Contrasting Cr(VI) and Cd(II) immobilization in contaminated soils by microalgae derived hydrochar

Heavy metal accumulation in soils poses serious environmental risks, particularly from Cr(VI) and Cd(II). Hydrochar is a promising amendment for in situ immobilization of heavy metals; however, the application of algae derived hydrochar and its ability to immobilize metals with contrasting properties remain insufficiently explored. Herein, hydrochar synthesized from Chlorella sp. was applied to Cr(VI) and Cd(II) contaminated soils to investigate its immobilization performance and underlying mechanisms. The algal hydrochar (HC) exhibited a substantially higher immobilization efficiency for Cr(VI) of 96.9% compared with 76.1% for Cd(II). Toxicity characteristic leaching procedure tests demonstrated that Cr(VI) was rapidly and stably immobilized, whereas the stabilization of Cd(II) was less pronounced. Aging experiments further confirmed that the immobilized metals remained largely stable under dry-wet and freeze-thaw cycles. Sequential extraction analysis revealed that HC amendment promoted the transformation of both Cr(VI) and Cd(II) from labile to more stable soil fractions, which was accompanied by a marked reduction in metal accumulation and translocation in plants. Microbial community analysis indicated that HC could partially alleviate Cr(VI)- and Cd(II)-induced disturbances to the soil microbial community, thus facilitating the gradual recovery of soil ecosystem resilience. Mechanistic investigations showed that the immobilization of Cr(VI) was mainly governed by electron-transfer-driven reduction to Cr(III) followed by surface complexation, whereas Cd(II) immobilization was dominated by electrostatic attraction and coordination with oxygen- and nitrogen-containing functional groups. These findings indicate that algal hydrochar can mediate metal-specific immobilization pathways in soil, highlighting its potential for the targeted remediation of Cr(VI)- and Cd(II)-contaminated soils.