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Environ Technc... .Sd . 1887 1187-119. ,21 3 Kinetics of (III) Oxidation to Chromium (VI) by Reaction with L Edmond Eery* and Dhanpat Ra! BatteDe. Pacific Northwest Laboratories. Richland, Washington 99352 compares a /TTT, _ „ ..„ othe.. o d t r .hav) Schroede. (9 . e e , . Le d ran . . , . m The kinetics of oxidation of aqueous Cr(III) to Cr(VI) Aovm that . 8igniricant fraction of the aqueous Crail)

in industrial waste materials oxidatioe .Th aqueou f (/0nt o f ) *»**.*s * Cr(in oxidizes )wa y db Cr(ni) is not appreciably affected by dissolved , J^fff^°^Tw»r ** ?^ ^Dmff1? *^ ^ indicating that Cr(III) reacts directly with /J-Mn02(s o )t Co(II Co(IIIo )t ) (11aqueoud )an s As(III As(Vo )t ) (72). produc catalyzey eb Cr(VIt no d d) reactionan • sP»Per wit"" ° presenhe 1 ,w oxyge t nexperimenta lkie dat-th n ao at the /J-MnO8(s) surface. In acidic solutions, the oxidation netics of aqueous Cr(HI) oxidation to Cr(VI) by reaction of aqueous Cr(III) is highly nonlinear with time, possibly with [0-MnO2(s)], primarily for acidic environ- because strong adsorptio producee th f no d anionic Cr(VI) ments 0-MnO2(se chose .W us o et thin )i s study because species limit amoune sth Cr(IIItf o ) species abl contaceo t t it is expecte primara e b o dt y Mn-containing soli coan di l active oxidizing sites on the j9-MnO2(s) surface. The fol- fly ashes and smelter wastes that are formed at high tem- lowing expressio derives ni d fro rate m th e H p dat r afo peratures , although it doe fort soisi no m h l environments. 3.0-4.7 that empirically account for the nonlinear rate by Additionally, 0-MnO2(s) is the most chemically pure and describin reactioe gth n rat fractioterme n ei th f so f no crystalline for MnO2(sf mo ) compare soio dt l forms such Cr(VI) to total Cr, /&, remaining in solution: • as (£-Mn02(s)] or cryptomalene fo-Mn02(s)J. d/ /df (s"1) •= Consequently, 0-MnO2(s) can be expected to have a lower L/J ,inm , I/, » , %

III 60 -

600 Hours 600 Figur . Effecte» Initiaf so l Cr(III) concentratio ) ratee (a th f so n no Cr(VI) formation and (b) fl-MnO^s) dissolution (open symbols) for pH V «=A/ 35. d 6an Cr(VImM.'10 ) rateFigure 4. (a th f s) .o n Effect. o e formatio2 SoO H ) p df (b s o symbold nan s indicat- rat0 e f eeth o 0-MnOj(s) dissolution (open symbols) for Wto * lKr [(XIII) X MnOJs 6 8. ] « ) dissolution measure 4.0dH p I .na Cr-fre e solution. . SoliM d symbols Indicate rate /M*iO,(sf so ) absolution measured In increaset no increasiny db initiae gth l Cr(m) concentratioCr-frene solutions. but were remarkably similar in magnitude as indicated by parallel rat /J-Mn02(sf eo ) dissolution measure increasee th d durin Cr(VIn si e gth ) concentrations (Figure 3a)n I . oxidation rate experiments was markedly increased in the contrast, the overall rate of /J-MnO2(s) dissolution was lower pH solutions (Figure 2b). Also, in Figure 2b, the decreased by increasing the initial Cr(III) concentration rates of/3-Mn02(s) dissolutio Cr-containine th n i g solu- (Figure 3b) rate .acidiTh f e o c 0-Mn02(s) dissolution ni tions (open symbols comparee )ar rate e purelCr-freth a f so o d t als s i y eo 0 solutioinclude4. H p Figurt n da i . e3b acidic 0-MnO2(s) dissolution that were measured in Cr-free A comparison of this rate to those hi Cr solutions (Figure solutions (solid symbols) for comparable pH and surface 3b) shows that concentrations of Mn increase in a linear area to solution volume ratios. Murray (IS) has shown that manner in the absence of dissolved Cr species but are MnO2(s) dissolves more rapidly in low-pH solutions. Rates increased initially and then slowed hi the Cr solutions, as of 0-Mn02(s) dissolution were significantly observe s morwa e previouslrapin d i r dfo y described experiments (Figure an initial period in the Cr solutions but eventually slowed 2b). The eventual depression in the initially rapid rates with increasing time to rates that were less than those of Cr(IH) oxidation and /J-MnO2(s) dissolution may be measured hi the Cr-free solutions hi which the rates of caused by a slow rate of desorption of the produced Cr(Vl) 0-MnO2(s) dissolution were observed to be approximately species from the 0-MnO2(s) surfaces. The zero point of linear over long periods (Figure 2b). This result implies charge for 0-MnO2(s) is reported to occur at pH 7.3 (±0.2) tha initian ta l reaction between CrdH 0-MnO2(sd )an ) (16), thus the anionic Cr(VI) species (HCrO CriV'd ran ) increase rate 0-MnO2(ssf th eo ) dissolution abov likel strongle e eb ar o thayt ty adsorbe acidin di c solutionse .Th caused purely by the acidic dissolution of 0-MnO2(s). direct adsorption of Cr(VQ species onto 0-Mn02(s) was However, afte periora rapif do d reaction Cr(VIe ,th ) re- foun increasee b o dt loweres d greatlwa H dp s froya 0 m8. action product adsorptiosn resuli 0 decreasa 2. i treactivith o e t nth experimenti eh f yo reportest thano e tar d here. 0-Mn02(s) and a consequent decrease in the rates of both We speculate that the strong adsorption of the anionic Cr(III) oxidation and 0-MnO2(s) dissolution. Cr(VI) species reduces the number of active sites on the Rates of Cr(IH) oxidation at pH 4.0 and A/V« 35.0 /J-Mn02(s) surface, thus causing a decrease in the reaction m2-L'1 for a range of initial Cr(III) concentrations indicate rates for both Cr(m) oxidation and 0-MnO2(s) dissolution similaa rrat dissolve e effec/J-MnO2(se f eth o th n f to o r dC ) ove rCr(VIe timth s ea ) concentration increases. Various dissolution (Figure 3). The oxidation rates of CrQI^wCTe^ ^ ^surface reactions between Cr and Mn species are Environ. Scl. Technol., Vol. 21, No. 12. 1987 1189 5' Table I. Reaction Stolchlometries Calculated from Slope* o pH = 10.1 of Plot* °f Mn(II) versus Cr(VI) by Least-Square* a pH = 8 3 Regression of the Rate Data from Figure* 2 and 3 ' PH = 6 3 slope, f o . no Crflll)A/V=35.. 6 Mn(II)/ /*'. 10- M Cr(VI)% pointH p *' «0 9 3.0 9.6 x 10-* 3.17 ± 0.54 97.6 10-X 6 * 9. 2.5 30.25 ± 3. 4 1198. 9 9 4.0 9.6 X 10-* 0.69 ± 0.16 95.6 X10-6 9. * 3 0.44. 0 ±0.111 4 89.8 10 4.0 1.9 X1Q-* 5.06 ±0.74 97.9 X1Q-8 4. * 0 1.54. 3 ±0.1811 . 98.6 9 4.0 9.6 X10-1 0.69 ±0.16 95.6 9 4.0 19.2 X 10-* 0.59 ± 0.18 92.3 9 4.0 38.5 X 10-* 0.28 ± 0.20 74.4 •Error limit* indicat standaro etw d deviations. o 200 400 600 can be expressed by the following reaction, which is rele- Hours vant to the experimental pH range of 3.0-4.7 (1), i.e. Figure 4. Rates of Cr(VI) formation In solutions containing the equivalen 10-X t 6 Cr(III*oM f9. ) precipitated as CrfOHMs) wflV hA/ CrOH2 *1.5|3-MnO2(s+ )— HCrO4 ' l.SMn2+ ) (1 * • 35.6 m2-L-». Plot dissolvef so n versudM s Cr(VI HCrO4"s )(a ) were possible, and it is difficult to separate clearly their effects made with the solution data from Figures 2 and 3 in an reaction o n rates from those cause adsorptioy db n pro- attemp comparto t experimentae eth l result stoie th -o st basicessee thesth f so n s o e experiments. However, con- chiometr reactiof yo Afte. n1 initian ra l time periof do sidering the adsorption behavior of the anionic Cr(VI) nonlinearity for 20-100 h, these plots did show linear re- species under acidic conditions, we expect that adsorption lationships between dissolved Mn(II) and dissolved Cr(VI), desorptiod an n reaction importane slopesse ar th t controllin,r bu twhicfo he shoulgth indicative db reactioe th f eo n rat Cr(IIIf eo ) oxidatio experimentae th r nfo l conditions stoichiometry, varie witd witan h Cr(IIIH hp ) concen- described here. 3.5d slopee an , th 0 , tratios3. wer H p ne t (TablA . eI) Cr(III) Precipitation addition .I rat e eth expero nt - significantll mo Mn(I yf 0 o highe 1. l Qo mo t r5 tha1. e nth iments conducte acidii dh c solutions, three experiment Cr(Vf o Qs rati o predicte reactioy db (Tabln1 indicatin, eJ I) - - g a6.3,8.3H tp 10.d ,1an were also conducted equive .Th - tha acidie tth c dissolutio 0-MnO2(s)f no , i.e. alent of 9.6 x lO"6 M aqueous Cr(IID was initially added . M n M ,„«. M .+ „ o ^ ,. o . , ,„. \J to eac thf ho e solutions whitisa t ,hbu gree ^ ^Oa(aq+ \- nO solis H2 ) )d+ wa (2 0-Mn02(sn -M H 2 )+ immediately precipitated, and the aqueous Cr(IH) con- occurred as a parallel reaction at thes~e M pH values. centrations decreased to low levels. These observations, More id o^ssolution of 0-MnO2(s) was^vident in the , withg aon i the. solubditydata for Cr(OH), which is far from the zero point of charge for 0-Mn02(s) ^ 0 (76)]3 7. . H Also[p possibilite ,th y exists that _ Mn(III) rather than Mn(IV) specie0-Mn02(se th n so ) surfac€ e is the dominant oxidant under some conditionsS ° . Clearly e ,th combined processes of Cr(III) oxidation, Cr(VI6 )6. adsorption ^ , Mn redox speciation, and 0-MnO2(s) acidic dissolution f mak determinatioe eth reactioe th f no n stoichiometrief so oxidation-reduction reactions that occur at the surfaces of manganese oxides extremely difficult Our data indicate 62 that a combination of reactions is probable under acidic conditions. Empirical Rate Expression. The experimental ob- g 6 I servations of (1) highly nonlinear reaction rates, (2) the ' 3.2 3.6 4.0 4.4 4.8 lac stronf ko g dependenc Cr(inr o H )p en concentrationo , _log (Cro, depressioe th ) (3 /J-MnO2(s f nd o an ) dissolutio_ . n r ratC n ei solutions compared to Cr5ree sections indicate that the ^J slow desorptio anionif no c Cr(Vl calculate)s specie^m toast-squarey db s- p fro e mth s regression thd e,an error fcnits Intfcate Mn02(s) surfac n importana s ei t rate-limitin standaro tw gd deviationste r pfo s abou slopee tth . Cr(III) oxidatio 0-Mn02(sy nb acidii )h c solutions- .Ad sorption of Cr(VI) species was found to occur in acidic + Cr(ni)]), and n is some positive number. By describing solutions and appeared to cause eventual decreases hi the the rate in terms of the fractional rate of Cr(VI) formation Cr(III) oxidation rate possibly by limiting the generation in eq 5, the rate-limiting effects caused by the adsorption of new reactive sites on the 0-Mn02(s) surface. As a result, of the Cr(VI) products onto 0-Mn02(s) are empirically the Cr(IH) oxidation rate was apparently more dependent taken into account. The inverse dependence on [Crt] is on CrfVT) concentration than on Cr(IID concentration. An a result of describing the rate in terms of fc, but is also empirical rate expressio bees nha n derive accouno dt r tfo confirme determinatioe th y db slopa f approxino f e o - the rate-limiting effecproducee th f to d Cr(VI) specie d s logarithan e ploa matelth initiae r f to th fo f ml1 yo - fractiona l is described below. This expression is only applicabl o et rat Cr(VIf eo ) formation (d/c,/dt) versu logarithe sth f mo acidic solutions for which we have the most rate data. In the initial total Cr concentration [Cr?] (Figure 5). The neutral and basic solutions, the precipitation of Cr(OH)3(s) rates plotted in Figure 5 were derived by normalizing the and the weaker adsorption of Cr(VT) species onto ft- Cr(VI) concentration data shown hi Figure 3a to total Cr MnO2(s) would require a different approach, but the small concentration to obtain /<> at successive times. The initial amount of rate data gathered for such conditions prevents rates were determined by fitting second-order polynomials us from formulating an applicable expression. to the fc, versus time data by a least-squares method and The rates of heterogeneous reactions that involve com- taking the derivatives of those polynomials at time equal plex phenomena at surfaces can be described, in to zero. some instances, by expressions that relate the reaction rate The rate expression hi eq 5 can be integrated with re- reactioe toth n affinity, practicallwhice b n hca y described givo t gpece, fc timo tt d ean differencee th y b s between steady-state reactant concen- tration actuad san l concentrations to some power (19) o .T 1/( /c,)"'1- 1 -I/O.- l)[C - /cr)"~ « r( J-'Ml- / V)kt ) (6 apply suc ratha e expressio rate Cr(VIf eth o o nt ) forma- tion, inecessars ti defino yt steady-statea e Cr(VI) con- centration. Howeverrate th el experimentsal ,n i e initia,e th th whers i l fractio, e/& Cr(VIf no tota)o t . lCr concentrations of Cr(VD continued to slowly increase, even Equation 6 expresses the relationship between fc, and the steady-statd an , h afte 0 lons r50 a es g a concentration s were time that shoul observee db rate th e n experimentdi f si never convincingly reached. To derive a rate expression, the proposed rate expression is valid. To use eq 6 to we assumed that all of the aqueous Cr(IH) in the exper- evaluate the rate of aqueous Cr(VI) formation, it is nec- imental solution would be oxidized to Cr(VI) by reaction essary to determine the value of n, the empirical order of with 0-Mn02(s), given sufficient time and /J-MnO2(s). the reaction. The value of n was determined from two sets Because the rate of Cr(IH) oxidation is dependent on the of initial rate experiments that are described below, and amount Of Cr(VI) produced, we express this assumption this value was tested against the rate data from longer term in fractioterme th f s Cr(VIo f no tota)o t l dissolve r dC experiment determino st wit6 q h accurace e eth e th f yo remainin solutionn gi followin,e /Crth y ,b g limit rate expression. The initial rate method as applied to this study involves fc, - [Cr(VI)]/[Cr(VI) + Cr(m)] -» 1.0 as t — » (4) the determination of the fractional rate of Cr(VI) forma- ,•,.', , ^_ A. , * . tion as a function of the initial ratio of Cr(VI) to total wher brackete eth s refe molao rt r concentration s i t delved san (/c,)r dC . These initial rates linearf ,e i b n ,ca tim secondsn ei . Using this assumption propose ,w e that though tangentias a f to l lines alon reactioe gth n pathway the following rate expression isf increase descriptivM rate sf e th o nonlinearl f eo y with time e assume.W d that Cr(III) oxidatio Cr(VIo nt reactioy )b n with 0-MnO2(s) anionie th c: Cr(VI ) species, which were adde solutioo dt n At i At - t(A /VUCr Hfl 0 - /„ V for /„ ^ 1 0 (5) to create specific values of fc, >" the fractional rate ex- dfc,/d t*CA/V)[Cr- J (1.0 fct) for/CrS1. } 015 s, immediately cause-liiidn a rate-limitineffec- tbe g effect be- where * is a rate constant with units of mol-nT**'1, [CrJ cause of the reduction of reaction affinity through ad- refers to the total dissolved Cr concentration (i.e., fCr(VD sorption onto 0-Mn02(s) just as if they were produced by AR302602 Environ. Scl. Technol. 198, 712 . 119.No Vol. 1.21 Table II. Summary of Initial Fractional Rate* of Cr(VI) 8 6 Formatio Functioa * na /*rf no * fa pH df'ct/dt x 10" B*. % 3.4n= 5 (±0.64) 0.0 3.0 2.7 (±0.6) 95.6 pH=4.0 0.05 3.0 2.3 (±0.2) 99.4 , A/V=71.3 '*8 = (±0.39 1. ) 0 3. 98.40.1 0 (±0.2x 9 1. ) 0 3. 96.80.2 0 0.40 3.0 0.6 (±0.2) 93.4 <$ 0 4. 3.0.2 0s (±0.6 ) 97.2 § (±0.36 2. ) 0 4. 98.90.0 5 0.10 4.0 1.9 (±0.4) 96.4 ' 2.9n= 1 (±0.84) 0.20 4.0 1.4 (±0.2) 98.2 7.8 pH =3.0 (±0.20 1. ) 0 4. 97.10.30 A/V=35.6 0.40 4.0 0.5 (±0.2) 89.7 71.* 3V m'-L'1/ 'A . 'Error limit* indicat standaro etw d devia- tions. ______7.4 I I I 0 0.08 0.16 0.24 Cr(IU) oxidation. If the initial reaction rate for fractional ' . Cr(VI) production, d/^/dt, is linear with time, n theca nn ~lo Cf' ) a ' (1° be determined from a plot of log (d/c,/dt) versus log (1 - Hgure 6. toj-tog plot of the fractional rate of Cr(VI) formation showing ft., follows )a s fro logarithmie m th stoPe» * s use i.e , *>t*n*co 5 d. t erro forq e e th r f m o »Bmft- " s Indicat standaro etw d ^ deviations abou slopese tth . (7) Table III. Summary of Experimental Condition*. Rate determines valuwa e n Th f eo d from suc log-loha g plot Conrtant* Linead .an r Correlation Coefficient* (9-11 Data by using the rate data obtained from two sets of expert- PointB °' R**"8*" Analy*e.) ment is0 n conductewhic4. d % h, an th0 R* iou x de3. H k P *a fractionaH t [Cr?* p A10 / ]V x l rate Cr(VIf so ) formation were measure separati dh - eex periment" *• s wit h* specific valuetha , ° * /£ t f rangeso d fro 0 m0. s sfoa m to 0.40 possibilite .Th ^ 35 2a(±o!8yw a exist 3 )£ s tha apparene tth J tJj rate-lim- itin (±0.52 g1. effect) Cr(VI f s4 o 82^35 decreasy 9)ma e a tm higher value0 4. s of /cn possibly because of greater saturation of adsorption sites 4.0 38.5 35.6 1.3 (±0.5) 84.6 j3-MnO2(s)n o extremele th t ,bu y slo 6 w (±0.89. reaction3 3. 35.) 8 s0 rate93.3- 0s under such conditions prevented the gathering of accurate data. *•* '•' **•* J'SJ*0'8! |J'1 The fractional rate experiments lasted a total of 6-8 h, JJ JJ ,;} SJSS 9O8 during which a total of 8-10 samples were withdrawn and 3.5 . 94 7^1 2^3 (±0/7) 904 (±0.80 3. ) l analyze7- 87.6 determino d t 8 9- change quantitee th th i e° h 4- y /CT-/C* with time. The change in /<>-/$, with time was observed *•* 9-* 7.1 2.8 (±2.0) 84.4 to be Knear for each experiment for the 6-8-h period; *•' *• " "J*1^ !" hence, the initial fractional rates were determined from JJ J-j 7Ji SSoS 877 the slope1 7 i.4(±o!5s calculate 8 )9; lineay db 83.r0 regression0 4 experie th f so - mental data of /CT-/CT at (A/ V)t and are given in Table 4.0 9J 3&6 l!2(±o!s> 8o!o The quantity A/ V was included in the independent 4.0 9.6 71.3 1.3 (±0.5) 75.'3 solution sample removede sar e assumeW . d thae tth

eq 7 gave slopes of 2.9 (±0.8) and 3.5 (±0.8) (Figure 6), and ** the value of n is taken to be the average of the two slopes, d/c,/dt * k(A/V) [Cr J"l(l - /&)" (*° •"> for /Cr ^1.0 (9) 3.2 (±0.8). In Figure 6, it is clear that there are some large errors associated with the rate data. These errors reflect where m n i .- s im- v fri i m t~^ i /x A o - . resuli ratscattea e e analyticaeth f tar dato n rd i aan l dif- ficulties encountered in determining the small changes in * - 2.0 (±1.6) X KT" mol-m »-s » (10) Cr(VI) concentrations that occurred during some of the Linear correlation coefficients, R\ are also listed in Table fractional rate experiments conducte provid o solutiont i d h I H indicatioa ea s linearit wite plote th e hf th n th o f sf o y o higher values of fc, such as 0.30 and 0.40. the left side of eq 8 versus t. The R* values range from magnitude Th 9-1r rate fo 1eth datconstantf e% o 98 determinea s o pointst wa 5 , ,7 k , indicatind - g thaem e tth frorearrange6 q me a da pirically derived rate expressio ni9 s give q generalle n ni y descriptive of the reaction rates observed in the experi- |1/(1 - /Cr)" - 1/(1 - /&r)"}/{2.2(A/ VHCrJ-1) - kt menta with acidic solutions. (8) The rate expression hi eq 9 can be used to calculate the time required to oxidize a specific amount of Cr(III) to valuA eacer fofo h r k rat e experiment conducte acidii dh c Cr(VI). Iconsidee fw hypotheticara l exampl 1.0-ka f eo g solutio determines nwa d fro slopee mth plotf so thf so e left masacidin a f so c waste material containing 0.0% t 5w versuthesd ) an (8 elistesidq , se e valuet f ar e n o dk i f s/}-MnO2(so ) wit specifiha c surface arem^g"5 a f ad o 1an Table ID. Using the average k from all of the experiments, saturated porosity of 20% , then the resulting surface area :he rate expression describing the fractional rate of Cr(VI) to volume ratio for /3-MnO2(s) is 10 m^L*1. With this value . 12 . No . T*chr.oi •j?1 Sr n * Vr < flR3Q2603 Table IV. Time Required for Fractional Conversion of a °f manganese oxides the oxidation of aqueous Cr(III) is 10-* M Cr(III) Solution to Cr(VI) for 1 kg of a 20% Porosity unlikely to occur. Containing 0.05 wt % £-MnO,(s) with a Specific Surface m*»r 0 Are5. f alo Acknowledgments fc, days /& days We thank I. P. Murarka, EPRI project manager, for his „ - 9 0 01 osfl- continual suppor interestd tan e thanW . Sass. kB . ,J 0;30 31 0'J5 41^, Schramke, and two anonymous reviewers for their com- ments and discussions and Jan Baer for her editorial re- integrate e ,„,„,„„. - th , d . an drat. e for- „eth expressiof m,..„.. o . * „ _n n give y nb V16W eq 8, the times required to oxidize 10,30,50, and 90% of Re«istry No" Mn°" 1313-13'9: Cr'744 <6 Huntcr: J H' Loutitr ; > K' Wate> >M r Res-mi'15' reducing conditions in the underlying soils where Cr(VI) .„ i? „ . » « „ « n r E. • n , .«<>* would bi reduced to Cr(ffl) and possibly precipitated as <6) ?*S|£,e*' Cr(OH)3<$ depending on pH(J). However, we expect that (7) Murray> j/w> GeocWm. Cosmochim. Acta 1975,39, other form manganesf so e oxide causy sma e more rapid 505-519. rate Cr(fflf so ) oxidation calculatione th d ,Bartlett) an (g s showJames; ,R Environ . J n ,. i B . Qual. 1979.8, 31-35. Tabl represeneV I tbase-lina e cas whicr efo h futur Schroeder) (9 Watere Lee. ; comF . C. , G ,-. ,D Air, Soil Pollut. 1975, parisons might be made. 4,355-365. (10) Nakayama, E.; Kawamoto, T.; Tiurubo, S.; Fuginaga, T. Conclusions Anal. Chim. Acta 1981,130,401-404. _ ...... (ID Crowther, D. L.; Dilkrd, J. G.; Murray, J. W. Ceochim. experimentr Ou s indicate tha oxidatioe tth aqueouf no s Cosmochim. Acta 1983,47,1399-1403. Cr(m) doe occut s surface-catalyzeno y rb d reactions with (12) Oscarson Mammer; LiawHuang; ; K. W. M. . ,W . , . ,D . P , V dissolved oxyge direcy b t ntbu reaction with Soi j3-Mn02(s). T l Sci 1983,47,644-648.. SocJ . ,. Am . althoug exace hth t reaction stoichiometry is complicated (13) Rai Zachara; GirvinEary ,; Moore; D. ; E. M. C. . ,. ,L . , 3 D , parallee th y b l rat acidif eo c 0-Mn02(sSchmidt; SassResch; ; M. . T. A. L . , B . . ,. C ,D R ) dissolutio d nan possible formatio intermediatf no e manganese oxid - ere "C*eochel^al &**««* of Chromium Species"; final report action products such as MnOOH(s). The extent of Cr(ffl) EA-4544; Electee Power Research Institute: PaloAlto,CA, oxidation is limited probably by anionic Cr(VI) adsorption ,... 5??^_».j «» W.K.I,_,_ M T.p.i.j—... T r.i?«i ijn- acidi»^:j:«c solution„«!..*:„« Cr(OH)i(i y j^^. b \>,E. j d Method. . -s«» an S D ,r?,ir\\i . S ) «..J^.v.^rs \ precipitatio foSkougstad) \/. rW* Determinatio Fiahman^ W . ,.M . n n: i J. . ,;M Frtedmann o fbrd; C. - . ,L neutral to alkaline solutions. It is important to note, Inorganic Substances in Water and Fluvial Sediments; however, that soil form Mn02(ssf o likele ) o yt e lower fj.S. Geological Survey: Washington 1979, ,DC ; Boo. k5 zero point chargf so foe3 r[e.g.2. J-MnO2(s H ,p ) 337-340 p (75p ) versu. s 0-MnO2(sr fo 3 7. ) (16) highed ]an r surface energies - ,im (15) ColloiMurray. J . W d . ,InterfacJ e Sci. 1974,46,357-371. plying that these forms may cause more rapid oxidation (16) Heaty, T. W^ Herring, A. P.; Fuerstenau, D. W. J. Colloid of Cr(III) than the 0-MnO2(s) used here. The high solu- Interface Sci. 1966,21,435-444. bilities of Cr(VI) hi oxidizing environments imply <17> Hem-J-D-'lAad'c-J- Geochim. Cosmochim. Acta 1983, attenuatee b y thama r tdC onl adsorptioy yb n Sl^fJj^fLi,,.. reaction. , s , r^^^t,-^, A**~ 1091 inoo_in*O 5A » ratiier^by.nredpitationofasdu^^ g» ^^^^^S^^S^Ss^^, reade Th y oxidatio Cr(inf no Cr(VIo )t manganesy )b e XTc!, Kirkpatrick Eds., J. ; , MineralogicaR l Societf yo oxides indicates the need to examine the and America: Washington, DC, 1981; Chapter 1. oxidation potential of both the waste materials and the underlying soils at sites where Cr-bearing wastes may be Received for review November 25,1986. Accepted July 20,1987. deposited e presenc.Th manganesf eo e oxides woul - din This researc funde Electris e hwa th y db c Power Researc- hIn dicate the potential for greater transport of aqueous Cr ttitute (EPRI) under Contract RP2485-03, titled "Chemical aa Cr(VI) specie groundwatersn si , wherea absence th n si e Attenuation Studies".

Environ. Sci. Technol.. Vol. 21. No. 12. 1987 1193 flR30260*4