Comparison of Two Iron Preloaded Spent Coffee Grounds Biochars in Aqueous Phosphate Adsorption

Excess phosphate (PS) removal from water can help reduce eutrophication and harmful algal blooms. Among various PS-adsorption technologies, waste-based biochar preloaded with iron has been proven effective. However, biochar enrichment with ferric sulfate was much less studied, despite sulfate’s benefits to plants. In this study, PS adsorption by spent coffee grounds (SCG) biochars with iron sulfate and chloride preload was closely compared. Biochar characterization by SEM, FTIR, and XRD indicated that differences in morphology, functional groups, and iron crystallization contributed to variations in PS adsorption. Biochar preloaded with iron chloride (FBC-2) achieved rapid PS adsorption and reached equilibrium quickly, while biochar preloaded with iron sulfate (FBC-3) reached equilibrium much more slowly. The PS-adsorption capacity of FBC-3 was stable under pH values from 3 to 9, while that of FBC-2 was more pH-sensitive. Hydrogen bonding in FBC-2 contributed to faster PS adsorption and pH sensitivity, while iron oxides formed in FBC-3 contributed to its wider pH tolerance. The two biochars both fit better with the pseudo-second-order kinetics model. The maximum theoretical PS-adsorption capacities were 78.14 mg/g for FBC-2 and 71.62 mg/g for FBC-3. PS-adsorption capacities of both biochars were stable under various salinities and co-existing ions.