Phosphorus and Nitrogen Adsorption Capacities of Biochars Derived from Feedstocks at Different Pyrolysis Temperatures

This study investigates the P and NO3− adsorption capacities of different biochars made from plant waste including rice straw (RSB), Phragmites communis (PCB), sawdust (SDB), and egg shell (ESB) exposed to a range of pyrolysis temperatures (300, 500 and 700 °C). Results indicate that the effect of pyrolysis temperature on the physiochemical properties of biochar varied with feedstock material. Biochars derived from plant waste had limited adsorption or even released P and NO3−, but adsorption of P capacity could be improved by adjusting pyrolysis temperature. The maximum adsorption of P on RSB700, PCB300, and SDB300, produced at pyrolysis temperature of 700, 300 and 300 °C, was 5.41, 7.75 and 3.86 mg g−1, respectively. ESB can absorb both P and NO3−, and its adsorption capacity increased with an increase in pyrolysis temperature. The maximum NO3− and P adsorption for ESB700 was 1.43 and 6.08 mg g−1, respectively. The less negative charge and higher surface area of ESB enabled higher NO3− and P adsorption capacity. The P adsorption process on RSB, PCB, SDB and ESB, and the NO3− adsorption process on ESB were endothermic reactions. However, the NO3− adsorption process on RSB, PCB and SDB was exothermic. The study demonstrates that the use of egg shell biochar may be an effective way to remove, through adsorption, P and NO3− from wastewater.