Recovery of Rare Earth Elements and Yttrium from Passive

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Recovery of Rare Earth Elements and Yttrium from Passive Remediation Systems of Acid Mine Drainage Carlos Ayora, Francisco Macías, Ester Torres, Alba Lozano, Sergio Carrero, Jose Miguel Nieto, Rafael Perez-Lopez, Alejandro Fernandez-Martinez, and Hiram A. Castillo-Michel Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b02084 • Publication Date (Web): 28 Jun 2016 Downloaded from http://pubs.acs.org on July 2, 2016

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Environmental Science & Technology

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Recovery of Rare Earth Elements and Yttrium from Passive Remediation Systems of Acid Mine Drainage

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Carlos Ayora (*,1), Francisco Macías (2), Ester Torres (1), Alba Lozano (1), Sergio Carrero (2),

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José-Miguel Nieto (2), Rafael Pérez-López (2), Alejandro Fernández-Martínez (3) and Hiram

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Castillo-Michel (4)

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(1) Institute of Environmental Assessment and Water Research, CSIC, c/ Jordi Girona 18,

7

08034

Barcelona,

Spain,

[email protected],

[email protected],

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[email protected], tel. 34-934006100, fax. 34-932045904

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(2) Department of Geology, University of Huelva, Campus ‘El Carmen’ s/n

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21071 Huelva, Spain, [email protected], [email protected],

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[email protected], [email protected], tel. 34-959219809, fax. 34-

12

959219810

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(3) Institute de Sciences de la Terre, CNRS & Univ. Grenoble Alpes, 1381 Rue de la Piscine,

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38041 Grenoble, France, [email protected], tel. 33-

15

476635197, fax. 33-476635201

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(4) European Synchrotron Radiation Facility, 6 rue Jules Horowitz, Grenoble, France, [email protected], tel. 33-476882948, fax. 33-476882785 (*) Corresponding author

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Abstract

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Rare Earth Elements and Yttrium (REY) are raw materials of increasing importance for modern

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technologies, and finding new sources has become a pressing need. Acid Mine Drainage (AMD)

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is commonly considered an environmental pollution issue. However, REY concentrations in

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AMD can be several orders of magnitude higher than in naturally occurring water bodies. With 1 ACS Paragon Plus Environment


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respect to shale standards, the REY distribution pattern in AMD is enriched in intermediate and

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valuable REY, such as Tb and Dy. The objective of the present work is to study the behavior of

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REY in AMD passive remediation systems. Traditional AMD passive remediation systems are

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based on the reaction of AMD with calcite-based permeable substrates followed by

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decantation ponds. Two column experiments simulating AMD treatment demonstrate that

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schwertmannite does not accumulate REY, which, instead, are retained in the basaluminite

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residue. The same observation is made in two field-scale treatments from the Iberian Pyrite

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Belt (IPB, SW Spain). Based on the amplitude of this process, and on the extent of the IPB, our

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findings suggest that the proposed AMD remediation process can represent a modest but

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suitable REY source. In this sense, the IPB could function as a giant heap leaching process of

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regional scale, in which rain and oxygen act as natural driving forces with no energy

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investment. In addition to the environmental benefits of its treatment, AMD is expected to last

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for hundreds of years, and therefore the total reserves are practically unlimited.

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Key-words: acid rock leaching, schwertmannite, basaluminite, fluorite, Iberian Pyrite Belt,

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sustainable mining, renewable resource.

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Table of Contents (TOC) Art AMD passive remediation treatment

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AMD

pH3.5 Fe Sulfide oxidation and rock leaching

REY ORE

pH>5.5 Al limestone

Treated water



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INTRODUCTION

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Rare Earth Elements (REE) together with Yttrium (REY) are essential raw materials for modern

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technological developments. Their most important uses include the manufacturing of

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permanent magnets for wind turbines, alloys for rechargeable batteries and jet engines, and

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phosphor light-emitting compounds for plasma, liquid crystal or light-emitting diodes. In 2011,

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global demand was 105 kt of REY oxides, and it is expected to grow to 160 kt by 2016.1 In

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general, global consumption of REY is expected to increase at a compound annual growth rate

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in excess of 5% from 2014 through 2020.2 This increasing demand is particularly evident for

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elements used in wind energy and electric vehicles, such as Dy and Nd. In the absence of

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drastic changes in the present-day technologies of reuse and recycling, increases of 700% and

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2,600% for Nd and Dy, respectively, are expected over the next 25 years.3

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Most mined REY deposits are located in carbonatites and other alkaline magmatic intrusive

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rocks. Additional resources of REY are adsorbed on clay deposits from the weathering and

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reworking of original primary igneous rocks. China dominates worldwide REY production. The

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Bayan Obo superlarge deposit currently accounts for approximately 90% of the REE

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production, with clays accounting for 6–7%.4 In response to the increasing global demand and

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the mining dominance of China, alternative sources of REY have become a necessity for other

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countries. Recycling in-use stocks can be an alternative source, especially for the “big four,”

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i.e., La, Ce, Nd and Pr. The availability of less abundant REE, however, continues to be a

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challenge.5

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Noack et al.6 reported a comprehensive study of the distribution of REE in groundwater, lakes,

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rivers and oceans. According to these authors, REE concentrations in natural waters with a

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dispersion of one to two orders of magnitude show medians of 5, 53, 71 and 170 pmol/L for

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the ocean, groundwater, rivers and lakes, respectively. A strong correlation between high REE


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concentrations and acidity has been reported both in surface and ground waters as well as in

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leaching studies of soils with different pH solutions.6-9 REE contents reported in acid mine

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drainage (AMD)9-12 and from our unpublished data consistently show a range between 4,000

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and 80,000 pmol/L, several orders of magnitude above the medians of natural waters (Figure

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1).

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When concentrations are normalized to the North American Shale Composite (NASC),13 the

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REY distribution patterns in AMD usually show a typical upward convex curvature, indicating

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enrichment in middle-REE.11,12,14-17 This enrichment is consistent with the middle-REE-depleted

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pattern shown by the residual oxides resulting from supergene alteration of sulfide

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deposits.7,18 Light and heavy REY enrichments have also been described and discussed in some

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AMDs.19,20,21,22

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Verplanck et al.23 noted that REE behave conservatively in two acid streams at a pH below 5.1

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and are partitioned into the solid phases at pH values between 5.1 and 6.1. These authors

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confirmed these observations by neutralizing six AMD samples in the laboratory. This result is

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also consistent with the decrease in REE observed downstream of the confluence of an AMD

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with a pristine tributary and the consequent pH increase from 3 to 6 in Lousal mine, Portugal.12

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In recent decades, passive remediation systems have been implemented to treat acid

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drainages from coal mines. Among these systems, constructed wetlands with an organic

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substrate to support bacterially mediated sulfate reduction are the most commonly used.24

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Due to the high land demand of wetlands, more compact alternative designs, mainly based on

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the neutralization of acidity (up to pH 6) by means of limestone dissolution, have been

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developed.25-28 These systems have been extended to high-acidity drainages from massive

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sulfide mines by increasing the porosity of the calcite substrate.29-31 In some cases, additional

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alkalinity (up to pH 9) to precipitate divalent metals is added by the dissolution of caustic



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magnesia. 32-33 All the passive treatments produce solid products that require disposal in

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classified repositories and long-term management. Following their behavior in streams,12,23 it is

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expected that REE will be removed during neutralization and accumulated in the solid

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precipitates of remediation systems.

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The objective of the present work was to study the behavior of REY in a multi-stage sequential

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treatment of AMD: 1) determining how much was removed from the solution at each step; 2)

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identifying the solid phases accounting for REY retention; and 3) pointing out the selectivity of

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retention of some REY with respect to the rest.To this end, AMD from a mine gallery (rock

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leaching) and a roasted pyrite tailing dam (minor rock proportion) were sequentially treated

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with two successive columns—the first of calcite and the second of caustic magnesia—over 30

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weeks . Both the pore water and solid phases were analyzed. The results were compared with

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those for a full-scale treatment, and the economic perspectives of exploitation are discussed.

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EXPERIMENTAL SECTION

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Water chemistry. Two different AMD samples from the Iberian Pyrite Belt (IPB), SW Spain,

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were used in the laboratory experiments. The first was sampled at the outflow of a gallery of

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the Monte Romero mine and represented a strong interaction with a shale enclosing rock. The

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other was obtained from the leaching from Almagrera mine tailing dam, closed in 2010, and

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consisted of a material originally dominated by pyrite with minor proportions of silicate

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minerals.

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Column setup. Four columns were constructed of transparent polymethyl methacrylate (9.6

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cm in inner diameter and 40 cm in height) and equipped with lateral ports placed at 5 cm

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intervals for the water sampling. The columns contained a perforated drain pipe and a 2.5 cm

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layer of glass beads (3 mm in diameter) at the bottom. Two columns were filled to a height of 6 ACS Paragon Plus Environment

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20 cm with dispersed alkaline substrate (DAS) consisting of calcite sand (0.5-2 mm) and fresh

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pine wood shavings (5-20 mm) mixed at a 1:1 ratio in weight.34 Calcite dissolution increases pH

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to 6 and trivalent metals (Al, Fe(III)) precipitate. Wood shavings provide an inert matrix to

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allocate the neo-formed solids preventing clogging. The other two columns were filled with a

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DAS consisting of caustic magnesia sand (0.5-1 mm) and fresh pine wood shavings at a 1:1 ratio

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in weight.34. Caustic magnesia dissolution rises pH to 9, and divalent metals (Zn, Mn, …)

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precipitate. A decantation vessel was located under each column.

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During the experiment, input water (i.e., AMD from Monte Romero and Almagrera) was fed by

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a peristaltic pump to the top of the columns, which were open to the atmosphere. The AMD

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flowed downward through the substrate and out of the drain pipe into the decantation vessel.

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The columns were operated at a constant rate of 0.7 mL/min (equivalent to 0.144 m3/m2·day),

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four times lower than the rates expected in a full-scale treatment.35 A sketch of the system is

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provided in Figure 2. Water samples were collected from the input container, the supernatant

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water, and the drain pipe approximately every two weeks. Samples from the intermediate

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sampling ports along the columns were taken monthly.

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Chemical and mineralogical analyses. The pH, redox potential and total alkalinity were

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measured in situ. Filtered samples (0.1 μm nylon) were acidified with HNO3 for analysis of

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major and trace elements by Inductively Coupled Plasma-Optical Emission Spectroscopy

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(ICPOES) and Inductively Coupled Plasma-Mass Spectroscopy (ICP-MS), respectively. Details of

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the analytical procedures are described in Section S1 of the Supporting Information (SI).

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Once the experiment was concluded, the columns were dismounted, divided in 2 cm-thick

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slices and dried at room temperature. The solid samples consisted almost completely of

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residual calcite and organic material (wood chips) with Al and Fe(III)-phases. The mineralogy



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was determined by X-Ray Diffraction (XRD) and Field Emission Scanning Electron Microscopy

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with Energy Dispersive Analysis (FESEM-EDS).

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To enable the study of the partitioning of Al, Fe, and REY into the solid phase in detail, the

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following sequential extraction procedure adapted from Torres and Auleda36 was applied to

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the solid residues: 1) water-soluble fraction, extracted with Milli-Q water; 2) basaluminite and

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calcite, extracted with 1 M ammonium acetate at pH 4.5/20°C; 3) low-crystalline

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Fe(III)oxyhydroxides, extracted with 0.2 M ammonium oxalate at pH 3/20°C; 4) crystalline

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Fe(III)oxides, extracted with 0.2 M ammonium oxalate at pH 3/80°; and 5) the residual fraction,

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digested with a mixture of concentrated HNO3 and HClO4. The detailed protocol can be found

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in the SI.

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Synchrotron-based μ-X-ray Fluorescence mapping (µXFM) analyses of selected samples were

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performed using the ID21 beamline at the European Synchrotron Radiation Facility in Grenoble

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(ESRF, France). For this experiment, the excitation energy was selected with the use of a Si 111

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double-crystal monochromator at 7 keV and the beam was focused to a lateral resolution of 1

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μm with a fixed curvature KB mirrors. The XRF signal was detected using an SGX 100 mm2

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active area SDD detector. 2D maps were acquired by raster scanning the sample areas with a

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step size of 1 µm2 and an acquisition time of 100 ms. The XRF spectrum of each pixel of the 2D

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images was analyzed with PyMCA software, and then elemental maps were obtained through

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a batch treatment.37

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RESULTS AND DISCUSSION

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The REY concentration in the Monte Romero AMD is twice that in the Almagrera AMD (Table

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1). This pattern is consistent with the observation that the concentration of REE in the country


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rock of the Sitai coal mine (China) is one order of magnitude higher than those of pyrite and

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coal samples.38

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As previously described for AMD treatments with the calcite-DAS reactive mixture, three main

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zones were visually distinguished in the first two columns of both the Almagrera and Monte

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Romero samples.29,31,35 From the input downstream, these zones are as follows: a red zone

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characterized by schwertmannite precipitation, a white zone characterized by basaluminite

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formation, and a gray zone consisting of gypsum and unreacted calcite remaining from the

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original reagent (Figure S1 of SI). After decantation, the solutions were treated with a MgODAS

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mixture. A white precipitate of gypsum and Zn(OH)2 was formed close to the water entrance in

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both treatment lines, with unreacted MgO in the rest of the column (Figure S1).

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Aqueous phase composition. Depth profiles of the two calcite-DAS columns and the first few

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centimeters of the MgO-DAS columns after 18 weeks of operation are shown in Figure 3. The

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numerical values are compiled in Tables S1 and S2 of the SI. The pH values at the outlet of both

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calcite columns ranged between 5.5 and 6. The pH increase occurred from the beginning of the

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alkaline interaction in the Monte Romero column, whereas it started to increase after 10 cm in

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the Almagrera column. This difference is attributed to the exhaustion of the calcite reagent in

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the second case due to the much higher acidity of the inflow water (Figure S1). Thus, according

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to the definition of Cravotta9, the trivalent-metal acidities of the inflow waters were 1780 and

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6200 mg/L CaCO3 for Monte Romero and Almagrera, respectively. In both treatment lines, the

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pH in the first decantation vessel rose to approximately 6.3 due to CO2 degassing.

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Downstream, the pH increased to 9.2 in the first 2 cm of both MgO-DAS columns (Figure 3).

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Finally, due to CO2 dissolution and aragonite precipitation, the pH decreased to 6.8 and 7.5 in

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the second decantation vessel of the Monte Romero and Almagrera columns, respectively.



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The dissolved Fe in the inflow water was primarily Fe(III) in both treatment lines. Most of the

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dissolved Fe(III) was precipitated in the top centimeters of the Monte Romero column,

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whereas it required 20 cm to precipitate in the Almagrera column due to the calcite exhaustion

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(Figure 3).

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Aluminum was more heavily depleted from the solution in the deeper zones than Fe(III). In

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Monte Romero, most Al was eliminated in the top 10 cm of the column, whereas 20 cm was

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required in the Almagrera column (Figure 3). In the Almagrera column, Al initially increased to

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concentrations higher than in the inflow water. This is a common observation in calcite-DAS

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treatments32 and can be attributed to basaluminite re-dissolution. Thus, once calcite is

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exhausted, schwertmannite precipitates at the expense of the alkalinity produced from the

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dissolution of the previously precipitated basaluminite. Silica followed a pattern similar to Al.

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Calcium concentrations increased in the depth interval where Al and Fe removal was highest,

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indicating that calcite dissolution was directly linked to Al- and Fe-hydrolysis and precipitation.

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As expected, the Mg concentration was constant throughout the calcite-DAS columns and

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increased to double in the MgO-DAS columns (Tables S1 and S2 of SI).

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Similarly to previous experiences,29, 32,33 Zn, Mn, Cd, Co and Ni did not change in concentration

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in the calcite-DAS but were completely removed in the top centimeters of the MgO-DAS

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columns (Figure 3 for Zn; Tables S1 and S2 for the other metals). The sulfate concentration was

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partially depleted due to the precipitation of gypsum.

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REY were removed below detection limits from both treated waters. The first REY decrease

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coincided with the Al precipitation front. Thus, approximately 35% of the REY in Monte

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Romero and 55% in Almagrera were captured by the Al-rich solid phase. Then, a second

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depletion occurred along the transit through the unreacted calcite section of the column: 15%

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in Monte Romero and 20% in Almagrera. A third drastic removal of REY occurred in the


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decantation vessel: 40% in Monte Romero and 25% in Almagrera. Finally, the remaining 10%

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and 5% REY were removed in the top centimeters of the MgO-DAS column in both treatment

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lines. Among the other trace elements analyzed, only Cu showed a removal pattern very

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similar to that REY (Figure 3). The small increase in concentration observed for Cu and REY in

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the pore water from the red zone is attributed to the dissolution of previously precipitated

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phases.

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Another interesting issue is the possible fractionation of REE along the different removal steps.

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Both AMD samples treated here show the characteristic enrichment in middle-REE with

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respect to the NASC. The sample from Monte Romero, which leaches metasediments, showed

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a negative Eu anomaly that was less evident in the seeps from the Almagrera tailings. The

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different REE were consistently depleted from the solution, and no preferential decrease in the

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concentration of a particular REE was observed along the different stages of treatment (Figure

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S2 of SI).

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Solid-phase composition. The distributions of the Fe, Al, ΣREY, Cu and Zn concentrations for

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each solid sample, obtained via sequential extraction, are shown in Figure S3 of the SI. The

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complete database, including the different REE and other trace elements, can be found in

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Tables S3 and S4.

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The results confirm the major zonation deduced from the pore water. Thus, from surface to

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bottom, Fe-rich and Al-rich zones with a distinct segregation between them develop in both

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calcite columns. The zones are closer in the Monte Romero column than in the Almagrera

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column due to the lower acidity of the former. According to the major mineralogy, Fe is mainly

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dissolved during steps III and IV of the sequential extraction (reductive dissolution of low

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crystalline and crystalline oxides). Aluminum is extracted in steps II and III (dissolution at pH



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4.5 and 3, respectively). The dissolution of a pure basaluminite phase synthesized in the

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laboratory shows the same pattern (Figure S2 of SI).

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REY and especially Cu match the Al zone in the Almagrera column. In the Monte Romero

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column, both Cu and REY are also present in the lower part of the calcite column and in the

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decantation vessel, where Al is absent. Finally, Zn (as well as Mn, Cd, Ni and Co) is mainly

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present in the decantation vessel and in the top 2 cm of the MgO columns and is released in

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steps I and II of the sequential extraction (dissolution of easily soluble phases in water or

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slightly acidic conditions and carbonates). It should be noted that the concentration in the

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decantation vessel is higher because the solid is entirely formed by precipitates with no calcite

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or wood shavings.

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REY and Cu retention mechanisms. Their high concentrations in pore water and their scarcity

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in the digestion solutions of the red zone indicate that no significant amounts of Cu and REY

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were retained in schwertmannite. This finding is consistent with the conservative behavior of

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Cu and REY in streams at pH below 5,12,23 but it contradicts the REE sequestration by ochre

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sludge described in wetlands, although no detailed description of the ochre phases is given.39

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The conservative Cu and REY behavior in the red zone of the treatments leads to a relevant

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consequence from a practical point of view: REY and Cu are not expected to disperse into

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schwertmannite precipitating along the stream beds before the AMD is treated. Therefore,

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both REY and Cu can be entirely recovered in the remediation systems.

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The first decrease in REY concentration occurred concomitantly with the Al decrease and

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basaluminite formation. No data on REY sorption in basaluminite are available in the literature.

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However, the synchrotron-based μ-XRF has enabled the elemental relative abundance to be

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determined and reveals correlations between the different elements. Thus, a distinct spatial

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correlation of Al and Ce is evident in solid samples from the basaluminite-rich zone of both


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columns (Figure 4A, B). This correlation strongly suggests that sorption and co-precipitation of

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REY in basaluminite occurs.

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In addition, Cu experienced a decrease with Al in both treatment lines studied. The adsorption

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of Cu in basaluminite was observed for the AMDs from the Thuringian slate mining area.40 The

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authors showed that approximately 40% of Cu was retained in Al precipitates when AMD

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mixed with neutral tributaries. Similar to our column experiments, no other divalent metal was

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retained in this mixing process.

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The REY and Cu concentrations in the pore water continued to decrease after Al was practically

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absent, suggesting that a mechanism of retention other than basaluminite precipitation was

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occurring. Indeed, DRX and FESEM-EDS observations showed the precipitation of gypsum

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inside the calcite column, the decantation vessel and the top 2 cm of the MgO columns.

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Together

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(Zn,Cu)Zn(OH)13[(Si,S)(O,OH)4] and small cubes of cuprite (Cu2O) were observed and identified

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by XRD analysis in samples downstream of the Al zone. These minerals can account for the Cu

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depletion in the pore water.

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In contrast to the basaluminite samples, µ-XRF mapping of samples from the end of the calcite

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columns and decantation vessels showed no correlation between REE and Al, which seemed to

273

be enriched in distinct discrete particles. Indeed, spherical aggregates and plates of REE-Ca-

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Fbearing phase (fluorite?) were also observed together with bechererite in samples from the

275

end of the calcite column, the decantation vessel and the top 2 cm of the MgO column of

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Monte Romero (Figure 4D, E). Fluorite was also identified by DRX (Figure S4 of SI).

277

Fluorite has been investigated as a host for REE in magmatic and hydrothermal deposits. Thus,

278

fluorite veins have been selected as a possible target for REE exploration in Australia,41 China,42

279

the USA44 and South Africa.43 However, to the authors’ knowledge, no REE-fluorite formation

with

gypsum,

aggregates

of

trigonal

pyramids

of

bechererite



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has been described at environmental temperatures. The precipitation of fluorite in the Monte

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Romero treatment has two causes. Firstly, calcite dissolution supplies the Ca2+ required for

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fluorite formation. However, no fluorite is found in the schwertmannite zone, where active

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calcite dissolution occurs. Secondly, in AMD, F- is entirely complexed with Al3+ as AlF2+ (Figure

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S5 of SI). When Al is precipitated as basaluminite, the complex becomes unstable and F is

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mainly found as the free anion F-; the saturation index of fluorite is reached, and the mineral

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precipitates. This process may occur in some pores of the Monte Romero column but is more

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acute in the decantation vessel. The Almagrera AMD is much richer in Mg, and a strong MgF+

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complex forms together with free F- when Al is depleted (Figure S5) and fluorite is scarcer.

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The pore water geochemistry was, in general, consistent with the observed mineralogy. Thus,

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the pore water from the deeper part of the calcite column of both treatment lines was

291

supersaturated in basaluminite and cuprite, in equilibrium with gypsum and subsaturated in

292

calcite (Figure 5). In partial disagreement with the observations, however, fluorite saturation

293

was only reached at the first decantation vessel of the Monte Romero line. This discrepancy

294

may have been due to the effect of REY on the fluorite solubility product, although no data on

295

this point have been found in the literature. Although supersaturation was exhibited by the

296

pore waters, no fluorapatite was observed under FESEM-EDS inspection, and moreover, no P

297

was detected in any step of the sequential extraction of the solid phase. Therefore, although it

298

cannot be completely ruled out, phosphates do not seem to play a significant role in REY

299

retention.

300 301

ECONOMIC IMPLICATIONS

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As shown above, no REY were partially lost in schwertmannite precipitates during the AMD

303

transit through streams or pre-treatment lagoons, and all dissolved REY reached the 14 ACS Paragon Plus Environment

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remediation system. Indeed, practically all REY were completely retained in the basaluminite

305

layer and the decantation vessels of the calcite-DAS passive remediation systems. Therefore,

306

the REY content can be entirely recovered from AMD. Several full-scale treatment systems are

307

under construction in the IPB. However, only one full-scale system at Mina Esperanza and a

308

pilot plant at Monte Romero were operating for more than one year until the calcite reagent

309

was exhausted.32,34 Both systems reproduced the same zonation pattern described for the

310

column experiments: schwertmannite, basaluminite and calcite-gypsum zones (Figure S5 of SI).

311

As described for column experiments, the segregation between zones was also distinct in the

312

two field cases. 32,34 The annual reserves and rate can be roughly approximated from the total

313

dissolved Al and REY concentrations in the AMD and the molar volume of basaluminite. An

314

example for the two sites studied is shown in Table 2. The rates calculated from the AMD

315

concentrations are similar to the analysis performed in the basaluminite residues excluding

316

wood shavings: 0.30% for Mina Esperanza and 1.44% for Monte Romero. It is interesting to

317

note that U and Th concentrations were very low (

Recovery of Rare Earth Elements and Yttrium from Passive

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