from sodium lignosulfonate obtained by phosphoric acid activation ... investigation showed that lignosulfonate is a suitable precursor for preparation of activated.
Impact of impregnation ratio on copper adsorption by carbon adsorbents from sodium lignosulfonate obtained by phosphoric acid activation Maryna S. MYGLOVETS1, Olga I. PODDUBNAYA1, Barbara GAWDZIK2, Magdalena SOBIESIAK2, Beata PODKOŚCIELNA2, Mykola M. TSYBA1, Alexander M. PUZIY2 1
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Institute for Sorption and Problems of Endoecology, NASU, 03164 Kyiv, Ukraine, Department of Polymer Chemistry, Maria Curie Skłodowska University, 20-614 Lublin, Poland
INTRODUCTION Lignosulfonates are by-products from the isolation of lignin from lignocellulosic materials during acidic pulping by sulphite process [1]. World production of lignosulfonates is about 1000000 tons per year, but only up to 2% are commercially used as technical surfactants and plasticizers with dispersing and stabilizing abilities in the oil drilling industry and concrete production [2]. However, lignosulfonates as plasticizers are facing strong competition from petroleum-based synthetic equivalents with superior performance. Thus, alternative use of lignosulfonate is crucial as a part of biorefinery concept. Previous investigation showed that lignosulfonate is a suitable precursor for preparation of activated carbon using phosphoric acid activation at different temperatures [3]. This communication focuses on impact of impregnation ratio on adsorption properties towards copper ions. MATERIALS AND METHODS Activated carbons were obtained by phosphoric acid activation of sodium lignosulfonate at temperatures 400-1000 °C using different impregnation ratios of 0; 0,75; 1 and 1,5. After carbonisation, carbons were extensively washed with hot water in Soxhlet extractor until neutral pH. The surface area and pore structure of sodium lignosulfonate-based activated carbons were characterized by N2 adsorption–desorption isotherms at 77 K. Surface groups were determined by potentiometric titration method conducted at 25 °C in thermostatic vessel under argon flow. Copper adsorption was studied from 0.001 M solution containing 0.1 M of NaCl at different pH. RESULTS AND DISCUSSION Carbon yield is about 40-55% at 400-800 °C with progressive decrease to 22-36% at higher temperatures. Addition of phosphoric acid increases carbon yield by 15% at 700-800 °C. With increasing impregnation ratio, the yield tends to increase at 0.75 followed by decreasing at higher amount of phosphoric acid. Porous structure was more developed for phosphoric acid activated carbons. BET surface area increases with impregnation ratio for carbons obtained at all temperatures. Ash content is about 4% for acid-free carbons obtained at all carbonisation temperatures. Addition of phosphoric acid causes increasing ash content with activation temperature. Also an increase of impregnation ratio gives raise of ash content by up to 14% at 1000 °C and IR=1.5. Potentiometric titration revealed the acidic character of all the phosphoric acid activated carbons. Total amount of surface groups increases with activation temperature reaching maximum of 3.3 mmol/g at 800 °C (Fig. 1). Calculated proton affinity distributions show existence of several types of acidic surface groups that could be assigned to phosphate, carboxylic, enol, lactone and phenolic groups. Large amount of very acidic surface groups in phosphoric acid activated carbons is responsible for high capacity to copper ions (Fig. 2). Copper adsorption increased with pH of equilibrium solution due to gradual deprotonation of surface sites. Significant copper adsorption is observed at pH as low as 2 due to high amount of very acidic phosphate surface groups. Copper adsorption is at maximum for samples obtained at 900 °C for all impregnation ratios. With increasing impregnation ratio up to 1 the adsorption of copper increases followed by decreasing at higher amount of phosphoric acid.
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Fig. 1. Selected proton-binding isotherms (left) and proton affinity distributions (right) of carbons obtained by phosphoric acid activation of sodium lignosulfonate at different temperatures.
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Fig. 2. pH dependence of copper adsorption on carbons obtained by phosphoric acid activation of sodium lignosulfonate at different temperatures (left) and IR (right).
CONCLUSIONS Activation by phosphoric acid of sodium lignosulfonate allows obtaining activated carbons with a high surface area and well-developed macro- and mesoporosity. In addition, they exhibit large amount of surface acidic groups. This investigation shows that lignosulfonate-based phosphoric acidactivated carbons have considerable promise for the purification of aqueous solutions by heavy metal ion removal. REFERENCES 1. Suhas, Carrott, P. J. M. and Ribeiro Carrott, M. M. L. (2007). ‘Lignin - from natural adsorbent to activated carbon: a review.’, Bioresour. Technol., Vol.98(12), pp. 2301–2312. 2. Gargulak, J. D. and Lebo, S. E. (1999). ‘Commercial use of lignin-based materials’, in Glasser, W. G., Northey, R. A., and Schultz, T. P. (eds.), Lignin: Historical, Biological and Materials Perspectives, ACS Symposium Series, pp. 304–320. 3. Myglovets, M., Poddubnaya, O. I., Sevastyanova, O., Lindström, M. E., et al. (2014). ‘Preparation of carbon adsorbents from lignosulfonate by phosphoric acid activation for the adsorption of metal ions’, Carbon, Vol.80, pp. 771–783.
