Stockholm June 1993.
Material deterioration at different process
conditions in waste deposits - Prestudy.
Erik Levlin
Water Resources Engineering
Royal Institute of Technology
Final report 1992/93 to:
Avfallsforskningsrĺdet AFR
Materialnedbrytning vid olika processbetingelser i en avfallsdeponi.
Omrĺde 4 Miljöanpassad deponeringsteknik, Diarienr. 314, dossienr.
230.
1. Background.
The
purpose of this project is to get a better understanding of the reaction
mechanisms and the interaction between the deterioration by corrosion of
metallic scrap and the deterioration of organic wastes. A waste deposit will be
supplied with a mixture of easily decomposed organic material, metal scrap,
plastics and other solid wastes. Deterioration of metallic scrap in a waste
deposit plays an important role for determining the leaching of metal ions from
a waste deposit.
The
deterioration of metallic scrap is controlled by corrosion processes, which
depend on the process conditions in the waste deposit. The process conditions
such as pH and redox potential are a result of the deterioration of organic
materials in the waste. To understand the mechanisms for deterioration of
metallic scrap, both the deterioration of metallic and organic material and the
interaction between them must be studied.
There
are both differences and similarities between the conditions in soil and the
conditions in a waste deposit. However, the experience achieved by studies of
corrosion in soil (Levlin 1992) can be useful for studying deterioration by
corrosion of metal scrap in a waste deposit. A waste deposit can be regarded as
a well drained soil, however, with a very high content of organic matter. Well
drained soils often have a small content of organic matter due to fast bacterial
deterioration.
2. Process conditions in a waste deposit.
The
process conditions in a waste deposit produced by bacterial degradation of
organic waste goes through different phases. According to Ehrig (1983),
Förstner (1988) and Calmano (1989) there are three phases; first an aerobic,
followed by an acid anaerobic and last a methane producing phase. Since freshly
landfilled waste contains a considerable amount of air, the first phase of the
degradation is aerobic. During the aerobic phase the organic material is
decomposed by bacteria, under production of carbon dioxide. Under aerobic
conditions high temperatures of 80° to 90° can be measured. The length of the
aerobic phase depends on the compaction of the landfill. When the oxygen is
consumed the anaerobic degradation takes place. The oxygen penetration is very
low in high density landfill and therefore the anaerobic processes dominate.
During the acid anaerobic phase fatty acids and carbon dioxide is produced. The
degradation of material can in both aerobic and anaerobic degradation, be
divided into two phases, a first phase with decreasing pH-value and a second
phase with increasing pH-value. Due to the production of organic acids the
pH-level decreases during the first phase. In the second phase the pH-value
increases again, when the organic acids are further depredated. In the
following methane producing phase these acids are converted to methane and
carbon dioxide.
Table
1 by Christensen and Kjeldsen (1998) shows the four major bacterial groups
involved in anaerobic waste degradation during anaerobic conditions including
the reactants and products of the bacterial processes. Figure 1 shows a sketch
of the degradation process. During the fermentative and acetogenic processes
hydrogen and organic acids are produced by the bacteria. These products are
consumed by the methanogenic and the sulphate reducing bacterias. The rate
depending process is the hydrolysis, which dissolves the organic matter. Figure
2 shows the variations with time for gas and leachate composition.
Table 1. Examples of important reactions for
four groups of bacteria involved in anaerobic
waste degradation (Christensen and Kjeldsen 1998).
|
Reactants converted to products |
|
|
Fermentative processes |
2CH3COOH
+ H2 + 2CO2 |
|
Acetogenic processes |
CH3COOH
+ CO2 + 3H2 |
|
Methanogenic processes |
CH4
+ 2H2O |
|
Sulphate reducing processes |
HS−
+ 4H2O |
|
HCOOH:
formic acid, CH3COOH: acetic acid, CH3CH2COOH:
propionic acid, CH3C2H4COOH: butyric acid, C6H12O6:
glucose, CH3OH: methanol, CH3CH2OH: ethanol,
C6H5COOH: benzoic acid, CH4: methane, CO2:
carbon dioxide, H2: hydrogen, SO42−:
sulphate, HS−: hydrogen sulphide, HCO3−:
hydrogencarbonate, H+: protone, H2O: water. |
|
Figure 1. Substrates and major bacterial groups involved in anaerobic waste degradation (Christensen and Kjeldsen 1998).
Figure 2. Illustration of developments in gas and leachate composition in a landfill (Christensen and Kjeldsen 1998).
3.
Factors influencing corrosion.
The
corrosion is mainly influenced by the following factors:
3.1
Supply of oxygen to the metal surface.
Corrosion of metal is an electrochemical reaction,
there oxidation of the metal is the anodic reaction and reduction of oxygen is
the cathodic reaction. The corrosion rate is often controlled by the supply of
oxygen to the metal surface and is therefore higher in aerobic than in
anaerobic conditions.
3.2 Hydrogen consumption
Hydrogen evolution is an cathodic reaction that may substitute oxygen reduction. As the electrode potential for hydrogen evolution is lower than for oxygen reduction the corrosion rate is small. However, the corrosion rate increases when hydrogen consumption by bacteria increases the hydrogen evolution. Table 1 shows that hydrogen is consumed in both methanogenic and sulphate reducing processes. That corrosion may occur due to hydrogen consumption by sulphate reducers was first proposed in 1930 by von Wolzogen Khür (1953). Later also the influence of hydrogen consumption by methane producers has been studied (Lorowitz et al 1992).
3.3 Formation of corrosion products on the
metal surface.
Metallic
materials are thermodynamical unstable. The corrosion is, however, stifled by a
layer of corrosion products on the metal surface. The corrosion rates are
controlled by kinetics of the corrosion processes, which mainly depend on that
kind of corrosion products that are deposited on the metal surface and the
solubility of the corrosion products. A layer of corrosion products can be
protective if the concentration of dissolved metal ions is smaller than 10-6
mol/l.
3.4 pH-value.
A lower pH-value increases the solubility of corrosion products thus decreasing the protective effect of the corrosion products. Also the hydrogen evolution increases and when the pH-value gets lower than 4 hydrogen evolution may substite oxygen reduction as cathode reaction.
3.5 Complex formation by organic acids.
Humus is compared with clay, chalk and sand the most corrosive (von Beackman and Schwenk 1971). Clay is more corrosive than chalk and sand. The pH-value of the humus layer in a podsol is very low due to organic acids such as humic and fulvic acids produces by biological degradation of the organic material (Wiklander 1976). The low pH-value and the complex formation of fulvic acids with metal ions dissolves the protective corrosion products and thereby increases the rate of corrosion.
3.6 Solubility of corrosion products.
The solubility of the corrosions products is high at low pH-values both in aerobic and in anaerobic process conditions. However, the solubility of the corrosion products of iron is highest in anaerobic process conditions while the solubility of the corrosion products of cadmium is highest in aerobic process case. Figure 3 (Bourg 1988) shows the solubility of iron, manganese, cadmiumand mercury, versus redox potential and pH-value. In the aerobic case is iron precipitated as ferric hydroxide, while cadmium in the anaerobic case is precipitated as cadmium sulphide. In the anaerobic case is corrosion of the metal controlled by protective properties of metal sulphide precipitations on the metal surface.
Figure
3. Solubility of iron, manganese, cadmiumand mercury, versus redox potential
and pH-value (Bourg 1988).
The corrosion mechanisms of iron in soil (Levlin 1991) may be used as an example. The corrosion in aerobic soil is stifled by the deposition of iron(III) oxide-hydroxide. During anaerobic conditions sulphate reducing bacteria produce hydrogen sulphide. the organic material is decomposed by sulphate and nitrate reducing bacterias. If the hydrogen sulphide concentration is larger than the ferrous ion concentration, the corrosion of iron may in this phase be stifled by deposition of ferrous sulphide. The ferrous ions in soil orginates from leaching of inorganic soil particles by humic acids.
3.7 Influence of metal sulphide precipitations.
In anaerobic˙ process conditions is metal sulphide precipitated on the metal surface and may thereby prevent corrosion. Figures 4 through 7 shows potential/pH equilibrium diagrams of the ternary systems metal-sulfur-water (Horváth and Novák 1964) for iron (figure 4), zink (figure 5), lead (figure 6) and cupper (figure 7). The hatched area corresponds to the stability domain of metal sulphide at a hydrogen sulfide concentration of 1 atmosphere.
|
|
|
|
Figure 4. Potential/pH equilibrium diagram of
the ternary system Fe-S-H2O (Horváth and Novák 1964). |
Figure
5. Potential/pH equilibrium diagram of the ternary system Zn-S-H2O
(Horváth and Novák 1964). |
|
|
|
|
Figure 6. Potential/pH equilibrium diagram of
the ternary system Pb-S-H2O (Horváth and Novák 1964). |
Figure
7. Potential/pH equilibrium diagram of the ternary system Cu-S-H2O
(Horváth and Novák 1964). |
|
The formula for metal sulphide precipitation
is: Me2+ + H2S →
MeS + 2H+ The pH-value for the solubility line for
metal sulphiIde increases at decreasing hydrogen sulphide concentrations
giving an decrease in the stability area. In the diagrams for iron, zinc and
lead is the solubilty line marked for the pH2S-values (the
negative loggaritm of the hydrogen sulphide concentration) 0, 2, 4 and 6.
Figure 8 shows the solubility lines for a number of metals in a pH versus
log(H2S) diagram. Even if the pH-level of the waste is abow
these pH-levels for˙ stable˙ sulphides, corrosion of the scrap may occur due
to pitting corrosion. Inhomogenities in the sorrounding waste may create an
uneven distribution of the corrosion current on the metal surface. At areas
there the anodic reaction dominates, metal sulphide precipitation may create
an localy decreased pH-level. This can be illustrated by a formula there the
species carrying the corrosion current is marked vertically: |
|
|
Figure 8. pH-value for the solubilty of metal sulphide versus logaritm of hydrogen sulphide concentration.
Me + H2S
MeS + 2H+ + 2Cl– 2e– |
The
corrosion current is assumed to be carried by chloride ions. The total current is
the sum of the contributions from the different ions. The mobility of the
chloride ions is 75 cm2/Ωekv and for the hydrogen ions 350 cm2/Ωekv.
The hydrogen ions make an equal contribution to the corrosion current when the
hydrogen ion concentration is 20% of the chloride ion concentration. If the
chloride ion concentration is 3% this gives the pH-level 1. As long as the pH
is above 1, the formula shows that an enrichment of hydrochloric acid occurs,
leading to a decreased pH-value. If the solubility line for metal sulphide lies
above pH 1 the metal sulphide is dissolved at the anode areas leading to
pitting corrosion. The metals that in figure 8 has a solubulity line above pH
1, such as metal scrap of zinc and iron, should therefore in anaerobic environment
be sensible to pitting corrosion. The metals that are dissolved in the anodic
areas are precipitated in the bulk of the waste there the pH-value is higher
than at the anodic areas. The pitting corrosion of the metal scrap should
therefore not lead to an increased metal ion concentration in the leachate.
4. Estimation of corrosion
The corrosion of metallic scrap in the waste is estimated to be largest when the pH-value is at minimum and the amount of organic acids is largest. Figure 2 shows that the concentratiion of zinc and iron is highest during this period. If the pH-value is low enough corrosion in the anaerobic case increases due to hydrogen evolution.
Since the methanogenic phase of degradation is longer than the acid phase, the total corrosion of the scrap may be controlled by process conditions occuring in the later methanogenic phase, in spite of a lower corrosion rate in this phase of degradation.
5. Experimental study.
This prestudy is proposed to be continued by an experimental study, where metallic materials are exposed to organic waste at different process conditions, aerobic as well as anaerobic, occurring in different phases during the deterioration of a waste deposit. The metals and process conditions have to be chosen from three main criteria;
- Which are the main process conditions during
the different phases after the deposition of waste?
- Which metals may create problems by leaching
from the waste deposit? This is a combined effect of the toxity of the metal
and the relative occurrence in the waste.
- Which corrosion products may be precipitated
and stifle the corrosion during the different process conditions?
The process conditions in the experiments may be controlled by exposing the metal for organic material containing different organic acids. Aerobic conditions are created by infiltration of air through the experimental cell. Anaerobic process conditions will be achieved by limited oxygen supply combined with oxygen consumption through bacterial degradation of organic material. There are corrosion due to two cases of process conditions, that has to be studied:
-
Corrosion during the lowest pH-value achieved during an early phase of
degradation of the waste.
- Corrosion in the methanogenic phase of
degradation. In this case corrosion due to hydrogen consumption by methanogenic
and sulphate reducing bacteria as well as the influence of metal sulphide
precipitation will be studied.
The corrosion of the metals can be studied with
ordinary electrochemical methods. The corrosion products precipitated on the
metal will be examined and analysed. Methods to maintain the corrosion products
produced under anaerobic conditions from oxidation before analysis must be
considered. The corrosion can also be determined by the weight reduce of the
metal.
6. References.
von Beackman, W and Schwenk, W (1971) Handbook
of cathodic protection, Portcullis Press
Bourg, A C M (1988) Metals in aquatic and
terrestrial systems: sorption, speciation, and mobilization, Chemistry and
biology of solid waste, dredged material and mine tailings, Springer Verlag,
Berlin, ISBN 3-540-18231-4, page 3-32.
Calmano, W (1989) Schwermetalle in kontaminierten
Feststoffen, Verlag TšV Rheinland, Cologne, ISBN 3-88585-664-6
Christensen, T H and Kjeldsen, P (1989) Basic
biochemical processes in landfills, Chapter 2.1 of sanitary landfilling:
process, technology and environmental impact, Academic Press, London, ISBN
0-12-174255-5.
Ehrig, H-J (1983) Quality and quantity of sanitary
landfill leachate, Waste management & research, Vol 1, page 53-68
Förstner, U (1988) Geochemische vorgänge in
abfalldeponien, Die geowissenschaften, Vol 6, No: 10, page 302-306
Horváth, J and Novák, M (1964) Potential/pH
diagrams of some Me-S-H2O ternary systems and their interpretation
from the point of view of metallic corrosion, Corrosion Science, Vol 4, page
159-178.
Levlin, E (1992) Corrosion of water pipe
systems due to acidification of soil and groundwater, Doctors thesis, Applied
electrochemistry and corrosion science, Royal Institute of Technology,
Stockholm, TRITA-TEK 1992:01, ISBN 91-7170-094-3
Levlin, E (1991) Corrosion of cast-iron in soil
- discussion of two theories, especially with respect to the H2O/Fe2+-ratio,
Water Resources Engineering, Royal Institute of Technology, TRITA-VAT-4912,
Stockholm
Lorowitz, W H, Nagle D P and Tanner, R S (1992)
Anaerobic oxidation of elemental metals coupled to methanogenesis by
methanobacterium thermoautrophicum, Environmental Science and Technology, Vol
26, No 8, page 1606-1610.
Kersten, M (1988) Geochemistry of priority
pollutants in anoxic sludges: cadmium, arsenic, methyl mercury and chlorinated
organics, Chemistry and biology of solid waste, dredged material and mine
tailings, Springer verlag, Berlin, ISBN 3-540-18231-4, page 170-213
Wiklander, L (1976) Marklära, Agricultural
college of Sweden
von Wolzogen Kühr, C A H and van der Vlugt L S
(1953) Aerobic and anaerobic iron corrosion in water mains. Journ. American
Water Works Association, January, page 33-46.
POST SCRIPTUM:
Since this report was written deposition of
incinerable waste has been prohibited and deposition of organic waste will be
prohibited from year 2005, which will change the process condition and also the
degradation of metallic scrap n future waste deposits.