Simultaneous reclaiming phosphate and ammonium from aqueous solutions by calcium alginate-biochar composite: Sorption performance and governing mechanisms

The eutrophication of aquatic environments has become one of the major environmental problems. To reduce nitrogen and phosphorus in water, industrial waste (distillers grains), agricultural waste (rice straw), and an invasive plant (Eupatorium adenophorum) were used to prepare ball-milled biochar (BMB), and further synthesize calcium alginate-biochar composite (CA-MB) for simultaneous adsorption of phosphate and ammonium. The scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR), Zeta potential, X-ray diffractometer (XRD), and BET specific surface area (SSA) analyzer were used to characterize the physicochemical properties of CA-MB. The effects of different conditions (pyrolysis temperature, dosage, solution pH, and coexisting ions) on the adsorption performance were studied. Combined with adsorption kinetics and isotherms, the co-adsorption performance and governing mechanisms of phosphate and ammonium onto CA-MB were explored. The results showed that the CA-MB prepared from distillers grains, rice straw and Eupatorium adenophorum had better adsorption performances on phosphate and ammonium at the pyrolysis temperatures of 600 °C (CA-MWB600), 300 °C (CA-MRB300), and 600 °C (CA-MEB600), respectively. The maximum adsorption capacities of CA-MWB600, CA-MRB300, and CA-MEB600 for phosphate and ammonium estimated with Langmuir isotherm model were 24.1, 31.8, 24.0 mg g−1 and 12.27, 10.15, 9.90 mg g−1, respectively. The main adsorption mechanisms for phosphate were electrostatic attraction, surface precipitation, and ligand exchange, while for ammonium were ion exchange and electrostatic attraction. The retention capacity of CA-MB for phosphate was better than ammonium. This study indicates that CA-MB can be used as an effective adsorbent for simultaneous recovery of phosphate and ammonium from water.