Synthesis of Biodegradable Superabsorbent Polymers from Carboxymethyl Cellulose/Humic Acid

Superabsorbent polymer (SAP) blend has been synthesized from carboxymethyl cellulose (CMC), humic acid, and aluminum sulphate octadecahydrate cross-linker. SAP is hydrophilic networks that can absorb and retain huge amount of water within their structures. Humic acid as starting material of polymer, was isolated from subgrade Batujai Dam by using IHSS method. Water Absorption Capacity (WAC) measurement, FTIR analysis, and agitation tests to investigate the cross-linking process and which of Al 3+ and SO4 2ions causes the crosslinking are carried out. Optimum cross-linking ratio of CMC and cross-linker appeared to be 2wt% corresponded to WAC determination. FTIR spectrum of CMC/humic acid blend and agitation test showed that CMC react with humic acid during polymerization process via Al 3+ ion.


INTRODUCTION
Recently interest in developing superabsorbent from biopolymer such as starch [1], cellulose and its derivates [2], and chitosan [3] has been increased due to their exceptional properties compared to petroleum-based polymers that is sustainable, biocompatible, biodegradable, renewable and nontoxic [4]. Owing to their three-dimensional polymeric networks that can absorb and retain large volumes of water, superabsorbents are widely used in many fields, such as hygienic products, drugdelivery systems, agriculture and horticulture [5].
Superabsorbent prepared through electrostatic complexing can be used for example crosslinking of CMC/HEC [2] and CMC/starch [1] with aluminium sulphate as cross-linking agents. Aluminium sulphate, with its cationic species in effective form of Al 3+ , Al(OH) 2+ and oligomeric species in aqueous solution when pH< 5, provides multivalent positive charges and abundant sites for crosslinking with anionic groups [6] such us NaCMC that containing -COO groups to form hydrogel structures. Application of humic acids (HA, one of the main fractions of humic substances) in agriculture as soil fertilizer and soil conditioner has been extensively discussed in the literature [7]. Containing of carboxylic and phenolic groups, humic acid provides favorable conditions for chemical reactions, biological activity and improves physical structure of soil, water holding capacity, pH buffering and accelerate transport of nutrients to plants [8]. Therefore, it was planned to synthesize a biodegradable superabsorbent with suitable for soil water conservation by crosslinking CMC and humic acid with aluminum ions. Optimum cross-linker concentration, water absorption capacity and FTIR analysis were also studied.

Materials
Humic acid used in this study were isolated from subgrade Batujai Dam, Lombok Tengah, West Nusa Tenggara. Analytical quality materials (analytical grade) for humic acid isolation include: NaOH (sodium hydroxide), HCl (hydrogen chloride), HF (hydrogen fluoride), AgNO 3 (silver nitrate), paper Whatman filter 42, pH indicator strip were obtained from Merck (Germany). Nitrogen gas, carboxymethyl cellulose sodium salt, aluminum sulphate octadecahydrate was used at reagent grade and could be available from commercial sources.

Isolation and Characterization of Humic Acid
Humic acid isolated and purifed by using recommended method of the International Humic DOI: 10.29303/aca.v1i2. 8 Nurul Ismillayli et al Substances Society [9]. Humic acid isolated then were characterized using FTIR spectroscopy.

Preparation of Cross-Linked CMC/HA Blend
Carboxymethyl cellulose sodium salt (10 g) was mixed with 1.0 L of distilled water (DW) in a large beaker using a magnetic stirrer. The solution was agitated for 1 hour at 70ºC. Then varying amounts of aluminum sulfate were added to the beaker to investigate the optimum crosslinkage, and the solution was allowed to mix for another 30 min. The solution was then spread on Teflon baking pans and dried at 50°C until a film is formed. The film was shredded with a blender and then ground into a powder with a mortar and pestle. Dried film of the cross-linked CMC, was crushed and dissolved in DW. Using magnetic hot plate, humic acid solution mixed with the cross-linked CMC gelatinezed for 30 minute at 70ºC. Result paste was dried overnight at 50ºC, crushed, and tested.

Investigation of the Active Ion
When aluminum sulfate octadecahydrate dissolve in water, the material dissociates into Al 3+ and SO 4 2-. Investigating the ions which enters the cross-linking reaction has done by adopting following procedure: pieces of the cross-linked CMC/starch blend immersed in hot water 70ºC for 30 minutes and then filtered. The extraction then divided into two parts. The first part is tested for sulfate group using BaCl 2 solution, while the second part is tested for Al 3+ ion using NaOH solution.

Water Absorption Capacity Measurement at Various Cross-linker Concentration
The accurately weighed CMC/HA superabsorbent powder in various amount of Al 3+ (0.1± 0.0001 g) was immersed in 500 mL DW for 2 h to reach swelling equilibrium. The swollen sample were then separated from the unabsorbed water by being filtered over 100-mesh nylon screen. The weight of the swollen sample was measured. The water absorption capacity (WAC) was calculated by the following equation, Where W 0 is dried sample weight (g) and W 1 is swollen sample weight (g) and W AC is water absorption capacity per gram of dried sample (g/g).

Isolation and Characterization of Humic Acid
HA was obtained from subgrade Batujai Dam after separating its humin and fulvic acid then purifying by using mixed HCl/HF solution to dissolves silica oxide mineral. Based on interpretation of the FTIR spectra of crude and pure HA in Table 1 can be seen that purification decreased the ash content, from 5.3 % to 1.3 %, indicated by decrease of SiO stretching in 914 cm -1 . Peak at 1388 cm -1 coupled with 1659 cm -1 peak suggests that crude HA has carboxylate groups, presumably because of the ash content [10] [11]. The peak was shifted to 1381 and 1653 cm -1 respectively after purification of HA. Intensity of this peak was decreased, while peak intensity around 1722 cm -1 increased at pure HA spectra due to carboxylate to carboxylic changes in HA. This is supported by result of ash content determination that purification decreased ash content from 5.60% to 1.13%. Based on FTIR spectra, HA isolated containing functional groups of COOH, phenolic -OH, aliphatic hydrocarbon and aromatic ring. OH stretching of alcohols or phenols asymmetric stretching of aliphatic C-H in -CH 2 -groups symmetric stretching of aliphatic C-H in -CH 2 -groups C=O stretching (carboxylic and carbonyl group) aromatic C=C and asymmetric C=O stretching in COOˉ groups nitro groups methyl asymmetric C-H bending OH deformation and C-O stretching in phenols and COOˉ groups C-O stretching of aryl ethers and OH deformation of COOH groups C-O stretching (alcohols and polysaccharide) SiO stretching ortho disubstituted out-of-plane = C-H bending vibration metal-humic acid interaction The IR spectra of the HA, CMC, and CMC/HA blend are shown in Fig. 1 (a-c), respectively. Comparing with the IR spectrum of CMC ( Fig. 1(b)), the absorption bands at 1608 cm -1 for the -COONa group shift to 1634 cm -1 , and the absorption bands at 1381 (OH bending vibration) and 1268 cm -1 (C-O HA(a stretching) disappeared in the IR spectrum of cross-linked CMC/HA (Fig. 1(c)). Comparing with the IR spectrum of HA ( Fig. 1(a) (phenolic C-O stretching of HA) disappeared in the spectrum of cross-linked CMC/HA superabsorbent ( Fig. 1(c)). The results obtained from IR analysis showed that the reaction of both CMC and HA occurs in carboxylic and phenolic groups of HA and -COO groups of CMC via cross-linker.

Investigation of the Active Ion
In order to suggest a reasonable interaction, knowing which ions enters the cross-linking reaction ( Al 3+ or SO 4 2-) is needed. Tested for sulfate group using BaCl 2 solution on extraction of cross-linked CMC/HA, shows dense white precipitate indicated that the sulphate group not share for blend formation.While, tested for Al 3+ ion using NaOH solution on extraction of cross-linked CMC/HA, shows the absence of Al +3 ion. It was confirmed with FTIR analysis that the Al 3+ ion share in the blend structure as cross-linker that bind the carboxylic or phenolic groups of HA with -COO groups of CMC.

Water Absorption Capacity Measurement at Various Cross-linker Concentration
The effect of the amount of Al 3+ ion on WAC was shown in Fig. 2. The max. of absorbency was at 2% of Al 3+ ion. Increasing cross-linker could increase the nodes of network and the cross-linker density, which is favorable to the super-absorbent absorbing and retaining fluid [12]. Low concentration of the cross-linker leads to low degree of cross linking, and it is hard for network structure to form, so the water absorbency is low. However, when it is higher than the best value, there are more cross-linking points and the pores become smaller in the network, which causes the macroscopic decrease of the absorbency [13].

CONCLUSION
A superabsorbent was successfully prepared using the cross-linking of CMC/HA with aluminum ions. The interaction was occurs in carboxylic and DOI: 10.29303/aca.v1i2. 8 Nurul Ismillayli et al phenolic groups of HA and -COO groups of CMC via Al 3+ ion as cross-linker. The mass ratio of Al 3+ ion to CMC could affect its water absorption capacity.