Powder mixture with given molar ratio Ca/P = 1.67 consisting of brushite (calcium hydrophosphate dihydrate) CaHPO4·2H2O, calcium oxalate monohydrate CaC2O4·H2O in form of whewellite and weddellite and some quantity of quasi-amorphous phase was obtained as a result of the interaction of hydroxyapatite powder Ca10(PO4)6(OH)2 with an aqueous solution of oxalic acid H2C2O4 at a molar ratio of Ca10(PO4)6(OH)2/H2C2O4 = 1:4 under mechanical activation conditions. This powder mixture was used to produce microporous monophase ceramics based on hydroxyapatite Ca10(PO4)6(OH)2 with aperient density of 1.25 g/cm3 after firing at 1200 oC. Microporosity of sintered ceramics was formed due to presence of particles with plate-like morphology, restraining shrinkage during sintering. Microporous ceramics based on hydroxyapatite Ca10(PO4)6(OH)2 with roughness of the surface as a consequence of the created microporosity can be recommended as a biocompatible material for the bone defects treatment and as a substrate for bone cell cultivation.

Halophytic plants are, by definition, well adapted to saline soils. However, even halophytes can face nutritional imbalance and accumulation of high levels of compounds such as oxalic acid (OA), and nitrate (NO₃^¯). These compounds compromise the potential nutritional health benefits associated with salt tolerant plants such as Portulaca oleracea. Thus, preventing the accumulation of non-nutritional compounds will allow plants to be grown in saline conditions as crops. To this end, two ecotypes (ET and RN) of Portulaca oleracea plants were grown under growth room conditions with two levels of salinity (0, 50 mM NaCl) and three ratios of nitrate: ammonium (0:100%; 33:66%; 25:75% NO₃^¯:NH₄⁺). The results showed that both ecotypes exposed to elevated NO₃^¯, showed severe leaf chlorosis, high levels of OA, citric acid, and malic acid, while plants of ecotype ET exposed to elevated NH₄⁺ concentrations (33% and 75%) and 50 mM NaCl displayed a marked reduction in OA content, increased total chlorophyll and carotenoid contents, crude protein content, total fatty acid (TFA) and α-Linolenic acid (ALA) thus enhancing leaf quality. This opens the potential to grow high biomass, low OA P. oleracae crops. Lastly, our experiments suggest that ecotype ET copes with saline conditions and elevated NH₄⁺ through shifts in leaf metabolites.

The direct Cr (VI) reduction process by oxalic acid was conducted and the results showed that the Cr (VI) was efficiently reduced by oxalic acid at high reaction temperature and high dosage of oxalic acid. The reduced product, Cr (III), was easily generated stable complex compounds (Cr(HC2O4)3) with oxalate, which displayed a negative effect on the reduction process. The high reaction temperature and high acidic medium could destroy the stable structure of a complex compound to release oxalate, and facilitate the reduction of Cr (VI). Generally, the present study provided a versatile strategy for Cr (VI) reduction, exhibiting a bright application future for real wastewater treatment.

Oxalate is a divalent organic anion that has implications across many biological and commercial processes. It is derived from plant sources, such as spinach, rhubarb, tea, cacao, nuts, and beans, and therefore is commonly found in raw or processed food products. Oxalate can also be made endogenously by humans and other mammals as a byproduct of hepatic enzymatic reactions. It is theorized that oxalate is useful for plants to store calcium and protect against herbivory. Clinically, oxalate is best known to be a major component of kidney stones, which commonly contain calcium oxalate crystals. Oxalate can induce an inflammatory response that decreases the immune system’s ability to remove renal crystals. When formulated with platinum as oxaliplatin (an anticancer drug), oxalate has been proposed to cause neurotoxicity and nerve pain. There are many sectors of industry that are hampered by oxalate, and some that depend on it. For example, calcium oxalate is troublesome in the pulp industry and the alumina industry as it deposits on machinery. On the other hand, oxalate is a common active component of rust removal and cleaning products. Due to its ubiquity, there is interest in developing efficient methods to quantify oxalate. Over the past four decades several diverse methods have been reported. These approaches include electrode-based detection, liquid chromatography or gas chromatography coupled with mass spectrometry, enzymatic degradation of oxalate with oxalate oxidase and detection of hydrogen peroxide as a product, and indicator displacement-based methods employing fluorescent or UV light-absorbing compounds. It has become clear that no single method will work best for every purpose. This review describes the strengths and limitations of each method, and may serve as a reference for investigators to decide which approach is most suitable for their work.