incubation

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Figure 1. As(V) removal by alginate beads doped with iron oxides (conditions: linear velocity, 1.16 m/h; bed volume, 50 mL; pH 7.0).

in CaCl2 solution (0.3 M) for 24 h. Following this procedure, calcium alginate beads were modified by iron oxides suspension, either doped or coated. Previous investigations have shown that calcium alginate beads (unmodified) have a minimal tendency to adsorb arsenic;21 therefore, they were not further examined in this study. Arsenic Removal by Calcium Alginate Beads Doped with Iron Oxides. Iron oxides were doped into the calcium alginate network during the preparation of beads as previously described. Two kinds of calciumiron alginate beads were prepared, depending on the concentration of the Fe(NO3)3 suspension used for iron doping, and tested against the removal of As(V). The results are presented in Figure 1. It can be noticed that the removal of arsenic was greatly affected by the Fe(NO3)3 concentration used for the creation and doping of iron oxides. Because adsorption processes are greatly dependent on the available amount of adsorbent,12 the extent of arsenic removal was increased when 0.05 M doped iron concentration ([Fe]dop) was used, as compared to the experiments performed with 0.025 M [Fe]dop. The amounts of doped iron oxides were 2.8 and 1.4 mg of Fe/g of wet alginate bead, respectively. In the first case, the breakthrough point was reached after the treatment of around 80 bed volumes. It has been reported that, for calcium-iron alginate beads (batch experiments), the retention of arsenic was around 1.7 µg of As/g of wet alginate bead when incubated for 2 h, which increased to 6.5 µg of As/g of wet bead when the incubation time was 24 h.21 In this study, the retention of arsenic up to the breakthrough point was found to be 4.75 µg of As/g of wet alginate bead or 1.75 µg of As/mg of Fe doped. These results can be considered satisfactory, because, in contrast to previously published results (batch equilibrium experiments), the solution passes only once through the sorptive bed during column operation, resulting in a short residence time (5.5 min). Furthermore, the results from continuous column experiments cannot be directly compared with those from batch experiments, because, in the first case, the operation terminates when the breakthrough point is reached and, in this case, it terminated when the effluent concentration exceeded 10 µg/L, rather than when the bed reached its maximum sorptive capacity. Arsenic Removal by Calcium Alginate Beads Coated with Iron Oxides. Apart from Ca-Fe-doped alginate beads, calcium alginate beads coated with iron oxides were also examined for the removal of arsenic.

Figure 2. As(V) removal by alginate beads coated with iron oxides (conditions: linear velocity, 1.16 m/h; bed volume, 50 mL; pH, 7.0; [As]o, 50 µg/L).

Figure 3. As(V) removal by alginate beads coated with iron oxides: influence of coating duration (conditions: linear velocity, 1.16 m/h; bed volume, 50 mL; pH, 7.0; [As]o, 50 µg/L; [Fe]coat, 0.3 M).

The preparation of this material was previously described, and it was performed after the placement of the calcium alginate beads in the column. During the coating procedure, most of the adsorbent was loaded onto the surface of the material, probably as a result of ion exchange between calcium and iron.21 When the coating was performed with the 0.3 M Fe(NO3)3 suspension, the amount of iron coated per gram of wet alginate bead corresponded to 0.63 mg of Fe/g of wet alginate bead, whereas when 0.025 or 0.1 M Fe(NO3)3 was used, the amounts of iron coated were 0.24 and 0.44 mg of Fe/g of wet alginate bead, respectively. The pronounced effect of the adsorbent quantity on the sorption efficiency was also confirmed by the results displayed in Figure 2. Arsenic removal was found to be more efficient when the coating was performed with the more concentrated iron oxide suspension, and the breakthrough point was reached after the treatment of almost 45 bed volumes. The maximum amount of arsenic sorbed onto the alginate beads was found to be 2.6 µg of As/g of wet alginate bead, or 4.4 µg of As/mg of Fe. During these experiments, the duration of the coating procedure was 3 h. To investigate the kinetics of iron coating onto the calcium alginate beads and the sequential effect on arsenic removal, the coating procedure was also performed for 24 and 96 h, and the results are presented in Figure 3. It can be noticed that, when the coating was performed for 24 h, the removal of arsenic

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Figure 4. As(V) removal by different types of modified alginate beads (conditions: linear velocity, 1.16 m/h; bed volume, 50 mL; pH, 7.0; [As]o, 50 µg/L).

was more efficient, in comparison with the 3-h coating duration; however, it was found to be almost equal to the results obtained when the coating was performed for 96 h. This finding can be directly correlated with the amount of iron oxide coated onto the surface of the calcium alginate beads, which was found to be almost equal for 24- and 96-h coatings (1.13 and 1.145 mg of Fe/g of alginate, respectively). From the obtained results, it was obvious that the sorption of arsenic could occur on both the interior and the exterior surface of the alginate beads, if this material were modified appropriately. Therefore, the best conditions for preparing modified alginate beads would be to use calcium alginate beads doped with Fe ([Fe]dop ) 0.05 M) and, after placing them in the operating column, to treat them with Fe [at an iron coating concentration ([Fe]coat) of 0.3 M for 24 h] to allow sufficient coating to occur. Arsenic Removal by Calcium Alginate Beads Doped and Coated with Iron Oxides. Arsenic removal was examined using alginate beads prepared by a combination of doping and coating with iron oxides and was compared with previous results (Figure 4). The removal of arsenic was found to be more efficient. The adsorbing agents (iron oxides) were immobilized on both the interior and exterior surfaces of the beads. The deposition of iron oxides onto a greater area contributes to the extension of the active surface for sorption. Subsequently, more efficient utilization of the respective adsorption sites can be achieved by the adsorbates. The amount of iron loaded onto this combined (doped and coated) material was found to be 3.9 mg of Fe/g of wet alginate bead, comparable to the amount of iron coated on sand (2-10 mg of Fe/g of sand, although efficient coating requires temperatures over 200 °C).14 The increased amount of loaded iron had a direct consequence on the efficiency of the sorption process, as the breakthrough point was reached after 120 treated bed volumes, which corresponds to 7.2 µg of As/g of wet alginate bead or 1.8 µg of As/mg of Fe loaded on the bead, which is comparable to previous findings.21 Further investigations regarding treatment efficiency included an examination of the residence time of the solution in the filtration bed. The residence time is a critical parameter in adsorption experiments are carried out, as the removal efficiency depends strongly on the contact time between the adsorbent (iron hydroxides) and the adsorbate (arsenic). The effect of residence time

Figure 5. Effect of linear velocity on As(V) removal by alginate beads doped and coated with iron oxides (conditions: bed volume, 50 mL; pH, 7.0; [As]o, 50 µg/L).

was investigated by applying different values of linear velocity, and the results are presented in Figure 5. As expected, the extent of adsorption decreased as the linear velocity was increased because of the resulting lower contact times. The contact time, or empty bed residence time (EBRT) is the time required for the liquid to fill the empty column: EBRT ) bed volume/volumetric flow rate of the liquid.23 The best results were obtained when the linear velocity was lowest examined, i.e., 0.6 m/h, with a corresponding residence time 10 min. In this case, the breakthrough point was reached after the treatment of almost 230 bed volumes, or if expressed in pore volumes, the breakthrough point was reached after the treatment of almost 700 pore volumes. The amount of arsenic sorbed was also found to increase and reached 13.8 µg of As/g of wet alginate bead. The increase in sorption capacity with increasing residence time indicates that the sorption process is controlled by intraparticle mass transfer, rather than by external mass transfer,23 thus the critical stage in the arsenic removal kinetics is the diffusion of liquid into the bed porosity. However, the main mechanism responsible for arsenic removal is the chemical reaction between iron and arsenic. As(V) is known to form stable coordination compounds with iron oxides, by specific adsorption, through the following reaction26

M-FeOH + H3AsO4 f M-Fe-H2AsO4 + H2O The product of this reaction is ferric arsenate, which has a very low solubility product (10-20 mol/L).8 These results can be considered as satisfactory regarding the applicability of the method, as they were obtained without any pretreatment of the solution (i.e., pH adjustment). Additionally, because no studies regarding the application of modified alginate beads in fixed beds for the removal of arsenic from contaminated water could be found in the literature, the direct comparison of the obtained results with other literature findings was not possible. Previous investigations regarding arsenic removal by adsorptive filtration have been performed by applying conventional iron oxide coated sand.12 These results were equally efficient regarding the number of bed volumes treated before the breakthrough point (200250 bed volumes), although the pH of the treated solution was adjusted to 3.5, which certainly is not typical of natural waters. It is well-documented that

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Figure 6. Comparison between As(V) and As(III) removal by alginate beads doped and coated with iron oxides (conditions: linear velocity, 0.6 m/h; bed volume, 50 mL; pH, 7.0; [As]o, 50 µg/ L).

Figure 7. Breakthrough curves of As(V) for different sorbent quantities (40, 80, or 160 g of alginate beads doped and coated with iron oxides) (conditions: linear velocity, 0.6 m/h; pH, 7.0; [As]o, 50 µg/L).

As(V) tends to be more strongly adsorbed at lower solution pH, because the extent of complex formation between arsenates and iron depends on the ratio of the concentration of arsenates to the concentration of OH-.27 The log K values (affinity constants) of hydrous ferric oxide and ferric arsenate are 3827 and 20.2,28 respectively, which indicates that OH- has a stronger affinity for Fe(III) than does arsenate, as both anions compete with Fe(III) for the same adsorption sites. Thus, by decreasing the pH, the tendency of the arsenate ions to enter the coordination sheath of the Fe(III) ions will increase. Therefore, if the pH value in the present study were adjusted to 5.0, the results could be further improved, but the applicability of this method would also be restricted, because the pH values typically encountered fall in the range 6-8. However, the efficiency of alginate was found to be equal to or greater than that of other polymeric materials applied in the adsorbing filtration process.29 Under similar conditions, the use of iron oxide coated polystyrene beads was found to reach breakthrough after the treatment of 60 bed volumes, and iron-coated polymerized high internal phase emulsion (poly HIPE) produced similar results. In this study, the effect of other constituents of natural waters was also examined. It was found that phosphates inhibit the sorption of arsenic on iron-coated polymeric beads, whereas other anions, such as chlorides, nitrates, and bicarbonates, have a smaller effect on arsenic removal. These results were consistent with the findings of previous studies,30 and it was considered that similar results would be obtained in the present study. The aforementioned results refer to the removal of pentavalent arsenic anions. The other main inorganic form of arsenic that is usually present in aquatic environments is trivalent arsenic (AsIII), which is mainly present under mildly anaerobic or anoxic conditions, such as those found in groundwaters.6 This fact aggravates the overall problem of arsenic removal, because trivalent arsenic anions are far more toxic than pentavalent ones.1 Therefore, experiments were also performed by spiking the samples with trivalent arsenic, to investigate the removal of As(III) separately using the same modified alginate beads (Figure 6). It was found that As(V) was removed from the aqueous stream more efficiently, whereas the removal of trivalent arsenic was not significant, reaching the breakthrough point after the treatment of only 45 bed

volumes. This difference in removal efficiencies of inorganic arsenic forms could be attributed to the speciation of inorganic arsenic in natural waters. Pentavalent arsenic in the pH range of interest for drinking water is present in its anionic forms of H2AsO4- and HAsO42- (pKa ) 2.19, pKb ) 6.94).31 Thus, it can be adsorbed more easily on iron oxides, which, in this pH range, are mainly present with the cationic monomeric form [Fe(OH)2+],32 because adsorption is facilitated by Coulombic interactions. In contrast, trivalent arsenic in the same pH range is mainly present in its nonionic form of arsenious acid (H3AsO3, pKa ) 9.22).31 Therefore, at pH 7, it is not dissociated. Although specific chemical interactions between Fe(III) and As(III) can occur, Coulombic interactions are unfavorable, and thus, sorption is decreased. These results indicate that, when both forms of inorganic arsenic are present in an aqueous stream, a preoxidation step is essential to achieve maximum overall arsenic removal. This oxidation step can be performed by the application of chemical reagents, such as ozone, hydrogen peroxide, chlorine, etc.,8,33,34 and biological processes could also be used to catalyze trivalent arsenic oxidation.22 Modeling Fixed-Bed (Column) Operation by the Application of the Bed Depth Service Time (BDST) Model. During operation of a fixed-bed adsorption column, the service time of the system can be related to the bed depth for a given set of experimental conditions.23 The main consideration when sizing adsorptive columns is to predict the service time until the effluent exceeds a predefined pollutant concentration. The BDST model relates the service time of a fixed bed to the height of the adsorbent material in the bed and, hence, to its total quantity, as it is directly proportional to the bed height for a specific column size. Therefore, sorbent quantity is used in preference to bed height (Figure 7).24 The BDST model was found to be sufficiently described by the equation

t)

(

)

qo Co 1 Mln - 1 ) RM + b CoV kCo Ct

(1)

where t is the service time (h), qo is the adsorption capacity (µg/g), Co is the initial concentration of adsorbate (µg/L), V is the applied flow rate (L/h), M is the quantity of sorbent inside the column (g), and Ct is the

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Table 1. Calculated Constants of Bed-Depth Service Time Equation for Adsorption of As(V) on Alginate Beads Doped and Coated with Iron Oxides pH of influent

species

7

As(V)

breakthrough (%)

k (10-3 L -1 min mol-1)

qo (µg of As/ g of alginate)

qo (µg of As/ mg of Fe)

correlation coefficient, r2

10 20 30

11.7 2.91 1.6

7.79 13.75 22.6

1.99 2.91 5.79

0.99 0.99 0.99

Figure 8. Iso-removal lines for As(V) sorption on alginate beads doped and coated with iron oxides at 10, 20, and 30% breakthrough (conditions: linear velocity, 0.6 m/h; pH, 7.0; [As]o, 50 µg/L). Definition of percent breakthrough (e.g., 10%): the point when the effluent has a concentration equal to 10% of the influent concentration.

amount of arsenic sorbed onto the modified alginate beads was found to be in good agreement with the experimental results. The amount of arsenic sorbed for 20% breakthrough was calculated as 13.75 µg of As/g of wet alginate bead, whereas the experimental calculations for the corresponding experimental conditions produced a sorptive capacity equal to 13.8 µg of As/g of wet alginate bead. The application of different flow rates of contaminated water through the column can be also calculated by multiplying the original slope (R) by the ratio between the original and the new flow rate, because a linear change in flow rate is not expected to have any effect on the intercept (b). The influence of the feed concentration of the metal from Co to a new value Cn can be determined by performing the following correcting calculations

()

Rnew ) Rold

respective effluent concentration of adsorbate at time t. From the iso-removal lines (plots of t versus M) (Figure 8), regarding column operation under constant experimental conditions (except sorbent quantity), the main parameters of the BDST model can be calculated. Subsequently, from the slope (R) of the respective lines, the adsorption capacity (qo) can be calculated, and from the intercept (b), the rate constant of adsorption (k) can be found. Table 1 contains the summarized results of these calculations. The application of the BDST model describing the sorption of arsenic onto (modified) alginate beads or onto iron oxides has not previously been reported. Therefore, the direct comparison of the obtained results with other related experimental work is not possible. Nevertheless, the calculated parameters regarding the theoretical

(2)

and

bnew ) bold

Figure 9. Operating line plot for fixed-bed alginate exhaustion rate vs minimum residence time for As(V) removal for 20% total breakthrough (conditions: pH, 7.0; [As]o, 50 µg/L; quantity of sorbent, 40 g; material, alginate beads doped and coated with iron oxides).

Co Cn

( )[

]

Co ln(Cn - 1) Cn ln(Co - 1)

(3)

Fixed-Bed Operational Optimization Using the Empty Bed Residence Time (EBRT) Model. The two major parameters in the design of an adsorber unit are the contact time (or empty bed residence time) and the adsorbent exhaustion rate. The definition of EBRT was mentioned earlier. The adsorption exhaustion rate is the mass of adsorbent used per volume of liquid treated at the breakthrough point.23 The EBRT model enables the calculation of two basic parameters in the design of fixed-bed columns: (a) the minimum EBRT required to achieve the desired effluent concentration at infinitely high adsorption exhaustion rate and (b) the minimum adsorbent exhaustion rate, which occurs when the EBRT is large enough, so that the exhausted adsorbent is in equilibrium with the influent and the total volume treated at breakthrough will not increase with increasing EBRT. These two parameters can be calculated by plotting the EBRT versus the adsorbent exhaustion rate for the preselected breakthrough concentration and for different flow rates, which provides different EBRTs for the same amount of adsorbent (Figure 9). This plot produces the so-called “operating line” of the process, from which the aforementioned parameters can be obtained from the respective intercepts.23 This study was directed toward removing arsenic from contaminated water, so the breakthrough concentration of 10 µg/L was selected. Therefore, the operating line of the modified alginate system was designed for 20% breakthrough for an initial As(V) concentration of 50 µg/L. From the data presented in Figure 9, it was found that the minimum EBRT required to achieve an effluent concentration of 10 µg/L was 76 s (for an adsorbent exhaustion rate 4.3 kg of modified alginate/dm3), and

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the minimum adsorption exhaustion rate after which the influent was in equilibrium with the exhausted adsorbent was 0.45 kg of modified alginate/dm3. Conclusions Alginate beads were applied in fixed beds as supports for hydrous ferric oxides and examined in the removal of arsenic from contaminated water. The operation was found to be controlled by intraparticle mass transfer, as, when the residence time was increased, the extent of adsorption also increased. For a residence time of 10 min, 230 bed volumes of As(V) solution were treated before the breakthrough point of 10 µg/L was reached. In comparison, when As(III) solution was used, only 45 bed volumes were treated before the breakthrough point, indicating the need for preoxidation when As(III) is also present in the aqueous stream. The results were further elaborated using the bed depth service time model, and it was found that, for 20% breakthrough, the theoretical bed sorptive capacity was 13.75 µg of As/g wet alginate bead or 2.91 µg of As/mg of Fe. The application of the EBRT model enabled the calculation of two basic parameters for the design of column operations. The minimum EBRT required for achieving a predefined effluent concentration (i.e., 10 µg/L) was 76 s, and the minimum adsorbent exhaustion rate after which the volume treated at breakthrough will not increase further with increasing EBRT was determined to be 0.45 kg of alginate/dm3. Acknowledgment This work was part of the Doctoral Dissertation of Mr. Katsoyiannis, which was financially supported by the Greek State Scholarship Foundation. Thanks are also due to Ms. F. Kiachidou (Chemist) for experimental collaboration. Literature Cited (1) Pontius, F. W.; Brown, G. K.; Chen, J. C. Health implications of arsenic in drinking water. J. Am. Water Works Assoc. 1994, 86 (9), 52. (2) Desesso, J. M.; Jacobson, C. F.; Scialli, A. R.; Farr, C. H.; Holson, J. F. An assessment of the developmental toxicity of inorganic arsenic. Reprod. Toxicol. 1998, 12 (4), 385. (3) Guidelines for Drinking Water Quality. Health Criteria and Other Supporting Information, 2nd ed.; World Health Organization (WHO): Geneva, Switzerland, 1996; Vol. 2, pp 940-949. (4) Drinking Water Directive 98/83/EEC; European Commission: Brussels, Belgium, 1998 (EC directive on “drinking water quality intended for human consumption”). (5) EPA. Implementation Guidance for the Arsenic Rule; Report EPA-816-D-02-005; Office of Ground Water and Drinking Water, U.S. Environmental Protection Agency, U.S. Government Printing Office: Washington, DC, 2002 (also available at www.epa.gov/ safewater/ars/implement.html, accessed Mar 2002). (6) Ferguson, J. F.; Gavis, J. A review of the arsenic cycle in natural waters. Water Res. 1972, 6, 1259. (7) Zouboulis, A. I.; Katsoyiannis, I. A. Removal of arsenates from contaminated water by coagulation-direct filtration. Sep. Sci. Technol. 2002, 37 (12), 2859. (8) Jekel, M. R. Removal of arsenic in drinking water treatment. In Arsenic in the Environment. Part 1: Cycling and Characterization; Nriagu, J. O., Ed.; John Wiley & Sons: New York, 1994; pp 119-130. (9) Kartinen, E. O.; Martin C. J. An overview of arsenic removal processes. Desalination 1995, 103, 79. (10) Waypa, J.; Elimelech, M.; Hering, J. Arsenic removal by RO and NF membranes. J. Am. Water Works Assoc. 1997, 89 (10), 102.

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Received for review May 23, 2002 Revised manuscript received September 9, 2002 Accepted September 14, 2002 IE0203835