Significant role of sludge-derived organic matter in enhancing Cr (III) immobilization by jarosite: Insights from molecular selective adsorption and co-precipitation.

The high mobility of chromium (Cr) caused by acid mine drainage (AMD) poses a serious threat to the aquatic environment. Weakly crystalline minerals can passivate Cr and modify its environmental behavior. Dissolved organic matter (DOM) plays an important role in synergizing the in-situ immobilization of Cr by minerals. However, few studies have explored the molecular-level impact of DOM on Cr immobilization by inducing weakly crystalline minerals transformation. Here, we investigated the effects and mechanisms of sludge-derived DOM (with rich molecular diversity) on Cr immobilization by jarosite (KJ, widely present in AMD). The results showed that sludge-derived DOM significantly changed the speciation distribution of Cr and Fe. The maximum enhancement of Cr immobilized via adsorption and precipitation was 63.43 % and 161.89 %, respectively. Synchronous fluorescence analysis showed that the fractionation of high-oxygenated aromatic compounds promoted Cr adsorption on KJ. FT-ICR-MS spectra suggested that low-oxygenated aromatic and aliphatic compounds promote Cr immobilization in secondary ferrihydrite by capturing free Cr, followed by coprecipitation with dissolved Fe ions. The molecular-level finding in this study will facilitate the deep understanding of the molecular selectivity of sludge organics in remediating heavy metal-contaminated environments, and is of great significance for realizing the resource utilization of enormous waste sludge.