Potential of mining, agro-industrial, and urban wastes for the remediation of acidic mine water

Mining is a crucial industry worldwide because of its economic and social importance. However, the increasing number of operating mines raises major concerns for health and the environment. The intense mining activity generates large quantities of wastes associated with several environmental problems. For example, the generation of acid mine drainages (AMD) by oxidation of sulphide ores stored in tailings deposits, leachates high concentrations of potentially harmful elements (PHEs), which poses severe pollution problems to the environment (aquatic and terrestrial ecosystems). This study evaluates the acid neutralisation capacity and the removal effectiveness of inorganic PHEs present in an AMD of different waste materials. This study is a first approach to future studies to develop pilot remediation studies using designed waste-derived Technosols. The waste used includes 4 mining wastes (iron oxide and hydroxide sludges [IO], marble cutting and polishing sludge [MS], gypsum spoil [GS], and carbonated waste from a peat extraction [CW]), 3 urban wastes (composted sewage sludge [WS], bio-stabilised material from municipal solid waste [BM], and vermicompost from pruning and gardening [VC]), and 3 agro-industrial wastes (2 solid olive-mill by-products [OW, OL] and composted greenhouse waste [GW]). All waste materials were spiked with the acidic water (AMD_L) prepared in the laboratory from the oxidation of pyritic tailings from the Aznalcóllar mine accident (1998). Afterward, they were stirred for 24 h and filtered, separating the waste (solid phase) from the leachate (liquid phase). In the leachate (AMD_L treated), pH_{(L) 1:5}, EC_{(L) 1:5}, and inorganic PHEs concentrations were measured, the latter by ICP-MS. The acidic water showed a strongly acidic character (pH_(L) ~ 2.89), high salinity (EC_(L) ~ 3.76 dS m^-1), and high concentrations of PHEs. Among them, As, Cd, Cr, Cu, Ni, Pb, Sb, Th, Tl, U, V, Y, and Zn stood out since they far exceed various legal limits widely used worldwide and/or because their high toxicity to humans, animals, plants or microorganisms. The most abundant were Zn (32.21 mg l^-1), Cu (6.24 mg l^-1), As (2.86 mg l^-1), Sb (0.82 mg l^-1), Pb (0.60 mg l^-1), and Cd (0.45 mg l^-1). All wastes were effective in neutralising the acidic pH_(L) of the AMD, as the leachates showed pH_(L) close to 7. In contrast, changes in the EC_(L) have been very irregular among the wastes used. In general, all wastes have been effective in adsorbing the PHEs. Inorganic wastes have been much more effective than organic ones, with adsorption efficiencies above 95% for many of the PHEs (particularly for those in higher concentrations). The waste with the best remediation behaviour were IO, CW, MS, GS, and VC. Conversely, GW and WS were the worst at removing PHEs present in AMD. Therefore, this study shows that many of wastes tested are suitable for the construction of Technosols from these wastes to prevent soil pollution by AMD discharge.