The Impact of Physical Properties on the Leaching of Potentially Toxic Elements from Antimony Ore Processing Wastes

International Journal of Environmental Research and Public Health

Article The Impact of Physical Properties on the Leaching of Potentially Toxic Elements from Antimony Ore Processing Wastes

Saijun Zhou 1,* and Andrew Hursthouse 2,3

1 College of Civil Engineering, Hunan University of Science and Technology, Xiangtan 411201, China 2 Hunan Provincial Key Laboratory of Shale Gas Resource Utilization, Hunan University of Science and Technology, Xiangtan 411201, China 3 School Computing, Engineering & Physical Science, University of the West of Scotland, Paisley PA1 2BE, UK * Correspondence: [email protected]

Received: 31 May 2019; Accepted: 30 June 2019; Published: 3 July 2019

Abstract: This study reports on the assessment of the impact of antimony mine wastes from Xikuangshan (XKS) Antimony Mine in Lengshuijiang City, Hunan Province. We focus on the leaching of a number of potentially toxic elements (PTEs) from residues from the processing of antimony ore. The PTE content of ore processing waste and solutions generated by leaching experiments were determined for a suite of PTEs associated with the ore mineralization. These were Sb, As, Hg, Pb, Cd and Zn. As anticipated, high concentrations of the PTEs were identiﬁed in the waste materials, far exceeding the standard background values for soil in Hunan Province. For Sb and As, values reached >1800 mg kg 1 and >1200 mg kg 1, respectively (>600 and >90 times higher than the soil · − · − background). The leaching of Sb, As, Hg, Pb, Cd and Zn decreased with an increase in grain size and leachable portions of metal ranged between 0.01% to 1.56% of total PTE content. Leaching tests identiﬁed the release of PTEs through three stages: a. alkaline mineral dissolution and H+ exchanging with base cation; b. oxidation and acid production from pyrite and other reducing minerals; and c. the adsorption and precipitation of PTEs.

Keywords: antimony ore processing wastes; particle size; leaching; potentially toxic elements

1. Introduction The exploitation of antimony by mining has created serious environmental pollution at a number of locations and has attracted international research interest in understanding the mechanisms driving impact [1–4]. The waste produced by mining not only occupies large areas of land, but also causes lasting environmental pollution, particularly by the harmful elements released after heavy rainfall [4–8]. China has the most abundant Sb resources in the world [9] and has been responsible for approximately 90% of all antimony metal produced globally over the past decades. The Xikuangshan (XKS) Mine in Hunan, Central South China is the world’s largest Sb mine, and has been reported to produce 25% of the world’s Sb. It has been in operation extracting and smelting for more than 120 years, which coupled with the large-scale mining operations has produced signiﬁcant residual materials open to the percolation of water. It has released signiﬁcant quantities of harmful elements like Sb, As, Hg, Pb, Cd and Zn to the surroundings, threatening the wider environment and human health [10–13]. When an excessive amount of Sb enters the human body, it causes diseases to the liver, skin, and respiratory and cardiovascular systems, or even cancer [14,15]. The element Sb and its compounds are listed as pollutants of priority interest by the United States Environment Protection Agency [16] and the European Union [17,18].

Int. J. Environ. Res. Public Health 2019, 16, 2355; doi:10.3390/ijerph16132355 www.mdpi.com/journal/ijerph Int. J. Environ. Res. Public Health 2019, 16, 2355 2 of 11

Up to now, studies on pollution form antimony ore extraction and processing are mainly focused on Sb in waste water, waste gas and waste solid residues from antimony beneficiation and metallurgy, the distribution and spatial characteristics of pollution, the factors affecting its migration and release, transformation of chemical forms and its biogeochemical behavior, etc. [19–23]. Few studies exist on the pollution caused from effects of the environment on waste rock dumps. A number of studies showed that the dissolution of Sb-bearing ore minerals is thought to contribute significantly to pollution in the local drainage zone. The kinetics of the mobilization of Sb from stibnite, Sb3O6OH and Sb2O3 under environmental conditions has been studied in detail by Biver and Shotyk [7,8]. Hu et al. [24,25] studied the release kinetics and mechanisms of Sb from stibnite and Sb2O3 under the irradiation of light. Guo et al. [5] studied the leaching characteristics of antimony smelting slag in XKS Sb Mine and found that smelting slag is an important source of Sb pollution in nearby farmland. This paper reports on the effect of environmental conditions on the ore processing waste rock stored at a the XKS Sb Mine in Hunan Province, Central South China. It assesses leaching processes affecting waste rock of different particle sizes, identifying changes in the leaching solution (e.g., pH, conductivity and PTE release). The impact of results on prediction of wider environmental and ecological risk are also addressed.

2. Material and Methods

2.1. Experimental Material We collected the Sb ore waste rocks for our experiments from an area of open waste storage near the northern part of the XKS Sb mine (E 111◦29034.8700–E 111◦29043.4500, N 27◦46006.7000–N 27◦46013.4100) in Lengshuijiang City, Hunan Province. The waste rock storage was over an area of 36,000 m2, with a total stockpile of about 850,000 tons. A series of 20 sampling sites (see Figure1) were selected during site walk. At each point samples were collected from within a square (1 m 1 m), with a composite sample × of one kilogram slag material collected at the center and its four equidistant points of the sampling site. The 20 samples were mixed uniformly, dried naturally, and then ground in a mill (XMQΦ240 90, × Jiangxi Ganye, China) to three grades [26] with particle sizes 0.075–0.15 mm (1), 0.15–0.425 mm (2) and 0.425–0.85 mm (3), for experimentation. The particle size distributions of antimony ore processing wastes are shown in Figure2.

Figure 1. Map showing location of sampling sites. Int. J. Environ. Res. Public Health 2019, 16, 2355 3 of 11 Int. J. Environ. Res. Public Health 2019, 16, x 3 of 11

45 41.4 40

30.5 30 28.1

mass percent (%) 15

0 0.075~0.15 0.15~0.425 0.425~0.85 Particle size (mm) Figure 2. Size of distributions of antimony ore processing wastes. Figure 2. Size of distributions of antimony ore processing wastes. 2.2. Test Instruments and Reagents 2.2. Test InstrumentsThe instrumental and Reagents techniques used in sample analysis were: X-ray diffraction (XRD, D8 Advance, Bruker AXS Germany), hydride generation-atomic fluorescence (AFS-9700, Beijing Haiguang, China), The instrumental techniques used in sample analysis were: X-ray diffraction (XRD, D8 Advance, oven heating (GZX-9246MBE, Shanghai Boxun), scanning electron microscopy (SEM, JSM-6380LV,JEOL, Bruker AXSJapan), Germany), energy-dispersive hydride spectrometry generation-atomi (EDS, GENESIS,c fluorescence EDAX, United States),(AFS-9700, pH metering Beijing (PB-10, Haiguang, China), ovenSartorius, heating Germany), (GZX-9246MBE, conductivity metering Shanghai (CCT-5320E, Boxun), Hebeiscanning Ruida, electron China), usemicroscopy of an ultra-pure (SEM, JSM- 6380LV, waterJEOL, machine Japan), (LBY-20, energy-dispersive Chongqing Owen spectromet Technologyry (EDS Co., Ltd., , GENESIS, Chongqing, EDAX, China), United and orbital States), pH meteringshaking (PB-10, (HY-5, Sartorius, Jiangsu Germany), Jintan, China). conductivity The reagents usedmetering (HNO 3(CCT-5320E,, HF and H2O2 )He werebei of Ruida, high purity China), use (China Chemicals Co., Ltd., Shanghai, China). All utensils are soaked in 10% nitric acid solution for of an ultra-pure water machine (LBY-20, Chongqing Owen Technology Co., Ltd., Chongqing, China), 48 h before use, and then rinsed with ultra-pure water. and orbital shaking (HY-5, Jiangsu Jintan, China). The reagents used (HNO3, HF and H2O2) were of high purity2.3. Basic(China Mineral Chemicals Composition Co., of SbLtd., Ore Shanghai, Waste Rocks andChina). PTE Determination All utensils are soaked in 10% nitric acid solution for 48The hours mineral before composition use, and of antimonythen rinsed ore with waste ultra-pure rocks was observed water. and analyzed by XRD. The PTEs were determined by AFS (Hydride Generation-atomic Fluorescence) after digestion. 2.3. BasicThe Mineral digestion Composition process involved: of Sb Ore Five Waste samples Rocks of and particle PTE sizeDetermination 1 (0.100 g) were placed in PTFE digestion vessels along with one blank sample; acid was added (5 mL of HNO3 and 0.5 mL HF), Theand mineral allowed composition to digest at 170 of °Cantimony for 12 h. After ore waste cooling, rocks 1 mL ofwas 30% observed H2O2 was and added analyzed and allowed by toXRD. The PTEs werereact determined for 30 min. A 10by mLAFS aliquot (Hydride of 5% nitric Generation-atomic acid solution was then Fluorescence) added and filtered after by digestion. with a The digestionpolyethylene process involved: film injection Five filter samples (0.2 µm). of The particle filtrate was size then 1 (0.100 diluted g) to 50were mL withplaced ultra-high in PTFE purity digestion water and kept at 4 ◦C before analysis by AFS. vessels along with one blank sample; acid was added (5 mL of HNO3 and 0.5 mL HF), and allowed to digest2.4. at 170 Experimental ℃ for 12 Methods hours. After cooling, 1 ml of 30% H2O2 was added and allowed to react for

30 minutes. TheA 10 leaching mL aliquot solution wasof 5% a mixture nitric of acid H2SO solution4 and HNO was3 at thethen volumetric added ratioand of filtered 3:1, diluted by with a polyethylenewith NaOH film injection and high purity filter water, (0.2 μ andm). its The initial filtrate pH value was is setthen at 4.98,diluted according to 50 to mL the rainfallwith ultra-high of purity waterLengshuijiang and kept City at [427 °C]. Firstly, before 25 analysis g waste rock by samples AFS. with diﬀerent particle sizes were loaded into a 1000 mL glass bottle with stopper grinding mouth respectively, and then the leaching solution (250 mL) 2.4. Experimentalwas added Methods into the bottle. The solid-liquid ratio was set at 1:10. The glass bottles with stopper were placed in a circular shaker with rotational speed of 120 5 rpm for the leaching test. ± The leachingThe leaching solution test was lasted a 12 mixture days. An of aliquot H2SO of4 and 10 mL HNO of leachate3 at the was volumetric taken at regular ratio intervals of 3:1, diluted with NaOHevery and day high to measure purity its water, pH value and and its conductivity; initial pH andvalue the concentrationis set at 4.98, of according PTEs (Sb, As, to Hg,the Pb,rainfall of Cd, Zn) was determined for each sample of leachate. The leachate consumed during the test was Lengshuijiang City [27]. Firstly, 25 g waste rock samples with different particle sizes were loaded into replaced by blank leaching solution to maintain the solid to liquid ratio. Three replicates were run in a 1000 mL glass bottle with stopper grinding mouth respectively, and then the leaching solution (250 mL) was added into the bottle. The solid-liquid ratio was set at 1:10. The glass bottles with stopper were placed in a circular shaker with rotational speed of 120 ± 5 rpm for the leaching test. The leaching test lasted 12 days. An aliquot of 10 mL of leachate was taken at regular intervals every day to measure its pH value and conductivity; and the concentration of PTEs (Sb, As, Hg, Pb, Cd, Zn) was determined for each sample of leachate. The leachate consumed during the test was replaced by blank leaching solution to maintain the solid to liquid ratio. Three replicates were run in parallel. The concentration of PTEs in leachate was determined by AFS-9700, and pH and conductivity measured. After leaching solid samples were analyzed by scanning electron microscopy-energy dispersive spectrometry (SEM-EDS), and element composition was analyzed. The dissolution of PTEs per unit mass is calculated by the following formula: Int. J. Environ. Res. Public Health 2019, 16, x 4 of 11

Qqv=⋅ (1)

RQc=×( / ) 100% (2)

In the formula, Q is the amount per unit mass of the PTEs dissolved (μg·kg−1); q is the concentration of the PTEs in the leachate (μg·L−1); v is the volume of the leachate (L); R is the amount of dissolution for each PTE; c is the concentration of PTE per unit mass of raw solid (μg/kg). Int. J. Environ. Res. Public Health 2019, 16, 2355 4 of 11 2.5. Quality Control parallel. The concentration of PTEs in leachate was determined by AFS-9700, and pH and conductivity To ensure the data accuracy in the analysis process and the stability of the testing instruments, measured. After leaching solid samples were analyzed by scanning electron microscopy-energy the standard reference soil (GBW07406) from the National Standard Material Center was digested dispersive spectrometry (SEM-EDS), and element composition was analyzed. The dissolution of PTEs and measuredper unit mass by isthe calculated same method by the followingas the antimony formula: ore waste rocks samples. The recovery rates of Sb, As and Hg in standard reference materials are 95–106%, 94–107% and 94–104%, respectively. At the same time, in each batch of analytical samples,Q =reagentq v blanks were added, and 20% of the samples(1) · were re-measured. The relative standard deviation (RSD) of re-measurement was less than 10%. R = (Q/c) 100% (2) × 3. ResultsIn and the formula,DiscussionQ is the amount per unit mass of the PTEs dissolved (µg kg 1); q is the concentration · − of the PTEs in the leachate (µg L 1); v is the volume of the leachate (L); R is the amount of dissolution · − 3.1. Basicfor each Mineral PTE; Compositionc is the concentration and Heavy of PTEMetal per Content unit mass of Sb of rawOre solidWaste (µ Rocksg/kg).

The2.5. QualityXRD results Control for samples of waste rocks are shown in Figure 3. The results show that the main minerals identified were quartz and calcite, and the samples also contained a small amount To ensure the data accuracy in the analysis process and the stability of the testing instruments, of stibnitethe standard and pyrite reference. As soil shown (GBW07406) in Table from 1, the the content National of Standard Sb and Material As in waste Center rocks was digested was very and high, −1 −1 reachingmeasured 1806.21 by mg·kg the same and method 1277.64 as the mg·kg antimony, respectively, ore waste rocks which samples. is 606 The and recovery 91 times rates higher of Sb, than As the soil backgroundand Hg in standard values reference for PTEs materials in Hunan are 95–106%,Province 94–107% [28]. Although and 94–104%, the respectively.Hg and Cd Atcontents the same were relativelytime, inlow each (1.44 batch mg·kg of analytical−1, 1.67mg·kg samples,−1), they reagent were blanks still were 16 and added, 21 times and 20% higher of the than samples the standard were value.re-measured. Those are potentially The relative hazardous standard deviation materials. (RSD) of re-measurement was less than 10%.

3. Results and Discussion Table 1. Average content and relative pollution levels of potentially toxic elements (PTEs) in waste materials.3.1. Basic Mineral Composition and Heavy Metal Content of Sb Ore Waste Rocks TheElement XRD resultsContent for samples(mg·kg−1 of) wasteBackground rocks are Value(mg·kg shown in Figure−1) [24]3. The Enrichment results show Quotient that the main mineralsSb identiﬁed were1806.21 quartz and calcite, and the samples 2.98 also contained a small 606 amount of stibnite and pyrite.As As shown1277.64 in Table 1, the content of Sb and 14 As in waste rocks was very 91 high, reaching 1806.21Hg mg kg 1 and 1277.641.44 mg kg 1, respectively, 0.09 which is 606 and 91 times higher 16 than the soil · − · − backgroundPb values for PTEs46.24 in Hunan Province [28]. Although27 the Hg and Cd contents 1.7 were relatively low (1.44Cd mg kg 1, 1.67mg1.67kg 1), they were still 16 and 0.079 21 times higher than the standard 21 value. Those · − · − are potentiallyZn hazardous616.91 materials. 95 6.5

1 3 6000

1.Quartz (SiO2) 2.Kaolinite (Al O ·2SiO ·2H O) 5000 2 3 2 2 3.Calcite (CaCO ) 3 4.Stibnite (Sb S ) 2 3 4000 5.Pyrite (FeS ) 2

1 3000

2000 Intensity (Counts)

1 1000 5 3 3 1 4 1 2

10 15 20 25 30 35 40 45 50 55 60 65 o 2-Theta ( ) FigureFigure 3. 3.XRDXRD analysis analysis of of typical oreore waste. waste. Int. J. Environ. Res. Public Health 2019, 16, 2355 5 of 11

Table1. Average content and relative pollution levels of potentially toxic elements (PTEs) in waste materials.

Element Content (mg kg 1) Background Value(mg kg 1)[24] Enrichment Quotient · − · − Sb 1806.21 2.98 606 As 1277.64 14 91 Hg 1.44 0.09 16 Int. J. Environ. Res. PublicPb Health 2019, 16 46.24, x 27 1.7 5 of 11 Int. J. Environ. Res. Public Health 2019, 16, x 5 of 11 Cd 1.67 0.079 21 3.2. Leaching Law Znof Antimony Ore 616.91 Waste Rocks with Different 95 Particle Size 6.5 3.2. Leaching Law of Antimony Ore Waste Rocks with Different Particle Size The3.2. change Leaching in Law leachate of Antimony pH Ore value Waste Rocksis an with impo Differentrtant Particle indicator Size for evaluating environmental The change in leachate pH value is an important indicator for evaluating environmental pollution from mining, and it also has a strong influence on the valence and fractionation of PTEs in pollution fromThe mining, change in and leachate it also pH valuehas a is strong an important influence indicator on forthe evaluating valence environmentaland fractionation pollution of PTEs in samples [29]. Conductivity can reflect the concentration of PTEs in leachate, and its trend can reflect samples from[29]. mining, Conductivity and it also can has reflect a strong the influence concentrat on the valenceion of andPTEs fractionation in leachate, of PTEs and in its samples trend [29 can]. reflect the intensityConductivity of interaction can reflectthe between concentration solid-liquid of PTEs in leachate,interface and of its trendleaching can reflect system. the intensity The results of for the intensity of interaction between solid-liquid interface of leaching system. The results for experimentationinteraction carried between out solid-liquid are shown interface in Figu of leachingre 4, Figure system. 5, The Figure results 6, for respectively. experimentation carried experimentationout are shown carried in Figures out are4,5, shown and6, respectively. in Figure 4, Figure 5, Figure 6, respectively.

400 400 380 380

) 360 -1

) 360 -1 cm · 340 s cm µ · 340 s µ 320 320 Granularity 1 GranularityGranularity 12 300 Granularity 2 Conductivity/( 300 Granularity 3

Conductivity/( Granularity 3 280 280 260 260 123456789101112 123456789101112Time/d Time/d Figure 4. The trend in pH of leaching liquid over time with different particle size of waste. Figure 4.Figure The 4.trendThe trendin pH in of pH leaching of leaching liquid liquid over over timetime with with di ﬀdifferenterent particle particle size of size waste. of waste.

8.6 8.6 8.4 Granularity 1 8.4 GranularityGranularity 12 8.2 GranularityGranularity 23 8.2 Granularity 3 8.0 8.0 7.8 7.8

pH 7.6

pH 7.6 7.4 7.4 7.2 7.2 7.0 7.0 6.8 6.8 123456789101112 123456789101112Time/d Time/d Figure 5.Figure The 5.conductivityThe conductivity of leaching of leaching liquid liquid over time time with with diﬀ differenterent particle particle size of size waste. of waste. Figure 5. The conductivity of leaching liquid over time with different particle size of waste. As shown in Figure4, the trend in pH of the leachate with di ﬀerent particle sizes was essentially As shownthe same. in It Figure can be divided4, the trend into three in pH stages: of the an leac initialhate increase with todifferent a maximum, particle a slow sizes decline was and essentially As shown in Figure 4, the trend in pH of the leachate with different particle sizes was essentially the same.stabilization. It can be Ondivided the 6th into day ofthree leaching, stages: the an pH initial of the leachateincrease for to particle a maximum, size 1 and a particle slow sizedecline and the same. It can be divided into three stages: an initial increase to a maximum, a slow decline and stabilization.2 increased On the rapidly 6th day to the of highest leaching, value, the with pH particle of the size leachate 1 = pH for 8.47, particle while for size particle 1 and size particle 3 the size 2 stabilization. On the 6th day of leaching, the pH of the leachate for particle size 1 and particle size 2 increased rapidly to the highest value, with particle size 1 = pH 8.47, while for particle size 3 the increased rapidly to the highest value, with particle size 1 = pH 8.47, while for particle size 3 the highest value of 7.98 was achieved on the 7th day. After that, the pH values began to decrease, with highest value of 7.98 was achieved on the 7th day. After that, the pH values began to decrease, with the leachate from particle size 1 and 2 dropping to their lowest points on the 9th day (particle size 2 the leachate from particle size 1 and 2 dropping to their lowest points on the 9th day (particle size 2 was pH 7.94), 0.21 lower than that of the leachate on the 6th day. Between the 10th day and the 12th was pH 7.94), 0.21 lower than that of the leachate on the 6th day. Between the 10th day and the 12th day, the pH values gradually stabilized, and the final leaching solutions were alkaline. day, the pH values gradually stabilized, and the final leaching solutions were alkaline. From Figure 5, it can be seen that the conductivity of the leachate decreased with the increase of From Figure 5, it can be seen that the conductivity of the leachate decreased with the increase of particle size. Over the first eight days, the leachate conductivity increased rapidly, but slowed and particle size. Over the first eight days, the leachate conductivity increased rapidly, but slowed and stabilized between Day 9 to Day 12. On the 12th day, the conductivity for all three wastes was nearly stabilized between Day 9 to Day 12. On the 12th day, the conductivity for all three wastes was nearly 400 μs·cm−1. 400 μs·cm−1. Int. J. Environ. Res. Public Health 2019, 16, 2355 6 of 11

highest value of 7.98 was achieved on the 7th day. After that, the pH values began to decrease, with the leachate from particle size 1 and 2 dropping to their lowest points on the 9th day (particle size 2 was pH 7.94), 0.21 lower than that of the leachate on the 6th day. Between the 10th day and the 12th day, the pH values gradually stabilized, and the ﬁnal leaching solutions were alkaline. From Figure5, it can be seen that the conductivity of the leachate decreased with the increase of particle size. Over the ﬁrst eight days, the leachate conductivity increased rapidly, but slowed and stabilized between Day 9 to Day 12. On the 12th day, the conductivity for all three wastes was nearly Int. J. 400Environ.µs cm Res.1 Public. Health 2019, 16, x 6 of 11 · −

1600 (a) 10 (b) 1500 Granularity 1 1400 Granularity 1 Granularity 2 9 Granularity 2 1300 ) Granularity 3 ) -1

-1 Granularity 3 1200 8 g·L g·L

µ 1100 µ 1000 7 900 800 6 700 5 600 500 4 400 300 3 Dissolved quantity of Sb ( Dissolved quantity of As ( 200 100 2

123456789101112 123456789101112 Time/d Time/d 0.7 20 (c) (d) 18 Granularity 1 Granularity 1 0.6 Granularity 2

) Granularity 2 ) 16

-1 Granularity 3 -1 Granularity 3 g·L g·L µ µ 14 0.5 (

12 of Pb Pb of 0.4 10

8 0.3 6 Dissolved quantity Dissolved quantity of Hg ( 0.2 4

123456789101112 123456789101112 Time/d Time/d 0.09 (e) 800 (f) 0.08 Granularity 1 700 Granularity 1 )

-1 Granularity 2

) Granularity 2 -1 0.07 Granularity 3 Granularity 3 g·L µ

g·L 600 ( µ 0.06 500 of Zn Zn of 0.05 400 0.04 300 0.03

Dissolved quantity 200

Dissolved quantity of Cd ( 0.02 100 0.01

123456789101112 123456789101112 Time/d Time/d FigureFigure 6. The 6. The soluble soluble portion portion of of PTEs PTEs over timetime from from di ﬀdifferenterent solid solid phase phase particle particle sizes. (sizes.a) Sb; ((ba)) As;Sb; (b) As; ((cc)) Hg; Hg; (d)) Pb;Pb; (e(e)) Cd; Cd; (f ()f Zn.) Zn.

As can be seen from Figure6, the leaching behavior for all six PTEs was variable from element to As can be seen from Figure 6, the leaching behavior for all six PTEs was variable from element element. It is clear that the portion solubilized decreases with the increase of particle size, following to element.the trend It inis clear surface that area the available portion to solubilized react. After de 12creases days of with leaching, the increase the cumulative of particle amount size, leached following the trend in surface area available to react. After 12 days of leaching, the cumulative amount leached for the six PTEs (calculated according to Formula 1 and calculated in 10 mL leaching solution of each sample) was in the order Sb > Zn > Pb > As > Hg > Cd. Figure 5a–f shows that the leaching amount of six heavy metals basically reached a stable value in 9–12 days. After 12 days of leaching, the total amounts leached from the waste are shown in Table 2. The leaching characteristics of heavy metals in antimony ore processing wastes are consistent with previous studies [26,30].

Int. J. Environ. Res. Public Health 2019, 16, x 7 of 11

Table 2. Total leaching amount and leaching rate of heavy metals from antimony ore waste rock with different particle sizes.

Int. J. Environ. Res. Public HealthTotal Leaching2019, 16, 2355Leached Total Leaching Leached Total Leaching of Leached 7 of 11 The Content of (Granularity 1) Portion (Granularity 2) Portion Granularity 3 Portion Metals (μg/kg) (μg/kg) (%) (μg/kg) (%) (μg/kg) (%) forSb the six45,155.25 PTEs (calculated486.128 according to1.076 Formula 1399.363 and calculated 0.884 in 10 mL leaching92.626 solution 0.205 of each sample)As 31,941.00 was in the order 3.219 Sb > Zn > Pb 0.010> As > Hg > 2.523Cd. Figure6a–f 0.008 shows that 1.295 the leaching amount 0.004 ofHg six heavy36.00 metals basically 0.207 reached a 0.575 stable value 0.188in 9–12 days. 0.522 After 12 days 0.168 of leaching, the 0.467 total Pb 1156.05 6.104 0.528 3.661 0.317 2.468 0.213 amountsCd leached41.75 from the 0.025 waste are shown 0.060 in Table2 0.018. The leaching 0.043 characteristics 0.011 of heavy metals 0.026 in antimonyZn 15,422.75 ore processing 241.152 wastes are consistent1.564 with206.703 previous studies1.340 [ 26,30]. 55.878 0.362

TableIt can 2.alsoTotal be leachingseen from amount Table and 2 that leaching the leaching rate of heavy of Sb metals and Zn from from antimony the waste ore wasterock with rock withdifferent particlediff erentsizes particlediffered sizes. greatly. The leaching of Sb was far below 18% [4]; the reason may be related to the differentThe Content pH of leachingTotal Leaching and associatedLeached solubiTotallity. Leaching The smallerLeached the particleTotal Leachingsize, the higherLeached the leaching ofof the Metals PTEs, while(Granularity the leaching 1) Portion of As, Hg(Granularity, Pb and Cd 2) wasPortion less affectedof Granularity by particle size.Portion (µg/kg) (µg/kg) (%) (µg/kg) (%) 3 (µg/kg) (%) 3.3.Sb Scanning 45,155.25 Electron Microscope 486.128 and Energy 1.076 Spectrum 399.363 Analysis 0.884 92.626 0.205 As 31,941.00 3.219 0.010 2.523 0.008 1.295 0.004 HgSEM and 36.00 EDS analysis 0.207 were used to 0.575 study antimo 0.188ny ore waste 0.522 rock (0.075–0.15 0.168 mm) before 0.467 and afterPb leaching, 1156.05 and the results 6.104 of the EDS 0.528 measurement 3.661 of waste rock 0.317 surface morphology 2.468 at different 0.213 Cd 41.75 0.025 0.060 0.018 0.043 0.011 0.026 stagesZn were 15,422.75 obtained. Figure 241.152 7 shows the 1.564 surface morphology 206.703 of the 1.340 waste before 55.878 and after leaching. 0.362 It can be seen that the surface morphology changed significantly. Before leaching, the surface was relatively flat, compact and less crystalline, while after leaching, the surface of the waste was broken, It can also be seen from Table2 that the leaching of Sb and Zn from the waste rock with di fferent with a small amount of granular sediment. particle sizes differed greatly. The leaching of Sb was far below 18% [4]; the reason may be related Table 3 shows EDS results for the waste before and after leaching, emphasizing the very to the different pH of leaching and associated solubility. The smaller the particle size, the higher the significant changes in the surface composition. Before leaching, the main elements on the surface of leaching of the PTEs, while the leaching of As, Hg, Pb and Cd was less affected by particle size. waste rock were O, Si and Al. The contents of target PTEs Sb, As, Hg, Pb, Cd and Zn in the waste were3.3. Scanning 1.42%, 0, Electron 0.07%, Microscope1.09%, 0.12% and and Energy 1.24%, Spectrum respectively. Analysis With the increase of leaching time, the content of Si on the surface of waste antimony ore rock decreased; that is, the content of Si-bearing SEM and EDS analysis were used to study antimony ore waste rock (0.075–0.15 mm) before and quartz or silicate minerals on the surface of waste rock decreased. The contents of PTE elements and after leaching, and the results of the EDS measurement of waste rock surface morphology at different oxygen increased and varied greatly, which indicates that the PTE on the surface of and inside the stages were obtained. Figure7 shows the surface morphology of the waste before and after leaching. waste diffuse into solution under erosion, and then precipitate with metal hydroxides and as It can be seen that the surface morphology changed significantly. Before leaching, the surface was complexes [31,32], and are adsorbed on the surface of waste rock. The increase of As content may relatively flat, compact and less crystalline, while after leaching, the surface of the waste was broken, have been due to the exposure of As encapsulated in the waste rock particles of antimony ore rock with a small amount of granular sediment. by acid leaching.

(a) (b)

Figure 7. 7. SEMSEM images images of antimony of antimony ore waste ore waste rocks rocksat different at different periods periods (a) before (a )leaching; before leaching;(b) after (leaching.b) after leaching.

Table3 shows EDS results for the waste before and after leaching, emphasizing the very signiﬁcant changes in the surface composition. Before leaching, the main elements on the surface of waste rock were O, Si and Al. The contents of target PTEs Sb, As, Hg, Pb, Cd and Zn in the waste were 1.42%, 0, 0.07%, 1.09%, 0.12% and 1.24%, respectively. With the increase of leaching time, the content of Si on the surface of waste antimony ore rock decreased; that is, the content of Si-bearing quartz or silicate minerals on the surface of waste rock decreased. The contents of PTE elements and oxygen increased Int. J. Environ. Res. Public Health 2019, 16, 2355 8 of 11 and varied greatly, which indicates that the PTE on the surface of and inside the waste diﬀuse into solution under erosion, and then precipitate with metal hydroxides and as complexes [31,32], and are adsorbed on the surface of waste rock. The increase of As content may have been due to the exposure of As encapsulated in the waste rock particles of antimony ore rock by acid leaching.

Table 3. Main elements of antimony ore waste rocks surface at different periods by EDS (%, mass fraction).

O Si Al Ca S Sb As Hg Pb Cd Zn Cu Mn K Fe Before leaching 46.71 43.14 2.44 1.24 0.86 1.42 0.00 0.07 1.09 0.12 1.24 0.54 0.08 0.24 0.91 After leaching 54.03 38.82 1.23 0.85 0.69 1.25 0.61 0.03 0.78 0.06 0.93 0.05 0.07 0.02 0.58

3.4. Leaching Mechanism of Antimony Ore Waste Rock During the leaching process, the leachate pH showed three distinct stages: An initial increase, decrease and stabilization. The stages also reflected differences in the chemical stability. When the pH rose (the first 6 days of leaching), the leachate conductivity increased rapidly, with increased H+ consumption by reactions—probably solubilizing minerals such a as calcite, with H+ in solution interacting with available cations (K+, Na+, Ca2+, Mg2+, Al3+)[33] (Equation (3)), resulting in the rapid increases in pH and conductivity.

n n+ + n n+ n+ R −X + nH R −H + X X = K/Na/Ca/Mg/Al (3) ↔ Most of the metal elements in antimony ore waste rock predominantly exist in a very stable residual fraction and organic-sulfide fraction. Other phases include the reducible fraction of iron and manganese oxide, the carbonate-bound fraction and exchangeable fraction [34]. Metal elements in carbonate-bound and exchangeable forms are easily soluble in water and acid solution [35]. The Sb associated with Carbonate-bound and exchangeable forms in the waste was released by Equations (4)–(9) below [4,36]. Sulfur species in the dissolution system also governed the dissolution equilibrium of Sb2S3 [37]. The oxidation of sulfides shifted the dissolution equilibrium of Sb2S3. The released sulfide from upon dissolution of stibnite can undergo oxidation quickly, as observed during previous experiments [7]. The associated As, Hg, Pb, Cd and Zn in the leachate were also due to the solubility of carbonate-bound and exchangeable forms in acid.

Sb S (s) + 6H O(l) 2Sb(OH) (aq) + 3H S(aq) (4) 2 3 2 ↔ 3 2 0 + Sb(OH) + 1/2O + 2H O 2Sb(OH)− + H (5) 3 2 2 → 6 H S + 2O H SO (6) 2 2 → 2 4 2 2HS− + 2O H O + S O − (7) 2 → 2 2 3 2 2 0 S O − + 4O 2SO − + 2S (8) 2 3 2 → 4 2 + 2HS− + 4O 2SO − + 2H (9) 2 → 4 During the decline in pH (7th–10th day), the conductivity of leachate continued to increase, but the rate of increase slowed, indicating that the exchange between H+ dissolution and basic cations was still strong at this time, but the intensity was decreasing. As leaching progressed, the portion of reducing minerals in the waste increased, and pyrite was oxidized (Equation (10)), producing H+ to reduce leachate pH, which conforms to the law of leachate pH reduction in the leaching system.

3+ 2 + FeS + 15O + 2H O 4Fe + 8SO − + 4H (10) 2 2 2 ↔ 4 When the pH stabilizes (Day 11–Day 12), the pH of the leaching solutions were 8.25 (granularity 1), 7.95 (granularity 2) and 7.67 (granularity 3). The slight alkalinity highlights the strong acid neutralization Int. J. Environ. Res. Public Health 2019, 16, 2355 9 of 11 capacity with the PTEs forming hydroxides or complexes after dissolution. During the storage of the waste, the PTEs dissolved and released will be reduced in their ability to migrate immediately, and in the long term the hazard potential is localized within the site, but sensitive to future change in environmental conditions [38].

4. Conclusions (1) The dissolution concentration of PTEs decreases with the increase of particle size, and the dissolution intensity of each element is diﬀerent. The maximum dissolution of Sb, As, Hg, Pb, Cd and Zn from 25 g samples of antimony ore waste rock were 486.128 µg, 3.219 µg, 0.207 µg, 6.104 µg, 0.025 µg and 241.152 µg, respectively, and the portions dissolved were 1.076%, 0.010%, 0.575%, 0.528%, 0.060% and 1.564%, respectively. (2) The changes in leachate pH for the diﬀerent particle sizes is consistent. The pH trends show three stages: An immediate rise, a fall and then stabilization. The main chemical reactions of the leaching system in each stage are the exchange of H+ with alkaline minerals and basic cations, the oxidation of reducing minerals such as pyrite to produce acid and the adsorption and precipitation of metallic elements. (3) The waste has a strong acid neutralization capacity. The PTEs after dissolution and precipitation will form hydroxides or complexes, with lower immediate capability to migrate, but will remain available for release under changing environmental conditions. Long term impact is still likely and needs to be considered for management of the site.

Author Contributions: Conceptualization, S.Z. and A.H.; methodology, S.Z.; software, S.Z.; validation, A.H.; formal analysis, S.Z.; investigation, S.Z.; resources, S.Z.; data curation, S.Z. and A.H.; writing—original draft preparation, S.Z.; writing—review and editing, A.H.; visualization, S.Z.; supervision, S.Z.; project administration, S.Z.; funding acquisition, S.Z. Funding: This study was financially supported by the National Natural Science Foundation of China (No.41672350), Chinese Postdoctoral Science Foundation (No.2018M632961), the scientific research project of the Hunan Provincial Education Department (No.18A184) and Doctoral Fund of Hunan University of Science and Technology (No.E57109). Conflicts of Interest: The authors declare no conflict of interest.

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