Phase partitioning of mercury, arsenic, selenium, and cadmium in Chlamydomonas reinhardtii and Arthrospira maxima microcosms.

As the global human population increases, demand for protein will surpass our current production ability without an increase in land use or intensification. Microalgae cultivation offers a high yield of protein, and utilization of wastewater from municipal or agricultural sources in place of freshwater for microalgae aquaculture may increase the sustainability of this practice. However, wastewater from municipal and agricultural sources may contain contaminants, such as mercury (Hg), cadmium (Cd), selenium (Se), and arsenic (As). Association of these elements with algal biomass may present an exposure risk to product consumers, while volatilization may present an exposure hazard to industry workers. Thus, the partitioning of these elements should be evaluated before wastewater can be confidently used in an aquaculture setting. This study explored the potential for exposure associated with Arthrospira maxima and Chlamydomonas reinhardtii aquaculture in medium contaminated with 0.33 µg Hg L-1, 60 µg As L-1, 554 µg Se L-1, and 30 µg Cd L-1. Gaseous effluent from microalgae aquaculture was analyzed for Hg, As, Se, and Cd to quantify volatilization. A mass balance approach was used to describe the partitioning of elements between the biomass, medium, and gas phases at the end of exponential growth. Contaminants were recovered predominantly in medium and biomass, regardless of microalgae strain. In the case of Hg, 48 ± 2% was associated with A. maxima biomass and 55 ± 8% with C. reinhardtii when Hg was present as the only contaminant, but this increased to 85 ± 11% in C. reinhardtii biomass when As, Se, and Cd were also present. A small and highly variable abiotic volatilization of Hg was observed in the gas phase of both A. maxima and C. reinhardtii cultures. Evidence presented herein suggests that utilizing wastewater containing Hg, Cd, Se, and As for microalgae cultivation may present health hazards to consumers.

Nutrient recovery of the hydrothermal carbonization aqueous product from dairy manure using membrane distillation.

Phosphorus is a crucial resource for the agricultural industry, but its limited supply requires recovery from waste materials before it is lost and leads to eutrophication. Dairy manure is rich with phosphorus, and the growth and consolidation within the dairy industry has led to dairy manure management becoming a significant concern. Hydrothermal carbonization (HTC) and membrane distillation (MD) were investigated as an alternative to treat dairy manure and recover nutrients, specifically phosphorus and nitrogen. HTC is a thermal treatment process that converts organic matter into a hydrochar analogous to a low-grade coal, and MD is a thermally-driven separation process that can utilize low-grade waste heat from HTC, thus the two processes are synergetic. A byproduct of the HTC process is the aqueous product (HAP) that contains the water-soluble nutrients and organic components of dairy manure. In this work, the efficacy of MD to concentrate the nutrients in the presence of dissolved organic carbon was assessed. Samples included synthetic nutrient-rich streams as well as HAP produced at HTC temperatures ranging from 200 °C to 260 °C. In each case, the nutrients were successfully concentrated in the feed loop with rejections >99%. Dissolved carbon was found to foul the MD membrane at levels proportional to its hydrophobicity, with little fouling observed for glucose and substantial fouling observed for HAP solutions created at higher temperatures.