Presence Of Low-molecular-weight Organic Acids Research Articles

Low molecular weight organic acids stabilise siderite against oxidation and influence the composition of iron (oxyhydr)oxide oxidation products.

Siderite (FeCO3) is an important reservoir of mineral-bound ferrous iron in non-sulfidic, reducing soils and sediments. It is redox sensitive, and its oxidation may facilitate the reduction of a range of pollutants, produce reactive oxygen species, or induce the formation of oxidation products with large surface areas for contaminant sorption. However, there is currently a limited understanding of the stability of siderite in complex environments such as soils and sediments. Here, we use a series of batch experiments complemented with thorough characterisation of mineral oxidation products to investigate the oxidation of siderite in the presence and absence of the low molecular weight organic acids (LMWOAs) citrate, tiron, salicylate, and EDTA as analogues for naturally occurring compounds or functional groups of natural organic matter that ubiquitously coexist with siderite. Our results show that siderite alone at pH 7.5 was completely oxidised to form ferrihydrite, nanocrystalline lepidocrocite, and nanocrystalline goethite in less than 6 hours. However, in the presence of LMWOAs, up to 48% of the siderite was preserved for more than 500 hours and the formation of goethite was inhibited in favour of ferrihydrite and lepidocrocite. Using experimental data from electron microscopy and chemical speciation modelling, we hypothesise that the siderite may be preserved through the formation of an Fe(III)-passivation layer at the siderite surface.

The Dissolution Mechanism of Low-Molecular-Weight Organic Acids on the Sillimanite.

The interaction between low-molecular-weight organic acids (LMWOAs) and minerals in nature has been widely studied; however, limited research has been conducted on the dissolution mechanism of sillimanite in the presence of different organic acids. In this study, the interaction between the sillimanite sample and LMWOAs (citric acid, oxalic acid, and citric/oxalic mixture) at the same pH was investigated. The dissolution rate of Si and Al was high during the initial reaction time, then slowed down in the presence of LMWOAs. The dissolution data for Si and Al from sillimanite in the LMWOAs fit well with the first-order equation (Ct = a(1 - exp(-kt))) (R2 > 0.991). The dissolution process of sillimanite in the organic acids was controlled by the surface chemical reaction step. The dissolution concentration of Si in aqueous citric acid was higher than that in oxalic acid. In contrast, the dissolution concentration of Al in oxalic acid was more than that in citric acid. The maximum concentrations of Si and Al in the presence of composite organic acids were 1754 μmol/L and 3904 μmol/L. The sillimanite before and after treatment with LMWOAs were studied using X-ray diffraction (XRD) and scan electron microscopy (SEM). These results are explained by the characterization of the sillimanite. Under the single acid solution, the (210) crystal plane with a high areal density of Al in sillimanite was easily dissolved by the oxalic acid, while the (120) in sillimanite with a high areal density of Si was more easily dissolved by citric acid. In the composite organic acids, the Si-O bond and Al-O bond in sillimanite were attacked alternately, leading to the formation of some deeper corrosion pits on the surface of sillimanite. The results are of interest in the dissolution mechanisms of sillimanite in the low-molecular-weight organic acids and the environmentally friendly activation of sillimanite.

Effects of low molecular weight organic acids with different functional groups on arsenate adsorption on birnessite

In an aquatic ecosystem, especially constructed wetlands receiving arsenic (As)-containing wastewater, the fate and mobility of As is influenced by manganese (Mn) oxides and organic matter. Although Mn oxides have been extensively investigated for As(V) adsorption, effects of low molecular weight organic acids (LMWOAs) with different functional groups on As(V) adsorption onto birnessite and underlying mechanisms remain elusive. In this study, LMWOAs with two carboxyl groups (including tartaric (TA), malate (MA), and succinic acids (SA) with two, one and zero hydroxyl groups, respectively) were used. Results showed that more As(V) was adsorbed on birnessite with the presence of LMWOA, indicating that the LMWOA promoted As(V) adsorption via birnessite-carboxyl-As(V) ternary complex. Before birnessite dissolution, TA and MA facilitated As(V) adsorption more efficiently than SA, indicating that hydroxyl group enhanced the coordination among carboxyl groups, As(V) and birnessite. However, within high TA/MA batches, As(V) concentrations decreased sharply and then gradually increased, but Mn(II) concentrations continuously increased, showing the initial reductive dissolution of birnessite promoted As adsorption, while further dissolution was conducive to As mobilization. This study identifies the mechanisms of As adsorption in the presence of LMWOAs and highlights the importance of functional groups in As fate and mobility in aqueous environments.

Effects of low-molecular-weight organic acids/thiols on hydroxyl radical production from natural siderite oxidation

Oxygenation of siderite produces hydroxyl radicals (•OH) that are capable of transforming the pollutants. Yet, the influence of low-molecular-weight organic acids (LMWOAs)/ thiols (LMWOTs) which are widely occurring in soils and waters on •OH production from the oxygenation process of siderite is poorly understood. Results of this study show that the presence of LMWOAs/LMWOTs significantly facilitated •OH production from siderite oxidation by O2 at pH 6–8. For instance, at pH 7, the cumulative •OH concentration within 24 h was only 1.3 μM for oxygenation of 1 g/L siderite, while it boosted to 19.7 and 7.2 μM, respectively, with the respective addition of 1 mM ethylenediaminotetraacetate and cysteine. The mechanisms responsible for the enhanced •OH production are associated with the complexation ability and redox activity of LMWOAs/LMWOTs. In the absence of LMWOAs/LMWOTs, •OH is mainly produced from the oxidation of adsorbed Fe(II) by O2, followed by the oxidation of structural Fe(II) on siderite surface. In the presence of LMWOAs, LMWOAs react with adsorbed Fe(II) and structural Fe(II) to produce complexed Fe(II). Therefore, the oxidation of LMWOAs complexed Fe(II) by O2 predominantly contributed to •OH production. Although the complexation ability of LMWOTs is weaker than that of LMOWAs, LMWOTs can reduce O2 to generate •O2− and H2O2, thus facilitating •OH production. Based on the proposed mechanisms, a kinetic model was developed to quantify the relative contribution of each pathway to •OH production. The addition of 1 mM citrate and cysteine in siderite suspension could increase the removal of trichloroethylene from <5% to 15% and 7%, respectively. Our study reveals the overlooked catalytic effects of LMWOAs/LMWOTs in enhancing •OH production from siderite oxidation by O2 under redox-dynamic conditions.

Effects of low molecular weight organic acids on adsorption of quinclorac by sepiolite.

The effects of low molecular weight organic acids (LMWOAs) on the adsorption of quinclorac by sepiolite were investigated using laboratory batch technique. Experiments were conducted with two natural sepiolite samples with different crystal structures and chemical compositions and high-purity sepiolite. The LMWOAs used were acetic, oxalic, and citric acid. And the adsorption mechanism was characterized using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR). Our analysis revealed that adsorption of quinclorac on α-sepiolite and β-sepiolite was inhibited in the presence of 4 mmol L-1 LMWOAs, whereas LMWOAs stimulated the adsorption of quinclorac in the high-purity sepiolite. Inhibition or stimulation varied across the different types of organic acids. The adsorption isotherms in the presence of 4 mmol L-1 LMWOAs were better explained by Freundlich and linear model. The effect of organic acid concentrations (0-32 mmol L-1) on the adsorption of quinclorac by the three sepiolite samples varies greatly depending on the type of organic acid and the property of sepiolite. FTIR, XRD, and XPS analyses showed that LMWOAs bound strongly to the Si-O bond structure, and Si-O-quinclorac-acetic acid (oxalic acid or citric acid) was formed on the surface of β-sepiolite. The adsorption of quinclorac by β-sepiolite was via hydrogen bond, complexation reactions, and charge transfer in the presence of LMWOAs. These results indicate that LMWOAs affect quinclorac adsorption through various interactions involving competition, electrostatic attraction, bridging action, and hydrogen bonding.

Impact of low-molecular weight organic acids on selenite immobilization by goethite: Understanding a competitive-synergistic coupling effect and speciation transformation

The interactions between low-molecular weight organic acids (LMWOAs) and selenium (Se) on mineral/water interfaces affect the release, immobilization and bioavailability of Se in nature. Herein, the effects of three environmentally relevant LMWOAs (i.e., oxalic (Oxa), succinic (Suc) and citric (Cit) acids) on Se(IV) adsorption to goethite under oxic conditions were investigated using batch experiments, speciation fractionation, and ATR-FTIR and XPS analyses. The LMWOAs exhibited a competitive-synergistic coupling effect on Se(IV) adsorption to goethite, which inhibited the adsorption rate of Se(IV) by 14.1, 13.3 and 8.0 times. However, immobilization of Se(IV) was simultaneously enhanced by 39.1%, 34.6% and 14.1% in the following order Oxa > Suc > Cit. The results obtained by fractionation of the adsorbed Se(IV) revealed that the enhancement was due to surface binding as well as speciation transformation from ligand-exchangeable Se(IV) into residual fractions, which increased by approximately 18% in the presence of the LMWOAs. The dissolution of goethite significantly improved due to the LMWOAs and decreased to different degrees as the concentration of Se(IV) increased. The monodentate mononuclear complexes (58.2%) and Lewis base sites bonded Se (41.8%) were the predominant surface species of Se(IV) in goethite-Se(IV) system. The ATR-FTIR and high-resolution XPS analyses demonstrated that the formation of ≡FeO(SeO)O-CO surface complexes (22.8–27.0%) occurred in the presence of LMWOAs, which could be closely correlated with the interface-mediated reduction of Se(IV). In addition, the predominant mechanism for the formation of residual Se is LMWOA specific, in which ferric selenite-like precipitation was dominant for Suc (10.6%) and Cit (11.6%) and reduction was dominant for Oxa (17.5%). Overall, LMWOAs play an important role in Se(IV) immobilization and speciation transformation and may facilitate understanding the Se bioavailability in rhizosphere soils under oxic conditions.

Effects of low-molecular weight organic acids on the transport of graphene oxide nanoparticles in saturated sand columns

The impact of low-molecular weight organic acids (LMWOAs) on the transport of graphene oxide (GO) nanoparticles in saturated quartz sand was investigated. The different LMWOAs such as acetic acid, glycolic acid, malonic acid, and tartaric acid were used in experiments. The effects of LMWOAs on the transport of GO were markedly dependent upon organic acid species. In general, the transport enhancement effects followed the order of tartaric acid > malonic acid > glycolic acid > acetic acid, the regular pattern might be related to amount and type of functional groups of LMWOAs. Additionally, the different enhanced ability of LMWOAs was determined by their molecular weight. In the presence of Na+, the main deposition mechanism was ascribed to steric hindrance and competition between LMWOA and GO for deposition sites on grain surfaces under acidic conditions (i.e., pH 4.0 and 5.0). Batch adsorption experiments indicated the extents of competitive adsorption between LMWOAs and GO on quartz sand. In addition, the DLVO theory was not applicable to describe the transport of GO in the presence of LMWOAs at pH 5.0. Nevertheless, electrostatic and steric repulsion, existing between GO and sand grains, were the most important deposition mechanisms under the neutral condition (i.e., pH 7.0). When Ca2+ was the main cation in the background solution, the transport enhancement effects followed quite similar order to those of Na+, mainly due to different complexing strength of organic acids.

Effects of low-molecular-weight organic acids on the acute lethality, accumulation, and enzyme activity of cadmium in Eisenia fetida in a simulated soil solution.

In the present study, the effects of low-molecular-weight organic acids (LMWOAs) on the toxicity of cadmium (Cd) to Eisenia fetida were investigated in a simulated soil solution. The LMWOAs protected E. fetida from Cd toxicity, as indicated by the increased median lethal concentration (LC50) values and the increased activity of superoxide dismutase. In addition, Cd concentrations in E. fetida decreased dramatically in the presence of LMWOAs. These results were likely because of the complexation between Cd and LMWOAs, which decreased the bioavailability and consequential toxicity of Cd to E. fetida. Notably, LMWOAs reduced Cd toxicity in decreasing order (ethylenediamine tetraacetic acid [EDTA] > citric acid > oxalic acid > malic acid > acetic acid), which was consistent with the decreasing complexation constants between LMWOAs and Cd. These results advance our understanding of the interactions between Cd and LMWOAs in soil. Environ Toxicol Chem 2017;36:1005-1011. © 2016 SETAC.

Impacts of low-molecular-weight organic acids on aquatic behavior of graphene nanoplatelets and their induced algal toxicity and antioxidant capacity.

Knowledge of the interaction between graphene-based materials and low-molecular-weight organic acids (LOAs) is essential to understand fate and effects of graphene-based materials in the aquatic environment, but this interaction remains poorly elucidated. In this study, the effects of LOAs on the physicochemical properties of graphene nanoplatelets (GNPs) in an aqueous medium and on the GNP toxicity to algae were studied. The unicellular green alga Scenedesmus obliquus was exposed to GNP suspensions in the presence of benzoic acid or gallic acid at various concentrations. The GNPs had smaller hydrodynamic sizes and the GNP suspensions were more stable and had higher or lower surface zeta potentials in the presence of LOAs than when LOAs were not present. The toxic effects in S. obliquus cultures incubated with GNP suspensions containing LOAs were related to the LOA concentration, and the presence of LOAs caused three effects: stimulation, alleviation, and synergistic inhibition. The intensities of the effects mainly correlated with the LOA concentration, the extent of agglomeration, and particle-induced oxidative stress. The results indicate that the environmental fates and toxicities of GNPs are strongly affected by the binding of GNPs to LOAs.

Effects of pH and low molecular weight organic acids on competitive adsorption and desorption of cadmium and lead in paddy soils

The bioavailability and ultimate fate of heavy metals in the environment are controlled by adsorption-desorption process. Batch equilibrium experiments were performed to assess the effects of pH and low molecular weight organic acids (LMWOAs) on competitive adsorption and desorption of cadmium and lead in paddy soils from China. The results indicated that both soils exhibited greater sorption capacity for lead (Pb) (1.37-1.61-fold) than cadmium (Cd) as estimated by the maximum sorption parameter (Q) from the Langmuir equation. The Langmuir bonding energy coefficient (b) and distribution coefficient (K (d)) were greater for Pb than for Cd, furthermore, b (binary) and K (d) (single) were greater than b (single) and K (d binary), indicating that competition for sorption sites promote the retention of both metals on more specific sorption sites. Both Cd and Pb desorption as a function of solution pH was characteristic of "S" pattern. The presence of LMWOAs inhibited Cd or Pb desorption at the low concentrations (≤0.1 mmol L(-1)) but promoted Cd and Pb desorption at higher concentrations (≥0.5 mmol L(-1) for citric acid and ≥1 mmol L(-1) for malic and oxalic acid). The two paddy soils had a greater d (Cd) than d (Pb) in the presence of LMWOAs, indicating that Cd desorption was more affected by the presence of LMWOAs in binary metal system.

Enhanced adsorption of fluoride from aqueous solution onto nanosized hydroxyapatite by low-molecular-weight organic acids

The effects of low molecular weight organic acids (LMWOAs) on the defluoridation capacity of nanosized hydrpxyapatite (nHAP) from aqueous solution were investigated. Defluoridation capacity of nHAP was enhanced in the presence of LMWOAs. The nHAP adsorbed LMWOAs on its surface, and the LMWOAs on nHAP were considered to be the newly formed active sites for fluoride adsorption. At low pH, the adsorbed LMWOAs were protonated, positively charged, and thus could attract fluoride anions by electrostatic attraction. Meanwhile, the protonated LMWOAs could also form hydrogen bonding with fluoride. At high pH, the LMWOAs on nHAP were deprotonated and existed as anions. These organic acid anions on nHAP could be exchanged with fluoride. It was evident that with increasing aqueous LMWOAs concentrations, the LMWOAs adsorbed onto nHAP increased, and subsequently the defluoridation capacity of the adsorbent increased. Results also indicated that the defluoridation capacity increased with increasing the contact time, while decreased with the increase of the adsorbent dose or solution pH. In the presence/absence of LMWOAs, the adsorption isotherms were well fitted by the Freundlich model (R2>0.98), and the sorption kinetics were well described by the pseudo-second-order model. Moreover, thermodynamic parameters indicated that the adsorption was spontaneous and endothermic.

Effects of Low‐Molecular‐Weight Organic Acids on Sorption–Desorption of Phenanthrene in Soils

The impact of low‐molecular‐weight organic acids (LMWOAs) on the sorption–desorption of phenanthrene, used as a representative of polycyclic aromatic hydrocarbons, was investigated with a laboratory batch technique. Experiments were conducted with three types of soil samples containing various organic components: citric, oxalic, and malic acids. The sorption of phenanthrene by soils could be well described by a linear‐type model irrespective of the addition of LMWOAs. The simulated distribution constant (Kd) and C‐normalized distribution constant (Koc) for phenanthrene sorption decreased significantly with the addition of LMWOAs to the soils (P &lt; 0.05). In contrast, the desorption amounts of phenanthrene were clearly enhanced by the addition of LMWOAs, and the impact of citric acid on the sorption–desorption of phenanthrene was generally more significant than those of oxalic or malic acids, probably due to their chemical structures and properties. Compared with the baseline soil samples, the desorption amounts of phenanthrene in the presence of LMWOAs were significantly decreased in soils after 60 d of cultivation (P &lt; 0.05), whereby the level of the fractional soil organic C content (foc, %) increased proportionally with the inhibition strength of phenanthrene desorption in the presence of LMWOAs. Based on these observations, we suggest that the availability of polycyclic aromatic hydrocarbons can be enhanced with suitable types and concentrations of LMWOAs.

Thallium dynamics in contrasting light sandy soils—Soil vulnerability assessment to anthropogenic contamination

The influence of different soil conditions and the presence of LMWOA (Low Molecular Weight Organic Acids) on anthropogenic Tl dynamics were discussed in this study. A shift from the “labile” to the residual fraction during the ageing was identified, indicating Tl incorporation into stable phases (e.g., illite and/or amorphous silicates). The increased water-soluble Tl concentration (1.8-fold, in maximum) after the split application of LMWOA (simulating root exudation) was observed in all soils; partial dissolution of relatively “insoluble” Tl-bearing phases (silicates and eventually oxides) in the presence of LMWOA is suggested. Thermodynamic modeling showed that Tl mobilization in the presence of citric and oxalic acids was indirect and could be attributed to complexation of major elements (Ca, Mg, Al) originating from the dissolution of various soil phases. On the contrary, H +-promoted dissolution by acetic acid was assumed as the predominant mechanism of Tl mobilization. Manganese(III,IV) oxides, illite and probably amorphous silicates were evaluated as the dominant phases responsible for Tl retention in the soils. In carbonate-rich soils, Tl coprecipitation with the newly formed carbonates seems to be an important factor influencing Tl release. Therefore, we suggest data on CEC, pH ZPC and soil mineralogy to be critical for assessment of Tl behavior in soil systems.

Relationship between lead uptake by lettuce and water-soluble low-molecular-weight organic acids in rhizosphere as influenced by transpiration.

The relationship between Pb uptake by leaf lettuce ( Lactuca sativa L.) and water-soluble low-molecular-weight organic acids (LMWOAs) in rhizosphere, as influenced by transpiration (high and low), has been studied. Studies were carried out by culturing lettuce plants grown for 2 weeks in pots filled with quartz sand mixed with anion-exchange resin and then for 30 days in a greenhouse. The potted lettuce plants were subjected to stress by the addition of Pb(NO 3) 2 solutions (100, 200, and 300 mg of Pb L (-1)) and by high and low transpiration treatments for another 10-day period. Blank experiments (without addition of Pb(NO 3) 2 solutions to the pots) were also run. There were no significant differences in the growth of the plants with the addition of Pb(NO 3) 2 solutions in either of the transpirations studies. Uptake of Pb by the shoots and roots of the plants was found to be proportional to the concentration of Pb solutions added, and more accumulation was observed in the roots than in the shoots at the end of days 3 and 10. High transpiration caused more Pb uptake than did low transpiration. One volatile acid (propionic acid) and nine nonvolatile acids (lactic, glycolic, oxalic, succinic, fumaric, oxalacetic, d-tartaric, trans-aconitic, and citric acids) in rhizosphere quartz sand or anion-exchange resin were identified and quantified by gas chromatography analysis with a flame ionization detector. The amount of LMWOAs in rhizosphere quartz sand or anion-exchange resin increased with higher amounts of Pb in quartz sand solution and also with longer duration of the study. The total quantities of the LMWOAs in the rhizosphere quartz sand or anion-exchange resin were significantly higher under high and low transpiration with a 300 mg of Pb L (-1) solution addition at the end of day 10. Compared with our previous related studies (published work), the present study shows that the presence of LMWOAs in rhizosphere does not significantly affect Pb uptake by lettuce plants under high and low transpiration. A physiological mechanism of the roots of lettuce plants governing the relationship between Pb contamination level and quantity of water-soluble LMWOAs in rhizosphere quartz sand and resin, as influenced by transpiration, was proposed.

Fenton Degradation of Organic Pollutants in the Presence of Low-Molecular-Weight Organic Acids: Cooperative Effect of Quinone and Visible Light

The influence of low-molecular-weight organic acids (LMWOAs), such as malonic acid, ethylenediaminetetraacetic acid, and oxalic acid, on the Fenton degradation of organic pollutants was examined under visible irradiation (lambda > 450 nm). The Fenton degradation of malachite green in the dark was completely blocked in the presence of LMWOAs. It was found that either visible light irradiation or the addition of hydroquinone could initiate the dye degradation, but the mineralization yield was almost zero. An important result was that the dye mineralization in the presence of LMWOAs could be achieved when both visible irradiation and hydroquinone were introduced. Similar results were obtained with colorless pollutants, such as benzyltrimethylammonium chloride and 2,4,5-trichlorophenol. We concluded that coupling visible irradiation and hydroquinone could be a strong and universal driving force in the Fenton reaction for the complete degradation and mineralization of organic pollutants, even in the presence of LMWOAs.

KINETICS OF CADMIUM RELEASE FROM SELECTED TROPICAL SOILS FROM KENYA BY LOW-MOLECULAR-WEIGHT ORGANIC ACIDS

The low-molecular-weight organic acids (LMWOAs) commonly present in root exudates may influence the mobility and bioavailability of Cd in soils through the formation of soluble Cd-organic complexes in the soil rhizosphere. However, little is known about the dynamics of Cd released by LMWOAs from highly weathered tropical soils where large amounts of phosphate fertilizers are applied to correct phosphorus deficiency. Cadmium is a contaminant of phosphate fertilizers. The release of Cd from selected tropical soils treated with the Idaho monoammonium phosphate (MAP) fertilizer or the Cd perchlorate-added MAP chemical reagent by LMWOAs (10 -3 M and 10 -2 M) was investigated at 25 °C and at an ionic strength of 0.1 M NaN0 3 solution. The LMWOAs used in this study were acetic, citric, fumaric, malic, oxalic, and succinic acids. The surface soils used in this study were obtained from main agricultural areas in Kenya varying widely in physicochemical properties. The amounts of Cd released from the natural and the treated soils varied with the soils, the treatments, and the nature of the LMWOA. The amount of Cd released from the soils in the presence of LMWOAs also increased with the log stability constant values of the Cd-LMWOA complexes, indicating that Cd was brought into solution by LMWOAs as Cd-LMWOA complexes. A parabolic diffusion equation provided the best fit to the Cd released by LMWOAs during the short reaction period of 0.25 to 1 h. The overall diffusion coefficient values of the Cd released from the natural soils and the treated soils by LMWOAs varied with the soil type and the nature of the LMWOAs. The results showed that the LMWOAs enhanced the rate of Cd released from the soils, especially in the monoammonium phosphate-treated soils. The continuous release of Cd from the soils by renewal of LMWOAs indicates the sustaining power of the soils to replenish the Cd labile pool of the soils. The results also indicate that LMWOAs commonly present in the root exudates play a vital role in the mobilization of Cd in tropical soils and, hence, may influence its uptake by the plants.