Hydrothermal Carbonization of Residual Algal Biomass for Production of Hydrochar as a Biobased Metal Adsorbent

Conversion of residual algal biomass to value-added products is essential for enhancing the economics of algae cultivation. Algal hydrochar produced via hydrothermal carbonization of lipid-extracted Picochlorum oculatum is a material rich in oxygen functional groups and carbon (up to 67.3%) and hence a promising candidate for remediation of wastewaters. The hydrothermal carbonization conditions were optimized and the adsorption capacity of the hydrochar was tested for metal removal. By the end of the remediation process, cumulative removal of Al3+, Cu2+, Fe2+, Mg2+, Mn2+, and Pb2+ reached 89, 98, 75, 88, 75, and 100%, respectively. The adsorption of all metals was found to follow pseudo second-order kinetics and the Langmuir isotherm. Overall, when hydrothermal carbonization is applied to lipid-extracted algae, it generates a promising biobased adsorbent with value-added potential in metal remediation.

Lipid-Extracted Algae as a Soil Amendment Can Increase Soil Salinization and Reduce Forage Growth

Algae as a biodiesel feedstock are more productive per unit area than traditional feedstock options, but currently algae production is not economical without high-value co-products. Lipid-extracted algae (LEA) may be useful as a soil amendment; however, research is needed to determine the feasibility and management strategies required. The objective was to determine salinity-associated effects of LEA as a soil amendment on a range of salt tolerant forages [foxtail millet (Setaria italica L.), pearl millet (Pennisetum glaucum L.), and a sorghum-sudangrass hybrid (Sorghum bicolor L. Moench)]. Forage seed were planted in columns containing sandy clay loam soil amended with the following: (1) control [nitrogen (N) and phosphorus (P) fertilizer added], (2) 1.5% LEA by weight, (3) 3.0% LEA, (4) 1.5% LEA + 1.5% wheat straw (WS), and (5) 1.5% WS (+N, +P). Seedling emergence and total herbage mass (HM) from sequential harvests was determined, along with forage mineral uptake. Soil pH and electrical conductivity (EC) were analyzed after the final harvest. Seedling emergence of pearl millet was negatively affected by LEA application, but not foxtail millet or sorghum-sudangrass. Pearl millet emergence was most reduced in 3.0% LEA treated soil, however, this treatment produced greater HM by the third harvest. Soil pH and EC were greater in 3.0% LEA-treated soil than the control and 1.5% WS treatment. Production of salt tolerant forages such as sorghum-sudangrass is possible in LEA-amended soil; however, with repeated applications soil salinity may reduce productivity and sustainability.

An Assessment of Gasification of Lipid-Extracted Algae by Thermodynamic Simulation

Thermodynamic simulation of gasification of Nannochloropsis gaditana (after lipid extraction and before lipid extraction) over a wide range of temperatures (800 – 1400 oC) was performed in this study. The reactor temperature variation with respect to the O2 to fuel ratio and the syngas composition with temperature were evaluated for gasification performance. The results showed the H2/CO ratio for lipid-extracted algae (LEA) was very similar to the gasification behavior of raw algae (RA) before lipid extraction. Only a slight variation was observed for the lower heating value (9.2 MJ/Nm3 for RA and 8.9 MJ/Nm3 for LEA at 1000 oC) of the syngas from the two feedstocks. The H2/CO ratio for both RA and LEA remained almost the same over the range of temperatures (800-1400 oC) under consideration. The cold gas efficiency was found between 82 % to 75 % for RA and 75 % to 69 % for LEA at 1000 oC and 1400 oC respectively. As the overall gasification performance did not degrade significantly after extraction of lipid from algae, the LEA holds remarkable potential as a gasification feedstock in a biorefinery set up.Journal of Chemical Engineering, Vol. 30, No. 1, 2017: 59-63