A large amount of agricultural waste was used to prepare cellulose (Cel) and then the surface was modified with PEI (Cel-PEI) using the microwave method. To be used as a metal adsorbent, the adsorption of Cr (VI) from an aqueous solution by Cel-PEI was measured using Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), and thermogravimetric analysis (TGA) techniques. The parameters of Cr (VI) adsorption in solution by the Cel-PEI adsorbent were as follows: the pH of the solution was 3, the concentration of the chromium solution was 100 mg/L, and the adsorption time was 180 min at 30 °C using 0.01 g of adsorbent. Cel-PEI had a Cr (VI) adsorption capacity of 106.60 mg/g, while the unadjusted Cel was 23.40 mg/g and the material recovery showed a decrease in efficiency of 22.19% and 54.27% in the second and third cycles, respectively. The absorption isotherm of chromium adsorption was also observed. The Cel-PEI material conformed to the Langmuir model with an R2 value of 0.9997. The kinetics of chromium adsorption showed that under pseudo-second-order analysis, with R2 values of 0.9909 and 0.9958 for Cel and Cel-PEI materials, respectively. The G° and H° values of the adsorption process were negative, indicating that the adsorption is spontaneous and that the adsorption process is exothermic. The efficient preparation adsorbent materials for Cr (VI) was achieved using a short microwave method that is low-cost and environmentally friendly for use in the treatment of Cr-contaminated wastewater.