Chromium (III) interactions in seawater through its oxidation kinetics

Marine Chemistry, 34 ( 1991 ) 29-46

29

Elsevier Science Publishers B.V., Amsterdam

Chromium (III) interactions in seawater through its oxidation kinetics M. Pettinea, Frank J. Millerob and T. La Nocea aWater Research Institute, C.N.R., Via Reno 1, Rome 00198, Italy bRosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149, USA (Received 7 November 1990; revision accepted 21 March 1991 )

ABSTRACT Pettine, M., Millero, F.J. and La Noce, T., 199 I. Chromium (III) interactions in seawater through its oxidation kinetics. Mar. Chem., 34: 29-46. The rates of oxidation of chromium(III) to chromium(VI ) with n202 have been measured in NaCI and the major sea salts at pH 8 and 25°C. The effect of aging the chromium(III) solutions before the addition of H202 has also been investigated for the same solutions. Borate was the only major component of seawater found to affect the oxidation in seawater. Like O H - , an increase in the B(OH)~- concentration causes the rates of oxidation to increase. The rate-determining steps in seawater involve two chromium (III) species C r ( H 2 0 ) 5 O H 2+ + n 2 0 2

kon

,products

Cr(HzO)4(OH) [B(OH)4] + +H202

ka

,products

and the rate equation is given by

dCr(III)/dt=-k3[Cr(III)] [H202] [ O H - ] [ a ( o n ) ~ ] °3 where k3 = k~/ [ n202 ] [ O H - ] [ B (OH)~- ] 0.3 (k~ is the pseudo-first-order rate constant ). Aging of the solutions causes the rates of oxidation to decrease and is affected by the concentration of borate, carbonate and magnesium ions. Borate causes a reduction of the aging effect, while carbonate and magnesium produce an increase of the aging effect. The Mg 2+ influence is much stronger than CO~- when these ions are present at their seawater levels in individual Na-Mg-C! and Na-CI-CO3 solutions. When CO 2- and Mg 2+ are present at the same time, the combined aging effect is less than that found in simple Na-Mg-Cl solutions. Since the aging effect is related to the precipitation of chromium(III), magnesium and carbonate ions will probably affect the solubility of chromium in natural waters.

INTRODUCTION

Chromium exists in natural waters in two main oxidation states, chromium (III) and chromium (VI), characterized by markedly different ehemi-

0304-4203/91/$03.50 © 1991 Elsevier Science Publishers B.V. All fights reserved.

30

M. PETTINE ET AL.

cal behaviour. The cycle of this element is affected by various complex and interrelated processes, which include chemical or photochemical redox transformations, precipitation/dissolution and adsorption/desorption reactions, as well as biological interactions. Thermodynamic and kinetic data for these processes are needed to interpret field results and to predict the environmental fate of chromium. At present, adequate experimental data are scarce. Despite the thermodynamic stability of chromium(VI) in seawater, chrom i u m ( I l l ) species have been found at analytically significant levels (Elderfield, 1970; Nakayama et al., 198 l a). The concentration of total dissolved chromium is at nanomolar levels in seawater and the C r ( V I ) / C r (III) molar ratios range from 1 to 20 in oxic waters (Murray et al., 1983; Nakayama et al., 198 l b). Chromium (III) speciation in seawater is dominated by chromium (III) hydrolysis products. Elderfield (1970), using available association constants, calculated that Cr (OH)~- was the most important hydroxyl complex in seawater. According to more recent chromium(Ill) hydrolysis constants (Rai et al., 1987) Cr (OH) J- contributes only marginally to the total soluble chromium (III) concentration in solution. In the pH range of 6.5-10.5, Cr (OH)3 is the dominant complex. C r ( O H ) 7 becomes dominant above pH 11.4 and Cr(OH)2+ is dominant in the pH range 3.8-6.3 (Rai et al., 1989). Ion pairing with specific ions other than O H - is not considered to be important. Elderfield (1970) calculated insignificant concentrations for ion pairs characterized by the general formula [ CrX( H 2 0 ) 5] ~3- zx~,where X = F - , CI -, Br- and SO42- and Zx is the charge of the anion. Mixed complexes, involving O H - and the major anions of seawater were reported to be more important, but represent only a small fraction of the total chromium(Ill). Elderfield (1970) calculated that [CrCI(H20)4OH + ] was <0.1% of the total chromium (III) species. Turner et al. ( 1981 ) calculated an abundance of < 1% for F - , CI-, SO 2- ion pairs in seawater at pH 8.2. Unfortunately, it is not possible to make reliable speciation calculations for ligands such as CO ]- or B (OH)~- because the stability constants are not available. It is well known that chromium (III) tends to form polynuclear species in solution (Faust and Aly, 1981; Spiccia and Marry, 1986 ), but conflicting reports appear in the literature on the presence of these species under environmental conditions (Stunsi and Marty, 1983; Rai and Zachara, 1986). The association constants of some polynuclear species [Cr2(OH) 4÷, C r 3 (OH)45+ , C r 4 (OH)6+ ] are, according to Rai et al. (1987), many orders of magnitude lower than previously reported. At the very low levels of chromium (III) in seawater, the formation of these polynuclear species does not occur (Van der Weijden and Reith, 1982; Rai et al., 1989). The role of organic ligands in chromium(III) speciation was first addressed by Nakayama et al. (1981c) and later questioned by Osaki et al. ( 1983 ). The latter authors suggested that the chromium fractions, labelled by

CHROMIUM (lll) INTERACTIONS 1N SEAWATER

31

Nakayama et al. ( 1981 c ) as inorganic, Cr (III ) + Cr (VI), and bound organic species, could be an oversimplification. The solubility control of chromium (III) in natural waters is not fully understood. Three candidates for this role include Cr (OH) 3(s), chromite (FeCr204) and the mixed hydroxide (CrxFel_x) (OH)3(s). In the first case the concentration should be 400 nM at pH 8.5, according to the hydrolysis constants reported by Baes and Mesmer ( 1976 ) or lower if the constant found by Rai et al. ( 1987 ) is used. They gave a value of - 6 . 8 6 as the upper limit for log Ks~3. In the second case the chromium solubility should be less than 10 -~° M at pH 8 (Murray et al., 1983). In the third case it is expected to be lower than predicted for Cr(OH )3 (Sass and Rai, 1987; Rai et al., 1989), over a wide pH range. Recently, Millero (1985, 1990) had stressed the importance of speciation on the oxidation of metals in natural waters. Pettine and Millero (1990) examined the oxidation kinetics of chromium (III) with H202 in NaC104 solutions, pointing out the role this oxidizing agent has in controlling the geochemical cycle of chromium in surface waters. They also found that chromium (III) aging affected the oxidation kinetics and the aging behaviour in seawater was different from the results in NaC104 solutions. With the aim of explaining the observed differences in the aging process and to examine how chromium (III) ionic interactions affect the process, we have examined the effects the major seawater ions have on the kinetics of oxidation of chromium (III) with H202. This paper presents these oxidation and aging results for chromium(Ill) in sea salts and provides evidence for previously unknown species. EXPERIMENTAL

Reaction vessel The reactions were studied in a 400 cm 3 glass thermostatically controlled vessel. The temperature was controlled to + 0.004 ° C with a Grant circulating bath. The vessel was closed with a glass cover in which an automatic teflon dispenser was screwed. The cover had three more openings which allowed N2 to be bubbled through the solutions, to monitor pH during the runs with a combination Radiometer electrode and to add the reactants. The solutions were stirred with a magnetic teflon-coated stirrer.

Chemicals and procedure All of the chemicals used were reagent or ultrapure grade. Oxidation runs were carried out in NaCI (0.1-0.5 M) solutions, in artificial seawater or real seawater. Artificial seawater (NaC1, Na2SO4, KCI, NaHCO3, KBr, NaF, MgC12, CaC12, SrC12 and B (OH)3 ) was prepared according to the method of

32

M. PETTINE ET AL.

Millero (1976). Real seawater samples were collected in the coastal waters of the Tyrrhenian Sea about 80 km south of Rome. After temperature equilibration at 25°C, 1.9/~M Cr(III) was added to a 250 cm 3 sample from a stock nitrate solution (0.019 M ) stabilized in l N nitric acid. The pH of the sample was previously adjusted with borate buffer and small amounts of dilute HC1 or NaOH solutions in order to have a final pH of 8.00 _+0.05 after the addition of chromium. In pure 0.5 M NaCl solutions, without borate buffer, the pH was maintained constant at pH 8.00 ± 0.05 by using a Radiometer pHstat system, which automatically added very small volumes o f a 10 -4 M NaOH solution during the oxidation runs. The total volume added, in this latter case, was lower than 0.5% of the total sample volume. After the desired aging time, H202 (447/~M) was added from H202 solutions prepared immediately before the experiments by dilution of a 35% ( w / w ) stock solution. The oxidation of c h r o m i u m ( I I I ) was followed by measuring the appearance of c h r o m i u m (VI). The chromium (VI) was measured on aliquots of 20 cm 3 taken from the reaction vessel and quenched by addition to a 20 cm 3 flask containing 0.2 c m 3 H2SO4 ( l -I- l ) and 0.5 cm 3 of diphenylcarbazide in acetone (20 g dm -3). The diphenylcarbazide method used is selective for chrom i u m (VI). In our experimental conditions the detection limit was 0.05 ~tM. H202 interferes in the chromium (VI) determination by the standard diphenylcarbazide m e t h o d (APHA, 1985), due to its ability to rapidly reduce chrom i u m (VI) in acidic solutions to c h r o m i u m ( I I I ) . The concentration of reagents needed to eliminate this interference was determined from results of a previous study on the effect of H202 on chromium (VI) determination by the above method (Pettine et al., 1988). The absorbance was measured at 540 nm on a Carlo Erba spectrophotometer using 5-cm cells. The pH was measured on the free proton scale with a Radiometer pH-meter. Tris buffer was used to standardize the electrode at 25°C in the ionic strength range of 0.1-0.7 M (Millero et al., 1987 ). Results obtained during this study stimulated our interest in determining the correlation between chromium and various metals in particulate matter. Particulate metals (chromium, iron, manganese, calcium and magnesium) were determined, after digestion of dried samples acidified with 3 cm 3 HNO3 Suprapur (Merk) and 2 cm 3 HCI Suprapur (Merk) in closed teflon bombs, by a Perkin Elmer Plasma 40 (ICP). RESULTS AND DISCUSSION

The rates of oxidation of chromium (III) with H 2 0 2 have been studied recently in NaCIO4 solutions under pseudo-first-order conditions with [ H 2 0 2 ] > [Cr(III) ] (Pettine and Millero, 1990). The pseudo-first-order rate constant (kl, m i n - ~ ) , at an excess concentration of H202, has been determined from

CHROMIUM(Ill) INTERACTIONSIN SEAWATER

d [ C r ( I I I ) ] / d t = -k~ [Cr(III) ]

33

(1)

where [Cr(III)]=[Cr(III)]o-[Cr(VI)], the subscript refers to the initial concentration. The oxidation kinetics of chromium (III) have been found to be strongly dependent on aging the solution before the addition of H 2 0 2 ( P e t t i n e and Millero, 1990). The aging effect was attributed to the formation Of polynuclear hydroxide species. Based on the recent work of Rai et al. ( 1987, 1989 ) the polynucleation process in pure NaC104 or NaC1 solutions involves the Cr (OH)3 species and the aging effect should be interpreted in terms of solubility of this species ( < 10 -6"86) being exceeded in these experiments by the addition of 1.9 pM Cr (III). To correct the rates of oxidation for aging, Pettine and Millero (1990) extrapolated the values of the pseudo-first-order rate constant (kl) to time zero by plotting the function k~ - 2 / 3 ) v e r s u s the aging time. The analysis of the slope and the intercept of this function under different experimental conditions provides information on the kinetics of the aging process as well as the effects of chromium (III) speciation. In our first series of measurements we examined the effect of the borate concentration used to buffer the solutions (Table 1 ). The pseudo-first-order rate constant (kl, rain -1 ) in pure 0.5 M NaC1 medium and in 0.5 M NaC1 with some of the borate concentrations used is shown in Fig. 1 as a function of aging time. In Fig. 2 the power function ki-2/3) is plotted against time. The variations of the slopes of the aging function and of the pseudo-firstorder rate constants extrapolated to time zero kl {,-o~ as a function of borate TABLE 1 Values of ki extrapolated to t = 0 and the slope of the aging function in 0.5 M NaCI at pH 8.00_+ 0.05, 25 °C and different borate concentrations (Cr ( Ill ) = 1.9 #M and H202 = 4.47 pM ) Borate (mM)

kj~,~o)

Slope

0.009 0.017 0.034 0.063 0.085 0.137 0.205 0.273 0.340 0.512 0.683

0.049 0.064 0.074 0.091 0.107 0.128 0.132 0.151 0.144 0.174 0.182

0.146 0.116 0.089 0.065 0.069 0.064 0.061 0.055 0.057 0.056 0.051

34

M. PETTINE ET AL.

0.20 0.16 ~ X

a" °) b)

__\

• c)

0.12 L~i~..

.

~

~

.

o d) • e)

~

O.08"~ ' ~~ ' ~ ' ~ ~ 0.04

0.00

210

4'0

610 t(min)

8101001

120

H202(447

Fig. 1. Values of kt obtained for the oxidation of 1.9 pM Cr(III) with #M) at 25 °C and pH 8.00_+0.05 as a function of aging time in pure 0.5 M NaC1 (e) and in 0.5 M NaC1 with different borate concentrations: (a) 0.683 mM; (b) 0.340 mM; (c) 0.137 raM; and (d) 0.034 mM.

18 14

6 2

o d)

2'0

;o

s'o

t(min)

8'0

1oo'

~,

12o

Fig. 2. Values of the function k1-2/3) for the oxidation of 1.9 ~M C r ( I I I ) with I-I202 (447/~M) at 25°C and pH 8.00+0.05 as a function of aging time in pure 0.5 M NaCI (a) and in 0.5 M NaCI spiked with different borate concentrations: (b) 0.034 mM; (c) 0.137 raM; (d) 0.340

mM; and (e) 0.683 mM.

concentration (M), shown in Figs. 3 and 4, have been fitted to eqns. (2) and

(3): I/slope=40.55+6.519 log[B(OH)4] sd=0.998 k,(t~o) =0.40+0.071 log[B(OH)4] sd=0.008

(2) (3)

In pure 0.5 M NaCI, without borate buffer, the oxidation kinetics of chrom i u m ( I l l ) with H202 were found to be first order with respect to pH, in agreement with previous results (Pettine and Millero, 1990) obtained in 0.1 M NaC104 solutions buffered with borate (Fig. 5 ). The best fit of log k~ (t~o) versus pH gives a slope equal to 0.93 _+0.05 in 0.1 M NaC104 in the presence of borate and 0.90 + 0.06 in pure 0.5 M NaCI. The shift of the line in pure 0.5

35

CHROMIUM ( I l l ) INTERACTIONS IN SEAWATER 20

16 14

q m 12

>

10 8

6 -5'.0

-~.5

-~.o Iog[B(OH)~" ]

-3'.5

-3.0

Fig. 3. Values of the inverse of the slope of the function k } - 2/3) vs. aging time plotted as a function of log [ B ( O H ) 2 ] ( pH = 8.00 + 0.05; 25 ° C; Cr ( III) = 1.9 #M; I ' [ 2 0 2 = 447 #M ).

0.20 0.16

0.12

..~ 0.08 0.04-.

0.00 -5'.0

-,4.5

-4.0'

-3'.5

-5.0

Iog[B(OH)~-] Fig. 4. Values Ofkl extrapolated to zero aging k~ (,~o~ as a function of the log[ B ( O H ) g ] for the oxidation of 1.9 #M Cr (III) with H202 (447 #M ) at pH 8.00 + 0.05 and 25 ° C.

M NaC1 is consistent with eqn. (3) and the concentration of boron used in previous experiments (4.5 10 -4 M). The decrease of the slope of the power function with increasing borate concentration (Fig. 3 ), as well as the increase of the extrapolated kl (t~o) values (Fig. 4) may be related to the formation of Cr(H20)sB(OH) 2+ o r C r ( H 2 0 ) ( 5 _ x ) ( O H ) x [ B ( O H ) 4 ] ( 2 - x ) + ion pairs. These species cause the oxidation rate to increase and the aging process to be slower. The rate of oxidation of chromium (III) with H 2 0 2 in NaCl medium with or without borate can be related to the differences in the rate of oxidation of the various hydroxyl- and borate-chromium (III) complexes, according to the following equilibria,

36

M. PETTINEETAL. 0.0

--0.8 0.4

/ ~ , o ) o

9 a¢. -1.2 -1.6 -2.0

715

8

8'.5 pH

9

D b)

i

915

10

Fig. 5. Values of the logarithm of k~ extrapolated to zero aging k, (,~o) in NaCIO4 0.1 M solutions buffered with 8× l0 -5 M borate (a) and in pure 0.5 M NaCl (b) as a function ofpH at 25°C (Cr(III)= 1.9/~M; H202 =447/IM).

Cr 3+ + H 2 0 2

kCr

,products

(4)

Cr ( O H ) 2 + + H2 O2 kO_~H products C r B ( O H ) ] + "FH202

kB

,products

(5) (6)

where the waters of hydration have been o m i t t e d for clarity. The overall rate constant in a given m e d i a buffered with borate is related to the individual rates (Millero, 1985) by

k[Cr(III) ]r=kcr[Cr3+ ]+kon[Cr(OH)2+ ]+kB[CrB(OH)24 + ]

(7)

Equation (7) can be expressed as k = kcr O~Cr-t- koH aCrOH 4" ka OlCrB

(8 )

where the values of aCrX are the molar fractions of c h r o m i u m ( I I I ) species. The latter equation gives

k/acr=kcr+koH(aCrOH/Olcr)+

ka (aCra/O~Cr)

(9)

A similar equation will be obtained for NaC1 solutions without borate,

k' / acr = kc~ + koH ( aCroH/ oto ) =kc~

+ koH(fl,/H+)

(10)

( 11)

where ac~ is the free c h r o m i u m (III) fraction in the pure NaC1 m e d i u m and fl, is the first hydrolysis constant for c h r o m i u m (III). As shown in our earlier work (Pettine and Millero, 1990) the value of kcr is small and the measured k' is related only to kou. By substituting eqn. ( 11 ) into eqn. (9) we have

37

CHROMIUM (111) INTERACTIONSIN SEAWATER

k/acr =kcr +kon(flt/[H + ] ) +ka(aCrS/aCr)

(12)

Unfortunately, even considering only one borate bound chromium species eqn. (12) cannot be solved for ka since the value flCrB needed to determine acrB and act is not known. A rough estimation of the stability constant for the interaction of chromium with borate may be based on the assumption that the oxidation kinetics of chromium (III) are affected by the reaction of borate ions with Cr(OH )2+, the most probable species interacting with H202 (Pettine and Millero, 1990 ), according to the equilibrium ]~CrOH,B

Cr(OH) 2+ + B ( O H ) 4

,Cr(OH) [B(OH)4] +

(13)

This reaction produces a mixed borate hydroxide ion pair which has a faster oxidation rate than the simple hydroxide. Thus, in the presence of borate the two species, Cr ( OH ) 2+ and Cr (OH) [ B ( OH ), ] +, would control the oxidation kinetics. The measured pseudo-first-order rate constant kl (,~o) in NaCI 0.5 M buffered with borate is related to the individual ki values of the reacting species by k, (,~0) [ Cr (III) ] r = kc~on [ CrOH 2+ ] + kooHa [ CrOHB + ]

(14)

which can be simplified to

kl(t~o) = kc~o. aCrOH +

kCrOHBOtCrOHB

( 15 )

The fractions of the ion pairs ai can be estimated from aC~OH = ( 1 + flc~OHa[B ( O H ) ; ] ) - '

(16)

aC~OHa = 1 -

(

OdCrOH

17 )

The substitution into eqn. ( 15 ) gives

kl(t~o) = kc~oH ( 1/(

1 + flOOHB[B(OH)4 ] ) ) +

kcroH B ( 1 -- ( 1/( 1 +

(18)

flOOHB[B ( O H ) 4 ] ) ) )

where k o o n is the pseudo-first-order constant in pure NaC1 solutions (=0.029). Since both #CrOHB and kCrOHB are not available, eqn. ( 18 ) has been solved by an iterative technique. Values offlCrOHB= 104 and kcrorm=0.20 gave the best fit of the measurements. The value of koonB is reasonable considering that the experimental measurements of kl (t~0) at the highest borate concentrations levels off to a value near 0.18. Figure 6 shows a comparison of the measured and calculated values of kl(t~o) obtained with flCrOHB=10 4 and kCrOH B = 0.20. The value of the association constant estimated for the equilibrium (13) supports the competition between the Cr(OH) 2+ and Cr(OH) [B(OH)4] +, but both these species are at very low concentrations under our experimental conditions. The higher order chromium (III) hydrox-

38

M. PETTINE ET AL. 0.20 0.16

o

~

•

0.12 t -~ 0.08

a • o) o b)

0.04 O.OOl 0

012

0!4 [8(0H)4 ] r'nM

01.6

0.8

Fig. 6. Values of experimental (a) k~ extrapolated to zero aging as a function of borate (mM) compared with the corresponding kt u-o) calculated (b) by assuming flCrOUB = 104and kcroHa= 0.20, respectively, for the association constant of the species Cr(H20)4(OH)[B(OH)4] + and its oxidation rate (pH = 8.00 _+0.05; 25 °C; Cr(III ) = 1.9/~M; H 2 0 2 --~447 pM ). ides, more i m p o r t a n t in c h r o m i u m (III) speciation, m a y not be directly involved in the oxidation process with H202. The oxidation kinetics of these species are slow and the fast hydrolysis equilibrium allows the more reactive species C r ( O H ) 2+ and C r ( O H ) [ B ( O H ) 4 ] + to be formed. The influence of borate on aging the c h r o m i u m (III) solutions, shown in Fig. 3, strongly supports a chemical interaction between c h r o m i u m and borate. The aging process still affects the c h r o m i u m solubility, even at high borate concentrations, and suggests the formation of species, such as C r ( H 2 0 ) 3 ( O H ) 2 B ( O H ) 4 , which do not react directly with H202, but are characterized by slower precipitation kinetics than Cr ( H 2 0 ) 3 ( O H ) 3. The borate ions do not appear to significantly influence the equilibrium chrom i u m (III) speciation or solubility, but to play an i m p o r t a n t kinetic role controlling the cycling of c h r o m i u m . Figure 7 shows the linear dependence of the values of log k l(t~o) versus log [ B ( O H ) ~- ]. The slope of 0.30 ___0.01 represents the kinetic order of the chrom i u m (III) oxidation reaction with respect to borate ions. Since it has been d e m o n s t r a t e d that the oxidation rate of c h r o m i u m (III) with H202 in the natural water p H range is first order with respect to [ H202 ] and [ O H - ~] (Pettine and Millero, 1990), the overall rate equation for the oxidation of chrom i u m (III) with H202 in seawater is given by d [Cr(III)]/dt=

- k 3 [ C r ( I I I ) ] [H202] [ O H - ] [B (OH)~- ]0.3

(19)

where k3=kl/[H20] [ O H ] [ B ( O H ) z ]o3 (k~ is the pseudo-first-order rate constant). Aging curves in NaC1 m e d i u m at different ionic strengths ( I = 0.05, 0.3 and

CHROMIUM(II1) INTERACTIONSIN SEAWATER

39

-0.7 -0.8

, ~ °

S" - 0 . 9 -

+ - I .09

-°~ -1,1 -1.2 -1,3

°

-1.4

-5:0

~

-4.5

-I.0

-5.5'

-3.0

log[B(OH)4-]

Fig. 7. Values of the logarithm of k~ extrapolated to zero aging as a function of log[B ( O H ) 2 ] (pH = 8.00 + 0.05; 25 ° C; C r ( I l I ) = 1.9/~M; H202 = 447/~M ).

0.5 M) in the presence of a boron concentration in the range 0.4-1.85 mM give similar values, within the experimental error, of both kl(t~o) and the slope of the aging function (see Table 2). Since k lct~o) was also found to be unaffected by ionic strength in NaC104 solutions (Pettine and Millero, 1990), it may be concluded that chloride ions do not significantly interact with chromium(III). Oxidation runs of chromium (III) with H202 in 0.5 M NaC1 (pH = 8.0 ) at two different NaHCO3 concentrations, 0.002 and 0.006 M, in the absence of boron (Table 2 ) give similar extrapolated values of kl at t = 0 within the experimental error (0.027 + 0.007 and 0.025 + 0.006, respectively). The slopes of the respective aging functions k~ -2/3) are 0.604 and 0.376. These values can be compared with the corresponding values of k~(t~o) =0.029 _+0.002 and the slope of 0.173 ___0.019 obtained in pure 0.5 M NaC1 solutions. These resuits indicate that carbonate only affects the aging process and not rates of oxidation. In 0.5 M NaC1 with 0.41 mM boron (corresponding to 0.0757 mM [B (OH)~- ] ) the same concentrations of NaHCO3 give similar values of kl at t = 0 (0.124+0.011 and 0.111 +0.005) and slopes of k1-2/3) versus time (0.290 ___0.015 and 0.174 + 0.008-see Table 2 ). The corresponding values in the same matrix without carbonate are 0.107 for k~ at t = 0 and 0.073 for the slope. These results confirm that carbonate ions only influence the aging function, the slope of which increases with decreasing concentrations of NaHCO3. This would indicate that the chromium (III) interaction with carbonate ions produces new species which speed up the precipitation process. The competition with borate ions lowers the increase of the precipitation kinetics as a result of the formation of chromium ( III )-carbonate species. The observation of Baloga and Early ( 1961 ) that the presence of carbonate ions tends to lower the oxidation rate of chromium could be related, based on our results, to the effect that the carbonate species have on the chro-

40

M. PETTINEET AL.

TABLE 2 Values of kl extrapolated to t = 0 and the slope of the aging function in different media a Medium

k, ~,~0)

Slope

NaC1 (0.5 M ) NaC1 (0.5 M ) + NaHCO3 (0.002 M ) NaCI ( 0.5 M ) + NaHCO3 (0.006 M ) NaCI ( 0.5 M ) + B (OH) 4(0.0757 mM ) NaCI(0.5 M) +B(OH)4(0.0757 m M ) + NaHCO3 (0.002 M ) NaCI(0.5 M) +B(OH)4(0.0757 r a M ) + NaHCO3(0.006 M) NaC1 (0.41 M ) + B (OH)4 (0.0757 mM) + SO4 (0.028 M) NaCI(0.041 M)+B(OH)4(0.0757 r a M ) + NaHCO3 (0.002 M ) + SO4 (0.028 M ) NaCI (0.05 M ) + B (OH)4 (0.341 mM) NaCI(0.3 M) +B(OH)4(0.341 mM) NaCI(0.5 M) +B(OH)4(0.341 mM) NAC1(0.347 M) + B(OH)4(0.341 raM) + Mg(0.051 M) NaCI(0.26 M) +B(OH)4(0.0757 m M ) + SO4(0.028 M ) +NaHCO3(0.002 M) + Mg(0.051 M) ASW Real SW

0.029 _+0.002 0.027 _+0.007 0.025 _+0.006 0.107 b 0.124+_0.011

0.173 _+0.019 0.604 _+0.098 0.376 _+0.068 0.073 b 0.290_+0.015

0.111 +-0.005

0.174_+0.008

0.105 _+0.006

0.067 _+0.007

0.105_+0.007

0.266_+0.012

0.166 _+0.050 0.172_+0.010 0.191 _+0.040 0.227 + 0.050

0.086 + 0.009 0.060_+0.001 0.075_+0.006 6.30 c

0.048+_0.005

0.55 _+0.054

0.050+-0.005 0.045 + 0.002

0.71 _+0.05 0.57 _+0.03

aMeasurements were made at pH = 8.00 + 0.05, 25°C, C r ( I I I ) = 1.9/~M, and H2OE= 447/~M. The ionic strength was fixed at 0.5 M, with the exception of the seawater samples and NaC1 solutions at

0.05 and 0.3 M. bValues calculated from eqns. ( 1 ) and (2). cValue calculated from eqn. (20).

mium (III) aging process. Sulphate ions, at the seawater concentration (0.028 M ), do not appear to play any significant role in the chromium (III) aging or the rate of oxidation. In 0.5 M NaCI+0.41 mM boron ( = 0 . 0 7 5 7 mM [B (OH)~- ] ) the slope of the power function is 0.067 _+0.007 and the extrapolated k~ value to t = 0 is 0.105 _+0.006 in the presence of 0.028 M SO4z- compared to 0.073 and 0.107, calculated, respectively, from eqns. (2) and (3) for the same matrix without sulphate (Table 2). The ineffectiveness of SO 2- on the oxidation rate and aging of chromium is also shown in NaC1 solutions containing borate and carbonate ions at the seawater level (Table 2 ). A concentration as high as 1 M of sulphate was necessary to obtain even a very small effect. These results are consistent with the experimental finding of Rai and Zachara (1986) that anions, such as CI-, SO4z- , do not form aqueous complexes with chromium (III). After the investigation of anions we also performed some kinetic runs in artificial seawater (ASW). Figure 8 shows that the power c u r v e k~ -2/3) ver-

CHROMIUM(Ill) INTERACTIONSIN SEAWATER

41

20 • o) o b)

16

v~ 12 8 4

I 10

i

20

I

30

i

I

40 50 tCmin)

I

I

60

70

80

Fig. 8. Values ofk1-2/3) as a f u n c t i o n o f a g i n g t i m e i n 0.5 M NaC1 b u f f e r e d w i t h 0.08 mM B(OH)~- (c), in 0.41 M NaCl b u f f e r e d w i t h 0.08 mM B(OH)~- a n d s p i k e d w i t h 0.002 M NaHCO3 and 0.028 M SO~- (b), a n d in a r t i f i c i a l s e a w a t e r (a) (pH=8.00+0.05; 25°C; Cr(III) = 1.9/~M and H202= 447/zM). 18

1° / 14

• o) o b)

12

• c)

i

8 ~

D 40

1~3 210 310

410 510 610 7'0 *(mio)

80

Fig. 9. Values of k} -2/3) as a function of aging time in 0.5 M NaCI buffered with 0.08 m M B (OH)~- (b), in 0.44 M NaC] buffered with 0.08 m M B(OH)~- and spiked with 0.02 M Ca 2+ (¢), and in 0.44 M NaC] buffered with 0.08 m M B(OH)~- and spiked with 0.02 M M g 2+ (a) (pH = 8.00 + 0.05; 25 °C; Cr (HI) = 1.9/~M; H202 = 447/~M ).

sus time in ASW has a completely different behaviour (slope= 0.71 ___0.05 and kl (t~o) =0.05 _+0.005 ) compared with 0.5 M NaC1 with 0.41 mM B, 0.002 M NaHCO3 and 0.028 M SO 2- (slope=0.266_+0.012 and k,(,_o)= 0.105 ___0.007).

This large difference in ASW compared with NaC1 in the aging and oxidation rate is due to the cations in seawater (Fig. 9 ). The addition of Ca 2+ at a concentration about double that in seawater (0.02 M) to NaC1 solutions buffered with 0.41 mM B ( = 0 . 0 7 5 7 mM [B(OH)~- ] ) does not produce any significant variation. On the contrary, the same molar concentration of Mg 2÷ causes a strong increase in the slope of the power function from 0.07 to 1.16. In order to get more information on the Mg 2÷ effect, the oxidation of chromium (III) with H202 was investigated as a function of aging time in NaC1

42

M. PETTINEET AL.

with different levels of Mg 2÷ concentrations at a constant ionic strength (0.5 M) and a boron concentration of 1.85 m M (corresponding to 0.341 m M [B(OH)4] ) (Fig. 10). Mg 2+ ions exert a strong effect on the aging process, causing it to become faster with an increase of the Mg 2÷/Cr(III ) molar ratio. The value of kl at t = 0 does not show a significant variation with increasing Mg 2+ concentration. The mean value 0.227 + 0.05 is within the experimental error similar to the value 0.191 + 0.04 obtained under the same experimental conditions (NaCI 0.5 M + boron 1.85 m M ) in the absence of Mg 2+. The Mg 2÷ appears to be one of the most important ions affecting the chromium (III) aging process. The variations of the slope of the aging curve as a function of Mg 2÷ concentration (M) have been fitted to the equation log(slope) = 3.32+ 1.951og[Mg 2÷ ]

sd =0.074

(20)

If we assume that the slope of the power function k~ -2/3) is proportional to the chromium (III) aging rate, then a plot of log (slope) against log [ Mg 2÷ ] gives the kinetic order of the reaction relative to the Mg2+-Cr(III) interaction. Figure 11 shows the second-order dependence of chromium (III) aging rate over a Mg 2+ concentration range from 0.005 to 0.04 M. At very low concentrations ( < 1 0 -2.5 M) Mg 2+ does not significantly affect the chrom i u m ( I l l ) aging (Fig. 11 ). At Mg 2÷ concentrations higher than 0.04 M, it was impossible to make reproducible kinetic measurements since the aging process was too fast. Under these conditions no c h r o m i u m ( I l l ) oxidation could be detected after a few seconds of aging. The strong and very fast interaction of chromium (III) with Mg 2+, demonstrated by these results, and the occurrence of the mixed solid phase [ CrxFe~ -x) ] (OH) 3, controlling chrom i u m (III) solubility in natural waters (Sass and Rai, 1987 ), would suggest the formation of a previously unknown solid, such as [CrxMg(~_x~t.5 ] (OH)3. 0.16

•

0.12.

Db)

\\ .~- - ,

o d)

• c)

•

Z

o)

~

• e)

0.08

0.04

0 0 1'0

3'0 4'05'0 60 7'0

1 0110

t(min)

Fig. 10. Values of k~ for the oxidation of 1.9/~M Cr(III ) with H202 (447/zM) as a function of aging time at pH 8.00 + 0.05 and 25 °C in NaCI solutions buffered with 1.85 mM B (OH)Z and spiked with different molar Mg 2÷ concentrations at 0.5 m ionic strength: (a) pure NaC1; (b) NaCl+0.0068 M Mg2+; (c) NaCI+0.0009 M Mg2+; (d) NaCI+0.0091 M Mg2+; (e) NaCl+0.0182 M Mg2+; and ( f ) NaCI+ 0.04 M Mg 2÷.

CHROMIUM(Ill ) INTERACTIONS IN SEAWATER

1,2

o

43

o

0,8

I

0

-0.4

-0'81.0

° i

1.5

2L.0 2t.5 -log Mg(rnol/I)

3L.0

i

3.5

Fig. l l. Values of the logarithm of the slope of ki-2/3) as a function of the logarithm of the molar Mg 2+ concentration (pH = 8.00 + 0.05; 25 ° C; Cr (H!) = 1.9 #M; H202 = 447/~M ).

This species controls the soluble forms of chromium ( I i I ) under our experimental conditions. It is worth noting that chromium, magnesium and iron have similar ionic radii, equal to 0.69, 0.65 and 0.64/i, respectively. The (Cr,Mg) (OH) 3 species appear to accelerate the aging process when the Mg 2+ concentration is high. This is coherent with the results of Sass and RaJ ( 198? ), showing that the solid solution of (Cr,Fe)(OH)3 is dependent on the mole fraction of chromium (HI) in the solid and that its precipitation/dissolution kinetics are rapid. According to Sass and Rai (1987), at the relatively low chromium mole fraction ( < 0.1 ) expected in natural environments the solubility of the (Cr,Fe) (OH)3 solid should be lower than Cr(OH)3. The possibility of chromium ( III) interaction with iron ([H) and Mg 2+ is also strengthened by the highly positive correlation existing between chromium and iron or magnesium in riverine paniculate matter shown in Fig. 12 (data pair = 20; RFe= 0.94 and RM, = 0.89 ). These results were obtained on particulate metals in the dried suspended matter of 0.45-/~m filtered samples collected in the lower stretch of the Po river. The monitoring study was carried out in the period September, 1988-March, 1990 in the framework of a project set up to evaluate the main pollutant loads to the Adriatic sea. It is worth noting that particulate calcium and manganese in the same samples (n = 20 ) did not show a significant correlation with particulate chromium(Rca=0.5! and RM,=0.19). NaC1 solutions (0.5 M), with borate, carbonate and magnesium concentrations at the seawater level, show a similar k~ value extrapolated to time zero as ASW or natural seawater samples: the kl at t = 0 arc, 0.048_+0.05, 0.050 + 0.005 and 0.045 _+0.002, respectively. The chromium (III) aging behaviour is also similar in the above media, the relative slopes are, 0.55 _+0.05, 0.71 + 0.05 and 0.5? _+0.03, respectively, for NaC1 solutions with borate, carbonate and magnesium concentrations at the seawater level, ASW and real SW.

44

M. PETTINE

260

ET AL.

o -

•

~

G

• :~

l•

"-~ 140 'J

,

100

~

60 5000

D o

,,.

I 10000

o

.

[] )

~

t 15000

I 20000

o b)

I 25000

I 30000

: 35000

40000

Mep(/~ g / g )

Fig. 12. Correlation between particulate chromium (I II ) and particulate Mg 2+ (a) or Fe 3+ (b) as found in suspended matter of the Po river about 70 km from its mouth.

The Mg 2÷ effect in seawater-like media is markedly lower than in simple NaC1 solutions buffered with borate. As already noted, at the concentration of Mg 2÷ present in seawater, we could not measure any oxidation of chromium (III), due to the fast kinetics of the aging process. The slope of the aging function should be, according to eqn. (20), 6.3 for [ Mg 2÷ ] = 0.051 M, while the observed experimental slope in seawater corresponds to a concentration of Mg 2÷ about 0.015 M. Since variations in the free fraction of Mg 2+ as well as in its speciation cannot justify such a reduction of the Mg 2+ effect and sulphate was shown not to interact with chromium (III), the only possible explanation is that carbonate is kinetically favoured to react with chromium (Ill), thus reducing the Mg 2+ effect. By considering the strong correlation found between chromium (III) and Mg 2+ in riverine particulate matter, one might also expect that the effect of Mg 2÷ becomes more important in equilibrium conditions. CONCLUSIONS

The results presented provide a new insight into how the major components of natural waters affect the kinetics of oxidation of chromium(Ill). Complexation of chromium (III) with borate appears to be possible. At present, however, our estimation of the association constant should only be treated as a good guess. Although the influence of borate on the speciation of chrom i u m ( I I I ) may be small, the effect on the rate of oxidation of chromium (III) is large. The persistence of the aging process in the presence of borate indicates that this anion has essentially a kinetic effect in slowing down the chromium (III) aging and increasing the rate of oxidation. It is worth noting that borate appears to be the only ion present in seawater which tends to

CHROMIUM(lIDINTERACTIONSINSEAWATER

45

lower the precipitation kinetics and to increase the oxidation rate of chromium. The interaction of chromium (III) with Mg 2+ strongly affects the aging process in simple NaC1 solutions buffered with borate, probably supporting the formation of a mixed solid phase (Cr,Mg) (OH)3, which controls the chromium solubility. In seawater-like media such an influence is markedly reduced due to the interaction of chromium (III) with carbonate ions. The interaction of chromium (III) with carbonate and Mg 2÷ tends to increase the aging process, providing new species (probably mixed species such as CrC O 3 O H and CrxMg(l_x)1.5 (OH)3 ) which increase the kinetics of the precipitation. Carbonate-chromium(Ill) species were not found to affect the extrapolated values of kl to t=0. The similar results obtained on the pseudofirst-order rate constant of chromium (III) oxidation with H202 in NaC1 0.5 M with borate, carbonate and magnesium concentrations at the seawater level, in ASW or natural seawater samples allow us to make a better estimation of the lifetime of chromium(Ill) in surface seawater. By considering a mean value of 0.048 for kl at zero aging time and a H202 concentration of 0.1/tM the half time of chromium (III) at 25 ° C and pH 8 should be 45 days, rather than 24 days as previously estimated (Pettine and Millero, 1990). ACKNOWLEDGEMENTS

F.J. Millero wishes to acknowledge the support of the Office of Naval Research and Oceanographic section of the National Science Foundation for this study. REFERENCES American Public Health Association, 1985. Standard Methods, 16th edn., Washington, DC, pp.

201-204. Baes, C.F. and Mesmer, R.E., 1976. The Hydrolysis of Cations. John Wiley, New York, 489 pp. Baloga, M.R. and Earley, J.E., 1961. The kinetics of the oxidation of Cr(III) to Cr(VI) by hydrogen peroxide. J. Am. Chem. Soc., 83: 4906-4909. Edlerfield, H., 1970. Chromium speciation in seawater. Earth Planet. Sci. Lett., 9: l 0-16. Faust, S.D. and Aly, O.M., 1981. Chromium. In: Chemistry of Natural Water. Ann Arbor Science, An Arbor, MI, pp. 376-398. Millero, F.J., 1976. Thermodynamic models for the state of metal ions in seawater. In: E.D. Goldberg (Ed.), The Sea, Ideas and Observations, Vol. 6, Chap. 17. Wiley and Sons, New

York, pp. 653-693. Millero, F.J., 1985. The effect of ionic interactions on the oxidation of metals in natural waters. Geochim. Cosmochim. Acta, 49: 547-553. Millero, F.J., 1989. Effect of ionic interactions on the oxidation of Fe (II) and Cu (I) in natural waters. Mar. Chem., 28: 1-18. Millero, F.J., 1990. Effect of speciation on the rates of oxidation of metals. In: J.W. Patterson and R. Passino (Eds.), Metals Speciation, Separation and Recovery, Vol. II, Lewis Publ., Illinois, pp. 125-145. Millero, F.J., Hershey, J.P. and Fernandez, M., 1987. The pK of TRIS in N a - K - M g - C a - C I SO4 brines-pH scales. Geochim. Cosmochim. Acta, 51:707-711. Murray, J.W., Spell, B. and Paul, B., 1983. The contrasting geochemistry of manganese and

46

M. PETTINE ET AL.

chromium in the eastern Pacific Ocean. In: C.S. Wong, E. Boyle, K.W. Bruland, J.D. Burton and E.D. Goidberg (Eds.), Trace Metals in Sea Water, NATO Conf. Ser. IV. Plenum Press, pp. 643-669. Nakayama, E., Tokoro, H., Kuwamoto, T. and Fujinaga, T., 1981a. Dissolved state of chromium in seawater. Nature, 290: 768-770. Nakayama, E., Kuwamoto, T., Tokoro, H. and Fujinaga, T., 1981b. Chemical speciation of chromium in sea water. Part 3. The determination of chromium species. Anal. Chim. Acta, 130: 247-254. Nakayama, E., Kuwamoto, T., Tsurubo, S., Tokoro, H. and Fujinaga, T., 1981c. Chemical speciation of chromium in sea water. Part 1. Effect of naturally occurring organic materials on the complex formation of chromium (III). Anal. Chim. Acta, 130: 289-294. Osaki, S., Osaki, T., Hirashima, N. and Takashima, Y., 1983. The effect of organic matter and colloidal particles on the determination of chromium(VI) in natural waters. Talanta, 30: 523-526. Pettine, M. and Millero, F.J., 1990. Chromium speciation in seawater: the probable role of hydrogen peroxide. Limnol. Oceanogr., 35: 730-736. Pettine, M., La Noce, T., Liberatori, A. and Loreti, L., 1988. Hydrogen peroxide interference in the determination of chromium (VI) by the diphenylcarbazide method. Anal. Chim. Acta, 209: 315-319. Rai, D. and Zachara, T.M., 1986. Geochemical behavior of chromium species. Research Project 2485-3, Battelle Pacific Northwest Laboratories, Richland, pp. 63. Rai, D., Sass, B.M. and Moore, D.A., 1987. Chromium (III) hydrolysis constants and solubility ofchromium(llI) hydroxide. Inorg. Chem., 26: 345-349. Rai, D., Eary, L.E. and Zachara, T.M., 1989. Environmental chemistry of chromium. The Science of Total Environment, 86:15-23. Sass, B.M. and Rai, D., 1987. Solubility of amorphous chromium(IlI)-iron(III) hydroxide solid solutions. Inorg. Chem., 26: 2228-2232. Spiccia, L. and Marty, W., 1986. Fate of active chromium hydroxide Cr(OH )33H20 in aqueous suspension. A study of chemical changes involved in its aging. Inorg. Chem., 25:266-271. Stunzi, H. and Marty, W., 1983. Early stages of the hydrolysis of chromium(Ill) in aqueous solution 1. Characterization of tetrameric species. Inorg. Chem., 22:2145-2150. Turner, D.R., Whitfield, M. and Dickson, A.G., 1981. The equilibrium speciation of dissolved components in freshwater and seawater at 25 °C and 1 atm pressure. Geochim. Cosmochim. Acta, 45: 855-881. Van Der Weijden, C.H. and Reith, M., 1982. Chromium (III)-chromium (VI) interconversions in seawater. Mar. Chem., 11: 565-572.