A wood-based fibrous chemisorbent

Abstract

A method and the conditions for fabricating a fibrous chemisorbent by graft polymerization of acrylamide on pine wood in the presence of a Fe 2+ —H 2 O 2 redox initiating system were developed. The maximum values of the yield of commercial copolymer (76.79%) and mass fraction of the graft component (53.49%) were attained at a 0.017 wt. % concentration of H 2 O 2 , 1.5% monomer, temperature of 60°C, and process time of 1 h. The process takes place with 97.85% efficiency and insignificant formation of homopolymer (1.4%). The sorption and kinetic properties of the samples with respect to mercury ions were established. It was found that sorbent with a 9.93% nitrogen content extracts 65-64% of Hg(II) ions from dilute and concentrated solutions and is equal to the industrial strongly acid sulfo cation exchanger Ky-2x8. Based on the data from IR spectroscopy and the linear anamorphoses of equilibrium sorption isotherms, the chemisorption mechanism of absorption of mercury(II) ions by wood with graft amide groups was established. Graft polymerization of unsaturated nitrogen-containing monomers, acrylamide (AA) in particular, on different natural matrices, for example, lignin, coconut shells, starch, and especially cellulose [1-4], has been relatively widely treated in scientific publications. However, the studies on synthesis of graft amino copolymers based on wood are limited. Moreover, modification of such lignocellulosic material with chemically active monomers, where it is possible to create fibrous ion exchangers that differ from ordinary sorbents by smaller geometric dimensions and developed external surface which determine their high capacity and kinetic characteristics, is very pressing. We synthesized a graft copolymer of wood and AA (Wd-AA) and studied its sorption properties with respect to mercury(II) ions. Air-dried pine sawdust of the 2 mm screen sizing fraction containing (in wt. %) 48.3% cellulose, 17.6% hemicellulose, 28.6% lignin, 5.0% extractives, 0.52% ash (including 0.48% insoluble and 0.04% water-soluble residue; pH of ash of 4.72) was used for modification. The elemental composition of the wood was (in %): 45.63 C, 6.10 H, 47.40 O, 0.87 N; moisture content of 9.85; dryness coefficient of 0.9015. Graft polymerization was conducted with the Fe 2+ —H 2 O 2 redox initiating system. The wood was held in a solution of Mohr’s salt with a 7.09 mg/liter concentration of Fe 2+ at pH = 5 and after washing off the excess metal ions, immersed in an aqueous solution containing a different amount of monomer and H 2 O 2 . The concentration of Fe 2+ in the polymer matrix was 2.52 mg/g. The product was purified of homopolymer — polyacrylamide (PAA) and unreacted AA with distilled water in a Soxhlet apparatus. The amount of ungrafted polymer was determined gravimetrically after precipitation of PAA from aqueous solution into ethanol or evaporation of the solution at low pressure. The characteristics of the graft copolymer and grafting parameters were: mass content of PAA in modified wood (W), degree of conversion of AA into homo-, graft polymer (C hp , C g ); the total conversion of monomer (C t ), grafting percentage (P g ) and efficiency (G E ) were calculated according to [5, 6]. The content of amide groups in the reaction products (E N ) was determined by titration with an accurate weighed portion of samples in a solution of acetic anhydride with 0.1 N solution of HClO 4 in glacial acetic acid. The amount of iron ions adsorbed in the polymer phase was established by atomic-absorption analysis and the nitrogen content in the graft copolymers was determined chromatographically. Sorption of Hg(II) was conducted in static conditions from 20 ml of a solution of Hg(NO 3 ) 2 ⋅2H 2 O (analytically pure) with copolymer weighing 0.05 g. The initial and equilibrium concentrations of metal ions in the solution was determined by