Page 210 - Materials Chemistry, Second Edition

Vadose Zone Soil Remediation 193

As shown in Table 5.2, the suggested C:N:P ratio is 120:10:1. (Some other ref-
erences suggest C:N:P = 100:10:1.) The ratio is on a molar basis. It means that
every 120 moles of carbon requires 10 moles of nitrogen and 1 mole of phos-
phorus. For bioremediation, a feasibility study is always recommended.
Determination of an optimal nutrient ratio should be part of the feasibility
study. If no other information is available, the ratio mentioned here can be
used. The example in this section will show that the amount of nutrients
needed is relatively small, and so is the cost. Nutrients are often dissolved in
water first, and this is then applied to the soil by spraying or irrigation.
To determine the nutrient requirements, the following procedure can be
used:

Step 1: Determine the mass of the organics present in the impacted soil.
Step 2: Divide the mass of organics by its molecular weight to find the
moles of the COCs.
Step 3: Multiply the moles of COCs from Step 2 by the number of Cs in
the compound’s formula.
Step 4: Determine the moles of nitrogen and phosphorus needed using
the optimal C:N:P ratio. For example, if the ratio is C:N:P =
120:10:1, then
Moles of nitrogen needed = (moles of carbon present) × (10/120)
Moles of phosphorus needed = (moles of carbon present) × (1/120)
Step 5: Determine the amount of nutrient needed.

Information needed for this calculation:

• Mass of the organic COCs
• Chemical formula of the COCs
• Optimal C:N:P ratio
• Chemical formula of the nutrients

Example 5.23: Determine the Nutrient Requirement
for Soil Bioremediation
The results of a feasibility study indicate that the excavated soil in a stockpile
(Example 5.22) is suitable for on-site aboveground bioremediation. The fea-
sibility study also determined the optimum C:N:P molar ratio to be 100:10:1.
Estimate the amount and cost of nutrients (in lbs) needed to remediate the
impacted soil.
Use the following assumptions in your calculation:

• Volume of excavated soil in piles = 375 yd 3
• Initial mass of gasoline in the piles = 158 kg