Mechanism of iron salt impregnated biowaste biochar with a prevailing free radical initiator for effective removal of toxic thallium from solution

•Combination of iron salts@BC with PS can efficiently remove Tl from solution.•FeCl3@BC-PS shows a marginally higher removal efficacy (96.5%) than Fe(NO3)3-PS (94.1%).•Iron salts@BC-PS removes Tl mainly by chemical adsorption and oxidation process.•SO4− and OH are the vital radicals for transforming Tl(I) into Tl(III).