This study reports the chemical modification of the Byrsonima Crassifolia biomass with citric acid to improve its sorption properties for the removal of cadmium and nickel ions from aqueous solutions under competitive sorption conditions (i.e., multicomponent solutions). The best operating conditions of the chemical modification process were identified using the signal-to-noise ratio to enhance the metal uptakes and to reduce the competitive sorption effects during the simultaneous removal of these metals using the modified biomass. Results indicated that both the sorption capacity and selectivity for heavy metal removal can be improved in multicomponent metal solutions. This improvement in sorption properties of Byrsonima crassifolia biomass is mainly related to an increment of the acidic functional groups on the biomass surface caused by the chemical reaction between citric acid and this lignocellulosic material. The methodology reported in this study can be used to increase the sorption properties of other biomasses for the effective removal of toxic pollutants from multicomponent solutions and for the synthesis of sorbents with tailored sorption properties.