*2.1. Materials*

Tannin (92%), 4-mercaptobenzoic acid (4-MBA, 97%), chloroauric acid tetrahydrate (HAuCl4·4H2O, 99%), silver nitrate (AgNO3, 99.8%), carbimazole (C7H10N2O2S, 98%), sodium chloride (NaCl, 99%), potassium carbonate (K2CO3, 99%), and nitric acid (HNO3, 68%) were obtained from National Pharmaceutical Group Chemical Reagents Company (Beijing, China). Standard solution of mercury (Hg), manganese (Mn), nickel (Ni), lead (Pb), copper (Cu), cadmium (Cd), iron (Fe), arsenic (As), sodium (Na), and chromium (Cr), were procured from Siyuan Chemical Glass Co., Ltd. (Zhenjiang, China), and all their purities were greater than 97%. Ultrapure water was used for all the experiments. All reagents used in the study were analytical grade unless stated otherwise.
