Catalytic effect of soil colloids on the reaction between CrVI and p-methoxyphenol

Article abstract of DOI:10.1016/S0269-7491(00)00028-2

Adsorption of Cr^VI and p-methoxyphenol (PMP) on soil colloids at different pH media was studied. The resulting k₁ and n of 1.89*10² and 0.53 (r² = 0.99) and k₂ and b of 0.13 and 1.25*10³ (r² = 0.96) were obtained from Freundlich (Q = k₁*Cⁿ_aq) and Langmuir [Q = k₂*b*C_aq/(1 + k₂*C_aq)] simulation equations, respectively, for Cr^VI adsorption on soil colloids (pH 4.20). The adsorption of PMP on soil colloids in pH 5.72 media was simulated by five different equations and the results indicated that the Fritz-Schluender one (r² = 1.00) was the most suitable among them. Adsorption quantity of Cr^VI and PMP on colloids increased with increasing acidity in the pH range of 3.5-9.0. Study of Cr^VI adsorption kinetics indicated that the adsorption equilibrium of Cr^VI was reached rapidly within 2 h. In pure aqueous solution, Cr^VI reduction by PMP was observed only when the media's pH was lower than 4.0. Oxidation and reduction reaction between Cr^VI and p-methoxyphenol obviously occurred when soil colloids were involved in this system, even at pH >= 7.0, which strongly suggested that minerals in soil colloids acted as catalysts to speed the reaction of Cr^VI and PMP. The oxidized product of PMP by Cr^VI, extracted by chloroform in acid media and analyzed by gas chromatography-mass spectrometry, was identified as benzoquinone. The reaction included two steps of one electron process.

Full text of DOI:10.1016/S0269-7491(00)00028-2

Environmental Pollution 111 (2001) 75±81
www.elsevier.com/locate/envpol
Catalytic e€ect of soil colloids on the reaction between CrVI
and p-methoxyphenol
Dong-mei Zhou, Huai-Man Chen *, Chun-rong Zheng, Cong Tu
LMCP, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, People's Republic of China
Received 16 March 1999; accepted 18 December 1999
``Capsule'': Adsorption of CrVI and p-methoxyphenol on soil colloids was a€ected by pH.
Abstract
Adsorption of CrVI and p-methoxyphenol (PMP) on soil colloids at di€erent pH media was studied. The resulting k1 and n of
1.89Â102 and 0.53 (r2=0.99) and k2 and b of 0.13 and 1.25Â103 (r2=0.96) were obtained from Freundlich ꢀQ ˆ k1Canq† and Langmuir
‰Q ˆ k2bCaq=ꢀ1 ‡ k2Caq†Š simulation equations, respectively, for CrVI adsorption on soil colloids (pH 4.20). The adsorption of PMP
on soil colloids in pH 5.72 media was simulated by ®ve di€erent equations and the results indicated that the Fritz-Schluender one
(r2=1.00) was the most suitable among them. Adsorption quantity of CrVI and PMP on colloids increased with increasing acidity in the
pH range of 3.5±9.0. Study of CrVI adsorption kinetics indicated that the adsorption equilibrium of CrVI was reached rapidly within 2 h.
In pure aqueous solution, CrVI reduction by PMP was observed only when the media's pH was lower than 4.0. Oxidation and reduction
reaction between CrVI and p-methoxyphenol obviously occurred when soil colloids were involved in this system, even at pH57.0, which
strongly suggested that minerals in soil colloids acted as catalysts to speed the reaction of CrVI and PMP. The oxidized product of
PMP by CrVI, extracted by chloroform in acid media and analyzed by gas chromatography±mass spectrometry, was identi®ed as
benzoquinone. The reaction included two steps of one electron process. # 2000 Elsevier Science Ltd. All rights reserved.
Keywords: CrVI; p-methoxyphenol; Catalytic reaction; Soil colloids; Pollution
1. Introduction
via adsorption/desorption between soil (or sediment)
and water, hydrolysis, photolysis and volatilization.
Furthermore, they will react chemically with other spe-
cies or be degraded by microorganisms (Kawaguchi and
Inagaki, 1994; Gonzalez and Ukrainezyk, 1996; Senesi
et al., 1997). So, the migration and conversion of phe-
nolic compounds will be in¯uenced by other coexisting
species.
As a heavy metal, CrVI is toxic to humans and other
living organisms when it is discharged into the environ-
ment. In acid medium, CrVI is unstable, and it will be
easily reduced to less toxic CrIII by some metal ions or
organic pollutants. Since the 1970s, migration and con-
version of CrVI in minerals and soil has attracted great
attention (Bartlett and Kimble, 1976; James and Bar-
tlett, 1983; Cifuentes, et al., 1996). Interaction of CrVI
and concrete particles indicated that the reduction of
CrVI was mainly caused by FeII ions (Wang et al., 1996),
and organic ligands and pH played very important roles
in the reaction of CrVI and FeII in aqueous media
(Buerge and Hug, 1998).
More and more organic pollutants and heavy metals
exist in the environment as a result of anthropogenic
activity. Usually, these pollutants appear not singly but as
co-contamination. Compared with individual pollutants,
combinations in the forms of organic±organic, organic±
inorganic and inorganic±inorganic contaminants are very
complex and usually produce more serious pollution.
Therefore, the study of combined pollutants rather than
single ones is more important for improved under-
standing of the behavior of pollutants in real systems
(Balsberg, 1989).
Most phenolic compounds were listed as priority
organic pollutants by the US Environmental Protection
Agency because of their large amounts, diverse forms
and high toxicity. These pollutants, once in environ-
ment, will undoubtedly interact with their surroundings
* Corresponding author. Fax: +25-3353590.
E-mail address: hmchen@mail.issas.ac.cn (Huai-Man Chen).
0269-7491/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0269-7491(00)00028-2

76
D.-M. Zhou et al. / Environmental Pollution 111 (2001) 75±81
However, the above research concerned CrVI behavior
Table 1
Characteristics of soil colloidsa
in the form of a single pollutant. Recently, interactions of
CrVI and organic pollutants, such as phenol and its deri-
vatives, were studied in aqueous media (Elovitz and Fish,
1994, 1995) and on metal oxides such as aluminum oxide,
goethite and titanium oxides (Deng and Stone, 1996a,b).
It was found that the reaction of CrVI and phenolic com-
pounds in pure aqueous solution occurred only in acid
media, and metal oxides could act as ecient catalytic
reaction centers for the reaction even when pH>7.0. Soil,
especially ``red soil'' of China containing large amounts of
Fe and Al oxides, can be imagined to act as the catalyst for
CrVI and organic compounds.
Here, the adsorption behavior of CrVI and p-methox-
yphenol (PMP) on soil colloids was studied. Meanwhile,
the interaction of CrVI and PMP was examined in the
presence of soil colloids over a wide pH range. The pro-
duct was analyzed by gas chromatography±mass spec-
trometry (GC±MS), and the reaction mechanism was
proposed according to our research results.
OM
(g/kg)
Fe2O3
(%)
SiO2
(%)
Al2O3
(%)
TiO2
(%)
CEC
(mmol/kg)
12.60
11.18
40.86
29.08
1.14
25.67
a
OM, organic matter; CEC, cation exchange capacity.
added into these tubes to perform the batch equilibrium
experiments besides the corresponding blank ones. The
pH of samples was adjusted by adding di€erent content
3
of 10 mol/l HCl or NaOH and equilibrated prior to
the addition of CrVI and PMP. The total volume was 10
ml, and all media contained 0.01 mol/l NaNO3. Samples
were shaken horizontally and continuously, then cen-
trifuged for 10 minutes at 1211Âg to examine the change
of CrVI and PMP content in solution. The centrifugates
were used to analyze the contents of CrVI and PMP.
Meanwhile, accurate pH values were measured by glass
pH electrode in the centrifugates. Experimental tempera-
ture was controlled at 22Æ0.5ꢁC in a constant temperature
room. The experimental results were the average values
of two replications.
2. Materials and methods
2.1. Reagents and materials
2.3. Chemical analysis
All reagents were of analytical grade, and deionized
water was used here in all experiments. Chloroform was
distilled prior to use, and the middle fraction (60ꢁC) was
collected.
CrVI analysis was performed by the colorimetric
method with diphenylcarbazide using a 721 visible-spec-
trometry (Shanghai Instrumental Factory, China) at
detection wavelength of 540 nm. Samples, used for the
analysis of PMP, were obtained by extracting the above
solution in acid media using 5.0 ml chloroform three
times (PMP cannot be ionized in acid media, which
resulted in a high extraction eciency). Then, phenol was
added as an internal standard, and the resultant samples
were condensed to 2.0 ml and ®ltered through a 0.3-mm
microporous cellulose ether ®ltration ®lm (Hailin Med-
ical Instrumental factory, Zhejian Province, China). The
analysis of PMP samples was carried out in a Finnigan-
SSQ710 MS connected to a Varian GC with a 60 mÂ0.32
mm i.d. DB-5 fused silica capillary column. The tempera-
tures of the procedure were 50ꢁC (1 min)±150ꢁC (10ꢁC/
min)±250ꢁC (20ꢁC/min)±250ꢁC (5 min). MS operated in
the electron impact mode at 70 ev. Signi®cant total-ion
chromatographic peaks were identi®ed by mass spectral
interpretation, and the mass ion distributions were com-
pared with a data system library (Elovitz and Fish, 1995).
Air-dried soil (red soil, Udic Ferrisol), sampled from
Jingxian, Jianxi province, China, was sieved through a
20-mesh screen (f=0.84 mm), then, a portion of soil
was weighed and soaked in deionized water and soni-
cated to disperse. Soil colloids with f<0.002 mm were
separated and collected from the solution of soil col-
loids according to Stokes' equation, and the soil colloids
were washed with saturated CaCl2 solution and then
with deionized water to diminish chloride ions. So, the
subsamples were Ca-saturated and reserved to use after
being dried under an infrared lamp. The total contents
of Fe, Al and Si oxides in the soil colloids were analyzed
using DCP (Direct Couple Plasma) -atomic absorption
spectroscopy (AAS) with fused Li2CO3-H3BO3 (Sheng
et al., 1983). Cation exchange capacity (CEC) of colloids
was determined with BaCl2-H2SO4 method (Institute of
Soil Science, Chinese Academy of Sciences, 1978). The
characteristics of soil colloids are shown as Table 1.
2.2. Experimental procedure
3. Results and discussion
Stock solutions of CrVI and PMP were prepared
before each experiment, and 0.100Æ0.002 g of soil col-
loids were weighed and placed into 50ml PVF plastic
tube. Then, 1.0, 2.0, 3.0, 4.0, 5.0, 7.0, 10.0, 15.0, 25.0
mg/l CrVI and 13.0, 26.0, 46.0, 72.0, 96.0 mg/l PMP were
3.1. Adsorption of CrVI on soil colloids
Soil colloids used here included a high content of Fe
and Al oxides, and its pH, determined with colloids to
water ratio of 1:100 under our experimental condition,

D.-M. Zhou et al. / Environmental Pollution 111 (2001) 75±81
77
was 6.52. The content of oxides, CEC and organic mat-
ter content are listed in Table 1. Meanwhile, these soil
colloids, as variable charged ones, showed a high bu€er
capacity to acid or base addition.
1992). The pH in¯uence on CrVI adsorption showed that
the adsorption quantity of CrVI would increase with
increasing acidity, as shown in Fig. 1(b), which could be
due to the OH+2 groups formed on the surface of col-
loids resulting in a strong adsorption to the anionic CrVI
(Aide and Cummings, 1997). Meanwhile, HCrO4 , domi-
nated at pH <4.6, is the major adsorption ions among
CrVI forms in aqueous solution (Deng and Stone, 1996a).
The change of CrVI adsorption is signi®cant with pH, and
no CrVI adsorption occurred when pH exceeds 9.0.
Fig. 1(a) shows the adsorption isotherm of CrVI on soil
colloids as well as the in¯uence of pH [Fig. 1(b)]. CrVI
adsorption increased with the increasing initial con-
centration and ®nally reached its equilibrium. Adsorption
isotherm of Fig. 1(a) was simulated using the Freundlich
equation ꢀQ ˆ k1C1=n† and Langmuir equation ꢀQ ˆ
k2bC=‰1 ‡ k2C†Š (b is the maximum adsorption quantity)
with the resulting k1 and n of 1.89Â102 and 0.53 (r2=0.99)
and k2 and b of 0.13 and 1.25Â103 (r2=0.96), respectively.
Therefore, it suggested that both of these two simulation
equations could be used to describe CrVI adsorption on
the soil colloids very well with high correlation coecient.
Moreover, CrVI, as an anion, showed a strong adsorp-
tion on these colloids, which can be ascribed to large
amounts of Al and Fe oxides included in these colloids
(Aide and Cummings, 1997).
3.2. Adsorption isotherm of PMP on soil colloids
Fig. 2 shows the adsorption isotherm of PMP on soil
colloids and the in¯uence of pH on this adsorption pro-
cess. Usually, organic chemicals are mainly adsorbed on
the organic matter of soil; also, it can be linked to the
surface of soil or mineral by hydrogen bonds or other
chemical bonds (Sawhney and Brown, 1989; Xing and
Pignatello, 1998). So, organic matter contained in soil
colloid would contribute to the major portion for the
adsorption of PMP; meanwhile, mineral surface of the soil
colloids can also contribute to the adsorption of PMP,
CrO24 adsorption is a surface complexation reaction,
possibly accompanied by protonation of the ligand at
the surface (Ainsworth et al., 1989; Mesuere and Fish,
Fig. 1. (a) Adsorption isotherm of CrVI on soil colloids (pH 4.20); (b) represents the in¯uence of pH on the adsorption of 10.0 mg/l CrVI
.
Fig. 2. (a) Adsorption isotherm of PMP on soil colloids (pH=5.72); (b) represented the in¯uence of pH on the adsorption 96.0 mg/l PMP.

78
D.-M. Zhou et al. / Environmental Pollution 111 (2001) 75±81
especially in weakly acid media due to the easy forma-
tion of hydrogen bonds.
cules. In addition, the ionization density of soil colloids
would be changed with pH simultaneously. In acid
medium, PMP was adsorbed on the soil colloids easily
because of the strong adsorption of molecular PMP on
colloids and the easy formation of hydrogen bond
between PMP and the ionized soil colloid surface.
PMP (pKa 10.21) can be ionized and its two forms,
ions and molecules, exist depending on pH. According
to the results from adsorption/desorption of phenolic
compounds on granular activation carbon (Mollah and
Robinson, 1996), we use ®ve adsorption equations,
including Langmuir ‰q ˆ kbCaq=ꢀ1 ‡ kCaq†Š; Freundlich
3.3. Adsorption and reaction kinetics of CrVI with and
without PMP
1
ꢀq ˆ kCnaq†; Radke-Prausnitz ‰q ˆ aCnaq=ꢀ1 ‡ bCanq †Š;
Mathews-Weber ‰q ˆ aCnaq=ꢀ1 ‡ bCanq†† and Fritz-
n1
aq
n2
aq
Schluender ꢀq ˆ aC =ꢀ1-bC †Š, to simulate the experi-
In pure aqueous systems, it was observed that the
chemical reaction of CrVI and PMP was relatively fast in
low pH media, but it would be signi®cantly slowed
down with the increase of pH, and no reaction occurred
at pH>4.7 (Deng and Stone, 1996a). However, CrVI
reduction by phenolic compounds was observed in the
presence of metal oxides, and the pH of reaction medium
can be greatly extended to a higher value.
Fig. 3(I) shows the adsorption kinetics of CrVI on the
soil colloids in di€erent pH media. In near neutral media
(pH 6.5), the change of CrVI in solution decreased rapidly
in the ®rst 2 h, then reached its adsorption equilibrium.
Meanwhile, lower pH, as discussed above, would result
in higher CrVI adsorption on soil colloids. When pH
was 4.20, CrVI adsorption equilibrium was shown in
Fig. 3(I)(c), and CrVI concentration in solution
decreased continuously with time, possibly due to the
slow reaction of CrVI and organic matter or other
reduced species of soil colloids (Wittbrodt and Palmer,
1996). Of course, this part is very small because of the
low content of organic matter and other reduced species
included in the soil colloids (12.60 g/kg).
However, when PMP was added in this system, the
changes of CrVI content in solution with time were sig-
ni®cantly di€erent from that obtained in the absence of
PMP, as shown in Fig. 3(II). In the ®rst 2 h, no di€er-
ence for CrVI content in pH 6.50 medium was observed
in the absence and presence of PMP, which indicated
that no obvious interaction occurred between CrVI and
PMP in this time window. Meanwhile, the presence of
PMP did not in¯uence CrVI adsorption. However, with
a low pH media or an extended time, an obvious decrease
mental data and compared the results correspondingly
here. The results are shown in Table 2. The total errors
are smaller from the modi®ed simulation equations,
which usually give the best description of equilibrium
adsorption of phenol derivatives (Radke and Prausnitz,
1972), rather than that from the simple ones.
Adsorption quantity of PMP would increase gradu-
ally with the decreasing pH along with the decrease of
ionized PMP. This result is consistent with that from the
adsorption of pentachlorophenol on six natural soils
0
and three synthetic soils with pH (Lagas, 1988). Koc
values (equilibrium constant) for phenolate are about
15±30 times lower than that from undissociated mole-
Table 2
The simulation results with adsorption equations
Equation
Simulation parameters
The relative
errors (%)a
Freundlich
k=1.87Â102,
7.59
100
n=0.54, r2=0.98
Langmuir
k=0.03, b=2.78Â103,
r2=1.00
Radke-Prausnitz
Mathews-Weber
Fritz-Schluender
a=7.62Â103, n= 0.22,
b=1.21Â102, r2=1.00
a=53.34, b=0.02,
n=1.13, r2=1.00
1.35
3.29
0.66
a=71.96, b= 0.01,
n1=0.93, n2=1.27, r2=1.00
a
One-hundred percent is described as the error of [(Obs Pred)2]
for Langmuir equation, and others were compared with the errors of
Langmuir's.
Fig. 3. Change of CrVI (C0=10.0mg/L) in aqueous solution with time in (I): (a) pH 6.50; (b) pH 5.20 and (c) pH 4.20; and (II) (I) +100.0 mg/l PMP.

D.-M. Zhou et al. / Environmental Pollution 111 (2001) 75±81
79
of CrVI in solution was observed even with pH exceeding
6.50. It cannot be due to the in¯uence of PMP on CrVI
adsorption with time (Mesuere and Fish, 1992); on the
contrary, it strongly suggested that some reactions
between CrVI and PMP occurred. The decrease of CrVI in
solution may be ascribed to the reduction of CrVI by
PMP along with the catalytic e€ect of the soil colloids.
The catalytic reaction is mainly due to metal oxides like
Fe2O3 and Al2O3 included in the soil colloids.
When CrVI was recovered from both the soil colloids
(using 0.1 M KH2PO4 to extract CrVI (James et. al.,
1995) and aqueous solution, almost no decrease of CrVI
was found after its adsorption on soil colloids without
PMP, or with PMP in aqueous media in the pH range
from 4.0 to 7.0, which suggested that almost no CrVI
chemical reaction occurred in these two systems (Fig. 4).
However, recovery of CrVI decreased very much in the
presence of PMP and soil colloids simultaneously,
obviously indicating a catalytic reaction occurred on the
surface of the soil colloids.
Fig.5(a) shows the adsorption isotherms of CrVI with
and without PMP. The experimental data was deter-
mined after equilibrium of 50 hours. It can be observed
from Fig. 5(a) that the apparent adsorption of CrVI in
the presence of PMP was greater than that obtained in
the absence of PMP. So, it also supports the conclusion
that the presence of soil colloids extended the reaction
of CrVI and PMP to a wide pH range. Using the
Freundlich equation ꢀQ ˆ kCn† to simulate isotherm
(a), the resulting simulation parameters were 1.01Â102
and 0.61 for k and n, respectively (r2=0.99). As for iso-
therm (b), the apparent adsorption should include two
parts: CrVI adsorption and CrVI reduction, so, it could
be assumed as follows:
Qapp refers to the total apparent adsorption of CrVI;
Qads and Qreac represent the quantity of CrVI adsorption
and of chemical reaction, respectively.
When this equation was used for the simulation of Fig.
5(b), a very good result was obtained with k2 of 1.95Â102
and n2 of 0.30 (r2=1.00). So, the decrease of CrVI in solu-
tion was quantitatively described as Qapp ˆ 1:01  102
C0:61 ‡ 1:95  102 C0:3. The ratio of CrVI adsorbed and
reduced was written as Qad=Qreac ˆ 0:52 C0:31
.
Plot of ln(CrVI)aq (mol/l) versus time (hours) [data
from Fig. 3(II)] gives linear curves with good correlation
coecient shown as Fig. 6. So, the reaction can be
described as
to this equation, the kobs values were 0.24Â10
dꢀCrVI†aq=dt ˆ kobsꢀCrVI†aq. According
2
1
h ,
0.51Â10 2 h 1 and 0.83Â10 2 h 1 in the media with pH
6.50, 5.20 and 4.20, respectively. In comparison with the
kobs of 9.78Æ1.28Â10 2 h 1 resulted from pure minerals
(Deng and Stone, 1996a), the reaction kinetic constant
of CrVI and PMP was smaller in soil colloids.
According to the reaction process (Deng and Stone,
1996a,b), four reactions in parallel contribute to overall
Qapp ˆ Qads ‡ Qreac ˆ k1Cn1 ‡ k2Cn2
ˆ 1:01  102C0:61 ‡ k2Cn2
Fig. 5. Apparent adsorption isotherms of CrVI on soil colloids without
(a) and with (b) 96.0 mg/l PMP (pH=5.72).
Fig. 4. Recovery of CrVI from colloids in di€erent pH with (a) 10.0
mg/l CrVI+soil coloids; (b) 10.0 mg/l CrVI+96.0 mg/l PMP and (c)
10.0 mg/l CrVI+96.0 mg/l PMP+soil colloids.
VI
aq
Fig. 6. ln(Cr ) as a function of time in pH (a) 4.20; (b) 5.20 and (c)
6.50. CrVI=10.0 mg/L, PMP=100.0 mg/l.

80
D.-M. Zhou et al. / Environmental Pollution 111 (2001) 75±81
PMP oxidation, yielding the four terms in the di€er-
ential equation listed below:
GC±MS chromatogram of standard sample indicated
that the retention time of internal standard (phenol) and
PMP were 602600 min and 1001700, respectively. However,
there was an additional new peak appeared at 502000
from the experimental sample after PMP reacted with
CrVI in the presence of soil colloids. This new peak
should be due to a new chemical formed by the reaction
of CrVI and PMP on the surface of soil colloids. Further
study exhibited that its molecular mass to charge ratio
was 108. As shown in Fig. 7, the resulting mass chro-
matogram and the standard mass chromatogram of
benzoquinone are entirely the same. So, it can be con-
cluded that the main reaction product of PMP by CrVI
is benzoquinone.
d‰PMPŠT=dt ˆ k1‰CrVIŠ ‰PMPŠ
aq
aq
‡ k2‰CrVIŠ ‰PMPŠ
ads
aq
ads
‡ k3‰CrVIŠ ‰PMPŠ
ads
aq
‡ k4‰CrVIŠ ‰PMPŠ
ads
where the subscripts of aq and ads refer to dissolved and
adsorbed species concentration, and the subscript T
stands for total of PMP oxidation. The ®rst part can be
ignored because no reaction of CrVI and PMP occurred
in aqueous solution in near neutral medium. Reaction
of CrVI and PMP is a surface-catalyzed one, and cata-
lysis arises from the adsorption of CrVI (Deng and
Stone, 1996a). So, the second part in the above equation
could also be neglected. Meanwhile, the adsorption of
PMP can be described using Fritz-Schluender as shown
The reaction of CrVI and phenolic compounds in
aqueous media was studied and the reaction mechanism
was proposed (Elotivz and Fish, 1995). The major pro-
ducts were benzoquinone and its derivatives. According
to the reaction product obtained here, the mechanism of
n1
n2
aq
above ‰q ˆ ac =ꢀ1 bc †Š. So, the above equation can
aq
be rewritten as below,
d‰PMPŠT=dt ˆ k3‰CrVIŠ ‰PMPŠ
ads
aq
‡ k4‰CrVIŠ ‰PMPŠ
ads
ads
d‰PMPŠ ˆ k3‰CrVIŠ ‰PMPŠ
ads
aq
‡ k4‰CrVIŠads  a‰PMPŠanq1=ꢀ1 b‰PMPŠ †
n2
aq
n3
d‰PMPŠT=dt ˆ k5‰CrVIŠ ꢀ‰PMPŠ
aq
aq
‡ k6‰PMPŠnaq1=ꢀ1 b‰PMPŠ ††
n2
aq
From the above equations, it can be found that decrease
of PMP is related with the contents of CrVI and PMP in
aqueous solution.
3.4. Analysis of PMP oxidation product and the possible
reaction mechanism
Fig.7. The resulting mass chromatogram of sample obtained by the
interaction of CrVI and PMP on the surface of soil surface (a) and the
standard mass chromatogram of benzoquinone (b).
The preparation and analysis of PMP samples by
GC±MS were described in the experimental section. The
Fig. 8. Proposed reaction mechanism of CrVI and PMP in the presence of soil colloids.

D.-M. Zhou et al. / Environmental Pollution 111 (2001) 75±81
81
a surface catalytic reaction for CrVI and PMP was
similarly proposed in Fig. 8.
Cifuentes, F.R., Lindemann, W.C., Barton, L.L., 1996. Chromium
sorption and reduction in soil with implications to bioremediation.
Soil Science 161, 4, 233±241.
Deng, B., Stone, A.T., 1996a. Surface-catalyzed chromium (VI) reduction:
reactivity comparisons of di€erent organic reductants and di€erent
oxide surfaces. Environmental Science and Technology 30, 2484±2494.
Deng, B., Stone, A.T., 1996b. Surface-catalyzed chromium (VI) reduc-
tion: the TiO2-CrVI-mandelic acid system. Environmental Science
and Technology 30, 463±472.
Elovitz, M.S., Fish, W., 1994. Redox interactions of CrVI and sub-
stituted phenols: kinetics investigation. Environmental Science and
Technology 28, 2161±2169.
4. Conclusions
Adsorption of CrVI and PMP on soil colloids as well
as pH in¯uence were studied, and the results exhibited
that these adsorption processes could be quantitatively
described with general simulation equations very well.
In addition, the adsorption quantities increased with
increasing acidity for both CrVI and PMP.
CrVI adsorption was very fast and it reached equili-
brium within 2 h in neutral media. Meanwhile, the
reaction of CrVI and organic matter or other reduced
species of soil colloids occurred mainly in acid media.
Soil colloids could act as the catalytic reaction center
for CrVI and PMP over a wide pH range, and the reac-
tion was described quantitatively using the related equa-
tions. The product of interaction of CrVI and PMP was
analyzed by GC±MS here and the result showed that the
major product of PMP was benzoquinone. The reaction
of CrVI and PMP in the presence of soil colloids included
two steps of one electron process.
Elovitz, M.S., Fish, W., 1995. Redox interaction of CrVI and sub-
stituted phenols: products and mechanism. Environmental Science
and Technology 29, 1933±1943.
Gonzalez, J.M., Ukrainezyk, L., 1996. Adsorption and desorption of
nicosulfuron in soils. Journal of Environmental Quality 25, 1186±
1192.
Institute of Soil Science, Chinese of Academy of Sciences, 1978. Phy-
sical and Chemical Analysis of Soil, Shanghai Sci. & Technol. Press,
Shanghai, China.
James, B.R., Bartlett, R.J., 1983. Behavior of chromium in soil: VII,
adsorption and reduction of hexavalent forms. Journal of Environ-
mental Quality 12 (2), 177±181.
James, B.R., Petara, J.C., Vitale, R.J., Mussoline, G.R., 1995. Hex-
avalent chromium extraction from soils: a comparison of ®ve
methods. Environmental Science and Technology 29, 2377±2381.
Kawaguchi, H., Inagaki, A., 1994. Kinetics of ferric ion promoted
photodecomposition of 2-chlorophenol. Chemosphere 28 (1), 57±62.
Lagas, P., 1988. Sorption of chlorophenols in the soils. Chemosphere
17 (2), 205±216.
Mesuere, K., Fish, W., 1992. Chromate and oxalate adsorption on
goethite. I. Surface complexation modeling of competitive adsorp-
tion. Environmental Science and Technology 26, 2365±2370.
Mollah, A.H., Robinson, C.W., 1996. Pentachlorophenol adsorption
and desorption characteristics of granular activated carbon-I. Iso-
therms. Water Research 30 (12), 2901±2906.
Acknowledgements
This program was ®nancially supported by the National
Natural Science Foundation of China (29877027) and the
Postdoctoral Science Foundation of China. The authors
thank two anonymous reviewers for their helpful com-
ments on this paper.
Radke, C.J., Prausnitz, J.M., 1972. Adsorption of organic solutes
from dilute aqueous solution on GAC. Ind. Engng. Chem. Fund.
11, 445±451.
Sawhney, B.L., Brown, K. (Eds.), 1989. Reactivity and Movements of
Organic Chemicals in Soils. SSSA Special Publication No. 22. Soil
Science Society of America Inc., Madison, Wisconsin, USA.
Senesi, N., Cara, P.L., Miano, T.M., 1997. Adsorption of imazethapyr
to amended and nonamended soils and humic acids. Journal of
Environmental Quality 26, 1264±1270.
References
Aide, M.T., Cummings, M.F., 1997. The in¯uence of pH and phos-
phate on the adsorption of chromium (VI) on boehmite. Soil Sci-
ence 162, 8, 599±603.
Sheng, R.S., Zheng, C.R., Qiang, Q.W., 1983. Simultaneous determi-
nation of multielement by DCP-AES. Chinese of Analytical Chem-
istry 11, 599±603.
Ainsworth, C.C, Girvin, D.C, Zachara, J.M., Smith, S.C., 1989.
Chromate adsorption on geothite: e€ects of aluminum substitution.
Soil Science Society American Journal 53, 411±418.
Weng, C.H., Huang, C.P., Allen, H.E., Leavens, P.B., Sanders, P.F.,
1996. Chemical interactions between Cr(VI) and hydrous concrete
particles. Environmental Science and Technology 30, 371±376.
Wittbrodt, P.R., Palmer, C.D., 1996. Reduction of Cr(VI) by soil
humic acids. European Journal of Soil Science 47, 151±162.
Xing, B., Pignatello, J.J., 1998. Competitive sorption between 1,3-
dichlorobenzene or 2,4-dichlorophenol and natural aromatic acids in
soil organic matter. Environmental Science Technology 32, 614±619.
Balsberg, P.A.M., 1989. Toxicity of heavy metals (Zn, Cu, Cd, Pb) to
vascular plants. Water Air and Soil Pollution 47, 287±319.
Bartlett, R., Kimble, J.M., 1976. Behavior of chromium in soils: II.
Hexavalent forms. Journal of Environmental Quality 5, 383±386.
Buerge, I.J., Hug, S.J., 1998. In¯uence of organic ligands on chro-
mium(VI) reduction by iron(II). Environmental Science and Tech-
nology 32, 2092±2099.