Preparation of bamboo biochars by low-temperature hydrothermal method and its adsorption of organics

Abstract: Biochar is a kind of carbon-rich solid material obtained from biomass resources, which can be used as source of energy, catalyst, adsorbent and soil conditioner. Bamboo is a kind of widely used biomass resource containing high fraction of lignocellulose, and bamboo waste from industrial processing is a kind of good raw material for producing biochars. Hydrothermal carbonization approach was applied at low temperatures to produce bamboo biochars, and the selected biochar samples were further modified by being immersed in the sodium hydroxide (NaOH) solution or calcined in nitrogen atmosphere. The effects of the preparation conditions on the physico-chemical properties of the bamboo biochars were investigated, and the adsorption properties of the biochars for 2-naphthol and Congo red from their aqueous solutions were evaluated. The results showed that the yields of the bamboo biochars from hydrothermal carbonization without modification were between 54.12% and 71.53%, and the functional groups containing oxygen such as -OH (3 200-3 600 cm-1), C=O (1 650-1 735 cm-1) and C-O (1 000-1 250 cm-1) were found to be rich on the surfaces of these biochars; the oxygen content of the bamboo biochar produced at 200 ℃ and 7 h was as high as 46.33%, and the surface area of the same carbon sample was measured to be 14.10 m2/g. The bamboo biochars modified by being immersed in the NaOH solution still showed rich oxygen-containing groups on their surfaces, high yields were also achieved with these samples, much larger surface area (33.22 m2/g) was obtained for the biochar sample produced at 200 ℃ and 7 h and modified by soaking in NaOH solution for 1 h, and the oxygen content of this carbon sample was 50.57%, which was higher than that of the sample before modification treatment. On the other hand, few functional groups were found on the surfaces of the bamboo biochars modified by calcination in nitrogen atmosphere, the yields of these biochar samples were much lower (between 12.99% and 16.21%), even higher surface area (52.39 m2/g) was observed for the biochar sample produced at 200 ℃ and 7 h and modified by calcining in nitrogen atmosphere at 700 ℃, the carbon content of this carbon sample was 75.87%, and the oxygen content of this sample decreased to 20.89%. All of the bamboo biochar samples without modification treatment could adsorb 2-naphthol and congo red from aqueous solutions, of which the sample produced at 160 ℃ and 3 h showed the largest adsorption capacity for 2-naphthol (50.3 mg/mL at 25 ℃, initial concentration of 0.5 mg/mL), and the sample produced at 200 ℃ and 7 h showed the largest adsorption capacity for Congo red (63.7 mg/mL at 25 ℃, initial concentration of 0.5 mg/mL), and these 2 kinds of bamboo biochar samples were chosen as the most promising adsorbents for the adsorptive removal of 2-naphthol and Congo red. Surface adsorption was inferred to be the predominant mechanism for the adsorption of Congo red on the surfaces of the bamboo biochars, while a complex mechanism involving partition was suggested for the adsorption of 2-naphthol on the biochars. The NaOH immersion method was favorable for obtaining biochars with higher adsorption capacities for both 2-naphthol and Congo red, and the bamboo biochar sample produced at 200 ℃ and 7 h and modified by soaking in NaOH solution for 1 h displayed the largest adsorption capacities for both 2-naphthol (62.3 mg/mL at 25 ℃, equilibrium concentration of 0.3 mg/mL) and Congo red (69.1 mg/mL at 25 ℃, equilibrium concentration of 0.3 mg/mL). However, calcination in nitrogen didn't show positive effects on the adsorption properties of the bamboo biochars especially for the adsorption of 2-naphthol. The results obtained show that bamboo biochars produced through hydrothermal carbonization can be applied as low cost adsorbents for adsorptive removal of 2-naphthol and Congo red from wastewater.