Ultrasound-assisted synthesis of carboxy-methyl lignin from sawdust based lignin as a sustainable source

Sustainable biomass offers potential for obtaining products with added value with most literature focusing on use of cellulose and hemicellulose. The current study focuses on development of lignin-based surfactant utilizing the lignin recovered from pre-treatment of sawdust. Parameters such as sodium chloroacetate to lignin ratio (1:1–4:1 w/w), temperature (60–100° C), reaction time (0.5–3 h), and NaOH concentration (1.5 M) were varied to understand the effect on synthesis using conventional method. Additionally, ultrasonic power (40–120 W) and duty cycle (30–70 %) were varied in the ultrasound assisted approach to establish the best synthesis conditions. The lowest value of critical micellar concentration (CMC) with conventional method was 0.194 at 3:1 sodium chloroacetate to lignin ratio, 90° C and 2 h, while the lowest values of CMC with ultrasound was 0.057 at the optimised parameters of 2:1 sodium chloroacetate to lignin ratio, temperature of 80° C in just 1 h period. FTIR and NMR analysis was also performed on the extracted lignin and the carboxymethylation product with or without ultrasonic treatment. It was clearly demonstrated that improved product with almost 70 % lower CMC value could be obtained using ultrasound with significant reduction in the synthesis time and requirement of sodium chloroacetate as well.