Extraction of Marine Chlorella sp. and Potential Applications of the Extracted Residue

Abstract

In this work, marine Chlorella sp. was explored in various perspectives from biomass preparation and extraction through applications of its extracted residue. The experiments were divided into three sections as follows. Section-I. Effect of drying methods for biomass preparation. Marine Chlorella sp. biomass cultivated in 25 m3 open pond was harvested by flocculation after 7 days, washed to remove contamination and vacuum filtered to prepare wet paste. This paste (90% moisture) was divided into five parts to be processed for; (I) fresh paste-FP, (II) stored wet paste-SWP, (III) sun drying-SD, (IV) oven drying-OD, and (V) freeze drying (FD). The total processing time (hours) accounted for biomass drying was observed as 72, 40 and 24 h with energy expanse of 14.07, 590 and 1094 MJ/kg for SD, OD and FD, respectively to obtain biomass with moisture content of 8-9%. The biochemical analysis showed that FD biomass had highest accumulated lipid (10.68%), protein (22.1%) and energy (275.4 kcal) while carbohydrates were slightly lower than OD and SD but significantly higher than FP biomass. Section-II. Optimization of extraction by ultrasonication. Effects of various extraction factors including temperature (30-40 °C), time (60-120 min), biomass to solvent ratio (1/10-1/25 g/ml), solvent to solvent ratio (ml/ml), extraction cycles and solvent type on lipid yield (LY) were investigated. The results showed that with single extraction and single solvent recovery (1-1-cycle) process the LYS from fresh and stored paste were 11.7% and 6%, respectively, while freeze-dried biomass produced an 18.5% LY. The energy consumption was 6,000 MJ/kg lipid for the wet route and 8,200 MJ/kg lipid for the dry route in the 1-1-cycle process. The LY of the 2-1-cycle process using methanol/hexane (2/1 v/v) with a biomass to solvent ratio 1/20 g/ml was 31% and considered as a base case scenario of this study, which is 40.3% and 9.7% greater than those of the 1-1-cycle and 2-2-cycle, respectively. At developed optimized condition extraction of OD and SD biomass shown 27% and 22% LY, which is 12.90% and 29.1% lower than the base case value. Extracted lipids from OD, SD and FD were further scrutinized for their biochemical composition, fatty acids, free fatty acids (FFA) and chlorophyll contents. Moisture in all samples was observed lower than 10% and the ash contents were recorded as 10-11%. The protein (5.9%-7.1%) contents were also almost similar. However, the crude lipid, which is main component for biodiesel production was found highest in SD extract (53.1%), followed by FD extract (46%) and lowest in OD extract (34.3%). The carbohydrates were 41.65%, 19.87% and 32.13%, while energy contents (calories) were calculated as 500, 570 and 585 for OD, SD and FD, respectively. Total fatty acids were observed between (63%-66%) in dried extracts. SD biomass seems superior among the lipid extract of biomass, however sun drying is time consuming process and not a suitable choice in tropical regions. Moreover, SD biomass has an odor. The FD biomass was rich green, with regular structure and no burning spots observed at its surface. Due to these features along with high lipid yield, high crude lipid contents and longer period storage capability it was selected for further processing. Section-III. Assessment of extracted marine Chlorella sp. residue (EMCR) potential for the recovery of energetic and non-energetic products with their applications. EMCR was recycled for (i) biochar production with its application for heavy metals and yellow dye-145 abatement and (ii) bio-oil production via microwave pyrolysis assisted with EMCR derived biochar as a microwave absorber (MA). The resulting biochars were enriched with O containing functional groups. They are attractive for removal of heavy metals and anionic dyes. The surface areas of biochar prepared at 450, 550 and 650 °C were 266 m2/g, 355 m2/g and 151 m2/g, respectively. BC-450 was applied for Cr(VI), Zn(II) and Ni(II) removal by conventional adsorption (CA) and ultrasonic adsorption (UA). UA was found 1.1-1.3 higher adsorption capacity than CA in much shorter time. The maximum adsorption capacity was found as 27.45 mg/g for Ni(II) in UA. BC 550 applied for yellow dye removal by ultrasonication was highly efficient and shown an equilibrium achievement at 1 min. with 99.9% removal efficiency. The EMCR was also subjected to microwave pyrolysis (MWP) for bio-oil production with investigation of temperature (350-450 °C), time (20-40 min) and MA loading (10- 30 wt.%) at fixed microwave power of 850 W. BC-450 prepared in earlier step was introduced as MA for the first time. The pyrolysis condition was optimized to obtain maximum bio-oil yield using the Response Surface Methodology (RSM) based on Central Composite Design (CCD). The optimum condition was 350 °C, 15% MA loading and 40 min, which yielded 46% bio-oil. The bio-oil mainly composed of nitrogenated (30.37%), phenols (17.56%), furans and aromatics (5.56%), esters (17.62%), acids (12.18%), alcohols (6.07%), ketones/aldehydes (2.88%), sugar (2.30%) and alkenes (0.5%) compounds. Although, a high N containing compounds restricting the bio-oil utilization as a fuel but it could be an attractive feedstock for chemical and petrochemical industry. The results showed a high feasibility of applying EMCR as the feedstock for biochar and bio-oil production. The EMCR derived biochar presented great efficiency as the microwave absorber. The recycling of EMCR could improve the environmental and economy of Chlorella based algal industry. Keywords: Marine Chlorella sp, ultrasonic, lipid extraction, extracted residue, biochar, Microwave pyrolysis, bio-oil