A comparison is presented using two microwave pyrolyzed biochars produced from agricultural biomass (shredded hemp stalk) and woody biomass (maple wood chips) for the removal of Pb (II) from aqueous solution in a batch adsorption study. Biochars were produced by microwave pyrolysis of 1.5 kg of each biomass at an average temperature of 600 ˚C in a stainless steel 309 reactor and then magnetized by mixing aqueous Fe³⁺/Fe²⁺ solutions with aqueous biochar suspensions, followed by treatment with NaOH. 

The magnetic biochars were characterized and the effects of pH, adsorbent dose, temperature, contact time and initial concentration of Pb (II) solution on their adsorption performance were investigated. The physico-chemical properties of the biochars significantly influence their adsorption capacities. For the cation and system under investigation, the adsorption capacity of magnetic hemp biochar was higher than that of magnetic maple biochar. The higher adsorption efficiency of hemp biochar was correlated to its higher polarity index, pH and zeta potential.