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 HMs 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, HMs, decomposition, and volatile characteristics. The HCl-pretreated 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. The favorable pyrolysis and gasification behaviors of PV were maintained via the HCl-pretreated PV, with the raised and narrowed temperature range of mass loss but with the increased energy demand for the decomposition. Compared with PV (276.74 kJ/mol), 5% HCl-pretreated 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 the HCl-pretreated 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. Unlike the two atmospheres, the acid pretreatments changed the temperature dependency of volatile products during the main reaction-temperature range. The volatile products which released from the pyrolysis and gasification at the temperature of maximum mass loss peak or 350 °C were collected. 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, and Pb at a low leachable rate. The best joint optimization was achieved with the combined settings of 5% HCl-pretreated PV or 10% H3PO4-pretreated PV at 10 °C/min in the N2 atmosphere. Overall, findings provide new insights into how to best manage and valorize post-harvest and HM-laden hyperaccumulators.