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Studies on Ferric Oxide Hydroxides
JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO.
185, 355–362 (1997)
CS964522
Studies on Ferric Oxide Hydroxides III. Adsorption of Selenite (SeO 20 3 ) on Different Forms of Iron Oxyhydroxides K. M. PARIDA, 1 B. GORAI, N. N. DAS,
AND
S. B. RAO
Regional Research Laboratory, Bhubaneswar-751 013, Orissa, India Received April 1, 1996; accepted July 25, 1996
Adsorption of selenite (SeO 20 3 ) on different polymorphic forms of iron oxyhydroxides and amorphous ferrihydrite was studied as a function of time, temperature, pH, and concentration of adsorbate(s) and adsorbent(s). Analysis of adsorption data indicates that the surfaces of all the forms of oxyhydroxides and ferrihydrite are heterogeneous in nature and that adsorption fits into a heterogeneous site binding model. The adsorption capacity of oxyhydroxides for SeO 20 follows the order b-FeOOH õ a-FeOOH õ g3 FeOOH õ d-FeOOH õ ferrihydrite. q 1997 Academic Press
INTRODUCTION
Selenium is a nutritional element with mixed blessings to animals, including humans, wherein it has both toxic effects if assimilated in excess and equally severe consequences when there is a nutritional deficient (1–4). In fact, there is a very narrow range between deficient and toxic levels of Se in animals, which necessitates a clear knowledge of the processes affecting Se distribution in the environment (1, 4). The behavior of selenium in the environment strongly depends on its oxidation state. In nature selenium exists in different oxidation states, viz., elemental selenium (Se 0 ), 20 selenite (SeO 20 ), and selenate (SeO 20 3 ), selenide (Se 4 ). Thermodynamic calculations indicate that selenite and elemental selenium should be found in reducing environments and selenate in oxidizing environments. However selenite, selenate, and organic selenide have been found to coexist in oxic seawater samples, suggesting the importance of kinetics in speciation (5). Aqueous selenium exists predominantly as selenate and selenite. One of the important processes regulating the concentra1 To whom correspondence should be addressed at Regional Research Laboratory, Council of Scientific & Industrial Research, Bhubaneswar 751 013, Orissa, India.
tion and mobility of selenium is the adsorption on solid surfaces. Oxides and oxyhydroxides of iron, aluminum and manganese, and soil organic matter are thought to play a significant role in sequestering elements because of their large surface area, strong affinity for many elements, and ubiquitous occurrence in soils and sediments (1–11). Further, the anion adsorption depends on pH, solid composition, surface site composition, adsorbate identity and concentration, ionic strength, competing adsorbates, and formation of solution complexes (9, 12, 13). The oxidation states of metals in anions also affect the adsorption (13). For example selenite [Se(IV)] shows a greater affinity for iron oxide surface than selenate [Se(VI)] (13). Since selenite is more mobile and readily transported in groundwater than other selenium species and as iron oxides and oxyhydroxides appear to be the major adsorbents of soil and no comparative study on adsorption of selenite on various polymorphs of iron oxyhydroxides and amorphous iron oxyhydroxides has been reported in the literature, we have investigated the adsorption of SeO 20 3 from its aqueous solutions as a function of time, pH, concentration of SeO 20 3 , adsorbents, and temperature, using different forms of iron oxyhydroxides ( a- b-, g-, and d-FeOOH) and ferrihydrite. The adsorption behavior of all these oxyhydroxides has also been compared and correlated with their surface properties. EXPERIMENTAL
Preparation and Characterization of Iron Oxhydroxides The various forms of iron oxyhydroxides ( a-, b-, g-, and d-FeOOH) and ferrihydrite were prepared by following the methods reported in Refs. (14–18) and were characterized by X-ray diffraction. XRD patterns confirmed the absence of any other crystalline impurities. The surface area was determined by nitrogen adsorption desorption (BET method) at liquid nitrogen temperature using a quantasorb (Quantachrome, U.S.A.). The surface hydroxyl groups of the oxyhydroxides were determined by the conventional KI method in DMF medium (19). The number of surface hy-
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TABLE 1 Surface Properties of Ferric Oxyhydroxides Forms of iron oxyhydroxides
Methods of preparation
Nsa (mmol/g)
a b g d Ferrihydrite
Atkinson et al. (1967) Parida (1987) Nauer et al. (1985) Kamoto (1968) Benjamin and Leckie (1981)
0.26 0.21 0.36 0.52 —
a
pHPZC
Surface area (m2/g)
Surface OH group 102 (meq/g)
{ { { { {
70.8 77.8 61.1 117 225
5.4 4.5 6.3 9.0 12.7
7.75 7.9 7.26 8.00 7.89
0.1 0.1 0.1 0.1 0.1
Indicates the total exchange capacity.
droxyl groups available for selenite adsorption was estimated by adsorption of H / or OH 0 from 0.1 mol dm03 HNO3 or NaOH using the method of Hohl and Stumm (20). Surface charges were measured by potentiometric acid– base titrations of oxyhydroxide by suspending 0.2 g of the sample in 50 ml of 0.1, 0.01, and 0.001 mol dm03 KNO3supporting electrolyte following the method of Parks and De Bruyn (21). The values of pHPZC (point of zero charge) were obtained from the plots of adsorption density of H / or OH 0 ions versus pH. The various physical properties of the samples are collected in Table 1.
Adsorption experiments were carried out in 100-ml stoppered conical flasks by taking appropriate amounts of sodium
selenite solution in KCl and iron oxyhydroxides/ferrihydrite. The pH of the solution was adjusted with either 0.1 mol dm03 KOH or HCl. The final volume was invariably kept at 20 ml and the ionic strength was maintained at 0.1 mol dm03 using KCl. Preliminary experiments revealed that about 2 h is required for any metal ion to reach the equilibrium concentration. The flasks were shaken mechanically at a particular temperature in a thermostatic bath and filtered through Whatman 42 filter paper, and the concentration of selenite in the filtrate was determined spectrophotometrically by following the procedure of Huseyin et al. (22), where 2-mercaptoethanol is used as the reductant and also as the ligand to form zero valent selenium complex. The percentage of selenium adsorbed was determined from the ratio of selenium between the solution and the particulate phases,
FIG. 1. Adsorption of selenite with time on a-FeOOH and d-FeOOH. (1) [Selenite] Å 6.8 mmol, d-FeOOH Å 500 mg/L; (2) and (3) [Selenite] Å 2.53 mmol, a-FeOOH Å 250 mg/L.
FIG. 2. Selenite adsorption density as a function of the equilibrium selenite concentration at pH 6.5. ( 1, 2 ) for a-FeOOH and ( 3, 4 ) for ferrihydrite.
Adsorption Experiments
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reached equilibrium in about 2 h. There was no further change in equilibrium concentration up to 24 h. Representative time plots showing the adsorption of selenite with varying time and pH are presented in Fig. 1. Interestingly in the case of d-FeOOH and ferrihydrite, the adsorption reaction was much faster and equilibrium was reached in less than 2 h. Effect of Temperature The adsorption of selenite on different forms of iron oxyhydroxides and ferrihydrite at a particular pH, concentration of adsorbate, and adsorbent decreases with increase in temperature from 298 to 308 K. Representative adsorption isotherms of selenite for a-FeOOH and ferrihydrite are illustrated in Fig. 2. An idea of the uniformity or nonuniformity of the surface site is obtained from the values of isosteric heats as a function of adsorption density of selenite. The isosteric heats of adsorption were calculated from adsorption isotherms at two temperatures using the Clausius–Clapeyron equation (23) DHT Å R ln[(C2 /C1 )]/(1/T 2 0 1/T 1 ), FIG. 3. Isosteric heat of adsorption as a function of the selenite adsorption density. 1, 2, 3, and 4 are for b-, a-, g- and d-FeOOH, respectively.
Seads (% of selenium adsorbed) Å
Sein 0 Seeq 1 100, Sein
[1]
where DHT is the isosteric heat of adsorption in kJ/mol at a given adsorption density, R is the gas constant, and C1 and C2 are the equilibrium concentrations of the ion at tempera-
where Sein and Seeq are the initial and equilibrium concentrations of selenium, respectively. All spectrophotometric measurements were made with a Chemito 2500 recording UV – visible spectrophotometer using 10-mm matched quartz cells. The pH of the solutions at the beginning and end of experiments was measured and the average pH values are reported. All pH measurements were made by an Elico Digital pH meter ( Model LI 120 ) using a combined glass electrode ( Model CL 51 ) . The pH meter was standardized with NBS buffers before any measurement. The adsorption experiments under varying conditions of pH ( 3 – 10 ) , adsorbate concentration ( 0.01 – 0.1 mmol dm03 ) , adsorbent concentration ( 0.2 – 1.2 g / L ) , and temperature ( 298 – 3087K ) were also carried out using all the samples. All the experiments were carried out in duplicate. However, the data were reproducible within {3% error. RESULTS AND DISCUSSION
Effect of Time Kinetic experiments under varying conditions indicated that the adsorption of selenite on different forms of iron oxyhydroxides and ferrihydrite was fairly rapid and
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FIG. 4. Adsorption of selenite as a function of equilibrium pH. [Selenite] Å 3.79 mmol and adsorbent Å 250 mg/L. 1, 2, 3, 4, and 5 are for a-, b-, g-, d-FeOOH and ferrihydrite, respectively.
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FIG. 5. Adsorption of selenite as a function of adsorbent concentration. 1, 2, 3, 4, and 5 are for a-, b-, g-, d-FeOOH and ferrihydrite, respectively.
tures T 1 and T 2 and at the same adsorption density, respectively. If the isosteric heat of adsorption is independent of temperature, the surface is taken to be homogeneous. It is found that the isosteric heat of adsorption decreases with increasing adsorption density (see Fig. 3), suggesting that the surface of the oxyhydroxides/ferrihydrite is heterogeneous (18). This decrease in isosteric heat of adsorption may be due to different types of adsorption sites or the interaction of adsorbing ions (24). Effect of pH The adsorption of selenite on different polymorphs of iron oxyhydroxides and ferrihydrite as a function of pH and at a fixed selenite concentration is shown in Fig. 4. The adsorption of selenite at a particular initial concentration is higher at lower pH and progressively decreases with the increase in pH in accordance with K1
0 S{OH / H / / SeO 20 3 } S{SeO 3 / H2O
can occur if the chemical (i.e., specific) component dominates the electrostatic component. Stumm et al. (26) have also reported that the adsorption edge is not necessarily linked to the pHPZC or charge characteristics of the solid since the free energy of adsorption is a combination of chemical and electrostatic effects. Effect of Adsorbent Concentration The influence of varying concentrations of iron oxyhydroxides on the adsorption of selenite at a particular pH is presented in Fig. 5. The increase in adsorption, which occurs with increasing particle concentration, is nothing but a direct consequence of a greater amount of available binding sites for selenite. The distribution coefficient ( KD ) for selenite and iron oxyhydroxides at pH 3 was calculated by KD Å {[Se(IV)]ads /[Se(IV)]diss }(1/CP ),
[2]
[4]
K2
S{OH / 2H / / SeO 20 3 } S{HSeO3 / H2O, [3] where S{OH is a surface hydroxyl group and S{SeO 30 and S{HSeO3 are the adsorbed selenite species. This is a characteristic of anion adsorption (9). A similar trend has been previously observed in the case of goethite, clays, iron oxide, manganese dioxide, and hydrous aluminum oxide (1, 3, 13, 25). The data in Fig. 4 indicate that a significant amount of selenite adsorbed on different forms of iron oxyhydroxides even at pH values greater than pHPZC of iron oxyhydroxides ( Ç8.0), the pH at which the surface is negatively charged. The adsorption of an anion on a negatively charged surface
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where CP is the solid concentration in kg/L and KD has the units of L/kg; [Se(IV)]ads and [Se(IV)]diss are the concentrations in the particulate and dissolved phases, respectively. The distribution coefficient (KD ) for selenite and various adsorbents increases with increasing adsorbent concentration (Fig. 6). If the surface is homogeneous, the KD values at a given pH should not change with particle concentration. It is clear from Fig. 7 that the binding ability of the surface, as reflected in KD values, shows a decreasing trend with increasing adsorption density, which is consistent with the isosteric heat of adsorption (Fig. 3). All these observations suggest that the surface of oxyhydroxides/ferrihydrite is heterogeneous.
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FIG. 6. Log KD as a function of adsorbent concentration at equilibrium pH 3.3. 1, 2, 3, 4, and 5 are for a-, b-, g-, d-FeOOH and ferrihydrite, respectively.
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FIG. 8. The adsorption of selenite by a-FeOOH as a function of equilibrium pH at different selenite concentrations. a-FeOOH Å 250 mg/L.
Effect of Initial Selenite Concentration
FIG. 7. Log KD values as a function of the selenite adsorption density. 1 and 2 are for g- and d-FeOOH, respectively.
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The influence of varying selenite concentration on its adsorption as a function of pH is presented in Figs. 8 – 12. The locations of adsorption edges at different initial concentrations do not vary significantly. However, at a particular pH, the percentage of adsorption decreases with an increase in selenite concentration. The values of distribution coefficient ( KD ) for g-FeOOH decrease as the total selenite concentration increases or as the adsorption density increases ( Figs. 13 and 14 ) . Similar observations were also made in all other cases. The influence of the total selenite concentration on the percentage of selenite adsorption and on the KD value is consistent with the heterogeneous model ( 13 ) . It has been reported that selenite adsorption on oxides – hydroxides produced two types of complexes, SHSeO3 and SSeO 30 ) , via a ligand exchange mechanism ( 4 ) . Generally a two-step mechanism is involved in the adsorption and desorption reactions: ( i ) the formation of outer-sphere surface complexes ( SOH 2/ – HSeO 30 and SOH 2/ – SeO 320 ) through electrostatic attraction between the surface hy/ droxyl groups ( S – OH ) and HSeO 30 , SeO 20 3 , and H . ( ii ) In the second step divalent and monovalent selenite anions replace a molecular of water from the active sites of iron oxyhydroxide to form inner-sphere surface complexes SSeO 03 and SHSeO 30 ) . The various adsorption – desorption reactions may be delineated as shown in Scheme I.
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FIG. 9. The adsorption of selenite by b-FeOOH as a function of equilibrium pH at different selenite concentrations. b-FeOOH Å 250 mg / L.
FIG. 11. The adsorption of selenite by d-FeOOH as a function of equilibrium pH at different selenite concentrations. d-FeOOH Å 250 mg / L.
FIG. 10. The adsorption of selenite by g-FeOOH as a function of equilibrium pH at different selenite concentrations. g-FeOOH Å 250 mg / L.
FIG. 12. The adsorption of selenite by ferrihydrite as a function of equilibrium pH at different selenite concentrations. Ferrihydrite Å 250 mg / L.
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ADSORPTION OF SELENITE SOH1¤ 2 HSeO2 ‹ k⁄
k‹ k2‹
SHSeOƒ‹ 1 H¤O
k2⁄
‹ SOH 1 2H1 1 SeO 22
Kfi
Kfl
k¤ k2¤
SOH1¤ 2 SeO ‹22 1 H1
k› k2›
SSeO2‹ 1 H1 1 H¤O
SCHEME I.≥SOH is the surface hydroxyl group.
FIG. 13. Log KD values (for g-FeOOH) as a function of equilibrium selenite concentrations at different equilibrium pH (cf., Fig. 10).
This mechanism suggests that the removal of selenite from solution by adsorption on oxyhydroxides and oxides surfaces is influenced by the surface hydroxyl group, pH, concentra-
tion of available binding sites, and the equilibrium selenite concentration which is consistent with our result in the case of all of the oxyhydroxides. From the foregoing discussion it is clear that under identical conditions the adsorption capacity of iron oxyhydroxide / ferrihydrite increases in the order b-FeOOH õ a-FeOOH õ g-FeOOH õ d-FeOOH õ amorphous ferrihydrite, which is more or less similar to the trends of surface area, surface hydroxyl group, and total exchange capacity. Thus the most disordered form, i.e., amorphous ferrihydrite, shows the highest capacity for selenite adsorption due to its high surface area and surface hydroxyl groups, although the pHPZC values of all the oxyhydroxides remain in a narrow range. ACKNOWLEDGMENTS The authors are thankful to the Director for his permission to publish this paper and to the Ministry of Environment and Forest, Government of India, for financial assistance.
REFERENCES
FIG. 14. Log KD values (for g-FeOOH) as a function of selenite adsorption density at different equilibrium pH (cf., Fig. 11).
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1. Ghosh, M. M., Cox, C. D., and Yuan-pan, J. R., Environ. Prog. 13, 79 (1994). 2. Wang, S., Misra, M., Reddy, R. G., and Milbourne, J. C., Residues and effluents, in ‘‘Proceeding and Environmental Considerations’’ (R. G. Reddy, W. P. Imrie, and P. B. Queneau, Eds.), The Minerals, Metals & Material Society, 1991. 3. Balistrieri, L., and Chao, T. T., Geochim. Cosmochim. Acta 54, 739 (1990). 4. Zhang, P., and Sparks, D. L., Environ. Sci. Technol. 24, 1848 (1990). 5. Measures, C. I., and Burten, J. D., Earth Planet Sci. Lett. 46, 385 (1980). 6. Cary, E. E., Wiecaonek, G. A., and Allaway, W. H., Soil Sci. Soc. Am. Proc. 31, 21 (1967). 7. Geering, H. R., Cary, E. E., Jones, L. H. P., and Allaway, W. H., Soil Sci. Soc. Am. Proc. 32, 35 (1968). 8. Levesque, M., Can. J. Soil Sci. 54, 63 (1974). 9. Benjamin, M. M., Hayes, K. F., and Leckie, J. O., J. Water Pollut. Control Fed. 54, 1472 (1982). 10. Merrill, D. T., Manzione, M. A., Peterson, J. J., Parker, D. S., Chow, W., and Hobbs, A. O., J. Water Pollut. Control. Fed. 58, 18 (1986). 11. Balistrier, L. S., and Murray, J. W., Geochim. Cosmochim. Acta 51, 1151 (1987). 12. Balistrieri, L. S., and Murray, J. W., Geochim. Cosmochim. Acta 46, 1253 (1982). 13. Balistrieri, L. S., and Chao, T. T., Soil Sci. Soc. Am. 51, 1145 (1987).
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14. Atkinson, R. J., Posner, A. M., and Quirk, J. P., J. Phys. Chem. 71, 550 (1967). 15. Parida, K. M., J. Mater. Sci. Lett. 6, 1476 (1987). 16. Nauer, G., Strecha, P., Brinda Koponik, N., and Liptay, G., J. Therm. Anal. 30, 813 (1985). 17. Okamoto, S., J. Am. Ceram. Soc. 51, 594 (1968). 18. Benjamin, M. M., and Leckie, J. O., J. Colloid Interface Sci. 79, 209 (1981). 19. Parida, K. M., Satapathy, P. K., Das, N. N., and Rao, S. B., Indian J. Chem. 34A, 632 (1995). 20. Hohl, H., and Stumm, W., J. Colloid Interface Sci. 55, 281 (1976).
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21. Parks, G. A., and de Bruyn, P. L., J. Phys. Chem. 66, 967 (1962). 22. Huseyin, A., Resat, A., and Izzet, T., Analyst 114, 1319 (1989). 23. Chiou, C. T., Shoup, T. D., and Porter, P. E., Org. Geochem. 8, 9 (1985). 24. Moore, W. J., ‘‘Physical Chemistry.’’ Prentice Hall, Englewood Cliffs, NJ, 1972. 25. Hingston, F. J., Posner, A. M., and Quirk, J. P., Adv. Chem. Ser. 79, 82 (1968). 26. Stumm, W., Huann, C. P., and Jenkins, S. R., Croat. Chim. Acta 42, 223 (1970).
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CS964522
Studies on Ferric Oxide Hydroxides III. Adsorption of Selenite (SeO 20 3 ) on Different Forms of Iron Oxyhydroxides K. M. PARIDA, 1 B. GORAI, N. N. DAS,
AND
S. B. RAO
Regional Research Laboratory, Bhubaneswar-751 013, Orissa, India Received April 1, 1996; accepted July 25, 1996
Adsorption of selenite (SeO 20 3 ) on different polymorphic forms of iron oxyhydroxides and amorphous ferrihydrite was studied as a function of time, temperature, pH, and concentration of adsorbate(s) and adsorbent(s). Analysis of adsorption data indicates that the surfaces of all the forms of oxyhydroxides and ferrihydrite are heterogeneous in nature and that adsorption fits into a heterogeneous site binding model. The adsorption capacity of oxyhydroxides for SeO 20 follows the order b-FeOOH õ a-FeOOH õ g3 FeOOH õ d-FeOOH õ ferrihydrite. q 1997 Academic Press
INTRODUCTION
Selenium is a nutritional element with mixed blessings to animals, including humans, wherein it has both toxic effects if assimilated in excess and equally severe consequences when there is a nutritional deficient (1–4). In fact, there is a very narrow range between deficient and toxic levels of Se in animals, which necessitates a clear knowledge of the processes affecting Se distribution in the environment (1, 4). The behavior of selenium in the environment strongly depends on its oxidation state. In nature selenium exists in different oxidation states, viz., elemental selenium (Se 0 ), 20 selenite (SeO 20 ), and selenate (SeO 20 3 ), selenide (Se 4 ). Thermodynamic calculations indicate that selenite and elemental selenium should be found in reducing environments and selenate in oxidizing environments. However selenite, selenate, and organic selenide have been found to coexist in oxic seawater samples, suggesting the importance of kinetics in speciation (5). Aqueous selenium exists predominantly as selenate and selenite. One of the important processes regulating the concentra1 To whom correspondence should be addressed at Regional Research Laboratory, Council of Scientific & Industrial Research, Bhubaneswar 751 013, Orissa, India.
tion and mobility of selenium is the adsorption on solid surfaces. Oxides and oxyhydroxides of iron, aluminum and manganese, and soil organic matter are thought to play a significant role in sequestering elements because of their large surface area, strong affinity for many elements, and ubiquitous occurrence in soils and sediments (1–11). Further, the anion adsorption depends on pH, solid composition, surface site composition, adsorbate identity and concentration, ionic strength, competing adsorbates, and formation of solution complexes (9, 12, 13). The oxidation states of metals in anions also affect the adsorption (13). For example selenite [Se(IV)] shows a greater affinity for iron oxide surface than selenate [Se(VI)] (13). Since selenite is more mobile and readily transported in groundwater than other selenium species and as iron oxides and oxyhydroxides appear to be the major adsorbents of soil and no comparative study on adsorption of selenite on various polymorphs of iron oxyhydroxides and amorphous iron oxyhydroxides has been reported in the literature, we have investigated the adsorption of SeO 20 3 from its aqueous solutions as a function of time, pH, concentration of SeO 20 3 , adsorbents, and temperature, using different forms of iron oxyhydroxides ( a- b-, g-, and d-FeOOH) and ferrihydrite. The adsorption behavior of all these oxyhydroxides has also been compared and correlated with their surface properties. EXPERIMENTAL
Preparation and Characterization of Iron Oxhydroxides The various forms of iron oxyhydroxides ( a-, b-, g-, and d-FeOOH) and ferrihydrite were prepared by following the methods reported in Refs. (14–18) and were characterized by X-ray diffraction. XRD patterns confirmed the absence of any other crystalline impurities. The surface area was determined by nitrogen adsorption desorption (BET method) at liquid nitrogen temperature using a quantasorb (Quantachrome, U.S.A.). The surface hydroxyl groups of the oxyhydroxides were determined by the conventional KI method in DMF medium (19). The number of surface hy-
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TABLE 1 Surface Properties of Ferric Oxyhydroxides Forms of iron oxyhydroxides
Methods of preparation
Nsa (mmol/g)
a b g d Ferrihydrite
Atkinson et al. (1967) Parida (1987) Nauer et al. (1985) Kamoto (1968) Benjamin and Leckie (1981)
0.26 0.21 0.36 0.52 —
a
pHPZC
Surface area (m2/g)
Surface OH group 102 (meq/g)
{ { { { {
70.8 77.8 61.1 117 225
5.4 4.5 6.3 9.0 12.7
7.75 7.9 7.26 8.00 7.89
0.1 0.1 0.1 0.1 0.1
Indicates the total exchange capacity.
droxyl groups available for selenite adsorption was estimated by adsorption of H / or OH 0 from 0.1 mol dm03 HNO3 or NaOH using the method of Hohl and Stumm (20). Surface charges were measured by potentiometric acid– base titrations of oxyhydroxide by suspending 0.2 g of the sample in 50 ml of 0.1, 0.01, and 0.001 mol dm03 KNO3supporting electrolyte following the method of Parks and De Bruyn (21). The values of pHPZC (point of zero charge) were obtained from the plots of adsorption density of H / or OH 0 ions versus pH. The various physical properties of the samples are collected in Table 1.
Adsorption experiments were carried out in 100-ml stoppered conical flasks by taking appropriate amounts of sodium
selenite solution in KCl and iron oxyhydroxides/ferrihydrite. The pH of the solution was adjusted with either 0.1 mol dm03 KOH or HCl. The final volume was invariably kept at 20 ml and the ionic strength was maintained at 0.1 mol dm03 using KCl. Preliminary experiments revealed that about 2 h is required for any metal ion to reach the equilibrium concentration. The flasks were shaken mechanically at a particular temperature in a thermostatic bath and filtered through Whatman 42 filter paper, and the concentration of selenite in the filtrate was determined spectrophotometrically by following the procedure of Huseyin et al. (22), where 2-mercaptoethanol is used as the reductant and also as the ligand to form zero valent selenium complex. The percentage of selenium adsorbed was determined from the ratio of selenium between the solution and the particulate phases,
FIG. 1. Adsorption of selenite with time on a-FeOOH and d-FeOOH. (1) [Selenite] Å 6.8 mmol, d-FeOOH Å 500 mg/L; (2) and (3) [Selenite] Å 2.53 mmol, a-FeOOH Å 250 mg/L.
FIG. 2. Selenite adsorption density as a function of the equilibrium selenite concentration at pH 6.5. ( 1, 2 ) for a-FeOOH and ( 3, 4 ) for ferrihydrite.
Adsorption Experiments
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reached equilibrium in about 2 h. There was no further change in equilibrium concentration up to 24 h. Representative time plots showing the adsorption of selenite with varying time and pH are presented in Fig. 1. Interestingly in the case of d-FeOOH and ferrihydrite, the adsorption reaction was much faster and equilibrium was reached in less than 2 h. Effect of Temperature The adsorption of selenite on different forms of iron oxyhydroxides and ferrihydrite at a particular pH, concentration of adsorbate, and adsorbent decreases with increase in temperature from 298 to 308 K. Representative adsorption isotherms of selenite for a-FeOOH and ferrihydrite are illustrated in Fig. 2. An idea of the uniformity or nonuniformity of the surface site is obtained from the values of isosteric heats as a function of adsorption density of selenite. The isosteric heats of adsorption were calculated from adsorption isotherms at two temperatures using the Clausius–Clapeyron equation (23) DHT Å R ln[(C2 /C1 )]/(1/T 2 0 1/T 1 ), FIG. 3. Isosteric heat of adsorption as a function of the selenite adsorption density. 1, 2, 3, and 4 are for b-, a-, g- and d-FeOOH, respectively.
Seads (% of selenium adsorbed) Å
Sein 0 Seeq 1 100, Sein
[1]
where DHT is the isosteric heat of adsorption in kJ/mol at a given adsorption density, R is the gas constant, and C1 and C2 are the equilibrium concentrations of the ion at tempera-
where Sein and Seeq are the initial and equilibrium concentrations of selenium, respectively. All spectrophotometric measurements were made with a Chemito 2500 recording UV – visible spectrophotometer using 10-mm matched quartz cells. The pH of the solutions at the beginning and end of experiments was measured and the average pH values are reported. All pH measurements were made by an Elico Digital pH meter ( Model LI 120 ) using a combined glass electrode ( Model CL 51 ) . The pH meter was standardized with NBS buffers before any measurement. The adsorption experiments under varying conditions of pH ( 3 – 10 ) , adsorbate concentration ( 0.01 – 0.1 mmol dm03 ) , adsorbent concentration ( 0.2 – 1.2 g / L ) , and temperature ( 298 – 3087K ) were also carried out using all the samples. All the experiments were carried out in duplicate. However, the data were reproducible within {3% error. RESULTS AND DISCUSSION
Effect of Time Kinetic experiments under varying conditions indicated that the adsorption of selenite on different forms of iron oxyhydroxides and ferrihydrite was fairly rapid and
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FIG. 4. Adsorption of selenite as a function of equilibrium pH. [Selenite] Å 3.79 mmol and adsorbent Å 250 mg/L. 1, 2, 3, 4, and 5 are for a-, b-, g-, d-FeOOH and ferrihydrite, respectively.
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FIG. 5. Adsorption of selenite as a function of adsorbent concentration. 1, 2, 3, 4, and 5 are for a-, b-, g-, d-FeOOH and ferrihydrite, respectively.
tures T 1 and T 2 and at the same adsorption density, respectively. If the isosteric heat of adsorption is independent of temperature, the surface is taken to be homogeneous. It is found that the isosteric heat of adsorption decreases with increasing adsorption density (see Fig. 3), suggesting that the surface of the oxyhydroxides/ferrihydrite is heterogeneous (18). This decrease in isosteric heat of adsorption may be due to different types of adsorption sites or the interaction of adsorbing ions (24). Effect of pH The adsorption of selenite on different polymorphs of iron oxyhydroxides and ferrihydrite as a function of pH and at a fixed selenite concentration is shown in Fig. 4. The adsorption of selenite at a particular initial concentration is higher at lower pH and progressively decreases with the increase in pH in accordance with K1
0 S{OH / H / / SeO 20 3 } S{SeO 3 / H2O
can occur if the chemical (i.e., specific) component dominates the electrostatic component. Stumm et al. (26) have also reported that the adsorption edge is not necessarily linked to the pHPZC or charge characteristics of the solid since the free energy of adsorption is a combination of chemical and electrostatic effects. Effect of Adsorbent Concentration The influence of varying concentrations of iron oxyhydroxides on the adsorption of selenite at a particular pH is presented in Fig. 5. The increase in adsorption, which occurs with increasing particle concentration, is nothing but a direct consequence of a greater amount of available binding sites for selenite. The distribution coefficient ( KD ) for selenite and iron oxyhydroxides at pH 3 was calculated by KD Å {[Se(IV)]ads /[Se(IV)]diss }(1/CP ),
[2]
[4]
K2
S{OH / 2H / / SeO 20 3 } S{HSeO3 / H2O, [3] where S{OH is a surface hydroxyl group and S{SeO 30 and S{HSeO3 are the adsorbed selenite species. This is a characteristic of anion adsorption (9). A similar trend has been previously observed in the case of goethite, clays, iron oxide, manganese dioxide, and hydrous aluminum oxide (1, 3, 13, 25). The data in Fig. 4 indicate that a significant amount of selenite adsorbed on different forms of iron oxyhydroxides even at pH values greater than pHPZC of iron oxyhydroxides ( Ç8.0), the pH at which the surface is negatively charged. The adsorption of an anion on a negatively charged surface
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where CP is the solid concentration in kg/L and KD has the units of L/kg; [Se(IV)]ads and [Se(IV)]diss are the concentrations in the particulate and dissolved phases, respectively. The distribution coefficient (KD ) for selenite and various adsorbents increases with increasing adsorbent concentration (Fig. 6). If the surface is homogeneous, the KD values at a given pH should not change with particle concentration. It is clear from Fig. 7 that the binding ability of the surface, as reflected in KD values, shows a decreasing trend with increasing adsorption density, which is consistent with the isosteric heat of adsorption (Fig. 3). All these observations suggest that the surface of oxyhydroxides/ferrihydrite is heterogeneous.
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FIG. 6. Log KD as a function of adsorbent concentration at equilibrium pH 3.3. 1, 2, 3, 4, and 5 are for a-, b-, g-, d-FeOOH and ferrihydrite, respectively.
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FIG. 8. The adsorption of selenite by a-FeOOH as a function of equilibrium pH at different selenite concentrations. a-FeOOH Å 250 mg/L.
Effect of Initial Selenite Concentration
FIG. 7. Log KD values as a function of the selenite adsorption density. 1 and 2 are for g- and d-FeOOH, respectively.
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The influence of varying selenite concentration on its adsorption as a function of pH is presented in Figs. 8 – 12. The locations of adsorption edges at different initial concentrations do not vary significantly. However, at a particular pH, the percentage of adsorption decreases with an increase in selenite concentration. The values of distribution coefficient ( KD ) for g-FeOOH decrease as the total selenite concentration increases or as the adsorption density increases ( Figs. 13 and 14 ) . Similar observations were also made in all other cases. The influence of the total selenite concentration on the percentage of selenite adsorption and on the KD value is consistent with the heterogeneous model ( 13 ) . It has been reported that selenite adsorption on oxides – hydroxides produced two types of complexes, SHSeO3 and SSeO 30 ) , via a ligand exchange mechanism ( 4 ) . Generally a two-step mechanism is involved in the adsorption and desorption reactions: ( i ) the formation of outer-sphere surface complexes ( SOH 2/ – HSeO 30 and SOH 2/ – SeO 320 ) through electrostatic attraction between the surface hy/ droxyl groups ( S – OH ) and HSeO 30 , SeO 20 3 , and H . ( ii ) In the second step divalent and monovalent selenite anions replace a molecular of water from the active sites of iron oxyhydroxide to form inner-sphere surface complexes SSeO 03 and SHSeO 30 ) . The various adsorption – desorption reactions may be delineated as shown in Scheme I.
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FIG. 9. The adsorption of selenite by b-FeOOH as a function of equilibrium pH at different selenite concentrations. b-FeOOH Å 250 mg / L.
FIG. 11. The adsorption of selenite by d-FeOOH as a function of equilibrium pH at different selenite concentrations. d-FeOOH Å 250 mg / L.
FIG. 10. The adsorption of selenite by g-FeOOH as a function of equilibrium pH at different selenite concentrations. g-FeOOH Å 250 mg / L.
FIG. 12. The adsorption of selenite by ferrihydrite as a function of equilibrium pH at different selenite concentrations. Ferrihydrite Å 250 mg / L.
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ADSORPTION OF SELENITE SOH1¤ 2 HSeO2 ‹ k⁄
k‹ k2‹
SHSeOƒ‹ 1 H¤O
k2⁄
‹ SOH 1 2H1 1 SeO 22
Kfi
Kfl
k¤ k2¤
SOH1¤ 2 SeO ‹22 1 H1
k› k2›
SSeO2‹ 1 H1 1 H¤O
SCHEME I.≥SOH is the surface hydroxyl group.
FIG. 13. Log KD values (for g-FeOOH) as a function of equilibrium selenite concentrations at different equilibrium pH (cf., Fig. 10).
This mechanism suggests that the removal of selenite from solution by adsorption on oxyhydroxides and oxides surfaces is influenced by the surface hydroxyl group, pH, concentra-
tion of available binding sites, and the equilibrium selenite concentration which is consistent with our result in the case of all of the oxyhydroxides. From the foregoing discussion it is clear that under identical conditions the adsorption capacity of iron oxyhydroxide / ferrihydrite increases in the order b-FeOOH õ a-FeOOH õ g-FeOOH õ d-FeOOH õ amorphous ferrihydrite, which is more or less similar to the trends of surface area, surface hydroxyl group, and total exchange capacity. Thus the most disordered form, i.e., amorphous ferrihydrite, shows the highest capacity for selenite adsorption due to its high surface area and surface hydroxyl groups, although the pHPZC values of all the oxyhydroxides remain in a narrow range. ACKNOWLEDGMENTS The authors are thankful to the Director for his permission to publish this paper and to the Ministry of Environment and Forest, Government of India, for financial assistance.
REFERENCES
FIG. 14. Log KD values (for g-FeOOH) as a function of selenite adsorption density at different equilibrium pH (cf., Fig. 11).
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1. Ghosh, M. M., Cox, C. D., and Yuan-pan, J. R., Environ. Prog. 13, 79 (1994). 2. Wang, S., Misra, M., Reddy, R. G., and Milbourne, J. C., Residues and effluents, in ‘‘Proceeding and Environmental Considerations’’ (R. G. Reddy, W. P. Imrie, and P. B. Queneau, Eds.), The Minerals, Metals & Material Society, 1991. 3. Balistrieri, L., and Chao, T. T., Geochim. Cosmochim. Acta 54, 739 (1990). 4. Zhang, P., and Sparks, D. L., Environ. Sci. Technol. 24, 1848 (1990). 5. Measures, C. I., and Burten, J. D., Earth Planet Sci. Lett. 46, 385 (1980). 6. Cary, E. E., Wiecaonek, G. A., and Allaway, W. H., Soil Sci. Soc. Am. Proc. 31, 21 (1967). 7. Geering, H. R., Cary, E. E., Jones, L. H. P., and Allaway, W. H., Soil Sci. Soc. Am. Proc. 32, 35 (1968). 8. Levesque, M., Can. J. Soil Sci. 54, 63 (1974). 9. Benjamin, M. M., Hayes, K. F., and Leckie, J. O., J. Water Pollut. Control Fed. 54, 1472 (1982). 10. Merrill, D. T., Manzione, M. A., Peterson, J. J., Parker, D. S., Chow, W., and Hobbs, A. O., J. Water Pollut. Control. Fed. 58, 18 (1986). 11. Balistrier, L. S., and Murray, J. W., Geochim. Cosmochim. Acta 51, 1151 (1987). 12. Balistrieri, L. S., and Murray, J. W., Geochim. Cosmochim. Acta 46, 1253 (1982). 13. Balistrieri, L. S., and Chao, T. T., Soil Sci. Soc. Am. 51, 1145 (1987).
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21. Parks, G. A., and de Bruyn, P. L., J. Phys. Chem. 66, 967 (1962). 22. Huseyin, A., Resat, A., and Izzet, T., Analyst 114, 1319 (1989). 23. Chiou, C. T., Shoup, T. D., and Porter, P. E., Org. Geochem. 8, 9 (1985). 24. Moore, W. J., ‘‘Physical Chemistry.’’ Prentice Hall, Englewood Cliffs, NJ, 1972. 25. Hingston, F. J., Posner, A. M., and Quirk, J. P., Adv. Chem. Ser. 79, 82 (1968). 26. Stumm, W., Huann, C. P., and Jenkins, S. R., Croat. Chim. Acta 42, 223 (1970).
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