Characteristics of lignin aerogel from OPEFB black liquor of kraft pulping process

Abstract

Lignin has been potentially used as a raw material to produce lignin based hydrogel and aerogel products. This research reported the procedure to recover and evaluate the lignin obtained from kraft black liquor of oil palm empty fruit bunches. The experimental analyses were conducted to validate the efficiency of isolated lignin, which was received from a kraft-based dissolving pulp production, to produce the lignin-gum hydrogel and aerogel process. The study also described the properties of lignin based hydrogel and aerogel. The optimum condition of the kraft pulping process was explored by the Box and Behnken Design to evaluate the quality of pulp and lignin separation. The mechanical properties of pulp sheets were investigated, including the ring crush index (RCI), tensile index (TI), and bursting index (BI). The quality of kraft pulp was compared to the soda pulp. It was found that the mechanical properties of pulp sheets obtained from the kraft pulp are better than that of pulp sheets obtained from the soda pulp. The pulp sheets from kraft pulping process contributed the higher RCI, TI, and BI which were 0.26 Nm/g, 6.58 Nm/g, and 1.43 kPa.m2/g, respectively. On the other hand, the soda pulp sheets had the RCI, TI, and BI of 0.16 Nm/g, 2.74 Nm/g, and 0.65 kPa.m2/g, respectively. Kraft process is one of the most frequently used methods for the delignification of wood pulping. The large quantities of lignin in black liquor are the unwanted by-product of pulping waste. Normally, the lignin can be precipitated from black liquor by the acidic method. Lignin from the OPEFBs black liquor was precipitated at pH 2, 3 and 4 to determine the optimal condition of lignin separation. The results showed that the isolated lignin from the kraft black liquor of OPEFBS was greater intensity at pH 3. It contributed the highest yield and purity of lignin as 26.02% and 90.35%, respectively. Moreover, acidic pH has effected on the decreasing of chemical oxygen demand (COD) for wastewater with black liquor content. COD reduction was up to 38% at pH 2 and also showed a tendency to decrease the color of wastewater after lignin separation. The properties of isolated lignin were evaluated to consider the possibility of their use as the raw material of lignin-agarose hydrogel. Lignin characteristics have been performed using Fourier transform infrared spectroscopy (FT-IR). The molecular weight distribution of isolated lignin was determined using gel permeation chromatography (GPC). The thermogravimetric analysis (TGA) and the differential scanning calorimetry (DSC) were used to determine the thermal degradation and the glass transition temperature, respectively. The glass transition temperature (T) values of the kraft lignin precipitation at pH 3.0 was the highest (131.81°C). The weight-average molecular weight (Mw), number-average molecular weight (Mn), and molecular weight distribution (polydispersity = Mw/Mn) of the kraft lignin precipitated at pH 3.0 were 1588, 1082, and 1.47, respectively. Lignin-agarose hydrogel was synthesized by using epichlorohydrin as the cross- linking agent. FTIR recognized that the crosslinking between lignin and agarose has happened at the bands of 3416 and 3353 cm1. The vibration peaks of the acetal, 1179-1178 cm1, was occurred in the lignin-based hydrogel. Gel strength of lignin-agarose hydrogel was characterized by texture personal analysis (TPA). The results demonstrated that the gel strength increased with increasing of lignin and ECH in agarose solutions. 5% lignin, 5% agarose and 10 ml ECH contributed the best gel formation and the excellent mechanical properties. Lignin-xanthan gum hydrogel was prepared by chemical crosslinking of epichlorohydrin (ECH). The characteristic peaks of the C-Cl bond (740 cm1) are not present in the XL hydrogel but appear in the FT-IR spectrum of the XL-ECH hydrogel. This is the characteristic of the XL-ECH hydrogel samples. The intensities of the acetal, C-O peak at 1193 cm21, in XL-ECH hydrogel were the main factor that impact on the chemical reaction of -OH group in xanthan gum which had a presence of a crosslinking agent. Lignin based aerogels can be prepared in the different processes including freeze drying and supercritical drying process. The A5L5E10-CPD aerogel obtained from supercritical drying process has the high surface area of 2.43 m2/g with the low density, which has the average pore sizes of 16.49 Å (1.65 nm). However, the aerogel from freeze drying contributed a nanoporous structure and minimize surface area. Finally, the strengths and weaknesses of lignin based aerogel were analyzed. It was found that the important weaknesses of lignin based aerogel is the higher cost of material used to prepare the aerogel, especially agarose. At the same time, the strength of lignin based aerogel is value-added of residual materials from the palm oil industry and the pulp and paper industry. It is also possible to use the aerogels in this research to further develop in the field of medical engineering and pharmaceutical industries in the future.