J. Fajtl 1 , R. Tichý 2
1
Faculty of Agriculture,
2
Faculty of Biological Sciences, University of South Bohemia, CZ
K. Wendt-Potthoff, K. Friese, M. Koschorreck, P. Herzsprung
Department of Inland Water Research, UFZ, D
ABSTRACT
Our study was set up to evaluate sulphate leaching from aerated mine lake sediment that was treated by different additions of NaOH. NaOH was added to the sediment to suppress biochemical oxidative processes. During aerobic cultivation of the sediment slurry, the total sulphate production (SO4
TOTAL
) was estimated from the sum of (1.) adsorbed sulphate on the solid phase of the sediment at given pH, from (2.) sulphate precipitated as gypsum, and from (3.) soluble sulphate, that was directly measured in the solution. SO4
TOTAL
was produced at higher concentrations from the sediment that was supplemented by NaOH (up to 0.34 mol/kg, dry sediment) than from the control (0.25 mol/kg) when SO4
TOTAL
at the end of the experiment (375 hours) are compared. However, the addition of 0.3-0.75 mol of concentrated NaOH per 1 kg of dry sediment can suppress sulphate production from aerated mine lake sediment for 10-100 hours of intensive sediment aeration.
Keywords: sulphate production, freshwater sediments, NaOH supplement
1. INTRODUCTION
Aeration of freshwater sediments rich in sulphurous compounds results in a production of sulphate containing leachates. Since this aeration can not be always prevented, measures are to be taken to mitigate this serious environmental problem (Maass and Miehlich, 1988). Among other methods, addition of NaOH is frequently proposed (Anonymus, 1983; Rose and Eliot, 2000).
Direct effect of NaOH is neutralization of acidity. However, numerous authors have been speculated that NaOH supplement to aerated sediments can have also an indirect mitigate effect. pH-neutral conditions are unfavourable for appropriate microbes responsible for biochemical sulphate production, e.g. thiobacilli.
The aim of the work is to evaluate sulphate production from aerated mine lake sediment that was supplemented by different NaOH levels. For this purpose, the total sulphate that was produced from the sediment (SO4
TOTAL
) during aeration experiment was estimated. SO4
TOTAL
was the sum of soluble sulphate (SO4
SOLUBLE
), sulphate adsorbed on the solid phase of the sediment
(SO4
ADSORBED
) and sulphate precipitated to a form of gypsum (G). Since SO4
SOLUBLE
was measured, SO4
ADSORBED
and G had to be evaluated for each measurement.
2. MATERIAL AND METHODS
Sediment sample was taken from 40 years old mine lake in Lusatica, Germany. 10 l of anoxic dark sediment with its interstitial water was transferred to the laboratory under Ar-atmosphere in a polyethylene bottle. Then, the sediment was stored at 6°C in the dark for 14 days. Before the experiments, original pore water was removed by a centrifugation at 3750 rpm for 30 minutes.
(for selected parameters of original sediment, see Table 1) Then, following experiments were done:

Sulphate adsorption/desorption. Since original sediment contained significant level of Ca, Ca was removed from the system by its acidification at pH 2 by concentrated HCl and centrifugation of the slurry at 3759 rpm for 30 minutes. The solid particles of the sediment were resuspended in distilled water at a solid/liquid ratio of 1/7.5 (kg of dry sediment/L of the liquid). This procedure protected to gypsum formation during the ads/desorption tests. For ads/desorption tests, pH of the slurries from 3 to 13 was adjusted by an addition of concentrated NaOH and/or HCl (Merck).
Prepared slurries were agitated (95 rpm) at 27°C in the dark on a horizontal shaker (GFL 30 17) for 1 hour. Then, pH was measured and samples for dissolved sulphate were taken.
SO4
ADSORBED
was calculated by comparing dissolved sulphate before and after equilibrium experiment. SO4
ADSORBED was put into the relation with pH by a following model:
SO4
ADSORBED
= A+(B/(1+(C*(EXP(D*pH))))) (mol/kg) (1) where A, B, C and D are specific model parameters that were fitted by non-linear regression analyses (Marquart least-square method) with Statgraphics version 2.6. program. (EXP is the base of natural logarithm).
To evaluate gypsum formation, calcium adsorption/desorption behaviour was estimated by an equilibrium experiment. For calcium ads/desorption tests, pH of the slurries from 3 to 13 was adjusted by an addition of concentrated NaOH and/or HCl. Prepared slurries were agitated (95 rpm) at 27°C in the dark on a horizontal shaker (GFL 30 17) for 1 hour. For each suspension, a solid/liquid ratio of 1/7.5 was kept. Then, pH was measured and samples for dissolved calcium were taken. Ca
ADSORBED
was calculated by comparing dissolved calcium before and after the experiment, since G formation was not allowed during this experiment. Ca
ADSORBED
was put into the relation with pH by a following model:
Ca
ADSORBED
= A+(B/(1+(C*(EXP(D*pH))))) (mol/kg) (2) where A, B, C and D are specific model parameters that were fitted by non-linear regression analyses (Marquart least-square method) with Statgraphics version 2.6. program. (EXP is the base of natural logarithm).
Then, G was calculated for each measurement of all variants with NaOH amendment (see following text) using Equation 3:
G = Ca
TOTAL
- Ca
SOLUBLE
- Ca
ADSORBED
(mol/kg) (3)
In this equation, total calcium level in the sediment (Ca
TOTAL
) and dissolved calcium (Ca
SOLUBLE
) were directly measured. Ca
ADSORBED was calculated from Equation 2 at given pH. G was estimated for measurements where gypsum was be formed. It was checked by equilibrium simulations using
ECOSAT program (Keizer and Riemsdijk, 1994). In systems where G was not formed, G was equal to zero in following calculations.
SO4
TOTAL
production during aerobic cultivation of NaOH supplemented sediment. The control
(i.e. sediment without any NaOH addition) and 12 treatments with amendment of 0.015 - 0.75 mol of concentrated NaOH per 1 kg of dry sediment were prepared at a solid/liquid ratio of 1/7.5.
All sediment treatments were put into Erlenmeyer flasks that were closed by cotton plugs. Then, they were agitated (95 rpm) on a horizontal shaker (GFL 30 17) at 27°C in the dark for 375 hours.
In each flask, pH was measured and samples for dissolved sulphate and calcium were taken.
Then, SO4
TOTAL
was calculated for each measurement using Equation 4:
SO4
TOTAL
= SO4
SOLUBLE
+ SO4
ADSORBED
+ G (mol/kg) (4)
Here, SO4
SOLUBLE
was directly measured. SO4
ADSORBED
at given pH was calculated from Equation
1. G was expressed from Equation 3 for each measurement.
Analyses. pH was measured directly in the flasks kept shaking with pH 325-A/SET 2 pH meter with pH-combined electrode SenTix 50 with hydrogen reference electrode.
Afterwards, 10ml of the slurry was taken, centrifuged at 3750 rpm for 30 minutes and filtered through sterile 0.45um pyrogen free filter (FP 030/1). Clean samples were stored under liquid nitrogen.

Samples for dissolved sulphate were 10 times diluted and acidified at pH 2. Then, they were measured by ion-chromatography method (ICA-5000 system ion-chromatograph, auto-sampler
ICA-5450).
Samples for dissolved calcium were acidified with 65% HNO3 (MERCK) at a final concentration of 2M. Then, Ca was measured by ICP/OES method (OPTIMA 3000, Perkin Elmer
AS 90).
Total Ca content in the sediment was detected by wet micro-wave digestion (MARS 5) at
100°C for 24 hours. The digests were analysed by ICP/OES method.
3. RESULTS AND DISCUSION
3.1. Evaluation of SO4
TOTAL
To evaluate SO4
TOTAL
, SO4
ADSORBED at given pH and G was calculated for each measurement, since SO4
SOLUBLE
was directly measured.
SO4
ADSORBED
decreased with pH increasing, see Figure 1. At pH 9-12.5, low concentrations of sulphate were adsorbed (0.01

0.005 mol/kg, dry sediment). In other words, SO4
ADSORBED
was removed into the solution. At pH<9, SO4
ADSORBED increased and reached 0.08 mol/kg at pH of 3.
Similar ads/desorption sulphate behaviour was also published by Rose and Elliot, 2000.
SO4
ADSORBED
at given pH was described by the model that is expressed in Equation 1. Model parameters are given in Figure 1. The correlation index r 2 yielded 0.954 for 48 data points.
To evaluate G formation, Ca
ADSORBED at given pH was estimated. Ca
ADSORBED
decreased with pH lowering, see Figure 2. At pH 7-13, Ca
ADSORBED
reached stable values of 0.06

0.01 mol/kg. At pH<7, Ca
ADSORBED
decreased and reached the value of 0.002 mol/kg by pH of 3.7. Ca
ADSORBED
at given pH was described by the model that is given in Equation 2. Model parameters are given in
Figure 2. The correlation index r 2 yielded 0.945 for 34 data points. Then, Equation 2 was used to estimate G for each measurement of all variants with NaOH supplements via Equation 3 (data not shown). At the same time, SO4
ADSORBED
at given pH (Equation 1) was used to evaluate SO4
TOTAL using Equation 4.
3.2. SO4
TOTAL
production during aerobic cultivation of NaOH supplemented sediment
Surprisingly, in the control, 0.25 mol of SO4
TOTAL
was generated from 1 kg of the sediment
(dry mass) till the end of the experiment. In other treatments with NaOH supplement, up to 0.34 mol/kg of sulphate was produced till the end of the experiment, see Figure 3. It means that NaOH supplement did not suppress final SO4
TOTAL
production from aerated sediment when SO4
TOTAL
at the end of the experiment are compared. Higher SO4
TOTAL
production in variants with higher
NaOH supplement was caused by different solubility of sulphides in the system that is mainly control by pH (Stumm and Morgan, 1996). Solubility of sulphides increases with pH raising. It means that higher NaOH supplement, followed by a neutralisation or alkalisation, resulted in better availability of sulphides for biochemical production of SO4
TOTAL
. Therefore, less sulphate could be produced in variants with lower NaOH addition.
However, SO4
TOTAL
was also produced in lower concentrations in some treatments with
NaOH amendment than from the control during the experiment. It happened during 10 -100 hours of aeration when NaOH supplement of 0.3-0.75 mol/kg (concentrated NaOH per 1 kg of dry sediment) significantly suppressed biochemical sulphate production: only 50-89% of SO4
TOTAL was produced compared to SO4
TOTAL
in the control. It is given in Figure 4 where SO4
TOTAL
in treatments with NaOH supplement was compared to SO4
TOTAL
in the control during the experiment and it was expressed in %. Due to sulphate desorption that was caused by pH increasing after NaOH supplement up to 200% SO4
TOTAL
was estimated till 10 hours of the experiment when 0.2-0.75 mol/kg of NaOH was added. After 100 hours of aeration till the end of

the experiment, SO4
TOTAL
was higher in all treatments with NaOH amendment up to 149% than in the control due to the effect of sulphate biochemical production.
4. CONCLUSIONS
Sediment can be only treated for 10-100 with NaOH supplement of 0.3-0.75 mol of concentrated NaOH per 1 kg of dry sediment.
5. REFERENCES
Anonymus.
1983. U.S. Environmental Protection Agency. Neutralization of Acid Mine
Drainage, Design Manual. USEPA-600/2-83-001, Cincinnati, OH.
Keizer, M. G., Riemsdijk, W. H. Solving Chemical Equilibrium Problems by means of
Computer-Aided Instruction. Department of Soil Science and Plant Nutrition, Wageningen
Agricultural University, September 1994 .
Maass, B., Miehlich, G. Die Wirrkung des Redoxpotentials auf die Zusammnesetzung der
Porenlosung in Hafenschlicks-feldern. Mitt. Dtsch.Bodenkunde Ges ., 1988 , 56 , 289-294.
Rose, S., Elliot, W.C. 2000. The effects of pH regulation upon the release of sulphate from ferric precipitates formed in acid mine drainage. Applied Geochemistry 15, 27-34.
Stumm, W., Morgan, J. J. ( 1996 ). Aquatic chemistry. Chemical Equilibria and Rates in
Natural Waters. John Wiley and sons, Inc ., New York.

0.1
SO4
ADSORBED
=0.0054+(0.0799/(1+(0.01290*exp
(0.7119*pH)
)))
0.08
0.06
0.04
0.02
r2=0.954
n=48
0
2 4 6 8 10 12 14 pH
ADSORBED
0.08
0.06
Ca
ADSORBED
=0.0578+(-0.0688/(1+(0.0005*exp
(1.6217*pH)
))) r2=0.948
n=34
0.04
0.02
0
2 4 6 8 10 pH
Figure 2. Adsorbed Ca (Ca
ADSORBED
) as a function of pH.
12 14
0.8
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
Control
Treatments with NaOH supplement
0.75
0.015 mol/kg of
NaOH
0.7
0.6
0.5
0.4
0.3
0.2
50-89%
90-109%
110-149%
150-200%
0.1
0 100 200
Time, hours
300 400
0
0 50 100 150 200 250 300 350 400
Figure 4. The amounts of SO4
TOTAL
produced from sediment treated with
NaOH, compared to SO4
TOTAL of the control, expressed in %.
Time, hours
Figure 3. Total sulphate produced from the sediment during the experiment.
Parameter
ORIGINAL SEDIMENT SUSPENSION
Average value
Dry mass, % pH
Eh, mV
Conductivity, mS/cm
% O
2
(8°C)
ORIGINAL PORE WATER
Fe 2+ , mg/l
Fe 3+ , mg/l
Ca, mg/l
SO
4
, mg/l
TOTAL CONTENTS, dry mass of the sediment
Ca, mol/kg
Fe, mol/kg
21
5.8
-25
2.24
0.8
836
32
246
1935
0.06
1.83

0.7

0.2

3

0.03

0.1

5

0.5

5

15
0.01
0.09