Advertisement

Cellulose

, Volume 24, Issue 3, pp 1427–1443 | Cite as

Grafting of arginine and glutamic acid onto cellulose for enhanced uranyl sorption

  • Mina N. El-Bohy
  • Yasser K. Abdel-Monem
  • Kamal A. Rabie
  • Nagdy M. Farag
  • Mohamed G. Mahfouz
  • Ahmed A. Galhoum
  • Eric Guibal
Original Paper

Abstract

The grafting of arginine and glutamic acid on cellulose (through an intermediary step of chlorination) allows improving uranyl sorption of the biopolymer. The sorbents (Arg-Cell and Glu-Cell) were characterized by elemental analysis, FTIR spectrometry, XRD, SEM-EDX analysis and TGA. The sorption efficiency increases with pH; this can be attributed to the deprotonation of carboxylic acid and amine groups and to the formation of polynuclear hydrolyzed uranyl species. Sorption isotherms (fitted by the Langmuir equation) show sorption capacities at saturation of the monolayer of 147 and 168 mg U g−1 for Arg-Cell and Glu-Cell, respectively (compared to 78 mg U g−1 for raw cellulose); maximum sorption capacities at equilibrium (experimental values) reach 138, 160 and 73.4 for Arg-Cell, Glu-Cell and cellulose, respectively. Uranyl sorption is endothermic and is spontaneous for amino acid derivatives of cellulose (contrary to exothermic for cellulose). Uptake kinetics for the different sorbents are fitted by the pseudo-second-order rate equation. Uranium can be desorbed using sulfuric acid solutions, and the sorbents can be recycled for a minimum of five cycles of sorption/desorption: the decrease in sorption capacities at the fifth cycle does not exceed 13%.

Keywords

Cellulose derivatives Uranyl sorption Sorption isotherms Uptake kinetics Thermodynamics Sorbent regeneration 

Notes

Acknowledgments

This work was financilally supported by the Nuclear Materials Authority, Egypt. The article is specially dedication to the memory of Prof. Dr. Ahmed Donia.

Supplementary material

10570_2017_1193_MOESM1_ESM.docx (1.5 mb)
Supplementary material 1 (DOCX 1554 kb)

References

  1. AbowSlama EHY, Ebraheem E, Sam AK (2014) Precipitation and purification of uranium from rock phosphate. J Radioanal Nucl Chem 299:815–818. doi: 10.1007/s10967-013-2703-8 CrossRefGoogle Scholar
  2. Agrawal YK, Shrivastav P, Menon SK (2000) Solvent extraction, separation of uranium (VI) with crown ether. Sep Purif Technol 20:177–183. doi: 10.1016/s1383-5866(00)00110-6 CrossRefGoogle Scholar
  3. Albert A (1952) Quantitative studies of the avidity of naturally occurring substances for trace metals. II. Amino-acids having three ionizing groups. Biochem J 50:690–697CrossRefGoogle Scholar
  4. Alekseeva OV, Bagrovskaya NA, Noskov AV (2015) Sorption of heavy metal ions by cellulose modified with fullerene. Russ J Appl Chem 88:436–441. doi: 10.1134/s107042721503012x CrossRefGoogle Scholar
  5. Anirudhan TS, Sreekumari SS (2010) Synthesis and characterization of a functionalized graft copolymer of densified cellulose for the extraction of uranium(VI) from aqueous solutions. Colloids Surf A 361:180–186. doi: 10.1016/j.colsurfa.2010.03.031 CrossRefGoogle Scholar
  6. Aytas S, Turkozu DA, Gok C (2011) Biosorption of uranium(VI) by bi-functionalized low cost biocomposite adsorbent. Desalination 280:354–362. doi: 10.1016/j.desal.2011.07.023 CrossRefGoogle Scholar
  7. Baes CF Jr, Mesmer RE (1976) Hydrolysis of Cations. Wiley, NYGoogle Scholar
  8. Bai J et al (2013) Equilibrium, kinetic and thermodynamic studies of uranium biosorption by calcium alginate beads. J Environ Radioact 126:226–231. doi: 10.1016/j.jenvrad.2013.08.010 CrossRefGoogle Scholar
  9. Basarir SS, Bayramgil NP (2013) The uranium recovery from aqueous solutions using amidoxime modified cellulose derivatives. IV. Recovery of uranium by amidoximated hydroxypropyl methylcellulose. Cellulose 20:827–839. doi: 10.1007/s10570-012-9845-7 CrossRefGoogle Scholar
  10. Berto S, Crea F, Daniele PG, Gianguzza A, Pettignano A, Sammartano S (2012) Advances in the investigation of dioxouranium(VI) complexes of interest for natural fluids. Coord Chem Rev 256:63–81. doi: 10.1016/j.ccr.2011.08.015 CrossRefGoogle Scholar
  11. Biswas S, Rupawate VH, Hareendran KN, Roy SB, Chakravartty JK (2015) Novel precipitation technique for uranium recovery from carbonate leach solutions. J Radioanal Nucl Chem 304:1345–1351. doi: 10.1007/s10967-014-3863-x CrossRefGoogle Scholar
  12. Cao Q, Liu Y, Kong X, Zhou L, Guo H (2013) Synthesis of phosphorus-modified poly(styrene-co-divinylbenzene) chelating resin and its adsorption properties of uranium(VI). J Radioanal Nucl Chem 298:1137–1147. doi: 10.1007/s10967-013-2500-4 CrossRefGoogle Scholar
  13. Ciolacu D, Ciolacu F, Popa VI (2011) Amorphous cellulose—Structure and characterization. Cellul Chem Technol 45:13–21Google Scholar
  14. Dongre VG, Janrao DM, Kamble VW (1998) Potentiometric studies on some ternary complexes of uranyl ion with pyridine carboxylic acids and some amino acids. Asian J Chem 10:730–734Google Scholar
  15. Donia AM, Atia AA, Moussa EMM, El-Sherif AM, El-Magied MOA (2009) Removal of uranium(VI) from aqueous solutions using glycidyl methacrylate chelating resins. Hydrometallurgy 95:183–189. doi: 10.1016/j.hydromet.2008.05.037 CrossRefGoogle Scholar
  16. Donia AM, Atia AA, Abouzayed FI (2012) Preparation and characterization of nano-magnetic cellulose with fast kinetic properties towards the adsorption of some metal ions. Chem Eng J 191:22–30. doi: 10.1016/j.cej.2011.08.034 CrossRefGoogle Scholar
  17. Dubinin MM, Zaverina ED, Radushkevich LV (1947) Sorption and structure of active carbons. I. Adsorption of organic vapors. Zh Fiz Khim 21:1351–1362Google Scholar
  18. Dubois MA, Dozol JF, Nicotra C, Serose J, Massiani C (1995) Pyrolysis and incineration of cationic and anionic ion-exchange resins - identification of volatile degradation compounds. J Anal Appl Pyrolysis 31:129–140. doi: 10.1016/0165-2370(94)00817-k CrossRefGoogle Scholar
  19. Erkaya IA, Arica MY, Akbulut A, Bayramoglu G (2014) Biosorption of uranium(VI) by free and entrapped Chlamydomonas reinhardtii: kinetic, equilibrium and thermodynamic studies. J Radioanal Nucl Chem 299:1993–2003. doi: 10.1007/s10967-014-2964-x CrossRefGoogle Scholar
  20. Foo KY, Hameed BH (2010) Insights into the modeling of adsorption isotherm systems. Chem Eng J 156:2–10. doi: 10.1016/j.cej.2009.09.013 CrossRefGoogle Scholar
  21. Freundlich HMF (1906) Uber die adsorption in lasungen. Z Phys Chem 57:385–470Google Scholar
  22. Galhoum AA, Mahfouz MG, Abdel-Rehem ST, Gomaa NA, Atia AA, Vincent T, Guibal E (2015a) Diethylenetriamine-functionalized chitosan magnetic nano-based particles for the sorption of rare earth metal ions Nd(III), Dy(III) and Yb(III). Cellulose 22:2589–2605. doi: 10.1007/s10570-015-0677-0 CrossRefGoogle Scholar
  23. Galhoum AA, Mahfouz MG, Atia AA, Abdel-Rehem ST, Gomaa NA, Vincent T, Guibal E (2015b) Amino acid functionalized chitosan magnetic nanobased particles for uranyl sorption. Ind Eng Chem Res 54:12374–12385. doi: 10.1021/acs.iecr.5b03331 CrossRefGoogle Scholar
  24. Galhoum AA, Mahfouz MG, Atia AA, Gomaa NA, Abdel-Rehem SS, Vincent T, Guibal E (2016) Alanine and serine functionalized magnetic nano-based particles for sorption of Nd(III) and Yb(III). Adv Environ Res 5:1–18. doi: 10.12989/aer.2016.5.1.001 CrossRefGoogle Scholar
  25. Gao Y, Yuan Y, Ma D, Li L, Li Y, Xu W, Tao W (2014) Removal of aqueous uranyl ions by magnetic functionalized carboxymethylcellulose and adsorption property investigation. J Nucl Mater 453:82–90. doi: 10.1016/j.jnucmat.2014.06.028 CrossRefGoogle Scholar
  26. Gharib F, Zare K, Cheraghali R (2004) Ionic strength dependence of formation constants: complexation of glutamic acid with uranium(VI) ion. Russ J Inorg Chem 49:949–954Google Scholar
  27. Gharib F, Shamel A, Lotfi F (2005) Ionic strength dependence of formation constants, complexation of glycine with dioxouranium(VI) ion. Rev Inorg Chem 25:361–371Google Scholar
  28. Gianguzza A, Pettignano A, Sammartano S (2005) Interaction of the dioxouranium(VI) ion with aspartate and glutamate in NaCl(aq) at different ionic strengths. J Chem Eng Data 50:1576–1581. doi: 10.1021/je050040o CrossRefGoogle Scholar
  29. Gok C, Aytas S (2009) Biosorption of uranium(VI) from aqueous solution using calcium alginate beads. J Hazard Mater 168:369–375. doi: 10.1016/j.jhazmat.2009.02.063 CrossRefGoogle Scholar
  30. Guibal E, Roulph C, Lecloirec P (1992) Uranium biosorption by a filamentous fungus Mucor miehei: pH effect on mechanisms and performances of uptake. Water Res 26:1139–1145. doi: 10.1016/0043-1354(92)90151-s CrossRefGoogle Scholar
  31. Guibal E, Saucedo I, Jansson Charrier M, Delanghe B, Lecloirec P (1994) Uranium and vanadium sorption by chitosan and derivatives. Water Sci Technol 30:183–190Google Scholar
  32. Ho YS, McKay G (1999) Pseudo-second order model for sorption processes. Process Biochem 34:451–465. doi: 10.1016/S0032-9592(98)00112-5 CrossRefGoogle Scholar
  33. Kabay N, Demircioglu M, Yayli S, Gunay E, Yuksel M, Saglam M, Streat M (1998) Recovery of uranium from phosphoric acid solutions using chelating ion-exchange resins. Ind Eng Chem Res 37:1983–1990. doi: 10.1021/ie970518k CrossRefGoogle Scholar
  34. Karve M, Pandey K (2012) Sorption studies of U(VI) on Amberlite XAD-2 resin impregnated with Cyanex272. J Radioanal Nucl Chem 293:783–787. doi: 10.1007/s10967-012-1726-x CrossRefGoogle Scholar
  35. Kausar A, Bhatti HN, MacKinnon G (2013) Equilibrium, kinetic and thermodynamic studies on the removal of U(VI) by low cost agricultural waste. Colloids Surf B 111:124–133. doi: 10.1016/j.colsurfb.2013.05.028 CrossRefGoogle Scholar
  36. Kelly SD, Kemner KM, Fein JB, Fowle DA, Boyanov MI, Bunker BA, Yee N (2002) X-ray absorption fine structure determination of pH-dependent U-bacterial cell wall interactions. Geochim Cosmochim Acta 66:3855–3871. doi: 10.1016/s0016-7037(02)00947-x CrossRefGoogle Scholar
  37. Kilislioglu A, Bilgin B (2003) Thermodynamic and kinetic investigations of uranium adsorption on amberlite IR-118H resin. Appl Radiat Isot 58:155–160. doi: 10.1016/s0969-8043(02)00316-0 CrossRefGoogle Scholar
  38. Kremleva A, Krueger S, Roesch N (2009) Role of aliphatic and phenolic hydroxyl groups in uranyl complexation by humic substances. Inorg Chim Acta 362:2542–2550. doi: 10.1016/j.ica.2008.11.021 CrossRefGoogle Scholar
  39. Krestou A, Pania D (2004) Uranium(VI) speciation diagrams in the UO2 2+/CO3 2−/H2O system at 25 °C. Eur J Miner Process Environ Prot 4:113–129Google Scholar
  40. Lagergren S (1898) About the theory of so-called adsorption of soluble substances. Kungliga Sven Vetenskapsakademiens 24:1–39Google Scholar
  41. Lagrange P, Schneider M, Lagrange J (1998) Complexes of oxovanadium(IV), dioxovanadium(V) and dioxouranium(VI) with aminoacids in aqueous solution. J Chim Phys Phys- Chim Biol 95:2280–2299CrossRefGoogle Scholar
  42. Langmuir I (1918) The adsorption of gases on plane surfaces of glass, mica and platinum. J Am Chem Soc 40:1361–1402CrossRefGoogle Scholar
  43. Lin W, Carboni M, Abney CW, Taylor-Pashow KML, Vivero-Escoto JL (2013) Uranium sorption with functionalized mesoporous carbon materials. Ind Eng Chem Res 52:15187–15197. doi: 10.1021/ie402646r CrossRefGoogle Scholar
  44. Ma HY, Hsiao BS, Chu B (2012) Ultrafine cellulose nanofibers as efficient adsorbents for removal of UO2 2+ in water. ACS Macro Lett 1:213–216. doi: 10.1021/mz200047q CrossRefGoogle Scholar
  45. Mahfouz MG, Galhoum AA, Gomaa NA, Abdel-Rehem SS, Atia AA, Vincent T, Guibal E (2015) Uranium extraction using magnetic nano-based particles of diethylenetriamine-functionalized chitosan: equilibrium and kinetic studies. Chem Eng J 262:198–209. doi: 10.1016/j.cej.2014.09.061 CrossRefGoogle Scholar
  46. Mansour RA, El-Menshawy AM, Eldesoky AM (2015) Separation of uranyl ion from different media using a new cellulose hydrazone: adsorption isotherms, kinetic and thermodynamic studies. Int J Adv Res 3:966–980Google Scholar
  47. Marczenko Z (1976) Spectrophotometric determination of elements. Ellis Horwood series in analytical chemistry. Ellis Horwood, ChichesterGoogle Scholar
  48. Metilda P, Sanghamitra K, Gladis JM, Naidu GRK, Rao TP (2005) Amberlite XAD-4 functionalized with succinic acid for the solid phase extractive preconcentration and separation of uranium(VI). Talanta 65:192–200. doi: 10.1016/j.talanta.2004.06.005 Google Scholar
  49. Monier M, Abdel-Latif DA (2013) Synthesis and characterization of ion-imprinted resin based on carboxymethyl cellulose for selective removal of UO2 2+. Carbohydr Polym 97:743–752. doi: 10.1016/j.carbpol.2013.05.062 CrossRefGoogle Scholar
  50. Naduparambath S, Purushothaman E (2016) Sago seed shell: determination of the composition and isolation of microcrystalline cellulose (MCC). Cellulose 23:1803–1812. doi: 10.1007/s10570-016-0904-3 CrossRefGoogle Scholar
  51. Nie X-Q, Dong F-Q, Liu N, Zhang D, Liu M-X, Yang J, Zhang W (2015) Biosorption and biomineralization of uranium(VI) from aqueous solutions by Landoltia punctata. SpectroscSpectral Anal 35:2613–2619. doi: 10.3964/j.issn.1000-0593(2015)09-2613-07
  52. Petitramel MM, Mosoni L (1982) Stability and visible absorption of glutamic-acid complexes with uranyl and neodymium ions. Fresenius Z Anal Chem 313:544–547. doi: 10.1007/bf00493679 CrossRefGoogle Scholar
  53. Peyvandi S, Faghihian H (2014) Biosorption of uranyl ions from aqueous solution by Saccharomyces cerevisiae cells immobilized on clinoptilolite. J Radioanal Nucl Chem 301:537–543. doi: 10.1007/s10967-014-3184-0 CrossRefGoogle Scholar
  54. Rahmani-Sani A, Hosseini-Bandegharaei A, Hosseini SH, Kharghani K, Zarei H, Rastegar A (2015) Kinetic, equilibrium and thermodynamic studies on sorption of uranium and thorium from aqueous solutions by a selective impregnated resin containing carminic acid. J Hazard Mater 286:152–163. doi: 10.1016/j.jhazmat.2014.12.047 CrossRefGoogle Scholar
  55. Rahmati A, Ghaemi A, Samadfam M (2012) Kinetic and thermodynamic studies of uranium(VI) adsorption using Amberlite IRA-910 resin. Ann Nucl Energy 39:42–48. doi: 10.1016/j.anucene.2011.09.006 CrossRefGoogle Scholar
  56. Ramanujam VV, Rengaraj K, Sivasankar B (1979) Studies on uranyl complexes. II. Unidentate carboxylate coordination in uranyl complexes of alpha-, beta;-, and gamma-amino acids: a polarographic study. Bull Chem Soc Jpn 52:2713–2716. doi: 10.1246/bcsj.52.2713 CrossRefGoogle Scholar
  57. Say R, Erosz A, Denizli A (2003) Selective separation of uranium containing glutamic acid molecular-imprinted polymeric microbeads. Sep Sci Technol 38:3431–3447. doi: 10.1081/ss-120023407 CrossRefGoogle Scholar
  58. Sebe G, Ham-Pichavant F, Ibarboure E, Koffi ALC, Tingaut P (2012) Supramolecular structure characterization of cellulose II nanowhiskers produced by acid hydrolysis of cellulose I substrates. Biomacromolecules 13:570–578. doi: 10.1021/bm201777j CrossRefGoogle Scholar
  59. Semnani F, Asadi Z, Samadfam M, Sepehrian H (2012) Uranium(VI) sorption behavior onto amberlite CG-400 anion exchange resin: effects of pH, contact time, temperature and presence of phosphate. Ann Nucl Energy 48:21–24. doi: 10.1016/j.anucene.2012.05.010 CrossRefGoogle Scholar
  60. Sharma PR, Varma AJ (2014) Thermal stability of cellulose and their nanoparticles: effect of incremental increases in carboxyl and aldehyde groups. Carbohydr Polym 114:339–343. doi: 10.1016/j.carbpol.2014.08.032 CrossRefGoogle Scholar
  61. Silva Filho EC, Lima LCB, Silva FC, Sousa KS, Fonseca MG, Santana SAA (2013a) Immobilization of ethylene sulfide in aminated cellulose for removal of the divalent cations. Carbohydr Polym 92:1203–1210. doi: 10.1016/j.carbpol.2012.10.031 CrossRefGoogle Scholar
  62. Silva Filho EC, Santos Junior LS, Fernandes Silva MM, Fonseca MG, Abreu Santana SA, Airoldi C (2013b) Surface cellulose modification with 2-aminomethylpyridine for copper, cobalt, nickel and zinc removal from aqueous solution. Mater Res Ibero-Am J Mater 16:79–87. doi: 10.1590/s1516-14392012005000147 Google Scholar
  63. Singer DM, Guo H, Davis JA (2014) U(VI) and Sr(II) batch sorption and diffusion kinetics into mesoporous silica (MCM-41). Chem Geol 390:152–163. doi: 10.1016/j.chemgeo.2014.10.027 CrossRefGoogle Scholar
  64. Singh KK, Pathak SK, Kumar M, Mahtele AK, Tripathi SC, Bajaj PN (2013) Study of uranium sorption using D2EHPA-impregnated polymeric beads. J Appl Polym Sci 130:3355–3364. doi: 10.1002/app.39582 CrossRefGoogle Scholar
  65. Solgy M, Taghizadeh M, Ghoddocynejad D (2015) Adsorption of uranium(VI) from sulphate solutions using Amberlite IRA-402 resin: equilibrium, kinetics and thermodynamics study. Ann Nucl Energy 75:132–138. doi: 10.1016/j.anucene.2014.08.009 CrossRefGoogle Scholar
  66. Stopa LCB, Yamaura M (2010) Uranium removal by chitosan impregnated with magnetite nanoparticles: adsorption and desorption. Int J Nucl Energy Sci Technol 5:283–289CrossRefGoogle Scholar
  67. Sun X, Yang L, Li Q, Zhao J, Li X, Wang X, Liu H (2014) Amino-functionalized magnetic cellulose nanocomposite as adsorbent for removal of Cr(VI): synthesis and adsorption studies. Chem Eng J 241:175–183. doi: 10.1016/j.cej.2013.12.051 CrossRefGoogle Scholar
  68. Sun Y, Zhang R, Ding C, Wang X, Cheng W, Chen C, Wang X (2016) Adsorption of U(VI) on sericite in the presence of Bacillus subtilis: a combined batch, EXAFS and modeling techniques. Geochim Cosmochim Acta 180:51–65. doi: 10.1016/j.gca.2016.02.012 CrossRefGoogle Scholar
  69. Sureshkumar MK, Das D, Mallia MB, Gupta PC (2010) Adsorption of uranium from aqueous solution using chitosan-tripolyphosphate (CTPP) beads. J Hazard Mater 184:65–72. doi: 10.1016/j.jhazmat.2010.07.119 CrossRefGoogle Scholar
  70. Tan L et al (2015) Synthesis of Fe3O4@TiO2 core-shell magnetic composites for highly efficient sorption of uranium (VI). Colloids Surf A 469:279–286. doi: 10.1016/j.colsurfa.2015.01.040 CrossRefGoogle Scholar
  71. Tashiro T, Shimura Y (1982) Removal of mercuric ions by systems based on cellulose derivatives. J Appl Polym Sci 27:747–756. doi: 10.1002/app.1982.070270235 CrossRefGoogle Scholar
  72. Včeláková K, Zusková I, Kenndler E, Gaš B (2004) Determination of cationic mobilities and pKa values of 22 amino acids by capillary zone electrophoresis. Electrophoresis 25:309–317. doi: 10.1002/elps.200305751 CrossRefGoogle Scholar
  73. Wang GH, Liu JS, Wang XG, Xie ZY, Deng NS (2009) Adsorption of uranium (VI) from aqueous solution onto cross-linked chitosan. J Hazard Mater 168:1053–1058. doi: 10.1016/j.jhazmat.2009.02.157 CrossRefGoogle Scholar
  74. Yousif AM, El-Afandy AH, Wahab GMA, Mubark AE, Ibrahim IA (2015) Selective separation of uranium(VI) from aqueous solutions using amine functionalized cellulose. J Radioanal Nucl Chem 303:1821–1833. doi: 10.1007/s10967-014-3688-7 Google Scholar
  75. Zagorodnyaya AN, Abisheva ZS, Sharipova AS, Sadykanova SE, Bochevskaya YG, Atanova OV (2013) Sorption of rhenium and uranium by strong base anion exchange resin from solutions with different anion compositions. Hydrometallurgy 131:127–132. doi: 10.1016/j.hydromet.2012.11.003 CrossRefGoogle Scholar
  76. Zhou L, Shang C, Liu Z, Huang G, Adesina AA (2012) Selective adsorption of uranium(VI) from aqueous solutions using the ion-imprinted magnetic chitosan resins. J Colloid Interface Sci 366:165–172. doi: 10.1016/j.jcis.2011.09.069 CrossRefGoogle Scholar
  77. Zhou Y, Wang X, Zhang M, Jin Q, Gao B, Ma T (2014) Removal of Pb(II) and malachite green from aqueous solution by modified cellulose. Cellulose 21:2797–2809. doi: 10.1007/s10570-014-0282-7 CrossRefGoogle Scholar
  78. Zhu ZW, Pranolo Y, Cheng CY (2016) Uranium recovery from strong acidic solutions by solvent extraction with Cyanex 923 and a modifier. Miner Eng 89:77–83. doi: 10.1016/j.mineng.2016.01.016 CrossRefGoogle Scholar

Copyright information

© Springer Science+Business Media Dordrecht 2017

Authors and Affiliations

  • Mina N. El-Bohy
    • 1
  • Yasser K. Abdel-Monem
    • 2
  • Kamal A. Rabie
    • 1
  • Nagdy M. Farag
    • 1
  • Mohamed G. Mahfouz
    • 1
  • Ahmed A. Galhoum
    • 1
    • 3
  • Eric Guibal
    • 3
  1. 1.Nuclear Materials AuthorityEl-Maadi, CairoEgypt
  2. 2.Chemistry Department, Faculty of ScienceMenoufia UniversityShebin El-Kom, MenoufiaEgypt
  3. 3.Ecole des mines Alès, Centre des Matériaux des Mines d’AlèsAlès CedexFrance

Personalised recommendations