Microalgae technology is a viable solution for environmental conservation (carbon capture and wastewater treatment) and energy production. However, the nutrient cost, slow-kinetics, and low biosorption capacity of microalgae hindered its application. To overcome them, algal-biochar (BC) can be integrated with microalgae to treat textile wastewater (TWW) due to its low cost, its ability to rapidly adsorb pollutants, and its ability to serve as a nutrient source for microalgal-growth to capture CO₂ and biodiesel production. Chlorella vulgaris (CV) and algal-BC were combined in this work to assess microalgal growth, carbon capture, TWW bioremediation, and biodiesel production. Results showed the highest optical density (3.70 ± 0.07 OD₆₈₀), biomass productivity (42.31 ± 0.50 mg L⁻¹ d⁻¹), and dry weight biomass production (255.11 ± 6.01 mg L⁻¹) in an integrated system of CV-BC-TWW by capturing atmospheric CO₂ (77.57 ± 2.52 mg L⁻¹ d⁻¹). More than 99% bioremediation (removal of MB-pollutant, COD, nitrates, and phosphates) of TWW was achieved in CV-BC-TWW system due to biosorption and biodegradation processes. The addition of algal-BC and CV microalgae to TWW not only enhanced the algal growth but also increased the bioremediation of TWW and biodiesel content. The highest fatty acid methylesters (biodiesel) were also produced, up to 76.79 ± 2.01 mg g⁻¹ from CV-BC-TWW cultivated-biomass. Biodiesel’s oxidative stability and low-temperature characteristics are enhanced by the presence of palmitoleic (C16:1) and linolenic (C18:3) acids. Hence, this study revealed that the integration of algal-biochar, as a biosorbent and source of nutrients, with living-microalgae offers an efficient, economical, and sustainable approach for microalgae growth, CO₂ fixation, TWW treatment, and biodiesel production. Graphical Abstract

The prospect of synthesizing biochar from agricultural wastes or by-products to utilize them as a promising adsorbent material is increasingly gaining attention. This research work focuses on synthesizing biochar from castor biomass (CBM) and evaluating its potential as an adsorbent material. Castor biomass-based biochar (CBCs) prepared by the slow pyrolysis process at different temperatures (CBC400 °C, CBC500 °C, and CBC600 °C for 1 h) was investigated for the adsorption of textile dye effluents (safranin). The pyrolysis temperature played a key role in enhancing the morphology, and the crystallinity of the biochar which are beneficial for the uptake of safranin. The CBC600 adsorbent showed a higher safranin dye removal (99.60%) and adsorption capacity (4.98 mg/g) than CBC500 (90.50% and 4.52 mg/g), CBC400 (83.90% and 4.20 mg/g), and castor biomass (CBM) (64.40% and 3.22 mg/g). Adsorption data fitted better to the Langmuir isotherm model than to the Freundlich isotherm model. The kinetics of the adsorption process was described well using the pseudo-second-order kinetic model. The study on the effect of the contact time for the adsorption process indicated that for CBC600, 80% dye removal occurred in the first 15 min of the contact time. After three regeneration cycles, CBC600 exhibited the highest dye removal efficiency (64.10%), highlighting the enhanced reusability of CBCs. The crystalline patterns, functional binding sites, and surface areas of the prepared CBCs (CBC400, CBC500, CBC600) were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, and Brunauer–Emmett–Teller surface area measurements, respectively.