Tab. 4

Summary of the pH value and the content of SO42+
Initial
Cycle 1
Cycle 2
Cycle 3
Cycle 4
pH
2
6.78
6.8
7.04
7.58
SO42+ [mg/l]
4.000
1.800
1.600
1.200
1.000
BIOMEDY
Applications BIOREM
Next
Prj 9809 - Soil Remediation in a Coke Processing Plant
The lab in field test was conducted in September 1998 on a small portion of ground in a coke processing plant. It specifically referred to the ground below the keeping site of a coke-gasification plant. The aspect of the soil was a damp viscous black mass with a strong pitch-tar smell characterized by qualitative and quantitative analyses in Tab. 1 and Tab. 2 while analysis of water extracted by the ground sample is summarized in Tab. 3.
Tab. 1

Qualitative Roentgen-phase analysis
Quartz SiO2
Mean Contents
Pyrite FeS2
Low Contents
Hematite Fe2SO3
Low Contents
Kaolinite (Clay usually containing impurities of alkaline metals and ferric oxides)
Low Contents
Hydrolysed-Alumino-Silicates group

Feldspar group

K[AlSi3O3]-Na[AlSi3O3]-Ca[AlSi3O3]
Low Contents
Sulfur
Mean Contents
Organic phase (bituminous type)
High Contents
Complex nontoxic dark blue cyanide (obtained at dry distillation of dark coal)

Fe43+[Fe2+(CN)6]3
Mean Contents
Thiocyanates nontoxic green composition

Fe2+(CNS)23H2O

Thiocyanates nontoxic ammonium salt

NH4NCS
Mean Contents
Tab. 2

Quantitative spectrum, plasma-photometric and chemical analysis
Element
Contents [%]
Element
Contents [%]
SiO2
22.8
 Sb
0.01
Al2O3
2.6
 As
< 0.01
MgO
0.12
 Zn
0.03
CaO
0.12
 Cd
< 0.001
Fe2O3
7.9
 Sn
0.01
MnO
0.085
 Ge
0.003
Ni
0.03
 Be
0.0003
Co
0.005
 Sc
0.0005
TiO2
0.2
 La
0.003
V
0.01
 Y
0.002
Cr
0.05
 P
< 0.08
Mo
0.003
 Na2O
0.52
W
0.003
 K2O
0.38
Zr
0.02
 Sgeneral
11.53
Cu
0.075
 SO3
4.75
Pb
0.02
 Corganic(*)
11.6

[g/kg of soil]
Ag
0.0005
Solid Hydro-carbons
Tab.3

Quantitative analysis of water samples extracted from the soil
Element
Contents [mg/l]
pH (medium reaction)
1.0
Fe2+
28
Fe3+
84
Ca2+
96
Mg2+
106
SO42-
4600
Corganic(*)
Not Measurable
(*) Includes:
Carbon (C)

Hydrogen (H)

Sulfur (S)

Oxygen (O)

Bituminous

In the same soil sample was also present residues of coke (consisting in carbon and ashes) floating in the water.
In addition it was performed an analysis of the salts extracted from the soil whit the aim to determine the exchange acidity of the soil through the pH value. The exchange acidity of the soil, defined as the H-ions passing into solution that increase the soil acidity, was:

pH = 0.79

More specifically the site was polluted by 116 g/kg of organic compounds with a dreadful acidity (pH=0.79) and with complex nontoxic cyanide and Thiocyanates under influence of sulfuric acid (H2SO4). Whit these values the site could be defined as "dead".

Chemical pre-treatment

Bacterial activity was quite excluded at these soil conditions and a pre-treatment was necessary in order to decrease the acidity of the soil. The pre-treatment consisted in liming the soil with 0.5 Kg of CaCO3 per square meter of soil (calculation). Such addition of lime had the scope to:

  • Reduce the acidity of the soil
  • Allow the transformation of the major part of ferricyanide Fe43+[Fe2+(CN)6]3 into easy soluble form of calcium ferrocyanade with nontoxic complex composition, with the decreasing of iron in the ground and with a positive factor for the all chemical structure of the soil.

Microbiological bioremediation

From a soil sample it was extracted a micro flora consisting of yeast bacteria and fungi. No reagents, genetic modification or any other manipulation was done. The concentration of microorganism was about 1010 counts/ml. Initially prevailed heterotrophic bacteria but later specialized oxidizing sulfur bacteria and yeast were the most active.

The test work was then carried out using two different methodologies.

A) In-situ stationary conditions

Microorganisms (bacteria, fungi, yeast) were injected in the soil that was watered up until the complete saturation. The process was repeated each 24 hours so reproducing a sort of "heap leaching". Measurement were carried out on the percolation water.

During the performance of the remediation it was constantly tracked the pH value. Also qualitative reactions for Thiocyanates and cyanides were kept under scrutiny.

After 14 days it was observed that:

  • On the surface of the ground appeared fungi as dotted colonies. At first prevailed penicillin, then started the spontaneous growth of white fungi.
  • The drain water changed its color from dark blue-green (Fe2+(CNS)23H2O) to slightly green, which show the Thiocyanates outcome.
  • The pH passed from 0.79 to 2.
  • After 30 days it was observed that:
  • The drain water color was indiscernible.
  • The pH passed to 3.2.
  • Concentration of sulfates was about 1000 mg/l.
  • The presence of mixture of Fe2+/3+ was detected.
  • Unfortunately the process went significantly slowly and was terminated after 6 months.

B) Ex-situ agitation mode

Polluted soil was loaded into a slowly rotating agitator where also microorganism culture was introduced (bacteria, fungi, yeast). The pulp was brought to the ideal solid to liquid ratio by adding water and agitated in cycles lasting one week each. Each cycle terminated with the downloading of the liquid fraction that was sent to analysis and further refilling with microorganisms and water. No reagents were ever added.

Tab. 4 summarizes the pH value and the content of SO42+ after each of the 4 cycles that were performed.
During the test was also monitored the presence of thiocyanates in the drain water that showed an initial activity where mixture of Fe 2+/3+ -> Fe 3+

The test was concluded with the complete purification from sulfides, thiocyanates, cyanide, and phenol. In particular:

  • The contents of the most resistant contaminant were reduced 10 times, from 116 g/Kg to residual of 11 g/Kg.
  • The acidity of the sample was reduced from pH 0,79 to pH 5.5.