Chemical and Biological Characterization of Wastewater Generated from Hydrothermal Liquefaction ofSpirulina

Abstract

Hydrothermal liquefaction (HTL) is an attractive method for converting wet biomass into petroleum-like biocrude oil that can be refined to make petroleum products. This approach is advantageous for conversion of low-lipid algae, which are promising feedstocks for sustainable large-scale biofuel production. As with natural petroleum formation, the water in contact with the produced oil contains toxic compounds. The objectives of this research were to: (1) identify nitrogenous organic compounds (NOCs) in wastewater from HTL conversion of Spirulina; (2) characterize mammalian cell cytotoxicity of specific NOCs, NOC mixture, and the complete HTL wastewater (HTL-WW) matrix; and (3) investigate mitigation measures to reduce toxicity in HTL-WW. Liquid-liquid extraction and nitrogen-phosphorus detection was used in conjunction with gas chromatography-mass spectrometry (GC-MS), which detected hundreds of NOCs in HTL-WW. Reference materials for nine of the most prevalent NOCs were used to identify and quantify their concentrations in HTL-WW. Mammalian cell cytotoxicity of the nine NOCs was quantified using a Chinese hamster ovary (CHO) cell assay, and the descending rank order for cytotoxicity was 3-dimethylamino-phenol > 2,2,6,6-tetramethyl-4-piperidone > 2,6-dimethyl-3-pyridinol > 2-picoline > pyridine > 1-methyl-2-pyrrolidinone > σ-valerolactam > 2-pyrrolidinone > ε-caprolactam. The organic mixture extracted from HTL-WW expressed potent CHO cell cytotoxic activity, with a LC(50) at 7.5% of HTL-WW. Although the toxicity of HTL-WW was substantial, 30% of the toxicity was removed biologically by recycling HTL-WW back into algal cultivation. The remaining toxicity of HTL-WW was mostly eliminated by subsequent treatment with granular activated carbon.

Characterization of aqueous phase from the hydrothermal liquefaction of Chlorella pyrenoidosa

“…Among the three samples, the sample ERS 1 contained the most species of organic acids while ERS3 contained the most species of N&O-heterocyclic compounds. Pham et al (2013) also observed the same trend and could correlate the presence of N&O-heterocyclic compounds to high toxicity. Tommaso et al (2015) examined the anaerobic degradability of the aqueous product from HTL of mixed-culture algal biomass grown in a wastewater treatment system, and it was reported that as low was the area related to N&O-heterocyclic compounds as high would be the anaerobic degradability.…”

“…These studies also found that HTL AP has some inhibitory effect on algal growth probably due to some toxic organic compounds, and therefore various rates of dilution of HTL AP or further treatment is needed. Pham et al (2013) found high inhibitory potential related to nitrogenous organic compounds in AP from HTL of microalgae Spirulina and observed that granular activated carbon was effective in removing the toxic effect from the effluent. These findings all leads to the questions including: what is the range of the organic and nutrient content/concentration in HTL AP?…”

An investigation of reaction pathways of hydrothermal liquefaction using Chlorella pyrenoidosa and Spirulina platensis

“…Additionally, Tables S3 and S4 illustrated that nitrogenous organic compounds (NOCs) were detected in all aqueous fraction samples, such as 2-pyrrolidinone (RT 7.58), 2-piperidinone (RT 27.03) and 3-pyridinol (RT 31.80), which are typical NOCs detected in the HTL process of microalgae [7,11,12,30]. The results indicated that many classes of NOCs with methyl groups were detected, such as pyrazine, methyl-(RT 7.58), 2,5-PyrrolidinSedione, 1-methyl-(RT 22.67) and 3-pyridinol, 6-methyl-(RT 31.51) in aqueous fraction of CP and SP.…”

“…Small amounts of 2-pyrrolidinone, 1-methyl-(RT 18.06) was also observed in the aqueous sample of CP at higher temperatures (300-320°C). According to Pham et al [7], the compounds with two or more methyl groups are more toxic than those with one or no alkyl substitutions, which enhance the resistance of the compound to biological degradation [34] and alter equilibrium partitioning of compounds between cell and medium [35]. In the present study, NOCs with two methyl groups were detected in aqueous samples, such as pyrazine, 2,5-dimethyl-(RT 9.13) for CP and SP, 3-pyridinol, 2,6-dimethyl-(RT 29.57) for CP at higher temperatures (280-320°C) and pyrazine, 3,-ethyl-2,5-dimethylfor SP at the temperature range of 200-320°C.…”

“…Hydrothermal liquefaction (HTL) of microalgae to produce biofuels have many benefits in terms of higher conversion efficiency due to the properties of sub/supercritical water as the reaction medium, without a drying pretreatment that can reduce the energy consumption [1][2][3]. Previous studies [4][5][6][7] proved that low-lipid microalgae could be successfully converted to bio-crude oil by hydrothermal liquefaction. There have been also many studies investigating the reaction mechanism of the HTL of microalgae.…”