Preparation and utilization of rice straw bearing carboxyl groups for removal of basic dyes from aqueous solution

One kind of potentially biodegradable cationic sorbent with high sorption capacity of basic dyes was prepared by thermochemically esterifying oxalic acid onto rice straw, and then the esterified rice straw was further loaded with sodium ion for enhancing its cationic sorption capacity. The sorption of two basic dyes, basic blue 9 (BB9) and basic green 4 (BG4), from aqueous solutions onto modified product was investigated. The effects of various experimental parameters (e.g. initial pH, sorbent dosage, dye concentration, ion strength, contact time) were examined and optimal experimental conditions were decided. The BB9 and BG4 removal ratios came up to the maximum value beyond pH 6. The 2.0 g/l or above of sorbent could almost completely remove BB9 and BG4 from 250 mg/l of dye solution. The ratios of BB9 and BG4 sorbed kept above 97% over a range from 50 to 250 mg/l of dye concentration when 2.0 g/l of sorbent was used. Increase in ion strength of solution induced decline of BB9 and BG4 sorption. The isothermal data fitted well to the Langmuir and Freundlich models. The sorption processes could be described by the pseudo-second-order kinetic model. The results in this research confirmed that the OA-modified rice straw was an excellent basic dye sorbent.