Electro-assisted adsorption of heavy metals from aqueous solutions by biochar

Electro-assisted adsorption was investigated for Pb²⁺, Cu²⁺ and Ni²⁺ removal using date seed biochar (DSB-Electro). Compared with pristine biochar, the results showed that DSB-Electro effectively increased the adsorption capacity of Pb²⁺, Cu²⁺ and Ni²⁺ by 21% to 94%. Significant differences were observed between Pb²⁺ and Cu²⁺ adsorption compared with Ni²⁺, which could be explained based on ion polarizing power. Under the same voltage, Ni²⁺ solution shows the highest electric conductivity; thereby more Ni²⁺ is transported to the biochar anode, giving them a greater chance to interact with the surface groups. Electro-assisted adsorption occurred rapidly as around 88% of Pb²⁺ and Ni²⁺ adsorbed within the first 3 h, while 96% of Cu²⁺ occurred within the first hour of contact. Reversing the polarity did not seem to cause significant desorption of the adsorbed ions as the amount released from reversing polarity was less than 38%, indicating that only a small fraction of the ions was held by the electrostatic charge introduced by the current. It was likely that the enhanced charge facilitated other adsorption mechanisms by bringing the ions in contact with the biochar initially via electrostatic force. Electro-assisted adsorption can improve the biochar economic feasibility for metals removal (particularly Ni²⁺) from industrial streams.