Lignin-derived biochar for high-performance electrochemical adsorption of Cd(II) from aqueous solutions

Lignin derived from papermaking black liquor is an abundant and economical biochar precursor, however, its direct pyrolysis products often exhibit limited adsorption capacity for heavy metal ions due to their inherent poor pore structure. In order to improve the adsorption ability of lignin-based biochar, an electrochemical adsorption strategy was used to significantly enhance its removal performance for heavy metal in this study. Using alkali lignin and sodium lignosulfonate as precursors, biochar electrodes were prepared via direct pyrolysis. Their physical and chemical properties were comprehensively compared, and their selective electrochemical adsorption efficiency for Cd 2+ in high-salt wastewater was systematically evaluated. The effect of the applied voltage was investigated, and 1.2 V was determined as the optimal operating voltage within the tested range. Under this condition, alkali lignin biochar (AL-BC) exhibited a maximum electrochemical adsorption capacity of 409.3 mg g −1 , significantly exceeding the 82.8 mg g −1 observed for sodium lignosulfonate biochar (LS-BC). The electrochemical process increased the adsorption capacity of AL-BC and LS-BC by about 6.95 and 3.53 times, respectively. Quantitative analysis of electrochemical adsorption mechanisms revealed that the selective capture of Cd 2+ on AL-BC was primarily governed by chemical adsorption mechanisms. Surface precipitation was the dominant contributor (67.1 %), followed by ion exchange (26.2 %), whereas EDLs adsorption (4.8 %) and surface complexation (2.0 %) played minor roles. This study revealed the potential of lignin biochar as a highly efficient and selective electrochemical adsorbent for heavy metal ions in wastewater, providing a new strategy for the high-value utilization of lignin.