reagent grade. Monopotassium phosphate (KH2PO4), sodium hydroxide (NaOH), sodium carbonate (Na2CO3), potassium nitrate (KNO3), potassium chloride
(KCl), chitosan (90% of deacetylation degree), zinc nitrate hexahydrate,
2-methylimidazole, and methanol were purchased from Tianjin Kermel
chemical reagent Co., Ltd. (Tianjin, China). Diatomite (D) was purchased
from Tianjin Damao Chemical Reagent Co., Ltd. (Tianjin, China).