2. ENVIRONMENTAL ChEMISTRy & TEChNOLOGy 2.1. Lectures

Chem. Listy, 102, s265–s1311 (2008) Environmental Chemistry & Technology
P37 ThE RECLAMATION OF CALCIuM
SuLPhATE SLuDGES by SuLPhATE-
REDuCING bACTERIA
ALEnA LUPTáKOVá a and MáRIA KUŠnIEROVá a
Institute of Geotechnics of Slovak Academy of Sciences, Watsonova
45, 043 53 Kosice, Slovak Republic,
luptakal@saske.sk
Introduction
Combustion of fossil fuels containing sulfur releases
sulfur oxides to the atmosphere. If lime or limestone scrubbing
desulfurizes combustion gases, calcium sulfates sludges
are generated and these must be disposed of. Many processes
for their treatment have been developed. Under appropriate
conditions these sulfate can be converted to sulfide by the
anaerobic bacterial sulfate reduction, which is the basic metabolic
process of sulfate-reducing bacteria (SRB).
The SRB represent a group of chemoorganotrophic
and strictly anaerobic bacteria that may be divided into four
groups based on rRnA sequence analysis 1 : Gram-negative
mesophilic SRB, Gram-positive spore forming SRB, thermophilic
bacterial SRB and thermophilic archaeal SRB. They
include genera like Desulfovibrio, Desulfomicrobium, Desulfobacter,
Desulfosarcina, Desulfotomacullum, Thermodesulfobacterium,
Archaeoglobus, etc.
Considering the inorganic or organic character of energy
source of SRB there are two types of anaerobic respiration
of sulfates authotrophic and heterotrophic 2 . SRB produce
a considerable amount of gaseous hydrogen sulfide, which
reacts easily in the aqueous solution with heavy metal, forming
metal sulfides that have low solubility. In the bacterial
anaerobic reduction of sulfates the organic substrate (lactate,
malate, etc.) or gaseous hydrogen is the electron donor and
sulfates is the electron acceptor.
The industrial technologies for the desulfurization
of combustion products produced during the generation
of electric energy by combustion of fossil fuels use limestone
(CaCO 3 ) as an absorption agent. The desulfurization
of combustion products proceeds in the absorber in several
stages. This process results in the formation of gypsum suspension
(CaSO 4 . 2H2 O) which is incorporated into the final
stored product – “stabilizate” – after being treated together
with other wastes (ash, burnt lime, desulphurization waste
water, etc.).
The objective of our study was to verify experimentally
the possibility of using gypsum contained in the above-mentioned
“stabilizate” as the source of electron acceptors for the
growth of SRB with the prospect of the recycling of desulfurization
agent – limestone.
Experimental
M i c r o o r g a n i s m s
A culture of SRB (genera Desulfovibrio and Desulfotomaculum)
was obtained from drinking mineral water
Gajdovka (locality Kosice-north, Slovak Republic). For the
s412
isolation and cultivation of these bacteria a selective nutrient
medium (DSM-63 – Postgate′s C medium) was used 3 .
L i q u i d P h a s e
The feed solution (the selective nutrient medium (DSM-
63 – Postgate’s C medium without sulfates) was prepared by
dissolving analytical grade salts such as: K 2 HPO 4 0.5 g dm –3 ,
nH 4 Cl 1 g dm –3 , CaCl 2 . 6H2 O 0.1 g dm –3 , MgCl 2 . 6H2 O
0.3 g dm –3 , C 3 H 5 O 3 na 2.0 g dm –3 , C 2 H 3 O 2 Sna 0.1 g dm –3
and C 6 H 8 O 6 0.1 g dm –3 in distilled water.
S o l i d P h a s e
The sample of “stabilizate“ from Vojany power plant
(Slovak Republic) was used in the experiments. Mineralogical
characterisation by X-ray Diffraction (XRD) showed the
presence of CaSO 4 40.84 %, SiO 2 22.70 %, Al 2 O 3 10.70 %,
Fe 2 O 3 4.26 % and CaO 3.00 %.
A n a l y t i c a l P r o c e d u r e s
A turbidimetric method was used to measure the concentration
of soluble sulfate ion concentrations in the
liquid phase 4 . Samples were centrifuged for 10 minutes at
10,000 rpm before performing the analysis. Digital pH-
meter GPRT 144 AGL was used. Qualitative changes of
“stabilizate” were performed by the qualitative X-ray diffraction
analysis using Dron-2 instrument and energy dispersive
spectrometry (EDS) analysis using instruments, which consisted
of a scanning electron microscope BS 300 and an X-ray
microanalyser EDAX 9100/60. Samples of solid phase were
dried and coated with gold before the EDS analysis.
B i o l o g i c a l U t i l i z a t i o n o f G y p s u m
f r o m “ S t a b i l i z a t e ”
Series of anaerobic tests were studied in a fed batch
reactor in the thermostat at 30 °C. Samples of “stabilizate”
were kept in static conditions for a period of 40 days at pH
7.5. The weight of “stabilizate“ was 20 g. The stock culture
of SRB was used as an inoculum (10 %, v/v). The total
volume of feed solution consisted of 200 ml distilled water
and 300 ml selective nutrient medium for SRB (DSM-63
– Postgate’s C medium without sulfates). The abiotic control
was carried out without the SRB application at the same
conditions. After 40 days the solid phase was filtered, dried
and analyzed using the qualitative X-ray analysis and energy
dispersive spectrometry (EDS) analysis.
Results
The formation of black precipitates and the sensorial detection
of classical strong H 2 S smell were observed after 3–4
days from the beginning of the process. These remarks were
not detected in the abiotic control until the end of the experiment.
Changes of sulphates concentration during the discontinuous
cultivation of sulphate-reducing bacteria using gypsum
contained in the “stabilizate” as the source of electron acceptors
for the growth of SRB are shown in Fig. 1. The results of
qualitative X-ray diffraction analysis of original “stabilizate”,