ARTICLE

J. Cent. South Univ. (2019) 26: 430-439

DOI: https://doi.org/10.1007/s11771-019-4015-9

Improvement of alkaline electrochemical characteristics of bauxite residue amendment with organic acid and gypsum

KONG Xiang-feng(孔祥峰)¹, LI Chu-xuan(李楚璇)¹, JIANG Jun(江钧)¹,HUANG Long-bin(黄隆斌)², HARTLEY William³, WU Chuan(吴川)¹, XUE Sheng-guo(薛生国)¹

1. School of Metallurgy and Environment, Central South University, Changsha 410083, China;

2. Centre for Mined Land Rehabilitation, Sustainable Minerals Institute, The University of Queensland, Brisbane, Qld 4072, Australia;

3. Crop and Environment Sciences Department, Harper Adams University, Newport, Shropshire,TF10 8NB, United Kingdom

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2019

Abstract: Neutralization of alkaline properties of bauxite residue (BR) by using organic acid and gypsum additions may effectively improve electrochemical properties and alleviate physicochemical barriers to ecological rehabilitation. Mineral acids, citric acid and hybrid acid–gypsum additions were compared for their potential to transform and improve zeta potential, isoelectric point (IEP), surface protonation and active alkaline —OH groups, which are critical factors for further improvement of physicochemical and biological properties later. Isoelectric points of untransformed bauxite residue and six transformed derivatives were determined by using electroacoustic methods. Electrochemical characteristics were significantly improved by the amendments used, resulting in reduced IEP and —OH groups and decreased surface protonation for transformed residues. XRD results revealed that the primary alkaline minerals of cancrinite, calcite and grossular were transformed by the treatments. The treatments of citric acid and gypsum promoted the dissolution of cancrinite. From the SEM examination, citric acid and gypsum treatments contributed to the reduction in IEP and redistribution of —OH groups on particle surfaces. The collective evidence suggested that citric acid and gypsum amendments may be used firstly to rapidly amend bauxite residues for alleviating the caustic conditions prior to the consideration of soil formation in bauxite residue.

Key words: bauxite residue; alkalinity regulation; organic acid; gypsum; electrochemical characteristic; soil formation in bauxite residue

Cite this article as: KONG Xiang-feng, LI Chu-xuan, JIANG Jun, HUANG Long-bin, HARTLEY William, WU Chuan, XUE Sheng-guo. Improvement of alkaline electrochemical characteristics of bauxite residue amendment with organic acid and gypsum [J]. Journal of Central South University, 2019, 26(2): 430–439. DOI: https://doi.org/ 10.1007/s11771-019-4015-9.

1 Introduction

Developing clean, economic and novel strategies for the management of bauxite residue (BR) is a key objective in order to balance the expanding alumina industry that has prompted multiple environmental hazards. Large quantities of highly alkaline bauxite residue are produced through refining activities from alumina refineries, with approximately 0.5–2 t of residue generated per ton of alumina product [1, 2]. The global accumulative inventory of BR has exceeded 4.4 billion tons, whilst increasing by an estimated 200 million tons every year [3]. Almost all BR continues to be deposited indefinitely in bauxite residue disposal areas (BRDAs) [4–6], with little progress in rehabilitation, particularly in temperate regions [7–9]. Novel technology and methodology are urgently required to rehabilitate BRDAs with sustainable ecosystems and plant communities, one of which is in situ soil formation to support colonization of plant communities. One of the critical requirements of in situ soil formation from the BR is aggregation and physical structural development, but is severely limited by the unfavourable mineralogy (e.g., Na-rich minerals), geochemistry (e.g., alkaline pH and extremely high soluble Na levels) and electrochemical characteristics (e.g., dominance of negative charges), apart from the lack of organic matter [10].

Bauxite residues typically contained hematite (Fe₂O₃), perovskite (CaTiO₃), gibbsite (Al(OH)₃), and diaspore (α-AlO(OH)) [11]. Secondary minerals (especially desilication products of precipitated aluminosilicates) generated from Bayer process may include cancrinite ([Na₆Al₆Si₆O₂₄]·

2[CaCO₃]), sodalite ([Na₆Al₆Si₆O₂₄]·[2NaX or Na₂X]), hydrogarnet (Ca₃Al₂(SiO₄)_x(OH)_12–4x), tri-calcium aluminate (TCA, Ca₃Al₂(OH)₁₂), oxides, carbonates, and hydroxides [12]. Neo-formed buffering minerals of cancrinite, sodalite, tri-calcium aluminate and hydrogarnet can possess high alkalinity within their inherent structures and ionized surface active —OH groups, resulting in high acid neutralizing capacity (ANC) and extremely alkaline conditions (pH>11) in the BR [13]. Furthermore, these minerals are fine grained, poorly-crystallized, and structurally complex, rich in adsorbing surface area and internal channel for ionic adsorption [7, 16, 14], which renders the extreme difficulties in removals of alkalinity and salinity and slow progress of the development of soil-like physicochemical properties and soil formation in the BR.

As a result, lowering the strongly alkaline pH conditions by means of various neutralization methods is the prerequisite to soil formation and plant establishment and primary succession [5]. Gypsum reactions in the BR precipitate hydroxides, carbonates and aluminates as calcium hydroxide (Ca(OH)₂), hydrocalumite (Ca₄Al₂(OH)₁₂·CO₃), tri-calcium aluminate (Ca₃Al₂(OH)₁₂, TCA) and calcite (CaCO₃) [10, 15]. The effectiveness of gypsum amendment is subject to the content of effective constituent (i.e., CaSO₄) and its solubility of gypsum, which determines the rate of Ca²⁺ release into solution and associated reactions.

Both organic and mineral acids may be used to achieve rapid neutralization of the strongly alkaline bauxite residue where waste acid sources may be available from adjacent chemical and food processing industry activities. Organic acids produced from microbial driven organic matter decomposition have been demonstrated to be effective in reducing pH by interaction with various states of alkalinity in both pore water and solids [16, 17]. However, the efficiency of the microbial process may not be feasible under field conditions in regions experiencing prolonged cold season and low rainfalls. Waste acid can react with OH^– and CO₃^2–, and alkaline oxides [18], leading to rapid pH reduction and depletion of alkaline materials. However, acid dosing of BR is controlled by a complicated series of chemistry reactions involving interaction between the solution phase and multiple solids [19, 20], which is profoundly affected by both the complexity of mineralogy and the various surface behaviors [21–23]. The relationship between residue surface electrochemical behavior and its complex mineralogy is pivotal to effective acid-reactions and neutralization. It is unknown how electro and/or surface chemistry and alkaline behavior develop in relation to the transformation of alkaline minerals in the acid reactions.

The present study focuses on electrochemical behavior of BR mineral systems transformed by acid (HCl, citric acid (H₃Cit) and H₂SO₄) reactions and their combinations with gypsum. The specific objectives were to 1) characterize the behavior of zeta potential, isoelectric point, surface protonation and alkaline groups following different acids dosing and gypsum additions, 2) identify alkaline speciation and particle distribution using X-ray diffraction and scanning electron microscope, 3) investigate the interaction of surface electrochemical characteristics and alkaline behaviors and attempt to understand BR acid-base chemistry.

2 Materials and method

2.1 Sample collection and processing

Freshly deposited BR samples were collected from ACC’s Pingguo Refinery (Aluminum Corporation of China), in south-west China (latitude 23°18′28.68″N, longitude 107°31′8.15″E). Three samples were collected that had a uniform distance of 5 m from one another to form a representative sample. Samples were air-dried for 5 d and then subsequently oven-dried at 60 °C for 3 d. The samples were then slightly disaggregated using a pestle and mortar, and sieved to retain the <2 mm fraction.

2.2 Neutralization of BR with acid and gypsum

The acid neutralization experiment was carried out by mixing BR with HCl (0.500 mol/L), H₂SO₄ (0.250 mol/L), and H₃Cit (0.167 mol/L), respectively, at 1.6 mL increment in conical flasks (100 mL). Un-transformed bauxite residue (UTBR) (10 g) was subsequently added into the flasks at an initial liquid to solid ratio of 5:1, of which the final volume was topped up to the same volume with Milli-Q water. There were three replicates in each treatment. The treatments were HCl transforming bauxite residue (HTBR), H₂SO₄ transforming bauxite residue (STBR), H₃Cit transforming bauxite residue (OTBR).

The gypsum effects were investigated in a separate test by admixing 0.2 g gypsum (CaSO₄·2H₂O, 2%) rapidly into the above acid-pretreated BR. All treatments were carried out in triplicate, which were designated as: HGTR (HCl and gypsum transforming bauxite residue), SGTR (H₂SO₄ and gypsum transforming bauxite residue), and OGTR (H₃Cit and gypsum transforming bauxite residue).

After applying the above treatments, all samples were placed on a shaker operating at 120 r/min (25 °C) and shaken for 60 d. Following this time period, samples were centrifuged at 4000 r/min for 20 min and residual solids were washed twice with Milli-Q water and air dried for 2 d. The samples were subsequently dried at 60 °C for 96 h. The dried residues were gently crushed in a mortar and sieved to retain the <48 μm fraction.

2.3 Sample characterization

Aliquots of dried residues were used for X-ray diffraction (XRD) analysis on a Bruker D8 Discover 2500 with a Cu K_α1 tube using a Sol-X detector. X-ray diffraction patterns were collected from 10° to 80° at a of 2θ step 0.04°and a 1°/min scan rate. The relative ratio method was used to quantify phases from XRD data.

Specific surface area (BET) was estimated by using a Quantachrome Quadrasorb S1-3MP auto- adsorption analyzer (employing liquid nitrogen adsorption) with the static volumetric technique (using t method). Liquid nitrogen was equilibrated with solid powder samples for 20 s, followed by degassing (20 min). Subsequently, samples were sequentially degassed at 150 °C for 1 h at 0.02–0.2 atm. Qswin analysis software was used to analyze adsorption isotherms to determine the BET.

Scanning electron microscopy (SEM) was conducted on a FET Quanta-200. Powder samples were placed on a Cu support plate with Au support film gun.

2.4 Isoelectric point determination

Isoelectric points (IEPs) of dried residues were measured by potentiometric titration using ultrasonic attenuation and electro-sonic acoustics in a Colloid Dynamics Acoustizer II. Before titration, the UTBR was washed twice with Milli-Q water to remove soluble alkalinity, dried and prepared as described in Section 2.2. Three replicates of the transformed bauxite residues (TBRs) and UTBR were respectively dispersed at 2.5% (mass fraction) in a 0.001 mol/L NaCl electrolyte, subsequently dispersed in an ultrasonic cell crusher noise- isolating chamber for 30 min, then rested for 10 min. The electrolyte pH was normalized to pH 10 from its initial resting pH (approximately 11–8) prior to introduction into the Acoustizer II. Immediately, specimens were stirred at 160 r/min for 3 min, forming a homogeneous suspension, and a potentiometric titration was performed from pH 10 to 5 (0.5 pH unit decrements) with 0.1 mol/L HCl.

3 Results

3.1 Mineralogy

The quantitative mineral phases of UTBR and TBRs from XRD analysis revealed phase transformations after acid treatment (Table 1). The alkaline phases from UTBR were cancrinite (Na₈Al₆Si₆O₂₄(CO₃)(H₂O)₂, 13.8%), calcite (CaCO₃, 2.1%), and grossular (Ca₃Al₂Si₃O₁₂, 5.2%), whilst Fe oxide (Fe₂O₃), Ti mineral (Ca(TiO₃)), Al hydroxides (α-AlO(OH) and Al(OH)₃), and calcium phases (Ca₃(Fe_0.87Al_0.13)₂(SiO₄)_1.65(OH)_5.4) were also identified (Figure 1). The change in cancrinite peaks could also be observed in XRD patterns (Figure 1). Cancrinite was present in both UTBR and TBRs, although concentrations in TBRs with mineral acids were decreased, with OTBR (10.1%) and OGTR (9.2%) further decreasing. Calcite peaks were identified in UTBR, but not in all TBRs. Similarly, grossular peaks were not observed in all TBRs (Figure 1). Gibbsite peaks were observed in TBRs and its precipitates were higher than that of UTBR. Calcium silicate minerals were detected in HGTR (2%), SGTR (2.2%), and OGTR (2.3%), but were not evident in HTBR, SGTR and OTBR. Gypsum was only detected in STBR and SGTR. Furthermore, a large proportion of amorphous minerals was detected in UTBR and all TBRs, but was substantially decreased in TBRs.

Table 1 Mineral compositions of bauxite residues transformed by acid dosing and gypsum supplementary

Figure 1 XRD patterns from bauxite residues transformed by acid dosing and gypsum supplementary: C–Cancrinite; Ca–Calcite; Gr–Grossular; Gi–Gibbsite; Gy–Gypsum; CS–Calcium silicate; D–Diaspore; H–Hematite; A–Andradite; P–Perovskite

3.2 Isoelectric point

Zeta-potential curves of UTBR and TBRs (Figure 2) show that the zeta-potential curve from UTBR was most steeply sloped during the potentiometric titration (zeta potential changed from 26.2 mV to –25.9 mV in pH range of 5–10). For OTBR and OGTR, the zeta potential curves had a shallower slope, which possessed a considerably lower potential change from 11.8 mV to –10.2 mV in the same pH range. The trends of HTBR, HGTR, STBR and SGTR were between that of UTBR and OTBR or OGTR, whilst the slopes of HTBR, HGTR, STBR and SGTR became successively shallower and their potentials correspondingly lower.

The isoelectric points (IEPs) of the UTBR and TBRs were calculated from the mean zeta potential curves (Figure 2) and statistical analysis (Figure 3). There were significant differences between mineral and organic acid-transformed residues, except for the HGTR and SGTR (Figure 3). The IEP of UTBR was significantly higher (P<0.05) compared to TBRs. A significant decrease was clear in the TBRs. The IEPs of HTBR, HGTR, STBR and SGTR were significantly lower (P<0.05) than that of UTBR,including HGTR and SGTR that had a similar IEP. The IEPs of OTBR and OGTR treated with organic acid and gypsum were significantly lowered, with larger decrease than those of mineral acid transformed residues (MTBRs).

Figure 2 Mean zeta-potential curves of un-transformed and transformed bauxite residues in NaCl electrolyte solution

Figure 3 Mean IEPs of un-transformed and transformed bauxite residues

3.3 Alkaline group

The proton exchange curves of UTBR and TBRs (Figure 4) suggested a clear distinction of acid adsorption to the surface of minerals. The UTBR titration curve provided a shallower exchange curve. UTBR produced the maximum acid adsorption, particularly at low pH. The exchange curves of TBRs were steeper, producing decreased acid adsorption. Especially, the curves of OTBR and OGTR had the steepest slope, with limited acid adsorption to their surfaces.

The concentration of active alkaline groups (—OH) on the surface of minerals may be calculated semi-quantitatively from the proton exchange curves. The semi-quantitative results (Figure 5) indicated significant differences between UTBR and all TBRs, except for HGTR and STBR. For UTBR, the initial —OH was significantly greater (0.78 mol H⁺/kg solid, P<0.05). A significant decrease was obvious in all TBRs. The —OH groups of HTBR, HGTR, STBR and SGTR were significantly lower (P<0.05) compared with UTBR, including HGTR and STBR that had a similar amount of —OH (approximately 0.30 mol H⁺/kg solid). The —OH groups of OTBR and OGTR treated with organic acid and gypsum were significantly lower, and the magnitudes of decreases were greater than MTBRs.

Figure 4 Mean proton exchange curves of un-transformed and transformed bauxite residues in NaCl electrolyte solution normalized to IEP

Figure 5 Mean surface alkaline groups of un-transformed and transformed bauxite residues

3.4 Particle micromorphology

SEM image of the UTBR (Figure 6(a)) revealed that it consisted of 0.1–0.5 μm particles in approximately 2–5 μm aggregates. The particles of UTBR were poorly-crystallized and relatively dispersed, without particular order. After acid and gypsum treatments, aggregate particles of MTBR (Figure 6(b)) were enriched and orderly distributed. SEM images of OTBR and OGTR (Figures 6(c) and (d)) demonstrated that it was composed of 0.2–1 μm particles in approximately 5–10 μm aggregates. The proportion of smaller grains significantly decreased in OTBR, but the small grains had almost disappeared in OGTR. The increased macro-aggregate particles of OTBR and OGTR were uniformly distributed.

4 Discussion

4.1 Mineral acids

The mineralogical differences among the UTBR and the HTBR and STBR were the alkaline mineral concentrations and precipitation of gibbsite. Mineral acid transformations of HTBR and STBR produced a shallow zeta-potential curve and lowered IEPs. These changes may be closely related to concentrations of alkaline minerals (cancrinite, calcite, grossular) (Table 1). Hydrochloric and sulfuric acid treatments partially dissolved cancrinite (Eq. (1)) and transformed calcite and grossular minerals (Eqs. (2) and (3)). The UTBR contained high concentrations of alkaline materials with a higher IEP compared to TBRs, whilst the HTBR and STBR contained low concentrations of alkaline minerals, resulting in a lower IEPs. Dissolution products of the soluble ions and neo-formed gibbsite mineral from the dissolution of cancrinite, calcite and grossular seemed not to influence the IEP. The two mineral acids didn’t transform the chemical speciation of Na (Na is in a tetrahedral structure surrounded by three O at 2.398 and one CO₃ at 2.701 in a trigonal pyramid) of cancrinite in HTBR and STBR [1]. Furthermore, the newly precipitated Al(OH)₃ lowered the IEPs in the HTBR and STBR due to its lower IEP (approximately 5.0) [24]. The shallower zeta potential curve and lower IEP may also be attributed to changes in particle size micromorphology, where macro-aggregates formed and fine grains decreased in HTBR and STBR (Figure 6).

Figure 6 High-resolution bright field SEM images of un-transformed and transformed bauxite residues:

Na₈Al₆Si₆O₂₄(CO₃)(H₂O)₂(s)+7H⁺(aq)+16H₂O(l)→8Na⁺(aq)+6Al(OH)₃(s)+6H₄SiO₄(aq)+

HCO₃^2–(aq)                          (1)

CaCO₃(s)+H⁺(aq)→Ca²⁺(aq)+CO₂(g)+H₂O(l)(2)

Ca₃Al₂Si₃O₁₂(s)+H⁺(aq)→3Ca²⁺(aq)+2Al(OH)₃(s)+3H₄SiO₄(aq)                      (3)

Un-transformed residue presented abundant alkaline groups, which decreased in the HTBR and STBR, and may be attributed to the dissolution and transformation of cancrinite, calcite and grossular during mineral acid additions. Additionally, mineral acids promoted a decrease in small particles and neo-formed aggregates. BET surface area results (Table 1) show that a slight increase occurred and that BET surface area may have weakly influenced surface protonation and alkaline group distribution on particle surfaces. It is more likely that precipitation of amorphous minerals accelerated this decrease, and improvement of alkaline minerals with structural defects determined the intra-particle diffusion [25, 26] that allows the surface adsorbed —OH to redistribute.

4.2 Organic acid

Organic acid treatments transformed OTBR and generated a shallower zeta-potential curve, leading to much reduced IEP, compared to UTBR and MTBRs. Cancrinite concentration in OTBR indicated that more cancrinite was dissolved into solution with citric acid amendment, precipitating as gibbsite (Eq. (4)). The dissolution mechanism of citric acid (Eq. (4)) was similar to that of mineral acids (Eq. (1)), except for the neo-formation of sodium citrate. The restriction mechanism of the dissolution reaction (Eq. (4)) was controlled by acid mass transfer [27]. Citric acid probably increased the rate constant and activation energy of cancrinite, thus activating the dissolution reaction of cancrinite [27]. Calcite and grossular minerals are susceptible to citric acid treatment, which were completely dissolved into solution phase (Eqs. (5) and (6)). Citric acid didn’t transform the chemical speciation of Na (tetrahedral Na structure) [1]. The dissolution of calcite and grossular and neo-formation of Al(OH)₃ led to the reduction of the IEP in the OTBR. SEM images also indicated that organic acid treatment caused larger stimulatory effects of macro-aggregation of particles than mineral acids, probably due to the organomineral interactions induced by the organic acid and lowered IEP.

Na₈Al₆Si₆O₂₄(CO₃)(H₂O)₂(s)+2C₆H₈O₇(aq)+

20H₂O(l)→2Na₃C₆H₅O₇·2H₂O(aq)+

6Al(OH)₃(s)+6H₄SiO₄(aq)+

Na₂CO₃(aq)               (4)

3CaCO₃(s)+2C₆H₈O₇(aq)→Ca₃(C₆H₅O₇)₂(aq)+

3CO₂(g)+3H₂O(l)                  (5)

Ca₃Al₂Si₃O₁₂(s)+2C₆H₈O₇(aq)→Ca₃(C₆H₅O₇)₂(aq)+

2Al(OH)₃(s)+3H₄SiO₄(aq)             (6)

2Ca²⁺⁽aq)+H₄SiO₄(aq)→Ca₂SiO₄(s)+4H⁺(aq),

extra Ca source, duration exceeded to 28 d  (7)

Citric acid transformation decreased protonation and surface adsorption of H⁺ may be a primary buffering effect. Mineral acid transformed residues showed reduced alkaline groups, but citric acid helped to reduce the surface alkaline groups further. Following citric acid addition, cancrinite, calcite and grossular products were similar to mineral acid results, and the difference in alkaline minerals between OTBR and MTBR was only the cancrinite concentration, suggesting that cancrinite transformation may reduce the surface active alkaline groups in OTBR. In addition, citric acid promoted a decrease in fine grains and the formation/distribution of macro-aggregates, whilst leading to a reduction in the BET surface area (sorption sites were correspondingly reduced,Table 1). These changes also reduced the distribution of —OH on particle surfaces. Furthermore, citric acid improved the structural defects of cancrinite and precipitated some amorphous minerals that re-distribute the surface active alkaline group.

4.3 Gypsum

Addition of gypsum to bauxite residue (HGTR, SGTR and OGTR) made their respective zeta-potential curves shallower, and IEPs correspondingly lower, compared to mineral and organic acid additions. Mineral and organic acids dissolved cancrinite and grossular minerals into solution whilst forming weakly alkaline Al(OH)₃ precipitates and orthosilicic acid polymers (Eqs. (1), (3), (4) and (6)). Gypsum addition resulted in the formation of calcium silicate precipitates (Eq. (7)); the orthosilicic acid polymers slowly reacted with Ca and promoted dissolution of cancrinite and grossular, further reducing their concentrations in HGTR, SGTR and OGTR. Neo-formed calcium silicate appears to reduce the IEPs of HGTR, SGTR and OGTR only according to the low IEP of silicate [28].

Gypsum didn’t change the tetrahedral Na structure [1], and further decreases in IEPs and zeta potentials for HGTR, SGTR and OGTR compared to those of HTBR, STBR and OTBR, which may be attributed to further dissolution of cancrinite in HTBR, STBR and OTBR. Additionally, SEM images (Figures 6(c) and (d)) revealed that gypsum not only promoted the formation of macro- aggregates in OGTR and accelerated this behavior (positive effect on Ca on particle flocculation [29], but also increased the distribution of macro- aggregates, which may also contribute to shallow the zeta potential curves whilst lowering IEP of OGTR.

Application of gypsum produced the steepest proton exchange curve in OGTR, suggesting that adsorption of H⁺ was the only buffering available. The surface active alkaline groups of HGTR, SGTR and OGTR were correspondingly lower than those of HTBR, STBR and OTBR, respectively. Following mineral and organic acid dosing, surface —OH groups were reduced, and furthermore, gypsum contributed to their reduction due to the additional Ca. Improved distribution of macro-aggregates and precipitation of amorphous minerals, shows that gypsum can improve particle characteristics, by reducing the BET surface area and decreasing surface sorption site density. Particle surface changes highlight the redistribution of —OH groups on surfaces in HGTR, SGTR and OGTR, further confirming gypsum’s positive effect on reducing surface protonation.

4.4 Implications for bauxite residue management

Management of bauxite residue is a critical and seriously challenging waste problem that is a major concern to the world alumina industry. Bauxite residue has poor chemical and physical characteristics (high alkalinity, pH and IEP, abundant alkaline groups, and fine-grained particles) that requires amelioration prior to safe disposal and/or some innovative options. Environmental management of BR has been previously focused on indefinite storage, with little consideration given to organic acid and gypsum applications to amending alkalinity, especially for improvement of electrochemical characteristics. Currently, management projects are moving gradually towards rehabilitation, revegetation and soil-formation to reduce environmental hazards associated with long-term clean disposal, and subsequently establishment of a vegetation cover over the long- term period.

Chemical and physical characteristics of BR (surface adsorption, proton exchange, pH buffering capacity, rheology and aggregation) are highly dependent on particle surface behaviors [24, 26]. Surface behavior and complex mineralogy often dominate BRDAs, diffusion of alkaline dust, leakage of alkaline compounds, alkalization of occupied land, and formation and overflow of efflorescence substances and all frequently occur at the on of BRDAs. Particle surface behavior of alkaline substances is pivotal to rehabilitation, forming a soil-like substrate. Therefore, understanding the effects of acid and gypsum additions on residue mineralogy, and the influence on IEP and —OH behavior, highlights the requirement for effective management of BRDAs.

These observations indicated that mineral acid and organic acids and gypsum application to alkaline transformations of BR were successful in improving electrochemical characteristics and decreasing alkalinity. The results of citric acid application are an important step in improving the surface characteristics and particle morphology, especially for IEP, —OH and aggregation, which may further increase the rate of revegetation. In addition, by-products of sodium citrate are easily biodegradable. Furthermore, citric acid generally originates from the fermentation of starch materials that can be directly fermented and conveniently sourced in BRDAs. Importantly, the metabolite produced from Aspergillus niger in xylogen-rich biomass is beneficial to BRDA surface enhancement. Therefore, amendment with citric acid may be considered a promising way forward for sustainable disposal of bauxite residue.

5 Conclusions

This work provides evidence for the improvement of electrochemical characteristics transformation of BR using three acids and gypsum, by reducing IEP and —OH and decreasing surface protonation associated with transformation of alkaline minerals and induction of micro-aggregate formation. Transformations of calcite, grossular and cancrinite, greatly affected zeta potential and proton exchange curve shifts; citric acid and gypsum additions activated the reaction of cancrinite that significantly decreased the IEP and —OH. Citric acid promoted macro-aggregate formation whilst increasing its distribution, revealing the beneficial improvement to the reduction of IEP and —OH. Importantly, citric acid and gypsum are novel amelioration strategies for improving surface electrochemical characteristics and correspondingly amending alkalinity for remediation of BRDAs.

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(Edited by YANG Hua)

中文导读

有机酸-石膏联合作用改善赤泥碱性电化学性能

摘要：本文开展了有机酸-石膏联合转化赤泥碱性电化学性能的研究，探究了有机酸-石膏作用过程中zeta电位、等电点、矿物表面质子化、表面碱性官能团—OH的转变及其对赤泥堆场生态修复相关理化性质的影响。为了更好地评价有机酸的转化效果，利用无机酸（盐酸、硫酸）进行对比分析。采用有机酸-石膏中和赤泥的碱性，可有效改善赤泥的碱性电化学性能，处理后的赤泥等电点和矿物表面质子化程度显著降低，矿物颗粒表面碱性官能团—OH的含量也明显减少。XRD结果揭示有机酸、无机酸、酸-石膏联合转化了赤泥中主要的碱性物相钙霞石、方解石、钙铝榴石，但有机酸-石膏联合作用进一步促进了钙霞石的溶解。从SEM图可以看出，有机酸-石膏联合作用改善了赤泥颗粒的分布，也有利于等电点的降低及矿物颗粒表面碱性官能团—OH的重新分布。有机酸-石膏联合调控赤泥碱性的效果非常显著，可进一步促进碱性的转化，对实现堆场赤泥的土壤化具有重要的作用。

关键词：赤泥；碱性调控；有机酸；石膏；电化学特性；赤泥土壤化

Foundation item: Projects(41877511, 41842020) supported by the National Natural Science Foundation of China

Received date: 2018-10-15; Accepted date: 2018-12-12

Corresponding author: XUE Sheng-guo, PhD, Professor; Tel: +86-13787148441; E-mail: sgxue70@hotmail.com, sgxue@csu.edu.cn; ORCID: 0000-0002-4163-9383