Acidic Digestion of Soils to Determine Mercury Concentration

Soils are notoriously heterogeneous and difficult to analyze. The authors developed a digestion method to extract mercury from clay soils using relatively inexpensive cold vapor atomic absorption spectroscopy. By this method, 99% of available mercury is measured.

Procedure

Scope and application

This procedure is used to determine total, bound inorganic, and organic mercury in clay soils. A representative sample is digested for 14 hr using hydrofluoric acid and aqua regia at 180 °C. Mercury analysis by cold vapor atomic absorption spectroscopy follows.

To ensure all readings are between 98%T and 100%T, up to six cooled aliquots may be combined under a fumehood into a single flask after digestion and prior to the addition of stannous chloride.

Sample preparation

Soils must be crushed, sieved, and rifled prior to analysis. A number 40 sieve should be used.

The recommended sample is 3.0 g soil per digestion vessel for a total of 18.0 g soil for each data point.

Equipment

Figure 1 - Laboratory equipment required for digestion procedure.

Figure 2 - Aerator assembly.

The equipment is shown in Figure 1 and includes six 100-mL Teflon (DuPont, Wilmington, DE) perfluoroalkoxy resin polymer (TPFA) digestion vessels, glycerin, electric hotplate, heavy-duty electric timer from the hardware store, mercury analyzer with cold vapor atomic absorption spectroscopy, 5-qt stainless steel pot, one 4-L flask, an aerator assembly consisting of one #9 stopper, 6-in. 1/8-in.-diam glass tube, 10-ft 1/8-in.-diam Teflon tubing, glass septum holder with replaceable septum and a glass frit (see Figure 2), syringe, and safety equipment (safety glasses, gloves, laboratory coat, and gas mask).

Reagents

  1. Boric acid solution—for each 18.0 g soil, 126.0 g boric acid and 1440 mL deionized water should be used.
  2. Aqua regia—162.0 mL concentrated hydrochloric acid is mixed with 54.0 mL concentrated nitric acid to form 216.0 mL aqua regia. Next, 36.0 mL aqua regia is poured carefully into each of the six digestion vessels.
  3. Hydrofluoric acid (49%)—36.0 mL hydrofluoric acid is poured into each of the six digestion vessels.
  4. Stannous chloride—9.0 g stannous chloride is mixed with 9.0 mL hydrochloric acid and diluted with deionized water to 90 mL total volume. For every 18 g of soil, 90 mL of stannous chloride solution is used.

Procedure

All solutions should be prepared under a fumehood. A gas mask and other safety equipment should be worn when mixing solutions. Hydrofluoric acid is extremely toxic.

The user should check the digestion vessels for damage, and discard ruptured seals and damaged parts. Ruptured seals should be replaced in usable vessels.

The soil sample should be weighed and placed in a digestion vessel.

Under the fumehood, 36 mL aqua regia and 36 mL hydrofluoric acid should be added to the digestion vessel. The digestion vessel should be sealed and placed in the protective sleeve.

Figure 3 - Digestion vessels inside pot on the hotplate.

The digestion vessel assembly is placed in the stainless steel pot. Four to six aliquots can be processed at a time, depending on the diameter of the pot and the dimensions of the hotplate (see Figure 3). No less than 2 in. and no more than 3 in. of glycerine should be added to the pot. The glycerine will corrode the nut assembly and may weaken the sleeve.

The pot and digestion vessels are placed on the hotplate in the fumehood. The glycerine must be held at a temperature greater than 180 °C and less than 200 °C for 14 hr. The timer is optional but highly recommended.

The glycerine should be allowed to cool to room temperature (approx. 4 hr) before removing the digestion vessels from the sleeves.

The mercury analyzer should be turned on and allowed to stabilize as per the manufacturer’s instructions.

The boric acid and stannous chloride solutions are prepared. It is imperative that a gas mask is used. The digestion vessels should be opened under the fumehood. The aliquots are poured into the 4-L flask.

The aerator assembly is attached to seal the flask; 90 mL stannous chloride is then added through the septum. The aliquot is analyzed immediately for mercury.

Indications >98%T are inaccurate because they fall close to the lower detection limit of the machine. The absorbent in the line should be checked after every analysis to verify that no moisture has collected in the line. The PTFE lines should be replaced after 20 aliquots.

The authors are with the University of Cincinnati, Department of Civil and Environmental Engineering, Baldwin Hall 765, P.O. Box 210071, Cincinnati, OH 45221-0071; tel.: 513-556-2225; fax: 513-556-2599; e-mail: [email protected].