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Study of Acid Treatment Conditions on the Water and Moisture Absorbency of Coconut Coir

The influence of chemical treatment on the properties of lignocellulosic fibers has been reported in many studies. This work analyzes the effect of acid treatment on the water and moisture absorbency of coconut coir. Multiple acidic treatment conditions have been considered for coconut coir via Design of Experimental (DOE) software to optimize its treatment and ensure maximum water and moisture absorbency. The compositional and structural changes of the treated samples were determined using X-ray diffraction (XRD), scanning electron microscope (SEM), X-Ray photoelectron spectroscopy (XPS), Fourier transform infrared (FT-IR), and Thermogravimetric analysis (TGA). The optimized acidic condition led to a treated coconut coir with only an 8% increment of water absorption, and a ~13% decrease of moisture absorption. Based on the results, acidic treatment is unsuitable for increasing coconut coir’s water and moisture absorbency.


I. INTRODUCTION
Recent research on utilizing waste materials has opened many investigative paths. The investigation of raw and chemically/physically treated waste materials could efficiently determine the best application for many waste materials. It could also optimize both costs and technical performances towards utilizing waste materials for multiple applications [1]. Taking into account the cost and environmental impact of most desiccant materials, researchers focus on employing agricultural wastes as desiccant materials [2]- [5]. There are plenty of local agricultural wastes [6] that can be recycled for increased economic benefits [7]. Many agricultural wastes containing lingocellulosic exhibited increased absorption properties post physical/chemical treatments. The most common chemical treatment methods include acid and alkali treatments [8], which improves the absorption properties of the samples via increased hydrogen bonding, porosities, and lignin removal (i.e., introducing hydrophobicity) [9]. Coconut coir (CC), which consists of lignin, cellulose, and hemi-cellulose natural polymers [10], obtained from the coconut husk (which is cheap and regarded as an agricultural waste material) was selected for further investigation. In this study, a DOE software has been employed to determine acid treatment conditional factors and evaluate treated samples' water and moisture absorption.

A. Acid Treated Coconut Coir
Coconut coir was purchased from a local market and used as-received after being cut into small pieces (1 cm). Sulfuric acid (analytical reagent) and silica gel were procured from Ajax Finechem Pty Ltd. As per literature for acid treatment, the concentration of the solution ranged between 0.4-8 M, for 30-180 min at 25-80 °C, which were all keyed into the Design of Experimental software (DOE) version 7 (Stat-Ease Corporation, USA), response surface mode, and central composite design software. The results of the DOE and samples for comparison purposes (results from DOE to proceed to the next steps) are presented in Table I.

III. WATER AND MOISTURE ABSORPTION ANALYSIS
The water absorption property of the samples was examined using ASTM D 570-98 to evaluate its performance. The result is the average of a minimum of 3 samples for each treatment condition (Table I). The samples' moisture absorption was then determined. A saturated salt-water system was prepared as per the standard practice outlined in ASTM E104-02. The moisture source used NaCl solution to maintain a constant humidity at different times at a fixed temperature of 28 ± 2 °C (Humidity= 75% -50%) [11]. The result is the average of a minimum of 3 samples for each treatment condition(s) ( Table I). Many samples completely lost its fibers' shape and strength post acidic treatment, rendering them unsuitable for water and moisture absorbency tests. The results confirmed that there is an ~8% increment of water absorption in the case of the best acid treated samples (Sample 6) relative to the raw CC, where the moisture absorption decreased by ~13%.

IV. CHARACTERIZATION
Thermogravimetric analysis (TGA, Perkin Elmer TG7) was used (temperature range of 30-800 °C under air flow) to determine the thermal stability of the prepared samples. The weight loss of the acid treated CC being less than that of the raw specimen was evident. Reference [12] pointed out that the increased thermal stability is due to the partial removal of lignin, hemicelluloses, and α-cellulose from the fiber caused by acid treatment. The TGA plots of the treated samples were compared at similar temperatures and times but different H 2 SO 4 concentrations (Fig. 1) increased molarity, from 0.4 to 1 M, increased the samples' thermal stability, while increasing the concentration from 1 to 4 M caused the samples' thermal stability to slightly decrease.  The FT-IR spectroscopic analysis (Perkin Elmer, Frontier) was used to identify the functional groups present in the raw coconut coir and the treated samples (Fig. 2).
The XRD plots of the prepared samples are shown in Fig. 3. The samples reported similar patterns. Coconut coir contains cellulose, hemicellulose, lignin, and crystalline cellulose. Peaks at ~16•, 22•, and 35• are attributed to cellulose [13], [14], which are crystalline due to hydrogen bonding and Van der Waals interactions [15], [16]. Variance in acidic treatment did not influence the crystallinity of cellulose.  Morphological studies of raw and treated coconut coir were conducted by SEM, using a Philips XL30 (Fig. 4.)  Scanning electron microscopy (SEM) images of the fibers' surfaces exhibited a layer of extractives being removed post acidic treatment. The acid-treated samples showed porous structural features relative to that of raw CC, with some of the pores ruptured and others breaking apart.
X-ray photoelectron spectroscopy was used to investigate changes to the chemical structure of the outermost 3-10 nm of the treated surfaces (Fig. 5.). The results showed that C and O are the predominant species in all of the tested samples. The presence of N and Si on the surface of some of the samples was also detected from their characteristic emission peaks. The N1s line, with a binding energy value of ~400 eV, corresponds to the -NH2 group in amino acids/proteins found in small amounts on the surface of some lignocellulose materials, while Si is an impurity. The standard binding energy of C1s is ~285 eV, while the binding energies of C1(C-C, C=C, C-H), C2 (-O, C-O-C), C3(C=O, O-C-O) and C4(O-C=O, C(=O) OH) are located at 285 eV, 286.7 eV, 288.3 eV, and 289.5 eV, respectively. Based on many reported results pertaining to the lingocellulosic materials [17], the amount of C1 in XPS represents the quantity of non-carbohydrate bindings in the materials, only present in lignin. On the other hand, C2, C3, and C4 are mainly present in carbohydrates. The result shows the shift from higher binding energies to lower binding energies (eV) in C1s with increasing concentrations of H 2 SO 4 , which means that the decreased carbohydrate in the treated samples could be attributed to increased removal or degradation of carbohydrate compared to the lignin in treated CC. It could also be used to explain the decreased water and moisture absorption due to increased concentration of H 2 SO 4 .
Typically, the standard electron binding energy of O1s is ~532 eV, exhibiting two valence states such as O1 (C=O) at ~531.4-532.3 eV and O2 (C-O) at ~533.0-534.0 eV. According to literature, O1 with low binding energy comes from lignin, while O2 with the high binding energy comes mainly from carbohydrates in lingocellulosic materials [18]- [20].

V. CONCLUSIONS
Although acid treatment of coconut coir changes its morphology and the composition of the fibers based on the treatment conditions, there are no significant improvements to its water and moisture absorbency properties. The results showed that acidic treatment cannot be regarded as a suitable treatment for water and moisture absorbency. The samples seem to show reduced carbohydrate at levels higher than that of lignin in coconut coir fibers and decreased active hydroxyl groups on their surfaces. Nilofar Asim was born in Tabriz, Iran in 1970. She obtained her BSc in in applied chemistry, MSc in inorganic chemistry from Tehran University (Iran) and PhD in materials science from National university of Malaysia.
She is presently working at Solar Energy Research Institute, a center of excellence for the research and development in solar energy technology, Universiti Kebangsaan Malaysia. She has been involved in the field of materials science for more than 17 years. Dr. Nilofar Asim's main contributions are in synthesis of nanomaterials (metal oxide and composite), photocatalysts, inorganic polymers and agriculture waste materials modification for new applications. She has published more than 70 research papers in journals and conferences.
Mohamad Hassan Amin is a retired a professor in material engineering (ceramics). Currently, he is employed as a research scientist with the Centre for Advanced Materials & Industrial Chemistry, within the School of Sciences at RMIT University, Australia.
Dr Amin possesses 15+ years of industrial and academic experience and expertise in the synthesis and characterisation of new materials, heterogeneous catalysis, nanomaterials, refractory ceramics, and CO2 reforming of methane for the production of syngas, methanol and liquid fuels. He has published 90+ refereed papers in leading material science and chemistry journals, and three chemistry textbooks, and also holds 18 patents. Previously he was a visiting fellow at the University of New