The removal of sulphate from mine water by precipitation as ettringite and the utilisation of the precipitate as a sorbent for arsenate removal

Bioregeneration of sulfate-laden anion exchange resin

Ion exchange technology removes ionic compounds from waters effectively but treatment of the spent regenerant is expensive. The bioregeneration of sulfate-laden strong base anion exchange resin was successfully tested using both pure and mixed sulfate-reducing bacterial cultures. The resin was first used for removal of sulfate from neutral (pH 6.7 ± 0.5) synthetic sodium sulfate solutions, after which the spent resin was regenerated by incubating with a viable sulfate-reducing bacterial culture in batch and column modes. In the batch bioregeneration tests, the achieved bioregeneration was 36-95% of the original capacity of the fresh resin (112 mg SO₄^2-/g) and it increased with regeneration time (1-14 days). The capacity achieved in the column tests during 24 hours of bioregeneration was 107 mg SO₄^2-/g after the first regeneration cycle. During the bioregeneration, sulfate was mainly reduced by the sulfate-reducing bacteria (approx. 60%), but part of it was only detached from the resins (approx. 30%). The resin-attached sulfate was most likely replaced with ions present in the liquid sulfate-reducing bacterial culture (e.g., HCO₃^-, HS^-, and Cl^-). During the subsequent exhaustion cycles with the bioregenerated resin, the pH of the treated sodium sulfate solution increased from the original 6.7 ± 0.5 to around 9. The study showed that biological sulfate reduction could be used for sulfate removal in combination with ion exchange, and that the exhausted ion exchange resins could be regenerated using a liquid sulfate-reducing bacterial culture without producing any brine.

Removal of High-Concentration Sulfate from Seawater by Ettringite Precipitation

Due to the worldwide scarcity of fresh water, seawater becomes an alternative base fluid in hydraulic fracturing for oil and gas production. However, the injection of seawater that contains high concentration of sulfate will induce the scale formation and thus reduce hydrocarbon production. One of the most effective ways to solve this problem is to remove sulfate ions from seawater before fracturing application. The objective of this study is to develop an effective and environment-friendly approach to remove sulfate ions from seawater based on coprecipitation of SO42− with NaAlO2 and CaO as ettringite (Ca6Al2(SO4)3(OH)12·26H2O). Residual sulfate concentration in treated seawater was determined when NaAlO2 and CaO dosed at different molar ratios to sulfate. Results showed the efficiency of sulfate removal was more than 90% (4290 ppm to ∼400 ppm) when Al : Ca : S = 2 : 6 : 1. It was found the sulfate precipitation completed in 15 mins with stirring under an alkaline condition (pH ≈ 12) and was not affected by temperature (15°C to 45°C). Increasing the Na+ concentration from 0 to 25,000 ppm in waters resulted in the increment of residual sulfate concentration from 250 to ∼600 ppm, decreasing the removal efficiency. Besides, the analysis of Ca2+ and Mg2+ in treated seawater showed the Ca2+ concentrations were on the similar level as that before the treatment and Mg2+ was removed in the precipitation process, which is beneficial to the application of the treated seawater. The morphology and element analysis of the collected precipitates showed that the ettringites were in a layered shape with composition between Ca6Al2(SO4)3(OH)12 and Ca4Al2(SO4)(OH)12 at the optimized chemical dosage; therefore, the developed ettringite precipitation method could effectively remove sulfate from seawater without toxic chemicals involved, which benefits seawater hydraulic fracturing in an economic way, and this contributes to water sustainability.

Long-Term Stabilization/Solidification of Arsenic-Contaminated Sludge by a Blast Furnace Slag-Based Cementitious Material: Functions of CaO and NaCl

Arsenic is a kind of element widely distributed in the environment that may pose a threat to the ecological environment and human health, while effective remediation and sustainable utilization of arsenic-containing sludge is a challenge. Based on stabilization/solidification blast furnace slag-based cementitious materials (BCMs), this study innovatively proposes to improve the arsenic (As) solidification efficiency and long-term stability by using the activation mode of CaO and NaCl. The effects of different factors on the properties of the BCM were measured by unconfined compressive strength (UCS) tests, X-ray diffraction, Fourier transform infrared spectroscopy, and scanning electron microscopy. The long-term stability and safety of the BCM were verified by leaching toxicity and improved three stage continuous extraction method (BCR) tests. Experimental results show that the addition of CaO provides conditions for the formation of ettringite (AFt), thus promoting the crystal growth of AFt. The addition of NaCl can promote the formation of Cl-AFt and play a good long-term stabilizing role. When the content of the alkali activator is 10% and the modulus is 1.0, the contents of CaO and NaCl are 10 and 1%, respectively. The BCM has the best efficiency in terms of UCS and As solidification. The UCS at 28 days was 5.4 MPa, and the leaching concentration of As was 0.309 mg/L, and the As solidification efficiency was up to 99.9%. In the improved BCR test, the proportions of residual and oxidizable states of arsenic increased by 19.6 and 13.5%, respectively, and the stability of heavy metals improved. These findings show that the BCM has good long-term stability and safety. Overall, this study shows that CaO and NaCl significantly increase the output of AFt and achieve the purpose of efficient and stable solidification of As by the BCM.

Removal of fluoride ions from aqueous solution by metaettringite

Fluoride contamination is a major problem in wastewater treatment. Metaettringite (which has previously shown enhanced anion adsorption) was investigated as a possible adsorbent to remove fluoride from low-concentration solution (25 mg-F/L). The fluoride removal properties of ettringite and metaettringite were first compared at pH 10, and metaettringite was found to be more effective. The dominant reaction mechanism for fluoride adsorption in metaettringite was found to be recrystallization of metaettringite by rehydration; this was accompanied by precipitation of calcium fluoride. The adsorption kinetics followed the pseudo-second order model. Metaettringite was also able to remove fluoride effectively in low pH environment (i.e., at pH 3.5). The influence of coexistence of sulfate ions in solution on the fluoride removal performance was investigated, and a small decrease in performance was noted. The residual fluoride concentrations obtained with higher doses of metaettringite were lower than those specified by the Japanese effluent standard (non-coastal areas: 8 mg-F/L; coastal areas: 15 mg-F/L). The fluoride removal capacity of metaettringite was compared with those of other solid materials. The observed maximum capacity was 174.7 mg-F/g-metaettringite. In the case of high fluoride concentration solution, the main removal mechanism will be changed to calcium fluoride precipitation. In general, metaettringite is regarded as promising material for fluoride removal in wastewater treatment.

Sulfate removal from lignite coal mine drainage in Thailand using ettringite precipitation

This research was studied the optimum conditions for sulfate removal from lignite coal mine drainage in Thailand by ettringite precipitation using Central Composite Design. The effects of Ca/S and Al/S ratio (mole basis), reaction time and temperature on the sulfate removal efficiency were investigated. The statistical analysis showed that the Al/S ratio and reaction time had a significant effect on sulfate removal efficiency. The sulfate removal efficiency increases when Al/S ratio and reaction time are increased. The Ca/S ratio in the range of 1-7 was found to have no significant effect on sulfate removal efficiency. The optimal reaction time obtained from the predicted equation was 6.14 h, while the Ca/S and Al/S ratios were fixed at 4 and 4.5, respectively, at ambient temperature. Under the optimum conditions, sulfate removal of 99.6 and 99.0% was achieved in both Lamphun and Lampang mine drainage, respectively, which is very close to the predicted value (100%). This study showed that the sulfate removal efficiencies of Lamphun and Lampang mine drainage sharply increased from 33.3 to 89.9% and 42.3 to 81.8%, respectively, when the temperature was increased from 25 to 80 ^○C during a 3 h reaction time. XRD results match well with a majority of ettringite and a minority of calcium carbonate when the precipitation was done at ambient temperature. However, monosulfate was found to be the majority when the precipitation was done at 80 ^○C due to the decomposition of ettringite to monosulfate at the high temperature.

The mechanism of hydrating and solidifying green mine fill materials using circulating fluidized bed fly ash-slag-based agent

This study focused on classifying and disposing Circulating fluidized bed (CFB) fly ashes from the level of its origin, and proposed an optimal formulation system for clinker-free cemented backfill materials. CFB fly ash-blast furnace slag (BFS)-based cemented backfill materials with unequal strength grades are used in different locations of the goaf that require more than 1 Mpa and 4 Mpa, respectively, and the leaching levels of all toxic components are lower than the underground III water quality standard limit when the additional amount of CFB fly ash does not exceed 60 wt.%. The stable S/S of Cl^- is due to the combined effect of chemical fixation of HCC and physical adsorption of the C-S-H/C-A-S-H phase. B2(20 wt.% CFB fly ash) exhibits more functional hydration products and higher degree of polymerization with the hydration age extension. Ettringite is the major effective product of CFB fly ash-BFS-based cemented system due to low level of chlorine environment and HCC transformation. CFB fly ash with appropriate active Al₂O₃ can dissolve and promote [AlO₄]^5- to substitute [SiO₄]^4- to form the C-A-S-H phase with longer chains and higher degree of polymerization with increase in Al/Si ratio of C-A-S-H/C-S-H phase.

Over-sulfated soils and sediments treatment: A brief discussion on performance disparities of biological and non-biological methods throughout the literature

High sulfate concentrations in industrial effluents as well as solid materials (excavated soils, dredged sediments, etc.) are a major hindrance for circular economy outlooks. SO₄^2- acceptability standards are indeed increasingly restrictive, given the potential outcomes for public health and ecosystems. This literature review deals with the treatment pathways relying on precipitation, adsorption and microbial redox principles. Although satisfactory removal performances can be achieved with each of them, significant yield differences are displayed throughout the bibliography. The challenge here was to identify the parameters leading to this variability and to assess their impact. The precipitation pathway is based on the formation of two main minerals (ettringite and barite). It can lead to total sulfate removal but can also be limited by aqueous wastes chemistry. Stabilizer kinetics of formation and equilibrium are highly constrained by background properties such as pH, Eh, SO₄^2- saturation state and inhibiting metal occurrences. Regarding the adsorption route, sorbents' intrinsic features such as the q_max parameter govern removal yields. Concerning the microbial pathway, the chemical oxygen demand/SO₄^2- ratio and the hydraulic retention time, which are classically evoked as yield variation factors, appear here to be weakly influential. The effect of these parameters seems to be overridden by the influence of electron donors, which constitute a first order factor of variability. A second order variability can be read according to the nature of these electron donors. Approaches using simple monomers (ethanol lactates, etc.) perform better than those using predominantly ligneous organic matter.

Sulfate removal from mine-impacted water by electrocoagulation: statistical study, factorial design, and kinetics

This work aimed to remove sulfate and acidity from mine-impacted water (MIW) via electrocoagulation (EC), a technique which stands as an advanced alternative to chemical coagulation in pollutant removal from wastewaters. The multiple electrochemical reactions occurring in the aluminum anode and the stainless steel cathode surfaces can form unstable flakes of metal hydroxysulfate complexes, causing coagulation, flocculation, and floatation; or, adsorption of sulfate on sorbents originated from the electrochemical process can occur, depending on pH value. Batch experiments in the continuous mode of exposition using different current densities (35, 50, and 65 A m^-2) were tested, and a statistical difference between their sulfate removals was detected. Furthermore, the intermittent mode of exposure was also tested by performing a 2²-factorial design to verify the combination with different current densities, concluding that better efficiencies of sulfate removal were obtained in the continuous mode of exposition, even with lower current densities. After 5 h of electrocoagulation, sulfate could be removed from MIW with a mean efficiency of 70.95% (in continuous mode of exposition and 65 A m^-2 current density), and this sulfate removal follows probable third-order decay kinetics in accordance with the quick drop in sulfate concentration until 3 h of exposure time, remaining virtually constant at longer times. Graphical abstract.

Facile synthesis and application of cellulosic coagulant from banana peels in cadmium-spiked water

Cellulosic coagulant with low crystallinity and surface charge of -19.2 mV were extracted from wet banana peels (WBE) using kitchen-blending method. Functionalization with ferric chloride and aluminium chloride yielded higher surface charge of -23.8 mV (mWBE). Both WBE and mWBE coagulants were used to target cadmium ions from aqueous solution. Coagulants and the floccules (WBEA and mWBEA) were characterized by XRD, FT-IR, zeta sizer nano series, and SEM/EDs. The amount of cadmium ion coagulated was determined using ICP-OES. The FTIR analysis revealed the functional groups involved in the coordination and subsequent removal of the metals ions around 1634 cm-1, ascribed to the C = O vibrational band of carbonyl group. Microscopic analysis revealed that the mWBE is porous and exhibited microfibers with rod-like morphology. The effects of parameters such as the initial concentration, coagulant dosage and solution pH were investigated. Coagulation results showed that 10 mg of WBE and mWBE could remove about 80% and 90% of the Cd2+ ions respectively. However; the difference in the performance of both materials does not justify the essence of surface modification. Therefore, WBE is considered more efficient and environmentally friendly. Notwithstanding, the performance of these coagulants in real environmental samples will confirm their robustness.

Sequential combination of nanofiltration and ettringite precipitation for managing sulfate-rich brines

The sequential combination of nanofiltration (NF) and ettringite precipitation to manage sulfate-rich brine is proposed. In this study, NF experiments clearly demonstrated that sulfate-containing wastewater was effectively concentrated by the NF process (concentrate factor, CF > 5) with insignificant membrane fouling. Ettringite precipitation was implemented as an alternative to lime precipitation to process sulfate-rich brine resulting from the NF operation. More than 93% of the sulfate ions were removed by ettringite precipitation, whereas lime precipitation removed less than 28% under the same conditions due to the difference in their solubility. However, with highly concentrated NF brine (CF > 5), the pH and sulfate concentration of the supernatant were higher than the discharge limit. Therefore, optional blending of the supernatant after ettringite precipitation with the NF permeate was proposed to satisfy the discharge limit for sulfate. The sequential operation consisting of NF and ettringite precipitation enables sulfate-rich wastewater to be treated effectively, minimizing its negative impact by reducing the brine volume and enabling the water to be reused.

Investigation into the semi-dynamic leaching characteristics of arsenic and antimony from solidified/stabilized tailings using metallurgical slag-based binders

The leaching characteristics of metallurgical slag-based binders (MSB) solidified/stabilized tailings containing arsenic (As) and antimony (Sb), were investigated via a series of semi-dynamic leaching tests using three kinds of leachant, for the simulation of actual leaching conditions. The effectiveness of solidification/stabilization (S/S) treatment was evaluated by measuring the observed diffusion coefficients (D_obs). It was found that MSB efficiently prevented As and Sb leaching, providing D_obs values in the range of 10^-15 to 10^-13 cm²/s and 10^-11 to 10^-9 cm²/s, respectively, with the exception that the leaching mechanism of As was dissolution rather than diffusion under acetic acid leaching conditions. Physical encapsulation was found to be the dominant mechanism for Sb immobilization, while the dominant mechanism of As immobilization was precipitation in the monolithic MSB S/S treated tailings (MST). Results showed that the concentrations of leached As, Sb, Ca and Si, were affected by leachant pH and total acidity as well as the MSB constituent ratio. The effect of these parameters may be attributed to the stability of hydration products and their influence on the buffering capacity and structure of matrices, and the leachant pH and total acidity having the greatest influence on leaching characteristics.

Recycling of Reverse Osmosis (RO) Reject Water as a Mixing Water of Calcium Sulfoaluminate (CSA) Cement for Brick Production

This study explored the possibility of using reverse osmosis (RO) reject water as a mixing water for producing cementitious bricks using calcium sulfoaluminate (CSA) cement along with gypsum, and it investigated the changes in the properties of CSA cement pastes when RO reject water was used. The results were compared with those obtained using purified water and seawater. Overall, the use of RO reject water improved the cement paste’s strength. Given that the use of RO reject water very slightly affected ettringite formation but more significantly influenced the Al2O3-Fe2O3-mono (AFm) phases (i.e., monosulfate, kuzelite, and Friedel’s salt) and amorphous aluminum hydroxide (AH3), the strength improvement was likely mainly due to the formation of Friedel’s salt rather than ettringite formation. This study also demonstrated that the use of RO reject water for brick production satisfied the Korean Standards (KS) F 4004 and toxicity characteristic leaching procedure (TCLP); thus, it is recommended to use RO reject water as a mixing water to produce CSA cement bricks for use in construction.

Improved methane production and sulfate removal by anaerobic co-digestion corn stalk and levulinic acid wastewater pretreated by calcium hydroxide

Levulinic acid wastewater containing high concentration of sulfate was generated while producing levulinic acid by straw depolymerization with dilute sulfuric acid. In this study, levulinic acid wastewater was pretreated by calcium hydroxide precipitation, then co-digestion of pretreated levulinic acid wastewater and corn stalk was conducted for the further removal of sulfate from levulinic acid wastewater and production of bioenergy. Effects of sulfate loading and substrate level on methane production and sulfate removal from co-digestion were investigated. The results showed that the highest methane production potential of 249.93 mL/g volatile solid (VS) was achieved when the sulfate loading is 0.31 g/L and the substrate level is 32.3 g/L, which was significantly higher than 182.53 mL/g VS achieved for mono-digestion of corn stalk. The sulfate removal of 86.82-98.10% was obtained when sulfate loading is >0.31 g/L, and the concentration of sulfate in the biogas slurry was <0.09 g/L after 28 days of anaerobic co-digestion regardless initial sulfate loading. The results of microbial community analysis demonstrated that the relative abundance of methanogenic bacteria (such as Methanoculleus and Methanosarcina) had increased significantly at sulfate loading of 0.31 g/L, and the relative abundance of sulfate-reducing bacteria (Desulfotomaculum) increased from 0.01% to 2.11% when the sulfate loading rose from 0.10 g/L to 1.47 g/L under the substrate level of 32.3 g/L. This means that co-digestion of corn stalk and levulinic acid wastewater after calcium hydroxide pretreatment (CHP) was beneficial for methane production and sulfate removal.

Synthesis and application of alginate immobilised banana peels nanocomposite in rare earth and radioactive minerals removal from mine water

This study describes the preparation, characterisation and application of pelletised immobilised alginate/montmorillonite/banana peels nanocomposite (BPNC) in a fixed-bed column for continuous adsorption of rare earth elements and radioactive minerals from water. The materials was characterised by Fourier transform infrared, X-ray diffraction and scanning electron microscopy analyses. Analyses indicated that the pellets are porous and spherical in shape. FT-IR analysis showed that the functional groups responsible for the coordination of metal ions were the carboxylic (-COO-) and siloxane (Si-O-Si and Si-O-Al) groups. XRD analysis showed two additional peaks which were attributed to alginate and montmorillonite. The influence of the initial concentration, bed depth and flow rate were investigated using synthetic and real mine water in order to determine the breakthrough behaviour of both minerals. The processed bed volume, adsorbent exhaustion rate and service time, were also explored as performance indices for the adsorbent material. Furthermore, the breakthrough data were fitted to both the Thomas and Bohart-Adams models. The BPNC exhibited high affinity for U, Th, Gd and La in the real mine water sample. However, studies may still be required using waters from different environments in order to determine the robustness of BPNC.

Impact of total carbon/sulfate on methane production and sulfate removal from co-digestion of sulfate-containing wastewater and corn stalk

During the process of preparing furfural by straw depolymerization with dilute sulfuric acid, large amounts of high temperature sulfate-rich organic wastewater were produced. It cannot be treated directly by anaerobic digestion and converted to bioenergy due to high concentrations of sulfate. In this study, anaerobic co-digestion of sulfate containing wastewater and corn stalk was performed at thermophilic conditions to investigate the influences of total carbon (TC)/sulfate (6, 16, 35 and 110) on methane production and sulfate removal. The results showed that the highest methane production of 260.14 mL g^-1 volatile solid (VS) was achieved at TC/sulfate of 35, which was significantly higher than 12.53 mL g^-1 VS obtained at TC/sulfate of 6. Moreover, the results of sulfate balance analysis showed a maximum sulfate removal of 93.43% was achieved at TC/sulfate of 16, and sulfate concentration in biogas slurry was less than 0.1 g/L regardless of TC/sulfate after 28 days of co-digestion. The microbial community was analyzed using 16S rDNA sequencing technology, the results showed that methane was mainly produced by Methanoculleus and Methanosarcina, and sulfate was removed via Desulfotomaculum, and the relative abundance of methanogenic archaea (MA) and sulfate reducing bacteria (SRB) were significantly correlated with methane production and sulfate removal. It can concluded that higher methane production and sulfate removal can be obtained by anaerobic co-digestion of sulfate containing wastewater and corn stalk at properly TC/sulfate.

A Comparison Study on the Arsenate Adsorption Behavior of Calcium-Bearing Materials

The calcium-bearing adsorbents are widely used in the treatment of arsenic-containing wastewater due to their excellent treatment effect and economy. In order to obtain high-efficient adsorbents for arsenate (As(V)) removal, the adsorption behavior of calcium oxide (CaO), calcium fluoride (CaF₂) and calcium carbonate (CaCO₃) on As(V) in aqueous solution at different concentrations were explored. The adsorption mechanism was also explored based on surface characteristics: morphology, specific surface area, as well as their effective calcium content. Not only that, the chemical stability of these materials was further studied. Results exhibited that the As(V) removal capability of these materials is in the following order, CaO > CaF₂ > CaCO₃. When CaO served as an absorbent, As(V) with initial concentration of 0.2 mg/L can be reduced to 0.383 × 10^-3 mg/L in 10 min. Moreover, the capabilities of CaO, CaF₂ and CaCO₃ for removing As(V) are positively correlated with their effective calcium content in aqueous solution, which provide the basis for selecting calcium-bearing materials with excellently comprehensive properties for the field of As(V) removal in aqueous solution. What's more, all three materials exhibit great chemical stability after adsorption of As(V).

A novel sulfate removal process by ettringite precipitation with aluminum recovery: Kinetics and a pilot-scale study

A novel sulfate removal process via ettringite precipitation was developed by dissolving ettringite and recycling Al³⁺ under low pH condition. Effects of solid to liquid ratios, pH and temperature on ettringite dissolution, Al recovery and transformation of precipitates were investigated by batch experiments. The optimum condition for Al recovery is pH =3.0, suspended solid of 9.8 g/L and temperature below 303 K. Ettringite dissolution consists of two stages, (i) rapid but inconsistent dissolution with the fastest release of sulfate, followed by calcium, and then Al(OH)₆^3-; (ii) slow dissolution of Al(OH)₆^3- core and gypsum precipitation. Dissolution of Al(OH)₆^3- core follows the first-order kinetics with activation energy of 41.18 kJ/mol, while gypsum re-precipitation follows the second-order kinetics with activation energy of 26.36 kJ/mol. Long-term results of pilot-scale systems for treatment of real flue gas desulfurization wastewater showed that the process achieved sulfate removal of 98.3%-99.5% and Al recovery above 98.4%, and converted 98.8% sulfate in ettringite to CaSO₄, which resulted in 66.0% of sludge reduction and improved sludge dewaterability. Economic evaluation shows that the process with Al recovery reduces cost of ettringite precipitation by 35.1%, and is highly feasible and cost-effective for industrial application of high-sulfate content wastewater treatment.

Reduction of undesirable element leaching from fly ash by adding hydroxylated calcined dolomite

Fly ash always contains many toxic elements which can be released into environment, thereby easily leading to environmental contaminations. In order to dispose fly ash safely, related strategies are needed. In this investigation, two kinds of hydroxylated calcined dolomites (HCD60 and HCD100) were used as the additives and compared with lime on the leachabilities of anionic species from fly ash. Both additives were found effective in reducing the leaching concentrations of these elements, which was better than that of only lime addition. Mg(OH)₂ and MgO were believed to play important roles in the hydration reaction of fly ash. In the presence of Mg(OH)₂ and MgO, there were more hydration products including calcium silicate hydrate, ettringite, hydrocalumite and other Layered double hydroxides (LDHs) generated which were effective candidates for anion removal. Thus, the final leaching results were controlled by these newly formed phases through adsorption, incorporation or encapsulation. On the other hand, compared with Mg(OH)₂, MgO can promote the formation of hydration products in a larger extent because of the hydration process of MgO into Mg(OH)₂. There was no systematic trend in the promotion of fly ash hydration by Mg(OH)₂ or MgO because it had a close relationship with the properties of original fly ash. Objectively, hydroxylated calcined dolomites can be promising candidate additives for reduction of toxic elements leaching from fly ash.

Sulfate removal from acid mine water from the deepest active European mine by precipitation and various electrocoagulation configurations

Sulfate removal from mine or process water is a key issue in the mining industry. In this paper, precipitation with lime (calcium oxide) was integrated with electrocoagulation for sulfate removal from Pyhäsalmi/Finland mine water. Sulfate precipitation with calcium oxide decreased the sulfate concentration from 13,000 mg/L to 1600 mg/L. Various current densities were applied to the pre-treated mine water with various electrodes and aluminium and iron anodes. It was found that 25 mA/cm² was the best tested current density for both anode types. At the second stage, this current density was used for different iron and aluminium anodes in various monopolar and bipolar configurations. It was found that this hybridisation is effective for sulfate removal, and that a bipolar configuration showed better results than the monopolar configuration. The best result was gained from 25 mA/cm² with a two aluminium and two stainless steel anode-cathode configuration and calcium oxide pre-treatment to reach pH 12. The removal efficiency reached 84.4% and 63.8% with aluminium anodes in bipolar and monopolar configurations, respectively. This setup was able to decrease sulfate concentrations from 13,000 mg/L to 250 mg/L, which meets mine water discharge limits. Kinetic studies showed that iron and aluminium anodes obey pseudo-second order kinetic. Finally, the energy consumption was calculated.

How to tackle the stringent sulfate removal requirements in mine water treatment-A review of potential methods

Sulfate (SO₄^2-) is a ubiquitous anion in natural waters. It is not considered toxic, but it may be detrimental to freshwater species at elevated concentrations. Mining activities are one significant source of anthropogenic sulfate into natural waters, mainly due to the exposure of sulfide mineral ores to weathering. There are several strategies for mitigating sulfate release, starting from preventing sulfate formation in the first place and ending at several end-of-pipe treatment options. Currently, the most widely used sulfate-removal process is precipitation as gypsum (CaSO₄·2H₂O). However, the lowest reachable concentration is theoretically 1500 mg L^-1 SO₄^2- due to gypsum's solubility. At the same time, several mines worldwide have significantly more stringent sulfate discharge limits. The purpose of this review is to examine the process options to reach low sulfate levels (< 1500 mg L^-1) in mine effluents. Examples of such processes include alternative chemical precipitation methods, membrane technology, biological treatment, ion exchange, and adsorption. In addition, aqueous chemistry and current effluent standards concerning sulfate together with concentrate treatment and sulfur recovery are discussed.

Removal of High-Concentration Sulfate Ions from the Sodium Alkali FGD Wastewater Using Ettringite Precipitation Method: Factor Assessment, Feasibility, and Prospect

The feasibility of removal of sulfate ions from the sodium alkali FGD wastewater using the ettringite precipitation method was evaluated. Factors affecting the removal of sulfate ions, such as NaAlO2 dosage, Ca(OH)2 dosage, solution temperature, anions (Cl−, NO3− and F−), and heavy metal ions (Mg2+ and Mn2+), were investigated, and the optimal experimental conditions for the removal of sulfate ions were determined. Experimental results indicate that the ettringite precipitation method can effectively remove SO42− with removal efficiency of more than 98%. All the investigated factors have influences on the removal of sulfate ions, and among them, the dosage of reagents, solution temperature, and fluoride ions have the strongest influence. In addition, the method can effectively synergistically remove F− and heavy metal ions with removal efficiencies of more than 90% and 99%, respectively; meanwhile, Cl− and NO3− also can be removed minimally by the method. The result of actual wastewater treatment shows that the method is feasible for treating high-concentration sulfate wastewater. The ettringite precipitation method has the potential to be a commercial application in the future.