# The Use of Construction Waste to Remediate a Thermally Active Spoil Heap
## 3 Results and Discussion
### Acidification of Tailings  
Many of the environmental problems associated with rock material containing sulphide are related to the oxidation of pyrite and the occurrence of spontaneous acidification. A large part of the total sulphur contained in tailings is in the form of pyrite (FeS\({}_{2}\)) and other sulphides. They oxidize to sulphuric acid in the presence of water and oxygen. As a result of weathering, the pH values of the spoil heap body can drop dramatically to a very low pH value (2.0 to 3.5). [PERSON] et al. states that this highly acidified water can also have a pH value \(<\) 3.0. During its seeping into the deeper parts of the spoil heap body, it dissolves the surrounding mineral matrix, and is thus enriched with various metals (Al, Mn, etc. ), cations and salts [40]. This chemical oxidation of pyrite (initiation reaction), where acidity is produced and thus Fe is released, can be in a simple way described as follows:  
\[2\;\mathrm{FeS}_{2}+7\;\mathrm{O}_{2}+\mathrm{H}_{2}\mathrm{O}\to 2\;\mathrm{Fe} ^{2+}+4\;\mathrm{H}^{+}+4\;\mathrm{SO}_{4}^{2-}\]  
The oxidation of pyrite is catalyzed by acidophilic bacteria such as _Acidithiobacillus ferrooxidans_, which are present in coal seams and are capable of functioning even in environments with very low oxygen content (less than 1.0% partial pressure). This means that as long as acidic water penetrates through the spoil heap, pyrite oxidation will continue to occur even deep inside the body, regardless of efforts to restore vegetation and to stabilize the surface. This bio-chemical oxidation of Fe\({}^{2+}\) to Fe\({}^{3+}\), which is influenced by the bacteria of _Acidithiobacillus ferrooxidans_, whose metabolism starts to be fully manifested only at pH \(<\) 3.0, can be described in a simple way as follows:  
\[4\;\mathrm{Fe}^{2+}+4\;\mathrm{H}^{+}+\mathrm{O}_{2}\xrightarrow[Acidithiobacillus ferrooxidans]{}4\mathrm{Fe}^{3+}+2\;\mathrm{H}_{2}\mathrm{O}\]  
The above presented reaction is a limiting factor in the oxidation of pyrite because the oxidation of Fe\({}^{2+}\) to Fe\({}^{3+}\) is slow under abiotic conditions. The chemical oxidation of pyrite by bacterially produced Fe\({}^{3+}\) ions, which remain in soluble form under acidic conditions and react chemically with the pyrite present to form sulphate and ferric ions, can be described as follows:  
\[\mathrm{FeS}_{2}+14\;\mathrm{Fe}^{3+}+8\;\mathrm{H}_{2}\mathrm{O}\to 2\; \mathrm{SO}_{4}^{2-}+16\;\mathrm{H}^{+}+15\;\mathrm{Fe}^{2+}\]  
At pH \(<\) 3.5, there is a chemical hydrolysis of Fe\({}^{3+}\), with subsequent acidification of the environment, because under these conditions Fe\({}^{3+}\) hydrolyzes to:  
\[[\mathrm{Fe(OH)}]^{2+}\rightarrow[\mathrm{Fe(OH)}_{2}]^{+}\rightarrow\mathrm{ Fe(OH)}_{3}\]  
\[\mathrm{Fe}^{3+}+3\;\mathrm{H}_{2}\mathrm{O}\leftrightarrow\mathrm{Fe(OH)}_{3}+3\; \mathrm{H}^{+}\]  
The above presented chemical reactions show that due to the acidic nature of the tailings, the leaching or dissolution of potentially hazardous metals can be much more intense if the tailings are improperly handled during spoil heap remediation. As a corrective measure, we therefore recommend increasing the neutralisation potential by mixing the tailings with DCW, especially in areas with higher pyrite concentrations or with the presence of jarosite, which can be an important indicator in this respect. Increasing the neutralisation potential may also lead to a slower kinetics of pyrite oxidation. Due to the total mass of pyrite in many tailing piles and the relatively slow rate of water movement through them, it can be expected that acid mine drainage (AMD) will be emitted for decades, if not longer [20]. This is the reason why any attempt to recultivate the surface of the spoil heap, e.g., by applying a layer of soil and subsequent forest recultivation is usually unsuccessful in the long term. The study of tailings acidification is also necessary, with regard to its massive use in the redevelopment and regeneration of the area affected by mining in mountainous landscapes.