Lignin and black liquor based composite hydrogels for the enhanced adsorption of malachite green dye from aqueous solutions: Kinetics, rheology and isotherm studies.

Effluent from the textile industry, mainly dye wastewater, pollutes the water bodies and the environment. Mostly, malachite green has been used as a staining reagent in biological studies and a dye in the leather, textile, and aquaculture sectors. At present, research is moving towards developing sustainable adsorption-based dye removal systems. The aim of present study was to develop hydrogel from bio-waste for efficient removal of dye and cost effective. Towards this, black liquor (contains mainly lignin and hemicellulose macromolecules) was separated from the corncob through kraft-pulp process. Subsequently, the black liquor (BL) and sodium alginate (SA) were blended in different weight ratios (1:4, 1:2, 3:4 and 1:1) and cross-linked with calcium ion to form a composite hydrogel. The optimized weight ratio of Black liquor/Sodium alginate hydrogel was found to be 1:4 which exhibits maximum adsorption capacity (Q = 650 mg/g) for the dye concentration of 1500 mg/l. Furthermore, a comparison study was conducted with alkaline lignin (LN)/ sodium alginate (SA)-based composite hydrogel to validate the adsorption capability of black liquor-based hydrogel at a weight ratio of 1:4. The influence of parameters such as temperature, pH, adsorbent dosage, initial dye concentration was investigated on the malachite green dye removal and also optimized. Both the hydrogels (BL/SA and LN/SA) were further characterized using Field emission scanning electron microscopy, Fourier-transform infrared spectroscopy, Brunauer-Emmett-Teller, X-ray diffraction, and Rheological techniques. The results showed that the maximum dye removal efficiency of 95.54 % and 94.06 % were observed for 0.25BL/SA and 0.25LN/SA hydrogels respectively in 12 min. The adsorption kinetics showed the pseudo-second-order fit and the adsorption isotherm established a high correlation with the Langmuir model. The reusability tests were performed on Black liquor/Sodium alginate and Alkaline Lignin/Sodium alginate hydrogels, which showed good regenerative performance up to four successive adsorption/desorption cycles. In addition, a thermodynamic study is carried out for adsorption of dye on hydrogel, which demonstrates that it is spontaneous and exothermic in nature. The synthesized composite bio-based hydrogels from this study showed fast and efficient removal of malachite green dye. This makes the hydrogel a potential candidate for the sustainable wastewater treatment.