Competitive Effect Of Phosphate Research Articles

Study of arsenic adsorption in calcareous soils: Competitive effect of phosphate, citrate, oxalate, humic acid and fulvic acid.

Arsenic (As) bio-availability in the soil is influenced by different organic and inorganic anions. In the present study, the effects of various competitive agents, including phosphate, citrate, oxalate, humic acid (HA), and fulvic acid (FA), on the adsorption of As in calcareous soils were investigated. The results revealed the presence of phosphate, citrate, and oxalate in soil has a significant impact on the arsenic retention (adsorption) in soil which increases the As bio-availability. The negative impact of the competing anions was increased at higher concentrations. The Double Site Langmuir (DSL) isotherm was best fitted to the adsorption data, which indicates that most of the As adsorbed on the low-energy surfaces (non-specific adsorption by oxides, clays, and clay-size calcite). Accordingly, in soil 1, the DSL predicted that, due to phosphate, citrate, and oxalate competition (at a concentration of 10mM), the adsorption capacity of the high- and low-energy surfaces decreased from 86.2 to 33.5, 82.1 and 61.3mg/kg and from 663 to 659, 335.8, and 303.5mg/kg, respectively, Moreover, after addition of phosphate, citrate, and oxalate to the soil-As system, the Langmuir constant of high-energy surfaces decreased from 0.686 to 0.074, 0.261, and 0.301L/mg, respectively. No regular trend was observed for the Langmuir constant of low-energy surfaces. Similarly, in soils 2, 3, and 4, the adsorption capacities of both high- and low-energy surfaces as well as the Langmuir constant of high-energy surfaces decreased by the addition of phosphate, citrate, and oxalate to the soil-As system. HA and FA did not have a significant effect on the As adsorption behavior. Phosphate, citrate, and oxalate, as interfering oxyanions, increased the As bio-availability in the calcareous soils by decreasing the As adsorption.

Effects of phosphate on the transport of graphene oxide nanoparticles in saturated clean and iron oxide-coated sand columns

In this study, transport behaviors of graphene oxide (GO) in saturated uncoated (i.e., clean sand) and goethite-coated sand porous media were examined as a function of the phosphate. We found that phosphate enhanced the transport of GO over a wide range of solution chemistry (i.e., pH 5.0–9.0 and the presence of 10 mmol/L Na+ or 0.5 mmol/L Ca2+). The results were mainly ascribed to the increase of electrostatic repulsion between nanoparticles and porous media. Meanwhile, deposition site competition induced by the retained phosphate was another important mechanism leading to promote GO transport. Interestingly, when the phosphate concentration increased from 0.1 to 1.0 mmol/L, the transport-enhancement effect of phosphate in goethite-coated sand was to a much larger extent than that in clean sand. The observations were primarily related to the difference in the total mass of retained phosphate between the iron oxide-coated sand and clean sand columns, which resulted in different degrees of the electrostatic repulsion and competitive effect of phosphate. When the background solution contained 0.5 mmol/L Ca2+, phosphate could be bind to sand/ goethite-coated sand surface by cation bridging; and consequently, promoted competition between phosphate and nanoparticles for deposition sites, which was an important mechanism for the enhanced effect of phosphate. Moreover, the DLVO theory was applicable to describe GO transport behaviors in porous media in the absence or presence of phosphate. Taken together, these findings highlight the important status and role of phosphate on the transport and fate of colloidal graphene oxide in the subsurface environment.

Competitive adsorption between phosphate and arsenic in soil containing iron sulfide: XAS experiment and DFT calculation approaches

The competitive adsorption between phosphate and arsenic (As) in natural soil and mineral systems is important for controlling subsurface contaminants. Phosphate is known to adsorb more strongly than As in Fe(III) oxide-based mineral systems. Here, X-ray absorption spectroscopy (XAS) and density functional theory (DFT) were employed to understand the fundamental reaction mechanisms of As and phosphate on an FeS surface. The competitive effect of phosphate in As(V)-contaminated soil systems differed with changing FeS concentrations. In soil batches with high FeS contents, As(V) was more strongly adsorbed than phosphate, and no additional As release was observed in the aqueous phase, in the presence of phosphate. Through the XAS measurement of laboratory batch samples, the reaction mechanisms could be comprehensively understood, including the reduction of As(V) to As(III), the secondary precipitation of sulfide minerals, and the adsorption mechanism of As(V) on the surface of FeS. The DFT calculations revealed the relative adsorption strengths on the FeS surface were As(V) > phosphate > As(III). Comparing the adsorption energies revealed that As(V) reacted more strongly with FeS than phosphate and As(III), and that the effect of phosphate co-occurrence was negligible in the FeS–As system, in line with the experimental measurements.

Adsorption of Antimonate by Gibbsite: Reversibility and the Competitive Effects of Phosphate and Sulfate

Core Ideas Antimonate adsorption by gibbsite is pH and ionic‐strength dependent. Antimonate adsorption is reversible in alkaline but hysteretic in acidic systems. Adsorbed Sb(V) generates a downward shift in the gibbsite isoelectric point. Both phosphate and sulfate decrease adsorbed Sb(V) concentrations. The triple‐layer model predicts Sb(V) adsorption using inner and outer sphere surface complexes. Antimony (Sb) is a potential environmental contaminant of emerging concern that occurs in soils in the Sb(V) oxidation state as the antimonate species Sb(OH)6−. In soils, metal oxyhydroxides play an important role in the immobilization of contaminants and in restricting bioaccessibility. One such mineral is gibbsite, which bears the reactive aluminol surface functional group. Both inner‐ and outer‐sphere surface complexation mechanisms have been inferred from conflicting Sb(V) adsorption findings involving aluminol‐bearing minerals. The objectives of this research are to characterize Sb(V) adsorption by gibbsite and to use the macroscopic findings to develop mechanistic adsorption models. Antimonate adsorption envelopes were developed using two equilibrium techniques: continuous pH titration and batch. The adsorption of Sb(V) decreases with increasing pH and increasing ionic strength, suggesting that outer‐sphere surface complexation is an important adsorption mechanism. However, Sb(V) retention is irreversible at pH <6 but is reversible in pH >7 systems, suggesting that inner‐sphere species may be significant in acidic environments. Adsorbed Sb(V) also generates a downward shift in the gibbsite isoelectric point, further supporting the formation of inner‐sphere Sb(V) surface complexes. Both PO4 and SO4 decrease adsorbed Sb(V) concentrations: PO4 throughout the pH 3 to 10 range and SO4 in pH <7 systems. The triple‐layer surface complexation model successfully predicts Sb(V) adsorption by using both outer‐sphere and inner‐sphere surface species. The triple‐layer model also predicts ligand adsorption in the competitive systems without the need for reoptimization.

Role of Fe(II), phosphate, silicate, sulfate, and carbonate in arsenic uptake by coprecipitation in synthetic and natural groundwater

Competitive effects of phosphate, silicate, sulfate, and carbonate on As(III) and As(V) removal at pH ∼7.2 have been investigated to test the feasibility of Fe(II) aq and hydroxylapatite crystals as inexpensive and potentially efficient agents for remediation of contaminated well-water, using Bangladesh as a type study. Arsenic(III) removal ∼50–55% is achieved, when Fe(II) aq oxidizes to Fe(III) and precipitates as Fe(OH) 3 at 25 °C and 3 h reaction time, in the presence of all the oxyanion. Similar results were obtained for well-water samples from two sites in Bangladesh. Heating at 95 °C for 24 h results in 70% As(III) uptake due to precipitation of magnesian calcite. A two-step process, Fe(II) oxidation and Fe(OH) 3 precipitation at 25 °C for 2 h, followed by magnesian calcite precipitation at 95 °C for 3 h, yields ∼65% arsenic removal while reducing the expensive heating period. In the absence of silicate, up to 70% As(III) uptake occurs at 25 °C. In all cases, As(III) was oxidized to As(V) in solution by dissolved oxygen and the reaction rate was probably promoted by intermediates formed during Fe(II) oxidation. Iron-catalyzed oxidation of As(III) by oxygen and hydrogen peroxide is pH-dependent with formation of oxidants in the Fenton reaction. Buffering pH at near-neutral values by dissolved carbonate and hydroxylapatite seeds is important for faster Fe(II) oxidation kinetics ensuring rapid coprecipitation of As as As(V) in the ferric phases.

Physicochemical Characterization of Granular Ferric Hydroxide (GFH) for Arsenic(V) Sorption from Water

Physical and chemical characterization of granular ferric hydroxide (GFH) [e.g., scanning electron micrographs (SEM), X‐ray diffraction (XRD) analysis, Brunauer‐Emmett‐Teller (BET) and Langmuir surface area measurements, pore size distribution, pH titration, and zeta potential measurements] were conducted to determine its performance as an adsorbent for trace arsenic(V) removal. Speciation diagrams for arsenate and phosphate were produced for the present system. The equilibrium adsorption isotherms were measured over initial arsenate concentrations ranging from 100–750 µg/L and the pH range of 4–9. The adsorption of arsenate was found to decrease as the pH of the solution was increased, thus giving the optimal adsorption of arsenate onto GFH at pH 4. Adherence to the Langmuir isotherm was found at all pHs for the arsenate adsorption. The competitive effect of phosphate on the uptake of arsenate at pH 4 by GFH was investigated, outlining the greater affinity of GFH for arsenate adsorption compared to phosphate. The kinetic performance of GFH was assessed and the results were analyzed by applying a particle diffusion model.

Effect of phosphate on the adsorption of 2,4-D on lepidocrocite

The competitive effect of phosphate on the adsorption of 2,4-D on an iron oxyhydroxide is studied. The adsorption isotherms of the herbicide with no phosphate present are strongly pH-dependent and are concave, suggesting a competition with solvent molecules. The presence of phosphate causes a strong decrease in 2,4-D adsorption and a progressive change in the isotherm from concave to convex. The relationships between the adsorbed amounts of both adsorbates show that in their regions of maximum slope the loss of one site for herbicide adsorption is accompanied by the adsorption of at least 10 phosphate groups, and this number increases as the amount of 2,4-D present decreases. It is shown that the proportion of sites lost for 2,4-D adsorption only depends on the amount of phosphate adsorbed and is independent of the total 2,4-D present in the system, and this conclusion can be explained in terms of the Stern model. The order of addition of both adsorbates has little effect on the final distribution of either adsorbate, although P adsorption seems to be slightly retarded by the presence of 2,4-D previously adsorbed. It is suggested that 2,4-D retention is mainly due to electrostatic forces.

Role of positive charge in the competitive effect of phosphate on sulfate sorption by an allophanic soil

The effect of phosphate (P) on the sorption of sulfate (S) by an allophanic soil with high surface positive charge was investigated in the pH range of 5.3-7.0 obtained by incubation with Ca(OH)2. The effect of P on S sorption was directly related to the effect of P on surface positive charge. At low soil pH, P reduced surface positive charge to a larger extent than at high pH and this was associated with a larger effect of P on S sorption.

COMPETITIVE INTERACTIONS OF PHOSPHATE AND MOLYBDATE ON ARSENATE ADSORPTION

The adsorption of arsenate by a Cecil clay was found to be suppressed by the competitive effects of phosphate and molybdate. Competitive Langmuir-type equations have been used in other studies to describe the adsorption data, but in this investigation the adsorption of arsenate alone and in binary mixtures followed a Freundlich-type of response. Competitive Freundlich-type isotherm equations have not been used extensively in soil chemistry studies. The usefulness of two competitive Freundlich-type equations that have been used to model the competitive interactions of dilute organic solutes by activated carbon was examined for application to arsenate adsorption by a Cecil clay as influenced by phosphate and molybdate. The equation of DiGiano et al. (1978) had the potential of being able to predict the effects of competition on adsorption based entirely on single-component data. The Sheindorf et al. (1981) equation required the collection of competitive data to derive a competitive coefficient. Both expressions were found to be useful, although there was no clear indication of which, if either, was the more accurate. The ability of both expressions to describe the data appeared to be limited to situations where the ratios of the equilibrium concentrations of As/P and As/Mo were greater than 20, i.e., when the equilibrium concentration of As was much greater than that of P or Mo. This limitation was partly attributed to the regression procedure used to calculate competitive coefficients.

Ion accumulation in bacterial systems: II. Properties of silicate-metabolizing cell-free extracts and particulate fractions from Proteus mirabilis

The ability of cell-free extracts and participate fractions from Proteus mirabilis to accumulate silicate was found to be independent from previous adaptation of the cells to silicate. Preparations from silicate-adapted material gave 20–30% better results with respect to the amount of silicate accumulated, but responded more sensitively to minor alterations in the external phosphate concentration. The competitive effect of phosphate was found to occur in cell-free extracts as well as in the particulate preparations. The energy-requiring accumulation of silicate can be supported by either substrate-oxidation (NADH, succinate), or ATP cleavage. While silicate-fixation represents an unspecific ion accumulation process, the specific choice of the suitable membrane-bound ions depends on the ADP present in the system. The respiration-dependent but unspecific accumulation process and the highly specific ADP-ATP reaction combined seemed to act as a control mechanism, regulating the alternative stimulation and depression of either process.