Converting and Valorizing Heavy Metal-Laden Post-Harvest Hyperaccumulator (Pteris Vittate L.) into Biofuel Via Acid-Pretreated Pyrolysis and Gasification

Abstract

Not only should post-harvest hyperaccumulators rich in heavy metals (HMs) be properly disposed to avoid secondary HM pollution, but also they should be valorized to enhance circular economy. This study aimed to characterize how As-hyperaccumulator (Pteris vittate L.) (PV) pretreated with HCl or H3PO4 affected its physicochemical, HM, decomposition, and volatile characteristics. 5% or 10% HCl-pretreated PV (HCl-PV) retained its original main components, physical properties, and chemical structures but introduced Cl to carbon chain, induced O loss, increased C content, and removed most minerals, in particular, alkali/alkaline earth metals (AAEMs). This in turn helped to maintain the favorable pyrolysis and gasification behaviors of PV, with the raised and concentrated weightlessness temperature range, but demanded more energy than did PV. Compared with PV (276.74 kJ/mol), 5% HCl-pretreated PV (5%HCl-PV) reduced activation energy of its pyrolysis (260.62 kJ/mol). The H3PO4 pretreatment destroyed carbon chain, loaded phosphorus oxygen group, and removed more organics and minerals in PV than did HCl-PV. This in turn allowed for an earlier start and finish of devolatilization stage, an easier breaking of potential energy barrier, and improvement of reaction favorability.   HCl-pretreated PV reduced the formation of ring-opening products, while H3PO4-pretreated PV emitted more aromatic compounds and selectively generated ketone, such as levoglucanone. More HMs were dissolved with the increased acid concentration, with HMs in HCl-pretreated PV being at a higher leaching concentration than those in the other treatments. H3PO4-pretreated PV retained As, Cd,  Pb at a low leachable rate. The best joint optimization was achieved with combined settings of 5%HCl-PV or 10% H3PO4-PV, 10 oC/min,  N2 atmosphere. Overall, findings provide new insights into how to best manage and valorize post-harvest and HM-laden hyperaccumulators.

Keywords: Hyperaccumulator, Acid pretreatment, Pyrolysis and gasification, TGA-IST-FTIR-GC/MS system, Arsenic, uranium