mination, the pH value was adjusted to 3 with 0.1 mol L−1 HCl or
0.1 mol L−1 NH3·H2O prior to use. The water samples were analyzed
within 2 weeks after collection.
The reference materials (GBW 08301 and GBW 08303) were
treated according to the following method: to oxidize organic matter
such as humic acid, the sample was digested by oxidizing
UV-photolysis in the presence of 1% H2O2 using a low pressure
Hg-lamp which was integrated in a closed quartz vessel [25], and
then the digested samples were immediately filtered through a millipore
cellulose nitrate membrane, pore size 0.45 m, acidified to
pH 3 with hydrochloric acid and stored in precleaned polyethylene
bottles prior to use [26].
2.4. Preparation of new sorbent
2.4.1. Synthesis of silica gel modified with
3-aminopropyltrimethoxysilane (APSG)
In order to remove any adsorbed metal ions and increase the
content of –OH, silica gel (SG) was activated by refluxing with
concentrated hydrochloric acid under stirring for 8 h, then the
activated silica gel was filtered and washed with doubly distilled
water to neutral and dried under vacuum at 70 ◦C for 8 h. To prepare
APSG, 10 g of activated silica gel was dispersed into 150 mL
toluene, and then 10 mL 3-aminopropyltrimethoxysilane was gradually
added into the solution with continuous stirring. The mixture
was refluxed for over night. The final product was filtered off,
washed with toluene, alcohol and diethyl ether, and then dried
under vacuum at 70 ◦C for 8 h.
2.4.2. Synthesis of silica gel modified with
2-(2-oxoethyl)hydrazine carbothioamide (SG-OHC)
The dried APSG was transferred into the flask, then 10 mL
chloroacetyl chloride and 100 mL of dried ethanol were added in
sequence. The reaction mixture was stirred for 4 h at 0–5 ◦C under
nitrogen atmosphere, filtered under vacuum, washed with ethanol
and dried under vacuum at 60 ◦C for 6 h. Finally, 1.4 g of thiosemicarbazide
was dissolved in 80 mL dimethyl sulfoxide (DMSO), and
5.0 g of the dried product was added to the solution. The mixture
reacted at 120 ◦C for 8 h. The resulting reaction product (SG-OHC)
was filtered, washed with a small amount of DMSO, ethanol and
dried in an oven at 80 ◦C for 8 h. The synthetic route of the SG-OHC
is schematically represented in Scheme 1.
2.5. General procedures
2.5.1. Batch method
Portions of 10 g mL−1 (1 mL) standard or sample solutions containing
Hg(II) were transferred into a 25 mL beaker, and the pH