- Email: [email protected]
Sorption of As(V) ions by akaganéite-type nanocrystals
Chemosphere 50 (2003) 155–163 www.elsevier.com/locate/chemosphere
Sorption of As(V) ions by akaganeite-type nanocrystals E.A. Deliyanni *, D.N. Bakoyannakis, A.I. Zouboulis, K.A. Matis Chemical Technology Division, Department of Chemistry, Aristotle University of Thessaloniki, P.O. Box 116, GR-54124 Thessaloniki, Greece Received 19 February 2001; received in revised form 30 April 2002; accepted 27 June 2002
Abstract A priority pollution problem, the removal of arsenate oxyanions from dilute aqueous solutions by sorption onto synthetic akaganeite (b-FeO(OH)) was the aim of the present study. This is an innovative inorganic adsorbent material prepared in the laboratory, following a new method of preparation. The effect of akaganeite and arsenate concentration, the contact time, temperature, solution pH value, and ionic strength variation on the treatment process was mainly investigated during this study. Typical adsorption isotherms were determined, which were found to fit sufficiently the typical Langmuir equation. The mechanism of sorption was examined by electrokinetic, X-ray diffraction, Fourier transmission infrared and scanning electron microscopy measurements. Promising results were obtained, due to the favourite characteristics of the adsorbent applied. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Arsenic; Arsenate; Akaganeite; Removal; Sorption; Adsorbent; Ionic strength
1. Introduction Arsenic occurrence and mobility in natural waters and its removal in water treatment have become the focus of increasing attention. Arsenic, a toxic and possible carcinogenic element (Matis et al., 1999a) is occurring in many natural waters, as well as in various industrial wastes, solid or liquid. It is one of the most toxic of contaminants found in the environment. It enters the environment from anthropogenic sources such as petroleum refineries, fossil fuel power plants, nonferrous smelting activities and from ceramics, semiconductors, pesticides and fertilizer production (Pierce and Moore, 1980). Leaching of arsenic into the groundwater may cause significant contamination. The most illustrative problem
*
Corresponding author. Tel.: +30-31-99-7766; fax: +30-3199-7759. E-mail address: [email protected] (E.A. Deliyanni).
created by this toxic element is certainly the example of Bangladesh crisis, i.e. the poisoning of potentially 70 million people from arsenic present in the water drawn from many wells, originally installed to solve shortages of drinking water supply (Lepkowski, 1998). Problems with arsenic in groundwaters have also been met in EU countries; pyrite (iron sulfide) and arsenopyrite minerals exist for example in Northern Greece, often associated with gold releasing arsenic to them (Kydros and Matis, 1995). Various techniques are employed for the removal of arsenic anion from water and wastewater. Precipitation, membrane processes, ion exchange and adsorption are the most common (Wilkie and Hering, 1996). A conventional method for arsenic removal from aqueous system is precipitation–coagulation with metal ions such as aluminium and ferric salts. However, the precipitation method is usually encumbered by problems of disposing toxic waste sludge, which is difficult to dry. To overcome this problem, ion exchange and adsorbent materials have been studied for their ability to remove arsenic anions.
0045-6535/03/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 5 - 6 5 3 5 ( 0 2 ) 0 0 3 5 1 - X
156
E.A. Deliyanni et al. / Chemosphere 50 (2003) 155–163
Iron oxides have been widely used as sorbents to remove contaminants from wastewater and liquid hazardous wastes. Removal has been attributed to ion exchange, specific adsorption to surface hydroxyl groups or coprecipitation. Hydrous ferric oxide is an important sorbent in both natural and engineering aquatic systems (Hayes et al., 1988). Different similar sorbent materials have been also used, including goethite (Matis et al., 1977; Paige et al., 1997) and ferric hydroxide (Matis et al., 1987). Another type of ferric products, perhaps less known, constitutes akaganeite (b-FeO(OH)). This sorbent was synthesized in the laboratory by precipitation from aqueous solution of Fe(III) chloride and ammonium carbonate, after a simple and easily reproducible method. Advantage of the sorbent, which found to be nanostructured, was its high surface area and narrow pore size distribution (Deliyanni et al., 2000). On the other hand the sorbent retained its high surface area and crystalline for long and even after its regeneration. Since the starting reagents are cheap and common, the sorbent can be characterized as a low cost and easily available sorbent.
2. Experimental Akaganeite (denoted hereafter as Ak) was prepared in the laboratory from an aqueous solution of iron(III) chloride salt (0.506 M) and ammonium carbonate (0.23 g dm3 ) as precipitating agent. The preparation of the gel was performed at 298 K. The ammonium carbonate solution was added dropwise at a constant flow rate (0:15 104 dm3 s1 ) under vigorous mechanical stirring at 600 rpm until pH 8 was achieved. The precipitate thus obtained was decanted in a dialysis tubing cellulose membrane and the latter was placed in a bath of distilled water. Upon standing, the anions of the suspension were removed by osmosis through the membrane. The resulting cake (on the membrane surface) was isolated from the mother liquor and freeze-dried. More details for the preparation have been previously published (Deliyanni et al., 2000). The obtained material appeared in the form of ultrafine powder. The BET surface area was 330 m2 g1 , and its average crystallite size was 2.6 nm, estimated by TEM (Deliyanni et al., 2000). All chemicals were reagent grade and they were used without further purification. All solutions were prepared with distilled water and all glassware was cleaned by soaking in 10% HCl and rinsed with deionised water. The arsenate stock solution was prepared from the dissolution of disodium hydrogen arsenate, Na2 HAsO4 7H2 O and acidified to the pH value of 3.5, in order to improve its stability and to avoid reduction to As(III) form. The examined concentrations were 5–20 mg l1 As(V). The pH of the suspension was modified, when required, by addition of nitric acid (0.1 M) or sodium
hydroxide solution (0.1 M) and was measured with a pH meter (Crison micropH 2002). The pH value of the akaganeite suspension (1 g l1 ) in the arsenic solution (10 mg l1 As(V)) was found to be 6, but when the background electrolyte (KNO3 ) was added, which was used for varying the ionic strength of akaganeite suspensions, the pH value was 7.5. The experiments for toxic ions removal from dilute aqueous solutions by the addition of this sorbent were carried out batchwise at ambient temperature, using deionised water and suitable conical flasks (100 ml sample volume), agitated with a reciprocal shaker (160 rpm) for 24 h. This contact time allows the dispersion of sorbent and metal to reach equilibrium conditions, as found during preliminary experiments. Constant temperature could also be maintained, when required. For the sorption experiments, akaganeite concentrations of 0.5–2 g l1 were usually applied. For the desorption experiments of As(V) from akaganeite, the sorbent sample was analytically determined to have previously adsorbed 20 mg l1 As(V). The elution experiments were performed for 24 h at 298 K. The residual arsenic, i.e. that remaining in solution after the application of solid/liquid separation of suspended solids by 0.45 m membrane filtration, was chemically analyzed. The molybdenum blue method was followed, using a double-beam UV–visible spectrophotometer (Hitachi Model U-2000) according to the appropriate standard method. Samples not analyzed on the same day of adsorption experiment, were acidified to about pH 1 with concentrated HCl and stored in acidwashed high-density polyethylene containers. All samples were analyzed within three days of collection. Total dissolved iron was analyzed by atomic absorption spectrophotometry (Perkin Elmer Model 2380), using a C2 H2 /O2 -flame. The quantity of arsenic sorbed onto synthetic akaganeite was expressed as percentage removal of arsenic, R%, except for the sorption isotherms, where it was expressed as mg of As per g of sorbent. In order to describe the adsorption results, the Freundlich and Langmuir equations were tested. The empirical Freundlich equation for dilute solutions is given by the relation: 1=n Qeq ¼ kCeq
where Qeq is the quantity of solute (As(V)) sorbed per unit weight of solid adsorbent (Ak), Ceq is the concentration of solute in the solution at equilibrium, k and 1/n are constants indicating the adsorption capacity and the adsorption intensity (1=n < 1). A familiar form of Langmuir equation for dilute solutions is given below: ðCeq =Qeq Þ ¼ 1=ðKQmax Þ þ Ceq ð1=Qmax Þ
E.A. Deliyanni et al. / Chemosphere 50 (2003) 155–163
where Qeq and Ceq are previously denoted, K is an energy term which varies as a function of surface coverage strictly due to variations in the heat of adsorption and Qmax is the maximum loading capacity. The electrokinetic measurements were conducted using a microelectrophoretic apparatus (type Mark II, Rank Brothers, UK) and expressed as zeta-potential values. The usually applied experimental conditions were 1 g l1 akaganeite, 10 mg l1 As(V), 0.1 M KNO3 , magnetic stirring for 30 min, contact time at temperature 300 K unless otherwise stated. The crystal structure of akaganeite, as well as that of this substance after sorption were examined by X-ray diffraction. Powdered XRD patterns were recorded with the help of a Siemens D500 X-ray diffractometer with autodivergent slit and graphite monochromator using CuKa radiation with scanning speed of 2° min1 . The characteristic reflection peaks (d-values) were matched, with JCPDS data files and the crystalline phases were identified. Infrared spectra of samples were received from 4000 to 200 cm1 in a KBr matrix with the help of a Perkin Elmer spectrophotometer. Scanning electron microscopy (SEM) images were used for qualitative surface characterization of akaganeite samples, which had to be previously covered with conductive gold. The instrument used was a JEOL JSM-6300 SEM.
157
Fig. 1. Dissolution of iron from akaganeite particles in different pH values for 24 h contact time.
3. Results and discussion 3.1. Arsenate removal The rate of adsorption of arsenate on akaganeite was fast with 90% completion after 3 h of stirring at pH 5 and ionic strength 0.1 M KNO3 . After 15 h, 99% of the maximum adsorption had taken place, thus 24 h was an adequate realistic time for equilibrium reasons. Adsorption solely due to electrostatic processes is usually very rapid, in the order of seconds (Pierce and Moore, 1982). The adsorption of arsenic in this paper was in the order of hours, which may indicate a specific adsorption. The pH of solution is generally known to play an important role in adsorption. The stability of akaganeite in suspensions (1 g l1 ) of varying pH values is shown in Fig. 1. The aqueous suspensions, following 24 h contact time, were filtered and analyzed for total redissolved iron. It can be seen that below pH 4, the concentration of ferric ions in aqueous phase was found to increase. Over this pH value the concentration of ferric ions in solution were found to be constant by near to zero, indicating the stability of akaganeite in this pH range. Arsenic adsorption onto akaganeite over the pH range from 4 to 12 was investigated for three different contact times. As it is shown in Fig. 2 the greater the contact time, the better adsorption removals were re-
Fig. 2. Effect of pH on As(V) removal for different contact time. Initial concentration of As(V) 20 mg l1 , sorbent concentration 2 g l1 , I ¼ 0:01 M KNO3 and temperature 298 K.
sulted; nevertheless, the differences were mainly observed at pH values greater than 7. It is known that at pH range between 3 and 6 pentavalent arsenic occurs mainly in the form of H2 AsO 4 , while the divalent anions HAsO2 4 dominate at higher pH values, between 8 and 10.5 (Matis et al., 1999a). 3.2. Influence of ionic strength The effect of ionic strength of solution on the sorption process of arsenate anions onto akaganeite is shown in Fig. 3. A common electrolyte, added at various concentrations (0–0.1 M KNO3 ) was applied for this investigation. As its concentration was increased, it was observed that the removal of arsenic was further improved (until saturation), when varying the amount of
158
E.A. Deliyanni et al. / Chemosphere 50 (2003) 155–163
Fig. 3. Effect of sorbent concentration on As(V) removal for different ionic strengths. Initial concentration of As(V) 20 mg l1 , pH ¼ 7:5, contact time 24 h and temperature 298 K.
Fig. 4. Effect of pH on As(V) removal for different ionic strengths. Initial concentration of As(V) 20 mg l1 , sorbent concentration 2 g l1 , contact time 3 h and temperature 298 K.
sorbent. At concentration of 0.1 M KNO3 it can be seen that the removal was always over 95%. The increased ionic strength of solution resulted in a marked shift in the position of the pH edge towards the alkaline region, and also improved the removal of arsenic in this range (between 7 and 12), as it is presented in Fig. 4. This improvement could be attributed to certain depression of negative solid surfaces charges in the alkaline region, caused by the presence of inorganic electrolyte, therefore enhancing the interactions between surface sites and arsenic oxyanions, as it was previously described for the case of Al oxides (Davis and Hem, 1989). For the lower (acidic) pH values (4.5–7), the adsorption percentage of arsenic removal was constant. Adsorption of As(V) over this pH range was found to be almost quantitative, unresponsive to ionic strength variations and not depended on pH values.
The effect of ionic strength on cation and anion adsorption onto oxides provides a measure of the relative bonding affinity of these ions for surface hydroxyl groups. For the case of anions, which bound relatively strongly to surface of oxides, the changing of ionic strength (0.001–1.0 M) was found to show little effect on fractional adsorption. In the case of more weakly bonding anions the amount adsorbed was reduced with increasing ionic strength (Hayes et al., 1988). These results are consistent with model predictions, if weakly bonding anions are modeled as ion-pair complexes and stronger bonding ions are modeled as inner-sphere surface coordination complexes. Therefore, it would be possible to distinguish between inner-sphere (coordination) and outer-sphere (ion-pair) anion surface complexes by studying the effect of ionic strength on anion partitioning. The results of the aforementioned experiments predicted a trend of increasing adsorption with increasing ionic strength, which was rather opposite to the above mentioned. The increased adsorption, with increasing ionic strength, could be attributed to cooperative effects in which adsorption of a particular adsorbate was enhanced rather, than inhibited in multi-adsorbate systems. In adsorption experiments with As(V), the presence of potassium cations could be the reason for the notified increased adsorption. The adsorption of As(V) above neutral pH values was lowered, due to the formation of negatively charged surface species. The apparent adsorption constants corresponding to the formation of these species were strongly affected by the electrostatic behaviour of akaganeite surface. Thus, in the absence of potassium cations as the pH values were increased above the point of zero charge (pzc) the coulombic contribution became increasingly unfavourable. However, the preliminary adsorption of potassium cations, resulted in a positive surface charge being maintained in this pH range, thus favouring anion adsorption. 3.3. Sorption isotherms In Fig. 5 the percentage removal of arsenic for different sorbent concentrations is presented, when the initial As(V) content was varied. It is obvious that increasing the amount of sorbent, the removal of arsenic was also increased. The prediction of sorption rate of pollutants under various conditions (pH, ionic strength etc.) onto natural or synthetic adsorbents has been commonly experienced using different models for engineering evaluation of examined materials. The experimental data of a typical sorption isotherm for akaganeite concentration of 0.5 g l1 is presented in Fig. 6(a). The same figure shows the individual isotherm represented by the Langmuir or Freundlich models. It is seen that both models fit the experimental data reasonably well. The values of correlation coefficient R2 , which
E.A. Deliyanni et al. / Chemosphere 50 (2003) 155–163
159
well as the correlation coefficients of the respecting straight lines. The maximum sorption capacity was found to be of the order of 120 mg As(V) per g of akaganeite at 298 K (1.79 mmol per g Ak or 0.16 mmol per mol iron), value which is comparatively higher than that previously reported (25 mg As(V) per g of goethite) for the pH value of 5 (Matis et al., 1999b), or with that of 40 mg As(V) per g of goethite for the pH value of 3 (Lepkowski, 1998). This advantage could be attributed to the higher surface area of akaganeite (having 330 m2 g1 as compared to the surface area of 132 m2 g1 of goethite), being important for an effective sorption to take place. Table 2 lists the Qmax values for different absorbents. The heat of adsorption was found to be 28 kcal mol1 (i.e. endothermic adsorption). Therefore, the overall reaction was favored by increasing temperature. In Fig. 7 arsenic elution was examined for the pH range from 2 to 12. For pH values higher than 6 although arsenic elution increased rapidly, it did not reached 100%, which may indicate a specific binding mechanism.
Fig. 5. Effect of initial arsenic concentration on As(V) removal for different sorbent concentrations. I ¼ 0:1 M KNO3 , pH ¼ 7:5, contact time 24 h and temperature 298 K.
is a measure of the goodness-of-fit, confirm the good representation of experimental data by these models, although better correlation was shown applying the Langmuir adsorption isotherm. The effect of three different temperatures (298, 308 and 318 K) was also examined. Plots using the Langmuir equation yielded straight lines for each of the three tested temperatures as presented in Fig. 6(b). Table 1 lists the calculated Langmuir and Freundlich constants, as
3.4. Process mechanism Specific adsorption of anions makes the surface of colloids more negatively charged. This results in a shift of the isoelectric point to lower pH value, i.e. more acidic. Therefore, lower isoelectric point of the system should be observed, if arsenate was specifically adsorbed
Fig. 6. (a) comparison between Freundlich and Langmuir isotherm and (b) adsorption isotherms using the linear Langmuir equation.
Table 1 Freundlich and Langmuir adsorption constants for arsenate Temperature (K) 298 308 318
Freundlich model
Langmuir model 2
k
1/n
R
2.4 3.6 5.2
0.30 0.28 0.27
0.964 0.992 0.934
Qmax (mmol/g)
K (L/mmol)
R2
1.79 2.24 2.89
32.7 49.0 81.8
0.995 0.999 0.999
160
E.A. Deliyanni et al. / Chemosphere 50 (2003) 155–163
Table 2 Comparison of Qmax values for different adsorbents Qmax mmol As(V)/g
pH
Adsorbent
Reference
1.79 2.01 1.79 1.34 0.34 1.25 0.15 0.37 0.59
7.5 4
Akaganeite Hydrous iron oxide Ferrihydrite Hydrous ferric oxide (HFO) AmFeOOH Goethite Fly ash Goethite Goethite
This paper Hsia et al. (1994) Fuller et al. (1992) Wilkie and Hering (1996) Pierce and Moore (1982) Rubin (1974) Diamantopoulos et al. (1993) Matis et al. (1999a,b) Matis et al. (1987)
4 7.5 5.6–6 7 5 3
Fig. 7. Effect of dispersion pH on percentage elution of As(V) from As-loaded akaganeite. Elution time 24 h and temperature 298 K.
Fig. 8. Comparison between the zeta-potential measurements (mV) of akaganeite particles in presence and absence of As(V).
on akaganeite. In Fig. 8 the electrokinetic measurements of the system under investigation is presented. The pzc
for akaganeite (Ak) was found to be initially 7.3, but when arsenic anions were adsorbed (Ak þ As), it decreased from this value to about 6. That means that the presence of arsenate ions reduced the negative zetapotential values of solely akaganeite particles in the pH range 3–11. Specific adsorption rather than a purely electrostastic adsorption process could be deduced from the drop of isoelectric point at the aqueous arsenate/ akaganeite interface. Positive identification of adsorbed arsenate species was provided using the FTIR technique. The infrared spectra of sodium arsenate, akaganeite containing adsorbed arsenate and akaganeite free of arsenate, are given in Fig. 9(a). The main absorption bands observed at 1653.5 and 849.8 cm1 were in close agreement with the reported spectrum of the feed, raw material (Na2 HAsO4 7H2 O), which has bands at 1636, 1177, 855, 720, 594 and 416 cm1 (Keller, 1996). Free arsenate ion has been indicated to contain two bands designated as v3 (878 cm1 ) and v4 (463 cm1 ) vibrations (Hsia et al., 1994). The absorption frequency of 850 cm1 was closely consistent with v3 values reported (Hsia et al., 1994). A shift of absorption bands should occur if the symmetry of the ion was lowered by interaction with akaganeite. Arsenate interacts with akaganeite surface by chemical bonding, as indicating by a comparison of the spectra before and after adsorption of arsenate. A new absorption band at 838.3 cm1 appeared, which was not observed in the original spectrum of akaganeite, and the band at 1653 cm1 was enhanced in the spectra of akaganeite after arsenate adsorption. These bands shifted slightly from the 850 cm1 peak observed in the spectrum of sodium arsenate. This was due to the fact that the symmetry of arsenate ion was lowered after the adsorption reaction. The spectra of akaganeite treated in aqueous solutions of three different arsenate concentrations (10, 50 and 100 mg l1 ) is presented in Fig. 9(b). A new absorption band at 838.3 cm1 appeared, which was not observed in the original spectrum of akaganeite, and which was enhanced as the concentration of the arse-
E.A. Deliyanni et al. / Chemosphere 50 (2003) 155–163
161
Fig. 9. (a) FTIR spectra of akaganeite (a), Na2 HAsO4 7H2 O (b) and akaganeite after arsenate sorption (c); (b) FTIR spectra of akaganeite treated in aqueous solutions of three different As(V) concentrations, (a) 10 mg l1 , (b) 50 mg l1 and (c) 100 mg l1 .
Fig. 10. X-ray patterns of akaganeite and akaganeite after As(V) sorption.
nate solution was increasing. From the FTIR spectrum analysis results, it was concluded that specific adsorption should occur at the aqueous arsenate/solid akaganeite interface. In Fig. 10 XRD patterns are presented for the sorbent, as well as for the sorbent after arsenic sorption at pH 7.5. The sorbent had the structure of the typical crystalline akaganeite. Following arsenic sorption, the loaded sorbent had the same structure of akaganeite. Two new bands appearing at 22° and 54° could be attributed to the formation of ferric arsenate (FeAsO4 ) precipitate. SEM photographs of the sorbent and of the sorbent after arsenic sorption, maintaining similar conditions with the experiments of adsorption isotherms, following filtration through a membrane and drying at room
Fig. 11. (a) SEM micrographs of akaganeite particles and (b) SEM micrographs of akaganeite particles after As(V) sorption.
162
E.A. Deliyanni et al. / Chemosphere 50 (2003) 155–163
temperature, are presented in Fig. 11. Comparing the akaganeite samples in the presence and absence of As(V) it was noticed that the unloaded sorbent particles were relatively smaller, i.e. the presence of As(V) caused a slight increase in size of akaganeite particulates, a fact that could be attributed to the additional formation of FeAsO4 precipitates (Nenov et al., 1994).
Akaganeite loses about 25–30% of its capacity with each regeneration so that it must be replaced after three or four regenerations. However the low cost, since the materials used for the preparation of akaganeite (aqueous solution of iron (III) chloride salt and ammonium carbonate) are cheap reagents and easily available, the simple preparation method and the relatively high capacity for arsenic, keep costs at an acceptable level.
4. Conclusions
References
Arsenic oxyanions removal from dilute aqueous streams is often of great interest. The adsorptive capacity of akaganeite was effective for arsenic oxyanions. The maximum load capacity was found to be about 100–120 mg As(V) per g of akaganeite, when 0.5 g l1 akaganeite was used at 298 K, which is higher in comparison with other sorbents. The amount of arsenate adsorption increased by lowering the pH of the system and by increasing the amount of the sorbent and the ionic strength of the system. For sorbent amount of 0.5 g l1 and ionic strength of 0.1 M (KNO3 ) the adsorption of arsenic was achieved equilibrium in the order of few hours, which possibly indicated a specific adsorption of arsenic species on the adsorbent. The zeta-potential of akaganeite and of akaganeite after arsenate adsorption was measured and FTIR spectra were received for the same conditions. Zetapotential measurements of the system containing As(V) indicated a lowering of isoelectric point of the akaganeite system, as a result of adsorbed arsenate. Arsenate absorption bands were found (by FTIR spectra) to develop and shift slightly on the spectrum of akaganeite, verifying that arsenate could specifically adsorbed on akaganeite surface. The influence of ionic strength was also examined and commented. Although the study of cooperative effects of existing adsorbates is by no means comprehensive, the observed results were consistent in comparison with model predictions, which suggest that electrostatic effects on oxide surfaces could, in certain instances, account for the enhancement of adsorption in multiadsorbate systems. Akaganeite has been shown to remove arsenic in a number of instances and it is believed to absorbs arsenic into its surface. Eventually the surface becomes saturated with arsenic and adequately removal is no longer accomplished. It is then necessary to be regenerated. This is done by subjecting akaganeite to a caustic bath, which appears to remove the surface layer of akaganeite and the arsenic absorbed into that layer. Akaganeite is then neutralized with a sulfuric acid rinse, and placed back into service. Regeneration of akaganeite is not complete; as it was presented in Fig. 8, 75% of arsenic was eluted after treatment with NaOH at pH 12.
Davis, J.A., Hem, J.D., 1989. The Environmental Chemistry of Aluminum. CRC Press, Boca Raton, FL. Deliyanni, E.A., Bakoyannakis, D.N., Zouboulis, A.I., Matis, K.A., Nalbandian, L., 2000. Akaganeite-type b-FeO(OH) nanocrystals: preparation and characterization. Micropor. Mesopor. Mater. 42, 49–57. Diamantopoulos, E., Ioannidis, S., Sakellaropoulos, G., 1993. As(V) removal from aqueous solutions by fly ash. Water Res. 12, 1773–1777. Fuller, C.C., Davis, J.A., Waychunas, G.A., 1992. Surface chemistry of ferrihydrite: Part 2. Kinetics of arsenate adsorption and coprecipitation. Geochim. Cosmochim. Acta 57, 2271–2282. Hayes, K.F., Papelis, Ch., Leckie, J.O., 1988. Modelling ionic strength effects on anion adsorption at hydrous oxide/ solution interfaces. J. Coll. Interf. Sci. 125, 717–726. Hsia, T.-H., Lo, S.-L., Lin, C.-F., Lee, D.-Y., 1994. Chemical and spectroscopic evidence for specific adsorption of chromate on hydrous iron oxide. Coll. Surf. A 85, 1–7. Keller, R.J., 1996. In: The Sigma Library of FT-IR Spectra, vol. 2. Sigma Chemical Co. Inc., St. Louis, MO. Kydros, K.A., Matis, K.A., 1995. Processing an auriferrous pyrite concentrate in the presence of reducing agents. Can. Met. Quart. 34, 15–20. Lepkowski, W., 1998. Arsenic crisis in Bangladesh. Chem. Eng. News 76, 27–29. Matis, K.A., Zouboulis, A.I., Malamas, F.B., Ramos Afonso, M.D., Hudson, M.J., 1977. Flotation removal of As(V) onto goethite. Environ. Pollut. 97, 239–245. Matis, K.A., Papadoyannis, I.N., Zouboulis, A.I., 1987. Separation of germanium and arsenic ions from effluents by flotation techniques. Int. J. Min. Process. 21, 83–92. Matis, K.A., Lehmann, M., Zouboulis, A.I., 1999a. Modelling sorption of metals from aqueous solution onto mineral particles: the case of arsenic ions and goethite ore. In: Misaelides, P. et al. (Eds.), Natural Microporous Materials in Environmental Technology. Kluwer Academic Publishers, The Netherlands, pp. 463–472. Matis, K.A., Zouboulis, A.I., Zamboulis, D., Valtadorou, A.V., 1999b. Sorption of As(V) by goethite particles and study of their flocculation. Water Air Soil Pollut. 111, 297–316. Nenov, V., Zouboulis, A.I., Dimitrova, N., Dobrevsky, I., 1994. As(III) removal from wastewaters using non-stoichiometric precipitation with ferric sulphate and adsorbing colloid flotation. Environ. Pollut. 83, 283–289. Paige, C.R., Snodgrass, W.J., Nicholson, R.V., Scharer, J.M., 1997. An arsenate effect on ferrihydrate dissolution kinetics under acidic oxide conditions. Water Res. 31, 2370–2382.
E.A. Deliyanni et al. / Chemosphere 50 (2003) 155–163 Pierce, M.L., Moore, C.B., 1980. Adsorption of arsenite on amorphous iron hydroxide from dilute aqueous solution. Environ. Sci. Technol. 14, 214–216. Pierce, M.L., Moore, C.B., 1982. Adsorption of arsenite and arsenate on amorphous iron hydroxide. Water Res. 16, 1247–1253.
163
Rubin, A.J., 1974. Chemistry of Water Supply. Ann Arbor Science, Michigan. Wilkie, J.A., Hering, J.G., 1996. Adsorption of arsenic onto hydrous ferric oxide: effects of adsorbate/adsorbent ratios and co-occurring solutes. Coll. Surf. A 107, 97–110.
Sorption of As(V) ions by akaganeite-type nanocrystals E.A. Deliyanni *, D.N. Bakoyannakis, A.I. Zouboulis, K.A. Matis Chemical Technology Division, Department of Chemistry, Aristotle University of Thessaloniki, P.O. Box 116, GR-54124 Thessaloniki, Greece Received 19 February 2001; received in revised form 30 April 2002; accepted 27 June 2002
Abstract A priority pollution problem, the removal of arsenate oxyanions from dilute aqueous solutions by sorption onto synthetic akaganeite (b-FeO(OH)) was the aim of the present study. This is an innovative inorganic adsorbent material prepared in the laboratory, following a new method of preparation. The effect of akaganeite and arsenate concentration, the contact time, temperature, solution pH value, and ionic strength variation on the treatment process was mainly investigated during this study. Typical adsorption isotherms were determined, which were found to fit sufficiently the typical Langmuir equation. The mechanism of sorption was examined by electrokinetic, X-ray diffraction, Fourier transmission infrared and scanning electron microscopy measurements. Promising results were obtained, due to the favourite characteristics of the adsorbent applied. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Arsenic; Arsenate; Akaganeite; Removal; Sorption; Adsorbent; Ionic strength
1. Introduction Arsenic occurrence and mobility in natural waters and its removal in water treatment have become the focus of increasing attention. Arsenic, a toxic and possible carcinogenic element (Matis et al., 1999a) is occurring in many natural waters, as well as in various industrial wastes, solid or liquid. It is one of the most toxic of contaminants found in the environment. It enters the environment from anthropogenic sources such as petroleum refineries, fossil fuel power plants, nonferrous smelting activities and from ceramics, semiconductors, pesticides and fertilizer production (Pierce and Moore, 1980). Leaching of arsenic into the groundwater may cause significant contamination. The most illustrative problem
*
Corresponding author. Tel.: +30-31-99-7766; fax: +30-3199-7759. E-mail address: [email protected] (E.A. Deliyanni).
created by this toxic element is certainly the example of Bangladesh crisis, i.e. the poisoning of potentially 70 million people from arsenic present in the water drawn from many wells, originally installed to solve shortages of drinking water supply (Lepkowski, 1998). Problems with arsenic in groundwaters have also been met in EU countries; pyrite (iron sulfide) and arsenopyrite minerals exist for example in Northern Greece, often associated with gold releasing arsenic to them (Kydros and Matis, 1995). Various techniques are employed for the removal of arsenic anion from water and wastewater. Precipitation, membrane processes, ion exchange and adsorption are the most common (Wilkie and Hering, 1996). A conventional method for arsenic removal from aqueous system is precipitation–coagulation with metal ions such as aluminium and ferric salts. However, the precipitation method is usually encumbered by problems of disposing toxic waste sludge, which is difficult to dry. To overcome this problem, ion exchange and adsorbent materials have been studied for their ability to remove arsenic anions.
0045-6535/03/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 5 - 6 5 3 5 ( 0 2 ) 0 0 3 5 1 - X
156
E.A. Deliyanni et al. / Chemosphere 50 (2003) 155–163
Iron oxides have been widely used as sorbents to remove contaminants from wastewater and liquid hazardous wastes. Removal has been attributed to ion exchange, specific adsorption to surface hydroxyl groups or coprecipitation. Hydrous ferric oxide is an important sorbent in both natural and engineering aquatic systems (Hayes et al., 1988). Different similar sorbent materials have been also used, including goethite (Matis et al., 1977; Paige et al., 1997) and ferric hydroxide (Matis et al., 1987). Another type of ferric products, perhaps less known, constitutes akaganeite (b-FeO(OH)). This sorbent was synthesized in the laboratory by precipitation from aqueous solution of Fe(III) chloride and ammonium carbonate, after a simple and easily reproducible method. Advantage of the sorbent, which found to be nanostructured, was its high surface area and narrow pore size distribution (Deliyanni et al., 2000). On the other hand the sorbent retained its high surface area and crystalline for long and even after its regeneration. Since the starting reagents are cheap and common, the sorbent can be characterized as a low cost and easily available sorbent.
2. Experimental Akaganeite (denoted hereafter as Ak) was prepared in the laboratory from an aqueous solution of iron(III) chloride salt (0.506 M) and ammonium carbonate (0.23 g dm3 ) as precipitating agent. The preparation of the gel was performed at 298 K. The ammonium carbonate solution was added dropwise at a constant flow rate (0:15 104 dm3 s1 ) under vigorous mechanical stirring at 600 rpm until pH 8 was achieved. The precipitate thus obtained was decanted in a dialysis tubing cellulose membrane and the latter was placed in a bath of distilled water. Upon standing, the anions of the suspension were removed by osmosis through the membrane. The resulting cake (on the membrane surface) was isolated from the mother liquor and freeze-dried. More details for the preparation have been previously published (Deliyanni et al., 2000). The obtained material appeared in the form of ultrafine powder. The BET surface area was 330 m2 g1 , and its average crystallite size was 2.6 nm, estimated by TEM (Deliyanni et al., 2000). All chemicals were reagent grade and they were used without further purification. All solutions were prepared with distilled water and all glassware was cleaned by soaking in 10% HCl and rinsed with deionised water. The arsenate stock solution was prepared from the dissolution of disodium hydrogen arsenate, Na2 HAsO4 7H2 O and acidified to the pH value of 3.5, in order to improve its stability and to avoid reduction to As(III) form. The examined concentrations were 5–20 mg l1 As(V). The pH of the suspension was modified, when required, by addition of nitric acid (0.1 M) or sodium
hydroxide solution (0.1 M) and was measured with a pH meter (Crison micropH 2002). The pH value of the akaganeite suspension (1 g l1 ) in the arsenic solution (10 mg l1 As(V)) was found to be 6, but when the background electrolyte (KNO3 ) was added, which was used for varying the ionic strength of akaganeite suspensions, the pH value was 7.5. The experiments for toxic ions removal from dilute aqueous solutions by the addition of this sorbent were carried out batchwise at ambient temperature, using deionised water and suitable conical flasks (100 ml sample volume), agitated with a reciprocal shaker (160 rpm) for 24 h. This contact time allows the dispersion of sorbent and metal to reach equilibrium conditions, as found during preliminary experiments. Constant temperature could also be maintained, when required. For the sorption experiments, akaganeite concentrations of 0.5–2 g l1 were usually applied. For the desorption experiments of As(V) from akaganeite, the sorbent sample was analytically determined to have previously adsorbed 20 mg l1 As(V). The elution experiments were performed for 24 h at 298 K. The residual arsenic, i.e. that remaining in solution after the application of solid/liquid separation of suspended solids by 0.45 m membrane filtration, was chemically analyzed. The molybdenum blue method was followed, using a double-beam UV–visible spectrophotometer (Hitachi Model U-2000) according to the appropriate standard method. Samples not analyzed on the same day of adsorption experiment, were acidified to about pH 1 with concentrated HCl and stored in acidwashed high-density polyethylene containers. All samples were analyzed within three days of collection. Total dissolved iron was analyzed by atomic absorption spectrophotometry (Perkin Elmer Model 2380), using a C2 H2 /O2 -flame. The quantity of arsenic sorbed onto synthetic akaganeite was expressed as percentage removal of arsenic, R%, except for the sorption isotherms, where it was expressed as mg of As per g of sorbent. In order to describe the adsorption results, the Freundlich and Langmuir equations were tested. The empirical Freundlich equation for dilute solutions is given by the relation: 1=n Qeq ¼ kCeq
where Qeq is the quantity of solute (As(V)) sorbed per unit weight of solid adsorbent (Ak), Ceq is the concentration of solute in the solution at equilibrium, k and 1/n are constants indicating the adsorption capacity and the adsorption intensity (1=n < 1). A familiar form of Langmuir equation for dilute solutions is given below: ðCeq =Qeq Þ ¼ 1=ðKQmax Þ þ Ceq ð1=Qmax Þ
E.A. Deliyanni et al. / Chemosphere 50 (2003) 155–163
where Qeq and Ceq are previously denoted, K is an energy term which varies as a function of surface coverage strictly due to variations in the heat of adsorption and Qmax is the maximum loading capacity. The electrokinetic measurements were conducted using a microelectrophoretic apparatus (type Mark II, Rank Brothers, UK) and expressed as zeta-potential values. The usually applied experimental conditions were 1 g l1 akaganeite, 10 mg l1 As(V), 0.1 M KNO3 , magnetic stirring for 30 min, contact time at temperature 300 K unless otherwise stated. The crystal structure of akaganeite, as well as that of this substance after sorption were examined by X-ray diffraction. Powdered XRD patterns were recorded with the help of a Siemens D500 X-ray diffractometer with autodivergent slit and graphite monochromator using CuKa radiation with scanning speed of 2° min1 . The characteristic reflection peaks (d-values) were matched, with JCPDS data files and the crystalline phases were identified. Infrared spectra of samples were received from 4000 to 200 cm1 in a KBr matrix with the help of a Perkin Elmer spectrophotometer. Scanning electron microscopy (SEM) images were used for qualitative surface characterization of akaganeite samples, which had to be previously covered with conductive gold. The instrument used was a JEOL JSM-6300 SEM.
157
Fig. 1. Dissolution of iron from akaganeite particles in different pH values for 24 h contact time.
3. Results and discussion 3.1. Arsenate removal The rate of adsorption of arsenate on akaganeite was fast with 90% completion after 3 h of stirring at pH 5 and ionic strength 0.1 M KNO3 . After 15 h, 99% of the maximum adsorption had taken place, thus 24 h was an adequate realistic time for equilibrium reasons. Adsorption solely due to electrostatic processes is usually very rapid, in the order of seconds (Pierce and Moore, 1982). The adsorption of arsenic in this paper was in the order of hours, which may indicate a specific adsorption. The pH of solution is generally known to play an important role in adsorption. The stability of akaganeite in suspensions (1 g l1 ) of varying pH values is shown in Fig. 1. The aqueous suspensions, following 24 h contact time, were filtered and analyzed for total redissolved iron. It can be seen that below pH 4, the concentration of ferric ions in aqueous phase was found to increase. Over this pH value the concentration of ferric ions in solution were found to be constant by near to zero, indicating the stability of akaganeite in this pH range. Arsenic adsorption onto akaganeite over the pH range from 4 to 12 was investigated for three different contact times. As it is shown in Fig. 2 the greater the contact time, the better adsorption removals were re-
Fig. 2. Effect of pH on As(V) removal for different contact time. Initial concentration of As(V) 20 mg l1 , sorbent concentration 2 g l1 , I ¼ 0:01 M KNO3 and temperature 298 K.
sulted; nevertheless, the differences were mainly observed at pH values greater than 7. It is known that at pH range between 3 and 6 pentavalent arsenic occurs mainly in the form of H2 AsO 4 , while the divalent anions HAsO2 4 dominate at higher pH values, between 8 and 10.5 (Matis et al., 1999a). 3.2. Influence of ionic strength The effect of ionic strength of solution on the sorption process of arsenate anions onto akaganeite is shown in Fig. 3. A common electrolyte, added at various concentrations (0–0.1 M KNO3 ) was applied for this investigation. As its concentration was increased, it was observed that the removal of arsenic was further improved (until saturation), when varying the amount of
158
E.A. Deliyanni et al. / Chemosphere 50 (2003) 155–163
Fig. 3. Effect of sorbent concentration on As(V) removal for different ionic strengths. Initial concentration of As(V) 20 mg l1 , pH ¼ 7:5, contact time 24 h and temperature 298 K.
Fig. 4. Effect of pH on As(V) removal for different ionic strengths. Initial concentration of As(V) 20 mg l1 , sorbent concentration 2 g l1 , contact time 3 h and temperature 298 K.
sorbent. At concentration of 0.1 M KNO3 it can be seen that the removal was always over 95%. The increased ionic strength of solution resulted in a marked shift in the position of the pH edge towards the alkaline region, and also improved the removal of arsenic in this range (between 7 and 12), as it is presented in Fig. 4. This improvement could be attributed to certain depression of negative solid surfaces charges in the alkaline region, caused by the presence of inorganic electrolyte, therefore enhancing the interactions between surface sites and arsenic oxyanions, as it was previously described for the case of Al oxides (Davis and Hem, 1989). For the lower (acidic) pH values (4.5–7), the adsorption percentage of arsenic removal was constant. Adsorption of As(V) over this pH range was found to be almost quantitative, unresponsive to ionic strength variations and not depended on pH values.
The effect of ionic strength on cation and anion adsorption onto oxides provides a measure of the relative bonding affinity of these ions for surface hydroxyl groups. For the case of anions, which bound relatively strongly to surface of oxides, the changing of ionic strength (0.001–1.0 M) was found to show little effect on fractional adsorption. In the case of more weakly bonding anions the amount adsorbed was reduced with increasing ionic strength (Hayes et al., 1988). These results are consistent with model predictions, if weakly bonding anions are modeled as ion-pair complexes and stronger bonding ions are modeled as inner-sphere surface coordination complexes. Therefore, it would be possible to distinguish between inner-sphere (coordination) and outer-sphere (ion-pair) anion surface complexes by studying the effect of ionic strength on anion partitioning. The results of the aforementioned experiments predicted a trend of increasing adsorption with increasing ionic strength, which was rather opposite to the above mentioned. The increased adsorption, with increasing ionic strength, could be attributed to cooperative effects in which adsorption of a particular adsorbate was enhanced rather, than inhibited in multi-adsorbate systems. In adsorption experiments with As(V), the presence of potassium cations could be the reason for the notified increased adsorption. The adsorption of As(V) above neutral pH values was lowered, due to the formation of negatively charged surface species. The apparent adsorption constants corresponding to the formation of these species were strongly affected by the electrostatic behaviour of akaganeite surface. Thus, in the absence of potassium cations as the pH values were increased above the point of zero charge (pzc) the coulombic contribution became increasingly unfavourable. However, the preliminary adsorption of potassium cations, resulted in a positive surface charge being maintained in this pH range, thus favouring anion adsorption. 3.3. Sorption isotherms In Fig. 5 the percentage removal of arsenic for different sorbent concentrations is presented, when the initial As(V) content was varied. It is obvious that increasing the amount of sorbent, the removal of arsenic was also increased. The prediction of sorption rate of pollutants under various conditions (pH, ionic strength etc.) onto natural or synthetic adsorbents has been commonly experienced using different models for engineering evaluation of examined materials. The experimental data of a typical sorption isotherm for akaganeite concentration of 0.5 g l1 is presented in Fig. 6(a). The same figure shows the individual isotherm represented by the Langmuir or Freundlich models. It is seen that both models fit the experimental data reasonably well. The values of correlation coefficient R2 , which
E.A. Deliyanni et al. / Chemosphere 50 (2003) 155–163
159
well as the correlation coefficients of the respecting straight lines. The maximum sorption capacity was found to be of the order of 120 mg As(V) per g of akaganeite at 298 K (1.79 mmol per g Ak or 0.16 mmol per mol iron), value which is comparatively higher than that previously reported (25 mg As(V) per g of goethite) for the pH value of 5 (Matis et al., 1999b), or with that of 40 mg As(V) per g of goethite for the pH value of 3 (Lepkowski, 1998). This advantage could be attributed to the higher surface area of akaganeite (having 330 m2 g1 as compared to the surface area of 132 m2 g1 of goethite), being important for an effective sorption to take place. Table 2 lists the Qmax values for different absorbents. The heat of adsorption was found to be 28 kcal mol1 (i.e. endothermic adsorption). Therefore, the overall reaction was favored by increasing temperature. In Fig. 7 arsenic elution was examined for the pH range from 2 to 12. For pH values higher than 6 although arsenic elution increased rapidly, it did not reached 100%, which may indicate a specific binding mechanism.
Fig. 5. Effect of initial arsenic concentration on As(V) removal for different sorbent concentrations. I ¼ 0:1 M KNO3 , pH ¼ 7:5, contact time 24 h and temperature 298 K.
is a measure of the goodness-of-fit, confirm the good representation of experimental data by these models, although better correlation was shown applying the Langmuir adsorption isotherm. The effect of three different temperatures (298, 308 and 318 K) was also examined. Plots using the Langmuir equation yielded straight lines for each of the three tested temperatures as presented in Fig. 6(b). Table 1 lists the calculated Langmuir and Freundlich constants, as
3.4. Process mechanism Specific adsorption of anions makes the surface of colloids more negatively charged. This results in a shift of the isoelectric point to lower pH value, i.e. more acidic. Therefore, lower isoelectric point of the system should be observed, if arsenate was specifically adsorbed
Fig. 6. (a) comparison between Freundlich and Langmuir isotherm and (b) adsorption isotherms using the linear Langmuir equation.
Table 1 Freundlich and Langmuir adsorption constants for arsenate Temperature (K) 298 308 318
Freundlich model
Langmuir model 2
k
1/n
R
2.4 3.6 5.2
0.30 0.28 0.27
0.964 0.992 0.934
Qmax (mmol/g)
K (L/mmol)
R2
1.79 2.24 2.89
32.7 49.0 81.8
0.995 0.999 0.999
160
E.A. Deliyanni et al. / Chemosphere 50 (2003) 155–163
Table 2 Comparison of Qmax values for different adsorbents Qmax mmol As(V)/g
pH
Adsorbent
Reference
1.79 2.01 1.79 1.34 0.34 1.25 0.15 0.37 0.59
7.5 4
Akaganeite Hydrous iron oxide Ferrihydrite Hydrous ferric oxide (HFO) AmFeOOH Goethite Fly ash Goethite Goethite
This paper Hsia et al. (1994) Fuller et al. (1992) Wilkie and Hering (1996) Pierce and Moore (1982) Rubin (1974) Diamantopoulos et al. (1993) Matis et al. (1999a,b) Matis et al. (1987)
4 7.5 5.6–6 7 5 3
Fig. 7. Effect of dispersion pH on percentage elution of As(V) from As-loaded akaganeite. Elution time 24 h and temperature 298 K.
Fig. 8. Comparison between the zeta-potential measurements (mV) of akaganeite particles in presence and absence of As(V).
on akaganeite. In Fig. 8 the electrokinetic measurements of the system under investigation is presented. The pzc
for akaganeite (Ak) was found to be initially 7.3, but when arsenic anions were adsorbed (Ak þ As), it decreased from this value to about 6. That means that the presence of arsenate ions reduced the negative zetapotential values of solely akaganeite particles in the pH range 3–11. Specific adsorption rather than a purely electrostastic adsorption process could be deduced from the drop of isoelectric point at the aqueous arsenate/ akaganeite interface. Positive identification of adsorbed arsenate species was provided using the FTIR technique. The infrared spectra of sodium arsenate, akaganeite containing adsorbed arsenate and akaganeite free of arsenate, are given in Fig. 9(a). The main absorption bands observed at 1653.5 and 849.8 cm1 were in close agreement with the reported spectrum of the feed, raw material (Na2 HAsO4 7H2 O), which has bands at 1636, 1177, 855, 720, 594 and 416 cm1 (Keller, 1996). Free arsenate ion has been indicated to contain two bands designated as v3 (878 cm1 ) and v4 (463 cm1 ) vibrations (Hsia et al., 1994). The absorption frequency of 850 cm1 was closely consistent with v3 values reported (Hsia et al., 1994). A shift of absorption bands should occur if the symmetry of the ion was lowered by interaction with akaganeite. Arsenate interacts with akaganeite surface by chemical bonding, as indicating by a comparison of the spectra before and after adsorption of arsenate. A new absorption band at 838.3 cm1 appeared, which was not observed in the original spectrum of akaganeite, and the band at 1653 cm1 was enhanced in the spectra of akaganeite after arsenate adsorption. These bands shifted slightly from the 850 cm1 peak observed in the spectrum of sodium arsenate. This was due to the fact that the symmetry of arsenate ion was lowered after the adsorption reaction. The spectra of akaganeite treated in aqueous solutions of three different arsenate concentrations (10, 50 and 100 mg l1 ) is presented in Fig. 9(b). A new absorption band at 838.3 cm1 appeared, which was not observed in the original spectrum of akaganeite, and which was enhanced as the concentration of the arse-
E.A. Deliyanni et al. / Chemosphere 50 (2003) 155–163
161
Fig. 9. (a) FTIR spectra of akaganeite (a), Na2 HAsO4 7H2 O (b) and akaganeite after arsenate sorption (c); (b) FTIR spectra of akaganeite treated in aqueous solutions of three different As(V) concentrations, (a) 10 mg l1 , (b) 50 mg l1 and (c) 100 mg l1 .
Fig. 10. X-ray patterns of akaganeite and akaganeite after As(V) sorption.
nate solution was increasing. From the FTIR spectrum analysis results, it was concluded that specific adsorption should occur at the aqueous arsenate/solid akaganeite interface. In Fig. 10 XRD patterns are presented for the sorbent, as well as for the sorbent after arsenic sorption at pH 7.5. The sorbent had the structure of the typical crystalline akaganeite. Following arsenic sorption, the loaded sorbent had the same structure of akaganeite. Two new bands appearing at 22° and 54° could be attributed to the formation of ferric arsenate (FeAsO4 ) precipitate. SEM photographs of the sorbent and of the sorbent after arsenic sorption, maintaining similar conditions with the experiments of adsorption isotherms, following filtration through a membrane and drying at room
Fig. 11. (a) SEM micrographs of akaganeite particles and (b) SEM micrographs of akaganeite particles after As(V) sorption.
162
E.A. Deliyanni et al. / Chemosphere 50 (2003) 155–163
temperature, are presented in Fig. 11. Comparing the akaganeite samples in the presence and absence of As(V) it was noticed that the unloaded sorbent particles were relatively smaller, i.e. the presence of As(V) caused a slight increase in size of akaganeite particulates, a fact that could be attributed to the additional formation of FeAsO4 precipitates (Nenov et al., 1994).
Akaganeite loses about 25–30% of its capacity with each regeneration so that it must be replaced after three or four regenerations. However the low cost, since the materials used for the preparation of akaganeite (aqueous solution of iron (III) chloride salt and ammonium carbonate) are cheap reagents and easily available, the simple preparation method and the relatively high capacity for arsenic, keep costs at an acceptable level.
4. Conclusions
References
Arsenic oxyanions removal from dilute aqueous streams is often of great interest. The adsorptive capacity of akaganeite was effective for arsenic oxyanions. The maximum load capacity was found to be about 100–120 mg As(V) per g of akaganeite, when 0.5 g l1 akaganeite was used at 298 K, which is higher in comparison with other sorbents. The amount of arsenate adsorption increased by lowering the pH of the system and by increasing the amount of the sorbent and the ionic strength of the system. For sorbent amount of 0.5 g l1 and ionic strength of 0.1 M (KNO3 ) the adsorption of arsenic was achieved equilibrium in the order of few hours, which possibly indicated a specific adsorption of arsenic species on the adsorbent. The zeta-potential of akaganeite and of akaganeite after arsenate adsorption was measured and FTIR spectra were received for the same conditions. Zetapotential measurements of the system containing As(V) indicated a lowering of isoelectric point of the akaganeite system, as a result of adsorbed arsenate. Arsenate absorption bands were found (by FTIR spectra) to develop and shift slightly on the spectrum of akaganeite, verifying that arsenate could specifically adsorbed on akaganeite surface. The influence of ionic strength was also examined and commented. Although the study of cooperative effects of existing adsorbates is by no means comprehensive, the observed results were consistent in comparison with model predictions, which suggest that electrostatic effects on oxide surfaces could, in certain instances, account for the enhancement of adsorption in multiadsorbate systems. Akaganeite has been shown to remove arsenic in a number of instances and it is believed to absorbs arsenic into its surface. Eventually the surface becomes saturated with arsenic and adequately removal is no longer accomplished. It is then necessary to be regenerated. This is done by subjecting akaganeite to a caustic bath, which appears to remove the surface layer of akaganeite and the arsenic absorbed into that layer. Akaganeite is then neutralized with a sulfuric acid rinse, and placed back into service. Regeneration of akaganeite is not complete; as it was presented in Fig. 8, 75% of arsenic was eluted after treatment with NaOH at pH 12.
Davis, J.A., Hem, J.D., 1989. The Environmental Chemistry of Aluminum. CRC Press, Boca Raton, FL. Deliyanni, E.A., Bakoyannakis, D.N., Zouboulis, A.I., Matis, K.A., Nalbandian, L., 2000. Akaganeite-type b-FeO(OH) nanocrystals: preparation and characterization. Micropor. Mesopor. Mater. 42, 49–57. Diamantopoulos, E., Ioannidis, S., Sakellaropoulos, G., 1993. As(V) removal from aqueous solutions by fly ash. Water Res. 12, 1773–1777. Fuller, C.C., Davis, J.A., Waychunas, G.A., 1992. Surface chemistry of ferrihydrite: Part 2. Kinetics of arsenate adsorption and coprecipitation. Geochim. Cosmochim. Acta 57, 2271–2282. Hayes, K.F., Papelis, Ch., Leckie, J.O., 1988. Modelling ionic strength effects on anion adsorption at hydrous oxide/ solution interfaces. J. Coll. Interf. Sci. 125, 717–726. Hsia, T.-H., Lo, S.-L., Lin, C.-F., Lee, D.-Y., 1994. Chemical and spectroscopic evidence for specific adsorption of chromate on hydrous iron oxide. Coll. Surf. A 85, 1–7. Keller, R.J., 1996. In: The Sigma Library of FT-IR Spectra, vol. 2. Sigma Chemical Co. Inc., St. Louis, MO. Kydros, K.A., Matis, K.A., 1995. Processing an auriferrous pyrite concentrate in the presence of reducing agents. Can. Met. Quart. 34, 15–20. Lepkowski, W., 1998. Arsenic crisis in Bangladesh. Chem. Eng. News 76, 27–29. Matis, K.A., Zouboulis, A.I., Malamas, F.B., Ramos Afonso, M.D., Hudson, M.J., 1977. Flotation removal of As(V) onto goethite. Environ. Pollut. 97, 239–245. Matis, K.A., Papadoyannis, I.N., Zouboulis, A.I., 1987. Separation of germanium and arsenic ions from effluents by flotation techniques. Int. J. Min. Process. 21, 83–92. Matis, K.A., Lehmann, M., Zouboulis, A.I., 1999a. Modelling sorption of metals from aqueous solution onto mineral particles: the case of arsenic ions and goethite ore. In: Misaelides, P. et al. (Eds.), Natural Microporous Materials in Environmental Technology. Kluwer Academic Publishers, The Netherlands, pp. 463–472. Matis, K.A., Zouboulis, A.I., Zamboulis, D., Valtadorou, A.V., 1999b. Sorption of As(V) by goethite particles and study of their flocculation. Water Air Soil Pollut. 111, 297–316. Nenov, V., Zouboulis, A.I., Dimitrova, N., Dobrevsky, I., 1994. As(III) removal from wastewaters using non-stoichiometric precipitation with ferric sulphate and adsorbing colloid flotation. Environ. Pollut. 83, 283–289. Paige, C.R., Snodgrass, W.J., Nicholson, R.V., Scharer, J.M., 1997. An arsenate effect on ferrihydrate dissolution kinetics under acidic oxide conditions. Water Res. 31, 2370–2382.
E.A. Deliyanni et al. / Chemosphere 50 (2003) 155–163 Pierce, M.L., Moore, C.B., 1980. Adsorption of arsenite on amorphous iron hydroxide from dilute aqueous solution. Environ. Sci. Technol. 14, 214–216. Pierce, M.L., Moore, C.B., 1982. Adsorption of arsenite and arsenate on amorphous iron hydroxide. Water Res. 16, 1247–1253.
163
Rubin, A.J., 1974. Chemistry of Water Supply. Ann Arbor Science, Michigan. Wilkie, J.A., Hering, J.G., 1996. Adsorption of arsenic onto hydrous ferric oxide: effects of adsorbate/adsorbent ratios and co-occurring solutes. Coll. Surf. A 107, 97–110.