Study on green closed-loop regeneration of waste lithium iron phosphate based on oxalic acid system

The recovery of valuable metals from used lithium batteries is essential from an environmental and resource management standpoint. However, the most widely used acid leaching method causes significant ecological harm. Here, we proposed a method of recovering Li and Fe selectively from used lithium iron phosphate batteries by using low-concentration organic acid and completing the closed-loop regeneration. Low-concentration oxalic acid is used to carry out PO43-, which is significantly less soluble in aqueous solution than Li, two-stage selective leaching Li, where the leaching rate of Li reaches 99 %, and the leaching rate of Fe is only 2.4 %. The leach solution is then decontaminated. The solubility of Li3PO4 in aqueous solution is much smaller than that of Li2C2O4, which was required to recover Li to change the pH and Li can be recovered as Li3PO4; Fe can be retrieved as FeC2O4·2H2O, and re-prepared into lithium iron phosphate.

The hemostatic performance and mechanism of palygorskite with structural regulate by oxalic acid gradient leaching

Palygorskite (Pal) is a naturally available one-dimensional clay mineral, featuring rod-shaped morphology, nanoporous structure, permanent negative charges as well as abundant surface hydroxyl groups, exhibiting promising potential as a natural hemostatic material. In this study, the hemostatic performance and mechanisms of Pal were systematically investigated based on the structural regulate induced by oxalic acid (OA) gradient leaching from perspectives of structure, surface attributes and ion release.In vitroandin vivohemostasis evaluation showed that Pal with OA leaching for 1 h exhibited a superior blood procoagulant effect compared with the raw Pal as well as the others leached for prolonging time. This phenomenon might be ascribed to the synergistic effect of the intact nanorod-like morphology, the increase in the surface negative charge, the release of metal ions (Fe3+and Mg2+), and the improved blood affinity, which promoted the intrinsic coagulation pathway, the fibrinogenesis and the adhesion of blood cells, thereby accelerating the formation of robust blood clots. This work is expected to provide experimental and theoretical basis for the construction of hemostatic biomaterials based on clay minerals.

Fly ash treatment via conventional and microwave-assisted organic acid leaching: kinetics and life cycle assessment

Heedless disposal of oil-based fly ash contributes to the contamination of the air, water, and soil. Acid leaching of industrial solid wastes is recognized as a versatile, cost-effective, and environmentally friendly solid waste treatment approach. The present study investigated the viability of conventional leaching (CL) and microwave-assisted leaching (MAL) of predominant heavy metals from Mazut-burnt fly ash. For this purpose, the practicality of four organic acids with various specifications (ascorbic, gluconic, citric, and oxalic acids) on the dissolution efficiency of fly ash components was examined. Utilization of oxalic acid led to achieving full V recovery, complete Fe removal, and Ni enrichment in the residue in both CL and MAL setups. The Ni content of the sample was enriched from 6% in the calcinated sample to 23.7% in the oxalic acid leaching residue. Using citric acid resulted in the co-extraction of V, Ni, and Fe with nearly 70% V, 50% Ni, and 89% Fe dissolved in CL. The dissolution efficiencies were slightly lower in MAL. Oxalic acid was selected as the most promising organic acid reagent for fly ash treatment, so its CL kinetics was studied and defined by the shrinking particle model. The model showed that the controlling steps in the leaching of V differ over time, changing from a chemical reaction before 60 min to fluid film diffusion or mixing afterward. The kinetic study proved MAL as an effective technique in overcoming the leaching kinetic barriers. A life cycle assessment study was conducted to determine the environmental impacts of the proposed process. Accordingly, the MAL using oxalic acid was the most environmentally friendly process among the studied ones, and the utilization of microwaves leads to the reduction of the leaching processes' environmental impacts by decreasing the processing time.

Regulatory-systemic approach in Aspergillus niger for bioleaching improvement by controlling precipitation

In the context of e-waste recycling by fungal bioleaching, nickel and cobalt precipitate as toxic metals by oxalic acid, whereas organic acids, such as citric, act as a high-performance chelating agent in dissolving these metals. Oxalic acid elimination requires an excess and uneconomical carbon source concentration in culture media. To resolve this issue, a novel and straightforward systems metabolic engineering method was devised to switch metabolic flux from oxalic acid to citric acid. In this technique, the genome-scale metabolic model of Aspergillus niger was applied to predicting flux variability and key reactions through the calculation of multiple optimal solutions for cellular regulation. Accordingly, BRENDA regulators and a novel molecular docking-oriented approach were defined a regulatory medium for this end. Then, ligands were evaluated in fungal culture to assess their impact on organic acid production for bioleaching of copper and nickel from waste telecommunication printed circuit boards. The protein structure of oxaloacetate hydrolase was modeled based on homology modeling for molecular docking. Metformin, glutathione, and sodium fluoride were found to be effective as inhibitors of oxalic acid production, enabling the production of 8100 ppm citric acid by controlling cellular metabolism. Indirect bioleaching demonstrated that nickel did not precipitate, and the bioleaching efficiency of copper and nickel increased from 40% and 24% to 61% and 100%, respectively. Bioleaching efficiency was evaluated qualitatively by FE-SEM, EDX, mapping, and XRD analysis. KEY POINTS: • A regulatory-systemic procedure for controlling cellular metabolism was introduced • Metformin inhibited oxalic acid, leading to 8100 ppm citric acid production • Bioleaching of copper and nickel in TPCBs improved by 21% and 76.

Rhizosphere environmental factors regulated the cadmium adsorption by vermicompost: Influence of pH and low-molecular-weight organic acids

Vermicompost is a promising amendment for immobilization of cadmium (Cd) in soils; however, its effectiveness can be influenced by rhizosphere environment conditions, such as pH and the presence of low-molecular-weight organic acids (LMWOAs). In this study, a batch experiment was conducted to examine the characteristics of Cd adsorption by vermicompost at different pH (pH = 3, 5, and 7) and after the addition of different LMWOAs (oxalic acid; citric acid; malic acid). Furthermore, a series of morphology and structural analyses were conducted to elucidate the mechanisms of observed effects. The results showed that the adsorption capacity of vermicompost for Cd increased as pH increased, and chemisorption dominated the adsorption process. Changes in pH altered adsorption performance by affecting the -OH groups of alcohol/phenol and the -CH2 groups of aliphatics. Further, the addition of oxalic acid promoted Cd adsorption, and the effect was concentration dependent. Modifying the verimicompost surface with more adsorption sites might be the main reason. Conversely, citric acid and malic acid showed the ability to inhibit Cd adsorption by vermicompost. Citric acid caused a blocking effect by covering flocculent substances on the vermicompost surface while reducing surface adsorption sites by dissolving mineral components such as iron oxides. However, the action of malic acid did not appear to be related to changes in morphology or the structure of vermicompost. Overall, the results of this study partially explain the limited effectiveness of Cd immobilization within the rhizosphere by vermicompost, and provide theoretical support for regulating rhizosphere environments to improve the effectiveness of vermicompost immobilization of Cd.

Mechanism of catalytic ozonation in expanded graphite aqueous suspension for the degradation of organic acids

In this study, expanded graphite (EG) was prepared by the oxidation and intercalation of the natural flake graphite using perchloric acid and potassium permanganate at different expansion temperatures (300, 400, 500, and 600°C), and were characterized by scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS). EG prepared at 500°C was found to be highly effective for the mineralization of oxalic acid aqueous solution during ozonation at pH 3, which was ascribed to the formation of hydroxyl radicals from the surface reaction of surface hydroxyl groups on EG with ozone. The performance of expanded graphite in this catalytic system was basically unchanged after three repeated use. The presence of Cl^-, SO42-, HPO42-/H2PO4- and NO3- could inhibit the degradation of oxalic acid in catalytic ozonation with EG. Degradations of oxamic acid and pyruvic acid in catalytic ozonation with EG were pH-dependent, which were lower than that of oxalic acid. The degradations of oxalic acid and oxamic acid were identified as mineralization process by the determination of TOC, while pyruvic acid may transform into organic products such as acetic acid by O₃/EG. Manganese ion (Mn²⁺) could promote the degradation of oxalic acid by O₃/EG at pH 3 because permanganate was produced by O₃/EG in oxalic acid solution and then reacted with oxalic acid readily at acidic pH. Catalytic ozonation by EG exhibited great application potential for the destruction of refractory organic compounds.

Facile and rapid determination of oxalic acid by fading spectrophotometry based on Fe(III)-sulfosalicylate as colorimetric chemosensor

Spectrophotometry is an economic and rapid method for detecting oxalic acid (OA), while the reported methods have some drawbacks, such as narrow linear range, long response time, delicate operation and required expensive reagents. Herein, we found that the as-synthesized Fe(III)-sulfosalicylate (FeSSA) could be used as an efficient colorimetric chemosensor to detect OA, and the established FeSSA-based fading spectrophotometry showed prominent advantages over the existing ones in detecting OA. The as-established method has wider linear range of 0.80-160 mg/L with regression coefficient ≥ 0.999, while the widest linear range is just 2.7-54 mg/L among the reported ones. Moreover, the method has low limit of detection (0.74 mg/L), extremely fast response (several seconds), satisfactory selectivity, high accuracy and precision. Most importantly, its reliability was further verified by employing it to determine OA concentration during the degradation process of organic pollutants. The measured OA concentration at any time interval was perfectly consistent with those determined by the well-recognized high performance liquid chromatography (HPLC). These confirmed that the FeSSA-based fading spectrophotometry is an efficient, simple, fast, accurate and economic method to determine OA in a wide concentration range.

Oxalic acid enhances bioremediation of Cr(VI) contaminated soil using Penicillium oxalicum SL2

Oxalic acid is the most abundant low molecular weight organic acid (LMWOA) in many environments and offers enormous prospects for treating Cr(VI) contamination. In this study, laboratory batch experiments were conducted to estimate the roles of oxalic acid in Cr(VI) removal by Penicillium oxalicum SL2. Oxalic acid changed the initial pH and provided a suitable condition for the growth of strain SL2 when the penicillium was applied to bioremediation of Cr(VI) contamination in alkaline soil. Gompertz model analysis indicated that initial pH affected the lag time of the growth curve of strain SL2. Scanning electron microscopy and scanning transmission X-ray microscopy analysis showed strain SL2 sufficiently contacted with contaminated soil and reduced Cr(VI) to Cr(III) in the hyphae. The results suggested that oxalic acid could enhance the bioremediation efficiency of strain SL2 though improving chromium bioleaching from the contaminated soil and strengthening Cr(VI) removal in the leaching solution. This study provided oxalic acid as a green reagent for stimulating Cr(VI) removal by strain SL2 and would expand knowledge on the roles of LMWOA in Cr(VI) bioremediation.

Manipulating Selectivity of Hydroxyl Radical Generation by Single-Atom Catalysts in Catalytic Ozonation: Surface or Solution

Hydroxyl radical-dominated oxidation in catalytic ozonation is, in particular, important in water treatment scenarios for removing organic contaminants, but the mechanism about ozone-based radical oxidation processes is still unclear. Here, we prepared a series of transitional metal (Co, Mn, Ni) single-atom catalysts (SACs) anchored on graphitic carbon nitride to accelerate ozone decomposition and produce highly reactive ·OH for oxidative destruction of a water pollutant, oxalic acid (OA). We experimentally observed that, depending on the metal type, OA oxidation occurred dominantly either in the bulk phase, which was the case for the Mn catalyst, or via a combination of the bulk phase and surface reaction, which was the case for the Co catalyst. We further performed density functional theory simulations and in situ X-ray absorption spectroscopy to propose that the ozone activation pathway differs depending on the oxygen binding energy of metal, primarily due to differential adsorption of O₃ onto metal sites and differential coordination configuration of a key intermediate species, *OO, which is collectively responsible for the observed differences in oxidation mechanisms and kinetics.

Behaviors of Organic Ligands and Phosphate during Biochar-Driven Nitrate Adsorption in the Presence of Low-Molecular-Weight Organic Acids

A batch experiment was conducted to examine the behavior of nitrate, organic ligands, and phosphate in the co-presence of biochar and three common low-molecular-weight organic acids (LMWOAs). The results show that citrate, oxalate, and malate ions competed with nitrate ion for the available adsorption sites on the biochar surfaces. The removal rate of LMWOA ligands by the biochar via adsorption grew with increasing solution pH. The adsorbed divalent organic ligands created negatively charged sites to allow binding of cationic metal nitrate complexes. A higher degree of biochar surface protonation does not necessarily enhance nitrate adsorption. More acidic conditions formed under a higher dose of LMWOAs tended to make organic ligands predominantly in monovalent forms and failed to create negatively charged sites to bind cationic metal nitrate complexes. This could adversely affect nitrate removal efficiency in the investigated systems. LMWOAs caused significant release of phosphate from the biochar. The phosphate in the malic acid treatment tended to decrease over time, while the opposite was observed in the citric- and oxalic-acid treatments. This was caused by re-immobilization of phosphate in the former due to the marked increase in solution pH over time.

High-efficiency stabilization of lead in contaminated soil by thermal-organic acid-activated phosphate rock

Phosphate rock powder (PR) has been shown to possess the potential to stabilize lead (Pb) in soil. Most of the phosphorus (P) minerals in the world are low-grade ores, making it difficult to achieve the expected stabilization effect on heavy metals. This study compared the changes in the phase composition and structure of PR and three kinds of activated phosphate rock powder (APR) (organic acid-activated PR, thermal-activated PR, and thermal-organic acid-activated PR). The stabilization effectiveness of APR on Pb-contaminated soil was evaluated by toxicity leaching procedure; the Pb products adsorbed on APR and stabilization mechanism of APR on Pb were analyzed. The results demonstrated that APR showed decreased crystallinity and 3.4-fold increase in specific surface area, and a 53.07% and 49.32% increase in soluble P content in oxalic acid-activated PR and citric acid-activated PR, respectively, when compared with those of PR. These changes improved the stabilization effect of APR on Pb-contaminated soil, in which oxalic acid-600 °C-activated PR showed the best effect, presenting 94.0-99.8% reduction in Pb leaching concentration following addition of 2-10% modifier. Product characterization after Pb adsorption on APR showed that Pb was adsorbed onto APR by forming fluoropyromophite precipitation with APR.

Oxalic acid activated bone meal for immobilization of Pb and Cd contaminated soils

Bone meal (BM) is a cost-effective and low-carbon material to remediate heavy metal contaminated soils. Moreover, its immobilization efficiency for heavy metals still requires improvement. This study aimed to assess the activation effect of oxalic acid on the BM to develop an oxalic acid-activated bone meal (ABM) for improving immobilization efficiency. Several series of tests, including the available phosphorus content test, toxicity characteristic leaching procedure (TCLP), modified European Community Bureau of Reference (BCR) sequential extraction procedure, and X-ray diffraction (XRD) analysis, are used to investigate the effect of activation on the immobilization ability and chemical speciation of lead (Pb) and cadmium (Cd) in soils and the different mechanisms of Pb/Cd immobilization using the ABM and BM. The results indicate that the ABM possesses a higher solubility than the BM. The activation of BM achieves optimal effect when using 1 mol/L oxalic acid solution with a liquid-solid ratio of 2:1. The TCLP and BCR test results show that the ABM significantly outperforms the BM in terms of Pb immobilization. The leaching concentration of Pb from ABM immobilized soils can meet regulatory limits in China and the USA, and it is also 30 to 75% lower than that from BM immobilized soils. Regarding Cd immobilization, ABM outperforms BM after 90 days of curing. The XRD analysis shows that heavy metal phosphates are the primary products of Pb and Cd immobilized by ABM, whereas heavy metal carbonates are the main products after the immobilization by BM.

Biochar-driven reduction of As(V) and Cr(VI): Effects of pyrolysis temperature and low-molecular-weight organic acids

Batch experiments were conducted to examine the differential effects of biochar pyrolysis temperature and low-molecular-weight organic acids on the reduction of As(V) and Cr(VI) driven by Pennisetum hydridum biochar. The results showed that pyrolysis temperature significantly affected the reducing strength of the biochar. Biochar produced at 500 °C had a stronger electron-donating capacity than did the biochars produced at 300 and 700 °C. In the co-presence of the biochar and a low-molecular-weight organic acid, arsenic and chromium behaved differently. Oxalic acid and malic acid tended to have better effects on enhancing biochar-driven Cr(VI) reduction, as compared to citric acid while the opposite was observed for biochar-driven As(V) reduction. Biochar produced at 300 °C was more favourable for Cr(VI) reduction, as compared to the higher-temperature biochars while the opposite was observed for As(V) reduction in the presence of low-molecular-weight organic acids. This may make the lower-temperature biochar ideal for remediating contaminated soils containing both As(V) and Cr(VI) since it could maximize Cr(VI) reduction while minimizing As(V) reduction.

Chemical structure, chain conformation and rheological properties of pectic polysaccharides from soy hulls

We obtained a new acidic soy hull polysaccharide (SHP-1) with a molecular weight (Mw) of 4.81 × 105 g/mol through ammonium oxalate and microwave assisted extraction. SHP-1 was mainly composed of galacturonic acid, galactose, rhamnose and arabinose (molar ratio = 46.59%:17.95%:14.77%:13.97%) with small amounts of fucose, glucose, mannose and xylose. The chemical structure was presumed to be of pectin-I type, consisting of 2/3 HGA and 1/3 RG-I. Furthermore, the rheological information and the chain morphology of SHP-1 were different in five solvents. Surfactant, salt and alkali solutions enhanced the solubility and flexibility of the polysaccharide, but the polysaccharide showed decreased fluidity under acidic conditions. The addition of ions and alkali increased the consistency coefficient of the solution, but the effect was far less than that of the cross-linking morphology. The structural and morphological information of purified SHP should aid in further study of its structure-function relationships and applications.

Formation of metal-organic ligand complexes affects solubility of metals in airborne particles at an urban site in the Po valley

Metals in atmospheric aerosols play potentially an important role in human health and ocean primary productivity. However, the lack of knowledge about solubility and speciation of metal ions in the particles or after solubilisation in aqueous media (sea or surface waters, cloud or rain droplets, biological fluids) limits our understanding of the underlying physico-chemical processes. In this work, a wide range of metals, their soluble fractions, and inorganic/organic compounds contained in urban particulate matter (PM) from Padua (Italy) were determined. Metal solubility tests have been performed by dissolving the PM in water and in solutions simulating rain droplet composition. The water-soluble fractions of the metal ions and of the organic compounds having ligand properties have been subjected to a multivariate statistical procedure, in order to elucidate associations among the aqueous concentrations of these PM components in simulated rain droplets. In parallel, a multi-dimensional speciation calculation has been performed to identify the stoichiometry and the amount of metal-ligand complexes theoretically expected in aqueous solutions. Both approaches showed that the solubility and the aqueous speciation of metal ions were differently affected by the presence of inorganic and organic ligands in the PM. The solubility of Al, Cr, and Fe was strongly correlated to the concentrations of oxalic acid, as their oxalate complexes represented the expected dominant species in aqueous solutions. Oxalates of Al represented ∼98% of soluble Al, while oxalates of Cu represented 34-75% of the soluble Cu, and oxalates of Fe represented 76% of soluble Fe. The oxidation state of Fe can strongly impact the speciation picture. If Fe is present as Fe(II) rather than Fe(III), the amount of Cr and Cu complexed with diacids can increase from 75% to 94%, and from 32% to 53%, respectively. For other metals, the solubility depended on the formation of soluble aquo-complexes, hence with a scarce effect of the organic ligands. An iron-oxalate complex was also directly detected in aerosol sample extracts.

A one-step acidification strategy for sewage sludge dewatering with oxalic acid

Sewage sludge dewatering is an efficient approach to reduce the volume of sludge for the subsequent disposal. In this study, a novel one-step acidification sludge dewatering method was developed with using oxalic acid as a conditioner. In laboratory-scale experiments with the dosage of 200 mg/g dry solid (DS), the normalized capillary suction time and the specific resistance to filtration were respectively decreased by 78.7% and 60.0% after 30 min of oxalic acid conditioning, much more efficient than those conditioned with sulfuric acid and hydrochloric acid at the same pH value. This superior dewatering performance was attributed to two factors. One was that oxalic acid could more efficiently promote the hydrolysis of polysaccharide, especially pectins, to release bound water. The other was that OA could dissolve more Fe³⁺ and Al³⁺, as well as form precipitate with Ca²⁺ in sludge, which may act as flocculants or co-precipitator for the subsequent sludge particles coagulation. In pilot-scale experiments, the water content of oxalic acid conditioned sludge cake was reduced to 60% under the optimum conditions, while the reagent cost was as low as 110.0 USD/t DS. This work provides a cost-effective and easy-operated sewage sludge disposal technique, and also sheds light on the potential of oxalic acid in environmental waste treatment.

Elucidation of the mechanisms into effects of organic acids on soil fertility, cadmium speciation and ecotoxicity in contaminated soil

The remediation effect of organic acids in heavy metal contaminated soil was widely studied. However, the comprehensive evaluation of organic acids on micro-ecological environment in heavy metal contaminated soil was less known. Herein, this experiment was conducted to investigate the impact of malic acid, citric acid and oxalic acid on soil fertility, cadmium (Cd) speciation and ecotoxicity in contaminated soil. Especially, to evaluate the ecotoxicity of Cd, high-throughput sequencing was used to investigate the soil bacterial community structure and diversity after incubation with organic acids. The results showed that obvious changes in soil pH were not observed. Whereas, the contents of available phosphorus (Olsen-P) and alkali hydrolysable nitrogen (Alkeline-N) evidently increased with a significant difference. Furthermore, compared to control, the proportion of acetic acid-extractable Cd increased by 3.06-6.63%, 6.11-9.43% and 1.91-6.22% respectively in the groups amended with malic acid, citric acid and oxalic acid, which indicated that citric acid did better in improving the availability of Cd than malic acid and oxalic acid. In terms of biological properties, citric acid did best in bacteria count increase, enzyme activities and bacterial community structure improvement. Accordingly, these results provided a better understanding for the influence of organic acids on the micro-ecological environment in Cd contaminated soil, based on physicochemical and biological analysis.

Effect of biodegradable chelators on induced phytoextraction of uranium- and cadmium- contaminated soil by Zebrina pendula Schnizl

This study investigated the effect of ethylenediamine-N,N'-disuccinic acid (EDDS), oxalic acid (OA), and citric acid (CA) on phytoextraction of U- and Cd-contaminated soil by Z. pendula. In this study, the biomass of tested plant inhibited significantly following treatment with the high concentration (7.5 mmol·kg-1) EDDS treatment. Maximum U and Cd concentration in the single plant was observed with the 5 mmol·kg-1 CA and 7.5 mmol·kg-1 EDDS treatment, respectively, whereas OA treatments had the lowest U and Cd uptake. The translocation factors of U and Cd reached the maximum in the 5 mmol·kg-1 EDDS. The maximum bioaccumulation of U and Cd in the single plants was 1032.14 µg and 816.87 µg following treatment with 5 mmol·kg-1 CA treatment, which was 6.60- and 1.72-fold of the control groups, respectively. Furthermore, the resultant rank order for available U and Cd content in the soil was CA > EDDS > OA (U) and EDDS > CA > OA (Cd). These results suggested that CA could greater improve the capacity of phytoextraction using Z. pendula in U- and Cd- contaminated soils.

The critical role of the surface iron-oxalate complexing species in determining photochemical degradation of norfloxacin using different iron oxides

In the past decades, in-situ generation of reactive oxygen species (ROS) in the photochemical iron oxides/oxalate system (UV/IOs/Ox) has drawn a lot of attentions, while understanding the reaction mechanism upon the solid-liquid surface complexing behaviors/species is still scarcely. In this study, comparative degradation of norfloxacin (NOR) was investigated in the UV/IOs/Ox systems adopting four different common iron oxides. It was found that the type of IOs would lead to rather different NOR degradation patterns following the order of goethite (α-FeOOH) > hematite (α-Fe₂O₃) > maghemite (γ-Fe₂O₃) ≈ magnetite (Fe₃O₄). •OH was the main ROS and effects of parameters (dosage of IOs, NOR and Ox, pH) on the pseudo-first-order k_obs(NOR) were evaluated in the four systems. Results showed that the surface structures of IOs instead of catalyst amounts would be more responsible for the degradation efficiency. Evolutions of •OH, H₂O₂ and Fe²⁺ indicated the surface interfacial reactions would also contribute for the NOR degradation, but depended on the type of IOs. ATR-FTIR examinations demonstrated that catalytic activity of IOs correlated highly (R² = 0.999) with the amounts of bidentate mononuclear iron-Ox surface complex but correlated poorly with the amounts of monodentate mononuclear and outer sphere complexes. Different intrinsic properties of IOs such as the species of surface hydroxyl groups (-OH) would result in changing the proportion of the three surface complexes. Besides, the minor reduction transformation of NOR by the carbon dioxide anion radical (CO₂^•-) was verified by density function theory (DFT) calculation, further confirmed the involvement of both •OH and CO₂^•- for NOR degradation.

Can biochar and oxalic acid alleviate the toxicity stress caused by polycyclic aromatic hydrocarbons in soil microbial communities?

It remains unclear whether biochar amendment can mediate changes in soil microbial communities caused by organic contaminants in the rhizosphere. In this study, phenanthrene-contaminated soil was amended with biochar and oxalic acid (OA) alone or in combination and incubated for 21 days. Phospholipid fatty acids (PLFAs) and high-throughput sequencing were used to evaluate shifts in bacterial and fungal community structure. Phenanthrene stress led to significant shifts in both soil bacterial and fungal community structure, in particularly, 82% of microbial phyla decreased in abundance. Biochar and/or OA improved the phenanthrene-polluted soil by positively mediating shifts in soil microbial communities stressed by phenanthrene. Specifically, biochar and/or OA led to the survival of certain microbial taxa that were inhibited by phenanthrene stress. In addition, many functional microbial individuals and genes participating in polycyclic aromatic hydrocarbon (PAH) degradation were positively stimulated by high phenanthrene stress and further stimulated by the simultaneous application of biochar and OA. Based on these findings, tandem biochar and rhizoremediation may be a feasible strategy for relieving PAH toxicity to soil microbial communities.

Recycling of the scrap LCD panels by converting into the InBO3 nanostructure product

Preparation of the value-added products from e-waste resources is an important step in the recycling process. The present paper aims to propose a methodology for the recovery of In from scrap LCD panel via preparation of InBO₃ nanostructure. Discarded LCD panel was subjected to a recycling process through crushing, milling, and oxalic acid leaching to prepare In₂(C₂O₄)₃·6H₂O. Through the leaching process, B(OH)₃ from glass part (alumina borosilicate) has been leached out along with indium oxalate hydrated. Further thermal treatment on these extracted materials at 600 °C could result in the formation of InBO₃ nanostructures with an average particle size of 20 nm. A multistep mechanism based on thermodynamic calculations for the recycling of the InBO₃ form extracted precursors was proposed. Graphical abstract.

Fabrication and characterization of a hierarchical porous carbon from corn straw-derived hydrochar for atrazine removal: efficiency and interface mechanisms

The excellent hierarchical porous carbon was fabricated from corn straw-derived hydrochar by chemical activation using potassium oxalate (K₂C₂O₄). SEM, BET, XPS, XRD, and Raman analysis were carried out for the characterization of the as-obtained samples. The morphology of the as-obtained porous carbon with hierarchical porous structures is made up of a large number of nano-particle aggregates and some nanosheet-like structures, possessing a super-large specific surface area (SSA, up to 2523 m² g^-1) with a large total pore volume of 1.464 cm³ g^-1. The as-fabricated carbon material rapidly removes atrazine in the first 3 h at the initial concentration of 20 mg L^-1 with an adsorption efficiency of 93.6%, which is faster and better than other representative materials reported previously. The acidic conditions are favorable for the atrazine adsorption onto the porous carbon. An efficient adsorbent was fabricated for environmental remediation, and in-depth insights into the interface mechanism between hierarchical porous carbon and atrazine are proposed. In addition, 95% of the adsorption capacity of MPC-1:3 can be recovered by simple annealing treatment.

Band gap tuning of g-C3N4 via decoration with AgCl to expedite the photocatalytic degradation and mineralization of oxalic acid

A series of functional organic-metal AgCl-decorated graphitic carbon nitride (AgCl-CN_x) composites were synthesized and applied for the degradation of oxalic acid (OA) under visible light. The highest photocatalytic activity was achieved with AgCl decoration ratio of 1.0 (denoted as AgCl-CN_1.0). The pseudo-first-order constant for OA degradation was 0.0722 min^-1 with the mineralization efficiency of 90.80% after 60 min reaction in the photocatalytic process with AgCl-CN_1.0. A variety of characterization techniques including Brunauer-Emmett-Teller, X-ray diffraction, scanning electron microscope, transmission electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectra, ultraviolet-visible diffuse reflectance spectra, photoluminescence, and Mott-Schottky were utilized to elucidate the physicochemical, microstructure, and optical properties contributing to the improvement of the photocatalytic performance. The results showed that AgCl-CN_1.0 had an oblate flaky erythrocyte-like structure with a moderate band gap energy of ~3.00 eV. In addition, the effects of the key parameters (i.e., AgCl-CN_1.0 dosage, initial OA concentration, solution pH, and presence of natural organic matter) on OA degradation were systematically investigated. Radical scavenger experiments indicated that photogenerated holes, electrons, superoxide anion radicals, and hydroxyl radicals were the dominant reactive species. Moreover, AgCl-CN_1.0 exhibited excellent stability and reusability for OA degradation without detectable Ag⁺ release in the solution over multiple reaction cycles. The efficient OA mineralization could be mainly ascribed to the moderate specific surface area, increased numbers of active sites, and effective interfacial charge transfer of AgCl-CN_1.0. Overall, the AgCl-CN_1.0 composite was demonstrated to be a highly efficient, stable, and recoverable photocatalyst.

Effects of oxalic acid on Cr(VI) reduction by phenols in ice

Since Cr(VI) is highly toxic, the environmental reduction of Cr(VI) to Cr(III) has attracted significant attention. Oxalic acid, a primary component of dissolved organic matter (DOM), is widely distributed throughout the natural environment but the reduction of Cr(VI) by oxalic acid is insignificant at the low concentrations present in the environment; however, the reduction of Cr(VI) is accelerated significantly in ice. In terms of combined pollution, Cr(VI) can coexist with other organic pollutants in the environment but the impact of organic pollutants on the reduction of Cr(VI), changes to the organic pollutants themselves, and the role of oxalic acid in these reactions are unknown. In this study, we investigated redox reactions between Cr(VI) and phenolic compounds in ice (- 15 °C) in the presence of oxalic acid and compared these to room temperature redox reactions in aqueous solutions (20 °C). While these redox reactions were negligible in aqueous solution, they were significantly accelerated in ice under acidic conditions, which was primarily attributed to the freeze concentration effect. Oxalic acid has two functions in these redox reactions; the first is to provide the H⁺ required for the reaction and the second is to serve as a reducing agent. When oxalic acid and phenolic pollutants coexist, Cr(VI) preferentially reacts with the phenolic compounds. Phenol, 4-chlorophenol (4-CP), and 2,4-dichlorophenol (2,4-DCP) were each demonstrated to reduce Cr(VI) in ice, but the reaction rate and overall reactivity of these three phenolic compounds are different.

Hydrological tracers, the herbicide metazachlor and its transformation products in a retention pond during transient flow conditions

Since decades, surface water bodies have been exposed to pesticides from agriculture. In many places, retention systems are regarded as an important mitigation strategy to lower pesticide pollution. Hence, the processes governing the transport of pesticides in and through a retention system have to be understood to achieve sufficient pesticide attenuation. In this study, the temporal dynamics of metazachlor and its transformation products metazachlor-oxalic acid (OA) and -sulphonic acid (ESA) were observed in an agricultural retention pond and hydrologic tracers helped to understand system-inherent processes. Pesticide measurements were carried out for 80 days after their application during transient flow conditions. During a short-term (3 days) experiment, the tracers bromide, uranine and sulphorhodamine B were used to determine hydraulic conditions, residence times and sorption potential. A long-term experiment with sodium naphthionate (2 months) and isotopes (12 months) provided information about inputs via interflow and surface-groundwater interactions. During transient conditions, high concentration pulses of up to 35 μg L^-1 metazachlor, 14.7 μg L^-1 OA and 22.5 μg L^-1 ESA were quantified that enduringly raised solute concentrations in the pond. Mean residence time in the system accounted for approximately 4 h showing first tracer breakthrough after 5 min and last tracer concentrations 72 h after injection. While input via interflow was confirmed, no evidence for surface-groundwater interaction was found. Different tracers illustrated potentials for sorption and photolytic degradation inside the system. This study shows that high-resolution sampling is essential to obtain robust results about retention efficiency and that hydrological tracers may be used to determine the governing processes.

Effects of soil environmental factors and UV aging on Cu2+ adsorption on microplastics

Microplastics (MPs) in natural environments have attracted lots of attention. Although the quantity of MPs present in terrene is much higher than that in aquatic environment, few studies have investigated the chemical behavior of MPs in terrestrial environment. This study investigate the Cu²⁺ (as a model heavy metal) adsorption capacity of six kinds of MPs (polyamide-6 (PA), polyethylene (PE), polystyrene (PS), polyethylene terephthalate (PET), polyvinyl chloride (PVC), polymethyl methacrylate (PMMA)) in batch adsorption experiments and the effects of different soil environmental factors, including pH and the presence of cations and low-molecular-weight organic acids (LMWOAs), as well as ultraviolet (UV) aging. The Cu²⁺ adsorption capacities of PA and PMMA were higher than those of other MPs and their maximum equilibrium adsorption capacities (estimated by the Langmuir adsorption equation) were 323.6 μg/g ± 38.2 and 41.03 ± 1.78 μg/g, respectively. The Cu²⁺ adsorption on MPs was affected by pH, and the greatest amount of Cu²⁺ adsorbed on PA and PMMA was observed at pH = 6 and pH = 7, respectively. The presence of Ca²⁺ or Mg²⁺ inhibited Cu²⁺ adsorption by MPs, due to competition for the adsorption sites. Moreover, Cu²⁺ adsorption by MPs was affected by various types of LMWOAs. The Cu²⁺ adsorption on PA was significantly reduced by citric acid, followed by oxalic acid, and oxalic acid was particularly evident for Cu²⁺ adsorption on PMMA. UV aging (200 h) had different effect on Cu²⁺ adsorption on MPs and it depends on the change of carbonyl index. Results demonstrate that soil environmental factors can change the ability of different MPs to adsorb Cu²⁺ and affect the transport of pollutants as carriers.

Oxalic acid activated phosphate rock and bone meal to immobilize Cu and Pb in mine soils

The contamination of soil by copper (Cu) and lead (Pb) is a serious concern because of its high health risk via the food chain. Oxalic acid-activated phosphate rock (APR) and bone meal (BM) were applied to Cu and Pb co-contaminated soil to investigate their efficacy in the immobilization of Cu and Pb. APR and BM were applied into the contaminated soil (158.8 mg/kg total Pb and 573.2 mg/kg Cu) at four levels of dosages (0.1%, 0.5%, 2%, and 4%) and incubated for one year. The results demonstrated that the acid exchangeable Pb fraction in the soil treated with APR and BM decreased compared to the control, while there was no noticeable change in the acid-exchangeable Cu fraction in the soil treated with either APR or BM. Meanwhile, the application of BM and APR increased the fraction of residual Cu and Pb in the polluted soils. Moreover, the addition of either APR or BM at the dose of 4% decreased the concentrations of CaCl₂-extractable Cu and Pb in the amended soil, and the percentages of that reduction in the APR amended soils were 56% and 91% and in BM amended soils were 67% and 64%, respectively. The immobilization of Cu and Pb by APR and BM might be induced by the increased soil pH and soluble P contents in the amended soils. In general, BM is more effective than APR on the immobilization of Cu in polluted soil, while APR had greater efficiency than BM on the immobilization of Pb when the levels of amendments were above 2%.

Organic ligand induced release of vanadium from the dissolution of stone coal oxide ore

The effects of low-molecular-weight dissolved organic matters (LMWDOMs) on the release of vanadium (V) under environmental conditions are part of a broader study on the environmental geochemistry behavior of V. Eight typical naturally occurring LMWDOMs with carboxyl, hydroxyl, and amidogen groups were chosen: citric acid, oxalic acid, EDTA, salicylic acid, catechol, glycine, cysteine, and glucose. The results showed that the release of V was largely promoted by LMWDOMs with carboxyl functional groups under acidic conditions and with catechol under basic conditions. In the presence of citric acid, oxalic acid, or EDTA at pH 4.0, the initial release rates of V were approximately 25-39 times greater than the rates in the control experiments; the steady release rates were 164, 95, and 49 times than the rates in the control experiments, respectively. For catechol, the release rate at pH 8.0 was approximately 20 times the rate at pH 4.0. Amino acids and alcohols had a minimal effect on the release of V. Ligand-promoted release rates of V were found primarily due to the faster detachment of surface complexes, the protonated sites from the mineral surface and the reduction of dissolved V (V) in the presence of citric acid, oxalic acid, EDTA, and catechol. This study helps understand the pollution risk of V in some mine areas and the fate of V in the environment.

Fabrication of Ag-doped ZnO by mechanochemical combustion method and their application into photocatalytic Famotidine degradation

Ag/ZnO nanocomposites are successfully synthesized at different Ag contents through simple, effective, high yield and low-cost mechanochemical combustion technique, with the addition of silver acetate to zinc acetate and oxalic acid mixture. The synthesized materials are characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron spectroscopy (SEM), BET surface area analysis, UV - visible diffuse reflectance spectroscopy (UV - DRS) and photoluminescence spectroscopy (PL). It is shown that the prepared nanocomposites are composed of metallic Ag⁰ and wurtzite ZnO. The photocatalytic performance of different composites is evaluated by the degradation of Famotidine (FMT) under UV irradiation. The results indicate it that the maximum photodegradation rate is obtained with 6 wt% metallic Ag-decorated ZnO, and it is 2.1 times better than that obtained with pure ZnO. The photocatalytic degradation of FMT with Ag/ZnO is affected by various parameters such as calcination temperature and time, doping concentrations and reusability. The Ag/ZnO demonstrates higher activity due to the reduction of electron - hole recombination and Ag⁰ metal catalyst. The possible photocatalytic degradation mechanism of FMT with Ag/ZnO is estimated from the scavenger test.

Multi-step column leaching using low-molecular-weight organic acids for remediating vanadium- and chromium-contaminated soil

In soil, vanadium (V) contamination is commonly concomitant with chromium (Cr) contamination, which poses potential risks to humans, animals, and plants due to the transfer of toxic metals and the increase in their concentrations via the food chain or through direct exposure. This study applied a multi-step column leaching process using low-molecular-weight organic acids (LMWOAs) to treat V-contaminated soil from a smelter site that contains 2015.1 mg V kg^-1 and 1060.3 mg Cr kg^-1. After leaching three times with an equivalent solution/soil ratio of 0.3 mL/g using 1.0 M oxalic acid solution, the total removal rates reached 77.2% and 7.2% for V and Cr, respectively, while the removal rates of the extractable fractions reached 118.6% and 99.2% due to the reduction in residual fraction (F₄) of toxic metals. Simultaneously, the distribution and redistribution of geochemical fractions of V and Cr were determined with a sequential extraction technique, and the greater proportion of potential mobile fractions of V (65.1%) may increase its leaching from soil relative to Cr (7.1%). In addition, a lower pH of the leaching agent increased the efficiency of the leaching process to an extent. Compared with batch extraction with a typical solution to soil ratio of 10 mL/g, multi-step column leaching used less agent and hence produced less wastewater. This strategy could reduce the mobilization and bioavailability of toxic metals, and potentially enhance in situ soil flushing for the remediation of V- and Cr- contaminated soil.

Hierarchical biomimetic BiVO4 for the treatment of pharmaceutical wastewater in visible-light photocatalytic ozonation

Photocatalytic ozonation is an attractive advanced oxidation process for wastewater treatment, but highly active catalysts with strong response to visible light are urgently needed to push forward its practical application. In this study, a hierarchical biomimetic monoclinic bismuth vanadate (BiVO₄) with leaves morphology was synthesized by a hydrothermal method, and employed as catalyst for oxalic acid and penicillin degradation in photocatalytic ozonation. The results show that the organics degradation was more efficient using leaves shaped BiVO₄ as catalyst than the bulk shaped one in photocatalytic ozonation and the synergy index is ranged from 2.8 to 3.3, indicating a superior positive synergistic effect between photocatalysis and ozonation. The higher activity of the leaves shaped BiVO₄ was probably attributed to the distinctive biomimetic morphology and preferable band structure with more negative CB potential. Mechanism studies suggested that the main reactive species were h⁺ and OH for the degradation of persistent oxalic acid in photocatalytic ozonation. In addition, the effect of ozone concentration and inorganic ions and reusability of the material were also intensively investigated.

Oxalate-enhanced solubility of lead (Pb) in the presence of phosphate: pH control on mineral precipitation

Here we study the precipitation of lead (Pb)-phosphate minerals over the pH range of 4.0 to 8.0 with and without oxalate, a ubiquitous and abundant low-molecular-weight organic acid derived from plants and microorganisms in environmental matrices. In the aqueous Pb-phosphate systems, phosphate precipitated Pb efficiently, reducing the dissolved Pb concentration below 1 μM at all the tested pH values, with the minimum solubility of about 0.1 μM measured at the intermediate pH of 6.0. The measured dissolved Pb and free Pb2+ ion activity were not in agreement with predictions from generally-accepted solubility products of the Pb phosphate minerals, particularly hydroxypyromorphite [Pb5(PO4)3OH]. Discrepancies between our measured Pb phosphate solubility products and older reported values are attributed to non-ideal behavior of these minerals (incongruent dissolution) as well as uncertainties in stability constants for soluble Pb-phosphate ion pairs. The presence of equimolar levels of oxalate and phosphate resulted in up to 250-fold increase in Pb solubility at acidic pH and about a 4-fold increase at pH 7.0, due to the strong suppression of Pb phosphate precipitation by oxalate and formation of soluble Pb-oxalate complexes. At pH 4.0 and 5.0, Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) identified a Pb-oxalate mineral phase as the only precipitate despite the presence of phosphate; in the absence of oxalate, Pb hydrogen phosphate, PbHPO4, stably formed under these acidic conditions. At pH 6.0 and greater, FTIR and XRD data revealed that Pb-phosphate [Pb3(PO4)2], and hydroxypyromorphite [Pb5(PO4)3OH] to a lesser extent, were the predominant precipitates both in the absence and presence of oxalate. Therefore, oxalate did not strongly interfere with Pb-phosphate mineral formation at aqueous pH greater than 6.0 but oxalate controlled Pb solubility at acidic pH values.

Combined effects of maize straw biochar and oxalic acid on the dissipation of polycyclic aromatic hydrocarbons and microbial community structures in soil: A mechanistic study

Whether the rhizodegradation of organic contaminants occurs in biochar- amended soil and its potential mechanisms have rarely been reported. Therefore, a study was conducted to investigate the combined effects of root exudates and biochar on the dissipation of polycyclic aromatic hydrocarbons (PAHs) and on the microbial community structures in soil. As a major component of the root exudates of ryegrass, oxalic acid (OA) significantly enhanced the dissipation of high- and low-ring PAHs in the studied soil with or without maize straw biochar amendment (p < 0.05). However, biochar alone enhanced only the dissipation of high-ring PAHs. The activities of three enzymes (urease, polyphenol oxidase and dehydrogenase) were the highest in soil amended with both maize straw biochar and 0.5 mg kg^-1 of OA. Moreover, soil microbial biomass and the abundances of genera and genes associated with PAH degradation were significantly enhanced with the tandem application of biochar and OA (p < 0.05). These changes led to a synergetic effect of biochar and OA on the shifts in microbial community structures and on the dissipation of PAHs, especially for high-ring PAHs. The results in this study suggested that a combined biochar-rhizosphere approach should be a feasible remediation strategy for PAH-contaminated soil.

Differential release of sewage sludge biochar-borne elements by common low-molecular-weight organic acids

Biochar materials originated from sewage sludge may contain elevated levels of potentially toxic elements. There was a lack of information on the mobility of biochar-borne elements, as driven by low-molecular-weight organic acids (LMWOAs) contained in plant root exudates. A batch experiment was conducted to examine the effects of three common LMWOAs on the release of major elements and trace elements with a focus on various potentially toxic trace elements. The results showed that substantial amounts of Al, Mn, Fe, K, Na and Mg were extracted from two sewage sludge-derived biochar materials by the LMWOAs. A much higher release rate of potentially toxic trace elements was observed in the presence of LMWOAs, as compared to reported data using extractants not encountered in root exudates. The LMWOA-driven releasibility of various potentially toxic trace elements was in the following decreasing order: Zn > Ni > Pb > Cu > Cr >Co = Cd. Other trace elements that are subject to mobilization in the presence of LMWOAs included B, Ba, In, Li and Sr except Ba under oxalic acid extraction. Among the three LMWOAs, oxalic acid showed a generally stronger capacity to mobilize these metals. The findings obtained from this study provides new information that can be used for better evaluating the phyto-availability of trace elements bound to sewage sludge-originated biochar materials.

Arsenic mobilization from iron oxides in the presence of oxalic acid under hydrodynamic conditions

Oxalic acid potentially enhances pump-and-treat for groundwater As remediation by accelerating mobilization. This study examines how oxalic acid mobilizes As from Fe(III)-oxide coated sand under hydrodynamic conditions. Four columns were packed with metal-substituted ferrihydrite or goethite to 1% Fe, presorbed to 50% As surface coverage, and reacted with pH = 2.2 artificial groundwater amended with 10 mM oxalic acid at 1 m day^-1. Arsenic elution was affected by both As and Fe speciation. Although the As(V) columns experienced faster substrate dissolution, As(V) elution was delayed by re-adsorption, whereas As(III) elution was rapid due to pH decrease that prevented re-adsorption. Cr-ferrihydrite and Ni-goethite dissolved both effectively initially but then diverged. The Cr-ferrihydrite columns experienced continuous stoichiometric Fe and Cr release, and As release could be sustained. The Ni-goethite columns initially experienced nonstoichiometric Fe and Ni release, and As release was extensive. Such release, however, was not sustained. We hypothesized that Ni-goethite contained sites with distinct reactivity, and oxalic acid targeted readily-dissolved, sorption-dense sites. Our data indicate that oxalic acid-enhanced pump-and-treat methods would be easier to apply to aquifers dominated by As(III), requiring less amendment to be injected; such oxalic acid-enhanced methods remove reactive sediment Fe and As, potentially preventing future groundwater contamination.

Competitive sequestration of Ni(II) and Eu(III) on montmorillonite: role of molar Ni:Eu ratios and coexisting oxalate

The competitive binding trends of Ni(II) and Eu(III) on montmorillonite in the absence/presence of Na-oxalate are explored by using batch sorption/desorption technique, speciation modeling, and X-ray diffraction (XRD) analysis. With a series of molar Ni:Eu ratios (i.e., 1:1, 5:1, 10:1, 1:5, and 1:10), the coexisting Ni(II) did not affect the sequestration behaviors and immobilization mechanisms of Eu(III). In contrast, the presence of Eu(III) obviously suppressed the sorption percentages of Ni(II) in the acidic pH range. Even though no obvious influence of Eu(III) on the macroscopic binding trends of Ni(II) was observed under alkaline conditions, the fraction of Ni(II) adsorbed by the inner-sphere complexation mechanism decreased and that of Ni(II) precipitation increased with rising molar Ni:Eu ratio. The coexisting Na-oxalate did not influence Eu(III) sorption, while inhibited the sorption of Ni(II). The XRD analysis indicated the potential formation of two Eu-oxalate precipitate phases (i.e., Eu₂(C₂O₄)₃·xH₂O(s)-1 and Eu₂(C₂O₄)₃·xH₂O(s)-2) at different pH values (4.0 and 6.5) and Na-oxalate concentrations (ranging from 0.5 to 5.0 mM). Interestingly, the Eu₂(C₂O₄)₃·xH₂O(s)-2 phase would be transformed into the Eu₂(C₂O₄)₃·xH₂O(s)-1 solid with the increase of Na-oxalate concentration. The research findings could provide essential data for evaluating the fate of coexistent Eu(III) and Ni(II) in the complicated aquatic environment.

Feasibility of nanoscale zero-valent iron to enhance the removal efficiencies of heavy metals from polluted soils by organic acids

Soil washing with natural chelators to remediate metal-contaminated soils has been gained attention by researchers. However, the abilities of the chelators to remediate the multiple metal polluted soils are less effective. This study employed zero-valent iron nanoparticle (nZVI) to enhance the removal efficiencies of citric (CA), tartaric (TA) and oxalic acids (OA), and evaluate their feasibility. Results showed that metal removal efficiencies increased with the increasing concentration of nZVI and soil-liquid ratio, decreased with the increasing solution pH. The kinetic simulation indicated that pseudo-first-order and pseudo-second-order models could be used for describing the washing processes. Additionally, metal removals were significantly improved by addition of nZVI (p < 0.05). The highest enhancements of soil Cd, Pb and Zn removals under solution pH of 4.0, soil-liquid ratio of 1:20 and washing time of 120 min reached 12.83% (OA- nZVI), 24.92% (CA-nZVI) and 11.64% (OA- nZVI) for mine soil, and 19.24% (TA- nZVI), 18.16% (CA-nZVI) and 8.93% (OA- nZVI) for farmland soil, respectively. After soil washing, the exchangeable forms and the environmental risks of residual metals were markedly diminished in soils. Therefore, the combinations of the organic acids and nZVI are the feasible practices to repair the soils contaminated by heavy metals.

Investigation on the hygroscopicity of oxalic acid and atmospherically relevant oxalate salts under sub- and supersaturated conditions

Oxalic acid (OxA) is an end product in the oxidation of many organic compounds, and therefore is ubiquitous in the atmosphere and is often the most abundant organic species in ambient aerosols. To better understand the hygroscopic properties of OxA under sub- and supersaturated conditions in the atmosphere, we investigated the hygroscopic growth and cloud condensation nuclei (CCN) activation ability of pure OxA and its salts using a hygroscopic tandem differential mobility analyzer (HTDMA) and cloud condensation nuclei counter (CCNC), respectively. OxA particles absorb water under >45% RH, suggesting that the initial phase state might be an amorphous solid. The measured hygroscopic growth factor (HGF) of OxA at 90% RH was 1.47. We found that the HGF of ammonium oxalate (NH4-Ox) was larger than that of OxA, whereas HGFs of sodium, calcium, and magnesium oxalates (Na-Ox, Ca-Ox, and Mg-Ox) were smaller than that of OxA particles. Potassium oxalate (K-Ox) behaved like a typical water-soluble inorganic salt, exhibiting deliquescence and efflorescence transitions at around 85% and 50% RH, respectively. Na-Ox exhibited strong activation capabilities among all the investigated salts, followed by NH4-Ox and K-Ox as inferred from the activation ratios (CCN/CN) against supersaturations (SS). On the other hand, Ca-Ox showed moderate activation ability and Mg-Ox showed poor CCN activation ability. We also observed significantly higher κCCN values compared to κHTDMA for pure OxA and its salts (NH4-Ox and Na-Ox), suggesting that the condensation of OxA into the aqueous phase occurs during water uptake. These findings improve the fundamental understanding of hygroscopic behaviors and phase states of oxalic acid and its salts under sub- and supersaturated conditions in the atmosphere and impacts of hygroscopicity on the direct and indirect effects of aerosol particles.

Choline Chloride-Oxalic Acid as a Deep Eutectic Solvent-Based Innovative Digestion Method for the Determination of Selenium and Arsenic in Fish Samples

An innovative and effective digestion method based on choline chloride (ChCl)-oxalic acid (Ox) deep eutectic solvent (DES) was proposed for the determination of Se and As in fish samples via electrothermal atomic absorption spectrometry (ETAAS). The impacts of different variables, including the composition and volume of ChCl-Ox, temperature, and acid addition, on analyte recovery were studied for optimization. In this procedure, an 80 mg sample was dissolved in a 1:2 molar ratio of ChCl-Ox at 105°C for 40 min, with the subsequent addition of 4.0 mL HNO3 (1.0 M) and further heating at the same temperature for about 5 min. Next, centrifugation was applied, and the supernatant solution was filtered, diluted to a known volume, and measured by ETAAS. The accuracy of the developed method was tested using a Standard Reference Material (NIST SRM 1946 Lake Superior Fish Tissue). The proposed DES-based digestion method was successfully applied to the simultaneous extraction of Se and As from fish samples.

Innovative findings about ferrous oxalate dihydrate crystallization in simulated dihydrate phosphoric acid product

The basic fundamentals of ferrous oxalate dihydrate (FeC₂O₄.2H₂O) crystallization including supersaturation, nucleation and crystal growth in simulated dihydrate phosphoric acid product with and without cetyl pyridinium chloride (CPC) additive were studied. Oxalic acid and ferrous sulfate heptahydrate crystals were mixed with dilute phosphoric acid (28% P₂O₅) at 60 °C and the turbidity of the reaction mixture was measured at different time intervals. Induction time of ferrous oxalate dihydrate crystals was calculated at different supersaturation ratios ranging from 2.5 to 6.7. With increasing the supersaturation ratio, the induction time decreased. The nucleation rates are 46.4 × 10²⁸ nuclei cm^-3 s^-1 and 50.2 × 10²⁸ nuclei cm^-3 s^-1 at supersaturation ratio 6.7 with and without CPC addition, respectively. The surface energy increases with CPC addition compared to the baseline. In addition, the formed crystals are modified from cubic shape to rod-like shape with increasing CPC dose.

Effects of Fe(III)-fulvic acid on Cu removal via adsorption versus coprecipitation

This study compared the sorption and extractability of Cu following adsorption (SOR) and coprecipitation(CPT). The effect of solution pH, Fe: organic carbon (OC) ratios and fulvic acid (FA) on the combined removal of Cu was investigated in the batch tests using Fe(III) precipitates as a sorbent. Transmission electron microscope (TEM) images demonstrated that the coexisting FA reduced the particle size of ferrihydrites as expected. Generally, more Cu was eliminated in coprecipitation compared with adsorption and the dissolved Cu left in solutions decreased as the pH increased, most of dissolved Cu was trapped at pH 6 and above. Meanwhile, the inhibition or promotion of Cu removal really depended on the different Fe: OC ratios. The addition of FA led to a further decrease of Cu concentrations in CPT systems with Fe/OC ratio of 1:3, however, Cu removal was hindered in the presence of FA in SOR systems. In the case of extraction experiments, the addition of l-malic acid (MA), oxalic acid (OA) and citric acid (CA) resulted in lower extractability of coprecipitated Cu than adsorption samples. The gaps in extractions were seemed to be a consequence of tight Cu binding in CPT products, and the more feasible desorption of Cu from the surface of SOR samples. Based on the results of Cu adsorption and coprecipitation, coprecipitation of Cu with ferrihydrites was the more effective Cu sequestration mechanism in the removal of Cu. These results are helpful to understand the complicated interactions among Fe(III), FA and Cu (II) in the natural environment.

Mineralization of organic pollutants by anodic oxidation using reactive electrochemical membrane synthesized from carbothermal reduction of TiO2

Reactive Electrochemical Membrane (REM) prepared from carbothermal reduction of TiO₂ is used for the mineralization of biorefractory pollutants during filtration operation. The mixture of Ti₄O₇ and Ti₅O₉ Magnéli phases ensures the high reactivity of the membrane for organic compound oxidation through ^•OH mediated oxidation and direct electron transfer. In cross-flow filtration mode, convection-enhanced mass transport of pollutants can be achieved from the high membrane permeability (3300 LMH bar^-1). Mineralization efficiency of oxalic acid, paracetamol and phenol was assessed as regards to current density, transmembrane pressure and feed concentration. Unprecedented high removal rates of total organic carbon and mineralization current efficiency were achieved after a single passage through the REM, e.g. 47 g m^-2 h^-1 - 72% and 6.7 g m^-2 h^-1 - 47% for oxalic acid and paracetamol, respectively, at 15 mA cm^-2. However, two mechanisms have to be considered for optimization of the process. When the TOC flux is too high with respect to the current density, aromatic compounds polymerize in the REM layer where only direct electron transfer occurs. This phenomenon decreases the oxidation efficiency and/or increases REM fouling. Besides, O₂ bubbles sweeping at high permeate flux promotes O₂ gas generation, with adverse effect on oxidation efficiency.

Siderophore-mediated iron removal from chrysotile: Implications for asbestos toxicity reduction and bioremediation

Asbestos fibers are highly toxic (Group 1 carcinogen) due to their high aspect ratio, durability, and the presence of iron. In nature, plants, fungi, and microorganisms release exudates, which can alter the physical and chemical properties of soil minerals including asbestos minerals. We examined whether exudates from bacteria and fungi at environmentally relevant concentrations can alter chrysotile, the most widely used asbestos mineral, and lower its toxicity. We monitored the release of iron from chrysotile in the presence of organic acid ligands and iron-specific siderophores derived from bacteria and fungi and measured any change in fiber toxicity toward peritoneal macrophages harvested from mice. Both fungal and bacterial siderophores increased the removal of iron from asbestos fibers. In contrast, organic acid ligands at environmentally relevant concentrations neither released iron from fibers nor helped in siderophore-mediated iron removal. Removal of plant-available or exchangeable iron did not diminish iron dissolution by both types of siderophores, which indicates that siderophores can effectively remove structural iron from chrysotile fibers. Removal of iron by siderophore lowered the fiber toxicity; fungal siderophore appears to be more effective than bacterial siderophore in lowering the toxicity. These results indicate that prolonged exposure to siderophores, not organic acids, in the soil environment decreases asbestos fiber toxicity and possibly lowers the health risks. Thus, bioremediation should be explored as a viable strategy to manage asbestos-contaminated sites such as Brownfield sites, which are currently left untreated despite dangers to surrounding communities.

The mineralization of oxalic acid and bio-treated coking wastewater by catalytic ozonation using nickel oxide

Coking wastewater after biological treatment still possesses potential environmental risk and should be mineralized further. This work focused on the mineralization of bio-treated coking wastewater using catalytic ozonation by NiO. First, oxalic acid, the typical by-product of advanced oxidation process (AOPs), was used to test the catalytic performance of NiOs, prepared by modified hydrothermal methods upon addition of different surfactants. This demonstrated that NiO upon addition of hexadecyltrimethylammonium (CTAB) had the best catalytic activity, due to its high concentration surface hydroxyl density and strong stability. Moreover, the best NiO was applied for the catalytic ozonation of bio-treated coking wastewater. Under our experimental conditions, the total organic carbon (TOC) removal reached 100% after 420 min. In addition, the spectroscopic analysis suggested that compounds with conjugated structures could be significantly removed by both ozonation and catalytic ozonation. Some of these substances were transformed into by-products with aliphatic C-C and O=C-O groups such as organic acids that can inhibit further mineralization.

Enhanced stabilization of Pb, Zn, and Cd in contaminated soils using oxalic acid-activated phosphate rocks

Phosphate amendments, especially phosphate rock (PR), are one of the most commonly used materials to stabilize heavy metals in contaminated soils. However, most of PR reserve consists of low-grade ore, which limits the efficiency of PR for stabilizing heavy metals. This study was to enhance the stabilization of heavy metals through improving the available phosphorous (P) release of PR by oxalic acid activation. Raw PR and activated PR (APR) were characterized by scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS), X-ray powder diffraction (XRD), Brunauer-Emmett-Teller (BET) surface analysis, and laser diffraction to determine the changes of structure and composition of APR. The stabilization effectiveness of lead (Pb), zinc (Zn), and cadmium (Cd) in soils by APR was investigated through toxicity leaching test and speciation analysis. The results indicated that after treatment by oxalic acid, (1) the crystallinity of the fluorapatite phase of PR transformed into the weddellite phase; (2) the surface area of PR increased by 37%; (3) the particle size of PR became homogenized (20-70 μm); and (4) the available P content in PR increased by 22 times. These changes of physicochemical characteristics of PR induced that APR was more effective to transform soil heavy metals from the non-residual fraction to the residual fraction and enhance the stabilization efficiency of Pb, Zn, and Cd than PR. These results are significant for the future use of low-grade PR to stabilize heavy metals.

Effect of low-molecular-weight organic acids on hematite dissolution promoted by desferrioxamine B

Siderophores, as strong chelators specific to iron, have been intensively studied in relation to the facilitation of biological iron acquisition from iron oxides. In this study, the dissolution of hematite in the presence of the siderophore desferrioxamine B (DFOB) and three low-molecular-weight organic acids (LMWOAs, i.e., oxalic, citric, or malic acid) was investigated at three pH conditions (3.0, 5.5, and 9.0). Hematite dissolution was pH-dependent and LMWOA-specific. The adsorption of DFOB on hematite was significantly higher at pH 9.0 than at the other pH values. The adsorption of oxalic acid on hematite, however, showed a descending trend as pH was increased, and adsorption of citric and malic acids was not significantly affected by pH. The Fourier transform infrared (FTIR) results also indicated the occurrence of these ligands' adsorption. After acidification, dissolved iron was detected only in suspensions of hematite pre-adsorbed with oxalic acid at pH 5.5 and 9.0 or pre-adsorbed with citric acid at pH 5.5, indicating that these LMWOAs promoted the formation of labile iron on the hematite surface. Based on previous research and the results of this study, a hypothetical model is proposed. These results provide insight into the effect of LMWOAs on the dissolution of hematite promoted by DFOB.

Fluorine-doped carbon nanotubes as an efficient metal-free catalyst for destruction of organic pollutants in catalytic ozonation

Metal-free carbon materials have been presented to be potential alternatives to metal-based catalysts for heterogeneous catalytic ozonation, yet the catalytic performance still needs to be enhanced. Doping carbon with non-metallic heteroatoms (e.g., N, B, and F) could alter the electronic structure and electrochemical properties of original carbon materials, has been considered to be an effective method for improving the catalytic activity of carbon materials. Herein, fluorine-doped carbon nanotubes (F-CNTs) were synthesized via a facile method and characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. The as-synthesized F-CNTs exhibited notably enhanced catalytic activity towards catalytic ozonation for the degradation of organic pollutants. The oxalic acid removal efficiency of optimized F-CNTs was approximately two times as much as that of pristine CNTs, and even exceeded those of four conventional metal-based catalysts (ZnO, Al₂O₃, Fe₂O₃, and MnO₂). The XPS and Raman studies confirmed that the covalent CF bonds were formed at the sp³ C sites instead of sp² C sites on CNTs, not only resulting in high positive charge density of C atoms adjacent to F atoms, but remaining the delocalized π-system with intact carbon structure of F-CNTs, which then favored the conversion of ozone molecules (O₃) into reactive oxygen species (ROS) and contributed to the high oxalic acid removal efficiency. Furthermore, electron spin resonance (ESR) studies revealed that superoxide radicals (O₂^-) and singlet oxygen (¹O₂) might be the dominant ROS that responsible for the degradation of oxalic acid in these catalytic systems.

Highly Simple Deep Eutectic Solvent Extraction of Manganese in Vegetable Samples Prior to Its ICP-OES Analysis

In the present work, simple and sensitive extraction methods for selective determination of manganese have been successfully developed. The methods were based on solubilization of manganese in deep eutectic solvent medium. Three deep eutectic solvents with choline chloride (vitamin B4) and tartaric/oxalic/citric acids have been prepared. Extraction parameters were optimized with using standard reference material (1573a tomato leaves). The quantitative recovery values were obtained with 1.25 g/L sample to deep eutectic solvent (DES) volume, at 95 °C for 2 h. The limit of detection was found as 0.50, 0.34, and 1.23 μg/L for DES/tartaric, DES/oxalic, and DES/citric acid, respectively. At optimum conditions, the analytical signal was linear for the range of 10-3000 μg/L for all studied DESs with the correlation coefficient >0.99. The extraction methods were applied to different real samples such as basil herb, spinach, dill, and cucumber barks. The known amount of manganese was spiked to samples, and good recovery results were obtained.

Development of phosphate rock integrated with iron amendment for simultaneous immobilization of Zn and Cr(VI) in an electroplating contaminated soil

This study aims to develop an amendment for simultaneous immobilization of Zn and Cr(VI) in an abandoned electroplating contaminated soil. Nature phosphate rock was first activated with oxalic acid (O-PR) and then combined with FeSO₄ or zero-valent iron (ZVI) for immobilization of Zn and Cr(VI) from aqueous solutions. Finally, the optimized approach showing the highest immobilization ability in solution was applied in an electroplating contaminated soil. The O-PR combined with FeSO₄ was more effective in simultaneously removing Zn and Cr(VI) than the O-PR integrated with ZVI within the tested solution pH range of 5.5-8.5. Both O-PR with FeSO₄ and with ZVI removed over 95% of Zn from the solution; however, only 42-46% of Cr(VI) was immobilized by O-PR with ZVI, while O-PR with FeSO₄ almost precipitated all Cr(VI). Moreover, there were 75-95% Zn and 95-100% Cr(VI) remaining in the exhausted O-PR with FeSO₄ solid after toxicity characteristic leaching test (TCLP) while the exhausted O-PR with ZVI solid only retained 44-83% Zn and 32-72% Cr(VI). Zinc was immobilized mainly via formation of insoluble Fe-Zn phosphate co-precipitates, while iron-induced reduction of Cr(VI) into stable Cr(OH)₃ or Cr_xFe_(1-x)(OH)₃ was responsible for Cr(VI) immobilization. Application of the O-PR integrated with FeSO₄ in the electroplating contaminated soil rapidly reduced the TCLP extractable Zn and Cr(VI) to below the standard limits, with decrease by 50% and 94%, respectively. This study revealed that combination of oxalic acid activated phosphate rock with FeSO₄ could be an effective amendment for remediation of Zn and Cr(VI) contaminated soil.

Selecting the best AOP for isoxazolyl penicillins degradation as a function of water characteristics: Effects of pH, chemical nature of additives and pollutant concentration

To provide new insights toward the selection of the most suitable AOP for isoxazolyl penicillins elimination, the degradation of dicloxacillin, a isoxazolyl penicillin model, was studied using different advanced oxidation processes (AOPs): ultrasound (US), photo-Fenton (UV/H₂O₂/Fe²⁺) and TiO₂ photocatalysis (UV/TiO₂). Although all processes achieved total removal of the antibiotic and antimicrobial activity, and increased the biodegradability level of the solutions, significant differences concerning the mineralization extend, the pH of the solution, the pollutant concentration and the chemical nature of additives were found. UV/TiO₂ reached almost complete mineralization; while ∼10% mineralization was obtained for UV/H₂O₂/Fe²⁺ and practically zero for US. Effect of initial pH, mineral natural water and the presence of organic (glucose, 2-propanol and oxalic acid) were then investigated. UV/H₂O₂/Fe²⁺ and US processes were improved in acidic media, while natural pH favored UV/TiO₂ system. According to both the nature of the added organic compound and the process, inhibition, no effect or enhancement of the degradation rate was observed. The degradation in natural mineral water showed contrasting results according to the antibiotic concentration: US process was enhanced at low concentration of dicloxacillin followed by detrimental effects at high substrate concentrations. A contrary effect was observed during photo-Fenton, while UV/TiO₂ was inhibited in all of cases. Finally, a schema illustrating the enhancement or inhibiting effects of water matrix is proposed as a tool for selecting the best process for isoxazolyl penicillins degradation.