Thermodynamic and performance analysis of algal-infused biochar discs for desalination of reverse osmosis reject water

• Algal-infused biochar discs removed 71.06% TDS from RO reject water • Adsorption was spontaneous with ΔG° ranging from -3742 to -4377 J/mol • Enthalpy of +35.42 J/mol indicated endothermic process • Entropy of -13.88 J/K·mol proposed the disorder at the solid-liquid interface • Composite discs enable low-cost desalination with 498.9 mg/g sorption capacity The increasing salinization of groundwater due to desalination reject streams poses a significant challenge to water sustainability. This study explores the potential of algal-infused biochar discs for effective salinity reduction, emphasizing their thermodynamic behavior. Biochar derived from groundnut shells was surface-modified and infused with algal biomass to enhance ion removal capacity. The adsorption process was evaluated through batch experiments, assessing key parameters such as initial total dissolved solids (TDS) concentration (1676 mg/L to 8728 mg/L), temperature (25°C to 55°C), and disc dosage (2 to 6 discs). Thermodynamic analysis revealed that the adsorption process was spontaneous, with Gibbs free energy (ΔG°) values decreasing from -3742 J/mol to -4377 J/mol, confirming favorable ion uptake. Enthalpy (ΔH°) was determined as 35.42 J/mol revealing an endothermic nature, while entropy (ΔS°) showed a value of -13.88 J/K·mol, indicating reduced randomness at the solid-liquid interface. Characterization studies, including Brunauer-Emmett-Teller (BET), Scanning Electron Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS), and Thermogravimetric Analysis (TGA), demonstrated changes in porosity, surface chemistry, and thermal stability post-salinity rduction, with BET surface area decreasing from 18.98 m²/g to 16.08 m²/g, indicating salt deposition within the biochar matrix. This study examined the ionic salinity reduction performance of algal biochar discs under optimal conditions of 25°C temperature, 6-disc dosage, and 150 minutes contact time across varying initial TDS levels. Removal efficiency of 71.06 % was achieved for an initial TDS concentration of 1676 mg/L, whereas a maximum sorption capacity was 498.9 mg/g was observed for an initial TDS concentration of 8728 mg/L. This study underscores the salinity reduction potential of biochar-algal discs as a sustainable approach for addressing salinity challenges.