Arie C. Besemera,
Jan M. Jettena,
Ted M. Slaghekb
a
SCA Hygiene Products, Zeist, The Netherlands
b
TNO Nutrition and Food Research, Zeist, The Netherlands
Improved Calcium Sequestering Carbohydrate Based Agents
The improved tandem oxidation sequence of starch with the aim of developing, e.g. degradable cobuilders suitable for laundry formulations as an alternative to the currently used polyacrylates is
described. The tandem oxidation of starch starts with sodium periodate treatment of granular starch
followed by oxidation of the dialdehyde product with sodium chlorite to dicarboxy starch. It turns
out that improving the periodate oxidation resulting in a more complete transformation of the
glucose moiety into the dialdehyde product yields, after sodium chlorite oxidation, a dicarboxy
starch with a calcium sequestering capacity of 3.3 mmol Ca2+ per gram of dicarboxy product. This
is an improvement of about 25% compared with current literature data.
Keywords: Starch oxidation; Co-builder; Metaperiodate; Sequestering
1 Introduction
Builders are an important class of detergent additives in laundry formulations. The function of
builders is to prevent deposition of insoluble inorganic salts (mainly calcium and magnesium salts)
onto the fabric and machine parts during the washing cycle. Since the phosphate-based builder
(sodium tripolyphosphate, STPP) was banned from detergent formulations, due to the
eutrophication of surface water [1-3], research has been conducted aimed at developing alternatives.
The most widely used alternative for the phosphate-based builders is zeolite NaA [4-6]. Although
the zeolite binds calcium and magnesium well, the binding activity of especially magnesium is
relatively slow and nowadays the zeolite is used in combination with a so-called co-builder. This
co-builder is usually a polyacrylate or a copolymer of acrylate and maleate [7-10] and its purpose is
to prevent the calcium and magnesium of forming an inorganic water-insoluble salt and allow
enough time for the zeolite to complex the metal ions. This system works well but due to the poor
biodegradability of the co-builders the use is under discussion [7].
Other research focused on the use of, e.g. nitrilotriacetic acid [4, 8] and biodegradable polyaspartic
acid and derivatives [11-16] as a substitute, but non of these substitutes made to the large volume
market.
Another route towards the development of environmentally friendly builders was focused on
oxidation of polysac-charides due to the renewable and biodegradable advantage. For instance
oxidation using the NO2/N2O4 system gave products where the primary hydroxyl group of the
glucose moiety in e.g. starch was oxidized [17-20]. Other
Correspondence: Arie C. Besemer, SCA Hygiene Products, P.O. Box 360, 3700 AJ Zeist, The
Netherlands. Phone: +31-30-6944859, e-mail: arie.besemer@sva.com.
oxidation systems are based on oxygen in combination with sodium hydroxide [21-22], or in
combination with metals from group VIII of the periodic table [23-25]. However, also in this case
the transfer to large volume markets was hampered.
A third lead towards the development of alternatives is based on the calcium binding properties of
oxydiacetate (ODA, Fig. 1) and related structures. This W-shaped structural element can be
generated from carbohydrates such as starch and cellulose after glycolic oxidation of the vicinal
diols of the glucose moiety [10, 26-31]. Several routes have been developed, namely:
Fig. 1. ODA Ca2+ complex.
- A one-step route using nitric acid and sulfuric acid in the presence of a vanadium catalyst [32],
- A one-step route using sodium hypochlorite or sodium hypobromite or a combination of these
two as the oxidizing agent [28-31, 33-39] and
- A two-step approach whereby the diols of the glucose moiety are reacted with sodium periodate
to obtain the dialdehyde intermediate product and subsequently the dialdehyde is reacted with
sodium chlorite to obtain the dicarboxy product [29-31, 33].
Although both routes should generate the same product, the one-step approach yields products with
a calcium sequestering capacity of at most 1.5 mmol Ca2+ per gram of oxidized product and the
two-step approach yields products with a calcium sequestering capacity up to 2.5 mmol Ca2+ per
gram of oxidized product [29-31].
In theory every two carboxy units are able to sequester one calcium ion. If all the glucose moieties
are oxidized, a maximum sequestering capacity of about 4.2 mmol Ca2+ per gram of oxidized
product can be obtained. In practice, it seems to be difficult to obtain a complete conversion of the
diol systems of the glucose moiety into dicarboxy units, but it shows that there is room for
improvement. This can be attained by using excess of periodate and applying longer reaction times.
For this reason we have investigated this reaction in more detail and the subsequent transformation
into the dicarboxy product.
It is well known that polysaccharides can be oxidized with periodic acid to yield the so-called
diaidehyde polysac-charide. Especially, diaidehyde starch has been manufactured as a wet-strength
additive for tissue-paper and furthermore can serve as a raw material for many purposes. Also
diaidehyde starch draws attention as a precursor of dicarboxy starch as described above.
Starch + periodate → diaidehyde starch + iodate
Much effort was spent in the development and scaling up of the chemicals involved [40-42]. As
periodate is an expensive chemical the manufacture of diaidehyde starch was directed towards
recovery of the spent periodate after reaction.
Next to the use of ozone [43] an attractive route was to regenerate periodate by electrochemical
means and indeed many papers have been published regarding this subject [44-53 and references
cited therein]. One of the most recent studies has been published by Veelaert et al. The work covers
among others a new electrochemical principle. Besides the electrochemical regeneration, in which
the conventional lead electrodes are used, an electro-dialysis separation step is applied. The method
has the advantage that contact of starch and diaidehyde starch
with the lead electrode is avoided and herewith contamination of the electrode and possible side
reactions.
Another subject concerns the kinetics and a mathematical description of the process. As expected,
in the initial phase, the reaction can be described as a second-order process, but after a certain
period the kinetics behavior seems to change. The explanation is rather simple, because the
aldehyde groups formed react with free glucose units. Veelaert et al. pointed out that the reactions
will be manifold. The reactions concern hemiacetal and hemialdal formation and since they are
reversible it implies that still the reaction can be practically completed, but that the reaction time for
completion is (much) longer than expected. An important conclusion is that even after several days
the reaction does not appear to be complete.
Floor et al. evaluated the periodate reaction, but the conclusions of this group do not match with
those of Veelaert et al. which were later described. The study of the latter group was more extended.
Floor et al. oxidized starch with sodium periodate at low temperature (277 K) and for 20 h and
concluded that the product was fully oxidized. However, based on the kinetic data a not-completely
oxidized product can be expected (as follows from the data provided by Veelaert et al.). It implies
that the dicarboxy starch product could not generate an optimal complexing product.
Veelaert et al. showed that aldehyde groups formed in the glucose polymer react (amongst others)
with free glucose units. The net result is that such blocked groups cannot react with periodate. If all
reaction equilibria are taken into account the conclusion can be drawn that on the long term starch
can be completely oxidized (> 98%), but
Fig. 2. Hemiacetal formation with adjacent glucose moieties.
much slower than to be expected on basis of a second order behavior (see Fig. 2).
The above conclusion has consequences for the earlier mentioned two-step approach towards the
synthesis of complexing agents. Especially the prolonged reaction time of the first step will result in
a more efficient transformation of starch into dialdehyde starch. Subsequently the dialdehyde
moiety is oxidized by sodium chlorite into the dicarboxy product. Due to the presence of ODA-type
structural elements in the polymer the sequestering capacity of the final product will be improved.
This paper describes experiments, which show relevance towards the postulated hypothesis.
2 Experimental 2.1 Materials
Potato starch was obtained from Avebe (Foxhol, The Netherlands), corn starch was supplied by
National Starch and Chemicals (Bridgewater, NJ, USA) sodium trimetaphosphate (>85%), sodium
periodate (>99%) and sodium chlorite (80%) were obtained from Aldrich (St. Louis, MO, USA).
Sodium hydroxide was purchased from Baker Chemicals (Phillipsburg, NJ, USA) as a 0.5 M solution. EDTA, calcium chloride and hydrogen peroxide were obtained from Merck (Darmstadt,
Germany). Polyacrylate was a technical product (Dick Peters, Chemische indus-trie, Denekamp,
The Netherlands). The pH stat equipment was a Titrino 719 S (Metrohm, Herisau, Switzerland).
2.2 Synthesis of the complexing agents
Two series of experiments were conducted to study the influence of reaction variables on the
properties of the final product (dicarboxy starch). In the first series the effect of the initial
temperature of the periodate oxidation was investigated. Besides, we investigated also the oxidation
of corn starch (25% amylose) and high amylose corn starch (70% amylose). Experimental data and
results are presented in Tabs. 1 and 2.
2.2.1 Dialdehyde starch
In 100 mL of water 5.05 g starch (dry, corresponding to 31 mmol anhydroglucose units) was
suspended. To the magnetically stirred suspension 6.9 g (32 mmol) periodate was added. The
reaction time and temperature were varied to investigate the effect of these variables on the final
properties of the dicarboxy starch (see Tabs. 1 and 2 for details). According to literature data,
conversion of starch has to be conducted at low temperature and in the
Tab. 1. Calcium binding capacity of periodate/sodium chlorite oxidized starches.
Tab. 3. SC values of some Ca2+ binders per gram of product as function of pH.
dark to prevent release of iodine. However, one patent states that the reaction may be carried out at
higher temperature [54]. We investigated this option and because the results showed that this route
is less attractive, we decided to investigate temperature trajects. The starch product was collected by
filtration and washed with water until iodate free. One of the reasons to heat the reaction mixture is
to have a faster and more complete conversion. However, it is to be expected that at higher
temperature side reactions may play a role in, e.g. reduction of the average molecular weight of the
final dicarboxy product.
2.2.2 Dicarboxy starch
The wet dialdehyde starch was suspended in water and after addition of hydrogen peroxide (8.0 mL
30%, w/w, 78 mmol) the mixture was placed in a cooling bath (ice or water of 20 °C, see Tab. 1 for
details about the respective experiments) to prevent excessive heating. In the course of a few hours
sodium chlorite (7.4 g, 80%, which corresponds to 65 mmol) was added. Whilst reaction the pH
was maintained at 5, which was done by addition of 0.5 M sodium hydroxide solution, controlled
by a pH stat. The addition of NaCIO2 was performed batchwise in such a way that as soon as the
rate of sodium hydroxide addition declined, a new batch of NaCIO2 (1 - 1.5 g) was introduced.
After one day reaction approximately 45 mmol NaOH was consumed (at pH 5). The reaction
mixture was subsequently brought to pH 10. The total consumption of NaOH was 60-62 mmol in
good agreement with the theoretical value (61 mmol). Next, the reaction mixture was desalted by
nanofiltration (flat membrane UTC 60 supplied by Toray, Tokyo, Japan). Finally, the solution was
concentrated to about 50 mL and freeze dried. From the product yield and calcium sequestering
capacity (Ca SC) were determined.
2.3 Numerical simulation
To obtain more insight into the reaction of starch with pe-riodate, the course of the reaction was
calculated using Facsimile, a numerical simulation model, developed by AERE (Harwell, UK). The
experiments as described by Veelaert (thesis, page 45) were taken as reference for the calculations.
2.4 Calcium sequestering capacity
The calcium binding capacity of the product was determined using a Ca2+-ion selective electrode
(Radiometer F2002) in combination with a calomel electrode (Radiometer K 401) as the reference.
For the calibration of the electrode aqueous solutions of 10-2, 10-3, 10-4, 10-5, and 10"6 M CaCI2
were used. The calibration curve (log U and CCa2+) was linear in the region between 10-2 and 10-5,
according to the specifications of the supplier. The sequestering capacity was measured by titration
as follows: small volumes (100 μL) of a 0.4 M CaCI2 solution were added to a solution of
approximately 100 mg material dissolved in 200 mL of water. After 2 min of stirring the resulting
Ca2+ concentration was measured. The pH was kept at 10.3. From the titration curve the amount
needed for lowering the concentration to 10-5 M Ca2+ was found by interpolation.
The calcium sequestering capacity is defined as the number of mmol Ca2+, that is coordinated by 1
g of sequestering agent at given pH until the concentration of non-bound Ca2+ is 10-5 M. This value
is considered to be the upper limit in the washing process at which no incrustation occurs.
The sequestering capacities of the dicarboxy starches were determined at different pH and
compared to those of currently used sequestrants (see Tab. 3 for results).
3 Results and Discussion
The theoretical approach on the preparation of dialdehyde starch is a follows:
Based on the data provided by Veelaert et al. [44-47], we have evaluated the results by a
mathematical approach (numerical simulation model). In this model we have considered three
reactions:
1 glucose + periodate → dialdehyde glucose units starch + iodate (reaction constant kf)
2 dialdehyde glucose unit + glucose unit → hemiacetal 1 (equilibrium with k2 k-2)
3 hemiacetal + glucose unit → hemiacetal 2 (equilibrium with k3 k-3)
Fig. 3. Kinetic profile [calculated (m) and experimental (exp)] of the periodate oxidation of starch.
The rate constant of the first reaction k1 =0.014 M-1·s-1 at 25 °C is found in Veelaerts thesis. The
other values were estimated by fitting the measured and calculated data. With k2=0.0175 M-1S-1 k-2
= 0.004 s-1 k3=10.0175 M-1 s-1 k-3=0.004 s-1. a good agreement between calculated and experimental
data was achieved. A result of a calculation vs. experiments is given in Fig. 3. It has to be noted that
the other experiments described by Veelaert with 0.062 M of periodate and with 0.192 M of starch
agreed very well.
From these results we concluded that to complete this reaction 10% excess of periodate is needed. It
follows from this approach that in Float's experiments not completely oxidized DAS has been used
to prepare dicarboxy starch as was already expected.
From the preliminary calculations, it can be concluded that a prolonged reaction time of the
periodate oxidation probably will result in products with higher calcium sequestering capacity. In
order to evaluate the above idea, experiments have been performed whereby the theoretical degree
of oxidation approaches 100% and, as can be seen from Tabs. 1 and 2, the net result is indeed a high
calcium sequestering capacity. Tab. 1 gives the results of experiments in which we used different
starches and in which emphasis was put on the influence of the conditions of the first oxidation step.
Tab. 2 gives results of experiments in which only potato starch was studied and the influence of
reaction time and temperature on the final properties of the product was established. The most important conclusion from the latter experiments is that a relatively short oxidation time can be taken,
provided that
the initial phase is conducted at low temperature. Then the reaction can be completed within 24 h.
Additional experiments should be done to investigate the effect of shorter reaction times.
It is also clear that at more rigorous conditions the SC as expected does not further increase. The
highest value for the SC is 3.35 mmol Ca2+/g, which exceeds the values found by Floor by
approximately 25%. Apparently this is due to the prolonged reaction time of the first step and improved conditions with regard to the periodate oxidation. With respect to the second step, we have
the feeling that the conditions are more or less optimal. From the experiments it can be seen that
using a larger amount of NaCIO2 or longer reaction time does not improve the calcium sequestering
capacity. It is also important to stress that the ODA-structure can only be attained when two
neighboring groups are oxidized. Part of our improvement is probably due to the fact that a higher
amount of oxidized glucose units is introduced. It should also be considered that more neighboring
groups can form the ODA-structure.
- There seems to be no difference in the performance of potato starch and corn starch.
However, there is a significant difference in yield. With maize starch the yield is much
lower and in view of the work-up procedure this points to degradative losses. With regard to
the periodate oxidation it can be concluded that prolonged reaction time (i.e. three days
instead of two days) and heating at the end of the reaction have no effect on the final
properties of the product.
- Although the periodate reaction can be completed within 4 h by conducting the reaction at
elevated temperature, the quality of the final product is less. It seems therefore better to start the
reaction at relatively low temperature and to raise the temperature after a substantial conversion into
dialdehyde has occurred. Further optimization is needed to establish the minimum reaction time.
- The reaction temperature of the second step does not influence the quality of the final product.
4 Conclusion
Dicarboxy starch (DCS) with a high calcium sequestering capacity (3.3 mmol Ca2+/g) can be
prepared from starch by consecutive oxidation with periodate and sodium chlorite/hydrogen
peroxide in high yield (> 95%). Because DCS is degradable in the environment and its binding of
Ca2+ is higher than that of the currently polyacrylate used in laundry formulations the application of
DCS as a co-builder in laundry detergents could be a "green" alternative. Interestingly, the material
can be applied at lower pH without significant loss of binding capacity contrary to other well known
complexing agents.
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(Received: March 17, 2003)
(Accepted: May 21, 2003)