Valorization potential of N-rich zeolite and moving bed biofilm reactor (MBBR) biomass in the revegetation of non-acid generating gold mine tailings.

Treatment of ammonia nitrogen (NH-N) in mine effluents generates N-rich residual materials, such as moving bed biofilm reactor (MBBR) biomass and spent zeolite. Using them as substitutes for mineral fertilizers in revegetation of mine tailings avoids disposal and contributes to a circular economy. The study evaluated the effect of MBBR biomass and N-rich zeolite amendments on above- and below-ground growth and foliar nutrient and trace element concentrations of a legume and several graminoid species grown on non-acid generating gold mine tailings. N-rich zeolite (clinoptilolite) was produced by treating saline (up to 60 mS/cm) synthetic and real mine effluents (250 vs 280 mg/L NH-N). A three-month pot experiment was conducted with a dose of tested amendments equivalent to 100 kg/ha N and compared to unamended tailings (as negative control), tailings with a mineral NPK fertilizer, and a topsoil (as positive controls). Higher foliar N concentrations were found in amended and fertilized tailings vs negative control, but N was less available in the zeolite treatments than in other tailings treatments. For all plant species, the mean leaf area and above-ground, root, and total biomasses were similar in the zeolite-amended tailings to the unamended tailings, while the MBBR biomass amendment resulted in similar above- and below-ground growth to the NPK fertilized tailings and the commercial topsoil. Trace metal concentrations in water leaching from the amended tailings remained low, but tailings amended with zeolite exported NO-N concentrations up to 10 times greater (>200 mg/L) after 28 days compared to all other treatments. Foliar Na concentrations in zeolite mixtures were six to nine times higher than in other treatments. The MBBR biomass is a promising potential amendment for revegetation of mine tailings. However, Se concentrations in plants after MBBR biomass amendment should not be underestimated, while Cr transfer from tailings to plants was observed.