ARSENIC REMOVAL FROM DRINKING WATER USING IRON OXIDE-COATED SAND
O. S. THIRUNAVUKKARASU1, T. VIRARAGHAVAN1∗ and K. S. SUBRAMANIAN2 1 Environmental Systems Engineering, Faculty of Engineering, University of Regina, Regina, Saskatchewan, Canada; 2 Product Safety Bureau, Health Canada, Ottawa, Ontario, Canada ∗ ( author for correspondence, e-mail: [email protected], fax: +1 306 585 4855)
(Received 22 August 2001; accepted 22 April 2002)
Abstract. This article describes experiments in which iron oxide-coated sand (IOCS) was used to − study the removal of both As(V) and As(III) to a level less than 5 µgL 1 in drinking water. Iron oxide-coated sand 2 (IOCS-2) prepared through high temperature coating process was used in batch and column studies to assess the effectiveness and suitability. The isotherm study results showed that the observed data fitted well with the Langmuir model, and the adsorption maximum for IOCS-2 at − pH 7.6 was estimated to be 42.6 and 41.1 µgAsg 1 IOCS-2 for As(V) and As(III), respectively. In the fixed bed column tests to study arsenic removal from the tap water, good performance of IOCS-2 was observed in respect of bed volumes achieved and arsenic removal capacity. Five cycles of column tests were conducted to evaluate the performance of IOCS-2, and arsenic was successfully recovered from the media through regeneration and backwash operations. High bed volumes (860 to 1403) up − to a breakthrough concentration of 5 µgL 1 were achieved in the column studies with tap water, and the bed volumes achieved in the studies with natural water (containing arsenic) were 1520. The results of both the batch and column studies showed that iron oxide-coated sand filtration could be − effectively used to achieve less than 5 µgL 1 As in drinking water.
Keywords: adsorption, arsenic removal, batch studies, column tests, drinking water, iron oxide, speciation
1. Introduction
The enforcement of stringent standards for arsenic in drinking water by the reg- ulatory agencies calls for pragmatic approach in developing a suitable and cost- effective technology to remove arsenic from drinking water. Arsenic, a cancer causing substance is predominantly present as inorganic species in natural water systems. In oxygen-rich environments where aerobic conditions persist, arsenate − 2− [As(V)] is prevalent and exists as a monovalent (H2AsO4 )ordivalent(HAsO4 ) anion, whereas, arsenite [As(III)] exists as an uncharged molecule (H3AsO3)and − anionic (H3AsO3 ) species in moderately reducing environment where anoxic con- ditions persist (Ferguson and Gavis, 1972). Though arsenic contamination of drinking water and the associated health risks of drinking arsenic contaminated water were reported in Taiwan three decades ago (Tseng et al., 1968), similar reports of arsenic contamination from many parts of the world (Karim, 2000; Burkel and Stoll, 1999; Koch et al., 1999; Chatterjee et al.,
Water, Air, and Soil Pollution 142: 95–111, 2003. © 2003 Kluwer Academic Publishers. Printed in the Netherlands. 96 O. S. THIRUNAVUKKARASU ET AL.
1995; Cebrian et al., 1983) are receiving significant attention at present and they are of major concern to many water utilities and regulatory agencies. The drinking water standard for arsenic has been lowered in many countries (Viraraghavan et al., 1999; Driehaus et al., 1998), and more recently the United States Environmental Protection Agency (USEPA, 2001) adopted a new arsenic standard for drinking water at 10 µgL−1. Several studies have demonstrated that arsenic removal can be achieved by various technologies (Viraraghavan et al., 1994), and particularly coagulation with ferric salts was found to be the most effective method in the case of large-scale water utilities (Scott et al., 1995; Cheng et al., 1994). Fixed bed treatment sys- tems such as adsorption and ion exchange are getting increasingly popular for arsenic removal in small-scale treatment systems because of their simplicity, ease of operation and handling, regeneration capacity and sludge free operation. Iron oxides, oxyhydroxides and hydroxides (all are called ‘iron oxides’) play an important role in a variety of industrial applications, including pigments for paint industry, catalyst for industrial synthesis and raw material for iron and steel industry (Cornell and Schwertmann, 1996). The application of iron oxide has been extended to remove metals from water and wastewater (Benjamin et al., 1996; Edwards and Benjamin, 1989); recently arsenic removal with iron oxides has been investigated (Raven et al., 1998; Driehaus et al., 1998; Joshi and Chaudhuri, 1996; Wilkie and Hering, 1996; Hsia et al., 1994; Pierce and Moore, 1982; Pierce and Moore, 1980). It is generally assumed that arsenate [As(V)] has a strong affinity than arsenite [As(III)] on iron oxide surfaces. However recent studies (Raven et al., 1998) showed that at high initial As concentration, arsenite adsorption on ferrihydrite was higher than arsenate adsorption throughout the pH range of 3– 11. In adsorption studies using hydrous ferric hydroxide (HFO), high adsorption of arsenite on the HFO was observed in the pH range of 4–9, and one of the reasons for the removal was attributed to the partial oxidation of As(III) on HFO surface (Wilkie and Hering, 1996). In column studies with iron oxide-coated sand (Benjamin et al., 1996), a complete removal of arsenite (75 µgL−1)intheinfluent was observed despite the fact that the influent contained 800 mg L−1 sulfate. Though the sorption of arsenic on HFO has been studied in detail (Wilkie and Hering, 1996; Hsia et al., 1994; Pierce and Moore, 1982; Pierce and Moore, 1980), only limited information is available on arsenic adsorption on to iron oxide- coated sand (IOCS). The USEPA had proposed ion exchange, activated alumina, reverse osmosis, modified coagulation/filtration, and modified lime softening as best available technologies (BAT) for arsenic removal from small water facilities, but it considered iron oxide-coated sand filtration as an emerging technology for arsenic removal, for which only limited information is available and more testing is necessary (USEPA, 1999, 2000). There is also a need to understand how different coating procedures affect arsenic removal and retention of the coating. Therefore it was considered necessary to examine the effectiveness of IOCS for arsenate and arsenite removal. In the present study, batch studies were conducted to study the ARSENIC REMOVAL FROM DRINKING WATER 97 extent of adsorption of arsenic on to IOCS-2. Column studies were conducted to study the removal of both As(III) and As(V) that were spiked to the required con- centration levels in tap water. A speciation technique was used with the isotherm and column studies on a natural water to speciate arsenite and arsenate present in the effluent.
2. Experimental Section
2.1. WATER AND STANDARD SOLUTIONS
Tap water from the City of Regina, Saskatchewan, Canada and natural water from Kelliher Water Treatment Plant, Kelliher, Saskatchewan, Canada were used in the batch and column studies. Kelliher water contains 177.3 µgAsL−1,andthema- jor physicochemical characteristics of the Regina tap water and Kelliher natural water are listed in Table I. Distilled (double) deionized water was used in the preparation of standard solutions and for dilution of the samples. As(V) stock solution (1000 mg L−1) was prepared by dissolving 4.164 g of sodium arsenate (Na2HAsO4·7H2O; Sigma Chemical, Ontario) in 1 L distilled water and was pre- −1 served with 0.5% trace metal grade HNO3 (Fisher Scientific, Ontario). One mg L of As(V) was prepared by pipetting 1 mL of stock solution into a 1 L volumetric flask, and then the solution was made up to 1 L with distilled water. One mg L−1 of As(III) stock solution was prepared by pipetting 1 mL of arsenic oxide (1000 mg L−1 reference solution; Fisher Scientific, Ontario) into a 1 L volumetric flask, and then the solution was made up to 1 L with distilled water. In both the cases [As(V) or As(III)], required working standards were prepared daily from the stock solution. All glassware and sample bottles were washed with a detergent solution, rinsed with tap water, soaked with 10% nitric acid for at least 12 hr, and finally rinsed with distilled water three times.
2.2. PREPARATION OF IRON OXIDE-COATED SAND 2 (IOCS-2)
In the preparation of iron oxide-coated sand, coating of iron oxide was achieved on the redflint filter sand purchased from Watergroup Canada Ltd., Regina, Saskat- chewan, Canada. Initially, the sand was sieved to a geometric mean size of 0.6 to 0.8 mm, acid washed (pH 1; 24 hr), rinsed with deionized distilled water three times and dried at 100 ◦C for 20 hr before use. Iron oxide-coated sand prepared and used in the present study was different than that of IOCS-1 (Thirunavukkarasu et al., 2001), and named as IOCS-2; it was prepared similar to that of the procedure by Benjamin et al. (1996), with modifications. Effective coating of iron oxide on the sand was achieved in two steps. In step 1, the solution containing a mixture of 80 mL of 2 M Fe(NO3)3·9H2O and 1 mL of 10 M NaOH was poured over 200 g dried sand placed in a heat resistant dish. After gentle agitation, the mixture was heated for 4 hr at 110 ◦C and then at 550 ◦C for 3 hr. Upon cooling, the coated sand 98 O. S. THIRUNAVUKKARASU ET AL.
TABLE I Water quality parameters
Parameters Tap watera Kelliher watera
pH 7.4–7.6 7.4 Iron 0.07 2.1 Manganese 0.02 1.2 Turbidity (NTU) 0.28 NT Chloride 12 NT Copper 0.001 0.04 Zinc <0.005 0.01 Lead <0.002 0.002 Cadmium <0.001 <0.001 Barium 0.073 0.011 Chromium <0.001 0.001 Chlorine (residual) 0.2 Not analyzed
− a All parameters except pH and turbidity are in mg L 1. was washed with deionized distilled water till the black colored fraction washed away. In step 2, the solution containing the same mixture of Fe(NO3)3·9H2Oand NaOH was poured over 100 g of the coated sand from step 1, and heated for 20 hr at 110 ◦C. After cooling, the sand was broken mechanically to separate the grains, and sieved. When exposed to room temperature (20 ◦C) the coated sand became moist, which was overcome by drying the sand at 110 ◦C for 4 hr followed by 20 hr at 20 ◦C (five cycles). Finally the iron oxide coating on the sand was found to be dry and IOCS-2 was stored in capped bottles.
2.3. BATCH STUDIES
Iron oxide-coated sand-2 (IOCS-2) was used in the batch isotherm studies to study the removal of arsenic in the natural water, and As(III) and As(V) spiked in tap water. A portable bench top platform shaker (Model Classic C2, New Brunswick Scientific Co. Inc, NJ, U.S.A.) with digital display on the control panel was used to conduct the batch studies. Raw water from well 3 at Kelliher, Saskatchewan, Canada has a high arsenic concentration; water was collected from well 3 using 18.9 L (5 U.S. gallon) low density polyethylene containers and studies were con- ducted immediately on receipt of samples in the laboratory. Isotherm studies were conducted by varying the mass of IOCS-2, and all experiments were conducted at a room temperature of 22±1 ◦C. In the experiments, the adsorbent was transferred to 250 mL Erlenmeyer flasks containing 100 mL of the sample, and samples were placed on the shaker and shaken at 175 rpm. After completion of the mixing period, ARSENIC REMOVAL FROM DRINKING WATER 99 samples from each flask were decanted and analyzed for residual As by graphite furnace atomic absorption spectrometry (GFAAS).
2.4. COLUMN STUDIES
Column studies were conducted using iron oxide-coated sand 2, and a glass column of 16 mm diameter and 400 mm height was used. The column tests were conducted at the normal pH (7.6) of the tap water in Regina, Canada; tap water supplemented with required concentrations of arsenic [As(III) or As(V)] was pumped through the packed column with a peristaltic pump (Model # 7553-70, Cole Parmer Instrument Company, Ontario). The column was packed with 57 mL of IOCS-2, ensuring enough head space to allow expansion of the media during back washing. The flow rate to the column was kept at 21.5 mL min−1 (5 m hr−1 or 2 gpm ft−2) for both As(III) and As(V) that yielded an EBCT of 2.65 min. Five cycles of down flow column tests were conducted to study the removal of As(III) and As(V) spiked to an initial concentration of 500 µgL−1 in Regina tap water at pH 7.6. In these column tests, the first cycle was run up to exhaustion for both As(III) and As(V), and for the remaining cycles the column test was run until the effluent arsenic level reached 100 µgL−1. Four liters of 0.3 M NaOH solution were used as a regenerant solution at the end of column test to regenerate IOCS-2, followed by backwashing with deionized water. A column test was also conducted using IOCS-2 to remove arsenic from the natural water collected at the Kelliher water Treatment Plant. The volume of IOCS- 2, EBCT, and flow rate were 21 mL, 1 min, and 21.5 mL min−1 (5 m hr−1 or 2 gpm ft−2), respectively. Samples from the column tests were collected at regular intervals and analyzed for residual As. In both the isotherm and column studies with natural water the effluent samples were speciated to find soluble As, As(III) and As(V) as per the speciation protocol. The speciation protocol used in the study was similar to that of Thirunavukkarasu et al. (2001), except that anion exchange resin (Dowex 1X8-50, Sigma Chemical Co., MO, U.S.A.) of 20–50 mesh size was used instead of 50–100 mesh size.
2.5. ANALYTICAL METHODS
2.5.1. Iron Content and Surface Area of IOCS The iron content of the IOCS-2 was determined by acid digestion as per the proced- ure described in AWWARF (1993). One gram of IOCS-2 was added to 50 mL of 10% HNO3 in a beaker, and the solution was heated on a hot plate until it boiled. At the end of 2 hr, the iron oxide attached to the sand surface was completely dissolved and the acid solution turned into yellow in color. At this point, the digestion was discontinued and the solution was made up to 1 L with distilled deionized water, filtered through 0.45 µm filter, and iron content was determined as per FerroVer method using Hach instrument (model DR/850, Hach Company, Loveland, CO, 100 O. S. THIRUNAVUKKARASU ET AL.
U.S.A.). The surface area of IOCS-2 was determined using Flowsorb 2300 (Micro- meritics Instrument Corporation, Georgia, U.S.A.), and single point surface area measurements were employed to determine the surface area of the samples.
2.5.2. Arsenic Analyses All samples were acidified with 0.3% HNO3 (trace metal grade), and analyzed for arsenic using Varian type SpectrAA-600 Zeeman GFAAS equipped with GTA 100- graphite tube atomizer and programmable sample dispenser. Pyrolytically coated notched partition graphite tubes (Varian Canada Inc., Toronto) were used in the experiments, and argon gas of ultrahigh purity (99.995%; Praxair Products Inc, Ontario) was used to sheath the atomizer and to purge internally. Arsenic hollow cathode lamp (Varian Canada Inc., Toronto) was used at a wavelength of 193.7 nm with a slit width of 0.5 nm. Palladium solution (1500 µgL−1) + magnesium nitrate (1000 µgL−1) solution was used as matrix modifier for calibration. The modifier solution was prepared fresh before calibrating the instrument. An external reference standard from National Water Research Institute (NWRI), Environment Canada, Ontario was used to verify the calibration.
3. Results and Discussion
3.1. PROPERTIES OF IOCS AND ARSENIC REMOVAL MECHANISM
The results of acid digestion studies showed that the iron content of IOCS-2 was 45 mg g−1, which was approximately 60% higher than the iron content of IOCS-1 used in the earlier studies by Thirunavukkarasu et al. (2001). The surface area of IOCS-2 (10.6 m2 g−1) was nearly double that of IOCS-1, and these results indicated that effective coating of iron oxide was achieved on the sand through a high tem- perature coating process. The surface area of both natural and synthetic iron oxides may range from a few to several hundred m2 g−1, and during heating at higher temperatures the surface area of the sample usually rises initially as expulsion of water leads to the development of porosity and falls on further heating (Cornell and Schwertmann, 1996). The point of zero charge (PZC) of iron oxides is in the pH range of 6–10 (Cornell and Schwertmann, 1996), and previous studies (Benjamin et al., 1996) showed that the pH of the PZC of IOCS was 9.8. The sorption of arsenite and arsenate decreases at pH values above the PZC of the oxide surface (Wilkie and Hering, 1996). The surface areas of goethite (α-FeOOH) and haematite (α-Fe2O3) are in the range of 8–200 and 2–90 m2 g−1, respectively, and thermodynamically these iron oxides are the most stable compounds among all the other iron oxides and usually the end products of transformations (dehydration and dehydroxylation) of many other iron oxides (Cornell and Schwertmann, 1996). The surface area of IOCS-2 was in the specified range of goethite and haematite that suggested that the iron oxide was probably a combination of goethite and haematite. ARSENIC REMOVAL FROM DRINKING WATER 101
The possible reactions that lead to the formation of both goethite (α-FeOOH) and haematite (α-Fe2O3) in the preparation of IOCS-2 may be as follows:
2Fe(NO3)3·9H2O+2H2O → 2Fe + 6 HNO3 +17H2O+1.5O2 (1)
2Fe+6HNO3 +17H2O+1.5O2 + 4 NaOH →
2 Fe(OH)2 + 2 HNO3 + 4 NaNO3 +19H2O+0.5O2 (2) dehydration → 2 Fe(OH)2 + 2 HNO3 + 4 NaNO3 +19H2O+0.5O2 dehydroxylation
α-Fe2O3 + 2 HNO3 +21H2O+4NaNO3 or
α-FeOOH + 2 HNO3 +20H2O+4NaNO3 (3)
Oxyanionic arsenic species such as arsenate and arsenite adsorb at oxyhydroxide surface by forming complexes with the surface sites (Edwards, 1994). The adsorp- tion is specific, which may involve the replacement of surface hydroxyl groups by the adsorbing ligand. The reactions between arsenic species and goethite may be represented as follows: − + → α-FeOOH + H2AsO4 +3H FeH2AsO4 +2H2O(4)
+ α-FeOOH + H3AsO3 +2H → FeH2AsO3+2H2O(5)
3.2. BATCH STUDIES
In the kinetic studies using IOCS-2, arsenic removal was studied at the pH values of 5, 6, 7.6 and 8.5 (Viraraghavan et al., 2000). The results of the kinetic studies using IOCS-2 (Viraraghavan et al., 2000) showed that 85–90% of arsenic was removed in the initial phase (1 hr) of contact, and more than 95% removal was obtained for both As(III) and As(V) in the pH range of 5 to 7.6 after 6 hr of contact. Isotherm studies were only conducted at the pH value of 7.6, and the contact time was 6 hr based on kinetic studies; the initial concentrations of As(III) and As(V) in the tap water were kept at 100 µgL−1. The experimental results from the studies were fitted to the Langmuir isotherm, and non linear estimation was performed using a statistical software (Statsoft Inc., 1997). The adsorption densities estimated by the model with arsenic concentration remaining in solution are shown in Figure 1. The estimated values of the model fitted well with the observed values; the isotherms obtained for the removal of both As(III) and As(V) at the pH value studied had convex curves, which are 102 O. S. THIRUNAVUKKARASU ET AL.
Figure 1. The Langmuir isotherm plot for arsenic adsorption onto IOCS-2.
typical for microporous adsorbents suggesting a favorable arsenic adsorption by iron-oxide coated sand. The separation factor ‘R’ (Hall et al., 1966) estimated from the Langmuir constant for all the isotherms was <1 indicating that arsenic adsorption can be modelled by the Langmuir isotherm. The t-test values showed that the coefficients were significant at 95% confidence level for all the model equations; the correlation coefficients (r) for the isotherms of As(III) and As(V) removal were 0.97 and 0.98, respectively. The adsorption maxima estimated from the Langmuir model at pH 7.6 were 42.6 and 41.1 µgAsg−1 IOCS for As(V) and As(III), respectively; by expressing the mass of IOCS-2 in terms of Fe content, the adsorption density at a residual As concentration of 5 µgL−1 at pH 7.6 was estimated at 0.6 and 0.32 mmol As mol−1 Fe (16 and 22 µgg−1 IOCS-2) for As(V) and As(III), respectively; these values were lesser than the values reported by Wilkie and Hering (1996) for arsenite and Driehaus et al. (1998) for arsenate, and the difference could be due to the fact that the As/Fe ratio maintained in their studies was high compared to the ratio maintained in the present study [0.001 for As(III) and 0.004 for As(V)]. In an isotherm study conducted with the natural water collected from Kelliher, the initial total As, soluble As, and particulate As were 177.3, 169.8, and 7.5 µg L−1, respectively. The speciation of raw water sample showed that the concentra- tion of As(III) was nearly 2.2 times higher than that of As(V), which indicated the dominance of As(III) species in the raw water. The concentration of As remaining in solution is shown in Figure 2. The results showed that less than 5 µgL−1 of ARSENIC REMOVAL FROM DRINKING WATER 103
Figure 2. Concentration of As remaining in the batch studies with the Kelliher water. soluble As was achieved in the effluent samples, which indicated the capacity of IOCS-2 to remove more than 95% of arsenic from the natural water.
3.3. COLUMN STUDIES
The concentrations of As(III) and As(V) remaining in solution with the bed volumes achieved for all the five cycles up to an effluent arsenic level of 100 µgL−1 are showninFigures3and4.Thebedvolumesachievedupto5µgL−1 of As(III) and As(V) in the first cycle were 1403 and 1244, respectively. A slight decrease in the number of bed volumes was observed in the subsequent cycles, and the reason could be due to the detachment of iron from the sand particles that escape along with the effluent during backwashing operations. The results of the column studies showed that the column performed better compared to the results reported earlier by Joshi and Chaudhuri (1996). High bed volumes (nearly fifteen times higher) were achieved for both As(III) and As(V) up to a breakthrough concentration of 5 µgL−1 in the present study compared to the values reported by Joshi and Chaudhuri, (1996). In the case of rapid sand filters in water treatment facilities, the filtration rate that is being maintained and widely accepted in the filtration process ranges between 5 and 7.5 m hr−1 (Kawamura, 2000). In the present study the filtra- tion rate maintained in the column tests was 5 m hr−1, whereas the rate maintained in the tests by Joshi and Chaudhuri, (1996) was 1 mL min−1 (0.6 m hr−1), which may be well below optimal filtration rate. Analysis of random effluent samples collected during the column test operations showed that iron concentration ranged between 0.1 and 0.14 mg L−1. Acid digestion results of IOCS-2, that was collected 104 O. S. THIRUNAVUKKARASU ET AL.
Figure 3. Concentration of As(III) remaining in solution in the column studies with the tap water.
Figure 4. Concentration of As(V) remaining in solution in the column studies with the tap water. ARSENIC REMOVAL FROM DRINKING WATER 105 from the column after completion of five cycles showed that approximately 92% of the iron content was firmly attached to the sand particles. Various mathematical models have been developed to predict the dynamic be- havior of the column; the model developed by Thomas, which is used in the design of fixed bed adsorption systems is as shown below (Reynolds and Richards, 1996):
Ce = 1 C0 + k − 1 exp Q (q0m C0V ) where
−1 Ce = effluent adsorbent concentration (µgL ); −1 C0 = influent adsorbent concentration (µgL ); k = Thomas rate constant (mL min−1.µg); Q = volumetric flow rate (mL min−1); −1 q0 = maximum solid phase concentration (µgg ); m = mass of adsorbent (g); V = throughput volume (mL). The results from the first cycle of the column tests were analyzed using the Thomas model and the estimated values of Ce/C0 by the Thomas model through non lin- ear estimation were compared with the observed values and these are shown in Figure 5. The maximum solid phase concentration (q0; overall adsorption capacity) es- timated from the Thomas model for As(III) and As(V) were 0.74 and 0.72 mg g−1 IOCS-2, respectively. The results showed that the bed volumes achieved (Table II) in all the cycles for As(III) removal using IOCS-2 were higher than those values obtained for As(V) removal. The arsenate adsorptive capacity of IOCS-2 (0.72 mg g−1 IOCS-2) estimated in the column study was nearly equal to the value (0.8 g kg−1 GFH) reported by Driehaus et al. (1998). In the fixed bed adsorber (two in- line columns) tests with granular ferric hydroxide (GFH) at a pilot plant facility (W) in Germany, they achieved high bed volumes up to an effluent As level of 10 µgL−1, where the natural water had a maximum initial arsenic concentration of 18 µgL−1. It is expected that high bed volumes could be achieved using IOCS-2, while operating under low initial As concentration. The arsenic removal capacity to 5 µgL−1 by IOCS-2 in the first cycle of column tests for As(III) and As(V) were 0.65 (0.41) and 0.52 mg cm−3 IOCS-2 (0.33 mg g−1 IOCS-2), respectively, and these values were higher than the values reported by Simms and Azizian, (1997). The arsenic removal capacity was calculated by the difference between the applied arsenic loading and the amount removed by IOCS- 2, and divided by the volume of IOCS-2 used in the column tests. In a pilot plant trial for arsenic removal from source water using activated alumina (AA) at pH 7.5, 106 O. S. THIRUNAVUKKARASU ET AL. ), mass of IOCS-2 (90 g), volume of IOCS-2 1 − ), flow rate (21.5 mL min 1 TABLE II − gL µ Summary of the results from column studies using IOCS-2 Tap water As(III)12345 12345 As(V) ) a 3 − of As 0 1 g) (mg cm − µ 1 1 − gL − ) from the Column cycles: µ 1 gL − µ In all the column cycles, the initial As concentration (500 Total throughputvolume (L) Maximum solid phaseconcentration, q 178(mg g 0.74 101 – 90 – 90 – 70 – 157 0.72 85 – 75 – 70 60 – – Regression coefficient, rBed volumes achievedup to 0.95 5 1403 – 1222 – 1132 1041 – 860 – 1244 1132 0.96 1041 – 996 894 – – – Arsenic removalcapacity of the media to 5 0.65 0.56 0.52 0.47 0.34 0.52 0.48 0.44 0.38 0.31 Thomas model ‘k’ value from theThomas model (mL min 0.22 – – – – 0.22 – – – – a (57 mL) and EBCT (2.7 min) were kept as the same. ARSENIC REMOVAL FROM DRINKING WATER 107
Figure 5. Thomas model values in the first cycle of column tests. they reported that the arsenic removal capacity of AA to 10 µgL−1 varied between 0.19 and 0.35 g As kg−1 AA at different empty bed contact times (3, 6, and 9 min). Arsenic recovered through the regeneration and backwash operations at the end of column tests is shown in Table III. Simpson’s rule (Chapra and Canale, 1988) was applied to determine the area under the curve, which was equated to arsenic concentration and the arsenic recovered at the end of each cycle was determined. In the case of column studies with IOCS-2, arsenite recovery efficiency ranged from 80 to 83%, whereas arsenate recovery efficiency was in the range of 84 to 87%. The results of column test (Figure 6) with Kelliher water using IOCS-2 showed that As(V) predominated after 28 hr of column operation, which indicated the oxidation of As(III) to As(V). The column continued to remove soluble As to a value less than 5 µgL−1 for a period of 28 hr, and the bed volumes achieved up to 5 µgL−1 of soluble As in the effluent was 1720.
3.4. APPLICATION TO PRACTICE
Simplicity and cost are the two major factors that should influence the selection of a treatment system for arsenic removal in small communities dependent on groundwater for drinking water supply. Although ion-exchange systems may be suitable for some communities in U.S.A. and Canada, such systems may not be appropriate for a number of small communities, especially in developing countries such as Bangladesh. Other simple and economical systems such as manganese greensand filtration and oxide-based filtration systems may offer the best choice. 108 O. S. THIRUNAVUKKARASU ET AL.
Figure 6. Arsenic species remaining in Kelliher water in the column studies.
TABLE III Arsenic recovered from the column studies after regeneration and backwash
Adsorbent Arsenic Column Total As As removed As recovered by As species test load to in the regeneration recovery cycles the column column tests and back wash efficiency (mg) (mg) (mg) (%)
IOCS-2 As(III) 1 89.0 85.2 68.0 80 2 50.3 50.1 40.4 81 3 45.2 45.0 36.9 82 4 45.2 45.0 37.2 83
As(V) 1 78.7 76.1 64.2 84 2 42.6 42.4 37.1 87 3 37.5 37.3 32.1 86 4 34.8 34.7 30.1 87 ARSENIC REMOVAL FROM DRINKING WATER 109
Since manganese greensand filtration systems may not be effective to meet more stringent standards below 25 µgL−1, filtration systems using iron oxide-coated sand would offer an alternative solution, because the present studies showed clearly that finished water arsenic levels below 5 µgL−1 could be consistently achieved. Simplicity as well as familiarity of local operators with simple sand filtration sys- tem gives the iron oxide-coated sand filtration a significant advantage in the choice of an alternative treatment system for arsenic removal.
4. Conclusions
Surface area analysis and acid digestion results showed that IOCS-2 had a higher surface area, and iron content than IOCS-1 due to the attachment of iron oxides on the sand, which was attained through a high-temperature coating process. Based on the surface area measurements, it was expected that the iron oxide coating on the sand might be a combination of goethite and haematite. The results of isotherm studies with IOCS-2 showed that the estimated values of the Langmuir model fitted well with the observed values at the pH value studied, and all the model equations were found to be statistically significant at 95% confidence level. Batch isotherm study results with both tap and natural water using IOCS-2 showed that less than 5 µgAsL−1 was achieved. In the batch studies with the Kelliher water, the speciation results showed that the raw water contained 4% particulate As and 96% soluble As, and the ratio of As(III)/As(V) in the raw water was 2.2:1. The column tests showed that high bed volumes and high arsenic removal capa- city were achieved with IOCS-2. The results of column studies showed that IOCS-2 was capable of removing As(III) and As(V) present in the water to a value less than 5 µgL−1. IOCS-2 can be regenerated and the arsenic retained in the media can be recovered by exposing the media to base solutions and through backwash oper- ations. The maximum solid phase concentration ‘q0’ estimated from the Thomas model for IOCS-2 was high, which indicated a good adsorption capacity of IOCS- 2. Acid digestion results showed that only 8% of iron was lost from IOCS-2 through treatment, regeneration, and backwashing operations, thereby demonstrating the applicability of IOCS-2 for the removal of arsenic in drinking water. In the case of commercial production of IOCS, industrial coating processes are expected to sig- nificantly reduce this loss. Overall, the results of the studies with IOCS-2 showed that it can be effectively used to achieve a low level of arsenic (less than 5 µgL−1) in drinking water supplies, particularly in small water facilities.
Acknowledgements
The authors acknowledge major support from Health Canada, Ottawa for this study. The first author expresses sincere thanks to the officials of Kelliher Water Treat- 110 O. S. THIRUNAVUKKARASU ET AL. ment Facility, Kelliher, Saskatchewan, Canada for their assistance in the collection of water samples.
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