Short-term experiment for the in situ stabilization of a polluted soil using mining and biomass waste.

Mining and biomass waste were used to remediate a brownfield affected by As, Cd, Cu, Pb and Zn pollution in a pilot scale experiment, and a plant used for phytoremediation purposes was used as an indicator of possible toxicological effects. To carry out the experiments, plots in field conditions were treated with magnesite (mining waste), magnesite-sludge compost, and magnesite-biochar respectively, while untreated soil was used as a control. The plots were then irrigated and left for one week, after which seeds of the ryegrass Lolium perenne L. were sown. Soil properties such as metal(loid) availability, pH, phosphorus availability, total nitrogen, organic carbon, and nutrients were monitored for two months. Finally, the ryegrass was harvested and pollutant concentrations were analyzed in the aerial parts. Magnesite proved to be an excellent amendment for metal(loid) immobilization, although the notable increase in soil pH and Mg content inhibited plant growth. However, the application of magnesite in combination with the sludge compost (rich in N and P) favored plant growth and also immobilized metals, although As availability increased. In contrast, the analysis of plants in this treatment revealed lower As and metal concentrations than those grown in the untreated soil. In turn, the application of magnesite and biochar was also effective in reducing metal(loid) availability; however, the plants did not grow under these conditions, probably due to the low N and P content of biochar. In this regard, the combined application of mining waste and sludge compost emerges as a useful nature-based solution for soil remediation in the context of the circular economy.

Comparison of the effectiveness of biochar vs. magnesite amendments to immobilize metals and restore a polluted soil.

Here we addressed the remediation of a soil severely contaminated by Cu, Cd, Pb and Zn. In this regard, we tested the capacity of magnesite and biochar, inorganic and organic soil amendments, respectively, to reduce metal availability and improve soil properties. To this end, 1-kg pots containing the polluted soil were amended with either magnesite or biochar. Metal availability and soil properties were then measured at days 15 and 75. Also, to evaluate the impact of the two treatments on plant growth, we conducted experimental trials with Brassica juncea L. and compost addition. Both amendments, but particularly magnesite, markedly decreased metal availability. Soil properties were also enhanced, as reflected by increases in the cation exchangeable capacity. However, plant growth was inhibited by magnesite amendment. This observation could be attributable to an increase in soil pH and cation exchange capacity as well as a high Mg concentration. In contrast, biochar increased biomass production but decreased the quantity of metals recovered when the plants are harvested. In conclusion, on the basis of our results, we propose magnesite as a suitable approach for stabilizing contaminated soils (or even spoil heaps) where revegetation is not a priority. In contrast, although biochar has a lower, but still significant, capacity to immobilize metals, it can be used to restore natural soil properties and thus favor plant growth.