Preparation of biochar adsorption material from walnut shell by supercritical CO₂ pretreatment

Highlights

1.	The poly-generation of biochar precursor and bio-oil could be realized by supercritical CO2.
2.	Prepared activated carbon had great physical performance and excellent dye adsorption capacity.
3.	The pretreatment process produced high-value organic liquid by-products.
4.	Mild pretreatment conditions were more advantageous.

Abstract

In order to treat dyes in the wastewater of the printing industry and to reutilize walnut shell (WS) waste generate economic benefits, supercritical carbon dioxide (SC-CO₂) pretreatment technology was developed to prepare porous biochar as a precursor for adsorption material. Orthogonal experiments were conducted at the temperatures of 200, 300, and 400 ℃ with durations of 20, 40, and 60 min, and a control group was set up using N₂ pretreatment. Then, KOH activation was employed to prepare biochar adsorption material. The biochars were analyzed and characterized using TGA, BET, SEM, FT-IR, and XRD, and the liquid and gas phase products of the pretreatment process were analyzed semi-quantitatively and quantitatively using GC–MS and gas chromatography. Methylene blue (MB) dye was selected as an indicator to measure the adsorption capacity of biochar, and adsorption kinetics were analyzed based on the data. The results indicate that pretreatment with SC-CO₂ effectively enhanced the performance and yield of the activated carbon. The highest specific surface area increased by 18%, and the maximum adsorption of MB increased by 23% compared to the N₂ control group. The yield increased by 8–262% and the specific surface area increased by 50–192% compared to the direct activation of walnut shell (WS). During the pretreatment processes for the preparation of biochar adsorption material with the best specific surface area, phenol-enriched bio-oil was produced as a by-product which has economic value.

Funding

National Natural Science Foundation of China(52242609)