Silica Elution of Activated Carbon

Description:
Si Elution of Activated Carbon After 10 Bed Volumes

Remarks:

Related document: NSTM 4.02 – Determination of the apparent density of granular activated carbon (GAC) according to ASTM D2854-89.

Scope

During the first BV after start up of a water filter, granular activated carbon (GAC) may release inorganics like Ca, Mg, Na, K and trace metals. For most applications these phenomena are acceptable; if not, HCl washed grades are an appropriate solution. However, leaching of Si occurs both from unwashed and HCl washed grades. In case of condensate water treatment, Si leaching can pose serious problems, especially for high pressure boiler systems.

This NSTM test describes the test procedure used to measure the leaching of Si from activated carbon.

Sample

Take a representative sample of approximately 1 litre "as is".

Apparatus

Water jacketed glass column (i.d. 3.5 cm, height 100 cm).
Water bath capable of maintaining a test temperature of 38°C, Haake F3 or equal.
pH/ORP controller with a calibrated pH electrode, Cole Palmer model 5656-00 or equal.
Dosing pump, ProMinent Gamma/4-W or equal.
Peristaltic pump, Watson Marlow 505U or equal.

Reagents

Demineralised water.
NaOH solution, 4 gram NaOH per litre demiwater.
HNO₃ (concentrated).
Dry, oil free compressed air.

Procedure

See appendix for the test unit.

Switch on the waterbath and set the column temperature and cistern temperature on 38°C. Use the pH controller and pH electrode to maintain the pH of demi water at 8.5 by means of dosing NaOH solution. Turn on the aeration of the water in the cistern. Before the test is started both temperature and pH of demi water must be stable.

Determine the apparent density of the sample according to NSTM 4.02. Weigh 0.5 litre sample (equal to back washed and drained volume) by multiplying the apparent density (g/l) by 0.5 and 0.88.

Boil the carbon in 1 litre of demi water for 5 minutes. Flush the carbon in the column using demi-water. Backwash the carbon at 50% bed expansion using 2 litre demi-water during approximately 2 minutes in order to remove fines.

Feed the column in up-flow direction with pH 8.5 demi water at a flow rate of 2 litre per hour by means of the peristaltic pump. The contact time (EBDCT) is 15 minutes. Check the flow rate by measuring the weight increase per time unit. Perform a blank test (empty column).

Take effluent samples after 1 (4 bed volumes), 2 (8 bed volumes) and 3 hours (12 bed volumes). Take an influent sample. After acidification to pH 1 with concentrated HNO₃, analyse for Si.

Calculation

Subtract the Si level measured for the blank test from the actual test. Plot the results of this corrected Si concentration (y-axis) against the bed volumes treated (x-axis). Draw a smooth curve through the data points and read the Si concentration level in the effluent after 10 bed volumes.

Report

Report the Si concentration after 10 bed volumes in µg/l.

Silica Elution of Activated Carbon