Due to their substantial biomass and rapid growth, emergent plants found in wetlands are viewed as excellent sources for biochar production, which has been demonstrated to serve as an effective substitute for absorbite in the effluent treatment. This article systematically contrasted the physicochemical properties of biochar derived from each section of Iris sibirica L. (designated as BC_R, BC_S, and BC_L) under identical pyrolysis conditions, in order to assess their effectiveness in removing cadmium (Cd) from effluent. Experimental results indicated that the removal efficiencies of Cd among the various biochars followed the order BC_S (19.92 mg/g) > BC_L (19.89 mg/g) > BC_R (13.22 mg/g). The removal of Cd²⁺ by biochar is primarily governed by chemisorption, as described by the Langmuir and Freundlich models. Moreover, different adsorption kinetic models, e.g., first-order kinetics, second-order kinetics, intra-particle diffusion, and the Elovich model, were performed to elucidate the adsorption process. Compared to BC_L and BC_R, the proportions of ion exchange and precipitation were more superior in BC_S, reaching 54% and 31%, respectively, which could serve as an effective adsorbent for metal ions, achieving the maximum adsorption capacity. In addition, precipitation (46%) was predominant during the Cd²⁺ adsorption process through BC_R. Therefore, BC_R was more suitable for the acidic wastewater treatment. This study provided an in-depth understanding of the cadmium removal behavior through biochar obtained from different part (roots, stems, and leaves) of wetland plants and introduced a new option for efficient utilization of waste biomass.

The substantial generation of hazardous, metal-enriched biomass residues poses significant risks of secondary contamination, presenting a critical bottleneck to the broader implementation of phytoremediation that urgently requires effective treatment solutions. This study addressed this challenge by pyrolyzing Pb-enriched biomass (BM _Pb ) across a temperature range (300 °C–700 °C) to produce Pb-enriched biochar (BC _Pb ), evaluating its efficacy for safe residue management. The results demonstrated that pyrolysis effectively reduced the volume of BM _Pb , and the produced BC _Pb significantly enriched and immobilized Pb. Element analysis revealed distinct stabilization mechanisms: Pb ₂ (P ₄ O ₁₂ ) and PbCO ₃ precipitation dominated Pb immobilization at 400 °C, whereas Pb ₃ (CO ₃ ) ₂ (OH) ₂ , Pb ₂ (P ₄ O ₁₂ ), and NaAlSiO ₄ became predominant at temperatures ≥500 °C. Sequential extraction of Pb (BCR) demonstrated a consistent decline in the more labile Pb fractions (exchangeable, F1, and reducible, F2) with increasing pyrolysis temperature, concurrent with a significant increasing in the stable fractions (oxidizable, F3, and residual, F4). Notably, the combined F1+F2 fraction decreased substantially (17% at 700 °C), while the stable F3+F4 fraction increased correspondingly (83% at 700 °C), indicating markedly reduced Pb bioavailability and ecological risk at elevated temperatures. Leaching tests confirmed that Pb release from all BC _Pb samples remained well below relevant regulatory thresholds when the pH higher than 2 (<9.98 mg·g ^-1 vs. 10.0 mg·g ^-1 ), with leaching susceptibility inversely related to pyrolysis temperature. Soil simulation experiments further indicated a conversion of bioavailable Pb (F1+F2) in BC _Pb -amended systems towards stable forms (F3+F4), confirming low ecological risk. Overall, these findings suggested that pyrolysis of BM _Pb at temperatures above 500 °C shows great promise as an effective and safe method for treating phytoremediation residues, demonstrating high stability and low ecological risk to both water and soil environments under most natural conditions, though careful management is required under extreme acidic scenarios.