Metal sericin complexation and ultrafiltration of heavy metals from aqueous solution
Graphical abstract
Introduction
Discharge of toxic organic compounds and metallic ions into sewage systems may damage the operational process of biological treatment plants [1]. The environmental and ecological requirements include low energy, cheap labor and small capital costs, but the conventional water treatment techniques are incapable to remove the metal ions from aqueous effluents up to the minimum desirable concentration. Secondly, technologies involving process of ion exchange, activated carbon adsorption, electrolytic removal are prohibitively expensive [2]. Application of pressure driven membranes is very common in order to segregate metal ions or organic dyes from industrial wastes or natural waters [2], [3], [4]. Of many separation techniques membrane driven technique is effective and compatible to other techniques regarding technical and economic feasibility [5]. Membrane separation process for instance, microfiltration, nanofiltration and ultrafiltration are only limited to retain the molecules of higher masses and unable to remove all the contaminants [6]. Removal of metallic ions can be accomplished by either reverse osmosis (RO) or at least nanofiltration on account of smaller size of target metal ions. Nonetheless, use of dense polymeric membranes in such techniques significantly enhances the capital and operating costs [7], [8]. Likewise, micellar enhanced ultrafiltration (MEUF) technique has been executed to eliminate ions and soluble organic solutes from aqueous environment using electrostatic attractive forces and hydrophobic–hydrophobic interactions [9], [10], [11], [12]. The negative aspect of MEUF is the transfer of monomeric surfactant molecules through the membrane to the aqueous stream namely permeate [13]. The idea of enhanced ultrafiltration (UF) works on the principle of metal–polymer binding to make macromolecular complexes that are hampered by the small pores of membrane on one side while the non-complexed metal ions pass through other side of employed membrane. The major concern is to find the suitable polymer for effective complexation with target metal ions [14], [15]. Over the past many years, variety of polymers have been used to remove metal ions from water for instance; polyelectrolytes, poly (ethylenimine) and poly (diallyl dimethyl ammonium chloride) [16]. Since water treatment technologies require benign and safe polymers, eventually researchers have drawn their attention to natural materials for example; clay materials, agricultural wastes, biomass, marine organism and biopolymer chitosan for the removal of heavy metal ions from aqueous solutions [16], [17]. The silkworm cocoons contain two major proteins, fibroin and gummy substance sericin that constitute 20–30% silk fiber and is water soluble glycoprotein [18]. In addition, sericin is specifically synthesized in the middle silk gland of the silkworm [19]. Sericin extracted from Bombyx mori, comprises a group of polypeptides with molecular mass of 20–400 kDa and has particularly rich serine content [20], [21]. Presence of high percentage of water-soluble amino acids (serine and threonine) in sericin makes it moisturizing agent in cosmetic industry [22]. We exclusively used this biopolymer (sericin) to entrap heavy metal ions by keeping in view its water solubility and remarkable property of chelation in our study as shown in Fig 1 [23]. A metal-sericin complex (MSC) of considerable size is formed on mixing in water. Sericin-enhanced metal ions were subjected to pass through the ultrafiltration membrane. On account of larger size MSC, trapped metal ions are hampered by the smaller pores of UF membrane. The pure stream of water on the other side of membrane is taken out as permeate. In this study we discussed in detail theatrical or physical behavior of metal-sericin complexation in ultrafiltration.
Section snippets
Metal-sericin complex (MSC) formation and pH effect
To discuss theoretically, we ignore other functional groups in sericin polymer and only consider role of most effective NH2 functional group with lone pair of electrons for chelation. A complex formation reaction between divalent metal ions (M) and amino (NH2) group of PDL sericin (S) can be written as:where M, are free metal ions in aqueous solution and S is repeating unit of PDL sericin. Charges have been ignored for understanding.
The amine (NH2) and carboxyl (COOH) group which are
Instrumentation
The assessment of pH effect on metal removal efficiency of sericin was studied. The measurement of pH was carried out with controlled pH analyzer which was standardized using buffer solutions of 2, 4, 6, 8, 10 and 12 pH values. Ultrafiltration experiments were performed in a cell made of steel and manufactured by Mechanical Engineering Department University of Waterloo, Canada. Three different trans-membrane pressures of nitrogen (N2) gas were applied: 10 psi, 20 psi and 30 psi at ambient
Effect of initial concentration of heavy metals
The effect of initial concentration on rejection percentage of heavy metal ions at constant amount (0.05%) of sericin for three different TMP has been shown in Fig 2. As it is clear, that by increasing the initial concentration, the R% decreases on account of gradual decline of trapping capacity of fixed amount of sericin. The values of J, on the other hand, for Pb2+ and Cu2+ virtually remain constant at 20 and 30 psi TMP with their gradual depression at 10 psi TMP by increasing initial metal
Conclusions
Rejection of heavy metals is the result of chelating efficiency of biopolymer that increases at elevated concentration of sericin and decreases by increasing the metal concentration at fixed amount of sericin in feed solution. On average, all metal ions showed high rejection range of 80–90% at lower TMP of 10 psi by sericin enhanced ultrafiltration technique. Effect of anions on R% and J is the result of greater solubility of salts in water and higher mass of two comparable salts of same metal
Acknowledgements
Authors acknowledge Higher Education Commission (HEC) Pakistan and Chemical Engineering Department, University of Waterloo, Canada for funding and lab facilities.
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