oa Azolla as a Feedstock for Bio-Refinery: Cultivation, Conversion and Application

Azolla is one of the world's fastest growing aquatic macrophytes, with a doubling time of only 2-5 days (Taghi-Ganji et al., 2005). It can be found on the surface of wetlands, ditches, ponds and rivers, native to the tropics, subtropics, and warm temperate regions of Africa, Asia, and the Americas (Costa et al., 2009) (Figure. 1). It has been intensively studied during the last few years for its potential uses as a green manure in rice fields, as a feed supplement for aquatic and terrestrial animals, as a human food, as medicine, as water purifier, as a biofertilizer, control of weeds and mosquitoes, to eradicate different metals, or to remove nitrogenous compounds from the water and the reduction of ammonia volatilization which accompanies the application of chemical nitrogen fertilizer. (Gregory, et al., 1997; Lumpkin, et al., 1980; Sadeghi, et al., 2013). Several other studies have also verified Azolla's potential as a biogas and hydrogen fuel source (Peters, 1976; Newton 1976, Van Hove, 1989).

These beneficial uses urged researchers to call it “green gold mine”. Seven extant Azolla species (A. filiculoides, A. caroliniana, A. mexicana, A. pinnata, A. nilotica, A. microphylla, A. rubra) are reported) Armstrong, 1998). Among the Azolla species, A. filiculoides is the only fern which is found in Anzali wetland, northern Iran (Sadeghi et al., 2012a, b; Sadeghi et al., 2013). A. Filiculoides also grows in southern South America, and western North America to Alaska. Also, it is on the US noxious weed list. Azolla has already covered about 50 percent of the 20,000 hectare Anzali wetlands. The overgrowth of Azolla is now considered as a serious issue in this unique ecosystem. Now, this useful plant is a harmful weed in water because it forms dense surface mats, interfering with boating, fishing, blocking irrigation channels and waterways and recreational activities as well as degrading water quality by reducing oxygen levels. It has been an increasing effort to maximize the beneficial properties and find new applications for that. So, Thermochemical conversion was applied on this biomass. Thermochemical processes depend on the relationship between heat and chemical action as a means of extracting and creating products and energy. Pyrolysis, gasification, and liquefaction which is conducted at a temperature of several hundred degrees Celsius are categorized in thermochemical processes. Currently, there are two principal processes for bio-oil production from biomass: pyrolysis and hydrothermal liquefaction (HTL). Besides biocrude oil as a liquid fuel, a special type of biochar which can be considered as an attractive byproduct was produced in HTL process. This carbonaceous material, which has received less attention can be potentially used for the removal of heavy metals contamination from wastewater (Liua, et al., 2009). The Cu at higher concentrations is a potentially toxic metal in the environment, mainly as the soluble form Cu²⁺. Obtained biochar from HTL of Azolla was used for Cu removal. The major concern in (HTL) is the high temperature and consequently high pressure of the process which results in high capital cost of equipment. So the process pressure and temperature should be reduced, but at a lower temperature, bio-oil yield is not high enough to make HTL economical for sustainable fuel production. For this purpose, ethanol was used as a solvent instead of water. Also, lipid extraction from Azolla was studied with Soxhlet extraction; because of its simplicity in operation, potential for scale up and safety. As our knowledge, to date, no study has reported an evaluation of the Azolla bio-refinery and also thermochemical conversion of Azolla for producing advanced fuel. This research was conducted in two parts: 1) cultivation and 2) conversion. The purpose of cultivation was to find the optimum requirements to reach the highest growth rate of Azolla under laboratory conditions. The results will help us to manage growing of this aquatic fern and also conserve wetland. This study showed, Humidity and pH are the most significant variables for achieving maximum Azolla growth rate. In conversion part; an all-out attempt was made to maximize the beneficial properties of the Azolla-Anabaena association and to find its new applications. After cultivation and harvesting of Azolla, this biomass was investigated as a feedstock for thermochemical conversion process. Lipid extraction, hydrothermal treatment, hydrothermal liquefaction and pyrolysis process were scrutinized aimed at utilizing Azolla as a biodiesel, liquid fertilizer, biocrude oil, biochar. Also, after these processes, liquid and solid residual were separated and analyzed for future utilization as liquid fertilizer and absorbent for heavy metal removal, respectively. This harmful weed with its fast growth rate and high potential products can be quite useful plant and can be fed into biorefinaries so that its removal will also conserve wetland. An all-out attempt has been made to maximize the beneficial properties of Azolla and find new applications for it. Reaching the highest growth rate under laboratory conditions will be conducive for managing this fast growing fern. Humidity and pH are the most significant variables for achieving a maximum Azolla growth rate. The dry mass growth rate of Azolla was. The maximum growing rate was achieved at; temperature: 22 °C, light = 20 Lux, humidity: 75% and pH = 6.4 with 2.1 days considered as doubling time. Lipid extraction, hydrothermal treatment, hydrothermal liquefaction and pyrolysis process were exerted upon Azolla for producing biodiesel, liquid fertilizer, biocrude oil, and biochar. Hydrous harvested Azolla converted to bio-crude oil at hydrothermal condition and Heat value increased from − 0.86 to 31.0 MJ/kg (yield = 39%). Bio-crude oil derived from pyrolysis had yields 29% with HHV =  33.2 MJ/kg. Pre-treatment at 180 °C before hydrothermal liquefaction at 300 °C was produced highest bio oil quality. Lipid content in Azolla was about 11.7%, which includes 38.07%, 39.49% and 19.81% saturated, monounsaturated and polyunsaturated fatty acids, respectively. The nitrogen rich liquid was produced at hydrothermal treatment. This harmful weed of overgrowth rate can be fed into biorefinaries so that its removal will also conserve wetlands.