2. Experimental2.1. Production of activated carbonsRice husk was obtained from local farmer near Changchun. Therice husk was mixed with phosphoric acid (50 wt%) in a mass ratio(rice husk to phosphoric acid) of 1:4, then the mixture was heated to500◦C and maintained 1 h, the obtained product was treated in dif-ferent ways: (1) it was washed with distilled water until neutral pH,the obtained solid product was denoted as P-AC; (2) it was washedwith hot distilled water (95◦C) until neutral pH, the resulting solidproduct was named as P95-AC; (3) the solid product was washedwith distilled water until neutral pH, then it was mixed with 3 wt%potassium hydroxide solution, the mixture was heated to boilingunder reflux condition, and maintained 1 h, then the solid productwas separated from liquid product by filtration, this solid prod-uct was labeled PK-AC. Another activated carbon was produced byKOH activation, the mass ratios of rice husk to KOH was 1:4, thereaction was conducted at 400◦C for 0.5 h, then 800◦C for 1 h, thecorresponding activated carbon was named K-AC.