Pyrolysis of industrial hemp biomass from contaminated soil phytoremediation: Kinetics, modelling transport phenomena and biochar-based metal reduction

Phytoremediation is the use of vegetation for the in situ treatment of contaminated environments. After plants have been used for phytoremediation of soils, their biomass can be used for example as value-added products or converted by thermochemical processes. Large-scale application of pyrolysis technology for phytoremediation biomass requires accurate predictive kinetic models and a characterization of the toxicity of the materials produced. The pyrolysis of industrial hemp (Cannabis sativa L.) was investigated on a laboratory scale by varying the process conditions and accurately modelled by considering four pseudo-components with first reaction order. The average value of the coefficients of determination is 0.9980. Biomass and biochar were characterized and the main components of the gas phase were monitored. We found Cd, Pb, and Zn in the roots, although in lower amounts than in the soil. Especially the leaves and stems showed negligible traces of these elements, so that these parts can be used directly, even if the hemp was grown on the polluted soil. After pyrolysis, the concentration of pollutants in the solid fraction decreased, which could be attributed to the reduction of metal oxides (or salts) to elemental form and subsequent evaporation. This pyrolysis process has the potential to treat heavy metal-rich biomass, with gas phase purification via condensation, yielding agricultural-grade biochar, CO-rich gas and a highly concentrated heavy metal stream in absorbent material.