Abstract
Bentonite clay is an abundant and low-cost adsorbent and silk fibroin, a naturally occurring protein, and both have a low capacity to remove lethal heavy metal ions from aqueous solution separately. To enhance their metal adsorbing capacity, a new silk fibroin-based bentonite composite was prepared for improving water quality by eliminating heavy metal ions i.e., lead, cadmium, mercury, and chromium. The as-synthesized composite shows better metal sorption capacity than either of them alone. To analyze their structural properties and characteristic functional groups, X-ray diffraction and Fourier-transform infrared spectroscopy were used. The specific surface area for silk/bentonite composite was about 4 m2/g that is smaller than the unmodified bentonite (23 m2/g) which indicates the impregnation of bentonite onto the silk fibroins. Scanning electron microscopy results shows the changes in morphology from plate aggregates to rosette like arrangements. The XRD results of clay/composite shows an increase in basal spacing (d001, from 1.55 to 3.34 nm) in comparison to pristine clay. FTIR results show the presence of organic moiety in SF clay composite. The mechanism of adsorption based on complex formation and ion exchange were proposed briefly. Various adsorption isotherms and kinetic models were applied for the removal of Pb(II), Cd(II), Hg(II), and Cr(VI). As the kinetic study was concerned, kinetic data fitted well to pseudo second order kinetics because experimental values of qe are much closer to the calculated values. The adsorption equilibrium was best studied by Langmuir isotherm whose regression coefficient values (0.985–0.995) are best when compared to Freundlich adsorption isotherm (0.954–0.990) and are indicative of homogeneity of adsorption sites on the SF/clay composite. The monolayer adsorption capacity for Cd(II), Pb(II), Hg(II), and Cr(VI) was found to be 11.35, 11.1, 10.5, and 10.2 mg/g, respectively.
Highlights
Silk (Bombyx mori) is a naturally occurring polymer made up of fibroin and glue like protein sericin, that holds the fibroins together (Zhang et al, 2009)
Fourier Transform Infrared (FTIR) Analysis In the FT-IR spectra of silk fibroin (Figure 1), three different types of peaks are associated with amide groups in various proteins
The band observed in the spectra of bentonite at 3,404 cm−1 is due to OH stretching of hydroxyl groups and water present in the mineral and at 1,632 cm−1 is due to OH bending vibration
Summary
Silk (Bombyx mori) is a naturally occurring polymer made up of fibroin and glue like protein sericin, that holds the fibroins together (Zhang et al, 2009). Silk (Bombyx mori) possess many ionizable groups on different amino acid residues on the side chain and its separation depends on the value of pH in surrounding medium as it is amphoteric in nature. Bentonite is used as an adsorbent for removal of metal ions because of its cation exchange capacity, larger surface area and adsorptive capacity for different organic and inorganic ions (Donat et al, 2005). Clay possesses the ability of adsorption of various hydrophilic substances, the modification of their surface can successfully enhance their capabilities (Liu, 2007; Monvisade and Siriphannon, 2009) In these inorganicorganic hybrids, mostly inorganic minerals are deposited in organic matrix by covalent crosslinking or hydrogen bonding. That is why the modification of bentonite by using silk fibroin polymer was performed to achieve better sorption capacity for heavy metals. To the best of our knowledge, this is the first attempt to use SF/bentonite clay composite for the sorption of heavy metal removal
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