Overuse of antibiotics has resulted in widespread contamination of the environment and triggered antibiotic resistance in pathogenic bacteria. Conventional wastewater treatment plants (WWTPs) are not equipped to remove antibiotics. Effluents from WWTPs are usually the primary source of antibiotics in aquatic environments. There is an urgent need for cost-effective, environment-friendly technologies to address this issue. Along with antibiotics, nutrients (nitrogen and phosphorus) are also present in conventional WWTP effluents at high concentrations, causing environmental problems like eutrophication. In this study, we tested vetiver grass in a plug flow reactor-based constructed wetland model in a greenhouse setup for removing antibiotics ciprofloxacin (CIP) and tetracycline (TTC), and nutrients, N and P, from secondary wastewater effluent. The constructed wetland was designed based on a previous batch reaction kinetics study and reached a steady-state in 7 days. The measured concentrations of antibiotics were generally consistent with the modeling predictions using first-order reaction kinetics. Vetiver grass significantly (p < 0.05) removed 93% and 97% of CIP and TTC (initial concentrations of 10 mg/L), simultaneously with 93% and 84% nitrogen and phosphorus, respectively. Results show that using vetiver grass in constructed wetlands could be a viable green technology for the removal of antibiotics and nutrients from wastewater.

Selenate is a common contaminant in agricultural drainage from areas with seleniferous soils. Microbes can convert selenate to elemental selenium nanoparticles, which may be recovered as a valuable resource. One challenge in the recovery of selenium from agricultural drainage is the coexistence of sulfate at very high concentrations. Studies have shown that sulfate may inhibit selenate reduction and lead to the precipitation of selenium sulfides, thus hindering the recovery of elemental selenium nanoparticles. It was found in this work that the hydraulic retention time (HRT) determined the effects of sulfate on selenate reduction by controlling the composition and spatial distribution of the selenium products in a H₂-based membrane biofilm reactor. At a HRT of 0.28 days, 99% of selenate was reduced to elemental selenium nanoparticles precipitated in the reactor and suspended in effluent, which was desirable for recovery. When the HRT was decreased to 0.14 days or smaller, selenium sulfides became the dominant particulate selenium product in the effluent. Interestingly, elemental selenium was always the dominant particulate selenium product in the biofilm regardless of HRT owing to further biological reduction of selenium sulfides to elemental selenium and sulfide. At HRTs of 0.28 and 0.14 days, famous selenate and selenite reducers such as Rhodocyclaceae (including Azospira oryzae) and Rhizobium sp. were enriched in the biofilm. Decreasing HRT to 0.07 days resulted in electron donor limitation, which further led to the biofilm dominated by Desulfobulbaceae that is well known for its ability to reduce sulfur and selenium oxyanions and the enrichment of sulfide-oxidizing bacteria.

Vetiver grass (Chrysopogon zizanioides) is well-known for its contaminant phytoextraction potential and its capacity to reduce soil erosion, owing to its massive, dense root system. However, the shoots are not major contributors to either of these processes, and are either not utilized at all or they become part of the waste stream. It is well-recognized that lignocellulosic biomass can serve as a source of raw material to produce second-generation bioethanol. This study investigated the simultaneous saccharification and fermentation (SSF) of acid–alkali pretreated vetiver (VG) shoots by Saccharomyces cerevisiae. Vetiver shoots were obtained from three sources: (1) shoots from VG grown in clean potting soil, (2) shoots from VG used for antibiotics phytoextraction from a constructed wetland setup, and (3) shoots from VG used for lead phytoextraction during soil remediation. Bioethanol yield from the shoots from clean soil was the highest (19.58 g/L), followed by the one used for lead phytoextraction (19.50 g/L) and the one used for antibiotics phytoextraction (19.17 g/L). Bioethanol yield and quality obtained from these three VG shoots was superior or similar to other C4 grasses used for bioethanol generation. This study successfully demonstrated that spent vetiver biomass after phytoextraction applications can be repurposed to generate high-quality bioethanol.