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Effective Cr(VI) Removal from Simulated Groundwater through the Hydrotalcite-Derived Adsorbent Yunfeng Xu, Jia Zhang,† Guangren Qian,*,† Zhong Ren,† Zhi Ping Xu,*,‡ Yueying Wu,† Qiang Liu,† and Shizhang Qiao‡ School of EnVironmental and Chemical Engineering, Shanghai UniVersity, Shanghai 200072, People’s Republic of China, and Australian Research Council (ARC) Centre of Excellence for Functional Nanomaterials, Australian Institute for Bioengineering and Nanotechnology and School of Engineering, The UniVersity of Queensland, Brisbane, QLD 4072, Australia
We investigated the feasibility of using calcined hydrotalcite (CHT) as the adsorbent of chromate to treat Cr(VI)-contaminated water through column tests under varied conditions. The column tests reveal that CHT can take up 34.3-44.7 mg(Cr)/g when the Cr(VI) concentration in the influent varies over a range of 50-200 mg/L (e.g., 0.96-3.85 mM) with pH 6-7 at 298 K. This uptake capacity is only reduced to 29.1 mg(Cr)/g when HCO3- (1.0 mM) and Cl- (1.0 mM) coexist in the influent. We note that the treated water is of high quality and is free of Cr(VI), with Mg and Al concentrations of 0.96 in all cases and the simulated curves fit well with the data points at CO/CI < 0.9 (see Figure 1). As listed in Table 1, both the initial sorption rate constant (κ0) and the deactivation rate constant (κd) are dependent on the adsorbent amount and the inlet Cr(VI) concentration used in the tests. The deactivation rate (κd) increases as the Cr(VI) concentration increases, but it decreases as the adsorbent amount in the column increases, as reflected by the steeper breakthrough curve when a small amount of CHT adsorbent is filled in the column. In contrast, the adsorption rate constant (κ0) is very similar in the three different Cr(VI) concentrations (W ) 53 g), but it decreases with the increase of the adsorbent amount used at [Cr(VI)] ) 50 mg/L increases. In our opinion, the dimensionless term [κ0(W/Q)] may more exactly reveal the initial adsorption rate. As indicated in Table 1, the column breakthrough parameters [κ0(W/Q)] and the effective adsorption amount (Qeff)) are almost constant ((20-30%) under varied conditions, showing a relatively stable performance in adsorbing Cr(VI) when CHT is used under the varied situations. Detail inspection indicates that term κ0(W/Q) is slightly smaller in the cases of 53 g of CHT filled in the column than in the other two situations (40 and 161 g). Based on eq 1, the smaller κ0(W/Q) term will result in a relatively longer breakthrough time, and thus a slightly higher effective adsorption amount (Qeff) (see Table 1). 3.3. Quality of Treated Water. The quality of treated water was also evaluated in terms of pH and leaching level of Mg and Al from the sorbent during the test. Figure 2 shows the concentrations of Mg, Al, and Cr(VI), as well as the pH, in the effluent over time. In the initial period of adsorption (0-24 h), it seems that a small portion of Mg and Al is leached out from the adsorbent into water, giving a Mg/Al concentration of 10-15 mg/L and a pH of 8-9. After this period, both Mg and Al leaching levels are limited, with the concentration below 5-10 mg/L, and pH stabilized at 6.5-7 (which is common in all cases), and [Cr(VI)] < MCL most of the time (see Table 1). When 161 g of adsorbent was used (Table 1), the treatment efficiency is even better, because tMCL (1547 h) is only 40 h shorter than t5 (1587 h). During the breakthrough period, the Cr(VI) concentration in the effluent gradually recovers with time. It is very interesting to note that Mg leaching after the
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Figure 3. Desorption profile of Cr(VI) from the Cr(VI)-adsorbed CHT collected from the column filled with 161 g of CHT: (a) 0.25 g of collected sample in 50 mL of 0.1 M NaHCO3 solution at 298 K and (b) 0.25 g of collected sample in 50 mL of 0.5 M NaHCO3 solution at 323 K.
Figure 2. Concentrations of Cr(VI) (denoted by open triangles, 4), Mg (denoted by open squares, 0), and Al (denoted by large solid circles, b), and the pH (denoted by small solid circles, •) in the outflow over the column filled with 53 g of CHT in the case of the inflow containing (A) 50 mg/L Cr(VI) and (B) 50 mg/L Cr(VI), 1.0 mM NaHCO3 and 1.0 mM NaCl.
breakthrough increases simultaneously from a few mg/L to 15-20 mg/L, until adsorption of the Cr(VI) from the stream no longer occurs, while the Al leaching is still limited ( 1.0). The increased amount (20-30 mg/L) is attributed to the leaching of adsorbed CrO42- through the exchange with HCO3-/CO32-, as discussed shortly in the following section. In brief, Mg and Al leaching occurs only in the initial period and is very much limited afterward, until the breakthrough; the pH is very stable at 6.5-7 after the initial adsorption period, and the residue Cr(VI) concentration is below the MCL level most of the time before 5% breakthrough (particularly, the case of 161 g of adsorbent in Table 1), indicating the high quality of the treated water. 3.4. Recovery of Cr(VI) and Regeneration of CHT. We have also found that Cr(VI) undergoes quick desorption from the collected Cr(VI)-CHT samples in NaHCO3 solution via the anion exchange process. As shown in Figure 3, ∼75% adsorbed
Cr(VI) was released within 0.5 h at room temperature in 0.1 M NaHCO3 solution, attributed to the higher affinity of carbonate for hydrotalcite.23 Desorption for 6 h only increases the percentage from ca. 75% to ca. 80%. We observed that, in the warm NaHCO3 solution (323 K) with a higher concentration (0.5 M), up to 90% Cr(VI) was released within 0.5 h. Here, heating accelerates the anion exchange and the high bicarbonate concentration facilitates the desorption of more chromate. In this way, most Cr(VI) can be quickly recovered. Afterward, the solid sample was collected and dried, with the phase of hydrotalcite-like carbonate. The dried solid sample could then be calcined into new adsorbent, such as the initial CHT at 673 K.23,25,29,30 Therefore, in a real application, we can build a CHT-filled column to remove Cr(VI) from the contaminated groundwater. Then, just before the breakthrough, we can replace the adsorbent and recover Cr(VI) as a valuable chemical via a desorption process. Finally, we can regenerate the adsorbent via further calcination. Further calcination progressively diminishes the adsorption capacity,31 but we can regenerate the adsorbent for a few times with enough high adsorption capacity. Therefore, recovery of Cr(VI) and regeneration of CHT minimize the operation cost. 3.5. Cr(VI) Adsorption Process. At pH 6-7, Cr(VI) mainly exists in the form of HCrO4- and CrO42- in the Cr(VI) solution (refer to Figure S1 in the Supporting Information). In the current column test, the calcined hydrotalcite is assumed to be reconstructed to the layered structure in the initial period, as represented by the following reaction: Mg2.25AlO3.75 +
( nx )A
n-
+ 3.75H2O f
Mg2.25Al(OH)6.5(An-)x/n(OH)1-x + xOH-
(2)
where An- denotes any anions that are available for the intercalation, including HCrO4-/CrO42-, and HCO3-, OH-, and Cl-. In exchange for other anions, some OH- is leached, which results in a higher pH (8-9) at the initial stage. The reconstruction of CHT to the layered structure normally takes half an hour to a few hours at room temperature.32 During the reconstruction of CHT, HCrO4-/CrO42- should be completely adsorbed by the bottom CHT in the column via adsorption/intercalation, and the similar adsorption mechanism is used in the batch test for Cr(VI) removal.25 On the other hand, most CHT (middle and top in
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Figure 4. XRD spectra of original HT (spectrum a); calcined HT (CHT) (the two asterisk-marked peaks are characteristic of a rock salt phase (MgO)) (spectrum b); and reconstructed CHT in water (spectrum c). Also shown are spectra from CHT adsorbents collected from the bottom (spectrum d) and top (spectrum e) of the column filled with 161 g of CHT.
the column) seems to purely undergo the layer reconstruction. The reconstruction involves a dissolution-deposition-diffusion process (the so-called “3-D process”), as proposed by Xu and Lu.33 The 3-D process consists of the dissolution of Mg2+ and Al(OH)4- from the mixed oxide in the initial step, deposition of these ionic species onto the particle surface to grow as a new layer of HT phase with intercalation of anions (OH- and CrO42-), and ion diffusion within and between the layers to form a better HT phase. After reconstruction, HCrO4-/CrO42- removal is continued via the quick adsorption onto the surface and then the ratedeterminant exchanging reaction when HCrO4-/CrO42- contact the reconstructed CHT bed: Mg2.25Al(OH)6.5(CrO42-)x/2(B-)1-x +
( y -2 x )CrO
2-
4
Mg2.25Al(OH)6.5(CrO4 )y/2(B)1-y + (y - x)B 2-
f -
(3)
where B- represents OH-, Cl-, HCO3-/CO32-, etc. When the exchange reaches the equilibrium in the lower bed, HCrO4-/CrO42- contact the higher reconstructed-CHT bed, being similarly adsorbed and exchanged, until it reaches the top of the column, where Cr(VI) starts the breakthrough. This is the main difference between the batch and column experiments, because such an exchanging reaction rarely occurs in the batch test. After the full breakthrough, the similar anion exchange will occur between the adsorbed CrO42- and anions such as HCO3-/CO32- (carbonate is always existing in a low concentration), so that some adsorbed CrO42- is replaced into the solution and the Cr(VI) concentration in the effluent after the breakthrough is higher than that in the influent, i.e., CO/CI > 1.0 (see Figure 1B), in particular, in the presence of 1.0 mM HCO3-/CO32- and 1.0 mM Cl- (see Figure 2B and content highlighted in Figure
Figure 5. FTIR spectra of samples collected from the bottom (spectrum a) and the top (spectrum b) of the column passed by the influent containing coexisting anions (53 g of CHT); from the bottom (spectrum c) and the top (spectrum d) of the column passed by the influent containing only chromate (161 g of CHT); and that of purchased hydrotalcite (spectrum e).
1B). Because HCO3-/CO32- has a stronger affinity for hydrotalcite-like materials than CrO42-,23 we can readily recover 70%-80% of the CrO42- from Cr(VI)-adsorbed CHT in NaHCO3 solution (see Figure 3). It is true that the layered structure is completely regenerated in pure water after 24 h, as clearly reflected by the appearance of all characteristic XRD peaks of hydrotalcite in Figure 4 (spectrum c). The purchased HT shows sharp XRD peaks (spectrum a), indicating a well-crystallized HT-phase with a layer spacing of 0.76 nm. After calcination at 673 K for 4 h, most of the HT phase is collapsed and transferred to a mixed oxide (MgO salt phase, spectrum b in Figure 4). The incomplete decomposition of HT to mixed oxide gives a weight loss of 41%, which is smaller than the estimated 45% calculated from Mg2.25Al(OH)6.5(CO3)0.5 · 2H2O. After 24 h of contact with water at room temperature, the layered structure is reconstructed. The layer spacing of the reconstructed hydrotalcite is 0.77 nm (spectrum c in Figure 4), revealing that the main interlayer anions are OH- as well as HCO3- (adsorbed CO2 from air). After the column test, the adsorbent was collected and dried. The samples collected at the bottom and on the top show a similar layered structure, with a layer spacing of 0.77 nm (see spectra d and e in Figure 4). This implies that after the breakthrough, HCO3-/CO32-, OH-, and Cl- (in the simulated case) are the major interlayer anions, and adsorbed chromate constitutes just a portion. This is because, if chromate is the dominant anion in the interlayer, then the layer spacing will be expanded to ca. 0.94 nm, as estimated from the addition of the hydroxide layer thickness (0.48 nm) and the height of CrO42- (0.46 nm, see Figure S2 in the Supporting Information).25 Supportively, the effective adsorption amount (Qeff) is also consistent with the XRD
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patterns. Theoretically, 1.0 g of CHT (MW ≈ 142 for Mg2.25AlO3.75) can adsorb 7.0 mmol of equivalent charges. Supposing that Qeff is 41 mg/g (i.e., 0.80 mmol/g, 1.60 equivalent charges/g in CrO42- form); thus, the portion of chromate in the interlayer is only 23%, in terms of the equivalent charge, which may not significantly expand the layer spacing as expected. In fact, the composition analysis indicates that the adsorbent collected from the column after the simulated test (50 mg/L Cr(VI), 1.0 mM HCO3-, and 1.0 mM Cl-) has an approximate chemical formula of Mg2.25Al(OH)6.5(CrO4)0.09(Cl,OH)0.1(CO3)0.36 · 2H2O, which corresponds to a Cr(VI) adsorption amount of 31.2 mg/g (experiment: 29.1 mg/g) and 1.60 wt % carbon (experiment: 1.62 wt %). This formula also reveals that CrO42- is only ca. 20%, in terms of both the equivalent change and the anion number in the interlayer. This was also supported by the FTIR spectra of Cr(VI)-adsorbed HTs. As shown in Figure 5, besides the major band at ∼3500 cm-1 (stretching vibration of O-H bonds in metal hydroxides and water molecules), the peak at 1370 cm-1 and the shoulder at ∼3000 cm-1 reveal that the major interlayer anion is CO32-/HCO3-.23 The characteristic vibrations of CrO42- (800-950 cm-1)34 are marginally noted as the shoulder of the band in 600-1000 cm-1. 4. Conclusion The calcined hydrotalcite (CHT) has been found to adsorb 34.3-44.7 mg (Cr)/g in the column tests under varied conditions. This effective Cr(VI) adsorption amount is only reduced to 29.1 mg (Cr)/g under the coexistence of HCO3(1.0 mM) and Cl- (1.0 mM) in the influent, which, together with the high quality of treated water, quick recovery of Cr(VI), and ready regeneration of the adsorbent, demonstrates that CHT can be used as an effective and regenerable adsorbent for the Cr(VI)-contaminated groundwater remediation. Acknowledgment This project is supported by National Nature Science Foundation of China (Nos. 20477024, 20677037 and 20877053), and the Key Subject of Shanghai Municipality (No. S30109). The support from the ARC Centre of Excellence for Functional Nanomaterials, funded by the Australia Research Council, under its Centre of Excellence Scheme, is also appreciated. Supporting Information Available: Chromate species distribution at different pH and drawing of chromate for size estimation. (PDF) This information is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) Chon, C. M.; Kim, J. G.; Moon, H. S. Kinetics of Chromate Reduction by Pyrite and Biotite under Acidic Conditions. Appl. Geochem. 2006, 21, 1469–1481. (2) Loyaux-lawniczak, S.; Lecomte, P.; Ehrhardt, J. Behavior of Hexavalent Chromium in a Polluted Groundwater: Redox Processes and Immobilization in Soils. EnViron. Sci. Technol. 2001, 35, 1350–1357. (3) Mukhopadhyay, B.; Sundquist, J.; White, E. Hydro-geochemical Controls on Removal of Cr(VI) from Contaminated Groundwater by Anion Exchange. Appl. Geochem. 2007, 22, 370–387. (4) Owlad, M.; Aroua, M. K.; Daud, W. A. W.; Baroutian, S. Removal of Hexavalent Chromium-contaminated Water and Wastewater: A Review. Water Air Soil Pollut. 2009, 200, 59–77. (5) Qafoku, N. P.; Dresel, P. E.; McKinley, J. P.; Liu, C.; Heald, S. M.; Ainsworth, C. C.; Phillips, J. L.; Fruchter, J. S. Pathways of Aqueous Cr(VI)
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ReceiVed for reView September 20, 2009 ReVised manuscript receiVed December 21, 2009 Accepted January 20, 2010 IE901469C
