Valorization of marine biomass and de-oiled seed cake for inorganic dye and organic pollutants removal from textile process water

This study investigated the valorization possibilities of indigenous marine biomass: blue-green microalgae, Chlorophyta (macroalgae), de-oiled Jatropha curcas, Moringa oleifera and Azadirachta indica (neem) seeds as bio-adsorbents. The de-oiled seed cake was obtained using n-hexane with a solid-to-solvent ratio of 1:6 for 1 h at 60 °C. These biomass feedstocks were chemically activated by 0.2 M phosphoric acid (H₃PO₄) at 120 ± 5 °C for 24 h, solar-dried and calcined at 650 °C for 2 h to prepare the desired carbon materials. Further, the set of bio-adsorbents has been comprehensively characterized to determine moisture content, ash content, bulk density, structural analysis, morphological analysis, particle size distribution and specific surface area. The characterization results were benchmarked with commercial activated carbon (AC), and the studied bio-adsorbents presented desired physicochemical characteristics for adsorption purposes. Therefore, the bio-adsorbents were applied against an inorganic dye, methylene blue (MB) dye from local textile process water in Pakistan. The adsorption capacity of blue-green microalgae, Chlorophyta macroalgae, J. curcas seed, M. oleifera seed and A. indica seed was obtained as 37.5 mg/g, 22.18 mg/g, 18.6 mg/g, 23.1 mg/g and 44.3 mg/g at the equilibrium point through batch adsorption, respectively. The adsorption data were fit to the linear, Freundlich and Langmuir isotherm models. Blue-green microalgae, Chlorophyta macroalgae, M. oleifera seed and J. curcas seed bio-adsorbents were the best fitted by the Freundlich model with R² = 0.9832, 0.9747, 0.9732 and 0.8786, respectively. A. indica seed bio-adsorbent was the best fitted by the linear isotherm model with R² = 0.9729. Besides, the bio-adsorbents set was as effective as commercial AC against organic pollutants such as chemical oxygen demand (COD), total nitrogen (TN) and total organic carbon (TOC) removal efficiency. The maximum COD, TN and TOC removal efficiency were obtained, by A. indica seed bio-adsorbent with 31.88%, 33.30% and 9.29%, respectively, while the COD, TN and TOC removal efficiency of commercial AC were 28.59%, 16.66% and 11.75%, respectively. Therefore, it can be concluded that the abundant marine biomass and de-oiled seed cake derived from A. indica seed as bio-adsorbents can be valorized to treat process water from local textile industries in Pakistan.