ARTICLE pubs.acs.org/IECR
High Sorptive Removal of Borate from Aqueous Solution Using Calcined ZnAl Layered Double Hydroxides Paulmanickam Koilraj and Kannan Srinivasan* Discipline of Inorganic Materials & Catalysis, Central Salt and Marine Chemicals Research Institute, Council of Scientific and Industrial Research (CSIR), GB Marg, Bhavnagar 364021, India
bS Supporting Information ABSTRACT: Borate uptake studies were carried out over both as-synthesized and calcined ZnAl layered double hydroxides (LDHs) containing carbonate or chloride as interlayer anions, with different Zn/Al atomic ratios (2, 3, and 4). Chloride-containing materials in both as-synthesized and calcined forms showed higher borate uptake than carbonate-containing forms. A maximum borate uptake of 31 mgB/g was achieved for the 400 °C heat-treated Zn2AlCl LDH material (referred here as Zn2AlClCLDH). The mechanism of borate uptake for heat-treated materials involves reconstruction as elucidated from PXRD, FT-IR, and SEM measurements. Interestingly, solid state transformation of tetraborate to monoborate anion occurred in the interlayer on drying for longer time. At a ratio of 4 g Zn2AlClCLDH per liter of 100 mgB/L solution, the uptake of borate reached 97% further, and at 5 mgB/L solution the borate concentration reduced below the value of 0.5 mgB/L recommended by the World Health Organization (WHO), using 1 g/L ratio of sorbent. The borate uptake adsorption isotherm fitted well to a monolayer Langmuir adsorption model. The influence of various parameters such as the mass of adsorbent, reaction time, borate concentration, temperature, and pH of the medium was studied. Constant borate uptake was observed over the pH range 37 owing to the buffering nature of LDH. The influence of the copresence of other anions on borate uptake by Zn2AlClCLDH depended on the charge to radius ratio of the anion. Cyclic adsorptiondesorption studies revealed that the material was recyclable. High uptake capacity along with recyclability of this material suggests the promise as a sorbent for wastewater remediation of borate.
1. INTRODUCTION Boron is an essential nutrient for plant growth. Deficiency studies in animals and humans have revealed some evidence that low intake of boron affects cellular function and the activity of other nutrients.1 However a high intake of boron may cause acute boron toxicity with nausea, headache, diarrhea, kidney damage, and death from circulatory collapse.2 Boron in the aquatic environment arises from the erosion of boron rich rocks, soil, and the earth’s crust.3 Concentrations of boron in groundwater throughout the world range widely from ∼0.3 to 100 mg/L. Boron is a significant constituent of seawater with an average concentration of 4.5 mg/kg. The World Health Organization (WHO) has recommended a provisional guideline for boron in drinking water of 0.5 mg/L.4 Borate exists in the aqueous solution mainly as boric acid and [B(OH)4] due to the buffering nature of the weak acid/weak base conjugate pair. B4 O7 2 þ 7H2 O T 2BðOHÞ3 þ 2½BðOHÞ4 BðOHÞ3 þ 2H2 O T ½BðOHÞ4 þ H3 Oþ BðOHÞ3 þ OH T ½BðOHÞ4 At low concentration ( CO32 (1.85 Å) ≈ CrO42 (2.40 Å) > OH (1.33 Å) > Cl (1.81 Å) > NO3 (1.89 Å) (values given in parentheses are the ionic radius of corresponding anions). Phosphate diminished the borate uptake drastically while the presence of nitrate did not affect the uptake. Further, no significant change in the uptake was seen when the ionic strength (in terms of NaCl concentration) of the medium was changed significantly (Figure S10, Supporting Information). This result suggests the utility of ZnAlCLDHs for borate uptake even in the presence of a very high concentration of chloride anion which is commonly prevalent in natural water systems. 3.8. Desorption and Recycle Studies. Desorption experiments were performed over the reconstructed Zn2Alborate LDH obtained after borate uptake in different desorbing media such as water, 1 M NaCl, 1 M NaOH, and 1 M Na2CO3. Among the media studied, water showed very low desorption of around 2 mgB/g (6%) while NaOH showed a maximum of 26 mgB/g (83%); however, in NaOH the material dissolved completely, probably due to the formation of soluble aluminum and zinc hydroxides.35 The amount of borate desorbed in NaCl and Na2CO3 was 23 mgB/g (74%) and 25 mgB/g (79%), respectively. As the stability of carbonate in the interlayer of LDHs is high, further adsorptiondesorption experiments were conducted by using NaCl solution as the desorbing medium. Figure 10 shows sequential adsorption/desorption of borate in 1 M NaCl solution for five cycles. As mentioned earlier, borate uptake is through reconstruction and desorption is through ionexchange with chloride ion. Such transformation of reconstruction and ion-exchange were ascertained through PXRD of materials (Figure S11, Supporting Information) acquired during the sequential adsorptiondesorption cycles. Although the adsorption of borate per gram of material decreased with the 6949
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Industrial & Engineering Chemistry Research number of cycles, the extent of desorption increased. After the fifth cycle, the borate uptake per gram of material decreased to nearly 45% of the first cycle. The decrease in borate uptake with an increase in the number of cycles is due to the increase in the formation of some irreversible ZnO during a repeated calcination and reconstruction process, as evidenced from the PXRD of material acquired during the sequential adsorptiondesorption cycles.
4. CONCLUSIONS Borate uptake by ZnAl LDHs having chloride or carbonate anions in the interlayer was studied at different Zn/Al atomic ratios (2, 3, and 4). The nature of interlayer anion played an important role in borate uptake. Borate uptake over as-synthesized chloride containing material was due to anion-exchange while the uptake over carbonate containing material was due to surface adsorption. Chloride-containing materials both in their as-synthesized and heat-treated forms showed higher borate uptake relative to carbonate-containing materials. Heating the as-synthesized LDHs to 400 °C gave materials that had higher borate uptake which increased with a decreasing Zn/Al atomic ratio. Lesser uptake was found for the calcined material that had carbonate as interlayer anion than for chloride albeit similar composition due to stronger affinity of carbonate with poorly crystalline ZnO. Among the samples studied, Zn2AlCl CLDH showed a maximum borate uptake of 31 mgB/g (∼2.87 mmolB/g), the highest uptake reported so far for any LDH material. Such high borate uptake for the heat-treated materials is attributed to reconstruction of the oxide to LDH phase, as demonstrated by PXRD, FT-IR, and SEM studies. An interesting, solid state transformation of [B4O5(OH)4]2 to [B(OH)4] anion occurred in the interlayer on drying for longer time (1 month) as elucidated from PXRD. The equilibrium sorption studies showed that borate uptake could well be described by a Langmuir monolayer adsorption. The maximum borate uptake was found in the pH range 89; slightly lower uptake was found in the pH range 48 where the material had some buffering capacity. Below pH 4 or above pH 11, the sorbent was unstable owing to leaching of Zn2þ or Al3þ ions, respectively. The impeding influence of the copresence of other anions on the borate uptake by Zn2AlClCLDH depended on the charge to radius ratio of the anion wherein phosphate showed a maximum while nitrate did not influence the uptake of borate. Sequential adsorptiondesorption studies showing a slight decrease in borate uptake with an increase in the number of cycles promise reusability of material. The high adsorption capacity and stability in a wide range of pH (410) along with reusability of material suggest that it could be efficiently explored for the uptake of borate in wastewater streams. ’ ASSOCIATED CONTENT
bS
Supporting Information. FT-IR of as-synthesized material, PXRD, FT-IR, and SEM of material after borate uptake, adsorption isotherm plots, ionic strength variation on borate uptake, zeta potential of calcined LDH in water, PXRD peak assignments of material recorded at different pH, PXRD of Zn2AlClCLDH calcined at different temperature, PXRD of Zn2AlClCLDH after borate uptake at 30 and 60 °C, and sequential adsorptiondesorption cycles. This material is available free of charge via the Internet at http://pubs.acs.org.
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’ AUTHOR INFORMATION Corresponding Author
*Tel.: þ91-278-2567760 ext. 703. Fax: þ91-278-2567562. E-mail:
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’ ACKNOWLEDGMENT S.K. thanks the Ministry of Environment & Forests (MoEF), New Delhi for the financial assistance rendered under Environment Research Program (F.No.19/55/2005-RE). The authors thank Analytical Sciences Discipline of CSMCRI, Bhavnagar, India, for their assistance in instrumental analysis. ’ REFERENCES (1) U.S. EPA (U.S. Environmental Protection Agency). EPA: 822-R08-013. Drinking Water Health Advisory For Boron; U.S. EPA, Office of Water, Office of Science and Technology: Washington, DC, 2008; http://www.epa.gov/safewater/ccl/pdfs/reg_determine2/healthadvisory_ ccl2-reg2_boron.pdf (accessed 30 September 2010). (2) Boron as a medicinal ingredient in oral natural health products, Natural Health Products Directorate, Health Canada, 2007; http:// www.hc-sc.gc.ca/dhp-mps/pubs/natur/boron-bore-eng.php (accessed 30 September 2010). (3) Woods, W. G. An introduction to boron: History, sources, uses, and chemistry. Environ. Health Perspect. 1994, 102, 5–11. (4) WHO. Guidelines for Drinking-Water Quality, 3rd ed.; World Health Organization: Geneva, Switzerland, 2008; Vol. I. Recommendations, p 186. (5) Parks, J. L.; Edwards, M. Boron in the environment. Crit. Rev. Environ. Sci. Technol. 2005, 35, 81–114. (6) Yılmaz, A. E.; Boncukcuoglu, R.; Yılmaz, M. T.; Kocakerim, M. M. Adsorption of boron from boron-containing wastewaters by ion exchange in a continuous reactor. J. Hazard. Mater. 2005, 117 221–226. (7) Yilmaz, A. E.; Boncukcuoglu, R.; Kocakerim, M. M. A quantitative comparison between electrocoagulation and chemical coagulation for boron removal from boron-containing solution. J. Hazard. Mater. 2007, 149, 475–481. (8) Liu, H.; Qing, B.; Ye, X.; Li, Q.; Lee, K.; Wu, Z. Boron adsorption by composite magnetic particles. Chem. Eng. J. 2009, 151, 235–240. (9) Liu, H.; Ye, X.; Li, Q.; Kim, T.; Qing, B.; Guo, M.; Ge, F.; Wu, Z.; Lee, K. Boron adsorption using a new boron-selective hybrid gel and the commercial resin D564. Colloids Surf. A 2009, 341, 118–126. (10) Geffen, N.; Semiat, R.; Eisen, M. S.; Balazs, Y.; Katz, I.; Dosoretz, C. G. Boron removal from water by complexation to polyol compounds. J. Membr. Sci. 2006, 286, 45–51. (11) Melnyk, L.; Goncharuk, V.; Butnyk, I.; Tsapiuk, E. Boron removal from natural and wastewaters using combined sorption/membrane process. Desalination 2005, 185, 147–157. (12) Ozturk, N.; Kavak, D.; Kose, T. E. Boron removal from aqueous solution by reverse osmosis. Desalination 2008, 223, 1–9. (13) Blahusiak, M.; Schlosser, S. Simulation of the adsorptionmicrofiltration process for boron removal from RO permeate. Desalination 2009, 241, 156–166. (14) Polat, H.; Vengosh, A.; Pankratov, I.; Polat, M. A new methodology for removal of boron from water by coal and fly ash. Desalination 2004, 164, 173–188. (15) Garcia-Soto, M. M. F.; Camacho, E. M. Boron removal by means of adsorption with magnesium oxide. Sep. Purif. Technol. 2006, 48, 36–44. (16) Karahan, S.; Yurdakoc, M.; Seki, Y.; Yurdakoc, K. Removal of boron from aqueous solution by clays and modified clays. J. Colloid Interface Sci. 2006, 293, 36–42. 6950
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