Procedure For Extraction of Biogenic Silica

modified from Conley et al., (1993)

Materials and Laboratory Equipment

• motar and pestle
• centrifuge
• 35 ml centrifuge tubes
• hotplate
• thermometer
• balance (analytical)
• 100 ml beaker
• pipette (25 ml)
• pipette (1 ml)
• ion chromatograph vials

Reagents

• Sodium hydroxide (NaOH), 0.1 M solution
• Distilled water
• Hydrochloric acid (HCl)

Extraction

Biogenic silica is extracted from sediment by dissolving samples in a strong basic solution. The extraction of biogenic silica from sediment dissolves all forms silica in the sample. The silica components in sediment can be separated between diatoms, sponge spicules, and silicate minerals (Conley et al., 1993). Distinguishing the source of biogenic silica is possible because of the different dissolution rates of the silica components. Diatoms have been shown to completely dissolve within the first two hours of digestion (DeMaster, 1981; Krausse et al., 1983). Sponge spicules are larger in size and have longer dissolution rates, and minerals have the longest dissolution rates.

Extractions run for 5-hours have been shown to yield the concentration of biogenic silica from diatoms (DeMaster, 1981; Conley et al., 1993)

Procedure

1. Dry samples either in 50°C oven or by freeze drying samples
2. crush in agate motar and pestle to eliminate clumps of sediment
3. weigh between 10 and 15 mg of sediment to the nearest 0.0001 g and transfer via weighing paper to 35 ml plastic centrifuge tubes
4. heat about 500 ml of 0.1 M NaOH to 85⁰ C
5. using a pipette, add 25 ml of heated NaOH to each sample tube.
6. write down precise time of step #5
7. shake each tube and place in an 85⁰ C bath of deionized water.
8. every 30 minutes for the next 5 hours, remove each centrifuge tube from the water bath and shake it vigorously.
9. at precisely 2 hours after step #5, shake each sample, place in centrifuge and spin for about 1 minute
10. draw off 1 ml of solution and place into a ion chromatograph vial
11. repeat steps 9 and 10, at 3 hours, 4 hours, and 5 hours since step #5
12. finally, add 1 ml of DI water to each sample.

Analysis

Analysis of aliquots is by ion chromatography. In order to obtain the concentrations of diatoms and sponge spicules contributing to the total biogenic silica of a sample, two slopes must be fit to the 24-hour digestion (Conley et al., 1993). The first slope is mainly a function of the dissolution of sponge spicules, and minerals; the second slope is the dissolution of only minerals (Conley et al., 1993). Using the slopes of these two lines, the amount of diatom biogenic silica and sponge spicule biogenic silica can be extrapolated. By fitting a line to the 2, 3, 4, and 5 hour extractions the slope of the sponge spicule and mineral components is determined. Extrapolating these best-fit-lines to the y-axis will give the values of the diatom silica, and the spicule silica. The amount of diatom silica can be interpreted by only using the y-intercept from 2, 3, 4, and 5 hours.