Useful nanoparticles from mining waste and acid mine drainage

Mining waste can generate environment pollution including acid mine drainage (AMD). AMD is dangerous for its surroundings and can pollute surface and groundwater it is contacting with. Nanomaterials are advanced materials used in all fields of human activity and development. However, their production is still expensive and may pollute the environment due to the chemicals’ used and/or production of the energy needed for their synthesis. A smart solution could be use of mining waste and AMD to produce nanomaterials with properties similar to the properties of nanomaterials obtained from clean chemicals. Thus both waste will be valorised / decontaminated and useful and needed materials produced. This paper presents production of nanoparticles and nanomaterials from mining waste and AMD with emphasis on iron- and copper-based materials, as well as some applications of the obtained materials.


Introduction
Mining and minerals beneficiation are fundamental activities that provide raw materials for the production of goods and services necessary for the daily life of every person, as well as for the sustainable development of all humanity.
However, large amounts of solid rocky waste and tailings are generated that, apart from their unaesthetic appearance, can be a source of dangerous environmental pollutants resulting from the leaching of usually present toxic metals, metalloids and/or non-metals under the influence of precipitation [1,2].This type of waste has been accumulated for hundreds and thousands years and some authors estimate that its yearly production rate is 350x109 t [3] while other give the value of 100x109 t waste from primary production of mineral and metal commodities [4].Although the mining waste disposal decreases in some well-developed countries, due to their ability to treat, recycle or use for energy recovery part of this waste this is not the case with all countries worldwide.Scientific and engineering community has already been working on the technologies for mining waste management [4,5] and for extraction of valuable metals from mining and mineral processing waste [6][7][8][9][10].
Mining activities and mine waste can be a source of mine drainages that can be four types (neutral, acid, basic and saline) in dependence of the ores composition and local hydro-geological 1254 (2023) 012063 IOP Publishing doi:10.1088/1755-1315/1254/1/012063 2 and physicochemical conditions.Among the mentioned types, acid mine drainage (AMD) represents the major concern.Generally, its formation can be presented by the following chemical equations [11,12]: The pyrite (FeS 2 ) oxidation can be facilitated by the presence of sulphur-oxidizing bacteria.The acidic nature of the AMD leads to the leaching of the ores that it is contacting with and thus different metals, such as aluminium (Al), cadmium (Cd), chromium (Cr), cobalt (Co), copper (Cu), manganese (Mn), nickel (Ni), lead (Pb), zinc (Zn), etc. and metalloids (for example, arsenic -As) appeared in the AMD, besides iron (Fe).AMD is dangerous for the surrounding environment or when it mixes with surface or groundwater bodies.The composition and characteristics of AMD depends on the local geological, hydro-geological and climate conditions, availability of pyrite ores and the rate of their oxidation.Without any claims for comprehensiveness, table 1 gives an idea about the ranges of some AMD parameters.It is difficult to prevent formation of AMD because it is a natural oxidation process accelerated by the increase in pyrite-bearing minerals surface area that is exposed to water and oxygen during mining operations [14].The already formed AMD is treated in active and/or passive systems.Constructed wetlands, limestone drains, permeable reactive barriers, and use of natural sulphate reduction processes are the main passive treatment methods.They could be more cost effective and environmentally friendly, compared to active methods, but are slow and depend on the site conditions [12].Most often the active treatment is based on addition of neutralizing and alkalizing reagents, mainly lime, limestone, magnesite, etc. [11,20].However, alkalization precipitation of metals-pollutants results in big amount of sludge.In addition, the metals in the sludge could be relatively easily mobilized by contacting with acidic or even fresh water.Thus, this type of treatment needs corresponding handling and storage.Adsorption is considered as a cost-effective and efficient method in treating AMD.Different adsorbents are used, among them natural mineral materials such as clays and zeolites.Their low mechanical strength and poor ion selectivity can be pointed as drawbacks.Membrane technologies such as nanofiltration and reverse osmosis possess high separation efficiency, automatic operation and produce good water quality.However they are still relatively expensive and need a preliminary treatment of the water in order to reduce membrane system pollution and unstable work [12].
Recovery of the valuable metals from AMD and solid mining waste is needed in order to achieve sustainable use of resources jointly with sustainable remediation and the environment protection.
Nanomaterials are materials that have at least one dimension at nanoscale.Due to their size, nanomaterials have a number of different properties compared to bulk materials of the same chemical composition [14].They are used in different areas such as electronics, power generation and storage, mechanical industries, water treatment, pharmaceutical and medical activities and products, etc. [21,22].Recently magnetic zeolite nanocomposites have been studied as reusable green adsorbents for removal of different pollutants from wastewater [23,24].
Two approaches are applied to synthesize nanoparticles (NPs).The "top-down" approach is based on a physical miniaturization of materials to the nanometer scale by applying highenergy inputs.Mechanical milling, laser ablation, ion sputtering, etc. are included in this approach.The "bottom-up" approach is based mainly on chemical reaction of precursors and precipitation of the obtained NPs or evaporation followed by deposition.Both techniques for NPs production are expensive and can pollute the environment by using chemicals and/or due to the production of the energy needed.The basis of the NPs biosynthesis is the use of biochemical redox reactions where the ion is transformed to its solid phase by using as chemicals extracts of different microbes and plants.This approach is considered environmentally-friendly, cost effective and scalable [21,25] but usually the NPs yield is low.
This paper aim is to present recovery of the valuable metals from mining waste and AMD under the form of useful nanoparticles and nanocomposites and some applications of the produced materials, which is a relatively new scientific field being developed in recent years.

Nanoparticles from mining waste
Mining waste contains diverse valuable metals and various technologies have been and are being developed and applied to extract metals.Recent 10-15 years with the burst of nanoscience and nanotechnologies the number of studies on NPs production from mining waste is also increasing.Mainly iron (Fe) mining waste has attracted the scientists' attention probably due to the two reasons: a) diverse unique properties of iron-based nanoparticles and especially of the magnetite nanoparticles (MNPs) and b) due to the mineral beneficiation technology usually this waste is rich in Fe that could be used as a cheap precursor for preparing MNPs.

Iron-based nanoparticles
Iron based materials and especially magnetite (Fe 3 O 4 ) nanoparticles have found wide application.Back in 2004 Hu and co-workers [26] prepared MNPs (∼ 10 nm) by sol-gel method and studied their ability to remove Cr(VI) from wastewater by adsorption.Adsorption on MNPs is regarded as an effective and economically vital method for removing different pollutants (such as metal ions, dyes) from water [20].Besides their use as adsorbents MNPs are used as catalysts, paint pigments, additives to ceramics, in electronic materials fabrication, ferrofluid technology, controlled drug delivery, etc. [20,27].
MNPs are able to remove most divalent and some trivalent metal ions (forming stable precipitates) from water (including AMD) at easy application and recovery by magnetic filtration.In addition, most of them do not change their magnetic properties when used as adsorbents and thus they can be reused [20].MNPs are synthesized by different methods, such as thermal decomposition, milling, chemical precipitation, sonochemical synthesis.Chemical precipitation is the most widely used approach because the produced MNPs are with controlled size and morphology.Co-precipitation of Fe 3+ and Fe 2+ (from aqueous solutions, at a ratio of 2:1) is usually applied to prepare the MNPs.
It is carried out under anaerobic alkaline (pH 9-12) conditions [27].The cost of iron precursor salts used is the main component contributing to the price of nanoparticles.For this reason, as a source of Fe ions, mining wastes from iron ore mining and processing are of interest for the production of MNPs.In addition, in recent years an increasing attention is paid to the synthesis of iron-based materials, especially aimed to be used in energy storage applications.[28] synthesized magnetic Fe 3 O 4 nano-powder by applying ultrasonic chemical co-precipitation in presence or absence of a surfactant sodium dodecyl sulfate (C 12 H 25 OSO 3 Na).They used high purity Fe that was separated from iron ore tailings by acidic leaching (with 37.5 wt.% hydrochloric acid -HCl).Further, hydrogen peroxide (H 2 O 2 ) was added to the filtrate (i.e. to the pregnant leach solution -PLS) in order to ensure that all iron exists as Fe 3+ .The solution was heated to 60 • C and concentrated ammonia solution (NH 4 OH) was added to achieve pH 3.2.Thus iron was separated from tailings and precipitated as Fe(OH) 3 .The precipitate formed was washed and re-dissolved in HCl.Measured amount of FeSO 4 •7H 2 O was added in order to ensure Fe 3+ and Fe 2+ molar ratio of 1.5:1.Then sodium hydroxide (NaOH) solution was added under ultrasonic agitation to form black precipitate of Fe 3 O 4 by the reaction

Materials based on iron oxides. Wu et al
The produced MNPs (with an average diameter of 15 nm) possessed high crystallinity and super-paramagnetism (74.86 emu g −1 saturation magnetization).Fe 3 O 4 NPs coated with C 12 H 25 OSO 3 Na were with uniform size and exhibited better dispersion, compared to uncoated.
Giri and co-authors synthesized MNPs using ferric iron ions obtained by treatment of tailing of iron ore industries [29].The material containing ≈39.01%Fe was digested in HCl solution to produce FeCl 3 solution.Concentrated NH 4 OH solution was added to produce Fe(OH) 3 that was washed, dried and dissolved in 50% HCl solution together with FeCl 2 •4H 2 O at Fe 3+ : Fe 2+ molar ratio of 2:1.Then NaOH solution was added to reach pH 11, under continuous stirring, nitrogen (N 2 ) atmosphere and 70 • C. Further, temperature was risen to 90 • C, sodium dodecyl sulphonate was added to stabilize the formed MNPs.After cooling to ambient temperature, the produced MNPs were magnetically separated, washed with acetone and distilled water and sonicated.The dispersed MNPs (from 8.3 to 23.0 nm) were used in adsorption studies.They were found effective in removing anionic (Congo red) and cationic (methylene blue) dyes from their aqueous solutions with adsorption capacities of 70.4 and 172.4 mg g −1 for methylene blue and Congo red, correspondingly.
Kumar et al prepared MNPs using as a Fe source the waste from the iron ore processing plant [30].The waste material contained 26.8% Fe 2 O 3 , 72.4% SiO 2 and 0.7% Al 2 O 3 .Metallic iron was added at the ratio of Fe to Fe 2 O 3 content of tailings = 1:4 and the mixture was subjected to ball milling followed by leaching with HCl solution.Thus, chloride solution was produced that contained iron (III) and iron (II) at ratio of 2:1 M. Urea was added to the solution and its hydrolysis at 95 • C released ammonia (NH 3 ) that alkalized the medium and black MNPs were formed in the suspension.After the suspension was cooled to room temperature, the NPs were removed by magnetic separation, washed and dried in a vacuum desiccator (at room temperature).The produced MNPs were coated with silver (Ag) nanoparticles by wet impregnation with aqueous silver acetate followed by washing and drying.The antibacterial activity of the prepared Ag-Fe 3 O 4 nano-composite was studied against Escherichia coli.Silver coated MNPs possessed antibacterial activity.Silver NPs anchored to the Fe 3 O 4 can be easily separated from the water by a suitable magnet in order to be reused.Thus, waste can be converted into useful resource and the produced product can be applied in water treatment.
Suh et al started from a low-grade iron ore and prepared MNPs (with a purity of 99.8%) and a Mg-rich solution that were used as a nano-adsorbent and a coagulant for water treatment, correspondingly [31].Aqueous HCl solution was used to leach the ore.After solid/liquid separation, 30% H 2 O 2 was added to convert all iron available in the solution to Fe 3+ ions.Impurities that passed to the PLS were removed by solvent extraction with tri-n-butyl phosphate as an extractant.Therefore, the amounts of Mg and Si that are able to inhibit the formation of MNPs, were reduced from 15.5 wt.% and 10.3 wt.% to less than 1.4 mg L −1 and 28.1 mg L −1 , respectively.As a result, the Fe content increased from 68.6 wt.% to 99.8 wt.%.This high-purity Fe 3+ solution was used to synthesize 5-15 nm MNPs.The high-purity Fe 3+ solution was mixed with ferrous solution (obtained by reducing one-third of the high-purity Fe 3+ solution with sodium borohydride -NaBH 4 aqueous solution) in the ratio of 2:1 and the obtained mixture was added to NaOH solution.The following reactions took place at 40 • C to produce MNPs: The synthesized NPs were washed with distilled water and separated by using a magnet.Thus, MNPs adsorbent can be produced in large quantities at low cost and at the same time the cost of iron-ore-wastewater treatment could be decreased.
Ren and co-authors [32] used ferrous sulphate heptahydrate (FeSO 4 •7H 2 O, 92.45%) that is an industrial by-product from titanium dioxide production by sulphuric acid (H 2 SO 4 ) method and pyrite (FeS 2 , 75.62%) that is a by-product of mineral processing plants.Ferrous sulphate was reduced with pyrite under N 2 protection to produce porous MNPs (with particle size of 25-50 nm).
Iron ore tailings (containing 43.47% Fe, 7.99% Si, 9.07% Ca, 3.8% Mg, and 2.4% Al) were ball milled and leached in mixture of 5 M H 2 SO 4 and 2 M NaCl at 85 • C [33].The use of H 2 SO 4 /NaCl hybrid lixiviant resulted in decreased Ca 2+ ions impurity in PLS because of precipitation of gypsum.Then Fe 3+ ions in the produced PLS were reduced by adding sodium thiosulfate pentahydrate (Na 2 S 2 O 3 •5H 2 O), under N 2 atmosphere, in such stoichiometric amount to obtain and maintain the Fe 3+ /Fe 2+ = 2. Further NH 4 OH was added to reach pH=9.5, under N 2 atmosphere at room temperature.Greenish black precipitate was formed that was heated at 85 • C and then the magnetic precipitate was gathered with a magnet, washed with distilled water and ethanol, and dried at 44 • C. The XRD and ICP-OES analyses showed that the produced magnetite nanoparticles (with an average diameter of 19±3 nm) were pure (impurities < 3%).The VSM analysis showed that the powder behaved as ferromagnetic at room temperature with a saturation magnetization of 63.27 emu g −1 .The proposed reduction-precipitation method is suitable and economic for producing MNPs when low amount of iron ions present in the tailings (< 25%).
Microorganisms have also been studied for their ability to produce iron oxide NPs from mining waste.The bacterium Rhodococcuserythropolis ATCC 4277 was used in stirred tank reactor to extract residual iron from rhomboclase (found in coal tailings) and to transform it 6 into magnetic nanoparticles [34].The prepared NPs were composed of β-Fe 2 O 3 and α-Fe 2 O 3 .The proposed process is environment-friendly and sustainable.

Other iron-based materials.
Two organic-inorganic hybrid materials with magnetic properties were synthesized, using as precursors the iron ions obtained from Fe ore mining tailings [35].The first precursor material consisted mainly of ferric sulphate.It was from iron mine tailings of a company.The second precursor resulted from the acid extraction of iron mine sludge gathered from the Doce River watercourse near the Fundão dam in Mariana, after its rupture in 2015.The hybrid materials were produced in the following way: CoCl 2 •6H 2 O and waste iron salts were dissolved in natural organic material (NOM)-rich water.Then the solution was alkalized to pH 9 with 1 M NaOH solution, the precipitate formed was washed and dried.SEM and TEM images pointed at the formation of nanostructures, while XRD analyses disclosed formation of the cobalt ferrite phase (CoFe 2 O 4 ).The hybrids obtained (NOM-CoFe 2 O 4 ) exhibited conversion rate of 99% of nitrophenol in short times (1-2 min).They also showed high performance in the adsorption of PAHs, achieving removals in the range of 75-80%.
Other study has presented synthesis of manganese ferrite (MnFe 2 O 4 ) from a low-grade mining waste bearing both iron and manganese [36].Initially the material was heated at 700 • C and then leached in H 2 SO 4 medium.Further, the leachate was purified and its composition adjusted to reach the Fe : Mn molar ratio of 2:1.The MnFe 2 O 4 synthesis was achieved through coprecipitation from the solution at temperature of 90 • C. The obtained nano-sized particles (45 nm) possessed a saturation magnetization of 51.03 emu g −1 and are candidates for energy storage devices.
Yao et al synthesized interconnected α-Fe 2 O 3 nanoparticles using tin ore tailings as Fe source [37].The ore tailings (containing 20.9% Fe) were leached in 1.5 mol L −1 H 2 SO 4 at 70 • C, the liquid-solid ratio of 4:1 (mL:g) for 90 min.Further, sodium carbonate (Na 2 CO 3 ), NaOH, and NH 4 OH solutions were added to the equal volumes of the PLS to precipitate iron hydroxide.The precipitate was washed and calcinated at 800 o C to obtain α-Fe 2 O 3 nanoparticles.The α-Fe 2 O 3 nanostructures synthesized by using NaCO 3 as precipitating reagent showed the best lithium storage performance with a reversible discharge capacity of 1146 mAh g −1 at 0.5 A g −1 after 300 cycles and stable discharge capacity of 377 mAh g −1 at a high current density of 4 A g −1 .The obtained results point at the possibility to use the ore tailings to produce high-value products.
Natural pyrite NPs were prepared by grinding pyrite taken from real mine waste to micro particles (< 50 µm), washing them with 0.5 M HNO 3 and additional grinding in ceramic mortar followed by ball milling in a planetary ball mill.
Thus produced NPs were rinsed with a mixture of ethanol and deionized water (50/50) and dried at 105 • C [38].The obtained NPs were studied for their ability to function as a catalyst for the activation of peroxymonosulfate (PMS) and peroxydisulfate (PDS) to oxidize tetracycline (TTC).A degradation of 98.3% and mineralization up to 46% of TTC were achieved using the produced pyrite catalyst.In addition, biochemical and histopathological assays pointed that nephrotoxicity and hepatotoxicity effect of TTC were decreased by 90% and 85% respectively.The pyrite NPs catalyst was used in four consecutive cycles with no significant decrease in process efficiency (<3% decrease in TTC removal).Furthermore, it was found that the pyrite presence in the water had not any significant toxic effects.The generalized route used for the preparation of MNP from mining waste is presented in figure 1.

Nanoparticles based on other metals
Mining waste material contains other valuable elements besides iron and have to be considered for their utilization in the NPs preparation.However, until our days no many efforts appeared in this direction.Wong-Pinto et al utilized a real effluent produced by the leaching of sulfide ore tailings as a copper source to synthesize copper nanoparticles (CuNPs) using the non-pathogenic bacteria P seudomonasstutzeri DSM 5190 as a bio-reducing agent [21].In order to extract the Cu from the chalcopyrite-bearing tailings, they were treated with H 2 SO 4 solution (120 g L −1 ) in presence of sodium nitrate (1 g L −1 ) that acted as an oxidizing reagent able to enhance Cu dissolution.A copper recovery of 50% was achieved in 8 h.Further, the PLS was purified by solvent extraction (using Acorga PT5050 diluted with Escaid 110) and then Cu was stripped with H 2 SO 4 solution (120 g L −1 ).The obtained solution was used as a precursor for CuNPs synthesis after raising its pH to 4 by addition of NaOH solution.Then, pellets of cultured P seudomonasstutzeri DSM 5190 were added to the Cu-bearing solution.Some of the produced CuNPs (1-2 nm) were attached to the pellets of the biomass, other were left in the solution which after drying produced Na 2 SO 4 -CuNPs mixture.The obtained products are intermediate.Different techniques have been proposed to separate CuNPs from the biomass (such as washing-vacuum filtration, density gradient centrifugation, sonication), which are relatively cheap and clean methods.
Recently a study appeared on the synthesis of selenium NPs (SeNPs) using mining waste that contained 6.11% Se, 4.97% Sb, 4.69% Pb, 4.12% Ag, 3.61% Cu and 70.01%barium sulphate (BaSO 4 ) [39].In order to prepare pure solution for SeNPs synthesis the following stages were taken: (a) gravity separation to remove BaSO 4 , (b) Leaching of the obtained concentrate by 3M nitric acid (HNO 3 ), (c) Chloride solution addition to precipitate Ag and Pb as insoluble salts, and (d) removal of other impurities such as tellurium, strontium and arsenic by different reagents addition, followed by solid/liquid separation.The purified solution was utilized as a precursor for the green synthesis of SeNPs using different fruit extracts with the orange extract being found to be the most efficient reducing and stabilizing reagent leading to the formation of stable SeNPs with particle size between 70 and 80 nm.

Nanoparticles from acid mine drainage
Recently various efforts have been done to handle AMD with the aim not only to decrease the content of hazardous elements that present in it but also to produce valuable minerals and metals by the AMD treatment.In this line, efforts to produce NPs have their place.Studies on the synthesis of iron based materials are prevailing but also other metals and minerals are aimed at.

Iron based materials
Efforts to use AMD to synthesize MNPs started also 15-17 years ago.One of the pioneering works was that of Wei and Viadero [27].They recovered ferric iron from AMD pumped out from abandoned underground coal mines.Hydrogen peroxide was added to the raw AMD to oxidize the eventually available Fe 2+ to Fe 3+ .Then the AMD pH value was raised from the natural 2.6 to 3.5-4.0by addition of 4 M NaOH solution.At this iron was precipitated as ferric hydroxide/oxyhydroxide.At pH 3.5 other metals remained in the solution.The precipitate was separated from AMD by centrifugation.The ferric precipitate was resolubilized and used as a ferric iron source.Reagent-grade FeSO 4 •7H 2 O was the ferrous iron source.Solution containing Fe 3+ : Fe 2+ = 2 : 1 was prepared under a N 2(g) atmosphere to prevent oxidation and mixed for a certain time to remove the dissolved oxygen.Further, pH was risen to 9.5 by addition of 6.4 M NH 4 OH solution.NPs were allowed to grow for 30 min, under N 2 atmosphere.The black precipitate (magnetite NPs with size of 10-15 nm) was separated from solution with the aid of an external magnetic field and washed.The prepared MNPs were of the same quality as the MNPs prepared using chemicals (pure iron salts) under the similar conditions.
Cheng et al studied microbial fuel cells that can be used to treat AMD and generate electricity [40].The dissolved oxygen reacts with Fe 2+ in AMD and iron oxide precipitates are produced that at frying are transformed to goethite (α-FeOOH) NPs with size in the range from 120 to 700 nm.The size of the produced NPs could be controlled by changing the conditions in the fuel cell -initial Fe 2+ concentration (50-1000 mg L −1 ), pH (4-7.5) and the cell current density (0.04-0.12 mA cm −2 ).
Kefeni et al initially optimized the conditions for synthesis of MNPs from analytical grade chemicals at low pH and temperature and then applied the optimized conditions to treat real AMD with the aim to produce valuable commercial NPs [20].They have proved that it is possible to synthesize Fe 3 O 4 and CoFe 2 O 4 from their corresponding binary salts.However, the direct application of the same conditions to both simulated and real AMD did not result in the MNPs formation.The reason was the presence of many other cations that can hinder that process.Beside iron oxides, Mn 3 O 4 , MnO 2 and ZnO were formed from real AMD.It is found that in order to produce MNPs from AMD higher pH and heat are needed in comparison with the model binary solutions.The study showed that NH 4 OH (aq) was better alkalizing reagent than NaOH solution.It is proved that if the AMD is treated with MNPs the formation of ferrites is accelerated which resulted in an increased magnetic moment of ferrite sludge produced.
A study was conducted with the aim to recover Fe 2+ and Fe 3+ from Fe 2+ rich acidic mine water that resulted from coal mining and washing processes, and to use the recovered Fe 2+ and Fe 3+ sludge as precursor for magnetite synthesis [11].Only real AMD and soda ash were used.A sequential and fractional precipitation procedure was used to recover different Fe-species.In order to precipitate Fe 3+ the pH value of the AMD was raised by Na 2 CO 3 addition to pH≥4.5 and to obtain Fe 2+ the pH was increased to pH≥8.3.Afterwards, the two recovered and collected 1254 (2023) 012063 IOP Publishing doi:10.1088/1755-1315/1254/1/0120639 sludges were used to synthesize magnetite.The reaction can be described as it follows: The formed magnetite (with a purity of 28%) was separated by centrifuging.The optimum conditions for the recovery of MNPs were mol ratio of Fe 3+ / Fe 2+ =2:1, pH≥10 and temperature ranging from 25 to 100 • C.
Moreira et al prepared acicular goethite nanoparticles from AMD and studied their ability to adsorb arsenate, phosphate and humic acids [41].The AMD was collected from a coal mine.It was treated using the following steps (the procedure is similar to that used at industrial scale to treat AMD): a) pre-neutralization by lime (Ca(OH) 2 ) addition to pH 2.7 to precipitate selectively aluminium hydroxides and CaSO 4 , b) addition of NaOH to pH 3.2 to precipitate acicular goethite nanoparticles (AGNs).The AGNs were washed, filtered, dried at 90 o C for 5 h and stored.The obtained AGNs were similar in composition to the product derived at industrial scale at treatment of that AMD.The size of the AGNs was 23 nm, the material specific surface area was 102 m 2 g −1 .The AGNs adsorbed humic acid (37.30mg C g −1 ), arsenate (19.91 mg As(V) g −1 ) and (12.98 mg PO 3− 4 g −1 ).Further, a study has been carried out with the aim to explore the use of AMD as a source of Fe 3+ ions as a precursor for synthesis of iron NPs and use of the produced NPs to remove pollutants from the same AMD [14].A coal mine AMD (containing 4219.14 mg L −1 iron and 21,317.79mg L −1 sulphate as the predominant anion) was used as feedstock.Initially the ferrous ion that present in the mine water was converted to ferric ion by addition of hydrogen peroxide solution: . In order to remove dissolved oxygen from the thus treated AMD, nitrogen gas was bubbled.Iron(III) ions that present in the mine water were reduced at room temperature using 0.5 M NaBH 4 solution to produce the iron nanoparticles.
The synthesized black particles were separated from the mixture by using a strong external magnet, washed and dried.Their size was 31.8 nm and BET surface area was 88±3.16 m 2 g −1 .For comparison iron NPs were synthesized under the similar conditions from pure chemicals as precursors.Their size and BET surface area were 28.05 nm and 91±3.08 mm 2 g −1 correspondingly.The morphology of NPs prepared by starting from the different precursors was the same.The NPs synthesized by using AMD have been applied to remove toxic elements that present in the raw AMD.An average removal efficiency of 75% has been achieved due to combination of the different properties of the produced NPs, namely chemical stability, high redox potential and specific surface area as well as the presence of easily accessible adsorption sites.The main reactions leading to the pollutants removal have been identified as adsorption, co-precipitation and reduction.
Another study has used plant extracts to recover iron NPs from AMD [42].Real wastewater from an industrial iron-ore processing company was used.Extracts of different plants were added to the wastewater at ratio of 1:1 v/v (determined as optimal in preliminary experiments) and temperature and contact time were varied.The precipitates formed were dried and annealed.Their analyses disclosed presence of maghemite-C (Fe 2 O 3 ) and magnetite (FeFe 2 O 4 ) nanoparticles.Best results at room temperature have been obtained when Eucalyptus globulus extract was used.In addition, the quality of treated mine water was improved and thus it was made reusable for other purposes.
Real AMD from a coal mine was used to synthesize goethite, hematite, and magnetite NPs [43].Recovery of Fe 3+ sludge and recovery of Fe 2+ sludge was achieved by selective precipitation.The pH of the AMD was increased from 2 to 4.5 by addition of 10% Na 2 CO 3 solution to obtain Fe 3+ rich precipitates / sludge.The filtrate from the sludge / water separation was used for ferrous (Fe 2+ ) sludge production by raising the pH from 4.5 to ≥ 8.5 via addition of 10% NaOH solution.For goethite synthesis the Fe 3+ rich slurry was heated at 80 • C and then dried at room temperature for 24 h.For hematite synthesis the Fe 3+ rich slurry was heated at 700 • C and then cooled to room temperature.Magnetite nanoparticles were synthesized by mixing the preliminary obtained Fe 3+ and Fe 2+ sludge at 2:1 M ratio and adding NaOH solution to reach pH > 9 under oxygen-free environment achieved by N 2 gas bubbling into the reactor.High purity (>99%, =100% for goethite) has been observed for the recovered Fe-species, which indicates that they are suitable for industrial applications.
A recent study has applied hydrazine as reductant in order to produce iron nanoparticles using iron-rich AMD solution as a ferric iron source [44].In order to prepare NPs 0.9 M hydrazine solution was added to AMD sample (4492.1 mg L −1 iron, pH=2.14)from a coal mine under stirring at a temperature of 70 • C. A brownish yellow precipitate was formed that was identified by the XRD analysis as goethite.The SEM images of the iron nanoparticles revealed spherical morphology.The HRTEM analysis showed nanoparticles with size 8.66±0.58nm and the XRF analysis results revealed that the samples were very rich in iron.It is concluded that Fe-rich AMD is a very good substitute material for commercial reagent grade salts for synthesizing iron NPs.
Another recent study proposed preparation of stabilized iron NPs from AMD applying rooibos tea extract as reagent used to reduce Fe 2+ /Fe 3+ to zero-valent Fe [45].Ambient temperature (∼25 • C), a rooibos tea extract dosage of 5 g L −1 , a pH of 6, and 6 h of reaction time are found as optimal conditions.The average particle size was 36 nm, as determined by TEM analysis and the NPs were stabilized by tea polyphenols that partially coated the surface of the nano-iron.When the synthesized iron NPs were used as a Fenton like catalyst for the degradation of textile dye (orange II sodium salt), 94% removal efficiency was achieved in only 30 min.

Copper based materials
Schaffie and Hosseini proposed a biosynthesis of semiconductor copper sulfide nanoparticles from mine wastewaters with the aid of Fusarium oxysporum fungus [18].Study was conducted with wastewater collected from a copper mine.The wastewater contained 56.75 mg L −1 Cu and its pH was 5.2.The bio-reduction of Cu ions occurred at 30 • C for 96 h.The analysis of the separated and washed nanoparticles revealed that they possessed a covelite composition and the particle size was in the range of 10-40 nm.For comparative purpose NPs were synthesized also using 10 −3 M CuSO 4 under the same conditions as when AMD was used.Analyses pointed that the properties of the NPs produced from AMD were the same as the properties of the NPs synthesized from the pure CuSO 4 solution.
Crane and Sapsford produced Cu nanoparticles from AMD using nanoscale zerovalent iron (nZVI) as a selective reducing reagent [16].The AMD used was collected from abandoned open pit Cu-Pb-Zn mine.Its pH was 2.67 and the copper concentration -45.41 mg L −1 .The used nZVI was prepared from FeSO 4 •7H 2 O by addition of 4 M NaOH solution.Two types of batch experiments were carried out -with unbuffered (pH 2.67 at t = 0) and pH buffered (pH < 3.1) AMD that was contacted with nZVI (with concentrations in the range of 0.1 -2.0 g L −1 ).It is found that addition of nZVI at concentrations ≥ 1 g L −1 to the unbuffered AMD lead to rapid and nearly total removal of Cu, Al and Cd from the water solution (> 99.9% removal within 1 h) through cementation (for Cu), precipitation and sorption to iron corrosion products (for Al and Cd).Other metals that present in the AMD were not immobilized.When buffered AMD was used Cu was selectively precipitated at addition of nZVI.The maximum removal of Cd and Al were only < 1.5% and < 0.5% correspondingly.Spherical nanoparticles (with diameter of 20-100 nm) were formed, containing up to 68 wt.% Cu, as revealed by HRTEM-EDS analysis.
The study showed that highly selective formation of Cu bearing nanoparticles from real AMD can be achieved by tuning the synthesis conditions.

Sulphides based materials
Below are presented some examples of metal sulphides NPs synthesis at contacting the AMD with sulphide ions produced by microorganisms.
Vitor et al studied the possibility to treat AMD at simultaneous bio-synthesis of zinc sulphide (ZnS) nanoparticles and ZnS / TiO 2 nanocomposites by using a combined process, developed by the members of this research group [19].It consists of two consecutive steps -water neutralization in a calcite tailing column, and treatment of the neutralized water in an anaerobic sulphate reducing bioreactor.That process was planned to use locally available wastes and/or natural cheap materials, with the aim to develop as simple as possible, working at normal pressure and ambient temperature environmentally friendly and economically viable treatment of an AMD.The AMD used was collected from the region of an abandoned copper mine.The neutralizing column was filled with a mixture of small pieces of calcite tailing (from a marble stone cutting and polishing industry) and coarse sand, in a 1:2 (w/w) ratio.For the second stage the Upflow anaerobic packed-bed reactor was filled in with coarse sand.It was inoculated with sulphate reducing bacteria (SRB).The effluent from the bioreactor contained biologically generated sulphide.The biosynthesis of ZnS was realized by allowing the effluent from the biotreatment process to flow into a reaction vessel, containing a zinc sulphate solution.Composites were prepared by adding commercial titanium oxide (TiO 2 ) powder to the zinc solution.Over 90% of the zinc initially available in the AMD was removed as ZnS nanoparticles (with an average size in the range of 29 -39 nm) or ZnS/TiO 2 .Water going out of the system complied with legal irrigation requirements.The results obtained confirmed the possibility of coupling the synthesis of ZnS nanoparticles and nanocomposites with the AMD bioremediation.
Kumar and Pakshirajan studied the ability of biomass with SRB from an anaerobic rotating biological contactor to remove metals -(Cu(II), Cd(II), Ni(II), Fe(II), Pb(II), Mn(II) and Zn(II)) from simulated AMD while simultaneously rendering them in metal-sulphide nanoparticles [15].The main mechanism of metal sulphide formation was the binding of metals to sulphide produced outside bacterial cell surface as a result of sulphate reduction by the bacteria.Heavy metals recovery by sulphide precipitation was over 70% for Cu and Pb.The recovery was a little bit lower for the other metals.Studies on the particle size distribution of the different NPs revealed that the size of NiS and ZnS nanoparticles was 15-17 nm, the size of CdS and CuS nanoparticles was 12-14 nm, the MnS NPs were 9-11 nm, while the size of FeS and PbS NPs was in the range 8-10 nm.

Conclusions and outlook
Studies carried out in recent 10-20 years have proved that: (i) Mine tailings can be successfully used as iron source for producing different nanoparticles: magnetite, interconnected α-Fe 2 O 3 , ferrites and their composites, as well as copper and selenium nanoparticles -all with properties and prospective applications similar to the properties and applications of nanoparticles and composites fabricated from clean chemicals.(ii) AMD can be utilized as an iron source for synthesizing magnetic, zerovalent iron, goethite and hematite nanoparticles, along with being a resource for copper and sulfide (copper, zinc, lead) nanoparticles and their nanocomposites.Besides NPs synthesis, remediation of the AMD is the other achieved positive result.The obtained materials showed ability to remove different pollutants from contaminated water.
The positive feature of the studies carried out is that most of them have used real waste material and AMD.
The following drawbacks could be pointed out: (i) The experiments are conducted at small laboratory scale, (ii) Most often solutions of strong acids are used as leaching reagents for mining waste, and strong alkaline solution (NaOH) is used as pH raising reagent in treating both the AMD and the PLS; (iii) Stabilization of the produced non-magnetic nanoparticles in different composites or on various carries (for example, clay, zeolite) is practically not studied, while it could contribute to easier separation and multiple use of those nanocomposites.
In short -production of nanoparticles and nanocomposites from mining waste and AMD seems promising environmentally friendly and feasible approach to minimize the negative impact of these waste streams and utilize them.However, there is still long way till their real implementation.

7 Figure 1 .
Figure 1.Generalized route for preparation of magnetic nanoparticles from mining waste.

Table 1 .
Concentration ranges of some metals and As in AMD from different mines worldwide.