Efficient Toluene Adsorption on Metal Salt-Activated Porous Carbons Derived from Low-Cost Biomass: A Discussion of Mechanism

Porous carbons (PCS) derived from sodium lignin sulfonate were activated by four common metal salts. The samples exhibit distinct characteristics of irregular, sunflower-like, interconnected sheet, and tine block morphologies under the impact of NaCl, CaCl₂, ZnCl₂, and FeCl₃, respectively (PCS-MCl_x). Surprisingly, the maximum and minimum specific surface areas are 1524 and 44 m²/g corresponding to PCS-ZnCl₂ and PCS-NaCl. All of the samples have plentiful functional groups; herein, PCS-NaCl and PCS-FeCl₃ are detected with the highest O and S contents (11.85, 1.08%), respectively, which signifies sufficient active sites for adsorption. These porous materials were applied in toluene adsorption from paraffin liquid and matched the Langmuir isotherm models well. Thus, the activation mechanism was discussed in detail. PCS-MCl_x has a completely different pyrolysis behavior according to thermogravimetry/derivative thermogravimetry (TG/DTG) analysis. It is speculated that H­[ZnCl₂(OH)] would have an etching effect on the carbon structure of PCS-ZnCl₂, and HCl or H₂SO₄, resulting from FeCl₃ hydrolysis and a reduction reaction, would be corrosive to the sodium lignin sulfonate (SLS) surface. Each metal salt plays a different role in activation. The devised method for the synthesis of porous carbons is green and economical, which is suited to mass production.