Hydropyrolysis of freshwater macroalgal bloom for bio-oil and biochar production: Kinetics and isotherm for removal of multiple heavy metals

In this study, hydropyrolysis was carried out using sodium carbonate to convert the green algal bloom into bio-oil, biochar, aqueous solution, and gases. The effect of supercritical conditions (400, 450, 500 °C) on the product yield, bio-oil composition, and structure, and functionalities of the biochar was determined. The high yield of biochar and bio-oil was reported at 400 °C. A significant reduction in bio-oil and increment in hydrocarbon content was reported on the elevation of temperature from 450 °C to 500 °C. After that, kinetic and isotherm analysis was investigated simultaneously to remove four heavy metals viz. Cu(II), Ni(II), Co(II), and Cd(II) from the mixture solution. Results show that kinetics data follow a pseudo-second-order kinetics model and adsorption isotherm is in better agreement with the Langmuir model, not with the Freundlich model. The maximum adsorption capacity was found 10.90, 5.74, 5.80, and 16.28 mg/g with the biochar prepared at 500 °C for Cu(II), Ni(II), Co(II), and Cd(II) metals, respectively. The current investigation provided a promising way for the utilization of freshwater algal bloom biomass for renewable products and simultaneously heavy metal removal from the water.