Novel electrochemical method to activate biochar derived from spent coffee grounds for enhanced adsorption of lead (Pb).

Biochar (BC) has received much attention as a promising adsorbent that can be exploited to remove heavy metals in domestic and wastewater. The adsorption capacity of BC is, however, relatively low compared to that of conventional adsorbents, and its performance is inversely proportional to its stability. Various chemical and physical methods have been tried to address these limitations, but BC activation still generates too much acidic or alkaline wastewater. Here we propose a novel electrochemical method and compare its lead (Pb) adsorption capacity to that of acid- and alkaline-based approaches. We found that electrochemical activation significantly increased the number of hydroxyl and carboxylic groups on the BC surface, which led to an increase in Pb absorption from 27 % (pristine BC) to 100 % because the oxygenated-functional groups contributed to the adsorption of Pb. Pb capacity was 1.36, 2.64, 3.31, and 5.00 mg g, corresponding to pristine, acidic, alkaline, and electrochemical activation, respectively. The Pb absorption capacity of electrochemically activated BC was also higher than that of acid- and alkali-activated BC, which we attribute to the observed increases in oxygen ratio and surface area. Moreover, the adsorption rate of BC after electrochemical activation was 190 times faster and its capacity was 2.4 times higher than that of pristine BC. These findings show that the electrochemical activation of BC results in greater adsorption capacity than conventional methods.