Integrated bioprocess for Se(VI) remediation using duckweed: Coupling selenate removal to biogas production

Abstract

The use of phytoremediation as a method for wastewater treatment or removal of pollutants is garnering significant interest and duckweed (DW), a free floating macrophyte, depicts significant potential for the removal of nutrients and toxic compounds from contaminated waters. The present work aimed to develop an integrated process for remediating selenate (Se(VI)) using DW biomass and subsequent use of Se(VI) enriched DW for biogas production. The main objective is to extend the application of selenium (Se) enriched DW biomass for biogas production. Se(VI) enriched DW biomass (Se-DW) gave higher methane production (48.38 ± 3.6 mL gCOD-1) than control DW biomass (C-DW) (24.46 ± 3.6 mL gCOD-1). To further enhance methane production, three pre-treatment approaches (acid, alkali and hydrothermal) were assessed and the solid and liquid fractions obtained after pre-treatment were used as a substrate. Pre-treatments increased biogas production in both Se-DW and C-DW than untreated conditions. Liquid fractions gave higher biogas production than solid fractions. In Se-DW, highest biogas production was observed in hydrothermal pre-treated Se-DW, while in C-DW, acid pre-treatment gave higher biogas production. Methane production was shown to be enhanced up to a Se(VI) concentration of 1.7 mg L-1, whereas a concentration beyond this lowered biogas production. There is a substantial release of Se oxyanions, selenite (Se(IV)) and selenate (Se(VI)), into the environment causing severe pollution of both terrestrial and aquatic environments. Se although is required in minute quantities for normal functioning of living beings, its excessive accumulation could lead to Se poisoning. Current work focuses on developing an integrated process for remediating selenate (Se(VI)) using duckweed biomass and subsequent use of Se(VI) enriched duckweed for biogas production. In a practical implication, this study shows a win-win scenario, where a variety of plant crops could potentially be used in constructed wetlands to uptake Se for bio-fortification. However, when these plant crops accumulate Se in higher concentrations, the harvested biomass could be effectively managed by co-digesting them with other feedstocks to obtain biogas.

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