Solubility, structure, and morphology in the co-precipitation of cadmium and zinc with calcium-oxalate

Morphology and composition of calcium oxalate monohydrate phytoliths in the bark of Betula ermanii (stone birch): Case study from Sakhalin Island

The morphology of calcium oxalate monohydrate precipitates (COM, Ca(C2O4)·H2O, P21/c, whewellite) occurring as crystals or intergrowths, as well as distribution of crystal-bearing idioblasts, have been studied for the first time in the bark of stone birch Betula ermanii from Sakhalin Island sampled in an area affected by mud volcanism and an unaffected typical forest environment taken for reference. The study addresses several issues (i) number and size of phytoliths and their distribution in different cell types; (ii) density of calcification in specific cells; (iii) habits of single crystals, twins, and complex intergrowths, as well as frequency of different morphologies and their relations. The trends of time-dependent morphological changes in separately analyzed crystals and intergrowths record the evolution of COM morphology from nuclei to mature grains. Of special interest are the nucleation sites and features of organic and inorganic seeds and nuclei for COM phytoliths. The precipitation process and crystal habits are mainly controlled by supersaturation, and it is thus important to constrain the Ca distribution patterns in different bark tissues. The B. ermanii samples were analyzed by several methods: scanning electron microscopy (SEM) for the distribution patterns and micromorphology of COM precipitates and bulk Ca content in bark; electron probe microanalysis (EPMA) for the mineral chemistry of COM precipitates; inductively coupled plasma optical emission spectrometry (ICP-OES) and inductively coupled plasma mass spectrometry (ICP-MS) for trace elements in bulk bark and wood. RESEARCH HIGHLIGHTS: The distribution and morphology of whewellite precipitates in the analyzed B. ermanii bark samples indicate that the aqueous solution was most strongly supersaturated with respect to the Ca(C2O4)·H2O solid phase at the parenchyma-sclerenchyma boundary, where most of the COM spherulites are localized and often coexist with large single crystals and contact COM twins.

Nickel as a modifier of calcium oxalate: an in situ liquid cell TEM investigation of nucleation and growth

Despite extensive research on the nucleation and growth of calcium oxalate (CaOx) crystals, there are still several challenges and unknowns that remain. In particular, the role of trace metal elements in the promotion or inhibition of CaOx crystals is not well understood. In the present study, in situ graphene liquid cell transmission electron microscopy (in situ GLC TEM) was used to observe real-time, nanoscale transformations of CaOx crystals in the presence of nickel ions (Ni2+). The results showed that Ni2+ form Ni-water complexes, acting as a shape-directing species, generating a unique morphology and altering growth kinetics. Transient adsorption of Ni-water complexes resulted in a metastable phase formation of calcium oxalate trihydrate. Atomistic molecular dynamics simulations confirmed that Ni2+ acts as a weak inhibitor which slows down the CaOx crystallization, elucidating that Ni2+ impacts small-sized CaOx clusters by bringing more water into the clusters. This work highlighted the intricacies behind the effect of Ni2+ on CaOx biomineralization that were made possible to discern using in situ GLC TEM.

Calcium Oxalate Crystals, the Plant 'Gemstones': Insights into Their Synthesis and Physiological Implications in Plants

From simple algal forms to the most advanced angiosperms, calcium oxalate (CaOx) crystals (CRs) occur in the majority of taxonomic groups of photosynthetic organisms. Various studies have demonstrated that this biomineralization is not a simple or random event but a genetically regulated coordination between calcium uptake, oxalate (OX) synthesis and, sometimes, environmental stresses. Certainly, the occurrence of CaOx CRs is old; however, questions related to their genesis, biosynthesis, significance and genetics exhibit robust evolution. Moreover, their speculated roles in bulk calcium regulation, heavy metal/OX detoxification, light reflectance and photosynthesis, and protection against grazing and herbivory, besides other characteristics, are gaining much interest. Thus, it is imperative to understand their synthesis and regulation in relation to the ascribed key functions to reconstruct future perspectives in harnessing their potential to achieve nutritious and pest-resistant crops amid anticipated global climatic perturbations. This review critically addresses the basic and evolving concepts of the origin (and recycling), synthesis, significance, regulation and fate vis-à-vis various functional aspects of CaOx CRs in plants (and soil). Overall, insights and conceptual future directions present them as potential biominerals to address future climate-driven issues.

From soil to cacao bean: Unravelling the pathways of cadmium translocation in a high Cd accumulating cultivar of Theobroma cacao L

The research on strategies to reduce cadmium (Cd) accumulation in cacao beans is currently limited by a lack of understanding of the Cd transfer pathways within the cacao tree. Here, we elucidated the transfer of Cd from soil to the nib (seed) in a high Cd accumulating cacao cultivar. Here, we elucidated the transfer of Cd from soil to the nib (seed) in a high Cd accumulating cacao cultivar through Cd stable isotope fractionation, speciation (X-Ray Absorption Spectroscopy), and localization (Laser Ablation Inductively Coupled Plasma Mass Spectrometry). The plant Cd concentrations were 10-28 higher than the topsoil Cd concentrations and increased as placenta< nib< testa< pod husk< root< leaf< branch. The retention of Cd in the roots was low. Light Cd isotopes were retained in the roots whilst heavier Cd isotopes were transported to the shoots (Δ ^114/110 Cd _shoot-root = 0.27 ± 0.02 ‰ (weighted average ± standard deviation)). Leaf Cd isotopes were heavier than Cd in the branches (Δ ^114/110 Cd _{IF3 leaves-branch} = 0.18 ± 0.01 ‰), confirming typical trends observed in annual crops. Nibs and branches were statistically not distinguishable (Δ ^114/110 Cd _nib-branch = -0.08‰ ± 0.06 ‰), contrary to the leaves and nibs (Δ ^114/110 Cd _{nib-IF3 leaves} = -0.25‰ ± 0.05 ‰). These isotope fractionation patterns alluded to a more direct transfer from branches to nibs rather than from leaves to nibs. The largest fraction (57%) of total plant Cd was present in the branches where it was primarily bound to carboxyl-ligands (60-100%) and mainly localized in the phloem rays and phelloderm of the bark. Cadmium in the nibs was mainly bound to oxygen ligands (60-90%), with phytate as the most plausible ligand. The weight of evidence suggested that Cd was transferred like other nutrients from root to shoot and accumulated in the phloem rays and phelloderm of the branches to reduce the transfer to foliage. Finally, the data indicated that the main contribution of nib Cd was from the phloem tissues of the branch rather than from leaf remobilization. This study extended the limited knowledge on Cd accumulation in perennial, woody crops and revealed that the Cd pathways in cacao are markedly different than in annual crops.

Preparation of novel chemically bonded ceramics with steel slag and potassium hydrogen oxalate

A novel chemically bonded ceramic (novel-CBC) is prepared based on the acid-base reaction of alkali metals in steel slag (SS) and oxalate anion (C₂O₄^2-) in potassium hydrogen oxalate (PO). The effects of SS/PO ratio and water-solid (W/S) ratio on the setting and compressive strength of novel-CBC were studied in this paper. Reaction products and microstructure of novel-CBC were characterized by X-ray diffractometer (XRD), field emission scanning electron microscope-energy dispersive spectroscopy (FESEM-EDS) and thermogravimetric analysis/differential scanning calorimetry (TG/DSC). An optimal formula is obtained at a SS/PO ratio of 3.0 and a W/S ratio of 0.20, which starts setting at 10 min and gives the strengths of 18.0, 25.0, 39.8 and 49.0 MPa at 1, 3, 7 and 28 days, respectively. The reactants from SS are mainly Ca-bearing phases, while only a small amount of RO phase is involved in reaction. The main reaction products of novel-CBC are calcium oxalate monohydrate (CaC₂O₄·H₂O; whewellite) crystals and agglomerates consisting of K, Mg, Al, Si and O elements. The unreacted Ca-bearing phase particle and RO phase residue are embedded in a mixture of abundant CaC₂O₄·H₂O crystals with smooth surfaces and a size of 0.5-1.0 µm with large amounts of the nanoscale agglomerates.

Modulation of the calcium oxalate dihydrate to calcium oxalate monohydrate phase transition with citrate and zinc ions

Higher concentrations of Ca2+ and Ox2− can form COD which then transforms to COM. Citrate forms a protective layer to inhibit COD transition; whereas Zn2+ substitutes Ca2+ sites to generate a stable COD structure that retards COM formation.

Synthesis and Characterization of (Ca,Sr)[C2O4]∙nH2O Solid Solutions: Variations of Phase Composition, Crystal Morphologies and in Ionic Substitutions

To study strontium (Sr) incorporation into calcium oxalates (weddellite and whewellite), calcium-strontium oxalate solid solutions (Ca,Sr)[C2O4]∙nH2O (n = 1, 2) are synthesized and studied by a complex of methods: powder X-ray diffraction (PXRD), scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy. Two series of solid solutions, isomorphous (Ca,Sr)[C2O4]·(2.5 − x)H2O) (space group I4/m) and isodimorphous Ca[C2O4]·H2O(sp.gr. P21/c)–Sr[C2O4]·H2O(sp.gr. P 1 - ), are experimentally detected. The morphogenetic regularities of their crystallization are revealed. The factors controlling this process are discussed.

Lead Solubility and Mineral Structures of Coprecipitated Lead/Calcium Oxalates

Low-molecular-weight organic acids such as oxalate, which are ubiquitous in the environment, can control the solubility and bioavailability of toxic metals such as Pb in soils and water by influencing complexation and precipitation reactions. Here, we investigated Pb solubility in relation to Pb-oxalate precipitation at pH 5.0 in the absence and presence of calcium (Ca), a common cation in environmental matrices. At Pb mole fractions less than 0.10, sequestration of Pb into Ca oxalate to form a solid solution substantially lowered Pb solubility relative to that of pure Pb oxalate to an extent inversely proportional to the Pb mole fraction. Small Pb/Ca solid-solution distribution coefficients at these low mole ratios was largely attributed to the stronger complexation of Pb compared to Ca with oxalate to form soluble metal-oxalate complexes, which in turn limited Pb incorporation into the Ca-oxalate crystal lattice. Characterization of the Pb/Ca-oxalate coprecipitates by X-ray diffraction, optical microscopy, and Fourier transform infrared spectroscopy revealed that the whewellite (Ca-oxalate monohydrate) structure was destabilized by substitution of small amounts of Pb into the lattice, and thus, the formation of the Ca-oxalate dihydrate (weddellite) was favored over the monohydrate. At Pb mole fractions above 0.20, discrete crystallites of Pb oxalate were identified. These new findings imply that Pb/Ca-oxalate coprecipitates in the presence of Ca could reduce the solubility of Pb in Pb-contaminated acid soils.

Bioaugmentation-assisted phytoremediation of lead and salinity co-contaminated soil by Suaeda salsa and Trichoderma asperellum

The combined application of plant Suaeda salsa and indigenous fungus Trichoderma asperellum on the treatment of a lead (Pb) and salinity (Na⁺ and Ca²⁺) co-contaminated soil was investigated by a flowerpot experiment. As demonstrated by plant growth and selected antioxidant parameters, S. salsa was able to tolerate and grow in the co-contaminated soil, especially bioaugmented with T. asperellum, which promoted plant growth (9-23% and 5-13% increases for plant height and fresh weight, respectively) and appeared to alleviate plant oxidative damage (7-85% and 7-49% decreases for plant malondialdehyde and peroxidase levels, respectively). The SDS-PAGE fingerprints indicated that the total protein contents of S. salsa were affected under Pb and salinity stresses. The interactions of Na⁺ and Ca²⁺ ions on the phytotoxicity of Pb remained hormesis phenomenon that low-dose alleviation and high-dose enhancement. The analysis of phytoextraction parameters and bioavailability demonstrated that Pb was mainly concentrated in plant roots and poorly translocated, indicating the phytostabilization served as a major repair pathway. On the contrary, the Na⁺ and Ca²⁺ ions were concentrated in plant by the following order: shoot > root. Moreover, bioaugmentation of planted soil with T. asperellum generally led to the 9-42%, 13-58%, and 19-30% decreases of plant Pb, Na⁺, and Ca²⁺ concentrations and translocations, respectively, as well as a 6-21% decrease of soil Pb bioavailability. This study provided a bioaugmentation-assisted phytoremediation technique to make up the deficiencies of the long-term remediation for heavy metals and salinity.

Cadmium and zinc bioaccumulation by Phytolacca americana from hydroponic media and contaminated soils

Hydroponic, greenhouse and field experiments were conducted to explore the potential of pokeweed (Phytolacca americana L.) to accumulate Zn and Cd from nutrient solutions and contaminated soils. The hydroponic results confirmed that this native species is a strong Zn and Cd bioaccumulator that does not experience severe phytotoxicity until quite high root and shoot concentrations, approaching 4000 and 1600 mg kg^-1 of Zn, and 1500 and 500 mg kg^-1 of Cd, respectively. These high Zn and Cd concentrations were accompanied by increased sulfur and lower manganese in both shoots and roots. However, in field and greenhouse trials with soils historically contaminated by a number of heavy metals including Zn and Cd, concentrations of Zn and Cd in shoots of P. americana reached concentrations less than 30% and 10%, respectively, of those achieved with hydroponics. The main constraint to phytoremediation of soils by P. americana was the low concentrations of Zn and Cd in soil solution. Pretreatment of the metal-contaminated soil by oxalic acid increased soluble Cd and Zn but failed to increase plant uptake of either metal, a possible result of higher solubility of competing metal ions (Cu, Mn) or low bioavailability of Cd and Zn-oxalate complexes.

Anhydrous cadmium oxalate polymorphism: a first principle study

The structure of γ-CdC₂O₄ has been theoretically refined, XRD spectrum and properties are in excellent agreement with experiments.

Cadmium associates with oxalate in calcium oxalate crystals and competes with calcium for translocation to stems in the cadmium bioindicator Gomphrena claussenii

Cd binds to oxalate crystals, where it replaces Ca in the vacuoles of a bioindicator plant Gomphrena clausenii.