Biochar vs magnesite amendments for metals immobilization: lab-scale and field experiments

The process of industrial change has resulted in the creation of so-called ‘brownfields’ across Europe, particularly in urban areas, in the industrial sections of cities. The need to recover these brownfields can be linked to the new European Commission program "Zero Wastes", that is, to restore or recondition these areas by applying amendments made with by-products or green elements. In this sense, the capacities of magnesite and biochar, inorganic and organic soil amendments respectively, were tested to reduce metal availability and improve the properties of a soil severely contaminated by Cu, Cd, Pb and Zn. To this end, two implementation steps were performed.

First, 1 kg pots containing the polluted soil were amended with either magnesite or biochar and then determined metal availability and soil properties at days 15 and 75 in a greenhouse experiment. In addition, to evaluate the impact of the two treatments on plant growth, the experimental trials were carried out using Brassica juncea L. and compost addition. Both amendments, but particularly magnesite, markedly decreased metal availability. Soil properties were also improved, as revealed by increases in the cation exchangeable capacity. However, plant growth was inhibited by magnesite amendment. This effect was probably due to an increase in soil pH, cation exchange capacity and a high Mg concentration. In contrast, biochar increased biomass production whereas decreased the content of metals harvested. Then, a field scale experiment was performed in situ by means of treating 1 ton of the soil with the magnesite and also with the biochar. Brassica juncea L. was used for testing the impact on plants, and the experiment was monitored at 3, 30 and 60 days from the beginning of the experiment. Similar results to the greenhouse experiment were obtained.

In conclusion, the results indicate that magnesite amendment may be suitable 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, its use emerges as a promising tool for restoring soil properties and thus favoring plant growth.

Acknowledgment

This work was supported by the research projects NANOBIOWASH CTM2016-75894-P (AEI/FEDER, UE) and NANOCAREM MCI-20-PID2019-106939GB-I00 (AEI/FEDER, UE).

Diego Baragaño obtained a grant from the “Formación del Profesorado Universitario” program, financed by the “Ministerio de Educación, Cultura y Deporte de España”.

Arenas-Lago D. thanks to his postdoc contract ED481D 2019/007 (Xunta de Galicia and Universidade de Vigo).