Feedstock type and pyrolysis temperature drive lead(II) adsorption on magnetite-impregnated biochar in aqueous solutions

Heavy metals and metalloids in industrial wastewater have long been a concern. Low-cost adsorbents such as biochar are increasingly being promoted for treating industrial wastewater. This study investigated the effects and associated mechanisms of magnetite (Fe₃O₄) activation on lead (Pb, II) removal from aqueous solutions by biochars pyrolyzed from sawdust, wheat straw, canola straw, and manure pellet at 300, 500, and 700 °C. Depending on feedstock type, increasing pyrolysis temperature enhanced Pb(II) adsorption in pristine biochars. However, Fe₃O₄ activation reduced Pb(II) adsorption by up to 86%, likely due to decreased ash content, pH, and exchangeable cations. Cation exchange and the relative proportion of Ca²⁺/Mg²⁺ to K⁺/Na⁺ dominated the adsorption process via co-precipitation and outer/inner-sphere complexation. This process, depending on the feedstock type, occurred on either heterogeneous or homogenous surfaces of the biochars, with the data fitting better to the Freundlich or Langmuir model. We conclude that feedstock type and pyrolysis temperature, rather than Fe₃O₄ activation, are more critical when designing biochars for Pb(II) removal from wastewater. Biochars with higher inorganic fractions and greater proportions of Ca²⁺/Mg²⁺ to K⁺/Na⁺ should be prioritized for Fe₃O₄ activation to optimize Pb(II) removal from aqueous solutions.