Residual Alkalinity Research Articles

Low-cost dealkalization of bauxite residue by gypsum coordinated with industrial iron salt: Mineralogical mechanisms and field practice

The sustained-release of alkalinity in bauxite residue hinders the soil formation and ecological restoration processes in disposal areas. Ferric salts (Fe3+), known for their rapid neutralization reaction process and straightforward operational simplicity, showed a synergistic advantage with gypsum (CaSO4) in the alkaline regulation of bauxite residue. Nevertheless, the mineralogical mechanisms of the reaction between Fe3+ and Ca2+ and alkaline minerals in bauxite residue remained unclear. The present study aimed to elucidate the geochemical stability mechanisms of sodalite and cancrinite, major alkaline minerals in bauxite residue, driven by Fe3+ and Ca2+ from a mineral structural perspective and the integrated restoration effect was demonstrated by the field experiment. Unreacted shrinking core model (USCM) calculations demonstrated that Fe3+ was able to enter the lattice structure of sodalite and cancrinite to replace Na+ and assisted Ca2+ in successfully replacing Na+ in cancrinite. Moreover, Fe3+ and Ca2+ facilitated the formation of insoluble coating layers on their surface. In the lattice structure of sodalite and cancrinite, the replacement reaction for Na+ was influenced by the competition between Fe3+ and Ca2+. The Ca2+ preferentially replaced Na+, while Fe3+ accumulated near the Si-O bonds, balancing the negative charges of the Si-O and Al-O tetrahedra structure and further inhibited alkaline mineral dissolution. The pH and EC of bauxite residue decreased from 10.52 to 7.60 and 712 μS/cm to 501 μS/cm, respectively, after a one-year field-scale amelioration by the integration of Fe3+ and Ca2+. These findings highlighted the synergistic roles of Fe3+ and Ca2+ in regulating the alkalinity of bauxite residue and provided a practical strategy for ecological restoration in disposal areas.

Mono e codigestão anaeróbia de resíduos de frutas e vegetais: Efeitos no rendimento de biogás e no biofertilizante

ABSTRACT The proper disposal of fruit and vegetable waste is essential to promote environmental sustainability, reduce the carbon footprint, improve soil quality, and contribute to the transition to a circular economy. This study examined the impact of incorporating dairy cattle wastewater (DCWW) into anaerobic co-digestion with fruit and vegetable residues and utilization of the resulting biofertilizer as a diluent in the anaerobic mono-digestion of fruit and vegetable residues (recycle). The specific biogas and methane production and the agronomic quality of the biofertilizer were evaluated in a semi-continuous trial. Two startup strategies were employed: initially, the reactors were entirely fed with inoculum (biofertilizer derived from DCWW), and fruit and vegetable residue feedstock was gradually introduced. Anaerobic co-digestion of fruit and vegetable residues with DCWW yielded the highest specific production of biogas and methane at 720 and 436 L kg-1 of volatile solids (VS), respectively. The addition of DCWW increases fruit and vegetable residue alkalinity by 249% and reduces the volatile acidity by 83.4%. Anaerobic mono-digestion of fruit and vegetable residues enhances macronutrient recovery in the biofertilizer through recycle. Logistically, directing fruit and vegetable residues to rural areas for anaerobic co-digestion with DCWW promotes agricultural biofertilizer use.

The effect of heating rate on microstructure and electrochemical performance of nickel-rich layered oxides cathode materials

Nickel-rich layered oxides have attracted significant attention as promising cathode materials for lithium-ion batteries due to their high specific capacity, rate capability, and relatively low cost. However, the material still faces challenges such as harsh synthesis conditions, easy cation mixing and large residual alkali on the surface, severely limiting its practical applications. To overcome these drawbacks, this study successfully synthesized a series of LiNi0.8Co0.1Mn0.1O2 (NCM) cathode materials with different heating rates. The results indicate that appropriately reducing the heating rate can improve the cycling performance of the material and reduce cation mixing and surface residual alkalinity. Specifically, among the four samples, the LiNi0.8Co0.1Mn0.1O2 sample (NCM-2 °C min-1) exhibites a lower Li⁺/Ni²⁺ mixed arrangement and fewer surface residual alkalis, demonstrating good cycling performance and rate capability. The NCM-2 °C min-1 sample provides an excellent initial discharge capacity of 210.4 mAh g⁻¹ under 0.1 C conditions, and it achieves the highest capacity retention rate (85.9%) after 100 cycles at 1 C, while the capacity retention rates for NCM-1 °C min-1, NCM-3 °C min-1, and NCM-4 °C min-1 are 75.4%, 75.2%, and 71.2%, respectively. NCM-2 °C min-1 sample also showed excellent rate performance, especially at high rates. The discharge capacity remains at 151.6 mAh g-1 at 10 C, which is much higher than that of other samples (149.3 mAh g-1 for NCM-1 °C min-1, 123.5 mAh g-1 for NCM-3 °C min-1, and 120.6 mAh g-1 for NCM-4 °C min-1). The research of synthesis technology has important guiding significance for the synthesis of high-stability high-nickel layered oxide materials.

Development of Enteric Diclofenac Sodium Microparticles Through a Spray-Drying Process Facilitated by Different Aqueous Dispersion Systems

The objective of this study was to prepare enteric diclofenac sodium microparticles using an aqueous dispersion system via spray-drying. Two aqueous-based solvent systems, phosphate buffer at pH 7.0 and ammonium hydrogen carbonate solution, were employed as the feed dispersion media in the spray-drying process based on the solubility characteristics of Eudragit® L100. At a drug-to-polymer ratio of 1:1, the optimal solids concentration in the feed dispersion was determined to be 2% w/v, as it enabled an approximately 5-30 µm smooth-surfaced spherical microparticle with a high production yield. Diclofenac sodium was efficiently encapsulated within the microparticles, existing as solid dispersion in a partially amorphous form. Notably, the spray-drying conditions utilized in this study obviated the need for further heating for microparticles prepared using ammonium hydrogen carbonate solution, as the residual ammonium could be completely eliminated during the spray-drying process. The two-stage biorelevant drug release profile of enteric microparticles demonstrated their ability to inhibit drug dissolution under acidic conditions while facilitating drug release under basic conditions. The phosphate buffer-based microparticles exhibited greater protection efficiency under acidic conditions compared to ammonium hydrogen carbonate-based systems, despite residual alkaline salt being present in the microparticles. These results validate the potential of the developed microparticles for use as an enteric drug delivery system.

Formation and Detriments of Residual Alkaline Compounds on High-Nickel Layered Oxide Cathodes.

High-nickel layered oxides LiNixM1-xO2 (x≥0.9) have emerged as promising cathode materials for automotive batteries due to their high energy density and lower cost. However, the formation and accumulation of surface alkaline compounds during storage hinder their mass production and commercialization. Here, a validated chemical method is employed to deconvolute and quantify the evolution of each residual lithium compound in four representative cathodes during ambient-air storage, viz., LiNiO2 (LNO), LiNi0.95Co0.05O2 (NC), LiNi0.95Mn0.05O2 (NM), and LiNi0.95Al0.05O2 (NA). Furthermore, the activation energy of the reaction between water and the cathode is determined by measuring the leached LiOH concentration at various temperatures. While residual lithium and time-of-flight secondary-ion mass spectrometry measurements collectively reveal that the air stability overall follows the trend of NM>NA≈NC>LNO, the aged NM exhibits the highest charge-transfer resistance and the worst electrochemical performance among the cathodes. In situ, X-ray diffraction and scanning transmission electron microscopy unveil that the aged NM is plagued by a large area of resistive spinel-like M3-xLixO4 phases, leading to aggravated particle reaction heterogeneity. Finally, a one-step recalcination method is demonstrated effective in fully restoring the degraded cathodes. This work provides insights into overcoming air sensitivity issues of high-Ni cathodes.

The formation of the carbonate system of circulating cooling water of the Rivne NPP and its influence on changes in the surface waters pH levels of the Styr river

The data on pH changes and corresponding equilibrium shifts of the carbonate system of process and return waters of the circulating cooling system (RCS) of the Rivne NPP during water treatment were analysed. In the additional cooling water previously clarified by liming, there is no dissolved carbon dioxide and no residual free alkalinity. The effects that occur during heating, cooling, and aeration in the RCS cause a shift in the equilibrium of the carbonate cooling water system with a decrease or increase in the content of carbon dioxide, bicarbonate, and carbonate ions, and pH level. The influence of the shift of the carbonate system and the changes in pH level during the discharge of return water from the Rivne NPP into a natural water body (Styr River) was analysed.

Performance Assessment of Wood Ash and Bone Char for Manganese Treatment in Acid Mine Drainage

Developing efficient methods for Mn separation is the most challenging in exploring innovative and sustainable acid mine drainage (AMD) treatments. The availability and capacity of certain waste materials for Mn removal warrant further exploration of their performance regarding the effect of process factors. This study addressed the influence of AMD chemistry (initial pH and concentrations of Mn, sulfate, and Fe), the solid/solution ratio, and the contact time on Mn separation by wood ash (WA) and bone char (BC). At an equivalent dose, WA displayed higher neutralization and Mn removal capacity over the initial pH range of 2.5–6.0 due to lime, dicalcium silicate, and fairchildite dissolution. On the other hand, at optimal doses, Mn separation by BC was faster, it was less affected by coexisting sulfate and Fe(II) species, and the carbonated hydroxyapatite structure of BC remained preserved. Efficient removal of Mn was feasible only at final pH values ≥ 9.0 in all systems with WA and at pH 6.0–6.4 using BC. These conclusions were confirmed by treating actual AMD with variable doses of both materials. The water-leaching potential of toxic elements from the AMD/BC treatment residue complied with the limits for inert waste. In contrast, the residue of AMD/WA treatment leached non-toxic quantities of Cr and substantial amounts of Al due to high residual alkalinity. To minimize the amount of secondary waste generated by BC application, its use emerges particularly beneficial after AMD neutralization in the finishing step intended for Mn removal.

Remediation of acid mine drainage and immobilization of rare earth elements: Comparison between natural and residual alkaline materials

Acid mine drainage (AMD) is a well-known source of toxic trace metals in freshwaters. Traditional passive treatment systems rely on AMD neutralization with limestone and removal of most common toxic transition metals such as Cu and Zn with little attention to rare earth elements (REE). Alkaline waste materials now receive increasing attention as low cost AMD treatment alternatives in the circular economy. This study was set up to identify the efficiency of alkaline waste materials remediating AMD and scavenging REE in addition to other toxic trace elements. An AMD sample was collected from a lixiviate coming from pyrite heaps in the Iberian Pyrite Belt (pH = 1.8, 30 μM ∑REY). The sample was treated with either blast furnace slag (BFS) generated during smelting of iron ore in a blast furnace or biomass ashes (BA) derived from combustion of biomass, thereby using analytical grade CaCO3, and NaOH as reference products. The batch alkalinization experiments were conducted by adding each alkaline material at an amount to obtain an equal pH to ≈6.5. The required amounts of the products were NaOH < CaCO3<BFS < BA in line with their acid neutralizing capacities. The largest removal of sulfate from water was obtained in the CaCO3 treatment suggesting gypsum precipitation which was lower with BA and BFS and virtually absent with NaOH, these trends were confirmed by SEM-EDX and XRD. Both BFS and BA removed more Fe than CaCO3 and NaOH. The REE elements were well removed by all treatments (>99%) and the remaining REE concentrations in the solutions were clearly lower than values for Cu and Zn. The Zn and Cu removals were not consistently high enough (except with NaOH) to meet environmental limits in the discharge waters. The largest efficiency for REE removals was obtained with CaCO3. Indirect evidence here suggests that gypsum is a better host for the trivalent REE than Fe(III) minerals in the precipitates. The ionic radii of trivalent REE are more similar to Ca2+ than to Fe3+, explaining the better potential of gypsum as REE host. This study showed also the potential of BFS as alkaline agent for the remediation of AMD in terms of its higher alkalinity generation potential as compared to BA, thus making BA less promising than BFS.

Salinisation of soils in semi‐arid endorheic zone: Case of Chott El Beida (Algeria)

AbstractChott El Beida, North of Algeria (Setif, 585 km2), is an endorheic zone located in a semi‐arid climate. It is among the first chotts listed under the Ramsar Convention on the Wetlands of International Importance. Numerous studies on wetlands in the Setif region have focused on hydrogeology, avifauna, and vegetation. However, only one study has been devoted to soil studies. This study focused on the pedological and geochemical aspects of salinity in this area. This study was conducted on a toposequence of 200 m comprising eight soil profiles. A set of 24 samples were collected from the top to a depth (0–200 cm) and were subjected to a series of physical and chemical analyses. The concentrations of the major elements were used to determine the chemical facies and calculate the saturation indices for each mineral and residual alkalinity using geochemical modelling with PHREEQC. This study shows that the salinity in Chott El Beida is very variable, varying from less saline to extremely saline, moving from the outside to the centre of the chott, with two types of saline profiles: the descending type towards the outside of the chott and the ascending type in the highly saline depression. Geochemical investigations demonstrated that the dominant facies is (Na‐Cl) which is in accordance with the neutral saline pathway. The phenomenon of salinisation in Chott El Beida is controlled mainly by the precipitation of minerals, namely, calcite, aragonite, dolomite, gypsum, and anhydrite, and the dissolution of evaporite minerals. The study of calcite residual alkalinity and generalised residual alkalinity showed that salinisation evolves in the neutral saline path without the risk of alkalinization.

Assessment of chemo-mechanical degradation of concrete sewer pipes through an integrated experimental approach

The chemo-mechanical degradation of unreinforced concrete sewer pipes applied in domestic service locations is assessed through a systematic, integrated experimental approach, considering 18 new and 35 used sewer pipes. The characteristics and environmental conditions of the sewer pipes are reported, and data obtained from surface condition classification, residual alkalinity tests and XRD analyses are combined to identify the type and degree of chemical attack of the used sewer pipes. Concrete material properties are determined by material tests on sewer pipe samples, providing quantitative insight into the age dependency of the Young’s modulus, compressive strength, tensile strength and mode I toughness. All relatively old pipes (installed in the 1920s and 1950s) show substantial chemical attack by biogenic sulphide corrosion (at the inner side of the pipe) and carbonation (at the outer side of the pipe). The time development of the corresponding corrosion depths on average follows a linear trend, whereby the corrosion rate for biogenic sulphide corrosion is about a factor of 1.3 larger than for carbonation. Due to these chemical processes, the mechanical properties of concrete may significantly depend on the age of the sewer pipe. In particular, the average compressive strength and average tensile strength decrease approximately linearly with the age of the pipe, in correspondence with relative reductions of, respectively, a factor of 1.7 and 1.5 over a period of almost 100 years. The values and time-dependent trends found for the concrete properties and corrosion depths of sewer pipes can serve as input for practical analyses and advanced numerical simulations on their bearing capacity and time-dependent degradation. The experimental results also emphasize the importance of regularly assessing the amount of chemical degradation of (especially older) in-situ sewer pipes, by determining the (decrease in) effective wall thickness via core sampling and laser profiling.

Recycling industrial alkaline solutions for soil stabilization by low-concentrated fly ash-based alkali cements

This study aims to evaluate the usability of a recycled alkaline solution, a residue from cleaning iron plates in the car manufacturing industry, to enhance the strength and stiffness of clayey soil by carrying out unconfined compressive strength (UCS), indirect tensile strength (ITS), ultrasonic pulse velocity (UPV) and microstructural (SEM, XRD) tests. High- and low- calcium fly ashes with percentages of 10% to 40% and 10% to 30%, respectively, were independently introduced into the alkaline activation process to determine the proper type and content of precursors at room temperature. Two mixtures of the residual cleaning solution blended with 2 M and 4 M NaOH solutions at a weight ratio of one were also utilized to determine how efficiently binary solutions could improve the performance of the parent clay soil. These experiments revealed that all mixtures of the residual alkaline solution work better to activate high-calcium fly ash as compared to the low-calcium fly ash in terms of both mechanical strength and stiffness. The sample of high-calcium fly ash with fly ash/solid and Na/ash ratios equal to 30% and 0.037, respectively, experienced the highest improvement. A microstructural analysis showed the formation of C-(A)-S-H and N–(C)–A–S–H gels in the corresponding sample, thus providing a considerably dense structure, which in turn significantly contributed to the enhancement of the maximum strength and stiffness of the treated soil. In fact, the high-calcium precursor proved to be the best candidate for a low-range alkaline activator because it contributes more calcium to a low-alkaline environment. An environmental impact assessment was also carried out to compare the CO2 emissions of clays treated with the 4 M NaOH binary solutions tested in this study with those conventionally stabilized by cement and lime and also those treated traditionally with commercial 2, 4, and 8 M NaOH. The samples in this study treated with environmentally friendly binary solutions (residual cleaning solution blended with 2 M and 4 M NaOH solutions at a weight ratio of one) performed better in terms of both CO2 emissions and strength gain compared to the samples stabilized with conventional materials.

The aggregation effect of organic matter on bauxite residue particles and its improvement mechanism

Introduction: Understanding organic amendment and the agglomeration of bauxite residue particles is vital to soil amelioration of bauxite residues. In this study, a pot culture experiment was conducted to illustrate the aggregation of organic amendment on bauxite residues particles and its improvement mechanism. The single organic amendment and its combination with soil inoculum were conducted to explore the aggregation effect of organic matter on bauxite residue particles, and its correlations with microbial rehabilitation.Methods: The dry- and wet-sieving method were used to obtain different sizes of aggregates. The concentrations of soil organic carbon and iron and aluminum (Fe/Al) oxides in the forms extractable by DCB (Fed/Ald) and oxalate (Fe0/Al0) were measured. Microbial rehabilitation after 180 days incubation was determined with the methods of Biolog Ecoplate™ and the high-throughput sequencing.Results and Discussion: The results showed that over 180 days incubation, the alkalinity of bauxite residues was significantly decreased with the organic amendment based on the value of pH, EC and ESP (down to 9.26, 0.61 m/cm and 55.5%, respectively, in HS3). Secondly, organic amendment significantly promoted microbial community establishment and ecological function recovery. Moreover, the MWD value of aggregates also increased to 0.73 mm from the initial 0.32 mm, companied with the increase of the Fe/Al oxidizes. The further Pearson relationship analysis and the characterization of EPMA and SEM indicated that the organic matter and Fe/Al oxides played important roles in cementing fine bauxite residues particles and increasing aggregates stability, while this aggregation process was accelerated by the establishment of microbial ecology in bauxite residues. Therefore, organic amendment was more implication in soil amelioration of bauxite residues other than the directive modifications on alkalinity.

Recovery of Lithium from Simulated Nanofiltration-Treated Seawater Desalination Brine Using Solvent Extraction and Selective Precipitation

ABSTRACT The world's seas and oceans contain vast amounts of lithium, but the low concentration hereof renders solvent extraction impractical for its recovery. By contrast, seawater desalination brine, after treatment by nanofiltration, contains a roughly tenfold greater concentration of lithium than raw seawater. Hence, lithium can be effectively recovered from such streams using solvent extaction. Compared with other techniques to sequester lithium from dilute solutions, solvent extraction offers the advantages of simple operations, robust and well-established technology and high recovery yields. Thus, we propose a solvent-extraction based process to recover lithium from seawater desalination brine, treated by nanofiltration. The first step comprises the removal of magnesium and calcium using methyltrioctylammonium neodecanoate in p-cymene. This is followed by a lithium extraction step using the extractants Mextral 54–100 and Cyanex 923 in Shellsol D70 diluent. The lithium extract is then scrubbed with water and stripped with hydrochloric acid. Subsequently, residual alkaline earth metals are removed with sodium hydroxide in ethanol and finally lithium is precipitated using sodium carbonate. The solvent extraction, scrubbing and stripping steps were demonstrated on mini-pilot scale in continuous countercurrent mode (in mixer-settlers), while the precipitation steps were demonstrated in batch. The process was found to have an overall yield of 74%, affording a lithium carbonate product with a purity of 97 wt%.

A review on reinforcement corrosion propagation in carbonated concrete – Influence of material and environmental characteristics

Low-binder eco-efficient concretes and carbon uptake are opportunities for mitigating CO2 emissions and reaching net zero goals. However, the risk of corrosion induced by carbonation can accelerate. Here, a meta-analysis of the results of corrosion current density (icorr) obtained from steel embedded in carbonated concrete was conducted considering the characteristics of concrete and environment. The literature review shows higher values of icorr than expected and reported in reference studies and standards at different exposure classes of relative humidity (RH). High levels of icorr were found in low- and high-binder-content concretes, different paste volumes, or estimated porosities. The concrete resistivity, which is less known, shows the best relationship with icorr in carbonated concrete. Resistivity reflects better moisture content, and pore saturation conditions at the steel-concrete interface than the external RH, although seldom reported. Research on the relationship between icorr during the propagation period, carbonation degree, microstructure, residual alkalinity, and resistivity for low-binder concretes is still scarce and needed.

Two-Step Solvothermal Process for Nanoarchitectonics of Metastable Hexagonal WO3 Nanoplates

Hexagonal tungsten trioxide (h-WO3) has shown great potential for application in electrochromic devices, gas sensors, battery electrodes, and as photo-catalysts. The h-WO3 structure features a unique large network of open hexagonal channels that allow for intercalation. The hydrothermal synthesis of h-WO3 using sodium tungstate dihydrate as a precursor is widely explored, however, the residual alkaline ions are difficult to eliminate during the synthesis. The solvothermal synthesis using tungsten hexachloride as starting materials largely avoids the use of alkaline ions, but the effect of various synthesis parameters is not well-understood yet. To resolve these ambiguities, this study provides a reliable route to obtain h-WO3 via solvothermal synthesis and dehydration annealing. The effects of precursor concentration, water content, synthesis temperature, and synthesis time, as well as dehydration temperature, on the as-synthesized crystal structure and crystal morphology are studied.

Effects of Sodic Water Irrigation and Neutralizing Amendments on Physiological, Biochemical, and Nutritional Quality Traits of Fodder Sorghum

This study was conducted at two farmers’ fields to assess the production potential and quality of summer fodder sorghum intervened between the rice-wheat cropping sequences (RWCS) on high residual alkalinity, i.e., residual sodium carbonate (RSC) water irrigation-induced sodic soil. The treatments were comprised of two field sites having different residual alkalinity [RSC ~5 me L−1 (RSC-1) and ~7 me L−1 (RSC-2) water irrigation in main plots, four neutralization strategies, i.e., control/unamended condition (N0), gypsum @ 7.5 t ha−1 (N1), pressmud @ 10 t ha−1 (N2) and gypsum @ 3.75 t ha−1 + pressmud @ 5 t ha−1 (N3) in sub plots and two varietal sequences of RWCS, i.e., salt tolerant varieties (CSR 30 basmati fb KRL 210) and traditionally grown varieties (PB 1121 fb HD 2967) of rice and wheat as sub–sub plots. Sorghum cv. Sugargraze (Advanta Company) was grown after the harvesting of wheat and cut for green fodder before transplanting rice during both years. Sorghum physiological and biochemical traits [relative water content (RWC), total chlorophyll content, photosynthetic rate (Pn), stomatal conductance (gS), transpiration rate (E), chlorophyll fluorescence (Fv/Fm), photon quantum yield [Y (II)] and K/Na ratio]; fodder quality traits [Crude protein (CP), and ether extract (EE)] and productivity [green fodder yield (GFY), dry matter yield, CP yield, EE yield and ash yield) and profitability (gross returns, net returns, benefit–cost ratio) significantly decreased with the increase in irrigation water RSC from 5 to 7 me L−1. Proline, total soluble sugar (TSS), total soluble protein (TSP), dry matter (DM), ash, neutral detergent fibre (NDF), acid detergent fibre (ADF), acid detergent lignin (ADL), neutral detergent insoluble CP (NDICP) and acid detergent insoluble CP (ADICP) decreased with increasing RSC of irrigation water. Sodicity neutralization considerably improved sorghum physiological adaptation mechanisms, fodder quality, productivity and profitability. The introduction of summer fodder sorghum between RWCS resulted in additional net returns (NR) (INR 13.64 to 20.79 × 103 ha−1). Our results indicate that pressmud proved a feasible alternative to replace and/or reduce the quantity of gypsum required for neutralization of RSC water irrigation. Growing summer fodder sorghum between RWCS along with neutralization of RSC water irrigation can increase the availability of quality green fodder during lean period and also increase the profitability of the rice-wheat cropping system in high residual alkalinity water irrigation conditions.

Sodium removal from bauxite desilication product (sodalite) aided by chelating effects of inorganic and organic acids

Dealkalization is a prerequisite to converting bauxite residue into non-hazardous materials that can be used for various upcycling applications. Structural alkali (Na+) lodged inside the densely packed aluminosilicate-cages of sodalite, the dominant desilication product from refining alumina, is a common culprit in the persistence of strong alkalinity of bauxite residue. The present study unravelled chemical and mineralogical processes involved in sodalite dealkalization, driven by organic and inorganic acids. These acids have different H+ dissociation coefficients and their anions have different chelation abilities with surface metal atoms of aluminosilicate minerals. The efficacy of sodium removal by exposure to the acids was found not only dependent on the acid strength (pKa), but also on the chelating property of dissociated conjugate anions. Following an initial H+-Na+ exchange, Na+ removal from sodalite was correlated with partial hydrolysis of aluminosilicate network and resultant chelating reactions with acid anions. The selection of organic and inorganic acids whose conjugate bases possess good chelating capability in the pH buffer zone 7–9 (e.g., oxalate or phosphate), would provide significant aid to the dealkalization process. The findings in this study are crucial in understanding the conversion of bauxite residue into a soil-like growth media (technosol) for sustainable mined land rehabilitation.

Blending bauxite residues with multiple byproducts improves capping materials for tailings storage facilities

Amelioration and management of large volumes of tailings resulting from alumina refining is a major challenge owing to the high alkalinity and salinity of residues. Blended byproduct caps are a potential new and more cost-effective approach to tailings management, where tailings are blended with other local byproducts in order to reduce pH, salinity and toxic elements. Here, alkaline bauxite residue was blended with four byproducts (waste acid, sewage water, fly ash and eucalypt mulch) to create a range of potential capping materials. We leached and weathered materials in the glasshouse with deionized water over nine weeks to investigate if byproducts on their own or in combination improved cap conditions. Combining all four byproducts (10 wt % waste acid, 5 wt % sewage water, 20 wt % fly ash and 10 wt % eucalypt mulch) achieved lower pH (9.60) compared to any byproduct applied individually, or un-remediated bauxite residue (pH 10.7). Leaching decreased EC by dissolving and exporting salts and minerals from the bauxite residue. Fly ash addition increased organic carbon (likely from non-combusted organic material) and nitrogen, while eucalypt mulch increased inorganic phosphorus. Addition of byproducts also decreased the concentration of potentially toxic elements (e.g., Al, Na, Mo and V) and enhanced pH neutralisation. Initial pH with single byproduct treatments was 10.4–10.5, which decreased to between 9.9–10.0. Further lowering of pH and salinity as well as increased nutrient concentrations may be possible through higher addition rates of byproducts, incorporation of other materials such as gypsum, and increasing leaching/weathering time of tailings in situ.

Fixed-bed column study for the remediation of the bauxite-liquid residue using acid-activated clays and natural clays

Large amounts of bauxite-liquid residue are generated during the production of aluminium, which has detrimental effects on human and environmental health. Currently, the primary goal of every alumina industry is to improve the wet disposal of bauxite-liquid residues into the environment using eco-friendly and cost-effective methods. Therefore, this study investigated the possibility of treating bauxite-liquid residue with natural clays (NCs) and acid-activated clays (AACs) using a fixed-bed column adsorption study. The chemical compositions and functional groups of clays and bauxite were studied using X-ray diffractometry (XRD), X-ray fluorescence (XRF), and Fourier transform infrared spectroscopy (FTIR) techniques. For iron adsorption, breakthrough curves were plotted by varying the adsorbent type in the fixed-bed column. The Bohart–Adams, Thomas, and Yoon–Nelson models were successfully fitted with the breakthrough curves. Two regeneration cycles revealed high regeneration efficiencies for both natural and acid-activated clays. Overall, the study found that AACs were the best candidates for treating bauxite-liquid residue when compared to NCs. For instance, the pH, temperature, electrical conductivity, total suspended solids, total dissolved solids, biochemical oxygen demand, turbidity, and total alkalinity of the bauxite-liquid residue were all significantly decreased below tolerance levels by using AACs. The AACs removed 92% of the iron in the bauxite-liquid residue. Lastly, our research shows that AACs can be used as an adsorbent to treat bauxite-liquid residue, making it less hazardous when it is disposed of into the environment.

Acidification and alkalinization pretreatments of biowastes and their effect on P solubility and dynamics when placed in soil

BackgroundSustainability concerns as well as recent increases in fertilizer prices exacerbates the need to optimise the use of biowastes as fertilizers. For this reason, we investigated how different pretreatments affect the P dynamics when biofertilizers are placed in the soil. MethodsSewage sludge (SS), sewage sludge ash (SS-ash), meat and bone meal (MBM), and the solid fraction of biogas digestate (BGF) were pretreated with H2SO4, NaOH, and Ca(OH)2 and incubated for 2 and 12 days, respectively, in a one-dimensional reaction system for detailed studies of the interactions in the biomaterial-soil interface and the soil adjacent to the placement zone. ResultsOur results showed that acidification and treatment with NaOH increased the P solubility of the biomaterials. The P loss from the biomaterial layer to the soil was correlated with water-extractable P in the biomaterials (0.659) and water-extractable P in the soil (0.809). Acidification significantly increased the total amount of P depleted from the biomaterial to the soil whereas NaOH pre-treatment did not. However, for NaOH-treated SS and SS-ash, the apparent recoveries were significantly higher compared to the acidification due to a decrease in soil P sorption capacity as the soil pH increased due to residual alkalinity in the biomaterials. ConclusionsAcidification showed promising results by increasing the P solubility of all the biomaterials, and the alkalinization of SS and SS-ash with NaOH by increasing the apparent recovery in the soil. However, further studies are needed to assess the effects of these treatments on plant growth and P uptake.