Silk fibroin: a promising bio-material for the treatment of heavy metal-contaminated water, adsorption isotherms, kinetics, and mechanism.

Silk is the strongest natural biopolymer produced by silk worms possessing superior adsorbent properties and thus extensively used in various applications. The present study involved the preparation of powder form of a silk fibroin materials and their application in adsorption of heavy metals, particularly, iron from aqueous solution. The morphological and structural characteristic properties of this promising materials were examined by using different analytical techniques. Batch experiments were conducted within feasible parametric ranges to understand the effect of dose, time, concentration, pH, and reusability. Silk fibroin was effective for iron adsorption over a wide range of pH 6 to 10. The adsorption removal efficiency of 98% was attained for removal of iron from contaminated water at moderate dose of 0.25 g and contact time of 60 min, which is unprecedented by considering the environment benign nature of the material. The data was examined in different isotherm models wherein it fitted best in Langmuir adsorption model. Similarly, Langmuir isotherm model, with R value of 0.984 and K 0.412 and maximum adsorption capacity as 12.82 mg g, suggests monolayer adsorption. Kinetic study with better R value of 0.941 represented the pseudo-second order kinetics governed by the chemisorption reaction. To understand the practical applicability of silk fibroin, the repeatability study up to 5 cycles were performed. The findings are very encouraging which confirmed the usage of silk fibroin as adsorbent for multiple cycles with marginal decrease in adsorption efficiency. Eventually, the material was tested for iron removal in real contaminated water which revealed its potential and selectivity for removal of iron in different matrix.