Simazine sorption to biochars produced by pyrolysis of corn straw at 200 ℃ (CS200) and 400 ℃ (CS400) as influenced by ionic strength, heavy metal (Cd²⁺) and pH was examined. The more carbonized biochar CS400 had higher sorption affinity (log K_oc) for simazine than CS200 because CS400 had stronger hydrophobicity, more aromatic fractions and larger surface area. Although H-bonding interaction could occur, the dominant sorption mechanisms for CS400 were hydrophobic binding, charge-transfer (π–π*) interaction and pore-filling, while for CS200 was partitioning. Increase of ionic strength and presence of Cd²⁺ had no significant effects on the sorption affinities for the both biochars, indicating no cationic exchange occurred between simazine and biochars and Cd²⁺ could not play the role of bridging ion. Lower pH facilitated for sorption of simazine to the both biochars. After loading simazine, significant decrease of zeta potentials for CS400 verified an important role of charge-transfer (π–π*) interaction between them, while only slightly decrease of zeta potentials for CS200 illustrated the partition-dominant mechanism. Nearly no change of zeta potentials for biochars sorbing simazine with and without Cd²⁺ demonstrated that specific adsorption was not dominant mechanism for Cd²⁺ sorption to biochars. Cation exchange might be the dominant mechanism for Cd²⁺ sorption to biochars, which also could well explain sorption more Cd²⁺ to CS200 than to CS400 and little effect of Cd²⁺ on sorption of simazine to biochars.