Reed biomass harvested from wetland constructed for water purification was modified into a biosorbent for Pb(II) removal from aqueous solution. The enhancement of Pb(II) adsorption by reed biosorbent depended not only on the types of reagent used for pretreatment, but also on the pH during the pretreatment process. The mechanisms, as elucidated using relational data obtained from Boehm titration, Fisher esterification and FTIR, involved the conversion of carboxylic groups into carboxylate groups, and proved the role of the carboxylate group, which occupied more than 800% in binding Pb(II). The Langmuir sorption isotherm of Pb(II) by R-NaOH-12 showed Qmax and b values of 0.082 mmol/g and 312.5 g/mmol, suggesting enough adsorption performance to reduce the concentration of Pb(II) to meet the range of WHO guidelines. The salinity of aqueous solution containing NaH2PO4 and NANO3 promoted the adsorption of Pb(II), while NaCl and Na2SO4 suppressed the adsorption capacity of Pb(II). The adsorption mechanism of reed biosorbent provides valuable insight on the pretreatment effects and the advantages of utilizing this plant as biosorbent for Pb(II) and other heavy metals.