Combination of citric acid and ferric chloride enhances cadmium accumulation in sweet sorghum facilitating phytoremediation

Sweet sorghum presents a promising candidate for cadmium (Cd) phytoremediation, combining notable Cd accumulation with the added-value of bioethanol production. To develop an efficient and cost-effective strategy for enhancing this process, we evaluated chemical amendments including ferric chloride (FeCl3), citric acid (CA), polyaspartic acid (PASP), and their combinations. Preliminary screening identified the integrated application of 0.3% FeCl3 and 0.3% CA as the most effective, enhancing Cd concentrations by 1.1–1.5-fold compared to other agents without biomass penalty. This synergistic effect was corroborated in a follow-up experiment with three times of applications, which increased shoot Cd accumulation by 1.4-fold relative to the control, without affecting dry weight. The FeCl3 and CA application significantly elevated soil electrical conductivity (EC), nitrate, ammonium, and DTPA-extractable Cd. Mantel and Pearson correlation analyses revealed that the enhanced Cd accumulation was positively correlated with increased soil EC, which itself was associated with higher nitrate, ammonium, and available Cd levels. In conclusion, the low-dose combination of FeCl3 and CA offers a practical, economical, and environmentally-sound strategy to improve sweet sorghum-based Cd phytoremediation.