The rapid development of industry brings about pollution as a negative effect. There is excessive release of heavy metals like copper and cadmium that affect the ecosystem functioning and biodiversity. Biomaterials can be used in the sorption of these harmful metal contaminants in wastewater. Lignin is a constituent of the cell walls of vascular plants of which 60 million tons of industrial lignin are generated annually. Most of it is used as low cost fuel while a small percentage is commercialized. For economic protection, lignin materials should be embraced more in adding value.
Researchers (Li-Qiu Hu, Lin Dai, Rui Liu, and Chuan-Ling Si) at Tianjin University of Science and Technology improved the biosorption capacity of alkaline lignin by copolymerization with acrylic acid and investigating the biosorption performances. Their research work is now published in Journal of Materials Science.
Raw lignin has rich polymeric groups that facilitate the uptake of metals such as Cu(II), Cd(II), Pb(II), Cr(III) and Cr(VI). However, studies done do not show satisfactory sorption capacity of raw lignin and hence the need for further improvement through chemical modification. Hence the radical graft copolymerization with acrylic acid. Acrylic acid (AA) is a monomer used in the preparation of adsorption materials. The incorporation of acrylic acid into materials introduces numerous carboxyl groups for metal binding
The research team initiated their studied by copolymerization of acrylic acid with alkaline lignin. Briefly, a mixture of alkaline lignin and dimethylformamide is magnetically stirred in a nitrogen atmosphere at room temperature. A solution of ammonium persulfate and dimethylformamide is then added. Acrylic acid and a solution of methylenebisacrylamide and dimethylformamide are added respectively. Polymerization starts in the addition of ammonium persulfate and acrylic acid. When the reaction finishes, it is precipitated with excess deionized water and purified with an acetone extraction.
The Lignin-graft-poly (acrylic acid) from the reaction has more binding sites beyond the external surface of the graft copolymers as the chains of the poly(acrylic acid) stretch in aqueous solutions due to repulsion of carboxyl groups in metal biosorption. The amount of acrylic acid had the most effect on biosorbent capacity while the synthetic time had the least effect. The biosorption uptake improved with an increase in the pH values. The biosorption capacity of raw alkaline lignin improved dramatically after it was grafted with acrylic acid. After six hours in a solution without pH control, the final solution pH is lower as the pKa of –COOH is in the range of 3.5-5.0 then preventing the dissociation of carboxyl groups and inhibiting further biosorption. The release of hydrogen ions in the biosorption decreases the pH of the solution.
The researchers observed adsorption of Cu(II) and Cd(II) was fast in the first half hour then it decreased gradually to reach the biosorption equilibrium. The bonding sites in the copolymer were initially countless but decreased due to the repulsion between adsorbed metal ions and those still in the solution. The complicated structure of Lignin-graft-poly(acrylic acid) also affected the biosorption time. The biosorption rate of Cu(II) was higher than that of Cd(II) due to its smaller ionic radius. The elevation of temperature increased the level of bisorption capacity and rate.
Raw lignin does not have effective biosorption qualities until it is copolymerizes with acrylic acid. The copolymer creates numerous binding sites that enhance the adsorption of copper and cadmium that may be pollutants in water. Copper is adsorbed more effectively than cadmium. The study advocates positive effects on the environment would be exerted by the preparation of Lignin-graft-poly (acrylic acid) for water treatment by adding value to the lignin produced in paper industries and enhancing the purification of water.
Hu LQ, Dai L, Liu R, Si CL. Lignin-graft-poly (acrylic acid) for enhancement of heavy metal ion biosorption. Journal of Materials Science. 2017 Dec 1;52(24):13689-99.
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