Efficiency of dye adsorption by biochars produced from residues of two rice varieties, Japanese Koshihikari and Vietnamese IR50404

Adsorption of Methylene Blue (MB) and Bromocresol Green (BG) were examined by using biochars produced from rice residues, straw and husk of Japanese Koshihikari and Vietnamese IR50404 rice varieties, to evaluate their capacity to adsorb dyes and possible adsorption mechanism. Cationic MB dye was more effectively adsorbed than anionic BG dye by all biochars examined. Vietnamese IR50404 biochars showed higher capacity of adsorption of two dyes than Japanese Koshihikari biochars, approximately about 1.5 times for MB and 1.7 times for BG. In varying pH from 2 to 10, alkaline condition increased adsorption of cationic MB dye - approximately by 27.7–33.5% for rice straw biochars and by 86.2–92.2% for rice husk biochars; albeit a slight decrease in adsorption of anionic BG dye by 27.2–32.9% for rice straw biochars and by 47–70.6% for rice husk biochars. The pH experimental results indicated the existence of negative charges in the biochars and their electrostatic interaction with dyes. The adsorption kinetic study supported intra-particle diffusion of dyes, proceeding via a complex mechanism consisting of both surface adsorption and intra-particle transport within the pores of biochars. Thermodynamic analysis of adsorption suggested that the process was spontaneous with negative ΔG 0 values and endothermic with positive ΔH 0 values (ΔG 0 = 35.27–41.94 kJ/mol; ΔH 0 = 3.92–23.69 kJ/mol). This paper discussed possible explanation of dye adsorption with physisorption through porous diffusion, hydrogen bonding, π-π interaction or π + -π interaction, common to both anionic and cationic dyes, with additional electrostatic interaction for cationic dyes with biochars in aqueous solution.