Geochemical Characteristics and Toxic Elements in Alumina Refining Wastes and Leachates from Management Facilities

A nationwide investigation was carried out to evaluate the geochemical characteristics and environmental impacts of red mud and leachates from the major alumina plants in China. The chemical and mineralogical compositions of red mud were investigated, and major, minor, and trace elements in the leachates were analyzed. The mineral and chemical compositions of red mud vary over refining processes (i.e., Bayer, sintering, and combined methods) and parental bauxites. The main minerals in the red mud are quartz, calcite, dolomite, hematite, hibschite, sodalite, anhydrite, cancrinite, and gibbsite. The major chemical compositions of red mud are Al, Fe, Si, Ca, Ti, and hydroxides. The associated red mud leachate is hyperalkaline (pH > 12), which can be toxic to aquatic life. The concentrations of Al, Cl−, F−, Na, NO32−, and SO42− in the leachate exceed the recommended groundwater quality standard of China by up to 6637 times. These ions are likely to increase the salinization of the soil and groundwater. The minor elements in red mud leachate include As, B, Ba, Cr, Cu, Fe, Ni, Mn, Mo, Ti, V, and Zn, and the trace elements in red mud leachate include Ag, Be, Cd, Co, Hg, Li, Pb, Sb, Se, Sr, and Tl. Some of these elements have the concentration up to 272 times higher than those of the groundwater quality standard and are toxic to the environment and human health. Therefore, scientific guidance is needed for red mud management, especially for the design of the containment system of the facilities.


Introduction
Red mud is an insoluble residue produced by aluminum oxide (alumina) refining process with the characteristics of very fine particle size and high alkalinity [1]. In 2015, 274 million tons of bauxite ore were mined globally for alumina production, with Australia (33%), China (20%) and Brazil (16%) being the leading bauxite producers in the world [2]. Approximately 60 million tons of red mud is produced each year globally, with 6 million tons of red mud occurring in China. However, approximately 85% of the red mud is stored in the on-site reservoirs near the alumina refining plant. The reservoir is constructed with dams for containment purposes, and the red mud is treated by a natural settlement process [3].
Currently, manufacturers often adopt the chemical ore dressing process for alumina refining. This process uses the alkali method (sodium hydroxide or sodium carbonates) to produce alumina from bauxite and can be divided into the Bayer process, the sintering method, and the combined method [4,5]. The main components of the red mud are hematite (Fe 2 O 3 ), calcite (CaCO 3 ), cancrinite (Na 6 CaAl 6 Si 6 (CO 3 )O 24 ·2H 2 O), hydrogarnet (Ca 3 AlFe(SiO 4 )(OH) 8 ), tricalcium aluminate (Ca 3 Al 2 (OH) 12 ), and sodalite ((Na 6 Al 6 Si 6 O 24 )·(2NaX or Na 2 X)) [4,6]. Red mud has strong alkalinity and salinity, with heavy metal elements, including arsenic (As), lead (Pb), zinc (Zn), copper (Cu), nickel (Ni), chromium (Cr), and vanadium (V). The salinity and alkalinity of red mud can affect plant growth and lead to deterioration of soil quality. The leakage or spill of red mud releases oxyanionic trace elements such as Cr, molybdenum (Mo), and V. These contaminants have been reported to be highly soluble under high pH conditions (material pH of red mud) in leaching tests [6,7], suggesting that red mud could be a potential sink of contaminants [8].
Red mud typically occupies land spaces for storage and further generates potential threats to the surrounding environment. Unlined or inappropriately lined red mud reservoirs could induce the leakage of leachate (i.e., bauxite liquor), leading soil swampiness and groundwater pollution [9,10]. Additionally, occasional dam failures damage infrastructure and contaminate the environment. For example, the catastrophic dam failure of the Ajkai Timfoldgyar Zrt red mud reservoir that occurred in Hungary released of 0.7 to 1 million m 3 of caustic red mud into the Torna River, which resulted in the loss of lives and seriously contaminated soil and water [6,[9][10][11].
Overall, the environmental risks are closely related to the soda content, alkalinity, and heavy metal content in the red mud. However, there is still a lack of knowledge regarding the mineralogy and chemical composition within red mud and its leachates in China. The objective of this study is to understand the potential contamination by red mud and its associated leachates from red mud management facilities. The chemical compositions of red mud from different resources and production process are summarized. As red mud leachate has essential effects on the environment, the leaching ability of red mud and its environmental impacts were studied through the analysis of the concentration of elements in leachates. Fresh red mud and its leachates were sampled from the major alumina manufacturers in China. The main chemical parameters, including pH, electrical conductivity (EC), redox potential (ORP), and elemental concentration, were evaluated through laboratory analytical methods.
The Al/Si ratio determines the alumina processing procedure. In general, Guangxi aluminum mines are bauxite deposits with a high Al/Si ratio (6.3-17.8). This type of bauxite can be utilized to extract alumina using the Bayer process. Shanxi, Shandong, Henan, Guizhou, and Sichuan aluminum mines, which account for over 98% of the reserves in China, are mainly bauxite deposits with a relatively low Al/Si ratio (3.3-9.4). These types of ores are middle/low-grade diaspore bauxites. They are usually challenging and energy consuming to process for alumina. Therefore, alternative methods modified from the traditional Bayer process, i.e., the sintering process and the Bayer-sintering combined process, have been developed for alumina refining [4].

The Sampling of Red Mud and Leachate
In this study, samples of red mud and leachates from five management facilities were collected from three provinces (i.e., Guangxi, Shangdong, and Henan) in China ( Figure 1). The details of the sampling are summarized in Table 2. Fresh red mud samples GX-A1 and GX-A2, and leachate samples GX-A2-L and GX-A2-L were collected from the same manufacturer but two adjacent reservoirs (A1 and A2, respectively) in Pingguo County, Guangxi Province. The average annual temperature of Pingguo County is 21.5 • C, and the annual precipitation reaches 1500 mm. Fresh red mud sample GX-B and leachate sample GX-B-L were collected in Jingxi County, Guangxi Province. The average annual temperature of Jingxi County is 19.1 • C, and the annual precipitation reaches 1636 mm. Red mud sample SD-A and its leachate sample SD-A-L were collected in Zibo, Shandong. The average annual temperature of Zibo is 13.5 • C, and the annual precipitation reaches 650 mm. Red mud sample HN-A and its leachate sample HN-A-L were collected in Xingyang, Henan Province. The average annual temperature is 14.3 • C, and the annual precipitation reaches 645 mm. Fresh red mud was sampled after pressure filtration, and the leachate was collected from the drainage pipe of the red mud reservoir.

The Sampling of Red Mud and Leachate
In this study, samples of red mud and leachates from five management facilities were collected from three provinces (i.e., Guangxi, Shangdong, and Henan) in China ( Figure 1). The details of the sampling are summarized in Table 2. Fresh red mud samples GX-A1 and GX-A2, and leachate samples GX-A2-L and GX-A2-L were collected from the same manufacturer but two adjacent reservoirs (A1 and A2, respectively) in Pingguo County, Guangxi Province. The average annual temperature of Pingguo County is 21.5 °C, and the annual precipitation reaches 1500 mm. Fresh red mud sample GX-B and leachate sample GX-B-L were collected in Jingxi County, Guangxi Province. The average annual temperature of Jingxi County is 19.1 °C, and the annual precipitation reaches 1636 mm. Red mud sample SD-A and its leachate sample SD-A-L were collected in Zibo, Shandong. The average annual temperature of Zibo is 13.5 °C, and the annual precipitation reaches 650 mm. Red mud sample HN-A and its leachate sample HN-A-L were collected in Xingyang, Henan Province. The average annual temperature is 14.3 °C, and the annual precipitation reaches 645 mm. Fresh red mud was sampled after pressure filtration, and the leachate was collected from the drainage pipe of the red mud reservoir.

Bulk Chemical Analysis
Specimens were analyzed using X-ray fluorescence (XRF) quantitative analysis (Shimadzu XRF-1800, Kyoto, Japan). The analytical method for silicate rocks was used, and specimens were analyzed

Bulk Chemical Analysis
Specimens were analyzed using X-ray fluorescence (XRF) quantitative analysis (Shimadzu XRF-1800, Kyoto, Japan). The analytical method for silicate rocks was used, and specimens were analyzed in duplicates. Five grams of each sample was air-dried for 24 h and ground to <0.075 mm with agate mortar and pestle. Loss on ignition tests were performed before XRF analysis by sintering samples at 900 • C using a muffle furnace (YSD-5-12T, Yaoshi Instrument Equipment Ltd., Shanghai, China). The LOI is used as an indicator for organic contents in the samples. XRF tests were conducted by fusing 0.9 g of calcined powder (i.e., specimen after LOI) with 1 g of NH 4 NO 3 oxidizer and 9.0 g of lithium borate flux (50%/50% Li 2 B 4 O 7 -LiBO 2 ) at 1050 • C into a flat molten glass disk. The specimens were then analyzed by XRF spectrometry.

Mineralogical Analysis
Quantitative X-ray diffraction analysis (Rigaku D/MAX-2005 X-ray diffractometer, Tokyo, Japan) was performed on the red mud samples to determine the major mineral phases. Specimens were placed in a desiccator for 24 h and ground to 0.075 mm with agate mortar and pestle. Cu Kα radiation was used, and each sample was placed in a 2-mm deep sample holder and scanned at 0.02 • intervals between 5 • to 50 • 2θ with a 2 s dwell time. Samples were scanned within 48 h after desiccation to minimize crystal dehydration.

Chemical and Mineral Compositions of Red Mud
The chemical and mineralogical compositions of red mud in China from this study and the literature are summarized in Tables 3 and 4, respectively. The major chemical components of red mud are SiO 2 , TiO 2 , Al 2 O 3 , Fe 2 O 3 , CaO, and Na 2 O. These components comprise 69.5-98.3% of the total mass of the red mud. Na mainly comes from the sodium hydroxide (NaOH), which is used to treat bauxite, while Al, Si, Fe, Ti, and other elements are derived from the parental bauxite. The secondary components are K 2  The SiO 2 content of red mud ranges from 8.4% to 32.5%. SiO 2 is leached from kaolinite (Al 2 Si 2 O 3 (OH) 4 ) in bauxite during alumina refining and exists in the form of hydrophane (SiO 2 ·nH 2 O) and sodium silicate (Na 2 SiO 3 ) [24]. In general, the red mud samples derived from Guangxi and Yunnan have lower SiO 2 contents (8.4-11.9%) than those from Henan, Shandong, Shanxi and Guizhou (up to 32.5%) ( Figure 2 and Table 3). This difference is mainly due to the different Si-Al ratio of bauxite ores. The Fe 2 O 3 content of red mud ranges from 2.5% to 37.0% (Table 3). Fe is mainly in the form of Fe(OH) 3 , which is the oxidation and hydration product of FeS 2 in bauxite. Fe(OH) 3 is unstable under strong alkaline and high-temperature conditions, and easily converts into goethite (FeOOH). In fresh red mud, Fe(OH) 3 and FeOOH may coexist, and FeO may exist in the form of siderite (FeCO 3 ). The Al 2 O 3 content of red mud ranges from 6.4 to 31.0% (Table 3), and Al 2 O 3 usually exists in two forms, i.e., NaAlO 2 and Al(OH) 3 , under strongly alkaline conditions. The CaO content ranges from 2.2% to 48.4% (Table 3), and CaO usually forms aragonite (CaCO 3 ) or calcite (CaCO 3 ). CaCO 3 can deposit and crystallize after the introduction of quicklime (CaO) and carbon dioxide (CO 2 ) during alumina production. The content of Na 2 O in red mud ranges from 2.3% to 16.2% (Table 3) and exists in pore solutions as free Na + . In addition, a series of saline deposits or colloid products, such as Na 2 CO 3 , NaHCO 3 , Na 2 SiO 3 , and NaAlO 2 , are formed during desiccation. The Ti 2 O content ranges from 0.05% to 7.0% (Table 3). Ti 2 O exists as rutile and anatase in bauxite.

The Hydrochemistry of Red Mud Leachate and its Major Elements
Hydrochemical parameters of the red mud leachate samples include pH, EC, ORP, and ionic strength are summarized in Table 5. The pH of red mud leachate ranges from 12.1 to 12.6, which exceeds the Integrated Wastewater Discharge Standard in China (IWDS) (GB8978-1996). The ionic strength ranges from 66.9 to 484.3 mM, and the EC value of red mud leachate ranges from 4.4 to 51.1 mS/cm which exceeds the standard (EC value less than 2.0 mS/cm) for reverse osmosis water treatment system. The ORP ranges from −110.0 to −29.0 mV, indicating a reducing state of the red mud leachate.

The Hydrochemistry of Red Mud Leachate and Its Major Elements
Hydrochemical parameters of the red mud leachate samples include pH, EC, ORP, and ionic strength are summarized in Table 5. The pH of red mud leachate ranges from 12.1 to 12.6, which exceeds the Integrated Wastewater Discharge Standard in China (IWDS) (GB8978-1996). The ionic strength ranges from 66.9 to 484.3 mM, and the EC value of red mud leachate ranges from 4.4 to 51.1 mS/cm which exceeds the standard (EC value less than 2.0 mS/cm) for reverse osmosis water treatment system. The ORP ranges from −110.0 to −29.0 mV, indicating a reducing state of the red mud leachate. Al, Ca, Na, K, Mg, Si, Cl − , F − , NO 3 2− , and SO 4 2− are the major elements and ions (defined based on the concentrations) in the red mud leachate (Figure 3 and Table 6). The maximum contamination levels (MCLs) of type (III) (for drinking water) groundwater of the Quality Standard for Groundwater in China (China (GW)) (GB/T 14848-2017) and US Environment Protection Agency Groundwater Quality Standard (USEPA (GW)) were used to evaluate the possible harmfulness of leachate to the environment and human health.  Al, Ca, Na, K, Mg, Si, Cl − , F − , NO3 2− , and SO4 2− are the major elements and ions (defined based on the concentrations) in the red mud leachate (Figure 3 and Table 6). The maximum contamination levels (MCLs) of type (III) (for drinking water) groundwater of the Quality Standard for Groundwater in China (China (GW)) (GB/T 14848-2017) and US Environment Protection Agency Groundwater Quality Standard (USEPA (GW)) were used to evaluate the possible harmfulness of leachate to the environment and human health.    The Al concentration in red mud leachate ranges from 118.3 to 1327.4 mg/L, which exceeds the recommended values set by USEPA (GW) (2 mg/L) and China (GW) (0.2 mg/L). Na concentrations in the red mud leachate range from 1200.5 to 10,650.0 mg/L, which is 6002.5 to 53,250 times higher than the recommended value by China (GW) (0.2 mg/L). Additionally, Cl − , as one of the main anions found in the leachate solution of red mud, has a concentration ranging from 551.4 to 6588.1 mg/L. This range is 2.2 to 26.4 times higher than the recommended value of USEPA (GW) and China (GW) (250 mg/L). The F − concentration in the leachate ranges from 88.0~299.6 mg/L. which is more than 44 times higher than the recommended value of USEPA (GW) (2 mg/L) and China (GW) (1 mg/L). The concentration of NO 3 2− ranges from 183.2-730.7 mg/L, which is 9.2 to 36.5 times higher than the recommended value of

The Minor and Trace Elements in Red Mud Leachate
The minor elements of the red mud leachates mainly include As, B, Ba, Cr, Cu, Fe, Ni, Mn, Mo, Ti, V, and Zn ( Figure 4 and Table 7). Elements, such as, Cr, and V, form oxyanions under alkaline conditions (pH >10) and are highly soluble in the water [39]. The concentration ranges of these elements are: 0.  Trace elements of the red mud leachate mainly include Ag, Be, Cd, Co, Hg, Li, Pb, Sb, Se, Sr, and Tl ( Figure 5 and Table 8 Trace elements of the red mud leachate mainly include Ag, Be, Cd, Co, Hg, Li, Pb, Sb, Se, Sr, and Tl ( Figure 5 and Table 8 . However, the MCLs of IWDS are much higher than those for drinking ground-water. The concentrations of the minor and trace elements, such as As, Cr, Ni, Mn, Zn, Be, Cd, Pb, Hg, and Se exceed the MCLs of IWDS; this indicates that red mud leachate is harmful to the environment and cannot be directly discharged into the river network.  Trace elements of the red mud leachate mainly include Ag, Be, Cd, Co, Hg, Li, Pb, Sb, Se, Sr, and Tl ( Figure 5 and Table 8  The reactions indicate that NaOH, Na 2 CO 3 , NaHCO 3 , and NaAl(OH) 4 are the major soluble alkalinity compounds in red mud. These compounds are formed mainly due to the addition of NaOH, CO 2 , and CaO during the alumina production process [4]. The main anions leading to the alkalinity of red mud in the leachate are: OH − , CO 3 4 ), and tricalcium aluminate (Ca 3 Al 2 O 6 ). However, the alkaline substances of red mud vary with the refining processes. The Bayer red mud is strongly alkaline [9]. Compared with other processes, more caustic soda is added in the Bayer process, resulting in much higher contents in sodium monoxide (Na 2 O) in the red mud, with approximately 9.7% being detected in the red mud sample of SD-A ( Table 2). In general, the major elements (including Al, Ca, K, Na, Mg, Si, Cl−, F−, NO 3 2− , and SO 4 2− ) all exceeded the recommended value of groundwater quality standards and could increase the risk of salinization of soil and water [7,9]. Al mainly exists as Al(OH) 4− under high pH (pH > 12) conditions, and the insoluble Al(OH) 3 in red mud can easily be converted into soluble Al(OH) 4− . NaOH is added during the refining process, therefore, a high concentration of Na is expected to be retained or dissolved in the leachate. In some cases, KOH is added during the refining process, such as in sample HN-A-L, and a higher content of K (1016.0 mg/L) in the leachate can be found than in leachates of other red muds (Table 6). Na or K is the top components for salinization of soil and water [9]. As one of the main anions found in the major elements of red mud leachate, Cl − is derived from CaCl 2 . CaCl 2 is used to precipitate soluble hydroxides, aluminates, and carbonate. By adding CaCl 2 , soluble alkaline substances can be converted to calcite (CaCO 3 ), hydrocalumite (Ca 4 Al 2 (OH) 12 ·CO 3 ), and aluminohydrocalcite (CaAl 2 (CO 3 ) 2 (OH) 4 · 3 H 2 O). The insoluble tricalcium aluminate (Ca(AlO 2 ) 2 ) existing in red mud will participate in forming soluble Al(OH) 4− ions [44][45][46]. This precipitation process aims to lower the pH and alkalinity of the leachate before discharge. The F − is mainly leached from the fluoride in red mud, derived from the bauxite ore. The F − content in red mud leachate is more than 44 times higher than the recommended value of USEPA (GW) and China (GW) (Figure 3, Table 6). The long-term intake of water with high fluoride concentration may cause bone fluorosis, dental fluorosis, osteoporosis, resulting in serious human health issues.
The concentrations of minor and trace elements, such as As, B, Cr, Fe, Ni, Mn, Mo, V, Zn, Ag, Be, Cd, Co, Hg, Pb, Sb, Se, and Tl, exceed the standards. The heavy metal elements, such as Cr, Fe, Ni, Mn, Mo, V, Zn, Ag, Co, Hg, and Pb, derived from the metal and its associated minerals in bauxite ore. These heavy metal elements could accumulate in soil and water. They increase ecological risks to crops, agricultural products, and groundwater, and also endanger human health through the food chain [47]. The impacts of red mud and its associated leachate on the surrounding environment largely depend on the management of red mud. Measures to prevent the leakage or spill of red mud and its leachate should be considered in the design of red mud management facilities, especially in the design of the containment system (especially, the liner) of the red mud reservoirs. Therefore, further studies are needed to investigate the compatibility between red mud leachate and liner materials, as the chemical environment, especially high pH and salinity conditions, could affect the hydraulic behavior and strength of the liner materials [48][49][50].

Conclusions
This study evaluated the chemical and mineralogical composition of red mud produced in China. Red mud and leachate samples were taken from five management facilities in Shandong, Guangxi, and Henan provinces of China. The chemical and mineralogical compositions of red mud and the major, minor and trace elements in the leachate were evaluated by laboratory analytical methods. The results of geochemical analysis on red mud from the literature were compared for a better understanding of the chemical characteristics of red mud.
Based on the findings of this study, the following conclusions and recommendations are drawn: The mineral and chemical compositions of red mud vary over different alumina refining processes and parental bauxite. Diaspore is the primary type of bauxite in China. Among the red mud produced by the Bayer, sintering, and combined methods, levels of Na 2 O and Al 2 O 3 are higher in the red mud from the Bayer method, and CaO is higher in red muds from the sintering method.