Inherent Metals of a Phytoremediation Plant Influence Its Recyclability by Hydrothermal Liquefaction

Plants used for phytoremediation of contaminated soil are often enriched in certain metals present in the soil. However, the role of the inherent metal content of the plants on their recycling by hydrothermal liquefaction (HTL) has not been considered in previous studies. The present study showed that Rhus chinensis plants grown in highly Pb-polluted soil can release inherent metals (such as Pb, K, Ca, Na, and Mg) into the HTL solution, further enhancing the production of formic and acetic acids and decreasing the yield of levulinic acid. Theoretical calculations using HTL reactions of model compounds showed that a low Pb content could enhance production of levulinic and formic acids via catalysis of the rehydration reaction for 5-(hydroxymethyl)furfural, while a high Pb content promoted the decomposition of levulinic acid to acetic acid. Fourier transform ion cyclotron resonance mass spectrometry analysis confirmed that Pb2+ preferentially promoted the depolymerization of macromolecular compounds with the lignin structure. In general, the inherent metals occurring in the phytoremediation plant influenced the production of organic acids during HTL recycling. Undoubtedly, the combination of phytoextraction and HTL reaction can present a practical pathway toward a sustainable soil remediation technology.