Can rare earth elements be recovered from abandoned mine tailings by means of electrokinetic-assisted phytoextraction?

Given the high impact of traditional mining, the recovery of rare earth elements (REEs) from hazardous waste materials could become an option for the future in accordance with the principles of the circular economy. In this work, the technical feasibility of REEs recovery from metal mine tailings has been explored using electrokinetic-assisted phytoremediation with ryegrass (Lolium perenne L.). Phytoextraction combined with both AC current and DC current with reversal polarity was applied (1 V cm-1, 8 h day-1) to real mine tailings containing a total concentration of REEs (Sc, Y, La, Ce, Pr, and Nd) of around 146 mg kg-1. Changes in REEs geochemical fractionation and their concentrations in the soil pore water showed the mobilization of REEs caused by plants and electric current; REE availability was increased to a higher extent for combined electrokinetic-assisted phytoextraction treatments showing the relevant role of plants in the process. Our results demonstrated the initial hypothesis that it is feasible to recover REEs from real metal mining waste by phytoextraction and that the performance of this technology can be significantly improved by applying electric current, especially of the AC type, which increased REE accumulation in ryegrass in the range 57-68% as compared to that of the treatment without electric field application.

A new hyperaccumulator plant (Spergularia rubra) for the decontamination of mine tailings through electrokinetic-assisted phytoextraction.

The screening of new effective metal hyperaccumulators is essential for the development of profitable phytoremediation projects in highly degraded environments such as mining areas. The goal of this research was to analyze the phytoextraction potential of the native plant Spergularia rubra to decontaminate and eventually recover metals (phytomining) from the mine tailings (belonging to an abandoned Pb/Zn Spanish mine) in which it grows spontaneously. To do so, the ability of this plant species to accumulate metals was evaluated both under natural conditions and through simple and electrokinetically assisted phytoextraction tests using alternating current and different combinations of voltage gradient (1/2 V cm-1) and application time (6/12 h per day). The complete duration of the greenhouse trial was 64 days, although alternating current was applied only during the last 14 days. The results obtained demonstrated the exceptional effectiveness of S. rubra for metal hyperaccumulation and growth without affecting toxicity in highly contaminated mining waste. Zn was the metal accumulated to a higher extent in the shoots, reaching concentrations up to 17,800 mg kg-1; Pb was mainly accumulated in the roots reaching a maximum concentration of 8709 mg kg-1. Cu and Cd were accumulated to a lesser extent but the bioconcentration factors were much >1. It has been proved that S. rubra is a hyperaccumulator species for Zn and Cd both in natural and greenhouse conditions and, very probably, Pb in wild conditions. The application of AC current did not significantly increase metal concentrations in plant tissues but it was able to increase the aerial biomass of S. rubra by 49.8 %. As a result, the phytoextraction yields of all metals were significantly improved as compared to wild conditions (up to 86 % for Zn). It could open new expectations about the economic viability of recovering high-value metals from mine tailings.

Analyzing the Role of Fe0 and Fe3+ in the Formation of Expanded Clay Aggregates.

The effect of the addition of Fe0 and Fe3+ on the formation of expanded clay aggregates was studied using iron-free kaolin as an aluminosilicates source. Likewise, the incorporation of cork powder as a source of organic carbon and Na2CO3 as a flux in the mixtures was investigated in order to assess its effect in combination with the iron phases. An experimental protocol, statistically supported by a mixture experiments/design of experiments approach, was applied to model and optimize the bloating index, density, absorption capacity, and mechanical strength. The process of expansion and pore generation and the associated decrease in density required the addition of iron, such that the optimum mixtures of these properties presented between 25 and 40 wt.% of Fe0 or Fe3+, as well as the incorporation of 3.5-5 wt.% of organic carbon. The addition of Fe3+ produced a greater volumetric expansion (max. 53%) than Fe0 (max. 8%), suggesting that the formation of the FeO leading to this phenomenon would require reducing and oxidizing conditions in the former and the latter, respectively. The experimental and model-estimated results are in good agreement, especially in the aggregates containing Fe0. This reinforces the application of statistical methods for future investigations.

Does environmental risk really change in abandoned mining areas in the medium term when no control measures are taken?

Studies regarding how environmental risk evolves in abandoned mining areas in the medium term have been seldom carried out. The answer to this question is not obvious despite it is essential in order to evaluate the need to take urgent control measures in these areas. Fifty-two samples corresponding to soils (from natural pasture and arable lands) and mine tailings were collected in the surroundings of an old Spanish Pb/Zn mine (San Quintín, Central Spain). Current concentrations of pseudo-total and available metal (loid)s (Pb, Zn, Cd, Cu, As and Ag) were determined and the environmental risk assessment (ERA) was conducted with these data and those corresponding to a sampling previously carried out in 2006. ERA was carried out by calculating the geoaccumulation index (Igeo), the pollution load index (PLI) and the potential ecological risk index (PER). Results demonstrated that Pb and Zn concentrations have increased in the soils of the plots surrounding the mining areas causing a moderate rise in most of the determined pollution indices between 2006 and 2020. It was especially significant in the pastureland areas, with increases up to 17% in the number of soil samples that reached the highest risk classification in 2020 as compared to those taken in 2006. The results obtained here demonstrate that the environmental risk can actually increase in a continuous way in abandoned mining areas despite the closure of the mining operation and the effect of the possible natural attenuation.

Manufacturing of lightweight aggregates from biomass fly ash, beer bagasse, Zn-rich industrial sludge and clay by slow firing.

A 70:30 black-white clay mixture (C) has been blended with different proportions of three wastes: bagasse from beer production (BG), a flux sludge used in galvanization (FS) and fly ash enriched to 60% with K2O from a biomass plant (FA). A complete characterization of the raw materials was performed. Mixtures of C and FA for 15, 17.5 and 20 wt% of K2O were blended with water, shaped into prismatic specimens, oven-dried, muffle-sintered at 1000-1200 °C, and finally crushed into lightweight aggregates (LWAs). The amounts of FA corresponding to K2O = 20% for a temperature, T, of 1000 °C (LWA variety C20K-1000) yielded the best technological properties: oven-dry density of 1.22 g/cm3, water absorption of 39.8% and open porosity of 48.6%. Therefore, other specimens containing BG and FS were sintered according to the same protocol considering K2O = 20% and T = 1000 °C. White aggregates were also obtained at 1000 °C (C-1000 variety) from C-mixture (without FS, FA and BG). The addition of BG (5,7 and 10 wt%) did not mean any improvement with respect to C20K-1000, and in the case of FS (25-40%), the development of red color was fostered, which could be interesting for gardening or ornamental purposes. A leaching test conducted on the aggregate with 40 wt% FS showed that the concentration of heavy metals was below the permitted environmental thresholds. Although the leachable Zn concentration of the aggregate (205 ppm) was reduced by 85% with respect to the original residue, it is still high enough for crops that demand this metal. The outcome of this investigation proves the usefulness of recycling agricultural and industrial wastes to obtain high quality LWAs for agriculture or gardening.

Performance of waste-based amendments to reduce metal release from mine tailings: One-year leaching behaviour.

A one-year leaching experiment has been conducted in order to assess the effectiveness of several amendments on metal immobilization in mine tailings from an old Pb/Zn mining area of Central Spain (San Quintín mine). Demineralized water was used as leaching solution, selecting doses equivalent to the annual rainfall conditions of the studied area. Columns with mine tailings without any amendment and others treated with 10% of sugar foam (SF), 15% of drinking water treatment sludge (DWS), 30% of paper mill sludge (PMS) and 15% of olive mill waste (OMW) were used. SF, DWS and PMS amendments increased the pH of leachates from values of approximately 4 to around neutrality. Additionally, the release of sulfate ions from the oxidation of pyritic residues was decreased in some extent by SF and DWS amendments. Metal leaching was effectively reduced by the amendments reaching overall decreases with respect to the unamended columns of 79-96% for Pb, 36-100% for Zn, 50-99% for Cu and 44-100% for Cd. The effect of the amendments in leachate pH, sulfate concentration and metal release from mine tailings was kept throughout the whole experimental period. Our results showed that the application of different organic and inorganic amendments based on by-products and waste materials may be a feasible alternative for the restoration of soils around abandoned metal mines.

Effect heating dwell time has on the retention of heavy metals in the structure of lightweight aggregates manufactured from wastes.

The main objective of this paper was to study how effective thermal treatment is in the retention of different heavy metals (HMs) within the structure of artificial lightweight aggregates (LWAs). These LWAs were manufactured by washing aggregate sludge and sewage sludge. The consequence of increasing the heating dwell time whilst manufacturing these LWAs was also determined. Partitioning of the HMs (Cr, Ni, Cu, Zn, Cd and Pb) was studied by means of the optimized BCR sequential extraction procedure. Then, the leaching ratio (LRx,y) was calculated. Thermal treatment was totally effective for immobilizing most of the elements studied except for a part of the non-residual Zn and Cd fraction which could volatilize, and the fractions of Pb which were water- and acid-soluble, weakly adsorbed, exchangeable, and oxidable. These were more highly concentrated in the LWAs than in the initial waste mixture. The effect of increasing heating dwell time on the retention of heavy metals in the LWAs depended on both the chemical element studied and the heating dwell time. This study is very important since certain rises in the heating dwell time caused a decrease in retention of some specific heavy metals in the LWAs. ABBREVIATIONS: BCR-SEP: optimized BCR sequential extraction procedure; b.d.l: below the detection limit; F1: weakly adsorbed, exchangeable and water- and acid- soluble fraction; F2: reducible fraction; F3: oxidable fraction; F4: residual fraction; HM: heavy metal; ICP-MS: inductively coupled plasma-mass spectroscopy; LOI: loss on ignition; LWA: lightweight aggregate; LWA-5: lightweight aggregate sintered for 5 min; LWA-10: lightweight aggregate sintered for 10 min; LWA-20: lightweight aggregate sintered for 20 min; LWA-30: lightweight aggregate sintered for 30 min; LRx,y: leaching ratio of the element x in the fraction y; n.e: not established; S: compressive strength; SS: sewage sludge; WA24h: water absorption after 24 hours; WAS: washing aggregate sludge; W75S25: mixture of 75% (wt) of the dried washing aggregate sludge and 25% (wt) of the dried sewage sludge; ρb: loose bulk density; ρd: dry particle density; ∑1 + 2 + 3: non-residual fraction; ∑1 + 2 + 3 + 4: total concentration; ∑2 + 3: reducible and oxidable fractions.

Development of lightweight aggregates from stone cutting sludge, plastic wastes and sepiolite rejections for agricultural and environmental purposes.

Three different wastes have been assessed for lightweight aggregate (LWA) manufacturing: granite and marble sludge (COR), sepiolite rejections (SEP) and polyethylene-hexene thermoplastics (P). A preliminary study of the physical and chemical properties of the raw materials was carried out to design proper batches. It was mixed 10% SEP with 90% COR to confer plasticity, and in turn, 0, 2.5, 5 and 10% (w/w) of P was added to check its suitability as a bloating agent. The mixtures were milled, kneaded with water, extruded, shaped into pellets, oven-dried and finally fired at 1100, 1125 and 1150 °C for 4, 8 and 16 min. The main technological properties of the aggregates related to bloating, density, porosity, loss on ignition, water absorption and compressive strength were measured. Scanning Electron Microscopy was used to study the microstructure of some LWAs. 23 out of 29 types of aggregate were lightweight, although neither bloating effect was observed, nor the typical cellular structure comprised of shell and core with relatively large pores was obtained, but a structure consisting of micropores and microchannels. The increase of temperature and time of firing involved a greater sintering, which in turn was translated into higher shrinkage, density and compressive strength values, but less porosity and water absorption. The addition of P did not involve any improvement, indeed it caused a significant decrease in compressive strength. The LWA sintered without P at the minimum time (4 min) and temperature of firing (1100 °C) was selected to assess its water suction capability. The results pointed out that this LWA could be suitable in hydroponics and/or water filtration systems, even better than the commercial LWA Arlita G3. A new and most environment-friendly perspective in LWA industry arises from here, promoting LWA production at relative low temperatures (prior to significant sintering occurs) and using non-plastic silty wastes instead of clays as major components.

Comparison of extractants used for the assessment of mercury availability in a soil from the Almadén mining district (Spain).

Single extraction methods have been extensively used to assess the availability of metals in polluted soils. This work focused on checking the feasibility of several chemicals, i.e. CaCl2, EDTA, diethylenetriaminepentaacetic acid (DTPA) and a low-molecular-weight organic acid mixture (rhizosphere-based method), to be used as extractants for mercury (Hg) in a soil from the Almadén mining district (Spain). Moreover, the effect of several experimental parameters, i.e. extraction time (0.5, 1, 2, 5, 16 and 24 h), concentration of extractant (0.01, 0.05, 0.1 and 1 M) and soil/extractant ratio (1:2, 1:5 and 1:10), on the amount of Hg extracted was investigated. The Hg extraction ability followed the descending order EDTA > rhizosphere-based method > DTPA ≈ CaCl2. This ranking was attributed to the higher complexation power of EDTA and organic acids. It was also found that extraction times between 2 and 5 h are required to avoid underestimation of mobile Hg and re-adsorption of the Hg previously extracted. Although some exceptions were found, Hg extraction efficiency was generally enhanced with higher extractant concentrations. Finally, the amount of Hg extracted by the four extractants increased with decreasing soil/extractant ratios.

EDTA and hydrochloric acid effects on mercury accumulation by Lupinus albus.

The efficiency of white lupine (Lupinus albus) to uptake and accumulate mercury from a soil polluted by mining activities was assessed in a pot experiment with chemically assisted phytoextraction. The mobilizing agents tested were ethylenediaminetetracetic acid (EDTA) and hydrochloric acid (HCl). Two doses of each amendment were used (0.5 and 1.0 g of amendment per kg of soil), and unamended pots were used as a control. Addition of HCl to the soil did not negatively affect plant biomass, while the use of EDTA led to a significant decrease in plant growth when compared to that found for non-treated pots, with plants visually showing symptoms of toxicity. The addition of hydrochloric acid increased root, shoot and total plant Hg uptake of white lupine by 3.7 times, 3.1 times and 3.5 times, respectively, in relation to non-amended plants. The greatest efficiency was obtained for the highest HCl dose. EDTA led to higher concentrations of total plant Hg than that found with the control, but, due to the aforementioned decrease in plant biomass, the Hg phytoextraction yield was not significantly increased. These results were attributed to the capability of both amendments to form stable Hg complexes. The concentration of Hg in the water of the soil pores after the phytoextraction experiment was very low for all treatments, showing that risks derived from metal leaching could be partially avoided by using doses and chemicals suitable to the concentration of metal in the soil and plant performance.

A Bending Test for Determining the Atterberg Plastic Limit in Soils.

The thread rolling test is the most commonly used method to determine the plastic limit (PL) in soils. It has been widely criticized, because a considerable subjective judgment from the operator that carries out the test is involved during its performance, which may affect the final result significantly. Different alternative methods have been put forward, but they cannot compete with the standard rolling test in speed, simplicity and cost. In an earlier study by the authors, a simple method with a simple device to determine the PL was presented (the "thread bending test" or simply "bending test"); this method allowed the PL to be obtained with minimal operator interference. In the present paper a version of the original bending test is shown. The experimental basis is the same as the original bending test: soil threads which are 3 mm in diameter and 52 mm long are bent until they start to crack, so that both the bending produced and its related moisture content are determined. However, this new version enables the calculation of PL from an equation, so it is not necessary to plot any curve or straight line to obtain this parameter and, in fact, the PL can be achieved with only one experimental point (but two experimental points are recommended). The PL results obtained with this new version are very similar to those obtained through the original bending test and the standard rolling test by a highly experienced operator. Only in particular cases of high plasticity cohesive soils, there is a greater difference in the result. Despite this, the bending test works very well for all types of soil, both cohesive and very low plasticity soils, where the latter are the most difficult to test via the standard thread rolling method.

Chemical and plant tests to assess the viability of amendments to reduce metal availability in mine soils and tailings.

The goal of this research was to assess the potential of several industrial wastes to immobilise metals in two polluted soils deriving from an old Pb/Zn mine. Two different approaches were used to assess the performance of different amendments: a chemical one, using extraction by ethylenediaminetetraacetic acid (EDTA), and a biological one, using Lupinus albus as a bio-indicator. Four amendments were used: inorganic sugar production waste (named 'sugar foam', SF), sludge from a drinking water treatment sludge (DWS), organic waste from olive mill waste (OMW) and paper mill sludge (PMS). Amendment to soil ratios ranged from 0.1 to 0.3 (w/w). All the amendments were capable of significantly decreasing (p < 0.05) EDTA-extractable Pb, Zn and Cu concentrations in the two soils used, with decreases in ranges 21-100, 25-100 and 2-100 % for Pb, Zn and Cu, respectively. The amendments tested were also effective in reducing the bioavailability of Pb and Zn for L. albus, which gave rise to a decrease in shoot metal accumulation by the lupine plants compared to that found in the control soil. That decrease reached up to 5.6 and 2.8 times for Pb and Zn, respectively, being statistically significant in most cases. Moreover, application of the OMW, DWS and SF amendments led to higher average values of plant biomass (up to 71%) than those obtained in the control soil. The results obtained showed the technology put forward to be a viable means of remediating mine soils as it led to a decrease in the availability and toxicity of metals and, thus, facilitated the growth of a vegetation layer.

Assessment of adsorption behavior of dibutyltin (DBT) to clay-rich sediments in comparison to the highly toxic tributyltin (TBT).

The sorption behavior of dibutyltin (DBT) to four types of natural clay-rich sediments as a function of pH and salinity was studied. The strongest affinity of DBT was found to the montmorillonite-rich sediment, which is characterized by the highest specific surface area and cation exchange capacity of the four used sediments. Kd values range between 12 and 40 (l/kg) on simulated marine conditions (pH 8, salinity 32%). A maximum of DBT adsorption was found at a salinity of 0% and pH 6. Desorption occurred over the entire studied pH range (4-8) when contaminated sediments interact with butyltin-free water. The maximum of desorption coincided with the minimum of adsorption, and vice versa. The results of DBT adsorption are compared with tributyltin (TBT), and the mechanism of the adsorption process is discussed.

Adsorption behavior of toxic tributyltin to clay-rich sediments under various environmental conditions.

The adsorption and desorption behavior of tributyltin (TBT) from aqueous solution to clay-rich sediments was studied under various conditions (pH, salinity) using the batch technique. Sediments containing illite, kaolinite, and montmorillonite in different proportions were used as sorbent materials. Several physicochemical parameters of the sediments (e.g., Brunauer-Emmett-Teller [BET] surface area, cation exchange capacity [CEC], total organic carbon [TOC]) were evaluated to assess the influence of sediment characteristics to the adsorption capacity for TBT Adsorption isotherms were linear over the concentration range of 100 to 1,000 ng(Sn)/ml. The adsorption coefficient (Kd) values range from 29 to 70 at the pH value generally found in marine systems (pH 8). The adsorption capacity shows a maximum in the range of pH 6 and 7. Salinity is also an important factor in controlling TBT adsorption. The strongest adsorption was observed at salinity of 0/1000, and it strongly decreases with increasing salinity. The adsorption mechanism is controlled by the properties of the clay minerals as well as the aquatic chemistry of TBT. Desorption takes place over the studied pH range (4-8) when contaminated samples interact with TBT-free water at given experimental conditions.