Four residues of uncommon crops—Coffea arabica fruit endocarp, coconut fruit endocarp, Eichhornia crassipes weed (EC), and Guadua angustifolia plant—and a commercial activated carbon (CA) are evaluated as Ni(II) and Cd(II) biosorbents. These materials are chosen for their abundance and economic importance in Colombia. Surface functional groups containing oxygen, such as carboxylic, lactonic, and phenolic, are quantified using the Boehm's method. The Ni(II) and Cd(II) adsorption are determined from dose curves. The highest removal percentage is obtained using EC: 74.31% and 95.77% for Ni(II) and Cd(II), respectively. The equilibrium isotherms are constructed at 28 °C and pH 6.6. The results are adjusted to the Langmuir model by nonlinear regression, showing a good fit, with R2 above 95%. The highest maximum adsorption capacity (XmL) is found with EC: 17.47 mg Ni(II)/g EC and 32.82 mg Cd(II)/g EC. The trend in the estimated XmL values is found to be consistent with their contents of total acidic surface functional groups. It is concluded that these lignocellulosic materials, without chemical and physic treatment, behave similarly or even equal to the CA.