In this work, twenty different physical and chemical methods were used to study the effect of pretreatment on Cr(VI) and Cd(II) biosorption by an aquatic plant (Ceratophyllum demersum) biomass. The H3PO4 boiled biomass had the highest biosorption capacity (45 mg/g) for Cr(VI). The NaOH boiled biomass showed a higher biosorption capacity for Cd(II) (63 mg/g). Dunnett's test was carried out to compare the biosorption capacity of raw biomass with those pretreated by H3PO4 and NaOH, and the results showed the better performance of pretreated samples. To explore the surface characteristics and explain the differences in biosorption behavior; SEM, FT-IR and elemental analysis were performed for raw biomass and those pretreated by boiling in H3PO4 and NaOH. This work is of great meaning in protecting natural environment by drawing on natural biomass, and it is a combination of nature and modern industry.