Elaboration and characterisation of novel low-cost adsorbents from grass-derived sulphonated lignin

This study investigated the use of water-soluble sulphonated lignin (SL) extracted from grass, which has not been used before as a precursor of activated carbon (AC). Chemical activation of SL with three dehydrating salts (ZnCl₂, KCl, Fe₂(SO₄)₃·xH₂O) at various salt concentrations (10%, 20%, 30% w/w), charring temperatures (600, 700 °C) and charring times (1, 2 h) has been carried out. The surface characteristics and removal efficiencies of cadmium, copper and zinc ions from aqueous solutions were affected by the activation conditions. The sulphonated lignin-based activated carbons (SLACs) with the highest specific surface area, total pore and micropore volume were produced at the lowest dehydrating salt concentration (10% w/w) and at 700 °C and 2-h charring. These optimal sulphonated lignin-based ACs 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 central composite design and surface response methodology of different SLACs characteristics showed that the optimal responses were achieved at the same operating conditions. These SLACs also achieved the highest removal efficiencies of Cd²⁺, Cu²⁺ and Zn²⁺ ions from aqueous solutions. The chemical activation had significantly increased the total porosity, microporosity and surface area of water-soluble SL. The activation mechanism depended on the used dehydrating salt where the porosity developed by the dehydration effect of ZnCl₂, and by a series of hydrolysis and redox reactions for the other two salts. The results of this research demonstrated that water-soluble SL has a great potential as a novel precursor for the production of activated carbons.