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.
To remove NO3-N from water, coconut shell biochar (CSB) was modified by a solution of FeCl3, a solution of AlCl3 and a mixture solution of FeCl3 and AlCl3 respectively. The obtained modified biochar with the best effect of NO3-N adsorption was screened out to explore the adsorption behavior and mechanism of NO3-N removal by batch experiments and kinetics and thermodynamics and correlated characterization. The results indicated that the mixture solution of FeCl3- and AlCl3- modified CSB (Fe-Al/CSB) showed the best adsorption performance for NO3-N removal. Iron and aluminum elements existed on the surface of Fe-Al/CSB in the form of FeOOH, Fe2O3, Fe2+, and Al2O3 respectively. The adsorption process could reach equilibrium in 20 min. An acidic condition was favorable for NO3-N adsorption. The presence of coexisting anions was not conducive for NO3-N adsorption. The quasi-second-order model and Freundlich model could be well fitted in the adsorption process. The maximum adsorption capacity of Fe-Al/CSB fitted by the Langmuir model could reach 34.20 mg/g. The adsorption of NO3-N by Fe-Al/CSB was an endothermic and spontaneous process. Ligand exchange and chemical redox reaction were the NO3-N adsorption mechanisms which led to NO3-N adsorption by Fe-Al/CSB.
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