Influence of colloid suspensions of humic acids upon the alkaline fading of carbocations

Article abstract of DOI:10.1002/poc.1317

The influence of humic substances (HSs) upon the alkaline hydrolysis of crystal violet (CV) and malachite green (MG) has been studied. Important inhibition of hydrolysis reaction has been reported. This inhibition has been explained in terms of association of reactants to the humic substances. Kinetic results have been modelized using the micellar pseudophase model. Copyright

Full text of DOI:10.1002/poc.1317

Research Article
Received: 4 October 2007,
Revised: 16 November 2007,
Accepted: 4 December 2007,
Published online in Wiley InterScience: 24 January 2008
(www.interscience.wiley.com) DOI 10.1002/poc.1317
Influence of colloid suspensions of humic
acids upon the alkaline fading of carbocations
a
b
b
b
c
´
´
M. Arias-Estevez , G. Astray , A. Cid , D. Fernandez-Gandara , L. Garcı´a-Rı´o
and J. C. Mejutob
*
The influence of humic substances (HSs) upon the alkaline hydrolysis of crystal violet (CV) and malachite green (MG)
has been studied. Important inhibition of hydrolysis reaction has been reported. This inhibition has been explained in
terms of association of reactants to the humic substances. Kinetic results have been modelized using the micellar
pseudophase model. Copyright ß 2008 John Wiley & Sons, Ltd.
Keywords: colloid suspensions; humic; crystal violet; malachite green; basic hydrolysis
organic xenobiotics (pesticides, PCBs, PAHs and many other
substances) are regarded micro-organisms, neglecting the role of
INTRODUCTION
Humic substances (HSs) make up a large portion of the organic
matter found in natural environments with a typical concen-
tration ranging from 0.1 to 200 mg Lꢀ1 dissolved organic
carbon.[1] HSs are anionic polyelectrolyte that represents
65–75% of the total organic matter in soils. Average molecular
weight ranges from 2 to 400 kDa, depending on the determi-
nation method.[2] An empirical formula for the basic structure of
HA has been proposed[3]: C72H43–95O30N4 ꢁ (0–38) H2O. They are
characterized spectrosocopicaly by a strong band of aliphatic
absorption (C—H) between 2980 and 2920 cmꢀ1 and other two
bands at 1720 and 1650 cmꢀ1 due to the presence of carbonyl
and carboxyl groups. HSs specific surface is around 900 m2 gꢀ1
and their negative surface charge presents a large dependence
with pH (surface charge increases on increasing pH). This surface
charge is mainly due to the deprotonation of carboxyl and phenol
groups (both of them represent the 85–90% of surface charge of
humic aggregates).[1] The presence of amino groups may justify
the presence of positive charges in the humic surfaces but as
compared with the negative charges due to carboxyl and phenol
groups, it is negligible. Full ionized the capacity of cationic
HSs. Amongst properties of HS possibly influencing their catalytic
activity can be mentioned in their ability to form micelles.[13–16]
There are few studies in which catalytic activity of HSs has been
compared.[11,12,17–19] In the same time the study of the catalytic
activity of HSs may be very important, considering their possible
role in the fate in biogeochemical cycling of elements and high
concentrations of HSs in natural environments. Velocity of
different reactions could be influenced by the presence of HSs.
Reactions of interest would be related to hydroxyl ion attack,
much studied to evaluate different aspects of micellar
catalysis.[20–23]
The alkaline fading of stable triarylmethyl carbocations
(Scheme 1) is a reaction with a long chemical tradition. The
rate for the process – in spite of being a cation-anion combination
– is slow enough for a conventional kinetic study, and in fact, the
reaction has become
a popular one for undergraduate
laboratories.[24,25] Reaction also takes place with nucleophiles
other than OHꢀ, a fact that has been used for the construction of
the familiar Ritchie Nþ nucleophilicity scale.[26–32] In spite of the
many studies made of the mechanism of these reactions, some
mechanistic essential details are still confused and the reaction
has become one of the main challenges to the reactivity-
selectivity principle.[33–44]
interchange (CIC) is 0.3–0.6 mol(þ) kgꢀ1
.
HSs can complex many environmental pollutants.[4,5]
In particular, they are able to complex heavy metals[6,7] (due
to presence of —COOH and —OH groups in their molecules), and
persistent organic xenobiotics[8,9] (due to presence of hydro-
phobic structures in their molecules), but also strongly interact
with mineral surfaces.[10] The interaction of HSs with xenobiotics
may modify the uptake and toxicity of these compounds by living
organisms, and affect the fate of pollutants in the environment. If
traditionally HSs have been considered as inert, then nowadays
many aspects of their reactivity change understanding about
their role in the environmental processes and rather they are
considered as reactive carriers, influencing fluxes of substances.
In the same time just the reactivity of HS has been comparatively
little analysed, although HSs contain many groups and structures
which in similar macromolecules are responsible for their
catalytic activity.[11,12] This question is of particular importance,
as far as commonly as the main factor responsible for removal of
*
Departamento de Qu´ımica F´ısica, Facultad de Ciencias, Universidad de Vigo,
32004, Ourense, Spain.
E-mail: xmejuto@uvigo.es
a
M. Arias-Estevez
Area de Edafolog´ıa y Qu´ımica Agr´ıcola, Facultad de Ciencias, Universidad de
Vigo, 32004 Ourense, Spain
´
´
b
c
G. Astray, A. Cid, D. Fernandez-Gandara, J. C. Mejuto
Departamento de Qu´ımica F´ısica, Facultad de Ciencias, Universidad de Vigo,
32004 Ourense, Spain
L. Garc´ıa-R´ıo
Departamento de Qu´ımica F´ısica, Facultad de Qu´ımica, Universidad de
Santiago de Compostela, 15706 Santiago, Spain
J. Phys. Org. Chem. 2008, 21 555–560
Copyright ß 2008 John Wiley & Sons, Ltd.

M. ARIAS-ESTEVEZ ET AL.
Scheme 1.
The reaction has also become a useful one for studying
chemical reactivity colloids and other micro-organized media.
One of the first studies on micellar catalysis and inhibition
referred to the alkaline fading of crystal violet (CV),[45] and these
reactions have been used later very often for studies in normal
micelles,[46–49] micelles in the presence of additives,[50–53] reverse
micelles or microemulsions[54–61] and clusters.[62] The reaction is
typically a ‘well-behaved’ one and has become an interesting
source of information on the role of the different factors that
affect overall reactivity in micelles of HSs (i.e. compartmentaliza-
tion of the reagents, characteristics of the humic aggregate as
reaction medium). The aim of the present work was to study the
catalytic activity of HSs in mentioned reactions and its
rationalization in the bases of the pseudophase model for
micellar catalysis.
Kinetic measurements and data analysis
Reaction kinetics was recorded at 25.0 ꢂ 0.1 8C in a Kontron
Uvikon 923 spectrophotometer equipped with thermostated cell
carriers (T ¼ 25 8C). The disappearance of the absorbance at
590 nm (CV) and 610 nm (MG) due to dye consumption was
followed. The initial concentration of dye (8.0 ꢃ 10ꢀ6 M for VC and
1.5 ꢃ 10ꢀ5 M for MG) was always much lower than that of the
other reagents. The kinetic data always fitted the first-order
integrated rate equation satisfactorily (r > 0.999; Eqn (1)); in what
follows, kobs denotes the pseudo first-order constant:
lnðAt ꢀ A Þ ¼ lnðAi ꢀ A Þ ꢀ kobs
t
(1)
1
1
where Ai, At and A are the absorbance at t ¼ 0, at a given time,
1
and at t ¼ 1, respectively.
EXPERIMENTAL
RESULTS AND DISCUSSION
Reactives
Basic hydrolysis of CV and MG in water
All reagents were Merck or Sigma-Aldrich products of the
maximum commercially available purity, and none required
further purification. All aqueous solutions were prepared by
weight using double-distilled degasified water.
Although the hydrolysis of CV and MG in homogeneous media
has been thoroughly studied[67] we examine the basic hydrolysis
of both carbocations to ensure good consistency in the
evaluations of the experimental results. Figure 1 shows the
influence of [NaOH] upon the pseudo first order rate constant of
both reactions. The rate constant (kw) for the basic hydrolysis of
CV in aqueous medium is kw ¼ (12.3 ꢂ 0.2) Mꢀ1 minꢀ1, and the
corresponding one value for basic hydrolysis of MG is
kw ¼ (181 ꢂ 6) Mꢀ1 minꢀ1. They are in good concordance with
the literature.
Humic substances isolation and characterization
HSs used in this study were isolated from soil.[63] The composition
of the HSs used was determined by CNH elemental analysis using
a Fisons EA-1108 elemental analyser (518 g kgꢀ1 C, 40 g kgꢀ1 N
and 40 g kgꢀ1 H). Its ash content was 61 g kgꢀ1. HSs functional
group distribution has been determined by the Schinitzer
methods.[64] The functional group distribution was: total acidity
5.99 mol kgꢀ1, COOH groups 3.30 mol kgꢀ1 and total OH groups
Association of dyes to humic substances
UV spectra of CV and malachite green (MG) have been carried out
in the presence and in the absence of humic aggregates. CV and
MG UV spectra are strongly influenced by the medium.[68–73]
2.91 mol kgꢀ1 [65,66]
.
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Copyright ß 2008 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2008, 21 555–560

REACTIVITY IN HUMIC AGGREGATES
Figure 3. Pseudo-first order kinetics for alkaline fading of CV in the
presence of HSs. [HSs] ¼ 24 mg Lꢀ1, [NaOH] ¼ 0.01 M. (*) Exponential fit
(*) linearized fit (Eqn (1)) [l ¼ 580 nm]
Figure 1. Influence of [NaOH] upon the pseudo first order rate constant
of basic hydrolysis of CV (*) and MG (*) in water. T ¼ 25 8C
Figure 2 shows the changes in the UV spectra of CV in the
presence of HSs. The increase in HSs concentration implies a
decrease in the absorbance value at l ¼ 580 nm. Also changes in
the spectrum shoulder (l ¼ ꢄ545) have been observed. The
addition of HSs results in changes of Maximum/Shoulder ratio. It
is well known that CV and MG spectrums have high sensibility to
the nature of the solvent and the chemical environment of the
molecule, hence, large solvatochromic effects were reported in
the literature[68–73] as well as the formation of ionic pairs between
the CV and different substrates.[74–77] In particular this result was
observed previously in the literature due to the sorption of CV to
other colloid aggregates.[78–81] Similar results (data not shown)
were found for MG. In both cases the association of dyes to the
HSs has been proved.
consumption of NaOH by HS, for each concentration of HSs the
influence of NaOH concentration on the basic hydrolysis of both
carbocations was investigated. Figure
3 shows a typical
Absorbance versus time trace for this reaction. Solid line
represents the satisfactory fit of Eqn (1) to experimental results.
The NaOH concentrations were 0.01, 0.02, 0.03 M for the
basic hydrolysis of CV and 0.001 M, 0.002 M and 0.003 M for
the basic hydrolysis of MG. These concentrations of NaOH imply
that all humic acid would be as sodium humate.
Figures 4 and 5 show the influence of NaOH concentration for
different HSs concentrations for CV and MG fading, respectively.
From the slope of these linear fits the apparent bimolecular rate
constant (k2,app) for our reactions is obtained.
The presence of HSs inhibits the basic hydrolysis of both
carbocations. Figures 6 and 7 show the influence of HSs upon the
apparent bimolecular rate constant for our reactions (k2,app). The
presence of HSs implies an inhibition of 4.5 times in the CV fading
and 24 times in the MG hydrolysis. This inhibition is due to the
association of carbocations to the HSs substances and it can be
explained in terms of pseudophase model.
Basic hydrolysis of dyes in the presence of HSs
The influence of humic aggregates on the alkaline fading of VC
and MG has been studied. The concentration of HSs has been
varied between 0 and 24 mg Lꢀ1. To minimize the possible
Figure 2. UV spectrum of CV in the presence of humic substances. Dot
line CHSs ¼ 0 mg Lꢀ1, dash-dot line CHSs ¼ 1.0 mg Lꢀ1, dash-dot-dot line
Figure 4. Influence of [NaOH] upon pseudo-first order rate constant for
alkaline fading of CV in the presence of HSs. (*) [HSs] ¼ 2mg Lꢀ1. (*)
[HSs] ¼ 4 mg Lꢀ1. (D) [HSs] ¼ 8 mg Lꢀ1
CHSs ¼ 2.0 mg Lꢀ1, dash line CHSs ¼ 4.0 mg Lꢀ1, solid line CHSs ¼ 8.0 mg Lꢀ1
.
[CV] ¼ 8.5 ꢃ 10ꢀ6 M
J. Phys. Org. Chem. 2008, 21 555–560
Copyright ß 2008 John Wiley & Sons, Ltd.
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M. ARIAS-ESTEVEZ ET AL.
Pseudophase model
Over the last years, our research group had carried out an
exhaustive study of reactivity in microheterogeneous media, and
the pseudophase model had been used for a satisfactory
quantitative interpretation of a large amount of kinetic results in
different microheterogenoeus aggregates (as micelles,[82–84]
microemulsions[85–94] or vesicles[95,96]). In HSs dispersions the
loci of reaction may be inside the HSs aggregate, in the aggregate
surface (at Stern Layer) or at the bulk solvent. The nature of
the HSs aggregate, like the interior of a micelle, formed by the
hydrophobic portion of the HSs, is not fully understood. But,
taking into account the knowledge of micelles, the centre of the
HSs aggregate would be similar to liquid hydrocarbon and that
water can penetrate the aggregate, so that part of the hydro-
carbon chains of HSs from the hydrophilic groups would be
exposed to the water. A carbocation adsorbed in the very inner
part of a micelle would be inert to hydroxide ion because the
anionic nucleophile is certainly absent in the hydrocarbon core.
The HSs surface is another possible site of carbocation
adsorption. The ionic portions of the HSs form highly charged
surfaces which attract small counterions from the solution in
order to relieve electrostatic repulsion. Thermal motion, which
tends to produce uniform distribution of the ions, disturbs the
ionic array resulting in the well-known electrical double layer.
Basic hydrolysis of carbocation can be sensitive to ionic strength
changes, so that the high concentration of ions at the double
layer would itself greatly affect the reaction. However, adsorption
of a dye at the surface of an anionic could result in considerable
rate enhancement because some ionic groups of HSs surface can
act as nucleophiles and promote himself the fading of the dye.
On the other hand, long-range electrostatic effects originating
from the charged HSs interface could well be a major factor
affecting hydrolysis rates inside this region resulting in an
inhibitory effect. Moreover, electrostatic considerations, taking
into account the negative surface charge of HSs, can justify the
absence of HOꢀ from the vicinity of HSs aggregate.[97–101]
Whereas, in the case that HOꢀ can approximate to HSs aggregate,
we must take into consideration the fact that HSs is a very
compact unit, then most of the interior water will be highly
structured and in these conditions is well known that basic
hydrolysis would be markedly impeded.
Figure 5. Influence of [NaOH] upon pseudo-first order rate constant for
alkaline fading of MG in the presence of HSs. (*) [HSs] ¼ 2 mg Lꢀ1. (*)
[HSs] ¼ 4 mg Lꢀ1. (D) [HSs] ¼ 8 mg Lꢀ1
Figure 6. Influence of [HSs] upon apparent bimolecular rate constant for
alkaline fading of CV
A third possibility is that the carbocations are not adsorbed by
the HSs aggregates and exist in the free water.
To decide the possible mechanism, we now direct our
attention to Figures 6 and 7 which show that HSs inhibit
hydrolysis of VC and MG. The remarkable rate inhibitions may be
explained by carbocation adsorption into the hydrocarbon centre
of the aggregate where presumably there is no anionic
nucleophile (HOꢀ) (vide supra). Alternatively, adsorption could
occur within the outer aqueous areas of the aggregate where the
hydroxide ion concentration might be greatly reduced because
of electrostatic factors originating at the HSs surface. The former
reason can be neglected taking into account the fact that both CV
and MG are ionic substrates. Hence, it would be more reasonable
to assume that both of them would not be located in the core of
the aggregate. The location of VC and MG in the Stern layer is the
most rationale location for our carbocations. The large inhibition
observed (Fig. 8) would probe also that the reaction occurs in the
bulk water. There are not any other reaction pathways for the
alkaline fading of carbocations by the attack of the nucleophilic
groups of HSs surface.
Figure 7. Influence of [HSs] upon apparent bimolecular rate constant for
alkaline fading of MG
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Copyright ß 2008 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2008, 21 555–560

REACTIVITY IN HUMIC AGGREGATES
CONCLUSIONS
The kinetic effect of the presence of HSs can be modelized using
the pseudophase model. The HSs are colloid dispersions in water
and their influence upon chemical reactivity can be rationalized
considering that HSs are micelle-like colloids. The observed
behaviour is similar to the observed one in traditional
micelles.[102,103]
Acknowledgements
M.A. thanks Ministerio de Educacion y Ciencia for a ‘Ramon y
Cajal’ research grant. The authors thank to Xunta de Galicia
´
(Conseller´ıa de Industria e Innovacion), project PGIDIT07P-
XIB383198PR.
Figure 8. Fit of experimental data to Eqn (3). (*) CV and (*) MG.
T ¼ 25 8C
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