**6. Physicochemical complexities behind coupling long-term reactive barriers with horizontally constructed sub-surface wetlands**

In a mine site located in the northern part of Chile, a pilot plant of AMD treatment designed by the company ISMP SpA consisting of LTPRB-HCSSW combined has been installed ideally to secure the water quality of surface waters for a timespan of a few years. It has been widely proved that different types of vegetation have absorption capabilities for different metals such as that shown in **Figure 12** which corresponds to Phragmites Australis.

The inlet pH was 5.0 and the aqueous solution flowrate is 1 *Ls*1. The pilot plant consisted of 30 m<sup>3</sup> effective volumes of LTPRB-HCSSW combined. The system has been designed in a way to promote and enhance the rate at which sulphate reduction reaction [Eq. (29)] is produced triggering as a secondary mechanism the heavy metal precipitation as metal sulphides. To accomplish this, the acidity is provided by introducing metallic scrap into the LTPRB. The dissolution of the metallic scrap by oxygen *Fundamentals and Practical Aspects of Acid Mine Drainage Treatment: An Overview from Mine… DOI: http://dx.doi.org/10.5772/intechopen.104507*

#### **Figure 12.**

*Picture of the AMD treatment pilot plant system implemented before being covered (a), and close-up to one of the Phragmites Australis used in the HCSSW (b).*

reduction is inhibited and it is expected to especially be driven by complexation reactions.

$$2\text{CH}\_2\text{O}\_{(s)} + \text{SO}\_{4(aq)}^{2-} + 2\text{H}\_3\text{O}\_{(aq)}^+ \leftrightarrow \text{H}\_2\text{S}\_{(g)} + 2\text{CO}\_{2(g)} + \text{3H}\_2\text{O}\_{(l)} \times \text{M}\_{(aq)}^{n+} + y\text{S}\_{(aq)}^{2-} \to \text{M}\_x\text{S}\_{(l)}\tag{28}$$

with

$$m\mathbf{x} - 2\mathbf{y} = \mathbf{0}$$

Metal sulphide precipitation, though, is required to be formed as much as possible at pH values where hydrolysis of sulphide ions is low which could be accomplished by evaluating the competitiveness for sulphide complexation within the system. Otherwise, the acidification of the aqueous phase could again take place following Eq. (30).

$$2\text{M}\_{(aq)}^{n+} + n\text{HS}\_{(aq)}^{-} + H\_2\text{O}\_{(l)} \rightarrow \text{M}\_2\text{S}\_{n(s)} + nH\_3\text{O}\_{(aq)}^{+} \tag{29}$$

Preliminary results indicate that LTPRB removes sulphate at between 10 and 30 times the rate reported for sulphate removal observed using wetlands only [92, 93]. The HCSSW allowed stabilising of the pH between 6 and 8. Preliminary computations indicate that the volume of control used is about one or two orders of magnitude lower than classic wastewater treatments. All these systems are complex by nature, but they could be engineeringdesigned from the beginning to enhance/inhibit reactions to avoid AMD. Now, considering the residence time of the AMD flowing through the system, is there any chance to adjust all these mechanisms to act standing alone at the appropriate rates enabling a wastewater treatment to last for a few years by itself keeping as much as possible the permeability of the porous media? This is certainly an opportunity still to be accomplished.

### **7. Conclusions**

Acid Mine Drainage (AMD) formation is yet a process ill-understood. The AMD occurs spontaneously, and it is highly dependent on the local atmospheric conditions which makes it difficult to predict any of its characteristics. Preliminary strategies aiming at forming caps around these solid wastes could be considered a good first step towards preventing the formation of AMD. Nevertheless, in cases where AMD is already formed new strategies for isolating the wastes need to be considered.

Although the precursors of AMD such as sulphide minerals, and notably pyrite, water and oxygen are known to be involved, the physical chemistry and the biology linked to its production need to be studied in more detail and integrated, particularly in long-term reaction kinetics of the different mechanisms taking place.

Several strategies have been suggested to treat AMD. The condition of mine closure takes this challenge to the next level requiring a solution that cannot be intensive in the use of personnel, energy, or reagents.

Strategies involving passive wastewater treatment technologies which attempt to somehow mimic natural systems look promising.

Nowadays, the difference between passive and active wastewater treatment has become a thin line. On one hand, even passive wastewater treatment strategies require to some point the involvement of human resources. On other hand, new long-term permeable reactive barriers have been pointed out as wastewater treatment strategies than can gather several aspects of passive treatment systems such as low maintenance requirements to work and, simultaneously, exhibit fast wastewater treatment kinetics. Some practical aspects associated with implementing long-term permeable barriers coupled with constructed wetlands were presented but improvements with regard to the efficiency of these strategies to remove sulphate, heavy metals and other contaminants are still a matter of study.

Finally, perhaps the most relevant conclusion that can be drawn from this chapter is that addressing AMD generation, prevention or treatment is in fact a multidisciplinary topic where the conjunction of many specialities occurs such as chemistry, physics, hydrology, microbiology, electrochemistry, among others.
