Vertical and Temporal Dynamics of Metal(loid)s during Soil Remediation with Sewage Sludge-Derived Amendments and Nanoscale Zero-Valent Iron

Valorising sewage sludge (SS) into soil amendments aligns waste management with circular-economy principles. Here, a three-layer outdoor macrocosm system was designed to track the temporal and vertical redistribution of metal(loid)s under field-representative conditions. Six 1 m 3 macrocosms containing polluted sandy soil overlying clean clayey soil and a sand drainage layer were amended with composted SS or SS-derived hydrochar, alone or combined with nanoscale zero-valent iron (nZVI), and monitored for one year. Hydrochar markedly increased porewater metal(loid) concentrations in the polluted layer: As increased from 77 µg L -1 in the control to 1493 µg L -1 at the beginning of experiment and peaked at 2679 µg L -1 at day 30, while Zn reached 307 mg L -1 . Evidence of downward contaminant redistribution was observed; after one year, the sand layer receiving hydrochar plus nZVI contained 43 µg L -1 Cd and 9.3 mg L -1 Zn, compared with 0.2 µg L -1 and 2.9 mg L -1 in the control. Composted SS caused smaller changes in contaminant mobility and reduced porewater Cd and Zn in the polluted layer by 27% and 35%, respectively. It also supported the highest Lolium perenne biomass and bacterial diversity, whereas both were lower with hydrochar. nZVI generally reduced metal(loid) mobility, particularly for As, although its effectiveness varied among elements. Importantly, the stratified system showed that decreasing contaminant concentrations in topsoil did not necessarily indicate immobilisation, as deeper layers revealed redistribution in some treatments. Considering both contaminant control and biological performance, composted SS showed a more favourable remediation outcome than hydrochar.