Rhizosphere dynamics of heavy metals during phytoremediation of multi-metal contaminated soil by Celosia argentea

The ability of hyperaccumulator plants to uptake and tolerate heavy metals (HMs) is critical for the phytoremediation of contaminated soils. Although previous research has primarily examined single-metal uptake mechanisms, the spatiotemporal dynamics of multiple HMs at the rhizosphere interface remain poorly understood. In this study, we investigated the 60-day behavior of cadmium (Cd), lead (Pb), zinc (Zn), and copper (Cu) at the root-porewater-soil interface of Celosia argentea Linn. Distinct spatial and temporal gradients of HM concentrations were observed in the rhizospheric soil zone, with the most pronounced changes occurring at the root surface (0 mm). Within 10 mm of the root, HM concentrations decreased by 4.5 %-18.9 %, reaching their maximum decline on day 45. Sequential extraction analyses showed that the Fe-Mn oxide-bound fraction was the most responsive pool, exhibiting reductions of 28.4 %-46.0 % for all four metals by day 45, accompanied by substantial redistribution within the 0-2 mm root zone. In porewater, HM concentrations in the rhizosphere displayed marked temporal fluctuations, characterized by an initial increase followed by a decrease, together with clear vertical stratification, with concentrations increasing with depth by 48.7 %-241.8 %. These HM dynamics were more pronounced in rhizospheric porewater than in bulk soil porewater and were significantly correlated (p ≤ 0.05) with sulfate, phosphorus, iron, and manganese concentrations. Iron plaque formation on root surfaces played a key role in regulating metal sequestration. The accumulation of Cd, Pb, and Zn in iron plaque increased initially and subsequently declined, whereas Cu accumulation showed a continuous increase over time. Notably, Pb accumulation was negatively correlated with that of Cd, Zn, and Cu in iron plaque, suggesting competitive interactions or selective binding processes. Collectively, these results provide mechanistic insights into the coupled mobilization and sequestration of multiple metals at the soil-porewater-root interface, highlighting the importance of rhizosphere-mediated processes during phytoremediation.