Doped carbon dots affect heavy metal speciation in mining soil: changes of dissimilated iron reduction processes and microbial communities

Abstract

Carbon dots (CDs) inevitably accumulate in soils due to their widespread use, affecting soil enzyme activities and microbial community structure. Soil microbial-mediated dissimilatory iron reduction processes will affect soil elemental cycling and heavy metal transport and transformation. Yet few studies have characterized changes in dissimilatory iron reduction and speciation transformation of heavy metals following application of CDs to the soil system. Here, the linkage between heavy-metal speciation changes and CD imposition was investigated by incubating two types of doped carbon dots (nitrogen-doped and nitrogen–phosphorus-doped) into mine soils. Results showed that the application of nitrogen–phosphorus-doped carbon dots (N,P-CDs) was beneficial to improving soil acidity. In addition, the application of CDs immobilized Pb and Zn but also increased the mobility of Cu in the soil. Furthermore, the addition of CDs increased Fe(II) and total Fe content, suggesting that it promoted dissimilated iron reduction. The activity of dehydrogenase was activated by CDs, while the activation of catalase was not significant. The application of CDs altered the structure and diversity of soil microbial communities with a dependence on the type of CDs and the time of incubation. Combined with the redundancy analysis (RDA) results, the microbial community was tightly related to pH, residual Zn concentration, Fe(II), and total Fe. The results of this study contribute to the understanding of the role of CDs in geochemical element cycling and remediation of heavy metal contaminated soils.