Chemical and physical characteristics of optimal synthesised activated carbons from grass-derived sulfonated lignin versus commercial activated carbons

• Sulfonated lignin-based ACs showed competitive chemical and physical properties.
• Chemical activation of sulfonated lignin increased its pore volume up to 73 times.
• Chemical activation of sulfonated lignin increased its surface are up to 64 times.
• Lignin-based ACs may act better in continuous adsorption process than commercial ACs.
• FTIR and SEM analysis of sulfonated lignin changed after chemical activation.

This study aims to evaluate the surface characteristics and chemistry of optimal activated carbons (ACs) synthesised from water-soluble grass-derived sulfonated lignin (SL) using three dehydrating salts (ZnCl2, KCl and Fe2(SO4)3·xH2O). The optimal AC synthesised by each dehydrating salt was chosen as the carbon that achieved the highest removal efficiency of Cd2+, Cu2+ and Zn2+ ions from aqueous solutions. These optimal sulfonated lignin-based activated carbons (SLACs) showed the highest surface areas, total pore and micropore volumes among all the synthesised ACs. These SLACs were named SLAC-ZC (optimal grass-derived SLAC activated by zinc chloride); SLAC-PC (optimal grass-derived SLAC activated by potassium chloride) and SLAC-FS (optimal grass-derived SLAC activated by ferric sulphate). The surface characteristics of two commercial activated carbons (CAC1 and CAC2) were also appraised for comparison purposes. The optimal SLACs showed similar, or even better, properties to the two CACs. The N2 adsorption/desorption isotherms showed that the micropore fraction of SLAC-ZC was greater and its mesopores were narrower than SLAC-PC. For SLAC-FS, the amount of adsorbed N2 was markedly lower than all other ACs and hence its values of BET surface area (ABET) and total pore volume (Vtotal) were the lowest. The iodine volume capacities of SLAC-ZC and SLAC-PC were higher than CAC1, suggesting that they could operate better in continuous adsorption processes. FTIR and SEM analysis illustrated that chemical activation had changed the surface chemistry of SL. Overall; synthesis of ACs from this novel precursor will add value to sulfonated lignin, which is considered as an industrial waste.

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