Silk adsorbent for green and efficient removal of methylene blue from wastewater

Silk, a naturally occurring proteinaceous biopolymer with remarkable adsorbent properties, has been employed in wastewater remediation. The sericin coating, functioning as a protective barrier and rendering fibres impervious to external chemical attacks and preventing their involvement in chemical reactions, was removed using a greener alternative to harness silk as an effective adsorbent. Subsequently, the silk fibres underwent intermittent microwave degumming to extract sericin, and the fibres were utilized for the adsorptive exclusion of the hazardous methylene blue (MB) dye. The comparative batch adsorption studies (kinetics and isotherm) between raw silk fibres and degummed fibres were performed to comprehend the role of degumming on fibre adsorption efficacy by varying operating conditions, including pH, time of contact, initial adsorbate and dosage of adsorbent. The paramount adsorption capacity of raw silk was observed to be 137.08 mg g^-1 and 179.14 mg g^-1 for degummed silk when adsorbate conc. was 100 ppm. The kinetics of adsorption obeyed pseudo-second order suggesting that the rate controlling step is chemisorptions, and data demonstrated greatest fit to Langmuir isotherm exhibiting mono-layer adsorption. Further, biodegradability was studied by mimicking natural environmental conditions where the raw and degummed silk fibres demonstrated 51% and 53% degradation, respectively, after 180 days. Overall, based on obtained results, this study highlights the suitability of silk as an effective as well as sustainable adsorbent for the exclusion of toxic methylene blue dye from wastewater.