Valorization of Phytoremediation Byproduct via Synthesis of Biodiesel from Cockspur Grass (Echinochloa crus-galli) Seed

Phytoremediation is an ecofriendly remediation practice for contaminated soils. This study evaluated the efficiency of cockspur grass (Echinochloa crus-galli) as a phytoremediation plant species by measuring the heavy metal(loid) uptake. In addition, for determining a viable route to valorize the phytoremediation byproduct, biodiesel was produced by the conversion of contaminated cockspur grass seeds through noncatalytic transesterification with a porous medium. From a series of experimental studies, this study confirmed that a substantial amount of heavy metal(loid)s was translocated into cockspur grass, and their removal efficiency after 3 weeks was as follows: Ni ≥ Cu > Co > Zn > Mn > Fe > As. Prior to the transesterification study, lipid content (11.6 wt %), a feedstock for biodiesel production, in cockspur grass seeds was estimated by the thermogravimetric analysis and the Soxhlet extraction test. Because the lipid extract from cockspur grass seeds contained a substantial amount of organic and inorganic impurities, biodiesel synthesis from a conventional transesterification method was negligible. However, biodiesel yield from noncatalytic transesterification using a porous medium reached 98% at 390 °C. The yield of biodiesel decreased at ≥400 °C due to biodiesel thermal cracking. This study also revealed that the noncatalytic transesterification of cockspur grass seeds into biodiesel was affected by the steric factors (collision frequency), evidenced by the carbon chain length of the acyl acceptor that was inversely proportional to the biodiesel yield.