Removal of NO₃-N in alkaline rare earth industry effluent using modified coconut shell biochar

Coconut shell biochar (CSB) was selected as raw material to obtain two kinds of modified biochars by pickling and iron modification. The pickling coconut shell biochar (PCSB) and pickling-iron modified coconut shell biochar (PICSB) were used as adsorbents to remove NO₃-N in alkaline rare earth industry effluent. The results showed that pickling smoothed the surface of CSB, and α-FeOOH was formed on the surface of PCSB because of FeCl₃ solution modification. Suitable adsorbent dosages of PCSB and PICSB were both 2.0 g/L. The NO₃-N adsorption process by PCSB and PICSB both reached equilibrium at 30 min. The quasi-first-order kinetic model shows good fit to the NO₃-N adsorption by PCSB. Whereas, the quasi-second-order kinetic model is more suitable for PICSB adsorbing NO₃-N. The adsorption mechanisms of PICSB for NO₃-N removal were ligand exchange and electrostatic attraction, and that of PCSB for NO₃-N removal was electrostatic attraction. The NO₃-N adsorption amounts of PCSB and PICSB decreased with increasing adsorption temperature and pH. The maximum NO₃-N adsorption amounts of PCSB and PICSB were 15.14 mg/L and 10.75 mg/L respectively with adsorbent dosage of 2.0 g/L, adsorption time of 30 min, adsorption temperature of 25 ± 1 °C, and initial solution pH of 2.01.