Although assisted phytoremediation using chemical treatments is a suitable technique for the removal of heavy metals (HMs), the estimation of this process using simple models is also crucial. For this purpose, a greenhouse trial was designed to evaluate the effectiveness of citric, oxalic, and tartaric acid on Cd, Pb, Ni, and Zn phytoremediation by maize and sorghum and to estimate this process using sigmoid HMs uptake model. Results showed that mean values of root and shoot dry weight and metals uptake, translocation factor (TF) of Pb and Zn, and uptake efficiency (UE) of Cd in maize were higher than sorghum but the TF of Cd and the phytoextraction efficiency (PEE) and UE of Pb in sorghum were higher than maize. Citric, oxalic, and tartaric acid significantly increased the UE of Pb by 17.7%, 22.5%, and 32.5%, respectively. Tartaric acid significantly increased the mean values of shoot dry weight, shoot Cd, Pb, and Ni uptake, and PEE of Pb and Ni, but decreased TF of Zn. The R2, NRMSE, and KM values indicated the ability of sigmoid HM uptake model in estimating HMs uptake in maize and sorghum treated with organic acids. Thus, tartaric acid was more effective than citric and oxalic acids to enhance phytoremediation potential. Sigmoid HM uptake model is suitable to estimate the HMs uptake in plants treated with organic acids at different growth stages.