The present invention relates to a superabsorbent composition comprising a cross-linked polysaccharide and wood flour or other lignocellulose-containing renewable raw material, wherein the cross-linked polysaccharide comprises polysaccharide chains, which are chemically cross- linked via ester bridges, wherein said ester bridges are spaced by an aliphatic linker L. The present invention further relates to a soil treatment product comprising the superabsorbent composition of the invention and at least one additional ingredient selected from the group consisting of fillers, nutrients, fertilizers, pesticides and combinations thereof, to a process for the manufacture of the superabsorbent composition of the invention, and to the use of the composition for agricultural applications.

Background of invention

Hydrogels are formed from superabsorbent polymers which can absorb and retain extremely large amounts of a liquid relative to their own mass. Such superabsorbent polymers are often also referred to as swellable polymers, hydrogel forming polymers, water absorbing polymers, gelforming polymers, and the like. Sometimes also the superabsorbent polymer in the dry form is referred to as hydrogel. In the context of the present invention, the term "hydrogel" will be used only in the context of the wetted state of a superabsorbent polymer, however, because in the dry state, the superabsorbent polymer is typically not present in the form of a gel, but in the form of a powder or a granulate having good flow properties.

An overview over superabsorbent polymers, their properties and methods of manufacturing them is provided by Frederic L. Buchholz and Andrew T. Graham in "Modern Superabsobent Polymer Technology", J. Wiley & Sons, New York, USA / Wiley VCH, Weinheim, Germany, 1997, ISBN 0-471 -1941 1 -5.

Superabsorbent polymers and compositions comprising superabsorbent polymers have become important materials for agricultural applications due to their capacity of absorbing large quanti- ties of water. By using the superabsorbent polymers and superabsorbent compositions for soil treatment, the physiological properties of soils can be improved by increasing their capacity to hold water, reducing erosion and runoff, reducing the frequency of irrigation, increasing the effi- ciency of the water being used, increasing soil permeability and infiltration, reducing the tendency of the soil to get compacted, and helping plant performance.

Most of the superabsorbent polymers used today are cross-linked synthetic polymers. They include, for example, polymers and copolymers based on acrylamide, which are not based on renewable raw material and which are insufficiently biodegradable.

For many applications, and in particular for agricultural applications, the biodegradation of the superabsorbent polymers is a preferred or required design variable to be addressed, however. In this context, polysaccharide-based and in particular cellulose-based superabsorbent polymers are considered highly attractive not only because of their biodegradability, but also because of the large availability of cellulose and the low cost of cellulose derivatives.

Depending on the polysaccharide derivatives used, a number of cross-linking agents and cata- lysts can principally be employed to form superabsorbent polymers thereof. Epichlorhydrin, aldehydes and aldehyde-based reagents, urea derivatives, carbodiimides and multifunctional car- boxylic acids are most widely used cross-linkers for polysaccharides. However, the water absorption capacity of these cross-linked polysaccharides is often not satisfying compared to synthetic superabsorbent polymers. Good swelling properties are e.g. described for cross-linked polysaccharides obtainable by using epichlorhydrin as cross-linking agent.

In this regard, C. Chang et al. describe superabsorbent hydrogels based on cellulose for smart swelling and controllable delivery, which are prepared from sodium carboxymethylcellulose (CMC) and cellulose in an NaOH/urea aqueous system by using epichlorohydrin (ECH) as cross-linker. C. Chang et al. conclude that the experimental results would prove that the cellulose/CMC hydrogels exhibit superabsorbent capacity and high equilibrium swelling ratio (C. Chang, B. Duan, J. Cai, L. Zhang, European Polymer Journal 46 (2010) 92-100).

CN 101445609 A relates to a hygroscopic cellulose hydrogel and a preparation method thereof. The hydrogel is prepared by dissolving carboxymethylcellulose and cellulose, respectively, in a water solution of NaOH/urea, in which the concentration of NaOH is 6-10 wt.-%, and the concentration of the urea is 4-12 wt.-% to obtain a NaOH/urea mixed water solution of 1 -5 wt.-% of the carboxymethylcelluluse and a NaOH/urea mixed water solution of 1 -5 wt.-% of cellulose; mixing the NaOH/urea mixed water solution of the carboxymethylcellulose and the NaOH/urea mixed water solution of cellulose according to the weight ratio 9:1 -1 :9 and stirring evenly to obtain a mixed solution; adding the mixed solution to a certain amount of the cross-linker epoxy- cholorpropane; and, after the cross-linking reaction, placing the mixed solution at the gelatin temperature until the hygroscopic cellulose hydrogel is formed, wherein the water absorbing capacity is 50-1000 g/g.

It should be emphasized however that epichlorhydrin is highly toxic in the unreacted state. Alt- hough unreacted chemicals are usually eliminated after cross-linking through extensive washing in distilled water, toxic cross-linking agents should be avoided, in order to preserve biocompati- bility of the resulting hydrogel, as well as to ensure an environmentally friendly sustainable production process. It is therefore desired to use alternative non-toxic cross-linking agents. At the same time, the polysaccharide-based superabsorbent polymers obtainable thereof should ex- hibit a satisfying water absorption capacity.

Accordingly, there remains a need for polysaccharide-based superabsorbent polymers obtainable from non-toxic cross-linking agents to further improve the safety of both, the final product and the manufacturing process. At the same time, it is desired to achieve a high water absorp- tion capacity, in order to make the polysaccharide-based superabsorbent polymers competitive with acrylate-based superabsorbent polymers, which are presently on the market for agricultural applications.

It is therefore an object of the present invention to improve the water absorption capacity of a polysaccharide-based superabsorbent polymer, which is biodegradable and environmentally friendly in that it is obtainable from non-toxic cross-linking agents.

Furthermore, it is an object of the present invention to provide a soil treatment product, which is biodegradable and environmentally friendly, and exhibits a satisfying water absorption capacity compared to non-biodegradable soil treatment products.

Summary of the invention

The above mentioned objects are achieved by providing a superabsorbent composition com- prising a cross-linked polysaccharide and wood flour or other lignocellulose-containing renewable raw material ("other lignocellulose-containing renewable raw material" is referred to as "OLM" in the following), wherein the cross-linked polysaccharide comprises polysaccharide chains, which are chemically cross-linked via ester bridges on each chain, wherein said ester bridges are spaced by a linker L.

It has surprisingly been found that by combining wood flour or OLM with a cross-linked polysaccharide in a composition according to the present invention, the water absorption capacity and the nutrient solution absorption capacity can be significantly improved compared to the water absorption capacity and nutrient solution absorption capacity of the cross-linked polysaccharide alone. It has been found that it is particularly advantageous if the cross-linked polysaccharide is formed by reacting the polysaccharide with the precursor P of the linker L as cross-linking agent in the presence of the wood flour or OLM and water. The superabsorbent compositions of the invention are then directly obtained from this reaction mixture and exhibit the advantageous effects as described herein.

It is noted that the water absorption capacity of the superabsorbent compositions according to the present invention is not only superior to the water absorption capacity of the cross-linked polysaccharide alone, but also also superior to the water absorption capacity of wood flour or OLM alone. Furthermore, it has surprisingly been found that the absorption capacity in terms of a nutrient solution is even improved more than threefold, if wood flour or OLM is combined with the cross-linked polysaccharide in the compositions of the present invention. Thus, the super- absorbent composition of the present invention is particularly advantageous in terms of the absorption capacity in terms of a nutrient solution.

Furthermore, it is an advantage of the composition of the present invention that the cross-linked polysaccharides are completely biodegradable to water and carbon dioxide, and the same ap- plies of course also to the wood flour or OLM. Furthermore, the cross-linked polysaccharide is obtainable not only from non-toxic polysaccharide derivatives, but also from non-toxic cross- linking agents because the formation of ester bridges does not require cross-linking agents, which contain epoxy groups or carbon-halide bonds as e.g. required for the formation of ether bridges with the hydroxyl groups of a polysaccharide. Furthermore, it should be emphasized as another advantage that ester bridges can be easily hydrolyzed in contrast to e.g. ether bridges. Therefore, the biodegradability is significantly improved.

According to the invention, the composition may be used as a component in a soil treatment product, e.g. for agricultural applications. Such a soil treatment product comprises the super- absorbent composition of the invention and at least one additional ingredient selected form the group consisting of fillers, nutrients, fertilizers, pesticides and combinations thereof, wherein the composition and the additional ingredient are preferably present in a weight ratio of from 80:20 to 20:80. The soil treatment product of the invention is biodegradable and environmentally friendly because the presence of an acrylate-based superabsorbent polymer can be completely avoided. At the same time a comparable water absorption capacity can be achieved due to the fact that wood flour or OLM is combined with the cross-linked polysaccharide in the superabsorbent composition contained in the soil treatment product.

The invention further relates to a process for preparing the superabsorbent compositions of the present invention comprising the steps of

1 ) dissolving the precursor P of the linker L in water to obtain a homogenous solution;

2) adding the wood flour or OLM and the polysaccharide to the solution of step 1 ) to obtain a highly viscous mixture;

3) allowing the mixture of step 2) to react at 20 to 30°C for 2 to 4 days;

4) keeping the mixture of step 3) at 2 to 8°C for 1 to 3 days;

5) heating the mixture of step 4) to 120 to 160°C for 2 to 4 hours to obtain a dried mixture.

Furthermore, the invention relates to the use of the superabsorbent compositions of the inven- tion for agricultural applications.

Figures

Figure 1 : Water absorption capacities of compositions, wherein the wood flour and the poly- saccharide chains of the cross-linked polysaccharide are present in a weight ratio 0/100, 40/60, 60/40, 80/20, 90/10 and 100/0.

Figure 2: Absorption capacities of compositions in terms of nutrient solutions, wherein the wood flour and the polysaccharide chains of the cross-linked polysaccharide are present in a weight ratio 0/100, 40/60, 60/40, 80/20, 90/10 and 100/0.

Detailed description of the invention

The superabsorbent composition according to the present invention comprises a cross-linked polysaccharide and wood flour or OLM, wherein the cross-linked polysaccharide comprises polysaccharide chains, which are chemically cross-linked via ester bridges, wherein said ester bridges are spaced by an aliphatic linker L.

In a preferred embodiment, the above mentioned superabsorbent composition according to the present invention comprises a cross-linked polysaccharide and wood flour. In another preferred embodiment, the above mentioned superabsorbent composition according to the present invention comprises a cross-linked polysaccharide and OLM. In yet another preferred embodiment, the above mentioned superabsorbent composition according to the present invention comprises a cross-linked polysaccharide and OLM, wherein the OLM is selected from the group consisting of hemp dust, flax dust, ground straw, ground olive stones, ground tree bark, waste material from pulp production, sugar beet peel, sugar cane waste, rice husks, cereal husks, ground hemp fibers, ground flax fibers, ground Chinese silvergrass fibers, ground coconut fibers, ground kenaf fibers, and ground wood fibers. The OLM is preferably hemp dust, flax dust, ground straw, ground olive stones, ground tree bark, waste material from pulp production, sugar beet peel, or sugar cane waste, more preferably hemp dust, flax dust, ground straw, or sugar cane waste, most preferably flax dust or ground straw, in particular flax dust.

In a preferred embodiment, the superabsorbent composition comprises a cross-linked polysaccharide and wood flour or OLM, wherein the cross-linked polysaccharide comprises polysaccharide chains, which are chemically cross-linked via ester bridges, wherein said ester bridges are spaced by an aliphatic linker L, wherein the ester bridges are formed by reacting hydroxyl groups of the polysaccharide chains with a precursor P of the linker L comprising at least two carbonyl containing functional groups suitable for forming an ester bridge spaced by the aliphatic linker L, wherein the precursor P of the linker L is a compound of the following general formu- la (I):

(I) wherein

(a) R ^a and R ^b are independently selected from the group consisting of -halo, -OH, -OR ¹ , -IMH2 and -N(R ¹ ) ₂ with R ¹ being -(Ci-C ₆ )alkyl or -C(=0)(Ci-C ₄ )alkyl, or

(b) R ^a -R ^b together represent an oxygen bridge -0-, and

L is the aliphatic linker, which is optionally substituted with at least one substituent selected from the group consisting of -OH, -OR ³ , -NH ₂ , -N(R ³ ) ₂ , -COOH, and -COOR ⁴ , with R ³ being -(Ci- C ₆ )alkyl and R ⁴ being -(Ci-C ₆ )alkyl or Na.

As used herein, the expression "chemically cross-linked via ester bridges, wherein said ester bridges are spaced by an aliphatic linker L" means that the polysaccharide chains have been cross-linked with each other by forming ester bridges to an aliphatic linker L. Accordingly, the polysaccharide chains are connected to each other in that each polysaccharide is connected to an ester bridge, and said ester bridges are connected to each other by the aliphatic linker L. Thus, two cross-linked polysaccharide chains of the cross-liked polysaccharide according to the invention can generically be illustrated as follows:

ester ester

bridge bridge polysaccharide polysaccharide

chain chain

It should be emphasized that each polysaccharide chain may form several ester bridges which are linked to ester bridges of other polysaccharide chains via the aliphatic linker L so that a cross-linked network of polysaccharide chains is established. As used herein, the term "aliphatic linker" refers to an alkyl, alkylene or alkynylene chain comprising from 1 to 10 carbon atoms, which may be linear or branched, and which may be unsub- stituted or substituted by at least one substituent selected from the group consisting of -OH, - OR ³ , -NH ₂ , -N(R ³ ) ₂ , -COOH, and -COOR ⁴ , with R ³ being -(Ci-C ₆ )alkyl and R ⁴ being -(Ci-C ₆ )alkyl or Na. Preferably, the aliphatic linker is a (Ci-Cs)alkyl chain, which may be linear or branched, and which is optionally substituted with at least one substituent selected from the group consisting of -OH, -OR ³ , -NH ₂ , -N(R ³ ) ₂ , -COOH, and -COOR ⁴ , with R ³ being -(Ci-C ₆ )alkyl and R ⁴ being -(Ci-Ce)alkyl or Na. More preferably, the aliphatic linker L is a linear (C ₂ -C ₄ ) alkyl linker, which is optionally substituted with -OH, -OR ³ and -COOH or -COOR ⁴ with R ³ being -(Ci-C ₃ )alkyl and R ⁴ being -(Ci-Cs)alkyl or Na. Most preferably, the aliphatic linker L is a linear C3-alkyl linker, which is optionally substituted with -OH and -COOH or -COONa. Particularly preferably, the aliphatic linker is a linear C3-alkyl linker, which is substituted with one substituent -OH at the C ₂ -atom and with one substituent -COOH or -COONa at the C ₂ -atom.

As used herein, the term "ester bridge" covers a -C(=0)-0- moiety. The carbonyl group -C(=0)- of said -C(=0)-0- moiety may either stem from the polysaccharide chain, if e.g. carboxymethyl- cellulose is used and is reacted with a precursor P of the linker L comprising at least two hy- droxyl groups as functional groups, or the carbonyl group -C(=0)- of said -C(=0)-0- moiety may stem from a precursor P of the linker L comprising at least two carbonyl containing functional groups suitable for forming an ester bridge, which then react with the hydroxyl groups of the polysaccharide chain to form the -C(=0)-0-moiety. Thus, there are two possibilities for the connectivity of two cross-linked polysaccharide chains in terms of their cross-linkage via the ester bridges spaced by the aliphatic linker, i.e. the following connectivity 1 or 2. polysaccharide polysaccharide polysaccharide polysaccharide chain chain chain chain

2

In a preferred embodiment of the present invention, the ester bridges are formed by reacting hydroxyl groups of the polysaccharide chains with a precursor P of the linker L comprising at least two carbonyl containing functional groups suitable for forming an ester bridge spaced by the aliphatic linker L.

Accordingly, it is preferred that the carbonyl group -C(=0)- of the -C(=0)-0-moiety in each es- ter bridge stems from the precursor P of the linker L. Thus, it is preferred that two cross-linked polysaccharide chains of the cross-linked polysaccharide according to the invention have the following connectivity 1.

polysaccharide polysaccharide

chain chain

It should be emphasized that ester bridges are particularly advantageous compared to e.g. ether bridges because they can be easily hydrolyzed, which improves the biodegradability of the cross-linked polysaccharides.

In view of the above, the term "precursor" has to be understood as a molecule comprising the linker L and at least two functional groups capable of forming the ester bridges with the polysaccharide. As already indicated above, said functional groups may either be hydroxyl groups or carbonyl containing functional groups suitable for forming an ester bridge. It is preferred, however, that the precursor P comprises at least two carbonyl containing functional groups suitable for forming an ester bridge. When preparing the cross-linked polysaccharides according to the present invention, the precursor P of the linker L may thus be employed as cross-linking agent.

As used herein, the term "carbonyl containing functional group suitable for forming an ester bridge" refers to a functional group, which comprises a -C(=0)- moiety and is activated for forming an ester bridge if reacted with a hydroxyl group. Preferably, said "carbonyl containing functional group suitable for forming an ester bridge" is represented by -C(=0)-R ^L , wherein R ^L is a leaving group, which is substituted upon reaction with a hydroxyl group. For example, R ^L may be selected from -halo, -OH, OR ¹ , -NH ₂ and -N(R ¹ ) ₂ with R ¹ being -(Ci-C ₆ )alkyl or -C(=0)(Ci- C4)alkyl. Since preferably at least two carbonyl containing functional groups suitable for forming an ester bridge are present in the precursor P, said groups may also be referred to as -C(=0)- R ^a , -C(=0)-R ^b , -C(=0)-R ^c , -C(=0)-R ^d , and so on. Two leaving groups together, e.g. R ^a and R ^b , may also together represent an oxygen bridge -0-, i.e. form a cyclic anhydride with two carbonyl groups, so that both carbonyl groups are activated for substitution by the one oxygen bridge -O-

In view of the above definition of the precursor P of said linker L as having at least two functional groups capable of forming the ester bridges of the polysaccharide, which are preferably carbonyl containing functional groups suitable for forming an ester bridge, a precursor P of the linker L may e.g. be described by the general formula (I)

(I)

In a preferred embodiment of the present invention, the precursor P of the linker L is a compound of the following general formula (I)

(I) wherein

(a) R ^a and R ^b are independently selected from the group consisting of -halo, -OH, -OR ¹ , and -N(R ¹ ) ₂ with R ¹ being -(Ci-C ₆ )alkyl or -C(=0)(Ci-C ₄ )alkyl, or

(b) R ^a -R ^b together represent an oxygen bridge -0-, and L is the aliphatic linker, which is preferably a linear or branched (Ci-Cs)alkyl chain, which is optionally substituted with at least one substituent selected from the group consisting of -OH, - OR ³ , -IMH2, -N(R ³ ) ₂ , -COOH, and -COOR ⁴ , with R ³ being -(Ci-C ₆ )alkyl and R ⁴ being -(Ci-C ₆ )alkyl or Na.

In a more preferred embodiment of the present invention, the precursor P is a dicarboxylic acid or a tricarboxylic acid, preferably a tricarboxylic acid, more preferably citric acid.

In this context, a dicarboxylic acid has to be understood as an aliphatic compound comprising two carboxylic acid groups -COOH as carbonyl containing functional groups -C(=0)-R ^a and - C(=0)-R ^b suitable for forming an ester bridge, and a linker L as defined above, wherein the optionally substituted alkyl, alkylene or alkynylene chain of the linker L is not substituted with a further carboxyl group such as an carboxylic acid group or an carboxylic acid ester group. On the other hand, a tricarboxylic acid has to be understood as an aliphatic compound comprising two carboxylic acid groups -COOH as carbonyl containing functional groups -C(=0)-R ^a and - C(=0)-R ^b suitable for forming an ester bridge, and a linker L as defined above, wherein the alkyl, alkylene or alkynylene chain of the linker L is mandatorily substituted with one -COOH group as further carbonyl containing functional group -C(=0)-R ^c suitable for forming an ester bridge, but is not substituted with a further carboxyl group such as an carboxylic acid group or an carboxylic acid ester group.

Preferably, the dicarboxylic acid is an aliphatic compound comprising

(i) a linear or branched (Ci-Cs)alkyl chain, which is optionally substituted with at least one substituent selected from the group consisting of -OH, -OR ³ , -IMH2, and -N(R ³ )2, with R ³ being - (Ci-C ₆ )alkyl, and

(ii) two carboxylic acid groups -COOH.

Preferably, the tricarboxylic acid has to be understood as an aliphatic compound comprising

(i) a linear or branched (Ci-Cs)alkyl chain, which is optionally substituted with at least one substituent selected from the group consisting of -OH, -OR ³ , -IMH2, and -N(R ³ )2, with R ³ being -

(Ci-Ce)alkyl, and mandatorily substituted with at least one -COOH group, and

(ii) two carboxylic acid groups -COOH.

Particularly preferred dicarboxylic acids according to the present invention are e.g. malonic acid, succinic acid (butanedioic acid), glutaric acid, adipic acid and pimelic acid. Further preferred dicarboxylic acids are C ₄ to C20 alpha, omega-dicarboxylic acids, for example C ₄ to C10 alpha, omega-dicarboxylic acids. Further preferred dicarboxylic acids are unsaturated dicarboxylic acids such as itaconic acid, maleic acid and fumaric acid. Preferred tricarboxylic acids are citric acid, isocitric acid, aconitic acid, propane-1 ,2,3-tricarboxylic acid (tricarballylic acid, carballylic acid) and trimesic acid. The most preferred precursor P of the linker L is citric acid. The chemical structure of citric acid is depicted below.

Citric acid can either exist in an anhydrous form or as a monohydrate. Both forms are encom- passed by the term "citric acid", when used in the context of the present invention.

As used herein, the term "polysaccharide" refers to long carbohydrate molecules of monosaccharide units joined together by glycosidic bonds. In this context, the term "polysaccharide chain" is used to indicate that a single carbohydrate molecule may be considered as chain of monosaccharide units. Considering that the repeating monosaccharide units in polysaccharides are preferably six-carbon monosaccharides, polysaccharides may e.g. be represented by the general formula (C6Hio05) _n , wherein 40≤ n < 3000. Examples of polysaccharides include starch, glycogen, cellulose and chitin. Examples of monosaccharides are glucose, fructose and glyceraldehydes. A preferred polysaccharide according to the present invention is cellulose, which is based on repeated glucose units bonded together by β-glycosidic bonds.

According to the present invention, the term "polysaccharide" also covers derivatives of polysaccharides. For example, the hydroxyl groups of the polysaccharides may partly be modified, so that the hydrogen atom is replaced by an alkyl group such as an ethyl or methyl group, a hydroxyalkyl group such as a hyroxypropyl group, or a carboxyalkyl group such as a carboxy- ethyl group or a carboxymethyl group or the alkali salts thereof.

In a preferred embodiment of the present invention, the polysaccharide chains contain car- boxy(Ci-C3)alkyl groups, preferably carboxyethyl or carboxymethyl groups, more preferably car- boxymethyl groups. Examples of natural polysaccharides containing carboxymethyl groups are inter alia alginate and pectin.

Accordingly, it is preferred that the hydroxyl groups of the polysaccharide chains are partly modified, so that the hydrogen atom is replaced by a carboxyethyl group or a carboxymethyl group or the alkali salts thereof, more preferably by a carboxymethyl group or the alkali salts thereof. If the carboxymethyl group is present in the form of its alkali salt, the carboxymethyl group is present in anionic form and an alkali metal is present in cationic form. Otherwise, the carboxymethyl group is present in protonated form. Accordingly, a carboxymethyl group may be represented by the formula -CH2COOH, if present in protonated form, or by the formula -ChbCOO-IV , wherein M represents an alkali metal, if in the form of the alkali salt. It is preferred according to the present invention that the polysaccharide comprises either protonated carboxymethyl groups or anionic carboxymethyl groups, preferably anionic carboxymethyl groups, more preferably sodiumcarboxymethyl groups. Most preferred polysaccharides according to the present are carboxymethylcellulose and sodiumcarboxymethylcellulose. Particularly preferably, the pol- ysaccharide is sodiumcarboxymethylcellulose.

Carboxymethylcellulose may be represented by the following general formula (A):

(A) wherein R either represents hydrogen or -CH2COOH and 40≤ n < 3000, preferably 60≤ n < 2000, more preferably 80≤ n < 800, and wherein at least one R in the chain represented by general formula (A) represents -CH2COOH.

Sodiumcarboxymethylcellulose may be represented by the following general formula (B):

(B) wherein R either represents hydrogen or -CH2COO ^" Na ⁺ and 40≤ n < 3000, preferably 60≤ n < 2000, more preferably 80≤ n < 800, and wherein at least one R in the chain represented by general formula (A) represents -CH2COOH. Carboxymethylcellulose and sodiumcarboxymethylcellulose are particularly advantageous compared to cellulose because of their solubility in water. However, also mixtures of these cellulose derivatives with cellulose itself are encompassed by the polysaccharides of the present invention.

The properties of carboxymethylcellulose and sodiumcarboxymethylcellulose depend on the substitution grade, i.e. the extent to which the hydroxyl groups are modified, so that the hydrogen atom is replaced by a carboxymethyl group or a sodiumcarboxymethyl group. In other words, it is the ratio of the number of substituents R in the above formulae (A) and (B), which represent a carboxymethyl group or a sodiumcarboxymethyl group, relative to the number of substituents R, which represent hydrogen. According to the present invention, a substitution grade of from 0.3 to 1.0, preferably from 0.5 to 0.9, more preferably from 0.7 to 0.8.

Thus, in a preferred embodiment of the present invention, the polysaccharide chains are carboxymethylcellulose with a substitution grade of from 0.3 to 1.0, preferably from 0.5 to 0.9, more preferably from 0.7 to 0.8. Alternatively, the polysaccharide chains are anionic carboxymethyl- cellulose, preferably sodium carboxymethylcellulose, with a substitution grade of from 0.3 to 1.0, preferably from 0.5 to 0.9, more preferably from 0.7 to 0.8.

With regard to the preferred polysaccharide chains and the preferred precursors P of the linker L according to the present invention, it is preferred that the cross-linked polysaccharide is based on carboxymethylcellulose with a substitution grade of from 0.3 to 1 .0, preferably from 0.5 to 0.9, more preferably from 0.7 to 0.8, or anionic sodium carboxymethylcellulose, preferably sodium carboxymethylcellulose, most preferably sodium carboxymethylcellulose with a substitution grade of from 0.3 to 1.0, preferably from 0.5 to 0.9, more preferably from 0.7 to 0.8, and a dicar- boxylic acid or tricarboxylic acid, preferably a tricarboxylic acid, more preferably citric acid as precursor P of the linker L. For example, the cross-linked polysaccharide may be based on carboxymethylcellulose or sodium carboxymethylcellulose with a substitution grade from 0.5 to 0.9 and a tricarboxylic acid as precursor P of the linker L. It is particularly preferred that the cross- linked polysaccharide is based on carboxymethylcellulose or sodium carboxymethylcellulose with a substitution grade from 0.7 to 0.8 and citric acid as precursor P of the linker L.

In other words, it is preferred that the cross-linked polysaccharide comprises carboxymethylcellulose chains, which are chemically cross-linked via ester bridges, wherein said ester bridges are spaced by a linear C3-alkyl linker, which is preferably substituted with -OH and -COOH or - COONa. It is more preferred that the cross-linked polysaccharide comprises carboxymethyl- cellulose chains with a substitution grade from 0.5 to 0.9, which are chemically cross-linked via ester bridges, wherein said ester bridges are spaced by a linear C3-alkyl linker, which is optionally substituted with -OH and -COOH or -COONa. It is particularly preferred that the cross- linked polysaccharide comprises carboxymethylcellulose chains with a substitution grade from

0.7 to 0.8, which are chemically cross-linked via ester bridges, wherein said ester bridges are spaced by a linear C3-alkyl linker, which is substituted with one substituent -OH at the C2-atom and with one substituent -COOH or -COONa at the C2-atom. In this context, it is also particular- ly preferred that the carbonyl group -C(=0)- of the -C(=0)-0- moiety of the ester bridge stems from the precursor P of the linker L, which corresponds to connectivity 1 indicated above.

Cross-linking of the polysaccharide chains with the precursor P of the linker L is preferably achieved by reacting the polysaccharide chains with the precursor P of the linker L, i.e. the cross-linking agent, in water in the presence of the wood flour or OLM. This provides the advantage that the compositions of the present application can be obtained in a one-pot reaction,

1. e. the polysaccharide chains to be cross-linked, the precursor P of the linker L and the wood flour or OLM are preferably mixed together in the desired weight ratios for the final composition, and then the cross-linking reaction is performed, and the superabsorbent composition is directly obtained from said reaction mixture. Thus, the composition of the invention is preferably directly obtainable from a mixture comprising the polysaccharide chains, the precursor P of the linker L and the wood flour or OLM.

With regard to the final composition, it is preferred that the cross-linked polysaccharide is based on certain weight ratios of the polysaccharide chains to the precursor P of the linker L. These weight ratios are preferably established in the reaction mixture for cross-linking the polysaccharide in the presence of the wood flour or OLM.

In a preferred embodiment of the present invention, the cross-linked polysaccharide is formed by reacting the polysaccharide with the precursor P of the linker L as cross-linking agent in a weight ratio of from 200:1 to 90:1 , preferably from 170:1 to 120:1 , more preferably from 160:1 to 130:1 , most preferably from 150:1 to 140:1 , in the presence of the wood flour or OLM and water. It is noted that cross-linking is typically achieved by heating a highly viscous mixture of the polysaccharide, the precursor P of the linker L to 120 to 160°C, preferably 140°C, for 2 to 4 hours, preferably 3 hours, in the presence of wood flour or OLM.

With regard to the final composition, certain weight ratios of the wood flour or OLM to the polysaccharide chains are also preferred according to the present invention. It is noted that said weight ratios are again preferably established already in the reaction mixture for cross-linking the polysaccharide in the presence of the wood flour or OLM. In a preferred embodiment of the present invention, the wood flour or OLM and the polysaccharide chains are present in the composition of the invention in a weight ratio of from 1 :99 to 80:20, preferably from 10:90 to 70:30, more preferably from 20:80 to 60:40, most preferably from 30:70 to 50:50, particularly preferably about 40:60. If the polysaccharide is selected to be carboxymethylcellulose, the wood flour or OLM and the carboxymethylcellulose chains are present in the composition of the invention in a weight ratio of from 1 :99 to 80:20, preferably from 10:90 to 70:30, more preferably from 20:80 to 60:40, most preferably from 30:70 to 50:50, particularly preferably about 40:60. In terms of the water absorption capacity, it is particularly preferred that the wood flour or OLM and the polysaccharide chains are present in the composition of the invention in a weight ratio of from 1 :99 to 50:50, preferably from 10:90 to 50:50, more preferably from 20:80 to 50:50, most preferably from 30:70 to 50:50, particularly preferably from 35:65 to 45:55. It is particularly preferred that the wood flour or OLM and the polysaccharide chains are present in the composition of the invention in a weight ratio of 40:60. Furthermore, it is particularly preferred that the polysaccharide is selected to be carboxymethylcellulose, and that the weight ratios of the wood flour or OLM to the polysaccharide chains are as indicated above.

In terms of the nutrient solution absorption capacity, it is particularly preferred that the wood flour or OLM and the polysaccharide chains are present in the composition of the invention in a weight ratio of from 1 :99 to 75:25, preferably from 10:90 to 70:30, more preferably from 20:80 to 70:30, still more preferably from 30:70 to 70:30, most preferably from 35:65 to 65:35, particularly preferably from 40:60 to 60:40. It is particularly preferred that the wood flour or OLM and the polysaccharide chains are present in the composition of the invention in a weight ratio of 40:60. Furthermore, it is particularly preferred that the polysaccharide is selected to be carboxymethylcellulose, and that the weight ratios of the wood flour or OLM to the polysaccharide chains are as indicated above.

In another preferred embodiment, the cross-linked polysaccharide and the wood flour or OLM are together present in the composition of the invention in an amount of at least 60 wt.-%, preferably at least 80 wt.-%, more preferably at least 90 wt.-%, most preferably at least 95 wt.-% based on the total weight of the composition.

With regard to the wood flour being present in the compositions of the present invention, it is noted the following. Wood flour is finely pulverized wood that has a consistency fairly equal to sand or sawdust, but can vary considerably, with particles ranging in size from a fine powder to roughly the size of a grain of rice. According to the present invention, the wood flour is preferably in the form of a fine powder. The term "wood flour" also comprises wood dust and sawdust.

In a preferred embodiment, the wood flour or OLM has a particle size distribution wherein at least 95% of the particles have a diameter below 2000 μηη, preferably below 1500 μηη, more preferably below 1000 μηη, particularly below 500 μηη. The particle size distribution is preferably as such that if a sieve analysis is performed, from 0 to 5 % of the particles have a diameter in the range of from 0 to 150 μηη, from 30 to 40% of the particles have a diameter in the range of from more than 150 μηη to 300 μηη, and from 55 to 65% of the particles have a diameter in the range of from more than 300 μηη to 500 μηη.

In another preferred embodiment, the wood flour or OLM has a bulk density of from 150 g/L to 250 g/L, preferably from 170 g/L to 250 g/L. In a preferred embodiment of the present invention, the wood flour is based on softwood, preferably spruce wood or fir wood. The source of the wood flour is important for its chemical composition as outlined in the following.

In another preferred embodiment of the present invention, the wood flour is based on hardwood, preferably ash wood, aspen wood, beech wood, birch wood, cherry wood, ebony wood, elm wood, eucalyptus wood, mahogany wood, maple wood, or oak wood.

Wood flour mainly comprises the following components: cellulose, hemicelluloses and lignin. Cellulose is an organic compound with the formula (C6Hio05) _n , a polysaccharide consisting of a linear chain of preferably 7000 to 15000 β(1→4) linked D-glucose units.

Unlike cellulose, hemicellulose, which is also a polysaccharide, consists of shorter chains, i.e. of 500 to 3000 sugar units. In addition, hemicellulose is a branched polymer, while cellulose is unbranched. Furthermore, hemicellulose contains not only one, but many different sugar monomers. For instance, besides glucose, sugar monomers in hemicellulose can include xylose, mannose, galactose, rhamnose, and arabinose. The composition of the units depends on the source of the hemicellulose. Xylose is in most cases the sugar monomer present in the largest amount, although in softwoods mannose can be the most abundant sugar. Not only regular sugars can be found in hemicellulose, but also their acidified form, for instance glucuronic acid and galacturonic acid can be present. Lignin is a cross-linked racemic macromolecule with molecular masses in excess of 10000 u. It is relatively hydrophobic and aromatic in nature. The degree of polymerisation in nature is difficult to measure, since it is fragmented during extraction and the molecule consists of various types of substructures that appear to repeat in a haphazard manner. There are three monolignol monomers, methoxylated to various degrees: p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol. These lignols are incorporated into lignin in the form of the phenylpropanoids p- hydroxyphenyl, guaiacyl, and syringyl, respectively. It is noted that lignin fills the spaces in the cell wall between cellulose, hemicellulose, and pectin components, especially in xylem trache- ids, vessel elements and sclereid cells. It is covalently linked to hemicellulose and, therefore, cross-links different plant polysaccharides, conferring mechanical strength to the cell wall and by extension the plant as a whole.

Due to the fact that lignin may be considered as cross-linking cellulose and hemicellulose in wood flour or OLM, wood flour or OLM may be understood as a specific cross-linked polysac- charide. Therefore, wood flour or OLM is often referred to as lignocellulose, wherein lignocellulose stands for a threedimensional network essentially consisting of cellulose, hemicellulose and lignin. However, it is noted that wood flours or OLM may also comprise other components than lignocellulose, such as resins, terpenes, phenols, chinones and the like, together in a minor amount of less than 5 wt.-% based on the total weight of the wood flour or OLM. According to the present invention, the term "wood flour" covers both, wood flour comprising lignocellulose alone and wood flour comprising lignocellulose in combination with other components being present together in an amount of less than 5 wt.-% based on the total weight of the wood flour. The same applies to OLM. It is noted that the advantageous effect provided by the wood flour or OLM is assumed to be attributable to the lignocellulose. However, the other components of the wood flour or OLM are not assumed to inhibit or deteriorate the advantageous effect of the lignocellulose.

In view of the above, it may be understood that the composition of the invention preferably comprises a cross-linked polysaccharide comprising polysaccharide chains, which are chemi- cally cross-linked via ester bridges, wherein said ester bridges are spaced by an aliphatic linker L, and wood flour or OLM comprising the cross-linked polysaccharide lignocellulose, and optionally other components being present together in an amount of less than 5 wt.-% based on the total weight of the wood flour or OLM. Preferably, the composition of the invention comprises cross-linked carboxymethylcellulose, wherein the carboxymethylcellulose chains have been cross-linked with citric acid as cross-linking agent, and lignocellulose. With regard to the term "lignocellulose", it is noted that lignocellulose preferably comprises from 45 to 55 wt.-% of cellulose, from 20 to 30 wt.-% of hemicellulose, and from 20 to 30% wt.-% of lignin based on the total weight of the lignocellulose, wherein these components are preferably together present in an amount of at least 99 wt.-% of the lignocellulose.

Similarly, wood flour or OLM preferably comprises from 45 to 55 wt.-% of cellulose, from 20 to 30 wt.-% of hemicellulose, and from 20 to 30% wt.-% of lignin based on the total weight of the lignocellulose, wherein these components are preferably together present in an amount of at least 94 wt.-% of the wood flour or OLM, and other components such resins, terpenes, phenols, chinones and the like may additionally be present.

Wood flour as described above is commercially available e.g. under the trade name "Linocel BK 40-90".

It has surprisingly been found that by combining the cross-linked polysaccharide comprising polysaccharide chains, which are chemically cross-linked via ester bridges, wherein said ester bridges are spaced by an aliphatic linker L, with wood flour or OLM in the compositions of the invention, the water absorption capacity can be improved significantly compared to the water absorption capacity of the cross-linked polysaccharide alone. For example, the water absorption capacity of cross-linked carboxymethylcellulose is improved remarkably, if wood flour or OLM and the carboxymethylcellulose chains are present in a weight ratio of about 40:60 after at least 2 days at a temperature of 20°C to 30°C.

It is noted that also an improvement of the water absorption capacity compared to the water absorption capacity of the wood flour or OLM alone can be observed. It has been found that the water absorption capacity of the compositions of the present invention reaches a maximum, if wood flour or OLM and the carboxymethylcellulose chains are present in a weight ratio of about 40:60. In a preferred embodiment of the present invention, the composition is capable of absorbing water in an amount of at least 50 g, preferably in an amount of at least 150 g, more preferably at least 250 g per gram of the composition, at a temperature of from 20°C to 30°C for an absorption time of at least 2 days. In another preferred embodiment of the present invention, the composition is capable of absorbing water in an amount of from 50 g to 400 g, preferably in an amount of from 150 g to 350 g, more preferably from 250 g to 320 g, most preferably from 300 g to 320 g per gram of the composition, at a temperature of from 20°C to 30°C for an absorption time of at least 2 days. Furthermore, it has surprisingly been found that by combining the cross-linked polysaccharide comprising polysaccharide chains, which are chemically cross-linked via ester bridges, wherein said ester bridges are spaced by an aliphatic linker L, with wood flour or OLM in the compositions of the invention, the absorption capacity in terms of a nutrient solution can be improved significantly compared to the absorption capacity in terms of a nutrient solution of the cross- linked polysaccharide alone. For example, the absorption capacity in terms of a nutrient solution of cross-linked carboxymethylcellulose is improved aboutthreefold, if the wood flour or OLM and the carboxymethylcellulose chains are present in a weight ratio of about 40:60 after at least 2 days at a temperature of 20°C to 30°C.

It is noted that also a significant improvement of the absorption capacity in terms of a nutrient solution compared to the absorption capacity in terms of a nutrient solution of the wood flour or OLM alone can be observed. For example, the absorption capacity in terms of a nutrient solution of wood flour or OLM is improved about eightfold, if wood flour or OLM and the carbox- ymethylcellulose chains are present in a weight ratio of about 40:60 after at least 2 days at a temperature of 20°C to 30°C. Again, it has been found that the absorption capacity in terms of a nutrient solution of the compositions of the present invention reaches a maximum, if wood flour or OLM and the cross-linked carboxymethylcellulose are present in a weight ratio of about 40:60. When looking at the absorption capacities in terms of a nutrient solution of the composi- tion of the invention compared the absorption capacities of wood flour or OLM and cross-linked carboxymethylcellulose alone, it can be seen that, if wood flour or OLM and the carboxymethylcellulose chains are present in a weight ratio of about 40:60, a synergistic effect can be observed because the absorption capacity is improved in the compositions of the invention to an extent, which is significantly more than additive.

As used herein, a nutrient solution is an aqueous solution, which typically comprises one or more nutrients selected from Ca, NO3, NH ₄ , S0 ₄ , K, Mg, CI, P0 ₄ , B, Mn, Cu, Zn and Mo. Preferably, the nutrient solution comprises Ca, NO3, NH ₄ , S0 ₄ , K, Mg, CI, P0 ₄ , B, Mn, Cu, Zn and Mo. More preferably, a nutrient solution comprises Ca in a concentration of 1000 μΜ, NO3 in a concentration of 2000 μΜ, NH ₄ in a concentration of 200 μΜ, S0 ₄ in a concentration of 651 μΜ, K in a concentration of 850 μΜ, Mg in a concentration of 325 μΜ, CI in a concentration of 300 μΜ, P0 ₄ in a concentration of 100 μΜ, B in a concentration of 8 μΜ, Mn in a concentration of 1 μΜ, Cu in a concentration of 0.2 μΜ, Zn in a concentration of 0.2 μΜ and Mo in a concentration of 0.2 μΜ.

In a preferred embodiment of the present invention, the composition of the invention is capable of absorbing a nutrient solution in an amount of at least 15 g, preferably at least 20 g, more preferably at least 50 g, most preferably at least 75 g per gram of the composition, at a temperature of from 20°C to 30°C for an absorption time of at least 2 days. In another preferred embodiment of the present invention, the composition is capable of absorbing a nutrient solution in an amount of from 15 g to 120 g, preferably in an amount of from 20 g to 100 g, more prefer- ably from 50 g to 90 g, per gram of the composition, at a temperature of from 20°C to 30°C for an absorption time of at least 2 days.

The present invention is also directed to a soil treatment product comprising the composition according to the present invention and at least one additional ingredient selected from the group consisting of fillers, nutrients, minerals, fertilizers, pesticides, herbicides, fungicides and combinations thereof, wherein the composition of the invention and the additional ingredient are preferably present in a weight ratio of from 80:20 to 20:80. Preferably, the composition according to the invention and the additional ingredient are together present in an amount of at least 90 wt- % based on the total weight of the composition.

The composition according to the present invention and the soil treatment product according to the present invention are suitable for agricultural applications. For this purpose, the composition as well as the soil treatment product are preferably present in dry granular form, wherein the granulates exhibit good flow properties.

In the context of agricultural applications, it is particularly advantageous that the composition and the soil treatment product of the invention exhibit a particularly high biodegradability. Preferably, the composition or soil treatment product is biodegradable in soil by at least 20%, preferably at least 30%, more preferably by at least 45%, most preferably by at least 50% at a tem- perature of from 20°C to 30°C after 140 days, wherein the percentage value is calculated from the CO2 formation compared to the carbon content of the tested amount of the composition or soil treatment product. In particular, the percentage value defines the amount of carbon in mg, which has been converted the carbon dioxide, compared to the amount of carbon in mg in the tested sample of the composition or soil treatment product, which may be determined by ele- mental analysis.

The present invention is also directed to a process for the manufacture of a composition according to the present invention comprising the steps of

1 ) dissolving the precursor P of the linker L in water to obtain a homogenous solution; 2) adding the wood flour or OLM and the polysaccharide to the solution of step 1 ) to obtain a highly viscous mixture;

3) allowing the mixture of step 2) to react at 20 to 30°C for 2 to 4 days; 4) keeping the mixture of step 3) at 2 to 8°C for 1 to 3 days;

5) heating the mixture of step 4) to 120 to 160°C for 2 to 4 hours to obtain a dried mixture.

Optionally, the process further comprises a step 6) of milling and sieving the dried mixture, in order to obtain a composition in the form of a powder.

In a preferred embodiment of the process of the present invention, the weight ratio of the polysaccharide and the precursor P of the linker L is from 200:1 to 90:1 , preferably from 170:1 to 120:1 , more preferably from 160:1 to 130:1 , most preferably from 150:1 to 140:1.

In another preferred embodiment of the process of the present invention, the weight ratio of the wood flour or OLM to the polysaccharide is from 1 :99 to 80:20, preferably from 10:90 to 70:30, more preferably from 20:80 to 60:40, most preferably from 30:70 to 50:50, particularly preferably about 40:60. In yet another preferred embodiment of the process of the present invention, e.g. if the focus for the resulting composition is on the water absorption capacity, the weight ratio of the wood flour or OLM to the polysaccharide is from 1 :99 to 50:50, preferably from 10:90 to 50:50, more preferably from 20:80 to 50:50, most preferably from 30:70 to 50:50, particularly preferably from 35:65 to 45:55 or about 40:60. In yet another preferred embodiment of the process, e.g. if the focus for the resulting composition is on the nutrient solution absorption capaci- ty, the weight ratio of the wood flour or OLM to the polysaccharide is from 1 :99 to 75:25, preferably from 10:90 to 70:30, more preferably from 20:80 to 70:30, still more preferably from 30:70 to 70:30, most preferably from 35:65 to 65:35, particularly preferably from 40:60 to 60:40, especially about 40:60. In another preferred embodiment, the polysaccharide is carboxymethylcellulose or sodium carboxymethylcellulose in each case with a substitution grade of from 0.3 to 1.0, preferably from 0.5 to 0.9, more preferably from 0.7 to 0.8.

In another preferred embodiment, the precursor P of the linker L is citric acid.

In another preferred embodiment, the wood flour is based on softwood, preferably spruce wood or fir wood.

Accordingly, it is particularly preferred that the process of the present invention is performed with carboxymethylcellulose or sodiumcarboxymethylcellulose with a substitution grade of from 0.7 to 0.8 as polysaccharide and with citric acid as precursor P of the linker L, wherein carboxymethylcellulose or sodiumcarboxymethylcellulose and citric acid are used in a weight ratio of from 150:1 to 140:1 , and wood flour or OLM and carboxymethylcellulose or sodium carbox- ymethylcellulose are used in a weight ratio of from 30:70 to 50:50, preferably from 35:65 to 45:55, or from 35:65 to 65:35, preferably from 40:60 to 60:40, and particularly preferably in a weight ratio of about 40:60.

In another preferred embodiment of the process of the present invention, the heat treating step is performed at a temperature of from 130°C to 150°C for a time period of 3 h.

The present invention is also directed to the use of the composition or the soil treatment product according to the present invention for agricultural applications. In this context, the composition or soil treatment product of the invention is preferably used for improving plant growth and crop yield.

In a preferred embodiment, plant growth is accelerated by using the composition or soil treat- ment product of the invention in that the weight of a plant in treated soil is increased by at least 20%, preferably by at least 30%, most preferably by at least 40% compared to the weight of a plant in untreated soil, wherein the percentage value corresponds to the weight increase of the dry weight of the plant in treated soil after 3 weeks cultivation at a temperature of from 20°C to 30°C compared to the plant in untreated soil.

In a preferred embodiment, the composition or the soil treatment product according to the present invention may be used for improving the physiological properties of soils. This may e.g. be achieved by increasing their capacity to hold water, reducing erosion and runoff, reducing the frequency of irrigation, increasing the efficiency of the water being used, increasing soil perme- ability and infiltration, reducing the tendency of the soil to get compacted, and helping plant performance. In particular, the composition or soil treatment product may be used for improving the physiological properties of plant soil, garden soil, meadow soil, lawn soil, forest soil, field soil, for preparing soils for cultivating plants, and for recultivating of fields, which have become deserted. It is preferred that the composition or the soil treatment product is used for absorbing and storing humidity in soils, e.g. in areas under cultivation of plants. Alternatively or additionally, it is preferred that the composition or the soil treatment product is used for improving the soil structure by loosening the soil. Furthermore, the soil treatment product may also be used for uniformly distributing nutrients, minerals and fertilizers, wherein the nutrients, minerals and fertilizers are preferably released in a controlled manner over a time period of at least one month. For the uses indicated above, the composition or the soil treatment product of the invention will preferably be added to the soil in an amount of 1 to 1000 kg/ha, preferably in an amount of 1 to 25 kg/ha field, or in an amount of from 0.1 to 100 kg/T soil. The invention is further illustrated by the examples, which are not to be understood as limiting the invention, however.

E X A M P L E S A. Determination methods

The following definitions of terms and determination methods apply for the above general description of the invention including the claims as well as to the below examples unless otherwise defined.

a1) Determining the water absorption capacity (tea bag analysis)

The water absorption capacity can be determined by the "tea bag analysis" using deionized water.

The superabsorbent composition is grinded and sieved, and the sieve fraction of 150 - 800 μηη is used for testing. The superabsorbent composition is dried and the residual moisture content is determined. 100 mg of the dry superabsorbent composition is placed in a first teabag 1 , and the teabag 1 is then sealed with a film sealer. Another 100 mg of the dry superabsorbent composition is placed in a second teabag 2, and the teabag 2 is then sealed with a film sealer. Both teabags 1 and 2 are placed in 700 ml deionized water and stored at ambient temperature.

Three further teabags 3, 4 and 5 without superabsorbent composition are also placed in 700 ml deionized water and stored at ambient temperature.

After 24 hours, the teabags 1 and 2 are taken out of the water and hanged out inclined for 10 minutes to let the water drain off. Then the weight of teabags 1 and 2 is determined. Similarly, teabags 3, 4 and 5 are taken out of the water and hanged out inclined for 10 minutes to let the water drain off. Then the weight of teabags 3, 4 and 5 is determined and the average weight Wo is determined. After that, teabags 1 and 2 are again placed in 700 ml deionized water and stored at ambient temperature.

After 48 hours, the teabags 1 and 2 are taken out of the water and hanged out inclined for 10 minutes to let the water drain off. Then the weight of teabags 1 and 2 is determined. After that, teabags 1 and 2 are again placed in 700 ml deionized water and stored at ambient temperature. After 168 hours, the teabags 1 and 2 are taken out of the water and hanged out inclined for 10 minutes to let the water drain off. Then the weight of teabags 1 and 2 is determined.

The weight of the absorbed water is determined for the absorption times of 24 hours, 48 hours and 168 hours as follows: Weight of absorbed water = Weight of teabag 1 - Weight of dry sample - Wo

Weight of absorbed water = Weight of teabag 2 - Weight of dry sample - Wo

Then, the weight of absorbed water is normalized to 1 g of dry superabsorbent composition. The results are provided as the weight of absorbed water in gram per weight of the dry super- absorbent composition in gram [g (water)/g(cross-linked polysaccharide or superabsorbent composition)] after 24, 48 and 168 hours, respectively.

a2) Determining the absorption capacity in terms of a nutrient solution (tea bag analysis)

A nutrient solution for determining the absorption capacity of said solution is prepared by mixing 4700 g deionized water with three stock solutions A, B and C as defined below, each in an amount of 100 g. The stock solutions A, B and C are prepared by dissolving the following amounts of nutrients in 5 L of deionized water, so that the concentrations provided in the right columns of the tables below are obtained.

Stock solution A:

Stock solution B:

Stock solution C:

Type of nutrient Amount of nutrient (gram) Concentration nutrient (μηη)

KH2PO4 3.40 5000 The absorption capacity in terms of the above described nutrient solution can then be determined by the "tea bag analysis" as described under a1 ) using the nutrient solution instead of water.

The weight of the absorbed nutrient solution is determined for the absorption times of 24 hours, 48 hours and 168 hours as follows:

Weight of absorbed nutrient solution = Weight of teabag 1 - Weight of dry sample - Wo

Weight of absorbed nutrient solution = Weight of teabag 2 - Weight of dry sample - Wo

Then, the weight of absorbed nutrient solution is normalized to 1 g of dry superabsorbent composition.

The results are provided as the weight of absorbed nutrient solution in gram per weight of the dry superabsorbent composition in gram [g (water)/g(superabsorbent composition)] after 24, 48 and 168 hours, respectively.

b) Determining the biological degradability (biodegradability)

The mineralization of the superabsorbent composition is measured using the method and the manometric measurement system described by Robertz, M. et al. ("Cost-effective method of determining soil respiration in contaminated and uncontaminated soils for scientific and routine analysis" published in: Wise, D.L., et al. (eds.) Remediation Engineering of Contaminated Soil, 573 - 582, Marcel Dekker Inc., New York, Basel, 2000). The carbon mineralization is expressed as the difference in the accumulated soil respiration (CO2 formation) with the superabsorbent composition added minus without the superabsorbent composition added. Per measuring unit, 50 g of dry soil is used to which water is added up to 50% of its maximum water holding capacity. The amount of the superabsorbent composition added is equivalent to 50 mg C determined by elementary analysis. The soil used is a light textured soil from Limburgerhof, Germany, with pH 6.8. The results are the average of 4 replicates.

c) Determining the acceleration of plant growth (cylinder test)

With the aid of the test described hereinafter, the effects of the inventive compositions on the shoot and root growth of corn plants (plant growth) can be measured. The superabsorbent composition to be studied (0.01 -10 g/kg) is added to a water-moistened plant substrate and mixed in until homogeneously distributed. To determine the blank value, correspondingly mois- tened quartz sand is used. Then five precultivated corn seedlings were planted into each pre- treated substrate and cultivated at ambient temperature for about 3 weeks, in the course of which the plants are watered with a compound fertilizer solution once per week. The plants are removed from the pots along with the roots, the roots are cleaned by washing and the plants are assessed for appearance and size. Then the shoot and root are separated from each other in each case and both parts are weighed to determine their fresh weight. The shoots and roots are subsequently dried to constant weight and their dry weights are determined. The final weights for the shoots and roots of 5 identically treated plants in each case are used to calculate the mean values for fresh and dry weights.

B. Examples: Example 1

Preparation of compositions comprising cross-linked carboxymethylcellulose and wood flour:

Compositions are prepared in one-pot reaction process, wherein the following ratios of wood flour (Lignocel BK 40-90) to carboxymethylcellulose (CMC 12000) are used: a) 0/100, b) 40/60, c) 60/40, d) 80/20, e) 90/10 and f) 100/0. Examples 1 a) and 1f) serve as comparative examples. The following amounts of components are used in the process.

The compositions are prepared by dissolving citric acid in water and adding lignocel BK 40-90 under stirring at 200 U/min to obtain a highly viscous mixture. Then, stirring is interrupted, and the mixture is allowed to stand for 3 hours at room temperature. Subsequently, the mixture is stored for 2 days at 5 °C in a fridge. Finally, the highly viscous mixture is heat treated for 3 hours at 140°C in a vacuum drying oven. The resulting product is milled and sieved to obtain sieve fraction of 125-150 μηη.

Elemental analysis of the compositions provides the following results.

C O H Na

Example Linocel BK/CMC12000

1 a) 022a-2 0/100 3h140°C 35.,6 49.2 5.5 7.8

1 b) 022b-2 40/60 3h140°C 40.76 47.2 5.86 4.68

1 c) 022c-2 60/40 3h140°C 43.34 46.2 6.04 3.12

1 d) 022d-2 80/20 3h140°C 45.92 45.2 6.22 1 .56 1 e) 022e-2 90/10 3h140°C 47.21 44.7 6.31 0.78

1f) 022f-2 100/0 3h140°C 48.5 44.2 6.4 0

The water absorption capacity and the nutrient solution absorption capacity are determined by the "tea bag analysis". The results are provided in Figures 1 and 2.