Oxalic acid

About: Oxalic acid is a research topic. Over the lifetime, 11584 publications have been published within this topic receiving 173263 citations. The topic is also known as: ethanedioic acid & H2ox.

Mercury removal from aqueous solutions of HgCl2 by heterogeneous photocatalysis with TiO2

Abstract: The photocatalytic removal of Hg(II) from aqueous solutions of HgCl 2 using TiO 2 as catalyst was studied. The influence of pH and the addition of methanol, formic acid and oxalic acid as sacrificial additives on the extent of Hg(II) adsorption and photocatalytic reduction was investigated. The results showed that the overall process strongly depended on pH, being enhanced as the pH was increased. At pH 10, an efficient removal of Hg(II) was achieved even in the absence of organic additives, attaining final mercury concentrations in solution at trace levels (μg L −1 ). In acidic conditions, the addition of sacrificial organic molecules significantly increased the rate and extent of aqueous Hg(II) removal. The nature and distribution of mercury products deposited on the catalyst was dependent on the reaction conditions. In the absence of additives, Hg 2 Cl 2 and Hg 0 were respectively identified in acidic and neutral/alkaline media as main reduced species on the titania surface. The addition of organic additives enhanced the photocatalytic reduction to Hg 0 . Comparison between adsorption and reaction results evidenced that it cannot be established a direct correlation between Hg(II) dark adsorption on the TiO 2 surface and the efficiency of Hg(II) photoreduction achieved.

Dissociation quotients of oxalic acid in aqueous sodium chloride media to 175°C

Abstract: The first and second molal dissociation quotients of oxalic acid were measured potentiometrically in a concentration cell fitted with hydrogen electrodes. The emf of oxalic acid-bioxalate solutions was measured relative to an HCl standard solution from 25 to 125°C over 25o intervals at nine ionic strengths ranging from 0.1 to 5.0 molal (NaCl). The molal dissociation quotients and available literature data were treated in the all anionic form by a five-term equation that yielded the following thermodynamic quantities at infinite dilution and 25°C: logK1a=−1.277±0.010, ΔH 1a o =−4.1±1.1 kJ-mol−1, ΔS 1a o =38±4 J-K−1-mol−1, and ΔC p,1a o =−168±41 J-K−1-mol−1. Similar measurements of the bioxalate-oxalate system were made at 25o intervals from 0 to 175°C at seven ionic strengths from 0.1 to 5.0m. A similar regression of the experimentally-derived and published equilibrium quotients using a seven-term equation yielded the following values at infinite dilution and 25°C: logK2a=−4.275±0.006, ΔH 2a o =−6.8±0.5 kJ-mol−1, ΔS 2a o =−105±2 J-K−1-mol−1, and ΔC p,2a o =−261±12 J-K−1-mol−1.

Enhancement of uranium phytoaccumulation from contaminated soils

Abstract: Chelation and complexation of uranium (U) and soil acidification were evaluated as practical ways to solubilize, detoxify, and enhance U accumulation by plants. Sunflower (Helianthus annuus) and Indian mustard (Brassica juncea) were selected as potential U accumulators for U phytoextraction in one U mine tailing soil (469 mg U kg -1 ) and nine acid and calcareous soils (pH 4.7 to 8.1) contaminated with different rates (100 to 600 mg U(VI)kg -1 ) of uranyl nitrate (UO 2 (NO 3 ) 2 .6H 2 O). To enhance U phytoextraction, organic chelates were added to soils alone or as complexed-U forms of CDTA, DTPA, EDTA, and HEDTA, and citric and oxalic acids at rates of 1 to 25 mmol kg -1 , to soils with 4-week old seedlings. Dry matter production, U concentration in shoots and roots, and soil pH were measured. Contaminated soils were also evaluated for U desorption and by fractionation. Uranium desorption was performed with 2 to 20 mmol kg -1 of citric acid, CDTA, DTPA, and HEDTA. Uranium fractions [(exchangeable, carbonate, manganese (Mn), iron (Fe), organic, and residual)] were determined after 4 weeks of incubation. Plant dry matter production and U accumulation varied with soil contamination rate, chelate, organic acid form and rate, and soil type. The highest U concentration was in plants growing in calcareous soils and the lowest in clayey acid soils with high Fe and Mn oxides and organic matter content. Addition of citric and oxalic acids increased U accumulation and U translocation to the shoots significantly. Addition of 20 mmol of citric acid kg -1 to loamy acid soils reduced the soil pH to below 5.0 and increased U concentration in shoots to 1400 mg U kg -1 or by 150-fold, but addition of complexed-U forms had little effect on U translocation to shoots. Citric acid was the most effective chelate in desorption and plant accumulation of U. Uranium phytoacumulation was limited to acid soils with low adsorptive potential and to alkaline soils with carbonate minerals.