Black-wattle tannin/kraft lignin H3PO4-activated carbon xerogels as excellent and sustainable adsorbents

Abstract

This work proposed new black-wattle tannin/kraft lignin H3PO4-activated carbon xerogels as sustainable and efficient adsorbents. The precursors were chosen based on their eco-friendly and cost-effective nature, aiming to achieve adsorbents with high adsorption capacities. Carbon xerogels were synthesized through polycondensation with formaldehyde and alkaline catalyst in a simple one-pot procedure. Activation was performed using H3PO4 in a tubular furnace (500 °C), under a nitrogen atmosphere. Results show that the inclusion of the kraft lignin led to changes in the morphology of the materials, facilitating the development of their porous structure and increasing specific surface area and pore volume. The best adsorbent (XLT 50 %) was synthesized using a 1:1 tannin/kraft lignin mass ratio. This material presented an adsorption capacity of nearly 1150 mg g−1 of methylene blue (pH = 5 and T = 298 K), which was linked to its high specific surface area of 1348 m2 g−1. The adsorption process followed the pseudo-second-order kinetic model, whereas the adsorption isotherms were best fitted by the Sips model. The XLT 50 % presented good reusability properties, maintaining its adsorption capacity for 3 cycles. Finally, the XLT 50 % presented good adsorptive properties toward other pollutants (methyl orange, 4-chlorophenol, and hexavalent chromium), indicating its versatility for adsorption processes.