Environ. Sci. Techno/. 1995, 29, 222-231
Chemical Kinetic Mechanism for Atmospheric Chromium C H R I S T I A N S E I G N E U R * AND E L P I D A CONSTANTINOU ENSR Consulting and Engineering, 1320 Harbor Bay Parkway, Suite 210, Alameda, California 94502
A chemical kinetic mechanism for the atmospheric chemistry of chromium has been developed. The chemistry of chromium is of particular interest because its oxidation state is believed to affect its toxicity. Chromium can be reduced from the hexavalent form to the trivalent form by several chemicals including trivalent arsenic, divalent iron, vanadium, and sulfur dioxide. Trivalent chromium can be oxidized to hexavalent chromium by reactions with manganese. Computer simulations were conducted for several scenarios using typical atmospheric concentrations. The simulation results suggestthattypical conditions favor the reduction of Cr(VI) to Cr(lll). Under some extreme conditions, Cr(lll) could be oxidized slowly to C.(VI). Experimental data of chromium concentrations in ambient air and precipitation are needed to evaluate these theoretical results.
Introduction Trace metals are emitted into the atmosphere from anthropogenic as well as natural (e.g., soil erosion) sources. Some of these trace metals are suspected to present some adverse health effects for humans. Chromium is considered to present potential adverse health effects through inhalation (1). However, the U S . Environmental Protection Agency has listed only the hexavalent form of chromium to be carcinogenic (2);the trivalent form is, therefore, considered to be significantly less toxic, presenting only noncarcinogenic adverse effects. Therefore, it is essential to determine the oxidation state of chromium if one wants to assess its potential health effects in a realistic manner. To this end, we have reviewed the chemical reactions that govern the atmospheric chemistry of chromium, and we have developed a chemical kinetic mechanism for its atmospheric chemistry. The two oxidation states of chromium commonlyfound in the environment are trivalent chromium [Cr(III)] and hexavalent chromium [Cr(VI)I. Divalent chromium [Cr(II)] is fairly unstable and rapidly oxidized to Cr(II1). Elemental chromium [Cr(O)lis also oxidized to Cr(II1)unless it is made passive by superficial oxidation. Our study of the atmospheric chemistry of chromium, therefore, focuses on its two prevalent forms: Cr(II1) and Cr(VI). Chromium is present in the atmosphere either in the solid phase or in the liquid phase. Atmospheric chromium is emitted from anthropogenic sources, which account for 60-70%, and from natural sources, which account for the remaining 30-40% ( I , 3). Average atmospheric concentrations of chromium range from 0.01 to 1.0 pg m-3 in rural and polluted urban areas, respectively ( 4 ) . The vapor pressure of chromium species is negligible so that gaseous chromium species do not exist at ambient atmospheric temperatures. The chemistry of chromium is therefore associated with particle and droplet chemistry. Chemical reactions that convert Cr(II1)to Cr(VI), and vice versa, will occur in the aqueous phase. Consequently, it is essential to know the concentrations of dissolved chromium in atmospheric particles or droplets, that is, to understand the solution/precipitation chemistry of chromium. We review first the solution chemistry of each chromium valence state, Le., Cr(II1) and Cr(VI). Then, we review the chemical transformations, which in solution can lead to the conversion of Cr(1II)to Cr(VI)and vice versa. Next, we describe the development of the chemical kinetic mechanism. Using this chemical mechanism, computer simulations are conducted for typical atmospheric conditions to provide insights into the conversion of chromium species between different oxidation states.
Review of the Atmospheric Chemistry of Chromium Solution Chemistry of Chromium(II1). Chromium(II1) may be present in the form of its oxide Cr203or as soluble chromium salts such as chromium sulfate Cr2(S0& (5). Chromium oxide is insoluble even in acidic solution (6) and is, therefore, not affected by chemical reactions in atmospheric aerosols or droplets. We focus here on the aqueous chemistry of soluble Cr(II1). * To whom correspondence should be addressed.
222 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 1, 1995
0013-936X/95/0929-0222$09.00/0
a
1994 American Chemical Society
TABLE 1
Equilibrium Chemistry of Cr(lll) equilibrium constant at 25 "C
equilibrium reaction
+ + + + + * + + + + = + + = +
Cr(OH)3(s) 3H+ S Cr3+ 3H20 Cr(0H)ds) 2H+ = CrOH2+ 2H20 H+ Cr(OH)2+ H20 Cr(OH)&) Cr(0H)ds)t Cr(0H)daq) Cr(0H)ds) H2O = Cr(OH)4- H+ 2Cr(OH)s(s) 4H+ Cr2(0H)z4+ 4H20 3Cr(OH)&) 5H+ Cr3(0H)d5+ 5H20 4Cr(OH)s(s) 6H+ 2 Cr4(OH)66+ 6H2O 2CrOH2+ C r ~ ( 0 H ) 2 ~ + Cr(N03)3(s) Cr3+ 3N03CrS04+ Cr3+ S042Cr2(SO4)3(s) 2Cr3+ 3sOa2C$+ CI- t CrC12+ CrC12+ CrC12+ CICrCI&) = CrC12+ CI2CrCIOH+ S CrC12+ Cr(OH)2+ CrC120H H+ CrC12+ H20 C6+ Br- = CrB2+ C$+ FCrF2+ CrF2+ F- t CrF2+ CrF2+ F CrF3
+
+ = + * = +
+ +
+
+
+
+ *
+ + + + =
+
ref
