Neutralization of Acid Sulfate Solutions Using Bauxite Refinery

Dec 24, 2012 - Sara J. Couperthwaite,* Dean W. Johnstone, Graeme J. Millar, and Ray L. Frost. Chemistry, Physics, Mechanical Engineering, Science, and...
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Neutralization of Acid Sulfate Solutions Using Bauxite Refinery Residues and Its Derivatives Sara J. Couperthwaite,* Dean W. Johnstone, Graeme J. Millar, and Ray L. Frost Chemistry, Physics, Mechanical Engineering, Science, and Engineering Faculty, Queensland University of Technology, GPO Box 2434, Brisbane Queensland 4001, Australia ABSTRACT: This investigation has shown that by transforming free caustic in red mud (RM) to Bayer hydrotalcite (during the seawater neutralization (SWN) process) enables a more controlled release mechanism for the neutralization of acid sulfate soils. The formation of hydrotalcite has been confirmed by X-ray diffraction (XRD) and differential thermalgravimetric analysis (DTG), while the dissolution of hydrotalcite and sodalite has been observed through XRD, DTG, pH plots, and ICP-OES. Coupling of all techniques enabled three neutralization mechanisms to be determined: (1) free alkali, (2) hydrotalcite dissolution, and (3) sodalite dissolution. The mechanisms are determined on the basis of ICP-OES and kinetic information. When the mass of RM or SWN-RM is greater than 0.08 g/50 mL, the pH of solution increases to a suitable value for plant life with aluminum leaching kept at a minimum. To obtain a neutralization pH greater than 6 in 10 min, the following ratio of bauxite residue (g) in 50 mL with a known iron sulfate (Fe2(SO4)3) concentration can be determined as follows: 0.04 g:50 mL:0.1 g/L of Fe2(SO4)3.

1.0. INTRODUCTION Acid sulfate soils (sulfide-bearing minerals) are an important environmental issue that needs to be addressed due to the serious implications that it poses on all ecosystems. Acid sulfate soils (ASS) have the potential to have significant implications on both soil and water quality. Acidic waters are caused by the oxidation (exposure to air) of sulfide-bearing minerals to sulfuric acid through a number of activities including mining, agriculture, flooding, dredging, and aquaculture.1−4 In soils, the lower pH reduces availability of nutrients to plants and in large concentrations removes iron, aluminum, and some other transition metals from soils to the point that they become toxic.3,5 The sulfuric acid produced also has the ability to dissolve metals contained in rocks, such as arsenic, cadmium, copper, lead, and zinc, which can poison groundwater making it unusable in drinking and agricultural purposes.5 Implications in waterways include fish and crustacean kills at low pH (2−4) and high aluminum concentrations, while plants and sedentary gilled organisms are either eradicated or stunted. In summary, these potentially toxic characteristics of acid sulfate soils have the ability to permanently damage the surrounding ecosystems and, therefore, require suitable treatment methods. The problem with ASS is increasing, over 40,000 km of Australia’s coastline has been affected by ASS.6 Flooding and increased mining activities, particularly dredging, have significantly contributed to the problem of ASS. Another large source of ASS is from operating and abandoned polymetallic sulfide mining sites that contain mine waste rock piles, quarries, and tailing dams. Heavy metal mining industries, such as aluminum, copper, nickel, tin, lead, and zinc, have significantly contributed to acid mine drainage (AMD) because they are located in sulfide ores. Of most concern is the sheer amount of tailings that is generated and stored at these sites (approximately 18 billion m3 per year), which is expected to almost double in the next decade.7 Acid rock drainage (ARD) also contributes slightly to acidification of waters and soils, which occurs when © 2012 American Chemical Society

sulfide minerals are exposed to weathering and react with water and oxygen to produce sulfuric acid. The remediation of contaminated sites can be extremely expensive, and therefore, a lot of focus is now on the utilization of industrial residue in contamination applications. There are a number of studies on the neutralization of acid mine drainage using alkaline industrial waste such as red mud from the alumina industry,8−20 along with byproducts from the coal industry21−23 and cementitious materials.24 The highly caustic composition of red mud slurry (45% liquor and 55% solid) makes this particular waste material an ideal candidate for acid sulfate soil remediation. Another advantage of using bauxite residue is the solid component (mixture of iron, aluminum, and titanium oxides) that is characterized as having a high surface area, surface reactivity, and exchange capacity that enables it to adsorb metals and metalloids.25−27 However, the variability in composition of bauxite residues (different bauxite ore sources and processing conditions) means the capacity of the residue to neutralize acid sulfate soils is highly variable. Bauxite residue contains three sources of alkalinity: (1) caustic liquor entrained in the residue, (2) calcium compounds formed from the addition of lime during the Bayer process, and (3) sodalite formed by the reaction of caustic soda and silica compounds, which traps caustic in sodalite cages.28−30 The neutralization rate of the above sources of alkalinity is highly variable and has been described as follows: (1) rapid neutralization of liquor, (2) rate is highly dependent on what temperature the calcium compound formed (can be neutralized in minutes or weeks), and (3) sodalite is initially neutralized at a rapid rate, but as it becomes less alkaline, the neutralization Received: Revised: Accepted: Published: 1388

June 19, 2012 December 8, 2012 December 23, 2012 December 24, 2012 dx.doi.org/10.1021/ie301618p | Ind. Eng. Chem. Res. 2013, 52, 1388−1395

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rate decreases.30 For these reasons, the overall neutralization process of acid sulfate soils will be highly dependent on the type of bauxite residue used. This investigation focuses on using seawater-neutralized bauxite refinery residues as a source of hydroxide ions for the neutralization of acid sulfate waters. Red mud is produced by the Bayer process, which involves the digestion of bauxite using elevated temperatures and concentrated caustic solutions. It is proposed to be an effective neutralizer of acid because of the residual caustic associated with the iron/aluminum hydroxide material. However, untreated red mud is classed as a hazardous material and is not suitable for transportation and handling. The seawater neutralization of red mud results in the formation of hydrotalcite (Mg6Al2(OH)16(CO32‑,SO42‑)•4H2O), which is stable in neutral and alkaline environments. The stability of hydrotalcite is due to the hydroxyl ions being locked up in the hydrotalcite structure, which can then be safely transported to affected sites. The formation of hydrotalcite (eq 1) occurs by the reaction of magnesium in seawater with aluminate ions (Al(OH)4−) and caustic (NaOH) in the associated liquor component of red mud. Hydrotalcite is stable in neutral and alkaline conditions but will dissociate in acidic solutions until neutral conditions are achieved. This investigation focuses on the neutralizing mechanism of acid sulfate solutions using red mud and seawater-neutralized red mud. The formation of Bayer hydrotalcite during the neutralization process has generally been overlooked as a mechanism for neutralization in acid sulfate soils, which is the main aim of this investigation. Therefore, the effect of hydrotalcite in seawater-neutralized red mud on the neutralization capacity of the residue is investigated.

and then allowed to stir for a further 2 h before being vacuum filtered. The precipitate was then placed in an oven overnight before being crushed to a fine powder. The treatment of iron sulfate solutions involved the addition of each material with a known mass to 50 mL (pipet) of Fe2(SO4)3 at varying, but known, concentrations. The pHs of all solutions were monitored using a calibrated TPS WP80 pH meter using a Sentek general laboratory probe and buffers 4 and 7. Each treatment was left for 30 min before the pH measurements were transferred to Excel. For ICP-OES analysis, each sample was syringed filtered before being analyzed. 2.2. Characterization of Bayer Precipitate. 2.2.1. X-ray Diffraction (XRD). X-ray diffraction patterns were collected using a Philips X’pert wide angle X-ray diffractometer, operating in step scan mode, with Cu Kα radiation (1.54052 Å). Patterns were collected in the range of 5°−85° 2θ with a step size of 0.02° and a rate of 30s per step. Samples were prepared as a finely pressed powder into aluminum sample holders. The Profile Fitting option of the software uses a model that employs 12 intrinsic parameters to describe the profile, instrumental aberration, and wavelength dependent contributions to the profile. 2.2.2. Thermogravimetric Analysis (TGA). Thermal decomposition of the hydrotalcite was carried out in a TA Instruments incorporated high-resolution thermogravimetric analyzer (series Q500) in a flowing nitrogen atmosphere (80 cm3/min). Approximately 50 mg of sample was heated in an open platinum crucible at a rate of 2.0 °C/min up to 1000 °C. The synthesized hydrotalcites were kept in an oven (85 °C) for 24 h before TG analysis. Thus, the mass losses are calculated as a percentage on a dry basis. 2.2.3. Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES). Acid sulfate solutions were analyzed using a VISTA-MPX CCD simultaneous ICP-OES instrument without dilution. A certified standard from Australian Chemical Reagents (ARC) containing 1000 ppm of aluminum, calcium, magnesium, and sodium was diluted to form a multi-level calibration curve and an external reference that was used to monitor instrument drift and accuracy of the results obtained. Results were obtained using an integration time of 0.15 s with 3 replications. Wavelengths used were as follows: Al (396.152), Ca (422.673), Mg (285.213), and Na (589.592).

2.0. MATERIALS AND METHODS 2.1. Preparation of Materials. The formation of acid sulfate soils in natural and mining scenarios is generally a result of the oxidation of iron sulfide ores to iron sulfate.1,2,4 Thus, AR grade iron sulphate will be used to emulate real acid sulphate soils. Red mud slurry was sourced from an Australian alumina refinery. Before the treatment of acid sulfate solutions (0.25g/L Fe2(SO4)3), red mud was vacuum filtered to remove most of the liquor component and then placed in an oven (85 °C) overnight. Dry red mud was manually crushed, using a mortar and pestle, to a fine powder (less than 500 μm in size) before being mixed with the acidic solutions. Seawater-neutralized red mud involved the addition of seawater to 100 mL of red mud slurry until a pH of 8.5 was obtained. The seawater-neutralized red mud was then vacuum filtered and placed in an oven overnight to dry. The same crushing process was used for seawater neutralized red mud as that described for dry red mud, which involved manual crushing and a 500 μm sieve. Bayer hydrotalcite was prepared by diluting saturated evaporative liquor (SEL) to give an aluminate concentration of 4 g/L (supernatant liquor, SNL). For 1 L of SNL, 20.72 g of sodium carbonate (Na2CO3) was dissolved in 500 mL of 18.2 MΩ water, to which 42.28 mL SEL was added and then filled to the mark. To synthesize Bayer hydrotalcite, 200 mL of SNL was placed into a 5 L beaker with an overhead stirrer (IKA, RW20) with a basic 4 propeller paddle placed into solution. The stirrer was set to 400 rpm to ensure uniform mixing, while seawater was pumped into the beaker using a Watson and Marlow 520U pump set to 1.5 rpm with marprene tubing diameter of 6.4 mm. The addition of seawater ceased once a pH of 8.5 was obtained

3.0. RESULTS 3.1. X-ray Diffraction. The mineralogical composition of red mud is highly complex, with some samples of red mud consisting of 15 different phases.31 The highly variable composition of red mud is due to a number of variables including the bauxite source and composition and the conditions used during the Bayer process to extract alumina. The general consensus of the composition of red mud has been found to be largely composed of iron oxides, primarily hematite (Fe2O3), goethite ((FeOOH), gibbsite (Al(OH)3), boehmite (AlOOH), other aluminum hydroxides, calcium oxides, titanium oxides (anatase and rutile), and aluminosilicate minerals (sodalite).14,32,33 The mineralogical complexity of bauxite residues has also been reported by Castaldi et al.34 The seawater neutralization process also causes the formation of hydrotalcite, hydrocalumite, and calcium carbonate (calcite and aragonite),27 which are also identified in the XRD patterns (not labeled in Figure 1). The amounts of these phases that form during the seawater neutralization process are insignificant to the amount of other phases that are originally found in red 1389

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confirming the dissociation of hydrotalcite (eq 2). XRD also identified the present of gibbsite, which is relatively insoluble at pH 6−8, and increases in more acidic and basic conditions (acidic, Al3+, AlOH2+, Al(OH)2+; pH 6−8, Al(OH)30; basic, Al(OH)4−).35 The overall XRD patterns of the acid sulfatetreated materials appear noisier and suggest that all mineralogical phases have been affected to some extent by the acidic conditions. However, the major phases of red mud, hematite, gibbsite, sodalite, and anatase are still present after acid treatment with a resultant pH of 7.5. Mg6Al 2(OH)16 (CO3)· 4H 2O(s) + 12H+(aq) → 6Mg 2 +(aq) + 2Al(OH)3(s) + CO2(g) + 15H 2O(l) (2)

3.2. Differential Thermalgravimetric Analysis (DTG). A single symmetric mass loss of 4.58%, centered at 74 °C is observed for red mud and is attributed to physically adsorbed water (Figure 2). A number of broader mass losses are

Figure 1. XRD pattern of untreated RM and SWN-RM and treated acid sulfate solutions with SWN-RM.

mud. Thus, their corresponding XRD patterns are weak in comparison to phases such as hematite and anatase. The major mineralogical phases present in this particular red mud and the corresponding seawater-neutralized red mud are hematite and gibbsite. The only significant difference between the neutralized and un-neutralized red mud is a low intensity broad peak at approximately 12° 2θ (Figure 1), which is characteristic of the d003 peak of hydrotalcite.35 The presence of this broad feature confirms that the neutralization process successfully forms hydrotalcite, shown in eq 1. Because of the crystallinity of the major mineralogical phases (hematite, anatase, and sodalite), other characteristic bands of hydrotalcite cannot be identified with any certainty. The weak intensity of this peak is due to the very small quantity of hydrotalcite in the red mud matrix, and although XRD does not provide irrefutable evidence of its existence, the chemistry behind the seawater neutralization process27 enforces the strong possibility that hydrotalcite has indeed formed.

Figure 2. Thermal gravimetric analysis of RM, SWN-RM, and SWNRM after treatment with a 0.25g/L Fe2(SO4)3 solution.

6MgCl2(aq) + 2NaAl(OH)4(aq) + 8NaOH(aq) + 0.5Na 2CO3(aq) + 0.5Na 2SO4(aq)

observed for seawater-neutralized red mud up to 209 °C with a total mass loss of 5.07%. However, the same mass loss attributed to the removal of adsorbed water at 75 °C (mass loss of 4.07%) is observed. The remaining 1.00% is believed to be due to the removal of interlayer water associated with the formation of Bayer hydrotalcite during the seawater neutralization process, eq 3.

→ Mg6Al 2(OH)16 (CO32 − , SO4 2 −) ·x H 2O(s) + 12NaCl(aq)

(1)

All seawater neutralized red mud samples used for the treatment of acid sulfate solutions resulted in the disappearance of the hydrotalcite peak at around 12° 2θ (Figure 1), 1390

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Mg6Al 2(OH)16 (CO3 , SO4 ) ·4H 2O(s) → Mg6Al 2(OH)16 (CO3 , SO4 )(s) + 4H 2O(g)

(3)

The primary mass loss for red mud is observed at around 241 °C and is attributed to the loss of chemically adsorbed water to the aluminum phases found in red mud such as boehmite and gibbsite. The third major mass loss is observed between 400 and 600 °C attributed to the decarbonation of calcite/aragonite (eq 4). This mass loss is common to all samples with slight increases being observed for SWN-RM due to the reaction of carbonate in Bayer liquor and calcium in seawater. Red mud shows a large mass loss of 2.61%, centered at 603 °C, which disappears upon the addition of seawater. It is believed this peak is due to CaCO3 formed after the addition of lime in the Bayer process and because high temperature formation has a higher decomposition temperature in red mud than the reformed CaCO3 in the seawater-neutralized sample. The DTG results also suggest that no significant changes occurred for calcium carbonate species. Ca(CO3)(s) → CaO(s) + CO2(g)

Figure 3. Neutralizing capacity of RM.

reached. This buffering region is due to the interaction of H+ with the solid components of RM, previously observed in the investigation by Rubinos et al.42 The reduced rate of neutralization could also be due to the slow release of caustic from sodalite cages and the slow dissolution of calcium aluminate species.30

(4)

An additional mass loss for seawater-neutralized red mud at 306 °C is attributed to the simultaneous dehydroxylation and decarbonation of hydrotalcite (eq 5).36−39 Sulfate anions, presumed to also be intercalated in the hydrotalcite interlayer due to the high concentrations in seawater, are also removed during this decomposition step.

H 2SO4(aq) + 2NaOH(aq) → 2H 2O(l) + Na 2SO4(aq)

H 2SO4(aq) + 2NaAl(OH)4(aq) → 2Al(OH)3(s) + 2H 2O(l) + Na 2SO4(aq)

Mg6Al 2(OH)16 (CO3 , SO4 )(s)

(7)

The neutralization curve of seawater-neutralized red mud (SWN-RM) shows a deviation in the shape of the curve compared to that of the RM neutralization curves (Figure 4).

→ MgAl2O4(s) + 5MgO(s) + (CO2 , SO2 )(g) + 8H 2O(g) + O2(g)

(6)

(5)

The differential thermogravimetric (DTG) curve of seawaterneutralized red mud recovered after the treatment of a synthetic acid sulfate solution corresponds with that of red mud (absence of a significant mass loss at around 306 °C). These results confirm that some of the neutralizing capacity of seawaterneutralized red mud is through the dissolution of hydrotalcite, which releases hydroxyl ions into solution that are then used to neutralize the acidic solution (eq 2).40 The neutralization of these acidic solutions through the dissolution of hydrotalcite is also confirmed by XRD results discussed previously. 3.3. Neutralizing Capacity. Ideal plant growth for most species requires the soil pH to be between 6 and 7.41 Therefore, this investigation is focused on neutralizing acid sulfate solutions to a pH of around 7. To obtain a neutralization pH greater than 7, the following ratio of bauxite residue (g) in 50 mL with a known iron sulfate (Fe2(SO4)3) concentration can be determined as follows: 0.04 g:50 mL:0.1 g/L of Fe2(SO4)3. Using this mass:concentration ratio, a pH of above 6 can be obtained within 10 min and begins to plateau at pH 7.5 after 60 min (Figure 3). The inflection point of all the neutralization curves occurs at a pH of approximately 4.8, suggesting that similar reactions are taking place for each run. Untreated red mud contains readily available caustic in the form of residual caustic and aluminate ions (Al(OH)4−), while fixed caustic is available in calcium aluminate species and sodalite cages. The neutralization of sulfuric acid is proposed to be simple acid/ base reactions (eqs 6 and 7). The free alkali and aluminate ions in RM reacts immediately with sulfuric acid and quickly increases the pH until the buffer region between pH 4 and 5 is

Figure 4. Neutralizing capacity of SWN-RM.

The lower mass samples showed a similar curve to that of RM with the same inflection point at 4.8; however, the inflection was a lot more defined. This suggests that a more controlled reaction is occurring and is clearly observed by (1) the beginning of the plateau starting at around 20 min and (2) the final pH is only approaching pH 7 compared to 7.5 for RM. The mechanism of neutralization for SWN-RM is believed to be more controlled because of the slower dissociation kinetics of hydrotalcite (eq 2). Once the dissociation of hydrotalcite occurs, the simultaneous dissolution of Al(OH)3 occurs at low pH (eqs 8, 9, and 10), which in turn neutralizes H+. The formation of hydrotalcite during the seawater neutralization 1391

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process is proposed to have dual benefits in the application of acid sulfate soils: (1) The neutralized red mud is safer for transportation because of caustic being fixed in the hydrotalcite structure, which in turn reduces the pH of seawater neutralized red mud to 9. (2) Hydroxide ions in Bayer hydrotalcite are in a more stable form than residual caustic on RM. When the mass of SWN-RM was increased (ratio remains the same), it can be more clearly be observed that a slower neutralization reaction is occurring (dissolution of hydrotalcite) with an inflection point of 4.60. The neutralization curves in Figure 5 show how the derivatives of RM can neutralize iron sulfate solutions in a more controlled and predicable manner. RM as a neutralization material results in sporadic increases in pH, especially when

comparing lower amounts of RM used to neutralize acid (0.005 and 0.01g). These sporadic increases are due to release of caustic from sodalite cages and calcium aluminate species. The use of SWN-RM shows a more controlled release of OH− to neutralize the acidic solution due to seawater reacting with caustic trapped in sodalite cages to form Bayer hydrotalcite before it is used in the treatment of acid sulfate solutions. It is proposed that the first increase in pH is due to the reaction of free caustic with H+ (fast kinetics). The second is due to dissolution of Bayer hydrotalcite (based on the amount available, rate of increase, and the absence of this small step in RM), then finally, the release of trapped caustic in sodalite cages until the complete dissolution of sodalite occurs (based on the slow dissolution kinetics of sodalite compared to hydrotalcite30 and the increase in sodium concentration over time−ICP-OES results). The main difference between RM and SWN-RM is the slower rate of neutralization in the first 5 min, whereby the neutralization steps are more clearly defined, thus making it possible to see a third neutralization step corresponding to the dissolution of hydrotalcite (second dissolution step). Bayer hydrotalcite neutralization curves are quite similar to the SWNRM curves, with the exception that multiple neutralization steps do not occur because the primary neutralization mechanism is dissolution rather than a combination of reactions described for RM and SWN-RM. This result does indicate that the controlled release in SWN-RM is related to the BHT precipitate that forms during the seawater neutralization process, which reduces free caustic in the entrained liquor and trapped caustic in the sodalite cages. A smaller amount of BHT material is required to achieve the same neutralization results as RM and SWN-RM because numerous compounds in RM do not participate in the neutralization of acidic conditions. 3.4. ICP-OES. The treatment of acid sulfate-affected waterways with red mud or its derivatives must ensure that the mechanism of neutralization is not causing a secondary effect, for example, the release of toxic levels of aluminum. The release of aluminum from RM is proposed to be due to the dissolution of gibbsite (eqs 8,43 9,45 and 1045) at low pH and to a lesser extent sodalite and calcium aluminate species, while SWN-RM would be a combination of gibbsite, sodalite, calcium aluminate species, and Bayer hydrotalcite (eq 2) dissolution. The rate of dissolution varies for all these species, with gibbsite having the greatest rate of dissolution. It should also be noted that aluminum solubility is amphoteric, which means in alkaline conditions, aluminate ions (Al(OH)4−) (eq 1145). The ANZECC guidelines describe the toxicity of aluminum to be greatest for the single unit aluminum specie (Al(OH)2+), which forms in the pH range 4.4−5.4 with maximum toxicity occurring between 5.0 and 5.2.43 Under very acidic conditions, the effect of the high H+ concentration is deemed more toxic than the release of aluminum ions. Because of the biodiversity in aquatic ecosystems, the pH and aluminum levels affect all species differently, and therefore, a set of optimal conditions cannot be achieved. However, current acceptable release limits of aluminum into waterways from industry has been reported to be 5 mg/L.44 The concentration of aluminum in solution after the treatment of a 0.25 M Fe2(SO4)3 solution for different amounts of RM, SWN-RM, and Bayer hydrotalcite (BHT) are outlined in Table 1.

Figure 5. Neutralization curves for varying amounts of (a) RM, (b) SWN-RM, and (c) BHT. 1392

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of solution is between 5.5 and 8.5, and thus the formation of the precipitate Al(OH)3 dominates. RM and SWN-RM were also tested in deionized water to determine the residual alkalinity of both materials. RM showed a significant rise in pH, and thus resulted in Al(OH)3 transforming to Al(OH)4− ions at pH values greater than 8.5. SWN-RM, on the contrary, did not show an increase in aluminum in solution even though the pH rose above 9.85. This can be explained by the formation of Bayer hydrotalcite (eq 1), which remains stable in alkaline conditions. The sodium content in iron sulfate solutions treated with RM and SWN-RM are similar and thus suggests that common sodium species exist in both RM and SWN-RM and participate in similar neutralization reactions. SWN-RM did show slightly lower sodium levels, and this is believed to be due to the formation of salt (NaCl) during the formation of Bayer hydrotalcite (BHT). However, sodium alkalinity is believed to be one of the major neutralizing contributors in both materials. Sodium concentrations in BHT are believed to be due to the dissociation of salt. The results show that a linear relationship exists for sodium in BHT treated solutions; therefore, it is simply salt impurities dissolving in solution. Seawater neutralization introduces significant amounts of magnesium and calcium into the red mud matrix; thus iron sulfate solutions treated with SWN-RM detected magnesium ions not previously observed for RM. It is thought the increase is predominantly due to the dissolution of Bayer hydrotalcite; however, because the increase in magnesium concentration is not linear, it indicates that a secondary specie is also dissociating, possibly brucite (Mg3(OH)6). Brucite has been reported as forming alongside Bayer hydrotalcite previously.26−28 Calcium released in solution for RM and SWNRM is due to calcium species other than CaCO3. Possible sources are residual lime, hydrocalumite (Ca 4 Al(OH)12·CO3·6H2O), and tricalcium aluminate (Ca3Al2(OH)12), which are commonly found in bauxite residues.28 These species are renowned for their instability in acidic conditions.46 Bayer hydrotalcite results show a rapid increase in magnesium concentration until a pH of 6.8 is obtained for 0.020 g of BHT. It is only in acidic conditions that the dissolution of hydrotalcite occurs, which is clearly observed in Table 1, whereby the release of aluminum into solution was observed. It is only for 0.001 g/50 mL that the pH is low enough (2.6) that Al(OH)3 dissociates to Al3+. The dissolution of hydrotalcite forms gibbsite, which is expected to dissociate at lower pH. The pH of all solutions, apart from 0.001 g, is high enough that gibbsite is still able to form, and thus no aluminum in solution is observed. The results also indicate the stability of hydrotalcite in neutral conditions, where the concentration of magnesium remains relatively unchanged between pH values 6.5 and 8. The absence of calcium in all solutions indicates that only CaCO3 (aragonite and calcite) forms in the neutralization of RM with seawater, and that hydrocalumite and tricalcium aluminate are not present in the BHT precipitate. Assuming there are no other magnesium species present in the precipitate, the Mg:Al ratio of this particular hydrotalcite is 2.76, calculated from the highly acidic solution with a pH of 2.61.

Al(OH)3(s) + H+(aq) ↔ Al(OH)2+(aq) + H 2O(l) pH 4.4−5.4

(8)

Al(OH)3(s) + 2H+(aq) ↔ Al(OH)2 +(aq) + 2H 2O(l) pH 3.0−4.7

(9)

Al(OH)3(s) + 3H+(aq) ↔ Al3 +(aq) + 3H 2O(l)

pH < 3.0 (10)

Al(OH)3(s) +

OH−(aq)

↔

Al(OH)4 −(aq)

pH > 8.5 (11)

Table 1. ICP-OES Analysis of Sodium, Calcium, Aluminum, and Magnesium after the Treatment of Iron Sulphate Solutions with RM, SWN-RM, and BHT concentration (mg/L) red mud untreated (g) in 50 mL of 0.25g/L Fe2(SO4)3 0.05 g/50 mL of H2O 0.005 g 0.010 g 0.020 g 0.040 g 0.080 g 0.120 g SWN red mud (g) in 50 mL of 0.25g/L Fe2(SO4)3

Na

Ca

Al

Mg

21.79 5.92 14.89 26.32 41.74 59.46 70.21

1.62 0.95 2.06 3.08 4.75 8.15 10.56

4.90 4.27 11.10 11.98 4.21 0.00 0.00

0.65 0.63 0.63 0.62 0.62 0.70 0.69

Na

24.39 0.05 g/50 mL of H2O 0.005 g 5.74 0.010 g 13.63 0.020 g 22.58 0.040 g 39.83 0.080 g 58.47 0.120 g 72.91 Bayer hydrotalcite (g) in 50 mL of 0.25g/L Fe2(SO4)3 Na 0.05 g/50 mL of H2O 0.001 g 0.012 g 0.020 g 0.025 g 0.040 g

5.14 0.00 1.08 1.84 2.54 4.06

Ca

Al

Mg

3.79 1.16 2.51 3.94 7.20 10.65 14.40

0.35 4.25 10.19 12.03 3.21 0.00 0.00

1.05 1.99 3.83 5.69 9.71 12.72 15.01

final pH 10.02 3.11 3.10 4.51 4.44 5.60 8.20 final pH

Ca

Al

Mg

9.85 3.02 2.94 4.06 4.51 5.8 7.35 final pH

0.03 0.00 0.00 0.00 0.00 0.00

0.00 1.39 0.00 0.00 0.00 0.00

5.39 3.85 28.62 30.26 30.71 32.38

9.47 2.61 5.8 6.87 7.34 7.85

The neutralization of iron sulfate solutions using varying amounts of RM and SWN-RM show a similar aluminum elemental release trend, which is highly dependent on the final pH of solution after 20 min. Red mud and the seawaterneutralized derivative both release increasing amounts of aluminum into solution as the quantity of each material increases. However, the increase in aluminum is not linear, and at amounts greater than 0.08g/50 mL, no aluminum is observed in the treated iron sulfate solution. A simple explanation for both the nonlinearity and absence of aluminum for greater amounts of material is simply due to the solubility of aluminum in different pH ranges. For RM and SWN-RM masses below 0.08g/50 mL (0.25g/L Fe2(SO4)3), the neutralization pH remains below 5.5, and thus the formation of Al(OH)2+, Al(OH)2+, and Al3+ dominate in solution. However, when the mass of RM or SWN-RM is greater than 0.08g/50 mL, the pH

4.0. CONCLUSIONS This investigation has shown that the neutralization of acid sulfate solutions using seawater-neutralized red mud has greater potential than untreated red mud. The formation of hydro1393

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talcite during the neutralization process creates a more controlled source of hydroxide ions for the neutralization of acidic solutions compared to the sporadic increase in pH observed for RM. It has been proposed that the release of trapped caustic in sodalite is responsible for erratic increases in pH, and by reacting seawater with RM, trapped caustic can be used in the formation of hydrotalcite, which results in a more controlled release of OH−. There are three mechanisms responsible for the neutralization of acid sulfate solutions: (1) free caustic, (2) hydrotalcite dissolution, and (3) slow dissolution of sodalite. Because of free caustic being trapped in the form of hydrotalcite, SWN-RM is safer to handle during transportation to affected sites. SWN-RM also has the benefit of being more predictable during neutralization, which will reduce the risk of overtreating an area to the point the soil becomes alkaline. Further investigations of the kinetics of the neutralization mechanism would provide more information on the implications of SWN-RM on affected acid sulfate soils, with the primary goal being the construction of predictive modeling software.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +61 7 3138 1804. Tel.: +61 7 3138 4766. E-mail: sara. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial and infra-structure support of Queensland University of Technology, Science and Engineering Faculty are gratefully acknowledged. The Australian Research Council (ARC) is also thanked for funding of instrumentation.

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