Metal(loid) immobilization effects and the potential mechanisms by biological crusts in a molybdenum tailings pond

Biological crusts (BCs) are usually initially colonized in the natural restoration of tailings pond and are recognized as “ecological engineers” in degraded ecosystems. The present study is to investigate the immobilization effects of metal(loid)s (Mo, Ni, Zn, Cd, Pb, Cr, Cu, and As) by the BCs and reveal the potential metal(loid) immobilization mechanisms.

Two successional stage BCs (cyanobacteria-moss mixed crust and moss crust) in a molybdenum tailings pond were sampled, and the bare tailings were set as the control.

Two BCs could effectively accumulate and immobilize metal(loid)s in tailings, and over 90% of immobilized metal(loid)s existed as the biosorbed and non-EDTA-exchangeable fractions. And the moss crust showed a higher metal(loid) accumulation capacity than that of mixed crust. The adsorption (clay minerals, organic functional groups), co-precipitation (Fe/Mn oxides, mineral lattice), and the aggregation of tailings particles by microbial mycelia were the potential mechanisms for BCs to immobilize metal(loid)s. In addition, BCs significantly enhanced the nutrient levels of the topsoil in tailings pond. The results of redundancy analysis (RDA) and Pearson’s correlation analysis (PCA) showed that among the physicochemical factors that affected the metal(loid) bioavailability in BCs, pH was positively correlated with DTPA-extracted metal(loid)s, while clay particles and TOC showed a negative correlation.

BCs shows the dual roles in improving nutrients and immobilizing metal(loid)s of tailings ponds, which enriches the ecological functions of BCs of tailings ponds and is helpful for remediating tailings ponds via promoting BCs formation with artificial strengthening measures.