Soil pollution by trace element is a worldwide concern. One possibility for the remediation of these polluted soils is the application of organic amendments. Nevertheless, organic amendments are very diverse and their properties need to be studied prior to their use. Biochar, the solid residue produced by thermal treatment of biomass under absence of oxygen (pyrolysis), has diverse properties depending on feedstock and pyrolysis conditions (Campos et al., 2020). In this work, biochars were produced under different pyrolysis conditions, pyrolysis reactor and feedstock. The continuous pyrolysis reactor required only 12 minutes of residence time of the feedstock in the reactor to produce biochars with similar properties than those pyrolysed during at least 1 h in the fixed bed reactor. Produced biochars were tested for metal adsorption in batch experiment and greenhouse experiment. Among the biochars used, rice husk biochar may be considered the better tool to reduce metal pollution, as it showed a maximum adsorption capacity of 30.7 and 19.3 mg g-1 for Cu2+ and Pb2+, respectively. The most promising biochars for degraded soils remediation (rice husk biochar and olive pit biochar) were used as amendment (8 t ha-1) at a field experiment. The plots were located in Aznalcóllar (Spain). These soils were polluted in April 1998 by the massive dumping of mine sludge contaminated with heavy metals, called the Aznalcóllar disaster. Additionally, biochar was buried in nylon bags to determine changes in biochar composition and physical properties due to the ageing process. The experiment lasted 22 months and soil properties, enzymatic activities and plant development were periodically analysed. The biochar potential for carbon sequestration was also analysed. Soils were sampled after 6, 12 and 20 months for determining microbial diversity using the Illumina Miseq technology of the 16S rRNA gene. The application of 8 t ha-1 of RHB and OPB (produced at the continuous pyrolysis reactor) at field recovered soil ecological functionality in the studied polluted soils. The studied biochars significantly adsorbed the trace elements present in the studied soils after 2 years. Nevertheless, in these highly multicontaminated soils, the application of 8 t ha-1 of biochar was not enough for an effective reduction in trace element bioavailability, which is explained by the existence of an equilibrium in such polluted soils. Campos P., Miller A.Z., Knicker H., Costa-Pereira M.F., Merino A. & De la Rosa J.M. (2020) - Chemical, physical and morphological properties of biochars produced from agricultural residues: Implications for their use as soil amendment. J. Waste Manag.,105, 256-267.

The former Spanish Ministry of Economy, Industry and Competitiveness (MINEICO) and AEI/FEDER are acknowledge for funding the project CGL2016-76498-R.

Abstract Congresso SGI-SIMP 2022. Geosciences for a sustainable future, 19th-21st september 2021 (Italia)

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