**5. Conclusions**

Ancient mining waste contains large amounts of sulfurous minerals with highly and concentrated toxic elements that, exposed to weathering, undergo hydrolysis and oxidation–reduction reactions and form AMD, that dissolves mineral phases and release As, Pb, Cd, Zn, Cu and Fe. AMD cause secondary minerals formation such as clays, gypsum, jarosite, ferrihydrite, hematite, among others; when minerals of ferric oxyhydroxide and gypsum are agglomerated and strongly cemented form hardpans that are important sinks mainly of As. Likewise, jarosite, during its growth, incorporates As (V) and Pb (II) in its structure simultaneously, achieving less aqueous solubility.

Due to AMD, the PTEs by runoff reach rainwater and, due to the prevailing pH, neutral to slightly basic in stream water currents, the highly concentrated metals and As are absorbed into the finest particles of the sediments (clays, oxyhydroxides and OM). Thus, the mobility and fate of As and metals in sediments and groundwater is strongly controlled by the sorption process, and extent of adsorption is influenced by the presence of OM. Metals such as As, Pb, Cd and Cu are adsorbed to oxyhydroxides by specific adsorption, by inner sphere strong bonds, causing reduced mobility. On the other hand, ion exchange, although it also takes place in most metals, is more representative of Zn, by outer sphere weak bonds, that cause high mobility. Cu, in the presence of OC, rather than adsorbed by internal sphere bonds, this is co-precipitated in the cavities of the ferrihydrite. Likewise, in sediments rich in sulfides, As can precipitate as rejalgar (As4S4) or co-precipitate in pyrite. The coprecipitated/preadsorbed HA in the ferrihydrite inhibit As binding, promoting the mobility of As (v) > As (III).

The risk that PTEs represent to human health can be inferred by their speciation or fractionation chemical perfil. Although sediments have high concentrations of toxic metals, only those found in fraction 1 and 2, soluble/interchangeable and carbonate, respectively, are those what present greater mobility, toxicity, and bioavailability in aquatic environments. However, the concentrations of As and metals in the distinct fractions could undergo changes mainly due to variations in the pH.

In addition, understanding of the physicochemical processes and mineralogy in tailings deposits could contribute to create more efficient protocol and alternatives to reduce mobility of PTEs in sediments and aquatic environments and consequently reduce effects on human beings.
