Graphitic bio-char and bio-oil synthesis via hydrothermal carbonization-co-liquefaction of microalgae biomass (oiled/de-oiled) and multiple heavy metals remediations

Highlights: • Microalgae cultivation in wastewater integrated HRAP system for biomass production. • HTCL of microalgae oiled/de-oiled biomass for bio-chars (graphitic 2H) and bio-oils. • Highly disordered graphitic and crystalline bio-chars due to high C/N and low C/H. • Bio-oils showed mostly prevailing aliphatic protons at upfield integration signals. • Kinetic models of adsorption behavior and models of adsorption isotherm of HMR. Thermochemical transformation of microalgae biomass into graphitic bio-chars entices as proficient bio-adsorbents for heavy metal contaminants. This study explores the synergistic impact of Chlorella sorokiniana on biomass generation and wastewater remediation in high rate algae pond (HRAP). Biomass produced was applied for hydrothermal carbonization-co-liquefaction (HTCL). The structural and morphological characteristics of HTCL products (i.e. bio-chars and bio-oils) have been systematically studied by XRD, Raman, FTIR, elemental analyzer, SEM, BET, and ¹H NMR spectroscopy. The crystallite size of the graphite 2H indexing planes was to be 4.65 nm and 14.07 nm in the bio-chars of oiled biomass (MB-OB) and de-oiled biomass (MB-DOB), respectively. The increase in the I_D/I_G ratio of MB-DOB indicated the highly disordered graphitic structure due to the appearance of carbonyl, hydroxyl, and epoxy functionalities in the line of high C/N and low C/H ratio. Also, the multiple heavy metals remediation of MB-DOB revealed better efficiency as ~100% in 720 min. The kinetics analysis shows the correlation coefficient of pseudo-second-order is well fitted compared to the pseudo-first-order. The Langmuir adsorption model signifies the adsorption of heavy metal ions in a monolayer adsorption manner. The study proposes the microalgae bio-char potential for multiple heavy metals remediation alongside bio-oils.