Monitoring of copper adsorption on biochar using spectral induced polarization method

Copper contaminant generated from mining and industrial smelting poses potential risks to human health. Biochar, as a low-energy and cost-effective biomaterial, holds value in Cu remediation. Spectral Induced Polarization (SIP) technique is employed in this study to monitor the Cu remediation processes of by biochar in column experiments. Cation exchange at low Cu²⁺ concentrations and surface complexation at high Cu²⁺ concentrations are identified as the major mechanisms for copper retention on biochar. The normalized chargeability (m_n) from SIP signals linearly decreased (R² = 0.776) with copper retention under 60 mg/L Cu influent; while m_n linearly increases (R² = 0.907, 0.852) under high 300 and 700 mg/L Cu influents. The characteristic polarizing unit sizes (primarily the pores adsorbing Cu²⁺) calculated from Schwartz equation match well with experimental results by mercury intrusion porosimetry (MIP). It is revealed that Cu²⁺ was driven to small pores (∼3 μm) given high concentration gradient (influent Cu²⁺ concentration of 700 mg/L). Comparing to activated carbon, biochar is identified as an ideal adsorbent for Cu remediation, given its high adsorption capacity, cost-effectiveness, carbon-sink ability, and high sensitivity to SIP responses - the latter facilitates its performance assessment.