Classifying the Potential of Biochars from Agricultural and Industrial Waste for the Recovery of Fe and Mn Mining Tailings

Various types of agricultural and industrial waste have been used for biochar production. However, a consistent method of biochar stratification for agronomic and environmental purposes is still lacking. The aim of this study was to propose and evaluate a method of stratification based on physical and chemical properties. A set of biochar samples was produced by pyrolysis and co-pyrolysis from two agricultural wastes (poultry litter - PL and swine manure -SM) and three industrial wastes (construction wood - CW, tyres - TR and PVC plastic - PVC) at a wide range of pyrolysis temperatures (300-700 degrees C). The proposed stratification considered the following criteria in order: (i) construction of a biochar viability indicator (BVI); (ii) classification of biochar potential based on fixed carbon, ash and volatile matter; (iii) joint use of biochar quality criteria and those established in the literature (e.g. International Biochar Initiative - IBI); and (iv) the restrictive conditions of the environment where the biochar might be applied, in this case mining tailings, aiming at plant development. Using the proposed method of stratification, PL+CW pyrolysed at 300 degrees C was chosen for the recovery of Fe and Mn mining tailings due to its intermediate fertility level and extremely low risk of environmental pollution. The proposed method allowed the identification of restrictions on biochar use, and most importantly, highlighted the potential of the most viable biochars for conditioning mining tailings, which hinder plant growth. However, despite being useful and efficient in biochar selection, the proposed methods are still dependent on the physico-chemical characteristics of the biochars.