Anal. Chem. 1996, 68, 740-745
Ion Chromatography-Photodiode Array UV-Visible Detection of Cr(III) Hydrolytic Polymerization Products in Pure and Natural Waters Farida Y. Saleh,* G. E. Mbamalu,† Q. H. Jaradat,‡ and C. E. Brungardt
Graduate Environmental Sciences Program, University of North Texas, Denton Texas 76203-3078
Hydrolytic polymerization of Cr(III) aqua ions was investigated in pure Milli-Q water and in surface waters, using Sephadex column fractionation followed by nonsuppressed ion chromatography (IC) with UV detection at λ 462 nm. Pure and natural waters spiked with 0.04 M Cr(III) were fractionated on Sephadex columns into four fractions using eluents with increasing ionic strength. Fractions were analyzed for total Cr by atomic absorption, and recoveries ranged from 94 to 101%. Fractions representing monomeric and low oligomeric Cr (III) species were subjected to IC using a low-capacity mixed resin column and a mobile phase consisting of 2 M NaClO4/0.02 M HClO4 at pH 4.50. Monomeric Cr(III) species were detected in the IC chromatograms of the freshly prepared Cr(III) solution with capacity factors (k′) ranging from 0.05 to 0.40. In the 3-days-aged samples, dimeric and trimeric peaks with k′ ) 1.09 and 1.70, respectively, were detected. Monomeric, dimeric and trimeric Cr(III) peaks collected from the IC preparative experiments were scanned between λ 200 and 600 nm, using a photodiode array detector. The UV-visible spectral characteristics of the monomer, dimer, and trimer confirmed their identities. The trimeric Cr(III) peak was more predominant in the IC chromatograms of the surface water and accounted for 6.5-35.9% of the total Cr(III) in the samples. The hydrolytic polymerization of metal ions is a fundamental process occurring in natural waters and biological systems and during ore formation. Multinuclear hydrolysis products of Cr(III) and other metallic cations are of almost universal occurrence in water solvent systems.1-5 Polyvalent metal ions in surface water and groundwater are known to hydrolyze, forming hydrates and hydroxides. Simple hydrolysis equilibria are usually very fast, but under conditions of saturation such as those occurring in hazardous waste sites, a sequence of hydrolytic polymerization and † Permanent address: Department of Chemistry and Physics, Johnson C. Smith University, Charlotte, NC 2821. ‡ Permanent address: Chemistry Department , Mu’tah University, Karak, Jordan. (1) Stumm, W.; Morgan; J. J. Aquatic Chemistry, 2nd ed.; Wiley Interscience: New York, 1981; p 656. (2) Martell, A. E.; Smith, R. M. Critical Stability Constants, Vol. 1; Plenum Press: New York, 1974; p 469. (3) Baes, C. F., Jr.; Mesmer, R. F. The Hydrolysis of Cations; John Wiley & Sons: New York, 1976; pp 211-219. (4) Morel, F. M.; Herring, J. G. Principles and Applications of Aquatic Chemistry; John Wiley and Sons: New York, 1993; Chapters 5 and 6. (5) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry, 4th ed.; Interscience Publishing: New York, 1980; p 1145.
740 Analytical Chemistry, Vol. 68, No. 5, March 1, 1996
condensation reactions leads to the formation of hydroxy polymers and, ultimately, to the formation of precipitates. Polynuclear complexes are often formed rather slowly. Many polynuclear hydroxy complexes are kinetic intermediates in the slow transition from free metal ions to solid precipitates and are thus thermodynamically unstable. Such polynuclear species could be of significance in the aquatic environment and may persist as metastable species for years. Speciation of Cr ions in dilute aqueous solution is a function of acid-base redox and complexation reactions. Under redox conditions normally found in natural waters and sediments, Cr(III) is the most stable and less toxic form. Hexavalent chromium forms a number of oxyacids or anions, such as hydrochromate (HCrO4-), dichromate (Cr2O72-), and chromate (CrO42-), all of which are quite soluble and quite toxic. Because of the carcinogenicity and toxicity of Cr(VI) species, numerous studies14-18 have recently been conducted on Cr speciation and the kinetics of Cr(III) oxidation to Cr(VI). Fortunately, the only naturally occurring oxidants of Cr(III) are manganese oxides.17
3MnO2 + H2O + 2Cr3+ h 3Mn2+ + Cr2O72- + 2H+ log Keq ) -10.16 The hexaaqua ion [Cr(H2O)6]3+ occurs in aqueous solution and in numerous salts. Between pH 4 and 9, inorganic Cr(III) species are hydrolyzed to form [Cr(OH)]2+ , [Cr(OH)2]+ , [Cr(OH)4]-, and [CrO2]- species. The extent of formation of each depends on the pH.6,7 The thermodynamic hydrolytic polymerization products of Cr(III) have been widely investigated since 1908.8-10 Earlier studies isolated two soluble dimeric species, the single and double bridged forms. Stunzi and Marty11 investigated the structures and stabilities of the dimeric [Cr2(OH)2]4+, trimeric [Cr3(OH)4]5+, and tetrameric [Cr4(OH)6]6+ oligomers. Their order of stability was dimer < trimer > tetramer, according to (6) Schmidt, R. L. Thermodynamic Properties and Environmental Chemistry of Chromium. Report PNL-4881; Battelle Pacific Northwest Laboratory: Richland, WA, 1984; p 43. (7) MINTEQA2/PRODAF2. A Geochemical Assessment Model for Environmental System, Version 3.11; U.S. Environmental Protection Agency: Athens, GA, 1990. (8) Finholt, J. E. Chemistry of Some Hydrolyzed Cr(III) Polymers. Report UCRL8879; Ph.D. Thesis, Lawrence Radiation Laboratory, University of California, Berkely CA, 1960. (9) Laswick, J. A.; Plain, R. A. J. Am. Chem. Soc. 1959, 81, 3564-3567. (10) Thompson, M.; Connick, R. E. Inorg. Chem. 1981, 20, 2279-2285. (11) Stunzi, H.; Marty, W. Inorg. Chem. 1983, 22, 2145-2150. 0003-2700/96/0368-0740$12.00/0
© 1996 American Chemical Society
Cr(OH)2+ + Cr(OH)2+ h Cr2(OH)24+
Milli-Q water surface lake water
dimer, log Kdimer ) 3.3 2+
Cr(OH)
+ Cr2(OH)3
3+
Table 1. Characteristics of the Experimental Watersa
5+
h Cr3(OH)4
trimer, log Ktrimer ) 4.5 Cr(OH)2+ + Cr3(OH)54+ h Cr4(OH)66+
pH at 25 °C conductivity (µΩ-1 cm-1) at 25 °C total alkalinity (mg/L CaCO3) total hardness (mg/L CaCO3) total sulfate (mg/L SO42-) total organic carbon (mg/L) a
5.50 ( 0.05
