**5. Practical aspects of semi-passive wastewater treatment: coupling long-term permeable reactive barriers with horizontally constructed sub-surface wetlands**

This section is devoted to the design of AMD treatment systems which can operate in mine closure situations, close to stand-alone and unassisted systems to treat wastewaters. These treatment paths attempt to gather many of the mechanisms previously revised acting simultaneously to clean up AMD streams. Since most of these mechanisms need to take place spontaneously, many of these wastewater treatment systems attempt to mimic nature. Particularly, this subchapter addresses some aspects of two of these systems: Long-Term Permeable Reactive Barriers (LTPRB) and Horizontally Constructed Sub-Surface Wetlands (HCSSW).

On one hand, wetlands are one of the preferred passive wastewater treatment strategies to be implemented as a tertiary wastewater treatment [81]. Although it is considered a mature technology by many authors, it is one of the most difficult systems to model and understand. The latter is not only because of the many physicochemical interactions simultaneously taking place between all the species belonging to the system but also because of the multiple roles the local biota may play. For instance, sulphate ions are difficult to reduce using inorganic species only [82]. Indeed, looking at the Pourbaix diagrams, sulphate ions are stable over the whole pH range either in their acid form or not. It has been pointed out, though, that such reduction can be accomplished at the surface of organic material where carbon has a pivotal role. Indeed, it is well known that anaerobic systems, as well as aqueous media set in contact with solid metals, promote the growth of sulphate-reducing bacteria [83]. Carbon, among all its functions such as respiration, fermentation, methanogenesis, denitrification, and iron reduction would have a key role in sulphate reduction [84]. Sulphate reduction reactions are summarised by Eqs. (27), (28).

$$\begin{aligned} 2\text{CH}\_3\text{CHOH}\text{COO}^-\_{(aq)} + \text{SO}^{2-}\_{4(aq)} + \text{H}\_3\text{O}^+\_{(aq)} \\ \rightarrow 2\text{CH}\_3\text{COO}^-\_{(aq)} + 2\text{CO}\_{2(g)} + 3\text{H}\_2\text{O}\_{(l)} + \text{HS}^-\_{(aq)} \end{aligned} \tag{26}$$

$$\text{CH}\_3\text{COO}\_{(aq)}^- + \text{SO}\_{4(aq)}^{2-} + 2\text{H}\_3\text{O}\_{(aq)}^+ \rightarrow 2\text{CO}\_{2(g)} + 4\text{H}\_2\text{O}\_{(l)} + \text{HS}\_{(aq)}^- \tag{27}$$

These reactions, though, are not in total agreement with classic electrochemical fundamentals. The standard electrode potential of the sulphate reduction is 220 mV vs SHE which is not fully consistent with the stability region of sulphate ions declared in Pourbaix diagrams [35]. Authors have indicated that such reduction is complex and involves metastable products [35, 85]. The reduction reaction would then consist of at least two reactions in series which are triggered by the sulphate activation by ATP sulphurylase increasing the potential to about 60 mV where the reduction from sulphate to sulphite is achieved. However, the reduction of sulphite to sulphide is yet not fully understood [86]. In addition, another disadvantage of these two reactions is, in principle, the production of carbon dioxide identified as a greenhouse gas. Additionally, authors have pointed out that the low performance in eliminating phosphorous may also be observed for other contaminants which usually increase the requirements in terms of residence time and/or surface lands available to implement these systems [87]. Whenever these systems are not available naturally, constructed wetlands are engineering-designed which can be implemented vertically or horizontally [84]. The latter corresponds to the case study to be described in the next section.

On the other hand, long-term permeable reactive barriers have captured interest from the scientific community since it houses several materials to treat wastewaters securing the correct quality of groundwater resources. The phenomena embedded in this type of strategy are mainly chemical or biological degradation, precipitation, and adsorption to immobilise contaminants [88]. Due to the similarities in dealing with organic matter between reactive barriers and wetlands, sulphate reduction bacteria can also be promoted in these systems. Additionally, the permeability of these systems needs to be secured. Unreactive or low-reaction alkali materials are used as a fixed bed introducing more reactive materials inside the pores that can range from specifically designed materials to wastes from other industries such as ferrihydrite-bearing soils or nanostructured calcium silicate adsorbent, among others [89, 90]. Since the growth of vegetation is not present in these systems, the permeability may be designed to avoid dead volumes or volumes with low mixing capabilities. Long term reaction kinetics is still a matter to do research on. Considering that a few reactions are associated to oxidation mechanisms by oxygen, and given the relatively low concentration of the gas, particularly in low permeability media, atmospheric corrosion perspective could improve the knowledge on these matters [91].
